Journal
SCIENCE
Volume 323, Issue 5917, Pages 1045-1048Publisher
AMER ASSOC ADVANCEMENT SCIENCE
DOI: 10.1126/science.1165675
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Funding
- NASA Astrobiology Drilling Program
- NSF Geobiology and Low Temperature Geochemistry
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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 (parts per thousand) and back to +2.5 parts per thousand over similar to 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.
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