Figure 8.4: A massive hydrate layer (left) obtained from fine-grained sediment recovered in a marine setting in the Gulf of Mexico, compared to (right) hydrate recovered in a coarse-grained gravel layer from an Arctic setting, the Mallik 2L-38 well, drilled in the Northwest Territories of northern Canada.


Barkley Canyon drops down the continental slope toward the abyss. Canyons are typically sites of underwater landslides as well as movement of sediment, organic material and nutrients to the deep ocean. Te upwelling of nutrients driving the region’s rich biodiversity makes the upper slope a crucial study area for scientific and environmental policy purposes. It also makes it a popular region for commercial fishing. One November day in 2000 the fishing vessel Ocean Selector


is trawling over the Barkley Canyon in an area of 800m water depth. (Trawls are fishing nets that are pulled along the bottom of the sea or in midwater at a specified depth.) When the captain pulls up the net, the crew see the most unusual catch: over a ton of strange white solid that fizzes and crackles on the deck. Tey shovel it overboard, fortunately without harm. Te crew has by accident discovered a large seafloor deposit of methane hydrate. If one of the seamen had lit his smoke it could have resulted in a disaster as the crackling was actually the release of significant amounts of methane gas. Methane is a fuel, and


despite being trapped in an ‘ice’, it will readily burn. For this reason, methane hydrates are known as burning ice. Tat day in November


introduced the scientific community to a new, massive seafloor outcrop of gas hydrate on a 500m-wide, 1 km-long plateau perched 150m above the canyon floor. In 2004, Canadian scientists explored and verified the site, and


262


the ROV revealed pinnacles cutting up through the seafloor. Massive outcrops are exposed in 1–2m high mounds covered by a thin veneer of sediment. A light yellow condensate fluid is present in the surrounding sediment and associated with the hydrate, causing yellow staining. With buoyant hydrate slabs on the seafloor reaching 7m in length and 3m in height and with only a thin sediment cover, the hydrate mass must continue deeper and be anchored below the surface. Although methane hydrate is known to be stable at the sea


floor for water depths greater than 500–600m at temperate latitudes, observed outcrops of hydrate at the sea floor are rare and somewhat poorly understood.


Figure 8.5: Methane gas hydrate stability zone (GHSZ) for permafrost (a) and marine (b) settings. The pressure-temperature phase boundary (idealised) is shown as a green curve, and the local thermal (assumed) gradients are in red. In permafrost, the GHSZ can begin at 100–300m depth and extend for hundreds of metres beneath the base of the permafrost (typically occurring at 150–600m depth). In marine sediments, the GHSZ typically begins below 300–600m and extends for hundreds of metres. The thickness of the marine GHSZ depends on sea floor water temperature (typically 3–4°C), salinity, geothermal gradient and depth.


Bill Winters, USGS


Courtesy SEG


Bill Winters, USGS


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