A 1,500 year south Australian rainfall record based on speleothem hydrological proxies

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AMOS National Conference
Cave drip water studies at Wombeyan Caves (34°19’S, 149°59’E) demonstrated a marked hydrochemical response to wet/dry phases (McDonald Et al. 2004; 2007). Geochemical Variations in three 20th Century coeval active Speleothems were able to be linked to the instrumental record. Subsequently geochemical relationships were investigated in a long record speleothem(WM7) which grew deeper within the same cave system. Obtaining a robust chronology proved to be challenging, due to the young age of the speleothem and very low uranium concentrations (~10 ppb) the use of U‚Aeseries disequilibrium dating was ineffective to produce a robust chronology. Chronology for WM7 was based on a dense sequence of DCF corrected ages using three different age-­‐depth models: Clam (Classical method), and Bacon and OxCal (Bayesian statistical approach) (Hua et al. 2012).The new chronology indicated that WM7 began growth around 4400 cal BP(171 mm). However, since sampling from 0-­‐50mm was most intensive, the model is based on this part of the stalagmite and indicates that the top 50 mm of WM7 grew during the past 1360 and 1740 years. An aridity index based on Sr,P, Y, La, and Ba shows that over the last 1,500 years several sustained episodes of wet/arid and otherwise variable phases have occurred. Two sustained wet phases ~ 700-­‐880 AD and ~ 900-­‐ 1250 AD were followed by ~ 400 years of variable wet/dry conditions, although from ~1300 to 1600 AD a drying trend is indicated, but punctuated by several wetter episodes. The last 200 years indicate sustained drying phases. The OE¥13C record is anomalous from ~ 1880 to present and attributed to the stalagmite’s recording of increasing contribution of fossil fuel to CO2 concentrations. Within the longer-­‐time scale oscillations, higher resolution (~ 2-­‐5 years) variability is evident, replicating the trend shown by modern annually resolved stalagmites at this site.
Caves, Uranium, Fossil fuels, Carbon dioxide, Oscillations, Washout
McDonald, J., Drysdale, R., Hua, Q., Hodge, E., Treble, P., Greig, A., Fallon, S., Lee, S., & Hellstrom, J. (2013). A 1,500 year south Australian rainfall record based on speleothem hydrological proxies. Paper presented at the 19th National Conference of the Australian Meteorological and Oceanographic Society (AMOS), 11-13 February 2013 at the Melbourne Convention and Exhibition Centre, Melbourne, Australia.