Calibrating changes across the Precambrian-Cambrian transition

Since Darwin first noted the seemingly abrupt appearance of fossils in Cambrian strata, the cause and tempo of the ‘Cambrian Explosion’ has remained one of Earth Science’s most provocative research topics. Contrary to Darwin’s hypothesis that this phenomenon reflected a largely eroded and imperfect geological record, scientists have spent the past 70 years unraveling a diverse Precambrian biosphere characterized by the deep roots of eukaryotic evolution. Now, it is generally agreed upon that the Neoproterozoic origins of animals and multicellularity are at least partly related to genetic challenges; however, the simultaneous explosion in body size, morphological features, and skeletonization in the Cambrian is likely the result of external “triggers”, the nature of which have been widely debated.
One of the environmental changes that has been observed in carbonate strata spanning the Ediacaran–Cambrian boundary is high frequency and high magnitude variability in carbon isotopic values (d13C). Although the causal mechanism(s) behind these d13C excursions are still debated, they have been generally assumed to chronicle primary changes in the net global fluxes of organic carbon burial and oxidation. Thus, secular changes in these isotopic records have been widely used to develop three-dimensional frameworks of ancient marine basins, quantitatively link changes in carbon burial to those in atmospheric oxygen, invoke shifts in the fluxes of isotopically distinct pools of carbon, and construct age models regionally and globally. Using data from multiple Ediacaran–Cambrian paleocontinents, my group members, collaborators, and I aim to test the hypothesis that the d13C excursions across the Ediacaran–Cambrian transition reflect the composition of dissolved inorganic carbon in global oceans and are buffered from post-depositional diagenetic effects. Here I will present radioisotopic and stable isotopic geochemical data that are grounded in robust geologic frameworks to discuss our current understanding of how to interpret d13Ccarb records from Ediacaran–Cambrian successions.