Transport Sediment Research Articles

Applicability of automatic image analysis in quartz-grain shape discrimination for sedimentary setting reconstruction

The quartz-grain shape is an important sedimentological attribute typically used to reconstruct depositional settings and sediment transport conditions and predict the duration of grain transport. A common assumption is that the extent and duration of eolian and fluvial transport are reflected in the increased sphericity and roundness of sand grains, and this study aims to verify this relationship. Automatic particle-shape image analysis was performed on quartz grains (0.8–1.0 mm) from three sedimentary environments: weathered with angular fresh grains (GNU), eolian with well-rounded grains (GRM), and fluvial with well-rounded grains (GEL). This enabled us to obtain four associated shape-description parameters: highly sensitive circularity (circularity HS), convexity, solidity, and aspect ratio. Subsequent principal component analysis and discriminant analysis of the parameters indicated that circularity HS (associated with the degree of roundness) best distinguished the shape of GNU, GRM, and GEL grains. However, the aspect ratio was more useful for analyzing the degree of grain sphericity. GRM and GEL grains, those with a high degree of roundness, had either low or high sphericity, which should be considered a characteristic of long-term reworking by eolian and fluvial processes. The presence of grains with low sphericity and high degree of roundness, such as those found in environments where transport prevails, should be considered a shape inheritance. Using both sphericity and roundness parameters facilitates an accurate interpretation of transport environments in terms of transport duration and sediment maturity. However, neglecting grain sphericity when analyzing these sorts of sedimentary settings may result in the misinterpretation of their true sedimentary settings.

Modeling study on fate of micro/nano-plastics in micro/nano-hydrodynamic flow of freshwater

As the risk of micro/nano-plastics (MPs/NPs) on marine ecosystems is reported, there is a growing interest in behaviors of MPs/NPs in freshwater. Thus, this study aims at developing a plastics fate model linked with a flocculation kinetics model, PsFM/FKM, to understand the vertical behaviors of MPs/NPs in freshwater. Based on the Population Balance approach, the model numerically predicts vertical transport (sedimentation, resuspension, and burial) and transformation (degradation and aggregation) in water and sediments. This study performed model simulations to validate model accuracy and precision as estimating temporal changes in MPs/NPs concentration in water and sediments, based on the aggregation process to form homo-aggregates between plastics and hetero-aggregates between plastics and SS in water. It confirmed that a significant parameter of the aggregation was collision frequency in water and that attachment efficiency influenced the aggregation rate occurring in the collision. The model emphasized the importance of attachment efficiency with the size-dependence and confirmed that the formed hetero-aggregates promoted the sedimentation process to settle down to the sediments. The model validity was demonstrated by comparing experiments and simulations for attachment efficiency. A further study improves the model and extends its applicability to various types of MPs/NPs, SS, microalgae, and metal hydrate salts.

ANALISIS ANGKUTAN SEDIMEN DASAR (BED LOAD) PADA SALURAN IRIGASI MATARAM YOGYAKARTA

In the last decade, the problem that has occurred in the Yogyakarta Mataram irrigation channel is the occurrence of sedimentation in the channel. This has an impact on reducing the cross-sectional discharge capacity of the canal and resulting in the supply of irrigation discharge to agricultural areas to be not optimal, so that agricultural productivity in the Mataram Irrigation Area will also not be optimal. The sediment transport (bed load) that occurs in an open channel can be approached using the empirical equation, including the Einstein, Meyer - Peter Muller and Frijlink equations. Sediment transport events that occur in the channel are stated based on the magnitude of the flow shear stress which exceeds the critical shear stress of the sediment particles. The quantity of sediment transport in the channel is stated on the logarithmic curve of the relationship between the froude number (fr) to the sediment transports (qb). The Curve explains that the increase in the froude number (fr) that occurs on each section of the channel will be directly proportional to the increase in the quantity of transport sediment (qb). The largest sediment transport occurred at the site of the Gambang and Nambongan channel section with a prediction of sediment transport of 3.57 m3/day and 3.67 m3/day, respectively. Thus, the potential for sediment transport that will settle in the downstream area is 3.67 m3/day.

Seismic characteristics and distributions of Quaternary mass transport deposits in the Qiongdongnan Basin, northern South China Sea

Mass transport deposits can cause marine geohazards and transport sediments to deep-water areas and act as hydrocarbon reservoirs and seals, which have been extensively examined throughout the last decade in the northern South China Sea. Based on 2D and newly acquired high-resolution 3D seismic data, as well as logging-while-drilling (LWD) data, we document the characteristics, distributions, sources, and triggering mechanisms of the Quaternary MTDs in the Qiongdongnan Basin (QDNB). Moreover, their impacts on deep-water channels, gas hydrate accumulation and marine geohazards are discussed. The results show that there are ten phases of Quaternary MTDs development in the study area. Among them, MTD3 covers the largest area, ~627 km2, while the smallest is MTD10 at ~89 km2. These MTDs are characterized by chaotic and translucent/transparent seismic facies and negative-phased seismic reflections on their seismic profiles. The transport directions of these MTDs mainly occur from the northwest to southeast, northeast to southwest, and southwest to northeast, with sources in the Northern Slope Area and Western Slope Area or Guangle Uplift Area. Based on an analysis of the tectono-sedimentary evolution of the QDNB, these Quaternary MTDs may have been triggered by an anomalously high sedimentation rate, increased slope gradient, fault reactivation, earthquakes, and sea level fluctuation. Among these processes, earthquakes and rapid sea level fluctuation could be the most critical triggering mechanisms. Furthermore, the MTDs primarily act as hydrocarbon seals in the study area, facilitating the accumulation in the shallow gas reservoirs and gas hydrates within the Quaternary strata. MTDs and related tsunamis are also major marine geohazards, which need in-depth integrated researches by multidisciplinary scientists.

Numerical modelling of alongshore variability in waves and wave-driven currents during the morphodynamic change of a laboratory beach

A non-hydrostatic phase-resolving numerical model (SWASH) is used to investigate the wave-driven nearshore hydrodynamics corresponding to new observations from a three-dimensional physical model in a wave basin. In the physical model, waves from a passing storm are simulated by varying the energy and duration of waves generated by a paddle over time and the beach morphology is surveyed in high spatial-resolution using a 3D laser scanner after each segment of the storm. An oblique incident wave angle introduces spatially variability in wave energy along the beach and results in morphologic differences during the storm, with active transport of beach sand by the wave-driven circulation. SWASH is validated using wave and current observations at each stage of the storm, indicating low root-mean-squared errors between model results and wave measurements, where the high-resolution model is able to predict wave shoaling, breaking and generation of the alongshore current. Model-data agreement is not as high for the velocity, suggesting spatial variability in bottom friction and horizontal mixing, and evolution of the bed in the physical model, could explain the differences. The detailed numerical model results indicate that the alongshore current is generated by wave breaking in the nearshore zone, and it increases in speed across a wider surf zone for increasing wave energy during the storm. This is associated with a bottom shear stress that exceeds the critical bed shear stress, with the potential to initiate motion and transport sediment in agreement with the experimental observations.

Analysis of the Influence of the Shibahu Channel on Water Level and Sediment in the Lower Reach Yellow River

It is necessary to study and analyze the Shibadu flow path as a prospective alternative flow path for the Yellow River diversion. In this paper, the established two-dimensional mathematical model is used to calculate the Shibadu flow path and analyze the water level characteristics of the Shibadu inlet channel and the influence of the ocean tide level on the upstream water level and on the sediment transport. From the calculation results and water-sediment characteristics, it can be seen that: the Shibahu diversion is located in a bend, so it has a high resistance to the upstream water flow, resulting in a small change in the upstream water level at the diversion, a high upstream water level, and a significant decrease in the water level after the diversion; the range of the spread of the sediment movement in the Shibahu is small, and coupled with the weak ocean dynamics, the ability to transport sediment to the outer sea is poor, resulting in the inlet sediment near the mouth gate. The variation of sediment entering the sea during the rising tide and the falling tide is the same and different: the sediment concentration increases at a certain distance during the rising tide, but not during the falling tide. Both cases are about 41km away from the entrance, sediment began to settle.

Transformation of sediment delivery and dispersal patterns controlled by relay-ramp evolution along the boundary fault of a lacustrine rift: The Eocene Shahejie formation, Dongying Sag, Bohai Bay Basin, NE China

The boundary fault of the Dongying Sag is divided into isolated and scattered segments and small-scale relay zones, which are important geomorphic features controlling sedimentation in the early stage of rift evolution. Previous studies have shown changes in facies in deposits of thousands of meters in thickness on fault-bounded margins, where deep-water fan systems are vertically stacked with delta deposits in the same way as in many basins around the world. However, changes in sedimentation processes and sedimentary systems and their relationship with the evolution of the main boundary fault remain a matter of debate. We focus on the transition from shallow-water to deep-water systems and their responses to relay-ramp evolution in the Yanjia Subsag of the Eocene Dongying Sag in the Bohai Basin. A relatively simple assemblage of local structures provides an ideal opportunity to investigate the impact of relay-ramp evolution on the early synrift deposits. An integrated dataset of 3D seismic volumes, cores and wireline logs is used to analyze the evolution of the Chennan Fault, to delineate thirteen lithofacies, and to recognize two depositional styles (including fan deltas and nearshore subaqueous fans) and their distribution during different stages of the relay-zone evolution. In the early stage of rifting, the depositional style changes, from the development of axial and transverse systems in the early to middle period to the development of transverse systems in the late period; this is mainly controlled by the evolution of the relay ramp. The change is accompanied by a rapid decrease in the rate of sediment supply, architectural changes in transport conduit from the relay ramp to a canyon, and local geomorphic changes that control transport routes, sediment delivery, and depositional locations. Floods ultimately become dominant transport mechanism with a new system, such that hyperpycnal flows form, leading to another transformation of the sedimentary systems from a fan-delta system to a nearshore subaqueous fan system. On the basis of these transformations in sedimentary systems we are able to propose new recommendations for reservoir exploration in continental rift basins, which include exploration potential, source reservoir cap assemblage, and spatiotemporal evolution of plays.

A mixed turbidite – contourite system related to a major submarine canyon: The Marquês de Pombal Drift (south‐west Iberian margin)

AbstractSynchronous interaction between bottom currents and turbidity currents has been reported often in channel–levée systems where the thickness of the turbidity currents exceeds that of the levées. Such interplay between along‐slope and down‐slope sedimentary processes is one of the mechanisms by which ‘mixed turbidite–contourite systems’ can originate. However, bottom currents flow over large areas of the seafloor, including continental slopes characterized by deeply incised submarine canyons rather than channel levées. In these cases, a direct interaction between along‐slope and down‐slope currents is, theoretically, unlikely to take place. In this study, oceanographic, swath bathymetry, multichannel seismic data and sediment cores are used to investigate a 25 km long, 10 km wide and up to 0.5 km thick deep‐sea late Quaternary deposit that sits adjacent to the north‐west flank of one of the major canyons in the North Atlantic, the São Vicente Canyon, in the Alentejo Basin (south‐west Iberian margin). The area receives the influence of a strong bottom current, the Mediterranean Outflow Water, which has swept the continental slope at different water depth ranges during glacial and interglacial periods. Architectural patterns and sediment characteristics suggest that this sedimentary body, named Marquês de Pombal Drift, is the result of the interaction between the Mediterranean Outflow Water (particularly during cold periods) and turbidity currents flowing along the São Vicente Canyon. Because the canyon is incised significantly deeper (ca 1.5 km) than the thickness of turbidity currents, an additional process, in comparison to earlier models, is needed to allow the interaction with the Mediterranean Outflow Water and transport sediment out of the canyon. In the São Vicente Canyon, and likely in other canyons worldwide, interaction of turbidity currents with contour currents requires intermediate nepheloid layers that export the finer‐grained fraction of turbidity currents out of the canyon at the boundary between major water masses.

Sulfuric acid vapor and sulfur dioxide in the atmosphere of Venus as observed by the Venus Express radio science experiment VeRa

The Venus Express radio science experiment VeRa provided more than 900 neutral atmospheric profiles between the years 2006 and 2014. About 800 of these could be used for an analysis of the radio signal absorption at X-Band (wavelength: 3.6 cm), which is mainly caused by sulfuric acid vapor within the Venus atmosphere. The absorptivity profiles were converted into sulfuric acid vapor profiles. The combined measurements from the entire Venus Express mission reveal a distinct latitudinal H2SO4(g) variation. A latitudinal gradient can be observed at the topside of the H2SO4(g) layer, which is located approx. 4 km higher at equatorial latitudes compared to polar latitudes. Regions of enhanced sulfuric acid vapor abundance were found at equatorial and polar latitudes. The highest H2SO4(g) values at equatorial latitudes show mean maximal values of more than 12 ppm at around 47 km altitude. At polar latitudes mean maximal values were found at around 43 km altitude and ranged from 9 to 12 ppm. Both latitudinal regions of increased sulfuric acid vapor abundance are clearly separated by a low abundance region located at mid-latitudes with values of 5 to 7 ppm.A simplified two-dimensional transport model was developed to study the formation processes of sulfuric acid vapor accumulation at equatorial and polar latitudes. It turned out that the H2SO4(g) accumulation observed at high latitudes can be explained by precipitation of H2SO4(l) droplets that evaporate into gaseous sulfuric acid upon entering lower (warmer) altitudes. The influence of wind transport on this formation process was minor. In contrast, the H2SO4(g) accumulation observed at equatorial latitudes could be reproduced in the model by oppositely directed mass transport (upward winds and sedimentation) as well as by simplified evaporation and condensation processes. The low H2SO4(g) abundance observed at mid-latitudes was reproduced by downward winds in the model calculations.The VeRa observations were additionally used to estimate the abundance of SO2 above the cloud bottom. A latitudinal dependence was found with highest values of 90 ± 60 ppm at equatorial latitudes, compared to 150 ± 50 ppm and 160 ± 50 ppm at southern and northern polar latitudes, respectively.Both the equatorial and polar regions displayed show large variability of the H2SO4(g) and SO2 abundances from observation to observation. A weak tidal influence is also visible in the sulfuric acid vapor abundance in the equatorial region. The northern polar H2SO4(g) abundance, as well as the southern and northern SO2 abundances, exhibit distinct long-term variations.

Management and seasonal impacts on vineyard soil properties and the hydrological response in continental Croatia

Soil degradation in vineyards is a global problem, indicating the need for the adoption of sustainable practices. Studies on the seasonal and long-term management impacts on soil properties and hydrological responses in vineyards are needed. Rainfall simulation experiments (60 mm h−1 for 30 min) and soil sampling were conducted on Cambisols in eastern Croatia during the dry and wet seasons in long-term grass-covered and tilled vineyards. The results show that bulk density (BD), soil organic matter (SOM), available phosphorous (P2O5), soil water content (SWC), mean weight diameter (MWD), and water-stable aggregates (WSA) were significantly higher in grass-covered plots than in tilled plots. WSA was significantly lower in the wet period, whereas the opposite trend was observed for SWC and MWD. The time to ponding (PT) and time to runoff (RT) were high (p < 0.05) in the tilled plot in the dry season and low in the wet season. The sediment concentration (SC), soil loss (SL), carbon loss (C loss), and P2O5 loss were significantly higher in the tilled plot than in the grass-covered plot. Runoff, SC, SL, C loss, and P2O5 loss were significantly lower in the dry period. Although the grass-covered plots exhibited high compaction, the stable structure reduced SL by 91% and 94% in the dry and wet seasons, respectively. The tilled plots increased disaggregation and the availability of sediments for transport, enhancing C loss (4.5 times higher) and P2O5 loss (6.4 times higher) compared with the values in the grass-covered plots. Tillage as an unsustainable practice was more pronounced in the wet season with 16.4, 10.1, and 11.1 times higher sediment, P2O5, and C losses, respectively, than those in the grass-covered plots. More sustainable practices are needed to decrease soil degradation and reverse these processes.

Changes of Flow and Sediment Transport in the Lower Min River in Southeastern China under the Impacts of Climate Variability and Human Activities

The Min River is the largest river in Fujian Province in southeastern China. The construction of a series of dams along the upper reaches of the Min River, especially the Shuikou Dam, which started filling in 1993, modified the flow processes at the lower Min River, leading to the significant increase in low-flows and slightly decrease in flood-flows. At the same time, reservoirs have more effects on the sediment transport process than flow process by trapping most sediment in the reservoirs, and greatly reduced the amount of sediment transporting downstream. Increase in vegetation cover also contributes to the decrease in sediment yield. The reduction in sediment together with excessive sand mining in the lower Min River resulted in the severe downward erosion of the riverbed. Using a reformulated elasticity approach to quantifying climatic and anthropogenic contributions to sediment changes, the relative contribution of precipitation variability and human activities to sediment reduction in the lower Min River are quantified, which shows that the sediment reduction is fully caused by human activities (including land use/land cover changes and dam construction).

Suspended particulate matter in a submarine canyon (Whittard Canyon, Bay of Biscay, NE Atlantic Ocean): Assessment of commonly used instruments to record turbidity

Nepheloid layers with elevated concentrations of suspended particulate matter (SPM) are found throughout the world's oceans. They are generated by both natural processes, involving resuspension of seabed sediment by bottom currents, and anthropogenic sediment resuspension due to bottom trawling, dredging and in the future potentially due to deep-sea mining. These nepheloid layers represent pathways of lateral SPM transport, including lithogenic and biogenic sediment, organic matter, (trace) metals, organic pollutants and plastics. For assessment of the dispersion of these materials, it is essential that SPM mass concentrations can be accurately quantified. However, this is not straightforward as the detected turbidity signal, which is used as a proxy for SPM mass concentration, not only depends on the concentration of particles, but also on physical characteristics of these particles, such as particle size, substance and shape. Here we present a comparative study of turbidity data to assess the potential implications different sensors have on the estimates of SPM mass concentration. Optical backscatter sensors (OBSs), transmissometers and both low- and high-frequency ADCPs were deployed simultaneously in the Whittard Canyon (North Atlantic Ocean), and water samples were collected for quantification of SPM mass concentration and ex-situ particle size analysis. We found that SPM mass concentrations inferred from the transmissometer are easily overestimated in the biologically productive surface layer due to higher light absorption by chlorophyll-bearing phytoplankton, compared to suspended detritic particles. Furthermore, we observed that depending on sensor type some particles are not, or less well, detected. This is due to differences in particle size sensitivities of these sensors towards the diverse range of particle sizes found in the Whittard Canyon, whereby the low-frequency ADCP was most sensitive for coarse-grained material and the high-frequency ADCP and OBSs most sensitive for fine-grained material. In future studies, we suggest to use a combination of different sensors as the use of only one type of sensor could potentially lead to misinterpretation and mis-quantification of particle transport processes and fluxes.

DINÂMICA HIDROSSEDIMENTOLÓGICA NA BACIA DO RIO GUABIROBA, GUARAPUAVA, PARANÁ

O presente trabalho apresenta os resultados de estudo sobre a dinâmica hidrossedimentológica em uma bacia hidrográfica rural, no Sul do Brasil, a partir da classificação e análise das curvas de histerese. A pesquisa avaliou vinte eventos hidrológicos monitorados na bacia do rio Guabiroba, em Guarapuava, Paraná. A classificação das curvas de histerese deu-se em função da dinâmica de comportamento da vazão e da concentração de sedimento em suspensão ocorrida nos eventos pluviométricos, em um dos tipos de modelos de histerese descritos na literatura. Os resultados obtidos demostraram ocorrer na bacia cinco tipo de modelo de histerese, principalmente, as do tipo clockwise loop e figure eight. A relação entre a vazão e a concentração de sedimento em suspensão total, na ascensão e recessão do hidrograma apresentou coeficiente de correlação positiva na maioria dos eventos. As características apresentadas nas diversas curvas de histerese na bacia hidrográfica são resultado da disponibilidade de sedimentos finos ao transporte fluvial, provenientes da erosão das vertentes, das margens fluviais, e de estradas rurais.

Critical Shifts in Trace Metal Transport and Remediation Performance under Future Low River Flows.

Exceptionally low river flows are predicted to become more frequent and more severe across many global regions as a consequence of climate change. Investigations of trace metal transport dynamics across streamflows reveal stark changes in water chemistry, metal transformation processes, and remediation effectiveness under exceptionally low-flow conditions. High spatial resolution hydrological and water quality datasets indicate that metal-rich groundwater will exert a greater control on stream water chemistry and metal concentrations because of climate change. This is because the proportion of stream water sourced from mined areas and mineralized strata will increase under predicted future low-flow scenarios (from 25% under Q45 flow to 66% under Q99 flow in this study). However, mineral speciation modelling indicates that changes in stream pH and hydraulic conditions at low flow will decrease aqueous metal transport and increase sediment metal concentrations by enhancing metal sorption directly to streambed sediments. Solute transport modelling further demonstrates how increases in the importance of metal-rich diffuse groundwater sources at low flow could minimize the benefits of point source metal contamination treatment. Understanding metal transport dynamics under exceptionally low flows, as well as under high flows, is crucial to evaluate ecosystem service provision and remediation effectiveness in watersheds under future climate change scenarios.

Field Study of Surface and Subsurface Bed Material Samplings to Estimate Gravel Bed Rivers Dimensions

One of the most common types of methods to design stable dimensions of a channel is empirical regime relationships to which morphological parameters of stable rivers are related. These relationships could be considered to be more realistic for the design purposes as they consider sediment transport in the rivers. Bed material gradation curve of rivers and river bed armouring development are key points to identify river characteristics and controlling factors in sediment transporting and eroding processes of gravel bed rivers, respectively. In this study, field work was initially carried out on several gravel bed rivers in Khuzestan and Chaharmahal provinces. 24 hydrometeorological stations in 17 river reaches were sampled to characterize bed material gradation curve. With regards to bed structure of gravel bed rivers, samplings were made by surface and volumetric methods. Thereafter, sediment samples were differentiated in three categories of surface, armour and sub-armour layers. The collected field data were originally applied to derive regime relationships by using multivariate regression analysis. The effect of bed structure was then studied in these relationships for the better understanding of that in the regime relationships. The regime relationships yield most appropriate estimated value for four variables equations in comparison with less variables equations.

Best management practices for coastal inlets

Coastal inlets separate individual barrier islands or barrier spits and adjacent headlands (Hayes and Fitzgerald 2013). Inlets modify longshore transport and store sediment in flood and ebb shoals leading to dynamic adjacent shorelines. For example, 80% to 85% of the beach erosion in Florida can be attributed to inlets (Dean 1991). In some cases, structured inlets are designed to trap sand in a preferred location to minimize interference with navigation and facilitate its removal through dredging. Sound coastal engineering practice requires that this sand be placed on adjacent eroding beaches (NRC 1995) to protect coastal resources. This paper provides a brief overview of coastal inlet management and identifies Best Management Practices (BMPs) intended to balance human needs for inlet navigation with the natural systems adjacent to tidal inlets. Today’s conservation measures, which are a result of considerable monitoring, numerical modeling, and other science-based methods, demonstrate that BMPs improve management of sand resources and reduce impacts associated with tidal inlet dredging. For some inlet conditions, BMPs include use of inlet sediment sinks as cost-effective and eco-friendly sand sources for beach nourishment projects located close to the inlet. For optimal coastal inlet management, the ASBPA Science and Technology Committee recommends the following BMPs and conservation measures: • Limit frequency and duration of impacts, • Follow environmental windows, • Implement regional sediment management, • Use beach-compatible sand, • Conduct pre-, during-, and post-dredging monitoring, • Modify dredging equipment/practices, and • Design rechargeable, low-impact inlet borrow sites.

Transport Sediment Analysis Using Bed-load: Case Study Bilibili Reservoir

This study conducted to analyze transport sediment using bed-load in the Bilibili reservoir case, South Sulawesi, Indonesia. The laboratory test has done to determine the basic characteristics of samples consisting of sediment concentration tests and specific gravity tests. Furthermore, the data analysis using Van Rijn formulation to calculate bed sediment discharge (Qb) and flow rate (Q), and analysis result found the relationship between bed sediment discharge and flow rate expressed in equation y = 12.314x1.4292 where y = x. Qb = Q. Therefore, the existing sediment transport amount of the Bilibili reservoir for 2014 to 2018 is 1.102 million tons, 1.426 million tons, 0.737 million tons, 1.277 million tons, and 0.895 million tons, respectively. Besides, total sediment is 216,9261.2 tons/year equivalent to 6,142 m3/year, and the percentage of bed sediment transport to total sediment is 70.5%. The analysis results also indicate an increase in bed sediment discharge due to an increase in river water discharge. The condition explains that increasing the flow rate causes the amount of transported sediment to increase.

Alluvial fans on volcanic islands: A morphometric perspective (São Vicente, Cape Verde)

The distribution, morphology and development of volcanic island alluvial fans are investigated using São Vicente from the Cape Verde volcanic chain (east-central Atlantic Ocean). This is a tectonically and volcanically inactive dryland setting, comprising an eroded Plio-Pleistocene composite volcano overlying a submarine shield volcano. We mapped 228 alluvial fans, quantifying fan (F) and catchment (C) morphological properties (area [A], gradient [G], relief [R], length [L]). Fans and catchments of varying size and gradient are distributed around the island coastal margins (6%) and along interior valley sides (94%). Regression results (i.e. +ve/−ve slopes of best fit lines) conformed to broader fan research but with weak correlations, and coefficient-exponent values outside, or at the lower end of values reported from non-volcanic settings. Analysis of fan area and gradient vs catchment area regression residuals revealed fans were dominated by small-steep gradient (28%) and large-low gradient forms (39%). Fan building space is a fundamental control, specifically linked to volcano structure, but with interplay between rock strength, base level, and climate. Fans around the volcanic edifice flanks are uncommon due to steep flank slopes. Where flank margin fans occur they are typically small and steep forms due to marine erosion. Fans along flank incised valleys are common. Large low gradient fans are associated with a major volcanic rift-aligned valley structure, with extensive erosion into altered and weathered edifice-core lithologies. Small-steep gradient fans are associated with edifice flank valleys. Steeper and narrower valley forms reflect incision into stronger lithologies of the composite volcano flanks, restricting fan building space. Fans located within the volcano central depression are amongst the largest and lowest gradient types, building into the eroded and unconfined edifice core space. Coalescence, apex-toe incision, toe erosion and catchment stream channel backfilling displayed by most fans suggest landscape erosion dominates during more arid periods (e.g. interglacials), with the dryland setting generating limited sediment and insufficient flood runoff for transport and fan deposition. Fan building likely occurs during wetter periods linked to Quaternary climate variability. Quaternary eustatic variations are relevant for the volcano edifice margins where sea level changes control fan building space and erosion.

Late Quaternary sedimentary processes in the central Arctic Ocean inferred from geophysical mapping

Cryospheric events in the Arctic Ocean have been largely studied through the imprints of ice sheets, ice shelves and icebergs in the seafloor morphology and sediment stratigraphy. Subglacial morphologies have been identified in the shallowest regions of the Arctic Ocean, up to 1200 m water depth, revealing the extent and dynamics of Arctic ice sheets during the last glacial periods. However, less attention has been given to sedimentary features imaged in the vicinity of the ice-grounded areas. Detailed interpretation of the sparse available swath bathymetry and sub-bottom profiles from the Lomonosov Ridge and the Amundsen Basin shows the occurrence of mass transport deposits (MTDs) and sediment waves in the central Arctic Ocean. The waxing and waning ice sheets and shelves in the Arctic Ocean have influenced the distribution of MTDs in the vicinity of grounding-ice areas, i.e. along the crest of Lomonosov Ridge. Due to the potential of Arctic sediments to hold gas hydrates, their destabilization should not be ruled out as trigger for sediment instability. Sediment waves formed by the interaction of internal waves that propagate along water mass interfaces with the bathymetric barrier of Lomonosov Ridge. This work describes the distribution and formation mechanisms of MTDs and sediment waves in the central Arctic Ocean in relation to grounding ice and internal waves between water masses, respectively. The distribution of these features provides new insight into past cryospheric and oceanographic conditions of the central Arctic Ocean.

Provenance studies of southern Tanzania river sediments: Heavy mineral signatures and U–Pb zircon ages

Detrital zircon U–Pb geochronology and heavy mineral data from four main rivers in coastal southern Tanzania - the Rufiji, Matandu, Mbwemkuru and Ruvuma rivers - are compared with the bedrock geology of their catchment and with data from the Mesozoic strata of the Mandawa Basin, also in coastal Tanzania. The objective is to evaluate the source-to-sink pattern of sedimentation through time, between the Mesozoic and present day.The recent river sediments display variations in both heavy mineral assemblages and U–Pb zircon populations, reflecting their different catchment areas. The Matandu and Mbwemkuru rivers transport sediments characterised by amphibole-dominated heavy mineral assemblages, whereas the great Rufiji and Ruvuma rivers are characterised by more stable heavy mineral assemblages. The detrital zircon populations show age peaks at c. 2900–2500, 2000–1800, 1000, 800 and 700–500 Ma, the common denominators are the Late Mesoproterozoic (c. 1000 Ma) and Late Neoproterozoic (700–500 Ma) age fractions. The Rufiji River and the Matandu River display similar zircon age distributions, both containing abundant Palaeoproterozoic zircons, interpreted as recycled from Karoo successions in a part of the Selous Basin which is drained by both rivers. The Mbwemkuru and Ruvuma rivers contain mainly Late Mesoproterozoic and Late Neoproterozoic aged zircons that were supplied from the Unango and Marrupa complexes and the Cabo Delgado Nappe Complex/Eastern Granulites. The more diverse zircon population in the Rufiji and Matandu river is likely reflects recycling of older sedimentary successions in addition to sediments supplied from basement lithologies.