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

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 D[CO DIC (as a simple model of boron speciation in seawater and incorporation into CaCO 3 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 [PO 32 4 ] (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.

Calibration of the B/Ca proxy in the planktic foraminifer Orbulina universa to Paleocene seawater conditions

“…In general, O. universa specimens grown under Paleocene seawater conditions have similar B/Ca ratios as those grown in “modern” seawater (Figure ). The trends observed in our Paleocene carbonate chemistry experiments are also consistent with those carried out previously in modern seawater [ Allen et al , , ; Henehan et al , ] in which B/Ca increases with pH and decreases with total DIC. When DIC is increased across a range of 1000–4000 μmol kg −1 (holding [B(OH) 4 − ] constant), B/Ca ratios decrease from 88 ± 6 to 42 ± 3 μmol/mol (Figure a).…”

“…Specifically, when modern culture calibrations for O. universa and T. sacculifer [ Allen et al , ] are used to interpret the B/Ca data of Penman et al [] in terms of peak‐PETM [B(OH) 4 − ]/DIC, the size of the B/Ca excursion translates to negative [B(OH) 4 − ]/DIC ratios for the PETM (Figure a). Only the calibration slopes of G. ruber [ Allen et al , ; Henehan et al , ] give a positive ratio, but the strong nonlinearity of this calibration at the low pH end in G. ruber (pink) (Figure S1 in the supporting information), as well as the much higher values in this species compared to M. velascoensis , lends low confidence to this interpretation.…”

“…Modern B/Ca proxy calibrations are not applicable to the PETM B/Ca excursion. (a) Calibrations for B/Ca versus pH in G. ruber (white [ Henehan et al , ] and pink [ Allen et al , ]), T. sacculifer [ Allen et al , ], and O. universa [ Allen et al , ] are shown (solid colored lines and symbols). The horizontal dashed black lines indicate average pre‐PETM and peak‐PETM B/Ca ratios from the M. velascoensis study of Penman et al [] (black diamonds).…”

Assessing the impact of diagenesis on δ11B, δ13C, δ18O, Sr/Ca and B/Ca values in fossil planktic foraminiferal calcite

“…However it should be noted that considerable uncertainty remains as to the importance of factors other than pH (and the B(OH) 4 À :HCO 3 À ratio) in determining the B content of calcite, including (but not limited to) the rate of precipitation (Gabitov et al, 2014;Mavromatis et al, 2015;Ni et al, 2007), temperature (Foster, 2008;Tripati et al, 2009;Yu et al, 2007b), salinity (Allen et al, 2011) and light intensity (Babila et al, 2014). Indeed the concept of K D as defined here (following Hemming and Hanson, 1992) may itself be at least partly incorrect (see Henehan et al, 2015).…”

“…The B/Ca of planktic foraminiferal tests also looked to be a promising candidate for reconstructing surface ocean pH (Yu et al, 2007b) or [CO 3 ] 2À (Foster, 2008). However, subsequent investigations indicate that the controls on B/Ca are not clear and thus, interpretation of B/Ca records (particularly in the geological record) may not be as straightforward as originally anticipated Babila et al, 2014;Henehan et al, 2015).…”