Precambrian

C

The Oceans and Atmosphere

The oceans and the atmosphere were most likely created by the process called volcanic outgassing, the release of immense quantities of gases during volcanic eruption. These gases, held around Earth by gravity, would have formed the first atmosphere. Scientists believe that carbon dioxide and water vapor were two of the major components of this early atmosphere. Because carbon dioxide absorbs heat, preventing much of it from escaping from the atmosphere to space, the Archean atmosphere was probably warmer than the present atmosphere. As the planet cooled, water vapor condensed, fell to the surface, and formed the first oceans.

D

Origin of Life

Life appears to have formed in the oceans about four billion years ago, probably from chemical reactions of molecules such as water, carbon dioxide, and methane. The first life forms for which there is fossil evidence are prokaryotes, one-celled organisms. These organisms lacked a nucleus and reproduced only by cell division. Today’s bacteria and cyanobacteria (often called blue-green algae) are descendants of these Archean ancestors.

Researchers believe that the earliest bacteria must have obtained food (energy) either from ingesting organic molecules or from chemosynthesis, a chemical reaction involving chemicals such as hydrogen sulfide. Not until about 3.6 billion years ago did cells evolve that could produce their own food by photosynthesis. The earliest evidence of photosynthetic organisms are stromatolites, which are domed and layered sedimentary structures formed by mats of filamentous one-celled cyanobacteria and trapped sediments.

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.

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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.