GEOCHRONOLOGICAL CONSTRAINTS ON NEOPROTEROZOIC GLACIATIONS AND THE RISE OF METAZOANS

A full understanding of the Neoproterozoic, a period characterized by extreme climatic fluctuations and major biological change, has been hindered by a lack of high-precision geochronological constraints. In particular the correlation and estimates of duration of global or “Snowball” glaciations has relied on physical stratigraphy, chemostratigraphy, and thermal subsidence models respectively which have given rise to much debate about the number, synchroneity and duration of glacial episodes. The close stratigraphic juxtaposition of the youngest (?) Neoproterozoic glaciogenic rocks and those preserving the earliest large animal fossils is suggestive of an environmental control on Metazoan evolution.

A review of available geochronological data indicates that there were more than three and perhaps as many as five periods of Neoproterozoic glacial deposition including rocks from Oman, Newfoundland, Namibia, NWT of Canada, and Idaho. However, in order to reliably test correlations, determine durations and assess magnitudes of the various glacial events, precise ages are essential. What is not known at present is whether the Neoproterozoic records one or more truly synchronous global glaciations, a number of ‘conventional’ high-latitude glacial episodes, or a combination of both? New U-Pb constraints on the age of the Gaskiers Formation glacial deposits and associated cap carbonate in Newfoundland, and overlying fossiliferous rocks in the same area indicate a short duration of glaciation followed in less than 5 million years by the first large Ediacaran fossils. This strongly supports a link between extreme climatic fluctuations and Metazoan evolution but the short duration is at odds with models for an ice-covered planet. A highly-resolved Neoproterozoic timescale will be required to understand the significance of glaciations and their effect on ocean and atmospheric chemistry and evolution.