Magnesium/Calcium ratios in benthic foraminifera from surface sediments

• Main Authors: Raitzsch, Markus, Kuhnert, Henning, Groeneveld, Jeroen, Bickert, Torsten
• Format: Dataset
• Language: English
• Published: PANGAEA 2008
• Subjects: B_LANDER; Bottom lander; Brazil Basin; Cape Basin; Cibicidoides mundulus; Magnesium/Calcium ratio; standard deviation; Continental slope off Brazil; Depth; bottom/max; sediment/rock; top/min; Equatorial Atlantic; Event label; GeoB1041-1; GeoB1105-3; GeoB1113-7; GeoB1118-2; GeoB1203-2; GeoB1216-2; GeoB1217-1; GeoB1218-1; GeoB1707-2; GeoB1710-2; GeoB1711-5; GeoB1712-2; GeoB1713-6; GeoB1715-3; GeoB1719-2; GeoB1720-4; GeoB1721-5; GeoB1726-2; GeoB1728-3; GeoB1729-2; GeoB2102-1; GeoB2105-3; GeoB2106-1; GeoB2107-5; GeoB2715-1; GeoB2718-2; GeoB2727-1; GeoB3602-2; GeoB3603-1; GeoB3803-1; GeoB3804-2; GeoB3807-1; North Atlantic Deep Water; North Atlantic
• Online Access: https://doi.pangaea.de/10.1594/PANGAEA.759564; https://doi.org/10.1594/PANGAEA.759564

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 d[CO3 2-] of bottom water exerts a significant control on dMg/Ca (temperature-corrected) of C. mundulus (dMg/Ca = 0.017*d[CO3 2-] -0.14), while dMg/Ca of P. wuellerstorfi is more likely to be governed by TCO2 (dMg/Ca = -0.007*TCO2 + 15). Since both d[CO3 2-] and TCO2 are closely linked to [CO3 2-], it is inferred that carbonate ion acts as secondary control, after temperature, on benthic shell Mg/Ca below -4°C. A drop in [CO3 2-] by 25 ?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.