International audienceUnderstanding the role of climate and humans in generating mountain slope instability is crucial because such instability influences downstream fluvial activity and is a major threat to societies. Here, we use the sedimentary archive of Lake Allos (southeastern France), a mountain lake in the European Alps, to characterize mountain flood deposits and vegetation dynamics over the past 7000 yr. Our results support the interpretation of a critical threshold in catchment sensitivity to erosion at 1700 calibrated (cal.) yr B.P. (A.D. 250) probably resulting from long-term, uninterrupted impacts of human activity. The frequency and severity of floods increased dramatically after this date. These results demonstrate that underestimation of human impacts over the Holocene may pose a challenge to a clear understanding of past climate changes because paleoflood records are highly likely to have been affected by geomorphic thresholds. Natural reforestation since the end of the 19th century does not appear to be sufficient to induce a flood regime comparable to that which occurred prior to 1700 cal. yr B.P. This poses the question as to whether forest restoration in high-altitude environments is liable to foster a return to a low-erosion regime over the next decades, or whether the overall severity of soil degradation has been such as to preclude a return to previous conditions

To cite this version:Yoann Poher, Philippe Ponel, Frederic Medail, Valérie Andrieu-Ponel, Frédéric Guiter. Holocene environmental history of a small Mediterranean island in response to sea-level changes, climate and human impact. Palaeogeography, Palaeoclimatology, Palaeoecology, Elsevier, 2017, 465, Part A, pp.247-263.

A multidisciplinary study (geomorphology, sedimentology and palynology) shows that the landscapes of the southwest coast of Corsica have been deeply modified by humans and the climate since 3000 BC. Significant and rapid landscape transformations are recorded between the Chalcolithic and the Middle Bronze Ages (3000–1300 BC). Several major (2.2 ka BC, 1.2 ka BC) and local (3000 BC) detrital events affected the Taravo Lower Valley in relation to global climatic changes and anthropic activities. The vegetation dynamics since 3000 BC show alternating phases of agriculture and abandonment until the complete disappearance of the original forest populations in the vicinity of the Canniccia Marshes. An early phase of Olea cultivation is recorded between 2900 and 2300 BC. Plant macro-remains indicate that cereals, vine and many species of Fabaceae were cultivated in the nearby of the archaeological sites during the middle to the late Chalcolithic Age. The event of 2.2 ka BC corresponds to an abandonment phase in the lower Taravo Valley. Pastoralism dominated agricultural activities between 2200 and 1700 BC. During Roman times, agriculture is characterized by olive and vine cultivation. A new peak of pastoralism and the cultivation of Castanea are noted during invasion times (500 to 1000 AD), showing that invasions didn’t disturb agricultural activities in the Taravo Valley. During the Pisa Period (end of the 9th C. to then end of 13th C. AD), pastoralism declined and vine and cereals were cultivated in the very nearby of the Canniccia Marshes. During the Genoa Period upwards (end of the 13th C. to 1769 AD), a decline in agriculture and a recrudescence of the forest (maquis and pine) are recorded, leading to the settlement of a present-day vegetal landscape dominated by an Erica arborea maquis.

Référence électroniqueElodie Brisset, Cécile Miramont, Frédéric Guiter, Fabien Arnaud, Edward Anthony, Claire Delhon, Frédéric Guibal, Rosine Cartier, Yoann Poher et Eric Chaumillon, « Données nouvelles sur la chronologie de la déglaciation dans la vallée du Haut-Verdon (lac d'Allos, Alpes françaises du sud) », Quaternaire [En ligne], vol. 25/2 | 2014, mis en ligne le 01 juin 2017, consulté le 18 décembre 2019. RÉSUMÉCet article présente les datations 14 C AMS obtenues dans la séquence sédimentaire du lac d'Allos (alt. 2 230 m, vallée du Verdon, Alpes du Sud, France). Ces données contribuent à la connaissance de la chronologie de la déglaciation à l'échelle des Alpes du Sud et permettent de revoir le cadre chronologique proposé antérieurement sur ce site. La sédimentation lacustre démarre entre 12 300 et 13 500 ans cal. BP (2 sigmas). Les deux stades glaciaires identifiés en aval du lac à 1 900 m et sur le verrou glaciaire à 2 300 m sont par conséquent plus anciens. Nous rejoignons ici les conclusions des travaux antérieurs sur les tourbières et lacs d'altitude qui, sur la base des données polliniques, suggéraient une déglaciation très précoce remontant au début du Tardiglaciaire. ABSTRACT A NEW CONTRIBUTION TO THE CHRONOLOGY OF THE DEGLACIATION IN THE UPPER VERDON VALLEY (LAKE ALLOS, SOUTHERN FRENCH ALPS)This study presents new 14 C AMS ages obtained from the sedimentary sequence of Lake Allos (elevation: 2,230 m, Verdon valley, Southern Alps, France). These ages provide new evidence on the last deglaciation over the Southern Alps, enabling us to revise the chronological framework previously established. Sedimentation in the lake began between 12,300 and 13,500 cal. yr BP (2 sigma interval). The two glacial stages identified in a cirque downstream of Lake Allos (at 1,900 m a.s.l.) and on the glacial threshold (at 2,300 m a.s.l.) are hence older. Our results are in good agreement with previous conclusions based on palynological evidence from high-elevations peat bogs and lakes that suggested an earlier deglaciation, at the onset of the Lateglacial.

The Jiroft valley, situated on banks of the Halil Rud developed as an important agricultural and trading center during the Early Bronze Age. Known for its famous steatite sculptures and clay pottery, the first settlement in Konar Sandal collapsed around 3rd millennium BCE. A second shorter, but major phase of occupation in the settlement occurred towards the end of 2nd millennium BCE. A 250-cm long peat sequence near the archaeological complex at Konar Sandal was investigated to reconstruct the humanenvironment history using palynological, sedimentological and geochemical data. With a basal age of 4 ka, the core traces the hydroclimatic changes and human activities that started just after large scale abandonment of Konar Sandal and extends from the late Bronze Age to the Mongol invasion. The results show that Jiroft had an arid dry climate dominated by the Saharo-Sindian open pseudo-savanna vegetation. However, due to human clearance and intensified agro-sylvo-pastoral activities, and climatic factors, the land-cover shifted from open xeric scrublands to a more open degraded landscape. The principal human occupation was cereal cultivation and herding. However, it is likely that during the more arid periods, communities retreated and abandoned agriculture, facilitating successional processes. Such droughts occurred around 4.0e3.8 ka and 3.4e2.8 ka and are related to the Siberian Anticyclonic system. Declining Artemisia and shrubs indicate milder climates ca. 3.8e3.4 ka and 2.8e0.6 ka. The latter period that started with the rule of the Persian empires (550e650 BCE), and continued through the Islamic era, coincides with intensive human activities, and the highest degradation of vegetation.

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