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Nitrogen concentrations and isotopic compositions of kerogens from Precambrian cherts have been determined for a series of samples from Western and Northern Australia, South and Central Africa and Northern America. The nitrogen concentrations range from 2 to 106 ppm with C/N elemental ratios between 31 and 590. Sample ages range from 3.5 to 0.7 Gyr. They display δ 15 N values ranging from −6 to +13‰, whereas δ 15 N values of Phanerozoic marine samples usually range from 0 to +10‰. The oldest samples (3.5 to 3.4 Gyr) appear to be isotopically lighter (down to −6.2‰) whereas the Early Proterozoic samples (2.0 Gyr) display δ 15 N values (from 0.3 to 10.1‰) similar to the Phanerozoic samples. Changes through geological time of the atmospheric nitrogen isotopic composition or a selective diagenetic preservation of nitrogen-bearing organic compounds cannot account for this isotopic shift. Today in the sea marine organic matter exhibits positive 15 N values reflecting the 15 N enrichment of the dissolved nitrate (NO − 3 ). Therefore it seems likely that, in the absence of atmospheric oxygen, NO − 3 was absent in Archean seas. An Archean nitrogen cycle is proposed for which the negative 15 N values reflect a metabolic isotopic fractionation in anoxic conditions with microorganisms using the reduced forms of nitrogen (N 2 , NH + 4 ). The increase of atmospheric oxygen after the Archean time would have encouraged the biological production of NO − 3 and its use as a source for organic nitrogen. In this respect, nitrogen isotopes in kerogens have recorded the evolution of redox-conditions on the Earth's surface. |
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