Mg/Ca and δ¹⁸O in multiple species of planktonic foraminifera from 15 Ma to Recent

Abstract. The ratio of the trace element Mg over Ca (Mg/Ca) and the oxygen isotopic composition (δ¹⁸O) of foraminiferal calcite are widely employed for reconstructing past ocean temperatures, although geochemical signals are also influenced by several other factors that vary temporally and spatially. Here, we analyze a global dataset of Mg/Ca and δ¹⁸O data of 59 middle Miocene to Holocene species of planktonic foraminifera from a wide range of depth habitats, many of which have never been analyzed before for Mg/Ca. We investigate the extent to which Mg/Ca and δ¹⁸O covary through time and space, and identify several sources of mismatch between the two proxies. Once the data are adjusted for long term non-thermal factors, Mg/Ca and δ¹⁸O are overall positively correlated in a way consistent with temperature being the dominant controller of both through space and time and across many different species, including deep-dwellers. However, we identify several species with systematic offsets in Mg/Ca values, to which multispecies calibrations should be applied with caution. We can track the appearance of such offsets through ancestor-descendent species over the last 15 million years and propose that the emergence of these offsets may be the geochemical expression of evolutionary innovations. We find virtually all of the Mg/Ca and δ¹⁸O-derived temperatures from the commonly used genera Globigerinoides and Trilobatus are within uncertainty of each other, highlighting the utility of these species for paleoceanographic reconstructions. Our results highlight the potential of leveraging information from species lineages to improve sea surface temperature reconstruction from planktonic foraminifera over the Cenozoic.