Variations In Erosion Rates Research Articles

Stream capture as driver of transient landscape evolution in a tectonically quiescent setting

Research Article| September 01, 2011 Stream capture as driver of transient landscape evolution in a tectonically quiescent setting Philip S. Prince; Philip S. Prince Department of Geosciences, Virginia Polytechnic Institute and State University (0420), Blacksburg, Virginia 24061, USA Search for other works by this author on: GSW Google Scholar James A. Spotila; James A. Spotila Department of Geosciences, Virginia Polytechnic Institute and State University (0420), Blacksburg, Virginia 24061, USA Search for other works by this author on: GSW Google Scholar William S. Henika William S. Henika Department of Geosciences, Virginia Polytechnic Institute and State University (0420), Blacksburg, Virginia 24061, USA Search for other works by this author on: GSW Google Scholar Author and Article Information Philip S. Prince Department of Geosciences, Virginia Polytechnic Institute and State University (0420), Blacksburg, Virginia 24061, USA James A. Spotila Department of Geosciences, Virginia Polytechnic Institute and State University (0420), Blacksburg, Virginia 24061, USA William S. Henika Department of Geosciences, Virginia Polytechnic Institute and State University (0420), Blacksburg, Virginia 24061, USA Publisher: Geological Society of America Received: 16 Dec 2010 Revision Received: 24 Mar 2011 Accepted: 03 Apr 2011 First Online: 09 Mar 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 © 2011 Geological Society of America Geology (2011) 39 (9): 823–826. https://doi.org/10.1130/G32008.1 Article history Received: 16 Dec 2010 Revision Received: 24 Mar 2011 Accepted: 03 Apr 2011 First Online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Philip S. Prince, James A. Spotila, William S. Henika; Stream capture as driver of transient landscape evolution in a tectonically quiescent setting. Geology 2011;; 39 (9): 823–826. doi: https://doi.org/10.1130/G32008.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract We use unique fluvial gravel deposits preserved atop a regional drainage divide to confirm the role of stream capture in driving ∼250 m of incision in the transient Roanoke River basin of the Appalachian Mountains (United States). Gravel provenance constrains the pre-capture position of the divide, indicating that ∼225 km2 of basin area were abruptly connected to the base level of the capturing stream. The resulting wave of incision is currently manifest as major knickzones separating adjusting reaches from relict headwaters resembling streams of the New River basin, from which the Roanoke River was captured. The unusual preservation of the unconsolidated gravels on small relict surfaces adjacent to bedrock gorges indicates extreme spatial variability in erosion rates within the Roanoke basin, which is the first documented example of a transient passive margin basin connected to a capture event by stranded fluvial debris. Our results show the potential for stream capture across an asymmetric drainage divide to drive major transient incision independent of external forcings, such as climate change or tectonic uplift. A continuation of this process will lead to eventual capture of ∼7000 km2 of the New River basin in the relatively near geologic future. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Understanding Earth’s eroding surface with 10Be

For more than a century, geologists have sought to measure the distribution of erosion rates on Earth’s dynamic surface. Since the mid-1980s, measurements of in situ 10Be, a cosmogenic radionuclide, have been used to estimate outcrop and basin-scale erosion rates at 87 sites around the world. Here, we compile, normalize, and compare published 10Be erosion rate data (n = 1599) in order to understand how, on a global scale, geologic erosion rates integrated over 103 to 106 years vary between climate zones, tectonic settings, and different rock types. Drainage basins erode more quickly (mean = 218 m Myr−1; median = 54 m Myr−1) than outcrops (mean = 12 m Myr−1; median = 5.4 m Myr−1), likely reflecting the acceleration of rock weathering rates under soil. Drainage basin and outcrop erosion rates both vary by climate zone, rock type, and tectonic setting. On the global scale, environmental parameters (latitude, elevation, relief, mean annual precipitation and temperature, seismicity, basin slope and area, and percent basin cover by vegetation) explain erosion rate variation better when they are combined in multiple regression analyses than when considered in bivariate relationships. Drainage basin erosion rates are explained well by considering these environmental parameters (R2 = 0.60); mean basin slope is the most powerful regressor. Outcrop erosion rates are less well explained (R2 = 0.32), and no one parameter dominates. The variance of erosion rates is better explained when subpopulations of the global data are analyzed. While our compilation is global, the grouped spatial distribution of cosmogenic studies introduces a bias that will only be addressed by research in under-sampled regions.

Variability in erosion rates related to the state of landscape transience in the semi‐arid Chilean Andes

ABSTRACTWe quantify erosion rates in the higher sectors of the Huasco Valley, in the Main Cordillera of the semi‐arid Andes of Chile, using elevation differences between three successive geomorphic markers (pediments and paleo‐valleys) and the present day valley. Available Ar‐Ar ages of Neogene pediments are used to estimate mean erosion rates for the three periods (16 to 13 My, 13 to 8 My, and following 8 My). The landscape of the Huasco Valley is in a transient state, as indicated by well‐preserved pediment surfaces in interfluves, valleys deeply incised by fluvial and glacial erosion and scarped head‐valleys that represent the current knickzones. Higher erosion rates (45–75 m/My) are calculated for the more recent period (< 8 My) during which deep incision developed compared to previous periods (6–31 m/My). Quantitative data indicate that glaciers had a much higher erosional capability than fluvial activity in the higher sectors of the Main Cordillera. Comparison with erosion rates calculated in other drainage basins of the Chilean Andes suggests that the variability of erosion rates depends on the landscape's transient erosive state. The landscape's geomorphologic response to the uplift of the Main Cordillera results in the retreat of a knickzone, for which retreat velocity depends on precipitation rate pattern and glacial erosion intensity. Copyright © 2011 John Wiley & Sons, Ltd.

Characterization and Response Evaluation of Detonation-Gun Sprayed Alumina-Titania Ceramic Composite

Erosion due to solid particles impact has a detrimental effect on the performance of components used in various applications. Hard facing is widely used to combat wear in such situations. D-gun coating is increasingly popular for such applications. Alumina-titania composite has been deposited on mild steel substrate. The coating has been characterized and their tribological responses have been evaluated. Resistance to erosion, evaluated through standard dry erosion test is highlighted in this work. An orthogonal full–factorial experimental matrix which includes all possible variables that contribute to brittle erosion was designed, conducted and validated. The variation of erosion rate with process parameters was mathematically modeled by using the regression analysis method.

Does decreasing paraglacial sediment supply slow knickpoint retreat?

In four rivers in western Scotland for which there is a well constrained record of relative base-level fall, the rate of postglacial bedrock erosion is quantifi ed by measuring the concentration of in situ cosmogenic 10Be on strath terraces downstream of headward retreating knickpoints. Along-channel gradients in 10Be exposure age show two distinct trends: upstream younging and constant age, which we interpret as diagnostic of knickpoint retreat and diffusive transport-limited incision, respectively. We show that bedrock channel incision and regional formation of strath terraces began shortly after deglaciation (ca. 11.5 ka), and that knickpoint retreat rates peaked in the early to mid-Holocene. Erosion rates have since decreased by two orders of magnitude, converging in the late Holocene to low rates independent of stream power per unit channel area. We infer this regional slowing in postglacial knickpoint retreat to be the result of the depletion of paraglacial sediment supply over the Holocene, leading to a defi ciency in “tools” for bedrock erosion. Our results imply that episodes of major fl uvial erosion may be in tune with glacial cycles, and that sediment depletion following glacial-interglacial transitions may be an important cause of bedrock erosion rate variations in rivers draining glaciated landscapes.

Stress and System Energy in Erosion Process for Brittle Materials

Erosion tests on high strengh refractory castables were carried out using SiC grains at impact angles of 30°,45°,60°,and 90° with the velocity of 5m/s.In order to study the variation in stress and system energy with impact angles during solid particle erosion process,a single particle erosion model was designed by means of three-dimensional explicit dynamic software ANSYS/LS-DYNA according to experiment parameters. The Johnson-Holmquist brittle ceramic model was employed to model the failure of target material. The impact angles varied from 15° to 90° in increments of 15°.The simulation results were compared with erosion rate values from experiments. The results show that the variation trends of both the maximum stress of targets and system total energy loss are in a good agreement with experiment data,which increaes with increasing impact angle. The variation of erosion rate as a function of impact angle can be explained by the variation of the maximum stress of target material. The rule “the maximum erosion of typical brittle material occurs at 90°” is confirmed by the view of energy analysis.

Timescale dependence of glacial erosion rates: A case study of Marinelli Glacier, Cordillera Darwin, southern Patagonia

[1] Erosion rates have been estimated for a number of glaciated basins around the world, mostly based on modern observations (last few decades) of sediment fluxes to fjords. We use time-constrained sediment volumes delivered by Marinelli Glacier (55°S), an outlet glacier of the Cordillera Darwin ice cap, southern Patagonian Andes, Tierra del Fuego, to determine erosion rates across different timescales. Sediment volumes are derived using a dense grid of high- and low-frequency single channel seismic data and swath bathymetry data along with piston and Kasten cores. Our results show dramatic differences in erosion rates over different timescales. Erosion rates at Marinelli Glacier diminish about 80% (or by factor of ∼5) with each ten-fold increase in the time span over which erosion rates are averaged: 29.3 mm/yr for the last 45 years, 5.3 mm/yr for the last 364 years, and 0.5 mm/yr for the last 12,500 years. These results indicate that modern sediment yields and erosion rates from temperate tidewater glaciers can exceed long-term values over the time of deglaciation after the Last Glacial Maximum (centennial and millennial timescales) by up to 2 orders of magnitude. In view of the low exhumation rates of Cordillera Darwin (∼0.07 mm/yr average for the last 30 Myr), modern erosion rates could be up to 3 orders of magnitude higher than rates over geological time. We conclude that the pattern of erosion rate variation with time reflects the sensitivity of glaciers to climate variability.

Tectonic geomorphology along the eastern margin of Tibet: insights into the pattern and processes of active deformation adjacent to the Sichuan Basin

Abstract We present a review and synthesis of the tectonic geomorphology along the eastern margin of the Tibetan Plateau adjacent to and north of the Sichuan Basin. Re-evaluation of spatial variations in the form of fluvial longitudinal profiles provides a refined image of the distribution of anomalously steep channels. Three new analyses demonstrate that these variations in channel steepness reflect variations in the locus and rate of differential rock uplift. First, measurements of channel width along trunk streams reveal systematic co-variations in channel hydraulic geometry and slope that suggests channels are dynamically adjusted to spatial variations in erosion rate. Second, recent determinations of the functional relationship between channel steepness indices and erosion rate allow a quantitative estimation of erosion rate from channel profile form. Third, comparison of rock uplift patterns to variations in the distribution of slip associated with the 2008 Wenchuan earthquake confirms that channel gradients reflect differential rock uplift. Our analysis suggests that reactivated fault systems adjacent to the Sichuan Basin are primarily responsible for accommodating differential rock uplift, but that rock uplift northward along the margin is not associated with active faults and is likely sustained by flow and thickening in the deep crust.

Quantifying rock uplift rates using channel steepness and cosmogenic nuclide–determined erosion rates: Examples from northern and southern Italy

Rock uplift rates can be difficult to measure over 103–105 yr time scales. If, however, a landscape approaches steady state, where hillslope erosion and rock uplift rates are steady and locally similar, then it should be possible to quantify rock uplift rates from hillslope erosion rates. Here, we test this prediction by comparing channel steepness index values and 10Be catchment-averaged erosion rates to well-constrained rock uplift rates in two landscapes in Italy. The first field area is the Romagna Apennines, northern Italy, where rock uplift rates are relatively uniform, between 0.2 and 0.5 mm/yr (regional mean 0.40 ± 0.15 [SE] mm/yr), and have been steady since 0.9 Ma. The second area is the region around northeastern Sicily and the southernmost Italian peninsula, where rock uplift rates are higher and exhibit a strong spatial gradient, from ∼0.7 to ∼1.6 mm/yr (regional mean 1.09 ± 0.13 [SE] mm/yr). In both regions, channel steepness indices and 10Be erosion rates vary directly with rock uplift rates. Although there is considerable variability in erosion rates, regionally averaged rates in both the northern (0.46 ± 0.04 [SE] mm/yr) and southern (1.21 ± 0.24 [SE] mm/yr) areas accurately measure rock uplift rates. Although channel steepness indices do not quantify rock uplift rates, they are useful for (1) identifying regional patterns of rock uplift, (2) identifying areas where uplift rates might be expected to be uniform, and (3) informing 10Be sampling strategies. This study demonstrates that, together, channel steepness and hillslope erosion rates can provide a powerful tool for determining rock uplift rates.

Modeling of hydroecological feedbacks predicts distinct classes of landscape pattern, process, and restoration potential in shallow aquatic ecosystems

It is widely recognized that interactions between vegetation and flow cause the emergence of channel patterns that are distinct from the standard Schumm classification of river channels. Although landscape pattern is known to be linked to ecosystem services such as habitat provision, pollutant removal, and sustaining biodiversity, the mechanisms responsible for the development and stability of different landscape patterns in shallow, vegetated flows have remained poorly understood. Fortunately, recent advances have made possible large-scale models of flow through vegetated environments that can be run over a range of environmental variables and over timescales of millennia. We describe a new, quasi-3D cellular automata model that couples simulations of shallow-water flow, bed shear stresses, sediment transport, and vegetation dynamics in an efficient manner. That efficiency allowed us to apply the model widely in order to determine how different hydroecological feedbacks control landscape pattern and process in various types of wetlands and floodplains. Distinct classes of landscape pattern were uniquely associated with specific types of allogenic and autogenic drivers in wetland flows. Regular, anisotropically patterned wetlands were dominated by allogenic processes (i.e., processes driven by periodic high water levels and flow velocities that redistribute sediment), relative to autogenic processes (e.g., vegetation production, peat accretion, and gravitational erosion). These anistropically patterned wetlands are therefore particularly prone to hydrologic disturbance. Other classes of wetlands that emerged from simulated interactions included maze-patterned, amorphous, and topographically noisy marshes, open marsh with islands, banded string-pool sequences perpendicular to flow, parallel deep and narrow channels flanked by marsh, and ridge-and-slough patterned marsh oriented parallel to flow. Because vegetation both affects and responds to the balance between the transport capacity of the flow and sediment supply, these vegetated systems exhibit a feedback that is not dominant in most rivers. Consequently, unlike in most rivers, it is not possible to predict the “channel pattern” of a vegetated landscape based only on discharge characteristics and sediment supply; the antecedent vegetation pattern and vegetation dynamics must also be known. In general, the stability of different wetland pattern types is most strongly related to factors controlling the erosion and deposition of sediment at vegetation patch edges, the magnitude of sediment redistribution by flow, patch elevation relative to water level, and the variability of erosion rates in vegetation patches with low flow-resistance. As we exemplify in our case-study of the Everglades ridge and slough landscape, feedback between flow and vegetation also causes hysteresis in landscape evolution trajectories that will affect the potential for landscape restoration. Namely, even if the hydrologic conditions that historically produced higher flows are restored, degraded portions of the ridge and slough landscape are unlikely to revert to their former patterning. As wetlands and floodplains worldwide become increasingly threatened by climate change and urbanization, the greater mechanistic understanding of landscape pattern and process that our analysis provides will improve our ability to forecast and manage the behavior of these ecosystems.

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.

Effects of Coating Thickness, Test Temperature, and Coating Hardness on the Erosion Resistance of Steam Turbine Blades

This paper experimentally examines the influence of coating thickness, test temperature, coating hardness, and defects on the erosion resistance of boride coatings, ion plating CrN coatings, and thermal spraying coatings. The results demonstrate that the erosion rate of coating can be reduced effectively by improving coating hardness and thickness with the absence of the cracks of coating during the coating process. In comparison with thermal spraying coatings, boride coatings and ion plating CrN coatings are more suitable for protecting steam turbine blades from solid particle erosion due to higher erosion resistance. However, blades cannot be protected effectively when coating is thinner than a critical value θcrit. Based on our results, it is recommended that the protective coating for the steam turbine blade should be thicker than 0.02 mm. In addition, the effect of temperature on erosion resistance of the coating is strongly dependent on the properties of transition layer between coating and substrate material. For the coating without pinholes or pores in the transition layer, the variation in erosion rate with temperature is consistent with that of uncoated substrate material. However, the erosion rate of coating descends with the elevation of test temperature when a lot of pinholes or pores are produced in the transition layer.

Integrated geomorphic and geodynamic modeling of a potential blind thrust in the San Francisco Bay area, California

Geometries and slip budgets of the faults in the San Francisco Bay area imply previously unrecognized fault linkages, including examples of blind thrust structures that appear to connect segments of strike-slip faults and accommodate along-strike variations in slip rate along these structures. Displacement along linking faults may be associated with the development of topography and also may serve as earthquake sources. In Marin County, California, systematic spatial patterns in landscape topography and geomorphic indices suggest that the region north of Mt. Tamalpais is experiencing differential rock uplift. We suggest that a blind thrust underlies the elevated area, creating the observed topography and possibly resolving a slip discrepancy between the Hayward and San Andreas Fault in this region. We have developed and implemented an integrative approach that combines observations from tectonic deformation and geomorphic properties to identify a potential blind thrust beneath Marin County. Elastic displacement modeling has been tested for compatibility with the blind thrust hypothesis and to assess the sensitivity of observables to fault geometry and orientation; from this, a set of plausible blind thrust structures are defined. We use a range of empirical relationships between channel steepness index and erosion rate to estimate spatial variations in erosion rate along Bolinas Ridge. By coupling these erosion estimates with elastic displacement fault modeling, we can use the resulting topographic envelopes to constrain the rate and duration of deformation. These constraints, along with spatial bounds on the possible fault models, are used to calculate potential seismic moment and moment magnitude. With an assumed recurrence interval of ~ 100 years, such blind thrusts can produce a M w ~ 6.3 earthquake, while a longer recurrence time (~ 1000 years) results in a maximum M w ~ 7.0 earthquake. Although such events are not likely to be catastrophic, they are large enough to cause significant strong ground motions in the San Francisco Bay area.

INFLUENCE OF AIR-JET EROSION ON THE DETONATION-GUN SPRAYED ALUMINA-ZIRCONIA CERAMIC COMPOSITE COATINGS

Erosion due to solid particles impact has a detrimental effect on the performance of components used in various applications. Hard facing is widely used to combat wear in such situations. D-gun coating is increasingly popular for such applications. Aluminazirconia composite has been deposited on mild steel substrate. The coating has been characterized and their tribological responses have been evaluated. Resistance to erosion, evaluated through standard dry erosion test is highlighted in this work. An orthogonal full–factorial experimental matrix which includes all possible variables that contribute to brittle erosion was designed, conducted and validated. The variation of erosion rate with process parameters was mathematically modeled by using the regression analysis method.

Terrain maps displaying hill-shading with curvature

Many types of maps can be created by neighborhood operations on a continuous surface such as provided by a digital elevation model. These most commonly include first derivatives slope or aspect, and second derivatives planimetric or profile curvature. Such variables are often used in geomorphic analyses of terrain. First derivatives also provide subtle enhancements to hill-shaded maps. For example, some maps combine oblique and vertical illumination, with the latter reflecting variations in slope. This study illustrates how second derivative maps, in conjunction with hill-shading, can cartographically enhance topographic detail. A simple conic model indicates that image-tone edges where slope or aspect varies by less than 0.5° are visible on curvature maps. Hill-shaded images combined with curvature enhance the continuity of naturally occurring tonal edges, especially in strongly illuminated areas. Variations in planimetric and profile curvature seem to be especially effective at highlighting convergent and divergent drainages and variations in erosion rate between or within sedimentary units, respectively. Shading curvature with consideration given to illumination models can add detail to hill-shaded terrain maps in a manner similar to cognitive models employed by map viewers.

Practical implications of quantifying ancient sedimentation rates in lakes

S ediments stored in lakes represent a valuable archive that can be used to reveal the erosion history of watersheds. A portion of the soil that is moved down gradient during runoff events is deposited in lakes, and the rate at which sediment accumulates should be proportional to the rate of erosion from the surrounding land. The ability to calculate historical sedimentation rates opens several opportunities for improved understanding and management of watershed processes. Examples include quantifying changes in erosion and deposition rates caused by anthropogenic or natural shifts in land use or by climate change. Erosion is difficult to quantify at the watershed scale because of the large degree of spatial and temporal variation in erosion rates. Changes in slope, vegetative cover, soil compaction and composition, and a host of other variables can result in a wide range of erosion rates over distances of a few meters. Erosion rates can likewise vary significantly with changes in rainfall intensity and time since the last precipitation event. Lakes receiving this variable influx have a normalizing effect that allows long-term changes in sediment accumulation rates, and thus changes in erosion at the watershed scale, to be identified. Changes in erosion at scales smaller …

Single-grain cosmogenic 21Ne concentrations in fluvial sediments reveal spatially variable erosion rates

We evaluated the hypothesis that the spatial variation in erosion in a catchment is reflected in the distribution of the cosmogenic nuclide concentrations in sediments leaving the catchment. Using published data and four new 10Be measurements in fluvial sediment collected from the outlets of small river catchments, we constrained the spatial variability of erosion rates in the Gaub River catchment in Namibia. We combined these catchment-averaged erosion rates, and the mean slope values with which they are associated, in a digital elevation model (DEM)–based analysis to predict distributions of cosmogenic 21Ne concentrations in the sediment leaving the Gaub catchment. We compared these synthetic distributions with the distribution of concentrations of cosmogenic 21Ne (21NeC) in 32 quartz fluvial pebbles (16–21 mm) collected from the catchment outlet. The 21NeC concentrations span nearly two orders of magnitude (2.6–160 × 106 atoms/g) and are highly skewed toward low values. The DEM-based analysis confirms this skew—the measured 21NeC distribution plots within the envelope of distributions predicted for the catchment. This match between measured and synthetic 21Ne distributions implies that the measured distribution is a signature of the spatial variation in erosion rates.

Erosion and landscape development decouple strontium and sulfur in the transition to dominance by atmospheric inputs

Weathering and leaching can progressively deplete the pools of soluble, rock-derived elements in soils and ecosystems over millennial time-scales, such that productivity increasingly relies on inputs from atmospheric deposition. This transition has been explored using strontium isotopes, which have been widely assumed to be a proxy for the provenance of other rock-derived elements. We compared rock versus atmospheric proportions of strontium to those for sulfur, a plant macronutrient, at several tropical forest sites in Hawaii and Costa Rica. Isotopic analyses reveal that sulfur is often decoupled from strontium in the transition to atmospheric dependence. Decoupling is likely the result of differences in chemical factors such as atmospheric input rates, mobility in the soil environment, and mineral weathering susceptibility. Strontium and sulfur decoupling appears to be accentuated by the physical process of erosion. Erosion rates are presumed to be high on the Osa Peninsula of Costa Rica, where the recent onset of rapid tectonic uplift has placed the landscape in a transient state. Decoupling is strong there, as erosion has rejuvenated the supply of rock-derived strontium but not sulfur. The landscape response to changes in tectonic uplift on the Osa Peninsula has produced decoupling at the landscape scale. Decoupling is more variable along a Hawaiian catena, presumably due to smaller scale variations in erosion rates and their influence on rejuvenation of rock-strontium inputs. These results illustrate how chemical and physical processes can interact to produce contrasting origins for different nutrient elements in soils and the ecosystems they support.

Interpreting erosion rates from cosmogenic radionuclide concentrations measured in rapidly eroding terrain

AbstractA combination of numerical analysis and 10Be concentrations measured in sediment samples from the high‐relief Torrente catchment, southern Spain, allows us to investigate the sampling requirements for determining erosion rates using cosmogenic nuclides in high‐relief, landslide‐dominated terrain. We use simple modelling to quantify the effect of particle spalling and/or landsliding on erosion rates determined using a cosmogenic in‐situ produced isotope. Analytical results show that the cosmogenic nuclide concentration of a surface experiencing regular detachment of a grain or block may be considered to be in steady state, and ‘in‐situ’ erosion rates estimated, when an appropriate number of spatially independent samples are amalgamated. We present equations that enable calculation of the number of bedrock samples that must be amalgamated for the estimation of mean erosion rates on an outcrop experiencing regular detachment of a grain or chip of thickness L every T years. Our findings confirm that mean catchment erosion rates may be reliably estimated from 10Be concentrations in fluvial sediment in high‐relief rapidly eroding terrain. These catchment‐wide integrated erosion rates can be calculated where erosion is primarily accomplished through shallow (<3 m) spalling processes; where deep‐seated (>3 m) landslides are the dominant mode of erosion only minimum erosion rates can be determined. Lastly, we present erosion rate measurements from the Torrente catchment that reveal variation of two orders of magnitude (0·03–1·6 m ka−1) quantifying the high degree of spatial variation in erosion rates expected within rapidly uplifting catchments. Copyright © 2006 John Wiley & Sons, Ltd.

Quantification of chemical weathering rates across an actively eroding hillslope

Chemical weathering rates for landscapes are difficult to quantify because the timescales over which weathering occurs are often unknown. In this study, we use timescales defined by prior cosmogenic nuclide analyses and a suite of geochemical measurements to calculate weathering rates in saprolite and soil and to determine how these rates vary across an eroding hillslope. We also estimate the relative contributions of solute and erosional mass loss to landscape lowering. Analyses were conducted on a soil-mantled hillslope developed on a granodiorite pluton in southern NSW, Australia. Mass loss in solution accounts for 35% to 55% of the total mass loss from the hillslope. Saprolite close to the soil–saprolite boundary is less weathered at the ridge than at distance from the ridge. The calculated flux of silica from the saprolite is 5 tons km− 2 yr− 1 and does not vary with distance from the ridge or overlying soil thickness. The assumption of steady state, but spatially variable erosion rates with a temporally constant thickness of soil and saprolite allows us to calculate the downslope variation in the rate of solute losses. Soil silica weathering rates initially increase from ∼12 tons km− 2 yr− 1 at the ridge crest to ∼20 tons km− 2 yr− 1 on the convex region of the hillslope, and then decrease to 13 tons km− 2 yr− 1 by the time soils have been transported 45–50 m. Soils closer to the surface are more weathered than deeper soils. This geochemical stratification suggests that the soil column does not become thoroughly vertically mixed as soils move down slope. Dependence of solute loss on transport distance may explain the previously noted discrepancy between the observed soil thickness and that predicted based on curvature. Variation of solute loss with transport distance may also contribute to the linear dependence of soil thickness on distance from the ridge.