Cave Monitoring to Determine the Controls on δ18O From a Modern Speleothem Record in Semi-arid SE Australia.

Abstract:

Cave monitoring provides a crucial link between hydrological and climate processes and how they may in turn effect speleothem δ¹⁸O calcite composition. This is particularly important in arid environments where potential evapotranspiration (PET) exceeds precipitation (P) and may dominate the δ¹⁸O signature. In this study we investigate climate-karst-cave interactions driving the δ¹⁸O variability in modern drip water samples from Cathedral Cave in semi-arid SE Australia and use this information to interpret a modern sub-annually resolved speleothem record. Over a two-year monitoring campaign we established that mean composition of δ¹⁸O in modern drip waters was enriched by up to 2.9‰, relative to the weighted mean annual rainfall. Furthermore, fluctuations in δ¹⁸O over time were controlled by two processes (1) evaporation in the unsaturated zone, resulting in heavier values, and (2) recharge during infiltration events, resulting in isotopically lighter values. Large soil moisture deficits in the soil zone must therefore be surpassed to initiate infiltration, resulting in relatively infrequent (0-3 per year) infiltration events. A series of infiltration experiments were conducted to elucidate water flow delivery mechanisms and water residence times. We found that the hydrological pathways and resulting mixing processes that occurred in the unsaturated zone determined the isotopic variability in the drip water δ¹⁸O between individual drip sites, and that potential storage in the unsaturated zone is generally greater than 6 months. We establish that the δ¹⁸O record in speleothems from this cave do not exclusively record rainfall composition or amount but are overlaid with an evaporative signal that is strongly affected by the time since the last infiltration event, local hydrogeology, cave climate and disequilibrium during calcite deposition. This hypothesis was tested by reconstructing the δ¹⁸O proxy record in a speleothem growing for ~50 years in the instrumental era. Our results have implications for the interpretation of speleothem records from semi-arid regions where PET>P.