Seasonal variability in B speciation and B/Ca in planktonic foraminifera from the Cariaco Basin, Venezuela
Abstract
We determined B/Ca on four planktonic foraminiferal species (Globigerinoides ruber, Globigerinoides sacculifer, Orbulina universa, and Globorotalia menardii) on a Thermo Element 2 high resolution ICP-MS. The material is from biweekly sediment trap samples collected in Cariaco Basin, Venezuela (10o30' N, 65o31' W) over a three year period between May 2003 and May 2006. The data are compared to local hydrography and water column chemistry to evaluate environmental controls on B incorporation into foraminiferal calcite. In addition, seasonal variability of B speciation in the foraminiferal calcite is assessed using 11B magic angle spinning (MAS) nuclear magnetic resonance (NMR) on samples of O. universa from January, February, and April 2007 and of G. menardii from December 2006. The B/Ca (μmol/mol) data displays clear depth stratification with the surface dwelling G. ruber having the highest B/Ca (202 - 455), followed by G. sacculifer (137 - 335) and the deeper dwelling G. menardii and O. universa having the lowest (102 - 281; 109 - 222 respectively), consistent with a decrease in pH with depth. The data also show a repeatable seasonal pattern with the highest values occurring when the water column is stratified in June and July and the lowest occurring during upwelling in December and January. However, none of the environmental variables have a strong correlation with B/Ca. The 11B MAS NMR O. universa data show a seasonal change in the speciation of B within the foraminiferal calcite. During January the boron is almost entirely incorporated (~ 90%) in a previously unrecognized trigonal form. However, in April only ~75% of the boron is in this trigonal form, whereas the rest of the boron is divided evenly between borate and boric acid. The observed trigonal form has a Cq of 3.0, which is similar to the theoretical value of 3.15 for the corner-sharing borate carbonate complex, B(OH2)CO3-. It is hypothesized that during calcification boron is converted to a borate carbonate complex, which is ultimately converted to either borate or boric acid with each equally likely. Our results support this hypothesis except that most of the boron does not complete this reaction, but remains in the intermediate borate carbonate complex form. The December G. menardii sample matches the January O. universa sample indicating there is no species difference with respect to the form of boron incorporated into the foraminiferal test.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFMPP51E..06W
- Keywords:
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- 0419 BIOGEOSCIENCES / Biomineralization;
- 1042 GEOCHEMISTRY / Mineral and crystal chemistry;
- 4944 PALEOCEANOGRAPHY / Micropaleontology;
- 4964 PALEOCEANOGRAPHY / Upwelling