Thallium-Hotazel

Deciphering the extent and fluctuation of ocean-atmosphere oxygenation during the 2.42-2.05 Ga Great Oxidation Episode

01/01/2026 to 31/08/2027

The evolution of atmospheric and marine oxygen levels on Earth is critical to understanding the planet’s habitability and biological complexity. Thallium (Tl) isotopes have emerged as a sensitive geochemical proxy for reconstructing past oceanic oxygenation, thanks to their strong association with manganese (Mn) oxide formation under oxygen-rich conditions. In particular, Tl isotopes can track variations in global burial fluxes of Mn oxides and thus indicate the extent of bottom-water oxygenation through time. This project builds upon recent advances, notably the work of Ostrander et al. (2024), who used Tl isotopes in Paleoproterozoic black shales from South Africa to reveal a coupled rise in atmospheric and marine oxygen ~2.3 billion years ago during the Great Oxidation Episode (GOE).

Despite significant progress, uncertainties remain about the scale and stability of surface oxygenation during the GOE, especially concerning the initial Mn deposition in the ~2.4 Ga Hotazel Formation and the subsequent Lomagundi carbon isotope excursion (~2.22–2.05 Ga). While the Hotazel Formation hosts the world's largest sedimentary Mn deposit, its mineralogy—dominated by Mn(II) and Mn(III) phases—and geochemical features such as Ce anomalies and Mo isotope values suggest complexities in its redox history. This raises questions about whether Mn(IV) oxides were initially deposited and to what degree marine oxygenation had advanced at that time. Similarly, the Lomagundi Event, traditionally seen as an “oxygen overshoot” based on extreme carbon isotope values, may not reflect a global rise in oxygen if these values were regionally restricted. Thus, clarifying the extent of deep-ocean and atmospheric oxygenation during both the onset and culmination of the GOE is critical.

To address these questions, the project will use Tl isotopes as a primary tool due to their unique redox sensitivity and relatively short residence time (~20 kyr), which allows them to record oxygen fluctuations on million-year timescales. Two main questions will be explored: (1) What was the original Mn mineral phase during Hotazel Mn deposition, and what were the implications for coeval marine and atmospheric oxygen levels? (2) How extensive was marine oxygenation during the Lomagundi Event, and did atmospheric O₂ reach near-modern levels?