Fits and Starts: What Regulates the Flow of Huge Ice Streams? Author(s): Sid Perkins Source: Science News, Vol. 171, No. 13 (Mar. 31, 2007), pp. 202-204 Published by: Society for Science & the Public Stable URL: http://www.jstor.org/stable/20055497 Accessed: 09/10/2009 11:02 Your use of the JSTOR archive indicates your acceptance of JSTOR's Terms and Conditions of Use, available at http://www.jstor.org/page/info/about/policies/terms.jsp. JSTOR's Terms and Conditions of Use provides, in part, that unless you have obtained prior permission, you may not download an entire issue of a journal or multiple copies of articles, and you may use content in the JSTOR archive only for your personal, non-commercial use. Please contact the publisher regarding any further use of this work. Publisher contact information may be obtained at http://www.jstor.org/action/showPublisher?publisherCode=sciserv. 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BY SID PERKINS eration that immobilized the Kamb Ice Stream magine the consternation that your high school eventually may have been triggered by cooling that occurred during the Little Ice physics teacher would have shown if, a lab during Age several centuries ago. demonstration, the little wheeled block placed on an inclined had violated the law of GOING DOWNHILL All the ice in Antarctica eventually flows plane gravity. Imag sea. as to the Just the water flowing off warm landmasses reaches ine the block sometimes speeding up, sometimes the sea via rivers, most of the ice spilling off Antarctica and Green land is carried ice streams. These massive flow slowing down, and sometimes stopping dead on the by glaciers typically are much faster than nearby ice does. Many ice streams unexplored, Scientists have faced a similar situation as slope. they've and others haven't been visited by scientists in decades. Some of these flows didn't even have names until 5 studied some of Antarctica's most massive glaciers. The about years ago. Many ice streams nourish broad regions of floating ice that researchers are eager to understand the behavior of these remain attached to the land at their upstream boundaries, or ice streams because they have considerable influence grounding lines. These floating masses, called ice shelves, connect to about 44 of the continent's coast. on sea levels worldwide. percent Some regions of ice flow much Scientists estimate that ice faster than others. For example, streams contribute about 90 per one section of an ice stream can cent of the ice flowing directly off flow hundreds of times faster Antarctica into a the surrounding than higher area nearby does. sea. However, "we can't now pre Several factors affect the flow dict how much icewill flow into rate of an ice stream, says the sea in the A. a future," says Ted Richard Hindmarsh, glaciolo a Scambos, glaciologist at the gist with the British Antarctic National Snow and Ice Data Cen Survey in Cambridge, England. ter in Boulder, Colo. The temperature of the ice plays a Some factors that influence ice big role. Ice flows more rap streams are well known, but oth idly at 0?C than does ice at ers are just being revealed. New -10?C, he notes, because viscos findings show complex aspects ity at the higher temperature is of ice streams that have yet to be only 10 percent of that at the incorporated into models of how lower temperature. such ice notes. behaves, Scambos The roughness and grade of New studies bolster the notion the terrain underlying the ice are that bodies ofwater beneath gla rial in ice streams flows much other major influences on ciers accelerate the streams by y highlands. Blue tint denotes flow rate. They affect the fric r warming the ice and lubricating year; red denotes speeds of tion at the base of the ice stream. ^ its flow. Other recent research In most locations, friction at g hints that immense volumes of the base of an ice stream changes ? sediment that the streams at case scrape off the continent pile up where slowly, if all. However, that's not the where these megaglac- S> the ice meets the sea. This sediment may act as a buttress to slow iersmeet the sea and are supported by the water rather than by I ice flow. underlying terrain. Near the grounding line, the effects of ocean tides | S Furthermore, Antarctic field studies suggest that the Kamb become readily apparent, says Sridhar Anandakrishnan, a glaciol- Ice Stream stopped in its tracks 150 years ago. The neighbor ogist at Pennsylvania State University inUniversity Park. g ingWhillans Ice Stream is slowing significantly and, at the cur For example, the tide influences how forcefully an ice stream J rent rate of shut down a more a cen deceleration, may little than presses against the shoreline terrain, Anandakrishnan notes. Some | from now. So come a ? tury far, scientists can't fully explain these unusual ice streams to halt twice a day, typically near low tide, when slowdowns. they weigh most heavily against the streambed. Later, when ris- g Factors that affect the flow rates of ice streams at some stream operate many ing tides lift of the ice shelf's weight, the ice surges gj time scales. Ocean tides produce daily and weekly variations in forward, thereby decreasing the friction on the ground just inland. ? at some flow speed locations, while the draining and filling of sub Because of this surge-and-stall behavior, scientists must care- ? lakes and the climate such as ice to their measurements with tidal to glacial cycles, ages, appear fully synchronize cycles accurately g cause variations over or an m decades, centuries, millennia. The decel estimate ice stream's overall velocity, says Anandakrishnan. 202 MARCH 31, 2007 VOL. 171 SCIENCE NEWS New data suggest that ocean tides can cause the velocity of an Immediately downstream of the lakes, the ice stream flows course a over ice stream to vary not only over the of day but also the at rates between 20 and 30 m/yr. That's faster than upstream course of a month. Glaciologist G. Hilmar Gudmundsson of the of the lakes because the stream's bottom surface soaked up heat across British Antarctic Survey in Cambridge, England, examined the during its several-millennium-long trip the lakes, says Rutford Ice Stream inwestern Antarctica. That stream is 150 kilo Studinger. Also, water flowing from the lakes, either in trickles meters long, 25 km wide, and as much as 3 km thick. or in occasional floods, probably lubricates the base of the ice Gudmundsson used global positioning system (GPS) equip stream. ment to measure the rate of ice flow at several points on the Rut Even farther downstream of the lakes, the ice zips along at an ford Ice Stream every 5 minutes between late December 2003 and impressive 100 m/yr. mid-February 2004. On average, the ice in that stream slides sea Coincidentally, Anandakrishnan and another team of scientists ward about 1meter each day, he found. report that they've discovered a shallow body of water beneath the However, ice-stream velocity varied significantly during the head of an ice stream inwestern Antarctica. During the 2002-2003 7-week period. At neap tides?the exceptionally small tides that field season, the researchers collected seismic data along a 16.7-km occur when the moon is in one of its quarter phases?the ice stream's stretch of a tributary of the Bindschadler Ice Stream. Taking advan peak velocity measured about 0.90 m/day at the grounding line. tage of the well-known vibration-transmitting characteristics of ice, During the highest tides of themonth?the so-called spring tides that the researchers inferred the characteristics of the rock and sedi occur when the moon is full or new?the ice stream clocked in at a ments that underlie the region. velocity of 1.15 m/day. ^^ ^^^^^^^^^^^ Under most of the sites that Gudmundsson'sreport, the ^^IC^^^S^^^^^^^^HRI they studied, a layer of sedi first to describe ment 5 to 20 m thick was fortnightly, ^?Jp^^M^^^BMBB|?| in sandwiched between the base tidally inducedvariations ^flHH|^^BRSSBHJ^^| ice-stream of the ice stream and the velocity,appeared J^^^P^^^HB^^^^^^I inthe Dec. 2006Nature. Anandakrish 28, ^?ff^BS??^??^^K^? bedrock, says variations nan. Some of the Similarbiweekly ^^^S^^^^^^^H^^j^^^^l segments in were streambed were lined with ice-stream velocity FhH^K^-|S|hHH|^HH| observed at all sites where where 5fl^^BMfe^i^^^ggl?|wfflMB well-packed sediment, Gudmundssonhad installed ?JKHBBR ^^SmIBk?Bp seismic waves of the type that evenat a transmits shear forces theGPS equipment, a^^K^^^^E^-t^^BBffi^S spot 40 km inlandof the ice ^HHH^HHH|^^^^^9|KSH through the ground traveled stream's line.See- about 1 km second. In grounding H|HH|S^^P^^^^9HH per a a other the ice rested ing tidaleffect at such dis- W^g^^-^BMES^?W^^^S i%wj sections, tance "was a total Wkm4?**?* on sediment, surprise," "~^H?99^wn@l loosely packed Gudmundsson.