Holocene history of environment, vegetation and human settlement on Catta Ness, Lunnasting, Shetland

Pollen, charcoal, chemical, physical, magnetic and tephra analyses of 14C-dated Holocene lake sediments from Dallican Water, Catta Ness, north-east Shetland, are presented and interpreted in the light of models of Holocene climatic change, the Shetland archaeological record, and local documentary evidence. The sequence was subdivided and analysed using principal components analysis, a numerical zonation of the pollen data using optimal and binary divisive techniques, and measures of palynological richness and rates of change between samples. Sedimentation began at 9900 years BP, and until 9620 BP consisted of minerogenic sediments eroded from raw soils. Vegetation was herbaceous until 9500 BP, when Betula woodlands began to develop. During the period 9620-7500 BP woodland diversified (Betula, Corylus avellana, Juniperus communis, Quercus, Alnus glutinosa and possibly Ulmus and Fraxinus excelsior) with tall-herb communities that included a high proportion of ferns. Lake sediments became predominantly organic as soils became more stable, with some leaching of cations, and erosion was reduced. From 7500 to 5400 BP tree cover changed little, but the herb and fern communities were replaced by plants of heathland and mires. Erosion increased slighting and charcoal accumulation was high. These changes suggest the presence of Mesolithic people, and, associated with them, a mammalian herbivore (possibly red deer), which reduced the herb and fern communities by grazing. After 5400 BP, vegetation passed through a series of successional stages, returning within a few hundred years to a type similar to that prevailing earlier than 7500 BP. Contemporaneously, erosion decreased as soils restabilized. Charcoal accumulation was low. We suggest that the herbivore had become extinct, and the islands became uninhabited. From c. 4800 BP, Calluna vulgaris and other mire plants increased, probably due to natural soil leaching and acidification. The extent of peatland greatly increased, sealing sources of supply of erodible material. Pollen and charcoal data suggest that people were present on the island from c. 4500 BP, supporting archaeological dating for occupation of Shetland by agricultural communities. The first phase of clearance on Catta Ness was c. 4000 BP, and lasted for c. 400 years. Woodland then increased to its former levels. Woodland was almost completely cleared from the area within 150 years c. 3120 BP, probably as a deliberate act to extend rough grazing. This dramatic event cannot be correlated with any known cultural changes in the Shetland archaeological record, and probably results from local human activity. Heath plants increased substantially, and sediments became more organic as erosion sources continued to diminish. Vegetation and land use have remained more or less constant for the past 3000 years, despite major cultural changes, such as the building of brochs c. 2000 BP and the arrival of Norse people c. 1150 BP. Documentary evidence suggests that the area of land under cultivation on Catta Ness has not changed within the last 350 years. This study has confirmed the former presence of Alnus glutinosa on Shetland, and indicated that Quercus was also present. Thus, the early Holocene woodlands of Shetland were, at least locally more diverse than had hitherto been realized. At least two layers of volcanic ash occur in the sediments, probably originating from Icelandic sources. A layer of basic ash c. 9300 BP is identified as the Saksunarvatn ash previously described from the Faroe Islands. A complex layer of acidic ash 4000 BP is probably part of the Hekla 4 ash fall. These horizons hold considerable potential as dating horizons within the Holocene of the north-east Atlantic, and contribute to understanding of the distribution of Holocene tephras in the area. This is the first complete Holocene tephra record to be obtained from the British Isles.