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doi:10.2204/iodp.proc.301.103.2005

Background, motivation, objectives, and general design of Expedition 301 CORKs

Fluid flow within the seafloor influences globally important processes such as the thermal evolution of oceanic plates; alteration of the lithosphere and the chemistry of flowing fluids; establishment and maintenance of subseafloor microbial ecosystems; diagenetic, seismic, and magmatic activity along plate-boundary faults; creation of ore and hydrate deposits both on and below the seafloor; and exchange of fluids and solutes across continental margins. Although the significance of these processes has been appreciated for decades, the fundamental physical, chemical, and microbiological parameters that control and are controlled by fluid flow in the crust have remained largely unquantified.

Subseafloor hydrogeology was discussed frequently during the multiyear planning process that led to the establishment of the Integrated Ocean Drilling Program (IODP). The initial science plan for IODP includes three primary themes (International Working Group, 2001). Subseafloor hydrogeology is an essential part of one of these themes (the Deep Biosphere and the Subseafloor Ocean) and is important to components of the other two themes as well (Solid Earth Cycles and Geodynamics and Environmental Change, Processes, and Effects). The initial science plan highlights technological developments that are essential for IODP researchers to achieve critical scientific goals, including tools needed to establish long-term subseafloor observatories. It is appropriate that the first expedition of IODP emphasizes new kinds of borehole experiments and is helping to expand the capabilities of long-term, subseafloor observatories.

The upper oceanic crust comprises the largest aquifer on Earth. Numerous drilling, surface, and submersible surveys have explored aspects of oceanic crustal hydrogeology over the last 40 y. Critical characteristics of crustal hydrologic systems include sizes of fluid, heat, and solute fluxes; the magnitude and dynamic behavior of fluid driving forces; the nature of source and sink terms; the quantitative impacts of fluid flow on physical properties; the extent and variability of crustal storage properties; and relations among these parameters and processes. One limitation on progress in the past has been the inability to determine subseafloor conditions after the perturbing effects of drilling have dissipated. Subseafloor observatories address this need and also make it possible to maintain a measurement and sampling presence for years beyond the drilling expedition, so that induced and naturally occurring events can be monitored and interpreted.

Expedition 301 was designed to address several of the topics listed above, in part by replacing and improving two existing borehole observatory systems and establishing two new systems. The "Expedition 301 summary" chapter describes overall expedition objectives and strategy. In this paper, we describe design and deployment of subseafloor observatories. Detailed planning for these Circulation Obviation Retrofit Kit (CORK) systems began during fall 2003, immediately after the expedition was scheduled. CORK systems deployed during this expedition were intended to be used for passive monitoring and sampling and as observation points for active (perturbation) experiments that are to be initiated during a future IODP expedition and subsequent dive programs. The CORKs were designed to include instruments for pressure and temperature measurements and to house downhole and seafloor fluid samplers and microbiological growth substrate, all within vertically discrete intervals.

During Expedition 301, we planned to create two new basement holes at Site U1301, one that penetrated the uppermost 50–100 m of basement and a deeper hole that penetrated 300–400 m of basement. These holes were to be instrumented with single- and multilevel CORK systems, respectively. We also planned to replace Ocean Drilling Program (ODP) Leg 168 CORKs in Holes 1026B and 1027C. The existing CORK body in Hole 1026B was leaking and there was no downhole instrumentation installed, whereas the CORK system in Hole 1027C was only instrumented to measure pressure and did not separate true basement rocks from overlying sediments and a sill. We ran out of time and materials to complete CORK replacement work in Hole 1027C, but we discuss plans for replacement of this system in this paper because it was an important part of Expedition 301 preparation and it will be replaced during the next drilling expedition to this area.

The Expedition 301 CORKs are similar in some ways to instruments deployed during ODP Leg 205 on the Costa Rica margin (Jannasch et al., 2003). Like those instruments, the Expedition 301 CORKs were placed inside 10¾ inch casing in a reentry cone (Fig. F1). An internal 4½ inch casing with one or two packer elements, inflated with pressurized seawater pumped through the CORK head with the drill string, was used to isolate one or more borehole intervals at depth. Unlike the Leg 205 CORKs, the Expedition 301 CORKs did not include spool valves at the well head, and two inner casing plugs in each CORK were held in place by gravity (lower plug) and friction (upper plug), with no positive latch mechanism, in an effort to assure a complete seal and simplify later recovery. In addition to putting in situ osmotic samplers and growth substrate at depth, isolated intervals are accessed for fluid sampling by stainless steel or Tefzel (microbiologically "cleaner") tubing that penetrates through one or more casing packers and end inside small-diameter screens, with fluid sampling systems placed near the top of the CORK at the seafloor. This design concept is based on a system developed for CORKs deployed during ODP Leg 196 (Nankai Trough), although these earlier CORK systems were much larger in diameter. We arranged to have a new umbilical built for use in multilevel CORKs in Holes 1027C and U1301B, along with the microbiology sampling line for use in the lowermost intervals of these holes, and surplus umbilicals from Legs 196 and 205 were brought out for use in Holes 1026B and U1301A, respectively.

These and other design features and deployment specifics are discussed in the rest of this paper. The Expedition 301 CORKs were visited by a remotely operated vehicle (ROV) 3 weeks after the end of drilling operations, and operational results from this expedition are also described.

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