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doi:10.2204/iodp.sp.321T.2009

Introduction

IODP Expedition 301 was part of a series of expeditions and experiments to quantify hydrogeologic, lithologic, biogeochemical, and microbiological properties, processes, and linkages on the eastern flank of the Juan de Fuca Ridge (Fig. F1). This effort included site surveys to map bathymetric relief, seismic reflection profiles to delineate sediment thickness, sediment cores to allow analysis of pore fluids, and heat flow measurements to determine patterns of lithospheric and hydrothermal heat loss. Operations during Expedition 301 included replacing of one existing subseafloor borehole observatory (CORK), drilling two basement holes and installing two new long-term observatories, coring the upper ~300 m of basement and shallow sediments above basement, and collecting in situ hydrogeologic and geophysical data from basement. Subsequent remotely operated vehicle (ROV) and submersible expeditions have serviced IODP observatories, collecting pressure and temperature data and fluid and microbiological samples and replacing components as needed to maintain these systems for future use. Another drilling expedition (approved by the IODP Science Planning Committee and awaiting scheduling by the Operations Task Force) is planned to emplace three more borehole observatories and initiate crosshole tests, and additional ROV and submersible expeditions will conduct long-term experiments and recover subseafloor data and samples.

Borehole observatories installed during IODP Expedition 301 were designed to seal open holes so that thermal, pressure, and chemical conditions could equilibrate following the dissipation of the drilling disturbance; to facilitate collection of fluid and microbiological samples and temperature and pressure data using autonomous samplers and data logging systems; and to serve as long-term monitoring points for large-scale crustal testing (Fisher et al., 2005b). Unfortunately, the CORKs installed in Holes U1301A and U1301B were not sealed as intended (Fisher et al., 2005a), and data and samples collected during subsequent ROV and submersible servicing operations have shown that both observatory systems are leaking. Operations during Expedition 321T will seal these observatories by putting cement into the reentry cones surrounding the CORK wellheads, allowing the remaining components of the full experimental program to be completed during subsequent drilling and submersible expeditions.

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