The large and systematic negative shifts in the δ13C values (>12‰) of carbonate-dominated rocks that preceded Neoproterozoic glacial successions have been interpreted to record a dramatic series of global environmental and evolutionary events. These values are widely considered to be marine rather than diagenetic in origin because stratigraphic patterns of change are systematic and reproducible from basin to basin, distinct in magnitude, and associated with recognizable stratigraphic markers such as glacial deposits. In contrast, diagenetic systems are commonly considered to have a more local and stochastic influence on δ13C values. Cores taken in Quaternary carbonate platform sediments, however, reveal a curious similarity in magnitude, thickness, and core to core reproducibility where diagenetic alteration has occurred in response to sea-level fall. Sea-level changes produced similar δ13C and δ18O stratigraphic records at globally disparate locations, which are unrelated to the global marine δ13C signal and bear no relation to the global carbon cycle. By analogy with the Pliocene–Pleistocene, we propose that spatial reproducibility of δ13C in some Neoproterozoic successions might be attributed to causes other than secular variation of the global carbon cycle, including diagenesis. This observation does not negate the stratigraphic utility of the carbon isotopic values, only the origin of the signal.