Great Britain is an island nation, with over 11,000 miles of coastline*. But the coastline is continually changing, due to the process of coastal erosion. A group of scientists have been using Diamond Light Source to study the weathering of shale cliffs in North Yorkshire to understand physical and chemical interactions on the molecular scale that could cause weakening of the cliffs , including the intriguing possibility that a novel microbial community might be responsible.
|Samples of Lias rocks, approx 20 cm across
At least 17% of the UK coast suffers from erosion, and the shale cliffs on the North East coast of England are particularly at risk. Previous studies that mapped and quantified erosion in this region using laser photogrammetry demonstrated the impact of the physical process of rocks detaching from the cliff, and the patterns of these overtime. However, these macroscale effects do not provide much of an indication of the initial events that cause the rocks to detach. A group led by Professor Charles Cockell at the Open University have been using the Microfocus Spectroscopy beamline at Diamond to study the chemical and physical micro-environment of the weathering shales, in particular the microbial community living on the surface, to try discover how this impacts on the macroscopic environment.
The shale contains pyrite, and there are micro-organisms are known to catalyse pyrite oxidation. If the microbial community inhabiting the shale is oxidising pyrite along fracture planes in the shale, this could lead to rock weakening and detaching from the cliff. The team collected samples from cliff faces close to the village of Staithes in Yorkshire, formed from Jurassic “Lias” rocks. The shale samples contained 5 – 95% pyrite by mass, and were collected from man-made caverns where they were relatively undisturbed since the caverns were created in the 1670s.
The samples were studied with a range of techniques, including scanning electron microscopy, to determine the physical characteristics of the surface, X-ray diffraction to determine the major minerals, and X-ray spectroscopy at Diamond to examine the oxidation states and order within the surface weathering layer of the shale. They also examined the DNA within the bacteria and found there to be a novel microbial community with low diversity. While they were unable to determine a precise cause and effect mechanism between the microbes and weathering of the pyrite in the shale, some of the microbes were very similar to organisms found in iron oxidation environments. There are also methods of biological weathering to which the micro-organisms could contribute.
“We have shown that the microbial community in the shale could be changing the chemical conditions on the surface of the shale, and therefore contributing to rock weathering. But the potential impact is not limited to coastal erosion. The oxidised pyrite can produce sulphuric acid which can damage the local environment. Shale is also involved in the long-term carbonate-silicate cycle, during which CO2 is consumed, reducing atmospheric CO2 concentrations, which is then compensated for by volcanic eruptions. Further study of the microbes and how they interact with the rock surface will allow us to understand the extent to which they are responsible for rock weathering and other effects on the macroscopic scale.”
Prof Charles Cockell, Open University
*According to the Ordnance Survey
Molecular Characterization and Geological Microenvironment of a Microbial Community Inhabiting Weathered Receding Shale Cliffs, Charles S. Cockell, David Pybus, Karen Olsson-Francis, Laura Kelly, David Petley, Nick Rosser, Kieren Howard and Fred Mosselmans, Microbial Biology DOI: 10.1007/s00248-010-9730-6
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