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.