Carbonates and iron formations as deep-time archives of biological and environmental changes

Diamictite with clasts of chert and banded iron formation from the Duitschland Formation (~2.4 Ga), recording one of the first global ice ages.

Carbonates and other authigenic lithologies, such as iron formations, are important archives for biological and environmental changes on Earth. In the Precambrian, this includes perturbations to the carbon and sulfur cycles, ‘Snowball’ Earth ice ages, and the evolving oxidation state of the ocean-atmosphere state. These changes are intimately coupled to biological innovations, such as oxygenic photosynthesis in the Archean, or metazoan organisms in the latest Proterozoic. However, limited understanding of depositional processes and settings, complex intrabasinal and global stratigraphic relationships, and a scarcity of reliable absolute age dates currently restrict global reconstructions and modelling in time and space.

We currently focus on the sedimentary record of the first significant oxygenation in surface environments, the so-called ‘Great Oxygenation Event’ (GOE) at around 2.4 Ga ago. While atmosphere and oceans on the early Earth were reducing, biological activity initiated the slow but irreversible accumulation of oxygen, which became detectable in sedimentary archives during the GOE. Oxygenation and global tectonics were linked to removal of reducing oxygen sinks, such as iron, manganese and greenhouse gases CH4 and CO2. As the latter likely maintained a global greenhouse on early Earth, their removal plunged the Earth into its first global ice age. All of these processes transformed surface environments.

Microbial carbonates capture some of the changes in environments across the transition from a reducing to an oxygenated ocean-atmosphere system. These are deep-water stromatolites about 2.5 Ga old.

This project studies one of the best-preserved reference sections for this interval, the Transvaal Supergroup in South Africa. A refined stratigraphic framework for South Africa will allow integration with strata worldwide as a foundation for better understanding of temporal and spatial variations in processes responsible for global oxygenation and glaciation. To this end, new stratigraphic sections are logged, correlated and dated using detrital zircons to establish a stratigraphic-depositional framework. Geochemical work (isotopes, redox tracers) focuses on authigenic units, such as carbonates, iron and manganese formations, to constrain redox conditions.

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