A Three-Dimensional Subseafloor Observatory Network for Cross-Hole, Hydrogeologic Experiments Established in the Northeast Pacific Ocean

The upper oceanic crust, composed mainly of basalt, comprises the largest aquifer on Earth. Global fluid fluxes through the upper oceanic crust are at least as large as the annual riverine flux to the ocean, and influence a diverse array of processes and properties, including the thermal state and evolution of oceanic plates; alteration of the lithosphere and the chemistry of flowing fluids; establishment and maintenance of vast subseafloor microbial ecosystems; and diagenetic, seismic, and magmatic activity along plate-boundary faults. Active experiments are needed in the oceanic crust to determine hydrogeologic properties at a crustal scale, and to quantify linkages between thermal, fluid, solute, and biological processes. The first expedition of the Integrated Ocean Drilling Program established two new subseafloor observatories within 3.5 Ma crust in the northeast Pacific Ocean, and replaced a previously-deployed observatory, in anticipation of planned cross-hole tests and related experiments. The new holes extend up to 320 m into basement and isolate distinct depth intervals. All of the new observatories are instrumented with autonomous temperature loggers, osmotic fluid samplers, and microbiological incubation substrate. Future experiments will include hydrogeologic tests to determine fluid transmission and storage properties, at cross-hole distances of 35 to 2200 m, tracer tests to quantify rates and modes of solute transport, and seismic experiments to elucidate relations between velocity and hydrogeologic anisotropy.