Sediment Mobility Research Articles

Herbal plants- and rice straw-derived biochars reduced metal mobilization in fishpond sediments and improved their potential as fertilizers

Fishpond sediments are rich in organic carbon and nutrients; thus, they can be used as potential fertilizers and soil conditioners. However, sediments can be contaminated with toxic elements (TEs), which have to be immobilized to allow sediment reutilization. Addition of biochars (BCs) to contaminated sediments may enhance their nutrient content and stabilize TEs, which valorize its reutilization. Consequently, this study evaluated the performance of BCs derived from Taraxacum mongolicum Hand-Mazz (TMBC), Tribulus terrestris (TTBC), and rice straw (RSBC) for Cu, Cr, and Zn stabilization and for the enhancement of nutrient content in the fishpond sediments from San Jiang (SJ) and Tan Niu (TN), China. All BCs, particularly TMBC, reduced significantly the average concentrations of Cr, Cu, and Zn in the overlying water (up to 51% for Cr, 71% for Cu, and 68% for Zn) and in the sediments pore water (up to 77% for Cr, 76% for Cu, and 50% for Zn), and also reduced metal leachability (up to 47% for Cr, 60% for Cu, and 62% for Zn), as compared to the control. The acid soluble fraction accounted for the highest portion of the total content of Cr (43–44%), Cu (38–43%), and Zn (42–45%), followed by the reducible, oxidizable, and the residual fraction; this indicates the high potential risk. As compared with the control, TMBC was more effective in reducing the average concentrations of the acid soluble Cr (15–22%), Cu (35–53%), and Zn (21–39%). Added BCs altered the metals acid soluble fraction by shifting it to the oxidizable and residual fractions. Moreover, TMBC improved the macronutrient status in both sediments. This work provides a pathway for TEs remediation of sediments and gives novel insights into the utilization of BC-treated fishpond sediments as fertilizers for crop production.

Testing the effective-discharge paradigm in gravel-bed river restoration

Restoration of an alluvial wet-meadow system was conducted in the late 1990s to reestablish hydraulic interactions between the river, floodplain, and groundwater to support aquatic–riparian ecosystem function. A single-discharge approach sized the bankfull channel dimensions to the effective discharge (Qe) and three degrees of channel widening relative to the Qe design were explored to identify which design attained dynamic equilibrium in the shortest time. The three experimental channel designs were implemented with bankfull widths of 96%, 157%, and 191%, respectively, of the Qe geometry. Response trajectories were documented for channel dimensions, sediment mobility, channel morphology, floodplain connectivity, and riparian vegetation for the three channel designs, and the efficacy of a single-discharge approach for restoration was examined.Analysis of 20 years of monitoring data and hydraulic modeling revealed that each design responded differently to the imposed initial channel conditions and evolved at substantially different rates. The design with bankfull dimensions most closely approximating Qe reached dynamic equilibrium within four years of restoration, whereas the moderately over-widened channel (57% larger) exhibited slower responses toward dynamic equilibrium for some metrics and did not fully attain the Qe design dimensions within the monitoring period. The extremely-over-widened channel (91% larger) mainly induced slow rates of bed deposition that are projected to take nearly 300 years for the bankfull dimensions to narrow to the Qe width. All reaches had low bed mobility (bankfull Shields stress < 0.03) 14 years after restoration, demonstrating the challenge of reducing the drivers of channel widening, while maintaining sufficient competence for bedload transport and a sustained supply of coarse bed material for salmonid habitat. Restoration that sizes the channel to Qe can provide rapid dynamic equilibrium, but is a first-order simplification of 1) channel dynamics and 2) the range of flows needed for restoring physical and biological processes in wet-meadow systems.

Wave-induced sediment and rhodolith mobility on a narrow insular shelf dominated by wave variability (Fernando de Noronha Archipelago, Brazil)

This study combines numerical modeling and empirical equations to define spatial fields of wave-induced sediment and rhodolith mobility in a narrow insular shelf that is subject to a seasonal wave regime: the Fernando de Noronha insular shelf. From analyses of the annual, summer and winter wave climates, we noticed an intense directional variability in swell waves coming predominantly from opposing directions between the studied seasons (northern swell in summer and southern swell in winter), reflecting higher wave energy on the north coast during summer and lower energy during winter. The longitudinal transport at the northern coast, which is predominantly oriented to the southwest, is also higher in the summer months and lower in the rest of the year. To describe the spatial variation in wave-induced mobility under summer and winter wave conditions, we consider empirical sediment movement equations, based on mean grain size and density values. Carbonate bioclastic sediments, which are predominant in the Fernando de Noronha shelf, are remobilized by waves throughout the entire shelf. However, in the nearshore zone, this movement occurs intensely for all sizes. The depth and shelf side affected by wave-induced mobility vary according to the wave climate variability that determines the direction and type of wave prevailing in the regime. When swell waves come from the north (summer scenarios), mobility is greater mainly on the northern shelf; however, when trade wind waves come from the southeast and swell waves come from the south (winter scenarios), greater movement is produced on the south face. Wave-induced sediment mobility occurs at depths of up to 60 m, near the shelf break, indicating the importance of the shelf morphology. The movement of rhodoliths, which presents a lower density and larger diameter than the sediments, eventually occurs under extreme wave conditions up to depths of approximately 30 m in areas that vary seasonally around the island according to the predominant wave direction. These characteristics are favorable to rhodolith development. With our results, we hope to better understand the wave action on the bottoms of high-energy island shelves by identifying the sediment movement patterns attributed to wave climate variability and morphological characteristics of the shelf.

Geomorphic responses of fluvial systems to climate change: A habitat perspective

AbstractFluvial systems provide a variety of habitats that support thousands of species including many that are threatened or endangered. Moreover, these habitats, which range from aquatic and riparian to floodplain, are important for the variety of ecosystem services they provide. In addition to water temperature and streamflow change, geomorphic change is important and warrants consideration as one of the several potential threats to these habitats posed by climate change. The geomorphic response of fluvial systems to global warming in temperate environments, for example, caused by an increase in the frequency and magnitude of floods, is important because geomorphology is a primary determinant of habitat availability and quality. Possible geomorphic responses include increased erosion and (or) deposition in the river channel, riparian zone, and floodplain with associated habitat implications. Geomorphic changes caused by global warming can be beneficial (e.g., increased habitat complexity) or detrimental (e.g., mortality caused by scour or burial) to biota. The ability of a species to respond to and survive disturbances, including geomorphic changes, will depend on the nature of the disturbances and the sensitivity and adaptive capabilities of the species. Post‐flood recovery often is rapid; however, for certain species (e.g., periphyton, macroinvertebrates), changes in community composition may persist. Increased flood frequency, sediment mobility, and associated geomorphic changes potentially will result in more frequent and persistent changes in habitat and community composition in the affected fluvial systems.

Observations of near-bed orbital velocities and small-scale bedforms on the Dutch lower shoreface

The lower shoreface, with water depths between about 8 and 20 m, forms the transition between the inner shelf and upper shoreface. Knowledge of the lower shoreface is essential, as it is – in many cases – the sediment source for the upper shoreface and beach. This paper presents new data of near-bed orbital velocities and small-scale bedforms at various depths and locations on the Dutch lower shoreface. Near-bed orbital velocities were beyond 1 m/s during high-energetic wave conditions. They increase with wave height and decrease with water depth, and can be reasonably well described by linear wave theory. Ripple heights range between 0.01−0.03 m and ripple lengths between 0.08−0.20 m. Ripple dimensions are controlled by wave mobility, with lower and shorter ripples for higher waves, and not so much by the currents. The Van Rijn (2007) formula generally overpredicts the ripple heights, and the variation with tidal currents in time. The measurements clearly indicate significant sediment mobility at the lower shoreface under higher wave events. It is yet unclear what this means for the net sand transport. This will depend on the subtle timing of sediment suspension, wave-mean currents and near-bed orbital velocities. It requires detailed modeling to determine lower shoreface net transport rates, and to unravel the controlling sand transport mechanisms.

Hysteretic Implications for Graded Bed Load Sediment Transport in Symmetrical Hydrograph Flows

Differential parametric values associated with bed load sediment transport, that result at the same discharge levels on the rising and falling limbs of a flood hydrograph, are usually defined as bed load hysteresis. This hysteresis in bed load sediment transport rates is of considerable interest in the field of fluvial hydraulics. Within this study, a series of well-defined, symmetrical hydrograph flows are generated over a graded, mobile sediment bed to fully examine the hysteresis of the resulting bed load sediment transport in terms of the threshold of motion, and differential bed load transport rates and bed load yields during the hydrographs. The experiments are conducted in a titling flume without sediment supply specified at the upstream inlet, thereby representing typical river reach conditions immediately downstream of a dam that are exclusively subject to net in-channel bed degradation from sediment transport initiated during flood events. Our results show that the fractional bed load transport of defined fine, medium and coarse size classes within the graded sediment bed generally display clockwise, no/mixed and counter-clockwise hysteresis patterns, respectively, with clockwise hysteresis most commonly found for the coarse size class mobilised by hydrographs with long durations. By contrast, counter-clockwise hysteresis is usually observed for fine size class transported by hydrographs with short durations. Accordingly, the corresponding reference stresses for each size class vary between different hydrographs and are primarily controlled by the hydrograph flashiness (i.e. unsteadiness) and magnitude (i.e. total water work). Moreover, it is shown that the hysteresis effect, particularly for those size classes and hydrograph combinations that result in clockwise and counter-clockwise behaviour, should be fully accounted for when reproducing bed load transport rates using separate-limb based method. Finally, we investigate the relative fractions of the overall bed load yields generated during the rising and falling limbs of all symmetrical hydrographs (i.e. the bed load yield ratio), which are found to be primarily dependent on bed load transport hysteresis. Finally, the relationship between the bed load yield ratio and the ratio of reference stresses for the fractional sediment motion of each size class on both limbs is found to follow a power law.

Beyond the light effect: How hydrologic and geomorphologic stream features control microbial distribution across pool sequences in a temperate headwater stream

AbstractMicrobial community composition varies across stream habitats. However, there is little understanding of how varying hydraulic and geomorphic factors influence microbial distribution along a succession of pools. This study examines how substrate, geomorphological and hydraulic habitat variables may drive bacterial community composition within different stream pool habitats of a temperate headwater stream. Microbial community structures from rock biofilm and sediment samples within each of the 10 selected stream pools of White Clay Creek, PA, were determined by high‐throughput sequencing of 16S rRNA genes. The grain size distribution, organic matter content, streamflow velocity, temperature regime and morphology of each pool were quantified to characterize the pool habitats' variability. Multivariate statistical analysis revealed significant differences in the microbial community composition linked to the substrate's stability within the pool units. Indeed, soft and more mobile sediments were dominated by heterotrophic bacteria, while photosynthetic microorganisms (e.g., microalgae and cyanobacteria) were mainly found on rock biofilm. The difference in the distribution of bacterial communities can be explained by variations in the local hydraulic (i.e., depth and velocity) and the thermal conditions (daily fluctuation, min and max). These results highlight the geomorphological and hydrologic drivers for small‐scale diversity in bacterial communities and provide a better understanding of how maintaining and promoting variability in streambed physical properties may enhance microbial diversity. Better integration of these drivers into stream restoration practices will allow the inclusion of microorganisms, the trophic levels that are usually overlooked but still play critical roles in stream ecology.

Rheology of mobile sediment beds in laminar shear flow: effects of creep and polydispersity

Classical scaling relationships for rheological quantities such as the $\mu (J)$ -rheology have become increasingly popular for closures of two-phase flow modelling. However, these frameworks have been derived for monodisperse particles. We aim to extend these considerations to sediment transport modelling by using a more realistic sediment composition. We investigate the rheological behaviour of sheared sediment beds composed of polydisperse spherical particles in a laminar Couette-type shear flow. The sediment beds consist of particles with a diameter size ratio of up to 10, which corresponds to grains ranging from fine to coarse sand. The data was generated using fully coupled, grain resolved direct numerical simulations using a combined lattice Boltzmann–discrete element method. These highly resolved data yield detailed depth-resolved profiles of the relevant physical quantities that determine the rheology, i.e. the local shear rate of the fluid, particle volume fraction, total shear and granular pressure. A comparison against experimental data shows excellent agreement for the monodisperse case. We improve upon the parameterization of the $\mu (J)$ -rheology by expressing its empirically derived parameters as a function of the maximum particle volume fraction. Furthermore, we extend these considerations by exploring the creeping regime for viscous numbers much lower than used by previous studies to calibrate these correlations. Considering the low viscous numbers of our data, we found that the friction coefficient governing the quasi-static state in the creeping regime tends to a finite value for vanishing shear, which decreases the critical friction coefficient by a factor of three for all cases investigated.

Oscillatory velocity in boundary layer over mobile sediment bed under asymmetric wave and current conditions

Oscillatory velocity in boundary layer over mobile sediment bed under asymmetric wave and current conditions

Approximate velocity formula over mobile sediment bed induced by velocity-skewed waves and current

A velocity formula is proposed for flow over a mobile sediment bed induced by velocity-skewed waves and current. The formula is obtained by a separation of waves and current velocities and requires seven free variables related to free stream velocity and sediment characteristics. The formula includes two parts: (1) a wave part consisting of the free stream velocity and defect function, which considers phase lead, wave boundary layer thickness, and mobile bed level, and (2) a current part, which changes the wave part through the boundary layer thickness and mobile bed level, influenced by wave eddy viscosity. The wave part is superior to the current part such that the boundary layer characteristics are mainly determined by the waves. The wave process is divided into four quarter stages to model the asymmetries of parameters in velocity and acceleration. Under velocity-skewed waves and current conditions, velocity asymmetry and residual velocity are accounted for in the formula, especially the offshore residual velocity with a large phase lag and mobile bed effect with a small phase lag. Phase lead is shown to be non-constant in velocity-skewed oscillatory sheet flow and corresponds to an extra offshore residual velocity.

Sounding Out the Carbon: The Potential of Acoustic Backscatter Data to Yield Improved Spatial Predictions of Organic Carbon in Marine Sediments

Marine sediments hold vast stores of organic carbon (OC). Techniques to spatially map sedimentary OC must develop to form the basis of seabed management tools that consider carbon-rich sediments. While the natural burial of carbon (C) provides a climate regulation service, the disturbance of buried C could present a significant positive feedback mechanism to atmospheric greenhouse gas concentrations. We present a regional Scottish case study that explores the suitability of integrating archived seafloor acoustic data (i.e., multibeam echosounder bathymetry and backscatter) with physical samples toward improved spatial mapping of surface OC in a dynamic coastal environment. Acoustic backscatter is a proxy for seabed sediments and can be collected over extensive areas at high resolutions. Sediment type is also an important predictor of OC. We test the potential of backscatter as a proxy for OC which may prove useful in the absence of exhaustive sediment data. Overall, although statistically significant, correlations between the variables OC, sediment type, and backscatter are relatively weak, likely reflecting a combination of limited and asynchronous data, sediment mobility over time, and complex environmental processing of OC in shelf sediments. We estimate linear mixed models to predict OC using backscatter and Folk sediment type as covariates. Our results show that incorporating backscatter in the model improves the precision of OC predictions by 14%. Backscatter discriminates between coarse and fine sediments, and therefore low and high OC regimes; however, was not able to discriminate amongst finer sediments. Although sediment type is a stronger predictor of OC, these data are available at a much lower spatial resolution and do not account for fine-scale variability. The resulting maps display varying spatial distributions of OC reflecting the different scales of the predictor variables, demonstrating a need for further methodological development. Backscatter shows considerable promise as a high-resolution predictor variable to improve the precision of surface OC maps, or to reduce the number of OC measurements required to achieve a specified precision. Applications of such maps have potential in improved C-stock estimates and the design of conservation and management strategies that consider marine sediments as valuable C stores.

Evaluation of the Potential Release Tendency of Metals and Metalloids from the Estuarine Sediments: Case Study of Raša Bay.

Assessing the environmental quality of coastal systems is important not only for the management and protection of such areas, but also for improving the quality of water resources. Since sediment itself can often be a source of certain toxic elements, in addition to information on the distribution of metals in the water column and in the sediment itself, it is useful to determine the bioavailable forms of individual elements, particularly toxic ones. In this study, water and sediment geochemical data were supplemented with oxyanion mobility in sediments estimated by diffusion gradients in thin film (DGTs). The data obtained indicate that the chemical composition of the water in the Raša River estuary primarily reflects the high input of suspended sediment from the catchment, the mixing of freshwater and seawater, and to a lesser extent the effects of anthropogenic activities. Although sediment composition is primarily determined by geological and hydrodynamic conditions in the catchment, it also indicates moderate enrichment in Co, Cr, Mo and Ni. In contrast, the distribution of oxyanions in sediment pore water indicates the influence of sediment as a source of some elements in the bottom water; e.g., sediment contributes to 40% of the arsenic bottom water budget. The obtained depth profiles of the oxyanion distribution in the sediment pore water indicate an early onset of suboxic to anoxic conditions in Raša Bay, which is prone to rapid sedimentation. All this demonstrates the need to consider the bioavailable forms of elements when assessing environmental quality, as the lack of such information can lead to an incomplete assessment, especially in dynamic coastal systems such as estuaries.

Seasonal variations of cadmium (Cd) speciation and mobility in sediments from the Xizhi River basin, South China, based on passive sampling techniques and a thermodynamic chemical equilibrium model

Understanding the speciation and mobilization mechanisms of potentially toxic metals in sediments is critical to aquatic ecosystem health and contamination remediation in urban rivers. In this study, chemical sequential extraction, a thermodynamic chemical equilibrium model (Visual MINTEQ ver. 3.1), diffusive gradient in thin films (DGT), and high-resolution dialysis (HR-Peeper) techniques were integrated to identify seasonal variations in cadmium (Cd) mobility in sulfidized sediments. Acid-soluble Cd was the dominant geochemical fraction in sediments, followed by residual, oxidizable, and reducible Cd. The DGT-labile Cd concentration was associated with various geochemical processes and was independent of the total concentration and geochemical fractionation of Cd in sediments. Sulfate reduction facilitated the formation of insoluble CdS and induced low Cd concentrations in sediment porewater. Sulfide oxidation was principally responsible for lowered porewater pH and elevated Cd concentrations in summer. Strongly acidic conditions promoted release of sediment Cd but might reduce the binding efficiency of Chelex resin gel for dissolved Cd, leading to underestimation of the mobility of sediment Cd. Sediments generally functioned as a sink for Cd in winter and shifted to acting as a source in summer, releasing Cd into the overlying water mainly as Cd-S complexes with high potential to migrate downstream.

A new seabed mobility index for the Irish Sea: Modelling seabed shear stress and classifying sediment mobilisation to help predict erosion, deposition, and sediment distribution

The seafloor is increasingly being used for siting renewable energy and telecommunication infrastructure as well as supporting key fisheries and biodiversity. Understanding seabed stability and sediment dynamics is, therefore, a fundamental need for offshore engineering and geoscience and biological studies. In this study we aim to quantify the levels of sediment mobility in the Irish Sea: an area of increasing socio-economic interest and subsequent seabed pressures. The temporal and spatial interaction between bathymetry, hydrodynamics and seabed sediments leads to a complex pattern of erosion, bedload transport and deposition which can affect seabed infrastructure and modify habitats. Information on current and wave conditions were obtained from numerical modelling to assess their role in generating seabed hydrodynamic conditions. These outputs were coupled with observed seabed grain-size data to predict the exceedance of sediment mobility thresholds by bed shear stress values for a period of one year according to empirical formulae. Exceedance frequency values were used to calculate a number of sediment disturbance and mobility indexes to allow for a robust assessment of sediment dynamics. Sediment in the Irish sea, on average, is being mobilised 35% of the time during the year, with 35% of the spatial area studied being mobilised over 50% of the time. Even in areas of low sediment mobilisation frequency (<5%), there are implications for bedform dynamics. The spatial patterns of the calculated sediment mobility are discussed in the context of current seabed geomorphology and the implications for both engineering and environmental considerations.

Mechanistic insights into trace metal mobilization at the micro-scale in the rhizosphere of Vallisneria spiralis

Mobilization of trace metals in the rhizosphere of macrophytes is controlled by root-driven chemical changes, especially the steep gradients of O2 and pH from the rhizosphere to bulk sediments. Here, the O2 and pH dynamics, and the distribution of trace metal, in the rhizosphere of Vallisneria spiralis were obtained using planar optodes and diffusive gradients in thin films, respectively. Radial O2 loss (ROL) and acidification occurred on all visible roots of V. spiralis and exhibited highly spatiotemporal dynamics depending on the root growth and various environmental conditions. Trace metals showed different mobilization mechanisms in the rhizosphere. ROL and produced Fe(III) (oxyhydr)oxides decreased the mobility of Fe, As, Co, V and W in the rhizosphere. However, Mn, Ni and Cu exhibited greater mobility in the rhizosphere than bulk sediments as a result of the oxidation of metal sulfide and proton-induced dissolution of minerals. In particular, Co and Ni presented increased activity at the interface between rhizosphere and bulk sediment, which was attributed to the redox dissolution processes of Fe and Mn as a result of ROL and rhizosphere acidification. These results provide new insights into the roles of macrophyte root-induced O2 and pH changes in controlling trace metal mobility in sediments.

Impact of Jordanelle Dam on sedimentological and ecohydrological regimes of the Provo River, Utah, USA

ABSTRACT The impact of large dams primarily built in the 1950s and 1960s has been extensively studied; more recent dam construction often occurs alongside habitat restoration activities, which have the potential to mitigate impacts. Jordanelle Dam on the Provo River, constructed in 1992, is economically and ecologically important to the region. This study aims to elucidate the effects of the dam on the sediment distribution and mobility of the river to inform future management of the system. Due to decreased flows, gravels (D16, D50) downstream of the dam are generally larger and subsurface grains are significantly coarser (p < 0.05), than those upstream of the dam. Further, the flow required to mobilize sediment is larger downstream of the dam and occurs rarely, leading to less mobile sediment and a more stable channel. The reduced mobility of sediment below the dam has decreased channel complexity. Much of the mobile sediment below the dam may be supplied from an unaltered river reach, rather than upstream mobilization. This sediment is vital to trout habitat, an important aspect of the recreation economy of the region. Monitoring changes in grain size distribution below the dam is necessary to prevent the loss of this valuable resource through diminished spawning habitat.

Experimentally obtained velocity and pressure fields of an open channel flow around a cylinder using RIM-SPIV

The quantification of velocity and pressure fields over streambeds is important for predicting sediment mobility and water exchange between stream and sediment interstitial spaces (Schmeeckle and Nelson, 2003; Tonina and Buffington, 2009). The latter is typically referred as hyporheic flow, which consists of surface water that flows through the streambed sediment pores (Tonina and Buffington, 2009). These fluxes are mainly driven by pressure gradients at the water sediment interface. In this paper, we report an experimental investigation of the time-averaged velocity and pressure field, quantified in a set of laboratory experiments using stereo PIV (Particle Image Velocimetry) with a non-toxic index-matched fluid, for an open channel flow around a barely submerged vertical cylinder as a model for plant stalk over a plane bed of coarse granular sediment, mimicking a stream gravel bed. This is the first time that such a velocity and pressure field is characterized experimentally for a free surface flow with irregular floor contour.

How sediment composition and flow rate influence downstream channel morphodynamics upon dam removal

AbstractThe process of dam removal establishes the channel morphology that is later adjusted by high‐flow events. Generalities about process responses have been hypothesized, but broad applicability and details remain a research need. We completed laboratory experiments focused on understanding how processes occurring immediately after a sediment release upon dam removal or failure affect the downstream channel bed. Flume experiments tested three sediment mixtures at high and low flow rates. We measured changes in impounded sediment volume, downstream bed surface, and rates of deposition and erosion as the downstream bed adjusted. Results quantified the process responses and connected changes in downstream channel morphology to sediment composition, temporal variability in impounded sediment erosion, and spatial and temporal rates of bedload transport.Within gravel and sand sediments, the process response depended on sediment mobility. Dam removals at low flows created partial mobility with sands transporting as ripples over the gravel bed. In total, 37% of the reservoir eroded, and half the eroded sediment remained in the downstream reach. High flows generated full bed mobility, eroding sands and gravels into and through the downstream reach as 38% of the reservoir eroded. Although some sediment deposited, there was net erosion from the reach as a new, narrower channel eroded through the deposit. When silt was part of the sediment, the process response depended on how the flow rate influenced reservoir erosion rates. At low flows, reservoir erosion rates were initially low and the sediment partially exposed. The reduced sediment supply led to downstream bed erosion. Once reservoir erosion rates increased, sediment deposited downstream and a new channel eroded into the deposits. At high flows, eroded sediment temporarily deposited evenly over the downstream channel before eroding both the deposits and channel bed. At low flows, reservoir erosion was 17–18%, while at the high flow it was 31–41%.

Quantifying arsenic post-depositional mobility in lake sediments impacted by gold ore roasting in sub-arctic Canada using inverse diagenetic modelling

Lake sediments are widely used as environmental archives to reconstruct past changes in contaminants deposition, provided that they remain immobile after deposition. Arsenic (As) is a redox-sensitive element that may be redistributed in the sediments during early diagenesis, for instance along with iron and manganese, and thus depth profiles of As might not provide a reliable, unaltered record of past deposition. Here, we use inverse diagenetic modelling to calculate fluxes of As across the sediment-water interface and interpret As sedimentary records in eight lakes along a 80 km transect from the Giant and Con mines, Northwest Territories, Canada. The sediment cores were dated using 210Pb methods and analyzed for solid-phase and porewater As, Fe, Mn and organic C concentrations. We reconstructed the history of As deposition by correcting for the varying mobility patterns and calculated contemporary As deposition fluxes. Correction for diagenesis was substantial for three of the eight lakes, suggesting that lakes with lower sedimentation rates, which allows longer residence of As within the reactive zones defined by the model, enhance the influence of diagenesis. Results show that solid phase As peaks coincides with the period of high emissions from past gold ore roasting activities. Results also show that sediments sustained present-day As fluxes to the water column of study lakes within 50 km of the mines, while sediment in study lakes further than 50 km acted as As sinks instead.

The world unique triangular shaped Jonah High in the Levant Basin, eastern mediterranean – Tectonic setting, stratigraphy and exploration considerations

A clear magnetic anomaly is evident across the central part of the Southern Levant Basin, coinciding with prominent structural features known as the Jonah High and Jonah Ridge. Jonah High is a triangular shaped, horst-like structure, bounded at each of its three rims by segments of steep dips and is capped by a prominent seismic marker. An attempt to drill through the cap resulted in a loss of circulation in the Lower Miocene. The well was side-tracked to the east of the cap and penetrated a Lower Miocene and Upper Oligocene deep-water section, as found in other wells in the basin. Few models were suggested since the 1980's to explain the combined magnetic anomaly, gravity and oddly shaped structure; they include basement high, intrusive body and shallow volcanic edifice. The upper part of the high has been suggested to be a carbonate build-up of either Mesozoic or Cenozoic age or a shale structure. This study takes advantage of recently released 3D surveys acquired over the Jonah High and the results of the well. It is suggested that the Jonah High was intruded during the Lower to Middle Miocene by allochthonous bodies; these form an equilateral triangle, measuring 265 sq. km at 6.5 km depth, that might be world unique. The interior of the upper part of Jonah High, from about 5 km to 6.5 km, is suggested to be composed of a deep-water Oligocene and Lower Miocene section and probably contains the Tamar Sands, the main gas-bearing reservoir in the basin. The deep part exhibits folded and tilted high amplitude strata that may be partly composed of old magmatic material that upwelled during the Cenozoic by mobile sediment acting under the regional contractional regime; these might be associated with the observed magnetic and gravity anomalies. Further exploration by better imaging and drilling together with analogue cases from other basins are required in order to solve this enigmatic feature.