Excess Lead-210 Research Articles

Sediment budget of a Maumee River headwater tributary: how streambank erosion, streambed-sediment storage, and streambed-sediment source inform our understanding of legacy phosphorus

ObjectiveWe described source and phosphorus (P) retention potential of soft, fine-grained, streambed sediment and associated phosphorus (sed-P) during summer low-flow conditions. Combining in-channel, sed-P storage with relative age provided context on relevance to western Lake Erie Basin management goals.MethodsIn 2019, rapid geomorphic assessment (30 reaches) compared streambed-sediment storage (S) to streambank erosion (E), providing annual sediment budgets (S:E). Streambed sediment (13 reaches) was fingerprinted and analyzed for sed-P. The P saturation ratio (PSR; four reaches) quantified potential sorption/desorption of dissolved P (DP) between the water column and streambed sediment. Analyses were supplemented with data from 2017 and 2021. The ratio of two fallout radionuclides, beryllium-7 (54-day half-life) and excess lead-210 (22.3 years), apportioned “new” sediment based on time since rainfall contact.ResultsStreambed sediment was mostly streambank (54–96%) for contributing areas > 2.7 km2; for upstream reaches, a larger percentage was apportioned as upland (cropland, pasture, forest, and road), with < 30% streambank. Streambank erosion correlated with contributing area; however, soil type (ecoregion), stream characteristics, and land use combined to drive streambed-sediment storage. Individual-reach S:E (accumulation of 0.01–35 years of streambank erosion) differentiated erosional and depositional in-channel environments. Most reaches indicated that 17–57% of sediment had recent contact with rainfall. Streambed-sediment PSR indicated a low potential for further sorption of DP from the water column; one reach was a P source when sampled.ConclusionSed-P was higher in streambed sediment than in source samples, which varied by land use and ecoregion. This indicates homogenization resulting from in-stream sorption of DP during sediment transport that occurs over multiple events.

Soil organic carbon and nitrogen reduction through cattle-path induced erosion in montane grasslands, KwaZulu-Natal, South Africa

The impact of daily cattle migration from homesteads to higher altitude pastures creates severe erosion in the montane grasslands of the predominantly subsistence agricultural rural communities of KwaZulu-Natal, Drakensberg, South Africa. This study quantifies the impact of a degraded cattle path at up, mid and downslope positions on SOC and N distribution in the soil profile and within the soil aggregates. An attempt to evaluate sites of erosion and deposition using excess lead-210 (210Pbex) to support our findings was conducted. On average, the degraded cattle path reduced SOC and N in the bulk soil (by 3–4 times, respectively) and was associated with 53% reduction in aggregate stability and a 14% increase in soil bulk density over the non-degraded reference site. These results reflect the loss of vegetation cover (correlated positively to SOC and N (r ≈ 0.94)), which were triggered by cattle grazing and trampling leading to top-soil loss. Cattle hoofs damage the grass and breakdown soil aggregates, exposing the fertile topsoil particles to detachment and consequential transportation via rill and sheet erosion. This is supported by the loss of 210Pbex in the topsoil of the degraded slope positions relative to the reference site. Consistent down core mixing of 210Pbex activity in degraded slope sites supports evidence of cattle mediated soil mixing. Our findings highlight the accelerated land degradation that results from uncontrolled grazing and movement of cattle on sloping lands in the Okhombe Valley. Developing an integrated management strategy co-led by local communities to develop proactive participatory sustainable land use practices is critical for long-term landscape maintenance and recovery in the region.

Tendency of soil erosion dynamics by coupling radioisotopes and RUSLE model on the Southeastern Tibetan Plateau in response to climate warming and human activity

Soil erosion has created landscape problems in many parts of the world and in particular in cold regions where the sensitive permafrost conditions have changed due to climate warming. Such a case occurred in the Tibetan Plateau (TP), which has been strongly affected by global warming and human activities. Monitoring technologies, like remote sensing and field surveys were used to explore soil erosion rates in the TP, but they were limited by the resolution and meteorological disturbance factors or the spatial and time scales. Here, we present for the first time 210Pbex (excess lead-210) and 137Cs (caesium-137) data of soils from the southeastern TP (SETP) covering an area of 640,000 km2. In the permafrost-dominant areas, the results show mean soil-erosion rates in the last 56–100 years that were relatively higher (1891 t·km−2·a-1) based on 210Pbex than those based on 137Cs (1623 t·km−2·a-1). Modelling results from the Revised Universal Soil Loss Equation (RUSLE) indicate relatively high mean soil erosion rates of 4363 and 4394 t·km−2·a-1 using a period covering the last 40 or 10 years respectively. Our data suggest accelerating erosion rates on the SETP that are linked to permafrost degradation, and glacier and snow melting due to accelerating global climate warming. The increase in ground surface temperature of ∼2 °C in the last four decades has further shifted the regional hydrology, affecting the degeneration of vegetation cover and a further increase in soil-erosion rates. However, our radionuclides data also expose low erosion rates in the seasonally frozen ground at some sampling sites which indicates the complex nature of erosion trends in cold regions that require careful adaptation of soil management.

Assessing the Impact of Climate Change on Soil Moisture and Soil Erosion in Northwest Morocco Based on Nuclear Techniques (Caesium-137 Re-sampling and Excess Lead-210 Sediment Dating) and Remote Sensing

Assessing the Impact of Climate Change on Soil Moisture and Soil Erosion in Northwest Morocco Based on Nuclear Techniques (Caesium-137 Re-sampling and Excess Lead-210 Sediment Dating) and Remote Sensing

Caesium-137 re-sampling approach and excess Lead-210 sediment dating to assess the impacts of climate change and agricultural practices on soil erosion and sedimentation in Northwest Morocco

Caesium-137 re-sampling approach and excess Lead-210 sediment dating to assess the impacts of climate change and agricultural practices on soil erosion and sedimentation in Northwest Morocco

A new approach for dating recent silting soils of debris flow deposits from the changes of 210Pbex concentration in surface layers

The 210Pbex accumulation-decay model is widely applied for sediment dating while not applicable for dating sediments derived from debris flows because the sources of excess lead-210 (210Pbex) are diverse and changeable. In this study, we developed a new model (n=-1λln1-C(n)Cr) for dating recent silting soils of debris flow deposits using changes in 210Pbex concentration in surface soil layers, where C(n) is the 210Pbex concentration in the top layer of the silting soil after n years since final sedimentation (Bq/kg), Cr is the 210Pbex concentration in the top layer at a reference site (no erosion or sedimentation in the long term), and coefficient λ = 0.03114 year−1 is the decay constant of 210Pbex. The model was validated with data obtained from the Hunshui Gully watershed in southwestern China. Six sampling sites with different sedimentation stages were selected in the Hunshui Gully watershed. The 210Pbex concentration increased from 51.20 ± 4.61 for the site with debris flow deposits in 2003 to 112.56 ± 7.88 Bq/kg for those in 1965. The derived time of the new model fits the actual value well, and the root-mean-square error of this new model was 3.23 years. Our results confirm that the fallout radionuclide 210Pbex can be applied for dating recent silting soils in debris flow deposits. However, further studies are needed to verify the model and determine the coefficients for various environments.

A worldwide meta-analysis (1977–2020) of sediment core dating using fallout radionuclides including &amp;lt;sup&amp;gt;137&amp;lt;/sup&amp;gt;Cs and &amp;lt;sup&amp;gt;210&amp;lt;/sup&amp;gt;Pb&amp;lt;sub&amp;gt;xs&amp;lt;/sub&amp;gt;

Abstract. Dating recent sediment archives (&lt; 150 years) constitutes a prerequisite for environmental and climatic reconstructions. Radiocaesium (137Cs) emitted during thermonuclear bomb testing (∼ 1950–1980) and nuclear accidents and the decrease in excess lead-210 (210Pbxs) with depth are often combined to establish sediment core chronology. Although these methods have been widely used during the last several decades, there is a lack of structured and comprehensive worldwide synthesis of fallout radionuclide analyses used for dating sediment cores in environmental and Earth sciences. The current literature overview was based on the compilation of 573 articles published between 1977 and 2020, reporting the collection of 1351 individual dating sediment cores (the dataset can be accessed at https://doi.org/10.1594/PANGAEA.931493; Foucher et al., 2021). This review was conducted in order to map the locations where 137Cs fallout events were detected. These included the thermonuclear bomb testing peak in 1963, the Chernobyl accident in 1986, the Fukushima accident in 2011, and 24 additional events identified at 112 sites that led to local or regional radioactive releases (e.g., Sellafield accidents, Chinese nuclear tests). When 210Pbxs records were used along with 137Cs data, detailed information on the 210Pbxs age–depth models were also synthesized. With the current growing number of studies analyzing sediment cores and the increasing interest in the deployment of sediment fingerprinting techniques including radionuclides as potential discriminant properties, this spatialized synthesis provides a unique worldwide compilation for characterizing fallout radionuclide sources and levels at the global scale. This synthesis provides in particular a reference of 137Cs peak attribution for improving the sediment core dating, and it outlines the main questions that deserve attention in future research as well as the regions where additional 137Cs fallout investigations should be conducted in priority.

Stream bank erosion as a source of sediment within New Zealand catchments

ABSTRACT Stream bank erosion has been anecdotally identified as an important source of sediment in New Zealand catchments, however, there have been few attempts to quantify its contribution. Here we use a radionuclide-based sediment tracing approach to determine the relative contribution of stream bank- and hillslope-derived sediment within three catchments in the upper North Island of New Zealand. Both lithogenic (radium-226 and radium-228) and fallout radionuclides (caesium-137 and excess lead-210) were used to differentiate sediment derived from stream bank and hillslope sources. The relative contribution of stream banks and hillslopes to fluvially transported suspended sediment were predicted using a mixing model approach. Our results indicate that both fallout and lithogenic radionuclides provide good source differentiation. We demonstrate that stream bank erosion can contribute very high proportions of sediment within New Zealand catchments. We used independent assessments of bank erosion from each of the study catchments to support the sediment source fingerprinting results. Further work is required to determine the spatial and temporal variability of the contribution of sediment from stream banks. Information on the importance of different sediment sources is needed to target limited catchment rehabilitation resources where they will have the most impact.

Deciphering human and climatic controls on soil erosion in intensively cultivated landscapes after 1950 (Loire Valley, France)

Intensification of agricultural practices during the second half of the 20th century has accelerated soil erosion around the world. Although this phenomenon has been widely investigated through a combination of monitoring or modelling at short timescales (<10 years), few records are available for reconstructing the trajectory of soil erosion during longer periods (i.e. the 20th century). Analysis of sediment deposits in reservoirs provides a valuable tool for reconstructing these trends and for identifying the driving factors that may have disturbed the sediment cascade after 1950. Accordingly, sediment cores (n = 5) were collected in a reservoir located at the outlet of an intensively cultivated lowland catchment (24.5 km²) representative of those found in central France. Natural (excess lead-210) and artificial radionuclides (caesium-137, americium-241) enable dating of these sedimentary sequences. The corresponding sediment accumulation and erosion rates were calculated for the 1928–2017 period. In addition, daily rainfall records, land use change and agricultural field patterns were reconstructed for the period comprised between 1950 and 2017, based on weather records, digitalized aerial images (n = 10) and agricultural census data (n = 6). Results showed substantial acceleration of erosion rates after 1928 (+500 %). This increase occurred simultaneously with major landscape changes that led to an increase in plot size (+465 %) and decrease of the surface occupied by grassland and fallow land (-93 %). Both parameters correlated strongly with the erosion rates reconstructed in this catchment (r = 0.87 and r = 0.95 for the plot size and grassland/fallow land surfaces, respectively). In addition, spectral analyses of daily rainfall records and mass accumulation rates, estimated with a high temporal resolution from the sediment core tomography scanner data, showed concomitant short (i.e., 1 year) and long-term (i.e., 16 years) cycles between mass accumulation rates and rainfall. Overall, this study demonstrated the long-term impact of human activities and rainfall dynamics on soil erosion. Between 1928 and 2017, erosion rates increased seven-fold in this lowland catchment, until reaching 31.5 t km−2 yr-1 by 2017. Although landscape modifications likely drove the pluri-decadal trends of erosion, this study has also demonstrated the major role played by rainfall intensity on annual sediment dynamics.

Linking soil redistribution to soil organic carbon using 210Pbex along different complex toposequences in a karst region, southwest China

Understanding the magnitudes and patterns of soil and associated soil organic carbon (SOC) redistribution is of great importance for assessing both soil degradation and rocky desertification in fragile karst ecosystems. However, to date reliable information on the application of excess lead-210 (210Pbex) for quantifying dynamics of SOC-soil redistribution relationship is limited on the hillslopes due to the highly heterogeneous landscape after land use change in karst systems. In this study, three typical toposequences (grassland, forests, and agroforestry) were selected in a typical karst graben basin of southwestern China. A total of 229 soil samples were collected along the toposequences to explore the soil redistribution process using a model coupled with measured 210Pbex, and then to ascertain the relationship of discharged 210Pbex to erosion-induced SOC. The results showed that most soil samples were significantly affected by soil erosion (reference value of 210Pbex: 5986 ± 756 Bq m−2). The output of the model demonstrated that the highest estimated net soil flux occurred in agroforestry (21.38 Mg ha−1 yr−1), followed by forests (8.39 Mg ha−1 yr−1) and grassland (−3.54 Mg ha−1 yr−1). Meanwhile, the SOC concentration showed a decreasing order of agroforestry > forest > grassland on the hillslopes, implying that while agroforestry help increase SOC sequestration potential, intensive human intervention also carries a high risk of soil erosion in karst regions. The fitting correlation of 210Pbex and SOC in surface soils displayed the decreasing order of grassland (R2 = 0.80) > forests (R2 = 0.69) > agroforestry (R2 = 0.42), indicating that fallout 210Pbex could be used directly as a tracer to quantify the dynamics of the SOC-soil redistribution relationship in grassland toposequence. Our results provide practical evidence of the potential of 210Pbex for quantifying SOC-soil redistribution along hillslopes with complex toposequences in a karst region.

Interactive effects of land use and soil erosion on soil organic carbon in the dry-hot valley region of southern China

Changes in land use can result in soil erosion and the loss of soil organic carbon (SOC). However, the individual contribution of different land use types on SOC variability as well as the combined impacts of land use and soil erosion are still unclear. The aims of the present study were to: (1) evaluate soil erosion and SOC contents under different land use types, (2) identify the influences of soil depth and land use on SOC content, and (3) determine the contribution of land use and soil erosion on SOC variability. We assessed the SOC and total soil nitrogen (TSN) contents under three types of land use in the dry-hot valley in southern China. Caesium-137 (137Cs) and excess lead-210 (210Pbex) contents were also measured to determine soil-erosion rates. Land use was found to significantly affect soil erosion, and erosion rates were higher in orchard land (OL) relative to farmland (FL), which is in contrast with previous study results. SOC and TSN contents varied significantly between the three land use types, with highest values in forest land (FRL) and lowest values in OL. SOC was found to decrease with decreasing soil depth; the highest rate of reduction occurred in the reference site (RS), followed by FRL and FL. The interaction between soil erosion and land use significantly impacted SOC in the soil surface layer (0–12 cm); the direct impact of soil erosion accounted for 1.5% of the SOC variability, and the direct or indirect effects of land use accounted for the remainder of the variability. SOC content in deep soil was mainly affected by factors related to land uses (89.0%). This quantitative study furthers our understanding on the interactive mechanisms of land use and soil erosion on changes in soil organic carbon.

A Preliminary Study of the Impacts of Shelter Forest on Soil Erosion in Cultivated Land: Evidence from integrated 137Cs and 210Pbex Measurements

The Three-North Shelter Forest Program (TNSFP) has played an important role in increasing the forest coverage and decreasing the wind erosion rate. Fallout radionuclides (FRNs), such as caesium-137 (137Cs) and excess lead-210 (210Pbex), have extensively been used as fast and economical tools to estimate erosion rates over a broad range of spatial and temporal scales. However, the 137Cs and 210Pbex conversion models used to estimate annual soil erosion rates under steady state conditions have not been used to predict corresponding rates following reforestation due to the land-use changes that have occurred since 1963 in China. This study investigated the influence of shelter forest on soil erosion of cultivated land (CL) by using 137Cs and 210Pbex in northern China. Opposing trends in the spatial variation were found between the 137Cs and 210Pbex inventories, which were due to their differing sources and mass balances within the soil profile. Exploiting this finding, we propose a conceptual 137Cs and 210Pbex variation model to detect depth variations between before and after reforestation. Wind erosion rate was 29.11 ± 14.42 Mg ha-1 yr-1 in the CL when estimated with 137Cs. However, estimation by the revised 210Pbex mass balance model showed that wind erosion rates were 68.55 ± 22.11 and -8.52 ± 47.32 Mg ha-1 yr-1 before and after the surrounding forest matured, respectively. The spatial variation in the 137Cs inventories mainly reflects the soil erosion patterns before the surrounding shelter forest matured, while the spatial variation in the 210Pbex inventories mainly reflects the soil erosion patterns after the surrounding shelter forest matured. Furthermore, the soil erosion rates that were deduced from the 210Pbex conversion model confirmed that the TNSFP played an important role in decreasing the soil erosion rates and intercepting sandstorms in northern China. Thus, the advantages and limitations associated with the use of FRNs for assessing wind erosion in the study area were also proposed. More attention should also be paid to the limitation of estimating soil erosion rates by using FRNs measurements in the TNSFP region of China.

Use of caesium-137 re-sampling and excess lead-210 techniques to assess changes in soil redistribution rates within an agricultural field in Nakhla watershed

The Nakhla watershed was one of the most affected areas by water erosion in Northwest Morocco. This phenomenon caused the sedimentation of the Nakhla dam, an important water reservoir that contributed to water supply in the region. The motivation behind our investigation is to know the evolution of the situation in this watershed. The objective of this study is to use fallout Caesium-137 and excess Lead-210 to assess changes in soil redistribution rates during the last century and the impact of recent soil conservation practices within an agricultural field in the Nakhla watershed. The Caesium-137 technique, which has proved its efficiency in estimating erosion rates over the last 50–60 years, was associated with the re-sampling approach in the current study to reveal the response of soil redistribution rates to land use change over the last fifteen years within an upland agricultural field in Northwest Morocco under Mediterranean conditions. The excess Lead-210 was used to generate data on soil movement over a longer time window of 100 years. Subsequently, the combined use of Caesium-137 re-sampling with excess Lead-210 techniques allowed for the estimation of soil erosion rates over three periods: 1954–2002, 1954–2017 and 1917–2017. Quantification of water erosion with excess Lead-210 indicated a rate of 27 t ha−1 yr−1. Based on Caesium-137 measurements, the comparative analysis of the soil redistribution estimates over two periods (1954–2002 and 1954–2017) shows that soil loss rates have significantly decreased from 36 t ha−1 yr−1 to 29 t ha−1 yr−1 during the period between the two sampling campaigns (2002–2017) due to the beneficial impact of agricultural practices. This decrease is consistent with the available records of sediment yield in the Nakhla dam during these periods. The main findings show that the sustainable agricultural practices and the implemented soil erosion control strategy, based on more frequent fallow with natural vegetation and olive plantations, were effective in reducing soil erosion in the investigated area.

Using reservoir deposits to reconstruct the impact of recent changes in land management on sediment yield and sediment sources for a small catchment in the Black Soil region of Northeast China

Sediment deposits in lakes and reservoirs can provide a basis for reconstructing the erosional response of the upstream catchment. Recent studies undertaken in the Loess Region of China have successfully used the flood couplets contained in the sediment deposits behind check dams to reconstruct records of annual sediment yield and document the impact of changes in land management. However, flood couplets are rarely found in other areas of the world and there is a need to develop alternative approaches to establish the chronology for sediment deposits. Lead-210 dating techniques offer such potential, by making it possible to establish the age of the individual sections of a core. This contribution reports a study of the sediment deposits accumulated in a reservoir at the outlet of a small 3.46 km2 catchment in the Black Soil region of Northeast China. This region suffered serious soil erosion during the last century and this resulted in changes in land management aimed at reducing erosion rates. The sediment deposits in the reservoir covered the period 1974–2004, when such changes occurred. The study was designed to explore the potential for using excess lead-210 to establish the chronology for the reconstruction and for extending the scope of such studies by using sediment source fingerprinting techniques to include complementary information on changes in sediment source. The study successfully reconstructed the record of annual sedimentation depth, a surrogate for sediment yield. This showed that the annual sediment yield from the catchment decreased by about 60% as a result of improved land management introduced in the late 1970s and early 1980s. Sediment source fingerprinting techniques were also successfully used to identify three primary sources, namely topsoil (interrill and rill) erosion from cultivated and uncultivated areas and gully erosion and apportion their contributions to the annual sediment yields. The findings indicated that interrill and rill erosion and gully erosion were both important sediment sources throughout the period covered, contributing 47.9% and 42.0% of the sediment, respectively, that gully erosion increased in importance during years with high magnitude events, and that there was a general trend for rill and interrill erosion to decrease and for gully erosion to increase over the period 1974–2004. The study is seen as demonstrating the potential for using lead-210 measurements in combination with sediment source fingerprinting techniques to reconstruct the impact of changes in land use and land management on erosion processes and sediment yields.

Using Atmospheric Fallout Radionuclides 137Cs and 210Pbxs to Identify Sources of Suspended Sediment in an Agricultural Watershed

Abstract. The use of atmospheric fallout radionuclides as sediment tracers can provide valuable information on sediment transport processes within watersheds. Research was conducted in cropland and mixed land use subwatersheds of the Pleasant Valley watershed (50 km2) in south-central Wisconsin to identify sources of suspended sediment using cesium-137 (137Cs) and excess lead-210 (210Pbxs). In the cropland-dominated subwatershed, contributions to suspended sediment from in-stream and agricultural sources ranged from 36% to 81% and from 19% to 64%, respectively. In the mixed land use subwatershed, contributions to suspended sediment from in-stream and agricultural sources ranged from 68% to 100% and from 0% to 32%, respectively. The relative contribution to suspended sediment from agricultural sources was greater in the cropland subwatershed compared to the mixed land use subwatershed. The phosphorus (P) concentration in suspended sediment and fine sediment deposited on the streambed varied from 601 to 1545 mg kg-1, and cropland areas were the dominant sources of P-enriched sediment in streams. The legacy P and sediment in this watershed can result in a lag time between implementation of best management practices and attainment of desired water quality improvement. Depending on the fingerprinting properties considered (metals vs. fallout radionuclides) and suspended sediment source considerations, the source apportionment results may vary. Keywords: Fallout radionuclides, Legacy sediment, Phosphorus, Sediment fingerprinting, Soil erosion, Streambanks.

Using 137Cs and 210Pbex measurements to explore the effectiveness of soil conservation measures in semi-arid lands: a case study in the Kouhin region of Iran

One of the most fragile agro-ecosystems in Iran is represented by dry farming lands on steep hillslopes that occupy ca. 54% of the national agricultural lands. Therefore, in order to reduce loss of fertile soil from these lands, it is important to apply effective soil conservation strategies. This study used cesium-137 (137Cs) and excess lead-210 (210Pbex) measurements to assess the effectiveness of soil conservation practices in controlling soil erosion in Kouhin, Qazvin Province of Iran. Soil samples were collected from two adjacent hillslopes with and without conservation agriculture practices. The managed site benefited from conservation practices, including controlled grazing, terraces, and contour farming for 50 years. The unmanaged site was under cultivation without agricultural conservation practices. At the managed site, both 137Cs and 210Pbex activities were measured. Only 137Cs activity was measured for the unmanaged site. Conservation practices performed at the managed site were effective in minimizing soil erosion, i.e., net soil erosion rates provided by 137Cs measurements were only 4.6 Mg ha−1 year−1 at the managed site, compared to 12 Mg ha−1 year−1 for the unmanaged site. By using the 210Pbex approach, net soil loss and sediment delivery ratio in the managed site were 12.3 Mg ha−1 year−1 and 60%, respectively. Soil redistribution magnitudes established from 137Cs inventories are associated with a period extending from 1963 to the time of sampling, while those obtained from 210Pbex measurements are related to the past 100 years. However, 210Pbex measurements are expected to be more sensitive to erosive events that occurred during the last 15 to 20 years and this could reflect the higher estimate of erosion rate provided by this radionuclide for the managed site. The increasing trend seems to suggest a possibility that the higher soil losses estimated by the 210Pbex measurements are a result of increasing climate variability during the last two decades. However, further investigation would be needed to confirm this hypothesis.

High-resolution investigation of event driven sedimentation: Northeastern Gulf of Mexico

Abstract A rapid sedimentation pulse in the northeast Gulf of Mexico, associated with the Deepwater Horizon blowout in 2010, provided a unique opportunity to investigate a depositional event in real time and at very high resolution. Sediment cores were collected annually (2010-2016) from four sites and sub-sampled at 2 mm resolution to identify and characterize the sedimentary signature, as well as geochronology and accumulation rates using excess Lead-210 (210Pbxs) and excess Thorium-234 (234Thxs). The “time-series” collection of sediment cores on an annual basis allowed for the identification of changes in sedimentation on monthly to annual time-scales, which define the depositional pulse (2010-2011), initial post-event sedimentary impact and response (2011-2012), and post-event stabilization and preservation (2013-2016). The 2010 depositional pulse was short-lived (

Excess Lead-210 and Plutonium-239+240: Two suitable radiogenic soil erosion tracers for mountain grassland sites

The expected growing population and challenges associated with globalisation will increase local food and feed demands and enhance the pressure on local and regional upland soil resources. In light of these potential future developments it is necessary to define sustainable land use and tolerable soil loss rates with methods applicable and adapted to mountainous areas. Fallout-radionuclides (FRNs) are proven techniques to increase our knowledge about the status and resilience of agro-ecosystems. However, the use of the Caesium-137 (137Cs) method is complicated in the European Alps due to its heterogeneous input and the timing of the Chernobyl fallout, which occurred during a few single rain events on partly snow covered ground. Other radioisotopic techniques have been proposed to overcome these limitations. The objective of this study is to evaluate the suitability of excess Lead-210 (210Pbex) and Plutonium-239+240 (239+240Pu) as soil erosion tracers for three different grassland management types at the steep slopes (slope angles between 35 and 38°) located in the Central Swiss Alps. All three FRNs identified pastures as having the highest mean (± standard deviation) net soil loss of −6.7 ± 1.1, −9.8 ± 6.8 and −7.0 ± 5.2 Mg ha-1 yr-1 for 137Cs, 210Pbex and 239+240Pu, respectively. A mean soil loss of −5.7 ± 1.5, −5.2 ± 1.5 and-5.6 ± 2.1 was assessed for hayfields and the lowest rates were established for pastures with dwarf-shrubs (−5.2 ± 2.5, −4.5 ± 2.5 and −3.3 ± 2.4 Mg ha-1 yr-1 for 137Cs, 210Pbex and 239+240Pu, respectively). These rates, evaluated at sites with an elevated soil erosion risk exceed the respective soil production rates. Among the three FRN methods used, 239+240Pu appears as the most promising tracer in terms of measurement uncertainty and reduced small scale variability (CV of 13%). Despite a higher level of uncertainty, 210Pbex produced comparable results, with a wide range of erosion rates sensitive to changes in grassland management. 210Pbex can then be as well considered as a suitable soil tracer to investigate alpine agroecosystems.

Integrating compound-specific δ13C isotopes and fallout radionuclides to retrace land use type-specific net erosion rates in a small tropical catchment exposed to intense land use change

Retracing net erosion rates linked to land use change in tropical agricultural catchments dominated by small-holder farmers is challenging, due largely to catchment heterogeneity and uncontrolled farming practices. To tackle this problem and to complement a preceding study (Brandt et al., 2016), we introduce here an advanced approach that integrates compound-specific δ13C isotopes (CSSI) and fallout radionuclides (FRN), Excess lead-210 (210Pbex) and Cesium-137 (137Cs) to estimate past net erosion rates of dominant land use types in the mountainous catchment Chieng Khoi (207ha, Northwest Vietnam). Spatially-integrated topsoil (0 to 2cm) samples of dominant land use types (e.g., protected and secondary forests, teak, fruit orchards, maize, cassava) were collected from at least three discrete plots of each land use type within the upland area (i.e., erosion sites) of the studied catchment. In the corresponding lowland area, a representative sediment profile was localized and divided into sections of 2 to 4cm for CSSI and FRN analysis. Samples for FRN reference data were taken from undisturbed areas in close proximity. At a soil deposition site near a lakeshore, 210Pbex data determined the age and sediment accumulation rates of 19 sediment layers to a depth of 38cm. Based on 210Pbex activity, maximum sediment accumulation rates of 127tha−1y−1 were calculated, corresponding to erosion rates of about 16tha−1y−1 for the total catchment area. CSSI data confirmed that maize and cassava were the most important erosion sources during a period of dramatic land use change (1987 to 2004), when forests were cleared and high-yielding maize hybrids were introduced. Based on integrated FRN and CSSI data, net erosion rates of maize and cassava reached maximum rates of 4.8tha−1y−1 (maize), 6.2tha−1y−1 (cassava). This major finding verified the potential of integrating FRN and CSSI to accurately estimate land use type-specific net erosion rates. In conclusion, determining past sediment budgets for specific land use types provides insight into the accelerating impact of specific land use change on soil retrogression and degradation. Such knowledge is of prime importance for effective soil conservation through evidence-based land management and decision making.

Quantifying fluvial (dis)connectivity in an agricultural catchment using a geomorphic approach and sediment source tracing

Purpose Catchments subject to land clearance, soil tillage and grazing suffer legacy effects from altered sediment and hydrological regimes and from changes in sediment connectivity between hillslopes and channels. Sediment dynamics are routinely investigated using the fallout radionuclides (FRNs) caesium-137 (137Cs) and excess lead-210 (210Pbex), which provide information regarding sediment sources and fluvial processes if source types are differentiated with confidence. Downstream transport, mixing and dilution of FRN-labelled fine sediment can obliterate the tracer signal from sources with low initial concentrations, so geomorphic evidence for downstream changes in sediment source types, mixing and fluvial (dis)connectivity should be used with tracers to ascertain the degree of sediment source variation and dominant fluvial processes.