Calibrating δ18O in Dromaius novaehollandiae (emu) eggshell calcite as a paleo-aridity proxy for the Quaternary of Australia

The large, flightless Australian bird, Dromaius novaehollandiae (emu), breeds at about the same time of year throughout its range, and fragments of its eggshells are commonly preserved in geological and archaeological archives spanning the Late Quaternary. The δ18O of eggshell calcite is set primarily by the δ18O of body water in the days prior to egg calcification. Changes in the δ18O of body water are dominantly determined by the δ18O of ingested water. We provide a calibration dataset of 334 δ18O analyses on late Holocene Dromaius eggshell from 21 sites across a wide range of environmental conditions and climate states. In regions where interannual rainfall variability is high, the average of 20 samples from different years is required to derive a δ18O estimate within ±1‰ of the millennial mean; fewer than 10 different-year samples is sufficient for less variable rainfall regions. Many climate variables in Australia are highly correlated with δ18O in eggshell, but the strongest correlation is with the average site-specific point-potential evapotranspiration (PPET) for February plus March, two summer months prior to egg calcification, which explains 93% of the variance in the mean δ18O of our calibration dataset. Eggshell δ18O can be used as a PPET (paleo-aridity) proxy in older samples providing four assumptions are closely approximated: (1) no difference in season of breeding over time, (2) breeding in most years, (3) little change in δ18O of precipitation over time, and (4) the bird's source water is exposed to evaporative processes. The reconstructed PPET derived from 150 δ18O measurements in an 80 ka time-series of eggshell from south-central Australia where these assumptions are satisfied is consistent with independent estimates of environmental water balance derived from changes in lake level and dustiness. The tight correlation between eggshell δ18O and PPET indicates that the average δ18O in time-series of Dromaius eggshell is a robust proxy to reconstruct changes in moisture balance through the Late Quaternary of Australia.