Investigating the Precambrian atmosphere, ocean and biosphere with selenium isotopes.

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Multicellular life is sustained by our oxygen-rich atmosphere, yet our planet's early history is marked by a lengthy anoxic period when such life could never have existed. Geological differences between the ancient and modern Earth show that there was insufficient oxygen in the early atmosphere to leave traces of oxidation that today are ubiquitous, such as the reddening of exposed iron-rich rocks. The transition from the ancient anoxic atmosphere to a partly oxygenated atmosphere occurred around 2400-2200 million years ago. At this point, oxygen levels were sufficient to produce a stratospheric ozone layer that protected life from ultraviolet radiation but oxygen levels were apparently insufficient to support respiration in animal-like life. Later, around 800-600 million years ago, oxygen levels evidently rose again and animal life subsequently appeared. Despite this general picture, the exact levels of oxygen that were reached after the first and second rises of oxygen are still a matter of disagreement. However, the level of oxygen is critical for determining basic aspects of the Earth, such as whether the deep ocean remained devoid of oxygen after the first rise of oxygen or exactly when there was enough oxygen to support widespread respiration in amoeba-like life or multicellular life. In fact, exactly why oxygen appeared about halfway through Earth's 4500 million-year history and increased later are still open questions. Microbes use certain chemical elements in their metabolism. Selenium is one such element, and when microbes convert selenium between different soluble forms or to an insoluble state to gain energy, they discriminate between different selenium isotopes. Selenium is only soluble when there is sufficient oxygen around, so the isotopic composition of selenium compounds in sediments should reflect the oxygenation of the environment. Moreoever, selenium has two soluble states depending upon the amount of oxygen, so we should be able to deduce the oxygen level to a degree that has hitherto proved elusive. We will analyse sedimentary rocks that sample the times before, after and during Earth's great oxic transitions. By looking for trends and changes in the selenium isotopes, we can deduce whether there was oxygen in the atmosphere or not, and characterise the magnitude of the oxygen level. Selenium gases are known to be released to the atmosphere from microbes. When these gases were in the ancient anoxic atmosphere, it is possible that atmospheric chemical reactions produced anomalous isotopic signals, or 'mass-independent fractionation' of isotopes. We will also test for this signal, which may prove a powerful signal of the first rise of oxygen. The discovery of selenium isotope signals in ancient rocks will also indicate the presence of groups of bacteria that use selenium. This inference will mean that such bacteria had evolved by the time that the rocks were deposited. If future genetic studies show how present-day selenium-utilizing bacteria relate to other bacteria, then we will advance the understanding of microbial evolution on Earth. The overall outcome of this work will be to help resolve the oxygenation state of the environment through Earth history. This issue is fundamental in understanding the co-evolution of life and the chemistry of the atmosphere, oceans and land surface.