The effects of temperature, salinity, and the carbonate system on Mg/Ca in Globigerinoides ruber (white): A global sediment trap calibration

The Mg/Ca of planktic foraminifera Globigerinoides ruber (white) is a widely applied proxy for tropical and sub-tropical sea-surface temperature. The accuracy with which temperature can be reconstructed depends on how accurately relationships between Mg/Ca and temperature and the multiple secondary controls on Mg/Ca are known; however, these relationships remain poorly quantified under oceanic conditions. Here, we present new calibrations based on 440 sediment trap/plankton tow samples from the Atlantic, Pacific and Indian Oceans, including 130 new samples from the Bay of Bengal/Arabian Sea and the tropical Atlantic Ocean. Our results indicate temperature, salinity and the carbonate system all significantly influence Mg/Ca in G. tuber (white). We propose two calibration models: The first model assumes pH is the controlling carbonate system parameter. In this model, Mg/Ca has a temperature sensitivity of 6.0 +/- 0.8%/degrees C (2 sigma), a salinity sensitivity of 3.3 +/- 2.2%/PSU and a pH sensitivity of -8.3 +/- 7.7%/0.1 pH units; The second model assumes carbonate ion concentration ([CO32-]) is the controlling carbonate system parameter. In this model, Mg/Ca has a temperature sensitivity of 6.7 +/- 0.8%/degrees C, a salinity sensitivity of 5.0 +/- 3.0%/PSU and a [CO32-] sensitivity of -0.24 +/- 0.11%/mu mol kg(-1). In both models, the temperature sensitivity is significantly lower than the widely-applied sensitivity of 9.0 +/- 0.69%/degrees C. Application of our new calibrations to down-core data from the Last Glacial Maximum, considering whole ocean changes in salinity and carbonate chemistry, indicate a cooling of 2.4 +/- 1.6 degrees C in the tropical oceans if pH is the controlling parameter and 1.5 +/- 1.4 degrees C if [CO32-] is the controlling parameter. (C) 2017 Elsevier B.V. All rights reserved.