| 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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