FIGURE 2.1 International Stratigraphic Chart showing the Precambrian. (From the International Commission on Stratigraphy; Courtesy F. Gradstein.)
dynamic processes of plate tectonics, new crust is created and old subducted, and there are few rocks still available that formed in the Paleoarchean. Currently the oldest known rocks are from the Acasta gneisses of the Great Slave Lake area in Canada (Northwest Territories), dated at 4.03-4.0 Ga (previously 3.96Ga; Bowring et al., 1989) based on uranium-lead isotopes (U-Pb) (Bowring and Williams, 1999) (FIG. 2.2). Slightly younger rocks are known from Isua and Akilia, West Greenland, including the oldest known sedimentary rocks, dated to ~3.82Ga (Manning et al., 2006). Although no older rocks have survived, several studies suggest that continental crust was present, based on the occurrence of detrital zircons, which were weathered out of older rock. The zircons have ages slightly older than 4.0 Ga (Iizuka et al., 2006) suggesting that weathering processes were also active at this time. Geochronology (dating) methods have improved significantly in recent years; however, the controversy over these very old rocks will remain, as they all have experienced a long and complex history of metamorphism (Kamber et al., 2001).
Although it was previously thought that the Earth formed over many hundreds of millions of years, the convergence of dates for the oldest continental crust with the age of the Earth itself suggests that continental crust began to form very soon after Earth accreted, perhaps within 30myr (Boyet and Carlson, 2005). What was the environment of Earth at that time? The composition of Earth's early atmosphere has long been a topic of debate. Stanley Miller, in his classic experiments in the 1950s (Miller, 1953), suggested that Earth's early atmosphere was a reducing one (high hydrogen content), composed of methane (CH4), ammonia (NH3), hydrogen (H2), and water (Miller, 1953). When a spark (e.g., from lightning) was introduced into this system in the laboratory, amino acids formed. The reducing environment hypothesis fell out of favor for many years, although it has resurfaced in a model that contains more CO2 than previous models (Tian et al., 2005) and seems to be the currently prevailing model. An important aspect of the presence of CO2 and especially methane in the early atmosphere is that they are both greenhouse gases. The early Sun is thought to have been ~30% less bright than today, an idea called the Faint Young Sun hypothesis; thus, some means of global warming was needed in the Archean for Earth to be hospitable to life (Kasting, 2005). Concentrations of greenhouse gases, especially methane, which is a much stronger greenhouse gas than CO2, would have been important to increase global temperature. These gases may have formed a haze-like atmosphere (Kasting, 2005), and it has been suggested that this haze may have protected early life from harmful ultraviolet rays before the ozone layer was formed. Oxygen is believed to have been only a very minor component of the atmosphere before ~2.4Ga. Most recent research suggests that the early Earth before ~3.2Ga was hot, perhaps as hot as 60-73°C (Lowe and Tice, 2007). Whatever the initial composition of the atmosphere on Earth, it must have affected the evolution of life and, in turn, was itself changed by the presence of organisms, as will be discussed in the following sections.
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