The time between the formation of the earth and the beginning of the Cambrian(about 570mya) is a 4000 my long period known as the Precambrian, this includes approximately 90% of geological time of which we know very little about as pre-Cambrian rocks are poorly exposed, many have been eroded or metamorphosed and fossils are seldom found.

The Precambrian has been divided into 3 Eons: 1.Hadean (4600-3800 mya of which there is no rock record) 2.Archean 3800-2500 mya) 3.Proterozoic 2500-570 mya.

The present atmosphere is greatly depleted in Ne, Xe and Kr which are inert gases that should be preserved in the atmosphere. This suggests that the earth's initial atmosphere was lost early on either by boiling away during the magma ocean event or by being carried away by intense solar wind in the early solar system. At the end of the Hadean the present atmosphere and hydrosphere began to develop from volcanic emissions. It was during the proterozoic that a critical change occurred in the atmosphere, when it changed from a trace oxygen content of the Archean atmosphere to above 15% oxygen by 1800 mya.

It is widely believed that this change was brought about by the emergence of cyanob

The BIF's can occur in sequences which range from <1 m to 200m thick and a few meters to many kilometres in length. A general and widely accepted definition of BIF is a iron rich chemical sediment with interbanded layers of iron rich silica and less iron rich silica, with an overall percentage of >15 ferric iron.

Not all scientists have accepted the validity of these observations or of their interpretation, Dimroth and Kimberley(1976) argued that at least some red beds antedate the end of BIF deposition, that Archean granites have paleoweathering profiles indicative of oxic environments, and that oxidised sulphur minerals (sulphates) occur in some of the oldest known sedimentary successions. All of these observations are correct, and we must ask whether they preclude the interpretation of Archean and earliest Proterozoic environments as oxygen poor. Holland (1984) believed the answer to be no. As the formation of red beds and oxidised weathering profiles on granitic substrates requires oxygen, but only in minute quantities- considerably less than is needed for aerobic metabolism. Also, marine sulphate does not require free oxygen as all as H2S can be photooxidized anaerobically to SO42- by photosynthetic bacteria, while the photochemical oxidation of volcanogenic S and SO2 to sulphate was probably a steady source of oxidised sulphur in the Archean oceans(Walker, 1983).

acteria which had adapted to create energy from the sun by photosynthesis(probably due to a shortage of raw materials for energy), as a result they had began to poison the earlier anaerobic bacteria or archea with their waste product; oxygen.

The origin sequence and connection of these events are the issues that are currently in debate. Most are in agreement that they are all linked and that O2 levels are a key factor in the formation of banded iron formation, the problems arise when trying to distinguish when and how O2 began to be produced, the q

Some common words found in the essay are:
Holland1991 Holland, Dimroth Kimberley1976, Archean O2, Formations BIF's, Iron Formations, Xe Kr, Precambrian BIF's, banded iron, iron formations, banded iron formations, 1800 mya, Formations Cloud, Banded Iron, archean earliest, earliest proterozoic, evolution atmosphere, archean earliest proterozoic, red beds, banded iron formation, oxygen levels, rock record, organic matter, 1 2% pal,
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