Evaluating the utility of B/Ca ratios in planktic foraminifera as a proxy for the carbonate system: A case study of Globigerinoides ruber. (4th April 2015)

• Record Type: Journal Article
• Authors: Henehan, Michael J.; Foster, Gavin L.; Rae, James W. B.; Prentice, Katy C.; Erez, Jonathan; Bostock, Helen C.; Marshall, Brittney J.; Wilson, Paul A.
• Is Part Of: Geochemistry, geophysics, geosystems. Volume 16:Number 4(2015:Apr.)
• Page Start: 1052
• Page End: 1069
• Publication Date: 2015-04-04
• Subjects: Geochemistry -- Periodicals; Geophysics -- Periodicals; Earth sciences -- Periodicals; 550.5
• DOI: 10.1002/2014GC005514
• Languages: English
• ISSNs: 1525-2027

Abstract

<abstract abstract-type="main"> <title>Abstract</title> <p>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[ <inline-formula><alternatives><inline-graphic mimetype="image" xlink:href="ark:/27927/pgjnf8pmmc" xlink:type="simple" xmlns:xlink="http://www.w3.org/1999/xlink" /><mml:math display="inline" altimg="urn:x-wiley:15252027:media:ggge20702:ggge20702-math-0001" overflow="scroll" xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msubsup><mml:mrow><mml:mtext>CO</mml:mtext></mml:mrow><mml:mn>3</mml:mn><mml:mrow><mml:mn>2</mml:mn><mml:mo>−</mml:mo></mml:mrow></mml:msubsup></mml:mrow></mml:math></alternatives></inline-formula>] ([ <inline-formula><alternatives><inline-graphic mimetype="image" xlink:href="ark:/27927/pgjnf8pmnw" xlink:type="simple" xmlns:xlink="http://www.w3.org/1999/xlink" /><mml:math display="inline" altimg="urn:x-wiley:15252027:media:ggge20702:ggge20702-math-0002" overflow="scroll" xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msubsup><mml:mrow><mml:mtext>CO</mml:mtext></mml:mrow><mml:mn>3</mml:mn><mml:mrow><mml:mn>2</mml:mn><mml:mo>−</mml:mo></mml:mrow></mml:msubsup></mml:mrow></mml:math></alternatives></inline-formula>]<sub>in situ</sub> – [ <inline-formula><alternatives><inline-graphic mimetype="image" xlink:href="ark:/27927/pgjnf8pmjb" xlink:type="simple" xmlns:xlink="http://www.w3.org/1999/xlink" /><mml:math display="inline" altimg="urn:x-wiley:15252027:media:ggge20702:ggge20702-math-0003" overflow="scroll" xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msubsup><mml:mrow><mml:mtext>CO</mml:mtext></mml:mrow><mml:mn>3</mml:mn><mml:mrow><mml:mn>2</mml:mn><mml:mo>−</mml:mo></mml:mrow></mml:msubsup></mml:mrow></mml:math></alternatives></inline-formula>]<sub>at saturation</sub>) correlating best with B/Ca ratios in benthic foraminifera, rather than pH, <inline-formula><alternatives><inline-graphic mimetype="image" xlink:href="ark:/27927/pgjnf8pmkv" xlink:type="simple" xmlns:xlink="http://www.w3.org/1999/xlink" /><mml:math display="inline" altimg="urn:x-wiley:15252027:media:ggge20702:ggge20702-math-0004" overflow="scroll" xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mfrac><mml:mrow><mml:mi mathvariant="normal">B</mml:mi><mml:msubsup><mml:mrow><mml:mo stretchy="false">(</mml:mo><mml:mtext>OH</mml:mtext><mml:mo stretchy="false">)</mml:mo></mml:mrow><mml:mn>4</mml:mn><mml:mo>−</mml:mo></mml:msubsup></mml:mrow><mml:mrow><mml:msubsup><mml:mrow><mml:mtext>HCO</mml:mtext></mml:mrow><mml:mn>3</mml:mn><mml:mo>−</mml:mo></mml:msubsup></mml:mrow></mml:mfrac></mml:mrow></mml:math></alternatives></inline-formula>, or <inline-formula><alternatives><inline-graphic mimetype="image" xlink:href="ark:/27927/pgjnf8pmg9" xlink:type="simple" xmlns:xlink="http://www.w3.org/1999/xlink" /><mml:math display="inline" altimg="urn:x-wiley:15252027:media:ggge20702:ggge20702-math-0005" overflow="scroll" xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mfrac><mml:mrow><mml:mi mathvariant="normal">B</mml:mi><mml:msubsup><mml:mrow><mml:mo stretchy="false">(</mml:mo><mml:mtext>OH</mml:mtext><mml:mo stretchy="false">)</mml:mo></mml:mrow><mml:mn>4</mml:mn><mml:mo>−</mml:mo></mml:msubsup></mml:mrow><mml:mrow><mml:mi>D</mml:mi><mml:mi>I</mml:mi><mml:mi>C</mml:mi></mml:mrow></mml:mfrac></mml:mrow></mml:math></alternatives></inline-formula> (as a simple model of boron speciation in seawater and incorporation into CaCO<sub>3</sub> would predict). Furthermore, culture experiments have shown that in planktic foraminifera properties such as salinity and [B]<sub>sw</sub> 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 <italic>G. ruber</italic> 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 [ <inline-formula><alternatives><inline-graphic mimetype="image" xlink:href="ark:/27927/pgjnf8pmht" xlink:type="simple" xmlns:xlink="http://www.w3.org/1999/xlink" /><mml:math display="inline" altimg="urn:x-wiley:15252027:media:ggge20702:ggge20702-math-0006" overflow="scroll" xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msubsup><mml:mrow><mml:mtext>PO</mml:mtext></mml:mrow><mml:mn>4</mml:mn><mml:mrow><mml:mn>3</mml:mn><mml:mo>−</mml:mo></mml:mrow></mml:msubsup></mml:mrow></mml:math></alternatives></inline-formula>] (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.</p> </abstract>