Description: Are longitudinal ice-surface structures on the Antarctic Ice Sheet indicators of long-term ice-flow configuration? Earth Surface Dynamics Discussions, 2, 911-933, 2014 Author(s): N. F. Glasser, S. J. A. Jennings, M. J. Hambrey, and B. Hubbard Continent-wide mapping of longitudinal ice-surface structures on the Antarctic Ice Sheet reveals that they originate in the interior of the ice sheet and are arranged in arborescent networks fed by multiple tributaries. Longitudinal ice-surface structures can be traced continuously down-ice for distances of up to 1200 km. They are co-located with fast-flowing glaciers and ice streams that are dominated by basal sliding rates above tens of m yr -1 and are strongly guided by subglacial topography. Longitudinal ice-surface structures dominate regions of converging flow, where ice flow is subject to non-coaxial strain and simple shear. Associating these structures with the AIS' surface velocity field reveals (i) ice residence times of ~ 2500 to 18 500 years, and (ii) undeformed flow-line sets for all major flow units analysed except the Kamb Ice Stream and the Institute and Möller Ice Stream areas. Although it is unclear how long it takes for these features to form and decay, we infer that the major ice-flow and ice-velocity configuration of the ice sheet may have remained largely unchanged for several thousand years, and possibly even since the end of the last glacial cycle. This conclusion has implications for our understanding of the long-term landscape evolution of Antarctica, including large-scale patterns of glacial erosion and deposition.

Description: Bedload transport controls intra-event bedrock erosion Earth Surface Dynamics Discussions, 3, 53-82, 2015 Author(s): A. R. Beer and J. M. Turowski Fluvial bedrock incision constrains the pace of mountainous landscape evolution. Fluvial erosion processes have been described with incision models that are widely applied in river reach and catchment scale studies. However, so far, no linked field data set at the process scale had been published that allows to assess model requirements and adequacy. Here, we evaluate the predictive power of various incision models on data on hydraulics, bedload transport and erosion recorded on an artificial bedrock slab installed in a steep mountain stream for a single bedload transport event. The influence of transported bedload on erosion rate (the "tools effect") is shown to be dominant while other effects are of minor importance. Hence, a simple temporal distributed incision model in which erosion rate is proportional to bedload transport rate is proposed for transient local studies. This model can be site-calibrated with temporally lumped bedload and erosion data and its applicability can be assessed by visual inspection of the study site. Basic discharge-based models like derivatives of the stream power model family however, are adequate to reproduce the overall trend of the observed erosion rate, at least for the event on hand. This is relevant for long-term studies of e.g. landscape evolution with no interest in transient local behaviour.

Description: Spatial distributions of earthquake-induced landslides and hillslope preconditioning in northwest South Island, New Zealand Earth Surface Dynamics Discussions, 3, 1-52, 2015 Author(s): R. N. Parker, G. T. Hancox, D. N. Petley, C. I. Massey, A. L. Densmore, and N. J. Rosser Current models to explain regional-scale landslide events are not able to account for the possible effects of the legacy of previous earthquakes, which have triggered landslides in the past and are known to drive damage accumulation in brittle hillslope materials. This paper tests the hypothesis that spatial distributions of earthquake-induced landslides are determined by both the conditions at the time of the triggering earthquake (time-independent factors), and also the legacy of past events (time-dependent factors). To explore this, we undertake an analysis of failures triggered by the 1929 Buller and 1968 Inangahua earthquakes, in the northwest South Island of New Zealand. The spatial extent of landslides triggered by these events was in part coincident (overlapping). Spatial distributions of earthquake-triggered landslides are determined by a combination of earthquake and local characteristics, which influence the dynamic response of hillslopes. To identify the influence of a legacy from past events, we use logistic regression to control for the effects of time-independent variables (seismic ground motion, hillslope gradient, lithology, and the effects of topographic amplification caused by ridge- and slope-scale topography), in an attempt to reveal unexplained variability in the landslide distribution. We then assess whether this variability can be attributed to the legacy of past events. Our results suggest that the 1929 Buller earthquake influenced the distribution of landslides triggered by the 1968 Inangahua earthquake. Hillslopes in regions that experienced strong ground motions in 1929 were more likely to fail in 1968 than would be expected on the basis of time-independent factors alone. This effect is consistent with our hypothesis that unfailed hillslopes in the 1929 earthquake were weakened by damage accumulated during this earthquake and its associated aftershock sequence, and this weakening then influenced the performance of the landscape in the 1968 earthquake. While our results are tentative, the findings emphasize that a lack of knowledge of the damage state of hillslopes in a landscape potentially represents an important source of uncertainty when assessing landslide susceptibility. Constraining the damage history of hillslope materials, through analysis of historical events, therefore provides a potential means of reducing this uncertainty.

Description: Millennial erosion rates across the Pamir based on 10 Be concentrations in fluvial sediments: dominance of topographic over climatic factors Earth Surface Dynamics Discussions, 3, 83-128, 2015 Author(s): M. C. Fuchs, R. Gloaguen, S. Merchel, E. Pohl, V. A. Sulaymonova, C. Andermann, and G. Rugel The understanding of erosion processes is fundamental to study the evolution of actively deforming mountain ranges, whereas the relative contributions tectonic and climatic factors and their feedbacks are debated. The Pamir is peculiar in both, high deformation rates induced by the India–Eurasia collision and its position at the transition between Westerlies and Monsoon. In order to contribute to this debate we quantify basin-wide erosion rates from cosmogenic 10 Be concentrations in modern river sediments measured by accelerator mass spectrometry. Sample locations represent the Panj basin at six sites along its trunk stream, and the major, east–west elongated tributary basins at five sites. An average erosion of ~0.64 mm yr −1 for the entire Pamir reveals a rapid landscape evolution. Erosion rates of tributary sub-basins highlight the strong contrast between the plateau (0.05 to 0.16 mm yr −1 ) and the Pamir margins (0.54 to 1.45 mm yr −1 ). The intensity of erosion is primarily ( R 2 of 0.81) correlated to slope steepness (0.75 quartiles) suggesting either tectonic uplift or base level lowering. Multiple linear regression reveals that precipitation may contribute also to the efficiency of erosion ( R 2 of 0.93) to a lesser extent. Dry conditions and low slopes hinders sediment transport and consequently, erosion on the plateau. The highest erosion coincides with the predominant winter precipitation from the Westerlies. The concentrated discharge during spring and early summer favors pronounced erosion along the north-western Pamir margin by driving the sediment flux out of the basins. The magnitude of erosion in Pamir is similar to rates determined in the south Himalayan escarpment, whereas climatic and tectonic conditions are very different. Millennial erosion does not balance the roughly ten times higher fluvial incision implying a transient landscape. We propose that river captures are responsible for the strong base level drop driving the incision along the Panj and consequently, initiate steep hillslopes that will contribute to high erosion at the Pamir margins. Precipitation may act as limiting factor to hillslope adjustment and consequently to erosion processes.

Description: Hitting rock bottom: morphological responses of bedrock-confined streams to a catastrophic flood Earth Surface Dynamics Discussions, 2, 1093-1128, 2014 Author(s): M. Baggs Sargood, T. J. Cohen, C. J. Thompson, and J. Croke The role of extreme events in shaping the earth's surface is one that has held the interests of Earth scientists for centuries. A catastrophic flood in a tectonically quiescent setting in eastern Australia in 2011 provides valuable insight into how bedrock channels respond to such events. Field survey data (3 reaches) and desktop analyses (10 reaches) with catchment areas ranging from 0.5 to 169 km 2 show that the predicted discharge for the 2011 event ranged from 400 to 900 m 3 s −1 , with unit stream power estimates of up to 1000 W m −2 . Estimated entrainment relationships predict the mobility of the entire grain size population and field data suggests the localised mobility of boulders up to 4.8 m in diameter. Analysis of repeat LiDAR data demonstrates that all reaches (field and desktop) were areas of net degradation via extensive scouring of mantled alluvium with a strong positive relationship between catchment area and normalised erosion ( R 2 = 0.8). The extensive scouring in the 2011 flood decreased thalweg variance significantly with the exposure of planar bedrock surfaces, marginal bedrock straths and bedrock steps, along with the formation of a plane-bed cobble morphology. Post-flood field data suggests a slight increase in thalweg variance as a result of the smaller 2013 flood, however the current nature and distribution of channel morphological units does not conform to previous classifications of upland river systems. This suggests that extreme events are significant for re-setting the morphology of in-channel units in such bedrock systems. As important, is the exposure of the underlying lithology to ongoing erosion.

Description: Groundwater seepage landscapes from local or distal sources in experiments and on Mars Earth Surface Dynamics Discussions, 3, 129-171, 2015 Author(s): W. A. Marra, S. J. McLelland, D. R. Parsons, B. J. Murphy, E. Hauber, and M. G. Kleinhans Theater-headed valleys can form due to groundwater sapping, but these valleys could also be the result of knick-point (waterfall) erosion generated by overland flow. This morphological ambiguity hampers the interpretation of such valleys on Mars, especially due to insufficient knowledge of material properties, but the climate implications are quite different. Instead of single-valley morphology, metrics of the entire landscape may provide diagnostic insight in the formative hydrological conditions. However, flow patterns and the resulting landscapes are different for different sources of groundwater and poorly understood. We aim to increase our understanding of the formation of the entire landscapes by sapping from different sources of groundwater and to provide a framework of landscape metrics of such systems to aid interpretation of such landscapes. We study sapping from local and distal sources of groundwater in sandbox experiments and combine our results with previous experiments. Key results are that groundwater piracy acts on distally-fed valleys, which results in a sparsely dissected landscape of many small and a few large valleys while locally-fed valleys result in a densely dissected landscape. In addition, distally-fed valleys grow into the direction of the groundwater source while locally-fed channels grow in a broad range of directions and have a strong tendency to bifurcate, particularly on flat horizontal surfaces. As an example, we apply these results to two Martian cases. The valleys of Louros Valles show properties of sapping by a local source and Nirgal Vallis shows evidence of a distal source, which is likely groundwater from Tharsis.

Description: Network response to internal and external perturbations in large sand-bed braided rivers Earth Surface Dynamics Discussions, 3, 197-250, 2015 Author(s): F. Schuurman, M.G. Kleinhans, and H. Middelkoop The intrinsic instability of bars, bifurcations and branches in large braided rivers is a challenge to understand and predict. Even more, the reach-scale effect of human-induced perturbations on the braided channel network is still unresolved. In this study, we used a physics-based model to simulate the hydromorphodynamics in a large braided river and applied different types of perturbations. We analyzed the propagation of the perturbations through the braided channel network. The results showed that the perturbations initiate an instability that propagates in downstream direction by means of bifurcation instability. It alters and rotates the approaching flow of the bifurcations. The propagation celerity is in the same order of magnitude as the theoretical sand wave propagation rate. The adjustments of the bifurcations also change bar migration and reshape, with a feedback to the upstream bifurcation and alteration of the approaching flow to the downstream bifurcation. This way, the morphological effect of a perturbation amplifies in downstream direction. Thus, the interplay of bifurcation instability and asymmetrical reshaping of bars was found to be essential for propagation of the effects of a perturbation. The study also demonstrated that the large-scale bar statistics are hardly affected.

Description: The periglacial engine of mountain erosion – Part 2: Modelling large-scale landscape evolution Earth Surface Dynamics Discussions, 3, 327-369, 2015 Author(s): D. L. Egholm, J. L. Andersen, M. F. Knudsen, J. D. Jansen, and S. B. Nielsen An increasing number of studies point to a strong periglacial control on bedrock erosion in mountain landscapes. Periglacial processes have also been suggested to control the formation of block-fields on high-elevation, low-relief surfaces (summit flats) found in many alpine landscapes. However, to which degree periglacial processes took part in accelerating global erosion rates in response to Late Cenozoic cooling still remains as an unanswered question. In this study, we present a landscape evolution model that incorporates two periglacial processes; frost cracking and frost creep, which both depend on the mean annual temperature (MAT) and sediment thickness. The model experiments allow us to time-integrate the contribution of periglacial processes to mountain topography over million-year time scales. It is a robust result of our experiments that periglacial frost activity leads to the formation of smooth summit flats at elevations dominated by cold climatic conditions through time periods of millions of years. Furthermore, a simplistic scaling of temperatures to δ 18 O values through the late-Cenozoic indicates that many of the highest summit flats in mid- to high-latitude mountain ranges can have formed prior to the Quaternary. The model experiments also suggest that cooling in the Quaternary accelerated periglacial erosion by expanding the areas affected by periglacial erosion significantly. A computational experiment combining glacial and periglacial erosion furthermore suggests that landscape modifications associated with glacial activity may increase the long-term average efficiency of the frost-related processes.

Description: The periglacial engine of mountain erosion – Part 1: Rates of frost cracking and frost creep Earth Surface Dynamics Discussions, 3, 285-326, 2015 Author(s): J. L. Andersen, D. L. Egholm, M. F. Knudsen, J. D. Jansen, and S. B. Nielsen With accelerating climate cooling in the late Cenozoic, glacial and periglacial erosion became more widespread on the surface of the Earth. The resultant shift in erosion patterns significantly changed the large-scale morphology of many mountain ranges worldwide. Whereas the glacial fingerprint is easily distinguished by its characteristic fjords and U-shaped valleys, the periglacial fingerprint is more subtle but potentially prevailing in some landscape settings. Previous models have advocated a frost-driven control on debris production on steep headwalls and glacial valley sides. Here we investigate the important role that periglacial processes also play in less steep parts of mountain landscapes. Understanding the influences of frost-driven processes in low-relief areas requires a focus on the consequences of an accreting soil-mantle, which characterizes such surfaces. In this paper, we present a new model that quantifies two key physical processes: frost cracking and frost creep, as a function of both temperature and sediment thickness. Our results yield new insights to how climate and sediment transport properties combine to scale the intensity of periglacial processes. The thickness of the soil-mantle strongly modulates the relation between climate and the intensity of mechanical weathering and sediment flux. Our results also point to an offset between the conditions that promote frost cracking and those that promote frost creep, indicating that a stable climate can only provide optimal conditions for one of those processes at a time. Finally, quantifying these relations also opens the possibility of including periglacial processes in large-scale, long-term landscape evolution models, as demonstrated in a companion paper.

Description: Tectonic geomorphology at small catchment sizes – extensions of the stream-power approach and the χ method Earth Surface Dynamics Discussions, 3, 689-714, 2015 Author(s): S. Hergarten, J. Robl, and K. Stüwe Quantitative tectonic geomorphology hinges on the analysis of longitudinal river profiles. The model behind almost all approaches in this field originates from an empirical relationship between channel slope and catchment size, often substantiated in form of the stream-power model for fluvial incision. A significant methodological progress was recently achieved by introducing the χ transform. It defines a nonlinear length coordinate in such a way that the inherent curvature of river profiles due to the increase of catchment sizes in downstream direction is removed from the analysis. However, the limitation to large catchment sizes inherited from the stream power approach for fluvial incision persists. As a consequence, only a small fraction of all nodes of a DEM can be used for the analysis. In this study we present and discuss some empirically derived extensions of the stream power law towards small catchment sizes in order to overcome this limitation. Beyond this, we introduce a simple method for estimating the adjustable parameters in the original χ method as well as in our extended approaches. As a main result, an approach originally suggested for debris flow channels seems to be the best approximation if both large and small catchment sizes are included in the same analysis.