It was a strange world that an international research team discovered in the middle of the Atlantic, some years ago. On a dive with the small submarine "Alvin" to the summit of the Atlantis Massif, scientists discovered a collection of white, sometimes strangely shaped chimneys. They called this location, which covers an area of 400 times 400 metres, "Lost City". A group of researchers is now returning to Lost City to investigate the structures more closely (1). Gretchen Bernasconi-Green from the ETH Institute of Mineralogy and Petrography (2), a member of the original expedition, is also taking part in the second.

No signs of magma

"Lost City" is located at latitude 30°N, 15 kilometres from the summit of the Mid-Atlantic Ridge. Some of the chimneys are as high as 60 metres and composed of carbonate and magnesium hydroxide minerals. The formation of the structures is due to leaky flows of warm water from under the seabed and precipitates carbonate from the sea water in these places. "What is special about Lost City is that this hydrothermal process is not driven by hot underground magma," explains Bernasconi. And she adds, "Rather, it can be put down to a mineralogical reaction." This is in marked contrast to other such chimneys, so-called "Black smokers", that have been formed by sulphide minerals and due to the magmatism of the Mid-Atlantic Ridge.

Making serpentine from olivine

The spot on which "Lost City" stands is where tectonic activity has pushed rock from below to the earth's mantle. Once these rocks come into contact with water, they start to cool down and a hydration process begins. The mineral olivine is transformed into serpentine and heat is released at this stage of the process. This, in its turn, induces a hydrothermal cycle, which leads to the formation of the white chimneys.

Until now other structures that can be compared to those of "Lost City" are hardly known. Nevertheless, the interest in the white chimneys is not merely idle curiosity, as Bernasconi says. "In the early stages of our planet's formation, in the Archaean period, such structures were probably quite common. As the hydrothermal water surrounding Lost City is very base and only about 40_75°C, these sedimentary deposits might have been an ideal habitat for early forms of life. Indeed, already on the first expedition, scientists discovered manifold microbial life.

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"Lost City" consists of numerous strange, white chimneys, some of which are 60 metres high (picture: G. Bernasconi-Green). large

Living in "Lost City"

In the coming weeks the team plans to make a total of 18 dives, once again with the submarine "Alvin" (3). The descents will start from the research vessel "Atlantis" which is run by the "Woods Hole Oceanographic Institution" (4) and a private US research institute. The scientists want to examine rock and water samples and, especially, the microbial life of "Lost City". Bernasconi herself, together with an ETH graduate student, wants to study hydrothermal processes. One thing she wishes to ascertain is the age of the rocks, which will help to determine how quickly the outer crust of the rock is transformed and the length of time that such systems stay active. Interested parties can keep up with the expedition via the internet. An internet link-up is planned so that secondary school pupils can put questions about the project to the scientists. A class from the Kantonsschule Rämibühl in Zurich will be participating in this dialogue for Switzerland.