A LASER ABLATION ICP-MS PROTOCOL FOR HIGH-RESOLUTION I/CA PROFILING IN CORALS

Corals are continuous, time-resolved archives of ambient seawater geochemistry and can extend climate records beyond direct monitoring. The I/Ca ratio may be a proxy for local oxygen depletion, but the current solution-based ICP-MS protocol limits temporal resolution. A protocol was developed for rapid analysis of coral I/Ca using laser ablation ICP-MS. First, iodine analyses were compared between two reference materials: coral JCp-1 and synthetic carbonate MACS-3. The influence of laser parameters (spot size, fluence, repetition rate, and scan speed) on iodine sensitivity from JCp-1 was evaluated to optimize laser settings for accurate and reproducible I/Ca calibration. Then, I/Ca ratios were measured from continuous transects along and across the growth axis of a Porites sp. We find that the repeatability and precision in measuring iodine, as well as other tracers (Sr, Mg, Ba, and U), are improved using the biogenic standard JCp-1 compared to MACS-3. In the coral sample, while there is fine-scale (< 250 μm) I/Ca variability in parallel transects, which may be due to biomineralization processes, large-scale features (> 500 μm) along the growth axis tend to correlate. Laser ablation ICP-MS has great potential for accurate, high-resolution I/Ca profiling in corals using JCp-1 as a suitable calibration standard. Because of compositional variability near centers of calcification, it is important to pay attention to how the laser transect is aligned relatively to the growth axes.