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Methane Seep Facies in the Low-Latitude Neoproterozoic Reynella Siltstone from Southern Australia: Implications for Marinoan Climate Change, Ocean Chemistry and the Methane Hydrate Reservoir

David Mrofka and Martin Kennedy
University of California, Riverside - Department of Earth Sciences
Riverside, CA USA email: [email protected]

Multiple carbonate horizons in the Reynella siltstone, equivalent to the glacigenic low-latitude (<12º) Elatina Formation (~635 Ma) are connected vertically through a network of m-scale chimneys and are associated with concentrations of smaller spar-filled vugs and tubes with vertical features showing evidence of disruption and fluidization. These and other facies are characteristic of methane seeps found today on the seafloor and in the geologic record. Carbonate horizons are interbedded with and eroded by tidal channels containing clasts of hardgrounds in channel-fill, demonstrating their synsedimentary formation. Authigenic pore-filling early diagenetic cements from methane seep hardgrounds display the greatest range of values of δ18O ever reported from marine carbonates: -27‰ to +12‰. Because the host siltstone has not been heated above 60ºC, such depleted δ18O values must be derived from highly depleted fluids, associated only with glacial meltwater in the modern environment. Enriched values in marine cements are unique and characteristic of pore-water fractionation during clathrate formation. This geochemical evidence suggests a mixing between glacial meltwater and clathrate-derived pore fluids at low-latitude during deglaciation of the Earth’s most severe ice age. The presence of methane seeps at low-latitudes implies that permafrost developed on exposed continental shelves during the Marinoan glacial low-stand, stabilized by low-temperatures or overlying ice. The extension of the methane clathrate/permafrost reservoir to low-latitudes could entail a ten-fold increase in the methane reservoir during the Marinoan ice age. Oxidation of this methane could have consumed oceanic O2 and SO42- during deglaciation impacting Neoproterozoic ocean chemistry, climate change and Metazoan evolution.

 

AAPG Search and Discovery Article #90070 © 2007 AAPG Foundation Grants in Aid