"When people think of mountains most bring to mind steep, towering slopes with jagged peaks, as in some parts of the Alps”, says John Jansen. “But many mountain ranges have significant areas that are flat-topped.” The Australian geoscientist with European roots is currently at the Institute of Earth and Environmental Science to pursue an exciting geological question: What actually causes flattish surfaces, or plateaux, to form in high mountains? The answer could reveal not only a lot about the history of mountains and their evolution, but also challenge a theory that has prevailed for more than a century.
“The existence of high plateaux has traditionally been interpreted as evidence for tectonic uplift,” says Jansen. “The flattish topography is usually thought to have formed long ago when that landscape was close to sea level followed by rapid tectonic uplift to its present-day high elevation”. But more recently, geoscientists have proposed alternatives to tectonism for explaining some plateaux. Climate is the other major process that shapes Earth’s surface, and at high-latitudes mountains have experienced a very long history of cold climate dating back more than 10 million years when Earth experienced accelerated cooling. “Cold climate processes, such as glacial erosion and frost action, might also have the capacity to develop flattish topography in situ without any involvement of tectonics”. The goal is now to test this hypothesis and to do so, this Australian has left his sunny homeland for a research project that will take him and his team to remote parts of the Scandinavian Mountains. “Nature is already diabolically complicated, so it often helps to study slightly simpler systems where fewer things are going on all at once. This helps with targeting the fundamental processes that drive landscapes”. Scandinavia presents a natural laboratory to examine the role of cold-climate processes, because the region has not been subject to large-scale tectonic uplift.
The mountain felsenmeer plateaux make for a rather hostile workplace, but Jansen seems to thrive in such environments. He clearly relishes the combination of intellectual and physical labour that comes with a career in geoscience: “We walk up steep mountains with heavy gear, dig holes, walk back down with even heavier packs filled with stones and sand for the lab; it’s insane but fun too”, he laughs. “Best of all are probably the weeks spent in far-flung places with comrades who can reliably work hard and have fun while doing so. This produces good science”.
He is also grateful for his good fortune. “I’ve been very lucky to have spent the years since gaining a PhD working with some very clever folks in many wonderful landscapes. I especially like dry, stony places, but seem always to wind up in cold, wet spots,” he jokes about Norway and Scotland, where he was based at the University of Glasgow for six years. The time in Glasgow signalled a major shift in Jansen’s interests. Up until that time he was primarily a fluvial geomorphologist (a specialist in desert rivers) and had barely seen a glacial landscape let alone worked on one. “The Scottish Highlands opened up a new world of processes associated with ice”. That work involved studying waterfalls and what they reveal about how rivers respond to glacio-isostatic rebound, which is the rapid uplift of Earth’s crust following ice sheet decay. “We applied cosmogenic nuclides in a neat way that allowed us to measure the rates at which knickpoints (waterfalls) were migrating upstream and therefore deepening valleys.” Cosmogenic nuclides by the way are formed in surface rocks due to bombardment by cosmic rays from exploding supernovae. Jansen describes that some of his work involves highly sophisticated technology and collaboration with geochemists and accelerator physicists. “Using cosmo is way out and especially funny as I can’t even operate a telephone properly!”.
The research in Scotland led naturally to Scandinavia. “Actually, it’s glacio-isostatic rebound that first got me interested in Scandinavia. The Scottish Highlands, around Loch Linnhe, are rebounding today at about 2 mm per year, which is fast, but northern Sweden is rocketing up at 10 mm per year!” Such uplift rates are among the fastest anywhere on Earth, but yield just a few hundred metres of uplift in total—nothing like the kilometres of uplift experienced by mountain ranges such as the Himalayas or the Andes. As it turned out, the Swedish rivers lack the erosional power to down-cut and counteract this uplift and instead get carried up with the rebounding landscape. Jansen found that most of the erosion, in fact, occurs under the ice during deglaciation when huge volumes of meltwater and sediment combine to cut bedrock gorges in just a century or two. “This was a surprising finding and we published the results last year in Nature Communications”.
Another string to Jansen’s research involves extreme events. In April he travelled to Nepal along with fellow Geohazards Group members: Amelie Stolle and Wolfgang Schwanghart to help with their project examining cataclysmic floods. “It’s not easy to imagine such a gigantic event, but roughly 5 cubic kilometres of gravel and sand were ejected from the Annapurna massif into the Pokhara valley—possibly involving a combination of earthquakes, landsliding, and the collapse of a temporary lake high up in the mountains”, he explains. “Most amazingly, this all happened just 800 years ago, which is just yesterday in geologic terms”. Such recent events are a reminder of the colossal upheavals that frequently characterise this part of our planet. Indeed, Jansen and his colleagues were fortunate to depart Kathmandu just days before the massive earthquake struck on 25th April.
Extreme erosion and deposition during deglaciation is one of Jansen’s major research themes. Along with Martin Margold at Stockholm University, he is documenting the size and timing of floods associated with the collapse of a huge ice-dammed lake in a remote part of Siberia. “There were several floods more than 200 m deep flowing down the Vitim-Lena river system to the Arctic Ocean just as the big ice sheets were breaking up at the end of the last two ice ages”. Big influxes of freshwater into the Arctic Ocean have the potential to trigger major climate feedbacks in the northern hemisphere. Jansen makes the point here that “We know that climate drives rivers and glaciers via precipitation, but in the case of the Siberian superfloods, surface processes might have driven changes in Earth’s climate system. It’s complicated and endlessly fascinating”.
The fieldwork in Russia also feeds Jansen’s political interests. “I’m very interested in totalitarianism; its development and aftermath, perhaps”, he suggests, “due to reading too much Orwell and Kundera as a teenager”. Russia was not an easy place to work back in 2012, but in light of more recent events we’ve put further work on hold until change finally comes and who knows when that will be”, he says grimly. Of course living in the former GDR offers another historical perspective, which clearly inspires Jansen too. “Living in amongst the last 80 years of German history means a lot to me. My father is Dutch but his father’s family came from Germany.”
Before moving to Potsdam last December, Jansen had been working in Scandinavia for some years already. After Glasgow he moved on to a postdoc at Stockholm University and it was there that he became familiar with the long-standing theory that sees Norway’s mountain plateaux as remnants of uplifted peneplains whose flatness was established close to sea level more than 100 million years ago. The flattish summit areas are termed the Paleic Surface, which literally means ancient, and Jansen agrees that “they probably do have very ancient roots, but I question whether such areas were ever close to sea level, and as for Mesozoic peneplains I think it’s fair to say that the evidence is rather thin”. “Such ideas go back to the work of W.M. Davis more than a century ago, yet still carry undue influence especially in some far corners of the geoscience community”.
In fact, Jansen questions the whole idea of describing erosional landscapes in terms of age. “It’s a question that’s more usefully framed in terms of erosion rates, not ages. When you think about it, nearly every part of a mountain landscape is eroding: some regions, like Taiwan, are eroding at more than five km per million years, whereas others like central Australia are eroding at less than one metre per million years”. Jansen suggests that the impulse for ascribing an age to a given landscape surface is outmoded, and lacks real meaning for contemporary landscapes, except in rare cases. “Advances with cosmogenic nuclides and thermochronometry mean that erosion rates can be determined very precisely over a wide range of timescales. This has been a revolutionary step forward for the geosciences and we are applying these new approaches to understand the evolution of the Scandinavian Mountains.”
The Scandinavian plateaux are certainly eroding slowly, but even slow erosion has an effect over very long time spans. Over the last 2-3 million years glaciers have cut valleys and fjords more than 2 km deep in some places, but the felsenmeer plateaux extending between deep valleys have been subject to intense frost action over more than ten million years. “We know that freeze-thaw processes break up rock and transport it downslope via a diffusion-like process known as frost creep”. “We’re calibrating mathematical models with field-based analyses and cosmogenic nuclide measurements to test the hypothesis that frost action can cause topographic smoothing over many millions of years. In other words, the plateaux topping high-latitude mountain ranges, such as in Norway and Greenland, might have developed via cold-climate processes and need not be linked to tectonics at all”.
Thanks to a BRAIN-Marie Curie Fellowship, Jansen joined the group of Oliver Korup, Professor for Geohazards at the Institute of Earth and Environmental Science, last December. “Oliver has assembled a talented group of researchers. Here in Potsdam there’s an expert in virtually every branch of the geosciences”, he enthuses. In addition, Jansen will be working with colleagues in Denmark, Sweden and Australia. This international project demands a cosmopolitan lifestyle with a lot of movement, but “most importantly”, he says, “I always try to travel with my old three-speed bike. It’s heavy like a tractor and perfect for snowy winter days and for blowing off steam in the woods nearby”.
The Researcher
John D. Jansen, Ph.D. studied geology at the Bendigo College. He received his doctoral degree at Macquarie University in Sydney in 2001. He worked at the University of Wollongong, the University of Glasgow, the University of New South Wales and the Stockholm University. Since December 2014 Jansen has been a Marie Skłodowska-Curie Fellow at the Institute of Earth and Environmental Science at the University of Potsdam.
Contact
Universität Potsdam
Institut für Erd- und Umweltwissenschaften
Karl-Liebknecht-Str. 24–25, 14476 Potsdam
E-Mail: john.jansenuuni-potsdampde
Text: Matthias Zimmermann/John D. Jansen
Online-Editing: Agnes Bressa
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