Decoding past rainfall trends from southwest Australian speleothems

Pauline Treble

Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia

Southwest Western Australia experienced a reduction in rainfall of around 10-20% that has persisted since about 1970.  This rainfall reduction has had significant consequences for dam levels and water-dependent ecosystems in the region.  Dr Treble is the chief investigator on a project to reconstruct the frequency, duration and intensity of past multi-decadal dry periods in the southwest Western Australian speleothem record for the past 1000 years.  Research during 2007 in Earth Environment by Dr Treble focused on constraining the climatic controls on cave drip water chemistry in the southwest of Western Australia.  Understanding the relationship between surface climate and cave hydrology/geochemistry are fundamental for unraveling past climatic information preserved in cave speleothems.

A key process in understanding the relationship between surface climate and the stalagmite geochemical trends is monitoring the real-time encoding of inter and intra-annual climate variations in the cave environment.  This was done by monitoring changes in the drip water hydrology and geochemistry, as well as soil moisture and rainfall events.  Interpretation of a 2.5 year long dataset has revealed that speleothem growth takes place primarily during the winter seasons driven by the partial pressure of cave CO₂ which in turn is related to the hydrology of the overlying rock.  Distinct annual cycles in cave drip water Mg/Ca and Sr/Ca are directly related to the amount and duration of winter rainfall entering the cave each year.  The existence of these annual cycles provides an important chronological tool as well as valuable seasonal information.  Importantly, this research demonstrated that the cave drip water chemistry responded to the winter rainfall deficit in 2005 which was the driest year on record for this region.

This research was partly aided by an Exchange Fellowship granted to Dr Treble by the British Council. Dr Treble travelled to the University of Birmingham in the UK to collaborate with Prof Ian Fairchild on this research topic.  Journal articles reporting the results of this study are currently in preparation.