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 I8 O. 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 CH 4 -H 2 S hydrate of which up to 10% of the gas is H 2 S.

Expedition 311 Synthesis: scientific findings

“…Teichert et al (2005) also proposed a mixing line (dotted black line in Fig. F9D) for the strontium isotope and concentration data for the southern Cascadia margin using seawater and bottom fluids collected from ODP Sites 1027 and 1026, similar to the previous mixing line proposed by Kastner et al (1995b).…”

“…It is also evident that the entire set of values from all sites is overprinted by diagenesis from carbonate precipitation (Fig. F9D), especially for Site U1327-a finding that was already noted by Kastner et al, (1995b) for nearby Site 889. Similar observations were made by Teichert et al (2005) for data from Leg 204.…”

“…• Strontium (Sr) and lithium (Li) components of the pore water following earlier results from Legs 146 and 204 (Teichert et al, 2005;Kastner et al, 1995b); and…”

“…At lower temperatures, lithium is partitioned into clay minerals, whereas it is leached into the pore fluid at temperatures >70°C. At the thermal gradients measured during Expedition 311 (~60°C/km), inferred in situ lithium leaching mainly occurs at depths >1 km (see also Kastner et al, 1995b). Thus, higher-than-seawater lithium concentrations found at shallower depths are indicative of an upwardmigrating deep fluid source.…”

“…Additional constraints on the deep fluid sources come from lithium and strontium data (e.g., Teichert et al, 2005;Kastner et al, 1995b). Here we use the measured concentration-depth profiles of lithium and strontium as well as 87 Sr/ 86 Sr ratios (Fig.…”