Regional investigation of extinction-recovery patterns during the Toarcian (Lower Jurassic) hyperthermal event

Project Introduction

The Toarcian Stage of the Lower Jurassic marks a period of extreme environmental perturbations, including changes to the global carbon cycle, rapid warming to ‘hyperthermal’ conditions, and increased continental weathering and run-off. These events have been linked to the eruption of the Karoo-Ferarr Large Igneous Province in the contemporary Southern hemisphere. In many of the marine sedimentary basins in the Northern hemisphere the hyperthermal conditions led to stagnation, consequent sediment and water column de-oxygenation and the deposition of distinctive organic-rich laminated black shales (linked effects often termed the Toarcian Oceanic Anoxic Event – TOAE). These challenging conditions caused an extinction event in the macrofauna (particularly the benthos) and in microfossil groups. Recovery from this event was slow (several millions of years) and occurred only when environmental conditions improved after the TOAE. The TOAE and associated pattern of extinction and recovery is well recorded in the Cleveland Basin of Yorkshire (Figure 1), and further afield in Germany and France. It has also recently been recognized globally, in, for example, Canada. However, in many marine basins in Southern Europe laminated black shale facies are either weakly developed, or absent, yet there are still distinct patterns of biotic turnover during the Toarcian Stage as well as evidence for rapid and extreme warming. This strongly indicates that while changes in marine oxygen levels during the Toarcian where regional in extent, warming was a global phenomenon and had a primary control on both environmental and biotic changes. As such, the Toarcian Stage has lessons to teach us about the consequences of anthropogenic global warming now and in the future.

Figure 1. Lower Toarcian organic-rich rocks cropping out at Hawsker Bottoms, North Yorkshire.

From a UK perspective the TOAE and associated extinctions and recovery has been extensively researched in the Cleveland Basin, but much less is known about the event in the rest of the UK. This is because whilst there is a narrow band of Toarcian outcrop in England from Yorkshire through the Midlands to the South-West, the rocks are poorly exposed. Toarcian rocks do crop out on the Dorset coast, but here they are present as a highly condensed, carbonate facies, quite unlike the expanded shales of the Yorkshire coast. However, there exists at the British Geological Survey (BGS) an unparalleled archive of rock cores from deep boreholes passing through the Lower Jurassic across the UK. The cores have been described sedimentologically and were split to recover macrofossils, which are now in the Registered Specimens collections in the BGS. For the Lower Jurassic time period tens of thousands of Registered Specimens exist in the collections, most of which have never been looked at since the original descriptions of the rock cores. Further, within the past few years there have been two excavations of Toarcian rocks from inland sites in Somerset: Strawberry Bank and near Seavington St Michael. Here the Toarcian is much more expanded than at the Dorset coast, but as limestone-marl carbonate rather than mainly limestone or shale facies. Both of these excavations have yielded considerable numbers of macro and microfossils, and bulk samples for processing.

Aims and outcomes

The primary aim of this project is to investigate regional patterns of benthic macrofossil extinction and recovery through the Toarcian hyperthermal event, by integrating existing data from the Yorkshire coast, with new data from rock cores in the BGS and excavated specimens from SW England. This will involve the student spending considerable amount of time at the BGS working through the Registered Specimen collections from the relevant cores from the Pliensbachian (the stage before the Toarcian) through the Toarcian times. Benthic macrofossils will be identified, and species range charts will be constructed and plotted against the established stratigraphy. Collections from the Somerset excavations are also available will be similarly dealt with, and bulk samples will be processed for additional fossils. All the benthic macrofossils in the study will be assigned to trophic guilds via an inferential modelling approach where key palaeoecological traits define the likelihood of predator-prey interactions and consumption. Meta-communities will then be analysed using trophic network modelling, so to investigate changes in ecosystem structure and function across the extinction and recovery intervals. The student will also analyse biogeographical recovery in the aftermath of the extinction event by incorporating data from Pliensbachian-Toarcian sections outside the UK, derived from the Paleobiology Database.

The expected results from the project will be: (i) obtaining a regional overview for the pattern(s) of extinction and recovery through the hyperthermal event; (ii) better understanding of how facies controlled these pattern(s); (iii) knowledge ecosystem structure and function changes across the extinction and recovery intervals; (iv) integration of UK derived Toarcian data with global data to understand the biogeography of recovery after the event.

Training

The project is interdisciplinary and the student will work within the Earth Surface Science Institute (ESSI) under the supervision of Dr Crispin Little (macrofossil palaeontology) and Dr Alex Dunhill (quantitative palaeontology). As a member of the Palaeo@Leeds group the student will have access to a broad spectrum of relevant expertise, which will be supplemented by an extensive range of research and personal development workshops delivered by DTP and the University of Leeds, from numerical modelling, through to managing your degree, and preparing for your viva. The student will also spend a considerable period of time at the British Geological Survey (https://www.bgs.ac.uk/) using the extensive rock core specimens at the Keyworth site, and will be able to interact with the project supervisor there, Dr Jim Riding, and other BGS staff. The student will also interact with the two supervisors who have access to material collected from the Somerset excavations, Dr Kevin Page (University of Exeter) and Matt Williams (Bath Royal Literary and Scientific Institution).

References

Danise, S., Twitchett, R.J., and Little, C.T.S. (2015) Environmental controls on Jurassic marine ecosystems during global warming. Geology 43:263-266.

Remírez, M.N., and Algeo, T.J. (2020) Carbon-cycle changes during the Toarcian (Early Jurassic) and implications for regional versus global drivers of the Toarcian oceanic anoxic event. Earth-Science Reviews 209: 103283. https://doi.org/10.1016/j.earscirev.2020.103283

Wignall, P.B., Newton, R.J. and Little, C.T.S. (2005) The timing of paleoenvironmental change and cause-and-effect relationships during the early Jurassic mass extinction in Europe. Am. J. Sci. 305: 1014-1032.