| Shallow gas generally refers to the gas in the strata within 1000 m of the seafloor,and its main component,methane,is an important greenhouse gas.The presence of shallow gas has an important impact on the marine sedimentary environment,marine engineering and climate environment.Estuaries and shelf areas are the main areas of shallow gas distribution.The Pearl River estuary,as the mouth of a large river,has a large amount of shallow gas in the nearshore sediments,but there are few related studies,and the distribution,burial depth,and seepage activities of shallow gas in the Pearl River estuary are not well understood.In recent years,due to the development of the Pearl River Basin and the construction of the Guangdong-Hong Kong-Macao Greater Bay Area,human activities have had a profound impact on the underwater geomorphology of the Pearl River Estuary.Therefore,two scientific questions are raised in this paper:(1)What are the characteristics of shallow gas distribution and seepage in the Pearl River Estuary?(2)Have human activities influenced the distribution and seepage of shallow gas?In this paper,the spatial distribution characteristics of shallow gas accumulation and seepage in the Pearl River Estuary are investigated by using high-resolution sub-bottom profiles and side-scan sonar image data in conjunction with radiocarbon dating data,sediment pore water and water chemistry parameters,and the influence of human activities.The following insights were obtained:(1)The reflection characteristics of shallow gas on shallow stratigraphic profiles are mainly acoustic gaps and turbidity spikes.By analyzing the sub-bottom profiles and side-scan sonar images,it is found that a large amount of shallow gas exists in the Holocene sediments of the Pearl River estuary and the adjacent inner shelf area,especially in the West Shoal of Lingdingyang and Maodaomen,where the shallow gas is widely distributed,continuous and very shallowly buried.(2)The top interface of shallow gas mainly spreads along the stratigraphic layer,but in some areas shallow gas has penetration.The shallow gas is distributed in different stages of the Holocene strata,indicating that the genesis of the shallow gas may be related to the early to late Holocene sediments.The high organic matter content in the Holocene sediments provides a possibility for the generation of shallow gas.(3)The generation and distribution of shallow gas are closely related to the sedimentation rate.In the high sedimentation rate zone,shallow gas is buried very shallow and continuous,while in the low sedimentation rate zone,the distribution of shallow gas is discontinuous and shallow gas does not exist in some areas.The high sedimentation rate area not only provides abundant carbon source for shallow gas generation,but also provides favorable reduction environment for shallow gas generation and preservation.(4)The shallow gas migration and seepage in the Pearl River estuary are active.The transport and seepage features mainly include gas chimneys,acoustic plumes,seabed domes and pockmarks.The gas chimneys in the sediments and the acoustic plume released into the water column are more spatially coincident,indicating that the shallow gas release activities in the Pearl River Estuary are ongoing.There is a correlation between the distribution of shallow gas burial depth and the distribution of methane concentration in the water column,indicating that shallow gas in the sediments may be the main source of high methane concentration in the estuarine water column.(5)By analyzing the sub-bottom profiles in the anthropogenic areas,we found that a large number of shallow gas seepage features are related to the disturbance of sediments by human activities,such as sand mining,channel dredging and bridge construction.The absence of shallow gas or the discontinuous state of the gas front in these areas suggest that human activities in the Pearl River estuary could lead to the release of shallow gas from Holocene sediments.Preliminary estimates of sand mining in the Pearl River Estuary suggest that approximately 4.8 t of pore water methane and at least 4×10~5 m~3 of free methane were released into the water column and atmosphere.Due to the abundant input of organic matter from terrestrial sources and active primary productivity,shallow gas is often widely distributed in the sediments of many large river deltas and estuaries.In addition,the release of methane from shallow areas will be absorbed by the water column in much smaller amounts compared to the deep ocean,and most of it can be directed to the atmosphere.Therefore,methane release in shallow water environment is of more concern.With the enhancement of human activities,the seepage pattern and seepage mechanism of shallow gas need continuous attention and further research. |
