TRACE ELEMENT GEOCHEMISTRY OF A STURTIAN CAP DOLOSTONE IN THE SCOUT MOUNTAIN MEMBER OF THE POCATELLO FORMATION: EVIDENCE FOR OCEAN ANOXIA FOLLOWING LOW LATITUDE GLACIATION

Carbonate deposition is widely accepted to have immediately followed each of the major low latitude glacial events of the Neoproterozoic. Cap dolostones sit directly upon glacial diamictite and provide an opportunity to examine ocean chemistry immediately following glaciation. The Neoproterozoic margins of Laurentia record at least two such cap dolostone depositions and both show unusual negative carbon isotope anomalies; however, few detailed trace metal geochemical studies have explored these carbonates. This study examines the trace element geochemistry of the Scout Mountain Member of the Pocatello Formation, Idaho, with a focus on geochemical indicators of redox state in the post glacial ocean. Redox state is related to oceanic mixing and geochemical cycling of trace elements. The Scout Mountain Member contains an approximately one meter thick, finely laminated, pink dolomite cap that sits directly above a glacial diamictite. The pink dolomite is commonly considered to be a cap dolostone and provides an opportunity to examine ocean chemistry on the margin of the Laurentia at the terminal stages of the Sturtian glaciation. This cap dolostone is of particular value because its depositional age is well constrained and it has undergone only limited post depositional alteration. Here, we use high resolution chemostratigraphy to show that the ocean was rapidly changing during cap emplacement. Normalized trace metal concentrations of Cd, Ce, Cr, Cu, Ni, Mn and Zn suggest that reducing, not euxinic, conditions prevailed during the initial deposition of the pink dolomite. The enrichments show a period of anoxia in the water column following glaciation which may result from the presence of an uncoupled ocean/atmosphere, a strongly stratified water column coupled with the decomposition of organic carbon, or microbially mediated sulfate reduction. This period of anoxia is brief and conditions quickly return to an environment of oxic deposition. These data are largely consistent with geochemical studies of younger Marinoan cap dolostones, but add new insight into ocean chemistry immediately after the Sturtian glaciation.