Denudation variability of the Sila Massif upland (Italy) from decades to millennia using ¹⁰Be and ^239+240Pu

Abstract

Landscapes and soils evolve in non‐linear ways over millennia. Current knowledge is incomplete as only average denudation (or erosion) rates are normally estimated, neglecting the temporal discontinuities of these processes. The determination of regressive and progressive phases of soil evolution is important to our understanding of how soils and landscapes respond to environmental changes. The Sila Massif (Italy) provides a well‐defined geomorphological and geological setting to unravel temporal variations in soil redistribution rates. We used a combination of in situ cosmogenic radionuclide measurements (10Be) along tor (residual rock) height profiles, coupled with fallout radionuclides (239+240Pu) in soils, to model soil denudation rates over the last 100 ka. We measured rates prior to the Last Glacial Maximum (LGM) of ≤30 t km−2 yr−1 (~0.036 mm yr−1). Following the LGM, during the transition from the Pleistocene to the Holocene, these rates increased to ~150–200 t km−2 yr−1 and appeared to be above soil production rates, causing regressive soil evolution. For the last ~50 years, we even describe erosion rates of ≥1,000 t km−2 yr−1 (~1.23 mm yr−1) and consider human impact as the decisive factor for this development. Consequently, the natural soil production rates cannot cope with the current erosion rates. Thus, a distinct regressive phase of soil formation exists, which will give rise to shallowing of soils over time. Overall, our multimethod approach traced denudation and erosion histories over geologic and human timescales and made a new archive to soil science and geomorphology accessible.

Climate and relief‐induced controls on the temporal variability of denudation rates in a granitic upland

“…Following Raab et al . (), we used the surface ages, and thus exhumation rates, of the individual tors to model the surface denudation rates (Figures b, d, f, b and d). We calculated average exhumation rates (≈ D Surface ) of 0.062 ± 0.037 mm yr ‐1 , 0.044 ± 0.023 mm yr ‐1 , 0.046 ± 0.036 mm yr ‐1 , 0.062 ± 0.035 mm yr ‐1 and 0.036 ± 0.031 mm yr ‐1 for Tors #1–#5, respectively (rock surface erosion rate of 0.002 mm yr ‐1 ).…”

“…Additionally, a relatively short warmer and drier climate pulse from 30 to 50 ka bp enabled wooded steppes to form here (Figure ; Allen et al ., ) which would have slightly balanced the denuding processes through a denser root network that would have withheld soil and sediment. Furthermore, soil development is strongly linked with volcanic ash influx during this time (Raab et al ., ). Fertilization through volcanic ash‐inputs (Pelle et al ., ; Raab et al ., , ) may have enhanced this development at the upland, and might help explain the low D Surface of Tor #2 (Figures d and b).…”

“…(a) Topographic map of the Sila Massif in southern Italy (after Raab et al ., ). The sampling site of the study area near the Cecita Lake is marked as a white box (Geoportale Nazionale, Ministero dell'Ambiente, Italy).…”

“…As a consequence of local long‐term deep weathering processes followed by exhumation, granitic boulder fields, tors and steep scarps have developed here which are represented by a large assortment of granitic outcrops of different sizes and shapes. Various forms depicted in the weathering scheme of Migoń () are present (Raab et al ., ). Tors and outcrops with chessboard‐like joint structures are found mostly along ridge crests but also occur in slightly undulating areas and isolated small topographic basins.…”

“…To understand landscape evolution more comprehensively, new approaches and ( in situ ) archives are needed that preserve the temporal signal of surface lowering over time. A recent study (Raab et al ., ) indicated that geomorphic forms of granite landscapes (Figure ) may preserve the denudation signal over time. Here, we explore the potential of a variety of granite landforms such as boulders, tors or scarps in more detail.…”