| 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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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?