implications of till deformation on glacier dynamics the dynamics of glacier motion are governed to a large extent by the properties of the basal interface in this paper we address the interaction of a glacier with a layer of till at its bed in an attempt to test whether our physical understanding of till is sufficient to explain general features of the observed flow field and changes in geometry of black rapids glacier alaska u s a we also investigate whether or not a till layer has a clear surface observable signature in the dynamics of the glacier towards this end we use a finite element ice flow model with a coulomb failure criterion within the basal till layer we find that simple till physics can be used to describe decadal seasonal and short term hours to days velocity variations and possibly uplift events mechanisms for each of these variations involve an increase in the extent of till at failure a transfer of shear stress across the bed and a consequent increase in ice deformation effective shape factors are calculated that permit a simple incorporation of this boundary condition into glacier response models our analyses however have not resulted in the identification of a clear and unique signature of a till layer in the surface dynamics of a glacier

rheology of till beneath storglaciaren sweden in order to study in situ the rheology of a deforming subglacial till various instruments were emplaced in till beneath storglaciaren sweden boreholes were used to gain access to the till beneath about 100 m of ice tiltmeters provided an estimate of the shear strain rate in the till two other instruments yielded measures of till strength in addition water pressures were recorded in boreholes and in the till a computer controlled distance meter provided an effectively continuous record of the surface velocity and data from frequent surveys of a stake network were used to estimate the mean basal drag based on a force balance calculation tilt rates varied directly with effective pressure so decreases in water pressure apparently increased the coupling between the glacier and the bed surface speed was either out of phase with tilt or varied independently of tilt thus increases in speed were apparently a consequence either of longitudinal coupling or of reduced coupling between the glacier and the bed they were not a result of till deformation till strength varied directly with effective pressure which is consistent with it being a mohr coulomb or frictional material the devices measuring till strength are presumed to have been pulled through the till at a speed that varied in phase with the surface speed but till strength did not vary systematically with surface speed this implies that the residual strength of the till is insensitive to strain rate thus the appropriate constitutive equation for till rheology may be of the form e e k where k is a constant this is consistent with experimental data reported in the geotechnical literature

effects of basal debris on glacier flow glacier movement is resisted partially by debris either within glaciers or under glaciers in water saturated layers in experiments beneath a thick sliding glacier ice containing 2 to 11 debris exerted shear traction of 60 to 200 kilopascals on a smooth rock bed comparable to the total shear traction beneath glaciers and contrary to the usual assumption that debris bed friction is negligible imposed pore water pressure that was 60 to 100 of the normal stress in a subglacial debris layer reduced shear traction on the debris sufficiently to halt its deformation and cause slip of ice over the debris slip resistance was thus less than debris shearing resistance

spatial patterns of glacier motion during a high velocity event haut glacier d arolla switzerland the surface motion of haut glacier d arolla switzerland was monitored at a high spatial and temporal resolution data are analyzed to calculate surface velocities surface strain rates and the components of the glacier force budget before during and after an early melt season speed up or spring event we investigate the extent to which variations in glacier motion can be attributed to hydrologically induced local forcing or to non local forcing transmitted via horizontal stress gradients enhanced glacier motion is dependent on a change in the spatial distribution of areas of high drag across the glacier

shear margins in glaciers and ice sheets analytical and numerical techniques are used to examine the flow response of a sloped slab of power law fluid power n subjected to basal boundary conditions that vary spatially across the flow direction as for example near an ice stream margin with planar basal topography the primary assumption is that basal shear stress is proportional to the basal speed time a spatially variable slip resistance the ratio of mean basal speed to the speed originating from shearing through the thickness denoted as r gives a measure of how slippery the bed is the principal conclusion is that a localized disturbance in slip resistance affects the basal stress and speed in a zone spread over a greater width of the flow in units of ice thickness h the spatial scale of spreading is proportional to a single dimensionless number r n r n 1 1 n 1 derived from n and r the consequence for a shear zone above a sharp jump in slip resistance is that the shearing is spread out over a boundary layer with a width proportional to r n for an ice stream caused by a band of low slip resistance with a half width of wh the margins influence velocity and stress in the central part of the band depending on r n in comparison to w three regimes can be identified which for n 3 are quantified as follows low r defined as r 3 1w for which the boundary layers from both sides bridge across the full flow width and the driving stress in the center is supported almost entirely by side drag intermediate r for which the driving stress in the center is supported by a combination of basal and side drag shear zones that are narrower than predicted on the basis of this theory r 3 would require localized softening of the ice to explain the concentration of deformation at a shorter scale

tidal flexure at ice shelf margins hinge zones accommodate the differential tidal movement between floating ice shelves which move with the sea tide and grounded ice sheets which do not observations on rutford ice stream and ronne ice shelf antarctica using the kinematic method of global positioning system have yielded the first continuous tidal displacement profiles from an ice sheet hinge zone the form of these tidal displacement profiles indicates that the flexure can be fit to an elastic beam model using a parameter space optimization technique the model matches the observed tidal deflection profile to within 5 cm which is similar to the observational uncertainty the same model and parameter fitting technique is then successfully applied to various other published and unpublished tidal displacement data from doake ice rumples bach ice shelf and ekstrom ice shelf antarctica and jakobshavns glacier greenland i conclude that the elastic beam model with a single value of the elastic modulus 0 88 0 35 gpa adequately describes almost all the data and so can usefully be applied elsewhere earlier studies of tidal dissipation in ice shelf hinge zones based on a viscous or transient creep rheology showed that perhaps 30 of global tidal energy dissipation could be occurring in ice shelf hinge zones this study suggests however that an elastic rheology may be equally appropriate in which case this estimation of the tidal energy dissipation is excessively high

ring shear studies of till deformation coulomb plastic behavior and distributed strain in glacier beds a ring shear device was used to study the factors that control the ultimate steady strength of till at high shear strains tests at a steady strain rate and at different stresses normal to the shearing direction yielded ultimate friction angles of 26 3 and 18 6 for tills containing 4 and 30 clay sized particles respectively other tests at steady normal stresses and variable shear strain rates indicated a tendency for both tills to weaken slightly with increasing strain rate this weakening may be due to small increases in till porosity these results provide no evidence of viscous behavior and suggest that a coulomb plastic idealization is reasonable for till deformation however viscous behavior has often been suggested on the basis of distributed shear strain observed in subglacial till we hypothesize that deformation may become distributed in till that is deformed cyclically in response to fluctuations in basal water pressure during a deformation event transient dilation ofdiscrete shear zones should cause a reduction in internal pore water pressure that should strengthen these zones relative to the surrounding till a process called dilatant hardening consequent changes in shear zone position when integrated over time may yield the observed distributed strain

the effect of the subglacial water pressure on the sliding velocity of a glacier in an idealized numerical model in order to interpret observed short term variations of the sliding velocity of a glacier the effect of a variable subglacial water pressure on the sliding velocity has been studied using an idealized numerical model in particular the transient stages of growing or shrinking water filled cavities at the ice bedrock interface were analysed it was found that the sliding velocity was larger when cavities were growing than when they had reached the steady state size for a given water pressure the smallest sliding velocities occurred while cavities were shrinking when cavitation is substantial a small drop of water pressure below the steady state value e g by 0 5 bar can temporarily cause backward sliding a limiting water pressure at which sliding becomes unstable is derived the consequences of more realistic assumptions than those of the model are discussed

application of image cross correlation to the measurement of glacier velocity using satellite image data abstract image to image cross correlation software is applied to pairs of digital satellite images to map the velocity field of moving ice this technique uses small scale glacial surface features such as crevasse scars and snow dunes as markers on the surface of the moving ice displacements of the surface features are mapped by selecting small image areas centered on distinct features or by dividing a large area of densely featured glacial surface into a grid of areas and searching a subsequent image for matching areas using a cross correlation algorithm interpolation of the peak correlation values allows the displacements to be measured to subpixel accuracy resulting in very precise velocity measurements cross correlation is also applied to provide image coregistration in areas devoid of bedrock exposures in such areas subtle large scale topographic undulations in the ice surface related to underlying bedrock structure may be correlated by using large image areas and low pass filtered images both types of applications are demonstrated using ice stream d and ice stream e in west antarctica as test areas a high resolution map of the velocity field of the central portion of ice stream e generated by the displacement measuring technique is presented the use of cross correlation software is a significant improvement over previous manually based photogrammetric methods for velocity measurement and is far more cost effective than in situ methods in remote polar areas

sediment deformation beneath glaciers rheology and geological consequences experiments beneath breidamerkurjokull in iceland have led to development of flow laws for the subglacial till relating strain rate to shear stress and effective pressure and assuming either bingham fluid or nonlinearly viscous fluid behavior water pressures in the till are less than ice pressures and it is suggested that this may lead to infiltration of ice into the sediment which inhibits sliding at the ice sediment interface where water pressures are equal or near to ice pressures infiltration does not occur and sliding may result a one dimensional theory of subglacial deformation is developed in which the empirical flow law is coupled with a model of subglacial hydrology and consolidation this predicts stable states in which subglacial sediment either does not deform or a dilatant deforming horizon forms with positive effective pressures at the ice bed interface or unstable states where zero or negative effective pressures are predicted time dependent analyses show that response times following perturbations of the system may be of the order of 103 years and thus that unsteady behavior may be normal on glaciers flowing over unlithified sediment beds it is suggested that the natural variability of material properties in subglacial sediment beds leads to the development of drumlins on the glacier bed it is suggested that unstable deformation at zero or negative effective stress leads to piping in subglacial sediments at the glacier terminus and the growth of sediment floored subglacial tunnels their frequency is that which is sufficient to draw down subglacial water pressures so as to prevent unstable deformation where they discharge large water volumes subglacial sediments flow laterally toward them producing tunnel valleys this sediment is then removed by water flowing along the axial tunnel tunnel valleys can be regarded as the equivalent in soft sediment areas of eskers in bedrock areas

evolution of subglacial water pressure along a glacier s length observations from along the length of bench glacier alaska usa show that the subglacial water pressure field undergoes a multiphase transition from a winter mode to a summer mode data were collected at the glacier surface the outlet stream and in a network of 47 boreholes spanning the length of the 7 km long glacier the winter pressure field was near overburden with low magnitude centimeter to meter scale and long period days to weeks variations during a spring speed up event boreholes showed synchronous variations and a slight pressure drop from prior winter values diurnal pressure variations followed the speed up with their onset associated with a glacier wide pressure drop and flood at the terminus stream diurnal variations with swings of up to 80 of overburden pressure were typical of mid summer several characteristics of our observations contradict common conceptions about the seasonal development of the subglacial drainage system and the linkages between subglacial hydrology and basal sliding 1 increased water pressure did not accompany high sliding rates 2 the drainage system showed activity characteristic of the spring season long before abundant water was available on the glacier surface 3 the onset of both spring activity and diurnal variations of the drainage system did not show a spatial progression along the length of the glacier

water pressure and basal sliding on storglaciaren northern sweden the subglacial hydrology of the ablation area of storglaciaren a small valley glacier in northern sweden is dramatically affected by a subglacial ridge or riegel water pressures above this riegel are relatively constant while down glacier from it they vary significantly the lower part of the glacier accelerates in response to peaks in basal water pressure the upper part may be weakly coupled to the lower part during these peaks a power law fit of observed basal water pressures and measured surface velocities yields where u s is the surface velocity and p e is the effective water pressure ice overburden pressure minus subglacial water pressure data from findelengletscher reported by iken and bindschadler 1986 yield an identical exponent and a coefficient one order of magnitude larger the similar exponent implies that the process producing the velocity variations on both glaciers is similar the variations in velocity are inferred to be due to hydraulic jacking on both glaciers

transient glacier response with a higher order numerical ice flow model in this paper a higher order numerical flowline model is presented which is numerically stable and fast and can cope with very small horizontal grid sizes 10 m the model is compared with the results from blatter and others 1998 on haut glacier d arolla switzerland and with the european ice sheet modelling initiative benchmarks huybrechts and others 1996 results demonstrate that the significant difference between calculated basal drag and driving stress profiles in a fixed geometry disappears when the glacier profile is allowed to react to the surface mass balance conditions and reaches a steady state dynamic experiments show that the mass transfer in higher order models occurs at a different speed in the accumulation and ablation areas and that the front position is more sensitive to migration compared to the shallow ice approximation

the importance of pressurized subglacial water in separation and sliding at the glacier bed l effet de l eau sous glaciaire sous pression sur le processus de glissement est examine a partir d un parametre appele indice de decollement du lit qui exprime les contraintes perpendiculaires au lit et la pression d eau les donnees proviennent de trois glaciers de formes differentes une loi puissance du type de weertman avec un exposant voisin de 3 modifiee pour tenir compte de l effet de la pression des eaux sous glaciaires apparait comme etant la meilleure

dynamic controls on glacier basal motion inferred from surface ice motion 1 current heuristic laws that relate the motion of glaciers due to sliding along the bed to the subglacial water pressure fail to reproduce variations in sliding speed on timescales of specific hydrologic events such as lake drainage rainfall or surging this may be due to the importance of subglacial cavity evolution and shifts in the glacier stress field both of which are not accounted for in typical sliding laws we use multiple time series of surface motion over a 66 day period at brei amerkurjokull iceland to infer changes in bed separation and longitudinal force budget we observe multiple distinct periods of increased surface motion and uplift corresponding to periods of rainfall and or increased temperatures we find consistent hysteresis and lags between motion and variations in both the bed separation and longitudinal stress gradient that we attribute to the redistribution of normal stresses at the bed during cavity growth increases in the longitudinal stress gradient suggest a downglacier stress transfer during increased basal motion that is consistent with increased drainage system efficiency toward the terminus our results suggest that the transient evolution of the subglacial drainage system and shifts in the glacier stress field are important controls on basal motion

recognition and characterisation of the aeolian component in soils in the girilambone region north western new south wales australia abstract characterisation of aeolian material in the girilambone region of new south wales 600 km nw of sydney using particle size analysis mineralogy geochemistry and micromorphological analysis shows that it consists of spheroidal highly abraded 70 m quartz grains although relatively pure aeolian accessions are found on top of tertiary leucitite flows the aeolian material is generally admixed with residual and transported soil to a depth of 0 3 m throughout the region this study has established a set of criteria for identifying the aeolian contribution to soils in the girilambone region of north western new south wales where it is difficult to distinguish these include a characteristic particle size of 70 m of the quartz grains highly abraded and spherical morphology presence of thin clay coats on well rounded grains and gradational patterns in particle size distribution mineralogy and geochemistry through the soil profile these types of soils with a high content of silt and fine sand do not readily form aggregates particularly where they have low organic content to improve the sustainability of cropping and pastoral activities on these soils their management should aim to maintain or increase structural stability for example by addition of organic material mineral exploration procedures should avoid the diluting aeolian component by sampling the 150 m fraction from surficial soils or material from below 0 3 m

coarse textured basal zones in thin loess deposits products of sediment mixing and or paleoenvironmental change abstract the purpose of this research was to characterize and interpret the coarse basal zones that are common in thin loess deposits that overlie coarser textured sediment to that end we sampled nine pedons in northeastern wisconsin and the upper peninsula of michigan each of which had formed in thin 55 cm loess over sandy glacial sediment at most of these sites the loess became noticeably coarser near the lithologic discontinuity the loess has a primary particle size mode in the coarse silt or fine very fine sand fraction 30 65 m and a secondary mode in the fine or medium sand fraction 200 400 m we attribute the secondary mode to mixing of underlying sands into the loess either during loess deposition or by post depositional pedoturbation in thin loess pedoturbation processes can penetrate into the underlying sediment facilitating mixing upward as far as 50 cm into the loess silt from the loess has also been mixed into the underlying sandy sediment alternatively in some pedons the loess itself coarsens with depth the particle size mode for the loess becomes increasingly coarser with depth this coarsening suggests that during loess deposition one or more of the following was occurring 1 wind velocities were decreasing over time 2 the textural character of the loess source area s were changing decreasing or 3 additional source areas with finer textured sediment became more dominant over time our research demonstrates the utility of detailed particle size data for detecting and interpreting the paleoenvironmental history of loess we also document the extent to which pedoturbation can impact the original textural characteristics of loess or any sediment that occurs as a thin surficial mantle

a new model of topographic effects on the distribution of loess a model of topographic influence on the regional transport and accumulation of loess in sparsely vegetated landscapes is proposed based on the critical role played by saltating eolian sand in the entrainment and long distance transport of finer grained suspended dust in this model long term accumulation of loess occurs mainly downwind of topographic obstacles that limit eolian transport of sand and associated re entrainment of dust regional and local patterns of distribution of late wisconsinan loess in parts of the upper mississippi river basin are closely related to the distribution of topographic obstacles to sand transport these cases of topographic influence occur in a region where evidence exists for sparse tundra like vegetation during the deposition of loess in the late wisconsinan similar topographic effects may not be evident further south in the missouri and mississippi river basins because vegetation density was generally sufficient to prevent the movement of eolian sand out of source valleys thus topographic obstacles and dense vegetation may have similar effects on loess distribution under different environmental conditions

scales and rates of glacial sediment removal a 20 km long 300 m deep trench created beneath breiðamerkurjokull during the little ice age

abrupt glacier motion and reorganization of basal shear stress following the establishment of a connected drainage system three episodes of strong basal motion occurred at trapridge glacier yukon territory canada on 11 june 1995 following the establishment of a connected subglacial drainage system responses to these spring events are noted in the records for 42 instruments and were recorded throughout the 60 000 m study area strong basal motion during the events is indicated by ploughmeter load bolt and vertical strain records and abrupt pressure changes in several transducer records denote damage caused by extreme pressure pulses these pressure pulses generated by the abrupt basal motion also resulted in the failure of seven pressure sensors records for pressure turbidity and conductivity sensors indicate that basal drainage patterns did not change significantly during the events geophone records suggest that the episodes of basal motion were precipitated by the gradual failure of a sticky spot following hydraulic connection of part of the study area this failure resulted in the transfer of basal stress to the unconnected region of the bed during the course of the events no evidence for strong basal motion is seen in the instrument records for several weeks following the events suggesting that the mechanical adjustments resulted in a stable configuration of basal stresses this event illustrates how unstable situations can be quickly accommodated by mechanical adjustments at the glacier bed

basal mechanics of ice stream b west antarctica 2 undrained plastic bed model based on the results of our studies of the physical conditions beneath ice stream b we formulate a new analytical ice stream model the undrained plastic bed model henceforth the upb model mathematically the upb model is represented by a non linear system of four coupled equations which express the relationships among ice sliding velocity till strength water storage in till and basal melt rate we examine this system of equations for conditions of ice stream stability over short timescales that permit holding ice stream geometry constant less than hundreds of years temporal variability is introduced into the upb model only by the direct dependence of till void ratio changes e t on the basal melting rate m r since till strength b e and ice stream velocity u b b change as long as till void ratio varies the first condition for ice stream stability is that of constant till water storage 0 the second condition for ice stream stability arises from the feedback between ice stream velocity till strength and the basal melting rate which depends on shear heating m r u b b this is the weak till condition which requires that in a steady state till strength is a small fraction of the gravitational driving stress b n 1 1 d the salient feature of the upb model is its ability to produce two thermo mechanically controlled equilibrium states one with a strong bed and slow ice velocities ice sheet mode and one with a weak bed and fast ice velocities ice stream mode this bimodality of basal conditions is consistent with the available observations of subglacial conditions beneath slow and fast moving ice in west antarctica basal conditions that do not correspond to these two steady states may occur transiently during switches between the two stable modes the upb model demonstrates that ice streams may be prone to thermally triggered instabilities during which small perturbations in the basal thermal energy balance grow leading to generation or elimination of the basal conditions which cause ice streaming

tidewater glaciers frontal flow acceleration and basal sliding a numerical glacier flow model finite element method is used to suggest the processes that control the flow behind the calving front of a tidewater glacier the model is developed for grounded calving glaciers and includes an effective pressure dependent sliding law the sliding law is implemented by adding a soft basal layer with a variable viscosity the model is applied on hansbreen a tidewater calving glacier in svalbard comparison between modeled surface velocities and observed velocity data shows good agreement we conclude that the flow of a grounded calving glacier can be modeled with an effective pressure dependent sliding law

two modes of accelerated glacier sliding related to water 1 we present the first glacier wide detailed measurement of basal effective pressure and related observations including bed separation to elucidate the role of water in sliding the hard bedded glacier instrumented in our study exhibited two phases of accelerated sliding motion apparently driven by separate mechanisms the first acceleration phase up to 5 fold increase in speed was closely tied to an increase in bed separation the faster second phase up to 9 fold increase in speed was related to an unusually high level of connectivity of subglacial waters we infer the first mode was related to cavity opening and the second mode was related to reduced ice contact with the bed glacier sliding over a hard bed is typically represented by sliding laws that include the effective basal pressure but neither sliding phase was accompanied by a simultaneous decrease in local or regional effective pressure

evidence for basal cavity opening from analysis of surface uplift during a high velocity event haut glacier d arolla switzerland high rates of surface uplift and horizontal velocities were measured during a hydrologically induced spring speed up event spatial patterns of surface uplift are analyzed to estimate components of vertical motion due to flow along an inclined bed and vertical strain areas are identified where surface uplift was most likely due in part to the opening or enlargement of subglacial cavities by bed separation results suggest a widespread enlargement of subglacial cavities during the event and survival of residual cavities after the event the spatial pattern of cavity enlargement closely matches previously identified axes of preferential subglacial drainage it is suggested that localized cavity opening along axes of preferential drainage may constitute the initial stage in the seasonal development of channelized subglacial drainage it is concluded that spatial and temporal variations in glacier motion may play an active role in determining the structure and rate of development of subglacial drainage during the summer melt season

waves of accelerated motion in a glacier approaching surge the mini surges of variegated glacier alaska u s a periods of dramatically accelerated motion in which the flow velocity increases suddenly from about 55 cm d to a peak of 100 300 cm d and then decreases gradually over the course of a day occurred repeatedly during june and july 1978 81 in variegated glacier alaska a surging type glacier that surged in 1982 83 these mini surges appear to be related mechanistically to the main surge the flow velocity peak propagates down glacier as a wave at a speed of about 0 3 km h over a reach of about 6 km in length it is accompanied by a propagating pressure wave in the basal water system of the glacier in which after a preliminary drop the pressure rises rapidly to a level greater than the ice overburden pressure at the glacier bed and then drops gradually over a period of 1 2 d usually reaching a new low for the summer the peak velocity is accompanied by a peak of high seismic activity due to widespread fresh crevassing it is also accompanied by a rapid uplift of the glacier surface amounting to 6 11 cm which then relaxes over a period of 1 2 d maximum uplift rate coincides with the peak in flow velocity the peak in accumulated uplift lags behind the velocity peak by 2 h the uplift is mainly due to basal cavitation driven by the high basal water pressure although the strain wave associated with the mini surge motion can also contribute basal cavitation is probably responsible for the pulse of high turbidity that appears in the terminal outflow stream in association with each mini surge in the down glacier reach where the mini surge waves are attenuating the observed strain wave corresponds to what is expected for the propagating pulse in flow velocity but in the reach of maximum mini surge motion the strain wave has a form quite different possibly related to special features in the mini surge initiation process from that point up stream the flow acceleration in the mini surges is due to enhanced basal sliding caused by the high basal water pressure and the consequent reduction of bed friction a preliminary velocity increase shortly before the pressure wave arrives is caused by the forward shove that the main accelerated mass exerts on the ice ahead of it and the resulting preliminary basal cavitation causes the drop in water pressure shortly before the pressure wave arrives the mini surge wave propagation is controlled by the propagation of the water pressure wave in the basal water conduit system the propagation characteristics result from a longitudinal gradient up glacier increase in hydraulic conductivity of the basal water system in response to the up glacier increase of the basal water pressure in the mini surge wave the mini surge waves are initiated in a succession of areas situated generally progressively up glacier during the course of the summer season in these areas presumably melt water that has accumulated in subglacial reservoirs is released suddenly into the basal water system immediately below generating a pressure rise that propagates down stream from there relationships of the mini surges to the main surge are seen in the role of high basal water pressure in causing the rapid glacier motion in both phenomena in the pulse propagation features of both and in the high outflow turbidity associated with both the mini surges of variegated glacier have a strong resemblance to movement and uplift events observed in unteraargletscher and findelengletscher switzerland this bears on the question whether the mini surges are a particular characteristic of surge type glaciers prior to surge

the record of continental dust from australia in tasman sea sediments abstract the flux of aeolian dust from australia to sediments of the tasman sea has increased during glacial stages of the last four glacial cycles low fluxes of dust from australia before 350 ka were followed by an increase in stage 10 and high fluxes in peak glacial periods thereafter the initiation of higher dust fluxes in stage 10 is consisten with previous estimates of the age of onset of aridity in the source area dust flux to the tasman sea is thought to record the intensity of deflation from the source area in interior southeastern australia which was marked by arid conditions and possibly greater windiness during the last glacial a transect of five cores from the central tasman sea revealed consistently low dust fluxes north of 33 s increasing southward to approximately 40 s glacial to interglacial movement of the northern boundary of the dust plume governed by the position of the subtropical ridge was less than 6 latitude probably closer to 3 350 km the position of the dust plume determined in this study and its response to climate change are significantly different from a previous reconstruction based on the measurement of quartz content of the non biogenic sediment fraction

instruments and methods direct measurement of sliding at the glacier bed sliding at the base of trapridge glacier yukon territory canada was measured using a drag spool we describe this simple and inexpensive instrument as well as its installation and operation from 1990 to 1992 seven sites were instrumented with drag spools at six of the sites basal sliding during the period of observation accounted for 50 70 of the total flow observed at the glacier surface the contribution from ice creep is known to be small so most of the remaining surface motion must be attributed to subglacial sediment deformation for the seventh site the observed sliding rate was 90 of the total flow an indication that the sliding contribution varies spatially across the bed diurnal variations in the response of one of our instruments appear to be correlated to subglacial water pressure fluctuations and are interpreted in terms of changes in sliding velocity rather than the opening and closing of basal cavities

seasonal changes in the morphology of the subglacial drainage system haut glacier d arolla switzerland dye tracing techniques were used to investigate the glacier wide pattern of change in the englacial subglacial drainage system of haut glacier d arolla during the ablation seasons of 1990 and 1991 analysis of breakthrough curve characteristics indicate that over the course of a melt season a system of major channels developed by headward growth at the expense of a hydraulically inefficient distributed system by the end of the melt season this channel system extended at least 3 3 km from the snout of the 4 km long glacier and drained the bulk of supraglacially derived meltwater passing through the glacier the upper limit of the channel system closely followed the retreating snowline up glacier rates of headward channel growth reached c 65 m d 1 although these rates decreased in the upper 1 km of the glacier where snowline retreat exposed a patchy firn aquifer it appears that the removal of snow with its high albedo and significant water storage capacity from the glacier surface resulted in a dramatic increase in the volume of runoff into moulins and in the peakedness of daily runoff cycles this induced transient high water pressures within the distributed drainage system which caused it to evolve rapidly into a channelised system it is therefore likely that at a local scale channel growth occurred down glacier from moulins and that the overall up glacier directed pattern of channel formation was caused by the retreating snowline exposing new moulins and crevasses to inputs of ice derived meltwater damping of diurnal melt inputs by storage in the firm aquifer accounts for the slowing of channel growth in the upper glacier 1998 john wiley sons ltd

surface melt induced acceleration of greenland ice sheet flow ice flow at a location in the equilibrium zone of the west central greenland ice sheet accelerates above the midwinter average rate during periods of summer melting the near coincidence of the ice acceleration with the duration of surface melting followed by deceleration after the melting ceases indicates that glacial sliding is enhanced by rapid migration of surface meltwater to the ice bedrock interface interannual variations in the ice acceleration are correlated with variations in the intensity of the surface melting with larger increases accompanying higher amounts of summer melting the indicated coupling between surface melting and ice sheet flow provides a mechanism for rapid large scale dynamic responses of ice sheets to climate warming

basal mechanics of ice stream b west antarctica 1 till mechanics data from laboratory geotechnical tests on till recovered from beneath ice stream b west antarctica at the upstream b camp hereinafter the upb till show that failure strength of this till is strongly dependent on effective stress but is practically independent of strain and strain rate these data support use of a coulomb plastic rheology in modeling of ice stream behavior and subglacial till deformation our testing program combined triaxial ring shear and confined uniaxial tests to investigate till strength and compressibility results show that the upb till follows closely coulomb s equation in which shear strength is a linear function of normal effective stress apparent cohesion near zero and internal friction angle equal to 24 till compressibility is best described by a logarithmic function that relates void ratio to normal effective stress in general the behavior of the upb till is consistent with other experimental evidence regarding mechanical behavior of granular materials based on our laboratory results we formulate the compressible coulomb plastic till model in which there are three interrelated primary state variables shear strength void ratio and normal effective stress this model is used in the second part of our study to simulate response of subglacial till to realistic effective stress forcings these simulations demonstrate that the model is capable of reproducing fundamental aspects of subglacial till kinematics 1 occurrence of tilt rate oscillations and negative tilt rates in tiltmeter records and 2 distributed till deformation to depths of 0 1 1 0 m beneath the ice base our laboratory and modeling results substantiate application of the compressible coulomb plastic model in simulations of the motion of ice stream b over its weak till bed

observations of a reversal in vertical and horizontal strain rate regime during a motion event on unteraargletscher bernese alps switzerland during a motion event on unteraargletscher bernese alps switzerland in spring 1996 surface velocities were measured up to eight times a day at four different locations along the central flowline using global positioning system equipment in addition accumulated vertical strains over the uppermost 50 and 100 m were measured at a location where the total ice thickness is 260 m the motion event was accompanied by high horizontal and vertical strain rates as compared to annual mean values a reversal in strain regime was observed with horizontal strain rates changing to extension while vertical strain rates became compressive this strain rate reversal coincided within the temporal resolution of the data with a maximum in vertical ice displacement at the surface within a day variations in vertical strain from 0 04 a 1 to 0 06 a 1 were observed over the uppermost 100 m vertical stretching is estimated to have contributed to at least 20 of the anomalous vertical ice movement at the surface there were significant differences between measured longitudinal strain averaged over a distance corresponding to a few ice thicknesses and measured vertical strain in spring 1997 a similar but more detailed set of measurements was collected at the same measuring site and vertical strain rates were found to vary non uniformly with depth with the largest values closest to the surface

spatial and temporal evolution of rapid basal sliding on bench glacier alaska usa we measured the surface velocity field during the summers of 1999 and 2000 on the 7 km long 185 m thick bench glacier alaska usa in the spring of both years a short lived pulse of surface velocity 2 4 times the annual mean velocity propagated up glacier from the terminus at a rate of 200 250md 1 displacement attributable to rapid sliding is 5 10 of the annual surface motion while the high velocity event comprised 60 95 of annual basal motion sliding during the propagating speed up event peaked at 6 14 cm d 1 with the highest rates in mid glacier continuous horizontal and vertical gps measurements at one stake showed divergence and then convergence of the ice surface with the bed as the velocity wave passed with maximum surface uplift of 8 16 cm high divergence rates coincided with high horizontal velocities suggesting rapid sliding on the up glacier side of bedrock steps initiation of the annual speed up event occurred during the peak in englacial water storage while the glacier was entirely snow covered basal motion during the propagating speed up event enlarges cavities and connections among them driving a transition from a poorly connected hydrologic system to a well connected linked cavity system sliding is probably halted by the development of a cnduit system

mechanical and hydrologic basis for the rapid motion of a large tidewater glacier 1 observations the data presented in part 1 of this paper meier et al this issue are here used to assess the role of water input output water storage and basal water pressure in the rapid movement of columbia glacier alaska consistently high basal water pressures mostly in the range from 300 kpa below to 100 kpa above the ice overburden pressure are responsible in an overall way for the high glacier flow velocities 3 5 9 m d 1 which are due mainly to rapid basal sliding caused by the high water pressure diurnal fluctuation in basal water pressure is accompanied by fluctuation in sliding velocity in what appears to be a direct causal relation at the upglacier observation site the water pressure fluctuation tracks the time integrated water input less a steady withdrawal as expected for the diurnally fluctuating storage of water in the glacier far from the terminus at the downglacier site the situation is more complex diurnal peaks in water level which are directly related to intraglacial water storage as well as to basal water pressure are shifted forward in time by 4 hours probably as a result of the effect of diurnal fluctuation in water output from the glacier which affects the local water storage fluctuations near the terminus because of the forward shift in the basal water pressure peaks which at the downglacier site lead the velocity peaks by 6 hours a mechanical connection between water pressure and sliding there would have to involve a 6 hour quarter period delay however the nearly identical nature of the diurnal fluctuations in velocity at the two sites argues for a single consistent control mechanism at both sites the velocity variations in nondiurnal speed up events caused by extra input of water on the longer timescale of several days are only obscurely if at all correlated with variations in basal water pressure but correlate well with water storage in the glacier it appears that small variations in water pressure 100 kpa sufficient to produce the observed velocity variations 15 30 are mostly masked by pressure fluctuations caused by spontaneous local reorganizations of the basal water conduit system on a spatial scale much smaller than the longitudinal coupling length over which basal water pressure is effectively averaged in determining the sliding velocity at the achieved level of observation the clearest though not complication free control variable for the sliding velocity variations is basal water storage by cavitation at the glacier bed

geochemical and microbiological fingerprinting of airborne dust that fell in canberra australia in october 2002 during the night of 22 23 october 2002 a large amount of airborne dust fell with rain over canberra located some 200 km from australia s east coast and at an average altitude of 650 m it is estimated that during that night about 6 g m 2 of aeolian dust fell we have conducted a vast number of analyses to fingerprint some of the dust and used the following techniques grain size analysis scanning electron microscope imagery major trace and rare earth elemental plus sr and nd isotopic analyses organic compound analyses with respective compound specific isotope analyses pollen extraction to identify the vegetation sources and molecular cloning of 16s rrna genes in order to identify dust bacterial composition dna analyses show that most obtained 16s rrna sequences belong mainly to three groups proteobacteria 25 bacteriodetes 23 and gram positive bacteria 23 in addition we investigated the meteorological conditions that led to the dust mobilization and transport using model and satellite data grain sizes of the mineral dust show a bimodal distribution typical of proximal dust rather than what is found over oceans and the bimodal aspect of size distribution confirms wet deposition by rain droplets the inorganic geochemistry points to a source along near the darling river in nw new south wales a region that is characteristically semiarid and both the organic chemistry and palynoflora of the dust confirm the location of this source area meteorological reconstructions of the event again clearly identify the area near bourke cobar as being the source of the dust this study paves the way for determining the export of australian airborne dust both in the oceans and other continents

what s that smell hydrogen sulphide transport from bardarbunga to scandinavia abstract on sep 9 2014 several incidences of foul smell rotten eggs were reported on the coast of norway in particular in the vicinity of molde and then on sep 10 in the interior parts of county vasterbotten sweden one of the theories that were put forward was that the foul smell was due to degassing of the bardarbunga volcano on iceland using satellite images gome 1 2 of the sulphur dioxide so 2 contents in the atmosphere surrounding iceland to estimate flux of so 2 from the volcano and an atmospheric transport model pello we vindicate this theory we argue that the cause for the foul smell was hydrogen sulphide originating from bardarbunga the model concentrations are also compared to so 2 concentration measurements from muonio finland

short term velocity variations on hansbreen a tidewater glacier in spitsbergen spatial and temporal variations of the flow of hansbreen a tidewater glacier in southern spitsbergen svalbard are investigated during summer 1999 surface flow velocities were measured in the ablation zone of hansbreen with a temporal reso lution of 3 4 hours short events with strongly increased surface velocities and a typical duration of 1 2 days were observed these speed up events are related to periods of strongly increased water input to the glacier due to rainfall or enhanced surface melt a close relation is found between the surface velocities and water pressure recorded in a moulin however there are indications from a short time lag between velocity and water pressure peak as well as from observed vertical surface uplifts that basal motion is related to basal water storage rather than directly to basal water pressure the observed short term velocity variations and associated processes on hansbreen are very similar to those observed on land based valley glaciers and suggest that the relevant mechanisms and physical processes that control the flow and its temporal variations are similar in contrast to the flow of land based glaciers sliding velocities on hansbreen are observed to be high all year round and velocities increase towards the calving front

basal conditions and glacier motion during the winter spring transition worthington glacier alaska u s a observations of the motion and basal conditions of worthington glacier alaska u s a during late winter and spring melt seasons revealed no evidence of relationship between water pressure and sliding velocity measurements included borehole water levels used as a proxy for basal water pressure surface velocity englacial deformation sliding velocity and time lapse videography of subglacial water flow and bed characteristics the boreholes were spaced 10 15 m apart six were instrumented in 1997 and five in 1998 in late winter the water pressure field showed spatially synchronous fluctuations with a diurnal cycle the glacier s motion was relatively slow and non cyclic in spring the motion was characterized by rapid diurnally varying sliding the basal water pressure displayed no diurnal signal but showed high magnitude fluctuations and often strong gradients between holes this transition in character of the basal water pressure field may represent a seasonal evolution of the drainage system from linked cavities to a network of isolated patches and conduits these changes occurred as the glacier was undergoing a seasonal velocity peak the apparent lack of correlation between sliding velocity and water pressure suggests that local scale water pressure does not directly control sliding during late winter or early in the melt season

the relationship between subglacial water pressure and velocity of findelengletscher switzerland during its advance and retreat findelengletscher switzerland advanced about 250 m between 1979 and 1985 and retreated thereafter subglacial water pressure surface velocity and surface strain rate were determined at several sites the measurements were made early in the melt seasons of 1980 1982 1985 and 1994 and in the autumn of 1983 and the winter of 1984 changes of surface geometry were assessed from aerial photographs the estimated basal shear stress changed little between 1982 and 1994 nevertheless large changes in the relationship of subglacial water pressure and surface velocity were observed which cannot be reconciled with the most commonly used sliding law unless it is modified substantially consideration of possible reasons indicales that a change in the subglacial drainage system occurred probably involving a change in the degree of cavity interconnection isolated cavities damp the variations in sliding velocity that normally result from changes in water pressure because the pressure in isolated cavities decreases as the sliding speed increases in contrast by transmitting water pressure fluctuations to a larger area of the bed interconnected cavities amplify the effect of water pressure fluctuations on sliding speed thus we suggest that an observed decrease in velocity for a given water pressure between 1982 and 1994 was a consequence of a decrease in the interconnectedness of the subglacial cavity system

time dependent basal stress conditions beneath black rapids glacier alaska usa inferred from measurements of ice deformation and surface motion observations of surface motion and ice deformation from 2002 03 were used to infer mean stress fields in a cross section of black rapids glacier alaska usa over seasonal timescales basal shear stresses in a well defined zone north of the center line orographic left were approximately 7 and 16 lower in spring and summer respectively than in winter correspondingly higher stresses were found near the margins these changes in the basal shear stress distribution were sufficiently large to cause mean surface velocities to be 1 2 and 1 5 times larger in spring and summer than in winter these results were inferred with a simple inverse finite element flow model that can successfully reproduce bulk surface velocities and tiltmeter data stress redistribution between the well defined zone and the margins may also occur over much shorter time periods as a result of rapidly changing basal conditions ice bed decoupling or enhanced till deformation thereby causing large variations in surface velocity and strongly influencing the glacier s net motion during summer

force balance along an inland tributary and onset to ice stream d west antarctica the transition from inland to streaming style ice flow near to and upstream from the onset to ice stream d west antarctica is investigated using the force balance technique basal drag provides the majority of the flow resistance over the study area but is substantially modified by non local stress gradients lateral drag increases with distance downstream balancing 50 100 of the driving stress at the onset longitudinal stress gradients lsg are also found to be significant an observation that distinguishes ice flow in this region from the inland and streaming flow regimes that bound it in which lsg are usually negligible lsg decrease the spatial variability in basal drag and sliding speed and increase the area of the bed over which frictional melting occurs overall lsg decrease the resistive influence of basal stress concentrations and increase the spatial uniformity of basal sliding these observations suggest that streaming flow develops as an integrated response to the physical interaction between the ice and its bed over an extended region upstream from the onset rather than being solely due to changes in basal characteristics at the onset an implication is that non steady flow behavior upstream from the onset may ultimately propagate downstream and result in non steady behavior at the onset

coupling between a glacier and a soft bed i a relation between effective pressure and local shear stress determined from till elasticity to predict the distribution of motion beneath glaciers on soft beds the strength of the coupling between the ice and the bed and its variation with effective pressure must be known a record of shear strain acquired with a tiltmeter emplaced in till beneath storglaciaren sweden indicates that fluctuations in water pressure cause variations in the local shear stress on the bed and that the bed deforms elastically in response to these variations to estimate the shear stress from the elastic component of the total shear strain the shear modulus of the till was measured in relaxation tests conducted in the laboratory with a ring shear device after accounting for the elastic compliance of the device these tests yielded shear moduli of about 1000 and 1800 kpa at confining pressures of 85 and 280 kpa respectively these values are comparable to those of other granular materials undergoing recoverable shear strains of the same magnitude the local shear stress on the till calculated by applying the measured shear moduli to the tilt record scales with p e 1 7 where p e is the effective pressure this relation implies that as p e decreases at the ice till interface shear stresses on the till are reduced and concentrated elsewhere on the bed perhaps where the till is absent or the glacier is frozen to the bed when compared with the strength of the till determined from ring shear tests this relation also accounts for the lack of permanent deformation at depth in the bed during periods of low p e and indicates that most basal motion was by sliding or ploughing

rapid retreat and acceleration of helheim glacier east greenland 1 a significant amount of the measured coastal thinning of the greenland ice sheet may be due to recent acceleration of outlet glaciers using remote sensing we measured two major periods of speedup on helheim glacier between 2000 and 2005 that increased peak speeds from approximately 8 to 11 km yr these speedups coincided with rapid retreats of the calving front totaling over 7 5 km the glacier also thinned by over 40 m from 2001 to 2003 retreat of the ice front appears to decrease resistance to flow and concentrates the gravitational driving force over a smaller area farther up glacier acceleration may be a delayed response to surface draw down and steepening of the glacier s main trunk if the 2005 speedup also produces strong thinning then much of the glacier s main trunk may un ground leading to further retreat citation howat i m i joughin s tulaczyk and s gogineni 2005 rapid retreat and acceleration of helheim glacier east greenland geophys res lett 32 l22502 doi 10 1029 2005gl024737

evolving force balance at columbia glacier alaska during its rapid retreat 1 changes in driving and resistive stresses play an essential role in governing the buoyancy forces that are important controls on the speed and irreversibility of tidewater glacier retreats we describe changes in geometry velocity and strain rate and present a top down force balance analysis performed over the lower reach of columbia glacier our analysis uses new measurements and estimates of basal topography and photogrammetric surface velocity measurements made between 1977 and 2001 while assuming depth independent strain sensitivity tests show that the method is robust and insensitive to small changes in the calculation parameters spatial distributions of ice speed show little correspondence with driving stress instead spatial patterns of ice speed exhibit a nonlinear correspondence with basal drag primary resistance to flow comes from basal drag but lateral drag becomes increasingly more important throughout the retreat which may account for observed increases in speed maximum basal drag is always located in a prominent constriction located 12 km upstream from the preretreat terminus once the terminus retreated into deep water off the terminal moraine marking the modern maximum extent the upstream location of this maximum basal drag helped to promote thinning and decrease effective pressure in the lower region by limiting replenishing ice flow from upstream an increase in both ice velocity and calving resulted initiating what appears to be an irreversible retreat

pedogenetic interpretations of particle size distribution curves for an alpine environment abstract the qilian mountains are generally capped by a productive silty soil layer wherever the environment allows as holocene loess is prevalent across the qilian mountains we assume that at least some of these fine sediments are derived from loess deposition and that soils across the region may be genetically linked to test these hypotheses nine pedons in different landscapes within a typical alpine watershed of the qilian mountains were sampled we also collected fine earth samples from glacier surfaces and crevices of glacial debris in the moraine zone the particle size distribution psd is used as a proxy for identifying aeolian fractions in soils the psd curves of all of the fine earth fractions examined are polymodal although three modal sizes of roughly 16 m 35 m and 80 m are found in almost all sediments in varying proportions these modal sizes have been identified as three sources of aeolian sediments in different transport systems the composition and thickness of loess sediments are related to local site conditions and may be related to the paleoenvironmental history of the site clastic debris and vegetation cover serve as dust traps during different stages of soil formation in this alpine environment and soils grow upward with accumulating loess sediment our study demonstrates that psd curves can be used to determine the origins and especially aeolian origins of soils furthermore this study provides insight into the pedology ecology and paleoenvironmental history of loess affected ecosystems in alpine areas of the qinghai tibetan plateau

aeolian dust contributions to soil of the namoi valley northern nsw australia abstract aeolian processes have been regarded as having little relevance to pedogenesis in the subhumid regions of eastern australia due perhaps to a perceived paucity of clear evidence of dust deposition and to difficulties of measuring an aeolian dust component within soils contrary to earlier opinions an aeolian soil component may occur in eastern australia as appreciable amounts of dust are deposited annually we report dust deposition rates and some dust characteristics at gunnedah and measured granulometric mineralogical and geochemical attributes of four soil profiles found in different physiographic areas of the namoi valley estimates of dust deposition rates indicate that a deposit at least 20 cm thick could have formed over the last 13 000 years the soil profile which most clearly indicates the presence of dust overlies basalt at bald hill here the topsoil contains a conspicuous population of particles with a modal diameter of 40 50 m and has been enriched in si by the addition of quartz which cannot have originated from the basalt or from colluvial or alluvial processes a clayey floodplain soil profile boonawa also contained a large dust particle population and also illite and moderate concentrations of k of possible aeolian origin two sandy profiles at round swamp displayed the same conspicuous dust particle populations and one exhibited an interstratified illite smectite component suggesting dust additions local aeolian processes were also indicated by the well sorted coarse sand sized particles of the round swamp east profile located on a low lunette adjacent to the swamp these fine grained quartz rich aeolian dust deposits differ considerably from the benchmark australian aeolian dust mantles which form the clay aggregated calcareous parna soils of southern new south wales

rigorous gps data processing strategies for glaciological applications global positioning system gps data are now routinely used for many glaciological applications in some common cases systematic errors are unmodelled at the data processing stage although they are often presumed insignificant in this paper i investigate these assumptions for three different scenarios 1 measurements on a moving glacier 2 measurements on a floating ice shelf and 3 precise height determination over large elevation ranges such as for aircraft positioning in lidar laser altimeter missions in each case systematic errors are shown to be present in the coordinate solutions that have a far greater magnitude than the formal error estimates produced by the gps processing software under certain conditions if these coordinate biases go undetected short and long term measurements of horizontal ice velocity or rates of ice thickness change may be erroneous and the coordinates could not be expected to match rigorously processed data or results from different processing techniques more rigorous processing strategies are discussed that allow for bias free parameter estimation

multidisciplinary characterization of the middle holocene eolian deposits of the elsa river basin central italy abstract in temperate regions eolian deposits of different natures are often pedogenically altered and mixed with underlying sediments the research reported in this paper identifies for the first time the presence of eolian deposits in soils of central tuscany and investigates the characteristics and origin five relict polycyclic paleosol profiles were studied p1 was situated in a natural dust trap a doline on top of an isolated limestone hill p2 and p3 were both situated on a limestone plateau and p4 and p5 on a mid and foot slope on schist the profiles were sampled for routine analyses iron forms heavy minerals major and trace elements and pollen spores and non pollen palynomorphs undisturbed samples were taken for micromorphological and sem analyses nine soil horizons were sampled for osl dating eolian deposits were dominant or abundant in the first layers of p1 p2 and p3 osl age determinations of soil horizons deriving from eolian parent material were middle holocene chemical and heavy mineral analyses indicated different possible sources including bare slopes alluvial fans and wide channels coming from the nearby streams that drain the middle tuscany ridge and the mounts of chianti marginal contribution of volcanic ashes was only found in p1 the research demonstrated that wind soil erosion accompanied water erosion and colluvial deposition during the middle holocene in the elsa river basin pollen spectra in particular indicated that soil degradation occurred in an environment showing signs of incipient desertification resulting from an increase of aridity in a land already strongly influenced by humans although central italy is currently considered to be only marginally affected by wind soil erosion a climate change which would imply increased arid conditions could trigger a new cycle of slope denudation wind erosion and loess deposition

basal perturbations under ice streams form drag and surface expression classical sliding theories consider ice sliding over obstacles which are much shorter than the thickness of overlying ice here we present a theory which considers form drag generated under ice streams by large obstacles such as subglacial bedforms which may have lengths comparable to ice thickness we also investigate how perturbations at the surface of an ice stream can be generated by such bedforms and develop a mathematical framework for separating the effects of such local kilometre scale variations in ice flow from the bulk flow of the ice stream

combined measurements of subglacial water pressure and surface velocity of findelengletscher switzerland conclusions about drainage system and sliding mechanism interpretation des mesures de pression dans les forages et des vitesses superficielles la comparaison avec les modeles suggere que l on a neglige le frottement du aux debris dans les modeles et qu il n existe jamais de chenaux cylindriques rectilignes