Uncovering unnoticed small-scale tsunamis: field survey in Lombok,Indonesia, following the 2018 earthquakes

Natural Hazards - In 2018, Lombok island, Indonesia, was hit by a series of destructive earthquakes that caused thousands of casualties and widespread material damage. In response to those events,...     

Head-on collision between positron acoustic waves in homogeneous and inhomogeneous plasmas

The head-on collision between positron acoustic solitary waves (PASWs) as well as the production of rogue waves (RWs) in homogeneous and PASWs in inhomogeneous unmagnetized plasma systems are investigated deriving the nonlinear evolution equations. The plasmas are composed of immobile positive ions, mobile cold and hot positrons, and hot electrons, where the hot positrons and hot electrons are assumed to follow the Kappa distributions. The evolution equations are derived using the appropriate coordinate transformation and the reductive perturbation technique. The effects of concentrations, kappa parameters of hot electrons and positrons, and temperature ratios on the characteristics of PASWs and RWs are examined. It is found that the kappa parameters and temperature ratios significantly modify phase shifts after head-on collisions and RWs in homogeneous as well as PASWs in inhomogeneous plasmas. The amplitudes of the PASWs in inhomogeneous plasmas are diminished with increasing kappa parameters, concentration and temperature ratios. Further, the amplitudes of RWs are reduced with increasing charged particles concentration, while it enhances with increasing kappa- and temperature parameters. Besides, the compressive and rarefactive solitons are produced at critical densities from KdV equation for hot and cold positrons, while the compressive solitons are only produced from mKdV equation for both in homogeneous and inhomogeneous plasmas.     

Geochemistry of subsurface Late Quaternary ironstones in Rajshahi and Bogra Districts,Bangladesh: implications for genetic and depositional conditions

The present study deals with the geochemistry of Late Quaternary ironstones in the subsurface in Rajshahi and Bogra districts, Bangladesh with the lithological study of the boreholes sediments. Major lithofacies of the studied boreholes are clay, silty clay, sandy clay, fine to coarse grained sand, gravels and sands with(fragmentary) ironstones. The ironstones contain major oxides, Fe_2 O_3*(*total Fe)(avg. 66.6 wt%), SiO_2(avg. 15.3 wt%), Al_2 O_3(avg. 4.0 wt%), MnO(avg. 7.7 wt%), and CaO(avg. 3.4 wt%). These geochemical data imply that the higher percentage of Fe_2 O_3* along with Al_2 O_3 and MnO indicate the ironstone as goethite and siderite, which is also validated by XRD data. A comparatively higher percentage of SiO_2 indicates the presence of relative amounts of clastic quartz and manganese-rich silicate or clay in these rocks. These ironstones also have significant amounts of MnO(avg. 7.7 wt%) suggesting their depositional environments under oxygenated condition. Chemical data of these ironstones suggest that the source rock suffered deep chemical weathering and iron was mostly carried in association with the clay fraction and organic matter. Iron concretion was mostly formed by bacterial build up in swamps and marshes, and was subsequently embedded in clayey mud.Within the coastal environments, the water table fluctuates and goethite and siderite with mud and quartz became dry and compacted to form ironstone.     

Trophic functioning of macrobenthic fauna in a tropical acidified Bornean estuary (Southeast Asia)

The trophic structure of a community is used to infer ecosystem functioning(e.g. energy transfer and nutrient cycling). Here the trophic structure of the benthic infaunal and epifaunal communities in the Brunei Estuary are characterized, and their distribution along an estuarine pH gradient is analyzed using univariate and multivariate techniques. This analysis revealed that surface deposit feeders(e.g., polychaetes) were numerically dominant within the infaunal communities whereas in the epifaunal communities filter feeders(e.g., bivalves) were highly abundant. Species richness for almost all trophic groups increased toward the lower estuary, except for omnivores in the epifaunal communities, which decreased markedly. Both Analysis of Variance(ANOVA) and Analysis of Similarities(ANOSIM) detected significant differences in the density of respective trophic groups among stations. Within infaunal communities, both Biological and Environmental procedure(BIO-ENV) and Canonical Correspondence Analysis(CCA) showed that trophic shifts were associated with environmental gradients. Surface-deposit feeders and omnivores were the most abundant macrobenthos of the upper estuary characterized by low salinity, low pH, and a higher percentage of mud particles. The proportion of filter feeders and carnivores increased with salinity/pH and sand. A more uniform distribution of trophic structure was found in the lower estuary, with high salinity and pH over sandy habitat. In contrast, within epifaunal trophic groups,the percentage of surface deposit feeders and omnivores declined, but filter feeders remarkably increased toward the sea. The proportion of carnivores remained similar at all stations. Non-Metric Multidimensional Scaling(nMDS) ordination for epifaunal trophic groups clearly demarcated higher salinity/pH stations from lower salinity/pH stations, suggesting different trophic compositions along the estuarine pH gradient.     

The Impact of the Degree of Aquifer Confinement and Anisotropy on Tidal Pulse Propagation

Oceanic tidal fluctuations which propagate long distances up coastal rivers can be exploited to constrain hydraulic properties of riverbank aquifers. These estimates, however, may be sensitive to degree of aquifer confinement and aquifer anisotropy. We analyzed the hydraulic properties of a tidally influenced aquifer along the Meghna River in Bangladesh using: (1) slug tests combined with drilling logs and surface resistivity to estimate Transmissivity (T); (2) a pumping test to estimate T and Storativity (S) and thus Aquifer Diffusivity (D_PT); and (3) the observed reduction in the amplitude and velocity of a tidal pulse to calculate D using the Jacob‐Ferris analytical solution. Average Hydraulic Conductivity (K) and T estimated with slug tests and borehole lithology were 27.3 m/d and 564 m²/d, respectively. Values of T and S determined from the pumping test ranged from 400 to 500 m²/d and 1 to 5 × 10^?4, respectively with D_PT ranging from 9 to 40 × 10⁵ m²/d. In contrast, D estimated from the Jacob‐Ferris model ranged from 0.5 to 9 × 10⁴ m²/d. We hypothesized this error resulted from deviations of the real aquifer conditions from those assumed by the Jacob‐Ferris model. Using a 2D numerical model tidal pulses were simulated across a range of conditions and D was calculated with the Jacob‐Ferris model. Moderately confined (K_top/K_aquifer < 0.01) or anisotropic aquifers (K_x/K_z > 10) yield D within a factor of 2 of the actual value. The order of magnitude difference in D between pumping test and Jacob‐Ferris model at our site argues for little confinement or anisotropy.     

Spatial and seasonal distribution of intertidal macrobenthos with their biomass and functional feeding guilds in the Naf River estuary,Bangladesh

The Naf River estuary is one of the most productive ecological ecosystems in Bangladesh providing an important feeding area for fishes and other aquatic animals. However, detailed information on macrobenthic communities is rarely available in this area. Our study focused on the seasonal and spatial variability of macrobenthic community structure with their biomass and functional guilds. In total, fortyseven taxa were identified under nine major groups and seven functional feeding guilds. Among macrobenthic taxa, Polychaeta was dominant in both seasons and all over the estuary contributing 60% of total benthos.The mean abundance of macrobenthos was higher during the pre-monsoon(2 972±1 994 inds./m~2) than the monsoon(1 572±361 inds./m~2) being maximum at mid-estuary region and minimum at upper and lower estuary regions. Of the diversity indices, density(P=0.01, P=0.003), the number of taxa(P=0.000 3,P=0.000 9) and Margalefs Index(P=0.000 4, P=0.001) of macrobenthos were significantly different among stations and seasons. Shannon-Weiner index(P=0.009, P=0.12) and Pielou's evenness index(P=0.03,P=0.14) had significant differences among stations but not among seasons. Cluster analysis suggested that distribution of macrofauna is strongly influenced by seasonality. Among the feeding types, carnivorous were found to be dominant at the upper and mid-estuarine regions, and herbivorous were at lower estuarine regions. Carnivorous species were dominant in both seasons. Omnivores have shown to be the most abundant feeding type in terms of biomass though herbivores were highest during monsoon.     

Water and sediment quality parameters in the Chalan Beel,the largest wetland of Bangladesh

A study was conducted to investigate the status of the water and sediment quality in the Chalan Beel——a major fresh water fish reservoir of the country for a period of one year from July 2007 to June 2008. The mean values of water quality parameters(depth: 214.73±152.22 cm, temperature 27.68±4.26℃, transparency 123±82 cm, p H 9.7±0.47, total alkalinity 137±42 mg/L, conductivity 307±147 μs/cm, total dissolved solids 183±89 mg/L, ammonia-N 0.27±0.39 mg/L, nitrate-N 0.09±0.07 mg/L, phosphate-P 2.01±2.53 mg/L) and sediment quality parameters(p H 7.21±0.35, organic matter 2.59±0.52%, total nitrogen 0.09±0.04%, available phosphorus 5.4±3.6 Meq./100 g and exchangeable potassium 0.43±0.14 Meq./100 g) were within the range recommended for most of the inland fishes of Bangladesh. Although the water and sediment quality parameters except ammonia and phosphate are in the suitable range, the overall results suggest that better management techniques should be practiced in order to overcome the declining trend of associated aquatic life(fauna and flora) of this important fresh water body of Bangladesh.     

Dynamics of arsenic adsorption in the targeted arsenic-safe aquifers in Matlab, south-eastern Bangladesh: Insight from experimental studies

Targeting shallow low-As aquifers based on sediment colour may be a viable solution for supplying As-safe drinking water to rural communities in some regions of Bangladesh and West Bengal in India. The sustainability of this solution with regard to the long-term risk of As-safe oxidized aquifers becoming enriched with As needs to be assessed. This study focuses on the adsorption behaviour of shallow oxidized sediments from Matlab Region, Bangladesh, and their capacity to attenuate As if cross-contamination of the oxidized aquifers occurs. Water quality analyses of samples collected from 20 tube-wells in the region indicate that while there may be some seasonal variability, the groundwater chemistry in the reduced and oxidized aquifers was relatively stable from 2004 to 2009. Although sediment extractions indicate a relatively low amount of As in the oxidized sediments, below 2.5 mg kg⁻¹, batch isotherm experiments show that the sediments have a high capacity to adsorb As. Simulations using a surface complexation model that considers adsorption to amorphous Fe(III) oxide minerals only, under-predict the experimental isotherms. This suggests that a large proportion of the adsorption sites in the oxidized sediments may be associated with crystalline Fe(III) oxides, Mn(IV) and Al(III) oxides, and clay minerals. Replicate breakthrough column experiments conducted with lactose added to the influent solution demonstrate that the high adsorption capacity of the oxidized sediments may be reduced if water drawn down into the oxidized aquifers contains high levels of electron donors such as reactive dissolved organic C.     

Provenance, tectonics and source weathering of modern fluvial sediments of the Brahmaputra–Jamuna River, Bangladesh: Inference from geochemistry

This present study describes the elemental geochemistry of fluvial sediments in the Kurigram (upstream) to Sirajganj–Tangail (downstream) section of the Brahmaputra–Jamuna River, Bangladesh, with the aim of evaluating their provenance, weathering and tectonic setting. Petrographically, the sediments are rich in quartz (68%), followed by feldspars (8.5%) and lithic grains (7%). The bulk sediment chemistry is influenced by grain size. Concentrations of TiO₂, Fe₂O₃, MgO, K₂O, P₂O₅, Rb, Nb, Cr, V, Y, and, Ce, Th and Ga slightly decrease with increasing SiO₂/Al₂O₃ and grain size, suggesting clay matrix control. In contrast, concentrations of CaO, Na₂O, Sr and Pb increase with increasing SiO₂/Al₂O₃ and grain size, suggesting residence of these substances in feldspar. Decrease in Zr as grain size increases is likely controlled both by clay matrix and heavy minerals. In addition, heavy minerals' sorting also influences Ce, Th, Y and Cr abundances in some samples. The sediments are predominantly quartzose in composition with abundant low-grade metamorphic and sedimentary lithics, low feldspars and trace volcanic detritus, indicating a quartzose recycled orogen province as a source of the sediments. Discriminant diagrams together with immobile element ratio plots show that, the Brahmaputra–Jamuna River sediments are mostly derived from rocks formed in an active continental margin. Moreover, the rare earth element ratios as well as chondrite-normalized REE patterns with flat HREE, LREE enrichment, and negative Eu anomalies indicate derivation of the sediments of Brahmaputra–Jamuna River from felsic rock sources of upper continental crust (UCC). The chemical indices of alteration suggest that Brahmaputra–Jamuna River sediments are chemically immature and experienced low chemical weathering effects. In the A–CN–K ternary diagram, most of the samples close to the plagioclase–K-feldspar join line and to the UCC plot, and in the field of various lithologies of Higher Himalayan Crystalline Series, suggesting that rocks in these series are likely source rocks. Therefore, the elemental geochemistry of the Brahmaputra–Jamuna River sediments is controlled mostly by mechanical breakdown of lithic fragments and subsequent preferential attrition of muscovite > albite > quartz.