Life thrived below solid ice shelf.

An area of Antarctic seafloor that until recently was covered by thick, floating ice is host to a several-millennia-old ecosystem apparently based on chemical nourishment, not sunshine, according to a recent underwater survey of the area.

More than 60 percent of the Larsen B ice shelf disintegrated and drifted away early in 2002 (SN: 3/30/02, p. 197). That ice mass had been floating, although attached to shore, for 10,000 to 12,000 years, says Scott E. Ishman, a marine geologist at Southern Illinois University in Carbondale. Before that time, during the last ice age, the ice reached all the way to the seafloor and scraped the bottom clear of sediments.

In March, Ishman and his colleagues studied a 5,500-square-meter plot of ocean bottom, 850 meters deep, in a glacier-scoured trough that oceanographers had spotted on an earlier research cruise.

The researchers found up to 70 percent of the seafloor area covered with a white mat of bacteria. The mat looks just like those found in other oceans at so-called cold seeps, where frigid, methane-rich waters ooze from beneath the seafloor, creating a nutrient-rich environment. In those areas, the methane derives from microbes living deep in sediments and consuming organic material there, says Ishman.

At the Antarctic site, however, there hasn't been enough time since the last ice age for much organic-rich sediment to accumulate. Therefore, the methane presumed to nourish the bacterial mat there probably originated in ancient, petroleum-rich strata beneath the seafloor, not from microbes. Other organisms higher in the food chain at the Antarctic site include clams and brittle stars, which are closely related to starfish, says Ishman. The scientists report their findings in the July 19 Eos.