Geochemical Evidence for Fluid Flow and Diagenesis at the Cascadia Convergent Margin

Abstract

Pervasive fluid advection is indicated in the geochemical depth profiles at four locations (five sites), two in the Vancouver Island (VI) sector and two in the Central Oregon (CO) sector of the Cascadia Convergent Margin. Diffusive fluid flow prevails at the VI Sites 888 and 889/890, whereas confined fluid flow dominates the CO Sites 891 and 892. Exactly the same deepseated fluid source (>1.5 to <4 km) was found in both areas, regardless of the fluid-flow regime. It is characterized by lower than seawater Cl concentration, higher than seawater Li, Ca, Sr, and silica concentrations, non-radiogenic Sr isotopes and is slightly depleted in I8O. The upward-advecting fluid that transports thermogenic hydrocarbons mixes with the pore fluids altered by diagenesis. The extent of mixing between these fluids varies spatially. At the CO sector of the Cascadia Margin where confined fluid flow prevails, the frontal thrust at Site 891 is presently not acting as a significant, but only as a leaky, fluid conduit, in contrast to the landward-dipping fault at Site 892. Organic matter-fueled diagenesis dominates over the depth-range drilled. An inverse relationship between sedimentation rate and both total organic carbon content (TOC) and the sulfate reduction rate is characteristic of this convergent margin. An important result of the bacterial degradation of organic matter is a widespread formation of diagenetic carbonates. The upwardmigrating fluid that is enriched in Ca, dissolved inorganic carbonate, and thermogenic hydrocarbons most likely favors the formation of epigenetic carbonates. Although solid gas hydrate was not recovered at the depth of either of the prominent bottom-simulating reflectors (BSRs), their presence is inferred from geochemical (and geophysical) measurements. The geochemistry, particularly the coincidence between pore fluids that have very low Cl concentrations and very high methane concentrations at these depths, indicates that primarily a methane hydrate occupies at least 15% of the pore space above the BSR at the VI Site 889, and at least 10% of the pore space above the BSR at the CO Site 892. The solid gas hydrate recovered at ~2 to 19 mbsf, Site 892, is a mixed CH4-H2S hydrate of which up to 10% of the gas is H2S.