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29-Sep-2019 04:30

) are easily destroyed in an oxidizing atmosphere; confirmation of a reducing atmosphere is provided by unoxidized grains of these minerals in 3.0-billion-year-old sediments.However, the presence of many types of filamentous microfossils dated to 3.45 billion years ago in the cherts of the Pilbara region suggests that photosynthesis had begun to release oxygen into the atmosphere by that time.The stability of liquid water (that is, its continuous presence on Earth) implies that surface seawater temperatures were similar to those of the present.Differences in the chemical composition of Archean and Proterozoic sedimentary rocks point to two different mechanisms for controlling seawater composition between the two Precambrian eons.The presence of fossil molecules in the cell walls of 2.5-billion year-old blue-green algae (cyanobacteria) establishes the existence of rare oxygen-producing organisms by that period.Oceans of the Archean Eon (4.0 to 2.5 billion years ago) contained much volcanic-derived ferrous iron (Fe) in BIFs.

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A major burst in the deposition of BIFs from 3.1 billion to 2.5 billion years ago—peaking about 2.7 billion years ago—cleared the oceans of ferrous iron.It was denoted by the first appearance of animal life (metazoans) requiring sufficient oxygen for the production of collagen and the subsequent formation of skeletons.Furthermore, in the stratosphere during the Precambrian, free oxygen began to form a layer of ozone (O The origin of Earth’s oceans occurred earlier than that of the oldest sedimentary rocks.Although many varied forms of life evolved and were preserved extensively as fossil remains in Cambrian sedimentary rocks, detailed mapping and examination of Precambrian rocks on most continents have revealed that additional primitive life-forms existed approximately 3.5 billion years ago.

Nevertheless, the original terminology to distinguish Precambrian rocks from all younger rocks is still used for subdividing geologic time.Precambrian time, period of time extending from about 4.6 billion years ago (the point at which Earth began to form) to the beginning of the Cambrian Period, 541 million years ago.Precambrian time encompasses the Archean and Proterozoic eons, which are formal geologic intervals that lasted from 4 billion to about 541 million years ago, and the Hadean Eon, which is an informal interval spanning from 4.6 billion to 4 billion years ago.Today, phosphate sediments are deposited primarily along the western side of continents.



A sedimentary rock is defined and what sedimentary rock formations look like. The formation of the sedimentary rocks sandstone, limestone, chalk, shale, mudstone.… continue reading »


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Precambrian time Precambrian time, period of time extending from about 4.6 billion years ago the point at which Earth began to form to the beginning of.… continue reading »


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Evidence for catastrophism is very widespread in the fossil record. Find out who are the 4 major fossils types.… continue reading »


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