COPS (CLIMATIC OSCILLATION PERIODS) AT THE CROSSROADS OF EARTH EVOLUTION

The Proterozoic eon began and ended with dramatic climatic events, which can be characterized as climatic oscillation periods (COPS). The "type" Paleoproterozoic succession, the Huronian Supergroup on the north shore of Lake Huron in Canada, contains evidence of three major glacial episodes, separated by rocks that mainly indicate vigorous weathering processes. For example the last and most widespread glacial unit (the Gowganda Formation) is overlain by one of the world's thickest supermature orthoquartzites - the Lorrain Formation. These enigmatic climatic events followed amalgamation of the late Archean supercontinent, Kenorland. Orogenic events that culminated in production of a supercontinent would have led to greatly enhanced weathering, drawdown of atmospheric CO2 and glaciation. The climatic oscillations recorded by Paleoproterozoic glaciations and the intervening strongly weathered deposits may be explained by feedback mechanisms, set in motion by supercontinentality. Evidence from paleosols, pyritic clastic sedimentary rocks, red beds, BIF's and C isotopes, supports gradual oxygenation of the atmosphere during and after the Paleoproterozoic COP. After an ice-free(?) period that spanned part of the Paleoproterozoic and the entire Mesoproterozoic eon, the Earth entered a second COP, following the Grenville tectonic events and the production of the supercontinent, commonly known as Rodinia. Many parallels can be drawn between the Neo- and Paleoproterozoic COP's. Both began after periods of supercontinent formation - the deposits of both COP's contain evidence of alternating frigid and "warm" conditions. Both saw an increased atmospheric oxygen content. The dramatic Neoproterozoic environmental changes may have played a critical role in the Cambrian evolutionary "explosion". Thus the two greatest transition periods in Earth's history, which signal the beginning and end of the Proterozoic eon, may be the result of plate tectonic processes - the formation of supercontinents - which in turn led to dramatic climatic oscillation periods (COPS), changes in atmospheric and hydrospheric composition and played a significant role in the evolution of life