Proterozoic Climates

Abstract

Turbulent climatic conditions, including glaciations, are recorded in rocks near the beginning and end of the Proterozoic Eon (2500–541 million years ago), with relative climatic stability during the “boring billion” (~1.8–0.8 billion years ago). Continents underwent repeated assembly and breakup, and there were episodic huge impacts of comets and asteroids. Long-term climatic influences include changes in solar radiation, Earth's rotation rate, atmospheric composition and internal temperature. Both periods of climatic upheaval were marked by widespread glaciations at low paleolatitudes and altitudes, each glacial episode being accompanied by a significant increase in atmospheric oxygen and followed by a warm period. There are two prominent hypotheses for Proterozoic glaciations at low paleolatitudes. The Snowball Earth Hypothesis posits that glaciers spread from polar regions to the equator by runaway albedo, when heat reflected from expanding polar ice sheets reached a tipping point and caused the planet to be encased in ice. The High Obliquity Hypothesis explains glaciers near sea level at low paleolatitudes by invoking a radically different climatic zonation related to high obliquity (tilt) of Earth's spin axis until near the end of the Proterozoic Eon. The cause of dramatic climatic changes near the beginning and end of the Proterozoic Eon remains uncertain, but the advent of “modern-style” plate tectonics, manifested as rifting (with initiation of glaciations on elevated fault blocks) during continental breakup, and as important changes in atmospheric pCO2 during volcanic episodes, may have played an important role. Climatic stability during the “boring” billion may reflect relative tectonic quiescence.