Main Sediment Source Research Articles

Plio-Pleistocene sedimentation and palaeogeographic reconstruction in the Poso Depression, Central Sulawesi, Indonesia: from a sea channel to a land bridge

The Poso Depression provides a record of Plio-Pleistocene environments and palaeogeography of Central Sulawesi. Outcrop-based sedimentological and provenance studies suggest that during the Pliocene the Poso Depression was a sea channel connecting Gorontalo and Bone bays formed in an asymmetric half-graben. The Pliocene history began with deposition of the Puna Formation with fan deltas at the eastern basin margin and channel complexes in the deep-water basin further east. Analyses of light and heavy minerals indicate the main sediment source was ultrabasic rocks in East Sulawesi with minor and intermittent magmatic and metamorphic input from West Sulawesi. Later, in the Middle to Late Pliocene, carbonates of the Poso Formation accumulated on the eastern basin margin. They are unconformably overlain by shallow marine glaucophane-rich siliciclastics of the Pleistocene Lage Formation that are associated with the rapid exhumation and uplift of the Pompangeo metamorphic complex. This uplift led to the development of a land bridge connecting western and eastern Sulawesi. The terrane evolution favoured increasing the area of exposed land due to rapid tectonic uplift, which when combined with the tropical climate, contributed to faunal speciation and dispersal in Sulawesi.

Assessing landscape features and geomorphic processes influencing sediment dynamics in a geomorphologically highly active Mediterranean agroecosystem: The upper Val d'Arda case study (Northern Apennines, Italy)

The management of Mediterranean Agroecosystems is crucial to prevent or mitigate sediment-related threats like soil erosion, water quality issues, and reservoir sedimentation. Hence, the identification of type, extent, and location of sediment sources and sinks, in relation to geological and landscape features, is a key prerequisite for identifying erosion hot spot areas and assessing sediment dynamics at catchment scale. This study was carried out in the upper Val d'Arda catchment (Northern Apennines, Italy). We applied geomorphological field mapping together with multi-temporal photointerpretation, GIS-based hydrological modelling, terrain analysis and photogrammetry. Our approach was aimed at exploring the main landscape features, as well as identifying the variety of processes influencing sediment dynamics. In this poorly studied area, we identified a high heterogeneity in terms of type and extent of sediment sources and related processes. Landslides are the most common processes contributing to the sediment yield. Their morphogenesis, extent and location are controlled mainly by lithology and topography. Large ancient earthflows have been the most important geomorphic factor in shaping the landscape, strongly influencing the surficial drainage pattern and the morphological evolution of the main fluvial systems. Upland rill-interrill erosion is another main sediment source, particularly on arable lands. Piping and gullying locally contribute to the development of badlands, and have a high potential for soil loss and sediment production. Finally, fluvial processes significantly contribute to the sediment yield with several active fluvial erosion scarps and bank erosion source areas. Alluvial sediments are locally stored in transient landforms such as bars or more permanent sinks such as floodplains or river terraces. Our study shows that an integrated methodology based on fieldwork, geomorphological mapping, GIS applications and proximal sensing methods is an effective approach to characterize complex geomorphic systems in the Northern Apennines.

Analysis of the sediment sources of flood driven erosion and deposition in the river channel of the Fu River Basin

Studying the characteristics of runoff and sediment processes and revealing the sources of sediment provide key guidance for the scientific formulation of relevant soil erosion protection measures and water conservancy development plans. In the current study, the flow and sediment data of five hydrological stations on the main stream of the Fu River Basin (FRB) from 2007 to 2018 were selected to identify flood events, explore the variation of sediment transport along the FRB, and clarify the sediment sources. The results found that the Jiangyou–Fujiangqiao section is the main source of sediment in the FRB during the flood season. The runoff volume and sediment load during flood events in the Jiangyou–Fujiangqiao section accounted for 35% and 145% respectively of that of Xiaoheba station. These results combined with the change of the sediment load before and after the 2008 Wenchuan Earthquake (May 12) show that the sediment in this section mainly comes from the Fu River tributary–the Tongkou River watershed. The calculation results for the sediment carrying capacity of the Fu River show that the main stream was in a state of erosion in theory. However, according to the calculation results for the interval sediment yield during flood events, the sediment load at the Xiaoheba station was smaller than that at the Shehong station upstream. The analysis indicates that this was not because of sediment deposition in the river channel, but because of sand mining in the river channel and sediment interception by water conservancy projects. If heavy rainfall occurs in the FRB, the sediment accumulated upstream will move downstream with the resulting flood, and the sediment yield in the FRB may further increase. These research conclusions can provide reference information for improving the prediction and management ability of soil and water loss in the FRB and scientific regulation of the Three Gorges Reservoir.

Depositional history of the subaqueous delta off the southeastern coast of Korea during the post-glacial period

The distal mud deposit (DMD) of the Korea Strait Shelf, located along the southeastern coast of Korea and detached from the main sediment source, the mouth of the Nakdong River, is considered a subaqueous delta system. The complex spatiotemporal changes in the stratigraphic architecture of the DMD were reconstructed, based on combined seismic and sediment core data, according to the three stages of post-glacial sea-level rise. During the early transgressive stage (from ca. 19 to 10.9 ka), transgression occurred at a relatively deeper northern outer shelf of the DMD, where Unit A began to be deposited in an aggradational stratal stacking pattern. During the late transgressive stage (ca. 10.9 to 8 ka), further sea-level rise provided sufficient accommodation for DMD, but an insufficient sediment supply from the Nakdong River might have prevented sedimentation. At the highstand stage (ca. 8 ka to the present), the increased supply of suspended sediments from the Nakdong River coupled with the entrance of the Tsushima Warm Current through the Korea Strait may have enabled the along-shore lobate progradation of Unit B on the flat and wide southern shelf. After appoximately 2 ka, as Unit B proceeded into the narrow, steep northern shelf, it started to prograde in a shore-normal direction, controlled by a rapid downslope decrease in near-bed shear stress. Over the last 700 years, sediments have been actively deposited at the depocenter of the DMD by frequent storm surges during the summer season. The reconstructed depositional history of the DMD demonstrates the importance of the pre-existing shelf morphology, along-shore currents, and storm surges in offshore sediment transport for a subaqueous delta in a microtidal shelf with a relatively small riverine sediment load.

Sediment source fingerprinting and the temporal variability of source contributions

Sediment source fingerprinting has been progressively developed and refined over the past 40 years or more and now represents a widely used and valuable technique, with important practical applications. However, relatively little attention has been given to the target samples and the extent to which they are able to provide meaningful information on short- or longer-term relative source contributions for a given study catchment. A key issue here is the inherent short- and longer-term temporal variability of source contributions and the extent to which such variability is taken into account by the target samples. The objective of this study was to investigate the temporal variation of source contributions from the Qiaozi West catchment, a small (1.09 km2) gully catchment located within the Loess Plateau of China. The target samples represented a suite of 214 spot suspended sediment samples collected during eight representative wet season rainfall events occurring over two years. A suite of geochemical properties was used as fingerprints and standard source apportionment calculations indicated that the gully walls contributed the most sediment (load-weighted mean 54.5%) and, together with cropland (load-weighted mean 37.3%), and gully slopes (load-weighed mean 6.6%) were the main sediment sources. The 214 individual target samples indicated that the contribution of cropland sources varied between 8.3% and 60.4%, gully walls between 22.9% and 85.8% and gully slopes between 1.1% and 30.7%, representing ranges of 52.1%, 62.9% and 29.6% respectively. In order to explore whether the temporal variability of source contributions demonstrated by the study catchment should be seen as typical, equivalent information was abstracted from 14 published studies for other catchments of varying size and located in different environments worldwide. This information demonstrated similar temporal variability of the relative contributions of the major sources, which were typically characterized by ranges of the order of 30–70%. The temporal variability associated with the estimates of relative source contributions provided by target samples has important implications for the uncertainty associated with such estimates derived using source fingerprinting techniques based on a limited number of target samples. Further attention needs to be directed to the design of sampling programmes used to collect such samples and to taking account of such uncertainty in source apportionment calculations.

Vegetation Cover Variation in Dry Valleys of Southwest China: The Role of Precipitation

Many ecological restoration projects have been carried out in Southwest China; however, changes in vegetation cover in the dry valleys of Southwest China have rarely been reported. With their unique characteristics of high temperatures and low humidity, these dry valleys have considerably lower vegetation cover than their neighboring areas, making them the main sediment sources of rivers in Southwest China. Thus, it is imperative to understand changes in vegetation cover in China’s dry valleys, as well as the effects of changes in precipitation, since water deficit is the dominant cause of obstructed plant growth. In this study, changes in fractional vegetation cover (FVC) in dry valleys in the period 2000 to 2020 were analyzed based on MODIS-NDVI data, and the effects of precipitation were also analyzed. Our results indicated that: (1) the long-term mean annual FVC values in the dry–hot valleys (DHVs), dry–warm valleys (DWVs), and dry–temperate valleys (DTVs) were 0.426, 0.504, and 0.446, respectively; (2) significant decreasing trends in FVC were mainly found in DHVs and DWVs that were distributed in the southwestern part of the dry valley region (DVR), which was mainly due to the decrease in precipitation; and (3) significant increasing trends were reported in DTVs of the Min River and the Baishui River, which was probably due to the increase in precipitation. By analyzing the temporal trends of FVC in dry valleys, this study highlighted the effects of precipitation on the dynamics of FVC and demonstrated that anthropogenic activities such as urbanization, land use changes, and hydro-power project construction may also have considerable effects on FVC in dry valleys. Overall, this study not only provides insights that might inform further detailed studies on the dynamics and mechanisms of vegetation cover, but could also provide valuable guidance for ecological restoration management in the dry valley region.

Stratigraphy, provenance, and timing of Neogene sedimentation in the western Valdés Basin, Patagonia. Accurate paleogeographic reconstructions as a key piece for andean-passive margin integration

With the aim to better understand the timing and geological evolution of the late Cenozoic stratigraphic record of the onshore, western Valdés Basin in northeast Chubut province (Patagonia, Argentina), we perform sedimentological, stratigraphic, and zircon U–Pb geochronologic analyses. The study includes the terrestrial Early Miocene Trelew Member of the Sarmiento Formation, the shallow marine Early Miocene Gaiman Formation, the shallow marine to estuarine Late Miocene Puerto Madryn Formation, and the Plesitocene fluvial conglomerates of the Rodados Patagónicos. Based on our data set, together with the recognition of relevant stratigraphic surfaces, we divide the succession into stratigraphically bounded units and define three cycles of sedimentation. The Lower Cycle rests on top of Surface 1, which comprise a long hiatus including the entire Oligocene Epoch. This cycle is in turn divided into two subunits separated by the transgressive, ravinement Surface 2. The Lower Cycle is separated from the overlying Middle Cycle (=Puerto Madryn Formation) by the flat and regionally extensive Surface 3. Finally, the latter is separated from the Upper Cycle (=Rodados Patagónicos) by Surface 4, a complex surface at the base of gravel deposits forming several terrace levels. Maximum depositional ages calculated from 10 zircon U–Pb samples indicate the succession between surface 1 and 4 extends from 21.5 to 2.7 Ma (Aquitanian-Piacenzian). The Lower Cycle is restricted to the 21.5–16 Ma interval (Aquitanian-Burdigalian); the Middle Cycle between 12 and 2.7 Ma (latest Serravalian-Piacenzian), with most of the new and previous ages clustering in the Tortonian; whereas the Upper Cycle is restricted to the Pleistocene. Although most of the analyzed sediments show fresh volcanic glass particles, older zircon ages are dominant, representing detrital components with a dominant peak of Permian-aged grains, with subordinated Devonian, Jurassic, Cretaceous, and Paleogene peaks. We interpret the analyzed succession as a condensed section compared to the offshore Valdés Basin depocenter area. Sedimentation in the study area was controlled by periods of slow subsidence, probably related to thermal subsidence and sediment compaction in the depocenter. Two main sources of sediments for the analyzed succession are envisaged: one represented by explosive volcanism located either in the Andes or in the North Patagonian Massif, the other through erosion of older rocks surrounding the North Patagonian Massif where late Paleozoic plutonic rocks are abundant. Particularly for the Middle and Upper cycles, an increase in Mesozoic contribution suggests erosion from sediments of the Cañadón Asfalto Basin. The new ages and stratigraphic analyses here reported are of major relevance for the integration of the onshore and offshore stratigraphies of the Valdés Basin which is key to connect the geologic evolution of the Andean and passive margin areas of southern South America. They are also of major relevance for the calibration of fossil communities, particularly for the abundant and diverse marine vertebrates preserved in the Burdigalian Gaiman Formation.

Tracing the dominant sources of sediment flowing towards Lake Victoria using geochemical tracers and a Bayesian mixing model

PurposeLake Victoria has been increasingly silting over the past decades, impacting water quality and loss of biodiversity. Sediment control strategies require information on the relative and absolute contributions of sediment from different sources. However, to date, there is no continuous monitoring of sediment flux or water quality in any of the tributaries, prohibiting an assessment of the scale of the problem. The aim of this study was to trace the dominant sources of riverine sediment using geochemical fingerprinting, thereby generating a knowledge base for improving land management and reducing sediment yields in Simiyu River catchment, one of the main contributing rivers to Lake Victoria.Materials and methodsGeochemical tracer concentrations were analyzed in transported sediment from the main river and two tributaries (riverbed sediments) and from soils in five dominant land use types (agricultural land, bush land, forest land, channel banks, and main river banks). Dominant sources to the Simiyu main river sediment were attributed using the Bayesian MixSIAR model.Results and discussionThe mixing model outputs showed that the Simiyu tributary was the dominant source of sediment to the Simiyu main river with 63.2%, while the Duma tributary accounted for 36.8%. Cultivated land was shown to be the main land use source of riverine sediment, accounting for 80.0% and 86.4% in Simiyu and Duma sub-tributaries, respectively, followed by channel banks with 9.0% in both sub-tributaries. Direct unmixing of the Simiyu main river sediment to the land use sources yielded 64.7% contribution of cultivated land and 26.5% of channel banks.ConclusionThe demonstrated application of sediment source tracing provides an important pathway for quantifying the dominant sources of sediment in the rivers flowing towards Lake Victoria. Eroded soil from agricultural areas is the biggest contributor to transported sediment in the Simiyu River. This information is vital for the design of catchment wide management plans that should focus on reducing soil erosion and sediment delivery from farming areas to the river networks, ultimately supporting both food security and water quality in the Lake Victoria Basin.

Sediment provenances shift driven by sea level and Indian monsoon in the southern Bay of Bengal since the last glacial maximum

The Tibetan Plateau uplift has induced the formation of the largest sediment source-sink system in the northeast Indian Ocean, which has become an ideal region for investigating land-sea interaction processes. However, many questions regarding sediment transport patterns and their controlling factors at different time scales remain unanswered. Therefore, in the present study, a gravity core named BoB-79, based on the southern Bay of Bengal (BoB) was selected to investigate sediment provenance shift and its corresponding mechanism to sedimentary environment change since the last glacial maximum (LGM). The clay mineral compositions are analyzed and the whole core sediments reveal a feature dominated by illite (~55%), followed by chlorite (~24%) and kaolinite (~17%), and the content of smectite (~4%) is the lowest. A trigonometric analysis of provenance discrimination of clay minerals showed that the Himalayas, together with the Indian Peninsula, represent the main sources of southern BoB sediments, and the last glacial period might have been controlled by the dominant Himalayan provenance, with an average contribution of approximately 90%. However, as a secondary source, the influence of the Indian Peninsula increased significantly during the Holocene, and its mean contribution was 24%, thus, indicating that it had a crucial effect on the evolution process of BoB. The sediment transportation pattern changed significantly from the LGM to the Holocene: in the last glacial period, the low sea level exposed the shelf area that caused the Ganges River connected with the largest submarine canyon in BoB named Swatch of No Ground (SoNG), and the Himalayan materials could be transported to the BoB directly under a strong turbidity current, thereby forming the deep sea deposition center with a sedimentation rate of 4.5 cm/kyr. Following Holocene, the sea level increased significantly, and the materials from multiple rivers around the BoB were directly imported into the continental shelf area. The intensive Indian summer monsoon dominated the transportation process of the terrestrial materials, thereby forming a deposition center in the shallow water area of the continental shelf northeast Indian Ocean; subsequently, the material flux relative to the input to the deep sea area decreased significantly, and the sedimentation rate in the southern BoB decreased to 1.7 cm/kyr.

Modulation of sediment load recovery downstream of Three Gorges Dam in the Yangtze River

The sediment load in the Yangtze River downstream of the Three Gorges Dam (TGD) has substantially declined in recent decades. The decrease is more profound below the TGD, e.g., a 97% decrease at Yichang, compared with that at the delta apex, 1200 km downstream, e.g., a 75% decrease, implying along-river sediment recovery. Two large river-connected lakes, i.e., Dongting and Poyang Lakes, may play a role in the re-establishment of the river’s morphodynamic equilibrium, but a quantitative data-based understanding of this interaction is not yet available. In this work, we collected a series of field data to quantify the sediment gain and loss in the river-lake system in the middle-lower Yangtze River, and evaluate the lake’s response to the reduction in riverine sediment supply. We find that Dongting Lake and Poyang Lake shifted from net sedimentation to erosion in 2006 and 2000, and back to a sedimentation regime again after 2017 and 2018, respectively. Natural morphodynamic adaptation and sand mining play an important role in the regime changes in the Dongting Lake whereas sand mining dominates the abrupt changes in the Poyang Lake. The Dongting and Poyang Lake contributed maximum by 38% (2015) and 17% (2006) (respectively) to the sediment recovery in the erosion regime, whereas the riverbed erosion dominates the main sediment source. These changes in the relative contribution of sediment sources also indicates a response time of ~ 20 years in the lakes towards a new equilibrium state. It is noteworthy that the lakes’ buffer effects may be overestimated as the supplied sediment from the lakes is rather small compared to the significant dam trapping in the upstream basin and sediment source from downstream degradation. The results imply that river management and restoration should take into account of the river-lake interactions and feedback impact at decadal time scales.

Fingerprinting Sediment Origin of the Silting Process of Urban Reservoirs

The silting process of urban reservoirs has been occurring in many regions across the world. However, identifying the main sources of sediment and controlling the silting process in urban reservoirs are still unsolved problems in many regions, mainly in developing countries such as Brazil. In this study, we identify which land use most influences the siltation of reservoirs, and how the different tributary streams contribute to this process in two urban reservoirs located in Campo Grande, Midwestern Brazil. Thus, we applied a sediment source fingerprinting (SSF) approach, associated with land use analysis, and the bathymetric data of reservoirs connected to the stream and drainage network, obtained between the years 2008 and 2018. The reduction in the volume and area of the reservoir during the study period were 45% and 39%, respectively. We found a proportional relationship between the reduction in the reservoir volume and the increase in the impermeable areas of the studied basin. We also noted that the sediments deposited in the reservoir originate from bare soil, banks, and bed in the proportions of 46.9%, 37.1%, and 17.2%, respectively. Our findings show that the use of bathymetric surveys and data on land use and land cover, associated with the source tracing technique, are useful alternatives to identifying sediment mobility in urban basins, especially in those where the drainage network is connected to water courses. We conclude that the factors that most contribute to the silting up of reservoirs are the erosion of banks and beds, sediment remobilization and the connectivity of the drainage network with water courses.

The integration of RUSLE-SDR lumped model with remote sensing and GIS for soil loss and sediment yield estimation

As a major contributor of land degradation, water borne soil erosion poses a threat to environmental and socioeconomic sustainability worldwide. Recently, various anthropogenic factors have aggravated soil erosion related problems in most of the humid and semi arid parts of the developing countries like India. Therefore, accurate quantification of the spatially distributed soil erosion along with the identification of erosion hot spots is of paramount significance for designing extensive management strategies to combat erosion hazard. The present study aimed to estimate mean annual soil loss in a tropical river basin such as Kumari river basin (KRB, area: 195125 ha) in the Chotanagpur plateau region in India. In addition, the study also quantified mean sediment yield and identified the main sediment sources within the basin. For these purposes, the present study introduced a raster based spatially distributed approach that integrated RUSLE and sediment delivery ratio (SDR) models with remote sensing (RS) and GIS. The results depicted that annual soil loss within the basin varied from 0 to 208.34 t ha – 1 yr – 1 with an average of 1.92 t ha – 1 yr – 1. The computed annual soil loss was about 364.05 × 103 tonnes, of these, 209.46 × 103 (∼58 %) were lost from only 8.93 % of the total basin area at intolerable rate. Similarly, the estimated sediment yield within the basin ranged from 0 to 64.53 t ha – 1 yr – 1 with a mean of 1.15 t ha – 1 yr – 1 and followed a similar distribution pattern of soil erosion. Being characterized by steep topography, the forest patches in the upstream part of the basin showed the most critical erosion risk. The methodology adopted in the current research displayed acceptable correctness and enabled spotting of most vulnerable areas of water erosion and critical sediment sources, initiating a comprehensive anticipating aid for the implementation of proper land management and soil conservation plans. This perspective is surmised to be helpful for extensive study of water erosion in other tropical river basins.

Воздействие отработанных месторождений россыпной платины на речную систему р. Вывенки (Камчатский край)

Geological exploration and mining operations have been conducted in the Vyvenka River basin on the Seinav-Galmoenan platinum deposit since 1994. The majority of the explored and prospective reserves are located in the valleys of spawning salmon tributaries, raising concerns about the environmental impact of mining activities. Despite the cessation of platinum mining in the 2010s, there has been a continuing impact on the river system in recent years, with extensive sediment plumes forming along the length of the rivers. This research aims to assess the dynamics of disturbed lands and determine the scale of sediment export into the river network from the territory of the ore mining sites of Vetvisty, Penisty, Ledyanoy, Yuzhny, and Levtyrinivayam. Field and satellite data were combined to analyze the suspended sediment discharge regime and the main sediment sources, i.e., the riverbank deformations along the mining sites. As of 2021, the total area of disturbed lands in the Vyvenka basin due to open-cast platinum mining is estimated to be 19.2 km2 corresponding to 0.15 % of the area of the Vyvenka River basin. Up to 75 % of the disturbed area consists of bare lands, making them prone to surface erosion. The largest amounts of sediment influx from the mining-affected river basins were observed in 2018 during the unevenly high freshet. The increased sediment discharge was due to the overflow of sedimentation ponds in certain areas. The study results indicate a prolonged anthropogenic impact on river ecosystems, manifested in the change of hydrological regime, activation of channel deformations, and the increased sediment influx to the river network from ore mining sites.

Sediment-Loading Processes in a Forested Catchment: Modeling and Observations

In order to investigate sediment-loading processes in a catchment, the daily time series of river discharge and sediment load were applied to a semi-distributed model, the Soil and Water Assessment Tool (SWAT). The time series of discharge and sediment load were obtained by monitoring the river stage and water turbidity of the Oikamanai River, Hokkaido, Japan, in the rainfall season (April-November) of 2011-2014. The catchment is forested (ca 90% area) but underlain by the Neogene sedimentary rocks with currently active faults and forest soils with tephra layers, which tend to frequently produce slope failure such as landslide and bank collapse by rainfall or snowmelt. The water turbidity, T, in ppm was converted into suspended sediment concentration, SSC, in g/L by applying the linear relationship between T and SSC. The acquisition of the time series of discharge, Q (m3/s) and sediment load, L (=Q·SSC in g/s) of the river allowed us to distinguish the fluvial sediment transport, accompanied by slope failure in the upstream, from that under no slope failure. The SWAT was used to simulate soil erosion and identify the region prone to the soil erosion in the Oikamanai River basin. The model’s results showed a satisfactory agreement between daily observed and simulated sediment load as indicated by the high Nash-Sutcliffe efficiency. This evidences that the upper mountainous region of the catchment provides a main sediment source, accompanied by slope failure.

Late Jurassic back‐arc extension in the Neuquén Basin (37°S): Insights from structural, sedimentological and provenance analyses

AbstractThe Middle Jurassic–Early Cretaceous evolution of the Neuquén Basin is traditionally attributed to a long phase of thermal subsidence. However, recent works have challenged this model. In view of this, we study the Late Jurassic Tordillo Formation, a non‐marine depositional unit that marks a shift to regional regression across the basin. Previous studies propose different causes for this regression, including the growth of the magmatic arc in the west, uplift in the south or extension in the north. We studied the Tordillo Formation in sections located at an intermediate position in the Neuquén Basin, in order to understand the tectonic processes active during sedimentation. We present evidence of normal faulting within the Tordillo Formation and the base of the overlying Vaca Muerta Formation. Some of these faults can be attributed as syndepositional. We characterize the Tordillo Formation as part of a distal fan‐playa lake depositional system with a contemporaneous western magmatic arc as the main source of sediment. When compared to the Late Triassic–Early Jurassic NE to NNE‐oriented rifting, which marks the opening of the Neuquén Basin, the Late Jurassic extension shows a switch in stress orientation; the latter is orthogonal to the north‐trending subduction zone. We interpret this change as a renewed phase of back‐arc extension induced by slab rollback along with minor distributed intraplate extension prior to opening of the South Atlantic Ocean.

Effect of Rill Development on Slope Erosion and Sediment Yield Based on Stereophotogrammetry Technology

Rill erosion is an important kind of slope erosion and the main source of sediment. Through simulated rainfall tests, the morphological characteristics of rill were quantified by stereophotogrammetry technology, and the relationship between rill development and sediment yield was studied. The results show that there was a positive correlation between sediment yield and slope and rainfall intensities. With the increase in rainfall duration, sediment yield first increased sharply and then decreased gradually after reaching the peak value, until it reached dynamic stability. With the increase in rainfall intensity and slope, the length, width, and number of rills increased significantly, with a maximum length of 2.58 m and a maximum width and depth of 9.7 and 2.2 cm. The rill density (RD) increased from 16.67% to 62.65%; rill fragmentation degree (RFD) increased from 16.67% to 100.00%; rill complexity (RC) increased from 10.62% to 30.84%, and rill width–depth ratio (RWDR) decreased from 15.82% to 56.28% with the increase in slope from 6° to 15° and rainfall intensity from 2.0 to 3.0 mm/min. There was a good nonlinear relationship between sediment yield and RC and RWDR (R2 = 0.89, NSE = 0.85, n = 10). This study could provide help for the quantification research of rill erosion mechanisms and provide reference for the measurement and scale transformations of soil erosion at different scales.

Exploratory radioisotope measurements suggest that in-stream erosion represents the main sediment source in a pristine, tropical rainforest in Costa Rica

Exploratory radioisotope measurements suggest that in-stream erosion represents the main sediment source in a pristine, tropical rainforest in Costa Rica

Runoff sediment characteristics affected by erosive rainfall patterns in a small watershed in karst areas of southwest China

Few studies have focused on observing long-term localized runoff and sediment yield at a small watershed scale in karst areas in southwest China. In addition, the study of soil erosion mechanisms have also been unsatisfactory. Using hydrological data from 2014 to 2019, this study investigated the rainfall, runoff, and sediment yields in the Mahuangtian small watershed, where covered an area of 2.26 km2. The results showed mean values of 124 t/(km2·a) and 13.8 mm for the sediment modulus and runoff depth, respectively. Using the K-medoids clustering method, 112 erosive rainfall events were classified into three types of rainfall patterns based on duration (TR), rainfall depth (R), and maximum 30-minute rainfall intensity (I30). Pattern A was characterized by the shortest TR, lowest R, and highest I30; pattern B was characterized by medium TR, R, and I30; and pattern C was characterized by the longest TR, highest R, and lowest I30. Pattern B caused the largest runoff and sediment yield in the watershed, implying that further attention should be focused on this rainfall pattern to control soil and water loss. In addition, hysteretic loop analyses of suspended sediment concentration and flood discharge suggest that complex and clockwise loops were dominant for all events. This study found that the deposition stored in the subterranean river was the main source of sediment for the watershed. The results of this study provide useful information regarding the relationship between rainfall and runoff sediment in a small karst watershed and advance research on soil erosion mechanisms in karst areas.

Assessment of Metallic Contamination in Soils and Sediments in the Bétaré-Oya Gold Artisanal Mine District, East-Cameroon

Mining is an activity that involves the use of chemicals and results in unsustainable and dangerous conditions for the miners, their families, the community and the surrounding ecosystem. This study was done to assess metallic contamination of soils and sediments in the Bétaré-Oya gold artisanal mine district, East-Cameroon. A total of thirty (30) samples of exploited soils, control soils and sediments were taken from the mining sites of Lom, Mbal and Mali. The concentrations of heavy metals and metalloids (Fe, Sr, Cu, As, Pb and Zn) were determined by Inductive Plasma Ionisation Source Atomic Emission Spectrometry. The results show that in the sediments metals such as: As (1.82), Cu (24.22), Pb (18.04) and Zn (65.69) have average concentrations slightly higher than the UCC and Average Shale reference values. In the exploited soil As (1.93), Cu (16.86) and Zn (96.22) have average concentrations slightly higher than the UCC and Average Shale reference values.While in the cotrol soils only As (1.62) and Zn (59.72) shows an average concentration slightly above the UCC reference values. The average concentrations of the different heavy metals and metalloids were then used to calculate the intensity of contamination in soils and sediments. The results of this calculation indicate that the Enrichment Factor (EF) of As, Sr, Zn and Pb indicate an extremely high enrichment in these elements. The geo-accumulation Index (Igeo) (22.85-25), the Contamination (CF) (1.47), the low values of the ecological risk indices suggest a low potential ecological risk and Pollution load index (PLI) (0.67) indicates extreme contamination with these elements in the soils and sediments. However, only As indicates a slight contamination (class 1) in soils and sediments. Gold panning activities and untreated mine tailings discharges would be considered as the main sources of sediment and soil pollution in Bétaré-Oya.

Landslides and Gullies Interact as Sources of Lake Sediments in a Rifting Context: Insights from a Highly Degraded Mountain Environment

Like many other lakes in the world, the interconnected Abaya and Chamo lakes in the Southern Main Ethiopian Rift are affected by rapid sediment accumulation. Although land degradation is a well-known issue in this part of the African continent, the main sediment sources, their spatial distribution and interaction in the Abaya–Chamo lakes’ basin have not yet been documented. Here, we present a systematic inventory, characterization, and spatial analysis of landslides and gullies as concentrated sediment sources, for four representative river catchments impacted by landscape rejuvenation. Using Google Earth imagery and field surveys, we mapped with high accuracy a total of 7336 gullies and 430 landslides. Recent landslides observed during the last decade were few, small and shallow, and appear to have played a minor role in the current sediment dynamics. Large landslides are old and inactive. Although they do not contribute to the current sediment budget, they contribute indirectly to landscape dynamics by favoring the occurrence of gullies. Overall, large percentages of severe to extremely degraded areas of gully erosion are located in rejuvenated landscapes, especially at the level of the old landslides. Many active gullies are connected to the river network, as such acting as the source of sediment. Our analysis is a step towards understanding the nature and control of anthropic activities on sediment production in the region. We also highlight the importance of considering the interactions between sediment sources and the connectivity of the geomorphological system.