Salinity bias on the foraminifera Mg/Ca thermometry: Correction procedure and implications for past ocean hydrographic reconstructions

Mg/Ca in foraminiferal calcite has recently been extensively used to estimate past oceanic temperatures. Here we show, however, that the Mg/Ca temperature relationship of the planktonic species Globigerinoides ruber is significantly affected by seawater salinity, with a +1 psu change in salinity resulting in a +1.6°C bias in Mg/Ca temperature calculations. If not accounted for, such a bias could lead, for instance, to systematic overestimations of Mg/Ca temperatures during glacial periods, when global ocean salinity had significantly increased compared to today. We present here a correction procedure to derive unbiased sea surface temperatures (SST) and δ 18 O sw from G. ruber T Mg/Ca and δ 18 O f measurements. This correction procedure was applied to a sedimentary record to reconstruct hydrographic changes since the Last Glacial Maximum (LGM) in the Western Pacific Warm Pool. While uncorrected T Mg/Ca data indicate a 3°C warming of the Western Pacific Warm Pool since the LGM, the salinity‐corrected SST result in a stronger warming of 4°C.