Satellite imaging techniques are shedding new light on an ancient map of the northeast Atlantic

The ornate map, seemingly crude by today’s standards, depicts sea monsters off the coast of Scotland, sinking galleons, sea snakes, and wolves urinating against trees.

When oceanographers from Plymouth Marine Laboratory and the University of Rhode Island compared a large group of swirls, shown on the chart off the east coast of Iceland, with thermal images from an Earth observation satellite they found the swirls corresponded almost perfectly with the Iceland-Faroes Front – where the Gulf Stream meets cold waters coming down from the arctic.

The cartographer, Olaus Magnus, an exiled Swedish priest living in Italy, had a dislike of blank canvases and covered every available space with ink. But Professor Tom Rossby, from the University of Rhode Island, believes 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, ’

’They are the earliest known description of large scale eddies in the ocean – these are huge bodies of water, 100km 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 work, part funded by the Natural Environment Research Council, came about while Tom was discussing the Iceland-Faroes Front at a workshop in Bergen, Norway. Shortly afterwards Tom was reading ’Cod’, the international bestseller by Mark Kurlansky. The book 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.’

Tom explained that in 1539 people did not understand the importance of some of the details in the map.

’The whorls in the map quickly lost significance. When the map was reissued in 1572 everything, including sea monsters, was faithfully reproduced, but the huge eddies had disappeared.’

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 preceding it or following until well into the seventeenth 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 oceans contain a similar richness of information. 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. The giant sea snakes and other monsters are taken directly from the imagination of fishermen and sailors, though some researchers suggest these monsters may reflect commercial tensions at the time between the Hanseatic League* and England. It has also been noted that the monsters are only attacking vessels from countries that had joined the reformation.

Dr Peter Millar from the Remote Sensing Group at Plymouth Marine Laboratory became involved in the project when Tom needed accurate thermal information on the water temperatures in the region.

’There is always a great deal of cloud over the northeast Atlantic so it is difficult to get a clear image of the whole region. I merged images over the course of a month to get a complete picture. 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,’ said Peter.

Peter explained that the waters coming up from the south can be as much as five degrees warmer than the cold currents from the north. At the point they meet these huge eddies form. Sailors would have been aware of these large rotations of water as they affected navigation. When a heavy, slow moving craft sailed through the front its course would be affected by as much three points on the 32 point compass used at the time.

Peter said, ’They would notice a change in colour of the water too. Warm water from the south is a darker blue than cold water. The temperature difference means marine life will vary across the front and, right 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 area to feed.’

Tom has started a new investigation of these fronts working collaboratively with the Institute of Marine Research in Bergen.

’We want to know more about the currents and movements of these waters. We are laying down subsurface drifters that will be tracked acoustically. This is to see how waters get drawn into the region and organise themselves into the front. These floats will tell us how these waters spread out into the Greenland Sea and Norwegian Sea.’