Banded Iron Formations and the

Banded Iron Formations and evolution of the atmosphere
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 cyanobacteria 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.
This essay will focus on the evolution of the atmosphere and its relation to the banded iron formations of the late Precambrian.
Banded Iron Formations
Cloud (1968) calls Banded Iron Formations, rhythmically banded chemical sediments of large, open water bodies that take different aspects but most characteristically consists of alternating layers of iron- rich and iron-poor silica. It is present in some of the oldest volcanic sequences (greenstone belts ca 2800 mya) and is a common... [continues]

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