Boron isotope-based seasonal paleo-pH reconstruction for the Southeast Atlantic - A multispecies approach using habitat preference of planktonic foraminifera

The boron isotopic composition of planktonic foraminiferal shell calcite (delta B-11(Cc)) provides valuable information on the pH of ambient water at the time of calcification. Hence, delta B-11(Cc) of fossil surfaced-welling planktonic foraminifera can be used to reconstruct ancient aqueous pCO(2) if information on a second carbonate system parameter, temperature and salinity is available. However, pH and pCO(2) of surface waters may vary seasonally, largely due to changes in temperature, DIC, and alkalinity. As also the shell fluxes of planktonic foraminifera show species-specific seasonal patterns that are linked to intra-annual changes in temperature, it is obvious that delta B-11(Cc) of a certain species reflects the pH and thus pCO(2) biased towards a specific time period within a year. This is important to consider for the interpretation of fossil delta B-11(Cc) records that may mirror seasonal pH signals. Here we present new Multi-Collector Inductively Coupled Mass Spectrometry (MC-ICPMS) delta B-11(Cc) coretop data for the planktonic foraminifera species Globigerina bulloides, Globigerinoides Tuber, Trilobatus sacculifer and Orbulina universa and compare them with delta B-11(borate) derived from seasonally resolved carbonate system parameters. We show that the inferred season-adjusted delta B-11(Cc)/delta B-11(borate) relationships are similar to existing calibrations and can be combined with published delta B-11(Cc) field and culture data to augment paleo-pH calibrations. To test the applicability of these calibrations, we used a core drilled on the Walvis Ridge in the Southeast Atlantic spanning the last 330,000 years to reconstruct changes in surface-water pCO(2). The reconstruction based on G. bulloides, which reflects the austral spring season, was shown to yield values that closely resemble the Vostok ice-core data indicating that surface-water pCO(2) was close to equilibrium with the atmosphere during the cooler spring season. In contrast, pCO(2) estimated from delta B-11(Cc) of O. universa, T. sacculifer and G. ruber that predominantly lived during the warmer seasons, exhibits up to similar to 50 ppmv higher values than the Vostok ice-core data. This is probably due to the higher austral summer and fall temperatures, as shown by Mg/Ca to be on average similar to 4 degrees C higher than during the cooler spring season, accounting for an increase in pCO(2) of similar to 4% per 1 degrees C. Our results demonstrate that paleo-pH estimates based on delta B-11(Cc) contain a significant seasonal signal reflecting the habitat preference of the recording foraminifera species. (C) 2018 Elsevier B.V. All rights reserved.