Science News - March 31, 2007

The wedges were laid down by ice streams during the last ice age, which ended around 10,000 years ago. Then, sea levels were more than 100 m lower than they are today, and the streams had to flow much farther to reach the ocean, says Anandakrishnan.

The sediment deposits were left behind as sea levels rose and the ice streams retreated. Up and down each ancient channel, they’re spaced about 50 km apart. Similar deposits probably lie beneath the floating ice shelves that rim much of Antarctica, but scientists can’t yet conduct surveys of those areas to confirm the features’ presence.

However, a seismic survey of the Whillans Ice Stream at its grounding line shows that a wedge of sediment is accumulating there. “It’s being replenished as we speak,” says Anandakrishnan.

The seismic reflections from the terrain beneath the ice stream sug- I SEE ECHOES — The broad, horizontal lines across the gest that the wedge is no more than bottom of the image depict the lower surface of Antarctica’s

30 m or so thick. It stretches at least Whillans Ice Stream. The faint, slanted line at lower right

8 km along the ice stream’s chan- traces the lower edge of the underlying sediment wedge, nel. Anandakrishnan says that the which slows ice flow from the continent. wedge may extend another 10 km or so offshore of the grounding line, but the water beneath the ice shelf interferes with the echoes of seismic-shear waves and so masks any underlying material. Anandakrishnan and his colleagues describe their findings in the March 30 Science.

Data also show that ice is piling up atop the sediment wedge at the grounding line, so the wedge appears to act as a speed bump for the ice stream, says Anandakrishnan. The extra weight stabilizes the location of the grounding line because modest rises in sea level—

increases of 5 m or less, the team’s models suggest—wouldn’t cause the ice stream to float free.

The wedge’s buttressing effect slows down the ice flow from the continent—an important role, to be sure. Because Antarctica holds about 90 percent of the world’s land-based ice, scientists have long been concerned about a surge in the rate of Antarctic ice reaching the ocean. There’s enough ice there to boost worldwide sea level by 60 m.

Global warming, which increases ice temperature and makes glaciers flow faster, may be only part of the problem. Computer models suggest another factor: Ice streams often accelerate when the ice shelf into which they flow disappears (SN: 2/3/01, p. 70), potentially boosting sea level. Indeed, five of the six large glaciers flowing into the Larsen A Ice Shelf accelerated soon after that mass of ice disintegrated early in 1995 (SN: 3/8/03, p. 149).

With the host of new findings— including the subglacial lakes that tend to accelerate ice streams and the sediment wedges that hold them

in check—researchers expect to improve their predictions of ice-stream behavior.

“These are incredible discoveries,” says Scambos. “They’re a big plus for modeling of the Antarctic ice sheet.”

Slowly but surely, scientists are discovering the factors that underlie previously puzzling megaglacier behavior. The ice streams aren’t defying gravity, after all. As Scambos says, “It’s all about the balance between friction and freezing.” ■

ANANDAKRISHNAN E T AL.

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males are still flirting with females and accepting contributions to the egg pile. Tests showed that females looked more favorably on a male with young eggs in the pile. Once the flirting period was over, however, the males tended to snack on the younger eggs.

Besides egg age, certainty of paternity can influence a 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 is more 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 couple has paired up, another male may dash over to it and release 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.

For three breeding seasons, Suzanne M. Gray of Simon Fraser University in Burnaby, British Columbia, watched several hundred 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 cannibalism rose almost sixfold, Gray and her colleagues report in the February American Naturalist.

This is the first time that anyone has shown cannibalism increasing 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 in working out the nuances of filial cannibalism. But that research demonstrates how dramatically ideas have changed since the days when eating the kids was considered just a crazy, stupid mistake. ■