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The mechanisms by which climate and vegetation affect erosion rates over various time scales lie at the heart of understanding landscape response to climate change. Plot-scale field experiments show that increased vegetation cover slows erosion, implying that faster erosion should occur under low to moderate vegetation cover. However, demonstrating this concept over long time scales and across landscapes has proven to be difficult, especially in settings complicated by tectonic forcing and variable slopes. We investigate this problem by measuring cosmogenic Be-10-derived catchment-mean denudation rates across a range of climate zones and hillslope gradients in the Kenya Rift, and by comparing our results with those published from the Rwenzori Mountains of Uganda. We find that denudation rates from sparsely vegetated parts of the Kenya Rift are up to 0.13 mm/yr, while those from humid and more densely vegetated parts of the Kenya Rift flanks and the Rwenzori Mountains reach a maximum of 0.08 mm/yr, despite higher median hillslope gradients. While differences in lithology and recent land-use changes likely affect the denudation rates and vegetation cover values in some of our studied catchments, hillslope gradient and vegetation cover appear to explain most of the variation in denudation rates across the study area. Our results support the idea that changing vegetation cover can contribute to complex erosional responses to climate or land-use change and that vegetation cover can play an important role in determining the steady-state slopes of mountain belts through its stabilizing effects on the land surface.
Deciphering the response of sediment routing systems to climatic forcing is fundamental for understanding the impacts of climate change on landscape evolution. In the Kangra Basin (northwest Sub-Himalaya, India), upper Pleistocene to Holocene alluvial fills and fluvial terraces record periodic fluctuations of sediment supply and transport capacity on timescales of 10(3) to 10(5) yr. To evaluate the potential influence of climate change on these fluctuations, we compare the timing of aggradation and incision phases recorded within remnant alluvial fans and terraces with climate archives. New surface-exposure dating of six terrace levels with in-situ cosmogenic Be-10 indicates the onset of incision phases. Two terrace surfaces from the highest level (T1) sculpted into the oldest preserved alluvial fan (AF1) date back to 53.4 +/- 3.2 ka and 43.0 +/- 2.7 ka (1 sigma). T2 surfaces sculpted into the remnants of AF1 have exposure ages of 18.6 +/- 1.2 ka and 15.3 +/- 0.9 ka, while terraces sculpted into the upper Pleistocene-Holocene fan (AF2) provide ages of 9.3 +/- 0.4 ka (T3), 7.1 +/- 0.4 ka (T4), 5.2 +/- 0.4 ka (T5) and 3.6 +/- 0.2 ka (T6). Together with previously published OSL ages yielding the timing of aggradation, we find a correlation between variations in sediment transport with oxygen-isotope records from regions affected by the Indian Summer Monsoon. During periods of increased monsoon intensity and post-Last Glacial Maximum glacial retreat, aggradation occurred in the Kangra Basin, likely due to high sediment flux, whereas periods of weakened monsoon intensity or lower sediment supply coincide with incision. (C) 2016 Elsevier B.V. All rights reserved.
The southernmost thrust of the Himalayan orogenic wedge that separates the foreland from the orogen, the Main Frontal Thrust, 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 reentrant 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 Be-10-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 similar to 10ka was followed by episodic reincision, which created four cut-and-fill terrace levels, the oldest of which (T3) was formed at 10.10.9ka. A vertical offset of 445m of terrace T3 across the JMT indicates a shortening rate of 5.60.8 to 7.51.1mma(-1) over the last similar to 10ka. This result suggests that thrusting along the JMT accommodates 40-60% of the total Sub-Himalayan shortening in the Kangra reentrant 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.
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 Be-10 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 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 Bolling/Allerod interstadial. Our study supports the view that many Himalayan glaciers advanced during the LGM, likely in response to global variations in temperature.
Despite remarkable tectonostratigraphic similarities along the Himalayan arc, pronounced topographic and exhumational variability exists in different morphotectonic segments. The processes responsible for this segmentation are debated. Of particular interest is a 30- to 40-km-wide orogen-parallel belt of rapid exhumation that extends from central Nepal to the western Himalaya and its possible linkage to a midcrustal ramp in the basal decollement, and the related growth of Lesser Himalayan duplex structures. Here we present 26 new apatite fission track cooling ages from the Beas-Lahul region, at the transition from the Central to the Western Himalaya (77 degrees-78 degrees E) to investigate segmentation in the Himalayan arc from a thermochronologic perspective. Together with previously published data from this part of the orogen, we document significant lateral changes in exhumation between the Dhauladar Range to the west, the Beas-Lahul region, and the Sutlej area to the east of the study area. In contrast to the Himalayan front farther east, exhumation in the far western sectors is focused at the frontal parts of the mountain range and associated with the hanging wall of the Main Boundary Thrust fault ramp. Our results allow us to spatially correlate the termination of the rapid exhumation belt with a midcrustal ramp to the west. We suggest that a plunging anticline at the northwestern edge of the Larji-Kullu-Rampur window represents the termination of the Central Himalayan segment, which is related to the evolution of the Lesser Himalayan duplex. Key Points
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 Rhone 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 Rhone Valley, an overdeepened glacial valley in the Swiss Alps, yields a total estimated sediment volume of 9711km(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.
The Indus River, one of Asia's premier rivers, drains the western Tibetan Plateau and the Nanga Parbat syntaxis. These two areas juxtapose some of the lowest and highest topographic relief and commensurate denudation rates in the Himalaya-Tibet orogen, respectively, yet the spatial pattern of denudation rates upstream of the syntaxis remains largely unclear, as does the way in which major rivers drive headward incision into the Tibetan Plateau. We report a new inventory of Be-10-based basinwide denudation rates from 33 tributaries flanking the Indus River along a 320 km reach across the western Tibetan Plateau margin. We find that denudation rates of up to 110 mm k.y.(-1) in the Ladakh and Zanskar Ranges systematically decrease eastward to 10 mm k.y.(-1) toward the Tibetan Plateau. Independent results from bulk petrographic and heavy mineral analyses support this denudation gradient. Assuming that incision along the Indus exerts the base-level control on tributary denudation rates, our data show a systematic eastward decrease of landscape downwearing, reaching its minimum on the Tibetan Plateau. In contrast, denudation rates increase rapidly 150-200 km downstream of a distinct knick-point that marks the Tibetan Plateau margin in the Indus River longitudinal profile. We infer that any vigorous headward incision and any accompanying erosional waves into the interior of the plateau mostly concerned reaches well below this plateau margin. Moreover, reported long-term (>10(6) yr) exhumation rates from low-temperature chronometry of 0.1-0.75 mm yr(-1) consistently exceed our Be-10-derived denudation rates. With averaging time scales of 10(3)-10(4) yr for our denudation data, we report postglacial rates of downwearing in a tectonically idle landscape. To counterbalance this apparent mismatch, denudation rates must have been higher in the Quaternary during glacial-interglacial intervals.
Climatic controls on debris-flow activity and sediment aggradation: The Del Medio fan, NW Argentina
(2016)
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 subhumid and arid climates and hosts the only active debris-flow fan within this intermontane valley. By combining Be-10 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.
In the high mountains of Asia, glaciers cover an area of approximately 115,000 km² and constitute one of the largest continental ice accumulations outside Greenland and Antarctica. Their sensitivity to climate change makes them valuable palaeoclimate archives, but also vulnerable to current and predicted Global Warming. This is a pressing problem as snow and glacial melt waters are important sources for agriculture and power supply of densely populated regions in south, east, and central Asia. Successful prediction of the glacial response to climate change in Asia and mitigation of the socioeconomic impacts requires profound knowledge of the climatic controls and the dynamics of Asian glaciers. However, due to their remoteness and difficult accessibility, ground-based studies are rare, as well as temporally and spatially limited. We therefore lack basic information on the vast majority of these glaciers. In this thesis, I employ different methods to assess the dynamics of Asian glaciers on multiple time scales. First, I tested a method for precise satellite-based measurement of glacier-surface velocities and conducted a comprehensive and regional survey of glacial flow and terminus dynamics of Asian glaciers between 2000 and 2008. This novel and unprecedented dataset provides unique insights into the contrasting topographic and climatic controls of glacial flow velocities across the Asian highlands. The data document disparate recent glacial behavior between the Karakoram and the Himalaya, which I attribute to the competing influence of the mid-latitude westerlies during winter and the Indian monsoon during summer. Second, I tested whether such climate-related longitudinal differences in glacial behavior also prevail on longer time scales, and potentially account for observed regionally asynchronous glacial advances. I used cosmogenic nuclide surface exposure dating of erratic boulders on moraines to obtain a glacial chronology for the upper Tons Valley, situated in the headwaters of the Ganges River. This area is located in the transition zone from monsoonal to westerly moisture supply and therefore ideal to examine the influence of these two atmospheric circulation regimes on glacial advances. The new glacial chronology documents multiple glacial oscillations during the last glacial termination and during the Holocene, suggesting largely synchronous glacial changes in the western Himalayan region that are related to gradual glacial-interglacial temperature oscillations with superimposed monsoonal precipitation changes of higher frequency. In a third step, I combine results from short-term satellite-based climate records and surface velocity-derived ice-flux estimates, with topographic analyses to deduce the erosional impact of glaciations on long-term landscape evolution in the Himalayan-Tibetan realm. The results provide evidence for the long-term effects of pronounced east-west differences in glaciation and glacial erosion, depending on climatic and topographic factors. Contrary to common belief the data suggest that monsoonal climate in the central Himalaya weakens glacial erosion at high elevations, helping to maintain a steep southern orographic barrier that protects the Tibetan Plateau from lateral destruction. The results of this thesis highlight how climatic and topographic gradients across the high mountains of Asia affect glacier dynamics on time scales ranging from 10^0 to 10^6 years. Glacial response times to climate changes are tightly linked to properties such as debris cover and surface slope, which are controlled by the topographic setting, and which need to be taken into account when reconstructing mountainous palaeoclimate from glacial histories or assessing the future evolution of Asian glaciers. Conversely, the regional topographic differences of glacial landscapes in Asia are partly controlled by climatic gradients and the long-term influence of glaciers on the topographic evolution of the orogenic system.