Evaluating the temperature dependence of δ44/40Ca along with δ18O, Mg/Ca, and Sr/Ca in calcite tests of multiple foraminifera species

Abstract

Preserved biomineral chemical compositions serve as valuable tools in constructing proxies for reconstructing past climates. A persistent effort exists to enhance and innovate temperature proxies, ensuring a comprehensive understanding of historical temperature variability—a vital Earth System parameter. Calcium isotopes (δ44/40Ca) in foraminifera have been debated to be sensitive to temperature and requires further refinement. In this study we present a comprehensive assessment of δ44/40Ca temperature sensitivity in foraminifera by utilizing concurrent δ44/40Ca, δ18O, Mg/Ca, and Sr/Ca ratios data on same samples. These coupled isotopic and elemental ratios were determined across various foraminifera species, including benthic (C. wuellerstorfi), thermocline planktic (G. truncatulinoides, G. inflata), and mixed layer planktic (G. ruber, G. bulloides, O. universa, G. siphonifera) from core-top and sediment trap samples from the Atlantic Ocean. The δ44/40Ca -T calibration included calcification temperatures derived from both δ18O and Mg/Ca, utilizing established δ18O -T and Mg/Ca-T relationships. The temperature sensitivity of δ44/40Ca in foraminifera is 0.042–0.058 ‰/°C using all species data from published work and this study (except data from <3 °C habitat temperature). We report foraminifera δ44/40Ca and temperature relationships: δ44/40Ca (‰) = 0.042 × T (°C) + 0.289 (r2 = 0.80, p < 0.0001), using temperatures derived from δ18O and Mg/Ca, respectively. Furthermore, we introduce a novel paleotemperature equation for foraminifera, employing a multi-regression approach that combines two temperature proxies, δ44/40Ca and Mg/Ca: SST (°C) = (9 ± 2.6) × δ44/40Ca + (3.28 ± 0.63) × Mg/Ca - (2.89 ± 1.09). This multi-proxy approach can mitigate the uncertainty associated with individual proxy measurements and allows cross-validation. Application of this multi-proxy relationship to an East Atlantic Site (GeoB1112) yields a glacial-interglacial temperature change of 4.2 ± 0.93 °C between MIS 1 and MIS 3 which is in agreement with published work.