Holocene climatic changes in the Western Mediterranean, from south-east France to south-east Spain

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Abstract

Holocene climatic changes along coastal regions from south-east France to south-east Spain were studied using pollen ratios. Comparing modern pollen rain, vegetation and climate along selected transects from the Atlantic Ocean to the Mediterranean, we obtained threshold values of two different ratios corresponding to the different climatic conditions along the transects. These pollen ratios and threshold values were employed to characterize the Holocene climatic changes from nine Mediterranean coastal sites. The results were compared with data from marine and continental pollen sequences distributed in the western Mediterranean basin, and with additional regional data independent of human activity: lake-level fluctuations, alpine glacier advance and retreat chronology, 14C anomaly and cooling phases in Eastern France and Central Europe. The role of anthropogenic activities and climate on the changes in vegetation is discussed. Six major changes in vegetation cover were identified. They correspond to aridification phases that occurred around 9500–9000 yr BP (10 900–9700 cal BP), 7500–7000 yr BP (8400–7600 cal BP), 4500–4000 yr BP (5300–4200 cal BP), 3700–3300 yr BP (4300–3400 cal BP), 2600–1900 yr BP (2850–1730 cal BP) and 1300–1000 yr BP (1300–750 cal BP). These arid episodes were regional responses to more global climatic changes and determined the changes in the vegetation cover. Humans undoubtedly enhanced the vegetation changes, but none the less had to adapt to these new climatic conditions.

Introduction

In the north-western Mediterranean, palynological data indicate the presence and even the dominance of deciduous trees at low altitude, close to the Mediterranean Sea during the early Holocene (Bernard, 1971, Laval and Médus, 1994, Pérez-Obiol, 1987, Pérez-Obiol, 1988, Planchais and Duzer, 1978, Planchais, 1982, Planchais and Parra Vergara, 1984, Planchais, 1985, Riera i Mora, 1994, Riera i Mora and Esteban Amat, 1994, Rivas-Martı́nez et al., 1990, Triat-Laval, 1978). Evergreen trees and shrubs were rare at these times. Before 6000 yr BP, steppes and Mediterranean vegetation are thought to have been restricted to the Near East (Huntley and Prentice, 1988, Prentice et al., 1996). From 6000 yr BP, and more particularly from 4500–4000 yr BP., marked changes occurred in the vegetation cover. Evergreen sclerophyllous trees and shrubs developed at the expense of deciduous trees, along with evidence of anthropogenic factors. Many authors have considered that this substitution and the establishment of the present Mediterranean vegetation were entirely the result of human activity (Bernard and Reille, 1987, Planchais and Duzer, 1978, Planchais, 1982, Pons, 1981, Pons and Quézel, 1998, Quézel, 1999, Reille and Pons, 1992, Triat-Laval, 1978, Vernet, 1973).

However, other authors consider that climatic factors were largely responsible for the major change in vegetation cover (Huntley and Prentice, 1988, Huntley et al., 1989, Huntley, 1990a, Huntley, 1990b, Kelly and Huntley, 1991). At 6000 BP, the dominance of temperate forests including temperate deciduous forests instead of the present xerophytic vegetation implies a combination of colder than present winters and wetter than present conditions during the growing season for temperate trees (Huntley and Prentice, 1988, Prentice et al., 1996). Variations in macroclimate would thus be the most probable causes of the changes (Kelly and Huntley, 1991). Human activity would have only accentuated their consequences on the vegetation during the second half of the Holocene (Magri, 1996), as suggested by recent studies on north-eastern Spain (Carrión and Dupré, 1996, Parra, 1994, Yll et al., 1995, Yll et al., 1997).

During the early Holocene, in the north-western Mediterranean, particularly in the Gulf of Lion area, the dominance of temperate deciduous oak forests including Alnus, Betula, Populus, Salix, Carpinus and Fagus (Jalut, 1995, Jalut et al., 1997, Planchais and Duzer, 1978, Planchais, 1982) does not seem compatible with the regular summer drought that characterizes the Mediterranean climate. A month is considered to be dry when the mean monthly precipitation expressed in mm (Pmm) is equal to, or lower than, twice the mean monthly temperature (T°C) expressed in degrees Celsius (Pmm2T°C) (Bagnouls and Gaussen, 1953). To explain this wide extension of temperate deciduous oak forests, it has been generally thought that a sub-humid Mediterranean climate prevailed in the north-western Mediterranean during the Holocene (Vernet and Thiébaut, 1987). This assumption is associated with the conventional opinion attributing the changes in the Mediterranean vegetation to human activity.

On the assumption that, during the early Holocene, the climate was not Mediterranean in the area of the Gulf of Lion, we propose a reinterpretation of Holocene palynological data distributed from south-eastern Spain to south-eastern France. We used these data to characterize and correlate the main changes in Holocene vegetation during this period and to determine, along the same gradient, the evolution of the Mediterranean climate. Changes in vegetation cover and climate are compared to the regional and global palaeoenvironmental changes.

Section snippets

Methodology

In the studied zone, our approach was based on comparisons between modern and fossil pollen data. Along two climatically well-defined phytogeographic transects (Fig. 1), pollen ratios were used to compare the vegetation cover and its pollen rain and to characterize, for each climatic zone, the corresponding pollen ratios. We then conducted a climatic reinterpretation of Holocene pollen data from Mediterranean coastal sites (Fig. 1, Fig. 2).

Deciduous broad-leaf versus evergreen sclerophyllous taxa pollen ratio

Fig. 3a and b show that under an oceanic climate (samples 1–6, transect 1; samples 1–7 transect 2), deciduous tree pollen predominates, and the pollen ratio is above 0.5 on a decimal logarithmic scale. Sample 1, near Bordeaux, reflects the presence of sclerophyllous taxa in the vicinity. According to Emberger (in Dajet and David, 1982), this station is Sub-Mediterranean. Values over 0.5 will be used to characterize a climate with a wet summer.

Under a Sub-Mediterranean climate with irregular

Selection of the sites

Along the north-western Mediterranean coast, from south-east France (La Trémie), to south-east Spain (Cabo de Gata), nine Mediterranean sites were studied (Fig. 1, Fig. 2). To limit the consequences on the vegetation history of local climatic conditions determined by altitude or exposure, only sea-level coastal sites were selected. Selection also took into account the various Mediterranean stages along the south–north gradient and the sites belonging to the different Mediterranean stages

Conclusion

The good correlations between our results and pollen data from the western Mediterranean, as well as with records independent of human influence, indicate the interest and reliability of pollen ratios for studying Holocene climatic changes. From south-east France to south-east Spain, six major changes in vegetation cover were identified.

They correspond to aridification phases that took place at ca. 9500–9000 yr BP (10900–9700 cal BP), 7500–7000 yr BP (8400–7600 cal BP), 4500–4000 yr BP (5300–4200 cal BP),

Acknowledgements

This research was supported by the Programs ‘Environment and Climate Program: ARIDUSEUROMED’, ‘Dynamique des Paysages’ and ‘Temps et Espace dans le Bassin de l'Aude du Néolithique ‘l'Age du Fer’, ATP ‘Grands travaux d'Archéologie métropolitaine’, CNRS, Sous Direction de l'Archéologie, under Prof. J. Guilaine. We express our gratitude to Dr. P. Ambert (UMR 150 CNRS) and J.J. Dedoubat (UMR 5552) for their help in the field work at Capestang; to J.J. Dedoubat who prepared the samples for pollen

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