Mechanisms of nitrogen isotope fractionation at an ancient black smoker in the 2.7 Ga Abitibi greenstone belt, Canada
Abstract
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.
Citation
Martin , A N , Stüeken , E E , Michaud , JA-S , Münker , C , Weyer , S , van Hees , E H P & Gehringer , M M 2024 , ' Mechanisms of nitrogen isotope fractionation at an ancient black smoker in the 2.7 Ga Abitibi greenstone belt, Canada ' , Geology . https://doi.org/10.1130/G51689.1
Publication
Geology
Status
Peer reviewed
DOI
10.1130/G51689.1ISSN
0091-7613Type
Journal article
Description
Funding: Funding for Martin, Münker, Weyer, and Gehringer was provided by the German Research Foundation (DFG) priority program “SPP-1833 Building a Habitable Earth,” and for Michaud by the DFG priority program “SPP-2238 Dynamics of Ore Metals Enrichment” (HO1337/49-1). Stüeken acknowledges support from a UK Natural Environment Research Council (NERC) Frontiers grant (NE/V010824/1) and a Leverhulme Trust grant (RPG-2022-313).Collections
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