DRAWDOWN OF A 15N ENRICHED C1 CHONDRITE PRECAMBRIAN ATMOSPHERE

Nitrogen isotopic compositions of kerogen separated from Precambrian black shales at prehnite-pumpellyite to greenschist facies are systematically 15N-enriched. Samples from the 2.7 Ga Superior Province, Canada; E. Dharwar craton of India; Penhalonga Formation, Botswana; and the Birimain Belt of Ghana, West Africa, yield d15N values of 13-20‰, compared to existing data of 2 to 6‰ in Phanerozoic counterparts. Paleoproterozoic equivalents have intermediate d15N of 7 to 12‰, implying a secular decrease in d15N of shale kerogen. The 15N-enriched nitrogen in Precambian black shales cannot be caused either by N-isotopic shifts accompanying metamorphism or Rayleigh fractionation given that: (1) there is no co-variation of d15N with N, C/N ratios, or metamorphic grade; and (2) the magnitude of shifts during progressive metamorphism of sedimentary rocks in previous studies is constrained at <1‰ to greenschist, and about 3‰ to amphibolite facies. It is possible that the 15N-enriched values stem from a different N-cycle in the Archean, with large biologically mediated fractionations, yet the magnitude of the fractionations observed exceeds any presently known, and chemoautotrophic communities tend to depleted values. Earlier results on Archean cherts show a range from –6‰ to 30‰. Given the temporal association of chert-BIF with mantle plumes the range is consistent with mixing between mantle N2 of -5‰, and the enriched marine reservoir identified in this study. The 15N-enriched Archean atmosphere-hydrosphere reservoir does not robustly constrain Archean redox-state. We attribute the 15N-enriched reservoir to a secondary atmosphere derived from CI-chondrite-like material and comets with d15N of +30 to +42‰. Shifts of d15N to its present atmospheric value of 0‰ can be accounted for by a combination of early growth of the continents with sequestration of atmospheric N2 into crustal rocks, and degassing of mantle N ~ -5‰. If Earth's surface environment became oxygenated at ~ 2 Ga, then there were no associated large N-isotope excursions.