Observational and experimental constraints on iodine cycling in low oxygen waters

Iodine is widely applied as a paleoredox proxy for ancient atmospheric and seawater O 2 accumulation because marine iodine is redox sensitive and its dissolved speciation is preserved in carbonate rocks. However, while the oxidized iodate (IO 3 - ) is observed to become depleted relative to the reduced iodide (I - ) in modern and ancient low oxygen settings, the mechanisms and relationship with oxygen, if any, remain unclear. To provide constraints on potential O 2 controls on the iodine biogeochemical cycle, we collected depth profiles and investigated iodine cycling in 2 sub-basins of the Baltic Sea (Fårö and Gotland Deep) and performed laboratory IO 3 - reduction experiments in cultures of the facultative bacterium Shewanella oneidensis at variable O 2 levels. Like other stratified euxinic systems, I - increases and IO 3 - declines at or below the chemocline. The Baltic Sea, however, has complex patterns between iodine species that are decoupled from O 2 , likely reflecting the unique hydrological and biogeochemical conditions of this stratified system. Notably, we observe significant IO 3 - depletion in the fully oxygenated surface layers of the Baltic Sea, a phenomenon uncommonly reported elsewhere. Our culture experiments provide evidence that S. oneidensis preferentially reduces O 2 over IO 3 - until O 2 levels are < 0.1 µM, which are only relevant in anoxic and suboxic (nitrogenous and manganous) zones of the Baltic Sea. We suggest that instead of in situ reduction in the sampled surface waters, the depleted surface water IO 3 - is a legacy effect of previous hypoxia either within surface waters and/or along the flow path sourcing surface waters. Since I - oxidizes relatively slowly, such legacy effects may reflect past shoaling of the oxycline or be at least partially the result of the frequency since the last Major Baltic Inflow (MBI) (in this case 7 years), which intermittently introduce oxygenated, IO 3 - -rich seawater from the North Sea. Together, our findings have important implications for secular IO 3 - and O 2 accumulation throughout Earth history. The microbial reduction experiments provide refined constraints on atmospheric O 2 accumulation at the Paleoproterozoic Great Oxidation Event (∼4 × 10 -4 of the present atmospheric level, or PAL), when IO 3 - initially accumulated. Further, comparison to the Baltic Sea provides evidence that O 2 and IO 3 - may become decoupled in redox-stratified and restricted basins even after atmospheric O 2 increases, which may contribute to observations of widespread muted IO 3 - accumulation even throughout the Phanerozoic.