Calcification and growth processes in planktonic foraminifera complicate the use of B/Ca and U/Ca as carbonate chemistry proxies
Salmon, Kate H.
Sexton, Philip F.
MetadataShow full item record
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)(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.
© The Author(s), 2016. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Earth and Planetary Science Letters 449 (2016): 372-381, doi:10.1016/j.epsl.2016.05.016.
The following license files are associated with this item:
Showing items related by title, author, creator and subject.
Boiteau, Rene M.; Greaves, Mervyn; Elderfield, Henry (American Geophysical Union, 2012-09-08)The rate of uranium accumulation in oceanic sediments from seawater is controlled by bottom water oxygen concentrations and organic carbon fluxes—two parameters that are linked to deep ocean storage of CO2. To investigate ...
The influence of differential production and dissolution on the stable isotope composition of planktonic foraminifera Erez, Jonathan (Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, 1978-11)Planktonic foraminifera from plankton tows, sediment traps and sediments from the central North Atlantic were studied in order to understand how they acquire their oxygen and carbon isotope compositions. Shallow dwelling ...
Keigwin, Lloyd D.; Bice, Marley; Copley, Nancy J. (American Geophysical Union, 2005-07-18)Monthly samples of stratified plankton tows taken from the slope waters off Cape Cod nearly 25 years ago are used to describe the seasonal succession of planktonic foraminifera and their oxygen isotope ratios. The 15°C ...