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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. 

This perspective reviews how atmospheric compositions, animals and marine algae evolved together to determine global ocean habitability during the past 500 million years.