River valleys come with a wide range of shapes, from narrow canyons to wide plains. We know very little about what controls their width. To first order, wide valleys occur in big rivers. Indeed, compilations show a relationship between water discharge and valley width. They also show that valleys narrow as valley walls get harder to erode. However, widths scatter over multiple orders of magnitude for the same water discharge and valley-wall lithology. Something more controls the shape of valleys.

To investigate these additional controls, we turned to paired river terraces. Paired river terraces preserve the geometry of past valley shapes at a single point along the river. Moreover, many terrace sequences can be linked to cycles of wet and dry climate. In that case, all terrace levels preserve valleys that were formed under similar climatic and lithologic conditions.

We compiled valley widths from 12 globally distributed – and climatically formed – terrace sequences. For all sequences, we find a very clear relationship between the height and width of the valley (The height refers to the height of the entire valley wall, not the height of individual terraces). This finding raises the following question: Does a process related to valley height impact the width of valleys?

Based on our observation, we propose a new model for valley formation. Rivers widen valleys by lateral erosion of the valley walls. The eroded sediment has to be removed before erosion can continue. At the same time, valley hillslopes are eroding and deliver sediment to streams. A linear relationship between valley width and valley height can be explained only when sediment supply from hillslopes and sediment removal by streams are in balance. Hence, sediment supply from hillslopes may limit valley widths.

The erosion of active mountain ranges exposes rocks to the surface of the Earth. Acidic rain- and soil waters slowly dissolve minerals in these rocks. Depending on the type of mineral, these “chemical weathering” reactions can either draw down CO₂ from the atmosphere or release CO₂. Therefore, uplift of different rock-types in mountain ranges can potentially affect Earth’s climate.

In our recent study, we wanted to investigate how rock-type affects the balance of CO2 drawdown and release in mountains. We collected waters from small streams on the eastern Tibetan Plateau. These streams drain regions with either metasedimentary or granitoid rocks. Moreover, the erosion rates of the mountains vary by more than two orders of magnitude. This contrast can be clearly seen in the shape of the landscape.

Across the erosion rate gradient, we find that granitoid lithologies have generally lower weathering rates than metasedimentary rocks. Using a mixing model, we can infer the carbon balance of these weathering reactions. For all lithologies, increasing erosion shifts weathering from CO2 drawdown to CO2 release. This shift is most dramatic for metasedimentary rocks.

Throughout the history of a mountain belt, different rock-types are exposed to the surface of the Earth. Our results suggest that changes in the exposure of rocks can alter the carbon cycle and earth’s climate in addition to changes in erosion rates.

Bufe, A., Cook, K.L., Galy, A., Wittmann, H., Hovius, N. (2022). The effect of lithology on the relationship between denudation rate and chemical weathering pathways. Evidence from the eastern Tibetan Plateau. Earth Surface Dynamics. 10(3), 513-530. Journal Link

New Paper led by Daphnee Tuzlak. With Joel Pederson and Tammy Rittenour

Underneath Yellowstone National Park sits one of the largest active super volcanoes on Earth. It has intrigued scientists and non-scientists for decades. A hot zone in Earth’s mantle below Yellowstone is generating magma, supports high topography, and makes the region one of the fastest deforming on the North American continent. Over more than 15 million years, the North American Plate has been slowly drifting southwest over the hot mantle and has left a track of volcanic centers that spans from Nevada to Montana. The hot mantle lifts the crust above it, but the abandoned volcanic centers in the wake of the hotspot track sink back down. Thus, uplift and subsidence from the hot spot interact with the mountain topography that formed millions of years before its arrival. If this complex setting isn’t enough on its own, repeated glaciations and rivers carved into the uplifting crust and formed an intricate topography.

We set out to study how uplift above the hotspot, subsidence in its wake, major crustal faults, and changes in climate shaped the landscape over the late Quaternary (the past 100 thousand years). To this end, we explored trunk drainages of the Snake River system that flow from high up on the uplifting center of the Yellowstone region into the subsiding Snake River Plain west of Yellowstone. By analyzing the patterns of steepness and energy along the Snake River and its tributaries and by estimating the age of abandoned terraces that mark the river’s history, we found that the Snake River has been episodically cutting into the uplifting mountains at an average rate of ~0.3 mm/y. We also found that the pattern of incision is not dominated by broad uplift of the crust above the Yellowstone hotspot but rather by the movement of individual faults and the subsidence of the Snake River Plain downstream. Thus, we shed light on the dominant tectonic processes that have shaped the landscape over the past 100,000 years.

This work was led by Daphnee Tuzlak and Joel Pederson at Utah State University. Sand samples from river terraces were dated together with and Tammy Rittenour at at the Utah State University Luminescence Lab. My work within this project was supported by an EarthScope AGeS Program geochronology student award funded by the National Science Foundation.

Tuzlak, K., Pederson, J.L., Bufe, A., Rittenour, T.M. (2021). Patterns of Incision and Deformation on the Flank of the Yellowstone Hotspot — Alpine Canyon of the Snake River, WY. Geological Society of America Bulletin. Journal Link.

I am excited to advertise a new paper on the chemical and physical erosion of the northern Apennines: Led by Erica Erlanger.

Rocks exposed to the surface of the Earth break down by physical processes (e.g. through river erosion, cracking under the influence of temperature etc.) and chemical processes (e.g. by dissolution in acidic water). Water, wind, and gravity transport rock fragments and form extensive sedimentary deposits. In turn, rivers carry the dissolved load into lakes and into the ocean, thereby influencing Earth’s chemical cycles. The relative importance of these physical and chemical denudation processes depends on the type of rock. For example, carbonate rocks dissolve much faster than silicates, but they can also be more resistant to physical breakdown. In our study, we asked how physical and chemical denudation are partitioned in mixed carbonate-silicate rock. To address this question, we went to the northern Apennines.

Compared to the Alps or the Himalaya, the northern Apennines are a young mountain range that exposes marine carbonates and silicate rocks. These rocks were deposited by turbidity currents and they experienced only limited burial and metamorphism. The Apennines therefore provide an opportunity to study the evolution of physical and chemical erosion in the early stages of mountain building.

We combined erosion rates from measurements of cosmogenic nuclides in river sediments with analyses of the dissolved load carried by rivers. Compared to more evolved siliciclastic mountain ranges, the Apennines have a larger relative chemical weathering flux; Most likely, due to the rapid dissolution of carbonate.

Interestingly, we also find that up to 90% of the dissolved carbonate re-precipitates as sediment grains. How can that be? We believe this phenomenon can be explained by the saturation of the river water with respect to calcium carbonate. When cool CO₂-laden acidic groundwater discharges into streams, the temperature and CO₂ equilibrate with the atmosphere. Warmer water with less CO₂ can dissolve less carbonate, and the excess precipitates. This mechanism converts a large fraction of the chemical flux back into sediment. As a result, the chemical flux out of the Apennines is not limited by the dissolution of minerals in the subsurface, but by the capacity of the stream to carry the dissolved carbonate; A surprising result.

Shoutout to: Erica Erlanger, Jeremy C. Rugenstein, Vincenzo Picotti, and Sean Willett.

[12] Erlanger E.D., Rugenstein, J.K.C., Bufe A., Picotti V., Willett, S.D. (2021). Controls on Physical and Chemical Denudation in a Mixed Carbonate-Siliciclastic Orogen. Journal of Geophysical Research: Earth Surface. 126(8), e2021JF006064. Journal Link (open access).

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.