Transient marine bottom water oxygenation on continental shelves by 2.65 billion years ago

Abstract

A growing body of evidence suggests that molecular oxygen (O₂) accumulated in some shallow marine environments beneath the effectively anoxic Archaean atmosphere 4.0 to 2.5 billion years (Ga) ago. Yet, the temporal and spatial distribution of these oxygen oases is not well known. Here we use thallium (Tl) isotope ratios, which are sensitive to manganese oxide burial, to place constraints on the timing and tempo of marine oxygen oases between about 2.65 Ga and 2.50 Ga. Lower-than-crustal authigenic ²⁰⁵Tl/²⁰³Tl ratios are common in shales from the approximately 2.65 Ga Jeerinah Formation (Western Australia) and the 2.50 Ga Klein Naute Formation (South Africa). Particularly low ²⁰⁵Tl/²⁰³Tl ratios are found at 2.50 Ga, coincident with a pronounced ‘whiff’ of O₂. These data can be explained by widespread seafloor manganese oxide burial, a scenario that requires persistent O₂ penetration into marine sediments beneath regionally extensive marine oxygen oases. By contrast, ²⁰⁵Tl/²⁰³Tl ratios from the 2.60–2.52 Ga Nauga Formation (South Africa) do not deviate from crustal values, suggesting an intervening period of muted seafloor Mn oxide burial. Our data suggest that O₂ accumulated over greater spatial extents and to greater depths than previously thought at about 2.65 Ga and that marine oxygenation was spatially and temporally dynamic well before the Great Oxidation Event began at about 2.4 Ga.