231Pa and 230Th contamination at zero age: a possible limitation on U/Th series dating of speleothem material

Analysis of 67 New Zealand speleothem specimens for both 230Th/234U and 231Pa/235U by alpha spectrometry showed that in most cases the age concordance between them was excellent. However, analysis of two growing speleothems less than 100 years old showed apparent near-concordant ages of 2 and 3 ka—both 230Th and 231Pa had been added to the speleothem, contrary to a basic assumption of the dating method. In other cases of discordance there was additional evidence that 230Th and sometimes 231Pa had been added to the growing speleothem and probably originated from erosion of old limestone by carbonate-rich waters. Calculation shows that typical 230Th and 232Th contents in carbonate-rich waters could easily be deposited in sufficient quantity in growing speleothems in realistic time intervals to produce the amounts found in the straw stalactites. The U contents of these speleothem samples were unusually low (42% contained less than 0.1 mg kg−1) but aragonitic samples contained 100× as much U.