title: Atmospheric record in the Hadean Eon from multiple sulfur isotope measurements in Nuvvuagittuq Greenstone Belt (Nunavik, Quebec).
abstract: Mass-independent fractionation of sulfur isotopes (S-MIF) results from photochemical reactions involving short-wavelength UV light. The presence of these anomalies in Archean sediments [(4-2.5 billion years ago, (Ga)] implies that the early atmosphere was free of the appropriate UV absorbers, of which ozone is the most important in the modern atmosphere. Consequently, S-MIF is considered some of the strongest evidence for the lack of free atmospheric oxygen before 2.4 Ga. Although temporal variations in the S-MIF record are thought to depend on changes in the abundances of gas and aerosol species, our limited understanding of photochemical mechanisms complicates interpretation of the S-MIF record in terms of atmospheric composition. Multiple sulfur isotope compositions (δ(33)S, δ(34)S, and δ(36)S) of the >3.8 billion-year-old Nuvvuagittuq Greenstone Belt (Ungava peninsula) have been investigated to track the early origins of S-MIF. Anomalous S-isotope compositions (Δ(33)S up to +2.2‰) confirm a sedimentary origin of sulfide-bearing banded iron and silica-rich formations. Sharp isotopic transitions across sedimentary/igneous lithological boundaries indicate that primary surficial S-isotope compositions have been preserved despite a complicated metamorphic history. Furthermore, Nuvvuagittuq metasediments recorded coupled variations in (33)S/(32)S, (34)S/(32)S, and (36)S/(32)S that are statistically indistinguishable from those identified several times later in the Archean. The recurrence of the same S-isotope pattern at both ends of the Archean Eon is unexpected, given the complex atmospheric, geological, and biological pathways involved in producing and preserving this fractionation. It implies that, within 0.8 billion years of Earth's formation, a common mechanism for S-MIF production was established in the atmosphere.
authors.affiliation: Centre de Recherches Pétrographiques et Géochimiques, Université de Lorraine, 54501 Vandœuvre-Lès-Nancy, France; Department of Earth and Planetary Sciences and GEOTOP, McGill University, Montreal, QC H3A 0E8, Canada; emilie@crpg.cnrs-nancy.fr.
authors.forename: Emilie
authors.initials: E
authors.lastname: Thomassot
authors.affiliation: Department of Earth Sciences, University of Ottawa, Ottawa, ON K1N 6N5, Canada; and.
authors.forename: Jonathan
authors.initials: J
authors.lastname: O'Neil
authors.affiliation: Department of Earth and Planetary Sciences and.
authors.forename: Don
authors.initials: D
authors.lastname: Francis
authors.affiliation: Laboratoire de Géochimie des Isotopes Stables, Institut de Physique du Globe de Paris, UMR 7154 CNRS, Université Paris Denis-Diderot, Sorbonne Paris Cite, 75005 Paris, France.
authors.forename: Pierre
authors.initials: P
authors.lastname: Cartigny
authors.affiliation: Department of Earth and Planetary Sciences and GEOTOP, McGill University, Montreal, QC H3A 0E8, Canada;
authors.forename: Boswell A
authors.initials: BA
authors.lastname: Wing
keywords: Nuvvuagittuq Greenstone Belt (Nunavik, Québec); early Archean sulfur cycle; early Earth surface environment; photochemistry