Thermally-driven circulation of seawater through flanks of mid-ocean ridges, in seafloor older than one million years, cools and alters the crust, and dominates thermal and chemical mass transport between the oceans and the seafloor. Despite the significance of these processes, little is known about the mechanics of ridge-flank circulation, including the physical and chemical nature of discharge and recharge sites, the pathways and lateral length scales of flow, and the depth extent of penetration into the oceanic crust. Global fluxes of major and trace elements through ridge flanks are also uncertain. We propose to investigate patterns of vigorous fluid circulation and associated fluxes on the eastern flank of Juan de Fuca Ridge. Results from a recent drilling transect in this area produced evidence for two important and unexpected ridge-flank hydrothermal processes: (1) upwelling of deeply-sourced hydrothermal fluids that penetrate an area also hosting shallow circulation, and (2) significant axis-parallel recharge in shallow basement, allowing recharging seawater to flow along strike over several to tens of kilometers. We will investigate the extent and efficiency of fluid, heat, and solute transport (a) across the strike of the basement relief (i.e., along a crustal aging profile, perpendicular to the spreading axis), (b) along the strike of the buried and partially buried basement ridges, including along deep axis-parallel faults, and (c) within the vicinity of conical basement edifices, probably small, long-extinct volcanoes, that penetrate through the nearly continuous sediment on the ridge flank. This work will document and evaluate these interpretations, to test competing hypotheses regarding the geometry of ridge-flank circulation, to constrain ridge-flank flux estimates, and to help select sites for a return of the drill ship for additional sampling and downhole experiments.
