The Effects of Temperature and Salinity on Mg Incorporation in Planktonic Foraminifera Globigerinoides ruber (white): Results from a Global Sediment Trap Mg/Ca Database

Abstract:

Mg/Ca in Globigerinoides ruber is arguably the most important proxy for sea surface temperature (SST) in tropical and sub tropical regions, and as such guides our understanding of past climatic change in these regions. However, the sensitivity of Mg/Ca to salinity is debated; while analysis of foraminifera grown in cultures generally indicates a sensitivity of 3 – 6% per salinity unit, core-top studies have suggested a much higher sensitivity of between 15 – 27% per salinity unit, bringing the utility of Mg/Ca as a SST proxy into dispute. Sediment traps circumvent the issues of dissolution and post-depositional calcite precipitation that hamper core-top calibration studies, whilst allowing the analysis of foraminifera that have calcified under natural conditions within a well constrained period of time. We collated previously published sediment trap/plankton tow G. ruber (white) Mg/Ca data, and generated new Mg/Ca data from a sediment trap located in the highly-saline tropical North Atlantic, close to West Africa. Calcification temperature and salinity were calculated for the time interval represented by each trap/tow sample using World Ocean Atlas 2013 data. The resulting dataset comprises >240 Mg/Ca measurements (in the size fraction 150 – 350 µm), that span a temperature range of 18 – 28 °C and 33.6 – 36.7 PSU. Multiple regression of the dataset reveals a temperature sensitivity of 7 ± 0.4% per °C (p < 2.2*10^-16) and a salinity sensitivity of 4 ± 1% per salinity unit (p = 2*10^-5). Application of this calibration has significant implications for both the magnitude and timing of glacial-interglacial temperature changes when variations in salinity are accounted for.