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. Expulsion of methane-charged fluid from accretionary systems concentrates gas hydrate along convergent plate boundaries. The northern Cascadia margin (offshore Vancouver Island, Canada) is a convergent margin with gas hydrate-bearing cold seeps composed of both thermogenic and microbial 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.

The isotopic and hydrocarbon composition of gas from the thermogenic gas hydrate site, Barkley Canyon, indicated this cold seep is connected to the landward Tofino Basin petroleum system. Barkley Canyon is the only known accretionary margin setting where thermogenic gas hydrate is exposed at the seafloor. The pore water and sediment geochemistry from distinct seafloor environments (e.g., clam colonies and bacterial mats) were evaluated to identify the fluid flux regimes and metabolic pathways that control how methane carbon was cycled by the microbial consortium. Evidence for coupling between anaerobic oxidation of methane (AOM) andmethanogenesis was inferred from the stable carbon isotope composition of the dissolved inorganic carbon (DIC) and methane. Lipid biomarker analysis indicated sulfate reducing bacteria also consumed other sources of organic matter. Coupling between AOM and methanogenesis was also inferred at the microbial gas hydrate cold seep, Bullseye vent. AOM operating within both of these seep systems displayed a remarkable capacity to quantitatively consume methane within the sedimentary system; thus limiting methane emissions to the water column.

The radiocarbon content from gas-hydrate bound methane from six distant oceanic locations was almost entirely of fossil origin. 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). Radiocarbon analysis of sedimentary carbon pools was utilized to reconstruct the depositional history around the Bullseye vent site, which appears to have been subjected to tectonic uplift and erosion since the termination of the last glacial maximum.

This study demonstrates the value of integrating geochemical, chronological and geophysical data to understand the sources and cycling of globally significant reservoirs of methane in cold seep systems. The unique isotopic (13C and 14C) signature of methane is conveyed to and recorded within the pore water and sediment systems. By coupling those patterns with temporal (i.e., radiocarbon) and spatial (i.e., seismic) records, a more comprehensive model for how the northern Cascadia margin is interconnected and how the gas hydrate system has evolved was developed.

http://www.vims.edu/library/Theses/Pohlman06.pdf