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Calcification and growth processes in planktonic foraminifera complicate the use of B/Ca and U/Ca as carbonate chemistry proxies

Salmon, Kate H.; Anand, Pallavi; Sexton, Philip F. and Conte, Maureen (2016). Calcification and growth processes in planktonic foraminifera complicate the use of B/Ca and U/Ca as carbonate chemistry proxies. Earth and Planetary Science Letters, 449 pp. 372–381.

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Although boron and uranium to calcium ratios (B/Ca, U/Ca) in planktonic foraminifera have recently received much attention as potential proxies for ocean carbonate chemistry, the extent of a carbonate chemistry control on these ratios remains contentious. Here, we use bi-weekly sediment trap samples collected from the subtropical North Atlantic in combination with measured oceanographic data from the same location to evaluate the dominant oceanographic controls on B/Ca and U/Ca in three depth-stratified species of planktonic foraminifera. We also test the control of biological, growth-related, processes on planktonic foraminiferal B and U incorporation by using foraminifer test area density (μg/μm2) (a monitor of test thickness) and test size from the same samples. B/Ca and U/Ca show little or no significant correlation with carbonate system parameters both within this study and in comparison with other published works. We provide the first evidence for a strong positive relationship between area density (test thickness) and B/Ca, and reveal that this is consistent in all species studied, suggesting a likely role for calcification in controlling boron partitioning into foraminiferal calcite. This finding is consistent with previous observations of less efficient discrimination against trace element ‘impurities’ (such as B), at higher calcification rates. We observe little or no dependency of B/Ca on test size. In marked contrast, we find that U/Ca displays a strong species-specific dependency on test size in all species, but no relationship with test thickness, implicating some other biological control (possibly related to growth), rather than a calcification control, on U incorporation into foraminiferal calcite. Our results caution against the use of B/Ca and U/Ca in planktonic foraminifera as reliable proxies for the ocean carbonate system and recommend that future work should concentrate on improving the mechanistic understanding of how planktonic foraminifer calcification and growth rates regulate boron and uranium incorporation into the test.

Item Type: Journal Item
Copyright Holders: 2016 The Authors
ISSN: 0012-821X
Project Funding Details:
Funded Project NameProject IDFunding Body
Understanding the Impact of Recent Ocean Acidification on Bio-Calcification (SE-10-139-PA)NE/I019891/1NERC (Natural Environment Research Council)
Johanna Resig Foraminifera FellowshipNot SetCushman Foundation
Oceanic Flux Program time-seriesOCE-1234292National Science Foundation
Bermuda Atlantic Time SeriesOCE-0801991National Science Foundation
Keywords: planktonic foraminifera; boron; uranium; proxy; sediment trap
Academic Unit/School: Faculty of Science, Technology, Engineering and Mathematics (STEM) > Environment, Earth and Ecosystem Sciences
Faculty of Science, Technology, Engineering and Mathematics (STEM)
Item ID: 46974
Depositing User: Philip Sexton
Date Deposited: 10 Aug 2016 11:09
Last Modified: 04 May 2019 16:33
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