[1] Univ Oxford, Sch Archaeol, Oxford OX1 2PG, England
[2] Univ Bristol, Sch Chem, Organ Geochem Unit, Biogeochem Res Ctr, Bristol BS8 1TS, Avon, England
[3] Univ Sheffield, Dept Archaeol, Sheffield, S Yorkshire, England
[4] British Geol Survey, NERC Isotope Geosci Lab, Keyworth NG12 5GG, Notts, England