Cracks and pore spaces within oceanic crust accommodate a volume of fluids equivalent to 2% of the ocean and a large majority (~90%) of hydrothermal fluid circulates through older, cooler crust that lacks strong temperature and chemical contrasts with oceanic bottom water. Such fluids are challenging to identify and access but they can be recovered from CORK (Circulation-Obviation Retrofit Kit) Observatories installed by the IODP, one of the few windows into the cool crustal environment. Fluids from the flank of the mid-Atlantic show the cool crust to be a net sink for oceanic dissolved organic carbon (DOC), an enormous reservoir of carbon with radiocarbon ages of 4,000 - 5,000 years. Carbon isotopic and molecular analyses of fluids collected over six years show that microbial activity drives the biogeochemistry of the cool crust. With unusual metabolic adaptations, organotrophs selectively degrade DOC with preference for the "younger", 13C-enriched and less molecularly degraded components. This leaves a more "aged", more molecularly degraded fraction to return to the deep ocean. The cool crust, therefore, may act as a continuous-flow subsurface bioreactor that helps maintain the deep ocean as a reservoir of 14C-depleted and refractory DOC.