On the combination of the planktonic foraminiferal Mg/Ca, clumped (Δ47) and conventional (δ18O) stable isotope paleothermometers in palaeoceanographic studies

Abstract

Assuming that foraminiferal clumped isotope (Δ47) values are independent of seawater salinity and pH, the combination of Mg/Ca, δ18O and Δ47 values, may in theory allow us to disentangle the temperature, salinity/δ18Osw and pH signals. Here, we present a new Mg/Ca-Δ47 dataset for modern planktonic foraminifera, from various oceanographic basins and covering a large range of temperatures (from 0.2 to 25.4 °C). These measurements were performed on the same samples and species as the ones used for the foraminiferal Δ47 calibration of Peral et al. (2018), allowing comparison between both Mg/Ca and Δ47 paleothermometers (excluding the two benthic foraminiferal data points). There is a good agreement between these two paleothermometers when the Mg/Ca-temperature is corrected for seawater salinity and pH, suggesting that foraminiferal Δ47 may not be influenced by salinity or pH. However, our results show that Δ47 temperature uncertainties still limit our ability to reconstruct pH and δ18Osw from the combination of Mg/Ca, δ18O and Δ47 in a useful manner. We also find that disagreements between Mg/Ca and Δ47 values in G. bulloides persist after correction for vital, salinity and pH effects, suggesting that other process(es) may also influence Mg/Ca in this species.This study also provides an updated I-CDES version of the previously published planktonic and benthic foraminiferal Δ47 calibration of Peral et al. (2018), covering a range of temperature from −2 to 25.4 °C.