| 04 Jul 2025
Quantifying erosion in a pre-Alpine catchment at high resolution with concentrations of cosmogenic 10Be, 26Al, and 14C
Abstract. Quantifying erosion across spatial and temporal scales is essential for assessing different controlling mechanisms and their contribution to long-term sediment production. However, the episodic supply of material through landsliding complicates quantifying the impact of the individual erosional mechanisms at the catchment scale. To address this, we combine the results of geomorphic mapping with measurements of cosmogenic 10Be, 26Al, and 14C concentrations in detrital quartz. The sediments were collected in a dense network of nested sub-catchments within the 12 km2-large Gürbe basin that is situated at the northern margin of the Central European Alps of Switzerland. The goal is to quantify the denudation rates, disentangle the contributions of the different erosional mechanisms (landsliding versus overland flow erosion) to the sedimentary budget of the study basin, and to trace the sedimentary material from source to sink. In the Gürbe basin, spatial erosion patterns derived from 10Be and 26Al concentrations indicate two distinct zones: headwater zone with moderately steep hillslopes dominated by overland flow erosion, with high nuclide concentrations and low denudation rates (~ 0.1 mm/yr), and a steeper lower zone shaped by deep-seated landslides, where lower concentrations correspond to higher denudation rates (up to 0.3 mm/yr). In addition, 26Al/10Be ratios in the upper zone align with the surface production ratio of these isotopes (6.75), which is consistent with sediment production through overland flow erosion. In the lower zone, higher 26Al/10Be ratios of up to 8.8 point towards sediment contribution from greater depths, which characterises the landslide signal. The presence of a knickzone in the river channel at the border between the two zones points to the occurrence of a headward migrating erosional front and supports the interpretation that the basin is undergoing a long-term transient response to post-glacial topographic changes. In this context, erosion rates inferred from 10Be and 26Al isotopes are consistent, suggesting a near-steady, possibly self-organised sediment production regime over the past several thousand years. In such a regime, individual and stochastically operating landslides are aggregate over time in a specific region of higher erosion with a higher average denudation rate. Although in-situ 14C measurements were also conducted, the resulting concentrations show a non-conclusive pattern.
Schmidt et al. sampled three different cosmogenic nuclides to test whether these can be used to disentangle specific erosion mechanisms and to what extent the episodic nature of erosion influences catchment‐average erosion rates. This is a highly valuable and detailed approach that challenges and tests several assumptions commonly (and perhaps underappreciatedly) made in studies based on catchment‐average cosmogenic nuclide sampling. The authors show that different morphological zones in the catchment are dominated by distinct processes, and that these differences are reflected in the 10Be/26Al ratios. Furthermore, they confirm that the assumptions of sediment mixing and constant sediment production are valid under the studied conditions. I find the approach, data, and interpretation valuable for advancing studies of landscape evolution and erosion processes, and I therefore consider this manuscript an excellent contribution to Esurf. Below are a series of minor comments that might help improve the manuscript:
The abstract makes it sound as though the 14C concentrations were erratic and did not exhibit a consistent signal. However, they do show the same downstream trend in denudation as 10Be and 26Al, albeit with higher rates and more variability. I comment more on this later, but overall, I would interpret the 14C results less conservatively. This leans more toward personal opinion and is not something the authors need to revise for publication.
The authors state that the Grübe catchment is large enough to ensure good sediment mixing despite the presence of deep‐seated landslides. From the background, however, it seems that these landslides are slow and episodic, only supplying minor amounts of sediments at a time. Are they truly comparable, in terms of providing shielded sediment, to a single large deep‐seated landslide event? I felt the background information on landslides was insufficient to fully assess this distinction.
L30: Typo. “aggregated”
L93: How long are the stable phases? Please relate them to the nuclide integration timescales.
L131: Please, indicate, which resampling method was used (bilinear, bicubic nearest, etc.).
L134-5: Please rephrase the sentence about the DEM correction — it is currently difficult to follow.
L142: It would be helpful to include the equation for the connectivity index.
L151: There are many ways to calculate ruggedness/roughness using different approaches and kernel sizes. Please add a brief explanation of how ruggedness is defined here.
L152: as any smoothing of stream elevations used for Ksn calculation? Or were raw elevations used for each pixel KSn, later averaged within 125 m of the stream?
L161: Diffusive hillslope transport is negligible? Seems surprising. Later in the discussion the manuscript talks about diffusive hillslope transport in the upper zone.
L199: Figure 1 legend says sampling occurred in 2017 and 2022; the caption says 2017 and 2020; the text says 2022 and 2023. Please clarify.
L213: How far above the stream was this sample taken? Was it from a recent (hundreds of years) deposit, or from something older, e.g., Pleistocene?
L271: Typo. “Influences”
Section 4.1: As a reader, I would appreciate some representative photos of the different zones to better judge the interpretations made. Especially, when it comes to the erosion process discussion.
L321: Since production rates increase with altitude, please put your reported concentration differences into the context of corresponding production rate differences between zones.
Table 1: Add a column indicating the zone of each sample to facilitate interpretation.
Figure 3: Consider plotting the 10Be–26Al and 10Be–14C ratios, either here or elsewhere, to support later discussion.
Figure 4: he basin outline color is hard to interpret. I assume this was done to avoid overlapping colors in nested catchments, but the outlines may need a much thicker rim. Since you used TopoToolbox, here’s a useful blog post on creating thicker outlines:
https://topotoolbox.wordpress.com/2021/12/17/making-beautiful-drainage-basin-outlines/
Section 5.3 – 14C interpretation:
L366: The 14C pattern appears relatively similar to the other nuclides, with some offset and one outlier in the middle section. In the upper section, it shows a similar difference between the two samples compared to the other two nuclides. This statement makes it look like the 14C is all over the place.
Other studies have reported an increase in erosion over the past 3000 years in the Alps (Andrič et al., 2020; Rapuc et al., 2024), which matches the integration time-span of 14C in this study. Given its sensitivity, 14C is more prone to variability from stochastic processes and shielding, especially with a small sample set. Personally, I would lean toward one of the authors’ proposed explanations for the 14C results, but I understand and respect their conservative interpretation.
L490. Typo. Adjust parentheses of citation.
Section 5.4 Please provide more specific (and, if possible, quantitative) discussion of the mechanisms affecting the low‐reach samples. How exactly would landslides and related cascading processes influence the 10Be/26Al ratios? Some simple calculations could strengthen this section. For example, using your mapping and published information on landslide depths and sediment volumes, can you estimate how much landsliding or storage over what timescales would be required to explain the measured concentrations?
Figure 7 shows that 10Be/26Al ratios in the fan samples roughly equal surface values of the upper catchment zone. If erosion rates are higher in the middle section, and given its larger quartz‐bearing area, one would expect the fan deposits to be dominated by that flux. Yet the ratios are lower in the fan than in the middle section. This discrepancy is not addressed — what do the authors think about this?
L557: Please clarify why limestone would lead to underestimation or distortion of denudation rates. Do you mean that rapid limestone erosion would not register in quartz‐based estimates? This seems obvious, so I found the list in lines 560–565 somewhat distracting. Perhaps streamline it.
L595: The time-scale of knickpoint migration and the integration time of the cosmogenic nuclides are very different, which is something that should be highlighted in this statement or elsewhere. With the measured erosion rates the authors could even think about calculating knickpoint propagation times.
Richard Ott
References
Andrič, M., Sabatier, P., Rapuc, W., Ogrinc, N., Dolenec, M., Arnaud, F., von Grafenstein, U., & Šmuc, A. (2020). 6600 years of human and climate impacts on lake-catchment and vegetation in the Julian Alps (Lake Bohinj, Slovenia). Quaternary Science Reviews, 227, 106043. https://doi.org/10.1016/j.quascirev.2019.106043
Rapuc, W., Giguet-Covex, C., Bouchez, J., Sabatier, P., Gaillardet, J., Jacq, K., Genuite, K., Poulenard, J., Messager, E., & Arnaud, F. (2024). Human-triggered magnification of erosion rates in European Alps since the Bronze Age. Nature Communications, 15(1), Article 1. https://doi.org/10.1038/s41467-024-45123-3