Sediment Profile Research Articles

Equilibrium partitioning behaviors and remobilization of trace metals in the sediment profiles in the tributaries of the Three Gorges Reservoir, China

Geochemical behaviors of trace metals in the sediment profiles are crucial for predicting the associated environmental risks in aquatic ecosystems. However, the comprehensive transport of trace metals under both equilibrium and dynamic conditions is still unclear under the changing hydrological regime. Here, the equilibrium partitioning behaviors and remobilization of five trace metals (Ni, Cu, Zn, As, and Pb) in sedimentary profiles within the tributaries of the Three Gorges Reservoir were explored by the partitioning coefficient (Kd), diffusive gradients in thin films (DGT), and DGT induced flux in sediments (DIFS) model. According to the Kd values, As posed the highest migration ability among the trace metals in the sediment profiles under equilibrium circumstances. Similarly, the dynamic processes of trace metals simulated by the DIFS model also suggested that As displayed the highest desorption rate despite having the lowest labile pool size. Moreover, all trace metals were classified as the “partially sustained” case, while the supply abilities of As and other trace metals were limited by the diffusion and the desorption kinetics, respectively. In addition, DGT-labile trace metals showed a diffusion trend from the sediment to the water column (except for Zn) at the sediment-water interface, indicating potential risks to water quality. Specifically, the equilibrium partitioning behaviors revealed the potential labile pool of trace metals in the solid phase, and the dynamic resupply process between the solid phase and porewater remained undetermined. In comparison, although DGT simulated the kinetic process of trace metals in the sediments, the labile pool of the trace metals could not be obtained. This study provided a holistic insight into the complementary trace metal behaviors under both equilibrium and dynamic conditions in the sediment and was beneficial to the water quality protection and internal pollution remediation in the aquatic environment.

Long-term shoreline evolution (Late-Holocene – 2018) of the Channel 15 coast (Samborombón bay, NE – Buenos Aires province, Argentina)

Here we present an investigation with the aim of reconstructing the shoreline evolution and geomorphological changes in the central zone of Samborombón bay (Channel 15, Buenos Aires province, Argentina) during the period 2700 cal yr BP to recent times. Palynological, geomorphological and historical imagery and proxy-data analyses were combined to reconstruct the shoreline evolution and paleo-environmental changes. Palynological material (pollen and non-pollen palynomorphs -NPP-) was recovered from a continuous 117 cm long sedimentary profile extracted from an outcrop at the northern margin of Channel 15. Relict landforms, including paleo- shorelines, were reconstructed by applying enhancing filters to a Spot 6 image from 2015. Historical shorelines were identified in high-definition images (aerial and satellite imagery) between 1968 and 2018, but also by using the marsh vegetation edge as a shoreline proxy. Shoreline change was quantified using the Net Shoreline Movement and Weighted Linear Regression algorithms in ArcMap 10.1. Results based on the palynological analysis for the period 2700–2740 BP, show the passage from subtidal-intertidal to supratidal environment with influence of brackish to fresh-waters, where paleo-shorelines imprint was recognized developing over a tidal plain that was grading to marshes. Results based on aerial and satellite imagery for the period 1968–2018 show shoreline retrocession only immediately at the north of the channel mouth, while the rest of the coast prograded toward the estuary. These processes indicate that new marshes – and, therefore, wetlands – are constantly developing at the edge of Samborombón bay.

Spatial Distribution of Bioavailable Metal Concentrations and Total Metal Concentration-depth Relationship along the Sediment Profile within Phuket Bay

Heavy metals in coastal sediments can adversely affect human health and the environment. The distribution and metal bioavailability of Pb and Zn in 21 sediment samples collected from Phuket Bay, Phuket, Thailand using the first-two steps of sequential extraction proposed by the Standards, Measurements and Testing programme (known as BCR) was determined. The results showed that Pb formed weak complexes contributing up to 11.2% to 33% of its total concentration (1.7 to 7.5 mg kg-1) in the first fraction (BCR1), while Zn in the BCR1 fraction ranged from 4.9% to 9.9%. The results suggest that Pb could easily enter the food chain and the main cause of heavy metal contamination is related to local anthropogenic activities and effects of urbanization in the region, such as the ferry terminal, boatyards, and other maritime activities. Meanwhile, the enrichment factors of the metals showed minor to moderately severe enrichment. The metal concentration-depth relationship along the sediment profile showed metal concentration in each layer of the sediment core ranging from 45.4 to 88 mg Zn kg-1 and from 12.7 to 44.5 mg Pb kg-1. Based on the changes in heavy metal accumulation in the sediment core, and the calculated the enrichment factor versus depth, these allowed us to understand the historical variability in pollutant linked to past activities in Phuket Bay.

REY in pore waters of sediments hosting Fe-Mn nodules of the Interoceanmetal exploration area in the Clarion-Clipperton Fracture Zone, NE Pacific

Our study is focused on REE and yttrium (REY) geochemistry of pore waters from core-box sediments. The samples were collected from the 0–5 cm, 10–15 cm, 25–30 cm, and 35–40 cm depth intervals of four stations of the eastern part of block H_22 of IOM license area of the Clarion-Clipperton Fracture Zone, NE Pacific. The REE studies in marine pore fluids were limited by analytical challenges. The pore water analysis we applied is based on a modern, improved analytical technique (ICP-MS, Perkin-Elmer SCIEX Elan DRC-e) with a cross-flow nebulizer and a spectrometer optimized (RF, gas flow, lens voltage) using a quadrupole cell in a DRC (Dynamic Reaction Cell) mode that allowed us to define the whole suite of REE. The ƩREY values of the samples vary from 4.05 μg/l to 106.34 μg/l. The REE content is at least one order of magnitude higher than the oceanic water. We followed the natural variations of La, Lu, Ce, and Y in absolute concentrations for station 3607. Cerium and Y are slightly enriched around the water-sediment interface, while La and Lu are enriched in the deeper layers. PAAS normalized REY patterns show a pronounced negative Ce/Ce* ratio together with a little MREE and HREY enrichment. The relatively “flat” REE patterns are typical for the shallow open ocean and characterize REE released from the organic matter degradation. We assume that the decomposition of and adsorption on organic matter and oxidation conditions are the main factors for REE fractionation in the pore water. The reason for some scatter in our REY data might be linked to bioturbation that has affected the sediment profiles.

Inclusive Hyper- to Dilute-Concentrated Suspended Sediment Transport Study Using Modified Rouse Model: Parametrized Power-Linear Coupled Approach Using Machine Learning

The transfer of suspended sediment can range widely from being diluted to being hyper-concentrated, depending on the local flow and ground conditions. Using the Rouse model and the Kundu and Ghoshal (2017) model, it is possible to look at the sediment distribution for a range of hyper-concentrated and diluted flows. According to the Kundu and Ghoshal model, the sediment flow follows a linear profile for the hyper-concentrated flow regime and a power law applies for the dilute concentrated flow regime. This paper describes these models and how the Kundu and Ghoshal parameters (linear-law coefficients and power-law coefficients) are dependent on sediment flow parameters using machine-learning techniques. The machine-learning models used are XGboost Classifier, Linear Regressor (Ridge), Linear Regressor (Bayesian), K Nearest Neighbours, Decision Tree Regressor, and Support Vector Machines (Regressor). The models were implemented on Google Colab and the models have been applied to determine the relationship between every Kundu and Ghoshal parameter with each sediment flow parameter (mean concentration, Rouse number, and size parameter) for both a linear profile and a power-law profile. The models correctly calculated the suspended sediment profile for a range of flow conditions (0.268 mm≤d50≤2.29 mm, 0.00105gmm3≤particle density≤2.65gmm3, 0.197mms≤vs≤96mms, 7.16mms≤u*≤63.3mms, 0.00042≤cˉ≤0.54), including a range of Rouse numbers (0.0076≤P≤23.5). The models showed particularly good accuracy for testing at low and extremely high concentrations for type I to III profiles.

Understanding source terms of anthropogenic uranium in the Arctic Ocean – First 236U and 233U dataset in Barents Sea sediments

This work reports the first dataset of 236U and 233U in sediment cores taken from the Barents Sea, with the aim to better understand the source terms of anthropogenic uranium in the Arctic region. Concentrations of 236U and 233U along with 137Cs, and 233U/236U atomic ratio were measured in six sediment profiles. The cumulative areal inventories of 236U and 233U obtained in this work are (3.50–12.7) × 1011 atom/m2 and (4.92–21.2) × 109 atom/m2, with averages values of (8.08 ± 2.93) × 1011 atom/m2 and (1.08 ± 0.56) × 1010 atom/m2, respectively. The total quantities of 236U and 233U deposited in the Barents Sea bottom sediments were estimated to be 507 ± 184 g and 7 ± 3 g, respectively, which are negligible compared to the total direct deposition of 236U (6000 g) and 233U (40–90 g) from global fallout in the Barents Sea. The integrated atomic ratios of 233U/236U ranging in (0.98–1.57) × 10−2 reflect the predominant global fallout signal of 236U in the Barents Sea sediments and the highest reactor-236U contribution accounts for 30 ± 14 % among the six sediment cores. The reactor-236U input in the Barents Sea sediments is most likely transported from the European reprocessing plants rather than related to any local radioactive contamination. These results provide better understanding on the source term of anthropogenic 236U in the Barents Sea, prompt the oceanic tracer application of 236U for studying the dynamics of the Atlantic-Arctic Ocean and associated climate changes. The 236U-233U benchmarked age-depth profiles seem to match reasonably well with the reported input function history of radioactive contamination in the Barents Sea, indicating the high potential of anthropogenic 236U-233U pair as a useful tool for sediment dating.

Macroinvertebrates as engineers for bioturbation in freshwater ecosystem.

Bioturbation is recognized as a deterministic process that sustains the physicochemical properties of the freshwater ecosystem. Irrigation, ventilation, and particle reworking activities made by biotic components on sediment beds influence the flow of nutrients and transport of particles in the sediment-water interface. Thus, the biogenic disturbances in sediment are acknowledged as pivotal mechanism nutrient cycling in the aquatic system. The macroinvertebrates of diverse taxonomic identity qualify as potent bioturbators due to their abundance and activities in the freshwater. Of particular relevance are the bioturbation activities by the sediment-dwelling biota, which introduce changes in both sediment and water profile. Multiple outcomes of the macroinvertebrate-mediated bioturbation are recognized in the form of modified sediment architecture, changed redox potential in the sediment-water interface, and elicited nutrient fluxes. The physical movement and physiological activities of benthic macroinvertebrates influence organic deposition in sediment and remobilize sediment-bound pollutants and heavy metals, as well as community composition of microbes. As ecosystem engineers, the benthic macroinvertebrates execute multiple functional roles through bioturbation that facilitate maintaining the freshwater as self-sustaining and self-stabilizing system. The likely consequences of bioturbation on the freshwater ecosystems facilitated by various macroinvertebrates - the ecosystem engineers. Among the macroinvertebrates, varied species of molluscs, insects, and annelids are the key facilitators for the movement of the nutrients and shaping of the sediment of the freshwater ecosystem.

Numerical investigation of sheet flow driven by a near-breaking transient wave using SedFoam

Sheet flow driven by a near-breaking transient wave is numerically investigated using SedFoam, a two-phase Eulerian sediment transport model in the OpenFOAM framework. SedFoam resolves the full profile of sediment transport processes within the bottom boundary layer based on closures for intergranular stresses and fluid–particle interactions. Compared with large-scale wave flume data, good agreements are obtained for streamwise flow velocity profiles, sediment concentration profiles, and the sheet flow layer thickness. Model results show near-bed velocity and sediment profile evolution within the sheet flow layer. Intense sediment suspension trailing the wave crest generates stable density stratification that dampens near-bed turbulent kinetic energy and contributes to decreasing bed shear stress. Results suggest that the buoyant flux dominates turbulent kinetic energy dissipation after the passing of the wave crest, coinciding with the reduction of bed shear stress. The instantaneous upper bound of the sheet flow layer exists in different log-law regimes under the transient wave, giving rise to a near-bed velocity profile that is highly dependent on the variable sheet flow layer wall unit. The effect of the profile shape parameter on bedload sediment transport is studied where the bedload predicted using the time-varying optimal profile shape parameter yields good agreement compared to the directly modeled bedload. Modeled sediment transport rates demonstrate that reduced bed shear stress caused by density stratification limits bedload and results in a suspended load-dominant mode.

Chemical Forms and Spatial Distribution of Phosphorus in the Sediment of Sihe River

Based on the study of the content, forms, and spatial distribution of phosphorus (P) in the surface and columnar sediments of the Sihe River, the relationships between the total phosphorus (TP) and various forms of P and the basic physical and chemical properties of sediment and their ecological significance were deeply discussed. The forms of P in the sediment were defined including soluble and loosely bound P (S/L-P), Al-bound P (Al-P), Fe-bound P (Fe-P), reductant soluble P (RS-P), Ca-bound P (Ca-P), and residual P (RES-P) using the method of selectively sequential extraction. The results indicated that ω (TP) was 421.84-1188.65 mg·kg-1 in the surface sediment. Among the six forms of P, the content of Ca-P was the highest, accounting for more than 40% of TP, followed by the content of Fe-P, accounting for more than 20% of TP. The content of S/L-P was the lowest, which only accounted for approximately 0.4% of TP. The contents of TP and various forms of P in the surface sediments of the downstream area of the Sihe River were higher than those of the upstream area of the river, which was induced by the discharge of industrial wastewater and domestic sewage in the urban areas nearby the downstream portion of the Sihe River. The contents of TP in the upper samples of the two sediment profiles were obviously higher than those in the bottom samples, indicating that the P pollution in the water environment of the Sihe River has been intensifying in recent years. Among all forms of P in the sediment profiles, Ca-P accounted for the largest proportion, followed by Res-P. The correlation analysis results showed that significant correlations were observed between Fe and Fe-P, Al and Al-P, Ca and Ca-P, and TOC and RS-P in the surface sediments; the same correlations had not been found in the sediment profiles. The calculated results of Fe and TP molar ratio indicated that the sediments of the Sihe River could further accumulate P. The percentage of bioavailable P (BAP) in sediment was 25%-50%.

Paleo-vegetation and climate variability during the last three millennia in the Ladakh, Himalaya

Unlike other parts of the world, the high-resolution past climate (monsoonal) record of Northwestern Himalaya is limited and such paucity of data on climatic records hinders the past climatic understanding which further reduces the possibility of future prediction. In this study, high-resolution centennial to millennial-scale regional vegetation changes and associated climatic variability (Indian Summer Monsoon) is being reconstructed from a sedimentary profile of Shakti Wetland, Ladakh Northwestern Himalaya deposited during the Late Holocene. The reconstructed climate and vegetation record is based on a multi-proxy approach which are pollen, non-pollen palynomorphs (NPP), stable isotopes (δ13Corg, δ15Nbulk), total organic carbon (TOC), and total nitrogen (TN) derived from sedimentary profile spanning the last ∼ 3200 cal yr BP. A substantial variability in the ISM strengths was found throughout the profile covering the time span of ∼ 3230–740 cal yr BP. The peak warm and wet condition occurred during ∼ 2890 to 2630 cal yr BP, inferred from the rise in an alpine meadow, dry steppe, and marshy taxa. This interval is characterized by relatively lower δ15N suggesting increased nitrification. The first evidence of Cerealia pollen indicated that human settlement near the study area might have initiated around ∼ 2700 cal yr BP. Pollen data show a mixed forest type vegetation from ∼ 2630–2130 cal yr BP. The increase in alpine taxa suggests a wet phase from ∼ 1280 to 740 cal yr BP which globally corresponds to the Medieval Climate Anomaly (MCA). In addition, spectral analysis on the δ13C proxy shows multiple periodicities during the last ∼ 3200 cal yr BP reinforcing the idea of solar control on climate variability of Ladakh.

Revisiting 228Th as a tool for determining sedimentation and mass accumulation rates

The use of 228Th has seen limited application for determining sedimentation and mass accumulation rates in coastal and marine environments. Recent analytical advances have enabled rapid, precise measurements of particle-bound 228Th using a radium delayed coincidence counting system (RaDeCC). Herein we review the 228Th cycle in the marine environment and revisit the historical use of 228Th as a tracer for determining sediment vertical accretion and mass accumulation rates in light of new measurement techniques. Case studies comparing accumulation rates from 228Th and 210Pb are presented for a micro-tidal salt marsh and a marginal sea environment. 228Th and 210Pb have been previously measured in mangrove, deltaic, continental shelf and ocean basin environments, and a literature synthesis reveals that 228Th (measured via alpha or gamma spectrometry) derived accumulation rates are generally equal to or greater than estimates derived from 210Pb, reflecting different integration periods. Use of 228Th is well-suited for shallow (<15 cm) cores over decadal timescales. Application is limited to relatively homogenous sediment profiles with minor variations in grain size and minimal bioturbation. When appropriate conditions are met, complimentary use of 228Th and 210Pb can demonstrate that the upper layers of a core are undisturbed and can improve spatial coverage in mapping accumulation rates due to the higher sample throughput for sediment 228Th.

Assessment of sediment profiles applying nuclear techniques: use of a nucleonic gauge in Panama Canal watershed

An industrial nuclear technique based on the use of an X-ray profiler was implemented to estimate the densities or concentrations of sediments present in an Atlantic maritime zone in the areas subjected to dredging under the governance of the Panama Canal Authority (ACP).The sediment profiles show in most areas there is a concentration of between 1.00-1.15 g/cm³ except for one area in particular, the density starts at 1.20 g/cm³ and even reaches values greater than 1.50 g/cm³; therefore, an already consolidated sediment is present, which, depending on the depth found. Values of 1.265 g/cm³, 1.297 g/cm³, 1.185 g/cm³ obtained by ACP previous studies are within the range of 1.20–1.30 g/cm³ measured with the nucleonic gauge. However, it should be noted that during the tests with the X ray profiler, sediment densities values greater than the aforementioned limit were also obtained that varying according at depths close to 12 m and 18 m with values reached up to 1.513 g/cm³ and 1.60 g/cm³, respectively. This demonstrates that sediment accumulation depends on depth. This nucleonic gauge is feasible technique for the study of the sedimentation phenomenon in channel basins and even in other projects nationwide.

Effective sediment depth for phosphorus release in a monomictic eutrophic lake

Sediments play an important role in phosphorus (P) dynamics in eutrophic lakes. P concentrations are higher in surface sediments compared to deeper layers, representing a pool of P that can be released into the water column. Effective sediment depth for P release is different among lakes and little is known about this depth. Through experimental evaluation of P release in sediment cores, characterization of sediment porosity and distribution of P fractions in the sediment profile from a monomictic eutrophic lake, we determined which sediment layers produce release of P. The release of P in sediment layers of different depths varied between 0.89 and 2.07 mg P/m2×d with the highest P flow found in the sediment layer of 0-15 cm. The porosity of the sediment strata varied between 68 and 80%, with higher porosity in the surface sediment and decreasing porosity with depth of the sediment profile. We found that P bound to Fe/Mn (BD-P), Al/Fe oxides (NaOH-SRP), and organic matter (NaOH-NRP) were higher in the layer of 0-15 cm than in deeper layers. Sediment porosity, P release, total P and concentrations of the different P species in the sediment profile declined with sediment depth, stabilizing at the depth of 15 cm in the sediment profile. Therefore, we conclude that the effective sediment depth for release of P in this monomictic eutrophic lake is found up to 15 cm.

The variations of labile arsenic diffusion driven by algal bloom decomposition in eutrophic lake ecosystems

The vertical labile arsenic (As) concentration and diffusion pattern variations in eutrophic lakes were investigated using in situ techniques of diffusive gradients in thin films (DGT) and high-resolution dialysis (HR-Peeper) in the typical eutrophic system of Lake Taihu in China. In addition, simulation experiments were used to reveal labile As distributions in sediment profiles under the influence of algae blooms and wind fluctuations. Our results indicated that eutrophication could lead to the migration and transformation of As fractions, including increased As bioavailability, as well as varied diffusion patterns. The sulfate released from algae decomposition reduced to H2S and formed FeS, which weak adsorbability contributed to the increased mobility of the As fractions. Meanwhile, further decomposition released a large quantity of algae-derived organic matter which competed with the adsorbed As, leading to more endogenous As migrating to the overlying water. Accordingly, the H2S production presented a likely explanation for the changed distribution of labile As and contributed to labile As concentrations in the sediment profiles significantly increasing at depths of −20 mm to −60 mm in the early stages of the simulation experiment. Moreover, the areas of enhanced diffusion patterns with high concentrations of As obviously expanded. However, following the complete decomposition of the algae, the organic matter component significantly changed, suggesting an explanation for the variations in distribution of labile As. All the diffusion pattern variations showed similar trends. Consequently, variation of labile As diffusion patterns could indicate the decomposition and eutrophication levels of freshwater ecosystems.

Stability and trace element composition of natural schwertmannite precipitated from acid mine drainage

Schwertmannite is an iron hydroxo sulfate secondary mineral (Fe8O8(OH)8-2x(SO4)x•nH2O; 1 ≤ x ≤ 1.75) that commonly precipitates in sulfate-rich acid and metalliferous drainage as Fe(II) oxidises to Fe(III). It displays a high capacity to incorporate a range of elements through substitution of metal cations for Fe and anionic species for inner-sphere and outer-sphere sulfate groups, and surface adsorption. Precipitates from the base to top of sediment profiles immediately downstream from the historic Sunny Corner sulfide deposit are dominated by schwertmannite, indicating the stability of this mineral over decadal time frames in the low pH and high sulfate conditions. Various mineral characterisation methods, coupled with hydroxylamine hydrochloride based sequential selective extractions, reveal two forms of schwertmannite co-existing and in similar proportions throughout the profiles. The first form of schwertmannite has a high S/Fe ratio and the pattern of release of Cu, Pb, Zn and As relative to Fe indicates such trace elements are mostly adsorbed to outer-sphere sites. By contrast, the second and more crystalline schwertmannite has lower S/Fe ratios and requires higher concentrations of hydroxylamine hydrochloride to dissolve. The second schwertmannite displays substantial substitution of arsenate for sulfate, with As/(As + S) molar ratios ranging from 0.1 to 0.6, and substantially higher concentrations of trace elements. Sequential precipitation of the two schwertmannites from the AMD appears more likely than transformation of one form to the other. At sites such as Sunny Corner, pH neutralisation or removal of sulfate from the AMD stream may result in destabilisation of the schwertmannite and release of associated trace metals.

Nitrogen burial characteristics of Quaternary sediments and its controls on high ammonium groundwater in the Central Yangtze River Basin

As the strata sedimentary process proceeds, considerable amounts of nitrogen (N) is buried in sediments, which controls the sources and fate of N in the “groundwater-sediment” system. However, there is little concern regarding N burial characteristics in continuous sediment profiles from surface layer to deep aquifer thus far. In this study, lithology, grain size, geochronology, exchangeable N contents and geochemical proxies of sediments were analyzed to reveal the controlling mechanisms of N burial characteristics in Quaternary sediments and to interpret the enrichment of N in groundwater of central Yangtze River Basin. The results demonstrated a similar distribution trend for buried N in two sedimentary cores, which were high in the surface layer and decreased to stable in the deep aquifer. Excessive exchangeable N (EX-N) contents in sediments were mainly attributed to geologic origin. The N burial characteristics were controlled by the evolution of depositional environment: sedimentary facies determined the concentrations of total organic nitrogen (TON), further affecting the mineralization capacity of sediments; while paleoclimate regulated the intensity of the N transformation processes, ultimately influencing the actual concentrations of EX-N in sediments. In addition, due to the fast accumulation of alluvial deposits after Last Glacial Maximum and rapid development of Jianghan Lake Groups during Holocene, abundant organic matter (with high TON contents) was buried in sediments, which were still able to produce more ammonium or nitrate, and further posing continuous threats to groundwater quality. This study provided a new interpretation for the formation of high-ammonium aquifer in terms of depositional evolution.

Seasonal variation in release characteristics and mechanisms of sediment phosphorus to the overlying water in a free water surface wetland, southwest China

Geochemical cycling of iron (Fe) mediated by sediment microbes drives the remobilization of phosphorus (P). Understanding the underlying mechanism is essential for the evaluation of P retention by wetlands. The diffusive gradients in thin film (DGT) and 16S rDNA sequencing techniques were combined to explore seasonal variations in the remobilization mechanism of sediment P in a free water surface wetland in southwest China. A significantly positive correlation between labile P and Fe concentrations was found from the sediment profiles, indicating coupled remobilization of Fe and P in the sediment. Fe-reducing bacterial genera, particularly Sphingomonas and Geothermobacter, were responsible for the reductive dissolution of Fe oxides and subsequent P release in sediment. The efflux of sediment P was higher in the rainy season (95 ± 87 ng cm−2 d−1) than in the dry season (39 ± 29 ng cm−2 d−1). Based on the significantly positive relationship between the efflux and total concentration of sediment P, we propose a promising regression equation for quantifying the release risk of sediment P. The Luoshijiang Wetland exhibited a higher release potential as indicated by a greater regression slope (0.558) compared to the other water bodies (0.055), which was mainly attributed to the lower labile Fe:P molar ratio in the sediment. Based on estimations of the diffusive flux of P at the sediment-water interface, sediment contributed more than 172 and 413 g of P per day to the water column in the dry and rainy seasons, respectively, accounting for 14.0% and 1.9% of the P mass in the surface water of the wetland.

Specific characteristics of the microbial community in the groundwater fluctuation zone.

Groundwater level fluctuation is a common natural phenomenon that causes alternate changes in oxygen, moisture, and biogeochemical processes in sediments. Microbes are sensitive to these environmental changes. Therefore, a specific microbial community is proposed to form in the groundwater fluctuation zone (GFZ). The vertical distributions of microbial abundance, diversity, and functional microbes and genes in sediment profiles were investigated, focusing on the GFZ, using high-throughput 16S rRNA gene sequencing, qPCR, and the Functional Annotation of Prokaryotic Taxa (FAPROTAX) approach. The relationships between chemical variables and microbial community structure were investigated by redundancy analysis (RDA). Results showed that the microbial abundance and microbial community richness and diversity were higher in the sediments of the GFZ. The nitrate reducers prefer to stay just below the groundwater level in the GFZ. The predominant microbes in the GFZ functioned as nitrifiers and Fe-oxidizers. The specific community in the GFZ is mainly related to NO3- and Fe(III) in the sediment. Consequently, the biochemical processes nitrification and Fe- and Mn-oxidation sequentially happen above the nitrate-reduction zone near the groundwater level in the GFZ. These results provide new knowledge in the biogeochemistry cycle of the GFZ and its disturbance on the vertical distribution and transport of biogenic elements and contaminants.

The Upper Kotlin clays of the Saint Petersburg region as a foundation and medium for unique facilities: an engineering-geological and geotechnical analysis

The article reviews the issues concerned with correctness of the engineering-geological and hydrogeological assessment of the Upper Kotlin clays, which serve as the foundation or host medium for facilities of various applications. It is claimed that the Upper Kotlin clays should be regarded as a fissured-block medium and, consequently, their assessment as an absolutely impermeablestratum should be totally excluded. Presence of a high-pressure Vendian aquifer in the lower part of the geological profile of the Vendian sediments causes inflow of these saline waters through the fissured clay strata, which promotes upheaval of tunnels as well as corrosion of their lining. The nature of the corrosion processes is defined not only by the chemical composition and physical and chemical features of these waters, but also by the biochemical factor, i.e. the availability of a rich microbial community. For the first time ever, the effect of saline water inflow into the Vendian complex on negative transformation of the clay blocks was studied. Experimental results revealed a decrease in the clay shear resistance caused by transformation of the structural bonds and microbial activity with the clay’s physical state being unchanged. Typification of the Upper Kotlin clay section has been performed for the region of Saint Petersburg in terms of the complexity of surface and underground building conditions. Fissuring of the bedclays, the possibility of confined groundwater inflow through the fissured strata and the consequent reduction of the block strength as well as the active corrosion of underground load-bearing structures must be taken into account in designing unique and typical surface and underground facilities and have to be incorporated into the normative documents.

Further investigations towards luminescence dating of diatoms

Following up on previous attempts to date diatom frustules, further investigations were made on 1) extracting diatom frustules devoid of inorganic luminescent grains, 2) developing an equivalent dose estimating protocol based on the diatomite luminescence characterization, and 3) testing the applicability of this protocol on two lacustrine profiles. Diatom frustules were extracted in such a way that they are almost devoid of non-biogenic polymineral grains, confirmed by field emission scanning electron microscope (FE-SEM) observation and energy dispersive X-ray (EDX) analysis. The presence of opal was confirmed by X-ray diffraction (XRD) spectrometric analysis. The optimized luminescence signal that could be used for equivalent dose estimation was blue stimulated UV emission with a preheat temperature of 200 °C. The thermoluminescence glow curve peaking at 245 °C might be the source of this signal. In this study the characteristic dose was found to be ∼1500 Gy. Two sediment profiles were explored for luminescence dating, fading rates and a-values were found different between profiles. This discrepancy can be resolved 1) by measuring luminescence characteristics across different regions, or/and 2) by using species-specific luminescence measurements. This attempt has yielded an encouraging set of luminescence ages, with diatom frustule ages comparable to fine grain polymineral ages.