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

Autor: Albert Benthien, Grit Steinhoefel, Markus Raitzsch, Klaus-Uwe Richter, Michal Kucera, Jelle Bijma
Rok vydání: 2018
Předmět:
Zdroj: Earth and Planetary Science Letters. 487:138-150
ISSN: 0012-821X
Popis: The boron isotopic composition of planktonic foraminiferal shell calcite ( δ 11 B Cc ) provides valuable information on the pH of ambient water at the time of calcification. Hence, δ 11 B Cc of fossil surface-dwelling planktonic foraminifera can be used to reconstruct ancient aqueous p CO 2 if information on a second carbonate system parameter, temperature and salinity is available. However, pH and p CO 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 δ 11 B Cc of a certain species reflects the pH and thus p CO 2 biased towards a specific time period within a year. This is important to consider for the interpretation of fossil δ 11 B Cc records that may mirror seasonal pH signals. Here we present new Multi-Collector Inductively Coupled Mass Spectrometry (MC-ICPMS) δ 11 B Cc coretop data for the planktonic foraminifera species Globigerina bulloides , Globigerinoides ruber , Trilobatus sacculifer and Orbulina universa and compare them with δ 11 B borate derived from seasonally resolved carbonate system parameters. We show that the inferred season-adjusted δ 11 B Cc / δ 11 B borate relationships are similar to existing calibrations and can be combined with published δ 11 B 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 p CO 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 p CO 2 was close to equilibrium with the atmosphere during the cooler spring season. In contrast, p CO 2 estimated from δ 11 B Cc of O. universa , T. sacculifer and G. ruber that predominantly lived during the warmer seasons, exhibits up 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 ∼4 °C higher than during the cooler spring season, accounting for an increase in p CO 2 of ∼4% per 1 °C. Our results demonstrate that paleo-pH estimates based on δ 11 B Cc contain a significant seasonal signal reflecting the habitat preference of the recording foraminifera species.
Databáze: OpenAIRE
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