Special Paper: Comparison of Post-Glacial Molluscan and Vegetational Successions from a Radiocarbon-Dated Tufa Sequence in Oxfordshire

Abstract

Pollen, molluscan, and sedimentological analyses have been undertaken from a tufa, which forms part of a near-complete Post-glacial succession from Sidlings Copse near Oxford. Local pollen and mollusc zones are defined and the times of arrival of critical species compared. It is shown that some pollen and mollusc zone boundaries coincide pre- cisely but that others do not, with some faunal changes occurring during periods of vegetational stability. A detailed chronology is provided by radiocarbon dating using acceler- ator mass spectrometry. Tufa formation began at about 9300 yr BP as hazel (Corylus avellana) woodland began to fill the previously open land- scape. An unshaded marsh initially developed on the surface of the tufa, which supported snails such as Vertigo angustior and Discus ruderatus (a boreo-alpine species now extinct in Britain). Hazel woodland gradually encroached and caused the replacement of the early marsh communities by woodland mollusc assemblages. Discus ruderatus was replaced by D. rotundatus at 8990 ? 90 yr BP, and subsequently Leiostyla anglica and then Spermodea lamellata appeared. At 7180 ? 85 yr BP lime (Tilia cordata), slightly preceded by alder (Alnus glutinosa), spread into the area. Oxychilus cellar- ius first appeared at this time. Lime and elm (Ulmus) declined at 5250 t 75 yr BP, and this was accompanied by changes in the molluscan fauna and vegetation suggesting a reduction of shading. The land snail fauna at this time was composed of an unusual assemblage dominated by Leiostyla anglica and Oxychilus cellarius. Tufa formation ceased shortly after 5065 ? 120 yr BP, perhaps due to hydrological changes initiated by human activity. Rates of tufa growth from 1.3 to 5.4 cm/100 years have been calculated from the radiocarbon dates. These accumulation rates are similar to those derived from the overlying organic sediments. The upper sediments are highly organic and contain very few molluscs, but pollen continues to occur, enabling reconstruction of vegetational history up to the present. The biostratigraphy of Sidlings Copse is compared with neighbouring sites and other sequences from British tufas.