Elevated and variable values of 13C in speleothems in a British cave system

δ 13C isotope variations in speleothems have been investigated for samples from the British Isles, where plants which use the Hatch-Slack or C4 photosynthetic pathway are not present. The range of δ 13C expected in speleothem carbonate formed in isotopic equilibrium with soil CO2 derived from the overlying C3 vegetation should thus fall in the range −12 to −6‰. Forty-one actively growing speleothem samples from low-discharge sites were analysed from Stump Cross Caverns, Yorkshire, England. Ten percent have δ 13C greater than −6%. In addition, a large range of δ 13C was observed (−8.06 ± 1.38‰, a 1 σ variability of 17%), with adjacent samples having δ 13C differing by a maximum of 4.74‰. Similar findings were obtained from a review of analyses of late Quaternary speleothem samples from the British Isles, with 75% of flowstone samples and 57% of high-flow stalagmite samples exhibiting elevated δ 13C. Three possible processes are proposed as possible causes of elevated δ 13C in speleothems. Firstly, fractionation may occur between the stalactite and stalagmite due to evaporation or degassing. Secondly, degassing of the groundwaters may have occurred within the aquifer before reaching the cave void, allowing release of some CO2 from the water whilst remaining saturated in calcium. Finally, the elevated δ 13C may be due to short water residence times in the soil, such that equilibrium between soil water and soil CO2 is not reached. Evidence presented here demonstrates that any one of these mechanisms may be important in the karst areas of the British Isles. Caution is needed before interpreting the δ 13C signal within speleothems in terms of palaeovegetation.