Record of climate change, Southern Uplands
|Stone, P, McMillan, A A, Floyd, J D, Barnes, R P, and Phillips, E R. 2012. British regional geology: South of Scotland. Fourth edition. Keyworth, Nottingham: British Geological Survey.|
Record of climate change
There is little doubt that the repeated glacial and periglacial episodes of the Pleistocene have left a significant imprint on the landscape. Across the south of Scotland there is good evidence for glaciation in the form of small mountain corries, and ‘U’ shaped valleys (P774194), overdeepened rock basins such as those of Loch Trool, Loch Doon and St Mary’s Loch, and numerous small lochans in ‘knoll and tarn’ topography on high ground. Spectacular hanging valleys include that of the Tail Burn which emanates from the moraine-encircled Loch Skene before tumbling down the 100 m waterfall of the Grey Mare’s Tail, 14 km north-east of Moffat (P001082). Modification of lowland areas occurred mainly through glacigenic deposition, which formed widespread, gently undulating, poorly drained, relatively featureless plains of till interspersed with mounds, ridges and terraces of outwash (glaciofluvial) sand and gravel. Subglacial processes produced swathes of the ice-moulded, drumlin topography and streamlined bedrock landforms that are so characteristic of much of Wigtownshire (P741207 and P912366) in the western Southern Uplands and Berwickshire in the east.
Evidence of global Quaternary environmental change has been found in deep-sea sediments, in cores of ice taken from the Greenland and Antarctic ice caps and from extended sequences of interbedded loess and organic deposits from continental Europe and Asia. The onset of glaciation in the Northern Hemisphere probably began in the Late Miocene, suggesting that the climate of the British Isles had begun to deteriorate long before the beginning of the Quaternary Period at 2.6 Ma, when ice-rafted debris first appears in the North Atlantic deep ocean record.
The frequency, rapidity and intensity of climatic change are key features of the Quaternary, with climate alternating between ‘glacial’ and ‘interglacial’ modes; at least 50 significant ‘cold–warm’ oscillations have been recognised. The driving force of climatic change is the long-term cyclical variation in the incidence of solar energy caused by the Earth’s orbital periodicities, which run over roughly 23 ka, 41 ka and 100 ka intervals. These fluctuations have been amplified substantially by additional factors involving physical, biological and chemical interactions together with ‘feedback loops’ between the atmosphere, oceans and ice-sheets. Of particular importance to the British Isles are changes in the position of the Gulf Stream. This north-eastward-flowing current of warm surface water is compensated by the return southwards of cold, dense water at depth. Sudden changes in this circulation pattern had an immediate and major impact on climate.
An invaluable proxy record of global climate is provided by the relative proportions of the two common isotopes of oxygen contained in the skeletons of calcareous microfossils recovered from deep ocean sediment cores. During glacial periods the oceans’ water becomes relatively enriched in the heavy isotope of oxygen (18O), and the marine isotope stages (MIS) thus obtained now provide a universal means of dividing the Quaternary (Figure 54). The oxygen isotope record indicates that during the early Quaternary, when glaciers possibly first developed in the Galloway Mountains, each of the principal cold–warm cycles lasted about 40 ka. Following a major change at about 780 ka BP there have been seven longer, more rigorous glacial– interglacial cycles, although substantial ice-sheets appear to have grown in only three or four of them in the Northern Hemisphere. Each of these glacial episodes lasted between 80 and 120 ka and was followed abruptly by an interglacial lasting 10–15 ka; the rapid deglaciations are described as ‘terminations’ (P912367). The glacial periods included long, cold intervals, termed ‘stadials’, and less cold, or even warm, ‘interstadials’ lasting for a few thousand years. Most terrestrial evidence preserved in southern Scotland relates to the last major glacial–interglacial cycle (Devensian and Holocene stages), but older deposits may be preserved locally.
High-resolution evidence from Greenland ice cores, coupled with the MIS record, confirms that dramatic climatic changes have occurred on the millennial (and possibly even decadal) scale. Some 24 interstadial intervals are now identified in the Devensian stage alone, compared with the five or six that were recognised previously from the pollen record and formalised in the traditional British chronostratigraphy (P912367). These short interstadials started with abrupt warming and typically cooled over a period of 1 to 3 ka, several being grouped together and superimposed on longer cycles during which temperatures declined gradually (P912368). The culmination of each overall cooling phase was a glacial readvance of regional extent coinciding with a massive discharge of icebergs into the North Atlantic (known as a ‘Heinrich event’), as indicated by the appearance of abundant ice-rafted debris in deep ocean sediment cores.
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