Category Archives: Obertraun


Caves of Ice: The Next Frontier in Paleoclimatology?

It’s early June in the Austrian Alps. Tourists in shorts sweat their way up a trail from the cable car above Lake Hallstatt. But the summer heat doesn’t stop a group of scientists from pulling on brightly colored jumpsuits over their hiking clothes at the entrance to Mammuthohle, one of the many limestone caves that riddle the Dachstein Massif. Lukas Plan, a geophysicist at the University of Vienna, straps on his headlamp and pauses to warn the crowd of researchers about the cave they are about to enter. It won’t just be chilly inside, he cautions; it will be an Alpine meat locker.

The crowd, part of the fourth international ice cave workshop organized by a network of European geophysicists and glaciologists, is gathered to visit the cave’s year-round ice formations.

Plan turns toward the tunnel in the mountainside and opens the metal door. A rush of wind bursts out. The group prepares to enter, hoping to read the history of the region’s climate in the cave’s ice.

Paleoclimatologists often turn to ice cores drilled from glaciers or ice sheets in Greenland or Antarctica. And like glaciers and polar ice caps, “ice caves are just one of several peculiar phenomena that show a reaction to climate,” explains glaciologist Stephan Gruber of the University of Zurich in Switzerland and an editor of the journal The Cryosphere. “A lot of people who do analyses on ice cores are quite keen to also analyze samples from ice caves,” he says.

Somewhat sparse research over the past few decades has shown that ice in temperate caves holds similar and complementary secrets to ice else- where. Because it’s a new field, however, researchers are still trying to figure out how to use cave ice as a climate record: Its complex formation history makes it less straightforward to study than polar ice. But when unlocked, the secrets in ice collected from caves in temperate regions allow climatologists to study the effects of global climate cycles in places where people actually live, such as Obertraun, Austria, where last summer’s cave excursion took place.

Researchers line up for a gondola that will take them close to Mammuthohle Cave in the Dachstein Massif. -Lucas Laursen

The cave next door

Cavers suit up ahead of exploring Mammuthohle. – Lucas Laursen

Since the first international ice cave workshop in Capus, Romania, six years ago, a core group of 35 to 50 researchers has met biannually to catalog caves and compare notes. So far, they have traveled from New Mexico to Italy to Russia to study centuries- to millennia- old cave ice.

The cave ice can take the form of giant icicles, looking much like stalactites. Or it can build up from the cave floor, as stalagmites do. Or it can pool and build massive, dirty ice cubes. All of these forms offer clues to a cave’s past climate, locked in layers of dirt, minerals and ice.

Because caves with ice are found in otherwise temperate climates where year-round ice — much less ice enduring millennia — is not expected, they must have special properties, geologists reason. Somehow the ice remains outside the annual tumult of the water cycle, preserved in an eddy of geological time.

“First of all, we have to understand how the ice caves work,” says Valter Maggi, a glaciologist at the University of Milan-Bicocca in Italy. “We have some caves with ice, and other caves at the same altitude or position without ice,” he says, while tramping through Mammuthohle. Researchers don’t know why, and he adds, they don’t know much about what is normal and what isn’t in ice-bearing caves.

Thus far, researchers have found that cave ice forms in many ways. In Austria, some mountain caves, such as Mammuthohle, have multiple entrances at different heights on a mountainside, generating a flow of cool, sinking air inside the cave. If the air moves quickly enough, it can freeze dripping or standing water, trapping clues about the outside weather at the time the water entered the cave. In Croatia, however, at Nestasna jama — a 105-meter-deep pit cave in the Dinaric Mountains — the process is different. There, a sinkhole with a shaded, vertical shaft entrance accumulates snow and cold air during the winter. During the rest of the year, any warm air that finds its way into the cave can rise up and escape, allowing the cave to sustain freezing temperatures on the inside. Any water that trickles in will freeze, thanks to the perennial cold. Other ice caves hint at even more formation processes, which researchers are only beginning to understand, Maggi says.

The next paleoclimate frontier, next door?

Excursion leader Lukas Plan, a geophysicist at the University of Vienna, leads the team into the cave. -Lucas Laursen

Ice is a great record of past climates because water contains a signature ratio of oxygen isotopes that changes from year to year. A wet year, a dry year, a cold year or a warm year each has its own signature. Glaciologists like Maggi have exploited these variations in ice cores from Greenland and Alpine glaciers. They have matched the variations against other indicators of the global climate history going back nearly a million years.

Glaciologists can use cave ice records in a similar manner. And if cave researchers can work back- ward from the layers of ice they find in caves to the conditions at the time the ice formed, they will be one step closer to understanding global climate change on a smaller scale, to see how it affects individual sites. “Let’s say we see a temperature drop at 500 years ago [in Mammuthohle]. Can we see the same temperature drop around 500 years ago elsewhere in Europe?” asks Aurel Perșoiu, a geology doctoral student at the University of South Florida in Tampa.

Once researchers have a temperature record from ice cores, they can compare cave ice clues to other climate proxies. Pollen trapped in lake sediments or glacial ice, for instance, reveals how climate affected forest cover at a certain time. Researchers can also use radiocarbon dating to pin down the pollen’s age. And some cave ice researchers think it may be possible to extract complementary information about past winter seasons from cave ice, much like tree ring thickness reveals the relative length of growing seasons from year to year, Gruber says. Yet another point of comparison between cave ice and other proxies is the ratios of isotopes in speleothems such as small calcite stalagmites, says Stein-Erik Lauritzen, a geologist at the University of Bergen in Norway.

Perșoiu and company have a lot of grunt work to do in figuring out just how ice accumulates in caves before they can reduce each cave’s complex history to a data point on a map though. Ice from Nestasna jama tells a different story from ice in wind tunnel caves like Mammuthohle, because each cave can host wildly variable conditions. What if some old ice melts away for a few years before new layers form, for example? What if ice blocks a windy tunnel, preventing it from cooling an icy chamber?

What can we learn?

These questions are on researchers’ lips during the Mammuthohle excursion that is part scientific workshop and part scientific tourism. Once in the cave, the crowd files along a paved path inside a dry limestone tunnel, headlamps bobbing wildly as they look around. In one chamber, Plan shows an example of the kind of changes cave ice can undergo. A thin wall of ice stands like translucent sheetrock at a construction site. A stream of warm air is melting its way through the wall, creating a circular hole. Ice covering the top of the hole, visible in photos from previous visits, is missing. One caver leans up against the ice with her headlamp and stands stock still, illuminating it. Another caver leans against a rock and takes a long exposure with a camera in the silence, preserving what’s left of the wall in digital form.

Inclusions in ice help researchers date the layers of ice. -Lucas Laursen

The disappearance of cave ice from the stratigraphic record, even if more ice accumulates in subsequent years, makes some scientists skeptical that it will ever be a serious climate history proxy. “I’m not really optimistic about getting a continuous record since there have been times when there was no ice in these caves,” says Dietmar Wagenbach, an environmental scientist at the University of Heidelberg in Germany, who attended the excursion. But, he notes, the very absence of new ice says something about the climate at those times as well. “This is an important signal … but you can only give this answer if you have a dating tool” for the ice surrounding the gaps in the record, he says.

Barbara May, a graduate student at Heidelberg working with Wagenbach, has tried to do just that with an ice core from the Eisriesenwelt cave, another ice cave in Austria. May drilled into the ice in 2007; on her first attempt, she could not link the visual changes she observed in her core’s layers to other climate records. She tried finding the age of small organic inclusions in the core using carbon-14, but that did not work either. “We only have the records against depth, not against age,” she says. “We’re trying to date the core, but it’s resisting.”

Shifting, changing ice

Collecting data in icy caves is no easier than analyzing it, but it’s a necessary evil if researchers hope to learn how cave ice forms and is preserved. The last stop of the tour in Mammuthohle reinforces how sensitive cave ice is and the need for understanding it better. Plan, who until then had simply walked up and down the line of visitors like a schoolteacher on a field trip, asks a couple of helpers to set up a pair of ladders and ropes. The group then shuffles along the top of an ice bulge to the ladder, which everyone climbs to get a close-up look at the stratigraphy in the ice.

At the top, mud mutes the ice’s colors, but away from the edge and the ladder, alternate layers of blue and white from different crystallization pat- terns appear. Thin layers of calcite give the ice block a gritty feel. As cavers climb down the ladder, Plan says from his rocky perch below, a few meters away from the ice wall, “The place where I’m standing, I think 20 or 30 years ago you could walk on the ice.” Knowing that the state of the ice is so variable is just one piece of the puzzle — it would be tempting to say that warmer outside temperatures are responsible, but as Plan and the other cave visitors know, the caves’ own microclimates could moderate the effects of the outside world.

Despite the challenges, Nenad Buzjak, a geographer at the University of Zagreb in Croatia, can sometimes be found with a laptop deep inside one of Croatia’s caves, dodging dripping ice. He plans to examine nearly 50 caves known to contain ice and snow, map them, study their interior climate, and measure the quantity and dynamics of the ice inside them. He’ll need that information to understand the relationship between the amount of energy moving in and out of the cave in the form of heat and wind and the formation or melting of ice inside. It’s a crucial step for anyone intent on reverse-engineering what ice in today’s caves can say about the past climate inside the cave.

Another key step will be trying to characterize the relationship between the outside weather and the weather inside a cave. Bulat Mavlyudov, a geographer at the Institute of Geography within the Russian Academy of Sciences in Moscow, has been trying to tabulate weather ’s effect on caves in the Ural Mountains in Russia using eyewitness reports and scientific measurements dating to 1703. The tables include the angle of the cave entrance, its altitude and latitude, and how much ice it has contained over the years, which can be compared to weather records. But, he notes, “we have a problem of drawing global conclusions from a few examples.”

In addition, says geophysicist Marc Luetscher of the University of Innsbruck in Austria, “monitoring caves and getting their ice mass history is inefficient because there is so much variation between caves and the way they have been measured. … But maybe we can model the relation between local weather and ice instead, from the few we know.” He and others are trying to create equations that use a cave’s shape, air flow into and out of the cave, and temperatures at the entrance and inside the cave to predict how much ice should build up or melt under various circumstances. But verifying the accuracy of such models could be slow going. “Even if we build these models, the next step is to test them by waiting around 10 years,” given that there are no climate models with high enough resolution to predict the weather outside a cave entrance, says Friedrich Obleitner, a glaciologist also at Innsbruck who is also involved in Austrian ice cave surveys.

Cave results, at a glacial pace

Cave ice may still be considered a sideshow in the circus of climate science, but useful results are beginning to accumulate.

A team led by Perșoiu submitted results from a 2,000-year-old ice core from Scărișoara, Romania, to the Journal of Geophysical Research not long after the expedition to Mammuthohle. The researchers used organic objects buried in cave ice, such as parts of trees, to estab- lish an estimated age-depth relationship for the ice and then calculated the age of the deepest ice. Other teams are using similar dating approaches. A Danish team led by now-retired Greenland ice core specialist Henrik B. Clausen from the University of Copenhagen dated ice in a Slovakian cave to A.D. 1357 by radio- carbon dating a bat preserved in the ice. These cave ice dates are much younger than polar ice cores, but still useful for improving our understanding of recent climate patterns, Luetscher says.

In St. Livres cave in Switzerland, Luetscher and some Swiss colleagues have used radiocarbon dating of pieces of spruce trunks and branches in a snow and ice outcrop to assign ages to seven corresponding layers of ice, spread over 1,200 years. Then they identified layers that sug- gested strong snow accumulation and gaps during which little or no snow accumulated. Parts of the pattern matched one well-known climate system: the North Atlantic Oscillation (NAO). Several gaps in the St. Livres ice matched warm periods in the NAO. But another gap had no match in the NAO. In a 2009 paper in Quaternary Research, Luetscher and his colleagues explained the anomaly by citing historical records of locals who shoveled out some snow and ice from the cave during that gap.

By seeking out such a sensitive climate archive close to inhabited areas, these researchers may be on the bleeding edge of paleoclimatology. Cave ice isn’t a perfect climate record — the ice can melt or the locals can shovel it out, and the weather can change, preventing new records from forming or erasing old ones — yet it may someday follow in the intellectual footprints of other natural archives, each of which had its skeptics before achieving widespread adoption as a climate proxy.

Difficulty of collecting cave ice data

Collecting data in icy caves is no easy task. In Eisriesenwelt cave in Austria, Barbara May, a graduate student at the University of Heidelberg in Germany, found that she couldn’t just borrow glaciology equipment and tunnel into a cave. For starters, the temperature is wrong: Caves tend to hover around freezing. When May and her colleagues fired up their cor- ing drills in 2007, the heat from the drill melted ice just long enough to refreeze it, jamming the drill. The team tried lubricating the drill with alcohol, but they worried that the lubricant could interfere with the chemical analysis of the core. None of this is a problem on glaciers, which tend to preserve colder temperatures beneath their surfaces.

Valter Maggi, a glaciologist at the University of Milan-Bicocca in Italy, and his colleagues have encountered similar logistical challenges. During one expedition to the Abisso sul Margine dell’Alto Bregai in the Central Italian Alps in 2000, they hit rocks embedded in the ice 1.2 meters deep, preventing them from obtaining a full ice core. They tried again in 2003 and encountered the same problem at 5.1 meters. They published their initial results in 2004 in Theoretical and Applied Karstology.

“Ice coring by hand is slow,” Maggi explains, and generators to power electric drills are heavy and hard to transport. Researchers also can’t stay in the caves too long. “Maybe it is a paradise for scientists, but it is a hell for cavers because it is freezing cold and you are always looking at ice blocks above your head,” jokes Nenad Buzjak, a geographer at the University of Zagreb in Croatia, while describing his own field work at the workshop in Obertraun.

This feature, including photos, first appeared in EARTH Magazine: [pdf].


Climate Scientists Shine Light on Cave Ice

EISRIESENWELT, AUSTRIA—Tracing his glove along a chalky layer in a house-size block of ice that lines this cave in the Austrian Alps, Michael Behm can feel all that is left of an ancient warm spell. The ice, likely formed over the decades or centuries as calcium-enriched rainwater trickled deep into the cave and froze, must have once warmed enough on top to melt and release a few years’ worth of the mineral, the Vienna University of Technology geophysicist explains.

Behm is one of a small but growing number of researchers who are investigating whether persistent ice that drapes the inside of some caves can reveal what the climate outside was once like. At a recent workshop in Austria, which included visits to several nearby ice caves, about 50 such scientists discussed this formidable challenge and reported tantalizing progress. One team, for example, has been using ice within a Romanian cave to reconstruct a 1000-year climate history of the local region.

By dating layers of cave ice that form from ponds of trapped rainwater and then analyzing the isotopes and other substances that make up those layers, these scientists aim to chart past changes in temperature, rainfall, and other climate indicators such as carbon dioxide levels. It’s a strategy paleoclimatologists have long pursued with ice cores taken from glaciers and polar caps, or with layers of lake sediment, but cave ice has proven much more difficult to study and interpret. Every cave is unique in how water trickles in and freezes, and researchers have to go to great lengths to establish whether the ice layers they see represent annual, seasonal, or other time scales of deposition. “Cave ice is a complicated business,” says Greenland ice core specialist Sigfús Johnsen of the University of Copenhagen.

But it’s one worth pursuing, say Behm and others at the Austrian workshop, as polar ice cores don’t offer insight into the climate histories of temperate regions, where most people live. “There’s an urgent need to look for ice archives outside the polar region,” says environmental scientist Dietmar Wagenbach of the University of Heidelberg in Germany, who attended the workshop.

Mountain glaciers aren’t the easy answer either, because they are less representative of the climate near settled areas. Glaciers require massive snowfall and long-lasting cold to form, but ice can form in caves and trap climate information even in regions that don’t normally frost over. A cave with small temperature differences at its entrances can develop strong internal winds, as air moves to equalize the cave’s internal temperature. If the wind gets fast enough, as in a constriction, it can also get cold enough to freeze standing or dripping water. Long before electric refrigerators, ancient Persians used the same principle to make ice.

The study of tree rings, lake sediments, and other so-called climate proxies has provided some history of the climate in temperate regions, but paleoclimatologists are still eager for other sources of data to help them predict how future changes in climate might alter these locations. And ice is a paleoclimatologist’s best friend because it can retain many different clues to the past climate. In polar regions, recently fallen snow slowly becomes compressed into a layer of ice whose ratios of hydrogen and oxygen isotopes reflect the atmospheric temperature at the time of condensation. Such ice can also trap grains of pollen and dust and air bubbles that reveal concentrations of gases such as carbon dioxide that are important to reconstructing past climates.

Climate studies of ice cores from glaciers and polar caps took off in the 1960s, and some of those cores may record climate histories going back a million years. Similar studies of cave ice, however, have lagged far behind. Speleology, the scientific study of caves, dates back to the 19th century, but most such work has concentrated on the geology of the caverns, or the unusual flora and fauna found within them. Emil Racoviță of Cluj University in Romania, a pioneering cave biologist, may have been among the first to recognize the record-keeping potential of cave ice. In a 1927 report on his exploration of the Scărișoara cave in Romania, he notes that such ice must have formed under different climatic conditions and that it would be good to “decipher the passionate enigmas of the history of the ice.” Decades later, Racoviță’s assistant Mihai Serban followed up on this idea, reporting in an Italian journal that he could measure certain isotopes in cave ice from Scărișoara, although the data didn’t provide climate clues.

The idea of doing paleoclimatology on cave ice attracted serious international attention about 11 years ago, when a pair of Canadian cavers—one a cave operator in Alberta and the other a geography master’s student at the University of Calgary—published a study that identified different mechanisms of cave-ice formation. They reported in the journal Boreas that by looking in the right places in caves and taking into account how the ice formed, they could extract paleoclimatic information from isotopes in the cave ice.

The idea gained more momentum from the first workshop on ice caves, held in 2004. But even now, few scientific papers have been published on the topic, as a variety of technical difficulties have stymied researchers. Small rocks in the ice have halted efforts to extract long ice cores. And Wagenbach’s student Barbara May notes that cave ice is actually too warm for drills designed for extracting ice cores from glaciers; melted and refrozen ice jammed her drill until she added a lubricant, which she fears could interfere with many analyses, including those of air bubbles, isotopes, and particles in the ice.

Teasing climate clues from cave ice isn’t as simple as extracting a clean core, however. The most commonly drilled cave ice mimics the tidy horizontal layers seen in sedimentary rock. A single layer, however, could represent either a year’s worth of rain that trickled into the cave or just a few months when a river poured in a torrent of water. Each process would preserve different clues to climate, so researchers need to determine how water has historically entered a cave. The size of the ice crystals, whether the ice froze from the bottom up or from the top down, whether it partly melted or eroded after solidifying—these and other nuances can affect measurements and require careful interpretation, Behm says.

That complexity is one reason cave-ice studies have progressed so slowly. Another is that the topic is just a sideline for most researchers. Behm, for example, uses ground-penetrating radar (GPR) to study Earth’s crust. He only recently began trying the technology on ice caves. “Cave ice is such a perfect insulator that the signal-to-noise ratio is very high for GPR,” he says.

In studies of four Austrian caves, Behm has found layers in the ice that reflect his radar signal. He suspects that they may be accumulations of calcite from periods when ice melted and released its dissolved minerals, a possible sign of outside climate changes. In June 2007, May drilled a 7-meter ice core from one of the caves, but so far her isotope studies haven’t nailed down firm dates along its length.

Others have had better luck. At the 2006 ice caves workshop, a Danish-Slovakian team reported extracting a 14-meter core that they dated at about 1250 years based on carbondating of a bat found in nearby ice. Sharp changes in the accumulation of ice from year to year make the core hard to interpret, but it does show signatures of some climate oscillations found in other records.

At this year’s workshop, geology Ph.D. student Aurel Perșoiu of the University of South Florida in Tampa described even more promising work with ice from the same Scărișoara cave Racoviță studied. Using a 6.5-meter ice core drilled from the cave, Perșoiu and colleagues are making an ambitious attempt to reconstruct the past 2000 years of the region’s climate history. By carbon-dating organic remains such as leaves and insects in the ice, the team has related core depth to age in the top half of the core. And they’ve used that ice to reconstruct temperature variations within the region. The trends appear to match other regional climate proxies such as lake sediments, pollen, and stalagmites, Perșoiu says. A paper describing their methods is under review at the Journal of Geophysical Research. “We are being helped by paleoclimatologists,” Perșoiu says. “They are pushing us toward more mainstream journals.”

Some researchers at the Austrian workshop compared the debate over the usefulness of cave ice to a similar one that took place decades ago over stalagmites, towers of limestone that build up in active caves. At first, paleoclimatologists derided the idea that stalagmites could hold valuable data, but now studies of “speleothems” have become standard practice (Science, 27 July 2007, p. 448).

To spread the word about cave ice to a broader audience, glaciologist Stephan Gruber of the University of Zürich in Switzerland is now overseeing a special issue of The Cryosphere devoted to research presented at the workshop. Improving communication with scientists in related fields will be vital; one workshop attendee wore a baseball cap with “Glaciologist” printed on the front to remind cave experts to cut the jargon. (Cave scientists favor logos featuring bats, their unofficial mascot.) Yet with results from Scă rișoara’s ice in the pipeline and a book project to translate the backlog of European cave-ice articles into English in the works, Perșoiu is optimistic about the field’s future. Cave-ice research, he predicts, “is about to reach a climax.”

See this feature as it appeared in Science Magazine: [html] or [pdf].