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Abstract The Early Mississippian (Tournaisian) positive δ13C excursion (mid-Tournaisian carbon isotope excursion [TICE]) was one of the largest in the Phanerozoic, and the organic carbon (OC) burial associated with its development is hypothesized to have enhanced late Paleozoic cooling and glaciation. We tested the hypothesis that expanded ocean anoxia drove widespread OC burial using uranium isotopes (δ238U) of Lower Mississippian marine limestone as a global seawater redox proxy. The δ238U trends record a large Tournaisian negative excursion lasting ∼1 m.y. The lack of covariation between δ238U values and facies changes and proxies for local depositional and diagenetic influences suggests that the δ238U trends represent a global seawater redox signal. The negative δ238U excursion is coincident with the first TICE positive excursion, supporting the hypothesis that an expanded ocean anoxic event controlled OC burial. These results provide the first evidence from a global seawater redox proxy that an ocean anoxic event drove Tournaisian OC burial and controlled Early Mississippian cooling and glaciation. Uranium and carbon modeling results indicate that (1) there was an ∼6× increase in euxinic seafloor area, (2) OC burial was initially driven by expanded euxinia followed by expanded anoxic/suboxic conditions, and (3) OC burial mass was ∼4–17× larger than that sequestered during other major ocean anoxic events.