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Planktic foraminifera test iodine to calcium ratios represent an emerging proxy method to assess subsurface seawater oxygenation states. Several core-top studies show lower planktic foraminifera I/Ca in locations with oxygen depleted subsurface waters compared to well oxygenated environments. The reasoning behind this trend is that only the oxidized species of iodine, iodate, is incorporated in foraminiferal calcite. The I/Ca of foraminiferal calcite is thought to reflect iodate contents in seawater. To test this hypothesis, we compare planktic foraminifera I/Ca ratios, obtained from plankton tows, with published and new seawater iodate concentrations from 1) the Eastern North Pacific with extensive oxygen depletion, 2) the Benguela Current System with moderately depleted oxygen concentrations, and 3) the well oxygenated North and South Atlantic. We find the lowest I/Ca ratios (0.07 µmol/mol) in planktic foraminifera retrieved from the Eastern North Pacific, and higher values for samples (up to 0.72 µmol/mol) obtained from the Benguela Current System and North and South Atlantic. The I/Ca ratios of plankton tow foraminifera from environments with well oxygenated subsurface waters, however, are an order of magnitude lower compared to core-tops from similarly well-oxygenated regions. This would suggest that planktic foraminifera gain iodine post-mortem, either when sinking through the water column, or during burial.
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Simultaneous Determination of I/Ca and Other Elemental Ratios in Foraminifera: Comparing Results From Acidic and Basic Solutions
Abstract The iodine to calcium ratio in carbonate (I/Ca) has been widely used to indicate ocean oxygenation level in the past. Given the volatility of iodine, I/Ca has been measured in alkaline solutions in previous studies. However, this limits the application of I/Ca with other element/Ca (El/Ca) proxies at the same time and in the same foraminifera because other El/Ca data are preferably analyzed in acidic solutions. This study assesses the reliability of I/Ca measurements in acidic solutions measured with other El/Ca as well as the effects of different sample pre‐treatments on measured foraminiferal I/Ca. Our results show that when samples are measured within hours of prepaparation, the pH of the final solution has an insignificant effect on I/Ca measurements of a carbonate reference material JCp‐1 and a multi‐element standard solution, consistent with the slow kinetics of iodine volatilization. We find, however, that low pH possibly reduces the measured I/Ca in foraminiferal tests in some samples. Our experiments also suggest a resolvable effect of reductive cleaning, yielding lower foraminiferal I/Ca compared to without reductive cleaning. The HNO3concentration used to dissolve foraminiferal shells has a negligible effect. Despite the different solution pHs and cleaning and dissolving methods, our core top planktic I/Ca data are able to differentiate well‐oxygenated from oxygen‐depleted waters in the upper ocean, and after correcting for cleaning effect, our data are generally consistent with the published studies that analyzed I/Ca without reductive cleaning and in basic solutions. This study shows that measurements of I/Ca within hours of sample dissolutions yield reliable planktic I/Ca data, while also allowing the acquisition of other El/Ca values for paleoceanographic studies.
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- Award ID(s):
- 1919716
- PAR ID:
- 10378571
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Geochemistry, Geophysics, Geosystems
- Volume:
- 23
- Issue:
- 11
- ISSN:
- 1525-2027
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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