Mid-to Late Holocene landscape changes in the Rioni Delta area (Kolkheti lowlands, W Georgia)

The Kolkheti lowlands (Colchis, Colchian plain) form the central part of the extensive coastal lowlands along the Black Sea coast of Georgia. Situated between the Greater and the Lesser Caucasus, favourable climatic conditions resulted in a constant human occupation of the region during the Holocene. However, due to continued deltaic sedimentation and alluviation of the river Rioni, the configuration and the environmental conditions of the coast and its hinterland have changed considerably; this was related to sea-level fluctuations of the Black Sea and variation of the sediment supply. This study presents new data on the Holocene coastal evolution of Western Georgia. Based on the geochemical and sedimentological analysis of sediment cores and trenches from the northern part of the Kolkheti lowlands, between the Black Sea and the rivers Rioni and Khobistsqali, and a robust chronology (14C and IRSL dating), our goals were (i) to document the chrono-stratigraphy along two coring transects; (ii) to decipher geographical and environmental changes along Georgia's Black Sea coast; and (iii) to trace the sea-level evolution of the study area. Based on the succession of eight facies, representing different depositional environments, our results suggest that significant environmental changes took place throughout the last eight millennia. At least since 5000 cal BC, the sedimentary record indicates the widespread existence of shallow lagoons. Floodplain-related fine-grained alluvium accumulated on top of the lagoonal stratum. The progradation of the delta plain between 3500 and 1500 cal BC was accompanied by the evolution of extensive swamps with peat formation. The data indicate a gradual and moderate sea-level rise since ∼6000 BC. Ultimately, this and follow-up studies may provide a valuable background for the understanding of the palaeogeographical context of ancient settlements in coastal Georgia.