Mg/Ca paleothermometry in high salinity environments

• Published in: Earth and Planetary Science Letters
• Main Authors: Hoogakker, B. A. A., Klinkhammer, G. P., Elderfield, H., Rohling, E. J., Hayward, C.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Elsevier 2009
• Subjects: 01 - Climate Change and Earth-Ocean Atmosphere Systems; Planktonic foraminifera
• Online Access: http://eprints.esc.cam.ac.uk/1100/; http://eprints.esc.cam.ac.uk/1100/1/Hoogakker_Eldefield_Hayward_ESPL_284_3-4_2009.pdf; http://www.sciencedirect.com/science/article/B6V61-4WMD2MN-3/2/ef54f7fef730deae02d04cc30f27c6f7; https://doi.org/10.1016/j.epsl.2009.05.027

Planktonic foraminiferal Mg/Ca ratios have become a fundamental temperature proxy in past climate reconstructions. However, in the highly evaporative seas of the tropics and subtropics, anomalously high planktonic foraminiferal Mg/Ca ratios arise, possibly linked to high salinities. The extent to which salinity affects Mg uptake into foraminiferal calcite remains disputed. Some studies suggest only minor salinity effects, whereas others suggest a dominant role. Here, we present new data from the highly saline (> 40) Red Sea, which separate pure foraminiferal calcite from other phases. The results show that high Mg/Ca ratios (7 to 13 mmol/mol), found by conventional analysis of planktonic foraminifera from a Red Sea sediment core, are not caused by increased Mg uptake into foraminiferal calcite in a high salinity setting (e.g. beyond those predicted by culturing studies), but instead result from secondary high Mg-calcite overgrowths. The overgrowths likely formed near the sediment-seawater interface, from CaCO3 supersaturated interstitial seawater.