THE LATE ARCHEAN BIOSPHERIC EXPLOSION

There is a growing body of evidence to suggest that the earth's atmosphere and biosphere have evolved in parallel with the supercontinent cycle. That is, the endogenic energy of the planet has played a major role in driving the biosphere forward to greater complexity. There were two significant stages in earth history when atmospheric oxygen levels rose significantly and the biosphere expanded rapidly. The most recent of these biospheric expansions (c. 1.0 to 0.5 Ga), which saw the appearance of metazoa and the “Cambrian Explosion”, has been studied in considerable detail. The earlier event, which extends across the Archean-Proterzoic boundary (c. 3.0 to 2.2 Ga), has received little attention.

Evidence of the existence of a biosphere before 3.0 Ga is limited, perhaps partly a result of preservation, but also because the biomass was small. Concretions, the mineralogical expression of microbial activity during the diagenesis of sediments, are abundant in late Archean and early Paleoproterozoic successions. They provide a sensitive barometer to biological activity in both the accumulating sediments and in the overlying water column and suggest a rapid expansion of the biosphere. In particular they indicate that there was an abrupt increase in planktonic biomass. The environment within the accumulating sediments was relatively oxygen rich suggesting that oxygen was being released into the oceanic water column well before stable isotope data indicates its appearance in the atmosphere. The fact that the biosphere exploded just as the first large continental mass began to form and the earliest intracratonic basins began to subside is unlikely to be coincidental (Lindsay and Brasier, 2002). The connection almost certainly relates to carbon burial but possibly also to the recycling of essential nutrients (notably P) and the opening up of vast shallow marine environments in which solar radiation was the only source of biospheric energy. The links between these events is significant as it suggests that for life to evolve in complexity it must do so in concert with an evolving dynamic planet.

References Lindsay, J.F. and Brasier, M.D., 2002, Did global tectonics drive early biosphere evolution? Carbon isotope record from 2.6 to 1.9 Ga carbonates of Western Australian basins. Precambrian Research, 114, 1-34.