On 16 April 2025, Assoc.Prof. Xinming Chen, School of Oceanography, Shanghai Jiao Tong University, published an article titled “Transient marine bottom water oxygenation on continental shelves by 2.65 billion years ago” as the first and corresponding author in Nature Geoscience. The collaborators include Chadline Ostrander (The University of Utah), Brian Kendall (University of Waterloo), Ariel Anbar (Arizona State University), Sune Nielsen (University of Lorraine), and Brett Holdaway and Jeremy Owens (Florida State University).

A growing body of evidence support the hypothesis that molecular oxygen (O₂) accumulated in surface oceans beneath an essentially O₂-free atmosphere in the Archean 4.0 to 2.5 billion years ago (Ga). However, the timing and spatial extent of oceanic oxygenation remain under-constrained with limited sections and proxy specificity. To constrain the tempo and extent of oceanic oxygenation, we measured authigenic thallium isotopic compositions (e²⁰⁵Tl_A) in black shales from the Jeerinah Formation, Hamersley Basin, Western Australia (~2.65 Ga) and the Nauga and Klein Naute Formations, Griqualand West Basin, South Africa (~2.6–2.5 Ga).

We observed multiple negative Tl isotope excursions (e²⁰⁵Tl_A < -2) in the ~2.65 Ga Jeerinah Formation and the ~2.50 Ga Klein Naute Formation. Particularly low e²⁰⁵Tl_A values (as low as -4.2 e-unit) were seen at ~2.50 Ga Klein Naute Formation, coincident with a pronounced “whiff” of O₂ at this time. These lower-than-crustal e²⁰⁵Tl_A values (< -2) are best interpreted by a pronounced burial of manganese oxides on continental shelves elsewhere in the ocean, which requires the persistent presence O₂ in bottom waters. Beyond these intervals, we observed e²⁰⁵Tl_A values indistinguishable from the upper continental crust, suggesting muted seafloor Mn oxide burial over 2.6-2.5 Ga.

Figure 1. Compilation of all thallium isotope compositions in 2.65–2.5 Ga shales.

The negative Tl isotope excursions from the Jeerinah Formation extend evidence of at least regional marine bottom water oxygenation on continental shelves backward by ~150 million years. Low e²⁰⁵Tl_A values from the ~2.50 Ga Klein Natue Formation from South Africa corroborate the evidence of at least regional water column oxygenation indicated by the broadly coeval Mt. McRae shale from Western Australia formed ~1000 km away. The lower e²⁰⁵Tl_A values at ~2.50 Ga compared to those of the ~2.65 Ga suggests that oceanic oxygenation is likely stronger at ~2.50 Ga. Integrating our data with previously published datasets (the 2.6-2.5 Ga Mt. McRae Shale) provides a consistent picture of temporally dynamic oceanic oxygenation during the runup to the Great Oxidation Event.

Link to the article: https://www.nature.com/articles/s41561-025-01681-9

Contributed by Assoc.Prof. Xinming Chen from Earth Evolution Laboratory.