Developing trace element proxy calibrations for reconstructing polar surface ocean hydrography based on laboratory-grown planktonic foraminifera Neogloboquadrina pachyderma

Proxy based records of past changes in the polar surface ocean-cryosphere-climate interactions can provide invaluable constraints on ongoing and future climate change. However, studying polar ocean palaeoceanography remains challenging largely due to a lack of robust proxy calibrations. For example, the commonly used foraminiferal Mg/Ca paleothermometer does not have a reliable calibration at polar conditions and there are currently limited trace element proxies for other environmental variables such as salinity or carbonate chemistry. In addition, Neogloboquadrina pachyderma, the dominant foraminifera species in polar regions, sometimes grows a thick calcite crust outside its main growth phase calcite with different geochemical composition. This poses a challenge to the Mg/Ca paleothermometer as the crusts have lower Mg/Ca than the ontogenetic calcite. To address this, we cultivated >1500 individual specimens of N. pachyderma over a wide range of temperatures (2 to 9°C), salinities (~30 to 36.5), pHs (~7.7 to 8.4 total scale), carbonate ion concentrations (~100-250 µmol/mol at stable and variable pH), and Ba concentrations (2-4 times natural). The experimental water was spiked with 135Ba to label laboratory-grown calcite. Elemental ratios in the specimens have been analysed using laser ablation mass spectrometry (LA-ICP-MS), providing high resolution elemental profiles of intra-shell variability. Our microscopic observations and element ratios results suggest the growth and addition of crust in all treatments, allowing, for the first time, laboratory-based proxy calibrations for N. pachyderma’s crust. A preliminary data analysis show variability in trace element ratios in relation to variable temperature, salinity, barium concentration and carbonate chemistry. Our results also indicate significant distinction in trace element ratios between crust and ontogenetic calcite components of the N. pachyderma tests when both are grown in culture. We are in the process of developing separate laboratory-based proxy calibrations for the crust and ontogenetic calcite which will significantly improve the applicability of the proxy calibrations as well as our understanding of crust formation in this species. We aim to present Mg/Ca-temperature calibrations for the crust and ontogenetic parts separately as well as detailing the respective effects of salinity and carbonate chemistry on Mg/Ca ratios.