Carbon isotopic composition of dissolved CO2, CO2 gas and methane in sediments from ODP Leg 172 sites

Carbon isotopic data of interstitial dissolved CO2 (CO2), CO2 gas, and methane show that a variety of microbial diagenetic processes produce the observed isotopic trends. Anaerobic methane oxidation (AMO) is an important process near the sulfate-methane interface (SMI) that strongly influences the isotopic composition of CO2 in the sulfate reduction and upper methanogenic zones, which in turn impacts methane isotopic composition. Dissolved CO2 and methane are maximally depleted in 13C near the SMI, where 13C values are as light as -31.8 and -101 PDB for CO2 and methane, respectively. CO2 reduction links the CO2 and methane pools in the methanogenic zone so that the carbon isotopic composition of both pools evolves in concert, generally showing increasing enrichments of 13C with increasing depth. These isotopic trends mirror those within other methane-rich continental rise sediments worldwide.