Why do some large igneous province eruptions create mass extinctions whilst others don’t? There is an emerging link between volcanically driven mass extinctions, such as those at the Permo-Triassic (P-T) and Triassic-Jurassic (T-J) boundaries, the development of anoxia, and low or very low concentrations of sulfate in the oceans. Is this link coincidental or could it partly explain why not all large volcanic eruptions cause severe extinction?
Sulfate is very important in ocean sediments because it’s a store of oxidising power that’s used by bacteria after oxygen has run out to oxidise organic carbon to CO2. A large proportion of the hydrogen sulphide produced by this sulfate reduction reaction is trapped in the sediment by reacting with iron to form pyrite. This prevents it from diffusing to the sediment surface and consuming dissolved oxygen. In the modern ocean sulfate is very abundant and so these bacteria can oxidise a lot of carbon within the sediment with minimal impact on the dissolved oxygen budget of the lower water column. When sulfate is lower however, a much greater proportion of the organic carbon is instead transformed to methane, which is not trapped in the sediment, and instead returns to the bottom waters of the oceans where it’s oxidised using oxygen (see Figure 1). This oxygen consumption can then drive anoxic conditions which are linked to extinction. Evidence for a high methane flux during a low sulfate interval has recently been found in high resolution carbon isotope records from Late Cretaceous bivalve shells (Hall et al, 2018; Witts et al, 2018). This additional oxygen demand in low sulfate oceans may form a new and powerful mechanism that makes a low-sulfate ocean more predisposed to widespread anoxia and marine extinction during volcanically driven warming events.
This project will focus on records and models of the sulfur cycle and its links to other biogeochemical cycles during volcanically driven warming events to answer the following questions:
1) How did ocean sulfate concentrations vary before, during and after the events?
2) How might the sulfur and carbon cycles interact to control the dissolved oxygen concentrations of the oceans during these events?
These questions will be applied to two case studies: One from an event with a proven anoxia-mass extinction link (likely to be the Permo-Triassic event); and one from a volcanic event of a similar magnitude where evidence for anoxia and its biological impact is far more limited (such as one of the Cretaceous oceanic anoxic events or the Palaeocene-Eocene thermal maximum). There may also be the opportunity to study other events as the project develops.
The student will help to develop two novel techniques to infer past ocean sulfate concentrations. These are based on the substitution of sulfate into phosphate (McArthur, 1985; Piper and Kolodny, 1987; Hough et al., 2006) and carbonate minerals in tiny organisms called foraminifera (Paris et al., 2014). Both phosphate deposits and foraminifera have a much more continuous record than more traditional sulfate deposits (evaporites) and can be dated with far greater precision, creating the potential for capturing fluctuations in ocean sulfate that occur on short timescales. These methods can then be applied to the selected events to determine marine sulfate levels, and the data can be used to develop a biogeochemical computer model to test how the ocean in this state might respond to a warming event. The modelling will build on current frameworks developed by the supervisory team (e.g. Mills et al., 2016; 2019) to add in the dynamics of the methane cycle and the effect on marine oxygen levels.
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