Banded Iron Formations and Evolution of the Atmosphere

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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 sediment type formed until approximately 1800 mya. Although some younger formations with similar structure can be found there is great distinctions between them and the BIF’s of the Precambrian.
The BIF’s can occur in sequences which range from 15 ferric iron.
Some people believe the iron to be of volcanic origin, weathered and transported into the oceans or e...

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...oss a threshold from one steady state to another.
Conclusion
The complexities and conflicting arguments involved in relation to the evolution of the atmosphere and its links to banded iron formation are hard to over emphasise. It is a topic which still has a long way to go and one which may never be conclusively understood due to the lack of evidence from the Precambrian rocks (or lack of rock record altogether).
However the general consensus seems to be that for a period of over a 1000 my until 1800 mya, conditions where favourable for chemical deposition of iron, there was distinct changes occurring in the atmosphere with rapid increase in free O2 coupled with a fundamental change in the evolution of early life from anaerobic bacteria to aerobic cyanobacteria.
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 question arises, which came first the oxygen producing bacteria or the oxygen causing life sensitive to it to adapt?

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