experiments on saltation of sand in water a high speed video system was used to study saltation of sand in an open channel flow high rates of image acquisition provided the information needed to resolve details of the lagrangian properties of particle motion saltation of sand is described in terms of statistical properties of particle trajectories such as mean values and standard deviations of saltation length height and streamwise particle velocity results are compared with available empirical data saltation height seems to be independent of particle size when made dimensionless with the particle diameter however the dimensionless saltation length appears to increase as the particle size decreases particle collision with the bed is also analyzed observations of this process indicate that most of the interactions between saltating particles and the bed are of the collision rebound type which contradicts previous discussions on the subject the estimated value of the dynamic friction coefficient is about half that proposed by bagnold i

the response of an idealized ocean basin to variable buoyancy forcing the response of an idealized ocean basin to variable buoyancy forcing is examined a general circulation model that employs a gent mcwilliams mixing parameterization is forced by a zonally constant restoring surface temperature profile which varies with latitude and time over a period p in each experiment 17 different values of p are studied ranging from 6 months to 32 000 yr the model s meridional overturning circulation moc exhibits a very strong response on all time scales greater than 15 yr up to and including the longest forcing time scales examined the peak to peak values of the moc oscillations reach up to 125 of the steady state maximum moc and exhibit resonance like behavior with a maximum at centennial to millennial forcing periods depending on the vertical diffusivity this resonance like behavior stems from the existence of two adjustment time scales one of which is set by the vertical diffusion and the other of which is set by the basin width furthermore the linearity of the response as well as its lag with the forcing varies with the forcing period the considerable deviation from the quasi equilibrium response at all time scales above 15 yr is surprising and suggests a potentially important role of the ocean circulation for climate even at milankovich time scales

predicting incipient motion including the effect of turbulent pressure fluctuations in the bed 1 incipient motion of particles on a river bed can in principle be understood and predicted from a balance of the forces acting on the particles on a natural river bed the exposure of particles to the flow is variable the effect of exposure on the initiation of motion including the case of deep embedding is studied in this paper so far the turbulence induced lifting force has been derived exclusively from examinations of the surface flow the understanding of destabilization processes of the riverbed is improved when turbulence induced vertical pressure gradients in the upper sediment layer are included this is particularly important for particles that lie in the lee of larger ones an analytical solution for the critical shields parameter of spherical particles is found including the effects of pressure fluctuations in the bed macroscale flow structures average bed slope and shallow flow existing laboratory data designed to test the effect of exposure and roughness conditions are in excellent agreement with the new model

the elements and richness of particle diffusion during sediment transport at small timescales the ideas of advection and diffusion of sediment particles are probabilistic constructs that emerge when the master equation a precise probabilistic description of particle conservation is approximated as a fokker planck equation the diffusive term approximates nonlocal transport it looks upstream and downstream for variations in particle activity and velocities whose effects modify the advective term high resolution measurements of bed load particle motions indicate that the mean squared displacement of tracer particles when treated as a virtual plume primarily reflects a nonlinear increase in the variance in hop distances with increasing travel time manifest as apparent anomalous diffusion in contrast an ensemble calculation of the mean squared displacement involving paired coordinate positions independent of starting time indicates a transition from correlated random walks to normal fickian diffusion this normal behavior also is reflected in the particle velocity autocorrelation function spatial variations in particle entrainment produce a flux from sites of high entrainment toward sites of low entrainment in the case of rain splash transport this leads to topographic roughening where differential rain splash beneath the canopy of a desert shrub contributes to the growth of a soil mound beneath the shrub with uniform entrainment rain splash transport often described as a diffusive process actually represents an advective particle flux that is proportional to the land surface slope particle diffusion during both bed load and rain splash transport involves motions that mostly are patchy intermittent and rarefied the probabilistic framework of the master equation reveals that continuous formulations of the flux and its divergence the exner equation represent statistically expected behavior analogous to reynolds averaged conditions key topics meriting clarification include the mechanical basis of particle diffusion effects of rarefied conditions involving patchy intermittent motions and effects of rest times on diffusion of tracer particles and particle borne substances this article is protected by copyright all rights reserved

stratal architecture and platform evolution of an early frasnian syn tectonic carbonate platform canning basin australia facies architecture and platform evolution of an early frasnian reef complex in the northern canning basin of north western australia were strongly controlled by syn depositional faulting during a phase of basin extension the margin attached hull platform developed on a fault block of precambrian basement with accommodation largely generated by movement along the mount elma fault zone recognition of major subaerial exposure and flooding surfaces in the hull platform from outcrop and drillcore has enabled comparison of facies associations within a temporal framework and led to identification of three stages of platform evolution stage 1 records initial ramp development on the hangingwall dip slope with predominantly deep subtidal conditions that prevented any cyclic facies arrangements this stage is characterised by basal siliciclastic deposits and a major deepening upward facies pattern that is capped by a sequence boundary towards the footwall north west and a major flooding surface towards the hangingwall stage 2 reflects the bulk of platform aggradation significant platform growth towards the hangingwall and the development of reef margins and cyclic facies arrangements thickening of this stage towards the hangingwall indicates that accommodation was generated by rotation of the fault block and overlying platform stage 3 records a major flooding and backstep of the platform margin the hull platform illustrates important elements of margin attached carbonate platforms in a half graben setting including i prominent but limited coarse siliciclastic input that does not have a major detrimental effect on carbonate production near the rift margin in arid to semi arid settings ii wedge shaped accommodation created by syn depositional rotation of fault blocks and tilting of the hangingwall dip slope resulting in shallow water facies and subaerial exposure up dip of the rotational axis and deeper water facies down dip and iii evolution of a ramp to rimmed shelf coincident with a sequence boundary flooding surface that is accelerated by tilting of the hangingwall dip slope during fault block rotation

flow resistance in ice covered alluvial channels this paper describes the influences of floating level ice covers on flow and bed load transport in alluvial channels and proposes a semi empirical procedure for calculating flow resistance in such channels the procedure can be applied in conjunction with several existing methods for estimating flow resistance in alluvial channels under open water conditions previously cover influence was poorly understood with the consequence that flow resistance calculations for ice covered alluvial channels entailed an inordinate amount of guesswork the cover influences described herein were determined from the results of experiments conducted with a laboratory flume the main findings of the flume experiments are that ice covers increase flow depth decrease bulk flow velocity increase dune length and significantly decrease bed load transport rate additionally it was found that sediment transport relationships developed for open water flow are applicable to ice covered flow provided they are used in conjunctio

the effect of environmental changes on the evolution and extinction of late devonian trilobites from the northern canning basin western australia the frasnian famennian virgin hills formation represents fore reef facies deposited as part of the extensive late devonian reef system that fringed the sw kimberley block in western australia it contains a rich trilobite fauna dominated primarily by proetids and to a lesser extent harpetids phacopids scutelluids and odontopleurids to date 49 taxa have been described 40 of these being restricted to the frasnian herein five frasnian taxa are described three in open nomenclature and two the new species telopeltis intermedia and otarion fugitivum evolutionary trends in the virgin hills trilobites are dominated by a reduction in body size and eye size and to a lesser extent a reduction in exoskeletal vaulting although recording no sedimentological signature the fauna was strongly affected by the two globally recognized kellwasser extinction events the first at the end of conodont zone 12 affected taxa at the species and genus level the second within zone 13b had a much greater impact on the fauna causing extinctions at the familial and ordinal levels evidence is presented to suggest that evolutionary trends in the trilobites during the late frasnian reflect selection for forms adapted to low nutrient conditions the two intensive kellwasser extinction episodes may reflect periodic massive inputs of nutrients from the terrestrial into the shallow marine environment

time dependent response to cooling in a beta plane basin the time dependent response of an ocean basin to the imposition of cooling or heating is examined in the context of a quasigeostrophic two layer model on the beta plane the focus is on the structure and magnitude of the vertical motion and its response to both a switch on forcing and a periodic forcing the model employed is a time dependent version of an earlier model used to discuss the intensification of sinking in the region of the western boundary current the height of the interface of the two layer model serves as an analog of temperature and the vertical velocity at the interface consists of a cross isopycnal velocity modeled in terms of a relaxation to a prescribed interface height an adiabatic representation of eddy thickness fluxes parameterized as lateral diffusion of thickness and the local vertical motion of the interface itself the presence of time dependence adds additional dynamical features to the problem in particular the emergence of low frequency weakly damped rossby basin modes if the buoyancy forcing is zonally uniform the basin responds to a switch on of the forcing by coming into steady state equilibrium after the passage of a single baroclinic rossby wave if the forcing is nonuniform in the zonal direction a sequence of rossby basin modes is excited and their decay is required before the basin achieves a steady state for reasonable parameter values the boundary layers in which both horizontal and vertical circulations are closed are quasi steady and respond to the instantaneous state of the interior as in the steady problem the flow is sensitive to small nonquasigeostrophic mass fluxes across the perimeter of the basin these fluxes generally excite basin modes as well the basin modes will also be weakly excited if the beta plane approximation is relaxed the response to periodic forcing is also examined and the sensitivity of the response to the structure of the forcing is similar to the switch on problem

upper devonian conodont zonation sea level changes and bio events in offshore carbonate facies lali section south china abstract the lali section guangxi province south china is composed of quiet water carbonates containing abundant conodonts which allowed recognition of 12 frasnian and 20 famennian biozones the kellwasser and annulata events are characterized by transgressive black carbonates while the hangenberg event is represented by black shale deposits other transgressive events recognized at lali include the middlesex nehden enkeberg and dasberg as well as the strunian transgression four regressive events were recognized on the basis of lithological and microfacies changes i e the linguiformis regression condroz event velifer regression and the derwer regression these sea level change events were of short duration and globally isochronous based on conodont correlations microfacies analyses and the abundance of palmatolepis indicate that the highest frasnian sea level lies in fz 12 13 upper rhenana zone whereas the highest famennian sea level was in the utahensis zone

scour holes and ripples occur below the hydraulic smooth to rough transition of movable beds scour holes often form in shallow flows over sand on the beach and in morphodynamic scale experiments of river reaches deltas and estuarine landscapes the scour holes are on average 2 cm deep and 5 cm long regardless of the flow depth and appear to occur under similar conditions as current ripples at low boundary reynolds numbers in fine sand and under relatively low sediment mobility in landscape experiments where the flow is only about 1 cm deep such scours may be unrealistically large have unnatural effects on channel formation bar pattern and stratigraphy this study tests the hypotheses that both scours and ripples occur in the same conditions and that the roughness added by sediment saltation explains the difference between the ripple dune transition and the clear water hydraulic smooth rough transition about 500 experiments are presented with a range of sediment types sediment mobility and obstructions to provoke scour holes or removal thereof to assess scour hole persistence most experiments confirm that ripples and scour holes both form in the ripple stability field in two different bedform stability diagrams the experiments also show that scours can be provoked by perturbations even below generalised sediment motion moreover the hydraulic smooth rough transition modified with saltation roughness depending on sediment mobility was similar in magnitude and in slope to ripple dune transitions given uncertainties in saltation relations the smooth rough transitions modified for movable bed are empirically equivalent to the ripple dune transitions these results are in agreement with the hypothesis that scours form by turbulence caused by localised flow separation under low boundary reynolds numbers and do not form under generalised flow separation over coarser particles and intense sediment saltation furthermore this suggests that ripples are a superposition of two independent forms periodic bedforms occurring in smooth and rough conditions plus aperiodic scours occurring only in hydraulic smooth conditions this article is protected by copyright all rights reserved

frasnian famennian biotic crisis undervalued tectonic control the prime cause of the late devonian kellwasser crisis culminating in a mass extinction event near the frasnian famennian f f boundary remains conjectural nevertheless rapid sea level fluctuations of uncertain origin during tectono eustatic highstand paired with repeated oceanic anoxia and climatic changes are usually thought at present to be one of the main immediate triggers the cathles hallam model of stress induced changes in plate density accompanying rapid rift formation furnishes an alternative for understanding the enigmatic sudden eustatic variations in the non glacial time late devonian tectonic extension causing rifting and volcanicity appears to be strongly marked in several regions of eurasia particularly in kazakhstan and eastern laurussia all larger devonian continents were more or less tectonically affected a subtle record of this tectonic rearrangement is implied even for distant and apparently quiet carbonate platforms in local extensional block faulting and tilting hydrothermal mineralisation geochemical anomalies and localized blooms of siliceous biota interpreting the late frasnian regressive transgressive pattern in terms of the cathles hallam tectono eustatic model two major rifting events are hypothesized one at about the beginning of the rhenana zone and a second principal pulse in the late linguiformis zone that encompassed the f f transition developing the veimarn milanovsky scenario of the global extensional pulse it is assumed that the key endogenous factors were related to episodic super plume activity the tectonically triggered changes climaxed in thermal and nutrient pulses and induced the stepdown ecosystem destabilization observed in the f f bio crisis minor cometary strike s might have eventually participated in this prolonged multicausal environmental stress mainly due to additional thermal shocks but perhaps effective on a regional scale only

the depositional record of the frasnian famennian boundary interval in a fore reef succession canning basin western australia abstract the effects of the global frasnian famennian f f biotic crisis on the shallow marine reef builders and associated biota in a laterally extensive late frasnian reef complex on the lennard shelf northern canning basin are recorded in the well exposed fore reef succession napier formation of the napier range the conodont defined f f boundary lies within a distinctive and mappable stratigraphic interval up to 15 m thick which separates highly fossiliferous frasnian from markedly less fossiliferous famennian carbonate and siliciclastic lithofacies marked lateral facies variation within the interval means that the actual boundary is not represented by a discrete bed or surface the interval is characterised by a range of lithofacies that record very low sedimentation rates on the slope and reworking and bioturbation of slope strata indicating a condensed interval with negligible sediment input from contemporaneous carbonate platform production siliciclastic lithofacies and abundant sponge debris are locally developed macrofossil content is generally very low but there is no evidence for anoxic or hypoxic conditions during deposition at a range of palaeowater depths allochthonous blocks of stromatoporoid bearing limestone are absent from the uppermost frasnian fore reef strata and stromatoporoid debris is significantly reduced prior to the end of the frasnian this suggests that extinction of the stromatoporoid reef margins occurred before the f f boundary with microbial communities dominating the platform margins prior to the famennian sedimentological and biostratigraphic evidence suggests that a series of sea level fluctuations in response to basin tectonism generated recurring or ongoing environmental stress in the stromatoporoid dominated reefal communities because of recurring reductions in living space influxes of siliciclastic sediment supply from the proximal highlands and quite possibly generation of toxic too saline or too nutrient rich lagoonal water these conditions were potentially superimposed on a global regime of incipient cooling climate leading to widespread glaciation in the late carboniferous

syndepositional fault control on lower frasnian platform evolution lennard shelf canning basin australia syndepositional faulting was a major control on internal platform stratigraphy in a frasnian reef complex on the southeastern lennard shelf northern canning basin australia by combining platform mostly backreef facies distributions with key stratal surfaces and biostratigraphic data we have developed a temporal framework for the hull range area platform evolution was controlled by normal faults via an initial tilt block geometry and subsequent differential subsidence and accommodation across the platform three third order flooding surface bounded platform phases are recorded basal shallow marine siliciclastic facies were deposited in topographic lows adjacent to the hanging wall and close to internal faults early carbonate deposition in dominantly deep subtidal environments suggests an overall ramp style setting that deepened toward the southeast this phase is capped by a sequence boundary represented by multiple paleokarst surfaces in the northwest that pass laterally into a major flooding surface to the southeast above which the platform expanded toward the mount elma painted rocks fault system the overall stacking pattern above the sequence boundary flooding surface is aggradational to progradational with higher frequency shoaling trends and locally developed exposure surfaces related to higher order relative sea level changes this second phase of platform growth ended with major flooding and a pronounced backstep of the leeward margin

three dimensional numerical model for flow and bed deformation around river hydraulic structures this paper describes a numerical model developed to simulate flow and bed deformation around river hydraulic structures the model solved the fully three dimensional reynolds averaged navier stokes equation expressed in a moving boundary fitted coordinate system to calculate the flow field with water and bed surfaces varying in time a nonlinear k e turbulence model was employed in order to predict flow near the structure where three dimensional flow is dominant the temporal change in bed topography was calculated by coupling a stochastic model for sediment pickup and deposition using a momentum equation of sediment particles in order to account for the effect of nonequilibrium sediment transport in validating the numerical model a spur dike and a bridge pier which are considered to be typical river engineering structures were selected by comparing the numerical results with observed laboratory experimental data the model was found to reproduce flow and scour geometry around these structures with

recognising the taghanic crisis in the devonian terrestrial environment and its implications for understanding land sea interactions the late mid devonian eday marl formation from the orcadian basin scotland is a terrestrial equivalent of the marine devonian taghanic crisis the eday marl from orkney contains a high resolution archive of climatic change controlled by the relative strength of the seasonal insolation this includes several distinct and discrete episodes of basin flooding as demonstrated by the deposition of lacustrine laminites bedded evaporites marginal sheet flood sands and marine influenced bioturbated sheet sands these flooding events are intercalated with intense and sustained episodes of aridity indicating that insolation was relatively weak and the monsoon climate was ineffective at drawing in seasonal rainfall recognition of these basin flooding events has enabled the definition of three new units the wha taing roeberry and berstane members the eday marl formation can be correlated offshore and also recognised within marginal facies as an aeolian intercalation an environmental hydrological and climatic interpretation is given for the different events recognised in the eday marl this sequence of events when combined with the palynological age dates enables a detailed correlation to be made with the marine tully formation of new york state this suggests that regressions forced by climatic aridity are significant for understanding the deposition of the shallow water carbonates of the tully formation in what is otherwise a clastic depositional system comparison with the pelagic marine record shows that the latter as yet gives only limited resolution of the taghanic crisis recognising the taghanic as a rapid alternation of cool arid and relatively hotter pluvial events provides a unifying explanation for the associated collapse in the terrestrial vegetation and the parallel faunal changes in the marine realm

on the connection between dense water formation overturning and poleward heat transport in a convective basin an isopycnal two layer idealized model for a convective basin is proposed consisting of a convecting interior region and a surrounding boundary current buoyancy and wind driven parameterized eddy fluxes govern the exchange between the two to balance the interior buoyancy loss the boundary current becomes denser as it flows around the basin geostrophy imposes that this densification be accompanied by sinking in the boundary current and hence by an overturning circulation the poleward heat transport associated with convection in the basin can thus be viewed as a result of both an overturning and a horizontal circulation when adapted to the labrador sea the model is able to reproduce the bulk features of the mean state the seasonal cycle and even the shutdown of convection from 1969 to 1972 according to the model only 40 of the poleward heat buoyancy transport of the labrador sea is associated with the overturning circulation an exact solution is presented for the linearized equations when changes in the boundary current are small numerical solutions are calculated for variations in the amount of convection and for changes in the remotely forced circulation around the basin these results highlight how the overturning circulation is not simply related to the amount of dense water formed a speeding up of the circulation around the basin due to wind forcing for example will decrease the intensity of the overturning circulation while the dense water formation remains unvaried in general it is shown that the fraction of poleward buoyancy or heat transport carried by the overturning circulation is not an intrinsic property of the basin but can vary as a result of a number of factors

impact of labrador sea convection on the north atlantic meridional overturning circulation abstract the overturning and horizontal circulations of the labrador sea are deduced from a composite vertical section across the basin the data come from the late spring early summer occupations of the world ocean circulation experiment woce ar7w line during the years 1990 97 this time period was chosen because it corresponded to intense wintertime convection the deepest and densest in the historical record suggesting that the north atlantic meridional overturning circulation moc would be maximally impacted the composite geostrophic velocity section was referenced using a mean lateral velocity profile from float data and then subsequently adjusted to balance mass the analysis was done in depth space to determine the net sinking that results from convection and in density space to determine the diapycnal mass flux i e the transformation of light water to labrador sea water the mean overturning cell is calculated to be 1 sv 1 sv 106 m3 s 1 as compared with a horizontal gyre of 18 sv the

oceanic teleconnections remote response to decadal wind forcing abstract the transhemispheric and interbasin response to time dependent wind forcing localized in the northern hemisphere of a single basin is examined using the reduced gravity shallow water equations in domains of simple geometry on decadal timescales the pressure on the eastern boundary fluctuates synchronously in both hemispheres and thus communicates a signal to latitudes distant from the forcing the signal then penetrates into the interior through westward radiation of rossby waves associated with the eastern boundary pressure fluctuation is a time dependent mass flux across the equator that in a single basin is balanced by a storage of mass in the unforced hemisphere two oceanic basins connected by a reentrant channel at the high latitude edge of the southern hemisphere are then considered again the forcing is confined to the northern hemisphere of one basin only in this geometry the time dependent mass flux across the equator of the forced basin is not entirely balanced within the same ba

sediment transport under ice cover a method for calculating sediment transport in ice covered flows is developed laboratory data on bed form frictional characteristics and sediment transport are obtained for equivalent free surface and covered flows comparison of the data leads to the conclusion that the lower layer in a covered flow can be treated as a free surface flow for the purpose of calculating bed load transport using the flow characteristics calculated from the k e turbulence model and a conventional bed load equation the sediment transport for covered flows can be obtained the method was verified using measured data

scheck breccias from devonian reef complexes of canning basin western australia abstract late frasnian and early famennian marginal slope deposits of the canning basin reef complexes include distinctive red open framework breccias characterized by red micritic clasts cemented by white calcite spar they closely resemble the adneter scheck breccias in the jurassic adnet limestone of the austrian alps scheck breccias from the canning basin are largely confined to red stromatolitic and terrigenous marginal slope sediments having depositional slopes of 10 14 which were deposited in water depths of some tens to hundreds of meters individual breccias beds are strongly lensoid with concave locally scoured bases they are between 10 and 100 cm 4 and 39 in thick and are composed predominantly of irregular red micritic clasts reworked from associated nodular limestones breccias are clast supported and may be inverse to normally graded normally graded or massive the proposed depositional mechanism began with slumping of muddy sediment containing abundant early cemented nodules this initiated bimodal density modified grain flows nodules were supported by dispersive forces and deposited by frictional freezing as the depositional slope decreased producing clast supported graded sequences the finer sand and mud fractions continued downslope as turbidity currents leaving behind minor perched mud trapped in the open framework breccias early cementation by radiaxial calcite prevented further sediment infilling these devonian scheck breccias are characteristic components of condensed sequences as shown by conodont dating they represent infrequent events only about one bed of breccia was deposited for every 10 m 33 ft of section in about 300 000 years over a vertical section of about 200 m 660 ft end of article last page 495

variability in bedform morphology and kinematics with transport stage bedform geometry is widely recognized to be a function of transport stage bedform aspect ratio height length increases with transport stage reaches a maximum then decreases as bedforms washout to a plane bed bedform migration rates are also linked to bedform geometry in so far as smaller bedforms in coarser sediment tend to migrate faster than larger bedforms in finer sediment however how bedform morphology height length and shape and kinematics translation and deformation change with transport stage and suspension have not been examined a series of experiments is presented where initial flow depth and grain size were held constant and the transport stage was varied to produce bedload dominated mixed load dominated and suspended load dominated conditions the results show that the commonly observed pattern in bedform aspect ratio occurs because bedform height increases then decreases with transport stage against a continuously increasing bedform length bedform size variability increased with transport stage leading to less uniform bedform fields at higher transport stage total translation related and deformation related sediment fluxes all increased with transport stage however the relative contribution to the total flux changed at the bedload dominated stage translation related and deformation related flux contributed equally to the total flux as the transport stage increased the fraction of the total load contributed by translation increased and the fraction contributed by deformation declined because the bedforms got bigger and moved faster at the suspended load dominated transport stage the deformation flux increased and the translation flux decreased as a fraction of the total load approaching one and zero respectively as bedforms washed out to a plane bed

structure and forcing of observed exchanges across the greenland scotland ridge abstractthe atlantic meridional overturning circulation and associated poleward heat transport are balanced by northern heat loss to the atmosphere and corresponding water mass transformation the

influences of time dependent precipitation on water mass transformation heat fluxes and deep convection in marginal seas abstractinfluences of time dependent precipitation on water mass transformation and heat budgets in an idealized marginal sea are examined using theoretical and numerical models the equations proposed by spall in 2012 are extended to cases with time dependent precipitation whose form is either a step function or a sinusoidal function the theory predicts the differences in temperature and salinity between the convective water and the boundary current as well as the magnitudes of heat fluxes into the marginal sea and across the sea surface moreover the theory reveals that there are three inherent time scales relaxation time scales for temperature and salinity and a precipitation time scale the relaxation time scales are determined by a steady solution of the theoretical model with steady precipitation the relaxation time scale for temperature is always smaller than that for salinity as a result of not only the difference in the form of fluxes at the surface but also the variation in the eddy transpor

iridium anomaly in the upper devonian of the canning basin western australia a moderate iridium anomaly about 20 times the local background has been found in upper devonian rocks in the canning basin it occurs at or near the frasnian famennian boundary which is known to be associated with a major massextinction event of global extent the anomaly occurs in an extremely condensed limestone sequence laid down under quiet deepwater conditions its occurrence suggests a causal link with some form of meteoroid impact moreover carbon isotope data indicate that a large reduction in biomass could have occurred at this level however the anomaly coincides with a stromatolite bed containing the fossil cyanobacterium frutexites iridium platinum iron manganese cobalt arsenic antimony and cerium are preferentially concentrated in filaments of this organism with concentrations ranging from two to five times that of the matrix it is possible that frutexites extracted these elements directly from seawater without the need for their derivation from an extraterrestrial source

tests of selected sediment transport formulas eleven existing sediment transport formulas were tested against the field data measured at two united states geological survey usgs gauging stations along the sacramento river in california whose bed material sizes are classified as those ranging from fine sand to coarse gravel they include the ackers white einstein brown engelund fredsoe engelund hansen inglis lacey karim meyer peter and mueller rijin schoklitsch toffaleti and yang formulas general flow and sediment input variables required by each transport function as well as dependent variables are summarized and detailed and measured hydraulic and sediment quantities including bed material size distributions are presented the computed sediment discharges are tabulated for comparison and are directly compared against the measured suspended sediment discharges the computed values however are found to deviate significantly from the measured values except for a very few cases the test results clearly demonstrate how difficult a ta

frasnian famennian mass extinction and cold water oceans suturing of laurussia and africa elimination of easterly tropical currents and consequent equatorial diversion of cold and possibly dysaerobic waters are suggested as prime causes of the frasnian famennian late devonian extinction these are explained by using a new paleogeographic model evidence for stepdown extinctions that took place from givetian through frasnian time is taken particularly from atrypoid brachiopods one of the most abundant representatives of the global tropical shelly marine benthos during the middle and late devonian only 5 of 17 frasnian genera or subgenera of atrypoids are known to be present in late frasnian sediments and possibly only a single genus spinatrypa was present at the frasnian famennian boundary evidence for stepdown extinction from other invertebrate and vertebrate groups in general corroborates that provided by brachiopods

influences of precipitation on water mass transformation and deep convection abstractthe influences of precipitation on water mass transformation and the strength of the meridional overturning circulation in marginal seas are studied using theoretical and idealized numerical models nondimensional equations are developed for the temperature and salinity anomalies of deep convective water masses making explicit their dependence on both geometric parameters such as basin area sill depth and latitude as well as on the strength of atmospheric forcing in addition to the properties of the convective water the theory also predicts the magnitude of precipitation required to shut down deep convection and switch the circulation into the haline mode high resolution numerical model calculations compare well with the theory for the properties of the convective water mass the strength of the meridional overturning circulation and also the shutdown of deep convection however the numerical model also shows that for precipitation levels that exceed this critical threshold the circulat

a model for flow over two dimensional bed forms when non cohesive sediment is set in motion by a unidirectional flow waves of sand often result moreover these waves typically are asymmetrical with steep lee faces that produce flow separation behind each wave a wake region forms which grows in height and decays in strength with distance downstream producing a near bottom acceleration the no slip condition at the bed however requires an internal boundary layer to form beneath the wake region which retards the flow there the interaction of these flow processes produces a local maximum in the boundary shear stress downstream of a bump even over an otherwise flat bed because the erosion rate is proportional to the stress divergence for bed load and weak suspended load transport erosion will occur upstream of this point and desposition will occur downstream of it thus influencing the bottom shape herein the equations of motion are solved for the accelerating internal boundary layer beneath a wake the resulting velocity and boundary shear str

river ice cover influence on sediment transportation at present and under projected hydroclimatic conditions a large number of rivers are frozen annually and the river ice cover has an influence on the geomorphological processes these processes in cohesive sediment rivers are not fully understood therefore this paper demonstrates the impact of river ice cover on sediment transport i e turbidity suspended sediment loads and erosion potential compared with a river with ice free flow conditions the present sediment transportation conditions during the annual cycle are analysed and the implications of climate change on wintertime geomorphological processes are estimated a one dimensional hydrodynamic model has been applied to the kokemaenjoki river in southwest finland the shear stress forces directed to the river bed are simulated with present and projected hydroclimatic conditions the results of shear stress simulations indicate that a thermally formed smooth ice cover diminishes river bed erosion compared with an ice free river with similar discharges based on long term field data the river ice cover reduces turbidity statistically significantly furthermore suspended sediment concentrations measured in ice free and ice covered river water reveal a diminishing effect of ice cover on riverine sediment load the hydrodynamic simulations suggest that the influence of rippled ice cover on shear stress is varying climate change is projected to increase the winter discharges by 27 77 on average by 2070 2099 thus the increasing winter discharges and possible diminishing ice cover periods both increase the erosion potential of the river bed hence the wintertime sediment load of the river is expected to become larger in the future copyright 2015 john wiley sons ltd

resolving two dimensional flow structure in rivers using large scale particle image velocimetry an example from a stream confluence large scale particle image velocimetry lspiv has emerged as a valuable tool for measuring surface velocity in a variety of fluvial systems lspiv has typically been used in the field to obtain velocity or discharge measurements in relatively simple one dimensional flow detailed two dimensional or three dimensional characterization of flow structure has been relegated to laboratory settings because of the difficulty in controlling piv limiting factors such as poor particle seeding the need for camera rectification and challenging field conditions in this study we implement a low cost lspiv setup using a high resolution action camera mounted above a stream confluence and water seeded with recycled landscape mulch time averaged 2d velocities derived from lspiv are compared with those measured with an acoustic doppler velocimeter adv in the camera s field of view we also assess the capabilities of this setup to resolve both turbulent and time averaged flow structures at a stream confluence our results reveal that even in challenging field conditions a basic lspiv setup can yield accurate data on velocity and resolve in detail the temporal evolution of flow structures on the surface of rivers the resulting dataset contains velocity information at high spatial and temporal resolution a significant advance in understanding flow processes at stream confluences our lspiv analysis provides support for previous numerical modeling studies that have distinguished between kelvin helmholtz and wake modes of turbulent behavior within the mixing interface at confluences this study shows that lspiv should be considered as both an alternative for traditional methods and a tool that can provide unprecedented levels of resolution of surface velocity patterns on rivers that can be used to evaluate numerical predictions of flow structure in complex fluvial environments this article is protected by copyright all rights reserved

dynamics of downwelling in an eddying marginal sea contrasting the eulerian and the isopycnal perspective abstractin this study we explore the downward branch of the atlantic meridional overturning circulation amoc from a perspective in depth space eulerian downwelling as well as from a perspectiv

under ice movement of cohesive sediments before river ice breakup a significant body of research exists on river hydraulics and sediment transport during open water conditions and to a lesser extent during the period of ice cover most of the ice related studies however are based on controlled laboratory experiments or field studies conducted under stable ice cover conditions they have largely ignored the most dynamic periods such as breakup when hydraulic conditions are most rapidly changing and energy levels are maximized moreover the entire pre breakup to ice clearance period is virtually devoid of even standard hydrometric measurements of suspended sediment largely because of safety and logistic problems some recent work has pointed to the formation of a sediment plume comprising fine grained sediments that develops before the main breakup fracturing of the ice cover this plume has been noted as being particularly ecologically significant because it can contain the winter long deposition of contaminants that preferentially attach to fine grained material unfortunately however because measurements of the critical parameters affecting sediment transport during these periods are rarely taken much uncertainty remains about the hydraulic forces that resuspend and transport sediments under an ice cover and particularly for cohesive fine grained sediments this paper describes a field experiment designed to broaden our understanding of sediment transport during this critical pre breakup period detailed measurements of river stage ice elevations flow velocity profiles and suspended sediment were taken over a 17 day period just before the 1998 river ice breakup at hay river northwest territories canada results indicated that just before breakup the shear stress which governs the beginning of sediment motion increases dramatically and drives the development of the under ice sediment plume of very fine grained cohesive sediments the shear stress in this case became critical at a mean under ice velocity of 0 4 m s copyright 2002 john wiley sons ltd

thermally forced transients in the thermohaline circulation abstractthe response of a convective ocean basin to variations in atmospheric temperature is explored using numerical models and theory the results indicate that the general behavior depends strongly on the frequency at which the atmosphere changes relative to the local response time to air sea heat flux for high frequency forcing the convective region in the basin interior is essentially one dimensional and responds to the integrated local surface heat flux anomalies for low frequency forcing eddy fluxes from the boundary current into the basin interior become important and act to suppress variability forced by the atmosphere a theory is developed to quantify this time dependent response and its influence on various oceanic quantities the amplitude and phase of the temperature and salinity of the convective water mass the meridional overturning circulation the meridional heat flux and the air sea heat flux predicted by the theory compare well with that diagnosed from a series of numerical model

bed load at low shields stress on arbitrarily sloping beds alternative entrainment formulation 1 according to the bagnold hypothesis for equilibrium bed load transport a necessary constraint for the maintenance of equilibrium bed load transport is that the fluid shear stress at the bed must be reduced to the critical or threshold value associated with incipient motion of grains it was shown in a companion paper seminara et al 2002 however that the bagnold hypothesis breaks down when applied to equilibrium bed load transport on beds with transverse slopes above a relatively modest value that is well below the angle of repose an investigation of this failure resulted in a demonstration of its lack of validity even for nearly horizontal beds the constraint is here replaced with an entrainment formulation according to which a dynamic equilibrium is maintained by a balance between entrainment of bed grains into the bed load layer and deposition of bed load grains onto the bed the entrainment function is formulated so that the entrainment rate is an increasing function of the excess of the fluid shear stress at the bed over the threshold value the formulation is implemented with the aid of a unique set of laboratory data that characterizes equilibrium bed load transport at relatively low shear stresses for streamwise angles of bed inclination varying from nearly 0 to 22 the formulation is shown to provide a description of bed load transport on nearly horizontal beds that fits the data as well as that resulting from the bagnold constraint the entrainment formulation has the added advantage of not requiring the unrealistically high dynamic coefficient of coulomb friction resulting from the bagnold constraint finally the entrainment formulation provides reasonable and consistent results on finite streamwise and transverse bed slopes even those at which the bagnold formulation breaks down completely

tepee shaped agglutinated microbialites an example from a famennian carbonate platform on the lennard shelf northern canning basin western australia distinctive metre scale antiformal structures are well developed in a famennian carbonate platform in the chedda cliffs area of the lennard shelf reef complexes the structures are distinguished by chevron shaped crests and thickened cores and contain abundant non skeletal allochems ooids pisoids peloids and intraclasts of silt to pebble size and variably developed laminations and fenestrae the internal morphology and pervasive occurrence of fenestral clotted and wavy laminated fabrics suggest that these structures are microbial mounds composed of agglutinated stromatolites and thrombolites microbial fabrics most probably originated through sediment trapping and binding by microbial mats with early lithification involving microbial calcification and cementation of trapped sediment the facies and stratigraphic context of the mounds support a shallow subtidal transitional backreef to reef flat setting however alone these mounds do not provide unequivocal environmental information other large antiformal structures in famennian platforms on the lennard shelf previously described as tepee structures show morphological similarities to the chedda cliffs mounds which suggests that these other structures may also be microbial mounds the presence of microbial mounds in platform successions further highlights the importance of microbial communities in the lennard shelf reef complexes

probability distributions of particle hop distance and travel time over equilibrium mobile bedforms the joint probability distribution of streamwise particle hop distance lateral particle hop distance and travel time constrains the relationships between topographic change and sediment transport

analysis of the time averaged properties of sediment motion in a local scour process 1 we examined the kinematics of sediment particles in the proximity of a vertical plate attached to the side wall of a pressurized duct the laboratory phenomenon investigated was archetypal of local scour near bridge abutments in covered rivers we conducted a clear water scour experiment where we measured the temporal evolution of the local erosion depth the latter was found to follow a logarithmic trend the concentration and velocity of the moving particles were measured on the basis of digital movies taken at various evolution stages of the process the spatial pattern of the sediment motion showed that the action of the principal vortex system becomes progressively more evident with the general lowering of the sediment transport intensity detected as the scour hole increases in time furthermore the phenomenology of local sediment transport was analyzed key findings of the research include 1 the time averaged values of the sediment transport rate are linked with those of concentration and velocity through relationships which are almost invariant regardless of the experimental time and of the location within the scour hole 2 intermittency and irregularity of the sediment fluxes increase as their mean intensity decreases 3 the relative weight of the temporal fluctuations of concentration and velocity in the time mean sediment transport activity is significant throughout the entire process potential implications of the present results for the modeling of local scour processes in pressurized flow conditions are discussed the considerations made are likely to have analogous applicability to local scour with a free surface since the large scale evolutions of the erosion depth in the two conditions may not be greatly different given the qualitative analogy of the present findings with earlier ones obtained for one dimensional bed load it is suggested that similar considerations may hold also for the small scale modeling of a wider class of sediment transport processes

erosion on an anaerobic seafloor significance of reworked pyrite deposits from the devonian of new york state abstract several devonian black shale units in new york state display conspicuous basal discontinuities marked by accumulations of exhumed and reworked pyrite nodules tubes and fossil steinkerns this coarse debris is overlain by and or interlayered with black laminated shale the most important level is the leicester pyrite member which occurs along a regional unconformity flooring black shale deposits of the basal genesee formation the detrital pyrite occurs in discontinuous lenses both on and slightly above this surface basal genesee beds geneseo shale member record westward overlap of basinal anoxic muds during a major marine transgression taghanic onlap the reworked pyrite deposit is regionally diachronous as indicated by conodont data and shingling of lenses with basal genesee beds which are successively younger from east to west pyrite chemically unstable in aerobic bottom settings was exhumed and concentrated on the sea floor during brief erosion episodes in a normally anaerobic environment lenses contain istinctive diagenetic pyrite structures e g fossil steinkerns burrow tubes derived from muds underlying the discontinuities erosional reworking of pyrite on the sea floor is shown by mechanical breakage of pyrite grains reorientation of geopetal stalactitic pyrite and compactional features plus alignment of pyritic tubes by bottom currents general absence of carbonate allochems in lenses is believed to reflect dissolution of carbonate following its exhumation episodic deep storm generated turbulence was probably important in exhuming and transporting pyrite however shoaling internal waves impinging the sediment starved basin margin slope may also have been important in scouring the bottom a model of internal wave erosion during the taghanic transgression is presented herein we feel that such erosion concentrated at or near the base of the pycnocline during a period of relative sea level rise offers a particularly good explanation for the observed juxtaposition of geneseo and other similar black shale deposits on erosional discontinuities the diachronous character of leicester lenses and associated black genesee muds above the unconformity suggests a transient rather than fixed basin axis allowing for progressive westward burial of the unconformity and assiciated remanie deposits by onlapping black muds older and younger occurrences of resedimented pyrite are briefly described from other devonian basinal units and are shown to have formed in about the same manner as the leicester reworked pyrite is believed to be common although seldom recognized in phanerozoic sediment starved basinal settings moreover detrital pyrite analogous to that described herein probably awaits discovery in certain modern marine basins

the trouble with shear stress abstract predicted bedload fluxes often have large errors partly because calculations usually do not include spatial and temporal flow variability sediment motion is driven by a temporal distribution of applied fluid forces which are often simplified into time averaged flow velocities or shear stresses in sediment transport equations spatial variations in these flow parameters are also often neglected given the nonlinearities in the relation between flow and sediment flux the reach averaged sediment flux calculated with an average velocity or shear stress will not necessarily equal the measured spatially averaged bedload flux such spatial and temporal flow variations are ignored with the commonly used reach averaged shear stress ds that is a function of the flow depth and channel slope to demonstrate the uncertainties with using reach averaged shear stresses and even local shear stresses to predict bedload transport rates we have three goals first we outline the derivations and assumptions involved with commonly used estimates of shear stress second we summarize the literature on the uncertainties involved with using shear stress and other space and time averaged flow parameters to predict sediment flux third we use a new data set from experiments to add further support to a growing body of literature that suggests ds may not be linked to the mechanics of bedload transport we show considerable uncertainties exist in estimating a reach averaged shear stress by comparing ds and near bed shear stresses calculated from velocity profiles reynolds stresses and local turbulence for example ds was systematically greater than these near bed stresses even for experiments without large bed roughness we also find that correlations between local time averaged sediment fluxes and local reynolds stresses or turbulence intensities are relatively poor the near bed time averaged local flow velocity has the strongest relation with the time averaged local bedload flux because it is closely related to the forces that actually drive sediment transport however sediment transport rates in individual experiments could still vary by orders of magnitude for a given value of this flow velocity suggesting that additional work is needed to identity a parameter or distributions of parameters that will best predict sediment fluxes

observations of dune interactions from dems using through water structure from motion abstract the role of three dimensionality 3d in modulating both flow and sediment transport remains poorly understood 3d bed elevation measurements are difficult to obtain due to irregular dune shapes and submergence using photogrammetric tools for topographic reconstruction has become popular in surface studies yet water refraction makes through water image capturing of submerged dunes still challenging we show that through water images and structure from motion sfm tools can produce accurate digital elevation models dems of a measured dune field with a high level of detail it is a low cost and non intrusive alternative for submerged bed elevation measurements we provide a detailed sfm workflow introduce and discuss the use of camera coordinates and underwater ground control points and processing steps needed to obtain high accuracy dems our method results in dems with mm resolution in controlled laboratory conditions new data for 23 dune field dems from 6 experiments were obtained and are presented the dune field dem time series provide insights on dune interaction processes in our experiments merging and defect creation were most common merging decreases the number of bedforms and defect creation is responsible for more bedforms dunes continue to interact and jostle after achieving equilibrium simple crests are much more frequently observed than irregular sinuous or double crests two dimensional 2d crest shapes were observed although crest shapes were generally of 3d nature observed dune migration rates are not steady and vary in time and space bedform splitting speeds up dune migration whilst neighbouring dunes tend to accelerate or decelerate we identify a cascade of processes and show that 3d dems at high frequency are required to resolve observed rapid deformations and interaction of dunes the identified dune interaction processes are inherent to sediment flux variability whereas the temporal component of the underlying sediment transport processes is still not well understood in relation to average transport

late devonian carbon isotope stratigraphy and sea level fluctuations canning basin western australia the upper devonian reef complexes of the canning basin contain some of the world s best exposed continuous stratigraphic sections through the frasnian famennian boundary the facies distribution and composition of these reef complexes record interactions among sea level changes sediment supply ocean chemistry and paleoecology changes in relative sea level produced spatial shifts in reef platform development and regional changes in sediment supply that can be correlated across facies boundaries using a combination of sequence stratigraphy biostratigraphy and carbon isotope stratigraphy during the lowstand interval below the frasnian famennian boundary the reef margin advanced down the reef slope in shallow water environments and siliciclastics locally dominated in the marginal slope environment compilation of a broad late frasnian to early famennian sequence stratigraphic framework for the canning basin demonstrates that transgressive intervals correlate to positive carbon isotopic excursions within the basin these isotopic shifts also can be correlated to time equivalent positive carbon isotopic excursions reported from transgressive intervals in europe thus the late frasnian transgressions in the canning basin were primarily eustatic rather than tectonic in origin and positive carbon isotopic signatures of the kellwasser horizons are globally correlative c 2002 elsevier science b v all rights reserved

boundary currents and watermass transformation in marginal seas the properties of watermass transformation and the thermohaline circulation in marginal seas with topography and subject to a spatially uniform net surface cooling are discussed the net heat loss within the marginal sea is ultimately balanced by lateral advection from the open ocean in a narrow boundary current that flows cyclonically around the basin heat loss in the interior is offset by lateral eddy fluxes originating in the boundary current the objectives of this study are to understand better what controls the density of waters formed within the marginal sea the temperature of the outflowing waters the amount of downwelling and the mechanisms of heat transport within the marginal sea the approach combines heat budgets with linear stability theory for a baroclinic flow over a sloping bottom to provide simple theoretical estimates of each of these quantities in terms of the basic parameters of the system the theory compares well to a series of eddy resolving primitive equation model calculations the downwelling is concentrated within the boundary current in both a diffusive boundary layer near topography and an eddy driven region on the offshore edge of the boundary current for most high latitude regions the horizontal gyre is expected to transport more heat than does the overturning gyre

open ocean convection observations theory and models we review what is known about the convective process in the open ocean in which the properties of large volumes of water are changed by intermittent deep reaching convection triggered by winter storms observational laboratory and modeling studies reveal a fascinating and complex interplay of convective and geostrophic scales the large scale circulation of the ocean and the prevailing meteorology two aspects make ocean convection interesting from a theoretical point of view first the timescales of the convective process in the ocean are sufficiently long that it may be modified by the earth s rotation second the convective process is localized in space so that vertical buoyancy transfer by upright convection can give way to slantwise transfer by baroclinic instability moreover the convective and geostrophic scales are not very disparate from one another detailed observations of the process in the labrador greenland and mediterranean seas are described which were made possible by new observing technology when interpreted in terms of underlying dynamics and theory and the context provided by laboratory and numerical experiments of rotating convection great progress in our description and understanding of the processes at work is being made

coherent eddies in the labrador sea observed from a mooring abstract during june november 1994 a mooring in the central labrador sea near the former ocean weather station bravo recorded a half dozen anomalous events that prove to be two different types of coherent eddies comparisons with simple analytical models are used to classify these events as coherent eddies on the basis of their velocity signatures the first clear examples of long lived convectively generated eddies are reported these four small radius 5 15 km eddies are exclusively anticyclonic with cold fresh middepth potential temperature and salinity s cores surrounded by azimuthal currents of 15 cm s 1 their s properties identify them unambiguously as the products of wintertime deep convection in the interior labrador sea compared with eddies in other regions these anticyclones are unusual for their strong surface expressions and composite s cores two warm cyclones are also seen these are larger radius 15 km than the anticyclones and about as energetic currents 15 cm s 1

spectral description of migrating bed forms and sediment transport this paper considers the problem of spatiotemporal bed topography evolution and sediment transport estimation in rivers with migrating bed forms of different types and sizes in statistical equilibrium conditions instead of resorting to bed form classification we propose to evaluate the evolution of multiscale bed topography as the integral of unit contributions defined through a space time fourier decomposition of bed elevations using joint 2 d spectra in the frequency and wave number domain a functional relationship between the length scales and the timescales in which migrating bed forms are decomposed is proposed and developed into a dimensionless expression for scale dependent convection velocities this formulation highlights the violation of taylor s hypothesis for migrating bed forms confirming statistically that larger bed forms travel slower as compared to smaller bed forms this phenomenological description leads to a spectral extension of the simons et al 1965 formula for sediment transport to incorporate a range of multiscale migrating features both the scaling of convection velocities and the spectral estimate of sediment transport rate were validated through extensive bed elevation data from laboratory experiments conducted at the st anthony falls laboratory for a range of froude numbers 0 2 fr 0 5 under varying discharge and bed material composition

the fluid dynamics of river dunes a review and some future research directions 1 dunes are present in nearly all fluvial channels and are vital in predicting flow resistance sediment transport and deposition within many rivers progress in understanding the fluid dynamics associated with alluvial dunes has been significant in the last 15 years and has witnessed huge advances in field laboratory and numerical investigations progress has been made in detailing the principal features of mean and turbulent flow over asymmetric dunes that possess flow separation in their leesides and how these forms affect both downstream boundary layer structure and stress partitioning over the dune additionally the links between sediment transport over dunes and instantaneous coherent flow structures are being increasingly understood with the feedback of dune three dimensionality upon flow and sediment dynamics over these bed forms beginning to be recognized as vital such research now provides an outstanding background for beginning to address areas of greater complexity that will enable a fuller understanding of these important natural features this review paper summarizes the principal features of mean and turbulent flow over alluvial sand dunes five areas are then highlighted and discussed as a possible focus for future research 1 the influence of dune leeside angle upon flow processes in the dune wake and downstream flow field 2 the influence of three dimensionality in dune shape upon the generation of turbulence and distribution of bed shear stress 3 flow field modification resulting from bed form superimposition and amalgamation 4 the scale and topology of dune related turbulence and its interactions with sediment transport and the flow surface and 5 the influence of suspended sediment on the dune flow field and dune morphology

downwelling in basins subject to buoyancy loss abstractrecent observational theoretical and modeling studies all suggest that the upper part of the downwelling limb of the thermohaline circulation is concentrated in strong currents subject to buoyancy loss near lateral boundaries this is fundamentally different from the traditional view that downwelling takes place in regions of deep convection even when resolving the buoyant boundary currents coarse resolution global circulation and climate models rely on parameterizations of poorly known turbulent mixing processes in this study the first direct measurements of downwelling occurring within a basin subject to buoyancy loss are obtained downwelling is observed near the basin s vertical wall within the buoyant boundary current flowing cyclonically around the basin although the entire basin is cooled large scale mean downwelling is absent in the basin interior laboratory rotating experiments are conducted to explicitly resolve the turbulent mixing due to convective plumes and the baroclinic ed

observations of the labrador sea eddy field this paper is an observational study of small scale coherent eddies in the labrador sea a region of dense water formation thought to be of considerable importance to the north atlantic overturning circulation numerical studies of deep convection emphasize coherent eddies as a mechanism for the lateral transport of heat yet their small size has hindered observational progress a large part of this paper is therefore devoted to developing new methods for identifying and describing coherent eddies in two observational platforms current meter moorings and satellite altimetry details of the current and water mass structure of individual eddy events as they are swept past by an advecting flow can then be extracted from the mooring data a transition is seen during mid 1997 with long lived boundary current eddies dominating the central labrador sea year round after this time and convectively formed eddies similar to those seen in deep convection modeling studies apparent prior to this time the topex poseidon altimeter covers the labrador sea with a loose net of observations through which coherent eddies can seem to appear and disappear by concentrating on locating and describing anomalous events in individual altimeter tracks a portrait of the spatial and temporal variability of the underlying eddy field can be constructed the altimeter results reveal an annual pulsation of energy and of coherent eddies originating during the late fall at a particular location in the boundary current pinpointing the time and place of the boundary current type eddy formation the interannual variability seen at the mooring is reproduced but the mooring site is found to be within a localized region of greatly enhanced eddy activity notably lacking in both the annual cycle and interannual variability is a clear relationship between the eddies or eddy energy and the intensity of wintertime cooling these eddy observations as well as hydrographic evidence suggest an active role for boundary current dynamics in shaping the energetics and water mass properties of the interior region

quantifying the dynamic evolution of graded gravel beds using particle tracking velocimetry abstractthe motion of graded gravels under steady and spatially uniform turbulent flow is investigated in laboratory conditions using particle tracking velocimetry ptv the gravel bed is subjected to flows approaching critical armor velocity and water worked until static armor is reached a digital particle tracking dpt algorithm based on image subtraction between subsequent frames is developed and applied on the particle scale as the graded gravel bed is water worked the dynamic evolution of graded gravels is quantified continuously for up to 8 h characteristics of particle motion are identified and movement patterns under complex flow conditions are analyzed particle tracking is applied to low frequency 0 67 fps and high frequency 30 fps data this study provides a new perspective on gravel particle transport distances trajectory sinuosity sediment transport rate fractional sediment transport rates particle movement patterns and instantaneous particle velocities

sediment transport part iii bed forms and alluvial roughness a method is presented that makes the classification of bed forms the prediction of the bed form dimensions and the effective hydraulic roughness of the bed forms feasible the proposed relationships are based on the analysis of reliable flume and field data a verification analysis using about 1 500 alternative reliable flume and field data shows good results in predicting the hydraulic roughness friction factor for field conditions the proposed method yields considerably better results than previously proposed methods which are reviewed here the proposed method has also been used to predict the flow depth and the total bed material load

detailed simulation of morphodynamics 1 hydrodynamic model we present a three dimensional high resolution hydrodynamic model for unsteady incompressible flow over an evolving bed topography this is achieved by using a multilevel cartesian grid technique that allows the grid to be refined in high gradient regions and in the vicinity of the river bed the grid can be locally refined and adapted to the bed geometry managing the cartesian grid cells and faces using a hierarchical tree data approach a ghost cell immersed boundary technique is applied to cells intersecting the bed topography the governing equations have been discretized using a finite volume method on a staggered grid conserving second order accuracy in time and space the solution advances in time using the fractional step approach large eddy simulation is used as turbulence closure we validate the model against several experiments and other results from literature model results for stokes flow around a cylinder in the vicinity of a moving wall agree well with wannier s analytical solution at higher reynolds numbers computed trailing bubble length separation angle and drag coefficient compare favorably with experimental and previous computational results results for the flow over two and three dimensional dunes agree well with published data including a fair reproduction of recirculation zones horse shoe structures and boiling effects this shows that the model is suitable for being used as a hydrodynamic submodel in the high resolution modeling of sediment transport and formation and evolution of subaqueous ripples and dunes

detailed simulation of morphodynamics 2 sediment pickup transport and deposition the paper describes a numerical model for simulating sediment transport with eddy resolving 3 d models this sediment model consists of four submodels pickup transport over the bed transport in the water column and deposition all based on a turbulent flow model using large eddy simulation the sediment is considered as uniform rigid spherical particles this is usually a valid assumption for sand bed rivers where underwater dune formation is most prominent under certain shear stress conditions these particles are picked up from the bed due to an imbalance of gravity and flow forces they either roll and slide on the bed in a sheet of sediment or separate from the bed and get suspended in the flow sooner or later the suspended particles settle on the bed again each of these steps is modeled separately yielding a physics based process model for sediment transport suitable for the simulation of bed morphodynamics the sediment model is validated with theoretical findings such as the rouse profile as well as with empirical relations of sediment bed load and suspended load transport the current model shows good agreement with these theoretical and empirical relations moreover the saltation mechanism is simulated and the average saltation length height and velocity are found to be in good agreement with experimental results

an estimate of the eddy induced circulation in the labrador sea regions of oceanic deep convection such as the labrador sea are prone to baroclinic instability the resulting geostrophic eddies play a crucial role in the post convection adjustment process which involves both rearrangement of mass so as to release available potential energy and exchange of heat and salt with the boundaries in this study it is proposed that the slumping of isopycnals associated with baroclinic instability drives an eddy induced overturning circulation consisting of a surface intensified flow transporting low salinity water from the boundary currents into the interior sinking motion in the interior and an outflow at depth transporting newly ventilated labrador sea water towards the boundaries typical eddy induced velocities estimated from hydrographic data are roughly 0 5 cm s for the surface inflow 1 m day for the vertical motion and 0 1 cm s for the deeper outflow in close agreement with those calculated in a numerical model

devonian events and correlations a tribute to the lifetime achievements of michael robert house 1930 2002 the devonian was a peculiar time in the phanerozoic evolution of the earth most continents including the large gondwana and laurussia cratons formed a pangea type assembly around the tropical prototethys and an increasingly hot global greenhouse climate prevailed with a complete lack of major ice sheets even in polar areas there was gradual and increasing flooding of the continents creating huge epicontinental seas that have no modern analogues under these conditions the plants finally conquered the land with the innovation of deep roots in the emsian the appearance of seed precursors and trees in the givetian and the spread of vegetation into dry uplands in the late famennian in the marine realm the largestknown phanerozoic tropical reef belts surrounded craton margins and tropical islands it was the time of the sudden radiation of early ammonoids of the earliest episodic blooms of calcareousshelled pelagic zooplankton tentaculitoids the rise to dominance of fishes mostly of armoured forms and with giants reaching 10 m in length but also including the first sharks and the appearance of earliest tetrapods in marginal settings however the tropical and subtropical areas reaching up to 458 latitude were hardly a paradise a combination of climatic plate tectonic magmatic and still poorly understood palaeoceanographic factors caused the recurrent sudden perturbation of stable ecological conditions by short term global events of variable magnitude e g house 1985 including two of the biggest mass extinctions that the earth s biosphere has experienced the upper kellwasser event at the frasnian famennian boundary and the hangenberg events at the close of the devonian the global and regional correlation of devonian rocks relies strongly on the available fossil record on the facies distribution of marker fossils on the recognition of eustatic sea level change in sequence stratigraphy and as a relatively new development on the application of stable isotope geochemistry and magnetic susceptibility the prototethys seaway fortunately allowed a free exchange of pelagic faunas through all oceans and seas in the warm water areas there was only moderate endemism in conodonts ammonoids pelagic ostracodes and tentaculitoids which are the main biostratigraphic marker groups low diversity benthic assemblages of the outer shelf also include many cosmopolitan groups but they differ strongly from fossil associations of the nearshore neritic and photic zones in the latter areas including reef and biostrome complexes the evolution and ecologically controlled distribution of brachiopods stromatoporoids rugose and tabulate corals and trilobites provides refined regional time frameworks but the faunas are mostly characterized by endemism that prevents simple correlation across oceans and seaways palynomorphs that were washed into the seas enable the correlation of marine fluvial limnic and terrestrial deposits the highly variable palaeoecology and taxonomic composition of the terrestrial neritic and pelagic faunal assemblages are the fundamental problems of cross facies correlations within the devonian apart from overlapping facies and palaeobiogeographic ranges of some key taxa event and physical stratigraphy are the major tools in reconstructing global changes in the devonian world at high level of time resolution the international subcommission on devonian stratigraphy sds was the first international union of geological sciences iugs subcommission to complete the formal designation of all its series and stage subdivisions but all ratified gssps global stratotype sections and points as the norm were defined in the pelagic facies realm their recognition on all continents and in all other facies belts is still a major task to be resolved as a first step to reach some progress in this wide international and multidisciplinary scientific field the devonian subcommission and the institut scientifique university mohammed v rabat especially with a major organizational input by its titular member and vice chairman prof ahmed el hassani organized in march 2004 an international symposium el hassani 2004a on devonian neritic pelagic correlation and events followed by an excursion with the same topic to the continually inaccessible dra valley of sw morocco el hassani 2004b

sediment transport part i bed load transport a method is presented which enables the computation of the bed load transport as the product of the saltation height the particle velocity and the bed load concentration the equations of motions for a solitary particle are solved numerically to determine the saltation height and particle velocity experiments with gravel particles transported as bed load are selected to calibrate the mathematical model using the lift coefficient as a free parameter the model is used to compute the saltation heights and lengths for a range of flow conditions the computational results are used to determine simple relationships for the saltation characteristics measured transport rates of the bed load are used to compute the sediment concentration in the bed load layer a simple expression specifying the bed load concentration as a function of the flow and sediment conditions is proposed a verification analysis using about 600 alternative data shows that about 77 of the predicted bed load transport rates are within 0 5 and 2 times the observed values

platform margins reef facies and microbial carbonates a comparison of devonian reef complexes in the canning basin western australia and the guilin region south china devonian reef complexes were well developed in western australia and south china but no detailed direct comparison has been made between reef building in the two regions the regions differ in several respects including tectonic stratigraphic and palaeoceanographic palaeogeographic settings and the reef building styles reflect minor differences in reef builders and reef facies similarities and differences between the two reef complexes provide insights into the characteristics of platform margins reef facies and microbial carbonates of both regions here we present a comparison of platform margin types from different stratigraphic positions in the late devonian reef complex of the canning basin western australia and middle and late devonian margin to marginal slope successions in guilin south china comparisons are integrated into a review of the reefal stratigraphy of both regions reef facies reef complex architecture temporal reef builder associations 2nd order stratigraphy and platform cyclicity in the two regions were generally similar where the successions overlap temporally however carbonate deposition began earlier in south china carbonate complexes were also more widespread in south china and represent a thicker succession overall platforms in the canning basin grew directly on precambrian crystalline basement or early palaeozoic sedimentary rocks but in south china carbonate complexes developed conformably on older devonian siliciclastic strata pre frasnian reef facies in south china had more abundant skeletal frameworks than in canning basin reefs of equivalent age and famennian shoaling margins containing various microbial reefs may have been more common and probably more diverse in south china however late devonian platform margin types have been documented more completely in the canning basin deep intra platform troughs deep depressions containing non carbonate pelagic sediments nandan type successions that developed along syndepositional faults characterize devonian carbonate platforms in south china but have no equivalent on the lennard shelf canning basin where inter reef areas were more shallow the south china platform to depression pattern was generally continuous from the lower to upper devonian indicating that many pre devonian tectonic features continued to exercise considerable effect through deposition localized fault controlled subsidence was an important factor in both regions but similarities in 2nd order aggradation progradation cycles suggest that eustasy was also an important control on the larger scale stratigraphic development of both regions

wind driven recirculations and exchange in the labrador and irminger seas it is demonstrated that recently observed cyclonic recirculation gyres in the irminger and labrador seas may be forced by the strong cyclonic wind stress curl that develops each winter seaward of the east coast of greenland idealized analytical and numerical models forced with such variable winds over a sloping bottom reproduce the essential aspects of the observed gyres strength location and horizontal and vertical length scales the communication between the forcing region in the irminger sea and the recirculation to the west is achieved by baroclinic topographic rossby wave propagation along potential vorticity contours the circulation is characterized as a time dependent stratified topographic beta plume for weak stratification as found in the subpolar north atlantic the recirculation strength exhibits only weak seasonal variability consistent with the observations even though the forcing is active only during the winter baroclinic rossby waves that develop when the wind forcing ceases in springtime interact with the bottom to provide a source of cyclonic vorticity that maintains the circulation until the wind strengthens again in the following winter

bedload transport of fine gravel observed by motion picture photography motion pictures taken at duck creek a clear stream 6 5 m wide and 35 cm deep near pinedale wyoming provide detailed quantitative information on both the modes of motion of individual bedload particles and the collective motions of large numbers of them bed shear stress was approximately 6 pa 60 dynes cm 2 which was about twice the threshold for movement of the 4 mm median diameter fine gravel bed material and transport was almost entirely as bedload the displacements of individual particles occurred mainly by rolling of the majority of the particles and saltation of the smallest ones and rarely by brief sliding of large angular ones entrainment was principally by rollover of the larger particles and liftoff of the smaller ones and infrequently by ejection caused by impacts whereas distrainment was primarily by diminution of fluid forces in the case of rolling particles and by collisions with larger bed particles in the case of saltating ones the displacement times averaged about 0 2 0 4 s and generally were much shorter than the intervening repose times the collective motions of the particles were characterized by frequent brief localized random sweep transport events of very high rates of entrainment and transport which in the aggregate transported approximately 70 of the total load moved these events occurred 9 of the time at any particular point of the bed lasted 1 2 s affected areas typically 20 50 cm long by 10 20 cm wide and involved bedload concentrations approximately 10 times greater than background the distances travelled during displacements averaged about 15 times the particle diameter despite the differences in their dominant modes of movement the 8 16 mm particles typically travelled only about 30 slower during displacement than the 2 4 mm ones whose speeds averaged 21 cm s 1 particles starting from the same point not only moved intermittently downstream but also dispersed both longitudinally and transversely with diffusivities of 4 6 and 0 26 cm 2 s 1 respectively the bedload transport rates measured from the films were consistent with those determined conventionally with a bedload sampler the 2 4 mm particles were entrained 6 times faster on finer areas of the bed where 8 16 mm particles covered 6 of the surface area than on coarser ones where they covered 12 even though 2 4 and 4 8 mm particles covered practically the same percentage areas in both cases the 4 8 and 8 16 mm particles in contrast were entrained at the same rates in both cases to within the statistical uncertainty the rates of distrainment balanced the rates of entrainment for all three sizes and were approximately proportional to the corresponding concentrations of bedload

upper devonian iridium anomalies conodont zonation and the frasnian famennian boundary in the canning basin western australia abstract two iridium anomalies have been identified near the frasnian famennian boundary upper devonian in the canning basin of western australia both anomalies are associated with the cyanobacterium frutexites and are located in marginal slope facies the first was identified in the virgin hills formation on the west flank of mcwhae ridge near the southeastern end of the devonian outcrop belt this anomaly initially identified as being from the famennian upper palmatolepis triangularis zone is now known to be from the early palmatolepis crepida zone on the basis of the presence of palmatolepsis crepida in the bed the second anomaly found in drill core from the napier formation just south of the napier range is older than the first and is from the frasnian montagne noire conodont zone 12 or 13 of klapper 1989 the palmatolepis rhenana zone of ziegler and sandberg 1990 these two iridium anomalies are thus significantly below and above the frasnian famennian boundary and are not associated with the extinction event in the palmatolepis linguiformis zone in the canning basin the association of two iridium anomalies with beds containing abundant frutexites microstromatolites indicates that the concentrations of iridium are probably associated with a process of organic concentration there is no evidence that either example is directly associated with an impact event

ice influences on channel stability insights from missouri s fort peck reach this paper presents insights from a comprehensive study of river ice influences on alluvial channel bathymetry and stability the study entailed unique wintertime fieldwork along the fort peck reach of the missouri river the insights show how ice formation presence and breakup can influence channel stability in several important ways especially when channels must convey substantial water flow during winter ice may hasten the migration of channel bends cause transient scour and sediment deposition during winter and induce cyclic shifts of flow thalweg through sinuous braided subreaches the insights are of direct significance for engineering activities along the missouri s fort peck reach and other alluvial channels subject to frigid winters they also are significant for understanding the frigid winter habitat of aquatic life in such channels

oxic facies and the late devonian mass extinction canning basin australia the close association of anoxic or dysoxic sedimentary rocks and the major late devonian frasnian famennian mass extinction has focused considerable attention on anoxia as the major cause or as a major factor in a multicausal scenario the record of the late devonian biotic crisis in the well known reef complexes of northwestern australia canning basin in contrast to many localities elsewhere does not display sedimentological evidence of anoxia through the frasnian famennian boundary interval analysis of continuous drill core through this interval has yielded three positive 13 c isotopic excursions only one of which coincides with total organic carbon toc maxima in our data multi element geochemical proxies suggest that toc maxima preceding positive shifts in 13 c most likely resulted from higher productivity caused by nutrient influx from continental weathering given the close association between toc maxima and regional relative sea level falls our interpretation supports the view that anoxia was not a fundamental driver of mass extinction and stresses the importance of integrated data sets and understanding regional controls on environmental changes and or stresses

local scour at vertical wall abutments under clear water flow conditions 1 new clear water tests on abutment scour are presented thirty one tests with sand bed and nine tests with lightweight pumice bed were carried out in a 28 m long 4 m wide flume tests typically lasted for 1 week or more vertical wall abutments with length to depth l d ratios from 9 up to 36 were used other test campaigns in the literature are added to the data focusing on abutments with 2 l d 100 the critical flow for scour inception at the abutment nose is found to decrease as l d increases the whole data are analyzed for searching the equilibrium scour depths with different methods the slope change in the plot of scour depth against the logarithm of time is the method selected for the group of tests run close to the threshold condition some trends regarding the dimensionless equilibrium scour depth are 1 dependence on the bed material specific gravity in the sense that pumice tests produce less scour and 2 increase of the scour depth as l d increases finally the morphology of the scour area in terms of volume and plan dimensions is given by new regression equations