IODP Expedition 301 Installs Three Borehole Crustal Observatories, Prepares for Three-Dimensional, Cross-Hole Experiments in the Northeastern Pacific Ocean

Abstract

Introduction and Goals The basaltic upper oceanic crust comprises the largestaquifer on Earth, containing a volume of water about equalto that currently stored in ice sheets and glaciers. Annualfluid fluxes through the upper oceanic crust are at leastas large as the global river flux to the ocean. Much of theseafloor is hydrogeologically active, but the majority ofthe fluid flow within oceanic crust occurs on ridge flanks,regions located kilometers or more from active seafloorspreading centers. Fluid circulation in these areas is drivenmainly by lithospheric heat rising from deep within theplate but is influenced by seafloor and basement topography,seismic and tectonic events, and tides.Subseaf loor f luid f low on ridge f lanks inf luences adiverse array of processes and properties, including thethermal state and evolution of oceanic plates, alteration ofthe lithosphere and crustal pore waters, establishment andmaintenance of vast subseaf loor microbial ecosystems,and diagenetic, seismic, and magmatic activity along plateboundaryfaults. Although numerous drilling expeditionsand surface and submersible surveys over the last severaldecades have focused on hydrogeologic phenomena, we stillknow relatively little about driving forces, property distributions,scales of flow, rates of flow, extent of compartmentalizationor isolation of distinct fluid-rock systems, or linksbetween hydrogeologic, geochemical, microbiological, andgeophysical processes. Progress through drilling has beenlimited in the past by the perturbing effects of boreholecreation on subseafloor thermal, pressure, chemical, andbiological conditions. Subseafloor observatories addressthis challenge by allowing the formation to recover fromdrilling perturbations, and also allow scientists to runpassive and active experiments for years to decades.IODP Expedition 301 was part of a multi-disciplinaryprogram designed to evaluate the formation-scale hydrogeologicproperties within oceanic crust, determine how fluidpathways are distributed within an active hydrothermalsystem, and elucidate relations between fluid circulation,alteration, microbiology, and seismic properties. Thecomplete experimental program will comprise two IODPexpeditions (the first having been Expedition 301, thesecond to be scheduled), an offset seismic experiment, andlong-term monitoring and cross-hole tests facilitated withsubmersible and remotely operated vehicle (ROV) expeditionsextending 6–10 years after the first IODP expedition.The experimental program will also take advantage ofopportunities related to a plate-scale network of long-termobservatories (NEPTUNE) currently being planned.

Keywords

• IODP Expedition 301