Calibration of Mg/Ca thermometry in planktonic foraminifera from a sediment trap time series

Abstract

Paired Mg/Ca and δ18O measurements on planktonic foraminiferal species (G. ruber white, G. ruber pink, G. sacculifer, G. conglobatus, G. aequilateralis, O. universa, N. dutertrei, P. obliquiloculata, G. inflata, G. truncatulinoides, G. hirsuta, and G. crassaformis) from a 6‐year sediment trap time series in the Sargasso Sea were used to define the sensitivity of foraminiferal Mg/Ca to calcification temperature. Habitat depths and calcification temperatures were estimated from comparison of δ18O of foraminifera with equilibrium calcite, based on historical temperature and salinity data. When considered together, Mg/Ca (mmol/mol) of all species, except two, show a significant (r = 0.93) relationship with temperature (T °C) of the form Mg/Ca = 0.38 (±0.02) exp 0.090 (±0.003)T, equivalent to a 9.0 ± 0.3% change in Mg/Ca for a 1°C change in temperature. Small differences exist in calibrations between species and between different size fractions of the same species. O. universa and G. aequilateralis have higher Mg/Ca than other species, and in general, data can be best described with the same temperature sensitivity for all species and pre‐exponential constants in the sequence O. universa > G. aequilateralis ≈ G. bulloides > G. ruber ≈ G. sacculifer ≈ other species. This approach gives an accuracy of ±1.2°C in the estimation of calcification temperature. The ∼9% sensitivity to temperature is similar to published studies from culture and core top calibrations, but differences exist from some literature values of pre‐exponential constants. Different cleaning methodologies and artefacts of core top dissolution are probably implicated, and perhaps environmental factors yet understood. Planktonic foraminiferal Mg/Ca temperature estimates can be used for reconstructing surface temperatures and mixed and thermocline temperatures (using G. ruber pink, G. ruber white, G. sacculifer, N. dutertrei, P. obliquiloculata, etc.). The existence of a single Mg thermometry equation is valuable for extinct species, although use of species‐specific equations will, where statistically significant, provide more accurate evaluation of Mg/Ca paleotemperature.