Catchment-averaged Erosion Rates Research Articles

Quartz luminescence sensitivity enhanced by residence time in the critical zone

The emerging use of quartz luminescence properties to characterize Earth-surface processes shows promise, with optically stimulated luminescence (OSL) sensitivity proposed as a valuable tool for provenance or sediment history tracing. However, the geologic processes that lead to quartz sensitization remain unclear. Here we study the impact of source rock and surface processes on the luminescence properties of quartz sand from bedrock and modern and Late Pleistocene alluvium generated from a mountainous catchment in northern Utah, USA. Continuous wave and linear modulated OSL are used to characterize the luminescence sensitivity and intensity of the fast-decay component. We compare the OSL sensitivity with sand-grain provenance and with proxies for surface processes such as topographic metrics, cosmogenic 10Be-derived erosion rates, chemical weathering indices, and magnetic susceptibility. Late Pleistocene sediment has low OSL sensitivity and a weak fast-decay component, similar to bedrock samples from the source area. In contrast, modern alluvium is dominated by the fast-decay component and has higher and more variable OSL sensitivity, with no clear relationship to upstream bedrock source. There is, however, an inverse relationship between OSL sensitivity and catchment-averaged erosion rates and a positive relationship with chemical weathering indices and magnetic susceptibility. These metrics suggest that the modern alluvium has experienced increased residence time in the shallow critical zone compared to the Late Pleistocene sediments. We suggest that changes in hillslope processes between the effectively wetter, cooler Pleistocene and the dryer, warmer conditions of the Holocene enhanced the luminescence properties. The results suggest that climatic controls on rates and processes of chemical and mechanical weathering and sediment transport and residence within the critical zone are encoded in the luminescence properties of quartz sand.

Fault activity in the San Gabriel Mountains, southern California, USA: Insights from landscape morphometrics, erosion rates, and fault-slip rates

Many studies use landscape form to determine spatial patterns of tectonic deformation, and these are particularly effective when paired with independent measures of rock uplift and erosion. Here, we use morphometric analyses and 10Be catchment-averaged erosion rates, together with reverse slip rates from the Sierra Madre−Cucamonga fault zone, to reveal patterns in uplift, erosion, and fault activity in the range front of the San Gabriel Mountains in southern California, USA. Our analysis tests two prevailing hypotheses: (1) the range front of the San Gabriel Mountains is at steady state, in which rock uplift balances erosion and topographic elevations are stable throughout time, and (2) that west-to-east increases in elevation, relief, erosion rate, and stream-channel steepness across the interior of the massif reflect a parallel reverse-slip rate gradient on the range-bounding Sierra Madre−Cucamonga fault zone. We show that although deviations from steady state occur, the range-front hillslopes and stream channels are typically both well-connected and adjusted to patterns in Quaternary uplift driven by motion on the range-front fault network. Accordingly, landscape morphometrics, 10Be erosion rates, and model erosion rates effectively image spatial and temporal patterns in uplift. Interpreted jointly, these data reveal comparable peak slip rates on the Sierra Madre−Cucamonga fault zone and show that they do not monotonically increase from west to east. Thus, the eastward-increasing gradients developed within the interior of the massif are not solely related to reverse slip on the range-front faults. Evaluated on shorter length scales (<10 km), morphometric data corroborate earlier descriptions of the Sierra Madre−Cucamonga fault zone as multiple individual faults or fault sections, with slip rates tapering toward fault tips. We infer that these patterns imply the predominance of independent fault or fault section ruptures throughout the Quaternary, though data cannot rule out the possibility of large, connected Sierra Madre−Cucamonga fault zone ruptures. Deeper in the hanging wall of the Sierra Madre−Cucamonga fault zone, secondary faults accommodate range-front uplift. Motion on these faults may contribute to active uplift of the highest topography within the massif, in addition to partly reconciling differences between geologic and geodetic Sierra Madre−Cucamonga fault zone reverse-slip rates. This study provides a new, unified perspective on tectonics and landscape evolution in the San Gabriel Mountains.

Steady-state forms of channel profiles shaped by debris flow and fluvial processes

Abstract. Debris flows regularly traverse bedrock channels that dissect steep landscapes, but our understanding of bedrock erosion by debris flows and their impact on steepland morphology is still rudimentary. Quantitative models of steep bedrock channel networks are based on geomorphic transport laws designed to represent erosion by water-dominated flows. To quantify the impact of debris flow erosion on steep channel network form, it is first necessary to develop methods to estimate spatial variations in bulk debris flow properties (e.g., flow depth, velocity) throughout the channel network that can be integrated into landscape evolution models. Here, we propose and evaluate two methods to estimate spatial variations in bulk debris flow properties along the length of a channel profile. We incorporate both methods into a model designed to simulate the evolution of longitudinal channel profiles that evolve in response to debris flow and fluvial processes. To explore this model framework, we propose a general family of debris flow erosion laws where erosion rate is a function of debris flow depth and channel slope. Model results indicate that erosion by debris flows can explain the occurrence of a scaling break in the slope–area curve at low-drainage areas and that upper-network channel morphology may be useful for inferring catchment-averaged erosion rates in quasi-steady landscapes. Validating specific forms of a debris flow incision law, however, would require more detailed model–data comparisons in specific landscapes where input parameters and channel morphometry can be better constrained. Results improve our ability to interpret topographic signals within steep channel networks and identify observational targets critical for constraining a debris flow incision law.

Delayed 10Be dilution in detrital quartz following extensive coseismic landsliding: A 2016 Kaikōura earthquake case study

Large earthquakes play a major role in the topographic evolution of active orogens. Earthquake induced landslides can alter the mass balance of mountain belts for decades to centuries depending on how landslide material is transported and stored. In this study, we analysed in-situ10Be concentrations in fluvial sediments to capture a time series of post-earthquake denudation rates of the Tūtae Putaputa|Conway River catchment, which experienced ∼13 × 106 m3 of landsliding in the 2016 Kaikōura earthquake. 10Be concentrations were determined for detrital samples from the Conway River at the rangefront of the Seaward Kaikōura Mountains, South Island, New Zealand, and near the catchment outlet over three sampling intervals, including after significant precipitation events, 1-2 yr following the earthquake. Our results indicate somewhat higher apparent erosion rates at the rangefront (0.41-0.58 mm/yr) when compared to erosion rates incorporating lower-relief sub-catchments at the river outlet (∼0.28 mm/yr). No changes were found over the same time period in the Hurunui River, a neighbouring catchment that experienced negligible coseismic landsliding during the Kaikōura event. Apparent catchment-averaged erosion rates in the Conway River have remained similar to long-term exhumation rates from previously published thermochronology studies. To explain our results, we explore the observed controls on catchment-wide 10Be concentrations in our study area, such as the spatiotemporal variability in landslide connectivity to the channel and storage within the catchment. Our results highlight that using in-situ cosmogenic 10Be in fluvial deposits as a proxy for changes in sediment fluxes of earthquake-affected landscapes is complex and highly site-specific.

Tectonic controls on Quaternary landscape evolution in the Ventura basin, southern California, USA, quantified using cosmogenic isotopes and topographic analyses

AbstractThe quantification of rates for the competing forces of tectonic uplift and erosion has important implications for understanding topographic evolution. Here, we quantify the complex interplay between tectonic uplift, topographic development, and erosion recorded in the hanging walls of several active reverse faults in the Ventura basin, southern California, USA. We use cosmogenic 26Al/10Be isochron burial dating and 10Be surface exposure dating to construct a basin-wide geochronology, which includes burial dating of the Saugus Formation: an important, but poorly dated, regional Quaternary strain marker. Our ages for the top of the exposed Saugus Formation range from 0.36 +0.18/-0.22 Ma to 1.06 +0.23/-0.26 Ma, and our burial ages near the base of shallow marine deposits, which underlie the Saugus Formation, increase eastward from 0.60 +0.05/-0.06 Ma to 3.30 +0.30/-0.41 Ma. Our geochronology is used to calculate rapid long-term reverse fault slip rates of 8.6–12.6 mm yr–1 since ca. 1.0 Ma for the San Cayetano fault and 1.3–3.0 mm yr–1 since ca. 1.0 Ma for the Oak Ridge fault, which are both broadly consistent with contemporary reverse slip rates derived from mechanical models driven by global positioning system (GPS) data. We also calculate terrestrial cosmogenic nuclide (TCN)-derived, catchment-averaged erosion rates that range from 0.05–1.14 mm yr–1 and discuss the applicability of TCN-derived, catchment-averaged erosion rates in rapidly uplifting, landslide-prone landscapes. We compare patterns in erosion rates and tectonic rates to fluvial response times and geomorphic landscape parameters to show that in young, rapidly uplifting mountain belts, catchments may attain a quasi-steady-state on timescales of <105 years even if catchment-averaged erosion rates are still adjusting to tectonic forcing.

Erosion rates and weathering timescales in the eastern Great Escarpment, South Africa

The eastern escarpment in South Africa has a combination of geology and climate that is unique in the entire Great Escarpment. Yet, no studies have been undertaken before to quantify the landscape changes at this locality. In this study, we assess the denudation history of the eastern escarpment using cosmogenic beryllium-10 (10Be) on quartz from rock outcrops and river sediments, and carry out uranium‑thorium‑helium ((U,Th)-He) and argon‑argon (40Ar/39Ar) dating of Fe-oxides and Mn-oxides, respectively, in the soils. The erosion rates obtained on the eastern escarpment vary from 1.8 m/Ma to 24 m/Ma and are similar in range to values from the entire Great Escarpment. We found that the catchment-averaged erosion rates of the gentle catchments above the eastern escarpment are lower, whereas those from steep catchments draining the escarpment edge are higher. We also determined that the catchment-average erosion rates of the eastern escarpment are similar to those of the western escarpment in Namibia, lower than those of the Drakensberg Escarpment and lower than those of the Lowveld adjacent to it. The 40Ar/39Ar dating of cryptomelane-bearing Mn nodules yield Palaeoproterozoic ages unrelated to the pedogenic processes, and might indicate that these nodules are not pedogenic, but inherited from weathered bedrock. The (U,Th)-He ages of goethite concretions range from 0.85 Ma to 1.05 Ma and they date the last period of intensive chemical weathering on the eastern escarpment, which coincides with the Mid-Pleistocene Transition from humid to more arid conditions on the southern African subcontinent. Given the low erosion rates above the escarpment, an area that experiences intensive chemical weathering, and the high erosion rates down the escarpment we conclude that at the eastern escarpment, pediplanation is a more important driver of denudation than peneplanation.

Millennial-scale erosion patterns of the northern Qinling Mountains, Central China: Implications for topographical evolution

The northern Qinling Mountains stretch from east to west in Central China. They have undergone distinct surface uplift during the Late Cenozoic and have become the geographical boundary between northern and southern China. To investigate the topographic evolution of the northern Qinling Mountains over millennial timescales and to explore the major controlling factors of relief development we have calculated 10Be-derived catchment-averaged erosion rates. Results show that the catchment-averaged erosion rates ranged between ~67.4 and 327 m Ma−1. These erosion rates are nonlinearly correlated with topographic parameters, indicating that topographic steepness controls the spatial variability of the erosion rates. Comparison of local erosion rates reveals that lithology and precipitation have limited influence. Instead, the spatial distribution of erosion in the northern Qinling Mountains is primarily controlled by active tectonics, which can be attributed to uplift along the Qinling Piedmont Fault and the Taibai Fault. Due to the intensive tectonic activity of the Qinling Piedmont Fault since the Late Miocene, the Heihe River Basin has developed as a transient landscape. Finally, comparison between the vertical fault slip rates of the Qinling Piedmont Fault and erosion rates indicates that the topography of the northern Qinling Mountains has undergone continuous uplift and growth since at least the Late Pleistocene. Our results suggest that the northeastward growth of the Tibetan Plateau has regulated the modern erosion of the northern Qinling Mountains and shaped their modern topography.

Erosion rates and patterns in a transient landscape, Grand Staircase, southern Utah, USA

AbstractCosmogenic 10Be concentrations in alluvial sediment are widely used to infer long-term, catchment-averaged erosion rates based on the assumption that the landscape is in mass-flux steady state. However, many landscapes are out of equilibrium over millennial time scales due to tectonic and climatic forcing. The Grand Staircase of the Colorado Plateau (North America) is a transient landscape, adjusting to base-level fall from the carving of the Grand Canyon, and is characterized by cliff-bench topography caused by differential erosion of lithologic units. The 10Be concentrations from 52 alluvial and colluvial samples, collected in nested fashion from five catchments, produced inferred erosion rates ranging from 20 to >3500 m/m.y. (or mm/k.y.). We attribute this high variance in part to lithologic-controlled steepness and hotspots of erosion related to cliff retreat along the White Cliffs (escarpment near Mt. Carmel Junction, Utah), as well as headward drainage expansion along the uppermost Pink Cliffs (escarpment within Bryce Canyon National Park). Results from the downslope Vermillion Cliffs (near Kanab) indicate lower erosion rates despite similar slope and rock types, suggesting knick-zone migration has passed that lower region of our study area. The 10Be concentrations measured along trunk streams systematically match local, subcatchment erosion rates, with muted influence from upstream sediment sources. This is consistent with intermittent sediment conveyance between cliff and bench terrain, with sediment storage and localized release associated with ephemeral arroyo systems in the region. Therefore, while detrital cosmogenic nuclide records in transient landscapes may not directly reflect upstream catchment-averaged erosion rates, 10Be inventories can provide insight into unsteady upslope-directed erosion and downslope-directed sediment conveyance in these dynamic landscapes.

Tipping the balance: Shifts in sediment production in an active rift setting

Landscapes in actively developing rifts respond to tectonic forcing over a similar time scale to that of fault array evolution (i.e., 105–106 yr). Consequently transient landscapes (i.e., not in topographic steady state) predominate, characterized by focused incision along extensional fault scarps and regional tectonic tilting of surface slopes across strike. Using a field-calibrated numerical model to explore the controls on landscape evolution across the Corinth rift, central Greece, we demonstrate that this tilting, although subtle, leads to a shift in dominant source area as well as a shift toward sediment-starved conditions within the basin. We show, by comparing model runs with and without imposing tectonic forcing, that the impact of active faulting on relief development along the most active Corinth rift margin locally increases erosion rates and footwall incision. However, the overall sediment flux from this margin is reduced because back-tilting lowers erosion rates in catchment headwaters. Conversely, the hanging-wall side of the rift, as it is downwarped, supplies relatively more sediment as rift-directed channel slopes increase even though the relief is decreasing. In summary, we show that tilting plays a key role in controlling the syn-rift sediment flux and, in a counterintuitive way, modifies the relationship between topographic relief and catchment-averaged erosion rates. Our results provide a new perspective on the origin and timing of sediment starvation relative to structural development in rifts.

Controls on the erosion of the continental margin of southeast Brazil from cosmogenic 10Be in river sediments

The Atlantic Ocean coast region of southeast Brazil contains two coast-parallel mountain ranges (the Serra do Mar and Serra da Mantiqueira) generated by tectonic activity pulses tens of millions years after the main continental rift event occurred around 120 Ma. Although the short-term erosion rates for the region are established, the relative importance of the factors controlling erosion is poorly constrained. We combine new and published catchment-averaged erosion rates (n = 48) using in situ-produced 10Be concentrations in quartz from river sediments to establish the regional erosion pattern. The river catchments are (i) escarpment topography, (ii) high-altitude low-relief and (iii) mixed topography, which record how escarpment fronts are migrating inland. Ocean-facing coastal escarpment catchments of the Serra do Mar (ε = 18–53 m/Ma) can be eroded approximately twice as fast as continent-facing escarpment catchments in the Serra do Mar and Serra da Mantiqueira (ε = 7–24 m/Ma). The correlation between the normalized channel steepness index (ksn) and slope angle indicates that river incision and hillslope erosion processes combine to maintain the high relief. The Serra do Mar catchments define a mean slope angle threshold indicating that landslides are the dominant erosional process when slope angles in excess of ~30°. Tectonic activity is low and plays no significant role in driving erosion. A first-order relationship between erosion rate and precipitation-temperature across the region implies that climate plays a key role in soil production, river incision and in triggering erosional processes. Although the high topographic relief is a pre-condition for the occurrence of significant erosion, the climatic condition is the outlining factor of the regional variation in erosion rates.

Coupling erosion and topographic development in the rainiest place on Earth: Reconstructing the Shillong Plateau uplift history with in-situ cosmogenic 10Be

The uplift of the Shillong Plateau, in northeast India between the Bengal floodplain and the Himalaya Mountains, has had a significant impact on regional precipitation patterns, strain partitioning, and the path of the Brahmaputra River. Today, the plateau receives the highest measured yearly rainfall in the world and is tectonically active, having hosted one of the strongest intra-plate earthquakes ever recorded. Despite the unique tectonic and climatic setting of this prominent landscape feature, its exhumation and surface uplift history are poorly constrained. We collected 14 detrital river sand and 3 bedrock samples from the southern margin of the Shillong Plateau to measure erosion rates using the terrestrial cosmogenic nuclide 10Be. The calculated bedrock erosion rates range from 2.0 to 5.6 m My−1, whereas catchment average erosion rates from detrital river sands range from 48 to 214 m My−1. These rates are surprisingly low in the context of steep, tectonically active slopes and extreme rainfall. Moreover, the highest among these rates, which occur on the low-relief plateau surface, appear to have been affected by anthropogenic land-use change. To determine the onset of surface uplift, we coupled the catchment averaged erosion rates with topographic analyses of the plateau's southern margin. We interpolated an inclined, pre-incision surface from minimally eroded remnants along the valley interfluves and calculated the eroded volume of the valleys carved beneath the surface. The missing volume was then divided by the volume flux derived from the erosion rates to obtain the onset of uplift. The results of this calculation, ranging from 3.0 to 5.0 Ma for individual valleys, are in agreement with several lines of stratigraphic evidence from the Brahmaputra and Bengal basin that constrain the onset of topographic uplift, specifically the onset of flexural loading and the transgression from deltaic to marine deposition. Ultimately, our data corroborate the hypothesis that surface uplift was decoupled from the onset of rapid exhumation, which occurred several millions of years earlier.

Influence of anthropogenic land‐use change on hillslope erosion in the Waipaoa River Basin, New Zealand

AbstractEuropean settlement of the Poverty Bay Region resulted in deforestation and conversion of > 90% of the landscape to pastureland. The resulting loss of vegetation triggered a rapid increase in hillslope erosion as widespread landslide complexes and gully systems developed on weak lithologic units in the Waipaoa Basin. To quantify the rate and volume of historic hillslope degradation, we used a 1956–2010 sequence of aerial photographs for a ~16 km2 catchment to map temporal changes in the spatial extent of active landslides. Then we created a ‘turf index’ based on the extent and style of pastoral ground disruption, which correlates with downslope velocity. Based on the movement of trees and other features, we assigned average velocities to the turf classes as follows: (1) minimal disrupted ground: 0.6 m/yr, (2) a mix of disrupted ground and intact blocks: 3.4 m/yr, and (3) no intact blocks or vegetation: > 6 m/yr. We then calculated the average annual sediment flux using these turf‐derived velocities, the width of the landslide‐channel intersection, and an average toe depth of 4.4 ± 1.3 m (mean ± standard deviation [SD]) from 37 field measurements. The resulting catchment averaged erosion rates are (mean ± SD): 29.9 ± 12.9 mm/yr (1956), 28.8 ± 13.7 mm/yr (1969), 13.4 ± 4.9 mm/yr (1979), 17.0 ± 6.2 mm/yr (1988), and 9.9 ± 3.6 mm/yr (2010). Compared with long‐term (post‐18 ka) erosion rates (1.6 mm/yr) and the long‐term uplift rate (~1 mm/yr) for this site, the 50‐year anthropogenically‐driven rate is an order of magnitude larger (~20 mm/yr). Previously, we measured an increase in erosion over the past 3.4 kyr (2.2 mm/yr), and here, we demonstrate this increase could be primarily due to human land‐use change – showing that a century of rapid erosion superimposed on the background geologic rate can profoundly skew the interpretation of erosion rates. Copyright © 2016 John Wiley & Sons, Ltd.

Post-tectonic landscape evolution of a coupled basin and range: Pinaleño Mountains and Safford Basin, southeastern Arizona

The Pinaleno Mountains and adjacent Safford Basin are a landscape defined by the extensional tectonics of the Basin and Range physiographic province. However, over the last ∼4 m.y., this coupled basin and range have been actively degrading in the absence of widespread regional extension. While rates of relief generation and upland erosion during active subsidence ca. 12–5 Ma are reflected in the geometry of the basin’s structure and the stratigraphy it contains, rates of post-tectonic landscape evolution from the Pliocene to the present have been heretofore unknown. We combined topographic analyses of the Pinaleno Mountains with cosmogenic nuclide–derived catchment-averaged erosion rates and burial dates of axial and piedmont deposits to quantify rates of post-tectonic landscape evolution and define a chronology for the last stages of deposition and subsequent incision in Safford Basin. In addition to constraining the timing of a deposit’s formation, cosmogenic nuclide burial dates provide paleo-upland erosion rates at the time of deposition. Erosion rates in the Pinaleno Mountains have been generally moderate over the past 4 m.y., ranging between ∼30 and 60 m/m.y. with no strong relationship to the drainage basins’ modern topography. A potential acceleration of erosion rates to 100–250 m/m.y. between 3.5 and 2 Ma correlates with an inferred period of enhanced precipitation as well as the arrival from upstream of the Gila River in Safford Basin sometime shortly before 2.8 Ma. Widespread incision of Safford Basin was under way by ca. 2 Ma, as recorded by the dissection of piedmont basin highstand deposits (Frye Mesa) and two intermediate Gila River terraces on the northeast margin of Safford Basin (dated to 1.8 Ma and 0.64 Ma). Gila River incision rates have ranged from 30 to 60 m/m.y. over the past 3 m.y. Paleo-upland erosion rates and modern millennial-scale upland erosion rates fall within the same range as incision rates of the Gila River in Safford Basin, suggesting that upland erosion rates are predominantly a function of base-level fall driven by axial incision. However, based on similarities between catchment-averaged erosion rates and topography from basins draining into the integrated Safford Basin and the still internally drained Sulphur Springs Basin to the south, it appears that upland erosion rates during the Quaternary are not being driven exclusively by regional incision rates.

Spatial variability of 10Be-derived erosion rates across the southern Peninsular Indian escarpment: A key to landscape evolution across passive margins

The persistence of significant topography in ancient, tectonically inactive orogenic belts remains one of the outstanding questions in geomorphology. In southern Peninsular India, the impressive topographic relief of the Western Ghat Mountains in tectonic quiescence since at least ca. 65 Ma has raised important questions concerning the long-term mechanism of topographic evolution. Quantifying the distribution of erosion in space and time is critical to understanding landscape evolution. Although the long-term erosion rates are reasonably well known, the short-term erosion rates and the relative importance of factors controlling erosion in southern Peninsular India are less well constrained. We present a new suite of catchment-averaged and local erosion rates using in situ produced 10Be concentrations in river sediments and exposed bedrock samples in southern Peninsular India. Catchment-averaged erosion rates vary from 9.6±0.8 mMa−1 in the highlands to 114.3±13.8 mMa−1 on the escarpment side. Bedrock erosion rates range from 2.4±0.2 mMa−1 in the ridge-top to 143.4±25.4 mMa−1 in active channel beds of the highlands. Catchment-averaged erosion rates derived from the across-escarpment, westward-draining catchments are significantly higher than those derived from the eastward-draining, over highland catchments. The difference indicates that long-term down-wearing of the highland proceeds at lower rates than in the escarpment zones. Catchment-averaged erosion rates are moderately correlated with mean hillslope angles and local relief whereas they are strongly correlated with catchment-averaged channel steepness index. This suggests that topographic steepness is the major control on the spatial variability of erosion while strong rainfall gradient is of minor importance in this area. 10Be-derived average erosion rates in highlands are consistent with previous long-term erosion rate estimated from thermochronometry. These results collectively point to large-scale steady-state topography, only decaying slowly with time. Steady state likely reflects the balance between erosion and isostatically driven uplift of the southern Peninsular India.

Shaping post-orogenic landscapes by climate and chemical weathering

Research Article| November 01, 2013 Shaping post-orogenic landscapes by climate and chemical weathering Oliver A. Chadwick; Oliver A. Chadwick * 1Department of Geography, University of California–Santa Barbara, Santa Barbara, California 93106, USA *E-mail: oac@geog.ucsb.edu. Search for other works by this author on: GSW Google Scholar Josh J. Roering; Josh J. Roering 2Department of Geological Sciences, University of Oregon, Eugene, Oregon 97403, USA Search for other works by this author on: GSW Google Scholar Arjun M. Heimsath; Arjun M. Heimsath 3School of Earth and Space Exploration, Arizona State University, Tempe, Arizona 85287, USA Search for other works by this author on: GSW Google Scholar Shaun R. Levick; Shaun R. Levick 4Max Plank Institute for Biogeochemistry, Hans-Knöll-Strasse 10, 07745 Jena, Germany Search for other works by this author on: GSW Google Scholar Gregory P. Asner; Gregory P. Asner 5Department of Global Ecology, Carnegie Institution for Science, Stanford, California 94305, USA Search for other works by this author on: GSW Google Scholar Lesego Khomo Lesego Khomo 6Department of Biological Sciences, University of Cape Town, Rondebusch 7701, South Africa Search for other works by this author on: GSW Google Scholar Author and Article Information Oliver A. Chadwick * 1Department of Geography, University of California–Santa Barbara, Santa Barbara, California 93106, USA Josh J. Roering 2Department of Geological Sciences, University of Oregon, Eugene, Oregon 97403, USA Arjun M. Heimsath 3School of Earth and Space Exploration, Arizona State University, Tempe, Arizona 85287, USA Shaun R. Levick 4Max Plank Institute for Biogeochemistry, Hans-Knöll-Strasse 10, 07745 Jena, Germany Gregory P. Asner 5Department of Global Ecology, Carnegie Institution for Science, Stanford, California 94305, USA Lesego Khomo 6Department of Biological Sciences, University of Cape Town, Rondebusch 7701, South Africa *E-mail: oac@geog.ucsb.edu. Publisher: Geological Society of America Received: 08 May 2013 Revision Received: 28 Jul 2013 Accepted: 29 Jul 2013 First Online: 09 Mar 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 © 2013 Geological Society of America Geology (2013) 41 (11): 1171–1174. https://doi.org/10.1130/G34721.1 Article history Received: 08 May 2013 Revision Received: 28 Jul 2013 Accepted: 29 Jul 2013 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 Oliver A. Chadwick, Josh J. Roering, Arjun M. Heimsath, Shaun R. Levick, Gregory P. Asner, Lesego Khomo; Shaping post-orogenic landscapes by climate and chemical weathering. Geology 2013;; 41 (11): 1171–1174. doi: https://doi.org/10.1130/G34721.1 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 The spacing of hills and valleys reflects the competition between disturbance-driven (or diffusive) transport on hillslopes and concentrative (or advective) transport in valleys, although the underlying lithologic, tectonic, and climatic controls have not been untangled. Here, we measure geochemical and geomorphic properties of catchments in Kruger National Park, South Africa, where granitic lithology and erosion rates are invariant, enabling us to evaluate how varying mean annual precipitation (MAP = 470 mm, 550 mm, and 730 mm) impacts hill-valley spacing or landscape dissection. Catchment-averaged erosion rates, based on 10Be concentrations in river sands, are low (3–6 m/m.y.) and vary minimally across the three sites. Our lidar-derived slope-area analyses reveal that hillslopes in the dry site are gentle (3%) and short, such that the terrain is low relief and appears highly dissected. With increasing rainfall, hillslopes lengthen and increase in gradient (6%–8%), resulting in less-dissected, higher-relief catchments. The chemical depletion fraction of hilltop regoliths increases with rainfall, from 0.3 to 0.7, reflecting a climate-driven increase in chemical relative to physical erosion. Soil catenas also vary systematically with climate as we observe relatively uniform soil properties in the dry site that contrast with leached sandy crests and upper slopes coupled with downslope clay accumulation zones in the intermediate and wet sites. The geomorphic texture of this slow-eroding, granitic landscape appears to be set by climate-driven feedbacks among chemical weathering, regolith fabric differentiation, hydrological routing, and sediment transport that enhance the vigor of hillslope sediment transport relative to valley-forming processes for wetter climates. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Neogene rejuvenation of central Appalachian topography: Evidence for differential rock uplift from stream profiles and erosion rates

The persistence of topography within ancient orogens remains one of the outstanding questions in landscape evolution. In the eastern North American Appalachians, this question is manifest in the outstanding problem of whether topographic relief is in a quasi-equilibrium state, decaying slowly over many millennia, or whether relief has increased during the late Cenozoic. Here we present quantitative geomorphic data from the nonglaciated portion of the Susquehanna River drainage basin that provide insight into these end-member models. Analysis of channel profiles draining upland catchments in the northern Valley and Ridge, Appalachian Plateau, Blue Ridge, and Piedmont provinces reveals that a large number of streams have well defined knickpoints clustered at 300–600m elevation but not systematically associated with transitions from weak to resistant substrate. Cosmogenic 10Be inventories of modern stream sediment indicate that erosion rates are spatially variable, ranging from ~5–30m/Myr above knickpoints to ~50–100m/Myr below knickpoints. Overall, channel gradients, normalized for drainage area, scale linearly with catchment-averaged erosion rates. Collectively, regionally consistent spatial relationships among erosion rate, channel steepness, and knickpoints reveal an ongoing wave of transient channel adjustment to a change in relative base level. Reconstructions of relict channel profiles above knickpoints suggest that higher rates of incision are associated with ~100–150m of relative base level fall that accompanied epierogenic rock uplift rather than a change to a more erosive climate or drainage reorganization. Channel response timescales imply that the onset of relative base level change predates ~3.5Ma and may have begun as early as ~15Ma. We suggest that adjustment of the channel network was likely driven by changes in mantle dynamics along the eastern seaboard of North America during the Neogene.

Multitemporal ALSM change detection, sediment delivery, and process mapping at an active earthflow

ABSTRACTRemote mapping and measurement of surface processes at high spatial resolution is among the frontiers in Earth surface process research. Remote measurements that allow meter‐scale mapping of landforms and quantification of landscape change can revolutionize the study of landscape evolution on human timescales. At Mill Gulch in northern California, USA, an active earthflow was surveyed in 2003 and 2007 by airborne laser swath mapping (ALSM), enabling meter‐scale quantification of landscape change. We calculate four‐year volumetric flux from the earthflow and compare it to long‐term catchment average erosion rates from cosmogenic radionuclide inventories from adjacent watersheds. We also present detailed maps of changing features on the earthflow, from which we can derive velocity estimates and infer dominant process. These measurements rely on proper digital elevation model (DEM) generation and a simple surface‐matching technique to align the multitemporal data in a manner that eliminates systematic error in either dataset. The mean surface elevation of the earthflow and an opposite slope that was directly influenced by the earthflow decreased 14 ± 1 mm/yr from 2003 to 2007. By making the conservative assumption that these features were the dominant contributor of sediment flux from the entire Mill Gulch drainage basin during this time interval, we calculate a minimum catchment‐averaged erosion rate of 0·30 ± 0·02 mm/yr. Analysis of beryllium‐10 (10Be) concentrations in fluvial sand from nearby Russian Gulch and the South Fork Gualala River provide catchment averaged erosion rates of 0·21 ± 0·04 and 0·23 ± 0·03 mm/yr respectively. From translated landscape features, we can infer surface velocities ranging from 0·5 m/yr in the wide upper ‘source’ portion of the flow to 5 m/yr in the narrow middle ‘transport’ portion of the flow. This study re‐affirms the importance of mass wasting processes in the sediment budgets of uplifting weak lithologies. Copyright © 2011 John Wiley & Sons, Ltd.

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.

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.

Calculation of the cosmogenic nuclide production topographic shielding scaling factor for large areas using DEMs

AbstractThe recent surge of applications using terrestrial cosmogenic nuclides (TCNs) to calculate catchment‐averaged erosion rates from isotopic concentrations in fluvial sediment, and the prospect of coupling TCN production functions with numerical surface process models (SPMs), necessitate a fast and accurate algorithm for the calculation of topographic shielding. Topographic shielding refers to the proportion of the incoming cosmic radiation that is shielded by the surrounding topography, the scaling factor being defined as the ratio of the unshielded (total minus shielded) to the total (or maximum) cosmic ray flux (i.e. the flux received by a horizontal, unobstructed surface). Topography contributes to the reduction of TCN production by obstructing a certain proportion of the incoming flux and by modifying the angle of incidence. Available algorithms calculate the proportion of obstructed radiation by dividing the horizon as seen by the sample (a grid cell in the case of a DEM), into arc segments (usually of equal length) for which the average obstruction heights expressed as zenith angles are calculated. The use of these methods is feasible only when dealing with a small number of isolated samples, since the identification of obstructions when dealing with an entire area is computationally very intensive. This paper describes a method that uses a relief shadow modelling technique to identify those areas of a DEM that are under shadow (i.e. shielded), and thus to account for the obstructed radiation. This method produces results that are very similar to those obtained using a direct implementation of available methods (maximum difference between results of c. 0·1). The method based on relief shadow modelling is also faster than a direct implementation of any available method and can be readily implemented in any GIS system with raster capabilities. Copyright © 2006 John Wiley & Sons, Ltd.