About: Erlanger et al., (2024), Nature Geosciences (link)

In our new paper, we use stream water chemistry in two river catchments of the central Apennines to infer the CO₂ fluxes from surficial weathering reactions as well as the CO₂ degassing from depths.

In the east, where the crust is thick and cold, carbon fluxes from silicate weathering dominate the carbon budget. In contrast, the western catchment is underlain by thin and hot crust. Here, carbon fluxes are dominated by CO₂ degassing from the crust and mantle, and these fluxes are up to 50-times higher than the carbon drawdown from silicate weathering.

You can check out a more detailed press-release by the GFZ here.

About: Xu et al., (2024), Environmental Science & Technology (link)

In a new paper spearheaded by Sen Xu from Tianjin University, we investiagate the role of warming for the export of dissolved inorganic carbon (DIC) in two major rivers that drain the eastern Qinghai−Tibetan Plateau.

In the Jinsha River that has 51% of its catchment underlain by continuous permafrost, DIC fluxes increase substantially over the past 40 years. Changes in river discharge play a negligible role for that increase in flux. Instead, the increase in DIC fluxes correlates most strongly with the temperature increase.

The Yalong river that is situated at lower elevation and has only 14% permafrost cover does not show a substantial increase in DIC fluxes. This observation suggest that the presence or absence of permafrost may strongly modulate the sensitivity of inorganic carbon fluxes to global warming.

About: Turowski et al., (2024), Earth Surface Dynamics (link)

What sets the width of river valleys? In a paper published today, we propose a new model for the width of river valleys. It considers valley width as a competition of lateral channel motion and the uplift and erosion of valley walls. Here is the equation:

W is valley width
q_L is the lateral sediment transport capacity
q_H is the lateral input of sediment from hillslopes
U is the uplift rate
W₀ is the channel belt width
W_C is the channel width

A dimensionless “mobility-uplift number, M_U” expresses that competition where:

The model implies that valley width varies between two extremes: At a minimum, valleys are as wide as the channel. Such narrow valleys occur where rivers drain rapidly uplifting landscapes. At a maximum, valleys encompass wide channel belts. These two extremes are connected by a logarithmic function of the mobility-uplift number.

Despite its conceptual simplicity, the model compares surprisingly well to several datasets including experiments and a large compilation of valley widths in the Himalaya.

This model explains valley width in a landscape that has reached steady state. How do valleys evolve over time and what sets the maximum width of valleys? We are working on these questions in an upcoming publication.

About: Bufe et al., (2024), Science (link)

In this new paper, we analysed weathering data from different mountain ranges. We found that silicates, carbonates and sulfides had different non-linear erosion sensitivities. The behaviors are very similar in all study areas. As a result, all datasets show that CO₂ drawdown from rock-weathering is at a maximum at moderate erosion rates of ~0.07 mm/yr.

You can read press-releases here with more info.

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.

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.

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.

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

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.

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.

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.

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).