15:45 〜 16:00
[BPT04-08] Boron isotope-based seasonal paleo-pH reconstruction for the Southeast Atlantic – Multispecies approach using habitat preference of planktonic foraminifera
キーワード：carbon cycle, ocean carbonate chemistry, atmospheric pCPO2, boron isotopes, seasonality, foraminifera
The boron isotopic composition of planktonic foraminiferal shell calcite (δ11BCc) is a reliable tracer for seawater pH. Hence, under the assumption that surface-water pCO2 is in equilibrium with the atmosphere, δ11BCc of fossil surface-dwelling planktonic foraminifera can be used to reconstruct ancient atmospheric pCO2. However, pH and pCO2 of surface waters may vary seasonally, largely due to changes in temperature, DIC, and alkalinity. As shell fluxes of planktonic foraminifera show species-specific seasonal patterns, linked to intra-annual changes in temperature, it is obvious that δ11BCc of a certain species reflects the pH and thus pCO2 biased towards a specific time period within a year. This is important to consider for δ11B coretop calibrations and for the interpretation of fossil δ11BCc records that mirror seasonal pH signals. Here we present δ11BCc coretop data for the planktonic foraminifera Globigerina bulloides, Globigerinoides ruber, Trilobatus sacculifer and Orbulina universa and compare them with δ11Bborate derived from seasonally resolved carbonate system parameters.
To test the applicability of the season-adjusted 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 pCO2. The reconstruction based on G. bulloides, reflecting the austral spring season, yields values closely following the Vostok ice-core data, indicating that surface-water pCO2 was close to equilibrium with the atmosphere during the cooler spring season. In contrast, pCO2 estimated from δ11BCc of O. universa, T. sacculifer and G. ruber that predominantly lived during the warmer seasons, exhibits up to ~58 ppmv higher values than the Vostok ice-core data, with larger deviations during interglacials than during glacials.
This is probably due to the higher austral fall temperatures, as shown by Mg/Ca to be on average 4.3 °C higher than during the cooler spring season, accounting for an increase in pCO2 of 4 % per 1 °C. Our results show that paleo-pH estimates based on δ11BCc contain a significant seasonal signal reflecting the habitat preference of the recording foraminifera species.
To test the applicability of the season-adjusted 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 pCO2. The reconstruction based on G. bulloides, reflecting the austral spring season, yields values closely following the Vostok ice-core data, indicating that surface-water pCO2 was close to equilibrium with the atmosphere during the cooler spring season. In contrast, pCO2 estimated from δ11BCc of O. universa, T. sacculifer and G. ruber that predominantly lived during the warmer seasons, exhibits up to ~58 ppmv higher values than the Vostok ice-core data, with larger deviations during interglacials than during glacials.
This is probably due to the higher austral fall temperatures, as shown by Mg/Ca to be on average 4.3 °C higher than during the cooler spring season, accounting for an increase in pCO2 of 4 % per 1 °C. Our results show that paleo-pH estimates based on δ11BCc contain a significant seasonal signal reflecting the habitat preference of the recording foraminifera species.