Let’s now turn our attention to earlier times, which are less well understood, but which constitute almost 90 percent of the history of the planet. The entire time span between the Earths formation at 4.6 Ga and the dawn of the Cambrian is termed (not surprisingly) the Precambrian climate eon. Before it was known to have harbored life, the Precambrian climate was treated as a single time unit. Since then, it has been subdivided into three eras: the Hadcan (4.5-3.8 Ga), the Archcan (3.8-2.5 Ga), and the Proterozoic (2.5-0.54 Ga). The Precambrian climate or Precambrian, like the Phanerozoic, appears to have been mostly warm. Indeed, if one takes the evidence at face value, the Earth has been ice-free for over 90 percent of its history. This is especially remarkable in light of the dimmer young Sun. All other things being equal, one might expect that the early Earth should have been cold.
. The geologic record becomes increasingly sparse as one goes further back in time. The observation that the Mid-Proterozoic Era was ice-free between about 2.2 and 0.8 Ga is probably statistically meaningful, as there are numerous exposed rocks of this age, so we should expect to see evidence for glaciation had it occurred. The earlier Archean era has fewer well-preserved rock outcrops, however, and so the absence of glacial deposits during most of this time could be purely an artifact.. The Precambrian climate eon does contain evidence for several glaciations. The best documented ones occurred during the Palcoprotcrozoic, around 2.4 Ga, and during the Neoproterozoic, around 0.6-0.75 Ga. The glaciation at 2.9 Ga is labeled with a question mark because it has been less thoroughly studied than the ones that came later. Firm evidence for glaciation is found on only one continent, Africa,’’ so it could have been a localized occurrence. By contrast, both the Paleoproterozoic and Neoproterozoic gla-ciations occurred on multiple continents at close to the same times.
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