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 1σ 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.