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.
Figure 1. Cartoon illustrating the possible link between low sulfate and the enhanced development of anoxia during a volcanic CO2 driven warming event. A) The carbon cycle in ocean sediments under ‘high’ sulfate conditions. Here methanogenesis is restricted to deeper in the sediment and diffusing methane is consumed by sulfate before it can reach the water column. The reducing power of organic carbon is retained in the sediment as reduced sulfur bound to iron (as the mineral pyrite), and not consumed by dissolved oxygen. B) The ocean sediment carbon cycle under ‘low’ sulfate conditions. Methane production occurs at a much shallower depth in the sediment and methane can diffuse back up to the water column and consume dissolved oxygen as it is oxidised.
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:
- How did ocean sulfate concentrations vary before, during and after the events?
- 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.
The project will have three main objectives to address the questions above:
- Develop and calibrate the proxies. The student will sample selected intervals in the range from the Jurassic to the present to produce estimates of ocean sulfate using the methods above. This time period encompasses a large increase in ocean sulfate recorded in other datasets which can be used for comparison. Foram estimates will be produced for Cenozoic sediments whilst phosphorite estimates will be produced for both the Mesozoic and Cenozoic. Pilot data indicates that these techniques provide good estimates of ocean sulfate concentration. Many of the necessary samples are held in collections either in Leeds or accessible via existing collaborations but additional sample sets must be collected during fieldwork in Bulgaria, plus a visit to either Morocco or Canada, to supplement collections of phosphorite material.
- Generate records of ocean sulfate concentration across two events. The student will use the knowledge they have gained from proxy development and calibration to apply appropriate methods to generate records of sulfate concentration across the selected events.
- Biogeochemical model development and application. The student will build a simple extension to current models of the carbon, sulfur and oxygen cycles, and then constrain this with published isotope records and the new sulfate concentration data.
Potential for high profile outcome
The project will explore a novel hypothesis for why some volcanic episodes are linked to greater biological and environmental perturbations than others and provide new insight into the detailed chemical evolution of the oceans, one of the most important components of Earth’s biogeochemical system. We therefore expect the project to result in a number of significant papers in the field with one or more being suitable for a high-impact journal (e.g. Nature, Nature Geoscience).
Training & skills
The successful candidate will be fully trained in a wide range of geochemical techniques providing a high level of specialist expertise. Uniquely, this project will involve, field work, analytical lab work and computer model development and all of these aspects will be supervised by experts in these techniques. No previous expertise of these techniques is required and the project can be tailored to the skill set of the researcher to some degree. Completing a PhD develops a broad array of transferable skills such as written communication, public speaking, project management, leadership, collaboration and perhaps most importantly, critical thinking. All of the analytical techniques are available in the School’s excellent laboratory suite. The student will benefit from being part of the Earth Surface Science Institute, and the Cohen Geochemistry and Palaeo@Leeds research groups. This organisational framework provides a broader supportive environment which allows the cross fertilisation of ideas and expertise. In addition to the bespoke training for the PhD, the student will have access to a wide range of other general training and support. Examples would include useful scientific and transferrable skills such as programming or statistics, time management, writing and giving presentations, and skills specific to a PhD programme such as managing your degree and preparing for your viva.
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