Organically bound iodine as a bottom-water redox proxy: Preliminary validation and application

Carbonate-associated iodine (I/Ca) has been used as a proxy of local, upper-ocean redox conditions, and has successfully demonstrated highly dynamic spatial and temporal patterns across different time scales of Earth history. To further explore the utility of iodine as a paleo-environmental proxy, we present here a new method of extracting organically bound iodine (I_org) from shale using volumes of samples on the order of tens of milligrams, thus offering the potential for high-resolution work across thin shale beds. The ratio of I_org to total organic carbon (I/TOC) in modern surface and subsurface sediments decreases with decreasing bottom-water oxygen, which may be used to reconstruct paleo-redox changes. As a proof of concept, we evaluate the I/TOC proxy in Holocene sediments from the Baltic Sea, Landsort Deep (IODP 347) and discuss those data within a framework of additional independent redox proxies, e.g., iron speciation and [Mo]. The results imply that I/TOC may be sensitive to hypoxic–suboxic conditions, complementary to proxies sensitive to more reducing, anoxic–euxinic conditions. Then, we test the usage of I/TOC in sediments deposited during Late Cretaceous, Cenomanian–Turonian Oceanic Anoxic Event (OAE) 2 from ~ 94 million years ago (Ma). We generated I/TOC and I_org records from six OAE 2 sections: Tarfaya (Morocco), Furlo (central Italy), Demerara Rise (western equatorial Atlantic), Cape Verde Basin (eastern equatorial Atlantic), South Ferriby (UK), and Kerguelen Plateau (southern Indian Ocean), which provide a broad spatial coverage. Generally, I/TOC decreases over the interval recorded by the positive carbon-isotope excursion, the global signature of OAE 2, suggesting an expansion of more reducing bottom-water conditions and consistent with independent constraints from iron speciation and redox-sensitive trace-metals (e.g., Mo). Relatively higher I/TOC values (thus more oxic conditions) are recorded at two high latitude sites for OAE 2, supporting previous model simulations (cGENIE) that indicated higher bottom water oxygen concentrations in these regions. Our results also indicate that organic-rich and oxygenated seafloors are likely a major sink of iodine and correspondingly influence its global seawater inventory.