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OXYGEN AND PROTEROZOIC EVOLUTION: AN UPDATE 22 tains 21% O2, there is widespread agreement that prior to the emergence of oxygenic cyanobacteria, PO2 must have been extremely low. Recent models by Canuto et al. (1983) and Kasting (1987) suggest that the prebiotic atmosphere contained no more than about 10-10 bar of molecular oxygenâenough to make hematite stable, but far too little to provide an effective ozone screen or to support aerobic metabolism. Indeed, a standard tenet of chemical evolution is that prebiotic chemistry could not have proceeded in environments containing significant amounts of O2. Increasingly well resolved phylogenetic trees (e.g., Woese, 1987) complement this perspective. These trees indicate that anaerobic organisms diverged earlier than aerobes, and that aerobes requiring high PO2 (i.e., large animals) appeared later than aerobes able to function in less oxic environments. It is, therefore, attractive to link biological to environmental history; however, the entire pattern of biological evolution can potentially be explained quite differently. The facts noted in the previous paragraph really only require a specified initial condition: that is, that the Earth's prebiotic atmosphere was essentially anoxic and that the first organisms were, therefore, anaerobic. There is no a priori reason why an early radiation of cyanobacteria could not have engendered an early and rapid increase in PO2 approximating or even exceeding today's levels. Very different controls would then have to be sought for the observed evolutionary patterns. Acceptance of what might fairly be called the Cloud model requires that three criteria be satisfied: 1. geochemical documentation of environmental change; 2. independent paleontological evidence for coeval evolutionary innovation; and 3. physiological, phylogenetic, and ecological reasons for linking criteria 1 and 2. In the following pages, we evaluate the rapidly accumulating data on oxygen and biological evolution during two intervals often inferred to have been critical junctures in the history of life: (1) early in the Proterozoic Eon (ca. 2000 Ma), when increases in PO2 above the Pasteur point are thought to have made possible the evolution of aerobic prokaryotes and mitochondria-bearing protists; and (2) the latest Proterozoic (ca. 600 Ma), when another substantial increase in PO2 may have made possible the initial evolution of macroscopic animals. THE EARLY PROTEROZOIC EON Geochemical Evidence for Atmospheric Change The Paleoproterozoic Era (2500 to 1600 Ma) was a time of profound environmental change (Cloud, 1968a, 1972: Holland, 1984). Two independent sedimentological observations have long been cited in support of the hypothesis that the atmosphere first accumulated significant amounts of oxygen during this interval. Banded iron formations (BIF), quintessentially Precambrian sediments composed of iron-bearing minerals and silica, are abundant in successions older than ca. 1900 Ma, but are rare in younger sequences (Figure 1.1). Continental red beds display an inverse distribution. The origin of marine iron Figure 1.1 A summary of geochemical and paleobiological data relevant to considerations of Paleoproterozoic evolution and environmental change (PAL = present atmospheric level).
OXYGEN AND PROTEROZOIC EVOLUTION: AN UPDATE 23 formations probably requires anoxic mid- and deep oceans for the storage and transportation of ferrous iron, while it is likely that red beds can form only when terrestrial or nearshore marine sediments come in contact with atmospheric oxygen. Thus, it has been reasoned that the BIF-red bed transition marks the rise of atmospheric oxygen. Complementary information comes from detrital uraninite in Archean and earliest Proterozoic alluvial rocks. Because this uranium mineral can survive prolonged transport only in media containing little or no oxygen, the lack of detrital uraninite deposits younger than ca. 2300 Ma also points toward a significant environmental transition (Figure 1.1; Roscoe, 1969; Grandstaff, 1980; Holland, 1984). Not all scientists have accepted the validity of these observations or of their interpretation (see, for example, Dimroth and Kimberley, 1976; Clemmey and Badham, 1982; Windley et al., 1984). It has been argued repeatedly that at least some red beds antedate the end of BIF deposition, that Archean granites have paleoweathering profiles indicative of oxic environments, and that oxidized sulfur minerals (sulfates) occur in some of the oldest known sedimentary successions. All of these observations are correct, and we must ask whether they preclude the interpretation of Archean and earliest Proterozoic environments as oxygen poor. The answer appears to be no. The formation of red beds and oxidized weathering profiles on granitic substrates requires oxygen, but only in minute quantities (see, for example, Holland, 1984; Pinto and Holland, 1988) âconsiderably less than is needed for aerobic metabolism. Marine sulfate does not require free oxygen at allâH2S can be photooxidized anaerobically to SO42- by photosynthetic bacteria, while the photochemical oxidation of volcanogenic S and SO2 to sulfate was probably a steady source of oxidized sulfur in the Archean oceans (Walker, 1983). Towe (1990) has specifically argued for the development of aerobic respiration early in the Archean and, therefore, for the presence of 1 to 2% PAL (present atmospheric level) O2 in the atmosphere since that time. The possibility that oxygen levels reached this physiologically important threshold so early is not contradicted by the sparse geochemical data available for early Archean rocks (see below); however, Towe's model suffers from the absence of Archean O2 sinks other than Fe2+ . We believe that the neglect of volcanic gases in his model casts significant doubts on the validity of his analysis. Other arguments against the Cloud model posit that the geochemical indicators of low PO2 during the Archean and earliest Proterozoic could be the result of burial digenesis, which generally acts to reduce minerals. Equally, it has been argued that oxide facies iron formations are themselves diagenetic replacements of carbonates. Neither of these views can sustain critical scrutiny. Although both oxidation and reduction can occur during diagenesis, there is ample evidence that at least some detrital uraninite and most iron formations have a primary sedimentary origin. New data from paleosols add quantitative rigor to arguments for Paleoproterozoic environmental change (Holland and Zbinden, 1988; Holland et al., 1989; Holland and Beukes, 1990). All paleosols younger than 1900 Ma that have been studied to date are highly oxidized. Fe2+ in the parent rocks of these paleosols was oxidized quantitatively, or nearly so, to Fe3+, and was retained in the paleosols as a constituent of Fe3+ oxides or hydroxides. This is demonstrated by the near constancy of the ratio of total Fe to Al2O3 and of total Fe to TiO2 within these paleosols and their parent rocks. Paleosols older than 1900 Ma that were developed on basaltic rocks have lost nearly all iron from their upper sections. Some of this lost iron was reprecipitated in the lower sections of the paleosols. There is some evidence that iron loss from pre-1900 Ma paleosols developed on granitic rocks was much less pronounced. These observations suggest that the O2 content of the atmosphere prior to 1900 Ma was insufficient to oxidize more than a small fraction of the iron developed in soils on basaltic rocks, but was sufficient to oxidize a good deal of the much smaller amount of iron in soils developed on granitic rocks (Figure 1.2). More detailed studies of paleosols are needed to confirm the generality of these observations. If confirmed, they can be used to assign a rough value of ca. 1% PAL to the O2 content of the atmosphere between about 2700 and 2200 Ma (Pinto and Holland, 1988). Pre Figure 1.2 Iron retention in Precambrian paleosols plotted in terms of Râthe ratio of the oxygen to CO2 demand in the weathering of parent rocksâand geological age. Parent rocks with R < 0.025 are granitic; those with higher R are basaltic.
