"I-CYCLE v1.0": A new model of global marine iodine cycling including suboxic and benthic processes

Abstract. Atmospheric iodine has significant impacts on air quality, climate and human health. Its primary atmospheric source is a reaction on the sea surface between ozone and aqueous iodide (I^-), which is naturally present in the ocean alongside iodate (IO₃^-) and organic iodine species. Here we describe a new global biogeochemical model of the marine iodine cycle, I-CYCLE, which predicts the distribution and cycling of iodide, iodate, phytoplankton associated iodine, detrital iodine and benthic iodine. I-CYCLE is embedded within the UKESM 1.0 ocean framework and driven by the MEDUSA biogeochemical model. In the model, the reduction of iodate to iodide is driven by uptake to plankton and subsequent release from particulate iodine reservoirs. Iodide is returned to iodate via two processes: biological oxidation coupled to ammonia oxidation, and a slow background reaction which may be abiotic. Below a specified oxygen threshold, iodate is reduced to iodide and the release of benthic and detrital iodine as iodide is amplified. Model parameters were tuned against observed iodide and iodate fields; the best-fit parameters are in good agreement with empirical estimates. The model replicates observed present day latitudinal and vertical trends in dissolved iodine speciation, as well as the impacts of oxygen minimum zones. Future iodine fields predicted for 2100 under the SSP585 scenario are presented.