Catchment-wide Denudation Research Articles

Denudation rates of small transient catchments controlled by former glaciation: The Hörnli nunatak in the northeastern Swiss Alpine Foreland

Extensive glaciers repeatedly occupied the northern Alpine Foreland during the Pleistocene and left a strongly glacially overprinted low slope landscape. Only few islands appeared as nunataks standing above the surface of the large piedmont glacier lobes. These nunatak areas kept their original shape, manifested in steep catchments with mean slopes up to 33°. Even though not glaciated, these catchments where significantly affected by base-level changes occurring as a consequence of phases of glacier advances and retreats. Both domains, the glacially eroded and non-eroded, are therefore prone to different mechanisms and time-scales of fluvial and colluvial re-adjustment.In this study we investigate these effects by exploring the spatial distribution and magnitude of denudation in the Hörnli region of the eastern Swiss Alpine Foreland in the present Interglacial. The area represents both domains in a relatively small area with largely uniform tectonic, lithologic and climatic conditions. The differences in Holocene landscape evolution are investigated using topographic analyses and catchment-averaged denudation rates derived from 10Be concentrations in fluvial quartz sand. We find that in formerly non-glaciated, fluvially dominated catchments close hillslope-channel coupling prevails and that these catchments yield high average denudation rates of 350 mm/ka. Glacially overprinted catchments yielded catchment-wide denudation rates an order of magnitude lower. These low denudation rates are hypothesized to be the consequence of both (i) a dominance of slow hillslope processes and (ii) admixture of high concentration, pre-LGM glacial sediment. This suggests that a) a careful field investigation must accompany the denudation rate studies and b) that the concept of area-weighted cosmogenic nuclide denudation rates must be considered in light of the predominant catchment processes.

Catchment wide denudation rates in debris flow dominated catchments

Catchment wide denudation rates in debris flow dominated catchments

Debris-flow–dependent variation of cosmogenically derived catchment-wide denudation rates

Catchment-wide denudation rates (CWDRs) obtained from cosmogenic nuclides are an efficient way to determine geomorphic processes quantitatively in alpine mountain ranges over Holocene time scales. These rate estimations assume steady geomorphic processes. Here we use a time series (3 yr) in the Aare catchment (central Swiss Alps) to test the impact of spatially heterogeneous stochastic sediment supply on CWDRs. Our results show that low-frequency, high-magnitude debris-flow events significantly perturb cosmogenic nuclide ( 10 Be, 14 C) concentrations and thus CWDRs. The 10 Be concentrations decrease by a factor of two following debris-flow events, resulting in a doubling of inferred CWDRs. The variability indicates a clear time and source dependency on sediment supply, with restricted area-weighted mixing of sediment. Accordingly, in transient environments, it is critical to have an understanding of the history of geomorphic processes to derive meaningful CWDRs. We hypothesize that the size of debris flows, their connectivity with the trunk stream, and the ability of the system to sufficiently mix sediment from low- and high-order catchments control the magnitude of CWDR perturbations. We also determined in situ 14 C in a few samples. In conjunction with 10 Be, these data suggest partial storage for colluvium of a few thousand years within the catchment prior to debris-flow initiation.

Quantifying denudation rates and sediment storage on the eastern Altiplano, Bolivia, using cosmogenic 10Be, 26Al, and in situ 14C

Denudation processes and sediment transfer are investigated in a high-elevation, low-relief environment (eastern Altiplano, Bolivia) using 10Be, 26Al, and in situ 14C analysis in fluvial sediments. Concentrations of the long-lived nuclides 10Be and 26Al yield consistently low catchment-wide denudation rates of ~3–29mmky−1 (integrating over 21–194ky), which reflect the low geomorphic gradients and the discontinuity of fluvial transport along the eastern Altiplano margin. No significant correlation is recorded between denudation rates of individual catchments and morphological basin parameters (slope, area, elevation). This is attributed to the overall little variability in morphology. The agreement between the denudation rates and published modern sediment discharge data suggests steady landscape evolution of the eastern Altiplano from the latest Pleistocene until today. While 10Be and 26Al provide long-term estimates on sediment production, in situ cosmogenic 14C is used to trace short-term sediment storage. In situ 14C concentrations are comparatively low indicating that 14C decayed during alluvial storage over at least the past ~11–20ky. We assume storage at shallow depth (2m) and consider the influence of soil-mantled hillslopes on the in situ 14C concentration. Our results illustrate the importance of sediment storage even over short distances and demonstrate the potential of in situ 14C to study sediment routing and transfer times within drainage systems. However, this study also demonstrates that the long-lived 10Be and 26Al nuclides can provide adequate estimates on long-term denudation rates even if sediment transport is not fast but interrupted by several thousands of years of storage.

Spatial variations of surface water chemistry and chemical denudation in the Erdalen drainage basin, Nordfjord, western Norway

This study conducted over an investigation period of six years (2004–2010) focuses on spatial variation in chemical components of drainage water from sub-areas within a confined drainage basin of 79.5km2 (Erdalen drainage basin, inner Nordfjord, western Norway) for detection of spatial diversity and relation to environmental factors. The Erdalen drainage basin is homogenously composed of gneisses, which is reflected in a homogenous relative (as a percentage) chemical composition of surface water across the entire drainage basin system. At the same time, the mean annual TDS values of surface water sampled in creeks draining defined subsystems within Erdalen show a rather high spatial variability. The main controls of this spatial variability are (i) differences in slope deposit/regolith thickness, (ii) differences in slope angle, (iii) differences in areal regolith cover, (iv) differences in vegetation cover, (v) differences in snow cover and ground frost conditions, and (vi) differences in elevation (ma.s.l.). Altogether, the mean annual TDS values in Erdalen are rather low, which can be explained by (i) the shallow thickness of regolith across the very steep drainage basin, (ii) the small percentage of surface areas showing a significant cover of regolith, (iii) the cool climate in the fjord landscape of western Norway and (iv) the weathering resistance of the predominant gneisses within Erdalen. The annual chemical denudation rates in Erdalen are in a similar range of magnitude to rates published for numerous other cold environment catchments worldwide. Despite the rather low mean annual TDS values, chemical denudation is a comparably important and spatially very variable denudational process and should therefore not be neglected when studying slope development as well as slope- and catchment-wide denudation rates and mass budgets in this kind of steep, mountainous cold climate drainage basins.

El Niño forcing on 10Be-based surface denudation rates in the northwestern Peruvian Andes?

High magnitude precipitation events provide large contributions to landscape formation and surface denudation in arid environments. Here, we quantify the precipitation-dependent geomorphic processes within the Rio Piura drainage basin located on the Western Escarpment of the northern Peruvian Andes at 5°S latitude. In this region, monsoonal easterly winds bring precipitation to the > 3000 m asl high headwaters, from where the annual amount of precipitation decreases downstream toward the Pacific coast. Denudation rates are highest in the knickzones near the headwaters, ~ 200–300 mm ky − 1 , and sediment discharge is limited by the transport capacity of the channel network. Every few years, this situation is perturbed by westerly, wind-driven heavy precipitation during El Niño events and results in supply-limited sediment discharge as indicated by bedrock channels. The detailed analysis of the stream-long profiles of two river basins within the Rio Piura catchment reveals a distinct knickzone in the transition zone between the easterly and westerly climatic influences, suggesting an En Niño forcing on the longitudinal channel profiles over at least Holocene timescales. Measured trunk stream catchment-wide denudation rates are up to ca. 300 mm ky − 1 and decrease successively downstream along the river profiles. Denudation rates of tributary rivers are ca. 200 mm ky − 1 near the plateau and show a stronger downstream decreasing trend than trunk stream rates. This suggests that the landscape is in a transient stage of local relief growth, which is driven by fluvial incision. This corroborates the results of paleoclimate studies that point towards higher El Nino frequencies during the past ca. 3000 years, leading to higher runoff and more erosion in the trunk channel compared to the hillslopes and thus growth of local relief. Downstream increases in channel gradient spatially coincide with the reaches of highest precipitation rates during El Niño events, we therefore interpret that Holocene landscape evolution has largely been controlled by climate. The ky-timescale of the 10Be data together with the transience of the landscape implies that El Niño events in northwestern Peru have occurred since at least the Holocene, and that adjustment to channel incision is still taking place.

Frost-cracking control on catchment denudation rates: Insights from in situ produced 10Be concentrations in stream sediments (Ecrins–Pelvoux massif, French Western Alps)

The potential tectonic and climatic controls on erosion rates in the European Alps and other mountain belts remain strongly debated. We have quantified denudation rates at catchment scales using in situ produced cosmogenic nuclides ( 10Be) in stream sediments, sampled at the outlets of twelve variously sized (27–1072 km 2) catchments of the Ecrins–Pelvoux massif (French Western Alps), with average elevations ranging from 1700 to 2800 m. Spatially-averaged denudation rates, corrected for potential shielding by Little Ice Age glaciers, vary from 0.27 ± 0.05 to 1.07 ± 0.20 mm/yr on millennial timescales. Our results exhibit a correlation ( ρ 2 = 0.56) between denudation rate and mean catchment elevation, in the absence of significant correlation with any other morphometric parameters (relief, slope, catchment size, hypsometry, etc). Although such variations in erosion rates have been previously linked to variations in tectonic uplift rate, the relatively small size and tectonic homogeneity of our study area exclude a strongly variable tectonic control. We interpret the increase in erosion rate with elevation as the effect of frost-controlled processes, which are strongly temperature-dependent. We use a one-dimensional heat-flow model driven by high-resolution instrumental temperature records from the study area to correlate the variability in denudation rates with the integral of the absolute temperature gradient within the frost-cracking window (− 3 to − 8 °C), a proxy of the frost-cracking intensity, for each catchment. The results imply that the efficiency of frost cracking constitutes a major control on catchment-wide denudation rates in the study area, explaining more than half the measured variability in these rates. Our study shows that present-day denudation of the Ecrins–Pelvoux massif is controlled by a climatically driven factor and suggests that frost-cracking processes impose an important control on the post-glacial topographic evolution of mid-latitude mountain belts.

Determining the growth rate of topographic relief using in situ-produced 10Be: A case study in the Black Forest, Germany

Abstract To determine how topographic relief in mountainous regions evolves through time we present a new approach that uses in situ -produced cosmogenic 10 Be to quantify (1) spatially averaged denudation rates of small watersheds and (2) local denudation rates of the ridge crests bounding these basins. The technique is applied to two catchments in the Black Forest, a forested mountain range with a local relief of a few hundred meters, which is typical for ranges in central Europe. Both the Acher and the Gutach catchments expose predominantly Carboniferous granite, and only minor amounts of high-grade gneiss and Triassic sandstone. The latter occurs on ridges defining the eastern boundaries of the catchments, above a regional unconformity. In the Acher and northern Gutach watersheds denudation rates of subcatchments derived from 10 Be concentrations in stream sediment range from 52 to 87 mm/ka and 59 to 91 mm/ka, respectively. In contrast, grus samples from the ridge crests bounding both watersheds yield lower denudation rates of 34 to 59 mm/ka. The differences in denudation rates for sample pairs from individual subcatchments and adjacent ridge crests reveals that topographic relief is growing at a mean rate of 24 ± 12 mm/ka (with the exception of the flat southwestern part of the Gutach catchment, where catchment-wide denudation rates are similar to the rate of ridge crest lowering). The inferred rates of denudation and relief growth are consistent with erosion rates calculated from the known thickness of Triassic to Lower Jurassic sediments, which were once present above the regional unconformity but have been largely eroded during the exhumation of the Black Forest. The onset of exhumation ∼ 19 Ma ago is constrained by thermal modelling of apatite fission track data, which suggest a cooling rate of ∼ 3 °C/Ma. Combined with a geothermal gradient of 30 to 40 °C/km this cooling rate yields an average exhumation rate of 75–100 mm/ka for the modelled apatite fission track data, which is comparable to spatially averaged denudation rates derived from cosmogenic 10 Be. Our new approach may help to determine whether tectonically active mountain ranges are in a topographic steady state, in which rates of rock uplift and denudation are equal, or if such a dynamic equilibrium has not yet been attained.

Topographic and lithologic control on catchment-wide denudation rates derived from cosmogenic 10Be in two mountain ranges at the margin of NE Tibet

Fault-bounded mountain ranges with along-strike variations in mean slope angle and local relief offer the opportunity to investigate how denudation depends on these topographic parameters. We determined catchment-wide denudation rates along two actively growing, fault-bounded mountain ranges (the Yumu and western Longshou Shan) from 10Be concentrations in quartz from stream sediments. Both ranges expose (1) low-grade metamorphic Paleozoic rocks in their center, and (2) unmetamorphosed Cretaceous red beds near the tips of the range-bounding thrust faults. Field observations document debris flows, shallow landsliding, and surface wash as dominant mechanisms of mass transport on the hillslopes. Denudation rates obtained for the Paleozoic metasediments range from ∼100 to ∼300mmka−1 and correlate linearly with mean slope angle and local relief. In contrast, Cretaceous sediments exhibit mainly higher and more variable denudation rates between ∼30 and ∼500mmka−1, although slope and relief values are lower for these catchments. The higher denudation rates in the Cretaceous red beds are interpreted as resulting from their lower mechanical strength as compared to the resistant Paleozoic rocks. This important lithologic control on denudation demonstrates that uniform catchment lithologies and rock strength are essential for deciphering the effect of topographic indices on the rate of denudation.

Complex multiple cosmogenic nuclide concentration and histories in the arid Rio Lluta catchment, northern Chile

AbstractTerrestrial cosmogenic nuclide (TCN) concentrations measured in river sediments can be used to estimate catchment‐wide denudation rates. By investigating multiple TCN the steadiness of sediment generation, transport and depositional processes can be tested. Measurements of 10Be, 21Ne and 26Al from the hyper‐ to semi‐arid Rio Lluta catchment, northern Chile, yield average single denudation rates ranging from 12 to 75 m Myr–1 throughout the catchment. Paired nuclide analysis reveals complex exposure histories for most of the samples and thus the single nuclide estimates do not exclusively represent catchment‐wide denudation rates. The lower range of single nuclide denudation rates (12–17 m Myr–1), established with the noble gas 21Ne, is in accordance with palaeodenudation rates derived from 21Ne/10Be and 26Al/10Be ratio analysis. Since this denudation rate range is measured throughout the system, it is suggested that a headwater signal is transported downstream but modulated by a complex admixture of sediment that has been stored and buried at proximal hillslope or terrace deposits, which are released during high discharge events. That is best evidenced by the stable nuclide 21Ne, which preserves the nuclide concentration even during storage intervals.The catchment‐wide single 21Ne denudation rates and the palaeodenuation rates contrast with previous TCN‐derived erosion rates from bedrock exposures at hillslope interfluves by being at least one order of magnitude higher, especially in the lower river course. These results support earlier studies that identified a coupling of erosional processes in the Western Cordillera contrasting with decoupled processes in the Western Escarpment and in the Coastal Cordillera. Copyright © 2008 John Wiley & Sons, Ltd.

Spatial and temporal variations in denudation of the Wasatch Mountains, Utah, USA

We evaluate spatial and temporal variations in denudation of the north-central Wasatch Mountains, Utah, by determining catchment-wide denudation rates with 10Be concentrations in alluvial sediment and comparing these rates with previously published data on rock uplift and exhumation of the range. Catchments draining the range front show relatively little variation in denudation rate (0.07–0.17 mm/yr), while steeper (mean hillslope gradient >30°) catchments in the core of the range show larger variation (0.17–0.79 mm/yr). We attribute the larger spatial variation in catchment-wide denudation in the core of the range to landsliding of hillslopes at threshold gradients; faster denudation in this region may signify landscape adjustment to late Pleistocene glaciations. The mean denudation rate for all catchments (0.2 mm/yr) is generally consistent with longer-term exhumation rates derived from thermochronometers and with shorter-term vertical fault displacement rates, suggesting that denudation of the north-central Wasatch has been roughly steady, or decreasing slightly, over the past 5 m.y. Although 10Be-based catchment-wide denudation rates are sensitive to localized geomorphic processes and events, overall, they appear to reflect the larger tectonic forces that have driven denudation of the Wasatch Mountains over longer time scales.

Tectonic uplift, threshold hillslopes, and denudation rates in a developing mountain range

Studies across a broad range of drainage basins have established a positive correlation between mean slope gradient and denudation rates. It has been suggested, however, that this relationship breaks down for catchments where slopes are at their threshold angle of stability because, in such cases, denudation is controlled by the rate of tectonic uplift through the rate of channel incision and frequency of slope failure. This mechanism is evaluated for the San Bernardino Mountains, California, a nascent range that incorporates both threshold hillslopes and remnants of pre-uplift topography. Concentrations of in situ–produced cosmogenic 10 Be in alluvial sediments are used to quantify catchment-wide denudation rates and show a broadly linear relationship with mean slope gradient up to ∼30°: above this value denudation rates vary substantially for similar mean slope gradients. We propose that this decoupling in the slope gradient–denudation rate relationship marks the emergence of threshold topography and coincides with the transition from transport-limited to detachment-limited denudation. The survival in the San Bernardino Mountains of surfaces formed prior to uplift provides information on the topographic evolution of the range, in particular the transition from slope-gradient–dependent rates of denudation to a regime where denudation rates are controlled by rates of tectonic uplift. This type of transition may represent a general model for the denudational response to orogenic uplift and topographic evolution during the early stages of mountain building.

Constraining landscape development of the Sri Lankan escarpment with cosmogenic nuclides in river sediment

Escarpments are prominent morphological features along high-elevation passive margins. Recent studies integrating geomorphology, thermochronology, and cosmogenic nuclide-based denudation rate estimates suggest a rapid phase of denudation immediately after the earliest stages of seafloor spreading, and subsequent slow denudation rates since. To constrain the geomorphic evolution of passive margins, we have examined the development of the Sri Lankan escarpment. Cosmogenic nuclide data on river sediment along a north–south transect across the southern escarpment reveal that the landscape is eroding ten times more rapidly in the escarpment zone (26 to 71 mm kyr − 1 ) than in the high-elevation plateau above it and in the lowland plain beneath it (2.6 to 6.2 mm kyr − 1 ). Unlike these low denudation rate areas, the escarpment denudation is strongly and linearly hill slope-dependent. This shows that denudation and retreat are tightly interlinked within the escarpment, which suggests that the escarpment is evolving by rift-parallel retreat, rather than by escarpment downwearing. Supporting evidence is provided by the morphology of rivers draining the escarpment zone. These have steep bedrock channels which show sharp and prominent knickpoints along their longitudinal profiles. It appears that fluvial processes are driving escarpment retreat, as rivers migrate headwards were they incise into the high-elevation plateau. However, the average catchment-wide denudation rates of the escarpment zone are low compared to the denudation rates that are estimated for constant escarpment retreat since rifting. In common with other escarpments worldwide, causes for this slow down can be tectonic change related to flexural bending of the lithosphere, climate change that would vary the degree of precipitation focused into the escarpment, or the decrease in the contributing catchment area, which would reduce the stream power available for fluvial erosion.

Limits to quantifying climate driven changes in denudation rates with cosmogenic radionuclides

The hypothesis that climate change significantly modifies the denudation history of an orogen can be tested by reconstructing catchment-wide denudation histories. Cosmogenic radionuclides measured in sediment deposits of known age (e.g. a sequence of fluvial terraces) offer one approach for quantifying catchment-wide paleo-denudation rates. We present a numerical model, and two case studies, that evaluate the utility of this technique for reconstructing different denudation histories. The concentration of cosmogenic beryllium-10 ( 10Be) is calculated for a sequence of sediments produced from climate driven variations in the mean denudation rate of a drainage basin at sea level and high latitude. The model accounts for the nucleonic and muonic production, and decay of 10Be as sediment is eroded and stored in deposits of different ages. Two scenarios for different climate driven denudation histories are considered: (1) A sinusoidal variation in the input denudation rate with free parameters of periodicity (23, 41, 100 kyr), amplitude (0.1, 0.5, 1.0), and mean denudation rate (0.01, 0.1, 1.0 mm yr − 1 ). (2) A climate driven denudation history based on oscillations of a global benthic foraminiferal oxygen isotope (δ 18O) record over the last 2 Myr. Results from both scenarios suggest cosmogenic radionuclides measured in a system with high denudation rates (> ∼ 0.5 mm yr − 1 ), longer periodicity (100 kyr), and low amplitude have the highest potential for reconstructing climate driven paleo-denudation rates. Unfortunately, the high denudation rates preferred in the theoretical consideration are not suitable for paleo-denudation rate determination due to overprinting of nuclides accumulated during denudation by post-depositional decay and irradiation. Hence, a compromise between the capability to determine changes in paleo-denudation rates and the determination of true denudation rates has to be accepted. Application of the model to previous data sets from northwest Europe and southwest North America produce a good agreement, but highlight the need for future work to consider how geomorphic processes (e.g. landsliding) respond to climate change and influence catchment-wide denudation rates. In addition, other climate proxies than the benthic foraminiferal δ 18O record should be used for specific sample locations.