Development and application of molybdenum isotopes as a tool for tracking the evolving redox state of the Precambrian ocean

Lead Research Organisation: Newcastle University
Department Name: Civil Engineering and Geosciences

Abstract

Oxygen is vital to sustain many forms of life on Earth. Unlike the present-day, when life first evolved the atmosphere and oceans contained essentially no oxygen. Various lines of evidence suggest that the oxygen content of the atmosphere only began to rise about 2.3 billion years ago. Until recently it was thought that this also led to oxygenation of the ocean (as in the present day). A more recent hypothesis suggests that, instead, the increase in oxygen led to the weathering of sulphide minerals on land, which resulted in increased riverine delivery of sulphur to the oceans. The oceans then became rich in toxic hydrogen sulphide rather than oxygen (similar conditions are found in the modern day Black Sea). In fact, the ocean may only have become oxygenated following a second, much later rise in oxygen about 700 million years ago. All of this has profound consequences for the evolution of the biosphere. It is in the oceans where early life first evolved and flourished. The early biosphere was dominated by bacteria and the first photosynthesising bacteria probably evolved at least by 2.7 billion years ago, before the first major rise in oxygen. One of the puzzles of the early biosphere is why this early evolution of oxygen-producing photosynthesisers did not lead to the rapid oxygenation of the surface Earth thereafter. It is also clear that higher life forms, such as plants and animals (and ultimately humans) only began to evolve much later when the oceans eventually became oxic. Why was there a delay in the oxygenation of the surface Earth? Why did the biosphere only evolve slowly early in Earth's history? One prominent recent hypothesis attributes these puzzling features of the ancient Earth to ocean chemistry. One of the key requirements of photosynthesising bacteria is nutrients, which are essentially the elements contained in fertilisers- phosphorous, nitrogen, and trace metals such as molybdenum (Mo). Before the oxygenation of the atmosphere, the oceans were probably rich in dissolved iron (which is soluble in oxygen-poor water), leading to the widespread precipitation of chemical sediments very rich in iron (so-called Banded Iron Formations or BIFs). These may have taken vital nutrients like phosphorous and trace metals with them, leaving very low concentrations behind for bacteria to use. After the initial oxygenation of the atmosphere, and particularly if the oceans became sulphidic, trace metals may also have been in scarce supply as many of them are precipitated in the presence of hydrogen sulphide. This is important as it may have limited photosynthesis and hence oxygen production, helping to explain the apparent delayed oxidation of the Earth's surface, and hence the slow evolution of the biosphere. However, these ideas remain controversial. Detailed studies are required to assess whether the conditions described above did in fact exist. It is also important to determine how widespread these conditions were and how they affected nutrient availability. This project will examine nutrient availability as recorded by BIFs, the global extent of the transition to a sulphide-rich ocean following the first rise in atmospheric oxygen, and the chemical evolution of the oceans in the subsequent period of Earth's history leading up to the major explosion of animal and plant life. The tool we will use is the isotopes of molybdenum. The oceanic chemistry of Mo, and specifically the processes by which it is removed from solution into sediments, is highly dependent on the chemical state of the oceans. Further, these removal processes have variable preferences for the different isotopes of Mo, which makes the record of Mo isotope variations in the rocks interpretable in terms of both the oxygenation state of the ancient oceans, and the availability of Mo as a nutrient. This research should ultimately provide a better understanding of the links between ocean chemistry and the evolution of life on Earth.

Publications

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Description Final Report (June 2010):



This project comprised three general sections, linked by a common goal to develop and apply molybdenum isotopes to establish how the chemistry of the ocean evolved during the early stages of Earth history. The research proceeded as planned, and the major findings are summarised below.



1. Experimental: The first phase of the experimental work involved evaluation of Mo isotope systematics during adsorption of Mo to a variety of Fe (oxyhydr)oxide minerals. The results demonstrate that Mo is extensively adsorbed to Fe (oxyhydr)oxides, and furthermore this process involves a large isotopic fractionation which varies dependent on the type of Fe mineral involved. Largest fractionations were observed for hematite and smallest fractionations were observed for ferrihydrite. These results provide a new dimension to our understanding of the global Mo cycle, and suggest that the entire range of Mo isotope fractionations observed in modern marine sediments can potentially be explained by adsorption to different Fe minerals, coupled with the differing behaviour of these minerals during diagenesis. A manuscript based on these results has been published in Geochimica et Cosmochimica Acta.

The second phase involved evaluation of Mo isotope fractionations under sulfidic conditions. A range of carefully controlled experiments were conducted which demonstrate extensive removal of Mo from solution as Fe (oxyhydr)oxide minerals are sulfidized. Mo isotope measurements are currently being performed and a manuscript is planned based on these results.



2. Modern sediment studies: Mo isotope systematics were evaluated for three contrasting marine settings; 1. Golfo Dulce, Costa Rica - an anoxic non-sulfidic tropical basin; 2. Aarhus Bay, Denmark - an oxic marine setting where the sediments become highly reducing close to the sediment water interface; 3. Gulmars Fjord, Sweden - an oxic basin where the sediments are rich in manganese minerals. All sediments were characterised in detail in terms of diagenetic biogeochemical processes and mineral transformations, in order to provide a robust framework for evaluation of the Mo isotope data. A range of Mo isotope fractionations were observed which, based on our experimental results, can be attributed to the dominant biogeochemical process occurring at a particular diagenetic stage and in a particular environment. These results place important constraints on our understanding of Mo isotope fractionations in ancient rocks and hence on the interpretation of these data for evaluating the spatial extent of different water column redox conditions in the past. Three manuscripts are currently being written for leading international journals based on these results.



3. Precambrian sediments: Application of Mo isotopes to the rock record focused on 2.1 to 1.8 billion year old (Ga) sediments from the Pine Creek region of Northern Australia. Based on Fe speciation measurements, the sediments document a transition from anoxic, Fe-rich conditions at 2.1 Ga to euxinic water column conditions at 1.8 Ga. This transition ties in with the end of the final global episode of banded iron formation deposition in Earth's history, providing important supporting evidence that the cessation of banded iron formation deposition was a result of titration of Fe from the water column (as pyrite) as the oceans became euxinic. The Mo isotope data allow us to provide constraints on the global extent of ocean euxinia, with the results suggesting that euxinic conditions were likely more widespread at this time than at any other time in Earth history. A manuscript based on these results is currently being prepared for submission to Geology.
Exploitation Route The research has already been widely used by other workers interested in the application of Mo isotopes.
Sectors Other