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

delta(13)C isotope variations in speleothems have been investigated for samples from the British Isles, where plants which use the Hatch-Slack or C-4 photosynthetic pathway are not present. The range of delta(13)C expected in speleothem carbonate formed in isotopic equilibrium with soil CO2 derived from the overlying C-3 vegetation should thus fall in the range -12 to -6 parts per thousand. Forty-one actively growing speleothem samples from low-discharge sites were analysed from Stump Cross Caverns, Yorkshire, England. Ten percent have delta(13)C greater than -6 parts per thousand. In addition, a large range of delta(13)C was observed (-8.06 +/- 1.38 parts per thousand, a 1 sigma variability of 17%) with adjacent samples having delta(13)C differing by a maximum of 4.74 parts per thousand. 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 delta(13)C. Three possible processes are proposed as possible causes of elevated delta(13)C 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 delta(13)C 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 delta(13)C Signal within speleothems in terms of palaeovegetation.