Evidence for Mediaeval soil erosion in the South Hams region of Devon, UK

A major theme of research into the causes of past and present soil erosion has been to determine the relative importance of climate and land-use change in influencing Holocene erosion rates. Previous work suggests that land-use change, especially the conversion of woodlands into agricultural land, is the main factor influencing long-term increases in soil erosion. A study of an extensive minerogenic sediment deposit in a wetland at Slapton Ley in Devon suggests that agricultural intensification occurred before the onset of sedimentation (a silty-clay layer c. 40 cm thick) in the valley-bottom wetland of the Slapton Sewage Works marsh. The base of the silty-clay layer lies at an altitude of between 2.2 and 2.6 m AOD and has been radiocarbon dated at two locations. Conventional radiocarbon ages (+/- 2 sigma) were 910 +/- 160 yr and 960 +/- 140 yr BP. Successful radiocarbon dating of the upper surface of this minerogenic layer at one location yielded a conventional radiocarbon age of 730 +/- 120 yr BP. Within the errors associated with radiocarbon dating, the onset of sedimentation appears to be associated with a period of climatic deterioration towards the end of the Mediaeval Climatic Optimum. While agriculture plays an important role in exposing unprotected soil at certain times of the year, an increase in the magnitude and frequency of wet and severe winters may have led to a substantial increase in the risk of erosion. Contemporary analogues serve to illustrate the complex relationships which may exist between agricultural practices, climate and weather conditions and to explain why erosion is often localized and episodic in nature.