LINKS BETWEEN THE EVOLUTION OF EARTH AND ITS BIOSPHERE, EVIDENCE FROM WESTERN AUSTRALIA

The Pilbara and Yilgarn Cratons of Western Australia preserve a comprehensive record of early Earth history extending back in time to 3.5 Ga. The two cratons were subsequently sutured by the Capricorn Orogeny at c. 2.0 Ga preserving an extensive record of this event in a series of basins that developed in association with the suture. The d13Ccarb record preserved in the late Archaean and early Palaeoproterozoic basins of Western Australia provides evidence indicating a stable carbon mass balance in the late Archaean and early Palaeoproterozoic suggesting tectonic quiescence and a well establish role for photosynthesis in the late Archean biosphere (Lindsay and Brasier, 2002). However, some time after c.2.5 Ga the d13Ccarb record became much more complex with positive to negative excursions occurring following glaciation at c.2.4 Ga and with a major positive excursion at c. 2.2 Ga. It seems likely that this early period of instability in the d13Ccarb record is similar to and as complex as that encountered in the Neoproterozoic. It is striking that these two major biospheric events coincide closely with supercontinent cycles.

The coincidence of carbon isotopic excursions and tectonism result from the sequestration of carbon during ocean closure with organic-rich passive margin sediments containing isotopically light carbon subducted into and stored in the lower crust and mantle thereby enriching the global ocean in isotopically heavy carbon and releasing oxygen to the atmosphere. A second stepwise increase in atmospheric oxygen in the Neoproterozoic may also have been connected with the assembly of Rodinia. This second event has been associated with the development of multicellular life and the evolutionary "Big Bang". This implies that the evolution of both the atmosphere, and the biosphere, may have been driven forward by planetary evolution, further implying that biospheric evolution has largely been driven by the dynamo of earth's tectonism. If this is so it has fundamental implications, not only for life on earth, but for the more general problems surrounding the likelihood of life having evolved on other planetary bodies.

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.