Open AccessBenthic foraminifer Mg/Ca anomalies in South Atlantic core top sediments and their implications for paleothermometry
Author(s) -
Raitzsch Markus,
Kuhnert Henning,
Groeneveld Jeroen,
Bickert Torsten
Publication year - 2008
Publication title -
geochemistry, geophysics, geosystems
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.928
H-Index - 136
ISSN - 1525-2027
DOI - 10.1029/2007gc001788
Subject(s) - benthic zone , geology , oceanography , bottom water , carbonate , seawater , mineralogy , chemistry , organic chemistry
We used modern epibenthic foraminifer tests of Cibicidoides mundulus and Planulina wuellerstorfi from South Atlantic core top sediments in order to establish Mg/Ca‐temperature relationships for the temperature range from 0 to 15°C. We obtained the following calibrations: Mg/Ca (mmol/mol) = 0.830*exp(0.145*BWT (°C)) for P. wuellerstorfi , and Mg/Ca (mmol/mol) = 0.627*exp(0.143*BWT (°C)) for C. mundulus . However, a number of tests, especially those bathed in North Atlantic Deep Water, revealed higher Mg/Ca ratios than predicted from the calibration. Our data suggest that Δ[CO 3 2− ] of bottom water exerts a significant control on ΔMg/Ca (temperature‐corrected) of C. mundulus (ΔMg/Ca = 0.017*Δ[CO 3 2− ] − 0.14), while ΔMg/Ca of P. wuellerstorfi is more likely to be governed by TCO 2 (ΔMg/Ca = −0.007*TCO 2 + 15). Since both Δ[CO 3 2− ] and TCO 2 are closely linked to [CO 3 2− ], it is inferred that carbonate ion acts as secondary control, after temperature, on benthic shell Mg/Ca below ∼4°C. A drop in [CO 3 2− ] by 25 μ mol/kg at 4 km water depth, as suggested for the Last Glacial Maximum, would decrease Mg/Ca by up to 0.4 mmol/mol, which leads to an underestimation of bottom water temperature by ∼3.5°C. Therefore our results indicate that the Mg/Ca thermometer should be used cautiously for benthic foraminifers where changes in the carbonate chemistry are present in the paleoceanographic record.