mesoscale model experimental forecasts of the haar of northeast scotland abstract a mesoscale model is used to simulate the diurnal evolution of sea fog off the northeast scottish coast observed on 27 april 1984 it is shown that the accuracy of the early part of the forecast is very dependent on the specification of the initial conditions if the initial description of the fog is sufficiently good the model can accurately erode it during the day and reform it in the following evening the dependence of the accuracy of the forecasts on vertical resolution is also discussed

holocene evolution of seasonal stratification in the celtic sea refined age model mixing depths and foraminiferal stratigraphy published stable isotopic oxygen carbon and preliminary foraminiferal data from a holocene vibrocore from the celtic sea northwest european continental shelf have been interpreted in terms of the progressive replacement of tidally mixed by seasonally stratified water the first study of the long term dynamics of seasonal stratification this study was hampered by poor age control and the foraminiferal data were based on processing with a 125 m sieve which has been shown not to recover critical small taxa we present here a new age model for this vibrocore bgs 51 07 199 based on 12 accelerator mass spectrometry 14c dates on foraminifera and molluscs radionuclide 137cs 40k profiles from the vibrocore and juxtaposed multicores and a complete foraminiferal stratigraphy based on sieving at 63 m together the three datasets enable a mixing model to be proposed which clarifies the resolution of the record which is confirmed as extending from the late glacial to the late holocene correction for autocompaction reveals an increase in sedimentation rate and mixing depth from the early to the late holocene a temporary increase in sedimentation rate at 6650 yr cal bp is attributed to migration of the productive frontal zone across the core site some time after 3000 yr cal bp sedimentation either slowed abruptly or ceased completely giving the modern mixed layer an apparent age of 3000 yr cal bp the mixed layer depth indicates strongly that the apparent transition to stratification during the early holocene is primarily a threshold change attenuated by bioturbation secondary attentuation is related to reworking the mixed layer model adopted suggests that this isotopic transition occured between 8990 and 8440 yr cal bp 8720 2 the foraminiferal analyses in the light of modern foraminiferal distributional data support the inference that the succession can be interpreted as a response to the progressive seasonal stratification of the celtic sea during the holocene the data highlight the value of key taxa as indicators of shelf palaeostratification in the geological record notably of textularia bocki and stainforthia fusiformis as mixed frontal and frontal stratified indicators respectively the critical change to this frontal assemblage occurs at the same depth as the isotopic threshold based on the mixing model these data affirm the earlier interpretation of the isotopic record as registering bottom water temperature and water column productivity changes driven by the evolution of seasonal stratification the clear association between grain size and sedimentation as well as these isotopic and faunal data indicates that the shelf record of highstand sedimentation is preferentially biased towards the preservation of sequences deposited under seasonally stratified rather than mixed water masses

a finite amplitude necking model of rifting in brittle lithosphere we formulate a mechanical model describing the formation of rifts as finite amplitude necking of an elastic plastic layer overlying a fluid substrate a perfectly plastic rheology is a continuum description of faulting in rift zones two important aspects of rift evolution are illustrated by this model the evolution of the rift width as extension proceeds and the finite strain that occurs a region at yield initially develops with a width determined by the thickness of the brittle layer and the internal deformation within this yield zone is proportional to the topographic slope as extension proceeds the surface within the rift subsides and the width of the subsiding yield zone decreases at any stage of rifting material in regions just outside the yield zone is deformed but no longer deforming the width of these deformed regions increases with increasing extension vertical forces due to the mass deficit of the rift depression will flex the elastic layer outside the yield zone creating flanking uplifts the external force required to maintain active rifting increases with the amount of lithospheric stretching indicating that rifting is a quasi static stable process because the yield zone will revert to elastic behavior if the external force causing extension is removed the model predicts that the rift depression and flanking uplifts will be preserved after extension stops our simple mechanical model demonstrates the inherent relationship among graben formation lithospheric thinning and rift shoulder uplift in rift zones

modern carbonate marine sands in estuaries of southwest england large deposits of marine sand occur in the taw camel and hayle estuaries of north devon and cornwall total carbonate concentrations reach 67 in the hayle 62 in the camel and 40 in the taw well mixed the sands contain modern skeletal debris of consistent composition which persists up to 18 km and 12 km landward from the mouths of the taw and camel estuaries although primarily a molluscan sand remains of barnacles bryozoans echinoids foraminifera sponge spicules decapods and coralline algae are common carbonate mineralogy provided by these components on a 100 carbonate basis consists of 45 aragonite 38 low mg calcite and 17 high mg calcite two estuaries of the southeast coast of devon the exe and teign contain smaller carbonate concentrations of up to 9 and 14 values are low there owing to dilution by readily eroded cliff talus from local exposures of new red sandstone these permo triassic sediments provide carbonate lithoclasts however which supplement the modern biogenic supply strontium values have been used to estimate the extent of carbonate lithoclasts supplied to the exe and teign the present source of marine sand of estuaries in southwest england is considered to be mostly near shore detritus supplemented by eroded cliff talus derived largely from local outcrops once this material enters the estuarine regime it is available as a supply of sediment for further up estuary transport

sediment transport predictions for the english channel using numerical models output from an hydrodynanmic numerical model salomon breton 1991 a b has been combined with various empirical formulae to derive sediment transport rates and directions the predictions are compared with radioactive sand tracer experiments demonstrating the applicability of gadd et al s 1978 expression the derived bedload transport pattern for the area is consistent with sedimentological and geomorphological evidence but provides improved resolution a bedload parting zone is identified for the central english channel this is well defined for sand movement but seabed material present over this area gravel is not predicted to move significantly the area is non erosive over which fine grained sediments are being transported a bedload convergence is located to the southwest of the dover strait not in the strait itself as proposed previously a transport pathway bypassing the convergence along the french coastline which is supported by radioactive tracer experiments could be responsible for sand transfer from the english channel to the southern north sea

sedimentary facies and evolution in the qiantang river incised valley eastern china abstract this paper deals with the sedimentary facies and evolution of the qiantang river qr estuary and the characteristics and formation of the incised valley sequences and the related shallow biogenic gas reservoir on the basis of analysis of over 500 cores the result shows that since the last glaciation the late quaternary formation of the qr estuary area underwent three stages 1 deep cutting stage 2 rapid filling stage and 3 burial stage the fall of global sea level during the last glacial maximum enhanced the fluvial gradient and river cutting resulting in the formation of the large scale qr and taihu incised valleys with the interfluve being exposed to air on both flanks of the incised valley fluvial terraces at the elevations are present near the present qr estuarine mouth corresponding to 60 70 90 100 and 115 125 m burial depths the valleys were filled rapidly with fluvial sediments during the post glacial period with the rise of sea level the river mouth migrated to landward and backwater and retrogressive aggradation was enhanced the qr and taihu incised valleys are associated with an early filling and transgressive channel infilling sequence formation and a late filling and transgressive floodplain estuary formation subsequently the qr valley was buried under estuarine marine and estuarine sand bar sediments from bottom to top the incised valley successions can be grouped into four sedimentary facies river channel floodplain estuary estuary shallow marine and estuary sand bar the thickness of the river channel infilling deposits is controlled mainly by base level rising backwater retrogressive aggradation and neotectonism further localized thickening took place where deeper scour pools were present in the incised valley or fluvial terraces were formed during the fall of elative sea level during the deposition of the floodplain estuary facies the conditions of sea level rise tidal regime sediment supply and accommodation space were suitable for the development of a tidal ridge system the sand lenses associated with this facies may represent a tidal ridge system in the incised valley at the later stage when the estuarine sand bars were formed the sedimentary conditions were no longer favourable resulting in absence of sand ridge deposits biogenic gas is stored in the floodplain estuary sand lenses of the incised valleys the changjiang river provides the major sediment supply for the qr estuary sand bar and the qr carried sediments constitute only a small portion of the deposits

seismicity and active tectonic processes in the ultra slow spreading lena trough arctic ocean summary with its remote location in the ice covered fram strait lena trough is a poorly known segment of the global mid ocean ridge system it is a prominent member of the ultra slow spreading mid ocean ridges but its spreading mechanisms are not well understood we relocalized teleseismically recorded earthquakes from the past five decades to identify tectonic processes in lena trough and the adjacent spitsbergen fracture zone fz during two cruises with rv polarstern in 2008 and 2009 we deployed seismic arrays on ice floes to record the local seismicity of lena trough we could identify and localize microseismic events which we assume to be present in the entire rift valley in contrast our relocalization of teleseismically recorded earthquakes shows an asymmetric epicentre distribution along lena trough with earthquakes occurring predominately along the western valley flanks of lena trough in 2009 february march several high magnitude earthquakes peaking in an mb 6 6 event occurred in an outside corner setting of the spitsbergen fz this is the strongest earthquake which has ever been recorded in fram strait and its location at the outside corner high of the ultra slow spreading ridge is exceptional comparing the seismicity with the magnetic anomalies and high resolution multibeam bathymetry we divide lena trough in a symmetrically spreading northern part and an asymmetrically spreading southern part south of the south lena fz we propose that a complex interaction between the former de geer megashear zone which separated greenland from svalbard starting at late mesozoic early cenozoic times and the developing rift in the southern lena trough resulted an increasing eastward dislocation towards the spitsbergen fz between older spreading axes and the recent active spreading axis which we believe to be located west of the bathymetric rift valley flanks in a wide extensional plain

mechanisms of lithospheric extension at mid ocean ridges summary we examine the extensional deformation of oceanic plates at mid ocean ridges especially within an axial yield zone where pervasive faulting occurs thermal models of ridges are developed which include the effects of lithospheric thickening on the mantle flow the heat of magmatic crustal accretion at the ridge axis and the hydrothermal cooling due to seawater circulation in the crust when hydrothermal circulation occurs the brittle lithospheric plate at slow spreading ridges could be as thick as 8 9 km thicker than the crust while the plate at fast spreading ridges is only 1 2 km for a typical slow spreading ridge several kilometres of plate thickening are expected within a distance of 15 km from the ridge axis when subjected to the extensional force due to horizontal stretching shear failure by normal faulting will occur pervasively in an axial zone where the lithospheric plate is the thinnest adopting perfectly plastic rheology as a continuum description of deformation on the distributed faults we obtain approximate solutions for the stress distribution in the yield zone within this yield zone sea floor topography increases significantly away from the ridge axis so that the resuiting gravity sliding force balances the differential horizontal extensional force due to the thickening of the lithospheric plate basal stresses induced by the viscously deforming asthenosphere could significantly influence the stresses inside the lithospheric plate only if the mantle viscosity beneath the ridge is on the order of 10 pa s significantly higher than generally accepted values of 10 pa s model calculations reveal that although the seafloor topography at the mid atlantic ridge at 13 15 on is regionally compensated it is locally supported by stresses in the lithospheric plate the deviation of the lithospheric plate from its thermal isostatic equilibrium position can be explained by necking due to plastic stretching in the axial yield zone and elastic deflection of strong plates outside the yield zone the best fit models require the yield zones to have a half width of 10 15 km we find systematic variations in the gravity and topography of the east pacific rise which indicate strong influence of plate spreading rate on the ridge thermal and mechanical structure at the 16 17 on area where the half spreading rate is 4 3 cm yr a prominent axial topographic high and an axial mantle bouguer gravity low exist implying a crustal or sub crustal low density body the gravity low disappears at the 20 21 n area where the half spreading rate is 3 6 cm yr as the plate spreading rate decreases from 4 3 cm yr at the 16 17 on area to 2 7 cm yr at the 22 23 on area axial ridge topography changes from higher than the thermal isostatic equilibrium position to lower than the isostatic position the low axial topography at the 22 23 n area can be explained by the existence of a low amplitude median valley

mid ocean ridges fine scale tectonic volcanic and hydrothermal processes within the plate boundary zone a first order model of spreading centers as idealized linear boundaries of crustal and lithospheric generation provides only a gross understanding of global scale plate kinematics as we attempt to understand the complexity of crustal and lithospheric structure of two thirds of the earth s surface it is becoming increasingly necessary to study the tectonic volcanic and hydro thermal processes within the spreading center plate boundary zone all ocean ic crust bears the imprint of these processes this review focuses on a few selected topics concernining the fine scale tectonics and geophysics of the active axial zone of mid ocean ridges with reference to associated volcanic and hydrothermal processes it draws heavily on recent studies that use deeply towed instrument packages multi beam bathymetric mapping ocean bottom instruments and alvin e g the famous amar rise and galapagos expeditions we begin with a review of the large scale structure of spreading centers we then take a close look at the axial neovolcanic zone and progress away from the axis through the active tectonic zones next we consider the characteristics of the axial magma chamber and associated hydrothermal actvity as well as the generation of magnetic anomaly stripes and their implications for crustal generation one of our findings is that the initial two dimensional model of volcanic and tectonic zones must be expanded upon to allow for variations

comparisons of measured and predicted sediment transport rates in tidal currents abstract this paper describes bedload transport measurements in a large scale sedimentary system on the british continental shelf steady flow sediment transport theories are applied to oscillatory tidal currents and various equations are compared with measured transport rates from tracer dispersion experiments the effects of waves on both measured and predicted rates are briefly considered and the predicted directions of sediment movement compared with those inferred from sedimentological data

large scale atmospheric forcing by southeast pacific boundary layer clouds a regional model study abstract a regional model is used to study the radiative effect of boundary layer clouds over the southeast pacific on large scale atmosphere circulation during august october 1999 with the standard settings the model simulates reasonably well the large scale circulation over the eastern pacific precipitation in the intertropical convergence zone itcz north of the equator and marine boundary layer stratocumulus clouds to the south in a sensitivity experiment with the radiative effect of liquid clouds south of the equator over the eastern pacific artificially removed boundary layer clouds south of the equator almost disappear and precipitation in the itcz is reduced by 15 20 indicating that the stratocumulus clouds over the southeast pacific have both local and cross equatorial effects examination of the differences between the control and sensitivity experiments indicates that clouds exert a net diabatic cooling in the inversion layer in response to this cloud induced cooling an in situ anom

causes and consequences of variations in faulting style at the mid atlantic ridge both volcanism and faulting contribute to the rugged topography that is created at the mid atlantic ridge mar and preserved off axis in atlantic abyssal hill terrain distinguishing volcanic from fault generated topography is essential to understanding the variations in these processes and how these variations are affected by the three dimensional pattern of mantle upwelling ridge segmentation and offsets here we describe a new quantitative method for identifying fault generated topography in swath bathymetry data by measuring topographic curvature the curvature method can distinguish large normal faults from volcanic features whereas slope methods cannot because both faults and volcanic constructs can produce steep slopes the combination of curvature and slope information allows inward and outward facing fault faces to be mapped we apply the method to sea beam data collected along the mar between 28 and 29 30 n the fault styles mapped in this way are strongly correlated with their location within the ridge segmentation framework long linear small throw faults occur toward segment centers while shorter larger throw curved faults occur toward ends these variations reflect those of active faults within the axial valley we investigate two different physical mechanisms that could affect fault interactions and thus underlie variations in abyssal hill topography at the mar in the first model only one fault is active at a time on each side of the rift valley each fault grows while migrating away from the volcanic center due to dike injection extension across the fault causes a flexural rotation of nearby inactive faults the amount of stress necessary to displace the fault increases as the fault grows when reaching a critical size the fault stops growing as fault activity jumps inward as a new fault starts its growth near the rift valley this model yields a realistic terracelike morphology from the rift valley floor into the rift mountains the relief is caused by the net rotation accumulated in the lithosphere from the active faults e g 10 reached 20 km from the active fault fault spacing is controlled by lithospheric thickness fault angle and the ratio of amagmatic to magmatic extension we hypothesize that this mechanism may be dominant toward ridge segment offsets an alternative model considers multiple active faults each fault relieves stresses as it grows and inhibits the growth of nearby faults causing a characteristic fault spacing such fault interactions would occur in a region of necking instability involving deformation over an extended area this mode of extension would drive a feedback mechanism that would act to regulate the size of nearby faults we hypothesize that this mechanism may be active in the relatively weak regions of strong mantle upwelling near segment midpoints causing the homogeneous abyssal hill fabric in these regions

the impact of waves and tides on residual sand transport on a sediment poor energetic and macrotidal continental shelf 2019 the authors the energetic macrotidal shelf off south west england was used to investigate the influence of different tide and wave conditions and their interactions on regional sand transport patterns using a coupled hydrodynamic wave and sediment transport model residual currents and sediment transport patterns are important for the transport and distribution of littoral and shelf sea sediments morphological evolution of the coastal and inner continental shelf zones and coastal planning waves heavily influence sand transport across this macrotidal environment median 50 exceedance waves enhance transport in the tidal direction extreme 1 exceedance waves can reverse the dominant transport path shift the dominant transport phase from flood to ebb and activate sand transport below 120 m depth wave tide interactions encompassing radiation stresses stoke s drift enhanced bottom friction and bed shear stress refraction current induced doppler shift and wave blocking significantly and nonlinearly enhance sand transport determined by differencing transport between coupled wave only and tide only simulations a new continental shelf classification scheme is presented based on sand transport magnitude due to wave forcing tide forcing and nonlinear wave tide interactions classification changes between different wave tide conditions have implications for sand transport direction and distribution across the shelf nonlinear interactions dominate sand transport during extreme waves at springs across most of this macrotidal shelf at neaps nonlinear interactions drive a significant proportion of sand transport under median and extreme waves despite negligible tide induced transport this emphasizes the critical need to consider wave tide interactions when considering sand transport in energetic environments globally where previously tides alone or uncoupled waves have been considered

hydrodynamics and sediment dynamics of the wash embayment eastern england water and sediment movement in the wash embayment has been determined from an extensive set of data consisting of tidal current readings and suspended sediment concentration measurements instantaneous and residual currents in the embayment show a distinct lateral inhomogeneity whereas vertically the water column is almost homogenous the central deep water area 30 40 m of the embayment is dominated by a residual landward water movement whilst on the margins the residual movement is seaward sediment is supplied predominantly in suspension from the north through the northern extremity of boston deep suspended sediment pathways are coincident with the spring tide water movements and the subtidal channels act as the main conduits approximately 6 8 106 tonnes yr 1 of suspended sediments are supplied to the embayment from offshore areas bedload sediment supply is of lesser importance 1 4 104 tonnes yr 1 whereas suspended sediment movement appears to be the dominant mode of transport throughout the embayment bedload transport is important in reforming the sea bed into a variety of bedforms which are particularly well developed on the margins of channels and shoals

sediment transport in the new york bight north american atlantic shelf vincent c e swift d j p and hillard b 1981 sediment transport in the new york bight north american atlantic shelf mar geol 42 369 398 aanderaa current meter data collected during the course of noaa s new york bight project are sufficiently extensive in time 135000 observation hours and space 41 stations to permit an estimate of the sediment transport in this area the analysis is based on mean flow observations the role of wave oscillatory currents in resuspending sediment is considered but not explicitly included in the sediment transport calculations an estimate of the potential sediment transport rate qb at each station for three different threshold velocities uth 12 16 and 20 cm s 1 were made using a bagnoldtype transport equation qb const uc uth 3 for uc uth qb 0 for uc uth where uc is the mean current speed comparison was made between the results from this simple equation and other bedload and suspended load transport rates which include wave data for a limited subset of the bight data this comparison indicates that the bagnold results are reasonable for the along isobath transport estimates but are not reliable for the cross isobath transport in the cross isobath direction waves appear to play a dominant role in determining the transport rate the comparison also indicates that the bagnold approach tends to overestimate transport in deep water in shallow water resuspension of sand and rotor pumping of current meters both may become important and sediment transport calculations may not be accurate fine sediment suspended transport is approximately double the potential bedload transport for fine sand 0 02 cm but does not dominate the total transport as is believed to occur on the washington shelf smith and hopkins 1972 the data show that the new york bight is a storm dominated shelf with the general southwestward drift of water 6 cm s 1 modulated by more intense storm induced flows the transport calculations indicate that the bulk of sediment transport occurs during the winter months when wind stress is at a maximum and the water column is well mixed two major transport provinces are present a the hudson shelf valley and b the rest of the shelf the shelf valley acts as a sink and a barrier to the continuity of the mainly southwestward transport of sediment 104 g cm 1 yr 1 along the shelf within the valley the potential transport of material is of the order of l05 g cm 1 yr 1 along the valley axis progressive vector transport paths for fine sediment indicate that removal of sediment out of the ends of the valley can occur rapidly however consider ation of the finesediment budget for the whole hudson shelf valley and of the attenua tion of wave activity with depth show that these results are not inconsistent with the presence of high mud concentrations in the valley up valley sediment rates predominate during the period for which data are available

atmospheric responses to kuroshio sst front in the east china sea under different prevailing winds in winter and spring abstractatmospheric responses to the kuroshio sst front in the east china sea under different prevailing winds are examined using high resolution observations and numerical modeling satellite data reveal a significant in phase relationship between sst and surface wind speed indicative of ocean to atmosphere influence the atmospheric response varies according to the relative surface wind direction with respect to the sst front orientation under the alongfront condition low high slp anomalies are found on the warmer colder flank of the front accompanied by surface wind convergence divergence enhanced precipitation and frequent cumulus convection appear over the warm kuroshio suggesting an atmospheric response extending into the free troposphere under the cross front condition when the air blows from cold to warm warm to cold sst divergence convergence is located directly over the sst front and its magnitude is proportional to the downwind sst gradient under such prevailing winds the

effects of the andes on eastern pacific climate a regional atmospheric model study abstract a regional atmospheric model is used to study the effects of the narrow and steep andes on the eastern pacific climate in the southern hemisphere cold season i e august october 1999 the model reproduces key climatic features including the intertropical convergence zone itcz north of the equator and an extensive low level cloud deck capped by a temperature inversion to the south blocking the warm easterly winds from south america the andes help maintain the divergence and temperature inversion and hence the stratocumulus cloud deck over the southeast pacific off south america in an experiment where the andean mountains are removed the warm advection from the south american continent lowers the inversion height and reduces the low level divergence offshore leading to a significant reduction in cloud amount and an increase in solar radiation that reaches the sea surface in march and early april 1999 the model simulates a double itcz in response to the seasonal warming on and south

tidal and surge modelling using differential quadrature a case study in the bristol channel the incremental differential quadrature method idqm was applied to a tidal and surge model of the bristol channel uk the method is considered as an alternative new numerical technique in the field of marine hydraulics and its performance was examined through this case study for validation of the simulated results tide gauge data along the bristol channel was used another well known 1d model mike11 and a quasi 3d model polcoms provided more insight into the flow pattern of the estuary and additional validation data similar to mike11 idqm is unconditionally stable and so time steps of around 45 min achieved good results for the bristol channel whereas for methods which are restricted to the cfl criterion e g explicit finite differencing schemes the time step is limited to around 3 min since there is no stability constraint in idqm the time step must be selected with reference to accuracy rather than stability the usefulness of idqm was also demonstrated by using a small number of grid points 11 along the 110 km length of the bristol channel to produce accurate results based on the results of this case study it is concluded that idqm can be successfully implemented for 1d modelling of tidal elevations and surges in non prismatic irregular channels

holocene shelf sea evolution offshore northeast england abstract spatial and temporal changes of tidally mixed and stratified water conditions during the holocene are poorly understood and constrained within the north sea region this study investigates the changing tidal conditions at a core site from offshore northeast england through the holocene a continuous holocene sedimentary record in british geological survey vibrocore 54 02 97 54 53 05 n 1 3 88 w water depth 55 m collected from a natural trough in the north sea has been investigated a multiproxy approach including the interpretation of magnetic susceptibility values bulk sedimentological characteristics benthic foraminifera faunal and isotopic data is adopted the chronology of this record is constrained by a total of ten accelerator mass spectrometry 14 c dates obtained from molluscs and mixed benthic foraminiferal samples the age derived from benthic foraminifera and molluscs obtained at the same core depth are statistically the same these results suggest that benthic foraminifera tests do not undergo significant post mortem reworking at this shallow site and that the foraminifera based proxy records are reliable sedimentary characteristics indicate that tidally mixed waters between 11 150 and 5869 calendar cal yr bp affected the core site the core site was bathed by tidally mixed water conditions indicated by the presence of ammonia batavus hofker with stratified conditions and associated frontal systems developing at or close to the core site between 5869 and 2032 cal yr bp based on the presence of stainforthia fusiformis williamson finally species that are indicative of fully stratified water such as bulimina marginata d orbigny hyalinea balthica schroter and s fusiformis record the transition from mixed frontal to stratified conditions at 1158 cal yr bp this site records a faunal change in the last 500 years that may be a consequence of fully stratified conditions and enhanced phytodetritus delivery to the sea floor alternatively supported to some extent by heavy metal concentrations the fauna may be responding to eutrophication of coastal waters through anthropogenic influences

sediment transport and dispersion in a cool temperate estuary and embayment saco river estuary maine usa abstract the saco river flows through a rock bound mid latitude estuary recognized as the primary source of sand to the saco bay barrier beach system to better constrain the conditions under which the saco river contributes sand to saco bay three moorings equipped with current meters and a ctd were deployed inside and outside the saco river jetty system from april to june 2005 these measurements characterized high discharge events and their competence for transporting sand outside the jetty system discharge values from 125 175 m 3 s 1 were identified as the threshold envelope at which the saco river transports sand in its bedload to saco bay put in an 89 year historical context with usgs stream gauge data the saco river is expected to contribute sand annually to the embayment but the amount of transport varies significantly from year to year the moored hydrographic data were augmented by geophysical langrangian drifter data and local wind observations to better ascertain nearshore dispersal patterns once sand exits the jettied river mouth these data suggest that transported sand responds to seasonably variable shelf conditions reacts to artifacts of the jetty system or exits saco bay the results and methods of this study suggest that relatively small rock bound estuaries are potentially significant sources of sand to adjacent beach systems and the inner shelf although the pathway for sand from river to shoreface to beach or inner shelf is complex and mediated by local meteorological and oceanographic conditions

an assessment and calibration of formulations for bagnold s bedload equation abstract bagnold s bedload equation in the form q u ucr 3 has frequently been employed to predict sediment mass transport rates in tidal flows and to assist with the interpretation of ancient tidal deposits however physical considerations suggest that the form q u2 u2cr u is more correct this latter formulation is therefore calibrated with experimental data and the coefficient of proportionality is shown to be strongly dependent on the sediment grain size

regional model simulations of marine boundary layer clouds over the southeast pacific off south america part ii sensitivity experiments abstract the sensitivity of a regional climate model to physical parameterizations and model resolution is investigated in terms of its simulation of boundary layer stratocumulus scu clouds over the southeast pacific specifically the physical schemes being tested include shallow cumulus convection subgrid vertical mixing cloud droplet number concentration cdnc and drizzle as described in part i the model with standard settings captures the major features of the boundary layer in the region including a well mixed marine boundary layer a capping temperature inversion scu clouds and the boundary layer regime transition from the well mixed layer near the coast of south america to a decoupled cloud layer over warmer water to the west turning off the shallow cumulus parameterization results in a dramatic increase in the simulated scu clouds while the boundary layer structure becomes unrealistic losing the decoupled regime over warm water with reduced penetrative mixing at the top of shallow cu

history and evolution of the north betic strait prebetic zone betic cordillera a narrow early tortonian tidal dominated atlantic mediterranean marine passage abstract the north betic strait located in the prebetic zone outermost part of the betic cordillera was a narrow seaway connecting the atlantic ocean and the mediterranean sea through the betic cordillera during the early late miocene strait facies consist of heterozoan bioclastic carbonates and mixed siliciclastics carbonates local fan delta deposits conglomerates also occur the north betic strait deposits exhibit huge trough cross bedded structures single troughs are up to 5 m high and point to the east in easternmost outcrops in central outcrops they reach up to 15 m in height and some tens of metres in length and point both to the east and to the west the westernmost outcrops show the largest cross bedded structures up to 20 m in height and some hundred of metres in length all pointing to the west the large scale cross bedding was generated by the migration of very large dunes moved by tides the largest structures are inferred to have formed at depths of 90 m depth estimations indicate that the north betic strait shallowed to the east and deepened to the west the miocene evolution towards the formation of the north betic strait can be divided into a series of steps the palaeogeography evolved from a south facing platform marginal to a northeastern relief to a wide open marine passage limited by a southern platform and finally to a tidal dominated strait the closing of the north betic strait is recorded by the presence of lagoonal silty sediments covered by a stromatolite layer and crowned by a red soil in the westernmost outcrops

a reassessment of sand transport paths in the bristol channel and their regional significance abstract data on bedform morphology sediment parameters heavy mineral abundance current measurements sand transport rates numerical models of shear stress sea bed drifters and wave activity are all considered it is concluded that the dominant transport of sand and gravel is westwards from the transverse bristol channel bed load parting the notion of easterly transport of sand apart from wave induced transport on the beach and in the nearshore zone by the flood tidal current in continuous zones along the northern and southern margins of the bristol channel is largely discounted however there are some small disconnected areas where easterly transport of sand is locally induced by coastal irregularities by sand bank relief and by a local deep the ground for up to 2 km north of scarweather sands provides a rare example of the net sand transport direction being determined by the weaker but substantially longer flowing flood tidal current a facies model of the deposits is outlined

a simple model for the fault generated morphology of slow spreading mid oceanic ridges we postulate that fluctuations in magmatic activity at mid oceanic ridges perturb the horizontal least principal stress across rift bounding normal faults leading to alternating phases of magmatic accretion which increases valley width and tectonic extension which results in the growth of inner rift wall topography fine scale bathymetrie surveys and earthquake fault plane solutions show that active normal faults at slow spreading ridges are moderately dipping approximately 45 planar features throughout the seismogenic oceanic lithosphere a simple quantitative model that includes flexural deformation of a 10 km thick elastic plate by slippage on 45 dipping normal faults can match the bathymetrie profiles across several slow spreading ridge segments comparison among dip distributions of normal faulting earthquakes at mid ocean ridges in the trench outer rise region and on continents suggests that most events from these three tectonic environments initiated at dips close to 45 raising unanswered questions about the mechanical conditions under which the faults originated

modeling bed load transport of coarse sediments in the great bay estuary new hampshire abstract current sea level and bed load transport are investigated in the lower piscataqua river section of the great bay estuary new hampshire usa a well mixed and geometrically complex system with low freshwater input having main channel tidal currents ranging between 0 5 and 2 m s 1 current and sea level forced by the m 2 m 4 m 6 tides at the estuarine mouth are simulated by a vertically averaged non linear time stepping finite element model the hydrodynamic model uses a fixed boundary computational domain and accounts for flooding drying of tidal flats by making use of a groundwater component inertia terms are neglected in comparison with pressure gradient and bottom friction terms which is consistent with the observed principal dynamic balance for this section of the system the accuracy of hydrodynamic predictions in the study area is demonstrated by comparison with four tidal elevation stations and two cross section averaged current measurements simulated current is then used to model bed load transport in the vicinity of a rapidly growing shoal located in the main channel of the lower system consisting of coarse sand and gravel the shoal must be dredged every five to eight years two approaches are taken an eulerian parametric method in which nodal bed load flux vectors are averaged over the tidal cycle and a lagrangian particle tracking approach in which a finite number of sediment particles are released and tracked both methods yield pathways and accumulations in agreement with the observed shoal formation and the long term rate of sediment accumulation in the shoal area

simulation of seasonal variation of marine boundary layer clouds over the eastern pacific with a regional climate model abstractthe seasonal cycle of marine boundary layer mbl clouds over the eastern pacific ocean is studied with the international pacific research center iprc regional atmospheric model iram the results show that the model is capable of simulating not only the overall seasonal cycle but also the spatial distribution cloud regime transition and vertical structure of mbl clouds over the eastern pacific although the modeled mbl cloud layer is generally too high in altitude over the open ocean when compared with available satellite observations the model simulated well the westward deepening and decoupling of the mbl the rise in cloud base and cloud top of the low cloud decks off the peru and california coasts and the cloud regime transition from stratocumulus near the coast to trade cumulus farther to the west in both the southeast and northeast pacific in particular the model reproduced major features of the seasonal variations in stratocumulus decks off the peru and california coasts includin

sediment transport under waves and tidal currents a case study from the northern bristol channel u k bed load transport under tidal currents and waves and waves alone is predicted from near bed self recording current meter data and observations from a wave rider buoy transport rates and paths are derived using seven different unidirectional transport formulae and those of bagnold 1963 bijker 1967 and madsen and grant 1976 for combined flows predicted rates are compared with the results of a fluorescent sand tracer study the data are available from swansea bay a high tidal and wave energy embayment on the british continental shelf sediment transport paths in this region had been determined previously using a variety of techniques including bedform orientation and sea bed drifter recovery patterns transport is enhanced under the superimposed effect of waves and in some circumstances the direction of the resultant vector is altered such storm induced catastrophic transport is different to that predicted under tidal currents alone this could provide a mechanism for long term sediment supply in some areas there is correspondence between bedform orientation and storm induced transport paths in others there is opposition

measurements of the reynolds stress components close to a marine sand bank abstract soulsby r l 1981 measurements of the reynolds stress components close to a marine sand bank mar geol 42 35 47 observations of turbulence have been made at a height of 65 cm above the sea bed in water 42 m deep on the edge of the skerries sand bank off southwest england the observations extended over a 4 h period from the time of maximum flood velocity to slack water electromagnetic current meters were used to measure the streamwise u cross stream v and vertical w velocity components while simultaneously a conventional current meter measured the speed and direction 5 m below the water surface ratios of u2 1 2 v2 1 2 and w2 1 2 to shear velocity agreed with accepted values for laboratory and atmospheric flows near to the surface the current was found to flow across the bank while near the bed it ran nearly parallel to the isobaths consequently the bottom current was directed 480 clockwise of the surface current the azimuth of the reynolds stress uw vw near the bed lay between these directions and was directed about 60 anticlockwise of the near bed current this balanced the cross stream as well as the streamwise shear a lateral shear stress uv associated with the bank was observed taken together with previous measurements of the velocity distribution over the bank this yielded a value for lateral eddy viscosity of 1 2 105 cm2 s 1 the scale over which this acts is of the order 1 km and the value is appreciably smaller than reported values deduced for larger scales the veering of the current acts to lessen the bottom current convergence associated with coriolis and curvature effects and may contribute to lateral sorting by preferentially carrying finer sediment toward the crest of the bank the lateral eddy viscosity coefficient may be useful in assessing the diffusion of sediment on or off the bank

basin dimensions and morphology as controls on amplification of tidal motions the early miocene north hungarian bay following the late aquitanian sea level fall tide influenced deposition started in the north hungarian bay an embayment in the paratethys open to the north east the relatively narrow funnel shape of the bay supported amplification of tidal movements resulting in the generation of strong tidal currents the length and the depth of the north hungarian bay and the connecting seaway through east slovakia fell into the tidal amplification window and thus fulfilled the conditions needed for resonant amplification of semidiurnal m2 tides tide influenced deposits were formed at both sides of the north hungarian bay they reflect dominant currents in opposite directions and of different strengths at either side of the basin this difference was the result of bottom tide interactions cyclonic anticlockwise residual currents were induced above the asymmetrical central depression in the bay and were superimposed upon the tidal currents producing an anticlockwise time and velocity asymmetrical current system the north hungarian bay and other examples show that amplification of tidal motions and formation of tide influenced deposits may occur if basin dimensions pass through the tidal amplification window this window represents ideal conditions for resonant or amphidromic amplification of tidal currents it determines an ideal length depth or width depth ratio relative to the wavelength of the astronomical tides thus signs of strong tidal influence in fossil basin fills can be used to reconstruct the dimensions length depth and width of such basins

numerical simulation of a synoptic event in the southern california bight 1 in the middle of march 2002 a synoptic upwelling event occurred in the southern california bight it was marked by a precipitous cooling of at least 4 c within 10 20 km of the coast by the end of the month the preevent temperatures had slowly recovered the regional oceanic model system roms is used to simulate the event with an atmospheric downscaling reanalysis for surface wind and buoyancy flux forcing lateral boundary conditions of temperature salinity velocity and sea level are taken from a global oceanic product barotropic tidal fields from a global barotropic model are imposed along the open boundaries the simulation reproduces well the upwelling process compared with observed data the sensitivity of the simulation is examined to wind resolution heat flux and tidal forcing the oceanic response to the different wind resolutions converges at the level of the 6 km resolution which is the finest scale present in the terrain elevation data set used in the atmospheric downscaling the combination of an analytical diurnal cycle in the solar radiation and the empirical coupling with the instantaneous roms sea surface temperature produces a similar oceanic response to the downscaled heat flux tidal effects are significant in the upwelling evolution due to the increase in wind energy input through a quasi resonant alignment of the wind and surface current probably by chance

isostatic compensation of tectonic features of the mid atlantic ridge 25 27 30 s we investigate the isostatic compensation of tectonic features along the mid atlantic ridge 25 27 30 s through a detailed examination of the relationship between bathymetry and gravity anomalies the study area includes three ridge segments spreading at an average full rate of 35 mm yr their flanks out to about 6 m y old crust and the intervening rio grande and moore fracture zones in a three dimensional analysis of gravity and sea beam bathymetry data we focus on crustal thickness variations and mantle density anomalies by removing from the observed fields the predicted contribution of simple crustal and mantle models positive residual gravity anomalies over the northern wall of the rio grande fracture zone indicate that in spots the crust is 2 3 km thinner than average there in contrast to the moore fracture zone where little thinning is observed the greater than average depths of the fracture zones as a whole are not locally compensated by thin crust but the deepest basins within the active transform parts of the fracture zones are partially compensated by thin crust or cooler mantle a narrow linear ridge that crosses one of the inactive branches of the moore fracture zone had been suggested to be the product of an episode of excess volcanism but is found to be underlain by thinner not thicker crust there is no indication of thicker crust beneath topographic highs at the inside corners of ridge transform intersections presumably these highs are dynamically maintained as must be the median valleys characteristic of most the length of the ridge segments judging from the absence of axial residual gravity anomalies there is a residual gravity low associated with an unusually shallow 15 to 20 km section of the ridge segment between the rio grande and moore offsets where the median valley virtually disappears our analysis suggests that while part of the anomalous elevation of this section of the ridge axis is attributable to excess volcanism and a thicker crust much of the elevation contrast is simply caused by the diminution of the dynamic mechanism responsible for median valley formation two dimensional fourier transforms of the gravity and bathymetry fields show that seafloor topography is strongly aligned parallel to either ridges or transforms while density anomalies are more randomly oriented this implies that the tectonic processes that control the gross ridge transform topography are not the dominant control on the magmatic processes that determine the upper mantle and crustal density structure statistical analysis of the coherence between the gravity and bathymetry fields indicates that the average effective elastic plate thickness in the study area is about 6 km

seasonal simulations of the planetary boundary layer and boundary layer stratocumulus clouds with a general circulation model abstract the ucla general circulation model gcm has been used to simulate the seasonally varying planetary boundary layer pbl as well as boundary layer stratus and stratocumulus clouds the pbl depth is a prognostic variable of the gcm incorporated through the use of a vertical coordinate system in which the pbl is identified with the lowest model layer stratocumulus clouds are assumed to occur whenever the upper portion of the pbl becomes saturated provided that the cloud top entrainment instability does not occur as indicated by arakawa and schubert cumulus clouds are assumed to originate at the pbl top and tend to make the pbl shallow by drawing on its mass results are presented from a three year simulation starting from a 31 december initial condition obtained from an earlier run with a different version of the model the simulated seasonally varying climates of the boundary layer and free troposphere are realistic the observed geographical and seasonal variations of stratocumulus cloudi

textural and compositional characterization of recent sediments along a paraglacial estuarine coastline maine usa abstract textural and mineralogical characteristics of bottom sediments collected from the lower kennebec river estuary maine and adjacent nearshore region were used to differentiate subtidal sedimentary environments five depositional environments defined on the basis of bottom morphology bathymetry and dominant sedimentary processes estuarine channel channel margin outer bar shoreface and offshore possess distinct sedimentological characteristics trends in textural characteristics of these environments and those along the adjacent barrier beach include 1 systematic decrease in mean grain size from the estuarine channel to the offshore facies 2 similarity between the outer bar and estuarine channel sands 3 increase in sediment sorting from the channel margin and offshore to shoreface facies and 4 fining of the low tide terrace beachface and berm samples away from the bar welding region the fine grained nature of the offshore facies is the result of deposition of suspended sediment delivered to the coast during large flood events whereas the coarse fraction is contributed through the attrition of carbonate tests of the local macrofauna results of compositional analyses reveal 1 a decrease in shell fragments from the estuarine channel 50 to the offshore

estimation of annual bedload flux in a macrotidal estuary bristol channel u k sedimentological data from the bristol channel indicate that mutually evasive ebb and flood dominant zones of net bedload transport are present to test this hypothesis long term observations of currents from the channel are combined with empirical formulae to estimate annual bedload transport rates under the combined influence of tidal currents and surface wind waves extrapolating these values over the ebb and flood dominant zones sand flux to the upper part of the channel is estimated at about 64 105 t yr 1 with supply and removal being of nearly equal magnitude such patterns of sand and water movement may be typical of tidally dominated estuaries and embayments

the obduction of equatorial 13 c water in the pacific identified by a simulated passive tracer the obduction of equatorial 13 degrees c water in the pacific is investigated using a simulated passive tracer of the consortium for estimating the circulation and climate of the ocean ecco the result shows that the 13 degrees c water initialized in the region 8 degrees n 8 degrees s 130 degrees 90 degrees w enters the surface mixed layer in the eastern tropical pacific mainly through upwelling near the equator in the costa rica dome and along the coast of peru approximately two thirds of this obduction occurs within 10 years after the 13 degrees c water being initialized with the upper portion of the water mass reaching the surface mixed layer in only about a month the obduction of the 13 degrees c water helps to maintain a cool sea surface temperature year round equivalent to a surface heat flux of about 6 0 w m 2 averaged over the eastern tropical pacific 15 degrees s 15 degrees n 130 degrees w eastern boundary for the period of integration 1993 2006 during el nino years when the thermocline deepens as a consequence of the easterly wind weakening the obduction of the 13 degrees c water is suppressed and the reduced vertical entrainment generates a warming anomaly of up to 10 w m 2 in the eastern tropical pacific and in particular along the coast of peru providing explanations for the warming of sea surface temperature that cannot be accounted for by local winds alone the situation is reversed during la nina years

mechanisms for the origin of mid ocean ridge axial topography implications for the thermal and mechanical structure of accreting plate boundaries to better understand the implications of ridge axis morphology for the thermal and mechanical structure of accreting plate boundaries we examine mechanisms for the creation of stress supported axial topography and its dependence on parameters such as spreading rate asthenosphere viscosity and plate thickness near the ridge axis three basic mechanisms are considered 1 asthenospheric upwelling in a narrow steady state conduit beneath a ridge axis 2 mantle flow stresses due to plate spreading extending ridge axis lithosphere or melt segregation and associated mantle compaction and 3 topography supported by moments due to horizontal extensional stresses in a thickening lithosphere if mid ocean ridge topography is due to asthenosphere rising in a narrow conduit at the ridge axis then the conduit structure cannot be thermally controlled as the associated thermal structure has relatively broad flat isotherms instead of a conduitlike shape mantle flow associated with plate spreading alone induces no stress supported topography thus the intuitive notion that diverging flow at the ridge axis due solely to plate spreading produces the axial valley is incorrect vertical mantle flow at the base of a stretching accreting lithosphere and mantle flow associated with melt segregation can however lead to stress supported axial topography however for these mantle flow related processes to produce significant amounts of axial relief the mantle viscosity beneath the ridge axis must be on the order of 1020 pa s 1021 p poise significantly higher than generally accepted values of 1018 pa s 1019 p horizontal extensional stresses in a strong brittle lithosphere that thickens with distance from the ridge axis can produce axial topography of the form and amplitude observed on slow spreading ridges a continuum idealization of deformation due to faulting is used to examine this mechanism this model predicts the axial valley width is controlled by the plate thickness near the ridge axis an 8 km thick plate near the ridge axis would produce the 30 km wide axial valley observed at slow spreading ridges an increased lithosphere thickness of only several kilometers over the half width of the axial valley can produce kilometer scale relief to examine whether a plate 8 10 km thick at the ridge axis that thickens by several kilometers within 15 km of the axis is plausible at a slow spreading ridge thermal models that include both the heat of magmatic crustal accretion on the ridge axis and upwelling mantle flow beneath the axis are formulated hydrothermal cooling that increases the effective conductivity by about a factor of 10 is required for an 8 km thick plate to exist at a slow spreading ridge axis however even with this enhanced hydrothermal cooling the plate thickness at a fast spreading ridge axis and the resulting horizontal force in the plate are too small to result in appreciable topography while mid ocean ridge axial topography is strongly influenced by spreading rate other factors also shape axial structure for example the anomalously shallow slow spreading reykjanes ridge has an axial structure more like that of a fast spreading ridge and the anomalously deep intermediate spreading australian antarctic discordance has a median valley axial structure typical of a slower spreading ridge in addition within a given spreading segment there is often a systematic deepening of the ridge axis toward transform offsets these non spreading rate dependent axial topographic variations may be produced by variations in magmatic heat input associated with crustal emplacement which shapes the thermal structure and lithosphere thickness near the ridge axis

sand and rhodolith gravel entrainment on the mid to outer shelf under a western boundary current fraser island continental shelf eastern australia abstract carbonate sediments on the east australian continental shelf off fraser island represent a transition from the warm water tropical carbonate deposits in the north i e the great barrier reef to cool water temperate carbonate deposits in the south on the outer shelf in 40 140 m water depth these deposits are characterised by pebble to cobble sized rhodoliths current meter data obtained from this area demonstrates that near bed southward flowing currents of up to 130 cm s occur episodically to depths of over 70 m these current events may last for several days and are probably related to intrusions of the east australian current a western boundary current onto the shelf flume experiments on different size classes of rhodoliths suggests that the threshold speed referenced to 100 cm above the bed u 100 cr for 10 mm diameter rhodoliths is of the order of 45 cm s whereas u 100 cr for 50 mm diameter rhodoliths is of the order of 80 cm s therefore the shelf currents measured are competent to initiate rhodolith movement percentage exceedence of these threshold speeds ranges from 1 to 10 surface waves also generate nearbed oscillatory flows which when combined with the steady current are considered to be important in initiating movement of the rhodoliths

mechanisms for linear sandbank formation and maintenance in relation to dynamical oceanographic observations abstract theories associated with flow in the vicinity of headlands and sandbanks together with proposed mechanisms for sandbank formation and maintenance are reviewed to exemplify water and sand movement around a sandbank on a tidally dominated shelf tidal currents in the vicinity of a headland associated linear sandbank are described these data are integrated with sedimentological information on bedforms and grain size distributions the sandbank under detailed investigation the scarweather sands is located along the northern coastline of the bristol channel uk which is an area susceptible to high energy inputs the maximum spring tidal range here is around 10m and a 6000km fetch is available for storm wave approach from the atlantic ocean but the area is 460km from the shelf break detailed observations are presented from 11 long term current meter stations located within 10km of the sandbank currents are described in terms of residual water circulation shallow water harmonics and tidal ellipse characteristics bedload transport paths are inferred from the distribution of bedforms grain size and m2 m4 tidal current interactions transport vectors are predicted from near bed current meter observations combined with empirically derived formulae it is demonstrated that water circulation and sand transport paths form part of a clockwise eddy system around the sandbank mechanisms controlling the location and the characteristics of this particular sandbank are shown to be explicable in terms of a combination of models for sandbank formation and maintenance the results of the present study are thought to be applicable directly to headland associated sandbanks elsewhere it is shown on the basis of theoretical considerations that the pattern of residual water movement and possibly sand transport around this sandbank could be extended to continental shelves on a global basis

simulating the transition from drizzling marine stratocumulus to boundary layer cumulus with a mesoscale model a case of coastal california summer season boundary layer cloud has been simulated with the u s navy coupled ocean atmosphere mesoscale prediction system and the results analyzed in the context of consistency with conclusions derived from large eddy simulation based les studies results show a pronounced diurnal cycle and fair agreement with satellite derived observations of liquid water path when drizzle processes are included a significant degree of mesoscale organization emerges in the form of cloud bands accompanied by a transition from a well mixed boundary layer topped by unbroken stratocumulus cloud into a more potentially unstable convective boundary layer regime the transition and the subsequent development of mesoscale variability is analogous to the drizzle induced cloud breakup produced in large eddy simulation studies the dynamics of the pure stratocumulus cloud are dictated by the model s subgrid parameterization while the more convective regime exhibits appreciable vertical velocities characteristic of an ensemble of cumulus updrafts the existence of convective updrafts is tied to a weak drizzle induced decoupling of the cloud and subcloud layer after which air of higher equivalent potential temperature ue can pool at the surface some similarities to the propagation of deep convection are also noted

sediment dynamics of the severn estuary and inner bristol channel net sediment transport pathways have been determined in many environments by studying grain size trends this approach is extended here to an inner shelf environment with improved statistical techniques patterns of net surficial sediment transport are proposed indicating that the area is dominated by up estuary transport exceptions exist in the form of radial mud transport at the river mouths and clockwise sand circulation around linear sandbanks six different sedimentary environments are defined on the basis of the shape of the transfer function and its relationship to the grain size distributions erosion takes place along the axis of the estuary dynamic equilibrium is reached around the linear sandbanks whilst accretion occurs at the head of the estuary and where there are riverine inputs the results are compared with numerical model outputs representing water movement and sediment transport over the seaward part of the study area there is agreement between the two approaches contradictions increase however towards the upper reaches of the estuary the discrepancies can be attributed to 1 inherent differences between patterns of water and sediment movement 2 difficulties in modelling intertidal boundary conditions 3 inadequate representation of river influences in the models and 4 time scale differences associated with sediment transport processes

dependence of ridge axis morphology on magma supply and spreading rate why do some mid ocean ridge axes have as their topographic expression a median valley 1 2 km deep and 15 30 km wide whereas others have an axial high 100 200 m high and 1 2 km wide in general median valleys exist at the axes of ridges spreading at half rates less than 20 25 mm yr 1 whereas axial highs are found at faster spreading ridges1 2 a recent crustal genesis model3 incorporating hydrothermal cooling and crustal accretion by means of a magma lens agrees well with observations of the spreading rate dependence of axial morphology1 2 the depth of the axial magma chamber3 4 and the thickness of the ridge axis lithosphere5 6 but there are several notable exceptions to the correlation between spreading rate and axial morphology such as the reykjanes ridge which spreads at l0mm yr 1 but has an axial high here we show that by extending the model of ref 3 to include variations in crustal thickness we can explain the observed dependence of axial morphology on crustal thickness and spreading rate our results suggest that the ultimate control on axial morphology is the thermal structure at the ridge axis which is a function of both spreading rate and magma supply

sand transport in the bristol channel bedload parting zone or mutually evasive transport pathways two different interpretations of sand transport pathways in a macrotidal estuarine system are assessed with respect to published numerical modelling results and current meter data the inclusion of residual currents in numerical models of m2 m4 tidal interactions yields ebb directed maximum shear stresses along the axis of the system with flood directed vectors located inshore it is demonstrated that there is no simple correlation between the directions of maximum bed stress and bedload transport particularly in nearshore zones large residuals may occur in such narrow nearshore zones which are estimated to be only 1 km in width in the bristol channel the available numerical model output field observations and sedimentological data sets from the bristol channel are considered to be explained in terms of the mutually evasive transport model ebb directed sand transport occurs along the central axis of the estuary whilst flood directed transport occurs within narrow inshore zones the bedload parting zone descriptive scheme which calls for ebb directed bedload flux in the absence of any flood directed flux inshore fails to provide a reasonable basis for the interpretation of holocene sand transport pathways and estuarine evolution transport paths around the u k defined previously as bedload parting zones may in many cases be interpreted more completely in terms of the mutually evasive transport model

structure and seismotectonics of the vema fracture zone atlantic ocean the structure and seismo tectonics of the vema fracture zone and the adjacent regions of its transverse ridge to the south and the mid atlantic ridge spreading centre to the north is discussed based on an analysis of seismic refraction profiles micro earthquake activity and gravity anomalies crustal structures were determined by analysis of wide angle seismic data from explosive and airgun sources recorded on arrays of ocean bottom seismometers micro earthquakes were recorded by six instruments for a period of 14 days in an array to the north of the ridge transform intersection immature crust similar to that reported by fowler 1976 from the famous area lies beneath the spreading centre the lower crust thins and normal mantle appears as the fracture zone is approached the thinnest crust lies beneath the northern end of the topographic deep at the fracture zone intersection this is also the locus of current microearthquake activity at sub bottom depths of 3 4 km which a composite fault plane solution suggests may originate on normal faults that parallel the trend of the local topography on the eastern wall of the median valley within the central region of the fracture zone valley the crust thickens but still retains an anomalous velocity structure seismic layer 3 is absent and crustal velocities are low this is probably the consequence of attenuated crustal accretion of faulting and cracking and of hydrothermal alteration the pattern of residual gravity anomalies after correction for topography for sediment thickness and for mantle temperature variations is consistent with the crustal structure defined by the seismic experiments the gravity measurements indicate that there is a broad thinning of the crust over 20 40 km in the vicinity of the fracture zone on which are superimposed localized regions of low and high densities to the south of the fracture zone the crust thickens and assumes a normal velocity structure including a mature seismic layer 3 the seismic moho is warped upwards under the transverse ridge suggesting a tectonic origin from buckling rather than uplift by serpentinization

long term survival of the axial valley morphology at abandoned slow spreading centers results of a lithospheric stretching model of mid ocean ridges suggest that the axial valley topography created at active slow spreading centers may be preserved for tens of millions of years after cessation of spreading we simulate the evolution of a slow spreading center using a finite element model that incorporates a temperature dependent rheology and discrete faulting model results show that significant valley morphology persists after regional extension ceases because the high lithospheric viscosities associated with a slow spreading center prevent ductile relaxation and because no effective mechanism exists to reverse rift bounding normal faults removal of the axial valley can only be accomplished by reversal of the regional extensional strains immediately upon cessation of active spreading however such a process does not seem likely these results suggest that the persistence of axial valleys at extinct spreading centers is consistent with a lithospheric stretching model for active slow spreading ridges

the paradox of the axial profile isostatic compensation along the axis of the mid atlantic ridge mantle bouguer gravity anomalies mba and bathymetry on three profiles covering more than 1000 km along the axis of the mid atlantic ridge mar are highly correlated suggesting that along axis topographic relief is locally compensated by variations in crustal thickness and or mantle density structure the quantitative relationship between topography and gravity on these profiles could be explained by the flexure of a thin narrow elastic strip representing the response to isostatic loads of an inner rift valley isolated from the rest of the plate by weak bounding normal faults the paradox is that across axis profiles show that the median valley is an uncompensated feature apparently created by a dynamic mechanism new extensive off axis coverage of the mar from 31 to 36 s shows that the high correlation does not persist outside the axial zone we suggest that the on axis correlation exists because the mechanism creating the median valley is controlled by the mantle thermal structure and along axis variations in crustal thickness both of which contribute to the mba if the mechanism is extension of a brittle ductile lithosphere the critical parameter controlling topographic relief is the thickness of the relatively stiff mantle layer immediately beneath the crust at the ridge axis a three dimensional 3 d thermal model incorporating passive mantle flow hydrothermal circulation the plate boundary geometry and variable magmatic heating associated with observed variations in crustal thickness predicts variations in the thickness of the stiff mantle layer that correlate with the observed axial topography we model the expected topography using cross axis sections of the 3 d thermal model and a 2 d finite element model of an extending lithosphere that incorporates temperature and strain dependent rheology as well as the flexural response of a thickening plate the predicted topographic signal produced by the combined dynamic and isostatic effects matches the amplitude of the observed axial bathymetry at the 18 mm yr 1 spreading half rate in our south atlantic survey the 2 d models also reproduce the observed rapid transition along the axis between rift valley no rift valley morphology we conclude that extensional forces acting on segmented oceanic lithosphere with varying rates of crustal production produce the highly variable morphology of the southern mar and the along axis correlation between mba and bathymetry

gravity anomalies over extinct spreading centers a test of gravity models of active centers gravity fields over active mid ocean ridges display long wavelength free air gravity highs with superimposed short wavelength anomalies over the spreading axis these axial anomalies are negative over slow spreading active ridges with amplitudes of typically 30 70 mgal and wavelengths of 40 60 km but decrease in amplitude and wavelength with increasing spreading rate until at fast spreading centers the anomalies are positive 10 20 mgal with wavelengths of about 20 30 km similar free air anomalies are observed over extinct spreading ridges and typically show the same paleo spreading rate dependency suggesting that a large component of the anomaly is not dynamically maintained calculation and removal of gravitational effects from basement relief results in significant residual gravity lows at the extinct ridges analyzed here even though the thermal effects associated with active spreading have diminished and no longer contribute to such short wavelength gravity lows the amplitudes of the residual lows at slow paleo spreading rates are 25 45 mgal with half widths of about 20 45 km and decrease with increasing spreading rate to about 5 mgal amplitude and 10 15 km half width over fast paleo spreading ridges the amplitudes and wavelengths of these residual anomalies are nearly identical to those of the residual anomalies observed over active spreading centers most previously proposed models of mid ocean ridge structure are unable to explain these anomalies and many are inconsistent with a least some aspect of the large body of knowledge currently available for mid ocean ridges this research provides a further constraint on geometric models of accretion as these models predict vastly different gravity fields results indicate that a frozen in low density accretionary structure is probably confined to within about 10 km of either side of the extinct axis this conclusion is consistent with the presence of a low density gabbroic root situated at shallow mantle depths beneath the ridge axis this root tends to narrow with increasing spreading rate producing the required residual gravity field variation

a moist pbl parameterization for large scale models and its application to subtropical cloud topped marine boundary layers abstract a new general purpose boundary layer parameterization that permits realistic treatment of stratocumulus capped boundary layers scbls with coarse vertical resolution is described it combines a 1 5 order turbulent closure model with an entrainment closure at the boundary layer top three different implementations of the entrainment closure in which the boundary layer height is respectively prognosed reconstructed from thermodynamic values at the grid points or restricted to lie on a flux level of the host model grid are tested in a single column modeling framework at both fine and coarse vertical resolution the first two approaches permit a stratocumulus top and base to lie between grid levels and evolve continuously with time but are more complicated to implement in a three dimensional model the model performs very well in cases of dry convection whatever the inversion implementation and the vertical resolution with 15 mb or better vertical resolution all approaches properly simulate

dating the growth of oceanic crust at a slow spreading ridge nineteen uranium lead zircon ages of lower crustal gabbros from atlantis bank southwest indian ridge constrain the growth and construction of oceanic crust at this slow spreading midocean ridge approximately 75 of the gabbros accreted within error of the predicted seafloor magnetic age whereas 25 are significantly older these anomalously old samples suggest either spatially varying stochastic intrusion at the ridge axis or more likely crystallization of older gabbros at depths of 5 to 18 kilometers below the base of crust in the cold axial lithosphere which were uplifted and intruded by shallow level magmas during the creation of atlantis bank

on the formation of sand banks of finite extent we consider a model where the fluid depth depends on both horizontal coordinates quasi steady depth uniform non divergent fluid flow is governed by inertial pressure and bottom frictional forces sand transport is proportional to the cube of the instantaneous current but augmented by a down slope component and by wind wave action and sand is conserved it is found that low parallel banks grow fastest so that in an extensive spatially uniform sea previous calculations for linear banks are appropriate the inclination of banks to the tidal currents can be interpreted in terms of similarly inclined deposition bands resulting from vorticity generation and advection in flow over a small isolated hump a small bump can evolve to an equilibrium bank typically after an initial rapid extension across the tidal currents provided that sand is sufficiently restricted and particularly if some wind wave action prevents growth up to the sea surface sand banks are likely to be in a late stage of evolution when the main change is a slow lengthening as the net current and transport along the bank side slows and turns around the bank end with net deposition the equilibrium is apparently stable except when there is an overall bed slope in the direction of the tidal currents or when sand is abundant

bedform migration in a mixed sand and cohesive clay intertidal environment and implications for bed material transport predictions abstract many coastal and estuarine environments are dominated by mixtures of non cohesive sand and cohesive mud the migration rate of bedforms such as ripples and dunes in these environments is important in determining bed material transport rates to inform and assess numerical models of sediment transport and geomorphology however these models tend to ignore parameters describing the physical and biological cohesion resulting from clay and extracellular polymeric substances eps in natural mixed sediment largely because of a scarcity of relevant laboratory and field data to address this gap in knowledge data were collected on intertidal flats over a spring neap cycle to determine the bed material transport rates of bedforms in biologically active mixed sand mud bed cohesive composition changed from below 2 vol up to 5 4 vol cohesive clay as the tide progressed from spring towards neap the amount of eps in the bed sediment was found to vary linearly with the clay content using multiple linear regression the transport rate was found to depend on the shields stress parameter and the bed cohesive clay content the transport rates decreased with increasing cohesive clay and eps content when these contents were below 2 8 vol and 0 05 wt respectively above these limits bedform migration and bed material transport was not detectable by the instruments in the study area these limits are consistent with recently conducted sand clay and sand eps laboratory experiments on bedform development this work has important implications for the circumstances under which existing sand only bedform migration transport formulae may be applied in a mixed sand clay environment particularly as 2 8 vol cohesive clay is well within the commonly adopted definition of clean sand

threshold of erosion of submarine bedrock landscapes by tidal currents since sea level stabilized 7000 yr bp shelf seas experiencing semi diurnal tides will have been affected by streaming four times per day if tidal erosion of bedrock were even only marginally efficient the 10 million streamings since then should have left geomorphological imprints we examine high resolution multibeam sonar data from three areas with extreme tidal currents the minas passage bay of fundy experiencing 8 knot surface tidal currents was surveyed in 2007 with a multibeam sonar in an area near to transverse dunes which are evidence for bedload transport the data show local overhanging surfaces near to the sediment rock contact potentially created by abrasion by saltating particles however they are uncommon in the straits of messina where surface currents reach 10 knots surveying revealed ridges lying oblique to the flow that are not obviously broken into separate outcrops by erosion in the bristol channel uk sonar data collected where currents reach 3 4 knots at 1 5 m above the bed reveal outcrops of limestone with superimposed sand dunes but only minor rounding of blocks holocene tidal currents have apparently been generally ineffective at eroding bedrock we examine this issue further by compiling extreme tidal streams around the uk and from them estimate shear stresses representing a macro tidal environment where peak surface currents reach 9 7 knots those data are compared with shear stresses in mountainous rivers where long term rates of erosion are comparable with tectonic uplift rates and are thus geomorphologically significant whereas river stresses reach 102 103 pa the largest tidal stresses are generally 101 and only rarely approach 102 pa too small for quarrying to operate generally however the vertical faces in the minas passage may represent the onset of abrasion given this limited evidence for abrasion we explore conditions in the geological past for tides that may have locally eroded bedrock

bottom currents sedimentation and ice sheet retreat facies successions on the mac robertson shelf east antarctica abstract measurements of water turbidity currents seafloor sediment samples and geophysical data document the sedimentary processes and the late quaternary sedimentary history of a continental shelf valley system on the east antarctic continental margin the valley is up to 1200 m in depth and strikes across the shelf it is interpreted as having formed by glacial erosion processes on the outer shelf sill of the valley northwestward offshore currents with speeds of up to 0 47 m s referenced to 100 cm above the seabed were recorded over a 10 month period such currents are competent to initiate bedload transport of medium sand and formation of small ripple sized bedforms which explains the occurrence of ripple cross bedding observed in some x radiographs of cores a nepheloid layer with concentrations of up to 3 5 mg l was noted at two stations and the available evidence suggests that density flows resulting from the sea ice brine rejection mechanism are competent to entrain shelf bottom sediments episodically sediment cores taken from this environment document the succession of facies resulting from retreat of glacial ice from the shelf during the holocene transgression under low sea level glacial conditions the ice sheet is interpreted to have been grounded on the outer shelf where it deposited a grounding line moraine rising sea level and global warming caused the ice shelf to retreat leaving a sub ice shelf glacial marine mud this is overlain by a laminated to massively bedded ice rafted debris rich facies interpreted as being deposited at the glacial calving front above this is a siliceous mud and ooze facies having a basal age of about 10 000 years bp characterising modern open marine conditions two types of iceberg keel marks on the outer part of the shelf valley are recognised from sidescan sonographs one relict arcuate type formed during the retreat of the ice shelf prior to 10 000 years bp and a second modern elongate type that is presently being formed near the shelf break by iceberg grounding processes this facies succession has been described from other antarctic shelf environments and is probably representative of facies deposited during holocene glacial retreat from the antarctic shelf

the relationship of sediment transport rates and paths to sandbanks in a tidally dominated area off the coast of east anglia u k sand transport measurements of bedload and suspended load in the sizewell dunwich banks area east anglia have shown that the suspended mode is dominant the depth integrated spring tidal residual is 5 66 g cm 1 sec 1 although the neap rate is only one fifth of this the calculated bedload transport rates also vary from 0 012 to 0 040 g cm 1 sec 1 correlating with changing meteorological conditions in order to predict the bedload sediment circulation pattern from midwater current meter measurements five sediment transport equations were calibrated using fluorescent dyed sand yalin s relationship gave the best estimates the bed shear stress was determined by extrapolating the velocity profile as a power law relationship with an exponent equal to 0 1 from midwater down to 2 m and as a lognormal profile from 2 m to the sea bed roughness length values appropriate to the substrate were used although bedload transport residuals are mainly to the south the banks trend northwards from the coast and have also elongated in this direction this is thought to be in response to the dominance of the suspended sediment transport it is suggested that a tidal residual eddy mechanism is responsible for the banks maintenance similar to the process operating in start bay devon the well documented westward movement of the banks is likely to be related to wave processes

parallel bands of seismicity at the mid atlantic ridge 12 14 n 1 seismicity distribution along the mid atlantic ridge mar monitored with the autonomous underwater hydrophone auh array shows a band of seismicity 70 km west of the axis between 11 9 n and 14 2 n available focal mechanisms at the off axis seismic band show extension directions consistent with the accommodation of the north american na and south american sa relative plate motion axial seismicity shows a gap at 14 n coinciding with the 14 n hotspot like region and the focal mechanisms near this region are consistent with spreading of the african af and the na or sa plates we speculate that the triple junction between the af na and sa plates is close to the 14 n seismic gap the westward continuation of the na sa plate boundary is unconstrained but could initiate at 13 n at the south end of the off axis seismic band or be diffuse between 12 n and 15 n

crustal structure of the mid atlantic ridge at 23 n from seismic refraction studies an explosive seismic refraction experiment using four ocean bottom hydrophone receivers was carried out along a 120 km long section of the mid atlantic ridge mar median valley at latitude 23 n immediately south of its intersection with the kane fracture zone the data are interpreted in terms of laterally homogeneous horizontally layered structures using travel time inversion and synthetic seismogram modeling techniques these solutions are combined using two dimensional ray tracing to produce a single model defining the major along axis structural changes occurring beneath this section of the median valley with the exception of slightly low layer 3 velocities the crust beneath several tens of kilometers of the mar median valley has all the characteristics of simple mature ocean crust including a well defined moho transition zone 8 km s upper mantle velocities and a total crustal thickness of 6 7 km no evidence is found for the presence of a steady state axial magma chamber in the crust or upper mantle and the excellent propagation characteristics and orderly amplitude versus range relationships imply the existence of a considerable degree of lateral homogeneity beneath the median valley over distances of several tens of kilometers along axis we infer from these observations that the basic seismic characteristics of the ocean crust are frozen in in a time period less than that separating the major volcanic injection events 10 000 50 000 years by hydrothermal circulation penetrating to the base of the crust along the 100 km long ridge segment studied here two major along axis structural changes occur the first is a 10 to 15 km wide along axis zone of lowered velocities in the lower crust centered beneath a major along axis topographic high this is interpreted to be the remnants of the most recent phase of injection that has temporarily left behind it a region of elevated temperatures and pervasive cracking and thus reduced velocities the second occurs abruptly at latitude 23 15 n and is accompanied by both a major change in rift valley and crestal mountain morphology and the apparent noncoincidence of the median valley and central magnetic anomaly north of this boundary the 30 km long ridge segment that abuts the kane fracture zone has a crustal thickness of 4 5 km and no distinctive layering to produce the characteristic amplitude patterns that typify mature oceanic crust interpretation of the cause of this structural change is uncertain but may be related to local tectonic events perhaps a recent 10 to 20 km ridge jump to the east

sand transport in the bristol channel east of bull point and worms head a bed load parting model with some indications of mutually evasive sand transport paths abstract we shall mainly consider the additional arguments put forward by harris and collins this issue in favour of substantial easterly sand transport in the nearshore zone of the bristol channel east of bull point and worms head and will conclude on the available evidence that the proposed nearshore sand transport routeways are neither continuous nor important a few available examples suggest that some local exceptions may occur where the mean rather than the maximum shear stress determines the net sand transport direction these are limited largely to the nearshore zone in broad terms therefore the bed load parting model is necessary to explain both the existence of the sand sheet west of the bristol channel with its west directed sand waves and the infill of the severn estuary with an intervening zone of scour and non deposition thus the central part of the bristol channel is almost clear of sand not because of a present shortage of sand but because sand has been continually swept from it the bed load parting model and the mutually evasive sand transport system are not exclusive of one another bed load partings exist mainly offshore while the mutually evasive system is found mainly in sand choked estuaries

laboratory investigations into the threshold of movement of natural sand sized sediments under unidirectional oscillatory and combined flows the threshold of movement of sediment obtained from sandbanks within the bristol channel uk is investigated under unidirectional oscillatory and combined flows the experiments were undertaken in a recirculating unidirectional laboratory flume containing an oscillating plate to simulate wave action with movement along the same axis as the unidirectional flows the sand samples consisted of cohesionless quartz grains with median grain sizes between 0 315 and 0 513 mm the experiments were performed under flow velocities measured at 2 cm above the bed ranging between 0 and 24 cm s 1 and oscillatory currents wave periods of 5 12 and 15 s ranging from 0 to 28 cm s 1 the critical conditions for the initiation of sediment movement were assessed by visual observation using the yalin criterion the results show that under unidirectional flow there is a slight overestimation of the threshold of naturally graded sediments derived on the basis of empirically derived threshold curves for artificially prepared sediments under similar flow conditions in the case of oscillatory flows the threshold for the natural sands is found to be higher than that predicted by previously derived empirical curves under combined flows wave period is shown to control threshold conditions with the unidirectional and oscillatory flow components combining in a linear fashion for long period 12 s and 15 s waves in contrast in the presence of short period 5 s waves the unidirectional and oscillatory components of the flow appear to decouple for high orbital velocities in both cases the effect of the wave period on threshold diminishes

evidence from gravity data for focused magmatic accretion along the mid atlantic ridge gravity data from the mid atlantic ridge between latitudes 27 50 n and 30 40 n show that the accretion of magma at the ridge is focused at discrete centres along the spreading axis large positive gravity anomalies equivalent to reductions of almost 50 in crustal thickness are observed over non transform discontinuities bounding spreading segments

a new parameterization for shallow cumulus convection and its application to marine subtropical cloud topped boundary layers part i description and 1d results abstract a new parameterization of shallow cumulus convection is presented the parameterization consists of a mass flux scheme based on a buoyancy sorting entrainment detrainment plume model the mass flux scheme is coupled to a 1 5 order turbulence closure model with an entrainment closure for convective boundary layers model performance is verified using single column model simulations at relatively high vertical resolution of pure trade cumulus convection and a cumulus to stratocumulus transition mixing rates cloud cover and vertical flux profiles as deduced from previously published large eddy simulation studies are well reproduced by the parameterization the model is used to demonstrate that height variations of lateral mixing rates can be successfully captured by a simple implementation of a buoyancy sorting mechanism in the updraft cloud model a companion paper describes the implementation of the scheme in a mesoscale model

boundary layer dynamics and sediment transport under storm and non storm conditions on the scotian shelf abstract near bed measurements of waves currents seabed responses and ripple migration rates were obtained using an instrumented tripod at a water depth of 39 m on the scotian shelf during the winter of 1992 93 these data and the grant and madsen s 1986 grant w d madsen o s 1986 the continental shelf bottom boundary layer annu rev fluid mech 18 265 305 combined flow boundary layer model are used to examine wave current interaction various sediment dynamic thresholds and sediment transport on an exposed high energy continental shelf the seabed was found to be rippled for most time of the deployment and thus the ripple enhanced combined skin friction shear velocity had to be used to determine the initiation of bedload transport under combined flows this indirectly indicates adequate predictions of the skin friction shear velocities inside the wave boundary layer by the grant and madsen model at high transport stages bedload roughness must be added to the grain size roughness to obtain a transportrelated combined shear velocity in order to predict correctly the onsets of saltation suspension and sheet flow transports otherwise the initiation of suspended load transport and the total sediment transport rates will be severely under estimated the comparison between the prediction by the grant and madsen 1986 model and the calculation of a quadratic stress law suggests that the total current shear velocity was enhanced by a factor of 2 3 due to the wave current interaction during storms while the apparent bottom roughness was increased by more than an order of magnitude the non linear coupling between waves and currents also causes a 20 increase of the combined skinfriction shear velocity inside the wave boundary layer this non linear coupling is most important when waves and currents are roughly equal in magnitude and the angles between them are less than 30 four sediment transport formulae were tested while the engelund hansen total load and yalin bedload methods did not perform well the einstein brown and bagnold formulae were found to respectively give reasonable predictions of the bedload and total load sediment transport rates under the observed combined flow conditions the predicted sediment transport direction is also in good agreement with the observed ripple migration direction the gsc sediment transport model sedtrans92 predicts that the net daily transport rates during the storms reached 822 kg m 1 day 1 and were 2 3 orders of magnitude higher than the non storm transport suggesting the dominance of storm processes in sediment transport on the scotian shelf these values are in good agreement with the results from sand tracer experiments conducted in this region we thus conclude that sedtrans92 can properly simulate boundary layer dynamics and sediment transport on continental shelves

classifying seabed sediment type using simulated tidal induced bed shear stress abstract an ability to estimate the large scale spatial variability of seabed sediment type in the absence of extensive observational data is valuable for many applications in some physical e g morphodynamic models knowledge of seabed sediment type is important for inputting spatially varying bed roughness and in biological studies an ability to estimate the distribution of seabed sediment benefits habitat mapping e g scallop dredging although shelf sea sediment motion is complex driven by a combination of tidal currents waves and wind driven currents in many tidally energetic seas such as the irish sea long term seabed sediment transport is dominated by tidal currents we compare observations of seabed sediment grain size from 242 irish sea seabed samples with simulated tidal induced bed shear stress from a three dimensional tidal model roms to quantitatively define the relationship between observed grain size and simulated bed shear stress with focus on the median grain size of well sorted seabed sediment samples we present predictive maps of the distribution of seabed sediment classes in the irish sea ranging from mud to gravel when compared with the distribution of well sorted sediment classifications mud sand and gravel from the british geological survey digital seabed sediment map of irish sea sediments digsbs250 this grain size tidal current proxy gstcp correctly estimates the observed seabed sediment classification in over 73 of the area

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

low sea level tidal origin of celtic sea sand banks evidence from numerical modelling of m2 tidal streams abstract numerical modelling of m 2 tidal streams in the celtic sea with a sea level lowered by 100 m indicates tidal currents about twice as strong as those at present such currents would have been sufficient to generate and maintain the linear tidal sand banks now found in that region as moribund bedforms furthermore an indicated clockwise rotation of the major axes of the tidal ellipses at a sea level lowered by 100 m with respect to the present day is in the correct sense to further the comparison with modern examples of actively maintained linear tidal sand banks

residual transport model in correlation with sedimentary dynamics over an elongate tidal sandbar in the gironde estuary southwestern france abstract this study qualitatively describes by a multidisciplinary approach hydrodynamic and residual sediment transport over a sandbar in a macrotidal environment the sandbar is elongated and attached to a headland that is located in the gironde estuary southwestern france bathymetric surveys side scan sonar sonograms grain size trends of surficial sediment and 2dh numerical simulations of current patterns were used to define a conceptual model for the dynamics of this sandbar the sandbar is oriented anticlockwise with respect to the main flow the channel oriented bar face is ebb dominated whereas the opposite face is flood dominated resulting in a clockwise residual transport the line of sediment flux convergence corresponds to the crest of the bar the residual flow over the bar appears to originate from the regional estuarine flow the dominant process responsible for the maintenance of the sandbar seems to be the depth averaged main tidal flow rather than secondary circulation due to headlands or the interaction between the tidal flow and the seabed to generalize this regional conceptual model a qualitative comparison is made with the theories of sandbar dynamics due to tidal currents pingree and maddock 1979 zimmerman 1981 huthnance 1982 and others the seabed stability approach developed by huthnance 1982 seems to be the most reliable theory to explain the saint georges bank dynamics even though many natural factors are not taken into account

a new parameterization for shallow cumulus convection and its application to marine subtropical cloud topped boundary layers part ii regional simulations of marine boundary layer clouds abstract the impact of physical parameterizations on simulations of cloud topped marine boundary layers is investigated using the fifth generation pennsylvania state university ncar mesoscale model mm5 three month mm5 simulations of the northeast and southeast pacific during june august 1987 are boundary forced with time varying ecmwf analyses runs with four planetary boundary layer pbl parameterizations already implemented in mm5 are compared with runs using new parameterizations of boundary layer turbulence and shallow cumulus convection shcu described in a companion paper numerous modifications to the mm5 that allow it to be used as a regional climate model are described the simulated 3 month mean shortwave cloud radiative forcing swcf and vertical structure of cloud topped boundary layers in the northeast pacific are sensitive to the pbl shallow convection schemes all four current mm5 pbl schemes the blackadar medium range forecast mrf burk thompson and gayno seaman schemes produc

on the roughness of mesozoic oceanic crust in the western north atlantic summary seismic reflection profiles from mesozoic oceanic crust around the blake spur fracture zone bsfz in the western north atlantic have been widely used in constraining tectonic models of slow spreading mid ocean ridges these profiles have anomalously low basement relief compared to crust formed more recently at the mid atlantic ridge at the same spreading rate profiles from other regions of mesozoic oceanic crust also have greater relief the anomalous basement relief and slightly increased crustal thickness in the bsfz survey area may be due to the presence of a mantle thermal anomaly close to the ridge axis at the time of crustal formation if so the intracrustal structures observed may be representative of an atypical tectonic regime

summertime post cold frontal marine stratocumulus transition processes over the eastern north atlantic abstractmarine boundary layer mbl cloud morphology associated with two summertime cold fronts over the eastern north atlantic ena is investigated using high resolution simulations from the weat

sand transport under the combined influence of waves and tidal currents an assessment of available formulae abstract various formulae are used to predict the transport of sand under the combined influence of waves and currents these approaches include the use of the unidirectional formulae of einstein 1950 frijlink 1952 yalin 1963 engelund and hansen 1967 sternberg 1972 ackers and white 1973 and gadd et al 1978 modified for the presence of waves in accordance with the techniques described by bijker 1967 and swart 1976 and those of bagnold 1963 madsen and grant 1976 and vincent et al 1981 predicted rates are compared with measurements based upon fluorescent sand tracer studies in an area of high tidal current and wave energy northern bristol channel u k in general predicted rates are lower than those measured and only the formulae of madsen and grant 1976 and the modified formula of sternberg 1972 appear to provide realistic estimates of transport under wave and tidally induced currents in comparison with the field measurements

the role of non uniform roughness in the formation of sand ribbons abstract sand ribbons typically are shallow water sedimentary features characterized as thin elongate bands of sand overlying a coarser lag sediment which is exposed between them the ribbons are oriented along the direction of the dominant current and are formed by a secondary circulation pattern that includes a small component of the boundary shear stress directed toward the center of the sand ribbons a perturbation expansion coupled with flume experiments indicates that this circulation pattern is linked to differences in the bottom roughness between the sand ribbons and the adjacent coarser strips the non uniform turbulence field arising from this roughness distribution drives a secondary helical flow the expansion indicates that there is a preferred ratio of sand ribbon spacing to water depth 4 1 for unstratified channel flow conditions the secondary flow pattern forces the downstream flow to readjust so that the boundary shear stress over the smooth sand strips is considerably smaller than the mean stress this serves to retard the movement of sand through such a system

extinct spreading center in the labrador sea crustal structure from a two dimensional seismic refraction velocity model the labrador sea contains a rare example of an abandoned mid ocean ridge where active accretion of oceanic crust ceased due to a change in the spreading geometry of lithospheric plates seismic refraction data were collected along a line which transversely crosses the extinct spreading center two dimensional analyses of the refraction data using ray tracing and synthetic seismogram techniques reveal major variations in crustal thickness and velocity in relation to the axis of the extinct spreading center in the extinct spreading center a crustal thickness of approximately 4 km is determined compared with 5 5 km for the flanks substantial lateral variations in p wave velocities of the upper and lower crust are observed with a marked decrease within the extinct spreading center low velocities are also observed in the uppermost mantle underlying the extinct spreading center and are interpreted as being the result of hydrothermal alteration the anomalously low crustal velocities and crustal thinning are attributed to a decreasing supply of partial melt and increasing degree of tectonism at the slow spreading rates preceding extinction

on the relationship between bedload and suspended sand transport on the inner shelf long island new york data obtained by a near bottom profiling concentration and velocity system pcv deployed in 10 m water depth at a site 1 km from the long island coastline are used to examine links between bedload and suspended sand concentrations calculations of bedload areal concentration c are based on the empirical results of vincent et al 1981 and use the theoretical formulations of grant and madsen 1978 1979 to describe the interaction between wave and current boundary layers suspended sand concentrations were obtained directly from an acoustic concentration meter acm average suspended sand profiles cz were found to fit closely to a log linear profile cz c1 1 a loge z z1 where c1 is the sand concentration at a height z1 1 cm from the bed and a is empirically determined as 0 22 0 005 a linear correlation is observed between the areal bedload concentration c the volume of bedload per unit area of the bed and the suspended sand concentration 1 cm above the bed c1 with a correlation coefficient of 0 82 significant at the 1 level and supports the hypothesis of einstein 1950 that bedload and suspended load are related through bedload concentration it is also shown that the sedment threshold criterion of komar and miller 1973 when expressed as a ratio here called the komar ratio can be used as a useful predictor for c1 under conditions where the wave orbital currents are much greater than the mean flow these relationships offer the opportunity for the calculation of both bedload and suspended sand transport rates from measurements of the steady current velocity and wave parameters combined with information defining the surficial sediments and local bottom topography