Response of Shallow Gas‐Charged Holocene Deposits in the Yangtze Delta to Meter‐Scale Erosion Induced by Diminished Sediment Supply: Increasing Greenhouse Gas Emissions

Abstract

AbstractRiver deltas have long been considered important carbon sinks. However, the presence of shallow gas and the processes of delta erosion caused by diminished sediment supplies could reduce the strength of the carbon sink. In this study, based on historical bathymetric data and a data set obtained from a seismic survey, we investigate the response of gas‐charged deposits in the Yangtze subaqueous delta to seabed erosion. A conservative estimate of the total methane reserves of 0.55–4.35 × 1011 mol was obtained in the delineated gassy area of ∼3,800 km2. The seismic and bathymetric data reveal a prominent erosional belt at water depths ranging from 5 to 20 m and extending from the southwestern to northeastern nearshore areas of the Yangtze subaqueous delta. Erosion is severe in the south and slight in the north due to differences in the hydrodynamic conditions, sediment erodibility, and sensitivity to sediment reduction. Seabed erosion reduces the thickness of the cap bed and the overburden pressure at the gas front, making it easier for gas to seep through the sediment column and bypass the anaerobic oxidation of methane. The good agreement between the elevated methane concentrations and the erosional belts and the spatial coincidence between the shallower gas front and pockmarks indicate that seabed erosion accelerates gas seepage activities. In the context of global deltaic degradation, the increasing greenhouse gas emissions from deltaic deposits are worthy of further attention.