BOX 2.3

SNOWBALL EARTH

During Snowball Earth events, biological productivity in the oceans collapsed for millions of years due to extensive freezing. The NAI supported much of the fieldwork by Paul Hoffman (Harvard University team) and his students—in Namibia, Spitsbergen, and northwestern Canada—that provided the high-resolution stratigraphic and geochemical data needed to test and refine the snowball hypothesis.The NAI also supported Samuel Bowring’s fieldwork that determined strong geochronometric constraints on the timing of Neoproterozoic ice ages. The Snowball Earth topic was an integral part of Harvard University’s 1998 NAI proposal, and much of that team’s efforts went into developing the concept into a truly multidisciplinary topic of great astrobiological importance. Although the severity of the historical glaciations is debated, theoretical Snowball conditions are associated with the nearly complete shutdown of the hydrological cycle. A recent result by Joseph Kirschvink and colleagues suggests that, during such long and severe glacial intervals, photochemical reactions would give rise to the sustained production of hydrogen peroxide, which is stored in the ice. The peroxide would then be released directly into the ocean and the atmosphere upon melting and could mediate global oxidation events in the aftermath of the Snowball. Low levels of peroxides and molecular oxygen generated during Archean and earliest Proterozoic non-Snowball glacial intervals could have driven the evolution of oxygen-using enzymes and thereby paved the way for the eventual appearance of oxygenic photosynthesis.


Bibliography


D. Condon, M.Y. Zhu, S. Bowring, et al., “U-Pb Ages from the Neoproterozoic Doushantuo Formation, China,” Science 308: 95-98, 2005.

G.P. Halverson, P.F. Hoffman, D.P. Schrag, et al., “Toward a Neoproterozoic Composite Carbon-Isotope Record,” Bulletin of the Geological Society of America 117: 1181-1207, 2005.

P.F. Hoffman and D.P. Schrag, “The Snowball Earth Hypothesis: Testing the Limits of Global Change,” Terra Nova 14: 129-155, 2002.

M.-C. Liang, H. Hartman, R.E. Kopp, J. Kirschvink, and Y.L. Yung, “Production of Hydrogen Peroxide in the Atmosphere of a Snowball Earth and the Origin of Oxygenic Photosynthesis,” Proceedings of the National Academy of Sciences 10: 18896-18899, 2006.

complex and phylogenetically diverse. Microbes in anoxic, deep sub-seafloor sediments respire at rates that are orders of magnitude slower than previously believed necessary to sustain life. Their metabolic pathways include new processes, such as the biological generation of ethane and propane.21-25 For additional details see Box 2.6.

  • Metal isotope tracers of environment and biology. Studies of the biological and abiological fractionation of metal isotopes, particularly the redox-sensitive elements molybdenum and iron, were motivated by astrobiology objectives to study Earth’s redox evolution and to find new signatures for life. This work has been supported by the NAI from its earliest days (e.g., Kenneth Nealson’s team at the Jet Propulsion Laboratory) and is the focus of the new team headed by Clark Johnson (University of Wisconsin). Iron-isotope geochemistry is now being pursued in about 30 laboratories across the globe.26-31 For additional details see Box 2.7.

  • Life without the Sun. NAI scientists from Princeton University and Indiana University have discovered deeply buried life in a South Africa gold mine that appears to thrive independent of the familiar surface biosphere, which is powered by sunlight. These microbes draw energy from hydrogen and sulfates produced when the decay of radioactive elements in the rocks disassociates water molecules.32 For additional details see Box 2.8.

  • Early wet Mars. NAI astrobiologists such as Jack Farmer (Arizona State University team), David Des Marais (Ames Research Center team), Andrew Knoll (Harvard University team), Mark Allen (JPL team), John Grotzinger (MIT team), and Bruce Jakosky (University of Colorado team) have played major roles in recommending landing sites, defining objectives and spacecraft operations, and interpreting data from current Mars orbiters



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