Gas hydrates and cold seeps
Gas hydrate deposit detection and instability
Chemosynthetic biological communities are evidence of the presence of reduced, chemical-rich fluids at the seafloor. When the fluids are generated at ambient temperatures, as opposed to high temperatures such as at hydrothermal vents, they are said to be “cold seeps”. Cold seeps have now been found in diverse locations, such as on canyon walls, on active continental margins, from limestone escarpments, and above hydrocarbon deposits.
Acoustic surveys also provide evidence of cold seeps, and of layers of gas hydrates (carbon-dioxide, methane, and other hydrocarbon gases frozen into an icy slush at high pressure and low temperature) within the sedimentary pile. Gas hydrates are large reservoirs of methane, which is a fossil fuel and green-house gas. Methane is also the compound required by methane oxidizing microbes whose by-product feeds the hydrogen-sulfide oxidizing microbial symbionts that feed the cold-seep biota. Instabilities of the gas hydrate reservoirs may be a consequence of slumping and earthquakes on a local scale, and of increasing ocean temperatures on a global scale.
Our research on gas hydrates and cold seeps
Distribution of chemosynthetic communities
MONTEREY BAY – We report the first quantitative evaluation of the distribution of seafloor chemosynthetic biological (cold seep) communities on a regional scale. The results are based on the analysis of video images and navigation from 792 benthic ROV dives conducted on the continental margin in Monterey Bay, California. These communities are common, occurring within 5% of the 25-m-square grid cells within which there have been bottom observations within 45 km of the bay’s head and within 9% of the visited cells that are below 550 m water depth. Although it has been previously assumed that these communities are associated with fluid seepage from faults, they are not more common within known fault zones. Surprisingly, the communities in Monterey Bay occur preferentially on steep slopes, which are commonly sites of recent erosion and sharpened geochemical gradients. We propose that anaerobic, methane-oxidizing Archaea will flourish in the subsurface under areas of recent erosion as sulfate from the overlying seawater diffuses into methane-bearing sediment. The hydrogen-sulfide they produce by anaerobic oxidation of methane will diffuse up toward the seafloor to support the chemoautotrophic bacterial mats, vesicomyid clams, and vestimentiferan worms observed in the video images.
Reference: C.K. Paull, B. Schlining, W. Ussler III, J.B. Paduan, D. Caress, and H.G. Greene (2005) Distribution of chemosynthetic biological communities in Monterey Bay, California, Geology 33(2): 85-88. [Abstract] [Article]
Slumping releases trapped gases
MONTEREY BAY – The scientific community is engaged in a lively debate over whether and how venting from the gas-hydrate reservoir and the Earth’s climate is connected. The various scenarios which have been proposed are based on the following assumptions: the inventory of methane gas-hydrate deposits is locally enormous, the stability of marine gas-hydrate deposits can easily be perturbed by temperature and pressure changes, enough methane can be released from these deposits to contribute adequate volumes of the isotopically distinct greenhouse gas to alter the composition of oceanic or atmospheric methane reservoirs, and the mechanisms exist for the transfer of methane from deeper geologic reservoirs to the ocean and/or atmosphere. However, some potential transfer mechanisms have been difficult to evaluate. Here, we consider the possibility of marine slumping as a mechanism to transfer methane carbon from gas hydrates within the seafloor into the ocean and atmosphere. Our analyses and field experiments indicate that large slumps could release volumetrically significant quantities of solid gas hydrates which would float upwards in the water column. Large pieces of gas hydrate would reach the upper layers of the ocean before decomposing, and some of the methane would be directly injected into the atmosphere.
Reference: C.K. Paull, P.G. Brewer, W. Ussler III, E.T. Peltzer, G. Rehder, D. Clague (2003) An experiment demonstrating that marine slumping is a mechanism to transfer methane from seafloor gas-hydrate deposits into the upper ocean and atmosphere, Geo-Marine Letters 22(4): 198-203. [Abstract] [Article]
Mapping of carbonate and hydrate outcrops
HYDRATE RIDGE – A new 30 kHz multibeam echo-sounder survey of part of the Oregon margin imaged Hydrate Ridge at a higher resolution than previous surveys conducted in the same area. The bathymetric data show that a northern ridge rises to 588 m and a southern ridge to 773 m. Moderate-to-high backscatter areas, interpreted to indicate patchy, discontinuous chemosynthetic communities, authigenic carbonate, or hydrate accumulations cover about 26 km2 of the summit area of the northern ridge. Similar moderate-to-high backscatter areas comprise about 4.9 km2 of the southern ridge. However, the southern ridge does have about 0.095 km2 of high-backscatter that is believed to be associated with nearly continuous carbonate and hydrate outcrop on the western edge of the summit. A site on the southern ridge where intercalated carbonate and hydrate deposits were previously recovered is within the region with only patchy outcrop.
Reference: D.A Clague, N. Maher, and C.K. Paull (2001) High-resolution multibeam survey of Hydrate Ridge, offshore Oregon, In: Natural Gas Hydrates: Occurrence, Distribution, and Detection, C.K. Paull and W.P. Dillon (eds), Geophysical Monograph 124, American Geophysical Union, 297-303.