Formation Waters Delineate Diverse Hydrogeologic Conditions at a Plate Scale: Eastern Flank of the Juan de Fuca Ridge. (23rd November 2022)

• Record Type: Journal Article
• Authors: Wheat, C. Geoffrey; Mottl, Michael J.; Fisher, Andrew T.; Hulme, Samuel
• Is Part Of: Geochemistry, geophysics, geosystems. Volume 23:Number 11(2022)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2022-11-23
• Subjects: ridge flank -- hydrothermal -- Juan de Fuca -- hydrogeology -- formation water -- pore water; Geochemistry -- Periodicals; Geophysics -- Periodicals; Earth sciences -- Periodicals; 550.5
• DOI: 10.1029/2022GC010665
• Languages: English
• ISSNs: 1525-2027

Abstract

Abstract: The chemical composition of formation waters within the upper basaltic crust were calculated or measured at 24 sites on the northwest portion of the Juan de Fuca (JDF) Plate using data from sediment pore waters, scientific boreholes, and seafloor springs. Formation waters differ in composition across this ridge‐flank region because of variations in water‐rock reactions and residence times, exchange rates with overlying sediment pore waters, and microbial processes along flow paths. We interpret spatial variations in the solute composition of formation waters to resolve areas that are geochemically distinct or similar, lateral trends that result from water transport, areas where water‐rock reactions in the deeper crust are apparent, and sites of seawater recharge and formation water discharge. We provide evidence for large‐scale lateral flow associated with two (mostly) buried basement ridges on ∼1.4 and ∼3.5 Ma seafloor, which are subparallel to the JDF spreading axis to the west. Between these two ridges, where the seafloor and the sediment‐basement interface are relatively flat, formation waters have undergone extensive exchange with overlying sediment pore waters, consistent with a long residence time. Basaltic outcrops provide sites of seawater recharge and hydrothermal discharge, sometimes through the same feature, highlighting the heterogeneous nature of hydrogeologic conditions and processes. This work provides a blueprint for future plate‐scale studies to assess, for example, geologic controls of crustal age, spreading rate, and sedimentation on subsurface hydrologic patterns. Plain Language Summary: Seawater flows through the upper volcanic crust like groundwater in aquifers on land. As seawater moves through basaltic rocks it becomes altered from reaction with crustal rocks and microbes and from mixing with other fluids. The magnitude of this flow is significant with discharge from the oceanic crust equivalent to the discharge of Earth's rivers to the ocean. This paper combines geochemical data from crustal formation waters at 24 locations across a vast area in the northeastern Pacific Ocean offshore of North America. These data characterize hydrologic processes including: subseafloor areas that are well connected or isolated from each other, places where crustal formation waters appear to travel tens of kilometers through the seafloor, and other areas where sites separated by just a few kilometers are poorly connected. We also document sites where basaltic outcrops allow cool seawater to recharge into the ocean crust, and other outcrops where reacted crustal waters discharge. By combining data collected over decades at multiple sites across a vast region, we developed a nuanced understanding of subsurface plate‐scale transport of crustal formation waters, improving our understand of how the oceanic crust evolves and the impact it has on the composition of seawater. Key Points: Systematic differences in the chemical composition of ridge‐flank, crustal waters define patterns of solute transport within a 10 4 km 2 area Some ridge‐flank regions are hydrogeologically isolated, and other areas are connected across distances of kilometers to tens of kilometers Hydrothermal discharge and recharge occur mainly though volcanic rock outcrops, with some outcrops both recharging and discharging