ALBERT

Description: The isotopic characteristics of CH4 (d13C values range from -101.3 per mil to -61.1 per mil PDB, and dD values range from -256 per mil to -136 per mil SMOW) collected during Ocean Drilling Program (ODP) Leg 164 indicate that the CH4 was produced by microbial CO2 reduction and that there is not a significant contribution of thermogenic CH4 to the sampled sediment gas from the Blake Ridge. The isotopic values of CO2 (d13C range -20.6 per mil to +1.24 per mil PDB) and dissolved inorganic carbon (DIC; d13C range -37.7 per mil to +10.8 per mil PDB) have parallel profiles with depth, but with an offset of 12.5 per mil. Distinct downhole variations in the carbon isotopic composition of CH4 and CO2 cannot be explained by closed-system fractionation where the CO2 is solely derived from the locally available sedimentary organic matter (d13C -2.0 per mil ± 1.4 per mil PDB) and the CH4 is derived from CO2 reduction. The observed isotopic profiles reflect the combined effects of upwards gas migration and decreased microbial activity with depth.

Description: Anaerobic methane oxidation (AMO) was characterized in sediment cores from the Blake Ridge collected during Ocean Drilling Program (ODP) Leg 164. Three independent lines of evidence support the occurrence and scale of AMO at Sites 994 and 995. First, concentration depth profiles of methane from Hole 995B exhibit a region of upward concavity suggestive of methane consumption. Diagenetic modeling of the concentration profile indicates a 1.85-m-thick zone of AMO centered at 21.22 mbsf, with a peak rate of 12.4 nM/d. Second, subsurface maxima in tracer-based sulfate reduction rates from Holes 994B and 995B were observed at depths that coincide with the model-predicted AMO zone. The subsurface zone of sulfate reduction was 2 m thick and had a depth integrated rate that compared favorably to that of AMO (1.3 vs. 1.1 nmol/cm**2/d, respectively). These features suggest close coupling of AMO and sulfate reduction in the Blake Ridge sediments. Third, measured d13CH4 values are lightest at the point of peak model-predicted methane oxidation and become increasingly 13C-enriched with decreasing sediment depth, consistent with kinetic isotope fractionation during bacterially mediated methane oxidation. The isotopic data predict a somewhat (60 cm) shallower maximum depth of methane oxidation than do the model and sulfate reduction data.

Description: A unique set of geochemical pore-water data, characterizing the sulfate reduction and uppermost methanogenic zones, has been collected at the Blake Ridge (offshore southeastern North America) from Ocean Drilling Program (ODP) Leg 164 cores and piston cores. The d13C values of dissolved CO2 (sum CO2) are as 13C-depleted as -37.7 per mil PDB (Site 995) at the sulfate-methane interface, reflecting a substantial contribution of isotopically light carbon from methane. Although the geochemical system is complex and difficult to fully quantify, we use two methods to constrain and illustrate the intensity of anaerobic methane oxidation in Blake Ridge sediments. An estimate using a two-component mixing model suggests that ~24% of the carbon residing in the sum CO2 pool is derived from biogenic methane. Independent diagenetic modeling of a methane concentration profile (Site 995) indicates that peak methane oxidation rates approach 0.005 µmol/cm**3/yr, and that anaerobic methane oxidation is responsible for consuming ~35% of the total sulfate flux into the sediments. Thus, anaerobic methane oxidation is a significant biogeochemical sink for sulfate, and must affect interstitial sulfate concentrations and sulfate gradients. Such high proportions of sulfate depletion because of anaerobic methane oxidation are largely undocumented in continental rise sediments with overlying oxic bottom waters. We infer that the additional amount of sulfate depleted through anaerobic methane oxidation, fueled by methane flux from below, causes steeper sulfate gradients above methane-rich sediments. Similar pore water chemistries should occur at other methane-rich, continental-rise settings associated with gas hydrates.

Description: A pressurized core with CH4 hydrate or dissolved CH4 should evolve gas volumes in a predictable manner as pressure is released over time at isothermal conditions. Incremental gas volumes were collected as pressure was released over time from 29 pressure core sampler (PCS) cores from Sites 994, 995, 996, and 997 on the Blake Ridge. Most of these cores were kept at or near 0ºC with an ice bath, and many of these cores yielded substantial quantities of CH4. Volume-pressure plots were constructed for 20 of these cores. Only five plots conform to expected volume and pressure changes for sediment cores with CH4 hydrate under initial pressure and temperature conditions. However, other evidence suggests that sediment in these five and at least five other PCS cores contained CH4 hydrate before core recovery and gas release. Detection of CH4 hydrate in a pressurized sediment core through volume-pressure relationships is complicated by two factors. First, significant quantities of CH4-poor borehole water fill the PCS and come into contact with the core. This leads to dilution of CH4 concentration in interstitial water and, in many cases, decomposition of CH4 hydrate before a degassing experiment begins. Second, degassing experiments were conducted after the PCS had equilibrated in an ice-water bath (0ºC). This temperature is significantly lower than in situ values in the sediment formation before core recovery. Our results and interpretations for PCS cores collected on Leg 164 imply that pressurized containers formerly used by the Deep Sea Drilling Project (DSDP) and currently used by ODP are not appropriately designed for direct detection of gas hydrate in sediment at in situ conditions through volume-pressure relationships.

Description: Drilling in the Cascadia accretionary complex enable us to evaluate the contribution of dehydration reactions and gas hydrate dissociation to pore water freshening. The observed freshening with depth and distance from the prism toe is consistent with enhanced conversion of smectite to illite, driven by increase in temperature and age of accreted sediments. Although they contain gas hydrate -as evidenced by discrete low chloride spikes- the westernmost sites drilled on Hydrate Ridge show no freshening trend with depth. Strontium data reveal that all the mélange samples contain deep fluids modified by reaction with the subducting oceanic crust. Thus we infer that, at the westernmost sites, accretion is too recent for the sediments to have undergone significant illitization. Our data demonstrate that a smooth decrease in dissolved chloride with depth cannot generally be used to infer the presence or to estimate the amount of gas hydrate in accretionary margins.

Description: 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.

Description: 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.

Description: 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.