Tales of a Thermochronologist


Antarctica is a continent of superlatives. It is the most remote and inaccessible landmass on Earth and is the coldest, windiest, highest, and driest of the seven large continents. An ice cap comprising nearly 70% of the world's fresh water covers approximately 98% of the antarctic surface. Clearly, there is no place on Earth less inhabitable and more hostile to working outdoors than Antarctica.

These are the facts I learned about Antarctica when I was a school boy. Like most young boys I was captivated by the intrepid stories of the early antarctic explorers, by the tales of expeditions by Scott, Amundsen, Shackleton, and Mawson. Being interested in earth sciences I started working in a mine and studying geology at the traditional mining university Bergakademie Freiberg. My favorite lessons in Freiberg were given by Prof. Joachim Hofmann, one of the few East German geologists who was able to participate in Russian antarctic expeditions. He worked for many years on the Lambert Graben, which cuts across the East Antarctic continental margin. This is a vast rift valley, just like the East African rift valley, but occupied by the largest outlet glacier in Antarctica. Fifteen years later, I too am fascinated by this same geological feature.

I grew up in the former GDR and initially saw little opportunity for ever "going south". However, when I finished my studies in 1991, the wall was no longer a barrier, and I was able to apply for a Ph.D. position advertised by Prof. Martin Olesch at the Universität Bremen. The topic of the project was to study the landscape history of Marie Byrd Land and northern Victoria Land, which lie along the Pacific coast of Antarctica. Fortunately for me, one of the tasks I was assigned was to go and collect the rock samples myself! And so my first experience of polar field work was to participate in GANOVEX VII (German Antarctic North Victoria Land Expedition) of the Bundesanstalt für Geowissenschaften und Rohstoffe ( BGR).

I soon learned that the field conditions in Antarctica are not optimum for comfort. It takes quite some effort to leave a cozy sleeping bag in an unheated tent in the morning, and it is not very tempting to enter it (now considerably less snug) again in the evening. The food--drinks, bread, eggs, and everything else--is frozen and must be warmed up: in the sleeping bag, on top of the hot coffee mug, in a warm water bath, or in any other way available. Not to mention the shower, which is surely a most bracing experience!

Although the conditions are harsh, working as a geologist in Antarctica is something really special. One of the most tragic, and most impressive, examples of this geological fascination was a sledge with 16 kilograms of rock samples, still intact, after being man-hauled for hundreds of kilometers by an exhausted polar party, that was found in 1912 beside the tent containing the bodies of Robert F. Scott, Edward Wilson, and "Birdie" Bowers. No other continent has a population that consists mainly of geologists, and nowhere else is every second hill named after a geologist. So what makes Antarctica such a particular Mecca for earth scientists given that less than 2% of its surface is ice free?

The secret is that rocks exposed across this measly 2% are in pristine condition, without vegetation or soil cover, and with no accompanying anthropogenic pollution. Antarctica is also a unique laboratory for studying glacial processes, and is the best archive by far of information on past climates. Moreover, Antarctica was the heart of Gondwana. All other fragments of the former supercontinent were successively detached from Antarctica, causing the Antarctic continent to be surrounded exclusively by passive (non-compressional) margins. Extensional forces acting at the margins, and radially throughout the antarctic interior, formed a particular type of mountain range, quite unlike the "usual" mountain ranges such as the Alps and the Himalayas.

Some of these mountains reach elevations that exceed the altitude of the highest European mountain peaks. Deciphering the long-term landscape evolution of these rift-related mountains is still in its infancy. The focus of my current research is to study these rift mountains using thermochronological methods such as fission track analysis. The technique makes use of accumulated lattice damage (fission tracks) that occurs in minerals due to the natural radioactive decay of trace amounts of uranium present in the mineral. These tracks are only preserved at temperatures below a critical threshold temperature that is specific for each mineral. When analyzing the number and lengths of fission tracks, one obtains information about the temperatures a rock sample has experienced in the past.

The temperature of Earth's crust increases systematically with increasing depth so that rocks deep within Earth are at higher temperatures than those near the surface. The process of erosion causes rocks at depth to be brought gradually to the surface and consequently they experience a gradual cooling as they move from deeper to shallower levels. Fission track analysis allows geologists to reconstruct the cooling history for rocks and consequently provides critical information about the rates of erosion.

A dense network of fission track data is necessary and needs to be complemented by, and combined with, a whole range of other types of geological and geophysical data to reveal the complete history of the antarctic rift mountains. This broad, integrative approach to geological research in Antarctica epitomises modern science more than perhaps any other field. This is not only because of the highly sophisticated methodologies applied and the complex logistics or equipment needed, but also because it requires a willingness and ability of scientists of many different disciplines, nationalities, and cultures to interact closely and constructively. Antarctic earth science sets a refreshing example of how international science can be done with mutual camaraderie, as I have been fortunate enough to experience during my antarctic expeditions.

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