The Proterozoic: The Earth in Transition

Growth of Continents 

The Proterozoic Eon spans roughly 2 billion years, from about 2.5 Ga to the beginning of the Cambrian Period at 542 Ma thus, it encompasses almost half of Earth’s history. During Proterozoic time, Earth’s surface environment changed from being an unfamiliar world of fast-moving plates, small continents, and an oxygen-poor atmosphere, to the more familiar world of slower plates, large continents, and an oxygen-rich atmosphere. First, let’s look at changes to the continents. New continental crust continued to form during the Proterozoic Eon, but at progressively slower rates in fact, by the middle of the eon, over 90% of the Earth’s continental crust had formed. As Archean proto-continents collided with each other and with volcanic island arcs and hot-spot volcanoes, still larger continents gradually assembled. Significantly, the interiors of these larger continents became isolated from heating by subduction-related igneous activity that happened along its margins. Interior regions, therefore, slowly cool and strengthen until they become very rigid and durable. Such a region of cold, relatively stable continental crust is called a craton. All cratons that exist today had formed by about 1 Ga (figure above); therefore, crust in cratons (the old parts of continents) ranges from about 3.85 Ga to about 1 Ga. 

To understand the character of a craton, let’s examine North America’s craton a bit more closely. We see that it consists of two regions (figure above). Throughout the shield, outcrops expose Precambrian “basement,” which consists of igneous and metamorphic rocks older than about 1 Ga. The landscape of the shield tends to have fairly low relief there are small hills and valleys, but no dramatic mountain ranges. Most of North America’s shield lies in Canada, so geologists refer to it as the Canadian Shield. Throughout the cratonic platform, which surrounds the shield and also underlies Hudson Bay, a blanket of Paleozoic or Mesozoic cover strata overlies the Precambrian basement. 

By using isotopic dating on samples from both outcrops and drill holes, geologists have been able to subdivide the basement of North America’s craton into distinct blocks or provinces, each of which has been given a name (figure above). It appears that the Canadian Shield consists of several Archean crust blocks sutured together by Proterozoic orogens. The basement of the cratonic platform in the United States, in contrast, grew when a series of volcanic island arcs and continental slivers “accreted” (attached) to the margin of the Canadian Shield between  1.8 and 1.6 Ga. In the Midwest, granite plutons intruded much of this accreted region, and rhyolite ash flows covered it, between 1.5 and 1.3 Ga. Successive collisions ultimately brought together most continental crust on Earth into a single supercontinent, named Rodinia, by around 1 Ga. The last major collision during the formation of Rodinia produced a large orogen called the Grenville orogen. 

If you look at a popular (though not universally accepted) reconstruction of Rodinia, you can identify the crustal provinces that would eventually become the familiar continents of today (figure above a). Several studies suggest that sometime between 800 and 600 Ma, Rodinia “turned inside out,” in that Antarctica, India, and Australia broke away from western North America and swung around and collided with the future South America, possibly forming a short-lived supercontinent that some geologists refer to as Pannotia (figure above b). The map of the Earth clearly changed radically during the Proterozoic. But that’s not all that changed fossil evidence suggests that this eon also saw important steps in the evolution of life. When the Proterozoic began, most life was prokaryotic, meaning that it consisted of single-celled organisms (archaea and bacteria) without a nucleus. Though studies of chemical fossils hint that eukaryotic life, consisting of cells that have nuclei, originated as early as 2.7 Ga, the first possible body fossil of a eukaryotic organism occurs in 2.1-Ga rocks, and abundant body fossils of eukaryotic organisms can be found only in rocks younger than about 1.2 Ga. Thus the proliferation of eukaryotic life, the foundation from which complex organisms eventually evolved, took place during the Proterozoic. The last half-billion years of the Proterozoic Eon saw the remarkable transition from simple organisms into complex ones. Ciliate protozoans (single-celled organisms coated with fibers that give them mobility) appear at about 750 Ma. 

A great leap forward in complexity of organisms occurred during the next 150 million years of the eon, for sediments deposited perhaps as early as 620 Ma and certainly by 565 Ma contain several types of multicellular organisms that together constitute the Ediacaran fauna, named for a region in southern Australia where fossils of these organisms were first found. Ediacaran species survived into the beginning of the Cambrian before becoming extinct. Their fossil forms suggest that some of these invertebrate organisms resembled jellyfish, while others resembled worms (figure above a). The evolution of life played a key role in the evolution of Earth’s atmosphere. Before life appeared, there was hardly any free oxygen (O2) in the atmosphere. With the appearance of photosynthetic organisms, oxygen began to enter the  atmosphere. But it was not until about 2.4 Ga that the concentration of oxygen in the atmosphere increased dramatically. This event, called the great oxygenation event, happened when other environments were no longer able to absorb or dissolve the oxygen produced by organisms, so the oxygen began to accumulate as a gas in air. As a result, the oceans became oxidizing environments that, for reasons described in chemistry books, could no longer contain large quantities of dissolved iron. Between 2.4 Ga and 1.8 Ga, huge amounts of iron settled out of the ocean to form colorful sedimentary beds known as banded iron formation (BIF). BIF consists of alternating layers of iron oxide minerals (hematite or magnetite) and jasper (red chert) as illustrated by the chapter opening photo. Radical climate shifts occurred on Earth at the end of the Proterozoic Eon. Specifically, accumulations of glacial sediments occur worldwide in late Proterozoic stratigraphic sequences. What’s strange about the occurrence of these sediments is that they can be found even in regions that were located at the equator during these times (figure above b). This observation implies that the entire planet was cold enough for glaciers to form at the end of the Proterozoic. Geologists still are debating the history of these global ice ages; in one model, glaciers covered the land and perhaps the entire ocean surface froze, resulting in snowball Earth (figure above c). The shell of ice cut off the oceans from the atmosphere, causing oxygen levels in the sea to drop drastically, so many life forms died off. What brought an end to snowball Earth conditions? According to one model, the icy sheath also prevented atmospheric CO2 from dissolving in seawater, but it did not prevent volcanic activity from continuing to add CO2 to the atmosphere. Earth would have remained a snowball forever, were it not for the addition of volcanic CO2, a greenhouse gas that traps heat in the atmosphere much as glass panes trap heat in a greenhouse. As the CO2 concentration increased, Earth warmed up and eventually the glaciers melted. 

Introducing the Phanerozoic Eon 

As the Proterozoic came to a close, Earth’s climate warmed and continents drifted apart life evolved and diversified to occupy the new environments that formed. Over a relatively short period of time, shells appeared and the fossil record became much more complete. This event defines the end of the Proterozoic Eon and therefore, of the Precambrian, and the start of the Phanerozoic Eon. Of note, geologists recognized the siginificance of this event long before they could assign it a numerical age (currently 542 Ma). The Phanerozoic Eon consists of three eras the Paleozoic (Greek for ancient life), the Mesozoic (middle life), and the Cenozoic (recent life). Geologists have divided the Mesozoic and Cenozoic each into three periods and the Paleozoic into six periods. In the sections that follow, we consider changes in the map of our planet’s surface (its paleogeography), as manifested by the distribution of continents, seas, and mountain belts, as well as life evolution that happened during the three eras.
Credits: Stephen Marshak (Essentials of Geology)

1 comment:

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