The carbon isotopic (delta (super 13) C) composition of bulk carbonate sediments deposited off the margins of four carbonate platforms/ramp systems (Bahamas, Maldives, Queensland Plateau, and Great Australian Bight) show synchronous changes over the past 0 to 10 million years. However, these variations are different from the established global pattern in the delta (super 13) C measured in the open oceans over the same time period. For example, from 10 Ma to the present, the delta (super 13) C of open oceanic carbonate has decreased, whereas platform margin sediments analyzed here show an increase. It is suggested that the delta (super 13) C patterns in the marginal platform deposits are produced through admixing of aragonite-rich sediments, which have relatively positive delta (super 13) C values, with pelagic materials, which have lower delta (super 13) C values. As the more isotopically positive shallow-water carbonate sediments are only produced when the platforms are flooded, there is a connection between changes in global sea level and the delta (super 13) C of sediments in marginal settings. These data indicate that globally synchronous changes in delta (super 13) C can take place that are completely unrelated to variations in the global carbon cycle. Fluctuations in the delta (super 13) C of carbonate sediments measured during previous geological periods may also be subject to similar processes, and global synchroneity of delta (super 13) C can no longer necessarily be considered an indicator that such changes are related to, or caused by, variations in the burial of organic carbon. Inferences regarding the interpretation of changes in the cycling of organic carbon derived from delta (super 13) C records should be reconsidered in light of the findings presented here.