Late Quaternary Deposits from Southeast Scotland. II. The Diatom Assemblage of a Marl Core

Analysis of the diatom content of limnic sediments from Linton Loch, Roxboroughshire, allowed a subdivision of the sequence into six diatom assemblage zones. Palynological evidence from the same core has been used to relate the sequence to the Late Quaternary chronozones of Din Moss. Fragilaria spp. are the dominant pioneer taxa in the basal clays of Late Devensian age, when a minerogenic environment persisted. With the onset of the Flandrian, biogenic precipitation of marl commenced and a planktonic community developed, together with an epiphytic flora dominated by Epithemia spp. Productivity, as indicated by diatom concentrations, remained low throughout the period of marl deposition, though the rate of accumulation was in excess of that for other Late Quaternary sediments in Britain. Consequent on anthropogenic disturbance in the catchment, productivity increased as muds and finally peat were deposited in the basin with a diatom flora dominated by Fragilaria spp. Possible reasons for the apparent competition between Epithemia spp. and Fragilaria spp. are discussed. The persistence of an alkaliphilous/alkalibiontic diatom flora suggests the maintenance of waters of high base-status throughout the Late Quaternary. The limited application of Centrales:Pennales and Araphidineae:Centrales ratios in a marl deposit and in a palaeoenvironmental context are discussed. The Linton Loch diatoms are compared with other sites. There are unique characteristics, which appear to relate to the existence of a marl environment, and do not conform with general trends of Late Quaternary lake evolution in Britain.