[Show abstract][Hide abstract] ABSTRACT: Explanations of the glacial-interglacial variations in atmospheric pCO2 invoke a significant role for the deep ocean in the storage of CO2. Deep-ocean density stratification has been proposed as a mechanism to promote the storage of CO2 in the deep ocean during glacial times. A wealth of proxy data supports the presence of a "chemical divide" between intermediate and deep water in the glacial Atlantic Ocean, which indirectly points to an increase in deep-ocean density stratification. However, direct observational evidence of changes in the primary controls of ocean density stratification, i.e., temperature and salinity, remain scarce. Here, we use Mg/Ca-derived seawater temperature and salinity estimates determined from temperature-corrected δ(18)O measurements on the benthic foraminifer Uvigerina spp. from deep and intermediate water-depth marine sediment cores to reconstruct the changes in density of sub-Antarctic South Atlantic water masses over the last deglaciation (i.e., 22-2 ka before present). We find that a major breakdown in the physical density stratification significantly lags the breakdown of the deep-intermediate chemical divide, as indicated by the chemical tracers of benthic foraminifer δ(13)C and foraminifer/coral (14)C. Our results indicate that chemical destratification likely resulted in the first rise in atmospheric pCO2, whereas the density destratification of the deep South Atlantic lags the second rise in atmospheric pCO2 during the late deglacial period. Our findings emphasize that the physical and chemical destratification of the ocean are not as tightly coupled as generally assumed.
Full-text · Article · Jan 2016 · Proceedings of the National Academy of Sciences
[Show abstract][Hide abstract] ABSTRACT: To deconvolve the effect of growth rate and temperature on the boron partitioning into calcite and its isotope fractionation, seeded calcite precipitation experiments were performed at a constant temperature and various growth rates and at a constant growth rate and various temperatures. We show that boron partitioning increases with increasing growth rate and decreases with increasing temperature. The B isotope fractionation between calcite and B(OH)4− increases with increasing growth rate favoring the lighter B isotope for incorporation into calcite whereas no effect of temperature was observed within the temperature range investigated (12 °C to 32 °C). At the lowest temperature and growth rate δ11B of the calcite almost equals that of B(OH)4− in solution. Applying the surface entrapment model (SEMO) of Watson and Liang (1995) to our data, we demonstrate that the observed effects of temperature and growth rate on B concentration can be explained by processes in the near surface layer of the calcite crystal.
[Show abstract][Hide abstract] ABSTRACT: The shallow water benthic foraminifer Amphistegina lessonii was grown in seawater of variable Li and Ca concentration and shell Li/Ca was determined by means of LA-ICPMS. Shell Li/Ca is positively correlated to seawater Li/Ca only when the Li concentration of seawater is changed. If the seawater Ca concentration is changed, shell Li/Ca remains constant. This indicates that Li does not compete with Ca for incorporation in the shell of A. lessonii. A recently proposed calcification model can be applied to divalent cations (e.g., Mg and Sr), which compete for binding sites of ion transporters and positions in the calcite lattice. By contrast, the transport pathway of monovalent cations such as Li is probably diffusion based (e.g., ion-channels), and monovalent cations do not compete with Ca for a position in the calcite lattice. Here we present a new model for Li partitioning into foraminiferal calcite which predicts our experimental results and should also be applicable to other alkali metals.
No preview · Article · Nov 2015 · Geochemistry Geophysics Geosystems
[Show abstract][Hide abstract] ABSTRACT: The calcite tests of foraminifera lie in marine sediments for thousands to millions of years, before being analysed to generate trace element and isotope palaeoproxy records. These sediments constitute a distinct physio-chemical environment from the conditions in which the tests formed. Storage in sediments can modify the trace element and isotopic content of foraminiferal calcite through diagenetic alteration, which has the potential to confound their palaeoceanographic interpretation. A previous study of G. tumida from the Ontong Java Plateau, western equatorial Pacific, found that preferential dissolution of higher-Mg chamber calcite, and the preservation of a low-Mg crust on the tests significantly reduced whole-test Mg/Ca and Sr/Ca [Brown and Elderfield, 1996]. Here, we revisit these specimens with a combination of synchrotron X-ray computed tomography (sXCT) and electron probe micro-analyses (EPMA) to re-evaluate the nature of their diagenetic alteration. The dissolution of higher-Mg calcite with depth was directly observed in the sXCT data, confirming the inference of the previous study. The sXCT data further reveal a thickening of the chemically and structurally distinct calcite crust with depth. We propose that these crusts have a diagenetic origin, driven by the simultaneous dissolution of high-Mg chamber calcite and precipitation of low-Mg crust from the resulting modified pore-water solution. While the breadth of the study is limited by the nature of the techniques, the observation of both dissolution and re-precipitation of foraminiferal calcite serves to demonstrate the action of two simultaneous diagenetic alteration processes, with significant impacts on the resulting palaeoproxy signals.
[Show abstract][Hide abstract] ABSTRACT: Reconstruction of intermediate water properties is important for understanding feedbacks within the ocean-climate system, particularly since these water masses are capable of driving high–low latitude teleconnections. Nevertheless, information about intermediate water mass evolution through the late Pleistocene remains limited. This paper examines changes in Antarctic Intermediate Water (AAIW), the most extensive intermediate water mass in the modern ocean through the last 400 kyr using the stable isotopic composition (δ 18 O and δ 13 C) and trace element concentration (Mg/Ca and B/Ca) of two benthic foraminiferal species from the same samples: epifaunal Planulina wuellerstorfi and infaunal Uvigerina peregrina. Our results confirm that the most reasonable estimates of AAIW temperature and [CO 2− 3 ] are generated by Mg/Ca U. peregrina and B/Ca P. wuellerstorfi , respectively. We present a 400 kyr record of intermediate water temperature and [CO 2− 3 ] from a sediment core from the Southwest Pacific (DSDP site 593; 40 • 30 S, 167 • 41 E, 1068 m water depth), which lies within the core of modern AAIW. Our results suggest that a combination of geochemical analyses on both infaunal and epifaunal benthic foraminiferal species yields important information about this critical water mass through the late Pleistocene. When combined with two nearby records of water properties from deeper depths, our data demonstrate that during interglacial stages of the late Pleistocene, AAIW and Circumpolar Deep Water (CPDW) have more similar water mass properties (temperature and δ 13 C), while glacial stages are typified by dissimilar properties between AAIW and CPDW in the Southwest Pacific. Our new [CO 2− 3 ] record shows short timescale variations, but a lack of coherent glacial–interglacial variability indicating that large quantities of carbon were not stored in intermediate waters during recent glacial periods.
Full-text · Article · Oct 2015 · Earth and Planetary Science Letters
[Show abstract][Hide abstract] ABSTRACT: The isotopic ratio and concentration of B in foraminiferal calcite appear to reflect the pH and bicarbonate concentration of seawater. The use of B as a chemical proxy tracer has the potential to transform our understanding of the global carbon cycle, and ocean acidification processes. However, discrepancies between the theory underpinning the B proxies, and mineralogical observations of B coordination in biomineral carbonates call the basis of these proxies into question. Here, we use synchrotron X-ray spectromicroscopy to show that B is hosted solely as trigonal BO3 in the calcite test of Amphistegina lessonii, and that B concentration exhibits banding at the micron length scale. In contrast to previous results, our observation of trigonal B agrees with the predictions of the theoretical mechanism behind B palaeoproxies. These data strengthen the use of B for producing palaeo-pH records. The observation of systematic B heterogeneity, however, highlights the complexity of foraminiferal biomineralisation, implying that B incorporation is modulated by biological or crystal growth processes.
[Show abstract][Hide abstract] ABSTRACT: During the last and penultimate glacial maxima, atmospheric CO2 concentrations were lower than present, possibly in part because of increased storage of respired carbon in the deep oceans. The amount of respired carbon present in a water mass can be calculated from its oxygen content through apparent oxygen utilization; the oxygen content can in turn be calculated from the carbon isotope gradient within the sediment column. Here we analyse the shells of benthic foraminifera occurring at the sediment surface and the oxic/anoxic interface on the Portuguese Margin to reconstruct the carbon isotope gradient and hence bottom-water oxygenation over the past 150,000 years. We find that bottom-water oxygen concentra- tions were 45 and 65 μmol kg−1 lower than present during the last and penultimate glacial maxima, respectively. We calculate that concentrations of remineralized organic carbon were at least twice as high as today during the glacial maxima. We attribute these changes to decreased ventilation linked to a reorganization of ocean circulation and a strengthened global biological pump. If the respired carbon pool was of a similar size throughout the entire glacial deep Atlantic basin, then this sink could account for 15 and 20 per cent of the glacial PCO2 drawdown during the last and penultimate glacial maxima.
No preview · Article · Dec 2014 · Nature Geoscience
[Show abstract][Hide abstract] ABSTRACT: The Mg/Ca compositions of benthic foraminifera from the superfamily Miliolacea have been studied to explore the use of these high-Mg foraminifera as a proxy for deep ocean conditions. Taxonomic analyses, relative abundance, and depth distributions of different Miliolacea species were carried out on a collection of core-top samples, covering a depth range of 131 m to 2530 m, along the Australian coast of the Timor Sea. Pyrgo sp., comprised of Pyrgo sarsi and Pyrgo murrhina was found to be the most suitable for proxy studies. Mg/Ca values of this group of foraminifera show a strong correlation with bottom water temperatures and carbonate ion saturation described by the linear relationship: Mg/Ca = 2.53(±0.22) × BWT + 0.129(±0.023) × Δ[CO32-] + 4.63(±0.53), within the -1 °C to 8 °C temperature range. Absolute Mg/Ca values of Pyrgo sp. calcite and their temperature sensitivity are similar to those observed for inorganic calcite, suggesting that Mg composition of Pyrgo sp. calcite is mainly controlled by inorganic processes. The Mg/Ca composition of Pyrgo sp. calcite provides a new tool for reconstructing both water temperature and carbonate ion saturation when combined with other proxies for one of these parameters. A down-core record from the Eastern Equatorial Pacific has been generated to illustrate how Mg/Ca values can be used for palaeoclimate studies. This down-core record shows large changes in Pacific bottom waters [CO32-] across glacial-interglacial transition, implying an increase in [CO32-] during the glacial period.
[Show abstract][Hide abstract] ABSTRACT: We present an improved method for accurate and precise determination of the boron isotopic composition (11B/10B) of carbonate and water samples using a mineral acid matrix and HR-ICP-MS. Our method for δ11B determination utilizes a micro-distillation based boron purification technique for both carbonate and seawater matrices. The micro-distillation method is characterized by low blank (⩽0.01 ng-B) and 99.8 ± 5.7% boron recovery. We also report a new ICP-MS method, performed in a hydrofluoric acid matrix, using a jet interface fitted Thermo® Element XR that consumes <3.0 ng-B per quintuplicate analyses (±0.5‰, 2σ, n = 5). A comparatively high matrix tolerance limit of ⩽50 ppb Na/K/Mg/Ca characterizes our ICP-MS method. With an extremely low procedural blank (⩽0.05 ± 0.01 ng-B) the present isotope method is optimized for rapid (∼25 samples per session) analysis of small masses of carbonates (foraminifera, corals) with low boron abundance and small volume water samples (seawater, porewater, river water). Our δ11B estimates of seawater (39.8 ± 0.5‰, 2σ, n = 30); SRM AE-120 (−20.2 ± 0.5‰, 2s, n = 33); SRM AE-121 (19.8 ± 0.4‰, 2s, n = 16); SRM AE-122 (39.6 ± 0.5‰, 2s, n = 16) are within analytical uncertainty of published values. We apply this new method to assess the impacts of laboratory handling induced sample contamination and seawater physio-chemical parameters (temperature, pH, and salinity) on marine carbonate bound δ11B by analyzing core-top planktonic foraminifera samples.
[Show abstract][Hide abstract] ABSTRACT: Changes in the Sr to Ca ratio of sea water have important implications for the interpretation of past climate. It has proven difficult to interpret Sr/Ca of foraminiferal calcite as a measure of seawater Sr/Ca or as reflecting the influence of deep water carbonate ion saturation (Δ[CO32−]) on the incorporation of Sr into benthic foraminiferal carbonate. Here, we address this issue by measurements of paired benthic foraminiferal Sr/Ca and B/Ca (a proxy for deep water Δ[CO32−]) for core-tops from the global ocean and three down cores at different settings during the Last Glacial–interglacial cycle. These new data suggest a significant control of deep water Δ[CO32−] on benthic foraminiferal Sr/Ca, and that down-core shell Sr/Ca variations can be largely accounted for by past deep water Δ[CO32−] changes. We conclude that seawater Sr/Ca has likely remained near-constant on glacial–interglacial timescales during the late Pleistocene, in agreement with model results. With due caution, benthic Sr/Ca may be used as an auxiliary proxy for deep water Δ[CO32−] if seawater Sr/Ca is constant.
No preview · Article · Aug 2014 · Quaternary Science Reviews
[Show abstract][Hide abstract] ABSTRACT: Ostracods are a prominent source of freshwater [1,2] and marine  palaeoproxies. The Mg/Ca and Sr/Ca ratios of their ‘calcitic’ carapace are used as proxies for salinity and temperature. Recently, a number of uncertainties have been raised regarding several aspects of our understanding of these proxies: the hydrological processes ostracods are used to reconstruct, the biology of the ostracoda , and the effect of carbonate ion concentration on trace elements [5,6].
We focus on the biological complexity of ostracods, which has the potential to confound the interpretation of their trace elements. Ostracods are crustaceans, and are thus more biologically complex than the ‘workhorse’ of the palaeoproxy community: foraminifera. Particularly, the ostracod carapace is a complex, organo-mineral structure , which has been shown to contain amorphous calcium carbonate (rather than calcite) in other crustaceans . We address a fundamental aspect of trace element incorporation in ostracods: the internal distribution, and elemental coordination of Mg, Na, S and Fe in the carapace.
We use Photo Emission Electron Microscopy (PEEM) to probe the distribution and coordination of trace elements in a Krithe ostracod carapace. This synchrotron X-ray spectroscopy technique allows the nano-scale analysis of specific elements within a structure, by targeting the binding energies of particular electron levels in the atom of interest. Although primarily designed for application in the materials and magentic sciences, we were able to adapt the technique to investigating carbonate biominerals, and demonstrate its potential in the field of palaeoproxy research.
This novel approach reveals a previously unobserved complexity in the incorporation of trace elements in the ostracod carapace, with significant implcations for the interpretation of ostracod palaeoproxies.
[Show abstract][Hide abstract] ABSTRACT: We report a new method for HR-ICP-MS based accurate and precise B/Ca determination from low mass natural carbonates (≤ 5 µg CaCO3), utilizing a mixed acid matrix (0.1 M HNO3 & 0.3 M HF) and accurate matrix matching technique. Our procedural B/Ca blank of 2.0 ± 1.0 µmol/mol, internal precision ≤ 1.0%, average within run external precision ≤ 4.0% (2σ), and rapid sample analysis (60 samples per day) make the method well suited for routine measurements. Established methods of B/Ca determination require ≥ 65 µg CaCO3 to achieve a comparable external precision of 3.5% (2σ). We report a B/Ca detection limit of 2 µmol/mol compared to ≥ 10 µmol/mol for previous methods, a five-fold improvement. The method presented here can determine a wide range of B/Ca (9.0 - 250 µmol/mol) in mass limited samples with considerable tolerance for matrix matching efficiency (≤ ± 30%). The long-term reproducibility of B/Ca measured on Cambridge in-house consistency standards containing < 20, ~85, and ~200 µmol/mol of B/Ca are ± 3.7% (2σ, n = 100), ± 3.9% (2σ, n = 150), and ± 3.2% (2s, n =180) respectively. A host of other trace element to Ca ratios can also be determined at comparable external precision from samples containing ≤ 5 µg CaCO3. This method is suitable for trace element analysis of single foraminifera shells.
[Show abstract][Hide abstract] ABSTRACT: Formation of Labrador Sea Water proper commenced about 7000 years ago during the Holocene interglacial. To test whether fresher surface water conditions may have inhibited Labrador Sea Water convection during the early Holocene we measured planktonic foraminiferal (Globigerina bulloides) oxygen isotopes (δ18O) and Mg/Ca ratios at Orphan Knoll (cores HU91-045-093 and MD95-2024, 3488 m) in the Labrador Sea to reconstruct shallow subsurface summer conditions (temperature and seawater δ18O). Lighter foraminiferal δ18O values are recorded during the early Holocene between 11000 and 7000 years ago. Part of these lighter foraminiferal δ18O values can be explained by increased calcification temperatures. Reconstructed seawater δ18O values were, however, still on average 0.5‰ lighter compared with those of recent times, confirming that fresher surface waters in the Labrador Sea were probably a limiting factor in Labrador Sea Water formation during the early Holocene.
No preview · Article · Mar 2014 · Journal of the Geological Society
[Show abstract][Hide abstract] ABSTRACT: A record of deep sea calcite saturation (Δ[CO32-]), derived from X-ray Computed Tomography based foraminifer dissolution index, XDX, was constructed for the past 150 ka for a core from the deep (4,157 m) tropical western Indian Ocean. G. sacculifer and N. dutertrei recorded a similar dissolution history, consistent with the process of calcite compensation. Peaks in calcite saturation (~15 µmol/kg higher than the present day value) occurred during deglaciations and early in MIS 3. Dissolution maxima coincided with transitions to colder stages. The mass record of G. sacculifer better indicated preservation than did that of N. dutertrei or G. ruber. Dissolution-corrected Mg/Ca-derived SST records, like other SST records from marginal Indian Ocean sites, showed coolest temperatures of the last 150 ka in early MIS 3, when mixed layer temperatures were ~4 oC lower than present SST. Temperatures recorded by N. dutertrei showed the thermocline to be ~4 oC colder in MIS 3 compared to the Holocene (8 ka BP).
No preview · Article · Mar 2014 · Geochemistry Geophysics Geosystems
[Show abstract][Hide abstract] ABSTRACT: The Mg/Ca ratio of foraminiferal calcite is a widely accepted and
applied empirical proxy for ocean temperature. The analysis of
foraminifera preserved in ocean sediments has been instrumental in
developing our understanding of global climate, but the mechanisms
behind the proxy are largely unknown. Analogies have been drawn to the
inorganic precipitation of calcite, where the endothermic substitution
of Mg for Ca is favoured at higher temperatures. However, evidence
suggests that foraminiferal Mg incorporation may be more complex:
foraminiferal magnesium is highly heterogeneous at the sub-micron scale,
and high Mg areas coincide with elevated concentrations of organic
molecules, Na, S and other trace elements. Fundamentally, the
incorporation mode of Mg in foraminifera is unknown. Here we show that
Mg is uniformly substituted for Ca within the calcite mineral lattice.
The consistency of Mg-specific X-ray spectra gathered from nano-scale
regions across the shell ('test') reveals that the coordination of Mg is
uniform. The similarity of these spectra to that produced by dolomite
shows that Mg is present in an octahedral coordination, ideally
substituted for Ca in a calcite crystal structure. This demonstrates
that Mg is heterogeneous in concentration, but not in structure. The
degree of this uniformity implies the action of a continuous Mg
incorporation mechanism, and therefore calcification mechanism, across
these compositional bands in foraminifera. This constitutes a
fundamental step towards a mechanistic understanding of foraminiferal
calcification processes and the incorporation of calcite-bound
palaeoenvironment proxies, such as Mg.
Full-text · Article · Dec 2013 · Earth and Planetary Science Letters
[Show abstract][Hide abstract] ABSTRACT: Salinity increase in the subtropical gyre system may have pre-conditioned the North Atlantic Ocean for a rapid return to stronger overturning circulation and high-latitude warming following meltwater events during the Last Glacial period. Here we investigate the Gulf Stream – subtropical gyre system properties over Dansgaard–Oeschger (DO) cycles 14 to 12, including Heinrich ice-rafting event 5. During the Holocene and Last Glacial Maximum a positive gradient in surface dwelling planktonic foraminifera δ18O (Globigerinoides ruber) can be observed between the Gulf Stream and subtropical gyre, due to decreasing temperature, increasing salinity, and a change from summer to year-round occurrence of G. ruber. We assess whether this gradient was a common feature during stadial-interstadial climate oscillations of Marine Isotope Stage 3, by comparing existing G. ruber δ18O from ODP Site 1060 (subtropical gyre location) and new data from ODP Site 1056 (Gulf Stream location) between 54 and 46 ka. Our results suggest that this gradient was largely absent during the period studied. During the major warm DO interstadials 14 and 12 we infer a more zonal and wider Gulf Stream, influencing both ODP Sites 1056 and 1060. A Gulf Stream presence during these major interstadials is also suggested by the large vertical δ18O gradient between shallow dwelling planktonic foraminifera species, especially G. ruber, and the deep dwelling species Globorotalia inflata at site 1056, which we associate with strong summer stratification and Gulf Stream presence. A major reduction in this vertical δ18O gradient from 51 ka until the end of Heinrich event 5 at 48.5 ka suggests site 1056 was situated within the subtropical gyre in this mainly cold period, from which we infer a migration of the Gulf Stream to a position nearer to the continental shelf, indicative of a narrower Gulf Stream with possibly reduced transport.
No preview · Article · Dec 2013 · Quaternary Science Reviews
[Show abstract][Hide abstract] ABSTRACT: The El Niño-Southern Oscillation (ENSO) is one of the most important components of the global climate system, but its potential response to an anthropogenic increase in atmospheric CO2 remains largely unknown. One of the major limitations in ENSO prediction is our poor understanding of the relationship between ENSO variability and long-term changes in Tropical Pacific oceanography. Here we investigate this relationship using palaeorecords derived from the geochemistry of planktonic foraminifera. Our results indicate a strong negative correlation between ENSO variability and zonal gradient of sea-surface temperatures across the Tropical Pacific during the last 22 ky. This strong correlation implies a mechanistic link that tightly couples zonal sea-surface temperature gradient and ENSO variability during large climate changes and provides a unique insight into potential ENSO evolution in the future by suggesting enhanced ENSO variability under a global warming scenario.
Full-text · Article · Nov 2013 · Nature Communications
[Show abstract][Hide abstract] ABSTRACT: The Mg/Ca ratio in foraminiferal calcite is one of the principal proxies used for paleoceanographic temperature recon-structions, but recent core-top sediment observations suggest that salinity may exert a significant secondary control on plank-tic foraminifers. This study compiles new and published laboratory culture experiment data from the planktic foraminifers Orbulina universa, Globigerinoides sacculifer and Globigerinoides ruber, in which salinity was varied but temperature, pH and light were held constant. Combining new data with results from previous culture studies yields a Mg/Ca-sensitivity to salinity of 4.4 ± 2.3%, 4.7 ± 1.2%, and 3.3 ± 1.7% per salinity unit (95% confidence), respectively, for the three foraminifer species studied here. Comparison of these sensitivities with core-top data suggests that the much larger sensitivity (27 ± 4% per salinity unit) derived from Atlantic core-top sediments in previous studies is not a direct effect of salinity. Rather, we suggest that the dissolution correction often applied to Mg/Ca data can lead to significant overestimation of tem-peratures. We are able to reconcile culture calibrations with core-top observations by combining evidence for seasonal occur-rence and latitude-specific habitat depth preferences with corresponding variations in physico-chemical environmental parameters. Although both Mg/Ca and d 18 O yield temperature estimates that fall within the bounds of hydrographic obser-vations, discrepancies between the two proxies highlight unresolved challenges with the use of paired Mg/Ca and d 18 O anal-yses to reconstruct paleo-salinity patterns across ocean basins. The first step towards resolving these challenges requires a better spatially and seasonally resolved d 18 O sw archive than is currently available. Nonetheless, site-specific reconstructions of salinity change through time may be valid.
Full-text · Article · Nov 2013 · Geochimica et Cosmochimica Acta