Carbon unusually rich in C13 (??C13 = +5.4 to +19.0 per mil relative to the Peedee belemnite carbonate standard of the University of Chicago) is characteristic of certain diagenetic limestones and dolomites in the Miocene Monterey Shale of California and the Nye Mudstone of Oregon. This heavy carbon may have originated through low-temperature equilibration between CO3- and CO2 in migrating carbonated waters or between CH4 and CO2 in natural gas. Light carbon (??C13 = -5.6 to -18.2 per mil) derived through nonequilibrium oxidation of organic matter also occurs in the carbonate of Monterey Shale in some localities, but at most places it is much less common than heavy carbon.
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Carbon-13-rich diagenetic carbonates in miocene formations of California and Oregon