V. Andrieu

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

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.

40Ar/39Ar dating on tephra of the Velay maars (France): implications for the Late Pleistocene proxy-climatic record

Abstract Pollen contents and magnetic records of the Pleistocene sedimentary sequences cored in the Praclaux and Lac du Bouchet maars (Velay region, French Massif Central) demonstrated that they have high potential as detailed climatic and environmental archives. Tephra layers interbedded in these sequences have been studied in order to perform isotopic dating. First, petrographic studies and 40 Ar/ 39 Ar ages have shown that these tephra layers are the witness of the same eruption from the Sancy volcano centre, located 100 km to the north. According to fourteen 40 Ar/ 39 Ar ages, carried out on sanidine and biotite grains, this eruption occurred 275±5 ka ago. This result allows to construct a time scale for a composite high resolution record of the climate covering the last 300,000 years and to establish an absolute chronology for the palaeoclimatic events recorded in these sequences. These tephra layers are interbedded in organic layers, deposited during an interstadial stage called the Amargiers interstadial which, according to its age, can be considered as equivalent to isotopic stage 9a. Moreover, the geographical extension of the tephra and its well-constrained age give to this tephra the interest of a chronostratigraphic marker for upper Quaternary sequences in southeastern France.

An oxygen isotope record of lacustrine opal from a European Maar indicates climatic stability during the Last Interglacial

The penultimate temperate period, 127–110 ka before present (BP), bracketed by abrupt shifts of the global climate system initiating and terminating it, is considered as an analogue of the Holocene because of a similar low global ice‐volume. Ice core records as well as continental and marine records exhibit conflicting evidence concerning the climate variability within this period, the Last Interglacial. We present, for the first time, a high‐resolution record of oxygen isotopes in diatom opal of the Last Interglacial obtained from the Ribains Maar in France (44°50′09″N 3°49′16″E). Our results indicate that the Last Interglacial in southwestern Europe was generally a period of climatic stability. The record shows that the temperate period was initiated by an abrupt warm event followed midway by a minor climatic transition to a colder climate. An abrupt isotopic depletion that occurs simultaneously with abrupt changes in pollen and diatom assemblages marks the end of the temperate period, and is correlative with the Melisey I stadial. Variations in the isotopic composition of lake‐water related to the isotopic composition of precipitation and evaporation dominate the biogenic opal oxygen isotope record.

Middle Pleistocene temperate deposits at Dingé, Ille-et-Vilaine, northwest France: pollen, plant and insect macrofossil analysis

Nine cores were taken from a damp depression at Dinge, Ille-et-Vilaine, northwest France. Analyses of the pollen, plant macrofossil and Coleoptera remains preserved in the same organic samples of two profiles suggest a temperate vegetation characterised by a mixed deciduous forest with mesophilous taxa (Carpinus, Fagus, Quercus) followed by a coniferous forest with Pinus and Picea. The determination of plant taxa to species was made either directly through the identification of plant macrofossil remains and pollen or indirectly through the identification of phytophagous Coleoptera specifically related to certain plants. Stratigraphical information derived from pollen, plant macrofossil and insect data indicates that this sequence may be correlated with a temperate episode older than the Eemian and younger than the Holsteinian, possibly the Bouchet 2 (Oxygen Isotope Stage 7c) or Bouchet 3 (Oxygen Isotope Stage 7a) temperate periods or the Landos Interglacial (Oxygen Isotope Stage 9 pro parte)