A detailed radiocarbon chronology obtained by accelerator mass spectrometry together with organic carbon and carbonate measurements on two Black Sea cores has been used to compare and contrast the burial fluxes of organic carbon in the Holocene sapropel and the modern sediment. At both deep-water and eastern-slope sites, the sapropel is separated from the modern facies by a variable thickness of compositionally homogeneous sediment with low levels of organic carbon and anomalously old radiocarbon ages. This homogeneous unit probably represents deposition by slumping or mudflow. The age limits of the sapropel are 1600–6600 B.P. at the deep-water site and 4000–6000 B.P. (radiocarbon years before 1950 A.D.) at the shallow-water site. The carbon accumulation rate in the deep-water sapropel is higher than that in the modern deep-water facies by a factor of 2 and is approximately the same as that in the modern sediment in shallow water. The revised chronology of sapropel formation and the differences in the carbon accumulation rates probably indicate that the sapropel was formed by increased production during the transition from the premodern lake to the modern marine phases of the Black Sea. This conclusion is consistent with the clear marine carbon-isotope signal in the organic matter in the sapropel in both cores (results to be reported elsewhere), in contrast to the mixed source of carbon in the other facies.