Controls on shallow gas distribution, migration, and associated geohazards in the Yangtze subaqueous delta and the Hangzhou Bay

Abstract

Shallow gas is generally extensively distributed in the Holocene muddy sediments and gas seepage has been increasingly reported to induce geohazards in coastal seas, but controls on gas distribution and migration remain elusive. This study explores gas distribution and migration in the Yangtze subaqueous delta and the Hangzhou Bay using high-resolution acoustic profiles and core data. Shallow gas is widely detected by the common presence of acoustic anomalous reflections including enhanced reflection, gas chimney, bright spot, acoustic blanking, and acoustic turbidity. The gas front depth is generally less than 17.5 m, and is meanly shallower in the Hangzhou Bay than in the Yangtze subaqueous delta because of relatively shallower water depth and coarser Holocene sediments in the Hangzhou Bay. Shallow gas is inferred to be a biogenic product, and its distribution is highly contingent on the Holocene stratal thickness and water depth. Active gas migration and seepages are evident, and recently increasing occurrences of gas seepage can be ascribed to global warming and seabed erosion due to sediment deficit. The findings warn us to pay more attention to the positive feedback loops of gas seepages with global warming and seabed erosion for the associated geohazard prediction and reduction, typically in the highly developed coastal regions.

Response of early diagenesis to methane leakage in the inner shelf of the East China Sea

“…The Yangtze River Delta (YRD) and surrounding areas are characterized by widespread occurrences of shallow gas within the muddy sediments (Hu et al, 2016;Qiu et al, 2018). Typically buried within a few meters to tens of meters, with most deposits found at water depths less than 100 meters (Li et al, 2010), these shallow gas accumulations are particularly concentrated in Hangzhou Bay, as evidenced by acoustic blanking in seismic profiles (Song et al, 2023). This coincides with a shoreward increase in methane concentrations observed in ECS shelf waters (Ye et al, 2016;Sun et al, 2018).…”

“…A shallower gas front allows for a greater upward diffusive flux of methane, ultimately causing an upward shift of the SMTZ (Mogolloń et al, 2013;Flury et al, 2016). Studies have shown that the gas front depth in Hangzhou Bay is shallower than that in the Yangtze subaqueous delta, with both reaching depths no greater than 17.5 m (Song et al, 2023). In some locations, the cap overlying shallow gas is very thin, and the presence of pockmarks on the seabed further suggests active gas seepage.…”

“…Due to sediment deficit, seafloor erosion has been widely reported in the Yangtze subaqueous delta (Yang et al, 2011;Guo et al, 2021), and has recently extended into Hangzhou Bay (Xie et al, 2013(Xie et al, , 2017. The seafloor erosion has been accused of increasing gas leakage in the Yangtze subaqueous delta (Chen et al, 2020) and Hangzhou Bay (Song et al, 2023). They further pointed out the mutual promotion effect between seafloor erosion and gas leakage at local scales (Chen et al, 2020;Song et al, 2023).…”

“…The seafloor erosion has been accused of increasing gas leakage in the Yangtze subaqueous delta (Chen et al, 2020) and Hangzhou Bay (Song et al, 2023). They further pointed out the mutual promotion effect between seafloor erosion and gas leakage at local scales (Chen et al, 2020;Song et al, 2023).…”

“…Massive methane leakage can trigger submarine landslides, and the released methane contributes to global warming, ocean acidification, and even potential mass extinctions. These chain actions are still less known, deserving more attention due to their adverse environmental impacts (Reeburgh, 2007;Rhee et al, 2009;Letcher, 2019;Chen et al, 2020;Song et al, 2023).…”