Carving Out a New Idea of the Past

There was bad news and good news aboard the U.S. Coast Guard icebreaking vessel Healy during a 2002 voyage to the Arctic Ocean.

The bad news was a disturbing lack of sea ice, which, combined with strong winds, prevented the vessel from maintaining a steady position in the Arctic’s Chukchi Sea. The harsh conditions hindered a multi-institutional research team’s ability to extract seafloor sediment cores, one of the principal goals of the expedition.

But the good news was that the ice-free seas gave the researchers a rare opportunity to make intricately detailed maps of the region’s seafloor. Neal Driscoll of Scripps Institution of Oceanography at UC San Diego and his colleagues employed an acoustical imaging device called CHIRP to penetrate the ocean floor and construct profiles of sediment layers and the geological history of the Chukchi Sea.

Driscoll, recent Scripps graduate Jenna Hill, and their colleagues from the University of Massachusetts, Woods Hole Oceanographic Institution, and Coastal Carolina University recently published findings from the cruise that could alter ideas of historical climate change, glacial meltwater processes, and how global climate models are constructed.

Using CHIRP data, the team examined a deep undersea valley in an area known as the Chukchi shelf. While the valley was shown to have been repeatedly eroded during periods of low sea level as expected, the team also surprisingly found erosion, or “downcutting,” during periods of sea level rise. Such evidence of deep carving during sea level rise has been identified in other seas, but is rare. The newly mapped sediment profiles provided a wealth of geological clues on how such an oddity might be formed.

The scientists further found surprises in determining the water source responsible for eroding such a large valley. A candidate could have been rainfall, but the arid climate of the region dispelled that notion, especially since precipitation in the region during the period was 10 percent less than today’s paltry rainfall totals. The researchers concluded that only one source could have produced enough water to carve the valley: glaciers.

“This is a deep channel—on the order of 40 to 60 meters — and now we know that it was carved when sea level was rising, which is unusual, and we are forced to believe it was shaped by glacial meltwater,” said Driscoll.

Traditional ideas of climate and sea level cycles assume they work in tandem, and most evidence of sea channel erosion proves the point. When sea level goes down, precipitation carves new valleys through erosion. But the new discovery shows that climate and sea level can act independently, with ice and glacier levels sometimes fluctuating separately from the rising and falling sea.

“Our group is trying to reconstruct the climate, sea level, and glacial history of the region to try to understand how it evolved through time,” said Driscoll. “Our new results show that climate and circulation patterns may have been slightly different than people have modeled.”

Unrelated to the research, Driscoll noted that during the expedition, the research party encountered a polar bear, presumably foraging, that attempted to make an uninvited boarding onto Healy. The imposing animal, more than 100 miles offshore, eventually tired in his attempts to climb the slippery side of the vessel and moved on. Driscoll said the encounter illustrates how the lack of sea ice has forced polar bears to greatly broaden their search for their normal food staple of seals and sea lions.

In addition to Driscoll and Hill, co-authors of the research paper published in Quaternary Research included Julie Brigham-Grette, Jeffrey Donnelly, Paul Gayes, and Lloyd Keigwin.

Funding for the study was provided by the National Science Foundation Office of Polar Programs, the Oak Foundation, and the Los Angeles chapter of the ARCS (Achievement Rewards for College Scientists) Foundation.

-- Mario C. Aguilera