G.-J. Reichart

NIOZ Royal Netherlands Institute for Sea Research, Burg, North Holland, Netherlands

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Publications (171)415.08 Total impact

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    ABSTRACT: The mode and vigor of the global oceanic circulation critically depend on the salinity of (sub)surface water masses advected to the lociof deep-water formation. Within the Atlantic meridional overturning circulation (AMOC), an important supplier of high-salinity waters is the Mediterranean Outflow Water (MOW), discharging into the North Atlantic via the Strait of Gibraltar. Despite its importance for the North Atlantic salinity budget, the long-term dynamics of MOW production have remained poorly understood. Here we present high-resolution records of bottom-current velocity from three drill sites within the Gulf of Cádiz that document a persistent low-latitude forcing of MOW flow speed over the past ~150 k.y. We demonstrate that the African monsoon is the predominant driver of orbital-scale MOW variability via its influence on the freshwater budget of the eastern Mediterranean Sea. Consequently, MOW formation fluctuates in concert with orbital precession overprinted by centennial-scale oscillations of high-latitude origin. We further document that Northern Hemisphere summer insolation minima stimulate maximal injection of MOW-derived salt into the North Atlantic, likely strengthening the intermediate AMOC branch. The direct coupling of MOW dynamics to low-latitude climate forcing represents a hitherto neglected process for propagating (sub)tropical climate signals into the high northern latitudes.
    Full-text · Article · Nov 2015 · Geology
  • K.A. Koho · L.J. de Nooijer · G.J. Reichart
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    ABSTRACT: The Mn/Ca of carbonate tests of living deep-sea foraminifera (Hoeglundina elegans, Bulimina aculeata, Uvigerina peregrina and Melonis barleeanus) were determined together with pore water manganese along a bottom water oxygen gradient across the lower boundary of the Arabian Sea Oxygen Minimum Zone. Although Mn has long been considered an indicator for contamination, new cleaning protocols and high-resolution laser ablation ICP-MS now allow the reliable analyses of test-associated Mn. Within locations, Mn incorporation between species varies as a function of their in-sediment depth preferences and associated pore water chemistry. Under well-oxygenated bottom water conditions, shallow infaunal species incorporate little Mn in their test, whereas the species collected from deeper habitats show elevated Mn concentrations. With decreasing oxygen contents pore water Mn concentrations and benthic foraminiferal in-sediment distribution change. Whereas Mn/Ca in shallow infaunal species responds moderately to bottom water oxygenation, Mn/Ca of the infaunal species M. barleeanus correlates well to oxygenation. Although high productivity results in a shallower redox cline within the sediment, pore water Mn is retained as long as the bottom water remains oxygenated. Under reduced bottom water oxygen conditions, Mn escapes to the overlying water column and test-associated Mn/Ca decreases also in the infaunal species. By combining pore water chemistry of Mn, calcitic Mn/Ca and foraminiferal ecology, a new conceptual model is presented (TROXCHEM3) that provides a framework for deconvolving past organic matter input and bottom water oxygenation.
    No preview · Article · Jun 2015 · Geochimica et Cosmochimica Acta
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    ABSTRACT: Most planktonic foraminifera migrate vertically through the water column during life, meeting a range of depth-related conditions as they grow and calcify. For reconstructing past ocean conditions from geochemical signals recorded in their shells, it is therefore necessary to know vertical habitat preferences. Species with a shallow habitat and limited vertical migration will reflect conditions of the surface mixed layer and short-term and mesoscale (i.e. seasonal) perturbations therein. Species spanning a wider range of depth habitats, however, will contain a more heterogeneous, intra-specimen variability (e.g. Mg / Ca and δ18O), which is less for species calcifying below the thermocline. Obtained single-chamber Mg / Ca ratios are combined with single-specimen δ18O and δ13C of the surface-water inhabitant Globigerinoides ruber, the thermocline-dwelling Neogloboquadrina dutertrei and Pulleniatina obliquiloculata, and the deep dweller Globorotalia scitula from the Mozambique Channel. Species-specific Mg / Ca, δ13C and δ18O data combined with a depth-resolved mass balance model confirm distinctive migration and calcification patterns for each species as a function of hydrography. Whereas single-specimen δ18O rarely reflects changes in depth habitat related to hydrography (e.g. temperature), measured Mg / Ca of the last chambers can only be explained by active migration in response to changes in temperature stratification. Foraminiferal geochemistry and modelled depth habitats shows that the single-chamber Mg / Ca and single shell δ18O are in agreement with each other and in line with the changes in hydrography induced by eddies.
    Full-text · Article · Apr 2015 · Biogeosciences
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    A. Mewes · G. Langer · S. Thoms · G. Nehrke · G.-J. Reichart · L.J. De Nooijer · J. Bijma
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    ABSTRACT: Mg / Ca ratios in foraminiferal tests are routinely used as paleotemperature proxies, but on long timescales, they also hold the potential to reconstruct past seawater Mg / Ca. The impact of both temperature and seawater Mg / Ca on Mg incorporation in Foraminifera has been quantified by a number of studies. The underlying mechanism responsible for Mg incorporation in foraminiferal calcite and its sensitivity to environmental conditions, however, has not been fully identified. A recently published biomineralization model (Nehrke et al., 2013) proposes a combination of transmembrane transport and seawater leakage or vacuolization to link calcite Mg / Ca to seawater Mg / Ca and explains inter-species variability in Mg / Ca ratios. To test the assumptions of this model, we conducted a culture study in which seawater Mg / Ca was manipulated by varying [Ca2+] and keeping [Mg2+] constant. Foraminiferal growth rates, test thickness and calcite Mg / Ca of newly formed chambers were analyzed. Results showed optimum growth rates and test thickness at Mg / Ca closest to that of ambient seawater. Calcite Mg / Ca is positively correlated to seawater Mg / Ca, indicating that it is not absolute seawater [Ca2+] and [Mg2+] but their ratio that controls Mg / Ca in tests. These results demonstrate that the calcification process cannot be based only on seawater vacuolization, supporting the mixing model proposed by Nehrke et al. (2013). Here, however, we suggest transmembrane transport fractionation that is not as strong as suggested by Nehrke et al. (2013).
    Full-text · Article · Apr 2015 · Biogeosciences
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    ABSTRACT: Holocene rapid climate change (RCC) events, such as the Little Ice Age (LIA), are thought to have influenced average annual temperatures only marginally, but to have affected winter temperatures relatively strongly. With summer temperatures relatively unaffected, reconstructing climate change at a seasonal resolution is crucial to fully capture Holocene climate variability. Mediterranean climate is highly seasonal, being influenced by the subtropical high-pressure belt in summer and the mid-latitude westerlies combined with outbreaks of polar winds in winter. We identified events of high- and low-detrital input to the Gulf of Taranto (Central Mediterranean Sea), anticipated to be linked to humid and dry conditions, respectively and, thereby, potentially reflecting seasonal contrasts. These events represent the Bronze Age (BA), Roman Humid Period (RHP), Medieval Climate Anomaly (MCA), LIA and present-day, and were selected for the analysis of single specimen Globigerinoides ruber (white) carbonate chemistry (Mg/Ca, δ18O and δ13C). The dynamic range found for these parameters for the measured single individuals in the most recent interval reflects the present-day seasonal contrasts in temperature and precipitation, albeit with a bias towards the summer season. These results are compared with high-resolution (< 15 years/sample) Sea Surface Temperature (SST) and Bottom Water Temperature (BWT) reconstructions based on the δ18O of G. ruber (white) and Mg/Ca of benthic foraminifer Hyalinea balthica. Although the seasonal temperature contrast remains relatively stable, significant winter cooling is observed during the BA and LIA. Connections between high-latitude climate (winter conditions) and low-latitude climate (summer conditions) appear not straightforward during RCC events. This results in changes in the moisture balance, and in shifts in seasonal dominance between RCCs. During the LIA, winter-like conditions (cold and humid) prevail throughout the year. In contrast, winters are dry and cold during the BA, and are accompanied by dry and warm summers, suggesting year-round aridity and a relatively high seasonal temperature contrast. This could have had a profound impact on early agriculture in Southern Italy.
    Full-text · Article · Jan 2015 · Palaeogeography Palaeoclimatology Palaeoecology
  • A. Mewes · G. Langer · S. Thoms · G. Nehrke · G.-J. Reichart · L. J. de Nooijer · J. Bijma
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    ABSTRACT: Mg/Ca ratios in foraminiferal tests are routinely used as paleo temperature proxy, but on long timescales, also hold the potential to reconstruct past seawater Mg/Ca. Impact of both temperature and seawater Mg/Ca on Mg incorporation in foraminifera have been quantified by a number of studies. The underlying mechanism responsible for Mg incorporation in foraminiferal calcite and its sensitivity to environmental conditions, however, is not fully identified. A recently published biomineralization model (Nehrke et al., 2013) proposes a combination of transmembrane transport and seawater leakage or vacuolization to link calcite Mg/Ca to seawater Mg/Ca and explains inter-species variability in Mg/Ca ratios. To test the assumptions of this model, we conducted a culture study in which seawater Mg/Ca was manipulated by varying [Ca2+] and keeping [Mg2+] constant. Foraminiferal growth rates, test thickness and calcite Mg/Ca of newly formed chambers were analyzed. Results showed optimum growth rates and test thickness at Mg/Ca closest to that of ambient seawater. Calcite Mg/Ca is positively correlated to seawater Mg/Ca, indicating that not absolute seawater [Ca2+] and [Mg2+], but the telative ratio controls Mg/Ca in tests. These results demonstrate that the calcification process cannot be based only on seawater vacuolization, supporting the mixing model proposed by Nehrke et al. (2013). Here we, however, suggest a transmembrane transport fractionation that is not as strong as suggested by Nehrke et al. (2013).
    No preview · Article · Dec 2014 · Biogeosciences Discussions
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    ABSTRACT: We present ecological and isotopic (δ18O and δ13C) data on benthic foraminifera sampled from 4 deep-sea stations in a pockmark field from the deep-water Niger delta (Gulf of Guinea, Equatorial Atlantic Ocean). In addition, a series of sedimentological and (bio)geochemical data are shown to back up foraminiferal observations. All stations are located within 1.2 km of each other, so prevailing oceanographic conditions can be assumed to be similar at each site. Two of the sites (GMMC-01 and GMMC-02) are located in a pockmark (named “pockmark A”) where current methane seepages were recorded by ROV observations. A third station (GMMC-03) is located in the topographic depression interpreted as a collapsed pockmark (named “pockmark B”). The fourth site (GMMC-04) is a reference station, without evidence of past or present seepages. Our observations show that degraded organic matter with low bio-availability is present at all stations with a preferential burial of organic compounds in topographic depressions (GMMC-03 station). Authigenic aragonite is abundant in surface sediments at stations GMMC-01 and -02. Its precipitation is likely related to high rates of methane oxidation during past seep events in episodically active pockmark A. In contrast, the absence of anaerobic methanotrophic Archaea (ANME) during the sampling period (November 2011) suggests that only moderate sulphide and methane oxidation take place close to the sediment-water interface. Compared to the reference site GMMC-04, living foraminifera at the collapsed and episodically active pockmarks show minor changes in terms of diversity, standing stocks and faunal composition. However, the δ13C signal of living and dead (but well-preserved) foraminiferal species (Ceratobulimina contraria, Melonis barleeanus, Uvigerina peregrina) is depleted in the episodically active pockmark A compared to the other stations. Overgrowth of authigenic carbonate on altered foraminifera generates an important shift to lower δ13C values. Dead faunas carry a complex time-averaged message, integrating taphonomic gains and losses related to the temporal variability of gas emission. They reveal major faunal differences that may be useful to detect gas hydrate seepages in different pockmark stages.
    Full-text · Article · Dec 2014 · Deep Sea Research Part I Oceanographic Research Papers
  • W. Feldmeijer · L. J. de Nooijer · G.-J. Reichart · G.M. Ganssen
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    ABSTRACT: Millennial scale climate variability punctuates the record of Northern Hemisphere glacials in the form of Dansgaard–Oeschger cycles. The coldest episodes within these cycles have a marked impact on the location of the Inter-tropical Convergence Zone and thereby the intensity and extent of the Asian monsoon system. Arabian Sea oceanography is closely linked to the monsoon cycle and responds rapidly to changes therein. Seasonally varying upwelling intensity, adhesive mixing and the resulting variability on sea surface temperature and salinity, however, make it challenging to precisely reconstruct the behaviour of the Indian monsoon system through the Dansgaard–Oeschger cycles. To reconstruct impact of millennial-scale changes in the monsoon on the Arabian Sea, paired single-specimen Mg/Ca and stable oxygen isotope analyses were performed on three species of planktonic foraminifera from the northern as well as the western Arabian Sea. Mismatches between Mg/Ca- and δ18O-derived temperatures reflect changes in salinity caused by regional variability in the hydrological cycle. Comparison of Mg/Ca-derived temperatures from Globigerinoides ruber, Globigerina bulloides and Neogloboquadrina dutertrei over Heinrich Event 4 and Interstadial 8 allows for reconstructing the seasonal range in temperatures (minima and maxima) and vertical stratification of the water column during these intervals. Results show that Arabian Sea surface water temperatures were lower during the North Atlantic Heinrich Event 4, and that temperatures in the northern and western Arabian Sea differed relatively much. This implies that the north east monsoon (i.e. boreal winter) was enhanced during this interval. In contrast, the summer upwelling related species G. bulloides reveals lower temperatures during Interstadial 8 in the western Arabian Sea, showing an enhanced summer monsoon during the warmers stages of millennial-scale climate variability. Together, these results show that summer and winter monsoon intensities were thus also anti-phased on a millennial scale. An intensified NE monsoon might have contributed to enhanced mixing of surface waters in the northern Arabian Sea.
    No preview · Article · Sep 2014 · Climate of the Past Discussions
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    L.J. de Nooijer · H.J. Spero · J. Erez · J. Bijma · G.J. Reichart
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    ABSTRACT: In this paper, we review the current understanding of biomineralization in Rotaliid foraminifera. Ideas on the mechanisms responsible for the flux of Ca2 + and inorganic carbon from seawater into the test were originally based on light and electron microscopic observations of calcifying foraminifera. From the 1980’s onward, tracer experiments, fluorescent microscopy and high-resolution test geochemical analysis have added to existing calcification models. Despite recent insights, no general consensus on the physiological basis of foraminiferal biomineralization exists. Current models include seawater vacuolization, transmembrane ion transport, involvement of organic matrices and/or pH regulation, although the magnitude of these controls remain to be quantified. Disagreement between currently available models may be caused by use of different foraminiferal species as subject for biomineralization experiments and/or lack of a more systematic approach to study (dis)similarities between taxa. In order to understand foraminiferal controls on element incorporation and isotope fractionation, and thereby improve the value of foraminifera as paleoceanographic proxies, it is necessary to identify key processes in foraminiferal biomineralization and formulate hypotheses regarding the involved physiological pathways to provide directions for future research.
    Full-text · Article · Aug 2014 · Earth-Science Reviews
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    ABSTRACT: The Paleocene-Eocene Thermal Maximum (PETM, ∼ 56 Ma) was a ∼ 200 kyr episode of global warming, associated with massive injections of 13C-depleted carbon into the ocean-atmosphere system. Although climate change during the PETM is relatively well constrained, effects on marine oxygen concentrations and nutrient cycling remain largely unclear. We identify the PETM in a sediment core from the US margin of the Gulf of Mexico. Biomarker-based paleotemperature proxies (methylation of branched tetraether-cyclization of branched tetraether (MBT-CBT) and TEX86) indicate that continental air and sea surface temperatures warmed from 27-29 to ∼ 35 °C, although variations in the relative abundances of terrestrial and marine biomarkers may have influenced these estimates. Vegetation changes, as recorded from pollen assemblages, support this warming. The PETM is bracketed by two unconformities. It overlies Paleocene silt-and mudstones and is rich in angular (thus in situ produced; autochthonous) glauconite grains, which indicate sedimentary condensation. A drop in the relative abundance of terrestrial organic matter and changes in the dinoflagellate cyst assemblages suggest that rising sea level shifted the deposition of terrigenous material landward. This is consistent with previous findings of eustatic sea level rise during the PETM. Regionally, the attribution of the glauconite-rich unit to the PETM implicates the dating of a primate fossil, argued to represent the oldest North American specimen on record. The biomarker isorenieratene within the PETM indicates that euxinic photic zone conditions developed, likely seasonally, along the Gulf Coastal Plain. A global data compilation indicates that O2 concentrations dropped in all ocean basins in response to warming, hydrological change, and carbon cycle feedbacks. This culminated in (seasonal) anoxia along many continental margins, analogous to modern trends. Seafloor deoxygenation and widespread (seasonal) anoxia likely caused phosphorus regeneration from suboxic and anoxic sediments. We argue that this fueled shelf eutrophication, as widely recorded from microfossil studies, increasing organic carbon burial along many continental margins as a negative feedback to carbon input and global warming. If properly quantified with future work, the PETM offers the opportunity to assess the biogeochemical effects of enhanced phosphorus regeneration, as well as the timescales on which this feedback operates in view of modern and future ocean deoxygenation.
    Full-text · Article · Jul 2014 · Climate of the Past
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    ABSTRACT: Unusual C35 to C38 alkenones were identified in mid-Holocene (8—3.5 kyr BP) sediments from a restricted estuary in southwest Florida (Charlotte Harbor). The distribution is dominated by a C36 diunsaturated (ω15,20) ethyl ketone, which is identical to the one present in Black Sea Unit 2 sediments. Other unusual alkenones were tentatively assigned as a C35:2 (ω15,20) methyl ketone, a C37:2 (ω17,22) methyl ketone and a C38:2 (ω17,22) ethyl ketone. In late Holocene sediments <3.5 kyr BP, the common C37 to C39 alkenones were found. Compound-specific 14C, 13C, and D isotope measurements were used to constrain the possible origin of the alkenones. Conventional radiocarbon ages of alkenones and higher plant-derived long chain n-alcohols indicate no significant difference in age between mid-Holocene alkenones and higher plant n-alcohols. Both alcohols and alkenones are offset versus calibrated ages of shell fragments in the same sediment core, which suggests they are pre-aged by 500—800 years, implying resuspension and redistribution of the fine-grained sedimentary particles with which they are associated. The hydrogen isotopic (δD) composition (-190 to -200‰) of the C37 and C38 alkenones in the late Holocene sediments is in line with values for coastal haptophytes in brackish water. However, the unusual C36 and C38 alkenones from the mid Holocene sediments are enriched in D (by ca. 100‰) vs. the late Holocene alkenones. Also, δ13C values of mid-Holocene alkenones were consistently offset compared to late Holocene alkenones (-21 to -22‰ and -22 to -23‰, respectively). We suggest that the alkenones in Charlotte Harbor are produced by a so far unknown alkenone-producing haptophyte.
    No preview · Article · May 2014 · Organic Geochemistry
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    ABSTRACT: The Mozambique Channel (MC) is one of the major source areas feeding into the Agulhas Current, thereby presenting a major upstream control on Agulhas leakage. The MC is characterized by passage of large anticyclonic eddies, resulting in temporal deepening of the thermocline. A long-term mooring and sediment trap array in the MC is used to couple in situ physico-chemical conditions with planktonic foraminiferal test calcite chemistry to develop a novel proxy for past eddy intensity. We used laser-ablation inductively coupled plasma-mass spectrometry (LA-ICP-MS) to analyze the Mg/Ca of single chambers of the surface-dweller Globigerinoides ruber and the thermocline-dwelling species Neogloboquadrina dutertrei and Pulleniatina obliquiloculata, as well as the deep-dwelling Globorotalia scitula. These specimen were also used to determine single specimen stable oxygen isotopes. Results show that reduced temperature stratification during eddy conditions can be recognized from comparing the Mg/Ca of G. ruber and N. dutertrei. A reduced offset in Mg/Ca-based temperatures between these two species closely follows the passage of an eddy and we, therefore, propose that this difference can be used to reconstruct past eddy frequency in the MC.
    No preview · Conference Paper · Apr 2014
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    L.J. de Nooijer · E.C. Hathorne · G.J. Reichart · G. Langer · J. Bijma
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    ABSTRACT: Biological activity introduces variability in element incorporation during calcification and thereby decreases the precision and accuracy when using foraminifera as geochemical proxies in paleoceanography. This so-called 'vital effect' consists of organismal and environmental components. Whereas organismal effects include uptake of ions from seawater and subsequent processing upon calcification, environmental effects include migration- and seasonality-induced differences. Triggering asexual reproduction and culturing juveniles of the benthic foraminifer Ammonia tepida under constant, controlled conditions allows environmental and genetic variability to be removed and the effect of cell-physiological controls on element incorporation to be quantified. Three groups of clones were cultured under constant conditions while determining their growth rates, size-normalized weights and single-chamber Mg/Ca and Sr/Ca using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). Results show no detectable ontogenetic control on the incorporation of these elements in the species studied here. Despite constant culturing conditions, Mg/Ca varies by a factor of ~ 4 within an individual foraminifer while intra-individual Sr/Ca varies by only a factor of 1.6. Differences between clone groups were similar to the intra-clone group variability in element composition, suggesting that any genetic differences between the clone-groups studied here do not affect trace element partitioning. Instead, variability in Mg/Ca appears to be inherent to the process of bio-calcification itself. The variability in Mg/Ca between chambers shows that measurements of at least 6 different chambers are required to determine the mean Mg/Ca value for a cultured foraminiferal test with a precision of ≤ 10%.
    Full-text · Article · Mar 2014 · Marine Micropaleontology
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    C. Caulle · K. A. Koho · M. Mojtahid · G. J. Reichart · F. J. Jorissen
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    ABSTRACT: Live (Rose Bengal stained) benthic foraminifera from the Murray Ridge, within and below the northern Arabian Sea oxygen minimum zone (OMZ), were studied in order to determine the relationship between faunal composition, bottom water oxygenation (BWO), pore water chemistry and organic matter (organic carbon and phytopigment) distribution. A series of multicores were recovered from a ten-station oxygen (BWO: 2-78 μM) and bathymetric (885-3010 m depth) transect during the winter monsoon in January 2009. Foraminifera were investigated from three different size fractions (63-125 μm, 125-150 μm and >150 μm). The larger foraminifera (>125 μm) were strongly dominated by agglutinated species (e.g. Reophax spp.). In contrast, in the 63-125 μm fraction, calcareous taxa were more abundant, especially in the core of the OMZ. On the basis of a principal components analysis, three foraminiferal groups were identified and correlated to the environmental parameters by canonical correspondence analysis. The faunas from the shallowest stations, in the core of the OMZ (BWO: 2 μM), were composed of "low oxygen" species, typical of the Arabian Sea OMZ (e.g. Rotaliatinopsis semiinvoluta, Praeglobobulimina sp., Bulimina exilis, Uvigerina peregrina type parva). These taxa are adapted to the very low BWO conditions and to high phytodetritus supplies. The transitional group, typical for the lower part of the OMZ (BWO: 5-16 μM), is composed of species that are tolerant as well to low-oxygen concentrations, but may be less critical with respect to organic supplies (e.g. Globocassidulina subglobosa, Ehrenbergina trigona). Below the OMZ (BWO: 26-78 μM), where food availability is more limited and becomes increasingly restricted to surficial sediments, cosmopolitan calcareous taxa were present, such as Bulimina aculeata, Melonis barleeanus, Uvigerina peregrina and Epistominella exigua. Miliolids were uniquely observed in this last zone, reflecting the higher BWO and/or lower organic input. At these deeper sites, the faunas exhibit a clear succession of superficial, intermediate and deep infaunal microhabitats, which can be linked to the deeper oxygen and nitrate penetration into the sediment.
    Full-text · Article · Feb 2014 · Biogeosciences
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    ABSTRACT: The Paleocene/Eocene Thermal Maximum (PETM, ~56 Ma) was a ~200 kyr episode of global warming, associated with massive injections of 13C-depleted carbon into the ocean-atmosphere system. Although climate change during the PETM is relatively well constrained, effects on marine oxygen and nutrient cycling remain largely unclear. We identify the PETM in a sediment core from the US margin of the Gulf of Mexico. Biomarker-based paleotemperature proxies (MBT/CBT and TEX86) indicate that continental air and sea surface temperatures warmed from 27-29 °C to ~35 °C, although variations in the relative abundances of terrestrial and marine biomarkers may have influenced the record. Vegetation changes as recorded from pollen assemblages supports profound warming. Lithology, relative abundances of terrestrial vs. marine palynomorphs as well as dinoflagellate cyst and biomarker assemblages indicate sea level rise during the PETM, consistent with previously recognized eustatic rise. The recognition of a maximum flooding surface during the PETM changes regional sequence stratigraphic interpretations, which allows us to exclude the previously posed hypothesis that a nearby fossil found in PETM-deposits represents the first North American primate. Within the PETM we record the biomarker isorenieratane, diagnostic of euxinic photic zone conditions. A global data compilation indicates that deoxygenation occurred in large regions of the global ocean in response to warming, hydrological change, and carbon cycle feedbacks, particularly along continental margins, analogous to modern trends. Seafloor deoxygenation and widespread anoxia likely caused phosphorus regeneration from suboxic and anoxic sediments. We argue that this fuelled shelf eutrophication, as widely recorded from microfossil studies, increasing organic carbon burial along continental margins as a negative feedback to carbon input and global warming. If properly quantified with future work, the PETM offers the opportunity to assess the biogeochemical effects of enhanced phosphorus regeneration, as well as the time-scales on which this feedback operates in view of modern and future ocean deoxygenation.
    No preview · Article · Dec 2013 · Climate of the Past Discussions
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    Full-text · Dataset · Nov 2013
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    Full-text · Dataset · Oct 2013
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    Full-text · Dataset · Oct 2013
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    J.C. Wit · L.J. de Nooijer · Mariette Wolthers · G.J. Reichart
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    ABSTRACT: Salinity and temperature determine seawater density, and differences in both thereby control global thermo- haline circulation. Whereas numerous proxies have been calibrated and applied to reconstruct temperature, a direct and independent proxy for salinity is still missing. Ideally, a new proxy for salinity should target one of the direct constituents of dissolved salt, such as [Na+] or [Cl−]. This study investigates the impact of salinity on foraminiferal Na / Ca values by laser ablation ICP-MS analyses of specimens of the benthic foraminifer Ammonia tepida cultured at a range of salinities (30.0–38.6). Foraminifera at lower salinities (30.0 and 32.5) added more chambers (10–11) to their test over the course of the culturing experiment than those maintained at higher salinities (36.1, 7–8 chambers, and 38.6, 6–7 chambers), suggesting that growth rates in this species are promoted by lower salinities. The Na / Ca of cultured specimens correlates significantly with seawater salinity (Na / Ca =0.22S–0.75, R2=0.96,p<0.01) and size. Values for Na / Ca and D_Na vary between 5.17 and 9.29 mmol mol−1and 0.12–0.16×10−3, which are similar to values from inorganic precipitation experiments. The significant correlation between test size and Na / Ca results from co-variation with salinity. This implies that foraminiferal Na / Ca could serve as a robust and independent proxy for salinity, enabling salinity reconstructions independent of calcitic δ18O.
    Full-text · Article · Oct 2013 · Biogeosciences
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    C. Caulle · K. A. Koho · M. Mojtahid · G. J. Reichart · F. J. Jorissen
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    ABSTRACT: Live (Rose Bengal stained) benthic foraminifera from the Murray Ridge, within and below the northern Arabian Sea Oxygen Minimum Zone (OMZ), were studied in order to determine the relationship between faunal composition, bottom-water oxygenation (BWO), pore-water chemistry and organic matter (organic carbon and phytopigment) distribution. A series of multicores were recovered from a ten-station oxygen (BWO: 2-78 μM) and bathymetric (885-3010 m depth) transect during the winter monsoon in January 2009. Foraminifera were investigated from three different size fractions (63-125 μm, 125-150 μm and > 150 μm). The larger foraminifera (> 125 μm) were strongly dominated by agglutinated species (e.g. Reophax spp.). In contrast, in the 63-125 μm fraction, calcareous taxa were more abundant, especially in the core of the OMZ, suggesting an opportunistic behaviour. On the basis of a Principal Component Analysis, three foraminiferal groups were identified, reflecting the environmental parameters along the study transect. The faunas from the shallowest stations, in the core of the OMZ (BWO: 2 μM), were composed of "low oxygen" species, typical of the Arabian Sea OMZ (e.g., Rotaliatinopsis semiinvoluta, Praeglobobulimina spp. , Bulimina exilis, Uvigerina peregrina typeparva). These taxa are adapted to the very low BWO conditions and to high phytodetritus supplies. The transitional group, typical for the lower part of the OMZ (BWO: 5-16 μM), is composed of more cosmopolitan taxa tolerant to low-oxygen concentrations (Globocassidulina subglobosa, Ehrenbergina trigona). Below the OMZ (BWO: 26-78 μM), where food availability is more limited and becomes increasingly restricted to surficial sediments, more cosmopolitan calcareous taxa were present, such as Bulimina aculeata, Melonis barleeanus, Uvigerina peregrina and Epistominella exigua. Miliolids were uniquely observed in this last group, reflecting the higher BWO. At these deeper sites, the faunas exhibit a clear depth succession of superficial, intermediate and deep-infaunal microhabitats, because of the deeper oxygen and nitrate penetration into the sediment.
    Full-text · Article · Sep 2013 · Biogeosciences Discussions

Publication Stats

3k Citations
415.08 Total Impact Points

Institutions

  • 2014-2015
    • NIOZ Royal Netherlands Institute for Sea Research
      Burg, North Holland, Netherlands
  • 1997-2015
    • Utrecht University
      • • Department of Earth Sciences
      • • Faculty of Geosciences
      • • Geochemistry
      • • Institute of Earth Sciences Utrecht (IVAU)
      Utrecht, Utrecht, Netherlands
  • 2003-2012
    • Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research
      • Department of Marine Biogeosciences
      Bremerhaven, Bremen, Germany
  • 2010
    • University of Melbourne
      • School of Earth Sciences
      Melbourne, Victoria, Australia
  • 2009
    • Radboud University Nijmegen
      • Department of Microbiology
      Nijmegen, Provincie Gelderland, Netherlands