Active fluid flow in convergent margins can have a profound effect on the shallow thermal structure and fluid content of the downgoing plate, the physical properties of the subduction zone interface, deformation style, and the transport of elements to the oceans, the volcanic arc, and the deeper mantle. Therefore, it is a major process for understanding the behavior of the seismogenic zone and the flux balance through the subduction zone.
Temporal hydrologic variations at convergent margins are likely intimately connected to the location, magnitude, and frequency of earthquakes as well as aseismic deformation in a subduction zone. In order to monitor those phenomena, two newly modified CORKs (called CORK-IIs hereafter) were installed for long-term sampling of fluids and gases as well as for monitoring of fluid pressure, temperature, and flow rate at critical horizons. Fluid sampling and physical monitoring is achieved by placing OsmoSamplers, temperature recorders, and ports for pressure measurements within a pressurized formation below a packer. In contrast to the initial CORK-II design, these installations monitor and sample processes only in the bottom section of a borehole, isolated from higher formations. An inflatable packer and a novel OsmoSampler seat inside the casing separate the lower hole from the upper portion of the cased borehole. The OsmoSampler seat was designed to maintain pressure within the pressurized horizon even during retrieval. This change was initiated to simplify estimates for fluid fluxes and to avoid the question of whether the entire cased borehole is chemically mixing (Wheat et al., 2000a) and to avoid pressure loss during sampler redeployments. Borehole OsmoFlowmeters were newly developed for this experiment to deduce relative rates and direction of fluid flow.
Significant design modifications to previously deployed OsmoSamplers (e.g., ODP Leg 168, Sites 1024 to 1027; Wheat et al., 2000a) improve the chances of recovering the OsmoSamplers in unstable formations and allow the samplers to be recovered and redeployed within pressurized horizons. Temperatures are continuously measured inside the OsmoSamplers using small autonomous data loggers (Pfender and Villinger, 2002). Pressure ports are located above and within the sealed interval, and pressures are measured with high-precision pressure gauges and recorded with a data logger located inside the CORK-II head, which sits inside the reentry cone at the seafloor (Shipboard Scientific Party, 2002b). The samplers are retrievable by wireline with the help either of a submersible or a remotely operated vehicle (ROV). Pressure data can be downloaded during submersible or ROV visits at the site by connecting a computer to the data logger via an underwater-mateable connector.
Leg 205 focused on the Costa Rica margin offshore the Nicoya Peninsula, building on ODP Leg 170 coring and logging at adjacent sites, and was designed to investigate the composition of the downgoing plate together with the thermal structure and hydrological activity across the Costa Rica margin (see the "Explanatory Notes" chapter, this volume). ODP Leg 170 drilling and heat flow studies (Kimura, Silver, Blum, et al., 1997; Silver et al., 2000) showed three distinct hydrological systems in the Costa Rica margin:
Based on existing data from Leg 170 and coring results from Leg 205 (see the "Explanatory Notes" and "Site 1254" chapters, this volume), we identified zones of interest for fluid sampling and installed two CORK-IIs: one CORK-II in Hole 1253A, ~0.2 km seaward of the deformation front in the upper part of an igneous section, and one CORK-II in Hole 1255A in the décollement zone, ~0.4 km arcward of the deformation front. First observations of temporal variations of fluid and gas chemistry as well as pressure and temperature from CORK-II deployments of Leg 205 will be available once the fluid and gas samples have been recovered 1-2 yr postcruise.
In the following sections we will describe the individual components of a CORK-II system, with emphasis on the plumbing system for pressure measurements, the osmotic fluid and gas samplers, and temperature data loggers, and we will also explain the operational procedures for installment. In-depth discussion of some of the technical details and the design philosophy can be found in the "Explanatory Notes" chapter of the Leg 196 Initial Reports volume, during which ACORKs were deployed in the Nankai Trough (Shipboard Scientific Party, 2002a) (see the "Site 1253" and "Site 1255" chapters, this volume).