Mountains do not disappear faster

Allegedly for five million years continuous increase in sediment deposits does not exist

The high sediment load of the river shows the strong effect of erosion below the Bernina glacier in Switzerland. © GFZ
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Erosion by water and ice as well as the chemical weathering of rocks of the earth's surface remove mountains over millions of years. That's not new. However, an alleged steady increase in the amount of sediment deposits that has been reported by many geoscientists for five million years does not exist. At any rate, this is the key finding of a new study by a Potsdam research team in the scientific journal Nature.

As reported by the team led by Jane Willenbring and Friedhelm von Blanckenburg of the German Research Center for Geosciences GFZ, the four-fold increase in sedimentation observed worldwide is a mere observation artefact. All hypotheses based on it must now be reviewed.

Stronger mountain growth and increased erosion effects

Eroded rock is carried away by rivers and glaciers and is found in the sediment of the oceans and on the continents in the vicinity of large mountains. The global measurement of the thickness of sediment layers has historically produced the result that the amount of sediment that has been deposited per period of time worldwide in the last five million years has continuously multiplied. To produce this surplus of sediment, the mountains would actually have to erode at the same speed.

Hypotheses for the cause of this phenomenon globally imply increased mountain growth and consequently increased erosion effects. But it could also have been the other way around: the mountains had existed long before with high relief, but only a climate shift such as the one that led to the glaciation about three million years ago, has cleared away the mountains faster and in the sequence let the weight relief rise again faster. However, none of the two hypotheses could be reasonably substantiated so far.

Erosion in the Alps: Monte Disgrazia, Italian Alps, 3, 678 meters. © FvBlanckenburg / GFZ

Carbon dioxide and rock weathering

They only lead to another paradox that geoscientists have been unable to solve so far. Reconstructions of the previous concentrations of the greenhouse gas carbon dioxide (CO2) of the atmosphere with indirect chemical and biological methods have shown that these have been fluctuating for over ten million years approximately by the value of the atmosphere even before the beginning of today's rapid increase in CO2 concentration. However, if erosion increases worldwide, the decomposition of rocks must also increase. display

But this consumes constantly low amounts of atmospheric carbon dioxide via the carbonic acid contained in the rain. Strictly speaking, this consumption is just as great as the delivery of this gas to the atmosphere by volcanoes. Over a period of millions of years, nature stabilizes the greenhouse effect of the atmosphere and thus the temperature. If, however, the depletion of CO2 had multiplied as a result of the high level of weathering, today the atmosphere would scarcely contain appreciable quantities of the greenhouse gas. The result would be an extremely cold earth on which all water would be frozen.

The global beryllium cycle F. v. Blanckenburg / GFZ

Beryllium 9 and sediment preservation

The GFZ scientist Willenbring and her colleague von Blanckenburg are now leaving this riddle. They found that the increase in sedimentation velocity is an artifact of observations: the closer geologists look, the more sediment deposits they discover. And you can look into the recent geological past better than in the history of a very long time ago, because even sediment does not always survive the geological change. The older it is, the less it is transmitted. Apparently, the geological sedimentation rate increases the more the geological time and the shorter the observation period. The phenomenon of the increase in sedimentation is thus not real, but merely reflects sediment conservation.

As evidence for this new interpretation, Willenbring and von Blanckenburg now report geochemical measurements in certain ocean deposits. In centimeter-thick iron-manganese crusts, which grow extremely slowly in the ocean over millions of years, the information about the input of substances into the oceans in the past is in the form of changeable metal concentrations. The two researchers now used the isotope of mass 9 of the rare element beryllium to determine the amount of sediment that enters the oceans via rivers. If erosion-induced sedimentation had increased, we would find more of this beryllium-9 in the younger layers of these crusts.

Beryllium 10 as a witness

To prove this effect, Willenbring and von Blanckenburg also used the very rare isotope beryllium-10. This is created in the atmosphere by cosmic radiation in equal amounts and is fed via the precipitation to the oceans. As the iron-manganese crusts grow, beryllium-10 is incorporated into the iron-manganese crust in constant amounts. If the ratio of beryllium-10 to beryllium-9 varies, this is only due to changes in the entry of the erosion-derived beryllium-9.

As measurements show, the ratio of the two isotopes in the iron-manganese crust of all oceans has barely changed in the last ten million years. As a consequence, according to the scientists only one conclusion remains: the erosion of the continents has been stable over the past few million years, there has never been an increase. Thus a geological assumption has fallen, but at the same time a puzzle has been solved.

(idw - German Research Center for Geosciences GFZ, 14.05.2010 - DLO)