Mg/Ca variation in planktonic foraminifera tests: implications for reconstructing palaeo-seawater temperature and habitat migration

The nature of compositional variability within the tiny calcitic shells (tests) that are precipitated by planktonic foraminifera has been investigated using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Systematic large and correlated variation of Mg/Ca, Mn/Ca, Ba/Ca and Zn/Ca but relatively uniform Sr/Ca are observed through the test walls of analysed species (Globigerinoides sacculifer, Globigerinoides ruber, Neogloboquadrina pachyderma and Neogloboquadrina dutertrei). Distinct chamber and chamber-wall layer compositions can be resolved within individual tests, and Mg/Ca compositional differences observed in sequentially precipitated test components of the different species analysed are consistent with seawater temperature changes occurring with habitat migration during their adult life-cycle stages. Estimated test calcification temperatures are in keeping with available seawater temperature constraints, indicating the potential for accurate seawater temperature reconstruction using LA-ICP-MS. Mg-rich (<1–6 mol% Mg) surface veneers that are also enriched in Mn, Ba, and Zn have been found on all species and all fossil tests, as well as on live-sampled tests of G. ruber, with the latter suggesting a possible biogenic origin. These Mg-rich surfaces bias bulk test compositions toward higher Mg/Ca values by between 5 and 35%.