The oxygenation of Earth’s surface fundamentally altered global biogeochemical cycles and ultimately paved the way for the rise of metazoans at the end of the Proterozoic. However, current estimates for atmospheric oxygen (O₂) levels during the billion years leading up to this time vary widely. On the basis of chromium (Cr) isotope data from a suite of Proterozoic sediments from China, Australia, and North America, interpreted in the context of data from similar depositional environments from Phanerozoic time, we find evidence for inhibited oxidation of Cr at Earth’s surface in the mid-Proterozoic (1.8 to 0.8 billion years ago). These data suggest that atmospheric O₂ levels were at most 0.1% of present atmospheric levels. Direct evidence for such low O₂ concentrations in the Proterozoic helps explain the late emergence and diversification of metazoans.

https://science.sciencemag.org/content/sci/346/6209/635.full.pdf

Low Mid-Proterozoic atmospheric oxygen levels and the delayed rise of animals

The oxygenation of Earth's atmosphere is thought to have occurred in two broad steps, but details of the process remain uncertain. Frei et al. use stable chromium (Cr) isotopes from banded iron formations, which are sedimentary rocks containing large amounts of oxygen as iron oxides, to track the presence of Cr(VI) in Precambrian oceans, providing a time-resolved picture of the oxygenation of Earth's atmosphere–hydrosphere system. Their data suggest a transient elevation in atmospheric and surface ocean oxygenation prior to the first great rise of oxygen 2.45 to 2.2 billion years ago (the Great Oxidation Event). Chromium is not fractionated in 1.88-billion-year-old banded iron formations, indicating a decline in atmospheric oxygen. The authors propose that the Great Oxidation Event did not lead to a unidirectional stepwise increase in atmospheric oxygen. It is thought that oxygenation of the Earth's atmosphere occurred in two broad steps, but details of the evolution of atmospheric oxygenation remain uncertain. Chromium (Cr) stable isotopes from banded iron formations are now used to track the presence of Cr(VI) in Precambrian oceans, providing a time-resolved picture of the oxygenation history of the Earth's atmosphere–hydrosphere system. Geochemical data1,2,3,4 suggest that oxygenation of the Earth’s atmosphere occurred in two broad steps. The first rise in atmospheric oxygen is thought to have occurred between ∼2.45 and 2.2 Gyr ago1,5, leading to a significant increase in atmospheric oxygen concentrations and concomitant oxygenation of the shallow surface ocean. The second increase in atmospheric oxygen appears to have taken place in distinct stages during the late Neoproterozoic era (∼800–542 Myr ago)3,4, ultimately leading to oxygenation of the deep ocean ∼580 Myr ago3, but details of the evolution of atmospheric oxygenation remain uncertain. Here we use chromium (Cr) stable isotopes from banded iron formations (BIFs) to track the presence of Cr(VI) in Precambrian oceans, providing a time-resolved picture of the oxygenation history of the Earth’s atmosphere–hydrosphere system. The geochemical behaviour of Cr is highly sensitive to the redox state of the surface environment because oxidative weathering processes produce the oxidized hexavalent [Cr(VI)] form. Oxidation of reduced trivalent [Cr(III)] chromium on land is accompanied by an isotopic fractionation, leading to enrichment of the mobile hexavalent form in the heavier isotope. Our fractionated Cr isotope data indicate the accumulation of Cr(VI) in ocean surface waters ∼2.8 to 2.6 Gyr ago and a likely transient elevation in atmospheric and surface ocean oxygenation before the first great rise of oxygen 2.45–2.2 Gyr ago (the Great Oxidation Event)1,5. In ∼1.88-Gyr-old BIFs we find that Cr isotopes are not fractionated, indicating a decline in atmospheric oxygen. Our findings suggest that the Great Oxidation Event did not lead to a unidirectional stepwise increase in atmospheric oxygen. In the late Neoproterozoic, we observe strong positive fractionations in Cr isotopes (δ53Cr up to +4.9‰), providing independent support for increased surface oxygenation at that time, which may have stimulated rapid evolution of macroscopic multicellular life3,4,6.

Fluctuations in Precambrian atmospheric oxygenation recorded by chromium isotopes

The distribution of chromium isotopes and redox-sensitive metals in the Nsuze palaeosol and in the Ijzermyn iron formation from the Pongola Supergroup, in South Africa, suggests that there were appreciable levels of atmospheric oxygen about three billion years ago, some 300–400 million years earlier than previous indications for Earth surface oxygenation. The first long-term oxygenation of the Earth's atmosphere — the Great Oxidation Event — is thought to have occurred around 2.3 billion years ago, although there is some geochemical evidence for transient atmospheric oxygenation as early as 2.7 billion years ago. Sean Crowe et al. examine the distribution of chromium isotopes and other metals that act as indicators for oxidative weathering in ancient soil and rock samples from South Africa. Their measurements suggest that oxygen-driven weathering was taking place 3 billion years ago, implying the presence of appreciable levels of atmospheric oxygen. This oxygen was probably produced by photosynthesis, indicating that cyanobacteria may have evolved by this time. This suggests an early evolution of oxygenic photosynthesis, 600 million years before the Great Oxidation Event. It is widely assumed that atmospheric oxygen concentrations remained persistently low (less than 10−5 times present levels) for about the first 2 billion years of Earth’s history1. The first long-term oxygenation of the atmosphere is thought to have taken place around 2.3 billion years ago, during the Great Oxidation Event2,3. Geochemical indications of transient atmospheric oxygenation, however, date back to 2.6–2.7 billion years ago4,5,6. Here we examine the distribution of chromium isotopes and redox-sensitive metals in the approximately 3-billion-year-old Nsuze palaeosol and in the near-contemporaneous Ijzermyn iron formation from the Pongola Supergroup, South Africa. We find extensive mobilization of redox-sensitive elements through oxidative weathering. Furthermore, using our data we compute a best minimum estimate for atmospheric oxygen concentrations at that time of 3 × 10−4 times present levels. Overall, our findings suggest that there were appreciable levels of atmospheric oxygen about 3 billion years ago, more than 600 million years before the Great Oxidation Event and some 300–400 million years earlier than previous indications for Earth surface oxygenation.

/pdf/atmospheric-oxygenation-three-billion-years-ago-1qrgu4mgha.pdf

Atmospheric oxygenation three billion years ago

Supplementary Materials for Low Mid-Proterozoic atmospheric oxygen levels and the delayed rise of animals

Abstract This study presents a new technique for high precision measurement of iron isotope ratios using high mass resolution MC-ICPMS on the ThermoFinnigan Neptune. A mass resolving power of about 8000–9000 was used to resolve the mass interferences of isobaric polyatomic ions (e.g., 40 Ar 16 O + on 56 Fe + , 40 Ar 16 OH + on 57 Fe + and 40 Ar 14 N + on 54 Fe + ) from the Fe isotopes and to produce “flat top” peak shapes with a plateau width of about 100 ppm in mass (Δm/m). The abundance sensitivity was determined to 50 ppm contribution of the 40 Ar 16 O + peak tail on the 56 Fe + peak. This enables Fe isotope measurements with a Fe-isotope/interference ratio of up to 1, without significant contribution from the tail of the interference peak. The performance of the technique was tested by determining the reproducibility and accuracy of synthetic Fe samples and spiked standards against the IRMM014 standard. Various interface setups, including wet and dry plasma techniques, and a wide range of concentrations (from 50 ppb to 10 ppm) have been used to demonstrate the flexibility, robustness and sensitivity of the technique. An external precision of ca. 0.10‰ for δ 56 Fe and ca. 0.15‰ for δ 57 Fe (2 S.D.) was routinely achieved. Measured and calculated delta values between spiked standards and the IRMM014 standard agree within uncertainties. In the investigated concentration range, precision and accuracy are independent of both the sample introduction system and the sample concentration. The new technique presented in this study is a robust and simple method to perform high precision Fe isotope measurements, and it allows to measure much lower sample concentrations compared to previous techniques.

Sonja Zink

Papers

The stable Cr isotope inventory of solid Earth reservoirs determined by double spike MC-ICP-MS

Isotopic fractionation and reaction kinetics between Cr(III) and Cr(VI) in aqueous media

A chromate-contaminated site in southern Switzerland – Part 1: Site characterization and the use of Cr isotopes to delineate fate and transport

Assessing the Cr(VI) reduction efficiency of a permeable reactive barrier using Cr isotope measurements and 2D reactive transport modeling.

Unraveling the partial failure of a permeable reactive barrier using a multi-tracer experiment and Cr isotope measurements