Carboniferous Giants and Mass Extinction

The Late Paleozoic Ice Age World

George McGhee Jr.

• Editorial: Columbia University Press
• Año: 2018
• ISBN:
  9780231543385
• Categoría: Scienze biologiche
• Categoría: Evoluzione

Picture a world of dog-sized scorpions and millipedes as long as a car; tropical rainforests with trees towering over 150 feet into the sky and a giant polar continent five times larger than Antarctica. That world was not imaginary; it was the earth more than 300 million years ago in the Carboniferous period of the Paleozoic era. In Carboniferous Giants and Mass Extinction, George R. McGhee Jr. explores that ancient world, explaining its origins; its downfall in the end-Permian mass extinction, the greatest biodiversity crisis to occur since the evolution of animal life on Earth; and how its legacies still affect us today.

McGhee investigates the consequences of the Late Paleozoic ice age in this comprehensive portrait of the effects of ancient climate change on global ecology. Carboniferous Giants and Mass Extinction examines the climatic conditions that allowed for the evolution of gigantic animals and the formation of the largest tropical rainforests ever to exist, which in time turned into the coal that made the industrial revolution possible—and fuels the engine of contemporary anthropogenic climate change. Exploring the strange and fascinating flora and fauna of the Late Paleozoic ice age world, McGhee focuses his analysis on the forces that brought this world to an abrupt and violent end. Synthesizing decades of research and new discoveries, this comprehensive book provides a wealth of insights into past and present extinction events and climate change.

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.

—Fielding, 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 partially—or almost entirely—froze 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 all—but 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 “snowball Earth.” 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 occurred—the 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 Venus—that 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 Earth’s 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 complex—and large—life-forms with aerobic metabolism, the GOE was good news indeed as these organisms need free oxygen. For the ancient anaerobic life-forms—the original inhabitants of Earth—the 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...