Report on Radiocarbon Analysis of Surface Sediments from the Fore-Arc Basin of Nankai Trough

Understanding the fate of methane carbon in anaerobic marine sediments is a critical component of evaluating the resource potential of hydrates and gas deposits in the ocean. Radiocarbon analysis of the total organic carbon (TOC) and total inorganic carbon (TIC) on 30 sediment samples from two multicores and six piston cores was performed to investigate the fate of methane carbon in sediment of the Nankal Trough. The TIC pool had consistently lower AC-14 values compared to the TOC pool. A small deficit was expected from source reservoir differences during the biogenic production of TOC and TIC during and after sediment deposition. However, a large difference was observed in some samples, probably generated by a fossil carbon source after sediment deposition. We suggest this source of fossil carbon is from anaerobically oxidized fissile methane. Using a C-14 based isotope-mixing model, we calculated the potential contribution of methane carbon to the TIC pool. Model inputs to TIC were a near radiocarbon dead methane end member (DELTA C-14(sub AOM)) from anaerobic oxidation of methane (AOM) and the DELTA C-14(sub TOC), which was normalized for reservoir effects and served as a proxy for the biogenic inorganic carbon (B IC) end member DELTA C-14(sub BIC). Two cores, presumed to represent control sites, indicate insignificant contributions from the methane end member. The TIC in one core appears to be almost entirely derived from AOM, and three cores show moderate levels of methane carbon input, with a preference toward higher fractions of methane in the TIC pool downcore. The patterns we observed in the data provide clear evidence that a combination of biogeochemical factors can influence DELTA C-14 of the TIC and TOC pools in the sediment Furthermore, the data suggests that the TIC pool in the sediment can incorporate a large amount of C from fossil methane sources.