the significance of ultra refracted surface gravity waves on sheltered coasts with application to san francisco bay ocean surface gravity waves propagating over shallow bathymetry undergo spatial modification of propagation direction and energy density commonly due to refraction and shoaling if the bathymetric variations are significant the waves can undergo changes in their direction of propagation relative to deepwater greater than 90 over relatively short spatial scales we refer to this phenomenon as ultra refraction ultra refracted swell waves can have a powerful influence on coastal areas that otherwise appear to be sheltered from ocean waves through a numerical modeling investigation it is shown that san francisco bay one of the earth s largest and most protected natural harbors is vulnerable to ultra refracted ocean waves particularly southwest incident swell the flux of wave energy into san francisco bay results from wave transformation due to the bathymetry and orientation of the large ebb tidal delta and deep narrow channel through the golden gate for example ultra refracted swell waves play a critical role in the intermittent closure of the entrance to crissy field marsh a small restored tidal wetland located on the sheltered north facing coast approximately 1 5 km east of the golden gate bridge

wave current bottom shear stresses and sediment resuspension in cleveland bay australia the bottom sediment resuspension is an important process in cleveland bay north australia hydrodynamics and suspended sediment concentrations ssc were measured from january to april 1993 during this field work two high sediment resuspension events were observed an iterative method of calculating the bottom shear stress under wave current motion has been applied using the calculated bottom shear stresses the resuspension mechanisms of bottom sediment in cleveland bay has been investigated by analysing data collected during two periods in january and march 1993 swell has been found to be the most important factor in sediment suspensions for both high ssc events wind waves have little effect on ssc levels tidal currents are too weak to stir up bottom sediment particles into the flow but affect the detailed ssc structure by tidal convection the critical shear stress for sediment resuspension in cleveland bay is about 2 0 n m2

tidal residual intertidal mudflat trim model and its applications to san francisco bay california abstract a numerical model using a semi implicit finite difference method for solving the two dimensional shallow water equations is presented the gradient of the water surface elevation in the momentum equations and the velocity divergence in the continuity equation are finite differenced implicitly the remaining terms are finite differenced explicitly the convective terms are treated using an eulerian lagrangian method the combination of the semi implicit finite difference solution for the gravity wave propagation and the eulerian lagrangian treatment of the convective terms renders the numerical model unconditionally stable when the baroclinic forcing is included a salt transport equation is coupled to the momentum equations and the numerical method is subject to a weak stability condition the method of solution and the properties of the numerical model are given this numerical model is particularly suitable for applications to coastal plain estuaries and tidal embayments in which tidal currents are dominant and tidally generated residual currents are important the model is applied to san francisco bay california where extensive historical tides and current meter data are available the model calibration is considered by comparing time series of the field data and of the model results alternatively and perhaps more meaningfully the model is calibrated by comparing the harmonic constants of tides and tidal currents derived from field data with those derived from the model the model is further verified by comparing the model results with an independent data set representing the wet season the strengths and the weaknesses of the model are assessed based on the results of model calibration and verification using the model results the properties of tides and tidal currents in san francisco bay are characterized and discussed furthermore using the numerical model estimates of san francisco bay s volume surface area mean water depth tidal prisms and tidal excursions at spring and neap tides are computed additional applications of the model reveal qualitatively the spatial distribution of residual variables

a case of simple magma mixing in the columbia river basalt group the wilbur creek lapwai and asotin flows saddle mountains formation three small sequential saddle mountains flows occupying similar areas on the columbia plateau were erupted over a short interval of time in the lewiston basin area the middle flow of the trio lapwai has intermediate mineralogy and lies on a straight mixing line between the other two flows for virtually all twenty five elements analyzed systematic changes in the ratios of incompatible elements demonstrate that these relationships are a result of magma mixing rather than either crystal fractionation or variable degrees of partial melting the lapwai flow has a composition approximately midway between the two homogeneous end members and is itself relatively homogeneous this implies efficient mixing between equal amounts of asotin and wilbur creek magmas and suggests that mixing was completed in a magma reservoir prior to eruption the wilbur creek and asotin end members have isotopic features which are believed to result from different degrees of assimilation of crustal material by magma derived from an enriched mantle source carlson 1984 the mixing processes described here cannot be related to that earlier mantle crust mixing process

field and laboratory resistivity monitoring of sediment consolidation in china s yellow river estuary traditional methods for in situ monitoring of the dynamic physical process within marine sediments are not efficient and effective enough and determination of the consolidation state of marine sediments in the yellow river estuary remains a challenging problem however the recent development of in situ testing technology for resistivity methodology creates new possibilities our combined analysis of laboratory and field experimental results demonstrates the consolidation process and the contemporaneous physical mechanical and electrical properties of marine sediments in the yellow river estuary and we discuss the feasibility of and that influence in situ monitoring of the sediment consolidation process using a multi electrode probe these results demonstrate that resistivity is a good indirect predictor of porosity which is the primary factor affecting resistivity behavior during the consolidation process values for empirical constants a and m for yellow river estuary sediment can be obtained based on archie s formula the relationship between resistivity and the geotechnical strength of silty sediments in the yellow river estuary shows that the penetration resistance determined by a light penetration test and undrained shear strength determined by a vane shear test are closely correlated to resistivity with a power function during the consolidation process of a uniform originated seabed this study confirms that resistivity monitoring using a multi electrode probe is a relevant method to estimate the degree and state of sediment consolidation in real time and in situ more work is needed to investigate the implications of this for the prediction and prevention of geological disasters in estuarine areas

the use of modeling and suspended sediment concentration measurements for quantifying net suspended sediment transport through a large tidally dominated inlet abstract sediment exchange at large energetic inlets is often difficult to quantify due complex flows massive amounts of water and sediment exchange and environmental conditions limiting long term data collection in an effort to better quantify such exchange this study investigated the use of suspended sediment concentrations ssc measured at an offsite location as a surrogate for sediment exchange at the tidally dominated golden gate inlet in san francisco ca a numerical model was calibrated and validated against water and suspended sediment flux measured during a spring neap tide cycle across the golden gate the model was then run for five months and net exchange was calculated on a tidal time scale and compared to ssc measurements at the alcatraz monitoring site located in central san francisco bay 5 km from the golden gate numerically modeled tide averaged flux across the golden gate compared well r 2 0 86 p value mckee et al 2013 this issue there was little variation from year to year exports were computed to be greatest during the wettest water year analyzed but only marginally so

sediment transport patterns in the san francisco bay coastal system from cross validation of bedform asymmetry and modeled residual flux abstract the morphology of 45 000 bedforms from 13 multibeam bathymetry surveys was used as a proxy for identifying net bedload sediment transport directions and pathways throughout the san francisco bay estuary and adjacent outer coast the spatially averaged shape asymmetry of the bedforms reveals distinct pathways of ebb and flood transport additionally the region wide ebb oriented asymmetry of 5 suggests net seaward directed transport within the estuarine coastal system with significant seaward asymmetry at the mouth of san francisco bay 11 through the northern reaches of the bay 7 8 and among the largest bedforms 21 for 50 m this general indication for the net transport of sand to the open coast strongly suggests that anthropogenic removal of sediment from the estuary particularly along clearly defined seaward transport pathways will limit the supply of sand to chronically eroding open coast beaches the bedform asymmetry measurements significantly agree up to 76 with modeled annual residual transport directions derived from a hydrodynamically calibrated numerical model and the orientation of adjacent flow sculpted seafloor features such as mega flute structures providing a comprehensive validation of the technique the methods described in this paper to determine well defined cross validated sediment transport pathways can be applied to estuarine coastal systems globally where bedforms are present the results can inform and improve regional sediment management practices to more efficiently utilize often limited sediment resources and mitigate current and future sediment supply related impacts

the composition and evolution of the continental crust rare earth element evidence from sedimentary rocks the composition of the present day upper crust inferred from the uniformity of sedimentary rock r e e rare earth element patterns is close to that of granodiorite a revised andesite model is used to obtain total crustal composition the lower crust is the composition remaining assuming that the upper crust one third of the total is derived from intracrustal partial melting the upper crustal r e e pattern has pronounced eu depletion eu eu 0 64 the lower crustal pattern has eu enrichment eu eu 1 17 and the total crust has no eu anomaly relative to chondritic abundances the eu depletion in the upper crust is attributed to retention of eu in plagioclase in the lower crust because plagioclase is not stable below 40 km 10 kbar the anomaly is intracrustal in origin the archaean upper crust has a different r e e pattern to that of the present day upper crust being lower in total r e e and la yb ratios and lacking an eu anomaly these data are used to infer the archaean upper crustal composition which resembles that of the present day total crust except that ni and cr contents are higher the archaean crustal composition can be modelled by a mixture of tholeiites and tonalite trondhjemites the latter have steep light r e e enriched heavy r e e depleted patterns consistent with equilibration with garnet and hence probable mantle derivation there is little reason to suppose that the archaean lower crust was different in composition from the upper crust except locally where partial melting episodes occurred the r e e evidence is consistent with isotopic and geological evidence for a low continental growth rate in the early archaean a massive increase to about 70 of the total crust between about 3000 and 2500 ma b p and a slow increase until the present day the change from archaean to post archaean r e e patterns in the upper crust is not isochronous but is reflected in the sedimentary rock r e e patterns at differing times in different continents on the basis of a model composition for the mantle 36 of the potassium 30 of uranium 15 of lanthanum and 3 of ytterbium are concentrated in the present continental crust this enrichment is related to ionic size and valency differences from common mantle cations e g mg fe pre 3 9 ga b p crusts were obliterated by meteorite bombardment no geochemical evidence exists for primordial anorthositic sialic or mafic crusts

a reconsideration of the mass balance of a portion of the ross ice shelf antarctica des estimations raisonnables du degre de diminution de la vitesse juste a l amont de la zone de glace reposant sur le sol indiquent qu il n est plus possible de conclure a un epaississement a l aide de la bande d ecoulement originale de thomas et bentley 1978 c est pourquoi une nouvelle bande d ecoulement situee plus a l est que celle de thomas et bentley et non influencee par quelque zone de glace reposant sur le sol a ete choisie le bilan de masse de cette bande d ecoulement n est pas significativement different de zero une difference de vitesse entre l epaississement et l accretion a la base depassant environ 0 2 m a 1 n est pas vraisemblable

seismic measurements reveal a saturated porous layer beneath an active antarctic ice stream seismic reflection studies recently conducted on ice stream b part of the marine ice sheet of west antarctica show a metres thick layer immediately beneath the ice in which both compressional p and shear s wave speeds are very low these low wave speeds imply that the material in the layer is highly porous and is saturated with water at a high pore pressure from this and from arguments presented in an accompanying paper1 to the effect that the layer would be too weak to support the shear stress exerted by the overlying ice we conclude that the layer is deforming and that the ice stream probably moves principally by such deformation

comment on columbia river volcanism the question of mantle heterogeneity or crustal contamination by r w carlson g w lugmair and j d macdougall abstract strontium and neodymium isotopic data on basalts from the columbia river province are consistent with an unfractionated source for the magmas and do not necessarily indicate large amounts of crustal contamination this conclusion is based on the observation that 1 1 the nd values of 99 of the basalts are positive and sharply bounded at nd 0 2 2 the trace element patterns can be modelled without crustal assimilation 3 3 there is no associated rhyolitic volcanism and 4 4 the isotopic ratios correlate with geographic position as well as chemical composition

the influence of oceanic swell on flows over an estuarine intertidal mudflat in san francisco bay in this study we examine the role that remotely forced ocean waves play in the hydrodynamics of an intertidal estuarine mudflat the observations indicate that long period 10 20 s ocean waves are a potentially important source of near bed energy and shear stress in this environment over a two week period in february 2001 we deployed an autonomous sontek hydra system on a mudflat in central san francisco bay and measured velocity and sediment concentration approximately 10 cm from the bed using an acoustic doppler velocimeter adv and an optical backscatter sensor obs the experiment continued through wet high tide and dry low tide periods over an entire spring neap cycle and thus included the variation of near bed velocity over a range of timescales results show that during large ebb tides tidally forced flows dominate the near bed dynamics during calm conditions wind waves dominate whenever the wind direction exposes the mudflat to wind coming off the bay from the south and southwest as occurs during winter storms during periods when tidal forcing is limited and wind waves are small remotely forced ocean swells become an important energy source these motions appear in the burst samples at frequencies between 0 1 and 0 04 hz and their energy correlates well q 0 8 with ocean swell measured from a buoy offshore of san francisco spectral analysis of data shows that the average energy of ocean waves per tide varied between 2 and 15 of total energy load moreover extreme values in the distribution of ocean waves bring episodic bursts of greater energy onto the estuarine mudflat which may influence local suspension of sediments 2003 elsevier ltd all rights reserved

origin of the deccan trap flows at mahabaleshwar inferred from nd and sr isotopic and chemical evidence abstract the deccan flows at mahabaleshwar are divisible into a lower and an upper group based on nd and sr isotopic ratios which define two correlated trends this distinction is supported by incompatible element ratios and bulk compositions the data reflect contamination in a dynamic system of magmas from an lil depleted e juv 8 mantle by two different negative e juv endmembers one undoubtedly continental crust the other either continental crust or enriched mantle the depleted mantle source anomalously high in 87 sr 86 sr may have been in the subcontinental lithosphere or a region of rising indian ocean morb mantle

glacial isostatic adjustment in response to changing late holocene behaviour of ice streams on the siple coast west antarctica the siple coast region of antarctica contains a number of fast flowing ice streams which control the dynamics and mass balance of the region these ice streams are known to undergo stagnation and reactivation cycles which lead to ice thickness changes that may be sufficient to excite a viscous solid earth response glacial isostatic adjustment gia this study aims to quantify siple coast ice thickness changes during the last 2000 yr in order to determine the degree to which they might contribute to gia and associated present day bedrock uplift rates this is important because accurate modelling of gia is necessary to determine the rate of present day ice mass change from satellite gravimetry recently published reconstructions of ice stream variability were used to create a suite of kinematic models for the stagnation related thickening of kamb ice stream since 1850 ad and a gia model was used to predict present day deformation rates in response to this thickening a number of longer term loading scenarios which include the stagnation and reactivation of ice streams across the siple coast over the past 2000 yr were also constructed and used to investigate the longer term gia signal in the region uplift rates for each of the ice loading histories based on a range of earth models were compared with regional gps observed uplift rates and an empirical gia estimate we estimate kamb ice stream to have thickened by 70 130 m since stagnation 165 years ago modelled present day vertical motion in response to this load increase peaks at 17 mm yr 1 i e 17 mm yr 1 subsidence for the weakest earth models tested here comparison of the solid earth response to ice load changes throughout the last glacial cycle including ice stream stagnation and reactivation across the siple coast during the last 2000 yr with an empirical gia estimate suggests that the upper mantle viscosity of the region is greater than 1 10 20 pa s when upper mantle viscosity values of 1 10 20 pa s or smaller are combined with our suite of ice load scenarios we predict uplift rates across siple coast that are at least 4 mm yr 1 smaller than those predicted by the empirical gia estimate gps data are unable to further constrain model parameters due to the distance of the gps sites from the study area our results demonstrate that late holocene ice load changes related to the stagnation and reactivation of ice streams on the siple coast may play a dominant role in defining the present day uplift signal however both the detailed earth structure and deglacial history of the region need to be better constrained in order to reduce uncertainties associated with the gia signal of this region

dual frequency altimeter signal from envisat on the amery ice shelf in antarctica radar altimeter measurements are sensitive to dielectric and penetration properties of the sensed medium snow such that the spacecraft s altitude can be biased since 2002 relatively low frequency radar measurements over the amery ice shelf east antarctica have been acquired using the envisat dual frequency altimeter at s 3 2 ghz and ku 13 6 ghz bands which penetrate a few meters into the firn the altimeter signal is however modified in summer by the presence of snowfilled crevasses indeed the specularity of the snow surfaces in summer makes the altimetric signal sensitive mostly to nadir echoes that increases the ratio between the crevasse signal and the surrounding ice shelf signal at nadir crevasses are distinguished by differences in backscattering behavior compared with the surrounding ice shelf signal crevasses are characterized by a strong backscatter coefficient at ku band and anomalies in the s band altitude estimation these two characteristics make snowfilled crevasses detectable by the dual frequency altimeter of envisat we first retrieve the geometric properties of the crevasses using a hyperbolic shape function created by strong crevasse backscatter in the ku waveform measurements from this retrieved crevasse signal and further waveform analysis we assess the properties of the snow surface and its sub surface the crevasse due to its small size compared to the altimeter footprint is found to be an excellent target to study snow properties of the ice shelf the anomalies in the s band altitude measurements over crevasses can then be explained by the presence of a double echo in the s band waveforms this echo is attributed to a reflection at the base of the snowbridge where we see evidence of sub surface echos in the individual altimeter waveforms based on this observation a methodology is developed to estimate the thickness of the snowbridge we calculate the penetration depths in the summer snow surface of the amery at ku band that is found to be around 6 m

west antarctic ice streams draining into the ross ice shelf configuration and mass balance extensive airborne radar soundings closely tied to accurately located ground stations made over a portion of the west antarctic inland ice sheet and the ross ice shelf have enabled us to map the boundaries of the ice streams and their flow bands on the ross ice shelf the surfaces of the active ice streams a and b are heavily crevassed but there are no visible crevasses on ice stream c however the existence of numerous crevasses at a depth of about 35 m implies that ice stream c ceased to be active about 250 years ago there is a complex zone at the head of ice stream b that suggests that ice stream b is currently widening and advancing toward the interior of the ice sheet the grounding lines between the inland ice and the ice shelf were mapped from the surface elevations using the requirement of hydrostatic balance and the character of the radar echoes the inland ice is more extensive than previously mapped and includes a large weakly grounded area downstream of ice streams a and b where the surface elevations are barely above hydrostatic equilibrium crary ice rise extends 70 km upstream of the previously mapped position to relate surface elevation data from satellite observations to sea level three geoidal models were tested we conclude that the gem 10c model is the best of the three for this section of antarctica a new map of surface elevation shows deep marginal troughs and longitudinal ridges along ice streams a and b but not on ice stream c whose surface contours show signs of stagnation ice stream c shows a strongly positive and ice stream b a significantly negative net balance rate ice streams a and d also show a significantly negative net flux whereas ice streams e and f do not the overall net balance of this part of the west antarctic inland ice is suggestively negative 23 15 km3 yr 1 from the measured flux into the ross ice shelf and previous measurements we calculate an average basal melt rate from beneath the ice shelf of 0 12 0 03 m yr 1

large scale ice flow over a viscous basal sediment theory and application to ice stream b antarctica recent seismic studies of ice stream b antarctica and field analysis of mid latitude glacial deposits suggest that deformable basal sediments e g water saturated till are important in determining ice sheet flow if the ratio of till viscosity to effective ice viscosity is small vertical shear associated with horizontal flow is confined to the deforming bed alone ice flow over a deformable bed is thus akin to that of floating ice shelves because ice shelves flow over inviscid seawater for some antarctic ice streams and possibly for portions of the late wisconsin ice sheets in north america and eurasia basal drag associated with deforming basal sediment does not induce significant vertical gradients of horizontal velocity instead basal drag affects the flow as if it were a horizontal body force balanced by longitudinal and transverse deviatoric stress gradients comparison of the observed flow of ice stream b to finite element simulations incorporating a viscous basal till suggest that a simple till theology is sufficient to explain the current velocity profile these simulations also highlight the importance of horizontal deviatoric stress in regions where driving stress and basal stress do not locally balance while bed deformation is critical to ice stream existence sensitivity tests suggest that ice shelf back pressure is still a crucial control affecting ice stream response to atmospheric and oceanic climate

ndsr isotopes petrochemistry and origin of the siberian flood basalts ussr the siberian flood basalt province sfbp of permo triassic age is one of the largest flood basalt provinces with an estimated area of exposure of 337 000 km2 average thickness of 1 km and a magma volume of 337 000 km3 forty seven basaltic rocks from two main subprovinces norilsk 5 10 of area thickness up to 3 km and putorana 90 95 of area thickness of more than 2 km were selected on the basis of petrography and volcano stratigraphic relation for major element analysis twentysix of these basalts twelve from norilsk and fourteen from putorana were analyzed for nd and srisotopic compositions the norilsk and putorana basalts show some contrasting behavior in terms of the ratios of the highly incompatible elements of ti p and k as a function of their mg the norilsk basalts are more variable suggesting the role of fractional crystallization assimilation in their evolution in contrast the putorana basalts show remarkable uniformity in their bulk chemical compositions there is no overall apparent correlation between the mg and the initial nd for the basalts most of the putorana and some norilsk lavas show a narrow range of initial nd between 0 and 2 5 at mg 0 53 suggesting that the bulk of the sfbp was derived from a mantle source with near chondritic ndisotopic signature our data and six previously published analyses define a hyperbolic array near the mantle correlation line in 87sr86sr t nd t space however the majority 70 of the volumetrically significant putorana basalts fall in the ranges of 0 to 2 5 and 0 7046 to 0 7052 for the initial nd and 87sr86sr respectively in nd and sr isotopic space most of the siberian basalts also fall within the field defined by the ocean island basalts data implying common mantle sources it is concluded that the sfbp originated by hotspot volcanism due to the rise of a large and relatively primitive lower mantle derived plume beneath the siberian continent

thermohaline circulation below the ross ice shelf a consequence of tidally induced vertical mixing and basal melting the warmest water below parts of the ross ice shelf resides in the lowest portion of the water column because of its high salinity vertical mixing caused by tidal stirring can thus induce ablation by lifting the warm but dense water into contact with the ice shelf a numerical tidal simulation indicates that vertically well mixed conditions predominate in the southeastern part of the sub ice shelf cavity where the water column thickness is small basal melting in this region is expected to be between 0 05 and 0 5 m yr and will drive a thermohaline circulation having the following characteristics high salinity shelf water at 1 8oc formed by winter sea ice production in the open ross sea flows along the seabed toward the tidal mixing fronts below the ice shelf and meltwater at 2 2oc produced in the wellmixed region flows out of the sub ice shelf cavity along the ice shelf bottom sensitivity of this ablation process to climatic change is expected to be small because high salinity shelf water is constrained to have the sea surface freezing temperature

a glaciological map of filchner ronne ice shelf antarctica a set of landsat 5 multispectral scanner mss images of filchner ronne ice shelf was commissioned by the institut fur angewandte geodasie and recorded over a short period in early 1986 a mosaic of digitally enhanced images was constructed using all available ground control in due course it is planned to use the data for the production of a series of digital image maps at a scale of 1 1 000 000 meanwhile a line map has been prepared at a scale of 1 2 000 000 to show the principal glaciological features the new mosaic shows much more detail than earlier landsat images and extends coverage to higher latitudes more of the poorly known southern boundary of the ice shelf can be identified but floating ice still extends beyond the limit of coverage extensive ice rumples are revealed many of them undetected in earlier imagery flow bands can be tracked without a break from their ice stream source to the ice front over distances of up to 800 km even minor tributaries and sheet flow from the inland ice sheet between the ice streams yield narrow but identifiable flow bands ice thickness velocity and mass flux data are reviewed in order to underline their dependence on good mapping

sediment transport in the san francisco bay coastal system an overview abstract the papers in this special issue feature state of the art approaches to understanding the physical processes related to sediment transport and geomorphology of complex coastal estuarine systems here we focus on the san francisco bay coastal system extending from the lower san joaquin sacramento delta through the bay and along the adjacent outer pacific coast san francisco bay is an urbanized estuary that is impacted by numerous anthropogenic activities common to many large estuaries including a mining legacy channel dredging aggregate mining reservoirs freshwater diversion watershed modifications urban run off ship traffic exotic species introductions land reclamation and wetland restoration the golden gate strait is the sole inlet connecting the bay to the pacific ocean and serves as the conduit for a tidal flow of 8 10 9 m 3 day in addition to the transport of mud sand biogenic material nutrients and pollutants despite this physical biological and chemical connection resource management and prior research have often treated the delta bay and adjacent ocean as separate entities compartmentalized by artificial geographic or political boundaries the body of work herein presents a comprehensive analysis of system wide behavior extending a rich heritage of sediment transport research that dates back to the groundbreaking hydraulic mining impact research of g k gilbert in the early 20th century

velocity of ice streams b and c antarctica the essential difference between ice stream and inland ice and the cause of ice streaming are not known in order to study these problems velocities have been measured on and near ice streams b and c by repeated tracking of transit satellites the results confirm the difference in mode of flow between fast on ice stream b and slow on the interstream ice ridges also as expected ice stream c is nearly stagnant which with other evidence of formerly fast flow confirms its recent slowing and current thickening the drainage of ice stream b has a negative balance equivalent to thinning by 0 15 0 05 m yr 1 over the entire catchment the lower portion of the ice stream is thickening at about 2 m yr 1 but the overall behavior is thinning this general thinning is probably partly due to progressive extension of the ice stream into inland ice the transition from inland ice to ice stream seems to occur irregularly both spatially and in time with rafts of inland ice being carried into the ice stream and slowly incorporated

rapid grounding line migration induced by internal ice stream variability observations indicate that the grounding line position of west antarctica is sensitive to both forced and unforced ice stream variabilities this study endeavors to characterize and understand unforced ice stream variability and associated grounding line migration we use a flowline ice stream model with an undrained plastic bed lateral shear stresses and a stretched grid refined in the grounding zone this model exhibits parameter space structure and hysteresis behavior similar to simpler ice stream models low prescribed temperature at the ice surface or weak geothermal heating produces thermal oscillations between active and stagnant phases as in previous spatially resolved ice flow models thermal activation propagates as an activation wave this model s fine resolution of the grounding zone allows for accurate simulations of transient unforced grounding line migration upstream of the grounding zone horizontal grid spacing of 1 km is required to accurately resolve activation waves activation waves induce the grounding line to migrate over 100 km at a rate that can exceed 1 km yr this is followed during the active phase by retreat which then continues for the duration of the stagnant phase grounding line retreat is the result of a negative mass balance near the grounding line but is not necessarily associated with negative mass balance for the entire ice stream in our simulations of internal variability the novel approach and experiments described in this study show that there can be large excursions in grounding line position in the absence of either external forcing or retrograde slopes

macro scale bed roughness of the siple coast ice streams in west antarctica subglacial bed conditions are key to understanding ice stream behaviour and evolution with bed roughness re ecting substrate composition and ow resistance here we present an analysis of bed roughness in the siple coast region of west antarctica from airborne radio echo sounding data the ice streams are associated generally with low bed roughness values which decrease downstream the bed of the slow owing ice stream c 10 000 km2 is also characterized by being smooth at all scales wavelengths ranging from 5 km to in excess of 40 km furthermore the bed is smooth either side of ice stream c this suggests the location of the ice stream is controlled by internal ice sheet dynamics rather than by bed morphology if the ice stream were encouraged to migrate laterally when active there would be little resistance offered by the subglacial morphology other inter ice stream regions are rough however indicating a subglacial topographic in uence on ice stream position bed roughness increases up ow of ice streams which unless the bed is modi ed may limit the inland migration of these systems copyright 2004 john wiley sons ltd

high 3he plume origin and temporal spatial evolution of the siberian flood basalts an olivine nephelinite from the lower part of a thick alkalic ultrabasic and mafic sequence of volcanic rocks of the northeastern part of the siberian flood basalt province sfbp yielded a 40ar 39ar plateau age of 253 3 2 6 million years distinctly older than the main tholeiitic pulse of the sfbp at 250 0 million years olivine phenocrysts of this rock showed 3he 4he ratios up to 12 7 times the atmospheric ratio these values suggest a lower mantle plume origin the neodymium and strontium isotopes rare earth element concentration patterns and cerium lead ratios of the associated rocks were also consistent with their derivation from a near chondritic primitive plume geochemical data from the 250 million year old volcanic rocks higher up in the sequence indicate interaction of this high 3he sfbp plume with a suboceanic type upper mantle beneath siberia

the bottom boundary layer of the bay stem plains environment of lower chesapeake bay field observations of bottom boundary layer processes in the bay stem plains environment of lower chesapeake bay depth 11 12 m were made using a bottom mounted tripod supporting electromagnetic current meters and a pressure sensor the prandtl von karman equation the inertial dissipation method and the grant and madsen wave current interaction model were applied to the field data to estimate time varying bed stress and to evaluate the likely contributions of waves and currents to total shear stress and to skin friction biogenic bed micromorphology dominated bed roughness a background signal of long period low amplitude swell was present for most of the time although these oscillations were too weak to agitate the bottom sediments interactions of the swell with tidal currents in the presence of a biogenically roughened bottom caused the hydraulic roughness height z 0 and drag coefficient c d sensed by the mean current to be significantly increased in addition internal waves with frequencies in the neighbourhood of the pycnoclinal brunt vaisala frequency period range approximately 60 100 s appeared intermittently and caused near bottom velocity fluctuations with amplitudes of over 10 cm s 1

observation and analysis of ice flow in the largest greenland ice stream we have applied satellite radar interferometry methods to map the velocity field of the recently discovered northeast greenland ice stream we have used these data in conjunction with ice thickness and accumulation data to determine that the ice stream is in balance within the measurement errors we used control methods to invert a finite element model of ice sheet flow constrained by the data to infer the ice stream s basal shear stress distribution our results reveal that flow in a section of the downstream end has much in common with the streaming flow of the ross ice streams of west antarctica e g a weak bed and fast flow in the presence of low driving stresses for several hundred kilometers along the middle of the ice stream the basal shear stress balances the driving stress in the upstream area where the ice stream is first visible in the velocity data the bed appears to be weak which may contribute to the initiation of the ice stream

reverse sediment transport induced by amplitude modulated waves abstract amplitude modulated waves can induce bedload and suspended load transports of sediment in the reverse direction of the wave propagation the reverse transport rate is mainly controlled by the magnitude of the bottom orbital velocity the mechanism is most effective in transporting silts and fine sands offshore but not clays the sorting characteristic may be amplified when non uniform size sediments are present in the sea bed the mechanism provides a feasible explanation for two shelf sedimentation patterns difficult to understand using conventional seaward sediment transport models

a millennium of variable ice flow recorded by the ross ice shelf antarctica an enhanced composite advanced very high resolution radiometer avhrr image is used to map flow stripes and rifts across the ross ice shelf antarctica the patterns of these flow related features reveal a history of discharge variations from the ice streams feeding the eastern part of the shelf the most profound variations are visible in the track of rifts downstream of crary ice rise flow stripe bends to the west of this ice rise and adjacent to steershead ice rise and changes in the northern margin of ice stream b the track of rifts downstream of crary ice rise indicates that the ice rise has existed for at least 700 years the character of this track changes about 350 km downstream indicating a rearrangement of flow patterns about 550 years ago the large bulge in the flow stripes to the west of crary ice rise is shown in detail with bent flow stripes extending for several hundred kilometers along flow this feature formed from the south possibly due to a change in the discharge of ice stream a the avhrr image documents a complex history associated with the shutdown of ice stream c with changes in the margins of ice stream c and the northern margin of ice stream b and the grounding of steershead ice rise with an associated bending and truncation of flow stripes landsat imagery shows a region that appears to be actively extending just downstream of the ice rise as the shelf continues to respond to recent changes in ice stream discharge we present a four stage flow history which accounts for the features preserved in the ice shelf

glacial and glaciofluvial deposits in the interlobate areas of the scandinavian ice sheet abstract rapid flow within ice streams took place in a broad marginal zone of the scandinavian ice sheet during deglaciation ice streams and interlobate zones have been characterised by analysis of glacial flow patterns and examination of sedimentary features of interlobate glacial and glaciofluvial deposits reconstructed glaciodynamic systems are compared with ice streams of modem ice sheets wide interstream areas of inactive ice were left between some adjacent ice streams where they separated during the retreat of the margin these areas are characterised by morainic hummocks composed of both till and glaciofluvial sediment and sections show indications of subglacial deformation observations from fennoscandia suggest that these interstream areas were sites of net accumulation of glacial and glaciofluvial sediment during deglaciation glaciofluvial sediment was deposited in subglacial tunnels or cavities simultaneously with till deformation and deposition resulting from subglacial melting substantial glaciofluvial complexes were formed in narrow interlobate joints where adjacent ice lobes coalesced a case study made in southern finland shows converging flow patterns of two ice lobes and intervening glaciofluvial deposits concentration of meltwater in interlobate zones was a consequence of ice sheet configuration thinner ice in interlobate zones caused convergence of supraglacial and subglacial drainage the crevassed and strain softened ice may also have allowed surface meltwater to penetrate down to the ice bed interface close to the ice margin steep ice velocity gradients in interlobate zones would have increased subglacial melting rates and this environment was favourable for establishment of a subglacial drainage network and deposition of glaciofluvial sediment

ice dynamics at the mouth of ice stream b antarctica field data collected at the mouth of ice stream b show that the flow dynamics of this region are distinctly different than either the major portion of the ice stream upstream or the ice shelf downstream surface slopes in this region are as low as ice shelf surface slopes yet with the exception of patches of ice which may be floating the ice is grounded basal shear stress is negligible the resistance to flow being partitioned between shear at the sides and longitudinal gradients of longitudinal and transverse stress the surface is generally crevasse free features similar to ice rises are observed upstream of the grounding line their origin is uncertain but they move at velocities comparable to the surrounding ice the flow is laterally extensive and longitudinally compressive but there are large local variations of the strain rate from the regional trends the boundary between the two major tributaries to ice stream b followed with the radar is characterized by a band of strain rates much smaller than average detailed measurements at the downstream b network highlight this local variability of strain rates but confirm that there is a strong correlation between surface topography and strain rates the strain rates indicate that the undulating topography is locally generated the lower elevation ice is thicker and moves faster a velocity profile across the crevassed northern margin shows that the decrease of velocity toward the edge is nearly linear a calculation of ice stream discharge at this location agrees closely with two rather rough estimates of balance flux and is considerably larger than a third estimate the discharge of ice stream b does not appear to be significantly out of balance with published estimates of total ice accumulation within the present catchment basin

ice ocean interaction on ronne ice shelf antarctica detailed glaciological studies have been completed at 28 sites lying on an approximate flow line extending 760 km across ronne ice shelf parameters measured at each location include ice velocity thickness principal strain rates surface elevation temperature and accumulation rate the data have been used in a steady state model to derive the basal mass flux and the temperature profile with depth at each site from the principles of mass and energy conservation these calculations indicate basal melting in excess of 1 m yr 1 over the first 100 km of the flow line downstream of the grounding line where the ice shelf is between 1200 m and 1600 m thick the maximum melt rates in this region occur near the inland margin and exceed 4 m yr 1 melting continues at a lesser rate over the next 200 km before freezing commences freezing then dominates up to the final 100 km before the ice front resulting in the accumulation of a layer of basal sea ice up to 50 m thick this is rapidly removed as melt rates increase to over 6 m yr 1 at the ice front the cumulative effect of this pattern of basal accumulation and ablation is the wastage by melting of 85 of the mass discharged across the grounding line before it reaches the ice front supporting evidence for a layer of saline ice underlying the ice shelf is provided by the strength of basal radar reflections observed during radio echo sounding of ice thickness in the region where mass balance calculations suggest an accretion of basal sea ice reflection coefficients are consistently low ranging from 6 db to 38 db most of these weak reflections are believed to originate from the true base of the ice shelf the additional energy loss being the result of increased attenuation of the radar signal within the saline layer the derived pattern of basal melting and freezing is consistent with a simple model of sub ice shelf oceanic circulation involving a deep thermohaline convection cell dense saline water which is formed over the continental shelf during winter when the sea surface freezes drains into the deepest parts of the sub ice shelf cavity at the inland margin where this water mass comes into contact with the ice shelf its temperature is 1 c above the local pressure freezing point melting of ice results producing a buoyant outflow of cold relatively fresh water along the ice shelf base basal freezing occurs towards the ice front where the ascending water becomes supercooled this circulation has important implications for the production of antarctic bottom water and for the response of the ice shelf to driving stresses through the temperature dependent viscosity of ice

distribution of glacial geomorphic features on the antarctic continental shelf and correlation with substrate implications for ice behavior surveys were conducted seaward of all the major drainage outlets of the antarctic ice sheet from the pennell coast north victoria land to marguerite bay antarctic peninsula the results show that the ice sheet extended onto the outer shelf glacial troughs occur offshore of all major glacial outlets where the substrate is crystalline bedrock ice flow tended to follow the structural grain of the bedrock deposited little sediment and eroded the underlying bedrock where ice flowed over relatively soft more easily eroded sedimentary strata the direction of ice flow was more directly offshore and depositional features characterize the sea floor in these areas the signature of the grounded ice consists of till deposits and large scale geomorphic features drumlins occur within the region of contact between crystalline and sedimentary substrates the different geological substrates are interpreted to have exerted a fundamental control on the behavior of past ice sheets the troughs in the areas of bedrock composed of sedimentary substrate are interpreted to have been occupied by relatively fast flowing ice ice streams and the troughs in the areas of crystalline substrate are interpreted to have been occupied by slower moving ice the area between these two zones was characterized by ice acceleration and is marked by drumlins

tidal flexure of jakobshavns glacier west greenland jakobshavns glacier a floating outlet glacier on the west greenland coast was surveyed during july 1976 the vertical displacements of targets along two profiles perpendicular to the fjord wall bounding the north margin of the glacier were analyzed to determine the effect of flexure caused by tidal oscillations within the fjord an analysis based on the assumption that vertical displacements of the glacier reflected pure elastic bending yielded the conclusion that the effective thickness of the ice i e the thickness which remained unaffected by surface and basal cracking and which behaved as a continuum was 160 m 2 6 km upglacier from the calving front and 110 m 0 6 km from the calving front an analysis based on the more realistic assumption that observed bending reflected elastic and viscoplastic deformation yielded the conclusion that the average effective thickness of the ice was 316 74 m 40 of the estimated 800 m total thickness 2 6 km from the calving front and 160 48 m 21 of the estimated 750 m total 0 6 km from the calving front a constitutive relationship appropriate for hard glide during flexure was used

ocean variability contributing to basal melt rate near the ice front of ross ice shelf antarctica basal melting of ice shelves is an important but poorly understood cause of antarctic ice sheet mass loss and freshwater production we use data from two moorings deployed through ross ice shelf 6 and 16 km south of the ice front east of ross island and numerical models to show how the basal melting rate near the ice front depends on sub ice shelf ocean variability the moorings measured water velocity conductivity and temperature for 2 months starting in late november 2010 about half of the current velocity variance was due to tides predominantly diurnal components with the remainder due to subtidal oscillations with periods of a few days subtidal variability was dominated by barotropic currents that were large until mid december and significantly reduced afterward subtidal currents were correlated between moorings but uncorrelated with local winds suggesting the presence of waves or eddies that may be associated with the abrupt change in water column thickness and strong hydrographic gradients at the ice front estimated melt rate was 1 2 0 5 m a 1 at each site during the deployment period consistent with measured trends in ice surface elevation from gps time series the models predicted similar annual averaged melt rates with a strong annual cycle related to seasonal provision of warm water to the ice base these results show that accurately modeling the high spatial and temporal ocean variability close to the ice shelf front is critical to predicting time dependent and mean values of meltwater production and ice shelf thinning

a numerical model of interactions between a polar ice stream and the ocean application to ice stream e west antarctica a time dependent numerical model has been developed to study the dynamics of a polar ice stream grounded below sea level the model was applied to ice stream e one of the two most active ice streams draining the ross sea sector of the west antarctic ice sheet the retarding force or back stress from the floating ross ice shelf needed to hold the model grounding line in dynamic equilibrium was found to compare closely with retarding force mapped from field data by earlier investigators this implies that the grounding line of ice stream e is in fact close to dynamic equilibrium neither advancing nor retreating rapidly if the ice stream and its catchment area are approximately in mass balance sensitivity tests showed that the model ice stream is more sensitive to changes in back stress from the ice shelf than to changes in the accumulation rate changes in back stress can be caused by changes in the average thickness of the ice shelf the model was used to simulate retreat of the ice stream from the edge of the continental shelf in the ross sea during the holocene period of rising sea level the sea floor was assumed to be isostatically depressed but to remain rigid during retreat the effect of the assumed retreat history of the ross ice shelf on the computed timing of grounding line retreat was investigated by considering several alternative ice shelf histories the results support the conclusion of earlier investigators that the ross ice shelf caused the grounding line to stop retreating near its present position after extensive retreat during the holocene in addition the timing of retreat of the calving front of the ross ice shelf exerted major influence on the timing of retreat of the ice stream grounding line

rapid sea level rise soon from west antarctic ice sheet collapse there has been recent concern over whether shrinkage of ice sheets in antarctica caused by climate change could lead to rapid global sea level rise in his perspective bentley discusses the processes at work in ice sheet behavior and concludes that in view of the evidence for ice sheet stability catastrophic changes are unlikely in the next century or two

deformation of till beneath ice stream b west antarctica the behaviour and possible instability of the west antarctic ice sheet depend fundamentally on the dynamics of the large ice streams which drain it model calculations show that most ice stream velocity arises at the bed1 2 and radar sounding has shown the bed to be wet3 but the basal boundary condition is not well understood seismic evidence from the upstream b camp upb on the siple coast of west antarctica4 shows that the ice stream there rests on a layer of unconsolidated sediment averaging 5 or 6 m thick in which the water pressure is only 50 kpa less than the overburden pressure because this thin layer occurs well inland beneath an active ice sheet and rests on a surface showing flutes4 characteristic of glacial erosion5 we presume that it is glacial till we propose here that deformation within the till is the primary mechanism by which the ice stream moves and we discuss implications of this hypothesis

comments on petrogeneses and the tectonic setting of columbia river basalts abstract data on major and a few trace element contents for specimens of five formational scale units of the columbia river basalt group are reviewed and compared with those for other flood basalts and the chyulu basalts of kenya flows of the grande ronde formation which constitutes more than two thirds by volume of the columbia river basalt group in several respects resemble calc alkaline rocks and they plot in the field for magmas erupted at destructive plate margins on a th hf ta diagram these features of grande ronde flows may be related to the anomalous setting of the columbia river basalt group as a whole calc alkaline characteristics are exhibited by other flood basalts but rarely to the degree shown by grande ronde flows few if any of the columbia river basalts represent primary magmas rather they generally have been affected by multi stage fractionation and perhaps also by interaction with lower crustal rocks their compositions also may reflect partial melting of fragments of subducted lithospheric slabs

century scale discharge stagnation and reactivation of the ross ice streams west antarctica 1 flow features on the surface of the ross ice shelf west antarctica record two episodes of ice stream stagnation and reactivation within the last 1000 years we document these events using maps of streaklines emerging from individual ice streams made using visible band imagery together with numerical models of ice shelf flow forward model experiments demonstrate that only a limited set of discharge scenarios could have produced the current streakline configuration according to our analysis whillans ice stream ceased rapid flow about 850 calendar years ago and restarted about 400 years later and macayeal ice stream either stopped or slowed significantly between 800 and 700 years ago restarting about 150 years later until now ice stream scenarios emphasized runaway retreat or stagnation on millennial timescales here we identify a new scenario century scale stagnation and reactivation cycles as well as lateral communication with adjacent ice streams through thickness changes on lightly grounded ice plains this introduces uncertainty into predictions for future sea level withdrawals by the west antarctic ice sheet which are based in part on recent slowing of whillans ice stream and the stagnant condition of kamb ice stream

processes driving estuary infilling by marine sands on an embayed coast abstract detailed measurements of waves currents and sediment transport from an intertidal sandflat at the mouth of a partially filled new zealand estuary in an embayment with low littoral drift are presented with a view to establishing how marine sands are transported into the estuary waves mobilised sediments during episodic events early in events when waves were energetic sediments were in motion throughout the tidal cycle under abating waves sediment motion became intermittent finally becoming restricted to periods around low tide when wave orbital motions were able to penetrate to the bed three components of the total sediment flux were estimated from the data and analysed 1 a wind driven current which presumably has a complicated spatial expression in the vicinity of the estuary mouth controlled the net i e integrated over event direction of flux of suspended sediment entrained by waves and advected by currents there was no evidence of non symmetry over the tidal cycle in the suspension process that might drive a net sediment flux however there is some suggestion that lag in growth and decay of the estuary wave field over the tidal cycle systematically enhances offshore transport 2 net bedload transport by waves and currents was directed onshore which was guaranteed by two mechanisms firstly increasing wave orbital current skewness as the tide ebbed turned bedload transport onshore late in the ebb tide against the current and secondly bedload peaked early in the flood tide when skewed waves were reinforced by the flooding current 3 correlation between seaward directed forced mean flow and enhanced suspension under groups of high waves resulted in persistent offshore flux of suspended sediment at infragravity frequencies suspended sediment flux carried by gravity waves was directed seawards on flood tides and counter to the waves and onshore on ebb tides and with the waves which was explained in terms of a simple vortex ejection model we conclude that sediment transport on the sandflat at the mouth of this partially filled estuary is governed by subtle interactions between waves and currents that vary over the tidal cycle some interactions such as those that drive bedload transport promote estuary infilling by marine sands and others such as those that drive suspended sediment flux at infragravity frequencies do the opposite

response of a marine ice sheet to changes at the grounding line abstract a numerical model was designed to study the stability of a marine ice sheet and used to do some basic experiments the ice shelf ice sheet interaction enters through the flow law in which the longitudinal stress is also taken into account instead of applying the model to some measured profile and showing that this is unstable as is common practice in other studies an attempt is made to simulate a whole cycle of growth and retreat of a marine ice sheet although none of the model sheets is particularly sensitive to changes in environmental conditions the question as to what might happen to the west antarctic ice sheet in the near future when a climatic warming can be expecied as a result of the co 2 effect seems to be open for discussion again from the results presented in this paper one can infer that a collapse caused by increased melting on the ice shelves is not very likely

integration of bed characteristics geochemical tracers current measurements and numerical modeling for assessing the provenance of beach sand in the san francisco bay coastal system abstract over 150 million m3 of sand sized sediment has disappeared from the central region of the san francisco bay coastal system during the last half century this enormous loss may reflect numerous anthropogenic influences such as watershed damming bay fill development aggregate mining and dredging the reduction in bay sediment also appears to be linked to a reduction in sediment supply and recent widespread erosion of adjacent beaches wetlands and submarine environments a unique multi faceted provenance study was performed to definitively establish the primary sources sinks and transport pathways of beach sized sand in the region thereby identifying the activities and processes that directly limit supply to the outer coast this integrative program is based on comprehensive surficial sediment sampling of the san francisco bay coastal system including the seabed bay floor area beaches adjacent rock units and major drainages analyses of sample morphometrics and biological composition e g foraminifera were then integrated with a suite of tracers including 87sr 86sr and 143nd 144nd isotopes rare earth elements semi quantitative x ray diffraction mineralogy and heavy minerals and with process based numerical modeling in situ current measurements and bedform asymmetry to robustly determine the provenance of beach sized sand in the region

three dimensional modelling of cohesive sediment transport in the loire estuary in the framework of a european research programme processes describing cohesive sediment transport including fluid mud movements and consolidation have been modelled and implemented in the three dimensional finite element model telemac 3d which solves free surface flow problems the model has been applied to several european estuaries and among them the loire estuary in france this application proved the ability of the code to reproduce typical estuarine features such as the formation of a turbidity maximum and the movements of fluid mud copyright 2000 john wiley sons ltd

assimilation fractional crystallization of polvadera group rocks in the northwestern jemez volcanic field new mexico pliocene polvadera group rocks in the northwestern rio grande rift marginal portion of the jemez volcanic field record the rapid transition from weakly alkaline lobato basalt magmatism 48 52 sio2 7 9 ma through calc alkaline lobato andesite and dacite 53 64 sio2 and tschicoma dacite rhyodacite magmatism 63 69 sio2 7 4 ma petrologically lobato andesite and dacite and tschicoma dacite rhyodacite represent a cogenetic suite of differentiates the la grulla plateau or lgp suite distinctive from the bulk of polvadera group rocks including tschicoma andesite increasing 87sr 86sr o ratios with differentiation within the lgp suite from 0 7051 54 sio2 to 0 7064 68 sio2 trace element variations and disequilibrium mineral assemblages suggest open system differentiation involving 87sr enriched upper crust a likely parental magma is the voluminous lobato basalt 87sr 86 o 0 7043 0 7050 which was erupted predominantly earlier and to the east toward the rift axis the best model for petrogenesis involves bulk assimilation of locally wide spread proterozoic 1 4 1 6 ga upper crustal granite by fractionally crystallizing lobato basalt assimilation fractional crystallization afc modeling of sr isotope and trace element variation depaolo 1981 indicates that 40 crystallization of lobato basalt accompanied by 10 addition of granite reproduces the observed geochemical and sr isotopic diversity neither magma mixing nor mafic recharge have complicated the magmatic evolution of the lgp suite crustal thickness and or retarded tectonism could have facilitated conditions necessary for evolution by afc to occur within the upper crust

in situ estimation of erosion and deposition thresholds by acoustic doppler velocimeter adv abstract field based estimations of bed shear stress have been made using sontek ysi 10 mhz advs a coustic d oppler v elocimeter at the kongsmark mudflat danish wadden sea in order to test if it was possible to estimate erosion and deposition thresholds in situ by use of unidirectional tidal and orbital wave currents the results were promising and erosion thresholds were in the same range as those observed in eromes erosion experiments carried out at the same site similarly the short term erosion rates which could be calculated matched closely those obtained with eromes and were in the same range as those published for a large annular flume sea carousel from another fine grained site this indicates that the erosion rates obtained with those two erosion instruments are reasonable estimates of the actual erosion taking place under natural tidal current and waves one advantage of the use of advs to characterize sediment erodibility is that measurements can be carried out for long periods which makes it possible to examine the temporal variation of erosion threshold erosion rate and sedimentation the maximum bed shear stresses which was observed during a strong storm in january 2005 was only 0 7 n m 2 which is three to four times lower than the highest thresholds measured by eromes experiments on sediment with biofilms during previous studies this shows that such strong biofilms at this site will not get eroded unless degraded or physically disrupted by some other mechanism a prerequisite for the determination of any of the erodibility and sedimentation parameters is that the actual bed level changes at the measuring site are known in detail this information is obtained automatically by the advs used in the present study accurate estimates of the bed shear stress under waves in very shallow water are difficult to obtain and in the present study a simple method based on instantaneous velocities and assumption of a logarithmic velocity profile was used further field studies are required of the boundary layer under these extreme conditions in order to refine this simple method of calculating bed shear stresses

stagnation of ice stream c west antarctica by water piracy the dynamic behavior of the west antarctic ice sheet is of interest because of the possibility that it may change and cause rapid sea level rise attention is focused on the fast moving and rapidly responding ice streams that drain the ice sheet one of these ice stream c largely stopped about a century ago and some models for this shutdown postulate negative feedbacks that would tend to stabilize the ice sheet here new data are presented indicating that the slowdown of the ice stream is restricted to its lower part and occurred because of loss of lubrication on localized sticky spots at the bed of the ice stream the increased friction probably arises from a topographic accident of the glacier bed that has directed lubricating water to the neighboring ice stream b together with slow drawdown of the ice sheet rather than from any general stabilizing feedbacks

field studies of bottom crevasses in the ross ice shelf antarctica analyse des crevasses observees par sondages radar aeriens et terrestres localisation de leurs points de formation

the estimation of shear stresses from near bed turbulent velocities for combined wave current flows abstract high frequency near bottom velocities measured by a combined pressure gauge and flow meter are analysed with a view to understanding and modelling waves and sediment transport in shallow waters data from five stations in different water depth wave and current conditions are compared wave tide surge and turbulence components have been separated from the bottom velocity data and are used to derive a friction velocity and examine the bottom stress parameterisation the horizontal component of turbulent velocity fluctuations near the bed is found to be well correlated with the bed shear stress from a combined wave and current model the model of christoffersen and jonsson christoffersen j b jonsson i g 1985 bed friction and dissipation in a combined current and wave motion ocean engineering 12 5 387 423 is in reasonable agreement with the observed data the observed turbulence can shed light on the wave bed shear stress as well as the shear stress due to the current a fitting procedure gives a derived nikuradse length scale for each station which is similar to that obtained independently from bed grain size estimates

use of in situ flume to quantify particle flux biodeposition rates and sediment erosion for an intertidal mudflat in relation to changes in current velocity and benthic macrofauna

the elevation history of ice streams and the spatial accumulation pattern along the siple coast of west antarctica inferred from ground based radar data from three inter ice stream ridges measurements of the surface and internal layer geometry from ice penetrating radar and global positioning system surveys on three inter ice stream ridges in west antarctica siple dome ridge de and ridge bc are examined with ice flow models to infer 1 the history of the divide position at each site and 2 the spatial pattern of accumulation across the ridges we find that the divide position is most steady at siple dome somewhat steady at ridge de and highly variable at ridge bc data from siple dome and ridge de show evidence for steady northward motion of the ice divide for the past few thousand years the layers beneath ridge bc suggest a 5 km northward shift of the divide position within the past several hundred years assuming the divide shifts are all due to changing elevation of the bounding ice streams we infer the relative elevation history for segments of ice streams b e the northward displacement of the divide for all ridges implies a progressive relative thinning of the ice streams from e to b with most dramatic recent thinning 100 m in 103 years of ice stream b relative to ice stream c analysis of the internal layer pattern across the ridges indicates a south north accumulation gradient with higher accumulation rates on the northern flanks of the ridges in all three cases the inferred accumulation distribution is nearly uniform on the northern flanks decreases sharply within a few ice thicknesses across the divides and then decreases gradually farther to the south the north south decrease is strongest for ridge de and weakest for ridge bc this spatial pattern and the reduction in gradient strength with distance from the amundsen sea is consistent with the hypothesis that storms from the amundsen sea carry moisture first south then west over west antarctica and deposit more snow on the windward side of ridges due to orographic lifting this pattern has been stable for at least the past several thousand years

antarctic ice streams a review an ice stream is a part of an inland ice sheet that flows rapidly through the surrounding ice the distinction between an ice sheet and an outlet glacier is clear in principle but muddy in practice many antarctic glaciers are intermediate in character the ross ice streams which flow through the west antarctic inland ice into the ross ice shelf are distinct in character differing even from other ice streams in the marine ice sheet of west antarctica their surface elevation profiles are low their bed slopes are low and smooth and their driving stresses diminish monotonically downglacier in transverse profile they are broader in relation to ice thickness and exhibit shallower subglacial troughs than other ice streams many models for the fast sliding of glaciers have been applied to the ross ice streams most have included in some form a reduction in basal drag resulting from a lesser effective than glaciostatic pressure at the bed most models are only semi physical at best so need to be tuned by fitting to assumed parameters at some place or places in the real ice sheet the recent discovery of a very small effective pressure beneath one ice stream consequently has led to some gross errors in the velocities predicted by the models the difficulty may be resolved if it is true as recent experiments suggest that ice stream b and by extrapolation other ross ice streams as well slide on a deforming bed that absorbs most or all of the differential motion between the ice and the bedrock