Modern speleothem oxygen isotope hydroclimate records in water-limited SE Australia

Dryland regions are generally projected to become drier under future climate change scenarios. Understanding the long-term natural variability of dryland regions via paleo-reconstructions is therefore highly desirable. The δ¹⁸O of two coeval modern speleothems from Cathedral Cave, Wellington, in semi-arid SE Australia are compared to the instrumental record to assess its efficacy as a proxy of past hydrological variability. Stalagmite δ¹⁸O was modulated by the frequency of recharge events and epikarst evaporation of storage water. Prolonged intervals between recharge events, such as droughts, resulted in higher stalagmite δ¹⁸O. Conversely, periods with more frequent recharge events and a positive water balance, resulted in lower δ¹⁸O. Disequilibrium cave processes are likely to be enhanced during dry conditions, although it is argued that these will modulate δ¹⁸O_spel in the same direction as epikarst evaporation, effectively amplifying the response of δ¹⁸O_spel. Extreme events, such as floods and droughts, were also captured in the stalagmite records, although potentially with a lag of several years. We verify that modern speleothems from semi-arid regions can be used to reconstruct hydroclimate due to variations in δ¹⁸O_spel modulated by karst processes. Such records are archives of past changes in recharge rather than precipitation amount or surface temperature, as is commonly applied to speleothem records from non-water-limited regions.