Displays

Alpine rivers have experienced considerable changes in channel morphology over the last century. Human disturbance and natural factors are the main drivers of changes in channel morphology that modify natural sediment and flow regimes at local, catchment and regional scale. River sediment loads are likely to increase due to increasing snow and glacier melt runoff, facilitated by climate changes. Additionally, channel erosion and depositional dynamics and patterns are influenced by sediment delivery from rock walls, hillslopes, and sediment in the forefields of retreating glaciers. Land cover changes may facilitate or obstruct runoff and soil degradation.

In order to reliably assess the magnitudes of the channel changing processes and/or their frequencies due to recent climate change, the investigation period needs to be extended to the last century, ideally back to the end of the Little Ice Age. Moreover, a high temporal resolution is required to account for the history of changes of channel morphology and for better detection and interpretation of related processes.

The increasing availability of digitised historical aerial images, together with advancements of digital photogrammetry, provides the basis for reconstructing and assessing long-term evolution of the surface, both in terms of planimetric mapping and generation of historical digital elevation models (DEMs). This work presents the temporal evolution of fluvial channel morphology in Kaunertal, Austria, spanning twenty periods from 1953 to 2019. Here we use photogrammetric analysis of recent and historical images, together with LiDAR and drone-based photogrammetric DEMs, to quantify the river changes in terms of channel incision, riverbank erosion, as well as the spatial patterns of channel erosion and deposition and the amounts of mobilized sediment. We show that geomorphic changes are mainly driven by deglaciation, i.e. glacier retreat, and sediment delivery from recently deglaciated steep lateral moraines. Overall, this work contributes to better understand the links between channel changes and climatic factors and highlights similarities and differences in the evolutionary trajectories of the main rivers in the catchment.

How to cite: Piermattei, L., Heckmann, T., Altmann, M., Rom, J., Fleischer, F., Stark, M., Haas, F., Pfeifer, N., and Becht, M.: Quantifying long term evolution of fluvial channel in glacier forefield , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16515, https://doi.org/10.5194/egusphere-egu2020-16515, 2020

In mountain ranges, widespread landsliding triggered by large earthquakes can mobilise large amounts of non-cohesive sediment and organic matter that can be transported by rivers during the post-seismic landscape relaxation phase. The timescales over which this occurs are likely to be decades, meaning that it is difficult to establish the controls on post-seismic sediment evacuation from modern-day case studies. River gauging station data, reservoir and lake sediments have been helpful to constrain the temporal dynamics of fine sediment evacuation. However, key unknowns remain, particularly with regard to the competition between sediment supply and river transport capacity in space and time. Here, we attempt to tackle this using a 2D morphodynamic approach by applying the numerical model Eros at the catchment scale. We aim to systematically investigate how the properties of landslide populations and the runoff intensity and variability combine to control fine sediment export as suspended load from storm events to years and decades. Our focus is on the Potters Creek catchment located in the Southern Alps of New Zealand, where the Alpine Fault can generate Mw 8 earthquakes and which has one of the highest precipitation rates measured in the world. The chosen tectonic scenarios encompass different earthquake shaking intensities that translate to various landslide densities. Landslide properties are randomly sampled from empirical scaling relationships and the mobilised sediment is introduced in the landscape using a runout algorithm. The runoff distribution is constrained by empirical data and applied as climate forcing of the simulations. Prior to the quantification of the sediment export, we set up a calibration phase to constrain the sediment entrainment and deposition laws against data measured in the West Coast of New Zealand. Subsequently, an exploration phase is developed to quantify the sediment evacuation sensitivity to climatic parameters and the earthquake-derived landslide distribution properties. We find that the post-seismic sediment discharge is strongly controlled by the amount of sediment supplied and the accessibility of the sediment to fluvial transport. These two properties control the power-law scaling relationship (intercept and slope) between daily sediment concentration and water discharge. Runoff intensity and the sequence of discharge events plays a central role on the export velocity of the fine sediment. Simulations show that fine sediment transport can rapidly (with year) return to apparent pre-disturbance levels, before experiencing a renewed wave of sediment at the catchment outlet from more distal sources. These simulations provide new insight on the common controls and complexities of the evacuation of fine sediment from earthquake-triggered landslides.

How to cite: Croissant, T., Hilton, R., Lague, D., Densmore, A., Howarth, J., Steer, P., Wang, J., and Davy, P.: Catchment scale simulations of the climatic regulation of fine sediment evacuation after widespread landsliding , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18645, https://doi.org/10.5194/egusphere-egu2020-18645, 2020

Large continental earthquakes can produce 10^4 – 10^5 years of erosion in a geological instant through coseismic landslide generation. Local erosion rates increase by an order of magnitude immediately after an earthquake, but rapidly return to background levels. The short-lived nature of the enhanced erosion rates is insufficient to clear the orogen of coseismic landslide material, which can remain stored for centuries to millennia. The sediment which remains affects topographic evolution and potential hazards until it is removed from the orogen. We examine the processes by which the 3km^3 of sediment, generated by the cosesimic landslides of the 2008 Mw7.9 Wenchuan Earthquake, move through and within catchments. Using 10 years of satellite imagery and literature derived values, we can, for the first time, describe and measure the export of sediment by fluvial erosion, debris flows and overland flow. We find that less than 15% of the sediment, produced by the earthquake, has transitioned from the hillslope through tributary channels (of order <5) into the major orogen draining rivers. The transport of sediment through tributary channels is controlled by the frequency and magnitude of debris flows, which transport 60% of the sediment. Fluvial undercutting of landslide deposits plays a minor role in controlling sediment export, likely due to the low stream power and coarse nature of the sediment in tributary channels. Our observations suggest that the long-term evolution of channels in these range front catchments may be governed by the stochastic delivery of earthquake derived sediment.

How to cite: Francis, O., Hales, T., Hobley, D., Fan, X., and Huang, R.: The fate of sediment after a large earthquake , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-891, https://doi.org/10.5194/egusphere-egu2020-891, 2019

Geologists frequently debate the origin of iconic river canyons, as well as the extent to which river canyons record climatic and tectonic signals. Fluvial and hillslope processes work in concert to control canyon evolution; rivers both set the boundary conditions for adjoining hillslopes and respond to delivery of hillslope-derived sediment. But what happens when canyon walls deliver boulders that are too large for a river to carry? Large blocks of rock derived from resistant hillslope strata have recently been shown to control the evolution of hillslopes and channels by inhibiting sediment transport and bedrock erosion. Here we present Blocklab, a 2-D model within the Landlab modeling toolkit that uses a hybrid discrete-continuum framework to track block transport throughout a river canyon landscape in horizontally layered rock. Our model reveals that internal negative channel-hillslope feedbacks control erosion dynamics and result in characteristic planview and cross-sectional river canyon forms. Surprisingly, while the presence of blocks in the channel initially slows incision rates, the subsequent removal of blocks from the oversteepened channel substantially increases incision rates. This interplay between channel and hillslope dynamics results in highly variable long-term erosion rates. These autogenic feedbacks can mask external signals, such as changes in rock uplift rate, complicating the interpretation of landscape morphology and erosion histories.

How to cite: Glade, R., Shobe, C., Anderson, R., and Tucker, G.: River canyon evolution governed by autogenic channel-hillslope feedbacks, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12761, https://doi.org/10.5194/egusphere-egu2020-12761, 2020

Landslides are key agents of sediment production and transport. Ongoing efforts to map and simulate landslides continuously improve our knowledge of landslide mechanisms. However, understanding sediment dynamics following landslide events is equally crucial for developing hazard mitigation strategies. An outstanding research challenge is to better constrain the dynamic feedbacks between landslides and fluvial processes.  Fluvial processes simultaneously (i) act as conveyor belts evacuating landslide-derived sediment and (ii) lower the hillslope’s base level triggering further landsliding. Landslides in turn can choke river channels with sediment, thereby critically altering fluvial responses to external tectonic or climatic perturbations.

Here, we present HYLANDS, a hybrid landscape evolution model, which is designed to numerically simulate both landslide activity and sediment dynamics following mass failure. The hybrid nature of the model is in its capacity to simulate both erosion and deposition at any place in the landscape. This is achieved by coupling the existing SPACE (Stream Power with Alluvium Conservation and Entrainment) model for channel incision with a new module simulating rapid, stochastic mass wasting (landsliding). 

In this contribution, we first illustrate the functionality of HYLANDS to capture river dynamics ranging from detachment-limited to transport-limited configurations. Subsequently, we apply the model to a portion of the Namche-Barwa massive in Eastern Tibet and compare simulated and observed landslide magnitude-frequency and area-volume scaling relationships. Finally, we illustrate the relevance of explicitly simulating stochastic landsliding and sediment dynamics over longer timescales on landscape evolution in general and river dynamics in particular under varying climatologic and tectonic configurations.

With HYLANDS we provide a hybrid tool to understand both the long and short-term coupling between stochastic hillslope processes, river incision and source-to-sink sediment dynamics. We further highlight its unique potential of bridging those timescales to generate better assessments of both on-site and downstream landslide risks.

How to cite: Campforts, B., Shobe, C. M., Steer, P., Lague, D., Vanmaercke, M., and Braun, J.: To slide or not to slide: explicit integration of landslides and sediment dynamics in a landscape evolution model, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13064, https://doi.org/10.5194/egusphere-egu2020-13064, 2020

Although more and more processes are discussed and discovered on the genesis and evolution of cave systems, the tiered karsts are often explained by a control of the base level evolution. In this classical model, the horizontal galleries are explained by a stability of the base level elevation. To the contrary, the shafts and network segments with steep slopes are related to incision periods with a base level lowering.

We use Terrestrial Cosmogenic Nuclide Geochronology to estimate burial ages of alluvium trapped in several caves of the Larzac plateau in Southern France. All the samples are collected in horizontal cave levels, sometimes located between steeper segments. Some caves are opened in river gorge walls, while others are located below the Larzac plateau not farther than 5km away from the river gorges.

The burial ages for the caves opening in the gorges are consistent with the incision rates given for the area and could be interpreted using the classical model. However, the cave within the plateau show a horizontal level with alluvium deposited 200m above the caves in the gorge with the same burial ages (~1 Myr). Since then, new shafts have been opened without alluvium and are hydrologically connected to the river by deeper[jfr1]  hypogenic galleries. The cave morphologies and the geochronological data suggest that the classical model fails to explain the horizontal levels in cave below the plateau. We postulate that the geometry of the caves in these limestone and dolomite plateaus are related to a previous period of ghost-rock and alteration roots formations. Without the opening of an efficient connection between this primokarst and the valley, no alluvium can flow through the cave. Therefore, we think that our burial ages constrain the emptying of the ghost-rocks leading to the genesis of the cave where water and possibly alluvium can flow through. Furthermore, these new finding explain why the horizontal levels in the caves are not clearly related to horizontal markers in the surface geomorphology and why large shafts (>100m) exist in the area without evidences of long periods of base level stability followed by large drop of the regional base level.

How to cite: Vernant, P., Malcles, O., Ritz, J.-F., Fink, D., Cazes, G., Fujioka, T., and Chéry, J.: First quantitative evidences of ghost-rock karstification controlling regional karst geometry, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9099, https://doi.org/10.5194/egusphere-egu2020-9099, 2020

In many lowland areas, fluvial incision is usually relatively slowly and another factors as the stratigraphical control would play a relevant role. In the lower Seine valley of Northern France, cave systems developed in the sub-horizontal Upper Cretaceous chalk of the Anglo-Paris Basin offer the potential to constrain the Quaternary evolution of the Seine valley and to test the role of speleo-inception theory of conduit development in the chalk aquifer. Six chalk caves, with a combined length of over 5.7 km were studied in detail. In each studied cave, data on the passage morphology, cave deposits (speleothem and sediments) and stratigraphical control were recorded. Cave levels were defined based on geomorphological evidence and altitudinal cave passage analyses. The chronology of cave development and abandonment was constrained by ten U-Th speleothem dates and 144 palaeomagnetic samples collected from laminated sediments within the caves. Four regional cave levels were identified at 10, 40, 75-80, and 85-90 m asl, showing 1% slope to the Seine estuary. Each cave level is formed by phreatic and epiphreatic conduits enlarged by paragenesis, showing branch work or maze patterns. Cave infill corresponds mainly to clayey to silty sediments that occupy the majority of the karst conduits. Locally, sands and pebbles occur, and speleothems are relatively scarce. Palaeomagnetic and U-Th data show that these cave levels developed sequentially from >1.06 ka to c. 300 ka, ca. 78% of them in relation to prominent Turonian, Coniacian and Santonian hardgrounds as well as sheet- and semi-tabular flint bands. Their age correlates with the estimated age of the lower river terraces from limited previously published OSL, palaeontological and U-Th dating, although new age data from the study cave improve the chronology of the higher-level river terraces. The combination of all this data suggests an initial slow rate of incision during the early Pleistocene, followed by a phase of more rapid river incision up to ~ 0.30 m·ka^-1 from ca. 1 to 0.7 Ma. Later, incision rates dropped to ~0.08 m·ka^-1 during Middle Pleistocene, and 0.05 m·ka^-1 since the beginning of the Upper Pleistocene. In conclusion, fluvial incision constitutes also a relevant speleogenic factor in low-gradient areas as the Seine Basin, where conduit development was favoured at sites where suitable lithological inception horizons intercept the contemporary base level.

How to cite: Ballesteros, D., Nehme, C., Farrant, A., Todisco, D., Sahy, D., Grappone, J. M., and Mouralis, D.: Evolution of Quaternary cave levels in low-relief karst regions: influence of fluvial incision and speleoinception of chalk caves in northern France, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18031, https://doi.org/10.5194/egusphere-egu2020-18031, 2020

Multi-level cave systems record the history of regional river incision in abandoned alluvium-filled phreatic passages which, mimicking fluvial terrace sequences, represent former phases of fluvial base-level stability. In this respect, cosmogenic burial dating of in cave-deposited alluvium (usually via the nuclide pair ²⁶Al/¹⁰Be) represents a suitable method to quantify the pace of long-term river incision. Here, we present a dataset of fifteen ²⁶Al/¹⁰Be burial ages measured in fluvial pebbles washed into a multi-level cave system developed in Devonian limestone of the uplifted Ardenne massif (eastern Belgium). The large and well-documented Chawresse system is located along the lower Ourthe valley (i.e. the main Ardennian tributary of the Meuse river) and spans altogether an elevation difference exceeding 120 m.

The depleted ²⁶Al/¹⁰Be ratios measured in four individual caves show two main outcomes. Firstly, computed burial ages ranging from ~0.2 to 3.3 Ma allows highlighting an acceleration by almost one order of magnitude of the incision rates during the first half of the Middle Pleistocene (from ~25 to ~160 m/Ma). Secondly, according to the relative elevation above the present-day floodplain of the sampled material in the Manants cave (<35 m), the four internally-consistent Early Pleistocene burial ages highlight an “anomalous” old speleogenesis in the framework of a gradual base-level lowering. They instead point to intra-karsting reworking of the sampled material in the topographically complex Manants cave. This in turn suggests an independent, long-lasting speleogenetic evolution of this specific cave, which differs from the per descensum model of speleogenesis generally acknowledged for the regional multi-level cave systems and their abandoned phreatic galleries. In addition to its classical use for inferring long-term incision rates, cosmogenic burial dating can thus contribute to better understand specific and complex speleogenetic evolution.

How to cite: Rixhon, G., Bourlès, D. L., Braucher, R., Peeters, A., and Demoulin, A.: Cosmogenic burial dating of in cave-deposited alluvium: unravelling long-term incision rates and complex speleogenesis in multi-level cave systems, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19598, https://doi.org/10.5194/egusphere-egu2020-19598, 2020

Mountain building results in high rates of erosion and the interaction of rocks with the atmosphere, water and life. The resulting geochemical transfers may steer the evolution of the global carbon cycle and Earth’s long-term climate. For decades, much attention has focused on the weathering of silicate minerals and associated carbon dioxide (CO₂) drawdown, and it is now understood that mountains are places where this reaction is most sensitive to changes in climate. However, the focus on silicate weathering belies a multi-faceted role for mountain building and erosion in the carbon cycle. Erosion also mobilises organic carbon from forests, transferring it to rivers and delivering it to long-lived sedimentary deposits, which results in an additional CO₂ sink. In some mountain belts, exhumation of sedimentary rocks and exposure to the oxygen-rich atmosphere and hydrosphere can release CO₂ by oxidation of rock organic carbon and sulfide minerals. These fluxes remain poorly constrained.

Here we take stock of our current understanding of all of these processes and the magnitude of their fluxes, focusing on insight from modern-river catchments. We find that the net CO₂ budget associated with erosion and weathering appears to be controlled by processes that are not widely considered in conceptual or numerical models, specifically the fluxes from organic carbon burial and oxidation, and sulfuric acid weathering reactions. We suggest that lithology plays a major role in moderating the impact of mountain building on the global carbon cycle, with an orogeny dominated by sedimentary-rocks tending towards CO₂ neutrality, or indeed becoming a CO₂ source to the atmosphere. Over the coming century, erosion-induced changes in CO₂ emissions from sedimentary rocks may result in a previously overlooked positive feedback on anthropogenic climate change.

How to cite: Hilton, R. and West, J.: A shifting view of erosion and the carbon cycle, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5624, https://doi.org/10.5194/egusphere-egu2020-5624, 2020

Landslides are the major erosional process in many orogens, and one of the most sensitive erosional process to tectonic and climatic perturbations. However, it remains extremely difficult to constrain long-term or past rates of landslide activity, and hence their contribution to long-term landscape evolution and catchment sediment fluxes, because the physical records of landsliding are often removed in <10² yrs. Here, we use the in-situ ¹⁰Be and in-situ ¹⁴C concentrations of recent landslide deposits and catchments from the Fiordland and the Southern Alps of New Zealand to: (a) estimate landslide frequencies over 10³-10⁴ yr timescales, which we compare against landslide inventories mapped from air photos (<10² yrs) to estimate changes in landslide activity, (b) quantify catchment-averaged erosion rates, and landslide’s contribution to those erosional fluxes, and (c) test whether paired ¹⁴C-¹⁰Be measurements can be used to trace erosional depth-provenance and identify transient erosion rate changes. We show that ¹⁰Be concentrations on landslide deposits can be used to estimate landslide recurrence intervals and frequency over 10³ yr timescales, and that ¹⁴C/¹⁰Be ratios reflect the depth-provenance of sediment, and possibly transient changes in erosion rates. The comparison of our ¹⁰Be-based long-term landslide frequencies with short-term published inventories suggests that landslide frequencies have increased towards the present by up to an order of magnitude. We compare sediment fluxes inferred from these long- and short-term landslide inventories with sediment flux estimates derived from ¹⁰Be catchment-averaged erosion rates, which allows us to examine fluctuations in erosion rate estimates from 10¹ to 10³ yrs timescales. 

How to cite: Roda-Boluda, D., Schilgen, T., Lupker, M., Hella, W., Jeff, P., Stefanie, T., and Aaron, B.: Examining erosion in New Zealand over millennial timescales using in-situ 10Be and 14C , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9309, https://doi.org/10.5194/egusphere-egu2020-9309, 2020

Mountain landscapes are shaped by hillslope and fluvial processes that remove and transport material and sediment. Developing proxies to map these processes through space and time is a key element in better understanding their distribution and drivers. Remotely sensed and satellite observations of Earth’s surface are greatly expanding the reach of geomorphologists and presenting a myriad of new opportunities to explore and quantify Earth surface processes. Synthetic aperture radar (SAR), in particular, promises to be a powerful tool for mapping and quantifying geomorphic processes. Here, we exploit a time series of coherence estimates between SAR images from the Copernicus Sentinel-1 mission. Coherence is the spatial correlation between two SAR images and is sensitive to changes in both the phase (elevation) and amplitude (surface backscatter) of the received radar signal. Geomorphic processes such as landsliding, hillslope slump, cobble movement, or alluvial sediment transport can result in loss of SAR coherence. In regions without significant vegetation or anthropomorphic input, we therefore propose that coherence loss is a proxy for surface sediment movement and geomorphic activity. We constructed time series of Sentinel-1 coherence images spanning three to five years for arid and semi-arid regions of the Argentinian Central Andes and the north-western Himalaya. Both regions are characterized by active tectonics and seasonal climatic gradients. The relatively short revisit time of the Sentinel-1 satellites (~2-4 weeks in our regions of interest) mean that we can not only map geomorphic activity averaged over multiple years, but observe intra-annual and seasonal differences throughout a given year. We are also able to compare interannual geomorphic responses during years with, e.g.,  relatively strong or weak monsoon seasons.

We couple our Sentinel-1 coherence time series with a compilation of published 10-Berrylium terrestrial cosmogenic nuclide basin-wide denudation rates from the Open Cosmogenic isoTOPe and lUmineScence (OCTOPUS) database. For basins with cosmogenic data, we derive temporal and spatial statistics of our coherence time series. Across regional gradients, the range of coherence within basins positively correlates to millennial denudation rates and to topographic metrics used to indicate long-term uplift (e.g., channel steepness). Outlying basins include those in which erosion is driven by large, deep-seeded landslides that occur over repeat times longer than our multi-year observation period. Our study suggests that a dense time series of interferometric coherence can be used as a proxy for surface sediment movement and landscape stability in vegetation-free settings at event to decadal timescales.

How to cite: Olen, S. and Bookhagen, B.: Synthetic aperture radar coherence as a proxy for geomorphic activity, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5939, https://doi.org/10.5194/egusphere-egu2020-5939, 2020

Glaciers are an effective agent of erosion and landscape evolution, capable of driving high rates of erosion and sediment production. Glacial erosion is therefore an important process mediating the effect of climate on erosion rates and tectonics. Further, as a source of sediment, glacial erosion also has implications for the carbon and silicate cycles, with the potential for longterm feedbacks.  Understanding the interaction of climate, tectonics, glacial erosion and topography will lead to more insight into how glaciers can impact these processes. Simple, analytical long-profile models of fluvial incision are fundamental in tectonic geomorphology and critical for addressing fluvial analogues of problems such as those posed above. The advantage of these simple long-profile models is that they can be applied when information about forcing and boundary conditions is minimal (e.g. in deep time), and they can aid in the development of intuition about how such systems respond in general to different forcing. While models of glacial erosion have existed for quite some time, they tend to be complicated and computationally expensive. Currently, analytical long-profile models do not exist for glacial systems. At the same time, the patterns of glacial erosion and sediment transport, and how these processes respond to climate is fundamentally different than fluvial systems, and cannot be addressed properly with purely fluvial models.

Building on previous work, we introduce several simplifications to make the equations for coupled glacier-fluvial long-profile models easier to use and show that these simplifications have minimal effect on the steady state solution. We then use these new equations to develop an analytical solution for glacier-fluvial long-profiles at erosional steady state. The solution provides glacier geometry, including length and slope, ice thickness, and overall orogen relief for a given uplift rate, rock erodibility, profile length and climatic conditions. To explore the effect of glaciation on the balance between climate, erosion and orogen geometry, we integrate this solution into a critical wedge orogen theory. We find that the total orogen relief should be closely tied to the equilibrium line altitude (ELA), in line with the glacial buzzsaw theory. In addition, our theory predicts that the geometry and average uplift rate of glaciated critical wedge orogens respond more sensitively to changes in climate than those dominated by fluvial erosion. We suggest that the lowered ELA during glacial maxima over the last few million years could have triggered narrowing of critical orogens, with an associated increase in uplift rates within the active orogen core. 

How to cite: Deal, E. and Prasicek, G.: Analytical long-profile models of coupled glacier-fluvial systems, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5510, https://doi.org/10.5194/egusphere-egu2020-5510, 2020

Landscapes evolve through interactions between subsurface processes that move and deform bedrock, and surface processes that redistribute mass through erosion, transport, and deposition of sediment. Sediment is composed of discrete particles that are produced from bedrock and modified during transport by physical and chemical weathering. Sediment particle attributes, including size, angularity, and durability, therefore depend on the climatic, tectonic, and lithologic factors that regulate weathering processes. These attributes, in turn, influence rates and modes of sediment transport, and the tools and cover effects that control rates of river incision into bedrock. Thus the production of sediment helps set the slopes of river channels and the relief structure of landscapes, making it central to the feedbacks between tectonics, climate, and erosion that create topography. Despite their importance, sediment particles are rarely included explicitly in landscape evolution modeling due to gaps in understanding of sediment production on hillslopes, the particle evolution that occurs on hillslopes and in channels, and the implications of sediment attributes for river incision into bedrock. Although these processes have been studied in isolation, they have not been combined together in a comprehensive model of the role of sediment in climate-tectonic-erosion feedbacks. 

Here we present results from a new, spatially-explicit model that predicts the evolution of individual particle attributes, including size, angularity, and durability. The model also predicts the resulting distributions of particle attributes as sediment from different sources is mixed, and as particles evolve during transport through catchments. The model has two components. The first predicts the initial particle attributes as sediments are produced from bedrock on hillslopes. The initial particle size distribution depends on the spacing of fractures and sizes of mineral grains in crystalline rocks, and on the spacing of bedding planes and the size of cemented particles in clastic sedimentary rocks. Initial size, as well as particle angularity and durability, are also influenced by chemical weathering, which depends on the fraction of soluble minerals, the local climate (parameterized as mean temperature and precipitation), and the residence time of bedrock as it is exhumed through the hillslope weathering engine.

The second model component quantifies how particles change as they are transported across hillslopes and through channel networks. Particle sizes are reduced by abrasion as a function of three factors: the potential energy lost in transport; particle angularity; and particle durability, which depends on initial rock tensile strength and subsequent loss of strength due to chemical weathering. Mass lost from abrasion of coarse particles is converted to sand and silt. Particles become less angular as a function of cumulative mass loss. However, high rates of energy loss on steep slopes cause fragmentation, which creates new coarse particles and resets particle angularity. Model relationships are parameterized using published data as well as newly acquired data from laboratory experiments and field studies in the Sierra Nevada, California. We couple the model with the saltation abrasion/bedrock river incision model to simulate evolution of river longitudinal profiles, and explore potential feedbacks between rock uplift, climate, and sediment production.

How to cite: Sklar, L. S. and Riebe, C. S.: A process-based model for production and evolution of sediment particles by physical and chemical weathering in mountain catchments, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13008, https://doi.org/10.5194/egusphere-egu2020-13008, 2020

Weathering in mountain landscapes produces sediment with size distributions that evolve as particles are transported down hillslopes, delivered to channels, and carried downstream. The evolving sizes influence rates of river incision into bedrock, which in turn set sediment residence times on hillslopes, with implications for the sizes of sediment produced by weathering. Hence, variations in sediment size are central to feedbacks that link climate, tectonics, and erosion in mountain landscape evolution. However, few studies have quantified how sediment sizes evolve during transport across catchments, focusing instead on rates of erosion and weathering. Yet recent modeling suggests that spatial variations in sediment size can lead to bias in erosion rates from conventional techniques, further highlighting the importance of understanding how sediment size evolves across landscapes.

Here we show how a more complete and unbiased picture of sediment production, weathering, and erosion can be obtained by combining field measurements of sediment size together with conventional geochemical proxies in an integrative model that accounts for spatial variations in erosion, weathering, and sediment mixing, while incorporating effects of both abrasion and fragmentation during transport in channels. Our measurements, from a catchment draining the steep eastern Sierra Nevada, California, include particle size distributions of sediment from widely distributed locations. These measurements represent sediment that is produced on hillslopes and delivered to channels, reflecting the combined effects of the initial sediment size distribution (set by bedrock fracture spacing) and subsequent weathering on slopes. Our measurements also include cosmogenic nuclide concentrations and apatite-helium ages in 11 size classes, from sand to boulders, sampled from the creek. The cosmogenic nuclides reveal residence times of sediment in the catchment, while the apatite-helium ages reveal source elevations of sediment eroded into the stream. When combined together, the cosmogenic nuclide and apatite-helium data can be used to quantify altitudinal variations in erosion rates and sediment size distributions.

Our measurements from catchment slopes indicate that hillslope sediment size decreases with decreasing elevation, reflecting altitudinal trends in physical, chemical, and biological weathering and producing downvalley fining in hillslope sediment supply. Cosmogenic nuclides in stream sediment decrease by two-fold with increasing particle size, indicating that erosion rates calculated using traditional techniques are sensitive to the size sampled from the creek. Apatite-helium ages suggest that the smallest and largest sizes sediment sizes in the stream originate from lower elevations, where slopes are gentler and soil-mantled. In contrast, coarse gravel and cobbles appear to originate from higher in the catchment, where slopes are steeper and bare bedrock is exposed. The differences in altitudinal trends in sediment size implied by the apatite-helium data and the direct observations from catchment slopes can be reconciled by accounting for particle fragmentation and abrasion during transport from hillslope sources to the sampling point in the creek. Our analysis indicates that each of the unique sources of information in our study are necessary for a complete and unbiased understanding of spatial variations in the production of sediment across the full range of sizes and their evolution during transport across the catchment.

How to cite: Riebe, C. S., Sklar, L. S., and Lukens, C. E.: Is more better? Sediment production, weathering, and erosion inferred from multiple geochemical proxies and comprehensive field measurements in mountain catchments, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13564, https://doi.org/10.5194/egusphere-egu2020-13564, 2020

Gully erosion seriously threatens farmland and causes soil loss. Inferring sediment transport paths in a gully system is important for understanding the mechanisms of gully erosion. The morphological method successfully applied in estimating bed-material transport in both one dimension and two-dimensions in rivers, for some decades, has yet to be applied to gully erosion. Here, we infer sediment transport paths in a gully system using the morphological method. Two catchments in the Loess Plateau of China were selected as study areas. Multi-temporal high-resolution Digital Elevation Models (DEMs) were acquired using structure-from-motion multiview-stereo (SfM-MVS) photogrammetry for determining morphological changes. Then, both 1D sediment transport and 2D sediment transport paths were calculated based on morphological changes and topographic attributes. The results showed that the use of 1D treatment leads to substantial local errors in transport rate estimates, to a degree related to the number of branch gullies. The 2D application showed that a large proportion of the total transport was actually concentrated into one main channel in steep areas, the proportion of transport in branches is substantial in lower relief areas.

How to cite: Dai, W., Lane, S. N., and Tang, G.: Inference of sediment transport pathways in a gully system using the morphological method, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13657, https://doi.org/10.5194/egusphere-egu2020-13657, 2020

Mountains regions are usually characterized, according to their geological and structural setting, by an articulated relief, where gravity and water-driven processes occur with an increasing intensity following glaciers retreat. Denudation processes affecting mountain slopes may vary according to local conditions controlled by different factors (e.g., lithology and structural setting of bedrock, climate, relief features).
The succession of slope stability and instability phases can be registered in the soil record as paleosols or buried surfaces. Therefore, an exhaustive investigation of soils and paleosols could provide information to infer the spatial-temporal variation of the denudation/deposition processes.
The main aim of this study is the reconstruction of the dynamic interplay between erosion and sedimentation that have been characterizing the landscape evolution of the Buscagna Stream hydrographic basin (Veglia-Devero Natural Park, Central-Western Italian Alps) during the Late Holocene. The basin is characterized by an evident asymmetry between the valley slopes in terms of lithology (calcschists on the southeastern slope versus ortogneiss, micaschists and spots of ultramafic rocks on the northwestern slope), and by a structural control on the relief. This differentiation is also responsible for the great landforms variability and the geomorphic dynamics dissimilarities between the slopes. 
In order to reconstruct the different dynamics affecting the slopes, 11 soil profiles were investigated by means of field and laboratory (on both mineral and organic constituents) characterizations; the soil profiles were selected in different morphological contexts, along two downslope transects on the two sides of the valley.
The results show that the investigated soil profiles are characterized by different soil units, identifiable by the presence of grain size discontinuities and/or stone lines or buried organic horizons. The presence of different pedological units underlines the occurrence of separate events of pedogenesis alternated to phases characterized by slope instability and intensification of denudation and related degradation/aggradation processes. Moreover, the soils recorded in a different way the instability phases occurred in the two opposite flanks of the hydrographic basins, underlining changes in predominant erosion processes, which are also related to the varying bedrock both in term of lithology and structural settings.
In particular, on the southeastern slope characterized by a calcschists parent material and by less steep slopes i) the gravity erosion processes are less intense; ii) the presence of vegetation cover and a developed soil promote the slope stability. Whereas, on the northwestern slope characterized by gneiss and micaschists and locally by ultramafic rocks and high relief energy i) the soils have recorded many instability phases in term of sequences of buried surfaces; ii) the presence of coarse slope deposits only partially colonized by vegetation predispose to slope instability. The characterization of soil mineral component underlines the presence of different material sources, linked to action of a variety of agents (e.g., gravity, water, snow, wind), which have contributed to landscape evolution in term of sediment erosion, transport and deposition.
Finally, this research highlights the role of soil as useful archive for retracing the geomorphological processes responsible for high altitude areas landscape evolution.

How to cite: Masseroli, A., Bollati, I., Pelfini, M., and Trombino, L.: Differentiation among geomorphological processes in a mountain hydrographic basin by means of soils analyses, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-749, https://doi.org/10.5194/egusphere-egu2020-749, 2019

The periglacial areas of the European Alps are characterised by rugged peaks and steep rockwalls with adjacent scree slopes that reflect high rates of rockfall activity. The current state of knowledge regards ice segregation as the dominant mechanism responsible for the disintegration of rock and associated destabilization of rockwalls. In the present work, we (1) monitored rock temperature in Alpine rock walls, (2) determined rock properties in the laboratory and (3) simulated frost weathering using purely temperature-driven models (Hales and Roering, 2007; Anderson et al., 2013) and physical-based models (Walder and Hallet, 1985; Rempel et al., 2016).

(1) We monitored rock temperature in 9 rockwalls in the Hungerli Valley and 10 in the Gaisberg Valley at altitudes between 2400 m and 3000 m between 2016 and 2019. Mean annual rock temperature is between -2.8 and 7.9°C and is strongly affected by snow cover, which ranges between 3 and 283 days.

(2) Lithologies comprise Mica Schist in the Gaisberg Valley and Schisty Quartz Slate with inclusions of Aplite and Amphibolite in the Hungerli Valley. Rock density, seismic and strength properties were quantified in the lab (Draebing and Krautblatter, 2019) to be included in physical-based frost weathering models.

(3) Frost weathering due to ice segregation can be expressed as cracking intensity, crack growth and porosity change. Our model results show that an annual maximum of cracking intensity, crack growth and porosity change within the first meter of rock depth in the study areas’ rockwalls. Although frost weathering is highly dependent on the thermal distribution inside a rock mass, our data demonstrate that lithological parameters strongly determine frost weathering due to their influence on water migration and fracture toughness. Furthermore, the results suggest that there is no relationship between average annual rock temperature, frost weathering and exposure, a tentative conclusion that is broadly contrary to prevailing consensus.

In conclusion, rock walls are exposed to strong thermo-mechanical stresses due to ice segregation, which leads to a disintegration of rock and lowering of stability. The present work lends support to other studies, which regard frost weathering as the dominant mechanism responsible for rockfall in mountain periglacial settings.

Anderson, R. S., Anderson, S. P., & Tucker, G. E.: Rock damage and regolith transport by frost: an example of climate modulation of the geomorphology of the critical zone, Earth Surface Processes and Landforms, 38(3), 299-316, 2013.

Draebing, D., & Krautblatter, M.: The Efficacy of Frost Weathering Processes in Alpine Rockwalls. Geophysical Research Letters, 46(12), 6516-6524, 2019.

Hales, T. C., & Roering, J. J.: Climatic controls on frost cracking and implications for the evolution of bedrock landscapes. Journal of Geophysical Research-Earth Surface, 112, F02033, 2007.

Rempel, A. W., Marshall, J. A., & Roering, J. J.: Modeling relative frost weathering rates at geomorphic scales. Earth and Planetary Science Letters, 453, 87-95, 2016.

Walder, J., & Hallet, B.: A Theoretical-Model of the Fracture of Rock During Freezing. Geological Society of America Bulletin, 96(3), 336-346, 1985.

How to cite: Mayer, T. and Draebing, D.: Modelling frost weathering processes and related stresses in Alpine rockwalls, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2160, https://doi.org/10.5194/egusphere-egu2020-2160, 2020

Debris flows are destructive mass movements in steep alpine torrents. Due to their high magnitudes and impact pressures economic goods and human lives are threatened in inhabited areas. The amount of entrained material depends largely on the mobilisable loose debris available for transport, which in turn controls debris-flow mobility and runout. However, still very limited data exists regarding rates and controls of sediment recharge in debris-flow channels.

In June 2015 an extraordinary rainfall event triggered a debris flow in the Roßbichelgraben torrent in southern Germany. Twelve terrestrial laser scan campaigns (> 450 scans positions) and nine temporally synchronised UAV surveys were carried out between June 2015 and September 2019. Both TLS and SfM-based photogrammetry reveal the temporal, spatial and seasonal sediment dynamic in the channel. A nearby meteorological station recorded the rainfall intensity in 10 min intervals. The results show that both terrestrial laser scanning and SfM-based photogrammetry provide equivalent erosion and deposition volumes (difference < 5%). Between June 2015 and September 2019 the channel was refilled with material of adjacent slopes and the above lying catchment (≈ 1.2 m³/d), whereby a higher activity was observed in summer than in winter. In addition, the activity decreased with elapsed time since the debris-flow event, as most over-steepened river banks failed shortly after the event and stabilised over time. Short, intense rainstorm events best explain the sediment dynamic in the channel (R² up to 0.9).

The results contribute to better understand the sediment dynamic in highly active debris-flow channels and allow for a more reliable estimation of potential debris-flow volumes.

How to cite: Dietrich, A., Keilig, K.-P., Stammberger, V., and Krautblatter, M.: A 4-D reconstruction of post debris-flow sediment dynamic inferred from multi-temporal terrestrial laser scanning and photogrammetry (Roßbichelgraben, Germany), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18023, https://doi.org/10.5194/egusphere-egu2020-18023, 2020

Gravity moves dry grains or blocks downhill in rockslides and rockfall. These mass movements can cause large boulders to saltate and impact with huge energies. Boulder impacts into bedrock surfaces should cause significant bedrock erosion, likely shaping the topography even in the absence of water.  Examples of potential rockfall-driven bedrock landforms include bedrock gullies on steep hillslopes, so-called plinth surfaces on caprock-topped mesa escarpments, and steep impact-crater slopes on planetary surfaces. Although grain impact processes have been incorporated into mechanistic models for fluvial and debris-flow incision, similar models for dry rockfall erosion have yet to be developed.

To explore the potential for dry rockfall to erode bedrock and shape the topography, we set up a discrete, cellular D16 dry grain saltation trajectory model accounting for particle saltation dynamics and evolving topography. We calibrated the model variables (i.e., particle hop angles, distances and velocities) for different grain sizes and hillslope angles using laboratory experiments of dry gravel transport over a tilted foam bed that served as an erodible bedrock analogue. We then explored the calibrated model for a broad range of hillslope angles, grain sizes and bedrock erodibilities.

Both model and experiments predict significant erosion due to rockfall-driven impacts. As the topography develops, alcoves (shell-shaped hollows) form near the upslope end of the model domain. These alcoves eventually overdeepen and fill with talus, preventing further erosion. Farther downslope, topographic feedbacks drive rockfall into incipient channels, which cause those channels to incise resulting in gullies. Overall, our work suggests that dry rockfall can be a significant bedrock incision process, and can lead to gully formation, even for hillslope angles that are significantly less than the angle of repose.

How to cite: Beer, A. R., Ulizio, T. P., Ma, Z., Fischer, J., and Lamb, M. P.: Bedrock Topographic Evolution from Rockfall Erosion, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12980, https://doi.org/10.5194/egusphere-egu2020-12980, 2020

In mountain environments, landslide sediment supply is one of the main factors that can affect fluvial morphodynamics. In settings underlain by clay-rich lithologies, where earthflows are the dominant agents of hillslope sediment transfer, limited quantitative information is available on the contribution of these processes to the sediment budget. This is a critical aspect both for addressing basic scientific questions on landscape evolution, as well for tackling more applied issues on river sediment management.

This study focuses on the mountain portion of the Sillaro River basin (138 km²), a fluvial system underlain by argillites and siltstones of the Ligurian domain, Northern Apennines (Italy). Here, earthflows are the most common landslide type. Through the compilation of a multi-temporal earthflow inventory (1954-2019), we aim to: (i) characterize earthflow source-to-sink sedimentary pathways, with special reference to sediment delivery to ephemeral and perennial streams; (ii) explore possible litho-topographic controls on earthflow size, frequency and recurrence; (iii) examine historical trend of earthflow activity in relation to rainfall variability and land use changes. Finally, the high and extended temporal resolution of the inventory, will offer the opportunity to test how relevant information could complement the existing inventory of the Emilia-Romagna region, for evaluating earthflow hazard and risk potential.

Data collection entailed inspection of 12 sequential aerial photo sets (1954, 1969, 1976, 1988, 1996, 2000, 2006, 2008, 2011, 2014, 2016, and 2018), through which earthflows were classified and mapped in GIS environment. This remotely-based activities were complemented by confirmatory field visits on a subset of most recent events. Overall, we have mapped a total of 506 earthflows, which collectively extend over an area of 4.1 km².

Preliminary results show that earthflow size (i.e., total disturbed area) ranges from 400 m² to 98000 m², with frequencies peaking around 10000 m². In terms of source-to-sink pathways, we find that earthflows chiefly tend to deliver sediment to ephemeral gully channels (61%) and perennial tributaries (25%). Whereas, 5% of the events remain on the slopes, and another 5% are buffered by roads and similar anthropogenic barriers. Only a very limited proportion of earthflows (4%) makes it directly to the Sillaro River main stem.

This work, as part of the projects BEDFLOW and BEFLOW PLUS, is partially funded by Fondazione Cassa di Risparmio in Bologna.

How to cite: Pittau, S., Berti, M., Daniele, G., Pizziolo, M., and Brardinoni, F.: A multi-temporal inventory for constraining earthflow source-to-sink pathways in the Sillaro River basin, Northern Apennines, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10391, https://doi.org/10.5194/egusphere-egu2020-10391, 2020

Bedrock landsliding provides a strong negative feedback on bedrock river incision by causing long-lived burial events and hence hiatuses in downcutting.  Nevertheless, rivers in tectonically active settings carve deep canyons despite being periodically inundated with immobile boulders. How is this possible? In this contribution, we explore the processes through which rivers incise bedrock canyons within the Franciscan mélange in the actively uplifting California Coast Range. The Franciscan mélange is well known for its “melting ice cream topography” in which slow-moving landslides (“earthflows”) festoon the walls of river canyons and deliver car- to house-sized boulders to channels.  

Analysis of valley widths and river long profiles over ∼19  km of Alameda Creek (185  km² drainage area) and Arroyo Hondo (200  km² drainage area) in central California shows a very consistent picture in which earthflows that intersect these channels deposit immobile boulders that force tens of meters of gravel aggradation for kilometers upstream, leading to apparently long-lived sediment storage and channel burial at these sites. In contrast, over a ∼30  km section of the Eel River (5547  km² drainage area), there are no knickpoints or aggradation upstream of locations where earthflows impinge on its channel. Neither boulder supply nor transport capacity explains this difference. Rather, we find that the dramatically different sensitivity of the two locations to landslide blocking is linked to differences in channel width relative to typical seasonal displacements of landslides. The Eel River is ∼5 times wider than the largest annual seasonal displacement. In contrast, during wet winters, earthflows are capable of crossing and blocking the entire channel width of Arroyo Hondo and Alameda Creek. Hence, by virtue of having wide valley bottoms, larger rivers are more likely to simply flow around the toes of earthflows.  

For the smaller rivers in our study area that are chronically buried in landslide debris, our field observations provide evidence for two processes that may allow periodic bedrock river incision. Narrow channels in the Franciscan mélange that are buried in debris can incise epigenetic gorges around the margins of boulder jams during periods of earthflow dormancy when boulders are no longer input into channels.  Alternatively, during periods of earthflow dormancy, abrasion (and hence size reduction) of boulders in place from suspended sediment may ultimately render boulders mobile.  

Without explicit representation of these three processes, modeling the coupling of hillslope and channel evolution in this setting is not possible. 

How to cite: Finnegan, N.: How rivers incise to survive periodic inputs of immobile landslide-derived boulders, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12867, https://doi.org/10.5194/egusphere-egu2020-12867, 2020

In the forested mountain areas tree uprooting plays important role among many other geomorphic processes. In some cases, during extreme wind events, large patches of forest may be destroyed, which causes transport of significant amount of sediment.

The aim of this research was to investigate magnitude of sediment transport during one intense windthrow event, which took place on 25 December 2013 in the Tatra Mountains, southern Poland. The research was conducted in three second- to third-order catchments (16-81 ha), in which 34 to 94 percent of their areas were affected by windthrow. This was achieved by combining field measurements and GIS analyses. During field work root plates located within selected research polygons were measured in order to recognize the amount of sediment transported by a single uprooted tree. Then, each root plate located in the investigated catchments was mapped in GIS software using high-resolution (40 mm) orthophoto. Based on this, total volume of sediment displaced by uprooted trees within each catchment was estimated. Next, taking into account directions of tree fall and slope inclination within each uprooted tree, sediment flux by windthrow event in 2013 was calculated.

In total 211 uprooted trees were measured in the field. Mean volume of measured root plates was 1.84 m³. It was assumed that half of that value is accounted for roots of a tree, thus on average 0.92 m³ of sediment was transported by each root plate. Analysis of the orthophoto allowed for identification of 4650 uprooted trees located in the investigated catchments. Most of the trees have fallen in downslope direction. Sediment flux by windthrow event in 2013 calculated for each catchment was 1.0–4.6 × 10^–3 m³ m^–1.

How to cite: Strzyżowski, D.: Magnitude of sediment transport due to extreme windthrow event in small catchments in the Tatra Mountains, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3351, https://doi.org/10.5194/egusphere-egu2020-3351, 2020

Rivers draining the Himalaya and feeding the Indo-Gangetic plain support around 10% of the world’s population. However, these rivers are also prone to frequent and often devastating floods such as the 2008 Kosi floods which displaced more than 2.5 million people. Changes in sediment supply from the Himalaya influence the magnitude and distribution of floods through changing capacity and routing respectively. Widespread landsliding following the 2015 Gorkha (Nepal) earthquake increased suspended sediment supply to the river network and is expected to result in some degree of coarse bedload aggradation and increased rates of channel migration at the mountain front. Given the significant amounts of channel aggradation observed in the aftermath of similar events, understanding the timescales of sediment transport following the 2015 Gorkha earthquake and the impact of any resulting sediment wave on flooding in the Gangetic plains is crucial. We track the gravel size fraction of the landslide sediment along the Kosi River (East Nepal) by mapping zones of sediment input from optical satellite imagery and constructing a time series of high-resolution channel cross-sections using an Acoustic Doppler Current Profiler (ADCP) in the years following the earthquake. We use these datasets to identify zones of channel aggradation and migrating sediment, and test whether the changes are consistent with the location of sediment sources (landslides) and magnitude of the monsoon floods with the aid of landslide inventories and flow data. While initial results show a marked increase in coarse sediment following the 2015 monsoon, we see little evidence of large-scale downstream migration of any sediment pulse, indicating the Gorkha landslides may have less of an impact on flood and sediment dynamics on the Indo-Gangetic plains than expected from comparison with similar events. We suggest that the Gorkha landslides may not be connected to the fluvial system to the same extent as for similar events and revegetated rapidly, and therefore did not release significant amounts of sediment into channels after the initial post-2015 monsoon pulse.

How to cite: Graf, E., Sinclair, H., and Attal, M.: Where does all the gravel go? Tracking landslide sediment from the 2015 Gorkha earthquake along the Kosi River, Nepal, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-455, https://doi.org/10.5194/egusphere-egu2020-455, 2019

Empirical observations and climate models simulations indicate an increase of intensity and frequency of extreme precipitation events triggering torrent hazards over the last 100 years. This trend is predicted to continue in the future, likely resulting in a rise of the frequency of hazardous torrential processes. That might lead to an increase of sediment-laden torrential flooding events, which are one of the most frequent geo-hazards in Austria.

Heavy rainfall, the availability of sediment, and the connectivity of sediment deposits are crucial factors for the occurrence and severity of hazardous hydro-geomorphic processes. To protect lives and infrastructure an effective design of protection measures depends on the analysis of past extreme events. Repeated topographic surveys, such as laser scanning campaigns, are used to assess hillslope-channel relationships and quantify geomorphic work of different geomorphic processes in torrent systems. The analysis of pre- and post-event high-resolution topographic data is important  for the understanding of sediment dynamics and changes in channel morphology. The aim of this study is to investigate the response and the amount of mobilised sediment from three different torrential catchments to extreme precipitation- and runoff events. 

The three study areas are located in the Niedere Tauern (Central Alps, Austria). The Schöttlbach catchment is dominated by mica-schist and the proportion of quaternary sediment is around 20 %. The Lorenzerbach and Schwarzenbach catchments are characterized by different gneiss, phyllite as well as schists and a quaternary sediment share of approximately 50%. In the last decade all three catchments were struck by heavy rainfall that triggered torrential events causing considerable damage to human settlements and infrastructure. 

The point clouds of the Lorenzerbach and Schwarzenbach catchment as well as the pre-event dataset of the Schöttlbach catchment were collected with an airborne laser scanning system. For the post-event point cloud of the Schöttlbach, a UAV-borne laser scanning system was used. All datasets differ in quality due to flight altitude, scan angle, point density and footprint diameter. In the course of this project a workflow is developed to analyse uncertainties and improve the comparability of datasets from different surveys. This is also necessary for a reliable Geomorphic change detection (GDC) analysis as well as the investigation of sediment dynamics and the estimation of erosion and deposition volumes. 

Finally the outcomes of the GCD analysis are compared with the results of event-documentations done by the Austrian Service for Torrent and Avalanche Control. The approach of Zedlacher (1986) is used to estimate sediment loads for 150-year flood events. However, preliminary results indicate that this approach underestimates sediment output during extreme events for all three catchments. Based on the analysis of terrain models and other available information, we aim to ‘update’ the empirical Zedlacher approach to improve sediment load estimation, with the overarching question whether intensified precipitation events under climate change conditions will cause a shift of the torrential systems towards higher sediment yields.

How to cite: Krenn, P., Kamp, N., Peßenteiner, S., Funder, M., and Sass, O.: Analysing the impacts of extreme precipitation events on geomorphic systems in torrential catchments; a comparative study from Upper Styria, Austria, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13255, https://doi.org/10.5194/egusphere-egu2020-13255, 2020

Glaciated mountains are zones of high sediment dynamics and at the same time very sensitive to climate change. In times of increased summer temperatures and high melt rates have been related to observed increase in sediment dynamics at various locations. However, this response seems to be highly variable also on regional scales indicating that controlling factors have yet not been fully identified and understood. Sediment output from glaciated catchments affects sediment budgets, streamflow ecology and hydropower generation. Data on sediment discharge from proglacial areas in the Alps is scarce. Knowledge on sediment responses to increasing temperatures and changing climates is crucial for river and reservoir management and climate change adaptation.

We contribute to this debate by quantifying sediment discharge from the Obersulzbachkees glacier, Hohe Tauern, Austria based on recent lake deposition volume. Located at the valley head of the Obersulzbach valley, the glacier experienced rapid degradation within the last 20 years and also showed high rates of sediment discharge. The formerly large single glacier disintegrated into five remaining parts and a large proglacial lake formed. Sediment discharge from these smaller glaciers is captured by the lakes and a huge delta has developed after retreat of ice from the lake. We quantified the lake and delta sediments using ground penetrating radar and sub-bottom profiling and revised our previous estimations by including new data increasing the accuracy of our finding. The Obersulzbachkees retreated by 400-800 m in distance between 1999 and 2019 and lost more than 3 km² of glacier area. Between 2007 and 2019 more than 600,000 m³ of sediments have been deposited within the lake delta only. We discuss sediment discharge from glacier to lake in relation to glacier retreat and climate conditions since lake formation and relate our findings to both changes in the catchment and runoff and sediment output dynamics from the lake.

How to cite: Otto, J.-C., Walk, V., Heine, E., and Keuschnig, M.: Sediment discharge from alpine glaciers in times of increased melt – an example from the Austrian Alps, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19789, https://doi.org/10.5194/egusphere-egu2020-19789, 2020

Synthetic Aperture Radar (SAR) amplitude measurements from spaceborne sensors are sensitive to surface roughness conditions near their radar wavelength. These data can be exploited to measure gravel-to-sand transitions and downstream gradients in grain size related to geomorphic setting in tectonically active high mountain environments at large spatial scales. The bedload of mixed sand- and gravel-bed rivers can be considered mixed smooth (compacted sand) and rough (gravel) surfaces. Here, we assess backscatter gradients over a large high-mountain alluvial river with aerially exposed sand and gravel bedload using X-band TerraSAR-X/TanDEM-X, C-band Sentinel-1, and L-band ALOS-2 PALSAR-2 radar scenes. In a first step, we compare backscatter response over vegetation-free endmember surfaces within the dry channel bed to assess expected responses and limitations of SAR roughness measurements. We then develop methods to extract smoothed backscatter gradients downstream along the channel using kernel density estimates. In a final step, the presence of sand and gravel bars is analyzed using Fourier frequency analysis, by fitting stretched exponential and power-law models to the power spectrum. We find a large range in backscatter depending on the heterogeneity of contiguous smooth- and rough-patches of bedload material. The SAR signal responds primarily to the fraction of smooth-sand bedload, but is further modified by gravel elements. The sensitivity to gravel is more apparent in longer wavelength (L-band) radar. Because the spatial extent of smooth sand bars is typically < 50 m, only higher resolution sensors (e.g., TerraSAR-X/TanDEM-X) are useful for power spectrum analysis. Our results show the potential for mapping sand-gravel transitions and local geomorphic complexity using SAR amplitude at the scale of large high mountain catchments with aerially exposed bedload.

How to cite: Purinton, B. and Bookhagen, B.: Multiband (X, C, L) radar amplitude analysis for a mixed sand- and gravel-bed river in the eastern central Andes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3943, https://doi.org/10.5194/egusphere-egu2020-3943, 2020

The clastic sediments and its aspect of provenance, weathering and erosion, tectonic setting, fluvial processes, paleoclimate and some other geological processes are better studied with the help of geochemical analysis. The changing geochemistry of sediment present in Himalayan river has been a great point of interest in sedimentary geochemistry because of its impact over Indian ocean chemistry and climate. In all Himalayan rivers, the Ganga and Yamuna Rivers are most important in global scenario due to their perennial nature, and peculiar flow and depositional characteristics. These two rivers had played important role in formation of Indo-Gangetic Plain during Quaternary period. Both the Ganga and Yamuna Rivers emerge from great Himalaya and carried the sediments from there to Bay of Bengal, India.

This causes sequential change in geochemistry of deposited sediments. The studied region is near by Mohand ridge and extend up to Balawali in Ganga River side and up to Kait in Yamuna River side. In this region rivers have high gradient channels and high flow speed condition. The channels are braided and have gravelly bed load. The converging channel system effects the geochemical constituent of river sediments.

The geochemical analysis of river bed sediments of both rivers by using XRF data analysis were carried out to find out the variation and effect of river bed morphology over geochemical constituents concentration. The prepared tectonic setting discriminant diagrams through plots log[K₂O/Na₂O] versus SiO₂ and [SiO₂/Al₂O₃] versus log[K₂O/Na₂O] indicate transitional tectonic setting from an active continental margin to a passive margin. The discriminant function plot indicates quartzose sedimentary provenance, and to some extent, the felsic igneous provenance, derived from weathered granite, gneissic terrain and/or from pre-existing sedimentary terrain. Further, by plotting SiO₂ versus other major elements plot reveals the changing concentration of major elements with respect to changing river bed morphology with 50-60 km length of both the rivers. In braided zone of river, there is sudden increase in SiO₂ concentration of river sediments. The gravels present in channel bed provide more resistance and tight pore spaces for flow of water which causes increase in abrasion phenomena. These vital change in geochemistry (which is from 65% to 81% for SiO₂ concentration) of sediments indicates about the major role play of braided zone gravel deposit. The changing bed morphology of river channel has vital effect on geochemical composition of deposited sediments.

How to cite: Satyam, G. P. and Dubey, R. K.: Influence of braided river bed morphology over concentration of geochemical constituents in river sediments: a case study of Ganga and Yamuna Rivers in and around Kait-Haridwar, Uttarakhand, India, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21371, https://doi.org/10.5194/egusphere-egu2020-21371, 2020

In active orogen area, the transient landscape with upstream-migrating knickpoints substitutes temporal evolution with spatial distribution, and thus offers an unique chance to understand the interaction of tectonics, surface processes, and climate change on various time and space scales. Correspondingly, river longitudinal profile, geomorphic surfaces, and sedimentary sequences generally act as the key archives of the knickpoint passage and regional landscape evolution. The gently-sloping southeast Tibet is in transient state, with abundant high-elevation but low-relief surfaces perched between deep gorges (up to 2-3 km in depth) incised by the Salween, Mekong, and Yangtze rivers. In this study, we carry out geomorphic analysis for the Lancang River at Yunlong reach, and focus on field investigation, Unmanned Aerial Vehicle (UAV) photogrammetric technique, and K-feldspar post IR-IRSL (pIR-IRSL) dating for fluvial terraces preserved on western bank of the Lancang River at Songdeng.

Our work reveals that the Yunlong reach is located at a steeper segment of the Lancang River, although it is below the main knickzone to the south of Weixi; most tributaries at this reach are in transient state with an adjusting and steeper reach, and has transmitted upstream some distance on western bank. Reconstruction of some tributary profiles with relict segment yield >1300 m incision on west bank, and 500-700 m incision on east bank. This elevation difference of reconstructed tributaries’ outlets may result from two separate phases of external perturbation, or local tectonic modification. Five levels of fluvial terraces T₅ to T₁ are preserved on western bank at Songdeng, with the bedrock strath of T₅ to T₂ at ~320-340 m, ~200-230 m, ~130-160 m, ~80-60 m high above the Lancang River. Terrace deposits transported by both the western-bank Songdeng tributary river and the Lancang main trunk are investigated to collect suitable fine-grained sediments for K-feldspar post IR-IRSL dating, and initial measurements yield age estimates at 530-240 ka. Correspondingly, fluvial incision rates since the Middle Pleistocene can vary from 0.6 mm/yr to 0.25 mm/yr with time, which may relate to one passage of the knickpoint along the Lancang main trunk. Reconstruction of the Songdeng River profile characterized with a slope-break knickpoint reveals ~1300 m incision at the confluence with the Lancang River. Assuming a constant and averaged incision value of 0.4 mm/yr since the knickpoint arrived the Songdeng confluence, the response time is estimated to be >3 Myr, which is consistent with the initiation of rapid cooling around 3 Ma by west-bank bedrock low-T thermal modeling published. Further work such as numerical modeling is needed to shed insight into the role of tectonics, surface processes, and climate change in shaping the landscape of southeast Tibet in late Cenozoic time.

How to cite: Zhang, J., Liu-Zeng, J., Yang, H., Ge, Y., Yao, W., Wang, W., and Li, Z.: Reconstructing aggradation and incision of the Lancang River (Upper Mekong) at Yunlong reach, southeast Tibet, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12497, https://doi.org/10.5194/egusphere-egu2020-12497, 2020

Alluvial fans represent complex landforms with the potential to record past environmental conditions. However, their decryption is difficult as their formation depends on a broad set of influences (catchment properties, climate, accommodation space, base level change). A comparison of alluvial fans in three (semi)arid regions aims to illuminate dominant controls on alluvial fan evolution.

Large scale alluvial fans in the semiarid to arid mountain areas of western Mongolia, southwestern USA, and the northern part of the Chilean Andes are controlled by different sediment supply. Geomorphological processes in these mountain ranges vary along altitudinal and latitudinal gradients and, additionally, due to climatic change during the late Quaternary. Alluvial fans in Mongolia (Gobi Altai and Mongolian Altai) are mainly formed during the Pleistocene. Higher terraces and alluvial fan generations can be dated to the penultimate glacial cycle. Sheet flow dominated as alluvial fan constructing process during the last Glacial. Since the late Glacial, debris flow accumulation and Holocene incision occurred (Lehmkuhl et al. 2018). Quaternary alluvial fans in mountain areas of the southwestern United States develop in three major settings related to the availability and nature of sediment transport. These include alluvial fans that develop in: i) glaciofluvial settings, ii) areas of tectonic uplift, and iii) regions dominated by periglacial processes. There is evidence for Pleistocene periglacial activity throughout the mountain ranges of the American Southwest in different elevations (Löhrer, 2008). Frost weathering in periods of higher moisture produces debris in the catchment areas and, thus, primarily governs the sediment supply of alluvial fans during the Pleistocene. In the semiarid Andes of northern Chile, alluvial fans form in similar glaciofluvial as well as fluvial settings in elevations above ~4000 m asl.

A comparison between these three (semi)arid systems shows that the main fluvial activity occurred during cold and semihumid phases of the Pleistocene resulting in an altitudinal lowering of periglacial processes, thus leading to a higher sediment supply. In addition, in all these regions higher lake levels occurred during the transition from glacial to interglacial periods, e.g. from the Pleistocene to the Holocene. Moister conditions during the transitions control the interplay between lake level variations and the fluvial activity.

Lehmkuhl, F., Nottebaum, V., Hülle, D. (2018): Aspects of late Quaternary geomorphological development in the Khangai Mountains and the Gobi Altai Mountains (Mongolia). Geomorphology 312:24-39. https://doi.org/10.1016/j.geomorph.2018.03.029

Löhrer, R. (2008): Reliefanalyse an Schwemmfächern und Fußflächen Südwesten der USA. Dissertation an der Fakultät für Georessourcen und Materialtechnik der RWTH Aachen, September 2008. Online Veröffentlichung der RWTH Aachen: http://darwin.bth.rwth-aachen.de/opus3/volltexte/2008/2504/

How to cite: Lehmkuhl, F., Nottebaum, V., Walk, J., and Stauch, G.: Quaternary paleoenvironmental change preserved in alluvial fans systems in semiarid to arid mountain areas: Examples from western Mongolia, western USA, and the Chilean Andes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3893, https://doi.org/10.5194/egusphere-egu2020-3893, 2020

The interaction between sedimentation/erosion and faulting represents one of the most intriguing topics in landscape and tectonics evolution. Recently, several studies attempted to unravel this issue but only few of them have been able to document the feedback between faulting and sedimentary loading. Here, we focus on how the sediment loading/unloading influences the dynamic of the faults system taking as study case the Fucino Basin in Central Apennines (Italy). The Fucino Basin represents a remarkable case study with respect to the other main extensional basins in the Apennines, because of its large dimension, rectangular shape, significant sediment thickness and more important, its endorheic nature throughout its evolution.

We present a detailed structural analysis all around the basin, investigating the kinematic and geometry of each main fault strand. The slickensides analysis reveals multiples families of slip-vectors and timing of activity which suggest a changing from N240° to N200° occurred during middle-Pleistocene. Moreover, using a simple isostatic model, we estimate that up to the 30% of the total geological displacement of the faults, which overall ranges from 0.5 to 3.5 km, is related to the sediments loading/unloading. Then, we demonstrate a positive feedback between sedimentation and faulting which may also lead to a re-organization in fault-kinematic related to a significant increasing in the vertical stress. Finally, we propose a conceptual model to support the permanent endorheic configuration of the Fucino basin, mainly related to a fault-slip increasing and kinematic changing due to the sediment loading.

How to cite: Lanari, R., Faccenna, C., Benedetti, L., Bellier, O., Menichelli, I., and Primerano, P.: Formation and persistence of extensional internally-drained basins: the case of the Fucino (Central Apennines of Italy), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6970, https://doi.org/10.5194/egusphere-egu2020-6970, 2020

The Northern Apennines of Italy are a young orogen comprised of mixed siliciclastic and carbonate lithologies. Young orogens are typically characterized by marine sedimentary sequences that contain important volumes of carbonate, which can dominate chemical weathering, as carbonate weathers a factor of 3 times faster than silicates. However, most models that address the interplay between erosion and weathering have focused on silicate lithologies.  Carbonate weathering is typically limited by the availability of acid rather than dissolution kinetics, and more tightly linked to soil and sub-surface CO₂ concentrations than silicate weathering. Therefore, it remains unclear if the same processes that control the partitioning of denudation between erosion and weathering in actively uplifting, silicate-rich lithologies are also active in orogens comprised of mixed carbonate-silicate lithologies. The partitioning of denudation between physical erosion and chemical weathering in mixed silicate-carbonate landscapes remains a fundamental knowledge gap that has implications for landscape development and the carbon cycle. Here we address two key questions: (1) how is the total denudation separated into carbonate and silicate fluxes, and (2) how is carbonate denudation partitioned into erosion and weathering in an active orogenic setting? We partition denudation fluxes from ¹⁰Be concentrations into carbonate and silicate chemical weathering and physical erosion fluxes, using major dissolved ions from water chemistry, the percent of carbonate sand from each catchment, and annual discharge measurements. Denudation fluxes in the Northern Apennines are dominated by physical erosion of both silicate and carbonate lithologies. Chemical weathering fluxes are 1-2 orders of magnitude lower than physical erosion fluxes and are dominated by carbonate dissolution. Despite a number of studies that have shown a strong positive correlation between denudation and chemical weathering fluxes, we find only a weakly positive correlation. Relative to a global dataset from silicate-rich orogenic settings, the Northern Apennines have similar denudation fluxes as the eastern side of the New Zealand Southern Alps. However, rivers from the Northern Apennines generally have higher total weathering fluxes relative to the Southern Alps, consistent with the exposure of a large volume of carbonate lithologies in the Northern Apennines.

How to cite: Erlanger, E., Caves Rugenstein, J., Bufe, A., Picotti, V., and Willett, S.: Partitioning the denudation flux between silicate and carbonate physical erosion and chemical weathering in the Northern Apennines, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12043, https://doi.org/10.5194/egusphere-egu2020-12043, 2020

Huge quantity of terrigenous particles was exported from oceanic island small rivers in delivering to the ocean (Dadson et al., 2003; Milliman and Syvitsky, 1992).  Quantity of river particles entering the ocean could be related to river basin area, elevation, erosion rate, and seismic activity.  However, limited data are available regarding differences between physical and chemical weathering on erosion and their effects on particles export from oceanic type of small rivers nor data on extreme event, the typhoon, and its effect on weathering at this setting. Here we report and quantify particles as well as dissolved materials export from an oceanic small river, the Lanyang River at the northeastern Taiwan, during typhoon period and those under normal weather condition.  Our objectives are to quantify river particles and dissolved components export during normal and typhoon period; to understand factors controlling their variations; to compare efficiency of chemical and physical weathering under extreme weather condition and those at normal condition.  River particles and dissolved components were sampled monthly and during typhoons at every four hours frequency and filtered, weighted for particle concentrations as well as chemical analyses of particle and dissolved compositions in lab.  Chemical analyses include solid and dissolve silica, aluminum, iron, sodium, calcium, magnesium, and potassium as well as dissolved chloride, sulfate, and alkalinity. River discharge data were from Taiwan Water Resources Agency and precipitation data from Taiwan Central Weather Bureau.

Our results demonstrated that typhoon is the primary mechanism in driving concentration variations of both dissolved phases and solid components in the study river.  Huge amount of precipitation flushed into river during typhoon, resulting in rapid dilution of dissolved components as well as rapid increase of suspended particles concentration in reaching hyperpycnal level.  During the period of rapid increase of particles in the river, shift of types of particles as well as dissolve components were observed.  TDS (total dissolved solid) represent a small portion of the materials export to the ocean.  TSM (total suspended matter) flushed out of river during typhoon represent a major fraction (85%) of the annual total particles, however, the amount of particles for each typhoon varied significantly (from ~10 to ~45%). 

How to cite: Chen, W., Hsieh, I., Lien, K., and Lin, S.: Typhoon and weathering processes on particles export to the ocean from a small river on the oceanic island of Taiwan, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8459, https://doi.org/10.5194/egusphere-egu2020-8459, 2020

Lake Baikal is the world’s largest (by volume), deepest, and oldest (30-40 Ma) lake. In the catchment, climate varies from arid to semi-arid to arctic-boreal with extreme seasonal and spatial differences in temperature and precipitation¹. Elevation ranges from 450-3000m, resulting in a large range of geomorphic settings. The catchment has also been affected by periodic Quaternary glaciations². Although the geology of the catchment is diverse and contains igneous, metamorphic and sedimentary rocks of Archean to Cenozoic ages, the most prominent lithologies are granitoids and gneisses with only minor carbonate contributions¹. Continuous lake sediment cores are available recording the Quaternary glacial cycles, and even dating back into the Miocene. Lake Baikal is therefore a promising site to study variation of silicate rock weathering in both space and time.

In preparation for paleo-studies, we constrain the present-day budget of the lake with respect to radiogenic weathering tracers (Nd, Pb, and Sr) and meteoric ¹⁰Be/⁹Be isotope ratios.  Nd, Sr, Pb, and their radiogenic isotope systems show different behaviors in Lake Baikal. Sr concentrations in the lake are similar to riverine inputs, reflecting conservative behavior of Sr and resulting in a uniform isotopic composition that is slightly higher than the average of riverine inputs (possibly due to loess inputs³). Pb concentrations are higher in the lake than in the major tributaries. The isotopic composition of both lake and rivers point to anthropogenic sources of Pb. In contrast, Nd concentrations in the lake are much lower than in the rivers. Nd isotopic compositions are similar in the central and southern basin but less radiogenic in the northern basin. Both ¹⁰Be and ⁹Be concentrations are much lower in Lake Baikal than in its tributaries, possibly indicating removal due to pH induced changes in dissolved-particulate partitioning⁴. This may also explain the contrast in Nd concentrations between rivers and the lake. ¹⁰Be/⁹Be ratios in the lake are slightly elevated compared to riverine  inputs, suggesting a potential role for dust and/or precipitation as a source for ¹⁰Be⁵. We will also compare silicate weathering fluxes derived from meteoric Be isotope ratios with those derived from major element concentrations and riverine discharges.

Taken together, these results highlight the importance of assessing modern processes at sediment core locations prior to interpreting variation in the past, and the benefits of using a suite of weathering proxies rather than relying on one: while Sr isotopes at any core location record changes to the chemistry of the whole lake (and the processes in its catchment), Be and Nd isotopes are likely biased to the inputs of the nearest rivers.

1. Zakharova et al. Chem. Geol. 214, 223–248 (2005).
2. Karabanov et al. Quat. Res. 50, 46–55 (1998).
3. Yokoo et al. Chem. Geol. 204, 45–62 (2004).
4. You et al. Chem. Geol. 77, 105–118 (1989).
5. Aldahan et al. Geophys. Res. Lett. 26, 2885–2888 (1999).

How to cite: Suhrhoff, T. J., Rickli, J., Christl, M., Vologina, E. G., Sklyarov, E. V., and Vance, D.: Weathering signals in Lake Baikal and its tributaries, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18195, https://doi.org/10.5194/egusphere-egu2020-18195, 2020

Chemical weathering at Earth’s surface releases soluble elements from rocks to streams and the oceans, interacting with the global carbon cycle along multiple pathways. The carbon budget of continental erosion is strongly dependent on the nature and relative importance of these pathways [1]. Weathering of silicate minerals with carbonic acid represents a long-term net sink of atmospheric CO₂. However, chemical weathering by other acids, such as pyrite oxidation-derived sulfuric acid, represents a net CO₂ source to the atmosphere [2]. Constraining the net balance of acids and lithology involved in weathering reactions is therefore paramount to budget the impact of chemical weathering on the carbon cycle. In this contribution, we present preliminary radiocarbon data measured on dissolved inorganic carbon (DI¹⁴C) from stream and spring waters in the central Himalaya of Nepal. DI¹⁴C is a promising tracer of the different chemical weathering reaction pathways [3], and DI¹⁴C values in the central Himalaya span across the natural spectrum. To constrain sulfate sources, measurements of δ³⁴S on dissolved sulfate complement this dataset [4], which also shows considerable variability ranging between -15 to +18 ‰. Inverting the dissolved ion composition and their isotopic constraints provide constraints on the proportions of carbonic and sulfuric acid weathering of silicates and carbonates. These results will then be compared with catchment lithological, geomorphological and climatic parameters.

[1] Berner and Berner, 2012 - Princeton University Press  

[2] Calmels et al., 2007 – Geology 35-11

[3] Blattmann et al., 2019 – Scientific Reports 9

[4] Turchyn et al., 2013 – EPSL 374

How to cite: Lupker, M., Märki, L., Paris, G., Blattman, T., Haghipour, N., and Eglinton, T.: Chemical weathering pathways in the central Himalaya – new constraints from DI14C and δ34S, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4480, https://doi.org/10.5194/egusphere-egu2020-4480, 2020

Recent years have seen an increasing number of studies suggesting that hypogene processes are more important in the origin of cave systems than previously thought. Recognizing such hypogene caves has important implications for e.g. paleohydrology and has been primarily based on morphological criteria, which to some degree are subjective and difficult to quantify. Apart from caves containing coarsely crystalline spar backed by evidence of elevated paleotemperatures based on isotopes and/or fluid-inclusion data, there are no well-established physico-chemical tools to validate a hypogene model for a given cave.

In a systematic approach we have studied a number of cave systems showing morphological features diagnostic of upwelling fluids, and examined the composition of the rock immediately behind the cave wall using small-diameter drill cores. We commonly observed two features in this wall rock: (1) an increase in porosity (partly later occluded by carbonate cement) and (2) a change in the rock colour (bleaching of initially grey rock, or reddening). We also identified dedolomitisation of the dolomite host rock, which may locally lead to the formation of boxwork. The most diagnostic feature, however, is a systematic shift in the carbon and/or oxygen isotopic composition along wall rock drill cores. None of these petrographic and geochemical features were observed in wall-rock cores of epigene caves, opening the door to use this approach in order to identify, and in some cases quantify, paleo-water-rock interactions associated with hypogene speleogenesis.

How to cite: Spötl, C., Dublyanky, Y., Koltai, G., and Plan, L.: Cave wall-rock alteration as an indicator of hypogene speleogenesis, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3908, https://doi.org/10.5194/egusphere-egu2020-3908, 2020

Sansha Yongle Blue Hole is an oceanic blue hole and is located at the northeastern edge of the Yongle Atoll, in the Xisha Islands of the northwestern South China Sea. The 301.19 m deep makes it to be the deepest known blue hole in the world. Despite the 3-D morphology, hydrochemical properties and chemocline of the blue hole have been comprehensive investigated, its karst formation process is still enigmatic. This study presented new acquired multi-channel seismic data across the Yongle Atoll and seismic data across the Sansha Yongle Blue Hole to describe the seismic reflection characteristics of the carbonate platform and the blue hole in extensive detail. Combined with the scientific wells drilled on the atoll, our results show that carbonate sequences including Lower Miocene, Middle Miocene, Upper Miocene, Pliocene and Quaternary developed on the platform. The magmatically intrusive activity and related magmatic hydrothermal fluid flows have been very active since 5.5 Ma around/on the Yongle Atoll, and may remain active on both slopes and the carbonate platform of the Yongle Atoll at present. Seismic profiles also show that the blue hole is characterized by chaotic seismic reflections which are easily distinguished from surrounding carbonate rocks with sub-parallel, continuous, low to medium amplitude, and low to medium seismic reflections. It seems that the depth of the blue hole is deeper than that measured according to the seismic images. The results of δ¹⁸O from scientific wells show that the phreatic extent in the Xisha Islands is from 14.75 – 38.89 m to 152.06 – 183.29 m. Therefore, different from other classic karstological blue holes formed by the phreatic dissolution processes, a hydrothermal – phreatic model with magmatic hydrothermal pipes and collapse of deep seated phreatic dissolution voids was proposed to describe the formation of the Sansha Yongle Blue Hole.

How to cite: Gao, J., Zhang, H., Wu, S., Ma, B., Chen, W., Han, X., Liu, G., and Tian, L.: Seismic expressions of the Yongle Atoll and the Sansha Yongle Blue Hole in the Xisha Islands, Northwestern South China Sea, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7210, https://doi.org/10.5194/egusphere-egu2020-7210, 2020

Sulfuric acid speleogenesis (SAS) has been widely recognized as one of the most aggressive processes involving carbonate dissolution and rapid formation of karst porosity under hypogenic conditions. Italian carbonate sequences, and especially those outcropping in the Central Apennines, host some of the best studied hypogenic SAS caves of Italy (such as Frasassi, Monte Cucco, Acquasanta Terme, just to mention the most famous).

The Cavallone-Bove cave system (CBS) is one of the longest natural caves in Abruzzo region (over 1 km of length) and opens at ca 1470 m asl in the Taranta Gorge, Majella Massif. The sulfuric-acid origin of this inactive hypogenic system has been previously proven by D’Angeli et al. (2019) using field evidences, secondary minerals and stable isotopes analysis. ⁴⁰Ar/³⁹Ar dating of alunite deposits suggested the SAS process occurred about 1.52 ± 0.28 Ma.

Both caves are characterized by a main sub-horizontal rounded or trapezoidal passage with only minor secondary branches and sub-vertical rift-conduits (feeders). Spatial geometry and arrangement of CBS conduits differs significantly from typical SAS water table caves, where complex anastomotic or maze network patterns are observed. Combining classical geological surveys, fracture stratigraphy and cave morphogenetical analysis we characterized the speleogenesis of the CBS. Field observations, remote sensing, detailed geological and geomorphological surveys were performed to characterize the structural evolution of the carbonate sequence hosting the caves, and to explain the relationship with the peculiar spatial and functional organization of CBS.

Our work highlights that lithostratigraphy and fractures pattern guide the development of karst macro-porosity in a specific stratigraphic interval within the Santo Spirito Formation, consisting mainly of layered micritic limestones, confined at the top by a chert interlayers dominant member. Through-going faults and fracture-clusters zones are identified as the main permeability pathways for ascending and laterally spreading H₂S fluids, influencing the spatial localization of conduits. These fluids reacted close to past water table in oxi-conditions, creating aggressive H₂SO₄. Sulphur stable isotopes signatures of secondary minerals suggest an origin for these H₂S bearing fluids from deep-seated Triassic evaporites interacting with hydrocarbons, thus migrated upward through a network of interconnected fractures. Permeability pathways for this vertical ascending flow were provided by NW-SE persistent strike-slip fault zones segmenting the eastern front of the Majella anticline structure and NNE-SSW striking fracture-clusters localized in the hinge zone of the fold.

How to cite: Pisani, L., Antonellini, M., D'Angeli, I. M., and De Waele, J.: Karst porosity development in layered and fractured carbonates: field evidences of structural control on sulfuric acid speleogenesis (Majella Massif, Italy), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3935, https://doi.org/10.5194/egusphere-egu2020-3935, 2020