A δ11B-pH calibration for the high-latitude foraminifera species Neogloboquadrina pachyderma and Neogloboquadrina incompta

Abstract

The boron isotopic composition of planktonic foraminifera is a powerful tool to reconstruct ocean pH and CO2 in the past. Applications to the high-latitude and polar oceans are however limited as robust calibrations between the δ11B of foraminifera and ocean pH in these regions are lacking. Here, we present a new empirical calibration for the high latitude Arctic species Neogloboquadrina pachyderma and the sub-polar to temperate species Neogloboquadrina incompta using towed specimens from the Labrador Sea, Baffin Bay, and the Nordic Seas. When paired with in situ hydrographic data, this approach allows us to avoid key assumptions used in traditional core top calibrations that are required to link shell geochemical composition to hydrographic conditions during their formation. We show that the foraminifera δ11B of the species analysed is well correlated with the δ11B of seawater borate ion. Further, the foraminiferal δ11B values are consistently lower than seawater equilibrium borate values, consistent with the interpretation of more acidic seawater in the microenvironment due to respiration. However, unlike published calibrations for non-spinose species to date the slope of the δ11B foraminifera to δ11B borate calibration is >1. We discuss several drivers of this higher sensitivity to pH and describe the possible role of vital effects in determining the boron isotopic composition of N. pachyderma and N. incompta. Finally, we apply the tow calibration to core top samples from the Nordic Seas to validate the calibration for use in the paleorecord.