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1.
The occurrence of tors within glaciated regions has been widely cited as evidence for the preservation of relic pre-Quaternary landscapes beneath protective covers of non-erosive dry-based ice. Here, we test for the preservation of pre-Quaternary landscapes with cosmogenic surface exposure dating of tors. Numerous granite tors are present on summit plateaus in the Cairngorm Mountains of Scotland where they were covered by local ice caps many times during the Pleistocene. Cosmogenic 10Be and 26Al data together with geomorphic relationships reveal that these landforms are more dynamic and younger than previously suspected. Many Cairngorm tors have been bulldozed and toppled along horizontal joints by ice motion, leaving event surfaces on tor remnants and erratics that can be dated with cosmogenic nuclides. As the surfaces have been subject to episodic burial by ice, an exposure model based upon ice and marine sediment core proxies for local glacial cover is necessary to interpret the cosmogenic nuclide data. Exposure ages and weathering characteristics of tors are closely correlated. Glacially modified tors and boulder erratics with slightly weathered surfaces have 10Be exposure ages of about 15 to 43 ka. Nuclide inheritance is present in many of these surfaces. Correction for inheritance indicates that the eastern Cairngorms were deglaciated at 15.6 ± 0.9 ka. Glacially modified tors with moderate to advanced weathering features have 10Be exposure ages of 19 to 92 ka. These surfaces were only slightly modified during the last glacial cycle and gained much of their exposure during the interstadial of marine Oxygen Isotope Stage 5 or earlier. Tors lacking evidence of glacial modification and exhibiting advanced weathering have 10Be exposure ages between 52 and 297 ka. Nuclide concentrations in these surfaces are probably controlled by bedrock erosion rates instead of discrete glacial events. Maximum erosion rates estimated from 10Be range from 2.8 to 12.0 mm/ka, with an error weighted mean of 4.1 ± 0.2 mm/ka. Three of these surfaces yield model exposure-plus-burial ages of 295− 71+ 84, 520− 141+ 178, and 626− 85+ 102 ka. A vertical cosmogenic nuclide profile across the oldest sampled tor indicates a long-term emergence rate of 31 ± 2 mm/ka. These findings show that dry-based ice caps are capable of substantially eroding tors by entraining blocks previously detached by weathering processes. Bedrock surfaces and erratic boulders in such settings are likely to have nuclide inheritance and may yield erroneous (too old) exposure ages. While many Cairngorm tors have survived multiple glacial cycles, rates of regolith stripping and bedrock erosion are too high to permit the widespread preservation of pre-Quaternary rock surfaces.  相似文献   

2.
Paul Muzikar 《Geomorphology》2008,97(3-4):407-413
The buildup of cosmogenic nuclides in an eroding surface can be used to infer erosion rates and exposure ages. This situation is often modelled by assuming that the erosion rate is constant in time. In many cases, however, the erosion is episodic: surface denudation occurs by the spalling off of slabs of rock at discrete times. We consider a stochastic model of such exfoliation processes, and compute the expected behavior of the cosmogenic nuclide concentration. We also consider a nonstochastic model, in which the exfoliation events occur at regularly spaced intervals. These two models represent extreme end members of the episodic spallation scenarios: in the first, the spallation events are uncorrelated in time, while in the second they are tightly correlated. Understanding how the nuclide concentration is related to the timing and the magnitude of these events is important in making geologic inferences.  相似文献   

3.
Cosmogenic nuclides produced in quartz may either decay or accumulate while sediment is moved through a river basin. A change in nuclide concentration resulting from storage in a floodplain is potentially important in large drainage basins in which sediment is prone to repeated burial and remobilization as a river migrates through its floodplain. We have modeled depth- and time-dependent cosmogenic nuclide concentration changes for 10Be, 26Al, and 14C during sediment storage and mixing in various active floodplain settings ranging from confined, shallow rivers with small floodplains to foreland-basin scale floodplains traversed by deep rivers. Floodplain storage time, estimated from channel migration rates, ranges from 0.4 kyr for the Beni River basin (Bolivia) to 7 kyr for the Amazon River basin, while floodplain storage depth, estimated from channel depth, ranges from 1 to 25 m.For all modeled active floodplain settings, the long-lived nuclides 10Be and 26Al show neither significant increase in nuclide concentration from irradiation nor decrease from decay. We predict a hypothetical response time after which changes in 10Be or 26Al concentrations become analytically resolvable. This interval ranges from 0.07 to 2 Myr and exceeds in all cases the typical residence time of sediment in a floodplain. Due to the much shorter half life of 14C, nuclide concentrations modeled for the in situ-produced variety of this nuclide are, however, sensitive to floodplain storage on residence times of < 20 kyr.The cosmogenic nuclide composition of old deposits in currently inactive floodplains that have been isolated for periods of millions of years from the river that once deposited them is predicted to either increase or decrease in 10Be and 26Al concentration, depending on the depositional depth. These conditions can be evaluated using the 26Al/10Be ratio that readily discloses the depth and duration of storage.We illustrate these models with examples from the Amazon basin. As predicted, modern bedload collected from an Amazon tributary, the Bolivian Beni River, shows no systematic change in nuclide concentration as sediment is moved through 500 km of floodplain by river meandering. In contrast, in the central Amazon floodplain currently untouched by the modern river system, low 26Al/10Be ratios account for minimum burial depths of 5 to 10 m for a duration of > 5 Myr.The important result of this analysis is that in all likely cases of active floodplains, cosmogenic 10Be and 26Al concentrations remain virtually unchanged over the interval sediment usually spends in the basin. Thus, spatially-averaged denudation rates of the sediment-producing area can be inferred throughout the entire basin, provided that nuclide production rates are scaled for the altitudes of the sediment-producing area only, because floodplain storage does not modify nuclide concentrations introduced from the sediment source area.  相似文献   

4.
Low-temperature apatite (U–Th)/He (AHe) thermochronology on vertical transects of leucogranite stocks and 10Be terrestrial cosmogenic nuclide (TCN) surface exposure dating on strath terraces in the Lahul Himalaya provide a first approximation of long-term (104–106 years) exhumation rates for the High Himalayan Crystalline Series (HHCS) for northern India. The AHe ages show that exhumation of the HHCS in Lahul from shallow crustal levels to the surface was ~ 1–2 mm/a and occurred during the past ~ 2.5 Ma. Bedrock exhumation in Lahul fits into a regional pattern in the HHCS of low-temperature thermochronometers yielding Plio-Pleistocene ages. Surface exposure ages of strath terraces along the Chandra River range from ~ 3.5 to 0.2 ka. Two sites along the Chandra River show a correlation between TCN age and height above the river level yielding maximum incision rates of 12 and 5.5 mm/a. Comparison of our AHe and surface exposure ages from Lahul with thermochronometry data from the fastest uplifting region at the western end of the Himalaya, the Nanga Parbat syntaxis, illustrates that there are contrasting regions in the High Himalaya where longer term (105–107 years) erosion and exhumation of bedrock substantially differ even though Holocene rates of fluvial incision are comparable. These data imply that the orogen's indenting corners are regions where focused denudation has been stable since the mid-Pliocene. However, away from these localized areas where there is a potent coupling of tectonic and surface processes that produce rapid uplift and denudation, Plio-Pleistocene erosion and exhumation can be characterized by disequilibrium, where longer term rates are relatively slower and shorter term fluvial erosion is highly variable over time and distance. The surface exposure age data reflect differential incision along the length of the Chandra River over millennial time frames, illustrate the variances that are possible in Himalayan river incision, and highlight the complexity of Himalayan environments.  相似文献   

5.
Jose Luis Antinao  John Gosse   《Geomorphology》2009,104(3-4):117-133
The distribution and age of large (> 0.1 km2) Pliocene to recent rockslides in the Chilean Cordillera Principal (32–34.5 S), the Southern Central Andes, has been analyzed to determine the rockslide triggering mechanisms and impact on regional landscape evolution. Most of the rockslides appear in the western Cordillera Principal and cluster along major geological structures. Variographic analyses show spatial correlation between rockslides, geological structures and shallow seismicity. A relative chronosequence was calibrated with existing 14C and 40Ar/39Ar dates and new cosmogenic nuclide exposure ages for selected rockslides. Rockslide-induced sediment yield was estimated with empirical relations for rockslide area distributions. Throughout the Quaternary, rockslides have delivered sediment to streams at rates equivalent to denudation rates of 0.10 ±0.06 mm a− 1, while estimates using short term (20 a) seismicity records are 0.3− 0.2+ 0.6 mm a− 1. The estimates of sediment transfer and the spatial distribution of rockslides reflect a landscape in which tectonic and geological controls on denudation are more significant than climate.  相似文献   

6.
Cover beds, widespread on hillslopes of temperate climate zones, represent layers of allochthonous material laterally transported by periglacial processes during the Late Pleistocene. Two soil sections comprised of cover beds from the Bavarian Forest, SE Germany, have been analysed for in situ-produced cosmogenic 10Be. Major changes in the nuclide concentration agree well with soil section boundaries defined by field observations and grain size analyses. Numeric modeling of these cosmogenic nuclide sections demonstrates that simple continuous erosion and regolith mixing models fail to explain the measured nuclide concentrations. Instead, the measured data can be best described by modeling an admixture of material such as loess or reworked allochthonous material, which have different nuclide concentrations.A comparison of cosmogenic nuclide concentrations from the two cover bed sections with concentrations from river bedload sediments of the Regen catchment reveals that cover bed formation might affect the result of basin-wide erosion rate determinations based on cosmogenic nuclides. Nuclide concentration of river bedload is potentially a binary mixture produced by (1) spatial erosion of the soil surfaces; and (2) spatially nonuniform incision into deep cover bed layers that contributes sediment low in nuclide concentration.  相似文献   

7.
Previous studies of chemical weathering rates for soil developed on glacial moraines generally assumed little or no physical erosion of the soil surface. In this study, we investigate the influence of physical erosion on soil profile weathering rate calculations. The calculation of chemical weathering rates is based on the assumption that soil profiles represent the integrated amount of weathering since the time of moraine deposition. The weathering rate of a surface subjected to denudation is the sum of the weathering loss from the existing soil profile added to the weathering loss in the material removed by denudation, divided by the deposition age. In this study, the amount of weathered material removed since moraine deposition is calculated using the denudation rate estimated from cosmogenic nuclide data and the deposition age of the moraine. Weathering rates accounting for denudation since moraine deposition are compared to weathering rates based on the assumption of no physical erosion and on the assumption of steady-state denudation for the Type Pinedale moraine ( 21 ka) and the Bull Lake-age moraine ( 140 ka) in the Fremont Lake Area (Wind River Mountains, Wyoming, USA). The total weathering rates accounting for denudation are 8.15 ± 1.05 g(oxide) m− 2 y− 1 for the Type Pinedale moraine and 4.78 ± 0.89 g(oxide) m− 2 y− 1 for the Bull Lake-age moraine, which are  2 to 4 times higher, respectively, than weathering rates based on the assumption of no physical erosion. The weathering rates based on denudation since moraine deposition are comparable or smaller than weathering rates assuming steady-state denudation. We find the assumption of steady-state denudation is not valid in depositional landscapes with young deposition ages or slow denudation rates. The decrease in weathering rates over time between the Type Pinedale and Bull Lake-age soils that is observed in the case of no physical erosion is decreased when the influence of denudation on the total weathering rates is taken into account. Fresh unweathered material with high reactive mineral surface area is continuously provided to the surface layer by denudation diminishing the effect of decreasing weathering rate over time.  相似文献   

8.
The past decade has seen the development and application of over a dozen new methods for quantitative age-determinations of geomorphic surfaces. Some surface exposure dating methods are numerical, including the accumulation of cosmogenic radionuclides 10Be, 14C, 26Al, 36Cl, and 41Ca, accumulation of cosmogenic stable nuclides 3He and 21Ne, 14C dating of organic matter encapsulated in rock coatings, and dendrogeomorphology. Calendar ages are obtained by dendrogeomorphological analysis. Calibrated ages can be obtained by analysis of rock-varnish chemistry, lichenometry, weathering, and soils. Various methods can be used in combination to overcome individual limitations. Whereas conventional methods provide age control on stratigraphic profiles, surface-exposure dating methods are especially suitable for geographic problems, such as analyzing not only temporal, but also spatial variations in the rates of geomorphic processes. [Key words: geomorphology, process, geochronology, cosmogenic nuclides, Quaternary, surface exposure dating, rock varnish, weathering, soils.]  相似文献   

9.
The New River crosses three physiogeologic provinces of the ancient, tectonically quiescent Appalachian orogen and is ideally situated to record variability in fluvial erosion rates over the late Cenozoic. Active erosion features on resistant bedrock that floors the river at prominent knickpoints demonstrate that the river is currently incising toward base level. However, thick sequences of alluvial fill and fluvial terraces cut into this fill record an incision history for the river that includes several periods of stalled downcutting and aggradation. We used cosmogenic 10Be exposure dating, aided by mapping and sedimentological examination of terrace deposits, to constrain the timing of events in this history. 10Be concentration depth profiles were used to help account for variables such as cosmogenic inheritance and terrace bioturbation. Fill-cut and strath terraces at elevations 10, 20, and 50 m above the modern river yield model cosmogenic exposure ages of 130, 600, and 600–950 ka, respectively, but uncertainties on these ages are not well constrained. These results provide the first direct constraint on the history of alluvial aggradation and incision events recorded by New River terrace deposits. The exposure ages yield a long-term average incision rate of 43 m/my, which is comparable to rates measured elsewhere in the Appalachians. During specific intervals over the last 1 Ma, however, the New River's incision rate reached 100 m/my. Modern erosion rates on bedrock at a prominent knickpoint are between 28 and 87 m/my, in good agreement with rates calculated between terrace abandonment events and significantly faster than 2 m/my rates of surface erosion from ancient terrace remnants. Fluctuations between aggradation and rapid incision operate on timescales of 104− 105 year, similar to those of late Cenozoic climate variations, though uncertainties in model ages preclude direct correlation of these fluctuations to specific climate change events. These second-order fluctuations appear within a longer-term signal of dominant aggradation (until 2 Ma) followed by dominant incision. A similar signal is observed on other Appalachian rivers and may be the result of sediment supply fluctuations driven by the increased frequency of climate changes in the late Cenozoic.  相似文献   

10.
In an actively deforming orogen, maintenance of a topographic steady state requires that hillslope erosion, river incision, and rock uplift rates are balanced over timescales of 105–107 years. Over shorter times, <105 years, hillslope erosion and bedrock river incision rates fluctuate with changes in climate. On 104-year timescales, the Marsyandi River in the central Nepal Himalaya has oscillated between bedrock incision and valley alluviation in response to changes in monsoon intensity and sediment flux. Stratigraphy and 14C ages of fill terrace deposits reveal a major alluviation, coincident with a monsoonal maximum, ca. 50–35 ky BP. Cosmogenic 10Be and 26Al exposure ages define an alluviation and reincision event ca. 9–6 ky BP, also at a time of strong South Asian monsoons. The terrace deposits that line the Lesser Himalayan channel are largely composed of debris flows which originate in the Greater Himalayan rocks up to 40 km away. The terrace sequences contain many cubic kilometers of sediment, but probably represent only 2–8% of the sediments which flushed through the Marsyandi during the accumulation period. At 104-year timescales, maximum bedrock incision rates are 7 mm/year in the Greater Himalaya and 1.5 mm/year in the Lesser Himalayan Mahabarat Range. We propose a model in which river channel erosion is temporally out-of-phase with hillslope erosion. Increased monsoonal precipitation causes an increase in hillslope-derived sediment that overwhelms the transport capacity of the river. The resulting aggradation protects the bedrock channel from erosion, allowing the river gradient to steepen as rock uplift continues. When the alluvium is later removed and the bedrock channel re-exposed, bedrock incision rates probably accelerate beyond the long-term mean as the river gradient adjusts downward toward a more “equilibrium” profile. Efforts to document dynamic equilibrium in active orogens require quantification of rates over time intervals significantly exceeding the scale of these millennial fluctuations in rate.  相似文献   

11.
利用原地生宇生核素测定暴露年代时,通常会假设地貌体侵蚀速率为0。研究表明,该假设会低估地貌体的真实暴露年代。搜集2009~2012年全球不同区域56个岩石样品的宇生核素10Be测年数据,探讨侵蚀速率为0对于侵蚀速率为0.5、1以及2 mm/ka的样品,在不同暴露尺度上对暴露年代计算的影响幅度。结果表明,对于1×104a尺度的样品暴露年代可能低估约0.5%,1%,2%;对于10×104a尺度的样品可能低估约5%,7%,20%;对于50×104a尺度的样品可能低估约40%,70%甚至100%以上。  相似文献   

12.
Depth profiling of in situ-produced cosmogenic 10Be and 26Al was applied to estimate the rate of long-term denudation on actively uplifting hillcrests made of weakly consolidated sandstone in the Boso Peninsula, Japan. The nuclide concentration in the topmost meter of the hillcrest has reached an equilibrium value owing to surface denudation at 90 mm/kyr. This rate is much smaller than the regional uplift rate, and is consistent with the current isolated residual topography of the hillcrest. By considering the epoch of emergence of the sandstone hillcrests, we estimate the summit denudation rate of the adjacent mudstone hill formation to be 720 ± 110 mm/kyr.  相似文献   

13.
《Basin Research》2018,30(2):169-186
Long‐term (106–7 yr) clastic sedimentary fluxes to the ocean provide first‐order constraints on the response of continental surfaces to both tectonic and climatic forcing as well as the supply that builds the stratigraphic record. Here, we use the dated and regionally correlated relict lateritic landforms preserved over Sub‐Saharan West Africa to map and quantify regional denudation as well as the export of main catchments for three time intervals (45–24, 24–11 and 11–0 Ma). At the scale of West Africa, denudation rates are low (ca. 7 m Myr−1) and total clastic export rate represents 18.5 × 103 km3 Myr−1. Export rate variations among the different drainage groups depend on the drainage area and, more importantly, rock uplift. Denuded volumes and offshore accumulations are of the same magnitude, with a noticeably balanced budget between the Niger River delta and its catchment. This supports the establishment of the modern Niger catchment before 29 Ma, which then provided sufficient clastic material to the Niger delta by mainly collecting the erosion products of the Hoggar hotspot swell. Accumulations on the remaining Equatorial Atlantic margin of Africa suggest an apparent export deficit but the sediment budget is complicated by the low resolution of the offshore data and potential lateral sediment supply from the Niger delta. Further distortion of the depositional record by intracontinental transient storage and lateral input or destabilization of sediments along the margin may be identified in several locations, prompting caution when deducing continental denudation rates from accumulation only.  相似文献   

14.
Sediment supplied by continental sources is commonly suspected to have exerted a strong influence on the development of canyons and other morphological features on the continental slopes, but rarely is the sediment supply known sufficiently quantitatively to test this link. Here, we outline an area where offshore morphology, in the western Ionian Sea, may be linked to estimated sediment fluxes produced by subaerial erosion in NE Sicily and SW Calabria. Shelves in this area are narrow (<1 km), and the bathymetry shows that rivers and adjacent submarine channels are almost directly connected with each other. Integrated topographic analyses were performed on a merged digital elevation model (DEM) of ASTER data for subaerial topography and multibeam sonar data for submarine bathymetry. Spatial variations in sediment fluxes from onshore erosion were assessed using a variety of methods, namely: long‐term sediment flux from Pleistocene uplift rates, decadal sediment flux from landslide occurrences and published long‐term exhumation rates from 10Be cosmogenic nuclide concentrations. Submarine channels associated with rivers delivering larger sediment fluxes have broad channels, high relief and smooth concave‐upward longitudinal profiles. Conversely, submarine channels that lie offshore small‐flux rivers have straight longitudinal profiles, low relief and steep gradients. Where river catchments supply a greater sediment flux offshore, shelves tend to be wider (ca. 400 m) and submarine channels have gentler gradients. In contrast, where catchments supply less sediment flux, shelves are narrow (250–300 m) and offshore channel gradients are steeper. The variation of submarine morphology with tectonic uplift rate was also studied, but we find that, unlike onshore terrains where tectonics is commonly an important factor influencing channel morphology, in the submarine landscapes, sediment flux appears to dominate here.  相似文献   

15.
Weathering pits 1–140 cm deep occur on granite surfaces in the Cairngorms associated with a range of landforms, including tors, glacially exposed slabs, large erratics and blockfields. Pit depth is positively correlated with cosmogenic exposure age, and both measures show consistent relationships on individual rock landforms. Rates of pit deepening are non‐linear and a best fit is provided by the sigmoidal function D = b1 + exp(b2+b3/t). The deepest pits occur on unmodified tor summits, where 10Be exposure ages indicate that surfaces have been exposed to weathering for a minimum of 52–297 ka. Glacially exposed surfaces with pits 10–46 cm deep have given 10Be exposure durations of 21–79 ka, indicating exposure by glacial erosion before the last glacial cycle. The combination of cosmogenic exposure ages with weathering pit depths greatly extends the area over which inferences can be made regarding the ages of granite surfaces in the Cairngorms. Well‐developed weathering pits on glacially exposed surfaces in other granite areas are potential indicators of glacial erosion before the Last Glacial Maximum.  相似文献   

16.
Understanding the dynamics of sediment generation and transport on hillslopes provides important constraints on the rate of sediment output from orogenic systems. Hillslope sediment fluxes are recorded by organic material found in the deposits infilling unchanneled convergent topographic features called hollows. This study describes the first hollow infilling rates measured in the southern Appalachian Mountains. Infilling rates (and bedrock erosion rates) were calculated from the vertical distribution of radiocarbon ages at two sites in the Coweeta drainage basin, western North Carolina. At each site we dated paired charcoal and silt soil organic matter samples from five different horizons. Paired radiocarbon samples were used to bracket the age of the soil material in order to capture the range of complex soil forming processes and deposition within the hollows. These dates constrain hillslope erosion rates of between 0.051 and 0.111 mm yr− 1. These rates are up to 4 times higher than spatially-averaged rates for the Southern Appalachian Mountains making creep processes one of the most efficient erosional mechanisms in this mountain range. Our hillslope erosion rates are consistent with those of forested mountain ranges in the western United States, suggesting that the mechanisms (dominantly tree throw) driving creep erosion in both the western United States and the Southern Appalachian Mountains are equally effective.  相似文献   

17.
The aim of this study was to evaluate four metrics to define the spatially variable (regionalised) hillslope sediment delivery ratio (HSDR). A catchment model that accounted for gully and streambank erosion and floodplain deposition was used to isolate the effects of hillslope gross erosion and hillslope delivery from other landscape processes. The analysis was carried out at the subcatchment (~ 40 km2) and the cell scale (400 m2) in the Avon-Richardson catchment (3300 km2), south-east Australia. The four landscape metrics selected for the study were based on sediment travel time, sediment transport capacity, flux connectivity, and residence time. Model configurations with spatially-constant or regionalised HSDR were calibrated against sediment yield measured at five gauging stations. The impact of using regionalised HSDR was evaluated in terms of improved model performance against measured sediment yields in a nested monitoring network, the complexity and data requirements of the metric, and the resulting spatial relationship between hillslope erosion and landscape factors in the catchment and along hillslope transects. The introduction of a regionalised HSDR generally improved model predictions of specific sediment yields at the subcatchment scale, increasing model efficiency from 0.48 to > 0.6 in the best cases. However, the introduction of regionalised HSDR metrics at the cell scale did not improve model performance. The flux connectivity was the most promising metric because it showed the largest improvement in predicting specific sediment yields, was easy to implement, was scale-independent and its formulation was consistent with sedimentological connectivity concepts. These properties make the flux connectivity metric preferable for applications to catchments where climatic conditions can be considered homogeneous, i.e. in small-medium sized basins (up to approximately 3000 km2 for Australian conditions, with the Avon-Richardson catchment being at the upper boundary). The residence time metric improved model assessment of sediment yields and enabled accounting for climatic variability on sediment delivery, but at the cost of greater complexity and data requirements; this metric might be more suitable for application in catchments with important climatic gradients, i.e. large basins and at the regional scale. The application of a regionalised HSDR metric did not increase data or computational requirements substantially, and is recommended to improve assessment of hillslope erosion in empirical, semi-lumped erosion modelling applications. However, more research is needed to assess the quality of spatial patterns of erosion depicted by the different landscape metrics.  相似文献   

18.
Landscapes in southeastern Australia have changed dramatically since the spread of European colonisation in the 19th century. Due to widespread forest clearance for cultivation and grazing, erosion and sediment yields have increased by a factor of more than 150. In the 20th century, erosion and sediment yield were reduced again due to an increasing vegetative cover. Furthermore, during the last decades, thousands of small farm dams were constructed to provide drinking water for cattle. These dams trap a lot of sediment, thereby further reducing sediment delivery from hillslopes to river channels. Changes in sediment delivery since European colonisation are documented in sediment archives. Within this study, these changing rates in hillslope erosion and sediment delivery were modelled using a spatially distributed erosion and sediment delivery model (WATEM/SEDEM) that was calibrated for Australian ecosystems using sediment yield data derived from sedimentation rates in 26 small farm dams. The model was applied to the Murrumbidgee river basin (30,000 km2) under different land-use scenarios. First, the erosion and sediment yield under pre-European land-use was modelled. Secondly, recent land-use patterns were used in the model. Finally, recent land-use including the impact of farm dams and large reservoirs was simulated. The results show that the WATEM/SEDEM model is capable of predicting the intensity of the geomorphic response to changes in land-use through time. Changes in hillslope erosion and hillslope sediment delivery rates are not equal, illustrating the non-linear response of the catchment. Current hillslope sediment supply to the river channel network is predicted to be 370% higher compared to the pre-European settlement period, yet farm dams have reduced this back to 2.5 times the pre-19th century values. The role of larger reservoirs is even more important as they have reduced the current sediment supply downstream to their pre-European values, thus completely masking the increased hillslope erosion rates from land-use change. However, the model does so far not include valley widening and sediment storage in river systems. Therefore, modelled rates of sediment delivery are lower than observed values.  相似文献   

19.
Studies on denudation rates can provide insight into the influence of climate change, tectonics, and human activities on landscape evolution. Research performed in Central Italy has shown considerable spatial variability of denudation rates in the major river basins. These studies have focused mainly on the Tyrrhenian side of the Italian peninsula, where Plio-Pleistocene marine deposits filling NW–SE elongated sedimentary basins have been uplifted during the Quaternary up to several hundreds of meters above present sea level. Small sub-catchments developed on clays are affected by sharp- and/or rounded-edged badlands (i.e. calanchi and biancane), representing denudation “hot spots” in the present-day morphoclimatic framework.In this paper, we analyze the relationships between indirectly estimated denudation rates at the catchment scale and field monitoring data at the hillslope scale. We attempt to better understand and quantify all hillslope processes that contribute to seasonal variability of denudation, to help with predicting the net input from “hot spots” to the overall estimated sediment yield at the basin outlets. At the hillslope scale, we discuss, in particular, the variability of denudation rates at calanchi and biancane badlands as a function of their different morphoevolution.  相似文献   

20.
A Karst Connection model for Grand Canyon, Arizona, USA   总被引：2，自引：1，他引：1  
C.A. Hill  N. Eberz  R.H. Buecher   《Geomorphology》2008,95(3-4):316-334
Along the Asturian coast of northern Spain an uplifted wave-cut platform extends for  100 km east–west. The steep cliff which bounds the gently seaward-dipping platform to the north increases in height from 30 m in the west to 100 m in the east and reflects the overall eastward increase in platform elevation. The southern edge of the 2–4 km-wide platform runs along the foothills of the Cantabrian Mountains, as constrained by a high-resolution digital elevation model. The marine platform, which was carved into deformed Paleozoic bedrock with abundant quartzite beds, is largely covered by weathered marine and continental sediments. Quartzite samples from flat bedrock outcrops which are currently not covered by sediment or soil yield cosmogenic nuclide concentrations (21Ne, 10Be and 26Al) that demonstrate a long and complex exposure history, including periods of burial with partial or complete shielding from cosmic rays. The combination of multiple cosmogenic nuclides yields a minimum age of 1–2 Ma for the platform. Taking into account (i) the horizontal and vertical extent of the platform, (ii) the high resistance to erosion of the quartzitic bedrock, and (iii) published data on the magnitude of past sea level fluctuations, we suggest that the wave-cut platform formed in the Pliocene. Subvertical faults cutting the platform at high angles to the coastline offset the southern edge of the platform by 20 to 40 m and reactivate the pre-existing anisotropy in the Paleozoic bedrock. Uplift and crustal deformation of the coastal region have occurred after platform formation in the Pliocene and may still be active. The slow deformation of the northern edge of the Iberian plate including the Cantabrian Mountains may result from the ongoing slow convergence at an incipient subduction zone extending along the coast of northern Spain.  相似文献   

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