Nature's Journal Club

Friedhelm von Blanckenburg

GFZ German Research Centre for Geosciences, Potsdam, Germany

A geochemist learns that mountain building does not accelerate rock weathering.

Mountain building has been deemed essential for stabilizing Earth’s climate over the scale of millions of years. As tectonic forces push mountains into the sky, they provide fresh rock surface that is degraded by the physical effects of rain and temperature change, and by chemical weathering as carbon dioxide is dissolved in rain to form carbonic acid. Atmospheric CO2 is thus consumed to convert rock into soil, which happens fastest where erosion rates are highest, exposing new rock to be weathered. Rivers then transport this carbon to the oceans where it is disposed of as carbonate sediment.

But many Earth scientists have questioned this story. Wouldn’t periods of high tectonic activity, such as the rise of the Himalayas, provide enough rock to consume so much CO2 that the planet would turn into an ice house?

Yet the consequences of mountain building are perhaps less profound than expected. The amount of weathering over time can be accurately measured on hill slopes using new geochemical methods that combine solute loss from soils with radioactive isotopes formed by cosmic rays to determine how long it takes for rocks to break down into soil. Two recent papers modelled the implications of this approach numerically. Surprisingly, the prediction is that weathering decreases rather than increases at erosion rates typical of high, active mountains (K. L. Ferrier and J. W. Kirchner Earth Planet. Sci. Lett. 272, 591–599; 2008; E. J. Gabet and S. M. Mudd Geology 37, 151–154; 2009). So hill-slope weathering in the Himalayas might do no more to withdraw CO2 than any actively eroding, mid-altitude mountain range found worldwide.

Perhaps geochemists have been looking in the wrong place. Does the CO2-consuming mineral decomposition thought to occur on high slopes actually happen on the floodplains below large, active mountains? We might need to take a closer look at these areas before we really understand the geological carbon cycle.

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