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Integrated Ocean Drilling Program (IODP) Expedition 311 has been designed to further constrain models for the formation of marine gas hydrate in subduction zone accretionary prisms. The objectives include characterizing the deep origin of the methane, its upward transport, its incorporation in gas hydrate, and its subsequent loss to the seafloor. The main attention of this expedition is on the widespread seafloor-parallel layer of dispersed gas hydrate located just above the base of the predicted stability field. In a gas hydrate formation model, methane is carried upward through regional sediment or small-scale fracture permeability, driven by the tectonic consolidation of the accretionary prism. The upward-moving methane is incorporated into the gas hydrate clathrate as it enters the methane hydrate stability zone. Also important is the focusing of a portion of the upward methane flux into localized plumes or channels to form concentrations of near-seafloor gas hydrate. The amount of gas hydrate in local concentrations near the seafloor is especially important for understanding the response of marine gas hydrate to climate change. The expedition includes coring and downhole measurements at a transect of five sites across the Northern Cascadia accretionary prism. The sites will track the history of methane in an accretionary prism from (1) its production by mainly microbiological processes over a thick sediment vertical extent, (2) its upward transport through regional or locally focused fluid flow, (3) its incorporation in the regional hydrate layer above the bottom-simulating reflector (BSR) or in local concentrations at or near the seafloor, (4) methane loss from the hydrate by upward diffusion, and (5) methane oxidation and incorporation in seafloor carbonate, or expulsion to the ocean.
This expedition builds on the previous Cascadia margin gas hydrate drilling of Ocean Drilling Program (ODP) Leg 146 and more recent ODP Leg 204 off Oregon. Important experiments for this proposal include (1) logging-while-drilling, (2) wireline logging, (3) intensive coring and subsampling, and (4) pressure core sampling (PCS/HYACINTH) of gas hydrate and fluid recovery under in situ conditions. For this expedition, we plan to carry out logging-while-drilling/measuring-while-drilling operations prior to coring operations.