Shallow gas in Holocene sediments of the Pearl River Estuary and the implication for anthropogenic effects on its release

High-resolution seismic and side-scan surveys were conducted to investigate shallow gas accumulation and seepage in the Holocene sediment of the Pearl River Estuary. Extensive shallow gas accumulation and active seepage were detected in the Holocene sediments of the estuary and adjacent shelf regions. Laterally continuous and extremely shallow gas fronts were observed in areas with relatively higher sedimentation rates, whereas shallow gas was absent or discontinuous in regions with the sediment with lower sedimentation rates. Gas accumulation was present at extremely shallow depths in the West Shoal, which is the recent depocentre location. The co-occurrence of shallow gas distributions and variations in the sedimentation rate was related to the role of sediment accumulation in organic carbon burial and preservation, both of which are required for gas generation and accumulation in sediment. In addition, the consistency between the distribution of high water column methane concentrations in the West Shoal, the super shallow gas accumulation pattern in the sediment profile, and active seepage indicate that shallow gas in the Holocene sediment is a methane source in the Pearl River Estuary. Further, considerable evidence of recent or relic seepage is present in areas that have experienced active anthropogenic disturbances, including sand mining, dredging, and bridge building. The disrupted and discontinuous gas fronts at these sites suggest that active shallow gas releases were influenced by anthropogenic activity.