Caves as observatories of the past

Posted 25 November 2013

Professor Andy Baker emerging from a cave.

UNSW Science researchers are studying Wellington Caves to uncover a record of past climate and environmental conditions, writes Professor Andy Baker.

New South Wales has an abundance of limestone caves and some of the state’s most famous and popular tourist attractions include caves such as Jenolan, Wee Jasper and Wellington, where visitors can marvel at the glistening stalagmite and stalactite formations.

Caves also hold a fascination for scientists interested in past climates and environments. Going deep underground to understand climate and environmental change on the surface might not seem logical, but caves have several unique properties. Firstly, they provide a relatively protected location to preserve material from the past. And secondly, the sediments and stalagmites in the caves can contain records of past climates and environments that can be more than thousands of years in length.

Our team based at UNSW has spent the last three years installing instruments, monitoring and calibrating a cave system so that we can better understand the climate and environmental record within it. With the support of Wellington Council, and funded by the Federal Government’s Groundwater Education Investment Fund, the team has worked at the Wellington Caves in Central West NSW. The council, which runs the Wellington Caves, is highly supportive of visits by education groups, and our results are available to all visiting school groups.

So what can you see at Wellington Caves, and what have we learnt?

Firstly, we have installed an automatic weather station above the caves, so that we can make direct links between the subsurface and surface climate. The weather station downloads data to the website http:/groundwater.anu.edu.au and is freely available (note: you download a datafile which needs some basic understanding of weather parameters to use).

Next, we monitor the water movement from the surface into the caves, as it is this infiltrating water that carries the majority of the climate and environmental information from the surface. Visitors to Cathedral Cave can see our network of drip-water measuring devices. Manufactured by a UK company, Stalagmate ©, drip loggers behave like a miniaturised drum, with the drip waters that fall on them triggering a vibration that is recorded by a data-logger. In NSW we have the biggest network of cave-drip water measurements in the world (we are making similar measurements in the Yarrangobilly Caves in the Snowy Mountains in collaboration with colleagues from Australia’s Nuclear Science and Technology Organisation).

Understanding how water moves from the surface to an individual cave is important, as flow routes can be quite complex. By measuring the drip rates in the caves, we can work out how long it takes water to get to the cave. At Cathedral Cave, it can take less than one day for water to start dripping near the entrance, but up to two weeks for water to reach the end of the cave at a depth of 30 metres below the ground surface.  A Nature Education article on the relationship between drip water hydrology and stalagmite records of climate and environment can be accessed (see links and resources below).

The oxygen story

So what are the records of past climate and environment that can be preserved in stalagmites? The most widely used are records that come directly from the chemistry that makes up more than 99% of most stalagmites.  When rainwater percolates through the soil, it picks up carbon dioxide, as soils have higher carbon dioxide concentrations than the atmosphere, due to soil microbial activity and root respiration. This weakly acid water dissolves limestone present in the soil and bedrock until it becomes saturated. In the cave, the opposite reaction occurs: the cave air has relatively low carbon dioxide concentration compared to the drip water, and the water degasses carbon dioxide and forms speleothems (from the latin, meaning cave deposits) such as stalagmites. So stalagmites are primarily composed of calcium carbonate, and the majority of researchers analyse the oxygen and carbon isotopes in the stalagmite carbonate using a mass spectrometer.

At UNSW, we have a specialist mass spectrometer to make these measurements. It is an expensive facility, but the advantage of measuring oxygen isotopes in stalagmites in particular is that the variability in oxygen isotopes originally comes from the composition of the oxygen isotopes in the rain that fell above the cave. By drilling stalagmite samples very carefully, small amounts of carbonate can be analysed and we can obtain very high resolution records of past climate. Depending on how fast a stalagmite is growing - a good rule of thumb is between 0.01 and 0.1 mm per year - we can sample at about annual resolution. With stalagmites growing for thousands or tens of thousands of years, they are effectively time capsules, preserving high resolution records of past climate change.

Heat, aridity and evaporation

At Wellington, we are testing some of the fundamental assumptions behind the use of oxygen isotopes in stalagmites as records of past rainfall (oxygen has stable isotopes of mass 16, 17 and 18, and the most abundant two isotopes, O-16 and O-18 are analysed). At a global scale and at the most basic level, rainfall near the equator has relatively more O-18 than rainfall at the poles. The heavier isotope is preferentially removed from the atmosphere as rain during the process of moisture transport from the equator to the poles. In Australia, that assumption would suggest that rainfall from the tropical north would have more O-18 than rainfall from frontal systems that come from the west.

However, once the rain reaches the surface, other processes can affect its oxygen isotopic composition. In particular, evaporation also affects oxygen isotopes, with the heavier isotope less likely to evaporate than the lighter isotope due to its greater mass. At Wellington, we can assess the importance of evaporation in cave processes, as evaporation vastly exceeds rainfall at this site. And it turns out that in the Australian context, a stalagmite’s oxygen would be more likely to be a record of the frequency of rainfall events – the longer the time periods between rainfall the more time the water has to be evaporated in the soil, in the shallow sub-surface and in the cave. Water in Wellington Cave drip waters has much more of the heavy oxygen isotope O-18 than the rainfall, which is only possible if the lighter O-16 isotope has been evaporated between falling as rain and emerging in the cave as drip-water.

The future

Our research at Wellington Caves is ongoing and diverse. As well as the research just described, we are just concluding a three year project to test biological markers of temperature from the fatty-acid membranes from soil and cave microbes, and are just starting a project to investigate the effects of wildfire on cave processes. As part of the Groundwater Education Investment Fund, we have also drilled and put instruments in boreholes across the Caves Reserve to understand the relationship between river and groundwater levels at the site. This data can also be viewed from the GEIF website. As part of the drilling program, we inadvertently discovered a new cave and deliberately drilled a 40-metre long core of limestone from the surface to below the depth of Cathedral Cave. Down-bore footage of the cave and the 40 metre bore can be viewed from the link below (warning: it is a large filesize) and core material can be accessed by visiting groups to Cathedral Cave.

This article by Professor Baker first appeared in the newsletter of the Science Teachers Association of NSW.

Links

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