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  • Wiley  (3)
  • GSA (Geological Society of America)  (1)
  • 1
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    Marine-controlled source electromagnetic study of methane seeps and gas hydrates at Opouawe Bank, Hikurangi Margin, New Zealand (2017)
    AGU (American Geophysical Union) | Wiley
    In:  Journal of Geophysical Research: Solid Earth, 122 (5). pp. 3334-3350.
    Details
    Publication Date: 2020-02-06
    Description: Marine controlled source electromagnetic (CSEM) data have been collected to investigate methane seep sites and associated gas hydrate deposits at Opouawe Bank on the southern tip of the Hikurangi Margin, New Zealand. The bank is located in about 1000 m water depth within the gas hydrate stability field. The seep sites are characterized by active venting and typical methane seep fauna accompanied with patchy carbonate outcrops at the seafloor. Below the seeps, gas migration pathways reach from below the bottom-simulating reflector (at around 380 m sediment depth) toward the seafloor, indicating free gas transport into the shallow hydrate stability field. The CSEM data have been acquired with a seafloor-towed, electric multi-dipole system measuring the inline component of the electric field. CSEM data from three profiles have been analyzed by using 1-D and 2-D inversion techniques. High-resolution 2-D and 3-D multichannel seismic data have been collected in the same area. The electrical resistivity models show several zones of highly anomalous resistivities (〉50 Ωm) which correlate with high amplitude reflections located on top of narrow vertical gas conduits, indicating the coexistence of free gas and gas hydrates within the hydrate stability zone. Away from the seeps the CSEM models show normal background resistivities between ~1 and 2 Ωm. Archie's law has been applied to estimate gas/gas hydrate saturations below the seeps. At intermediate depths between 50 and 200 m below seafloor, saturations are between 40 and 80% and gas hydrate may be the dominating pore filling constituent. At shallow depths from 10 m to the seafloor, free gas dominates as seismic data and gas plumes suggest.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1002/2016JB013702
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 2
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    Paleo-Fluid Expulsion Influencing Contouritic Drift Formation on the Chatham Rise, New Zealand (2018)
    Wiley
    In:  Basin Research, 30 (1). pp. 5-19.
    Details
    Publication Date: 2021-02-08
    Description: The Chatham Rise is located offshore of New Zealand's South Island. Vast areas of the Chatham Rise are covered in circular to elliptical seafloor depressions that appear to be forming through a bathymetrically controlled mechanism, as seafloor depressions 2-5 km in diameter are found in water depths of 800-1100 m. High resolution P-Cable 3D seismic data were acquired in 2013 across one of these depressions. The seafloor depression is interpreted as a mounded contourite. Our data reveal several smaller buried depressions (〈20-650 m diameter) beneath the mounded contourite that we interpret as paleo-pockmarks. These pockmarks are underlain by a complex polygonal fault system that deforms strata and an unusual conical feature. We interpret the conical feature as a sediment remobilization structure based on the presence of stratified reflections within the feature, RMS amplitude values and lack of velocity anomaly that would indicate a non-sedimentary origin. The sediment remobilization structure, polygonal faults and paleo-depressions are indicators of past subsurface fluid flow. We hypothesize that the pockmarks provided the necessary topographic roughness for formation of the mounded contourites thus linking fluid expulsion and deposition of contouritic drifts.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1111/bre.12237
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 3
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    Gas-controlled seafloor doming (2015)
    GSA (Geological Society of America)
    In:  Geology, 43 (7). pp. 571-574.
    Details
    Publication Date: 2019-10-24
    Description: The upward migration of gas through marine sediments typically manifests itself as gas chimneys or pipes in seismic images and can lead to the formation of cold seeps. Gas seepage is often linked to morphological features like seabed domes, pockmarks, and carbonate build-ups. In this context, sediment doming is discussed to be a precursor of pockmark formation. Here, we present parametric echosounder, sidescan sonar, and two-dimensional seismic data from Opouawe Bank, offshore New Zealand, providing field evidence for sediment doming. Geomechanical quantification of the stresses required for doming show that the calculated gas column heights are geologically feasible and consistent with the observed geophysical data. The progression from channeled gas flow to gas trapping results in overpressure build-up in the shallow sediment. Our results suggest that by breaching of domed seafloor sediments a new seep site can develop, but contrary to ongoing discussion this does not necessarily lead to the formation of pockmarks.
    Type: Article , PeerReviewed , info:eu-repo/semantics/article
    Format: text
    DOI: 10.1130/G36596.1
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    OceanRep: Article in a Scientific Journal - peer-reviewed
  • 4
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    A quantitative assessment of methane cycling in Hikurangi Margin sediments (New Zealand) using geophysical imaging and biogeochemical modeling (2016)
    AGU (American Geophysical Union) | Wiley
    In:  Geochemistry, Geophysics, Geosystems, 17 (12). pp. 4817-4835.
    Details
    Publication Date: 2019-02-01
    Description: Takahe seep, located on the Opouawe Bank, Hikurangi Margin, is characterized by a well-defined subsurface seismic chimney structure ca. 80,500 m2 in area. Sub-seafloor geophysical data based on acoustic anomaly layers indicated the presence of gas hydrate and free gas layers within the chimney structure. Reaction-transport modeling was applied to porewater data from 11 gravity cores to constrain methane turnover rates and benthic methane fluxes in the upper 10 m. Model results show that methane dynamics were highly variable due to transport and dissolution of ascending gas. The dissolution of gas (up to 3761 mmol m−2 yr−1) dwarfed the rate of methanogenesis within the simulated sediment column (2.6 mmol m−2 yr−1). Dissolved methane is mainly consumed by anaerobic oxidation of methane (AOM) at the base of the sulfate reduction zone and trapped by methane hydrate formation below it, with maximum rates in the central part of the chimney (946 and 2420 mmol m−2 yr−1, respectively). A seep-wide methane budget was constrained by combining the biogeochemical model results with geophysical data and led to estimates of AOM rates, gas hydrate formation and benthic dissolved methane fluxes of 3.68 × 104 mol yr−1, 73.85 × 104 mol yr−1and 1.19 × 104 mol yr−1, respectively. A much larger flux of methane probably escapes in gaseous form through focused bubble vents. The approach of linking geochemical model results with spatial geophysical data put forward here can be applied elsewhere to improve benthic methane turnover rates from limited single spot measurements to larger spatial scales.
    Type: Article , PeerReviewed
    Format: text
    DOI: 10.1002/2016GC006643
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    OceanRep: Article in a Scientific Journal - peer-reviewed
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