Fluid and chemical flux in and out of sediments hosting methane hydrate deposits on Hydrate Ridge, OR, II: Hydrological processes

摘要

Multiple, long-term measurements of aqueous and chemical flux through regions of active fluid seeps and gas vents on Hydrate Ridge, Cascadia accretionary prism, were accomplished during 1998 and 1999 as part of the international TECFLUX project. These measurements indicate that flow is highly heterogeneous in both time and space with areas of inflow, outflow, and outflow of fluids of both altered and seawater-like composition. While tectonics is the dominant underlying driving force in fluid expulsion, we suggest that much of the shallow expression of this flow is modulated by more complex hydrological processes. These dynamic processes may include gas expulsion-driven pumping and aqueous entrainment in migrating gas, buoyancy-driven fracturing of overlying sediments,, rapid changes in the permeability distribution due to injection of gas and the formation of gas hydrates, migrating flow conduits, and tidally driven flow oscillations. By placing the flow rate observations obtained during the two field seasons in the contex of the distribution and nature of the seepage sites, we develop a conceptual model of the spatial and temporal interactions between various processes that appear be active at Hydrate Ridge. We further discuss: (1) evidence for the existence of a subsurface gas transport system and discuss the mechanisms which maintain it, (2) estimates of the in situ permeability of seep settings and evidence for, and the mechanisms controlling temporal changes in permeability, and (3) changes in seep output fluid chemistry which may related to the above hydrological processes.