Caves in the Leeuwin Naturaliste Ridge reveal climate history

The Cape to Cape Track traverses through the incredible Leeuwin Naturaliste National Park. The national park runs along a ridge with a bedrock of ancient granite gneiss, topped with Tamala Limestone, which is home to some of the most beautifully decorated caves on the planet. This limestone is some of the youngest rock in the world, having formed less than 2 million years from wind swept sand dunes cementing together. Being home to 100’s of the karst features that have formed through the porous rock, the ridge that runs between Cape Naturaliste and Cape Leeuwin resembles something similar to swiss cheese.

Caves within the ridge have recently become of great interest to those studying climate science, as the beautiful formations growing inside these magical caverns are well preserved, being protect from the elements influencing the world above ground. The decorations found within caves are collectively given the name of ‘speleothems’, and they all begin as a result of heavy rainfall that can penetrate through the forest floor above. The rainwater picks up carbon from the rotting organic material, turning it slightly acidic, which allows it dissolve the limestone as it moves through the pores like water through a sponge. After several months the supersaturated limestone solution will reach the ceiling of the cave and seep through just one drop at a time. As a drop of water enters the cave atmosphere, it degasses which allows the water to redeposit the dissolved limestone as calcium carbonate or Calcite crystal. The most simplest form of speleothem is cave straws, which form as a thin ring of Calcite is deposited around the drop of water over and over again. When water travels around the outside of a straw a thicker stalactite is born, and when water drips to the floor of the cave, a stalagmite begins to grow.

Calcite crystal in Calgardup Cave

Calcite crystal in Calgardup Cave

Some of the caves in the Leeuwin Naturaliste Ridge have been open to public for tourism since the early parts of the 20th century, and in 1911 Moondyne Cave down near Augusta received enough foot traffic to warrant the installation of a boardwalk. More than 80 years later when the boardwalk was replaced in 1992, it was found that a stalagmite had grown upon the timber. A cave guide thought that he would keep this blob in hope that it might be of use to someone one day. This stalagmite has led the way to groundbreaking research, led by Dr Pauline Treble. Before now, dating speleothems relied on uranium dating which would give an approximate age to within 100 years. The stalagmite from Moondyne Cave had both a birth and death certificate, allowing Treble to match the recorded environmental conditions with the chemical composition in the layers of the stalagmite that had formed by the cave drip water.

Dr Pauline Treble with some cross sections of a speleothems. Photo credit: ANSTO

Dr Pauline Treble with some cross sections of a speleothems. Photo credit: ANSTO

From around 1970 onwards, the southwest region of Western Australia has experience a 15% decline in annual rainfall and by testing layers in the stalagmite sample from Moondyne Cave for different oxygen isotopes Treble was able to conclude that when more rain falls, the amount of oxygen-18 compared to oxygen-16 also falls. Treble also uncovered clear evidence of annual climatic cycles by analysing trace elements in the speleothem layers. This information has put Treble at the forefront of some of the longest cave monitoring programs around the world and one of the most important places for her research is merely kilometres from the Cape to Cape Track! It also opened up many doors for other scientists to further study not only climate, but also bushfire history, alongside other aspects of cave science.

Treble’s research is currently focussed around creating a paleoclimatological record for the southwest and to make predictions as to how the trend of declining rainfall may impact our lives and the environment in the future. The rain we receive in the southwest region is driven by westerly winds and it is evident that these westerly winds have shifted southward since the 1970’s taking the rain with them. There are many factors that are thought to influence the southerly migration of the westerlies, of which some are natural and some are theorised to be influenced by us, increasing greenhouse gases in the atmosphere for example. Using large speleothems from nearby Golgotha Cave, Treble is now attempting to map out the rainfall history of the area to eventually deduce how big of an effect our climatic changes might have on the westerly winds.

Entrance to Golgotha Cave. Photo credit: The Australian National University

Entrance to Golgotha Cave. Photo credit: The Australian National University

Ultimately, the research conducted in the cave’s of the Leeuwin Naturaliste National Park is bound to have an astounding impact on our understanding of where we live. With Treble’s incredible research being fed into climate models to forecast and map to world future potential climate, many climate change sceptics might be able to appreciate how things could change and what we can do work with those changes.