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1.
In order to assess current understanding of meteoric 10Be dynamics and distribution in terrestrial soils, we assembled a database of all published meteoric 10Be soil depth profiles, including 104 profiles from 27 studies in globally diverse locations, collectively containing 679 individual measurements. This allows for the systematic comparison of meteoric 10Be concentration to other soil characteristics and the comparison of profile depth distributions between geologic settings. Percent clay, 9Be, and dithionite-citrate extracted Al positively correlate to meteoric 10Be in more than half of the soils where they were measured, but the lack of significant correlation in other soils suggests that no one soil factor controls meteoric 10Be distribution with depth. Dithionite-citrate extracted Fe and cation exchange capacity are only weakly correlated to meteoric 10Be. Percent organic carbon and pH are not significantly related to meteoric 10Be concentration when all data are complied.The compilation shows that meteoric 10Be concentration is seldom uniform with depth in a soil profile. In young or rapidly eroding soils, maximum meteoric 10Be concentrations are typically found in the uppermost 20 cm. In older, more slowly eroding soils, the highest meteoric 10Be concentrations are found at depth, usually between 50 and 200 cm. We find that the highest measured meteoric 10Be concentration in a soil profile is an important metric, as both the value and the depth of the maximum meteoric 10Be concentration correlate with the total measured meteoric 10Be inventory of the soil profile.In order to refine the use of meteoric 10Be as an estimator of soil erosion rate, we compare near-surface meteoric 10Be concentrations to total meteoric 10Be soil inventories. These trends are used to calibrate models of meteoric 10Be loss by soil erosion. Erosion rates calculated using this method vary based on the assumed depth and timing of erosional events and on the reference data selected.  相似文献   

2.
Understanding the evolution of geochemical and geomorphic systems requires measurements of long-term rates of physical erosion and chemical weathering. Erosion and weathering rates have traditionally been estimated from measurements of sediment and solute fluxes in streams. However, modern sediment and solute fluxes are often decoupled from long-term rates of erosion and weathering, due to storage or re-mobilization of sediment and solutes upstream from the sampling point. Recently, cosmogenic nuclides such as 10Be and 26Al have become important new tools for measuring long-term rates of physical erosion and chemical weathering. Cosmogenic nuclides can be used to infer the total denudation flux (the sum of the rates of physical erosion and chemical weathering) in actively eroding terrain. Here we review recent work showing how this total denudation flux can be partitioned into its physical and chemical components, using the enrichment of insoluble tracers (such as Zr) in regolith relative to parent rock. By combining cosmogenic nuclide measurements with the bulk elemental composition of rock and soil, geochemists can measure rates of physical erosion and chemical weathering over 1000- to 10,000-year time scales.  相似文献   

3.
Cosmogenic nuclide-based denudation rates and published erosion rates from recent river gauging in the Napo River basin (Peruvian Amazonia) are used to decipher erosion and sedimentation processes along a 600 km long transect from the headwaters to the lowlands. The sediment-producing headwaters to the Napo floodplain are the volcanically active Ecuadorian Andes, which discharge sediment at a cosmogenic nuclide-based denudation rate of 0.49 ± 0.12 mm/yr. This denudation rate was calculated from an average 10Be nuclide concentration of 2.2 ± 0.5 × 104 at/g(Qz) that was measured in bedload-derived quartz. Within the Napo lowlands, a significant drop in trunk stream 10Be nuclide concentrations relative to the Andean hinterland is recorded, with an average concentration of 1.2 ± 0.5 × 104 at/g(Qz). This nuclide concentration represents a mixture between the 10Be nuclide concentration of eroded floodplain deposits, and that of sediment eroded from the Andean hinterland that is now carried in the trunk stream. Evidence for addition of sediment from the floodplain to the trunk stream is provided by published decadal-scale sediment flux measurements from gauging stations operated in the Napo basin, from which an increase from 12 × 106 t/yr at the outflow of the Andes to ～47 × 106 t/yr at the confluence with the Solimões (upper Amazon River) is recorded. Therefore, approximately 35 × 106 t of floodplain sediment are added annually to the active Napo trunk stream. Combined with our nuclide concentration measurements, we can estimate that the eroded floodplain deposits yield a nuclide concentration of ～0.9 × 104 at/g(Qz) only. Under steady state surface erosion conditions, this nuclide concentration would translate to a denudation rate of the floodplain of ～0.47 mm/yr. However, we have no geomorphologic explanation for this high denudation rate within the low relief floodplain and thus suggest that this low-nuclide concentrated sediment is Andean-derived and would have been deposited in the floodplain at a time when erosion rates of the Andes were elevated. Therefore, the recently eroded floodplain sediment provides an Andean “paleo denudation rate” of 1.2 mm/yr that was calculated for high Andean production rates. A likely period for elevated erosion rates is the LGM, where climate and vegetation cover of the Andes differed from that of the Holocene. A possible cause for the erosion of the floodplain is the tectonic uplift of the Eastern Andes, which progressively shifts the Napo River northwards. Hence, the river cuts into ancient lowland sediment, which is admixed to the Andean sediment carried in the main Napo River.  相似文献   

4.
Cosmogenic nuclide concentrations measured on abandoned fan surfaces along the Mojave section of the San Andreas Fault suggest that sediment is generated, transported, and removed from the fans on the order of 30-40 kyr. We measured in situ produced cosmogenic 10Be, and in some cases 26Al, in boulders (n = 15), surface sediment (n = 15), and one depth profile (n = 9). Nuclide concentrations in surface sediments and boulders underestimate fan ages, suggesting that 10Be accumulation is largely controlled by the geomorphic processes that operate on the surfaces of the fans and not by their ages.Field observations, grain-size distribution, and cosmogenic nuclide data suggest that over time, boulders weather into grus and the bar sediments diffuse into the adjacent swales. As fans grow older the relief between bars and swales decreases, the sediment transport rate from bars to swales decreases, and the surface processes that erode the fan become uniform over the entire fan surface. The nuclide data therefore suggest that, over time, the difference in 10Be concentration between bars and swales increases to a maximum until the topographic relief between bars and swales is minimized, resulting in a common surface lowering rate and common 10Be concentrations across the fan. During this phase, the entire fan is lowered homogeneously at a rate of 10-15 mm kyr−1.  相似文献   

5.
In contrast to active tectonic settings, little is known about the potential feedback between surface processes and climate change in tectonically inactive cratonic regions. Here, we studied the driving forces of erosion and landscape evolution in the Kruger National Park in South Africa using cosmogenic nuclide dating. 10Be‐derived catchment‐wide erosion rates (~2 and ~10 mm ka?1) are similar in magnitude to erosion and rock uplift elsewhere in South Africa, suggesting that (1) rock uplift is solely the isostatic response to erosion and (2) the first‐order topography is likely of Cretaceous age. The topographic maturity is promoted by widespread exposure of rocks resistant to erosion. Our data, however, suggest that local variations in rock resistance lead to transient landscape changes, with local increases in relief and erosion rates.  相似文献   

6.
Long-term retreat rates of Puget Sound's unconsolidated sediment shorelines have been difficult to quantify, and little systematic research has been completed to constrain retreat in this area. We put forward a new application of cosmogenic 10Be exposure dating to assess long-term shoreline retreat on Whidbey Island, WA by dating lag boulders exposed on the shore platform as the shoreline erodes. Production of 10Be in shoreline boulders is modulated by both tidal submergence and topographic shielding from the retreating bluff. By modeling the combined effect of these variables on 10Be production, the timing of exposure can be determined and used to calculate long-term (103–104 yr) bluff retreat rates. In rare cases, retreat rates are underestimated due to inherited 10Be. Within the study area, average retreat rates ranged between 0 and 8 cm yr? 1. Our results demonstrate the utility of cosmogenic nuclides for determining long-term shoreline retreat rates in areas with thick sediment cover, where large numbers of samples can be collected, and where the pre-depositional history of the boulders is uncomplicated.  相似文献   

7.
We demonstrate that cosmogenic nuclide surface exposure dating can be used to provide the first well-constrained age for a Fiordland bedrock surface that was created by coastal erosion and has since been uplifted. Tight clustering of 10Be and 26Al apparent exposure ages between 102-119 kyr on a terrace with strandline at 65 ± 8 m gives a last interglacial age of terrace formation of 130-120 ka, and an uplift rate of 0.52 ± 0.08 mm/yr. Apparent exposure ages from a higher (92-130 m), more incised region of remnant coastal morphology fall in the range 53-111 kyr. The anomalously low ages and large variance demonstrate that weathering and fluvial or rockfall erosion rates are too extreme at the higher sites to determine an age of coastal erosion. Sea level samples have apparent exposure ages in the range 2-11 kyr, with an uncertainty of about 3 kyr. This is consistent with surface exposure during the present sea level high-stand, indicates minimal inheritance of ancient cosmogenic nuclides, and is in accord with geomorphic arguments. Mean 26Al/10Be ratios of 6.6 for each sample set is consistent with the actively exhuming late Quaternary tectonic setting. Large boulders and gently convex rocky outcrops formed during coastal erosion preserve surfaces that are least modified during later uplift, and are hence the best sites for determining the age of coastal erosion.  相似文献   

8.
This study provides the first attempt to combine terrestrial (in situ) cosmogenic nuclide (10Be) surface exposure dating with Schmidt hammer relative-age dating for the age estimation of Holocene moraines at Strauchon Glacier, Southern Alps, New Zealand. Numerous Schmidt hammer tests enable a multi-ridged lateral moraine system to be related to three late-Holocene ‘Little Ice Age’-type events. On the basis of cosmogenic 10Be ages, those events are dated to c. 2400, 1700, and 1100 years ago. Linear age-calibration curves are constructed in order to relate Schmidt hammer R-values to cosmogenic 10Be ages. The high explanation yielded reveals the causal link between both data sets. The potential of combining both methods in a ‘’multiproxy approach’ is discussed alongside possible future improvements. Terrestrial cosmogenic nuclide dating delivers absolute ages needed as fixed points for Schmidt hammer age-calibration curves. The Schmidt hammer technique can be used to crosscheck the boulder surfaces chosen for surface exposure dating by terrestrial cosmogenic nuclides. It should, therefore, reduce the number of samples necessary and costs.  相似文献   

9.
By using accelerator mass spectrometry, we measured 10Be (T1/2 = 1.5 Ma) concentrations in nine Ivory Coast (IVC) tektites, in six soil samples collected near the Bosumtwi impact crater, the likely source region, and in a depth profile taken through a 23 g moldavite. In the core of the moldavite sample we also measured an upper limit on the 36Cl (T1/2 = 0.3 My) concentration. The average 10Be concentration in IVC tektites of (22 ± 11) × 106 atom/g exceeds reasonable limits for a meteoritic component or cosmic-ray production in situ after tektite formation. The 10Be must be meteoric, which implies that IVC tektites formed from soils or sediments. Corrected to the time of formation (ToF) 1.07 Ma ago and for a small in situ component, the average 10Be concentration of (35 ± 7) × 106 atom/g (1 − σ mean) is considerably lower than those of contemporary Bosumtwi soils, ∼250 × 106 atom/g, or of Australasian tektites at their ToF, 0.8 Ma B.P. near Lake Bosumtwi today the soil column is only ∼1 m thick. If the landscape was similar 1.07 Ma ago, then the total thickness of the tektite formation zone probably did not exceed 10 m. With increasing depth below the surface of the moldavite, the 10Be concentrations decrease rapidly owing to the presence of a surface component, probably of recent origin. The main interior mass of the sample contains ∼0.8 × 106 atom 10Be/g and fewer than 0.1 × 106 atom 36Cl/g, little of which can be meteoritic. Although not definitive, consideration of several possible cosmic-ray exposure histories suggests that about half the interior 10Be has a meteoric origin, which if corrected to the time of formation yields a concentration compatible with those measured in typical contemporary soils. The observations are consistent with the formation of three of the four main tektite groups from surface soils or sediments.  相似文献   

10.
7Be在土壤侵蚀示踪中的应用研究进展   总被引：7，自引：0，他引：7  
7Be是一种自然产生的放射性核素，由对流层和同温层大气中的氧和氮在宇宙射线作用下分裂生成，其半衰期为53．3天，7Be通过连续的干湿沉降作用到达土表，被土壤颗粒紧密吸附，其土壤环境化学行为与137Cs相似，因此，可以用于示踪某种特定土地利用方式或侵蚀事件下的土壤侵蚀作用的强度，空间分配以及季节性变化，7Be技术可与其它反映中长期土壤侵蚀的放射性核素（137Cs,210Pb)示踪技术相结合，共同表征某个流域的土壤侵蚀状况，对7Be示踪技术现有的土壤侵蚀速率定量估算模型进行了简述，并提出了各自的局限性及今后的研究方向。  相似文献   

11.
In situ-produced cosmogenic nuclides provide a means for quantitative evaluation of a wide range of weathering and sediment transport processes. Although these nuclides have received attention for their power as geochronometers of surface exposure, it may be argued that they are more broadly suited for study of surface processes. In many environments, they may be used to evaluate collapse, erosion, burial, bioturbation, and creep, as well as providing a qualitative basis for distinguishing allochthonous from autochthonous materials. In addition, these nuclides can provide quantitative information on rates of erosion on scales of landforms and drainage basins. Here, we review the systematics of cosmogenic nuclide production within the Earth's surface, and present field examples demonstrating the utilization of in situ-produced cosmogenic nuclides distributions for evaluation of a range of soil evolution processes. To cite this article: E.T. Brown et al., C. R. Geoscience 335 (2003).  相似文献   

12.
10Be和26Al在地表形成和演化研究中的应用   总被引：28，自引：4，他引：24       下载免费PDF全文
地球表面丰富多样的地形、地貌是地球内外动力相互作用的结果。宇宙射线辐射地表岩石、沉积物等产生的10Be和26Al放射性核素（半衰期分别为1．5Ma和0．71Ma）,它们的浓度主要取决于由地理位置和地形、地貌条件所决定的宇宙射线辐射的通量和时间,从而能够记录地表的形成和演化历史。因此,宇宙成因核素是研究地表形成历史和作用过程的有力工具。  相似文献   

13.
原地生成宇宙成因核素测年技术在计算暴露年龄和侵蚀速率时,一直默认样品所在位置不随时间发生高程变化,从而核素生成速率不因高程变化而改变。在构造稳定区,这样的假设是合理的。在构造活动区,往往会因构造运动,地表样品高程发生改变。大冰盖地区（如南极）的冰川消融或加厚也会引起地壳均衡反弹,从而导致样品高程变化,而核素的生成速率将随高程变化而改变。本文对连续抬升情况下,地表样品中宇宙成因核素浓度与生成速率加速率间的计算关系进行了三种方法的数学推导,并提出如何利用单块样品中两种宇宙成因核素（以10Be和26Al为例）浓度计算地表抬升速率,最后指出已往利用宇宙成因核素方法对构造活动区的侵蚀速率的计算存在高估。本文首次提出利用地表在接近稳态侵蚀状态下的单块样品的两种宇宙成因核素直接计算地表持续抬升速率的方法,从而将原地生成宇宙成因核素方法和构造运动学研究直接联系起来。  相似文献   

14.
In situ cosmogenic 14C (in situ 14C) analysis from quartz‐bearing rocks is a novel isotopic tool useful for quantifying recent surface exposure histories (up to ～25 ka). It is particularly powerful when combined with longer‐lived cosmogenic isotopes such as 10Be. Recent advances in the extraction of in situ 14C from quartz now permit the routine application of this method. However, only a few experiments to calibrate the production rate of in situ 14C in quartz have been published to date. Here, we present a new in situ 14C production rate estimate derived from a well‐dated debris flow deposit in the Southern Alps, New Zealand, previously used to calibrate 10Be production rates. For example, based on a geomagnetic implementation of the Lal/Stone scaling scheme we derive a spallogenic production rate of 11.4 ± 0.9 atoms 14C (g quartz)?1 a?1 and a 14C/10Be spallogenic production rate ratio of 3.0 ± 0.2. The results are comparable with production rates from previous calibrations in the northern hemisphere. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   

15.
Quartz, SiO2, a pure mineral with tight crystal structure, is widespread in rocks and soil. Cosmic rays produce10Be (t 1/2=1·5×106, yr) and26Al (t 1/2=7·05×105 yr) in quartz exposed at or near the earth’s surface. The use of accelerator mass spectrometry permits measurement of these nuclides in samples exposed at sea level for typical periods.In situ production makes interpretation relatively straightforward. Potential applications include age determination, measurement of erosion and deposition rates, and use as a tracer for continental weathering processes.  相似文献   

16.
青藏高原的剥蚀与构造抬升*   总被引：8，自引：5，他引：8  
利用宇宙成因核素10 Be和26 Al对西昆仑和可可西里北部地表基岩的剥蚀速率进行了测定,得到的结果是:西昆仑的平均剥蚀速率为12m/Ma, 可可西里北部的平均剥蚀速率为15m/Ma。裂变径迹和宇宙成因核素这两种手段所得到的平均剥蚀速率的时间尺度是不同的,前者得到的是几个至数十百万年的平均剥蚀速率,而后者得到的是十几至几十万年的平均剥蚀速率。比较通过这两种手段得到的青藏高原北部和中部的平均剥蚀速率可以发现其平均剥蚀速率从20Ma以来的100m/Ma以上减少到了最近几十万年以来的10m/Ma,我们认为这一剥蚀速率下降的趋势反映了青藏高原从中新世到第四纪晚期构造活动的减弱,据此推断青藏高原北部和中部的隆升应该主要发生在第四纪晚期之前,而不是在最近的几十万年。  相似文献   

17.
New radiocarbon ages for Sierra Nevada deglaciation, the first 10 Be measurements from the Laurentide terminal moraine, and calculations based on paleomagnetic field strength have the potential to substantially improve the accuracy of cosmogenic age estimates. Specifically, three new constraints apply to the interpretation of measured abundances of in situ produced cosmogenic 10Be and 26Al: (1) A suite of minimum-limiting radiocarbon dates indicates that the Sierra Nevada was deglaciated at least several thousand years earlier than assumed when Nishiizumi et al. (1989) first calibrated 10Be and 26 Al production rates based on polished bedrock surfaces in the range, with retreat beginning by 18,000 cal yr B.P. and completed by 13,000 cal yr B.P. (2) Concentrations of 10Be in moraine boulders and glacier-polished bedrock in New Jersey show little variance (10%, 1σ) and can be used to calculate a preliminary 10Be production rate (integrated over the past 21,000-22,000 cal yr B.P. at 41°, 200-300 m altitude) that is about 20% lower than currently accepted. (3) Calculations of the effect of past geomagnetic field-strength variations on production rates suggest that the use of temporally averaged production rates may generate age errors of >20%; however, cosmogenic exposure ages can be corrected for this effect, although the corrections currently are imprecise. Many previously reported late-Pleistocene 10Be and 26Al exposure ages are probably too young and are less accurate and less precise than implied by reported uncertainties. The discrepancy between accepted production rates and those calculated from Laurentide exposures, when considered together with the Sierran deglacial chronology and the model results, suggest that correlations between cosmogenic and other numerical ages, especially for brief events like the Younger Dryas and Heinrich events, will not be robust until temporal variations and the altitude/latitude scaling of production rates are fully understood and quantified at levels comparable to current analytic uncertainties (3%).  相似文献   

18.
The formulation of watershed management strategies to protect water resources threatened by soil erosion and sedimentation requires a thorough understanding of sediment sources and factors that drive soil movement in the watershed. This paper describes a study of medium-term water-driven soil erosion rates in a mountainous watershed of the Shihmen Reservoir in Taiwan. A total of 60 sampling sites were selected along a hillslope. At each sampling site, the inventory 137Cs activity was determined and then calculated with the diffusion and migration model to derive soil erosion rates. The rates are one to two orders of magnitude lower than estimates using the Universal Soil Loss Equation, a soil erosion model often used in Taiwan. Results of multiple regression analysis indicate that the spatial variability of soil erosion rates is associated with the relative position of a sampling site to the nearest ridge and soil bulk densities (r 2 = 0.33, p < 0.01). Finally, the patterns of soil redistribution rates on the hillslope follow the 137Cs hillslope model as soil erosion increases in the downslope direction. No deposition site is found at footslope because soil deposition is swept away by regular flooding along the stream channel. This study is an important first step in using 137Cs as a tracer of soil redistribution in mountainous watersheds of Taiwan.  相似文献   

19.
The statistical distributions of cosmogenic nuclide measurements from moraine boulders contain previously unused information on moraine ages, and they help determine whether moraine degradation or inheritance is more important on individual moraines. Here, we present a method for extracting this information by fitting geomorphic process models to observed exposure ages from single moraines. We also apply this method to 94 10Be apparent exposure ages from 11 moraines reported in four published studies. Our models represent 10Be accumulation in boulders that are exhumed over time by slope processes (moraine degradation), and the delivery of boulders with preexisting 10Be inventories to moraines (inheritance). For now, we neglect boulder erosion and snow cover, which are likely second-order processes. Given a highly scattered data set, we establish which model yields the better fit to the data, and estimate the age of the moraine from the better model fit. The process represented by the better-fitting model is probably responsible for most of the scatter among the apparent ages. Our methods should help resolve controversies in exposure dating; we reexamine the conclusions from two published studies based on our model fits.  相似文献   

20.
This paper investigates the denudation rates in the Quadrilátero Ferrífero, Minas Gerais State (Brazil). The aim is to compare chemical weathering rates from measurements of solute fluxes in rivers and long-term mean erosion rates deduced from in situ-produced cosmogenic 10Be concentrations measured in fluvial sediments. Both water samples and sediments were collected in fifteen stations (checkpoints) located in four hydrographic basins with low anthropogenic perturbations.Depending of the type of substratum, three degrees of chemical denudation rates from water samples are observed: (i) high rates in marbles; (ii) medium rates in schists, phyllites, granites, gneisses and migmatites; (iii) low rates in quartzites and itabirites. Preliminary results of long-term erosion rates deduced from in situ-produced 10Be are comparable with those of chemical rates.  相似文献   

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