Valley of the dead … seals

Remote mummified remains tell story about past ecology and climate change

Antarctica has existed in a deep freeze for millions of years. But even the world’s coldest continent isn’t as unyielding as it might seem. Ice sheets shrink and expand over time, as does the sea ice that encircles Antarctica.

These variations in the ice cover signal changes in the climate or environmental conditions, such as fluctuations in ocean circulation. Scientists use a standard suite of methods to measure and interpret such changes, from collecting ice and sediment cores to making observations with GPS and satellites.

And then there is the team hunting for mummified seals in the McMurdo Dry Valleys.

“We’re looking for two things: Changes in the ecology of the species and changes in the ecosystem. Sometimes we’re using the seals as sampling buoys. Sometimes we’re actually trying to figure out what they’re doing,” explained Paul Koch, a professor of Earth and planetary sciences at the University of California, Santa Cruz, where he also serves as dean of the Division of Physical and Biological Sciences.

Koch and co-principal investigator Brenda Hall, a professor of Glacial and Quaternary Studies at the University of Maine, have led a field team to the relatively ice-free McMurdo Dry Valleys region for the last two Antarctic summers.

Each year they’ve collected small samples of fur, skin, bone, whisker and even bits of teeth from long-dead seals that had somehow wormed their way many kilometers inland from the cold waters of McMurdo Sound and the Ross Sea.

Hall, Koch, and colleagues had previously focused on the presence of numerous mummified elephant seals along the beaches of Victoria Land. The discovery of major colonies so far south suggested that milder climate conditions existed between 7,000 and 1,000 years ago before the population declined and went locally extinct about 400 years ago – a finding that was published in 2009 in the journal PLoS Genetics. [See previous article — Cradle to grave: Study provides insight into evolution and extinction of vanished elephant seal colony.]

Today, extensive sea ice and land fast ice exists along the 300-kilometer-long coast where, at its height, the elephant seal population included more than 220,000 individuals based on DNA evidence recovered mainly from fur and skin.

While some elephant seals feed in sea ice, they will not travel across ice shelves, meaning that ice-free conditions must have existed that allowed the animals to access the Victoria Land beaches during the part of the year when they would normally come ashore to breed and molt.

Then the climate appeared to shift to a colder and icier regime about a thousand years ago.

Koch, Hall and their team are now shifting their focus inland to the valleys where for reasons still unknown, some species of seals – crabeater, Weddell and leopard – have wandered. Their mummified remains may offer clues about the changes in environmental conditions that occurred a thousand years ago, as well details about the past ecology of these animals – and their prospects for the future.

“This research was largely spurred by that elephant seal project,” Koch said. “It was pretty clear that the disappearance of the elephant seals signaled an environmental change.”

Added Hall, the paleoclimate expert, “It might also suggest that what we think of as normal today might only have come into existence in the last thousand years or so ago.”

The researchers, including postdocs, graduate students and undergraduates, spent about a month each year in field camps in the McMurdo Dry Valleys, where this season they collected specimens from about 200 crabeater seals, 50 Weddell seals and 10 leopard seals – a proportion that somewhat corresponds to the existing populations today.

Koch said many of the mummified seals had been assumed to be crabeater seals (Lobodon carcinophaga), the most populous seal today and one of the most abundant wild large mammals on Earth. They prefer the seasonal pack ice as a habitat versus Weddell seals (Leptonychotes weddellii), which are more comfortable in thicker sea ice attached to the shore.

“For these seals here now, the big change would be the presence or absence of the land-fast ice,” Koch noted. Were crabeaters even more populous when the elephant seal colonies were thriving during the period when less ice was present between 7,000 and 1,000 years ago? What would happen to them in the future should such conditions return?

“[Crabeater seals are] a big chunk of the ecosystem. Understanding how they respond to this perturbation is very important,” Koch said. “They’re a key player. They’re the ones I’m most interested in.”

The third pinniped under study, the solitary leopard seal (Hydrurga leptonyx), is a formidable predator that keeps the crabeater population in check, eating about 80 percent of them. Little is known about their ecology.

“They’re much easier [to study] dead than alive,” Hall quipped.

DNA evidence from wayward seals may shed light on past climactic conditions

Ancient DNA preserved in the mummified corpses can provide details about the population size and diversity. Different isotopes of various elements, particularly nitrogen and carbon, can provide details on diet.

The latter is based on the idea that you are what you eat. In this case, the seals would have absorbed carbon and nitrogen isotopes of their prey. The ratio of those isotopes would remain in any tissue samples.

For example, nitrogen isotope ratios indicate how far up or down an animal feasts on the food web. The rare Nitrogen-15 isotope becomes more concentrated at higher trophic levels, so that scientists would be able to determine if crabeater seals, for instance, ate more fish in the past versus more krill today. What might the diet of ancient leopard seals reveal?

“Even today, despite their reputation, leopards eat a surprising amount of krill,” Koch noted, referring to the tiny crustacean that is a keystone species in the Antarctic food web.

And today it’s still unclear what caused the environment to change a millennium ago. The event roughly coincides with a cooling period in Europe known as the Little Ice Age, but there is much debate on whether there is a link between those events, according to Hall.

It’s even possible, Koch and Hall noted, that ocean circulation may have fluctuated, forcing water only a few degrees above freezing to the surface, which would have melted sea ice.

“It could be more of an ocean-driven phenomenon,” Koch said.

What the researchers are finding might help test that theory, Hall added.

In fact, many of the glaciers near McMurdo Station are retreating today. Not on the scale seen in West Antarctica or Greenland, but a few hundred meters in recent decades.

“As they’re retreating, they’re revealing elephant seals. It’s returning to a glacial configuration that existed before,” Hall said.

The ice also helped preserve the elephant seal mummies and other remains, some 7,000 years old. The freeze-dried, fragile bodies found in the McMurdo Dry Valleys aren’t nearly as old – maybe 2,000 years at the far end of the scale.

“Mummies exposed on the surface break down very fast,” Koch said. “They blow apart in the wind.”

Jonathan Nye, a former undergraduate under Koch, is working on a project to establish a scale that measures the rate at which the carcasses degrade and turn back into dust based on previous research.

“We went out and tried to classify what we found based on the scale,” he explained. “It’s fairly linear.”

Bone fragments offer the best chance to find older material, Koch added.

Actually locating the mummified seals turned out to be relatively easy – and sometimes quite startling. Some of the animals managed to work their way 20 kilometers or more inland, across “impossible terrain” that required the scientists to hike over glaciers.

“Some of them did go over the rocks, and you find a lot of them at dead ends, which is pretty sad,” Hall said. “You go up to the glacier margin or to a cliff and you’re almost certain to find seals. …They got there and that was it.”

Koch said it’s unclear what drove the seals to what amounts to certain death. The Dry Valleys, with just a few ice-covered lakes and ephemeral streams, are devoid of water. Life there is on the microscopic scale – worm-like nematodes and bacteria.

In the end, it could just be happenstance.

“It could just be noise and represent a small fraction of the population,” Koch said. “It may just be that one or two seals per year walked the wrong way.”

The researchers, however, hope they’re on the right track to understanding the region’s past environment and ecology thanks to a few wayward seals.

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