(Table 1) Crystal sizes of measured samples from the Bush Hill region in the Gulf of Mexico and from ODP Leg 204 at the Hydrate Ridge offshore Oregon

The grain sizes of gas hydrate crystallites are largely unknown in natural samples. Single grains are hardly detectable with electron or optical microscopy. For the first time, we have used high-energy synchrotron diffraction to determine grain sizes of six natural gas hydrates retrieved from the Bush Hill region in the Gulf of Mexico and from ODP Leg 204 at the Hydrate Ridge offshore Oregon from varying depth between 1 and 101 metres below seafloor. High-energy synchrotron radiation provides high photon fluxes as well as high penetration depth and thus allows for investigation of bulk sediment samples. Gas hydrate grain sizes were measured at the Beam Line BW 5 at the HASYLAB/Hamburg. A 'moving area detector method', originally developed for material science applications, was used to obtain both spatial and orientation information about gas hydrate grains within the sample. The gas hydrate crystal sizes appeared to be (log-)normally distributed in the natural samples. All mean grain sizes lay in the range from 300 to 600 µm with a tendency for bigger grains to occur in greater depth. Laboratory-produced methane hydrate, aged for 3 weeks, showed half a log-normal curve with a mean grain size value of c. 40 µm. The grains appeared to be globular shaped.

Dyfamed-benthos time series observations

In 1990, a benthic component to the DYFAMED (dynamics of fluxes in the Mediterranean) program, the DYFAMED-BENTHOS survey, was established to investigate the possible coupling of benthic to pelagic processes at a permanent station in >2700 m water depth, 52 km off Nice, France. Surface sediment was first sampled at different periods of the year to assess the importance of the biological compartment (particularly metazoan meiofauna) and its relation to seasonally varying particulate matter input to the sea floor (estimated by measuring surface sediment particle size and porosity, as well as chloroplastic pigments, organic carbon, nitrogen and calcium carbonate contents). Beginning in 1993, surface sediment was sampled at an average interval of 1.4 months for over five consecutive years using multicorers. Biogeochemical techniques such as deployments of a free-vehicle benthic respirometer and a near-bottom sediment trap, along with analyses of sediment vertical profiles for dissolved oxygen, nutrients and dissolved metals in the porewater, were developed in conjunction with more extensive biological analyses to characterize the recycling of organic matter, and ultimately increase our understanding of the oceanic carbon cycle. This article provides the scientific background and motivation for the development of the on-going DYFAMED-BENTHOS survey, the general characteristics of the benthic site, as well as a detailed description of the sampling design applied from late 1990-2000.