A comment on tectonics and the future of terrestrial life*/reply

In our recent paper (Lindsay and Brasier, 2002) we evaluated stable carbon isotope data from 474 samples of platform carbonates collected from the Late Archaean and early Paleoproterozoic Hamersley Basin and associated basins of Western Australia. The data are consistent and compare well with data from rocks of similar age on other major ancient continental blocks (cf. Karhu and Holland, 1996; Bau et al., 1999). The data clearly reflect a global signal, which has been attributed to the oxygenation of earth’s atmosphere and hydrosphere. The conspicuously bimodal nature of the secular carbon curve suggests that the global reduced carbon reservoir has grown episodically (see Knoll and Canfield, 1998 for a summary). This in turn has been taken to suggest that the atmosphere was oxygenated in a stepwise fashion (Des Marais et al., 1992) as a result of episodic burial of carbon during large scale tectonic cycles (supercontinent cycles; Des Marais, 1994; Lindsay and Brasier, 2000, 2002). When viewed in the larger context of earth history, there are several converging lines of evidence that suggest that carbon burial and oxygenation are linked to planetary evolution and that this in turn could have driven biospheric evolution. Not only is oxygenation an important component in the evolution of complex life but the recycling of biolimiting nutrients, especially phosphorus, is also important (Brasier and Lindsay, 1998). Without this driving mechanism we conjecture that the biosphere would enter a prolonged stasis and ultimately face extinction. Gerstell and Yung (2002) raise a number of interesting and provocative points concerning the future of the earth and its biosphere but we point out that the focus of our conclusions (Lindsay and Brasier, 2002) was not the future of life on earth as they imply, but the early history of life on earth or even on smaller planets such as Mars. A planet with the mass of the Earth has the ability to retain a significant atmosphere and, with it, a hydrosphere, together with enough endogenic energy resources to maintain an active plate tectonic regime. The earth’s plate tectonics regime has been sustained and prolonged by the role of water in lowering solidus temperatures (Hodder, 1986). Crustal evolution requires the formation of vast volumes of granite, which in turn is dependent upon the subduction of hydrated oceanic crust (Campbell and Taylor, 1983). Mars is considerably smaller than earth and maintains only a thin atmosphere and, early in its history, a limited hydrosphere. It has been argued that Mars is more moon-like than earth-like with high cratering densities on the older highland regions, while the northern lowlands with their lower density of impact craters could be regarded ! Corresponding author E-mail address: [email protected] (J.F. Lindsay). Precambrian Research 118 (2002) 293"/295