TH-230/U-234 AND C-14 DATING OF A LATE PLEISTOCENE STALAGMITE IN LOBATSE-II CAVE, BOTSWANA

A late Pleistocene stalagmite from a karst cave in southeastern Botswana has been used to compare the C-14 dating method on speleothem carbonates with the Th-230/U-234 method. An age discrepancy between the two methods indicates that at least one of the dating series does not reflect the true growth ages. The deviation is smallest in the youngest part of the stalagmite, where 21,600 U-series years correspond to 17,800 C-14 BP. This is in accordance with the C-14 and Th-230/U-234-dating results on Barbados corals (Bard et al., 1990). Subsequently, the deviation increases rapidly with age, resulting in a discrepancy of 20,000 years at the Th-230/U-234 age of 50,000 years. Whilst the C-14 results exhibit a reversed order in the middle of the sequence, the Th-230/U-234 age estimates are stratigraphically ordered. We consider the U-series data reliable and conclude that these results od indeed reflect true calendar years. The C-14 age estimates are probably a result of the post-depositional introduction of C-14, but may also reflect, to some extent, variations in the atmospheric C-14 content. This study indicates that C-14 dating of speleothem carbonate is problematic.