Iodine-to-calcium ratios in deep-sea scleractinian and bamboo corals

• Published in: Frontiers in Marine Science
• Main Authors: Yun-Ju Sun, Laura F. Robinson, Ian J. Parkinson, Joseph A. Stewart, Wanyi Lu, Dalton S. Hardisty, Qian Liu, James Kershaw, Michèle LaVigne, Tristan J. Horner
• Format: Article in Journal/Newspaper
• Language: English
• Published: Frontiers Media S.A. 2023
• Subjects: biogenic carbonate; iodate incorporation; seawater oxygenation proxy; dissolved oxygen; marine redox state; Science; Q; General. Including nature conservation; geographical distribution; QH1-199.5; Pacific; North Atlantic
• Online Access: https://doi.org/10.3389/fmars.2023.1264380; https://doaj.org/article/27cde835d55740d88cbf05752e9f9963

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.