Integrated Ocean Drilling Program, United States

Integrated Ocean Drilling Program Expedition 301 is the first part of a two-expedition, multidisciplinary program to evaluate the formation-scale hydrogeologic properties within oceanic crust; determine how fluid pathways are distributed within an active hydrothermal system; establish linkages between fluid circulation, alteration, and microbiological processes; and determine relations between seismic and hydrologic anisotropy. During Expedition 301, we replaced one existing borehole observatory penetrating the upper oceanic crust on the eastern flank of the Juan de Fuca Ridge and established two new observatories penetrating to depths as great as 583 meters below seafloor or 318 m into basement. We sampled sediments, basalt, fluids, and microbial samples; collected wireline logs; and conducted hydrogeologic tests in two basement holes. Shore-based studies will help us to learn where microbiological communities live in the crust and how these communities cycle carbon, alter rocks, and are influenced by fluid flow paths. During a follow-up 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 new 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 importance of scaling on hydrologic properties. This work is helping us to understand the nature of permeable pathways, the depth extent of circulation, the importance of permeability anisotropy, and the significance of hydrogeologic barriers in the crust.