rapid gravitational adjustment of horizontal shear flows the evolution of a horizontal shear layer in the presence of a horizontal density gradient is explored by three dimensional numerical simulations these flows exhibit characteristics of both free shear flows and gravity currents but have complex dynamics due to strong interactions between the turbulent features of each vertical vortices produced by horizontal shear are tilted and stretched by the gravitational adjustment rapidly enhancing vorticity shear intensification at frontal convergences produces high wavenumber vertical vorticity and the slumping of the density interface produces horizontal kelvin helmholtz vortices typical of a gravity current the interaction between these instabilities promotes a rapid transition to three dimensional turbulence the flow development depends on the relative time scales of shear instability and gravitational adjustment described by a parameter gamma where the limits gamma rightarrow infty and gamma rightarrow 0 represent a pure gravity current and a pure mixing layer respectively the growth rate of three dimensional instability and the mixing increase for smaller gamma when gamma is sufficiently small there are two distinct regimes an early period of during which the interface grows rapidly followed by horizontal diffusive growth numerical results are consistent with field observations of tidal separation flows in the haro strait farmer pawlowicz jiang dyn atmos oceans vol 36 2002 pp 43 58 including the magnitude of downwelling vertical currents horizontal scales of surface vortex features and mixing rate

the cretaceous tertiary boundary stratotype section at el kef tunisia how catastrophic was the mass extinction abstract the cretaceous tertiary k t boundary stratotype section at el kef tunisia represents the most complete and expanded sedimentary record across this important mass extinction horizon presently known high resolution analysis of planktic foraminifera in two outcrops el kef i stratotype and el kef ii along with comparisons between planktic and benthic foraminifera calcareous nannofossils ostracods pollen and spores and dinoflagellates indicate that major changes across the k t boundary are registered only in benthic and planktic foraminifera and calcareous nannofossils biotic changes in benthic foraminifera are unique to el kef and similarly shallow continental shelf sections and appear to be the result of a sea level regression in the latest maastrichtian followed by a sea level rise across the k t boundary that was accompanied by expansion of the local oxygen minimum zone omz biotic changes in planktic foraminifera appear partly related to these conditions also but in general reflect more global oceanographic changes for instance species extinctions are gradual and selective as observed in k t sections worldwide rather than random and abrupt although there is a 69 decline in species richness between 25 cm below and 10 cm above the k t boundary only rare species disappeared their combined relative abundance constitute less than 20 of the total population about 52 of these extinct taxa 8 of the population are large ornate morphologically complex tropical subtropical forms that lived at or below the thermocline no planktic foraminifera from this depth range survived the k t boundary event all survivor taxa were surface dwellers living within the photic zone their relative abundance 80 dominates both cretaceous and early tertiary populations these data indicate that the k t biotic record in the shallow continental shelf section at el kef was significantly influenced by local conditions which combined with the latest maastrichtian sea level regression and subsequent sea level rise resulted in shallowing of the local omz relative to the sea surface shallowing of the local omz lead to the selective disappearance of benthic faunas and may have adversely affected the surviving photic zone dwellers the selective nature of species extinctions however appear to be related partly to long term global oceanographic changes which were accelerated at the k t boundary possibly by a bolide impact

forcing mechanisms for mid cretaceous black shale formation evidence from the upper aptian and lower albian of the vocontian basin se france calcareous nannoplankton palynomorph benthic foraminifera and oxygen isotope records from the supraregionally distributed niveau paquier early albian age oceanic anoxic event 1b and regionally distributed niveau kilian late aptian age black shales in the vocontian basin se france exhibit variations that reflect paleoclimatic and paleoceanographic changes in the mid cretaceous low latitudes to quantify surface water productivity and temperature changes nutrient and temperature indices based on calcareous nannofossils were developed the nutrient index strongly varies in the precessional band whereas variations of the temperature index reflect eccentricity since polar ice caps were not present during the mid cretaceous these variations probably result from feedback mechanisms within a monsoonal climate system of the mid cretaceous low latitudes involving warm humid and cool dry cycles a model is proposed that explains the formation of mid cretaceous black shales through monsoonally driven changes in temperature and evaporation precipitation patterns the lower albian niveau paquier which has a supraregional distribution formed under extremely warm and humid conditions when monsoonal intensity was strongest bottom water ventilation in the vocontian basin was diminished probably due to increased precipitation and reduced evaporation in regions of deep water formation at low latitudes surface water productivity in the vocontian basin was controlled by the strength of monsoonal winds the upper aptian niveau kilian which has a regional distribution only formed under a less warm and humid climate than the niveau paquier low latitude deep water formation was reduced to a lesser extent and or on regional scale only the threshold for the formation of a supraregional black shale was not reached the intensity of increases in temperature and humidity controlled whether black shales developed on a regional or supraregional scale at least in the vocontian basin the increased preservation of organic matter at the sea floor was more significant in black shale formation than the role of enhanced productivity

lobe cleft instability in the buoyant gravity current generated by estuarine outflow gravity currents represent a broad class of geophysical flows including turbidity currents powder avalanches pyroclastic flows sea breeze fronts haboobs and river plumes a defining feature in many gravity currents is the formation of three dimensional lobes and clefts along the front and researchers have sought to understand these ubiquitous geophysical structures for decades the prevailing explanation is based largely on early laboratory and numerical model experiments at much smaller scales which concluded that lobes and clefts are generated due to hydrostatic instability exclusively in currents propagating over a nonslip boundary recent studies suggest that frontal dynamics change as the flow scale increases but no measurements have been made that sufficiently resolve the flow structure in full scale geophysical flows here we use thermal infrared and acoustic imaging of a river plume to reveal the three dimensional structure of lobes and clefts formed in a geophysical gravity current front the observed lobes and clefts are generated at the front in the absence of a nonslip boundary contradicting the prevailing explanation the observed flow structure is consistent with an alternative formation mechanism which predicts that the lobe scale is inherited from subsurface vortex structures

three dimensional turbulent bottom density currents from a high order nonhydrostatic spectral element model overflows are bottom gravity currents that supply dense water masses generated in high latitude and marginal seas into the general circulation oceanic observations have revealed that mixing of overflows with ambient water masses takes place over small spatial and time scales studies with ocean general circulation models indicate that the strength of the thermohaline circulation is strongly sensitive to representation of overflows in these models in light of these results overflow induced mixing emerges as one of the prominent oceanic processes in this study as a continuation of an effort to develop appropriate process models for overflows nonhydrostatic 3d simulations of bottom gravity are carried out that would complement analysis of dedicated observations and large scale ocean modeling a parallel high order spectral element navier stokes solver is used as the basis of the simulations numerical experiments are conducted in an idealized setting focusing on the startup phase of a dense water mass released at the top of a sloping wedge results from 3d experiments are compared with results from 2d experiments and laboratory experiments based on propagation speed of the density front growth rate of the characteristic head at the leading edge turbulent overturning length scales and entrainment parameters results from 3d experiments are found to be in general agreement with those from laboratory tank experiments in 2d simulations the propagation speed is approximately 20 slower than that of the 3d experiments and the head growth rate is 3 times as large thorpe scales are 1 3 1 5 times as large and the entrainment parameter is up to 2 times as large as those in the 3d experiments the differences between 2d and 3d simulations are entirely due to internal factors associated with the truncation of the navier stokes equations for 2d approximation

high stress paleoenvironment during the late maastrichtian to early paleocene in central egypt biostratigraphic mineralogical geochemical and isotopic analyses of the gebel qreiya section in the asyut basin of central egypt indicate a depositional environment interrupted by periods of erosion due to local tectonic activity exacerbated by eustatic sea level fluctuations and by high stress environmental conditions akin to those normally experienced during the cretaceous tertiary boundary transition during the late maastrichtian 66 8 65 4 ma this region experienced a breakdown of the biologically mediated surface to bottom gradient of the 13c 12c ratio with planktic 13c values 0 2 0 8 lighter than benthic values planktic foraminiferal species diversity was reduced by more than 50 with faunal assemblages dominated 75 90 by the opportunistic disaster species guembelitria cretacea which alternate with abundance of small low oxygen tolerant heterohelicids heterohelix navarroensis h dentata h globulosa this prolonged breakdown in ocean primary productivity occurred during a time of global climate cooling and sea level regressions at 66 8 and 65 5 ma though clay mineralogy suggests that locally low seasonality warm wet tropical and subtropical conditions prevailed the high detrital influx suggests that the biologically high stress environment was primarily linked to the existing shallow shelf conditions in southern egypt and possibly to local tectonic activity and restricted circulation a normal carbon isotope gradient was briefly re established during the short climate warming and rising sea level between 65 4 and 65 2 ma a time of increased species diversity peak abundance of rugoglobigerinids and common heterohelicids during the last 200 000 years of the maastrichtian increased precipitation and terrestrial runoff increased phyllosilicates and kaolinite and increasing total organic carbon values are associated with heterohelix dominated planktic foraminiferal assemblages the k t boundary is marked by a red clay layer and ir anomaly of 5 4 ppb during the early danian planktic foraminiferal populations and stable isotope data indicate that similarly fluctuating high stress conditions prevailed in central egypt as elsewhere in the marginal eastern tethys

groundwater level changes in a coastal dune sea level fluctuations and shoreline movement on a sandy beach abstract it has been suggested earlier 1 that groundwater conditions may contribute to low frequency shoreline fluctuations with recurrence intervals of 0 5 10 years that proposition has been examined by comparison of a 7 year record of groundwater levels from coastal dune lands with mean sea level fluctuations and shoreline changes from the adjacent coast in an original analysis of western australian records 2 the annual cycles of sea level and shoreline change were observed to respond in an inverse manner such that a 1 0 cm rise in sea level approximately corresponds with a 1 0 m decrease on each profile in the present analysis the ratio ranges from 110 1 to 125 1 for oscillations in the annual to triennial band the ratio is less approximately 80 1 for the 0 5 year cycle these results are close to those reported from the original analysis in other respects however the sea level effect may be in dispute there is better agreement between time series describing the groundwater and shoreline fluctuations than between sea level and beach change the annual oscillation of the groundwater table is within 2 of being the inverse of that for the shoreline in the annual cycle sea level reaches its maximum in early winter during may june whereas the groundwater peak is highest during september october at a time when the beach is in its most depleted state

the cretaceous paleogene boundary in jordan abstract the cretaceous paleogene k pg boundary in jordan is marked by a major depositional hiatus that differs in magnitude from place to place due to variable structural movements of the syrian arc fold belt that resulted in irregularity of jordan levant depositional basin after the deposition of maastrichtian succession to elucidate the nature of this hiatus fieldwork was carried out at a number of locations including lithofacies and stratigraphic analysis and a multi proxy study of microplanktonic biostratigraphy calcareous nannofossilis and planktonic formaminifera however the duration of this hiatus extended over latest maastrichtian and early danian stages this is based on the absence of the planktonic foraminifera pseudoguembelina hariaensis cf3 pseudoguembelina palpebra cf2 plummerita hantkeninoides cf1 guembelitria cretacea p0 parvularugoglobigerina eugubina p zones and parasubbotina pseudobulloides p1a subzones and the coeval calcareous nannofossils nephrolithus frequens and markalius inversus zones we estimate that in the paleo lows areas an unexpected 3 96 ma hiatus is present conversely in swell areas the duration of the hiatus represents the entire danian selandian interval and revealed an unexpected 10 33 ma hiatus especially in the central part of jordan subsequently a marked transgression took place over the whole of jordan which resulted in the prevalence of deep water conditions zones p4 or equivalent np7 8 this caused the deposition of a retrogradational parasequence set of middle shelf pelagic marl and chalk during a rapid relative rise of sea level a correlative hiatus and time gap have also been reported in different parts of the arabian and african plates indicating that jordan was influenced by regional tectonics that combined with the latest maastrichtian sea level fall resulted in a long term sub marine hiatus and or non deposition of sediments a combination of sea level changes and tectonic uplift are the likely cause for such a long gap in the sediment record in the region

upper cretaceous sequence stratigraphy of the galala plateaux western side of the gulf of suez egypt abstract based on the study of lithology biostratigraphy and analysis of facies this study deduces the occurrence of nine third order upper cretaceous cenomanian maastrichtian depositional sequences which are bounded by eleven sequence boundaries of geographic extent in the galala plateaux situated along the western side of the gulf of suez egypt some of the recorded sequence boundaries e g cenomanian turonian turonian coniacian intra santonian and campanian maastrichtian sequence boundaries are not yet confirmed to eustatic sea level changes fourteen main mixed carbonate siliciclastic facies associations were recognized on a lateral transition between inner to outer ramp settings with apparent subsidence towards the south the tectonic position of the galala plateaux played an important role in the profound complexity of facies changes and the configuration of depositional sequences representing variable time gaps

trends in caribbean paleoproductivity related to the neogene closure of the central american seaway abstract this study tests the hypothesis that the late miocene to early pliocene constriction and closure of the central american seaway cas connecting the tropical atlantic and east quatorial pacific eep caused a decrease in productivity in the caribbean due to decreased coastal upwelling and an end to the connection with high productivity tropical pacific waters the present study compared paleoceanographic proxies for the interval between 8 3 and 2 5 ma in 47 samples from south caribbean odp site 999 with published data on eep dsdp site 503 proxies for site 999 include the relative abundance of benthic foraminiferal species representing bottom current velocity and the flux of organic matter to the sea floor the ratio of infaunal epifaunal benthic foraminiferal species and benthic foraminifer accumulation rates bfars in addition we calculated resistant planktic foraminifers species and used the previously published sand fraction and benthic carbon isotope values from site 999 during early shoaling of the isthmus 8 3 7 9 ma the caribbean was under mesotrophic conditions with little ventilation of bottom waters and low current velocity the pre closure interval 7 6 4 2 ma saw enhanced seasonal input of phytodetritus with even more reduced ventilation and enhanced dissolution between 6 8 and 4 8 ma during the post closure interval 4 2 2 5 ma in the caribbean paleoproductivity decreased current velocity was reduced and ventilation improved while the seasonality of phytodetrital input was reduced dramatically coinciding with the establishment of the atlantic pacific salinity contrast at 4 2 ma our data support the hypothesis that late miocene constriction of the cas at 7 9 ma and its closure at 4 2 ma caused a gradual decrease in paleoproductivity in the caribbean consistent with decreased current velocity and seasonality of the phytodetrital input

rapid water table fluctuations within the beach face implications for swash zone sediment mobility abstract to incorporate groundwater infiltration exfiltration in the description of swash zone sediment transport it is required that the details of groundwater dynamics within the beach face be clarified field measurements of the vertical pore pressure structure within the bed identify capillarity effects as the primary mechanism driving rapid and relatively large magnitude water table fluctuations within the swash zone when the upper extent of the fully saturated capillary fringe coincides with the beach face surface wave runup produces near instantaneous increase in pore pressures across the capillary fringe corresponding to a rapid rise of the phreatic surface i e the surface where pore pressure atmospheric pressure to the sand surface therefore counter to previous conclusions in the literature the rapid rise and fall of the water table under the swash zone do not equate to regions of the beach face alternating between states that favor sediment deposition unsaturated and erosion saturated similar reports that rapid fluctuations of the water table within the beach face correspond to rapid rates of vertical flow and hence bed fluidization are also a misinterpretation field measurements and a careful consideration of saturation and pore pressure characteristics within the beach face demonstrate that rapid water table rise is associated with minute downwards swash infiltration rather than rapid upwards groundwater exfiltration the cause of the pressure fluctuations and phreatic surface oscillations is the alternating appearance and disappearance of meniscuses at the sand surface which generate pressure head fluctuations of decimeters due to the addition of millimeters of water

high resolution sequence stratigraphy of the maastrichtian ypresian succession along the eastern scarp face of kharga oasis southern western desert egypt a thick maastrichtian ypresian succession dominated by marine siliciclastic and carbonate deposits of the regionally recognized nile valley and garra el arbain facies associations is exposed along the eastern escarpment face of kharga oasis located in the western desert of egypt the main objectives of the present study are i to establish a detailed biostratigraphic framework ii to interpret the depositional environments and iii to propose a sequence stratigraphic framework in order to constrain the palaeogeographic evolution of the kharga sub basin during the maastrichtian ypresian time interval the biostratigraphic analysis suggests the occurrence of 10 planktonic zones two in the early maastrichtian cf8b and cf7 four in the palaeocene p2 p3 p4c and p5 and four in the early eocene e1 e2 e3 and e4 recorded zonal boundaries and biostratigraphic zones generally match with those proposed elsewhere in the region the stratigraphic succession comprises seven third order depositional sequences which are bounded by unconformities and their correlative conformities which can be correlated within and outside egypt these depositional sequences are interpreted as the result of eustatic sea level changes coupled with local tectonic activities each sequence contains a lower retrogradational parasequence set bounded above by a marine flooding surface and an upper progradational parasequence set bounded above by a sequence boundary parasequences within parasequence sets are stacked in landward stepping and seaward stepping patterns indicative of transgressive and highstand systems tracts respectively lowstand systems tracts were not developed in the studied sections presumably due to the low relief ramp setting the irregular palaeotopography of the dakhla basin which was caused by north east to south west trending submerged palaeo highs and lows together with the eustatic sea level fluctuations controlled the development and location of the two facies associations in the kharga oasis the nile valley open marine and garra el arbain marginal marine

a note on the structure of the head of an intrusive gravity current the head of an intrusive flow advancing along the interface between two fluids is studied experimentally when the two layers are of equal depth and the density of the intrusion is the mean of the two densities the dependence of the flow on the interface thickness and the depth of the intrusion is determined when the interface is very thin the flow is similar to the nominally inviscid gravity currents observed by britter simpson 1978

maastrichtian to early paleocene sea level changes and climatic evolution on the southeastern tethys margin abstract an integrative multi proxy approach based on lithology microfacies bulk rock composition total organic carbon toc and 13ccarb was used to estimate southeastern tethys sea level fluctuations during the deposition of the maastrichtian danian oil shales in jordan clay mineral assemblages were used as proxies to determine climatic variation induced sediment supply during the cretaceous paleogene boundary k pg interval the results from two cores os 02 jafr basin southeast jordan os 28 azraq hamza basin central east jordan reveal four major intervals of distinctive bulk rock composition reflecting sea level changes interval i earliest maastrichtian with a low sea level and the formation of dolomites and a high detrital quartz input predates the oil shales intervals ii and iii in core os 02 and interval ii in core os 28 represent the highest relative sea level following the early maastrichtian sea level rise on the arabian platform intervals iii core os 02 and ii core os 28 coincide with the highest organic matter deposition interval iv core os 02 and intervals iii and iv core os 28 relate to the sea level regression transgression cycle in the latest maastrichtian through danian and are marked by a large drop in the toc content increased terrestrial influx during sea level lows is evident from two peaks intervals of major negative 13ccarb values in core os 28 based on clay mineralogy it can be differentiated between warm humid warm cool seasonally alternating and cool arid climates when bulk rock composition is used as a sea level proxy clay mineral assemblages from both cores reveal the dominance of warm and humid climate or alternatively warm cool seasonal alternations during high sea levels ii and iii core os 02 ii and iv core os 28 while periods of arid and cool climate correspond to lower sea levels the sea level changes and climatic variations deduced from the jordanian oil shale successions are in agreement with the long term variations from the egyptian western desert sections southeastern tethys and the tunisian el kef section southweastern tethys moreover the sea level changes identified relate to the regional changes on the arabian platform during the maastrichtian danian

idealized numerical simulations of the interactions between buoyant plumes and density currents idealized numerical experiments using a large eddy simulation les model are performed to examine the fundamental dynamical processes associated with the interactions between buoyant plumes and density currents the aim of these simulations is to provide insight into the rapid changes in the structure of plumes that may be observed during the passage of density current phenomena such as thunderstorm outflows sea breeze fronts or intense cold fronts the les model results indicate that when the ambient winds are calm the vertical velocity in the plume decreases with the passage of the density current but that when the ambient winds oppose the motion of the density current a significant increase in vertical velocity in the plume may occur temporarily in the latter case the pressure perturbation and the associated region of horizontal convergence that lead the head of the density current interact with the tilted plume causing the base of the plume to become vertical and resulting in a dramatic increase in vertical velocity within the plume this basic dynamical behavior occurs over a relatively broad range of parameters provided the characteristic velocity in the density current taken as the densimetric speed exceeds the ambient wind speed when this is the case the interaction is dominated by the effect of the density current on the buoyant plume such that the plume is essentially advected as a passive tracer by the flow due to the density current and the increase in vertical velocity depends on the inverse of the convective froude number of the buoyant plume

numerical simulation of thunderstorm outflow dynamics part i outflow sensitivity experiments and turbulence dynamics abstract in this first paper of a two part series a two dimensional numerical model is developed and used to investigate the dynamics of thunderstorm outflows by focusing only on the outflow and using essentially inviscid equations and high spatial resolution we are able to explicitly represent important physical processes such as turbulent mixing to simplify interpretation of the results the model atmosphere used in all experiments is calm and dry adiabatic this approach allows us to establish basic characteristics of modeled outflows in simple physical settings and provides a foundation for future studies using more realistic environments all simulated outflows are initialized by prescribing a controlled horizontal flux of cold air into the model domain through a lateral boundary in a series of sensitivity tests we examine three parameters of the cold air source region 1 the vertical temperature deficit profile 2 the magnitude of the temperature deficit and 3 the cold air depth by hol

environmental significance of foraminiferal assemblages dominated by small sized ammodiscus and trochammina in triassic and jurassic delta influenced deposits abstract the sediment packages analyzed for benthic foraminifera consist of mudstones with interbedded sandstones deposited in shallow delta influenced shelf to deltaic environments the sections are located in spitsbergen the barents sea northern north sea and yorkshire and range in age from late triassic to middle jurassic salient features of the foraminiferal successions are 1 the assemblages consist entirely or dominantly of agglutinated taxa 2 the faunal diversities are extremely low 3 the dominant genera are ammodiscus and trochammina 4 the species are generally of small size compared to usual dimensions within the genera the features listed above suggest that the assemblages were adapted to restricted conditions clearly divergent from those of a normal marine shelf where the main limiting factors were low salinity and reduced amount of dissolved oxygen in unstable storm influenced environments evidence for environmental conditions is obtained from modern analogues although the large evolutionary changes in foraminifera during post jurassic time make it difficult to find such analogues additional information is derived from functional morphology sedimentary features and paleogeography the analyzed sediment packages show close faunal similarities suggesting opening of a marine pathway which connected the paleo arctic ocean with the western european shelf seas in early jurassic a depositional biofacies model of the small sized ammodiscus trochammina assemblages envisages a delta influenced shelf environment where high freshwater influx would have created a density stratified water column with a tendency to develop hypoxic conditions in its deeper parts the depth interval between fair weather and storm wave base the offshore transition zone is indicated as the habitat of the small sized ammodiscus trochammina assemblages in this zone benthic biota would have been stressed by intermittent periods with moderate hypoxia combined with lowered salinity and storm impacts

mean sea level and beach width variation at scarborough western australia abstract a sixteen year record of beach width change at six profile stations on scarborough beach in the perth metropolitan area of western australia was analysed by eliot and colleagues their results described long term beach progradation measured over the sixteen years and identified cyclic beach changes with periods of 0 5 1 0 3 5 and 7 0 yrs and a long term secular progradational trend in this paper the beach width measurements from scarborough are compared with twelve years of mean sea level record from fremantle western australia approximately 18 km south of scarborough beach the annual cycles of each respond in an inverse manner such that a 1 cm rise in mean sea level corresponds with a 1 m decrease in the beach width the sea level rise leads the beach width retreat by approximately one fifth of a cycle a long period oscillation of 3 5 yrs in the mean sea level shows a similar 100 1 ratio for beach gradation with a lead of approximately one tenth of a cycle however the secular trend in beach width change was not evident in the mean sea level record indicating that the progradation is tied to other processes such as a northerly sediment drift related to prevailing sw swell and associated current activity

inside the head and tail of a turbulent gravity current gravity currents are an important buoyancy driven flow in environmental geophysical and industrial settings turbulence and mixing is commonplace in these flows but is typically overlooked in theoretical models and predictions sher woods j fluid mech vol 784 2015 pp 130 162 have quantified the velocity and density structure in turbulent gravity currents by combining high quality experimental data with new theory their insights are set to stimulate significant advances in the area

efficiency of eigenfunctions for discriminant analysis of subaerial non tidal beach profiles abstract a one year set of subaerial beach profiles from the southern mediterranean beaches of israel measured at fortnightly intervals was analysed by applying eigenfunctions and computations of scour and fill the eigenfunction associated with the first eigenvalue is shown not to be efficient for beach categorization however the second eigenfunction effectively classified three subenvironments of the exposed beaches these constitute the upper foreshore the stable segment and the upper backshore a model of subaerial beach types based on the second eigenfunction is confirmed by scour and fill indices and by morphological characteristics

analysis and direct numerical simulation of the flow at a gravity current head part 1 flow topology and front speed for slip and no slip boundaries direct numerical simulations are performed of gravity current fronts in the lock exchange configuration the case of small density differences is considered where the boussinesq approximations can be adopted the key objective of the investigation is a detailed analysis of the flow structure at the foremost part of the front where no previous high resolution data were available for the simulations high order numerical methods are used based on spectral and spectral element discretizations and compact finite differences a three dimensional simulation is conducted of a front spreading along a no slip boundary at a reynolds number of about 750 the simulation exhibits all features typically observed in experimental flows near the gravity current head including the lobe and cleft structure at the leading edge the results reveal that the flow topology at the head differs from what has been assumed previously in that the foremost point is not a stagnation point in a translating system rather the stagnation point is located below and slightly behind the foremost point in the vicinity of the wall the relevance of this finding for the mechanism behind the lobe and cleft instability is discussed in order to explore the high reynolds number regime and to assess potential reynolds number effects two dimensional simulations are conducted for reynolds numbers up to about 30 000 for both no slip and slip i e shear stress free boundaries it is shown that although quantitative reynolds number effects persist over the whole range examined no qualitative changes in the flow structure at the head can be observed a comparison of the two dimensional results with laboratory data and the three dimensional simulation provides evidence that a two dimensional model is able to capture essential features of the flow at the head the simulations also show that for the free slip case the shape of the head agrees closely with the classical inviscid theory of benjamin

large scale laboratory investigation into the effect of varying back barrier lagoon water levels on gravel beach morphology and swash zone sediment transport abstract it is generally accepted that the effect on beach morphology of the interactions between swash motion and the beach groundwater table depends on the elevation of the latter relative to that of the sea specifically a low beach groundwater table is considered to enhance onshore sediment transport and beach accretion while the reverse is thought to hold for a high beach groundwater table here we use the results of a carefully designed series of tests conducted in the delta flume the netherlands as part of the barrier dynamics experiment bardex to unequivocally demonstrate the validity of this assertion during the experiment a 4 m high and 50 m long gravel barrier d 50 11 mm was constructed within the central region of the 250 m long and 7 m deep wave flume enabling a back barrier lagoon to be located at its landward side the groundwater table in the beach was manipulated by lowering and raising the lagoon level and the gravel beach was subjected to variable wave conditions and sea lagoon levels using as the initial beach profile the morphology resulting from 90 min of wave forcing with the sea and lagoon at the same level the identical wave forcing with a low lagoon level consistently resulted in onshore sediment transport in the swash zone and beach accretion whereas the identical wave forcing with a high lagoon level consistently caused offshore sediment transport in the swash zone and beach erosion the divergent beach morphological responses for the low and high lagoon levels could not be attributed to changes in the bulk swash hydrodynamics i e based on single point velocity measurements arising from a difference in the amount of swash infiltration into the unsaturated beach instead application of a sediment transport model that accounts for the effects of in exfiltration suggested that the onshore sediment transport and beach accretion for the low lagoon tests mainly resulted from enhanced bed shear stresses during the wave uprush due to infiltration into the saturated part of the beachface due to the large hydraulic conductivity of gravel k o 0 1 m s 1 the effect of altered shear stresses due to in exfiltration may be an important factor in sediment transport in the swash zone of gravel beaches

temporal changes in foraminiferal faunas and impact of pollution in sandebukta oslo fjord abstract sandebukta oslo fjord norway is an inlet with depths up to 70 m characterised by estuarine water stratification an influx of pollutants and a mudflat in the inner part the distribution of its foraminiferal fauna displays essentially an estuarine pattern but also reveals significant subrecent changes as demonstrated by means of 9 sediment cores supplemented by a larger number of sea bed surface samples the faunal changes have taken place during the last 100 years and are summarized as follows 1 upwards increase in size of the total population shown by the sediment cores 2 reduction in the relative frequency of calcareous foraminifera upwards in the cores so that the recent fauna of the inlet consists mainly of arenaceous species 3 alterations in the species composition of populations most pronounced in deeper waters faunal comparisons with outer parts of the fjord system suggest that these changes are at least partially induced by pollution originating from human activity in addition post mortem destruction of foraminiferal tests is considered as a possibly important modifying factor requiring further study an important additional reason for the faunal turnover found in the mudflat seems to be successive shallowing during progradation

marine shelf to paralic biofacies of upper triassic to lower jurassic deposits in spitsbergen abstract the upper triassic to lower jurassic kapp toscana group of spitsbergen consists of mudstones sandstones and shales deposited in offshore marine to paralic conditions in an extensive shelf embayment the study focuses on microfossil based biofacies features mainly foraminifera but also palynomorphs combined with sedimentary and selected geochemical data in relation to transgressive regressive developments the analysed material includes field logs and sediment samples originating from the juvdalskampen section central spitsbergen the benthic foraminiferal succession reveals three principal associations 1 assemblages of mixed agglutinated and calcareous components typified by medium to high diversities indicating offshore shelf conditions 2 assemblages consisting dominantly or entirely of agglutinated forms with low diversity attributed to hyposaline conditions 3 highly impoverished agglutinated assemblages or barren samples suggesting extremely brackish to freshwater environments the marine influence is highest in the lower and upper parts and lowest in the middle part of the kapp toscana group as indicated both by foraminiferal and palynomorph proxies the potential of the boron concentration as a salinity indicator is demonstrated by its co variance with biofacies features the boron content increases with increasing salinity the kapp toscana group consists of two depositional sequences the storfjorden sequence carnian to early norian starts with a transgressive systems tract formed in offshore shelf conditions with normal marine salinity and terminates at the maximum flooding event which is succeeded by a highstand systems tract deposited in paralic hyposaline to freshwater prodelta to delta plain environments the overlying wilhelmoya sequence middle norian to bajocian is punctuated by several hiatuses and was deposited in hyposaline waters on a sediment starved prodelta shelf platform

beach groundwater dynamics abstract an understanding of the interaction between surface and groundwater flows in the swash zone is necessary to understand beach profile evolution coastal researchers have recognized the importance of beach watertable and swash interaction to accretion and erosion above the still water level swl but the exact nature of the relationship between swash flows beach watertable flow and cross shore sediment transport is not fully understood this paper reviews research on beach groundwater dynamics and identifies research questions which will need to be answered before swash zone sediment transport can be successfully modelled after defining the principal terms relating to beach groundwater the behavior measurement and modelling of beach groundwater dynamics is described research questions related to the mechanisms of surface subsurface flow interaction are reviewed particularly infiltration exfiltration and fluidisation the implications of these mechanisms for sediment transport are discussed

variation in subaerial beach sediment volume on a small sandy beach over a monthly lunar tidal cycle abstract three major patterns of sediment movement along 0 5 m deep horizontal segments of beachface on a small sandy beach at coledale new south wales were determined by empirical orthogonal function eof analysis of a 36 day record of beach change the fundamental beach response is identified by the first eigenfunction mode the mode describes sediment transfers occurring over the beachface between the low tide zone and the berm crest as well as sediment movement along the beach the alongshore translocations occurred about a nodal point located approximately 0 75 of distance along the beach from its southern end transfers across the beach were established by comparing the time series associated with the eigenfunctions describing alongshore transfers of sediment in different segments the slice to slice comparisons showed an increasing phase down the beachface from the berm crest segment to the mid tidal zone the phase difference was approximately 2 5 days per 0 5 m segment the fundamental beach response and secondary sediment transfers typical of the supratidal zone occur with an apparent 28 30 day lunar tide cycle they were unrelated to short term variations in the incident wave amplitude lower down the beachface the time series associated with the eigenfunction describing sediment translocations in the mid tidal zone is aperiodic the irregularities can be linked to particular process events however similar beach changes were not always related to similar process conditions and they were not simply related to wave regime fluctuations the patterns of sediment movement along coledale beach were consistent with similar patterns reported from elsewhere by clarke and eliot 1982 1983a in that they were closely related to variation in the nearshore water circulation system in particular nodal points identified by the eof analysis commonly occurred on the beachface landward of inshore regions where bars most frequently developed the antinodal positions were similarly associated with regions where rip channels are frequently formed

the intrusion of fluid mechanics into geology during the last few years a number of fluid dynamicists often in collaboration with geologists have been investigating problems suggested by geology and creating the new field of geological fluid mechanics the research has uncovered many new fluid mechanical phenomena and the studies have ranged widely over theoretical contributions laboratory experiments and confirmatory field observations this paper presents an overview of part of the field with a strong emphasis on the novel fluid mechanical concepts the presentation includes discussion of some of the effects of cooling and crystallization and the resulting convective motions the flow of gravity currents at both high and low reynolds number and the erosion of solid retaining walls by hot turbulent flows

the vertical structure of new england coastal fronts abstract aircraft data from the new england winter storms experiment newsex are used to examine the vertical structure of four new england coastal fronts the aircraft made multiple passes at varying elevations through the coastal fronts the observations were projected onto cross sections normal to the coastal fronts using frontal orientations determined from surface analyses and frontal velocities determined from a least squares fit to multiple frontal penetrations the frontal zones were found to be roughly 200 m wide rising sharply from the ground to a height of 150 m to 400 m over distances of 0 5 to 2 5 km before leveling off and rising gradually toward the mountains the underlying pools of cold air were similar in form and flow patterns to cold air damming structures observed elsewhere along the east coast the coastal fronts formed the leading edges of the cold pools air on the warm side of the fronts was neutrally stratified having been recently heated by the warm gulf of maine this warm a

a numerical study on sea land breezes as a gravity current kelvin helmholtz billows and inland penetration of the sea breeze front abstract a two dimensional nonhydrostatic compressible and dry numerical model was developed by using high spatial resolution and an appropriate turbulence model fine structure and dynamics of the sea breeze head were investigated kelvin helmholtz kh instability was found to occur in the foremost part of the head consequently kh billows were generated there and grew in amplitude while travelling backward relative to the advancing front along the zero velocity boundary the kh billows entrained the upper air into the sea breeze the resultant turbulent mixing and wave perturbations induced a top friction force acting on the sea breeze head structure of the kh billow and the wavelike motion induced by it near the ground were also investigated and compared with the results obtained in laboratory density currents linear theories and observations the kh instability khi did not occur at all stages of the sea breeze and this resulted in a diurnal variation of the fine structure and dynamics of

short term environmental changes in the cretaceous tethyan ocean micropalaeontological evidence from the early albian oceanic anoxic event 1b to better understand the linkage between climate and ocean circulation under greenhouse conditions we have studied calcareous nannofossils palynomorphs and benthic foraminifera from the early albian oceanic anoxic event 1b black shale oae 1b in the vocontian basin se france we propose that monsoonal activity resulting from precessional forcing and modulated by eccentricity driven temperature changes represents the driving factor of oae 1b formation in low latitudes with the onset of oae 1b increasingly warm and humid conditions stronger winds and enhanced terrestrial input led to higher surface water productivity therefore increased productivity is an important factor for oae 1b formation in the vocontian basin however monsoonally forced productivity changes are a regional climate signal only the supraregional occurrence of the oae 1b is probably due to a reduction of deep water formation in the low latitudes under extremely warm and humid conditions leading to enhanced preservation of organic matter

relationship of benthic foraminiferal diversity to paleoproductivity in the neogene caribbean abstract diversity trends 8 3 2 5 ma in caribbean deep sea foraminifera as interpreted from the indices fisher s and the information index h s parallel paleoproductivity proxies benthic foraminiferal infaunal epifaunal species ratio benthic foraminifer accumulation rates and flux of organic carbon to the seafloor paleoproductivity never reached a eutrophic threshold value above which we would predict opposite trends of high paleoproductivity and low diversity consistent with stressful conditions instead results are similar to those from other studies of oligotrophic settings that show a positive and statistically significant correlation between paleoproductivity and diversity the correlations between current intensity and watermass oxygenation with diversity are negative and statistically significant these results are also borne out by principal component analysis pca intervals of increased relative abundance of epistominella exigua a proxy for seasonality of phytodetrital input to the seafloor coincide with increased diversity suggesting that pulsed paleoproductivity enhanced the diversity signal in the caribbean diversity and paleoproductivity peaked in the caribbean at 7 9 ma and declined thereafter due to the cut off of the nutrient rich pacific deep waters

biostratigraphic implications of the maastrichtian lower eocene sequence at the north gunna section farafra oasis western desert egypt abstract eleven planktic foraminiferal zones are established in the maastrichtian lower eocene sediments at the north gunna section farafra oasis western desert egypt the study section is composed of five rock units the khoman dakhla tarawan esna and farafra formations a hiatus at the k t boundary including the upper maastrichtian lower paleocene abathomphalus mayaroensis parasubbotina pseudobulloides zones has been detected moreover most of the esna formation in the study area is of early eocene age m subbotinae a pentacamerata zones rather than late paleocene as commonly observed in the neighbouring areas the zonal scheme developed here is correlated with other sequences and the p zonation of blow 1979 and berggren and miller 1988 are discussed

tail structure and bed friction velocity distribution of gravity currents propagating over an array of obstacles the bed friction velocity distribution and sediment entrainment potential of boussinesq compositional gravity currents propagating over a series of obstacles and over a smooth surface respectively are analysed based on high resolution three dimensional large eddy simulations the investigation focuses on the parameter regime for which currents with a high volume of release go through an extended slumping phase with approximately constant front velocity tokyay constantinescu meiburg j fluid mech vol 672 2011 570 605 under these conditions a quasi steady regime is reached between consecutive obstacles that is similar to the steady regime observed for constant density channel flows over bottom obstacles at a given location this quasi steady regime is reached in the tail of the current after the passage of the front and the associated hydraulic jumps reflected from the first few downstream obstacles a double averaging procedure is employed to characterize the global changes in the structure of the tail region between currents with a high volume of release propagating over smooth surfaces and over obstacles reynolds number induced scale effects on the flow and turbulence structure within the tail region are discussed in some detail the presence of this quasi steady regime is significant since the simulations with obstacles show that most of the sediment is entrained by the tail of the current rather than by its front a detailed analysis of the effects of the obstacle shape on the quasi steady mean flow and turbulence structure is presented which provides insight into why gravity currents over dunes can entrain more sediment than gravity currents over ribs of comparable size finally the bed friction velocity distributions and the potential to entrain sediment are compared for a compositional current with a high volume of release during the slumping phase and a current with a low volume of release for which transition to the buoyancy inertia phase occurs a short time after the release of the lock gate

benthic foraminiferal response to relative sea level changes in the maastrichtian danian succession at the dakhla oasis western desert egypt the maastrichtian danian benthic foraminiferal diversity and assemblages through sequence stratigraphy were studied at dakhla oasis egypt benthic foraminifera numbers bfn high flux species and characteristic benthic foraminiferal species and genera distribution are also incorporated to assess palaeobathymetry palaeoenvironment and palaeoproductivity all these proxies are then taken together to construct a sea level curve and interpreted in terms of regional tectonics climate and eustasy data suggest a remarkably highly equitable benthic environment deposited in a brackish littoral and or marsh setting with moderate to low oxygen conditions and reduced salinity oligotrophic possibly due to increased precipitation and terrestrial runoff the interrupted dominance of calcareous forms and high organic flux species suggests occasional marine incursions and high palaeoproductivity due to local upwelling the inferred sea level curve replicates the global eustatic curve and suggests that the curve is more influenced by the prevailing climate and global eustasy rather than by regional tectonics the post cretaceous palaeogene boundary displays improvement in the environment in terms of diversity and number of species and specimens with a marked reduction in the abundance of high organic flux species during early paleocene danian time indicating a shift from a more mesotrophic open marine environment to much reduced oligotrophic conditions

the campanian paleoenvironment inferences based on benthic foraminifera from northeastern iran abstract the benthic foraminifera from the campanian rocks globotruncanita elevata to globotruncana aegyptiaca zones exposed in the kopeh dagh basin northeast iran are studied quantitatively to reconstruct the campanian paleoenvironment based on unconstrained heriarchial clustering nonmetric multidimensional scaling nmds and analysis of similarity anosim three statistically significant benthic foraminiferal assemblages are noted paralabamina lunata lower and upper haplophragmoides spp and gyroidinoides nitida based on benthic foraminiferal marker species the following paleodepths are assigned lower p lunata upper bathyal upper p lunata upper middle bathyal boundary haplophragmoides spp and g nitida outer neritic the p lunata assemblage can be further subdivided into a lower coryphostoma incrassata dominated interval whereas the upper interval is p lunata dominated and reflects a change in the quality of food the upper experienced higher input of fresh phytodetritus the overall p lunata assemblage reflects a stable bottom water environment the haplophragmoides spp assemblage is characterized by reduced paleoproductivity and environmental instability whereas the g nitida assemblage is marked by increasing oligotrophy within a well oxygenated bottom water and ventilated basinal conditions the partial least squares regression yielded significant correlations with planktic positive proxy for sea level and agglutinated species negative proxy for shallow depths and low organic flux the reduced major axis regression models also yielded significant negative correlations with agglutinated and infaunal species nutrient availability suggesting that these factors were responsible for the inferred benthic foraminiferal distribution patterns two globally comparable shallowing trends are also noted one in the middle of the r calcarata zone haplophragmoides spp assemblage and the other in the g aegyptiaca zone gyroidinoides nitida assemblage that coincide with kca6 and kca7 sequence boundaries and lowstands respectively for the kca6 event the occurrence of spiroplectammina haplophragmoides spp assemblage represents a shallow water outer neritic fauna invasion event within a deeper upper bathyal setting paralabamina lunata assemblage and for the kca7 event a similar shallowing from upper middle bathyal boundary to outer neritic depths is noted g nitida assemblage and attributed to eustasy and basinal paleohighs and lows

end cenomanian benthic foraminiferal extinctions and oceanic dysoxic events in the northwestern pacific ocean abstract the second largest cretaceous extinction event coincided with an oceanic anoxic event at the cenomanian turonian c t boundary this high resolution analysis of upper bathyal benthic foraminifera in central hokkaido japan indicates that the extinction event occurred in the latest cenomanian over a period of 0 2 m y 93 6 93 4 ma analysis of the dissolved oxygen index based on calcareous benthic foraminiferal wall thickness size and morphology shows that low oxygen 0 1 1 5 ml l conditions developed during a period of 0 15 m y before the c t boundary and 1 5 m y after the minimum oxygen condition 0 1 1 ml l prevailed twice during this interval similarly benthic foraminiferal extinctions occurred twice in concert with initiation of the two minimum oxygen levels coincidence of the two extinction horizons with developemt of low oxygen conditions suggests that the main cause of the extinctions was formation of dysoxic intermediate water this report suggests that the global c t boundary extinction event occurred in a stepwise fashion during the latest cenomanian 0 2 0 5 m y due to development of widespread oceanic anoxic dysoxic conditions within intermediate water masses