Effects of Past Global Change on Life (1995)

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OXYGEN AND PROTEROZOIC EVOLUTION: AN UPDATE 30 1990). During this same interval, the 87Sr/86Sr ratio of carbonates is anomalously low (∆87Sr as low as -500; Veizer et al., 1983; Derry et al., 1989; Asmeron et al., 1991). Figure 1.6 Summary of geochemical and paleobiological data relevant to considerations of Neoproterozoic evolution and environmental change. Filled triangles indicate ice ages; Fe indicates iron formation deposition; the cross marks a major extinction of large, morphologically complex protistan fossils; asterisks indicate present-day values for the carbon isotopic composition of diagenetically stabilized carbonates (left margin) and the strontium isotopic composition of seawater (right margin). The positive carbon isotope anomalies indicate a substantial increase in the proportional rate of organic carbon burial in the Neoproterozoic oceans. Insofar as oxygen generation is dependent on the burial of photosynthetically produced organic matter, this may signal a significant Neoproterozoic oxygen increase. This is not necessarily the case, however, because increased introduction of H2, CO, and/or other reduced materials at midocean ridges and/or from terrestrial volcanoes could have balanced the oxygen generated by the burial of excess organic carbon. Interpretation of the Sr isotopic composition of seawater remains problematic; however, the extremely low ∆87Sr values of carbonates that also exhibit anomalous 13C enrichment may well indicate a high rate of hydrothermal input into seawater during this interval. Recent models of Neoproterozoic environmental change by Knoll and Walker (1990) and Derry et al. (1992), although differing in assumptions and procedures, both suggest that during the ca. 850 to 600 Ma interval of unusual carbon and strontium isotopic signatures, PO2 remained relatively low. Both models further suggest that during latest Proterozoic time, when Sr isotopic ratios in seawater increased from their lowest to nearly their highest values in the past 1000 m.y. PO2 may have increased significantly. The relatively rapid change in the isotopic composition of Sr occurred just prior to the Ediacaran radiation. The Knoll/Walker and Derry et al. models indicate that a latest Proterozoic PO2 increase is plausible, but not that it is proven. There are as yet no direct quantitative data indicating a change in PO2. One indirect line of evidence that supports the idea of latest Proterozoic oxygen increase in the sulfur isotopic record. The sulfur isotopic composition of 850 to 600 Ma sulfates does not move antithetically to the carbon curve, as during much of the Phanerozoic. Antithetic movement was established in latest Proterozoic times—during the brief but eventful interval when the isotopic composition of Sr in seawater shifted; Ediacaran metazoans radiated; and intriguingly, most of the large, morphologically complex protists that characterize the 850 to 600 Ma microplankton record disappeared (Figure 1.4D; Knoll and Butterfield, 1989; Zang and Walter, 1989). The substantial shift in the isotopic composition of marine sulfate recorded at this time indicates a marked shift of the sedimentary sulfur reservoir toward pyrite (Claypool et al., 1980; François and Gerard, 1986)—a shift that contributed further to the production rate of O2. Summary of the Latest Proterozoic Record The Cloud model links the diversification of macroscopic animals to new evolutionary opportunities attendant on increasing PO2. Certainly, there can no longer be any doubt that the period immediately prior to the Ediacaran radiation was a time of marked environmental fluctuation. This may have included a PO2 increase, although quantification of Neoproterozoic oxygen levels and even the O2 levels at which Ediacaran-grade animals were able to function remains uncertain. As in the case of Paleoproterozoic atmospheric change and evolution, we can claim to have only two of the three required pieces of the puzzle in place; however, specific attention can now be focused on paleosols and other features of the Neoproterozoic rock record in a concentrated effort to understand the pattern of atmospheric change at or just before the first appearance of macroscopic metazoans. CONCLUSIONS In the Phanerozoic geological record, environmental change is often associated with extinction. Its link to