Characterizing the marine iodine cycle and its relationship to ocean deoxygenation in an Earth System model

Iodine abundance in marine carbonates (as an elemental ratio with calcium – I:Ca) is of broad interest as a proxy for local/regional ocean redox. This connection arises because the speciation of iodine in seawater—in terms of the balance between iodate (IO3-) and iodide (I-)—is sensitive to the prevalence of oxic vs. anoxic conditions. However, although I:Ca ratios are being increasingly commonly measured in ancient carbonate samples, a fully quantitative interpretation of this proxy is hindered by the scarcity of a mechanistic and quantitative framework for the marine iodine cycle and its sensitivity to the extent and intensity of ocean deoxygenation. Here we present and evaluate a representation of marine iodine cycling embedded in an Earth system model (‘cGENIE’) against both modern and paleo observations. In this, we account for IO3- uptake and reduction by primary producers, the occurrence of ambient IO3- reduction in the water column, plus the re-oxidation of I- to IO3-. We develop and test a variety of different mechanistic relationships between IO­3- and I- against an updated compilation of observed dissolved IO­3- and I- concentrations in the present-day ocean. In optimizing the parameters controlling previously proposed mechanisms behind marine iodine cycling, we find that we can obtain broad matches to observed iodine speciation gradients in zonal surface distribution, depth profiles, and oxygen deficient zones (ODZs). We also identify alternative, equally well performing mechanisms which assume a more explicit mechanistic link between iodine transformation and environment. This mechanistic ambiguity highlights the need for more process-based studies on modern marine iodine cycling. Finally, because our ultimate motivation is to further our ability to reconstruct ocean oxygenation in the geological past, we conducted ‘plausibility tests’ of our various different model schemes against available I:Ca measurements made on Cretaceous carbonates – a time of substantially depleted ocean oxygen availability compared to modern and hence a strong test of our model. Overall, the simultaneous broad match we can achieve between modelled iodine speciation and modern observations, and between forward-proxy modelled I:Ca and geological elemental ratios supports the application of our Earth system modelling in simulating the marine iodine cycle to help interpret and constrain the redox evolution of past oceans.