Cu isotopes in marine black shales record the Great Oxidation Event Ernest Chi Frua,1, Nathalie P. Rodrígueza, Camille A. Partinb, Stefan V. Lalondec, Per Anderssond, Dominik J. Weisse, Abderrazak El Albanif, Ilia Rodushking, and Kurt O. Konhauserh aDepartment of Geological Sciences and Bolin Centre for Climate Research, Stockholm University, SE-10691 Stockholm, Sweden; bDepartment of Geological Sciences, University of Saskatchewan, Saskatoon, SK, Canada S7N 5E2; cCNRS-UMR 6538 Laboratoire Domaines Océaniques, Institut Universitaire Européen de la Mer, Université de Bretagne Occidentale, Technopôle Brest-Iroise, Place Nicolas Copernic, Plouzané 29280, France; dDepartment of Geosciences, Swedish Museum of Natural History, SE-104 05 Stockholm, Sweden; eDepartment of Earth Science and Engineering, Royal School of Mines, Imperial College, London SW7 2BP, United Kingdom; fInstitut de Chimie des Milieux et Matériaux de Poitiers, Université de Poitiers UMR 7285-CNRS, 86073 Poitiers, France; gALS Scandinavia AB, ALS Laboratory Group, S-977 75 Lulea, Sweden; and hDepartment of Earth and Atmospheric Sciences, University of Alberta, Edmonton, AB, Canada T6G 2EG Edited by Mark H. Thiemens, University of California, San Diego, La Jolla, CA, and approved March 10, 2016 (received for review December 17, 2015) The oxygenation of the atmosphere ∼2.45–2.32 billion years ago variations in black shale pyrite after 1.5 Ga coincides with oceans (Ga) is one of the most significant geological events to have af- becoming even less Fe-rich due to waning submarine hydrothermal fected Earth’s redox history. Our understanding of the timing and activity (8–13). To provide a new and complementary perspective processes surrounding this key transition is largely dependent on on water column redox evolution surrounding the GOE, we the development of redox-sensitive proxies, many of which remain turned to sedimentary copper (Cu) isotope compositions, known unexplored. Here we report a shift from negative to positive cop- to respond to redox shifts tuned to Fe and sulfur (S) cycling. δ65 65 per isotopic compositions ( CuERM-AE633) in organic carbon-rich Preferential sequestration of Cu by iron oxides (e.g., refs. 14– shales spanning the period 2.66–2.08 Ga. We suggest that, before 16) precipitated from ferruginous Archean oceans may have 2.3 Ga, a muted oxidative supply of weathering-derived copper enriched seawater in residual 63Cu. We can thus hypothesize that, enriched in 65Cu, along with the preferential removal of 65Cu by 65 before the GOE, massive Archean iron oxide precipitation in the iron oxides, left seawater and marine biomass depleted in Cu but 65 63 form of BIF preferentially buried Cu, enriching residual sea- enriched in Cu. As banded iron formation deposition waned and water in 63Cu. Conversely, oxidative continental weathering of continentally sourced Cu became more important, biomass sam- sulfides, such as chalcocite (Cu S) and chalcopyrite (CuFeS ), pled a dissolved Cu reservoir that was progressively less fraction- 2 2 should have increased the supply of dissolved Cu(II) and de- ated relative to the continental pool. This evolution toward heavy livered more 65Cu-rich runoff to the oceans (17–27) after the δ65Cu values coincides with a shift to negative sedimentary δ56Fe GOE. Oxidative dissolution of Cu(I) sulfide minerals to dissolved values and increased marine sulfate after the Great Oxidation Cu(II) entails Cu isotope fractionation between the reservoirs by Event (GOE), and is traceable through Phanerozoic shales to modern – ‰ Δ65 marine settings, where marine dissolved and sedimentary δ65Cu up to 1.2 3.1 [ Cu(Cu(II)–Cu(I) sulfide)] (26). Moreover, Cu values are universally positive. Our finding of an important shift isotope fractionations between surface/groundwater, secondary in sedimentary Cu isotope compositions across the GOE provides Cu minerals, and primary Cu sulfide sources all enrich the aque- 65 ‰ new insights into the Precambrian marine cycling of this critical ous phase in Cu, yielding fractionations up to 12 between micronutrient, and demonstrates the proxy potential for sedimen- dissolved Cu and its source (26). Riverine supply of waters 65 tary Cu isotope compositions in the study of biogeochemical cycles enriched in Cu as the result of continental sulfide oxidation (1–12) and oceanic redox balance in the past. should have influenced the highly productive post-GOE marginal paleoceanography | trace metals | copper cycling | Precambrian | Significance Proterozoic EARTH, ATMOSPHERIC, AND PLANETARY SCIENCES Redox-sensitive transition metalsandtheirisotopesprovidesome hortly after the Archean–Paleoproterozoic boundary, the of the best lines of evidence for reconstructing early Earth’soxy- SGreat Oxidation Event (GOE) ∼2.45–2.32 billion years ago genation history, including permanent atmospheric oxygenation (Ga) marks the first permanent accumulation of oxygen in following the Great Oxidation Event (GOE), ∼2.45−2.32 Ga. We Earth’s atmosphere (1–4). Evidence for the GOE comes from a show a shift from dominantly negative to permanently positive convergence of proxies, including the evolution of redox-sensitive copper isotope compositions in black shales spanning ∼2.66−2.08 element concentrations and their isotopic compositions in marine Ga. We interpret the transition in marine δ65Cu values as reflecting sedimentary rocks. Accompanying the GOE was an increase in the somecombinationofwaningbandedironformationdeposition oxidative weathering of a relatively untapped continental sulfide (which removes heavy Cu) and increased oxidative delivery of Cu reservoir and the onset of sulfide-rich (euxinic) water column from continental sulfides (which supplies heavy Cu). Both pro- conditions in marginal marine settings caused by bacterial sulfate cesses are ultimately related to increased oxidative weathering reduction of an increasing oceanic sulfate pool (1–8). These events and a progressive increase in sulfate and sulfide availability ac- are reflected in the transition from Neoarchean oceans charac- companying the GOE. Our results provide insights into copper cy- ’ terized by abundant iron oxide precipitation and the deposition of cling and bioavailability coupled to Earth s oxygenation history. banded iron formations (BIF) to relative BIF scarcity and in- Author contributions: E.C.F. designed research; E.C.F., N.P.R., C.A.P., and S.V.L. performed creased global marine pyrite burial for much of the Paleoproter- research; E.C.F., C.A.P., S.V.L., P.A., A.E.A., I.R., and K.O.K. contributed new reagents/ ozoic (4–13). analytic tools; E.C.F., N.P.R., C.A.P., S.V.L., P.A., D.J.W., and K.O.K. analyzed data; and Light iron (Fe) isotope enrichments in sedimentary pyrite older E.C.F., N.P.R., C.A.P., S.V.L., D.J.W., and K.O.K. wrote the paper. than 2.4 Ga records oceans strongly affected by BIF deposition The authors declare no conflict of interest. and dissimilatory Fe(III) reduction, whereas generally near-zero to This article is a PNAS Direct Submission. positive Fe isotope compositions between 2.3 Ga and 1.8 Ga reflect 1To whom correspondence should be addressed. Email: [email protected]. near-complete drawdown of the Fe reservoir by enhanced oxidation This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. 56 and/or marine pyrite burial (8, 13). The lack of substantial δ Fe 1073/pnas.1523544113/-/DCSupplemental. www.pnas.org/cgi/doi/10.1073/pnas.1523544113 PNAS | May 3, 2016 | vol. 113 | no. 18 | 4941–4946 Downloaded by guest on September 30, 2021 marine basins. We can thus hypothesize that the GOE should be A 102 accompanied by a transition to seawater that is enriched in 65Cu sampled interval relative to continental sources, similar to the modern ocean (28). This 63Cu, in turn, would have become incorporated into plank- 100 GOE tonic biomass that scavenged Cu from seawater. After the GOE, increased oxidative weathering of a previously unweathered con- -2 tinental sulfide pool (5) should have progressively delivered 10 isotopically heavy riverine Cu(II) to the oceans as iron oxide molar Cu/Ti deposition waned in the face of increasingly common oxic or 10-4 euxinic conditions. Collectively, this sequence of events predicts that sedimentary 65 δ Cu values should have become increasingly positive across the 10-6 3500 3000 2500 2000 1500 1000 500 0 GOE. To test this hypothesis, we selected a suite of well-char- 102 acterized, low metamorphic grade black shale samples deposited B before, during, and after the GOE, spanning ∼2.7–2.1 Ga. The samples span the Transvaal Supergroup in South Africa and the 100 Francevillian Series in Gabon, both of which have been widely used for reconstructing key intervals in Earth’s oxygenation (1, 56 13 -2 7, 29–31). Our coeval δ Fe (redox-sensitive proxy), δ Corg 10 (biological activity proxy), and δ65Cu records, spanning Late Ar- chean iron-rich to Paleoproterozoic sulfide-rich marginal ocean con- molar Cu/Ti -4 ditions, provide new insight into the Precambrian cycling of Cu, a 10 critical micronutrient, as well as the proxy potential of δ65Cu for tracing marine paleoredox. -6 10 3500 3000 2500 2000 1500 1000 500 0 Results and Discussion C 102 Temporal Changes in Sedimentary Cu Dynamics on the Evolving Earth. Changes in the sources, sinks, and mechanisms of Cu delivery to 100 Archean and Paleoproterozoic oceans would have affected both biological activity and Cu isotopic composition. To interpret the 65 δ Cu data, we first reconstruct temporal Cu sedimentary concen- 10-2 trations and the relative importance of iron and sulfide sinks from the BIF and black shale archives (Fig. 1). The shale and BIF records molar Cu/Ti of authigenic Cu enrichment (Fig. 1 A and B), normalized to detrital 10-4 input using Ti (e.g., refs. 5, 22, and 32), are remarkably static, showing insignificant change on either side of the GOE. Authigenic 10-6 Cu enrichment in BIF is ∼1,000 times higher relative to black shale 3500 3000 2500 2000 1500 1000 500 0 (Fig.