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

Abstract

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 (14 C and IRSL dating), our goals are (i) to document the chronostratigraphy 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 finegrained 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 the area.

Bronze Age settlement mounds on the Colchian plain at the Black Sea coast of Georgia: A geoarchaeological perspective

“…As for the Orulu mounds, a 14 C age estimate from an alluvial layer proves that these conditions existed since the 3rd millennium B.C. ORU3 reveals the alternation of fluvial and alluvial deposits down to 12 m b.s., which suggests that these conditions had already established during mid‐Holocene times, as observed in the central part of the Kolkheti lowlands (Laermanns et al., ).…”

“…In addition, determining the type and purpose of the settlements mounds–i.e., constructed or cumulative–remains challenging. In general, the warm and humid climate and the swampy conditions at that time (Connor et al., ; de Klerk et al., ; Laermanns et al., ), the rather short accumulation phase of Ergeta 1 and Orulu 2 (Figure ), and the similar ages at the base of Orulu 1 and Orulu 2 all suggest an intentional construction of the mounds within a short time period. This contradicts the nature of formation of tells with their successive accumulation in space and time, e.g., in Thessaly with >20,000 m² (Runnels et al., ) and in western Anatolia with ≈16,000 m² and 5000 m² (Stock et al., ).…”

“…The relative sea level (RSL) reached nearly its current state (between −1.5 and 0 m below mean sea level) about 3000 B.C. (Giosan et al., ; Brückner, Kelterbaum, Marunchak, Porotov, & Vött, ; Fouache et al., ; Kelterbaum et al., ; Laermanns et al., ), and subsequent sea‐level oscillations of ≈1 m might have affected the settlement conditions in the marshlands close to the coast. However, it can be considered to have had only a minor effect on the areas of the investigated mounds, which are located at an altitude between ≈4 and 11 m a.s.l.…”

“…Due to the presence of extensive wetlands during the mid‐Holocene, and the transformation from open lagoons to an alluvial plain that took place between the 4th and the mid‐2nd millennium B.C. (Laermanns et al., ), favorable conditions for the occupation of the Colchian lowlands seem to have developed only shortly before the 2nd millennium B.C.…”

“…In coastal Georgia, extensive back‐barrier areas were mainly silted up by fluvial sediments; they changed between 3000 and 1500 B.C. from open lagoons to swampland and peat formations (Laermanns et al., ).…”