Long-term peat thickness from cosmogenic 26 Al and 10 Be, Hautes Fagnes, Belgian Ardennes

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 the use of paired 26 Al and 10 Be to simultaneously determine denudation rates and peat thicknesses averaged over multiple glacial-interglacial cycles. We report cosmogenic 26 Al and 10 Be 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 26 Al / 10 Be ratios are lower than expected for steadystate 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 26 Al and 10 Be concentrations. Modelled 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.