Texas A&M University, United States

Integrated Ocean Drilling Program (IODP) Expedition 301 to the eastern flank of the Juan de Fuca Ridge was the first part of a multidisciplinary program designed to evaluate the formation-scale hydrogeologic properties within oceanic crust; determine how fluid pathways are distributed within an active hydrothermal system; and elucidate relations between fluid circulation, crustal stratigraphy, alteration, microbiology, and seismic properties. The complete experimental program will comprise two IODP expeditions (the first having been Expedition 301, the second to be scheduled), an offset seismic experiment, and long-term monitoring and cross-hole tests facilitated with submersible and remotely operated vehicle expeditions extending 6-10 y after Expedition 301. During Expedition 301, we replaced one existing borehole observatory penetrating the upper oceanic crust and established two new observatories penetrating to depths as great as 583 meters below seafloor, or 318 m into basement. The observatories include long-term temperature and pressure logging systems, as well as fluid sampling and microbiology growth instruments. We also sampled sediments, basalt, pore fluids, and microbial communities; collected wireline logs; and conducted hydrogeologic tests in two basement holes. Shore-based studies include investigations of sediment and rock physical, chemical, and magnetic properties and will help us to learn where microbiological communities live in the crust and how these communities cycle carbon, alter rocks, and respond to fluid flow paths. During a second drilling expedition, we will conduct the first multidimensional, cross-hole experiments attempted in the oceanic crust, including linked hydrologic, microbiological, seismic, and tracer components. After completion of drillship operations, we will initiate multiyear tests using the three-dimensional network of subseafloor observatories, allowing us to examine a much larger volume of the crustal aquifer system than has been tested previously. By monitoring, sampling, and testing within multiple depth intervals, we can evaluate the extent to which oceanic crust is connected vertically and horizontally; the influence of these connections on fluid, solute, heat, and microbiological processes; and the scale-dependence of hydrologic properties.