International Ocean Discovery Program (IODP) Expedition 357 recently drilled 17 holes into the Atlantis Massif, the ocean core complex that hosts the serpentinite-dominated Lost City Hydrothermal field. Multiple types of fluid samples were collected before, after, and during drilling to characterize the biogeochemical regime of the system. These samples collectively represent the first window into the habitats of the Atlantis Massif subsurface. Preliminary data indicates that low concentrations of H2 are present across a ~11 km wide region of the Atlantis Massif, potentially pointing to an extensive, cold, H2-rich serpentinite subsurface. I propose a biogeochemical analysis of the fluids collected on IODP Expedition 357, including measurements of the concentrations of amino acids and their D/L ratios as an indication of chemolithoautotrophy and/or heterotrophy, organic acid concentrations to investigate potential abiotic production, helium isotopes as a proxy for potential mantle input, and dissolved organic carbon (DOC) and its 14C content to track bulk changes and to identify potential incorporation of mantle carbon. These data will be compared to DNA sequences of simultaneously collected samples by fellow Expedition 357 science party members with the goals of characterizing the extent and activity of the subsurface microbial communities.