Impact of climate changes on vegetation and human societies during the Holocene in the South Caucasus (Vanevan, Armenia): A multiproxy approach including pollen, NPPs and brGDGTs

Résumé En

Relationships between steppe vegetation, human practices and climate changes in the past are crucial to disentangle human development in Eurasia. In this frame, our study investigates (1) modern pollen-vegetation relationships and (2) changes in vegetation, human activity and climate in the Holocene record of Vanevan peat (south-eastern shore of Lake Sevan, Armenia), using a multiproxy approach including sediment geochemistry (XRF), pollen, Non-Pollen Palynomorphs (NPPs), and branched Glycerol Dialkyl Glycerol Tetraethers (brGDGTs). Climate reconstructions are provided by (1) water-level changes, (2) brGDGTs, and (3) pollen transfer functions (multi-method approach: Modern Analogue Technique, Weighted Averaging Partial Least Squares regression, Random Forest, and Boosted Regression Trees). Modern pollen assemblages are selected along an altitudinal transect in Armenia. They show a dominance of Chenopodiaceae in semi-desert/steppe regions while meadows steppes, subalpine, and alpine meadows are dominated by Poaceae. Past vegetation is characterized by steppes dominated by Poaceae surrounded during the Mid-Holocene (8200-4200 a cal BP) by scarce open woodlands. Humans have influenced the local vegetation, mainly through their agricultural practices present since 5200 a cal BP with several intensification steps. Our reconstruction indicates a climate shift from a cold and arid Early Holocene toward a warmer and more humid Mid-Late Holocene. An aridification trend marks the last 5000 years causing a drop in water level, which allowed humans to live and cultivate on Lake Sevan shores. Arid events are recorded at 6.2 ka, 5.2 ka, 4.2 ka and 2.8 ka a cal BP, which are commonly related to multi-centennial-scale variations of Westerlies activity (North Atlantic Oscillation). Through our temperature reconstruction, we can assign (1) the 5.2 and 2.8 ka events as being cold and probably related to a strong Siberian High, and (2) the 4.2 ka event as being warm associated with high Arabian subtropical pressures in the South Caucasus and the Near East. Our study suggests a significant impact of these arid events on the Lake Sevan shore populations and they are consistent with cultural phases in the South Caucasus, thus showing the impact of climatic variations on cultural, land use and occupation mode development in this crossroad region between Europe, Africa and Asia.