Underground detective reveals secrets of stalagmites

Inside Moondyne Cave, on the south-west coast of Western Australia, stalactites hang in clusters from the ceiling – delicate crystal straws resting alongside stout organ pipes. Tapered stalagmites rise like pointy hats from the cave floor.

In the early 20th century, Moondyne’s ghostly structures attracted enough tourists that, in 1911, cave managers installed a boardwalk to control the flow of people through the cave. In 1992, a local tour company ripped up the rotting old wooden boardwalk to replace it with new timber. In the decommissioning, workers found a stalagmite about the size of a standard paper clip sprouting from a piece of the old wood. They shelved the odd little souvenir, thinking maybe someday it would be of use.

They were right. In 2001, a doctoral student from Australia National University named Pauline Treble visited several caves in Western Australia looking for research samples. Treble thought of herself as an underground detective. A palaeoclimatologist, she was studying caves as a way of reconstructing Earth’s past.

A cave management employee pulled out the Moondyne splinter with its conjoined stalagmite. “Is this of any interest to you?” he asked.

Treble’s eyes gleamed as she realised how special this stalagmite was. It was a cipher that could reveal how 20th-century climate had encoded itself in the formation of this hunk of mineral.

Moondyne would alter the course not just of Treble’s project, but of her entire field.

going just a few centimetres into the structure transports you far back in time.

Caves store permanent records of temperature and rainfall patterns. Rainwater filters through the ground, drips into the cave and evaporates, depositing minerals. Gradually, stalactites and stalagmites – collectively known as speleothems – build up in layers, like the drip castles kids construct at beaches. The outside of each one is modern. But going just a few centimetres into the structure transports you far back in time. “These speleothems just sit here, quietly dripping away for thousands of years in the dark, silent witness to these climate and environmental changes going on around them,” says Treble.

Coaxing climatic history out of those silent witnesses depends on knowing exactly how far back in time a given layer goes. Scientists figure out the age of speleothem samples using radioactive dating, usually on the uranium that seeps into them as they grow. As uranium ages, it decays, at a regular and predictable rate, into thorium. By measuring how much uranium remains in a given speleothem relative to how much thorium has appeared, scientists can learn how old the sample is.

The trouble is that radioactive dating is inherently fuzzy. Researchers can only approximate the age of a cave deposit to within about 100 years. That’s why the boardwalk speleothem was so valuable. Treble knew when it had first started to grow – the boardwalk’s 1911 grand opening was a kind of birth certificate – and she had its RIP date too: 1992 when the boardwalk was torn up.

Treble could now check to see how well the known environmental conditions between 1911 and 1992 matched up with the speleothem’s chemical composition. She could determine whether – and if so, how – speleothems encrypted information about rainfall and climate.

In the years since she stumbled on her boardwalk barnacle, Treble – now a research scientist at the Australian Nuclear Science and Technology Organisation – has become a pre-eminent cave palaeoclimatologist. Leading some of the world’s longest-running cave monitoring programs, she’s deduced how historical climate changes have affected rainfall in Australia and the transformations future climate change is likely to bring. Her methods have also helped set a course for cave researchers examining shifting climates elsewhere in the world.

Treble was a curious kid, and not unlike a cave herself: she observed, absorbed and interpreted the environment around her. She still does in her work and her hobby. When she’s not playing with her data or kids, she paints. She’s just had her first exhibition – portraits of her children, aged seven and nine – and has moved on to still life. “To reproduce a vase is all about careful, careful observation,” she says. “That’s the same with science; it’s not about going in with a biased idea of how the world should work.”

Treble wasn’t always so keen on caves. When she was a child, her older brother was into spelunking. Seeing photographs of him on underground adventures, she thought: “That would be terrifying.” The idea of going beneath Earth’s surface held no appeal. She did, though, discover a taste for climbing rocks – scaling high cliffs wasn’t nearly as scary as venturing underground.

At the University of Sydney, Treble started out in chemical engineering. But when she discovered the geography department, she was hooked. She was fascinated by the idea that it was possible to reconstruct the planet’s past climate. When a professor showed her two Tasmanian speleothems and suggested she could use them to look for evidence of ice ages, her future was clinched.

Working with the Tasmanian speleothems led Treble underground for the first time. There, her childhood fear disappeared. Here was a fairyland of wall-to-wall crystal and stalagmites so big they resemble fossilised trees. Beneath the Earth’s surface, Treble felt safe.

And she liked getting to know a hidden part of the planet – her artist’s eye eager to see the world beyond its surface level.

While caves are geriatric, cave palaeoclimatology is just a Gen-Xer, born in 1968 when New Zealand palaeoclimatologists Chris Hendy and Alexander Thomas Wilson published the first cave decoder manual, outlining how scientists could trace the history of surface climate from underground. “At the time, there were very few terrestrial records you could reconstruct,” says Treble. Tree rings tell some of the story but trees are seldom old on a geologic time scale. Ice cores are old but primarily come from Earth’s far north and south. Caves, though – caves are everywhere, and their insides are both very changeable and very, very old.

Speleothems encode information about the water that begat them. We think of water as having a straightforward formula: two hydrogen atoms and one oxygen atom. But in rainwater, oxygen normally comes in two main forms: isotopes that carry either 16 or 18 neutrons, like twins with slightly different weights. Generally, when the world is cooler, rain contains more light oxygen-16 relative to heavy oxygen-18. When it’s warmer, oxygen-18 catches up.

As Hendy and Wilson explained, that relationship between temperature and water chemistry meant that scientists could reconstruct the history of a region’s temperature fluctuations by using speleothems. If modern temperatures were shifting in a way that was out of proportion with past fluctuations, that most likely indicated a human cause.

Hendy and Wilson’s approach opened up a new era of climate research. Now, scientists at sites all over the world could piece together clues from right beneath their feet to better understand how the world once was and what it might become.

Eventually, though, it became clear that Hendy and Wilson’s model was incomplete. It assumed that the only factor that changed water’s oxygen isotope composition was temperature. But other factors, researchers were discovering, also affected oxygen isotopes. As air masses move inland, for example, they lose comparatively more heavy oxygen, and the rain they drop reflects that. The same thing happens as weather systems move toward more extreme latitudes.

By the late 1990s, Treble says, researchers suspected that speleothems “are actually archives of past rainfall, rather than temperature”. When more rain falls, the amount of oxygen-18 compared to oxygen-16 also falls. In dry periods, heavy oxygen makes a comeback. But just how those shifts affected the makeup of speleothems was largely theoretical. Moondyne’s boardwalk speleothem gave Treble a chance to test the theory against real rainfall patterns.

Western Australia, the stalagmite’s home region, had seen a 15% decrease in rainfall between 1970 and the early 2000s. Now Treble could test whether that dry period was reflected, year for year, in her speleothem’s shifting oxygen isotopes It was an opportunity not to be missed.

Stop – drop everything, she told herself. Focus on this.

In the Moondyne speleothem’s layers, she found clear evidence of annual cycles: different trace metals deposited with predictable regularity as the calendar flipped forward. She could match the passing seasons to the chemicals’ shifting concentrations, like matching a stack of photos to the dates they were taken by looking at clues such as snow on the ground, freshly mown grass and autumn leaves. In speleothems, the clues came from trace elements such as magnesium, sodium and barium which change with the seasons. And in the season-subdivided speleothem, she could detect more of the heavy oxygen appearing when the dry period began in the 1970s. Sure enough, it steadily rose as more precipitation fell.

Treble recapitulates that moment. “Yes, here it is!” she says, throwing her fist through the air. Victory! Her real-world observations backed an idea that until then had existed mostly on paper.

She became the envy of her young colleagues who were struggling to find samples. At one conference after she presented her work, another student approached her.

“That’s my PhD,” he said, visibly dejected. “That’s what I’ve been trying to do.”

“Oh, well,” said Treble. Bad luck.

Because academic positions in her field were scarce in Australia, in 2003 Treble accepted a post-doc at the University of California, Los Angeles. But she always longed to return home. “In my heart, I knew I had this great story for the south-west of Western Australia,” she says. “There’s just no palaeo records for that region and it was just dying to be done.”

That story, which she has continued to investigate, was about how the westerly winds have evolved over the millennia and what effect that has had on Western Australia’s water supply. The westerlies are responsible for what little precipitation Western Australia gets. But since around 1970, these winds have shifted southward, taking the rain clouds with them.

Scientists believe a number of factors have contributed to the westerlies’ migration, says Treble. Some of the variability is likely natural, and ozone depletion around Antarctica has also probably played a role, she says. Scientists also theorise that the westerlies are sensitive to increases in greenhouse gases, whose concentrations in the atmosphere continue to mount. Just how sensitive, though, is an open question.

Treble’s work in Western Australia’s caves is premised on the idea that if she can figure out just how changeable rainfall was throughout the region’s history – as reflected in speleothems – then she can deduce how big an effect climatic changes have on the westerlies. That information could help shape forecasts for how life in Western Australia might alter in the coming decades: how little winter water crops will get, for example, and how much the water supply to nearby Perth could dwindle. It can also feed into the climate models other scientists use to forecast future climatic shifts.

Even while she was still in California, Treble believed such work could also make climate change real for people back home, by revealing how a global and long-term process is playing out in the here and now. “This is one of the current problems with people struggling to understand the impact of climate change on their lives,” says Treble. “They’re only seeing climate or rainfall through one lifetime. They don’t have an appreciation of how much things could change.” But caves hold the whole history of those changes within themselves – if only she could decode it. She began writing grants to return home.

treble always believed her work could make climate change real for people back home.

In 2005, with a grant from Land and Water Australia, Treble set up what would become one of the world’s longest cave-monitoring programs. The rocky formations are to past climates what the Rosetta Stone was to deciphering hieroglyphics: Treble and colleagues can look for the seasonal and climatic signatures in older speleothem samples and understand when it rained and how hard, and so how far the westerlies wandered back then.

Her most extensive monitoring operation is in a cave called Golgotha, about 20 kilometres north of Moondyne. Caves in this area lie beneath native vegetation, rather than agricultural fields or private properties. That means Treble can study how caves such as Golgotha respond chemically to climate change in the absence of other human-made environmental influences.

What’s more, water penetrates the porous rock in the region easily, building speleothems up quickly and packing them with information about the recent world above. Treble’s team is currently reconstructing the past 8,000 years of rainfall, to build that baseline of how the rainfall and the westerlies have evolved long-term, how the current dry period compares, and what future shifts in the wind might mean for Western Australia.

Inside Golgotha, Treble’s equipment is a constant presence, as much as the otherworldly stalactites and stalagmites that carry the cave’s story. The site is a five-hour flight from Treble’s home base near Sydney, and she only makes the trip there about once a year. Anne Wood and Elizabeth McGuire of the Department of Parks and Wildlife make sure the day-to-day data keep coming. When Treble does scramble into the cave, wearing full-body coveralls, her brown hair tied back and a lamp beaming from her forehead, there is a military focus about her. Her collaborator Ian Fairchild at the University of Birmingham remembers a 10-day field trip he once took with Treble. The experience was intense. “Pauline is a bit of a stickler,” Fairchild says. “We had to work every day, and there were no stops. We were allowed some wine, I think, at the end of the day on the last day.”

He means “stickler” in a nice way. “Pauline is certainly a careful scientist – very methodical, very high ethical values,” he says. Where others have cut bureaucratic corners, she sticks to the book – monitoring drip sites for years to figure out which will be most scientifically useful, sending regulators documentation along the way and waiting for permission before taking physical samples. That quality, Fairchild says, has helped keep her in the good graces of the land management agencies that hold the keys to the underground castles.

The massive Golgotha dataset also attracts scientists with different interests. Some geostatisticians from the University of Lausanne and Western Sydney University recently offered to use lasers to make a 3-D map of Golgotha’s water paths. Treble pounced on the opportunity.

The map led to an important discovery: where tree roots have cracked rock at the surface, rainwater funnels into the cave below. But those same roots then change the composition of the water, sucking up nutrients – including the same trace metals that palaeoclimatologists use as climate clues. If an oak tree happens to be growing directly above a speleothem, that scrambles the story. And unscrambling, as with an egg, isn’t really possible. That, Treble realised, means it’s crucial to find speleothems that are free of such blemishes.

Tree roots aren’t the only force of nature that can skew cave readings, as Treble’s group recently discovered. Taking readings at a different Western Australian cave called Yonderup, Treble couldn’t see the oxygen signature of the rainfall changes she knew had occurred between 2005 and 2011.

She shelved the Yonderup data, figuring the samples must have become contaminated – although by what, she didn’t know. But she did have a suspicion: a fire had burned above Yonderup in 2005. Somehow that fire must have affected the development of the cave formations. But how?

She didn’t get an answer until a few years later when a University of New South Wales geoscience student, Gurinder Nagra, approached her and collaborator, Andy Baker, in need of a project. They put the budding cave detective to work finding out how the fire had affected the water flows.

Nagra, with help from others in Treble and Baker’s group, discovered that when a fire burns above a cave, water evaporates at a higher rate. In response, the cave’s heavy oxygen rises in a way that is – so far –indistinguishable from the increase due to rainfall changes. Cave sites that have sat silent under conflagrations, then, provide a blemished picture of climate.

When scientists don’t know these imperfections exist, Treble says, they see the data as smooth and simple: temperature changes affect water chemistry – the end.

Treble has helped reveal just how many warts there are in the data. Her science imitates her art.

By revealing the caves’ imperfections, she has forced her field towards greater realism, just as detailing people’s blemishes and hairs, makes them more human. Embracing this increasingly gritty portrait of caves, Treble plans to keep probing speleothems to read the past, put the present in context and help predict the future.