Smithsonian National Museum of Natural History Geologic Time The Story of a Changing Earth
Presented by the Department of Paleobiology.
The Archean
Eon Overview
Early Continents and Oceans
The First Life on Earth
Changes in the Atmosphere
Acasta gneiss, one of the oldest known rocks on Earth.
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Banded Iron Formation
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Foundational Concepts
Dating Methods
Earth Processes
Life Processes
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Changes in the Atmosphere
Stromatolites and other microfossils provide important evidence for the transformation of the oxygen-poor atmosphere of the Archean, into one that was oxygen-rich in the Proterozoic, like the atmosphere today. Some microorganisms that build modern stromatolites are capable of photosynthesis and release free oxygen into the ocean. Chemical traces of microorganisms, known as biomarkers, show that photosynthetic organisms had evolved by 2.7 billion years ago, but they were probably not present during the early Archean.

During the Archean and early Proterozoic Eons, the deep oceans had large volumes of dissolved iron. This combined with oxygen, possibly produced by photosynthetic microorganisms, to produce iron oxides along the continental margins. This “rust” was concentrated in sedimentary deposits known as banded iron formations. These rocks are mined worldwide for iron ore. Once all the dissolved iron in ocean waters had precipitated out as iron oxide, oxygen from photosynthetic microorganisms was free to escape to the atmosphere. The first oxygen-rich atmosphere developed in the Proterozoic Eon.

Eon Overview | Early Continents and Oceans | The First Life on Earth
Changes in the Atmosphere

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