GM7.2

GM7
Erosion, Weathering, and Sediment Transport in Mountain Landscapes 

Mountain belts are characterized by the fastest rates of physical erosion and chemical weathering around the world, making them one of the best places to observe sediment production (e.g. erosion, weathering) and transport processes. In these settings, varied processes such as rockfall, debris flow, hillslope failure, glacial and periglacial erosion, fluvial erosion, transport and deposition, and chemical weathering operate, often simultaneously, over a wide range of temporal and spatial scales.

As a result, tracking the interactions between denudation, climatic forcing, tectonic activity, vegetation and land use is complex. However, these feedbacks affect both long- and short-term natural surface processes, landscape development, and human interactions with the environment. Many of these processes also pose serious threats to the biosphere, mountain settlements and infrastructure. Therefore, understanding and quantifying rates of erosion, weathering, and deposition within mountain landscapes is a challenging, but crucial research topic in Earth surface processes.

We welcome contributions that (1) investigate the processes of production, mobilisation, transport, and deposition of sediment in mountain landscapes, (2) explore feedbacks between erosion and weathering due to natural and anthropogenic forcings, and (3) consider how these processes contribute to natural hazards specific to mountain landscapes. We invite presentations that employ observational, analytical or modeling approaches in mountain environments across a variety of temporal and spatial scales. We particularly encourage early career scientists to apply for this session.

Convener: Erica ErlangerECSECS | Co-conveners: Elizabeth DingleECSECS, Emma GrafECSECS, Eric DealECSECS, Apolline MariottiECSECS
Presentations
| Mon, 23 May, 08:30–11:50 (CEST)
 
Room -2.32/33

Presentations: Mon, 23 May | Room -2.32/33

Chairpersons: Erica Erlanger, Apolline Mariotti, Emma Graf
08:30–08:33
08:33–08:40
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EGU22-10378
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Presentation form not yet defined
Robert Hilton, Mateja Ogric, Mathieu Dellinger, Guillaume Soulet, Sebastian Klotz, Jordon Hemingway, Alexandra Turchyn, Caroline Le Bouteiller, and Christian Schiffer

Deforestation of steep mountain forests can result in a large increase in physical erosion rates. A growing body of work has highlighted that erosion may set the pace of oxidative weathering of sulfide minerals, which results in the production of sulfuric acid and can release CO2 from carbonate minerals in rocks to the atmosphere. However, the role of land use change on this CO2 release pathway has not been assessed, primarily due to the lack of measurements to isolate this driver over other potential environmental controls (e.g. temperature, hydrology). Here we study the stream water chemistry of two marl-dominated catchments, the Laval (0.86 km2) and the Brusquet (1.07 km2), in the Draix-Bléone Critical Zone Observatory, Provence, France. The Laval has very high rates of physical erosion (sediment yields of 8,700 t km-2yr-1 and 15,800 t km-2yr-1 in 2016-2017 and 2017-2018, respectively) that result from the combination of deforestation, bare rock surfaces, weak rocks and the hydroclimatic setting. In contrast, the Brusquet catchment was reforested at the end of the 19th Century and has much lower present day sediment yields (45 t km-2 yr-1 and 492 t km-2 yr-1 in 2016-2017 and 2017-2018, respectively).

We collected samples from every storm event and during flow over two water years in each catchment. We measure the major ions and the sulfur and oxygen isotopic composition of dissolved sulfate (SO4). In both catchments cation partitioning shows a dominance of carbonate (>70%) over silicate weathering. The stable sulfur isotopic signature suggests sulfide oxidation is the dominant source of sulfate in these catchments. Examination of dissolved ion rations (HCO3/∑Cat+, SO4//∑Cat+) shows that sulfuric acid governs mineral dissolution, rather than carbonic acid, accounting for 90±6% and 63±9% in the Laval and Brusquet, respectively.

In the highly erosive Laval catchment, the estimated CO2 release from sulfide oxidation coupled to carbonate weathering was very high, at 22.1±7.1 tC km-2 yr-1. We also find evidence for seasonal changes in sulfate flux which suggest that the rates are moderated by changes in air temperature, with elevated sulfide oxidation rates in summer. These observations support independent measurements in the shallow weathering zone of the Laval catchment, that shows an increase in CO2 release from sulfide oxidation with temperature. In marked contrast, the CO2 release estimated in the reforested Brusquet catchment is 4 to 5 x lower (at 4.6±0.8 tC km-2 yr-1) and the fluxes are not seasonally moderated (i.e. not temperature controlled). We suggest this relates to changes in the supply of mineral surfaces to the shallow, oxygenated weathering zone. Reforestation could result in a marked decrease in the release of carbon from rock to the atmosphere in areas where sulfide and carbonate minerals outrcop, and make the resultant fluxes less sensitive to changing climate.

How to cite: Hilton, R., Ogric, M., Dellinger, M., Soulet, G., Klotz, S., Hemingway, J., Turchyn, A., Le Bouteiller, C., and Schiffer, C.: Reforestation drastically reduces CO2 release from sulfide oxidation and its climatic sensitivity – Insight from a paired catchment approach at the Draix-Bleone Observatory, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10378, https://doi.org/10.5194/egusphere-egu22-10378, 2022.

08:40–08:42
08:42–08:49
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EGU22-515
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ECS
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Virtual presentation
Anupam Samanta, Gyana Ranjan Tripathy, Rajani Panchang, B Nagender Nath, Ravi Bhushan, and Nisha Bharti

Geochemistry of marine sediments serves as a reliable proxy for past continental erosion patterns. Available studies on paleo-erosion although have evaluated the impact of natural factors (climate, vegetation, basin slopes) on erosion rates, the impact of anthropogenic forces on the land surface processes has received limited attention. Recognizing this, major and trace elemental compositions of a sediment core (SSK40/GC06) from the eastern Arabian Sea have been investigated to reconstruct the temporal change in erosional pattern, if any, during the late Holocene period. The chronology of the core has been established using the AMS-14C age of foraminifera samples. The SSK40/GC06 sediments are mostly clayey to silty in nature, with minimal change in size with depth. Depth profiles of Al-normalized elemental ratios and patterns of rare earth elements point to an insignificant change in relative sediment contributions from the major provenances. In contrast, temporal trends of multiple weathering indices show a significant increase in chemical erosion since ~ 1600 yr BP to the present. Past climatic and sedimentary records show no major change in the southwest monsoon regime during this period. However, existing studies based on sedimentary records from peninsular India show a synchronous rise in the C4 vegetation, which possibly is linked to human-induced agricultural practices. This increase of C4 vegetation with shallow root systems may erode young soils more efficiently, which in turn expose fresh minerals for chemical weathering in basins. The observed trends, along with available past vegetation records, show a strong linkage between chemical erosion and anthropogenic activity.

How to cite: Samanta, A., Tripathy, G. R., Panchang, R., Nath, B. N., Bhushan, R., and Bharti, N.: Linkage between chemical erosion and human-induced vegetation pattern during the late Holocene period, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-515, https://doi.org/10.5194/egusphere-egu22-515, 2022.

08:49–08:51
08:51–08:58
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EGU22-7187
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ECS
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On-site presentation
Aaron Bufe, Jeremy Rugenstein, and Niels Hovius

The uplift and erosion of active mountain ranges and the consequent weathering of minerals modulates the global carbon cycle and impacts Earth’s climate on geologic timescales. However, the link between erosion and weathering is complex because weathering rates can be limited by the supply of minerals to the weathering zone, by the supply of acidic fluids, or by the kinetics of mineral weathering. Existing approaches that model the carbon cycle over geologic timescales assume that with increasing erosion rates, weathering transitions from a supply limit where weathering rates scale linearly with erosion to an ‘acid’ or a kinetic limit where weathering is insensitive to erosion. Alternative models fit a single power-law to the relationship between erosion and weathering across multiple orders of magnitude. The validity of these two approaches remains difficult to assess at the landscape scale because existing data do not cover all limits or because co-variation between runoff and erosion obscures the driver of changes in weathering rates. Here, we compile five datasets of solute concentrations in streams that span well-defined erosion rate gradients in relatively uniform lithologies and with limited or well-constrained variations in runoff. Across 2-3 orders of magnitude of erosion rates, and for both metasedimentary and granitic lithologies, we find that silicate weathering rates are insensitive to erosion rates. In turn, weathering of sulfide and carbonate minerals increase with erosion rates, consistent with a limitation by mineral supply. However, contrary to existing models of supply-limited weathering, we observe a non-linear increase of sulfide and carbonate weathering rates with erosion. These new findings suggests that supply-limited and kinetically limited zones of weathering co-exist within a single landscape across multiple orders of magnitude of erosion rate. The distribution of these zones is most likely controlled by erosion processes. As a consequence, existing weathering models that assume a linear relationship between erosion and weathering at the supply limit may overestimate the sensitivity of weathering rates to erosion and underestimate the impact of climate on these reactions, with implications for the effect of mountain building on the carbon cycle and on Earth’s climate.

How to cite: Bufe, A., Rugenstein, J., and Hovius, N.: Erosion and chemical weathering of siliceous rocks at the supply and kinetic limits, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7187, https://doi.org/10.5194/egusphere-egu22-7187, 2022.

08:58–09:00
09:00–09:07
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EGU22-975
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On-site presentation
Peter Clift and Tara Jonell

Does uplift and erosion of the Himalaya-Tibetan Plateau drive Cenozoic global cooling? We tested this classic hypothesis put forward by Raymo and Ruddiman (1992) that suggests enhanced erosion in the Himalaya-Tibetan Plateau drove long-term Cenozoic global cooling through the chemical weathering of siliciclastic sediment. Here we examine three Asian marginal drainage systems (the Indus, Mekong and Pearl) where marine scientific drilling has yielded detailed seismic surveys and geochemical datasets that critically permit sediment mass flux and therefore chemical weathering flux budgets to be made. By compiling suitable bedrock endmember compositions for the fresh bedrock sources and proximal modern river sediments, it is possible to calculate the chemical weathering flux and relative CO2 consumption rates for each drainage system into the early Miocene. We correct for evolving provenance of sediment delivered to the offshore and test the sensitivity of our calculations to selected bedrock endmembers, in light of the abundant mafic bedrock exposed Indus and Mekong systems. The Indus shows a progressive shift away from erosion of the Karakoram to the Himalaya. Appropriate Upper Continental Crust endmembers were further validated using data compiled from the GEOROC database. Regardless of which endmembers were used, calculations demonstrate that the total rate of CO2consumption decreased by 50% between ~16 and 5.3 Ma, especially within the NW Himalaya as erosion slowed and provenance shifted away from mafic arc units in the suture zone. This direct test of the uplift-erosion-weathering hypothesis establishes that chemical weathering fluxes in this region did not increase during the Neogene and cannot be responsible for the drawdown of atmospheric CO2 during that time period. Either additional mechanisms have been driving global cooling since 16 Ma or CO2 consumption via chemical weathering is taking place in other areas outside the Himalaya-Tibetan Plateau.

How to cite: Clift, P. and Jonell, T.: Erosion and Weathering of the Himalaya and Tibetan Plateau is not the Cause of Neogene Global Cooling, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-975, https://doi.org/10.5194/egusphere-egu22-975, 2022.

09:07–09:09
09:09–09:19
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EGU22-2808
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ECS
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solicited
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On-site presentation
Odin Marc

Landsliding is the dominant process of sediment production in active mountain range. Various geophysical forcing are triggering widespread landsliding over different timescales.

Though less continuous than climatic forcing, shallow earthquakes are an important trigger of mass wasting. PGA not only controls the spatial density of landslides (Meunier 2007) but also their size (Valagussa 2019) and their location in the landscape (Milledge 2019), with implications still to be explored. Combining simple seismological scalings with landscape characterization allows to relate EQ sequences to landslide flux, over various timescales (Marc et al., 2015, 2016, 2017).

Intense rainfall is another recurrent trigger. Landslide catalogue associated with rainfall events are multiplying and showing specific relations with rainfall and topography (Marc et al 2018), in particular, landslide scar appear unrelated to saturation index (Emberson et al., 2021), and in well constrained cases they seem to relate better to rainfall anomaly (ie relative to local extreme rainfall) than to total event rainfall (Marc et al 2019a, in Review).

These triggers can be combined for a given mountain range, such as the Nepalese Himalayas (Marc et al., 2019b). Here we combined a fine characterization of the landslide frequency size distribution for EQ and monsoon rainfall to derive a long-term erosional flux. Accounting for a distribution of EQ properties and the frequency of giant paleolandslides we obtain an erosion rate consistent with the thermochronometric exhumation rate. Recent work has shown that the monsoon strength variations could also modulate the landslide response and that landsliding caused by extreme daily rainfall could also be isolated (Jones 2021), which could allow to refine and better understand how this budget has changed in the past.

Last, we remind that, large, deep seated landslides remain an important and understudied challenge. Indeed, they are poorly correlated to rainfall properties (either the monsoon (Marc et al 2019b, Jones 2021) or the rainfall anomaly from large typhoons (Marc et al., 2019a). However, they may be a dominant contributor to erosion and sediment volumes, and their control, likely related to mechanical strength degradation, should be investigated urgently.

How to cite: Marc, O.: Understanding landscape mass-wasting in response to geophysical forcings to constrain long-term erosional budgets, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2808, https://doi.org/10.5194/egusphere-egu22-2808, 2022.

09:19–09:24
09:24–09:31
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EGU22-10417
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Virtual presentation
Christoff Andermann, Albert Galy, Sabrina Hennig, Bernhard Zimmermann, Edward T. Tipper, Erica Erlanger, Kristen L. Cook, Anja Schleicher, Liane Benning, and Niels Hovius

Glacier Lake Outburst Floods GLOFs are well known for their destructive powers and far-reaching consequences. These events are typically very difficult to predict, have a short duration, and often destroy installations in- and close to the river, making observational insights into GLOFs very rare. This limits our process understanding of such events. Here, we will present new observational data of the 2016 Bhotekoshi-Sunkoshi GLOF in Nepal. This data provides new insights on the erosion and weathering processes linked to violent floods and their aftermath.

In the night of the 5th of July 2016, a GLOF roared down the Bhotekoshi-Sunkoshi, originating from China, and causing severe damage along the river for roughly 50 km. This flood was recorded by an array of seismometers, allowing us to reconstruct the hydrodynamics of this event. Furthermore, 3 hydrological monitoring and sampling stations along the river captured the event. These stations cover the entire river length from Barabise, located in the affected upstream reach, to Kurkot situated halfway down, to Chatara, located at the outlet to the Gangetic foreland. For all 3 stations, we present daily resolved river discharge, suspended sediment flux and major element geochemistry for a month before and after the event. The dataset is complemented by mineralogical analysis of the suspended sediments and trace element analysis of the dissolved load.

We observed a clear enhanced suspended sediment signal that is carried through the entire river system. Sediment concentrations dilute from >30g/l at Barabise to ~ 7g/l at Chatara. At Barabise, sediment concentration decays in an exponential manner over roughly 14 days, mimicking the seismically deduced bedload activity. At the same time, we observed a clear peak in total dissolved chemistry of 50-70 mg/l above background. However, not all major elements contribute equally to this peak, with K+ and HCO3- being the major ions of this signal. Surprisingly, F- also has a well sustained peak, tripling the background concentration from ~0.07 to 0.21mg/l and mimicking the suspended load. This F- peak is present all the way down to Chatara, despite an almost 2 order of magnitude increase in discharge. We identified muscovite as potential source where F- can replace OH- in the crystal lattice. This is supported by the mineralogical and trace element analysis, e.g. Rb+ replacing K+ in muscovite. We propose that the very high energy sediment transport during the flood, together with a complete reorganization of the river bed, caused violent abrasion, leading to mechano-chemical dissolution of weakly bound F- from the fresh muscovite surfaces. In order to test our hypothesis, we were able to reproduce the signal in a circular flume.

This very fast evolution of F-, an element generally little considered in weathering processes, highlights the role of mechanical rock grinding and crushing in chemical dissolution processes in rivers. Furthermore, we show for the first time the application of F- as a tracer for catastrophic floods that can project well beyond the actual flood affected area and provides valuable insights on the chemical processes of such extreme events.

How to cite: Andermann, C., Galy, A., Hennig, S., Zimmermann, B., Tipper, E. T., Erlanger, E., Cook, K. L., Schleicher, A., Benning, L., and Hovius, N.: Erosion and weathering forensics of a catastrophic glacial lake outburst flood in Nepal, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10417, https://doi.org/10.5194/egusphere-egu22-10417, 2022.

09:31–09:33
09:33–09:40
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EGU22-10932
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ECS
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Virtual presentation
Anil Kumar, Pradeep Srivastava, and Rahul Devrani

Rivers in the Himalaya have extensively been used as a tool to understand the mechanism of valley aggradation and incision resulting due to precipitation variability and ongoing deformation. The suture zone tectonics control the landscape building along the upper Indus River, however, intensified monsoon encroaches on the region and modify the surface processes. The valley is filled with outwash fans and aggradation pulses at ~ 52 ka, ~ 28 ka and ~16 ka when the monsoon was strengthened. At the younger time, during ~13 to ~9 ka, the Indus valley has suffered an incision phase marked by exposure of bedrock near the Indus-Zanskar confluence.

The present study constrained the paleodischarge of the Indus River during periods of established river aggradation, incision and flood-time to understand climatic settings during the enhanced sediment-load modulated by the increased discharge. At the valley filling time (47–23 ka), clast geometric data of the largest imbricated clasts from the riverbed as well as the aggraded sequences were utilized to calculate discharges. Sand-silt couplets marked as slack water deposits (SWDs) of age 14–10 ka at Indus-Zanskar confluence were used to constrain the paleodischarges during net river incision. The catchment-scale paleodischarge derived from valley fill sequences and SWDs ranges from 834±47 to 4457±253 and 19030 to 47954 cumecs. Incision-time discharges were three to ten-fold higher than from the aggradation time observed that the aggradation in the Himalayan rivers occurred in transient time (33–21 ka and 17–14 ka) when the sediment load in the rivers increased just after the glaciation. Hence, the aggradation in the Indus River has occurred when the sediment to water ratio was higher and the river carrying capacity has reduced, subsequently, the incision was initiated when sediment to water ratio reduced and the river sediment carrying capacity increases during post-glacial climatically wet phase (early Holocene).

How to cite: Kumar, A., Srivastava, P., and Devrani, R.: Late Quaternary floods and their control on aggradation and incision in the Indus River, Ladakh Himalaya, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10932, https://doi.org/10.5194/egusphere-egu22-10932, 2022.

09:40–09:42
09:42–09:49
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EGU22-12197
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On-site presentation
Kristen Cook, Wolfgang Schwanghart, Bijaya Puri, Christoff Andermann, and Basanta Raj Adhikari

The Himalayas experience frequent fluvial disasters due to precipitation-driven floods, as well as events such as glacial lake outburst floods and landslide lake outburst floods. In the past years, these events have received much attention in order to understand and estimate their likelihoods and potential magnitudes. In addition to inundation-related impacts, geomorphic processes such as sediment transport, bank erosion, and hillslope feedbacks have been addressed by numerous studies at different timescales, demonstrating that sediment mobilization, transport, and deposition can become a dominant driver of fluvial hazard and risk.

The combined impact of hydrological and geomorphological processes is exemplified by the 2021 disaster on the Melamchi River in central Nepal. A flood event on 15 June caused wide-spread aggradation along tens of km of river corridor, with up to 15 meters of deposition in the town of Melamchi Bazar, destruction of the intake of the Melamchi-Kathmandu drinking water tunnel, and extensive modification of the river channel. This modification led to increased terrace erosion, slope destabilization, and amplified impacts of later floods. A second flood event on 31 July caused further aggradation and extensive damage. Overall, sediment deposition in these events was on the order of 50-75 million m3. One source of these deposits can be identified in the upper headwaters of the Melamchi Khola at an elevation of ~3600m, where ~100 million m3 of sediment was stored behind a paleo-landslide dam.  Destabilization of the landslide deposit during the June event has led to ~1 km of headward incision and the rapid evacuation of both landslide and paleo-lake fill sediment.

The Melamchi event highlights the importance of understanding controls on sediment storage and mobility in mountain catchments, and of recognizing the potential hazards involved once these sediments are entrained and routed downstream. In particular, situations where a large amount of relatively fine-grained mobile sediment is trapped behind a landslide dam, moraine, or other temporary dam have the potential for catastrophic sediment transport events. To better understand these potential hazards and their spatial patterns, we have conducted a Himalaya-wide assessment to identify, classify, and predict the occurrence of such features, which we term sediment bombs. A statistical model of sediment bomb locations identifies mean hillslope gradient and glacier extent during the last glacial maximum as predictors of their occurrence, suggesting that post-glacial adjustment may promote their formation in steeply incised valleys. Our analysis suggests that the conditions that led to the Melamchi disaster are more widespread than previously recognized, and that these upstream reservoirs of potentially mobile sediment should be considered in assessments of fluvial hazards.

How to cite: Cook, K., Schwanghart, W., Puri, B., Andermann, C., and Adhikari, B. R.: Himalayan sediment bombs – generalizing from the 2021 Melamchi Khola disaster, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12197, https://doi.org/10.5194/egusphere-egu22-12197, 2022.

09:49–09:51
09:51–09:58
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EGU22-12816
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ECS
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Presentation form not yet defined
Positive feedback between rates of rock mass weathering and landscape lowering through fluvial and hillslope erosion in the High Himalaya of Bhutan
(withdrawn)
Larissa de Palézieux, Kerry Leith, and Simon Loew
09:58–10:00
Coffee break
Chairpersons: Erica Erlanger, Apolline Mariotti, Emma Graf
10:20–10:27
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EGU22-3072
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ECS
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Presentation form not yet defined
Laura Quick, Richard Williams, Richard Boothroyd, Trevor Hoey, Pamela Tolentino, Esmael Guardian, Migo Reyes, and Catherine Sabilo

River morphology results from sediment transport and deposition, which are both a consequence of water flow. Episodic variation in natural (e.g. typhoons, earthquakes, volcanoes) and anthropogenic (e.g. gravel mining, river bank protection) sediment supply drives changes in riverbed levels and planform morphology of rivers. These geomorphic changes determine channel capacity and flow routing, and thus associated flood risk to surrounding people and property.

Despite the significance of variation in riverbed levels and channel position for flood risk, geomorphological processes are commonly overlooked in flood risk mapping. Tropical gravel bed rivers, like those observed in the Philippines, are particularly dynamic; flood risks arising from sedimentation and erosion should be assessed and incorporated into flood risk management to mitigate the impact of flooding on welfare and the economy.

Here we use new high resolution (0.5 m) repeat topographic surveys from 2014, 2019 and 2020 of the Bislak River in northwest Luzon Island to quantify annual and multi-year sediment budgets (i.e. erosion and deposition) and geomorphic change over single and multiple wet seasons.

Changes in flood risk related to geomorphic change (natural and anthropogenic) is tested using repeat two-dimensional hydraulic modelling to see whether observed geomorphic changes result in altered discharge routing and flood inundation extent in the region.

How to cite: Quick, L., Williams, R., Boothroyd, R., Hoey, T., Tolentino, P., Guardian, E., Reyes, M., and Sabilo, C.: Geomorphic change as a driver of flood risk in a tropical gravel bed river, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3072, https://doi.org/10.5194/egusphere-egu22-3072, 2022.

10:27–10:29
10:29–10:36
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EGU22-3594
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ECS
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On-site presentation
Boris Gailleton, Luca Malatesta, Jean Braun, Guillaume Cordonnier, and Benoit Bovy

Source to sink systems involve processes happening at very large timescales and the current state of the earth only represents a brief snapshot of it through space and time. These processes encompass any of those involved in the removal, transport and deposition of material from sources to sinks. The relatively stable incising valleys of the upland source landscapes allow for very efficient mathematical expressions to model their evolution over geological timescales. The transport part of the system is more challenging to study and resists radical simplification. Softer and lower-relief compared to their upstream counterparts, they are much more dynamics and undergo convoluted cycles of erosion/deposition/remobilisation only leaving sparse clues. The presence (or absence) of materials from identified provenance in the transport zone is commonly used to interpret landscapes connectivity through time. However, recent studies suggest that the dynamic nature of the transport section can make apparent provenance data ambiguous and misleading.

In this contribution, we leverage the ability of a newly developed cellular automata landscape evolution modelling framework, CHONK, to investigate the transport zone. CHONK is a landscape evolution modelling framework combining advantages of cellular-automata methods with common eulerian ones. Equations are implemented in a cell referential and cells are processed in a lake-aware multiple flow topological order. Because everything happens within a cell before communicating with downstream landscape, this framework allows fine tracking of sediment provenance through space and time, unconditionally to which law is implemented, or how complex the landscape structure.

We set up a basic mountain range with its foreland, exhume a discrete pluton of different rock type and track the pathways of the sediments through time, including in a stratigraphy allowing remobilisation. We show how, even in this simplistic setting, complex and stochastic patterns of sediment pathways arise.  We explore at which degree these patterns are predictable and at extents they are stochastic – in other word we track the variability of the proportion of sediment coming from the pluton at given locations through time. Finally, adding more complexity to the settings with variable tectonic and climatic cycles, intermediate sink (lakes) and heterogeneous lithologies, we offer a perspective on the framework’s potential to study the role of sediment flux in shaping the landscape and its record in novel ways.

How to cite: Gailleton, B., Malatesta, L., Braun, J., Cordonnier, G., and Bovy, B.: (Un)predictability of sediment pathways in the transport sections of Source to Sink systems, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3594, https://doi.org/10.5194/egusphere-egu22-3594, 2022.

10:36–10:38
10:38–10:45
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EGU22-6474
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ECS
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On-site presentation
Jesse Zondervan, Martin Stokes, Anne Mather, Matt Telfer, Sarah Boulton, Jan-Pieter Buylaert, Mayank Jain, Andrew Murray, and Mhamed Belfoul

We demonstrate how assumptions about strath terrace formation affect the interpretation of climatic control on landscapes, calculation of incision and rock uplift rates, and recommend strategies for geochronological sampling and interpretation. An innovative approach to OSL dating terrace gravels allows us to produce a detailed ~200 kyr history of punctuated river incision and strath terrace formation spanning two stratigraphic landform levels in the High Atlas Mountains (NW Africa). Extensive preservation and exposure of strath-top gravels, within a post-orogenic setting unaffected by eustatic influences, enables the derivation of rates of base-level fall, integrated over periods of strath-top deposition, metastable equilibrium, and incision, that are consistent with independently constrained regional rock uplift rates. Combining a punctuated river incision model with our well-constrained terrace formation history allows us to demonstrate how assumptions concerning Quaternary river incision and deposition can lead to the problematic Sadler Effect, an apparent dependence of incision rates on measured time interval. Subsequently, we demonstrate that an approach to reinterpreting previously published data using the punctuated incision model, even when combined with limited terrace age data, results in more consistent and parsimonious conclusions about rates of river incision, rock uplift and base-level lowering across the mountain belt. Our recommendations for sampling strategies to constrain rock uplift rates require samples to be taken just above the strath surface, and in addition towards the top of the deposit for river incision rates. In a setting with punctuated river incision and strath terrace formation, both rock uplift and incision rates require burial dates, as exclusive use of abandonment ages will not yield constraints on accurate rates of rock uplift or incision. Furthermore, we find that only with multiple along-stream locations and multiple burial dates in each terrace deposit, could a reliable climatic signal be extracted; this signal would not have shown up in terrace abandonment ages such as those derived from cosmogenic exposure dates. The demonstrated effects of assumptions about strath terrace formation, and the recommended approaches for sampling and interpretation, have implications for those attempting to constrain palaeoclimatic, tectonic, and geomorphic histories from strath terrace records in regions exhibiting punctuated river incision.

How to cite: Zondervan, J., Stokes, M., Mather, A., Telfer, M., Boulton, S., Buylaert, J.-P., Jain, M., Murray, A., and Belfoul, M.: Punctuated river incision: implications for deriving climate signals, river incision and rock uplift rates from Quaternary strath terraces, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6474, https://doi.org/10.5194/egusphere-egu22-6474, 2022.

10:45–10:47
10:47–10:54
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EGU22-9714
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ECS
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On-site presentation
Ron Nativ, Jens Turowski, Liran Goren, Jonathan Laronne, and J. Bruce H. Shyu

Large boulders with a diameter of up to several tens of meters are globally observed in mountainous bedrock channel environments. Recent theories suggest that high concentrations of boulders are associated with changes in channel morphology. However, data are scarce and ambiguous, and process-related studies are limited. Here we present data from the Liwu River, Taiwan, showing that channel width and slope increase with boulder concentration. We apply two mass balance principles of bedrock erosion and sediment transport and develop a theory to explain the steepening and widening trends. Five mechanisms are considered and compared to the field data. The cover effect by immobile boulders is found to have no influence on channel width. Channel width can partially be explained by boulder control on the tools effect and on the partitioning of the flow shear stress. However, none of the mechanisms we explored can adequately explain the scattered width data, potentially indicating a long-timescale adjustment of channel width to boulder input. Steepening can be best described by assuming a reduction of sediment transport efficiency with boulder concentration. We find that boulders represent a significant perturbation to the fluvial landscape. Channels tend to adjust to this perturbation leading to a new morphology that differs from boulder-free channels. The general approach presented here can be further expanded to explore the role of other boulder-related processes.   

How to cite: Nativ, R., Turowski, J., Goren, L., Laronne, J., and H. Shyu, J. B.: Influence of Boulders on Channel Width and Slope in Bedrock Rivers, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9714, https://doi.org/10.5194/egusphere-egu22-9714, 2022.

10:54–10:56
10:56–11:03
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EGU22-2475
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Virtual presentation
Carole Petit, Thibaut Cardinal, Yann Rolland, Laurence Audin, and Régis Braucher

River channels and hillslopes are shaped by the joint action of localized, vertical fluvial incision along the channel and of diffuse surface creep, landslides or rock falls on the adjacent slopes, the latter being often gathered under the generic term of hillslope processes. The interplay between river incision and hillslope processes is responsible for various landscape forms, from smooth, low-relief areas to sharp and deeply incised domains. In areas where river incision is the dominant erosive process, determining the gradual exposure of river gorge walls  by cosmogenic radionuclide dating permits to estimate the long-term (several ka) local incision rate. However, strongly cohesive rocks like massive limestones or sandstones may be prone, from time to time, to abrupt and localized degradation by rock falls. On a gorge wall, besides resetting the exposure age signal on the area where a block has been detached, rock fall events also produce debris that transiently protect the bedrock from river incision and the bottom of the gorge wall from cosmic radiations. In some extreme cases, rock fall events can even lead to the formation of epigenetic gorges. Although gorge walls appear as good markers of river incision, the random occurrence of rock falls may therefore add complexity to the interpretation of exposure ages, to the point where the actual river incision imprint is barely discernable. In this presentation, we simulate the 1D evolution of topography and Cosmogenic Radionuclides Exposure (CRE) ages on a gorge wall progressively formed by river incision and randomly reshaped by rock falls, in order to evaluate the imprint of these events on the CRE signal. We then discuss the implications of these models on the sampling strategy and on the interpretation of previously dated river gorges in the Southern French Alps and Provence.

How to cite: Petit, C., Cardinal, T., Rolland, Y., Audin, L., and Braucher, R.: How to interpret Cosmogenic Radionuclides Exposure (CRE) ages on river-incised gorges undergoing rock falls: application to Southern French Alps and Provence, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2475, https://doi.org/10.5194/egusphere-egu22-2475, 2022.

11:03–11:05
11:05–11:12
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EGU22-4328
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ECS
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On-site presentation
Thibaut Cardinal, Carole Petit, Yann Rolland, Laurence Audin, Swann Zerathe, Stéphane Schwartz, Pierre Valla, Régis Braucher, and Aster Team

As one of the major erosive processes acting at Earth’s surface, fluvial incision is highly sensible to tectonic, isostatic and climatic variations. In order to better understand what is the timing and driving mechanism(s) for Late-Quaternary incision in the Southwestern Alps, we focused on bedrock gorges and measured in situ-produced 36Cl concentrations along several river-polished gorge walls in the external Southwestern French Alps. Unlike previously dated river gorges in the study area, these newly dated catchments lie out of the previously glaciated domains during the last glacial periods, which makes them suitable to quantify fluvial incision dynamics in a non-glacial environment.

Cosmic-ray exposure dating results (ranging from 1 to 85 ka), compared to previous literature results in nearby catchments with glacial imprint and combined with a morphometric analysis, allow us to highlight three catchment groups related to different incision dynamics: (i) Group A with very high (≈5 mm/yr) and recent (post-10 ka) incision rates that reflect recent topographic readjustment of glaciated catchments by fluvial and hillslope processes; (ii) Group B, including gorges that are directly or indirectly connected to the glacial processes, showing increased incision rates (1-3 mm/yr) during the paraglacial period after the Last Glacial Maximum (ca. 20 ka), possibly related to an increase in sediment yield and water runoff following glacier retreat; (iii) Group C with slow and steady incision rates (<1 mm/yr over the last ca. 30 kyr), which do not seem to reflect any impact of any climatic variations (except the humid Holocene phases) and that are comparable to previously estimated long-term denudation and rock-uplift rates in the area.

In catchments with glacial imprints, the climatic impact on fluvial incision is evidenced through high amplitudes changes that hinder the long-term (background) tectonic signal. Despite this, our results suggest the influence of long-term tectonic on geomorphic processes for sites disconnected from the glacial influence, showing that fluvial bedrock gorges can provide insightful constraints on both long-term tectonic and short-term climatic forcing.

How to cite: Cardinal, T., Petit, C., Rolland, Y., Audin, L., Zerathe, S., Schwartz, S., Valla, P., Braucher, R., and Team, A.: Fluvial bedrock gorges as markers for Late-Quaternary tectonic and climatic forcing in the French Southwestern Alps , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4328, https://doi.org/10.5194/egusphere-egu22-4328, 2022.

11:12–11:14
11:14–11:21
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EGU22-5718
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ECS
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On-site presentation
Sophie Norris, John Gosse, Romain Millan, Jeremie Mouginot, Antoine Rabatel, Mathieu Morlighem, Matthew Bolton, Julia Fast, and Richard Alley

Glacial erosion has often been parameterized as proportional to glacier sliding velocity, while the role played by local geology, hydrology and climate remain largely unquantified. As a result, our understanding of the links between global climate, tectonics and glacial erosion is limited. To address this shortcoming, we present a comprehensive synthesis of previously published Quaternary glacial erosion rates from six different measurement techniques integrated over 10-2 to 106 years: (i) instrumental measurements beneath active glaciers, (ii) sediment fluxes derived from meltwater streams or (iii) ice-marginal deposits, (iv) terrestrial cosmogenic nuclide dating (TCN), (vi) luminescence thermochronometry, and (v) relief generation of chronologically constrained surfaces. Our synthesis includes 1065 empirical data points and 465 erosion rates from ice sheets, ice caps, and topographically confined glaciers that range over six orders of magnitude, between 10-4 and 100 mm yr-1. Using a filtered dataset of contemporary erosion rates, we apply machine learning tools, using available environmental, glaciological, and geological datasets to assess the dominant controls on subgroups of nominal data categories.  On a global scale, while glacial sliding velocity is an important control, we also discover equally strong or stronger correlations between other glaciological, environmental, and lithological parameters and glacial erosion rate, some of which have not been previously documented. 

 

How to cite: Norris, S., Gosse, J., Millan, R., Mouginot, J., Rabatel, A., Morlighem, M., Bolton, M., Fast, J., and Alley, R.: Glacial erosion: controls and global distribution, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5718, https://doi.org/10.5194/egusphere-egu22-5718, 2022.

11:21–11:23
11:23–11:30
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EGU22-7380
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Virtual presentation
Luca Dimuccio, Maurizio Zambaldi, Diego E. Angelucci, Thierry Aubry, Nelson Rodrigues, and Lúcio Cunha

The geomorphological and sedimentary dynamics related to cold-climate conditions that occurred during Last Glacial Period constitute one of the most relevant factors in the recent relief evolution in mainland Portugal. Among them, low-altitude cold non-glacial processes (periglacial) are well-known north of Lisbon, from coastal areas to the inland regions of Hercynian Massif, through the carbonate massifs of central Portugal. However, a comprehensive lithostratigraphic and chronological characterization of relict periglacial stratified slope deposits, on different bedrocks (carbonate, schistose, and granitic), is still largely missing.

This study focuses on a ~3-m-thick deposit excavated at the Fariseu 9 archaeological site, located on the left-bank of the Côa River in the Iberian plateau (“Meseta” - northeast Portugal) and featuring Upper Palaeolithic and Neolithic remains. Standard facies and fabric analysis, coupled by soil-micromorphological and petrographic observations, were performed. Luminescence dating and laser diffraction particle-size analysis of the <2 mm fraction are in progress. Nonetheless, based on the stratigraphic correlations with three nearby sedimentary successions (G-81/82, G-92/93, and Rock Art Panel 1 profiles), dates by thermoluminescence, luminescence and radiocarbon, a chronology from the Lateglacial to the Younger Dryas for the studied deposit can be assumed.

The sedimentary dynamics are mainly related to mass wasting on a schistose (phyllite) bedrock, associated with frost-shattering. In the field, alternating openwork (or clast-supported) layers of large angular clasts, locally imbricated and organized into planar sedimentary structures, and intercalated beds with variable percentages of sand and silt, with limited evidence of weathering, are observed. Clast fabric is strongly slope-oriented both in the fine-grained and in the open-work stony layers, although the imbrication may show varying polarity and inclination among different beds. Some microfeatures, such as a concentric or circular pattern of quartz grains and rock fragments, silt cappings developed onto rock fragments, and reworked clay coatings, may be considered indicators of debris-flow mechanisms combined with discontinuous frost action (freeze-thaw cycles). In the intercalated fine-grained beds, instead, the fabric characteristics and the presence of preferred oblique orientation of elongated grains point to water runoff and solifluction, which may be indicative of still cold and relatively more humid conditions. In addition, a well-defined erosive unconformity between the Pleistocene and Holocene records is observed in one of the thin sections.

The occurrence of these relict slope landforms and deposits at low-altitudes (125-130 m), and the evidence of associated periglacial dynamics with three main sedimentary mechanisms that dominate the emplacement of the studied sediments (debris-flow, runoff, and solifluction), are used to identify different cold stages at a local scale. Based on these preliminary data, we suggest that a high frequency of freeze-thaw cycles would have occurred during these stages, along with a seasonal renewal of a thin snow cover, whereas permafrost was absent. The combination of intersecting geomorphological, sedimentological, petrographic, and geochronological analyses of relict periglacial stratified slope deposits in the Côa Valley region, therefore, led to the reconstitution of late Pleistocene and early Holocene environmental changes for this area, allowing to discuss the relative importance of the forcing factors responsible for such land evolution.

How to cite: Dimuccio, L., Zambaldi, M., Angelucci, D. E., Aubry, T., Rodrigues, N., and Cunha, L.: Facies and microfacies characterization of relict periglacial stratified slope deposits in the Côa Valley region (northeast Portugal), EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7380, https://doi.org/10.5194/egusphere-egu22-7380, 2022.

11:30–11:32
11:32–11:39
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EGU22-10084
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ECS
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On-site presentation
Donovan Dennis and Dirk Scherler

Frost-cracking is believed to set rates of erosion in cold, bedrock hillslopes where temperatures are below freezing for a substantial portion of the year. Nevertheless, studies from the European Alps, North American Cordillera, and the Himalaya suggest that permafrost-thaw-induced rockfalls, a consequence of (warming) temperatures, may also contribute non-trivially to the long-term erosion rate.

 

Here, we present an assessment of bedrock, hillslope erosion rates and mechanisms in cold regions using a global dataset compiled from published literature. We additionally report 23 new bedrock hillslope erosion rates from across the European Alps estimated using in-situ cosmogenic 10Be. The hillslopes and glacial headwalls sampled span elevations from 2100 m to 4040 m, and include diverse lithologies, thermal regimes, and glaciation histories. Our new 10Be-based erosion rates range over several orders of magnitude, from 0.01 mm yr-1 to 1.5 mm yr-1, and integrate over timescales from 400 to 30000 years. To evaluate the relative rates of frost-cracking at both our field sites and those within the global compilation dataset, we calculate site-specific frost-cracking intensities using modern ground surface temperatures and modelled paleoclimatic conditions. Comparing these calculated frost-cracking intensities against the measured erosion rates suggests that frost-cracking alone may not be the rate-limiting erosional process in cold, alpine regions. We likewise observe no correlation between hillslope angle and erosion rate. Our compiled dataset does suggest, however, that erosion rates estimated using methodologies that integrate over long timescales are lower than those which integrate over short timescales, highlighting the potential for stochastic events, like rockfalls due to permafrost thaw, and/or paraglacial adjustment processes, to bias erosion rate estimates which integrate over short timescales.

How to cite: Dennis, D. and Scherler, D.: Controls on Bedrock Hillslope Erosion in Cold, High-Alpine Regions, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10084, https://doi.org/10.5194/egusphere-egu22-10084, 2022.

11:39–11:41
11:41–11:48
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EGU22-3757
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ECS
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Virtual presentation
Duna Roda-Boluda, Taylor Schildgen, Hella Wittmann, Stefanie Tofelde, Aaron Bufe, Jeff Prancevic, and Niels Hovius

The interplay of tectonics, climate, and erosion controls mountain topography, modulates Earth’s climate, and regulates the fluxes of sediments and solutes across Earth’s surface. Understanding and quantifying the complex processes behind this interplay remains one of the key challenges in the Earth Sciences. At the scale of mountain ranges, glacial erosion, river incision, and landsliding are the only processes that have been observed to be capable of balancing rapid rock-uplift rates. These processes are thought to link tectonics and climate through the influence of mountain elevations on orographic precipitation or glaciation.

The western Southern Alps of New Zealand (WSA) are one of the fastest-eroding ranges on Earth, where erosion has long been thought to be dominated by landsliding and glacial scouring. However, previous erosion studies in the WSA have been restricted to very few catchments, to decadal timescales, or to below the tree line, which has prevented evaluating the variables and processes controlling erosion at the orogen-scale over longer timescales. Here, 20 new in-situ 10Be catchment-averaged denudation rates, which mostly range between ~0.6-9 mm/yr, allow us to examine the controls and spatial distribution of denudation. We find that the proportion of catchment area within the 1500-2000 m elevation window explains >70% of the variability in denudation rates, more than any other variable. In the WSA, this elevation range is where temperatures most commonly fluctuate between -3ºC to -8ºC in the presence of water (the frost-cracking window), and includes the zone of recent glacial retreat and permafrost degradation. Our data hence suggests that temperature-controlled peri- and paraglacial erosional processes can balance some of the fastest rock-uplift rates on Earth, of several mm/yr. Therefore, these processes, which are also elevation-dependent, can play an important, but previously overlooked role in linking tectonics and climate and limiting mountain elevations.

 

 

How to cite: Roda-Boluda, D., Schildgen, T., Wittmann, H., Tofelde, S., Bufe, A., Prancevic, J., and Hovius, N.: Temperature-controlled erosion dominates in the Western Southern Alps of New Zealand, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3757, https://doi.org/10.5194/egusphere-egu22-3757, 2022.

11:48–11:50