Geology

Snowball or Slushball?

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Science  01 Nov 2002:
Vol. 298, Issue 5595, pp. 925-927
DOI: 10.1126/science.298.5595.925e

The provocative snowball Earth hypothesis states that, during the Neoproterozoic Era, ice covered Earth's entire surface, including its oceans, for several periods lasting from 5 to 10 million years. In this scenario, global glaciation, recorded in worldwide stratigraphic sections from the era, would have continued until sufficient atmospheric CO2 had accumulated to initiate melting of the glaciers, an event marked by the sudden appearance of a characteristic suite of carbonates in the rock record. Occurring at an early stage in metazoan evolution, the global deep-freeze would have exerted a devastating impact on organisms of the time and perhaps have helped to shape the subsequent metazoan radiations of the late Neoproterozoic and early Cambrian.

One prediction of the snowball Earth hypothesis is the virtual shutdown of Earth's hydrologic cycle. Leather et al. examined a 1.5-km-thick succession of Neoproterozoic glacial and nonglacial rocks in Oman, overlain by the characteristic cap carbonate. They found evidence for pulsed glacial advances and retreats, with a highly active hydrological cycle, that appear more reminiscent of the high-frequency glaciations of the Pleistocene than of the long stasis envisioned in snowball Earth. This geologic evidence may necessitate a reconsideration of some of the key assumptions underlying the hypothesis. — SW

Geology30, 891 (2002).

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