ALBERT

Description: [1]  As rivers transport water and sediment across Earth's surface they radiate elastic and acoustic waves. We use seismic and infrasound observations during a controlled flood experiment (CFE) in the Grand Canyon to show that three types of fluvial processes can be monitored from outside the channel. First, bed-load transport under conditions of evolving bed mobility is identified as the dominant seismic source between 15 – 45 Hz. Two lower-frequency seismic bands also excited by the CFE exhibited greater power increases, and are consistent with source processes related to fluid rather than sediment transport. The second fluvial seismic source is inferred to be fluid tractions on the rough riverbed, which drive the maximum seismic power increase at 0.73 Hz, but do not excite infrasound. Waves at the fluid-air interface are suggested as a third source, which generates a common 6 – 7 Hz peak in seismic and infrasound responses to the CFE.

Description: High Asian glacial landscapes have large variations in topographic relief and the size and steepness of snow accumulation areas. Associated differences in glacial cover and dynamics allow a first-order determination of the dominant processes shaping these landscapes. Here we provide a regional synthesis of the topography and flow characteristics of 287 glaciers across High Asia using digital elevation analysis and remotely sensed glacier surface velocities. Glaciers situated in low-relief areas on the Tibetan Plateau are mainly nourished by direct snowfall, have little or no debris cover, and have a relatively symmetrical distribution of velocities along their length. In contrast, avalanche-fed glaciers with steep accumulation areas, which occur at the deeply incised edges of the Tibetan Plateau, are heavily covered with supraglacial debris, and flow velocities are highest along short segments near their headwalls but greatly reduced along their debris-mantled lower parts. The downstream distribution of flow velocities suggests that the glacial erosion potential is progressively shifted upstream as accumulation areas get steeper and hillslope debris fluxes increase. Our data suggest that the coupling of hillslopes and glacial dynamics increases with topographic steepness and debris cover. The melt-lowering effect of thick debris cover allows the existence of glaciers even when they are located entirely below the snow line. However, slow velocities limit the erosion potential of such glaciers, and their main landscape-shaping contribution may simply be the evacuation of debris from the base of glacial headwalls, which inhibits the formation of scree slopes and thereby allows ongoing headwall retreat by periglacial hillslope processes. We propose a conceptual model in which glacially influenced plateau margins evolve from low-relief to high-relief landscapes with distinctive contributions of hillslope processes and glaciers to relief production and decay.

Description: Thick sedimentary fills in intermontane valleys are common in formerly glaciated mountain ranges, but difficult to quantify. Yet, knowledge of the fill-thickness distribution could help to estimate sediment budgets of mountain belts and to decipher the role of stored material in modulating sediment flux from the orogen to the foreland. Here we present a new approach to estimate valley-fill thickness and bedrock topography based on the geometric properties of a landscape using artificial neural networks. We test the potential of this approach following a four-tiered procedure. First, experiments with synthetic, idealized landscapes show that increasing variability in surface slopes requires successively more complex network configurations. Second, in experiments with artificially filled natural landscapes, we find that fill volumes can be estimated with an error below 20 %. Third, in natural examples with valley-fill surfaces that have steeply inclined slopes, such as the Unteraar and the Rhône glaciers in the Swiss Alps, for example, the average deviation of cross-sectional area between the measured and the modeled valley fill is 26 % and 27 %, respectively. Finally, application of the method to the Rhône Valley, an overdeepened glacial valley in the Swiss Alps, yields a total estimated sediment volume of 97 ± 11 km 3 and an average deviation of cross-sectional area between measurements and model estimates of 21.5 %. Our new method allows for rapid assessment of sediment volumes in intermontane valleys while eliminating most of the subjectivity that is typically inherent in other methods where bedrock reconstructions are based on digital elevation models.

Description: Despite a large number of dated glacial landforms in the Himalaya, the ice extent during the global last glacial maximum (LGM) from 19 to 23 ka is only known to first order. New cosmogenic 10 Be exposure ages from well-preserved glacially polished surfaces, combined with published data, and an improved production-rate scaling model allow reconstruction of the LGM ice extent and subsequent deglaciation in the Chandra Valley of NW India. We show that a 〉1000-m-thick valley glacier retreated 〉150 km within a few thousand years after the onset of LGM deglaciation. By comparing the recession of the Chandra Valley Glacier (CVG) and other Himalayan glaciers with those of northern and southern hemisphere glaciers, we demonstrate that post-LGM deglaciation was similar and nearly finished prior to the Bølling/Allerød interstadial. Our study supports the view that many Himalayan glaciers advanced during the LGM, likely in response to global variations in temperature.

Description: Fluvial fill terraces preserve sedimentary archives of landscape responses to climate change, typically over millennial timescales. In the Humahuaca Basin of NW Argentina (Eastern Cordillera, southern Central Andes), our 29 new optically stimulated luminescence ages of late Pleistocene fill terrace sediments demonstrate that the timing of past river aggradation occurred over different intervals on the western and eastern sides of the valley, despite their similar bedrock lithology, mean slopes, and precipitation. In the west, aggradation coincided with periods of increasing precipitation, while in the east, aggradation coincided with decreasing precipitation or more variable conditions. Erosion rates and grain-size dependencies in our cosmogenic 10 Be analyses of modern and fill-terrace sediments reveal an increased importance of landsliding compared to today on the west side during aggradation, but of similar importance during aggradation on the east side. Differences in the timing of aggradation and the 10 Be data likely result from differences in valley geometry, which causes sediment to be temporarily stored in perched basins on the east side. It appears as if periods of increasing precipitation triggered landslides throughout the region, which induced aggradation in the west, but blockage of the narrow bedrock gorges downstream from the perched basins in the east. As such, basin geometry and fluvial connectivity appear to strongly influence the timing of sediment movement through the system. For larger basins that integrate sub-basins with differing geometries or degrees of connectivity (like Humahuaca), sedimentary responses to climate forcing are likely attenuated.

Description: Spatially variable response of Himalayan glaciers to climate change affected by debris cover Nature Geoscience 4, 156 (2011). doi:10.1038/ngeo1068 Authors: Dirk Scherler, Bodo Bookhagen & Manfred R. Strecker Controversy about the current state and future evolution of Himalayan glaciers has been stirred up by erroneous statements in the fourth report by the Intergovernmental Panel on Climate Change. Variable retreat rates and a paucity of glacial mass-balance data make it difficult to develop a coherent picture of regional climate-change impacts in the region. Here, we report remotely-sensed frontal changes and surface velocities from glaciers in the greater Himalaya between 2000 and 2008 that provide evidence for strong spatial variations in glacier behaviour which are linked to topography and climate. More than 65% of the monsoon-influenced glaciers that we observed are retreating, but heavily debris-covered glaciers with stagnant low-gradient terminus regions typically have stable fronts. Debris-covered glaciers are common in the rugged central Himalaya, but they are almost absent in subdued landscapes on the Tibetan Plateau, where retreat rates are higher. In contrast, more than 50% of observed glaciers in the westerlies-influenced Karakoram region in the northwestern Himalaya are advancing or stable. Our study shows that there is no uniform response of Himalayan glaciers to climate change and highlights the importance of debris cover for understanding glacier retreat, an effect that has so far been neglected in predictions of future water availability or global sea level.

Description: [1]  Erosion in the Himalaya is responsible for one of the greatest mass redistributions on Earth, and has fueled models of feedback loops between climate and tectonics. Although the general trends of erosion across the Himalaya are reasonably well known, the relative importance of factors controlling erosion is less well constrained. Here, we present 25 10 Be-derived catchment-average erosion rates from the Yamuna catchment in the Garhwal Himalaya, northern India. Tributary erosion rates range between ~0.1-0.5 mm yr -1 in the Lesser Himalaya and ~1-2 mm yr -1 in the High Himalaya, despite uniform hillslope angles. The erosion-rate data correlate with catchment-average values of 5-km radius relief, channel steepness indices, and specific stream power, but to varying degrees of non-linearity. Similar non-linear relationships and coefficients of determination suggest that topographic steepness is the major control on the spatial variability of erosion, and that 2-3-fold differences in annual runoff have only minor effects in this area. Instead, the spatial distribution of erosion in the study area is consistent with a tectonic model in which the rock-uplift pattern is largely controlled by the shortening rate and the geometry of the Main Himalayan Thrust fault (MHT). Our data support a shallow dip of the MHT underneath the Lesser Himalaya, followed by a mid-crustal ramp underneath the High Himalaya, as indicated by geophysical data. Finally, analysis of sample results from larger main-stem rivers indicates significant variability of 10 Be-derived erosion rates, possibly related to non-proportional sediment supply from different tributaries and incomplete mixing in main-stem channels.

Description: 10 Be-derived catchment average erosion rates from the Himalaya and Eastern Tibet show different relationships with normalized channel steepness index ( k sn ) , suggesting differences in erosional efficiency of bedrock river incision. We used a stochastic-threshold stream power model (SPM) to explore to what extent this observation can be explained by differences in the mean and variability of discharge between the two regions. Based on the analysis of 199 daily discharge records (record lengths 3-45 years; average 18.5 years), we parameterized monsoonal discharge with a weighted sum of two inverse gamma distributions. During both high- and low-flow conditions, annual and inter-annual discharge variability is similarly low in each region. Channel widths for 36 rivers indicate, on average, 25% wider streams in Eastern Tibet than in the Himalaya. Because most catchments with 10 Be data are not gauged, we constrained mean annual discharge in these catchments using gridded precipitation data sets that we calibrated to the available discharge records. Comparing 10 Be-derived with modeled erosion rates, the stochastic-threshold SPM explains regional differences better than a simple SPM based on drainage area or mean annual runoff. Systematic differences at small k sn values can be reconciled with k sn -dependent erosion thresholds, whereas substantial scatter for high k sn values persists, likely due to methodological limitations. Sensitivity analysis suggests that changes in the duration or strength of summer monsoon precipitation have the largest effect on erosional efficiency, while changes in monsoonal discharge variability have almost no effect. The modeling approach presented in this study can in principle be used to assess the impact of precipitation changes on erosion.

Description: In the Central Andes, several studies on alluvial terraces and valley fills have linked sediment aggradation to periods of enhanced sediment supply. However, debate continues over whether tectonic or climatic factors are most important in triggering the enhanced supply. The Del Medio catchment in the Humahuaca Basin (Eastern Cordillera, NW Argentina) is located within a transition zone between sub-humid and arid climates and hosts the only active debris-flow fan within this intermontane valley. By combining 10 Be analyses of boulder and sediment samples within the Del Medio catchment, with regional morphometric measurements of nearby catchments, we identify the surface processes responsible for aggradation in the Del Medio fan and their likely triggers. We find that the fan surface has been shaped by debris flows and channel avulsions during the last 400 years. Among potential tectonic, climatic, and autogenic factors that might influence deposition, our analyses point to a combination of several favorable factors that drive aggradation. These are in particular the impact of occasional abundant rainfall on steep slopes in rock types prone to failure, located in a region characterized by relatively low rainfall amounts and limited transport capacity. These characteristics are primarily associated with the climatic transition zone between the humid foreland and the arid orogen interior, which creates an imbalance between sediment supply and sediment transfer. The conditions and processes that drive aggradation in the Del Medio catchment today may provide a modern analog for the conditions and processes that drove aggradation in other nearby tributaries in the past.

Description: The southernmost thrust of the Himalayan orogenic wedge that separates the foreland from the orogen, the Main Frontal Thrust (MFT), is thought to accommodate most of the ongoing crustal shortening in the Sub-Himalaya. Steepened longitudinal river-profile segments, terrace offsets, and back-tilted fluvial terraces within the Kangra re-entrant of the NW Sub-Himalaya suggest Holocene activity of the Jwalamukhi Thrust (JMT) and other thrust faults that may be associated with strain partitioning along the toe of the Himalayan wedge. To assess the shortening accommodated by the JMT, we combine morphometric terrain analyses with in-situ 10 Be-based surface exposure dating of the deformed terraces. Incision into upper Pleistocene sediments within the Kangra Basin created two late Pleistocene terrace levels (T1 and T2). Subsequent early Holocene aggradation shortly before ~10 ka was followed by episodic re-incision, which created four cut-and-fill terrace levels, the oldest of which (T3) was formed at 10.1 ± 0.9 ka. A vertical offset of 44 ± 5 m of terrace T3 across the JMT indicates a shortening rate of 5.6 ± 0.8 to 7.5 ± 1.1 mm.a -1 over the last ~10 ka. This result suggests that thrusting along the JMT accommodates 40-60% of the total Sub-Himalayan shortening in the Kangra re-entrant over the Holocene. We speculate that this out-of-sequence shortening may have been triggered or at least enhanced by late Pleistocene and Holocene erosion of sediments from the Kangra Basin.