Molluscan successions from two Holocene tufas near Northampton, English Midlands
To investigate the molluscan successions from two Holocene tufa sequences in the English Midlands to establish whether similar patterns of colonization occur and to document how they relate to other successions reported from southern Britain and elsewhere. Location
Courteenhall and Weston Favell near Northampton, English East Midlands. Methods
Close serial sampling and quantitative analyses have allowed detailed faunal successions to be reconstructed for each site. Radiocarbon dating of Cepaea shells by AMS has provided a chronology for each site. Results
Remarkable similarities were found to exist between the two sites. The record from Courteenhall extended back to the end of the Devensian late-glacial and continued until about 6000 yr BP. The sequence from Weston Favell was shorter, commencing at the beginning of the Holocene and ending about 7500 yr BP. Both successions show a progressive replacement of open country assemblages by those of woodland. At Courteenhall arctic-alpine species, such as Columella columella (Martens) and Vertigo genesii (Gredler), were present in these early open-ground communities. Both sites supported extremely rich (> 30 species) land snail assemblages during the mid Holocene. This richness cannot be matched at any single site in Britain today. These mid Holocene assemblages include species such as Spermodea lamellata (Jeffreys), Leiostyla anglica (Wood) and Vertigo alpestris Alder that are rare or absent today in southern Britain. Discus ruderatus (Férussac), which has a modern boreo–montane range, occurred at Courteenhall between about 9700 and 8750 yr BPbut was also present in two consecutive samples just below a level dated at about 7000 yr BP, its latest known occurrence in Britain. These assemblages have no modern analogue because the ranges of several species (e.g. Discus ruderatus and Leiostyla anglica) never overlap today. General conclusions
During the Holocene land snails appear to have colonized southern Britain in a relatively ordered sequence. The history of some species is unpredictable and they may occur at different sites at different times. For others, the general pattern shows greater consistency, enabling the recognition of a series of mollusc zones applicable over large areas of southern Britain. These Northamptonshire sites also contribute to the debate surrounding the notion of a ‘late Holocene tufa decline’. Tufa formation began unusually early (c. 10,280 yr BP) at Courteenhall but it had ceased to form at both sites by 6000 year (maybe by 7500 yr BPat Weston Favell). There is no evidence for late Holocene tufa formation at either site, a pattern consistent with many other British sites. Rates of tufa growth from 2.7 to 7.0 cm 100 years–1, calculated from the radiocarbon dates from Courteenhall, are comparable with accumulation rates established from other Holocene tufas.