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Science 20 February 2009:
Vol. 323. no. 5917, pp. 1045 - 1048
DOI: 10.1126/science.1165675
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Reports

Isotopic Evidence for an Aerobic Nitrogen Cycle in the Latest Archean

Jessica Garvin,1 Roger Buick,1* Ariel D. Anbar,2,3 Gail L. Arnold,2 Alan J. Kaufman4

The nitrogen cycle provides essential nutrients to the biosphere, but its antiquity in modern form is unclear. In a drill core though homogeneous organic-rich shale in the 2.5-billion-year-old Mount McRae Shale, Australia, nitrogen isotope values vary from +1.0 to +7.5 per mil ({per thousand}) and back to +2.5{per thousand} over ~30 meters. These changes evidently record a transient departure from a largely anaerobic to an aerobic nitrogen cycle complete with nitrification and denitrification. Complementary molybdenum abundance and sulfur isotopic values suggest that nitrification occurred in response to a small increase in surface-ocean oxygenation. These data imply that nitrifying and denitrifying microbes had already evolved by the late Archean and were present before oxygen first began to accumulate in the atmosphere.

1 Department of Earth and Space Sciences and Astrobiology Program, University of Washington, Seattle, WA 98195–1310, USA.
2 School of Earth and Space Exploration, Arizona State University, Tempe, AZ85287, USA.
3 Department of Chemistry and Biochemistry, Arizona State University, Tempe, AZ 85287, USA.
4 Department of Geology, University of Maryland, College Park, MD20742, USA.

* To whom correspondence should be addressed. E-mail: buick{at}ess.washington.edu

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