UNUSUAL ISOTOPIC PHENOMENA IN WELSH QUARRY WATER AND CARBONATE CRUSTS

Modern terrestrial speleothem-like calcareous deposits in streams drai ning a disused lime quarry on Black Mountain, South Wales have anomalo usly negative delta(18)O and delta(13)C compositions compared with oth er similar European deposits. Black Mountain water chemistry is unusua l only in its locally very high pH (> 11.5) and carbonate ion concentr ations. The high pH is caused by dissolution of lime spoil, resulting in high OH- concentrations. This high alkalinity causes uptake of atmo spheric CO2 and strong fractionation of both carbon and oxygen stable isotopes, resulting in calcite precipitates with unusually negative is otopic compositions. Since shifts in delta(18)O of < 1% are highly sig nificant for Holocene palaeoclimate reconstructions, depletions caused by hyperalkaline waters must be avoided. While extreme lime spoil con tamination should be obvious, less heavily affected sites will record smaller fractionation effects and might escape detection. Even small d epletions from low-level contamination will have large affects on pala eotemperatures based on carbonate crust delta(18)O values.