Letter | Published:

Dynamics of a Snowball Earth ocean

Nature volume 495, pages 9093 (07 March 2013) | Download Citation

Abstract

Geological evidence suggests that marine ice extended to the Equator at least twice during the Neoproterozoic era (about 750 to 635 million years ago)1,2, inspiring the Snowball Earth hypothesis that the Earth was globally ice-covered3,4. In a possible Snowball Earth climate, ocean circulation and mixing processes would have set the melting and freezing rates that determine ice thickness5,6, would have influenced the survival of photosynthetic life4,5,7,8,9, and may provide important constraints for the interpretation of geochemical and sedimentological observations4,10. Here we show that in a Snowball Earth, the ocean would have been well mixed and characterized by a dynamic circulation11, with vigorous equatorial meridional overturning circulation, zonal equatorial jets, a well developed eddy field, strong coastal upwelling and convective mixing. This is in contrast to the sluggish ocean often expected in a Snowball Earth scenario3 owing to the insulation of the ocean from atmospheric forcing by the thick ice cover. As a result of vigorous convective mixing, the ocean temperature, salinity and density were either uniform in the vertical direction or weakly stratified in a few locations. Our results are based on a model that couples ice flow and ocean circulation, and is driven by a weak geothermal heat flux under a global ice cover about a kilometre thick. Compared with the modern ocean, the Snowball Earth ocean had far larger vertical mixing rates, and comparable horizontal mixing by ocean eddies. The strong circulation and coastal upwelling resulted in melting rates near continents as much as ten times larger than previously estimated6,7. Although we cannot resolve the debate over the existence of global ice cover10,12,13, we discuss the implications for the nutrient supply of photosynthetic activity and for banded iron formations. Our insights and constraints on ocean dynamics may help resolve the Snowball Earth controversy when combined with future geochemical and geological observations.

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Acknowledgements

We thank B. Rose for comments. This work was supported by the NSF Climate Dynamics P2C2 programme, grant number ATM-0902844 (to E.T. and Y.A.). E.T. thanks the Weizmann Institute for its hospitality during parts of this work. Y.A. thanks the Harvard EPS department for a most pleasant and productive sabbatical visit.

Author information

Affiliations

  1. Department of Solar Energy and Environmental Physics, The Blaustein Institutes for Desert Research, Ben-Gurion University of the Negev, Midreshet Ben-Gurion, 84990, Israel

    • Yosef Ashkenazy
  2. Institute of Earth Sciences, The Hebrew University of Jerusalem, Jerusalem, 91904, Israel

    • Hezi Gildor
  3. Alfred-Wegener-Institut für Polar- und Meeresforschung, 27515 Bremerhaven, Germany

    • Martin Losch
  4. Earth and Planetary Sciences, Harvard University, 20 Oxford Street, Cambridge, Massachusetts 02138, USA

    • Francis A. Macdonald
    • , Daniel P. Schrag
    •  & Eli Tziperman
  5. School of Engineering and Applied Sciences, Harvard University, 20 Oxford Street, Cambridge, Massachusetts 02138, USA

    • Daniel P. Schrag
    •  & Eli Tziperman

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Contributions

Y.A. and E.T. formulated the problem and performed the model runs and analysis, F.A.M. and D.P.S. contributed to the geological motivation and interpretation, M.L. and H.G. helped with the model set-up, and all authors contributed to the writing of the manuscript.

Competing interests

The authors declare no competing financial interests.

Corresponding authors

Correspondence to Yosef Ashkenazy or Eli Tziperman.

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https://doi.org/10.1038/nature11894

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