The Peach 4 debrite is the most recent in a series of large scale Pleistocene MTDs within the Barra fan on the northwest British continental margin. Geophysical data indicate that Peach 4 was formed through a combination of blocky and muddy debris flows and affects an area of ~700 km 2 . BGS core sample 56 -10 36, located directly over the Peach 4 debrite, provides a minimum age of 14.68 ka cal BP for the last major failure. An upwards fining turbidite sequence in BGS core sample 56 -10 239 is associated with increased As and S concentrations, indicators of diagenetic pyrite which forms under anoxic conditions. It is proposed that As and S concentrations may provide a method of distinguishing between contourite and turbidite sedimentation, although further research is required. Keywords Submarine mass movement • submarine landslide • mass transport deposit • debrite • debris flow • turbidite • Barra fan • British ice sheet • arsenic • sulphur • pyrite
Regional Setting and Sedimentation on the Barra FanThe Peach 4 debrite is located on the northwest British continental margin within the Barra fan. Sedimentation on the fan is governed by a combination of down-slope and across-slope mechanisms. The primary across slope process is driven by contouritic
Naturally occurring gas hydrates have recently become one of the most interesting and rapidly expanding research topics in the geologic oceanographic and engineering sciences. These frozen mixtures of hydrocarbon gas (mostly methane) and water occur over vast areas of the ocean floor in special temperature and pressure regimes. They are important because they: (a) represent an enormous potential energy source, (b) have been implicated as drivers of global climate change by releasing large volumes of methane (a greenhouse gas) into the atmosphere, and (c) present potential engineering geology problems for man's operations in deep water and may be placed in the category of a geohazard. However, the identification of gas hydrates in the shallow subsurface from remotely sensed acoustic data is not straight-forward in complex geological provinces like the continental slope of the northern Gulf of Mexico and essentially no definitive data on seabed methane flux from dissociation of gas hydrates occurring at or near the seabed exists in the scientific literature.Repeated direct observations (manned submersible) of sites in the deepwater northern Gulf of Mexico where gas hydrates have been identified as exposures on the seafloor indicate that hydrates can form and decompose within the return observation period of 1year. A 1-year in situ data collection experiment reported on in this document was designed to produce data for developing a better understanding of surficial gas hydrate formation and decomposition cycles and their possible links to oceanographic forcing. Current meter data and records of gas emission from a site of exposed gas hydrate suggest a coupling of temperature excursions of < 2 C and pulses of gas emission (gas hydrate decomposition). Increases in water tmeperature are strongly coherent with north-south pulses in the current field. Higher frequency cycles of gas emission also occur at the tidal time scale. However, these small variations are superimposed on longer-term events of 1-2 weeks. These in situ collected data indicate that intrusions of Loop Current water are not needed to force dissociation of surface and near-surface gas hydrates. Surface sediments associated with areas characterized by exposures of gas hydrate have extremely variable geotechnical properties. Much of this variability relates to shell content and products of early diagenesis.
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