DEVELOPING AN ENHANCED CHRONOLOGY FOR THE TERMINAL EDIACARAN-CAMBRIAN TRANSITION ON A GLOBAL SCALE

The terminal Ediacaran Period represents a critical time interval marked by significant changes in the global climate, ocean chemistry and the evolution of life, which then set the stage for the Cambrian evolutionary radiation. The Ediacaran–Cambrian (E-C) transition constitutes of a series of events, including the last of the Neoproterozoic glaciations, the advent of complex multicellular life, a major perturbation in the global carbon cycle, and the first appearance of calcified metazoans. These culminated in the remarkable radiation of the bilaterian fauna and the birth of many of the modern phyla in the early Cambrian.

Understanding the mechanisms of co-evolution of life and environment during the E-C interval requires a robust chronostratigraphic framework on the global scale. In the past dozen years, significant developments in single-zircon U-Pb ID-TIMS analysis, including zircon pre-treatment by the chemical abrasion method and the EARTHTIME standards and protocols, have greatly improved the precision and accuracy of ash bed geochronology. These have enhanced our ability to resolve the tempo of geologic and biotic events during the 40 m.y.-long interval of terminal Ediacaran. Recent and ongoing high-precision U-Pb geochronology by the CA/ID-TIMS method has shown that the ca. 580 Ma Gaskiers glaciation lasted less than 1.7 m.y. (shorter than 340 k.y. locally), and it may not have had a significant impact on the rise of complex, eukaryotic life, whose earliest documented evidence (Ediacaran biota) dates back to ca. 571 Ma. The subsequent appearance of calcified metazoans (Cloudina and Namacalathus) followed the largest known carbon isotope excursion in the stratigraphic record (Shuram excursion) although its exact timing and duration remain inadequately constrained. Finally, the extinction of both the Ediacaran assemblages and the early calcified metazoans precedes the radiation of bilaterian fauna at the onset of Cambrian.

The reliance of the E-C boundary age on U-Pb geochronology that predated the EARTHTIME standards or single-zircon analysis calls for a reevaluation (or repeat) of the legacy data from Oman and Namibia. Geochronology in progress along with new interpretations of the E-C boundary point to a new boundary age that is ~1.5 m.y. younger than the presently accepted value.