What’s on : Lectures

Muddy Bottoms: response of Jurassic seafloors to palaeoenvironmental change

18 Jul 2023
Start time
2:30 PM
Yorkshire Museum
Dr Bryony Caswell, University of Hull
Muddy Bottoms: response of Jurassic seafloors to palaeoenvironmental change

Event Information

Muddy Bottoms: response of Jurassic seafloors to palaeoenvironmental change

Dr Bryony A. Caswell, University of Hull, UK

This talk will consider changes on seafloors, during the Toarcian and Kimmeridgian stages of the Jurassic. I will present on my past and new work asking how the marine ecosystems recorded within these mudrocks responded to palaeoenvironmental changes, including global warming and ocean deoxygenation, in the Cleveland and Wessex basins. I will explore how this compares to the changes occurring in the present-day and what it can teach us about how marine communities and ecosystems might respond in the future ocean.

Lecture to be held in the Tempest Anderson Lecture Theatre, Yorkshire Museum,
YO1 7DR at 2.30pm


Image: Geological exposure of the Whitby Mudstone Formation at Port Mulgrave, UK


Member’s report

The afternoon started with the presentation of the John and Anne Phillips geological mapping prize for the University of Hull to Louis Chambers for his mapping of Yr Wyddfa/Snowden. The prize is sponsored by the YPS and was presented by Catherine Brophy, the YPS Chair.

Dr Caswell gave her talk on what past climate change can tell us about current and future environments. Understanding the complex interactions between the earth’s rock cycle, atmosphere, hydrosphere and biosphere are critical for predicting future global changes.

She has for many years studied the changes in the seafloors during the Toarcian stage (Early Jurassic, 183 to 175 million years ago (Ma)) to be found in the shales of the Whitby area.  This stage began with the Toarcian Oceanic Anoxic Event which is evident in the black shales of the area. These are oil-source rocks, organic rich and high in carbon formed in a fully marine open basin below storm level (up to 200m deep) with evidence that burrowing stops abruptly with no (anoxic) or inadequate (hypoxic) supply of oxygen in the water column, then the next 40 meters above show complete absence of any life on the seafloor.

A hyperthermal event (sudden warming of the planet in geological time of between 7 and 13 degrees C) resulted in numerous interacting effects including global warming, enhanced weathering, sea-level change, carbonate crisis, marine anoxia (complete loss of oxygen) and biotic crisis.  Initial warming through a Large Igneous Province (massive volcanic activity) is suspected which released vast quantities of Carbon Dioxide and Methane and this fed into run-away warming through methane hydrates and possible deterioration of marine perma-frost regions. Changes on land need further research.

This was a second order extinction event, compared to the most extreme end-Permian or end Cretaceous extinctions.

The 40 metres is considered to represent 900 000 years which it took for some recovery as shown in the absence then gradual reappearance of trace fossils. Life came back slowly and the number of species became the same as before, but they were not the same species. There was a general reduction in size for the rest of the Toarcian.

There was a regional, less extreme de-oxygenation event during the Kimmeridgian stage of the Late Jurassic over a 6 million year period.

Dr Caswell gave an account of current and predicted changes to marine environments and the processes that control them.  The IPCC predict global temperature changes of between 0.5 and 7.5 degrees C by 2100. Temperature rises and carbon dioxide increases have already produced a reduction of oxygen in the water column. Nutrients and sewage feeding into rivers and seas produce excessive fertilisation and algal blooms which lead to anoxia. Upwelling of nutrients through increased temperature differentials also produces hypoxia. There has been a 2% decline in oxygen in 50 years. A much larger decrease is predicted by 2070.

There have been changes in body size in fish with a decline of 20% expected by 2100. Water acidification will lead to dissolution of calcium carbonate in marine life. Some of this is irreversible.  Species are expected to invade towards the poles, threatening the extinction of polar species.

While current carbon dioxide levels at 400 parts per million by volume (ppmv) are much lower than in the Toarcian, the more significant aspect is the much greater rate at which this is rising in modern times.

She finished with an appeal for essential targets for conservation and management of marine environments.

Paul Thornley