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

Nitrogen concentrations and isotopic compositions of kerogens from Precambr ian cherts have been determined for a series of samples from Western and No rthern Australia, South and Central Africa and Northern America. The nitrog en 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 thousa nd. 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) simil ar to the Phanerozoic samples. Changes through geological time of the atmos pheric nitrogen isotopic composition or a selective diagenetic preservation of nitrogen-bearing organic compounds cannot account for this isotopic shi ft. Today in the sea marine organic matter exhibits positive N-15 values re flecting 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 condition s 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 nitr ogen. In this respect, nitrogen isotopes in kerogens have recorded the evol ution of redox-conditions on the Earth's surface. (C) 1999 Elsevier Science B.V. All rights reserved.