Thesis (M.A.)--New Mexico State University, 1967. / Vita. Bibliography: leaves 64-70.

Thesis (Ph. D.)--University of Oregon, 2001. / Typescript. Includes vita and abstract. Includes bibliographical references (leaves 145-170). Also available for download via the World Wide Web; free to University of Oregon users.

Thesis (M.Ag.Sc.)--Dept. of Agricultural Biochemistry and Soil Science, Waite Agricultural Research Institute, University of Adelaide, 1973.

Thesis (M.S.)--West Virginia University, 1998. / Title from document title page. "November 12, 1998." Document formatted into pages; contains viii, 191 p. : ill. (some col.), col. maps. Includes abstract. Includes bibliographical references (p. 71-84).

Thesis (Ph.D.)--Baylor University, 2005. / Includes bibliographical references (p. 108-119).

Thesis (Ph. D.)--University of Texas at El Paso, 2008. / Title from title screen. Vita. CD-ROM. Includes bibliographical references. Also available online.

Thesis (Ph.D.)--Baylor University, 2008. / Includes bibliographical references (p. 118-134).

Thesis (M.S.)--Bowling Green State University, 2006. / Document formatted into pages; contains xiv, 177 p. : ill., maps. Includes bibliographical references.

No description available.

M.Sc. / The evolution of oxygen in the Earth's atmosphere during the early Precambrian has been a subject of debate for many years. Two fundamental models oppose another. The one by Cloud, Holland and co-workers suggests that the atmosphere was essentially anoxic until about 2.2Ga and then became highly oxygenated due to a sudden rise in oxygen levels. The, other advocated by Dimroth, Kimberley and Ohmoto suggests that the atmosphere was oxygenated as early as 3.5Ga. The most crucial assumption for the Cloud-Holland model for the evolution of atmospheric oxygen is that the 2.2-2.3Ga Hekpoort paleosol formed under reducing atmospheric conditions. However, regional field, drill core, petrographic and geochemical investigations of the Hekpoort paleosol during this study clearly show that the Hekpoort paleosol in fact represents an oxidised lateritic weathering profile. In addition, the Hekpoort paleosol correlates well to the oxidised saprolites below the Gamagara/Mapedi erosion surface in the Northern Cape Province. The basis for theassumption by Holland and co-workers that a dramatic rise in atmospheric oxygen levels took place at 2.2Ga thus falls away. During this study extensive red beds, belonging to the Dwaal Heuvel Formation were discovered directly above the Hekpoort paleosol in the Pretoria Group in Botswana and the western Transvaal area. The red beds show two stages of development, firstly fluvial and then deltaic. The red beds are correlated with the Gamagara/Mapedi red beds in Griqualand West. Apart from this evidence for highly oxygenated conditions immediately above the Hekpoort/Ongeluk lavas, hematitic ferricrete, pisolitic mudclast conglomerate and hematitic oolitic ironstones were also found in the Timeball Hill Formation underlying the Hekpoort lava. Oolitic ironstones are developed over an area of more than 100 000 km2. Several different types of oolites are developed within the oolitic ironstone which contains up to 73wt% Fe203. The ferricrete and hematitic pisolitic mudclast conglomerate contain oncolites. These ferricretes, pisolitic mudclast conglomerate and oolitic ironstones suggest that the atmosphere was already highly oxidising between 2.4 and 2.45Ga, prior to deposition of the Hekpoort lava. Pretoria Group rocks that were deposited in close contact with the atmosphere show no evidence for an anoxic atmosphere. It is suggested that atmospheric oxygen levels may have fluctuated through time but at the same time increased in a steplike manner during deposition of the Transvaal Supergroup. However, at this moment in time we do not have enough information available to develop a quantitative model for the evolution of atmospheric oxygen. New age data available on the Hekpoort/Ongeluk lava unit indicate that it may be 2.395Ga old i.e. some 200Ma older than thought earlier. Thus, the atmosphere could have been highly oxygenated in very early Paleoproterozoic times. Uraninite, pyrite and siderite present in older Archean sedimentary rocks do, however, argue for more reducing atmospheric conditions at that time. Both the Cloud-Holland and Dimroth-Ohmoto models of atmospheric oxygen development are therefore in need of revision.