Study shows air temperature influenced African glacial movements

Changes in air temperature, not precipitation, drove the expansion and contraction of glaciers in Africa’s Rwenzori Mountains at the height of the last ice age, according to a Dartmouth-led study funded by the National Geographic Society and the National Science Foundation.

The results – along with a recent Dartmouth-led study that found air temperature also likely influenced the fluctuating size of South America’s Quelccaya Ice Cap over the past millennium — support many scientists’ suspicions that today’s tropical glaciers are rapidly shrinking primarily because of a warming climate rather than declining snowfall or other factors. The two studies will help scientists to understand the natural variability of past climate and to predict tropical glaciers’ response to future global warming.

The most recent study, which marks the first time that scientists have used the beryllium-10 surface exposure dating method to chronicle the advance and retreat of Africa’s glaciers, appears in the journal Geology. A PDF is available on request.

Africa’s glaciers, which occur atop the world’s highest tropical mountains, are among the most sensitive components of the world’s frozen regions, but the climatic controls that influence their fluctuations are not fully understood. Dartmouth glacial geomorphologist Meredith Kelly and her team used the beryllium-10 method to determine the ages of quartz-rich boulders atop moraines in the Rwenzori Mountains on the border of Uganda and the Democratic Republic of Congo. These mountains have the most extensive glacial and moraine systems in Africa. Moraines are ridges of sediments that mark the past positions of glaciers.

The results indicate that glaciers in equatorial East Africa advanced between 24,000 and 20,000 years ago at the coldest time of the world’s last ice age. A comparison of the moraine ages with nearby climate records indicates that Rwenzori glaciers expanded contemporaneously with regionally dry, cold conditions and retreated when air temperature increased. The results suggest that, on millennial time scales, past fluctuations of Rwenzori glaciers were strongly influenced by air temperature.

Current ice melt rate in Pine Island Glacier may go on for decades

A study of the Pine Island Glacier could provide insight into the patterns and duration of glacial melt.

The Pine Island Glacier, a major outlet of the West Antarctic Ice Sheet, has been undergoing rapid melting and retreating for the past two decades. But new research by an international team including researchers from Lawrence Livermore National Laboratory shows that this same glacier also experienced rapid thinning about 8,000 years ago.

Using LLNL’s Center for Accelerator Mass Spectrometry to measure beryllium-10 produced by cosmic rays in glacially transported rocks, Lawrence Livermore researchers Bob Finkel and Dylan Rood reported that the melting 8,000 years ago was sustained for decades to centuries at an average rate of more than 100 centimeters per year. This is comparable to modern-day melting rates.

The findings indicate that modern-day melting and thinning could last for several more decades or even centuries. The research appears in the Feb. 20 issue of Science Express.

“Pine Island Glacier has experienced rapid thinning at least once in the past. Once set in motion, rapid ice sheet changes in this region can persist for centuries,” said Finkel, one of the authors of the new findings.

Ice mass loss from the Pine Island-Thwaites sector dramatically contributes to the sea level of the West Antarctic Ice Sheet. The Pine Island Glacier is currently experiencing significant acceleration, thinning and retreat. The rate of thinning from 2002-2007 on the grounding line (the part where the glaciers export the ice down the continent and lose contact to the ground and become a floating ice shelf) was between 1.2 meters per year and 6 meters per year.

The change is likely tied to the increased influx of warm water to the cavity under the ice shelf at the glacial front.

Dramatic changes over longer timescales — from centuries to millennia — are somewhat limited, so there is considerable uncertainty associated with model projections of the future evolution of timing and ice loss of the Pine Island Glacier. Current geological research is tied to the grounding line retreat across the continental shelf. However, little is known about the terrestrial thinning history and how the ice stream evolved from 8,000 years ago to the onset of present-day thinning.

The team found that there was a direct correlation from glacial-geological samples consisting of cobblestones and granite boulders from the Hudson Mountains to rapid thinning in the Pine Island Glacier system about 8,000 years ago.

“The melting of the Pine Island Glacier at a rate comparable to that over the past two decades is rare but not unprecedented,” Rood said. “Ongoing ocean-driven melting of the glacial ice shelf in current times may result in continued rapid thinning and ground line retreat for several more decades or even centuries.”

New study determines more accurate method to date tropical glacier moraines

The Quelccaya Ice Cap, the world's largest tropical ice sheet, is rapidly melting. -  Meredith Kelly
The Quelccaya Ice Cap, the world’s largest tropical ice sheet, is rapidly melting. – Meredith Kelly

A Dartmouth-led team has found a more accurate method to determine the ages of boulders deposited by tropical glaciers, findings that will likely influence previous research of how climate change has impacted ice masses around the equator.

The study appears in the journal Quaternary Geochronology. A PDF of the study is available on request.

Scientists use a variety of dating methods to determine the ages of glacial moraines around the world, from the poles where glaciers are at sea level to the tropics where glaciers are high in the mountains. Moraines are sedimentary deposits that mark the past extents of glaciers. Since glaciers respond sensitively to climate, especially at high latitudes and high altitudes, the timing of glacial fluctuations marked by moraines can help scientists to better understand past climatic variations and how glaciers may respond to future changes.

In the tropics, glacial scientists commonly use beryllium-10 surface exposure dating. Beryllium-10 is an isotope of beryllium produced when cosmic rays strike bedrock that is exposed to air. Predictable rates of decay tell scientists how long ago the isotope was generated and suggest that the rock was covered in ice before then. Elevation, latitude and other factors affect the rate at which beryllium-10 is produced, but researchers typically use rates taken from calibration sites scattered around the globe rather than rates locally calibrated at the sites being studied.

The Dartmouth-led team looked at beryllium-10 concentrations in moraine boulders deposited by the Quelccaya Ice Cap, the largest ice mass in the tropics. Quelccaya, which sits 18,000 feet above sea level in the Peruvian Andes, has retreated significantly in recent decades. The researchers determined a new locally calibrated production rate that is at least 11 percent to 15 percent lower than the traditional global production rate.

“The use of our locally calibrated beryllium-10 production rate will change the surface exposure ages reported in previously published studies at low latitude, high altitude sites and may alter prior paleoclimate interpretations,” said Assistant Professor Meredith Kelly, the study’s lead author and a glacial geomorphologist at Dartmouth.

The new production rate yields beryllium-10 ages that are older than previously reported, which means the boulders were exposed for longer than previously estimated. Prior studies suggested glaciers in the Peruvian Andes advanced during early Holocene time 8,000 -10,000 years ago, a period thought to have been warm but perhaps wet in the Andes. But the new production rate pushes back the beryllium-10 ages to 11,000 -12,000 years ago when the tropics were cooler and drier. Also during this time, glaciers expanded in the northern hemisphere, which indicates a relationship between the climate mechanisms that caused cooling in the northern hemisphere and southern tropics.

The findings suggest the new production rate should be used to deliver more precise ages of moraines in low-latitude, high-altitude locations, particularly in the tropical Andes. Such precision can help scientists to more accurately reconstruct past glacial and climatic variations, Kelly said.

Study finds earlier peak for Spain’s glaciers

Jane Willenbring (upper right) takes samples to date a boulder in Spain's Bejar mountain range. Her findings helped show that ancient glaciers in the region reached their maximum size several thousands of years earlier than once believed. -  University of Pennsylvania
Jane Willenbring (upper right) takes samples to date a boulder in Spain’s Bejar mountain range. Her findings helped show that ancient glaciers in the region reached their maximum size several thousands of years earlier than once believed. – University of Pennsylvania

The last glacial maximum was a time when Earth’s far northern and far southern latitudes were largely covered in ice sheets and sea levels were low. Over much of the planet, glaciers were at their greatest extent roughly 20,000 years ago. But according to a study headed by University of Pennsylvania geologist Jane Willenbring, that wasn’t true in at least one part of southern Europe. Due to local effects of temperature and precipitation, the local glacial maximum occurred considerably earlier, around 26,000 years ago.

The finding sheds new light on how regional climate has varied over time, providing information that could lead to more-accurate global climate models, which predict what changes Earth will experience in the future.

Willenbring, an assistant professor in Penn’s Department of Earth and Environmental Science in the School of Arts and Sciences, teamed with researchers from Spain, the United Kingdom, China and the United States to pursue this study of the ancient glaciers of southern Europe.

“We wanted to unravel why and when glaciers grow and shrink,” Willenbring said.

In the study site in central Spain, it is relatively straightforward to discern the size of ancient glaciers, because the ice carried and dropped boulders at the margin. Thus a ring of boulders marks the edge of the old glacier.

It is not as easy to determine what caused the glacier to grow, however. Glaciers need both moisture and cold temperatures to expand. Studying the boulders that rim the ancient glaciers alone cannot distinguish these contributions. Caves, however, provide a way to differentiate the two factors. Stalagmites and stalactites – the stony projections that grow from the cave floor and ceiling, respectively – carry a record of precipitation because they grow as a result of dripping water.

“If you add the cave data to the data from the glaciers, it gives you a neat way of figuring out whether it was cold temperatures or higher precipitation that drove the glacier growth at the time,” Willenbring said.

The researchers conducted the study in three of Spain’s mountain ranges: the Bejár, Gredos and Guadarrama. The nearby Eagle Cave allowed them to obtain indirect precipitation data.

To ascertain the age of the boulders strewn by the glaciers and thus come up with a date when glaciers were at their greatest extent, Willenbring and colleagues used a technique known as cosmogenic nuclide exposure dating, which measures the chemical residue of supernova explosions. They also used standard radiometric techniques to date stalagmites from Eagle Cave, which gave them information about fluxes in precipitation during the time the glaciers covered the land.

“Previously, people believe the last glacial maximum was somewhere in the range of 19-23,000 years ago,” Willenbring said. “Our chronology indicates that’s more in the range of 25-29,000 years ago in Spain.”

The geologists found that, although temperatures were cool in the range of 19,000-23,000 years ago, conditions were also relatively dry, so the glaciers did not regain the size they had obtained several thousand years earlier, when rain and snowfall totals were higher. They reported their findings in the journal Scientific Reports.

Given the revised timeline in this region, Willenbring and colleagues determined that the increased precipitation resulted from changes in the intensity of the sun’s radiation on the Earth, which is based on the planet’s tilt in orbit. Such changes can impact patterns of wind, temperature and storms.

“That probably means there was a southward shift of the North Atlantic Polar Front, which caused storm tracks to move south, too,” Willenbring said. “Also, at this time there was a nice warm source of precipitation, unlike before and after when the ocean was colder.”

Willenbring noted that the new date for the glacier maximum in the Mediterranean region, which is several thousands of years earlier than the date the maximum was reached in central Europe, will help provide more context for creating accurate global climate models.

“It’s important for global climate models to be able to test under what conditions precipitation changes and when sources for that precipitation change,” she said. “That’s particularly true in some of these arid regions, like the American Southwest and the Mediterranean.”

When glaciers were peaking in the Mediterranean around 26,000 years ago, the American Southwest was experiencing similar conditions. Areas that are now desert were moist. Large lakes abounded, including Lake Bonneville, which covered much of modern-day Utah. The state’s Great Salt Lake is what remains.

“Lakes in this area were really high for 5,000-10,000 years, and the cause for that has always been a mystery,” Willenbring said. “By looking at what was happening in the Mediterranean, we might eventually be able to say something about the conditions that led to these lakes in the Southwest, too.”

Sea level rise: New iceberg theory points to areas at risk of rapid disintegration

In events that could exacerbate sea level rise over the coming decades, stretches of ice on the coasts of Antarctica and Greenland are at risk of rapidly cracking apart and falling into the ocean, according to new iceberg calving simulations from the University of Michigan.

“If this starts to happen and we’re right, we might be closer to the higher end of sea level rise estimates for the next 100 years,” said Jeremy Bassis, assistant professor of atmospheric, oceanic and space sciences at the U-M College of Engineering, and first author of a paper on the new model published in the current issue of Nature Geoscience.

Iceberg calving, or the formation of icebergs, occurs when ice chunks break off larger shelves or glaciers and float away, eventually melting in warmer waters. Although iceberg calving accounts for roughly half of the mass lost from ice sheets, it isn’t reflected in any models of how climate change affects the ice sheets and could lead to additional sea level rise, Bassis said.

“Fifty percent of the total mass loss from the ice sheets, we just don’t understand. We essentially haven’t been able to predict that, so events such as rapid disintegration aren’t included in those estimates,” Bassis said. “Our new model helps us understand the different parameters, and that gives us hope that we can better predict how things will change in the future.”

The researchers have found the physics at the heart of iceberg calving, and their model is the first that can simulate the different processes that occur on both ends of the Earth. It can show why in northern latitudes-where glaciers rest on solid ground-icebergs tend to form in relatively small, vertical slivers that rotate onto their sides as they dislodge. It can also illustrate why in the southernmost places-where vast ice shelves float in the Antarctic Ocean-icebergs form in larger, more horizontal plank shapes.

The model treats ice sheets-both floating shelves and grounded glaciers-like loosely cemented collections of boulders. Such a description reflects how scientists in the field have described what iceberg calving actually looks like. The model allows those loose bonds to break when the boulders are pulled apart or rub against one another.

The simulations showed that calving is a two-step process driven primarily by the thickness of the ice.

“Essentially, everything is driven by gravity,” Bassis said. “We identified a critical threshold of one kilometer where it seems like everything should break up. You can think of it in terms of a kid building a tower. The taller the tower is, the more unstable it gets.”

Icebergs do have a tendency to form before that threshold though, Bassis suspects, due to cracks that are already there-either formed when capsizing bergs crash into the water and send shockwaves through the surrounding ice, or when melted water on the surface cuts through. The former is believed to have led to the Helheim Glacier collapse in 2003. The glacier had begun to retreat slowly in 2002, but suddenly gave way the following year when the thinner ice had broken away, exposing a thicker ice coast.

The latter-melted water pools-are occurring more frequently due to climate change, and they’re believed to have played a role in the rapid disintegration of the Antarctica’s Larsen B ice shelf, which crumbled over about six weeks in 2002.

When the researchers added random cracks to their model, it could mirror both Helheim and Larsen B.

A third feature is also required for the most dramatic ice collapses to occur. Icebergs can’t float away and make room for more icebergs to break off the main sheet unless the system has access to open water. So areas that border deep, unobstructed ocean rather than fjords or other waterways are at greater risk of rapid ice loss. The researchers point to the Thwaites and Pine Island glaciers in Antarctica and the Jakobshavn Glacier in Greenland, which is already retreating rapidly, as places vulnerable to “catastrophic disintegration” because they have all three components.

“The ice in those places gets thicker as you go back. If our threshold is right, then if these places start to retreat as you expose the thicker calving font, they’re susceptible to catastrophic breakup,” Bassis said.

Retreat of the current ice coasts in these places areas could occur via melting or iceberg calving.

Old maps and dead clams help solve coastal boulder mystery

This is the boulder ridge around the coastline of the Aran Islands. New research finds that storm waves have formed these ridges, despite the contention of some researchers that only a tsunami would have enough power to do this. -  Ronadh Cox
This is the boulder ridge around the coastline of the Aran Islands. New research finds that storm waves have formed these ridges, despite the contention of some researchers that only a tsunami would have enough power to do this. – Ronadh Cox

Perched atop the sheer coastal cliffs of Ireland’s Aran Islands, ridges of giant boulders have puzzled geologists for years. What forces could have torn these rocks from the cliff edges high above sea level and deposited them far inland?

While some researchers contend that only a tsunami could push these stones, new research in The Journal of Geology finds that plain old ocean waves, with the help of some strong storms, did the job.

And they’re still doing it.

The three tiny Aran Islands are just off the western coast of Ireland. The elongated rock ridges form a collar along extended stretches of the islands’ Atlantic coasts. The sizes of the boulders in the formations range “from merely impressive to mind-bogglingly stupendous,” writes Dr. Rónadh Cox, who led the research with her Williams College students. One block the team studied weighs an estimated 78 tons, yet was still cut free from its position 36 feet above sea level and shoved further inland.

Armed with equations that model the forces generated by waves, some researchers have concluded that no ordinary ocean waves could muster the force necessary to move the largest of the boulders this high above the ocean surface and so far inland. The math suggests the rocks in the ridges could only have been put there by a tsunami.

The equations tell one story. The islands’ residents tell another. According to some locals, enormous rocks have moved in their lifetimes, despite the fact that there hasn’t been a tsunami to hit the islands since 1755.

“Unless you have little green men from mars doing this on the quiet, it must be storm waves,” Cox said.

While the anecdotes from residents are interesting, Cox and her team went in search of more concrete evidence. The clincher came when the team compared modern high-altitude photos of the coastline to set of meticulous maps surveyed in 1839. The 19th century surveyors, who Cox describes as “possibly the most anal men on the planet,” carefully mapped not only the boulder ridges, but all of the criss-crossing stone walls that farmers built between fields. The researchers digitized the maps and overlaid them on the modern images, using the walls to line the two up accurately.

“Not only did they map every wall, they did it right. The maps aren’t even off by even a meter.”

The overlay of the new photos with the old maps shows definitively that sections of the ridges have moved substantially since

1839-nearly 100 years after the most recent tsunami. Some sections moved inland at an average rate of nearly 10 feet per decade. In some places, the ridge had run over and demolished field walls noted on the old maps.

Other lines of evidence corroborate residents’ accounts of recent movement. When the boulders were ripped from the bedrock, tiny clams that live in cracks and crevices sometimes came along for the ride. Using radiocarbon dating, Cox and her team found that some of the rocks have been pulled from the coastline within the last 60 years. What’s more, the researchers have been photographing sections of the ridge during each field season since 2006, and they’ve documented movement from year to year.

So what of the equations that point to tsunami as the only possible earth mover?

“We’ve eliminated tsunami and I think we can rule out little green men,” Cox said. “What that says is our equations aren’t good enough.”

Cox thinks the characteristics of the Aran Island shoreline are throwing off the calculations. The Aran cliffs rise nearly vertically out of the Atlantic, leaving very deep water close to the shore. As waves slam into the sheer cliff, that water is abruptly deflected back out toward the oncoming waves. This backflow may amplify subsequent waves. The result is an occasional storm wave that is much larger than one would expect.

“In this kind of environment these would be less rare,” Cox said. “You only need a couple of them to move these rocks around. The radiocarbon data show that not only are some boulders moving in recent years, but also that some of them have been in the ridges for hundreds and even a couple of thousand years. Accumulated activity of rare large-wave events over that time could certainly build these structures”

Cox plans to add a physicist to her research team in the near future to try to shed some light on the wave dynamics on the islands, but it’s clear from the evidence the team has already gathered that storm waves can do more than some researchers thought.

Following the devastating Indonesian tsunami in 2004, there has been renewed interest in learning about how a tsunami can change the landscape. Cox’s findings have important implications for that research.

“There’s a tendency to attribute the movement of large objects to tsunami,” she said. “We’re saying hold the phone. Big boulders are getting moved by storm waves.”