Pressemitteilung der University of California, Santa Cruz, vom 10. Juli 2015:
(eine deutsche Übersetzung gibt es auf EIKE)
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Study finds surprisingly high geothermal heating beneath West Antarctic Ice Sheet
UC Santa Cruz team reports first direct measurement of heat flow from deep within the Earth to the bottom of the West Antarctic ice sheet
The amount of heat flowing toward the base of the West Antarctic ice sheet from geothermal sources deep within the Earth is surprisingly high, according to a new study led by UC Santa Cruz researchers. The results, published July 10 in Science Advances, provide important data for researchers trying to predict the fate of the ice sheet, which has experienced rapid melting over the past decade.
Lead author Andrew Fisher, professor of Earth and planetary sciences at UC Santa Cruz, emphasized that the geothermal heating reported in this study does not explain the alarming loss of ice from West Antarctica that has been documented by other researchers. „The ice sheet developed and evolved with the geothermal heat flux coming up from below–it’s part of the system. But this could help explain why the ice sheet is so unstable. When you add the effects of global warming, things can start to change quickly,“ he said.
High heat flow below the West Antarctic ice sheet may also help explain the presence of lakes beneath it and why parts of the ice sheet flow rapidly as ice streams. Water at the base of the ice streams is thought to provide the lubrication that speeds their motion, carrying large volumes of ice out onto the floating ice shelves at the edges of the ice sheet. Fisher noted that the geothermal measurement was from only one location, and heat flux is likely to vary from place to place beneath the ice sheet.
„This is the first geothermal heat flux measurement made below the West Antarctic ice sheet, so we don’t know how localized these warm geothermal conditions might be. This is a region where there is volcanic activity, so this measurement may be due to a local heat source in the crust,“ Fisher said.
WISSARD Project
The study was part of a large Antarctic drilling project funded by the National Science Foundation called WISSARD (Whillans Ice Stream Subglacial Access Research Drilling), for which UC Santa Cruz is one of three lead institutions. The research team used a special thermal probe, designed and built at UC Santa Cruz, to measure temperatures in sediments below Subglacial Lake Whillans, which lies beneath half a mile of ice. After boring through the ice sheet with a special hot-water drill, researchers lowered the probe through the borehole until it buried itself in the sediments below the subglacial lake. The probe measured temperatures at different depths in the sediments, revealing a rate of change in temperature with depth about five times higher than that typically found on continents. The results indicate a relatively rapid flow of heat towards the bottom of the ice sheet.
This geothermal heating contributes to melting of basal ice, which supplies water to a network of subglacial lakes and wetlands that scientists have discovered underlies a large region of the ice sheet. In a separate study published last year in Nature, the WISSARD microbiology team reported an abundant and diverse microbial ecosystem in the same lake. Warm geothermal conditions may help to make subglacial habitats more supportive of microbial life, and could also drive fluid flow that delivers heat, carbon, and nutrients to these communities.
According to coauthor Slawek Tulaczyk, professor of Earth and planetary sciences at UC Santa Cruz and one of the WISSARD project leaders, the geothermal heat flux is an important value for the computer models scientists are using to understand why and how quickly the West Antarctic ice sheet is shrinking.
„It is important that we get this number right if we are going to make accurate predictions of how the West Antarctic ice sheet will behave in the future, how much it is melting, how quickly ice streams flow, and what the impact might be on sea level rise,“ Tulaczyk said. „I waited for many years to see a directly measured value of geothermal flux from beneath this ice sheet.“
Melting ice shelves
Antarctica’s huge ice sheets are fed by snow falling in the interior of the continent. The ice gradually flows out toward the edges. The West Antarctic ice sheet is considered less stable than the larger East Antarctic ice sheet because much of it rests on land that is below sea level, and the ice shelves at its outer edges are floating on the sea. Recent studies by other research teams have found that the ice shelves are melting due to warm ocean currents now circulating under the ice, and the rate at which the ice shelves are shrinking is accelerating. These findings have heightened concerns about the overall stability of the West Antarctic ice sheet.
The geothermal heat flux measured in the new study was about 285 milliwatts per square meter, which is like the heat from one small LED Christmas-tree light per square meter, Fisher said. The researchers also measured the upward heat flux through the ice sheet (about 105 milliwatts per square meter) using an instrument developed by coauthor Scott Tyler at the University of Nevada, Reno. That instrument was left behind in the WISSARD borehole as it refroze, and the measurements, based on laser light scattering in a fiber-optic cable, were taken a year later. Combining the measurements both below and within the ice enabled calculation of the rate at which melt water is produced at the base of the ice sheet at the drill site, yielding a rate of about half an inch per year.
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Wetter-Center.de berichtete seinerzeit über die Studie. Auch Bild der Wissenschaft beschrieb das Phänomen in einem Beitrag vom 10. Juli 2015:
Wärmefluss unterm Eisschild
Die mächtigen Eismassen des Westantarktischen Eisschildes werden kontinuierlich von unten erwärmt: Wärmeströme aus dem heißen Inneren der Erde fließen direkt auf die Eisbasis zu – und das in überraschender Menge, wie Forscher nun festgestellt haben. Sie haben erstmals eine direkte Wärmefluss-Messung unter dem Eisschild durchgeführt und dabei den ungewöhnlich starken Heizungseffekt entdeckt. Dieser könnte erklären, warum der seit Jahren rapide schmelzende Eisschild so instabil ist.
Weiterlesen bei Bild der Wissenschaft.
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Am 8. Dezember 2015 legte die Washington University in St. Louis mit einer weiteren Pressemitteilung zum Thema nach:
Hot rock and ice: Volcanic chain underlies Antarctica
Seismic maps of the mantle will improve predictions of giant ice sheet’s fate
Planetary scientists would be thrilled if they could peel the Earth like an orange and look at what lies beneath the thin crust. We live on the planet’s cold surface, but the Earth is a solid body and the surface is continually deformed, split, wrinkled and ruptured by the roiling of warmer layers beneath it. The contrast between the surface and the depth is nowhere starker — or more important — than in Antarctica. What is causing the mysterious line of volcanoes that emerge from the ice sheet there, and what does it mean for the future of the ice? “Our understanding of what’s going on is really hampered because we can’t see the geology,” said Andrew Lloyd, a graduate student in earth and planetary sciences in Arts & Sciences at Washington University in St. Louis. “We have to turn to geophysical methods, such as seismology, to learn more,” he said.
Lloyd helped deploy research seismometers across the West Antarctic Rift System and Marie Byrd Land in the austral summer of 2009-10. He then returned in late 2011 and snowmobiled more than 1,000 miles, living in a Scott tent, to recover the precious data. The recordings the instruments made of the reverberations of distant earthquakes from January 2010 to January 2012 were used to create maps of seismic velocities beneath the rift valley. An analysis of the maps was published online in the Journal of Geophysical Research: Solid Earth on Nov. 12, 2015 (doi:10.1002/2015JB012455). This is the first time seismologists have been able to deploy instruments rugged enough to survive a winter in this part of the frozen continent, and so this is the first detailed look at the Earth beneath this region.
Not surprisingly, the maps show a giant blob of superheated rock about 60 miles beneath Mount Sidley, the last of a chain of volcanic mountains in Marie Byrd Land at one end of the transect. More surprisingly, they reveal hot rock beneath the Bentley Subglacial Trench, a deep basin at the other end of the transect. The Bentley Subglacial Trench is part of the West Antarctic Rift System and hot rock beneath the region indicates that this part of the rift system was active quite recently.
A volcanic mystery
Mount Sidley, the highest volcano in Antarctica, sits directly above a hot region in the mantle, Lloyd said. Mount Sidley is the southernmost mountain in a volcanic mountain range in Marie Byrd Land, a mountainous region dotted with volcanoes near the coast of West Antarctica. “A line of volcanoes hints there might be a hidden mantle plume, like a blowtorch, beneath the plate,” said Doug Wiens, PhD, professor of earth and planetary sciences and a co-author on the paper. “The volcanoes would pop up in a row as the plate moved over it.” “But it’s a bit unclear if this is happening here,” he said. We think we know which direction the plate is moving, but the volcanic chain is going in a different direction and two additional nearby volcanic chains are oriented in yet other directions. “If this was just a plate moving over a couple of mantle plumes, you’d expect them to line up, as they do in the Hawaiian Islands,” he said. Although the hot zone’s shape is ill-defined, it is clear there is higher heat flow into the base of the ice sheet in this area, Wiens said.
Deeper than the Grand Canyon
The most interesting finding, Lloyd said, is the discovery of a hot zone beneath the Bentley Subglacial Trench. The basin is part of the West Antarctic Rift System, a series of rifts, adjacent to the Transantarctic Mountains, along which the continent was stretched and thinned. The old rock of East Antarctica rises well above sea level, but west of the Transantarctic Mountains, extension has pulled the crust into a broad saddle, or rift valley, much of which lies a kilometer below sea level. “If you removed the ice, West Antarctica would rebound, and most of it would be near sea level. But the narrower and deeper basins might remain below it,” Lloyd said. “The Bentley Subglacial Trench, which is the lowest point on Earth not covered by an ocean, would still be a kilometer and a half below sea level if the ice were removed.”
Because the West Antarctic Rift is hidden, less is known about it than about other famous rift systems such as the East African Rift or, in the United States, the Rio Grande Rift. “We didn’t know what we’d find beneath the basin,” Wiens said. “For all we knew it would be old and cold. “We didn’t detect any earthquakes, so we don’t think the rift is currently active, but the heat suggests rifting stopped quite recently.” In this way, it resembles the Rio Grande Rift, which is also no longer active but has yet to cool completely. A period of diffuse extension created the rift valley in the late Cretaceous, roughly 100 million years ago, Lloyd said, and more focused extension then created deep basins like the Bentley Subglacial Basin and the Terror Rift in the Ross Sea. “This period of more focused extension likely occurred in the Neogene,” Lloyd said. “If it’s still hot there, it might also be hot under other basins in the rift system.”
Will the heat flow grease the skids?
The rift system is thought to have a major influence on ice streams in West Antarctica. “Rifting and ice flow occur on completely different time scales,” Lloyd said, “so rifting is not going to suddenly make the ice sheet unstable. “But to accurately model how quickly the ice is going to flow or the rock to rebound, we need to understand the ‘boundary conditions’ for ice models, such as heat flow from the mantle,” he said. “Seismic surveys like this one will help inform models of the ice sheet,” Wiens said. “Modelers need an estimate of the heat flow, and they need to know something about the geological conditions at the bottom of the ice sheet in order to estimate drag. Right now, both of these factors are very poorly constrained.”
While heat flow through the Earth’s crust has been measured at at least 34,000 different spots around the globe, in Antarctica it has been measured in less than a dozen places. In July 2015, scientists reported the heat flow at one of these spots was four times higher than the global average. Ever since then, scientists have been wondering why the reading was so high. “Recent extension in the Bentley Subglacial Trench might explain these readings,” Wiens said. The next big problem, he said, is to understand the structure under the Thwaites and Pine Island glaciers, which lie closer to the coastline than the Bentley Subglacial Trench. These two glaciers have been described as the ‘weak underbelly’ of the ice sheet because surges in the ice flow there could theoretically cause the rapid disintegration of the entire West Antarctic ice sheet. During the 2014-2015 Antarctic field season, Lloyd helped deploy another 10 seismic stations that together with seismometers deployed by the British will map the underside of this key area.
