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.

Methane and hydrogen sulfide in porewater of sediment cores from the upwelling area off Namibia

Porewater concentrations of sulfate, methane, and other relevant constituents were determined on four sediment cores from the high productivity upwelling area off Namibia which were recovered from the continental slope at water depths of 1300 and 2000 m. At all four stations a distinct sulfate-methane transition zone was observed several meters below the seafloor in which both sulfate and methane are consumed. Nutrient porewater concentration profiles do not show gradient slope changes at the depths of the transition zones. Flux calculations carried out on the basis of the determined porewater profiles revealed that anaerobic methane oxidation accounts for 100% of deep sulfate reduction within the sulfate-methane transition zone and consumes the total net diffusive sulfate flux. A significant contribution of organic carbon oxidation to the reduction of sulfate at these depths could, therefore, be excluded. We state that porewater profiles of sulfate with constant gradients above the transition zones are indicative for anaerobic methane oxidation controlling sulfate reduction.

Bitumoids and methane in bottom sediments of the northeast Black Sea

Complex investigations of recent and ancient Black Sea sediments from the outer shelf, continental slope, and deep-water basin of the Russian Black Sea sector have been carried out. Samples were collected during Cruise 100 of R/V Professor Shtokman organized by the P.P. Shirshov Institute of Oceanology (March 2009) and expedition of UZHMORGEO (summer 2006). Rates of the main anaerobic processes during diagenesis (sulfate reduction, dark CO2 assimilation, methanogenesis, and methane oxidation) were studied for the first time in sediment cores of the studied area. Two peaks in the rate of microbial processes and two sources of these processes were identified: the upper peak near the water-sediment contact is related to solar energy (OM substrate of the water column) and the lower peak at the base of ancient Black Sea sediments with high(>1 mmol) methane concentration related to energy of anaerobic methane oxidation. New labile OM formed during this process is utilized by other groups of microorganisms. According to experimental data, daily rate of anaerobic methane oxidation is many times higher than that of methanogenesis, which unambiguously indicates migration nature of the main part of methane.