Top Sediment Layer Research Articles

Deciphering source, degradation status and temporal trends of organic matter in a himalayan freshwater lake using multiproxy indicators, optically stimulated luminescence dating and time series forecasting

The present study assessed source, degradation status, and temporal trends of organic matter up to ∼3700 years Before Present (BP) in Dal Lake sediment cores using multiproxy indicators, optically stimulated luminescence (OSL) dating, and time series forecasting. Bulk organic parameters such as total organic carbon and total nitrogen indicated their increasing trend in upper sediment layers from ∼476 to ∼73 years BP. Similarly, the C/N ratio revealed varying sources of organic matter with values ranging from 2.26 to 13.05, depicting an increasing trend up to ∼1811 years BP and decreasing significantly from ∼2002 years BP. Amino acid and hexosamine concentrations decreased with depth, dominated by aspartic acid, glycine, glutamic acid, alanine, glucosamine, and galactosamine in recent sediments (∼73 years BP) of the lake. The positive degradation index (DI) confirmed fresh organic matter in top sediment layers (∼396 years BP) of the lake, showing a strong positive correlation with reactivity index (RI) and total organic carbon. However, a negative correlation was observed between DI and RI with Ox/Anox index. Furthermore, the Gluam/Galam, Asp/β-Ala and Glu/γ-Aba ratios revealed enhanced organic matter degradation with age, indicating prokaryotic origins, microbial activity changes, and redox conditions within the lake. Multivariate analysis revealed increasing autochthonous and allochthonous inputs, domestic effluent discharges, agricultural runoff, redox conditions, and microbial processes as the possible contributing factors affecting the dispersal pattern of amino acids and nutrients in Dal Lake. Time series forecasting using the ARIMA model effectively confirmed the future temporal dynamics of amino acids, and organic parameters, indicating potential ecological shifts and external influences affecting the degradation of organic matter in Dal Lake. This work provides novel insights into the diagenetic alterations and biogeochemical processes essential for understanding the organic matter cycling and guiding the conservation efforts for Dal Lake and beyond.

Isotopic labeling of nanoparticles for the evaluation of their environmental fate in mesocosm experiments

Mesocosm systems simulating floodplain areas are essential for the understanding of the environmental fate and effects of engineered nanoparticles (ENPs). In such mesocosm studies, the quantification of different types of nanoparticles coexisting in natural systems and containing the same element is often challenging. Such coexistence is expected e.g., for ENPs simultaneously released into the environmental systems. Despite the relevance of the coexistence, little is known about combined behavior and effects of ENPs in the environment. In this study, we developed a method for the quantification and differentiation of silver nanoparticles enriched by 109Ag isotope (109Ag-NPs) and sulfidized silver nanoparticles with natural isotopic distribution (S-Ag-NPs) in water, soil, and sediment and applied it to evaluate the environmental fate of these nanoparticles introduced simultaneously together with gold (Au-NPs) and titanium dioxide (TiO2-NPs) nanoparticles into mesocosms simulating an aquatic-terrestrial transition zone. High nanoparticle recoveries determined in water, sand, and soil spiked with nanoparticle mixtures indicate that the application of isotopically enriched ENPs will allow their differentiation from other nanoparticles containing the same element in environmental compartments even at the concentrations in the range of natural background. The co-accumulation of 109Ag-NPs, S-Ag-NPs, and Au-NPs in the top layer of sediment and soil and in biofilms observed in mesocosm studies suggests that these compartments can act as effective sinks for these ENPs. We suggest that the hetero-aggregation between different co-occurring ENPs and their high affinity to biota are major mechanisms controlling their fate in the aquatic-terrestrial transition zone. A high co-enrichment of 109Ag-NPs, S-Ag-NPs, Au-NPs, and TiO2-NPs in/on algae, biofilms, leaves, and amphipods suggests an enhanced risk of biomagnification. The findings of this study will contribute to a better understanding of the fate of ENPs and their combined effects in environmental compartments, where the simultaneous presence of diverse nanoparticles is expected.

How can the microbial community in watershed sediment maintain its resistance in the presence of shifting antibiotic residuals?

The widespread presence of antibiotics in global watershed environments poses a serious threat to public health and ecosystems. It is essential to examine the resistance of microbial communities in watershed environments in response to shifting antibiotic residues. Sediment samples were collected from seven sites across a watershed, encompassing surface sediment (0–10 cm) and bottom sediment (30–40 cm) depths. The aim was to replicate exposure scenarios to different antibiotics (oxytetracycline (OTC) and sulfadiazine (SD)) at varying concentrations (0, 10, and 100 μg/L) in sediment overlying water, within controlled laboratory settings. The study findings revealed significant variations in the microbial community structure of sediments between different treatments, with distinct differences observed in the upper stream and top sediment layers compared to the sediments located downstream and in the bottom layers. After the introduction of antibiotics, a significant decrease in microbial nodes was observed in the genus-level co-occurrence network analysis of the bottom sediment layer, particularly in the OTC treatment groups. In contrast, the downstream region displayed more robust correlations among the top 20 genera than the upstream area. There was no significant variance observed in the expression of Antibiotic resistance genes (ARGs), consisting of tetracycline resistance genes (tetC, tetG, tetM, tetW, and tetX) and sulfonamide resistance genes (sul1, sul2, and sul3), between sediments in the top and bottom layers. Nevertheless, downstream samples exhibited significantly higher levels of ARGs when compared to upstream samples. Network correlation analysis indicated notably lower correlations between ARGs and bacterial genera in sediments from upstream or surface layers compared to those in downstream or deeper layers. Moreover, correlations in the sediments from surface layers and upstream regions showed a decreasing trend with increasing SD exposure concentrations, while those in deeper layers and downstream areas remained relatively stable. The presence of antibiotics notably enhanced the correlation between sediment properties and ARGs, particularly emphasizing associations with total carbon, nitrogen, and sulfur content. However, the introduction of SD and OTC resulted in a decrease in the influence of these sediment factors on microbial community functions related to sulfur and nitrogen metabolism, as indicated by KEGG (Kyoto Encyclopedia of Genes and Genomes) annotation. The research provided empirical evidence on how microbial resistance responds to changes in antibiotics in sediment samples taken from various depths and locations within a watershed. It emphasized the urgent need for heightened awareness of the movement and alteration of antibiotic resistance patterns in watershed ecosystems.

Polyunsaturated long-chain alkenones in the lake sediments of North-Minusinsk valley (southern Siberia) as a paleo-indicator of climate change

The research of biochemical thermo- and salt-sensitive markers - long-chain alkenones in sediments of lakes of the North Minusinsk valley is an important contribution to the understanding of climatic changes in this region. We found that the total concentration of alkenones in the top layers of bottom sediments of saline lakes increases significantly at a salinity of about 20 g l-1. Also, it was found that the average chain length of alkenones in the surface sediments rises with increasing salinity and thus can be used as an indicator of salinity, and the ratio of alkenones C37/C38 varies widely, suggesting that this index can be used as a paleo-indicator of salinity. The high correlation between ,  and  indices with salinity indicates their potential for paleosalinity reconstruction. The obtained functional dependences were additionally verified in the core of bottom sediments of Lake Utichie-3 and confirmed the possibility of using alkenones as a paleo-indicator of climatically determined salinity changes in endorheic lakes. The taxonomic composition of haptophyte algae and alkenone composition in the studied lakes were comparable to lakes in prairie regions of North America.

Potentially Harmful Elements Content in Soil and Stream Sediments in Southwestern Districts of Katowice (Southern Poland) – Geochemical Record of Historical Industrial Plants’ Activity

Abstract The aim of the research was to assess the quality of topsoils and stream sediments in the districts of the Katowice City of southern Poland in the Upper Silesian Industrial Agglomeration, influenced primarily by the exploitation of hard coal deposits and metallurgy. The task was carried out using indicators used to assess anthropogenic soil contamination (contamination factor CF and the Igeo geoaccumulation index), as well as ecotoxicological indicators used to determine the quality of stream sediments (Threshold Effect Concentration TEC and Probable Effect Concentration PEC). In order to indicate the most polluted areas, geochemical maps of selected elements in soil and stream sediments were developed. The material comes from the top layer of soil (0.0–0.3 m) and stream sediments located in the districts of the Katowice City. The work took into account selected potentially harmful elements (PHEs). The concentrations of elements were measured using the ICP-AES method, except for Hg (the CV-AAS method). The values of the CF and Igeo indicators allowed for the indication of high concentrations of most metals, arsenic, and sulfur in the topsoil layer, significantly exceeding the geochemical background values caused by the historical exploitation of Zn-Pb and zinc ores and ferrous metallurgy. The values of the TEC and PEC indicators classify the sediments into categories that may threaten the environment and living organisms.

The Effects of Water Column Dissolved Oxygen Concentrations on Lake Methane Emissions—Results From a Whole‐Lake Oxygenation Experiment

AbstractLakes contribute 9%–19% of global methane (CH4) emissions to the atmosphere. Dissolved molecular oxygen (DO) in lakes can inhibit the production of CH4 and promote CH4 oxidation. DO is therefore often considered an important regulator of CH4 emissions from lakes. Presence or absence of DO in the water above the sediments can affect CH4 production and emissions by (a) influencing if methane production can be fueled by the most reactive organic matter in the top sediment layer or rely on deeper and less degradable organic matter, and (b) enabling CH4 accumulation in deep waters and potentially large emissions upon water column turnover. However, the relative importance of these two DO effects on CH4 fluxes is still unclear. We assessed CH4 fluxes from two connected lake basins in northern boreal Sweden where one was experimentally oxygenated. Results showed no clear difference in summer CH4 emissions attributable to water column DO concentrations. Large amounts of CH4 accumulated in the anoxic hypolimnion of the reference basin but little of this may have been emitted because of incomplete mixing, and effective methane oxidation of stored CH4 reaching oxic water layers. Accordingly, ≤24% of the stored CH4 was likely emitted in the experimental lake. Overall, our results suggest that hypolimnetic DO and water column CH4 storage might have a smaller impact on CH4 emissions in boreal forest lakes than previous estimates, yet potential fluxes associated with water column turnover events remain a significant uncertainty in lake CH4 emission estimates.

Development of a Marine Radioactivity Expert System based on the Extended Marine Food Web: Experiments with Initial Bottom Contamination

An extended version of an expert system was developed as an effort to provide technical support to decision makers for the initial prompt assessment of environmental impacts of marine radioactivity, considering both pelagic and benthic marine food webs. The preliminary version of this system was firstly developed to determine the transfer of radionuclides through a pelagic marine food web (Kim et al, 2020). This system's approach was similar to the Cornell Mixing Zone Expert System (CORMIX) developed by Jirka (1995), as it used idealized settings for the physical/hydrodynamic coastal conditions. Existing or extended systems were executed by taking into account different kinds of radioactivity sources. The initial contamination of bottom sediment is considered in the present study, while the direct discharge to the coastal ocean from the nuclear power plant was considered in the early study. Assuming initial contamination, sensitivity experiments of temporal <sup>137</sup>Cs variations without/with horizontal advection and diffusion (EXP-WOHAD and EXP-WHAD, respectively) were conducted. In EXP-WOHAD, a total of six test cases were constructed, considering a range of vertical diffusion coefficients and sediment fluxes at the water-sea bed interface. The EXP-WHAD experiments were classified into two categories with six cases each, considering the horizontal diffusion and advection processes. The EXP-WOHAD results show that the vertical diffusion processes in the bottom sediment result in the exponential decay of the <sup>137</sup>Cs concentrations in the top sediment layer through upward and downward diffusive fluxes, producing pulse-like time variations in <sup>137</sup>Cs in the water column and the middle sediment layer. These sediment fluxes affect the peak height and its occurrence time in the water column and the middle sediment layer. From EXP-WHAD, it has been confirmed that part of <sup>137</sup>Cs diffused into the water column experiences horizontal dispersion and thereafter deposition to the bottom sediment. It is noted that due to the limited diffusion to the water column, the <sup>137</sup>Cs concentrations in benthic organisms are larger than those in pelagic organisms.

The effect of Zostera noltei recolonization on the sediment mercury vertical profiles of a recovering coastal lagoon

Mercury's extreme toxicity and persistence in the environment justifies a thorough evaluation of its dynamics in ecosystems. Aveiro Lagoon (Portugal) was for decades subject to mercury effluent discharges. A Nature-based Solution (NbS) involving Zostera noltei re-colonization is being tested as an active ecosystem restoration measure. To study the effect of Zostera noltei on the sediment contaminant biogeochemistry, seasonal (summer/winter) sediment, interstitial water and labile mercury vertical profiles were made in vegetated (Transplanted and Natural seagrass meadows) and non-vegetated sites (Bare-bottom area). While no significant differences (p > 0.05) were observed in the sedimentary phase, Zostera noltei presence reduced the reactive/labile mercury concentrations in the top sediment layers by up to 40% when compared to non-vegetated sediment, regardless of season. No differences were found between vegetated meadows, highlighting the fast recovery of the contaminant regulation ecosystem function provided by the plants after re-colonization and its potential for the rehabilitation of historically contaminated ecosystems.

Pola Sebaran Echinodermata Pada Ekosistem Lamun di Pulau Rhun Maluku Tengah Provinsi Maluku

Echinoderms are benthic fauna whose existence has an impact on the fertility level of the bottom substrate. Aside from being a deposit feeder which destroys most of the sediment, it also causes the process of oxygenation of the top layer of sediment. The purpose of this study was to determine the distribution pattern of echinoderms in the seagrass ecosystem from February to March 2021 on Rhun Island, Central Maluku. This research was carried out on Rhun Island, Banda Naira District. The research location was divided into three stations based on the habitat characteristics at each station. Station I (a beach of fine sand and rocks with a wide expanse of seagrass and a sandy and gravel substrate), Station II (a rocky beach, a fairly wide seagrass with a gravel and sandy substrate). Station III (fine sand beach, seagrass spreaded unevenly and not too dense with gravel and rocky substrate). The results showed that 4 classes of Echinoderms were found with 307 individuals. The physical and chemical environmental factors state that it still met the safe limits for Echinodermata life. The distribution of Echinodermata had a clustered and uniform pattern. Key words: benthic; Echinodermata; Rhun Island; seagrass.

Effects of near-bed turbulence on microplastics fate and transport in streams

Quantifying the impact of hyporheic exchange is crucial for understanding the transport and fate of microplastics in streams. In this study, we conducted several Computational Fluid Dynamics (CFD) simulations to investigate near-bed turbulence and analyze vertical hyporheic exchange. Different arranged spheres were used to represent rough and permeable sediment beds in natural rivers. The velocities associated with vertical hyporheic flux and the gravitational force were compared to quantify the susceptibility of microplastics to hyporheic exchange. Four scenario cases representing different channel characteristics were studied and their effects on microplastics movements through hyporheic exchange were quantitatively studied. Results show that hyporheic exchange flow can significantly influence the fate and transport of microplastics of small and light-weighted microplastics. Under certain conditions, hyporheic exchange flow can dominate the behavior of microplastics with sizes up to around 800 μm. This dominance is particularly evident near the sediment-water interface, especially at the top layer of sediments. Higher bed porosity enhances the exchange of microplastics between water and sediment, while increased flow conditions extend the vertical exchange zone into deeper layers of the bed. Changes in the bedform lead to the most pronounced vertical hyporheic exchange, emphasizing the control of morphological features on microplastics transport. Furthermore, it is found that sweep-ejection events are prevailing near the bed surface, serving as a mechanism for microplastics transport in rivers. As moving from the water column to deeper layers in the sediment bed, there's a shift from sweeps dominance to ejections dominance, indicating changes of direction in microplastics movement at different locations.

New data on mobility of transuranium elements in sediments of the Yenisei River

The inflow of transuranium elements to the Yenisei River was previously associated with the production of weapons-grade plutonium at the Mining-and-Chemical Combine (MCC, Zheleznogorsk, Russia), but the source of transuranium elements in the River today is fabrication of MOX fuel that started recently at the MCC. The current study presents results of sequential chemical extraction of radionuclides from sediment samples collected in 2014 and 2020 in two areas near the MCC discharge site and compares these results with the data obtained previously by sequential extraction of sediments collected during 1999–2007. Over the study period, the strength of binding of 137Cs and 60Co in the Yenisei River sediments was high (up to 100%) and remained so, while the percentages of 241Am and 152Eu in residual solids after sequential extraction increased considerably and the percentage of 239,240Pu in residual solids decreased in samples from all study areas. In samples collected at the position located close to the MCC discharge site, the percentages of the strongly bound 241Am and 152Eu as well as 239,240Pu were lower than in the samples from the other positions. The study demonstrated an enormous increase in 239,240Pu activity concentration in the top sediment layers collected at all positions in 2020 relative to 2014. In the same period, as literature data suggest, 239,240Pu activity concentrations also increased in aquatic organisms of the Yenisei River, which can be indicative of the growing potential bioavailability of plutonium in the aquatic ecosystem, which could be caused by the presence of the mobile form of plutonium in the routine discharges from the MCC.

A rationale for higher ratios of CH4 to CO2 production in warmer anoxic freshwater sediments and soils

A rationale for higher ratios of <scp>CH₄</scp> to <scp>CO₂</scp> production in warmer anoxic freshwater sediments and soils

Estimating seabed properties with a residual convolutional network

Deep learning can assist in characterizing seabeds using sources of opportunity such as shipping noise. While previous work focused on seabed classification, this study uses a residual convolutional neural network to find individual seabed properties. The training data were labeled with sound speed, density, attenuation, and thickness of the layer values of the top sediment layer. A comparison was made between predictive capabilities of ResNet-18 networks when trained to learn a single parameter and those trained to simultaneously learn multiple parameters. For stiff parameters—those with high information content in the data—learning an individual parameter performed better. These single parameter predictions are fundamentally different from a geoacoustic inversion for one parameter. In geoacoustic inversion, all other parameters are held at a fixed value. In deep learning, variability in all other parameters is contained in the training data, but the network focuses on features in the data related to a single property. The trained networks are applied to ship noise measured during the 2017 Seabed Characterization Experiment. [Work supported by the Office of Naval Research and the National Science Foundation’s REU program.]

Responses of coastal sediment organic and inorganic carbon to habitat modification across a wide latitudinal range in southeastern China

Coastal wetlands are important to the global carbon (C) budget and climate regulation. Plant invasion and aquaculture reclamation have drastically transformed China’s coastal wetlands, but knowledge of the effects on sediment carbon remains limited. We sampled top layer sediments (0–20 cm) in 21 coastal wetlands in southeastern China across the tropical-subtropical climate gradient, that have experienced the same sequence of habitat transformation from native mudflats (MFs) to Spartina alterniflora marshes (SAs) then to aquaculture ponds (APs). We measured the sediment carbon contents and ancillary physicochemical parameters. Landscape change from MFs to SAs increased sediment organic carbon (SOC) but decreased sediment inorganic carbon (SIC) content, whereas conversion of SAs to APs resulted in the opposite changes. Based on stepwise regression analysis, ammonium concentration and particle size distribution were the common factors that affected changes in SOC between habitat types, whereas for SIC it was ammonium and chloride concentrations. Habitat change affected SOC to a larger degree than SIC. Overall, invasion of MFs by SAs increased total carbon storage in the top sediment by 22%, or 6.6 × 106 g C ha−1; conversion of SAs to APs decreased it by 9.7%, or 3.5 × 106 g C ha−1. Our results showed the differential effects of different habitat modification scenarios on the sediment carbon pools and help assess how landscape-scale change affects terrestrial carbon budget and emission in the context of global climate change.

Evaluation of modified Cenchrus americanus starch as disintegrant in formulation of chlorpheniramine maleate tablets

The aim of this research was to investigate the disintegrant properties of modified Cenchrus americanus starch (MCS) in chlorpheniramine maleate tablets formulation. The matured seeds of C. americanus were steeped in 4 L of 0.25 % w/v sodium hydroxide at 5 °C for 24h, washed, soaked in distilled water (30 ± 2 °C) for 24 h, and wet milled. The slurry was washed with distilled water through a 200 mesh screen, and the filtrate left undisturbed for 18 h. The top layer sediment was scrapped off, and the starch re-suspended with water. Further washing and sedimentation was done until the top layer of the starch became white. The starch was dried at 40 °C, defatted using Petroleum-Ether, air dried, sized (125µm) and characterized using standard methods. The MCS was used at 5, 10 and 15 % w/w as disintegrant in the formulation of chlorpheniramine maleate granules using the wet granulation method. Primogel at similar concentrations was used as comparing standard. Characterization of the granules for densities and flowability were done. Chlorpheniramine maleate tablets compressed from these granules were evaluated for uniformity of weight, hardness, disintegration time, assay and dissolution. Results showed poor flow properties for the MCS powder while the chlorpheniramine granules exhibited good flow. The tablets showed minimal variation in weight, good hardness (6.06±0.10 - 8.07±1.31 kgF), disintegration time (&lt; 7 min), friability (&lt; 1 %), and dissolution of more than 80 % within 30 min. The MCS compared favourably with primogel as disintegrant in chlorpheniramine tablets formulation and met with British Pharmacopoeia set limits.

Learning both a value and uncertainty label for seabed parameters from ship noise data

Because the seabed impacts sound propagation in the ocean, machine learning is being used for both seabed classification and to obtain estimates of individual seabed properties. This paper proposes a method to simultaneously estimate these properties and an associated uncertainty label. A residual neural network is trained and validated using synthetic ship noise spectrograms generated with a range-independent normal mode sound propagation model and a ship noise source spectrum. The data set includes 140 seabeds: In each, the top sediment layer has a random thickness and properties randomly chosen from five sets of bounds, which roughly correspond to clay, mud, sand, silt, and gravel. For each of the 140 sediments, a random selection of 405 combinations of ship speed, closest-point-of-approach range, and source depth are used resulting in 22k data samples. Each data sample is labeled with the true values of the sediment parameters as well as a label for the uncertainty level of each. The uncertainty levels are obtained from the Fisher information and qualify of the information content in the ship spectrogram about each parameter. Examples of how the residual neural network learns to perform regression for the parameter value and the uncertainty level will be presented.

Phosphorus removal by aquatic vegetation in shallow eutrophic lakes: a laboratory study.

Eutrophication in inland lakes is occurring frequently with the rapid urbanization, the increases in human population, and the intensive agricultural activities. Traditional management programs focusing on external nutrient reduction failed in recovery of certain aquatic environments where internal nutrient releases are substantial. In this study, we evaluated the effects of aquatic vegetation in altering the phosphorus concentrations in a shallow, eutrophic lake through laboratory flume experiments. Our measurements demonstrated that aquatic vegetation could effectively lower the phosphorus levels in the water column, and the average reduction reached 90% for submerged vegetation and 80% for emergent vegetation. The experimental results showed that the submerged vegetation was effective to reduce phosphorus concentrations in the top and mid layers of pore waters and sediments. Differently, the emergent vegetation would assimilate more phosphorus in the bottom layer due to its deep root distributions. The flowing-water environment favored phosphorus removal for emergent vegetation, while the submerged vegetation was more functional in static waters according to our observations. The flux results showed that phosphorus transports from water columns to leaves, roots to leaves, and sediments to roots were all inhibited in flowing-water environments for submerged vegetation. Oppositely, the fluxes of emergent vegetation groups were all enhanced in flowing waters. Our experiments could inform ecosystem management concerning the potentials of aquatic vegetation in nutrient removal at regional and lake-wide scales.

Disclosing the ecological implications of heavy metal disturbance on the microbial N-transformation process in the ocean tidal flushing urban estuary

Anthropogenic source of heavy metals in the estuary-coastal sediment could cause serious environmental concerns. Thus, much needs to be concerned about the linkages between bacterial community, microbial nitrogen processes and heavy metals accumulation in the ocean tidal flush urban estuary where the river meet the sea. In this study, Illumina sequencing analysis and quantitative PCR (qPCR) were adopted to detect the effect of heavy metals on the bacterial community and nitrogen transformation genes from vertical sediment profiles in the four tidal flush estuaries. Results indicated that bacterial community and structure varied greatly from the vertical and spatial distribution. The vertical distribution of heavy metals varied greatly in the different groups. It was concluded that the accumulation of heavy metals mainly occurred in the medium layer of the depth from 40 cm to 70 cm. Heavy metals were found to pose a significant influence on the bacterial community and nitrogen transformation genes. The abundance of denitrification genes was highly associated with Cr, Zn, As and the abundance of amoA was vulnerable to the influence of Ni. Both denitrification and nitrification genes were greatly affected by C and N content, which indicated that the enhanced nitrogen transformation activities mainly occurred in the top layer sediment. Our results indicated that the bacterial community and nitrogen transformation functional genes were sensitive to the variation of heavy metals and nutrient content, which will provide new insight into the microbial N-cycling process influenced by pollutant and show a potential strategy for discharge management in the circumstances of estuary-coastal zones.

Seabed Classification Using a Convolutional Neural Network on Explosive Sounds

The Seabed Characterization Experiment 2017 yielded a rich set of environmental and acoustical data, and subsequent geoacoustic inversions have estimated seabed properties. Seventeen of these seabed parameterizations are now used to define seabed classes, and a convolutional neural network (CNN) is trained to select a seabed class using explosive sounds. The CNN is trained on synthetic 1-s pressure time series and then applied to measured data samples from a single pressure sensor. While the environmental variability in the training data impacts the seabed classification, physical insights are gained by considering the classification results as a function of the sound speed ratio across the sediment–water interface and the interval velocity of the top sediment layer. These results indicate that the selected seabed classes for the data samples with longer propagation distances consistently have similar sediment–water sound speed ratios that are less than unity, while the data samples with the shortest propagation distances consistently have higher interval velocities. These classification results indicate that the CNN has learned physical features associated with acoustic sound propagation and points to future work that needs to be considered if the seabed classes have acoustically distinct signatures in the data.

Sedimentation of metals in Sundarban mangrove ecosystem: Dominant drivers and environmental risks.

Metal contamination from upstream river water is a threat to coastal and estuarine ecosystem. The present study was undertaken to unveil sedimentation processes and patterns of heavy metal deposition along the salinity gradient of a tropical estuary and its mangrove ecosystem. Sediment columns from three representative sites of differential salinity, anthropogenic interference, and sediment deposition pattern were sampled and analyzed for grain size distribution and metal concentrations as a function of depth. Sediments were dominantly of silty-medium sand texture. A suite of fluvial and alluvial processes, and marine depositional forcing control the sediment deposition and associated heavy metal loading in this estuary. The depth profile revealed a gradual increase in heavy metal accumulation in recent top layer sediments and smaller fractions (silt + clay), irrespective of tidal regimes. Alluvial processes and long tidal retention favor accumulation of heavy metals. Enrichment factor (0.52-15), geo-accumulation index (1.4-5.8), and average pollution load index (PLI = 2.0) indicated moderate to higher heavy metal contamination status of this estuary. This study showed that alluvial processes acted as dominant drivers for the accumulation of metals in sediments, which prevailed over the influence of marine processes. Longer tidal retention of the water column favored more accumulation of heavy metals. Metal accumulation in the sediments entails a potential risk of bioaccumulation and biomagnification through the food web, and may increasingly impact estuarine ecology, economy, and ultimately human health.