Modern terrestrial speleothem-like calcareous deposits in streams draining a disused lime quarry on Black Mountain, South Wales have anomalously negative d18O and d13C compositions compared with other similar European deposits. Black Mountain water chemistry is unusual only in its locally very high pH (> 11.5) and carbonate ion concentrations. The high pH is caused by dissolution of lime spoil, resulting in high OH– concentrations. This high alkalinity causes uptake of atmospheric CO2 and strong fractionation of both carbon and oxygen stable isotopes, resulting in calcite precipitates with unusually negative isotopic compositions. Since shifts in d18O of < 1° are highly significant for Holocene palaeoclimate reconstructions, depletions caused by hyperalkaline waters must be avoided. While extreme lime spoil contamination should be obvious, less heavily affected sites will record smaller fractionation effects and might escape detection. Even small depletions from low-level contamination will have large effects on palaeotemperatures based on carbonate crust d18O values.
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|Published - 1997