Nonthermal Influences on Mg/Ca in Planktonic Foraminifera: A Review of Culture Studies and Application to the Last Glacial Maximum

Planktonic foraminiferal Mg/Ca is one of the most widely applied sea surface temperature proxies. While the influence of salinity on Mg/Ca has led the accuracy of Mg/Ca‐temperatures to be questioned, the effect of seawater carbonate chemistry (pH) is seldom accounted for down‐core. Using published data sets, we review controls on Mg/Ca in laboratory cultures of planktonic foraminifera Globigerinoides ruber (white), Trilobatus sacculifer, Globigerina bulloides, and Orbulina universa. All are characterized by a temperature sensitivity of ~6% per °C and a salinity sensitivity of ~4% per salinity unit; all except T. sacculifer are sensitive to carbonate chemistry (−5 to −9% per 0.1 pH units). We demonstrate the down‐core manifestation of these sensitivities using data spanning the last deglaciation and an Earth System Model forced with last glacial maximum conditions. While the effects of salinity are relatively minor, seawater carbonate chemistry exerts a large bias on Mg/Ca temperature if unaccounted for; however, as local pH changes beyond the effect of atmospheric CO2 are relatively small across most of the ocean (less than ±0.05 pH units, 2σ), atmospheric CO2 can be used to accurately correct Mg/Ca temperatures. We present protocols to correct Mg/Ca for pH down‐core using either atmospheric CO2 or (preferably) boron isotopes in a new software package “MgCaRB.”