Uppermost Sediment Layer Research Articles

Ártéri üledékek mikroműanyag tartalma az Alsó-Tisza egy kanyarulata mentén

Plastics get into the environment legally or illegally, and their amount is increasing continuously because their usage is constantly increasing. Microplastics (< 5 mm) have several morphological types: fragments can be formed by physical weathering of larger plastic objects; fibres usually originate from washing synthetic clothes; while pearls used in cosmetics get into the environment by means of communal waste. Our aim is to analyse the sedimentation pattern of microplastics on various floodplain forms along a natural meander of the Lower Tisza River, Hungary. The uppermost sediment layer (0–5 cm) of the floodplain forms contains 0–1800 microplastics/kg. The sediments of the swales (458–1300 particles/kg) and the point bar sediments (220–1600 particles/kg) are the most polluted, but the samples from the low-lying clay pits are also highly contaminated (818–880 particles/kg). The active point bar and the natural levee have lower levels of microplastic contamination (100–620 particles/kg). In the natural levee along the channel, microplastics are present to a depth of 50 cm (1753–4800 particles/kg), while in the distal parts of the floodplain only the upper 0–10 cm sediment layer is polluted. Plastic fibres are the most common plastic types in the samples, clearly indicating that they originate from treated or untreated wastewater.

Increasing Trends of Legacy and Emerging Organic Contaminants in a Dated Sediment Core From East-Africa

Temporal trends of industrial organic contaminants can show how environmental burdens respond to changes in production, regulation, and other anthropogenic and environmental factors. Numerous studies have documented such trends from the Northern Hemisphere, while there is very limited data in the literature from sub-Saharan Africa. We hypothesized that the temporal trends of legacy and contemporary industrial contaminants in sub-Saharan Africa could greatly differ from the regions in which many of these chemicals were initially produced and more extensively used. For this purpose, a dated sediment core covering six decades from a floodplain system in urban Dar es Salaam, Tanzania, was analysed. The samples were analysed for selected legacy persistent organic pollutants (POPs) [polychlorinated biphenyls (PCBs) and polybrominated biphenyl ethers (PBDEs)] and chemicals of emerging concern (CECs) [alternative brominated flame retardants (aBFRs), chlorinated paraffins (CPs), and dechloranes]. All groups of chemicals showed a steep increase in concentrations towards the uppermost sediment layers reflecting the more recent years. Concentrations of the individual compound groups in surface sediment were found in the order CPs >> aBFRs ∼ ∑25PBDEs > dechloranes ∼ ∑32PCBs. Time trends for the individual compounds and compound groups differed, with ∑32PCBs showing presence in sediments since at least the early 1960s, while some CECs first occurred in sediments corresponding to the last decade. Investigations into potential drivers for the observed trends showed that socioeconomic factors related to growth in population, economy, and waste generation have contributed to increasing concentrations of PBDEs, aBFRs, CPs, and Dechlorane Plus. Further monitoring of temporal trends of industrial organic contaminants in urban areas in the Global South is recommended.

Seasonal sedimentation in saline Lake Shira (Siberia, Russia) and meromixis: implications for regional paleoclimate reconstructions

Abstract. Detailed studies of processes of sediment record formation are necessary for accurate sediment-derived paleo-environmental reconstructions. We conducted the multi-year seasonal sediment evolution of sediment fluxes of chemical elements, organic and inorganic carbon, total nitrogen, and photosynthetic pigments for a period from 2012 to 2017. In 2013, 2016, 2017, and 2018, we estimated the contents of photosynthetic pigments in the uppermost sediment layers frozen in situ with a freeze-corer. In 2015 and 2016, a rare event of transition from meromixis (i.e. long-term hypolimnetic anoxia) to holomixis was observed, which was accompanied by the temporary disappearance of hydrogen sulfide from the water column in spring and a decrease in hydrogen sulfide in other seasons compared to the meromictic state. We have demonstrated that okenone and Mo in the Lake Shira sediments reflect the presence of hydrogen sulfide in the water column. However, the okenone showed smoothened multi-year dynamics without a pronounced seasonal one. Therefore, the okenone can be a proxy of sulphidic conditions in photic zone and weakly depend on seasons whereas Mo can be used as an indicator of winter periods when analyzed in cores with annual resolution. Sedimentation fluxes of other substances showed typical seasonal dynamics with a minimum in winter and a maximum in late summer and autumn. All chemical elements in the sedimentation flow can be roughly divided into those associated with organic matter and terrigenous-chemogenous. The components of the second group showed a pronounced peak of sedimentation in autumn 2012 and summer 2017 presumably due to the increased amount of precipitation at that time. This demonstrates the relationship between the terrigenous components and the climate humidity for this lake. Besides, it reflects the irregularity of annual varves composition.

Biological productivity and glaciomarine sedimentation in the Central Basin of the northwestern Ross Sea since the last glacial maximum

This study documents multi-proxy data representing surface water productivity and AMS 14C dates of box (BC3) and gravity (GC2) cores in the Central Basin of the northwestern Ross Sea. Based on AMS 14C dates, a comparison of sediment properties between BC3 and GC2 reveals that BC3 records the complete Holocene (i.e., interglacial) history, which is correlated to the uppermost part of GC2. The lithostratigraphic succession of GC2 consists of the repetition of contrasting layers distinguished by the productivity proxies. In contrast to the uppermost sediment layer (i.e., interglacial), the subsurface sediment layer (i.e., deglacial) is distinctly characterized by very high biogenic components. Such pronounced biogenic remnants in the deglacial sediments are not explained exclusively by in situ enhanced productivity in the surface water. Our results, thus, suggest that eroded and reworked shelf sediments from a previous interglacial period enriched in biogenic components by the advancing ice sheet might be transported through the melt-water plumes from the grounding line to the Central Basin, to provide high geochemical properties of deglacial sediments. Thus, growth and retreat of the grounded ice sheet played an important role in glaciomarine sedimentation change in the Central Basin of the northwestern Ross Sea.

Combined Use of Zirconium-Modified Bentonite Capping and Calcium Nitrate Addition to Control the Release of Phosphorus from Sediments

In this study, sediment incubation experiments were carried out to investigate the efficiency and mechanism of the control of phosphorus (P) release from sediments. The results showed that under anoxic conditions, P could be released from the sediment into the pore water first and then the dissolved P in the pore water could be transported into the overlying water, leading to high concentrations of soluble reactive P (SRP) and diffusive gradient in thin-films (DGT)-labile P in the overlying water. However, the combined use of calcium nitrate (CN) addition and zirconium-modified bentonite (ZB) capping could effectively control the release of P from sediment, resulting in the low concentrations of SRP and DGT-labile P in the overlying water. Furthermore, the combined use of CN addition and ZB capping could significantly decrease the concentrations of SRP and DGT-labile P in the sediment. In addition, the combined utilization of CN addition and ZB capping also could result in a reduction of redox sensitive P (BD-P) in the uppermost sediment layer. The reduction of pore water SRP, DGT-labile P, and BD-P in sediment solids is of great importance to the control of sediment-P liberation by the combined use of CN addition and ZB capping. The reduction efficiency of overlying water SRP by combined CN addition/ZB capping technology was higher than that of single CN addition technology. Compared to that of single CN addition technology, the reduction efficiencies of pore water SRP, SRP diffusion flux across the sediment/overlying water interface (SWI), and BD-P in the sediment by the combined use of CN addition and ZB capping were also higher. The combined technology based on CN addition and ZB capping had a higher reduction efficiency of overlying water SRP during the late stage of sediment remediation than the single technology based on ZB capping, and the former had higher reduction efficiencies of pore water SRP, DGT-labile P, and SRP diffusion flux across the SWI and apparent P diffusion flux through the SWI than the latter. The results of this work indicate that the combined use of CN addition and ZB capping is a very promising method for the control of P release from sediments.

Assessment of potential impact of invasive vegetation on cohesive sediment erodibility in intertidal flats

In-situ erodibility experiments were conducted to reveal the effects of vegetation on sediment stability in an intertidal flat. Spartina alterniflora (Spartina), one of the most widespread types of vegetation, led to complexity in sediment erodibility. The long stems and leaves of Spartina, which grew to approximately 156 cm from May to November 2019, were effective in trapping suspended sediments in the water columns, eventually promoting the deposition of approximately 2.3 cm within its communities. Sediments eroded by increasing bed shear stress (τb) mainly originated from sediments that were adhered to the stems and leaves of Spartina (May: 76%; November: 54%). They protected subsequent bed erosion against τb. However, this was only an apparent effect because the Spartina caused the erosion rate (E) to stagnate by suppressing the outflow of eroded mass from the bed. As the protective effect of the stems was removed, the uppermost sediment layers in the Spartina communities became more vulnerable to erosion by τb, with the initial erosion thresholds lowered to 0.1 Pa (May) and 0.05 Pa (November). Despite continuous sedimentation by sediment trapping, the sediment bed in Spartina communities had not been consolidated under repeated tidal inundation, showing no distinct development of the critical shear stress for erosion. Thus, the differences in E between cases with or without Spartina's stems reached approximately 1.22 × 10−6 (May) and 1.83 × 10−6 kg m−2 s−1 (November) at τb = 0.6 Pa. Results suggest that the increase in thickness of erodible layers mainly contributed more than sediment volume fraction to the enhancement of erosion potential with τb. This study highlights the necessity to assess both positive and negative effects of Spartina on the stability of sediment beds in intertidal flats.

СРЕДНЕ-ВЕРХНЕЧЕТВЕРТИЧНЫЕ ОТЛОЖЕНИЯ СЕВЕРНОГО СЕКТОРА КОНТУРИТОВОЙ СИСТЕМЫ КОНТИНЕНТАЛЬНОГО СКЛОНА ПАТАГОНИИ (ЮГО-ЗАПАДНАЯ АТЛАНТИКА)

Lithological and micropaleontological (planktonic and benthic foraminifers, diatoms) studies in four sediment cores revealed major patterns of lithostratigraphy and biostratigraphy of contourites and gravitites in the uppermost sediment layer within the depth range 3500–4300 m of the northern sector of the Patagonian continental slope contourite system. It is ascertained that in conditions of low terrigenous material supply from the land and high, although variable bottom contour current velocities of waters of Antarctic origin, hemipelagic calcareous nanno-foraminiferal ooze deposited in the Holocene over middleupper Pleistocene contourites in the southern part of the study area (cores AI–2609, AI–2610). The Holocene calcareous surface layer is absent in the northern part of the study area (cores AI–2611, AI–2612). Middle-Upper contourites and gravitites are unevenly enriched in diatoms and radiolarians here, up to formation of slightly siliceous-terrigenous diatomaceous mud. Reworked Neogene foraminifers and diatoms evidence for erosion of Neogene deposits bottom currents and gravity flows.

Changes in deep-sea benthic foraminiferal fauna caused by turbidites deposited after the 2011 Tohoku-oki earthquake

The impact of the 2011 Tohoku-oki earthquake and subsequent turbidites on the deep-sea environment was investigated by conducting sedimentological, radionuclide, and benthic foraminiferal analyses of core sediments collected from water depths of 3230 and ~3570 m, 5 and 17 months after the earthquake. Sedimentological profiles and patterns of excess 210Pb, 134Cs, and 137Cs indicate that the analyzed sediment core records deposits before the earthquake, turbidite deposits just after the earthquake, and deposits after the Fukushima Daiichi Nuclear Power Plant (FNPP1) accident, which caused the release of a large amount of radioactive material four days after the earthquake. Core 4W-2012 F-RN was collected at 3585 m water depth 17 months after the earthquake. Within this core, an event deposit layer (6.5 cm thick) consisting of turbidites contains highly diverse and possibly reworked foraminiferal assemblages. Overlying these turbidites are sediments with high 134Cs concentrations that were deposited after the FNPP1 accident. Overall foraminiferal and species abundances show a marked decrease in the sediments above the deposited turbidites. The abundance of opportunistic and competitive species shows a gradual upward increase in the uppermost sediment layers, and these species are dominant in the top 1 cm of the core. Our findings demonstrate that although foraminiferal assemblages in the sediment overlying the turbidites related to the 2011 earthquake might include allochthonous assemblages, turbidites deposited after the earthquake caused a marked decrease in benthic foraminiferal diversity and allowed opportunistic species to colonize in a low-diversity assemblage within 17 months. These foraminiferal faunal changes, together with analysis of sedimentological structure, provide valuable information for identifying these types of sedimentation events in core sediments from the deep-sea floor.

Seasonal changes in foraminiferal assemblages along environmental gradients in Adventfjorden (West Spitsbergen)

Foraminifera are numerically abundant components of the benthic fauna; thus, they are very important compounds in glaciomarine environments. However, ongoing oceanographic changes may influence their distribution, abundances and diversity differently at different times of the year. In the present study, we aimed to assess seasonal environmental changes in an Arctic fjord (Adventfjorden, W Spitsbergen) and the structure and distribution of benthic foraminiferal assemblages during four seasons (January, May, August, November). The study revealed that the benthic foraminiferal assemblages in Adventfjorden are susceptible to environmental changes related to enhanced inflow of shelf-transformed water (STW). We distinguished three zones in the fjord: (1) the deeper outer part influenced by STW, with living specimens staying in the few uppermost layers of sediment, (2) the shallower interior influenced by freshwater outflows and high sediment load, resulting in the dominance of opportunistic glaciomarine taxa constantly buried in the sediment, and (3) the central part, where the seasonal variation in the foraminifera assemblage reflects the interplay between the influence of riverine inputs and STW. Our study also revealed that the presence of STW in Adventfjorden favours an early spring appearance of juvenile foraminifera; however, the majority of the specimens are later withered and do not contribute to the summer pool of mature individuals. On the other hand, the majority of the mature specimens can survive during the winter because of sufficient organic matter stocks and year-long ice-free conditions.

Dynamics of Sulfate-Reducing Bacteria Community Structure in Surface Sediment of a Seasonally Hypoxic Enclosed Bay.

We herein report on the dynamics of a sulfate-reducing bacteria (SRB) community structure in the surface sediment of a seasonally hypoxic enclosed bay for two consecutive years (2012 and 2013). The uppermost (0–5 mm) and subsurface (5–10 mm) layers of sediment were examined with a terminal-restriction fragment length polymorphism (T-RFLP) analysis based on the dissimilatory sulfite reductase (dsrA) gene. The SRB community significantly differed between the two sediment layers over the sampling period. This difference was mainly attributed to operational taxonomic units (OTUs) that were unique to either of the sediment layers. However, nearly 70% of total OTUs were shared between the two layers, with a few predominating. Therefore, no significant shift was observed in the SRB community structure under varying dissolved oxygen (DO) conditions in bottom water overlying the sediment surface. An additional analysis of 16S rRNA gene amplicon sequences, conducted for three uppermost sediment samples (July, August, and September in 2012), revealed that Desulfococcus, a member of SRB with high tolerance to oxygen, was the predominant Deltaproteobacteria across the uppermost sediment samples. Based on the predominance of shared OTUs across the SRB community in the sediment (0–10 mm) regardless of bottom-water DO, some SRB that are physiologically tolerant of a wide range of DO conditions may have dominated and masked changes in responsive SRB to DO concentrations. These results suggest that the SRB community structure in the enclosed bay became stable under repeated cycles of seasonal hypoxia, but may be compromised if the severity of hypoxia increases in the future.

Effect of TiO2 and CeO2 nanoparticles on the metabolic activity of surficial sediment microbial communities based on oxygen microelectrodes and high-throughput sequencing

Environmental concerns regarding the potential ecological risks of metallic oxide nanoparticles (MNPs) in aquatic ecosystems are increasing; sediment is considered a sink for these MNPs. Although several studies have studied the potential impact of MNPs on microbial communities in freshwater and estuarine sediments, limited information is available regarding the influence of MNPs on the metabolic activity of surficial sediment microbial communities and related biogeochemical conditions. To address these issues, a microcosm approach was established to study the metabolic response of surficial sediment microbial communities to a single addition of TiO2 or CeO2 NPs (5 mg/L) using oxygen microelectrodes, enzyme activity measurements, and high-throughput sequencing. Rapid sedimentation of MNPs (regardless of NP type) was observed in freshwater samples, and most (up to 85%) accumulated in surface sediments (<5 mm). Microelectrode profile measurements in pre-incubated sediments treated with MNPs showed that the oxygen concentration decreased at a slower rate with increasing sediment depth compared to that in untreated controls. Biological oxygen consumption in the uppermost sediment layer (0–1500 μm) was significantly inhibited by MNPs, as calculated from steady-state microprofiles, with CeO2 NPs resulting in enhanced acute toxicity than TiO2 NPs. High-throughput sequencing showed that MNP exposure increased the bacterial diversity and altered the bacterial community structure, regardless of NP type. The abundance of three dominant bacterial genera, Methylotenera, Cytophagceae_uncultured (classified as an aerobic bacterium), and Cyanobacteria_norank (a facultative bacterium), was markedly reduced by MNPs, which was primarily responsible for inhibiting microbial-mediated oxygen consumption in surficial sediments. In summary, short-term exposure to MNPs negatively affected the metabolic activity of benthic microbial communities, which could influence the biogeochemical functions along the sediment-water interface.

Benthic nematode biodiversity of the Abzu, Tiamat and Michael Ivanov mud volcanoes located along the SWIM fracture zone (Gulf of Cadiz)

Studies that focus on meiofaunal assemblages of deep-sea mud volcanoes show an unpredictable abundance and diversity in a clear response to the different environmental conditions of the seeped sediment. The mud volcanoes Abzu, Tiamat and M. Ivanov (ATI), are located along the SWIM1 fracture zone, in front of the accretionary wedge of the Gulf of Cadiz (AWGC). The geological setting and the fluid geochemical characteristics of the ATI mud volcanoes are different from those located within the AWGC. The main aim of this study is to describe and compare the spatial and vertical distributions of the meiofauna and nematode assemblages from the ATI mud volcanoes, the Porto mud volcano located in the AWGC, and a non-seep site (Site 2) as reference. The pore-water on the uppermost sediment layers has compositions close to the near-bottom seawater. The meiofauna abundances were generally lower and the vertical distribution of the assemblages showed a typical pattern, gradually decreasing towards depth. The lack of spatial patterns of the standing stocks contrasts with the spatial variability of diversity and biomass, related to the differences in the nematode assemblages that are distinct between ATI, Site 2 and the Porto mud volcano. The ATI and Site 2 assemblages are similar to deep-sea non-seep habitats, and are clearly coupled with the environmental conditions of the bottom seawater. No evidence of seep conditions favouring the development of specialised fauna were found. The lower diversity and the presence of higher dominance species could be driven by distinct seepage conditions of the Porto mud volcano.

Land-use and hydroperiod affect kettle hole sediment carbon and nitrogen biogeochemistry

Kettle holes are glaciofluvially created depressional wetlands that collect organic matter (OM) and nutrients from their surrounding catchment. Kettle holes mostly undergo pronounced wet-dry cycles. Fluctuations in water table, land-use, and management can affect sediment biogeochemical transformations and perhaps threaten the carbon stocks of these unique ecosystems. We investigated sediment and water of 51 kettle holes in NE Germany that differ in hydroperiod (i.e. the duration of the wet period of a kettle hole) and land-use. Our objectives were 1) to test if hydroperiod and land management were imprinted on the isotopic values (δ13C, δ15N) and C:N ratios of the sediment OM, and 2) to characterize water loss dynamics and kettle hole-groundwater connectivity by measuring the stable δ18O and δD isotope values of kettle hole water over several years. We found the uppermost sediment layer reflected recent OM inputs and short-term processes in the catchment, including land-use and management effects. Deeper sediments recorded the degree to which OM is processed within the kettle hole related to the hydroperiod. We see clear indications for the effects of wet-dry cycles for all kettle holes, which can lead to the encroachment of terrestrial plants. We found that the magnitude of evaporation depended on the year, season, and land-use type, that kettle holes are temporarily coupled to shallow ground water, and, as such, kettle holes are described best as partially-closed to open systems.

Effects of drying on phosphorus uptake in re-flooded lake sediments.

Sediment drying associated with large water level fluctuations is an increasingly common feature of temporary streams and lakes worldwide. Drying-induced sediment aeration and re-flooding periodically alter redox conditions, and therefore stimulate redox-sensitive processes influencing phosphorus (P) binding forms. We experimentally tested the effects of drying on P binding forms, and the P sorption potential, by drying and re-flooding lake sediments in the laboratory. Wet and dried fine sediments were re-flooded in columns, and the overlying water was continuously re-stocked to a constant P concentration. We measured changes in P forms, P uptake rates, and the pore water dynamics in each column over 36 weeks. Drying decreased the fraction of stable P, stimulated the mineralization of organic P, and increased the proportion of labile and reductant-soluble forms. Drying of sediment furthermore reduced its P sorption affinity and capacity by up to 32% in batch equilibrium experiments, and led to a fourfold increase in sediment compaction which increased P uptake rates by a factor of 1.7 in sediment column experiments. Compaction due to drying also induced the development of a sharp gradient below which P was mobilized. These results indicate that in fine sediments, a single drying event can result in the transformation of P components into more labile forms which accumulate in the uppermost sediment layer, therefore raising the potential for a pulsed P release under reducing conditions.

A sedimentological model of organic-matter preservation and phosphogenesis in the Miocene Monterey Formation at Haskells Beach, Goleta (central California)

At Haskells Beach, west of Santa Barbara (CA, U.S.A.), the upper part of the Miocene Monterey Formation is predominantly composed of organic-rich mudstone, which is interstratified with phosphatic laminae and lenses. Minor lithologies consist of dolomite, volcanic ash, porcelanite, chert and condensed phosphate. These sediments date from the end of the Serravallian and almost the entire Tortonian (11.05–7.85Ma) based on calcareous nannofossils and the δ13Corg record, which correlates well with the global record. Sediments have total organic-carbon values between 2.75 and 9.15wt.% (average value=6.48 wt.%; n=61). Rock-Eval analyses show the dominance of type-II kerogen. The sediment accumulation rate varied between 5.9 and 84.7m/Ma. Correspondingly, organic-carbon accumulation rates range between 0.08 and 1.43 g/cm2/ky (average value=0.48 g/cm2/ky). Total phosphorus contents in the organic-rich mudstone vary between 0.001 and 3.48 wt.% and Corg/Porg ratios show high values of up to 4727 (average value=2232), whereas Corg/Ptotal values are low (up to 185; average value=52). This indicates that early diagenetic organic-matter degradation was a source of phosphate in phosphogenesis. In addition, other sources of phosphate were necessary in order to explain the abundance of phosphate relative to organic matter such as the transfer of dissolved inorganic phosphate into the sediments. Slumps, angular unconformities, erosive surfaces, reworked clasts and nodules, and condensed phosphatic layers suggest that hydrodynamic conditions were important and likely variable, leading to frequent erosion and sediment reworking. Under these circumstances, organic-matter was predominantly delivered during gravity-flow events, which were followed by longer periods of low sediment accumulation and phosphogenesis of the uppermost sediment layer. Associated pore occlusion by phosphate minerals may have considerably enhanced the preservation of organic-matter. During the early Late Miocene, a progressive change is observed from the mixed accumulation of organic, phosphate, biosilica and carbonate sediments, to the predominant accumulation of biosilica-rich sediments. This change is related to more intense upwelling, a shift of the upwelling center to the Californian coastline, and a leveling out of basin topography leading to a more even spread of biosilica.

Element remobilization, "internal P-loading," and sediment-P reactivity researched by DGT (diffusive gradients in thin films) technique.

Labile P, Fe, and sulfide with the high spatial resolution in sediment porewater of five sites (A-E) of Dianchi Lake (China) were measured at same locations using AgI/Chelex-100, Chelex-100, and ferrihydrite DGT (diffusive gradients in thin films) probes. DGT derived P/Fe/S concentrations in sediment porewater on millimeter or sub-millimeter scale in order to reveal the element remobilization process and the mechanism of "internal P-loading" of sediments in Dianchi Lake. Decomposition of alga biomass in the uppermost sediment layer and the reductive dissolution of Fe-bound P in the anoxic sediment were the two main processes causing P release. A dynamic numerical model-DIFS (DGT-induced flux in sediments) was used to assess sediment-P reactivity (capacity of solid pool and rate of transfer) and P release risk by kinetic parameter-T C (1089∼20,450 s), distribution coefficient-K d (167.09∼502.0 cm(3) g(-1)), resupply parameter-R (from 0.242 to 0.518), and changes of dissolved/sorbed concentration, R and M at the microzone of DGT/porewater/sediment.

Interlinked seasonal variation in biogenic nutrient fluxes and pore-water nutrient concentrations in intertidal sediments

The seasonal variation in biogenic fluxes of NH4 +, PO4 3− and SiO2 calculated from the nutrient excretion rates of dominant bivalves (Ruditapes philippinarum and Arcuatula [=Musculista] senhousia), and pore-water nutrient (NH4 +, PO4 3−, SiO2 and NO3 − + NO2 −), sedimentary acid-volatile sulfide (AVS) and benthic chlorophyll-a (Chl-a) concentrations was assessed on an intertidal sandflat in the Seto Inland Sea (Japan) from summer 1994 to autumn 1995. In spite of the large variability between sampling dates and stations, significant correlations between biogenic nutrient fluxes and pore-water nutrient concentrations were found, suggesting a seasonal linkage between bivalve-mediated biological processes and chemical features of sediments. This linkage was stronger in surface (0–0.5 cm) than subsurface (0.5–2 cm) sediments, consistent with the autoecological characteristics of R. philippinarum and A. senhousia inhabiting the uppermost sediment layer. Significant temporal variation in pore-water NO3 − + NO2 −, sedimentary AVS and benthic Chl-a concentrations was also found, which was related to both occasional extreme events (e.g., dystrophy) and alternating periods of production and decomposition. This study may serve twofold in (1) contributing to unravel the ecological structure and functioning of natural tidal flats, and the scale of seasonal variability in biotic and sedimentary parameters and (2) providing useful information for assessing the effectiveness of the physico-chemical and biological structure of artificial tidal flats which are growing in number and extension worldwide.

Trace-fossil assemblages with a new ichnogenus in “spotted”

Abstract Highly-bioturbated “spotted” limestones and marls (Fleckenmergel-Fleckenkalk facies) of the Early Jurassic, which were deposited in broad and recurrent deep-shelf habitats of the Northern Tethys, are characterized by rare benthic carbonate-producing macroinvertebrates. To address this paradox, we analyse trace-fossil assemblages in a ~85 m-thick succession of Pliensbachian spotted deposits (Zliechov Basin, Western Carpathians). They are dominated by infaunal and semi-infaunal deposit-feeders, with 9 ichnogenera and pyritized tubes of the semi-infaunal foraminifer Bathysiphon, being dominated by Chondrites, Lamellaeichnus (new ichnogenus), and Teichichnus. Lamellaeichnus, represented by a horizontal basal cylindrical burrow and an upper row of stacked convex-up gutters, was produced by a mobile deposit-feeder inhabiting shallow tiers because it is crossed by most other trace fossils. We show that the spotty appearance of the deposits is generated by a mixture of (1) dark, organic-rich shallow- and deep-tier traces (TOC = 0.16-0.36), and (2) light grey, organic-poor mottled or structurless sediment (TOC = 0.09-0.22). The higher TOC in shallow-tier burrows of Lamellaeichnus demonstrates that uppermost sediment layers were affected by poor redox cycling. Such conditions imply a limited mixed-layer depth and inefficient nutrient recycling conditioned by hypoxic bottom-waters, allowed by poor circulation and high sedimentation rates in depocenters of the Zliechov Basin. Hypoxic conditions are further supported by (1) dominance of trace-fossils produced by infaunal deposit feeders, (2) high abundance of hypoxiatolerant agglutinated foraminifer Bathysiphon, and (3) high abundance of Chondrites with ~0.5 mm-sized branches. Oxygen-deficient bottom-conditions can thus simultaneously explain the rarity of benthic carbonate-producing macroinvertebrates and high standing abundance of tolerant soft-shell and agglutinated organisms in spotted deposits.

Benthic foraminiferal community changes and their relationship to environmental dynamics in intertidal muddy sediments (Bay of Cádiz, SW Spain)

The temporal and vertical variability of live intertidal benthic foraminifera was studied in parallel to environmental variables such as porewater and overlying water nutrients, salinity, temperature, sedimentary pigments and organic carbon. About 80% of benthic foramini - fera were concentrated in the upper 0�4 mm sediment layer, which was also the maximum oxygen penetration depth under light conditions as determined by oxygen microsensor measurements. Significant changes in the composition of the foraminiferal community over the course of the study were found in the surface layer (0�2 mm), whereas no differences were detected in the deepest sediment layers (10�30 mm). Univariate measures, such as abundance, species richness and diver- sity showed few differences over time and little relation to environmental variables. On the other hand, multivariate regression analysis for the whole community assemblage showed that the significant community composition changes over time in the surface layer could be related to fluctuations of several environmental variables, mainly those related to sediment food availability. The macroalgal bloom that occurred during the winter months had no detectable effect on benthic foraminiferal abundance or community patterns. Overall, the intertidal foraminiferal community in the Bay of Cadiz showed a highly dynamic character and a clear vertical distribution which could be effectively captured by studying the uppermost sediment layer at sub-centimetric vertical resolution.

Historical environmental pollution trend and ecological risk assessment of trace metals in marine sediments off Adyar estuary, Bay of Bengal, India

Geochemical, mineralogical and textural analyses were carried out in core sediments off Adyar estuary, Bay of Bengal, India to record the contamination trend from urban and industrial activities during the historical past. Quartz, feldspar, kaolinite, chlorite and illite were the main lithogenic and clay minerals; carbonate was the predominant biogenic mineral. Trace metals (Fe, Al, Cu, Cr, Ni, Pb and Zn) indicate more enrichment in the surface sediment layers due to recent anthropogenic activities. The mean anthropogenic factor (AF) values for trace metals in core sediments decreased in the following order: Cr > Ni > Zn > Cu > Pb. The pollution load index (PLI) values in Adyar core sediments ranged from 1 to 1.25 with an average of 1.07. Based on AF, PLI, and sediment quality guidelines values for trace metals, significant metal enrichment and ecological risk were obtained in upper-most sediment layer. Multivariate statistical methods such as correlation matrix, principal component analysis and cluster analysis were carried out to find the relationships among the texture size, metals and minerals. The pollution of Adyar estuarine sediments was started in the 1960s, responding to the rapid economic development in Chennai coastal and Adyar estuarine region in the last five decades. Despite these high concentrations in the upper layer, development and expansion of industries are still continuing. The stricter regulations for the discharge and remediation of sediments are urgent for the conservation of environments and human health.