All posts by aaronbufe@gmail.com

New paper: Weathering on the Qinghai-Tibet Plateau

About: Xu et al., (2024), Geochimica et Cosmochimica Acta (link)

A multitude of different minerals are exposed at the surface of the Earth. Under the influence of acid waters, these minerals slowly dissolve and transform. These ‘chemical weathering’ reactions release nutrients, and they change move carbon between rocks, water, and the atmosphere. Rivers collect elements dissolved in soils. Therefore, we can use river chemistry to study the weathering reactions that occur within landscapes.

Deep Gorge carved by the Tongtian River – tributary to the Yangtze River – and located on the steep eastern margin of the Qinghai-Tibet Plateau

In large drainage basins that host many different rock-types, it can be a challenge to interpret the chemistry of rivers. In particular evaporite (“salt”) minerals can strongly dominate the weathering budget, and their contribution is difficult to distinguish from that of silicate, carbonate, or sulfide minerals.

Proportions of different minerals contributing to river solutes on the Qinghai-Tibet Plateau

In our work, we used a series of isotopes and major element chemistry to obtain a weathering budget in the headwaters of three of the largest rivers in the world – the Yangtze, Mekong, and Salween Rivers. We then analyzed how this weathering budget depends on erosion rates, rainfall and permafrost extent. We found that mountain building and attendant erosion play a major role in weathering of the studied rivers. Erosion boosts weathering reactions that may move CO2 from the rock-record to the atmosphere.

Chumar River with a high load of red suspended sediment located on the Qinghai-Tibet Plateau

New paper: How do paraglacial and periglacial processes control erosion?

About: Roda-Boluda et al., (2023), JGR-Earth Surface (link)

Spectacular view along the Poerua River showcasing steep hillslope and the high relief of the Southern Alps of New Zealand

Where rocks are uplifted, they get eroded by wind, water, ice, and gravity. Erosion creates large volumes of sediment that are transported from the mountains to sedimentary basins. The rates of erosion are fundamentally driven by mountain uplift. However, the climate can also impact the breakdown and movement of rock. For example, heavy and sustained precipitation can trigger landslides, glaciers grind their bases to a fine powder, and cycles of freezing and thawing can efficiently break down solid bedrock. Geologists currently debate how climate affects erosion on the scale of an entire mountain range.

Bare Hillslopes with scree – subject to periglacial erosion

The Southern Alps of New Zealand are a fantastic place to dig deeper into the link between climate and erosion. Along a narrow range, metamorphosed sandstones are lifted up at multiple millimeters per year – making the Southern Alps one of the fastest deforming mountain ranges on the planet. A relief of over 3000 m captures the westerly winds and leads to heavy rains on the Western Southern Alps with yearly precipitation of 2 – 10 meters. Moreover, the Southern Alps are subject to so-called “paraglacial” (conditioned by recently retreated glaciers) and “periglacial” (in a zone where temperatures fluctuate around 0ºC) erosion processes: Where glaciers recently retreated, hillslopes have become unstable and temperatures around freezing cause efficient freeze-thaw cycles.

During one month in the field, we sampled sand from a number of rivers that drain the Western Southern Alps. Measuring the concentration of cosmogenic beryllium-10, we estimated the average erosion rate upstream of each sample point. Then, we studied how erosion rates vary with different topographic and climatic parameters.

Plot suggesting a correlation between erosion and peri- and paraglacial erosion processes that are most active between ~1500 – 2000 m in the study area

We found that erosion rates were highest in those rivers that had a substantial portion of their catchment at an elevation of 1500 – 2000m. At these elevations para- and periglacial processes are particularly strong in the Southern Alps. In contrast, rainfall and erosion rates did not correlate well.

Overall, the pattern of erosion is set by the uplift of the rocks. However, our data suggest that these erosion rates can be modulated substantially by processes related to freeze-thaw and glacier retreat.

Moving 1000 kg of sand for analysis of cosmogenic nuclide concentrations

Move to the LMU Munich

I am thrilled to announce my start as an assistant professor at the Ludwig Maximilian University of Munich. I am looking forward to building a research program and teaching new generations of geoscientists. Please browse this website and contact me for information about our work and opportunities to join the group.

New paper: Controls on the width of river valleys

Valley shapes in the Morača river, Montenegro (left) and the Naryn river, Kyrgyzstan (right)

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.

Valley width as a function of drainage area of streams in the French Alps with large scatter for a given drainage area. Adapted from Langston & Temme 2019, GRL

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.

Relationship between valley width and valley height

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?

A terrace sequence in the Quebrada del Toro, Argentina, showing sediment transport slopes from upper terrace levels into the stream

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.

Valley width as a balance between sediment supply from hillslopes (Eh) and the transport capacity of the stream (qL)

New paper: The role of lithology in weathering active mountain ranges

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.

The steep peaks of the Gonga Shan erode at rates of almost 10 millimeters per year. Photo: Niels Hovius

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.

Rolling hills on the Eastern Tibetan plateau are lowered at rates of less than 0.1 millimeters per year. Photo: Kristen Cook

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.

With increasing erosion, weathering shifts from drawing down CO2 to releasing CO2 for all lithologies. The shift is most prominent in the 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: Quaternary deformation on the southern flank of the Yellowstone Hotspot

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.

Overview of the Study Area. Volcanic centers are marked in light red. White-shaded area mark the extent of Ice during the Last Glacial Maximum (~13,000 – 20,000 years ago). White dotted line is the outline of Yellowstone National Park. Fine and solid white lines are state borders.

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.

Fluvial gravels overlying tilted bedrock on a terrace along the Hoback River; tributary to the Snake River

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.

Fault scarp of the Hoback fault (~3.5 m-high) on a ~14,000 year-old fluvial terrace indicating average slip of ~0.25 mm/year

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.

New paper: Weathering and erosion of a mixed carbonate-silicate landscape

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.

The Lima River

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.

Weathering and total denudation in the Apennines compared to older and more siliciclastic mountain ranges

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).

Too much good food may have been involved

EGU 2021

I will be at EGU this year. Please come and check out our work:

vPICO: Steady-state valley width revealed by alluvial terrace sequences
Monday 26th of April 15:44–15:46:
Stefanie Tofelde, Aaron Bufe, and Jens M. Turowski
Link

Session: Processes and timescales of sediment production, transport, and deposition from source to sink
Tuesday 27th of April 11:00 – 12:30: Session
Convener: Oliver Francis | Co-conveners: Aaron Bufe, Lisa Harrison, Stefanie Tofelde
Link

vPICO: Co-variation of silicate, carbonate, and sulphide weathering drives release of CO2 with erosion
Wednesday 28th of April 09:21–09:23:
Aaron Bufe, Niels Hovius, Robert Emberson, Jeremy K.C. Rugenstein, Albert Galy, Hima J. Hassenruck-Gudipati, and Jui-Ming Chang
Link

vPICO: Erosion rates of the New Zealand Southern Alps reflect long-term tectonics and transient climate
Wednesday 28th of April 09:30–09:32:
Duna Roda-Boluda, Taylor Schildgen, Hella Wittmann-Oelze, Stefanie Tofelde, Aaron Bufe, Jeff Prancevic, and Niels Hovius
Link

New paper: Mountain uplift and release of CO2 from weathering of sediment

Sediment and solute transport in a Taiwanese stream (Photo: A. Bufe)

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.

Effect of erosion on the net CO2 drawdown and release from weathering

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.

Brown bedrock seepage typical for pyrite weathering (Photo: K. Cook)

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.

Sampling stream water (Photo: R. Emberson)

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.

(Absence of) correlation between sulfate from sulfide weathering and carbonate (blue) and silicate weathering (red)

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.

A pet in front of our accommodation (Photo: A. Bufe)

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.