global ice sheet system interlocked by sea level abstract denton and hughes 1983 quaternary research 20 125 144 postulated that sea level linked a global ice sheet system with both terrestrial and grounded marine components during late quaternary ice ages summer temperature changes near northern hemisphere melting margins initiated sea level fluctuations that controlled marine components in both polar hemispheres it was further proposed that variations of this ice sheet system amplified and transmitted milankovitch summer half year insolation changes between 45 and 75 n into global climatic changes new tests of this hypothesis implicate sea level as a major control of the areal extent of grounded portions of the antarctic ice sheet thus fitting the concept of a globally interlocked ice sheet system but recent atmospheric modeling results manabe and broccoli 1985 journal of geophysical research 90 2167 2190 suggest that factors other than areal changes of the grounded antarctic ice sheet strongly influenced southern hemisphere climate and terminated the last ice age simultaneously in both polar hemispheres atmospheric carbon dioxide linked to high latitude oceans is the most likely candidate shackleton and pisias 1985 atmospheric carbon dioxide orbital forcing and climate in the carbon cycle and atmospheric co2 natural variations archean to present e t sundquest and w s broecker eds pp 303 318 geophysical monograph 32 american geophysical union washington d c but another potential influence was high frequency climatic oscillations 2500 yr it is postulated that variations in atmospheric carbon dioxide acted through an antarctic ice shelf linked to the grounded ice sheet to produce and terminate southern hemisphere ice age climate it is further postulated that milankovitch summer insolation combined with a warm high frequency oscillation caused marked recession of northern hemisphere ice sheet melting margins and the north atlantic polar front about 14 000 14c yr b p this permitted renewed formation of north atlantic deep water which could well have controlled atmospheric carbon dioxide w s broecker d m peteet and d rind 1985 nature london 315 21 26 combined melting and consequent sea level rise from the three warming factors initiated irreversible collapse of the interlocked global ice sheet system which was at its largest but most vulnerable configuration

the late devensian 22 000 bp irish sea basin the sedimentary record of a collapsed ice sheet margin the late devensian 20 ka bp glacial geology of the irish sea basin 4000 km2 is an event stratigraphy recording the entry of marine waters into a glacio isostatically depressed basin and the rapid retreat of the irish sea glacier as a tidewater ice margin marine limits occur up to 140 m o d across much of the central basin the ice margin was uncoupled from its bed exposing a subglacially scoured topography to glaciomarine processes the irish sea glacier was a major drainage conduit of the last british ice sheet calving of the marine ice margin resulted in fast flow surging of ice streams recorded by drumlin fields around the northern basin margin and tunnel valleys rapid evacuation of the basin may have stranded large areas of dead ice in peripheral zones e g cheshire shropshire lowlands and initiated the collapse of the ice sheet thick wedges of ice contact glaciomarine sediments were deposited during ice retreat as morainal bank complexes by successive tidewater ice margins stabilized at pinning points around the irish sea coast where morainal banks occur on the seaward side of drumlin swarms there is a clear sequential relationship between rapid ice loss from calving ice margins the development of fast flowing ice streams drumlinization and the pumping of subglacial sediment to tidewater raised delta complexes are locally associated with marine limits along the high relief coastal margins of wales east central ireland and the lake district associated valley infill complexes record downslope resedimentation of heterogenous sediments into the marine environment during ice retreat co eval offshore deposits are represented by well stratified glaciomarine complexes that infill a subglacially scoured topography that shows networks of tunnel valleys glaciomarine mud drapes occur well to the south of the maximum limit of grounded ice in the basin e g north devon scilly islands southern ireland the age of these distal sediments previously mapped as pre devensian tills is constrained by amino acid ratios basin rebound following deglaciation was rapid with over 100 m recovery in 3 ka and was followed by a low marine still stand peat accumulating in offshore areas now as much as 55 m below sea level has been drowned by the postglacial eustatic rise in sea level the glacio sedimentary model identified in this paper involving rapid ice retreat and related sedimentation triggered by rising relative sea level suggests that isotatic downwarping is an important mechanism for deglaciating continental shelves

detection of small scale heterogeneities at the inner core boundary 2018 elsevier b v the hemispherical pattern of seismic velocities in the uppermost inner core has been described at the global scale in many studies in this paper we investigate shorter scale heterogeneities in the vicinity of the inner core boundary icb using differential travel time and amplitude measurements between the pair of core phases pkp df and pkikp seismic events occurring close to sumatra and recorded at usarray stations in the distance range 132 142 are used to investigate the velocity and attenuation structure at the icb and in the uppermost 100 km of the inner core beneath eastern russia in this distance range pkp df and pkikp are difficult to separate due to their proximity in travel time we present a new method based on the envelope of the seismic signal to identify them following which we measure differential travel time residuals and amplitude ratios by comparison with synthetics computed using the direct solution method which allows us to take into account the correct phase shift of the pkikp we found that in our region of study the compressional velocity is slower in the uppermost inner core than the ak135 model however the sampled region is located in the eastern hemisphere generally considered as faster and more attenuating than the ak135 model this suggests significant regional departures from the simple hemispherical pattern reported in the literature we also deduce from the amplitude ratios that the attenuation is stronger than the global average at the top of the inner core in this region as expected from its location in the eastern hemisphere our results confirm the presence of a low attenuation zone at the top of the inner core followed by a highly attenuating layer centered around 35 km depth in the inner core as found in the eastern hemisphere in previous studies this zone is however thinner 15 km and more attenuating q reaching values less than 50

microimpact phenomena on australasian microtektites implications for ejecta plume characteristics and lunar surface processes abstract detailed investigations of the microimpact phenomena on australasian microtektites from four samples from the central indian basin reveal an array of features such as very low velocity captured droplets welded projectiles angular fragments and dust craters generated by projectiles defining an oblique trajectory high velocity pitless craters and the conventional hypervelocity craters with well defined central pits and radial and concentric cracks found commonly on lunar surface materials the microimpacts are a consequence of interparticle collisions within the ejecta plume as suggested by their chemistry subsequent to a major impact and therefore reveal processes inherent in an impact generated plume all the impact phenomena observed here have taken place while the targets and projectiles were in flight and are therefore secondary impacts in lunar terms however some of the resultant features are analogous to lunar micro craters attributed to primary impacts by cosmic dust therefore ballistic sedimentation on the moon is likely to contain plume collisional debris as well

seismic velocity and attenuation structures in the top 400 km of the earth s inner core along equatorial paths 1 we study seismic velocity and attenuation structures in the top 400 km of the earth s inner core based on modeling of differential traveltimes amplitude ratios and waveforms of the pkikp pkikp phases observed at the epicentral distance range of 120 141 and the pkpbc pkikp phases observed at the distance range of 146 160 along equatorial paths our data are selected from the seismograms recorded in the global seismographic network from 1990 to 2001 and many regional seismic networks the observed pkikp pkikp and pkpbc pkikp phases exhibit distinctive east west hemispheric patterns 1 at the distance ranges of 131 141 and 146 151 pkikp phases arrive about 0 3 s earlier than the theoretical arrivals based on the preliminary reference earth model prem for the pkikp phases sampling the eastern hemisphere 40 e 180 e of the inner core and about 0 4 s later for those sampling the western hemisphere 180 w 40 e at the distance range of 151 160 pkikp phases arrive about 0 7 s earlier than the predicted arrivals based on prem for those sampling the eastern hemisphere and about 0 1 s later for those sampling the western hemisphere 2 amplitude ratios of the pkikp pkikp phases at the distance range of 131 141 and of the pkikp pkpbc phases at the distance range of 146 151 are in general smaller for the pkikp phases sampling the eastern hemisphere than for those sampling the western hemisphere at distances greater than 151 the pkikp pkpbc amplitude ratios become indistinguishable for the two hemispheres these observations can be best explained by two different types of seismic velocity and attenuation models along equatorial paths one for each hemisphere in the top 400 km of the inner core for the eastern hemisphere the velocity structure has a velocity increase of 0 748 km s across the inner core boundary icb a small velocity gradient of 0 0042 km s 100 km in the top 235 km followed by a steeper velocity gradient of 0 1 km s 100 km extending from 235 km to 375 km and a velocity gradient of 0 01 km s 100 km in the deeper portion of the inner core the attenuation structure has an average q value of 300 in the top 300 km and an average q value of 600 in the deeper portion of the inner core for the western hemisphere the velocity structure has a velocity increase of 0 645 km s across the icb and a velocity gradient of 0 049 km s 100 km in the top 375 km the attenuation structure has an average q value of 600 in the top 375 km of the inner core our results suggest that the inner core hemispheric variations in velocity extend deeper than 375 km below the icb and the top 235 km of the inner core in the eastern hemisphere is anomalous compared to the rest of the inner core in having a small velocity gradient high velocity and high attenuation

insights into spatial sensitivities of ice mass response to environmental change from the searise ice sheet modeling project ii greenland searise greenland

modeling ice sheets from the bottom up abstract three facts should guide ice sheet modeling 1 ice height above the bed is controlled by the strength of ice bed coupling reducing ice thickness by some 90 percent when coupling vanishes 2 ice bed coupling vanishes along ice streams that end as floating ice shelves and drain up to 90 percent of an ice sheet 3 because of 1 and 2 ice sheets can rapidly collapse and disintegrate thereby removing ice sheets from earth s climate system and forcing abrupt climate change the first model of ice sheet dynamics was developed in australia and applied to the present antarctic ice sheet in 1970 it treated slow sheet flow which prevails over some 90 percent of the ice sheet but is the least dynamic component the model made top down calculations of ice velocities and temperatures based on known surface conditions and an assumed basal geothermal heat flux in 1972 joseph fletcher proposed a six step research strategy for studying dynamic systems the first step was identifying the most dynamic components which for antarctica are fast ice streams that discharge up to 90 percent of the ice ice sheet models developed at the university of maine in the 1970s were based on the fletcher strategy and focused on ice streams including calving dynamics when ice streams end in water these models calculated the elevation of ice sheets based in the strength of ice bed coupling this was a bottom up approach that lowered ice elevations some 90 percent when ice bed coupling vanished top down modeling is able to simulate changes in the size and shape of ice sheets through a whole glaciation cycle provided the mass balance is treated correctly bottom up modeling is able to produce accurate changes in ice elevations based on changes in ice bed coupling provided the force balance is treated correctly truly holistic ice sheet models should synthesize top down and bottom up approaches by combining the mass balance with the force balance in ways that merge abrupt changes in stream flow with slow changes in sheet flow then discharging 90 percent of the ice by ice streams mobilizes 90 percent of the area so ice sheets can self destruct and thereby terminate a glaciation cycle

on the pulling power of ice streams gravity wants to pull an ice sheet to the center of the earth but cannot because the earth s crust is in the way so ice is pushed out sideways instead or is it the ice sheet sees nothing preventing i t from spreading out except air which is much less massive than ice therefore does not ice rush forward to fill this relative vacuum does not the relative vacuum suck ice into i t because nature abhors a vacuum if so the ice sheet is not only pulled downward by gravity i t is also pulled outward by the relative vacuum this pulling outward will be most rapid where the ice sheet encounters least resistance the least resistance exists along the bed of ice streams where ice bed coupling is reduced by a basal water layer especially if the ice stream becomes afloat and the floating p art is relatively unconfined around its perimeter and unpinned to the sea floor ice streams are t herefore fast currents of ice that develop near the margins of an ice sheet where t hese conditions exist because of these condit ions ice streams pull ice out of ice sheets and have pulling power equal to the longitudinal gravitational pulling force multiplied by the ice stream velocity t hese boundary conditions beneath and beyond ice streams can be quantified by a basal buoyancy factor that provides a life cycle classification of ice streams into inception growth mature declining and terminal stages during which ice streams disintegrate the ice sheet surface profiles of ice streams are diagnostic of the stage in a life cycle and hence of the vitality of the ice sheet

neodymium and strontium isotopic study of australasian tektites new constraints on the provenance and age of target materials nd and sr isotopic studies of australasian tektites provide information on the age and provenance of the target materials and allow us to characterize the target area and the impact process leading to tektite formation nd values of australites splash form indochinites and muong nong type indochinites are indistinguishable within analytical uncertainty and average 11 5 u depleted mantle nd model ages fall within the narrow range of 1490 1620 ma indicating that the source material was derived dominantly from a proterozoic crustal terrane sr values are variable and are correlated with the geographic location of the tektite samples analyses of four muong nong type or layered indochinites from a single locality in eastern thailand yield an isochron age of 167 12 ma 2 a correlation of rb sr fractionation with sr model ages indicates that the last major rb sr fractionation event experienced by the target materials occurred 175 15 ma ago we interpret this age as the time of deposition of sedimentary target rocks and consider the compositional layering observed in muong nong type tektites to reflect compositional variability inherited from jurassic sediments the nd and sr isotopic data provide evidence that all australasian tektites were derived from a single sedimentary formation with a narrow range of stratigraphic ages close to 170 ma we suggest that all of the australasian tektites were derived from a single impact event and that the australites represent the upper part of a melt sheet ejected at high velocity whereas the indochinites represent melts formed at a lower level in the target material which were distributed closer to the area of impact the impact site is inferred to be within an area of jurassic sedimentary bedrock which spans the geopolitical boundaries between northern cambodia southern laos and southeastern thailand

a detailed chronology of the australasian impact event the brunhes matuyama geomagnetic polarity reversal and global climate change a mechanism had been recently proposed to show how an impact event can trigger a geomagnetic polarity reversal by means of rapid climate cooling we test the proposed mechanism by examining the record from two high sedimentation rate 8 11 cm kyr deep sea sediment cores odp sites 767 and 769 from marginal seas of the indonesian archipelago which record the australasian impact with well defined microtektite layers the brunhes matuyama polarity reversal with strong and stable remanent magnetizations and global climate with oxygen isotope variations in planktonic foraminifera both odp cores show the impact to have preceded the reversal of magnetic field directions by about 12 kyr both records indicate that the field intensity was increasing near the time of impact and that it continued to increase for about 4 kyr afterwards furthermore the oxygen isotope record available from sediments at odp site 769 shows no indication of discernible climate cooling following the impact the microtektite event occurred in the later part of glacial stage 20 and was followed by a smooth warming trend to interglacial stage 19 thus the detailed chronology does not support the previously proposed model which would predict that a decrease in geomagnetic field intensity resulted from a minor glaciation following the impact event we conclude that the evidence for a causal link between impacts and geomagnetic reversals remains insufficient to demonstrate a physical connection

calving processes and the dynamics of calving glaciers abstract calving of icebergs is an important component of mass loss from the polar ice sheets and glaciers in many parts of the world calving rates can increase dramatically in response to increases in velocity and or retreat of the glacier margin with important implications for sea level change despite their importance calving and related dynamic processes are poorly represented in the current generation of ice sheet models this is largely because understanding the calving problem involves several other long standing problems in glaciology combined with the difficulties and dangers of field data collection in this paper we systematically review different aspects of the calving problem and outline a new framework for representing calving processes in ice sheet models we define a hierarchy of calving processes to distinguish those that exert a fundamental control on the position of the ice margin from more localised processes responsible for individual calving events the first order control on calving is the strain rate arising from spatial variations in velocity particularly sliding speed which determines the location and depth of surface crevasses superimposed on this first order process are second order processes that can further erode the ice margin these include fracture propagation in response to local stress imbalances in the immediate vicinity of the glacier front undercutting of the glacier terminus by melting at or below the waterline and bending at the junction between grounded and buoyant parts of an ice tongue calving of projecting submerged ice feet can be regarded as a third order process because it is paced by first or second order calving above the waterline first order calving can be represented in glacier models using a calving criterion based on crevasse depth which is a function of longitudinal strain rate modelling changes in terminus position and calving rates thus reduces to the problem of determining the ice geometry and velocity distribution realistic solutions to the problem of modelling ice flow therefore depend critically on an appropriate choice of sliding law models that assume that basal velocities are controlled by basal drag can replicate much of the observed behaviour of calving glaciers with grounded termini but an important limitation is that they cannot be used to model floating glacier termini or ice shelves alternative sliding laws that parameterise drag from the glacier margins provide more flexible and robust ways of representing calving in ice sheet models such models can explain a remarkable range of observed phenomena within a simple unifying framework including downglacier increases in velocity and strain rates where basal and or lateral drag diminishes flow acceleration in response to thinning through time the tendency for glaciers to stabilise at pinning points in relatively shallow water or fjord narrowings the constraints on ice shelf stability and the contrasts in calving rates between tidewater and freshwater calving glaciers many unresolved issues remain however including the role played by the removal of backstress in the acceleration of retreating calving glaciers and the controls on melting at and below the waterline

pkp travel times at near antipodal distances implications for inner core anisotropy and lowermost mantle structure abstract previous studies from pkp ab df differential travel times at large distances suggest that the central part of the inner core is very anisotropic these differential times however can be affected greatly by strong heterogeneity in the lowermost mantle here we examine a unique data set of pkp travel times from global digital and analog stations at near antipodal distances where the effects of both inner core anisotropy and mantle heterogeneity are the greatest our results show that the ab df residuals for the polar paths are consistently larger than those of the equatorial paths by over 3 4 standard deviations we also measured df and ab absolute times and found that the df residuals are negatively correlated with the ab df residuals while the ab residuals have a much weaker correlation with the ab df residuals we compare several mantle models with the data our results suggest that the mantle structure can explain part of the residuals of the equatorials paths but cannot explain the polar path anomalies these results strongly suggest that most of the ab df anomalies for the polar paths are likely from the inner core anisotropy and not from mantle heterogeneity assuming a uniform cylindrical anisotropy model the average inner core anisotropy amplitude is about 2 5 on the other hand equatorial paths from events in the west pacific and recorded at south america show a steep azimuthal change in ab df times about 4 s over a 60 azimuthal range the sharp change is well predicted by existing p and s tomographic models the azimuthal change occurs as the ab paths sweep across the great slow anomaly in the central pacific the high correlation between the p and s velocities suggests that the central pacific slow anomaly may be of thermal origin

melting induced stratification above the earth s inner core due to convective translation several observed properties of earth s solid inner core and liquid outer core remain enigmatic including an east west seismic asymmetry and the presence of a roughly 250 kilometre thick layer of reduced seismic velocity at the base of the outer core alboussiere et al now suggest a model that can produce both of these features based on experiment and numerical modelling they propose that simultaneous crystallization and melting at opposite sides of earth s inner core surface can result in a translational mode of thermal convection within the inner core which also introduces an asymmetry between hemispheres these authors show that simultaneous crystallization and melting at the surface of the earth s inner core can result in a translational mode of thermal convection within the inner core producing the observed stratified layer of reduced seismic velocity at the base of the outer core the dynamical model they propose also introduces an asymmetry between hemispheres that may explain the enigmatic east west asymmetry in seismic properties of the inner core in addition to its global north south anisotropy1 there are two other enigmatic seismological observations related to the earth s inner core asymmetry between its eastern and western hemispheres2 3 4 5 6 and the presence of a layer of reduced seismic velocity at the base of the outer core6 7 8 9 10 11 12 this 250 km thick layer has been interpreted as a stably stratified region of reduced composition in light elements13 here we show that this layer can be generated by simultaneous crystallization and melting at the surface of the inner core and that a translational mode of thermal convection in the inner core can produce enough melting and crystallization on each hemisphere respectively for the dense layer to develop the dynamical model we propose introduces a clear asymmetry between a melting and a crystallizing hemisphere which forms a basis for also explaining the east west asymmetry the present translation rate is found to be typically 100 million years for the inner core to be entirely renewed which is one to two orders of magnitude faster than the growth rate of the inner core s radius the resulting strong asymmetry of buoyancy flux caused by light elements is anticipated to have an impact on the dynamics of the outer core and on the geodynamo

regional variation of inner core anisotropy from seismic normal mode observations earth s solid inner core is surrounded by a convecting liquid outer core creating the geodynamo driving the planet s magnetic field seismic studies using compressional body waves suggest hemispherical variation in the anisotropic structure of the inner core but are poorly constrained because of limited earthquake and receiver distribution here using normal mode splitting function measurements from large earthquakes based on extended cross coupling theory we observe both regional variations and eastern versus western hemispherical anisotropy in the inner core the similarity of this pattern with earth s magnetic field suggests freezing in of crystal alignment during solidification or texturing by maxwell stress as origins of the anisotropy these observations limit the amount of inner core super rotation but would be consistent with oscillation

impact microcrater morphology on australasian microtektites scanning electron microscopy of 137 australasian microtektites and fragments from 4 sediment cores in the central indian ocean reveals more than 2000 impact generated features in the size range of 0 3 to 600 m 2 dominant crater number densities in m and sub m sizes therefore tektite producing impacts can lead to the generation of microcraters that mimic those found on lunar surface materials and for the lunar rocks to qualify as reliable cosmic dust flux detectors their tumbling histories and lunar surface orientations have to be known precisely

relationship between tidewater glacier calving velocity and water depth at the calving front an analysis of the relationship between iceberg calving rates and water depth has been completed for 22 tidewater glaciers a linear relationship provides reasonable accuracy with a correlation coefficient of 0 85 for all tidewater glaciers examined whether they be polar or temperate the polar glaciers have a slightly lower calving rate for a given water depth this relationship indicates a lower calving rate for water depths over 50 m than determined by brown and others 1982 it is based only on glaciers or ice streams and cannot be applied to ice shelves

geometrical force balance in glaciology 1 the analytical force balance traditionally used in glaciology relates gravitational forcing to ice surface slope for sheet flow and to ice basal buoyancy for shelf flow it is unable to represent stream flow as a transition from sheet flow to shelf flow by having gravitational forcing gradually passing from being driven by surface slope to being driven by basal buoyancy downslope along the length of an ice steam this is a serious defect because ice streams discharge up to 90 of ice from ice sheets into the sea the defect is overcome by using a geometrical force balance that includes basal buoyancy represented by the ratio of basal water pressure to ice overburden pressure as a source of gravitational forcing when combined with the mass balance the geometrical force balance provides a holistic approach to ice flow in which resistance to gravitational flow must be summed upstream from the calving front of an ice shelf this is not done in the analytical force balance and it provides the ice thinning rate required by gravitational collapse of ice sheets as interior ice is downdrawn by ice streams

changes in abundance and nature of microimpact craters on the surfaces of australasian microtektites with distance from the proposed source crater location abstract over 4600 australasian microtektites from 11 sediment cores along an n s transect in the central indian ocean have been investigated optically for microimpact features on their surfaces detailed scanning electron microscope examination of 68 microtektites along this transect shows 4091 such features these samples are located between approximate distances of 3900 5000 km from the suggested impact site in indochina and therefore constitute distal ejecta the morphology of the microimpacts seems to show distinct variations with distance from the source crater the total number of microcraters on each microtektite decreases drastically from north to south indicating systematic decrease in the spatial density of the ejecta and decrease in collisional activity between microtektites with distance from the proposed source crater location closer to the proposed source crater location the microcraters are predominantly small few m pit bearing with radial and concentric cracks suggestive of violent interparticle collisions the scenario is reverse farther from the source crater with smaller numbers of impacted microtektites due to increased dispersion of the ejecta and the microcraters are large and shallow implying gentle collisions with larger particles these observations provide systematic ground truth for the processes that take place as the ejecta of a large oblique impact which generated the australasian tektite strewn field is emplaced the microimpacts appear to take place during the descent of the ejecta and their intensity and number density decrease as a function of the spatial density of the ejecta at any given place and with distance from the source region these features could help understand processes that take place during ejecta emplacement on planets with substantial atmosphere such as mars and venus

surficial glaciology of jakobshavns isbrae west greenland part i surface morphology jakobshavns isbrae is a large fast moving ice stream outlet glacier in west greenland which ends at a floating calving front it drains about 6 5 of the area of the greenland ice sheet studies of its surface morphology are described in this paper the surface is relatively steep 0 01 0 03 and the thickness is large up to 2600 m along the center line clarke and echelmeyer 1989 indicating very high driving stresses 200 300 kpa the ice stream is about 6 km wide and 85 90 km long all of which is in an area of surface melting the base of the ice stream and of much of the drainage area is below sea level marginal crevasse zones have a width on the order of the width of the ice stream itself unique surficial features are ice blisters and lakes the latter have a sequence of ogive like features on their floating ice cover which can be used to determine velocity there is a pinning point near the terminus which may act as a stabilizing influence possibly playing a role in halting at least temporarily a recent retreat of the terminus ice thickness estimates at the terminus lead to a flux which is less than previously assumed by others e g bindschadler 1984 pelto and others 1989 when estimating jakobshavns isbrae s drainage basin to be nearly in balance the driving stresses on jakobshavns isbrae are an order of magnitude higher than those of the ice streams of west antarctica its crevasse patterns are much less localized its relatively unconfined terminus is more comparable to that of relatively unbuttressed ice streams such as pine island and thwaites glaciers than it is to other west antarctic ice streams which terminate in large confined ice shelves

the flow of fireweed rock glacier alaska u s a fireweed rock glacier is a large rock glacier in south central alaska u s a it flows relatively fast with velocities up to 3 5 ma 1 and exhibits both seasonal and annual velocity variations some of which are related to periodic terminus calving and increased rainfall our analysis reveals that motion is likely concentrated in a pseudo rectangular channel within the larger parabolic channel with a shear plane at 27 m depth there is likely motion along the shear plane as well as internal deformation above it we estimate that the ice rock mixture is up to seven times softer than clean glacier ice with a temperature of 2 c calving at the terminus is an important component of the mass balance of this rock glacier

a coupled marine ice stream ice shelf model theorie des variations climatiques du pleistocene a aujourd hui liees a la dynamique des couvertures marines de glace couplee aux glaces libres flottantes on neglige les effets thermodynamiques et on etudie les processus de flottaison en relation avec les rapports de densite entre la place et l eau de mer

jakobshavns glacier drainage basin a balance assessment maximum and minimum estimates are made of the drainage basin feeding the jakobshavns glacier by using surface elevation maps derived from seasat altimetry benson s 1962 net balance measurements are used to calculate the balance flow within the basin comparisons of the balance flux at the terminus with estimates of actual flux suggest the basin is in overall equilibrium or slightly thickening this agrees with measurements along the nearby egig traverse balance velocities accelerate rapidly within 100 km of the coast farther upstream balance velocities are consistent with both measured velocities along the egig traverse and calculated deformation velocities it is estimated that jakobshavns glacier discharges between 4 8 and 7 6 of the annual net balance over greenland and drains between 3 7 and 5 8 of the ice sheet area

investigation of surface melting and dynamic thinning on jakobshavn isbrae greenland jakobshavn isbrae is the most active glacier in greenland with an annual discharge of about 30 km 3 of ice and it is one of the few recently surveyed glaciers to thicken between 1993 and 1998 despite locally warm summers repeated airborne laser altimeter surveys along a 120 km profile in the glacier basin show slow sporadic thickening between 1991 and 1997 suggesting a small positive mass balance but since 1997 there has been sustained thinning of several m a 1 within 20 km of the ice front with lower rates of thinning further inland here we use weather station data from the coast and the ice sheet to estimate the effects on surface elevation of interannual variability in snowfall and surface melt rates and thus to infer the temporal and spatial patterns of dynamic thinning these show the glacier to have been close to balance before 1997 followed by a sudden transition to rapid thinning initially confined to the lower reaches of the glacier below about 500 m elevation but progressively spreading inland until between 1999 and 2001 thinning predominated over the entire surveyed region up to 2000 m elevation if this continues the glacier calving front and probably its grounding line will retreat substantially in the very near future

depth extent of hemispherical inner core from pkp df and pkp cdiff for equatorial paths abstract differential travel times and amplitude ratios of pkp df and pkp cdiff partly including pkp bc for equatorial paths at epicentral distances from 150 to 160 are examined to discuss the radial structure of the uppermost 500 km of the inner core by using seismograms with a predominant frequency of approximately 0 5 hz 649 measurements are successfully collected including 295 and 354 readings for the eastern and western hemispheres respectively forward modeling with the reflectivity method suggests that the best fit velocity structures in the eastern and western hemispheres are 0 5 faster and 0 9 slower respectively than an average structure at the inner core boundary icb and that the velocity perturbations become linearly closer to the average with depth finally coinciding with the average at 400 km and 250 km below the icb respectively if ak135 is adopted for the structure above the icb in the eastern hemisphere the q values in the uppermost 250 km of the inner core and that in the range from 300 to 450 km below the icb are approximately 180 and 1000 respectively in contrast the q value in the western hemisphere is almost constant at approximately 300 400 in the uppermost 450 km of the inner core

landform modification by palaeo ice streams in east central ireland in eastern ireland subglacial bedforms including drumlins and rogen moraines were modified by headward erosion along two ice streams which had over lapping flow tracks the ice streams which hadtidewater termini are dated by geochrono metric and morphostratigraphic methods to 5 15 0 14 ckyr bp castleblaney ice stream and 13 8 14 ckyr bp armagh ice stream bedforms along ice stream tracks show a morphological continuum which reflects a down ice increase in the degree of modification by ice stream activity i e resulting in unmodified remoulded overprinted cross cut streamlined bedforms the down ice changes inbedform types are inferred to relate to changes in subglacial drainage and sediment transport mechanisms bedform and sedi mentary evidence suggest that discrete subglacial meltwater channels which developed up ice changed in a down ice direction to unchannelized flows which deepened towards the ice margin meltwater release from subglacial cavities and produced by strain heating at sheared ice stream margins probably helped support ice stream flow which ended as the volume of subglacial meltwater discharge decreased dated millennial scale cycles of ice activity may be related to instability at tidewater margins followed by complex thermal and hydraulic responses within the ice mass

greenland ice sheet response to external forcing kinematic wave modeling is used to evaluate possible responses of the greenland ice sheet to changes in its surface mass balance in the approach followed here the reference state is defined based on measured velocity and discharge flux along the central flow line of petermann glacier in the northwest and perturbations on this state are considered the results indicate that significant rates of thickness change can occur immediately after the prescribed change in surface mass balance but adjustments in flow rapidly diminish these rates to a few centimeters per year at most full adjustment of the ice sheet requires times of the order of 1000 years the instability mechanism known as the jakobshavn effect is discussed and based on observational evidence as well as results from prior modeling studies it is concluded that this is an unlikely mechanism for destabilizing major drainage basins of the greenland ice sheet

regionally heterogeneous uppermost inner core observed with hi net array studies of velocity structure in the earth s inner core and its volumetric variations illuminate our understanding of inner core dynamics and composition here we use an extensive number of seismograms recorded by the hi net array to construct complete empirical curves of pkpbc pkpdf differential traveltimes the nature of these curves implies that significant variations in traveltimes are accumulated during the passage of pkpdf waves through the uppermost inner core and rules out outer core structure effects uniform cylindrical anisotropy of a plausible strength in the uppermost inner core can also be ruled out as a cause of the observed traveltime variations because the range of sampled ray angles is too narrow the configuration and strength of inhomogeneities from a recent tomographic model of the lowermost mantle cannot account for the observed traveltime variations therefore we infer that either variations of p wave isotropic velocity on the scale of about hundred km and less are present in the uppermost inner core or the material may be organized in distinctive anisotropic domains and both of these features may be superimposed on long wavelength hemispherical structure if the former holds true the absolute magnitude of required p wave velocity perturbations from referent values is 0 60 0 10 in the quasi eastern and 1 55 0 15 in the quasi western hemisphere 0 85 0 05 and 1 10 0 10 respectively with the lowermost mantle correction the existence of these variations is a plausible physical outcome given that vigorous compositional convection in the outer core and variations in heat exchange across the inner core boundary may control the process of solidification

derived bedrock elevations strain rates and stresses from measured surface elevations and velocities jakobshavns isbrae greenland jakobshavns isbrae 69 10 n 49 59 w drains about 6 5 of the greenland ice sheet and is the fastest ice stream known the jakobshavns isbrae basin of about 10 000 km 2 was mapped photogrammetrically from four sets of aerial photography two taken in july 1985 and two in july 1986 positions and elevations of several hundred natural features on the ice surface were determined for each epoch by photogrammetric block aerial triangulation and surface velocity vectors were computed from the positions the two flights in 1985 yielded the best results and provided most common points 716 for velocity determinations and are therefore used in the modeling studies the data from these irregularly spaced points were used to calculate ice elevations and velocity vectors at uniformly spaced grid points 3 km apart by interpolation the field of surface strain rates was then calculated from these gridded data and used to compute the field of surface deviatoric stresses using the flow law of ice for rectilinear coordinates x y pointing eastward and northward and curvilinear coordinates l t pointing longitudinally and transversely to the changing ice flow direction ice surface elevations and slopes were then used to calculate ice thicknesses and the fraction of the ice velocity due to basal sliding our calculated ice thicknesses are in fair agreement with an ice thickness map based on seismic sounding and supplied to us by k echelmeyer ice thicknesses were subtracted from measured ice surface elevations to map bed topography our calculation shows that basal sliding is significant only in the 10 15 km before jakobshavns isbrae becomes afloat in jakobshavns isfjord

seismic velocity structure in the earth s outer core 1 we constrain seismic velocity structure in the earth s outer core by analyzing differential travel times waveforms and amplitude ratios of several pairs of core phases we use differential travel times and waveforms of pkpbc pkikp at 144 148 pkikp pkikp at 120 141 pkikp pkpbdiff at the pkp caustics distance range 141 145 and differential travel times and amplitude ratios of pkpab pkpbc at 146 159 to avoid the influence of the inner core anisotropy in velocity we only use the pkikp pkikp and the pkpbc pkikp observations whose pkikp phases sample the inner core along the equatorial paths paths with their ray angles being larger than 35 from the earth s rotation axis these observations show the following characteristics 1 both the observed pkpbc pkikp and pkikp pkikp differential travel times show a distinct east west hemispheric pattern pkikp phases arrive about 0 7 s earlier for those sampling the eastern hemisphere 40 e 180 e than those sampling the western hemisphere 180 w 40 e 2 the observed differential pkikp pkpbdiff travel time residuals also exhibit a hemispheric difference pkikp pkpbdiff differential travel times are about 0 9 s larger for those sampling the western hemisphere than those sampling the eastern hemisphere and 3 both the observed pkpab pkpbc differential travel times and pkpbc pkpab amplitude ratios show scatter overall these observations can be best explained by two one dimensional p velocity models one for each hemisphere at the bottom of the outer core the seismic data sampling the eastern hemisphere can be explained by prem at the bottom of the outer core while those sampling the western hemisphere can be explained by a lower velocity gradient at the bottom of the outer core which has reduced velocities relative to prem linearly increasing from 0 at 200 km above the inner core boundary icb to 0 35 at the icb different velocity gradients at the bottom of the outer core indicate that there may exist a compositional difference and or a large scale temperature difference there and that inner core formation processes may be different between the two hemispheres different inner core formation processes may produce different geometric inclusions of melt in the top of the inner core and thus may provide an explanation to the hemispheric variation of seismic velocity and attenuation in the top of the inner core the seismic data also suggest that the variation of seismic velocity in the tangential cylinder of the outer core if it exists is less than 0 1 we use a compressional wave tomographic model to study the travel time delays caused by the seismic heterogeneities in the mantle the tomographic model is unable to explain the scatter of the travel time data suggesting the existence of unknown strong small scale seismic heterogeneities in the mantle

experimentation in planetary geology the reasons for conducting experimentation are as varied as the topics that are studied whether a specific parameter must be quantified or an attempt is being made to duplicate a peculiar process all experiments are unified by a common ultimate objective to provide a tangible link between theory and observation although a suitable theory is often nonexistent at the time of the experiment experimentation in planetary geology assumes particular importance because of the factors inherent in the field of study itself often enormous spatial scales timescales rivaled only by those of cosmology and the remoteness of the objects of study in addition any geoscientist who has entertained thoughts of developing a realistic mathematical model of even the simplest of processes rapidly becomes acquainted with the intimidating complexity of geological phenomena experimentation is still in many cases the only effective means of learning more about some natural mechanisms

independent estimate of velocity structure of earth s lowermost outer core beneath the northeast pacific from pkikp pkpbc differential traveltime and dispersion in pkpbc the presence of a low velocity layer at the base of earth s outer core has been proposed however the seismic profile of the basal layer indeed has been poorly constrained in previous seismic studies the model parameters of the layer are substantially nonunique and there are tradeoffs between the seismic velocity of the layer and the properties of the mantle and inner core a more tightly constrained profile of the layer helps further examine the composition and dynamics of the layer in this study we obtained the p wave velocity profile of the basal layer beneath the northeast pacific based on two new seismic observations by analyzing seismograms of the hi net in japan the new observations are particularly sensitive to the layer structure and are relatively insensitive to the structure of the other parts of the earth 1 the frequency dispersion in p waves that graze or are diffracted at the inner core boundary pkpbc and pkpc diff and 2 differential traveltimes between the p waves reflected from the inner core boundary pkikp and those that turn above the boundary pkpbc the resulting velocity model of the lowermost outer core called f layer velocity model for the western hemisphere fvw has p wave velocities that lie between those of ak135 and the preliminary reference earth model prem and a velocity gradient that is slightly gentler than that of prem models with a uniform p wave velocity value within the layer are not supported by the observations for the region investigated which appears not to support the presence of a thick basal layer that is fe rich and dense there

the structure of the base of the outer core inferred from seismic waves diffracted around the inner core 1 we systematically searched for seismograms of waves diffracted around the inner core pkpcdiff from all the temporary seismic arrays with data currently available at the iris dmc as well as some permanent regional seismic arrays including f net in japan and grf in germany to assemble the largest high quality pkpcdiff database ever created pkpcdiff waves preferentially sample the base of the outer core and so contain important clues about earth structure in this region we measured pkpdf pkpcdiff differential traveltimes and pkpcdiff pkpdf amplitude ratios in the distance range of 154 160 and modeled the observations using grid searches and full wave theory synthetic seismograms the optimum model found by fitting the differential traveltimes has relatively low velocity at the base of the outer core as in ak135 which is consistent with many previous traveltime studies however the optimum model found by fitting the amplitude ratios pkpcdiff pkpdf does not exhibit this feature and instead is closer to prem the discrepancy may be explained by two likely causes one is that small scale topography or roughness on the icb tends to scatter energy away from pkpcdiff waves by generating trailing coda waves the other is that there exists a thin layer with relatively low q at the base of the outer core this might be expected if there are suspended solid particles at the base of the outer core as proposed decades ago both mechanisms could generate smaller pkpcdiff amplitudes without significantly affecting pkpcdiff traveltimes

seismic structure near the inner core boundary in the south polar region 1 it is crucial to obtain good spatial coverage of seismic data points for better understanding the earth s core but the core beneath the polar regions remains largely unexplored we analyzed differential traveltimes and amplitude ratios of core phases whose raypaths run beneath antarctica for determining the vp and qp structure near the inner core boundary in the south polar region the model we obtained south polar region model spr is described relative to the preliminary reference earth model prem as follows a 0 05 km s lower vp value at the top of the inner core a 1 5 times steeper vp gradient in the upper 300 km of the inner core a smaller qp 300 in the upper 300 km of the inner core and a 0 04 km s lower vp at the bottom of the outer core the vp values of spr in the lowermost outer core lie between those of prem and ak135 being closer to those of ak135 the lowermost outer core vp inside the tangent cylinder is thus close to the global average in the upper inner core spr has lower vp than ak135 and prem the spr vp profile is close to that of previous models for the western hemisphere although most of our data sample the eastern hemisphere of the inner core our results indicate that the inner core does not have a simple hemispherical variation as usually supposed our data support an eyeball shaped high vp anomaly with compressional velocity higher than in 1 d reference earth models concentrated to a smaller region beneath eastern asia

impact microcraters on an australasian microtektite microcraters attributable to impact have been discovered on an australasian microtektite from a core in the central indian basin the craters resemble lunar microcraters and those generated during impact experiments the largest crater here which has a welded promontory is unique the projectiles that produced the impacts defined varying trajectories and velocities ranging from hypervelocity to low velocity a few 10 m s the impacts took place while the microtektite was in flight at an elevated target temperature this is the first observation of the microimpact phenomenon on a microtektite

uppermost inner core heterogeneity from differential travel times of pkikp vs pkp bdiff and pkp cdiff phases we evaluate the average p wave velocity perturbations of the earth s uppermost inner core ic by analyzing two pairs of core phases pkikp pkp bdiff 456 observations in the distance range of 138 5 142 and pkikp pkp cdiff 1215 observations at 155 162 the former is most sensitive to inner core velocities in the depth range of 350 590 km and the latter from 0 to 100 km below the inner core boundary the differential travel time residuals exhibit the roughly east west hemispheric pattern especially beneath the pacific ocean here the separation between the two quasi hemispheres is near the 150 w meridian from pkp bdiff observations but possibly rotated toward a nw se orientation from pkp cdiff alternately a velocity heterogeneity could be present here in agreement with the previously reported pkikp vs pkpbc observations assuming the conventional definition for the ic quasi hemispheres i e the qeh between 0 and 180 e and the qwh between 0 and 180 w the quasi eastern hemisphere exhibits perturbations of 1 17 0 04 from pkikp pkp bdiff and 0 19 0 01 from pkikp pkp cdiff the corresponding perturbations in the quasi western hemisphere are of 0 06 0 06 and 0 19 0 02 respectively an anomalous ic zone beneath the indian ocean is not conspicuous in our pkikp pkp cdiff observations beneath the north atlantic the separation between the ic quasi hemispheres appears to be located between 60 w and 30 w from the pkp bdiff observations but it remains unresolved from the pkp cdiff data due to scarce observations here

theoretical calving rates from glaciers along ice walls grounded in water of variable depths calving has been studied for glaciers ranging from slow polar glaciers that calve on dry land such as on deception island 63 0 s 60 6 w in antarctica through temperate alaskan tide water glaciers to fast outlet glaciers that float in fiords and calve in deep water such as jakobshavns isbrae 69 2 n 49 9 w in greenland calving from grounded ice walls and floating ice shelves is the main ablation mechanism for the antarctic and greenland ice sheets as it was along marine and lacustrine margins of former pleistocene ice sheets and is for tide water and polar glaciers

tropical sea surface temperatures and the earth s orbital eccentricity cycles the tropical oceanic warm pools are climatologically important regions because their sea surface temperatures ssts are positively related to atmospheric greenhouse effect and the cumulonimbus cirrus cloud anvil such a warm pool is also present in the central tropical indian ocean paleo ssts estimated by the radiolarian transfer functions for the last 200 1400 kiloyears ka in a deep sea sediment core from this warm pool fluctuated within 2 75 c 26 14 28 89 c around the threshold temperature 27 c for the super greenhouse effect it is possible that this sst variation might have led to the corresponding fluctuation in the greenhouse effect and the cirrus cloud anvil in the geological past these paleo ssts also exhibited cyclicities at 100 and 400 ka corresponding to the earth s orbital eccentricity cycles results therefore imply that the tropical indian ocean warm pool persisted during the quaternary and the paleo ssts fluctuating at the orbital eccentricity frequencies might have modulated the intensity of greenhouse effect

new sites of australasian microtektites in the central indian ocean implications for the location and size of source crater 1 we have identified 15 new australasian microtektite sites along a transect roughly n s in the central indian ocean these locations in addition to the existing 46 sites total to 61 microtektite sites in the oceans we carried out regression analysis of a selected area between 12 26 n and 98 112 e at 0 5 intervals using three microtektite data sets separately 41 sites 46 sites number of sites known up to the years 1994 and 2006 respectively and the entire data set of 61 sites this area was chosen because on the basis of various lines of evidence several investigators suggested impact craters for the strewn field within this broad area the 41 site data set defines an impact site at about 12 n and 106 e whereas the contours joining the highest values of the square of the correlation coefficient by using 46 site and 61 site data sets define an area located in ne thailand central laos the center of this site is at 18 n and 104 e therefore with progressive increase in microtektite data the location of the impact site seems to get defined more rigorously on the basis of the equations for the concentration of the ejecta and the distance from the source region the calculated crater size has a diameter range between 33 and 120 km interestingly the lower size limit of 33 km has been found closer to ground truth by verification from the microtektite data of the ivory coast strewn field

ground based interferometric radar for velocity and calving rate measurements of the tidewater glacier at kronebreen svalbard this paper presents the first attempt to measure the calving rate and flow velocity of the front of a glacier using ground based high range resolution interferometric radar at a high temporal rate 2 hz the radar operating at 5 75 ghz is located 4 km from the calving front of kronebreen svalbard the radar measures a horizontal width of 700 m of the front and a range of 300 m the latter includes the calving front and 250 m up glacier we find that the glacier surface provides permanent scatterers so spatially continuous movements at the front and at locations further up glacier can be tracked for 8 hours we measured a large increase in velocity at the calving front which extended 30 m up glacier while further up glacier the velocity was stable 2 5 m d 1 during the entire period two large calving events were identified during this survey removing 25 m of ice in the radar range direction normal to the ice front the velocity of the front slowed to a stable 2 5 m d 1 after the events during another data series the velocities were stable 2 5 m d 1 and no calving events were identified the method is promising for carrying out studies of processes at a calving front as it provides spatially continuous high frequency accurate velocities from a safe distance to the glacier

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

the jakobshanvs effect the jakobshavns effect may have been a significant factor in hastening the collapse of palaeo ice sheets with the advent of climatic warming after 18 000 years ago and may precipitate partial collapse of the present day greenland and antarctic ice sheets following co2 induced climatic warming in the decades ahead the jakobshavns effect is observed today on jakobshavns glacier which is located at 69 10 n on the west coast of greenland the jakobshavns effect is a group of positive feedback mechanisms which allow jakobshavns glacier to literally pull ice out of the greenland ice sheet at a rate exceeding 7 km a across a floating terminus 800 m thick and 6 km wide the pulling power results from an imbalance of horizontal hydrostatic forces in ice and water columns at the grounding line of the floating terminus positive feedback mechanisms that sustain the rapid ice discharge rate are ubiquitous surface crevassing high summer rates of surface melting extending creep flow progressive basal uncoupling progressive lateral uncoupling and rapid iceberg calving

hemispherical structure in inner core velocity anisotropy 1 we investigate the hemispherical pattern in inner core velocity anisotropy using a new and independent high quality data set of pkpbc pkpdf and pkpab pkpdf body wave observations our data show no evidence for a tilted anisotropy axis with respect to the earth s rotation axis and is significantly better fit by a model with 4 4 anisotropy in the western hemisphere and only 1 0 in the eastern hemisphere than by a model of uniform anisotropy we carry out variance minimization and find the boundaries between the hemispheres at lines of constant longitude at 14 e and 151 w variance minimization enables us to extract the imprint of hemispherical structure from all of the data and not just polar paths resulting in boundaries which are nearly 30 from the results of previous studies the high quality of the data allows us to provide robust evidence that the isotropic velocity in the eastern hemisphere is 0 2 lower than in the western hemisphere in the top 660 km of the inner core our data set also suggests an increase in inner core anisotropy in the eastern hemisphere from 1 to around 6 at depths deeper than 660 km indicating that the hemispherical pattern in anisotropy may disappear at greater depths the presence of hemispherical structure rules out mechanisms for creating anisotropy which are unable to sustain longitudinal variations in the inner core furthermore steady inner core superrotation of the order of 0 1 year would eradicate the hemispherical differences though inner core oscillation would still be permissible

intricate heterogeneous structures of the top 300 km of the earth s inner core inferred from global array data i regional 1d attenuation and velocity profiles abstract we apply a waveform inversion method based on simulated annealing to complex core phase data observed by globally deployed seismic arrays and present regional variation of depth profiles of attenuation and velocity for the top half of the inner core whereas measured attenuation parameters exhibit consistent trends for data sampling the eastern hemisphere of the inner core for the western hemisphere there is a remarkable difference between data sampling the inner core beneath africa w1 and beneath north america w2 obtained attenuation profiles suggest that intricate heterogeneities appear to be confined in the top 300 km the profile for the eastern hemisphere has a high attenuation zone in the top 150 km that gradually diminishes with depth conversely for the western hemisphere the profile for w1 shows constant low attenuation and that for w2 represents a gradual increase from the inner core boundary to a peak at around 200 km depth velocity profiles obtained from differential traveltimes between pkp df and pkp cd bc phases for the eastern and western hemispheres are respectively about 0 8 faster and 0 6 slower than the reference model at the top of the inner core and the difference nearly disappears at about 200 km depth our result suggests the presence of intricate quasi hemispherical structures in the top 200 300 km of the inner core

mapping tide water glacier dynamics in east greenland using landsat data landsat multispectral scanner and thematic mapper images were co registered for the kangerdlugssuaq fjord region in east greenland and were used to map glacier drainage basin areas changes in the positions of tide water glacier termini and to estimate surface velocities of the larger tide water glaciers statistics were compiled to document distance and area changes to glacier termini the methodologies developed in this study are broadly applicable to the investigation of tide water glaciers in other areas the number of images available for consecutive years and the accuracy with which images are co registered are key factors that influence the degree to which regional glacier dynamics can be characterized using remotely sensed data three domains of glacier state were interpreted net increase in terminus area in the southern part of the study area net loss of terminus area for glaciers in upper kangerdlugssuaq fjord and a slight loss of glacier terminus area northward from ryberg fjord local increases in the concentrations of drifting icebergs in the fjords coincide with the observed extension of glacier termini positions ice surface velocity estimates were derived for several glaciers using automated image cross correlation techniques the velocity determined for kangerdlugssuaq gletscher is approximately 5 0 km a 1 and that for kong christian iv gletscher is 0 9 km a 1 the continuous presence of icebergs and brash ice in front of these glaciers indicates sustained rates of ice front calving

large fluctuations in speed on greenland s jakobshavn isbrae glacier it is important to understand recent changes in the velocity of greenland glaciers because the mass balance of the greenland ice sheet is partly determined by the flow rates of these outlets jakobshavn isbrae is greenland s largest outlet glacier draining about 6 5 per cent of the ice sheet area and it has been surveyed repeatedly since 1991 ref 2 here we use remote sensing data to measure the velocity of jakobshavn isbrae between 1992 and 2003 we detect large variability of the velocity over time including a slowing down from 6 700 m yr 1 in 1985 to 5 700 m yr 1 in 1992 and a subsequent speeding up to 9 400 m yr 1 by 2000 and 12 600 m yr 1 in 2003 these changes are consistent with earlier evidence for thickening of the glacier in the early 1990s and rapid thinning thereafter our observations indicate that fast flowing glaciers can significantly alter ice discharge at sub decadal timescales with at least a potential to respond rapidly to a changing climate

a simple holistic hypothesis for the self destruction of ice sheets abstract ice sheets are the only components of earth s climate system that can self destruct this paper presents the quantitative force balance for bottom up modeling of ice sheets as first presented qualitatively in this journal as a way to quantify ice bed uncoupling leading to self destruction of ice sheets hughes 2009a rapid changes in sea level and climate can result if a large ice sheet self destructs quickly as did the former laurentide ice sheet of north america between 8100 and 7900 bp thereby terminating the last cycle of quaternary glaciation ice streams discharge up to 90 percent of ice from past and present ice sheets a hypothesis is presented in which self destruction of an ice sheet begins when ubiquitous ice bed decoupling quantified as a floating fraction of ice proceeds along ice streams this causes ice streams to surge and reduce thickness by some 90 percent and height above sea level by up to 99 percent for floating ice so the ice sheet undergoes gravitational collapse ice collapsing over marine embayments becomes floating ice shelves that may then disintegrate rapidly this floods the world ocean with icebergs that reduce the ocean to atmosphere heat exchange thereby triggering climate change calving bays migrate up low stagnating ice streams and carve out the accumulation zone of the collapsed ice sheet which prevents its recovery decreases earth s albedo and terminates the glaciation cycle this sequence of events may coincide with a proposed life cycle of ice streams that drain the ice sheet a first order treatment of these life cycles is presented that depends on the longitudinal force balance along the flowbands of ice streams and gives a first approximation to ice bed uncoupling at snapshots during gravitational collapse into ice shelves that disintegrate thereby removing the ice sheet the stability of the antarctic ice sheet is assessed using this bottom up approach

relationship of d structure with the velocity variations near the inner core boundary 1 variations in regional differential times between pkikp i and pkikp i have been attributed to hemispheric p velocity variations of about 1 in the upper 100 km of the inner core referred to as hic the top of the inner core appears relatively fast beneath asia where d is also fast an alternative interpretation could be the lateral variation in p velocity at the lowermost outer core hoc producing the same differential times to resolve this issue we introduce the diffracted pkp phase near the b caustic bdiff in the range of 139 145 epicenter distances and the corresponding differential times between bdiff and pkikp and pkikp as observed on broadband arrays due to the long wavelength nature of bdiff we scaled the s wave tomography model with k values k dlnvs dlnvp to obtain large scale p wave velocity structure in the lower mantle as proposed by earlier studies waveform synthetics of bdiff constructed with small k s predict complex waveforms not commonly observed confirming the validity of large scaling factor k with p velocity in lower mantle constrained at large scale the extra travel time constraint imposed by bdiff helps to resolve the hoc hic issue our preliminary results suggest k 2 for the lowermost mantle and support hic hypothesis an important implication is that there appears to be a relationship of d velocity structures with the structures near the inner core boundary via core dynamics

fast tidewater glaciers some iceberg calving outlet glaciers flow continuously at speeds normally associated with surging glaciers arid exhibit dramatic instability scenarios related to those suggested for marine ice sheets no temperate tidewater glaciers are known to have floating termini but many polar and subpolar tidewater glaciers do the fast flow of temperate calving glaciers is almost entirely due to basal sliding and appears to be a function of the effective pressure on the bed which may approach zero and the longitudinal back stress on the terminus the terminus boundary condition the calving relation is imperfectly known yet is vital to the dynamics of these glaciers calving relations for grounded tidewater glaciers have been suggested on empirical grounds but have not been rigorously tested the calving relations for floating termini are virtually unknown this together with the imperfect understanding of basal sliding inhibits confidence in our understanding of the stability of these glaciers columbia glacier alaska is an instructive example because observations have been made on the major changes in its geometry calving rate and dynamics that have occurred in less than 10 years the calving flux has increased more rapidly than the glacier flux causing thinning and retreat as a result the ice velocity has increased markedly the short term velocity changes relate to changes in back pressure ice recession tidal changes and the flux of water injected to the bed these results have relevance to the mechanisms of basal sliding glacier surges and the stability of marine ice streams

seismic structures in the earth s inner core below southeastern asia documenting seismic heterogeneities in the earth s inner core ic is important in terms of getting an insight into its history and dynamics a valuable means for studying properties and spatial structure of such heterogeneities is provided by measurements of body waves refracted in the vicinity of the inner core boundary icb here we investigate eastern hemisphere of the solid core by means of pkpbc pkpdf differential travel times that sample depths from 140 to 360 km below its boundary we study 292 polar and 133 equatorial residuals measured over the traces that probe roughly the same volume of the ic in both planes equatorial residuals show slight spatial variations in the sampled ic volume mostly below the level of 0 5 whereas polar residuals are up to three times as big direction dependent and can exhibit higher local variations the measurements reveal fast changes in seismic velocity within a restricted volume of the ic we interpret the observations in terms of anisotropy and check against several anisotropy models few of which have been found capable of fitting the residuals scatter we particularly quantify the model where a dipping discontinuity separates fully isotropic roof of the ic from its anisotropic body whereas the depth of isotropy anisotropy transition increases in southeast direction from 190 km below southeastern asia off the coast of china to 350 km beneath australia another acceptable model cast in terms of localized anisotropic heterogeneities is valid if 33 largest polar measurements over the rays sampling a small volume below southeastern asia and the rest of polar data are treated separately this model envisages almost isotropic eastern hemisphere of the ic at least down to the depth of 360 km below the icb and constrains the anisotropic volume only to the ranges of north latitudes from 18 to 23 east longitudes from 125 to 135 and depths exceeding 170 km the anisotropy strength in either model is about 2 further effective pursuit of the models presents challenges in terms of resolution and coverage and basically requires a significant dataset extension

hemispherical variations in seismic velocity at the top of the earth s inner core knowledge of the seismic velocity structure at the top of the earth s inner core is important for deciphering the physical processes responsible for inner core growth1 2 3 previous global seismic studies4 5 6 7 8 9 have focused on structures found 100 km or deeper within the inner core with results for the uppermost 100 km available for only isolated regions10 11 12 here we present constraints on seismic velocity variations just beneath the inner core boundary determined from the difference in travel time between waves reflected at the inner core boundary and those transmitted through the inner core we found that these travel time residuals observed on both global seismograph stations and several regional seismic networks are systematically larger by about 0 8 s for waves that sample the eastern hemisphere of the inner core 40 e to 180 e compared to those that sample the western hemisphere 180 w to 40 e these residuals show no correlation with the angle at which the waves traverse the inner core this indicates that seismic anisotropy is not strong in this region and that the isotropic seismic velocity of the eastern hemisphere is about 0 8 higher than that of the western hemisphere

intermittent thinning of jakobshavn isbrae west greenland since the little ice age rapid thinning and velocity increase on major greenland outlet glaciers during the last two decades may indicate that these glaciers became unstable as a consequence of the jakobshavn effect hughes 1986 with terminus retreat leading to increased discharge from the interior and consequent further thinning and retreat to assess whether recent trends deviate from longer term behavior we measured glacier surface elevations and terminus positions for jakobshavn isbrae west greenland using historical photographs acquired in 1944 1953 1959 1964 and 1985 these results were combined with data from historical records aerial photographs ground surveys airborne laser altimetry and field mapping of lateral moraines and trimlines to reconstruct the history of changes since the little ice age lia we identified three periods of rapid thinning since the lia 1902 13 1930 59 and 1999 present during the first half of the 20th century the calving front appears to have been grounded and it started to float during the late 1940s the south and north tributaries exhibit different behavior for example the north tributary was thinning between 1959 and 1985 during a period when the calving front was stationary and the south tributary was in balance the record of intermittent thinning combined with changes in ice marginal extent and position of the calving front together with changes in velocity imply that the behavior of the lower parts of this glacier represents a complex ice dynamical response to local climate forcings and interactions with drainage from the interior

instability of the antarctic ross sea embayment as climate warms collapse of the antarctic ice sheet since the last glacial maximum 18 000 years ago is most pronounced in the ross sea embayment which is partly ice free during antarctic summers thereby breaching the o ring of ice shelves and sea ice surrounding antarctica that stabilizes the ice sheet the o ring may have vanished during early holocene 5000 to 3000 b c roman 1 to 400 a d and medieval 900 to 1300 a d warm periods and reappeared during the little ice age 1300 to 1900 a d we postulate further collapse in the embayment during the post 1900 warming may be forestalled because east antarctic outlet glaciers nail the ross ice shelf to the transantarctic mountains so it can resist the push from west antarctic ice streams our hypothesis is examined for byrd glacier and a static ice shelf using three modeling experiments having plastic viscous and viscoplastic solutions as more data and improved modeling became available observed crevasse patterns were not reproduced a new research study is needed to model a dynamic ross ice shelf with all its feeder ice streams outlet glaciers and ice calving dynamics in three dimensions over time to fully test our hypothesis the required model must allow accelerated calving if further warming melts sea ice and discerps the ice shelf calving must then successively pull the outlet glacier nails so collapse of the marine west antarctic ice sheet proceeds to completion

glacier fjord and seismic response to recent large calving events jakobshavn isbrae greenland 1 the recent loss of jakobshavn isbrae s extensive floating ice tongue has been accompanied by a change in near terminus behavior calving currently occurs primarily in summer from a grounded terminus involves the detachment and overturning of several icebergs within 30 60 min and produces long lasting and far reaching ocean waves and seismic signals including glacial earthquakes calving also increases near terminus glacier velocities by 3 but does not cause episodic rapid glacier slip thereby contradicting the originally proposed glacial earthquake mechanism we propose that the earthquakes are instead caused by icebergs scraping the fjord bottom during calving

abrupt climatic change related to unstable ice sheet dynamics toward a new paradigm ice streams are fast currents of ice that drain some 90 of present day ice sheets and probably drained a similar percent of ice from former ice sheets ice streams may have life cycles akin to but much longer than surge cycles of certain mountain glaciers the floating tongues of marine ice streams coalesce to form floating ice shelves that buttress marine ice sheets and the grounded lobes of terrestrial ice streams stabilize terrestrial ice sheets ice shelves and ice lobes form during the life cycles of ice streams and disintegrate rapidly to initiate new life cycles successive life cycles continue until the ice sheet collapses a new paradigm is postulated in which abrupt climatic change is driven by iceberg outbursts associated with life cycles of marine ice streams and dust storms associated with life cycles of terrestrial ice streams for pleistocene northern hemisphere ice sheets these mechanisms regulated north atlantic surface temperatures and salinity and perhaps planetary albedo and atmospheric co2 and thereby drove climatic change which tended to be abrupt because ice stream life cycles began and ended abruptly ice streams proliferated along the margins of northern hemisphere ice sheets after they advanced beyond the crystalline shields of north america and eurasia and spread over softer sedimentary rocks overlain by thick ice cemented sediments that progressively thawed especially along river valleys and marine troughs an increasing fraction of these ice sheets was then subjected to the unstable life cycles of ice streams as seen in the pronounced spikiness of the late glacial oxygen isotope stratigraphy in the greenland ice core at dye 3 life cycles of greenland ice streams during the holocene seem to be linked to episodes of regional climatic cooling most recently the little ice age when superimposed on the present milankovitch hemicycle of reduced insolation over the north atlantic this could abruptly initiate a new worldwide glaciation cycle in the aftermath of present day co2 greenhouse warming proliferation of antarctic ice stream life cycles during the period of greenhouse warming may also trigger global climatic change by forcing a reorganization of circumpolar thermohaline circulation in the southern ocean

heterogeneity and anisotropy of earth s inner core seismic observations provide strong evidence that earth s inner core is anisotropic with larger velocity in the polar than in the equatorial direction the top 60 80 km of the inner core is isotropic evidence for an innermost inner core is less compelling the anisotropy is most likely due to alignment of hcp hexagonal close packed iron crystals aligned either during solidification or by deformation afterward the existence of hemispherical variations used to be controversial but there is now strong evidence from both seismic body wave and normal mode observations showing stronger anisotropy less attenuation and a lower isotropic velocity in the western hemisphere two mechanisms have been proposed to explain the hemispherical pattern either a inner core translation wherein one hemisphere is melting and the other is solidifying or b thermochemical convection in the outer core leading to different solidification conditions at the inner core boundary neither is yet able to explain all se