Banded iron formations and the
Banded iron formations and theBanded 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 exhaled from fumeroles
Relation to the atmosphere
The link between Banded Iron Formations (or BIF’s) and this change in oxygen levels is a close one, as BIF’s appeared about 2600 mya and continued until about 1800 mya. These deposits of marine hematite and quartz represent a precipitation of dissolved iron from sea water as the dissolved oxygen content of the water increased. After 1800 mya, Banded iron formations are rare, but terrestrial red beds are common for the first time suggesting that iron is being oxidised and precipitated in soils and rocks on land in the source area of sediments instead of being dissolved and carried into the oceans in...
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