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The distribution of dissolved iodine in seawater is sensitive to multiple biogeochemical cycles, including those of nitrogen and oxygen. The iodine-to-calcium ratio (I/Ca) of marine carbonates, such as bulk carbonate or foraminifera, has emerged as a potential proxy for changes in past seawater oxygenation. However, the utility of the I/Ca proxy in deep-sea corals, natural archives of seawater chemistry with wide spatial coverage and radiometric dating potential, remains unexplored. Here, we present the first I/Ca data obtained from modern deep-sea corals, specifically scleractinian and bamboo corals, collected from the Atlantic, Eastern Pacific, and Southern Oceans, encompassing a wide range of seawater oxygen concentrations (10–280 μmol/kg). In contrast to thermodynamic predictions, we observe higher I/Ca ratios in aragonitic corals (scleractinian) compared to calcitic corals (bamboo). This observation suggests a strong biological control during iodate incorporation into deep-sea coral skeletons. For the majority of scleractinian corals, I/Ca exhibits a covariation with local seawater iodate concentrations, which is closely related to seawater oxygen content. Scleractinian corals also exhibit notably lower I/Ca below a seawater oxygen threshold of approximately 160 μmol/kg. In contrast, no significant differences in I/Ca are found among bamboo corals across the range of oxygen concentrations encountered (15–240 μmol/kg). In the North Atlantic, several hydrographic factors, such as temperature and/or salinity, may additionally affect coral I/Ca. Our results highlight the potential of I/Ca ratios in deep-sea scleractinian corals to serve as an indicator of past seawater iodate concentrations, providing valuable insights into historical seawater oxygen levels.
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A Laser Ablation ICP‐MS Protocol for High‐Resolution Iodine‐to‐Calcium Ratio (I/Ca) Analysis on Corals
ABSTRACT RationaleCorals are continuous, time‐resolved archives of ambient seawater geochemistry and can extend climate records beyond direct monitoring. The iodine‐to‐calcium (I/Ca) ratio may be a proxy for local oxygen depletion in corals, but the current solution‐based ICP‐MS protocol limits sampling resolution. A protocol was developed for rapid analysis of coral I/Ca using laser ablation ICP‐MS. MethodsTwo reference materials, a powdered coral (JCp‐1) and a synthetic carbonate (MACS‐3), were compared for precision in measuring Sr, Mg, I, Ba, and U. Then, the influence of laser parameters (spot size, fluence, repetition rate, and scan speed) on iodine sensitivity from the reference material was evaluated to optimize laser settings for accurate and reproducible I/Ca calibration. Then, I/Ca was measured in line scans along and across the ambulacrum in aDiploria labyrinthiformiscoral. ResultsWe find that JCp‐1 has greater precision in measuring iodine, as well as other traces, compared to MACS‐3. At a 10 Hz repetition rate, spot sizes from 150 to 85 μm obtained concentrations in agreement with certified values, but higher repetition rates overestimated iodine concentrations from JCp‐1. Certain scan speeds and fluence can introduce noise, likely due to matrix effects, but the signal‐to‐noise ratio can be improved by adjacent‐average filtering. Using this simple data filtering routine and optimized laser settings, the highest resolution for accurate I/Ca analysis is < 100 μm. While the fine‐scale (< 250 μm) I/Ca variabilities in parallel transects in a coral sample likely resulted from biomineralization processes, large ‐scale features (> 500 μm) along the ambulacrum tend to correlate. ConclusionsLA‐ICP‐MS has great potential for accurate, high‐resolution I/Ca profiling in corals using JCp‐1 as a calibration standard. Because of compositional variability near centers of calcification, it is important to pay attention to how the laser transect is aligned relative to skeletal elements, which may incorporate iodine differently.
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- Award ID(s):
- 2121445
- PAR ID:
- 10571248
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Rapid Communications in Mass Spectrometry
- Volume:
- 39
- Issue:
- 9
- ISSN:
- 0951-4198
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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