Chemical evolution of the proterozoic biosphere

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The surface of the present day Earth is characterised by high levels of oxygen. This is vital to sustain many forms of life on Earth. By contrast, when life first evolved the atmosphere and oceans contained essentially no oxygen. Under these conditions, only certain types of bacteria were able to survive. Various lines of evidence suggest that the oxygen content of the atmosphere only began to rise about 2.3 billion years ago, but at this time the atmosphere still contained much lower amounts of oxygen than at present. Until recently it was thought that this also led to oxygenation of the ocean (as in the present day), which is the environment where early life formed. However, higher life forms such as algae (and ultimately humans), only began to evolve much later. Recently it has been proposed that the oceans did not become oxygenated after the initial rise in atmospheric oxygen. Instead, the increase in oxygen led to the weathering of sulfide minerals on the land, which resulted in increased riverine delivery of sulfur to the oceans. The oceans then became rich in hydrogen sulfide rather than oxygen (similar conditions are found in the modem day Black Sea). Hydrogen sulfide is highly toxic to many life forms, and thus the development of an ocean rich in hydrogen sulfide helps to explain the much later evolution of higher life forms. In fact, the ocean may only have became oxygenated following a second, much later rise in oxygen. This second rise in oxygen broadly coincides with an 'explosion' of life on Earth, and thus indicates a dose link between oxygenation and biological evolution. However, the idea that the oceans contained hydrogen sulfide for a long period of Earth's early history is highly controversial, and further studies are required to test whether such conditions did in fact exist. It is also important to determine how widespread these conditions were (i.e. was the entire ocean rich in hydrogen sulfide), and to determine the precise time when the ocean eventually became oxygenated. Fortunately, it is possible to answer these questions by a detailed chemical examination of rocks which were deposited in the oceans at this time. For a large period of Earth's early history, a type of rock called a 'banded iron formation' was deposited in the oceans. These are rocks which contain a large proportion of iron-rich minerals, and such rocks do not form in the modem environment. It is believed that these rocks could only form in an ocean containing very low amounts of oxygen. Some time after the rise in atmospheric oxygen around 2.3 billion years ago the deposition of banded iron formations abruptly stopped. This project will examine rocks formed during the final stages of the deposition of banded iron formations on Earth. Such rocks are available for scientific research largely because of the previous drilling of rock cores from significant depths below the Earth's surface, in order to find suitable sites for the economic exploitation of minerals. By examining the type of iron-and sulfur-containing minerals in banded iron formations and in overlying oceanic sediments, the nature of the change in ocean chemistry at this time will be evaluated. In addition, the length of time that such conditions lasted will be explored by examining rocks deposited between the two periods of rising atmospheric oxygen. New techniques will be developed and applied to all of the rocks studied, with a particular aim to identify the changing nature of ocean chemistry on a global scale. This research should ultimately provide a better understanding of the links between atmospheric oxygen, ocean chemistry, and the evolution of life on Earth. In doing so, a better understanding of the conditions necessary for life to exist elsewhere in the universe will be achieved.