U-Pb Dating Small Buried Stalagmites from Wonderwerk Cave, South Africa: a New Chronometer for Earlier Stone Age Cave Deposits

Wonderwerk Cave, in the Northern Cape, South Africa, contains a long and much studied sequence of archaeological sediments and stone tool industries and is particularly known for its Earlier Stone Age (ESA) deposits. The paucity of well-dated ESA sites in South Africa, as well as question of the Fauresmith, seen as a transitional industry between the ESA and Middle Stone Age, make Wonderwerk Cave an important site. Uranium-lead (U-Pb) dating of speleothems is a relatively new chronometer, and ages for samples as small and fragmentary as the little buried stalagmites from Wonderwerk Cave help establish it as an imminent dating technique for archaeological sites. The key to successful U-Pb dating of these relatively young, buried stalagmites is thorough pre-screening. Laser ablation scans proved invaluable in identifying layers within the stalagmite suitable for U-Pb dating, as well as providing detailed trace element data, which exclude the possibility of diffusion absorption of U in or out of the samples and confirm the integrity of the U-Pb ages. The relationship between the small stalagmites and the sediments surrounding them is paramount to interpreting the ages. Given that stalagmites need a stable cave floor to form on, these samples will predate the sediments below them and are most likely coeval with the sediments surrounding them and as such provide close to maximum ages for these deposits. WW23 from stratum 5 at the base of excavation 2 has a U-Pb age of 0.548 +/- 0.027 Ma. Two samples from stratum 10 from excavation 1, WW2 and WW4, have U-Pb ages of 0.734 +/- 0.069 and 0.839 +/- 0.026 Ma, respectively. These ages are precise enough to become a powerful tool to be used in pinning palaeomagnetic sequences and hint at a younger age of similar to 780 ka to 1.07 Ma for the older deposits, but a much larger data set is needed before this can be done with confidence. As it stands, these two ages suggest that there may be significant temporal variation within the stratum 10 deposits across the excavation area, and further work will be needed to constrain the chronology of the ESA deposits at Wonderwerk Cave. However, the addition of a new chronometer in the form of U-Pb dating adds an important new line of evidence for this complex task. As such, this technique will have broad applications for ESA cave sites older than 500 ka and beyond the limit of conventional uranium-thorium (U-Th) dating.