secondary circulation in a region of flow curvature relationship with tidal forcing and river discharge 1 on the basis of a 301 day acoustic doppler current profiler mooring in an estuarine tidal strait the strength and structure of secondary circulation in a region of flow curvature is related to variations in tidal forcing and river discharge during low flow conditions the structure of secondary flow is consistent with a centrifugally forced helical flow with bottom flow toward the inside of the bend and surface flow toward the outside of the bend the strength of secondary flow increases linearly with tidal range and is consistent with a vertical eddy viscosity that is linearly dependent on tidal current speed during times of high river discharge the strength of secondary flow is significantly reduced and its vertical structure undergoes a fundamental change over the spring neap cycle during spring tides the classic helical flow pattern is evident albeit weaker than during low flow conditions however during neap tides a more complex two cell structure is evident the change between these two states occurs with a spring neap transition in the subtidal flow indicating that it is also accompanied by changing stratification simple scaling analysis suggests that during weakly stratified conditions secondary circulation will influence stream wise dynamics and dispersion for channels with widths on order or less than 0 1 h cd where h is the water column depth and cd is a quadratic bottom drag coefficient in contrast during highly stratified conditions lateral excursions due to secondary flows are limited to approximately one tenth of the channel s width and are an ineffective lateral mixing agent

constraints on hydraulic parameters and implications for groundwater flux across the upland estuary interface abstract analyses of independent laboratory and field scale measurements from two sites on sapelo island georgia reveal heterogeneity in hydraulic parameters across the upland estuary interface regardless of the method used short duration pumping tests amplitude attenuation of tidal pumping data sediment grain size distributions and falling head permeameter tests we obtain hydraulic conductivity of 10 4 m s 1 for the fine grained well sorted clean sands that make up the upland areas proximal to the upland estuary boundary the tidal pumping analyses and permeameter tests suggest that hydraulic conductivities decrease by more than two orders of magnitude a result consistent with the presence of a clogging layer such a clogging layer may arise due to a variety of physical chemical or biological processes the extent and orientation of the layers of reduced hydraulic conductivity near the upland estuary boundary influence the nature of the aquifer s response to tidal forcing where the lower conductivity layer forms a relatively flat creek bank tidal pumping produces a primarily mechanical response in the adjacent aquifer where the creek bank is nearly vertical there is a more direct hydraulic connection between the tidal creek and the adjacent aquifer the clogging layer likely contributes to the development of complicated flow pathways across the upland estuary boundary effective flow paths calculated from tidal pumping data terminate within the marsh beyond the boundary of the upland aquifer suggesting a diffuse regime of groundwater discharge in the marsh we postulate that in many settings submarsh flow may be as important as seepage faces for groundwater discharge into the marsh estuary complex

analytical solution of a model of contaminant transport in the advective zone of a river abstractan analytical solution of a model of contaminant transport in the advective zone of rivers is presented and evaluated an existing model that accounts for transport in the advective zone by dividing the channel into flowing and stagnant zones is solved with laplace transforms for the case of a gaussian pulse injected into the center of the channel the effects of the two main parameters of the model the fraction of the channel occupied by the stagnant zone and a transfer coefficient are consistent with expectations from the theory of shear dispersion another parameter which is related to the streamwise width of the initial pulse determines whether a separate pulse appears in the tracer response curves a procedure for determining the parameters from temporal moments of measured concentration curves is described and applied to measurements in the advective zone of a mountain stream predictions from the model of reichert and wanner fit the measurements especially the peak concentration better

the crystalline units of the high himalayas in the lahul zanskar region northwest india metamorphic tectonic history and geochronology of the collided and imbricated indian plate in the high himalayan belt of northwest india crustal thickening linked to palaeogene collision between india and eurasia has led to the formation of two main crystalline tectonic units separated by the syn metamorphic miyar thrust the high himalayan crystallines sensu stricto hhc at the bottom and the kade unit at the top these units are structurally interposed between the underlying lesser himalaya and the very low grade sediments of the tibetan nappes they consist of paragneisses orthogneisses minor metabasics and chiefly in the hhc leucogranites the hhc registers a polyphase metamorphism with two main stages designated as m1 and m2 a metamorphic zonation with high temperature recrystallization and migmatization at middle structural levels and medium temperature assemblages at upper and lower levels in contrast the kade unit underwent a low temperature metamorphism rb sr and u th pb isotope data point to derivation of the orthogneisses from early palaeozoic granitoids while the leucogranites formed by anatexis of the hhc rocks and were probably emplaced during miocene time most of the complicated metamorphic setting is related to polyphase tectonic stacking of the hhc with the cooler kade unit and lesser himalaya during the himalayan history however a few inconsistencies exist for a purely himalayan age of some ml assemblages of the hhc as regards the crustal derived leucogranites the formation of a first generation mixed with quartzo feldspathic leucosomes was possibly linked to melt lubricated shear zones which favoured rapid crustal displacements at upper levels they intruded during stage m2 and the latest movements along the syn metamorphic miyar thrust but before juxtaposition of the tibetan nappes along the late metamorphic zanskar fault

observations of stratified turbulent mixing in an estuary neap to spring variations during high river flow abstract observations of turbulent mixing stratification and currents were made with a free falling microstructure profiler and a 600 khz adcp in the hudson river off manhattan in may 1994 the ship was anchored in a relatively straight and uniform channel about 15 m deep over 4 days of intense observations from 19 to 24 may 1994 the progression from neap tides to spring tides caused a severe reduction in stratification from an initial top to bottom salinity difference of 18 to 3 6 while low gradient richardson numbers ri occurred only in the weakly stratified bottom layer on flood during neap tides low ri spanned the water column on the second half of ebb during spring tides turbulent dissipation rates varied roughly inversely to ri during neap conditions turbulent mixing was intense only in the bottom layer on flood where eddy diffusivities k reached more than 10 2 m 2 s 1 while mixing was weak in the centre of the halocline near 5 m during spring tides strong mixing occurred throughout the water column during the latter part of ebb with typical k 5 10 3 m 2 s 1 and typical vertical salt flux of 0 0015 kg m 2 s 1 if it existed in isolation this flux would eliminate the haline stratification on a time scale of 4 h ebb time bands of low ri and intense mixing showed increasing phase lag with increasing distance from the bottom the spring tide ebb time internal turbulent stress was relatively uniform with depth with a typical magnitude of 0 4 pa 4 dyne cm 2 turbulent overturning thorpe scales ranged from below 1 cm in the neap tide halocline to a few metres in the weakly stratified bottom layer on flood the strong spring tide ebb time mixing had thorpe scales of typically 0 5 m throughout the water column ozmidov scales tracked thorpe scales except in the weakly stratified layer above the bottom

air content paleo record in the vostok ice core antarctica a mixed record of climatic and glaciological parameters under present day climatic conditions the air content of ice shows a high sensitivity to the atmospheric pressure and hence to the elevation at the surface of the ice sheet this observation has been used to infer past ice sheet thickness variations of antarctica and greenland a high resolution air content profile more than 1000 measurements covering approximately the last 200 000 years was obtained along the 2546 m long vostok ice core three analytical techniques were used leading to consistent results which show large amplitude and rapid air content variations the vostok results support thicker thinner ice in the central part of east antarctica during warm cold periods

evolution of the himalaya abstract the compression and attendant deformation of a thick and vast sedimentary prism formed since early riphean times on the northern continental margin of the indian craton gave rise to the himalaya mountains as a result of convergence and collision of the indian and asian plates the oceanic trench sediments tectonically implanted with sea floor material and intimately associated with calc al kaline volcanics in the narrow sindhu tsangpo belt extending from kohistan through dras leh darchen mansarovar to shigatse and beyond represent the subduction island arc complex which developed south of the dynamic southern margin of the asian continent and was welded to the colliding indian plate during the late eocene to oligocene period this complex is fringed to the north by a wide zone of andean type granitic bodies the evolution of the himalayan orogen is closely connected with the development of the present day andaman nicobar indonesia island arc subduction system in the southeast and the makran ranges oman trench in the southwest the evolution of the himalaya was accomplished in four major phases of tectonic upheaval during the late cretaceous to palaeocene karakoram phase late eocene to oligocene malla johar phase middle miocene to pontian sirmurian phase and late pliocene to middle pleistocene siwalik phase while the karakoram phase marks the convergence of continents and the malla johar phase represents the collision and subduction it was during the sirmurian upheaval that the main tectonic features developed and the himalaya acquired its distinctive structural complexion

a model for low pressure facies metamorphism during crustal thickening low pressure metamorphic facies i e high t p ratios are widespread in a wide range of tectonic settings explanations offered for the occurrence of these facies include extensional and or magmatic models however these fail to explain that the low p facies metamorphism is commonly coeval with a phase of pervasive crustal thickening with t p ratios increasing during or slightly lagging behind the thickening we propose an alternative explanation based on the approximate synchroneity of crustal thickening and erosion thinning of the mantle lithosphere

the effect of dead zones on longitudinal dispersion in streams on montre que la presence d une distribution aleatoire de zones d eau morte peut etre traitee par la methode des moments en choisissant une moyenne composite appropriee application a l etude de la dispersion d un contaminant dans un ecoulement de couette en conduite et dans des ecoulements de poiseuille

orbital tuning chronology for the vostok climate record supported by trapped gas composition abstract we present data on the o2 n2 ratios of trapped gas samples for the entire length of the vostok climate record as in other cores o2 n2 ratios in these samples are less than the atmospheric ratio by a small and variable amount because o2 is selectively excluded during the gas trapping process and because o2 is also preferentially lost in poorly preserved core samples samples younger than 150 ka have large and variable o2 depletions samples older than 200 ka have o2 n2 ratios that replicate well and vary smoothly with depth we plot o2 n2 ratios of well replicated samples older than 160 ka using a chronology derived by matching the 18o of paleoatmospheric o2 18oatm to northern hemisphere june insolation on this timescale o2 n2 varies coherently with local 78 s summertime insolation based on time series analysis of the o2 n2 record and the dynamics of snow metamorphism at the surface we conclude that summertime insolation influences physical properties of ice grains that control the degree of o2 exclusion during bubble closeoff o2 n2 in vostok is thus arguably a property that records local summertime insolation and can be used to test independent chronologies for the core we show that the 18oatm chronology supported by the coincidence of o2 n2 ratios with insolation is also compatible with recent radiometric dating of corals from high sea stands we further successfully test the 18oatm tuning chronology by showing that it predicts a chronology for the gisp2 core which is essentially indistinguishable from the standard gisp2 chronology and therefore in excellent agreement with the radiometric chronology of hulu cave china an accurate chronology for the vostok ice core is now in place

mesoscopic and magnetic fabrics in arcuate igneous bodies an example from the mandi karsog pluton himachal lesser himalaya field microstructural and anisotropy of magnetic susceptibility ams data from the palaeozoic mandi karsog pluton in the lesser himalayan region reveal a concordant relationship between fabric of the proterozoic host rock and the granite the pluton displays a prominent arcuate shape on the geological map the margin parallel mesoscopic and magnetic fabrics of the granite and warping of the host rock fabric around the pluton indicate that this regional curvature is either synchronous or pre dates the emplacement of the granite body mesoscopic fabric magnetic fabric and microstructures indicate that the northern part of the pluton preserves its pre himalayan magmatic fabric while the central and southern part shows tectonic fabric related to the tertiary himalayan orogeny the presence of nw se trending aplitic veins within the granite indicates a post emplacement stretching in the ne sw direction shear sense indicators in the mylonites along the margin of the pluton suggest top to the sw shearing related to the himalayan orogeny based on these observations it is envisaged that the extension that gave rise to this rift related magmatism had a ne sw trend that is normal to the trend of the aplite veins subsequently during the himalayan orogeny compression occurred along this same ne sw orientation these findings imply that the regional curvature present in the himachal lesser himalaya is in fact a pre himalayan feature and the pluton has formed by filling a major pre himalayan arcuate extension fracture

u th pb systematics and ages of himalayan leucogranites south tibet abstract the age and origin of five leucogranites from the high and tethys himalaya and two country rock gneisses were investigated by u pb dating of zircon fractions and single grains and fractions of monazite additionally th u concentrations in whole rock powders and isotopic compositions of pb in leached k feldspars were determined monazites yield ages of 16 8 0 6 m y for the nialam migmatite granite 15 1 0 5 m y for the lhagoi kangri granite 14 3 0 6 m y for a granite from mt everest and 9 8 0 7 m y and 9 2 0 9 m y for two varieties of the maja granite these data together with monazite ages of 21 9 0 2 and 24 0 0 4 m y determined earlier on the makalu granite 1 substantiate a period of intracontinental granite emplacements from 24 to 9 m y ago i e from uppermost oligocene to late miocene times such a period of plutonic activity is consistent with the view that all these granites result from intracrustal melting following the collision of india with eurasia furthermore the individual ages together with structural relationships between granites and country rocks suggest that granite formation and tectono metamorphism occurred as alternating and strongly related processes with a periodicity of 7 to 9 m y inherited lead components present in all granite zircons point to large proportions of precambrian material in the magma source regions up to 2200 m y old th u systematics between monazite and country rocks indicate that u has been leached from most of the granites after crystallisation of monazite zircon dating of the kangmar granite gneiss which occurs in a window through the tethys himalayan sediments shows that this pluton transformed to a gneiss during the alpine orogeny crystallised in lowermost palaeozoic times 562 4 m y ago

rate controlling processes in the creep of polar ice influence of grain boundary migration associated with recrystallization abstract information on the deformation modes and recrystallization processes in polar ice sheets was obtained thanks to the analysis of the ice structure along two deep ice cores from greenland and antarctica it is shown that the deformation of polar ice at low stresses is produced by intracrystalline slip accommodated by grain boundary migration gbm a deformation model based on an equilibrium between work hardening and recovery processes has been developed the decrease in dislocation density is due to both gbm associated with grain growth and the formation of boundaries by recrystallization the value of the dislocation density along the byrd and grip ice cores obtained thanks to the model is about 1 10 11 m 2 this value is in agreement with data from synchrotron x ray diffraction on samples taken along the core this deformation model can account for the deformation of polar ice at low stresses it is shown that the flow law with a stress exponent lower than 2 can be related to the efficiency of gbm as a recovery process

modification of fabric in pre himalayan granitic rocks by post emplacement ductile deformation insights from microstructures ams and u pb geochronology of the paleozoic kinnaur kailash granite and associated cenozoic leucogranites of the south tibetan detachment zone himachal high himalaya the present day south tibetan detachment std of higher himalaya is a system of low angle normal faults in the himachal high himalaya the std hanging wall is characterized by the presence of s type per aluminous paleozoic 475 ma granite called the kinnaur kailash granite kkg this granite is later intruded by cenozoic leucogranites 18 ma in vicinity of the std zone in this work microstructures anisotropy of magnetic susceptibility ams and u pb geochronology were carried out on the kkg and the leucogranites with an aim to a understand the conditions of fabric development and b decipher the tectonic relationship between deformation along the std and the evolution of these granites microstructural features and magnetic anisotropy indicate that the granites are intensely deformed in vicinity of the std and preserve their emplacement related fabric in the interior parts it is inferred that close to the std zone fabrics of both the kkg and the leucogranite are tectonic and are modified by the cenozoic 20 ma right lateral slip and extensional tectonics magnetic fabric in the interior parts of the kkg is related to its emplacement indicating that original fabric was preserved u pb geochronology of zircons from two samples of the kkg yields crystallization age of 477 6 3 4 and 472 4 ma the leucogranite gives a crystallization age of 18 5 0 6 ma zircons from the kkg also reveal signatures of a deformation event 20 6 2 3 ma at its rim it is inferred that deformation of the external rim of the kkg and crystallization of the leucogranites are synchronous and triggered by ductile deformation along the std

zircon u pb ages and lu hf isotopes of metagranitoids from the subansiri region eastern himalaya implications for crustal evolution along the northern indian passive margin in the early paleozoic

a new device for the measurement of air content in polar ice a new device for measuring air content in polar ice has been designed built and tested with 22 samples of antarctic ice the new technique is based on the barometric method which implies 1 air extraction under vacuum by melting refreezing of the ice sample placed in a calibrated cell and 2 accurate air pressure and temperature measurements the new apparatus simplifies the experimental procedures decreases the duration of measurement and can eventually be used in the field it provides results with an accuracy equivalent to or better than other methods previously used

cycle of vertical and horizontal mixing in a shallow tidal creek 1 an experimental study of vertical mixing and along channel dispersion parameterized in terms of horizontal mixing near the mouth of the duplin river a tidal creek bordered by extensive intertidal salt marshes on sapelo island georgia was carried out over several spring neap cycles in the fall of 2005 vertical mixing is modulated on both m4 and fortnightly frequencies with maximum turbulent stresses being generated near the bed on periods of maximum flood and ebb and propagating into the water column showing a linear dependence with depth values are significantly greater on spring tide than on neap horizontal mixing evaluated by salt fluxes is driven and dominated by tidal dispersion which is also modulated by the fortnightly spring neap cycle net export of salt from the lower duplin is shown to be due to residual advection modified by upstream tidal pumping the tidal dispersion coefficient exhibits a pulsating character with greater values on spring tide followed by smaller values on neap tide

estuarine boundary layer mixing processes insights from dye experiments a series of dye releases in the hudson river estuary elucidated diapycnal mixing rates and temporal variability over tidal and fortnightly time scales dye was injected in the bottom boundary layer for each of four releases during different phases of the tide and of the spring neap cycle diapycnal mixing occurs primarily through entrainment that is driven by shear production in the bottom boundary layer on flood the dye extended vertically through the bottom mixed layer and its concentration decreased abruptly near the base of the pycnocline usually at a height corresponding to a velocity maximum boundary layer growth is consistent with a one dimensional stress driven entrainment model a model was developed for the vertical structure of the vertical eddy viscosity in the flood tide boundary layer that is proportional to u 2 n where u and n are the bottom friction velocity and buoyancy frequency above the boundary layer the model also predicts that the buoyancy flux averaged over the bottom boundary layer is equal to 0 06n u 2 or based on the structure of the boundary layer equal to 0 1nblu 2 where nbl is the buoyancy frequency across the flood tide boundary layer estimates of shear production and buoyancy flux indicate that the flux richardson number in the flood tide boundary layer is 0 1 0 18 consistent with the model indicating that the flux richardson number is between 0 1 and 0 14 during ebb the boundary layer was more stratified and its vertical extent was not as sharply delineated as in the flood during neap tide the rate of mixing during ebb was significantly weaker than on flood owing to reduced bottom stress and stabilization by stratification as tidal amplitude increased ebb mixing increased and more closely resembled the boundary layer entrainment process observed during the flood tidal straining modestly increased the entrainment rate during the flood and it restratified the boundary layer and inhibited mixing during the ebb

pan african magmatism and sedimentation in the nw himalaya correlation of early palaeozoic pan african 500 50 ma granites that intruded the chail salkhala haimanta formations in the lesser himalaya zanskar crystallines and lower taglang la of tso morari crystallines in the northwestern himalaya is based on the field relationship tectonic setting mineralogical and geochemical characteristics and isotope dating of the granites these granite plutons exhibit identical petrographical and geochemical character the mineralogical composition of the granites is quite similar consisting of quartz k feldspar plagioclase feldspar biotite muscovite garnet tourmaline cordierite andalusite and sillimanite fibrolite the granite which are massive and inequigranular in the core of the plutons show strongly foliated character indicating development of ductile shear zone at the margins these are peraluminous s type granites having high a cnk value 1 presence of normative corundum rounded shape of zircon and high initial sr ratio suggest crustal source of the granites mantle normalized spider diagram exhibits similar characters for all these granitoids the intrusion of the pan african granites mark an abrupt end of the sedimentation that continued virtually uninterrupted from palaeoproterozoic the sudden break in sedimentation towards the terminal phases of the lower cambrian has been observed in almost all parts in lesser as well as the tethys himalaya occurrences of large number of plutons along different tectonic belts of northwestern himalaya are indicative of widespread tectono thermal event during early palaeozoic 500 50 ma the bracketing of the two features like the break in sedimentation during post late cambrian and the intrusion of granites around 500 50 ma is considered to be the result of a strong diastrophic orogenic event correlatable to the late phases of the pan african orogeny in africa

bubbly ice densification in ice sheets i theory dry snow on the surface of polar ice ice sheets is first densified and metamorphosed to produce firn bubbly ice is the next stage of the transformation process which takes place below the depth of pore closure this stage extends to the transition zone where due to high pressures and low temperatures air trapped in bubbles and ice begins to form the mixed air clathrate hydrates while the gas phase progressively disappears here we develop a model of bubbly ice rheology and ice sheet dynamics taking into account glacier ice compressibility the interaction between hydrostatic compression of air bubbles deviatoric uniaxial compressive deformation of the ice matrix and global deformations of the glacier body is considered the ice matrix pressure and the absolute load pressure are distinguished similarity theory and scale analysis are used to examine the resultant mathematical model of bubbly ice densification the initial rate of bubble compression in ice sheets appears to be relatively high so that the pressure density relaxation process takes place only 150 200 m in depth below pore close off to reach its asymptotic phase wherein the minimal drop between bubble and ice pressures is governed by the rate of loading ice accumulation this makes it possible to consider densification under stationary present day conditions of ice formation as a special case of primary interest the computational tests performed with the model indicate that both ice porosity and bubble pressure profiles in ice sheets are sensitive to variations of the rheological parameters of pure ice however only the bubble pressure profile distinguishes between the rheological properties at low and high stresses the porosity profile at the asymptotic phase is mostly determined by the air content in the ice in the companion paper lipenkov and others 1997 we apply the model to experimental data from polar ice cores and deduce through an inverse procedure the rheological properties of pure ice as well as the mean air content in holocene and glacial ice sediments at vostok station antarctica

height changes over subglacial lake vostok east antarctica insights from gnss observations height changes of the ice surface above subglacial lake vostok east antarctica reflect the integral effect of different processes within the subglacial environment and the ice sheet repeated gnss global navigation satellite systems observations on 56 surface markers in the lake vostok region spanning 11 years and continuous gnss observations at vostok station over 5 years are used to determine the vertical firn particle movement vertical marker velocities are derived with an accuracy of 1 cm yr or better repeated measurements of surface height profiles around vostok station using kinematic gnss observations on a snowmobile allow the quantification of surface height changes at 308 crossover points the height change rate was determined at 1 5 mm yr thus indicating a stable ice surface height over the last decade it is concluded that both the local mass balance of the ice and the lake level of the entire lake have been stable throughout the observation period the continuous gnss observations demonstrate that the particle heights vary linearly with time nonlinear height changes do not exceed 1 cm at vostok station and constrain the magnitude of spatiotemporal lake level variations icesat laser altimetry data confirm that the amplitude of the surface deformations over the lake is restricted to a few centimeters assuming the ice sheet to be in steady state over the entire lake estimates for the surface accumulation on basal accretion melt rates and on flux divergence are derived

proterozoic sedimentation and pan african geodynamic development in the himalaya abstract the purana or proterozoic lower cambrian continental margin sedimentary succession of the lesser himalaya comprises three lithological divisions a lower argillo arenaceous group a middle carbonate succession and an upper argillo calcareous assemblage the poorly sorted basal sediments consisting of wacke slate alternations of the lower group were emplaced by currents including turbidity currents which in the early stage flowed northwards from the aravalli and satpura mountains of peninsular india lenticular deposits of diamictite within the flysch succession represent submarine slides penecontemporaneous basaltic lavas and associated tuffs interbedded with shallow water arenites making up the bulk of the upper part of the lower group were related to rifting of the basins the overlying dolomites and dolomitic limestones were laid down on a very shallow stable shelf and are characterised by columnar branching stromatolites and lenticular units of crystalline magnesite and associated talc lenticular diamictites towards the top of the upper group of slates and limestone are harbingers of tectonic movements culminating towards the end of the early cambrian in an upheaval which abruptly terminated sedimentation throughout the lesser himalaya and peninsular india and caused a pronounced interruption of sedimentation in the tethyan domain in the northern himalaya the cambro ordovician 500 25 ma granites intruding the lesser himalayan thrust sheets of lower proterozoic metamorphic rocks and the upwarped northern front of the himalaya signify an important tectonic event that terminated the purana proterozoic lower cambrian sedimentation in the indian subcontinent this seems to be related to the tectonic movements that occurred towards the close of the late proterozoic or early cambrian

residual circulation in shallow estuaries 2 weakly stratified and partially mixed narrow estuaries long wave theory is used herein to analyze circulation in weakly stratified and partially mixed estuaries unlike the highly stratified systems considered in part 1 the flows considered here have only a minimal tidal frequency internal wave component these estuaries may therefore be modeled as the sum of weakly interacting barotropic and baroclinic modes the dominant factors driving the residual flow are finite amplitude barotropic effects in weakly stratified estuaries and a combination of barotropic effects and steady horizontal density gradient forcing in partially mixed estuaries the dominant vertical exchange mechanism in the weakly stratified case is bottom boundary induced turbulence and that in partially mixed estuaries is believed to be random internal wave interactions a model of the columbia river estuary under weakly stratified conditions accurately predicts the observed residual velocity and salinity fields and the tidal amplitude at which a transition to a highly stratified state takes place the partially mixed state is unstable in the columbia river estuary because the tides and buoyancy input are both too strong

bubbly ice densification in ice sheets ii applications a mathematical model for simulating the densification of bubbly glacier ice is used to interpret the following experimental data from the vostok central antarctica ice core two ice porosity profiles obtained by independent methods and a bubble pressure profile obtained by direct measurements of air pressure within individual bubbles the rheological properties of pure polycrystalline ice are deduced from the solution of the inverse problem the model and the inferred ice flow law are then validated using porosity profiles from seven other ice cores drilled in antarctica and greenland in the temperature range from 55 to 20 c the following expression is adopted for the constitutive law 2e 1 2 exp q 1 t s 1 t r s where e and are the effective strain rate and stress respectively is the creep exponent taken as 3 5 r s is the gas constant and t t s is the temperature standard temperature the numerical values obtained for the linear and non linear viscosities are 1 2 9 1 3 mpayear and 2 0 051 0 019 mpa year and the apparent activation energy q is confirmed to be 60 kj mole 1 the corresponding flow law is in good agreement with results of both mechanical tests and independent estimations based on the analysis of different natural phenomena associated with glacier ice deformation when the model is constrained by the porosity and bubble pressure profiles from vostok the mean air content in holocene ice is inferred to be about 0 088 cm 3 g 1 the corresponding mean air pressure in bubbles at the end of pore closure is about 0 083 mpa whereas the atmospheric pressure at this depth level would be 0 063 mpa the influence of the climatic change on the ice porosity profile is discussed it resulted in an increased air content in ice at vostok during the last glacial maximum 0 096 cm 3 g 1

observational evidence on the stability of the hydro glaciological regime of subglacial lake vostok 1 combined geodetic geophysical and glaciological in situ measurements are interpreted regarding surface height changes over subglacial lake vostok and the local mass balance of the ice sheet at vostok station repeated gps observations spanning 5 years and long term surface accumulation data show that the height of the lake surface has not changed over the observation period the application of the mass conservation equation to purely observational data yields an ice mass balance for vostok station close to equilibrium

enhanced one dimensional modeling of transport in rivers longitudinal dispersion of substances transported in rivers is caused by variations of the advective velocity over the river cross section and by transverse mixing this two dimensional nature of the process is not accounted for by the usually applied one dimensional advection diffusion equation which therefore is not valid in the so called initial period immediately downstream of a substance input site an enhanced one dimensional model is presented based on a very much simplified transverse velocity profile with a flow velocity of zero near the river banks and a constant value in the center of the river dispersion is modeled by the exchange of mass between the two zones in which no transverse spatial gradients are considered dispersion can be estimated in this model by two dimensionless parameters that vary only moderately from one river to another the behavior of the model in the asymptotic range and in the initial period the limitations of the model and its advantages compared to the one and two dimensional models are discussed the main advantage of the model is that it is not more complex than the one dimensional model but still yields a good description of substance transport for about 80 of the initial period

regressive transgressive sedimentation in the ordovician sequence of the spiti tethys basin himachal pradesh india abstract the spiti basin comprises about 10 000 m of more or less fossiliferous cambrian to cretaceous sedimentary succession in the tethyan tibetan himalaya the ordovician strata in this basin consist of dominant sandstone with minor mudstone siltstone facies and locally developed basal conglomerate and persistent topmost minor carbonates this sequence has been regarded variously as continental and or marine to stretch out the environment of deposition a detailed study on field characteristics palaeocurrents petrography and reported marine fauna and trace fossils was carried out and suggests three major depositional environments viz fluvial transitional and marine the fluvial facies assemblage is characterised by fine to coarse grained multistoried sheets of sublithic arenite with abundant planar cross stratification and low mudstone content and is interpreted as having formed in a braided river environment the generalised palaeoslope is towards ne and nw the transitional facies comprises fine to medium grained sublithic to subarkosic arenites having sheet and lenticular sandstones with small to medium scale trough and planar cross stratification and subordinate herringbone cross stratification sigmoidal reactivation surfaces flaser and tidal bedding etc these features indicate the influence of both non marine fluvial and marine tidal flat environments the marine facies consists of fine to medium grained subarkose sublithic arenite and quartz wacke as well as sandy dolomicrite in the upper part the sandstone facies are generally multistoried having a sheet geometry and are interstratified with mudstone the important sedimentary structures include trough planar and hummocky cross stratifications and parallel lamination the palaeoflow direction is generally towards ne and nw with some minor fluctuations toward the south and west landward side these characteristic features reveal deposition in the nearshore shore zone three different stages of regression and transgression are recognised stage i resulted in the deposition of fluvial facies on the earlier marine domain cambrian representing a regressive phase caused by differential uplift and subsidence probably triggered by magmatic intrusion and or regional tectonics stage ii represents channelising of the fluvial system on tidal flats marking an initiation of transgression finally stage iii denoting the development of shore zone facies caused by basin subsidence and a change in sea level is interpreted as a transgressive phase

observations of turbulence in a partially stratified estuary the authors present a field study of estuarine turbulence in which profiles of reynolds stresses were directly measured using an adcp throughout a 25 h tidal day the dataset that is discussed quantifies turbulent mixing for a water column in northern san francisco bay that experiences a sequence of states that includes a weak ebb and flood that are stratified followed by a strong and eventually unstratified ebb and flood these measurements show that energetic turbulence is confined to a bottom mixed layer by the overlying stratification examination of individual reynolds stress profiles along with profiles of richardson number and turbulent froude number shows that the water column can be divided into regions based on the relative importance of buoyancy effects using the measured turbulence production rate p the dissipation rate e is estimated the observed turbulence had values of e nn 2 20 all of the time and e nn 2 200 most of the time suggesting that the observed motions were buoyancy affected turbulence rather than internal waves however at times turbulent froude numbers in much of the upper water column were less than one indicating important stratification effects taken as a whole the data show that stratification affects the turbulent velocity variance q2 most severely that is observed reductions in u9w9 are largely associated with small values of q2 rather than with a dramatic reduction in the efficiency with which turbulent motions produce momentum fluxes finally the dataset is compared to predictions made using the popular mellor yamada level 2 5 closure these comparisons show that the model tends to underestimate the turbulent kinetic energy in regions of strong stratification where the turbulence is strongly inhomogeneous and to overestimate the turbulent kinetic energy in weakly stratified regions the length scale does not appear to compensate for these errors and as a result similar errors are seen in the eddy viscosity predictions it is hypothesized that the underestimation of q2 is due to an inaccurate parameterization of turbulence self transport from the near bed region to the overlying stratification

dynamic recrystallization and texture development in ice as revealed by the study of deep ice cores in antarctica and greenland the preferred c axis orientation of ice from polar ice sheets develops essentially as a result of intracrystalline slip but dynamic recrystallization appears to alter the kinetics of the development of deformation textures and is at high temperature at the origin of recrystallization textures the purpose of this work is to obtain a better understanding of recrystallization processes that occur in polar ice sheets and to clarify the relationship between dynamic recrystallization and textures the study was based on two deep ice cores from greenland and antarctica the greenland ice core project grip and vostok ice cores the structure along the grip core displays normal grain growth in the first 100 m of the ice sheet and rotation recrystallization and migration recrystallization near the bottom only grain growth and rotation recrystallization appear to occur in the vostok ice core the transition between these recrystallization regimes was studied estimating for interglacial ice the evolution with depth of the dislocation density this calculation has shown the efficiency of grain boundary migration for the absorption of dislocations at vostok the highest value of the dislocation density is found at a depth of about 1000 m and the continuous decrease in the dislocation density below this depth is related to the increase of the grain boundary migration rate it is shown that the driving force required to initiate migration recrystallization is not reached in interglacial ice at vostok the observed textures were compared with those predicted by the self consistent approach recrystallization textures are interpreted by assuming that the less stressed grains i e the best oriented for basal slip are favored by the size advantage of subgrains the recrystallization textures are compared with those of other materials

physical and climatic parameters which influence the air content in polar ice abstract under present day climatic conditions the air content of polar ice v generally shows a high sensitivity to the atmospheric pressure and hence to the surface elevation of the ice sheet wherethe ice is formed the results presented here are from sixteen different sites fourteen in antartica one in greenland and one in the yukon territories canada and they allow a better understanding of the parameters which influence the air content in ice it is demonstrated the v can be expressed very simply as a function of the atmospheric pressure p i the temperature t i and the porous volume of ice v i at which the air in the firn becomes isolated in terms of pressure from the atmosphere during the process of pore close off our results confirm a v i increase with temperature and show no clear v i dependence on snow accumulation rate the possible non linearity of the v i t i relation we obtain could be due to a wind influence on v i and or to a second order effect of the accumulation rate our v i results are compared to measurements of the volume of closed pores versus depth in the firn to close we discuss the parameterization that we can obtain to interpret the air content variations observed in deep ice cores over periods covering glacial interglacial transitions in terms of paleoclimatic conditions prevailing at the surface of the ice sheet

dynamics of willapa bay washington a highly unsteady partially mixed estuary results from 3 yr of hydrographic time series are shown for willapa bay washington a macrotidal partially mixed estuary whose river and ocean end members are both highly variable fluctuating ocean conditions alternations between wind driven upwelling and downwelling and intrusions of the buoyant columbia river plume are shown to force order of magnitude changes in salinity gradients on the event 2 10 day scale an effective horizontal diffusivity parameterizing all up estuary salt flux is calculated as a function of riverflow results show that willapa s volume integrated salt balance is almost always far from equilibrium at very high riverflows the top 15 of observations the estuary loses salt on average while at all other riverflow levels it gains salt under summer low riverflow conditions in fact the effective diffusivity k is large enough to drive a net increase in salinity that is 3 6 times the seaward river driven salt flux this diffusion process is amplified not damped by increased tidal forcing contrary to the expectation for baroclinic exchange furthermore k varies along the length of the estuary as 5 of the rms tidal velocity times channel width a scaling consistent with density independent stirring by tidal residuals to summarize willapa s event and seasonal scale variability a simple diagnostic parameter space for unsteady estuarine salt balances is presented a generalization from the hansen and rattray steady state scheme

fractionation of gases in polar ice during bubble close off new constraints from firn air ne kr and xe observations gas ratios in air withdrawn from polar firn snowpack show systematic enrichments of ne n2 o 2 n2 and ar n2 in the firn ice transition region where bubbles are closing off air from the bubbles in polar ice is correspondingly depleted in these ratios after accounting for gravitational effects gas in the bubbles becomes fractionated during the process of bubble close off and fractionation may continue as ice cores are stored prior to analysis we present results from firn air studies at south pole and siple dome antarctica which add ne kr and xe measurements to the suite of observations ne o2 and ar appear to be preferentially excluded from the shrinking and occluding bubbles and these gases therefore accumulate in the residual firn air creating a progressive enrichment with time and depth in firn air early sealing of gases by thin horizontal impermeable layers into a nondiffusive zone or lock in zone greatly enhances this enrichment a simple model of the bubble close off fractionation and lock in zone enrichment fits the data adequately the model presumes that fractionation is caused by selective permeation of gas through the ice lattice from slightly overpressured bubbles the effect appears to be size dependent because ne o2 and ar have smaller effective molecular diameters than n2 and fractionation increases strongly with decreasing size ne is fractionated 34 2 times more than o2 in south pole firn air and reaches an enrichment of 90 in the deepest sample the large atoms kr and xe do not appear to be fractionated by this process despite the large size difference between the two gases suggesting a threshold atomic diameter of 3 6a above which the probability becomes very small that the gas will escape from the bubble these findings have implications for ice core and firn air studies that use gas ratios to infer paleotemperature chronology and past atmospheric composition

sedimentary evidence of a cambro ordovician orogenic event in the northwestern himalaya abstract the late proterozoic early paleozoic sequence of the tethys himalaya provides evidence of an orogenic episode close to the cambrian ordovician boundary late pan african event pre middle cambrian sediments were deposited on a terrigenous tidal flat gradually passing upward to a peritidal carbonate platform the succession shows an overall transgressive trend which is reproduced at smaller scale by second and third order coarsening upward cycles terminated by the landward shift of carbonate facies during highstands of sea level sandstone petrography suggests a passive continental margin setting for this part of the sequence in middle cambrian times the carbonate platform underwent gradual drowning and outer shelf slope shales with intercalated tuffaceous material erupted from a nearby immature volcanic arc were deposited the shales are overlain by basinal turbidites attesting to the growth of a sandstone lobe enclosed in lobe fringe siltstones in the middle ordovician unconformable alluvial fan conglomerates and braid plain sandstones derived from uplifted sedimentary sequences indicate erosion of a newly formed orogenic belt comparison with early paleozoic and recent counterparts allows to interpret the cambro ordovician sedimentary evolution in terms of foreland basin development the thermal subsidence of a passive margin temporarily reduced by the passage of a peripheral bulge was enhanced strongly by flexural downwarping towards a subduction zone collision of a buoyant continental margin at the trench hampered subduction rapid uplift ensued with formation of a foreland fold thrust belt shedding sedimentary detritus into the adjacent molasse basins

air bubbles and clathrate hydrates in the transition zone of the ngrip deep ice core abstract field studies of air bubble and clathrate inclusions in the ngrip ice core reveal two stages of clathrate formation in the transition zone translucent primary clathrates with rough surfaces and irregular shapes formed from individual bubbles split up and form smaller specimens deeper down in the transition zone 1100 m clathrate metamorphosis sets in giving rise to the formation of transparent secondary clathrates with smooth and regular often poly hedral or faceted shapes rod like crystals of great length 2 mm often show signs of separation into smaller units that fragment into smaller specimens these findings imply that the simplified picture of one bubble forming one clathrate has to be modified

radiometric approach for estimating relative changes in intraglacier average temperature we investigate the degree to which ultrahigh frequency radio emission can be used to estimate subsurface physical temperature in the polar ice sheets we combine electromagnetic emission forward models with plausible models of depth dependent physical properties in the ice sheet temperature models are parameterized with variables including accumulation rate geothermal heat flux and surface temperature scattering is parameterized using empirical observations of grain growth combined with measured densities electromagnetic absorption is modeled using dielectric dispersion processes and semiempirical models based on observations our models illustrate that information about east antarctic ice sheet temperature from near the surface to near the base can be gleaned from ultrawideband radiometer data based on our modeling study we illustrate an instrument concept to measure ice sheet temperature profiles comprising a novel ultrawideband radiometer

a 150 000 year climatic record from antarctic ice during much of the quaternary the earth s climate has undergone drastic changes most notably successive glacial and interglacial episodes the past 150 kyr includes such a climatic cycle the last interglacial the last glacial and the present holocene interglacial a new climatic time series for this period has been obtained using 18 o data from an antarctic ice core

the precambrian cambrian boundary problem and its prospects northwest himalaya india the precambrian cambrian boundary problem is studied in an areno argillaceous sequence in the kashmir and spiti valley northwest himalaya india in kashmir a rich and diversified microbiota cryptarchs and algae of the late precambrian and low lower cambrian trace fossils are recorded in the spiti valley the yield of microbiota is poor and the trace fossils are late lower cambrian modified journal abstract

spatial variation of englacial radar attenuation modeling approach and application to the vostok flowline 1 constraining the spatial variation of englacial radar attenuation is critical for accurate inference of the spatial variation of the englacial and basal properties of ice sheets from radar returned power here we evaluate attenuation models that account for spatial variations in ice temperature and chemistry and test them along the flowline that passes through the vostok ice core site antarctica the simplest model often used but rarely valid assumes a uniform attenuation rate everywhere along the flowline so that total attenuation is proportional to ice thickness the next simplest model uses spatially varying temperatures predicted by an ice flow model and assumes uniform chemistry additional models account for spatially varying chemistry using englacial stratigraphy we find that the roundtrip attenuation to the bed can easily differ by 10 db or more between the uniform attenuation rate model and models that account for variable ice temperature such differences are sufficient to confound the delineation of dry and wet beds also including spatial variations in chemistry produces smaller differences 10 db but the magnitude of these differences depends on the relative importance of dry and wet deposition of impurities in the past accounting for dry deposited impurities requires ice flow modeling and results in larger differences from all other models which assume uniform chemistry or wet deposition only these results indicate that modeling the spatial variation of attenuation requires a spatially varying temperature model in order to infer bed conditions from bed returned power accurately and that both ice core data and radar stratigraphy are also strongly desirable

flow at ice divide triple junctions 1 three dimensional full stokes modeling 1 ice domes are either axisymmetric high points along ridges or ridge triple junctions we model time dependent isothermal flow near triple junctions solving the full set of mechanical equations with a nonlinear power law rheology forcing is applied through the boundary conditions which affect flow patterns at outlets where such forcing is purely axisymmetric an axisymmetric dome is formed if a threefold symmetry in the forcing is applied the axisymmetric dome breaks up into three ridges subtending angles of 120 sets of experiments where the forcing was not exactly threefold symmetric by angle or by amplitude caused the triple junction to migrate to a new steady state here in steady state the ridges join the triple junction at nearly 120 but one ridge curves to satisfy the boundary forcing the slope pattern in the immediate dome vicinity depends only on a dimensionless parameter which is a function of the ice consistency the accumulation and the rheological power law index attempts to replicate the topography around summit greenland obtained a good fit with n 3 at a triple junction the dome is really distinct from the surrounding ridges contrary to the highest point of a single ridge divide as a consequence the raymond effect is at its strongest at the dome and weakens considerably over one ice thickness as one moves away from the flow center along the ridges leaving the dome the raymond effect is still present and decreases with the ratio of the flow across and along the ridge in the vicinity of the dome horizontal strain rates vary strongly from uniaxial to biaxial large scale effects represented in our model as fluxes at boundaries seem to be the primary controls on dome position and shape

simple relations for the close off depth and age in dry snow densification a physical model for the snow firn densification process salamatin and others 2006 and martinerie and others 1992 1994 correlation for the firn density at the pore closure are employed to perform a scale analysis and computational experiments in order to deduce simplified relations for the close off depth and ice age in quasi stationary ice formation conditions the critical snow density at which ice grain rearrangement stops is used to take into account variability of snow structures subjected to densification the results obtained are validated on a representative set of ice core data from 22 sites which covers wide ranges of present day temperatures and ice accumulation rates a simple analytical approximation for the density depth profile is proposed