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While it is recognised that the nature of the sediment is a major driver of benthic macro-infaunal community structure, it is also true that diverse environmental factors determine the distribution and composition of sediments. Among those factors are depth, tidal stress and seasonal stratification of the water column. In the Celtic Sea an area of seabed approximately 20 km wide and 125 km long was selected in which variation in water depth, stratification, primary production and current velocity were minimised, but which contained sediments ranging from fine muds to coarse gravelly sands. 55 stations were sampled across the area using a box-corer. At each station a comprehensive suite of sediment and biogeochemical measurements were made. Macrofauna were identified and weighed. Of the stations sampled, four had been chosen as focal sites for a study of relationships between benthic biogeochemistry and sediment type. Relationships between variation in environmental and sediment variables and macrofaunal community structure were analysed using a range of non-parametric multivariate techniques. Environmental variables were discriminated into situational variables that broadly encapsulate potential drivers of spatial heterogeneity in the benthos such as depth and fishing effort, and in-situ variables that were measured at each site concurrently with the sampling of the macrobenthos, such as sediment properties and biogeochemical measurements. Among the former, analyses tended to identify the importance of average shear stress and depth in explaining observed variation in benthic community structure, even though the area had been chosen to minimise variation in those factors. Analyses using in-situ measurements of sedimentary conditions at each site identified very fine sand content (correlated with average shear stress) as the most important explanatory variable. Most of the measured biogeochemical variables varied with sediment structure, particularly reflecting differences between finer-grained sediments with higher organic content (generally from deeper areas) and coarser sediments with lower organic content. While clear spatial heterogeneity in sediments and associated biogeochemical variables could be demonstrated, spatial variation in benthic abundance and biomass was less clear. Benthic community structure varied significantly with sediment type, but did not vary closely with the in-situ environmental variables measured at the same sites. This may indicate that the samples collected were too small to accurately characterise the benthic assemblage at each site, or that most species inhabiting the area inhabit a range of sediment types, or that processes which are not reflected in sediment or biogeochemical measurements are also important determinants of benthic community structure. • 55 stations were sampled in the Celtic Sea using a box corer. • Stations represented a range of environmental and sediment conditions. • Infaunal microbenthic abundance and biomass were determined. • Infaunal communities varied weakly with sediment type. • Variation in communities did not closely match measured environmental variables. [ABSTRACT FROM AUTHOR]

Sediment transport over Sandy Point dredge pit in the northern Gulf of Mexico during a cold front event in November 2014 was examined using a finely resolved numerical model. The Delft3D model was used to perform numerical experiments that simulate the effect of wind-generated waves, wind-driven currents, river discharge, and tides on sediment dynamics in Sandy Point dredge pit. The hydrodynamics and sediment models were validated and calibrated using field data of current, wave, water level, and suspended sediment concentration. Two potential sources of sediment were examined: fluvial sediment from the Mississippi River and resuspended sediments from the seabed. Results showed that during a cold front, shear stress from wave motions played a significant role in the resuspension of sediments in Sandy Point dredge pit. The maximum cold front-related wave impact on sediment resuspension could increase near-bed sediment concentration in Sandy Point dredge pit by 20–50 times. In addition, the results suggest that the primary source of sediment for Sandy Point dredge pit during a cold front was resuspension from the ambient seabed due to increased bottom shear stress by wind-induced waves and strong southward wind-driven currents. Currents dispersed sediments from the Mississippi River passes and inhibited riverine sediment supply from Sandy Point dredge pit. Results also showed that cold fronts contribute 16%–24% of the annual sedimentation in Sandy Point dredge pit. • A coupled wave-current-sediment model was used to study the sandy dredged pit. • The combined effect of waves and currents on sedimentation was assessed. • The reverse bottom current inside the pit was identified. • Bottom boundary layer was significantly affected by waves. • Sediment accumulation occurs mostly on the north of the pit. [ABSTRACT FROM AUTHOR]

Compound deltas, composed of a subaerial delta plain and subaqueous clinoform, are common termini of large rivers. The transition between clinoform topset and foreset, or subaqueous rollover point, is located at 25–40-m water depth for many large tide-dominated deltas; this depth is controlled by removal of sediment from the topset by waves, currents, and gravity flows. However, the Mekong Delta, which has been classified as a mixed-energy system, has a relatively shallow subaqueous rollover at 4–6-m depth. This study evaluates dynamical measurements and seabed cores collected in Sep 2014 and Mar 2015 to understand processes of sediment transfer across the subaqueous delta, and evaluate possible linkages to geometry. During the southwest rainy monsoon (Sep 2014), high river discharge, landward return flow under the river plume, and regional circulation patterns facilitated limited sediment flux to the topset and foreset, and promoted alongshore flux to the northeast. Net observed sediment fluxes in Sep 2014 were landward, however, consistent with hypotheses about seasonal storage on the topset. During the northeast rainy monsoon, low river discharge and wind-driven currents facilitated intense landward and southwestward fluxes of sediment. In both seasons, bed shear velocities frequently exceeded the 0.01–0.02 m/s threshold of motion for sand, even in the absence of strong wave energy. Most sediment transport occurred at water depths <14 m, as expected from observed cross-shelf gradients of sedimentation. Sediment accumulation rates were highest on the upper and lower foreset beds (>4 cm/yr at <10 m depth, and 3–8 cm/yr at ~10–20 m depth) and lowest on the bottomset beds. Physically laminated sediments transitioned into mottled sediments between the upper foreset and bottomset regions. Application of a simple wave-stress model to the Mekong and several other clinoforms illustrates that shallow systems are not necessarily energy-limited, and thus rollover depths cannot be predicted solely by bed-stress distributions. In systems like the subaqueous Mekong Delta, direction of transport may have a key impact on morphology. [ABSTRACT FROM AUTHOR]

The San Jorge Gulf is an ecosystem of great economic importance in the southern hemisphere. It is a breeding and spawning area for various fish and crustacean species. In particular, the Pleoticus muelleri shrimp in different stages of its life cycle and the benthic system are linked. Therefore, in recent decades, it has been of interest for this shrimp resource to carry out interdisciplinary studies focused on the properties of the marine substrate. The concentration of single domain + multidomain magnetite, paramagnetic minerals, and organic matter of seabed sediments vary widely within a study area of about 60400 km2. Values of specific magnetic susceptibility (χ), anhysteretic ratio (χ ARM /χ), total organic carbon (TOC), and total nitrogen (TN) range between 18.3–323.1 × 10−8m3kg−1, 1.5–13.2, 0.14–1.58%, and 0.02–0.19%, respectively. The spatial correlations between TOC and TN with χ (ρ K = −0.72 and −0.59, respectively) and χ ARM /χ (ρ K = 0.90 and 0.71) for these seabed sediments provide good magnetic proxies, which can be determined readily and with low cost. The spatial distribution of geochemical variables and magnetic proxies shows well-delimited areas concerning coastal, inner gulf, eastern, and northeastern zones, which could benefit the management of shrimp resource. [Display omitted] • Total organic carbon and total nitrogen are inversely correlated with magnetic susceptibility χ. • Diverse benthic communities are related to higher TOC and lower χ values. • Well-delimited areas by magnetic proxies could benefit shrimp fishery management. [ABSTRACT FROM AUTHOR]

Seafloors of unconsolidated sediment are highly dynamic features; eroding or accumulating under the action of tides, waves and currents. Assessing which areas of the seafloor experienced change and measuring the corresponding volumes involved provide insights into these important active sedimentation processes. Computing the difference between Digital Elevation Models (DEMs) obtained from repeat Multibeam Echosounders (MBES) surveys has become a common technique to identify these areas, but the uncertainty in these datasets considerably affects the estimation of the volumes displaced. The two main techniques used to take into account uncertainty in volume estimations are the limitation of calculations to areas experiencing a change in depth beyond a chosen threshold, and the computation of volumetric confidence intervals. However, these techniques are still in their infancy and, as a result, are often crude, seldom used or poorly understood. In this article, we explored a number of possible methodological advances to address this issue, including: (1) using the uncertainty information provided by the MBES data processing algorithm CUBE, (2) adapting fluvial geomorphology techniques for volume calculations using spatially variable thresholds and (3) volumetric histograms. The nearshore seabed off Warrnambool harbour – located in the highly energetic southwest Victorian coast, Australia – was used as a test site. Four consecutive MBES surveys were carried out over a four-months period. The difference between consecutive DEMs revealed an area near the beach experiencing large sediment transfers – mostly erosion – and an area of reef experiencing increasing deposition from the advance of a nearby sediment sheet. The volumes of sediment displaced in these two areas were calculated using the techniques described above, both traditionally and using the suggested improvements. We compared the results and discussed the applicability of the new methodological improvements. We found that the spatially variable uncertainty derived from the CUBE algorithm provided the best results (i.e. smaller confidence intervals), but that similar results can be obtained using as a fixed uncertainty value derived from a reference area under a number of operational conditions. [ABSTRACT FROM AUTHOR]

While the nature of the suspended load above steep, wave-induced, sand ripples is of practical importance, it also raises intriguing questions about the relative mixing efficiencies of sediment and momentum above the seabed. It has been widely accepted that the mixing efficiency of sediment is substantially greater than that of momentum. But, hitherto, this has not been explained clearly in terms of the underlying, detailed physical mechanisms which revolve around the generation and ejection of sediment-laden vortices at the ripple crest, and their subsequent advection by the flow. A two-dimensional discrete-vortex, particle-tracking research model, with the parameter settings corresponding to a well-documented laboratory experiment, is used here to represent these processes. Both the modelled and also experimental flow and concentration fields are described in detail, together with the horizontally (ripple-) averaged fields, and the cycle-mean, ripple-averaged fields. From these considerations, the ratio ( β ) of the sediment diffusivity to the eddy viscosity, or the inverse of the Schmidt number, is then determined. It is found that β is larger than unity, in fact between 1.3 and 3.1 for two different computational approaches (based on harmonics and exponential fitting) for the model and data. These values for β agree well with previous results reported in the literature. This research elucidates, from fundamental principles related to spatio-temporal correlations between concentration and velocity, the improved efficiency of sediment mixing compared with momentum mixing in the vortex layer above rippled beds and its key role in determining suspension profiles in such flows. [ABSTRACT FROM AUTHOR]

This study investigates the relationship between spatial and temporal patterns of wave-driven sediment mobility events on the U.S. East Coast continental shelf and the characteristics of the storms responsible for them. Mobility events, defined as seafloor wave stress exceedance of the critical stress of 0.35 mm diameter sand (0.2160 Pa) for 12 or more hours, were identified from surface wave observations at National Data Buoy Center buoys in the Middle Atlantic Bight (MAB) and South Atlantic Bight (SAB) over the period of 1997–2007. In water depths ranging from 36–48 m, there were 4–9 mobility events/year of 1–2 days duration. Integrated wave stress during events (IWAVES) was used as a combined metric of wave-driven mobility intensity and duration. In the MAB, over 67% of IWAVES was caused by extratropical storms, while in the SAB, greater than 66% of IWAVES was caused by tropical storms. On average, mobility events were caused by waves generated by storms located 800+ km away. Far-field hurricanes generated swell 2–4 days before the waves caused mobility on the shelf. Throughout most of the SAB, mobility events were driven by storms to the south, east, and west. In the MAB and near Cape Hatteras, winds from more northerly storms and low-pressure extratropical systems in the mid-western U.S. also drove mobility events. Waves generated by storms off the SAB generated mobility events along the entire U.S. East Coast shelf north to Cape Cod, while Cape Hatteras shielded the SAB area from swell originating to the north offshore of the MAB. [ABSTRACT FROM AUTHOR]

We extend a numerical model to explore the morphodynamics of intertidal flats, with a specific focus on the sorting dynamics of sand and mud. We investigate the effect of tidal currents, wind waves, sediment properties, external sediment supply, flocculation and initial bed composition on the cross-shore profile shape and sediment sorting of intertidal flats. Consistent with existing analytical theories and benchmark simplified numerical solutions, the equilibrium cross-shore profile of intertidal flats simulated by the extended model is convex-up when tidal currents dominate and it progressively becomes concave-up when the strength of wind waves increases. The equilibrium profile is influenced by the external sediment supply which can lead to the seaward advance of intertidal flats. In line with field observations, mud tends to deposit on the upper intertidal flats when wind waves are relatively weak, while sand is mainly distributed on the middle and lower tidal flats. When wind waves are strong, both sand and mud are more easily resuspended and eroded, resulting in a noticeably concave-up profile near the high water mark. The initial bed composition (e.g., percentage of mud and sand fractions) is also found to play an important role: the intertidal flat is more convex-up in a muddier environment. Numerical modeling demonstrates that sediment properties (e.g., critical shear stress for erosion, settling velocity) and flocculation can pronouncedly influence the sediment sorting dynamics by modifying the initiation threshold and the advection distance of entrained sediments. Application of the extended model to a natural study site indicates a qualitative agreement with field observations. [ABSTRACT FROM AUTHOR]

Since her shipwreck in 1771, the wooden vessel Vrouw Maria has been lying on the seabed at 41 m depth on the border between the Archipelago Sea and the northern Baltic proper, off the southern coast of Finland. The wreck lies in an upward position in a right angle to the dominating bottom current, which is potentially affecting seabed topography, sediment characteristics and zoobenthic communities both upstream (NE) and downstream (SW) of the wreck. This multidisciplinary study attempts to clarify abiotic and biotic patterns and processes in the vicinity of the wreck by combining field investigations with physical simulation studies in the field and in the laboratory. Multibeam echo-sounder techniques were utilised to generate a map of the wreck area and sediment grab samples were taken to characterize the sediment type and its zoobenthic community. A medium-sized field experiment generated data on the accumulation of sediment organic matter in the presence and absence of a current and/or a barrier on the seafloor and a small-scaled laboratory study was conducted to simulate scour forming processes. The results showed that a deeper basin, scour area, with the dimensions 150×300 m, was present downstream of the wreck and there was also a smaller scour area upstream of the wreck. Similar traces on the bottom were simulated in the laboratory. The organic content (recent mud) and the proportion of finer sediment were more pronounced in areas closer to both sides of the wreck. These results were in turn imitated in a field experiment, where the accumulation of organic matter in the sediment increased significantly downstream, when a current was interrupted by a barrier. Regarding zoobenthos in the wreck area, 11 taxa and a mean total abundance of 773 individuals per m 2 were registered. The dominant species were Macoma balthica and Marenzelleria sp., which together made up >80% of the total number of individuals. Multivariate data analyses showed significant differences in community structure between upstream and downstream sides of the wreck and these differences were basically expressed as more individuals upstream of the wreck of the seven most common species. Univariate data analyses also showed significantly more species and higher total abundance in the scour area upstream of the wreck compared to the downstream scour area. The weakened communities downstream of the wreck were possibly due to accumulation of organic matter and more frequent occurrence of hypoxia. The results are useful for describing bottom areas around long-term physical barriers/artificial reefs, as background measurements ahead of possibly increased archaeological diving activities, sediment excavations or measures taken to raise the wreck to the surface, as well as for any attempts to preserve the wreck in situ. [ABSTRACT FROM AUTHOR]

Abstract: Methane in the sediments of the Bay of Puck occurs in the form of free gas bubbles and is released from the sediments into near-bottom water in the form of a flux. The sediments of the Bay of Puck also contain methanogens whose biological activity results in the production of methane. Research carried out in the coastal areas of the bay along the Hel Peninsula proved the existence of a methane flux ranging from 0.81mmolm−2 d−1 to 33.41mmolm−2 d−1 in 2011, while in 2010 ranged from 0.91mmolm−2 d−1 to 49.15mmolm−2 d−1. Seasonal and annual fluctuations were also observed. Other factors contributing to the change in methane flux are water temperature and other environmental factors. An increase in temperature causes an increase in the flux. Fluctuations of the flux within individual sampling sites were also observed. A test performed with specific methanogenic inhibitor—sodium dodecyl sulfate (SDS) unexpectedly revealed higher methane concentrations (10–13%) in samples to which SDS was added. We speculate that these higher methane concentration result from the lysis of methane-forming bacteria cells when exposed to SDS. [Copyright &y& Elsevier]

Sediment mapping is important for understanding the physical processes, the impact of human activity, and the conditions for marine life on the seabed. For this purpose, the seabed classification tool QTC IMPACT analyses statistical variations in single-beam echo sounder data. QTC was applied in a large and physically diverse area of the Norwegian Channel, between 59°30′N and 61°N, to produce a new sediment map and to verify the QTC algorithm. The results were interpreted using ground truth (grain size analyses of 40 gravity cores and five grab samples), multi-beam echo sounder bathymetry (MBES), and seismo-acoustic profiles. Surficial sediments were divided into five classes: (1) mud and silt, (2) a variety of clay, silt and sand, (3) sandy mud with gravel, (4) sand with gravel, and (5) clay and sandy clay. Along the Norwegian coast, where MBES imagery shows evidence of glacial erosion, the surficial sediment distribution is variable. The echo shape analysis of QTC did not produce a natural partition of the data, and statistical assumptions did not always hold. Sediment classification was therefore sensitive to the choice of cluster algorithm. However, QTC produced the most physically plausible results on a large scale compared to other cluster algorithms. Class boundaries were consistent with supporting data. One exception is a transition from muddy to sandy sediments not visible in seismo-acoustic data. A possible explanation is that seabed fluid seepage and water current erosion cause sand particle transport into the western part of the channel. The study confirms the capability of QTC in a complex environment, but there are some possible improvements. [ABSTRACT FROM AUTHOR]

The seafloor is increasingly being used for siting renewable energy and telecommunication infrastructure as well as supporting key fisheries and biodiversity. Understanding seabed stability and sediment dynamics is, therefore, a fundamental need for offshore engineering and geoscience and biological studies. In this study we aim to quantify the levels of sediment mobility in the Irish Sea: an area of increasing socio-economic interest and subsequent seabed pressures. The temporal and spatial interaction between bathymetry, hydrodynamics and seabed sediments leads to a complex pattern of erosion, bedload transport and deposition which can affect seabed infrastructure and modify habitats. Information on current and wave conditions were obtained from numerical modelling to assess their role in generating seabed hydrodynamic conditions. These outputs were coupled with observed seabed grain-size data to predict the exceedance of sediment mobility thresholds by bed shear stress values for a period of one year according to empirical formulae. Exceedance frequency values were used to calculate a number of sediment disturbance and mobility indexes to allow for a robust assessment of sediment dynamics. Sediment in the Irish sea, on average, is being mobilised 35% of the time during the year, with 35% of the spatial area studied being mobilised over 50% of the time. Even in areas of low sediment mobilisation frequency (<5%), there are implications for bedform dynamics. The spatial patterns of the calculated sediment mobility are discussed in the context of current seabed geomorphology and the implications for both engineering and environmental considerations. • A 2D hydrodynamic model and a spectral wave model were developed to assess sediment mobilisation in the Irish Sea. • Sediment mobility indices calculated for combined wave-current sediment mobilisation. • 35% of the sediment coverage spatially within the study area is mobilised 50% of the year, some areas higher than 90%. • Only 2% of the study area was calculated as experiencing 0% sediment mobility. • Sediment mobility maps can be used in siting offshore infrastructure and to inform effective marine management. [ABSTRACT FROM AUTHOR]

Abstract: We examined the spatial distribution of sedimentary reactive iron (FeR) and manganese (MnR) along the continental shelf near the mouth of the Umpqua River, Oregon (USA). A well-defined muddy (silt+clay) depocenter of fluvial origin characterizes this part of the Oregon margin. Reactive Fe and Mn contents are elevated within the silt-rich landward edge of the depocenter. Away from this depocenter, sediments are predominantly sandy both along the inner-shelf (<∼100m depth) and mid-shelf (∼100–150m depth) and have lower concentrations of reactive metals compared to the depocenter. Sediments are also muddy along the slope (>∼150m depth) and have elevated FeR and MnR. Based on their correlation with sediment grain size, it appears that FeR and to a lesser extent MnR, are associated with mud size sediments. Reactive metal concentration is also positively correlated with organic carbon (OC) content, indicating a potentially common source. Seabed sediments from five other small, mountainous river systems (Klamath, Eel, Navarro, Russian, and Salinas) located south of Umpqua show the same general relationship between FeR and OC. Although both FeR and MnR exhibit similar relationships to grain size and OC, the relationships with MnR exhibit considerable scatter. Comparison of Umpqua River suspended sediment data with the seabed data suggests that MnR is more prone to loss from sediment particles during transit to the seabed as compared to FeR, and this difference explains why FeR maintains a reasonably tight relationship with organic carbon and particle size along the seafloor relative to MnR. [Copyright &y& Elsevier]

Abstract: The Minas Basin, the eastern end of the Bay of Fundy, is well known for its high tide ranges and strong tidal currents, which can be exploited to extract electricity power. The properties of the tidally-induced sediment transport in the Minas Basin, where significant changes in tidal processes may occur due to a recently proposed tidal power project, have been studied with a three-dimensional hydrodynamic model, an empirical bed load sediment transport model and surface sediment concentrations derived from the remotely-sensed images. The hydrodynamic model was evaluated against independent observational data, which include tidal elevation, tidal current (in the full water column and bottom layer), residual current profile and tidal asymmetry indicators. The evaluation shows that the model is in good agreement with the observations. The sediment transport includes two components, bed load and suspended particulate load. The bed load is calculated using the modelled bottom shear stress and the observed grain size data. The estimated features of bed load transport roughly agree with the observed patterns of the erosion and deposition in the Minas Basin and Cobequid Bay. The transport of the suspended load is estimated using the modelled velocity fields and the surface sediment concentration derived from remote-sensing images. The comparisons between the modelled results and the limited observations illustrate that the observed directions of suspended sediment transport are basically reproduced by the model. The modelled net suspended sediment input into the Minas Basin through Minas Passage is 2.4×106 m3 yr−1, which is comparable to the observed value of 1.6×106 m3 yr−1. The variations of the bed load and the suspended load in space and time are also presented. The total net transport, defined as the mean value of the sum of bed and suspended load transports during the tidal cycle, shows strong spatial variability. The magnitude of the transport flux ranges from 0.1 to 0.2kgm−1 s−1 in Minas Channel and Minas Passage, 0.1kgm−1 s−1 in Cobequid Bay, to 0.01kgm−1 s−1 in the central Minas Basin and Southern Bight. In Minas Channel, the sediment transport follows the structure of the tidal residual circulation, which features a large anticlockwise gyre. The sediment in Minas Passage moves eastward and deposits into the central Minas Basin. However, the sediment from the eastern part of the Basin moves westward and deposits in the central Minas Basin as well. In the Cobequid Bay, sediment moves eastward and deposits in the upper bay. [Copyright &y& Elsevier]

Abstract: We investigated the spatial distribution and composition of microbenthos in the seafloor sediments from 48 stations in the Yellow Sea using epifluorescence microscopy and quantitative protargol staining techniques. The bacterial abundance ranged from 2.4×108 to 1.9×109 cellscm−3 in the wet sediment, about three orders of magnitude higher than that of phototrophic (PNFs, from 6.4×105 to 8.8×106 cellscm−3) and heterotrophic nanoflagellates (HNFs, from 5.8×104 to 5.9×106 cellscm−3) and four orders of magnitude higher than that of cyanobacteria (from 2.3×104 to 2.3×106 cellscm−3) in the upper 5cm of sediments. The abundance of diatoms varied greatly, from 3–1.1×105 cellscm−3 in the upper 8cm of sediments, whereas those of heterotrophic microflagellates (HMFs, 1–182cellscm−3) and ciliates (1–221cellscm−3) were less varied and lower. The biomass partitioning indicates the primary importance of benthic bacteria (50.3μgCcm−3 on average), followed by PNFs (40.7μgCcm−3), HNFs (19.3μgCcm−3), and finally by cyanobacteria (8.8μgCcm−3). Benthic diatoms (0.8μgCcm−3), ciliates (0.15μgCcm−3), and HMFs (0.03μgCcm−3) contribute relatively small fractions to the total biomass of the microbenthos. About 95% of diatoms, 77% of ciliates, and 56% of HMFs were distributed in the upper 2cm of sediments, whereas no distinct vertical distributions were observed for bacteria, cyanobacteria, PNFs, and HNFs. The microbenthos are quantitatively important in the shallow seafloor, wherein their main components have an average abundance three orders of magnitude higher than the corresponding planktonic organisms in the same sea area. Our estimates indicate that pico-sized phytobenthos might contribute a large proportion to the primary production. Benthic ciliates and heterotrophic flagellates contribute about 90% to the estimated combined metabolic rate of micro- and meiobenthic consumers in the whole sea area, with nanoheterotrophs accounting for the majority. The data suggest the potential for the rapid primary and secondary production of microbenthos and detrital utilization in the shallow seafloor sediments of the Yellow Sea. [Copyright &y& Elsevier]

Abstract: Concerns about human-induced degradation of marine ecosystems are presently being translated into European Directives aimed at a restoration of healthier seas. However, anthropogenic alterations are ancient and data on the pristine ecosystem are lacking, making the evaluation of their cumulative effects in the long run challenging. In the Belgian and Dutch parts of the North Sea, a historic data-set (the Gilson’s archive) exists to describe various compartments of the marine ecosystem in the first decade of the 20th century (1899–1914). The bulk of samples were acquired in the nearshore (up to 10 nautical miles off the coast) and further offshore in the Hinder banks area. In this contribution, we focus on the sediment information, which consists of 2200 visual descriptions of geo-referenced seafloor samples and 690 archived sub-samples, to reconstruct the seafloor composition 100 years ago. Estimates of mud levels, sand grain size, shell debris levels as well as occurrence of gravel, pebbles and cobbles were derived from the descriptions and standardized. Mud levels and sand grain-size categories were calibrated by means of grain-size analysis of archived samples (n=383 analyses). Large patches of very high mud levels (50–100%) used to occur close to the shore, along the central and eastern coast. The visual sand grain-size categories display a significant trend in their median grain-size, enabling us to identify distinct patches of fine (<250μm) and medium (250

Abstract: Appropriate descriptions of turbulence are important in predicting sediment transport and seabed evolution. We use here a three-dimensional model to investigate the impacts of sediment-induced stratification, erosion parameterization, and turbulence closures on sediment transport and morphological simulations. The model is implemented in the Proudman Oceanographic Laboratory Coastal Ocean Modelling System (POLCOMS), which couples a hydrodynamic model to the General Ocean Turbulence Model (GOTM) and to a sediment transport model. This sediment transport model calculates both suspended sediment concentrations and bed evolution. A large number of turbulence closures can be tested and we focus on the methods used to calculate the turbulent length scale and the stability functions. Different modelling scenarios are assessed against experimental data of velocity profiles, suspended sediment concentrations, and trench migration in a laboratory flume. Numerical results show that the processes investigated have little impact on the flow velocity profiles, in spite of some differences on the bed shear stress. The only exception is the one-equation turbulence closure, which cannot reproduce velocity profiles appropriately. Sediment-induced stratification, erosion parameterization, and turbulence closures can all have significant impacts on the suspended sediment concentration and the bed evolution. Model-data comparisons are found to be particularly sensitive to the method used to calculate the turbulent length scale. [Copyright &y& Elsevier]

Abstract: Georges Bank is one of the world’s most productive marine ecosystems, but the lack of accurate broad-scale sediment maps presently limits habitat assessments and spatial fisheries management. From 1999 to 2009 we surveyed 36,669km2 of Georges Bank using 2.8 and 0.6m2 quadrats viewed with live underwater video (video quadrats). The sediment types observed in 61,604 quadrats were used to map and evaluate spatial structure of local surficial sediment coarseness, dominance, heterogeneity, and maximum size characteristics at a 1km2 spatial resolution. Sand dominated sediment covered 62% of the study area, and there was a logarithmic decline in coverage by larger, coarser and more heterogeneous sediments. Gravel dominated sediments covered 38% of the study area and were more than twice as abundant as previously estimated. A 12,890km2 swath of gravel dominated seabed stretched from Cape Cod to northeastern Georges Bank consistent with estimates of prehistoric glacial extent. Within the swath there were 14 large gravel outcrops (15–2743km2) . This work increases the spatial resolution of sediment information available for habitat assessments and spatial fisheries management on Georges Bank by two orders of magnitude. The four sediment characteristics we evaluated support further detailed investigations of the Bank’s benthos, including the influences of surficial sediment characteristics on species and community distributions, and more spatially accurate estimates of seabed roughness. Finally, this work demonstrates the use of video quadrats as an alternative to traditional grab sampling and modern acoustic sampling for continental shelf-scale mapping. [Copyright &y& Elsevier]

Abstract: The irregular seafloor of the narrow Irish Sea on the NW European Shelf has been documented over several decades. From recently collected swath bathymetry data, very large trochoidal, nearly symmetrical sediment waves are observed in many parts of the Irish Sea and appear similar to those described from other continental shelf seas in North America that were covered by glacigenic sediments during the Last Glacial Maximum. Swath multibeam and single beam bathymetry data, backscatter intensity, shallow seismic imagery, video footage and sediment cores from the Irish Sea high sediment waves have been integrated to identify their genesis with reference to present and past hydrodynamic variability. From cross-sectional profiles over asymmetrical sediment waves in the Irish Sea the direction of asymmetry is used to map residual bed stress directions and associated bedload transport paths. Irish Sea peak bed stress vectors were generated using a two-dimensional palaeo-tidal model for the NW European shelf seas and compare well with the observations. Tidally induced bed stresses are modelled to have increased between 7–10ka BP, to be nearly symmetrical in magnitude and to have reversed in dominant direction on a millennial scale. These environmental conditions during the post-glacial marine transgression are suggested here to help comprehend the construction of the very large sediment waves, with local variations due to differences in sediment grain size, sediment supply, water depth and intensified currents due to seafloor slopes. Model parameterisation using an open ocean boundary with time-dependent tidal changes and the implementation of high-resolution bathymetric information will improve future models of small-scale bed shear stress patterns and improve the predictive value of such modelling efforts. [Copyright &y& Elsevier]

Ripples and small-scale bedforms are ubiquitous in shallow water environments under the combined action of currents and waves. Small scale processes linked to their formation and migration are interconnected with sediment transport at larger scales (e.g. tens of metres to kilometres), both resulting in and being affected by large scale sediment transport and geomorphological evolution. The lower shoreface provides a key link between coasts and continental shelves, but the contribution of ripples and small-scale bedforms to sediment transport in this region has yet to be fully addressed. This work presents a study of sediment dynamic processes on the lower shoreface in the presence of small-scale bedforms. Observations were made during the winter of 2017 on the lower shoreface of Perranporth Beach, which is in the south west of the UK and exposed to Atlantic waves. The analysis of morphological expressions and the variability of ripples under waves, currents and wave-current conditions are assessed. Ripple morphology and associated dynamics are analysed for their potential contribution to the exchange of sediment between the lower and the upper shoreface. In the present study it was observed that even though ripples were evolving depending on the wave-current forcing, little ripple migration was observed due to low wave skewness. The implication is that ripple migration and bedload transport are only a small contribution to onshore sediment transport under low to moderate energy conditions. However, during more energetic conditions, ripples were washed out and the wave skewness increased, resulting in onshore sediment transport under a sheet flow regime. This suggests that ripple formation and migration can have little impact on the cross-shore supply of sediment from the lower shoreface to the upper shoreface and that more energetic wave conditions are required to significantly transport sediment towards the beach. • High-resolution observations on the seabed reveal 2D and 3D wave vortex ripples in a macrotidal sandy lower shoreface. • The morphological ripple transitions provide assessment of sediment transport regime thresholds. • The hypothesis of ross-shore sediment supplied from the lower to the upper shoreface through ripple migration is not supported. [ABSTRACT FROM AUTHOR]

Abstract: A qualitative and semi-quantitative study of recent dinoflagellate cysts has been undertaken in the NW part of Aegean Sea, Thermaikos Gulf (Eastern Mediteranean), before (September 2001), during (October 2001) and after 120 days (February 2002) of intensive trawling activities. This is the first survey of recent dinoflagellate cysts from Greek marine coastal environments. Sediment samples were collected with a corer and the vertical distribution of the cysts was studied at five different layers, from 0 to 10cm. Dinoflagellate cysts were both abundant and diverse. Cysts were found over the whole sampling area and periods, with concentrations ranging between 247–3202cystscm−3. Thirty-six cyst types were encountered, of which 32 were identified to species level, representing 12 genera. It seems that significant local resuspension, related to the onset of the trawling period and stirring up of the sediment, contributed to mixing of the upper layers, resulting to more homogenous cyst profiles in the sediment. Viable cysts constituted 16–60% of the total cyst abundance. The abundance peaks of viable cysts within the subsurface sediment layers, observed during the undisturbed period, disappeared during October. In February, the reduction of cyst concentration was associated to a loss of viable cysts, whilst the ratio of viable/empty cysts ranged between 0.30 and 0.67. The abundance of the different dinoflagellate species, in their active form, was monitored in order to detect any relationship between the concentration of cysts in the top 10cm of sediment and blooms of algae in the water column. Cysts of potentially toxic species, causing Paralitic Shellfish Poisoning (PSP), such as Alexandrium cf. tamarense, A. cf. affine, A. cf. minutum, as well as Gymnodinium catenatum, were detected in the cyst survey. [Copyright &y& Elsevier]

Abstract: The East Antarctic continental shelf has had very few studies examining the macrobenthos structure or relating biological communities to the abiotic environment. In this study, we apply a hierarchical method of benthic habitat mapping to Geomorphic Unit and Biotope levels at the local (10s of kilometers) scale across the George V Shelf between longitudes 142°E and 146°E. We conducted a multi-disciplinary analysis of seismic profiles, multibeam sonar, oceanographic data and the results of sediment sampling to define geomorphology, surficial sediment and near-seabed water mass boundaries. Geographic information system models of these oceanographic and geophysical features increase the detail of previously known seabed maps and provide new maps of seafloor characteristics. Kriging surface modeling on data includes maps to assess uncertainty within the predicted models. A study of underwater photographs and the results of limited biological sampling provide information to infer the dominant trophic structure of benthic communities within geomorphic features. The study reveals that below the effects of iceberg scour (depths >500m) in the basin, broad-scale distribution of macrofauna is largely determined by substrate type, specifically mud content. In waters within the direct influence of glacial ice (depths &#60;500m) on the banks, scouring by icebergs is a strong limiting factor in the distribution of macrobenthos. In areas protected from iceberg scour disturbance, such as on the outer shelf banks and slope, the direction and speed of oceanic currents are the likely dominant abiotic factor in the broad-scale distribution of macrofauna. This hierarchical method of benthic habitat mapping could be applied circum-Antarctic for comparison against other geographic areas, and would assist authorities responsible for developing ecosystem-based plans by identifying the different types of marine habitats and their associated biological communities at varying scales on the Antarctic shelf. [Copyright &y& Elsevier]

Abstract: Geological studies offshore of Wrightsville Beach, North Carolina reveal subtle large-scale regions of coarse sand with gravel and shell hash (widths between 100 and 200m and negative relief of ∼1m) that trend obliquely to the coast. It was previously suggested that these regions serve as conduits for sand exchange between the shoreface and inner shelf during storm-associated downwelling. Consequently they were classified as rippled scour depressions. More recently, the role of alongshore flows and self-organization as a result of inhibited settling of fine sand has been discussed. In this study, 45 days of near-bed current measurements were analyzed using benthic boundary layer and sediment transport models to examine the role of along- and cross-shore flows in driving sediment transport at this site. The wind climate was found to be a dominant influence on near-bed flows. Six distinct sediment transport events were recognized. During these events, sediment transport models show that bedload transport is directed mainly in the cross-shore direction, while suspended sediments are directed alongshore to the southwest. Current observations during these sediment transport events provide no evidence of cross-shore sediment transport caused by steady downwelling currents. Instead, benthic boundary layer model results are used to show that differences in bed roughness between the coarse areas of the seabed within the “rippled scour depressions” and the finer areas of the inner shelf are more pronounced during increasingly energetic wave and current conditions. The enhanced difference in roughness results in increased turbulence intensities over coarse regions inhibiting the deposition of the fine sand that is resuspended over the shelf during these events relative to finer areas over the shelf. Findings from this study contribute to explaining the observed long-term persistence of these features. [Copyright &y& Elsevier]

Abstract: The impact of human interventions on water and sediment fluxes from the Colorado River into the Gulf of California is still poorly understood. The erosion of the deltaic structure and the adjacent tidal flats has caused the dispersion and re-deposition of metal-rich clayey mud along the Baja California coast and alternate sediment sources are thought to have gained importance. The sediments in the Northern Gulf of California (NGC) are studied geochemically in order to determine the probable sources and sinks of sediments, metals and phosphorus in the region. Four geochemical factors are identified in the NGC: terrigenous silts and clays, biogenic carbonates, barite and phosphate. Unlike other sediments in the NGC, the Colorado River-derived silts and clays (Fe ≤ 2.8%) are characterised by their high concentrations of labile inorganic-P (269μgg−1); in these sediments ∼27% of the bulk concentrations of P is found in the exchangeable, Fe and Mn redox-sensitive and Al-oxihydroxide fractions. Apatite is the main P bearing fraction in the sediments, and except in one sample, organic-P is considered to be dominantly refractory. Localised, probably detrital, apatite and barite sources are identified directly adjacent to the Sonora coast. The anomalous enrichment of apatite in one sample with uncommonly high concentrations of labile (non-refractory) P suggests a probable authigenic mineralization in the eastern coast of the NGC. The high abundance of Ba in the Midriff Island region may indicate biogenic sources for this element there. Except for three samples from shallow carbonate bays and the deeper Delfín Basin, the sediments in the NGC contain >90% of Al and Ba and >70 % of Fe, Cu, Cr, Co, Zn and La in residual fractions. The chondrite normalised rare-earth element geochemistry is indicative of typical upper continental crust provenance of the sediments from the NGC. [Copyright &y& Elsevier]

Abstract: The ephemeral Santa Clara River delivers large amounts of freshwater and sediment to the eastern Santa Barbara Channel during brief, episodic discharge events. This discharge into the channel was characterized here with shipboard measurements during floods of 1997 and 1998. Within approximately 1-km of the river mouth, the river discharge quickly stratifies into a freshened, turbid surface plume and a bottom nephloid layer. Observations immediately off the Santa Clara River mouth on a peak day of river discharge revealed that sediment rapidly settled from the freshened surface waters, as suspended sediment in the freshened surface plume contained only 6% of the sediment mass expected if the sediment mixed conservatively. On the two subsequent days the reduction of sediment mass in the surface plume continued at 50% per day. These observations suggest that river sediment undergoes rapid initial settling within 1-km of the river mouth, followed by somewhat slower rates of settling. Although we did not measure sedimentation or bottom boundary layer processes, our mass balance results suggest that almost all of the river sediment either escapes along or deposits upon the inner shelf seabed. [Copyright &y& Elsevier]

Energy is required to maintain a particle heavier than water in suspension in the sea. In equilibrium, energy must be supplied at a rate equal to the product of the weight of the particle and its sinking velocity. If the energy supply rate is known, it becomes possible to predict the total weight, and hence the concentration of suspended particles in the sea. This principle has been applied to a model of fine suspended sediments in the Irish Sea. The model is used to test the hypothesis that the supply of turbulent kinetic energy (TKE) alone controls both the spatial pattern and seasonal changes in concentration. TKE is supplied by the tide and wind, and a fixed fraction of this energy is used to keep sediment in suspension. There is assumed to be a readily available supply of fine sediments at the sea bed, which rises into suspension as soon as TKE becomes available, and sinks down again when it is not. The model incorporates 2-layer thermal stratification and vertical mixing between layers but there is no horizontal advection or diffusion. The model is driven by smoothly varying wind speeds and heating over the year and spatially varying tidal stirring. The particles have a constant settling velocity of 10 m day−1, and a density twice that of water. A fraction 0.00006 of the total (tide+wind) TKE energy input is used to stir the sediment. The results of the model are in agreement with both the observed spatial pattern and the seasonal variation of fine suspended sediments in the surface waters of the Irish Sea to within the limits of measurement accuracy and natural variability. This energy model performs at least as well as conventional stress-driven models in this regard. We therefore conclude that the general behaviour of fine suspended sediments in the Irish Sea can be explained by TKE supply alone. Other factors, such as changes in particle size and density over the year, can be used for fine tuning but are not necessary to give a broadly correct picture. The result is that we have a simple, physically coherent, method of predicting the turbidity (and hence the light regime) of a shelf sea from physical inputs alone. [Copyright &y& Elsevier]

Past major changes in sea level have had a significant influence on global- and shelf sea tidal dynamics. Some of these changes are preserved in sedimentary records from the shelf seas, and so appropriate proxy data have the potential to constrain tidal model outputs over the recent geological past. Tidal models which simulate the evolution of tide-dependent parameters over geological timescales are fundamental to understanding the response of the tides to sea-level rise and climate change. This study explores a potential new sedimentary proxy for validating past shelf sea tidal dynamics, interrogating the relationship between tidally-modulated bed shear stress and seabed sediment grain size at discrete sediment core locations over the northwest European shelf seas. Radiocarbon-dated sediment grain size profiles were generated for four British Geological Survey UK shelf sediment vibrocores, spanning a range of physical environments. Changes in observed sediment grain size through time were compared with simulated changes in tidal-induced bed shear through time, using temporal and spatial outputs from the most recently developed palaeotidal model of the Northwest European shelf seas. Although a positive correlation between observed grain size and simulated bed shear stress was observed at three of the four sediment cores sites, no robust relationship could be quantified. The palaeotidal model output failed to resolve the details of the actual sediment dynamics, since only tidal-induced bed shear stresses were considered. Wave processes were neglected, and the model was not sensitive enough to constrain simulated past tidal conditions at point locations; rather it is suitable for examining general trends. There remains a need to develop new proxies for past shelf sea hydrodynamic conditions which can be used to constrain numerical model output of tidal currents at regional scales. • We explore a new proxy for validating models of past shelf sea tidal dynamics. • Age-constrained seabed sediment data for 5 marine cores for NW Europe presented. • Evolutions of median grain size compared with simulated tidal-induced bed stress. • General trends in sediment grain size and simulated bed shear stress were observed. • The tidal model did not fully resolve the intricacies of observed sediment dynamics. [ABSTRACT FROM AUTHOR]

Normal incidence echo data (bottom reflection) can provide acoustic reflectivity estimates for seabed classification and predicting sediment properties with the use of seabed sediment models. However, the accuracy of normal-incident reflection coefficients significantly affects the geoacoustic inversion result. In this study, a multi-core platform with an acoustic recording system, which can perform a near-scope photographical survey of the seafloor, was used to acquire sound data and collect surficial sediment samples in the southwestern sea off Taiwan. The acquired acoustic data was used to analyze the potential errors in acoustic reflectivity estimation, which were induced by erroneous assumptions in acoustic propagation and sea surface reflection. The subsequent data analysis led to the following conclusions. First, theoretical spherical spreading loss compensation is appropriate in seabed reflection coefficient estimates, and relatively small uncertainties in propagation occur. Second, the assumption of perfect reflection of the sea surface is invalid for seabed reflection coefficient estimates. The resulting errors caused by sea-surface effects are described in detail. Moreover, the collected seabed reflection coefficients were used to invert the sediment properties, which were verified using core samples and camera images. • Accuracy of acoustic reflectivity estimation is essential in determining seabed sediment properties. • Normal-incident acoustic reflectivity can be estimated by multiple echoes from seabed. • Seabed reflection coefficients and sediment properties can be underestimated owing to sea surface scattering. • Sea-surface effects should be considered for estimating seabed reflection coefficients via multiple echoes from seabed. [ABSTRACT FROM AUTHOR]

Abstract: Bottom sediments were collected on a muddy tidal flat in Willapa Bay (southwestern Washington State) and analyzed for grain size using a Coulter Multisizer. The disaggregated inorganic grain size (DIGS) distributions obtained from the sediment were then analyzed using conventional methods including median diameter, d50, the largest 25% of the grain size in the population, d75, skewness, and kurtosis. In addition, the inverse model of Curran et al. (2004) was used to provide process-based parameters for description of the size distributions. To examine small-scale spatial differences in grain size, the results of the analysis were plotted against seabed elevation, obtained from USGS LiDAR data. Results reveal a strong inverse correlation between the mass fraction deposited to the seabed as flocs, which is called the “floc fraction”, and elevation on the tidal flat but failed to show any correlation with conventional grain-size descriptors. The dependence of floc fraction on elevation arises mainly due to the differences in size distributions between secondary tidal channels and tidal flats. The extent of flocculation in sediments deposited in the channels of the Shoalwater tidal-flat system is seasonal. Greater precipitation in the winter months is associated with periods of increased suspended-sediment concentrations, which favours more extensive flocculation and the formation and maintenance of low-strength, high-water-content muds. During the summer dry season, lower suspended-sediment concentrations lead to reduced floc fractions in the channels, while the size distributions on the flats resemble those in winter. [Copyright &y& Elsevier]

Abstract: Tidal flats with fluvial input can have ample fine-grained sediment supply, and yet the Skagit River tidal flats remain mostly sandy. Therefore, hydrodynamic mechanisms must efficiently export mud off the flats. This study evaluates short-term deposition and long-term burial of mud on an intertidal flat and links seabed deposits to sediment-transport processes. To investigate the fine-grained sediment-transport mechanisms within the sandy Skagit River tidal flats, seabed and water-column observations were collected seasonally from 2008 to 2009 under varying river discharge conditions. Little mud was observed deposited on the seabed. Spatially, mud was most common near channels and on the seaward flat edge. Its presence was temporary, and was typically observed following high river discharge. Strongest current velocities and high suspended-sediment concentrations were measured during spring tidal cycles with peaks in the late ebb. Maximum velocities were observed within channels, permitting for rapid transport of fine-grained particles to the seaward edge of the flat and export beyond. These physical processes also act to mobilize the mostly sandy seabed, which is reworked at various frequencies: on a semi-diurnal tidal timescale, both channel and flat seabeds are reworked to 1–2cm; and over a decadal timescale, lateral channel migration acts to rework the seabed to 1–2m, making the limited mud deposits available for export. Overall, channel networks on sandy tidal flats with fluvial input not only deliver fine-grained sediment to the flat, but also aid in the export of suspended sediment at ebb tides and in the release of muddy deposits through reworking of the seabed. [Copyright &y& Elsevier]