Spatial and temporal variations in rockwall erosion rates derived from cosmogenic ¹⁰Be in medial moraines at five valley glaciers around Pigne d'Arolla, Switzerland

Rockwall erosion in high-alpine glacial environments varies both temporally and spatially. Where rockwalls flank glaciers, changes in debris supply and supraglacial cover will modify ice ablation. Yet, quantifying spatiotemporal patterns in erosion across deglaciating rockwalls is not trivial. At five nearby valley glaciers around Pigne d'Arolla in Switzerland, we derived apparent rockwall erosion rates using 10Be cosmogenic nuclide concentrations ([10Be]) in medial moraine debris. Systematic downglacier sampling of six medial moraines that receive debris from rockwalls with differing orientation, slope, and deglaciation histories enabled us to assess rockwall erosion through time and to investigate how distinct spatial source rockwall morphology may express itself in medial moraine [10Be] records. Our dataset combines 24 new samples from medial moraines of Glacier du Brenay, Glacier de Cheilon, Glacier de Pièce, and Glacier de Tsijiore Nouve with 15 published samples from Glacier d'Otemma. For each sample, we simulated the glacial debris transport using a simple debris particle trajectory model to approximate the time of debris erosion and to correct the measured [10Be] for post-depositional 10Be accumulation. Our derived apparent rockwall erosion rates range between ∼ 0.6 and 10.0 mm yr−1. Whereas the longest downglacier [10Be] record presumably reaches back to the end of the Little Ice Age and suggests a systematic increase in rockwall erosion rates over the last ∼ 200 years, the shorter records only cover the last ∼ 100 years from the recent deglaciation period and indicate temporally more stable erosion rates. For the estimated time of debris erosion, ice cover changes across most source rockwalls were small, suggesting that our records are largely unaffected by the contribution of recently deglaciated bedrock of possibly different [10Be], but admixture of subglacially derived debris cannot be excluded at every site. Comparing our sites suggests that apparent rockwall erosion rates are higher where rockwalls are steep and north-facing, indicating a potential slope and temperature control on rockwall erosion around Pigne d'Arolla.</p

Climatic and geologic controls on suspended sediment flux in the Sutlej River Valley, western Himalaya

The sediment flux through Himalayan rivers directly impacts water quality and is important for sustaining agriculture as well as maintaining drinking-water and hydropower generation. Despite the recent increase in demand for these resources, little is known about the triggers and sources of extreme sediment flux events, which lower water quality and account for extensive hydropower reservoir filling and turbine abrasion. Here, we present a comprehensive analysis of the spatiotemporal trends in suspended sediment flux based on daily data during the past decade (2001–2009) from four sites along the Sutlej River and from four of its main tributaries. In conjunction with satellite data depicting rainfall and snow cover, air temperature and earthquake records, and field observations, we infer climatic and geologic controls of peak suspended sediment concentration (SSC) events. Our study identifies three key findings: First, peak SSC events (≥ 99th SSC percentile) coincide frequently (57–80%) with heavy rainstorms and account for about 30% of the suspended sediment flux in the semi-arid to arid interior of the orogen. Second, we observe an increase of suspended sediment flux from the Tibetan Plateau to the Himalayan Front at mean annual timescales. This sediment-flux gradient suggests that averaged, modern erosion in the western Himalaya is most pronounced at frontal regions, which are characterized by high monsoonal rainfall and thick soil cover. Third, in seven of eight catchments, we find an anticlockwise hysteresis loop of annual sediment flux variations with respect to river discharge, which appears to be related to enhanced glacial sediment evacuation during late summer. Our analysis emphasizes the importance of unconsolidated sediments in the high-elevation sector that can easily be mobilized by hydrometeorological events and higher glacial-meltwater contributions. In future climate change scenarios, including continuous glacial retreat and more frequent monsoonal rainstorms across the Himalaya, we expect an increase in peak SSC events, which will decrease the water quality and impact hydropower generation

The story of a summit nucleus: hillslope boulders and their effect on erosional patterns and landscape morphology in the Chilean Coastal Cordillera

While landscapes are broadly sculpted by tectonics and climate, on a catchment scale, sediment size can regulate hillslope denudation rates and thereby influence the location of topographic highs and valleys. In this work, we used in situ 10Be cosmogenic radionuclide analysis to measure the denudation rates of bedrock, boulders, and soil in three granitic landscapes with different climates in Chile. We hypothesize that bedrock and boulders affect differential denudation by denuding more slowly than the surrounding soil; the null hypothesis is that no difference exists between soil and boulder or bedrock denudation rates. To evaluate denudation rates, we present a simple model that assesses differential denudation of boulders and the surrounding soil by evaluating boulder protrusion height against a two-stage erosion model and measured 10Be concentrations of boulder tops. We found that hillslope bedrock and boulders consistently denude more slowly than soil in two out of three of our field sites, which have a humid and a semi-arid climate: denudation rates range from ∼5 to 15 m Myr−1 for bedrock and boulders and from ∼8 to 20 m Myr−1 for soil. Furthermore, across a bedrock ridge at the humid site, denudation rates increase with increasing fracture density. At our lower-sloping field sites, boulders and bedrock appear to be similarly immobile based on similar 10Be concentrations. However, in the site with a Mediterranean climate, steeper slopes allow for higher denudation rates for both soil and boulders (∼40–140 m Myr−1), while the bedrock denudation rate remains low (∼22 m Myr−1). Our findings suggest that unfractured bedrock patches and large hillslope boulders affect landscape morphology by inducing differential denudation in lower-sloping landscapes. When occurring long enough, such differential denudation should lead to topographic highs and lows controlled by bedrock exposure and hillslope sediment size, which are both a function of fracture density. We further examined our field sites for fracture control on landscape morphology by comparing fracture, fault, and stream orientations, with the hypothesis that bedrock fracturing leaves bedrock more susceptible to denudation. Similar orientations of fractures, faults, and streams further support the idea that tectonically induced bedrock fracturing guides fluvial incision and accelerates denudation by reducing hillslope sediment size.</p

Relative importance of fluvial and glacial erosion in shaping the Chandra Valley, western Himalaya, India

Abstract HKT-ISTP 2013 B

Time scale bias in erosion rates of glaciated landscapes

Deciphering erosion rates over geologic time is fundamental for understanding the interplay between climate, tectonic, and erosional processes. Existing techniques integrate erosion over different time scales, and direct comparison of such rates is routinely done in earth science. On the basis of a global compilation, we show that erosion rate estimates in glaciated landscapes may be affected by a systematic averaging bias that produces higher estimated erosion rates toward the present, which do not reflect straightforward changes in erosion rates through time. This trend can result from a heavy-tailed distribution of erosional hiatuses (that is, time periods where no or relatively slow erosion occurs). We argue that such a distribution can result from the intermittency of erosional processes in glaciated landscapes that are tightly coupled to climate variability from decadal to millennial time scales. In contrast, we find no evidence for a time scale bias in spatially averaged erosion rates of landscapes dominated by river incision. We discuss the implications of our findings in the context of the proposed coupling between climate and tectonics, and interpreting erosion rate estimates with different averaging time scales through geologic time

Cosmogenic 10BE and 26AL studies of the rising star site, Cradle of Humankind, South Africa: mystery of the true denudation rates

Based on 10Be denudation rates previously found (3.6 m/Ma, [2]; 3.44 m/Ma, [1]), the landscape across the CoH is considered old and eroding slowly. High erosion rates similar to our results (5.13 - 15.02 m/Ma) for chert bedrock are ascribed to fast river incision or a recent partial collapse event [1,2]. In contrast, we think our high outcrop erosion rates reflect true denudation and low apparent values from soil samples indicate long retention of quartz on surface, while dolomite is largely removed in solution. The quartz then experiences periods of burial and reworking in caves and river terraces, resulting in low 26Al/10Be ratios

Paleo-denudation rates suggest variations in runoff drove aggradation during last glacial cycle, Crete, Greece

Fluvial aggradation and incision are often linked to Quaternary climate cycles, but it usually remains unclear whether variations in runoff or sediment supply or both drive channel response to climate variability. Here we quantify sediment supply with paleo-denudation rates and provide geochronological constraints on aggradation and incision from the Sfakia and Elafonisi alluvial-fan sequences in Crete, Greece. We report seven optically stimulated luminescence (OSL)and ten radiocarbon ages, eight 10Be,and eight 36Cl denudation rates from modern channeland terrace sediments. For five samples, 10Be and 36Cl were measured on the same sample by measuring 10Be on chert and 36Cl on calcite. Results indicate relatively steady denudation rates throughout the past 80kyr, but the aggradation and incision history indicates a link with climate shifts. At the Elafonisi fan, we identify four periods of aggradation coinciding with Marine Isotope Stages (MIS) 2, 4, 5a/b, and likely 6, and three periods of incision coinciding with MIS 1, 3, and likely 5e. At the Sfakia fan, rapid aggradation occurred during MIS 2 and 4,followed by incision during MIS 1. Nearby climate and vegetation records show that MIS 2, 4, and 6 stadials were characterized by cold and dry climates with sparse vegetation, whereas forest cover and more humid conditions prevailed during MIS 1, 3, and 5. Our data thus suggest that past changes in climate had little effect on landscape-wide denudation rates but exerted a strong control on the aggradation-incision behaviour of alluvial channels on Crete. During glacial stages, we attribute aggradation to hillslope sediment release promoted by reduced vegetation cover and decreased runoff; conversely, incision occurred during relatively warm and wet stages due to increased runoff. In this landscape, past hydroclimate variations outcompeted changes in sediment supply as the primary driver of alluvial deposition and incision

Glacial isostatic uplift of the European Alps

Following the last glacial maximum (LGM), the demise of continental ice sheets induced crustal rebound in tectonically stable regions of North America and Scandinavia that is still ongoing. Unlike the ice sheets, the Alpine ice cap developed in an orogen where the measured uplift is potentially attributed to tectonic shortening, lithospheric delamination and unloading due to deglaciation and erosion. Here we show that ∼90% of the geodetically measured rock uplift in the Alps can be explained by the Earth's viscoelastic response to LGM deglaciation. We modelled rock uplift by reconstructing the Alpine ice cap, while accounting for postglacial erosion, sediment deposition and spatial variations in lithospheric rigidity. Clusters of excessive uplift in the Rhône Valley and in the Eastern Alps delineate regions potentially affected by mantle processes, crustal heterogeneity and active tectonics. Our study shows that even small LGM ice caps can dominate present-day rock uplift in tectonically active regions

Erosion rate maps highlight spatio-temporal patterns of uplift and quantify sediment export of the Northern Andes

Erosion rates are widely used to assess tectonic uplift and sediment export from mountain ranges. However, the scarcity of erosion rate measurements often hinders detailed tectonic interpretations. Here, we present 25 new cosmogenic nuclide-derived erosion rates from the Northern Andes of Colombia to study spatio-temporal patterns of uplift along the Central and Eastern Cordillera. Specifically, we combine new and published erosion rate data with precipitation-corrected normalized channel steepness measurements to construct high-resolution erosion rate maps. We find that erosion rates in the southern Central Cordillera are relatively uniform and average ∼0.3 mm/a. In the northern Central Cordillera rapidly eroding canyons dissect slowly eroding, low-relief surfaces uplifting since 8.3+ 3.7 - 2.6 Ma, based on a block uplift model. We interpret that persistent steep slab subduction has led to an erosional steady-state in the southern Central Cordillera, whereas in the northern Central Cordillera, Late Miocene slab flattening caused an acceleration in uplift, to which the landscape has not yet equilibrated. The Eastern Cordillera also displays pronounced erosional disequilibrium, with a slowly eroding central plateau rimmed by faster eroding western and eastern flanks. Our maps suggest Late Miocene topographic growth of the Eastern Cordillera, with deformation focused along the eastern flank, which is also supported by balanced cross-sections and thermochronologic data. Spatial gradients in predicted erosion rates along the eastern flank of the Eastern Cordillera suggest transient basin-ward migration of thrusts. Finally, sediment fluxes based on our erosion maps, suggest that the Eastern Cordillera exports nearly four times more sediment than the Central Cordillera. Our analysis shows that accounting for spatial variations in erosion parameters and climate reveals important variations in tectonic forcing that would otherwise be obscured in traditional river profile analyses. Moreover, given relationships between tectonic and topographic evolution, we hypothesize that spatio-temporal variations in slab dip are the primary driver of the dynamic landscape evolution of the Northern Andes, with potentially superposed effects from inherited Mesozoic rift structures