A joint research team led by Assoc. Prof. WANG Changle from the Institute of Geology and Geophysics of the Chinese Academy of Sciences (IGGCAS) has found that oxygen (O2) remained at extremely low levels in the ocean-atmosphere system for most of Earth's history, and showed that a rise in oxygen occurred in step with the evolution and expansion of complex eukaryotic ecosystems.
This work was published in PNAS.
Researchers from McGill University, Georgia Institute of Technology, University of California Riverside, and Yale University were also involved in the study.
The relationships between surface oxygenation and evolutionary ecology of early complex life remain highly debated, due to the uncertainty of the oxygen (O2) level of the ocean-atmosphere system during the mid-Proterozoic, 1.9 billion-0.9 billion years ago.
There are several geochemical indices from ancient sedimentary rocks that have been employed as proxies for mid-Proterozoic atmospheric O2 level, predominantly the abundance or isotopic composition of redox-sensitive trace metals in marine carbonate rocks or shales.
However, atmospheric O2 levels estimated from these proxies are often characterized by high uncertainties, suffer from apparently conflicting inferred O2 level histories, or a combination of the two factors. For this reason, a robust, mechanistically understood and sensitive proxy for mid-Proterozoic O2 levels is required.
To gain insight into the O2 levels of mid-Proterozoic shallow seawater, the researchers analyzed the iron (Fe) isotope compositions of Proterozoic and Phanerozoic ironstones from different localities, due to the fact that ironstones feature ooids and are deposited in shallow marine environments.
In addition, compared to paleoredox proxies, the Fe isotopic composition of Fe-rich chemical sedimentary rocks is expected to be strongly rock-buffered and resistant to post-depositional alteration.
The mid-Proterozoic ironstones possess highly variable and mostly positive Fe isotope compositions, whereas the younger equivalents display a narrow range of isotopic compositions, which are indistinguishable from those of hydrothermal and igneous ferrous Fe, or Fe(II), sources. These characteristics indicate that the partial oxidation of dissolved Fe(II) was characteristic of Proterozoic shallow marine environments, whereas younger ironstones formed via complete oxidation of Fe(II).
Based on the results of the Fe(II) oxidation kinetic modelling approach, partial Fe(II) oxidation in mid-Proterozoic ironstones were most compatible with shallow seawater O2 contents below ~5 μmol/kg, corresponding to atmospheric O2 levels below ~1% present atmospheric level.
Furthermore, the ironstone data suggested a change in surface O2 levels in the Tonian Period, and narrowed down the timing of Neoproterozoic Oxygenation Event to between ca. 900 million-750 million years ago, coincident with an apparent rise in eukaryotic ecosystem complexity.
The ironstone record provides new support for the idea that surface O2 levels were changing in step with eukaryotic evolution in the Proterozoic, and reveals that Earth is capable of stabilizing at low atmospheric oxygen levels, with implications for the exploration of exoplanet biosignatures.
