III.

The Proterozoic Eon

During the Proterozoic Eon, Earth’s cooling rate probably decreased, and the mechanics of plate tectonics began to slow down and operate much as they do today. Large mountain chains formed when continents collided. The geologic record shows that thick successions of quartz-rich sandstones, shales, and limestones were, for the first time, widely deposited on the continents. Researchers also hypothesize that during the Proterozoic Eon, continents broke up and re-formed several times.

A.

Rocks and Mineral Deposits

A very important mineral deposit from the early Proterozoic Eon is the red and gray rock called banded iron formation. The bands in this rock occur in alternating layers: The gray bands are fossil-bearing chert (silica) and the red bands are hematite (iron oxide). These rocks indicate that no free oxygen was present in the atmosphere when they were formed. Researchers hypothesize that soluble iron, weathered from rocks, was transported across continents and deposited in shallow seas where it reacted with oxygen and formed hematite (nonsoluble iron), which settled on the ocean floor.

Economic minerals formed during the Proterozoic Eon include chromium found in the Bushveld Complex (a very large area of Proterozoic igneous rock) in South Africa and lead, uranium, and gold from ancient island arc (chains of islands) deposits, which are associated with subduction zones.

B.

Oxygen in the Atmosphere

Beginning about 1.7 billion years ago, banded iron formations disappeared from the geologic record and red beds (red-colored sediments) began to appear. Red beds are sedimentary rocks made of iron-bearing sand and mud eroded from rock deposited on land. The rock is red because it contains iron that has been oxidized (rusted). Its presence on land means that the eroding rock was exposed to free oxygen in the atmosphere. Scientists believe that this oxygen was produced during photosynthesis of the increasingly abundant microorganisms in the sea. Researchers have also found fossil soils that contain oxidized iron in their upper layers, another indicator of higher levels of oxygen in the atmosphere.

While oxygen increased in the atmosphere, carbon dioxide decreased, as more photosynthesis took place. This is shown in the geologic record by the presence of limestone, which is made of calcium carbonate, one of the by-products of photosynthesis. From evidence found in some Precambrian rocks, it is fairly certain that several periods of widespread glaciation occurred on the continents. Such evidence indicates that the atmosphere was cooling during the Proterozoic Eon.

C.

Proterozoic Life

Eukaryotes first appeared during the Proterozoic Eon. Eukaryotes are more advanced than prokaryotes because they have a nucleus enclosed in a nuclear membrane. They also have organelles, specialized organs for functions such as respiration, photosynthesis, and food storage. Most important, eukaryotes are able to reproduce sexually, which means that early eukaryotic organisms were able to achieve genetic diversity and adapt to and survive environmental changes. Eukaryotes are found in the form of advanced algae and bacteria from about 2 billion years before present. By 1.7 billion years before present, eukaryotes were widespread, as indicated by the microfossil record in Proterozoic cherts. By the middle Proterozoic Eon, stromatolites deposited by cyanobacteria occurred in greater numbers than at any other time. By 1 billion years before present, cyanobacteria and a great variety of protozoa (single-celled animals) were the dominant organisms on the Earth.

Between 1 billion and 800 million years ago, the oxygen levels in the oceans and the atmosphere were high enough for multicellular organisms to evolve. Because these organisms were soft-bodied, they were not well preserved as fossils; they are found only as molds or casts in rocks. The first Proterozoic fossils of multicellular organisms discovered by scientists were the Ediacaran fauna, originally found in shallow-water marine sediments in the Ediacara Hills of southern Australia. Since this discovery, these fossils have been found on all continents, indicating that they were quite widespread during the Proterozoic Eon.