Anoxic Transition Metal Isotope Geochemistry

It is the complex interaction of physical, chemical and biological processes that defines the natural environment which we experience and which affects our daily lives. Earth scientists are interested in understanding the contribution of these diverse processes to the evolution of the global environment throughout Earth's history to the present day. Over the last 60 years geochemists have made use of the chemical properties of the different isotopes of elements like carbon, oxygen sulfur and hydrogen to investigate and understand the biogeochemistry of the atmosphere, ocean and the Earth's immediate subsurface. Isotopes are atoms of the same element but with different atomic masses. Different isotopes of the same element display subtle differences in chemical behaviour which allow them to be used to trace the progress and importance of biogeochemical processes. Thus the isotopic composition of natural materials can tell us about the composition of the chemical reactants from which those materials formed, the environment in which they formed and can often define the processes which formed them. In the last decade advances in analytical geochemistry have made available new techniques to investigate the behaviour of the isotopes of metals like iron and copper in the natural environment. This branch of science has become one of the most rapidly expanding and exciting areas of the Earth Sciences. Geochemists have produced and published analyses of natural materials from geological samples ranging from rocks formed over 3 billion years ago to samples actually being formed in the present day. Many of these analyses come from metal sulfide minerals like pyrite and chalcopyrite. They have recorded patterns in the metal isotope data which may help us to understand the influence of early life and the rise of oxygen in the primitive atmosphere, the processes forming economic mineral deposits and the ways in which bacteria contribute to the chemistry and environmental mobility of metals in sediments. Our problem is that in order to meaningfully interpret these data, we need to understand in detail the fundamental chemical processes that control the geochemical behaviour of metal isotopes in the environment. This project will investigate the behaviour of iron and copper isotopes during chemical reactions with sulphur. Much of the Earth's history and most of its subsurface is characterised by anoxia, i.e. the absence of oxygen. In such environments the chemistry of metals is usually controlled by their reactions with sulfur in the form of sulfide. We will use a laboratory based experimental approach to follow the behaviour of iron and copper isotopes during i) the formation of recently discovered dissolved metal sulfide materials called clusters, ii) the formation geologically important sulfide minerals like pyrite and chalcopyrite and iii) the exchange of iron and isotopes between sulfide minerals and solution. We test the hypothesis that it is the nature of the chemical reaction (the 'mechanism', as chemists refer to it) which controls the isotopic composition of metal sulfide materials. We will use our experimental results to identify whether the iron and copper isotope compositions of natural sulfide materials reflect solely the composition of the materials from which they formed, or whether their composition is also determined by the formation processes itself. The results will be of fundamental importance to isotope geochemists and applicable directly to studies of metal isotopes in a wide variety of natural systems of all geological ages.