Hydrothermal Contributions to Elevated Dissolved Organic Carbon Concentrations and Cycling Along the Mid‐Atlantic Ridge

Hydrothermal systems along the Mid‐Atlantic Ridge (MAR) are spatially variable sources of dissolved organic carbon (DOC) to the deep ocean, with implications for carbon cycling and trace‐metal transport. We present DOC measurements from four research cruises focusing on major vent fields at Menez Gwen, Rainbow, Lost City, Broken Spur and TAG (25–40°N). Distinct DOC patterns were observed across sites. At Rainbow and Menez Gwen, DOC maxima (>1,000 μM) were decoupled from Fe and 3 He plumes, indicating diffuse DOC sources and potential chemosynthetic production. In contrast, CTD casts intercepting buoyant plumes at TAG and Rainbow showed elevated DOC (90–140 μM) coincident with Fe and 3 He, consistent with direct hydrothermal inputs and possible Fe–ligand complexation. No DOC enrichment was detected at Broken Spur. These results demonstrate that hydrothermal vents have highly variable magnitudes and sources of DOC, leading to spatially heterogeneous impacts on deep‐ocean carbon cycling and metal‐organic interactions.