High-Resolution Analysis of the Nature and Volume of Gas Hydrate and Carbonate Mineralization Across the Oregon Margin Accretionary Complex

 

 

 

   

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Gas Hydrates and Methane Carbonates

 

TECFLUX
 

COAS logo, Active Tectonics Lab Geomar, submarine research logo Monterey Bay REasearch Institute logo tecflux gas hydrate research project logo
 

High-Resolution Analysis of the Nature and Volume of Gas Hydrate and Carbonate Mineralization Across the Oregon Margin Accretionary Complex

 

C. Goldfinger, J. Johnson, M. Torres, A. Trehu

Oregon State University College of Oceanic and Atmospheric Sciences

Abstract

Gas hydrates are solid substances composed of cages of water that enclose a low- weight molecular gas, mainly methane.  Natural gas hydrates and free gas are common within seafloor sediments of the world's continental margins where water depths exceed 300 to 500 m.  Estimates on the potential amount of carbon locked in gas hydrate systems generally range from 1015 to 1016 kg of carbon, such that they may constitute the largest single form of fossil fuel hydrocarbons on the planet (e.g. Kvenvolden 1995). Destabilized gas hydrate beneath the seafloor could have broad implications for regional sediment stability, as well as for global climate change (MacDonald, 1990: Gornitz and Fung, 1994). Investigators have suggested that dissociation of methane hydrates at the close of the Pleistocene increased post-glacial warming (Kvenvolden, 1988b; Nisbet, 1990).  Paull et al., (1990) argued that lowered sea level during the Pleistocene triggered hydrate dissociation, causing destabilization of slopes and massive methane release, thus acting as a limiting factor in glacial growth and further global cooling.  The effects on global climate of the massive methane source under continental margins remains largely speculative because relatively little is known about the effects of changes in the hydrate layer on slope stability, and most importantly, the total volume of methane is poorly known.

 

bathymetry, tecflux hydrate study area, cascadia margin

Figure 1. Shaded relief bathymetry of the Hydrate Ridge region. Contour interval is 100 m and bathymetric grid is 100 m pixel resolution. Inset shows Pacific Northwest bathymetry and topography (Haugerud, 1999) and the location of Hydrate Ridge region on the lower continental slope of the Cascadia accretionary prism. Hydrate Ridge is a NE-SW trending thrust ridge with northern and southern summits; (NHR) Northern Hydrate Ridge; (SHR) Southern Hydrate Ridge. The ridge is located ~10 km from the deformation front (DF) and bordered on the west and east by slope basins (HRB-W) Hydrate Ridge Basin-West and (HRB-E) Hydrate Ridge Basin-East. ODP (Ocean Drilling Program) site 891 on the crest of the first accretionary ridge (FAR) and site 892 on NHR are shown. Daisy Bank (DB) is also shown. Click for larger image

 

This project addressed the nature, distribution and hydrogeochemistry of gas hydrate, free gas and carbonate by-products in a transect across a continental margin. Theproject had two components.  The first sought to define the spatial and volumetric carbonate and gas hydrate presence in a corridor spanning a gas hydrate province from the lower continental slope to the landward limit of hydrate stability.  We used both sidescan sonar, high-resolution seismic reflection, and had planned to use a semi autonomous coring drill rig to map the surface and shallow subsurface extent of hydrates, and carbonates associated with fluid venting. We conducted a high-resolution, low-frequency sidescan sonar/bathymetry survey of a transect across the Cascadia margin centered on ODP Site 892 at the crest of Hydrate Ridge, on the central Oregon Cascadia margin. The 30 kHz sonar can penetrate the thin sediments that drape the authigenic carbonate by-products of methane charged fluid venting.  The sonar data can be corrected for the reflectance effects of topography to produce imagery that maps areas of high reflectance.  Previous experience has shown that high reflectance in the study can be correlated with venting-related carbonates.  These data, when related to detailed structural analysis of the corridor,aallowed us to asses the distribution of fluid venting using the carbonate proxy, and the roles of focused and diffuse fluid flow.
   

bathymetry, seamarc sidescan sonar image, tecflux hydrate study area, cascadia margin

Figure 2. The SeaMARC 30 sidescan sonar mosaic overlain on 100m shaded relief bathymetry. The SM30 survey is gridded at 1 m pixel resolution. Notice the three categories of backscatter present across the region (inset map). Category I backscatter is characterized as blotchy to circular with apparent surface roughness, category II backscatter is blotchy to circular with no apparent surface roughness, and category III streaky to continuous with variable surface roughness. The three regions of each backscatter category are shown in the inset and depict locations in the survey where each category is dominant. Note the coincidence of most of the category III backscatter with the regions of greatest slope. The major geologic and geographical features are labeled as follows; (HRB-W) Hydrate Ridge Basin-West; (NHR) Northern Hydrate Ridge; (SHR) Southern Hydrate Ridge; (HRB-E) Hydrate Ridge Basin-East; (DB) Daisy Bank. Click for larger image

 

To maximize our seafloor resolution and in order to image carbonates buried by a thin veneer of hemipelagic mud we chose the low frequency (30 kHz), deep-towed, SeaMARC 30 (SM 30) sidescan sonar system, operated by Williamson and Associates in Seattle, Washington.  The sonar was towed at a depth of ~200 m above the seafloor and collected data in ~3.0 km swaths across the entire region and ~1.5 km swaths across the crest of Hydrate Ridge.  The frequency on the port side is 27 kHz and on the starboard side 30 kHz.  The gain of the sonar was adjusted manually in 10, 3dB steps to gain approximately equal record intensity across the survey.  Navigation was by Sonardyne USBL (Ultra-Short BaseLine acoustic positioning).  The R/V New Horizon was used to tow the SM 30 at 2-3 knots.  The sidescan images were acquired and processed using Triton Elics International (TEI) Isis sonar processing software, and ultimately georeferenced and gridded at 1 m pixel resolution for the entire survey using Erdas Imagine software.

bathymetry, seamarc sidescan sonar image, tecflux hydrate study area, cascadia margin

 

Figure 3. (A) SeaMARC 30 coverage and ground truth across Hydrate Ridge. TVG (tvgrab) and OFOS (tv-camera tow) tracks from Sonne Leg 143 are shown. Mud volcanoes MV1, MV2, and MV3 are shown (see Other Fluid Venting Manifestations) as well as the Southern Hydrate Ridge (SHR) pinnacle (note the acoustic shadow on the imagery). The intermediate backscatter on SHR represents seafloor gas hydrate as observed in ALVIN dives (Tones et al., 1999). (B) OFOS track 216 across NHR (from Bohrmann et al., 2000). The diagram was constructed from deep towed video observations of the seafloor. Note the coincidence of the chemoherm carbonates and carbonate crusts with the regions of highest backscatter along the track. (C) A bottom type map constructed from ALVIN observation on NHR (from Tones et al., 1999). Again note the coincidence of high backscatter on the survey and the carbonates observed on the seafloor. (D) ALVIN photograph of the carbonate on the SHR pinnacle (photo courtesy of Marta Tones, Oregon State University). Notice the large fracture in the middle of the image. Click for larger image

 

 

bathymetry, seamarc sidescan sonar image, tecflux hydrate study area, cascadia margin

Figure 4. (A) SeaMARC 30 coverage at the western edge of HRB-E. Notice the circular category II high backscatter patches. Tv-camera tow tracks and the gravity and multicorer sites described in the text are shown. (B) Methane distribution in surface sediment multicorer samples taken at some of the backscatter patches shown in (A). Samples taken at a bright backscatter patch (50148175-1B-MUC) yielded high methane, at a dark backscatter patch (S0143/31-1A-MUC) lower methane, and at a low backscatter reference site, at the center of HRB-E (S0143!63-1A-MUC), an even lower methane concentration. (C) Methane distribution in two surface sediment gravity core samples (locations shown in A). High methane was found in both gravity core samples; one taken from a dark backscatter patch (S0143/32-2-SL) and a second from a bright backscatter patch (S0143/35-1SL). Gravity core (S0148-76-SL) taken at a very bright backscatter patch recovered carbonates and gas hydrates. Tv-camera tow track S0148/9 also documented authigenic carbonate at the surface (on the same backscatter patch sampled by gravity core S0148/76-SL). The remaining tv-camera tows documented only sediments and bacterial mats at the surface, suggesting that some of the high backscatter patches may be caused by authigenic carbonates andlor gas hydrates slightly buried by hemipelagic sediment.

 

ALVIN image, bathymetry, seamarc sidescan sonar image, tecflux hydrate study area, cascadia margin

 

 

 

 

 

Figure 5. ALVIN image of chemosynthetic clamm community and carbonate debris on the crest of Hydrate Ridge.

 

 

 

 

 

 

 

 

 

 

 

 

interpretation of sidescan, seamarc sidescan sonar image, tecflux hydrate study area, cascadia margin

 

 

 

Figure 6. Schematic diagrams depicting the environments likely responsible for each
of the backscatter categories. Suggested pathways responsible for the delivery of fluids
to the shallow subsurface and gas hydrate stability zone (BSR labeled) are shown in the
lower block diagram. Fluids migrate and accumulate at structural highs like Hydrate
Ridge and the first accretionary ridge (FAR) and to a lesser extent on the eastern slope
east of HRB-E. The structural highs are local sites of tension and it is likely faults and
fractures permeate their crests. Category I backscatter occurs in this environment and, in
the presence of abundant gas hydrate, fluid flow responsible for the carbonate
precipitation at the surface is characterized as structurally influenced and gas hydrate
related. Category II backscatter occurs on the eastern slope, east of HRB-E. It is
characterized by variable intensity and a patchy distribution of backscatter patterns,
suggestive of sporadic seafloor venting. It also lies in a region of the wedge that is mildly
deformed, thus structural control on fluid expulsion sites is unlikely. Fluid flow
responsible for this backscatter is likely diffuse and related to disruptions in the
underlying gas hydrate. Category III backscatter across the Daisy Bank fault zone
suggests long-term deep-seated fluid flow. Its location above the hydrate stability zone
and the linear backscatter patterns suggest this region is dominated by structurally
controlled non-gas hydrate related fluid flow.

 

 

 

 

shaded views of the structure of Hydrate ridge, Cascadia margin

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 7. Details of gravity−driven extension/compression paired structures at the crest of Hydrate Ridge. A. Uninterpreted shaded relief image of Simrad EM−300 data. Low−angle sun shading is used here to illuminate subtle morphology. Shading is from the southeast, the view is from the southwest, depth contours are in meters. B. The ridge crest is in tension due to the bending moment resulting from growth of the main anticline. The ridge crest is under greatest tension at its north end, where throw on the underlying thrust is greatest. Tension resulting from the decreased radius of curvature has driven extension of the ridge top. Blocks sliding westward (downslope) are moving on the base of the gas hydrate layer (BSR), and compress at their downslope ends as pressure ridges. It is one of these pressure−ridges that was previously interpreted as a thrust fault, and was drilled as ODP Site 892. Gas pits have formed in areas of maximum extension. Bioherm development is mainly on the hanging wall blocks. Smaller slumps have subsequently modified the main scarp. Ticks indicate headwall, compressional faulting shown by sawteeth. (Click for larger image)

 

 

 

Conclusions
Based on seafloor mapping using high resolution SeaMARC 30 sidescan sonar data coupled with seafloor observations, samples, and subsurface geologic mapping the following conclusion can be made: (1) the blotchy to circular category I high backscatter present on Hydrate Ridge is indeed authigenic carbonate, (2) the category II high backscatter present along the eastern side of the survey is likely authigenic carbonate, with some gas hydrate, slightly buried by hemipelagic sediment, (3) both category I and II high backscatter sites represent carbonates that may have precipitated from destabilized gas hydrate and show an intimately linked gas hydrate carbonate system, however on Hydrate Ridge, direct fluid migration from depth also likely contributes to the carbonate precipitation, (4) breached anticlines serve not only to trap and aid in the migration of fluids and gases through the sediment column, but also serve as escape pathways, providing a local extensional environment at their crests for fluid expulsion, (5) diffuse gas hydrate related fluid flow is likely responsible for the category II high backscatter on the eastern slope, east of HRB-E, (6) category III backscatter associated with the Daisy Bank fault zone is likely derived from deep seated fluids that have a long history of escape along the fault and are likely unrelated to the destabilization of gas hydrate, and (7) the abrupt decrease of pockmark fields and category II backscatter patches on the eastern edge of the survey area may serve to delineate the landward limit of gas hydrate stability across this region.

 


Acknowledgements
We thank the 1999 R/V New Horizon crew, the shipboard scientific party, and Williamson and Associates of Seattle, WA for their hard work during the SeaMARC 30 data acquisition and processing phase.  We also thank Gerhard Bohrmann (GEOMAR), for helpful comments and suggestions during the sidescan sonar cruise and use of the R/V Sonne Leg 143 data, Peter Linke (GEOMAR) for use of the R/V Sonne SO148 OFOS data, and Marta Torres (OSU) for use of her ALVIN observations on Hydrate Ridge.  Additional thanks to Jason Chaytor (OSU) for compiling the 50 m bathymetric grid used to make the slope map and Anne Tréhu and Matt Arsenault (OSU) for assistance with the 1989 Digicon seismic data.  This manuscript also benefited from very thorough and thoughtful reviews by Nathan Bangs, David Piper, and an anonymous reviewer.  GEOMAR collaborators on cruises SO143 and SO148 were funded through the TECFLUX projects by the Federal Ministry of Science and Technology (BMBF; grants 03GO/143/148).   Funding for the TECFLUX project at OSU for the data collection and processing of the SM30 data was provided by NSF Award #9731023. Acknowledgement is also made to the Donors of The Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research.

 

The TECFLUX project was a component of a multi-institution collaboration
scientists Oregon State University (USA) at GEOMAR (Germany) and the Monterey
Bay Research Institute (MBARI, USA) .

 

Publications

Johnson, J.E., Goldfinger, C., Bangs, N.L., Tréhu, A.M. and Chevallier, J., In revision, Structural vergence variation and clockwise block rotation in the Hydrate Ridge region, Cascadia accretionary wedge offshore Oregon.

 

Johnson, J.E., 2005, Deformation, fluid venting, and slope failure at an active margin gas hydrate province, Hydrate Ridge Cascadia accretionary wedge, [PhD thesis], Oregon State University, Corvallis, Oregon, 159 pp. Plate 1 Plate2

Johnson, J.E., Goldfinger, C., and Suess, E., 2003, Geophysical constraints on the surface distribution of authigenic carbonates aross the Hydrate Ridge region, Cascadia margin: Marine Geology, v. 202, p. 79-110.

Johnson, J.E., Goldfinger, C, Trehu, A.M., Bangs, N., Torres, M.E., and Chevallier, J., 2005, North-south variability in the history of deformation and fluid venting across hydrate ridge, Cascadia margin, Proceedings of the Ocean Drilling Program Scientific Results, Volume 204.