Sedimentary rocks are mostly formed under water. Earth’s early atmosphere and oceans contained little or no free oxygen. Iron dissolves readily in water that has no free oxygen.
Alternations of iron oxides and chert (silica) have been found in sedimentary rocks dated to more than 1800 mya. It is generally thought that these banded iron formations were formed in seawater when oxygen released by cyanobacteria combined with iron dissolved in Earth’s oceans and formed a thin layer on the substrate. Each band is similar to a varve to the extent that the banding is assumed to result from cyclic variations in available oxygen.
Evidence confirming the oxygenation of the oceans is found in red beds, which derive their colour from flecks of iron oxides that form within sands when the groundwaters that wash over them contain oxygen. Red beds are common only in sedimentary successions after about 2200 mya.
The oldest known eukaryote cell was discovered in an abandoned mine in Michigan in rocks dated at 1900 million years old. The fossils are corkscrew-shaped and closely resemble later algae. The Michigan corkscrews are huge, nearly a centimetre long. To survive, the eukaryotic cells required at least one percent of the atmosphere to be oxygen.
After 1800 mya the free oxygen in the oceans (the lives of fishes depend on it) had reached a level at which seawater could no longer hold dissolved iron; banded iron formations (BIFs) could no longer form and the concentration of oxygen in the atmosphere began to increase.
From about 1600 mya the advent of sexual reproduction led to a rapid increase in a diverse, perhaps unrelated group of spherical microfossils called acritarchs, probably the remains of single-celled organisms. The acritarchs declined at the end of the Devonian but made a weak recovery during the Jurassic and continued through the Cretaceous and Tertiary.
The sediments record a major glaciation between 720 and 635 mya. When conditions improved the ice melted and sea levels rose. Low coasts were flooded and new sediments accumulated. At the Ediacara Hills in South Australia, there are rocks that contain fossils of soft bodied organisms much more advanced than any that had occurred previously. Almost all the fossils are cnidarians (‘nettle like’: jellyfish, soft coral, stony coral, sea anemones). Sea pens, which look like plants but like corals eat floating animals, were attached to the seafloor. Some of the fossils are worms that moved across the seafloor, others are unlike any living animals.
In 1959 Martin Fritz Glaessner (1906-89) recognised that similar fossils from Australia, Namibia and England were part of a single, widespread biota that characterises the end of the Proterozoic, and he coined the term ‘Ediacara’ (also called ‘Vendian’). The majority of the fossils occur 635-542 mya, after that the Ediacara biota seem to have become extinct.
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