An Assessment of the Precambrian/Cambrian Transition Events on the Basis of Carbon Isotope Records

Abstract

Renewed interest in the isotope events at the Precambrian/Cambrian (PC/C) transition has led to a recent proliferation of high resolution δ13C records acquired from sedimentary carbonate sections that are stratigraphically continuous. These δ13C records generally show a bimodal distribution of values, with a 13C-enriched mode in the end-Precambrian and a sharp transition to a 13C-depleted mode at, or slightly above, the inferred PC/C boundary. The time-bound δ13C excursions have considerable potential as chronostratigraphic markers which are independent of the controversial biostratigraphic definitions currently used to delineate the PC/C boundary. The relation between δ13C shifts and oceanic fertility changes is reasonably well known but the circumstances under which these changes have occurred at the PC/C boundary are unclear. Using quantitative arguments derived from a simplified carbon cycle model, an ocean stratification event is proposed to have occurred during the latest Vendian followed by a turnover and a return to a ventilated ocean in the lowest Cambrian. The exact relationship between the changing rates of ocean ventilation and the seemingly contemporaneous faunal turnovers remains to be explored.