the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 12 Jan 2026
Long-term peat thickness from cosmogenic 26Al and 10Be, Hautes Fagnes, Belgian Ardennes
Abstract. Upland peatlands are a major terrestrial carbon reservoir that may play an important role in the global carbon cycle. However, knowledge of upland peatlands before the Holocene remains speculative because of the poor long-term preservation potential of peat in upland environments. Here, we explore using paired 26Al and 10Be to simultaneously determine denudation rates and peat thicknesses averaged over multiple glacial-interglacial cycles. We report cosmogenic 26Al and 10Be concentrations in quartz from saprolite underlying the modern peat cover along a hillslope transect and from stream sediment in the Hautes Fagnes, an upland peatland in the Belgian Ardennes. The measured 26Al/10Be ratios are lower than expected for steady-state denudation under the modern peat cover, which we interpret as evidence of thicker peat in the past. To quantify long-term average peat thicknesses and denudation rates and identify secular changes in overburden, we inverse-model the measured 26Al and 10Be concentrations. Modeled denudation rates of the saprolite, reflecting landscape lowering rates, are exceptionally low (0.3–4.9 tons km-2 yr-1, equivalent to approximately 0.1–1.9 m Myr-1). The median probability long-term overburden thicknesses exceed modern overburden thicknesses by 190–350 g cm-2 along the hillslope transect, approximately equivalent to 1.8–3.4 m of saturated peat. Peat degradation from historical land use, including peat extraction, drainage, and afforestation, may explain much of the discrepancy. Inverse-modeling of the sample with the slowest denudation rate, and thus the longest near-surface residence time of quartz and signal integration timescale, suggests that a secular increase in overburden thickness, potentially reflecting the onset of peat cover, coincided with mid-Pleistocene uplift of the Ardennes. These results demonstrate the utility of cosmogenic nuclides in inferring the long-term history of peat cover where geomorphic process rates are slow and differential radioactive decay is non-negligible.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Earth Surface Dynamics.
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2026-36', Anonymous Referee #1, 21 Jan 2026
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RC2: 'Comment on egusphere-2026-36', Richard Ott, 20 Feb 2026
Moore et al. present an interesting study trying to infer past peat thickness, denudation, and timing of peat accumulation in the Belgian Ardennes. The study is well written and in most parts also very thorough. However, parts of the methods were brief and thus I could not follow exactly what the authors did to run their MCMC inversion. Based on my understanding of what the authors did, I have concerns that the recovered parameters and the conclusions might be biased.
Inversion of CRN concentrations:
My major concern is the application of the MCMC inversion for this study. The methods section on the MCMC application was brief, and thus, I might have misunderstood something. It was not clear what parameters were inverted for, what the prior ranges were, and whether samples were inverted jointly or separately. The way I understood the manuscript, the authors use the MCMC to constrain the parameters of denudation rate, initial overburden thickness, overburden thickness, and time of change, in equation 5 INDIVIDUALLY for every hillslope sample. Thus, there are 4 unknowns for every sample (D, z1, z2, t), but only two equations with data (10Be equation and 26Al equation). This is an undetermined problem and cannot have a unique solution without additional information.
This can be somewhat seen in the lower panels of figure S1, where one would expect all parameter combinations that can explain the data to fall onto a curve. They do so for panel D , but the MCMC apparently did not explore the entire space for denudation rates. From personal experience I know that these MCMC random walks sometimes tend to cluster one side of the prior parameter space—for reasons I don’t understand—, leading to the effect that they don’t map the full line of parameter combinations. However, in theory, with 3 parameters one can calculate curves of valid parameter combinations analytically from the two nuclide build-up equations.
The only reason to apply a MCMC to this problem would be to invert all samples together, hoping that differences in the sample parameter combinations provide additional information. This is the approach that Hippe et al., 2021 used for 3 parameters and 2 nuclide equations. However, they unfortunately did not provide synthetic tests to prove that the inversion can still reliably recover synthetic parameters (I think it’s great that the authors of this study included such a test!). My experiences from personally working with similar inversions for cosmogenic nuclide erosion histories is that the inversion only partially recovers initial conditions, for combined samples with 2 nuclides and 3 parameters.
A simple fix could be to hold values constant—assume a denudation rate and/or zero initial peat thickness—and then calculate the other two parameters for every sample, resulting in a range of plausible parameters.
This being said, I apologize, if I misunderstood part of the manuscript and the samples were indeed jointly inverted. In that case, some method clarification and a better documented synthetic text with several samples would be nice.
Other comments:
Production rate scaling. The authors use Stone 2000 scaling. However, compared to the last few million years the geomagnetic field is comparatively strong at the moment. This may lead to an underestimation of long-term production rates. Maybe given the other uncertainties in the inversion this effect is small enough to be neglected. In that case, the authors could add a brief statement explaining their reasoning.
The 10Be and 26Al measurement data need to be reported in more detail to ensure reproducibility. Currently, only the final concentrations are reported in table 1. However, the authors should report all relevant data that were used to convert AMS measurements to concentrations (AMS ratios, sample weights, carrier concentration, blank values, etc.).
I was confused as to why the authors only focus on Si for the weathering rates. Why weren’t the total dissolved solids considered? Why was the standard CDF equation modified to Si, given that supposedly all other major elements were measured by ICP-OES?
Would be nice to add a picture of a soil pit and sample location within the pit.
Attenuation length: The authors calculated a separate peat attenuation length. This makes a lot of sense. However, the peat depth according to the authors is only 0.2-2.1 m at the moment. Meaning that a lot of production will occur under the peat and thus have a different attenuation length. Does the forward model in the inversion use both attenuation lengths (for peat and below peat)?
L13: Seems like there’s a word missing after “explore”.
L122: What was the grain size of the stream sediment samples?
L159: Here or in the cosmogenic nuclide result table, report the carrier concentration.
L202: I suggest reporting the exponential parameters here or in the supplement. Otherwise, interested people would need to dig through the code.
L214: Missing i- for production pathway
L237: I presume the authors mean denudation rate, instead of erosion.
L440: Please, elaborate on your argument. The authors note the “nature of these lenses” make them unlikely to have had sufficient shielding effect. This is a vague statement that could be explained better.
For questions about this review, feel free to get in contact.
Richard Ott
Citation: https://doi.org/10.5194/egusphere-2026-36-RC2
Data sets
Geochemistry data from the Hautes Fagnes, Belgian Ardennes Angus Moore https://doi.org/10.5281/zenodo.18018526
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General comments
Peatlands are important carbon reservoirs at the Earth’s surface; their shrinking or thickening, partially affected by human activities, can contribute to carbon release or sequestration and thus exert an impact on climate to some extent. This manuscript by Moore et al. focuses on the evolution of upland peat thickness in a slowly eroding area in Belgium, using paired 10Be-26Al in underlying saprolite and nearby stream sediments. The 10Be-26Al concentrations suggested lower values compared to steady-state conditions at current peat thickness, hinting at a thicker cover in the past. The authors attributed the shrinkage of the peatland to enhanced human activities. Note that I am not an expert in peatlands or burial dating, and thus I may not be able to comment on part of the paper. In general, the utilization of cosmogenic nuclides in tracking peat thickness based on its shielding effect on underlying saprolite seems to be a novel application. Nevertheless, such an application relies on some assumptions, as stated in the manuscript, and the degree of uncertainty they may introduce is difficult to quantify, which might affect the robustness of the data interpretation. I hope the authors find the comments below useful during their revision process.
Specific comments
1. Does this application rely on an assumption of time-invariant saprolite denudation? However, if peat thickness can vary—for example, from being completely removed to a thick cover—the denudation rate may also vary to a large extent, ranging from fast erosion by runoff to slow erosion due to the shielding effect of the peat cover?
2. The study area is characterized by a MAT of 6.7 °C. As such, I would assume that the temperature in the LGM could be very close to the freezing point (mentioned in the manuscript). In this case, even if there was no glacial coverage (please provide literature) in the past (e.g., 1 Myr), periglacial processes could play an important role in surface denudation, particularly during past glacial periods. How does this process potentially affect your modeled denudation rate?
3. Eq. (2). The calculation of the long-term denudation rate using water chemistry is indeed inappropriate. Beyond the timescale bias and effect of mineralogical sorting mentioned by the authors, the solute sources can be derived from peat, quartzite, and argillite, while the solid source only includes one source (either quartzite or argillite). This discrepancy might make the application of Eq. (2) for denudation rate calculation problematic.
4. Line 188. Is there a seasonal change in water content, and how does it affect your estimate of atomic mass?
5. Line 292: The large difference in immobile element ratios between saprolite and bedrock is indeed worrisome. But I agree it is difficult to find a convincing explanation.
6. Conclusions: Lines 577-578. Since the authors argue that the removal of peat is mainly caused by human land use in the last few hundred years, why not use a shorter-half-life nuclide such as 137Cs (Jelinski et al., 2019, JGR), which may be more appropriate for this purpose? Note that I do not mean to suggest measuring an additional nuclide, but rather to provide a short paragraph on future directions.
Technical corrections
7. Fig. 1: Perhaps add a colorbar of elevation for the inset map of Belgium.
8. Table 1: Depth to top or bottom. What does top or bottom mean? Top of the peat or top of the saprolite?