Nitrogen isotope ratios of kerogens in Precambrian cherts: a record of the evolution of atmosphere chemistry?

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 delta(15)N values ranging from -6 to 13 parts per thousand, whereas delta(15)N values of Phanerozoic marine samples usually range from 0 to +10 parts per thousand. The oldest samples (3.5 to 3.4 Gyr) appear to be isotopically lighter (down to -6.2 parts per thousand) whereas the Early Proterozoic samples (2.0 Gyr) display delta(15)N values (from 0.3 to 10.1 parts per thousand) 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 N-15 values reflecting the N-15 enrichment of the dissolved nitrate (NO3-). Therefore it seems likely that, in the absence of atmospheric oxygen, NO3- was absent in Archean seas. An Archean nitrogen cycle is proposed for which the negative N-15 values reflect a metabolic isotopic fractionation in anoxic conditions with microorganisms using the reduced forms of nitrogen (N-2, NH4+). The increase of atmospheric oxygen after the Archean time would have encouraged the biological production of NO3- 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. (C) 1999 Elsevier Science B.V. All rights reserved.