(carbon isotopesystratigraphyypaleontologyyEdiacaran fauna)

Detailed correlations of ancient glacial deposits, based on temporal records of carbon and strontium isotopes in seawater, indicate four (and perhaps five) discrete ice ages in the terminal Proterozoic Eon. The close and repeated stratigraphic relationship between C-isotopic excursions and glaciogenic rocks suggests that unusually high rates of organic carbon burial facilitated glaciation by reducing atmospheric greenhouse capacity. The emerging framework of time and environmental change contributes to the improved resolution of stratigraphic and evolutionary pattern in the early fossil record of animals. In 1964, W. B. Harland (1) proposed that an ice age of global extent occurred near the end of the Proterozoic Eon. He recognized a worldwide distribution of tillites and associated lithologies beneath strata bearing Ediacaran fossils and considered that these rocks were deposited during surface refrigeration more pronounced than the Pleistocene ice age. Debate engendered by Harland’s hypothesis continues to the present. Are geographically scattered tillites demonstrably synchronous? How many ice ages occurred, if indeed they compose discrete events? And, does late Cenozoic climatic change provide an appropriate model for more ancient glaciations? The demonstration that the carbon and strontium isotopic composition of seawater varied systematically through the Neoproterozoic Era [1000–543 million years ago (Ma); refs. 2, 3] provides a stratigraphic framework with the potential to resolve fundamental questions of tillite correlation. At the same time, chemostratigraphic data may constrain hypotheses to explain Neoproterozoic climatic oscillations. The C-isotopic record through this interval is characterized by unusual 13C enrichment in carbonates and organic matter, punctuated by excursions to unusually low d13C values. Some, and perhaps all, of these strong negative excursions are associated with glaciogenic rocks (4, 5). If general, this suggests that Neoproterozoic climatic change was mediated, at least in part, by oceanographically driven variations in carbon cycling. Here we present new data from Neoproterozoic successions in northwestern Canada and Spitsbergen, focusing on glacial intervals not previously sampled, and we use integrated stratigraphic techniques to predict worldwide correlations of ancient tillites. The resulting chronostratigraphic framework provides an improved perspective on the relationship between terminal Proterozoic environmental change and early animal evolution. Chemostratigraphy of Tillite-Bearing Successions: