PerspectiveOcean Science

Who Threw That Snowball?

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Science  31 Oct 2003:
Vol. 302, Issue 5646, pp. 791-792
DOI: 10.1126/science.1091464

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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 COthan would occur today, amplifying any initial cooling until the snowball was well underway.