OS31D-01 INVITED
Massive gas hydrate occurrences in fractured systems: Combined observations from deep drilling campaigns at the Cascadia margin, Krishna-Godhavari Basin, and Ulleung Basin
Natural gas hydrates are observed in various forms, such as (1) disseminated in the pores of sediments, (2) as small nodules or veins within fine-grained sediments, and (3) as a massive unit composed mainly of solid gas hydrate with minor amounts of sediment. The more massive occurrences are mainly associated with cold vents or fractured systems and gas hydrate can occur near or at the seafloor. Over the past decade several prominent marine cold vent settings have been drilled as part of the Ocean Drilling Program (ODP) Legs 164 (Site 996, Blake Ridge) and Leg 204 (Sites 1249 and 1250) at the southern Cascadia Margin, Integrated Ocean Drilling Program (IODP) Expedition 311 (Site U1328) off Vancouver Island on the northern Cascadia Margin, as well as the India National Gas Hydrate Program (NGHP) Expedition 01 (Site NGHP-01-10) within the Krishna-Godavari Basin, and the Korea Ulleung Basin Gas Hydrate (UBGH) Expedition 01 (Site UBGH-01-9, and -10) in the East Sea. In all these settings, the drilling campaigns provided a vast amount of Logging-while drilling (LWD) and wire-line log data together with conventional and pressure core data. Common observations at all cold vents visited are the unique abundance of fracture-control on the gas hydrate formation and an extreme degree of heterogeneity of the gas hydrate abundance across the area affected by the cold vents. The high abundance of massive gas hydrate in cold vents makes core-recovery with conventional tools challenging and sometimes even a safety hazard, resulting in significantly degraded sample quality if sediments can be recovered at all. Results of adjacent boreholes cannot easily be correlated laterally, making volumetric assessments of gas hydrate concentrations difficult. Volumetric assessments are further challenged by the unique seismic character of the cold vents, which often are seismically transparent, which is why these vents are often described as blank or wash-out zones. Electrical resistivity log data often exhibit large fluctuations with values larger than several hundred ohm-m. Fracture- filling gas hydrate, often occurring with steep dips of the fractures, can lead to anisotropic effects and erroneous resistivity and acoustic measurements. The nature of the gas hydrate occurrence in grain- displacing form is a challenge to the conventional theories typically used to calculate gas hydrate concentrations, where gas hydrate is modeled to form as a pore-filling medium (e.g. effective medium theory or Archie's law). Applying these techniques within cold vent settings requires caution and gas hydrate concentrations derived from these techniques may be in error.
OS31D-02 INVITED
Multiscale Heterogeneity in Methane Flux Regimes Between and Within Major Marine Gas Hydrate Provinces
More than a decade after the first dedicated Ocean Drilling Program (ODP) expedition focused on characterizing conditions within a major gas hydrate province, sufficient data are now available from ODP/IODP, other drilling, and numerous site survey activities to draw meaningful conclusions about variations in methane flux regimes within and between key passive (Blake Ridge, Gulf of Mexico, Indian margin) and active (Cascadia, Chile, Costa Rica, Nankai, and New Zealand) margin gas hydrate systems. Here I report on the analysis of a large data set that summarizes direct and indirect indicators of methane flux acquired at various spatial scales (from discrete, widely-spaced, deep boreholes to closely-spaced conventional piston cores) in many of the world's major marine hydrate provinces. Within a given province, the methane flux variations inferred from the data provide a partial explanation for the patchiness of bottom simulating reflectors (BSRs) and underscore the challenges associated with applying standard flux indicators to interpret methane dynamics at seafloor seeps. Comparison of flux indicators between provinces shows that the prevailing assumption of a 1:10 relationship between the sulfate-methane interface (SMI) depth and the directly-detected top of gas hydrate occurrence is largely inadequate. Instead, I propose using BSR depth (where available) and SMI depth, two parameters that should be directly related to methane flux in simple marine systems and that can be easily obtained during standard site survey activities, to estimate the depth to the top of the gas hydrate zone and thence the thickness of the hydrate-bearing interval. This thickness is critical for refining estimates of the quantity of hydrate-trapped methane in the global marine reservoir and for assessing how much of that methane might reach the ocean-atmosphere system under certain climate and seafloor deformation scenarios. Deviations from the proposed relationship between SMI and BSR depths are most pronounced proximal to seafloor seeps, but may also be related to microbial generation of methane within the hydrate stability zone or significant hydrologic (e.g., advection), geologic (e.g., slumping), or lithologic (e.g., variegated sedimentary section) heterogeneity.
OS31D-03
Active gas Venting at the Landward Limit of Hydrate Stability Offshore Svalbard
Most climate models predict rapid warming in the Arctic during the next few decades. This warming is expected to lead to warming of the seabed and destabilisation of submarine methane hydrates in the Arctic, with possible additional release of methane into the atmosphere. The part of the submarine methane hydrate system that is most sensitive to such warming is the region where the base of the hydrate stability field intersects the seabed. We report initial results from a multidisciplinary cruise in August-September 2008 from such a region on the western margin of Svalbard, where gas hydrate-related bottom-simulating reflectors (BSRs) are widespread and numerous seabed pockmarks are present. We acquired high- resolution ocean-bottom seismometer, multi-channel seismic reflection, subbottom profiler and sidescan sonar data, swath bathymetry, sediment cores, water column samples, and surface air samples. An extensive field of gas flares in the water column was imaged with 38 kHz sonar where the seabed is less than 370 m deep, just shallower than where the base of the hydrate stability field is predicted to intersect the seabed. Temperature and salinity measurements and water sampling indicate that the ocean water in these flares is well mixed and has enhanced concentrations of methane. The BSR is present at water depths of 700 m and greater, but cannot be traced into the region of gas flares. However, widespread high-amplitude, reversed- polarity seismic reflectors beneath the hydrate stability field in 500-800 m water depth indicate the presence of free gas at depth. A zone of acoustic scattering was observed beneath the region of gas flares, extending down slope to the region of high-amplitude reflectors. Such a zone is consistent with the presence of pathways for gas migration. Several discrete fluid-escape structures were imaged that appear to penetrate through the hydrate stability field to the seabed. Our observations suggest localised gas venting through the hydrate stability field has occurred in postglacial times and ongoing gas venting is occurring at its landward limit.
OS31D-04
The Significance of Methane Carbon to Bulk Sedimentary Organic and Inorganic Carbon Pools Across the Porangahau Ridge, New Zealand
Sediment and pore water geochemistry was studied along a ~7km transect across the Porangahau Ridge located on the northeastern coast of New Zealand. Trends in methane, sulfate, dissolved and total inorganic carbon (DIC and TIC), and total organic carbon (TOC) concentration and stable carbon isotope ratios (δ13C) of methane, DIC, TIC and TOC were compared at 12 locations. Piston core samples were taken on, and landward and seaward of the ridge. At the landward base of the ridge, elevated methane concentration at anomalously shallow depths was observed. This observation coincided with geophysical indicators (seismic, heatflow) of upward migrating fluids, possibly associated with dissociating methane hydrates escaping to surface sediments at this location. Across the transect, methane δ13C ranged from -107.9‰ to -44.3‰. Minimum values were observed in cores on the landward flank of the ridge (average -80.7‰). Given the distinctly 13C-depleted signature of methane in high concentration samples, methane δ13C was used to track in situ oxidation and ultimate incorporation of methane into bulk organic (TOC) and inorganic (DIC and TIC) carbon pools. DIC δ13C and TIC δ13C nearest to the landward base of the ridge averaged -28.7‰ and -6.4‰ respectively. These averages are substantially 13C-depleted compared to the whole transect average (-19.7‰ and 0.47‰ respectively) indicating that methane carbon contributed significantly to the inorganic carbon pool at this location. 13C-depleted DIC likely resulted from in situ methane oxidation and precipitation of authigenic carbonates likely explains the appearance of 13C- depleted TIC at this location. Although TOC concentration was elevated on the seaward side of the ridge, average TOC δ13C (-23.0‰) was 13C-depleted in proximity to the landward flank of the ridge compared to the whole transect average (-21.9‰). These data suggest that there is a large degree of heterogeneity in the supply of methane to surface sediments along the Porangahau Ridge, but in some locations methane carbon may contribute more significantly to sedimentary carbon cycling than planktonic or terrestrial carbon sources.
OS31D-05
Geomicrobial characterization of sediment bacterial communities across the Porangahau Ridge, New Zealand
Bacterial diversity in sediments from the Porangahau Ridge (northeastern coast of New Zealand) was studied using length heterogeneity-polymerase chain reaction (LH-PCR). LH-PCR, a small subunit rRNA gene (SSU rDNA) fingerprinting method, provides data on the distribution and relative abundance of operational taxonomic units (OTUs) representing individual phylotypes (species or strains). LH-PCR data from sediments obtained along an approximately 7km transect across the ridge were compared to determine the degree of spatial heterogeneity in community composition. Generally, the greatest diversity was found throughout piston core samples collected on the landward flank of the ridge. Lowest diversity was observed on the ridge apex, and at the landward base of the ridge where elevated methane flux to surface sediments was observed. Piston core samples penetrated the sulfate-methane transition zone (SMT) at only three locations. At the SMT for each of these locations, a significant reduction in overall diversity was observed, and an OTU not observed in other samples accounted for a significant percent of OTU abundance. This OTU matches one documented to dominate community composition in SMT samples from a methane seep at another distant location, and may represent the same uncultured relative of the sulfate-reducing Desulfosarcina variabilis subgroup previously observed. Given that bacterial community structure may be influenced by location and/or supply of terrestrial vs. in situ substrates, LH-PCR data will be explored in relation to geochemical data (sulfate, methane, bulk inorganic and organic carbon pools, chloride and total dissolved sulfides). This analysis will reveal which abiotic factors are responsible for structuring bacterial communities at this location.
OS31D-06
Temporal Variability in Pore-Fluid Chemistry at a Gulf of Mexico Gas Hydrate Site
Temporal variability in dissolved ions and gases was assessed in gas hydrate bearing sediments using a specialized Pore-Fluid Array (PFA) sampler. The PFA is a seafloor probe that consists of an interchangeable instrument package that houses OsmoSamplers, long-term pore-fluid samplers; a specialized low-dead volume fluid coupler; and eight sample ports along a 10-meter sediment probe shaft. The PFA was deployed at Mississippi Canyon 118, a Gulf of Mexico hydrate site, to test the hypothesis that pore-fluid chemistry records hydrate formation or decomposition events and reflects local seismic activity. A 160 day record was acquired from the overlying water (OLW) and 1.3 meters below seafloor (mbsf). Fluids were measured for dissolved chloride, sulfate, and light hydrocarbon (C1– C4) concentrations and dissolved inorganic carbon plus methane stable carbon and hydrogen isotope ratios. The overall formation or decomposition of gas hydrates could not be determined from any significant changes in the observed chloride and sulfide concentrations, but changes in other light hydrocarbons show some interesting patterns. Methane concentrations at 1.3 mbsf averaged 4 mM over the deployment and d13C-CH4 values (-32.4±3.4‰) indicated a thermogenic origin. The synchronous timing of an anomalous 14 mM methane spike and a nearby earthquake (Mw=5.8) suggests a significant methane flux out of sediments due to local tectonic activity.
OS31D-07
Methane flux and sulfate reduction variations in the continental margin offshore southwestern Taiwan
Methane gas is an important greenhouse gas. It can affect global climate if large amounts of methane gas release to the atmosphere. During migration, methane will be consumed via anaerobic methane oxidation (AOM) before reaching seawater or the atmosphere (Hinrichs et al, 1999). High methane concentration associated with gas hydrate will enhance sulfate reduction and AOM, resulting in steep sulfate gradients and shallow sulfate-methane interface (SMI) (Borowski et al., 1999). Therefore, sulfate profile is useful to estimate methane flux if AOM become the predominant process in sediment. During our TowCam survey, methane seep-related seafloor features, especially authigenic carbonate buildups, chimney structures and chemosynthetic communities, were found in the continental margin offshore southwestern Taiwan. In order to understand methane flux and sulfate reduction variations in this study area, piston cores and gravity cores were collected on board the r/v Ocean Research I. Sediment samples collected were analyzed for methane, sulfate, dissolved sulfide, pyrite-sulfide, organic carbon, and carbonate content. Spatial variations of methane concentration and sulfate reduction were found in this study area. Shallow SMI and high methane concentration were found on the front of the accretional wedge close to the deformation front (Yung-An lineament). Some SMI depth is shallower than 1m. Methane and dissolved sulfide concentration were as high as 10mM at some locations. Concentration of methane and dissolved sulfide decreased while SMI depth increase away from the accretional wedge front from west to east. C-13 isotope depleted authigenic carbonate and mussel tissues found on gas seep area showed that methane is the major carbon source and the AOM is an important biogeochemical process in the study area. The range of sulfate and methane flux calculated by the Fick's law were 1.80-219 mmole/m2/yr and 0.01-87 mmole/m2/yr, respectively. Sulfate flux, however, was higher than methane flux, indicating that sulfate reduction was not controlled only by the AOM, but also through oxidizing sedimentary organic carbon. Sulfate flux in this study area was higher than those of the Blake Ridge (Dickens, 2001, Borowski et al., 1999), the Namibia (Niewohner et al., 1998) and the West Argentine Basin (Hensen et al., 2003), probably reflecting methane flux seeping from beneath in this study region.