Towards calcification site and seawater carbonate chemistry reconstruction from boron incorporation into large benthic foraminifera
Abstract
Empirical calibrations between the carbonate system and the boron concentration or isotopic composition of biogenic carbonates have been successfully utilised in order to reconstruct past changes in (e.g.) seawater pH. More recently, understanding of inorganic boron incorporation into aragonite has been applied to infer the carbonate chemistry of the calcification site in corals which, like foraminifera, is different to that of the ambient seawater as these organisms raise the calcification site pH in order to promote CO2 diffusion and aid precipitation. In order to investigate 1) whether the B/Ca ratio of tropical shallow-dwelling benthic foraminifera can be used as an archive of past changes in seawater carbonate chemistry, and 2) if boron incorporation in cultured individuals can constrain the degree to which the calcification site carbonate chemistry differs from the surrounding seawater, we present B/Ca data of Operculina ammonoides, a large benthic species that likely lacks the ability to modify the Mg/Ca ratio of the calcifying space. Cultures at constant DIC and variable pH, and vice versa, enable the relative importance of specific carbonate system parameters to be investigated, whilst the secondary sensitivity of boron incorporation to a wide number of additional factors (temperature, salinity, seawater [Mg] and [Ca]) was also determined. We show that B/Ca ratios in this species are a sensitive recorder of the carbonate system, with only a minor influence of the other investigated variables. Moreover, boron incorporation into Operculina broadly appears to conform to what is known about fractionation into inorganic calcite, with the caveat that constraining some secondary controls such as crystal growth rate is challenging in foraminifera. Nonetheless, this raises the possibility that constraints may be placed on the carbonate chemistry at the site of calcification using B/Ca data.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFMPP43A..01E
- Keywords:
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- 0419 Biomineralization;
- BIOGEOSCIENCESDE: 0473 Paleoclimatology and paleoceanography;
- BIOGEOSCIENCESDE: 4924 Geochemical tracers;
- PALEOCEANOGRAPHYDE: 4994 Instruments and techniques;
- PALEOCEANOGRAPHY