Improving polar ocean temperature reconstructions with crust-lamellae specific Mg/Ca-temperature calibrations and improved understanding of its non-thermal forcers

Reconstructing past ocean-cryosphere interactions can provide crucial insight to the ongoing rapid climate change in the polar regions and beyond. However, there are large uncertainties in existing proxies commonly used in downcore reconstructions, including a lack of low-temperature (<9°C) culture-based Mg/Ca-temperature calibrations for planktic foraminifera. In the polar oceans the foraminiferal assemblage is not diverse and commonly dominated by Neogloboquadrina pachyderma, yet there is limited understanding of non-thermal influences on this proxy in this species. N. Pachyderma also precipitates a thick low-Mg/Ca crust over its inner higher Mg/Ca lamellar calcite, that contributes to uncertainties and inaccuracies in high-latitude palaeotemperature reconstructions.

To address this, we cultured N. pachyderma across a 2 to 9°C temperature range, at a range of salinities (29.8–36.6), and carbonate chemistry conditions with both co-varying and decoupled pH (7.65–8.4) and [CO₃^2-] (64–243 µmol/kg) and analysed their trace element composition using Laser Ablation Inductively Coupled Plasma Mass Spectrometry.

We present a new method that distinguishes the crust and lamellar calcite using trace element profiles from both cultured and fossil shells. This allowed us to show distinct geochemical signals in the crust and lamellar calcite of laboratory-grown N. pachyderma, including lower Mg/Ca, Na/Ca, and B/Ca in the crust compared to the lamellar calcite. We present new Mg/Ca-relationships, with independent calibrations for the crust and lamellar calcite. The temperature calibrations extend the lower range of culture-based Mg/Ca-calibrations down to 2°C. Furthermore, we show significant and opposing pH and [CO₃^2-] influences on Mg/Ca when these variables are decoupled and no statistically significant influence of salinity on Mg/Ca. Crust and lamellar calcite element/Ca are found to have different sensitivities to changing environmental conditions. Our results also show that environmental conditions control the crust-lamellar proportions and shell thickness which has implications for both downcore reconstructions and ongoing ocean acidification and warming.

Overall, our findings suggests that the crust and lamellar calcite precipitate via contrasting biomineralisation strategies and/or varying precipitation rates, leading to distinct geochemical compositions and different sensitivities to changing environmental conditions. We propose that distinguishing the two components and applying Mg/Ca-environmental relationships with separate calibrations for the crust and lamellar calcite will substantially reduce uncertainties in high-latitude palaeoceanographic reconstructions. We are now in the process of applying these methods and relationships to Quaternary sediment records from the central Arctic Ocean and the Nordic Seas.