It has been long known that the oceanic crust is the largest aquifer on Earth. However, relatively little is known about how this aquifer influences biogeochemical cycles in the deep ocean. Recent studies suggest that in some seafloor settings, crustal aquifer fluids are replete with oxygen or nitrate, and provide the overlying sediments with additional oxidants. These environments provide a unique opportunity to examine how the availability of oxidants might influence sedimentary biogeochemical processes, in particular recalcitrant organic matter degradation. Specifically, deeper sediments at the North Pond appear to be replenished by oxygen through an upward flux of recharged crustal fluids. Thus, the North Pond represents an ideal natural laboratory to study the fate of recalcitrant organic carbon present in deep anoxic sediments that are being replenished with oxygen and other electron acceptors from below through recharged basement fluids. The proposed project will shed light on the degree to which the availability of oxidants supports the transformation of recalcitrant organic carbon by microbes in these deep subsurface sediments. The assessment of the metabolic potential of crustal microbial communities solidly aligns with the C-DEBI research themes, and our studies in particular are aimed at furthering our understanding of metabolic activity in the deep subseafloor biosphere and the limits of life across the range of habitats encompassing the subsurface biosphere.