CONCLUSION

The dissolved carbon dioxide and methane pools within marine, continental-rise pore waters are geochemically coupled to one another through anaerobic methane oxidation and CO₂ reduction. In methane-rich settings, these processes cause consistent trends in isotopic composition that show: (1) maximum depletion of ¹³C of the both pools near the sulfate-methane interface; and (2) increasing proportions of ¹³C in the CO₂ pool with increasing depth in the shallow methanogenic zone. At locations with sufficiently deep holes, ¹³C values of CO₂ may become more enriched in ¹²C with increasing depth, moving in the direction of the carbon isotopic composition of sedimentary organic matter.