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

Abstract

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.

A high-precision interpretable framework for marine dissolved oxygen concentration inversion

“…We selected three consecutive dates of MDOC minima (Figures 10A-C) and three consecutive dates of MDOC maxima (Figures 10D-F) to analyze the impact direction and magnitude of each marine elements within their respective value ranges at these critical climate nodes. We referenced the research by El Bilali et al (2023) in our analysis, identifying the critical value ranges at which the direction of the influence of marine elements on the MDOC climatological normals changes. The analysis provides the following insights: (i) Salinity has the most significant impact on MDOC climatological normals, followed by pH in typical cases, with temperature being less significant, and chlorophyll concentration having the least impact; (ii) Salinity below 23.81 psu has a positive impact on the MDOC climatological normals, while levels above 29.70 psu have a negative impact; (iii) Temperatures below 12.64°C have a positive effect on the MDOC climatological normals, whereas temperatures above it have a negative effect; (iv) pH above 7.80 positively impacts the MDOC climatological normals, while pH below it has a negative impact; (v) Chlorophyll concentrations above 5.50 µg/L positively affect the MDOC climatological normals, whereas in other circumstances a negative impact occurs.…”

Speciation and cycling of iodine in the subtropical North Pacific Ocean

“…Iodine is also an element of interest in paleoclimatology, with iodine-to-calcium ratios in marine carbonates being widely interpreted as proxies for the oxygenation state of the ocean through geological time (e.g. Lu et al, 2018;Lu et al, 2020;Sun et al, 2023). In seawater, iodine is primarily found in its reduced and oxidized anionic forms, iodide (I − ) and iodate (IO 3 − ), respectively.…”

The Marine Iodine Cycle, Past, Present and Future

“…Iodine is also an element of interest in paleoclimatology, with iodine-to-calcium ratios in marine carbonates being widely interpreted as proxies for the oxygenation state of the ocean through geological time (e.g. Sun et al, 2023). In seawater, iodine is primarily found in its reduced and oxidized anionic forms, iodide (I − ) and iodate (IO 3 − ), respectively.…”

“…In this Research Topic, three studies have scrutinized the validity of various carbonate archives as records of marine iodate levels. Specifically, the archives studied were planktic foraminifera (Winkelbauer et al, 2023), deep-sea bamboo and scleractinian corals (Sun et al, 2023), and ancient bulk carbonate rocks (He et al, 2022). An important finding is that, while benthic and planktic foraminifera from core-tops are known to record iodate variations of the overlying water column (reviewed in Hoogakker et al, 2024), foraminifera from plankton tows demonstrated littleto-no iodine incorporation regardless of local iodate abundance (Winkelbauer et al, 2023).…”