Research paperPore water geochemistry in shallow sediments from the northeastern continental slope of the South China sea

•Pore water geochemistry anomalies in the study area are consistent with bottom simulating reflector (BSR) distribution.•The sulfate-methane transition (SMT) can be inferred with combination of concentration gradients and isotope composition.•The reactive transport model demonstrates a causative link between shallow pore water and deep gas amount.•Heterogeneous lithology, which due to complex tectonics, may be the reason for variable methane fluxes.

The northeastern continental slope of the South China Sea is one of the promising areas for gas hydrates, which is characterized by its variable topography and complex tectonics. In this paper, two gravity piston cores were sampled from the shallow sediments in the vicinity of the region where various morphologies of gas hydrate was recovered during the second gas hydrate expedition of Guangzhou Marine Geological Survey. Conventionally, the identification of gas hydrate was primarily dependent on the bottom simulating reflector (BSR). Apart from BSR, the anomalies in pore water geochemistry, which are highly related to various microbially-mediated processes and features of fluid from deep hydrocarbon reservoir, are also helpful for the gas hydrate exploration. In this study, upward methane flux may be responsible for distinct characteristics of pore waters observed at the two sample sites. A reactive transport model was accordingly established to compare contrastive fluid features and to estimate the upward methane flux. Since fluid migration is typically associated with complex tectonic settings of the study area, it is proposed that heterogeneous distribution of gas hydrates along the continental margin should be well considered.