4 minute read
Going underground
OUR ENVIRONMENT Going underground
Understanding Australia’s past climate
Given the importance of water resources in Australia, surprisingly, there is relatively little information about past variability of rainfall across this continent.
Over the past century there are good instrumental records of climatic and rainfall variability but beyond this point, data is very scarce and we must therefore turn to other archives, such as speleothems, to fill in knowledge gaps.
This information is extremely valuable as it can be used as a way to anticipate the availability of water resources in the future, potential future climate trends (wetting or drying phases) and ultimately help inform management of limited natural resources.
This study is part of a larger effort by ANSTO’s speleothem research team to understand past climate and rainfall variability during Australia’s recent past. Part of this research is aimed at interpreting the climate and rainfall records over the past 2000 years in Australia. The first stage of this research is to assess how past records are recorded in speleothems by conducting cave monitoring.
Cave deposits, or speleothems, are mineral accumulations formed by calcium-rich water flowing into underground caverns. The isotopic and geochemical record is preserved in speleothem layers and these time series can be used as proxies for past environmental change.
Speleothems are particularly useful archives as growth layers can be analysed, for example using mass spectroscopy to provide ratios of oxygen-18 and oxygen-16, revealing records of past rainfall amount and frequency. They can also be precisely and accurately dated by measuring isotopes of uranium and thorium in the speleothem fabric on a Multi-Collector Isotope Ratio Mass Spectrometer.
One study is currently taking place at Yarrangobilly Caves in the Snowy Mountains, New South Wales — an important area as it provides approximately 25 per cent of the head waters into the Murray-Darling Basin. The limestone deposit contains a system of about 400 caves managed by the National Parks and Wildlife Service (NPWS). Geologists suggest that the caves were formed about 440 million years ago.
The research team monitored drip waters, as well as climate variables (such as precipitation, temperature, barometric pressure and soil moisture) from a custom-built weather station directly above the cave.
In the cave, dripping water was automatically recorded using Stalagmate® drip loggers, devices similar to a miniature, watertight, plastic drum, that records each drip from the vibration measured each time a water droplet hits its surface. Fourteen sites are part of a network of 50 such devices placed throughout the cave, one of the largest such studies in the world. Interpreting the data provided by the Stalagmate® loggers provided a unique way to classify and understand water flow from the surface to the cave.
Water from the surface carries the majority of the climate and environmental information ultimately recorded in the speleothem. Researchers found that the soil moisture content may be more important than the amount of rainfall in controlling hydrological flow paths. Although rainfall is essential for groundwater recharge, it was the antecedent soil moisture saturation (i.e. wet or dry preconditions) that controlled whether water from individual rainfall events reached the underground cave system.
The researchers identified five different drip water responses to surface climate and were surprised to see different flow patterns in drips in close proximity to each other (e.g. 1 m away). The five types of response were due to the many possible water flow paths from the surface to the cave, with water potentially stored in both the soil and in fractures and solution pockets in the limestone, for varying amounts of time before reaching the cave. These findings were published in the Journal of Hydrology. In another study published in Hydrology and Earth System Sciences, the researchers identified that the dripwater trace elements were linked to the outside precipitation, which is influenced heavily by the El Niño Southern Oscillation in this region.
Most importantly, the research demonstrates that speleothems can have very individual relationships to the surface climate due to the specific water flow route. This information has allowed researchers to identify which speleothems are most suitable to be analysed for proxy records of past environmental change.
The next phase of this research is to use this hydrological understanding to interpret modern speleothem records which grew over the instrumental era (last 100 years) to reveal how the environmental signal is preserved in the speleothem fabric over a period of known climate variability.
Pictured
Monika Markowska in Harrie Wood Cave, Yarrangobilly, New South Wales. Photo courtesy Dr Cath Jex.
Contact
monika.markowska@ansto.gov.au carol.tadros@ansto.gov.au
COLLABORATORS
1
ANSTO
3
University of New South Wales
RESEARCH FACILITY / TECHNIQUE
Centre for Accelerator Science (CAS) Isotopic tracing techniques
2
4
National Parks and Wildlife Service Wellington Council
University of Melbourne
Canberra
RESEARCHER TEAM
Monika MARKOWSKA
Dr Pauline TREBLE
Carol TADROS
Prof Andy BAKER Dr John HELLSTROM George BRADFORD
Sydney
Wollongong
