Sediment biogeochemistry of northern Cascadia margin shallow gas hydrate systems

• Main Author: Pohlman, John W.
• Format: Others
• Language: English
• Published: W&M ScholarWorks 2006
• Subjects: Biogeochemistry
• Online Access: https://scholarworks.wm.edu/etd/1539616813; https://scholarworks.wm.edu/cgi/viewcontent.cgi?article=2380&context=etd

Methane contained in gas hydrate is a significant component of the global organic carbon inventory. Describing the methane sources supporting these systems and the mechanisms that control the distribution of methane in marine sediments are critical elements in evaluating the resource potential, climate change implications and geologic hazards associated with gas hydrate. The northern Cascadia margin (offshore Vancouver Island, Canada) is a convergent margin with gas hydrate-bearing cold seeps composed of both thermogenic (Barkley Canyon) and microbial (Bullseye vent) gas sources. Gas hydrate and sediment cores were collected from each of these settings to examine the sources that sustain the gas hydrate and the biogeochemical processes that control the flux and cycling of methane carbon within the cold seep system. Gas hydrate bound methane from Barkley Canyon, Bullseye vent and four other oceanic locations was primarily synthesized from "fossil" organic matter devoid of radiocarbon. Recognizing that the global gas hydrate reservoir is comprised of fossil carbon may alter our understanding of how gas hydrate methane influences the radiocarbon content of other ocean carbon pools (e.g., dissolved organic carbon). Analysis of gas and oil samples from the thermogenic gas hydrate site, Barkley Canyon, indicated the thermogenic hydrocarbons were generated from a source rock with a low hydrocarbon producing potential. The pore water and sediment geochemistry from Barkley Canyon and Bullseye vent were evaluated to identify fluid flux regimes and the metabolic pathways that control how methane carbon was cycled by the microbial consortium. Evidence for coupling between anaerobic oxidation of methane (AOM) and methanogenesis was inferred from the stable carbon isotope composition of the dissolved inorganic carbon (DIC) and methane at both locations. Lipid biomarker analysis further indicated sulfate reducing bacteria consumed organic matter sources in addition to methane. This study demonstrated the value of integrating geochemical, chronological and geophysical data to understand the sources and cycling of methane in cold seep systems and provided a more comprehensive model for how the northern Cascadia margin is interconnected and how the gas hydrate system has evolved.