Quantification of the sources of sedimentary organic carbon at methane seeps: A case study from the South China Sea

Abstract

Large amounts of methane are stored along continental margins in the form of methane hydrate. Methane hydrate is sensitive to environmental change, resulting in the release of substantial quantities of methane upon its destabilization. In the marine subsurface, microorganisms consume most of the methane in the sulfate-methane transition zone and convert a certain amount of methane into organic matter. Burial of such organic matter deriving from methane oxidation represents a long-term carbon sink, therefore mitigating the effect of a prominent greenhouse gas on global warming. However, the controls on the formation and consumption of sedimentary organic matter at marine seeps remain poorly constrained, impeding accurate quantification of carbon burial at seeps and its role in the marine carbon cycle. To gain new insight on the effect of seeps on carbon burial, sediments from two seep sites of the South China Sea (Site F, Haiyang 4) and a nearby reference site (Jiulong canyon) were analyzed for total organic carbon contents (TOC), δ13CTOC and Δ14C values, total inorganic carbon contents (TIC) and δ13CTIC values, as well as organic nitrogen (N) contents. Depth distributions reveal that the TOC at seeps (Site F, 0.55% ± 0.08%, Haiyang 4, 0.67% ± 0.11%) is higher than in sediments not affected by seepage (Jiulong canyon, 0.50% ± 0.10%). The enrichment of total sulfur, high TIC, and negative δ13CTIC values in sediments from Site F and Haiyang 4 agrees with locally prominent sulfate-driven anaerobic oxidation of methane (AOM) and resultant precipitation of authigenic carbonate. At the two seep sites, the sediments are characterized by more negative δ13CTOC and Δ14C values and a lower TOC/N ratio, indicating a contribution of the microbial fixation of 13C-depleted methane to the local pool of sedimentary organic matter. A linear increase in 14C ages of organic carbon with sediment depth at the reference site indicates steady depositional conditions, whereas the 14C age of organic carbon at seeps is typified by contributions of methanotrophy-derived carbon. A Δ14C mass balance approach shows that carbon derived from fossil methane accounts for at least 10 to 20% of the organic carbon preserved in the sediments. By comparing seeps, hydrate-bearing sediments, modern coastal sediments, and Archaean rocks, it becomes apparent that the carbon stable isotope composition of organic carbon in seep sediments is mainly controlled by the rate of methane oxidation and marine primary productivity.