Mechanisms of nitrogen isotope fractionation at an ancient black smoker in the 2.7 Ga Abitibi greenstone belt, Canada

• Published in: Geology (Boulder) 2024-03, Vol.52 (3), p.181-186
• Main Authors: Martin, A.N., Stüeken, E.E., Michaud, J.A.-S., Münker, C., Weyer, S., van Hees, E.H.P., Gehringer, M.M.
• Format: Article
• Language: English
• Subjects: Atmosphere; Belts; Copper; Enrichment; Fluids; Fractionation; Hydrothermal flow; Isotope fractionation; Mineralization; Nitrogen; Nitrogen isotopes; Sediment; Sediments; Sulphides; Volcanic deposits; Zinc
• ISSN: 0091-7613; 1943-2682
• DOI: 10.1130/G51689.1

The biological nitrogen (N) cycle on early Earth is enigmatic because of limited data from Archean (meta-)sediments and the potential alteration of primary biotic signatures. Here we further investigate unusual 15N enrichments reported in 2.7 Ga meta-sediments from the Abitibi greenstone belt, Canada, purportedly related to a 15N-enriched Archean atmosphere. Given that sediments from this region are contemporaneous with large-scale volcanogenic massive sulfide deposits, we utilize Cu and Zn contents to trace the effects of hydrothermal circulation on N isotope fractionation. We show that high δ15Nbulk values as high as +23‰ are associated with Cu-Zn mineralization, whereas unmineralized organic-rich shales exhibit much lower δ15Nbulk and δ15Nkerogen values. Moreover, we find a large offset between δ15Nbulk and δ15Nkerogen of as much as 17‰ and relate this to the addition of organic-bound N during the late-stage emplacement of organic-rich veins. We conclude that the previously reported high δ15N values are most parsimoniously explained by biotic and abiotic mechanisms rather than a 15N-enriched atmosphere. Crucially, both mechanisms require the presence of NH4+ in hydrothermal fluids, supporting the hypothesis that hydrothermal discharge was an important nutrient source for Neoarchean marine life.