Nuclide Concentrations Research Articles

Is it possible to quantify pebble abrasion and velocity in rivers using terrestrial cosmogenic nuclides?

[1] Attrition rates and mean transport velocities of coarse fluvial sediments averaged over long periods of time (100 a) are yet to be fully quantified. A knowledge of long-term transport rates would allow us to predict the response of fluvial systems to changes in external conditions, while determining clast attrition rates would improve our understanding of fluvial abrasion processes. The concentration of terrestrial cosmogenic nuclides (TCNs) in boulders can be modified during river transport by attrition and temporary exposure, which are two competing processes. In order to evaluate the potential of TCNs to quantify these processes, a stochastic model of TCN concentration evolution in clasts of different sizes is developed from exhumation on a hillslope to a sampling point in a river. The model includes shallow landslides or movement in a regolith and episodic transport of clasts in a sediment layer on the river bed. We study the downstream evolution of TCN concentrations for a population of clasts sourced from a localized TCN-rich lithology on a hillslope. When attrition is strong, the model predicts that the variance and maximum of TCN concentrations decrease downstream. Such a trend is possible only if the dominant hillslope erosion corresponds to shallow landslides. Natural variability of hillslope processes can hamper the trend. A way to limit the scatter is to sample the biggest clasts. On the contrary, if clast attrition is small and river transport slow, TCN concentrations increase downstream. In this case, combining the TCN concentrations of clasts gathered at several river stations should provide a method to estimate their mean transport rate. Our results offer guidelines to interpret the downstream evolution of TCN concentration in pebbles and at the surface of boulders.

Numerical Tools for the Evaluation of Super-Compacted Radioactive Waste Residues

Numerical modeling and simulations of radioactive emissions from nuclear waste residues are important for the assessment of the long-term hazard potential and safety properties. The specific radioactive waste residues considered in this paper are dried and super-compacted metallic remains after sheering and dissolving spent nuclear fuel elements from commercial NPP’s in the process of fuel reprocessing. They consist mainly of hulls and end pieces of the fuel assemblies and some additional technological metallic waste from the reprocessing itself. Because of activation and some small percentage of spent fuel material adhering to hulls, these rad-waste residues are classified as medium active waste. The goal of this work is to develop numerical tools to evaluate the final repository relevant properties of rad-waste residues. Simulating the gamma- and neutron-emission spectra allows numerical studying the effect of varying the concentration, activity and geometrical position of key nuclides in a heterogeneous material embedding.

A metrological approach to measuring40Ar* concentrations in K-Ar and40Ar/39Ar mineral standards

In geochronology, isotopic ages are determined from the ratio of parent and daughter nuclide concentrations in minerals. For dating of geological material using the K-Ar system, the simultaneous determination of 40Ar and 40K concentrations on the same aliquot is not possible. Therefore, a widely used variant, the 40Ar/39Ar technique, involves the production of 39Ar from 39K by neutron bombardment and the reliance on indirect natural calibrators of the neutron flux, referred to as “mineral standards.” Many mineral standards still in use rely on decades-old determinations of 40Ar concentrations; resulting uncertainties, both systematic and analytical, impede the determination of higher accuracy ages using the K-Ar decay system. We discuss the theoretical approach and technical design of a gas delivery system which emits metrologically traceable amounts of 40Ar and will allow for the sensitivity calibration of noble gas mass spectrometers. The design of this system is based on a rigorous assessment of the uncertainty budget and detailed tests of a prototype system. A number of obstacles and proposed resolutions are discussed along with the selection of components and their integration into a pipette system.

Control of geomorphic processes on 10Be concentrations in individual clasts: Complexity of the exposure history in Gobi-Altay range (Mongolia)

The dating of alluvial landforms by cosmogenic nuclides requires distinguishing the pre-deposition inheritance from the post-deposition history of the clasts in the studied marker. Moreover, estimating catchment-scale erosion rates from the concentrations of cosmogenic nuclides in active alluvia requires a good knowledge of the local/regional relationships between rock exhumation and transport through space and time. This is still poorly known for timescales of tens of thousand years. In order to document the evolution of clast exhumation and transport rates through time, we analyze in situ 10Be concentrations in boulders and cobbles from hillslopes to outlet of an arid mountainous catchment located in Gobi-Altay, Mongolia, strongly affected by global climatic changes during the Pleistocene–Holocene period. Samples were collected on bedrock, abandoned alluvial deposits, active colluvia and alluvia. Our results show a large 10Be scattering in the active river bed, consistent with a low and discontinuous catchment erosion rate dominated by mass wasting and fluvial incision. On the contrary, pre-exposure signal within abandoned terraces is much more homogeneous, consistent with climatic pulses responsible of strong erosional events on hillslopes and rapid fluvial transport. These results show that exhumation/transport processes at the catchment scale vary in style and intensity through time as a consequence of climatic oscillations. The occurrence of abrupt climatic changes during short periods of time recorded by 10Be concentrations in abandoned alluvia raise questions about the temporal applicability of catchment erosion rates derived from cosmogenic nuclide concentrations measured in sediments of active rivers. On the other hand, strong and short erosion events limit and homogenize the pre-exposure 10Be signal in associated deposits like debris-flows, making them particularly suitable markers for dating in active tectonic and paleoclimatic studies.

Coupled thermo-hydro-mechanical-migratory model for dual-porosity medium and numerical analysis

A coupled thermo-hydro-mechanical-migratory model of dual-porosity medium for saturated-unsaturated ubiquitous-joint rockmass was established, in which the stress field and the temperature field were single, but the seepage field and the concentration field were double, and the influences of sets, spaces, angles, continuity ratios, stiffnesses of fractures on the constitutive relationship of the medium were considered. Also, the relative two-dimensional program of finite element method was developed. Taking a hypothetical nuclear waste repository as a calculation example, the case in which the rockmass was unsaturated dual-porosity medium and radioactive nuclide leak was simulated numerically, and the temperatures, negative pore pressures, saturations, flow velocities, nuclide concentrations and principal stresses in the rockmass were investigated. The results show that the negative pore pressures and nuclide concentrations in the porosity and fracture present different changes and distributions. Even though the saturation degree in porosity is only about 1/10 that in fracture, the flow velocity of underground water in fracture is about three times that in porosity because the permeability coefficient of fracture is almost four orders higher than that of porosity. The value of nuclide concentration in fracture is close to that in porosity.

Noble gases in the Xinjiang (Armanty) iron meteorite––A big object with a short cosmic‐ray exposure age

Abstract– We measured the concentrations and isotopic ratios of the cosmogenic noble gases He, Ne, and Ar in the very large iron meteorite Xinjiang (IIIE). The 3He and 4He data indicate that a significant portion of the cosmogenic produced helium has been lost via diffusion or in a recent impact event. High 22Ne/21Ne ratios indicate that contributions to the cosmogenic 21Ne from sulfur and/or phosphorous are significant. By combining the measured nuclide concentrations with model calculations for iron meteorites we were able to determine the preatmospheric diameter of Xinjiang to 260–320 cm, which corresponds to a total mass of about 70–135 tons. The cosmic‐ray exposure age of Xinjiang is 62 ± 16 Ma, i.e., relatively short compared to most of the other iron meteorites. With the current database we cannot firmly determine whether Xinjiang experienced a complex irradiation history. The finding of 3He and 4He losses might argue for a recent impact event and therefore for a complex exposure.

Recycling of Amazon floodplain sediment quantified by cosmogenic 26Al and 10Be

The modern Amazon trunk stream and tributaries contain substantial amounts of sediments that were formerly stored for 1–3 m.y. in floodplains. By reworking of floodplain deposits, this sediment is being incorporated into Amazonian streams. We conclude this from using the ratio of in situ−produced cosmogenic 26 Al to 10 Be in detrital sediment from rivers draining the sediment-producing Andean, Guyana, and Brazilian shield areas, as well as the largest Amazonian lowland rivers. Samples from the modern Amazon River yield 26 Al/ 10 Be ratios between ∼3.8 and 5.5, and thus represent mixing between a non-buried sediment end member and a formerly buried sediment end member. The non-buried end member is sourced in the Andes with a ratio of ∼6.5, reflecting recent erosion of hillslopes exposed to cosmic rays, and the buried end member is potentially represented by deeply stored floodplain sediment as old as Miocene age, with a ratio of ∼3, that was partially shielded from cosmic rays. This simple picture of binary mixing is complicated by grain-size effects. Coarser-grained sediment from non-Andean settings (lowlands, cratonic shields) contains slightly more formerly buried material, whereas finer-grained lowland sediment mostly records non-buried 26 Al/ 10 Be ratios and nuclide concentrations that are indicative of an Andean source. Our assessment shows an incorporation of formerly buried, old sediment of central Amazonian floodplains to the fresh, Andean-derived sediment mainly carried in the Amazon trunk stream. We interpret our results to mean that the Amazon regularly avulses into these old deposits that crop out in more distal vicinities to the main river and temporarily leaves its current Holocene channel.

Dating inset terraces and offset fans along the Dehshir Fault (Iran) combining cosmogenic and OSL methods

SUMMARY Be and 36 Cl cosmic ray exposure (CRE) and optically stimulated luminescence (OSL) dating of offset terraces have been performed to constrain the long-term slip rate of the Dehshir fault. Analysis of cosmogenic 10 Be and 36 Cl in 73 surface cobbles and 27 near-surface amalgamscollectedfrominsetterracesdemonstratestheoccurrenceofalowdenudationrateof 1mM a −1 and of a significant and variable inheritance from exposure prior to the aggradation of these alluvial terraces. The significant concentrations of cosmogenic nuclides measured in the cobbles collected within the riverbeds correspond to 72 ± 20 ka of inheritance. The mean CREageofthesurfacesamplescollectedontheolderterraceT3is469 ±88kabuttheanalysis of the distribution of 10 Be concentration in the near-surface samples discard ages older than 412 ka. The mean CRE age of the surface samples collected on terrace T2 is 175 ± 62 ka but the 10 Be depth profile discard ages older than 107 ka. For each terrace, there is a statistical outlier with a younger age of 49.9 ± 3.3 and 235.5 ± 35.4 ka on T2 and T3, respectively. The late sediments aggraded before the abandonment of T2 and inset levels, T1 b and T1a, yielded OSL ages of, respectively, 26.9 ± 1.3, 21.9 ± 1.5 and 10.0 ± 0.6 ka. For a given terrace, the OSL ages, where available, provide ages that are systematically younger than the CRE ages. These discrepancies between the CRE and OSL ages exemplify the variability of the inheritance and indicate the youngest cobble on a terrace, that minimizes the inheritance, is the most appropriate CRE age for approaching that of terrace abandonment. However, the upper bound on the age of abandonment of a terrace that is young with respect to the amount of inheritance is best estimated by the OSL dating of the terrace material. For such terraces, the CRE measurements are complementary of OSL dating and can be used to unravel the complex historyofweatheringandtransportinthecatchmentofdesertalluvialfans.Thiscomprehensive set of dating is combined with morphological offsets ranging from 12 ± 2t o 380± 20 m to demonstrate the Dehshir fault slips at a rate in the range 0.9 mm yr −1 –1.5 mm yr −1 .T he variable inheritance exemplified here may have significant implications for CRE dating in arid endorheic plateaus such as Tibet and Altiplano.

Dating Pliocene lacustrine sediments in the central Jordan Valley, Israel — Implications for cosmogenic burial dating

Cosmogenic burial dating of sediments is usually used at sites with relatively simple or known exposure–burial histories, such as in caves. In an attempt to extend the applicability of the method to other common geological settings (i.e. the dating of late Neogene sedimentary formations), where much less is known about the exposure–burial history, we apply the cosmogenic burial method on Pliocene–early Pleistocene (1.5–4.5Ma) lacustrine sediments in the central Jordan Valley, Israel. 26Al, 10Be, and 21Ne concentrations in quartz were obtained from a 170m tectonically-tilted section. Assuming fast burial and no post-burial production we obtained burial ages which range between 3.5 and 5.3Ma. Integrating simple geological reasoning and the cosmogenic nuclide data, post burial production is found to be insignificant. We also found that the samples contain two distinct populations of grains (chert and quartz) from two different sources which experienced different pre-burial exposure histories. The cosmogenic nuclide concentrations in the samples are in accordance with those expected for the mixing of two sources, and the burial ages computed for both end members agree. Theoretical calculations of two-source mixing show that initial 26Al/10Be ratios are depressed relative to the expected surface ratios and may result in burial ages overestimated by as much as 500ka. Using ages derived from cosmogenic nuclides, independent age constraints, and magnetostratigraphy we correlate the bottom of the section to the Cochiti Normal magnetic subchron (4.19–4.30Ma) within the Reverse Gilbert chron, and the top of the section to the Reverse subchron at the top of the Gilbert chron (3.60–4.19Ma).

The Galactic Cosmic Ray Intensity over the Past 106–109 Years as Recorded by Cosmogenic Nuclides in Meteorites and Terrestrial Samples

Concentrations of stable and radioactive nuclides produced by cosmic ray particles in meteorites allow us to track the long term average of the primary flux of galactic cosmic rays (GCR). During the past ∼10 Ma, the average GCR flux remained constant over timescales of hundreds of thousands to millions of years, and, if corrected for known variations in solar modulation, also during the past several years to hundreds of years. Because the cosmic ray concentrations in meteorites represent integral signals, it is difficult to assess the limits of uncertainty of this statement, but they are larger than the often quoted analytical and model uncertainties of some 30%. Time series of concentrations of the radionuclide 10Be in terrestrial samples strengthen the conclusions drawn from meteorite studies, indicating that the GCR intensity on a ∼0.5 million year scale has remained constant within some ±10% during the past ∼10 million years. The very long-lived radioactive nuclide 40K allows to assess the GCR flux over about the past one billion years. The flux over the past few million years has been the same as the longer-term average in the past 0.5–1 billion years within a factor of ∼1.5. However, newer data do not confirm a long-held belief that the flux in the past few million years has been higher by some 30–50% than the very long term average. Neither does our analysis confirm a hypothesis that the iron meteorite data indicate a ∼150 million year periodicity in the cosmic ray flux, possibly related to variations in the long-term terrestrial climate.

Cosmogenic isotope burial dating of fluvial sediments from the Lower Rhine Embayment, Germany

Cosmogenic isotope burial dating, using 10Be and 26Al, was applied to Plio–Pleistocene fluvial successions from the Lower Rhine Embayment, Germany. The approach consists of three principal steps: (1) measurement of cosmogenic nuclides in depth profiles, (2) modelling of hypothetical nuclide concentrations based on a first-order conceptualisation of the geological context and the principal succession of depositions and subsequent erosional and burial phases, and (3) using parameter estimation to identify values for the a priori unknown model parameters (burial age, initial nuclide concentrations, terrace erosion rates) that result in minimal disagreement between hypothetical and measured nuclide concentrations. The Late Pliocene Kieseloolite Formation was dated to 3650 ± 1490 ka and the Early Pleistocene Waalre Formation to 900 ± 280 ka. The unconformably overlying Upper Terrace Formation revealed ages of 740 ± 210 ka and 750 ± 250 ka for the two different sites. These findings are in good agreement with independent age control derived by bio-, magneto-, and litho-stratigraphy. Furthermore, identifiability and uncertainty analysis reveal a relationship between burial depth and sensitivity of isotope concentrations to burial age and erosion rate. Our results indicate that using shallower buried samples would enable a considerably more robust estimation of the burial age and the terrace erosion rate. Uncertainties arose mainly from nuclide measurements, and not from the uncertainties derived from modelling or insufficient knowledge of nuclide production and decay properties.

Comparison between correlated sampling and the perturbation technique of MCNP5 for fixed-source problems

Correlated sampling and the differential operator perturbation technique are two methods that enable MCNP (Monte Carlo N-Particle) to simulate small response change between an original system and a perturbed system. In this work the performance of the MCNP differential operator perturbation technique is compared with that of the MCNP correlated sampling method for three types of fixed-source problems. In terms of precision of predicted response changes, the MCNP perturbation technique outperforms correlated sampling for the problem involving variation of nuclide concentrations in the same direction but performs comparably with or even underperforms correlated sampling for the other two types of problems that involve void or variation of nuclide concentrations in opposite directions. In terms of accuracy, the MCNP differential operator perturbation calculations may predict inaccurate results that deviate from the benchmarks well beyond their uncertainty ranges for some of the test problems. However, the accuracy of the MCNP differential operator perturbation can be improved if the midpoint correction technique is used.

Air mass origin and its influence on radionuclide activities ( 7Be and 210Pb) in aerosol particles at a coastal site in the western Mediterranean

Studies of radionuclide activities in aerosol particles provide a means for evaluating the integrated effects of transport and meteorology on the atmospheric loadings of substances with different sources. Measurements of aerosol mass concentration and specific activities of 7Be and 210Pb in aerosols at Málaga (36° 43′ 40″ N; 4° 28′ 8″ W) for the period 2000–2006 were used to obtain the relationships between radionuclide activities and airflow patterns by comparing the data grouped by air mass trajectory clusters. The average concentration values of 7Be and 210Pb over the 7 year period have been found to be 4.6 and 0.58 mBq m −3, respectively, with mean aerosol mass concentration of 53.6 μg m −3. The identified air flow types arriving at Málaga reflect the transitional location of the Iberian Peninsula and show significant differences in radionuclide activities. Air concentrations of both nuclides and the aerosol mass concentration are controlled predominantly by the synoptic scenarios leading to the entrance of dust-laden continental flows from northern Africa and the arrival of polar maritime air masses, as implied by the strong correlations found between the monthly frequencies of the different air masses and the specific activities of both radionuclides. Correlations between activity concentrations and precipitation are significant though lower than with air masses.

Degradation of glacial deposits quantified with cosmogenic nuclides, Quartermain Mountains, Antarctica

AbstractMany glacial deposits in the Quartermain Mountains, Antarctica present two apparent contradictions regarding the degradation of unconsolidated deposits. The glacial deposits are up to millions of years old, yet they have maintained their meter‐scale morphology despite the fact that bedrock and regolith erosion rates in the Quartermain Mountains have been measured at 0·1–4·0 m Ma−1. Additionally, ground ice persists in some Miocene‐aged soils in the Quartermain Mountains even though modeled and measured sublimation rates of ice in Antarctic soils suggest that without any recharge mechanisms ground ice should sublimate in the upper few meters of soil on the order of 103 to 105 years. This paper presents results from using the concentration of cosmogenic nuclides beryllium‐10 (10Be) and aluminum‐26 (26Al) in bulk sediment samples from depth profiles of three glacial deposits in the Quartermain Mountains. The measured nuclide concentrations are lower than expected for the known ages of the deposits, erosion alone does not always explain these concentrations, and deflation of the tills by the sublimation of ice coupled with erosion of the overlying till can explain some of the nuclide concentration profiles. The degradation rates that best match the data range 0·7–12 m Ma−1 for sublimation of ice with initial debris concentrations ranging 12–45% and erosion of the overlying till at rates of 0·4–1·2 m Ma−1. Overturning of the tills by cryoturbation, vertical mixing, or soil creep is not indicated by the cosmogenic nuclide profiles, and degradation appears to be limited to within a few centimeters of the surface. Erosion of these tills without vertical mixing may partially explain how some glacial deposits in the Quartermain Mountains maintain their morphology and contain ground ice close to the surface for millions of years. Copyright © 2010 John Wiley & Sons, Ltd.

Relative radiological risks derived from different TENORM wastes in Malaysia

In Malaysia technologically enhanced naturally occurring radioactive materials (TENORM) wastes are mainly the product of the oil and gas industry and mineral processing. Among these TENORM wastes are tin tailing, tin slag, gypsum and oil sludge. Mineral processing and oil and gas industries produce large volume of TENORM wastes that has become a radiological concern to the authorities. A study was carried out to assess the radiological risk related to workers working at these disposal sites and landfills as well as to the members of the public should these areas be developed for future land use. Radiological risk was assessed based on the magnitude of radiation hazard, effective dose rates and excess cancer risks. Effective dose rates and excess cancer risks were estimated using RESRAD 6.4 computer code. All data on the activity concentrations of NORM in wastes and sludges used in this study were obtained from the Atomic Energy Licensing Board, Malaysia, and they were collected over a period of between 5 and 10 y. Results obtained showed that there was a wide range in the total activity concentrations (TAC) of nuclides in the TENORM wastes. With the exception of tin slag and tin tailing-based TENORM wastes, all other TENORM wastes have TAC values comparable to that of Malaysia's soil. Occupational Effective Dose Rates estimated in all landfill areas were lower than the 20 mSv y(-1) permissible dose limit. The average Excess Cancer Risk Coefficient was estimated to be 2.77×10(-3) risk per mSv. The effective dose rates for residents living on gypsum and oil sludge-based TENORM wastes landfills were estimated to be lower than the permissible dose limit for members of the public, and was also comparable to that of the average Malaysia's ordinary soils. The average excess cancer risk coefficient was estimated to be 3.19×10(-3) risk per mSv. Results obtained suggest that gypsum and oil sludge-based TENORM wastes should be exempted from any radiological regulatory control and should be considered radiologically safe for future land use.

Naturally Occurring Radionuclides of Paper Ashes and Their Effect in the Environment

The concentrations of naturally occurring radionuclides (226Ra, 228Ra, 228Th, and 40K) in ashes of papers such as magazines and newspapers were determined from the nuclide concentrations in the papers and the ash contents of the papers. The average 226Ra, 228Ra, 228Th, and 40K concentrations in the 34 ashes were respectively 27, 68, 75, and 75Bq kg−1. The radium equivalent activities of the ashes were calculated to evaluate the hazard of &gannma;-ray radiation from the ashes in the environment. A copying paper sample showed a high radium equivalent activity of 602Bq kg−1. However, the average radium equivalent activity was 140Bq kg−1 and was lower than the level that causes an environmental health problem.

Measurement of natural activities of238U,232th and40k in tin ore

Tin ore is widely used in many industrial purposes. Such as tin ore contain natural radioactive nuclides at various concentrations. If this ore contain high concentrations of natural radioactive nuclides, workers handling them might be exposed to significant levels of radiation. Therefore it is important to determine the radioactive nuclides in this ore. The natural radionuclides ( 238 U, 232 Th and 40 K) and their daughter products contents of tin ore have been estimated in gamma-ray spectrometry and their levels using 70% Hyper-Pure` Germanium (HPGe) detector. The mean activities due to the three radionuclides ( 238 U, 232 Th and 40 K) were found to be 40 ± 3, 19 ± 2.2 and 97 ± 19 Bqkg −1 , respectively. The radium equivalent activity varied from 77.87 ± 11-12.03 ± .27 Bqkg −1 . The representative external hazard index values for the corresponding Tin Ore are also estimated. The results of this assessment obtained by the gamma-ray spectroscopic analysis, have indicated that the levels of natural radioactivity were lower than the international recommended limits.

Natural radionuclide distribution in quartzite sands of East Mediterranean Region (Turkey)

The study examined 15 sites in the Ovacik-Kargicak (Mersin, Turkey) open pit mines and obtained two quartzite samples from each. Gamma-ray spectrometric measurements were undertaken on the quartzite rocks and sands to quantify the concentrations of the natural radionuclides 226Ra, 232Th and 40K. In addition, the mean radioactivity concentrations of these natural nuclides were calculated to be 6.94, 7.34 and 140.05 Bqkg−1. Also the mean radioactivity concentrations of the natural nuclides in old Ovacik quartzite sands 226Ra, 232Th and 40K were calculated to be 9.85, 10.54, and 226.40 Bqkg−1, respectively.

Transmutation detectors

We have designed a new type of detectors, called transmutation detectors, which can be used primarily for neutron fluence measurement. The transmutation detector method differs from the commonly used activation detector method in evaluation of detector response after irradiation. Instead of radionuclide activity measurement using radiometric methods, the concentration of stable non-gaseous nuclides generated by transmutation in the detector is measured using analytical methods like mass spectrometry. Prospective elements and nuclear reactions for transmutation detectors are listed and initial experimental results are given. The transmutation detector method could be used primarily for long-term measurement of neutron fluence in fission nuclear reactors, but in principle it could be used for any type of radiation that can cause transmutation of nuclides in detectors. This method could also be used for measurement in accelerators or fusion reactors.

Sediment production and transport from in situ-produced cosmogenic 10Be and river loads in the Napo River basin, an upper Amazon tributary of Ecuador and Peru

Abstract 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 Solimoes (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.