Environmental Evolution of the Archean-early Proterozoic Earth

Abstract

Although, with regard to most parameters, the rock record shows little evidence of an environment on the early Earth vastly different from that of today, there are several biologically important aspects of the environment that are likely to have changed. These include: (1) The length of the day. Throughout the Phanerozoic, day-length is known to have gradually increased as a result of dissipation of tidal energy; in the Precambrian the length of the day was probably quite different from that of the present. (2) The average surface temperature of the planet and the precipitation and weather that it controls. These are likely to have changed as a result of changing solar luminosity and atmospheric composition. (3) The partial pressure of carbon dioxide in the atmosphere. As a result of man's activities, this parameter is changing today; it seems quite likely to have changed in the past in response to biological and geological evolution. (4) The partial pressure of oxygen in the atmosphere. At present, the level of atmospheric oxygen is sustained by oxygen-producing photosynthetic organisms; prior to their origin, the oxygen content of the atmosphere should have been much lower. (5) The availability to organisms of important nutrient elements, including biologically useable nitrogen and dissolved sulfur species. The solubility of many such nutrients depends on their oxidation state; their availability may thus have varied with the partial pressure of atmospheric oxygen. Nitrogen in a biologically useable form is chiefly produced today either via lightning-mediated oxidation reactions in the atmosphere or by nitrogen-fixing microorganisms; it thus may have been in short supply prior to the origin of nitrogen-fixing forms of life. This chapter discusses the changes that may have occurred in these biologically important aspects of the environment. To the extent possible the discussion is based on geological evidence, but because this evidence is commonly indirect and incomplete, it must be interpreted and extrapolated with the help of theory. Much of the theory that we shall invoke is either a gross simplification of the real world, or extremely speculative, or both. It is not yet possible to reach many firm conclusions concerning the environmental evolution of the Archean-Early Proterozoic Earth. Our tentative conclusions should, therefore, be accepted with caution (and with regard to the history of some biologically important environmental parameters, not even tentative conclusions have yet been reached).

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