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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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