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Abstract: Basin-wide erosion rates can be determined through the analysis of in situ-produced cosmogenic nuclides. In transient landscapes, and particularly in mountain catchments, erosion and transport processes are often highly variable and consequently the calculated erosion rates can be biased. This can be due to sediment pulses and poor mixing of sediment in the stream channels. The mixing of alluvial sediment is one of the principle conditions that need to be verified in order to have reliable results. In this paper we perform a field-based test of the extent of sediment mixing for a ∼42 km2 catchment in the Alps using concentrations of river-born 10Be. We use this technique to assess the mechanisms and the spatio-temporal scales for the mixing of sediment derived from hillslopes and tributary channels. The results show that sediment provenance and transport, and mixing processes have a substantial impact on the 10Be concentrations downstream of the confluence between streams and tributary channels. We also illustrate that the extent of mixing significantly depends on: the sizes of the catchments involved, the magnitude of the sediment delivery processes, the downstream distance of a sample site after a confluence, and the time since the event occurred. In particular, continuous soil creep and shallow landsliding supply high 10Be concentration material from the hillslope, congruently increasing the 10Be concentrations in the alluvial sediment. Contrariwise, a high frequency of mass-wasting processes or the occurrence of sporadic but large-magnitude events results in the supply of low-concentration sediment that lowers the cosmogenic nuclide concentration in the channels. The predominance of mass-wasting processes in a catchment can cause a strong bias in detrital cosmogenic nuclide concentrations, and therefore calculated erosion rates may be significantly over- or underestimated. Accordingly, it is important to sample as close as possible to the return-period of large-size sediment input events. This will lead to an erosion rate representative of the “mass-wasting signal” in case of generally high-frequency events, or the “background signal” when the event is sporadic. Our results suggest that a careful consideration of the extent of mixing of alluvial sediment is of primary importance for the correct estimation of 10Be-based erosion rates in mountain catchments, and likewise, that erosion rates have to be interpreted cautiously when the mixing conditions are unknown or mixing has not been achieved. [Copyright &y& Elsevier]

The article presents a study which examines the terrestrial cosmogenic nuclide (TCN) impact in determining the rates of the catchment-wide denudation in Rio Lluta catchment in northern Chile. It discusses the utilization of TCN samples which are split for noble gas analysis and degasses through step-wise heating. The result shows the contrast of the palaeodenuation rates with the previous TCN-derived erosion rates from the bedrock exposures particularly in the lower river course.

Hellas basin acts as a major sink for the southern highlands of Mars and is likely to have recorded several episodes of sedimentation and erosion. The north-western part of the basin displays a potentially unique Amazonian landscape domain in the deepest part of Hellas, called “banded terrain”, which is a deposit characterized by an alternation of narrow band shapes and inter-bands displaying a sinuous and relatively smooth surface texture suggesting a viscous flow origin. Here we use high-resolution (HiRISE and CTX) images to assess the geomorphological interaction of the banded terrain with the surrounding geomorphologic domains in the NW interior of Hellas to gain a better understanding of the geological evolution of the region as a whole. Our analysis reveals that the banded terrain is associated with six geomorphologic domains: a central plateau named Alpheus Colles, plain deposits (P1 and P2), reticulate (RT1 and RT2) and honeycomb terrains. Based on the analysis of the geomorphology of these domains and their cross-cutting relationships, we show that no widespread deposition post-dates the formation of the banded terrain, which implies that this domain is the youngest and latest deposit of the interior of Hellas. Therefore, the level of geologic activity in the NW Hellas during the Amazonian appears to have been relatively low and restricted to modification of the landscape through mechanical weathering, aeolian and periglacial processes. Thermophysical data and cross-cutting relationships support hypotheses of modification of the honeycomb terrain via vertical rise of diapirs such as ice diapirism, and the formation of the plain deposits through deposition and remobilization of an ice-rich mantle deposit. Finally, the observed gradual transition between honeycomb and banded terrain suggests that the banded terrain may have covered a larger area of the NW interior of Hellas in the past than previously thought. This has implications on the understanding of the evolution of the deepest part of Hellas. [ABSTRACT FROM AUTHOR]

Abstract: The quantification of geomorphic process rates on the outcrop- and the orogen-scale is important to describe accurately the interaction between the relative effects of erosion, tectonics and climate on landscape evolution. We report single and paired cosmogenic nuclide (10Be, 26Al and 21Ne) derived erosion rates and exposure ages on hillslope interfluves from the tectonically active western central Andes that show a distinct spatial variation. A positive correlation of erosion rates with elevation and present-day rainfall rates is observed. Erosion rates at lower altitudes–the hyperarid Coastal Cordillera and the Western Escarpment with the northern part of the Atacama Desert–are extremely low and of the order of 10–100 cm/My (nominal exposure ages 1–6 My). In contrast, erosion rates at higher altitudes–the semiarid Western Cordillera–range up to 4600 cm/My (nominal exposure ages 0.02–0.1 My). This latter average long-term bedrock erosion rate record, suggested to be coupled to an orographically controlled pattern of rainfall, is also reflected in the pattern of denudation rates derived from a short-term decadal record of limited sediment yield data. Specifically, denudation rates calculated from sediment flux data are of a similar order of magnitude as erosion rates deduced from long-lived cosmogenic nuclides from bedrock hillslope interfluves of the Western Cordillera. Nevertheless, the production and the supply of sediment from the western Andean slope are very limited. Analysis of multiple cosmogenic nuclides allows simultaneous determination of erosion rates and exposure ages but also reveals complex exposure histories of non-bedrock samples, such as boulders or amalgamated clast samples. Notably, this study shows that saturation of nuclides, usually assumed in studies where only a single nuclide is analyzed, is rather the exception than the rule, as revealed by erosion island plots. Constant erosion that started much later than the formation age of the rocks or episodic erosion by spalling can partially explain non-steady-state concentrations and more complicated exposure scenarios. Furthermore, the use of multiple nuclides with different half-lives allowed us to infer that no significant variations in long-term erosion rates have occurred and that at the Western Escarpment erosion rates have been low and constant for most of the late Neogene. Nevertheless, the time intervals necessary to reach steady-state concentrations for cosmogenic nuclides can be quite different from those needed for landscapes to reach steady state. [Copyright &y& Elsevier]

Abstract: Quantification of the volumes of sediment removed by rock–slope failure and debris flows and identification of their coupling and controls are pertinent to understanding mountain basin sediment yield and landscape evolution. This study captures a multi-decadal period of hillslope erosion and channel change following an extreme rock avalanche in 1961 in the Illgraben, a catchment prone to debris flows in the Swiss Alps. We analyzed photogrammetrically-derived datasets of hillslope and channel erosion and deposition along with climatic and seismic variables for a 43year period from 1963 to 2005. Based on these analyses we identify and discuss (1) patterns of hillslope production, channel transfer and catchment sediment yield, (2) their dominant interactions with climatic and seismic variables, and (3) the nature of hillslope–channel coupling and implications for sediment yield and landscape evolution in this mountain basin. Our results show an increase in the mean hillslope erosion rate in the 1980s from 0.24±0.01myr−1 to 0.42±0.03myr−1 that coincided with a significant increase in air temperature and decrease in snow cover depth and duration, which we presume led to an increase in the exposure of the slopes to thermal weathering processes. The combination of highly fractured slopes close to the threshold angle for failure, and multiple potential triggering mechanisms, means that it is difficult to identify an individual control on slope failure. On the other hand, the rate of channel change was strongly related to variables influencing runoff. A period of particularly high channel erosion rate of 0.74±0.02myr−1 (1992–1998) coincided with an increase in the frequency and magnitude of intense rainfall events. Hillslope erosion exceeded channel erosion on average, indicative of a downslope-directed coupling relationship between hillslope and channel, and demonstrating the first order control of rock–slope failure on catchment sediment yield and landscape evolution. [Copyright &y& Elsevier]