Updated iodine cycle constraints and parameters in the cGENIE model: implications for modern iodine redox transformation and paleoredox reconstruction

Iodine abundance in sedimentary marine carbonates represents a potentially powerful proxy for local surface ocean redox due to its high sensitivity to dissolved oxygen concentration. Comparisons between iodine redox proxy records throughout Earth's history, when combined with an iodine cycle configured in the Grid ENabled Integrated Earth system modelling (cGENIE) — a 3-D earth system model including dynamic ocean circulation and biogeochemical cycling — can potentially enable reconstruction of spatial variability in ancient ocean redox. However, quantitative applications of the iodine proxy and modelling have been hindered by considerable uncertainties regarding the rates and mechanisms of transformation of iodine species in low-oxygen waters. To build on these past efforts and provide new insights into the processes regulating iodate reduction in low oxygen ocean settings, we tested potential links with the oxygen, nitrogen and carbon cycles using compiled marine iodine speciation and related elemental distributions as well as rates and mechanisms of iodine redox transformation from existing literature. We used statistical analyses to estimate correlations and thresholds between modern iodine and other elemental cycles for sensitivity analysis and to refine the iodine cycling parameterization for the cGENIE Earth system model. We then compared model results with the observed distribution of iodine species in the modern ocean in order to assess the effectiveness of updated parameters. The difference between model results and observations exposes the possibility of variable factors driving iodine redox transformation between ocean basins, which require experimental and field-based ground truthing. Our new constrains and updated parameterization have implications for integrating sedimentary carbonate iodine records and other paleoceanographic proxies during periods of Earth history defined by air-sea disequilibrium and stark marine spatial gradients in oxygen concentration, such as those inferred for much of the Proterozoic.