In some calcium carbonate-bearing sandstones from the edge of the continental shelf off the northeast United States, the ??C13 range is from -30 and -60 per mil for both aragonite and high-magnesium calcite. The ??C13 of co-existing shells of Modiolus sp. is normal (+1.7 to -2.7 per mil). The ??O18 values of around +3.5 per mil in all samples suggest deposition at temperatures around 0??C. Quaternary methane oxidized either chemically or microbiologically to carbon dioxide is the probable source of carbon in these carbonates.
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Methane-derived marine carbonates of pleistocene age