The culprit is believed to be something called “Circumpolar Deep Water,” which, in the upside-down world of the South Pole, is a layer of warm salty water (warm being a relative term, as it’s only slightly above the freezing point) that lies deeper than a very cold water layer nearer to the surface. The idea is that this Circumpolar Deep Water has been sneaking up onto the continental shelves that surround Antarctica and lapping at the bases of marine glaciers, melting them from below.
From a scientific perspective, what we’re talking about is one of the hardest phenomena to observe on the Earth. You have to somehow be in Antarctica; you have to be able to cross a vast field of sea ice; and then, you have to be able to travel hundreds of meters under the freezing ocean surface to boot. This is, needless to say, not a task very well suited for humans — not even even when they have extremely expensive ocean vessels equipped with subsea robots.
A new study in Geophysical Research Letters, though, documents a new and ingenious way of solving this problem using marine mammals that dive to the relevant depths, and, in effect, turning them into scientists by proxy.
To study the behavior of Antarctic waters, the research team, led by PhD student Xiyue Zhang of Caltech, turned to something called MEOP — Marine Mammals Exploring the Oceans Pole to Pole. The program has installed non-invasive or scientific instruments, called tags, on southern elephant seals — the world’s largest pinnipeds — and other marine mammals in both the Arctic and Antarctic. And the result is 300,000 measurements of key ocean variables, and scores of scientific publications.
The program is part of a much broader trend toward using animals on land and at sea as monitors of the environment — a trend that has been enabled by technological advances that have led to sensor technologies that are so small as not to be invasive. “New technology has brought the study of animal movement into the realm of big data, and exponential increases in data volumes are expected to continue in the coming decade,” wrote one researcher in the journal Science last year, when the outlet published twin articles devoted to the subject of using animals as observing systems.
When it comes to the current research, the seals are outfitted with tiny sensors atop their heads, which do not interfere with their swimming behavior. “It’s really cool, and it usually lasts for about six months, and then when they molt, it falls off,” Zhang said. The sensors send data about depth, temperature and salinity of the waters the seals are swimming through.
Southern elephant seals are amazing creatures — they can stay underwater for as much as two hours and dive to depths of more than a mile. They swim down to collect food at the seafloor of Antarctic continental shelves, living off of fish and squid, spending the vast majority of their lives in and under the water.
The current study used nearly 20,000 dive measurements, from elephant and Crabeater seals, to study the water characteristics in the Bellingshausen Sea, off the coast from a string of glaciers that separate the Antarctic Peninsula from the fast melting Amundsen Sea region of West Antarctica. Particularly important is that many of the measurements were taken in winter, when the region is even more inaccessible than usual to human observers.
Beneath the surface, the Bellingshausen Sea is home to a feature called the Belgica Trough, an extremely deep underwater canyon that was dug into the seabed long ago by a vast stream of ice when Antarctica was even more massive than it is today (and the Earth, most assuredly, colder than at present).
When ice filling the trough retreated, it left behind a subsea landscape that reaches depths of more than a kilometer, and extends inland toward the Antarctic mainland for 250 kilometers. That’s more than deep enough for Circumpolar Deep Water to enter the trough and potentially travel toward the base of ice shelves and glaciers along the Bellingshausen coast — such as the Venable glacier, whose ice shelf has been observed to be melting steadily of late.
Sure enough, the study found that in general, Circumpolar Deep Water rests offshore between 400 and 800 meters below the ocean surface — but it was also making its way into the Belgica Trough, albeit at somewhat cooled temperatures (but still, warm enough to melt ice shelves). More specifically, the research found that the warm water entered the trough on its eastern side, formed a large and swirling undersea cyclone as it mixed with cooler water, and then traveled toward the icy coast. Then, it traveled back again and exited on the western side.
“What we found is that near the ice shelf, in the continental shelf in general, the water is as warm as about 1 degree C beneath the surface,” Zhang said. “And given that the melting point of seawater is about -1.8 degrees, given that there’s salinity in the water, this is already about 3 degrees warmer than the melting point of ice in that region. So this tells us that the heat of the ocean is able to melt the ice shelf in the region.”
Overall, then, the study certainly validates — via seals — the idea that warm water can get up onto the continental shelf and near the marine glaciers of the Bellingshausen Sea. Granted, in a region where little about the ocean is known, there is much more to discover. One key thing the study could not show was whether there was a trend, over time, in the waters’ behavior.
But for more research, well, there’s plenty more seal data available.
“Given that we have a lot more data produced by these seals, I think people could use more data to study the Southern Ocean,” Zhang said.
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