Geochemical characteristics of sediment pore water in Haima area of the South China Sea: An indication of cold seeps

The Haima cold seeps are located in the southeastern part of Qiongdongnan Basin, which is one of the two active cold seeps found in the South China Sea. We analyzed the contents of anions and cations, dissolved inorganic carbon (DIC) and its carbon isotopic composition, and Sr and Ba contents of sediment pore water in two piston gravity columns at the Rov2 and PC3 cores in Haima Cold Seeps. Results show that the DIC contents and δ¹³C_DIC values of pore water in the two cores had a significant "mirror" relationship. With the increase of depth, the DIC contents of the two cores gradually increased (Maximum DIC content of Rov2 and PC3: 27.4 and 8.5 mM, respectively). In contrast, the δ¹³C_DIC values had a negative excursion (Minimum values for the two cores: −54.63‰ and −48.93‰, respectively). Combined with the sulfate depth profile characteristics of pore water, the sulfate-methane interface (SMI) in Rov2 and PC3 cores was located at ~485 and ~410 cm, respectively. The upward methane fluxes in Rov2 and PC3 cores were estimated to be 67.4 and 97.2 mol m⁻² ka⁻¹, respectively. The very low δ¹³C_DIC values in pore water near SMI are indicative of the occurrence of AOM (anaerobic oxidation of methane) interaction and its contribution to DIC. In Rov2 core, authigenic carbonate minerals are dominated by high-Mg calcite, and the Ca²⁺, Mg²⁺ and Sr²⁺ showed similar trends to those of SO₄²⁻. Near the SMI, with the depletion of SO₄²⁻, the mineralization of organic matter released large amounts of Ba²⁺ and PO₄³⁻ into the pore water. The geochemical characteristics of pore water could help us effectively identify the early diagenesis in seepage activity area, and are indicative of migration and transformation of materials under the influence of AOM.