A reinterpretation of the Lateglacial environmental history of the Isle of Skye,Inner Hebrides,Scotland

This paper presents a major revision of the Late Devensian Lateglacial environmental history of the Isle of Skye, Scotland, based upon a combination of geomorphological, biostratigraphical and radiocarbon evidence. The distribution of glacial and periglacial landforms, and of raised shorelines, suggests that there was only one extensive readvance of local glaciers in southern Skye following the wastage of the Late Devensian ice sheet. Pollen-stratigraphic evidence from 10 sites inside and 4 sites outside the mapped ice limits indicates that this readvance occurred during the Loch Lomond Stadial. At that time over 180km² of the uplands of south-central Skye were covered by glacier ice, a much more extensive glaciation than previously envisaged. Palynological evidence from four Lateglacial profiles implies that degree of exposure to strong westerly winds was the principal factor determing vegetational contrasts on the island, and that regional differences in vegetational type were less pronounced than has hitherto been suggested. The glacial and palaeobotanical reconstructions reported here are more compatible with Lateglacial data from the Scottish mainland and Hebridean islands than were the previously-published accounts for the Isle of Skye.     

Late Devensian and Flandrian palaeoenvironmental changes on the Scottish continental shelf west of the Outer Hebrides

Examination of two radiocarbon-dated vibrocores taken from south of St Kilda at a water depth of about 155 m, a short distance within the maximum position of the Late Devensian (Dimlington Stadial) ice sheet, suggests that the St Kilda Basin became free of glacier ice after 15250 yr BP. Sedimentation in a shallow, low energy, high arctic, muddy environment continued until after 13500 yr BP. There followed a higher energy temperate episode during which water depths were roughly about 40 m: this is correlated with the latter part of the Windermere Interstadial and with the warmer interval known in shallow Scottish seas about or a little before 11 000 yr BP. The Loch Lomond (Younger Dryas) Stadial is marked in the vibrocores by the return of muddy sediments and a cold-water fauna. Relatively shallow water conditions seem to have persisted into the earliest Flandrian, when the water depth was still roughly 60 m, corresponding to a sea-level in the area 90–100 m below present. It is suggested that pack ice was widespread in the northeast Atlantic before the Windermere Interstadial and also during the Loch Lomond Stadial, when it transported shards of Icelandic volcanic ash into the St Kilda basin. Estimates of sea-surface temperature for the last part of the Windermere Interstadial are close to those derived from the deep-sea record for the same period.     

Lacustrine ostracodes as stable isotope recorders of late-glacial and Holocene environmental dynamics and climate

Oxygen- and carbon-isotopic signatures of benthic ostracodes from lake sediments from climate sensitive regions in the Alpine region, Central Europe, the north-central USA, the Chilean Altiplano and Patagonia, Argentina, are used to characterize lake system processes and to reconstruct climate patterns of the past 16,000 years. The case studies provide examples that highlight different aspects of the broad application of isotope stratigraphies, and provide keys for the interpretation of complex lacustrine records.The integration of stable-isotope stratigraphy, sedimentology, and ecological information from ostracode assemblages is a new tool that acquires climate information from the indirect views of climate series provided by lake sediments. This tool (1) identifies lake system characteristics, (2) confines which isotopic signatures are controlled by which processes in the lake system and/or in the catchment, and (3) defines which signatures are ultimately controlled by climate change. If sudden shifts in the isotopic composition occur concomitantly with changes between sedimentological units, then the isotopes reflect first of all changes in catchment hydrology that may be ultimately controlled by climate. Also, if ostracode ¹⁸O and ¹³C values show the same timing and direction of shifts, then this indicates a major change in the hydrological budget of the lake.The case studies presented here show that coupled isotopic signatures may be used to track hydrological changes related to meltwater and deglaciation, shifting rivers and ground water sources, and changes in precipitation mechanisms and patterns. Values of ¹⁸O from large lakes with short water residence time, low evaporation rates and homothermic bottom waters provide records of past temperatures of precipitation. The ¹³C values reflect changes in the ratio of C₃:C₄ plants in the catchment. They indicate shifts in modes of organic decay in the surface sediments that can be linked to a change in hydrodynamics within a lake. The ¹³C values also allow detection of the input of volcanically charged ground waters providing large quantities of ¹⁴C-free CO₂ that hinders accurate ¹⁴C chronology. General climate trends for the sites in the Americas indicate a dry mid-Holocene punctuated by moist spells, and show a general increase in moisture during the past approximately 4000 years, interrupted by recurring droughts. This hints at an interhemispheric connection and a common driving mechanism.Environmental isotopes from high-resolution lake sediments thus provide an ideal tool to identify and characterize the regional impact and magnitude of global climate change. This tool contributes to a better understanding of regional climate change and its driving mechanisms and thus provides the type of information needed to improve climate models. Environmental isotopes provide more information than just moisture balance and airmass history if they are integrated with the detailed sedimentological and ostracode ecological evidence, and understanding for the component system. Thus environmental isotopes serve to a better understanding of the climate signal archived in lake records and represent an essential contribution to Global Change research and Earth System Science.     

The Weichselian Late-glacial in southwestern Europe (Iberian Peninsula,Pyrenees, Massif Central,northern Apennines)

This paper presents a summary of Late-glacial environmental changes in southwestern Europe (lberian Peninsula, Pyrenees, Massif Central and the northern Apennines). The emphasis is on palaeoclimatic interpretations inferred from key sites in the region from which the most detailed records are available and which have been radiocarbon dated. The earliest evidence for climatic improvement following the end of the last glacial stage is dated to ca. 15 ka BP and is found at a few sites only. By 13 ka BP, a more widespread and marked climatic improvement is evident, although it is difficult to be precise in the timing and magnitude of the event. There are significant variations in detail between the Late-glacial records, but evidence for a significant cooling correlated with the Younger Dryas event is widespread throughout the region. Just two sites in the region provide evidence for an earlier, less emphatic phase of climatic cooling, which is tentatively equated with the ‘Older Dryas’ of continental northern Europe. Dry conditions appear to have predominated throughout the region in the later part of the Younger Dryas and the early Holocene.     

Late Weichselian vegetation,climate, and floral migration at Liastemmen,North Rogaland,south-western Norway

Palynological results from Liastemmen indicate a tripartite division of the Late Weichselian. In the pleniglacial period, from deglaciation ca. 14000 BP to ca. 13000 BP, Artemisia-dominated pioneer vegetation on disturbed, mineral-soil was strongly influenced by cold winters and katabatic winds. The Late Weichselian Interstadial (ca. 13000 BP-ca. 11000 BP) comprises a Salix-shrub consolidation phase, and from ca. 12700 BP a tree-birch phase. In the last 500 years of this period July and January means are estimated to about 16°C and between ?2°C and ?6°C, respectively. In the Younger Dryas Stadial (ca. 11000 BP-ca. 10200 BP) Artemisia-dominated vegetation returns. Three brief climatic deteriorations (ca. 12 250 BP, 11 700 BP, and 11 300 BP), unfavourable to woody vegetation on humus soils, are demonstrated within the interstadial. Critical climatic factors include cool winters and strong winds, exposing vegetation and soil to frost, drought, and erosion. The oldest and strongest oscillation, probably involving local deforestation, is correlated with the ‘Older Dryas deterioration’. Boreal-circumpolar, eurasiatic, and arctic-alpine plants dominated the late-glacial flora. For the majority of the late-glacial taxa a northward migration is demonstrated. This may also apply for Papaver radicatum, Pinguicula alpina, and Primula scandinavica, all with bicentric distributions in Norway today.     

Postglacial vegetation, climate, and fire history along the east side of the Andes (lat 41-42.5°S), Argentina

The history of the low-elevation forest and forest-steppe ecotone on the east side of the Andes is revealed in pollen and charcoal records obtained from mid-latitude lakes. Prior to 15,000 cal yr BP, the vegetation was characterized by steppe vegetation with isolated stands of Nothofagus. The climate was generally dry, and the sparse vegetation apparently lacked sufficient fuels to burn extensively. After 15,000 cal yr BP, a mixture of Nothofagus forest and shrubland/steppe developed. Fire activity increased between 13,250 and 11,400 cal yr BP, contemporaneous with a regionally defined cold dry period (Huelmo/Mascardi Cold Reversal). The early-Holocene period was characterized by an open Nothofagus forest/shrubland mosaic, and fire frequency was high in dry sites and low in wet sites; the data suggest a sharp decrease in moisture eastward from the Andes. A shift to a surface-fire regime occurred at 7500 cal yr BP at the wet site and at 4400 cal yr BP at the dry site, preceding the expansion of Austrocedrus by 1000-1500 yr. The spread of Austrocedrus is explained by a shift towards a cooler and wetter climate in the middle and late Holocene. The change to a surface-fire regime is consistent with increased interannual climate variability and the onset or strengthening of ENSO. The present-day mixed forest dominated by Nothofagus and Austrocedrus was established in the last few millennia.     

A 14,300-year-long record of fire-vegetation-climate linkages at Battle Ground Lake, southwestern Washington

High-resolution macroscopic charcoal analysis was used to reconstruct a 14,300-year-long fire history record from the lower Columbia River Valley in southwestern Washington, which was compared to a previous vegetation reconstruction for the site. In the late-glacial period (ca. 14,300-13,100 cal yr BP), Pinus/Picea-dominated parkland supported little to no fire activity. From the late-glacial to the early Holocene (ca. 13,100-10,800 cal yr BP), Pseudotsuga/Abies-dominated forest featured more frequent fire episodes that burned mostly woody vegetation. In the early to middle Holocene (ca. 10,800-5200 cal yr BP), Quercus-dominated savanna was associated with frequent fire episodes of low-to-moderate severity, with an increased herbaceous (i.e., grass) charcoal content. From the middle to late Holocene (ca. 5200 cal yr BP to present), forest dominated by Pseudotsuga, Thuja-type, and Tsuga heterophylla supported less frequent, but mostly large or high-severity fire episodes. Fire episodes were least frequent, but were largest or most severe, after ca. 2500 cal yr BP. The fire history at Battle Ground Lake was apparently driven by climate, directly through the length and severity of the fire season, and indirectly through climate-driven vegetation shifts, which affected available fuel biomass.     

Glacial deposits at the Boyne Bay Limestone Quarry,Portsoy, and their place in the late Pleistocene history of northeast Scotland

The glacial deposits at the Boyne Bay Limestone Quarry near Portsoy, a key Quaternary Site of Special Scientific Interest, comprise (i) a sandy, partly weathered diamicton (Craig of Boyne Till Formation, CBTF) resting on decomposed bedrock, (ii) a central, variably glaciotectonised assemblage of dark clay, diamicton and sand, with rafts of sand and weathered diamicton (Whitehills Glacigenic Formation, WGF), and (iii) an upper dark sandy diamicton (Old Hythe Till Formation, OHTF). The CBTF was probably derived from the west or southwest, and the WGF from seawards. Structures within the OHTF conform to deposition by east‐ or southeast‐moving ice from the Moray Firth, but some erratics indicate derivation from the south. The CBTF is believed to pre‐date the last (lpswichian) interglacial, but the WGF and OHTF both post‐date the early Middle Devensian, and are probably of Late Devensian age. It is proposed that the OHTF was deposited by ice from inland which was directed eastwards near the coast by a vigorous glacier in the Moray Firth, and that the complex, Late Devensian glacial history of the south coast of the Moray Firth as a whole is the result of the interplay of these two contemporary ice‐masses. British Geological Survey. © NERC 2000.     

Middle Devensian (Weichselian) coleopteran assemblages from Earith,Cambridgeshire (UK) and their bearing on the interpretation of ‘Full glacial’ floras and faunas

Macroscopic plant remains from middle Devensian organic lenses, exposed in a gravel pit near Earith in the valley of the River Great Ouse in Cambridgeshire, were the object of an extensive study by Frances Bell in the late 1960s, which has been much referred to in the subsequent literature. The floras from each sampling site have been taken to be characteristic of Full‐glacial conditions in general. Coleopteran assemblages taken at the same time from exactly the same sampling sites show that at least two periods of markedly different climatic regimes were involved. The first of these (? the older) was characterised by a cold and continental climate with mean July temperatures at or below 10d°C and mean January/February temperatures about −23°C. The second climatic regime was characterised by warm summers with mean July temperatures about 16°C and mean January/February temperatures close to −5°C. In spite of this temperate climate, pollen analysis shows that no trees grew in Britain at that time. This treeless but temperate episode in the middle of the Devensian (Weichselian) is discussed in the context of coleopteran assemblages from other sites in southern England that probably date from the same period. Copyright © 2000 John Wiley & Sons, Ltd.     

A Pleistocene succession from beneath Chelford Sands at Oakwood Quarry,Chelford, Cheshire

A thin sequence of in situ fossiliferous gravels and silts overlain by a glacigenic bed is described from a temporary exposure in a working silica sand quarry. The sequence directly overlies unweathered Mercia Mudstone. Locally the bedrock surface forms a shallow palaeovalley some 15 m deep and almost 1 km wide. The sequence in question lies close to the valley axis. The silts, pollen, plant macrofossils, Mollusca, Coleoptera, and Ostracoda assemblages each suggest a similar environment of sedimentation: a shallow pool within a treeless open landscape. Much of the bedrock within the working quarry appears to be draped by the Oakwood till and this unit is co-extensive with the glacigenic sediments over the silts and gravels. In parts at least, the till has been subject to redeposition and its upper surface has scattered ventifacts. Above this wind-deflation horizon lies the Chelford Sands Formation, in the middle of which is the Chelford Interstadial stratotype, the Farm Wood member. The biota preserved in the gravels and silts suggest a stadial prior to the Early Devensian Chelford Interstadial. This is the first unambiguous stratigraphic evidence from Cheshire of a glacial event antedating the Chelford Interstadial.