During the Carboniferous (359-259 Ma), Scotland lay near the equator in a position akin to central Africa, and therefore had a hot climate. The northern parts of Scotland were upland areas and the southern parts of Scotland were lower lying. Further to the north and west lay the North American continent, to which Scotland was joined, and to the south of what we know as England, lay a large ocean. During the Carboniferous, sea level rose and fell several times. There were times when southern Scotland lay underwater, covered by a shallow, warm sea. At other times this area lay above sea level.
When below sea level, the area was covered in sand and mud that was washed into the sea, leading to the formation of rocks such as sandstone, mudstone, and shale. Limestone also formed in part through the accumulation of dead sea creatures, but also through the precipitation of calcium carbonate in the tropical conditions. The shallow tropical seas of Carboniferous Scotland, much akin to the present-day Bahamas, were full of life and much of this life became preserved as fossils in the rocks, including shellfish, corals, crinoids, sharks and other fish. When above sea level, the land was covered by tropical swamps, where forests of large trees, that were very unlike modern trees, flourished. Giant-sized centipedes, dragonflies and spiders ruled the landscape along with amphibian and early reptiles, the forerunners of the dinosaurs. Over time, the remains of trees that had fallen into the swamps were buried deeply and compacted, eventually forming peat, and finally coal (Figure 1).
Volcanic activity was another important feature of the Carboniferous (Figure 2). The continent at this time was being ‘stretched’, allowing magma to well up from the mantle to form many volcanoes. The remains of these volcanoes are preserved as intrusive rocks (e.g., Figure 3) and lava flows (e.g., Figure 4) that dot the landscape of southern Scotland. These rocks comprise classical sites, such as Arthur’s Seat, Edinburgh.
Although the stratigraphic relationships of intrusive and extrusive volcanic rocks and the Carboniferous sedimentary rocks of Scotland are good, the absolute age and temporal framework of the rocks is poor. Lack of an accurate and precise temporal framework essentially leaves the Carboniferous rocks of Scotland ‘floating’ in time with reference to the Geological Time Scale. Without an absolute chronology the rates of climate change and evolution of flora and fauna preserved within the Carboniferous sediments throughout Scotland cannot be assessed.
The aim of this PhD project is to date, using high precision radio-isotope geochronology, the Carboniferous volcanic rocks located throughout the Midland Valley (Figure 2). The data will be used to construct a robust temporal framework that can place these sequences of rocks within the global Geological Time Scale and constrain the ages of fossil-bearing sediments. To push the limits of geochronology the project has support from Thermo Scientific (CASE Partner) and the student will get access to the latest state-of-the-art mass spectrometer technology. In addition, isotope geochemistry (co-supervisors Ellam and McPherson) will be used to understand the formation and evolution of the Carboniferous magmatic systems of Scotland.
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Figure 1: Spireslack is a stunning exposed section of an important coal-bearing sequence, unparalleled in Scotland. More than 1 km long, the 80-metre-deep canyon is cut into a gently dipping succession of Carboniferous-age strata that includes economically valuable coal, ironstone and oil shale.
Figure 2: Volcanic rocks of Carboniferous and Permian age in the Midland Valley, Scotland.
Figure 3: Dumbarton rock is the remains (basaltic plug) of a Carboniferous volcano.
Figure 4: Exposure of ca. 30 lava flows of Carboniferous age, Western Campsie Fells from Strathblane.