Subseafloor Cross‐Hole Tracer Experiment Reveals Hydrologic Properties, Heterogeneities, and Reactions in Slow‐Spreading Oceanic Crust

The permeability, connectivity, and reactivity of fluid reservoirs in oceanic crust are poorly constrained, yet these reservoirs are pathways for about a quarter of the Earth's heat loss, and seawater‐rock exchange within them impact ocean chemical cycles. We present results from the second ever cross‐hole tracer experiment within oceanic crust and the first conducted during a single expedition and in slow‐spreading crust west of the Mid‐Atlantic Ridge at North Pond. Here we employed boreholes that were drilled by the Integrated Ocean Drilling Program (Sites U1382 and U1383) that were instrumented and sealed. A cesium salt solution and bottom seawater tracer experiment provided a measure of the minimum Darcy fluid velocity (2 to 41 m/day) within the upper volcanic crust, constraining the minimum permeability of 10−11 to 10−9 m2. We also document chemical heterogeneities in crustal fluid compositions, rebound from drilling disturbances, and nitrification within the basaltic crust, based on systematic differences in borehole fluid compositions over a 5‐year period. These results also show heterogeneous fluid compositions with depth in the borehole, indicating that hydrothermal circulation is not vigorous enough to homogenize the fluid composition in the upper permeable basaltic basement, at least not on the time scale of 5 years. Our work verifies the potential for future manipulative experiments to characterize hydrologic, biogeochemical, and microbial process within the upper basaltic crust. Plain Language Summary Seawater flows within the oceanic crust, much like groundwater flows though permeable aquifers within continental crust. As seawater flows through the oceanic crust it dissolves some minerals while precipitating others. These biogeochemical reactions coupled with subsurface seawater flow affect the distribution of heat, solutes, and microbial populations within the oceanic crust, and such reactions and transport can impact oceanic processes. To assess hydrologic and biogeochemical processes that occur within the oceanic crust west of the Mid‐Atlantic Ridge, we conducted the second ever borehole‐to‐borehole tracer experiment within oceanic crust; however, similar experiments are commonplace in continental settings. On the basis of this tracer experiment and prior sampling of these boreholes, we determined that the upper several hundred meters of volcanic oceanic crust are highly permeable and support microbial nitrification; however, fluids within