J.-L. de Beaulieu

Comparison of terrestrial and marine records of changing climate of the last 500,000 years

A broad correspondence between long pollen sequences and the deep-sea oxygen isotope record has been noted for some time, but there has been little effort to explore just how similar the two types of evidence are in terms of their overall structure on glacial-interglacial timescales and also how they may differ. These questions have profound importance both for how we view the stratigraphic record of changing climate in different regions and for our understanding of the climate system. Here we link the four longest European pollen records and derive a terrestrial sequence of vegetation events and a coherent stratigraphic scheme for the last 500,000 years. Comparison of the terrestrial and marine records shows good agreement, but it also reveals that the pollen sequences contain a higher degree of climate sensitivity than the oxygen isotope record. In addition, it suggests that neither an oxygen isotope record nor a Milankovitch-forced ice volume model may provide an appropriate template for fine-tuning the terrestrial record and that better chronologies will depend on an improved understanding of controls on sedimentation rates in individual sedimentary basins

The last climatic cycle at La Grande Pile (Vosges, France) a new pollen profile

A new pollen diagram has been made from an old unanalyzed core from La Grande Pile (GP XX). Correlations with the previous work of Woillard (1975–1978) leads to the following conclusions: the end of the Eemian was marked by a minor climatic change prior to the Melisey I stadial; the Ognon II and Ognon III interstadials are not consistent, and correspond to the level of reworking of mesophilous taxa during the last Pleniglacial; the limitations of the botanical interpretation of pollen spectra from the last Pleniglacial are underlined; lastly, Bruckenthalia and Larix are identified for the first time at La Grande Pile. The respective roles of these taxa following the Eemian are additional arguments for correlating the St-Germain I and II with the Early Weichsel interstadials in the north of middle Europe.

The climate in Western Europe during the last Glacial/Interglacial cycle derived from pollen and insect remains

Using the pollen sequence of La Grande Pile XX (France), we review problems with the application of transfer functions in paleoclimatic reconstructions. One of them is to find modern analogues for the herbaceous vegetation of the cold periods. We propose a method to distinguish between steppes and tundra vegetations for which the moderns are only partial analogues of the glacial periods. Another method to solve these problems is based on constraining by insect remains. The two methods provide coherent reconstructions. The results are also compared with other paleodata. There is a good correlation with the six cold Heinrich events between 70 and 15 ka B.P. A cooling event during the Eemian period (marked by high percentages of Taxas) at about 125 ka B.P. needs still to be correlated with high resolution ocean and ice cores.

Pollen analytical biostratigraphy of the last five climatic cycles from a long continental sequence from the Velay region (Massif Central, France)

In the volcanic region of Velay (Massif Central, France), lake sediment sequences derived from maar craters situated close to one another (Ribains, Praclaux and Lac du Bouchet) have been correlated on the basis of tephrostratigraphical analyses. This has enabled the construction of a sequence (2980 pollen spectra) that begins during the glaciation preceding the Holsteinian and ends in the present. This sequence covers the period from 450 ka, i.e. from the end of marine oxygen isotope stage 12, to the present time, and includes five climatic cycles. The study of a second core from the Praclaux site completed the long Velay sequence. The biostratigraphy of the whole sequence is described in detail and illustrated by a synthetic and simplified pollen diagram. Thirteen temperate forest episodes (interglacials or interstadials) and as many cold periods (glacials or stadials) are defined. Each temperate episode is characterised by particular forest development and vegetation dynamics, and all of them, with the exception of the Holocene, begin and end with a Pinus forest. The vegetation dynamics observed during the Holocene resemble more closely those recorded during the interstadials than during the interglacials. Copyright

Was the climate of the Eemian stable? A quantitative climate reconstruction from seven European pollen records

The aim of the present study is to estimate the range of the climatic variability during the Eemian interglacial, which lasted about 10,000 years (marine isotopic stage 5e). The modem pollen analogue technique is applied to seven high resolution pollen records from France and poland to infer the annual precipitation and the mean temperature of the coldest month. The succession of pollen taxa and the reconstructed climate can be interpreted coherently. The warmest winter temperatures are centred in the first three millennia of the Eemian interglacial, during the mixed oak forest phase with Quercus and Corylus as dominant trees. A rapid shift to cooler winter temperatures of about 6 degrees to 10 degrees C occurred between 4000 and 5000 years after the beginning of the Eemian, related to the spread of the Carpinus forest. This shift is more obvious for the reconstructed temperatures than for precipitation and is unique and irreversible for the whole Eemian period. Following this climatic shift of the Eemian, variations of temperature and precipitation during the fast 5000 years were only slight with an amplitude of about 2 degrees to 4 degrees C and 200 to 400 mm/yr. The estimated temperature changes were certainly not as strong as those reconstructed for the stage 6/5e termination or the transition 5e/5d. This is consistent with the constantly high ratio of tree pollen throughout the Eemian, indicative of a succession of temperate forest types. This gradual transition between different forest landscapes can be related to intrinsic competition between the species rather than to a drastic climatic change

Establishing a terrestrial chronological framework as a basis for biostratigraphical comparisons.

The palynological signature of interglacial deposits in the fragmentary European terrestrial record has often been used as the basis for determining their chronostratigraphical position and ultimately their age. This has placed emphasis on the presence/absence and abundance of certain characteristic taxa, but given the lack of continuous stratigraphies and independent chronologies, it has been difficult to assess the extent to which this strategy has produced reliable schemes. Here, an alternative approach is adopted whereby a chronological framework is developed for long and continuous pollen sequences from southern Europe. This in turn allows the emergence of a complete stratigraphical scheme of major vegetation events for the last 430 thousand years (ka) and the evaluation of the stage record of different taxa and their potential diagnostic value for biostratigraphical correlation. The comparison shows distinct similarities among some temperate stages of the terrestrial equivalent complexes of Marine Isotope Stages (MIS) 5 and 7 and also of MIS 9 and 11, but examination of combined records of taxa provides a possibility to differentiate between individual stages. A numerically-derived dichotomous key for the terrestrial stages based on the palynological records of 10 taxa is presented. Carpinus, Fagus, Abies, Pterocarya and Buxus emerge as the best ‘indicator pollen types’ because of their variable behaviour from one stage to the next, possibly a result of their late expansion within a temperate stage or reduced genetic variability. The analysis shows that the palynological signature of a temperate deposit can constrain the range of chronostratigraphical possibilities, but vegetation and palynological variability arising from local factors could result in difficulties in making a definite assignment at individual sites.

Pollen analysis of a long upper Pleistocene continental sequence in a Velay maar (Massif Central, France)

Abstract A new 20 m sequence is presented which is based on 279 spectra derived from a coring at Le Bouchet lake (Massif Central, France). The vegetational history of this region is described back to the end of the last Interglacial (Ribains interglacial). The two Preglacial (early Glacial) forested interstadials (St-Geneys 1 and St-Geneys 2) are evidence for the first time in the Massif Central. In spite of the altitude of the site (1200 m), they suggest a quite temperate vegetation, with the same mesophilous character as during the Holocene. The Pleniglacial appears to be threefold, with two cold episodes (Lower and Upper Pleniglacial) divided by a long complex period of moderate climatic improvement. The vegetation history of this mountain region is compared with that evidenced for the same period on the basis of the sequence of Les Echets at a lower altitude (267 m). Botanical and climatic inferences are discussed.

Recent contributions to the climatology of the last glacial-interglacial cycle based on French pollen sequences

Abstract New pollen data derived from three long French sequences and methodological improvements in the methods used to reconstruct past climates are summarized here, after a short presentation of the study sites and pollen diagrams. Estimated climate parameters during the major episodes of the last 140,000 years are shown to be coherent between sites. Analysis of the seasonality variations provides much additional information. In particular it is shown that the advection from the ocean has governed the last climatic cycle on the European continent.

History of the Würm and Holocene vegetation in western velay (Massif Central, France): A comparison of pollen analysis from three corings at Lac du Bouchet

Abstract Comparisons between three pollen diagrams from cores sampled at the bottom of the Lac du Bouchet (Massif Central, France) permit a synthetic biostratigraphical sequence to be established. Particular attention is paid to the lower part of the borings that partly covers the Middle Wurm. The complexity of this period appears clearly. Although it is not possible to characterize real interstadials, there existed a succession of short periods during which arboreal taxa could immigrate; however, they were soon replaced by a steppe vegetation. The last presence of Picea in the region is also evidenced. A new study of the Limagne mire was made in order to characterize the Lateglacial, very badly represented at Le Bouchet. The Holocene, previously described in the region in a study by Beaulieu, Pons and Reille, shows a marked landnam at the beginning of the Subboreal. Further borings within a cooperation program with the C.C.E. should make it possible in the near future to obtain more detailed information regarding the beginning of the Wurm.

Holocene Palaeoenvironments at the Timberline in the Taillefer Massif, French Alps: A Study of Pollen, plant Macrofossils and Fossil Insects

In order to reconstruct the history of the tree-line fluctuations, an integrated palaeoecological study of pollen, plant macrofossils and insect fossils was made in two peat-bogs containing Pinus uncinata trunks in the Taillefer Massif (Isére, France) at an altitude of 2100 m (i.e., above the present timberline). There is apparently an absence of a local woodland during the Lateglacial. After a hiatus corresponding to the beginning of the Holocene, the local presence of P. uncinata is recorded from at least 7500 BP onwards. There are also indications of Abies woodland in the vicinity of the plateau during the Atlantic chronozone. Significant changes in regional ecosystems are recorded from 5000 BP in an expansion of Fagus and Pinus cembra, and the first pollen evidence for human activities. Plant macrofossils and fossil insects (mainly Coleoptera) significantly improved the interpretations based on pollen analysis. Pinus uncinata disappeared from the plateau at about 2000 BP, slightly before the expansion of Picea at lower altitude. Pollen data imply that the timberline recession may be due to the exploitation of the forest by man. However, a lowering of the snow-line coinciding with the decline of Pinus uncinata is attested by the appearance of the insect Helophorus glacialis. It is therefore probable that there was a combination of human action and climatic impact, the importance of the latter having often been underestimated up to now.