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Atmospheric Record in the Hadean Eon from Multiple Sulfur Isotope Measurements in Nuvvuagittuq Greenstone Belt (Nunavik, Quebec)

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Atmospheric Record in the Hadean Eon from Multiple Sulfur Isotope Measurements in Nuvvuagittuq Greenstone Belt (Nunavik, Quebec) Atmospheric record in the Hadean Eon from multiple sulfur isotope measurements in Nuvvuagittuq Greenstone Belt (Nunavik, Quebec) Emilie Thomassota,b,c,1, Jonathan O’Neild, Don Francisb, Pierre Cartignye, and Boswell A. Wingb,c aCentre de Recherches Pétrographiques et Géochimiques, Université de Lorraine, 54501 Vandœuvre-Lès-Nancy, France; bDepartment of Earth and Planetary Sciences and cGEOTOP, McGill University, Montreal, QC H3A 0E8, Canada; dDepartment of Earth Sciences, University of Ottawa, Ottawa, ON K1N 6N5, Canada; and eLaboratoire 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 Edited by Mark H. Thiemens, University of California, San Diego, La Jolla, CA, and approved December 7, 2014 (received for review October 15, 2014) Mass-independent fractionation of sulfur isotopes (S-MIF) results wide intervals in time and are shared among a variety of rock- from photochemical reactions involving short-wavelength UV forming environments. light. The presence of these anomalies in Archean sediments [(4–2.5 High-resolution measurements on Neoarchean samples (2.5– billion years ago, (Ga)] implies that the early atmosphere was free of 2.8 Ga) (13, 14) have provided evidence for temporal variation the appropriate UV absorbers, of which ozone is the most important of the multiple S-isotopic signal reflecting changes in the relative in the modern atmosphere. Consequently, S-MIF is considered some proportions of atmospheric gases of volcanogenic and biogenic of the strongest evidence for the lack of free atmospheric oxygen origin. The S-isotopic signal then appears as a robust metric of before 2.4 Ga. Although temporal variations in the S-MIF record are atmospheric composition that reflects both the photolytic reac- thought to depend on changes in the abundances of gas and aerosol tions involving S-bearing phases and their transfer to the sedi- species, our limited understanding of photochemical mechanisms ments. The present study focuses on the origins and extent of this complicates interpretation of the S-MIF record in terms of atmospheric atmospheric memory in the sedimentary record. To probe the δ33 δ34 composition. Multiple sulfur isotope compositions ( S, S, and sensitivity of early Archean photochemical processes to major δ36 > S) of the 3.8 billion-year-old Nuvvuagittuq Greenstone Belt surface perturbations, including emergence of dominant bio- (Ungava peninsula) have been investigated to track the early origins genic elemental cycling, intense volcanic activity, bombardment of S-MIF. Anomalous S-isotope compositions (Δ33Supto+2.2‰) con- of Earth’s surface by bolides, and evolution of the solar flux, we firm a sedimentary origin of sulfide-bearing banded iron and silica- examined the multiple sulfur isotope record in some of the oldest rich formations. Sharp isotopic transitions across sedimentary/igneous metasediments found on Earth. The samples were collected in lithological boundaries indicate that primary surficial S-isotope com- > positions have been preserved despite a complicated metamorphic the 3.8-Ga Nuvvuagittuq Greenstone Belt (NGB) (15) located history. Furthermore, Nuvvuagittuq metasediments recorded coupled on the eastern coast of Hudson Bay (Ungava Penninsula, variations in 33S/32S, 34S/32S, and 36S/32S that are statistically indistin- Northeastern Superior Province of Canada). guishable from those identified several times later in the Archean. Geological Setting The recurrence of the same S-isotope pattern at both ends of the Archean Eon is unexpected, given the complex atmospheric, geolog- The NGB exposes a sequence of metavolcanic rocks, ultramafic ical, and biological pathways involved in producing and preserving cumulates, and metasediments [primarily banded iron formations this fractionation. It implies that, within 0.8 billion years of Earth’s (BIF) and silica-rich formations (Si formation)] hosted in an iso- formation, a common mechanism for S-MIF production was estab- clinal synform refolded into a south-plunging synform (Supporting lished in the atmosphere. Significance Nuvvuagittuq Greenstone Belt (Nunavik, Québec) | early Archean sulfur cycle | photochemistry | early Earth surface environment Sulfur isotopes in ancient Earth sediments have peculiar compo- sitions resulting from photochemical reactions of sulfur-bearing ultiple sulfur isotopes have become the tool of choice for gaseous species with UV photons in the anoxic early Earth’sat- Mprobing atmospheric evolution during the first 2.5 billion mosphere. High-precision measurements of sediments from dis- years of Earth history. In particular, mass-independent frac- tinct localities have revealed that these isotopic anomalies are tionation of sulfur isotopes (S-MIF) (1) is a characteristic feature sensitive to the abundances of atmospheric gases. They thus of the geologic record before 2.32 gigayears ago (Ga) (2), after appear as promising metrics of ancient atmospheric composition. which S-MIF is almost completely lacking (3). Photochemistry in In the present study, we focus on the origin and extent of this atmospheric memory. We analyzed the multiple S-isotope com- alow-O2 atmosphere is often considered as the dominant cause of S-MIF (1), and models for production and preservation of S-MIF positions of some of the oldest metasediments preserved on −5 Earth (originating from the >3.8-Ga Nuvvuagittuq Greenstone anomalies suggest that an increase in O2 beyond ∼10 times present atmospheric level (PAL) (4) would quench the production Belt, Canada) and found that this unusual atmospheric finger- of S-MIF signals in the geological record. While the abrupt nature print has been present since the end of the Hadean, the earliest eon of Earth history. of the Archean/Proterozoic transition reflects a high-sensitivity EARTH, ATMOSPHERIC, response to atmospheric O , the temporal variability of the S-MIF AND PLANETARY SCIENCES 2 Author contributions: E.T. and B.A.W. designed research; E.T., J.O., D.F., P.C., and B.A.W. record before about 2.45 Ga indicates a complex interplay among performed research; E.T. and P.C. contributed new reagents/analytic tools; E.T., J.O., D.F., atmospheric composition (5–8), magnitude (9) and oxidation state P.C., and B.A.W. analyzed data; and E.T., J.O., D.F., P.C., and B.A.W. wrote the paper. (10) of sulfur fluxes into the atmosphere, accessible photochemical The authors declare no conflict of interest. pathways (11), microbial sulfur cycling in the surface environment This article is a PNAS Direct Submission. (10), and atmospheric solar flux (12). Despite the broad range of 1To whom correspondence should be addressed. Email: [email protected]. geologic and biologic controls that apparently influence the S-iso- This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. topic record, first-order isotopic patterns are present that span 1073/pnas.1419681112/-/DCSupplemental. www.pnas.org/cgi/doi/10.1073/pnas.1419681112 PNAS | January 20, 2015 | vol. 112 | no. 3 | 707–712 Downloaded by guest on September 26, 2021 Information and Fig. S1). Samples from the predominant lithology in metasediments igneous rocks the belt [mafic amphibolites known as the Ujaraaluk unit (ref. 15 and BIF Supporting Information)] as well as cogenetic ultramafic cumulate Si-formation 142 144 147 144 define a compact Nd/ Nd versus Sm/ Nd correlation. If 2 BIF margins and greenstones interpreted as an isochron, the slope of this correlation corresponds Ultramafics +0.15 Gabbros and felsic band to an age of 4.388 /−0.17 Ga (16), which would represent the 1.5 Tonalite emplacement age of the volcanic protolith to the Ujaraaluk unit (see S (‰) Supporting Information for details on the Sm/Nd systematics and 1 33 a discussion of this aspect). This Hadean age for the Ujaraaluk unit Δ is strongly supported by the presence of gabbroic sills intruding the 0.5 Ujaraaluk unit that yield a 143Nd/144Nd versus 147Sm/144Nd isochron age of 4.115 ± 0.1 Ga (16). Zircon from thin trondhjemitic intrusions (which sometimes occur as concordant horizons within the 0 0 40 80 120 160 gabbroic sills) provide a minimum closure age of the U-Pb system D (m) (distance to the closest contact between sediments and igneous horizons) [>3.75 Ga (17–20)]. These age relationships are consistent with the -0.5 inferred intrusive relationships among the throndhjemite bands, the gabbro sills, and the amphibolite (15). Aluminous lithologies within Δ33 the NGB contain mineral assemblages indicative of upper amphib- Fig. 1. Plot of sulfur mass-independent fractionation ( S) versus dis- tance to the nearest contact between proposed metasediments and met- olite facies metamorphism (Supporting Information), consistent with ∼ aigneous lithologies (D). BIF (filled orange diamonds) and Si formation inferences from cation exchange thermometry [ 640 °C (21)]. Pre- (unfilled orange diamonds) all carry significant S-MIF anomalies, which cise U-Pb depth profiling of zircon grains indicates that this meta- confirm their sedimentary origin (when not plotted, the errors bars are morphic grade was likely attained at ∼2.7Ga(21).Therelativeorder smaller than the symbols). Metaigneous lithologies (gabbros, blue circles; of the BIF, Si formation, and volcanogenic horizons is symmetric ultramafic sills, green circles; surrounding tonalite, pink circles) do not across the synform, suggesting that the current lithological se- show significant nonzero Δ33S. Small but discernable
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