Carbon isotopic disequilibrium in polar planktonic foraminifera and its impact on modern and Last Glacial Maximum reconstructions

Abstract

Calibration studies of Neogloboquadrina pachyderma (sinistral coiling, hereinafter referred to as N. pachyderma (s.)) in the South Atlantic raise questions about the interpretation of this species' carbon isotope composition as a paleoceanographic tracer of Southern Ocean surface nutrient contents. Carbon isotope disequilibrium between δ13C of total CO2 (TCO2) of seawater and δ13C of N. pachyderma (s.) increases systematically between 70° and 40°S. Several effects could cause the observed carbon isotope disequilibrium, including a combination of foraminiferal diet, calcification temperature, and carbonate ion chemistry. Combining these corrections to the δ13C of N. pachyderma (s.) improves the reconstruction of the δ13C of seawater TCO2 between 0 and 200 m in the modern South Atlantic. With these corrections applied, the δ13C for equilibrium calcite reconstructed from N. pachyderma (s.) at the Last Glacial Maximum is very similar to preindustrial values, suggesting that one cannot rule out the possibility that surface nutrient concentrations in the South Atlantic Ocean were comparable to today. However, the magnitude of the uncertainties associated with these corrections make it difficult to assess absolute paleonutrient concentrations with much confidence.