The Biogeochemical Processes, The Activity, And The Formation Time Of The Pockmarks In The Southwestern Xisha Uplift, Northwestern South China Sea

Serving as an indicator of fluid seepage from seabed sediments, pockmarks are ubiquitous along continental margins worldwide. Geochemical analysis was done upon the sediment and porewater samples taken from the pockmark field in the southwestern Xisha Uplift, northwestern South China Sea. Combined with reactiontransport modeling, our purposes are to understand the main biogeochemical processes and their reaction rates, to infer current pockmark activity, and to estimate the minimum age of pockmark formation as well as possible formation mechanism.Porewater sulfate in C9 and C19 is primarily consumed by OSR. For C14, the reduced sulfate concentration in pore water is caused by OSR within 0-0.66 mbsf; The sulfate loss is triggered by both OSR and AOM from 0.66 to 3.7 mbsf; AOM predominantly controls the shape of the sulfate concentration-depth profile from 3.7 mbsf to the bottom of the sediment core. The relatively low sulfur isotope fractionation factor provides additional evidence for the possible occurrence of AOM in C14. The SMI depth and methane flux are estimated to be ~14.3 mbsf and ~0.0144 mol·m-2·yr-1, respectively, depending on the sulfate concentration-depth profile of C14 below 3.7 mbsf. Authigenic carbonate minerals might have precipitated in the three investigated sediment cores, and high Mg-calcite may be predominant in C14 sediments. The δ13CDIC values in the pore waters of C9 and C19 are a mixture of δ13C of organic matter and the δ13CDIC of bottom water, while four possible end members, including the δ13CDIC of bottom water, δ13C of organic matter, the δ13C of methane, and the 13C-enriched DIC from below SMI, contribute to the δ13CDIC values in the pore waters of C14. Based on the concentration-depth profiles of the selected pore water species in the investigated cores, the pockmark from which C19 was recovered is inferred to be inactive and in its quiescence period. The pockmark where C14 was collected may currently be in a sluggish activity state and methane-bearing fluid is weakly seeping from subsurface sediments.Through the measurement of pore water concentrations of Cl-, Na+, and K+ as well as δ18O values of pore water and dissolved sulfate obtained from shallow sediments of pockmark field in the southwestern Xisha Uplift, SCS, we discover compelling indicators for gas hydrate dissociation-Cl- concentration decrease coupled with δ18O values increase. Moreover, we precluded the possibility that the anomalies of pore water Cl- concentrations and δ18O values are due to clay mineral dehydration by incorporating investigation of Na+ and K+ concentration profiles. The difference between δ18O values of sulfate in the reference core(C9) and the target core(C14) may be an additional indication for gas hydrate occurrence. Gas hydrates occupy 6%–10% and 7%–15% of pore-space volume within sediments, respectively, based on the deviation of Cl- concentrations and δ18O values from the baselines. Consequently, the pockmark field in the southwestern Xisha Uplift, northern SCS, is a potential area exhibiting favorable prospects for gas hydrate exploration.Porewater geochemical profiles in sediments sampled at a reference core(C9) and pockmark core(C14) were simulated using a non-steady-state reaction-transport model in order to estimate the time when pockmark activity ceased, which represents the minimum age of the pockmark. The model suggests that the pockmarks formed at least 39 kyr B. P. and the termination of fluid seepage may be ascribed to gas hydrate stabilization or complete depletion when sea level reached a relatively high-stand. We were also able to show that POC flux and reactivity increased during the Holocene due to enhanced primary and export production related to the monsoon. This is, to our knowledge, the first study that uses a reaction-transport model to constrain the cessation of fluid flow at pockmarks. The proposed method could be a significant tool in illuminating temporal evolution of fluid seepage in pockmarks because most pockmarks on continental margins are presently dormant, especially if a reliable agecontrol is lacking.