Tales From the Earth: the Palaeoclimates of Boranup

There’s no denying that climate change will have a massive impact on Margaret River. Over the past 50 000 years, locals have witnessed dramatic changes to landscape and ecology, and there are likely many more to come.

Within the caves of the Leeuwin-Naturaliste Ridge, Margaret River holds a record of climate change spanning over a million years. There is a fossil record that includes the arrival of the first Australians, the onset of the last ice-age, and the current warm period known as the Holocene. Scientists have studied the extinctions of megafauna; reconstructed past climates (‘palaeoclimates’), examined how people adapted to change, and are now working on reading past rainfall patterns laid down in the drip-stone stalagmites. Much of this research has been carried out in Boranup.

Boranup is a small area of forest 20 km south of Margaret River. It has become famous for the beauty and size of its karri trees: one of the tallest tree species in the world. There is something truly awesome about being surrounded by giants with pale gleaming trunks, that light up like paper lanterns when the sun shines out. The forest grows on ancient limestone dunes, and the trees stretch their roots down into cave systems famous for their beautiful crystal decorations.
But Boranup hasn’t always been a karri forest. During the last 50 000 years, there have been massive glaciations, fluctuating sea levels, changes in rainfall and vegetation, and multiple extinctions.

The limestone of the Ridge is itself the result of changes in the ancient climate. During an ice-age that began just over a million years ago, extensive sand plains were exposed on the continental shelf. The prevailing south-westerly wind blew it inland, creating mobile dune systems. Over time, these were consolidated by the action of rainfall and plant growth, into the soft limestone that characterises much of coastal Western Australia. Caves are then formed when rainwater reacts with surface plant material, to make a dilute carbonic acid which dissolves the limestone.

Ancestors of the Wadandi people used some of these caves as shelters. Caves are not only warm and cosy. They act as ideal preservation environments, holding a record of human lives in the form of hearths, bone material from meals, stone tools, and occasionally even personal ornaments such as bone beads. They also preserve a record of vegetation and fauna. From this scientists have reconstructed the changing environment, along with glimpses of how people responded and adapted over time.

Mammoth Cave was the first to be excavated. In 1904 cave custodian Tim Connolly found a giant bone when he was building a boardwalk. The bone was quickly identified as belonging to an extinct giant kangaroo (simosthenurus) and led to the formal excavation of the cave by museum geologist Ludwig Glauert. He extracted 38 cubic metres of bone material from the cave, and named three new species of megafauna. Unfortunately, scientific method was then unsophisticated and he removed the entire deposit, destroying valuable information in the process. Then, in the 1950s, Glauert sent a group of Perth zoologists, including my father, to Margaret River in search of more megafauna fossils.

One of the sites they surveyed was a small rockshelter in Boranup. They named it Devil’s Lair for the amount of bone material and disturbance left behind by tasmanian devils. No megafauna was found in Devils Lair, but signs of human occupation were reported to archaeologist Charlie Dortch.

Charlie began excavating the rockshelter in the early 1970s. Radio-carbon dates came back at around 30 000 years, the first of such extreme age on the Australian continent. After years of excavation and painstaking analysis, the rockshelter eventually yielded a date of over 45 000. The site had been used sporadically until around 6000 BP (before present), when the entrance was blocked, and a layer of calcite capstone sealed the deposit. People had used the cave before, during, and after the last glacial maximum, and the site has yielded a huge amount of information about past climates and human adaptation.

During most of the period Devils Lair was in use, the ocean was much further away than it is as present. It reached a maximum distance of 40km west during the peak of the last glaciation 17 000 years ago. The exposed continental shelf was similar to the Swan Coastal Plain: open woodland, swamps, coastal heath, and estuaries. The climate was drier, and the forest type more arid and open. As the climate warmed, the sea rose, drowning the coastal plains and reaching the current shoreline around 6000 BP.

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Changing sea levels along the Leeuwin-Naturaliste coast. Image from ‘Devil’s Lair, a study in Prehistory’ by Charles Dortch and published by the WA Museum 1984.

People would have lived, hunted and foraged on the vanished coastal plains. There is concrete evidence for this in the form of chert tools found at many sites along the ridge. Chert is no longer available in the region and it is surmised that the stone was quarried from locations now drowned by the sea. This is also reflected in Wadandi legends about rising seas, and the Nyitting, or the ‘great cold’.

But sea level wasn’t the only change: with warmer temperatures and a maritime climate, local rainfall increased. This had a major impact on the vegetation around Margaret River, but as the Devils Lair deposit was sealed during the recent warming period, another source of information needed to be found.

In the 1990s, Charlies’ son Joe excavated Tunnel Cave, on the northern outskirts of Boranup. He analysed charcoal in the floor deposit, finding that until around 10 000 BP the forest at the entrance was dominated by jarrah and banksia. Around 10 to 12 000 BP rainfall increased, and karri started to appear in the record. It selectively replaced the jarrah/banksia woodland, resulting in the mosaic of vegetation that we see around Boranup today.

Jarrah-banksia woodland near Tunnel Cave

The Karri trees have transformed the landscape, creating a ‘closed’ forest habitat, which supports a different range of species than the open jarrah forest that dominated the Ridge for thousands of years. Dortchs fauna analysis of the open-forest period indicates a higher presence of dryland species; such as lizards, the black-footed rock wallaby (or petrogale), the bettong (bettongia leseur), and the arid region bandicoot (perameles). These species disappear from the cave deposits around 7000 BP.

They followed into extinction many other animals including wombats, koalas, giant pythons, and the megafauna, which vanished between 50 000 and 30 000 years ago. Why the giant marsupials became extinct is still being debated, but the main factors are the long-term drying of the Australian continent, and an unknown degree of human impact.

Not only have the karri created a different type of habitat, they have changed the cave systems that lie beneath the forest. The caves are not just geological entities, they are an ongoing conversation between rainfall, vegetation, and limestone. Karris have a habit of dropping vast amounts of bark, creating a heavy layer of leaf litter, and a soil rich in humus. More leaf litter means more carbon material for the rainwater to turn into acid, thus accelerating the processes of cave formation. They also have a physical impact as their large root systems creep down into the water table and use large quantities of water. The physical intrusion of roots opens the way for more water to enter, destabilising the rock and even making new entrances.

But now, the rainfall is again diminishing. The underwater streams that form the cave systems are drying out, and watertables have dropped dramatically. It is still uncertain whether this is part of a long term change, or a shorter term rain cycle. In some parts of Boranup, the karri trees are beginning to show signs of water stress, with limbs dying off at the tips.

The landscape is constantly changing. Our climate in the south-west has been deceptively stable for a few thousand years, but changes have already begun. The ancestors of the Wadandi learnt to adapt to major long term shifts caused by global weather patterns. Now, we are all faced with sudden upheavals that we ourselves contribute to as a consequence of our daily lives. By learning from the tales Earth has to tell, we may be in a better position to share that wisdom with the future.