Evaluating the utility of B/Ca ratios in planktic foraminifera as a proxy for the carbonate system: a case study of Globigerinoides ruber

M.J. Henehan, G.L. Foster, J.W.B. Rae, K.C. Prentice, J. Erez, H.C. Bostock, B.J. Marshall, P.A. Wilson

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36 Citations (Scopus)
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Abstract

B/Ca ratios in foraminifera have attracted considerable scientific attention as a proxy for past ocean carbonate system. However, the carbonate system controls on B/Ca ratios are not straightforward, with Δ[ CO32-] ([ CO32-]in situ - [ CO32-]at saturation) correlating best with B/Ca ratios in benthic foraminifera, rather than pH, B(OH)4-/HCO3-, or B(OH)4-/DIC (as a simple model of boron speciation in seawater and incorporation into CaCO3 would predict). Furthermore, culture experiments have shown that in planktic foraminifera properties such as salinity and [B]SW can have profound effects on B/Ca ratios beyond those predicted by simple partition coefficients. Here, we investigate the controls on B/Ca ratios in G. ruber via a combination of culture experiments and core-top measurements, and add to a growing body of evidence that suggests B/Ca ratios in symbiont-bearing foraminiferal carbonate are not a straightforward proxy for past seawater carbonate system conditions. We find that while B/Ca ratios in culture experiments covary with pH, in open ocean sediments this relationship is not seen. In fact, our B/Ca data correlate best with [ PO43-] (a previously undocumented association) and in most regions, salinity. These findings might suggest a precipitation rate or crystallographic control on boron incorporation into foraminiferal calcite. Regardless, our results underscore the need for caution when attempting to interpret B/Ca records in terms of the ocean carbonate system, at the very least in the case of mixed-layer planktic foraminifera.

Original languageEnglish
Pages (from-to)1052-1069
JournalGeochemistry, Geophysics, Geosystems
Volume16
Issue number4
Early online date4 Apr 2015
DOIs
Publication statusPublished - Apr 2015

Keywords

  • Globigerinoides ruber
  • B/Ca ratios
  • Planktic foraminifera
  • pH proxy
  • Trace elements
  • Proxy calibration

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