Mg/Ca and Sr/Ca ratios in planktonic foraminifera: Proxies for upper water column temperature reconstruction

Abstract

Reliable temperature estimates from both surface and subsurface ocean waters are needed to reconstruct past upper water column temperature gradients and past oceanic heat content. This work examines the relationships between trace element ratios in fossil shells and seawater temperature for surface‐dwelling foraminifera species, Globigerinoides ruber (white) and Globigerina bulloides, and deep‐dwelling species, Globorotalia inflata, Globorotalia truncatulinoides (dextral and sinistral) and Pulleniatina obliquiloculata. Mg/Ca and Sr/Ca ratios in shells picked in 29 modern core tops from the North Atlantic Ocean are calibrated using calculated isotopic temperatures. Mg/Ca ratios on G. ruber and G. bulloides agree with published data and relationships. For deep‐dwelling species, Mg/Ca calibration follows the equation Mg/Ca = 0.78 (±0.04) × exp (0.051 (±0.003) × T) with a significant correlation coefficient of R2 = 0.74. Moreover, there is no significant difference between the different deep‐dwellers analyzed. For the Sr/Ca ratio, the surface dwellers and P. obliquiloculata do not record any temperature dependence. For the Globorotalia species, the thermo dependence of Sr/Ca ratio can be described by a single linear relationship: Sr/Ca = (0.0182 (±0.001) × T) + 1.097 (±0.018), R2 = 0.85. Temperature estimates with a 1 sigma error of ±2.0°C and ±1.3°C can be derived from the Mg/Ca and Sr/Ca ratios, respectively, as long as the Sr geochemistry in the ocean has been constant through time.