POLLEN AND DIATOM ANALYSES OF LATE‐QUATERNARY DEPOSITS IN THE BLELHAM BASIN, NORTH LANCASHIRE

Abstract

SummaryInvestigations have been made of Late‐Weichselian and Flandrian deposits in Blelham Tarn, Lancashire and other parts of its basin. Pollen and diatom analyses have been made of several cores and samples taken from the inflows. Pollen analysis has been used as a basis for chronology. The vegetational history of the region and past diatom flora of the tarn and ‘kettle‐hole’ are discussed. During the Late‐Weichselian, in pollen zones I and III, the vegetation was dominated by herbs of open habitats, whilst in pollen zone II birch woodland developed. The diatom flora consisted mainly of non‐planktonic forms with Fragilaria spp. dominant. In the Early‐Flandrian (pollen zones IV–VI) birch woodland developed and was succeeded by a mixed oak forest. Differences are shown in the pollen spectra from the ‘kettle‐hole’, marginal and open water regions of the tarn. The planktonic/non‐planktonic (P/NP) diatom ratio slowly increased in the tarn. However, dominance was retained by the non‐planktonic forms. This increase in P/NP ratio did not occur in the ‘kettle‐hole’ region. A new diatom variety Melosira distans var. blelhamensis appeared in the flora. At the opening of pollen zone VIIa Alnus rapidly increased, and for the first time planktonic diatoms became dominant in the open waters of the tarn. In littoral regions non‐planktonic forms, in particular Achnanthes minutissima var. cryptocephala, remained dominant. Sphagnum‐sedge peat accumulated in the ‘kettle‐hole’. Diatoms are scarce in this peat. During the Late‐Flandrian (pollen zone VIIb to the present day) the local vegetation was an older mixed oak forest. However, more open vegetational conditions gradually developed as shown by the increase in herbaceous pollen. Dominance was held in the open waters by the planktonic diatoms. The littoral regions favoured the growth of Achnanthes minutissima var. cryptocephala. Sphagnum‐sedge peat continued to accumulate in the ‘kettle‐hole’. Other limnological aspects considered include the tarn's trophic condition, productivity, alkalinity, the transparency of its waters and tarn level changes. In the past the tarn covered a much wider area. Field evidence shows that man has interfered with the drainage system.