Climatic catastrophes in Earth history: two great Proterozoic glacial episodes

Authors


G. M. Young, Department of Earth Sciences, Western University, London, Ontario, N6A 5B7, Canada. E-mail: gyoung@uwo.ca

Abstract

Near the beginning and end of the Proterozoic Eon (2.5 Ga–542 Ma) the Earth went through dramatic climatic perturbations. The Palaeoproterozoic (Huronian) glaciations are best known from the Canadian Shield where there is evidence of at least three such episodes. Glacial deposits of comparable age are also known from Fennoscandia, South Africa and Western Australia. In the type area, the Huronian glacial deposits are preserved in an ancient rift system that preceded break-up of the supercraton, Kenorland, whereas those in the southern hemisphere may have been deposited in a foreland basin setting. Detailed correlations between the two hemispheres must await more geochronological data. Following a long period (~1.5 Ga) with little evidence of glaciation, the climatic upheavals of the Neoproterozoic Era began. The two most widespread glacial events are known as the Sturtian and Marinoan. The Neoproterozoic glaciations also took place on a supercontinent (Rodinia). Some were accompanied by unexpected rock types such as dolomitic cap carbonates and iron formations, both of which show evidence of hydrothermal influence. Major influences on surface temperatures on Earth include solar luminosity (increasing throughout geological history) and the concentration of atmospheric greenhouse gases such as CO2 (generally diminishing with time). It is suggested that the two great Proterozoic climatic oscillation periods resulted from perturbations of the balance between these two variables, triggered by drawdown of atmospheric CO2 during intensive weathering of supercontinents. A weathering-related negative feedback loop resulted in multiple glaciations with intervening warm periods. Climatic stability only returned after the supercontinent broke apart and reduced continental freeboard moderated continental weathering. Copyright © 2012 John Wiley & Sons, Ltd.

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