Barras Christine

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Barras
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Christine
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  • Article
    Temperature calibration of Mg/Ca ratios in the intermediate water benthic foraminifer Hyalinea balthica
    (American Geophysical Union and the Geochemical Society, 2011-04-01) Rosenthal, Yair ; Morley, Audrey ; Barras, Christine ; Katz, Miriam E. ; Jorissen, Frans ; Reichart, Gert-Jan ; Oppo, Delia W. ; Linsley, Braddock K.
    Core top samples from Indonesian and northeast Atlantic depth transects were used to calibrate Mg/Ca and δ18O in tests of the calcitic benthic foraminifer Hyalinea balthica to bottom water temperature between 4°C and 13°C. This shallow infaunal species is primarily abundant in neritic to upper bathyal sediments (<600 m). Both linear and exponential calibrations suggest a temperature sensitivity of ~12% per °C that is ~4 times higher than observed in other species of deep-sea benthic foraminifera. Culture experiments support the core top calibration. We find no discernible effect of salinity and saturation on Mg/Ca. Comparison between the measured benthic foraminiferal δ18O and predicted equilibrium values suggests that on average H. balthica δ18O is 0.64‰ ± 0.13‰ lower than predicted from the equilibrium composition. To test the reliability of using paired H. balthica Mg/Ca and δ18O measurements for reconstructing seawater δ18Osw and salinity, we apply this calibration to another depth transect from Cape Ghir off NW Africa, which was not included in the calibration. Based on error analysis of the calibration data and this validation test, we show that the uncertainty of reconstructing bottom water temperature and salinity from paired Mg/Ca and δ18O measurements of H. balthica is better than ±0.7°C and ±0.69 practical salinity scale, respectively. The small uncertainties allow for the reconstruction of seawater density to better than 0.3σθ units, which is precise enough for the identification of specific water masses and reconstruction of changes in their properties. We propose that the relatively high Mg content and temperature sensitivity of H. balthica might be due to minor, biologically mediated contribution of high-Mg calcite to the primarily low Mg calcite test, which is influenced by the ambient temperature. This hypothesis, if correct, suggests that benthic species with relatively high Mg/Ca may be better suited for deepwater temperature reconstructions than species that have thus far been more commonly used.
  • Article
    Foraminiferal Mn/Ca as bottom-water hypoxia proxy: an assessment of Nonionella stella in the Santa Barbara Basin, USA
    (American Geophysical Union, 2021-10-18) Brinkmann, Inda ; Ni, Sha ; Schweizer, Magali ; Oldham, Véronique E. ; Quintana Krupinski, Nadine B. ; Medjoubi, Kadda ; Somogyi, Andrea ; Whitehouse, Martin J. ; Hansel, Colleen M. ; Barras, Christine ; Bernhard, Joan M. ; Filipsson, Helena L.
    Hypoxia is of increasing concern in marine areas, calling for a better understanding of mechanisms leading to decreasing dissolved oxygen concentrations ([O2]). Much can be learned about the processes and implications of deoxygenation for marine ecosystems using proxy records from low-oxygen sites, provided proxies, such as the manganese (Mn) to calcium (Ca) ratio in benthic foraminiferal calcite, are available and well calibrated. Here we report a modern geochemical data set from three hypoxic sites within the Santa Barbara Basin (SBB), USA, where we study the response of Mn/Caforam in the benthic foraminifer Nonionella stella to variations in sedimentary redox conditions (Mn, Fe) and bottom-water dissolved [O2]. We combine molecular species identification by small subunit rDNA sequencing with morphological characterization and assign the SBB N. stella used here to a new phylotype (T6). Synchrotron-based scanning X-ray fluorescence (XRF) imaging and Secondary Ion Mass Spectrometry (SIMS) show low Mn incorporation (partition coefficient DMn < 0.05) and limited proxy sensitivity of N. stella, at least within the range of dissolved [O2] (2.7–9.6 μmol/l) and Mnpore-water gradients (2.12–21.59 μmol/l). Notably, even though intra- and interspecimen Mn/Ca variability (33% and 58%, respectively) was only partially controlled by the environment, Mn/Caforam significantly correlated with both pore-water Mn and bottom-water [O2]. However, the prevalent suboxic bottom-water conditions and limited dissolved [O2] range complicate the interpretation of trace-elemental trends. Additional work involving other oxygenation proxies and samples from a wider oxygen gradient should be pursued to further develop foraminiferal Mn/Ca as an indicator for hypoxic conditions.
  • Article
    Through the eDNA looking glass: Responses of fjord benthic foraminiferal communities to contrasting environmental conditions
    (Wiley, 2023-03-27) Brinkmann, Inda ; Schweizer, Magali ; Singer, David ; Quinchard, Sophie ; Barras, Christine ; Bernhard, Joan M. ; Filipsson, Helena L.
    The health of coastal marine environments is severely declining with global changes. Proxies, such as those based on microeukaryote communities, can record biodiversity and ecosystem responses. However, conventional studies rely on microscopic observations of limited taxonomic range and size fraction, missing putatively ecologically informative community components. Here, we tested molecular tools to survey foraminiferal biodiversity in a fjord system (Sweden) on spatial and temporal scales: Alpha and beta diversity responses to natural and anthropogenic environmental trends were assessed and variability of foraminiferal environmental DNA (eDNA) compared to morphology‐based data. The identification of eDNA‐obtained taxonomic units was aided by single‐cell barcoding. Our study revealed wide diversity, including typical morphospecies recognized in the fjords, and so‐far unrecognized taxa. DNA extraction method impacted community composition outputs significantly. DNA extractions of 10 g sediment more reliably represented present diversity than of 0.5‐g samples and, thus, are preferred for environmental assessments in this region. Alpha‐ and beta diversity of 10‐g extracts correlated with bottom‐water salinity similar to morpho‐assemblage diversity changes. Sub‐annual environmental variability resolved only partially, indicating damped sensitivity of foraminiferal communities on short timescales using established metabarcoding techniques. Systematically addressing the current limitations of morphology‐based and metabarcoding studies may strongly improve future biodiversity and environmental assessments.
  • Article
    Benthic foraminiferal Mn/Ca as low‐oxygen proxy in Fjord sediments
    (American Geophysical Union, 2023-05-03) Brinkmann, Inda ; Barras, Christine ; Jilbert, Tom ; Paul, K. Mareike ; Somogyi, Andrea ; Ni, Sha ; Schweizer, Magali ; Bernhard, Joan M. ; Filipsson, Helena L.
    Fjord systems are typically affected by low‐oxygen conditions, which are increasing in extent and severity, forced by ongoing global changes. Fjord sedimentary records can provide high temporal resolution archives to aid our understanding of the underlying mechanisms and impacts of current deoxygenation. However, such archives can only be interpreted with well‐calibrated proxies. Bottom‐water oxygen conditions determine redox regime and availability of redox‐sensitive trace elements such as manganese, which in turn may be recorded by manganese‐to‐calcium ratios (Mn/Ca) in biogenic calcium carbonates (e.g., benthic foraminifera tests). However, biological influences on Mn incorporation (e.g., species‐specific Mn fractionation, ontogeny, living and calcification depths) are still poorly constrained. We analyzed Mn/Ca of living benthic foraminifera (Bulimina marginata, Nonionellina labradorica), sampled at low‐ to well‐oxygenated conditions over a seasonal gradient in Gullmar Fjord, Swedish West coast (71–217 μmol/L oxygen (O2)), by laser‐ablation ICP‐MS. High pore‐water Mn availability in the fjord supported Mn incorporation by foraminifera. B. marginata recorded contrasting Mn redox regimes sensitively and demonstrated potential as proxy for low‐oxygen conditions. Synchrotron‐based scanning X‐ray fluorescence nanoimaging of Mn distributions across B. marginata tests displayed Mn/Ca shifts by chambers, reflecting bottom‐water oxygenation history and/or ontogeny‐driven life strategy preferences. In contrast, Mn/Ca signals of N. labradorica were extremely high and insensitive to environmental variability. We explore potential biologically controlled mechanisms that could potentially explain this species‐specific response. Our data suggest that with the selection of sensitive candidate species, the Mn/Ca proxy has potential to be further developed for quantitative oxygen reconstructions in the low‐oxygen range.