Soil characteristics and erosion in the UNESCO Geopark Estrela, Portugal.

In Europe, a high soil erosion risk is modelled for the Mediterranean area such as the Iberian Peninsula (e.g., EEA, 2009), while actual field data often lacks behind. Here we present the first ^239+240Pu soil erosion results (last ~60 years) in the UNESCO Geopark Estrela, Portugal. We investigated soils in a former vastly glaciated and a non-glaciated area. We hypothesized that erosion rates in relatively young areas (max. about 16–20 kyr) will be distinctly higher than in old areas (several 100 kyr). We assumed that soil structure, organic matter and weathering degree in younger (natural) soils are still less favourable and do not yet protect efficiently enough soils from erosion. Besides soil erosion, we explored the weathering degree of the soil material using chemical weathering indices, determined the soil surface age using meteoric ¹⁰Be and looked at a broad set of physico-chemical soil characteristics of these two landscape settings.

A glimpse of our first Plutonium results indicates that the differences between these two settings are rather minor. Soil erosion rates in these natural conditions (Geopark) predominantly depend on slope. With increasing slope angle, a maximum soil erosion rate of ~1600 [t km^-2 yr^-1] is reached. Not surprisingly, the age estimates of the soils within the formerly glaciated area confirmed the start of formation after the beginning of ice-decay. The formerly glaciated area is depleted in C and N compared to the never glaciated area. In the never glaciated area, a higher soil weathering degree is found by multiple weathering indices and an overall lower SiO₂ content. Although past glacial activities rejuvenated the soil material (expressed by a lower weathering degree) and affected the soil organic matter content, soil erosion susceptibility does not seem to be higher compared to never glaciated areas. Under natural conditions, a quasi-steady state with respect to soil erosion seems to be reached fairly before 20 kyrs.