Carbon isotope sequences at Ocean Drilling Program Site 1143, South China Sea, reveal a long-term cyclicity of ~500 kyr that is superimposed on the glacial cycles and is present in long δ13C sequences from all oceans. The Quaternary δ13C record is punctuated by four δ13C maximum events: δ13Cmax-I, which began in marine isotope stage (MIS) 3 around 50--60 kyr ago, δ13Cmax-II (MIS 13, 0.47--0.53 Ma), δ13Cmax-III (MIS 27--29, 0.97--1.04 Ma) and δ13Cmax-VI (MIS 53--57, 1.55--1.65 Ma). As the same cyclicity is also found in carbonate curves, the δ13Cmax events must denote major reorganization in the carbon reservoir of the global ocean. They also are associated with major changes in glacial cyclicity, such as the Mid-Brunhes Event following δ13Cmax-II and the Mid-Pleistocene Revolution following δ13Cmax-III, which in turn were associated with expansion of the ice sheets. From a carbon perspective, therefore, the Quaternary period has passed through three major stages, and each appears to represent a further step in ice cap development.