Ocean Sulfate and Earth's Surface Evolution
Dr Robert Newton (SEE), Dr Ben Mills (SEE), Dr Tracy Aze (SEE), Prof. Liane Benning (GFZ)Project partner(s): GFZ, Germany (CASE)Contact email: R.J.Newton@Leeds.ac.uk
Ocean sulfate is an underappreciated control on the evolution of Earth’s surface environment. Sulfate concentrations have changed a lot over the course of Earth’s history, but the effect on the biogeochemical cycles of other elements and the links to evolutionary events has received relatively little attention. It’s well known that the burial of organic carbon and pyrite controls the amount of oxygen in Earth’s atmosphere. Sulfate is both the most important oxidant for organic matter in modern ocean sediments (after dissolved oxygen), and its abundance in seawater controls exerts a control on pyrite burial. It can account for up to 80% of organic matter oxidation in present day continental shelves where most organic carbon is buried, but how would this change if its concentration were 10% of modern and what feedbacks would this induce? A low sulfate ocean also promotes methane production in marine sediments, a process proposed to have important implications for the amount of oxygen in bottom waters. It’s less well known that sulfate exerts a strong control on the primary carbonate mineralogy precipitated in the oceans and so is central to transitions between calcite and aragonite seas. Work at Leeds is now also connecting low ocean sulfate with several mass extinction events and their associated profound carbon cycle perturbations, but we don’t know how these observations may be connected.
We are starting to appreciate that ocean sulfate concentrations may be able to change very quickly (geologically speaking) during evaporite deposition and dissolution events, but the possible effects of such rapid change are poorly understood and we have little evidence with which to test this idea. This leaves us with two linked problems which form the focus of this PhD.
1) How has ocean sulfate changed on both short and long timescales? Current datasets and methods are inadequate to answer this question so we need methods for determining ancient ocean sulfate concentrations that can be used at higher temporal resolutions. This project will investigate two new methods for determining marine sulfate concentrations.
2) What effects do changing ocean sulfate concentrations have on the operation of other biogeochemical cycles such as carbon? This project will explore these effects using biogeochemical modelling.
Initial work will focus on the time period from the start of the Jurassic to the present, with fieldwork in Bulgaria and Morocco. This time period encompasses an approximately three-fold increase in ocean sulphate and contains several interesting carbon cycle perturbations such as the early Toarcian and Palaeocene-Eocene Thermal Maximum. To understand the implications of these records the student will use various modelling approaches to explore the controls on the oceanic sulphur cycle and its effect on the marine carbon cycle and sedimentary methane production.
To address the need for high resolution records of past-ocean sulfate, the student will explore two novel methods based on the substitution of sulphate into phosphate and carbonate minerals. Both phosphate deposits and foraminifera have a much more continuous record than evaporites and can be dated with far greater precision.
The project offers excellent training in practical and numerical geochemical techniques in research groups well known for their supportive research culture and world leading profile. The findings of the project will have broad implications for the co-evolution of Earth’s surface chemistry and life.
Related undergraduate subjects:
- Chemical engineering
- Earth science
- Earth system science
- Environmental science
- Geological science
- Natural sciences
- Physical geography
- Physical science