1 | Harbingers of the Late Paleozoic Ice Age |
The late Paleozoic ice age lasted for ~67 m.y. [million years] in eastern Australia, and as such, it was the longest-lived icehouse interval in the Phanerozoic.
âFielding, Frank, Birgenheier, et al. (2008, 55)
ICE AGES IN EARTH HISTORY
We know that at least seven ice ages have occurred in the past 4,560 million years, time periods during which the Earth partiallyâor almost entirelyâfroze over. The history of the Earth is divided into four eons: the Hadean, Archaean, Proterozoic, and Phanerozoic, from oldest to youngest (for the geologic timescale, see table 1.1). These eons are themselves divided into smaller time units, the eras, and they are divided into still smaller time units, the periods (table 1.1), much as our calendar years are divided into months, weeks, and days. We know from fossil evidence that ancient bacterial life was present on the Earth 3,450 million years ago, during the Paleoarchaean Era, and we have geochemical evidence that indicates that life was present even earlier during the Eoarchaean Era, some 3,830 million years ago (McGhee 2013; Nutman et al. 2016). The first four of the known ice ages occurred during the Proterozoic Eon, much later in time than the appearance of life on Earth, and three more ice ages occurred even later during the Phanerozoic Eon; thus life on Earth has successfully survived them allâbut at a cost. Many of the ice ages are associated with periods of extinction and large losses of biological diversity, a topic that will be explored in more detail in the next section of this chapter.
TABLE 1.1 The geologic timescale and ice ages.
Eon | Era | Period | Time of Onset (Ma) | Ice Ages (ICE) |
Phanerozoic | Cenozoic | Quaternary | 2.59 | ICE |
| | Neogene | 23.03 | ICE |
| | Paleogene | 66.0 | ICE |
| Mesozoic | Cretaceous | 145.0 | |
| | Jurassic | 201.3 | |
| | Triassic | 252.2 | |
| Paleozoic | Permian | 298.9 | ICE |
| | Carboniferous | 358.9 | ICE |
| | Devonian | 419.2 | ICE |
| | Silurian | 443.8 | |
| | Ordovician | 485.4 | ICE |
| | Cambrian | 541 | |
Proterozoic | Neoproterozoic | Ediacaran | 635 | ICE |
| | Cryogenian | 850 | ICE |
| | Tonian | 1,000 | |
| Mesoproterozoic | Stenian | 1,200 | |
| | Ectasian | 1,400 | |
| | Calymmian | 1,600 | |
| Paleoproterozoic | Statherian | 1,800 | |
| | Orosirian | 2,050 | |
| | Rhyacian | 2,300 | ICE |
| | Siderian | 2,500 | |
Archaean | Neoarchaean | | 2,800 | |
| Mesoarchaean | | 3,200 | |
| Paleoarchaean | | 3,600 | |
| Eoarchaean | | 4,000 | |
Hadean | | | 4,560 | |
Source: Timescale modified from Gradstein et al. (2012).
Note: Ma = millions of years before the present for the start of each time unit listed.
A major difference between the four Proterozoic and three Phanerozoic glacial episodes is reflected in their formal names, given in table 1.2. The four Proterozoic episodes are called snowball Earths, whereas the three Phanerozoic episodes simply are called ice ages. The snowball-Earth glaciations were much more geographically extensive than any seen in the Phanerozoic in that continental-covering, sea-level-reaching ice sheets extended from the poles of the planet all the way down to the equator. From space, the entire planet may have looked like one giant snowball, hence the name âsnowball Earth.â The first known snowball Earth, the Huronian, occurred some 2,300 million years ago during the Rhyacian Period (tables 1.1 and 1.2). Surprisingly, it is now thought that life may have triggered not only this massive freezing of the Earth during the Rhyacian but also the first mass extinction in Earth history. How could this be?
TABLE 1.2 Ice ages in Earth history.
Ice Age | Position in Geologic Time (table 1.1) |
A. Phanerozoic ice ages: |
1. Cenozoic Ice Age | Late Paleogene to Recent |
2. Late Paleozoic Ice Age | Late Devonian to Late Permian |
3. End-Ordovician Ice Age | Late Ordovician |
B. Proterozoic ice ages: |
1. Gaskiers Snowball Earth | Ediacaran |
2. Marinoan Snowball Earth | Cryogenian |
3. Sturtian Snowball Earth | Cryogenian |
4. Huronian Snowball Earth | Rhyacian |
About 200 million years earlier than the Huronian freezing, at the beginning of the Siderian Period of the Paleoproterozoic Era (table 1.1), an event of major importance in the evolution of life on Earth occurredâthe Great Oxygenation Event, or GOE for short. The very atmosphere of the Earth has been radically transformed by the presence of life. The original atmosphere of the Earth was probably very similar to that of its sister rocky volcanic planets Mars and Venusâthat is, composed mostly of carbon dioxide. The atmosphere of Mars today is 95 percent carbon dioxide and the atmosphere of Venus is 97 percent, whereas the atmosphere of the pre-industrial-age Earth was only 0.03 percent carbon dioxide.1 Both anaerobic and aerobic photosynthesizing bacteria2 actively remove carbon dioxide from the atmosphere and use the carbon to form complex hydrocarbons for food. Thus, on Earth, life has been removing carbon dioxide from the atmosphere for the last 3,830 million years, contributing to the transformation of the Earthâs atmosphere to its present carbon-dioxide-depleted state.
The aerobic photosynthesizing cyanobacteria not only remove carbon dioxide from the atmosphere but also add oxygen to the atmosphere.3 About 2,500 million years ago, the oxygen-producing activity of the ancient cyanobacteria was finally to have its first major impact on the atmosphere of the Earth: it triggered the GOE. For the future evolution of complexâand largeâlife-forms with aerobic metabolism, the GOE was good news indeed as these organisms need free oxygen. For the ancient anaerobic life-formsâthe original inhabitants of Earthâthe GOE was a disaster because oxygen is a poison to them. The first mass extinction in the history of life on Earth probably occurred 2,500 million years ago, when vast unknown numbers of species of anaerobic bacteria a...