Snowball earth: The African legacy.

The stratigraphic association of late Precambrian glacial deposits with shallow-marine carbonate successions, which are biased toward the warmer parts of the surface ocean, was known in Africa and elsewhere since the 1930's, but geologists under-appreciated it as a paradox in physical climatology. A physical explanation for global freeze-up emerged from one-dimensional energy-balance climate models in 1969, after which it was widely discussed and refined. In 1981, it was suggested that a global freeze-up could be self-terminating after millions of years through slow accumulation of atmospheric CO 2 , a greenhouse gas. The idea of a self-reversing global freeze-up, or 'snowball Earth,' was first applied geologically by Kirschvink (1992) based on paleomagnetic evidence in South Australia. He cited no paper in physical climatology and geologists remained unaware of developments in that science until after 2000. A self-reversing global freeze-up involves strong predictions in duration, synchroneity and termination state that have now been tested in the geological record. Inflections in growth rate and disciplinary diversity of peer-reviewed papers in 1997 implies that climate physics and geology were galvanized to test both the Siderian (ca 2430 Ma) and Cryogenian (717−635 Ma) snowball Earth hypotheses by new field research conducted in South Africa and northern Namibia, respectively. • A Siderian snowball Earth (SBE) was first inferred from African paleomagnetic data. • Tandem Cryogenian SBEs were most forcefully posed by African carbonate stratigraphy. • SBEs have been tested by geochronology, geochemistry, sedimentology & climate models. • SBEs followed supercontinent break-up and flood-basalt volcanism on the paleoequator. • Preexisting polar−alpine biota survived SBEs and were ancestral to later surface life. [ABSTRACT FROM AUTHOR]