Recharge variability in Australia's southeast alpine region derived from cave monitoring and modern stalagmite δ¹⁸O records

Oxygen isotopic (δ¹⁸O) variations in stalagmite records have the potential to provide new insights about past climates beyond the instrumental record. This paper presents the first high-resolution oxygen isotope time series of three coeval stalagmite records from the alpine region of south-eastern Australia covering the period 1922–2006 CE. We use extended surface and cave monitoring datasets, petrographic investigation, modelled recharge time series and farmed calcite precipitates to assess the controls on speleothem δ¹⁸O and investigate the coherence between three records from Harrie Wood Cave. The drip water response to recent interannual rainfall variability shows that cave drip water Cl⁻, δ¹⁸O and drip rate display a clear response to an increase in rainfall recharge. It is demonstrated that stalagmites from the same drip sites also record variability in interannual recharge, where an increase in δ¹⁸O values is observed with lower recharge, while a decrease in δ¹⁸O values correspond to higher recharge amounts. The three stalagmite δ¹⁸O records are in broad agreement, showing common responses to relatively higher recharge between 1945 and 1995 CE and the low recharge periods between 1937 and 1945 CE (World War II drought) and late 1996 to 2006 CE (beginning of the Millennium Drought). However, differences in the magnitude of the relative response of each stalagmite δ¹⁸O record varies. Based on evidence from our cave monitoring study and farmed calcites, we conclude that the differences between the three stalagmite records is attributed to variability in the contribution of preferential flows during recharge events and the store reservoir volume supplying the drip site. When the δ¹⁸O decreases in response to enhanced recharge, the speleothem δ¹³C also decreases, and this is interpreted to reflect a soil respiration response to changes in soil moisture availability due to recharge. Hence, stalagmite δ¹⁸O from the Australian alpine region can be applied to reconstruct periods of relatively higher and lower rainfall recharge and thus extend our knowledge of the timing and relative magnitude of droughts as well as past periods of higher recharge in this region.