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Project Progress

We have continued to accumulate a variety of geochemical evidence in Precambrian rocks suggesting the atmosphere and oceans became oxygenated and the oceans and land were colonized by diverse organisms by 3.2 billion years ago. This date is at least 1 billion years earlier than most geoscientists have accepted. The investigated rock types include: (a) marine shales and greywackes; (b) paleosols; (c) red beds; (d) banded iron formations; (e) ores (volcanogenic massive sulfides, uraninite); and (f) igneous rocks (volcanics and plutonics). These rocks range from 3.4 to 1.8 billion years in age, and they were systematically collected from the Kaapvaal Craton in South Africa, the Pilbara-Hamersley district in Australia, the Abitibi greenstone belt in Canada, and the Lake Superior district in Canada/USA. The types of geochemical investigations have included: (a) organic geochemistry of organic matter (carbon isotopes, C/P/N/H ratios, C content); (b) Fe geochemistry (Fe/Al, Fe/Ti, Fe³⁺/Fe²⁺); (c) sulfur geochemistry (sulfur isotopes, S/C/Fe ratio, S content); (d) rare earth element geochemistry (Ce anomaly); and (e) trace element geochemistry (Mo, U, V, etc).

We have conducted laboratory experiments to determine the dissolution rates of some important redox-sensitive minerals (i.e., uraninite and pyrite) as a function of O₂ content and pH of solutions. These data have become essential in relating the geochemical data on natural rocks (see above) to the atmospheric O₂ level.

Based on the analyses of laboratory data on the kinetics of oxidation of organic matter and of the carbon contents of marine sediments, we have established a quantitative model for the geochemical cycle of oxygen. We have suggested that the atmospheric O₂ level has been maintained within ±50% of the present level since the emergence of oxygenic photosynthesis by two major negative feedback mechanisms. One is an increase in the burial flux of organic matter in marine sediments with decreasing pO₂, and the other is a decrease in the O₂ consumption flux during soil formation with decreasing pO₂.