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Carbonate sediments of nonglacial Cryogenian (659 to 649 Ma) and early Ediacaran (635 to 590 Ma) age exhibit large positive and negative delta C-13(carb) excursions in a shallow-water marine platform in northern Namibia. The same excursions are recorded in fringing deep-sea fans and in carbonate platforms on other paleocontinents. However, coeval carbonates in the upper foreslope of the Namibian platform, and to a lesser extent in the outermost platform, have relatively uniform delta C-13(carb) compositions compatible with dissolved inorganic carbon (DIC) in the modern ocean. We attribute the uniform values to fluid-buffered diagenesis that occurred where seawater invaded the sediment in response to geothermal porewater convection. This attribution, which is testable with paired Ca and Mg isotopes, implies that large delta C-13(carb) excursions observed in Neoproterozoic platforms, while sedimentary in origin, do not reflect the composition of ancient open-ocean DIC.