Mud Layer Research Articles

Anatomy of a ‘suspended’ seafloor in the dense brine waters of the deep hypersaline Urania Basin

When micron-sized particles are suspended into the dense brine waters of a deep hypersaline basin, in the absence of biological packaging, their settling velocities should be greatly attenuated. We observed this phenomenon in the anoxic brine water of the Urania Basin in the eastern Mediterranean. We found that the deepest part of this basin is filled with a high-density, about 110 m thick, homogenous fluid mud layer mainly containing microfossils of coccoliths and nannoplankton of Pleistocene and Pliocene age that float in a hot methane-laden brine overlaid by a ‘clear’ brine. The top of the fluid mud forms a secondary ‘suspended seafloor’ that has been stable for at least the last two decades since the discovery of the basin. The geochemical characteristics of the fluid mud (e.g. lower salinity than the clear brine, methane-rich) point to a deep source for the fluids, while the age of the microfossils suggests that the floating biogenic particles came from shallower, post-Messinian pelagic deposits such as those exposed along the walls and the rim of the Urania Basin. We hypothesize that catastrophic gravity failures and mass mobilization of the post-Messinian sedimentary layers triggered by earthquake activity are one of the main sources of the sediment in the fluid mud. Because of both, the dense brine fluids, the continuous mixing by thermal convection and the very fine grained nature of the particles, the re-mobilized sediments have never settled, and stay suspended as if they were frozen in time. The prolonged residence time of the supersaturated fluids and the associated microbial activity have also fostered the precipitation of the authigenic gypsum crystals and micron-sized authigenic carbonates that occur mixed with the fluid mud sediments.

Combined analysis of sulfur and carbon contents, and foraminifer as paleoenvironmental indicators in tidal flat sediments on Miura Peninsula, Japan

Reconstruction of natural hazards from tidal flat sediments is essential for assessment of coastal hazards, and is based on knowledge of recent sediments. This study therefore examined recent sediments deposited on a narrow, muddy tidal flat on Miura Peninsula, metropolitan Tokyo, Japan, by means of sedimentological analysis, C, N, and S elemental analysis, carbonate content, 137Cs dating, and analysis of benthic foraminifera (species composition and degree of preservation). 137Cs profiles of four sediment cores of ∼20 cm in length showed that the sediments were deposited after the release of 137Cs by the accident at the Fukushima No. 1 Nuclear Power caused by the Tohoku-oki tsunami at March 2011. Results of C/S ratios of mud fractions indicated that the intertidal deposits in the central bay are characterized by low C/S ratios (4.9–11.4) and the bayhead deposits of the upper intertidal and supratidal zones by higher ratios (13.9–24.9). A fluid–mud layer identified in the central bay had the same C/S ratios and carbonate contents as marine sediments, indicating that the fluid-mud deposits probably resulted from resuspension of mud caused by storm-wave action after March 2011. Faunal analysis of benthic foraminifera showed that very low density of foraminifera can be regarded as one of the defining characteristics of fluid-mud deposits.

Sedimentation Process of Rambatan Formation in Larangan Brebes, North Serayu Range, Central Java

DOI:10.17014/ijog.6.2.141-151Rambatan Formation in the western part of North Serayu Basin, Brebes, Central Java, comprises generally flysch facies of turbidite sediments deposited in a deep marine environment. This formation is equivalent to Merawu Formation found in the eastern part of the basin and deposited in the environment of tidal flat to subtidal. The turbidite sediments were highly controlled by a rapid downward movement taking place continuously during Early to Late Miocene. The variation of the depositional environment has been the object of this research which aims to understand the sedimentation process of Rambatan Formation in this type locality with a modern turbidite approach. Rambatan Formation was deposited in N13-N19, as a deep marine sediment channel, turbidite, and deep marine tidal zone. The sedimentation was affected by gravity flow and contourite. The sediments on N13-N14 were marked by turbidite sediments until Middle N17. The sediment supply increased on Middle N17, as a sediment filler on a channel marked by contourite mud layer (muddy slump) and debris flow, with sources from the north. The increase of sediment supply was followed by an environmental transformation from a deep marine channel into deep marine tidal area. In N19, the sediments were redeposited as turbidite sediment, starting with debris flow in Middle N18.

The nesting habits of Chalybion bengalense (Dahlbom) (Hymenoptera: Sphecidae)

ABSTRACT The nesting habits of Chalybion bengalense are described. Copulation takes place at the nesting site. For nesting, the female uses old nests of Sceliphron and various eumenids, preexisting holes, and trap nests. The wasp cleans the nest cell and provisions it with paralyzed spider prey, 8–32 per S. madraspatanum (Fabricius) cell or 30–60 per trap nest cell. The female lays an egg on one of her prey, frequently the second or the third brought to the nest. Chalybion bengalense closes the nest cells with mud and on the cell cap adds an extra layer of lime, mud, cow dung, resinous material, or rotting plant. The total time from egg to emergence of the adult wasp is 27–34 days. The longevity of adults is 15–47 days for males and 21–49 days for females. Chalybion bengalense overwinters as prepupa from mid November to early April of the following year.

Using transient temperature–depth profiles to assess vertical groundwater flow across semi-confining layers in the Chianan coastal plain aquifer systeme, southern Taiwan

The quantification of vertical groundwater fluxes across semi-confining layers is fundamental to evaluate groundwater recharge and discharge rates to and from aquifer systems. Methods to estimate vertical groundwater fluxes from temperature–depth profiles have been available since the 1960s. While some methodologies assume steady-state conditions, changes in land-surface temperatures as well as hydrogeological conditions can lead to transient heat flow conditions. Indeed, many studies have indicated that transient temperatures in deeper confined aquifers are widespread. A study is presented that uses transient-temperature–depth curves obtained from groundwater observation wells in the Chianan coastal plain in southern Taiwan. In this area, sedimentary aquifer systems consist of a stack of alternating sand and mud layers, over several hundred meters in thickness. Groundwater has been abstracted from these aquifers for decades, resulting in large hydraulic gradients between the shallow and deeper aquifers. Hence, vertical groundwater flow is likely enhanced across finer-grained, semi-confining units. A set of temperature–depth profiles is available from this area. Constrained by these profiles, numerical models of one-dimensional transient heat transfer were used to infer vertical fluxes of 3.3 × 10−8 to 3.9 × 10−8 m/s using thermal data from 2013 to 2016. An analytical solution was also employed that assumes steady-state conditions. Calculated fluxes using the latter approach were lower, at approximately 1.1 × 10−8 to 1.6 × 10−8 m/s. The study suggests that vertical fluxes derived from using Bredehoeft and Papadopulos’s analytical solutions result in underestimates of actual vertical seepage rates across aquitards.

A Study of Wave Dissipation Rate and Particles Velocity in Muddy Beds

The interactions between free surface waves and layers of cohesive sediments including wave height attenuation and mud movement are of great importance in coastal and marine engineering. In this study, the results from a new analytical model were compared with those from literature experimental works and analytical models in terms of wave height dissipation rate and mud velocity. It was found that the new model provided good agreements in the case of coexisting waves and currents, while the literature model of Ng (explained in Section 2 of the text) —assuming the mud layer as a highly viscous layer with high shear rates—matched well with the experimental data for high viscosity (mud viscosity, νm = O [0.01 m2/s]). In addition, it was found that the new model is able to successfully simulate particles velocity in the presence of co-current.

Evaporitic carbonates in the pre-salt of Santos Basin – Genesis and tectonic implications

Abstract Since the first discoveries of oil accumulations in the carbonate reservoirs of the Lower Cretaceous Barra Velha Formation of the Santos Basin, their genesis has been the subject of intense discussion. Different depositional models have been proposed, with some authors advocating a microbial origin, while others an abiotic origin as travertines or travertine-like rocks. This search for a modern carbonate analogue for these unusual rocks may have disregarded an intriguing alternative: an evaporative model supported by our petrographical and geochemical data. Evaporitic sediment patterns in endorheic basins, in a depositional framework of brine drawdown, are dominated by ephemeral salt pan hydrology. Bedded salt crusts form a stack of subaqueous aligned crystal beds at the sediment-water interface; salt grows diagenetically within saline and mud layers beneath the dry pan surface; finally laminated sediments are deposited. We observe a similar cyclic pattern in the pre-salt core we described: beds of shrub-like crystals and diagenetic spherulites growing in magnesium clay matrix are followed by laminites. The model we propose attempts to contribute solving the puzzle of Pre-Salt carbonate deposition by integrating sedimentology and geochemistry with basin tectonics. Our observations and basin tectonics point to the exciting possibility that the sag phase carbonates of the Santos-Campos basin were deposited as the initial/transitional stage of a closed sub-sea-level evaporite basin whose rising salinities culminated in the deposition of halite and bittern salts (carnallite, sylvite, tachyhydrite). Basinwide evaporites are singular events in the geological record. They form when a giant sub-sea-level basin becomes a sump of continental, marine and hydrothermal fluids, producing unexpected brine compositions in unusual depositional settings present in ancient evaporitic basins but absent from most modern ones. The novelty in this paper is that we apply to pre-salt carbonate deposition the sedimentology, hydrology and geochemistry of ancient evaporitic basins. With this concept in mind, we propose a geochemical model for the unusual pre-salt carbonate precipitation of the Santos Basin. Regarding the Walvis-Rio Grande volcanic high as a topographic barrier during the Aptian, we suggest that the main sources of calcium to the Barra Velha paleolake were hydrothermal brines, mostly from infiltrating seawater that reacted at depth with the basalts of the volcanic barrier. Hyperextension of the lithosphere, with a shallow Moho, increased geothermal gradients to levels favorable for the generation of hydrothermal CaCl2 brines, a process facilitated by the already elevated Ca++ ion and low sulfate concentration of the Cretaceous ocean. These CaCl2-rich fluids would pass through the volcanic barrier into a sub-sea-level paleolake in the rift basin. The high salinity paleolake would become saturated in calcium bicarbonate/carbonate as internally drained NaHCO3-bearing alkaline waters, formed by physical/chemical weathering of penecontemporaneous basaltic rocks, mix with the CaCl2-bearing hot brines percolating through the barrier, producing a hybrid brine (2NaHCO3 + CaCl2 = Ca(HCO3)2 + 2NaCl). Such a depositional system, integrating the geochemical-depositional controls of sag phase carbonates with those of the South Atlantic evaporites, has important implications for unravelling the genesis of these exotic deposits.

Vector Acoustic Analysis of Time-Separated Modal Arrivals From Explosive Sound Sources During the 2017 Seabed Characterization Experiment

The Intensity Vector Autonomous Recorder (IVAR) is a system that records four coherent channels of acoustic data continuously: one channel for acoustic pressure and three channels associated with a triaxial accelerometer from which acoustic particle velocity is obtained. IVAR recorded the vector acoustic field in broadband signals originating from Signal, Underwater Sound (SUS) (Mk-64) charges deployed at 5–13-km range from the fixed IVAR site (mean depth 74.4 m) as part of the 2017 Seabed Characterization Experiment (SBCEX) designed to study the acoustics of fine-grained muddy sediments. Sufficient geometric dispersion at these ranges permitted unambiguous identification of up to four modes as a function of frequency for frequencies less than 80 Hz. From time–frequency analysis of the dispersed arrivals, a single mode ( $n$ ) and single-frequency ( $f_{i})$ properties are identified at peaks in the narrowband scalar field, with time dependence corresponding to mode group speed. At these time–frequency addresses, four quantities derived from the vector acoustic measurements are formed by coherent combination of pressure and velocity channels: first, modal phase speed; second, circularity, a measure of the normalized curl of active intensity; third, depth-dependent mode speed of energy; and fourth, vertical component of reactive intensity normalized by scalar intensity. A means to compute these quantities theoretically is provided, and a comparison of model results based on a notional geoacoustic representation for the SBCEX experimental area consisting of a single low-speed mud layer over a half-space area versus a Pekeris representation based on the same half-space shows a striking difference, with the field observations also clearly at variance with the Pekeris representation. A fundamental property of mode 2, observed at the IVAR location, is a change in sign for circularity and vertical reactive intensity near 37 Hz that is posited as a constraint observation for mode 2 that must be exhibited by any geoacoustic model that includes a low-speed mudlike layer applied to this location.

The interpretative significance of ripple-derived sedimentary structures within an upper Neogene fluvial succession of central Poland

Abstract Sedimentary structures discussed in the present study are genetically linked to ripples that consist of pure sand or alternating sand and mud layers. All types of ripple-related structures, such as climbing-ripple cross-lamination and heterolithic bedding, i.e., flaser, wavy and lenticular (nodular), have been identified for the first time in fluvial strata that have been characterised previously as commonly massive. These small-scale bedforms, produced by migrating ripples, have been documented in a fluvial channel of late Neogene age in central Poland. The abundance and co-occurrence of the structures discussed and their spatial distribution provide evidence of their formation under very low-energy conditions, when flow velocity changed markedly, but was often significantly less than 0.5 m/s. Therefore, these ripple-derived sedimentary structures are here recognised as typical of channel fills of an anastomosing river.

System response to gas production from a heterogeneous hydrate accumulation at the UBGH2-6 site of the Ulleung basin in the Korean East Sea

We investigate the feasibility of production from a layered marine gas hydrate reservoir using the properties and conditions corresponding to the UBGH2-6 site of the Ulleung Basin in the Korean East Sea. The work expands and furthers previous investigations in support of a proposed field test. The target system is location in deep water and consists of 13 m of alternating hydrate-bearing sand and soft mud layers and will be produced using a vertical well. We assess production potential during a 14-day field test, examine sensitivity to heterogeneity in permeability, porosity, and initial hydrate saturation, and assess the geomechanical response of the system to short-term production. Producing gas from the system appears to be technically feasible, however, low production rates and relatively large water production rates are expected during the field test. Expected subsidence and reservoir compaction is limited given the current data and the short timeframes of the production test.

Submarine groundwater discharge site in the First Salpausselkä ice-marginal formation, south Finland

Abstract. Submarine groundwater discharge (SGD) has been implicated as a significant source of nutrients and potentially harmful substances to the coastal sea. Although the number of reported SGD sites has increased recently, their stratigraphical architecture and aquifer geometry are rarely investigated in detail. This study analyses a multifaceted dataset of offshore seismic sub-bottom profiles, multibeam and side-scan sonar images of the seafloor, radon measurements of seawater and groundwater, and onshore ground-penetrating radar and refraction seismic profiles in order to establish the detailed stratigraphical architecture of a high-latitude SGD site, which is connected to the Late-Pleistocene First Salpausselkä ice-marginal formation on the Hanko Peninsula in Finland. The studied location is characterized by a sandy beach, a sandy shore platform that extends 100–250 m seaward sloping gently to ca. 4 m water depth, and a steep slope to ca. 17 m water depth within ca. 50 m distance. The onshore radar and offshore seismic profiles are correlated based on unconformities, following the allostratigraphical approach. The aquifer is hosted in the distal sand-dominated part of a subaqueous ice-contact fan. It is interpreted that coarse sand interbeds and lenses in the distal fan deposits, and, potentially, sandy couplet layers in the overlying glaciolacustrine rhythmite, provide conduits for localized groundwater flow. The SGD takes place predominantly through pockmarks on the seafloor, which are documented on the shore platform slope by multibeam and side-scan sonar images. Elevated radon-222 activity concentrations measured 1 m above seafloor confirm SGD from two pockmarks in fine sand sediments, whereas there was no discharge from a third pockmark that was covered with a thin organic-rich mud layer. The thorough understanding of the local stratigraphy and the geometry and composition of the aquifer that have been acquired in this study are crucial for successful hydrogeological modelling and flux studies at the SGD site.

Study on Behavior of Turbid Water around Dredged Navigation Channel at River Mouth

Field measurements were carried out in the port of Niigata to elucidate sediment transport processes around navigation channel and turning basin in the port at river mouth. The measurements include salinity, suspended sediment concentration and in-situ mud density at several monitoring points along the navigation channel in the port. The survey successfully captured the formation of three layers structure during a river flood condition with high turbid fresh water in the upper layer and low turbid sea water in the middle layer. Furthermore, high concentrated mud or fluid mud layer was also found in the lowest layer at the deeper dredged channel area. Physical model experiments were also carried out with a circulating flume to elucidate the fluid mud transport on the slope at the edge of step-like bathymetry. The experiments indicate the behavior of near bed turbid water depends on the overlying flow conditions.

Effect of a soft sediment layer over elastic basement on normal mode dispersion

The effect of shear on dispersion of acoustic normal modes was investigated in a previous study (Potty and Miller, 2010). Modal dispersion was calculated using a bottom model consisting of a liquid layer over an elastic basement. The modal travel times corresponding to the Airy Phase regions were found to be extremely sensitive to shear. Simple inversion schemes were developed to estimate the shear speed in the sediment by comparing theoretical predictions with experimental data. Modal dispersion characteristics of broadband data collected during experiments conducted in Middle Atlantic Bight and New England Mud patch were analyzed, and bottom shear speeds were estimated. The estimated shear speeds were also compared with shear speeds calculated from core data. In this study, the bottom model will be revised to include a soft sediment layer over the elastic basement. The modal dispersion will be calculated using this bottom model corresponding to the New England Mud patch environment using propagation models such as ORCA and KRAKENC. The effect of the addition of the soft mud layer on dispersion of lower order modes will be investigated and presented. [Work supported by the Office of Naval Research.]

Kinetics of organic carbon mineralization and methane formation in marine sediments (Aarhus Bay, Denmark)

Abstract Sediments were sampled at nine stations on a transect across a 7–10 m thick Holocene mud layer in Aarhus Bay, Denmark, to investigate the linkages between CH4 dynamics and the rate and depth distribution of organic matter degradation. High-resolution sulfate reduction rates determined by tracer experiments (35S-SRR) decreased by several orders of magnitude down through the mud layer. The rates showed a power law dependency on sediment age: SRR (nmol cm−3 d−1) = 106.18 × Age−2.17. The rate data were used to independently quantify enhanced SO42− transport by bioirrigation. Field data (SO42–, TCO2, T13CO2, NH4+ and CH4 concentrations) could be simulated with a reaction-transport model using the derived bioirrigation rates and assuming that the power law was continuous into the methanogenic sediments below the sulfate-methane transition zone (SMTZ). The model predicted an increase in anaerobic organic carbon mineralization rates across the transect from 2410 to 3540 nmol C cm−2 d−1 caused by an increase in the sediment accumulation rate. Although methanogenesis accounted for only ∼1% of carbon mineralization, a large relative increase in methanogenesis along the transect led to a considerable shallowing of the SMTZ from 428 to 257 cm. Methane gas bubbles appeared once a threshold in the sedimentation accumulation rate was surpassed. The 35S-measured SRR data indicated active sulfate reduction throughout the SO42− zone whereas quasi-linear SO42− gradients over the same zone indicated insignificant sulfate reduction. This apparent inconsistency, observed at all stations, was reconciled by considering the transport of SO42− into the sediment by bioirrigation, which accounted for 94 ± 2% of the total SO42− flux across the sediment-water interface. The SRR determined from the quasi-linear SO42− gradients were two orders of magnitude lower than measured rates. We conclude that models solely based on SO42− concentration gradients will not capture high SRRs at the top of the sulfate reduction zone if they do not properly account for (i) SO42− influx by bioirrigation, and/or (ii) the continuity of organic matter reactivity with sediment depth or age.

Tsunami history over the past 2000 years on the Sanriku coast, Japan, determined using gravel deposits to estimate tsunami inundation behavior

At Numanohama Marsh on the rocky Sanriku coast in northeast Japan, we identified 17 sand layers with sedimentary structures characteristic of event deposits (i.e., a sharp erosional contact with the underlying unit, a normal and/or inverse grading structure, parallel/cross lamination, mud clasts, and/or a thin mud layer). Abundant marine nannofossils, coccoliths, and the small catchment area in the narrow valley suggest that these event layers were likely formed by inundations during tsunamis or storms. Among the 17 sand layers, nine contained gravels derived from the beach, riverbed, or slope areas. Six event layers contained all three of these gravels, suggesting a strong inflow current that transported round pebbles from the beach, erosion of wall rock in the valley during the inflow and outflow phases, and a backwash current that caused sub-angular riverbed gravel to accumulate in lowland areas. These six layers were traced up to 490 m inland from the coastline (present elevation: 7 m above mean sea level) and were several cm thick at almost every survey point. We concluded that these six event layers are tsunami deposits, and dating results revealed that they correlate with the 2011 Tohoku-oki earthquake; the 1933 and 1896 Sanriku-oki earthquakes; any of the 1763 Aomori-oki, 1793 Miyagi-oki, and 1856 Aomori-oki earthquakes; the 1611 Tohoku-oki earthquake; and the 869 Tohoku-oki (Jogan) earthquake. In addition, changes in the sedimentary environment inferred from diatom assemblages were well correlated with the deposition of three of the event layers. Two historical earthquakes — the 1454 and 1611 Tohoku-oki earthquakes — have been previously suggested as being the immediate predecessors of the 2011 Tohoku-oki earthquake, and our dating results favor the 1611 earthquake. Although the source region of the 869 Tohoku-oki (Jogan) earthquake has been estimated to have been off the Fukushima and Miyagi prefectures, our results suggest that the source region extended further north along the Japan Trench.

Stability of mud-water interface under long surface waves

Fluid mud often exists in coastal areas with an interface separating it from its upper water layer. When a surface wave propagates over a bed covered with water and fluid mud, it will cause an interfacial wave of the mud-water interface, which damps the surface wave and results in mass transport of fluid mud. Most researches about wave attenuation and mass transport of fluid mud are based on the assumption that the mud-water interface is unbroken. This assumption excludes the breaking interfacial waves that are known as an important mechanism responsible for mass and momentum transport between the two fluids. When the surface wave is long, its velocity field, which also serves as basic flows, may be susceptible to the Kelvin-Helmholtz (K-H) instability if the shears at the interface are strong enough. In the present paper, the critical conditions for the K-H instability to occur for the mud-water interface is investigated via linear stability analysis and numerical simulation. It is found that, for a K-H instability to occur, the Stokes boundary layer thickness induced by a surface wave must be large enough to penetrate the fluid mud layer and produce a strong shear at the interface. Meanwhile, a critical condition is found for a long surface wave to cause breakup of mud-water interface through K-H instability. This is practically instructive for waterway and harbor construction and protection because it predicts that a thicker mud layer is harder to be taken away by a surface wave.

Mass Stabilization as reinforcement of organic soils

The decreasing number of places suitable for constructing buildings forces people to creatively develop newer methods of soil reinforcement. One of these methods is the deep soil mixing. This technology has been firstly developed and applied in Japan in the 1970s. Initially, it was used to create DSM (Deep Soil Mixing) columns. In the subsequent years, it was also developed in Scandinavia. Over time, the deep mixing technology was modified and developed, and in addition to the wet method, also the dry method was started to be used, while in addition to the cement binder, also lime binders and fly ashes were used. Technologies consisting of the deep mixing of cement with soil are very popular due to the wide range of applications and relatively low implementation costs. The method of Mass Stabilization (MS) is a soil reinforcement method that is analogical to DSM and it consists of mixing large volumes of soil with cement. This article describes the method of dry Mass Stabilization of organic soils. It cites the analyzed laboratory tests of soil-cement material manufactured in MS technology. The tests included the creation of 140 material samples, and subsequently the performance of compression strength test on them, along with the registration of stress path. The main aspect of these tests consisted of increase in the primary deformation modulus over time, depending on the amount of applied cement. Also, an example of the project to strengthen the layer of aggregate mud under the floor in the hall is demonstrated. The reinforcement was implemented in the MS technology.

Effect of self-weight consolidation on a hydro-sedimentological model for the Río de la Plata estuary

The effect of the consolidation process on the morphodynamics and fine sediment dynamics of the Río de la Plata estuary is explored through a circulation-wave-sediment transport model. The consolidation model is calibrated based on settling column experimental data. Different simulations are done in order to initialize the mud layer distribution and to investigate the impact of different erosion parameter assumptions on the modeled sediment dynamics. Finally a two-year simulation is done with and without the consolidation process and realistic hydrodynamic forcings. Considering the consolidation process, the model correctly reproduces measured vertical density profiles in the Montevideo Bay access channel. The simulated suspended sediment dynamics behavior in Montevideo Bay with the consolidation process provides a more realistic deposition pattern in regard to the dredging activities.

Beneficial use of dredged sediment to enhance salt marsh development by applying a ‘Mud Motor’

We test an innovative approach to beneficially re-use dredged sediment to enhance salt marsh development. A Mud Motor is a dredged sediment disposal in the form of a semi-continuous source of mud in a shallow tidal channel allowing natural processes to disperse the sediment to nearby mudflats and salt marshes. We describe the various steps in the design of a Mud Motor pilot: numerical simulations with a sediment transport model to explore suitable disposal locations, a tracer experiment to measure the transport fate of disposed mud, assessment of the legal requirements, and detailing the planning and technical feasibility. An extensive monitoring and research programme was designed to measure sediment transport rates and the response of intertidal mudflats and salt marshes to an increased sediment load. Measurements include the sediment transport in the tidal channel and on the shallow mudflats, the vertical accretion of intertidal mudflats and salt marsh, and the salt marsh vegetation cover and composition. In the Mud Motor pilot a total of 470,516 m3 of fine grained sediment (D50 of ∼10 μm) was disposed over two winter seasons, with an average of 22 sediment disposals per week of operation. Ship-based measurements revealed a periodic vertical salinity stratification that is inverted compared to a classical estuary and that is working against the asymmetric flood-dominated transport direction. Field measurements on the intertidal mudflats showed that the functioning of the Mud Motor, i.e. the successful increased mud transport toward the salt marsh, is significantly dependent on wind and wave forcing. Accretion measurements showed relatively large changes in surface elevation due to deposition and erosion of layers of watery mud with a thickness of up to 10 cm on a time scale of days. The measurements indicate notably higher sediment dynamics during periods of Mud Motor disposal. The salt marsh demonstrated significant vertical accretion though this has not yet led to horizontal expansion because there was more hydrodynamic stress than foreseen. In carrying out the pilot we learned that the feasibility of a Mud Motor depends on an assessment of additional travel time for the dredger, the effectiveness on salt marsh growth, reduced dredging volumes in a port, and many other practical issues. Our improved understanding on the transport processes in the channel and on the mudflats and salt marsh yields design lessons and guiding principles for future applications of sediment management in salt marsh development that include a Mud Motor approach.

Dynamics of sediments on a glacially influenced, sediment starved, current-swept continental margin: The SE Grand Banks Slope off Newfoundland

The SE Grand Banks Slope is unusual on the glaciated eastern Canadian margin in that it was remote from ice stream and glacial ice of the Laurentide Ice Sheet (LIS). It thus allows an analysis of the role of contour currents and landslides in sculpting the continental margin, processes that are largely masked where downslope proglacial sediment supply dominated. Detailed oxygen isotope, geotechnical, pXRF, and bulk sediment geochemical analyses were made on seven piston cores and were placed in context using multi-beam bathymetry and high-resolution seismic reflection profiles. Cores have a record of sedimentation back to MIS6 preserved in autochthonous sediment and slide blocks, contourites and mass-transport deposits (MTDs). Detrital‑carbonate-rich Heinrich layers (H) are present throughout the succession, but red mud layers of glacial meltwater origin from ice-streams in the SE sector of the LIS are identified only during MIS2 and late MIS3. Both layers allowed high-resolution correlation between cores. Sediments on the upper slope above H2 are condensed and the section above H1 is <50 cm thick, indicating that the powerful Labrador Current winnowed the section above H2. In contrast to the upper slope, approximately 3 to 5 m sediments were recorded in the mid-slope above H2 suggesting that the Labrador Current was weaker. Regional stratigraphic variations compared to Flemish Pass and Orphan Basin to the north suggest that NE Newfoundland sources of sediment played an important role and may have been previously underestimated. This study demonstrates that across-margin thickness variations reflect the character and distribution of the Labrador Current sediment transport system through time.Five horizons with MTDs with headscarps generally up to 25 m high are recognized in the upper 50–70 m. Thick MTDs are blocky in cores and in their seismic and bathymetric expression resemble spreads, with local thin mud clast conglomerates that ran out as debris flows over the contemporaneous seabed. Geotechnical data suggest that the seabed is stable under static conditions, although there is geological evidence for fluid escape and geotechnical evidence for underconsolidation. Weak layers along which failure took place are found somewhere around H3–H4 and at or near the top of H5 and MIS6. The low plasticity and liquid limit of the H layers make these layers susceptible to failure during cyclic loading. Episodic earthquakes are the most likely cause of failure.