FERMOR LECTURE: THE INFLUENCE OF LIFE ON THE FACE OF THE EARTH: GARNETS AND MOVING CONTINENTS

Abstract

The Hadean Earth (before c. 4 Ga) was abiotic, possibly steering a bumpy course between brief periods of hot inferno after meteorite impacts, and long episodes of Norse icehell. The earliest Archaean life would probably not have been planet-altering, but restricted to particular habitats. One of the first may have been hot regions around hydrothermal systems where redox contrasts between ocean water and magmatic fluids could be exploited. Molecular evidence suggests that with the evolution of anoxygenic photosynthesis, life became able to occupy wider regions, although focused in the vicinity of hydrothermal systems. Oxygenic photosynthesis by cyanobacteria allowed life fully to occupy the planet, not only forming coastal microbial mats but also possibly inhabiting the broad oceans with abundant photosynthetic bacterial picoplankton, underlain by deeper archaeal picoplankton. In the Belingwe belt, Zimbabwe, textural and isotopic evidence suggests that a complex microbial ecology existed in the late Archaean (2.7 Ga), which was essentially modern in its biochemical abilities and which sequestered into the biosphere the same fraction of primitive carbon emitted from mantle as today. To do this, by the late Archaean the biological productivity must have been significant; not necessarily as large as today, but capable of managing the global carbon budget. When this began is unknown, possibly earlier than 3.5 Ga ago. The controls on the oxidation state of the late Archaean atmosphere-ocean system are not self-evident. Although inorganic controls dominate the long-term balance, short-term biological management of the air may have been crucial. Methane may have played a major role in the pre-metazoan biosphere. The modern atmosphere is a biological construct: oxygen and its reverse, carbon dioxide, are managed by rubisco; nitrogen, its oxides and hydrides mainly by nitrifying and denitrifying bacteria, with a small input from lightning in an oxygen-rich atmosphere; and water (itself the most important greenhouse gas) by its complex interdependence with other greenhouse gases and albedo, including clouds. Earth's air is highly improbable. In controlling surface temperature a subtle interplay between organic and inorganic controls has operated, perhaps to the extent that it is invalid to ask which was the dominant factor. But there is a reasonable uniformitarian argument that life has constructed the air in the past as now, and that, within the broad constraints of the physical setting, this biologically shaped atmosphere has been the dominant control on the planet's surface temperature. In turn, the surface temperature has been one of the various controlling factors on the tectonic evolution of the planet. Thus to a significant extent life has helped shape the physical evolution of the planet.