Testing the links between mass extinctions, volcanism and ocean chemistry

Why do some large volcanic eruptions create mass extinctions whilst others don’t? There is an emerging link between the volcanically driven mass extinctions, such as those at the Permo-Triassic and Triassic-Jurassic boundaries, the development of anoxia, and low concentrations of sulfate in the oceans (He et al, 2020). Is this link coincidental or could it partly explain why not all large volcanic eruptions cause severe extinction?

Ocean sediments are key components of marine biogeochemical cycles and host a suite of important reactions driven by organic matter and facilitated by bacteria. Sulfate-reducing bacteria oxidise organic carbon to CO2 after oxygen has run out, whilst deeper in the sediment, methanogenic bacteria produce methane from the organic carbon that remains. Unchecked this methane could diffuse upwards to the water column and consume dissolved oxygen, however, sulfate can also be used by third group of bacteria to oxidise methane, thus preventing it from reaching the water column (see Figure 1). In the modern ocean sulfate is very abundant and the combined effect of these microbial communities is to limit both methane production (by consuming organic carbon), and release (by consuming methane itself), largely preventing any impact on dissolved oxygen. When sulfate is lower however, a much greater proportion of the organic carbon is transformed to methane, and much more of this methane is able to escape into the water column where it’s oxidised using oxygen (see Figure 1). This oxygen consumption can then speed up the transition to anoxic conditions which are linked to extinction. Evidence for a high methane flux impacting bottom water oxygen during a low sulfate interval has recently been found in the chemistry of 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. 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:

  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).

The student will work on 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) 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 simple 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 links to the methane cycle and its effect on marine oxygen levels.


The project will have three main objectives to address the questions above:

  1. Proxy development. 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. We have pilot data that 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 and either Morocco or Canada, to supplement collections of phosphorite material.
  2. 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.
  3. 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. The project therefore has the potential to generate a number of significant papers with one or more being suitable for high-impact journals.

Training & skills

The successful candidate will be fully trained in a wide range of geochemical techniques to develop a high level of 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.


He, T., Dal Corso, J., Newton, R. J., Wignall, P. B., Mills, B. J. W., Todaro, S., Di Stefano, P., Turner, E. C., Jamieson, R. A., Randazzo, V., Rigo, M., Jones, R. E., and Dunhill, A. M., 2020, An enormous sulfur isotope excursion indicates marine anoxia during the end-Triassic mass extinction: Science Advances, v. 6, no. 37, p. eabb6704.

Hall, J.L.O., Newton, R.J., Witts, J.D., Francis, J.E., Hunter, S.J., Jamieson, R.A., Harper, E.M., Crame, J.A., and Haywood, A.M., 2018, High benthic methane flux in low sulfate oceans: Evidence from carbon isotopes in Late Cretaceous Antarctic bivalves. Earth and Planetary Science Letters, 497: 113-122.

Horita, J., Zimmermann, H., Holland, H.D., 2002. Chemical evolution of seawater during the Phanerozoic: Implications from the record of marine evaporites. Geochimica et Cosmochimica Acta, 66(21): 3733-3756.

McArthur, J.M., 1985. Francolite geochemistry–compositional controls during formation, diagenesis, metamorphism and weathering. Geochimica et Cosmochimica Acta, 49(1): 23-35.

Mills, B.J.W., Belcher, C.M., Lenton, T.M. & Newton, R.J. 2016. A modeling case for high atmospheric oxygen concentrations during the Mesozoic and Cenozoic. Geology 44, 1023-1026.

Mills, B.J.W., Krause, A.J., Scotese, C.R., Hill, D.J., Shields, G.A.and Lenton, T.M. et al. Modelling the long-term carbon cycle, atmospheric CO2, and Earth surface temperature from late Neoproterozoic to present day. 2019. Gondwana Research 67, 172-186.

Paris, G., Fehrenbacher, J.S., Sessions, A.L., Spero, H.J., Adkins, J.F., 2014. Experimental determination of carbonate-associated sulfate δ34S in planktonic foraminifera shells. Geochemistry, Geophysics, Geosystems, 15(4): 1452-1461.

Piper, D.Z., Kolodny, Y., 1987. The stable isotopic composition of a phosphorite deposit: d13C, d34S, and d18O. Deep Sea Research Part A. Oceanographic Research Papers, 34(5-6): 897-911.

Witts, J.D., Newton, R.J., Mills B.J.W., Wignall, P.B., Bottrell, S.H., Hall, J.L.O., Francis, J.E. and Crame, J.A., 2018. “The impact of the Cretaceous–Paleogene (K–Pg) mass extinction event on the global sulfur cycle: Evidence from Seymour Island, Antarctica.” Geochimica et Cosmochimica Acta 230: 17-45.