Award: OCE-1130146

Seawater flows through the oceanic crust, much like groundwater flows through the continental crust. Aquifers exist in both crustal types. On land aquifers are bisected by drilling to extract groundwater for drinking and irrigation and in some locations aquifers reach the surface where springs are sampled, bottled, and distributed. In the oceanic crust only a few boreholes exist to study aquifer conditions, and even few springs have been located. In the ocean the most of the fluid flow though the oceanic crust occurs on ridge flanks, far from the influences of magmatic heat on mid-ocean spreading centers. Here seawater flows into the ocean crust, warms (2-20 degrees Celsius), and reacts with crustal minerals. This flow removes ~25% of the lithospheric heat and circulates a volume of seawater that is similar to the flux of water from rivers to the oceans. Thus, given even a small change in composition of the circulating seawater from water-rock or microbial reactions within oceanic crustal aquifers, the net impact could affect the current and past composition of the ocean. However, even with this vast flow of fluid, until this grant, no springs that are typical of such flows have ever been directly sampled. This award was designed to specifically study one such typical aquifer where preliminary data pointed to the presences of springs at Dorado Outcrop, a basalt edifice on 23 M.y. old seafloor of the Cocos Plate, eastern Pacific Ocean. Two expeditions to this site were undertaken, one to map the area and locate springs and the second to bring new sampling techniques to collect pristine spring fluids. Geochemical analysis of fluids that discharge from Dorado Outcrop are only slightly altered from bottom seawater; however, given the magnitude of flow on a global scale, these data coupled with estimates of the global fluid discharge from cool ridge-flank systems result in fluxes of Rb, Mo, V, U, and Li that are ≥10% of the riverine flux. In addition, spring fluids have ~50% less dissolved oxygen than bottom seawater. Some of this loss is due to diffusion to the overlying sediment pore waters. The remainder is consumed in the basaltic crust, presumable by microbial activity, since abiotic reactions are sluggish at these cool temperatures and the residence time of seawater in the crust at this site is about two years. This is a clear indication that microbes affect the evolution of the basaltic crust. Lastly, the crustal age of this location coupled with the presence of dissolved oxygen within the oceanic crust demonstrates that permeable pathways within the upper oceanic crust can remain oxic for tens of millions of years. This work demonstrates the potential for cool ridge-flank hydrothermal systems to influence crustal alteration, the subseafloor biosphere, and geochemical budgets of the global ocean. Last Modified: 10/28/2016 Submitted by: C. Geoffrey Wheat