OS33A-1302
An overview of the latest results of cold seep research along the Hikurangi Margin, New Zealand
Prior to 2006, the knowledge about cold seeps around New Zealand was based mainly on accidental recovery of seep fauna or methane-derived carbonates by fishermen and flares in echo sounders. Lewis and Marshall (1996) compiled these findings, providing the first details on 13 seep sites. Four of those are located at the Hikurangi Margin along the east coast of New Zealand's North Island. Since then, three international cruises in 2006 and 2007 enhanced our knowledge considerably about methane seepage along the Hikurangi Margin, an area which has in places very strong BSRs. Two cruises on RV TANGAROA in 2006 focused on extensive reconnaissance work as well as fauna sampling, geochemical pore water analyses and CTD casts including water sampling for methane analyses. Several new seep sites were discovered during these cruises. Using these data, very detailed investigations in four main working areas could be performed during a 10-weeks expedition with RV SONNE (SO191). All research topics currently discussed in the scientific community were addressed using state-of-the-art equipment (e.g. deep- tow side-scan and ROV-deployments). Fourteen institutes from seven countries were involved. Echosounder and sidescan surveys unmistakably revealed active seep sites by detecting bubbles in the water column and carbonate precipitation at the seafloor forming massive chemoherm complexes. These complexes are associated with typical seep fauna like tube worms, bivalve mollusk species (Calyptogena, Bathymodiolus),and bacterial mats. At the fringe of these chemoherms dark sediment patches were observed which exihibit a novel seep habitat dominated by dense beds of two new species of heterotrophic ampharetid polychaetes. Bubble release was visually observed at several sites and recorded in the backscatter of various acoustic devices. At one site (680m water depth) very strong, pulsing outbursts could be observed repeatedly with methane fluxes of 20 to 25 l/min (60 to 74 mol/min). Intense CTD sampling and onboard methane analyses revealed that at least three of the areas are actively venting methane with an upper boundary at about 500 m, due to a density barrier. ADCP data indicate tide-dependent changes in current speed and direction. Delta-13C values of dissolved methane range from -71 to -19 permil, reflecting bacterial oxidation of methane in the water column, with a removal rate of 38 nM/day (or 11 to 19%/day). Equilibrator surveys, analyzing the sea surface and atmospheric methane concentrations show no significant oversaturation and fluxes for the entire studied area of the Hikurangi Margin. Extensive pore-water measurements, including in situ measurements during lander deployments, were aimed at evaluating flux rates of dissolved geochemical species and free gas. These measurements revealed that the dark sediment patches represent a remarkable seep habitat because of its very high methane fluxes and total oxygen consumption rates. Detailed seismic and controlled-source electromagnetic surveys allowed quantification of gas hydrates and regional estimates of fluid-flow focusing and the impact on the gas hydrate stability and BSR occurrence. Furthermore, the geophysical data imaged fluid pathways under seeps and indicated that more seep sites could be found at the seafloor. In 2006 and 2007, 23 new seep sites have been identified and visually observed, which resulted in a total of 31 seeps sites for the Hikurangi Margin. With more cruises proposed, this number is likely to increase.
OS33A-1303
Seeps along the Hikurangi Margin - a comparison of four different sites
Methane plays a crucial role in the global bio-geo-system. Hence, cold seeps, where methane leaves the seafloor and enters the water column, are an important issue of present-day research. The cruise SO191-1 in 2007 to the Hikurangi Margin, east of New Zealand's North Island, was dedicated to the investigation of local and regional transport processes of methane and gas hydrate deposits east of New Zealand. The Hikurangi Margin varies greatly along its axis. However, methane seep sites seem to accumulate on crests of thrust faulted ridges along the middle slope. Geophysical data from four different research areas reaching from Builders Pencil close to Hawke Bay to the area of Wairarapa near Cook Strait allow a comparison between seep sites, their setting and their distribution. The seep sites are analysed with regard to their seismic characteristics, their appearance on Parasound echosounder profiles, their bathymetric relief and the monitored methane concentration. The methane, of biogenic origin, is probably captured within the sedimentary layers and transported upwards with the uprising fluids from the subduction channel or fluids from within the sediments due to sediment compaction. Fluid escape from the seafloor is partly governed by seafloor topography. In some regions, like Uruti, fluid seepage is restricted to areas of bathymetric highs. Whereas in other regions, e.g. Wairarapa and Omakere Ridge fluid escapes the seafloor at bathymetric highs, as well as along small faults through horizontally layered sedimentary sequences. Varying subseafloor structures related to fluid escape, e.g. bright spots and large fault systems, hint at different fluid escape mechanisms, regionally, as well as locally. Along the entire Hikurangi Margin fluid escape is governed by gravitational forces, enhanced fluid escape occurs during low tides due to the smaller hydrostatic pressures. The comparison of seep sites in different setting and different areas at the Hikurangi Margin provides valuable insight into the distribution of methane, the pathways of methane within the gas hydrate stability zone and the influence of sediments and tectonic setting on the development of seep structures.
OS33A-1304
Submeter Mapping Of Methane Seeps By ROV Observations And Measurements At The Hikurangi Margin, New Zeeland
During R.V. Sonne cruise SO191-3, part of the "New (Zealand Cold) Vents" expedition, RCMG deployed their CHEROKEE ROV "Genesis" on the Hikurangi Margin. This accretionary margin, on the east coast of New Zealand, is related to the subduction of the Pacific Plate under the Australian Plate. Several cold seep locations as well as an extensive BSR, indicating the presence of gas hydrates, have been found at this margin. The aims of the ROV-work were to precisely localize active methane seeps, to conduct detailed visual observations of the seep structures and activity, and to perform measurements of physical properties and collect samples at and around the seep locations. The ROV allowed first ever visual observations of bubble- releasing seeps at the Hikurangi Margin. Seeps were observed at Faure Site and LM-3 in the Rock Garden area, at a flat to moderately undulating sea floor where soft sediments alternate with carbonate platforms. Bubble-releasing activity was very variable in time, with periods of almost non-activity (5 bubbles/second) alternating with periods of violent outbursts (190 bubbles/second). Bubbles sizes ranged from less than 5 mm to more than 20 mm. At Faure Site, bubble release was monitored over a period of 20 minutes, resulting in the observation of 6 outbursts, each lasting 1 minute at a 3 minute interval. These violent outbursts were accompanied by the displacement and resuspension of sediment grains and the formation of small depressions showing what is possibly an initial stage of pockmark formation. At the LM-3 site only some small bubbling seeps were observed near a large carbonate platform covered by Bathymodiolus mussels, Calyptogena shells and tube worms. Sediment-temperature measurements, in both areas, were largely comparable with the bottom-water temperature except at LM-3, at a site densely populated by polychaetes, where anomalous low sediment-temperature was measured. Overall, both seep areas are very confined in space and bottom-water sampling revealed that the released methane has a microbial signature.
OS33A-1305
Focused Fluid Flow and its Relation to Gas Hydrate Distribution on the Outer Accretionary Wedge, Southern Hikurangi Margin, New Zealand: Evidence from Seismic Data
Large quantities of fluids are predicted to be expelled from subduction margins. We present seismic images from the outer accretionary wedge on the southern Hikurangi Margin offshore New Zealand. The data show high-amplitude anomalies beneath a thrust ridge, the Porangahau Ridge, that are most likely caused by free gas above the regional level of bottom simulating reflections (BSRs). While we cannot rule out buoyancy- driven gas invasion, several observations let us favor upwarping of the base of gas hydrate stability (BGHS) due to advective heat flow as cause for these anomalies. Estimates of advection rates indicate that the most significant source of fluids is likely to be compaction of subducted sediments. We roughly estimate that ~15% of fluid expulsion across the margin may take place at this ridge, demonstrating the significance of focusing of fluid expulsion in subduction margins. Furthermore, a north-to-south progression of the degree of upwarping of the BGHS and the lack of a pronounced heatflow anomaly in seafloor thermal data point towards transience of fluid expulsion. The amplitude anomalies disappear further south and develop into a BSR gap, which we interpret as evidence for gas depletion. Such several-hundred-meter wide BSR gaps occur beneath other ridges on this margin and we speculate they may mark locations of focused fluid flow. On regional scales i.e., ignoring such relatively small gaps, BSRs coincide with deeply rooted thrusting in the accreted wedge. Low coherency, decrease of amplitudes, and possible lowering of frequencies in the seismic data indicate elevated attenuation in the thrusted sections possibly caused by free gas and/or scattering associated with deformation. The source of methane for hydrate formation on this margin is thought to be mainly biogenic. We propose that the regional link between thrusting and BSR occurrence reflects the opening of fluid conduits that facilitate de-watering of the wedge. Upward migrating fluids "pick up" gas generated in the temperature window for biogenic methane generation in the upper ~2 km beneath the seafloor. This gas is available for BSR and hydrate formation at locations where fluids are being channeled to the seafloor, such as the thrust ridges.
OS33A-1306
Gas Hydrate Distribution From Seismic and Controlled Source Electromagnetic Data at Porangahau Ridge, Southern Hikurangi Subduction Margin
High concentrations of gas hydrate are inferred to be present at Porangahau Ridge, a thrust ridge within the accretionary wedge of the southern Hikurangi Subduction Margin, New Zealand. We present a very-high resolution 2D velocity model of Porangahau Ridge and surrounding geological structures developed from a combination of conventional techniques and horizon-based velocity analysis of 05CM-38 --- a 120 km long multi-channel seismic line perpendicular to the margin. Anomalous velocities in this model differentiate between the presence of free gas (anomalously slow velocity) and gas hydrate (anomalously fast velocity). This high-resolution velocity model has been used to derive a preliminary acoustic impedance section, and to produce a high-quality migrated seismic section which reveals complex near-surface faulting not previously resolved, and more clearly defines a band of high-amplitude reflections above the regional bottom simulating reflections (BSRs). Interpretation of this migrated seismic section provides new insight into the nature of faulting and its relationship to fluid flow and gas hydrate formation on the margin. Controlled source electromagnetic (CSEM) surveying is sensitive to strongly resistive gas hydrate and free gas near the seafloor. CSEM data acquired along a transect across Porangahau Ridge (coincident with 05CM-38 seismic profile) show anomalously high apparent resistivities above the high-amplitude reflection bands we observe. We use 1D-inversion results constrained by depth-converted seismic horizons to test a possible correlation between layered resistivity structure, p-wave velocity and reflection strength, suggesting the presence of a gas hydrate sweet spot.
OS33A-1307
The transient nature of heat and fluid flux on the Porangahau Ridge, New Zealand
Reflection seismic dip lines across the Porangahau Ridge on the Hikurangi Margin of Northeast New Zealand exhibit a bottom simulating reflection (BSR), suggesting the presence of methane hydrate. Below the crest of the Porangahau Ridge, the BSR shoals significantly, suggesting a strong thermal anomaly at depth. However, marine heat flow measurements with a 3m probe show linear thermal gradients at the shallowest point of the BSR, suggesting a little if any present day advective flux. Concave downward thermal profiles, indicative of greater flux, are seen on the landward flank of the ridge, offset from the point where the BSR is shallowest. Finite element modeling of heat and fluid flux suggests that this apparent disparity could be due to temporal changes in the location of the fluid flux. We estimate the re-establishment of linear thermal gradients (a conduction only regime) is relatively fast for the top three meters of seafloor (a few decades). Assuming the shoaling of the BSR is due to thermal perturbation from fluid advection, it could be a result of flux that ceased several thousand years ago, and the methane hydrate is in the process of re-forming. Results imply that seafloor expulsion on this ridge may be significantly variable on a time scale of thousands of years.
OS33A-1308
34S/32S and 18O/16O ratios of dissolved sulfate from interstitial water samples above gas hydrate bearing sediments of IODP Expedition 311, Cascadia
Microbially mediated sulfate reduction affects the isotopic composition of dissolved and solid sulfur species in marine sediments. Although several details of the fractionation process remain controversial, the overall process is well understood and can be described as the sum of several mass dependent fractionations during the stepwise reduction of sulfate to sulfide. Experiments and field data show that the 18O/16O of sulfate is also modified in the presence of sulfate-reducing microorganisms. Here we use a reaction transport model to analyze these processes and to constrain the rates of organotrophic versus methanotrophic sulfate reduction. Our results show that even in cases where sulfate concentrations decline in a linear fashion, up to 50% of all sulfate is consumed by organotrophic sulfate reduction.
OS33A-1309
Study of Marine Gas Hydrates on the Northern Cascadia Margin: Constraints from Logging, Seismic Interpretation and Geochemistry (IODP Expedition 311)
The study area of Integrated Ocean Drilling Program (IODP) Expedition 311 is located at the accretionary prism of the northern Cascadia subduction zone. The project utilizes data and results from IODP Expedition 311 and a more recent coring and seismic expedition in 2008 conducted at the northern Cascadia Margin. A transect of four deep drilling sites (U1325, U1326, U1327, and U1329) across the entire margin was established in 2005 to study the occurrences and formation of gas hydrate in accretionary complexes. The four transect sites represent different stages in the evolution of gas hydrate across the margin. The primary objectives of this project are to constrain geologic models for the formation of gas hydrate in subduction zone accretionary prisms by (1) study the formation of natural gas hydrate in marine sediments, (2) determine the mechanisms that control the nature, magnitude and distribution of the gas hydrate occurrence, (3) find the pathways enabling the upward fluid migration, (4) examine the effect of gas hydrate on the physical properties of the host sediment. This project uses conventional wire-line and Logging-While-Drilling data to calculate gas hydrate concentrations. Correlation of seismic data and logging data is then achieved by generating synthetic seismograms to define the detailed gas hydrate occurrence at each of the 4 transect drilling sites. Results from the geophysical analysis are complemented by geochemical pore fluid data to further constrain gas hydrate formation characteristics. Detailed comparison of seismic and log data suggests major lithological control on the gas hydrate occurrence and concentration (especially observed at Sites U1325 and U1326). Individual reflectors and abundant faults can be traced across the drill sites that could act as migration pathways for fluids (and gas) explaining the high gas hydrate concentrations at relatively shallow depths of less than 100 meters below seafloor at Site U1326. However, contradicting evidence for non-fluid-migration related gas hydrate occurrences are identified from the void-gas isotopic composition at Site U1326 and U1327, where mainly in- situ gas hydrate formation has been postulated. This contradiction has yet not been resolved.
OS33A-1310
Methane budget of the down-current plume from Coal Oil Point seep field, Santa Barbara Channel, California
Previous research indicates that 5.5-9.6 x 106 mol/d (90-150 t/d) of methane are emitted from the seafloor into the coastal ocean near Coal Oil Point (COP), Santa Barbara Channel (SBC), California. Methane concentrations and biologically-mediated oxidation rates were quantified at 12 stations in a 198 km2 area down-current from COP during the SEEPS"07-Cruise with the R/V Atlantis. A ship-board Acoustic Doppler Current Profiler (ADCP) recorded current velocity patterns simultaneously with water sampling. The observed methane distribution matches the cyclonic gyre which is the normal current flow in this part of the Santa Barbara Channel - pushing water to the shore near the seep field and then broadening the plume while the water turns offshore further from the source. A methane budget was calculated using a box model, with budget terms including methane burden, sea-air flux, oxidative loss, and flux in and out of the 51 km3 box. The results indicate a 0.6% loss via sea-air exchange and a 1.5% loss due to microbial oxidation. The majority of the methane is advected in and out of the box. This data enables a calculation of the amount of dissolved methane emitted from the COP seep field, and when combined with published measurements of bubble flux, allows for a revision of the total methane flux from the COP seeps. Revised estimates for the dissolved methane flux for COP are 5.5 x 106 mol/d, raising the total COP methane release to 7.4-11.5 x 106 mol/d (120-180 t/d). These results represent a snapshot, but serve as a base for the first complete dissolved methane budget of the water column above a seep site in the marine realm.
OS33A-1311
Velocity Modelling Results Of A Pockmark In The Nyegga Region, Norwegian Sea
Pockmarks are a common indicator of fluid flow through the seafloor at continental margins. They are thought to be of global significance as pathways for the escape of methane from the sediments to the water column beneath continental margins, and also as habitats for chemosynthetic communities of biota. Their mechanism of formation and internal dynamics remain poorly constrained, partly due to a lack of proper three dimensional imaging of their internal structure. Numerous fluid escape features provide evidence for an active fluid-flow system in the Nyegga region of the Norwegian continental margin. In June-July 2006 we conducted a high-resolution seismic experiment using ocean bottom seismometers (OBS) to investigate the detailed 3D structure of a 250-m-wide pockmark in this region, named G11 and hence to determine the distribution of gas and gas hydrate in and around the pockmark . An array of 14 OBS were deployed across the pockmark. Shots fired from two 35 cu. in. mini GI guns were recorded on these OBS and on a near surface hydrophone streamer. Shot and OBS locations were determined with c. 1 m uncertainty using water wave arrivals. The OBS and reflection data reveal many interesting features of the subsurface geology of the chimney. A group of bright reflectors underlies the pockmark at a travel time of c. 1.4 s and at deeper depths some of the reflectors show strong attenuation indicating the presence of gas in the sediments. A pipe ascends from this gas charged zone to where it terminates in the investigated G11 pockmark. We have analysed data from five OBS lying along a line running NE-SW across the pockmark, using both raytracing and reflection tomography. The raytraced forward model, which incorporates signals from six subsurface reflectors, shows, a gradual increase in velocity between the seafloor and the gas charged zone, lying c. 300 m depth below the seabed. The travel-time fit is improved significantly if velocities in the pipe beneath the pockmark are higher than those in the surrounding sediments. The maximum velocity anomaly is c. 6%. An initial tomographic inversion using signals from a single subsurface reflector at the top of the gas-charged zone also recovers a velocity anomaly of c. 6%. This velocity anomaly may be attributed to the presence of increased saturations of methane hydrate beneath the pockmark.
OS33A-1312
Active seafloor seeps with associated methane and methane hydrate bearing sediments on the Mid-Norwegian margin, Nyegga: a multidisciplinary geological, geochemical and biological study
The deep-water area at Nyegga, on the Mid-Norwegian continental margin, has the last few years been a significant laboratory for studies of active seep structures. However, of the more than 200 pockmarks that recently have been identified within this region only seven of them have been described. The structures have a pronounced topographical relief, from -10 m to +10 m, and are associated with gas bearing sediments and a regional BSR (Bottom Simulating Reflector). It also seems that the most active pockmarks are located above deeply rooted vertical pipe structures. The geological processes that have created these complex structures are strongly influenced by high biological and geochemical activity. The pockmarks also appear to be at different maturity level in terms of the biological and microbiological community. Through the Norwegian gas hydrate project, GANS, a broad multidisciplinary geophysical, geological, geochemical and biological study has been carried on Nyegga. We will present preliminary results from these recent field studies.
OS33A-1313
Shale Tectonics in the Continental Slope and Rise Regions of Krishna-Godavari Basin, Bay of Bengal: Implication in Gas-Hydrate Exploration
Increased oil and gas exploration activity has led to a detailed investigation of the deep offshore and adjacent slope regions of Mahanadi, Krishna-Godavari (KG) and Cauvery basins, which are categorized as promising petroliferous basins along the eastern continental margin of India. The high sedimentation rate, thick sedimentary prism, and deeply buried mobile shale strata favor shale tectonics in KG basin which is manifested in the form of large extensional growth faults in the shelf and upper slope regions, and mud diapirs and toe-thrusts in the deep offshore regions of KG basin. Multichannel seismic reflection data depict the acoustic signatures akin to imbricate thrust faults, escarpment, mud diapirs, and intraslope basins in the KG deep offshore. The multibeam swath bathymetry mosaic and the sub-bottom profiler (SBP) datasets confirm their surface and subsurface manifestations. Some of the structural elements are buried under large scale mass transport deposits. The shallow deposits associated with the shale tectonics structures like sliding/slumping deposits, debrites, sediment creep deposits, turbidites, pelagites/hemipelagites, and interlayered debrites and turbidites are inferred from SBP echo facies analysis and microtopography. Shallow structures associated with shale tectonics, and their spatial distribution is a precursor in understanding the subsurface occurrence of gas hydrate deposits. We believe that the shale tectonics structures are largely responsible for the distribution of gas hydrate deposits in KG-offshore basin, and the inferred toe-thrust and mud diapiric zones are favorable locales for gas hydrate accumulation. The results of recent drilling/coring in the KG-offshore (May-Aug, 2006) show the presence of thick accumulation of gas hydrate in the vicinity of the region associated with mud diapiric and toe-thrust fault zones. Further, the study of geophysical data and analysis of long sediment cores collected onboard Marion Dufresne (May, 2007) in the mud diapiric and toe-thrust regions suggest paleo-expulsion of methane and sulfidic fluid from the seafloor. However, the cores collected in the intraslope basin do not show any indications of methane venting. We prepared a regional tectonics map that illustrates the distribution of shale structures and shallow depositional environments. This map may serve as a better constraint in understanding the genesis and occurrence of gas hydrate deposits in KG-basin.
OS33A-1314
Structural interpretation of large-scale faulting in the Krishna-Godhavari Basin offshore India to define the deep-plumbing system and gas migration pathways for gas hydrate formation
The Krishna-Godhavari (KG) basin, located on the east coast of India is within a passive-margin setting with a lateral (coastal) extent of 500km, and was targeted during the India National Gas Hydrate Program (NGHP) Expedition 01 in 2006. A 3-D seismic reflection data set crossing Site NGHP-01-05 within the KG-basin was closely investigated to identify migration pathways from a gas source at about 2 km depth to the near-surface sediments in the top 100 to 200 meter below seafloor (mbsf), where gas hydrates are present. The occurrence of bright spots at the base of and within the expected gas hydrate stability zone indicate the presence of gas hydrate, which were also directly observed in coring and inferred from data at Site NGHP- 01-05. However, no continuous bottom simulating reflector (BSR) was identified. The seismic data show that the area around Site NGHP-01-05 is dominated by three groups of faults, including one set of thrust faults (set 1), which represent the main gas migration pathway. Gas hydrate accumulates on the hanging wall of the thrust faults and the deep gas moves along the faults towards the shallow sediments. The enhanced reflection amplitudes away from the thrust-pinch-outs and the amplitude blanking zones indicate the presence of free gas that accumulated near the base of the gas hydrate stability field. The second group of faults, are reverse faults and also connect to the same area of gas hydrate accumulation as the thrust faults of group one, but are not tapping into a deeper source of free gas. The third group of faults (normal) is not continuously present in the entire 3-D seismic data area. Despite the fact that they appear to connect into the same deep gas source as the faults of groups one and two, there is no evidence in the seismic data within the top 100 to 200 mbsf for the presence of gas hydrate, suggesting that the third set of faults is not a major pathway for gas migration (compared to set 1). However, there may still be gas hydrate in the area where the set-3 faults are pinching out at the seafloor, but concentrations may be too low to generate prominent seismic signatures.
OS33A-1315
The characteristics of gas hydrates recovered from shallow sediments at Umitaka spur, Eastern margin of the Sea of Japan
Visible (massive, nodular, etc) gas hydrate samples were recovered in shallow sediments at the Umitaka spur, Eastern margin of the Sea of Japan, during investigation UT06 in 2006. These samples, all associated with a gas plume, occurred in dark gray silty clay. Systematic analyses were applied to characterize these hydrate samples. As examined by powder X-ray diffraction, all of the samples studied are sI gas hydrate. The results obtained by a confocal Raman and solid 13C NMR indicate that the hydrate samples are predominantly composed of methane, with a minor amount of hydrogen sulfide identified as well in the Raman spectra. DSC (differential scanning calorimeter) results suggest that several phases with different gas compositions coexist in the same sample, indicative of a heterogeneous distribution in composition. The carbon isotope composition of methane in the hydrate ranges from -36.2 to -40.3 per mil, showing the characteristic feature of gas of thermogenic origin.
OS33A-1316
Seismic Interpretation Based of the Gas Hydrate Bearing Filed off Joetsu, Eastern Margin of Japan Sea
Umitaka Spur and Joetsu Knoll are gas hydrate sites in the Joetsu Basin, off Joetsu, eastern margin of Japan Sea. Vents, plumes, hydrates outcrops, bacterial mats and carbonate crusts are direct evidence of gas hydrates. Mounds and pockmarks are also observed over these gas hydrate sites in a strict relation with methane hydrate plumes. A 2D Single Chanel Seismic (SCS) survey was conducted in 2006, 2007 and 2008 cruises using R/V Natsushima of JAMSTEC, and shows indirect indications of gas hydrate deposits, confirmed by the surface and near surface studies. Gas chimneys, BSR's, amplitude and "pull up" structures are observed. The BSR's occurs at around 0.15~0.20ms below sea floor on both Umitaka Spur and Joetsu Knoll. Heat flow measurements by piston cores and push cores indicate a gradient around 10°C/100m, giving a very good indication about the base of gas hydrate stability zone (BGHSZ) at around 150mbsf. Coupled SCS and heat flow data infer the seismic velocity on the shallow sediments as 1500~1700m/s. Double BSR's appear on the northeastern part of Joetsu Knoll. It may represent a seismic signal of sea level changes, indicating that that the BGHSZ was shallower in the past. In this study we identified deep and shallow faults and fractures, which are pathways for gas migration from deep reservoirs. Structural maps of the sea floor and other levels show at least two different directions (N-S and NW-SW). This pattern is likely to control gas migration, gas hydrate distribution, and also the occurrence of plumes, mounds and pockmarks on the spur and knoll.
OS33A-1317
Ocean Bottom Gamma-Ray Anomaly Around Methane Seeps Related to Gas Hydrate- Bearing Zone in The Eastern Margin of Japan Sea and Off Southwest Taiwan
JAMSTEC has conducted the ocean bottom gamma-ray measurement using ROVs and Submersibles since 1997. Gamma-ray spectrometer utilizes 3-inch spherical NaI(Tl) scintillator and the signal processor including DA converter in a pressure case. After processing data, we get total count rate (intensity value: count per second (cps)) of gamma ray and contents of K, U-, and Th-series radionuclides. The sensor was equipped to the side of the sample basket or foot of ROVs and submersibles, and always touches the seafloor when ROVs completely landed. Their results are posted on JAMSTEC website as a database. On the basis of past achievements, we present the results of the ocean bottom gamma-ray measurement at the methane seep sites related to gas hydrate off Joetsu in the eastern margin of Japan Sea and off southwest Taiwan. Off Joetsu: A number of mounds, large pockmarks (20 - 50 m deep and 200 - 500 m across), gas plumes, and gas hydrate are found at water depth of 900 - 1000 m in the Umitaka Spur and the Joetsu Knoll. Gamma-ray intensity values are 50 - 70 cps in normal muddy seafloor. On the other hand, the intensity values are 100 - 200 cps around methane venting sites, bacteria mats, and 'collapsed hydrate zone' which has an undulating, rugged seafloor with carbonate nodules and gravels. Contents of each radionuclide are also high. Low U/Th ratio suggests that there is less contribution of Rn accompanied with a recent fault activity. Off southwest Taiwan: Large, dense chemosynthetic communities, associated with carbonate pavements, were discovered at water depth of about 1100 - 1200 m on the top of the Formosa Ridge. Gamma-ray intensity values in normal muddy seafloor (120 - 150 cps) are higher than those around Japan. Since Th-series radionuclide easily absorbs other particles, it is commonly included in surface sediments. This may cause higher content of Th-series radionuclide in normal muddy seafloor. On the other hand, anomaly of gamma-ray intensity (200 - 300 cps) shows a methane seep activity from subsurface. It is hard to pinpoint the location and occurrence of seeps and fault without biotic activities. The ocean bottom gamma-ray measurement is one of the effective tools for the exploration of seeps and faults, though their values are different among areas and are not quantitative.
OS33A-1318
Gas-hydrate associated acoustic features off SW Taiwan: results from a deep-tow sidescan sonar and sub-bottom profiler survey
In the offshore area of southwest Taiwan, the gas-hydrate features in terms of BSR (bottom simulating reflector) are widely distributed. Although there is no real sample of gas-hydrate, geophysical and geochemical signatures indicate highly potential of gas-hydrate reservoir. To have a better understanding of gas-hydrate potential, we have conducted a deep-tow survey in a lowest magnetization area off SW Taiwan where the water depth is generally around 1500 m. Our survey area is roughly located near a deformation front (northward extension of the Manila Trench) where a structure named Yuan-An Lineament (YAL) bisects our survey area. During the survey, we used Edgetech 120/410 kHz sidescan sonar and 1-6 kHz sub-bottom profiler and generally kept the tow-fish at 30 m above the seafloor. To the east of YAL, the area belongs to active accretionary prism. Here we found wide distribution of carbonate in terms of strong acoustic reflection which suggests large methane gas had interacted with sulfate. In this case, the acoustic penetration is very low and diffraction phenomenon is obvious in sub-bottom profiler data. In contrast, to the west of YAL, the sedimentary layers can be clearly identified. The acoustic blanking can be traced along a layer as thick as 30 m, which suggests that this area the potential gas-hydrate is possibly well reserved. However, in some places, the possible gas-hydrates (blanking areas) have outcropped to the seafloor.
OS33A-1319
Halogen Profiles in Gas Hydrate Potential Area Offshore SW Taiwan
Recent studies showed very shallow depths of sulfate methane interface (SMI) and high methane concentrations in the cored sediments from the gas hydrate potential areas offshore southwestern Taiwan. This confirms very high methane flux degassing from seafloor in the area. Furthermore, the d13C data of methane and C1/(C2+C3) ratio indicate that the methane in this area is mainly derived from biogenic source. In this study, we carry on the measurement of halogen concentration (Br and I) in the pore water to help understanding the early diagenesis processes in this studied area and better constrain the methane source. From our results, dissolved I- concentration increases rapidly with depth in most profiles of cored sediments. The largest slope could be up to 0.1054 mmol/m, which is about one to two order of magnitude larger than that obtained from the Nankai Trough. These profiles thus suggest that the influx of fluid was carrying iodine with methane from deep sources, which may be from dissolved gas hydrates. Furthermore, the I-/Br- ratios are higher than the organic matter from general marine sediment. This again supports that the dissolved halogen not only from local marine sediments but also from other sources.
OS33A-1320
The carbon isotopes of DIC and methane gas from gas hydrate potential area offshore SW Taiwan
Two possible processes could cause the sulfate reduction which is usually found in marine sediments: (1) the bacteria consume sedimentary organic matter with sulfate reduction (or organic matter degradation) oxidation of sedimentary organic matter; (2) anaerobic methane oxidation (AMO). d13C data of dissolved inorganic carbon (DIC) in the interstitial water are considered to be able to distinguish above mentioned two processes. Therefore, the carbon isotopes of methane and DIC of cored samples from three sites: GT1 (at the active margin), GT39B (at the offshore mud volcanoes), and GT44 (at the passive margin), respectively in the gas hydrate potential area offshore SW Taiwan, will be analyzed in this study to better constrain the methane sources and chemical processes occurred in the region. The d13C-methane ranges from -90 to -80 permil at site GT1, indicating a biogenic source. Nevertheless, the values of -63 to -45 permil at site GT39B show that the methane gas is mainly derived from a thermogenic source at that site. Meanwhile, the carbon isotopic characteristics of DIC at the depth of sulfate-methane interface indicate that AOM process played a major role at sites GT1 and GT39B. However, the effect of organic matter degradation cannot be ignored due to the high sedimentation rate in the area. Furthermore, the role of each process at each site will be discussed in this study.
OS33A-1321
Volcanic and pockmark structure in the northern margin of the South China Sea
The opening of the northern South China Sea is suggested to be started from late Eocene (ca. 37 Ma). After the spreading, the oceanic crust in the northern South China Sea was intruded by post-spreading volcanism. An obvious evidence is that sea mounts are widely distributed in the northern margin of the South China Sea. To better understand the transition from continental crust to oceanic crust, we have collected data of multi- beam bathymetry, backscatter and sub-bottom profiles in several marine geophysical cruises. New bathymetric data show that the margin in the southwest of our study area is relatively steeper than in the northeast. Several canyons have cut through the continental slope. In contrast, the slope is quite diffusive in the northern portion. The sea mounts are concentrated near the Tung-Sha (Pratas) island. The presence of the sea mounts in this area suggests that the post-spreading volcanism had occurred in the continental- ocean transition zone. Some large pockmarks (diameter ~500m) are also identified on the slope. These pockmarks seem to in a lineament parallel to the continental slope. We suggest that the pockmarks were created by upward migrated gas. The newly backscatter images show that the deep ocean area (depth > 4000m) has higher backscatter values than other places in the study area. The sea mounts and the axe of canyons also show higher backscatter values.
OS33A-1322
HyFlux - Part I: Regional Modeling of Methane Flux From Near-Seafloor Gas Hydrate Deposits on Continental Margins
HyFlux - Part I: Regional modeling of methane flux from near-seafloor gas hydrate deposits on continental margins MacDonald, I.R., Asper, V., Garcia, O., Kastner, M., Leifer, I., Naehr, T.H., Solomon, E., Yvon-Lewis, S., and Zimmer, B. The Dept. of Energy National Energy Technology Laboratory (DOE/NETL) has recently awarded a project entitled HyFlux: "Remote sensing and sea-truth measurements of methane flux to the atmosphere." The project will address this problem with a combined effort of satellite remote sensing and data collection at proven sites in the Gulf of Mexico where gas hydrate releases gas to the water column. Submarine gas hydrate is a large pool of greenhouse gas that may interact with the atmosphere over geologic time to affect climate cycles. In the near term, the magnitude of methane reaching the atmosphere from gas hydrate on continental margins is poorly known because 1) gas hydrate is exposed to metastable oceanic conditions in shallow, dispersed deposits that are poorly imaged by standard geophysical techniques and 2) the consumption of methane in marine sediments and in the water column is subject to uncertainty. The northern GOM is a prolific hydrocarbon province where rapid migration of oil, gases, and brines from deep subsurface petroleum reservoirs occurs through faults generated by salt tectonics. Focused expulsion of hydrocarbons is manifested at the seafloor by gas vents, gas hydrates, oil seeps, chemosynthetic biological communities, and mud volcanoes. Where hydrocarbon seeps occur in depths below the hydrate stability zone (~500m), rapid flux of gas will feed shallow deposits of gas hydrate that potentially interact with water column temperature changes; oil released from seeps forms sea-surface features that can be detected in remote-sensing images. The regional phase of the project will quantify verifiable sources of methane (and oil) the Gulf of Mexico continental margin and selected margins (e.g. Pakistan Margin, South China Sea, and West Africa Margin) world-wide by using the substantial archive of satellite synthetic aperture radar (SAR) images. An automated system for satellite image interpretation will make it possible to process hundreds of SAR images to increase the geographic and temporal coverage. Field programs will quantify the flux and fate of hydrate methane in sediments and the water column.
OS33A-1323
HyFlux - Part II: Subsurface sequestration of methane-derived carbon in gas-hydrate- bearing marine sediments
The recently funded DOE/NETL study "HyFlux: Remote sensing and sea-truth measurements of methane flux
to the atmosphere" (see MacDonald et al.: HyFlux - Part I) will combine sea surface, water column and
shallow subsurface observations to improve our estimates of methane flux from submarine seeps and
associated gas hydrate deposits to the water column and atmosphere along the Gulf of Mexico continental
margin and other selected areas world-wide. As methane-rich fluids rise towards the sediment-water
interface, they will interact with sulfate-rich pore fluids derived from overlying bottom water, which results in
the formation of an important biogeochemical redox boundary, the so-called sulfate-methane interface, or
SMI. Both methane and sulfate are consumed within the SMI and dissolved inorganic carbon, mostly
bicarbonate (HCO3-) and hydrogen sulfide are produced, stimulating authigenic carbonate
precipitation at and immediately below the SMI. Accordingly, the formation of authigenic carbonates in
methane- and gas-hydrate-rich sediments will sequester a portion of the methane-derived carbon. To date,
however, little is known about the quantitative aspects of these reactions. Rates of DIC production are not
well constrained, but recent biogeochemical models indicate that CaCO3 precipitation rates may be as
high as 120 μmol cm-2a-1. Therefore, AOM-driven carbonate precipitation must be
considered when assessing the impact of gas-hydrate-derived methane on the global carbon cycle.
As part of HyFlux, we will conduct pore water analyses (DOC, DIC, CH4, δ13CDIC,
δ13CDOC, δ13CCH4, δ18O, and δD isotope ratios) to
evaluate the importance of authigenic carbonate precipitation as a sequestration mechanism for methane-
derived carbon. In addition, sediment and seafloor carbonate samples will be analyzed for bulk sedimentary
carbonate (δ13C and δ18O) and bulk sedimentary organic matter (δ13C
and δ15N), as well as sulfur, bulk mineralogy, texture and morphological features, and carbonate
stable isotopes. We will then combine observational, geochemical, microbiological, and mathematical
methods to assess the effectiveness of authigenic carbonate precipitation as a sink for methane-derived
carbon under varying environmental conditions. Results of water column flux analysis, air-sea flux modeling,
and sediment and pore water analysis will contribute to the development of a working model for quantifying
regional fluxes of gas-hydrate-derived methane from the subsurface to the water column and atmosphere.
OS33A-1324
Natural gas hydrates from different marine environments. Physical and geochemical characterization.
During the last past decade, IFREMER has participated to numerous surface/diving oceanographic cruises to investigate fluid circulation, cold seep discharges and gas hydrate formation on continental margins. The development of sampling tools, instrumentations and laboratory apparatuses related to gas hydrate study is shown. The instrumentation path has then been widen to investigate on a variety of natural gas hydrate properties as well as the associated pore waters and gas bubbles. The field deployable instrumentations include PEGAZ for sampling of gas bubbles, chemical tracers measurement like methane concentration for determining anomalies in the seawater column above mud volcanoes and pockmarks. Besides, laboratory development was mainly focused on Raman Spectroscopy, high resolution powder X-ray synchrotron diffraction, gas chromatographic techniques, Inductive coupled plasma mass-spectometry. Now we are enlarging our expertise by modeling. During those research activities, emphasis was given to the understanding of the geochemical processes related to the formation, stability and migration of marine gas hydrates. A lot of various gas hydrate specimens have been collected and studied at both sea and laboratory for their origin, formation and stability in different environments at different temperature/pressure conditions. High-resolution powder X-ray synchrotron diffraction and UV-Raman spectroscopy techniques are shown to be efficient and powerful tools to determine the hydrate structures (I, II, H). Gas and isotopic composition of gas hydrates is important for determining the gas origin (thermogenic, bacterial or mixing). The chemical signature of the hydrate waters help to understand the influence of pore waters on the hydrate composition. In areas where gas bubbles are present, stability field of natural gas hydrates may be obtained from bubble composition, providing information on the thickness of the hydrate layer in the sediment. This work will most focus on the description of the different technologies and studies developed at the Laboratoire de Géochimie et Métallogénie at IFREMER in the area of marine gas hydrate. It includes the ocean-deployable instrumentation, the laboratory development on both shore and ship, the modeling aspect on gas hydrate. Some achievements will be shown for illustrating the efficiency and reliability of each development.Various gas hydrates specimens issued from the Congo-Angola basin, the Nigeria basin, the Norvegian margin, the Marmara Sea will be shown and physical and geochemical results will be discussed and compared.
OS33A-1325
Authigenic Carbonate Constructions From Deep Sea Cold Seeps
Various examples of authigenic carbonates from modern marine cold seeps have been described worldwide on active or passive continental margins. They may be associated or not with gas hydrates. After a decade of microbiological and geochemical studies, we know that in cold seep marine environments, methane and other hydrocarbon compounds contained in the ascending fluids are oxidized as CO2 by a microbial consortium of sulfate reducing bacteria and methanotrophic archea. The Anaerobic Oxidation of Methane (AOM) represents the main microbial process driving the precipitation of authigenic carbonate crusts and concretions within the subsurface anoxic sediments. This explains why sea floor is often hardened by carbonate constructions at the sites of active methane seepage. The lateral and vertical extensions of these carbonate constructions are controlled by the balance between the intensity of the venting fluid flux and the ability of microbial communities to oxidize methane and to reduce sulfate. At steady state, the microbial filter transforms the totality of the emitted methane and generates carbonate; however, the efficiency of this filter can be counteracted by high methane flux, so that methane can escape in the water column and eventually reach the atmosphere ; numerical modelling of carbonate crust formation has shown that bioturbation and sedimentation rates are also important factors that control fluid and methane flow rates and thus carbonate precipitation at cold seep sites. The carbonate constructions observed at the sea floor exhibit various morphologies : massive to porous crusts, cm to meters thick, forming large pavements or fragmented slabs, circular chimneys, irregular concretions corresponding to cemented bioturbations. These hard substrates are often colonized by fixed organisms as polychaetes, tube-worms, molluscs, as well as by an abundant vagile fauna. The authigenic carbonates represent very useful archives to reconstruct the story of the seep activity. Their mineralogy, geochemical and isotopic compositions depend on the composition of the fluids and thus provide information of the origin of these fluids. It is noteworthy to mention that carbonate minerals are generally dominated by aragonite and Mg-calcite although dolomite and ankerite may become the unique species as in the carbonate chimneys of the Gulf of Cadiz. All cold seep carbonates are characterized by very low δ13C values down to - 60 permil clearly indicating that they were methane-derived products. The lipid biomarkers that are entrapped in the carbonate network provide the complementary information of the composition of the microbial communities that were involved during the diagenetic processes, AOM, sulfate reduction and methanogenesis.
OS33A-1326
Seep Carbonates From Tubeworm- and Mussel-Associated Environments at Atwater Valley, Northern Gulf of Mexico
During 2006 and 2007, MMS and NOAA jointly supported cruises where fifteen hydrocarbon seep sites at greater than 1000 m water depth on the lower Louisiana slope in the Gulf of Mexico (GOM) were explored. These sites contain numerous authigenic carbonates as well as high-density communities of tubeworms and mussels. However, the integrated petrographic and geochemical characterization of the tubeworm- and mussel-associated carbonates remains poorly known. Here, a comparative study of petrographic and geochemical features of the carbonate samples from tubeworm- and mussel- associated environments was approached for an active and complex seep site at Atwater Valley lease block 340 (AT 340) at 2200 m water depth in the GOM. The carbonate morphologies include concretion, blocky and massive carbonates up to several meters in size, and irregularly shaped carbonates, some of them are displaying high porosity. Some are highly brecciaed with aragonite layers of varying thicknesses lining fractures and voids. Lithologically, the carbonates are microcrystalline Mg-calcite, calcite and aragonite containing peloids, clasts and shell fragments. The carbon isotopic composition of carbonates varies narrowly, ranging from -46.45 ‰ to - 60.81 ‰, indicating 13C-depleted carbon source probably methane of microbial origin. But the common trend is that the tubeworm-associated carbonates have more depleted δ13C values when compared to mussel-associated carbonates. A similarly small variability of δ18O values (+3.12 ‰ to +5.09 ‰) demonstrates the temperature and/or fluid composition did not change greatly during carbonate. The total content of rare earth elements (REE) of the 5% HNO3-treated solution of the carbonates is from 6.54 ppm to 29.41 ppm. The shale-normalized REE patterns show slightly positive Ce anomalies, suggests that the carbonates precipitated under anoxic conditions. The possible factors (i.e. habitat of chemosynthetic animals, depth of carbonate precipitation and the flow rate of the seeping fluids) controlling the petrographic and geochemical characterization of those carbonates will be discussed by comparing data obtained on typical samples from tubeworm- and mussel-associated environments.
OS33A-1327
Seafloor Deformation in the South Caspian Sea: A Potential Proxy for Gas Hydrate Dissociation and Climate Change
The presence of buried gas hydrates in the South Caspian Sea has been interpreted using 2-D seismic reflection data. Evidence for buried gas hydrates consists of a shallow (300-500 m below seafloor) comparatively high velocity zone approximately parallel to the seafloor bounded by a positive-polarity reflector at the top and a high-amplitude negative-polarity reflector at the base. The position of gas hydrates fall within the stability field predicted for the South Caspian Basin. New industry quality 3-D seismic data are being used to determine the relationship between the presence of gas hydrates and an approximately 2,500 km2 late-Pleistocene zone of seafloor deformation and submarine slumping in the South Caspian Sea, named the Absheron Allochthon. Well-logs from the South Caspian Sea offshore Azerbaijan are being correlated with the 3-D seismic data to determine the age and origin of the Absheron Allochthon. The history of sea level changes in the Caspian Sea in the past ~700 ka emphasizes a major ~100 m drop in sea level during the latest deglaciation (late Pleistocene). Preliminary results show that the Absheron Allochthon may have formed through catastrophic failure of the western South Caspian continental slope as a result of dissociation of underlying buried gas hydrates. If this proves to be true, then repeated, remarkably rapid global warming events in the Caspian Sea during the late Quaternary were likely due to rapid marine gas hydrate (clathrate) dissociation rather than exhalation from wetlands.
OS33A-1328
Physicochemical characteristics of marine gas hydrates from two different geographic areas: A case study of two different structures
In recent years, marine gas hydrate has become a focus of extensive research. Those research activities have considerably increased the body of knowledge about not only the properties of gas hydrate itself, but also its interactions with its natural environment. However more need to be done on marine gas hydrate in order to achieve a better understanding of its formation and its stability, as well as the origin of the hydrate formers. This is of immense importance for both a fundamental point of view and the petroleum industry. In fact, such investigations are useful for the determination of slope stability on continental margins. Furthermore, considering the increasing world energy demand, such studies can widen the list of data for the development of new technologies to produce natural gas from marine gas-hydrate deposits. The work presented here is devoted to the geo-physicochemical study of two different gas hydrate sample. The first gas hydrate has been sampled off the Turkey coast in the Marmara Sea. This area is well known for its large number of natural gas fields. The sampled gas hydrate is of structure II and contains a significant amount of heavy hydrocarbons. Its compositional analysis has been performed. Besides, a sample of this gas hydrate has been fully dissociated then reformed in order to understand its formation mechanism into the natural sediment. This study has been carried out by monitoring the change in the gas phase composition. A detailed description of this experiment will be given. The second hydrate sample come from the Niger Basin and has a biogenic signature. Therefore, the mixture of hydrate former contains nearly exclusively methane and leads to structure I. Its physicochemical characteristics will be presented. One conclusion that could be drawn from this study is that multi-component gas leads to complex hydrates, featured by a heterogeneous composition along a same core. An explanation of this behaviour using the chemical fractionation phenomenon in relation with the geological context will be discussed.
OS33A-1329
Marine Gas Hydrates in Thin Sands From in Situ Biogenic Methane
Methane in marine gas hydrate deposits is generated either (1) within the gas hydrate stability zone (GHSZ) by in situ biogenic processes or (2) below the GHSZ and transported upward by fluid advection. This work examines whether in situ methane generation can explain hydrate occurrences within sand layers of mud- dominated continental margin sequences. For example, at IODP Site U1325 (Exp. 311, Cascadia margin), gas hydrate occupies 30-60% of the pore space in thin sand layers (~5 cm thick), with no clear evidence of hydrate in the adjacent marine mud intervals (2.5 m thick on average). This work models the formation of gas hydrates in marine sediments and accounts for in situ generation and diffusion of methane. Under free-space conditions, hydrates form when the dissolved methane concentration exceeds the methane solubility for bulk water. In porous media, however, theory and experiments show that gas hydrate formation is inhibited in small pores. Thus, methane concentration may exceed the free-space solubility in fine-grained marine muds, whereas in sand layers the pores are large enough for gas hydrate to form as in free-space conditions. To model the effect of pore size, consider a 2.5 m thick mud layer sandwiched between two sand layers. Methane concentration at the top and bottom of the mud layer (in the thin sands) is fixed to free-space solubility. As these sediments are buried, in situ biogenic activity increases their methane concentration. Once concentration in the mud layer exceeds free-space solubility, methane will be transported by diffusion into the upper/lower bounding sand layers. The rate of diffusion is fast compared to the rate of biogenic methane generation, and therefore the maximum methane concentration in excess of free-space solubility within the mud layer will always remain very small (<1% of solubility). Even a minor inhibition of gas hydrate formation in fine-grained mud will cause any in situ biogenic methane to be transported into the adjacent sands, where it forms hydrate. As a consequence, the inhibition of gas hydrate formation in marine mud and the diffusive transport of methane into thin sands can result in large concentrations of hydrate (~50% of pore volume) in thin sand layers even though in situ biogenic methane would produce only small amounts of hydrate (~1% of pore volume) if it formed in the mud layers.