Precipitation changes in the Mediterranean basin during the Holocene from terrestrial and marine pollen records: a model–data comparison

Climate evolution of the Mediterranean region during the Holocene exhibits strong spatial and temporal variability. The spatial differentiation and temporal variability, as evident from different climate proxy datasets, has remained notoriously difficult for models to reproduce. In light of this complexity, we examine the previously described evidence for (i) opposing northern and southern precipitation regimes during the Holocene across the Mediterranean basin, and (ii) an east-to-west precipitation gradient or dipole during the early Holocene, from a wet eastern Mediterranean to dry western Mediterranean. Using quantitative climate information from marine and terrestrial pollen archives, we focus on two key time intervals, the early to mid-Holocene (8000 to 6000 cal yrs BP) and the late Holocene (4000 to 2000 yrs BP), in order to test the above mentioned hypotheses on a Mediterranean-wide scale. Palynologically derived climate information is compared with the output of regional-scale climate-model simulations for the same time intervals. Quantitative pollen-based precipitation estimates were generated along a longitudinal gradient from the Alboran (West) to the Aegean Sea (East); they are derived from terrestrial pollen records from Greece, Italy and Malta as well as from pollen records obtained from marine cores. Because seasonality represents a key parameter in Mediterranean climates, special attention was given to the reconstruction of season-specific climate information, notably summer and winter precipitation. The reconstructed climatic trends corroborate a previously described north-south partition of precipitation regimes during the Holocene. During the early Holocene, relatively wet conditions occurred in the south-central and eastern Mediterranean region, while drier conditions prevailed from 45° N northwards. These patterns reversed during the late Holocene, with a wetter northern Mediterranean region and drier conditions in the east and south. More sites from the northern part of the Mediterranean basin are needed to further substantiate these observations. With regard to the existence of a west-east precipitation dipole during the Holocene, our pollen-based climate data show that the strength of this dipole is strongly linked to the seasonal parameter reconstructed: Early Holocene summers show a clear east-to-west gradient, with summer precipitation having been highest in the central and eastern Mediterranean and lowest over the western Mediterranean. In contrast, winter precipitation signals are less spatially coherent. A general drying trend occurred from the early to the late Holocene; particularly in the central and eastern Mediterranean. However, summer precipitation in the east remained above modern values, even during the late Holocene interval. Pollen-inferred precipitation estimates were compared to regional-scale climate modelling simulations based on the HadAM3 GCM coupled to the dynamic HadSM3 and the high-resolution regional HadRM3 models. Climate model outputs and pollen-inferred precipitation estimates show remarkably good overall correspondence, although many simulated patterns are of marginal statistical significance. Nevertheless, models weakly support an east to west division in summer precipitation and there are suggestions that the eastern Mediterranean experienced wetter summer and winter conditions during the early Holocene and wetter summer conditions during the late Holocene. The extent to which summer monsoonal precipitation may have existed in the southern and eastern Mediterranean during the mid-Holocene remains an outstanding question; our model, consistent with other global models, does not suggest an extension of the African monsoon into the Mediterranean. Given the difficulty in modelling future climate change in Southern Europe, more simulations based on high resolution global models and very high resolution regional downscaling, perhaps even including transient simulations, are required to fully understand the patterns of change in winter and summer circulation patterns over the Mediterranean regio

Un Coléoptère "marin" mythique redécouvert sur l'aire d'adhésion du Parc national de Port-Cros: Brachemys brevipennis (Laporte de Castelnau, 1838) (Coleoptera, Malachiidae) sur la presqu'île de Giens (Hyères, Var)

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L’intérêt de la paléoécologie du Quaternaire pour reconstruire les anciens environnements

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Dynamique du paléoenvironnement de la vallée montagnarde de la Garonne (Pyrénées centrales, France) de la fin des temps glaciaires à l'actuel

The palaeoenvironment dynamics of the upper watershed of the Garonne river have been established from the end of the glacial times until the present-day on the basis of palynologicval, sedimentological and isotopical analysis. During the pleniwurm, the regional vegetation consisted of a steppe with poaceae. The two cold episodes suggested by the fall of delta 18 0 slightly before 1750+ -300 bp and between 15800+ -270 bp and 15100+ -300 bp are the equivalent of the cold episodes recorded in the oceans around 18000 bp and 15000 bp. During the oldest dryas, between 15100+ -300 bp and 13490+ -220 bp, a steppe with artemisia established itself locally, as a result of a climate warming, also suggested by higher delta 18 0 values. The beginning of the bolling was characterised by the establishment of a sparse woodland with betula and a bushland with juniperus. During the lateglacial interstadial, the notable increase of the delta 18 0 and the delta 13 c suggests markedly warmer climatic conditions. At the beginning of the holocene, a medio-european type oak forest penetrated with corylus and ulmus established itself. The atlantic was characterized by the expansion of an oak forest with tiliaL'étude palynologique, sédimentologique et isotopique de sondages effectués dans la vallée montagnarde de la Garonne a permis de reconstituer l'histoire du paléo-environnement depuis la fin de la déglaciation wurmienne. Au Wurm final, la végétation régionale était une steppe à poacées. Peu avant 17650+ -330 bp et entre 15800+ -270 bp et. 4. 0 + -300 bp, le delta 18 o enregistre deux phases de refroidissement qui sont les analogues de celles révélées par les océans vers 18000 bp et 15000 bp. Entre 15100+ -300 bp et 13490+ -220 bp, une steppe à armoises s'installe sous l'effet d'un réchauffement du climat (progression du delta 18 o). Au Bolling, le genévrier suivi du bouleau s'installe. Pendant l'interstade tardiglaciaire, date 11590+ -270 bp, l'évolution du delta 18 o et du delta 13 c traduit un important réchauffement du climat. Au début de l'holocène se met en place une chênaie à noisetiers et ormes, qui s'enrichit en tilleuls à l'Atlantique. A la fin de cette période, les premiers landnams sont notés. Au subatlantique, l'apparition du noyer est l'indice de l'agriculture gallo-romaine et celle du sarrasin de l'agriculture médiévale. A la fin du 19eme siècle, la montagne est reboisée en résineux (pins, épicéas