holocene vegetation

tephra

key sites

Dallican Water (HU498674) (see map)

A B C

Tephra particles in sediments from Dallican Water. A, 703 cm depth B, 523 cm depth C, 503 cm depth

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Alnus glutinosa

Dallican Water

Images and text originally from http://www.kv.geo.uu.se/scotland/scotland.html

Significance: this site on Catta Ness, Lunnasting has provided pollen, charcoal, chemical, physical, magnetic and tephra analyses that allow a detailed reconstruction of environmental change through the Holocene (Bennet, et al., 1992).

Sedimentation in the lake began around 9900 years BP, and until 9620 years BP consisted of minerogenic sediments eroded from raw soils (see Fig. 3). Vegetation was herbaceous until 9500 years BP, when Betula woodlands began to develop (see Fig. 4). During the period 9620-7500 years 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 (see Fig. 4). Lake sediments became predominantly organic as soils became more stable, with some leaching of cations, and erosion was reduced.


From 7500 years BP to 5400 years BP, tree cover changed little, but the herb and fern communities were replaced by plants of heathland and mires. Erosion increased slightly 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 years 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 years BP Contemporaneously, erosion decreased as soils re-stabilized. Charcoal accumulation was low. We suggest that the herbivore had become extinct, and the islands became uninhabited.

From about 4800 years 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 about 4500 years BP, supporting archaeological dating for occupation of Shetland by agricultural communities. The first phase of clearance on Catta Ness was at about 4000 years BP, and lasted for about 400 years. Woodland then increased to its former levels.

Woodland was almost completely cleared from the area within 150 years at about 3120 years 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 at about 2000 years BP and the arrival of Norse people at about 1150 years 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 at about 703 cm depth (about 9300 years BP) is identified as the Saksunarvatn ash previously described from the Faeroe Islands. A complex layer of acidic ash at 523 cm and 503 cm depth (about 4000 years BP) is probably part of the Hekla 4 ash fall. These horizons hold considerable potential as dating horizons within the Holocene of the northeast 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.

Keith Bennett, Professor e-mail: Keith.Bennett@geo.uu.se University of Uppsala