Mid-Pleistocene cave fills, megafaunal remains and climate change at Naracoorte, South Australia: towards a predictive model using U-Th dating of speleothems

Abstract

The limestone caves of the Naracoorte region in South Australia contain extensive deposits of megafauna-rich sediments and intercalated cave formation (speleothems). High-precision thermal ionisation mass spectrometry (TIMS) U/Th dating of flowstones directly associated with several large deposits reveals a distinct cyclicity in the timing of sediment and flowstone depositional events in the Naracoorte caves. This pattern parallels a cyclical alternation of 'Wet Phases', and intervening periods with a water deficit over the last 500 ka (Ayliffe et al., 1998. Geology 26, 147). Dates from flowstone interlayers in the fossil deposits coincide with the massive speleothem growth events which characterise Wet Phases. Hiatuses in flowstone deposition correlatable through several cave systems imply that water deficits were initiated by regional aridity during full glacial and then extended into the succeeding warmer, wetter interglacials. Clastic deposits in caverns with restricted entrance shafts correlate with hiatuses, suggesting many of these deposits contain fauna representative of full glacial and interglacial climates. Caverns with small openings have often been sealed from the surface and dating has constrained the length of accumulation episodes in several fossil deposits, one to less than 20 ka. The hydrological regimes and environmental conditions inferred from the timing of speleothem deposition have been used to develop a model for cyclic sediment and bone accumulation in the caverns at Naracoorte over the last 400 ka. The giant Victoria Fossil Chamber deposit accumulated prior to 200 ka and because its entrance is large it may contain faunas representative of all climatic phases. There has been no apparent change in faunal diversity at Naracoorte over at least three glacial-interglacial cycles of the Middle Pleistocene. (C) 2000 Elsevier Science B.V.