Late Pleistocene stalagmite growth in Wolkberg Cave, South Africa

摘要

Little is known about the sequence of climate and environmental change in southern Africa during the last glacial period, in spite of the intimations from records, such as Antarctic ice cores and archaeological sites, that very marked changes took place which would have had profound effects on vegetation and animal distributions across the sub-continent. High-resolution, (semi-) continuous climate and environmental records can be extracted from suitable cave speleothems. Speleothems are reasonably abundant in southern Africa, but their occurrence is patchy in time and space and the records can be difficult to interpret. Here we report our assessment of the stalagmite W5 from Wolkberg Cave in the northeastern part of South Africa, as an archive for glacial-period climatic and environmental shifts. The cave is located at 1450 m asl, in the dolomitic limestones of the Transvaal System in an area currently dominated by C-4 grass vegetation. Nine U/Th dates show growth from 58 to 46 ka, and a second brief phase ca. 40 ka, indicating that the available moisture was sufficient to allow speleothems to form. The delta O-18 and delta C-13 values along the growth axis show variability in the order of 2 parts per thousand for the former, while variability in the latter is characterized by a shift from values near -2 parts per thousand in the older section to +2 parts per thousand or more in the younger part. These high delta C-13 values are probably the combined result Of CO2 degassing of the percolating soil water prior to the carbonate precipitation in the cave chamber, the increasing dominance Of C-4 over C-3 vegetation, and the high percentage of aragonite towards the stalagmite's top. The retrieved data point towards increasingly drier and colder conditions during the growth period of the stalagmite. Furthermore, the high-frequency variations of delta O-18 values indicate the presence of short term climate oscillations that are probably linked to shifts of the Intertropical Convergence Zone.