THE NATURE AND EVOLUTION OF THE EARLY PALEOZOIC WORLD

Despite much progress on several fronts in recent years, many of the intriguing questions remain on the nature and evolution of the Earth during the Early Paleozoic (Cambrian through Silurian; c.540-410 Ma). The late phases of Rodinia breakup and Gondwana assembly were followed by continental dispersal and some further reassembly (Baltica to Laurentia). Much uncertainty remains on the positions of many peri-Gondwana terranes, which is critical for establishing the role of key gateways that impacted the climate system. The high level of volcanic activity is expressed especially in the occurrences of extensive ash-falls across much of the Iapetus Ocean and its borderlands (Arenigian-Caradocian) and voluminous volcanics (Darriwilian-Caradocian) in the New Brunswick Appalachians, both types of occurrences being possibly the largest single events in the Phanerozoic. These may be related to a superplume event in the Middle Ordovician for which there are several lines of supporting evidence (high sealevels, black shales, Sr isotope excursions, lack of magnetic reversals). The Early Paleozoic is a remarkable time in the evolution of life, notably with the Cambrian explosion, the radiation of the Paleozoic Fauna in the early-mid Ordovician, the terminal Ordovician mass extinction, and the rise of lacustrine and terrestrial biota. The controls and precise processes involved in these are not well understood. New geochemical and isotopic data and modelling have advanced interpretations of the changing paleoceanography and paleoclimatology, especially those from greenhouse to icehouse conditions in the Late Ordovician-Early Silurian and the apparent paradox of a glaciation at times of up to 16X CO2. More quantitative values are needed for atmospheric CO2, O2 levels and oceanic nutrient values. There is likely a strong correlation between increased water vapour and greenhouse climates with the most extensive epeiric seas in the Phanerozoic, and a close linkage between transgressive/regressive cycles and climate and faunal change.