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Geochemical and Biomarker Study of the Gas Hydrate and Petroleum System of the Northern Cascadia Margin: John W. Pohlman

Project Title: Geochemical and Biomarker Study of the Gas Hydrate and Petroleum System of the Northern Cascadia Margin
Mendenhall Fellow: John W. Pohlman, (508) 457-2213,
Duty Station: Woods Hole, MA
Start Date: March 4, 2007
Education: Ph.D. Chemical Oceanography, School of Marine Science, College of William and Mary, 2006
Research Advisors: Deborah Hutchinson, (508) 457-2263,; Timothy Collett, (303) 236-5731,

Project Description: Gas hydrate, a crystalline form of gas (primarily methane) and water, occurs in continental slope margins and arctic permafrost environments in concentrations that have the potential to influence future energy supplies and the global carbon cycle.  Integrated Ocean Drilling Program (IODP) Expedition 311 was dedicated to understanding the evolution of an entire gas hydrate system within the northern Cascadia margin accretionary complex (fig. 1).  Results from this margin-perpendicular transect have challenged the simple model that gas hydrate is concentrated where gas-charged fluids ascend into the gas hydrate stability zone.  Results from that program suggest local methane solubility, fluid advection rates, and availability of suitable host material are also important controlling factors.

The purpose of this project is to determine to what extent organic matter controls the occurrence, distribution and fate of gas hydrate methane within this accretionary margin.  At the most fundamental level, the origin of all methane in gas hydrate is buried organic matter.  The mechanisms for liberating methane from organic matter are either microbial processes occurring in the upper 1000 m below the seafloor, high temperature pyrolysis at greater depth where temperatures exceed ~120°C, or some combination of both.  The age and depositional origin of the organic matter influence the reactivity of the organic matter, which, in turn, influences the rates of methanogenesis.  To determine how the cycling of organic matter and the occurrence of gas hydrate are related, organic geochemical data will be interpreted within the context of proxies used for estimating gas hydrate sediment saturation.  Stable isotope (d13C and dD) data from reactive carbon pools in the gas hydrate and dissolved fluid phases will be used to infer the methanogenic pathways (for example, thermogenic-vs-microbial).  Shifts in the isotopic signatures along the transect will provide unique insight into the maturation of the organic carbon reservoirs and its ability to generate enough methane to saturate the pore fluids (a requisite for gas hydrate formation).  Furthermore, the concentration and isotopic composition of lipid biomarkers from microbes associated with specific carbon cycles will be analyzed to constrain the role of chemotaxonomic groups in specific carbon cycles.

Northern Cascadia margin, offshore Vancouver
      Island (Canada)   Figure 1.  Northern Cascadia margin, offshore Vancouver Island (Canada).  The presence of gas hydrate has been seismically inferred beneath 50% of the continental margin.  The study sites for this project are the IODP Expedition 311 drilling transect and two cold seeps (Bullseye vent and Barkley Canyon).

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Last modified: 16:08:32 Thu 13 Dec 2012