tide supported gravity flows on the upper delta front fraser river delta canada abstract three anomalous events aes were recorded off the fraser river canada during the freshet of 2008 and are ascribed to quasi continuous gravity flows these flows transport warm low salinity fraser river waters to the upper delta front of the fraser delta to at least 50 m water depth all flows occurred during the freshet and spring ebbing tide and were characterized by deposition of fine grained sand and silt beds with likely high water contents these sediments were susceptible to resuspension by the subsequent flood tide current producing very high near bed suspended sediment concentrations ssc with enough excess density to generate hyper concentrated flows our observations show that these episodic events result from the combination of high river discharge high ssc and strong tides and therefore are likely to occur on the delta fronts of other large tide dominated and tide influenced rivers in shallow water basins

palaeoenvironmental reconstruction and biostratinomic analysis of the jurassic yanliao lagerstatte in northeastern china abstract the middle upper jurassic yanliao lagerstatte contains numerous exceptionally preserved fossils of aquatic and land organisms including insects salamanders dinosaurs pterosaurs and mammaliaforms despite extensive study of the diversity and evolutionary implications of the biota the palaeoenvironmental setting and taphonomy of the fossils remain poorly understood we reconstruct both the palaeoenvironment of the daohugou area one of the most famous yanliao fossil areas and the biostratinomy of the fossils we use high resolution stratigraphic data from field investigation and excavations to document in detail the stratigraphic succession lithofacies facies associations and biostratinomic features of the lagerstatte our results show that frequent volcanic eruptions generated an extensive volcaniclastic apron and lake s in the studied area the frequent alternation of thin lacustrine deposits and thick volcaniclastic apron deposits indicates either that the studied area was located in the marginal regions of a single lake where the frequent influx of volcaniclastic apron material caused substantial fluctuations in lake area and thus the frequent lateral alternation of the two facies or that many short lived lakes developed on the volcaniclastic apron most terrestrial insects were preserved in the laminated normally graded siltstone claystone and tuff facies that form many thin intervals with deposits of graded sandstone siltstone and tuff in between within each interval the terrestrial insects occur in many laminae associated with abundant aquatic organisms but are particularly abundant in some laminae that directly underlie tuff of fallout origin most of these terrestrial insects are interpreted to have been killed in the area adjacent to the studied palaeolake s during volcanic eruptions their carcasses were transported by influxes of fresh volcaniclastic material primarily meteoric runoff and possibly minor distal pyroclastic flow into the palaeolake s and were buried in palaeolake deposits prior to extended decay probably due to a combination of rapid vertical settling ash fall and water turbulence

is scaling in turbidite deposition consistent with forcing by earthquakes abstract two hypotheses have been suggested to explain the statistical distribution of bed thickness in turbidite sequences that it is a reflection of turbidite triggering by earthquakes occurring haphazardly in time and that it is controlled by the intrinsic nonlinear dynamics of a sedimentary system composed of many downslope transport events that are correlated over a broad range of scales of length and time in an attempt to distinguish between these hypotheses we have applied five tests to two previously described oligocene turbidite sequences from the izu bonin basin of the western pacific ocean we conclude that turbidite deposition in the izu bonin region is not the outcome of a self organized process nor of a low dimensional chaotic system but rather is consistent with turbidite emplacement by flows triggered by earthquakes at least some recent turbidites are known to have been deposited by such flows and the tests we have applied demonstrate that no other process is required to explain the statistics of the thickness and the timing of emplacement of beds in turbidite sequences if turbidites are indeed related to earthquakes then changes in the rate of turbidite emplacement may be used to place important constraints on paleoseismicity

patterns of deposition from experimental turbidity currents with reversing buoyancy turbidity currents are turbulent sediment laden gravity currents which can be generated in relatively shallow shelf settings and travel downslope before spreading out across deep water abyssal plains because of the natural stratification of the oceans and or fresh water river inputs to the source area the interstitial fluid within which the particles are suspended will often be less dense than the deep water ambient fluid consequently a turbidity current may initially be denser than the ambient sea water and propagate as a ground hugging flow but later reverse in buoyancy as its bulk density decreases through sedimentation to become lower than that of the ambient sea water when this occurs all or part of the turbidity current lofts to form a buoyant sediment laden cloud from which further deposition occurs deposition from such lofting turbidity currents containing a mixture of fine and coarse sediment suspended in light interstitial fluid is explored through analogue laboratory experiments complemented by theoretical analysis using a box and cloud model particular attention is paid to the overall deposit geometry and to the distributions of fine and coarse material within the deposit a range of beds can be deposited by bimodal lofting turbidity currents lofting may encourage the formation of tabular beds with a rapid pinch out rather than the gradually tapering beds more typical of waning turbidity currents lofting may also decouple the fates of the finer and coarser sediment depending on the initial flow composition the coarse fraction can be deposited prior to or during buoyancy reversal while the fine fraction can be swept upwards and away by the lofting cloud an important feature of the results is the non uniqueness of the deposit architecture different initial current compositions can generate deposits with very similar bed profiles and grading characteristics highlighting the difficulty of reconstructing the nature of the parent flow from field data it is proposed that deposit emplacement by lofting turbidity currents is common in the geological record and may explain a range of features observed in deep water massive sands thinly bedded turbidite sequences and linked debrites depending on the parent flow and its subsequent development for example a lofting flow may lead to a well sorted largely ungraded or weakly graded bed if the fines are transported away by the cloud however a poorly sorted largely ungraded region may form if during buoyancy reversal high local concentrations and associated hindered settling effects develop at the base of the cloud

the depositional architecture and facies variability of shelf deltas in the eocene battfjellet formation nathorst land spitsbergen deltas are commonly classified according to their plan view morphology as either river dominated tide dominated or wave dominated however most deltas form under the mixed influence of these processes commonly with laterally varying process regimes it has also become clear that there is a mismatch between the plan view morphology and internal facies composition in some deltas combined outcrop and subsurface data from the eocene battfjellet formation spitsbergen provide an example of ancient shelf deltas that formed under mixed influence internally these shelf deltas are characterized by wave dominated facies that are normally associated with strike extensive nearly linear shoreline sandstones however the formation comprises partially overlapping sandstone bodies of limited lateral extent 20 km in any direction this stacking pattern is attributed to frequent autogenic lobe switching that caused localized and rapid transgressions such processes typify fluvial dominated deltas and occur less commonly in wave dominated ones thus there is an apparent mismatch between inferred plan view morphology and internal facies composition it is argued that the battfjellet deltas were flood dominated and prograded mainly during periods of high fluvial discharge however reworking of the fluvial flood facies by fair weather and storm waves as well as longshore currents resulted in a wave dominated facies character delta lobes undergoing auto retreat were particularly prone to reworking by basinal processes including tidal currents it is suggested that repeated delta progradation from inner shelf settings towards the outer shelf and shelf edge was aided by high sediment supply rather than relative falls in sea level as previously suggested this interpretation is supported by i the lack of major facies dislocations and extensive sub aerial unconformities and ii an overall relative rise in sea level as evidenced by an overall low angle 0 8 to 1 2 ascending shoreline trajectory the latter results from the combined effect of basin subsidence eustatic highstand and sediment compaction

the importance of double diffusion to the settling of suspended material the vertical fingering motions which can occur when warm salty water overlies colder fresher water are important to the vertical flux of salt in the ocean an analogous and not uncommon situation is that where turbid water overlies clearer and denser water for the case of an originally sharp horizontal interface between the two water layers associated as an example with an interflow due to a warm muddy river entering a stratified lake the resulting downward sediment flux is compared with that occurring by way of gravitational settling rules are proposed to ascertain when the fingering effect could be important to sedimentation for example it seems that fingering could be of consequence in the situation where the sediment has a diameter of 2 m and the upper layer is about 1 c warmer than the clear lower layer when the upper layer concentration is 1 ppm the effect of fingering on the residence time of a gravitationally stable turbid layer is calculated such layers may be more short lived than commonly thought

assessment of wave height distributions using an extensive field database abstract the present paper investigates the short term statistical distribution of wave heights specifically some of the most commonly applied wave height distributions are assessed using field measurements the latter comprise of wave radar observations from 10 different locations in the north sea and cover water depths between 7 7 m and 45 m in total the field database includes more than 200 million waves making it one of the largest of its kind in this water depth regime in using these data the accuracy of existing wave height distributions has been examined and guidance is provided concerning the best performing models and their domain of applicability additionally insights concerning the influence of key met ocean parameters are also provided taken together the results in this paper present an overview of the statistical behaviour of wave heights in finite water depths as observed in the field

shear instability in a highly stratified estuary abstract shear instability is found to be the principal mechanism of vertical exchange within the pycnocline of a salt wedge estuary a field program involving high resolution velocity and density measurements as well as high frequency acoustic imagery allowed direct comparison of instantaneous richardson number distributions to the occurrence of shear instability the theoretical stability threshold of 0 25 is consistent with the measurements based on estimates of gradients that contain the mean as well as fluctuations due to internal waves an effective stability threshold based on mean gradients is found to be approximately one third reflecting a significant contribution of internal wave shear the integral effect of the mixing process is to homogenize the gradients of velocity and density producing linear profiles of these quantities across the pycnocline a turbulent prandtl number of unity is suggested by the vertical distributions of velocity and density during periods of active vertical mixin

which triggers produce the most erosive frequent and longest runout turbidity currents on deltas subaerial rivers and turbidity currents are the two most voluminous sediment transport processes on our planet and it is important to understand how they are linked offshore from river mouths previously it was thought that slope failures or direct plunging of river flood water hyperpycnal flow dominated the triggering of turbidity currents on delta fronts here we re analyse the most detailed time lapse monitoring yet of a submerged delta comprising 93 surveys of the squamish delta in british columbia canada we show that most turbidity currents are triggered by settling of sediment from dilute surface river plumes rather than landslides or hyperpycnal flows turbidity currents triggered by settling plumes occur frequently run out as far as landslide triggered events and cause the greatest changes to delta and lobe morphology for the first time we show that settling from surface plumes can dominate the triggering of hazardous submarine flows and offshore sediment fluxes

finite dispersal of a separative nepheloid plume by an internal hydraulic jump in a tropical mountainous river estuary this paper investigates the dynamics of an internal hydraulic jump in a river plume and associated suspended sediment dispersal field investigations were undertaken into the river plume generated by the herbert river australia following a moderate flood event induced by cyclone fritz in 2004 the forced plume experiences an abrupt transition from supercritical to subcritical via an internal hydraulic jump as defined by a mode 1 internal froude number computed using the phase speeds from the taylor goldstein equation the hydraulic theory of a two layer stratified flow was used to identify the plume shape and the mechanical energy loss within the jump the hydraulic jump energy loss is primarily transferred to the buoyancy driven potential energy uplifting the river plume intense stratification decreases the bottom stress damping the resuspension therefore a separative nepheloid dispersal system occurs at the jump section both the upper and lower nepheloid flows are confined to the inner shelf but have different dispersal behaviors and mechanisms the upper nepheloid flow which is primarily controlled by advection and settling satisfies an exponential decay law of the total suspended sediment concentrations versus the offshore distance the lower nepheloid flow dominated by deposition is detached seaward near the lift off point of the river plume a turbidity front associated with the jump may accumulate a large quantity of suspended sediments enhancing sediment release from the river plume these findings will promote in depth understanding of both the cross shelf sediment dispersal and muddy deposit on the shelf

from source to sink a new perspective on the past dynamics of the murray canyon group from benthic foraminiferal communities abstract we present fossil benthic foraminiferal assemblage data from marine sediment core ss02 06 gc2 located in the abyssal plain of the murray canyon group offshore south australia the sedimentological characteristics indicate the presence of turbidite deposits showing classical bouma like sequences dated between 40 and 12 cal ka bp these results confirm the previous interpretation of the observed large deep water holes in the abyssal area where the core was sampled as being gouged by surges of high energy turbidity currents the presence of good indicator taxa and unique assemblages occupying specific bathymetric depths allows the determination of the source origin of the sediments making the turbidites three distinct faunal groups are found 1 mostly shelf species 2 mostly bathyal species and 3 mostly abyssal species in the sediment core these groups present a quasi systematic succession with nearly all bouma like sequences starting with the dominance of bathyal species in the coarse grained base followed by the dominance of shallow species in the silty part and finally with abyssal species in the clays to explain such phenomena turbidites triggered by mixed hyperpycnal hypopycnal flow processes and turbidity currents during periods of river floods are considered for the first time within the murray canyon group they are mostly related to periods of increased fluvial discharges during wet phases in the murray darling basin

measurement of size and settling velocity of suspended aggregates on the northern california continental shelf abstract a high resolution video camera attached to a miniature sediment trap was used to make time lapse images of suspended aggregates settling through the water column the video trap system was placed 2 m above the seabed in the strataform study area off eureka california and was programmed to take a 7 second time sequence of settling particles every 6 h over a 4 month period the camera field of view within the trap is 15 mm and the minimum particle resolution is approximately 130 m data analysis was carried out with a video frame grabber and digital imaging software to determine the particle size shape and settling velocity results of the first deployment of this system september 1995 show that particle sizes elliptical nominal diameter determined from the video images ranged from 130 to 740 m with a modal size of 300 m the average form factor ratio of short to long semi axis of the aggregates was 0 71 measured settling velocities varied from 0 09 to 8 13 mm s in terms of weight percent or mass the median size of the aggregate distribution is 600 m analysis of settled particles collected from the sediment trap shows disaggregated or component grains ranging from 1 0 to 63 m with a modal size of 26 m and a median size of 11 m additionally an analysis was carried out to separate the relative importance of particle shape and excess density on settling velocity and these results were compared to observations using the equations for settling velocity of spheres and prolate spheroids and substituting published relationships for size versus excess density into these equations the settling velocity for combinations of shape and excess density has been calculated results show that differences between sphere and prolate spheroid with major axis both parallel and perpendicular to flow approximations caused less than a 13 difference in computed settling velocity in contrast the various size excess density relationships used to compute settling velocity caused order of magnitude deviations from measured values in all cases however the approximations of settling velocity for aggregates provide a significantly better fit to data than approximations based on disaggregated sediment size distribution

long shelf sediment transport and storm bed formation by cyclone winifred central great barrier reef australia three major sediment facies belts occur on the great barrier reef shelf an inner shelf prism of holocene terrigenous sand and mud a patchy middle shelf veneer of palimpsest muddy shelly calcsand 1 m thick above a weathered pleistocene erosion surface and an apron of modern carbonate mud and sand around reefs of the main reef tract these sediments were mobilized and strongly affected by the passage of severe tropical cyclone winifred central pressure 958 hpa which traversed the central great barrier reef shelf on february 1 1986 re surveys after the passage of cyclone winifred showed that i a storm induced shell lag and or a normally graded bed of terrigenous fine sand mud had been deposited at depths down to 20 m ii middle shelf longitudinal bedforms were widespread at depths of 28 35 m and comprised a furrowed substrate of palimpsest muddy shell gravel surmounted by 40 150 m wide ribbons of quartzose and bioclastic sand up to 15 cm thick iii fields of northward facing 1 2 m wavelength megaripples were present adjacent to zones of ribbons and in the alleys between ribbons iv five days after the cyclone mud was still settling from suspension and large parts of the inner shelf were bathed in muddy hypopycnal river plumes some of which reached 30 km seaward to the inner edge of the reef tract and v suspended mud derived in part by unmixing of the seabed was present throughout the shelf water column and a seaward thinning mud drape up to 40 cm thick had accumulated on and seawards of the inshore sediment prism tapering seawards to a few mm thick only over most of the middle shelf it is inferred that storm waves and currents associated with the passage of cyclone winifred caused widespread unmixing of bottom sediments offshore long shelf transport of bedload sand ribbons and megaripples was affected by powerful shelf parallel currents of velocity 1 3 m s as the storm passed the graded seaward thinning sand mud bed was deposited over the inner middle shelf on the middle shelf bioturbation and downward mixing of the mud drape started immediately after the passage of the cyclone and was well advanced after 3 months in this way the ephemeral seabed features produced by passage of a cyclone are gradually degraded and the storm layer stratigraphy becomes incorporated within a thin middle shelf veneer of muddy shelly calcsand

particle interaction in fjord suspended sediment abstract glacial flour enters the surface layer of howe sound a fjord in southwestern british columbia as a sediment plume which moves quickly down inlet while gradually mixing with the sea water flocculation occurs in the lower brackish waters of the surface layer although mixing and diffusion are the dominant processes for sediment to enter the marine water once in the underlying marine water zooplankton pelletization and biologic agglomeration of inorganic floccules predominate this generates a response in the order of days between the two layers in terms of sedimentation of particulate matter most particles settling in the fjord environment fall as 1 sand and silt grains containing attached clay particles 2 clay clasts possibly related to river mudballs 3 mineral bearing fecal pellets from pelagic zooplankton 4 large grain inorganic and colloidal floccules and 5 inorganic biogenic agglomerates analysis of sediment trap data has led to the following conclusions 1 water turbidity cannot be used to infer the downward flux of particles 2 size distributions of sediment deposited on the sea bed are a function of non lognormal size distributions from particles composing sub laminae

the influence of diffusive convection on sedimentation from buoyant plumes abstract diffusive convection driven by the differential diffusion of density altering fluid properties may enhance the scavenging of particles from natural buoyant plumes for single phase fluid solute heat systems this phenomenon has been extensively studied because salt fingering generated at the oceanic thermocline is a major mechanism of salt transport in the oceans however the influence of this process on particle laden plumes for example fluvial plumes in lakes and estuaries volcanic clouds and seafloor hydrothermal plumes is largely unknown in this paper we present direct experimental measurements of the interfacial particle flux at the plume base which can be applied to predict particle scavenging from natural buoyant plumes particle flux is measured using a light attenuation technique employing a chain of photodiodes which average concentration over a large number of fingers the results are in good general agreement with earlier studies based on finger velocity flux measurements cover a wide range of conditions from those where diffusive convection dominates to those where settling and diffusive convection are of a similar magnitude for very small particles double diffusive salt finger theory is applicable to two component particulate systems as suggested by earlier studies green t 1987 the importance of double diffusion to the settling of suspended material sedimentology 34 319 331 two component diffusive convection theory is extended to three components in order to predict particle scavenging from marine fluvial plumes which involve the diffusion of sediment salt and heat for larger particles which settle significantly the flux can be approximated by adding the double diffusive and settling fluxes a theory to predict particle transport through the lower layer and sedimentation at the bed is developed based on the observation of strong convection below the plume application of our theory and experimental results indicates that while double diffusion may significantly influence the longitudinal distribution and vertical sorting of deposits from lacustrine plumes the diffusive convection process is generally insignificant in marine plumes observational evidence for lacustrine double diffusion based on water column measurements is presented

contrasting styles of shelf sediment transport and deposition in a ramp margin setting related to relative sea level change and basin floor topography turonian cretaceous western interior of central utah usa abstract the turonian lower ferron sandstone and juana lopez members of the mancos shale in the western interior foreland basin form a series of regressive shoreface to shelf depositional complexes sourced from the sevier fold thrust belt to the west continuous dip parallel outcrop exposures around the northern and eastern edges of the tertiary san rafael anticline sra in central utah provide an insight into the origins of anomalous marine mudstone encased sandstones lower ferron strata consist of both coarsening and thickening upward and sharp based shoreface successions interpreted to have been deposited as sequential highstand and falling stage systems tracts overlying these shallow marine deposits is a surface that marks sediment bypass that is correlated down dip into coarse grained cross bedded sandstones interpreted as the deposits of channelised turbidity flows these turbidites are considered to be the products of sustained hyperpycnal flows exiting rejuvenated lowstand rivers based on sustained flow indicators such as dune scale cross bedding absence of genetically related slumps and delta front deposits in up dip areas surfaces of sediment bypass coarser grain sizes than the underlying shoreface deposits and regional palaeogeography the flows are interpreted to have been ignitive in proximal areas down a tectonically induced slope in the uplifted area of the farnham dome the change in depositional style from falling stage to lowstand systems tracts is attributed to increased rates of sediment supply to the shoreline promoting hyperpycnal flows a flooding surface caps the interval and separates the lower ferron sandstone from the overlying juana lopez member this interval in contrast consists only of heterolithic parallel and ripple laminated graded sandstone mudstone couplets interpreted to have been deposited by turbulent storm induced geostrophic flows the couplets are arranged into bundles and are interpreted as parasequences which in turn are arranged into a progradational parasequence set deposited during a period of sea level highstand divergent sole mark and bedform palaeocurrents from both members suggest slight eastward deflection of the turbulent flows off the proto sra further evidence for this topography is inferred from an absence of turbidites along the western side of the structure the inferred influence of local basin floor structures on sedimentation of these units has important implications for structural models for this basin for generalized sequence stratigraphic models applied to ramp type foreland basin sequences and for elucidating the origin of anomalous marine mudstone encased sandstone bodies

depositional characteristics and reservoir potential of paleogene sediment gravity flow deposits on a faulted slope of the zhanhua sag bohai bay basin china abstract deep water sandstones related to sediment gravity flows are becoming an increasingly important exploration domain in continental rift basins in china however ambiguities remain regarding the initiation evolution and deposition of sediment gravity flows on faulted slopes where step faults are commonly developed to address the uncertainties related to these processes grain size analysis lithofacies analysis logging interpretation and seismic interpretation were employed to study paleogene sediment gravity flow deposits from well cores on a faulted slope in the zhanhua sag of the bohai bay basin twelve lithofacies related to sediment gravity flows were recognized and attributed to intrabasinal sediment gravity flows isgfs induced by slope failures and extrabasinal sediment gravity flows esgfs generated by floods active faults were important triggers for isgfs on the faulted slope in the zhanhua sag the fault activity rate had a prominent role in determining the volume of re transported sediments which further influenced the evolution of the resulting flows travelling across multiple faults downslope isgfs generally underwent considerable velocity losses with the result that almost all sediments were deposited in shallow water moreover unconsolidated isgf deposits could be transported again to generate sediment gravity flows obscuring the depositional characteristics of isgfs esgfs originated from rivers during floods and involved the transformation of cohesive into turbulent hyperpycnal flows in humid and warm climates active sag border faults favoured the generation of long lived esgfs these flows usually travelled along intra sag faults and therefore considerable volume of sediments could be transferred into deep water the resulting thick bedded hyperpycnites contributed substantially to the reservoirs of bonan oilfield meanwhile short lived esgfs usually generated in the regions where sag border faults were extinct they generally travelled down the faulted slope and deposited sediments in shallow water these features suggest that the initiation evolution and deposition of sediment gravity flows on the faulted slope are mainly controlled by faults and climate hyperpycnites can serve as excellent reservoirs for hydrocarbon accumulation and they are probably common in other sags of the bohai bay basin

settling driven convection a mechanism of sedimentation from stratified fluids convection driven by sediment particles may play an important role in sedimentation from the base of buoyant hypopycnal plumes for example fluvial plumes in stratified estuaries and lakes black smokers on the ocean floor volcanic clouds and coastal currents in addition to the well known double diffusive convection mechanism another mode of convective instability development is by settling across the density interface we performed laboratory experiments to investigate this fingering convective instability mechanism and its effect on particle distribution in the water column and deposition at the bed a simple theoretical model of finger formation at a fluid density interface is developed based on an analogy with thermal plume formation at a flat heated plate this model which involves a thickening interface layer that becomes gravitationally unstable relative to the ambient fluid is in good agreement with measurements of finger size and instability wavelength from visualization experiments since fingering at the density interface drives larger scale convection in the fluid below a mass balance model of the lower layer assuming strong mixing i e uniform sediment concentration is successfully applied to predict sediment concentration in the water column and deposition at the bed strong mixing can be assumed since convective velocities are usually much greater than the particle fall velocities as convection proceeds the sediment concentrations in the two layers approach each other and convection will die out using the model equations we develop analytical expressions for the time when convection ceases and the portion of sediment remaining in the water column

hyperpycnal plume formation from riverine outflows with small sediment concentrations a series of laboratory experiments has been conducted in order to elucidate the sediment induced mixing processes accompanying riverine outflows specifically the discharge of a warm fresh particle laden fluid over a relatively dense cool brine in a parameter regime analogous to recently acquired field measurements hypopycnal surface plumes were subject to a convective instability driven by some combination of heat diffusing out of the warm fresh sediment laden plume and particle settling within it convection was robust in the presence or absence of intense turbulence at sediment concentrations as low as 1 kg m 3 and took the form of millimetre scale sediment laden fingers descending from the base of the surface plume a consequence of the convective instability of the original hypopycnal plume is the generation of a hyperpycnal bottom riding flow the experiments presented here indicate that natural river outflows may thus generate hyperpycnal plumes when sediment concentrations are 40 times less than those required to render the outflow heavy relative to the oceanic ambient the resulting hyperpycnal plumes may play an important role in transporting substantial quantities of sediment to the continental slope and beyond

sedimentation from flocculated suspensions in the presence of settling driven gravitational interface instabilities we experimentally examine sedimentation from a freshwater suspension of clay flocs overlying saltwater in the presence of gravitational instabilities the study seeks to determine 1 if flocculation hampers or alters interface instability formation 2 how the removal rates of sediment from the buoyant layer compare to those predicted by individual floc settling and 3 whether or not it is possible to develop a model for effective settling velocity the experiments were conducted in a tank at isothermal conditions all experiments were initially stably stratified but later developed instabilities near the interface that grew into downward convecting plumes of fluid and sediment throughout we measured sediment concentration in the upper and lower layers floc size and plume descent rates the data showed that flocculation modifies the mixture settling velocity and therefore shifts the mode of interface instability from double diffusive what one would expect from unflocculated clay to settling driven leaking and rayleigh taylor instability formation removal rates of sediment from the upper layer in the presence of these instabilities were on the same order of magnitude as those predicted by individual floc settling however removal rates were found to better correlate with the speed of the interface plumes a simple force balance model was found to be capable of reasonably describing plume velocity based on concentration in the buoyant layer this relation coupled with a critical grashof number and geometry relations allowed us to develop a model for the effective settling velocity of the mixture based solely on integral values of the upper layer

convective instability in sedimentation linear stability analysis 1 convective sedimentation in a stably stratified saltwater is studied using the linear stability analysis convective sedimentation is known to occur due to the double diffusive mechanism and the settling driven mechanism in this study a semi empirical closure of sediment diffusivity based on the long range hydrodynamics effect is adopted the sediment phase can act as either the slow or fast diffusing agent in the double diffusive system for the given salt diffusivity moreover the settling driven effect is proportional to the square of the sediment diameter via stoke settling law we consider sediment concentration grain size in the upper freshwater layer to be in the range of 0 1 to 39 4 g l 2 to 60 m which is on top of a saltwater layer with salinity 35 ppt linear stability analysis allows us to identify the dominant mechanism that triggers the instability the growth rate and the resulting characteristic finger width model results suggest that for fine sediment with grain diameter smaller than 10 m settling velocity 0 09 mm s double diffusive mechanism controls the instability and the resulting sediment finger size is of millimeter scale for the given threshold of growth rate of o 0 01 s 1 the minimum sediment concentration is about 8 15 g l for grain size greater than or around 10 m the settling driven mechanism dominates and instabilities occur at sediment concentration as low as o 0 1 g l with centimeter scale fingers our findings may contribute to a better understanding on the observed rapid sediment removal in the plume of small mountainous rivers

a submarine canyon as a climate archive interaction of the antarctic intermediate water with the mar del plata canyon southwest atlantic abstract the mar del plata canyon is located at the continental margin off northern argentina in a key intermediate and deep water oceanographic setting in this region strong contour currents shape the continental margin by eroding transporting and depositing sediments these currents generate various depositional and erosive features which together are described as a contourite depositional system cds the mar del plata canyon intersects the cds and does not have any obvious connection to the shelf or to an onshore sediment source here we present the sedimentary processes that act in the canyon and show that continuous holocene sedimentation is related to intermediate water current activity the holocene deposits in the canyon are strongly bioturbated and consist mainly of the terrigenous sortable silt fraction 10 63 m without primary structures similarly to drift deposits we propose that the mar del plata canyon interacts with an intermediate depth nepheloid layer generated by the northward flowing antarctic intermediate water aaiw this interaction results in rapid and continuous deposition of coarse silt sediments inside the canyon with an average sedimentation rate of 160 cm kyr during the holocene we conclude that the presence of the mar del plata canyon decreases the transport capacity of aaiw in particular of its deepest portion that is associated with the nepheloid layer which in turn generates a change in the contourite deposition pattern around the canyon since sedimentation processes in the mar del plata canyon indicate a response to changes of aaiw contour current strength related to late glacial holocene variability the sediments deposited within the canyon are a great climate archive for paleoceanographic reconstructions moreover an additional involvement of hemi pelagic sediments indicates episodic productivity events in response to changes in upper ocean circulation possibly associated with holocene changes in intensity of el nino southern oscillation

in situ observations of floc settling velocities in glacier bay alaska abstract in situ floc settling velocities and diameters of particles ranging in size from 0 63 to 5 05 mm equivalent circular diameter were measured under a buoyant discharge plume by deploying a bottom tripod mounted floc camera assembly fca in tarr inlet glacier bay alaska these observations were used to estimate floc effective densities three results emerge from this work first fits of settling velocity and effective density to diameter are consistent with expressions published for other environments suggesting that common controls on floc size and settling velocity operate across diverse marine environments second the raw data show considerable scatter with upper and lower 95 prediction intervals on settling velocity and excess density differing by about a factor of 7 analysis of sources of error suggests that the variability is caused by differences in component grain composition among flocs and turbulent stirring within the stilling box third bin averaged effective densities and settling velocities are highly correlated with diameter thus while it is not possible based on diameter to predict accurately the settling velocity of a single floc it is possible to estimate the mean settling velocity of a population of like sized flocs

river plume sedimentation modeling for sequence stratigraphy application to the eel margin northern california abstract sediment deposition in the coastal zone from riverine input is of key importance in understanding the construction of sediment sequences through time a turbulent jet model is used to simulate sediment input to the continental margin from a riverine source turbulent jet models track the momentum which is the first order force carrying most of the sediment flowing into the ocean the froude number at the river mouth remains supercritical during floods of the eel river justifying the use of a jet model for the initial dispersion of the sediment laden plume sediment is deposited from the plume prior to the centerline froude number decreasing to its critical value empirically derived first order removal rate constants are used for each grain size in an advection diffusion equation to predict the areal extent of sediment deposition one strength of using a jet model is its ability to simulate deposits on a daily time step over intermediate time scales thousands of years using a limited number of input parameters a number of simulations of the eel river plume are shown for conditions of the present day and 18 000 yr bp during the last glacial maximum the model results for modern conditions show that a majority of the deposition occurs within 20 km north of the river mouth and is typically confined to within 10 km of the coastline corresponding well with the shelf sands on the eel margin however the initial mud deposit from a 1995 flood was farther offshore and northward additional advection of the mud fraction must occur after it leaves the surficial plume and prior to its initial deposition the model runs of 18 000 yr bp with 100 m lower sea level wetter and colder climate and the river mouth assumed to be at the canyon show that the plume is able to transport sand silt and clay out of the canyon and onto the continental slope

sedimentary and rock magnetic signatures and event scenarios of deglacial outburst floods from the laurentian channel ice stream abstract eastern canadian margin sediments bear testimony to several catastrophic deglacial meltwater discharges from the retreating laurentide ice sheet the reddish brown plumite layers deposited on the levees of the laurentian fan valleys have been recognized as indications of multiple outburst floods between heinrich events 2 and 1 five event layers have been consistently recorded in three new gravity cores retrieved on the sw grand banks slope and comply with the previously published laurentian fan core md95 2029 the apparently huge extent of these outburst plumes around the laurentian fan as well as their causes and consequences are investigated in this study using physical properties rock magnetic and grain size analyses together with seismoacoustic profiling we provide the first detailed 14c ages of the outburst event sequence and discuss their recurrence intervals in the context of regional ice retreat compared to the hemipelagic interlayers event layers have overall uniform and systematic changes of rock magnetic properties hematite contents increase over time and proximally while magnetite grain sizes fine upwards and spatially away from the fan based on the sediment composition and load we argue that these plumites were formed by recurrent erosion of glacial mud deposits in the laurentian channel by meltwater outbursts three alternative glaciological scenarios are evaluated in each case the provenance of the transported sediment is not an indicator of the precise source of the meltwater

submarine canyon head morphologies and inferred sediment transport processes in the alias almanzora canyon system sw mediterranean on the role of the sediment supply abstract canyon head regions are key areas for understanding the shelf to canyon sedimentary dynamics and assessing the predominant hydrodynamic and sedimentary processes shaping their morphology high resolution multibeam bathymetry surveys conducted on the shelf upper slope and along the main thalwegs of the alias almanzora submarine canyon system were used to recognize the specific morphological features of the various canyon heads and infer the main sediment transport mechanism operating through them off the almanzora river different canyon head morphologies were observed at 65 90 m water depth incising the outer prodeltaic deposits a direct connection from the river supply is evidenced by the presence of a large radial field of sediment waves on the prodelta presumably related to hyperpycnal flows during flash flood events and their continuation towards trains of cyclic steps that merge into the almanzora canyon head processes linked to seepage and groundwater sapping also seems apparent in the prodeltaic deposits which results in pockmark alignments and formation of elongated narrow canyon heads at the southern limit of the almanzora prodelta another type of canyon head results from the formation and merging of linear gullies these have been interpreted as the morphological expression of the distal off shelf transport of flood related hyperycnal flows however most of the canyon heads of the main tributaries of this canyon system i e garrucha cabrera and alias are found at much shallower water depths 7 20 m they generally show a continuation in land with rivers or intermittent creeks known as ramblas that also experience flash flood events but some of them are disconnected from any river source these canyon heads cut into the infralittoral prograding wedge displaying feeder channels that tend to oriented against the direction of the prevalent long shore current some canyon head tributaries show sediment waves along their axis interpreted as the result of hyperpycnal flood events or storm induced turbidity flows this study focusses on the diversity of sediment transport processes that can co exist in the various canyon tributaries of a given submarine canyon system stressing on the importance of the canyon heads location with respect to the principal sedimentary source which ultimately determines their main geomorphological traits

enhanced sediment scavenging due to double diffusive convection abstract river water intruding into a lake or ocean typically results in a surface layer of warm sediment laden water overriding relatively dense cold fluid though this system is stable with respect to density instability may arise as a result of the different diffusivities of sediment and heat a series of experiments were conducted which indicated that double diffusive convection ddc was responsible for rapid sedimentation of slowly settling particles a simple theory based upon relevant physical processes is developed that describes this behavior it also incorporates the previously neglected effects of ambient stratification on the strength of the ddc application of the theory and our results to field data taken from oceanic river plumes demonstrates that the flux due to double diffusive sedimentation dds can be at least as strong as flocculation enhanced gravitational settling

the role of saltwater and waves in continental shelf formation with seaward migrating clinoform continental shelves have generally been interpreted as drowned coastal plains associated with the allogenic effect of sea level variation here without disputing this mechanism we describe an alternative autogenic mechanism for subaqueous shelf formation driven by the presence of dissolved salt in seawater and surface waves we use a numerical model describing flow hydrodynamics sediment transport and morphodynamics in order to do this more specifically we focus on two major aspects 1 the role of saltwater in the subaqueous construction of continental shelves and 2 the transformation of these shelves into seaward migrating clinoforms under the condition of repeated pulses of water and sediment input and steady wave effects but no allogenic forcing such as sea level change in the case for which the receiving basin contains fresh water of the same density as the sediment laden river water the hyperpycnal river water plunges to form a turbidity current that can run out to deep water in the case for which the receiving basin contains sea water but the river contains sediment laden fresh water the hypopycnal river water forms a surface plume that deposits sediment proximally this proximate proto shelf can then grow to wave base after which wave supported turbidity currents can extend it seaward the feature we refer to is synonymous with near shore mud belts

hyperpycnal transport of carbonaceous sediment example from the upper devonian rhinestreet shale western new york u s a abstract the upper devonian rhinestreet shale of the western new york region of the appalachian basin contains centimeter scale composite beds comprised of a thin black shale layer overlain by several cm of gray shale that are interpreted to have been deposited from hyperpycnal turbidity currents evidence for this explanation includes 1 the presence of planar parallel and cross lamination within some layers and 2 zr al profiles consistent with an inversely graded lower unit overlain by normally graded deposits diminished k al values of the inferred hyperpycnites suggest that they were generated during periods of increased precipitation and consequent intensified continental weathering of the source region within the tectonically active acadian highlands the transition to a wetter climate reflected in these deposits perhaps a response to astronomically induced variations in tropical monsoonal dynamics would have increased the frequency and magnitude of floods within the catskill delta complex thereby enhancing the likelihood that hyperpycnal flows would have been produced organic matter contained within the hyperpyncites probably reflects the entrainment of carbonaceous detritus perhaps much of it in the form fecal pellets from estuarine and shallow marine settings during flood related discharges post depositional oxygen depletion within the organic rich sediment and related bacterial sulfate reduction resulted in the precipitation of abundant diagenetic pyrite intense bioturbation of gray shale intercalated with the dark layers suggests that oxygenated conditions prevailed during accumulation of the sturgeon point section alternatively these deposits reflect a history of repeated early diagenetic burn down events that resulted in the strongly reworked nature of the organic lean gray shale and the sharp upper contacts of the black shale layers the present study offers an alternative explanation of thin black shale beds hosted within organic deficient successions beyond models involving marked changes in productivity on the one hand and the establishment of a stratified water column on the other

a numerical investigation of convective sedimentation 1 understanding the fate of riverine sediment in the coastal environment is critical to the health of the coastal ecosystem and the changing morphology one of the least understood mechanisms of initial deposition is the convective sedimentation of hypopycnal plumes this study aims at investigating convective sedimentation by means of a numerical model for fine sediment transport solving the non hydrostatic reynolds averaged navier stokes equations for stratified turbulent flow model validation is sought by comparison to laboratory results for turbidity and saline currents over a changing slope the model is shown to be capable of predicting both the upstream supercritical and the downstream subcritical flows the numerical model is then utilized to study convective sedimentation and its depositional and mixing characteristics by analyzing model results of more than 40 runs for different inlet sediment concentration density ratio settling velocity particle reynolds number rep and inlet velocity height inlet reynolds number re four distinct flow regimes are revealed for large we observe divergent plumes with significant deposits near the inlet for intermediate and large rep intense convective fingers are predicted which are only marginally affected by ambient shear flow further reducing the density ratio or rep gives weak convective fingers that are significantly affected by the ambient shear flow eventually no convective fingers are observed during the computation for very small or rep sediment deposits in the divergent plume and intense convective finger regimes are relatively insensitive to re deposit increases with re in the weak convective finger regime

relative importance of processes that govern the generation of particulate hyperpycnal flows simple mathematical approaches are used to investigate the relative importance of salinity temperature dissolved load stratification of suspended load and estuarine mixing on the direct formation of particulate hyperpycnal currents from river outflow specifically the critical concentration of suspended sediment for hyperpycnal flow formation is related to a reference concentration that does not take any of these processes into account the results show that the most important process in reducing this critical concentration is estuarine mixing which can reduce it by several orders of magnitude the other processes are relatively secondary but can all reduce the critical concentration by about one order of magnitude examples from rivers where hyperpycnal flows are known to form illustrate the influence of the described processes

climate forcing of fine grained deep marine systems in an active tectonic setting middle eocene ainsa basin spanish pyrenees abstract a multi proxy approach to understand environmental change in deep time was undertaken on about 150 m of core from a 230 m long middle eocene core from the ainsa basin spanish pyrenees representing deep marine siliciclastic sediments using detailed sedimentary logging high resolution multi element xrf geochemistry total organic carbon and stable carbon isotopes the well a6 was drilled as part of an industry university consortium through siltstones fine very fine grained sandstone turbidites and hemipelagic structureless mudstones that were deposited as overbank and off axis deposits from a sandy submarine fan and interfan deposits for comparative mineralogy between the sandstone turbidites and siltstones scanning electron microscopy sem and x ray diffraction xrd were undertaken on selected samples the sandstone turbidites show enrichment of detrital elements such as si zr and ti that can be linked to greater quartz and heavy mineral content compared with adjacent siltstones structureless hemipelagic mudstones comprise mainly clay minerals and carbonate we interpret the sandstone turbidites as from hyperpycnal flows during high river sediment discharge whilst hemipelagic mudstones resulted from the suspension fall out of hypopycnal flows cyclostratigraphic analysis of the core reveals milankovitch cyclicity at frequencies of 0 03 cycles m short eccentricity 0 09 cycles m obliquity 0 15 cycles m precession couplet and 0 19 cycles m precession couplet orbital parameters appear to have controlled the cyclic delivery of coarser grained sediment by turbidity currents two equally plausible depositional models both as milankovitch driven can explain the cyclical changes in the deep marine sediments 1 climatic cycles with humid periods of enhanced chemical weathering increased storminess and greater riverine run off leading to high sediment flux to the deep basin as sandstone turbidites 2 climatic cycles with cooler conditions linked to high frequency small scale eustatic sea level fluctuations with lowstand shelf edge delta progradation resulting in greater volumes of coarse detrital sediment to the seafloor by hyperpycnal flows this study provides an insight into the likely depositional effects of orbitally induced climate change on the nature and delivery of terrigenous sediment into deep marine environments

facies architecture and stratigraphic evolution of a river dominated delta front turonian ferron sandstone utah u s a abstract facies architectural analysis of a river dominated deltaic parasequence in the turonian ferron sandstone member near factory butte utah was conducted to determine the hierarchical spatial arrangement of mouth bars and their role in constructing the delta five architectural elements were identified using 21 measured sections paleocurrent measurements and bedding diagrams they are 1 prodelta pf 2 frontal splay fs 3 detached distal bars dbs 4 distributary mouth bars dmbs and 5 terminal distributary channels tdcs lower in the parasequence detached bars consist of proximally located cross bedded and quasi parallel laminated subaqueous bars that onlap the delta front slope and grade distally into finer grained distal delta front splays these bars translate downstream in a progradational aggradational degradational trajectory and are overlapped by adjacent bars forming a bar assemblage these bars are interpreted to have been deposited by descending hyperpycnal flows as a decelerating axial jet deposition of these bars occurs at the delta front to prodelta chokepoint presumably due to flow deceleration the distal bar assemblage is capped by an extensive but discontinuous shale drape suggesting local abandonment upper facies assemblages show a distinct switch to predominantly cross bedded distributary mouth bars corresponding to a decrease in water depth an increase in friction and a likely transition into planar versus axial jets this is indicated by aggrading cross bedded distributary mouth bars that overlie the distal bars laterally the mouth bar deposits are interstratified with terminal distributary channel deposits with prominent scour surfaces at the base separate distributary channels and their bars coalesce ultimately to build the entire delta lobe the detached distal bars prograde locally west to northwest and lack fair weather wave formed sedimentary structures suggesting a local embayment along the regionally northeast prograding shoreline low levels of ichnofaunal diversity indicate a stressed river dominated setting at the base of the section frontal splays and detached bars dominated by storm driven river flood deposits are deposited over muddy prodelta facies detached bars are likely initiated about 500 meters distal of the distributary channel in water depths of 10 30 meters based on the thickness of the parasequence once this subaqueous platform is constructed by the distal bar assemblage more proximal distributary mouth bars fill the remaining shallow water accommodation as the system continues to shallow distributary channels override and scour the bars reflecting reduced accommodation

in situ measurements of velocity structure within turbidity currents 1 turbidity currents are thought to be the main mechanism to move 500 000 m3 of sediments annually from the head of the monterey submarine canyon to the deep sea fan indirect evidence has shown frequent occurrences of such turbidity currents in the canyon but the dynamic properties of the turbidity currents such as maximum speed duration and dimensions are still unknown here we present the first ever in situ measurements of velocity profiles of four turbidity currents whose maximum along canyon velocity reached 190 cm s two turbidity currents coincided with storms that produced the highest swells and the biggest stream flows during the year long deployment

evaluating along strike variation using thin bedded facies analysis upper cretaceous ferron notom delta utah thin bedded delta front and prodelta facies of the upper cretaceous ferron notom delta complex near hanksville in southern utah usa show significant along strike facies variability primary initiation processes that form these thin beds include surge type turbidity currents hyperpycnal flows and storm surges the relative proportion of sedimentary structures generated by each of these depositional processes events has been calculated from a series of measured sedimentological sections within a single parasequence ps6 1 which is exposed continuously along depositional strike for each measured section sedimentological data including grain size lithology bedding thickness sedimentary structures and ichnological suites have been documented parasequence 6 1 shows a strong along strike variation with a wave dominated environment in the north passing abruptly into a fluvial dominated area then to an environment with varying degrees of fluvial and wave influence southward and back to a wave dominated environment further to the south east the lateral facies variations integrated with palaeocurrent data indicate that parasequence 6 1 is deposited as a storm dominated symmetrical delta with a large river dominated bayhead system linked to an updip fluvial feeder valley this article indicates that it is practical to quantify the relative importance of depositional processes and determine the along strike variation within an ancient delta system using thin bedded facies analysis the wide range of vertical stratification and grading sequences present in these event beds also allows construction of conceptual models of deposition from turbidity currents i e surge type turbidity currents and hyperpycnal flows and storm surges and shows that there are significant interactions and linkages of these often paired processes

early muddy deposits along the gulf of lions shoreline a key for a better understanding of land to sea transfer of sediments and associated pollutant fluxes large quantities of sediment introduced by rivers especially during flood events lead to the formation of prodeltaic structures characterized by fine grained deposits in the proximal area of the shelf according to their mean grain size it is largely admitted in the literature that these deposits mainly stand below the storm wave base in the gulf of lions nw mediterranean however our data show that these muddy sediments also exist above the storm wave limit in the uppermost unit of the holocene prism the purpose of this work is therefore to draw a detailed sedimentological and geochemical portrait of these units and to discuss the processes involved in the sedimentation and remobilization of these sediments our results show that the fine particles accumulate earlier in the inner shelf in front of the nearby rivers in association with the chemical elements which are commonly associated with this grain size fraction e g organic carbon radionuclides and heavy metals this accumulation probably mainly occurs during storm and flood events the early deposits are then exposed to resuspension events and can actively participate to the distribution of fine grained sediment in the entire margin by advective transfers via the benthic nepheloid layer these fine grained nearshore prodeltas are a common feature of the mediterranean deltaic systems and mark the frontal processes that occur at the continent sea interface of non tidal seas our results especially point out the potential use of heavy metals as markers of the tracking of riverine sediments which is naturally an important condition when evaluating the ultimate fate of land derived pollutants in the marine realm but also more fundamental studies on the understanding of sedimentological processes and reconstruction of margin stratigraphies should consider these deposits as functional units for the distribution of platform sediments

tide modulated hyperpycnal flows off the huanghe yellow river mouth china few hyperpycnal flows have ever been observed in marine environments although they are believed to play a critical role in sediment dispersal within estuarine and deltaic depositional systems the paper describes hyperpycnal flows observed in situ off the huanghe yellow river mouth their relationship to tidal cycles and the mechanisms that drive them simultaneous observations at six mooring stations during a cruise off the huanghe mouth in the flood season of 1995 suggest that hyperpycnal flows observed at the river mouth are initiated by high concentrations of sediment input from river and modulated by tides hyperpycnal flows started near the end of ebb tides when near bottom suspended sediment concentration ssc increased rapidly and salinity decreased drastically an inverse salt wedge the median grain size of suspended particles within the hyperpycnal layer increased causing strong stratification of the suspended sediments in the water column towards the end of flood tides the hyperpycnal flow attenuated due to frictions at the upper and lower boundaries of the flow and tidal mixing which collapsed the stratification of the water column both sediment concentration and median grain size of suspended particles within the bottom layer significantly decreased the coarser sediment particles were deposited and the hyperpycnal flows stopped the intra tidal behaviors of hyperpycnal flows are closely associated with the variations of ssc salinity and stratification of the water column as nearly 90 of riverine sediment is delivered to the sea during the flood seasons when hyperpycnal flows are active hyperpycnal flows at the huanghe mouth and the river s high sediment loads have caused rapid accretion of the huanghe delta copyright 2010 john wiley sons ltd

cyclone driven deep sea injection of freshwater and heat by hyperpycnal flow in the subtropics china 973 program 2009cb421200 program of introducing talents of discipline to universities b07034 academia sinica thematic program afobi taiwan nsc 98 2116 m 001 005

nile floods recorded in deep mediterranean sediments clastic mud beds rich in continental organic matter are observed recurrently in the nile deep sea turbidite system they formed during flooding periods of the river similar to those that induce sapropel formation and occurred during periods of increased density stratification of the eastern mediterranean the very fine grained flood deposits are intercalated within pelagic sediments sapropels and bouma type turbidites these flood deposits form by the successive reconcentrations of surface hypopycnal plumes by convective sedimentation which in turn generate a fine grained low energy hyperpycnal flow sea level high stands seem also to favor hypopycnal plume formation and increase clastic mud bed formation consequently these muddy clastic beds provide a direct link between deep marine sedimentary records and continental climatic change through flood frequency and magnitude

damaging sediment density flows triggered by tropical cyclones the global network of subsea fibre optic cables plays a critical role in the world economy and is considered as strategic infrastructure for many nations sediment density flows have caused significant disruption to this network in the recent past these cable breaks represent the only means to actively monitor such flows over large oceanic regions here we use a global cable break database to analyse tropical cyclone triggering of sediment density flows worldwide over 25 yrs cable breaking sediment density flows are triggered in nearly all areas exposed to tropical cyclones but most occur in the nw pacific they are triggered by one of three sets of mechanisms tropical cyclones directly trigger flows synchronous to their passage as a consequence of storm waves currents and surges cyclones also trigger flows indirectly with near synchronous timing to their passage as a consequence of peak flood discharges last cyclones trigger flows after a delay of days as a consequence of the failure of large volumes of rapidly deposited sediment no clear relationship emerges between tropical cyclone activity i e track frequency and intensity and the number of sediment density flows triggered this is a consequence of the short period of observation however expansion of the cable network and predicted changes to cyclone activity in specific regions increases the likelihood of increasing numbers of damaging flows

benthic foraminifera from capbreton canyon revisited faunal evolution after repetitive sediment disturbance abstract at a 650 m deep site in the axis of capbreton canyon an 18 cm thick turbidite was deposited in december 1999 during subsequent campaigns an almost monospecific fauna of the benthic foraminifer technitella melo considered as a pioneer species was found in may 2000 in 2001 this fauna had disappeared and was replaced by an exceptionally rich fauna strongly dominated by the opportunistic species bolivina subaenariensis we present sedimentological radionuclide and foraminiferal data of new cores sampled in 2005 and 2011 taken with the aim to study the further evolution of the benthic ecosystem cores sampled in 2005 show that in the canyon axis a new ca 5 cm thick turbidite has been deposited the live benthic foraminiferal faunas were much poorer than in 2001 but still had a high dominance and low diversity although less extreme than in 2001 we conclude that in the canyon axis benthic foraminiferal faunas remain in an early stage of ecosystem colonization it appears that the very thick 1999 turbidite marks an exceptional event the uncommonly rich faunas observed in 2001 could be a response to the concentration of organic rich material in the fine grained top of this deposit in 2011 cores were sampled at a slightly different site on the lower canyon flank the sedimentary sequence here is marked by the absence of coarse turbidite layers although some levels show slightly increased grain size and lower 210 pb xs activities indicative of an admixture with advected older sediments live foraminiferal faunas are much more equilibrated as shown by their higher diversity lower dominance and deeper penetration into the sediment all these characteristics are indicative of a much more stable ecosystem dead faunas are present throughout the core indicating that the levels with slightly elevated grain size are not typical turbidites resulting from hyperpycnal currents which are characterized by levels barren of foraminifera but denote other more long term sedimentary processes leading to the advection of older material such as bottom nepheloid layers or repetitive fine grained turbidite deposits due to small scale slumping the comparison of live and dead fauna shows that at both sites the foraminiferal turnover rates are fairly low at the lower canyon flank site sampled in 2011 the foraminiferal faunas are renewed every 1 5 2 years such a fairly long foraminiferal lifespan corresponds to earlier estimates but is surprisingly high for the opportunistic taxa that dominate the faunal assemblages in these unstable and food enriched submarine canyon settings

deltaic infill of a deglaciated arctic fjord east greenland sedimentary facies and sequence stratigraphy abstract the deltaic fill of a deglaciated side entry fjord in western jameson land east greenland shows the record of an interplay between regional and local controls including climate and relative sea level change onshore and offshore topography and sediment yield a four phase model is suggested for the delta development and fjord filling at the deglaciation stage phase 1 the newly flooded fjord floor primarily accumulated sediment from reworked glacial material these deposits were relatively thin and draped the uneven sea floor topography the subsequent onset of paraglacial delta sedimentation phase 2 was characterized by a high meltwater runoff and extreme sediment yield and corresponded to the highstand and gradual fall of relative sea level a gilbert type delta with a short distributary plain formed at the head of the tributary fjord which received a very high amount of fine grained sediment the stage of enhanced delta progradation phase 3 was characterized by decreased sediment yield and runoff and a rapid fall of relative sea level delta progradation was fast because of rapidly decreasing accommodation space and a solitary river channel deeply incised the delta the equilibrium stage phase 4 spanned most of holocene time and involved a lowered rate of relative sea level fall seasonal runoff supplied limited amounts of sediment to the river mouth and the delta front became increasingly reworked by wave action and longshore drift the side entry fjord a small deglaciated valley invaded by the sea was filled chiefly by highstand and regressive systems tracts whereas relatively little sea floor aggradation was associated with the transgressive systems tract the stratigraphic pattern documented in this paper contrasts with the pattern usually reported from nonglaciated incised valley systems which are filled mainly by the transgressive systems tract typically involving a retreating estuarine system the pattern also differs from some large gradually deglaciated fjord valleys containing thick transgressive glaciomarine deposits accumulated during glacier retreat the study demonstrates the large variability of valley fills and illustrates the importance of topography sediment supply and climate as well as the style of deglaciation on the sediment filling of a fjord valley

frequency of exceptional nile flood events as an indicator of holocene hydro climatic changes in the ethiopian highlands abstract climate conditions in africa have varied substantially during the late quaternary with alternating humid and arid periods controlled mainly by the african monsoonal regime however the duration and termination of the last african humid period 14 6 ka bp and its internal climatic variability are still debated using a laminated sequence from the nile deep sea fan we reconstruct for the first time the monsoon induced frequency of exceptional nile floods at centennial resolution during the african humid period by combining sedimentological geochemical and microscopic tools and comparing our record with two proximal piston cores and with regional paleoclimatic records we show a the occurrence of recurrent high energy floods between 10 1 and 9 ka bp during the height of the african humid period b a shift in the hydro climatic regime as early as 9 ka bp with a progressive reduction in flood frequency and magnitude until 8 2 ka bp likely related to a southward migration of the monsoon rainfall belt c a drastic reduction of flood activity between 8 2 and 7 8 ka bp d an unstable ethiopian nile hydrological system from 7 8 ka bp followed by a further decrease in river runoff until 4 ka bp the occurrence of a stepwise hydro climatic deterioration over the ethiopian highlands from 9 ka bp brings into question the climatic linkages and feedbacks between low and high latitudes during the early to mid holocene and in particular around the 8 2 ka bp north atlantic cooling event our unique record of flood frequency at centennial resolution therefore allows us to draw new insights on fluvial and geomorphic feedbacks of the nile hydrologic system to monsoonal regimes during a period of major environmental shifts

sulfur isotopes in coal constrain the evolution of the phanerozoic sulfur cycle sulfate is the second most abundant anion behind chloride in modern seawater and its cycling is intimately coupled to the cycling of organic matter and oxygen at the earth s surface for example the reduction of sulfide by microbes oxidizes vast amounts of organic carbon and the subsequent reaction of sulfide with iron produces pyrite whose burial in sediments is an important oxygen source to the atmosphere the concentrations of seawater sulfate and the operation of sulfur cycle have experienced dynamic changes through earth s history and our understanding of this history is based mainly on interpretations of the isotope record of seawater sulfates and sedimentary pyrites the isotope record however does not give a complete picture of the ancient sulfur cycle this is because in standard isotope mass balance models there are more variables than constraints typically in interpretations of the isotope record and in the absence of better information one assumes that the isotopic composition of the input sulfate to the oceans has remained constant through time it is argued here that this assumption has a constraint over the last 390 ma from the isotopic composition of sulfur in coal indeed these compositions do not deviate substantially from the modern surface water input to the oceans when applied to mass balance models these results support previous interpretations of sulfur cycle operation and counter recent suggestions that sulfate has been a minor player in sulfur cycling through the phanerozoic eon

enhanced sedimentation beneath particle laden flows in lakes and the ocean due to double diffusive convection the transport rate of particles beneath sediment laden overflows and interflows in lakes and in the ocean can be enhanced by double diffusive and settling driven convection in previous experiments with sediment laden fluid overlaying a saline layer visual measurements could only be made in the optically clear lower layer hence there was difficulty distinguishing the two processes hindering predictions of when enhanced sedimentation occurs we used an acoustic doppler velocimeter adv to measure velocities and turbulence above and below the initial sediment salt interface the velocity of the sediment fingers in the lower layer were always larger than the stokes settling velocity of the particles leading to an asymmetry in the flow field of the two convective layers sediment fingers only dominated when there were marginal density differences between the two layers we conclude that double diffusive sediment fingers control sedimentation beneath interflows in most lakes whereas settling driven convection is dominant in most oceanic overflows

desert flash floods form hyperpycnal flows in the coral rich gulf of aqaba red sea abstract running rivers are very scarce in globally distributed hyperarid and arid coastlines 28 000 km worldwide and it is ephemeral rivers that carry most terrestrial sediment into the sea in these regions however there is very little information regarding the contribution of terrestrial sediment and dynamics of transport of sediment that enter marine basins from these rivers one hyperarid region the gulf of aqaba received an exceptional number of flashflood events during the winter of 2012 2013 the results illustrate for the first time how the high volume of flashflood sediment influences the distribution of coral reefs dwarfs the contribution of airborne dust elevates floodwater densities to produce hyperpycnal flows despite highest ocean salinities and is subsequently transported to the deep basin where it may be preserved as a climate archive

dispersion of disposed dredged slurry in the meso tidal changjiang yangtze river estuary to understand the dispersal pattern of sediment plume and its controlling processes a field experiment of concentrated slurry dispersal created by a dredger was conducted in the changjiang yangtze river estuary during the 2002 flood season an acoustic suspended sediment concentration profiler and an acoustic doppler profiler were deployed to simultaneously observe suspended sediment concentrations ssc and tidal currents at the pre selected sections shortly following the release of dredged materials water sampling grab sampling and shallow coring were simultaneously carried out to obtain the ssc and grain size texture high resolution ssc profiler observations showed that two distinct sediment plumes middle level and near bed plumes occurred during the intermediate tidal phase between the spring and neap due to differential settling of the sediment mixture whereas only a benthic plume occurred due to rapid flocculation settling during the neap tide three subsequent stages can be identified during the dispersal of the sediment plume 1 initially stable stage before the release 2 unstable stage shortly following the release as a settling cloud and 3 stable stage after the formation of a primary lutocline or a benthic plume enhanced mixing due to oscillatory shear flows could raise only the elevation of the lutocline in the slurry but could not enhance the transport capacity of suspension in the presence of high concentration the fate of bottom sediment plume was controlled by the bottom stress independent of the interfacial mixing

linking river flood dynamics to hyperpycnal plume deposits experiments theory and geological implications turbid river plumes entering ocean or lake water of lesser density i e hyperpycnal plumes can plunge to form turbidity currents providing an important link between terrestrial sediment sources and marine depositional sinks a leading hypothesis suggests that hyperpycnal plume deposits accurately record the rising and falling discharge of a flooding river in terms of sediment size grading bedform sequence and deposit thickness which if correct has significant implications for unraveling river dynamics reservoir potential and earth history from marine event beds herein we present one of the first experimental flume studies aimed at testing this hypothesis results indicate that depth averaged hyperpycnal plume velocities can be uncorrelated or even anti correlated with river discharge at certain seabed locations because of translation of the plunge point resulting from temporal variations in discharge and sediment concentration through the duration of a river flood an advection length scale of settling sediment is found to be an important control on hyperpycnal plume deposits where coarse sediment sand is most likely to record multiple flow accelerations and decelerations related to plunge point translation even for a river flood with a single peaked hydrograph in contrast fine sediment mud is relatively insensitive to local plunge point dynamics and is most likely to preserve directly rising and falling river discharge finally it was found that the necessary fluvial sediment concentration to form a plunging plume can be much larger than the concentration typically used assuming density equivalence because of deposition upstream of the plunge point

limnology sedimentology and hydrology of a jokulhlaup into a meromictic high arctic lake a large ice dammed lake drained catastrophically into lake tuborg ellesmere island beginning on 25 july 2003 limnological sedimentological and hydrological parameters were recorded before during and after this event for several weeks prior to the jokulhlaup water overtopped the ice dammed lake and flowed into lake tuborg s fresh water basin a shallow sill separates the freshwater basin from a larger deeper basin containing 25 psu practical salinity units salt water the sill blocked underflows from entering the saltwater basin before the jokulhlaup the ice dammed lake drained completely and catastrophically when englacial or subglacial conduits developed and a glacier portal formed 980 m from the lake tuborg shore marking the beginning of the jokulhlaup the level of lake tuborg increased by 7 6 m in 84 h this jokulhlaup is the largest known to have occurred in the high arctic and the largest witnessed in canada since 1947 strata of very cold water flowed above the chemocline for about 14 km from the sill to the southwest end of the lake the cold strata turbulently mixed with underlying salt water allowing for saltwater flocculation of suspended sediment causing rapid settling the saltwater layer very slightly freshened and cooled close to the sill near surface sediments derived from the jokulhlaup are coarse and laminated however no erosion occurred toward the distal end of the lake where a fining upward unit with a coarse base was deposited resume un grand lac retenu par des glaces s est draine de maniere catastrophique dans le lac tuborg de l ile ellesmere le 25 juillet 2003 les parametres limnologiques sedimentologiques et hydrologiques ont ete mesures avant durant et apres cet evenement durant plusieurs semaines avant le jokulhlaup de l eau se deversait par dessus le barrage de glace du lac et s ecoulait dans le bassin d eau douce du lac tuborg un seuil peu profond separe le bassin d eau douce d un bassin plus grand et plus profond contenant de l eau a une salinite d environ 25 usp unites de salinite pratique avant le jokulhlaup le seuil empechait des courants de fond d entrer dans le bassin d eau salee le lac retenu par les glaces s est draine completement et de maniere catastrophique lorsque des canalisations dans ou sous la glace se sont de veloppees et qu un portail glaciaire s est forme a 980 m des berges du lac tuborg marquant ainsi le debut du jokulhlaup le niveau du lac tuborg s est eleve de 7 6 m en 84 heures ce jokulhlaup est le plus grand connu qui se soit produit dans le grand nord et le plus gros a etre vu au canada depuis 1947 une couche d eau tres froide coulait par dessus le chemocline sur une distance d environ 14 km depuis le seuil jusqu a l extremite sud ouest du lac la couche froide s est melee de maniere turbulente avec l eau salee sous jacente engendrant une floculation des sediments en suspension dans l eau salee et une sedimentation rapide la couche d eau salee est devenue legerement plus douce et plus froide les sediments de surface provenant du jokulhlaup sont grossiers et lamines a proximite du seuil toutefois il ne s est pas produit d erosion a l extremite distale du lac ou s est deposee une unite a base grossiere et a granodecroissance vers le haut

powerful unconfined turbidity current captured by cabled observatory on the fraser river delta slope british columbia canada turbidity currents are one of the main sediment transport processes on earth yet are notoriously difficult to monitor directly this article presents the first direct and high bandwidth observation of a turbidity current using a cabled sea floor observatory on 5 june 2012 a platform on ocean networks canada located in 107 m of water on the fraser river delta slope was displaced downslope and severed from its data cable the platform weighed ca 1000 kg in water the event took place during high river discharge high tides and rapid sediment accumulation on adjacent upslope areas of the sea floor data recorded as it tumbled downslope allow a reconstruction of the flow which is inferred to have been an unconfined turbidity current lines of evidence indicate that the flow came in as a bed hugging wedge and built up to between 1 m and 4 m in height as the head passed through comparison with laboratory data suggest that the flow was initially supercritical while the adjacent slope offset to the north clearly exhibits change over an annual resurvey period the bathymetry directly at the event location show no resolvable change over a period from seven months before the event to one month after sediment cores collected after the event were pervasively biototurbated and they contain no obvious deposit connected with this event the remarkable aspects of this research follow the flow was powerful enough to carry a 1 tonne platform and sever a heavily armoured cable the current occurred on the unchannelized open slope this powerful event failed to cause discernible seabed elevation change the flow was triggered by tidal conditions the event was detected by a purpose designed cabled observatory thus providing high bandwidth data and also alerting researchers in real time to mount follow on investigations

distinguishing sediment waves from slope failure deposits field examples including the humboldt slide and modelling results abstract migrating sediment waves have been reported in a variety of marine settings including submarine levee fan systems floors of fjords and other basin or continental slope environments examination of such wave fields reveals nine diagnostic characteristics when these characteristics are applied to several features previously attributed to submarine landslide deformation they suggest that the features should most likely be reinterpreted as migrating sediment wave fields sites that have been reinterpreted include the humboldt slide on the eel river margin in northern california the continental slope in the gulf of cadiz the continental shelf off the malaspina glacier in the gulf of alaska and the adriatic shelf a reassessment of all four features strongly suggests that numerous turbidity currents separated by intervals of ambient hemipelagic sedimentation deposited the wave fields over thousands of years a numerical model of hyperpycnal discharge from the eel river for example shows that under certain alongshore current conditions such events can produce turbidity currents that flow across the humboldt slide serving as the mechanism for the development of migrating sediment waves numerical experiments also demonstrate that where a series of turbidity currents flows across a rough seafloor i e numerical steps sediment waves can form and migrate upslope hemipelagic sedimentation between turbidity current events further facilitates the upslope migration of the sediment waves physical modelling of turbidity currents also confirms the formation and migration of seafloor bedforms the morphologies of sediment waves generated both numerically and physically in the laboratory bear a strong resemblance to those observed in the field including those that were previously described as submarine landslides

dynamic sedimentary environments of an arctic glacier fed river estuary adventfjorden svalbard i flux deposition and sediment dynamics abstract the spatial patterns of the transport settling and deposition of sediments were studied in a small arctic glacier fed river estuary adventfjorden west spitsbergen svalbard salinity and temperature suspended solids concentration and particle fluxes in the water column were measured at seven stations situated on the tidal flat formed at the river mouth the delta slope and along the fjord axis down to the fjord mouth turbidity currents were measured in situ using a current meter combined with a turbidity meter the sediments collected with a niemisto gravity corer were x rayed and analyzed for granulometric composition the results of the major sampling campaign undertaken in july 2002 were supplemented with winter hydrological data from january 1996 since the river is frozen in winter no fresh water or fluvial sediment supply is observed in the fjord water column a hypopycnal plume of brackish water 1 5 m thick extends in summer as far as 0 8 km from the river mouth the highest concentration 826 mg l 1 and vertical flux over 1000 g m 2 day 1 of suspended solids were noted at the edge of the tidal flat and over the upper slope of the delta both concentrations and the solid particle flux decreased as distance from the river mouth increased local peaks of turbidity in the intermediate and deep water layers in the central fjord indicated the presence of hyperpycnal plumes tidal flat sediments are rhythmically laminated by the tidally controlled resuspension redeposition cycles and occasionally eroded by catastrophic events such as intense storms or winter ice cover scouring the steep inclination of the bottom of the slope 15 19 promotes the gravity driven processes of sediment transport debris flows frequent resuspension and redeposition events can be traced on x radiographs and in the granulometric composition of the sediment cores collected on the slope turbidity currents are observed throughout the fjord although their intensity and frequency of occurrence change as distance from the river mouth increases the near bottom hyperpycnal flows transport the finest sediment over the slope and into the low energy environment of the central basin of adventfjorden the current study shows that the patterns of sediment storage and sedimentary dynamics in a glacier fed river estuary are driven by gravity flows and turbidity currents rather than by the patterns of the vertical sedimentation of suspensions in the water column

modern sedimentary processes at the heads of martinez channel and steffen fjord chilean patagonia abstract chilean fjord sediments constitute high resolution archives of climate and environmental change in the southern andes to interpret such records accurately it is crucial to understand how sediment is transported and deposited within these basins this issue is of particular importance in glaciofluvial martinez channel and steffen fjord 48 s due to the increasing occurrence of glacial lake outburst floods glofs originating from outlet glaciers of the northern patagonian icefield hence the bathymetry of the head of martinez channel and steffen fjord was mapped at high resolution and the grain size and organic carbon content of grab sediment samples were examined results show that the subaquatic deltas of baker and huemules rivers at the head of martinez channel and steffen fjord respectively are deeply incised up to 36 m by sinuous channels the presence of sediment waves and coarser sediments within these channels imply recent activity and sediment transport by turbidity currents although several triggering mechanisms are possible we argue that elevated river discharge and the associated relatively high suspended sediment loads is the main cause of the occurrence of turbidity currents at the head of these fjords and are of key importance in shaping the fjord s subaquatic morphology this study shows that the heads of martinez channel and steffen fjord are dynamic sedimentary environments with rapidly migrating channels it highlights the importance of site selection and multi coring in any future project aimed at reconstructing environmental change using the sediments of either fjord

on the links between a river s hyperpycnal plume and marine benthic nepheloid layer in the wake of a typhoon abstract in 2010 two moorings each configured with a ctd and an adcp one with an additional non sequential sediment trap nsst were deployed in the head region of the gaoping submarine canyon 3 days after the typhoon induced peaks of the runoff and suspended sediment concentration ssc of the gaoping river in southern taiwan our data show a demarcation between tidal and hyperpycnal regimes in the temperature salinity and flow fields the latter existed in the first 5 days out of the 18 day deployment as defined by higher water density due to high ssc several lines of evidence indicate the presence of the tail end of a hyperpycnal turbidity current htc including the retention of warm water near the canyon floor high ssc down canyon directed residual flow and its vertical structure and high terrestrial fraction larger than 70 of the organic particles carried by the flow the decreasing mass flux during the deployment is also an indication of a waning htc our findings also show that the vertical flow structure and the direction of the gravity driven down canyon htc were retarded by the instantaneous up canyon directed tidal oscillations in the submarine canyon

wave modified turbidites combined flow shoreline and shelf deposits cambrian antarctica sandstone tempestite beds in the starshot formation cen tral transantarctic mountains were deposited in a range of shoreline to shelf environments detailed sedimentological analysis indicates that these beds were largely deposited by wave modified turbidity currents these currents are types of combined flows in which storm generated waves overprint flows driven by excess weight forces the interpreta tion of the tempestites of the starshot formation as wave dominated turbidites rests on multiple criteria first the beds are generally well graded and contain bouma like sequences like many turbidites the soles display abundant well developed flutes they also contain thick divisions of climbing ripple lamination the lamination however is dominated by convex up and sigmoidal foresets which are geometries identical to those produced experimentally in current dominated com bined flows in clear water finally paleocurrent data support a tur bidity current component of flow asymmetric folds in abundant con volute bedding reflect liquefaction and gravity driven movement and hence their orientations indicate the downslope direction at the time of deposition the vergence direction of these folds parallels paleocur rent readings of flute marks combined flow ripples and a number of other current generated features in the starshot event beds indicating that the flows were driven down slope by gravity the wave component of flow in these beds is indicated by the presence of small to large scale hummocky cross stratification and rare small two dimensional ripples wave modified turbidity currents differ from deep sea turbidity cur rents in that they may not be autosuspending and some proportion of the turbulence that maintains these flows comes from storm waves such currents are formed in modern shoreline environments by a com bination of storm waves and downwelling sediment laden currents they may also be formed as a result of oceanic floods events in which intense sediment laden fluvial discharge creates a hyperpycnal flow event beds in the starshot formation may have formed from such a mechanism oceanic floods are formed in rivers of small to medium size in areas of high relief commonly on active margins the starshot formation and the coeval douglas conglomerate are clastic units that formed in response to uplift associated with active tectonism sedi mentological and stratigraphic data suggest that coarse alluvial fans formed directly adjacent to a marine basin the geomorphic conditions were therefore likely conducive to rapid fluvial discharge events asso ciated with storms the abundance of current dominated combined flow ripples at the tops of many starshot beds indicates that excess weight forces were dominant throughout deposition of many of these beds

hyperpycnal sediment discharge from semiarid southern california rivers implications for coastal sediment budgets southern california rivers discharge hyperpycnal river density greater than ocean density concentrations of suspended sediment 40 g l according to buoyancy theory during flood events mostly during el nino southern oscillation enso conditions because hyperpycnal river discharge commonly occurs during brief periods hours to occasionally days mean daily flow statistics often do not reveal the magnitude of these events hyperpycnal events are particularly important in rivers draining the transverse range and account for 75 of the cumulative sediment load discharged by the santa clara river over the past 50 yr these events are highly pulsed totaling only 30 days 0 15 of the total 50 yr period observations of the fate of sediment discharge although rare are consistent with hyperpycnal river dynamics and the high likelihood of turbidity currents during these events we suggest that much of the sediment load initially bypasses the littoral circulation cells and is directly deposited on the adjacent continental shelf thus potentially representing a loss of immediate beach sand supply during particularly exceptional events 100 yr recurrence intervals flood underflows may extend past the shelf and escape to offshore basins

sediment records of the influence of river damming on the dynamics of the nelson and churchill rivers western hudson bay canada during the last centuries two gravity cores 778 and 780 sampled at the nelson river mouth and one 776 at the churchill river mouth in western hudson bay canada were analyzed in order to identify the impact of dam construction on hydrology and sedimentary regime of both rivers another core 772 was sampled offshore and used as a reference core without a direct river influence core chronology was established using 14c and 210pb measurements cores 778 and 780 show greater variability than the others and the physical chemical magnetic and sedimentological properties measured on these cores reveal the presence of several hyperpycnites indicating the occurrence of hyperpycnal flows associated with floods of the nelson river these hyperpycnal flows were probably caused by ice jam formation which can increase both the flow and the sediment concentration following the breaching of such natural dams however these hyperpycnites are only observed in the lower parts of cores 778 and 780 it was not possible to establish a pr

arctic and antarctic submarine gullies a comparison of high latitude continental margins abstract submarine gullies are common features of high latitude continental slopes and over the last decade have been shown to play a key role in continental margin evolution submarine erosion downslope sediment transport slope deposits and the architecture of petroleum reservoirs however the processes that form these gullies the timescales over which they develop and the environmental controls influencing their morphology remain poorly constrained we present the first systematic and comparative analysis between arctic and antarctic gullies with the aim of identifying differences in slope character from which we infer differences in processes operating in these environments quantitative analysis of multibeam echosounder data along 2441 km of the continental shelf and upper slope and morphometric signatures of over 1450 gullies show that six geomorphically distinct gully types exist on high latitude continental margins we identify distinct differences between arctic and antarctic gully morphologies in the arctic data sets deep relief 30 m gully incision depth at 50 m below the shelf edge and shelf incising gullies are lacking these differences have implications for the timescales over which the gullies were formed and for the magnitude of the flows that formed them we consider two hypotheses for these differences 1 some antarctic gullies developed through several glacial cycles and 2 larger antarctic gullies were formed since the last glacial maximum as a result of erosive flows i e sediment laden subglacial meltwater being more abundant on parts of the antarctic margin over longer timescales a second difference is that unique gully signatures are observed on arctic and on antarctic margins environmental controls such as the oceanographic regime and geotechnical differences may lead to particular styles of gully erosion observed on arctic and antarctic margins

channelled erosion through a marine dump site of dredge spoils at the mouth of the puyallup river washington state usa abstract channels are relatively common on river mouth deltas but the process by which they arise from river sediment discharge is unclear because they can potentially be explained either by negatively buoyant hyperpycnal flows produced directly from the river outflow or by flows generated by repeated failure and mobilisation of sediment rapidly deposited at the delta front channels eroded through a dump site of dredge spoils are described here from multibeam and older sonar data collected in commencement bay at the mouth of the puyallup river shallow channels on the seaward upper surface of the dump site away from any flows that could have been produced by delta front failures suggest that at least some hyperpycnal flows were produced directly from the positively buoyant river outflow up to 200 m from the edge of the river mouth platform the form of channel bed erosion is revealed by the longitudinal shape of the main eroded channel compared with the adjacent dump site profile it suggests that the channel evolved by its steep front retreating rather than by simple vertical entrenchment or diffusive like evolution of the profile a geometry interpreted as evidence that repeated failure of the bed occurred in response to shear stress imposed by bottom travelling flows model calculations based on shear strengths back calculated from the geometry of channel wall failures suggest that if the main channel were eroded solely by hyperpycnal flows their generation was remarkably efficient in order to create flows vigorous enough to cause channel bed failure besides the sediment concentration and discharge characteristics that have been considered to dictate the ability of rivers to produce hyperpycnal flows it is suggested that the timing of floods with respect to the tidal cycle should also be important because extreme low tides may be needed to ensure that coarse sediment is transferred vigorously to the edge of river mouth platforms

recent sediment transport and deposition in the lisbon setubal and cascais submarine canyons portuguese continental margin abstract recent sediment transport and deposition in the lisbon setubal and cascais submarine canyons portuguese continental margin were investigated on the basis of water column profiles of suspended particulate matter records of near bottom currents and settling fluxes of particulate matter obtained with benthic landers and analysis of surface sediments the results show that fine grained predominantly lithogenic sediment derived from adjacent shelf areas accumulates in the upper reaches of the canyons sediment deposited further down in the middle and lower reaches of the canyons is essentially similar to the hemipelagic sediment found on the adjacent continental slope indicating that down canyon transport of sediment from the upper to the lower canyon is limited tidal currents measured at various depths in the lisbon setubal canyon appear sufficiently strong to resuspend and transport sediment but net up canyon flow of the bottom water may retain shelf derived lithogenic sediment in the upper canyon sediment gravity flows which in the nearby nazare canyon are an effective mechanism for down canyon sediment transport appear rare in the lisbon setubal canyon and probably also in the cascais canyon turbidity current events recorded in the sedimentary record of the canyons may correspond to seismic events of 1969 and 1755 ad

sediment laden fresh water above salt water nonlinear simulations when a layer of particle laden fresh water is placed above clear saline water both double diffusive and rayleigh taylor instabilities may arise the present investigation extends the linear stability analysis of burns meiburg j fluid mech vol 691 2012 pp 279 314 into the nonlinear regime by means of two and three dimensional direct numerical simulations dns the initial instability growth in the dns is seen to be consistent with the dominant modes predicted by the linear stability analysis the subsequent vigorous growth of individual fingers gives rise to a secondary instability and eventually to the formation of intense plumes that become detached from the interfacial region the simulations show that the presence of particles with a stokes settling velocity modifies the traditional double diffusive fingering by creating an unstable nose region in the horizontally averaged profiles located between the upward moving salinity and the downward moving sediment interface the effective thickness l s l c of the salinity sediment interface grows diffusively as does the height h of the nose region the ratio h l s initially grows and then plateaus at a value that is determined by the balance between the flux of sediment into the rose region from above the double diffusive rayleigh taylor flux out of the nose region below and the rate of sediment accumulation within the nose region for small values of h l s leqslant o 0 1 double diffusive fingering dominates while for larger values h l s geqslant o 0 1 the sediment and salinity interfaces become increasingly separated in space and the dominant instability mode becomes rayleigh taylor like a scaling analysis based on the results of a parametric study indicates that h l s is a linear function of a single dimensionless grouping that can be interpreted as the ratio of inflow and outflow of sediment into the nose region the simulation results furthermore indicate that double diffusive and rayleigh taylor instability mechanisms cause the effective settling velocity of the sediment to scale with the overall buoyancy velocity of the system which can be orders of magnitude larger than the stokes settling velocity while the power spectra of double diffusive and rayleigh taylor dominated flows are qualitatively similar the difference between flows dominated by fingering and leaking is clearly seen when analysing the spectral phase shift for leaking dominated flows a phase locking mechanism is observed which intensifies with time hence the leaking mode can be interpreted as a fingering mode which has become phase locked due to large scale overturning events in the nose region as a result of a rayleigh taylor instability

preconditioning and triggering of offshore slope failures and turbidity currents revealed by most detailed monitoring yet at a fjord head delta rivers and turbidity currents are the two most important sediment transport processes by volume on earth various hypotheses have been proposed for triggering of turbidity currents offshore from river mouths including direct plunging of river discharge delta mouth bar flushing or slope failure caused by low tides and gas expansion earthquakes and rapid sedimentation during 2011 106 turbidity currents were monitored at squamish delta british columbia this enables statistical analysis of timing frequency and triggers the largest peaks in river discharge did not create hyperpycnal flows instead delayed delta lip failures occurred 8 11 h after flood peaks due to cumulative delta top sedimentation and tidally induced pore pressure changes elevated river discharge is thus a significant control on the timing and rate of turbidity currents but not directly due to plunging river water elevated river discharge and focusing of river discharge at low tides cause increased sediment transport across the delta lip which is the most significant of all controls on flow timing in this setting