Valérie Andrieu-Ponel

An attempt at correlation between the Velay pollen sequence and the Middle Pleistocene stratigraphy from central Europe

Abstract Long continuous lacustrine sequences constitute on the continent a precious tool for coupling the long term continental environmental changes with deep sea and ice core records, using the ‘count from the top’ method. Moreover, they can contribute to long distance correlation on the continent itself and thus help to classify discontinuous sedimentary records. Palynostratigraphical correlation is proposed here between the Velay long sequence and the late Middle Pleistocene series from Central Europe with special attention to temperate episodes. The similarities between the Praclaux and the Holsteinian Interglacial suggest that they are both contemporaneous with marine isotopic stage (MIS) 11, even if absolute dates are still divergent. The evidence of two major warm periods with an interglacial type of vegetation between the Holsteinian and the Eemian is confirmed by the Velay record.

High-resolution record of climate stability in France during the last interglacial period

The last interglacial period (127–110 kyr ago) has been considered to be an analogue to the present interglacial period, the Holocene, which may help us to understand present climate evolution. But whereas Holocene climate has been essentially stable in Europe, variability in climate during the last interglacial period has remained unresolved, because climate reconstructions from ice cores, continental records and marine sediment cores give conflicting results for this period. Here we present a high-resolution multi-proxy lacustrine record of climate change during the last interglacial period, based on oxygen isotopes in diatom silica, diatom assemblages and pollen–climate transfer functions from the Ribains maar in France. Contrary to a previous study, our data do not show a cold event interrupting the warm interglacial climate. Instead, we find an early temperature maximum with a transition to a colder climate about halfway through the sequence. The end of the interglacial period is clearly marked by an abrupt change in all proxy records. Our study confirms that in southwestern Europe the last interglacial period was a time of climatic stability and is therefore still likely to represent a useful analogue for the present climate.

Rapid ecosystem response to abrupt climate changes during the last glacial period in western Europe, 40-16 ka

We present a high-resolution and independently dated multiproxy lake sediment record from the paleolake at Les Echets in southeastern France that displays synchronous changes in independent limnic and terrestrial ecosystem proxies, in concert with millennial-scale climate oscillations during the last glacial period. Distinct lake-level fluctuations, low lake organic productivity, and open, treeless vegetation indicate cold and dry conditions in response to Heinrich events. Alternating phases of higher and low lake organic productivity, stratified surface waters and long-lasting lake ice cover, decreased or increased catchment erosion, and tree-dominated or herb-dominated vegetation resemble Dansgaard-Oeschger interstadialstadial variability. Transitions between different ecological states occurred in as little as 40–230 yr and seem to have been controlled by the position of the Polar Front. Ecosystem response after 30 ka suggests that local climate conditions became more important. Our results demonstrate that all parts of the terrestrial system responded to the abrupt and dramatic climatic changes associated with Dansgaard-Oeschger and Heinrich events, and that regional factors modulated ecosystem response.

Vegetation dynamics in north-western Mediterranean regions: Instability of the Mediterranean bioclimate

Abstract Pollen and plant macrofossil records from a selection of sites in Spain, France and Italy are used to explore the origin and the development of the Mediterranean vegetation. The role of evergreen and summergreen taxa is discussed. The expansion of sclerophyllous trees during the Pleistocene interglacials is well correlated with the orbital forcing. During the Holocene, the large number of data illustrates latitudinal and longitudinal differences in vegetation dynamics. Multidisciplinary studies identify short terms climatic fluctuations. The debate is still open between those who attribute an increase of sclerophyllous forests during the late Holocene to a trend toward aridity and those who consider this dynamic as human-induced.

Indian summer monsoon variations could have affected the early Holocene woodland expansion in the Near East

Postglacial expansion of deciduous oak woodlands of the Zagros—Anti-Taurus Mountains, a major biome of the Near East, was delayed until the middle Holocene at ~6300 cal. yr BP. The current hypotheses explain this delay as a consequence of a regional aridity during the early Holocene, slow migration rates of forest trees, and/or a long history of land use and agro-pastoralism in this region. In the present paper, support is given to a hypothesis that suggests different precipitation seasonalities during the early Holocene compared with the late Holocene. The oak species of the Zagros—Anti-Taurus Mts, particularly Quercus brantii Lindl., are strongly dependent on spring precipitation for regeneration and are sensitive to a long dry season. Detailed analysis of modern atmospheric circulation patterns in SW Asia during the late spring suggests that the Indian Summer Monsoon (ISM) intensification can modify the amount of late spring and/or early summer rainfall in western/northwestern Iran and eastern Anatolia, which could in turn have controlled the development of the Zagros—Anti-Taurus deciduous oak woodlands. During the early Holocene, the northwestward shift of the Inter-Tropical Convergence Zone (ITCZ) could have displaced the subtropical anticyclonic belt or associated high pressure ridges to the northwest. The latter could, in turn, have prevented the southeastward penetration of low pressure systems originating from the North Atlantic and Black Sea regions. Such atmospheric configuration could have reduced or eliminated the spring precipitation creating a typical Mediterranean continental climate characterized by winter-dominated precipitation. This scenario highlights the complexity of biome response to climate system interactions in transitional climatic and biogeographical regions.

Palaeoenvironments and cultural landscapes of the last 2000 years reconstructed from pollen and Coleopteran records in the Lower Rhône Valley, southern France:

For the first time, a high-resolution pollen/Coleoptera joint analysis is performed on a late Holocene sedimentary sequence located in the Lower Rhône Valley. 14C dates validated by pollen data show that the bottom of the sequence is contemporaneous with the Greco-Roman period whereas the top is attributed to the present. This sequence yielded very rich pollen and insect assemblages, enabling a detailed reconstruction of the palaeoenvironment succession during the two last millennia around the site itself and more widely in the Arles plain. The very low pollen representation of trees and the near absence of tree-dependent Coleoptera suggest a marked deforestation of the area. The abundance of dung-beetles and nitrophytes is also in keeping with a strong grazing impact throughout the sequence. Three agricultural phases reflecting a growing level of human activities are identified. Phase 1 is contemporaneous with Celto-ligurian, Greek and Roman civilizations in Provence. At this time the forest cover was already largely destroyed, and pastoralism, cultivation of cereals, olives, vines and walnuts, was practised. Phase 2 is contemporaneous with a period spanning the Merovingian time and the Upper Middle Ages. It is characterized by increased agro-pastoral activities, probably related to the establishment of a monastic community at the Montmajour Abbey and to the settlement of farmers on the nearby Castellet hill. The major characteristic of agricultural phase 3 is the very high pollen percentages of Dipsacus fullonum or teasel, which was formerly extensively cultivated for cloth teasing. This early cultivation of an industrial plant, dated at La Calade to the twelfth century, is recorded for the first time in Provence. It may be connected with craft industries performed by monks at Montmajour Abbey. The sedimentological data suggest a succession of two stability phases interrupted by three flood phases. This interpretation agrees both with insect data, the fluctuating abundances of which are certainly connected with the alternation of ground submersion and dry periods, and with pollen data, marked by the impact of the floods upon the marshy veg etation. However, no clear climatic signal is recorded.

Towards the reconstruction of the Holocene vegetation history of lower Provence : two new pollen profiles from Marais des Baux

Two new14C-dated Holocene pollen profiles from Marais des Baux, Bouches-du-Rhône, France, are presented. The record begins in the Younger Dryas, when the vegetation consisted mainly of grasses and mugwort (Artemisia). The Preboreal was marked by a transitory expansion of pine forests and was followed by the establishment of a rich deciduous oak-forest that included hazel (Corylus) and elm (Ulmus). During the Boreal, hazel played a dominant role within the oak-forest. The oak forests, which includedQuercus ilex, achieved a major expansion during the Atlantic period. The Subboreal was characterised by the regional establishment of, firstly, fir (Abies) and then beech (Fagus). The spread and expansion of beech coincides with the first clear evidence for farming. Agricultural activities brought about the decline of deciduous oak-forest. During the Subatlantic, forests in the vicinity of Marais des Baux were cleared for farming. Cereal growing, which included rye cultivation, was of considerable importance. Three noteworthy characteristics that serve to differentiate the Holocene vegetation history of the low-lying Provence region from other French regions are as follows: 1) the early establishment (from the onset of Preboreal) of low altitude mixed forest; 2) the expansion during the Subboreal of fir and beech in low altitude areas with a Mediterranean climate and, 3) the exceptional taxonomic richness of the pollen assemblages (120 identified taxa) and the presence of borealalpine and Euro-Siberian taxa that no longer exist in Lower Provence.

6700 yr sedimentary record of climatic and anthropogenic signals in Lake Aydat (French Massif Central)

A 19 m long sedimentary record retrieved in Lake Aydat (French Massif Central) covers the last 6700 yr at a high resolution. A multiproxy approach (density, magnetic susceptibility, XRF, Rock-Eval, pollen and non-pollen palynomorph analyses and a historical archives study) was used to characterise and propose a model of sedimentation. The high deposition rate results from the combined effects of the high suspension load of the river, autochthonous production and the narrow shape of the incised fluvial valley dammed by a lava flow c. 8550 years ago. Two contrasted periods (6700±200 to 3180±90 cal. BP, and 1770±60 cal. BP to present) were characterized. The lower unit (mid Holocene) displays a fine and regular lamination and holds a single, major, flood deposit. This unit is capped by an erosive mass-wasting deposit triggered c. 1770±60 cal. BP. The upper unit (late Holocene) is made of organic-rich and fine-grained faintly laminated sediment, with numerous interbedded flood deposits and diatom blooms. The sedimentation was principally controlled by climatic forcings until c. 1100 cal. BP, accompanied by detrital events linked to human activities around the lake. Then, a more detrital input attested by numerous and recurrent flood deposits can be linked to the intensification of a persistent anthropogenic impact on the catchment. Two phases of lake eutrophication are highlighted: 1200–1130 cal. BP, as a result of increased anthropogenic pressure, and the current phase that could have started c. 150 cal. BP.

Sedimentary cannabinol tracks the history of hemp retting

Hemp (Cannabis sp.) has been a fundamental plant for the development of human societies. Its fibers have long been used for textiles and rope making, which requires prior stem retting. This process is essential for extracting fibers from the stem of the plant, but can adversely affect the quality of surface waters. The history of human activities related to hemp (its domestication, spread, and processing) is frequently reconstructed from seeds and pollen detected in archaeological sites or in sedimentary archives, but this method does not always make it possible to ascertain whether retting took place. Hemp is also known to contain phytocannabinoids, a type of chemicals that is specific to the plant. Here we report on the detection of one of these chemicals, cannabinol (CBN), preserved in a sediment record from a lake in the French Massif Central covering the past 1800 yr. The presence of this molecule in the sedimentary record is related to retting. Analysis of the evolution of CBN concentrations shows that hemp retting was a significant activity in the area until ca. A.D. 1850. These findings, supported by pollen analyses and historical data, show that this novel sedimentary tracer can help to better constrain past impacts of human activities on the environment.

Chironomid responses (Insect: Diptera) to Younger Dryas and Holocene environmental changes in a river floodplain from northern France (St-Momelin, St-Omer basin)

Lateglacial to Holocene subfossil chironomids from the St-Omer basin at St-Momelin (northern France) were analysed. At the end of the Lateglacial, stratigraphic and chironomid sequences indicate a shallow, oligotrophic and slow-flowing freshwater stream, with abundant aquatic macrophytic vegetation. Cold-water adapted taxa indicate a cold climate. During the early Holocene, from about 9500 to 7700 conventional radiocarbon yr BP, climate improvement is marked by an increase in warm-water adapted taxa and a decrease in cold-water fauna. However, the cold-water fauna persist, probably because of both the instability and continentality of the climate. During the mid Holocene, from about 7700 to 6000 BP, the marine transgression strongly influenced the rivers hydrological regime and consequently the chironomid assemblages. At that time, eutrophic, warm and shallow stagnant-water conditions occured throughout the basin. From 6000 to 3200 BP, the St-Momelin basin was subjected to a marine regression. Chironomids indicate a hydrological regime marked by an alternance of phases with either a dominance of rheophilous or limnophilous fauna. During this period, equivalent percentages of warm- and cold-adapted taxa may be explained by cold-water input in context of a temperate climate.