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Medieval sea chart was in line with current thinking
(Filed: 04/05/2004)

The 16th-century Carta Marina, complete with sea monsters, gives an accurate location for dangerous eddies. Roger Highfield reports

A satellite image of the north-east Atlantic has revealed that medieval cartographers knew much more about ocean currents than was thought.

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.

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.

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.

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.

"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.

"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.

"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.

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.

"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."

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.

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.

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."

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.

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.

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.

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.

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.

Carta Marina
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