Evidence in Trace Elements and Fe3+/Fe2+ Ratios of Archaean and Early Proterozoic Shales for the Early Development of Oxic Atmosphere

Abstract

A currently popular model for the evolution of atmospheric oxygen, termed the Cloud-WalkerKasting-Holland (C-W-K-H) model, postulates essential absence of free O2 in the atmosphere prior to 2.2 Ga, then a dramatic rise of p% from ~ 1% PAL (present atmospheric level) during the period between 2.2 and 1.9 Ga. Another major rise in pO2 to nearly the present level at -600 Ma has been suggested by some researchers. An implication of this theory is that episodic rises in O2 were major causes for the evolution of organisms, such as the appearance of eukaryotes at ~2.0 Ga and of metazoa and H2S-oxidizing bacteria at ~600 Ma. An alternative model for atmospheric evolution, termed the Dimroth-Ohmoto (D-O) model, postulates an essentially constant atmospheric pO2 level (within _+50% of PAL) since ~4 Ga. According to this model, the 02 requirement for all aerobic organisms (>10% PAL?) has been satisfied since the early stage of Earth's history, and the evolution of aerobic organisms was not caused by changes in the atmospheric pO2 level. Ohmoto (1997) has recently summarized some geochemical data and concepts that support the D-O model. They include: (1) isotopic (Pb, Sr, Nd, and HI) and trace element (REE, Nb/U) data on igneous and metamorphic rocks of Archaean ages that support the Armstrong's (1981) crustal growth model postulating an essentially constant volume for the continental crust since ~4.0 Ga; (2) isotopic (O, Nd and Sr) and elemental data on Banded Iron Formations (BIFs), suggesting that major BIFs were formed by submarine hydrothermal fluids locally discharged within anoxic basins surrounded by oxic oceans; (3) micro-scale variations in the sulphur isotopic compositions of pyrite crystals in 3.4-2.2 Ga shales, suggesting that they were formed by sulphate-reducing bacteria in sulphate-rich oceans; (4) textural and paragenetic data on detrital grains of uraninite and pyrite in quartz-pebble conglomerates of >2.2 Ga age, suggesting that these minerals were mostly diagenetic or hydrothermal, rather than detrital, in origin; (5) Fe3+/Ti and FeZ+/Ti ratios of pre-2.2 Ga palaeosols, suggesting soil formation under an oxic atmosphere; (6) carbon isotopic compositions of organic matter in 3.8-2.0 Ga sedimentary rocks, suggesting that temporal and spatial fluctuations in redox state of sedimentary basins, much like those occurring in modem Earth, were common during this period; and (7) ~a3C data on marine carbonates and TOC data on shales of various geologic ages, suggesting essentially constant rates of production and consumption of atmospheric oxygen since at least 3.5 Ga. Here I present other lines of supportive evidence for the D-O model, which come from our own chemical analyses of more than 300 samples of shales of 3.8-0.5 Ga in age.