Contrasting Soil Dynamics in a Formerly Glaciated and Non-Glaciated Mediterranean Mountain Plateau (Serra Da Estrela, Portugal)

Abstract

After deglaciation, the landscape and freshly exposed sediments or bedrock tend to reach a new quasi steady-state. Since soil properties change over time, soil erosion rates are expected to change in parallel. Depending on the environmental conditions, slope stability attains a new equilibrium earlier or later. So far, only few data is available on how soil erosion rates compare between surfaces of different ages. Furthermore, short-term processes that often are caused by human impact may overprint the longer-term erosion signal and a distinction between surfaces of different ages becomes difficult, particularly in Mediterranean areas where human impact is obvious not only for the last few decades but for many millennia. We determined soil dynamics and characteristics among two end-member sites ('young' vs. 'old' geomorphic surface) at the Serra da Estrela (Portugal). We applied a multi-isotope approach (10Be,δ13C, 239+240Pu) and the principles of the percolation theory to disentangle short-term from long-term soil redistribution rates. Soil dynamics of a formerly glaciated ('young', maximum glacial extent at 22 – 30 ka BP) and non- glaciated ('old') area were identified. The formerly glaciated area has soils with a degree of lower weathering, lower carbon content and a higher soil density compared to soils of the non-glaciated area. Surprisingly, the non-glaciated area and thus old surface had the highest short-term (decades) soil erosion rates in the range of 900–1700 [t km-2 yr-1]. As a consequence, these soils are in degrading conditions. The younger soils, however, exhibited short-term (last few decades) soil deposition rates between 130 and 230 [t km-2 yr-1]. In contrast to the initial theory, the younger soils seemed to be closer to equilibrium than the older soils. Human impact, frequent bush fires and animal grazing are probable causes for the higher degrading at the formerly non-glaciated area. The average long-term (millennia) soil redistribution rates, however, corresponded to the expected lower rates at the young and higher at the old surface. The combined use of meteoric 10Be, δ13C, 239+240Pu and their distribution along the soil profiles enabled us to disentangle soil mixing process. It is hypothesised that the slightly higher elevation and formerly glaciated sites experienced cryoturbation effects over a longer period of time than the older, non-glaciated soils where for the last few decades, cryoturbation was less active or absent. We also show that decadal soil redistribution rates may be up to one order of magnitude higher than the long-term rates. These trade-offs in erosion and soil formation control landscape dynamics of Mediterranean uplands.