Can the growth of mountains and their erosion influence Earth’s climate over thousands to millions of years by changing the concentration of carbon-dioxide (CO₂) in the atmosphere? The answer to this question appears to be yes, but whether the growth of mountains increases or decreases atmospheric CO₂ has been a matter of debate. The chemical weathering of rocks is one of the key processes behind this link between erosion and the carbon cycle. In Taiwan, we found that at low erosion rates, weathering of sedimentary rocks sequesters carbon from the atmosphere, but at high erosion rates, it releases CO₂ at a rate that is two- to ten-times higher than the CO₂-drawdown.

In actively growing mountain ranges, fresh rocks are brought up to the surface by tectonic uplift and erosion. Exposed to circulating acidic water, the rocks are weathered chemically, and this weathering can have very different effects on Earth’s climate depending on the mineralogy of the rocks. For example, the alteration of silicate minerals by carbonic acid (CO₂ dissolved in water) fuels the precipitation of calcium-carbonate (CaCO₃), and binds the carbon on geologic timescales. Conversely, where sulfide minerals, such as pyrite, and carbonates occur, the opposite happens. When pyrite comes into contact with water and oxygen, it forms sulfuric acid, and the dissolution of carbonate minerals with sulfuric acid produces CO₂.

In our study, we quantified how erosion processes that expose fresh rocks to weathering affect the balance between CO₂ emission and drawdown. To this end, we visited the southern tip of Taiwan. Taiwan is an island of extremes: located at a subduction zone within the northwestern Pacific, severe earthquakes and typhoons repeatedly strike the region and change the landscape, sometimes catastrophically. This has made Taiwan a prime target for many geoscience studies. Interestingly for us, erosion rates vary across the island. Whereas the center of the island has been standing tall for several millions of years, the southern tip has just emerged from the sea and is characterized by a low relief. As a consequence, the center of the island erodes up to a thousand times faster than the far south– an ideal place to study the role of erosion on chemical weathering. Moreover, the sedimentary rocks of southern Taiwan are typical of many young mountain ranges around the world, containing mostly silicate minerals with some carbonate and pyrite.

We sampled rivers that drain areas of the mountains with different erosion rates. From the dissolved solutes in the rivers, we estimated the proportion of sulfide, carbonate, and silicate minerals involved in weathering, and the amount of CO₂ that is sequestered and released by these weathering reactions. In the southernmost part of Taiwan, silicate weathering and atmospheric CO₂ sequestration dominates. However, farther north, where mountains are eroding faster, carbonate and sulfide weathering dominate and CO₂ is released. Thus, it appears that chemical weathering in Taiwan, this most active of mountain belts, is a net emitter of CO₂ to the atmosphere. Our data also suggest that weathering of different phases interacts: Sulfuric acid boosts carbonate weathering but buffering of the acid – most likely by carbonates – appears to prevent silicate weathering from increasing as well.

This story may change where sediments that are eroded from the mountains are trapped in vast alluvial plains, such as along the foot of the Himalaya or the Alps. Here, silicate weathering dominates and sequesters CO₂. In addition, mountain building and erosion exposes not only sedimentary rocks with pyrite and carbonate, but also igneous rocks with many fresh silicates that weather quickly. Thus, our results from Taiwan have to be integrated with additional studies to unravel the global effect of mountain uplift on weathering and the carbon cycle.

A big shout-out to all involved colleagues, Check out the links to their websites: Niels Hovius, Robert Emberson, Jeremy Rugenstein, Albert Galy, Hima Hassenruck-Gudipati, and Jui-Ming Chang.

Bufe, A., Hovius, N., Emberson, R., Rugenstein, J.K.C., Galy, A., Hassenruck-Gudipati, H., Chang, J-M. (2021). Co-variation of silicate, carbonate and sulfide weathering drives CO₂ release with erosion. Nature Geoscience. 14(4), 211-216. Journal Link. PDF.