Vanished elephant seal colonies indicate Ross Ice Shelf survived warmer climate in recent past
Posted February 4, 2007
As a geologist who studies paleoclimate, Brenda Hall generally uses glaciers to help her reconstruct climate change through history.
But the University of Maine researcher and three members of her team spent the month of January picking through the beaches between McMurdo Sound and Terra Nova Bay looking for the remains of long-dead southern elephant seals. Hall believes the presence of colonies along Victoria Land as recently as a thousand years ago indicate the region was warmer than it is today.
“We’re interested in them because they shouldn’t be there,” Hall said during an interview at McMurdo Station shortly before heading into the field for five weeks. “Elephant seals don’t live in the Ross Sea today.”
The reason they don’t call the Ross Sea region home is the extensive sea ice cover now locked to the coast. Simply put: elephant seals don’t care much for sea ice. But evidence of colonies existing in the not-too-distant past means there was little or no sea ice during the part of the year when the elephant seals would normally come ashore to breed and molt, according to Hall.
The absence of sea ice tells Hall that it was likely warmer in the relatively recent geologic past. How much warmer, she can’t say, but evidence suggests the Ross Ice Shelf was still intact throughout the period that the colonies existed.
“Basically we’re able to say: it got warmer, and the Ross Ice Shelf survived,” Hall said. “We came to realize these elephant seals can give us a climate indicator.”
The connection is an important one for scientists studying climate change and attempting to model how Antarctica’s ice mass will respond to warming temperatures in the next century. Some research suggests the Ross and the Filchner-Ronne ice shelves are the keystones preventing the collapse of the West Antarctic Ice Sheet, Hall said. Their disintegration would open the floodgates for glaciers to empty out the marine-based ice sheet.
“If a big ice shelf were to give way, the results could be catastrophic,” said George Denton, a University of Maine colleague, in a previous press release from the university on the published findings, which appeared in The Proceedings of the National Academy of Sciences in June.
“Through her discovery of elephant seal remains over a widespread area where they do not exist today, [Hall] shows evidence not only that a warming occurred, but that the Ross Ice Shelf survived that event. It’s important because it speaks to the staying capacity of the ice shelf in the face of global warming,” Denton said.
The most recent warming period occurred between 1,000 and 2,500 years ago, Hall noted. “We have pretty good evidence of that time period,” she said.
A second significant warming might have occurred even earlier, between 4,000 and 5,000 years ago with evidence of elephant seal activity as much as 7,000 years in the past. This season, the group is looking for more samples for radiocarbon dating that will help it pinpoint this second period.
When Hall and colleagues first discovered the skin and hair from the locally extinct elephant seal colonies about 10 years ago, she had been on a different mission. She was studying the retreat of ice sheets through the Ross Sea and was literally combing the coast for organic material to help date when the beaches had formed. It took a couple of years before the scientists could genetically connect the tiny fragments of skin and fur to elephant seals.
Southern elephant seals are sub-Antarctic mammals, with huge breeding colonies on Macquarie Island and South Georgia. Macquarie Island lies about halfway between Antarctica and Australia and is the closest place today to the extinct Ross Sea colonies, according to Hall.
Rus Hoelzel, at Durham University in the United Kingdom, is one of two elephant seal specialists working on the team. He is a molecular biologist performing the DNA work on the samples that Hall and others in the group have brought back from previous trips.
Hoelzel said his initial work in the project involved identifying the remains of the elephant seals. Now his focus is determining what the genetics of the population can tell the team about the colonies’ history in the context of environmental change over the last 5,000 years.
“We can use genetic diversity and simulation modeling to estimate the pattern of demographic expansions and contractions over time, and thereby compare population dynamics with patterns of climate change,” Hoelzel explained via e-mail. “We will also use the approximately 5,000-year time span to estimate a more accurate mutation rate than would otherwise be possible, and this will facilitate our estimations of population demographics.
“We will also compare the ancient samples with modern populations to discover if seals from the Ross Sea may have dispersed to one or more of the island populations,” he added. A genetic link may exist between the Ross Sea seals and the modern colonies on Macquarie Island, he said, but that hypothesis is still being tested.
Hall and Paul Koch, co-principal investigator from the University of California, Santa Cruz, will head as far north as Terra Nova Bay to join Italian colleagues who study Adélie penguins. The penguins serve as another biological indicator for sea ice, Hall said.
Sea ice is a double-edged sword for Adélies. Big sea ice years present foraging problems. However, scientists working in the Antarctic Peninsula have identified a strong link between the abundance of krill, a major food source for Adélies, and the presence of sea ice.
Because they have different sea ice tolerances, the penguins and elephant seals rarely share the same space, which is proving true along coastal Victoria Land.
“It turns out when you combine different species, because each species has a slightly different tolerance to sea ice, you can get a more complete picture of sea ice extent and duration,” Hall said.
This is the second and final field season for the three-year grant from the National Science Foundation. Hall and colleagues will use the third year to crunch the data and see how it fits into the larger picture of global climate change.
Hall said it’s difficult to predict just how hardy the ice shelves will prove to be, though she does not believe the Earth has reached the tipping point yet for the type of climate change that would cause major de-glaciation of Antarctica. In the end, it will depend on just how much the temperature really rises over the long term.
“It’s hard to tell; you don’t know what’s going to happen to temperatures,” she said. “The Ross Ice Shelf managed to survive warmer-than-present temperatures 2,000 years ago, but its fate in the future may depend on just how warm temperatures get.”
NSF-funded research in this story: Brenda Hall, University of Maine; and Paul Koch, University of California, Santa Cruz.