| Summary: | International audience Geochemical proxies in marine sediment archives are often utilized to reconstruct changes in paleoenvironmental conditions. Understanding the role of the corresponding modern deep biosphere associated with sediments and the impacts these microbial communities have on the geochemistry enhance our ability to reconstruct past changes in ocean circulation. Additionally, in sedimentary systems where basalts of different ages, composition, and alteration interact with the deep biosphere, fluid-rock interactions my result in additional hydrothermal alteration of sediment-based geochemical proxies. Here, we present geochemical data collected during IODP Expedition 395C to analyze the diffusive transfer of ions between basement and sediment in several drill sites within the Mid-Atlantic Ridge/Reykjanes Ridge and Iceland hotspot interaction system. Through observed changes in gradient profiles of redox sensitive compounds (e.g. SO4, which is highest at the seawater-sediment interface and at the sediment-basement interface, and CH4 which varies between sample sites) and other ions as a measure of conservative exchange, crustal fluid conditions are extrapolated and compared to other ridge-flank upper oceanic crust ecosystems. Total organic carbon and cell biomass in sediment samples provide a window into patterns in deep biosphere communities associated with sedimentation changes. Cell biomass contained within basement basalt tracks variability in hydrothermal fluid circulation and resulting basalt alteration. The distribution of sites along a transect spanning the Mid-Atlantic Ridge and Iceland hotspot interaction system presents a unique interdisciplinary look at evidence for diffusion of solutes across the basalt-sediment interface and provides a means to quantify deep subsurface biomass in a range of off-axis hydrothermal settings.
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