BENTHIC FLUXES FROM SEDIMENTS HOSTING METHANE HYDRATE DEPOSITS AT HYDRATE RIDGE, CASCADIA

Extensive deposits of methane hydrate characterize Hydrate Ridge in the Cascadia margin accretionary complex. Samples collected with benthic instrumentation and from Alvin push cores reveal a complex hydrogeologic system where fluid and methane fluxes vary by several orders of magnitude at sites separated by only a few meters.

We identified three distinct fluid regimes: 1) Sites of methane gas ebullition, where the bulk of the flow occurs through localized channels with gas velocities reaching 4 x10⁹ cm yr^-1. 2) Extensive bacterial mats that overlay sediments capped with methane hydrate, where fluid flow rates range from 10 to 250 cm yr^-1. 3) Clam colonies, where bottom seawater flows into the sediments for at least some fraction of the time at rates typically less than 2 cm yr^-1. Away from the active gas-release sites, fluid flows calculated from pore water models are in agreement with estimates using published flowmeter data and numerical model calculations, and do not include the potential contribution of bio-pumping by vent biota. Methane fluxes out of mat-covered sites range from 10 to 100 mmol m^-2 day^-1, whereas at clam sites the methane flux is less than 1 mmol m^-2 day^-1.

The processes leading to gas discharge seem to be different in the northern and southern summits of the ridge. The northern summit represents a more mature stage in the evolution of these ridges as evidenced by extensive carbonate pavement and upward deflections of the BSR at a thrust fault. In contrast, the southern region is less evolved, exhibiting only limited and localized carbonate build up and at most minor departures in BSR depth from that predicted by the seawater/gas hydrate phase boundary. Both at the north and south, the methane is generated at, and transported from, deep sediment sequences. At the north, the gas discharge appears to be driven by pressure changes on a deep gas reservoir, and it is released episodically at a rate of ~6 x 10⁴ mol day^-1 following tidal periodicity. In contrast, observations at the southern summit indicate that gas discharge there is driven by more localized phenomena, possibly associated with destabilization of massive gas hydrate deposits at the seafloor.