HR: 1340h
AN: PP13A-1030    [Abstracts]
TI: Iron Speciation using X-ray Absorption Spectroscopy of Neoproterozoic Cap Carbonates from the Pocatello Formation (Idaho)
AU: * Meyer, E E
EM: emeyer@dartmouth.edu
AU: Quicksall, A N
EM: Andrew.Quicksall@dartmouth.edu
AU: Bostick, B C
AB: Carbonate deposition immediately follows each of the major low latitude glacial events of the Neoproterozoic. Cap carbonates sit directly upon glacial diamictite and provide an opportunity to examine ocean chemistry immediately after glaciation. The Neoproterozoic western margins of Laurentia record at least two such cap carbonates and show unusual trace element geochemistry. Here, we use synchrotron based X-ray absorption spectroscopy (XAS) to examine the speciation of iron in Neoproterozoic rocks of the Scout Mountain Pink Dolomite member of the Pocatello Formation (ID). Speciation of redox sensitive metals such as iron can be used in conjunction with trace element data to provide a highly sensitive record of marine sediment depositional environments and diagenesis. Iron redox state is related to oceanic mixing and types and intensity of biological productivity - both of these processes should be disrupted during low latitude glaciation. The unusual and rapidly-changing climatic conditions during low latitude glaciations may lead to highly variable conditions of ocean oxidation due to decoupling between the ocean and atmosphere, a strongly stratified water column coupled with the decomposition of organic carbon, or microbially-mediated sulfate reduction. The pink dolomite shows elevated iron concentrations at its base. These concentrations return to background values over only a few centimeters of section. The elevated iron concentrations may result from a range of redox states during deposition followed by sequestration in a variety of mineral phases. Under reducing conditions Fe(II) is prevalent and will accumulate in sulfide phases. Under suboxic conditions a mixture of Fe(II) and Fe(III) may exist and the deposition of magnetite will occur. In a well oxygenated water column iron is oxidized to Fe(III) and precipitates as hematite or goethite. XAS was utilized to probe the average bonding environment of iron. Linear combination fitting of iron mineral standards was used to find bulk iron mineralogy. The resulting data suggest that absorption spectroscopy is a useful tool for unraveling rapid changes in redox state during sedimentary deposition. It provides valuable insight into the rapid and highly unusual changes in ocean chemistry at the terminus of the Sturtian glaciation.
DE: 4924 Geochemical tracers
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2007 Fall Meeting