Reconstructing Holocene shore displacement and Stone Age palaeogeography from a foredune sequence on Ruhnu Island, Gulf of Riga, Baltic Sea

Abstract

Holocene shore displacement and the palaeogeography of Late Mesolithic and Late Neolithic settlements on Ruhnu Island, Gulf of Riga, were reconstructed using foredune sequence luminescence dating, sedimentological data supported by ground-penetrating radar analysis, and GIS-based landscape modelling. The foredune ridges consist of very well to well sorted fine- to medium-grained aeolian sand and are underlain by seaward dipping foreshore sediments. The studied sequence of 38 ridges was formed between 6.91±0.58ka and 2.54±0.19ka ago, and represents a period of falling relative sea level. Foredune plain progradation, with average rates of 0.3–0.6m per year, was controlled by isostatic land uplift, which caused a continuous withdrawal of shorelines to lower elevations. The dated foredune succession was used to reconstruct the coastal palaeogeography of the island. Palaeogeographical reconstructions show that during two phases of Late Mesolithic habitation, at ca. 7.2cal.ka BP and 6.2cal.ka BP, seal hunters settled the coastal zone of Ruhnu Island. Based on tool material and pottery type they could have originated from Saaremaa Island, which according to palaeoreconstruction of the Gulf of Riga, was located approximately 70km northwest of Ruhnu Island during the Late Mesolithic. Later signs of human occupation, radiocarbon dated to ca. 4.7cal.ka BP, were from the centre of the island, hundreds of metres away from the shore at about 8m above its contemporary sea level. This Late Neolithic habitation shows a clearly different pattern than earlier coastal settlement, and suggests a shift in subsistence strategy towards agriculture and animal husbandry.