Geoffrey Lemdahl

Synchronized terrestrial-atmospheric deglacial records around the North Atlantic

On the basis of synchronization of three carbon-14 (14C)-dated lacustrine sequences from Sweden with tree ring and ice core records, the absolute age of the Younger Dryas-Preboreal climatic shift was determined to be 11,450 to 11,390 ± 80 years before the present. A 150-year-long cooling in the early Preboreal, associated with rising Δ14C values, is evident in all records and indicates an ocean ventilation change. This cooling is similar to earlier deglacial coolings, and box-model calculations suggest that they all may have been the result of increased freshwater forcing that inhibited the strength of the North Atlantic heat conveyor, although the Younger Dryas may have begun as an anomalous meltwater event.

Quantification of biotic responses to rapid climatic changes around the Younger Dryas — a synthesis

To assess the presence or absence of lags in biotic responses to rapid climatic changes, we: (1) assume that the δ18O in biogenically precipitated carbonates record global or hemispheric climatic change at the beginning and at the end of the Younger Dryas without any lag at our two study sites of Gerzensee and Leysin, Switzerland; (2) derive a time scale by correlating the δ18O record from these two sites with the δ18O record of the GRIP ice core; (3) measure δ18O records in ostracods and molluscs to check the record in the bulk samples and to detect possible hydrological changes; (4) analyse at Gerzensee and Leysin as well as at two additional sites (that lack carbonates and hence a δ18O record) pollen, plant macrofossils, chironomids, beetles and other insects, and Cladocera; (5) estimate our sampling resolution using the GRIP time scale for the isotope stratigraphies and the biostratigraphies; and (6) summarise the major patterns of compositional change in the biostratigraphies by principal component analysis or correspondence analysis. We conclude that, at the major climatic shifts at the beginning and end of the Younger Dryas, hardly any biotic lags occur (within the sampling resolution of 8–30 years) and that upland vegetation responded as fast as aquatic invertebrates. We suggest that the minor climatic changes associated with the Gerzensee and Preboreal oscillations were weakly recorded in the biostratigraphies at the lowland site, but were more distinct at higher altitudes. Individualistic responses of plant and animal species to climatic change may reflect processes in individuals (e.g. productivity and phenology), in populations (e.g. population dynamics), in spatial distributions (e.g. migrations), and in ecosystems (e.g. trophic state). We suggest that biotic responses may be telescoped together into relatively short periods (50 to 150 years), perhaps disrupting functional interactions among species and thus destabilising ecosystems.

Temperature gradients in northern Europe during the last glacial-Holocene transition (14-9 14C kyr BP) interpreted from coleopteran assemblages.

Late-glacial and early Holocene climatic conditions have been reconstructed for northern Europe using the mutual climatic range (MCR) palaeoclimate method based on fossil coleopteran assemblages. Altogether, beetle faunas from 77 sites have been analysed ranging from Ireland in the west to Poland and Finland in the east, and MCR estimates calculated. The results are plotted on 16 maps, each representative of a selected time-slice covering the period from 14.5 14C kyr BP to 9.0 14C kyr BP. Eight of the maps show the MCR estimates of Tmax (mean temperature of the warmest month) derived from each site for which data are available, while the remainder show estimated Tmax isotherms interpolated from these values. It can be demonstrated that at times the thermal climate was fairly uniform throughout the study area, whereas at others temperature gradients were much steeper than they are in the region today. There also appears to be a distinct contrast between cold periods, when contours trended NW–SE, and warmer periods, when contours trend W–E or even NE–SW. The pattern of climatic changes that emerges is shown to be very different from the traditional view that has been used up to now as a template for classifying Late-glacial climatic events on a wide, even global, scale. The suddenness and intensity of changes in the thermal climate may have been partially responsible for an apparent lack of equilibrium between the flora and fauna of the time and the physical environment in which they lived.

Lateglacial and Early Holocene insect assemblages from sites at different altitudes in the Swiss Alps—implications on climate and environment

Insect analyses from four sites at differentaltitudes in the SwissAlps are presented. The dominant insect group considered here is Coleoptera (beetles), though other insect orders are also recorded ...

The Krakenes late-glacial palaeoenvironmental project

Kråkenes is the site of a small lake on the west coast of Norway that contains a long sequence of late-glacial sediments. The Younger Dryas is well represented, as a cirque glacier developed in the catchment at this time. This site offers unique opportunities to reconstruct late-glacial environments from independent sources of evidence; physical evidence (glacial geomorphology, sedimentology, palaeomagnetism, radiocarbon dating), and biological evidence from the remains of animals and plants derived from both the terrestrial and aquatic ecosystems. This report describes the background to the site, and the international multidisciplinary project to reconstruct late-glacial and early Holocene environmental and climatic changes at Kråkenes.

Regression coefficients of thermal gradients in northwestern Europe during the last glacial-Holocene transition using beetle MCR data.

Palaeotemperature estimates obtained from 74 sites in northern Europe, and collectively spanning approximately the last 45 000 yr (radiocarbon time-scale), have been compiled as a major component o ...

The role of tree composition in Holocene fire history of the hemiboreal and southern boreal zones of southern Sweden, as revealed by the application of the Landscape Reconstruction Algorithm: Implications for biodiversity and climate-change issues

We present a quantitative reconstruction of local forest history at two sites, Stavsåkra (hemiboreal zone) and Storasjö (southern boreal zone), in southern Sweden (province of Småland) to evaluate possible causes of contrasting Holocene fire histories in mid- and late Holocene. The Landscape Reconstruction Algorithm (LRA) is applied to evaluate between-site differences in the relative abundance of deciduous trees and Pinus (pine) and landscape/woodland openness during the Holocene. The LRA estimates of local vegetation abundance are compared with other proxies of local vegetation, that is, plant and beetle remains. The LRA results suggest that Pinus was a major tree taxon in the woodlands of Storasjö during mid- and late Holocene, while Tilia (linden) and Betula (birch) were dominant at Stavsåkra. The contrasting fire histories are shown to be strongly related to between-site differences in tree composition during mid-Holocene, 4000–2000 bc in particular. The archaeological/historical and beetle data indicate contrasting land uses from c. 1000 bc (late Bronze Age/early Iron Age), grazing in open Calluna heaths at Stavsåkra and woodland grazing at Storasjö. Between-site differences in fire history during late Holocene were likely due to different land-use practices. Between-site differences in tree composition in mid-Holocene are best explained by local climatic and geological/geomorphological differences between the hemiboreal and southern boreal zones of Småland, which might also be the primary cause of between-site differences in land-use histories during late Holocene. Maintenance of biodiversity at the landscape scale in the study area requires that existing old pine woodlands and Calluna heath are managed with fire and cattle grazing. Further climate warming might lead to higher probabilities of climate-induces fire, in particular in pine-dominated woodlands.

Historical land-use and landscape change in southern Sweden and implications for present and future biodiversity.

The two major aims of this study are (1) To test the performance of the Landscape Reconstruction Algorithm (LRA) to quantify past landscape changes using historical maps and related written sources, and (2) to use the LRA and map reconstructions for a better understanding of the origin of landscape diversity and the recent loss of species diversity. Southern Sweden, hemiboreal vegetation zone. The LRA was applied on pollen records from three small bogs for four time windows between AD 1700 and 2010. The LRA estimates of % cover for woodland/forest, grassland, wetland, and cultivated land were compared with those extracted from historical maps within 3-km radius around each bog. Map-extracted land-use categories and pollen-based LRA estimates (in % cover) of the same land-use categories show a reasonable agreement in several cases; when they do not agree, the assumptions used in the data (maps)-model (LRA) comparison are a better explanation of the discrepancies between the two than possible biases of the LRA modeling approach. Both the LRA reconstructions and the historical maps reveal between-site differences in landscape characteristics through time, but they demonstrate comparable, profound transformations of the regional and local landscapes over time and space due to the agrarian reforms in southern Sweden during the 18th and 19th centuries. The LRA was found to be the most reasonable approach so far to reconstruct quantitatively past landscape changes from fossil pollen data. The existing landscape diversity in the region at the beginning of the 18th century had its origin in the long-term regional and local vegetation and land-use history over millennia. Agrarian reforms since the 18th century resulted in a dramatic loss of landscape diversity and evenness in both time and space over the last two centuries leading to a similarly dramatic loss of species (e.g., beetles).

Late-glacial and early-Holocene Coleoptera assemblages as indicators of local environment and climate at Kråkenes Lake, western Norway.

Thirty-six Coleoptera (beetle) taxa and other insects were identified from the late-glacial and early-Holocene sediments at Kråkenes Lake. Compared with other Scandinavian late-glacial sites, this is a rather sparse record. The water beetles found in the Allerod are characteristic of a poorly vegetated clear-water lake. The terrestrial fauna is indicative of dwarf-shrub and moss vegetation. A marked decline in the number of species at the start of the Younger Dryas was rather rapid, probably over less than 80 calendar yrs. No obligate tundra species replaced the Allerod fauna. Most of the Younger Dryas is virtually devoid of beetles. The increase in numbers and diversity of both aquatic and terrestrial species at the Younger Dryas/Holocene transition is very rapid. After an initial pioneer stage, beetles associated with dwarf-shrub heath and willow scrub appeared, but no obligate tree or forest taxa were recorded.Mutual Climatic Range (MCR) temperature reconstructions suggest that the Allerod was colder and more continental than present. The near absence of beetles in the Younger Dryas probably reflects very cold conditions. A rapid temperature rise at the start of the Holocene resulted in a warmer and more continental climate than present.

Biotic responce to climatic changes during the time span 13 000 - 10 000 BP. - A case study from SW Sweden.

Palaeoecological studies based on analysis of pollen, plant macro-fossils and insect remains at a stratigraphic reference site, covering the time span 13,000–10,000 yrs B.P., in SW Sweden (Hakulls Mosse, province of Skane) are interpreted against the two dominant palaeoclimatic models: the palaeobotanical model (Iversen) and the palaeoentomological model (Coope). An important time lag is found in the vegetation response to climatic improvement after deglaciation, which means that the biotic changes are best explained against the palaeoentomological model. This implies optimal summer temperatures 13,000–12,500 followed by a gradual deterioration until 11,000 when a sudden drop of temperature leads to a minimum between 11,000 and 10,500. A distinct rise of summer temperature around 10,500 is confirmed although the time lag in the vegetation response is ab. 300 years.