Award: OCE-1260353

This multi-PI from multiple institutions project began about a decade ago with initial planning and developmental engineering with the primary objective of improving our knowledge about the largest aquifer system in the world, the upper ocean crust. In addition to advancing scientific knowledge, this study will help other studies of the deep microbial biosphere and the thermal and the chemical evolution of the solid earth. Finally, it also utilized and further developed subseafloor boreholes (CORKs) installed by the Ocean Drilling Program (ODP and its subsequent reincarnations, including IODP—the Integrated Ocean Drilling Program). The UCSB portion was dedicated to a multi-borehole tracer (chemical dye) experiment that began in August 2010 during IODP Expedition 327. During that expedition, two new boreholes were drilled in 3.5 million year old seafloor oriented N20°E, 100 km east of the Endeavor Segment on the Juan de Fuca Spreading Ridge in the Northeastern Pacific (Holes #1362A and #1362B) and installed with 3rd generation CORKs. A 24-hour pumping test was conducted in Hole #1326B prior to the installation of its CORK. For 20 hours of the pumping test, sulfur hexafluoride (SF6) tracer was added to the injection water. Although a strong greenhouse gas, SF6 is an ideal tracer in this setting because it is non-reactive, can be measured at low concentrations using a gas chromatograph in small samples (~1 ml), and background concentrations in natural waters are extremely low (< 1 fmol/L; 1 fmol = 1 x 10-15 mol). Samples were collected from four boreholes (Holes #1301A, #1362B; #1362A; and #1026B) using Osmosamplers in gas tight copper tubes. Osmosamplers are passive pumps that collect about 1 ml of borehole water per day. These samplers are designed to exploit osmosis by separating fresh and saline water reservoirs with semipermeable membranes, collecting and storing samples for years in remote environments without electrical power or moving parts. In addition to serving as an educational and research tool for three B.S., one M.S., and one Ph.D. students, major results include: 1) detecting tracer at all four boreholes, 2) verifying a dominant natural flow direction from south to north, 3) quantifying the effective porosity of the upper ocean crust aquifer, and 4) confirming the suspected anisotropy of the field location. The tracer study showed that 1) the effective porosity is very low (<<1%), which means that hydrothermal flow is restricted to <<1% of the aquifer volume and 2) the effective porosity parallel is at least 10 times higher than perpendicular to the spreading ridge, presumably due to larger and more extensive faulting in the ridge parallel direction. These results have important implications for the hydrogeology of the upper ocean crust on ridge flanks, and for fractured and heterogeneous water-rock systems more broadly, including those high-temperature systems near seafloor spreading centers and in the continental crust. Last Modified: 09/12/2017 Submitted by: Jordan F Clark