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
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 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.
Pre-recorded talk for the Fall Meeting of the American Geophysical Union 2020.
In this work, we present new water-chemistry data from streams in southern Taiwan that span a 2-3-fold erosion rate gradient to study the link between erosion and chemical weathering.
Landslides are not only a natural hazard, they also erode entire mountain ranges. The mobilization of rock and soils has far reaching implications for the concentration of CO2 in the atmosphere and it influences Earth’s climate. This February, we spent four weeks in the Southern Alps of New Zealand to investigate how landslides influence the global carbon cycle. We, that is Dr. Erica Erlanger – freshly graduated from the ETH Zurich and now a postdoc at the GFZ – and Alexander Gessner, Master student at the FU Berlin.
Team photo!
The Southern Alps of New Zealand are one of the fastest evolving mountain ranges on the planet. Uplift rates here are several millimeters per year, producing steep hillslopes that are rapidly eroded. Close to 100% of this erosion occurs by landsliding, and the fresh, fine rock mass in landslide deposits creates efficient reactors for chemical weathering. Chemical weathering is the dissolution of minerals by acidic groundwater. Much of the acid in the groundwater stems from dissolving atmospheric CO2 in water. Where this acid dissolves silicate rocks, calcium, magnesium, and bicarbonate (HCO3-) ions are produced that are washed into the ocean by rivers. In the ocean, these ions provide the ingredients for the formation of carbonate rocks (e.g. CaCO3) which effectively locks up the atmospheric CO2 into the rock record. By sampling springs from landslide deposits, we aim to build a model for how landslides influence chemical weathering in mountain regions. This project is part of the EU-funded Marie Skłodowska-Curie project WetSlide.
Landslides in the Western Southern Alps
Landslides have another important impact on the carbon cycle, because they strip soil and vegetation from hillslopes. The soil and vegetation contain organic carbon and all carbon that makes it to the ocean can get locked up in rocks on the ocean floor. On the bare hillslopes, new soils can build up and lock up carbon from the atmosphere. On this trip, we sampled soils and measured how they grow over time by studying landslides that occurred anywhere from 1- to 1000 years ago.
Landslide scars and deposits showing various degree of re-vegetation. Haremare Creek
From the movement of rivers, to the generation of
catastrophic landslides and the evolution of entire landscapes, many processes
that shape the surface of the earth are characterized by a high degree of
variability; variability that is not linked to environmental factors, but to complex
internal dynamics. Describing such complexity and variability requires
stochastic models that describe processes probabilistically and large datasets to
calibrate these models. What can we do when nature presents to us only one of
the many possible evolutions of a highly complex system?
In this paper, we describe a framework to calibrate stochastic models of morphodynamic systems with a single time-series of data. By “morphodynamic system” we refer to a system that is characterized by changes in shapes or position of objects. Rivers that are moving back and forth across a floodplain are a great example for a morphodynamic system that is characterized by complex internal dynamics. Here, we demonstrate the framework using an experiment of braided rivers moving in a flume. Yes, this is the same experiment that we used in our last paper to study the average behavior of lateral channel movements (Link). Here, we are interested in the variability.
In simple terms, the framework consists of generating a large number of “synthetic” time-series from a stochastic model. These synthetic time-series will vary depending on the input parameters to the model. We calibrate these parameters by finding model outputs that are statistically equivalent to the data. One of the key aspects of the framework is the choice of statistical tests to compare the data to the model. We propose three statistical tests to compare the behavior of channel movements in model and datasets, but these statistical comparisons are modular and can be adapted or expanded to suit the studied morphodynamic system.
Hoffimann, J., Bufe, A., Caers J. (accepted). Morphodynamic Analysis and Statistical Synthesis of Geomorphic Data: Application to a Flume Experiment. Journal of Geophysical Research: Earth Surface. Journal link.
If you are looking for a Master’s thesis project at the interface of geomorphology and geochemistry, I would like to draw your attention to an opportunity in the Geomorphology group of the German Research Center for Geosciences (GFZ) in Potsdam.
The chemical weathering of rocks on Earth’s surface is one of the cornerstones of the carbon cycle and controls atmospheric CO2 concentrations on geologic timescales. Weathering has traditionally been described by models of in-situ production and chemical alteration of regolith and soils. However, recent observations clearly indicate that in rapidly eroding mountain ranges bedrock landslides dominate the production and storage of fresh, unweathered sediment and that landslides may strongly influence weathering fluxes.
Within a recently-funded EU project (Link), we are investigating the impact of landslide erosion on chemical weathering. I am looking for a motivated student to tackle one of several open questions in this project. For example, how does chemical weathering in landslide deposits evolve through time? How important is the removal of a topographic load for fracture formation in the landslide scar area and the chemical weathering in mountain hillslopes? If you are more interested in geomorphology, you could also work on the distribution of residence times of rocks in landslide deposits. In other words, on what timescale are rocks in landslide deposits either removed from a hillslope or developed into a deposit that, for the purposes of chemical weathering, is indistinguishable from a soil?
The project will include
fieldwork in New Zealand, and you will be collecting and analyzing the
chemistry of seepage waters and/or characterizing landslide volumes and grain
sizes in the field and with drone imagery.
If this project sparks
your interest, or if you have any other questions, please contact me at abufe@gfz-potsdam.de.
Today, I start my new position as a Marie Skłodowska-Curie Fellow at the German Research Center for Geosciences! The office is the same, but I will embark on a new exciting project: WetSlide. We aim to develop a model for weathering of rocks in landslide deposits. Keep your eyes open for more updates and a new project website with more information.
Hillslopes with old landslide scars in the Poerua catchment, New Zealand
Natural lowland rivers tend to erode their banks and migrate across an alluvial surface. In our new paper, we use data from experiments to develop a model for lateral channel migration rates of braided streams. Surprisingly, we find that the direct influence of sediment discharge on migration rates is relatively weak, and that the main controls on migration rates are the water discharge and the channel bank height. Of course, the channel bank height itself is influenced by water and sediment discharges – this is where our results need to be combined with models for the long-profile evolution of streams, which leaves exciting new research avenues ahead.|
Bufe, A., Turowski, J.M., Burbank, D.W., Paola, C., Wickert, A.D., Tofelde, S. (accepted) Controls on the lateral channel migration rate of braided channel systems in coarse non-cohesive sediment. Earth Surface Processes and Landforms, Journal Link
I am excited to see our new paper on alluvial channel response to environmental perturbations published today in Earth Surface Dynamics. In this paper we present results from physical experiments of channels that were subject to perturbations of water and sediment discharges. We demonstrate that combining terrace geometries with information on (1) the timescales of terrace formation and/or (2) the sediment discharge from the river system, allows to distinguish between water and sediment discharges as the driver for river incision.
Tofelde, S., Savi, S., Wickert A.W., Bufe, A., Schildgen, T. (2019). Alluvial channel response to environmental perturbations: Fill-terrace formation and sediment-signal disruption. Earth Surface Dynamics, 7(2), 609-631. Journal Link
This is a schematic diagram of the changes expected in (a&b) river morphology and (c-f) the sediment output from an alluvial river during a transient phase of incision (a, c & e) or aggradation (b, d & f). Panels (c-f), show the upstream sediment input (orange line) and water input (blue line) and the downstream sediment output (colored circles). Importantly, a phase of incision can be due to a decrease in the sediment input into the channel, or an increase in the water input. The topography is similar in both cases but the pattern of sediment output is very different. Therefore, using sedimentary archives that record such sediment output together with terrace records can yield more information about the driver behind a change than each one of the records by itself.
EGU is happening and I hope to see some of you there! This year, I will:
Co-convene a session Erosion, chemical weathering and sedimentation in mountain landscapes Orals: Wed, 10 Apr, 08:30–12:30, 14:00–15:45, Room D3. Posters: Thu, 11 Apr, 08:30–10:15, Hall X2.
Give a solicited talk Bufe et al. Temporal changes in rock uplift rates of folds in the foreland of the Tian Shan and the Pamir from geodetic and geologic data. Tue, 09 Apr, 17:45–18:00, Room K1. (Link)
Co-author talks Roda-Boluda et al. Examining landslide recurrence intervals and landslide-derived sediment fluxes with 10Be concentrations and grain size distributions: preliminary results from the Fiordland and the Southern Alps, New Zealand. Wed, 10 Apr, 09:45–10:00 Room D3. (Link)
Tofelde et al. Fill-terrace formation and sediment-signal disruption in response to environmental perturbations. Wed, 10 Apr, 15:15–15:30, Room D3. (Link)
Hoffimann-Mendes et al. Morphodynamic Analysis and Statistical Synthesis of Geomorphic Data. Wed, 10 Apr, 16:15–16:30, Room 0.96. (Link)
