<p> 1 Cu isotopes in marine black shales record the Great Oxidation Event</p><p>2</p><p>3 Ernest Chi Fru1*, Nathalie Pérez Rodríguez1, Camille A. Partin2, Stefan Lalonde3, Per</p><p>4 Andersson4, Dominik Weiss5, Abderrazak El Abani6, Ilia Rodushkin7, Kurt Konhauser8.</p><p>5</p><p>6 1Stockholm University, Department of Geological Sciences and Bolin Centre for Climate</p><p>7 Research, SE-10691, Stockholm, Sweden</p><p>8 2University of Saskatchewan, Department of Geological Sciences, 114 Science Place,</p><p>9 Saskatoon, SK S7N 5E2, Canada.</p><p>10 3CNRS–UMR 6538 Laboratoire Domaines Océaniques, Institut Universitaire Européen de la</p><p>11 Mer, Université de Bretagne Occidentale, Place Nicolas Copernic, Plouzané 29280 France.</p><p>12 4Swedish Museum of Natural History, Department of Geosciences, Box 50 007 </p><p>13 SE-104 05 Stockholm, Sweden.</p><p>14 5Imperial College, Department of Earth Science and Engineering, Royal School of Mines,</p><p>15 London SW7 2BP, UK.</p><p>16 6Université de Poitiers UMR 7285-CNRS, Institut de Chimie des Milieux et Matériaux de</p><p>17 Poitiers - 5, rue Albert Turpin (Bât B35) 86073 Poitiers cedex.</p><p>18 7ALS Laboratory Group. ALS Scandinavia AB. Aurorum 10. S-977 75 Luleå, Sweden.</p><p>19 8Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta</p><p>20 T6G 2EG, Canada. </p><p>21</p><p>22 *Corresponding author: [email protected]</p><p>23</p><p>24</p><p>1 25 Abstract</p><p>26 The oxygenation of the atmosphere ~2.45–2.32 billion years ago (Ga) is one of the most</p><p>27 significant geological events to have affected Earth’s redox history. Our understanding of the</p><p>28 timing and processes surrounding this key transition is largely dependent on the development</p><p>29 of redox-sensitive elemental proxies, many of which remain unexplored. Here we report a</p><p>65 30 shift from negative to positive copper isotopic composition (δ CuERM-AE633) in organic carbon–</p><p>31 rich shales spanning the Great Oxidation Event (GOE), after which they maintain positive</p><p>32 values comparable to modern shales. The data cover the period 2.66 to 2.08 Ga, and suggest</p><p>33 that prior to 2.3 Ga, preferential removal of 65Cu by Fe(III)(oxyhydr)oxides left seawater and</p><p>34 marine biomass depleted in 65Cu but enriched in 63Cu. As banded iron formation (BIF)</p><p>35 deposition waned, biomass sampled a dissolved Cu reservoir that was progressively less</p><p>36 fractionated relative to the continental pool. Consequently, δ65Cu values ≤0 disappear from</p><p>37 the shale record after ~2.3 Ga, when many proxies converge on permanent atmospheric</p><p>38 oxygenation. This shift towards heavy δ65Cu values is traceable through Phanerozoic shales to</p><p>39 modern marine settings, where marine dissolved and sedimentary δ65Cu are universally</p><p>56 40 positive, coinciding with a shift to negative sedimentary δ Fe values and increased marine</p><p>41 sulfate after the GOE. The evolving δ65Cu composition is suggested to provide a more</p><p>42 straightforward explanation for ferruginous versus sulfidic conditions than iron isotope</p><p>43 compositions during the GOE transition. The permanent shift in sedimentary Cu isotope</p><p>44 compositions provides new insights into Precambrian marine cycling of this critical</p><p>45 micronutrient.</p><p>46</p><p>47</p><p>2 48 Significance statement</p><p>49 Redox-sensitive transition metals and their isotopes provide some of the best lines of evidence</p><p>50 for reconstructing early Earth’s oxygenation history, especially related to permanent atmo-</p><p>51 spheric oxygenation after ~2.4 Ga. We show a shift from dominantly negative to permanently</p><p>52 positive copper isotope compositions in black shales spanning the height of banded iron for-</p><p>53 mation deposition, the Great Oxidation Event, and the shift from ferruginous to more oxic and</p><p>54 euxinic seawater conditions with enhanced oceanic sulfate, ~2.66-2.08 Ga. This shift is re-</p><p>55 flected in black shale iron isotope compositions, but copper isotopes appear to provide a better</p><p>56 sensitivity towards distinguishing iron- vs. sulfide-rich marine conditions in the sedimentary</p><p>57 record. Our results provide new insights into copper bioavailability coupled to Earth’s oxy-</p><p>58 genation history.</p><p>59</p><p>60</p><p>3 61 /body</p><p>62 Introduction</p><p>63 Shortly after the Archean-Paleoproterozoic boundary, the Great Oxidation Event (GOE)</p><p>64 ~2.45-2.32 billion years ago (Ga) marks the first permanent accumulation of oxygen in Earth-</p><p>65 ’s atmosphere (1-4). Evidence for the GOE comes from a convergence of proxies, including</p><p>66 the evolution of redox–sensitive element concentrations and their isotopic compositions in</p><p>67 marine sedimentary rocks (see refs 2 & 4 for reviews). Accompanying the GOE was an in-</p><p>68 crease in the oxidative weathering of a relatively untapped continental sulfide reservoir (1-12)</p><p>69 and the onset of near–continental margin and intracratonic basin sulfide-rich (euxinic) water</p><p>70 column conditions caused by bacterial sulfate reduction of an increasing oceanic sulfate pool</p><p>71 (4, 6-12). These events are reflected in the transition from Neoarchean oceans characterized</p><p>72 by abundant iron(III)(oxyhydr)oxide precipitation and the deposition of banded iron forma-</p><p>73 tions (BIF), to relative BIF scarcity and increased global marine pyrite burial for much of the</p><p>74 Paleoproterozoic (6-17).</p><p>75 Light iron (Fe) isotope enrichments in sedimentary pyrite older than 2.4 Ga records</p><p>76 oceans strongly affected by BIF deposition and dissimilatory Fe(III) reduction, while general-</p><p>77 ly near-zero to positive Fe isotope compositions between 2.3-1.8 Ga reflect near-complete</p><p>78 draw-down of the Fe reservoir by enhanced oxidation and/or marine pyrite burial (12, 18).</p><p>79 The lack of substantial δ56Fe variations in black shale pyrite after 1.5 Ga coincides with</p><p>80 oceans becoming even less Fe-rich due to waning submarine hydrothermal activity (12, 13,</p><p>81 16, 17). To provide a new and complementary perspective on water column redox evolution</p><p>82 surrounding the GOE, we turned to sedimentary copper (Cu) isotope compositions, known to</p><p>83 respond to redox shifts tuned to Fe and sulfur (S) cycling. This relationship predicts that the</p><p>84 preferential sequestration of 65Cu by Fe(III)(oxyhydr)oxides (e.g., 19, 20) precipitated from</p><p>85 ferruginous Archean oceans may have enriched seawater in residual 63Cu. Conversely, oxida-</p><p>4 86 tive continental weathering of sulfides, such as chalcocite (Cu2S) and chalcopyrite (CuFeS2),</p><p>87 should have increased the supply of dissolved Cu(II) and delivered more 65Cu–rich runoff to</p><p>88 the oceans (21-31) after the GOE. Oxidative dissolution of Cu(I) sulfide minerals to dissolved</p><p>65 89 Cu(II) entails Cu isotope fractionation between the reservoirs by up to 1.2-3.1‰ (∆ Cu(Cu(II)–</p><p>90 Cu(I) sulfide)) (30). Moreover, Cu isotope fractionation between surface/groundwater, secondary</p><p>91 Cu minerals, and primary Cu sulfide sources, all enrich the aqueous phase in 65Cu, yielding</p><p>92 fractionations of up to 12‰ between dissolved Cu and its source (30). Riverine supply of wa-</p><p>93 ters enriched in 65Cu, as the result of continental sulfide oxidation (1-17), should have influ-</p><p>94 enced the highly productive post-GOE continental margins and intracratonic basins. We can</p><p>95 thus hypothesize that the GOE should be accompanied by a transition to seawater that is en-</p><p>96 riched in 65Cu relative to continental sources, similar to the modern ocean (32). </p><p>97 We further hypothesize that before the GOE, massive Archean Fe(III)(oxyhydr)oxide</p><p>98 precipitation in the form of BIF preferentially buried 65Cu, enriching residual seawater in 63Cu.</p><p>99 This 63Cu, in turn, would have become incorporated into planktonic biomass that scavenged</p><p>100 Cu from seawater. After the GOE, increased oxidative weathering of a previously unweath-</p><p>101 ered continental sulfide pool (7) should have progressively delivered isotopically heavy river-</p><p>102 ine Cu(II) to the oceans as iron oxide deposition waned in the face of increasingly common</p><p>103 oxic or euxinic conditions. Collectively, this sequence of events predicts that sedimentary</p><p>104 δ65Cu values should have become increasingly positive across the GOE. To test this hypothe-</p><p>105 sis, we selected a suite of well-characterized, low metamorphic grade black shale samples de-</p><p>106 posited before, during, and after the GOE, spanning ~2.7-2.1 Ga. by other studies. The sample</p><p>107 suite spans the Transvaal Supergroup in South Africa and the Francevillian Series in Gabon,</p><p>108 both of which have been widely used for reconstructing key intervals in Earth’s oxygenation,</p><p>109 and their continental margin setting and depositional redox conditions have been previously</p><p>56 13 110 established (1, 9, 33, 34, 35). Our coeval δ Fe (redox sensitive proxy), δ Corg (biological ac-</p><p>5 111 tivity proxy) and δ65Cu records, spanning Late Archean iron-rich to Paleoproterozoic sul-</p><p>112 fide-rich marginal ocean conditions, provide new insight into the relative importance of fer-</p><p>113 ruginous vs. sulfidic conditions across the GOE as well as the Precambrian cycling of Cu, a</p><p>114 critical micronutrient.</p><p>115</p><p>116 Results and Discussion </p><p>117 Temporal changes in sedimentary Cu dynamics on the evolving Earth: Changes in the</p><p>118 sources and mechanisms of Cu delivery to Archean and Paleoproterozoic oceans would have</p><p>119 affected both biological activity and Cu isotopic composition. To interpret the δ65Cu data, we</p><p>120 first reconstruct temporal Cu sedimentary concentrations and the relative importance of iron</p><p>121 and sulfide sinks from the BIF and black shale archives (Fig. 1). The shale and BIF records of</p><p>122 authigenic Cu enrichment (Figs. 1A and 1B), normalized to detrital input using Ti (e.g., refs 7,</p><p>123 26, 36), are remarkably static, showing little significant change on either side of the GOE.</p><p>124 Authigenic Cu enrichment in BIF is ~1000 times higher relative to black shale (Fig. 1C). Af-</p><p>125 ter the GOE, BIF deposition decreased by approximately three orders of magnitude in terms</p><p>126 of tonnage (Fig. 1D), likely associated with declining hydrothermal activity and increasing</p><p>127 marine sulfate content resulting from enhanced oxidative continental sulfide weathering (11,</p><p>128 12, 37-40). Consequently, during peak BIF deposition, BIF likely exerted a stronger influence</p><p>129 on the concentration and isotope composition of dissolved Cu than black shale deposits.</p><p>130 Moreover, Cu/Ti molar ratios in black shale generally fall within an order of magnitude of the</p><p>131 upper continental crust (Fig. 1C), implying that black shales are not a strong Cu sink and</p><p>132 therefore not expected to significantly influence the Cu isotope budget to the same degree as</p><p>133 iron oxide-rich sediments. This fundamental shift in Cu exit channels likely contributed to the</p><p>134 Cu isotope evolution of seawater and pelagic sediments across the GOE (see below).</p><p>135</p><p>6 136 Temporal changes in sedimentary Cu isotopic compositions: Predominantly negative</p><p>137 δ65Cu values in ferruginous late Archean oceans increase from –0.55 to +0.74‰ by ~2.3 Ga,</p><p>138 after which δ65Cu values ≤0 disappear from our shale record (Fig 2A; Table 1; Fig. S1). This</p><p>139 irreversible upward stratigraphic change, which points to secular modification of seawater</p><p>140 chemistry from the Late Archean to the Paleoproterozoic, is significant at the 95% confidence</p><p>65 13 141 interval (ANOVA, p<0.05). δ Cu values appear to plot inversely to δ Corg with respect to</p><p>142 time (Fig. 2B), suggesting co-variance in Cu and C cycling in ancient oceans.</p><p>143 The preferential adsorption of dissolved 65Cu by Fe(III)(oxyhydr)oxides yields a</p><p>65 144 ∆ Cusolution-Fe(III)(oxyhydr)oxides fractionation of up to ~1‰ (19, 20). During late Archean peak BIF</p><p>145 deposition at ~2.48 Ga, ~4.5×1012 mol Fe/yr may have been removed by iron oxide sinks,</p><p>146 thereby also removing ~1×108 moles of Cu from the ocean annually (41). Residual</p><p>147 bioavailable Cu in the water column would have been left enriched in the lighter isotope 63Cu.</p><p>148 This appears reflected in the Neoarchean data, where black shales reach the lowest δ65Cu</p><p>149 values in our dataset. With the dramatic reduction of BIF deposition by a factor of 1000</p><p>150 following the GOE (Fig. 1D), a shift to heavy dissolved marine 65Cu enrichment appears</p><p>151 inevitable. Shale δ65Cu values (following seawater; see below) may readily be explained by</p><p>152 Raleigh distillation across important shifts in the proportion of marine Cu exiting via iron</p><p>153 oxide sinks (Fig. 3). Similar isotopic distillation of Cu occurs today in supergene weathering</p><p>154 environments (30). Indeed, starting from seawater with a modern Cu isotopic composition of</p><p>155 ~0.5‰, δ65Cu values as low as those observed between 2.66 to 2.44 Ga cannot be achieved</p><p>156 during adsorption to iron oxides in the absence of open-system behavior (Fig. 3)</p><p>157 Moving into the GOE, increasing sedimentary δ65Cu relative to the Neoarchean may</p><p>158 also be partially explained by an increasing supply of isotopically heavy Cu via continental</p><p>159 sulfide weathering (22-31) that became more important after 2.4–2.3 Ga, when the</p><p>160 atmosphere was permanently oxygenated. Abiotic and biotic oxidation of Cu minerals by</p><p>7 65 65 161 molecular oxygen produces a solution enriched in Cu, with a ∆ Cusolution-source mineral value of up</p><p>162 to ~3‰ (21-23, 27, 30). Cu isotope fractionation during precipitation of secondary Cu</p><p>163 minerals further enriches the aqueous phase in 65Cu, leading to a cumulative fractionation of</p><p>164 up to 12‰ between dissolved Cu and its source (30). While shifts in both source Cu isotopic</p><p>165 composition and the importance of iron oxide sinks may have influenced the sedimentary</p><p>166 δ65Cu record, the Cu enrichment record in both shales and BIF are relatively static (Fig. 1A,</p><p>167 1B), which indicates a more important role for the latter.</p><p>168 The adsorption and uptake of dissolved Cu by bacteria appears to favour the light</p><p>65 169 isotope at acidic pH. However, a negligible isotopic effect (i.e., ∆ Cusolution-phytoplankton≈0) is</p><p>170 consistently observed in the pH range of modern oceans (20, 32, 42). Although the reasons for</p><p>171 this are unclear, our data imply that the light Neoarchean δ65Cu values are best explained by</p><p>172 inheritance from a seawater component that was isotopically enriched in 63Cu due to ~200</p><p>173 million years of intense BIF deposition that preferentially removed 65Cu (Fig. 4A). This</p><p>174 isotopically light seawater Cu was probably passively adsorbed by biomass and preserved</p><p>175 alongside sedimentary organic matter since active biological Cu assimilation was likely</p><p>176 limited until after the GOE (43). After the GOE, the reduction in BIF deposition, coupled to</p><p>177 increased acid rock drainage on land related to the oxidation of sulfide minerals (7), changed</p><p>178 the Cu isotopic composition of seawater to permanently positive values. </p><p>13 179 In our sample suite, the increasing depletion of δ Corg with time is in agreement with</p><p>180 reports for marine sedimentary rocks during this period (4). For instance, the anomalously</p><p>13 181 light δ Corg values of the FD Formation in the Francevillian Series have previously been</p><p>182 linked to massive oxidation of organic matter and methanotrophy (9, 44, 45). The appearance</p><p>65 183 of this unique δ Corg at a time linked to an expanding sulfur reservoir (Fig. S3) associated</p><p>184 with microbial sulfate reduction (9, 46), suggests the existence of a potential anaerobic</p><p>185 methane oxidation pathway coupled to sulfate reduction. However, the absence of a</p><p>8 186 corresponding shift in the δ65Cu signal from that seen at the height of the GOE, ~2.3 Ga, when</p><p>65 187 δ Corg was much higher, suggests that biological processes responsible for generating the</p><p>65 65 188 distribution of δ Corg through time leaves no visible imprint on the δ Cu signal. Together</p><p>189 with the proposition that the changing Fe(III)(oxyhydr)oxides reservoir exerted a major</p><p>190 influence on Cu bioavailability, the data assert a similar view in the modern ocean where</p><p>191 biological activity appears to play a negligible part in the δ65Cu composition of marine waters</p><p>192 (32). Not much is known on the microbial composition of the early oceans, whether similar to</p><p>193 now or not, but a recent study suggests many cyanobacteria carry primitive Precambrian</p><p>194 genes that may have been active in autotrophic CO2 fixation (47) </p><p>195</p><p>196 Paired Cu and Fe isotope records of marine redox evolution: In the Transvaal Supergroup,</p><p>197 a narrow heavy range in δ56Fe (+0.21 to +0.29 ‰) is recorded by ~2.66-2.5 Ga in shales (Fig.</p><p>198 2C; Fig. S2). This decreases to negative values by ca. 2.45 Ga, and evolves to slightly positive</p><p>199 values with greater variability (–0.22 to +0.32‰) for Timeball Hill shales deposited during</p><p>200 the GOE interval (Fig. 2C; Fig. S2). In the Francevillian Series, more oxic conditions</p><p>201 dominated ~2.15-2.12 Ga (9), during which bulk δ56Fe remains slightly heavy (+0.23‰ to</p><p>202 +0.49‰) in the FB1 and FB2 Formations (Fig. 2C; Fig. S2). Soon after, at ~2.084 Ga, δ56Fe</p><p>203 values display broader variability during euxinic conditions (9) in the FD Formation of the</p><p>204 Francevillian series, attaining values as low as -0.55‰ (Fig. 2C; Fig. S2). </p><p>205 The δ56Fe values are used in an attempt to underline the sensitivity of δ65Cu systematics</p><p>206 as a potential GOE transition proxy. This is because δ56Fe values in pyrites are argued to</p><p>207 characterize three distinct stages across the GOE transition and beyond (12), where the δ56Fe</p><p>208 Archean signal controlled by mass deposition of BIF, contains variable and generally negative</p><p>209 values. An intermediate early Proterozoic stage gives way to an interval with higher and</p><p>210 stable values linked to continental weathering (12). However, the trajectory of bulk δ56Fe</p><p>9 211 evolution from positive to near-zero values between 2.66 and 2.45 Ga, gradually increasing</p><p>212 from 2.45 to 2.15 Ga, and returning to more negative and variable values at 2.08 Ga, is</p><p>213 complex, suggesting an inverse relationship between bulk δ56Fe and pyrite-specific δ56Fe (12).</p><p>214 This in part is likely due to the local nature of the bulk Fe sink but also to the fact that</p><p>215 different environmental processes can generate similar Fe isotopic compositions (12, 18, 42,</p><p>216 48). Independent constraints on water column and early diagenetic redox conditions that are</p><p>217 available from our sample suite (1, 9, 33, 34, 35, 49) significantly narrow the processes likely</p><p>218 responsible for the δ56Fe compositions. The positive values preserved under ferruginous</p><p>219 conditions ca. 2.66 Ga most likely reflect precipitation of a small portion of the dissolved Fe</p><p>220 pool under weakly oxidizing conditions (12). Near-zero and variable values ~2.45–2.15 Ga</p><p>221 likely represent a more important proportion of the local dissolved Fe pool being deposited as</p><p>222 iron oxides or sulfides with increasing environmental oxygenation (12, 49). And a return to</p><p>223 bulk negative values under euxinic conditions at ca. 2.08 Ga (9) is best explained by</p><p>224 enrichment of light isotopes into iron sulfides formed in the water column or during early</p><p>225 diagenesis (48).</p><p>226 Taken together, the evolution of the sedimentary δ65Cu composition in the Late Archean</p><p>227 versus post-GOE Paleoproterozoic marine sedimentary deposits is more straightforward in</p><p>228 interpretation, since it agrees with the δ56Fe shifts of the pure pyrite proxy (12) than observed</p><p>229 for bulk (Figs 2A & C). The transition to positive δ65Cu values in the Timeball Hill Formation</p><p>230 and the Francevillian FD Formation, coincidental with expanding euxinia generated through</p><p>231 bacterial sulfate reduction (1, 9), is also recorded by a positive δ56Fe transition in sedimentary</p><p>232 pyrites (12), validating bulk δ65Cu as a potential GOE transition proxy. Moreover, the higher</p><p>233 consistency of the δ65Cu signal compared to bulk δ56Fe further highlights its value in</p><p>234 identifying the GOE transition in sedimentary deposits, supporting the idea that the δ56Fe of</p><p>10 235 pyrites captures the protracted nature of the GOE, culminating in the muting of negative black</p><p>236 shale δ65Cu at 2.3 Ga.</p><p>237</p><p>238 Implications for Cu cycling at the Archean-Proterozoic boundary: The complete</p><p>239 disappearance of δ65Cu values ≤0 after 2.3 Ga suggests that Cu isotopes captured a uni-</p><p>240 directional shift in the oxygenation of the atmosphere-ocean system across the GOE. Protein</p><p>241 structure phylogenomics suggest the global emergence of large-scale biological Cu utilization</p><p>242 between ~2.4-2.2 Ga—i.e., during the GOE (43). Evidence from biological assimilation by</p><p>243 modern ocean microbial populations (32, 50, 51) suggests that microbial Cu assimilation does</p><p>244 not fractionate Cu isotopes, and thus should not influence the Cu isotope composition of</p><p>245 seawater nor be expected to drive Cu isotopic evolution from Archean to Paleoproterozoic</p><p>246 seawater. Increased riverine Cu and sulfate supply as the result of sulfide mineral weathering</p><p>247 across the GOE was accompanied by a decrease in Fe(III)(oxyhydr)oxide precipitation after</p><p>248 the GOE, as evident from the dramatic reduction in BIF deposition (Fig. 1D) and increased</p><p>249 precipitation of sulfide minerals (12, 15-17) after the GOE. Indeed, our data indicate an</p><p>250 increase in bulk δ34S values beginning ~2.15 Ga (Table 1 & Fig. S2), coincident with the</p><p>251 general expansion of the marine sulfate budget after the GOE (1-4, 6, 8, 11). The isotopically</p><p>252 heavy Cu associated with an increasing continental supply would have been actively</p><p>253 incorporated into biological enzymes and/or adsorbed to phytoplankton cell surfaces (20, 32).</p><p>254 As mentioned above, modern ocean and experimental studies suggest a net zero isotopic shift</p><p>255 during the assimilation of dissolved δ65Cu (~0.5-0.9‰) by phytoplankton biomass. Instead,</p><p>256 the isotopically heavy δ65Cu values in post-GOE shales are most parsimoniously explained by</p><p>257 continental weathering processes (21-30) delivering dissolved riverine Cu with a δ65Cu</p><p>258 signature of up to ~0.7‰ (30, 52-54) to the oceans. </p><p>11 259 The simple explanation for a shift from light Late Archean to permanently heavy</p><p>260 Proterozoic bulk δ65Cu may be complicated by episodes in the Neoarchean record, 2.66-2.5</p><p>261 Ga, when mass balance between BIF Cu and Shale Cu may have approached equilibrium,</p><p>262 suggested by coeval Cu/Ti signals (Fig. 1C). This may corrupt the simplicity of the δ65Cu</p><p>263 model because it is possible that the very light δ65Cu values produced during these episodes</p><p>264 may be reflecting the formation of Cu sulfides when the black shale and BIF Cu sinks are not</p><p>265 markedly different. Reduction in the Fe(III)(oxyhydr)oxide sink across the GOE would</p><p>266 therefore make black shale sulfides a major Cu sink, swinging the authigenic δ65Cu signal to</p><p>267 heavier values difficulty to distinguish from a new continental source carrying a similar</p><p>268 signal. However, the only evidence of when considerable change in the sulfur reservoir may</p><p>269 have impacted Cu content coincides with an expanding sedimentary sulfur reservoir and a</p><p>270 concomitant reduction in the Fe(III)(oxyhydr)oxides reservoir, after 2.4 Ga (Fig. S3). The</p><p>271 persistently low bulk sulfur signal before this time is taken to mean that Cu concentrations</p><p>272 were regulated mainly by a fluctuating but broadly larger Fe(III)(oxyhydr)oxide reservoir, not</p><p>273 sulfide. </p><p>274 The fact that contemporary dissolved seawater δ65Cu is isotopically heavier by 1.5‰</p><p>275 relative to riverine inputs is further linked to fractionation against 65Cu during adsorption to</p><p>276 authigenic ferromanganese marine precipitates (52-54). This view seems to contradict the fact</p><p>277 that experimental and riverine Fe(III)(oxyhydr)oxides preferentially remove 65Cu. However,</p><p>278 the isotopically-light Cu in modern ferromanganese deposits appears associated with Mn(IV)</p><p>279 instead of Fe(III) (52-54). Since birnessite has been shown to preferentially remove the light</p><p>280 Cu isotope (53, 54), this may further explain why after the GOE sedimentary marine δ65Cu</p><p>281 values became permanently positive, since Mn(IV) oxides are only precipitated in the</p><p>282 presence of free oxygen (55). In the Archean oceans, Mn(IV) precipitation was generally</p><p>283 inhibited, but became significant across the GOE, exemplified by the deposition of Earth’s</p><p>12 284 largest Mn(IV) oxide deposits ~2.4-2.2 Ga (55). Similarly, our sample set records an increase</p><p>285 in Mn content from the Archean (Fig. S4) and our predominantly positive δ65Cu</p><p>286 Paleoproterozoic values, though lower (Table 1), fall within the modern marine sedimentary</p><p>287 range (32). </p><p>288 Similar and mostly positive δ65Cu values, of up to 0.49‰, characterize Cambrian</p><p>289 marine shales genetically linked to the oxidative weathering of Precambrian basement rocks</p><p>290 (56). The Cambrian shales accumulated under euxinic marine conditions characterized by</p><p>291 intense bacterial sulfate reduction in a lagoonal transgressive environment (56, 57). These</p><p>292 sulfidic depositional conditions are similar to the situation under which the Francevillian FD</p><p>293 and Timeball Hill formations were deposited (1, 9). If direct microbial production and</p><p>294 consumption of organic matter has a negligible effect on marine δ65Cu dissolved values (e.g.,</p><p>295 32), then it is reasonable to assume that early microbial diagenetic processing of sedimentary</p><p>296 organic matter would have simply recycled and preserved the Cu isotopic signatures</p><p>297 originating from organic matter in the marine water column. Although the generation of</p><p>298 secondary minerals enriched in 63Cu through fluid migration in reworked clastic sediments has</p><p>299 been noted (58), such metasomatic processes are not known to have affected our samples (1,</p><p>300 9, 33, 34). Moreover, metasomatism is often local and, therefore, cannot explain the secularly</p><p>301 consistent sedimentary δ65Cu pattern from ~2.66-2.3 Ga or the reproducibility of positive</p><p>302 sedimentary δ65Cu values after 2.3 Ga.</p><p>303</p><p>304 Conclusions </p><p>305 Across the GOE boundary, we observe a consistent trend from negative δ65Cu and positive</p><p>306 δ56Fe values during ferruginous conditions, towards heavy δ65Cu and light δ56Fe signatures in</p><p>307 post-GOE sulfidic sedimentary successions. We interpret the transition in marine δ65Cu</p><p>308 values, from negative to positive across the GOE, to reflect a progressive increase in sulfide</p><p>13 309 availability after the GOE, itself related to increased oxidative continental weathering, and in</p><p>310 turn, to waning BIF deposition. Our dataset reveals a disappearance of negative δ65Cu values</p><p>311 from the black shale record after ~2.3 Ga, when the GOE was at its height, and supports a</p><p>312 change from low marine sulfate conditions during the Archean to sulfate-rich oceans after the</p><p>313 GOE. The gradual change in our δ65Cu values continues for ~150 million years after the</p><p>314 disappearance of mass independent sulfur isotope fractionation at ~2.45 Ga (3, 4), a key</p><p>315 marker of the GOE (59), re-affirming that the GOE was a protracted event that took place</p><p>316 between 2.45-2.3 Ga (7). </p><p>317</p><p>318 Methods</p><p>319 Bulk Cu and Fe isotope measurements were performed at ALS Scandinavia on a multicollect-</p><p>320 or inductively coupled plasma mass spectrometer (MC-ICP-MS) at ALS Scandinavia. See</p><p>321 supplementary information for details of sample preparation. A Neptune Plus was used at me-</p><p>322 dium resolution mode. A standard-sample bracketing technique was used during the measure-</p><p>323 ment. The reported error is 2σ based on long time reproducibility. Prior to the analysis of cop-</p><p>324 per stable isotopes, the Cu fraction was diluted to 0.5 Cu mg l-1 (where possible) followed by</p><p>325 addition of 2 mg l-1 Zn for mass bias correction. The Fe fraction was diluted to 2 mg l-1. A plot</p><p>326 of δ57/56Fe vs. δ56/54Fe reveals strictly mass-dependent fractionation, confirming interference-</p><p>327 free measurement of all three isotopes. Online mass bias correction was achieved by spiking</p><p>328 the samples with nickel (internal standard) to 4 mg l-1 (60). Values are reported in per mil (‰)</p><p>329 according to equation 1 and 2. A detailed description of the operating conditions and measure-</p><p>330 ment, as well as reproducibility and accuracy data is reported in 61.</p><p>331</p><p>65 65 63 65 63 332 δ Cu = [( Cu/ Cu)sample/( Cu/ Cu)ERM-AE633 - 1] × 1000 (1) </p><p>56 56 54 56 54 333 δ Fe = [( Fe/ Fe)sample/( Fe/ Fe)IRMM-014 - 1] × 1000 (2)</p><p>14 334</p><p>335 Bulk stable isotopes of C, S and N were analysed at the Stable Isotope Laboratory (SIL) of the</p><p>336 Department of Geological Sciences, Stockholm University as described in supplementary</p><p>337 information. </p><p>338</p><p>339</p><p>15 340 Acknowledgements</p><p>341 We will like to thank Hans Schöberg for help with setting up methods for Cu isotope analysis</p><p>342 and Karin Wallner for help in the clean lab at the Swedish Museum of Natural History.</p><p>343 Thanks to François Gauthier Lafaye for his logistic support. This work was initiated by a</p><p>344 Marie Curie fellowship (Grant No: PIEF.GA-2010-276475) and partly by funds from the</p><p>345 European Research Council (Grant No: 336092) and the Bolin Centre for Climate Change. </p><p>346</p><p>347 Author contribution</p><p>348 E.C.F. Designed the project; N.P.R. and E.C.F. conducted research. N.P.R. and I.R.</p><p>349 performed isotopic analysis. S.L., C.P. and K.K. compiled Cu data through time. A.E.A.</p><p>350 provided rock samples. P.A. and D.W. contributed to Cu isotopic analysis methods. N.P.R.,</p><p>351 P.K., S.L., C.P., K.K. and E.C.F. analysed data. E.C.F. wrote the paper. All authors</p><p>352 contributed in editing the manuscript. K.K. would like to thank the Natural Sciences and</p><p>353 Engineering Research Council of Canada (NSERC) for their financial support.</p><p>354</p><p>355</p><p>356</p><p>357</p><p>358</p><p>359</p><p>360</p><p>361</p><p>362</p><p>363</p><p>364</p><p>16 365 References</p><p>366 1. Bekker A, et al. 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C) their enrichments compared (black shale in</p><p>544 green, banded iron formation in red), relative to the average continental crust (white</p><p>545 line). D) banded iron formation tonnage over time, reproduced from reference 16. </p><p>546</p><p>65 13 56 65 547 Fig. 2. Box and whisker isotopic plots (δ C, δ Corg, δ Fe) across the GOE interval. A) δ Cu</p><p>548 values in ~2.7 to 2.0 Ga black shales, showing a transition to heavier values with the on-</p><p>13 56 549 set of the GOE. B) and C), corresponding δ Corg and δ Fe values, respectively.</p><p>550</p><p>551 Fig. 3. Rayleigh isotope distillation of δ65Cu between seawater and iron oxide sinks. Black</p><p>552 shale δ65Cu values are assumed to reflect contemporaneous seawater composition (32).</p><p>553 An experimentally–determined 65Cu value of 0.75‰ between seawater and Fe(III)</p><p>554 (oxyhydr)oxides (19, 20) was used to calculate the isotopic composition of Cu</p><p>555 sequestered into iron oxides and residual isotopic composition of seawater, assuming an</p><p>556 initial δ65Cu of seawater of 0.5‰ in the absence of BIF deposition, similar to modern</p><p>557 (32). </p><p>558</p><p>559 Fig. 4. Conceptual model for Late Archean and Early Proterozoic marine cycling and isotope</p><p>560 composition of Cu. A) During the late Archean, the marine Cu cycle lacked oxidative</p><p>561 weathering inputs, and was strongly influenced by iron oxide deposition B) During the</p><p>25 562 early Proterozoic, isotopically heavy Cu was supplied by continental weathering of</p><p>563 sulfides, while iron oxide sinks waned, both promoting 63Cu depletion from seawater</p><p>564 and pelagic sediments. Fe-OB stands for iron-oxidizing bacteria.</p><p>26</p>
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