Pleistocene Periglacial Slope Deposits Research Articles

Quantification of the effect of loess admixture on soil hydrological properties in sandy slope deposits

The admixture of loess in soils formed in sandy parent materials has considerable impact on pedogenesis and soil ecological functions. This study aimed to evaluate these effects on soil hydrology by quantifying the relationships between loess content and soil hydrological properties in a sandstone landscape covered by Pleistocene periglacial slope deposits in central Europe. Studied properties were saturated soil water capacity, field capacity, permanent wilting point, available water capacity, macro-porosity, matrix porosity and saturated hydraulic conductivity. The effect of loess addition on soil hydrological properties differed between topsoil (pedogenic A horizons) and subsoil (pedogenic B and C horizons). In the subsoil, the studied soil hydrological properties were mainly controlled by loess content. By contrast, in the topsoil the effects of loess content on soil hydrological properties were largely masked by the effects of soil organic matter (reflected by soil organic carbon, SOC). The enhancing effect of SOC on soil hydrological properties was most prominent for saturated soil water capacity and was also significant for all other parameters except permanent wilting point. After the effects of SOC were accounted for, the residual effects of loess on soil hydrological properties were the same in both topsoils and subsoils, with one exception: Saturated hydraulic conductivity decreased with increasing loess content for subsoils, but not for topsoils. This study highlighted the ecohydrological significance of loess admixture in coarse-textured soils, especially for subsoils with low SOC contents. However, for coarse-textured topsoils, SOC content plays the dominant role in affecting soil hydrological properties.

Loess in Pleistocene periglacial slope deposits and Holocene colluvium of European low mountain ranges: Mixing processes and spatial variations

Loess, as an allochthonous material, plays a significant role in the formation of Pleistocene periglacial slope deposits (PPSD), and thus also for Holocene soil formation, in the central European low mountain range. Although the differing loess proportions in different layers of the PPSD have been well recognized, quantitative distribution and mixing processes of loess in PPSD are still not fully understood. Detailed particle-size distribution was applied for quantifying loess proportions in PPSD in a sandstone area in central Germany. The bimodal pattern of the particle-size distribution curves clearly reflected loess and sandstone-weathering material as two components of the loess-affected layers, which allowed for the quantification of these two components by applying a two-endmembers unmixing model. By inspecting variations of particle-size distribution curves both vertically and spatially, we gained insights into the mixing processes of loess in PPSD and Holocene colluvium. Our results suggest that the loess component in upper layers most likely results from the incorporation of preexisting loess. Holocene colluvium and upper layers were thoroughly homogenized by various earth-surface processes. Radiocarbon dating of charcoal indicated strong human impact during historic times. Close correlation between the overall thickness of loess-containing layers and the depth-weighted loess content pointed to the erosional-depositional history at each site. Our research demonstrates the utility of detailed particle-size data for detecting and interpreting geomorphological processes and landscape evolution. It shows that the spatial distribution patterns of loess are subjected not only to topographic factors controlling primary loess deposition, but also to erosional-depositional histories and anthropogenic impact.

Short Communication. Quantification of loess proportions in Pleistocene periglacial slope deposits and Holocene colluvium using grain‐size data by laser diffractometry

AbstractLoess proportions in slope deposits have rarely been quantified, due to the lack of quantification methods. In this study, we used laser granulometry to analyze grain‐size distributions in slope deposits of a Buntsandstein landscape in central Germany. The consistent loess contents obtained from two independent models corroborate the validity of the calculated loess proportions and the applicability of the new R package EMMAgeo. Yet, some limitations in transferability need to be considered.

Chronostratigraphy of silt-dominated Pleistocene periglacial slope deposits on Mt. Ślęża (SW, Poland): Palaeoenvironmental and pedogenic significance

Slope deposits with aeolian silt admixture are a widespread parent material of soils in the temperate zone but may be neglected when rates of soil production are quantified. The concept of periglacial cover beds differentiates slope deposits with or without aeolian silt admixture; yet there is a remaining debate on processes and the timing of their formation. A previous study done by us at Mt. Ślęża, SW Poland, concluded that slope deposits with variable aeolian silt admixture, or its lack, have a significant influence on the pathway of soil formation. The present work builds upon this finding, by adding further granulometric and micromorphological data from three representative profiles along a toposequence, in order to refine our understanding of local slope deposits and soil formation. Additionally, seven numerical ages using luminescence dating provide a chronological framework for our reconstructions and allow linking the forming processes of these pedosedimentary records to regional palaeoenvironmental conditions. The oldest aeolian deposits are of Middle Pleistocene age (>280 ± 19 ka) with interlayered palaeosol (marine isotope stage [MIS] 9 or older). Late Pleistocene slope deposits encompass the maximum loess thickness and are dated to MIS 2. Luminescence ages from the upper layers indicate shallow reworking, which we tentatively correlate to the Younger Dryas (YD). Two profiles with thick loess mantles have strong clay illuviation features, presumably formed during the Holocene. However, weak clay illuviation in the third profile with a thin loess mantle (having an age of YD) over granite regolith seems to have occurred before the Holocene, as only fragmented clay coatings (probably MIS 2 pedogenesis) could be found.

Field Trip D (27 September 2018): characteristics and development of the Mesozoic–Tertiary weathering mantle and Pleistocene periglacial slope deposits in the Hintertaunus mountainous region

Abstract. The Devonian slates and sandstones of the Rhenish Massif were subject to deep and intensive weathering under (sub)tropical climate conditions during the Cretaceous, the Paleogene and the Neogene, which caused the development of a weathering mantle (regolith) >100 m thick, consisting of kaolinitic saprolite and paleosols as well as correlated sediments. Especially the tectonic uplift of the Rhenish Massif and climate change during the Neogene and the Pleistocene led to a vast denudation of the weathering mantle. Only in less uplifted areas of the mountainous region did thick remnants of saprolites remain, and they were covered by Neogene sediments as well as Quaternary periglacial slope deposits. As the kaolinitic weathering products serve as raw materials for the clay industry, unique exposures are available in the Hintertaunus which offer impressive insights into the landscape development of the past ∼80 million years: the excursion proceeds from Giessen to Limburg and further south and southwest to the eastern and western Hintertaunus area. At site 1 near the village of Langhecke, characteristics and properties of the fresh, unweathered slates will be demonstrated. Excursion sites 2 and 3 are situated near the village of Eisenbach. In two open-cast clay mines, both a terrestrial and a semi-terrestrial saprolite from silt slate, covered by periglacial layers, are exposed. Properties and genesis will be discussed on the basis of morphological characteristics and mineralogical and geochemical analyses, as well as isovolumetric elemental mass balances. At site 4 a former basalt quarry near the village of Biebrich exposes a Paleogene Plinthosol above saprolite. The autochthonous paleosol was preserved below Upper Oligocene basalt tuff and periglacial layers. Site 5 is situated within a huge pit for mining of Upper Oligocene to Miocene quartz gravel near the village of Wasenbach. A Miocene Plinthosol developed from alluvial sediments on top of the gravel beds and was covered by periglacial slope deposits. At nearly all sites the basal layers of the periglacial cover beds consist of kaolinitic paleosol/saprolite material, which has an important influence on the site properties of the Holocene soils.

Genesis and evolution of regoliths: Evidence from trace and major elements and Sr-Nd-Pb-U isotopes

Regoliths encompass different materials from the fresh bedrock to the top of the organic horizons. The occurrence and evolution of these materials are determined by deposition, erosion and weathering processes that are specific for each region. The origin and interaction of the regolith layers are still important issues in the study of critical zone functioning. Studies that attempt to understand the processes responsible for the structure and the evolution of regoliths often focus either on the soil or on the bedrock compartment, and often from a narrowly focused approach using just a single tool such as major element patterns or mineralogy. This limits the understanding of the complete regolith composition and evolution. The present study proposes the combination of mineralogy, major and trace element concentrations, and Sr-Nd-Pb-U isotopes as an extremely useful procedure for tracking the spatial and temporal origin of regolith materials and understanding their stratification. This multi-tracer approach was applied on a regolith profile from the south-western edge of the Rhenish Massif, which is characterised by the occurrence of slate bedrocks and overlying Pleistocene Periglacial Slope Deposits (PPSD). The data allowed the differentiation of three compartments within the regolith: 1) the organic compartment, where low 206Pb/207Pb and 87Sr/86Sr isotopic ratios reflected the impact of atmosphere-derived components; 2) the PPSD compartment, whose upper horizons were characterised by the presence of atmosphere-derived particles rich in Cd, Sn, Sb, Hg and Pb and by relatively low 206Pb/207Pb and 87Sr/86Sr ratios, whereas the lower horizons showed Nb enrichments in their matrix and a mineralogical composition pointing to ancient volcanic events; and 3) the weathered slate compartment, whose weathering progression and genetic relationship with the PPSD were particularly well tracked with 87Sr/86Sr and 143Nd/144Nd isotope ratios, (234U/238U) activity ratios and positive Ce anomalies. The results prove the efficiency of radiogenic isotope measurements combined with the analysis of major and trace elements to complete regolith studies. Moreover, these isotopes and elements are shown to be potential geochemical tracers of exchange processes at the water-mineral interface. This approach allows for the first time the description of an evolution scheme of a complete slate regolith, which covers a large part of the Rhenish Massif.

Formation and transformation of Pleistocene periglacial slope deposits in the Spessart Mountains (Hesse, Germany)

The bedrock of the Spessart Mountains is covered by Pleistocene periglacial slope deposits (PPS- Ds). These slope deposits are supposed to have formed in a periglacial environment by a variety of processes. For this study soil profiles in the Spessart Mountains were analyzed with respect to features elucidating processes forming and transforming PPSDs. Field and laboratory analyses were complemented by micro- morphological investigations to gain more detailed information. The results verify aeolian sedimentation, solifluction and cryoturbation as most important formation processes. Furthermore, several findings sug- gest the influence of additional genetically active processes. PPSDs provide the parent material for Holocene soil formation. In addition, they experience alteration by bioturbation and anthropogenic influence on relief and soil. Pedogenic transformation is dependent on the PPSDs material composition. Particularly the content of silt-sized allochthonous material controls the efficacy of silicate weathering and depth of soil development. The coincidence of pedogenic boundaries and layers often reported in the literature is partly falsified: Eluviation and illuviation of clay is not separated at the boundary between upper and intermediate layer. PPSDs experience strong alteration by bioturbation: This process may transform characteristics that could point to formation processes. Furthermore, thin sec- tion analyses support the distinction of Holocene colluvium and PPSDs.

Applicability of ground‐penetrating radar as a tool for nondestructive soil‐depth mapping on Pleistocene periglacial slope deposits

AbstractSoil depth plays a decisive role in determining soil properties in mountainous regions for ecological site assessment. To evaluate the use of ground‐penetrating radar (GPR) for fast and high‐resolution mapping within mountainous regions, we examined the possibilities and limitations of GPR to determine soil depth over bedrock and to delineate individual substrate layers formed during the Pleistocene in a periglacial environment (Pleistocene periglacial slope deposits, PPSD). Selected catenae in representative subregions of the study area (Dill catchment, SE Rhenish Massif, Germany) have been successfully mapped using GPR. A practicable method was developed using a 400 MHz antenna to reach a mean penetration depth of 1.5 m and to map different substrates and layers of PPSD based on calibrations of the GPR at soil pits along 12 catenae. Colluvium, the three types of PPSD layers, as well as the in situ bedrock could be distinguished in most sections of the GPR surveys. Characteristic GPR facies caused by intrinsic material properties of the different substrates, such as stone content and soil moisture content, could be distinguished in different geomorphologic and lithological settings. A layer‐based velocity distribution was determined for characteristic substrate layers at soil pits enabling us to considerably enhance the accuracy of soil‐depth prediction. Compared to traditionally surveyed soil profiles, our results demonstrate an accuracy of layer thickness surveying within a standard deviation of approx. 0.1 m. It is demonstrated that the combination of GPR with conventional soil‐pit mapping is an efficient and valid method to produce high‐resolution data of substrate distribution.

Character, Age, and Ecological Significance of Pleistocene Periglacial Slope Deposits in Germany

Periglacial slope deposits (PSD) of Pleistocene age are one of the most widely distributed sediment types in nonglaciated mid-latitude regions. As diamictons of different genesis (geli-solifluction, cryoturbation, eolian transportation) they have a wide range of physical, chemical and mineralogical characteristics which are mainly controlled by bedrock geology, including saprolite. This paper reviews concepts used in Germany to study PSD stratigraphy and describes the different PSD units encountered (Upper Head, Middle Head, Lower Head). The paper is relevant to the understanding of PSDs in other mid-latitude regions. Classical and new results concerning the age of PSDs show differences in the interpretation of Upper Head genesis. In terms of their ecological significance, case studies from eastern Bavaria illustrate the influence of PSDs on the character and intensity of slope hydrological processes as well as on the formation and distribution of soils. Presently important research questions include (1) the establishment of a genetically based classification of PSDs, (2) the intensification of dating studies on PSDs, and (3) the clarification of hydrological processes in PSDs.

GPR-based detection of Pleistocene periglacial slope deposits at a shallow-depth test site

Pleistocene periglacial slope deposits (PPSD) almost completely cover the mountainous regions of Germany and are the parent material for soil formation. It is therefore necessary to map their thickness and spatial distribution. The mapping of periglacial slope deposits can be performed more quickly than conventional probing by drilling or soil profile analysis if ground penetrating radar (GPR) is used. Because the use of GPR to map soils or soil depth depends to a large degree on the suitability of soil properties, the performance of GPR in mapping layered substrates was evaluated at a test site with varying water content and substrate properties. We focus on the efficiency of GPR in mapping loess-rich layers above layers with a high stone content within a depth range of 1 m. Different materials such as loess, metabasalt and slates, including heavily weathered slates (saprolite) were studied. The test site was equipped with several time domain reflectometry (TDR) probes to monitor soil water content changes for the period of one year. Systematic investigations of the reflection patterns were performed with 100, 200, 400, 500 and 900 MHz antennae with respect to the quality of measurement, dielectric coefficient and soil water content of different material combinations. It became clear that the 400 MHz antenna is most suitable for creating images of loess and stone layers at the depth range of 1 m and under natural climatic conditions. The results prove that the most important effect the substrates at the test site have on GPR is produced by the differences in water content of bordering layers, making the soil water capacity of the materials a decisive parameter for evaluating GPR performance. The water content dependent dielectric permittivities show higher values for saprolite (heavily weathered slate) compared to loess by a factor of 1.3 to 2, with water content increasing from 15 to 40 vol.% respectively. In doing so we discovered the dielectric permittivity of loess to be 1.2 to 1.5 times higher than that of unweathered slate at the same water content. We were able to locate horizontal layer boundaries at the test site starting at a minimum of 0.5 vol.% water content difference and a reflection coefficient r ≥ 0.02 between the layers. The results obtained at the test site show the ability of GPR to detect layering in PPSD at different water contents and help to predict the performance of GPR measurements in mapping of PPSD.

Saprolite, soils, and sediments in the Rhenish Massif as records of climate and landscape history

The Rhenish Massif, at the western boundary of Germany, consists predominantly of Devonian slates and siltstones, which over large areas have been altered to saprolite. The distribution and characteristics of saprolites, Pleistocene periglacial slope deposits (PPSD), Holocene colluvium and soils were investigated along a 2.5 m deep gas pipeline ditch in 20 sections with an overall length of 19 km.The results are used for landscape history reconstruction. Under warm-humid conditions of the Upper Mesozoic and Lower Tertiary, intensive weathering led to formation of a planation surface covered by thick saprolites and kaolinitic soils. During Pleistocene glaciations, the area experienced periglacial conditions that resulted in erosion of the soils and parts of the saprolites. Several layers of PPSD developed. During the Holocene no significant slope processes occurred, until human deforestation led to accelerated hillslope erosion. As a result, today the PPSD are partly eroded or covered by Holocene colluvium in many places. The sediment distribution follows distinct rules. Generally, the sediment thickness increases downslope. Concave slope sections show a substantial increase in thickness and number of preserved sediment layers. The soils developed in PPSD on saprolite are primarily Planosols, while Cambisols and Luvisols predominate in PPSD on fresh rock.

Application of ground‐penetrating radar to determine the thickness of Pleistocene periglacial slope deposits

AbstractWide areas of the mountainous regions of Germany have rock covered by Pleistocene periglacial slope deposits (PPSD), formed by gelifluction during the cold periods of the ice ages in non‐glaciated areas. The PPSD provide the parent material for soil development, and their physical characteristics affect several stabile soil properties. Because the PPSD play a significant ecological role, we studied the spatial distribution and properties of the PPSD in order to assess the distribution of the stabile soil properties. The high stone content of the PPSD greatly hinders augering and digging. Hence, we tested the use of ground‐penetrating radar (GPR) as a potentially time‐saving, non‐destructive method to determine the thickness of the PPSD. In several study areas of the Rhenish Massif, GPR investigations of single soil profiles and soil transects along an exposed gas‐pipeline ditch were carried out. The GPR images were compared to the actual thickness of the layers of the PPSD exposed in the profiles and the ditch. In the GPR images usually at least one distinct boundary could be identified, which occurs at the transition between the loose material and the hard rock, mostly ranging between 50 and 150 cm depth. In some cases, in which stone content changed abruptly between different layers of the PPSD, also the boundaries between these layers could be identified in the GPR image. On the other hand, in areas where remnants of the Mesozoic‐Tertiary weathering mantle are preserved, the boundary between the saprolite and the overlying basal layer of the PPSD is ambiguous or not at all visible.