Coupling of organic carbon and nitrogen isotopes in cold seep carbonates and its implication for the anaerobic oxidation process of methane
The microorganisms that drive the anaerobic oxidation of methane(AOM)not only assimilate methane and other hydrocarbons as carbon sources,but also assimilate nitrogen,leading to the significant depletion of 13C and 15N in these microorganisms.However,there is still a lack of δ15N data in cold seep environments to directly confirm the involvement of AOM in the nitrogen cycle process.In this study,we analyzed the contents of total nitrogen,total inorganic nitrogen and total organic nitrogen and their δ15N in authigenic carbonate rocks from five cold seep sites on the Gulf of Mexico seafloor and compared them with the contents of total inorganic carbon and total organic carbon and δ13C data obtained in previous studies.We found that theδ15N of total nitrogen showed a positive correlation with the δ13C of total organic carbon only at some of the sites.However,after excluding the interference of inorganic nitrogen,δ15N of total organic nitrogen and δ13C of total organic carbon showed positive correlation at all sites.Therefore,we propose that coupling of isotopes of organic carbon and organic nitrogen is closely related to methane seepages and is promising to serve as an effective indicator for tracing the past submarine methane seepages and AOM.
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