Iodine is a redox sensitive element that transforms between its two stable forms—iodate (IO3-) and iodide (I-)—depending on the redox state of the seawater. Because only the oxidized form of iodine, IO3-, in incorporated into the carbonate lattice that is precipitated from seawater, iodine to calcium (I/Ca) ratios in marine carbonates are therefore used as a paleoredox proxy. However, the current understanding towards mechanisms of IO3- redox transformations is limited, hindering redox interpretations based on the I/Ca proxy. The overall goal of this dissertation is to understand the redox cycling of iodine in marine low-O2 settings and hence provide insights for quantitative redox reconstruction based on secular variation of I/Ca records. Chapter 2 focuses on the iodine cycle in modern marine low-O2 settings. Samples were collected from two modern anoxic basins: the Baltic Sea and Siders Pond. Iodine speciation was measured, including IO3-, I-, dissolved organic iodine (DOI) and total dissolved iodine (TI) of these samples. IO3- was depleted in both the anoxic basins, and was even low within the surface layer where dissolved O2 was saturated. In addition, non-conservative features were observed throughout the water column. A significant I- flux from the sediments contributed to excessive iodine observed in the bottom of Siders Pond. In the Baltic Sea, net I- removal from the water column near the chemocline was observed. The causation of such I- removal requires further investigation. The decoupling of IO3- and O2 in relatively small water bodies in the surface anoxic basins indicates IO3- accumulation requires ventilation within the oxygenated open ocean waters. Chapter 3 calibrates the iodine cycle built into an Earth System model (cGENIE). Four processes were simulated: IO3- uptake and release of I- through the biological pump, the reduction in ambient IO3- to IO3- in the water column, and the re-oxidation of I- to IO3-. New I- oxidation and IO3- reduction parameters were incorporated into cGENIE. The model performance was evaluated against both modern and paleo-observations. The iodine cycle parameterizations optimized through model-data comparison replicates the general trends of iodine speciation gradients, including the zonal surface distribution, depth profiles, and oxygen-deficient zones (ODZs). The best-performing parameters were selected to simulate IO3- distribution in a Cretaceous model configuration as a case study. The broad match between the simulated IO3- and the carbonate I/Ca observation emphasizes the potential of using these parameters to interpreting and constraining redox variation in past oceans. Chapter 4 provides insights into the secular variation of marine IO3- through Earth history. Two approaches were conducted targeting understanding IO3- accumulation in seawater: (1) microbial IO3- reduction experiment under low but controlled O2, and (2) Earth System modelling. The model bacterial strain Shewanella oneidensis MR-1 started reducing IO3- when dissolved O2 in its medium decreased to 0.1μM. This O2 threshold provides the minimum estimate of dissolved O2 in association with the first appearance of IO3- accumulation in seawater during the Great Oxygenation Event (GOE). Earth system modelling indicates that IO3- accumulation in the surface ocean is a function of atmospheric O2 and ocean nutrient levels (PO4). A low I/Ca baseline in the Proterozoic could be best explained as the combined results of low atmospheric O2 (< 3% present atmospheric level, PAL) and low PO4 (< 10% present ocean level, POL). The transition of the low I/Ca baseline from the Proterozoic low levels to the modern-like values observed throughout the Paleozoic may require shifts in both oxygenation and nutrient availability. Together, this chapter demonstrates that changes in IO3- steady-state values through Earth history reflects fundamental changes in the Earth System, including O2 and other non-redox factors that have been previously overlooked.
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