Newly drilled sediment cores could reveal how fast the Antarctic ice sheet will melt

VIENNA—Braving a raft of icebergs, a scientific drill ship has recovered the first deep sediment cores from the Amundsen Sea, where the massive Antarctic ice sheet is rapidly melting. The 800-meter-long records, described for the first time last week at the annual meeting of the European Geosciences Union here, contain several million years of history of the West Antarctic Ice Sheet. “It’s a very strategic place to drill,” says Florence Colleoni, a paleoclimate and ice sheet modeler at the National Institute of Oceanography and Experimental Geophysics in Sgonico, Italy.

Geoscientists suspect the ice sheet has previously collapsed, raising sea level around the world by several meters. By searching the cores for clues to the timing and extent of these dramatic, ancient events—and the ocean conditions that led to them—scientists can better assess how much the ocean might rise now and how quickly, Colleoni says. “This is really important for our future.”

The West Antarctic Ice Sheet is particularly vulnerable to melting from warming ocean waters because its base lies below sea level. Computer models vary in their predictions of how quickly it will disappear, but some predict it will be responsible for driving up global sea levels by a meter or more over the next century. To improve those models, scientists want to learn about the behavior of the ice sheet during the mid-Pliocene, 3 million to 4 million years ago, when temperatures were like today’s, says Rob DeConto, a glaciologist at the University of Massachusetts in Amherst who studies the ice sheet. The new cores, he says “will have a lot to bring to the table.”

The JOIDES Resolution, a research ship operated by the International Ocean Discovery Program, spent January to March in the Amundsen Sea, off the coast of West Antarctica. The hope was to drill sediment cores in five places, ranging from the continental rise toward the shallower waters of the continental shelf. Unfortunately, the ship could not reach the drill sites closer to Antarctica because it is not equipped to travel through ice and sea ice (which grows and retreats each year) extended farther out than usual. Even in the open water, avoiding the many icebergs meant less time to drill in other places. “We simply had bad luck this year,” says Karsten Gohl, a geophysicist at the Alfred Wegener Institute in Bremerhaven, Germany, and a leader of the cruise. “Despite the icebergs, we still got fantastic cores.”

The cores were retrieved from the two sites in deeper water. At each site, the JOIDES Resolution had to drill multiple holes—whenever an iceberg drifted too close for comfort, the crew aborted the drilling—but they eventually collected hundreds of meters of sediment. One set of cores, from seven holes, was drilled through a drift, a dune on the sea floor. Currents sweeping around the continent continuously deposited sediment there and with those records geologists can determine whether Earth was in an ice age. During glacial periods, the currents tend to be vigorous, leading to thin layers of fine sediments. But when Earth is warmer and the ice sheet disappears, the ocean circulations around Antarctica relax and sediment is deposited without obvious fine layers.

Crucially, the sediment cores preserve a complete history: The Amundsen Sea at that site is 4000 meters deep, so the seafloor dune was safe from storms that can erode sediment in shallower water. “The fact that you have a continuous record from this margin is really exceptional,” Colleoni says. Tiny fossils indicate that the oldest parts of the core date back to the late Miocene, about 6 million years ago, dates that were confirmed by the patterns of magnetism recorded in the sediments.

The other site, which yielded cores from four holes, is some 60 kilometers away from the first one and close to a submarine channel. When deep currents travel down such channels, they often carry and deposit sediment that was eroded from nearby land. (The currents passing over the drift, in contrast, are thought to carry sediment from much farther away.) Researchers can trace the origin of sediment by studying the mineralogy and chemistry of individual grains, comparing them to rocks on land. By combining various lines of evidence, the team hopes to infer when the ice sheet retreated.

The cores from both sites also contain pebbles or larger stones that were transported from the continent by icebergs. An abundance of such stones is another, more straightforward sign of a retreating ice sheet that is calving many icebergs.

A key question is how the waxing and waning of the ice sheet correlates with records of ocean temperatures, which the researchers can estimate from the abundance of certain chemical isotopes in the fossilized shells of tiny organisms called foraminifera.

The research team expects to spend the next 2 years coaxing this history out of the cores. But their analysis of them began on board the ship, as soon as they emerged from the drill rig. “The JOIDES Resolution is an extraordinary floating lab,” says Julia Wellner, a sedimentologist at the University of Houston in Texas who co-led the expedition. An x-ray machine was installed on board for the first time to look inside the cores for pebbles dropped by melting icebergs. In addition to counting the pebbles, the team has found a variety of layers, sediment types, and patterns of deposition that reveal the conditions of the ocean through time. “That was the truly exciting thing about this cruise,” Wellner says. “I had a little bit of dread that we would drill 800 meters of the same thing.”