Pore-water dissolved inorganic carbon sources and cycling in the shallow sediments of the Haima cold seeps, South China Sea

Pore-water geochemical compositions provide the insights into seafloor cold seep activities and biogeochemical processes of methane-derived carbon in sediments. In this study, six sediment push cores were collected by manned submersible at the center and edge of the active Haima cold seeps, and a nearby background site. Pore-water compositions of the SO42−, Ca2+, Mg2+, Sr2+, NH4+, dissolved inorganic carbon (DIC), and δ13C-DIC at each sediments core were measured. The very shallow depths of sulfate–methane transition zone (SMTZ) suggest that high methane flux and significant anaerobic oxidation of methane (AOM) activities occur in the shallow sediments. Reaction-transport modelling results show that the relative contributions of AOM and organoclastic sulfate reduction (OSR) consuming sulfate are 84.1% and 15.9% at site HM-2-6, and 94.6% and 5.4% at site HM-3-3, respectively. Furthermore, extremely low δ13C-DIC values at the central sites HM-2-6 (−45.67‰) and HM-3-3 (−53.87‰) indicate an external methane-derived DIC. Based on the δ13C mass balance, we quantify the proportions of pore-water DIC derived from organic matter degradation and AOM as 24.7% and 65.3% at site HM-2-6, and 9.3% and 80.7% at site HM-3-3, respectively. An increase in pore-water DIC concentration leads to carbonate precipitation. Plots of Sr/Ca vs. Mg/Ca ratios reveal that the carbonate phases in sediments are dominantly high Mg-calcite in this region. Our study demonstrates that AOM can serve as a dominant biogeochemical process through incorporation of methane-derived carbon into dissolved and solid inorganic carbon phases, significantly influencing DIC cycling within the shallow sediments of the Haima cold seep area.