Controls on stable isotope and trace metal uptake in Neogloboquadrina pachyderma (sinistral) from an Antarctic sea-ice environment

The polar foraminifera Neogloboquadrina pachyderma (sinistral) dominates assemblages from the high latitude Southern Ocean, which plays a key role in determining past climate due to the tight linkage between Antarctic temperature and atmospheric CO2. Here, we use N. pachyderma (s.) harvested from sediment traps off the West Antarctic Peninsula to construct a seasonal time series for the calibration of calcite proxies in a high latitude seasonal sea-ice environment where temperature is decoupled from other environmental parameters. We have used a combination of δ18OCaCO3 and δ 13CCaCO3 to decipher the calcification temperature and salinity, which reflect that N. pachyderma (s.) live in surface waters throughout the year, and at the ice–water interface in austral winter. Further, our results demonstrate that the uptake of trace metals into N. pachyderma (s.) calcite is influenced by secondary environmental conditions in addition to temperature during periods of sea-ice cover. We propose an elevated carbonate ion concentration at the ice– water interface resulting from biological utilisation of CO2 could influence calcification in foraminifera. Our calculations suggest that for N. pachyderma (s.) Mg/Ca, Sr/Ca ratios and Li/Ca ratios are linear functions of calcification temperature and [CO3 2−]. N. pachyderma (s.) Mg/Ca ratios exhibit temperature sensitivity similar to previous studies (~10–20%/°C) and a sensitivity to [CO32−] of ~1%/μmol kg−1. Sr/Ca ratios are less sensitive to environmental parameters, exhibiting ~5% increase/°C and ~0.5%/10 μmol kg−1. The relationship between Li/Ca ratios and both temperature and [CO32−] is less significant with ~10% increase in Li/Ca ratio/°C and 10 μmol kg−1.We show how a multi-proxy approach could be used to constrain past high latitude surface water temperature and [CO3 2−].

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