Element/Calcium ratios in middle Eocene samples of Oridorsalis umbonatus from Ocean Drilling Program Site 1209

Culturing studies and empirical-based calibrations suggest that elemental ratios in benthic foraminifera can be used as proxies to reconstruct past variations in bottom water temperature and saturation state (Δ[CO₃^2−]). However the mechanism(s) linking elemental ratios to Δ[CO₃^2−] are poorly constrained. We present middle Eocene records of Oridorsalis umbonatus Li/Ca, B/Ca, Mg/Ca and Sr/Ca from Ocean Drilling Program Site 1209. We apply calibrations developed from core top samples to estimate middle Eocene variations in intermediate water Δ[CO₃^2−]. The fidelity of bottom water Δ[CO₃^2−] reconstructions based on single element ratios are assessed by comparing the X/Ca-based reconstructions to each other and to carbon cycle proxy records (benthic foraminifera δ¹³C, organic carbon content, foraminifera dissolution indices), and a seawater δ¹⁸O reconstruction for Site 1209. Discrepancies in the reconstructed Δ[CO₃^2−] values for the middle Eocene based on these different metal ratios suggests that there are still gaps in our understanding of the parameters influencing X/Ca. The downcore record of O. umbonatus Mg/Ca does not exhibit any similarities with the Li/Ca, B/Ca and Sr/Ca records, suggesting that bottom water Δ[CO₃^2−] is not the dominant influence on Mg/Ca ratios for this species. This hypothesis is supported by the coefficients of multiple linear regression models on new and published Mg/Ca data.