Guy Jalut

Mid-Holocene vegetation and climatic history of the Iberian Peninsula

The aim of this research is to study the climate and vegetation history in the western Mediterranean, in the Iberian Peninsula, during the middle Holocene through pollen analysis. The origin of the deposits varied from the most xeric to more mesic Mediterranean environments. The timing, extent, and progress of the establishment of the Mediterranean climate have a degree of variability depending on the biogeographical region. Analyses of several pollen sequences reveal climatic transformations in the flora and vegetation between 7000 and 4000 cal. yr BP. Pollen concentrations have been used in some sequences to evaluate the order of magnitude in biomass changes through time. Three main spatial and taxonomic responses could be assumed: (1) in littoral regions, deciduous broadleaf trees were frequently dominant and then replaced by sclerophyllous and evergreen forests; (2) in continental regions and sub-Mediterranean mountains, the dominance of pine throughout the whole Holocene signals a change of less magnitude; and (3) in southeastern semiarid Mediterranean regions, the main changes are reflected by alternation between steppe and shrub communities. The emplacement of the Mediterranean climate is reflected in an aridification process. A temporal first approach of Holocene climatic changes is proposed: a humid phase (12 000—7000 cal. yr BP), a transition phase (7000—5500 cal. yr BP) and an aridification phase (5500 cal. yr BP—Present). According to archaeological data, natural changes in the forests favoured the environments suitable for human settlements, farming and sheepherding.

Holocene vegetational history of the Apuseni mountains, central Romania

From palynological investigations in the Apuseni mountains (Transylvania, Romania), a chronology of forest development is proposed. The data are compared with others from the Romanian Carpathians and the surrounding countries. After their appearance between 11,200 and 10,190 cal BP, Picea and Corylus were dominant up to 6450 cal BP. The extension phases of Carpinus, Fagus and Abies, respectively, began at ca 6450 BP, 4500 BP and 4100 cal BP. Occurrences of their pollen are recorded from about 7800 cal BP. First evidence of human impact appeared during the 7800–7425 cal BP period and first cultivations at ca 6820 cal BP. Between 4500 and 2750 cal BP deforestation and agriculture were limited, but increased around 1935 and 695–660 cal BP. r 2002 Elsevier Science Ltd. All rights reserved.

Holocene climatic changes in the western Mediterranean: installation of the Mediterranean climate

Abstract In the western Mediterranean during the Holocene, summer drought occurred progressively from south to north. In south eastern Spain, between latitudes 36 and 39 °N, it prevailed from about 10 000 BP and a first arid episode is recorded around 7,000 yr BP. A Mediterranean climate reached a latitude 39–40 °N near 7 000 BP and near 41 °N before 5 000 BP. The climatic change that began near 4 500–4 200 BP, resulted in changes in the seasonal pattern of precipitation and the installation of summer drought between about 3 300 and 1 000 BP in the zone 40 °–44°N.

A palaeoecological approach to the last 21 000 years in the pyrenees: The peat bog of Freychinede (alt. 1350 m, Ariege, South France)

Local palaeoclimatic and palaeoecological reconstructions in the Ariege Pyrenees were applied to palynological and sedimentological investigations of the Freychinede peat bog (alt. 1350 m), complemented by a geomorphological study of the neighbouring area. Correlations are possible between local glacial evolution, deposition in the lake and the history of the vegetational cover for the period from about 21 300 B.P. to 13 000 B.P. The altitude of the cirque floors indicates mean July temperatures probably ranging between 6.5°C and 8.5°C from 21 300 B.P. to 15 000 B.P.; 7°C and 9°C from 15 000 B.P. to 13 150 B.P.; 7.5°C and 9.5°C from 13 150 B.P. to the beginning of the final transitional phase. Based on the comparison of the palynological data with the modern plant communities of the area, especially with the subalpine and alpine communities, the local climate would have been favourable to the development of Boreo-arctic plants at the altitude of the lake during the cold phase. The Postglacial period begins near 10 650 B.P. with the extension of Quercus. Abies spread at about 7000 B.P. followed by Tilia cordata. From 5000 to 3900 B.P., Fagus spread moderately, but its strongest development occurred after about 3900-3800 B.P., when human activity began on the mountain forest and many beech—fir forests developed. Middle Ages, in many valleys of Ariege, the mountain and subalpine forests were destroyed. The modern landscape has large treeless zones with scattered recent populations of beech where fir is beginning to develop.

Differential human impact and vegetation history in two adjacent Pyrenean valleys in the Ariège basin, southern France, from 3000 B.P. to the present

Detailed palynological studies in two adjoining French Pyrenean valleys, complemented by the study of archives, demonstrate that under similar climatic conditions, the forest history of each valley from the Bronze Age to present time was essentially determined by socio-economical constraints, possibly modified by natural characteristics such as topography. The studies show why the expansion of Fagus (beech) at c. 4000 B.P. was asynchronous on the northern slope of the Pyrenees and emphasize the effects of the human impact on the recent lowering of the tree-line.

Did Pyrenean glaciers dance to the beat of global climatic events? Evidence from the Würmian sequence stratigraphy of an ice-dammed palaeolake depocentre in Andorra

Abstract The ice-marginal depositional sequence of La Massana provides a chronostratigraphic benchmark for reconstructing the Würmian glacial evolution of the Valira catchment in Andorra, SE Pyrenees. The sedimentary record of Andorra confirms the asynchronous chronology of glacier fluctuations in different parts of the Pyrenean mountain range. A major ice recession occurred at the end of Marine Isotope Stage (MIS) 4. High magnitude valley-glacier fluctuations during MIS 3 constitute another important finding. Major readvances occurred toward the end of MIS 3, whereas MIS 2 (in particular, the global Last Glacial Maximum, or LGM) featured sharp contrasts in ice recession rates between Pyrenean valleys. Substantial distances separated MIS 4 glacier fronts (the Würmian maximum ice extent, or MIE) from those reached during the global LGM, in contrast to situations in the eastern Pyrenees, where Würmian MIE and global LGM ice fronts nearly coincided. Overall, the Valira glaciers reveal patterns that are more similar to those recorded elsewhere in the western and central Pyrenees than in the eastern Pyrenees. The rapid fluctuations recorded by Andorran glaciers during the second half of the Würm also suggest a response to global forcing events such as Heinrich events in the North Atlantic.