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Abstract
This paper reviews the Precambrian history of atmospheric oxygen, beginning with a brief discussion of the possible nature and magnitude of life before the evolution of oxygenic photosynthesis. This is followed by a summary of the various lines of evidence constraining oxygen levels through time, resulting in a suggested history of atmospheric oxygen concentrations. Also reviewed are the various processes regulating oxygen concentrations, and several models of Precambrian oxygen evolution are presented. A sparse geologic record, combined with uncertainties as to its interpretation, yields only a fragmentary and imprecise reading of atmospheric oxygen evolution. Nevertheless, oxygen levels have increased through time, but not monotonically, with major and fascinating swings to both lower and higher levels.
Key Words O^sub 2^, atmosphere, evolution, Precambrian, ocean, banded iron formation
DEDICATION
This manuscript is dedicated to the memory of Robert M. Garrels, one of the fathers of modern low-temperature geochemistry. His accomplishments were extraordinary (consult Berner 1992), but of importance here were his pioneering insights into global biogeochemical cycles and oxygen regulation. The landmark book Evolution of Sedimentary Rocks (Garrels & Mackenzie 1971) carefully documented and quantified the processes involved in sediment and rock cycling at Earth's surface. The approach forwarded in the book inspired a generation of global biogeochemical models. The processes controlling atmospheric oxygen (O^sub 2^) concentrations were also eloquently developed by Garrels & Perry (1974), who also include a precise presentation of the controlling reactions. Rather presciently, Garrels et al. (1973) predicted the occurrence of anoxygenic phototrophic Fe-oxidizing bacteria, later discovered by Widdel et al. (1993). Garrels et al. (1973) furthermore described a complete carbon cycle based on Fe, a topic explored in more detail below.
Sadly, my contact with a number of bright young geobiology students reveals that much of Garrels' work has slipped from view. I blame this on the evolution of scholarship and the heavy reliance of young workers (and indeed all of us) on internet-accessible reference sources, which have a limited memory. This dedication is directed toward putting the name Garrels back on the table where it belongs.
INTRODUCTION
The evolution of oxygen-producing cyanobacteria was arguably the most significant event in the history of life after the evolution of life itself. Oxygen is a potent oxidant whose accumulation into the...