Mg/Ca, Sr/Ca and stable isotope from planktonic foraminifera <i>T. sacculifer</i>: testing a multi-proxy approach for inferring paleo-temperature and paleo-salinity

Abstract

<jats:p>Abstract. Over the last decades, sea surface temperature (SST) reconstructions based on the Mg/Ca of foraminiferal calcite have frequently been used in combination with the δ18O signal from the same material, to provide estimates of δ18O of the water (δ18Ow), a proxy for global ice volume and sea surface salinity (SSS). However, because of error propagation from one step to the next, better calibrations are required to increase accuracy and robustness of existing isotope and element to temperature proxy-relationships. Towards that goal, we determined Mg/Ca, Sr/Ca and the oxygen isotopic composition of Trilobatus sacculifer (previously referenced as Globigerinoides sacculifer), collected from surface waters (0–10 m), along a North-South transect in the eastern basin of the tropical/subtropical Atlantic Ocean. We established a new paleo-temperature calibration based on Mg/Ca, and on the combination of Mg/Ca and Sr/Ca. Subsequently, a sensitivity analysis was performed in which, one, two, or three different equations were considered. Results indicate that foraminiferal Mg/Ca allow for an accurate reconstruction of surface water temperature. Combining equations, δ18Ow can be reconstructed with a precision of about ± 0.5 ‰. However, the best possible salinity reconstruction based on locally calibrated equations, only allowed reconstruction with an uncertainty of ± 2.49. This was confirmed by a Monte Carlo simulation, applied to test successive reconstructions in an ideal case, where explanatory variables are known. This simulation shows that from a pure statistical point of view, successive reconstructions involving Mg/Ca and δ18Oc preclude salinity reconstruction with a precision better than ± 1.69 and hardly better than ± 2.65, due to error propagation. Nevertheless, a direct linear fit to reconstruct salinity based on the same measured variables (Mg/Ca and δ18Oc) was established. This direct reconstruction of salinity lead to a much better estimation of salinity (± 0.26) than the successive reconstructions. </jats:p>