Iodine incorporation into dolomite: Experimental constraints and implications for the iodine redox proxy and Proterozoic Ocean

Abstract

Iodine concentrations in sedimentary carbonate minerals are used as a proxy to reconstruct the redox landscape in shallow oceans throughout Earth history. The proxy [I/(Ca + Mg)] assumes that only the oxidized form of iodine, iodate (IO3-), is incorporated into carbonate minerals, and thus its presence in ancient carbonate rocks suggests their formation under oxic conditions where iodate prevails. This assumption has been experimentally tested in calcite but not in dolomite, despite dolomite being the host mineral for much of the ancient iodine record. Here, high-temperature (170–200 ℃) experiments are employed to investigate the fundamental controls on iodine incorporation in dolomite. The experiments included: 1.) the dolomitization of iodine-containing aragonite sediments in iodine-free Mg-Ca-Cl solutions to explore the effects of dolomitization by reducing pore fluids and 2.) dolomitization of iodine-free aragonite in solutions containing either iodate or iodide using various concentrations and temperatures. The I/(Ca + Mg) in natural aragonite samples dolomitized in iodate-free solutions—mimicking diagenesis in anoxic pore fluids—retain only a small proportion of the original iodine. This suggest that zero or low I/(Ca + Mg) in natural dolomites could be the result of dolomite formation and/or subsequent recrystallization in anoxic pore fluids during diagenesis and does not necessarily indicate an anoxic water column. The results of the dolomitization of the iodine-free aragonite demonstrate that iodate is incorporated into dolomite, whereas iodide is not, validating the iodine proxy, and suggesting that non-zero I/(Ca + Mg) in natural dolomites likely reflect the presence of iodate, and thus free oxygen in the dolomitizing fluid. I/(Ca + Mg) in dolomite covaries with [I] in solution, thus permitting determination of a partition coefficient for iodate (Diodate). Conversely, there is no discernable effect on iodine incorporation from reaction temperature at 170–200 ℃. The results also show that I/(Ca + Mg) in dolomite decreases with increasing dolomite stoichiometry (mol % MgCO3) and cation ordering, both of which increase with reaction time, suggesting that iodine content in dolomite, and thus Diodate, is a function of the chemical and structural characteristics of dolomite. Application of experimentally derived Diodate to previously published I/(Ca + Mg) values bolster interpretations that Proterozoic [IO3-]—and hence likely local O2—was maintained at low levels until the Neoproterozoic Oxygenation Event, when oxygen levels became high enough to support and stabilize the accumulation of iodate in seawater.