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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. 

Abstract Body size is an essential factor in an organism's survival, and when paired with paleoenvironmental proxies, size trends can provide insights into a lineage's evolutionary responses to changing environmental conditions. This study explores the diversity and body-volume trends of dacryoconarid tentaculitoids, globally abundant marine zooplankton, in the Devonian of the Appalachian Basin (eastern United States), spanning the late Givetian through the middle Frasnian punctata carbon isotope excursion. Using statistical approaches to model trends, we find evidence of a gradual, within-lineage reduction in styliolinid adult body sizes starting at the Givetian-Frasnian boundary. This reduction is followed by a significant decrease in both adult and initial chamber volumes during the punctata excursion. At the Givetian-Frasnian boundary, annulated forms (nowakiids) become rare and smooth forms (styliolinids) begin to dominate the assemblage. Using pre-existing geological and geochemical data sets, we consider environmental factors, including sea level, anoxia, nutrient availability, and temperature, as potential drivers of body-size reductions. Bottom-water anoxia most likely did not influence body-size trends of this pelagic group, but frequent water-column overturning in the Frasnian or other exchange between deep and shallow water may have affected taxonomic composition, favoring styliolinids. Sea-surface temperature correlates inversely with body size, suggesting that warming beginning in the early Frasnian may have contributed to gradual, long-term size reductions. Rising temperatures through the middle Frasnian may have led to the disappearance of dacryoconarids in the northern Appalachian Basin after the excursion. 

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