a method for building 3d models of barchan dunes abstract the distributions of barchan dunes are usually represented by digital terrain models dtms overlaid with digital orthophoto maps given that most regions with barchan dues have low relief a 3d map obtained from a dtm may ineffectively show the stereoscopic shape of each dune the method of building 3d models of barchan dunes using existing modeling software seldom considers the geographical environment as a result barchan dune models are often inconsistent with actual dtms and incompletely express the morphological characteristics of dunes manual construction of barchan dune models is also costly and time consuming considering these problems the morphological characteristics of barchan dunes and the mathematical relationships between the morphological parameters of the dunes such as length height and width are analyzed in this study the methods of extracting the morphological feature points of barchan dunes calculating their morphological parameters and building dune outlines and skeleton lines based on the medial axes are also presented the dune outlines skeleton lines and part of the medial axes of dunes are used to construct a constrained triangulated irregular network c and arcengine are employed to build 3d models of barchan dunes automatically experimental results of a study conducted in tengger desert show that the method can be used to approximate the morphological characteristics of barchan dunes and is less time consuming than manual methods

palaeogeography of atlantic canada 13 0 kyr abstract we combine isobase maps with a digital terrain model of atlantic canada to map coastlines from 13 14c kyr bp to the present at 13 14c kyr bp there are ridges of high relative sea level rsl values over newfoundland and the maritime provinces and a re entrant of low values in the gulf of st lawrence this pattern persists well into the holocene and reflects crustal response to the slow wasting of ice caps that persisted in newfoundland and the maritime provinces for up to five millennia after the removal of ice from the gulf of st lawrence by a migrating calving embayment the palaeogeographic reconstructions reveal an archipelago on the outer shelf from grand bank to the continent that persisted from 13 14c kyr bp until ca 8 14c kyr bp much of the magdalen shelf was exposed but the magdalen islands were never connected to the mainland prince edward island was initially separated from the mainland became connected after 11 14c kyr bp and was separated again just before 6 14c kyr bp when northumberland strait formed the reconstructions are highly sensitive to relatively small changes in isobase values especially on the shallow banks on the continental shelf

sand wave fields beneath the loop current gulf of mexico reworking of fan sands extensive fields of large barchan like sand waves and longitudinal sand ribbons have been mapped by deep towed seamarc ia sidescan sonar on part of the middle and lower mississippi fan that lies in about 3200 m of water the area is beneath the strongly flowing loop current the bedforms have not been adequately sampled but probably consist of winnowed siliciclastic foraminiferal sands the size about 200 m from wingtip to wingtip and shape of the large barchans is consistent with a previously observed peak current speed of 30 cm s measured 25 m above the seabed the types of small scale bedforms and the scoured surfaces of chemical crusts seen on nearby bottom photographs indicate that near bed currents in excess of 30 cm s may sometimes occur at the time of the survey the sand transport direction was to the northwest in the opposite direction to the loop current but consistent with there being a deep boundary current along the foot of the florida escarpment some reworking of the underlying sandy turbidites and debris flow deposits is apparent on the sidescan sonar records reworking by deep sea currents resulting in erosion and in deposits characterised by coarsening upwards structures and cross bedding is a process that has been proposed for sand found in cores in shallower parts of the gulf of mexico this process is more widespread than hitherto supposed

dunes on the skeleton coast namibia south west africa geomorphology and grain size relationships simple and locally compound transverse and barchanoid dunes dominate the 2000 km2 skeleton coast dunefield in northwestern namibia south west africa dune height and spacing are closely correlated r 0 89 and decrease across the dunefield from southwest to northeast with an accompanying change from transverse to barchanoid ridges and ultimately barchans the dunes are aligned transverse to the dominant strong south and south southwest onshore winds alignment patterns indicate that surface roughness changes between coastal plain and dunes cause dune forming winds to swing to the right over the dunes but resume their original direction beyond grain size and sorting vary at three scales the dune the dune landscape and through the dunefield overall the sands derived from three localities by deflation from beaches supplied by vigorous longshore drift become progressively finer and better sorted across the dunefield paralleling changes in dune height and spacing a statistically significant relationship r 0 65 was established between dune spacing and the phi grain size of the coarser fraction of the dune sands demonstrating the importance of the protective effects of coarse grains and suggesting that the morphometry of simple transverse dunes may be controlled by the scale of turbulence associated with the threshold wind speed required to move the coarsest fraction of the dune sand

the morphology of barchan shaped sand banks from western torres strait northern australia abstract large barchan shaped sand deposits have been observed in the north west of torres strait these deposits share characteristics of both subaerial barchan dunes and subaqueous sand banks a study of satellite imagery indicate that the deposits migrate in the direction indicated by their horns 10 15 m west per year and that sediment is shed from their horns features that are characteristic of barchan dunes however the orientations of sand dunes superimposed upon the sand banks indicate the presence of mutually evasive channels and circulation of sediment around the sand bank a characteristic of subaqueous sand banks the presence of mutually evasive channels is the criteria used to categorise the deposits as sand banks barchan forms are known to exist in regions with limited sediment supply and unidirectional current or wind regimes in the torres strait both these criteria are met previous work has demonstrated the presence of a net westward current through the torres strait that is driven by the southeast trade winds the relatively high displacement of the wind driven currents during the trade wind season relative to the monsoon appears to provide the necessary unidirectional regime to form barchans the low and typically eastwards displacement of the residual monsoon season current appears to have a negligible affect on the barchan form while seasonal wind driven currents appear to maintain the barchan shape of the sand banks tidal currents actively maintain mutually evasive channels observed by variations in dune orientation on the sand banks a sediment starved environment combined with bedload transport attributed to both wind driven and tidal currents is concluded to create a unique hydrodynamic environment where sand banks can attain a barchan form

morphology and composition of submarine barchan dunes on the scotian shelf canadian atlantic margin abstract submarine barchan dunes have been mapped on browns bank scotian shelf using multibeam sonar bathymetry and backscatter strength the morphology of subaerial barchans has been studied for almost a century but the advent of multibeam sonar technology now enables a quantitative investigation of their submarine counterparts the browns bank submarine barchans occur at a depth of 60 70 m and are crescentic in planform reaching almost 700 m in horn width and 5 m in height the barchans are convex to the se with steep lee faces to the nw indicating a dominant nw flowing current the barchans overlie a widespread gravel lag covered elsewhere with little or no sand obstacle marks emanate from the lee faces of the barchans and represent a lack of sand deposition and exposure of gravel lag on the sea floor the browns bank submarine barchan sediment texture is gravelly sand or sandy gravel and is primarily composed of subrounded to well rounded quartz grains the allometric relationship between submarine barchan slip face height and distance between horns is markedly different from that of subaerial barchans for the same dune height barchan horn width is about ten times greater in the marine environment the superimposition of megaripples on the stoss slopes of the submarine barchans suggests that the barchans are active and therefore represent an engineering risk to sea floor infrastructure current observations and models indicate that seasonal mean current strength is less than the critical velocity required for barchan migration however the barchans may be active under higher velocity storm induced currents repetitive multibeam sonar mapping is required to detect if barchan migration is occurring over longer time scales

classification of seafloor geology using multibeam sonar data from the scotian shelf abstract we describe a method for classifying multibeam sonar data and illustrate the method using data collected with a simrad em1000 sonar on the scotian shelf canada the method involves comparing various attributes of the bathymetry and backscatter with typical examples of specified seafloor types and computing their statistical degree of similarity sediment ponds are identified as areas of low echo amplitude and low topographic gradient and curvature ridges and troughs are identified by fitting a paraboloid to patches of the bathymetry once the sediments ridges and troughs are located we are able to use the database to extract orientations and other characteristics of these features for example the orientation of the topography can be computed from the paraboloid surface and rose diagrams of the ridge and trough orientations reflect the fabric of up turned cambro ordovician sedimentary beds outcropping in this area the unconsolidated sediments ponded within small basins have low topographic gradients with consistent tilts towards the southeast mean 133 which we interpret as due to offshore transport of holocene sediments possibly in response to storms

the laurentide and innuitian ice sheets during the last glacial maximum the late wisconsinan advance of the laurentide ice sheet started from a middle wisconsinan interstadial minimum 27 30 14 c ka bp when the ice margin approximately followed the boundary of the canadian shield ice extent in the cordillera and in the high arctic at that time was probably similar to present ice advanced to its late wisconsinan stage 2 limit in the northwest south and northeast about 23 24 14 c ka bp and in the southwest and far north about 20 21 14 c ka bp in comparison to some previous reconstructions of ice extent our current reconstruction has substantially more late wisconsinan ice in the high arctic where an innuitian ice sheet is generally acknowledged to have existed in the atlantic provinces where ice is now thought to have extended to the continental shelf edge in most places and on eastern baffin island where ice probably extended to the fiord mouths rather than to the fiord heads around most of the ice margin the late wisconsinan maximum ice extent either exceeded the extent of earlier wisconsinan advances or it was similar to the early wisconsinan advance ice marginal recession prior to 14 14 ck a bp occurred mainly in deep water and along the southern terrestrial fringe however heinrich event 1 probably drew down the entire central ice surface at 14 5 14 c ka bp sufficiently to displace the labrador sector outflow centre 900 km eastward from the coast of hudson bay the onset of substantial ice marginal recession occurred about 14 14 c ka bp in the northwest southwest and south but not until about 10 11 14 c ka bp in the northeast and in the high arctic thus the period of maximum ice extent in north america generally encompasses the interval from b24 21 to 14 14 c ka bp or considerably longer than the duration of the lgm defined as occurring during a period of low global sea level as well as during a time of relative climate stability b18 14 c ka bp the interval of advance of much of the laurentide ice sheet to its maximum extent between b27 14 c ka bp and b24 14 c ka bp coincides with a suggested interval of rapid fall in global sea level to near lgmlevels r 2001 elsevier science ltd all rights reserved

late wisconsinan ice retreat from the scotian shelf the late middle to late wisconsinan laurentide ice sheet terminated at the edge of the scotian shelf ca 21 ka in close temporal agreement with the h 2 heinrich event of the north atlantic stratigraphy the ice began its recession along the western edge of the shelf prior to 20 ka by 15 ka the ice front had retreated in a northeastward direction exposing much of the western half of the shelf although ice appears to have remained on the inner shelf north of a morainal system for another 2 k y the retreat was probably controlled by an ice divide along the axis of the mainland until after 16 ka at which time an ice flow pattern began to emanate from local remnant ice centers on the mainland late wisconsinan ice seems to have remained on much of the eastern portion of the shelf as late as 12 14 5 ka and probably deposited several outer bank moraines younger dryas ice probably was present at isolated land and coastal areas for a short period ca 10 5 ka but its offshore influence appears to have been restricted to floating ice and storm related events

control of alongshore oriented sand spits on the dynamics of a wave dominated coastal system holocene deposits northern gulf of lions france article the maguelone shore extends along the northern coast of the gulf of lions west of the rhone delta and east of high gradient coastal streams that have provided most of the clastic sediments to the gulf of lions margin since the early miocene this 10 km wide area is constituted by a small coastal watershed 15 km long in low lying carbonate hills a kilometre wide marsh a sandy beach barrier and a shoreface experiencing local low sedimentation rates this onshore offshore transition zone is a key area to understand the relationships between erosion by pass deposition and preservation processes unlike the neighbouring rhone deposits that display numerous thick deltaic sequences related to a continuous sediment flux the maguelone shore exhibits peculiar sedimentary bodies the analysis of about 250 km of new vhr seismic profiles acquired over the study area 9 short cores and outcrops data allowed us to determine the three dimensional high resolution geometry of the coastal system tracts from quaternary to present day this revealed dominant denudation processes in the upstream catchments associated with the formation of incised valleys during quaternary lowstands in addition this study investigates the erosion and deposition mechanisms controlled by littoral hydrodynamics that have been taken place on this shore since the last transgression in particular metre high and hundreds of metre long sandy bodies constructed by alongshore hydrodynamics were identified and their effect on the morphodynamics of the whole littoral system is discussed the maguelone shore can be used as an experimental study for onshore offshore dynamics under the influence of microtidal wave current hydrodynamics and very low river derived sediment flux during a eustatic cycle geometry of system tracts across flood plain to shoreface is usually considered to be controlled by relative sea level changes and is successfully analysed with 2d sequence stratigraphy cross shore models it is observed that in areas like the maguelone shore traditional sequence stratigraphy methods might not be fully relevant and need to integrate alongshore hydrodynamical processes

the m2 tide and its residual on the outer banks of the gulf of maine abstract tidal computations are reported for the gulf of maine with emphasis on its seaward banks georges bank browns bank nantucket shoals and the nearshore region off cape sable the model is 3d and nonlinear with quadratic vertical viscosity stratification is ignored the objective is to establish a theoretical baseline for tidal and residual currents on detailed topography against which seasonal changes may be examined two triangular meshes are used the base case corresponds to earlier studies with roughly 6 km resolution on the banks a more refined and extended mesh provides resolution at 2 3 km in critical areas and has boundaries farther removed from the banks both meshes provide very good fits with observed m2 elevations comparison of computed and observed m2 current profiles at 76 sites reveals general agreement that improves with resolution for the refined mesh and the 48 sites with records exceeding 60 days average deviations for m2 ellipse parameters are 6 1 4 7 cm s 1 for the

the contour current flow along the southern iceland faeroe ridge as documented by its bedforms and asymmetrical channel fillings abstract the iceland faeroe ridge is a natural barrier to the exchange of water masses between the norwegian greenland sea and the north atlantic basin the main overflow of cold and dense water from the norwegian greenland sea through the faeroe channels results in the generation of an intense contour current flow below 600 m water depth along its southern flank side scan sonar surveys revealed an extensive pattern of various bedforms with current indicators matching well with contour flow directions previously inferred from oceanographic measurements in the shallow ridge crest area only minor current induced bedforms were observed reflecting the relatively weaker intensities of overflow across the ridge in addition to the observed bedforms surveys of a nearby ridge parallel channel system also located on the southern flank of the iceland faeroe ridge document an asymmetrical sedimentation pattern with northern channel flanks displaying condensed sedimentation this asymmetrical sedimentation pattern is attributed to the influence of the coriolis force which is the ultimate driving mechanism for the generation of contour current systems

quaternary geology and surficial sediment processes browns bank scotian shelf based on multibeam bathymetry abstract browns bank is located on the glaciated continental shelf off southern nova scotia geological mapping of browns bank is based on interpretation of multibeam bathymetric and backscatter data in conjunction with 220 line km of seismic reflection profiles and sidescan sonograms sea floor sediment samples and bottom photographs the fundian moraine part of the previously identified end moraine system on the continental shelf off nova scotia is a prominent west east flat topped multi lobate ridge identifiable in the multibeam bathymetric data and in geophysical records the fundian moraine was subjected to erosion in the surf zone during sea level rise in depths less than 100 m a series of north south roughly parallel till ridges continuous in the subsurface with the fundian moraine is interpreted as interlobate moraines suggesting formation by a tidewater glacier the browns bank moraine connected to the fundian moraine is evidence of a newly identified grounded ice position farther seaward than the latter bedforms including obstacle marks comet marks sand waves and megaripples are evidence of a vigorous anticyclonic current pattern on browns bank the combination of multibeam bathymetric imagery with high resolution geoscientific information represents a powerful technique for sea floor geological investigations

toward a model for airflow on the lee side of aeolian dunes the interaction between dunes and the primary wind results in a complex pattern of secondary airflow on the lee side of dunes from 15 dunes studied during transverse flow conditions at padre island in texas white sands in new mexico and the algodones in california distinct flow regions can generally be recognized with the overall flow structure comparing favourably to that proposed for subaqueous bedforms downwind of dunes with flow separation is a back flow eddy that extends about four dune brink heights downwind from the brink of the dune beyond the separation cell the velocity profiles can be divided into regions based upon segments separated by kinks in the velocity profiles the interior is an area above the dunes of relative high wind speed but low velocity gradient beneath the interior is the wake which consists of two layers the upper wake exhibits an uppermost portion where the flow decelerates while the remainder exhibits accelerating flow so that the overall velocity gradient decreases downwind the lower wake exhibits low velocity gradients and wind speeds that accelerate downwind at all heights but primarily near the top of the layer thereby causing the velocity gradient to increase downwind at about eight dune heights downwind the upper and lower wakes equilibrate to a single profile with the kink between them no longer apparent the lowest recognizable region is the internal boundary layer it is recognized by a relatively steep velocity gradient below the wake and never exceeds a few tens of centimetres in height for our data set because of acceleration and increasing shear stress within this layer interdune flats are at least potentially erosional overall the wake and internal boundary layer show a downward transfer of momentum from upper regions so that the flow recovers where flow separation does not occur simple flow expansion down the lee face causes flow deceleration

hydrodynamic significance of variable ripple morphology across deep water barchan dunes in the faroe shetland channel a combination of high resolution 30 khz sidescan sonar and seafloor photography has been used to image barchan dunes and sand ripples in the faroe shetland channel the barchan dunes display body lengths and horn to horn widths of up to 120 m and the horns point downcurrent to the southwest the seafloor surrounding the dunes is variable with gravel patches sand with gravel streaks and uniform rippled sand all observed the pattern of sand ripples across the dune surface is strongly controlled by the dune morphology on the lower stoss slopes of the dunes straight crested transverse ripples gradually merge upslope into sinuous crested transverse ripples most of the upper dune surface is dominated by short crested linguoid ripples this sequence represents an increase in height above seafloor and increasing flow velocity the overall pattern of ripples across the dune surface indicates that the bottom current flow is modified by the dune morphology with flow concentrated over the dune crest and along the inner edge of the horns it is also possible to estimate flow velocity across the dune field using published analogues and short term current meter observations generally published examples of submarine barchans suggest a flow velocity of 40 80 cm s this range of values is in agreement with near bed current meter measurements in the study area which indicate peak flow velocities of 50 60 cm s however in the longer term higher flow speeds may be possible especially considering the presence of erosional furrows adjacent to the dune field

seasonal circulation on the western and central scotian shelf abstract a realistic representation of 3d seasonal circulation and hydrography on the western and central scotian shelf has been obtained from historical observations and a combination of diagnostic and prognostic numerical models with forcing by tides wind stress and baroclinic and barotropic pressure gradients the major current features the southwestward nova scotian and shelf edge currents and partial gyres around browns and sable island banks are found to persist year round but with significant seasonal changes comparison with current meter observations shows good agreement for the browns bank southwest nova scotia and inner shelf regions and poorer agreement in the sable island bank and shelf edge regions where current and density observations are sparser and tidal influences weaker there is significant spatial structure in the seasonal circulation and hydrography and in the underlying dynamical processes on the shelf scale there are substantial changes in stratification potential energy

the shape of the barchan dunes of southern morocco we present detailed shape measurements of several barchan dunes in southern morocco near la youne using these data we disprove the concept of shape invariance of barchan dunes of different sizes nevertheless some parts of the barchan dune scale and we try to distinguish these from non scaling ones furthermore we point out the importance of the exact position of the brink for the overall dune shape finally we propose a simple model for dune shape using our data to determine the model parameters despite its simplicity this model is able to explain the difference between barchans where the crest and the brink coincide and those where they are separated typical side slopes can be obtained and the basic properties of the evolution of a barchan dune can be described

numerical modelling of plio quaternary slope evolution based on geological constraints a case study from the caramanico valley central apennines italy evidence of deep seated gravitational slope deformations dsgsd and of large prehistoric landslides is fairly widespread within the central apennines italy these gravity induced processes accompanied the intense plio quaternary uplift phases that affected the mountain chain in this study a multidisciplinary approach has been adopted in order to better constrain the relationship between the tectonic evolution and the gravitational morphogenesis of a typical apennine morphostructure such as the caramanico valley for this purpose a conceptual model of the morphostructural evolution of the area has been reconstructed on the basis of geo logical constraints derived by the integration of detailed geological structural and geomorpholo gical surveys with available literature data based on this evolutionary model a multistage numerical modelling using the finite difference method code flac 6 0 has been performed in order to i evaluate the effect of the uplift related morphological changes of the valley slope system and ii assess the role of the horizontal vertical stress ratio variations due to geodynamic regime shifts the results of the numerical model show a good fit with the actual geomorphical evidence and also confirm the presence during some evolutionary stages of stress strain conditions compatible with those necessary to produce the massive rock slope failures testified by the presence of large palaeo landslide deposits

bedforms of the canadian eastern seaboard a comparison with global occurrences this paper presents a compilation of sedimentary bedforms identified from acoustic and seismic records collected over the southeastern canadian continental shelf in order to make the compilation it was found necessary to firstly review and synthesise the various terminologies used to describe the bedforms found and thereafter to relate the subsequent terminology to any diagnostic generic characteristic identifiable from the records sand waves are considered to be those forms that are composite in nature and are generated solely by the action of superimposed megaripples these latter features are identifiable adopting the classification of dalrymple et al 1978 which proves adequate for the acoustic differentiation of 2 d and 3 d types though we observed a greater coherence in bedform spacing of the 2 d types than reported hitherto sand ridges were adequately defined and recognised using the description of swift et al 1978 with respect to storm generated ridges and that of off 1963 and others with respect to tidal current ridges such ridges appear not to be uniquely described by ratios of their metrics the bedforms on the southeastern canadian continental shelf are abundant and wide spread virtually all types of bedforms recognisable on the u s shelf are present to the north the majority of these bedforms appear to be storm generated and are considered intermittently active many of the sand waves s s have no measureable relief though retaining their distinctive spatial geometry both active and moribund sand ridges were found of these ridges both tidal and storm generated types were identified the storm generated ridges were further broken down into shoreface connected ridges and offshore ridges a metrical analysis of the various bedforms show that a differentiation into stable bed phases can be made using height length ratios provided the bedforms are active and fully developed there is some overlap between megaripples and small sand waves and indeed between 2 d and 3 d megaripples yet the distinction is good between sand waves and sand ridges even moribund types sand ribbons in general can be confused with moribund sand waves and megaripples a detailed morphological examination is required to distinguish each feature data from sand ridges of the eastern canadian shelf combined with data from similar features found globally shows that ridge heights tend to cluster into three distinctive modes these modes are at heights of 7 13 and 27 m the first mode is correlated with storm generated ridges the second with tidal current ridges and the third possibly with submerged barrier islands

anomalous sea floor backscatter patterns in methane venting areas dnepr paleo delta nw black sea the relation between acoustic sea floor backscatter and seep distribution is examined by integrating multibeam backscatter data and seep locations detected by single beam echosounder this study is further supported by side scan sonar recordings high resolution 5 khz seismic data pore water analysis grain size analysis and visual sea floor observations the datasets were acquired during the 2003 and 2004 expeditions of the ec funded crimea project in the dnepr paleo delta area northwestern black sea more than 600 active methane seeps were hydroacoustically detected within a small 3 96 km by 3 72 km area on the continental shelf of the dnepr paleo delta in water depths ranging from 72 m to 156 m multibeam and side scan sonar recordings show backscatter patterns that are clearly associated with seepage or with a present dune area seeps generally occur within medium to high backscatter areas which often coincide with pockmarks high resolution seismic data reveals the presence of an undulating gas front i e the top of the free gas in the subsurface which domes up towards and intersects the sea floor at locations where gas seeps and medium to high backscatter values are detected pore water analysis of 4 multi cores taken at different backscatter intensity sites shows a clear correlation between backscatter intensity and dissolved methane fluxes all analyzed chemical species indicate increasing anaerobic oxidation of methane aom from medium to high backscatter locations this is confirmed by visual sea floor observations showing bacterial mats and authigenic carbonates formed by aom grain size analysis of the 4 multi cores only reveals negligible variations between the different backscatter sites integration of all datasets leads to the conclusion that the observed backscatter patterns are the result of ongoing methane seepage and the precipitation of methane derived authigenic carbonates mdacs caused by aom the carbonate formation also appears to lead to a gradual self sealing of the seeps by cementing fluid pathways horizons followed by a relocation of the bubble releasing locations

sedimentary environment of the faroe shetland and faroe bank channels north east atlantic and the use of bedforms as indicators of bottom current velocity in the deep ocean in the northeast atlantic much of the deep cold water flow between the norwegian sea and the main north atlantic basin passes through the faroe shetland and faroe bank channels generating strong persistent bottom currents capable of eroding and transporting sediment up to and including gravel a large variety of sedimentary bedforms including scours furrows comet marks barchan dunes sand sheets and sediment drifts is documented using sidescan sonar images seismic profiles seabed photographs and sediment cores from the floor of the channel published information on current velocities associated with the various bedforms has been used to reconstruct the pattern of bottom currents acting on the channel floor the results broadly reflect the current pattern predicted on the basis of regional oceanographic observations but add considerable detail the internal consistency of the results suggests that the methods used are robust giving confidence in the fine detail of the observed bottom current structure bottom current velocities in the range 1ae0 m s 1 are indicated by the range of observed bedforms with the strongest currents associated with south west transport of norwegian sea deep water nsdw at water depths of 800 1200 m the main nsdw flow forms a relatively narrow core that follows the base of the faroes slope this core follows the 90 change in trend of the faroes slope at the junction between the faroe shetland and faroe bank channels the strongest currents within the nsdw core are found over the shallowest sill in the faroe shetland channel and in the narrowest part of the channel immediately downstream of the sill and are generated by topographic constriction of the flow eastward flow of deep water along the northern flank of the wyville thomson ridge suggests a complex current pattern with some recirculation of deep water within the deep faroe bank channel basin the observations suggest that coriolis force is the main agent controlling the westward deflection of the nsdwinto the faroe bank channel contradicting a previous suggestion that this was controlled by the topography of the wyville thomson ridge

tide induced bedload transport pathways in a multiple sand ridge system offshore of hainan island in the beibu gulf northwest south china sea modern sand transport pathways in a multiple sand ridge system are elusive and have rarely been studied in recent years we report herein a field with four en echelon linear sand ridges offshore of hainan island in the beibu gulf and describe the distribution and morphology of these sand ridges in detail for the first time dune crest comparisons and seismic profiles are also interpreted to assess sand transport over the dunes based on a delft 3d model regional tidal currents and tide induced bedload transport in the multiple sand ridge system were simulated to provide insights into the related bedload transport paths the results show that bedload transport and residual flows are mostly directed to the south on the east side of the sand ridges and to the north on the west side and these differences coincide with dune asymmetries and migrations cross ridge transport is weak and mostly converges on the crests of sand ridges from the two flanks the bedload transport is unbalanced on either flank of the sand ridges thus leading to the asymmetry and crest kinks of the sand ridges a distinct correlation is not observed between the net cross ridge sand transport and sand ridge asymmetry in this system cross swale transport is significant between the neighboring sand ridges and helps construct the bedload transport circulation in the swales sand accretion also occurs in the swales and benefits from the southwestward bedload transport from the north end of sand ridge 3 distinct evidence has not been found for closed bedload transport circulation around sand ridges in this multiple sand ridge system although limited clockwise current vortices develop on several sand ridges additional field observations and simulations are required to characterize the suspended load transport in a multiple sand ridge system and the effects of local sand supply conditions on sand transport also need further evaluation copyright c 2018 john wiley sons ltd

width height and slope relationships and aerodynamic maintenance of barchans the allometric relationship between the height of barchan dunes and horn to horn widths is re examined the addition of new data from barchans in the namib desert and elsewhere confirms the relationship that the height of a barchan is approximately one tenth of the width of the dune height and width of the dune are closely linked to the three dimensional morphology of the dune in particular the relatively constant mean angles of around 11 at which the side slopes of the dune are maintained we suggest that the 3 d barchan form is directly controlled by aerodynamic processes constant feedback between three dimensional stoss face windward shape of the dune slipface height and horn to horn width and shear stress produces an equilibrium dune form

mechanisms of strain accommodation in plastically deformed zircon under simple shear deformation conditions during amphibolite facies metamorphism abstract this study documents the strain accommodation mechanisms in zircon under amphibolite facies metamorphic conditions in simple shear microstructural data from undeformed fractured and crystal plastically deformed zircon crystals are described in the context of the host shear zone and evaluated in the light of zircon elastic anisotropy our work challenges the existing model of zircon evolution and shows previously undescribed rheological characteristics for this important accessory mineral crystal plastically deformed zircon grains have axis oriented parallel to the foliation plane with the majority of deformed grains having axis parallel to the lineation zircon accommodates strain by a network of stepped low angle boundaries formed by switching between tilt dislocations with the slip systems 010 and 1 10 110 and rotation axis 001 twist dislocations with the rotation axis 001 and tilt dislocations with the slip system 001 and rotation axis 010 the slip system 1 10 110 is newly described for zircon most misorientation axes in plastically deformed zircon grains are parallel to the xy plane of the sample and have 001 crystallographic direction such behaviour of strained zircon lattice is caused by elastic anisotropy that has a direct geometric control on the rheology deformation mechanisms and dominant slip systems in zircon young s modulus and p wave velocity have highest values parallel to zircon 001 axis indicating that zircon is elastically strong along this direction poisson ratio and shear modulus demonstrate that zircon is also most resistant to shearing along 001 thus 001 axis is the most common rotation axis in zircon the described zircon behaviour is important to take into account during structural and geochronological investigations of poly metamorphic terrains geometry of dislocations in zircon may help reconstructing the geometry of the host shear zone s large scale stresses in the crust and possibly the timing of deformation if the isotopic systems of deformed zircon were reset

the mean and seasonal circulation off southwest nova scotia abstract long term measurements off southwest nova scotia reveal the following features of the mean circulation a a westward longshore coastal current 4 10 cm s 1 b an anticyclonic gyre around brown bank 5 15 cm s 1 and c an upwelling circulation off cape sable 1 2 cm s 1 at bottom the gyre circulation appears permanent but the coastal current and upwelling exhibit annual variations of the same order as the means there is a distinct annual signal in the longshore transport at cape sable maximum westward in winter whereas the mean transport 0 14 106 m4 s 1 is consistent with both geostrophic estimates and budget requirements in the gulf of marine strong seasonal cycles are also found in the salinity and density fields at cape sable which appear to be controlled both by buoyancy input from the coastal current and local mixing effects a linear diagnostic model indicates that the primary dynamical balance for the circulation is between a longshore pressure gradient and longshore mean