Award: OCE-1260548

The focus of this project was to complete a multi-year study of fluid flow through the oceanic crust. Such flow, much like groundwater flow through continental crust, redistributes one quarter of Earth's heat loss and affects the chemical composition of the Ocean. Like groundwater flow through continental crust, flow though the oceanic crust occurs though aquifers, which are sandwiched between impermeable layers. In the case of the oceanic crust the permeable layer is the upper basaltic crust, which is made of pillow lavas that can, for example, form large lava tubes like tthose observed in Hawaii. The impermeable overlying sediment and underlying sheet lava flows confine fluid flow to the layer of pillow lavas. Fluids within the oceanic crust are exchanged with seawater at basaltic outcrops, such as seamounts and other smaller basaltic features that penetrate the sediment and represent local topographical highs. This work was the first to directly test the hydrologic properties and processes of the oceanic crust with chemical tracers. These tracers were injected into one borehole that penetrated the sediment and into the upper basaltic layer and monitored in other boreholes that also penetrated into the upper basaltic layer. Fluids within the borehole were collected for four years using novel continuous flow fluid samplers (OsmoSamplers) and were monitored with temperature and pressure sensors. The focus of this award was to recover the samplers that were placed within the boreholes and on the boreholes (wellhead) using the submersible Alvin. Samplers deployed on the wellhead tap into the upper basaltic crust through tubes that were deployed with the borehole infrastructure and run the length of the borehole to specific depths within the basaltic portion of the oceanic crust. Once the samplers were recovered we engage in analyzing thousands of samples representing a time series from the time of deployment to the time of recovery. Analyses of fluid collected from the downhole and wellhead samplers (OmoSamplers) reveales that tracer traveled from the distribution borehole to each of the monitoring boreholes. Results indicate that much of the flow occurs through < 1% of the crust, suggesting highly permeable pathways direct fluid circulation though the oceanic crust. This result has ramification for the extent of reaction and transport in the oceanic crust and its combined affects on the chemical composition of the Ocean and the evolution of the oceanic crust as reactions between the circulating seawater and basaltic rock alter the circulating fluids and the rock though which it flows. This work also has ramification for utilizing oceanic boreholes as a source to power to seafloor instruments and monitoring devices and to sequester carbon dioxide. Last Modified: 09/08/2017 Submitted by: C. Geoffrey Wheat