Nebria rufescens, a tundra beetle
A shallow water diatom
Significance: this cliff-top site preserves a detailed record of vegetation change on Burra that extends from deglaciation and into the middle Holocene. The organic sediments are overlain by cliff top storm deposits.
The site is on the small island of Kletta and shows many interesting features. A former lake bed is unusually accessible due to continuing coastal erosion but this erosion threatens to remove entirely the sediment sequence.
The uppermost deposits form a low ridge or berm. The faces show crudely layered gravel and sand, with angular clasts of schist. The mix of clast-supported and matrix-support layers and sand lenses is typical of cliff-top storm deposits found elsewhere on Shetland. The material reflects wave- and air-throw of debris during one or more major storms. The lower part of the rubble is loaded into upper part of the organic deposits, with angular clasts found within deformed beds of mud. This feature implies that the organic sediment, now truncated, was wet when the storm deposits were laid down, suggesting drainage of the lake due to cliff retreat in a major storm. The youngest date on the underlying, eroded sediment is ~4000 cal yr, a minimum age for the storm that breached the northern rim of the lake basin. It is worth remembering that the site lies around 20 m above sea level so the height of the storm waves must have been enormous.
The schist floor in front of the lake sediments has been stripped by wave action and shows excellent striated surfaces trending towards 304º, with loose erratic boulders.
The lake sediments are around 1m thick. They comprise beds and laminae of silts and sands and include layers of organic mud or gyttja. Some laminae are rich in leaves of the dwarf shrubs Salix herbacea and Betula nana, indicating that this exposed site once supported scrub woodland. The entire sequence has been closely sampled to identify pollen, diatoms and invertebrates that can be used to reconstruct the environmental history of the site (Whittington et al, 2003; Robinson, 2004).
The site was free of ice and supporting vegetation between 15.7 and 14.4 thousand calendar years ago. A tundra landscape existed at this time, supporting dwarf hazel and birch but the presence of remains of caddis flies indicates that temperatures were close to those of the present. Around 13 thousand years ago conditions deteriorated, ushering in the Loch Lomond Stadial. Arctic beetles appear, together with diatoms typical of tundra lakes. At 11,500 years ago these cold indicators disappear, marking the start of the Holocene. Warmth-loving beetles arrive, now restricted to areas south of the Highland Line in Scotland. Birch woodland developed but had disappeared by around 10,000 years ago, perhaps as a result of exposure to strong winds. A maritime grassland was then developed which persisted thereafter.