- A satellite image of the north-east Atlantic has revealed
that medieval cartographers knew much more about ocean currents than was
thought.
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- The ornate Carta Marina, published in 1539, appears crude
by today's standards, depicting sea monsters off the coast of Scotland,
sinking galleons, sea snakes, and wolves urinating against trees.
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- But when oceanographers examined a large group of swirls
and whorls drawn off the south-east of Iceland, complete with ships, a
giant fish and red sea serpent, they found it corresponded with the Iceland-Faroes
Front - where the Gulf Stream meets cold Arctic waters, causing huge swirling
eddy currents that could sweep a ship off course.
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- The earliest known reference of its kind, which suggests
generations of seafarers including the Vikings were aware of ocean eddies,
is reported in the journal Oceanography by a team from the Plymouth Marine
Laboratory and the University of Rhode Island.
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- The cartographer, Olaus Magnus, an exiled Swedish priest
living in Italy, covered the map with ink. But Prof Tom Rossby, from Rhode
Island, believes that not every elaborate quill stroke was artistic licence.
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- "Their location, size and spacing seem too deliberate
to be purely artistic expression. Nowhere else on the chart do these whorls
appear in such a systematic fashion," he said.
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- "They are the earliest known description of large
scale eddies in the ocean - these are huge bodies of water, 100 kilometres
in diameter, that turn slowly. It seems the lines were deliberately drawn
to aid navigation.
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- "We know mariners were aware of these fronts but
they would not have the tools to quantify them nor the means to express
them," he said.
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- The discovery, from research part-funded by the Natural
Environment Research Council, followed a discussion of the Iceland-Faroes
Front at a workshop in Bergen, Norway. Shortly after the meeting Prof Rossby
read Cod, the international bestseller by Mark Kurlansky, which contains
an illustration of the Carta Marina.
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- "When I turned the page and saw the map I said,
'holy s**t! These are identical to our satellite images'. I don't think
I would ever have registered this had I not been in Bergen."
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- Dr Peter Miller from the Remote Sensing Group at Plymouth
Marine Laboratory provided more accurate satellite information on water
temperatures. "Things got exciting when I was able to provide Tom
with an image of the eddy field. The data confirmed Tom's theory that the
swirls on the map were not artistic licence," he said.
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- The satellite image shows how waters from the south,
shown in orange and red, can be as much as five degrees warmer than the
cold currents from the north, marked in purple.
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- At the point they meet, these huge eddies form, revealed
as a blue border. "Sailors would have been aware of these large rotations
of water as they affected navigation," he said. "They would notice
a change in colour of the water too. The cold currents to the north are
generally greener than the Atlantic water to the south due to a greater
abundance of plankton."
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- At the front, deep nutrient-rich waters move up to the
surface supporting phytoplankton and grazing zooplankton. "This ready
food supply brings pilot whales and other marine creatures to the front
to feed," said Dr Miller.
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- The Carta Marina took 12 years to complete and contains
an extraordinary amount of information. The list of towns, lakes and regions
is far more comprehensive than any map before well into the 17th century.
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- It is one of the first maps to give Finland and parts
of Russia roughly correct proportions and it is the first map to fully
portray the Baltic Sea, the Finnish Gulf and the Gulf of Bothnia in the
north.
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- Northern Scotland, the Hebrides, Orkneys, Faroes and
Greenland are described in detail but so, oddly, is a non-existent island,
Tile. This island may be related to the mythical northern community Thule.
To the ancient Greeks, Thule was the northernmost habitable region of the
world. Curiously, its location on the map puts it near St Kilda in the
Hebrides.
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- The map reveals details of shipping routes at the time
and warns sailors of drift ice in the north - illustrated by a stranded
polar bear on a floe. Whales, sea lions, walruses, crabs and lobsters are
also depicted.
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