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Archaea are widespread in marine sediments, but their occurrence and relationship with natural salinity gradients in estuarine sediments is not well understood. This study investigated the abundance and diversity of Archaea in sediments at three sites [Brightlingsea (BR), Alresford (AR) and Hythe (HY)] along the Colne Estuary, using quantitative real-time PCR (qPCR) of 16S rRNA genes, DNA hybridization, Archaea 16S rRNA and mcrA gene phylogenetic analyses. Total archaeal 16S rRNA abundance in sediments were higher in the low-salinity brackish sediments from HY (2–8 × 107 16S rRNA gene copies cm−3) than the high-salinity marine sites from BR and AR (2 × 104–2 × 107 and 4 × 106–2 × 107 16S rRNA gene copies cm−3, respectively), although as a proportion of the total prokaryotes Archaea were higher at BR than at AR or HY. Phylogenetic analysis showed that members of the ‘Bathyarchaeota’ (MCG), Thaumarchaeota and methanogenic Euryarchaeota were the dominant groups of Archaea. The composition of Thaumarchaeota varied with salinity, as only ‘marine’ group I.1a was present in marine sediments (BR). Methanogen 16S rRNA genes from low-salinity sediments at HY were dominated by acetotrophic Methanosaeta and putatively hydrogentrophic Methanomicrobiales, whereas the marine site (BR) was dominated by mcrA genes belonging to methylotrophic Methanococcoides, versatile Methanosarcina and methanotrophic ANME-2a. Overall, the results indicate that salinity and associated factors play a role in controlling diversity and distribution of Archaea in estuarine sediments. Archaea populations in the River Colne Estuary, UK sediments change with increasing salinity gradient from populations dominated by methanogenic Euryarchaeota and ‘Bathyarchaeota’ (MCG) in brackish sediments to putatively ammonia-oxidising Thaumarchaeota and MCG in marine sediments.

Pyrosequencing and metagenomic profiling were used to assess the phylogenetic and functional characteristics of microbial communities residing in sediments collected from the estuaries of Rivers Oujiang (OS) and Jiaojiang (JS) in the western region of the East China Sea. Another sediment sample was obtained from near the shore far from estuaries, used for contrast (CS). Characterization of estuary sediment bacterial communities showed that toxic chemicals potentially reduced the natural variability in microbial communities, while they increased the microbial metabolic enzymes and pathways. Polycyclic aromatic hydrocarbons (PAHs) and nitrobenzene were negatively correlated with the bacterial community variation. The dominant class in the sediments was Gammaproteobacteria. According to Kyoto Encyclopedia of Genes and Genomes (KEGG) enzyme profiles, dominant enzymes were found in estuarine sediments, which increased greatly, such as 2-oxoglutarate synthase, acetolactate synthase, inorganic diphosphatase, and aconitate hydratase. In KEGG pathway profiles, most of the pathways were also dominated by specific metabolism in these sediments and showed a marked increase, for instance alanine, aspartate, and glutamate metabolism, carbon fixation pathways in prokaryotes, and aminoacyl-tRNA biosynthesis. The estuarine sediment bacterial diversity varied with the polluted river water inputs. In the estuary receiving river water from the more seriously polluted River Oujiang, the sediment bacterial community function was more severely affected.

Reservoir quality prediction in deeply buried reservoirs represents a complex challenge to geoscientists. In sandstones, reservoir quality is determined by the extent of compaction and cementation during burial. During compaction, porosity is lost through the rearrangement and fracture of rigid grains and the deformation of ductile grains. During cementation, porosity is predominantly lost through the growth of quartz cement, although carbonate and clay mineral growth can be locally important. The degree of quartz cementation is influenced by the surface area of quartz available for overgrowth nucleation and thermal history. Clay grain coats can significantly reduce the surface area of quartz available for overgrowth nucleation, preventing extensive cementation. Using a coupled-effect compaction and cementation model, we have forward-modelled porosity evolution of surface sediments from the modern Ravenglass Estuary under different maximum burial conditions, between 2000 and 5000 m depth, to aid the understanding of reservoir quality distribution in a marginal marine setting. Seven sand-dominated sub-depositional environments were subject to five burial models to assess porosity-preservation in sedimentary facies. Under relatively shallow burial conditions (<3000 m), modelled porosity is highest (34 to 36%) in medium to coarse-grained outer-estuary sediments due to moderate sorting and minimal fine-grained matrix material. Fine-grained tidal flat sediments (mixed flats) experience a higher degree of porosity loss due to elevated matrix volumes (20 to 31%). Sediments subjected to deep burial (>4000 m) experience a significant reduction in porosity due to extensive quartz cementation. Porosity is reduced to 1% in outer estuary sediments that lack grain-coating clays. However, in tidal flat sediments with continuous clay grain coats, porosity values of up to 30% are maintained due to quartz cement inhibition. The modelling approach powerfully emphasises the value of collecting quantitative data from modern analogue sedimentary environments to reveal how optimum reservoir quality is not always in the coarsest or cleanest clastic sediments.

The elemental concentration and grain size composition (sand, silt and clay) of 63 bulk estuarine sediment samples from the bank of the Hau River in Mekong Basin were analyzed by neutron activation analysis and laser-diffraction particle size analyzer in this study. Principal component analysis was used to investigate the association between elemental concentration and particle size composition in sediment samples. The first principal component (PC) represents the alluvial composition in the sediment. Correlation tests showed that the elemental concentrations in the first PC (As, Co, Cr, Cs, Fe, Rb, Sb, Sc and Zn) have been strongly positively correlated with clay and silt contents and negatively correlated with sand content. The second PC represents the minerals composition of the sediment. The elemental concentration in the second PC (Ce, Eu, Hf, La, Sm, Ta, Tb, Th, U, and Yb) had a lower correlation with the grain size than the elemental concentration in the first PC. The third PC, which includes Br and Na, shows deposits coming from marine organisms and chemical precipitation in the ocean that had not correlated with the grain size.

In the quest to use modern analogues to understand clay mineral distribution patterns to better predict clay mineral occurrence in ancient and deeply buried sandstones, it has been necessary to define palaeo sub-environments from cores through modern sediment successions. Holocene cores from Ravenglass in the NW of England, United Kingdom, contained metre-thick successions of massive sand that could not be unequivocally interpreted in terms of palaeo sub-environments using conventional descriptive logging facies analysis. We have therefore explored the use of geochemical data from portable X-ray fluorescence analyses, from whole-sediment samples, to develop a tool to uniquely define the palaeo sub-environment based on geochemical data. This work was carried out through mapping and defining sub-depositional environments in the Ravenglass Estuary and collecting 497 surface samples for analysis. Using R statistical software, we produced a classification tree based on surface geochemical data from Ravenglass that can take compositional data for any sediment sample from the core or the surface and define the sub-depositional environment. The classification tree allowed us to geochemically define ten out of eleven of the sub-depositional environments from the Ravenglass Estuary surface sediments. We applied the classification tree to a core drilled through the Holocene succession at Ravenglass, which allowed us to identify the dominant paleo sub-depositional environments. A texturally featureless (massive) metre-thick succession, that had defied interpretation based on core description, was successfully related to a palaeo sub-depositional environment using the geochemical classification approach. Calibrated geochemical classification models may prove to be widely applicable to the interpretation of sub-depositional environments from other marginal marine environments and even from ancient and deeply buried estuarine sandstones.

Distribution of metals in different binding phases of estuarine sediments provides chemically significant description of metal–sediment interactions. This study describes the influences of ligand field stabilization energy (LFSE), Jahn–Teller effect, and water exchange rate (k ₋w) on metal distribution in different binding phases of estuarine sediments. It was found that Cu had highest affinity for organic binding phases in the studied sediments followed by Ni and Pb. However, Pb showed strong association with Fe/Mn oxide phases followed by Ni and Cu. Faster k ₋w of Cu (II) (1 × 10⁹ s⁻¹) increased the rate of complex formation of Cu²⁺ ion with ligand in the organic phases. The Cu–ligand (from organic phase) complexes gained extra stability by the Jahn–Teller effect. The combined effects of these two phenomena and high ionic potential increased the association of Cu with the organic phases of the sediments than Ni and Pb. The smaller ionic radii of Ni²⁺ (0.72 Å) than Pb²⁺ (1.20 Å) increase the stability of Ni–ligand complexes in the organic phase of the sediments. High LFSE of Ni(II) (compared with Pb²⁺ ions) also make Ni-organic complexes increasingly stable than Pb. High k ₋w (7 × 10⁹ s⁻¹) of Pb did not help it to associate with organic phases in the sediments. The high concentration of Pb in the Fe/Mn oxyhydroxide binding phase was probably due to co-precipitation of Pb²⁺ and Fe³⁺. High surface area or site availability for Pb²⁺ ion on Fe oxyhydroxide phase was probably responsible for the high concentration of Pb in Fe/Mn oxyhydroxide phase. Increasing concentrations of Cu in organic phases with the increasing Cu loading suggest that enough binding sites were available for Cu in the organic binding phases of the sediments. This study also describes the influence of nature of sedimentary organic carbon (terrestrial and marine derived OC) in controlling these metal distribution and speciation in marine sediment.

Microorganisms are known to play fundamental roles in the biogeochemical cycling of carbon in the coastal environments. To get to know the composition and ecological roles of the archaeal communities within the sediments of the Pearl River Estuary, Southern China, the diversity and vertical distribution of archaea in a sediment core was reported based on the 16S rRNA and mcrA genes for the first time. Quantitative PCR analysis revealed that archaea were present at 10 6 –10 7 16S rRNA gene copies/g (wet weight) in the sediment core, and the proportion of mcrA versus 16S rRNA gene copies varied from 11 to 45%. 16S rRNA gene libraries were constructed and analyzed for the top layer (0–6 cm), middle layer (18–24 cm), sulfate-methane transition zone (SMTZ, 32–42 cm), and bottom layer (44–50 cm) sediments. The results indicated that Miscellaneous Crenarchaeotal Group (MCG) was the main component in the sediments. The MCG archaea could be further divided into six subgroups: MCG-A, B, C, D, E, and F. On the other hand, mcrA sequences from methanogens related to the order Methanomicrobiales and ANME-2 methanotrophs were detected in all sediment layers. Taken together, our data revealed a largely unknown archaeal community in which MCG dominated within the Pearl River estuarine sediments, while methanogens and methane-oxidizing archaea putatively involving in methane metabolism, were also found in the community. This is the first important step towards elucidating the biogeochemical roles of these archaea in the Pearl River Estuary.

Regular release of sediment from reservoir has been increasingly adopted as a strategy for sustainable management. Here, we use a process‐based morphodynamic model to simulate the estuarine sediment dynamics impacted by turbidity current venting implemented by the Shihmen Reservoir during three typhoon events in 2008. Upon validation with the post‐event bathymetries, the model hindcasts reveal that mud releasing can be effective in mitigating reservoir siltation, yet may be a suboptimal strategy for alleviating coastal sediment deficit. A vast majority of the released muds were delivered through the estuary and exported to offshore by flood advection, wave dispersion, and tidal flushing. The flood‐driven sands, sourced mainly from downstream tributaries, were instead the major contributor to coastal sediment budget. However, mud mantling (covering and immobilizing sand deposits by the reservoir‐released muds) reduced sand availability and thus sand delivery to the coast. For the present case, 25% of the released muds were deposited along the way, presence of these mud covers reduced sand delivery by 15%, compared to a hypothetical scenario of clear‐water flood releases. The relative sand transport deficit is found to increase linearly with the degree of bed mud saturation, 1–D/R, with D/R the ratio of single‐event mud deposit to release. Given broad relevance to global reservoirs encountering the problems of siltation and coastal sediment deficit, our findings highlight that sustainable management needs to look beyond just a bulk amount of sediment, but it is critical to consider how different sediment fractions are interacting and impacted by human activities. Key Points Increasingly adopted mud release strategy is effective to mitigate reservoir siltation yet suboptimal to alleviate coastal sediment deficit Flood‐driven tributary‐sourced sands dominate supply to coastal sediment budget yet sand delivery is reduced by mantling of released muds Sand delivery deficit (relative to sand delivery of clear‐water flood release scenario) increases linearly with degree of bed mud saturation

Heavy metal pollution of sediments is a growing concern in most parts of the world, and numerous studies focussed on identifying contaminated sediments by using a range of digestion methods and pollution indices to estimate sediment contamination have been described in the literature. The current work provides a critical review of the more commonly used sediment digestion methods and identifies that weak acid digestion is more likely to provide guidance on elements that are likely to be bioavailable than other traditional methods of digestion. This work also reviews common pollution indices and identifies the Nemerow Pollution Index as the most appropriate method for establishing overall sediment quality. Consequently, a modified Pollution Index that can lead to a more reliable understanding of whole sediment quality is proposed. This modified pollution index is then tested against a number of existing studies and demonstrated to give a reliable and rapid estimate of sediment contamination and quality.

The extent to which small plastics and potentially associated compounds are entering coastal food webs, especially in estuarine systems, is only beginning to be realized. This study examined an estuarine reach at the mouth of urbanized Chollas Creek in San Diego, California to determine: 1) the extent and magnitude of microplastics pollution in estuarine sediments and fish, 2) the extent and magnitude of SVOC contamination in estuarine fish, and 3) whether fish preferentially ingested certain types of microplastics, when compared with the microplastic composition of creekbed sediments. Surface sediments (0-5 cm depth) contained about 10,000 small plastic pieces per m2, consisting mostly (90%) of fibers, and hard and soft pieces. Nearly 25% of fish contained small plastics, but prevalence varied with size and between species. Of the 25 types of small plastics found in sediment, fish preferred about 10 types (distinct colors and forms). Several SVOCs, both water soluble and sediment-associated compounds, were found in the two species of fish tested. This study revealed that a species' natural history may influence contamination levels, and warrants further study to better understand the pathways of plastics and associated contaminants into and throughout coastal food webs, and the potential health risks for small and/or low-trophic level organisms.