The Drama of Iceland's Subglacial Lakes
Subglacial lakes, large bodies of unfrozen water beneath ice masses, are relatively unexplored components of glacial geology that have important consequences for ice-sheet dynamics, proglacial environments, and climate change. For instance, the drainage of subglacial lakes in Iceland has produced numerous outbursts, locally called jökulhlaups, which constitute some of the largest floods witnessed by humankind. A recent presentation by an Earthwatch Institute-supported scientist reveals the unique geology and far-reaching implications of Iceland's subglacial lakes.
Dr. Andrew J. Russell, Keele University, presented his findings at the American Association for the Advancement of Science 2001 annual meeting in San Francisco on Saturday, February 17. "Subglacial Lakes of Iceland" reported on the results of Russell, Óskar Knudsen of Klettur Consulting Engineers, and Dr. Fiona S. Tweed of Staffordshire University, including field research sponsored by three years of Earthwatch Institute support.
Geologists have become increasingly interested in subglacial lakes since the discovery of more than seventy lakes under the East Antarctic ice sheet, including Lake Vostok, a body of water the size of Lake Ontario covered by a sheet of ice four kilometers thick. Many lesser subglacial lakes are found under smaller glaciers, often associated with thin floating ice shelves, and apparent subglacial lakes on other planets have drawn attention to the universal characteristics of these lakes. But the occurrence of glaciers directly on top of active volcanoes in Iceland represents a special case of subglacial lakes with spectacular results.
"In Iceland, volcanic activity can occur beneath several thousand feet of glacier ice, providing the opportunity for the filling of subglacial volcanic calderas as well as rapidly creating temporary ice-dammed lakes," said Russell. "Although similar processes may have occurred in the Antarctic, Iceland provides a highly active and accessible natural laboratory for studying volcanically generated subglacial lakes."
Russell's work concentrates on the downstream impact of subglacial lake outburst floods, jökulhlaups, on glacier margins and proglacial outwash plains. In particular, a 1996 jökulhlaup at Skeidararjokull glacier washed tons of sediment and ice into the ice-marginal environment, causing millions of dollars of damage in destroyed bridges, roads, and power lines. The flood event involved the drainage of nearly 4 cubic kilometers of water with a peak flow rate of 50,000 cubic meters of water per second, resulting in the transport of ice blocks up to 40 meters in diameter, the creation of a one-kilometer-wide outwash fan, and the formation of kettle holes 10 meters deep. Russell is working on the impact of even larger jökulhlaups on the northern margins of the Vatnajökull ice cap, possibly associated with repeated large subglacial eruptions during the Holocene, resulting in flood channels 12 kilometers wide.
An important aspect of jökulhlaups is the enormous amount of sediment these floods bring to the outwash plain and surrounding marine environment, a virtue of the active volcanoes they lie upon. One of the most significant finds of Russell and his colleagues is that the 1996 jökulhlaup deposited large quantities of sediment within the Skeidararjokull glacier. Surprisingly, this sediment was rapidly frozen into the glacier by flowing super-cooled water, an extreme example of a mechanism which may also operate more slowly at other times. This mechanism of sediment transport in glaciers and ice sheets could be crucial for the determining historic sediment flux and resulting landforms and the sedimentary record.
"At present most theories of debris entrainment involve the relatively slow accretion of thin layers of sediment and sediment-rich water at the glacier bed," said Russell. "Our research on the 1996 jökulhlaup has shown how large quantities of sediment can be entrained to high elevations within glaciers by high pressure jökulhlaup water exiting the glacier via hydro-fractures."
Subglacial lake outburst floods may also have a significant impact on ocean circulation and climate. Coupling of ocean and atmospheric circulation is recognized as a major driving force for regional and global climate changes, and recent research has shown that large freshwater inputs can be crucial in determining circulation of the North Atlantic Ocean. Historical jökulhlaups in Iceland have resulted in meltwater influxes of up to 10 cubic kilometers, and large ice-dammed lakes of up to 30 cubic kilometers are known to have drained during deglaciation. Freshwater inputs from Icelandic jökulhlaups, or cycles of jökulhlaup activity during deglaciation, may have contributed to ocean circulation collapse in the geological past, and may have implications for future climate change.
Russell's paper was part of a symposium titled "Subglacial Lakes: A Planetary Perspective," which he helped organize with his colleague, Dr. Amir Mokhtari Fard of the American University of Beirut. Fard has also been supported by Earthwatch Institute, and his paper at the symposium includes findings from field expeditions in Sweden supported by Earthwatch in 1996-7. In "Subglacial Lake Outbursts," Fard elaborates on the relationship between climate change, subglacial rebound uplift following deglaciation, and earthquakes, and their impacts on subglacial lakes. The symposium, including papers about subglacial lakes in Iceland, Sweden, Antarctica, and on Mars, will form the basis for a book edited by Fard, to be published in Europe next year.
"The red line between all these studies is the similarity of the morphology of subglacial lake basins regardless of time and space; a truly planetary resemblance," said Fard. "The outstanding difference, however, is the fantastic variations documented in the scale of morphologic features. Also, one can see from these examples how the living Earth is responding to climate changes in a dramatic way."
Russell's work on jökulhlaups and the glacial geology of Iceland continues this season with the support of Earthwatch Institute, which provides volunteer field assistants as well as funding.
"Earthwatch volunteers have provided invaluable field assistance," said Russell. "They have been involved in primary data collection such as surveying, sediment analysis, ground-penetrating radar work both on the glacier and in the immediate proglacial area. Earthwatch support has allowed our team to make systematic observations and measurements of our field area from year to year, providing an unparalleled opportunity to investigate the impact of these large floods on glacier margins and proglacial areas."
Earthwatch Institute is an international nonprofit organization which supports scientific field research worldwide by offering members of the public unique opportunities to work alongside leading field scientists and researchers. The Institute's mission is to engage people worldwide in scientific field research and education to promote the understanding and action necessary for a sustainable environment.