Figure 9. Drawing illustrating the major diagenetic reactions and possible response of authigenic carbonates is shown. A. The typical pore-fluid profiles produced by the major diagenetic reactions occurring in the upper part of marine sedimentary sections: (1) microbially mediated oxidation of organic matter by sulfate reduction, and (2) microbially mediated anaerobic methane oxidation (AMO). Both processes increase the alkalinity (by increasing DIC) of pore fluids and stimulate carbonate precipitation. B. Illustration of how the sedimentary record of carbonate minerals precipitated as a consequence of AMO will exhibit negative ¹³C values. Large negative excursions in the ¹³C value of carbonates at depth may reflect paleo sulfate/methane interfaces where AMO has been focused, leaving a ¹³C-depleted signature on the bulk carbonate.