variability in the mass flux of the ross ice streams west antarctica over the last millennium we synthesize previously published remote sensing observations radar data and model output to obtain a 1000 year ice flow history for the siple coast ice stream system in west antarctica to investigate the timing and magnitude of changes in mass flux the synthesized history shows significant short term variability in ice stream shear margin and grounding line position due to internal variability of the coupled system the chronology highlights the interplay between adjacent ice streams which implies that the behavior of any individual ice stream should not be examined in isolation furthermore individual events cannot be fully interpreted without an understanding of the broad scale long term variability in the ice sheet in the context of this millennium scale history we interpret the relatively recent stagnation of kamb ice stream kis as just one stage in the thermodynamic cycle of an ice stream in this region the changes in mass balance that result from the kis stagnation may thus be viewed as century scale noise relative to the longer term trend understanding and characterizing this noise is a necessary step before accurate model based predictions of ice sheet mass balance for the next century can be made

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

bed topography and subglacial landforms in the onset region of the northeast greenland ice stream the northeast greenland ice stream negis is an important dynamic component for the total mass balance of the greenland ice sheet as it reaches up to the central divide and drains 12 of the ice sheet the geometric boundary conditions and in particular the nature of the subglacial bed of the negis are essential to understand its ice flow dynamics we present a record of more than 8000 km of radar survey lines of multi channel ultra wideband radio echo sounding data covering an area of 24 000 km2 centered on the drill site for the east greenland ice core project egrip in the upper part of the negis catchment our data yield a new detailed model of ice thickness distribution and basal topography in the region the enhanced resolution of our bed topography model shows features which we interpret to be caused by erosional activity potentially over several glacial interglacial cycles off nadir reflections from the ice bed interface in the center of the ice stream indicate a streamlined bed with elongated subglacial landforms our new bed topography model will help to improve the basal boundary conditions of negis prescribed for ice flow models and thus foster an improved understanding of the ice dynamic setting

a variational approach to ice stream flow ice sheets are susceptible to the formation of ice streams or narrow bands of fast flowing ice whose high velocities are caused by rapid sliding at the contact between ice and the underlying bed based on recent geophysical work which has shown that the sliding motion of ice streams may be described by a coulomb friction law we investigate how the location of ice streams depends on the geometry of an ice sheet and on the mechanical properties of the underlying bed more generally this problem is relevant to the flow of thin films with coulomb or solid friction laws applied at their base by analogy with friction problems in elasticity we construct a variational formulation for the free boundary between ice streams where bed failure occurs and the surrounding ice ridges where there is little or no sliding this variational problem takes the form of a non coercive variational inequality and we show that solutions exist provided a force and moment balance condition is satisfied in that case solutions are also unique except under certain specialized circumstances which are unlikely to arise for a real ice sheet further we show how the variational formulation of the ice flow problem can be exploited to calculate numerical solutions and to simulate the effect of changing ice geometry and bed friction on the location and velocities of streaming flow lastly we study the effect of ice shelf buttressing on the flow of ice streams whose spatial extent is determined by the yield stress distribution of the bed in line with previous studies of ice shelf buttressing we find that the removal of an ice shelf can cause an ice stream feeding the ice shelf to speed up considerably which underlines the important role ice shelves may play in controlling the dynamics of marine ice sheets

a different perspective for the mars rover opportunity site fine grained consolidated hematite and hematite coatings 1 since 2001 there have been two parallel interpretations of mars global surveyor thermal emission spectrometer tes observations of sinus meridiani which are 1 coarse grained gray hematite is the only spectral match and 2 fine grained hematite with particles closer than wavelength fine intimate hematite e g coating ferricrete is a better match but coarse hematite is also viable the tes team interpreted the spectra as consistent only with a large deposit 750 km 350 km of coarse hematite 5 10 m grain size coarse hematite is considered strong evidence for longstanding water which led to the decision to land the rover opportunity there on the other hand the aerospace lpi remote sensing team argued that fine intimate hematite can better match tes spectra a thin coating 5 10 m thick and a low exposure 5 could cause the observed signatures the distinction is important because 1 it is unknown whether fine grained hematite implies abundant water 2 fine intimate hematite may explain the non detection of coexisting aqueous alteration minerals and the lack of hematite wind streaks 3 current hematite abundance maps may instead map the surface texture 4 coatings may be of astrobiology interest 5 studies are needed to determine whether visible infrared spectra can definitively distinguish fine intimate from coarse hematite

model experiments on the evolution and stability of ice streams this is the publisher s version also available electronically from http www igsoc org annals 23

positive mass balance of the ross ice streams west antarctica we have used ice flow velocity measurements from synthetic aperture radar to reassess the mass balance of the ross ice streams west antarctica we find strong evidence for ice sheet growth 26 8 gigatons per year in contrast to earlier estimates indicating a mass deficit 20 9 gigatons per year average thickening is equal to 25 of the accumulation rate with most of this growth occurring on ice stream c whillans ice stream which was thought to have a significantly negative mass balance is close to balance reflecting its continuing slowdown the overall positive mass balance may signal an end to the holocene retreat of these ice streams

a continuum mixture model of ice stream thermomechanics in the laurentide ice sheet 1 theory we employ a continuum mixture framework to incorporate ice streams in a three dimensional thermomechanical model of the laurentide ice sheet the ice mass is composed of a binary mixture of sheet ice which deforms by viscous creep and stream ice which flows by sliding and or sediment deformation at the bed dynamic and thermal evolutions are solved for each component in the mixture with coupling rules to govern transfer between flow regimes we describe two different transfer mechanisms 1 creep exchange the nourishment of ice streams by viscous creep inflow from the surrounding ice sheet and 2 bed exchange the activation growth and deactivation of ice streams perpetrated by transfers of bed area between flow constituents this paper develops the underlying mixture theory we express the governing equations for mass momentum and energy balance in a form suitable for direct incorporation in existing numerical models of ice thermomechanics a companion paper in this issue explores mixture and ice stream behavior in applications with the laurentide ice sheet

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

origin and consequences of cyclic ice rafting in the northeast atlantic ocean during the past 130 000 years deep sea sediment cores recovered from the northeast atlantic ocean were examined in order to elucidate the influence of the earth s orbital parameters on major ice rafting analyses of coarse grained ice rafted debris and planktonic foraminifers revealed a strong reaction to the precession signal since 130 000 yr b p dropstone layers have been deposited each half period of a precessional cycle 11 000 1000 yr ice rafting occurs during times of winter minimum summer maximum insolation and summer minimum winter maximum insolation in the first case high summer insolation forces meltwater discharge from the ice sheets into the polar seas which subsequently enhances formation of sea ice during the winter in the second case growth of continental ice enhances iceberg production which also leads to a salinity reduction of surface seawater both situations result in a southward penetration of polar water thus the marine record of dropstones documents ice rafting not only during weichselian stades but also during cold events within interstades the regularity of ice rafting yields a useful framework to calibrate and elucidate climatic changes not only in the region of the north atlantic ocean but also in remote areas such as the pacific ocean and the antarctic

a theory for ice till interactions and sediment entrainment beneath glaciers and frozen onto or melted from the glacier base can achieve a steady state that is in balance with the rate that latent heat is transported to or from the basal interface at constant n when a gradual increase in heat flow from the glacier base causes the rate of melting to decrease h increases and continues to do so when the heat flow is great enough to produce freezing as freezing becomes more rapid and h increases further the rate of fluid supply to the glacier base reaches a maximum when the effective permeability is sufficiently reduced by the partial ice saturation in the fringe larger h can be achieved with slower freezing at the glacier base but steady states with larger h are unstable the maximum rate of fluid supply to the glacier base is greater at lower n higher temperature gradients and for sediments with higher permeabilities unsteady behavior can lead to large changes in h when there is a mismatch between the rate that latent heat can be extracted and the rate that fluid is supplied to the glacier base transient behavior driven by abrupt changes in n is characterized by rapid variations in freezing rate followed by slower adjustments to h that are limited by the timescale for the conduction of latent heat the resulting patterns of sediment deformation are expected to commonly be distributed over finite depth ranges even when shear is perfectly localized at any single instant in time

binge purge oscillations of the laurentide ice sheet as a cause of the north atlantic s heinrich events ice rafted debris in sediment cores from the north atlantic suggests that the laurentide ice sheet lis periodically disgorged icebergs in brief but violent episodes which occurred approximately every 7 000 years here i propose that heinrich events i e what these episodes are called were caused by free oscillations in the flow of the laurentide ice sheet which arose because the floor of hudson bay and hudson strait is covered with soft unconsolidated sediment that forms a slippery lubricant when thawed the proposed heinrich event cycle has two phases the growth phase occurs when the sediment is frozen and the lis is stranded immobile on a rigid bed the volume of the lis slowly grows during this phase at a rate dictated by snow accumulation the purge phase occurs when the basal sediment thaws and a basally lubricated discharge pathway i e an ice stream such as those which occur in west antarctica today developes through hudson strait the volume of the lis rapidly equilibrates to the reduced basal friction during this phase by dumping icebergs into the labrador sea the periodicity t k sl2g 2 7000 years of the proposed heinrich event cycle is a function of the thermal conductivity and diffusivity of ice k and respectively the atmospheric sea level temperature sl in degrees celsius and the excess geothermal heat flux defined by g g k where is the atmospheric lapse rate and g is the geothermal heat flux agreement between the predicted t and the apparent periodicity implied by the marine record is the main virtue of the free oscillation mechanism i propose an alternative mechanism in which heinrich events are forced by periodic variations in external climate is implausible because periodic atmospheric temperature perturbations are strongly attenuated with depth in an ice sheet

permanent fast flow versus cyclic surge behaviour numerical simulations of the austfonna ice cap svalbard a large part of the ice flux within ice caps occurs through spatially limited fast flowing units some of them permanently maintain fast flow whereas others operate in an oscillatory mode characterized by short lived active phases followed by long quiescent phases this surge type behaviour results from intrinsic rather than external factors thus complicating estimates of glacier response to climate change here we present numerical model results from austfonna an ice cap on svalbard that comprises several surge type basins previous studies have suggested a thermally controlled soft bed surge mechanism for svalbard we systematically change the parameters that govern the nature of basal motion and thereby control the transition between permanent and oscillatory fast flow surge type behaviour is realized by a relatively abrupt onset of basal sliding when basal temperatures approach the pressure melting point and enhanced sliding of marine grounded ice irrespective of the dynamic regime the absence of considerable volumes of temperate ice both in the observed and simulated ice cap indicates that fast flow is accomplished by basal motion over a temperate bed given an idealized present day climate the equilibrium ice cap size varies significantly depending on the chosen parameters

physical conditions at the base of a fast moving antarctic ice stream boreholes drilled to the bottom of ice stream b in the west antarctic ice sheet reveal that the base of the ice stream is at the melting point and the basal water pressure is within about 1 6 bars of the ice overburden pressure these conditions allow the rapid ice streaming motion to occur by basal sliding or by shear deformation of unconsolidated sediments that underlie the ice in a layer at least 2 meters thick the mechanics of ice streaming plays a role in the response of the ice sheet to climatic change

interaction and variability of ice streams under a triple valued sliding law and non newtonian rheology 1 ice streams are regions of fast flowing glacier ice that transport a significant portion of the total ice flux from present ice sheets the flow pattern of ice streams can vary both temporally and spatially in particular ice streams can become stagnant and change their path we study the dynamics of ice streams using an idealized model of an isothermal and power law viscous ice flow that includes horizontal lateral shear stresses the basal sliding law is assumed to be triple valued we investigate the spatiotemporal patterns formed because of the flow over a flat bed fed from an upstream mass source the ice flows from the mass source region through one or two gaps in a prescribed upstream topographic ridge which restricts the flow leading to the formation of one or two ice streams we find a relation between the parameters of the ice rheology and the width of the ice stream shear margins and show how these parameters can affect the minimum width of an ice stream we also find that complex asymmetric spatiotemporal patterns can result from the interaction of two ice streams sharing a common mass source the rich spatiotemporal variability is found to mostly be a result of the triple valued sliding law but non newtonian effects are found to play a significant role in setting a more realistic shear margin width and allowing for relevant time scales of the variability

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

reactivation of kamb ice stream tributaries triggers century scale reorganization of siple coast ice flow in west antarctica ongoing centennial scale flow variability within the ross ice streams of west antarctica suggests that the present day positive mass balance in this region may reverse in the future here we use a three dimensional ice sheet model to simulate ice flow in this region over 250 years the flow responds to changing basal properties as a subglacial till layer interacts with water transported in an active subglacial hydrological system we show that a persistent weak bed beneath the tributaries of the dormant kamb ice stream is a source of internal ice flow instability which reorganizes all ice streams in this region leading to a reduced positive mass balance within decades and a net loss of ice within two centuries this hitherto unaccounted for flow variability could raise sea level by 5 mm this century furthermore better constraints on future sea level change from this region will require improved estimates of geothermal heat flux and subglacial water transport

heinrich events massive late pleistocene detritus layers of the north atlantic and their global climate imprint 1 millennial climate oscillations of the glacial interval are interrupted by extreme events the so called heinrich events of the north atlantic their near global footprint is a testament to coherent interactions among earth s atmosphere oceans and cryosphere on millennial timescales heinrich detritus appears to have been derived from the region around hudson strait it was deposited over approximately 500 250 years several mechanisms have been proposed for the origin of the layers binge purge cycle of the laurentide ice sheet jokulhlaup activity from a hudson bay lake and an ice shelf buildup collapse fed by hudson strait to determine the origin of the heinrich events i recommend 1 further studies of the timing and duration of the events 2 further sedimentology study near the hudson strait and 3 greater spatial and temporal resolution studies of the layers as well as their precursory intervals studies of previous glacial intervals may also provide important constraints

dynamics of ice stream temporal variability modes scales and hysteresis 1 understanding the mechanisms governing temporal variability of ice stream flow remains one of the major barriers to developing accurate models of ice sheet dynamics and ice climate interactions here we analyze a simple model of ice stream hydrology coupled to ice flow dynamics and including drainage and basal cooling processes analytic and numerical results from this model indicate that there are two major modes of ice stream behavior steady streaming and binge purge variability the steady streaming mode arises from friction stabilized subglacial meltwater production which may also activate and interact with subglacial drainage the binge purge mode arises from a sufficiently cold environment sustaining successive cycles of thinning induced basal cooling and stagnation low prescribed temperature at the ice surface and weak geothermal heating typically lead to binge purge behavior while warm ice surface temperature and strong geothermal heating will tend to produce steady streaming behavior model results indicate that modern siple coast ice streams reside in the binge purge parameter regime near a subcritical hopf bifurcation to the steady streaming mode numerical experiments exhibit hysteresis in ice stream variability as the surface temperature is varied by several degrees our simple model simulates heinrich event like variability in a hypothetical hudson strait ice stream including dynamically determined purge time scale till freezing and basal cooling during the binge phase these findings are an improvement on studies of both modern and paleo ice stream variability and provide a framework for interpreting complex ice flow models

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

temperate ice in the shear margins of the antarctic ice sheet controlling processes and preliminary locations abstract the flux of grounded ice from the antarctic ice sheet ais primarily occurs through narrow fast flowing outlet glaciers and ice streams shearing generates heat in the lateral glacier margins where there is a transition from fast flow to near stagnant ice or rock this heat warms the ice and can form zones of water saturated porous ice at the melting temperature known as temperate ice here we derive a one dimensional thermomechanical model to infer which ais shear margins may contain temperate ice our model provides expressions for the critical shear strain rate at which a zone of temperate ice initiates and the thickness of the temperate zone both of these quantities are described by two nondimensional numbers the brinkman and peclet numbers that are functions of observable parameters such as shear strain rate surface temperature surface accumulation rate and ice thickness the development of temperate zones is incited by shear strain rates brinkman number and suppressed by the advection of cold ice from the surface peclet number which scales with surface accumulation rate we also include a parametrization for lateral advection which can suppress the formation of temperate ice applying recent observations and outputs from a regional climate model we show that many active glacier shear margins could contain temperate ice the spatial distribution and thickness of temperate zones are controlled largely by shear strain rates and surface accumulation rates

subglacial conditions during and after stoppage of an antarctic ice stream is reactivation imminent borehole observations from the base of the west antarctic ice sheet wais reveal the presence of a 10 to 15 m thick accretionary basal ice layer in the upstream area of kamb ice stream kis this ice layer has formed over a time of several thousand years by freeze on of subglacial water to the ice base and has recorded during this time basal conditions upstream of its current location analysis of samples and videos sequences from boreholes drilled to the bottom of kis confirms that kis stoppage was due to basal freeze on and that relubrication of the ice stream is well underway these results further suggest that ice stream cyclicity may be shorter than expected 1000s of years and that a restart of kis may be imminent within decades to centuries

the great whirl observations of its seasonal development and interannual variability 1 in situ measurements are used together with sea surface height data to study the development and variability of the great whirl gw a large quasi stationary anticyclone that appears off the coast of somalia during the southwest monsoon season we find that anticyclonic circulation indicative of the gw appears on average in april almost two months before the onset of the southwest monsoon winds this early initiation is coincident with the arrival of annual rossby waves at the western boundary with the onset of the monsoon winds in early june the gw proper intensifies quickly remaining at its peak throughout july august and september and dissipating about one month after the winds have died the gw is present for an average 166 30 days per year and the position of its northern flank close to 9 n coincides with the latitude of zero wind stress curl the intraseasonal variability of the gw is intense as a result of mutual advection with one to three flanking cyclones which accompany the gw 70 of the time and tend to circulate clockwise around it there is no consistent seasonal pattern for the development or dissipation of the gw although movement to the southwest is common toward the end of the season the gw of 1995 deepened from 200 m in june to over 1000 m in september and strengthened from a swirl transport of 10 to 60 sv cool waters in its core resulted from advection via the somali current and some local vertical mixing

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

effect of precursor mineralogy on the thermal infrared emission spectra of hematite application to martian hematite mineralization 1 observations from the thermal emission spectrometer tes instrument aboard the mars global surveyor mgs spacecraft led to the discovery of two isolated deposits of gray crystalline hematite located in meridiani planum and aram chaos and several smaller deposits in valles marineris several pathways for formation of these hematite deposits have been proposed involving both aqueous and nonaqueous processes this work uses the precise shape and position of spectral features in the martian hematite thermal emission spectrum to constrain hematite formation pathways thermal infrared emission spectra x ray powder diffraction patterns mossbauer spectra and transmission electron microscope tem photomicrographs were obtained for synthetic and natural hematite samples derived by 1 dehydroxylation of fine grained goethite and 2 oxidation of magnetite collectively the instrumental analyses show that the mineralogical composition and crystal morphology of precursor samples and the time and temperature conditions under which decomposition to hematite occur determine the crystallinity and crystal morphology of the hematite product comparison of laboratory and mgs tes spectra shows that the martian hematite spectra correspond closely with a synthetic hematite sample derived by pseudomorphic and topotactic dehydroxylation of goethite at 300 c spectra of goethite precursor samples dehydroxylated at higher temperatures provide increasingly poor fits spectra of hematite samples derived by high temperature thermal oxidation of magnetite are also poorer fits to the martian hematite spectrum thermal emission spectra of goethites heated at lower temperatures are characterized by the spectral signatures of both hematite and goethite and are not consistent with the martian spectra the characteristic that distinguishes the synthetic hematite sample with the mars like spectral signature from the other synthetic hematite samples is the high proportion of crystal surfaces that are crystallographic 001 faces c faces for the former but not the latter the high proportion of 001 face area results because the largest surface of the lath shaped hematite particles is the 001 face as determined by tem thus a possible formation pathway for hematite in meridiani planum aram chaos and valles marineris is precipitation of goethite from aqueous solutions as lath shaped crystals possibly as a stain cement and or massive deposit followed by pseudomorphic and topotactic dehydroxylation to hematite at temperatures below 300 c

the influence of subglacial hydrology on the flow of kamb ice stream west antarctica 1 ice streams on the siple coast west antarctica have a complex history of flow because their basal motion is governed by time varying basal conditions although the mechanical interaction between ice and till is well established very little is known about the potential effect of regionally scaled water transport in a basal water system which has only recently become apparent to investigate the combined effect of hydrological and mechanical processes we developed the hydrology ice and till model in which ice flow is coupled to a coulomb plastic till layer and a basal water system consisting of discrete conduits when the model is applied to kamb ice stream kis results confirm that it is capable of oscillating between fast and stagnant modes of flow we show that when subglacial conduits are disregarded or do not extend to the grounding line the oscillatory behavior of the ice stream is governed by the basal thermal regime when conduits extend to the grounding line the modelled ice stream oscillation period is increased peak speeds are reduced and oscillations may ultimately cease if the volume of water supplied is sufficiently high three different hydrological states characterize the behavioral patterns of ice flow and these states are distinguished by conditions at the grounding line modelled ice stream velocities were found to oscillate with fast and slow periods typically lasting a few hundred years although varying according to hydrological activity our results indicate that kis could reactivate this century given its hydrological setting and 170 years of stagnation

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

large scale instabilities of the laurentide ice sheet simulated in a fully coupled climate system model 1 heinrich events related to large scale surges of the laurentide ice sheet represent one of the most dramatic types of abrupt climate change occurring during the last glacial here using a coupled atmosphere ocean biosphereice sheet model we simulate quasi periodic large scale surges from the laurentide ice sheet the average time between simulated events is about 7 000 yrs while the surging phase of each event lasts only several hundred years with a total ice volume discharge corresponding to 5 10 m of sea level rise in our model the simulated ice surges represent internal oscillations of the ice sheet at the same time our results suggest the possibility of a synchronization between instabilities of different ice sheets as indicated in paleoclimate records index terms 1827 hydrology glaciology 1863 1620 global change climate dynamics 3309 3344 meteorology and atmospheric dynamics paleoclimatology 5416 planetology solid surface planets glaciation citation calov r a ganopolski v petoukhov m claussen and r greve largescale instabilities of the laurentide ice sheet simulated in a fully coupled climate system model geophys res lett 29 24 2216 doi 10 1029 2002gl016078 2002

thermally controlled glacier surging bakaninbreen in svalbard and trapridge glacier in yukon territory canada are two prominent examples of surging glaciers which are thought to be controlled by their thermal regime both glaciers have developed large bulges which have propagated forward as travelling wave fronts and which are thought to divide relatively stagnant downstream cold based ice from faster moving warm based upstream ice additionally both glaciers are underlain by a wet metres thick layer of deforming till we develop a simple model for the cyclic surging behaviour of these glaciers which interrelates the motion of the ice and till through a description of the subglacial hydrology we find that oscillations surges can occur if the subglacial hydrological transmissivity is sufficiently low and the till layer is sufficiently thin and we suggest that these oscillations are associated with the development and propagation of a travelling wave front down the glacier we therefore interpret the travelling wave fronts on both trapridge glacier and bakaninbreen as manifestations of surges in addition we find that the violence of the surge in the model is associated with the resistance to ice flow offered by undulations in the bed and the efficiency with which occasional hydrological events can release water accumulated at the glacier sole

response of subglacial sediments to basal freeze on 1 theory and comparison to observations from beneath the west antarctic ice sheet 1 we have constructed a high resolution numerical model of heat water and solute flows in sub ice stream till subjected to basal freeze on the model builds on quantitative treatments of frost heave in permafrost soils the full version of the clapeyron equation is used hence ice water phase transition depends on water pressure osmotic pressure and surface tension the two latter effects can lead to supercooling of the ice base this supercooling in turn induces hydraulic gradients that drive upward flow of pore water which feeds the growth of segregation ice onto the freezing interface this interface may progress into the till and form ice lenses if supercooling is sufficiently strong hence the ice segregation process develops a stratified basal ice layer in our model a high basal temperature gradient 0 054 c m 1 triggers ice stream stoppage and the loss of basal shear heating leads to relatively high basal freeze on rates 3 5 mm a 1 in response the subglacial till experiences comparatively rapid consolidation till porosity can decrease from 40 to 120 kpa approximately within one century basal supercooling arising from redistribution of solutes and ice water interfacial effects amounts to ca 0 35 c below the pressure melting point fine grained till is in our model associated with widely spaced thick ice lenses coarse grained till yields thinner ice lenses that are more closely spaced our model results compare favorably although not in all details with available observational evidence from borehole studies of west antarctic ice streams

a continuum mixture model of ice stream thermomechanics in the laurentide ice sheet 2 application to the hudson strait ice stream episodic exportations of ice rafted debris to the north atlantic in the late pleistocene suggest quasiperiodic ice streaming or surging activity on the northeastern margin of the laurentide ice sheet much of this efflux of ice may originate from an ice stream issuing from hudson strait and tapping into core regions of the laurentide in hudson bay labrador and the foxe basin applying the continuum mixture theory outlined by marshall and clarke this issue we model the thermomechanical evolution of the hudson strait ice stream in a three dimensional finite difference model of the laurentide ice sheet our simulations focus on internal dynamics of the ice stream under thermal regulation of basal flow we find surge cycles of stream activity interspersed with quiescent periods where the ice stream is frozen to the bed modeled surge durations vary from 105 to 3260 years while surge periodicities range from 585 to 22 410 years with pervasively warm or cold internal temperature distributions in the ice ice streams can also establish modes of permanent activity or inactivity under thermal regulation our most vigorous ice streams produce peak values of approximately 0 03 sv of freshwater flux to the north atlantic from basal meltwater and iceberg production associated ice stream velocities in this maximum case approach 6700 m yr 1 the total ice volume mobilized in a single surge event is equivalent to a global sea level rise of 0 04m in the most tranquil surge and almost 0 6m in the most extreme case these velocities and sea level impacts are an order of magnitude less than those predicted by macayeal 1993a b and only our most exuberant streams approach the iceberg flux estimates of dowdeswell et al 1995 we propose that the sediment load of icebergs emanating from hudson strait in a surge event may exceed expectations from contemporary icebergs

significant groundwater contribution to antarctic ice streams hydrologic budget satellite observations have revealed active hydrologic systems beneath antarctic ice streams but sources and sinks of water within these systems are uncertain here we use numerical simulations of ice streams to estimate the generation flux and budget of water beneath five ice streams on the siple coast we estimate that 47 of the total hydrologic input 0 98 km3 yr 1 to whillans wis mercer mis and kamb kis ice streams comes from the ice sheet interior and that only 8 forms by local basal melting the remaining 45 comes from a groundwater reservoir an overlooked source in which depletion significantly exceeds recharge of the total input to bindschadler bis and macayeal macis ice streams 0 56 km3 yr 1 72 comes from the interior 19 from groundwater and 9 from local melting this contrasting hydrologic setting modulates the ice streams flow and has important implications for the search for life in subglacial lakes

ice streams in the laurentide ice sheet identification characteristics and comparison to modern ice sheets this paper presents a comprehensive review and synthesis of ice streams in the laurentide ice sheet lis based on a new mapping inventory that includes previously hypothesised ice streams and includes a concerted effort to search for others from across the entire ice sheet bed the inventory includes 117 ice streams which have been identified based on a variety of evidence including their bedform imprint large scale geomorphology topography till properties and ice rafted debris in ocean sediment records despite uncertainty in identifying ice streams in hard bedrock areas it is unlikely that any major ice streams have been missed during the last glacial maximum laurentide ice streams formed a drainage pattern that bears close resemblance to the present day velocity patterns in modern ice sheets large ice streams had extensive onset zones and were fed by multiple tributaries and where ice drained through regions of high relief the spacing of ice streams shows a degree of spatial self organisation which has hitherto not been recognised topography exerted a primary control on the location of ice streams but there were large areas along the western and southern margin of the ice sheet where the bed was composed of weaker sedimentary bedrock and where networks of ice streams switched direction repeatedly and probably over short time scales as the ice sheet retreated onto its low relief interior several ice streams show no correspondence with topography or underlying geology perhaps facilitated by localised build up of pressurised subglacial meltwater they differed from most other ice stream tracks in having much lower length to width ratios and have no modern analogues there have been very few attempts to date the initiation and cessation of ice streams but it is clear that ice streams switched on and off during deglaciation rather than maintaining the same trajectory as the ice margin retreated we provide a first order estimate of changes in ice stream activity during deglaciation and show that around 30 of the margin was drained by ice streams at the lgm similar to that for present day antarctic ice sheets but this decreases to 15 and 12 at 12 cal ka bp and 10 cal ka bp respectively the extent to which these changes in the ice stream drainage network represent a simple and predictable readjustment to a changing mass balance driven by climate or internal ice dynamical feedbacks unrelated to climate or both is largely unknown and represents a key area for future work to address

regional and hemispheric circulation patterns in the northern hemisphere winter or the nao and the ao 1 two different scales of teleconnection patterns are found over the euro atlantic region during the winter one is a regional seesaw between iceland and the azores and the other is a hemispheric seesaw between the mediterranean and the polar siberia region the north atlantic oscillation nao and the arctic oscillation ao indices are sensitive to both modes although the nao index reflects the more regional one the principal process producing a hemispheric seesaw pattern in the surface pressure is changing in vertical winds in the polar troposphere driven by wave forcings upward motion in the polar region induces meridional flows near the surface especially around the norwegian sea and bering strait which induce wavy patterns in the surface pressure the importance of distinguishing the two modes in understanding recent trends is also addressed

energy balance of ice streams analysis of the cross flow transmission of force from the central parts of a well lubricated ice stream to its margins shows that there is a corresponding shift in the lateral location of motion induced heat generation the rate of basal heat generation in the center can be substantially smaller than the local rate of potential energy loss given by driving stress times the speed of downslope motion the basal heating is a maximum for an intermediate level of lubrication for which speed is about 40 of the speed over a frictionless bed and base stress is about 25 of the driving stress stable and unstable balances between meltwater production and drainage on the bed are identified a stable steady state with a speed less more than that giving maximum heat generation is termed drainage production limited since an increase in speed would lead to increased decreased basal melting and must need not be balanced by increased drainage it is shown that gradual evolution of the basal water drainage system and the factors affecting basal melting can cause discontinuous jumps between fast and slow moving states a simplified analysis applied to six cross sections of west antarctic ice streams b d e and rutford ice stream shows them to be diverse in the level of support from the sides and corresponding shift of mechanical heating sideward from their central parts the cross sections of ice stream b near upstream b may be production limited because of especially high lubrication and related support from the sides cross sections in the upper part of ice stream d ice stream e and rutford ice stream are in a drainage limited condition substantial reduction of basal heat generation by side drag in most cases and expected high heat flow into the basal ice associated with low thickness in some cases tends to favor basal freezing nevertheless all of the examined cross sections except one are expected to experience basal melting with a modest geothermal heat flux density of 60 mw m or less in some cases the lower part of ice stream b is an exception where the analysis indicates that geothermal flux density must exceed 80 100 mw m to maintain melting if this high geothermal flux is not present then the base of the lower part of ice stream b may be freezing which would suggest continued deceleration of this part of ice stream b

spatial complexity of ice flow across the antarctic ice sheet ice streams control the discharge of ice from the interior of the antarctic ice sheet to the coast a map of flow convergence suggests that ice stream flow is subject to a mechanical regulation that limits flow orthonormal strain rates

response of subglacial sediments to basal freeze on 2 application in numerical modeling of the recent stoppage of ice stream c west antarctica 1 ross ice streams supply over 90 of the ice volume flowing out of the ross sector of the west antarctic ice sheet wais stoppage of ice stream c isc ca 150 years ago appears to have pushed this sector of wais from negative into positive mass balance joughin and tulaczyk 2002 we propose an explanation for the unsteady behavior of isc using a new numerical ice stream model which includes an explicit treatment of a subglacial till layer when constrained by initial conditions emulating prestoppage geometry dynamics and mass balance of isc the model yields a rapid 100 years stoppage of the main ice stream trunk the stoppage is triggered by basal freeze on which consolidates and strengthens the subglacial till our numerical simulations produce results consistent with a number of existing observations for example continuing activity of the two tributaries of isc the model always yields rapid stoppage unless we specify ice stream width that is smaller than its prestoppage values maximum of 80 km we conjecture that if isc was active for at least a few thousand years before slowdown its width was significantly smaller than today to sustain the long active phase ice stream width is a key control that helps determine whether ice stream flow is sustainable over a long term our work indicates that the recent stoppage of ice stream c could have been part of inherent ice stream cyclicity and it leaves open the possibility that other active ice streams may evolve in the future toward rapid shutdowns

thermally driven migration of ice stream shear margins ice stream shear margins are the lateral boundaries of narrow fast flowing bands of ice within an ice sheet we develop a theory for the migration of shear margins over time driven by viscous dissipation of heat within the ice focusing on widening of the ice stream the location of the margin is modelled as a transition from a cold to a temperate ice sheet bed and simultaneously as the transition from no slip to free slip at the same location the temperature field in the ice is affected by intense shear heating as well as by the migration velocity of the margin i e by the widening rate of the ice stream if migration is too fast there is little time for the ice to warm up and the margin remains cold causing the bed to freeze this suppresses widening conversely if the migration speed is too slow the ice in the margin warms up causing the bed on the far side of the cold temperate transition to reach the melting point and migration to speed up using a wiener hopf method we show that for a given far field shear stress geothermal heat flux and ice geometry there is a single migration velocity that balances the two effects and permits widening at a steady rate this velocity increases with the far field lateral shear stress imposed by the ice stream which controls shear heating in the margin our results also indicate that i a region of temperate ice must form in the margin and that ii lateral advection of ice may play a significant role in controlling migration speeds

slow slip events on the whillans ice plain antarctica described using rate and state friction as an ice stream sliding law the whillans ice plain wip antarctica experiences twice daily tidally modulated stick slip cycles slip events last about 30 minutes have sliding velocities as high as 0 5 mm s 15 km yr and have total slip 0 5 m slip events tend to occur during falling ocean tide just after high tide and just before low tide to reproduce these characteristics we use rate and state friction which is commonly used to simulate tectonic faulting as an ice stream sliding law this framework describes the evolving strength of the ice bed interface throughout stick slip cycles we present simulations that resolve the cross stream dimension using a depth integrated treatment of an elastic ice layer loaded by tides and steady ice inflow steady sliding with rate weakening friction is conditionally stable with steady sliding occurring for sufficiently narrow ice streams relative to a nucleation length stick slip cycles occur when the ice stream is wider than the nucleation length or equivalently when effective pressures exceed a critical value ice streams barely wider than the nucleation length experience slow slip events and our simulations suggest that the wip is in this slow slip regime slip events on the wip show a sense of propagation and we reproduce this behavior by introducing a rate strengthening region in the center of the otherwise rate weakening ice stream if pore pressures are raised above a critical value our simulations predict that the wip would exhibit quasi steady tidally modulated sliding as observed on other ice streams this study validates rate and state friction as a sliding law to describe ice stream sliding styles

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

a theory of glacier surges we propose a model of glacier flow that is capable of explaining temperate glacier surges the laws of conservation of mass and momentum are supplemented by the prescription of a sliding law that gives the basal shear stress as a function of the basal velocity u and the effective pressure n the effective drainage pressure n is determined by a simple study of the subglacial hydraulic system following rothlisberger we determine n nr for the case of drainage through a single subglacial tunnel alternatively following kamb we find that the corresponding theory for a linked cavity drainage system yields n nk nr furthermore the stability of each drainage system depends on the velocity u such that for large enough u there is a transition from tunnel to cavity drainage consequently one can write n n u we then find that the sliding law u is multivalued and hence so also is the flux depth relation q q h an analysis of the resulting system of equations is sketched for large enough accumulation rates a glacier will undergo regular relaxation oscillations resembling a surge the surge is triggered at the point of maximum stress from this point two hydraulic transition fronts travel up and down glacier to calculable boundary points the speed of propagation is the order of 50 metres an hour at these fronts the tunnel drainage system collapses and a high water pressure cavity drainage system is installed this activated zone has high velocities and quickly relaxes surges to a quasi equilibrium state this relaxation is much like opening a sluice gate in that a large wave front propagates forward behind this wave front the velocity can decay oscillatorily and thus the flow can be compressive we conclude with some discussion of the effects of seasonal variation and of prospects for the current theory s applicability to soft bedded glaciers

basal hydraulic system of a west antarctic ice stream constraints from borehole observations pressure and tracer measurements in boreholes drilled to the bottom of ice stream b west antarctica are used to obtain information about the basal water conduit system in which high water pressures are developed these high pressures presumably make possible the rapid movement of the ice stream pressure in the system is indicated by the borehole water level once connection to the conduit system is made on initial connection here also called breakthrough to the basal water system the water level drops in a few minutes to an initial depth in the range 96 117 m below the surface these water levels are near but mostly somewhat deeper than the flotation level of about 100m depth water level at which basal water pressure and ice overburden pressure are equal which is calculated from depth density profiles and is measured in one borehole the conduit system can be modelled as a continuous or somewhat discontinuous gap between ice and bed the thickness of the gap has to be about 2 mm to account for the water level drop on breakthrough and about 4 mm to fit the results of a salt tracer experiment indicating downstream transport at a speed of 7 5 mm s 1 the above gap conduit model is however ruled out by the way a pressure pulse injected into the basal water system at breakthrough propagates outward from the injection hole and also by the large hole to hole variation in measured basal pressure which if present in a gap conduit system with 2 or 4 mm would result in unacceptably large local water fluxes an alternative model that avoids the e objections called the gap opening model involves opening a gap as injection proceeds starting with a thin film the injection of water under pressure lifts the ice mass a round the borehole creating a gap 3 or 4 mm wide at the ice bed interface evaluated quantitatively the gap opening model accounts for the volume of water that the basal watery system accepts on breakthrough which obviates the gap conduit model in order to transport basal meltwater from upstream it is then necessary for the complete hydraulic model to contain also a network of relatively large conduits of which the most promising type is the canal conduit proposed theoretically by walder and fowler 1994 flat low conduits incised into the till 0 1 m deep and perhaps 1 m wide with a flat ice roof the basal water pressure data suggest that the canals are spaced 50 300 m apart much closer than r tunnel would be the deepest observed water level 117 m i the most likely to reflect the actual water pressure in the canals corresponding to a basal effective pressure of 1 6 bar in this interpretation the shallower water levels are affected by loss of hydraulic head in the narrow passageway s that connect along the bed from borehole to canal s once a borehole has frozen up and any passageways connecting with canals have become closed a pressure sensor in contact with the unfrozen till that underlies the ice will measure the pore pressure in the till given enough time for pressure equilibration this pressure varies considerably with time over the equivalent water level range from 100 to 113 m basal pressure sensors 500 m apart report uncorrelated variations whereas sensors in boreholes 25 m apart report mostly but not entirely well correlated variations of unknown origin in part of the record remarkable anticorrelated variations are interspersed with positively correlated one and there are rare abrupt excursions to extreme water levels as deep as 125m and as shallow as 74 m a diurnal pressure fluctuation intermittently observed may possibly be caused by the ocean tide in the ross sea the lack of any observed variation in ice stream motion when large percentagewise variations in basal effective pressure were occurring according to our data suggests that the observed pressure variations are sufficiently local and so randomly variable from place to place that they are averaged out in the process by which the basal motion of the ice stream is determined by an integration over a large area of the bed

infrared stealthy surfaces why tes and themis may miss some substantial mineral deposits on mars and implications for remote sensing of planetary surfaces 1 infrared spectral observations have yielded many important insights into the nature of mars however spectral effects tied to surface roughness and particle size can make mineral deposits substantially more difficult to detect and identify than is generally known including for current investigations the effects hold for mars and other planets but here we focus on the impact on the exploration of mars laboratory spectra of pure smooth surfaced minerals have previously been used to conclude that the 1996 global surveyor thermal emission spectrometer tes and the 2001 mars odyssey thermal emission imaging system themis will detect a mineral deposit that covers 10 of a pixel those laboratory spectra have formed the foundation for modeling and interpretations of tes themis data and for development of the 2003 mars rover minites here for the first time we compare tes themis foundation data to laboratory field and unique airborne spectra of rough surfaces we demonstrate that substantial well crystalline regional mineral deposits including rock outcrops can be 100 exposed and remain undetected at the sensitivity of tes themis we then detail the physics which apply to all geologic materials previous studies have demonstrated the complicating physical effects for solids versus particulates for a range of minerals including oxides sulfates carbonates and each of the seven silicate classes we identify three cases of spectral behavior that can impact interpretations in case 1 physical effects cause a target to be undetectable by a given infrared instrument infrared stealthy in case 2 variations in spectral contrast mimic abundance variations which confounds abundance mapping in case 3 alteration of the spectral band shape impacts compositional interpretations optically smooth materials will be well represented in tes themis minites identifications conversely if a material forms an optically rough surface or sufficiently small particles on mars then it will be comparatively underrepresented or missed surface roughness occurs on three broad scales and roughness at the grain scale can be a primary variable in whether a mineral is detectable nondetection of certain key minerals e g carbonates can significantly impact geologic and climatic interpretations and landing site selection the tes themis detection threshold may be 10 coverage on the basis of specific laboratory samples but for field measurements that prediction represents an idealized lower bound interpretations built on nondetection should be revisited after defining the uncertainties in detection capabilities for specific instruments as applied to expected mineral surface textures on mars including at the grain scale the impact on future measurement strategies including minites should also be assessed

numerical investigations of the slow down of whillans ice stream west antarctica is it shutting down like ice stream c we investigate whether the recent slow down of whillans ice stream wis west antarctica may lead to its complete stoppage in the near future using a numerical model basal resistance to ice stream motion is represented by a continuous till layer whose strength changes in response to basal melting and freezing we implement a basal drainage system which acts to hinder ice stream stoppage through supply of extra water to those parts of the bed that are experiencing freezing the ice module is a standard flowline model with parameterized key out of plane effects raymond 1996 the most important result of our modeling effort is that we never obtained a slow down that would not be followed by a complete stoppage within 5100years wis slow down canbe avoided in the model if large basal water input rates are assumed to satisfy basal freezing in com parison thetestedperturbationsinwiswidthandmassbalancehada relativelysmalleffect onthetendencyof the simulated ice streamto slowdown these resultsunderscorethe need for morequantitativeconstraints ontheefficiencyof sub ice streamwaterdrainage we con jecture that the present day slow down of wis will evolve to shut down in the next few decades unless anadditionof basalwaterprevents freeze on drivenbed strengthening

the pacific center of action of the northern hemisphere annular mode real or artifact the leading empirical orthogonal function eof of the sea level pressure slp field referred to as the arctic oscillation ao or northern hemisphere annular mode nam consists of a dipole between the polar cap region and the surrounding zonal ring centered along 458n embedded within the outer ring are centers of action over the euro atlantic and pacific sectors in which slp fluctuates in phase that the observed slp fluctuations at these two centers of action are virtually uncorrelated raises the question of whether the pacific center in the annular mode could be an artifact of eof analysis it is argued that sea level pressure fluctuations at the pacific and euro atlantic centers of action of the ao nam would be more strongly correlated were it not for the fact that slp variability over the north pacific is dominated by a pattern in which fluctuations over the north atlantic and north pacific are inversely related evidence of the coexistence of such a pattern which resembles an augmented version of the pacific north american pattern is presented

the marginal shear stress of ice stream b west antarctica to ascertain whether the velocity of ice stream b west antarctica may be controlled by the stress in its marginal shear zones the snake and the dragon we undertook a determination of the marginal shear stress in the dragon near camp up b by using ice itself as a stress meter the observed marginal shear strain rate of 0 14 a 1 is used to calculate the marginal shear stress from the flow law of ice determined by creep tests on ice cores from a depth of 300 m in the dragon obtained by using a hot water ice coring drill the test specimen orientation relative to the stress axes in the tests is chosen on the basis of c axis fabrics so that the test applies horizontal shear across vertical planes parallel to the margin the resulting marginal shear stress is 2 2 0 3 10 5 pa this implies that 63 100 of the ice stream s support against gravitational loading comes from the margins and only 37 0 from the base so that the margins play an important role in controlling the ice stream motion the marginal shear stress value is twice that given by the ice stream model of echelmeyer and others 1994 and the corresponding strain rate enhancement factors differ greatly e 1 2 vs 10 12 5 this large discrepancy could be explained by recrystallization of the ice during or shortly after coring estimates of the expected recrystallization time scale bracket the 1 h time scale of coring and leave the likelihood of recrystallization uncertain however the observed two maximum fabric type is not what is expected for annealing recrystallization from the sharp single maximum fabric that would be expected in situ at the high shear strains involved 20 experimental data from wilson 1982 suggest that if the core did recrystallize the prior fabric was a two maximum fabric not substantially different from the observed one which implies that the measured flow law and derived marginal shear stress are applicable to the in situ situation these issues need to be resolved by further work to obtain a more definitive observational assessment of the marginal shear stress

on the teleconnectivity of the arctic oscillation the term arctic oscillation ao has recently been introduced to describe the leading structure of slp variability over the northern hemisphere a key feature of the ao is its zonally symmetric appearance with a primary center of action over the arctic and opposing anomalies in midlatitudes does the ao s annular appearance result from significant temporal correlations between slp anomalies at distant longitudes the results presented indicate that the temporal coherence between the arctic and midlatitudes is strongest over the atlantic sector with weak correlations between the atlantic and pacific midlatitudes both on intraseasonal and interannual time scales during the past 50 yrs hence the annular character of the ao is more a reflection of the dominance of its arctic center of action than any coordinated behavior of the atlantic and pacific centers of action in the slp field the ao is nearly indistinguishable from the leading structure of variability in the atlantic sector e g the north atlantic oscillation their temporal correlation is 0 95 for monthly data

on the origin of planetary scale extratropical winter circulation regimes abstract sectorial and planetary scale winter circulation regimes are studied and the relationship between them is investigated in order to find how much the simultaneous occurrence of sectorial regimes contributes to the occurrence of hemispheric regimes the strategy is based on the multivariate gaussian mixture model the number of components in the model is estimated using two approaches the first one is based on arguments from order statistics of the mixture proportions and the second uses a more severe test based on reproducibility the procedure is applied next to the 500 hpa height field over the north pacific the north atlantic and the northern hemisphere using the empirical orthogonal function state space two highly significant regimes are found in each case namely the pacific north america pattern pna north atlantic oscillation nao for the hemisphere pna for the pacific sector and nao for the atlantic sector the sectorial regimes reflect mainly blocking and no blocking flows t

arctic oscillation or north atlantic oscillation abstract the definition and interpretation of the arctic oscillation ao are examined and compared with those of the north atlantic oscillation nao it is shown that the nao reflects the correlations between the surface pressure variability at its centers of action whereas this is not the case for the ao the nao pattern can be identified in a physically consistent way in principal component analysis applied to various fields in the euro atlantic region a similar identification is found in the pacific region for the pacific north american pna pattern but no such identification is found here for the ao the ao does reflect the tendency for the zonal winds at 35 and 55 n to anticorrelate in both the atlantic and pacific regions associated with the nao and pna because climatological features in the two ocean basins are at different latitudes the zonally symmetric nature of the ao does not mean that it represents a simple modulation of the circumpolar flow an increase in the ao or nao implies stro

numerical simulations of cyclic behaviour in the parallel ice sheet model pism numerical experiments are conducted on a synthetic topography with a three dimensional thermomechanically coupled ice sheet model the parallel ice sheet model pism within the model combined stress balances are connected to evolving thermodynamics and hydrology the sensitivity of cyclic behaviour to changes in sliding law parameters and the climate input is studied multiple types of oscillations were found with strong variations in both amplitude and frequency a physical description is given in which these variations and transitions from one oscillation type to another are linked to the interplay of stresses heat transport and hydrological processes high frequency oscillations period 114 169 years which are shown to have a major impact on ice velocities and a small effect on the ice volume are related to variations in the water distribution at the base low frequency cycles period 1000 years which have a major impact on both velocities and ice volume are linked to changes in the thermal regime oscillation characteristics are shown to be strongly sensitive to changes in sliding law parameters and the prescribed surface temperature and mass balance incorporating a surface height dependence of the mass balance is shown to provide an additional feedback which may induce longperiod oscillations

velocity and stress fields in grounded glaciers a simple algorithm for including deviatoric stress gradients a new and efficient algorithm for computing the three dimensional stress and velocity fields in grounded glaciers includes the role of deviatoric stress gradients a consistent approximation of first order in the aspect of ratio of the ice mass gives a set of eight field equations for the five stress and three velocity components and the corresponding boundary conditions a coordinate transformation mapping the local ice thickness on to unity and approximating the derivatives in the horizontal direction by centred finite differences yields five ordinary differential and three algebraic equations this allows use of the method of lines starting the integration with prescribed stress and velocity components at the base and a simple iteration procedure converges rapidly the algorithm can lie used for a wide range of stress strain rate relations as long as strain only depends on deviatoric and shear stresses and on temperature sensitivity tests using synthetic and realistic ice geometries show the relevance of normal deviatoric stresses in the solutions for the velocity components even for ice sheets stress and velocity fields may deviate substantially from the widely used shallow ice approximation

dynamic patterns of ice stream flow in a 3 d higher order ice sheet model with plastic bed and simplified hydrology 1 predicting ice sheet mass balance is challenging because of the complex flow of ice streams to address this issue we have coupled a three dimensional higher order ice sheet model to a basal processes model where subglacial till has a plastic rheology and evolving yield stress the model was tested for its sensitivity to regional water availability first with an assumed undrained bed the ice stream oscillates between active and stagnant phases solely as a result of thermodynamic feedbacks occurring at the ice till interface however the velocity amplitude decreases over time as insufficient basal meltwater causes the ice stream to gradually thicken and enter a slow flowing ice sheet mode second we assume that the till is able to assimilate water from a hypothetical regional hydrological system this leads to significantly different long term behavior as a continuously oscillating ice stream mode is maintained the extra water incorporated in the till leads to higher velocities triggering stronger thermodynamic feedbacks between the ice and till layer results also suggest that fast flowing ice streams may be modulated by till properties as a result of the duration of thermal conditions during the preceding stagnant phase similarly till properties beneath stagnant ice streams are influenced by basal conditions during the preceding fast flow phase our findings support the inference that ice streams are strongly influenced by the presence of a regional hydrological system underscoring the need to accurately describe the coupling between ice dynamics basal conditions and regional subglacial hydrology in ice sheet models

volcanic eruptions large scale modes in the northern hemisphere and the el nino southern oscillation abstract the author analyzes the impact of 13 major stratospheric aerosol producing volcanic eruptions since 1870 on the large scale variability modes of sea level pressure in the northern hemisphere winter the paper focuses on the arctic oscillation ao and the north atlantic oscillation nao to address the question about the physical nature of these modes the hypothesis that the phase of the el nino southern oscillation enso may control the geographical extent of the dominant mode in the northern hemisphere is also investigated as well as the related possibility that the impact of the eruptions may be different according to the phase of enso the author finds that both the ao and the nao are excited in the first winter after the eruptions with statistical significance at the 95 level both the signal and the significance are larger for the nao than for the ao the excitation of the ao and the nao is connected with the excitation of a secondary mode which resembles an augmented pacific north ame

a new hematite formation mechanism for mars the origin of hematite detected in martian surface materials is commonly attributed to weathering processes or aqueous precipitation here we present a new hematite formation mechanism that requires neither water nor weathering glass rich basalts with martian meteorite like chemistry high feo low al2o3 oxidized at high 700 and 900 c temperatures in air and co2 respectively form thin 1 m hematite coatings on their outermost surfaces hematite is manifested macroscopically by development of magnetism and a gray metallic sheen on the glass surface and microscopically by fe enrichment at the glass surface observed in element maps visible and nearinfrared thermal infrared and raman spectroscopy confirm that the fe enrichment at the oxidized glass surfaces corresponds to hematite mineralization hematite formation on basaltic glass is enabled by a mechanism that induces migration of fe2 to the surface of an oxidizing glass and subsequent oxidation to form hematite a natural example of the hematite formation mechanism is provided by a hawaiian basalt hosting a gray metallic sheen that corresponds to a thin hematite coating hematite coating development on the hawaiian basalt demonstrates that martian meteorite like feo contents are not required for hematite coating formation on basalt glass and that such coatings form during initial extrusion of the glassy basalt flows if gray hematite originating as coatings on glassy basalt flows is an important source of martian hematite which is feasible given the predominance of igneous features on mars then the requirement of water as an agent of hematite formation is eliminated

results from the ice sheet model intercomparison project heinrich event intercomparison ismip heino results from the heinrich event intercomparison heino topic of the ice sheet model intercomparison project ismip are presented ismip heino was designed to explore internal large scale ice sheet instabilities in different contemporary ice sheet models these instabilities are of interest because they are a possible cause of heinrich events a simplified geometry experiment reproduces the main characteristics of the laurentide ice sheet including the sedimented region over hudson bay and hudson strait the model experiments include a standard run plus seven variations nine dynamic thermodynamic ice sheet models were investigated one of these models contains a combination of the shallow shelf ssa and shallow ice approximation sia while the remaining eight models are of sia type only seven models including the sia ssa model exhibit oscillatory surges with a period of 1000 years for a broad range of parameters while two models remain in a permanent state of streaming for most parameter settings in a number of models the oscillations disappear for high surface temperatures strong snowfall and small sediment sliding parameters in turn low surface temperatures and low snowfall are favourable for the ice surge cycles we conclude that further improvement of ice sheet models is crucial for adequate robust simulations of cyclic large scale instabilities

processes controlling the downstream evolution of ice rheology in glacier shear margins case study on rutford ice stream west antarctica ice rheology governs how glaciers flow and respond to environmental change the rheology of glacier ice evolves in response to a variety of mechanisms including damage heating melting and the development of crystalline fabric the relative contributions of these rheological mechanisms are not well understood using remotely sensed data and physical models we decouple the influence of each of the aforementioned mechanisms along the margins of rutford ice stream a laterally confined outlet glacier in west antarctica we show that fabric is an important control on ice rheology in the shear margins with an inferred softening effect consistent with a single maximum fabric fabric evolves to steady state near the onset of streaming flow and ice progressively softens downstream almost exclusively due to shear heating the rate of heating is sensitive to local shear strain rates which respond to local changes in bed topography as ice is squeezed through the basal trough the impact of shear heating on the downstream evolution of ice rheology in a laterally confined glacier suggests that the thermoviscous feedback wherein faster ice flow leads to higher rates of shear heating further softening the ice is a fundamental control on glacier dynamics

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

evidence for platy hematite grains in sinus meridiani mars 1 midinfrared 1600 200 cm 1 spectral data received from the mars global surveyor thermal emission spectrometer mgs tes have provided evidence for a large hematite bearing fe2o3 deposit in sinus meridiani mars we report here the results of a laboratory spectroscopic investigation of 24 hematite samples including polycrystalline hand samples massive and schistose textures single crystal hand samples and particle size fractions single crystal and polycrystalline discrete particles laboratory midinfrared analyses of crystallographically oriented hematite samples suggest that the hematite emission in sinus meridiani sm is predominantly from the crystallographic c face of hematite this observation implies the presence of platy hematite particles with the plate face being the crystallographic c face the observations are consistent with a formational model where the platy gray hematite originated as an iron oxide chemically precipitated from fe rich aqueous and or hydrothermal solutions on early mars that was buried recrystallized to platy hematite and subsequently reexposed as lenses of schistose hematite in a friable consolidated stratigraphic unit unconsolidated platy hematite particles are also likely to be present as a physical weathering product of the schistose hematite lenses

widespread complex flow in the interior of the antarctic ice sheet it has been suggested that as much as 90 of the discharge from the antarctic ice sheet is drained through a small number of fast moving ice streams and outlet glaciers fed by relatively stable and inactive catchment areas here evidence obtained from balance velocity estimates suggests that each major drainage basin is fed by complex systems of tributaries that penetrate up to 1000 kilometers from the grounding line into the interior of the ice sheet this finding has important consequences for the modeled or estimated dynamic response time of past and present ice sheets to climate forcing

reconsideration of the true versus apparent arctic oscillation abstract the physical reality of the arctic oscillation ao or northern annular mode is considered the data used are mainly the monthly mean sea level pressure slp a schematic figure is first presented to illustrate the relationship between the north atlantic oscillation nao pacific north american oscillation pna system and the ao negative correlation mode between the atlantic and the pacific ao ncm system although the nao pna apparent ao ncm and true ao ncm systems give rise to the same eofs the probability density functions for the time coefficients of the two leading modes are different therefore the discrimination of the two systems is possible several pieces of evidence indicate that in the real world the nao pna and the ao ncm are located on almost the same plane in phase space this means that the nao pna and ao ncm systems have the same variations on the plane in common implying that when the nao pna system is real the ao ncm is unlikely to be real simple independent compon

weak bands within ice stream b west antarctica kilometer scale variations in ice velocity and surface topography are used to investigate the style of glacier deformation in the main body of ice stream b west antarctica the pattern is very different from that reported for other glaciers for the 250 km 2 area studied on ice stream b most of the observed deformation occurs within two narrow bands in which there is large across flow compression and slow lateral shearing the bands underlie valleys in the ice surface topography measured upward displacement of ice adjacent to the rapidly compressing bands appears to be linked to the creation of the ice stream s topography the most likely cause for the observed pattern of strain rates and surface topography and their changes over time is deformation guided by longitudinal bands of ice with an aligned crystal fabric

first use of an airborne thermal infrared hyperspectral scanner for compositional mapping in may 1999 the airborne thermal infrared hyperspectral imaging system spatially enhanced broadband array spectrograph system sebass was flown over mormon mesa nv to provide the first test of such a system for geological mapping several types of carbonate deposits were identified using the 11 25 m band however massive calcrete outcrops exhibited weak spectral contrast which was confirmed by field and laboratory measurements because the weathered calcrete surface appeared relatively smooth in hand specimen this weak spectral contrast was unexpected here we show that microscopic roughness not readily apparent to the eye has introduced both a cavity effect and volume scattering to reduce spectral contrast the macroroughness of crevices and cobbles may also have a significant cavity effect the diminished spectral contrast is important because it places higher signal to noise ratio snr requirements for spectroscopic detection and identification this effect should be factored into instrumentation planning and interpretations especially interpretations without benefit of ground truth sebass had the required high snr and spectral resolution to allow us to demonstrate for the first time the ability of an airborne hyperspectral thermal infrared scanner to detect and identify spectrally subtle materials

basal topographic controls on the stability of the west antarctic ice sheet lessons from foundation ice stream using observations of basal topography ice thickness and modern accumulation rates we use theory and a dynamic flowline model to examine the sensitivity of antarctica s foundation ice stream to changes in sea level accumulation and buttressing at the grounding line our sensitivity studies demonstrate that the steep upward sloping basal topography inland from the grounding line serves to stabilize retreat of the ice stream while the upward sloping submarine topography downstream from the grounding line creates the potential for significant advance under conditions of modest sea level lowering and or increased accumulation rate extrapolating from foundation ice stream many nearby weddell sea sector ice streams are in a similar configuration suggesting that the historical and projected responses of this sector s ice streams may contrast with those in the amundsen or ross sea sectors this work reaffirms that the greatest concerns for rapid west antarctic ice sheet wais retreat are locations of reverse slopes muted basal topography and limited lateral support

the arctic oscillation signature in the wintertime geopotential height and temperature fields the leading empirical orthogonal function of the wintertime sea level pressure field is more strongly coupled to surface air temperature fluctuations over the eurasian continent than the north atlantic oscillation nao it resembles the nao in many respects but its primary center of action covers more of the arctic giving it a more zonally symmetric appearance coupled to strong fluctuations at the 50 hpa level on the intraseasonal interannual and interdecadal time scales this arctic oscillation ao can be interpreted as the surface signature of modulations in the strength of the polar vortex aloft it is proposed that the zonally asymmetric surface air temperature and mid tropospheric circulation anomalies observed in association with the ao may be secondary baroclinic features induced by the land sea contrasts the same modal structure is mirrored in the pronounced trends in winter and springtime surface air temperature sea level pressure and 50 hpa height over the past 30 years parts of eurasia have warmed by as much as several k sea level pressure over parts of the arctic has fallen by 4 hpa and the core of the lower stratospheric polar vortex has cooled by several k these trends can be interpreted as the development of a systematic bias in one of the atmosphere s dominant naturally occurring modes of variability

sedimentary and structural evolution of a relict subglacial to subaerial drainage system and its hydrogeological implications an example from anglesey north wales uk subglacial drainage systems exert a major control on basal sliding rates and glacier dynamics however comparatively few studies have examined the sedimentary record of subglacial drainage this is due to the paucity of modern analogues the limited recognition and preservation of upper flow regime deposits within the geological record and the difficulty of distinguishing subglacial meltwater deposits from other meltwater sediments e g glacier outburst flood deposits within this study the sedimentological and structural evolution of a subglacial to subaerial ice marginal proglacial drainage system is examined particular emphasis is placed upon the genetic development and preservation of upper flow regime bedforms and specifically recognising them within a subglacial meltwater context facies are attributed to subglacial meltwater activity and record sedimentation within a confined but progressively enlargening subglacial channel system produced under dune to upper flow regime conditions bedforms include rare large scale sinusoidal bedding with syn depositional deformation produced by current induced traction and shearing within the channel margins subglacial sedimentation culminated with the abrupt change to a more ephemeral drainage regime indicating channel abandonment or a seasonal drainage regime retreat of the ice margin led to the establishment of subaerial drainage with phases of sheet flow punctuated by channel incision and anastomosing channel development under diurnal ablation related seasonal discharge the presence of extensive hydrofracture networks demonstrate that proglacial groundwater levels fluctuated markedly and this may have influenced later overriding of the site by an ice stream

rapid ice flow rearrangement induced by subglacial drainage in west antarctica ice streams are corridors of rapid ice flow draining the ice sheets they can exhibit astonishing spatial variability on annual to centennial time scales we propose that changes in the subglacial drainage of meltwater could induce these sudden rearrangements of ice streams we develop a two dimensional thermomechanical model representing an ice stream cross section and couple it to a plastically deforming bed with spatially variable meltwater influx we find that where ice flows over deformable sediments and lacks significant topographic control the efficiency of subglacial water drainage exerts direct control on the velocity location and width of ice streams this implies that meltwater percolation at the meter scale could have a significant effect on the short term variability in ice loss from a continental scale ice sheet we verify our model against previous analytical results and validate it against surface observations from the siple coast of west antarctica

seismic observations of transient subglacial water flow beneath macayeal ice stream west antarctica 1 new seismic observations of harmonic tremors beneath macayeal ice stream west antarctica are reported each of the two tremor events that we recorded during a six week period had sustained arrival of 3 hz energy for approximately 10 minutes during that time the source location migrated a few kilometers the harmonic nature of the tremors is interpreted as the result of resonance in subglacial water filled cracks and conduits the duration monochromatic nature and movement of the tremor indicate that the source mechanism is likely flow in the subglacial water system resulting from the discharge from a small subglacial lake our results suggest that the subglacial water system produces repeated small outburst floods with possible implications for ice stream dynamics

melting and freezing beneath the ross ice streams antarctica we have estimated temperature gradients and melt rates at the bottom of the ice streams in west antarctica measured velocities were used to include the effects of horizontal advection and strain heating in the temperature model and to determine shear heating at the bed our modeled temperatures agree well with measured temperatures from boreholes in regions of steady flow we find that ice stream tributaries and the inland ice account for about 87 of the total melt generated beneath the ross ice streams and their catchments our estimates indicate that the ice plains of whillans ice stream and ice stream c even when active have large areas subject to basal freezing confirming earlier estimates that import of water from upstream is necessary to sustain motion the relatively low melt rates on whillans ice stream are consistent with observations of deceleration over the last few decades and suggest a shutdown may take place in the future possibly within this century while there are pockets of basal freezing beneath ice streams d and e there are larger areas of basal melt that produce enough melt to more than offset the freezing which is consistent with inferences of relatively steady flow for these ice streams over the last millennium

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