turbulence structure over two dimensional bed forms implications for sediment transport turbulence measurements from the flow over two dimensional fixed dune shapes are presented along with analysis and discussion of the ramifications of the observations for transport of sediment as bed load over bed forms the spatial structure of the local transport rate determines the shape and stability of bed forms such as ripples and dunes and the transport of sediment is a highly nonlinear process that is profoundly affected by the statistics of the temporal fluctuations in the near bed flow field the measurements presented herein show strong spatial evolution of the joint probability distribution of the streamwise and bednormal fluctuating velocity components unlike measurements in uniform boundary layers these distributions and the higher moments of the velocity do not scale with the local shear velocity indicating that it is probably inappropriate to use the shear stress to characterize the sediment flux this conclusion is supported by observations of sediment flux over a dune

the geochemistry of rhenium and osmium in recent sediments from the black sea abstract the os isotopic composition of recent sediments from the black sea are reported with the concentrations of re os and major and minor elements osmium concentrations range from 0 23 to 0 69 ppb and re concentrations range from 21 to 85 ppb concentrations of both elements are large relative to average crustal material and are positively correlated with organic carbon 187 os 186 os ratios range from 5 7 to 7 1 all of these values are less radiogenic than the estimated 187 os 186 os ratio of seawater and average continental crust material variations in the os isotopic composition of these sediments are attributed to mixing of a hydrogenous component 187 os 186 os ratio between 7 and 9 and a less radiogenic detrital component 187 os 186 os ratio between 1 and 4 a mixing model is used to determine the fraction of os in the sediment associated with each component and the burial fluxes of re and os in the black sea are calculated in addition the data are used to estimate the magnitude of systematic error which such initial os isotopic heterogeneity may introduce into re os age determinations of ancient organic rich sediments

osmium in the rivers there is a large uncertainty in our understanding of the behavior of osmium during weathering and transport into deep oceans and the osmium budget of the oceans the problem stems chiefly from the lack of osmium data on the dissolved load in the rivers and in the estuaries in this study the concentration and isotopic composition of osmium have been determined in three north american rivers the mississippi the columbia and the connecticut and one river draining central europe and flowing into the baltic sea the vistula osmium concentration in the mississippi and the vistula is about 45 femto mol kg 1 it is about 14 and 15 femto mol kg 1 for the connecticut and the columbia respectively the 187 os 186 os ratios estimated for the mississippi and the vistula are 10 4 and 10 7 respectively for the connecticut and the columbia 187 os 186 os 8 8 and 14 4 respectively of all the rivers examined the mississippi is by far the largest supplying 1 6 of the total annual world river flow its osmium isotopic composition is identical to the upper mississippi valley loesses esser and turekian 1993a indicating 1 congruent dissolution of the bedrock and 2 little or no impact of anthropogenic sources on the osmium isotopic composition of the dissolved load the latter observation indicates that the upper limit of the anthropogenic input in the dissolved osmium load of the mississippi outflow is about 250 g yr 1 while the osmium concentration of the vistula is high the isotopic composition does not appear to have been affected by substantial pollution the river data can be used to put limits on the mean residence time of osmium in the oceans t os and on the osmium budget of the oceans if the bulk river influx of dissolved osmium into the oceans is similar to that of the mississippi we get a value for the net riverine inflow of osmium of 1680 mol yr 1 if there were no sequestering of osmium in the estuaries this would give a value of t os of 1 3 10 4 years as the estuaries may trap a significant amount of river osmium this is a lower limit so that t os 1 3 x 10 4 years using the iridium data by anbar et al 1996 the os ir ratio for the vistula is 2 8 we infer that there is no large fractionation between osmium and iridium during the transport of these elements from the continents into the oceans it follows that the high os ir ratio 22 observed in seawater must be related to the different rain out mechanisms of these elements in the estuaries or in the oceans

pb isotopes in surficial pelagic sediments from the north atlantic we measured pb isotopic composition and concentration in sediment samples close to the sea water interface in 6 box cores from the ne atlantic 2 box cores from the sargasso sea and one from the us continental shelf the anthropogenic pb input to marine sediments due to the increase of pb contamination over the ocean during the last century can be identified in all these cores in the eastern part of the atlantic i e in regions under aeolian influence from europe pb pollution can be recognized using its distinctive unradiogenic composition clearly different from the upper crustal values commonly found in pre holocene sediments in contrast pb pollution in regions influenced by north american sources can be identified only in detailed concentration profiles because the american pb pollution has an isotopic composition much closer to that of the natural detrital pb input coming from weathering of the continental crust pb excess inventories are in good agreement with fluxes estimated from sedimenttrap data and with the time record of pb contamination increase given by analyses in coral growth bands inventories of pb contamination to the sediments of the mud patch american shelf are tenfold higher 84 g cm2 than those to hatteras and bermuda abyssal plains 4 3 and 2 8 g cm2

formation of ripples in erodible channels formation of ripples and dunes in erodible channels is analyzed by using the hydrodynamic model developed by kobayashi and madsen this issue applying the continuity equation of sediment the propagation velocity of the wavy bed is expressed in terms of the net fluxes of bed load along the wavy bed and suspended sediment out of the bed load transport layer the spatial variation of the bed load transport rate is derived by analyzing the balance of forces acting on a representative bed load particle on a sloping bed the net flux of the suspended sediment out of the bed load transport layer is obtained by solving the diffusion equation for the suspended sediment near the bed the stability analysis predicts that a perturbation that is either fairly short or long relative to the depth of flow will initially grow depending on whether the flow is hydraulically smooth or fully rough thus the model explains why ripples are not observed in experiments involving fairly coarse sediments since in these cases the flow is rough turbulent

numerical computation of sand dune migration with free surface flow 1 this paper presents a morphodynamic model for free surface flows over bed forms the model reproduces bed form evolution and migration in a physically based manner the flow model component of the coupled morphodynamic model which is two dimensional in the streamwise and vertical directions explicitly treats unsteadiness and nonhydrostatic effects comparison with measurements shows that the flow model accurately simulates both mean flow and turbulence structure induced by bed forms under free surface flow the flow model performance is enhanced by the use of a high order godunov scheme known as the cubic interpolated pseudoparticle cip technique to compute the advection phase of the navier stokes equation in addition an enhanced k turbulence closure with nonlinear terms is incorporated in the present model in order to allow better predictions of reynolds stress anisotropy in regions with flow separation performance of the nonlinear k model is compared with standard k closure in the context of the flow as well as morphodynamic simulation nonequilibrium bed load sediment transport is treated using a eulerian stochastic formulation of the sediment exchange process in terms of pickup and deposition functions the proposed approach for sediment transport explicitly considers the local flow variability during morphodynamic computation model results show that the vertical grid distribution pattern affects the simulation result particularly for bed form morphodymics the model successfully simulates some important physical features of bed form evolution such as amalgamation and asymmetric geometry the simulation results are in agreement with laboratory experiments as well as some commonly used prediction methods for geometric characteristics of bed forms

the osmium isotopic composition of organic rich marine sediments abstract the osmium os concentration and 187 os 186 os ratio of several recent marine organic rich sediment samples from three widely separated sites have been measured os concentrations range from 0 095 to 0 212 ppb and 187 os 186 os ratios range from 8 2 to 8 9 the calculated fraction of hydrogenous os exceeds 78 in all samples thus the 187 os 186 os ratio of these samples reflects os isotopic composition of seawater the small range in measured 187 os 186 os ratio indicates that the os isotopic composition at these sites is fairly homogeneous the large magnitude of the os burial flux at these sites indicates the os burial in association with organic rich sediments is an important sink in the marine cycle of os these data also suggest that ancient organic rich sediments may provide a record of past variations in the os isotopic composition of seawater

sequence stratigraphic analysis and the origins of tertiary brown coal lithotypes latrobe valley gippsland basin australia abstract the methods of sequence analysis have been applied to the onshore gippsland basin and in particular to the latrobe valley group coal measures which include up to five coal seams each exceeding 100 m in thickness the methods appear to provide new depositional concepts to the evolution of these seams and the development of coal lithotypes in the eastern half of the latrobe valley evidence for marine transgressions into the coal measures are recorded in most of the interseam sediment splits by the presence of contained foraminifera and dinoflagellates to the west inland these splits pinch out into continuous coal however they can be followed westwards as enhanced organic sulphur levels along sharply defined boundaries between light coal lithotypes below and dark coal lithotypes above the dark lithotype immediately overlying each of these boundaries contains the highest sulphur value and warmer climate pollen assemblages sluiter et al 1995 this volume colorimeter and lithotype logging strongly supports an upwards lightening cyclicity to coal colour at 12 20 m intervals through the approx 100 m thick seams with cycle boundaries defined at sharp planar to undulating surfaces the lightening upward lithotype cycles together with their unique boundary conditions i e enhanced organic sulphur levels warm climatic indicators and laterally equivalent marine clay splits are interpreted as parasequences and parasequence boundaries respectively each major coal seam can comprise up to five parasequences and is interpreted to represent deposition during an outbuilding high stand systems tract at one of several maximum periods of tertiary coastal onlap the top of each major seam shows evidence of truncation erosion on a regional scale and these surfaces are interpreted to represent the sequence boundaries the major seams are usually conformably underlain by marine clays and extensive aquifer sands being deposits of the late transgressive systems tracts the low stands and early parts of the transgressive systems tracts appear not to be represented in the latrobe valley due to its more basin margin location but are probably present down dip in the equivalent marine facies of the seaspray group stratigraphic correlation of the sequence boundaries identified in the coal measures to the adjacent internationally dated marine seaspray group provides a basis for chronostratigraphic correlation of the coal successions to the coastal onlap charts of haq et al exon mesozoic cenozoic chronostratigraphic chart version january 1988 and august 1989 from this dating it appears that each major seam is confined to high stands of third order eustatic cycles it therefore follows that the lithotype cycles parasequences that comprise each seam are related to fourth order eustatic cycles by analogy all the coal cycles may have developed under subtropical conditions as ombrogenous forested peat swamps in a similar manner to the holocene though tropical swamps of indonesia

unsteady flow in straight alluvial streams part 2 transition from dunes to plane bed the effect of suspended sediment on the modification of individual dunes in unsteady channel flow is analyzed the model is an extension of that developed by fredsoe 1979 in which the change of the dune dimensions was analyzed assuming that the transport of sediment mainly occurs as bed load by including suspended sediment it is possible to describe dune behaviour at high sediment transport rates for which the dunes are washed out so the bed at sufficiently high bed shear stress becomes plane the model is compared with laboratory experiments and the agreement is reasonable the effect of temperature variations on dune dimensions is incorporated in the analysis

osmium isotopes and silver as tracers of anthropogenic metals in sediments from massachusetts and cape cod bays high concentrations of osmium os and silver ag and low 187os 186os ratios in boston sewage make these elements sensitive tracers of the influence of sewage on marine sediments in massachusetts and cape cod bays pristine marine sediments have ag concentrations more than 200 times lower than sewage sludge os concentrations 10 40 times lower and 187os 186os ratios six times higher surface sediments from both massachusetts and cape cod bays exhibit both high ag concentrations and low 187os 186os ratios indicating the influence of sewage particles on marine sediments in this region extends some 70 km from the point of sewage release in detail the distribution of os and ag do not support a model of simple physical mixing of sewage particles with normal marine sediments deviations from the mixing model may be the result of fractionation of os and ag in the marine environment and or independent temporal variations in the os and ag content of the waste stream the results of this investigation suggest that osmium isotopes may be widely applicable as tracers of the influence of sewage on sediments in estuarine environments and that subtle variations in the isotopic composition of os in the waste stream may help constrain the sources of os and other metals delivered to the environment

ripple and dune formation in rivers a linear stability analysis for dune and ripple formation is presented that implements a rotational two dimensional flow model valid in the smooth as well as in the transitional and rough flow regimes sediment is assumed to be transported as bedload disregarding the role of suspension therefore the main mechanism driving instability for both ripples and dunes is the phase lag between bed shear stress and bed elevation ripples are shown to be confined to relatively low values of the shields parameter and of the particle reynolds number for higher values of the shields parameter and of the particle reynolds number and thus of the froude number and of the roughness reynolds number ripples are replaced by dunes the present analysis ultimately allows for a successful unification of the theories of dune and ripple formation and for a clarification of the debated role of ripples on the formation of dunes a good agreement between predicted and observed wavelengths for both ripples and dunes is found

numerical model of turbulent flow over sand dune a two dimensional numerical model based on solution of the reynolds averaged navier stokes equations and the k turbulence model which takes into account sand grain roughness is developed to describe the flow in a fixed dune bed channel the model predicts the velocity and turbulence fields as well as the pressure and friction distributions along the dune the details of the flow in the separation eddy are calculated the model predictions are in general agreement with existing detailed experimental data in a rectangular channel with two dimensional dunes of typical but regular shape the calculated pressure and friction distributions enable determination of the resistance components of the channel without further empiricism however compared to a well known and representative semiempirical engineering formula for the prediction of resistance of natural dune bed channels the two dimensional numerical model predicts significantly different contributions of friction and pressure to the total resista

mechanics of flow over ripples and dunes a simple predictive model for velocity and boundary shear stress fields over two dimensional bedforms of finite amplitude is presented herein and the results of that model are compared with flume data the production of a wake due to separation of the flow near the bedform crest and the subsequent interaction of this wake region with a developing nonuniform boundary layer downstream of the flow reattachment point are treated using the method of mclean and smith 1986 their model is modified in order to take account of the flow response to short wavelength obstructions more accurately and to include the effects of wakes from upstream bedforms as these wakes coalesce in the interior the theory is also extended to include the region between the bedform crest and the reattachment point thereby providing a spatially complete field of velocity and shear stress results of the calculations are found to be in good agreement with laboratory measurements taken over immobile bedforms using a laser doppler velocimeter our finite amplitude flow model is shown to yield accurate predictions of the fluid dynamical effects ultimately responsible for the morphology of well developed natural bedforms

sedimentary environments within a glaciated estuarine inner shelf system boston harbor and massachusetts bay abstract three modern sedimentary environments have been identified and mapped across the glaciated boston harbor estuary and adjacent inner shelf of massachusetts bay by means of an extensive set of sidescan sonar records and supplemental bathymetric sedimentary subbottom and bottom current data 1 1 environments of erosion and nondeposition appear on the sonographs either as patterns with isolated reflections caused by outcrops of bedrock glacial drift and coastal plain rocks or as patterns of strong backscatter caused by coarse grained lag deposits sediments in these environments range from boulder fields to gravelly sands with megaripples inside the harbor areas of erosion or nondeposition are found primarily near mainland and insular shores and within constricted tidal channels whereas on the shelf they are present over extensive areas of hummocky topography near the coast and atop local bathymetric highs offshore 2 2 environments of sediment reworking are characterized on the sonographs by patterns with patches of strong to weak backscatter caused by a combination of erosional and depositional processes these environments have diverse grain sizes that range from sandy gravels to muds within the harbor the locations of reworked sediments are uncorrelated with the bottom topography but on the shelf they are found on the lower flanks of bathymetric highs within broad lows and in relatively deep water 30 50 m 3 3 environments of deposition are depicted on the sonographs as uniform patterns of weak backscatter these areas contain relatively fine grained muddy sands and muds inside the harbor depositional environments are found over extensive subtidal flats and within sheltered depressions whereas on the shelf they are restricted to broad lows mainly in deep water the extreme patchiness of modern sedimentary environments within the boston harbor massachusetts bay system reflects the interaction between the irregular bottom topography and both geologic and oceanographic processes the estuarine part of the system is an effective trap for fine grained detritus because of its protected nature low wave climate and large supply of sediments the open shelf however is largely mantled by winnowed and sorted sediments as a result of erosion during past sea level fluctuations sediment resuspension and transport by modern waves and currents and a spatially variable supply of fine grained sediments

the osmium isotopic composition of the continental crust osmium isotopic compositions and concentrations have been determined for deltaic and continental shelf sediments from three major rivers the amazon changjiang and mississippi and for two loesses from the upper mississippi river valley the loesses have heavy mineral and clay compositions characteristic of mississippi river and delta sediment concentrations range from 15 to 120 pg os g and 187os 186os ratios range from 8 2 to 10 9 sediments which are polluted surface sediment from the changjiang estuary or which have scavenged metals from seawater gulf of mexico slope surface sediment and distal amazon shelf sediment have high osmium concentrations and less radiogenic isotopic compositions purely terrigenous sediments have 15 90 pg os g and 187os 186os 10 0 10 9 north american loess and mississippi delta sediment osmium isotopic compositions are indistinguishable we believe that 187os 186os 10 11 is a good average for currently eroding upper continental crust these sediments have neodymium isotopic compositions close to the global mean for river sediment terrigenous 187os 186os is significantly more radiogenic than seawater 187os 186os 8 6 requiring a nonriverine source of osmium to seawater terrigenous osmium is also significantly more radiogenic than bulk pelagic clay osmium we use the improved estimate of terrigenous 187os 186os to recalculate the flux of extraterrestrial osmium to pacific marine sediment if the residence age of osmium in an upper crustal region is similar to the neodymium model age of a sediment derived from that region a model dependent upper crustal 187re 186os ratio can be calculated for the model ages of the sediments analyzed in this study tdm 1 5 1 7 ga model 187re 186os ratios are 340 360 and can be used to correct for bias resulting from the preferential erosion of young crust the 187os 186os ratio of 2 2 ga crust with 187re 186os 400 is 14 2 we have also used the data from this study to estimate upper crustal osmium and rhenium concentrations of 50 pg g and 390 pg g respectively

sources of osmium isotopes in manganese nodules abstract we have measured the 187 os 186 os and os concentrations of ferromanganese crusts forming on serpentinized peridotites from the atlantic ocean and also report major element data for these samples and six manganese nodules previously analyzed for os variations in the 187 os 186 os ratios of manganese nodules are the result of two processes operating on different spatial scales the large scale variations result from the impression of intense local os inputs from continental and oceanic sources on a generally homogeneous oceanic 187 os 186 os ratio fine scale variations may be understood as the result of the mixing of a constant fine grained debris flux of meteoritic os imposed on varying amounts of os precipitated from seawater

high gradients of lead isotopic composition in north east pacific upwelling filaments substantial differences in stable lead isotopic composition in coastal waters of the north east pacific have been inferred from a single vertical profile taken near western canada1 here we present measurements of lead isotopes in two upwelling filaments off central california 39 n 124 w which confirm that such marked differences exist the distinct filamentary structures appear in association with surface jets which transport cold upwelled water hundreds of kilometres offshore2 surface water ratios of stable lead isotopes 204pb 206pb 207pb 208pb within the core of the first filament for example 206pb 207pb 1 17 contrasted with those of surrounding coastal waters 206pb 207pb 1 19 and those of industrial lead aerosols from the united states 206pb 207pb 1 22 contrasts in the isotopic composition indicated that the lead within the core was primarily derived from aeolian inputs of asian industrial lead 206pb 207pb 1 16 to remote waters which were then upwelled near the north american coastline high gradients of lead isotopic composition in coastal waters were confirmed by surface and sub surface measurements surrounding a second upwelling filament these gradients illustrate the complexity of trace element cycles in coastal waters and the offshore fluxes within the ephemeral filaments demonstrate significant influences of coastal dynamics on composition within the ocean gyre

the stability of ripples and dunes abstract the stability of ripples and dunes in flows of finite depth are investigated for the linear small amplitude case and also for the more realistic finite amplitude situation for the former perturbation approach is taken leading to the general result that without the inclusion of some lag between the sediment transport rate and the boundary shear stress all wavelengths are unstable this results from an upstream shift in the stress relative to the bedform leading to deposition at the crest by assuming a realistic lag based on the mechanics of bed load transport a fastest growing wave at ripple scales results similarly if suspended sediment were included longer lags result and a dune scale instability could result some investigators have invoked a gravitational effect arguing that sediment transport is enhanced downslope and retarded upslope this creates an effective downstream shift in the transport rate and some argue that both ripple and dune instabilities result however a realistic evaluation of this effect shows that it is much too small to be effective naturally occurring bedforms are of finite amplitude typically they are asymmetrical often causing flow separation this highly non linear process creates a vastly different flow environment from that assumed in the initial stability case a wake like flow structure develops downstream of the separation zone and an internal boundary layer develops beneath the complex interaction of these two different flow regimes as well as the effect induced by the topography of the bedform itself produces a shear stress field that dictates the stability of the feature itself

fluid and sediment interaction over a plane bed a mathematical model is developed for the fluid and sediment interaction in an erodible channel the model is based on the conservation of mass and momentum for the fluid and sediment the interaction between the fluid and sediment as well as the interaction of sediment particles moving in the vicinity of the bed is described in the model an analysis is performed to derive the vertical distributions of the fluid velocity and sediment concentration over a plane bed in a wide open channel the analysis yields the friction factor due to the combined effects of grain roughness and sediment movement as well as the rate of sediment transport the developed model agrees with existing data for plane beds the predicted rate of transport of coarse sediment agrees with the meyer peter and mullen formula however detailed measurements of the fluid velocity and sediment concentration over a plane bed are required for a rigorous verification of the developed model