Stratified Soil on Dictyonema Shales in the Area of the Last Cover Glaciation (Southern Ladoga Region): Properties, Paleogeographic Interpretation, and Radiocarbon Dating Issues

The morphological features at the macro-, meso-, and microlevels and the properties of a unique virgin dark-humus soil (Phaeozem) developed from locally redeposited dictyonema shales overlain by a thin layer of late glacial silty loam and fine sand of eolian genesis have been studied in the Valdai glaciation area, in the Lyubsha River valley (right tributary of the Volkhov River, Leningrad oblast). The eolian sediments are enriched with carbonate gravel and glauconite material. Local displacement of the Dictyonema shales in the Lyubsha section is confirmed by the data of spore-pollen analysis of the layers at the base of the section, where the dictyonema shales are underlain by the Cambrian sand. The pollen spectra of these layers correspond to tundra vegetation. The shale thickness was subjected to cryogenic cracking intensifying from the lower to the upper horizons of the profile according to the micromorphological data. The soil developed under subaerial conditions beginning from the Late Pleistocene. The Holocene pedogenesis has affected the upper allochthonous stratum and the upper layer of the shales (AUcа horizon), in which structuring and loosening of the shale material are noticeable along with the presence of shale fragments with undisturbed microstructure. Such a weak manifestation of soil formation on dictyonema shales is determined by the high buffer capacity of the parent material and the predominance of the difficultly oxidizable fraction in the organic matter of shales. The determined radiocarbon age of the organic matter of dictyonema layers, which varies from the lower to the upper horizons within ~36–24 ka cal BP (the Lyubsha section) and ~35–26 ka cal BP (the Kirchhoff Hill section), does not reflect the real age of soil formation. The organic matter of these soils is heterogeneous and heterochronous. Both the relict organic matter of shales and the younger organic matter of pedogenic origin contribute to the radiocarbon age of the samples. This should be taken into account in interpretation of the results of radiocarbon dating.