Elemental and isotopic response of different carbon components to anaerobic oxidation of methane: A case study of marine sediments in the Shenhu region, northern South China Sea

Abstract

• Evidences show that the SMTZs are located at 300–360 and 440–480 cmbsf in core SH-CL52. • Under high methane flux and short methane-seep duration conditions, carbon isotope of TOC can indicate the location of SMTZs. • In the SMTZ, the positive excursion of TOC isotope is possibly due to the relatively high methanogenic rate. • TOC isotope has the potential to be a novel proxy of SMTZ. In the cold seep region, the carbonate rocks/nodules help to reveal the source of methane and gas-hydrate decomposition. Conventionally, authigenic carbonate rocks/nodules are great proxy of AOM, but it only forms under an intermediate methane flow rate. It is thus vital to explore another carbon component proxy of AOM. In this paper, we investigate different carbon component features from a 5.2 m-long sediment core (SH-CL52) in the Shenhu region. We constrain the location of SMTZ based on the natural remanent magnetization (NRM) characteristic, pyrite content, TS/TOC and sulfur isotope, after that we discuss carbon component’s isotopic response to AOM. Pyrite increases in 300–360 and 440–480 cmbsf, negative (NRM AF60mT - NRM AF80mT )/NRM values occurrence in 300–360 and 440–480 cmbsf, bulk-sediment δ 34 S values have positive excursion in 300–360 and 440–480 cmbsf and pyrite δ 34 S positive excursions appear in 300–360 and 440–480 cmbsf. All these reflect that at least two methane release events (MREs) are identified in 300–360 and 440–480 cmbsf in the core SH-CL52, respectively. In these zones, δ 13 C TC , δ 13 C TIC , δ 13 C FC and δ 13 C FGC variety is not discernable, but δ 13 C TOC has positive excursions which are most likely interpreted to the relatively higher methanogenesis rate in SMTZs. Our results suggest that δ 13 C TOC represents a potential proxy of SMTZ under high methane flux and shortduration condition.