Anthropogenic drivers for exceptionally large meander formation during the Late Holocene

Large-amplitude meanders may form in low-energy rivers despite generally limited mobility in theses systems. Exceptionally large meanders which even extend beyond the valley sides have developed in the Overijsselse Vecht river (the Netherlands) between ca. 1400 CE (Common Era) and the early 1900s, when channelization occurred. Previous studies have attributed the enhanced lateral dynamics of this river to changes in river regime due to increased discharges, reflecting climate and/or land-use alterations in the catchment. This paper focuses on local aspects that may explain why exceptionally large meanders developed at specific sites. Through an integrated analysis based on archaeological, historical, and geomorphological data along with optically stimulated luminescence dating, we investigated the relative impact of three direct and indirect anthropogenic causes for the local morphological change and enhanced lateral migration rates: (1) lack of strategies to manage fluvial erosion; (2) a strong increase in the number of farmsteads and related intensified local land use from the High Middle Ages onwards; and (3) (human-induced) drift-sand activity directly adjacent to the river bends, causing a change in bank stability. Combined, these factors led locally to meander amplitudes well beyond the valley sides. Lessons learned at this site are relevant for management and restoration of meandering rivers in similar settings elsewhere, particularly in meeting the need to estimate spatial demands of (restored) low-energy fluvial systems and manage bank erosion.</p

Insight into the dynamics of a long-runout mass movement using single-grain feldspar luminescence in the Pokhara Valley, Nepal

Mass movements play an important role in landscape evolution of high mountain areas such as the Himalayas. Yet, establishing numerical age control and reconstructing transport dynamics of past events is challenging. To fill this research gap, we bring luminescence dating to the test in an extremely challenging environment: the Pokhara Valley in Nepal. This is challenging for two reasons: (i) the optically stimulated luminescence (OSL) sensitivity of quartz, typically the mineral of choice for dating sediments younger than 100 ka, is poor, and (ii) highly rapid and turbid conditions during mass movement transport hamper sufficient OSL signal resetting prior to deposition, which eventually results in age overestimation. Here, we first assess the applicability of single-grain feldspar dating of medieval mass movement deposits catastrophically emplaced in the Pokhara Valley. Second, we exploit the poor bleaching mechanisms to get insight into the sediment dynamics of this paleo-mass movement through bleaching proxies. The Pokhara Valley is a unique setting for our case study, considering the availability of an extensive independent radiocarbon dataset as a geochronological benchmark. Single-grain infrared stimulated luminescence (IRSL) signals were measured at 50 ∘C (IRSL-50) and post-infrared infrared stimulated luminescence signals at 150 ∘C (pIRIR-150). Our results show that the IRSL-50 signal is better bleached than the pIRIR-150 signal. A bootstrapped minimum age model (bMAM) is applied to retrieve the youngest subpopulation to estimate the paleodose. However, burial ages calculated with this paleodose overestimate the radiocarbon ages by an average factor of ∼23 (IRSL-50) and ∼72 (pIRIR-150), showing that dating of the Pokhara Formation with a single-grain approach was not successful for most samples. Some samples, however, only slightly overestimate the true emplacement age and thus could be used for a rough age estimation. Large inheritances in combination with the scatter in the single-grain dose distributions show that the sediments have been transported under extremely limited light exposure prior to deposition, which is consistent with the highly turbid nature of the sediment-laden flood and debris flows depositing the Pokhara gravels. To investigate the sediment transport dynamics in more detail, we studied three bleaching proxies: the percentage of grains in saturation 2D0 criteria, the percentage of best-bleached grains (2σ range of bMAM-De) and the overdispersion (OD). None of the three bleaching proxies indicate a spatial relationship with runout distance of the mass movement deposits. We interpret this as evidence for the lack of bleaching during transport, which reflects the catastrophic nature of the event. While not providing reliable burial ages of the Pokhara mass movement deposits, single-grain feldspar dating can potentially be used as an age range finder method. Our approach shows the potential of luminescence techniques to provide insights in sediment transport dynamics of extreme and rare mass movement events in mountainous regions.</p

NCR-days 2008 : 10 years NCR: November 20-21

De verschillende subthema’s van de NCR-dagen 2008, (i) Stroomgebied en Overstromingsrisico management (ii) Hydrologie en (iii) Geomorfodynamica en Morfologie, dekken een groot gedeelte van het hedendaagse onderzoek dat in Nederland op rivierkundig gebied wordt uitgevoerd

Evaluación de los procesos de formación de suelo en terrenos graníticos del Mediterráneo

Soil is one of the most important natural systems that support the development of life and is part of fundamental processes that make our planetary ecosystem work. It is well known that the factors that intervene in the formation of the soil are climate, bedrock, relief, biota and time. However, despite the numerous studies related to soil formation, little is known about these processes due to the immense complexity in which they are integrated. This study identifies and investigates in detail the different soil-forming processes in the study area located in a rocky Mediterranean landscape. Specifically, the processes that have been studied have been biological such as bioturbation, physical such as erosiondeposit and physical weathering. These processes are related to the spatial distribution of carbon stored in the soil, which has also been part of this study. The spatial distribution of carbon stocks in the soil is highly variable in the study area, as well as bulk density and stoniness. The variable that controls the spatial distribution of the stoniness is the slope, demonstrating the influence of the erosion. The spatial distribution of the carbon content and carbon stocks in the soil depend on vegetation cover and insolation. Bioturbation has been studied in soil profiles at different depths along a steeply inclined slope using the IRSL (Infrared Stimulated Luminescence) luminescence technique used on sand sized feldspar grains for the first time for this purpose. This technique has made it possible to identify the age of the sample at different depths and an effective soil mixing rate that takes into account only those grains that have ever been exposed to the soil surface. This has allowed, for the first time, to identify the border between the mobile regolith and the saprolite because the grains close to the border have never visited the surface.The soil age at different depths previously calculated has been used to determine the bioturbation, erosion and deposition rates along the slope through a new analytical solution of the advection-diffusion equation. Physical weathering has been assessed in the horizons of ten soil profiles by comparing rock fragmentation field data with different models. In this way, the patterns governing rock fragmentation were established in six profiles. The rest of the profiles did not fit any of these models because they are characterized by armouring effect on the surface, i.e. the surface horizon contains larger size of rocks than the lower one due to the removal of fine particles produced by erosion. The study presented here is a great advance in the knowledge of these soil-forming processes and can be implemented in other studies of the same type. In the future, this study could be compared with the results that would be obtained from erosion rates using other methods (10Be or 137Cs), completed with an estimation of the chemical processes in the study area and, finally, useful for the calibration and validation of both the new soil formation model that will be developed linked to the work developed in this thesis, as explained in chapter six, and other soil formation and landscape evolution models to better understand the connection between pedology and geomorphological processes.El suelo es uno de los sistemas naturales más importantes que sustentan el desarrollo de la vida y forma parte de procesos fundamentales que hacen funcionar nuestro ecosistema planetario. Es bien sabido que los factores que intervienen en la formación del suelo son el clima, la roca madre, relieve, biota y el tiempo. Sin embargo, a pesar de los numerosos estudios relacionados con la formación del suelo, poco es conocido sobre estos procesos debido a la inmensa complejidad en la que se integran. Este estudio identifica y profundiza en el conocimiento de varios procesos formadores del suelo en la zona de estudio situada en un ambiente Mediterráneo. Específicamente, los procesos que se han estudiado han sido biológicos como la bioturbación, físicos como la erosión-depósito y la meteorización física. Estos procesos están relacionados con la distribución espacial de carbono almacenado en el suelo, la cual también ha formado parte de este estudio. La distribución espacial del carbono almacenado en el suelo ha resultado ser muy variable en la zona de estudio, caracterizada por ser rocosa, al igual que la densidad aparente y la pedregosidad. Las variables que controlan la distribución espacial de la pedregosidad es la pendiente, demostrándose la influencia de la misma en la erosión. La distribución espacial del contenido de carbono y carbono almacenado en el suelo dependen de la cobertura vegetativa y de la insolación. La bioturbación se ha estudiado en perfiles de suelo a distintas profundidades a lo largo de una ladera con pronunciada pendiente, a través de la técnica de luminiscencia IRSL (Infrared Stimulated Luminescence) usada en granos de feldespato tamaño arena por primera vez para dicho fin. Esta técnica ha permitido identificar la edad de la muestra a diferentes profundidades y una tasa efectiva de mezcla del suelo que tiene en cuenta sólo los granos que alguna vez vieron la luz en la superficie del suelo. Esto ha hecho posible que, por primera vez, se haya identificado el borde entre el regolito móvil y el saprolito debido a que los granos cercanos a esta franja nunca han visitado la superficie. La edad del suelo a diferentes profundidades anteriormente calculada ha sido usada para determinar las tasas de bioturbación, erosión y depósito a lo largo de la ladera a través de una nueva solución analítica de la ecuación advección-difusión. La meteorización física ha sido evaluada en los horizontes de diez perfiles de suelo comparando los datos de campo de fragmentación de rocas con diferentes modelos. De esta forma, se establecieron los patrones que rigen la fragmentación de partículas en seis perfiles. El resto de los perfiles no se ajustaron a ninguno de estos modelos por estar caracterizados por el efecto armouring en la superficie, es decir, el horizonte superficial contiene un tamaño de rocas mayor que el inferior debido a la eliminación de las partículas finas producida por la erosión. El estudio presentado aquí es un gran avance para el conocimiento de estos procesos formadores del suelo y puede ser implementado en otros estudios de la misma índole. En el futuro, este estudio podrá ser comparado con los resultados que se obtendrían de tasas de erosión usando otros métodos (10Be or 137Cs), ser completado con una estimación de los procesos químicos en el área de estudio y, por último, podría ayudar a la calibración y validación tanto del nuevo modelo de formación de suelo que se pretende desarrollar vinculado al trabajo desarrollado en esta tesis, tal y como se explica en el capítulo seis, como de otros modelos de formación de suelo y evolución de paisaje para entender mejor la conexión entre la pedología y los procesos geomorfológicos

Contribution of laser altimetry images to the geomorphology of the Late Holocene inland drift sands of the European Sand Belt

The paper explores the possibilities of applying the analysis of laser altimetry images to Dutch drift sands. All along the European Sand Belt, which stretches from Great Britain to the Ural Mountains, Late Glacial cover sands, river dunes and other ice–age deposits were reactivated as drift sand during the Holocene. New insights were obtained in three aspects of drift–sands geomorphology. First, the variety in forms of drift–sand landscapes is often described as chaotic. Laser altimetry images show that complex clusters are formed elongated in the direction of the prevailing SW wind and consisting of three zones which correspond to the successive aspects of the aeolian process: deflation, transport and deposition (dune formation). In densely populated areas, this structure has been ruined by human activities. Second, contrary to common belief, the drift–sand cells expanded against the prevailing SW wind whereas the characteristic comb dunes at the opposite NE edge remained fixed by vegetation. Third, the authors questioned the view that drift sands are due to anthropogenic activities. The origin of drift sands can best be explained by the climate with violent storms in the first part of the past millennium

Towards timing and stratigraphy of the Bronze Age burial mound royal tomb (Königsgrab) of Seddin (Brandenburg, northeastern Germany)

This study uses an integrated multi-method geoarcheological and geochronological approach to contribute to the understanding of the timing and stratigraphy of the monumental burial mound royal tomb (Königsgrab) of Seddin. We show that the hitherto established radiocarbon-based terminus post quem time frame for the construction of the burial mound of 910–800 BCE is supported by optically stimulated luminescence (OSL) dating. The radiocarbon samples were obtained from a substrate directly underneath the burial mound which supposedly represents the late glacial/Holocene soil that was buried below the structure. We use sedimentological (grain-size analyses) and geochemical analyses (element analyses, carbon, pH, and electric conductivity determinations) to reassess and confirm this hypothesis. In addition to the burial age associated with the last anthropogenic reworking during construction of the burial mound, the OSL dating results provide new insights into the primary deposition history of the original substrates used for the structure. In combination with regional information about the middle and late Quaternary development of the environment, our data allow us to provide a synoptic genetic model of the landscape development and the multiphase stratigraphy of the royal tomb of Seddin within the Late Bronze Age cultural group “Seddiner Gruppe” of northern Germany. Based on our initial experiences with OSL dating applied to the sediments of a burial mound – to the best of our knowledge the first attempt in Europe – we propose a minimal invasive approach to obtain datable material from burial mounds and discuss related opportunities and challenges

Modern impacts on an ancient landscape, the piedmont plain in southwest Turkmenistan

The piedmont plain in southwestern Turkmenistan has experienced a millennia-long settlement history despite prevailing arid climates. One of the prerequisites for the various agricultural efforts was irrigation. Most of the water used for irrigation measures came from the adjacent Kopet Dag mountain chain. This situation changed with the introduction of the Karakum canal in the middle of the 20th century. The present study evaluates the rich irrigation history of the piedmont plain by investigating two small catchments that drain the eastern ranges of the Kopet Dag. Within their catchments, geomorphological and hydrological analyses were conducted. We present several Optically Stimulated Luminescence and 14-C dating results that add to the understanding of the landscape history from the Pleistocene until modern ages. Moreover, modern climatological and hydrological data were analyzed that show a remarkable drop in runoff from the Kopet Dag since the 1960s