Abstract:

Since the onset of agricultural activity on loess-covered landscapes, the natural circulation of matter on hillslopes has been profoundly altered. Disturbance of the natural vegetation cover by agricultural practices has increased the susceptibility of soil material to detachment and transport by raindrop splash, sheet wash, and rill erosion. Today, nearly all agricultural loess areas in Poland are affected by water-induced soil erosion, resulting in the accumulation of colluvial deposits at the footslopes.
The highest rates of soil redistribution occur during intense rainfall events and rapid snowmelt. During such episodes, only part of the eroded material is deposited locally, whereas most of it is transported into river valleys. Between these extreme events, soil is gradually redistributed over short distances by sheet wash and agricultural tillage. The proportion of sediment retained on slopes and accumulated at their foot depends on slope geometry, land use, and climatic conditions. Ephemerally drained dry valleys are widespread within the Polish loess and contain the thickest Holocene colluvial sequences. In many studied profiles, these deposits overlie buried soils that developed under natural environmental conditions prior to the onset of agricultural activity. Less frequently, colluvial units are separated by younger buried soils representing periods of hillslope stability. Consequently, Holocene colluvial deposits record phases of accelerated soil erosion associated with agricultural land use, whereas buried soils document intervals of reduced erosion and landscape stabilization.
A major challenge in the investigation of Holocene colluvial deposits is the absence of diagnostic sedimentological features allowing individual colluvial units to be correlated with archaeologically recognized phases of agricultural activity. In areas subjected to repeated cycles of cultivation, reconstruction of hillslope history based solely on lithological characteristics is therefore extremely difficult. Colluvial deposits with similar sedimentological properties may have accumulated during different periods, while deposits of different lithology may have formed contemporaneously. Until now, the chronological framework of colluvial sequences has been based mainly on radiocarbon dating of buried soils underlying the colluvium and, occasionally, on archaeological artefacts recovered from their basal parts.
Optically Stimulated Luminescence (OSL) dating offers one of the most promising approaches for establishing the chronology of Holocene colluvial deposits. Radiocarbon dating of soil organic matter within colluvium is often unreliable because much of the organic material has been redeposited. Likewise, the age of isolated organic macroremains, such as charcoal fragments, does not necessarily correspond to the time of sediment deposition. In contrast, OSL dating enables direct determination of the depositional age of sediments based on quartz grains and can be applied to samples collected from any part of a sediment profile.
This presentation summarizes the results of OSL dating of colluvial deposits collected from loess profiles located in southern Poland, where agricultural activity has persisted since the Neolithic and where the archaeological record is well established. For the youngest deposits, OSL chronology is complemented by analyses of the fallout radionuclides 137Cs and excess 210Pb (210Pbex) as well as by dendrochronology. The presentation will discuss the physical principles of OSL dating, analytical procedures, and the main limitations associated with the interpretation of luminescence ages. The obtained results demonstrate that, despite certain limitations, OSL provides the only method currently available for direct dating of Holocene colluvial deposits and constitutes a powerful tool for reconstructing the history of soil erosion in agricultural landscapes.