Sand-drift and Soil Formation Along an Exposed North Atlantic Coastline: 14,000 Years of Diverse Geomorphological, Climatic and Human Impacts

Abstract

This paper sets out the history of aeolian deposition and soil formation over the last 14–15,000 calendar years along the exposed and gale-prone Atlantic coastlines of the Southern Isles of the Outer Hebrides of Scotland. Detailed palaeoenvironmental interpretation and chronological control have been provided by field-mapping, soil micromorphological analyses, and radiocarbon and optically stimulated luminescence (OSL) dating programmes. Two dominant types of aeolian sands are described in this region. Quartzose sands are shown to originate mainly as periglacial wind-blown sands derived from glacial and periglacial deposits of Late Devensian age which were formerly exposed on the present continental shelf. This aeolian deposition did, however, continue into the Early Holocene, with re-working, perhaps by slope-wash, taking place as recently as the 4th millenniumbp. Quartzose sands mantle valley floors and hillslopes. By the end of the Devensian and Early Holocene, a variety of soil types with extensive horizonation, had developed upon them, including podzolic brown earths, peaty gleys and peat soils.Radiocarbon and OSL dating demonstrate that the carbonate sands of the machair sand sheets which characterize the present Atlantic shorelines began to accumulate very early for a European shoreline: from approximately 8700 calendar years ago on Benbecula and North Uist; whereas further south, deposition began later. OSL-dating suggests that non-sequences and diastems are common. In greater part these machair sand sheets are best interpreted as a composite of separate slivers of sand reflecting small-scale and local episodes of sand drift, rather than distinct layers of region-wide distribution which might reflect past major storms or significant palaeoclimatic fluctuations. The new evidence presented here provides little support for the following explanatory geomorphic or stratigraphic models which have previously sought to explain the development of these coastal machair landscapes: “continuous development”; “layer cake”; or “step-wise prograding coastal barrier”. Qualified support is given for the explanations that emphasize (i) the long-term, spasmodic input and re-cycling of sands; and (ii) that the present smoothed landscapes and distinctive machair grassland vegetation are essentially recent in age and reflect the interactions of natural and human processes in a stormy environment characterized by re-working.Widespread layers of thick organic materials within the carbonate sands in the study area which have previously been regarded as dune palaeosols, dating to the Bronze Age and Iron Age, are suggested by soil-micromorphological analysis to be in significant part, anthropogenic in origin. They are organically-enriched layers or midden-soils, the consequence of human activities: notably the disposal of organic wastes and other forms of soil modification during times of significant settlement in these coastal sands. These periods of relatively intense human activity are therefore shown to be associated, at least locally, with periods of “soil” formation and stability; not universal instability and dune drift along these shorelines. In general, these extensive layers of midden-soil do not necessarily imply periods of decreased storminess. Relatively widespread soil formation in these coastal dunes did, however, take place in the period between 400 and 100 years ago: essentially within the period of increased storminess associated with the “Little Ice Age”.It is suggested that at some point(s) within the following periods of time (broadly defined in calendar yearsbp), notable sand drift affecting the quartzose sands, the carbonate sands or both, took place within the study area: 13,700–13,000; 11,000–10,500; 9000–8300; 7500–7000; 6900–6400; 5800–4200 (one of the most notable episodes of drift in the Outer Hebrides, as well as elsewhere on the coast of north-west Europe); 3800–3300; 1700–1300; 600–200 (with individual great storms being recorded in the 17th and 18th centuries); and 100–present-day (limited drift associated with “machair stratification”).