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Science 31 October 2003:
Vol. 302. no. 5646, pp. 791 - 792
DOI: 10.1126/science.1091464
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Perspectives

Also see the archival list of Science's Compass: Enhanced Perspectives

OCEAN SCIENCE:
Enhanced: Who Threw That Snowball?

David Archer

Between 1000 and 540 million years ago, Earth is believed to have undergone several episodes during which much of Earth's surface was frozen. Geological evidence provides some insights into the mechanisms behind these "Snowball Earth" episodes. However, as Archer explains in his Perspective, it has remained unclear why these episodes occurred during this time and not at any other time in Earth's history. Ridgwell et al. have identified a mechanism that may help to answer this question. At the time of the Snowball episodes, plankton that precipitate calcium carbonate in the open ocean had not yet evolved, and only shallow-ocean calcium carbonate deposition occurred. In this case, a drop in sea level would have driven a larger excursion in ocean pH and atmospheric CO2 than would occur today, amplifying any initial cooling until the snowball was well underway.

The author is in the Department of Geophysical Sciences, University of Chicago, Chicago, IL 60637, USA. E-mail: archer{at}geosci.uchicago.edu

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Science. ISSN 0036-8075 (print), 1095-9203 (online)