Unraveling erosion in the Andes

Marine deposits reflect geological processes on land

Sediment flags in the sea show how the rivers transport erosion material from the Andes. © SeaWiFSProject, NASA / Goddard Space Flight Center and ORBIMAGE
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Andean erosion in the last 24, 000 years has crucially depended on the amount of rainfall that fell there. Scientists have found this out in the analysis of marine sediments off the Chilean coast. They were also able to show that ocean sediments are very well suited to trace the environmental history of the South American Cordillera since the height of the last ice age.

In addition, the sediments show that, especially in the southern Andes, the precipitation amounts between hot and cold periods varied considerably, according to the researchers in the current issue of the science magazine Geology.

The Andes are one of the most imposing mountain ranges worldwide. They cover 7, 500 kilometers along the South American Pacific coast. The mountains are unfolded by the collision of two earth plates. For about 150 million years, the Pacific Nazca plate has been slipping under the continental South American plate. Seaward, this crumple zone is characterized by up to eight kilometers deep underwater ditch. From this deep-sea trench, the Andes rise steeply at heights of almost seven kilometers.

Creeping erosion

Like every mountain range, the Andes are also subject to a gradual erosion. Rain and meltwater penetrate into fine hairline cracks, freeze, expand and burst the rock. As this process - like the formation of the ore itself - stretches over millions of years, it is not easy to determine erosion rates.

In the geolaboratory of the research vessel SONNE, the sediment cores embedded in gray plastic cylinders are examined. © MARUM, University of Bremen

"We wanted to find out to what extent the erosion rate differs between hot and cold periods, " explains Professor Dierk Hebbeln from MARUM in Bremen, who was involved in the study together with Potsdam scientists. "That's why we sampled ocean deposits on two expeditions with the research vessel SONNE off the Chilean coast, " says marine geologist Hebbeln. Based on the sediment cores obtained from water depths of up to 4, 000 meters, it was determined in the domestic laboratories how much material is deposited in front of the Chilean coast per thousand years, ie by how many centimeters the seabed grows during this period. display

"Of course, we had to clean up these so-called sedimentation rates by the proportion of deposits that originate from the sea itself, after all we were only interested in the erosion debris from the Andes, " explains Hebbeln.

Precipitation is crucial

For the period since the peak of the last ice age, the scientists found that erosion on land and sedimentation rates in the sea were significantly influenced by rainfall. For example, over the last 24, 000 years, Chilean wetlands have had an average of ten times more erosion than they had to the north before the extremely dry Atacama desert.

More deposits during the cold period

If one compares the erosion between the last ice age and the present hot period, it can be seen that the deposition rates during the cold season were up to three times higher than in the current warm period. "That is due to a climate-related shift in precipitation areas, " notes Hebbeln. "Today, the westerlies only bring significant rainfall to the south of Chile.

During the last ice age, however, the Antarctic ice sheet expanded. As a result, climatic zones, including the rain-bringing westerly wind zone, moved north for about 550 kilometers towards central Chile. "

Patagonian ice sheet with important role

According to the results of the researchers, the marine deposits also reflect regional climatic phenomena: they show that the sedimentation patterns in southern Chile were influenced not only by increased precipitation but also by the Patagonian ice sheet that covered this region about 20, 000 years ago.

"We show with our work that the marine deposits very well reflect the geological processes on land", summarizes Hebbeln: "Worldwide, many sedimentary cores stored in mountain science institutes are stored in front of mountainous coastlines also trace the erosion history of other mountains - without having to organize a new ship expedition. "

(idw - MARUM_Forschungszentrum Ozeanr nder, University of Bremen, 14.02.2007 - DLO)