Arguments for an abiotic origin of low-molecular weight organic compounds in deep-sea hot springs are compelling owing to implications for the sustenance of deep biosphere microbial communities and their potential role in the origin of life. Theory predicts that warm H 2 -rich fluids, like those emanating from serpentinizing hydrothermal systems, create a favorable thermodynamic drive for the abiotic generation of organic compounds from inorganic precursors. Here, we constrain two distinct reaction pathways for abiotic organic synthesis in the natural environment at the Von Damm hydrothermal field and delineate spatially where inorganic carbon is converted into bioavailable reduced carbon. We reveal that carbon transformation reactions in a single system can progress over hours, days, and up to thousands of years. Previous studies have suggested that CH 4 and higher hydrocarbons in ultramafic hydrothermal systems were dependent on H 2 generation during active serpentinization. Rather, our results indicate that CH 4 found in vent fluids is formed in H 2 -rich fluid inclusions, and higher n-alkanes may likely be derived from the same source. This finding implies that, in contrast with current paradigms, these compounds may form independently of actively circulating serpentinizing fluids in ultramafic-influenced systems. Conversely, widespread production of formate by ΣCO 2 reduction at Von Damm occurs rapidly during shallow subsurface mixing of the same fluids, which may support anaerobic methanogenesis. Our finding of abiogenic formate in deep-sea hot springs has significant implications for microbial life strategies in the present-day deep biosphere as well as early life on Earth and beyond.abiotic organic synthesis | hydrothermal systems | methane | formate | fluid-vapor inclusions S eawater-derived hydrothermal fluids venting at oceanic spreading centers are a net source for dissolved carbon to the deep sea, with vent fluid carbon contents directly tied to the sustenance of the subseafloor biosphere (1). Highly reducing fluids rich in dissolved H 2 , such as those discharging from serpentinizing hydrothermal systems, are of particular interest because of the potential for abiotic reduction of dissolved inorganic carbon ðΣCO 2 = CO 2 + HCO − 3 + CO 2− 3 Þ to organic compounds (2-6) and their potential role as precursor compounds for prebiotic chemistry associated with the origin of life (7). Although there is increasing evidence that supports an abiotic origin for CH 4 and other low-molecular weight organic compounds in ultramafic-hosted hydrothermal systems (8-10), the physical conditions, reaction pathways, and timescales that control abiotic organic synthesis at oceanic spreading centers remain elusive. Working models for the formation of abiotic CH 4 and other hydrocarbons observed in vent fluids involve reduction of ΣCO 2 and/or CO through Fischer-Tropsch-type processes during active circulation of seawater-derived hydrothermal fluids that are highly enriched in dissolved H 2 because of serpentinization of...