Minerals have recently been identified as a primary host for organic carbon (OC) within marine sediments. This strong physical and chemical carbon-mineral association is believed to reduce, and in some cases completely eliminate, the bioavailablilty of this carbon for microbial life. The paucity of information regarding the nature of this carbon-mineral association and the composition of the hosted carbon, however, precludes our ability to predict the ultimate fate of this OC and its involvement in deep subsurface life. Here, we addressed this knowledge gap by using a suite of bulk and spatially-resolved geochemical and mineralogical techniques to characterize OC-mineral associations within the deep subsurface. We characterized sediment samples collected on the 2014 North Atlantic long coring expedition (KN223) in the western subtropical North Atlantic that included three geochemically distinct long cores to a depth of 24-30 m and spanned OC-limited oxic to anoxic sediments. We find measurable and relevant OC concentrations throughout the sediment cores, that decreases linearly over ~25 meters burial depth, from ~0.15 to 0.075 mol OC/kg solid. OC within the sediments is compositionally complex on both a macro- and micro-scale, spanning a gradient of lability even at depth. Proteins are observed throughout the sediment depth profiles, where they appear to constitute a substantial fraction of the TOC. Correspondingly, a low C:N ratio is observed, consistent with proteinaceous carbon within the sediments. In sum, these findings point to a substantial mineral-hosted OC reservoir within the deep subsurface that may fuel the deep biosphere and select for protein-based heterotrophy.