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Language: German with an English summary
The Rudolfstollen, an air raid shelter tunnel in Urfahr, Austria, was dug into the rocks (Urfahrwänd) of a distinctive Danube gap at the western margin of the city of Linz almost at the end of World War II. It is a remarkable, historically but above all scientifically interesting subterranean object. The tunnel is more than one kilometre long and marks a dark chapter of Austria's recent history. Yet, after decades of undisturbed development without interference by man, Rudolfstollen offers us the opportunity to study the "recapture" of an artificial object by nature and the phenomena involved. As opposed to many similar tunnels, where decay is the predominant stage of development, very few traces of decay can be found in the Rudolfstollen. On the other hand, during the last decades vast parts of the tunnels have become decorated by respectable dripstones, large flowstone covered surfaces and a variety of smaller speleothems that stand up to any comparison with the alpine karst caves.
But all these speleothems owe their significance less to their forms or beauty than to their genesis: According to the geological and hydrogeological set-up and the present results of investigations, speleothem development in the Rudolfstollen is not based on the classic mechanism of limestone dissolution by CO2-rich seepage water and precipitation of CaCO3 in the cavity but on an intensive weathering of Ca-rich feldspars of the bedrock ("pearl gneiss") and precipitation of the carbonate after buffering of the seepage waters before the tunnel is reached. This phenomenon has not been recorded in Austria so far. According to the hydrogeological findings, the last of the above mentioned criteria is due to the flowing distance of underground water from the higher parts of the Pöstlingberg area to the Rudolfstollen – a distance surprisingly long in terms of time and space. Direct drainage, however, does not play an important part. The growth rate of the speleothems is many times higher than that of alpine karst caves.
Unlike the eastern and central parts of the tunnel, the western part is rather dominated by brown and blackish flowstones as well as black, usually thin, up to 10 cm long tubular stalactites (straws). Here the extremely acidic (pH 2.6) seepage water is dripping off very slowly, its chemistry indicating a locally restricted but intense weathering of pyrite that induces a mobilisation of iron and manganese as well as of various heavy metals, and – above all – aluminium.
Another one of Rudolfstollen's special features is the occurrence of "root stalagmites", meshes of hair roots which grow from intruding roots upwards aiming towards the dripping water in cavities close to the entrance. There are only ten such sites known in Austria so far, and Rudolfstollen was the second one to be documented. Shape and activity of root stalagmites change with varying water supply, e.g. when water stops dripping or starts dripping from another point. From the discovery (2007) until 2011 the first root stalagmite grew steadily to 8 cm of height, and new, somewhat exotic looking specimens developed; since 2012, however, most of them have been degrading.
The fauna of the Rudolfstollen seems unspectacular at first sight but shows remarkable variation: 74 taxa have been identified so far, but, as might be expected, no troglobionts.
The subterranean climate is well balanced in most parts of the tunnel in terms of time and space due to the favourable morphological and geological framework and also due to the mostly tight shut-off. Not least because of this, apart from now being used as a valuable "under ground laboratory", the Rudolfstollen may well serve other purposes, e.g. tourism or health/speleotherapy.