Stable isotope record (δ18O and δ13C) of a Naracoorte Caves speleothem from before and after the Last Interglacial

Abstract

Stable isotope measurements (δ18O and δ18C) of speleothem SCS-10 from the Naracoorte Caves record a combination of ancient soil hydrology, palaeoclimatic conditions, and isotopic fractionation during speleothem formation. U-Th ages as well as macroscopic examination show two distinct growth phases separated by a period of speleothem dissolution (earlier phase of 179-162 ka and later phase of 114–96 ka). Sampling traverses along growth lamellae indicate significant isotopic differences between the centre and distal part of the lamellae (disequilibrium due to CO2 degassing). Carbon and oxygen isotopic data from the earlier growth phase (down axis of speleothem core) show pronounced co-variation, albeit with greater variation of carbon values. Comparisons of speleothem isotopic values with global isotopic records show reasonable correspondence to SPECMAP trends. An enhanced δ18O signal in the speleothem could be due to ocean water and soil hydrology signals combining colder, enriched ocean water and a drier, enriched soil signature, compared with warmer, depleted ocean water and a wetter, depleted soil signature. Carbon isotopes largely co-vary with oxygen; enriched carbon values probably reflect low soil activity during drier, cooler times and depleted carbon values probably reflect a soil carbon signature dominated by C3 vegetation.