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  • 1
    Publication Date: 2018-02-20
    Description: Submersible investigations of the Cascadia accretionary complex have identified localized venting of methane gas bubbles in association with gas hydrate occurrence. Acoustic profiles of these bubble plumes in the water column in the vicinity of Hydrate Ridge offshore Oregon provide new constraints on the spatial distribution of these gas vents and the fate of the gas in the water column. The gas vent sites remained active over the span of two years, but varied dramatically on time scales of a few hours. All plumes emanated from local topographic highs near the summit of ridge structures. The acoustic images of the bubble plumes in the water column disappear at water depths between 500 to 460 m, independent of the seafloor depth. This coincides with the predicted depth of the gas hydrate stability boundary of 510 to 490 m, suggesting that the presence of a hydrate skin on the bubble surface prevents them from rapid dissolution. The upper limit of the acoustic bubble plumes at 460 m suggests that dissolution of the residual bubbles is relatively rapid above the hydrate stability zone.
    Type: Article , PeerReviewed
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  • 2
    Publication Date: 2018-03-16
    Description: To constrain the fluxes of methane (CH4) in the water column above the accretionary wedge along the Cascadia continental margin, we measured methane and its stable carbon isotope signature (δ13C-CH4). The studies focused on Hydrate Ridge (HR), where venting occurs in the presence of gas-hydrate-bearing sediments. The vent CH4 has a light δ13C-CH4 biogenic signature (−63 to −66‰ PDB) and forms thin zones of elevated methane concentrations several tens of meters above the ocean floor in the overlying water column. These concentrations, ranging up to 4400 nmol L−1, vary by 3 orders of magnitude over periods of only a few hours. The poleward undercurrent of the California Current system rapidly dilutes the vent methane and distributes it widely within the gas hydrate stability zone (GHSZ). Above 480 m water depth, the methane budget is dominated by isotopically heavier CH4 from the shelf and upper slope, where mixtures of various local biogenic and thermogenic methane sources were detected (−56 to −28‰ PDB). The distribution of dissolved methane in the working area can be represented by mixtures of methane from the two primary source regions with an isotopically heavy background component (−25 to −6‰ PDB). Methane oxidation rates of 0.09 to 4.1% per day are small in comparison to the timescales of advection. This highly variable physical regime precludes a simple characterization and tracing of “downcurrent” plumes. However, methane inventories and current measurements suggest a methane flux of approximately 3 × 104 mol h−1 for the working area (1230 km2), and this is dominated by the shallower sources. We estimate that the combined vent sites on HR produce 0.6 × 104 mol h−1, and this is primarily released in the gas phase rather than dissolved within fluid seeps. There is no evidence that significant amounts of this methane are released to the atmosphere locally.
    Type: Article , PeerReviewed
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  • 3
    Publication Date: 2019-02-15
    Description: The Hikurangi Subduction Margin was the recent focus of two IODP expeditions seeking to explore the cause and effect of slow slip earthquake generation at this plate boundary. Characterising the stress field across the Hikurangi Subduction Margin is a crucial element of to understanding the relationship between the contemporary in-situ stress state, active and inactive structures along the subduction front, and fluid pressures and the observed spatial variation in subduction behaviour. Existing stress observations rely on earthquake focal mechanisms and limited onshore borehole data from industry wells on the overriding plate. Reported pore pressures within the over-riding plate are often close to vertical stress magnitudes at shallow depths. Variability of in-situ stress orientations occur along strike of the subduction trench, with a subduction trench parallel SHmax in the south transitioning to a plate motion parallel, trench-oblique SHmax further north. This spatially correlates with observed changes in subduction interface coupling and earthquake behaviour. Here we present new stress field orientation data acquired from resistivity image logging carried out in IODP Expedition 372 using the logging while drilling GeoVision Resistivity tool. We report Shmin orientations from borehole breakout observations of N-S at Site U1518 near the deformation front, and NW-SE from Site U1519 within the upper plate. These data represent the first estimates of stress field orientation (from drilling data) in the outer forarc, near the deformation front of the Hikurangi Margin, an area characterised by shallow slow slip.
    Type: Conference or Workshop Item , NonPeerReviewed
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  • 4
    Publication Date: 2017-07-31
    Description: Gas hydrate samples from various locations in the Gulf of Mexico (GOM) differ considerably in their microstructure. Distinct microstructure characteristics coincide with discrete crystallographic structures, gas compositions and calculated thermodynamic stabilities. The crystallographic structures were established by X-ray diffraction, using both conventional X-ray sources and high-energy synchrotron radiation. The microstructures were examined by cryo-stage Field-Emission Scanning Electron Microscopy (FE-SEM). Good sample preservation was warranted by the low ice fractions shown from quantitative phase analyses. Gas hydrate structure II samples from the Green Canyon in the northern GOM had methane concentrations of 70–80% and up to 30% of C2–C5 of measured hydrocarbons. Hydrocarbons in the crystallographic structure I hydrate from the Chapopote asphalt volcano in the southern GOM was comprised of more than 98% methane. Fairly different microstructures were identified for those different hydrates: Pores measuring 200–400 nm in diameter were present in structure I gas hydrate samples; no such pores but dense crystal surfaces instead were discovered in structure II gas hydrate. The stability of the hydrate samples is discussed regarding gas composition, crystallographic structure and microstructure. Electron microscopic observations showed evidence of gas hydrate and liquid oil co-occurrence on a micrometer scale. That demonstrates that oil has direct contact to gas hydrates when it diffuses through a hydrate matrix.
    Type: Article , PeerReviewed
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  • 5
    Publication Date: 2017-08-22
    Description: Two newly developed coring devices, the Multi-Autoclave-Corer and the Dynamic Autoclave Piston Corer were deployed in shallow gas hydrate-bearing sediments in the northern Gulf of Mexico during research cruise SO174 (Oct–Nov 2003). For the first time, they enable the retrieval of near-surface sediment cores under ambient pressure. This enables the determination of in situ methane concentrations and amounts of gas hydrate in sediment depths where bottom water temperature and pressure changes most strongly influence gas/hydrate relationships. At seep sites of GC185 (Bush Hill) and the newly discovered sites at GC415, we determined the volume of low-weight hydrocarbons (C1 through C5) from nine pressurized cores via controlled degassing. The resulting in situ methane concentrations vary by two orders of magnitudes between 0.031 and 0.985 mol kg− 1 pore water below the zone of sulfate depletion. This includes dissolved, free, and hydrate-bound CH4. Combined with results from conventional cores, this establishes a variability of methane concentrations in close proximity to seep sites of five orders of magnitude. In total four out of nine pressure cores had CH4 concentrations above equilibrium with gas hydrates. Two of them contain gas hydrate volumes of 15% (GC185) and 18% (GC415) of pore space. The measurements prove that the highest methane concentrations are not necessarily related to the highest advection rates. Brine advection inhibits gas hydrate stability a few centimeters below the sediment surface at the depth of anaerobic oxidation of methane and thus inhibits the storage of enhanced methane volumes. Here, computerized tomography (CT) of the pressure cores detected small amounts of free gas. This finding has major implications for methane distribution, possible consumption, and escape into the bottom water in fluid flow systems related to halokinesis.
    Type: Article , PeerReviewed
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  • 6
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    PANGAEA
    In:  Supplement to: Klapp, Stephan A; Bohrmann, Gerhard; Kuhs, Werner F; Murshed, Mangir M; Pape, Thomas; Klein, Helmut; Techmer, Kirsten S; Heeschen, Katja U; Abegg, Friedrich (2010): Microstructures of structure I and II gas hydrates from the Gulf of Mexico. Marine and Petroleum Geology, 27(1), 116-125, https://doi.org/10.1016/j.marpetgeo.2009.03.004
    Publication Date: 2020-01-18
    Description: Gas hydrate samples from various locations in the Gulf of Mexico (GOM) differ considerably in their microstructure. Distinct microstructure characteristics coincide with discrete crystallographic structures, gas compositions and calculated thermodynamic stabilities. The crystallographic structures were established by X-ray diffraction, using both conventional X-ray sources and high-energy synchrotron radiation. The microstructures were examined by cryo-stage Field-Emission Scanning Electron Microscopy (FE-SEM). Good sample preservation was warranted by the low ice fractions shown from quantitative phase analyses. Gas hydrate structure II samples from the Green Canyon in the northern GOM had methane concentrations of 70-80% and up to 30% of C2-C5 of measured hydrocarbons. Hydrocarbons in the crystallographic structure I hydrate from the Chapopote asphalt volcano in the southern GOM was comprised of more than 98% methane. Fairly different microstructures were identified for those different hydrates: Pores measuring 200-400 nm in diameter were present in structure I gas hydrate samples; no such pores but dense crystal surfaces instead were discovered in structure II gas hydrate. The stability of the hydrate samples is discussed regarding gas composition, crystallographic structure and microstructure. Electron microscopic observations showed evidence of gas hydrate and liquid oil co-occurrence on a micrometer scale. That demonstrates that oil has direct contact to gas hydrates when it diffuses through a hydrate matrix.
    Type: Dataset
    Format: application/zip, 2 datasets
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  • 7
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    Unknown
    PANGAEA
    In:  Supplement to: Heeschen, Katja U; Hohnberg, Hans-Jürgen; Haeckel, Matthias; Abegg, Friedrich; Drews, Manuela; Bohrmann, Gerhard (2007): In situ hydrocarbon concentrations from pressurized cores in surface sediments, Northern Gulf of Mexico. Marine Chemistry, 107(4), 498-515, https://doi.org/10.1016/j.marchem.2007.08.008
    Publication Date: 2020-01-18
    Description: Two newly developed coring devices, the Multi-Autoclave-Corer and the Dynamic Autoclave Piston Corer were deployed in shallow gas hydrate-bearing sediments in the northern Gulf of Mexico during research cruise SO174 (Oct-Nov 2003). For the first time, they enable the retrieval of near-surface sediment cores under ambient pressure. This enables the determination of in situ methane concentrations and amounts of gas hydrate in sediment depths where bottom water temperature and pressure changes most strongly influence gas/hydrate relationships. At seep sites of GC185 (Bush Hill) and the newly discovered sites at GC415, we determined the volume of low-weight hydrocarbons (C1 through C5) from nine pressurized cores via controlled degassing. The resulting in situ methane concentrations vary by two orders of magnitudes between 0.031 and 0.985 mol kg**-1 pore water below the zone of sulfate depletion. This includes dissolved, free, and hydrate-bound CH4. Combined with results from conventional cores, this establishes a variability of methane concentrations in close proximity to seep sites of five orders of magnitude. In total four out of nine pressure cores had CH4 concentrations above equilibrium with gas hydrates. Two of them contain gas hydrate volumes of 15% (GC185) and 18% (GC415) of pore space. The measurements prove that the highest methane concentrations are not necessarily related to the highest advection rates. Brine advection inhibits gas hydrate stability a few centimeters below the sediment surface at the depth of anaerobic oxidation of methane and thus inhibits the storage of enhanced methane volumes. Here, computerized tomography (CT) of the pressure cores detected small amounts of free gas. This finding has major implications for methane distribution, possible consumption, and escape into the bottom water in fluid flow systems related to halokinesis.
    Type: Dataset
    Format: application/zip, 5 datasets
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  • 8
    Publication Date: 2020-01-18
    Type: Dataset
    Format: text/tab-separated-values, 123 data points
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  • 9
    Publication Date: 2020-01-18
    Type: Dataset
    Format: text/tab-separated-values, 42 data points
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  • 10
    facet.materialart.
    Unknown
    PANGAEA
    In:  Supplement to: MacDonald, Ian R; Bohrmann, Gerhard; Escobar, E; Abegg, Friedrich; Blanchon, P; Blinova, Valentina N; Brueckmann, Warner; Drews, Manuela; Eisenhauer, Anton; Han, X; Heeschen, Katja U; Meier, Felix; Mortera, Carlos; Naehr, T; Orcutt, B; Bernard, B; Brroks, J; de Farágo, M (2004): Asphalt volcanism and chemosynthetic life, Campache Knolls, Gulf of Mexico. Science, 304(5673), 999-1002, https://doi.org/10.1126/science.1097154
    Publication Date: 2020-01-18
    Description: In the Campeche Knolls, in the southern Gulf of Mexico, lava-like flows of solidified asphalt cover more than 1 square kilometer of the rim of a dissected salt dome at a depth of 3000 meters below sea level. Chemosynthetic tubeworms and bivalves colonize the sea floor near the asphalt, which chilled and contracted after discharge. The site also includes oil seeps, gas hydrate deposits, locally anoxic sediments, and slabs of authigenic carbonate. Asphalt volcanism creates a habitat for chemosynthetic life that may be widespread at great depth in the Gulf of Mexico.
    Type: Dataset
    Format: text/tab-separated-values, 31 data points
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