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

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http://www.jstor.org FITS AND STARTS What regulates the flow of huge ice streams? BY SID PERKINS

eration that immobilized the Kamb 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 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 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. "For such '*S||pHpiMB^^B Along kilometer-long trates of the the how sensitively the ^^^^^^^B^^^^|jSBH|H! portion streambed, Antarctic ice sheet reacts to discovered series of scientists received no reflec fEifS ?ntly subglacial environmentalchanges." ^^^^^^^^^^^mE^^S?BRu? tions of shear waves at all?a a sign that pocket of water sat SLIPPIN'SLIDE Scientists beneath the ice stream. The have long presumed that an ^^^^^^^^^^^( time lag in the echoes of other can move more was m ice stream rapidly when water lubricates its base. seismic waves indicated that the water between 5 and 10 Field studies and data gathered by satellites now directly demon deep, Anandakrishnan and his team report in theMarch Geology. strate the link between ice streams and subglacial water. There's evidence of recent water movement beneath the ice a The Recovery Ice Stream, which drains a 1-million-square-kilo stream. In 2005, researchers reported that kilometers-wide meter area of eastern Antarctica, penetrates farther into the con region of ice upstream of the seismic survey sank about 30 cen same a tinent's interior than any other ice stream, says Michael Studinger, timeters during a 24-day period in 1997. In the interval, a glaciologist at Columbia University's Lamont-Doherty Earth broad region about 10 km downstream of the survey site rose about new Observatory in Palisades, N.Y. By combining satellite obser 50 cm. That change reflects a surge of about 10 million cubic vations and field data gathered decades ago, Studinger and his meters of water, the team calculated. colleagues have identified four immense subglacial lakes beneath Occasional floods and the long dry spells in between may con the ice stream. tribute to the irregular behavior of ice streams, says Scambos. Together, the lakes stretch across the upper reaches of the Recov ery Ice Stream's catchment area, a distance of more than 350 km. WEDGE ISSUE As ice streams wend their way to the sea, they They cover an area of about 13,300 km2, about half the size of scour the landscape, shattering the underlying bedrock into par North America's Lake Erie. Ice over the lakes is flat, featureless, ticles ranging from boulders to fine grounds called rock flour. Some and more than 3 km thick, the researchers report in the Feb. 22 of this material becomes embedded in the lowermost layers of ice. Nature. When the stream reaches the sea, some of the particles stay in < Upstream of the newly discovered lakes, ice velocities clock in the icebergs that break off and float away. The particles eventually | at only about 2 m/yr, says Studinger. Then, where the ice stream melt and scatter the detritus across distant seafloors. Other mate ? flows onto a lake, it suddenly speeds up. rial remains onshore and lines the streambed. Much of the rest is o Over a lake, the flowing ice floats on water and experiences bulldozed off the continent by the advancing ice and ends up just no 5 almost friction, the researchers note. The acceleration stretches offshore. i and thins the ice slightly at the upstream lakeshore, creating a dis Observations of the continental shelf beneath the open water 2 a tinctive, broad trough few meters deep. Similarly, when the ice off Antarctica have revealed many such deposits. Each is typi | reaches the downstream shore of the lake, it decelerates in response cally between 5 and 15 km long, tens of meters high, and wedge ^ to the friction there. That slowdown generates a small ridge in the shaped, with the narrow portion of the wedge pointing toward o ice stream's surface. shore.

WWW.SCIENCENEWS.ORG MARCH 31, 2007 VOL. 171 203 cause The wedges were laid down by ice streams during the last ice increases of 5 m or less, the team's models suggest?wouldn't age, which ended around 10,000 years ago. Then, sea levels were the ice stream to float free. more than 100 m lower than they are today, and the streams had The wedge's buttressing effect slows down the ice flow from the sure. to flow much farther to reach the ocean, says Anandakrishnan. continent?an important role, to be Because Antarctica holds The sediment deposits were left behind as sea levels rose and the about 90 percent of the world's land-based ice, scientists have long a in the ice streams retreated. Up and down been concerned about surge each ancient channel, they're spaced rate of Antarctic ice reaching the about 50 km apart. Similar deposits ocean. There's enough ice there to m. probably lie beneath the floating ice boost worldwide sea level by 60 shelves that rim much of Antarctica, Global warming, which increases but scientists can't yet conduct sur ice temperature and makes glaciers veys of those areas to confirm the flow faster, may be only part of the features' presence. problem. Computer models suggest However, a seismic survey of the another factor: Ice streams often into at its ground accelerate when the ice shelf ing line shows that a wedge of sed which they flow disappears (SN: iment is accumulating there. "It's 2/3/01, p. 70), potentially boosting as we sea level. five of the six being replenished speak," says Indeed, large Anandakrishnan. glaciers flowing into the Larsen A The seismic reflections from the Ice Shelf accelerated soon after that ? terrain beneath the ice stream sug ISEE ECHOES The broad, horizontal lines across the mass of ice disintegrated early in gest that the wedge is no more than bottom of the image depict th< lower surface of Antarctica's 1995 (SN: 3/8/03, p. 149). 30 m or so thick. It stretches at least i/h?t?aii^ Stre^rn.f be faii slanted?n? at io?t?r right With the host of new findings traces the lower of the u sediment 8 km along the ice stream's chan edge idertying wedge, including the subglacial lakes that nel. Anandakrishnan says that the which slows Ice flow from the continent. tend to accelerate ice streams and wedge may extend another 10 km or the sediment wedges that hold them so offshore of the grounding line, but thewater beneath the ice shelf in check?researchers expect to improve their predictions of ice interferes with the echoes of seismic-shear waves and so masks any stream behavior. underlying material. Anandakrishnan and his colleagues describe "These are incredible discoveries," says Scambos. "They're a big their findings in the March 30 Science. plus for modeling of the Antarctic ice sheet." Data also show that ice is piling up atop the sediment wedge at Slowly but surely, scientists are discovering the factors that so as a streams the grounding line, the wedge appears to act speed bump for underlie previously puzzling megaglacier behavior. The ice the the ice stream, says Anandakrishnan. The extra weight stabilizes the aren't defying gravity, after all. As Scambos says, "It's all about location of the grounding line because modest rises in sea level? balance between friction and freezing."

(continued from page 201) males are still flirting with females and accepting contributions SCIENCE to the egg pile. Tests showed that females looked more favor NEWS ably on a male with young eggs in the pile. Once the flirting was to on period over, however, the males tended snack the younger eggs. can a Besides egg age, certainty of paternity influence father's choice of edible offspring. Earlier tests had suggested that when another male tries to fertilize the eggs during spawning, the brood guarding male ismore likely to turn cannibal. New work shows that an intruding male has this effect even on a fish that doesn't take care of his eggs. The colorful males of Telmatherina sarasinorum compete fiercely to spawn with the drab females of Indonesia's Lake Matano. Even after a cou ple has paired up, another male may dash over to it and release So many topics, one great scienceweekly. sperm that might reach at least some of the eggs. The original male then might understandably make the best of a bad job and get lunch. Subscribe today!www.sciencenews.org For three breeding seasons, Suzanne M. Gray of Simon Fraser University in Burnaby, British Columbia, watched several hun 1-800-552-4412 dred encounters of courting T. sarasinorum. If a second male showed up, the chances tripled that the original male would turn around right after spawning and gobble some of the brood off the lake bottom. If a third male showed up, the chances of egg canni SCIENCE SCIENCE SCIENCE balism rose almost sixfold, Gray and her colleagues report in the NEWS NEWS NEWS February American Naturalist. This is the first time that anyone has shown cannibalism increas incoming! ing as evidence of cuckoldry increases, says Gray. In calculating the difference between the effects of one bounder and of two, scientists may be making a fine distinction inworking family portrait out the nuances of filial cannibalism. But that research demon strates how dramatically ideas have changed since the days when was a eating the kids considered just crazy, stupid mistake.

204 MARCH 31, 2007 VOL. 171 SCIENCE NEWS