气候变化领域集成服务门户

In 2014, the discovery of seafloor mounds leaking methane gas into the water column in the northwestern Barents Sea became the first to document the existence of nonpermafrost-related gas hydrate pingos (GHPs) on the Eurasian Arctic shelf. The discovered site is given attention because the gas hydrates occur close to the upper limit of the gas hydrate stability, thus may be vulnerable to climatic forcing. In addition, this site lies on the regional Hornsund Fault Zone marking a transition between the oceanic and continental crust. The Hornsund Fault Zone is known to coincide with an extensive seafloor gas seepage area; however, until now lack of seismic data prevented connecting deep structural elements to shallow seepages. Here we use high-resolution P-Cable 3-D seismic data to study the subsurface architecture of GHPs and underlying glacial and preglacial deposits. The data show gas hydrates, authigenic carbonates, and free gas within the GHPs on top of gas chimneys piercing a thin section of low-permeability glacial sediments. The chimneys connect to faults within the underlying tilted and folded fluid and gas-hydrate-bearing sedimentary rocks. Correlation of our data with regional 2-D seismic surveys shows a spatial connection between the shallow subsurface fluid flow system and the deep-seated regional fault zone. We suggest that fault-controlled Paleocene hydrocarbon reservoirs inject methane into the low-permeability glacial deposits and near-seabed sediments, forming the GHPs. This conceptual model explains the existence of climate-sensitive gas hydrate inventories and extensive seabed methane release observed along the Svalbard-Barents Sea margin.

Plain Language Summary Gas hydrates (concentrated hydrocarbon gases in cages of ice) are stable within high pressure and low temperature. At boundary conditions, minor increases in ocean temperature may trigger gas hydrate decay and the possibility that gas hydrates may dissociate due to future warming causes particular awareness. We present observations of a hydrate system expressed as 450-m wide and 10-m high gas hydrate pingos (seabed mounds bearing gas hydrates). The geological conditions controlling the formation of these shallow gas hydrate accumulations have not been previously investigated. Along a 700-km region that coincides with a regional fault system, the Hornsund Fault Zone, more than 1,200 seeps releasing gas from the seafloor have been observed. Linkage of this fault zone to the methane hotspots has been hypothesized but never supported by empirical data. Combining new and published data, we postulate the major preconditions for GHP development: geologically constrained focused release of methane from 55-65 million-year-old rocks, modern gas hydrate stability conditions, and a drape of muddy bottom sediments favorable for heaving due to hydrate growth. We observe a clear relationship between this methane system and the regional fault system, which potentially demonstrates a typical scenario of fault-controlled methane migration across the Svalbard-Barents Sea margin.