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doi:10.2204/iodp.proc.327.106.2011

Background and objectives

The eastern flank of the Juan de Fuca Ridge has been the target of numerous mapping, heat flow, sediment coring, seismic, drilling (Leg 168 and Expedition 301), and submersible expeditions (e.g., Davis et al., 1992; Davis, Fisher, Firth, et al., 1997; Wheat et al., 2004; Fisher, Urabe, Klaus, and the Expedition 301 Scientists, 2005; Hutnak et al., 2006). The central goal of these studies has been resolving coupled hydrogeologic, geochemical, and microbiological processes and properties within a ridge-flank hydrogeologic system where fluids, heat, and solutes are driven by lithospheric cooling and focused by areas of basement exposure. Determining rates and patterns of hydrothermal circulation in the ocean crust in this area is essential for developing a process-based understanding of how these systems work.

There are three basement outcrops located 100 km east of the active spreading center on the Juan de Fuca Ridge on 3.5–3.6 Ma seafloor and surrounded by turbidites and hemipelagic sediment that is 250–600 m thick: Papa Bare, Mama Bare, and Baby Bare (Davis et al., 1992; Underwood et al., 2005) (Fig. F1). These three outcrops are locations where altered seawater from ocean crust vents to the overlying ocean. Studies of Baby Bare outcrop suggest a total fluid discharge rate of 2–10 L/s (Wheat et al., 2004). In contrast, Grizzly Bare outcrop is a large basaltic feature 52 km to the south, where heat flow, seismic, and modeling studies indicate that cold seawater enters the crust (Fisher et al., 2003; Hutnak et al., 2006). These studies suggest a northeastern direction of fluid flow in basement from Grizzly Bare to Baby Bare outcrop, perhaps enhanced by the tectonic and structural fabric of the crust, which imparts azimuthal anisotropy in permeability (Wheat et al., 2000; Fisher et al., 2008). As at other locations where ridge-flank hydrothermal circulation is guided by basement outcrops, seawater is hypothesized to enter basaltic basement, flow laterally within the crust and become warm, react with basalt, exchange solutes with overlying pore waters in the sediment, and support microbial processes (Fisher and Wheat, 2010). Thermal and chemical changes between recharge and discharge sites are expected to be systematic, with temperatures that rise from 2° to 64°C and a fluid composition that loses dissolved oxygen, nitrate, and magnesium while gaining calcium. The time required for transition from oxic seawater to a hydrothermal fluid in this setting is not known with confidence but is likely to be tens to hundreds of years (Fisher et al., 2003; Hutnak et al., 2006).

Site U1363 (prospectus Sites GRB-1A, GRB-2A, and GRB-3A) is located adjacent to the northeastern edge of Grizzly Bare outcrop (Fig. F1) along reflection seismic Line GeoB00-170 (Fig. F2), co-located with a transect of shallow heat flow measurements oriented radially away from the outcrop (Zühlsdorff et al., 2005; Hutnak et al., 2006). If Grizzly Bare outcrop is a site of regional hydrothermal recharge, as hypothesized, fluid and microbiological composition at the base of the sediment column close to the outcrop should look much like that near the seafloor, whereas the chemical composition of pore fluids and microbiological communities from the middle of the sediment section should show the influence of diffusion and digenetic reactions as a function of depth, temperature, and distance from the outcrop.

Upper basement near Grizzly Bare outcrop is projected to be ~30°C where the sediment is ~200 m thick, only 700 m from exposed basalt, suggesting that seawater warms quickly as it enters basement and flows laterally. Laboratory experiments at low temperatures suggest that little change occurs on a timescale of a year at similar temperatures. This fluid also exchanges solutes via diffusion with overlying sediment pore water, which mainly influences elements that are highly reactive in the sediment (e.g., silica, nutrients, ammonium, sulfate, manganese, and iron). Microbial metabolic processes should also contribute to fluid alteration during lateral flow in basement.

The hypothesis that Grizzly Bare outcrop is a site of regional hydrothermal recharge was tested during Expedition 327 by collecting a transect of sediment cores from the northeastern edge of the outcrop, at locations where the sediment is ≤250 m thick. A transect of holes extending away from Grizzly Bare outcrop should show evidence for initial warming and losses of oxygen and nitrate in basement fluids. Many of the major ions should be little changed close to the outcrop, even if temperatures are elevated, because of the slow rate of reaction and the short residence time. The microbial population initially should be predominantly that found in bottom seawater. Farther from Grizzly Bare, as dissolved oxygen and nitrate are depleted, we anticipated finding a decrease in sulfate in fluids at the sediment/basalt interface as a result of diffusive loss to the overlying sediment. Additional changes were anticipated in other major and minor elements and in the microbial communities seen in sediments immediately above basement.

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