Soil erosion and organic carbon export by wet snow avalanches

Many mountain belts sustain prolonged snow cover for parts of the year, although enquiries into rates of erosion in these landscapes have focused almost exclusively on the snow-free periods. This raises the question of whether annual snow cover contributes significantly to modulating rates of erosion in high-relief terrain. In this context, the sudden release of snow avalanches is a frequent and potentially relevant process, judging from the physical damage to subalpine forest ecosystems, and the amount of debris contained in avalanche deposits. To quantitatively constrain this visual impression and to expand the sparse literature, we sampled sediment concentrations of <i>n</i> = 28 river-spanning snow-avalanche deposits (snow bridges) in the area around Davos, eastern Swiss Alps, and inferred an orders-of-magnitude variability in specific fine sediment and organic carbon yields (1.8 to 830 t km⁻² yr⁻¹, and 0.04 to 131 t C km⁻² yr⁻¹, respectively). A Monte Carlo simulation demonstrates that, with a minimum of free parameters, such variability is inherent to the geometric scaling used for computing specific yields. Moreover, the widely applied method of linearly extrapolating plot scale sample data may be prone to substantial under- or overestimates. A comparison of our inferred yields with previously published work demonstrates the relevance of wet snow avalanches as prominent agents of soil erosion and transporters of biogeochemical constituents to mountain rivers. Given that a number of snow bridges persisted below the insulating debris cover well into the summer months, snow-avalanche deposits also contribute to regulating in-channel sediment and organic debris storage on seasonal timescales. Finally, our results underline the potential shortcomings of neglecting erosional processes in the winter and spring months in mountainous terrain subjected to prominent snow cover

Carbon formation in catalytic partial oxidation of methane on platinum: Model studies on a polycrystalline Pt foil

A polycrystalline Pt foil has been investigated as model catalyst in methane catalytic partial oxidation to synthesis gas. It is demonstrated that a substantial amount of carbonaceous deposits forms on the Pt foil upon reaction light-off blocking a large fraction of Pt surface atoms. By using in situ Raman spectroscopy and quantitative spectral analysis the evolution and spatial distribution of these carbonaceous compounds with reaction temperature and reaction time have been characterized. The chemical composition of the carbon material changes from highly reactive and strongly disordered directly after reaction light-off to highly ordered, oxidation and steam reforming resistant after several hours time on stream at 800 °C reaction temperature. Remarkably the carbon distribution at the Pt surface was found to be inhomogeneous and related to the nature of the microcrystals forming the polycrystalline foil in a yet unknown manner

A global database of historic glacier lake outburst floods

Ongoing atmospheric warming has accelerated glacier mass loss in many mountain regions worldwide. Glacier lakes trap part of the glacial meltwater and have increased by about 50 % in number and area since the 1990s. Some of these glacier lakes may empty catastrophically and pose hazards to mountain communities, infrastructure, and habitats. Such glacier lake outburst floods (GLOFs) have caused millions of dollars of damages and fatalities and are one of many concerns about future changes in the magnitude, frequency, and impacts of processes of a shrinking mountain cryosphere. Consistently compiled inventories are thus vital to assess regional and local trends in GLOF occurrence, hazard, and risk. To this end, we studied 769 literature and internet sources and developed a standardized database with 57 attributes that describe and quantify the location, dam type, size, timing, and impacts of GLOFs in nine glaciated mountain regions. Our GLOF inventory also includes details about the lake area before and after the outburst for 391 cases that we manually mapped from optical satellite images since 1984. In total, we compiled 3151 reported GLOFs that occurred in 27 countries between 850 and 2022 CE. Most GLOFs have been reported in NW North America (26 %) and Iceland (19 %). However, the reporting density in our inventory varies. During the 20th century alone, the number of yearly documented GLOFs increased 6-fold. Less than one-quarter of all reported cases feature hydrodynamic characteristics such as flood peak discharge or volume or estimates of loss and damage. Our inventory more than doubles the number of reported GLOFs in a previous global inventory, though gaps in attributes remain. Our data collection process emphasizes the support of local experts in contributing previously undocumented cases, and we recommend applying protocols when reporting new cases. The global database on historic GLOFs is archived at https://doi.org/10.5281/zenodo.7330344 (Lützow and Veh, 2023a) and regularly updated at http://glofs.geoecology.uni-potsdam.de/ (last access: 9 May 2023).</p

Reactor for In-Situ Measurements of Spatially Resolved Kinetic Data in Heterogeneous Catalysis

The present work describes a reactor that allows in-situ measurements of spatially resolved kinetic data in heterogenous catalysis. The reactor design allows measurements up to temperatures of 1300 ±C and 45 bar pressure, i.e. conditions of industrial relevance. The reactor involves reactants flowing through a solid catalyst bed containing a sampling capillary with a side sampling orifice through which a small fraction of the reacting fluid (gas or liquid) is transferred into an analytical device (e.g. MS, GC, HPLC) for quantitative analysis. The sampling capillary can be moved with ¹m resolution in or against flow direction to measure species profiles through the catalyst bed. Rotation of the sampling capillary allows averaging over several scan lines. The position of the sampling orifice is such that the capillary channel through the catalyst bed remains always occupied by the capillary preventing flow disturbance and fluid bypassing. The second function of the sampling capillary is to provide a well which can accommodate temperature probes such as a thermocouple or a pyrometer fiber. If a thermocouple is inserted in the sampling capillary and aligned with the sampling orifice fluid temperature profiles can be measured. A pyrometer fiber can be used to measure the temperature profile of the solid catalyst bed. Spatial profile measurements are demonstrated for methane oxidation on Pt and methane oxidative coupling on Li/MgO, both catalysts supported on reticulated a-Al2O3 foam supports

Measurement and analysis of spatial reactor profiles in high temperature catalysis research

Spatial reactor profile measurements are a novel tool in chemical reaction engineering research. In this technique species concentrations or molar flow rates, phase temperatures and spectroscopic information are measured as function of the axial coordinate in a continuous flow tubular reactor. The obtained spatial gradients can be analyzed in terms of kinetic and mechanistic information about the reaction under study. The advantage of the spatial profile technique is that transient data are obtained at steady state and that it can be applied at temperature and pressure conditions relevant for industrial application. After a detailed description of the method various application examples are discussed such as methane catalytic partial oxidation on rhodium and platinum coated foam catalysts, methane oxidative coupling in the gas phase and oxidative dehydrogenation of ethane to ethylene on a supported molybdenum oxide catalyst. It is demonstrated how information about film transport limitation and reaction pathways can be extracted. The importance of spatial reactor profiles for validation of microkinetic models is highlighted for gas phase methane oxidative coupling at elevated pressure. Finally the idea of spatially resolved Raman spectroscopy using an optical fiber sensor is demonstrated and key parameters such as spatial resolution and position accuracy are determined

Seasonal logging, process response, and geomorphic work

Deforestation is a prominent anthropogenic cause of erosive overland flow and slope instability, boosting rates of soil erosion and concomitant sediment flux. Conventional methods of gauging or estimating post-logging sediment flux often focus on annual timescales but overlook potentially important process response on shorter intervals immediately following timber harvest. We resolve such dynamics with non-parametric quantile regression forests (QRF) based on high-frequency (3 min) discharge measurements and sediment concentration data sampled every 30–60 min in similar-sized (&sim;0.1 km2) forested Chilean catchments that were logged during either the rainy or the dry season. The method of QRF builds on the random forest algorithm, and combines quantile regression with repeated random sub-sampling of both cases and predictors. The algorithm belongs to the family of decision-tree classifiers, which allow quantifying relevant predictors in high-dimensional parameter space. We find that, where no logging occurred, &sim;80% of the total sediment load was transported during extremely variable runoff events during only 5% of the monitoring period. In particular, dry-season logging dampened the relative role of these rare, extreme sediment-transport events by increasing load efficiency during more efficient moderate events. We show that QRFs outperform traditional sediment rating curves (SRCs) in terms of accurately simulating short-term dynamics of sediment flux, and conclude that QRF may reliably support forest management recommendations by providing robust simulations of post-logging response of water and sediment fluxes at high temporal resolution

Catalytic partial oxidation of methane on platinum investigated by spatial reactor profiles, spatially resolved spectroscopy, and microkinetic modeling

Spatially resolved profile measurements, Raman spectroscopy, electron microscopy, and microkinetic modeling have been used to study the catalytic partial oxidation of methane on Pt. The measured species profiles through Pt coated foam catalysts exhibit a two-zone structure: an abrupt change in reaction rates separates the fast exothermic oxidation chemistry at the entrance of the reactor from the slow endothermic reforming chemistry. Spatially resolved Raman spectroscopy and electron microscopy confirm that the position of the mechanistic change could be correlated with Pt transportation and formation of carbonaceous deposits blocking the majority of active Pt sites in the reforming zone. The species profiles were simulated using a pseudo-2D heterogeneous model, which includes heat and mass transport limitations, and two state-of-the-art chemical kinetic mechanisms. Although both mechanisms are in quantitative agreement with the oxygen profiles, the two mechanisms differ substantially in their predictions of the branching ratio between partial and complete oxidation, as well as surface site coverages. The experimentally observed change in reaction rates is attributed to carbon formation, which the mechanisms are unable to reproduce, since they do not include carbon–carbon coupling reactions