Monday, October 18, 2010

Seals with sensors on their heads help map ocean

Some parts of the sea floor near the west side of the Antarctic Peninsula are much deeper than previously thought, according to new data.

And that could have a significant impact on the melting of the ice shelf on the west side of the Antarctic.

“I run numerical models of how changing ocean temperatures affect melting ice shelves,” explains Laurie Padman, an oceanographer and senior scientist at Earth and Space Research, a non-profit organization based in Seattle, Wash.

And to do that Padman needs accurate information on the depth of the ocean floor around this very fragile area. But up until recently maps of the ocean floor have not been very accurate, he explained.

This has all changed thanks to the data from a study of the elephant seal habitat that was conducted by Daniel Costa, a marine biologist at the University of California, Santa Cruz.

Dan Costa started tagging seals with sensors 25 years ago in an effort to record information about the animal's behavior and ability to adapt to changes in its environment. When it became clear that the seals dove way deeper and more frequently than had been thought, researchers realized that the sensors could collect more than just biological data.

Southern elephant seals dive 400 to 600 meters (about 440 to 650 yards)--and occasionally as deep as 1000 meters--during an average 60 dives per day. In the Southern Ocean around Antarctica, these depths harbor ocean circulation data that are of great interest to scientists who study climate change.

The Southern Ocean, which encircles Antarctica, holds cold, dense water that acts as a driving force in global ocean circulation. "A lot of the planet's weather is driven by what happens in the Southern Ocean," Costa stated. During the winter season, sea-ice formation around Antarctica essentially doubles the continent's size. Because sea-ice extent and thickness has a significant influence on global climate, understanding the rate at which sea ice forms is crucial to understanding the effects of climate change on the polar regions.

But one huge scientific hurdle remains: Much of the ocean beneath the Antarctic sea ice lays unobserved--a significant shortcoming given that the Southern Ocean covers about 19 million square kilometers. Although satellites can detect surface characteristics of sea ice, in situ data that reveal temperature and salinity at depth are needed to determine formation rates.

Now over 50 huge sea creatures are bringing back answers.

During the winter of 2004 to 2005, Costa and his colleagues from Australia, France, Germany and the United Kingdom tagged 58 southern elephant seals at four sub-Antarctic islands with oceanographic sensors. The sensor, which houses a pressure transducer, attaches to a section of fur on the seal's head. As the seal surfaces after a dive, the sensor records the dive depth and duration and takes measurements of water temperature and salinity. Each dive is stored in the sensor's memory, and when the seal surfaces the data are transmitted to a satellite that passes over the Antarctic region about every hour. While doing this, his seals also came up with a plethora of data on the depth of the waters surrounding the Antarctic. "For every dive, we're getting answers to two questions: What is the animal doing? And what oceanography can we learn?" Costa says.

Data collected by the seals have provided a thirty-fold increase in hydrographic profiles--charts that show ocean temperature and salinity--from the sea-ice zone. "Measurements collected by these predators have allowed the Southern Ocean's high-latitude fronts to be mapped in regions not sampled by other instruments," Costa writes in a paper published this week in the Proceedings of the National Academy of Sciences.

Several studies have been done to ensure that the sensors have no effect on the seals' behavior. "We've seen alpha males go through an entire breeding season with one of these tags on and seen no effects on his ability to mate," Costa says. The sensors fall off when the seals go through their annual molt (shedding their fur), so a seal never wears one for more than a year.

This year, seals tagged with sensors also helped uncover important information about the Wilkins Ice Shelf, a plate of floating ice about the size of the state of Vermont, on the Antarctic Peninsula, which began disintegrating in late February. According to Costa, tagged seals have been visiting the ice shelf since its breakup, bringing back data that show a previously unidentified tongue of warm water that may have contributed to the shelf's disintegration.

From Costa's point of view, the seal-tagging projects have created a unique partnership between biologists and oceanographers. "We're collecting a tremendous amount of information on these animals and their environment, and the added value is that the data are also of great value to oceanographers who study climate change. It's a win-win situation," he says.

Originally, Costa was tracking elephant seals so he could understand their environment and make predictions on how their physical conditions might change over time and how their behavior might change over time.

The gathered data was able to help Padman put together a “much better map” detailing the depth of the ocean floor, thanks to the seals. (Left: Wilkin ice shelf)
“I need to have accurate water-depth information for my models of ocean circulation to be accurate,” said Padman. “If the water depth is wrong then I get the wrong answer for how much warm water is getting to the ice shelf.”
The map and the data produced with the help of the seals is part of a study to be published in the journal Geophysical Research Letters. Costa and Padman are co-authors of the report.

The elephant seal information showed that the troughs around the west coast of Antarctica were much deeper than previously thought – about 300 meters deeper, Padman said.

Previous maps had placed the water depth at 400 meters in some spots, but the seals were diving down to 700 meters in some locations around the west coast, the oceanographer said.

The new depth data of those troughs and the warm water that flows through them could mean that the ice shelf on the west side of Antarctica is melting much faster than previously estimated. And if that ice is melting faster than expected, it could have a profound effect on
the rising sea levels that are predicted to occur over the next century.

The area surrounding the west side of the Antarctic Peninsula is already seeing a lot of thinning and melting of ice, said Padman. A similar thing is happening around Greenland. “These are the two areas that will contribute a lot of water to rising sea levels over the next century,” he said.

Padman and others are interested in getting a fix on just how deep the troughs across the continental shelf by this region of the Antarctic are so they can determine how much warm water flows through them and up to the ice shelves.

The amount of warm water, which comes from the edge of the Antarctic Circumpolar current, (left) that comes up will determine how quickly the ice shelves melt, he explained.

“We are trying to identify the places where the effects of global climate change might be more important,” Padman said.

If the rate of ice and water coming off the Antarctic is accelerating at a faster rate than previously thought, he added, sea levels could rise much faster than previously predicted.

The Star, "Seals with sensors on their heads help map ocean", accessed October 14, 2010
Popular Mechanics, "Sensor-Laden Super Seals Dive Deep for New Global Warming Data", accessed October 14, 2010

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