Methane hydrate at DSDP Site 84-570

Deep Sea Drilling Project (DSDP) studies at Site 570 on the landward slope of the Middle America Trench off Guatemala allow for the first time a quantitative estimate of the methane hydrate content in the massive mudstones deposited there. Drilling across the Guatemalan transect on DSDP Legs 67 and 84 has resulted in the greatest number of visual observations of gas hydrate in any marine area. At Site 570, a 1.5-m-long section of massive methane hydrate was unexpectedly cored in an area where none of the usual signs of gas hydrate in seismic records were present. The sediment section is similar to that recovered at the other eight sites off Guatemala, but drilling at Site 570 may have penetrated through a fault zone that provided the space for accumulation of massive gas hydrate. The methane hydrate was analyzed using the following well logs: density, sonic, resistivity, gamma-ray, caliper, neutron porosity, and temperature. The density, sonic, and resistivity logs define a 15-m-thick hydrated zone within which a 4-m-thick nearly pure hydrate section is contained. The methane gas content ranges from 240 m**3 to 1400 m**3 per m**2 of lateral extent; and if the body extends a square kilometer, its total volume of stored gas could be from 240*10**6m**3 to 1400*10**6m**3. Because the acoustic impedance of hydrate calculated from the sonic and density logs shows no anomalous values, the shape and extent of the hydrate body cannot be defined in seismic records. Thus the body is theoretically nonreflective in contrast to the base of the hydrate reflection. The base of the gas hydrate reflection is presumed to be the result of the velocity contrast between sediment containing gas hydrate and sediment containing free gas.

Isotopic composition of interstitial fluids and origin of methane at DSDP Leg 84 Holes

CH4 and CO2 species in pore fluids from slope sediments off Guatemala show extreme 13C-enrichment (d13C of -41 and +38 per mil, respectively) compared with the typical degree of 13C-enrichment in pore fluids of DSDP sediments (d13C of - 60 and + 10 per mil). These unusual isotopic compositions are believed to result from microbial decomposition of organic matter, and possibly from additional isotopic fractionation associated with the formation of gas hydrates. In addition to the isotopic fractionation displayed by CH4 and CO2, the pore water exhibits a systematic increase in d18O with decrease in chlorinity. As against seawater d18O values of 0 and chlorinity of 19 per mil, the water collected from decomposed gas hydrate from Hole 570 had a d18O of + 3.0 per mil and chlorinity of 9.5 per mil. The isotopic compositions of pore-fluid constituents change gradually with depth in Hole 568 and discontinuously with depth in Hole 570.

Geotechnical properties of sediments at DSDP Leg 84 Holes

The geotechnical characteristics of 22 sediment samples from Leg 84 sites were studied in an effort to associate these with processes active along the Middle America slope and with sedimentation mechanisms. Geotechnical properties measured include water content, porosity, bulk density, Atterberg limits, consolidation characteristics, permeability, and vane shear strength. A majority of samples obtained from Sites 565, 568, and 570 show significant disturbance resulting from degassing. This disturbance apparently results in underconsolidation, although other mechanisms such as excess pore pressures generated from the subduction process can also contribute to this state. Overconsolidated sediments were found at Sites 565, 566, and 569. The overconsolidated sediments at Sites 565 and 569 may result from downslope transport mechanisms rearranging and stressing the sediment mass under consideration. The sediment condition at Site 566 is probably a result of eroded overburden: an estimated 87 m of overlying sediments may have been removed. Geotechnical and permeability relationships with depth are consistent with those found for other hemipelagic sediments of silty clay to clayey silt textures.

Mineralogy and diagenesis of slope sediments at DSDP Leg 84 holes

The distribution and composition of minerals in the silt and clay fraction of the fine-grained slope sediments were examined. Special interest was focused on diagenesis. The results are listed as follows. (1) Smectite, andesitic Plagioclase, quartz, and low-Mg calcite are the main mineral components of the sediment. Authigenic dolomite was observed in the weathering zones of serpentinites, together with aragonite, as well as in clayey silt. (2) The mineralogy and geochemistry of the sediments is analogous to that of the andesitic rocks of Costa Rica and Guatemala. (3) Unstable components like volcanic glass, amphiboles, and pyroxenes show increasing etching with depth. (4) The diagenetic alteration of opal-A skeletons from etching pits and replacement by opal-CT to replacement by chalcedony as a final stage corresponds to the typical opal diagenesis. (5) Clinoptilolite is the stable zeolite mineral according to mineral stability fields; its neoformation is well documented. (6) The early diagenesis of smectites is shown by an increase of crystallinity with depth. Only the smectites in the oldest sediments (Oligocene and early Eocene) contain nonexpanding illite layers.