Senckenberg Research

Fossil rain tells a story

Monte Leone, 3552 m, view from the central Swiss Alps.
Monte Leone, 3552 m, view from the central Swiss Alps, footwall of the Simplon Fault Zone, Lepontine Alps. (Copyright: M.Campani)

 

 

The Alps have been influencing precipitation patterns in Europe and Central Asia for 15 million years. Researchers from the Biodiversity and Climate Research Centre (BiK-F) have extracted this information, as well as many other facts, from their studies of "fossilized raindrops". The isotope compositions of ancient raindrops are preserved in water contained in solid rock.

Thin section photograph of deformed white mica in a ductile shear zone.
Thin section photograph of deformed white
mica in a ductile shear zone. In such
environments fractures may transport surface
derived water either from rain or snow to
larger depths. The oxygen or hydrogen
isotope ratios of such waters can be
recovered from hydrous minerals by means
of stable isotope geochemistry.
(Copyright: Andreas Mulche)

They may appear ‘rock solid’, but even the highest peaks of the Central Alps are quite young – in geological terms. Scientists from the Biodiversity and Climate Research Centre (BiK-F ) have compared the isotope patterns of water in rocks from the Alps and their foothills in order to determine how high the mountains were in the past. It appears that 15 million years ago Europe’s highest peaks were already about as high as they are now.

What kind of geological and palaeoclimatic information can we extract from ancient raindrops even today?

The snow-capped summits of the earth seem to have an unlimited capacity to impress us. As indeed do stories of research and discovery, in which observation of living beings and geological formations have led to theories about how the earth came into being and brought forth such a huge variety of species. Mount Everest, Nanga Parbat and Mont Blanc are much more than interesting postcard motifs. The high mountain ranges have always represented key regions that exert great influence on the interrelated realms of the geosphere, the atmosphere and the biosphere. Despite the huge significance of mountains for our climate, the topographic history of most of the mountain ranges on the planet is largely unknown – and therefore the same applies to the many and various interactions that occur between the processes on the surface and climate dynamics.

Odd though it may seem at first, the story of how the mountain ranges developed can be read from fossilized raindrops. A recent study on palaeoaltimetry, i. e. modelling of the historical heights of mountain ranges, shows that the current morphology of the Central Alps is the result of a process that has been going on for at least 15 million years. Many Swiss peaks were between 2,850 and 3,350 metres high.At that time, when the rate at which the mountains were growing (being pushed up through the collision of Africa with Europe) exceeded the degree to which they were being eroded, the Alps were even higher than they are today.

How much do the Alps influence the Eurasian climate?

Camoscellahorn, 2612 m, central Swiss Alps, panorama of the Simplon Fault Zone.
Camoscellahorn, 2612 m, central Swiss Alps, panorama
of the Simplon Fault Zone.

High mountain ranges form a natural hindrance to humid masses of air, influencing the climate on both sides of the so-called ‘orographic barrier’. Here, the decisive factors are the height of the range and the degree to which it spreads out. Therefore the latest findings being established by Prof. Dr. Andreas Mulch’s working group allow conclusions to be drawn about precipitation patterns in southern Europe and Eurasia, and hence indirectly about the development conditions prevailing for whole ecosystems in the Mediterranean region as well. In the case of the Alps, this means that for the last 15 million years, precipitation that has originated in the Atlantic and is on its way towards Central Europe and Eurasia has been significantly influenced by the presence of the Alps, and this has had a corresponding effect on the Eastern Mediterranean climate. It appears that the Mediterranean region, recognized as a water shortage hotspot in relation to global warming, has already been subjected to arid spells in the past. Therefore for any forecast involving future water shortage scenarios for this region, an understanding of palaeological precipitation dynamics is of great importance.

Raindrops as witness es of the past

The method used by the BiK-F scientists is relatively simple: It is possible to deduce how high mountains have been in the course of their development by analysing the isotope composition of the precipitation at various altitudes. The isotope composition becomes ‘locked’ in the stone, and with it information about precipitation quantities and content, amongst other things. This analysis approach depends upon the fact that isotopes of both oxygen and hydrogen occur that have differing masses. With increasing altitude, the proportion of the heavy isotopes to be found in the precipitation declines in a predictable manner. The proportion of the heavy (18O, D) isotopes to the light (16O, H) isotopes in the rock therefore provides a direct indication as to the altitude at which the precipitation occurred. This enables us to determine the relative difference in the height of the high mountains to the Alpine foothills 15 million years ago.

Palaeoclimatic reconst ruction: an increasingly imp ortant research focus at BiK-F

A junior research group headed by Dr. Eva Niedermeyer is conducting research on tropical monsoon systems in Africa and Indonesia based on the isotope composition of the precipitation. In order to better understand the fluctuations in monsoon intensity, the scientists are looking back into the past. There are two benefits to this: On the one hand it is possible to determine the natural variability of tropical systems – i. e. uninfluenced by human activity – and on the other hand the underlying mechanisms can be discovered. In contrast to high mountain regions, the isotope concentration in tropical precipitation is mainly determined by the intensity of the rain.

But how can this information be stored over thousands or even millions of years? Here, the BiK-F researchers avail themselves of a property of leaf wax in higher land plants. These ‘biomarkers’ consist of extremely stable compounds (such as long-chain fatty acids, alkanes and alcohols) that resist microbiological degradation and can therefore remain unchanged over very long periods of time. As the hydrogen found in plants originates mainly from precipitation water, the hydrogen isotope composition of leaf waxes allows us to draw conclusions about the intensity of the precipitation at the time when the plants were subjected to monsoon rain.

Leaf wax, soils, sediments and even rocks – all of these allow us to study the climatic and precipitation history of our planet through the implementation of geochemical isotopic methods.

  

Author

 

Prof. Dr. Andreas Mulch 

Prof. Dr. Andreas Mulch was appointed as joint professor between Senckenberg and the Goethe University Frankfurt in 2010 and since then is Co-director of the LOEWE Biodiversity and Climate Research Centre (BiK-F). As member of the Board of Directors in Senckenberg he is in charge of developing the Senckenberg science program. Following his studies in geology and isotope geochemistry he spent several years in the U.S. and was appointed as professor at the University of Hannover in 2006. His research interests center around the interaction of Earth surface processes and climate with a particular focus on the reconstruction of the topography of the world’s highest mountain ranges and continental rainfall patterns.

 

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