U-PB ISOTOPE CONSTRAINTS ON THE EVOLUTION OF ATMOSPHERIC O₂

A number of geochemical indicators of the oxidation state of the atmosphere suggest strongly that the level of O2 rose rapidly at about 2.25 Ga. Paleosols have shown convincing evidence for this. Trace elements variations in carbonaceous shales can also be used as an indicator of the oxidation state of the atmosphere. Phanerozoic carbonaceous shales are frequently enriched in elements such as Mo, U, Re, V and Cr because these elements are stabilized as oxyanions in oxygenated surface and ground waters, transported to the oceans in solution and then in part re-precipitated in carbonaceous shales of local anoxic basins. However, under the anoxic atmospheric conditions early in Earth history, these elements were highly insoluble so they could not be effectively transported to the ocean and re-precipitated in high concentrations in early anoxic basins. One potential problem with ancient sediments is that they have usually seen substantial alteration and metamorphism and therefore using concentration data from such samples to infer atmospheric O2 evolution have been viewed with some skepticism. We have therefore carried out a Pb-isotope study of black shales that are both older and younger than 2.3 Ga. Pb isotopes are particularly well suited to establish the behavior of U in such rocks because we have two coupled U-Pb isotopic systems. Thus, an independent estimate of U in ancient black shales can be obtained from Pb isotopes alone. While there are some complexities in the Pb-isotope data, they do in general support a rapid increase in the release of U to the ocean in 2.2 Ga and younger samples due to onset of oxidative weathering at that time.