Lake sediments in northern Scotland

Abstract

A survey of deep-water sediments in 11 lakes in northern Scotland showed that only under certain conditions does a complete and conformable series of deposits accumulate. In lochs exposed to strong winds there may be no permanent settling of organic sediments in water depths of up to 50 m. Three lake cores (representative of three regions of northern Scotland), which proved to be complete and conformable profiles, were analysed in detail for pollen and certain chemical elements; one was also analysed for diatoms. A series of $^{14}$C dates was obtained for two of these profiles. Changes in pollen content were found to be very consistently related to changes in sediment composition. Pollen zones were defined in terms of characteristic taxa, and variance in sediment composition was expressed as the first component of a Principal Components Analysis; changes in this first component invariably coincided with pollen zone boundaries based on changes in pollen spectra. This close relationship is explained as the consequence of the derivation of these lake sediments from soils on the catchments. Two important features of soil history emerged from this study: first, the general impoverishment of soils, water and biota due to leaching during the early millennia of the 15 000 years covered by these profiles, and secondly the changes in pollen spectra which accompanied chemical evidence for the development of peat on certain of the western catchments since ca. 3000 B.C. Complete analysis of a core from Loch Sionascaig in Wester Ross (pollen, chemical and diatom) has provided a history of the soils and the aquatic and terrestrial vegetation of the Inverpolly National Nature Reserve. Pollen analysis of cores from two small lochs supplemented the evidence from Loch Sionascaig as to late-glacial and early post-glacial vegetation history in the far north-west Highlands. Chemical and biological evidence for the history of blanket peat formation on the Sionascaig catchment which was provided by the lake sediments has been supplemented by examination and pollen analysis of four profiles from blanket peat, in all of which the presence of timber within the peat confirms the evidence of the pollen diagrams that pine-birch forest was present in this region during the period between ca. 2400 and 1500 B.C. Complete late-glacial profiles were found in three lochs in Region 1, the far north-west Highlands, and in Loch Tarff above the Great Glen. Our results confirm and extend those from Loch Droma (Kirk & Godwin 1963), proving that those sites in northern Scotland were free of ice throughout the Late-Weichselian period. The sequence of pollen zones - A (Rumex zone), B (woody plants zone) and C (Artemisia zone) - resembles that which has been found at many sites in Highland West Britain, including the closely dated site at Blelham Bog in the Lake District (Pennington & Bonny 1970). In Region 2, the mountainous part of Wester Ross near Beinn Eighe, lake profiles indicate the input of thick and barren laminated sediments, impenetrable to the corers used, during the final cold phase of the Late-Weichselian (Younger Dryas time). During the preceding Late-Weichselian interstadial, which on the evidence from Loch Droma had begun by ca. 10 870 B.C., pollen spectra indicate that the vegetation of northern Scotland must have been Empetrum heath, with juniper present at some sites but not at others, and no trees present. Pre-interstadial pollen spectra (Rumex zone) resemble plant assemblages found on immature soils. Chemical analyses indicate continuous and uninterrupted processes of soil maturation through pre-interstadial into interstadial time - accumulation of humus, leaching, and chemical weathering with formation of clay minerals. Pre-interstadial and interstadial diatom assemblages at Loch Sionascaig include many alkaliphilous species and some now characteristic of eutrophic habitats; the present acid poverty of this lake and its catchment must be the result of removal of soil bases by late- and post-glacial leaching. At all sites where it is present, pollen zone C (Artemisia) corresponds with sediment of very low organic content, and simultaneous changes in pollen and chemical composition at the boundaries of this zone are interpreted as the results of pronounced and synchronous environmental (climatic) changes at the beginning and end of Younger Dryas time. Above the Artemisia zone, pollen spectra, diatoms and chemical analysis all indicate rejuvenation of soils by the effects of the post-interstadial cold period; the biological evidence points to a repetition, at the opening of the post-glacial period, of pre-interstadial to interstadial plant successions, but in a much shorter time. $^{14}$C dating at two sites shows that the spread of birch forest in northern Scotland was delayed for up to 1000 years after its establishment in northern England. On the evidence of ESR spectra of the humic acid in the Loch Sionascaig sediment, soils in northern Scotland had become acid before the arrival of forest. Post-glacial pollen diagrams are divided into a series of Regional Pollen Zones for northern Scotland; in north-west Scotland the boundaries of these zones have been dated by $^{14}$C at two sites and the pollen zones correlated with chronozones. Early post-glacial Empetrum and juniper zones are followed by a birch-hazel zone; from ca. 6000 B.C. onwards the birch-hazel pollen assemblage is replaced progressively by pine-birch. Surviving birch and birch-hazel woods round Loch Sionascaig are interpreted as the relics, on dry flush slopes, of a forest type which was widespread there before 6000 B.C. Chemical evidence suggests that between ca. 6000 and 4400 B.C. pine and pine-birch woods were growing on comparatively dry mineral soils, but from ca. 4400 B.C. the appearance of alder pollen is accompanied by evidence for solutional transport of iron and manganese from increasingly waterlogged soils. By 3000 B.C. formation...