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The Precambrian's Second Half: Proterozoic Time


Proterozoic Gneiss

Gneiss of mid-Proterozoic age

Andrew Alden photo

The second half of Precambrian time is marked by a slow evolution of the Earth toward its modern appearance. But this period had its own precarious and bizarre events, just as did the Archean and Hadean eons before it.

The Proterozoic Eon (2500–542 Ma)

The Proterozoic is Earth's longest chapter. Early during this time, traces of modern plate tectonics appear in the rocks. In the late Proterozoic, the first proper animals and plants arose. In between occurred several crises that involved the whole world.

The anaerobic Earth of the earlier Archean Eon was not sustainable, as cyanobacteria gradually poisoned the environment with their waste product of oxygen. Dissolved iron in the sea ran out, and subduction of the crust in plate tectonics brought oxidized material into the mantle, slowly changing its chemistry. Finally free oxygen gained a persistent presence in the air and water, threatening all anaerobic life. This event, called the Great Oxygen Event and similar names, appears to have reached its climax about 2450 Ma. However, there is a lively debate about the details and timing. It appears that Earth teetered on the edge of an oxygen catastrophe for many millions of years, with small amounts of oxygen in different parts of the air and ocean.

New, more complex organisms arose that could live in the oxidizing environment—the eukaryotes. These later gave rise to the animals and the plants, and the ancient anaerobes retreated. Today they thrive underground, still constituting the majority of Earth's biomass. With that, the elements of a balanced biogeochemical cycle were in place.

Carbon-containing gases were pulled from the air, cooling the planet, and in late Proterozoic time the world's first ice ages occurred. These appear to have covered the whole planet deep in ice, only to be melted as volcanic activity sent new carbon dioxide aloft. There appear to have been several of these "Snowball Earth" episodes in the later Proterozoic, during the Cryogenian Period.

The plate-tectonic motions of the continents are poorly known during the Proterozoic, but around 1000 Ma a supercontinent formed that is given the name Rodinia. It broke up starting around 750 Ma, and as continental fragments reached the north and south poles they probably contributed to the great ice ages. In the latest Proterozoic a new supercontinent called Pannotia came together.

Also around this time, multicelled animals evolved for the first time. Because none of them had hard parts, their fossils are very rare. A well-preserved fossil site in the Ediacara Hills of Australia has given its name to the Ediacaran Period, right at the end of the Proterozoic.

The Precambrian's End

What ended the Precambrian was a new wrinkle in the carbon cycle whose origin is still under discussion. Whether from evolutionary progress or from changes in geochemistry, animals gained the ability to build hard body parts of carbonate minerals.

At this point, at the start of the Cambrian Period, Earth had taken on its current form—in the life-filled oceans and oxygenated atmosphere. Coevolution of biosphere and lithosphere, over billions of years, led to that point. Anaerobes and oxygen-breathers had evolved complementary chemical cycles, and biogenic carbonates entered the plate-tectonic cycle of the crust and upper mantle with new efficiency.

Soon two more developments launched the Phanerozoic into the rapid pace of change it has had ever since the close of the Proterozoic: the development of burrowing animals that cultivated the seafloor (giving rise to bioturbation) and the invasion of the dry land. With the new dynamism in Earth's evolutionary and biogeochemical processes, there was no looking back.

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