Description: Two newly developed coring devices, the Multi-Autoclave-Corer and the Dynamic Autoclave Piston Corer were deployed in shallow gas hydrate-bearing sediments in the northern Gulf of Mexico during research cruise SO174 (Oct-Nov 2003). For the first time, they enable the retrieval of near-surface sediment cores under ambient pressure. This enables the determination of in situ methane concentrations and amounts of gas hydrate in sediment depths where bottom water temperature and pressure changes most strongly influence gas/hydrate relationships. At seep sites of GC185 (Bush Hill) and the newly discovered sites at GC415, we determined the volume of low-weight hydrocarbons (C1 through C5) from nine pressurized cores via controlled degassing. The resulting in situ methane concentrations vary by two orders of magnitudes between 0.031 and 0.985 mol kg**-1 pore water below the zone of sulfate depletion. This includes dissolved, free, and hydrate-bound CH4. Combined with results from conventional cores, this establishes a variability of methane concentrations in close proximity to seep sites of five orders of magnitude. In total four out of nine pressure cores had CH4 concentrations above equilibrium with gas hydrates. Two of them contain gas hydrate volumes of 15% (GC185) and 18% (GC415) of pore space. The measurements prove that the highest methane concentrations are not necessarily related to the highest advection rates. Brine advection inhibits gas hydrate stability a few centimeters below the sediment surface at the depth of anaerobic oxidation of methane and thus inhibits the storage of enhanced methane volumes. Here, computerized tomography (CT) of the pressure cores detected small amounts of free gas. This finding has major implications for methane distribution, possible consumption, and escape into the bottom water in fluid flow systems related to halokinesis.

Description: Drilling on Hydrate Ridge, offshore Oregon, during ODP Leg 204 enabled us to investigate fabrics of gas hydrate samples in a wide depth range of the gas hydrate stability zone (GHSZ). X-ray computerized tomographic imaging on whole-round samples, frozen in liquid nitrogen, revealed that layered gas hydrate structures are related to variable processes occurring at different sediment depths. Shallow gas hydrates often form layers parallel or sub-parallel to bedding and also crosscut sedimentary strata and other gas hydrate layers, destroying the original depositional fabric. The dynamic processes interacting with this complicated plumbing system in this shallow environment are responsible for such highly variable gas hydrate fabrics. Gas hydrate layers deeper in the sediments are most often dipping with various angles, and are interpreted as gas hydrate precipitates filling tectonic fractures. These originally open fractures are potential candidates for free gas transportation, and might explain why free gas can rapidly emanate from below the bottom-simulating reflector through the GHSZ to the seafloor.