Simultaneous Determination of I/Ca and Other Elemental Ratios in Foraminifera: Comparing Results From Acidic and Basic Solutions

Xiaoli Zhou, Anya V. Hess, Kaixuan Bu, Takuya Sagawa, Yair Rosenthal

Research output: Contribution to journalArticlepeer-review

3 Scopus citations

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 HNO3 concentration 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.

Original languageEnglish (US)
Article numbere2022GC010660
JournalGeochemistry, Geophysics, Geosystems
Volume23
Issue number11
DOIs
StatePublished - Nov 2022

All Science Journal Classification (ASJC) codes

  • Geophysics
  • Geochemistry and Petrology

Keywords

  • foraminiferal I/Ca
  • method development
  • redox proxy

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