Abstract:
Methane reservoirs and seeps are an active component
of the continental margin carbon budget and represent a
poorly characterized pathway for reduced carbon cycling
and methane input to the atmosphere. Active gas seeps from
three shelf settings on the Cascadia Continental Margin off
Oregon and Northern California contain nearly pure
methane with a heavy carbon isotope composition (-29 to -35%₀). An extensive study of the gas seep at Coquille
Bank, Oregon, revealed a warm, buoyant pore fluid
associated with the pockmark. As methane enters the
water column above these seeps in a steady gas stream, a
fraction escapes directly to the atmosphere while the
balance dissolves into local seawater. Measured oxidation
rates are too slow for significant local oxidation within the
water column near the seep. Large mats of pink and white
bacteria, including Beggiatoa spp. are found around the
vent, demonstrating the activity of sulfide oxidizers in this
ecosystem.
