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Evolution of Atmospheric O2, Climate, and the Terrestrial Biosphere (dm)

The major goal of the research carried out by Ohmoto’s group is to understand the connection between the chemical evolution of the atmosphere and oceans and the biological evolution in the oceans and land. Specific questions include: (1) the pO2 history of the atmosphere and the causes for its evolution; (2) the oxidation history of oceans; (3) the controlling mechanisms for atmospheric pO2 and pCO2; (4) the timings of emergence of important organisms in the oceans and on land; and (5) the influence of atmospheric and oceanic chemistry, and of biological activity, on the geochemical cycles of important elements. These questions have been approached from field-related geochemical investigations of a variety of rocks (e.g., paleosols, shales, carbonates, banded iron formations, uraniferous conglomerates) of 3.5 â?? 1.8 Ga in age, laboratory experiments on redox-sensitive minerals; and computer simulations. Satisfactory progress has been made in all fronts; new and important discoveries have been made to strengthen the theory that postulates the very early (~4.0 Ga ) development of an oxygenated atmosphere and a complex biosphere.

Paleosol Project: Geochemical investigation on 2.6 Ga paleosols in the Eastern Transvaal district of South Africa resulted in a discovery of remnants of indigenous cyanobacterial mats that developed on mineral surfaces during soil formation. This discovery has been widely publicized in news media as evidence for the oldest life on land, and as evidence for the early development of an ozone shield and an oxygenated atmosphere. During fieldwork in Finland during the Summer of 2000, we discovered several outcrops of paleolaterites of ~2.3 Ga in age. Formation of laterites, which are soils highly enriched in ferric iron, requires an oxygen-rich atmosphere and organic acids that were generated by soil organisms. Therefore, this discovery also supports the theory that an oxygen-rich atmosphere and the terrestrial biomass developed before 2.3 Ga.

Shales and Carbonates Project: A very large number of shale and carbonate samples were collected by us during the past 10 years from the Kaapvaal Craton in South Africa (3.5 â?? 2.0 Ga), the Pilbara â?? Hamersley district in Australia (3.5 â?? 2.4 Ga), southern Ontario district, Canada (2.75 â?? 1.9 Ga), and from other areas in the world. Detailed investigations on the mineralogical and geochemical characteristics (both inorganic and organic) of these samples have been carried. An important finding from these investigations is that the geochemical cycles of redox-sensitive elements (e.g., C, S, N, Fe, Mn, Mo, U, V, Ce) through the atmosphere â?? hydrosphere â?? lithosphere have been basically the same since at least ~3.5 Ga. This finding also supports the model of an early rise of atmospheric oxygen. The results of carbon and sulfur isotope analyses of the above samples suggest the divergence of organisms to produce aerobic organisms (e.g., cyanobacteria, methanotrophs, and eukaryotes) and anerobic organisms (sulfate-reducing bacteria, methanogenic bacteria) occurred before 3.5 Ga.

Banded Iron Formations (BIFs) Project: Banded iron formations, especially of the Lake Superior-type BIFs, have been considered by many previous researchers as the best evidence for an anoxic atmosphere prior to ~2.0 Ga. Our preliminary investigations on the geochemical characteristics of the Algoma-type BIFs, which are more common than the Superior-type BIFs, suggest that they formed in deep oceans by mixing of Fe2+-bearing hydrothermal solutions and O2-bearing, deep ocean water. “The oxygenated deep oceans” requires the atmospheric pO2 to be greater than 20-50 % of the present level. Because the Algoma-type BIFs formed through geologic history, our finding also suggests that the atmospheric p O2 level has been essentially the same since ~4.0 Ga.

Uraninite Project: The presence of detrital grains of uraninite, pyrite and siderite in some quartz-pebble conglomerates of pre-2.4 Ga age have been used by many researchers as strong evidence for an anoxic atmosphere, because these minerals have been considered to be unstable under an O2-rich atmosphere but stable under an O2-poor atmosphere. However, the results of laboratory experiments and theory suggest that the dissolution rates of uraninite and siderite are in fact faster in an O2-poor and CO2-rich environment compared to an O2-rich and CO2-poor environment; these minerals eventually dissolve out after a long exposure to rainwater under all pO2 conditions. Therefore, the presence of these minerals in some unusual sedimentary rocks (quartz pebble conglomerates) cannot be used as a measure of atmospheric oxygen level.

Modeling of the Atmospheric Evolution: We have carried out a quantitative evaluation of the various geochemical parameters that influence the production and consumption fluxes of O2 and CO2 through the atmosphere – , ocean , sediment, crust-, and soil reservoirs. Based on this, we have demonstrated that the atmospheric pO2 level is likely to have remained (and will remain) within ±50 % of the present level as long as the oxygenic photosynthetic organisms are active. The atmospheric pO2 level has been regulated primarily by the coupling of two negative feedback mechanisms. One is an increasing (or decreasing) flux of organic carbon burial in marine sediments, responding to a decreasing (or increasing) pO2 in the atmosphere. The other is an increasing (or decreasing) flux of O2 consumption by soils, responding to an increasing (or decreasing) pO2. Using the kinetic equations developed in the above study, we have also been carrying out simulation of the long-term (>1 billion years) evolution of atmospheric pO2 and pCO2 and of sedimentary chemistry, especially of the contents and isotopic compositions of carbon in marine sediments (carbonates and organic C), under a variety of scenarios for the evolution of the continental crust and the mantle degassing. This is the first attempt by any researcher to quantitatively link the evolution of the atmosphere, oceans, continents and mantle. Our preliminary computations suggest that the pO2 was likely to have risen essentially to the present level within 50 million years of the first appearance of cyanobacteria about 4 Ga ago and remained about this level while the pCO2 gradually decreased from ~1000 times the present level. This assumes that the degassing of H2 and other reducing gases decreased from about 8 times the present flux and the continental volume increased from ~10 % of the present value over a 4 billion year period.