ALBERT

Beschreibung: The isotopic compositions of interstitial waters collected from Hydrate Ridge during Ocean Drilling Program Leg 204 were measured to evaluate the fluid evolution of this accretionary prism. At shallow depths, the dissolved Cl- concentrations and dD and d18O values of the interstitial water reflect changes in the salinity and the isotopic compositions of seawater from the Last Glacial Maximum to the present. The presence of disseminated gas hydrates, which is well identified by discrete low Cl- anomalies within the gas hydrate stability zone, is accompanied by high dD and d18O values of the freshened fluids. This is consistent with incorporation of heavy isotopes into the gas hydrate lattice, which is also apparent in the signals observed at the ridge summit. Here, massive gas hydrate formation in the upper 20 meters below seafloor leads the formation of brines with dissolved Cl- concentrations as high as 1400 mM. The interstitial waters sampled near massive gas hydrates at the ridge summit are extremely depleted in D and 18O. Clay mineral dehydration within the deep prism results in a progressive decrease in Cl- and dD with depth. Dehydration temperature estimates based on those data likely suggest a progressive increase in the temperature of isotopic fractionation between clay and water with distance from the prism toe. The oxygen isotope data probably reflect the combined effects of clay dehydration, carbonate precipitation, and alteration of oceanic basement; however, there are not enough data to constrain the relative contribution of these processes to the observed signals.

Beschreibung: We report dissolved sulfide sulfur concentrations and the sulfur isotopic composition of dissolved sulfate and sulfide in pore waters from sediments collected during Ocean Drilling Program Leg 204. Porewater sulfate is depleted rapidly as the depth to the sulfate/methane interface (SMI) occurs between 4.5 and 11 meters below seafloor at flank and basin locations. Dissolved sulfide concentration reaches values as high as 11.3 mM in Hole 1251E. Otherwise, peak sulfide concentrations lie between 3.2 and 6.1 mM and occur immediately above the SMI. The sulfur isotopic composition of interstitial sulfate generally becomes enriched in 34S with increasing sediment depth. Peak d34S-SO4 values occur just above the SMI and reach up to 53.1 per mil Vienna Canyon Diablo Troilite (VCDT) in Hole 1247B. d34S-Sigma HS values generally parallel the trend of d34S-SO4 values but are more depleted in 34S relative to sulfate, with values from -12.7 per mil to 19.3 per mil VCDT. Curvilinear sulfate profiles and carbon isotopic composition of total dissolved carbon dioxide at flank and basin sites strongly suggest that sulfate depletion is controlled by oxidation of sedimentary organic matter, despite the presence of methane gas hydrates in underlying sediments. Preliminary data from sulfur species are consistent with this interpretation for Leg 204 sediments at sites not located on or near the crest of Hydrate Ridge.

Beschreibung: The Blake Ridge region lies on the passive margin of southeastern North America and contains a large amount of methane gas hydrate. The methane and methane gas hydrate are predominantly biogenic, apparently produced by CO2 reduction. Reflection seismics indicate that bottom-simulating reflectors (BSRs) enclose ~55,000 sq. km, with high-amplitude BSRs covering ~26,000 sq. km. Ocean Drilling Program (ODP) Leg 164 drilled three deep holes on a 10-km-long transect (Sites 994, 995, and 997; water depth 2770-2798 m). Based on sampling and geochemical, thermal, seismic, and borehole geophysical measurements, gas hydrates are most likely present between ~190 and 450 m in sediment column. Gas hydrate is most often disseminated throughout the sediment column, although concentrations occur within specific sedimentary horizons, within supposed fault zones, and at the base of gas hydrate stability (BGHS) where methane recycling produces more pervasive concentrations of gas hydrate. Estimates of gas hydrate inventory are based on a variety of methods including geochemical proxies, vertical seismic profiling, electric logging, and measurements of in situ methane. Over the entire sediment column, at least ~2-4% of pore space volume (1-2% sediment volume) is occupied by methane gas hydrates, but average and maximum estimates are 5.4% and 12%, respectively. Extrapolation of vertical gas hydrate and methane inventory over the area containing high-amplitude BSRs yields estimates of 67-406 Gt (gigatons, 10**15 g) of methane gas hydrate (or 9-52 Gt of methane) and 2.6-27 Gt of methane occurring as gas bubbles below the BGHS. Average values are 185 Gt of gas hydrate and 24 Gt of methane as gas hydrate. Any gas hydrate occurring outside the area underlain by BSRs (as suggested by geochemical evidence) or that associated with low-amplitude BSRs may increase these estimates by an unknown factor. Various data give conflicting pictures of mass transport with Blake Ridge sediments. The data can be reconciled by viewing the upper sedimentary section (〈~150 m) as dominated by diffusion, and the lower section characterized by buoyant advection (migration) of gaseous methane with both modes of transport overprinting generally low rates of pore-fluid movement (~20 cm/ky). Methane migration seems necessary to produce observed gas hydrate distribution and inventory estimates. Accumulation of gas hydrate in the Blake Ridge sediments depends on the amount of methane leaving the system versus the amount of methane entering the gas hydrate stability zone (GHSZ) over geologic time. Although there are some point sources of methane loss from the sediments (e.g., seafloor seeps, ODP Site 996) of unknown magnitude, most of the documented methane loss occurs through diffusion and consumption at the sulfate-methane interface (SMI) by anaerobic methane oxidation (AMO; ~2?10**8 mol/year). Methane entering the GHSZ at a rate of ~1.3?10**9 mol/year indicates a methane-trapping efficiency of ~85%. 129I measurements suggest that the Blake Ridge system has accumulated gas hydrate over as much as 55 million years.

Beschreibung: Sediments at the southern summit of Hydrate Ridge display two distinct modes of gas hydrate occurrence. The dominant mode is associated with active venting of gas exsolved from the accretionary prism and leads to high concentrations (15%-40% of pore space) of gas hydrate in seafloor or near-surface sediments at and around the topographic summit of southern Hydrate Ridge. These near-surface gas hydrates are mainly composed of previously buried microbial methane but also contain a significant (10%-15%) component of thermogenic hydrocarbons and are overprinted with microbial methane currently being generated in shallow sediments. Focused migration pathways with high gas saturation (〉65%) abutting the base of gas hydrate stability create phase equilibrium conditions that permit the flow of a gas phase through the gas hydrate stability zone. Gas seepage at the summit supports rapid growth of gas hydrates and vigorous anaerobic methane oxidation. The other mode of gas hydrate occurs in slope basins and on the saddle north of the southern summit and consists of lower average concentrations (0.5%-5%) at greater depths (30-200 meters below seafloor [mbsf]) resulting from the buildup of in situ-generated dissolved microbial methane that reaches saturation levels with respect to gas hydrate stability at 30-50 mbsf. Net rates of sulfate reduction in the slope basin and ridge saddle sites estimated from curve fitting of concentration gradients are 2-4 mmol/m**3/yr, and integrated net rates are 20-50 mmol/m**2/yr. Modeled microbial methane production rates are initially 1.5 mmol/m**3/yr in sediments just beneath the sulfate reduction zone but rapidly decrease to rates of 〈0.1 mmol/m**3/yr at depths 〉100 mbsf. Integrated net rates of methane production in sediments away from the southern summit of Hydrate Ridge are 25-80 mmol/m**2/yr. Anaerobic methane oxidation is minor or absent in cored sediments away from the summit of southern Hydrate Ridge. Ethane-enriched Structure I gas hydrate solids are buried more rapidly than ethane-depleted dissolved gas in the pore water because of advection from compaction. With subsidence beneath the gas hydrate stability zone, the ethane (mainly of low-temperature thermogenic origin) is released back to the dissolved gas-free gas phases and produces a discontinuous decrease in the C1/C2 vs. depth trend. These ethane fractionation effects may be useful to recognize and estimate levels of gas hydrate occurrence in marine sediments.