Major Sediment Source Research Articles

Reconstruction of Chemical Weathering Intensity and Asian Summer Monsoon Evolution in the Red River Basin Over the Past 36 kyr

AbstractThe Red River is located at the intersection of the Indian and East Asian monsoons. Understanding the evolution history of the monsoon in the Red River basin helps to comprehend the evolution of Asian Summer Monsoon. Herein, we present the high‐resolution major elements and Sr‐Nd isotopic ratios of the terrigenous sediment component for a well‐dated core retrieved from the northern South China Sea slope, aiming to identify the sediment provenance and reconstruct the silicate chemical weathering intensity since the past 36 kyr. The Red River was confirmed as the major sediment source through Sr‐Nd isotopic ratios. The silicate chemical weathering was affected by the rapid changes of sea level and monsoon, and thus it is difficult to indicate the regional paleoenvironment by the weathering degree. The principal component 1 composed of TFe2O3, Al2O3, K2O and MgO was shown to represent the fine‐grained sediment discharge, and employed as a proxy for the summer monsoon rainfall. Global cooling events/stages were identified through the proxies, including the Last Glacial Maximum, Heinrich events, Younger Dryas, and 8.2 ka, as well as the enhanced summer monsoon stages, including the Bølling/Allerød and early Holocene. Based on the comparison with the neighboring paleoclimate indicators, the region summer monsoon of the Red River basin was found to be governed by the Indian summer monsoon, and the summer insolation at low latitudes might be the primary forcing force on the orbital scale.

Impact of River Dams on Littoral Cells Located Adjacent to the River Mouth

Terrestrial sediment is a major source of sediment to all coasts. Suspended sediment is carried away by the rivers and supplied to the coast to maintain sediment budget. The construction of dams across the rivers arrest sediment behind it and affect the sediment budget of littoral cells along the coast. Reduction in sediment supply induces ecological as well as geomorphological changes along the shoreline. Coastal erosion may accelerate due to reduced sediment influx. With the growing number of cross-river dams and water diversion projects, it has become a major concern before the scientific community to measure, understand and find solutions to multi-fold geo-environmental problems that are arising out of river damming. The present study aims to find out the impact of dams on the coast. It examines how the changes in the suspended sediment supply of an Indian river impact the coast in terms of loss of area due to erosion. Temporal analysis of geomorphological changes along the shoreline in relation to sediment influx holds immense importance to coastal management essential for the sustainable life and livelihood of coastal communities. Scientific investigation into the impact of river dams on the coastal environment is likely to provide a strong ground to reconsider the way present basin development projects function. Areal changes in littoral sediment cells adjacent to the river mouth have been quantified and correlated with changes in sediment influx. Changes along the shorelines have been detected through multispectral satellite images of Landsat belonging to different dates. Image processing and quantification of changes have been performed in QGIS 3.14 “Pi” platform. Virtual raster, raster calculator, field calculator and other required tools in QGIS were used during image processing.

PETROLOGY AND GEOCHEMISTRY OF THE LATE CRETACEOUS PAB FORMATION, WESTERN SULAIMAN FOLD- THRUST- BELT, PAKISTAN: IMPLICATIONS FOR PROVENANCE AND PALEO-WEATHERING

Late Cretaceous sandstone succession of the Pab Formation in western Sulaiman Fold Thrust belt Pakistan was investigated for petrology and bulk rock chemistry to determine its source terrain, paleo-weathering and tectonic setting. The formation is mainly comprised of sandstone with reddish to maroon color shale and arenaceous limestone. Texturally, the sandstone is fine to coarse grained, sub-angular to well-rounded and moderately to well sorted. The sandstone is petrologically and geochemically classified as quartz arenite to sub lithic arenite. The detritus was mainly derived from plutonic acidic source. QtFL and QmFLt suggests that recycled orogeny and Craton Interior setting were major sources of sediments. Geochemical models support that the detritus was derived from quartzose sedimentary source terrain, suggest deposition in a passive continental margin setting. Average values of chemical indices are CIA 59% CIW 67% and CIV 12.70%, which suggest moderate to high degree of chemical weathering in source area, that may reflect humid climate condition in the source area. The petrographic study and geochemical models demonstrate that the Pab Formation is mostly composed of mature sandstone from acidic plutonic and low-grade metamorphic rocks terrain in recycled and Craton Interior setting deposited on western passive margin of Indian plate in Tethys Ocean.

Sediment Yield in Dam-Controlled Watersheds in the Pisha Sandstone Region on the Northern Loess Plateau, China

Soil erosion has become the dominant environmental issue endangering sustainable development in agriculture and the ecosystem on the Loess Plateau. Determination of watershed soil erosion rates and sediment yields is essential for reasonable utilization of water resources and soil loss control. In this study, we employed unmanned aerial vehicles (UAVs) and structure-from-motion (SfM) photogrammetry to determine the sediment yields in 24 dam-controlled watersheds in the Pisha sandstone region of the northern Loess Plateau. High differences in total sediment were trapped before the check dams due to their running periods and sediment yields. The estimated specific sediment yield ranged from 34.32 t/(ha∙a) to 123.80 t/(ha∙a) with an average of 63.55 t/(ha∙a), which indicated that the Pisha sandstone region had an intense soil erosion rate. Furthermore, the modified Sediment Distributed Delivery (SEDD) model was applied to identify the erosion-prone areas in the watersheds, and the sediment retained in the check dams were used for model calibration. The performance of the model was acceptable, and the modeling results indicated that the steep Pisha sandstone was the major sediment source for the watersheds, accounting for approximately 87.37% of the sediment yield. Catchment area, erosive precipitation, and badland proportion were the key factors for sediment yield in the dam-controlled watersheds of the Pisha sandstone region, according to multiple regression analyses. These findings indicated that the modified SEDD model is very efficient in identifying spatial heterogeneities of sediment yield in the watershed but requires comprehensive calibration and validation with long-term observations. The Pisha sandstone region is still the key area of soil erosion control in the Loess Plateau, which needs more attention for soil and water conservation due to high sediment yield.

Spatiotemporal Patterns of Fractional Suspended Sediment Dynamics in Small Watersheds

AbstractUnderstanding suspended sediment dynamics and their driving factors is essential for effective watershed management. In this study, the spatiotemporal patterns of suspended sediment were identified for 11 sub‐watersheds of the Goodwin Creek, Mississippi, USA, for the period 1982–2002 using wavelet transforms. The continuous wavelet transform (CWT) results of streamflow time series showed that a near‐continuous annual periodicity influenced all stations over the study period, yet an overall decline occurred in wavelet spectral power of sediment load during the 1990s. This decline in sediment load is a result of the indirect effect of decreased cultivation on reducing the rate of peak flow and thus fluvial erosion processes. The CWTs of fine sediment (clays and silts) concentration (FSC) time series showed that factors other than land‐use (e.g., ponds, ephemeral gullies, and bed composition) better explain the variations in FSC. A comparison of CWTs of FSC at two stations with different bed composition in their channels showed that sandy channels are less responsive to erosion control measures compared to gravely channels indicating that they are major sources of sediment. A change in phase relationship between flow and FSC for the gravel‐dominated channel was associated with the transition in upland land‐use indicating a change in sediment supply mechanisms. The CWTs of sand concentrations reflected the spatiotemporal impact of bank materials, the growth cycle of riparian vegetation, and in‐channel sand availability on sand dynamics. While CWT‐based analysis is sensitive to input conditions, the method is able to provide valuable outcomes for distinguishing patterns in complex temporal datasets.

Quantifying production rates and size fractions of parrotfish-derived sediment: A key functional role on Maldivian coral reefs.

Coral reef fish perform numerous important functional roles on coral reefs. Of these, carbonate sediment production, as a by‐product of parrotfish feeding, is especially important for contributing to reef framework construction and reef‐associated landform development. However, only limited data exist on: (i) how production rates vary among reef habitats as a function of parrotfish assemblages, (ii) the relative importance of sediment produced from eroded, reworked, and endogenous sources, or (iii) the size fractions of sediment generated by different parrotfish species and size classes. These parameters influence not only overall reef‐derived sediment supply, but also influence the transport potential and depositional fate of this sedimentary material. Here, we show that parrotfish sediment production varies significantly between reef‐platform habitats on an atoll‐margin Maldivian reef. Highest rates of production (over 0.8 kg m−2 year−1) were calculated in three of the eight platform habitats; a rubble‐dominated zone, an Acropora spp. dominated zone, and a patch reef zone. Habitat spatial extent and differences in associated parrotfish assemblages strongly influenced the total quantities of sediment generated within each habitat. Nearly half of total parrotfish sediment production occurred in the rubble habitat, which comprised only 8% of the total platform area. Over 90% of this sedimentary material originated from eroded reef framework as opposed to being reworked existing or endogenously produced sediment, and comprised predominantly coral sands (predominantly 125–1000 µm in diameter). This is comparable to the dominant sand types and size fractions found on Maldivian reef islands. By contrast, nearly half of the sediment egested by parrotfish in the Acropora spp. dominated and patch reef habitats resulted from reworked existing sediments. These differences between habitats are a result of the different parrotfish assemblages supported. Endogenous carbonate production was found to be insignificant compared to the quantity of eroded and reworked material. Our findings have important implications for identifying key habitats and species which act as major sources of sediment for reef‐island systems.

Variations in runoff, sediment load, and their relationship for a major sediment source area of the Jialing River basin, southern China

AbstractInvestigation of the variations in runoff, sediment load, and their dynamic relation is conducive to understanding hydrological regime changes and supporting channel regulation and fluvial management. This study is undertaken in the Xihanshui catchment, which is known for its high sediment‐laden in the Jialing River of the Yangtze River basin, southern China, to evaluate the change characteristics of runoff, sediment load, and their relationship at multi‐temporal scales from 1966 to 2016. The results showed that runoff changed significantly for more months, whereas the significant changes in monthly sediment load occurred from April to September. The contributions of runoff in summer and autumn and sediment load in summer to their annual value changes were greater. Annual runoff and sediment load in the Xihanshui catchment both exhibited significant decreasing trends (p < 0.05) with a significant mutation in 1993 (p < 0.05). The average annual runoff in the change period (1994–2016) decreased by 49.58% and annual sediment load displayed a substantial decline with a reduction of 77.77% in comparison with the reference period (1966–1993) due to climate change and intensive human activity. The power functions were satisfactory to describe annual and extreme monthly runoff–sediment relationships, whereas the monthly runoff–sediment relationship and extreme monthly sediment‐runoff relationship were changeable. Spatially, annual runoff–sediment relationship alteration could be partly attributed to sediment load changes in the upstream area and runoff variations in the downstream region. Three quantitative methods revealed that the main driver for significant reductions of annual runoff and sediment load is the human activity dominated by soil and water conservation measures, while climate change only contributed 22.73%–38.99% (mean 32.07%) to the total runoff reduction and 3.39%–35.56% (mean 17.32%) to the total decrease in sediment load.

A Comprehensive Assessment of Sediment Dynamics in the Barataria Basin (LA, USA) Distinguishes Riverine Advection from Wave Resuspension and Identifies the Gulf Intracoastal Waterway as a Major Sediment Source

Barataria (LA, USA) is an interdistributary deltaic basin that has experienced extensive marsh loss. The fate of these marshes is strongly controlled by the concentration of total suspended sediments (TSS) in the adjacent open bay water. This parameter, however, is poorly constrained, thus limiting the ability to predict the future marsh evolution. Here we investigate the open bay water sediment dynamics using three complementary approaches: monthly field surveys at 37 locations along the bay axis over 22 years, remote sensing over 20 years, and turbidity time series at 5 locations over 4 years. Monthly field sampling and remote sensing reveal that the TSS is highly seasonal and spatially variable. In particular, the sediment delivered through the Gulf Intracoastal Waterway (GIWW) during the flood season (January–April) dominates the TSS signal in mid-Barataria. This sediment originates mainly from the Atchafalaya River and is advected for more than 100 km before reaching Barataria. Because of a hysteresis in sediment transport, the TSS in the river water, and hence in the GIWW, is not uniquely related to the river discharge. As such, late flood season discharges do not contribute much sediment to Barataria. We estimate that the GIWW delivers on average 0.21 Tg of sediment per year to mid-Barataria. A different sediment input is instead present in lower Barataria, where river-borne sediment exiting the “birdfoot” of the Mississippi Delta is advected by easterly winds, especially during spring. At all five stations within Barataria, turbidity time series reveal that the daily sediment dynamics is mainly associated with local wind wave resuspension. River inputs and wave resuspension contribute nearly equally to the yearly averaged TSS, of about 80 mg/l on average. The GIWW, built in the 1920s, is an important but overlooked source of sediments for Barataria that can serve as a present-day analogue for the proposed Mississippi River sediment diversions.

Livestock and Small-Scale Irrigation Scheme Interactions as a Key Source for Sedimentation during the Irrigation Season: Four Case Studies from South-West Shoa, Ethiopia

Small-scale irrigation (SSI) development can play a major role in Ethiopia’s economic development, but sedimentation is a major threat to its sustainability. The focus of the dominant discourse around the sedimentation of SSI schemes lies in upstream catchment protection during the rainy season, neglecting both protection against erosion through overland flow along the margins of the canal network and sedimentation caused by livestock disturbances. Remedies against the latter causes of sedimentation during the irrigation season have been ineffective due to erroneous assumptions regarding its cause. This study aimed to identify the sources and extent of sedimentation in SSI schemes. The accumulated sediment in the canal pre-irrigation season was measured from four SSI schemes and suspended sediment samples during irrigation season were collected from one SSI scheme. The accumulated sediment in the canal pre-irrigation season was measured from four SSI and suspended sediment samples during irrigation season were collected from one SSI scheme. The extent of sedimentation in the canals during the pre-irrigation season in relation to canal capacity was 100% of lined and unlined canals in abandoned, 68% in a lined, and 84% unlined canals in heavily sedimented and 38% in a lined and 46% of unlined canals lightly sedimented schemes. Livestock interactions with the SSI schemes were found to be the major sediment source before and during the irrigation, hence, attention should be given to integrating livestock as a part of the system.

Progradation of a middle Eocene carbonate slope system, Assamoud Field, Sirte Basin, north central Libya – Implications on the dynamics of lateral growth of isolated carbonate platforms

This study investigates the internal configuration and stacking patterns of well-defined prograding clinothems within the middle Eocene Gialo Formation in the Assamoud Oilfield of the Sirte Basin through an integrated analysis of wire-line log and three-dimensional seismic data. The clinothem interval forms a highstand systems tract that is bounded by a downlap surface at the base and a sequence boundary at the top. The prograding clinothems occur over an area of at least 8 km wide along strike and at least 9.5 km along depositional dip. They range in height from 240 to 330 m and have a N progradation direction. Nine clinothems were identified on the basis of stacking patterns of log facies. Individual clinothems are 47–139 m thick and consist of a thin shale unit at the base overlain by thick limestone units. The clinothem interval is interpreted as a slope system and can be divided into two packages on the basis of their log facies, seismic characteristics, and position along the platform: a proximal package composed of high-angle sigmoidal clinothems and a distal package with low-angle sigmoidal pattern. The maximum slope angle of the proximal package is 24° and that of the distal package is 16°. The decrease of slope angle may be caused by an increased amount of siliciclastic mud. The progradation direction indicates that the major sediment source, which is most likely the platform top and margin, was located to the south of the study area. The sediments entered the slope from the platform margin and were redistributed downslope by surface currents driven by southerly probably Sirocco-like winds. The evolution of the platform was controlled by several factors. A pre-existing isolated platform to the south generated the topographic relief between the platform top and slope. Sea-level rise generated accommodation space, whereas sea-level fall caused subaerial exposure and terminated progradation. Subsidence caused a sea-level rise and enhanced accommodation space for platform expansion. Prevailing currents determined the direction of platform lateral growth. The results provide insights on the dynamics of carbonate platform lateral growth and may serve as an analog for future studies on carbonate slope systems.

Provenance of the Phuquoc Basin fill, southern Indochina: Implication for Early Cretaceous drainage patterns and basin configuration in Southeast Asia

Cretaceous basins cover large parts of mainland SE Asia. Yet, the development during the Cretaceous is poorly understood including the establishment of the Cretaceous basin configuration. Based on detrital zircon and heavy mineral analysis, this study investigates the provenance of Lower Cretaceous deposits in the Phuquoc Basin, exposed in Cambodia and SW Vietnam and extending into the Gulf of Thailand. Our results document five major zircon populations with 130–145 Ma, 145–200 Ma, 200–270 Ma, 420–470 Ma, 1800–1900 Ma ages and two subordinate 2400–2700 Ma, 750–1000 Ma age clusters. These ages resemble those of the Qinling-Dabie and the Songpan-Ganzi orogenic belts and differ from other Asian terranes. These ages suggest that the Qinling-Dabie and Songpan-Ganzi orogenic belts were major sources of sediment for the Phuquoc Basin in the Early Cretaceous. Heavy mineral compositions including the presence of almandine, often co-occurring with silimanite and epidote, suggest metamorphic and granitic constituents in the hinterland, compatible with these orogenic belts being the source.The detrital zircon age pattern of the Phuquoc Basin closely resembles the zircon ages in Khorat and the Sichuan basins, suggesting a common sediment source, and bears great similarities with mid-Cretaceous strata in the Lanping-Simao Basin. These basins share a remarkable lithostratigraphic similarity and are confined by erosion. Taken altogether, this supports that the Phuquoc, Khorat, Sichuan, and probably the Lanping-Simao basins, formed part of a Cretaceous basin covering much of central SE Asia segregated by Late Cretaceous and Cenozoic denudation and translation. The absence of 130–145 Ma and 145–200 Ma aged zircons in the Lanping-Simao Basin suggests that another drainage system sprang from the Songpan-Ganzi Orogenic Belt and fed into this basin before merging with the Qinling-Dabie Orogenic Belt drainage system south of the Sichuan Basin. The combined drainage system likely passed through the Khorat Basin before entering the Phuquoc Basin.

Variability in fluvial suspended and streambed sediment phosphorus fractions among small agricultural streams.

Agriculture is a major source of sediment and particulate phosphorus (P) inputs to freshwaters. Distinguishing between P fractions in sediment can aid in understanding its eutrophication risk. Although streams and rivers are important parts of the P cycle in agricultural catchments, streambed sediment and especially fluvial suspended sediment (FSS) and its P fractions are less studied. To address this knowledge gap, seasonal variations in FSS P fractions and their relation to water quality and streambed sediment were examined in three Swedish agricultural headwater catchments over 2 yr. Sequential fractionation was used to characterize P fractions in both streambed sediment and FSS. All catchments had similar annual P losses (0.4-0.8kg ha-1 ), suspended solids (124-183mg L-1 ), and FSS total P concentrations (1.15-1.19mg g-1 ). However, distribution of P fractions and the dominant P fractions in FSS differed among catchments (p<.05), which was most likely dependent on differences in catchment geology, clay content, external P sources, and flow conditions. The most prominent seasonal pattern in all catchments was found for iron-bound P, with high concentrations during low summer flows and low concentrations during winter high flows. Streambed sediment P fractions were in the same concentration ranges as in FSS, and the distribution of the fractions differed between catchments. This study highlights the need to quantify P fractions, not just total P in FSS, to obtain a more complete understanding of the eutrophication risk posed by agricultural sediment losses.

Modelled estimates of fine sediment and particulate nutrients delivered from the Great Barrier Reef catchments

The eWater Source modelling framework has been modified to support the Great Barrier Reef (GBR) Dynamic SedNet catchment modelling concept, which is used to simulate fine sediment and particulate nutrient generation, loss, and transport processes across GBR catchments. Catchment scale monitored data sets are used to calibrate and evaluate models. Model performance is assessed qualitatively and quantitatively. Modelling predicts that approximately half of generated sediment is delivered to the GBR lagoon; the remainder is deposited on floodplains, trapped in reservoirs or lost through other minor processes (e.g. irrigation extractions). Gullies are the major source of sediment, with comparable contributions from hillslopes and streambanks. Hillslope sources are considered the major source of particulate nutrients across the GBR catchments. We demonstrate that using locally developed, customised models coupled with a complementary monitoring program can produce credible modelled estimates of pollutant loads and provide a platform for testing catchment scale assumptions and scenarios.

Elucidating intra-storm variations in suspended sediment sources using a Bayesian fingerprinting approach

There is an increasing need for reliable information on the origin of fine-grained sediment transported by river systems, but critically, at temporal scales that reflect the timing and pattern of sediment mobilization and delivery in response to effective precipitation events. Temporal variability is likely to be more pronounced in steep mountainous catchments. Accordingly, the main objective of this study was to investigate the temporal variability of sediment sources during and between flood events, in a mountainous catchment in Iran, using a geochemical fingerprinting approach and Bayesian un-mixing model. Potential sediment sources were classified as either tributary sub-basin spatial end members or individual source types represented by surface soils and channel banks. A total of 34 geochemical elements, elemental ratios and weathering indices were measured as potential tracers on a total of 155 samples, including 40 target samples collected across the hydrographs of four flood events at the overall catchment outlet. Virtual mixture tests, used to compare the computed source estimates with known source mixtures, revealed better fits in the case of spatial, rather than source type apportionment. The Bayesian un-mixing model results revealed pronounced intra-storm variations in the relative contributions from both the spatial sources and source types, but with Quaternary surface soils and channel banks being dominant at different stages of the hydrographs. For all the suspended sediment samples collected throughout the three small to medium size events, the Babazid, middle upstream sub-catchment, was the major (84%–99%) sediment source while for the largest sampled event, suspended sediment samples predominantly (>98%) originated from the Kalkoshan, the most southern sub-catchment. By investigating both spatial sources and source types, temporal variations in the dominant sources were revealed. Quaternary geology and channel banks were identified as the main sediment sources. By combining instantaneous suspended sediment loads through the sampled flood events (across all floods) and using the corresponding source proportions derived from the MixSIR un-mixing model, channel banks and channel bank and surface soils generated 2911 kg and 34,171 kg of suspended sediment, respectively. Whilst the use of spatial source end members is practical in medium-size drainage basins, integration with estimates of sediment source type contributions is more informative to managers in selecting relevant sediment mitigation methods. The intra-storm results underscore the need to manage sediment loss from both surface soils and channel banks.

Quantifying the Extent of Eroding Streambanks in Iowa

AbstractStreambank erosion is a major source of sediment to rivers and it is important to find locations where severely eroding streambanks are occurring in watersheds. In this study, a new approach to mapping eroding streambanks at a regional scale was conducted using high‐resolution Light Detection and Ranging (LiDAR) elevation data. By calibrating field mapping studies to LiDAR derivative data, the extent of severely eroding streambanks in third‐ to sixth‐order streams and rivers found in Iowa was estimated. Study results suggest that over 35,200 km of streambanks may be severely eroding, or approximately 41% of all third‐ to sixth‐order streambanks. The extent of streambank erosion was not uniform across the state with more erosion occurring in hillier western and southern Iowa compared to flatter and more recently glaciated northern Iowa. Streambank erosion was related to greater bank heights and was more prevalent (as a percent of the total stream length) in larger fifth‐ and sixth‐order rivers. Overall, despite limitations in the study related to the collection and extrapolation of the LiDAR dataset, study results provide new evidence for the magnitude of the streambank erosion problem in Iowa and likely apply to similarly glaciated regions of the United States Midwest.

Causes of Variations in Sediment Yield in the Jinghe River Basin, China

The Jinghe River remains the major sediment source of the Yellow River in China; however, sediment discharge in the Jinghe River has reduced significantly since the 1950s. The objective of this study is to identify the causes of sediment yield variations in the Jinghe River Basin based on soil and water conservation methods and rainfall analyses. The results revealed that soil and water conservation projects were responsible for half of the total sediment reduction; sediment retention due to reservoirs and water diversion projects was responsible for 1.3% of the total reduction. Moreover, the Jinghe River Basin has negligible opportunity to improve its vegetation cover (currently 55% of the basin is covered with lawns and trees), and silt-arrester dams play a smaller role in reducing sediment significantly before they are entirely full. Therefore, new large-scale sediment trapping projects must be implemented across the Jinghe River Basin, where heavy rainfall events are likely to substantially increase in the future, leading to higher sediment discharge.

Effect of low-head dams on reach-scale suspended sediment dynamics in coarse-bedded streams

River infrastructure is one of the primary threats to riverine ecosystems globally, altering hydromorphological processes and isolating habitats. Instream barriers and low-head dams can have significant effects on system connectivity, but despite this, very few empirical studies have assessed the impacts of these structures on suspended sediment transport. Through a paired turbidity study over a 20-month monitoring period we investigated the differences in suspended sediment flux above and below two low-head dams in the south-east of Ireland. Using sediment balance as a proxy for sediment storage, results showed that a net-export of sediment from the study reach occurred for 68% of the high-flow events analysed. As the primary controls on sediment dynamics at the downstream reach depend on sediment availability from upstream, we argue that these results indicate the presence of a substantial local source of sediment between monitoring stations that cannot be explained by natural intra-reach erosional processes. Here we hypothesise that as sediment supply from the catchment becomes exhausted, the structure's impounded zone (typically considered a depositional area) provides a major sediment source to the downstream reach. Our rationale is that if sediment trapped behind the weir is periodically available for transportation at the rates and frequencies observed in this study, then we can infer that both structures must be trapping sediment under lower flows.

Modeling soil erosion between 1985 and 2014 in three watersheds on the carbonate-rock dominated Guizhou Plateau, SW China, using WaTEM/SEDEM

The Guizhou Plateau, SW China is largely underlain by carbonate rocks. Because soils are thin, soil loss remains a serious problem despite low erosion rates. Further understanding the impacts of changes in rainfall, land use and differences in topography on sediment yield and delivery may assist in the development of suitable policies to reduce soil erosion on the plateau. A spatially distributed soil erosion and sediment delivery model (WaTEM/SEDEM) was applied to investigate temporal–spatial changes in soil erosion between 1985 and 2014 in three watersheds (Dadukou (DDK), Caopingtou (CPT) and Gaoche (GC)) located in the southwest Guizhou Plateau. The WaTEM/SEDEM model was calibrated and validated using data on sediment yields measured at the watershed scale. The total sediment yield (SY) and soil erosion modulus (SEM) firstly decreased followed by an increase, whereas the sediment delivery ratio (SDR) remained almost unchanged over the 30-year period. The major sediment source was dry farmlands. SY was the highest in the largest DDK watershed. The highest SEM occurred in the CPT watershed due to steep terrain and high ratio of dry farmland areas on steeper slopes. SEM was the lowest in the GC watershed where slope gradient and ratio of dry farmland on steeper slopes are low. SDR was the highest in the GC watershed because of its topographic characteristics. SEM was sensitive to precipitation fluctuations in the GC, DDK and particularly in the steep and intensively eroded CPT watershed, while changes in dry farmland ratio influenced the SEM in the CPT and DDK watersheds but not in the gentle and mildly eroded GC watershed. Changes in forest ratio had significant impacts on SEM only in the GC watershed. Since responses of soil erosion to variations or differences in the main impact factors differ in the different watersheds, soil conservation strategies should be watershed specific.

Sediment budget of the Yellow River delta during 1959–2012, estimated from morphological changes and accumulation rates

The Yellow River (Huanghe) is a major sediment source of sediment to the Bohai Sea and Yellow Sea, resulting in the formation of a large and dynamic delta complex. However, the river sediment discharge reaching the delta has decreased dramatically since the middle 1960s. This study examines the sediment accumulation and morphological changes of the delta complex during 1959–2012 using a repeated bathymetry dataset, high-resolution seismic profiles, and sediment cores obtained in 2012 and 2013. Sediment accumulation rates were determined using the downcore concentrations of 137Cs and 210Pb. The average accumulation rates calculated for the delta complex range from 6.9–24 cm y−1 on the delta front slope, 1–2 cm y−1 at the toe of the slope, and < 1 cm y−1 in the northeastern shelf area. Morphological changes and sediment accumulation rates show that 1) the Yellow River delta has entered a destruction phase after 1996, 2) the amount of erosion of the whole delta reaches 210 MT y−1 of sediment, which is at least required to maintain the whole delta morphology, 3) three-quarters (487 MT y−1) of the Yellow River-derived sediment was deposited in the subaerial and subaqueous delta during 1959–2012, and 4) Whereas, one-quarter (144 MT y−1) was transported to the Bohai Sea, and as much as one fifth (117 MT y−1) was transported further offshore to the Yellow Sea, passing through the Bohai Sea during this period. The source of these sediments is a combination of the Yellow River sediment discharge and the erosion of the previous Yellow River delta lobes. Despite the sharp decline in the Yellow River sediment discharge over the past five decades, the offshore dispersal of the river discharge and eroded sediment to the Yellow Sea was more than 100 MT y−1 on average. Results of this study show that the erosion of the abandoned delta lobes can sustain the supply of sediment to remote offshore depocenters even after decades of declining river sediment discharge.

Late Silurian to early Devonian development of the Chingiz accretion arc, West Junggar: insights into accretion arc evolution in the Central Asia Orogenic Belt

ABSTRACT Early Palaeozoic evolution of the Kazakhstan orocline in the Central Asian Orogenic Belt (CAOB) is key to reconstructing the Palaeozoic tectonic framework in Central Asia. This study presents field mapping, geochemistry, and geochronological data from the Wulasitai area of northern West Junggar to constrain the subduction polarity and major tectonic events in the Chingiz arc of the Kazakhstan orocline during the Early Palaeozoic. The mapping area outcrops imbricated coherent turbidite slices and mélanges that consist of chert, limestone, silt, sandstone, and conglomerate blocks in mudstone matrix, representing an OPS (Ocean Plate Stratigraphy) mélange in an accretionary complex. Existing fossil ages from these blocks range from the Ordovician to Silurian, while our new detrital zircon U-Pb samples from the turbiditic matrix yield maximum depositional ages (MDTs) from 428 ± 2 Ma to 450 ± 1 Ma. Detrital zircon ages of ~510 to 430 Ma in the mélange suggest the Chingiz arc to the south of the study area as a major sediment source, providing an indirect constraint on the polarity of subduction. We suggest that the northern part of the Chingiz arc may be underlain by southward subduction, with the accretionary complex located on the Wulasitai area of the northern West Junggar. The Wulasitai mélange is overlain depostionally by volcanic rocks carrying depleted HFSE and enriched LILE, LREE. The andesite and tuff are dated to 415 ± 5 Ma, 414 ± 3 Ma, and 402 ± 6 Ma, which we interpret as reflecting the development of the volcanic arc onto the previous subduction complex. This model implies the generation of the arc (accretion arc) as the slab rolling back and trench retreating, which may play an important role in the evolutionary history of CAOB.