Le Petit Âge Glaciaire en milieu semi-aride : le bassin versant du Gialias (Chypre) et ses relations avec l'occupation des sols
Résumé
Historical and geological data have revealed a fresh and humid climatic phase between ca. A.D. 1550 and A.D.1850 (Lamb 1977, 1979, Grove 1988, Le Roy Ladurie 1983, 2003, Bond et al. 1997), linked to the North Atlantic Oscillation. This period, known as the Little Ice Age (LIA) has had a strong influence on the Mediterranean basin. The Eastern Mediterranean and more particularly the Near East is however the boundary area of the NAO influence (Bolle 2003). From a geomorphologic aspect, the LIA in Europe can be characterised by high flood frequency, intense detrital activity, and fluvial metamorphosis (Bravard 1989, Salvador et al. 1993, Huhmann et Brückner 2002, Pichard 1995, Zielhofer et al. 2002, Faust et al. 2004, Thorndycraft et Benito 2006). Agricultural activities and land use have probably enhanced its impact (Le Roy Ladurie 1983, Bravard 2000, 2002) maybe causing famine in some areas(Xoplaki et al. 2001). On the other hand, the LIA geomorphological impact in Eastern Mediterranean is poorly known. Some authors describe a similar evolution to the European one (Vita Finzi 1968, 1969, Lespez 2003, Issar 2003). Others show a decrease in flood frequency during the LIA and link this event to a diminution of the monsoon's influence (Laronne 1990). Arid hydrosystems are characterised by sporadic and violent flood events (flash floods) known to have the highest sedimentation rates of all floods (Karcz 1972, Phillips 2002). At first, flash flood could be suspected to be less frequent during the humid period of the LIA. This led us to examine the morphogenic potential of the LIA in semi-arid areas. The aim of this paper is to estimate if and how the climatic variations of the LIA have affected the fluvial system of the Cyprus island. Chronology and nature of flooding episodes are measured in order to understand interactions between fluvial dynamics, demography and land-use. The Gialias is a 936 km² watershed located within eastern Mesaoria (fig. 1A), the semi-arid central plain of Cyprus. This rivers has a rank of 7 (Stralher classification, fig. 1B). Fourteen (natural and archaeological) trenches, located in the median part of the watershed, have been excavated and are presented here. All of them are floods deposits organized in three alluvial terraces (fig. 1C), the last one beeing relatived to the LIA. The most complete sedimentary record is given by the Frankish mills of Palaeomylos (fig. 2 and 3). Two types of deposit, the mainstream and the flood plain sediment, have been studied. Mainstream deposits are composed of pebbles, gravels and coarse sands showing evidence of fast aggradation from the XIVth century to after the XVIth century (fig.3). Interstratified facies indicates an evolution of the fluvial environment to braided channels (fig. 3). Floodplain deposits can be divided in two facies, one with beddings (parallel beds : LP, or vertically sorted : GV) and the other with uniform facies. The former are related to a thick water column and long-lasting flood sequence (Visher 1969, Woodroffe 2002, Devillers 2005). The latter is a signature of flash flood events (Picard and High 1973, Blair 2000, Devillers 2005). Archaeological remains enabled us to date the LP and GV facies between the XVIth and the XIXth century, suggesting the disappearance of flash floods during the LIA (fig. 4A and 4B). Rock magnetism measurements (magnetic susceptibility A/m²/kg) prove that, during theses LIA events, longitudinal solid matter flows are more important (fig. 4A and 4B). Other sedimentary records confirm and make clear the chronology measured in the Palaeomylos site (fig. 5). The strongly increased sedimentation rate since the XVIth century correlates well with the decrease in flash flood events (fig. 4 and 5). All the data from the site of Paleomylos clearly illustrate an increase in stream and flood flow, in the frequency of flood events, but reveals also a decrease in the thickness of flood deposits and a fluvial change to a braided system (fig. 6). Consequently, the LIA effects described in western Mediterranean areas have also been identified in Cyprus where they have been proved to influence flood intensity, nature and frequency. Despite the fact that flood deposits are thinner during the LIA, their high frequency rate made the LIA a particular morphogenic period in this semi-arid watershed of Cyprus. Comparison of fluvial records with historical data reveal that settlement abandonment, harvest loss by grasshopper invasions and sever droughts are present before and after the LIA period and give a more precise image of environmental change and its consequence on the rural societies living in semi-arid lands (fig.6). From a geoarchaeological point of view, the LIA has been described as an environmental crisis (Jorda et Provansal 1996, Roberts 1998). This paper demonstrates that, in this environmental context, the negative effects of the LIA on populations (high sedimentation rates, floods...) could be minor in comparison with the droughts, harvest losses and flash flooding occurring just before and after the LIA, leading to important population migration. Therefore the definition of the LIA as a environmental crisis should be redefined and geographically specified.
Le Petit Âge Glaciaire (PAG) est décrit par divers auteurs comme une variation climatique fraîche et humide caractérisée par une augmentation de l'activité détritique et de la fréquence des crues au sein des hydrosystèmes de tout rang. Cette période coïncide d'autre part avec une importante exploitation agricole des sols. La conjonction de ces deux phénomènes a mené au constat de crise climato-anthropique des environnements durant cette période pour l'Europe occidentale. Cet article analyse le fonctionnement de l'hydrosystème du Gialias, dans l'île de Chypre, dans un contexte climatique de type méditerranéen semi-aride. La qualification et la quantification de la variabilité des dépôts de crue, par le croisement de la sédimentologie, la micropédologie et la minéralogie magnétique, sont datées par AMS 14C et de nombreuses données archéologiques. Les changements environnementaux sont mis en relation avec l'occupation des sols, connue par les sources écrites et archéologiques. Les résultats montrent l'existence d'une forte accumulation entre le XIVe et le XIXe siècle, formée par des crues de plaine de type " méditerranéen tempéré ", plutôt qu'à des " crues éclair ". Les rapports entre les variations hydro-sédimentaires et le peuplement justifient une redéfinition de la notion de crise environnementale durant le Petit Âge Glaciaire pour ce type de milieu.
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