Recently, Dr. Wang Jiliang from the Department of Deep-sea Geophysics and Resources, in collaboration with the Qingdao Institute of Marine Geology of the China Geological Survey, the Norwegian Institute for Energy Technology, and the CNOOC Research Institute, published a research paper titled "Submarine fluid flow system feeding methane emission in the northern South China Sea" in the journal Basin Research. The study investigates the structural characteristics and developmental evolution of the submarine fluid system that supports methane emission in the Qiongdongnan Basin, revealing the control of submarine landslides on methane emissions.

Methane emission is an important way for material cycling and energy exchange across the Earth's spheres, supporting unique chemosynthetic ecosystems. The methane released from seafloor can lead to ocean acidification and even affect global climate change. Recent surveys indicate that methane emission is widespread in deep-sea environments (water depth > 500 m). Compared to shallow seas, deep-sea methane in the strata first passes through the hydrate stability zone before reaching the seafloor. Traditionally, it was thought that methane forms hydrates within the stability zone, which reduces the permeability of the strata and hinders the vertical migration of methane. However, there is still insufficient understanding of the processes and mechanisms by which geologic methane crosses the hydrate stability zone.

To address this issue, this study focuses on methane emissions developed on the Songnan Low Uplift in the Qiongdongnan Basin. Based on three-dimensional seismic and logging data, it was found that the fluid flow system supporting methane emissions consists of two parts: an upper and a lower section. Below the hydrate stability zone in the study area, three large gas clouds were developed, which contain faults. Within the hydrate stability zone above the gas clouds, 28 fluid pipes were identified penetrating through three vertically stacked landslide deposits to reach the seafloor. The study suggests that the faults within gas clouds are vertically connected to pipes, together forming a submarine fluid flow system that supports methane emissions.

Figure 1. Fluid flow systems supporting methane emissions in the deep water area of the Qiongdongnan Basin.

The study found that in the hydrate stability zone, it is the low-permeability slope deposits rather than the hydrate layers that seal the underlying free gas. Three consecutive slope failures occurring on the seafloor have caused the hydrate stability zone to migrate rapidly upward, leading to the dissociation of hydrates near the original base of the gas hydrate stability zone, while large amounts of hydrates cannot be generated in a short time above the new base to form effective seals. Free gas enters the hydrate stability zone and migrates upward, accumulating at the bottom of low-permeability slope deposits, building-up overpressure. When this pressure reaches a critical threshold, hydraulic fracturing occurs, forming fluid pathways and resulting in methane leakage. The existence of multiple slope failures suggests that this process is repetitive and may lead to intermittent activity of cold seeps. The study demonstrates that slope failures play a controlling role in seabed cold seep activity, with low-permeability slope deposits acting like "valves" that regulate the release of methane from the seafloor.

Figure 2. Schematic model showing submarine landslides regulates methane emissions

The first author and corresponding author of the paper is Dr. Jiliang Wang from the Institute of Deep-Sea Science and Engineering, Chinese Academy of Sciences, and Dr. Ang Li  from the Qingdao Institute of Marine Geology, China Geological Survey, is a co-corresponding author. This research was jointly funded by the Youth Innovation Promotion Association of the Chinese Academy of Sciences, the Hainan Province High-Level Talent Project, the National Natural Science Foundation, and the Laoshan Laboratory Project.