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

• Published in: Geochemistry, geophysics, geosystems : G3 geophysics, geosystems : G3, 2022-11, Vol.23 (11), p.n/a
• Main Authors: Zhou, Xiaoli, Hess, Anya V., Bu, Kaixuan, Sagawa, Takuya, Rosenthal, Yair
• Format: Article
• Language: English
• Subjects: Acidity; Analytical methods; Calcium; Carbonates; Chemical elements; Chemical properties; Chemicophysical properties; Cleaning; Elements; Foraminifera; foraminiferal I/Ca; Iodine; Kinetics; method development; Oceans; Oxygen; Oxygenation; Ratios; redox proxy; Reliability; Shells; Upper ocean
• ISSN: 1525-2027; 1525-2027
• DOI: 10.1029/2022GC010660

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. Plain Language Summary The ratio of chemical elements iodine to calcium (I/Ca) in carbonate has been widely used to indicate the amount of oxygen in modern and ancient oceans. Because iodine is unstable in acidic solutions, I/Ca ratios have been measured in alkaline solutions. However, acquiring I/Ca ratios in this way limits the usage of other element to calcium ratios (El/Ca) that can reflect other physical and chemical properties of the ocean such as temperature and acidity, because these El/Ca ratios are commonly measured in acidic solutions. Here we assess the possibility of producing reliable I/Ca with other El/Ca data in acidic solutions. Our results suggest that I/Ca in fo