Pockmark activity inferred from pore water geochemistry in shallow sediments of the pockmark field in southwestern Xisha Uplift, northwestern South China Sea

Abstract

Pockmarks are widespread on the seabed offshore southwestern Xisha Uplift, northwestern South China Sea. Some of them are so enormous that they are rare worldwide, but their activities were previously poorly known. We collected three gravity-piston cores from this pockmark field, one (C9) from outside, but in close proximity to, a giant pockmark and the other two (C14 and C19) from inside two giant pockmarks. The geochemistry of the pore waters, including SO42−, K+, Ca2+, Mg2+, Sr2+, dissolved inorganic carbon (DIC), δ13CDIC values, and δ34S values of sulfate, was analyzed to elucidate the biogeochemical processes associated with sulfate consumption and to evaluate the current pockmark activity. The sulfate concentration–depth profiles of C9 and C19 are predominantly in response to organoclastic sulfate reduction (OSR), whereas the sulfate concentrations of C14 exhibit three zones of different concentration gradients resulting from varying proportions of contributions from OSR and anaerobic oxidation of methane (AOM). The estimated lower sulfur isotope fractionation factor suggests a higher microbial sulfate reduction rate in C14 than in C9 as a possible consequence of AOM. Based on the sulfate concentration gradient of C14 below 3.7 mbsf, the depth of the sulfate–methane interface (SMI) and the methane diffusive flux in C14 are calculated to be ∼14.3 mbsf and ∼0.0144 mol m−2 yr−1, respectively. The pore-water Mg/Ca and Sr/Ca weight ratios suggest that high Mg-calcite is in equilibrium with respect to pore water or has recently precipitated from pore water in C14. The δ13CDIC values of C9 and C19 are derived from a binary mixture of δ13C from organic matter and of δ13CDIC from bottom water, while DIC from bottom water, OSR, and AOM, concomitant with 13C-enriched DIC from below the SMI possibly all contribute to the DIC pool below 0.66 mbsf in C14. The integrated analysis of pore water geochemistry implies that the pockmark from which C19 was collected may be inactive while the pockmark from which C14 was obtained may currently be sluggish in activity with methane-bearing fluid weakly seeping from subsurface sediments.