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Cold water poured on cooling theory

Mt Everest, by Martin Lack FGS

Geologists find no evidence for increased erosion after uplift of Himalayan Plateau, casting doubt upon this as a cause of Cenozoic global cooling, reports Sam Shead*.

Geoscientist Online 15 June 2010

As the world’s great mountain ranges have been uplifted, the Earth’s continents have been subject to higher rates of erosion and thus increased weathering. An increase in erosion results in a greater flux of material into the world’s oceans. About 20% of the global suspended and dissolved sediment today derives ultimately from the Himalaya and the Andes.

Cenozoic global cooling can be linked to this sedimentation flux because carbon dioxide is extracted from the atmosphere it combines with water to form a weak carbonic acid, which acts as a weathering agent. Silicate weathering products remain high in carbon, which is thus extracted from the atmosphere and sequestered beneath the sea. The heightened drawdown of atmospheric carbon, particularly after the uplifting the Himalayas, has been cited as a potential cause for the Cenozoic change from “greenhouse” to “icehouse” conditions on Earth.

A controversial study by Jane Willenbring and Friedhelm von Blanckenburg1 (GFZ, Potsdam) suggests that the rise of mountain ranges has not in fact altered the amount of eroded material being transferred from the continents to the oceans. They show, by contrast, that the global flux of eroded material has remained roughly constant throughout the Late Cenozoic (the last 40 million years). They further suggest that the observed increase in sedimentation flux over the last five million years has been caused by bias in observation and measurement.

It is possible to measure erosion rates at points in the past by analysing the mass of sediments in basins. However, a previously neglected complication arises because eroded material is often subject to subsequent re-erosion. The timescale here is of key importance, becasue the probability of a sediment body suffering such recycling increases with age. Willenbring and von Blanckenburg claim that once this additional erosion is taken into account, erosion rates on the continents turn out to have remained roughly constant.

However, this conclusion implies that CO2 drawdown through silicate weathering also remained constant. The authors test for this by using beryllium isotopic evidence from deep-sea sediments spanning the past 10 million years of Earth history. These studies suggest that silicate weathering has indeed remained constant; though the major uplift phases (not to mention a large amount of Cenozoic cooling) occurred earlier than 10Ma. For that reason, Willenbring and von Blanckenburg face the challenge of extending their record further back in time.

The authors conclude that the global climate cooling in the Late Cenozoic had no link to geologically recent mountain uplift – a view is supported by a number of CO2 archives, which also suggest little change in CO2 atmospheric concentrations during the past 20 million years.

The results seem paradoxical. We know that weathering is more intense at higher altitudes – how therefore is it possible that global sedimentation flux and silicate weathering to remain constant for 10 million years while large mountain uplift was taking place?

One possible explanation is that there could have been a coeval decrease in erosion outside uplifted regions. There is also a possible climatic link, such as a reduction in water runoff brought about by drier conditions and the growth of large stable ice sheets. These factors would essentially counterbalance the effect of increased erosion in uplifted regions.

More evidence is needed but if the authors are right, and are able to show that their findings also applied during the last 40Ma, then the implications could seriously undermine some of the most basic concepts in our understanding of climate and surface processes.


  1. Long-term stability of global erosion rates and weathering during late-Cenozoic cooling by Jane K. Willenbring & Friedhelm von Blanckenburg Nature 465, pp 211–214 13 May 2010.

* Sam Shead, Royal Holloway University of London, is working as an intern for Geoscientist. He is about to take up a place at the Cardiff University School of Journalism.