DOE/NETL Methane Hydrate Projects
 
Characterizing the Response of the Cascadia Margin Gas Hydrate Reservoir to Bottom Water Warming Along the Upper Continental Slope Last Reviewed 6/1/2015

DE-FE0013998

Goal
The goal of this project is to investigate the response of the Washington margin gas hydrate system to the contemporary warming of bottom water along the upper continental slope.

Performer
University of Washington – Seattle, Washington

Background
The up-slope limit of hydrate stability represents one of the most climate-sensitive boundaries for the global hydrocarbon reservoir. Compared to other climate-sensitive gas hydrate accumulations—including those associated with thinning Arctic permafrost—continental slope hydrates are located in close proximity to actively circulating seawater. This close physical association promotes hydrate dissociation over relatively short timescales (i.e., periods of tens of years vs. 100s to 1000s of years for other climate-sensitive deposits) in response to modest seawater warming at intermediate depth. Documenting the vulnerability of these hydrates to ocean warming and quantifying the fate of methane during transit through the sediment and water column are high priorities and have implications for the global ocean-atmosphere inventory of greenhouse gases. This hydrate-derived flux could contribute to ocean acidification and hypoxia through microbial oxidation of methane, initiate large-scale collapse of continental slopes producing coastal tsunamis, and increase the emission of methane-derived CO2 from the ocean to the atmosphere.

This project focuses on the upper limit of gas hydrate stability along the Washington segment of the Cascadia margin. The Washington margin has been the focus of an impressive array of recent scientific initiatives and programs including Earthscope, the Plate Boundary Observatory, the Ocean Observatories Initiative, GeoPRISMS, the ARRA Cascadia Initiative, as well as several large National Science Foundation research projects including the COAST 2-D multi-channel seismic (MCS) survey on the R/V Langseth in 2012, the Johnson/Solomon heat flow and fluid flux experiment on the R/V Atlantis off Grays Canyon in August 2013, and multiple ocean bottom seismic deployments in 2012, 2013, and 2014. Because of this high level of scientific activity, many of the parameters associated with the distribution and stability of methane hydrates are already well-characterized making the Washington margin a rich target area to examine the response of methane hydrate to environmental changes.

Washington margin bathymetry map identifying key sites
Washington margin bathymetry map identifying key sites. Yellow circles are methane plume sites. Numbers next to plumes are the water depth of the emission sites. Yellow dashed box is the R/V Langseth MCS survey that identified large areas of bottom-simulating reflectors (BSRs) (Holbrook et al., 2012). Broad yellow line is schematic trackline for the planned 2014 expedition following 500 m contour.Blue boxes are areas for detailed conductivity, temperature, and depth (CTD); water sampling; and coring sites.

Impact
This project constitutes one of the first field programs outside the Arctic focused primarily on the response of a methane hydrate system at the upper limit of gas hydrate stability to environmental change. This detailed study of gas hydrate and methane dynamics in a mid-latitude margin, one that is highly susceptible to the warming of bottom water, is relevant to current research priorities that have been identified by the gas hydrate science community on the response of methane hydrate systems to climate change. Understanding this response to climate forcing, and quantifying the flux and sinks of methane associated with these hydrate occurrences, is important for constraining the significance of methane/gas hydrate dynamics to the global ocean-atmosphere system and how this process contributes to hypoxia and ocean acidification.

Accomplishments

  • Compiled all relevant and available MCS profiles, swath bathymetry, high-resolution CTD profiles, and acoustic backscatter data on the Washington margin.
  • Converted MCS data to P-wave Velocity (Vp) vs. depth profiles for estimating sediment porosities.
  • Compiled sediment core archives from legacy coring programs on Washington margin to determine sediment lithology in order to provide guidance regarding the distribution and partitioning of sediments into turbidites and pelagic sediments.
  • Characterized decadal scale temporal variations of bottom water temperature along the upper continental slope.
  • Established the distribution of gas hydrates and geothermal gradients along the Washington margin.
  • Reviewed existing compilations of seep sites and archive mid-water column sonar data to characterize the depth distribution of seeps along the Cascadia margin.
  • Identified methane emission sites extending from the deformation front to the shelf along the Cascadia margin.
  • Integrated archived data into numerical simulations to model ocean warming and methane hydrate dissociation along the upper continental slope of Washington.
  • Used numerical simulations to provide quantitative estimates of the response of hydrate stability associated with modern environmental change. Simulation results show a downslope retreat of the gas hydrate stability zone (GHSZ) along all three simulated profiles over the past 40 years, indicating that the upslope limit of the GHSZ is sensitive to contemporary bottom water warming along the Washington margin.
  • Characterization of long-term bottom water warming trends and simulation results are detailed in the American Geophysical Union Geophysical Research Letter 10.1002/2014GL061606.
  • Results from pre-cruise data analysis and numerical simulations provided the context for a systematic geophysical and geochemical survey of methane seepage along the upper continental slope of the Washington margin.

    Bathymetry map of archive data locations along the Washington continental slope
    Bathymetry map of archive data locations along the Washington continental slope showing conductivity-temperature-depth (CTD) observations as orange dots, bathymetric transects (P1–P3) as blue stars, and known methane seep sites as green triangles. (Hauntala et al. 2014)

  • Conducted a 10-day research expedition on the R/V Thompson from October 10-19, 2014. The field program exceeded expectations by accomplishing the following sampling, data collection, and analysis:
    • Sampled nine seep sites and two background sites
    • Imaged the entire upper continental slope of Washington to generate seafloor slope maps
    • Imaged 22 active bubble plumes with high resolution techniques
    • Deployed 39 gravity cores and 2 piston cores
    • Collected 20 full water column CTD profiles
    • Sub-sampled >300 whole-round cores for pore water geochemistry
    • Collected ~400 water samples for geochemical analyses
    • Analyzed water samples for pore water salinity, pH, alkalinity, and bottom water C1-C4 concentrations
  • Geochemical analysis is ongoing to characterize Cl and SO4 concentrations as well as δD, δ18O, and δ13CDIC stable isotope ratios.
  • Porosity analyses have been completed on all cores.
  • Preliminary results indicate that pore waters are 65% fresher than seawater which could be a result of gas hydrate dissociation, meteoric water, and/or clay dehydration at depth. Work is ongoing to determine the relative contribution from each fluid source. Preliminary results also indicate that the sulfate-methane transition zone varies from site to site ranging from 5 centimeters to >2 meters below the surface.

Current Status (June 2015)
Entering into the final phase of the project, The University of Washington continues post-cruise data and sample analysis. Post-processing of ship-board geophysical data, including acoustic Dopler current profiler, CTD, and 3.5 kHz profiles, is underway. Geochemical analysis of pore water and water column sub-samples is ongoing to determine Cl, SO4, and C1-C4 hydrocarbon concentrations as well as δD, δ18O, and δ13CDIC stable isotope ratios. Continued analysis, integration, and publication will be the major focus of the final phase of this project.

Project Start: October 1, 2013
Project End: September 30, 2016

Project Cost Information:
Phase 1 – DOE Contribution: $360,808, Performer Contribution: $200,000
Phase 2 – DOE Contribution: $270,161, Performer Contribution: $0
Planned Total Funding:
DOE Contribution: $630,969, Performer Contribution: $200,000

Contact Information:
NETL – Stephen Henry (stephen.henry@netl.doe.gov or 304-285-2083)
University of Washington – Dr. Evan Solomon (esolomn@uw.edu or 206-221-6745)
University of Washington – Dr. H. Paul Johnson (johnson@ocean.washington.edu or 206-543-8474)

Additional Information:

Research Performance Progress Report [PDF-289KB] January - March, 2015

Research Performance Progress Report [PDF-496KB] October - December, 2014

Research Performance Progress Report [PDF-482KB] July - September, 2014

Research Performance Progress Report [PDF-473KB] April - June, 2014

Research Performance Progress Report [PDF-472KB] January - March, 2014

Research Performance Progress Report [PDF-473KB] October - December, 2013

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