Experimental constraints on biotic and abiotic drivers of near surface-marine iodine redox transformations at the Bermuda Atlantic Time Series

Abstract:

Iodine speciation in the surface ocean can be directly linked to in situ processes of primary productivity by phytoplankton, diatoms, and algae. Profiles outlining the concentration of iodide (I^-) and iodate (IO₃^-) in the surface ocean with depth and latitude are well known, however, the rates and mechanisms of IO₃^- production—the most prevalent iodine species—remain unclear. In oxic euphotic waters, iodine speciation is in disequilibrium, with a peak in I^- concentration relative to IO₃^- at the ocean’s surface, hinting that oxygen is unlikely the driving oxidant in these reactions. Instead, it is thought that stronger oxidants, such as reactive oxygen species (e.g., superoxide, hydrogen peroxide), produced by marine bacteria or other biological catalysts may aid in iodide oxidation at the sea surface. To test potential drivers of iodine redox reactions in euphotic waters, shipboard incubations were performed as part of the Bermuda Atlantic Time Series (BATS) in the Sargasso Sea in September of 2018. Experimental treatments tested the importance of biological catalysts via filtered and unfiltered controls as well as the effects of several abiotic factors, including depth (euphotic vs subphotic), light/dark constraints, and the presence of superoxide thermal source (SOTS), hydrogen peroxide (H₂O₂), and manganese chloride (MnCl₂) on iodide (I^-) and iodate (IO₃^-) redox rates. The speciation of iodine was tracked over time using spectrophotometric methods as well as coupled ion-exchange chromatography and Inductively Coupled Plasma Mass Spectrometry (ICP-MS) in incubation samples spiked with an iodine radiotracer ¹²⁹I^- (half-life of 15.7 million years). Our depth profile measurements reaffirmed iodate and iodide concentration from previous studies at BATS and the incubation controls and sensitive radiotracer technique provide new insights on the drivers of seawater iodine speciation trends in oxic seawater more broadly.