Using multi-proxy approach to constrain temporal variations of methane flux in methane-rich sediments of the southern South China Sea

Abstract

Methane seeps are widespread on the seafloor in continental margins. However, the responses of methane flux and diagenetic processes at cold seeps to drastic environmental changes in the geological history are still poorly understood. Here, we report the geochemical composition of porewaters and sediments in a sediment core (CL3A) collected from methane-rich sediments in the southern South China Sea, in order to assess the variations in upward methane flux since the last glacial period. A sediment horizon at depths of ca. 510–660 cm below seafloor just above the current sulfate-methane transition zone (SMTZ) is identified, which shows geochemical anomalies including high total inorganic carbon (TIC) and total sulfur (TS) contents, extremely negative δ13CTIC values and positive sulfur isotopic ratios of acid insoluble sulfur (δ34Sacid-insoluble). We propose that the diagenetic records of sulfate-driven anaerobic oxidation of methane within a paleo-SMTZ were recorded by the accumulation of authigenic carbonate and iron sulfide within this zone. Such geochemical pattern implies a fixation of the SMTZ at a certain depth for a prolonged time, which is estimated as ~13,000 years. The stagnation of the SMTZ is probably attributed to a large decline in sedimentation rate during the transition of last glacial/interglacial period, which is supported by the 14C ages throughout the core. Moreover, a non-steady-state modelling for the concave-up depth profiles of porewater species suggests that a recent enhancement in upward methane flux has lasted for at least 550 years. In summary, this study suggests that the fixation of SMTZ during the glacial/interglacial transitions in dynamic sedimentation regimes can lead to the accumulation of large amount of authigenic carbonate and iron sulfide in continental margins and should be considered in global carbon and sulfur biogeochemical cycles.