Smithsonian National Museum of Natural History Geologic Time The Story of a Changing Earth
Presented by the Department of Paleobiology.
The Proterozoic
Eon Overview
Earth's Crust as a Platform for Prokaryotic Life
Eukaryotes and the First Multicellular Life Forms
Changes in the Earth's Atmosphere
Proterozoic Mountains and Glaciers
Rangea from the Ediacaran Fauna, Namibia.
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First eukaryotes
Ediacaran Fauna 1
Ediacaran Fauna 2
Ediacaran Fauna 3
Ediacaran Fauna 4
Banded Iron Formations
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Foundational Concepts
Dating Methods
Earth Processes
Life Processes
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Changes in the Atmosphere
Earth’s early atmosphere contained only small amounts of free oxygen, probably produced entirely by the reaction of sunlight with water vapor from volcanoes. The oxygen-rich atmosphere that evolved later, and upon which oxygen-breathing life now depends, was a result of the origin of photosynthesis. During the Precambrian, vast numbers of single-celled algae and cyanobacteria living in the seas eventually released enough oxygen to transform the environment. The oldest evidence of cyanobacteria dates to 2.7 billion years ago, although oxygen did not begin to build up in the environment until about 2.3 billion years ago. During the transition from oxygen-poor to oxygen-rich atmosphere, the first banded iron formations may have formed.

Banded iron formations are silica-rich rocks that show alternating thin layers of dark and red iron-rich rock. They are the most economically important deposits of iron ore. The silica probably was dissolved from volcanic ash and rock, and the iron came from sea floor vents or the weathering of iron-rich volcanic rocks. In the absence of free oxygen, iron dissolves in water. This must have occurred throughout the Archean, resulting in ocean waters that contained a great deal of dissolved iron. In the Proterozoic, however, the dissolved iron bonded with oxygen released into ocean water by photosynthesizing cyanobacteria to form magnetite (Fe3O4). This magnetite was then deposited on the ocean floor. The alternating layers in banded iron formations are thought to reflect the alternation of oxygen-rich and oxygen-poor conditions on the sea floor.

A vast amount of iron dissolved in the oceans was available to react chemically with oxygen, which kept oxygen from accumulating in the ocean and atmosphere. Once all of the dissolved iron was used up, the oxygen released by photosynthetic organisms could escape directly into the atmosphere. As gaseous oxygen built up, the atmosphere began to change from one that was chemically reducing to one that was oxidizing (i.e., rust-forming), like today’s. Iron weathered from basaltic volcanoes was oxidized on land before it reached the oceans. This resulted in the formation of red beds. The red color of these rocks comes from the particular variety of iron mineral precipitated on land, mostly hematite (Fe2O3).

Thus, the history of Earth’s early crust also tells the story of its early atmosphere. Banded iron formations were precipitated from about 3.1 to about 2 billion years ago—most (92%) during the Proterozoic between 2.5 and 2 billion years ago. Until all the available iron had been deposited in banded iron formations, oxygen could not build up in the atmosphere. Red beds appeared only after free oxygen was released into the atmosphere, beginning about 2.0 to 1.8 billion years ago. They are still being formed today.

Eon Overview | Earth's Crust as a Platform for Prokaryotic Life | Eukaryotes and the First Multicellular Life Forms | Changes in the Atmosphere | Proterozoic Mountains and Glaciers

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