Cu isotopes in marine black shales record the Great Oxidation Event

Abstract

The oxygenation of the atmosphere ∼2.45-2.32 billion years ago (Ga) is one of the most significant geological events to have affected Earth's redox history. Our understanding of the timing and processes surrounding this key transition is largely dependent on the development of redox-sensitive proxies, many of which remain unexplored. Here we report a shift from negative to positive copper isotopic compositions (δ(65)CuERM-AE633) in organic carbon-rich shales spanning the period 2.66-2.08 Ga. We suggest that, before 2.3 Ga, a muted oxidative supply of weathering-derived copper enriched in (65)Cu, along with the preferential removal of (65)Cu by iron oxides, left seawater and marine biomass depleted in (65)Cu but enriched in (63)Cu. As banded iron formation deposition waned and continentally sourced Cu became more important, biomass sampled a dissolved Cu reservoir that was progressively less fractionated relative to the continental pool. This evolution toward heavy δ(65)Cu values coincides with a shift to negative sedimentary δ(56)Fe values and increased marine sulfate after the Great Oxidation Event (GOE), and is traceable through Phanerozoic shales to modern marine settings, where marine dissolved and sedimentary δ(65)Cu values are universally positive. Our finding of an important shift in sedimentary Cu isotope compositions across the GOE provides new insights into the Precambrian marine cycling of this critical micronutrient, and demonstrates the proxy potential for sedimentary Cu isotope compositions in the study of biogeochemical cycles and oceanic redox balance in the past.

Keywords: Precambrian; Proterozoic; copper cycling; paleoceanography; trace metals.

Fig. 1.

Copper distribution in banded iron formations and black shales throughout Earth history (see Supporting Information and Datasets S1 and S2 for details). (A and B) molar Cu/Ti ratios in shales (A) and banded iron formations (B). (C) Their enrichments compared (black shale in green, BIF in red), relative to the average continental crust (white line). (D) BIF tonnage over time, data from ref. .

Fig. 2.

Box and whisker isotopic plots (δ⁶⁵C, δ¹³C_org, δ⁵⁶Fe) across the GOE interval. (A) The δ⁶⁵Cu values in ∼2.7–2.0 Ga black shales, showing a transition to heavier values with the onset of the GOE. (B and C) Corresponding δ¹³C_org (B) and δ⁵⁶Fe (C) values. See Supporting Information for details of analysis.

Fig. S1.

Individual black shale δ⁶⁵Cu values over the studied interval.

Fig. 3.

Rayleigh isotope distillation of δ⁶⁵Cu between seawater and iron oxide sinks. Black shale δ⁶⁵Cu values are assumed to reflect contemporaneous seawater composition (28). An experimentally determined Δ⁶⁵Cu value of 0.75‰ between seawater and iron oxides (–16) was used to calculate the isotopic composition of Cu sequestered into iron oxides and residual isotopic composition of seawater, assuming an initial δ⁶⁵Cu of seawater of 0.5‰ in the absence of BIF deposition, similar to modern (28).

Fig. 4.

Conceptual model for Late Archean and Early Proterozoic marine cycling and isotope composition of Cu. (A) During the Neoarchean, the marine Cu cycle lacked oxidative weathering inputs and was strongly influenced by iron oxide deposition. (B) During the Early Proterozoic, isotopically heavy Cu was supplied by continental weathering of sulfides, while iron oxide sinks waned, both promoting ⁶³Cu depletion from seawater and pelagic sediments. Fe-OB, iron-oxidizing bacteria.

Fig. S2.

Chemostratigraphic isotope composition trends for analyzed shales in the Francevillian Series (A) and Transvaal Supergroup (B).

Fig. S3.

Relationship between S and Cu/Fe molar ratios in analyzed dataset, expected to track changes in sedimentary S content after the GOE, because marine sulfate content is believed to increase during this transition.

Fig. S4.

Sedimentary Mn concentrations in the iron formations (A) and shales (B) screened for Cu isotope trends in this study.