Pore fluid compositions and inferred fluid flow patterns at the Haima cold seeps of the South China Sea

Fluid flow patterns at cold seeps provide insights into the mechanism and influence of methane emission into the ocean, which is critical in its environmental impact assessment. Here, we report pore fluid compositions of three similar to 8 m long piston cores (QDN-14A, QDN-14B and R1) collected from the newly-discovered active Haima cold seeps on the northwestern slope of the South China Sea. Reaction-transport models were further applied to quantify related biogeochemical processes and to reveal the patterns of fluid flow. Extremely low delta C-13 values (< -52 parts per thousand) of dissolved inorganic carbon (DIC) near the sulfate-methane transition in the three cores suggest that anaerobic oxidation of methane is the predominant biogeochemical process. The presence of small pieces of gas hydrates along with negative anomalies of porewater chloride and sodium concentrations reflects gas hydrate dissociation. Nearly invariable concentrations of sulfate, DIC, and calcium on a meter-scale were observed in the uppermost part of the sediment cores QDN-14A and QDN-14B. This irrigation-like feature is inferred to result from enhanced methane flux in QDN-14A and QDN-14B. We infer that lateral migration of methane-rich fluids from R1 site to QDN-14A and QDN-14B sites together with upward migrated methane is responsible for the enhanced methane flux. This speculation is supported by the occurrence of gas hydrates which might have clogged the fluid channel in the seepage center (R1) and driven the transportation of methane-bearing fluid along a coarser sediment layer in surrounding sediments (QDN-14A and QDN-14B). The proposed scenario is further demonstrated using a non-steady-state modeling that reconstructed the porewater irrigation-like feature assuming an increased methane flux. The modeling result predicts that gas-hydrate formation in core R1 started at least 150 yr B.P. The proposed fluid flow pattern within a localized seep site may have a great implication for understanding the heterogeneity of sedimentary records.