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Distributional variations in marine crenarchaeotal membrane lipids: a new tool for reconstructing ancient sea water temperatures?
Abstract
It has recently been shown that membrane lipids of marine crenarchaeota, a ubiquitous and abundant component of plankton, occur in relatively high concentrations in recent and ancient sediments. In this study we investigated the environmental controls on the relative distribution of these lipids in surface sediments. We especially focussed on temperature, as it is known from the thermophilic genetic relatives of marine crenarchaetoa that the composition of their membrane strongly depends on growth temperature. Indeed, a significant linear correlation (r2=0.92) is found between the number of cyclopentane rings in sedimentary membrane lipids derived from marine crenarchaeota and the annual mean sea surface temperatures. This suggests that the mechanism of physical adaptation of their membrane compositions to temperature is identical to that of their thermophilic relatives. In turn, archaeal lipid distributions in sediments may thus allow the reconstruction of sea water temperatures of ancient marine environments.
... Another organic SST proxy is TEX 86 (TetraEther indeX of tetraethers consisting of 86 carbon atoms; Schouten et al., 2002), which is based on nonhydroxylated isoprenoid glycerol dialkyl glycerol tetraethers (iGDGTs; Figure S1) produced by Thaumarchaeota-formerly Group I Crenarchaeota (Schouten, Hopmans, & Sinninghe Damsté, 2013, and references therein;Sinninghe Damsté et al., 2002). Thaumarchaeota is a major group of ammonia oxidizing archaea (Herndl et al., 2005;Ingalls et al., 2006;Karner et al., 2001;Könneke et al., 2005;Wuchter et al., 2006) which substantially if not majoritarily contributes to iGDGT and TEX 86 signals in open oceans (Besseling et al., 2020;Zeng et al., 2019). ...
... Thaumarchaeota is a major group of ammonia oxidizing archaea (Herndl et al., 2005;Ingalls et al., 2006;Karner et al., 2001;Könneke et al., 2005;Wuchter et al., 2006) which substantially if not majoritarily contributes to iGDGT and TEX 86 signals in open oceans (Besseling et al., 2020;Zeng et al., 2019). Core top and mesocosm studies have shown that TEX 86 increases with increasing SST (e.g., Kim et al., 2008Kim et al., , 2010Schouten et al., 2002Schouten et al., , 2007Tierney & Tingley, 2015;Wuchter et al., 2004) and the first applications of TEX 86 as an SST proxy date from more than a decade ago (Huguet, Kim, et al., 2006;Schouten et al., 2003). ...
... In practice, such a scenario rarely occurs and both calibration and stratigraphic studies are performed in parallel. This has been the case for the TEX 86 SST proxy with core top calibrations (e.g., Kim et al., 2008Kim et al., , 2010Schouten et al., 2002;Tierney & Tingley, 2015), mesocosm experiments Wuchter et al., 2004), and stratigraphic studies (e.g., Castañeda et al., 2010;Darfeuil et al., 2016;Hollis et al., 2019, and references therein;Huguet, Kim, et al., 2006;Schouten et al., 2003;Seki et al., 2009). Furthermore, the history of the fields of paleoclimatology and paleoceanography includes many examples for which geochemical proxies were first or mainly studied in a stratigraphic context, before reaching a better understanding with a proper calibration of observed variations versus the climatic parameter of interest. ...
RI‐OH′ and RI‐OH (ring index of hydroxylated tetraethers) are two novel organic paleothermometers which could either complement or replace more established paleothermometric proxies, such as UK′37 (C37 ketone unsaturation ratio) and TEX86 (TetraEther indeX of tetraethers consisting of 86 carbon atoms). Despite a few promising attempts, the paleothermometric potential of RI‐OH′ and RI‐OH is not fully constrained. Here we present new high‐resolution temperature records over the 160–45 ka BP (before present = year 1950 CE) period using four organic proxies (RI‐OH′, RI‐OH, TEX86, and UK′37) from three deep sea sediment cores located in a north‐south transect along the Iberian Margin. We analyzed all organic proxies from a single set of lipid extracts to optimize proxy‐proxy comparisons and phase relationship studies. RI‐OH′ responds to Dansgaard‐Oeschger and Heinrich events, better resembles UK′37 than TEX86, and better records the influence of (sub)polar waters during Heinrich events than does RI‐OH. While RI‐OH′ gives realistic sea surface temperatures and latitudinal gradients coherent with those from independent paleothermometers, a more extensive RI‐OH′‐temperature calibration for the North Atlantic is clearly needed. However, the absence of a significant warm bias in RI‐OH′‐based temperatures compared to a shallow sea site suggests that endemic, deep‐dwelling archaeal communities affect TEX86 but not RI‐OH′ in the Iberian Margin. TEX86 leads RI‐OH′ and UK′37 during four Heinrich‐like events, potentially due to background fluxes from deep waters for nonhydroxylated tetraethers summed with primary productivity dependent fluxes from surface waters for all investigated lipid classes. Relationships with Greenland temperatures further support RI‐OH′‐based paleothermometry.
... Rudists from the Provence and Jura-Bas-Dauphiné platforms in SE France (Sausset/Cluses: 29°-32° N) provide sclerochronological mean oxygen isotope (δ 18 O sclero ) values ranging between − 3.9 (uppermost Lower Barremian) and − 0.3‰ (lowermost Lower Aptian). The majority of upper Barremian δ 18 O sclero values from the Provence cluster around a value of − 2.1‰. ...
... Rudists from the Provence and Jura-Bas-Dauphiné platforms in SE France (Sausset/Cluses: 29°-32° N) provide sclerochronological mean oxygen isotope (δ 18 O sclero ) values ranging between − 3.9 (uppermost Lower Barremian) and − 0.3‰ (lowermost Lower Aptian). The majority of upper Barremian δ 18 O sclero values from the Provence cluster around a value of − 2.1‰. Lower Aptian (pre-OAE1a) rudists inhabiting both proto-Atlantic (Ericeira: 27° N) and Tethyan (Kanfanar: 23° N) shallow-water settings show δ 18 O sclero values near to − 3.4‰. ...
... The majority of upper Barremian δ 18 O sclero values from the Provence cluster around a value of − 2.1‰. Lower Aptian (pre-OAE1a) rudists inhabiting both proto-Atlantic (Ericeira: 27° N) and Tethyan (Kanfanar: 23° N) shallow-water settings show δ 18 O sclero values near to − 3.4‰. At Kanfanar, rudist shell material ascribed to the onset of OAE1a (chemostratigraphic segment C4) 38 provides a δ 18 www.nature.com/scientificreports/ ...
Late Cretaceous sea surface temperatures (SST) are, amongst others, traditionally reconstructed by compiling oxygen isotope records of planktonic foraminifera obtained from globally distributed pelagic IODP drill cores. In contrast, the evolution of Early Cretaceous SSTs is essentially based on the organic TEX 86 palaeothermometer, as oxygen-isotope data derived from well-preserved ‘glassy’ foraminifer calcite are currently lacking. In order to evaluate the extraordinary warm TEX 86 -derived SSTs of the Barremian to Aptian (130–123 Ma) subtropics, we present highly resolved sclerochemical profiles of pristine rudist bivalve shells from Tethyan and proto-North Atlantic shallow water carbonate platforms. An inverse correlation of seasonal ontogenetic variations in δ ¹⁸ O rudist and Mg/Ca ratios demonstrates the fidelity of oxygen isotopes as palaeotemperature proxy. The new data shows moderate mean annual SSTs (22–26 °C) for large parts of the Barremian and Aptian and transient warm pulses for the so-called Mid-Barremian Event and Oceanic Anoxic Event 1a (reaching mean annual SSTs of 28 to 30 °C). A positive shift in mean annual oxygen-isotope values (δ ¹⁸ O: ≤ − 0.3‰) coupled with invariant Mg/Ca ratios at the Barremian–Aptian boundary points to a significant net loss of ¹⁶ O in Tethyan shallow-marine settings. As the positive oxygen-isotope rudist shell values are recorded immediately beneath a major superregional hiatal surface, they are interpreted to be related to a major cooling phase and potential glacio-eustatic sea-level lowering. Our new sclerochemical findings are in clear contrast to open ocean SST records based on TEX 86 , which indicate exceptionally warm Barremian to earliest Aptian subtropical oceans and weak meridional SST gradients.
... GDGTs are cell-membrane lipids synthesized by many bacteria and archaea, including marine planktic Thaumarchaeota (Schouten et al., 2002). In marine environments, the proportions of specific GDGTs are related to the water temperature in which they were synthesized, and are considered to be stable in marine sediments over long time periods e perhaps tens of millions of years (Schouten et al., 2002;de Bar et al., 2019). ...
... GDGTs are cell-membrane lipids synthesized by many bacteria and archaea, including marine planktic Thaumarchaeota (Schouten et al., 2002). In marine environments, the proportions of specific GDGTs are related to the water temperature in which they were synthesized, and are considered to be stable in marine sediments over long time periods e perhaps tens of millions of years (Schouten et al., 2002;de Bar et al., 2019). The concentrations of three isoprenoidal GDGTs (iGDGTs, numbered 1 to 3, and an isomer of crenarchaeol), each containing 86 carbon atoms but having different structures, were used to define the original TEX 86 index (Schouten et al., 2002): ...
... In marine environments, the proportions of specific GDGTs are related to the water temperature in which they were synthesized, and are considered to be stable in marine sediments over long time periods e perhaps tens of millions of years (Schouten et al., 2002;de Bar et al., 2019). The concentrations of three isoprenoidal GDGTs (iGDGTs, numbered 1 to 3, and an isomer of crenarchaeol), each containing 86 carbon atoms but having different structures, were used to define the original TEX 86 index (Schouten et al., 2002): ...
Quaternary interglacial climates are often used as analogues for how the Antarctic Ice Sheet will respond to future climate warming. Southern Ocean marine sediments provide an important paleoclimate archive in this respect. Sea surface temperature (SST) reconstructions in the Southern Ocean depend exclusively on the fossils or geochemical signatures of planktic organisms, but the strengths of these SST proxies remain poorly quantified in this region. To improve confidence in paleoclimate reconstructions, Part 1 of this two-part study evaluates the reliability of Southern Ocean SST proxies employed at Quaternary glacial-interglacial time scales, focusing on three key potential problems: advection/dispersion, seasonality, and non-thermal influences. We find that foraminifera assemblages and long-chain alkenones likely provide the most reliable SST reconstructions in this region. Diatom assemblages and the Globigerina bulloides Mg/Ca ratio are considered to be 'moderately' reliable. Both are subject to potentially significant non-thermal influences, and diatom assemblages are likely modified by species-dependent advection as they sink to the sea floor. Nevertheless, diatoms are valuable at higher latitudes, since alkenones and foraminifera assemblages lose sensitivity below~1 to 2 C. Dinocyst assemblages, radiolarian assemblages, GDGTs and Neogloboquadrina pachyderma Mg/Ca are considered the least reliable in the Southern Ocean, due to weak calibrations, poorly-constrained non-thermal influences, and/or strong advection bias. We note that the seasonality of all proxies remains poorly constrained. Overall, Southern Ocean SST reconstructions using the recommended proxies and calibrations should be robust when averaging across multiple sites and proxy types, but should be treated with caution when analysing spatial variability, a small number of sites, or a single proxy type. Quantifying the effect of advection should be a priority for all planktic groups employed in Southern Ocean paleoclimate reconstructions.
... A total of 81 samples with an average time resolution of around~25-50 kyr (Fig. 2) were measured for total organic carbon (TOC) concentration and bulk organic carbon isotopes (δ 13 C org ), resolving the entire Weissert Event. We also present a detailed (N = 48 samples) sea surface temperature (SST) profile derived from the organic glycerol dialkyl glycerol tetraether (GDGT) paleothermometry TEX 86 23 . 65 samples were used for calcareous nannofossil investigation to refine the original biostratigraphy 22 and 31 samples were investigated for benthic foraminifera assemblages to reconstruct paleowater depths. ...
... Samples were analysed at the University of Cologne using an Agilent 1290 Infinity ultra high permormance liquid chromatography (HPLC) paired with an Agilent GDGT 6460 Triple Quadrupole MS system. Published chromatographic conditions were applied 58 in selected ion monitoring mode, following previous work for GDGTs identification 23,26 . Peak areas were integrated and calibrated against the internal C 46 standard. ...
... The TEX 86 index (Eq. 1) was calculated as originally defined 23 : ...
The Weissert Event ~133 million years ago marked a profound global cooling that punctuated the Early Cretaceous greenhouse. We present modelling, high-resolution bulk organic carbon isotopes and chronostratigraphically calibrated sea surface temperature (SSTs) based on an organic paleothermometer (the TEX86 proxy), which capture the Weissert Event in the semi-enclosed Weddell Sea basin, offshore Antarctica (paleolatitude ~54 °S; paleowater depth ~500 meters). We document a ~3–4 °C drop in SST coinciding with the Weissert cold end, and converge the Weddell Sea data, climate simulations and available worldwide multi-proxy based temperature data towards one unifying solution providing a best-fit between all lines of evidence. The outcome confirms a 3.0 °C ( ±1.7 °C) global mean surface cooling across the Weissert Event, which translates into a ~40% drop in atmospheric pCO2 over a period of ~700 thousand years. Consistent with geologic evidence, this pCO2 drop favoured the potential build-up of local polar ice. Modelling and sea surface temperature proxy data from the Weddell Sea document a 3–4 °C drop coinciding with the Early Cretaceous Weissert Event. Temperature data worldwide confirm a 3.0 °C global mean surface cooling, equivalent to a ~40% drop in atmospheric pCO2, favouring local polar ice.
... Archaeal isoprenoidal glycerol dialkyl glycerol tetraethers (isoGDGTs), sensitive to temperature change and relatively resistant to degradation processes, are wellpreserved in marine sediments (Huguet et al., 2008;Schouten et al., 2013). Hence, isoGDGTs are considered to be valuable 85 tools for reconstructing ocean temperatures (Schouten et al., 2002). ...
... Isoprenoidal GDGTs are archaeal membrane lipid-derived proxies and valuable tools for reconstructing ocean temperatures (Schouten et al., 2002). These specific lipids, preserved in marine sediments, are sensitive to temperature change and relatively resistant to degradation processes (Huguet et al., 2008;Schouten et al., 2013). ...
... These specific lipids, preserved in marine sediments, are sensitive to temperature change and relatively resistant to degradation processes (Huguet et al., 2008;Schouten et al., 2013). Schouten et al. (2002) found that the number of 345 cyclopentane rings in sedimentary GDGTs is correlated with surface water temperatures and developed the first archaeal lipid paleothermometer TEX 86 , a ratio of certain GDGTs, as a sea surface temperature (SST) proxy. ...
The importance of Southern Ocean sea ice has come into the focus of polar research in the last couple of decades. Especially in West Antarctica, where sea ice has declined, its distribution and evolution play a critical role for the stability of nearby ice shelves. Organic geochemical analyses of marine surface sediments from the West Antarctic continental shelves permit a biomarker-based reconstruction of sea surface conditions in these vulnerable areas. We analysed highly branched isoprenoids (HBIs), such as the sea-ice proxy IPSO25 and phytoplankton-derived HBI-trienes, but also phytosterols and isoprenoidal glycerol dialkyl glycerol tetraethers (GDGTs), which are established tools for the reconstruction of primary productivity and sea surface temperatures, respectively. The combination of IPSO25 with a phytoplankton marker results in the semi-quantitative sea-ice index PIPSO25, which provides useful reconstructions of sea-ice conditions, avoiding misleading over- or underestimations of sea-ice cover. Comparisons of the biomarker-based sea-ice distribution patterns and GDGT-based temperatures with (1) sea-ice distributions obtained from satellite observations and (2) estimated sea-ice patterns and SSTs deduced from modelled data are in reasonable agreement, but also highlight specific aspects that need to be considered when interpreting biomarker data. We further discuss IPSO25 concentrations in the vicinity of ice shelves, where elevated values could be related to the occurrence of ice shelf basal melt water and platelet ice under landfast sea ice.
... A main focus has been on the isoprenoidal glycerol dialkyl glycerol tetraethers (isoGDGTs) produced by archaea. The degree of cyclization of isoGDGTs is correlated with temperature in both laboratory cultures (De Rosa et al., 1980) and natural samples (Schouten et al., 2002;Kaur et al., 2015). This forms the basis for the TEX 86 paleothermometer (Schouten et al., 2002 that is widely used to reconstruct sea surface temperature (SST) across a range of time scales and as far back as the early Jurassic (e.g., Hertzberg et al., 2016;Robinson et al., 2017;Petrick et al., 2018). ...
... The degree of cyclization of isoGDGTs is correlated with temperature in both laboratory cultures (De Rosa et al., 1980) and natural samples (Schouten et al., 2002;Kaur et al., 2015). This forms the basis for the TEX 86 paleothermometer (Schouten et al., 2002 that is widely used to reconstruct sea surface temperature (SST) across a range of time scales and as far back as the early Jurassic (e.g., Hertzberg et al., 2016;Robinson et al., 2017;Petrick et al., 2018). However, other factors can also influence the degree of cyclization of isoGDGTs under specific conditions (Pearson et al., 2004;Elling et al., 2015). ...
... Some studies adapted pre-existing calibrations to develop regional models for the Antarctic polar environment. Shevenell et al. (2011) modified the calibration model of Schouten et al. (2002) and suggested a regional calibration for the Antarctic Peninsula. More recently, Park et al. (2019) studied the seasonal effect on the GDGT-based thermometry from the Antarctic Polar Front. ...
Isoprenoidal glycerol dialkyl glycerol tetraethers (isoGDGTs) are archaeal biomarkers. In many settings, the degree of cyclization of isoGDGTs is correlated with temperature, forming the basis of the TEX86 paleothermometer that is widely used to reconstruct sea surface temperature (SST) across a range of time scales. However, the application of TEX86 to the polar regions is relatively limited and there is currently no consensus on which calibration is best suited for polar environments. In addition, application of TEX86 to the polar regions is complicated by uncertainty regarding the source of organic matter input in coastal polar environments. We tested five different calibrations for TEX86 in marine sediments from the Antarctic coastal region of Admiralty Bay near King George Island, using four short cores that span the second half of the 20th century. We also explored the possible sources of organic matter in these cores using sterol biomarkers. Best results for TEX86 were obtained using a quadratic calibration. The TEX86 signal presented a strong seasonal signal and best matched reanalysis temperatures of the austral spring season (Oct-Nov-Dec). The most abundant compounds observed in the sediments were the sterols cholest-5-en-3β-ol and 24-ethylcholest-5-en-3β-ol, the fatty alcohols C16 and phytol, and isoGDGT-0, indicating a dominant marine origin of the organic matter. Differences in their vertical distributions suggests that some compounds (such as cholest-5-en-3β-ol and phytol) may have had different sources over the evaluated period. Together our results indicate that TEX86 can be used to reconstruct SSTs in the Antarctic coastal region.
... SST+0.092. Different alkenone calibration equations would yield the same pattern of SST changes, which is also true for the TEX 86 proxy (Schouten et al., 2002). Alkenone C 37 concentration is the sum of C 37:2 and C 37:3 . ...
... The relevant GDGT-based proxies, TEX 86 (tetraether index of 86 carbon atoms, Schouten et al., 2002), BIT (Branched and Isoprenoid Tetraether Index, Hopmans et al., 2004), and ACE (ratio of Archaeol to Caldarchaeol index, Turich and Freeman, 2011;Wang et al., 2013) are calculated as follows: ...
The Pliocene climate, characterized by sustained global warmth and weakened Walker Circulation, might be a potential analogue of our future climate. How Asian Summer Monsoon (ASM) evolved over this period and its controlling mechanisms remain controversial, which limits our understanding of future monsoon behavior. Here, we present alkenone- and GDGT-based records reconstructed from the northern coast of the South China Sea, interpreted to document the strength of ASM-induced upwelling/mixing effect, to infer Late Miocene-Pliocene ASM variability. Cooler local sea surface temperatures occurred between ∼5.6 million years ago (Ma) and 4.3 Ma, corresponding to Pliocene warmer conditions, effectively capturing the cooling signal induced by summer upwelling. Our multiple biomarker records consistently indicate three phases of Late Miocene-Pliocene ASM evolution: weak ASM strength at ∼6.5–5.6 Ma, substantially enhanced strength at 5.6–4.3 Ma, and reduced strength but with a slightly strengthening trend after ∼4.3 Ma. The three features have been reported collectively from previous terrestrial and marine records. The close correspondence of the ASM strength to tropical temperature before ∼4.3 Ma suggests a dominant tropical temperature control, whereas the increasing ASM strength after that, opposite to temperature changes, implies that the developed Walker Circulation started to influence ASM strength over the Late Pliocene. Hence, tropical temperature and Walker Circulation appear to have exerted competing roles in the Late Miocene-Pliocene ASM evolution.
... Archaea, as one of three domains of life in addition to Bacteria and Eukaryota (Woese et al., 1990), play a pivotal role in the geochemical cycles in marine environments (Biddle et al., 2006;Lipp et al., 2008;Offre et al., 2013), where their habitable zone extends from surface waters to at least~2 km below the seafloor (Inagaki et al., 2015;Santoro et al., 2019). Due to the ubiquitous distribution of Archaea in marine environments, their membrane lipids have been widely used in both paleoenvironmental (e.g., Schouten et al., 2002;Wö rmer et al., 2014;Elling et al., 2019) and modern biogeochemical studies (Hinrichs et al., 1999;Elvert et al., 2000Elvert et al., , 2005Biddle et al., 2006;Schubotz et al., 2011). Archaeal membranes primarily comprise glycerol phytanyl diethers (e.g. ...
... Our dataset also implies that the post-depositional production of caldarchaeol should be taken into consideration when applying the crenarchaeol to caldarchaeol tetraether (CCaT) index (cf. Wö rmer et al, 2014), a cautionary note consistent with early suggestions by Schouten et al. (2002) that resulted in the exclusion of this compound in the TEX 86 proxy. ...
Archaea occupy an important niche in the global carbon cycle and their lipids are widely used as indicators of environmental conditions in both paleoenvironmental and modern biogeochemical studies. The principal sources of archaeal lipids in marine sediments are benthic archaea, fossil remnants of planktonic archaea, and allochthonous sources such as soils. However, the relative contributions of these sources to the sedimentary lipid pool have not been comprehensively constrained, complicating a mechanistic understanding of archaeal lipid proxies. In order to provide insights into the relative contributions of these sources and identify signals derived from sedimentary activity, we performed a systematic survey of stable carbon isotopic compositions (δ¹³C) of both core and intact archaeal lipids via analyses of their phytanyl (Phy) and biphytanyl (BP) moieties in diverse marine sediments. The sample set consisted of 44 sediment horizons from the Mediterranean and adjacent basins and represented diverse sources of organic matter and depositional conditions. Complementary geochemical data enabled the comparison of lipid distributions and carbon isotopic signatures with prevailing redox conditions. The δ¹³C of tricyclic BP (BPcren) from the core and intact forms of crenarchaeol ranged from -19.1 to -28.6‰ and –18.1 to –27.4‰, respectively. δ¹³C values of core and intact BPcren did not differ, suggesting that intact crenarchaeol is either a fossil relic from planktonic archaea or a product of lipid recycling by benthic archaea, as opposed to being synthesized de novo by sedimentary archaea. δ¹³C values of BP0 derived from core and intact forms of glycerol and butanetriol dibiphytanyl glycerol tetraethers (GDGTs and BGDTs, respectively), but predominantly from caldarchaeol (GDGT-0), ranged from –19.4 to –32.0‰ and –20.9 to –37.0‰, respectively. In contrast to BPcren, intact-lipid derived BP0 was often ¹³C-depleted relative to its core counterpart, consistent with in situ production by sedimentary archaea. This relative depletion was most pronounced in sulfate reduction zones, likely due to heterotrophic activity. Core and intact archaeol exhibited the largest ranges in δ¹³C values, from –21.6 to –42.1‰ and –22.7 to –58.9‰, respectively. This strong ¹³C–depletion relative to both total organic carbon and dissolved inorganic carbon is consistent with mixtures of functional sources of sedimentary chemolithoautotrophic, methanotrophic, methanogenic and heterotrophic archaea. Based on the substantial ¹³C-depletion of BPcren and BP0 in samples in the vicinity of the Rhône River delta relative to a distal marine reference site, we infer that the terrestrial soil contribution of archaeal lipids to these sediments is as high as 43%. Hence, terrestrial input of archaeal lipids, including their intact forms, can be substantial and suggests caution when using existing molecular proxies aimed at constraining riverine input. In summary, our comparative isotopic analysis of sedimentary core versus intact archaeal lipids improves the apportionment of their diverse sources and confidence in distinguishing in situ lipid production by sedimentary archaea.
... Instead, they show a gradual 50 transition from low to moderately high values, which can be attributed to the integration of GDGTs from a range of shallow to intermediate depths. Likewise, TEX86 (Equation 1) may reflect a depth-integrated surface to subsurface signal (SubST), that is sensitive to temperature variability at and below the surface ocean (Kim et al., 2008(Kim et al., , 2012Schouten et al., 2002;Tierney and Tingley, 2015). (1). ...
... However, the targeted depth interval and weight distribution of the temperature integration remain subject of discussion Laepple, 2016, 2017;Tierney et al., 2017). The correlation of core top TEX86 to SST is not significantly stronger than to SubST, as obtained from a wide range of depth intervals (Ho and Laepple, 2016;Schouten et al., 2002). Therefore, the sensitivity of sedimentary TEX86 to https://doi.org/10.5194/cp-2021-92 ...
TEX86 is a paleothermometer based on Thaumarcheotal glycerol dialkyl glycerol tetraether (GDGT) lipids and is one of the most frequently used proxies for sea-surface temperature (SST) in warmer-than-present climates. However, the calibration of TEX86 to SST is controversial because its correlation to SST is not significantly stronger than that to depth-integrated surface to subsurface temperatures. Because GDGTs are not exclusively produced in and exported from the surface ocean, sedimentary GDGTs may contain a depth-integrated signal that is sensitive to local subsurface temperature variability, which can only be proved in downcore studies. Here, we present a 15 Myr TEX86 record from ODP Site 959 in the Gulf of Guinea and use additional proxies to elucidate the source of the recorded TEX86 variability. Relatively high GDGT[2/3] ratio values from 13.6 Ma indicate that sedimentary GDGTs were partly sourced from deeper (> 200 m) waters. Moreover, late Pliocene TEX86 variability is highly sensitive to glacial-interglacial cyclicity, as is also recorded by benthic δ18O, while the variability within dinoflagellate assemblages and surface/thermocline temperature records (Uk’37 and Mg/Ca), is not primarily explained by glacial-interglacial cyclicity. Combined, these observations are best explained by TEX86 sensitivity to sub-thermocline temperature variability. We conclude that the TEX86 record represents a depth-integrated signal that incorporates a SST and a deeper component, which is compatible the present-day depth distribution of Thaumarchaeota and with the GDGT[2/3] distribution in core tops. The depth-integrated TEX86 record can potentially be used to infer SST variability, because subsurface temperature variability is generally tightly linked to SST variability. Using a subsurface calibration with peak calibration weight between 100–350 m, we estimate that east equatorial Atlantic SST cooled by ~4.5 °C between the Late Miocene and Pleistocene. On shorter timescales, we use the TEX86 record as an Antarctic Intermediate Water (AAIW) proxy and evaluate climatological leads and lags around the Pliocene M2 glacial (~3.3 Ma). Our record, combined with published information, suggests that the M2 glacial was marked by AAIW cooling during an austral summer insolation minimum, and that decreasing CO2 levels were a feedback, not the initiator, of glacial expansion.
... It should also be noted that basal metabolism involves ATPrequiring pathways that are essential for cell survival, with a significant proportion of these pathways being protein synthesis, which maintains potential energy gradients across membranes 20 . Furthermore, increasing temperatures have also been positively linked to a higher production of cell wall proteins 27 and to a change in the composition of cell structural membranes 28 . The fact that temperature has a regulating effect on the cell biosystems machinery suggests that cellular physiological adaptations, including qualitative and quantitative variations in the cell wall proteins 27,28 , would result in a higher relative abundance of (inferred) cell structure and cell wall biosynthesis pathways. ...
... Furthermore, increasing temperatures have also been positively linked to a higher production of cell wall proteins 27 and to a change in the composition of cell structural membranes 28 . The fact that temperature has a regulating effect on the cell biosystems machinery suggests that cellular physiological adaptations, including qualitative and quantitative variations in the cell wall proteins 27,28 , would result in a higher relative abundance of (inferred) cell structure and cell wall biosynthesis pathways. As expected, we recorded an increase in relative abundances of cell structure and cell wall biosynthesis pathways as temperatures increased northward along the transect. ...
Global oceanographic monitoring initiatives originally measured abiotic essential ocean variables but are currently incorporating biological and metagenomic sampling programs. There is, however, a large knowledge gap on how to infer bacterial functions, the information sought by biogeochemists, ecologists, and modelers, from the bacterial taxonomic information (produced by bacterial marker gene surveys). Here, we provide a correlative understanding of how a bacterial marker gene (16S rRNA) can be used to infer latitudinal trends for metabolic pathways in global monitoring campaigns. From a transect spanning 7000 km in the South Pacific Ocean we infer ten metabolic pathways from 16S rRNA gene sequences and 11 corresponding metagenome samples, which relate to metabolic processes of primary productivity, temperature-regulated thermodynamic effects, coping strategies for nutrient limitation, energy metabolism, and organic matter degradation. This study demonstrates that low-cost, high-throughput bacterial marker gene data, can be used to infer shifts in the metabolic strategies at the community scale.
... GDGTs d glycerol dialkyl glycerol tetraethers d are a group of biomarker compounds synthesized by microbial cell membranes that can be preserved in geological archives (Schouten et al., 2013). Also, isoprenoid GDGTs (isoGDGTs) produced by archaea as well as branched GDGTs (brGDGTs) produced by bacteria have formalized structures which encode environmental information (Schouten et al., 2002;Weijers et al., 2007a,b;Tierney et al., 2010;Wang et al., 2019;He et al., 2020;Wang et al., 2021). Sea surface temperature reconstructions have been made using TEX 86 (TetraEther index of 86 carbon atoms), an index established using isoGDGTs. ...
... Sea surface temperature reconstructions have been made using TEX 86 (TetraEther index of 86 carbon atoms), an index established using isoGDGTs. (Schouten et al., 2002;Liu et al., 2009b;Zhang et al., 2014). In terrestrial systems, Weijers et al. (2007a) have calibrated the relationship between soil brGDGTs and MAT (Mean Air Temperature), which has promoted the use of brGDGTs for temperature reconstruction in terrestrial environments (Weijers et al., 2007b;Peterse et al., 2011), and later 5-methyl isomers of brGDGTs isolated with improved mass spectrometry separation techniques further clarified the temperature dependence of brGDGTs (De Jonge et al., 2014;Ding et al., 2015). ...
Available online xxx Handling Editor: P Rioual Keywords: Daye lake Water depth reconstruction Precipitation pattern Little ice age %Cren a b s t r a c t Alpine lakes are natural rain gauges, and reconstructing changes in their water level is a key to understanding the regional hydrological environment, climate change and vegetation evolution. Precipitation in the northern and the southern parts of the eastern monsoon region of China exhibits a centennial-scale inverse relationship over the past 2000 years; however, there is substantial uncertainty regarding the temporal range of this dipolar pattern. In order to better understand this north-south pattern of precipitation variation and its driving mechanism, we analyzed isoGDGTs biomarker compounds in a sediment core from alpine Daye Lake, in the Qinling Mountains, in the north-south climatic transition zone of eastern China. Measurements of %Cren were used to reconstruct changes in lake level over the past 2000 years. The results show that, from 240 to 1300 CE, prior to the Little Ice Age, the lake-level changes were consistent with the precipitation record for the northern part of eastern China, with the lake reaching its highest level of 25 ± 7.17 m at 555 CE; subsequently, the lake fell to its lowest level of 12 ± 7.17 m at 1030 CE. During the Little Ice Age, the water level maintained an increasing trend, especially during the last three centuries, when it remained above 20 ± 7.17 m, which is consistent with the precipitation record from southern China. The results indicate that the climatically transitional Qinling region has a complex history of climate change. During the early part of the record (240e1300 CE), the level of Daye Lake and the East Asian summer monsoon precipitation were in phase, controlled mainly by the strength of the East Asian summer monsoon. In contrast, since the Little Ice Age (1300 CE to the present), under the influence of ENSO, the westward extension and southward retreat of the West Pacific Subtropical High caused the rain belt to shift southward, decreasing the water vapor supply to the Qinling Mountains. The ascent of moisture-bearing air over the Qinling Mountains resulted in orographic rainfall, while the weakening of evaporation during the Little Ice Age reduced the evaporation of water vapor and also contributed to the continued rise of the level of Daye Lake. The abundant precipitation in the Qinling region during the Little Ice Age provided water resources to sustain human activities in the downstream Weihe Plain, but was also a major cause of flooding.
... Sediment samples were freeze-dried, homogenised and 2 g of dry sediment weighed. The GDGT fraction was extracted with DCM: MeOH (9:1) on a Dionex accelerated solvent extractor (ASE), following the method outlined in Schouten et al. (2002). This fraction was subsequently passed through aluminium oxide (Al 2 O 3 ) columns, and spiked with a deuterated C 46 standard (Huguet et al., 2006), monitoring m/z 748. ...
... A one-point calibration was used to quantify GDGTs using the internal standard. The analysis was by HPLC-MS, following Baker et al. (2019) and Schouten et al. (2002). Peak areas were integrated following the methods described in Weijers et al. (2007). ...
The alpine area of the Australian mainland is highly sensitive to climate and environmental change, and potentially vulnerable to ecosystem tipping points. Over the next two decades the Australian alpine region is predicted to experience temperature increases of at least 1 °C, coupled with a substantial decrease in snow cover. Extending the short instrumental record in these regions is imperative to put future change into context, and potentially provide analogues of warming. We reconstructed temperatures, using a lipid biomarker palaeothermometer technique, and mercury flux changes for the past 3500 years from the sediments of Club Lake, a high-altitude alpine tarn in the Snowy Mountains, southeastern Australia. Using a multi-proxy framework, including pollen and charcoal analyses, high-resolution geochemistry, and ancient microbial community composition, supported by high-resolution ²¹⁰Pb and AMS ¹⁴C dating, we investigated local and regional ecological and environmental changes occurring in response to changes in temperature. We find the region experienced a general warming trend over the last 3500 years, with a pronounced climate anomaly occurring between 1000 and 1600 cal yrs. BP. Shifts in vegetation took place during this warm period, characterised by a decline in alpine species and an increase in tree and grass taxa which co-occurred with an increase in regional fire activity. Given the narrow altitudinal band of Australian alpine vegetation, any future warming has the potential to result in the extinction of alpine species, including several endemic to the area, as treelines are driven to higher elevations. These findings suggest strong conservation efforts will be needed to protect the vulnerable alpine environments from the combined threats of climate changes, fire and invasive species.
... Low cyclic diversity of GDGTs in the Amundsen and Scotia seas could be due to differences in the synthesis of these lipids by the source Thaumarchaeota. The relationship between ocean temperature and the cyclicity of GDGTs has been firmly established, with increasing ocean temperatures correlated with increasing relative abundance of GDGTs with two or more cyclopentane moieties (Schouten et al., 2002(Schouten et al., , 2007Kim et al., 2008Kim et al., , 2010. However, Kim et al. (2010) note some differences between subtropical and subpolar oceans, with cren playing a more important role in temperature reconstructions in the subtropics than in polar oceans, suggesting that there may be differences in membrane adaptation strategies of Thaumarchaeota. ...
The West Antarctic Ice Sheet (WAIS) is one of the largest potential sources
of future sea-level rise, with glaciers draining the WAIS thinning at an
accelerating rate over the past 40 years. Due to complexities in calibrating
palaeoceanographic proxies for the Southern Ocean, it remains difficult to
assess whether similar changes have occurred earlier during the Holocene or
whether there is underlying centennial- to millennial-scale forcing in
oceanic variability. Archaeal lipid-based proxies, specifically glycerol
dialkyl glycerol tetraether (GDGT; e.g. TEX86 and TEX86L), are
powerful tools for reconstructing ocean temperature, but these proxies have
been shown previously to be difficult to apply to the Southern Ocean. A
greater understanding of the parameters that control Southern Ocean GDGT
distributions would improve the application of these biomarker proxies and
thus help provide a longer-term perspective on ocean forcing of Antarctic
ice sheet changes. In this study, we characterised intact polar lipid (IPL)-GDGTs, representing (recently) living archaeal populations in suspended
particulate matter (SPM) from the Amundsen Sea and the Scotia Sea. SPM
samples from the Amundsen Sea were collected from up to four water column
depths representing the surface waters through to Circumpolar Deep Water
(CDW), whereas the Scotia Sea samples were collected along a transect
encompassing the sub-Antarctic front through to the southern boundary of the
Antarctic Circumpolar Current. IPL-GDGTs with low cyclic diversity were
detected throughout the water column with high relative abundances of
hydroxylated IPL-GDGTs identified in both the Amundsen and Scotia seas.
Results from the Scotia Sea show shifts in IPL-GDGT signatures across
well-defined fronts of the Southern Ocean. Indicating that the
physicochemical parameters of these water masses determine changes in
IPL-GDGT distributions. The Amundsen Sea results identified GDGTs with
hexose-phosphohexose head groups in the CDW, suggesting active GDGT synthesis
at these depths. These results suggest that GDGTs synthesised at CDW depths
may be a significant source of GDGTs exported to the sedimentary record and
that temperature reconstructions based on TEX86 or TEX86L
proxies may be significantly influenced by the warmer waters of the CDW.
... Both types of GDGTs are ubiquitous and occur in a variety of environments, such as oceans , lakes (Russell et al. 2018), soils (Weijers et al. 2007;Peterse et al. 2012), peats (Naafs et al. 2017b) and speleothems (Yang et al. 2011;Blyth & Schouten 2013). Therefore, GDGT-based proxies (such as TEX 86 , MBT/ CBT, MBT'/CBT) are widely used to quantify past temperature changes within different natural archives (Schouten et al. 2002;Peterse et al. 2011;Sinninghe Damsté et al. 2012;Krause et al. 2019). Compared to the methods used for reconstructing past temperature changes in the marine environment, techniques for continental temperature reconstruction are comparatively few. ...
The nature of Holocene temperature changes is controversial because of the apparent discrepancy between global temperature reconstructions and climate modelling results. Here we present evidence indicating that the discrepancy can be attributed to the combination of seasonal biases in the proxy temperature records and insufficient understanding of the mechanisms within climate models. We obtained independently dated records of mean annual air temperature (MAAT) based on glycerol dialkyl glycerol tetraethers (GDGTs) from Bangong Co in the western Qinghai‐Tibetan Plateau (QTP) and two loess‐palaeosol sections in the northeastern QTP. One of the most prominent features in the composite MAAT record is the relatively cool Middle Holocene, ~2 °C cooler than the Early and Late Holocene. The MAAT variations are consistent with orbitally forced modelling results that show increasing regional temperatures since the Middle Holocene, but they contrast with global temperature reconstructions that mainly reflect summer temperature variations. Our findings suggest that changes in boreal summer insolation that led to variations in precipitation patterns and vegetation dynamics were the key factor modulating MAAT during the Holocene. Overall, our study sheds light on possible approaches to solving the Holocene temperature conundrum by considering internal feedbacks within the climate system.
... Given the importance of ocean thermal forcing to the dynamics of AIS marine margins and grounding lines, we will also evaluate the relationship between sea-surface temperature (SST) and IBRD flux. Because the alkenone unsaturation index (U k′ 37 ) is likely not applicable (Shevenell et al., 2011), we will extract isoprenoid and branched glycerol diether glycerol tetraether (GDGT) lipids to measure the TEX 86 ratio (Schouten et al., 2002) and convert it to SST using published calibrations (e.g., Kim et al., 2008). ...
International Ocean Discovery Program Expedition 382, Iceberg Alley and Subantarctic Ice and Ocean Dynamics, investigated the long-term climate history of Antarctica, seeking to understand how polar ice sheets responded to changes in insolation and atmospheric CO2 in the past and how ice sheet evolution influenced global sea level and vice versa. Five sites (U1534–U1538) were drilled east of the Drake Passage: two sites at 53.2°S at the northern edge of the Scotia Sea and three sites at 57.4°–59.4°S in the southern Scotia Sea. We recovered continuously deposited late Neogene sediments to reconstruct the past history and variability in Antarctic Ice Sheet (AIS) mass loss and associated changes in oceanic and atmospheric circulation.
The sites from the southern Scotia Sea (Sites U1536–U1538) will be used to study the Neogene flux of icebergs through “Iceberg Alley,” the main pathway along which icebergs calved from the margin of the AIS travel as they move equatorward into the warmer waters of the Antarctic Circumpolar Current (ACC). In particular, sediments from this area will allow us to assess the magnitude of iceberg flux during key times of AIS evolution, including the following:
The middle Miocene glacial intensification of the East Antarctic Ice Sheet,
The mid-Pliocene warm period,
The late Pliocene glacial expansion of the West Antarctic Ice Sheet,
The mid-Pleistocene transition (MPT), and
The “warm interglacials” and glacial terminations of the last 800 ky.
We will use the geochemical provenance of iceberg-rafted detritus and other glacially eroded material to determine regional sources of AIS mass loss. We will also address interhemispheric phasing of ice sheet growth and decay, study the distribution and history of land-based versus marine-based ice sheets around the continent over time, and explore the links between AIS variability and global sea level.
By comparing north–south variations across the Scotia Sea between the Pirie Basin (Site U1538) and the Dove Basin (Sites U1536 and U1537), Expedition 382 will also deliver critical information on how climate changes in the Southern Ocean affect ocean circulation through the Drake Passage, meridional overturning in the region, water mass production, ocean–atmosphere CO2 transfer by wind-induced upwelling, sea ice variability, bottom water outflow from the Weddell Sea, Antarctic weathering inputs, and changes in oceanic and atmospheric fronts in the vicinity of the ACC.
Comparing changes in dust proxy records between the Scotia Sea and Antarctic ice cores will also provide a detailed reconstruction of changes in the Southern Hemisphere westerlies on millennial and orbital timescales for the last 800 ky. Extending the ocean dust record beyond the last 800 ky will help to evaluate dust-climate couplings since the Pliocene, the potential role of dust in iron fertilization and atmospheric CO2 drawdown during glacials, and whether dust input to Antarctica played a role in the MPT.
The principal scientific objective of Subantarctic Front Sites U1534 and U1535 at the northern limit of the Scotia Sea is to reconstruct and understand how intermediate water formation in the southwest Atlantic responds to changes in connectivity between the Atlantic and Pacific basins, the “cold water route.” The Subantarctic Front contourite drift, deposited between 400 and 2000 m water depth on the northern flank of an east–west trending trough off the Chilean continental shelf, is ideally situated to monitor millennial- to orbital-scale variability in the export of Antarctic Intermediate Water beneath the Subantarctic Front. During Expedition 382, we recovered continuously deposited sediments from this drift spanning the late Pleistocene (from ~0.78 Ma to recent) and from the late Pliocene (~3.1–2.6 Ma). These sites are expected to yield a wide array of paleoceanographic records that can be used to interpret past changes in the density structure of the Atlantic sector of the Southern Ocean, track migrations of the Subantarctic Front, and give insights into the role and evolution of the cold water route over significant climate episodes, including the following:
The most recent warm interglacials of the late Pleistocene and
The intensification of Northern Hemisphere glaciation.
... They include archaeal isoprenoid (iso) GDGTs and bacterial branched (br) GDGTs that differ in alkyl chain structures and the number of cyclopentyl moieties (Supplementary Fig. S1; Schouten et al., 2013). Proxies developed based on GDGTs have been widely used in paleoclimate and paleoecology reconstruction due to their wide occurrence in a variety of geological settings (Sinninghe Damsté et al., 2000;Schouten et al., 2002;Weijers et al., 2007;Naafs et al., 2017;Dong et al., 2019;Lu et al., 2019). For example, tetraether index of 86 carbon atoms (TEX 86 ), based on isoGDGTs, was initially developed and applied to marine environments Kim et al., 2010), and later its application was extended to large lakes with minor terrigenous organic input (Powers et al., 2010). ...
Glycerol dialkyl glycerol tetraethers (GDGTs) are widespread but unique membrane-spanning lipids of many Archaea and some Bacteria. However, their specific biological sources and the associated environmental controls on their distribution remain unclear, especially in lacustrine settings, hindering our understanding of these compounds and their application as environmental proxies. Here we investigated the GDGT distributions across a large bottom water dissolved oxygen (DO) gradient (0.10–7.20 mg/L) using surface sediments of Lake Yangzonghai, a warm monomictic lake in southwestern China. We show that the distributions of both branched (br) and isoprenoidal (iso) GDGTs co-vary significantly with bottom water DO concentration. The relative abundances of tetramethylated brGDGTs, 5-methyl penta- and hexamethylated brGDGTs and isoGDGT-0 show a stepwise increase as bottom water DO concentration decreases. On the other hand, the relative abundances of 6-methyl penta- and hexamethylated brGDGTs and crenarchaeol exhibit a stepwise decline with a decrease in bottom water DO concentration. Genetic data indicate these DO-induced changes in GDGT parameters are related to changes in the bacterial and archaeal communities across the oxycline in the lake. For example, the high abundance of isoGDGT-0 in low DO samples coincides with a high abundance of methanogenic archaea and Bathyarchaeota. We propose that brGDGT-producing bacteria might include a diversity of other phyla in addition to the proposed source organism acidobacteria; different groups of anaerobic and aerobic bacteria are likely to contribute to the increased abundance of 5-methyl brGDGTs and 6-methyl brGDGTs in low and high DO zones, respectively. Consequently, the MBT′ and MBT′6ME brGDGT indices display strong correlations with DO concentration. Importantly, the MBT′5ME index is not significantly influenced by changes in DO concentration, suggesting that this index might be more resilient to these impacts and more suitable to reconstruct temperature in lake systems.
... Given the importance of ocean thermal forcing to the dynamics of AIS marine margins and grounding lines, we will also evaluate the relationship between sea-surface temperature (SST) and IBRD flux. Because the alkenone unsaturation index (U k′ 37 ) is likely not applicable (Shevenell et al., 2011), we will extract isoprenoid and branched glycerol diether glycerol tetraether (GDGT) lipids to measure the TEX 86 ratio (Schouten et al., 2002) and convert it to SST using published calibrations (e.g., Kim et al., 2008). ...
International Ocean Discovery Program Expedition 382, Ice- berg Alley and Subantarctic Ice and Ocean Dynamics, investigated the long-term climate history of Antarctica, seeking to understand how polar ice sheets responded to changes in insolation and atmo- spheric CO2 in the past and how ice sheet evolution influenced global sea level and vice versa. Five sites (U1534–U1538) were drilled east of the Drake Passage: two sites at 53.2°S at the northern edge of the Scotia Sea and three sites at 57.4°–59.4°S in the southern Scotia Sea. We recovered continuously deposited late Neogene sed- iments to reconstruct the past history and variability in Antarctic Ice Sheet (AIS) mass loss and associated changes in oceanic and at- mospheric circulation.
The sites from the southern Scotia Sea (Sites U1536–U1538) will be used to study the Neogene flux of icebergs through “Iceberg Alley,” the main pathway along which icebergs calved from the mar- gin of the AIS travel as they move equatorward into the warmer wa- ters of the Antarctic Circumpolar Current (ACC). In particular, sediments from this area will allow us to assess the magnitude of iceberg flux during key times of AIS evolution, including the follow- ing:
• The middle Miocene glacial intensification of the East Antarctic Ice Sheet,
• The mid-Pliocene warm period,
• The late Pliocene glacial expansion of the West Antarctic Ice
Sheet,
• The mid-Pleistocene transition (MPT), and
• The “warm interglacials” and glacial terminations of the last 800
ky.
We will use the geochemical provenance of iceberg-rafted detri- tus and other glacially eroded material to determine regional sources of AIS mass loss. We will also address interhemispheric phasing of ice sheet growth and decay, study the distribution and history of land-based versus marine-based ice sheets around the continent over time, and explore the links between AIS variability and global sea level.
By comparing north–south variations across the Scotia Sea be- tween the Pirie Basin (Site U1538) and the Dove Basin (Sites U1536 and U1537), Expedition 382 will also deliver critical information on how climate changes in the Southern Ocean affect ocean circulation through the Drake Passage, meridional overturning in the region, water mass production, ocean–atmosphere CO2 transfer by wind- induced upwelling, sea ice variability, bottom water outflow from the Weddell Sea, Antarctic weathering inputs, and changes in oce- anic and atmospheric fronts in the vicinity of the ACC.
Comparing changes in dust proxy records between the Scotia Sea and Antarctic ice cores will also provide a detailed reconstruc- tion of changes in the Southern Hemisphere westerlies on millen- nial and orbital timescales for the last 800 ky. Extending the ocean dust record beyond the last 800 ky will help to evaluate dust-climate couplings since the Pliocene, the potential role of dust in iron fertil- ization and atmospheric CO2 drawdown during glacials, and whether dust input to Antarctica played a role in the MPT.
The principal scientific objective of Subantarctic Front Sites U1534 and U1535 at the northern limit of the Scotia Sea is to recon- struct and understand how intermediate water formation in the southwest Atlantic responds to changes in connectivity between the Atlantic and Pacific basins, the “cold water route.” The Subantarctic Front contourite drift, deposited between 400 and 2000 m water depth on the northern flank of an east–west trending trough off the Chilean continental shelf, is ideally situated to monitor millennial- to orbital-scale variability in the export of Antarctic Intermediate Water beneath the Subantarctic Front. During Expedition 382, we recovered continuously deposited sediments from this drift span- ning the late Pleistocene (from ~0.78 Ma to recent) and from the late Pliocene (~3.1–2.6 Ma). These sites are expected to yield a wide array of paleoceanographic records that can be used to interpret past changes in the density structure of the Atlantic sector of the Southern Ocean, track migrations of the Subantarctic Front, and give insights into the role and evolution of the cold water route over significant climate episodes, including the following:
• The most recent warm interglacials of the late Pleistocene and • The intensification of Northern Hemisphere glaciation.
... Paleoclimatic reconstructions covering several glacial/interglacial cycles in the Eastern Mediterranean are mostly based on marine records from around the Mediterranean Sea (Emeis et al., 2003;Martrat et al., 2004) or continental records from lakes, palynomorphs and speleothems (Tzerdakis et al., 2006;Djamali et al., 2008;Badertscher et al., 2011;Gasse et al., 2011;Francke et al., 2015). The sea surface temperature (SST) of the Black Sea changed with 2-3 • C during stadial/interstadial transitions (20-65 kyr BP; Wegwerth et al., 2015) based on the TEX 86 , a temperature proxy based on changes in the relative abundance of isoprenoid glycerol dialkyl glycerol tetraether (GDGT) membrane lipids produced by temperature-sensitive archaea in the water column (Schouten et al., 2002). In another study from the Dead Sea, temperature and precipitation changes during the Holocene were estimated to be in the order of 4-5 • C and up to 150 mm/year, respectively, based on pollen . ...
We here estimate past temperature and hydroclimate variations in eastern Anatolia for marine isotope stages 4 to 1 (MIS4 to MIS1) respectively using branched glycerol dialkyl glycerol tetraethers (brGDGTs) and the hydrogen isotopic composition of n-C29 leaf-wax n-alkanes (δ²Hwax) stored in the sedimentary record of Lake Van (Turkey). Our millennial-scale lipid biomarker records reflect warm and wet conditions during interstadials/interglacials and colder and dryer periods with increased evapotranspiration and aridity during stadials/glacials. The degree of methylation of the 5-methyl brGDGTs (MBT´5me) indicates increasing mean annual air temperatures (MAT) during stadial/interstadial transitions, that characterize Dansgaard-Oeschger events, and during the last glacial termination.
Based on the effects of changes in precipitation amount on apparent enrichment factors between the δ²H of the C29 n-alkanes and δ²H of precipitation, a total increase in annual precipitation of about 200 mm during transitions from stadials to interstadials, and of 300–350 mm during glacial-interglacial transitions can be determined, in line with previous paleoclimate reconstructions for the Eastern Mediterranean.
High sterol concentrations in sediments deposited during warm and humid interstadials reflect increases in lake level, vegetation density and algal blooms, whereas lower values of the branched versus isoprenoid tetraethers ratio (BIT) likely reflects the increased niche of Thaumarchaeota resulting from enhanced windiness and mixing of the water column during colder periods. This quantitative hydroclimate reconstruction from Lake Van (Turkey), as it is optimally situated to respond to North Atlantic climate change via changes in the large-scale wind fields shed light into millennial-scale global climate variability.
... Given the importance of ocean thermal forcing to the dynamics of AIS marine margins and grounding lines, we will also evaluate the relationship between sea-surface temperature (SST) and IBRD flux. Because the alkenone unsaturation index (U k′ 37 ) is likely not applicable (Shevenell et al., 2011), we will extract isoprenoid and branched glycerol diether glycerol tetraether (GDGT) lipids to measure the TEX 86 ratio (Schouten et al., 2002) and convert it to SST using published calibrations (e.g., Kim et al., 2008). ...
International Ocean Discovery Program Expedition 382, Iceberg Alley and Subantarctic Ice and Ocean Dynamics, investigated the long-term climate history of Antarctica, seeking to understand how polar ice sheets responded to changes in insolation and atmospheric CO2 in the past and how ice sheet evolution influenced global sea level and vice versa. Five sites (U1534–U1538) were drilled east of the Drake Passage: two sites at 53.2°S at the northern edge of the Scotia Sea and three sites at 57.4°–59.4°S in the southern Scotia Sea. We recovered continuously deposited late Neogene sediments to reconstruct the past history and variability in Antarctic Ice Sheet (AIS) mass loss and associated changes in oceanic and atmospheric circulation.
The sites from the southern Scotia Sea (Sites U1536–U1538) will be used to study the Neogene flux of icebergs through “Iceberg Alley,” the main pathway along which icebergs calved from the margin of the AIS travel as they move equatorward into the warmer waters of the Antarctic Circumpolar Current (ACC). In particular, sediments from this area will allow us to assess the magnitude of iceberg flux during key times of AIS evolution, including the following:
The middle Miocene glacial intensification of the East Antarctic Ice Sheet,
The mid-Pliocene warm period,
The late Pliocene glacial expansion of the West Antarctic Ice Sheet,
The mid-Pleistocene transition (MPT), and
The “warm interglacials” and glacial terminations of the last 800 ky.
We will use the geochemical provenance of iceberg-rafted detritus and other glacially eroded material to determine regional sources of AIS mass loss. We will also address interhemispheric phasing of ice sheet growth and decay, study the distribution and history of land-based versus marine-based ice sheets around the continent over time, and explore the links between AIS variability and global sea level.
By comparing north–south variations across the Scotia Sea between the Pirie Basin (Site U1538) and the Dove Basin (Sites U1536 and U1537), Expedition 382 will also deliver critical information on how climate changes in the Southern Ocean affect ocean circulation through the Drake Passage, meridional overturning in the region, water mass production, ocean–atmosphere CO2 transfer by wind-induced upwelling, sea ice variability, bottom water outflow from the Weddell Sea, Antarctic weathering inputs, and changes in oceanic and atmospheric fronts in the vicinity of the ACC.
Comparing changes in dust proxy records between the Scotia Sea and Antarctic ice cores will also provide a detailed reconstruction of changes in the Southern Hemisphere westerlies on millennial and orbital timescales for the last 800 ky. Extending the ocean dust record beyond the last 800 ky will help to evaluate dust-climate couplings since the Pliocene, the potential role of dust in iron fertilization and atmospheric CO2 drawdown during glacials, and whether dust input to Antarctica played a role in the MPT.
The principal scientific objective of Subantarctic Front Sites U1534 and U1535 at the northern limit of the Scotia Sea is to reconstruct and understand how intermediate water formation in the southwest Atlantic responds to changes in connectivity between the Atlantic and Pacific basins, the “cold water route.” The Subantarctic Front contourite drift, deposited between 400 and 2000 m water depth on the northern flank of an east–west trending trough off the Chilean continental shelf, is ideally situated to monitor millennial- to orbital-scale variability in the export of Antarctic Intermediate Water beneath the Subantarctic Front. During Expedition 382, we recovered continuously deposited sediments from this drift spanning the late Pleistocene (from ~0.78 Ma to recent) and from the late Pliocene (~3.1–2.6 Ma). These sites are expected to yield a wide array of paleoceanographic records that can be used to interpret past changes in the density structure of the Atlantic sector of the Southern Ocean, track migrations of the Subantarctic Front, and give insights into the role and evolution of the cold water route over significant climate episodes, including the following:
The most recent warm interglacials of the late Pleistocene and
The intensification of Northern Hemisphere glaciation.
... IsoGDGTs are produced by several different species of archaea (for a review, see Schouten et al., 2013). In the marine realm, their structural diversity relates to sea surface temperature (Schouten et al., 2002;Kim et al., 2010), and based on their down-core variation in marine sediments, paleotemperature records of varying geological age have now been generated for several world regions (e.g. Sluijs et al., 2006;Castañ eda et al., 2010;dos Santos et al., 2010). ...
Branched glycerol monoalkyl glycerol tetraethers (brGMGTs) are membrane-spanning lipids which were initially identified in marine settings and have more recently been found in peats and lake sediments. In the latter settings, their abundance relative to that of the branched glycerol dialkyl glycerol tetraethers (brGDGTs) appears to increase under warmer climate conditions. Furthermore, their relative distribution in bottom sediments from an altitudinal transect of East African lakes has been linked to local mean annual air temperature (MAAT). To shed light on their sources and identify potential environmental factors driving their production in lakes, we investigated their occurrence and distribution in the water column, surficial lake-bottom sediments, and catchment soils of Lake Chala, a permanently stratified (meromictic) tropical crater lake in East Africa. Data from the water column comprise both suspended particulate matter (SPM) collected monthly at eight depth intervals between 0 and 80 m for 17 consecutive months, and settling particles collected monthly at a depth of 35 m over a period of 53 months. In catchment soils, brGMGTs are either relatively scarce or absent. In the water column, their occurrence is dominated by four of the seven known brGMGTs and mostly limited to the anoxic zone. Whereas the spatial distribution and concentration of brGMGTs in SPM were strongly influenced by the seasonal cycle of strong water-column stratification alternating with deep mixing, this trend is not consistent on the multi-year timescale of the settling-particles record. Moreover, their relative distribution in both SPM and settling particles did not respond to temperature variation during the studied interval. In contrast to the water column, surficial lake-bottom sediments contain the full suite of known brGMGTs in a distinct and spatially uniform fractional abundance, strongly suggesting that the majority of brGMGT production takes place there. BrGMGT-inferred MAAT for the Chala area reasonably estimates measured MAAT and conforms to the East African lake brGMGT calibration. Our results have important implications for the use of brGMGTs as proxy for MAAT, in that outside of the equatorial zone the presumably greater offset between bottom-water temperature and MAAT may create a bias towards cold-season air temperature. Regardless, the more singular source of brGMGTs in lake sediments is an advantage for paleotemperature reconstruction compared to brGDGTs, which several studies now indicate to have a mixed aquatic and terrestrial source of which the relative contribution cannot easily be disentangled or assumed to have been constant through time.
... They include archaeal isoprenoid (iso) GDGTs and bacterial branched (br) GDGTs that differ in alkyl chain structures and the number of cyclopentyl moieties (Supplementary Fig. S1; Schouten et al., 2013). Proxies developed based on GDGTs have been widely used in paleoclimate and paleoecology reconstruction due to their wide occurrence in a variety of geological settings (Sinninghe Damsté et al., 2000;Schouten et al., 2002;Weijers et al., 2007;Naafs et al., 2017;Dong et al., 2019;Lu et al., 2019). For example, tetraether index of 86 carbon atoms (TEX 86 ), based on isoGDGTs, was initially developed and applied to marine environments Kim et al., 2010), and later its application was extended to large lakes with minor terrigenous organic input (Powers et al., 2010). ...
Branched glycerol dialkyl glycerol tetraethers (brGDGTs) are bacterial membrane lipids that have been extensively used as tools for paleoclimate reconstruction. Temperature and pH were thought to be two major factors affecting the distributions of brGDGTs; however, the effects of other factors on the distribution of brGDGTs and related temperature and pH proxies in soil environments remain unclear. Here, we investigated iso- and br-GDGT distributions in three soil profiles (SPs) developed on bedrocks consisting of hornfels (HP), granite (GP), and basalt (BP), respectively. The downward environmental factors in these SPs differ, thus providing a chance to evaluate their effects on the distribution of brGDGTs. In the three SPs, the methylation degree for 5-methyl and total brGDGTs (MBT′5ME and MBT′), respectively, varied significantly downwards despite minor changes in soil temperature, highlighting a non-temperature effect. The depth-dependence of these proxies appears to be caused by the oxygen level. The MBT′ for 5- or 6-methyl brGDGTs (MBT′5ME and MBT′6ME), or total brGDGTs (MBT′), increased as the oxygen level decreased, suggesting tetramethylated brGDGTs (Ia, Ib, and Ic) would increase in abundance with decreased oxygen level. The two pH proxies, CBT and IR6ME, both decreased at lower oxygen levels. Such regulation of brGDGT compositions in response to oxygen availability could be explained by the changing bacterial communities that have different oxygen preferences. The possible oxygen level impact on the distribution of GDGTs could result in brGDGT-inferred temperature skewed towards higher values and pH towards lower values with decreased oxygen level. This finding may also have implications for the mechanism of soil aridity impact on the brGDGT-based proxies in semi-arid and arid regions, which is of profound importance for paleotemperature and hydroclimate reconstruction in loess-paleosol sequences. Increasing soil aridity could lead to changes in brGDGT-based proxies similar to that observed at the higher oxygen level, i.e., lower MBT′, higher CBT, and higher IR6ME. As soil aridity is generally associated with better aeration and higher oxygen level, we argue that changes in oxygen level might be the real cause of the bias in brGDGT-based proxies in soils from semi-arid and arid regions.
... The distribution of the iGDGTs is dominated by GDGT-0 (Fig. 2), comprising more than 90% of all iGDGTs. Previous studies have shown that the number of cyclopentane rings in GDGTs from surface sediments substantially increases with increasing lake temperature (Schouten et al., 2002), hence the predominance of GDGT-0 is not a surprise in Arctic lakes (MAT = À8.4 ± 1.7°C, 1960-2020, Inuvik weather station). However, the absence of crenarchaeol is surprising, and the observed distribution differs from those of permafrost samples of Kusch et al. (2019), and more closely resembles those found in the deep anoxic regions of Lake Chala, Africa and Lake Rotsee, Switzerland (Naeher et al., 2014a). ...
Glycerol Dialkyl Glycerol Tetraethers (GDGTs) are ubiquitous biomolecules whose structural diversity or isotopic composition is increasingly used to reconstruct environmental changes such as air temperature or pCO2. Isoprenoid GDGTs (iGDGT), in particular GDGT-0, are biosynthesized by a large range of Archaea. To assess the potential of GDGT-0 as a tracer of past methane cycle variations, three sediment cores from the Mackenzie River Delta have been studied for iGDGT and diploptene concentration, distribution and stable carbon signature. The absence of crenarchaeol, high GDGT-0 vs crenarchaeol ratio values, and ¹³C-enriched carbon signature of GDGT-0 indicate production by acetoclastic methanogens as well as heterotrophic Archaea. The oxidation of methane seems to be dominated by bacteria as indicated by the high abundance of ¹³C-depleted diploptene. Branched GDGTs (brGDGT), thought to be produced by heterotrophic bacteria, are dominated by hexa- and penta-methylated 5- and 6-methyl compounds. The presence of 5,6-methyl isomer IIIa’’ points towards in situ production of brGDGTs, with only a minor input from soil branched GDGT brought by the Mackenzie River. Carbon isotopic compositions of brGDGTs are in agreement with heterotrophic producers, likely living during summer. The reconstructed temperatures using a global lake calibration reflect recorded summer air temperature (± 2 °C) during the last 60 years, and further highlight the absence of warming in summer in this region during the last 200 years. Oxygen availability and connection time to the Mackenzie River also seem to control the distribution of brGDGT with an increase in 6-methyl and 5,6-methyl isomers during periods of increased anoxia.
... A number of organic geochemical proxies based on microbial lipids have been developed for palaeoclimate reconstruction (Eglinton and Eglinton, 2008;Luo et al., 2019;Meyers, 1997;Schouten et al., 2013). Three lipid biomarker based indices, TEX 86 (Kim et al., 2008;Schouten et al., 2002), U 37 K 0 (Brassell et al., 1986;Haug et al., 2005;Prahl and Wakeham, 1987;Sachs et al., 2001) and LDI (de Bar et al., 2020;Naafs et al., 2012;Rampen et al., 2012) have become important tools for determination of past sea surface temperature (SST). However, the above-mentioned proxies are generally applied in marine settings and biomarker based proxies for terrestrial environments, especially for temperature, remain relatively scarce. ...
3-Hydroxy fatty acids (3-OH-FAs), derived from Gram-negative bacterial outer membranes, have received recent attention for their potential as new terrestrial pH and temperature proxies for palaeoclimate studies. Initial studies from altitudinal transects of contemporary soils - correlating bacterial 3-OH-FA compositions to air temperature and pH - have shown promising results. But the geographical extent of recent calibrations is limited. In this study, we analyse 3-OH-FA lipid distributions in 186 globally distributed soil samples to study the environmental factors controlling the relative distribution of the 3-OH-FA isomers. Our sample-set covers a wide range of temperatures (-0.4 to 27°C) and pH (3.6 to 9.2). For the global compilation we find that the ratio of anteiso to normal 3-OH-FAs of the C15 or C17 homologues (RAN15 or RAN17) shows a strong linear relationship with mean annual air temperature (MAAT) (R²=0.48, p < 0.001 and R² = 0.41, p < 0.001, respectively). Additionally, the negative logarithm of the ratio of the summed iso and anteiso to the total amount of normal 3-OH-FAs (RIAN) is also strongly anticorrelated with the soil pH (R² = 0.66, p < 0.001). However, we find that for our 3-OH-FA based proxies there are significant differences in slope and intercept of the linear corrections at regional scales. Thus local or regional calibrations are likely preferable (at this stage of 3-OH-FA proxy development) for application to specific palaeoclimate archives. We also explore the relationship of 3-OH-FA isomer fractional abundances to environmental parameters using machine learning tools (a Gaussian Process (GP) emulator). This confirms the first order relationships to environmental parameters highlighted by the empirical equations and also derives several alternative GP emulator models for reconstructing MAAT and pH which give higher R² values (0.66 for MAAT; 0.63 for pH) and lower RSME values (3.5°C for MAAT; 0.76 for pH) compared to simple linear regressions at the global scale. We compare our 3-OH-FA based indices with bacterial branched glycerol dialkyl glycerol tetraethers (brGDGTs) based indices from the same soil samples. At a global scale RAN15 and RAN17 show negative correlations with the MBT′5ME-MAAT (MBT′5ME, methylation index of 5-methyl branched tetraethers) (r = -0.59, p < 0.001 and r = -0.42, p < 0.001, respectively), whilst RIAN shows strong linear correlations with the cyclisation ratio of branched tetraethers (CBT) (r = 0.77, p < 0.001). Similar to 3-OH-FA based temperature proxies, GDGT based temperature proxy MBT′5ME also showed different regional calibrations. Our new field-based correlations demonstrate the broad physiological response of Gram-negative bacterial cell membranes to external environmental changes on a global scale. We suggest that 3-OH-FA based proxies have widespread potential for palaeoenvironmental studies to estimate past MAAT and soil pH, but that regional/ local and context specific calibrations may need to be applied.
... However, by lumping compounds of various types and abundances into the denominator, the approach can also dampen meaningful trends and obscure important relationships, especially for less abundant molecules. This adverse effect has been recognized for other lipid biomarkers and has led to, for example, the exclusion of crenarchaeol from the TEX 86 index (Schouten et al., 2002) and tetra-unsaturated alkenones from the U K 37 index (Prahl and Wakeham, 1987). Numerous ratio-based indices have been developed for brGDGTs that similarly exclude low-abundance (e.g., MBT ;Peterse et al., 2012) or problematic (e.g., MBT 5Me ; De Jonge et al., 2014a) compounds. ...
Distributions of branched glycerol dialkyl glycerol tetraethers (brGDGTs) are frequently employed for reconstructing terrestrial paleotemperatures from lake sediment archives. Although brGDGTs are globally ubiquitous, the microbial producers of these membrane lipids remain unknown, precluding a full understanding of the ways in which environmental parameters control their production and distribution. Here, we advance this understanding in three ways. First, we present 43 new high-latitude lake sites characterized by low mean annual air temperatures (MATs) and high seasonality, filling an important gap in the global dataset. Second, we introduce a new approach for analyzing brGDGT data in which compound fractional abundances (FAs) are calculated within structural groups based on methylation number, methylation position, and cyclization number. Finally, we perform linear and nonlinear regressions of the resulting FAs against a suite of environmental parameters in a compiled global lake sediment dataset (n = 182). We find that our approach deconvolves temperature, conductivity, and pH trends in brGDGTs without increasing calibration errors from the standard approach. We also find that it reveals novel patterns in brGDGT distributions and provides a methodology for investigating the biological underpinnings of their structural diversity. Warm-season temperature indices outperformed MAT in our regressions, with the mean temperature of months above freezing yielding the highest-performing model (adjusted R2 = 0.91, RMSE = 1.97 ∘C, n = 182). The natural logarithm of conductivity had the second-strongest relationship to brGDGT distributions (adjusted R2 = 0.83, RMSE = 0.66, n = 143), notably outperforming pH in our dataset (adjusted R2 = 0.73, RMSE = 0.57, n = 154) and providing a potential new proxy for paleohydrology applications. We recommend these calibrations for use in lake sediments globally, including at high latitudes, and detail the advantages and disadvantages of each.
... Glycerol dialkyl glycerol tetraethers (GDGTs) are a kind of membrane-spanning lipids from bacteria and archaea, which are common in many natural archives, including soils (Weijers et al., 2007;Peterse et al., 2012), marine sediments (Schouten et al., 2002), lakes (Sun et al., 2011;Russell et al., 2018;Zhao et al., 2021), as well as peat deposits (Weijers et al., 2006;Huguet et al., 2010;Naafs et al., 2017). Previous studies have shown that the distribution of bacterial branched GDGTs (brGDGTs) is related to environmental factors, such as mean annual temperature (MAT) and soil pH (Weijers et al., 2007;Peterse et al., 2012;Naafs et al., 2017). ...
Meteorological data indicate that the increase in temperature at high-elevation sites with the background of global warming is much higher than the global average. However, there are few studies trying to evaluate temperature changes in high elevational areas over longer time scales. Here, we use brGDGTs in a sediment core collected from Hongyuan Peatland (3503 m a.s.l.), to reconstruct an independently-dated, high-resolution (~100 years), quantitative record of mean annual temperature (MAT) over the past 13,300 yr BP on the eastern Tibetan Plateau. Our data, consisting with other quantitative local reconstructions, show a peak warmth in the middle Holocene (6100–5600 yr BP). Together with other published MAT records from ~30° N in China, we find that the timing of the Holocene Thermal Maximum (HTM) in high-elevation sites lagged that in low-elevation sites. We also find that the warming rate in early Holocene was faster at a higher elevation site, by comparing with another MAT reconstruction using the same archive and the same proxy. We hypothesize the delayed HTM at high-elevation areas was probably caused by the offset of remaining regional glaciers and/or snow cover on the Tibetan Plateau. The enhanced warming with elevation can be ascribed to snow albedo and latent heat release. Our new results may imply a faster high-elevation temperature rise with an on-going global warming, considering the disappearing regional glaciers on the Tibetan Plateau.
... To reconstruct sea (sub-) surface temperature (SST), we applied the TEX 86 (TetraEther indeX of 86 carbon atoms) proxy (Schouten et al., 2002), which is based on the temperature-dependent cyclisation of isoprenoidal glycerol dialkyl glycerol tetraethers (GDGTs) produced by thaumarchaeotal membrane lipids. GDGTs were extracted from pow-dered and freeze-dried sediments using an accelerated solvent extractor. ...
Antarctic continental ice masses fluctuated considerably during the Oligocene “coolhouse”, at elevated atmospheric CO2 concentrations of ∼600–800 ppm. To assess the role of the ocean in the Oligocene ice sheet variability, reconstruction of past ocean conditions in the proximity of the Antarctic margin is needed. While relatively warm ocean conditions have been reconstructed for the Oligocene offshore of Wilkes Land, the geographical extent of that warmth is unknown. In this study, we reconstruct past surface ocean conditions from glaciomarine sediments recovered from Deep Sea Drilling Project (DSDP) Site 274 offshore of the Ross Sea continental margin. This site, located offshore of Cape Adare is ideally situated to characterise Oligocene regional surface ocean conditions, as it is situated between the colder, higher-latitude Ross Sea continental shelf and the warm-temperate Wilkes Land margin in the Oligocene. We first improve the age model of DSDP Site 274 using integrated bio- and magnetostratigraphy. Subsequently, we analyse organic walled dinoflagellate cyst assemblages and lipid biomarkers (TEX86, TetraEther indeX of 86 carbon atoms) to reconstruct surface palaeoceanographic conditions during the Oligocene (33.7–24.4 Ma). Both TEX86-based sea surface temperature (SST) and microplankton results show temperate (10–17 ∘C ± 5.2 ∘C) surface ocean conditions at Site 274 throughout the Oligocene. Oceanographic conditions between the offshore Wilkes Land margin and Cape Adare became increasingly similar towards the late Oligocene (26.5–24.4 Ma); this is inferred to be the consequence of the widening of the Tasmanian Gateway, which resulted in more interconnected ocean basins and frontal systems. Maintaining marine terminations of terrestrial ice sheets in a proto-Ross Sea with offshore SSTs that are as warm as those suggested by our data requires a strong ice flux fed by intensive precipitation in the Antarctic hinterland during colder orbital states but with extensive surface melt of terrestrial ice during warmer orbital states.
... The iGDGTs are a suite of membrane-spanning lipids, produced by thermophilic and non-thermophilic archaea, which are often found preserved in sedimentary archives (DeLong et al., 1998;Schouten et al., 2000;Schouten et al., 2013). The number of cyclic moieties within the isoprenoid chains, described by the TEX 86 index (TetraEther indeX with 86 carbon atoms) (Schouten et al., 2002(Schouten et al., , 2003, correlates positively with temperature, both in culture (Wuchter et al., 2004) and in marine core-top calibrations (Kim et al., 2008(Kim et al., , 2010Tierney and Tingley, 2014), providing an important paleo-sea surface temperature (SST) proxy. TEX 86 has also been calibrated for lakes, and several different lacustrine calibrations are available (e.g. ...
Isoprenoid glycerol dialkyl glycerol tetraethers (iGDGTs) are commonly preserved molecular biomarkers of archaea whose distributions can be used to reconstruct past temperature, and possibly, methane and nitrogen cycling. To date, iGDGT systematics have not been widely investigated in Arctic lacustrine environments. Here, we analyze iGDGTs in sediments of Lake El'gygytgyn, located in the Russian Arctic, to reconstruct the paleoenvironmental conditions from the Pliocene to today using TEX86 and other indices. The TEX86‐inferred temperature history shows a long‐term warming trend, in stark contrast to other Arctic records and other proxies from Lake El'gygytgyn, suggesting that non‐temperature factors obfuscate the use of TEX86 at this site. Other GDGT‐based indices including the BIT Index, ΔRing Index, Methane Index and the GDGT‐0/crenarchaeol ratio suggest that TEX86 is strongly influenced by archaeal community changes. The most significant community shifts are observed c. 2.4 Ma and record an increase in Euryarchaeota production and/or a decrease in Thaumarchaeota production, which was driven by the establishment of permafrost and perennial lake ice during the early Pleistocene. Overall, this study demonstrates an important interpretative framework for iGDGTs in lacustrine systems and describes variations in Arctic climate and lake biogeochemistry over timescales of thousands to millions of years.
... TEX 86 is a relatively new SST proxy method based on the ratio of different glycerol dialkyl glycerol tetraethers (GDGTs) with 86 carbons, which comprise membrane lipids in marine Crenarchaeota (Schouten et al., 2002). It has the benefit of not relying on δ 18 O sw assumptions. ...
On geologic time scales, Earth has fluctuated between greenhouse and icehouse climates. Understanding the mechanisms responsible for these disparate climate states provides valuable insight into long-term climate forecasts. During the Quaternary (2.6-0 Ma), there were a series of large glaciations. The pacing of these glacial cycles is often attributed to orbitally controlled high-latitude summer insolation, because it influences the amount of ice melt. However, this relationship is not well reflected in ice-volume records. For instance, in the early Pleistocene (2.6-0.8 Ma), glacial cycles oscillated mainly with obliquity while summer insolation varied most strongly with precession. Here, Earth system model simulations show that a combination of albedo feedbacks, seasonal offset of precession forcing, and orbital cycle duration differences amplified the ice-volume response to obliquity relative to precession; these results help explain the paradox of the early Pleistocene glacial cycles. Another enigma of Quaternary is the transition from 41 to 100 kyr glacial cycles with ~50 m greater sea level variability, which arose despite little change in CO2 or orbital forcing. The regolith hypothesis provides a potential explanation for this transition. It posits that glacial cycles gradually eroded pre-existing high-latitude regolith, causing a change in ice sheet response to orbital forcing as the ice bed transitioned from low-friction sediment to high-friction bedrock. Earth system model results provide support for the regolith hypothesis; only with reduced basal sliding does the 100 kyr ice-volume signal of the late Pleistocene (0.8-0 Ma) appear in the simulated ice-volume cycles. In contrast to the Quaternary, the Cretaceous (145-66 Ma) was a greenhouse climate. Nevertheless, evidence suggests significant climate changes occurred during this period, including a dramatic cooling from the Cenomanian (100-94 Ma) to Maastrichtian (72-66 Ma). Here, two Earth system models and a compilation of proxy records are used to explore the hypotheses that Late Cretaceous (100-66 Ma) cooling was in response to changes in geography or CO2. Results show that a decrease in CO2 is necessary to explain the proxy identified cooling across the Late Cretaceous. However, tectonic evolution caused substantial regional climate changes that must be considered when interpreting proxy records.
... The TEX86 proxy is based on the empirically-derived temperature sensitivity in the ratios of a suite of membrane lipids (GDGTs) produced by single-celled thaumarchaeotal organisms (Schouten et al., 2002(Schouten et al., , 2007Shimada et al., 2002;Wuchter et al., 2004;Kim et al., 2008Kim et al., , 2010. ...
Anthropogenic influence on modern climate and the environment is unambiguous and bears profound implications for agriculture, water availability, and natural resource management. These influences are global in extent and require critical examinations of past climatic and environmental perturbations in the geological record to predict the magnitude of anticipated changes (e.g. surface temperature increase) that will impact humanity in the future. This dissertation explores the use of existing and novel geochemical proxies for environmental information ranging from atmospheric circulation patterns of the Pleistocene/Holocene to coastal marine temperatures and volcanism of the Late Cretaceous. The initial focus of this research has centered on the application of carbonate clumped isotope (Δ47) paleothermometry to marine mollusk fossils in order to reconstruct marine temperatures preceding, during, and after the Cretaceous-Paleogene (K-Pg) extinction. The K-Pg boundary interval serves as an important analogue to modern and future climate projections, with atmospheric CO2 concentrations estimated at ~400 to 1100 ppm (compared to an annual average of 402.8 ppm in 2016) and a world lacking significant continental ice sheets. The K-Pg boundary is also known for pervasive terrestrial and marine extinctions including the demise of dinosaurian groups. These methods revealed Δ47-derived coastal marine temperatures from the ancient Mississippi Embayment and Atlantic Coast ranging from 7 to 25 °C, which compare closely to modern values in the range of 10 to 29 °C. The similarity between modern and reconstructed paleotemperatures suggests that the surface temperatures required at the subtropics for an unglaciated world may be closer to those observed in the modern than originally believed and implies a shallow equator-to-pole thermal gradient. Cold paleotemperatures along the Atlantic Coast near New Jersey (3 to 14 °C) may provide evidence for the presence of the Gulf Stream current, and subsequent North Atlantic return flow as early as the Campanian/Maastrichtian. The temperature ranges reconstructed for these sites also compare well to marine temperatures determined using other proxy methods, with the exception of the TEX86 organic geochemical temperature proxy, which may be seasonally biased towards summer temperatures. Coupled with the temperatures reconstructed from Δ47 values, we have developed a unique proxy for environmental mercury concentrations, [Hg], in the past using the same mollusk fossils from before, during, and after the K-Pg boundary interval. This work represents the first deep-time application of [Hg] records in biogenic carbonates. We have measured both [Hg] and Δ47 values from globally-distributed specimens across a latitudinal range from 70 °N to 67 °S. We found covariation between elevated marine temperatures (~7 to 10 °C excursions) and peak [Hg] between 17 to 42 ng g-1 (relative to a background of ~0 to 5 ng g-1) immediately prior to the K-Pg extinction and coinciding with the onset of the main eruptive phase of the Deccan Traps Large Igneous Province (LIP). Evidence of a global signal of volcanogenic emissions (e.g. CO2 and gaseous elemental Hg) and changing climate prior to the K-Pg boundary suggest causality and serve to clarify the role of the Deccan Traps in terms of extinction patterns. This work also reveals the potential of [Hg] in biominerals at other critical intervals of the geological record in addressing questions relating to LIP volcanism and environmental mercury cycling.
... The TEX 86 index was calculated using the following equations proposed by Schouten et al. (2002): ...
Isoprenoid glycerol dialkyl glycerol tetraethers (isoGDGTs) derived from archaea are lipid biomarkers that exhibit high sensitivity to changes in water temperature, leading to the widespread application of the isoGDGT-based tetraether index of 86 carbon atoms (TEX86) in surface seawater temperature (SST) reconstruction. However, there remain some uncertainties regarding the robustness of TEX86 under changing water conditions (e.g., variations in water depth, oxygen and pH). Here, we analyzed isoGDGTs in suspended particles at different depths of the East China Sea (ECS) during summer 2020, aiming to constrain the applicability of the TEX86 proxy in coastal waters. Our data showed that the isoGDGTs were mainly derived from planktonic Thaumarchaeota, as revealed by the low ratio of GDGT-0/crenarchaeol (<0.5). The vertical distribution of isoGDGT concentration depicted a downward increase from the surface to the bottom. This observation was likely shaped by Thaumarchaeota, which regulate the extent of ammonia oxidation based on the availability of ammonium. The occurrence of maximal isoGDGT concentrations in the bottom layer suggests that the isoGDGTs in sediments are mainly controlled by bottom archaeal production rather than surface archaeal production. By reanalyzing the published isoGDGT data of surface sediments in the ECS inner shelf, we found that the sedimentary TEX86 relates much better to the annual mean bottom seawater temperature (BST) than to the annual mean SST, indicating that sedimentary TEX86 is more inclined to be a proxy for the BST in the shallow ECS. In addition, the positive bias of TEX86 driven by low dissolved oxygen and low pH was observed under the pycnocline, indicating that the application of TEX86 to reconstruct seawater temperature should be carefully appraised in coastal environments with strong water column stratification.
... Quantification was carried out by integrating the peak area of [M + H] + ions in the extracted ion chromatogram and comparing with the C 46 internal standard. We then calculated the ACE, BIT, and TEX 86 indices using equations as given below: ACE archaeol archaeol+caldarchaeol×10 × 100 (Turich and Freeman 2011;Wang et al., 2013), BIT I+II+III I+II++III+cren (Hopmans et al., 2004), TEX 86 GDGT 2 +GDGT 3 +cren′ GDGT1+GDGT2+GDGT3+cren′ (Schouten et al., 2002). ...
Coastal regions of the northern South China Sea (SCS) strongly interact with the Asian monsoon circulation (AMC). Thus, variations of sea surface temperature (SST) here are newly suggested to document AMC changes in an effective manner, but additional physical parameters of oceanic conditions, probably also in relation to the AMC system, remain poorly understood. In this study, we analyzed glycerol dialkyl glycerol tetraethers (GDGTs) from a well-dated sediment core YJ, retrieved at the northern SCS coast, to further scrutinize the intrinsic response of water column to winter AMC strength. It shows that within the time frame of past ∼1,000 years, the tetraether index of lipids with 86 carbon atoms ( TEX 86 ) and published alkenone ( U 37 K ′ ) temperature records together confirm a reduced thermal gradient during the Little Ice Age (LIA), in comparison to that during the Medieval Climate Anomaly (MCA). Considering concurrent variations of the branched and isoprenoid tetraether (BIT) and the ratio of archaeol to caldarchaeol (ACE), for example, with decreased values (
... The study of archaeal lipid membranes and the mechanisms of biosynthesis conveys information about what ancient cell membranes might have looked like and provide the basis for the Tex86 historical temperature proxy that is extensively used to predict surface temperatures in the Cenozoic era (Schouten et al., 2002). ...
Simple, robust and versatile LC-MS based methods add to the rapid assessment of the lipidome of biological cells. Here we present a versatile RP-UHPLC-MS method using 1-butanol as the eluent, specifically designed to separate different highly hydrophobic lipids. This method is capable of separating different lipid classes of glycerophospholipid standards, in addition to phospholipids of the same class with a different acyl chain composition. The versatility of this method was demonstrated through analysis of lipid extracts of the bacterium Escherichia coli and the archaeon Sulfolobus acidocaldarius. In contrast to 2-propanol-based methods, the 1-butanol-based mobile phase is capable of eluting highly hydrophobic analytes such as cardiolipins, tetraether lipids and mycolic acids during the gradient instead of the isocratic purge phase, resulting in an enhanced separation of cardiolipins and extending the analytical range for RPLC.
... For molecular structures, see Fig. A1. GDGTs have garnered much attention due to their potential as molecular proxies for environmental conditions; the relative abundance of brGDGTs versus isoGDGTs has been used to qualitatively estimate soil-derived carbon input into marine sediments , while the internal distribution of iso-and brGDGT isomers carries information on aquatic and soil conditions (e.g., Schouten et al., 2002;Powers et al., 2004Powers et al., , 2010Liu et al., 2013;Coffinet et al., 2014;Yang et al., 2016). For example, the distribution of different brGDGTs, parameterized as the methylation of branched tetraethers (MBTs) and cyclization of branched tetraethers (CBTs) indices (Peterse et al., 2012;De Jonge et al., 2014a;Naafs et al., 2017), has been found to correlate with mean annual continental air temperature (MAT) and soil pH (Weijers et al., 2007), respectively. ...
Understanding controls on the persistence of soil organic matter (SOM) is essential to constrain its role in the carbon cycle and inform climate-carbon cycle model predictions. Emerging concepts regarding the formation and turnover of SOM imply that it is mainly comprised of mineral-stabilized microbial products and residues; however, direct evidence in support of this concept remains limited. Here, we introduce and test a method for the isolation of isoprenoid and branched glycerol dialkyl glycerol tetraethers (GDGTs)-diagnostic membrane lipids of archaea and bacteria , respectively-for subsequent natural abundance radio-carbon analysis. The method is applied to depth profiles from two Swiss pre-Alpine forested soils. We find that the 14 C values of these microbial markers markedly decrease with increasing soil depth, indicating turnover times of millennia in mineral subsoils. The contrasting metabolisms of the GDGT-producing microorganisms indicates it is unlikely that the low 14 C values of these membrane lipids reflect het-erotrophic acquisition of 14 C-depleted carbon. We therefore attribute the 14 C-depleted signatures of GDGTs to their physical protection through association with mineral surfaces. These findings thus provide strong evidence for the presence of stabilized microbial necromass in forested mineral soils.
... Another organic tracer to reconstruct past SST is based on the quantification of the average number of cyclopentane rings found in glycerol dialkyl glycerol tetraethers (GDGTs) of archaea membrane lipids. An index, called TEX86, was deduced after analyzing the GDGTs distribution in marine surface sediments in comparison to annual mean SSTs (Schouten et al., 2002). ...
... A means to better constrain future climate change is to study past climate states and transitions using proxy records obtained from climate archives. Organic temperature proxies (U K′ 37 , TEX 86 , LDI) [3][4][5] have proved to be indispensable in this context. These tools are frequently employed in marine sediment sequences to reconstruct past sea surface temperatures and have been used to establish high-resolution palaeotemperature records from all parts of the oceans [5][6][7] and in sediments dating back to the Early Jurassic (~200 Ma) 8 . ...
Understanding Earth’s response to climate forcing in the geological past is essential to reliably predict future climate change. The reconstruction of continental climates, however, is hampered by the scarcity of universally applicable temperature proxies. Here, we show that heterocyte glycolipids (HGs) of diazotrophic heterocytous cyanobacteria occur ubiquitously in equatorial East African lakes as well as polar to tropical freshwater environments. The relative abundance of HG26 diols and keto-ols, quantified by the heterocyte diol index (HDI26), is significantly correlated with surface water temperature (SWT). The first application of the HDI26 to a ~37,000 year-long sediment record from Lake Tanganyika provides evidence for a ~4.1 °C warming in tropical East Africa from the last glacial to the beginning of the industrial period. Given the worldwide distribution of HGs in lake sediments, the HDI26 may allow reconstructing SWT variations in polar to tropical freshwater environments and thereby quantifying past continental climate change.
Sea surface temperature (SST) reconstructions based on isoprenoid glycerol dialkyl glycerol tetraether (isoGDGT) distributions from the Eocene southwest (sw) Pacific Ocean are unequivocally warmer than can be reconciled with state-of-the-art fully coupled climate models. However, the SST signal preserved in sedimentary archives can be affected by contributions of additional isoGDGT sources. Methods now exist to identify and possibly correct for overprinting effects on the isoGDGT distribution in marine sediments. We here use the current proxy insights to assess the reliability of the isoGDGT-based SST signal in 69 newly analysed and 242 re-analysed sediments ODP Site 1172 (East Tasman Plateau, Australia) following state-of-the-art chromatographic techniques, in context of paleo-environmental and paleoclimatologic reconstructions based on dinoflagellate cysts. The resulting ~130 kyr-resolution Maastrichtian-Oligocene TEX86-based SST record confirms previous conclusions of anomalous warmth in the early Eocene sw Pacific and remarkably cool conditions during the mid-Paleocene. Dinocyst diversity and assemblages show a strong response to the local SST evolution, supporting the robustness of the TEX86 record. Soil-derived branched GDGTs stored in the same sediments are used to reconstruct mean annual air temperature (MAAT) of the nearby land using the MBT'5me proxy. MAAT is consistently lower than SST during the early Eocene, independent of the calibration chosen. General trends in SST and MAAT are similar, except for: 1) an enigmatic absence of MAAT rise during the Paleocene-Eocene Thermal Maximum and Middle Eocene Climatic Optimum, and 2) a subdued middle–late Eocene MAAT cooling relative to SST. Both dinocysts and GDGT signals suggest a mid-shelf depositional environment with strong river-runoff during the Paleocene-early Eocene, progressively becoming more marine thereafter. This trend reflects gradual drying and more pronounced wet/dry seasons in the northward drifting Australian hinterland, which may also explain the subdued middle Eocene MAAT cooling relative to that of SST. The overall correlation between dinocyst assemblages, marine biodiversity and SST changes suggests that temperature exerted a strong influence on the surface-water ecosystem, probably in part through sea level changes caused by steric effects. Finally, we find support for a potential temperature control on compositional changes of branched glycerol monoalkyl glycerol tetraethers (brGMGTs) in marine sediments. It is encouraging that a critical evaluation of the GDGT signals confirms the vast majority of the generated data is reliable. However, this also implies the high TEX86-based SSTs for the Eocene sw Pacific, and the systematic offset between absolute TEX86-based SST and MBT'5me-based MAAT estimates remain unexplained.
Environmental conditions strongly influence fish growth and size. Temperature, salinity and oxygen can have negative effects on growth and size when species’ tolerances are exceeded. Climate modelling predicts that persistent oxygen minimum zones (OMZs), where dissolved oxygen concentration is less than 0.5 mll⁻¹, will expand globally, in addition to increases in ocean temperature. The persistent oxygen minimum zone in a warming Arabian Sea is one of only three in the world and, as such, provides a window into the potential responses of fish fauna to decreasing oxygen levels in the global seas. Fork lengths for 19,633 fish of five economically important species (Argyrops spinifer, Drepane longimana, Epinephelus diacanthus, Lethrinus nebulosus and Pomadasys commersonnii) were measured in four regions, during four seasons and at four depth strata in the Arabian Sea in order to test the influence of these factors, as proxies for the OMZ, on mean length and length-frequency distributions. The five species showed three general responses. The first group includes E. diacanthus and P. commersonnii which were largest in the south, largest during the Northeast Monsoon and did not vary in size with depth, possibly taking advantage of rich food sources for recruits but with southern populations associated with the OMZ comprised of larger individuals. The second group was comprised of D. longimana and L. nebulosus which were smaller in shallow depths and showed no influence of region or season and appear relatively insensitive to the OMZ. These two species show typical distribution in relation to depth, with larger individuals found in deeper waters. Finally, A. spinifer, was smallest in the south and in deep waters but showed no effect of season. This may indicate a sensitivity to the OMZ where larger individuals are neither found in the south nor at depth. The results of kernel density estimates largely mirrored these results. The environmental variables were significant but explained little of the deviance in fork length, with the exception of temperature which explained approximately 40% and 26.9% of the variation in length of E. diacanthus and A. spinifer, respectively. These groupings may reflect differential species sensitivities to the OMZ which is strongest in the southern regions during the SW Monsoon. This highlights the need for careful consideration of responses to expanding OMZs, particularly with how this translates into fisheries productivity.
Oxygen isotopes are widely used in palaeoenvironmental and palaeoclimatic studies as they record variations in the precipitation temperature of biogenic carbonates and phosphates. Problems associated with the preservation state of fossils, selection of the proper temperature equation, vital effects occurring during biomineralization, habitat effects of organisms as well as salinity, bathymetry and water circulation changes limit, however, the applicability of oxygen isotopes to reconstruction of ancient environmental settings. The progress of oxygen isotope studies, temperature calculations and ambiguities of the isotope record are discussed in this paper. The same applies to the methods of retrieving reliable temperature signals and the record of water chemistry changes based on well-preserved calcareous and phosphatic fossils. Sometimes neglected importance of sedimentological and faunistic data associated with sea-level changes and salinity variations is emphasised as an important tool for refinement of the temperature trends of epeiric sedimentary basins. In addition, published case datasets and new laboratory techniques, including micro-area and clumped isotope analyses, are presented to demonstrate examples and prospective ways of extension of the scope of palaeoenvironmental research. The provided information may be used in discussion and a critical review of published oxygen isotope data and their palaeoenvironmental interpretations.
Climate models predict Africa will warm by up to 5°C in the coming century, stressing African societies. To provide independent constraints on model predictions, this study compares two notable reconstructions of East African temperatures to those predicted by Paleoclimate Model Intercomparison Project (PMIP3) and transient TraCE (Transient Climate Evolution) simulations, focusing on the Mid-Holocene (MH, 5–8 kyr B.P.). Reconstructions of tropical African temperature derived from lake sedimentary archives indicate 1–2.5°C of warming during the MH relative to the 20th century, but most climate models do not replicate the warming observed in these paleoclimate data. We investigate this discrepancy using a new lake proxy system model, with attention to the (potentially non-stationary) relationship between lake temperature and air temperature. We find amplified lake surface temperature changes compared to air temperature during the MH due to heightened seasonality and precessional forcing. Lacustrine processes account for some of the warming, and highlight how the lake heat budget leads to a rectification of the seasonal cycle; however, the simulated lake heating bias is insufficient to reconcile the full discrepancy between the models and the proxy-derived MH warming. We find further evidence of changes in mixing depth over time, potentially driven by changes in cloud cover and shortwave radiative fluxes penetrating the lake surface. This may confound interpretation for glycerol dialkyl glycerol tetraethers (GDGT) compounds which exist in the mixed layer, and suggests a need for independent constraints on mixed layer depth. This work provides a new interpretive framework for invaluable paleoclimate records of temperature changes over the African continent.
South Asian precipitation amount and extreme variability are predicted to increase due to thermodynamic effects of increased 21st-century greenhouse gases, accompanied by an increased supply of moisture from the southern hemisphere Indian Ocean. We reconstructed South Asian summer monsoon precipitation and runoff into the Bay of Bengal to assess the extent to which these factors also operated in the Pleistocene, a time of large-scale natural changes in carbon dioxide and ice volume. South Asian precipitation and runoff are strongly coherent with, and lag, atmospheric carbon dioxide changes at Earth’s orbital eccentricity, obliquity, and precession bands and are closely tied to cross-equatorial wind strength at the precession band. We find that the projected monsoon response to ongoing, rapid high-latitude ice melt and rising carbon dioxide levels is fully consistent with dynamics of the past 0.9 million years.
Fossilized lipids preserved in sedimentary rocks offer singular insights into the Earth’s palaeobiology. These ‘biomarkers’ encode information pertaining to the oxygenation of the atmosphere and oceans, transitions in ocean plankton, the greening of continents, mass extinctions and climate change. Historically, biomarker interpretations relied on inventories of lipids present in extant microorganisms and counterparts in natural environments. However, progress has been impeded because only a small fraction of the Earth’s microorganisms can be cultured, many environmentally significant microorganisms from the past no longer exist and there are gaping holes in knowledge concerning lipid biosynthesis. The revolution in genomics and bioinformatics has provided new tools to expand our understanding of lipid biomarkers, their biosynthetic pathways and distributions in nature. In this Review, we explore how preserved organic molecules provide a unique perspective on the history of the Earth’s microbial life. We discuss how advances in molecular biology have helped elucidate biomarker origins and afforded more robust interpretations of fossil lipids and how the rock record provides vital calibration points for molecular clocks. Such studies are open to further exploitation with the expansion of sequenced microbial genomes in accessible databases. Lipid biomarkers provide insights into the nature and history of the Earth’s microbial life. In this Review, Summons et al. discuss how advances in molecular biology have helped elucidate lipid biomarker origins and afforded robust interpretations of fossil lipids, and explore key examples of how genomics-enabled advances in lipid biomarker science are illuminating the history of microbial life on the Earth.
The Mid-Pleistocene Transition (MPT) is a widely recognized global climate shift occurring between approximately 1,250 to 700 ka. At this time, Earth's climate underwent a major transition from dominant 40 kyr glacial-interglacial cycles to quasi-100 kyr cycles. The cause of the MPT remains a puzzling aspect of Pleistocene climate. Presently, there are few, if any, continuous MPT records from the Arctic yet understanding the role and response of the high latitudes to the MPT is required to better evaluate the causes of this climatic shift. Here, we present new continental biomarker records of temperature and vegetation spanning 1,142 to 752 ka from Lake El'gygytgyn (Far East Russia). We reconstruct warm-season temperature variations across the MPT based on branched glycerol dialkyl glycerol tetraethers (brGDGTs) using the MBTʹ5ME proxy. The new Arctic temperature record does not display an overall cooling trend during the MPT but does exhibit strong glacial-interglacial cyclicity. Spectral analysis demonstrates persistent obliquity and precession pacing over the study interval and reveals substantial sub-orbital temperature variations at ~900 kyr during the first “skipped” interglacial. Interestingly, Marine Isotope Stage (MIS) 31, which is widely recognized as a particularly warm interglacial, does not exhibit exceptional warmth at Lake El'gygytgyn. Instead, we find that MIS 29, 27 and 21 were as warm or warmer than MIS 31. In particular, MIS 21 (~870 to 820 ka) stands out as an especially warm and long interglacial in the continental Arctic while MIS 25 is a notably cold interglacial. Throughout the MPT, Lake El'gygytgyn pollen data exhibits a long-term drying trend, with a shift to an increasingly open landscape noted after around 900 ka (Zhao et al., 2018), which is also reflected in our higher plant leaf wax (n-alkane) distributions. Although the mechanisms driving the MPT remain a matter of debate, our new climate records from the continental Arctic exhibit some similarities to changes noted around the North Pacific region. Overall, the new organic geochemical data from Lake El'gygytgyn contribute to expanding our knowledge of the high-latitude response to the MPT.
While Pleistocene glacial-interglacial cycles are commonly associated with strong waxing and waning of Northern Hemisphere ice sheets, the response of the Antarctic ice sheet and regional changes in oceanographic and environmental conditions to Pleistocene climate dynamics remain poorly constrained. We present a reconstruction of sea-ice cover, sea surface temperature and primary productivity off the Ross Sea margin (Adare Basin at the slope of the Drygalski Basin) during the marine isotope stages (MIS) 9 to 5 (350–70 thousands years ago, encompassing Terminations IV to II). Our multiproxy study relies on micropaleontology (diatoms, dinoflagellate cysts, benthic foraminifers), organic and inorganic geochemistry proxies (carbon and nitrogen isotopes, lipid biomarkers, XRF-data), and sedimentology (IRD) obtained from deep-sea core AS05-10. For each glacial-interglacial transition a clear succession of events can be observed: (near-)permanent sea ice cover during glacial stages is followed by ice-shelf break-up with episodic ice-free areas and surface water stratification. Notably, ice-shelf break-up precedes the increase in air temperature as measured in the Vostok ice core for each glacial-interglacial transition. Generally, air temperature over Vostok starts rising once sea-ice cover at site AS05-10 has significantly decreased, becoming seasonal, as indicated by the diatom species composition. This is also reflected by the high diatom productivity and increased water mixing at site AS05-10, which is indicative of its proximity to the Marginal Ice Zone. At the onset of Termination II (MIS6 to 5), high export productivity and dysoxic bottom water conditions occurred, while water temperature increased about 5 °C. During each interglacial spring/summer sea-ice cover is most reduced, and highest productivity occurs. Following each interglacial, the warm and cold fluctuations match the sawtooth character of the temperatures over Vostok. This record illustrates that at the Ross Sea margin, sea surface conditions and (export) productivity were strongly influenced by the natural climate variability of the Pleistocene. In light of this, current global warming may lead to increased ice-shelf break-up, water column stratification and shifts in the position/size of the Marginal Ice Zone with implications for algal species composition and diversity, and for primary productivity.
The Δ47 (paleo)thermometer has opened a new avenue to determine carbonate formation temperatures independent of the oxygen isotopic composition of the fluid from which the carbonate crystallized. A major limitation of this thermometer is related to kinetic effects if homogeneous isotopic equilibrium is not attained during carbonate precipitation. Dual clumped isotope thermometry – the high-precision analysis of Δ48 along with Δ47 in CO2 evolved from phosphoric acid digestion of carbonates – makes it possible to resolve temperature from the kinetic information recorded in an individual carbonate phase. Therefore, it provides a new opportunity to identify (bio)mineralization pathways and to determine carbonate formation temperatures devoid of a kinetic bias, based solely on isotopic analysis of a single carbonate phase.
Identification of the nature and extent of kinetic effects as well as the reconstruction of accurate formation temperatures requires knowledge of the position of equilibrium in Δ47 vs Δ48 space. Here, we present Δ47 and Δ48 data of carbonates that were previously considered as having crystallized closest to equilibrium in a temperature range of 8 to 1100 °C. Across this range, the temperature dependences of Δ47 and Δ48 are best expressed by the following fourth order polynomials of 1/T:
Δ47 (CDES 90) (‰) = 1.038 (−5.897 1/T − 3.521 10³ 1/T² + 2.391 10⁷ 1/T³ − 3.541 10⁹ 1/T⁴) + 0.1856
Δ48 (CDES 90) (‰) = 1.028 (6.002 1/T − 1.299 10⁴ 1/T² + 8.996 10⁶ 1/T³ − 7.423 10⁸ 1/T⁴) + 0.1245
with CDES 90 representing the Carbon Dioxide Equilibrium Scale at a reaction temperature of 90 °C. In its entire temperature range, our Δ47 (CDES 90) - T - relationship agrees within 2 ppm with two previous Δ47 (I-CDES) - T - relationships reported by Jautzy et al. (2020) and Anderson et al. (2021). Accuracy of our proposed Δ47 (CDES 90) − Δ48 (CDES 90) equilibrium relationship is independently confirmed by additional dual clumped isotope data of experimental and geothermal carbonates which precipitated from potentially equilibrated dissolved inorganic carbon pools at a temperature range of 25–100 °C. Furthermore, we reprocessed original dual clumped isotope data of natural carbonates (Bajnai et al., 2020) and compared their composition to the position of equilibrium in Δ47 vs Δ48 space. These results corroborate preliminary evidence that the hydration/hydroxylation reactions became rate-limiting during the calcification of a speleothem-like sample, a warm water coral, a cold water coral and a brachiopod, finally evoking significant departures of carbonate-Δ47 and -Δ48 from dual clumped isotope equilibrium.
An anti-clumped Δ48 value of −419 (±16) ppm (95% confidence interval level) is obtained for a technical calcite that was precipitated by the injection of CO2 into a Ca(OH)2-saturated solution. Its negative Δ48 value largely arises from a combinatorial effect, i.e. the carbonate oxygen derives from two sources with different bulk isotopic compositions. Besides the identification of the nature and the extent of (bio)mineralization kinetics and the reconstruction of carbonate formation temperatures unbiased by kinetics, dual clumped isotope analysis, therefore, allows tracing the isotopic heterogeneity of oxygen pools contributing to carbonate formation.
Branched glycerol dialkyl glycerol tetraethers (brGDGTs) are bacterial membrane lipids that are ubiquitous in the environment. Although the exact source mechanism is unknown, the distribution of brGDGTs in mineral soils, peats, and lake sediments is correlated with temperature through a decrease in the degree of methylation with increasing temperature. This empirical observation forms the basis of the brGDGT paleothermometer, one of the most important and widely used organic proxies to reconstruct terrestrial temperatures in the past. However, a mechanistic understanding to underpin this empirical correlation between the degree of methylation of brGDGT lipids and temperature is lacking, hindering a holistic understanding of the brGDGT paleothermometer as well as the membrane dynamics of their bacterial producers. To address this, here we present the first molecular dynamics simulations of membranes consisting of brGDGTs. Using intact polar lipid (IPL) brGDGTs with two sugar headgroups, our simulations demonstrate that increasing the degree of methylation modulates membrane order and packing, rendering the membrane less rigid and more fluid. These results indicate that the empirically observed correlation between the degree of methylation and temperature allows brGDGT-producing bacteria to maintain adequate membrane fluidity. Our simulations provide the first molecular simulation data to support the hypothesis that the brGDGT paleothermometer is based on homeoviscous adaptation.
A large and highly dynamic aquatic system called Paratethys governed important elements of the middle and late Miocene (15.97–5.33 Ma) hydrology in western Eurasia. So far, the impact of the vast Paratethys water body on the Eurasian climate, however, is not yet understood. Here we apply biomarker analyses coupled to compound-specific hydrogen and carbon isotope data to track changes in sea surface temperature, mean annual air temperature, hydrological budget and vegetation changes to reconstruct long-term western Eurasian climate conditions between 12.7 and 7.65 Ma in the Black Sea region. Biomarker data from Panagia (Russia) indicate the presence of three exceptionally evaporative intervals peaking at 9.65, 9.4 and 7.9 Ma. These peaks in evaporation relate to aridity, parallel increasing fire activity and are associated with changes in vegetation. Carbon isotope and pollen data support the evidence of an increase in C4 plants associated with these dry intervals. At 9.66 Ma, alkenone producing algae appear in the basin and thrive for the subsequent two million years. Cumulative fluctuations in both hydrology and surface temperature of Paratethys might have enhanced rainfall seasonality in western Eurasia as a response to changes in evaporation over the Paratethys basin. Our combined data suggest a strong regional imprint on overall climate patterns, dominated by basin dynamics causing Paratethys volume and surface reduction. Collectively, the presented biomarker results provide evidence of severe droughts affecting the late Miocene circum-Paratethys region, leading to a direct impact on the evolution of biota in the basin and its surroundings.
Thaumarchaeota and Thermoplasmatota are the most abundant planktonic archaea in the sea. Thaumarchaeota contain tetraether lipids as their major membrane lipids, but the lipid composition of uncultured planktonic Thermoplasmatota representatives remains unknown. To address this knowledge gap, we quantified archaeal cells and ether lipids in open ocean depth profiles (0–200 m) of the North Pacific Subtropical Gyre. Planktonic archaeal community structure and ether lipid composition in the water column partitioned into two separate clusters: one above the deep chlorophyll maximum, the other within and below it. In surface waters, Thermoplasmatota densities ranged from 2.11 × 10 ⁶ to 6.02 × 10 ⁶ cells/L, while Thaumarchaeota were undetectable. As previously reported for Thaumarchaeota, potential homologs of archaeal tetraether ring synthases were present in planktonic Thermoplasmatota metagenomes. Despite the absence of Thaumarchaeota in surface waters, measurable amounts of intact polar ether lipids were found there. Based on cell abundance estimates, these surface water archaeal ether lipids contributed only 1.21 × 10 –9 ng lipid/Thermoplasmatota cell, about three orders of magnitude less than that reported for Thaumarchaeota cells. While these data indicate that even if some tetraether and diether lipids may be derived from Thermoplasmatota, they would only comprise a small fraction of Thermoplasmatota total biomass. Therefore, while both MGI Thaumarchaeota and MGII/III Thermoplasmatota are potential biological sources of archaeal GDGTs, the Thaumarchaeota appear to be the major contributors of archaeal tetraether lipids in planktonic marine habitats. These results extend and confirm previous reports of planktonic archaeal lipid sources, and further emphasize the need for Thermoplasmatota cultivation, to better characterize the membrane lipid constituents of marine planktonic Thermoplasmatota, and more precisely define the sources and patterns of archaeal tetraether lipid distributions in marine plankton.
Branched glycerol dialkyl glycerol tetraethers (brGDGTs) are a family of bacterial lipids widely used for temperature and pH reconstructions in terrestrial settings. 3-hydroxy fatty acids (3-OH FAs) with 10 to 18 carbon atoms, produced by Gram-negative bacteria, have been recently proposed as independent and complementary proxies of temperature and pH in terrestrial environments. Nevertheless, the correlations between mean annual air temperature (MAAT)/pH and bacterial lipid (brGDGTs/3-OH FAs) distribution show a large degree of scatter, as the relative abundance of these lipids is influenced by factors other than temperature and pH. A full understanding of the environmental parameters influencing bacterial lipid distribution in soils is required to increase the reliability of the temperature and pH proxies based on these compounds in terrestrial environments. The aim of this work was to determine and quantify the cumulative effect of environmental parameters on the distribution of both brGDGTs and 3-OH FAs along a well-documented composite altitudinal transect in the French Alps (234-2,748 m). Redundancy analysis revealed that the influence of local parameters (pH and to a lesser extent soil moisture and grain size, related to vegetation and soil types) on brGDGT and 3-OH FA distribution amounted to 48.1% and 26.1%, respectively, and was predominant over MAAT. This likely explained the weak or lack of relationships between MAAT and brGDGT-/3-OH FA-based indices in this region. The identification of lipids whose fractional abundance is correlated with MAAT or pH allowed the development of local calibrations with MAAT/pH applicable in the French Alps which are representative of highly contrasted microenvironments, reflecting different types of soil and vegetation. The present study highlighted the importance of constraining the environmental factors affecting the distribution of 3-OH FAs and brGDGTs in terrestrial settings prior to any paleoenvironmental reconstruction. Such an approach should be reproduced in other sites, where local factors could also strongly influence the bacterial lipid distribution.
A previous report of high levels of members of the domain Archaea in Antarctic coastal waters prompted us to investigate the ecology of Antarctic planktonic prokaryotes. rRNA hybridization techniques and denaturing gradient gel electrophoresis (DGGE) analysis of the bacterial V3 region were used to study variation in Antarctic picoplankton assemblages. In Anvers Island nearshore waters during late winter to early spring, the amounts of archaeal rRNA ranged from 17.1 to 3.6% of the total picoplankton rRNA in 1996 and from 16.0 to 1.0% of the total rRNA in 1995. Offshore in the Palmer Basin, the levels of archaeal rRNA throughout the water column were higher (average, 24% of the total rRNA) during the same period in 1996. The archaeal rRNA levels in nearshore waters followed a highly seasonal pattern and markedly decreased during the austral summer at two stations. There was a significant negative correlation between archaeal rRNA levels and phytoplankton levels (as inferred from chlorophyll a concentrations) in nearshore surface waters during the early spring of 1995 and during an 8-month period in 1996 and 1997. In situ hybridization experiments revealed that 5 to 14% of DAPI (4',6-diamidino-2-phenylindole)-stained cells were archaeal, corresponding to 0.9 x 104 to 2.7 x 104 archaeal cells per ml, in late winter 1996 samples. Analysis of bacterial ribosomal DNA fragments by DGGE revealed that the assemblage composition may reflect changes in water column stability, depth, or season. The data indicate that changes in Antarctic seasons are accompanied by significant shifts in the species composition of bacterioplankton assemblages and by large decreases in the relative proportion of archaeal rRNA in the nearshore water column.
Most of the organic matter sequestered in Recent and ancient marine sediments is ultimately derived from organic matter biosynthesized by marine organisms inhabiting the surface waters of the oceans and transported to the seafloor as particulate organic matter (POM). However, most of the organic matter produced in the upper ocean is recycled in the upper few hundred meters of the water column (the epipelagic zone). Only a small fraction of the particulate material produced in the euphotic zone sinks into deeper waters (the mesopelagic and bathypelagic zones). In turn, only a small fraction of this sinking material survives transport to the seafloor to be preserved in the sediments. Extensive alteration of organic matter in the water column and at the sediment-water interface can yield sedimentary organic matter having a chemical composition markedly different from that of the material originally biosynthesized. A major goal of marine organic geochemists is to understand the qualitative and quantitative changes which occur in the water column and at the sediment-water interface (Wakeham and Lee, 1989). In this chapter we will discuss recent advances in our understanding of water column processes which influence the cycling of organic matter in the ocean.
Free and "bound" long-chain alkenones (C37:2 and C37:3) in oxidized and unoxidized sections of four organic matter-rich Pliocene and Miocene Madeira Abyssal Plain turbidites (one from Ocean Drilling Program site 951B and three from site 952A) were analyzed to determine the effect of severe post depositional oxidation on the value of UK'37. The profiles of both alkenones across the redox boundary show a preferential degradation of the C37:3 compared to the C37:2 compound. Because of the high initial UK'37 values and the way of calculating the UK'37 this degradation hardly influences the UK'37 profiles. However, for lower Uk'37 values, measured selective degradation would increase Uk'37 up to 0.17 units, equivalent to 5°C. For most of the UK'37 band-width, much smaller degradation already increases UK'37 beyond the analytical error (0.017 units). Consequently, for interpreting the UK'37 record in terms of past sea surface temperatures, selective degradation needs serious consideration.
Variations in sea-surface temperatures over the past 500,000 years are inferred from the relative abundance behaviour of two organic compounds, C37 alkenones over the upper 8 metres of a sediment core from the eastern equatorial Atlantic. This molecular record, ascribed to contributions from prymnesiophyte algae, correlates well with the variations in the delta18 signal for the calcareous skeletons of certain planktonic foraminifera, thus providing the first demonstration of a new stratigraphical technique, which may be especially valuable where methods based on carbonate delta18 fail.
Stable oxygen and carbon isotope measurements on biogenic calcite and aragonite have become standard tools for reconstructing past oceanographic and climatic change. In aquatic organisms, 18O/16O ratios in the shell carbonate are a function of the ratio in the sea water and the calcification temperature. In contrast, 13C/12C ratios are controlled by the ratio of dissolved inorganic carbon in sea water and physiological processes such as respiration and symbiont photosynthesis. These geochemical proxies have been used with analyses of foraminifera shells to reconstruct global ice volumes, surface and deep ocean temperatures,, ocean circulation changes and glacial-interglacial exchange between the terrestrial and oceanic carbon pools. Here, we report experimental measurements on living symbiotic and non-symbiotic plankton foraminifera (Orbulina universa and Globigerina bulloides respectively) showing that the 13C/12C and 18O/16O ratios of the calcite shells decrease with increasing seawater [CO32-]. Because glacial-period oceans had higher pH and [CO32-] than today, these new relationships confound the standard interpretation of glacial foraminiferal stable-isotope data. In particular, the hypothesis that the glacial-interglacial shift in the 13C/12C ratio was due to a transfer of terrestrial carbon into the ocean can be explained alternatively by an increase in ocean alkalinity. A carbonate-concentration effect could also help explain some of the extreme stable-isotope variations during the Proterozoic and Phanerozoic aeons.
Although detailed temporal studies of marine planktonic cyanobacteria have been reported, relatively little is known about variation of other marine picoplankton groups on time scales on the order of months to years. In this study, we followed vanation in relative abundance of planktonic archaeal rRNA at 5 depths in the upper 300 m of the Santa Barbara Channel (SBC) over 32 mo. Small subunit rRNA targeted oligonucleotide probes were used to quantify archaeal, bacterial, and eucaryal rRNA relative abundance. Archaea subgroups were quantified using probes specific to 2 planktonic archaeal groups, termed G1 and G11 The archaeal rRNA signal in the upper 20 m was characterized by intermittent 'blooms' that coincided with increases in the relatlvc ab.undance of GII rRNA as well as decreases in chlorophyll a (chl a). At greater depths, archaeal rRNA abundance was consistently ele-vated, approaching bacterial rRNA abundance. The G1 rRNA accounted for the majority of the deeper archaeal rRNA signal throughout the time series. Consistent with a previous report, the 2 groups of archaea had maximal rRNA abundance at different depths. The lnajolity of the variability in the G1 archaeal slgnal in the upper 75 m (77 %) could be attributed to a positive relat~onship with nutrients and negative relationships with prokaryotic abundance, chl a, and temperature; although, at most, 41% of the variability at each depth could be explained. Only 21 % of the variability in the G11 rRNA signal could be explained by the variables included in the model. Leucine incorporation rates and, to a lesser extent, prokaryote abundance, were highly correlated with chl a and POC levels. Only weak relation-ships were observed between prokaryote abundance or leucine incorporation and incredses in tem-perature. The results of linear regressions between prokaryote abundance and leucine incorporation suggest that top-down ecological controls may regulate prokaryotic assemblages in this dynamic coastal system. The distribution of the 2 archaeal groups suggested that they responded independently to environmental conditions, are physiologically different, and likely participate in different environ-mental processes.
RNA from Lake Michigan sediment was hybridized with a DNA probe for archaeal 16S rRNA. There was a peak of archaeal rRNA abundance in the oxic zone and another immediately below it. Six contributing species were identified by PCR amplification of extracted DNA with primers specific for archaeal rDNA: two related to Methanosarcina acetivorans and four related to marine crenarchaeotal sequences. rRNA quantification using a DNA probe specific for this crenarchaeotal assemblage showed it is most abundant in the oxic zone, where it accounts for about 10% of total archaeal rRNA.
We compared the phylogenetic compositions of marine planktonic archaeal populations in different marine provinces. Samples from eight different environments were collected at two depths (surface and aphotic zone), and 16 genetic libraries of PCR-amplified archaeal 16S rRNA genes were constructed. The libraries were analyzed by using a three-step hierarchical approach. Membrane hybridization experiments revealed that most of the archaeal clones were affiliated with one of the two groups of marine archaea described previously, crenarchaeotal group I and euryarchaeotal group II. One of the 2,328 ribosomal DNA clones analyzed was related to a different euryarchaeal lineage, which was recently recovered from deep-water marine plankton. In temperate regions (Pacific Ocean, Atlantic Ocean, and Mediterranean Sea) both major groups were found at the two depths investigated; group II predominated at the surface, and group I predominated at depth. In Antarctic and subantarctic waters group II was practically absent. The clonal compositions of archaeal libraries were investigated by performing a restriction fragment length polymorphism (RFLP) analysis with two tetrameric restriction enzymes, which defined discrete operational taxonomic units (OTUs). The OTUs defined in this way were phylogenetically consistent; clones belonging to the same OTU were closely related. The clonal diversity as determined by the RFLP analysis was low, and most libraries were dominated by only one or two OTUs. Some OTUs were found in samples obtained from very distant places, indicating that some phylotypes were ubiquitous. A tree containing one example of each OTU detected was constructed, and this tree revealed that there were several clusters within archaeal group I and group II. The members of some of these clusters had different depth distributions.
The ocean's interior is Earth's largest biome. Recently, cultivation-independent ribosomal RNA gene surveys have indicated a potential importance for archaea in the subsurface ocean. But quantitative data on the abundance of specific microbial groups in the deep sea are lacking. Here we report a year-long study of the abundance of two specific archaeal groups (pelagic euryarchaeota and pelagic crenarchaeota) in one of the ocean's largest habitats. Monthly sampling was conducted throughout the water column (surface to 4,750 m) at the Hawai'i Ocean Time-series station. Below the euphotic zone (> 150 m), pelagic crenarchaeota comprised a large fraction of total marine picoplankton, equivalent in cell numbers to bacteria at depths greater than 1,000 m. The fraction of crenarchaeota increased with depth, reaching 39% of total DNA-containing picoplankton detected. The average sum of archaea plus bacteria detected by rRNA-targeted fluorescent probes ranged from 63 to 90% of total cell numbers at all depths throughout our survey. The high proportion of cells containing significant amounts of rRNA suggests that most pelagic deep-sea microorganisms are metabolically active. Furthermore, our results suggest that the global oceans harbour approximately 1.3 x 10(28) archaeal cells, and 3.1 x 10(28) bacterial cells. Our data suggest that pelagic crenarchaeota represent one of the ocean's single most abundant cell types.
Biogeochemical and stable carbon isotopic analysis of black-shale sequences deposited during an Albian oceanic anoxic event (approximately 112 million years ago) indicate that up to 80 weight percent of sedimentary organic carbon is derived from marine, nonthermophilic archaea. The carbon-13 content of archaeal molecular fossils indicates that these archaea were living chemoautotrophically. Their massive expansion may have been a response to the strong stratification of the ocean during this anoxic event. Indeed, the sedimentary record of archaeal membrane lipids suggests that this anoxic event marks a time in Earth history at which certain hyperthermophilic archaea adapted to low-temperature environments.
Intact core tetraether membrane lipids of marine planktonic Crenarchaeota were quantified in water column-suspended particulate matter obtained from four depth intervals (∼70, 500, 1,000 and 1,500
m) at seven stations in the northwestern Arabian Sea to investigate the distribution of the organisms at various depths. Maximum
concentrations generally occurred at 500 m, near the top of the oxygen minimum zone, and the concentrations at this depth
were, in most cases, slightly higher than those in surface waters. In contrast, lipids derived from eukaryotes (cholesterol)
and from eukaryotes and bacteria (fatty acids) were at their highest concentrations in surface waters. This indicates that
these crenarchaeotes are not restricted to the photic zone of the ocean, which is consistent with the results of recent molecular
biological studies. Since the Arabian Sea has a strong oxygen minimum zone between 100 and 1,000 m, with minimum oxygen levels
of <1 μM, the abundance of crenarchaeotal membrane lipids at 500 m suggests that planktonic Crenarchaeota are probably facultative anaerobes. The cell numbers we calculated from the concentrations of membrane lipids are similar
to those reported for the Central Pacific Ocean, supporting the recent estimation of M. B. Karner, E. F. DeLong, and D. M.
Karl (Nature 409:507-510, 2001) that the world's oceans contain ca. 1028 cells of planktonic Crenarchaeota.
The basic structure and stereochemistry of the characteristic glycerol dibiphytanyl glycerol tetraether (GDGT) membrane lipid of cosmopolitan pelagic crenarchaeota has been identified by high field two-dimensional (2D)-NMR techniques. It contains one cyclohexane and four cyclopentane rings formed by internal cyclisation of the biphytanyl chains. Its structure is similar to that of GDGTs biosynthesized by (hyper)thermophilic crenarchaeota apart from the cyclohexane ring. These findings are consistent with the close phylogenetic relationship of (hyper)thermophilic and pelagic crenarchaeota based 16S rRNA. The latter group inherited the biosynthetic capabilities for a membrane composed of cyclopentane ring-containing GDGTs from the (hyper)thermophilic crenarchaeota. However, to cope with the much lower temperature of the ocean, a small but key step in their evolution was the adjustment of the membrane fluidity by making a kink in one of the bicyclic biphytanyl chains by the formation of a cyclohexane ring.
This prevents the dense packing characteristic for the cyclopentane ring-containing GDGTs membrane lipids used by hyperthermophilic crenarchaeota to adjust their membrane fluidity to high temperatures.
We have analysed alkenones in 149 surface sediments from the eastern South Atlantic in order to establish a sediment-based calibration of the U37K′ paleotemperature index. Our study covers the major tropical to subpolar production systems and sea-surface temperatures (SST’s) between 0° and 27°C. In order to define the most suitable calibration for this region, the U37K′ values were correlated to seasonal, annual, and production-weighted annual mean atlas temperatures and compared to previously published culture and core-top calibrations. The best linear correlation between U37K′ and SST was obtained using annual mean SST from 0 to 10 m water depth (U37K′ = 0.033 T + 0.069, r2 = 0.981). Data scattering increased significantly using temperatures of waters deeper than 20 m, suggesting that U37K′ reflects mixed-layer SST and that alkenone production at thermocline depths was not high enough to significantly bias the mixed-layer signal. Regressions based on both production-weighted and on actual annual mean atlas SST were virtually identical, indicating that regional variations in the seasonality of primary production have no discernible effect on the U37K′ vs. SST relationship. Comparison with published core-top calibrations from other oceanic regions revealed a high degree of accordance. We, therefore, established a global core-top calibration using U37K′ data from 370 sites between 60°S and 60°N in the Atlantic, Indian, and Pacific Oceans and annual mean atlas SST (0–29°C) from 0 m water depth. The resulting relationship (U37K′ = 0.033 T + 0.044, r2 = 958) is identical within error limits to the widely used E. huxleyi calibrations of Prahl and Wakeham (1987) and Prahl et al. (1988) attesting their general applicability. The observation that core-top calibrations extending over various biogeographical coccolithophorid zones are strongly linear and in better accordance than culture calibrations suggests that U37K′ is less species-dependent than is indicated by culture experiments. The results also suggest that variations in growth rate of algae and nutrient availability do not significantly affect the sedimentary record of U37K′ in open ocean environments.
Over the last decade the number of paleoceanographic studies presenting
and discussing alkenone-derived sea surface temperature (SST) records
and carbon isotope ratios of alkenones
(δ13Calkenones) has increased rapidly. This
short review paper summarizes the state of knowledge about SST
variability determined by alkenone paleothermometry in sediment trap
studies and investigations of marine sediments for timescales of single
to millions of years. In particular from the evaluation of Quaternary
records, characteristic temperature pattern have emerged that mismatch
with SST curves estimated by other methods, e.g., foraminiferal stable
oxygen isotopes and transfer functions. In future studies the spatial
distribution of such variance patterns typically observed in alkenone
SST curves should be checked in order to better understand their meaning
for ocean circulation changes and a possible relationship to global
climate change. On the other hand, certain physical and biological
processes have the potential to put the reliability of alkenone SST
curves into question. Accordingly, examples for possible effects of
bioturbational mixing, lateral transport of remote alkenones, diagnetic
alteration and degradation of alkenone composition, biogeographic and
temporal diversity changes in alkenone-producing algae, as well as the
influence of changes in salinity and nutrient conditions on the alkenone
paleotemperature signal, are briefly mentioned, highlighting the need
for more detailed future studies on these issues. Finally, a short
summary on the status of the use of the
δ13Calkenones signal as a proxy for past
variations in the carbon dioxide pressure (PCO2) in ocean
surface waters and related problems is given.
Qualitative analyses of lipid component parts (core lipids, phospholipid-polar head groups, and glycolipid-sugar moieties) without separation of individual lipids were done for further 14 strains of methanogens. The results confirmed the conclusion of our previous paper (system. Appl. Microbiol. 16, 342 1993) that the distribution of Lipid component parts was characteristic to taxonomic groups of methanogens at a family or genus level. Our previous and present analyses of lipid component part distribution of methanogens supported the division of the order Methanomicrobiales into two new orders Methanomicrobiales and "Methanosarcinales" proposed by Boone et al. based on 16S rRNA analyses (Methanogenesis: Ecology, Physiology, Biochemistry, & Genetics, 1993, pp 35-80). The whole results also phenotypically supported the establishment of new families "Methanocaldococcaceae" and "Methanosaetaceae" and new genera "Methanothermococcus", "Methanocaldococcus", "Methanoignis", and "Methanosalsus" proposed by Boone et al.
To a certain degree, all sea surface temperature (SST) proxies suffer
from uncertainty due to signal preservation problems and to
perturbations by other environmental variables. However, a multiproxy
reconstruction is justified because most biases should, in principle,
cancel out as they are specific to the biological groups and/or to the
analyzed chemical species. Such a multiproxy strategy has proven crucial
for calibrating these proxies and is also useful in assessing downcore
profiles. Until now there have been only a dozen deep-sea cores where it
is possible to compare directly alkenone-based SSTs with other
paleotemperature records. From this still limited data set the broad
picture is of general agreement for the amplitude of changes at low and
middle latitudes. The few observed discrepancies suggest that
alkenone-based temperatures are more comparable to those based on Mg/Ca
ratios than those obtained through statistical analysis of the
foraminiferal distribution. More systematic differences are noted at
higher latitudes, where alkenones indicate warmer temperatures during
the glacial period than transfer functions based on foraminifera or
diatoms. Several reasons have been invoked to resolve the mismatch, but
more "ground truthing" work is needed before deciding which temperature
changes are the most accurate.
METHANE is an important component of the global carbon cycle1 and a potent greenhouse gas2,3. Surface ocean waters are typically supersaturated with dissolved methane relative to atmospheric equilibrium, presumably as a result of in situ microbial methane production4–8. Because methanogenic bacteria are strict anaerobes9and surface ocean waters are highly oxygenated, the observation of methane supersaturation has been termed the 'oceanic methane paradox'10. Although methanogenic bacteria have been isolated from oceanic particulate matter, faecal pellets and zooplankton11–14, no data are available on in situ rates of methane formation in these microenvironments. During a series of experiments in the North Pacific ocean, we have identified a previously unrecognized component of the oceanic methane cycle. We find that methane is associated with sinking particles, presumably as a dissolved constituent of the interstitial fluids of particulate biogenic materials, which exchanges with the water column as particles sink. This phenomenon provides a mechanism for the active transport in the water column of an otherwise passive, dissolved species. The particle-to-seawater methane flux that we measure is sufficient to replace all of the methane present in the upper water column in about 50 days and to produce the characteristic methane supersaturations in less than a month. We suggest that particulate production and transport may also be relevant to the redistribution and cycling of other bioreactive compounds in the marine environment.
Stable oxygen and carbon isotope measurements on biogenic calcite and
aragonite have become standard tools for reconstructing past
oceanographic and climatic change. In aquatic organisms,
18O/16O ratios in the shell carbonate are a
function of the ratio in the sea water and the calcification
temperature. In contrast, 13C/12C ratios are
controlled by the ratio of dissolved inorganic carbon in sea water and
physiological processes such as respiration and symbiont photosynthesis.
These geochemical proxies have been used with analyses of foraminifera
shells to reconstruct global ice volumes, surface and deep ocean
temperatures,, ocean circulation changes and glacial-interglacial
exchange between the terrestrial and oceanic carbon pools. Here, we
report experimental measurements on living symbiotic and non-symbiotic
plankton foraminifera (Orbulina universa and Globigerina bulloides
respectively) showing that the 13C/12C and
18O/16O ratios of the calcite shells decrease with
increasing seawater [CO32-]. Because
glacial-period oceans had higher pH and [CO32-]
than today, these new relationships confound the standard interpretation
of glacial foraminiferal stable-isotope data. In particular, the
hypothesis that the glacial-interglacial shift in the
13C/12C ratio was due to a transfer of terrestrial
carbon into the ocean can be explained alternatively by an increase in
ocean alkalinity. A carbonate-concentration effect could also help
explain some of the extreme stable-isotope variations during the
Proterozoic and Phanerozoic aeons.
Small live individuals of Globigerinoides sacculifer which were cultured in the laboratory reached maturity and produced garnets. Fifty to ninety percent of their skeleton weight was deposited under controlled water temperature (14° to 30°C) and water isotopic composition, and a correction was made to account for the isotopic composition of the original skeleton using control groups. Comparison of. the actual growth temperatures with the calculated temperature based on paleotemperature equations for inorganic CaCO 3 indicate that the foraminifera precipitate their CaCO 3 in isotopic equilibrium. Comparison with equations developed for biogenic calcite give a similarly good fit. Linear regression with (1965) equation yields: where t is the actual growth temperature and Is the calculated paleotemperature. The intercept and the slope of this linear equation show that the familiar paleotemperature equation developed originally for mollusca carbonate, is equally applicable for the planktonic foraminifer G. sacculifer . Second order regression of the culture temperature and the delta difference ( 18 Oc - 18 Ow ) yield a correlation coefficient of r = 0.95: and 18 Ow are the estimated temperature, the isotopic composition of the shell carbonate and the sea water respectively. A possible cause for nonequilibnum isotopic compositions reported earlier for living planktonic foraminifera is the improper combustion of the organic matter.
A series of long-chain (C37, C38, C39), primarily di- and tri-unsaturated methyl and ethyl ketones, first identified in sediments from Walvis Ridge off West Africa and from the Black Sea1, has been found in marine sediments throughout the world2. The marine coccolithophorid Emiliania huxleyi and members of the class Prymnesiophyceae are now the recognized sources of these compounds3,4. Experiments with laboratory cultures of algae showed the degree of unsaturation in the ketone series biosynthesized depends on growth temperature2,5, a physiological response observed for classical membrane lipids6. Brassell and co-workers2,7 thus proposed that systematic fluctuations in the unsaturation of these alkenones noted down-core in sediments from the Kane Gap region of the north-east tropical Atlantic Ocean and correlated with glacial-interglacial cycles provide an organic geochemical measure of past sea-surface water temperatures. Using laboratory cultures of E. huxleyi, we have calibrated changes in the unsaturation pattern of the long-chain ketone series versus growth temperature. The calibration curve is linear and accurately predicts unsaturation patterns observed in natural particulate materials collected from oceanic waters of known temperature. We present evidence supporting the proposed palaeotemperature hypothesis2,7 and suggesting absolute 'sea-surface temperatures' for a given oceanic location can be estimated from an analysis of long-chain ketone compositions preserved in glacial and interglacial horizons of deep-sea sediment cores.
The plasma membrane of Caldariella acidophila, an extreme thermophilic archaebacterium, is characterized by unusual bipolar lipids. They are based on two C40 ω-ω′ biphytanyl residues, with up to four cyclopentane rings per chain linked to either two glycerols (symmetric lipid) or to one glycerol and to one branched-chain nonitol (asymmetric lipid). When C. acidophila is grown at various temperatures, these lipids show a degree of cyclization of the biphytanyl components which increases as the environmental temperature increases. The rôle of cyclization in determining the temperature adaptation is studied on three lipid samples presenting four, five and six cyclopentane rings per molecule, respectively. Differential scanning calorimetry on the dry asymmetric sample as well as conductance and capacitance measurements on black films have been performed. Both sets of measurements indicate the presence of thermal transitions, three in the hydrated compounds, two in the dry system. The latter are shifted towards higher temperature values as the number of cycles increases. Calorimetric measurements show that two of these transitions are strictly related to the presence of nonitol-containing polar heads. In fact, only a single 10-fold higher transition is detected in the homologous lipid bearing two glycerol polar heads, in the dry as well as in the hydrated form. It is suggested that the two higher-temperature thermal transitions, observed on warming the sample, are induced by the breaking of hydrogen bonds between the nonitol-containing polar heads. By contrast, the lower temperature transition, present only in the hydrated compound and similar to that exhibited by the symmetrical sample, is due to a partial melting of the hydrophobic core. The large change in capacitance observed near the higher transition points by lowering temperature would thus correspond to variations in the dielectric constant due to formation of hydrogen bonds.
We investigated the distributions and 13C values of biomarkers for Archaea associated with anaerobic methane oxidation in disparate settings throughout two Eastern Mediterranean mud dome fields. All major classes of archaeal lipids are present in the studied sediments, including isoprenoid glycerol diethers, isoprenoid glycerol dialkyl glycerol tetraethers, and irregular isoprenoid hydrocarbons. Of the compounds present, many, including a novel glycerol tetraether and sn-3-hydroxyarchaeol, have not been previously reported for settings in which methane oxidation is presumed to occur. Archaeal lipids are depleted in 13C, indicating that the Archaea from which they derive are either directly or indirectly involved with methane consumption. The most widespread archaeal lipids are archaeol, PMI, and glycerol tetraethers, and these compounds are present at all active sites. However, archaeal lipid abundances and distributions are highly variable; ratios of crocetane, PMI, and hydroxyarchaeol relative to archaeol vary from 0 to 6.5, from 0 to 2, and from 0 to 1, respectively. These results suggest that archaeal communities differ amongst the sites examined. In addition, carbon isotopic variability amongst archaeal biomarkers in a given mud breccia can be as large as 24 , suggesting that even at single sites multiple archaeal species perform or are supported by anaerobic methane oxidation.
We evaluate anaerobic oxidation of methane (AOM) in the Black Sea water column by determining distributions of archaea-specific glyceryl dialkyl glyceryl tetraethers (GDGTs) and 13C isotopic compositions of their constituent biphytanes in suspended particulate matter (SPM), sinking particulate matter collected in sediment traps, and surface sediments. We also determined isotopic compositions of fatty acids specific to sulfate-reducing bacteria to test for biomarker and isotopic evidence of a syntrophic relationship between archaea and sulfate-reducing bacteria in carrying out AOM. Bicyclic and tricyclic GDGTs and their constituent 13C-depleted monocyclic and bicyclic biphytanes (down to −67) indicative of archaea involved in AOM were present in SPM in the anoxic zone below 700 m depth. In contrast, GDGT-0 and crenarchaeol derived from planktonic crenarchaeota dominated the GDGT distributions in the oxic surface and shallow anoxic waters. Fatty acids indicative of sulfate-reducing bacteria (i.e., iso- and anteiso-C15) were not strongly isotopically depleted (e.g., −32 to −25), although anteiso-C15 was 5 more depleted in 13C than iso-C15. Our results suggest that either AOM is carried out by archaea independent of sulfate-reducing bacteria or those sulfate-reducing bacteria involved in a syntrophy with methane-oxidizing archaea constitute a small enough fraction of the total sulfate-reducing bacterial community that an isotope depletion in their fatty acids is not readily detected. Sinking particulate material collected in sediment traps and the underlying sediments in the anoxic zone contained the biomarker and isotope signature of upperwater column archaea. AOM-specific GDGTs and 13C-depleted biphytanes characteristic of the SPM in the deep anoxic zone are not incorporated into sinking particles and are not efficiently transported to the sediments. This observation suggests that sediments may not always record AOM in overlying euxinic water columns and helps explain the absence of AOM-derived biomarkers in sediments deposited during past periods of elevated levels of methane in the ocean.
Marine bacteria often dominate the plankton biomass and are responsible for much of the cycling of organic matter, but bacterial diversity is poorly understood because conventional identification methods (requiring culturing) miss about 99% of the organisms. Recent advances permit characterization of microbial communities by analysis of 16S ribosomal RNA gene sequences directly from biomass without the need to culture the organisms; such studies from surface ocean samples have found only eubacteria, not archaebacteria (or Archaea), which are profoundly different. Here we report 16S rRNA sequences obtained from Pacific Ocean bacterioplankton samples collected from depths of 100 m and 500 m. Among these we found sequences only distantly related to those of any organisms previously characterized by 16S rRNA sequences, with similarities to the nearest such relatives (extreme thermophiles) approximately the same as those between animals and plants. We suggest that these sequences are from a previously undescribed archaebacterial group that may have diverged from the ancestors of characterized organisms very early in evolution.
Archaea (archaebacteria) constitute one of the three major evolutionary lineages of life on Earth. Previously these prokaryotes were thought to predominate in only a few unusual and disparate niches, characterized by hypersaline, extremely hot, or strictly anoxic conditions. Recently, novel (uncultivated) phylotypes of Archaea have been detected in coastal and subsurface marine waters, but their abundance, distribution, physiology and ecology remain largely undescribed. Here we report exceptionally high archaeal abundance in frigid marine surface waters of Antarctica. Pelagic Archaea constituted up to 34% of the prokaryotic biomass in coastal Antarctic surface waters, and they were also abundant in a variety of other cold, pelagic marine environments. Because they can make up a significant fraction of picoplankton biomass in the vast habitats encompassed by cold and deep marine waters, these pelagic Archaea represent an unexpectedly abundant component of the Earth's biota.
A method combining normal phase high performance liquid chromatography (HPLC) with positive ion atmospheric pressure chemical ionization mass spectrometry (APCI-MS) was developed for the analysis of intact glycerol dialkyl glycerol tetraethers (GDGTs) in archaeal cell material and sediments. All GDGTs previously reported to occur in the thermophilic archaeon Sulfolobus solfataricus could be identified based on their mass spectra and retention time. Positive ion mass spectra consisted of abundant protonated molecules and fragment ions corresponding to loss of water and the glycerol moiety. In addition, two novel GDGTs representing alternative combinations of biphytanyl moieties were observed. Using this method, the tetraethers present in the thermophilic archaeon Metallosphaera sedula and two sediment samples were characterized. This rapid method will greatly contribute to the establishment of the sedimentary record of these compounds and increase our understanding of archaea and their occurrence in widely different environments.
Isoprenoid glycerol dialkyl glycerol tetraethers (GDGTs) and branched glycerol dialkyl diethers are main membrane constituents of cultured hyperthermophilic archaea and eubacteria, respectively, and are found in environments with temperatures >60 degrees C. Recently, we developed a new technique for the analysis of intact core tetraether lipids in cell material and sediments. The application of this technique to recent sediments shows that known and newly identified isoprenoid and branched GDGTs are widespread in low-temperature environments (<20 degrees C) and are structurally far more diverse than previously thought. Their distribution indicates the ubiquitous environmental presence of as yet uncultivated, nonthermophilic organisms that may have independently evolved from hyperthermophilic archaea and eubacteria. The structures of some of the new GDGTs point to the hybridization of both typical archaeal and eubacterial biosynthetic pathways in single organisms.
Five types of molecular species of C40 isoprenoid chains, having different numbers of cyclopentane rings, were detected in the ether core lipid of Thermoplasma acidophilum. The average cyclization number of the hydrocarbon chains in the lipids increased with increasing growth temperatures.
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