Gas hydrate on the northern Cascadia margin: regional geophysics and structural framework

Riedel, Michael; Willoughby, Eleanor C.; Chen, M. A.; He, Tingting; Novosel, I.; Schwalenberg, Katrin; Hyndman, R. D.; Spence, G. D.; Chapman, N. Ross; Edwards, R. N.

doi:10.2204/iodp.proc.311.109.2006

Cited by 24 publications

(19 citation statements)

References 74 publications

(105 reference statements)

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“…This result is in good agreement with the general depth of the BSR at LGF on the upper continental margins of >200 m (e.g. 240 mbsf on the northern Cascadia margin; Riedel et al, 2006; 200 mbsf on the Svalbard margin; Hustoft et al, 2009). However, GH can still be formed at a lower thickness of the GHSZ if fluid flow and/or gas ebullition are involved (e.g.…”

Section: Derivation Of the Transfer Functionsupporting

confidence: 84%

A transfer function for the prediction of gas hydrate inventories in marine sediments

et al. 2010

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Abstract.A simple prognostic tool for gas hydrate (GH) quantification in marine sediments is presented based on a diagenetic transport-reaction model approach. One of the most crucial factors for the application of diagenetic models is the accurate formulation of microbial degradation rates of particulate organic carbon (POC) and the coupled formation of biogenic methane. Wallmann et al. (2006) suggested a kinetic formulation considering the ageing effects of POC and accumulation of reaction products (CH 4 , CO 2 ) in the pore water. This model is applied to data sets of several ODP sites in order to test its general validity. Based on a thorough parameter analysis considering a wide range of environmental conditions, the POC accumulation rate (POCar in g/m 2 /yr) and the thickness of the gas hydrate stability zone (GHSZ in m) were identified as the most important and independent controls for biogenic GH formation. Hence, depth-integrated GH inventories in marine sediments (GHI in g of CH 4 per cm 2 seafloor area) can be estimated as: The transfer function gives a realistic first order approximation of the minimum GH inventory in low gas flux (LGF) systems. The overall advantage of the presented function is its simplicity compared to the application of complex numerical models, because only two easily accessible parameters need to be determined.

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Section: Derivation Of the Transfer Functionsupporting

confidence: 84%

A transfer function for the prediction of gas hydrate inventories in marine sediments

et al. 2010

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“…Water density (ρ f ) varied based on in situ pore fluid temperature [Sharqawy et al, 2010;Nayar et al, 2016], salinity of 33.5 g kg À1 , and seafloor depth (Z w ) and Z BSR . We assume a pure methane gas composition [Davis et al, 1990;Kastner et al, 1995] and pore water salinity at the BSR depth similar to ocean salinity of 33.5 g kg À1 [Riedel et al, 2006;Liu and Flemings, 2006;Kastner et al, 1995]. BSR pressure and salinity provide a BSR temperature using an empirically derived stability relationship [Tishchenko et al, 2005].…”

Section: Heat Flow Estimates From Bottom-simulating Reflectorsmentioning

confidence: 99%

Thermal environment of the Southern Washington region of the Cascadia subduction zone

2017

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Eleven recently collected multichannel seismic (MCS) profiles from the Cascadia Open‐Access Seismic Transects experiment offshore Washington State are used to characterize the distribution of bottom‐simulating reflectors (BSRs) from seaward of the deformation front onto the continental shelf of the Cascadia Subduction Zone. The 11 MCS lines consisted of nine lines perpendicular and two lines parallel to the Cascadia margin covering a 100 km along‐strike region of the accretionary wedge. From these MCS profiles we generated a 3‐D view of the Cascadia margin thermal structure by interpreting 40,232 individual BSR picks in terms of temperature and heat flow. Overall BSR‐derived heat flow values decrease from approximately 95 mW m−2 10 km east of the deformation front to approximately 60 mW m−2 located 60 km landward of the deformation front. Anomalously low heat flow values near 25 mW m−2 on a prominent midmargin terrace indicate recent sediment failure within the accretionary prism. Localized differences between BSR heat flow and numerical models reflect an estimated regional mean vertical fluid flow of +0.53 cm yr−1 for the survey area, with localized fluid flow approaching a maximum of +3.8 cm yr−1. Distinct finite element models for the nine MCS profiles perpendicular to the deformation front reproduce BSR heat flow values, producing an overall root‐mean‐square misfit of 10.2 mW m−2. At the deformation front, the incoming oceanic sediment/crust interface temperatures vary from 164°C to 179°C, indicating the updip limit of the Cascadia seismogenic zone.

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“…To estimate the volume and mass of gas hydrate that could dissociate, we assume a 5 m thick sulfate reduction zone in 1970, with gas hydrate occurring only below this depth. Assuming an average porosity of 0.63 in the upper 15 m [Riedel et al, 2006], and an average gas hydrate saturation of 5% of the sediment pore space, approximately 129-164 m 3 of gas hydrate per meter of margin could dissociate. Thus, warming along the Washington upper continental slope between 1970 and 2013 has the potential to dissociate 0.12 to 0.15 Gg Geophysical Research Letters 10.1002/2014GL061606 of gas hydrate per meter assuming a Structure I hydrate density of 9 × 10 5 g m À3 (Table 1).…”

Section: Impact Of Seafloor Warming On the Methane Hydrate Reservoirmentioning

confidence: 99%

“…This rate (~0.5 Tg CH 4 yr À1 ), represents the annualized release of a volume roughly quadruple the amount released during the Deepwater Horizon spill and~11% of the yearly flux of methane into the Black Sea [Reeburgh, 2007]. [Riedel et al, 2006] and an assumed average gas hydrate saturation of 5% of the pore space. c Calculated from the volume considering a methane hydrate density of 9 × 10 5 g/m…”

Section: Projected Gas Hydrate Dissociation Through 2100mentioning

confidence: 99%

Dissociation of Cascadia margin gas hydrates in response to contemporary ocean warming

Hautala

Solomon

Johnson

et al. 2014

Geophysical Research Letters

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Gas hydrates, pervasive in continental margin sediments, are expected to release methane in response to ocean warming, but the geographic range of dissociation and subsequent flux of methane to the ocean are not well constrained. Sediment column thermal models based on observed water column warming trends offshore Washington (USA) show that a substantial volume of gas hydrate along the entire Cascadia upper continental slope is vulnerable to modern climate change. Dissociation along the Washington sector of the Cascadia margin alone has the potential to release 45-80 Tg of methane by 2100. These results highlight the importance of lower latitude warming to global gas hydrate dynamics and suggest that contemporary warming and downslope retreat of the gas hydrate reservoir occur along a larger fraction of continental margins worldwide than previously recognized.

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Gas hydrate on the northern Cascadia margin: regional geophysics and structural framework

Cited by 24 publications

References 74 publications

A transfer function for the prediction of gas hydrate inventories in marine sediments

A transfer function for the prediction of gas hydrate inventories in marine sediments

Thermal environment of the Southern Washington region of the Cascadia subduction zone

Dissociation of Cascadia margin gas hydrates in response to contemporary ocean warming

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