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 its unoxidised form. Thus, it has been reasoned that the BIF-red bed transition marks the rise of atmospheric oxygen. Complimentary information comes from detrital uraninite in Archean and earliest Proterozoic alluvial rocks.
Because this uranium mineral can survive prolonged transport only in media containing little or no oxygen, the lack of detrital uraninite deposits younger than 2300 ma also points towards a significant environmental transition (Roscoe,1969). 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). Towe (1990) has specifically argued for the development of aerobic respiration early in the Archean and, therefor, for the presence of 1 to 2% PAL (present atmospheric level) O2 in the atmosphere since that time.
The possibility that oxygen levels reached this physiologically important threshold so early is not contradicted by the sparse geochemical data available for early Archean rocks, Towe’s model however suffers from the absence of Archean O2 sinks other than Fe2+. Some believe that the neglect of volcanic gases in his model casts significant doubts on the validity of his analysis. Knoll(1979) believed increases in atmospheric oxygen were probably occasioned by increases in primary productivity and/or decreased rates of oxygen consumption.
He believes the increase from very low O2 levels to 1 to 2% PAL may have been related to productivity increases associated with rapid growth and stabilising of the continents during the late Archean and earliest Proterozoic, in contrast, Cameron(1983) stated that ‘the later increase to 15% PAL does not seem to be related to a major tectonic event. The high oxygen level in today’s atmosphere must be related to the role of PO2 in the maintaining of redox balance of the atmosphere-biosphere-ocean-lithosphere system’. The nature of the connection is still in dispute.
It is believed that atmospheric PO2 determines the concentration of O2 in surface ocean water, but the influence of the O2 concentration in seawater on the burial efficiency of organic matter within marine sediments seems to be slight. Nutrients are a more likely link between PO2 and the burial rate of organic matter, and hence between PO2 and rates of long term O2 generation(Betts and Holland,1991). Holland constructed a plausible argument that links the marine geochemistry of PO43- to that of iron and hence to the O2 content of the atmosphere today.
He believes that if this argument is true then the history of atmospheric O2 may have been controlled by a complicated feedback system involving the marine geochemistry of iron and phosphorus. This would explain the rapid increase in PO2 around 2100 mya as marking the passage of the system across 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.
