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

• Published in: Geochemistry, geophysics, geosystems : G3 Vol. 21; no. 1
• Main Authors: C. Geoffrey Wheat, Keir Becker, Heinrich Villinger, Beth N. Orcutt, Trevor Fournier, Anne Hartwell, Claudia Paul
• Format: Journal Article
• Language: English
• Published: Wiley 01-01-2020
• Subjects: hydrothermal circulation; Mid‐Atlantic Ridge; nitrification; ridge flanks
• ISSN: 1525-2027
• DOI: 10.1029/2019GC008804

Abstract 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.