effects of a perovskite post perovskite phase change near core mantle boundary in compressible mantle convection 1 numerical simulations of compressible isochemical mantle convection are used to investigate the effect of the perovskite to post perovskite phase transition at around 2700 km depth which has recently been discovered by high pressure experiments and ab initio calculations on the convective planform temperature and heat transport characteristics of the mantle the usual phase transitions at 410 km olivine spinel and 660 km spinel perovskite are also included the exothermic post perovskite phase change at 2700 km depth destabilizes the lower thermal boundary layer increasing the heat flow increasing interior mantle temperature and increasing the number and time dependence of upwelling plumes the resulting weak highly time dependent upwellings also have a smaller horizontal spacing than the plumes that occur in the absence of the phase transition while the influence of post perovskite phase change may be smaller than that of some other complexities such as compositional stratification it appears to have an important enough effect that it should not be ignored in dynamical studies of mantle convection

outlet glacier response to forcing over hourly to interannual timescales jakobshavn isbrae greenland the loss of the floating ice tongue on jakobshavn isbrae greenland in the early 2000s has been concurrent with a pattern of thinning retreat and acceleration leading to enhanced contribution to global sea level these changes on decadal timescales have been well documented here we identify how the glacier responds to forcings on shorter timescales such as from variations in surface melt the drainage of supraglacial lakes and seasonal fluctuations in terminus position ice motion and surface melt were monitored intermittently from 2006 to 2008 dual frequency gps were deployed 20 50 km upstream of the terminus along the glacier center line gaps in surface melt measurements were filled using a temperature index model of ablation driven by surface air temperatures recorded during the same time period our results corroborate the premise that the primary factors controlling speeds on jakobshavn isbrae are terminus position and geometry we also observe that surface speeds demonstrate a complex relationship with meltwater input on diurnal timescales velocities closely match changes in water input however on seasonal timescales a longer more intense melt season was observed to effectively reduce the overall ice flow of the glacier for the whole year

d shear velocity heterogeneity anisotropy and discontinuity structure beneath the caribbean and central america the d region in the lowermost mantle beneath the caribbean and central america is investigated using shear waves from south american earthquakes recorded by seismic stations in north america we present a large scale composite study of volumetric shear velocity heterogeneity anisotropy and the possible presence of a d discontinuity in the region our data set includes 328 s sdiff sks differential travel times 300 scs s differential travel times 125 s sdiff and 120 scs shear wave splitting measurements and 297 seismograms inspected for scd the seismic phase refracted from a high velocity d layer broadband digital data are augmented by high quality digitized analog wwssn data providing extensive path coverage in our study area in all data from 61 events are utilized in some cases a given seismogram can be used for velocity heterogeneity anisotropy and discontinuity analyses significant mid mantle structure possibly associated with the ancient subducted farallon slab affects shear wave travel times and must be corrected for to prevent erroneous mapping of

tide modulated ice flow variations drive seismicity near the calving front of bowdoin glacier greenland glacier microseismicity is a promising tool to study glacier dynamics however physical processes connecting seismic signals and ice dynamics are not clearly understood at present particularly the relationship between tide modulated seismicity and dynamics of calving glaciers remains elusive here we analyze records from an on ice seismometer placed 250 m from the calving front of bowdoin glacier greenland using high frequency glacier flow speed measurements we show that the microseismic activity is related to strain rate variations the seismic activity correlates with longitudinal stretching measured at the glacier surface both higher melt rates and falling tides accelerate glacier motion and increase longitudinal stretching long term microseismic monitoring could therefore provide insights on how a calving glacier s force balance and flow regime react to changes at the ice ocean interface

is the long wavelength geoid sensitive to the presence of postperovskite above the core mantle boundary 1 the analysis of seismic data represents today the primary tool in the search for the presence of postperovskite in the lowermost mantle d 00 this work aims at testing whether the inversion of gravitational data can also contribute to the detection of postperovskite in d 00 w e assume that the transition from perovskite to postperovskite is accompanied by a reduction in viscosity and test the effects of such viscosity change on the prediction of the dynamic geoid with a numerical model of subducted lithosphere our results show that the long wavelength component of the geoid is very sensitive to the presence of postperovskite areas in d 00 especially if their viscosity is significantly lower than the viscosity of the surrounding perovskite and if these areas are located close to density

sudden increase in tidal response linked to calving and acceleration at a large greenland outlet glacier 1 large calving events at greenland s largest outlet glaciers are associated with glacial earthquakes and near instantaneous increases in glacier flow speed at some glaciers and ice streams flow is also modulated in a regular way by ocean tidal forcing at the terminus at helheim glacier analysis of geodetic data shows decimeter level periodic position variations in response to tidal forcing however we also observe transient increases of more than 100 in the glacier s responsiveness to such tidal forcing following glacial earthquake calving events the timing and amplitude of the changes correlate strongly with the step like increases in glacier speed and longitudinal strain rate associated with glacial earthquakes the enhanced response to the ocean tides may be explained by a temporary disruption of the subglacial drainage system and a concomitant reduction of the friction at the ice bedrock interface and suggests a new means by which geodetic data may be used to infer glacier properties citation de juan j et al 2010 sudden increase in tidal response linked to calving and acceleration at a large greenland outlet glacier geophys res lett 37 l12501

ice streams as the arteries of an ice sheet their mechanics stability and significance ice streams are corridors of fast ice flow ca 0 8 km year within an ice sheet and are responsible for discharging the majority of the ice and sediment within them consequently like the arteries in our body their behaviour and stability is essential to the well being of an ice sheet ice streams may either be constrained by topography topographic ice streams or by areas of slow moving ice pure ice streams the latter show spatial and temporal patterns of variability that may indicate a potential for instability and are therefore of particular interest today pure ice streams are largely restricted to the siple coast of antarctica and these ice streams have been extensively investigated over the last 20 years this paper provides an introduction to this substantial body of research and describes the morphology dynamics and temporal behaviour of these contemporary ice streams before exploring the basal conditions that exist beneath them and the mechanisms that drive the fast flow within them the paper concludes by reviewing the potential of ice streams as unstable elements within ice sheets that may impact on the earth s dynamic system d 2002 elsevier science b v all rights reserved

origin of seismic anisotropy in the d layer inferred from shear deformation experiments on post perovskite phase abstract seismic anisotropy is one of the significant features in the d layer of the earth and is thought to be derived from the lattice preferred orientation lpo of constituent materials or shape preferred orientation spo of heterogeneous materials such as melt and inclusions recent experimental and theoretical studies strongly suggest that the d layer consists mainly of a mgsio3 post perovskite phase together with ferro periclase to understand the anisotropy in the d layer we have conducted a series of simple shear deformation experiments at high temperature and pressure on polycrystalline cairo3 as an analogue of mgsio3 and measured the lpo of the post perovskite phase crystallographic orientation analysis of the deformed post perovskite phase showed strong lpos with the dominant slip system being 100 010 calculation of the elastic wave velocities considering the effect of lpos of post perovskite and ferro periclase showed as azimuthal and polarization anisotropies in the horizontal shear plane where the velocity of horizontally polarized s wave is considerably faster than that of vertically polarized s wave thus the seismic anisotropy observed in the d layer can be reasonably explained by the lpo of the mixture of post perovskite and ferro periclase where the lpo may result from the horizontal shear flow generated by the mantle convection

the stability and structure of primordial reservoirs in the lower mantle insights from models of thermochemical convection in three dimensional spherical geometry large scale chemical lateral heterogeneities are inferred in the earth s lowermost mantle by seismological studies we explore the model space of thermochemical convection that can maintain reservoirs of dense material for a long period of time by using similar analysis in 3 d spherical geometry in this study we focus on the parameters thought to be important in controlling the stability and structure of primordial dense reservoirs in the lower mantle including the chemical density contrast between the primordial dense material and the regular mantle material buoyancy ratio thermal and chemical viscosity contrasts volume fraction of primordial dense material and the clapeyron slope of the phase transition at 660 km depth we find that most of the findings from the 3 d cartesian study still apply to 3 d spherical cases after slight modifications varying buoyancy ratio leads to different flow patterns from rapid upwelling to stable layering and large thermal viscosity contrasts are required to generate long wavelength chemical structures in the lower mantle chemical viscosity contrasts in a reasonable range have a second order role in modifying the stability of the dense anomalies the volume fraction of the initial primordial dense material does not effect the results with large thermal viscosity contrasts but has significant effects on calculations with intermediate and small thermal viscosity contrasts the volume fraction of dense material at which the flow pattern changes from unstable to stable depends on buoyancy ratio and thermal viscosity contrast an endothermic phase transition at 660 km depth acts as a filter allowing cold slabs to penetrate while blocking most of the dense material from penetrating to the upper mantle

simultaneous volume measurements of post perovskite and perovskite in mgsio3 and their thermal equations of state abstract simultaneous volume measurements of mgsio 3 post perovskite ppv and perovskite pv were performed in a diamond anvil cell dac combined with synchrotron x rays an externally heated dac was used in addition to a laser heated dac for the volume measurement experiment at high temperatures the volume data were collected in the stability field of post perovskite from 115 to 130 gpa the temperature generated in the externally heated and the laser heated dacs for the volume measurement were up to 832 and 2330 k respectively using two different but complementary heating techniques we collected the data at a wide temperature range from 300 to 2330 k the obtained p v t data for ppv and pv were fitted to a third ordered birch murnaghan equation of state eos for a precise comparison of the volume between the two phases the eoss were constructed based on the same pressure scale of mgo the simultaneous volume measurements and the volumes calculated from the determined eoss demonstrate that the volume difference between ppv and pv of about 1 5 is almost constant with increasing temperature to 4000 k at the transition at the base of the mantle this density difference corresponds to a temperature anomaly of 1300 k without the phase transition due to the very small thermal expansivity of minerals which has a significant effect on mantle dynamics the thermal expansivity contrast between the top and the bottom of the mantle is a factor of 3 6 from a mantle convection study this value suggests that huge and hot plumes are formed at the core mantle boundary

tidal forcing of basal seismicity of ice stream c west antarctica observed far inland the seismicity rate beneath the downglacier 85 km of ice stream c west antarctica is modulated by the tide the tide beneath the ross ice shelf modifies the force balance of the ice stream basal environment enough to change the rate of basal microearthquake generation by an order of magnitude this tidal forcing travels up the ice stream as an attenuating wave at approximately 1 6m s 1 and is detectable 85km from the grounding line we successfully model this behavior as an elastic ice stream underlain by a viscous substrate of viscosity and thickness hb and calculate that the substrate has an apparent stiffness hb of o 108 pa s m 1 this finding suggests that the conditions of the till layer at the bed of ice stream c are similar to those of ice stream b and that the reason for the recent stagnation of ice stream c is other than loss of till we further find that the ice stream at the grounding line is more strongly affected by ice shelf processes than by the basal shear stress

a doubling of the post perovskite phase boundary and structure of the earth s lowermost mantle a new model of the deep earth allows geophysicists to measure variations in earth s temperature near the boundary between the rocky mantle and core using a recently discovered phase change in the lowest part of the earth s mantle where atoms rearrange themselves in crystals under extreme pressure and temperature the model also explains odd features of this part of the mantle such as patchy regions that reflect seismic waves generated by large earthquakes and inferences of dense partial melting combined with observations the results imply very large variations in temperature providing further evidence for circulation of material throughout the earth s mantle the thermal structure of the earth s lowermost mantle the d layer spanning depths of 2 600 2 900 kilometres1 is key to understanding the dynamical state and history of our planet earth s temperature profile the geotherm is mostly constrained by phase transitions such as freezing at the inner core boundary or changes in crystal structure within the solid mantle that are detected as discontinuities in seismic wave speed and for which the pressure and temperature conditions can be constrained by experiment and theory a recently discovered phase transition at pressures of the d layer2 3 4 is ideally situated to reveal the thermal structure of the lowermost mantle where no phase transitions were previously known to exist here we show that a pair of seismic discontinuities observed in some regions of d can be explained by the same phase transition as the result of a double crossing of the phase boundary by the geotherm at two different depths this simple model can also explain why a seismic discontinuity is not observed in some other regions and provides new constraints for the magnitude of temperature variations within d

thermoelastic properties of post perovskite phase mgsio3 determined experimentally at core mantle boundary p t conditions we have performed in situ x ray diffraction measurements on mgsio3 up to 144 5 gpa and 2535 k to obtain a precise experimental determination of the elastic properties of the post perovskite polymorph of mgsio3 at high pressure and temperature the complete dataset is comprised of 24 data points at ambient temperature and an equal number at high temperature fitted physical properties are the room temperature bulk modulus the gruneisen parameter with its volume dependency the thermal expansion with its temperature dependency and the linear incompressibilities as well as thermal expansions and their temperature dependency on each axis elastic anisotropy is observed the structure being much more compressible along the b axis and less compressible along the c axis in agreement with theoretical calculations we also demonstrate with the present study that experiments can provide precise information on the elasticity of complex materials at extreme pressure and temperature conditions

mechanisms of fast flow in jakobshavn isbrae west greenland part iii measurements of ice deformation temperature and cross borehole conductivity in boreholes to the bedrock at a site on the ice sheet adjacent to the jakobshavn ice stream in west greenland ice deformation rates and temperatures have been measured in boreholes to the bedrock at 830 m depth enhanced deformation rates were recorded just below the holocene wisconsin transition at 680 m depth a 31 m layer of temperate ice and the temperature minimum of 22 c at 520 m depth were detected the good agreement of these data with results of a two dimensional thermomechanically coupled flow model implies that the model input is adequate discrepancies between modelled and measured temperature profiles on a flowline at the ice stream centre have been attributed to effects not accounted for by the model we have suggested that the convergent three dimensional flow leads to a vertical extension of the basal ice entering the stream a thick basal layer of temperate and wisconsin ice would explain the fast flow of this ice stream as a test of this hypothesis the new core borehole conductivity cbc method has been used to compare conductivity sequences from the ice stream to those of the adjacent ice sheet the correlation thus inferred suggests that the lowest 270 m of the ice sheet correspond to the lowermost 1700 m of the stream and consequently that the lower part of the ice stream has experienced a very large vertical extension

tidally controlled stick slip discharge of a west antarctic ice a major west antarctic ice stream discharges by sudden and brief periods of very rapid motion paced by oceanic tidal oscillations of about 1 meter acceleration to speeds greater than 1 meter per hour and deceleration back to a stationary state occur in minutes or less slip propagates at approximately 88 meters per second suggestive of a shear wave traveling within the subglacial till a model of an episodically slipping friction locked fault reproduces the observed quasi periodic event timing demonstrating an ice stream s ability to change speed rapidly and its extreme sensitivity to subglacial conditions and variations in sea level

ice stream d flow speed is strongly modulated by the tide beneath the ross ice shelf the flow velocity of ice stream d west antarctica has been measured to vary by a factor of three over the course of a day these fluctuations are measured at the grounding line as well as upstream of the grounding line in the ice plain of ice stream d the diurnal velocity fluctations appear to be driven by the tide beneath the ross ice shelf these results suggest that there is significant and heretofore poorly understood influence of the ocean tide and of the ice shelf on the dynamics of ice stream flow

21st century evolution of greenland outlet glacier velocities earlier observations on several of greenland s outlet glaciers starting near the turn of the 21st century indicated rapid annual scale and large 100 increases in glacier velocity combining data from several satellites we produce a decade long 2000 to 2010 record documenting the ongoing velocity evolution of nearly all 200 of greenland s major outlet glaciers revealing complex spatial and temporal patterns changes on fast flow marine terminating glaciers contrast with steady velocities on ice shelf terminating glaciers and slow speeds on land terminating glaciers regionally glaciers in the northwest accelerated steadily with more variability in the southeast and relatively steady flow elsewhere intraregional variability shows a complex response to regional and local forcing observed acceleration indicates that sea level rise from greenland may fall well below proposed upper bounds

effects of low viscosity post perovskite on thermo chemical mantle convection in a 3 d spherical shell 1 numerical simulations of thermo chemical multi phase mantle convection in a 3 d spherical shell are performed to determine how a low viscosity of post perovskite affects dynamics and structures in the deep mantle low viscosity post perovskite weakens the deepest part of slabs allowing them to more effectively spread over the core mantle boundary cmb and it also results in a greater volume of basalt segregating both of which increase the size of dense chemical piles the horizontal lengthscale of regions of pooled slab material and the steepness of piles edges in composition and phase consistent with the existence of steep sharp sided edges found in seismic analyses cmb heat flux is strongly enhanced in regions of low viscosity post perovskite consistent with a theoretical prediction and both cmb and surface heat flux are increased on average by a low viscosity of post perovskite which could have important implications for the evolution of earth s core and mantle

short term flow dynamics of a retreating tidewater glacier leconte glacier alaska u s a an analysis of motion in the terminus region of leconte glacier alaska u s a delineates mechanisms that are important to tidewater glacier stability this glacier is undergoing rapid retreat since 1995 it has retreated 2 km and thinned 125 m at the present location of the terminus ice velocities in this region are quite high at the terminus they exceed 27 m d 1 our analysis reveals that fluctuations in speed and surface elevation are forced by ocean tides surface melt and precipitation the near terminus ice speed is 180 out of phase with the tide such that high tide corresponds to low speed smaller melt forced diurnal variations in speed are present throughout the lowermost 7 km speed ups in excess of 10 of the mean often occur after rainstorms as a result of increased basal water pressure and storage but the relation is not simple the velocity does not vary over the spring and summer seasons

spatial and temporal variations in greenland glacial earthquake activity 1993 2010 1 glacial earthquakes are anomalous earthquakes associated with large ice loss events occurring at marine terminating glaciers primarily in greenland they are detectable teleseismically and a proper understanding of the source mechanism may provide a remote sensing tool to complement glaciological observations of these large outlet glaciers we model teleseismic surface wave waveforms to obtain locations and centroid single force source parameters for 121 glacial earthquakes occurring in greenland during the period 2006 2010 we combine these results with those obtained by previous workers to analyze spatial and temporal trends in glacial earthquake occurrence over the 18 year period from 1993 2010 we also examine earthquake occurrence at six individual glaciers comparing the earthquake record to independently obtained observations of glacier change our findings confirm the inference that glacial earthquake seismogenesis occurs through the capsize of large newly calved icebergs we find a close correspondence between episodes of glacier retreat thinning and acceleration and the timing of glacial earthquakes and document the northward progression of glacial earthquakes on greenland s west coast over the 18 year observing period our results also show that glacial earthquakes occur when the termini of the source glaciers are very close to the glacier grounding line i e when the glaciers are grounded or nearly grounded

nusselt rayleigh number scaling for spherical shell earth mantle simulation up to a rayleigh number of 10 9 abstract an investigation of the power law relationship between nusselt number nu and rayleigh number ra for earth s convecting mantle is presented the nu ra relationship was calculated from the results of a model with three dimensional spherical geometry and free slip boundary conditions both basally and internally heated convection has been examined for nu ra a ra was found to be 0 294 0 004 for basally heated systems which is lower than the value of 1 3 suggested by conventional boundary layer theory the exponent 0 337 0 009 for internally heated systems when the internally heated ra is converted to a basally heated equivalent for comparison the influence of the method used to calculate was also considered with particular attention paid to high ra as an example of the significance of 0 29 rather than 1 3 a ra of 1 0 9 results in a surface heat flux which is 32 lower within the range of ra used in this study there is no evidence that changes at high ra therefore that mechanism cannot be used to moderate mantle temperature when projecting back to early earth conditions the differing planform of basally and internally heated models was shown to result in different scaling relationships between root mean square surface velocity and ra for the two modes of heating in particular a much lower surface velocity for internally heated cases relative to equivalent ra basally heated cases

recent changes on greenland outlet glaciers aircraft laser altimeter surveys during the 1990s showed near coastal parts of the greenland ice sheet to be thinning despite slow thickening at higher elevations the ice sheet lost mass to the ocean many outlet glaciers thinned more rapidly than could be explained by increased melting during the recent warmer summers indicating dynamic imbalance between glacier velocity and upstream snow accumulation results from more recent surveys presented here show that thinning rates have increased in most coastal regions for almost half of the surveys these increases might have resulted from increases in summer melting but rapid thinning on others is indicative of dynamic changes that increased with time in particular thinning rates on the three fastest glaciers increased to tens of m a 1 after 2000 and other observations show an approximate doubling in their velocities the deep beds of these glaciers appear to have a strong influence on rates of grounding line retreat and thickness change with periods of glacier acceleration and rapid thinning initiated by flotation and break up of lightly grounded glacier snouts or break up of floating ice tongues near simultaneous thinning of these widely separated glaciers suggests that warming of deeper ocean waters might be a common cause nearby glaciers without deep beds are thinning far more slowly suggesting that basal lubrication as a result of increased surface melting has only a marginal impact on greenland outlet glacier acceleration

using surface velocities to calculate ice thickness and bed topography a case study at columbia glacier alaska usa information about glacier volume and ice thickness distribution is essential for many glaciological applications but direct measurements of ice thickness can be difficult and costly we present a new method that calculates ice thickness via an estimate of ice flux we solve the familiar continuity equation between adjacent flowlines which decreases the computational time required compared to a solution on the whole grid we test the method on columbia glacier a large tidewater glacier in alaska usa and compare calculated and measured ice thicknesses with favorable results this shows the potential of this method for estimating ice thickness distribution of glaciers for which only surface data are available we find that both the mean thickness and volume of columbia glacier were approximately halved over the period 1957 2007 from 281m to 143m and from 294km 3 to 134km 3 respectively using bedrock slope and considering how waves of thickness change propagate through the glacier we conduct a brief analysis of the instability of columbia glacier which leads us to conclude that the rapid portion of the retreat may be nearing an end

ice melange dynamics and implications for terminus stability jakobshavn isbrae greenland 1 we used time lapse imagery seismic and audio recordings iceberg and glacier velocities ocean wave measurements and simple theoretical considerations to investigate the interactions between jakobshavn isbrae and its proglacial ice melange the melange behaves as a weak granular ice shelf whose rheology varies seasonally sea ice growth in winter stiffens the melange matrix by binding iceberg clasts together ultimately preventing the calving of full glacier thickness icebergs the dominant style of calving and enabling a several kilometer terminus advance each summer the melange weakens and the terminus retreats the melange remains strong enough however to be largely unaffected by ocean currents except during calving events and to influence the timing and sequence of calving events furthermore motion of the melange is highly episodic between calving events including the entire winter it is pushed down fjord by the advancing terminus at 40 m d 1 whereas during calving events it can move in excess of 50 103 m d 1 for more than 10 min by influencing the timing of calving events the melange contributes to the glacier s several kilometer seasonal advance and retreat the associated geometric changes of the terminus area affect glacier flow furthermore a force balance analysis shows that large scale calving is only possible from a terminus that is near floatation especially in the presence of a resistive ice melange the net annual retreat of the glacier is therefore limited by its proximity to floatation potentially providing a physical mechanism for a previously described near floatation criterion for calving

the response of petermann glacier greenland to large calving events and its future stability in the context of atmospheric and oceanic warming this study assesses the impact of a large 2010 calving event on the current and future stability of petermann glacier greenland and ascertains the glacier s interaction with different components of the climate and ocean system we use a numerical ice flow model that captures the major aspects of the glacier s mass budget the resistive forces controlling glacier flow and includes dynamic calving satellite observations and model results show that the recent break off of 25 of the floating tongue did not result in a significant glacier speed up due to the low lateral resistance of this relatively wide and thin ice tongue we demonstrate that seasonal speed up at petermann glacier is mainly driven by meltwater lubrication rather than freeze up conditions in the fjord results also show that sub shelf ocean melt may have a profound effect on the future stability of petermann glacier emphasizing the urgent need for more observations and a better understanding of fjord temperature variability and circulation

tidal modulation of ice shelf buttressing stresses ocean tides influence the flow of marine terminating glaciers observations indicate that the large fortnightly variations in ice flow at rutford ice stream in west antarctica originate in the floating ice shelf we show that nonlinear variations in ice shelf buttressing driven by tides can produce such fortnightly variations in ice flow these nonlinearities in the tidal modulation of buttressing stresses can be caused by asymmetries in the contact stress from migration of the grounding line and bathymetric pinning points beneath the ice shelf using a simple viscoelastic model we demonstrate that a combination of buttressing and hydrostatic stress variations can explain a diverse range of tidal variations in ice shelf flow including the period phase and amplitude of flow variations observed at rutford and bindschadler ice streams

thermal diffusivity and conductivity of olivine wadsleyite and ringwoodite to 20 gpa and 1373 k abstract we present results of lattice thermal diffusivity measurements on mg0 9fe0 1 2sio4 olivine and its high pressure polymorphs wadsleyite and ringwoodite under mantle conditions we used the angstrom method on cylindrical samples in multianvil apparatus at pressures up to 20 gpa and temperatures up to 1373 k because of the fine polycrystallinity of the specimens 30 40 m or less there is strong scattering absorption of light and suppression of radiative transport so that the lattice vibrational component is the dominant heat transfer mode lattice thermal conductivities were calculated from the thermal diffusivity results using heat capacity and equation of state data olivine thermal conductivities are consistent with previous results obtained at 1 atm e g j am ceramic soc 38 1954 107 j geophys res 77 1972 6966 science 283 1999 1699 thermal conductivity increases by approximately 30 at the transition from olivine to wadsleyite corresponding to the 410 km discontinuity and a further but smaller increase may occur at the transition from wadsleyite to ringwoodite for each of these phases lattice conductivity closely follows a t 1 2 dependence on temperature t phys rev 119 1960 507 j geophys res 79 1974 703 brown 2002 if such a dependence applies to other silicates and complex crystals there should be a useful way to estimate conductivities at high temperatures from room temperature measurements

the mantle wedge s transient 3 d flow regime and thermal structure arc volcanism volatile cycling mineralization and continental crust formation are likely regulated by the mantle wedge s flow regime and thermal structure wedge flow is often assumed to follow a regular corner flow pattern however studies that incorporate a hydrated rheology and thermal buoyancy predict internal small scale convection ssc here we systematically explore mantle wedge dynamics in 3 d simulations we find that longitudinal richter rolls of ssc with trench perpendicular axes commonly occur if wedge hydration reduces viscosities to 1 1019 pa s although transient transverse rolls with trench parallel axes can dominate at viscosities of 5 1018 1 1019 pa s rolls below the arc and back arc differ subarc rolls have similar trench parallel and trench perpendicular dimensions of 100 150 km and evolve on a 1 5 myr time scale subback arc instabilities on the other hand coalesce into elongated sheets usually with a preferential trench perpendicular alignment display a wavelength of 150 400 km and vary on a 5 10 myr time scale the modulating influence of subback arc ridges on the subarc system increases with stronger wedge hydration higher subduction velocity and thicker upper plates we find that trench parallel averages of wedge velocities and temperature are consistent with those predicted in 2 d models however lithospheric thinning through ssc is somewhat enhanced in 3 d thus expanding hydrous melting regions and shifting dehydration boundaries subarc richter rolls generate time dependent trench parallel temperature variations of up to 150 k which exceed the transient 50 100 k variations predicted in 2 d and may contribute to arc volcano spacing and the variable seismic velocity structures imaged beneath some arcs

changes in the velocity structure of the greenland ice sheet using satellite radar interferometry observations of greenland we detected widespread glacier acceleration below 66 north between 1996 and 2000 which rapidly expanded to 70 north in 2005 accelerated ice discharge in the west and particularly in the east doubled the ice sheet mass deficit in the last decade from 90 to 220 cubic kilometers per year as more glaciers accelerate farther north the contribution of greenland to sea level rise will continue to increase

in situ measurement of texture development rate in cairo3 post perovskite the rate of crystallographic preferred orientation cpo development during deformation of post perovskite is crucial in interpreting seismic anisotropy in the lowermost mantle but the stability field of mgsio3 post perovskite prevents high strain deformation experiments being performed on it therefore to constrain the rate of cpo development in post perovskite we deformed cairo3 a low pressure analogue of mgsio3 post perovskite in simple shear at 3 2 gpa and 400 c to a shear strain of 0 81 from x ray diffraction patterns acquired during deformation we invert for cpo as a function of strain by comparing the cpo that develops with visco plastic self consistent vpsc models we constrain the critical resolved shear stresses crss of the non primary slip systems in cairo3 to be of order 6 times stronger than the primary 100 010 slip system this value is significantly less than has been assumed by previous studies and if applicable to mgsio3 implies that seismic anisotropy in the d layer develops slower than has previously been assumed

elastic anisotropy of d predicted from global models of mantle flow in order to test the hypothesis that seismic anisotropy in the lowermost mantle is caused by the development of a post perovskite lattice preferred orientation and that anisotropy can thus be used as a probe of the dynamics of the mantle s lower boundary layer an integrated model of texture generation in d is developed this is used to predict the elastic anisotropy of the lowermost mantle as probed by global anisotropic tomographic inversions the model combines the current 3d mantle flow field with simulations of the deformation of post perovskite polycrystalline aggregates different descriptions of single crystal plasticity can lead to model results which are anti correlated to each other in models where post perovskite deformation is accommodated by dislocations moving on 010 or 100 patterns of anisotropy are approximately correlated with the results of tomographic inversions on the other hand in models where dislocations move on 001 patterns of anisotropy are nearly anti correlated with tomographic inversions if all the seismic anisotropy in d extracted from global anisotropic inversions is due to the presence of a lattice preferred orientation in post perovskite in the lowermost mantle and if the results of the tomographic inversions are not strongly biased by the sampling geometries these results suggest that in contrast to ideas based on the 1d anisotropic signal deformation of post perovskite in the lowermost mantle may be accommodated by dislocations moving on 010 or 100 alternatively a significant portion of the anisotropic signal may be caused by mechanisms other than the alignment of post perovskite crystals

fortnightly variations in the flow velocity of rutford ice stream west antarctica ice sheets like that of antarctica lose much of their ice via fast flowing ice streams hilmar gudmundsson has monitored the flow in one such stream the rutford ice stream in west antarctica over a period of two months the findings are a surprise surface flow velocities here fluctuate by as much as 20 over a period of two weeks perhaps under the influence of the spring neap fortnightly tidal cycle although the mechanism that couples these tides to ice flow has yet to be elucidated these observations caution against the use of velocity measurements over limited periods to infer long term changes in flow rate monitoring of the flow of an ice sheet in antarctica finds that the surface velocity oscillates by as much as 20 per cent every two weeks the spring to neap tidal cycle seems to be the cause although the mechanism through which these tides couple to ice flow has yet to be elucidated these observations caution against the use of velocity measurements over limited periods to infer long term changes in flow rate most of the ice lost from the antarctic ice sheet passes through a few fast flowing and highly dynamic ice streams1 quantifying temporal variations in flow in these ice streams and understanding their causes is a prerequisite for estimating the potential contribution of the antarctic ice sheet to global sea level change2 3 here i show that surface velocities on a major west antarctic ice stream rutford ice stream vary periodically by about 20 per cent every two weeks as a result of tidal forcing tidally induced motion on ice streams has previously been thought to be limited to diurnal or even shorter term variations4 5 6 7 8 9 the existence of strong fortnightly variations in flow demonstrates the potential pitfalls of using repeated velocity measurements over intervals of days to infer long term change

lattice thermal conductivity of mgsio3 perovskite and post perovskite at the core mantle boundary abstract thermal conductivity is essential for controlling the rate of core heat loss and long term thermal evolution of the earth but it has been poorly constrained at the high pressures of earth s lowermost mantle we measured the lattice component of thermal diffusivity heat transport by scattering of phonons of both mgsio 3 perovskite pv and post perovskite ppv at high pressures of up to 144 gpa and at room temperature lattice thermal conductivity of pv dominant lowermost mantle assemblage obtained in this study is about 11 w m k while ppv bearing rocks exhibit 60 higher conductivity since such pv value is comparable to the conventionally assumed lowermost mantle conductivity our findings do not significantly alter but support the recent notion of high core mantle boundary heat flow along with a young inner core and high temperatures in the early deep earth

fast glacier flow ice streams surging and tidewater glaciers fast flowing glaciers are unusual but important jakobshavns glacier in western greenland illustrates this point figure 1 considered to be the world s fastest glacier its maximum observed flow rate measured in midsummer near the floating terminus is 8360 m yr 1 lingle et al 1981 except for a tidal influence near the terminus the flow is believed to be fairly steady bindschadler 1984

lower mantle dynamics with the post perovskite phase change radiative thermal conductivity temperature and depth dependent viscosity the new post perovskite phase near the core mantle boundary has important ramifications on lower mantle dynamics we have investigated the dynamical impact arising from the interaction of temperature and depth dependent viscosity with radiative thermal conductivity up to a lateral viscosity contrast of 10 4 on both the ascending and descending flows in the presence of both the endothermic phase change at 670 km depth and an exothermic post perovskite transition at 2650 km depth the phase boundaries are approximated as localized zones we have employed a two dimensional cartesian model using a box with an aspect ratio of 10 within the framework of the extended boussinesq approximation our results for temperature and depth dependent viscosity corroborate the previous results for depth dependent viscosity in that a sufficiently strong radiative thermal conductivity plays an important role for sustaining superplumes in the lower mantle once the post perovskite phase change is brought into play this aspect is especially emphasized when the radiative thermal conductivity is restricted only to the post perovskite phase these results revealed a greater degree of asymmetry is produced in the vertical flow structures of the mantle by the phase transitions mass and heat transfer between the upper and lower mantle will deviate substantially from the traditional whole mantle convection model streamlines revealed that an overall complete communication between the top and lower mantle is difficult to be achieved 2005 elsevier b v all rights reserved

submarine melting of the 1985 jakobshavn isbrae floating tongue and the triggering of the current retreat 1 photogrammetric reanalysis of 1985 aerial photos has revealed substantial submarine melting of the floating ice tongue of jakobshavn isbrae west greenland the thickness of the floating tongue determined from hydrostatic equilibrium tapers from 940 m near the grounding zone to 600 m near the terminus feature tracking on orthophotos shows speeds on the july 1985 ice tongue to be nearly constant 18 5 m d 1 indicating negligible dynamic thinning the thinning of the ice tongue is mostly due to submarine melting with average rates of 228 49 m yr 1 0 62 0 13 m d 1 between the summers of 1984 and 1985 the cause of the high melt rate is the circulation of warm seawater thermal forcing of up to 4 2 c beneath the tongue with convection driven by the substantial discharge of subglacial freshwater from the grounding zone we believe that this buoyancy driven convection is responsible for a deep channel incised into the sole of the floating tongue a dramatic thinning retreat and speedup began in 1998 and continues today the timing of the change is coincident with a 1 1 c warming of deep ocean waters entering the fjord after 1997 assuming a linear relationship between thermal forcing and submarine melt rate average melt rates should have increased by 25 57 m yr 1 sufficient to destabilize the ice tongue and initiate the ice thinning and the retreat that followed

glacier acceleration and thinning after ice shelf collapse in the larsen b embayment antarctica 1 ice velocities derived from five landsat 7 images acquired between january 2000 and february 2003 show a two to six fold increase in centerline speed of four glaciers flowing into the now collapsed section of the larsen b ice shelf satellite laser altimetry from icesat indicates the surface of hektoria glacier lowered by up to 38 6 m in a six month period beginning one year after the break up in march 2002 smaller elevation losses are observed for crane and jorum glaciers over a later 5 month period two glaciers south of the collapse area flask and leppard show little change in speed or elevation seasonal variations in speed preceding the large post collapse velocity increases suggest that both summer melt percolation and changes in the stress field due to shelf removal play a major role in glacier dynamics

grounding line migration and high resolution calving dynamics of jakobshavn isbrae west greenland 1 we performed three field campaigns in 2004 2007 and 2010 at the southern margin of the jakobshavn isbrae west greenland in order to infer flow velocities and their changes from photogrammetric time lapse imagery with a temporal resolution of 20 min and a spatial spacing of about 30 m on the glacier surface area wide analysis of more than 3000 three dimensional trajectories at individual glacier positions allow for both the mapping of the grounding line and the detailed observation of flow variations during major calving events from 2004 to 2010 the grounding line of jakobshavn isbrae retreated 3 5 0 2 km considering previously published results the grounding line retreat amounts to 6 km since 1985 the glacier has an ephemeral floating tongue that can establish during the readvance of the glacier front and break apart after large calving events observations of a major calving event show that an acceleration of flow velocities coincides with the onset of the break up during which flow velocities of up to 70 m d can be reached moreover large vertical displacements of the glacier front in the order of 15 m and lowering of 8 m at positions 500 m beyond the calving front were observed 2 days before the calving event after the break up the glacier slowly adjusts to the new boundary conditions within the next 4 5 days flow velocity variations caused by calving were detected up to 1 km upstream only which indicates that individual calving events have no immediate effect on the large scale glacier dynamics

tidal influence on rutford ice stream west antarctica observations of surface flow and basal processes from closely spaced gps and passive seismic stations high resolution surface velocity measurements and passive seismic observations from rutford ice stream west antarctica 40 km upstream from the grounding line are presented these measurements indicate a complex relationship between the ocean tides and currents basal conditions and ice stream flow both the mean basal seismicity and the velocity of the ice stream are modulated by the tides seismic activity increases twice during each semi diurnal tidal cycle the tidal analysis shows the largest velocity variation is at the fortnightly period with smaller variations superimposed at diurnal and semi diurnal frequencies the general pattern of the observed velocity is two velocity peaks during each semi diurnal tidal cycle but sometimes three peaks are observed this pattern of two or three peaks is more regular during spring tides when the largest amplitude velocity variations are observed than during neap tides this is the first time that velocity and level of seismicity are shown to correlate and respond to tidal forcing as far as 40 km upstream from the grounding line of a large ice stream

observing calving generated ocean waves with coastal broadband seismometers jakobshavn isbrae greenland we use time lapse photography modis satellite imagery ocean wave measurements and regional broadband seismic data to demonstrate that icebergs that calve from jakobshavn isbrae greenland can generate ocean waves that are detectable over 150 km from their source the waves which are recorded seismically have distinct spectral peaks are not dispersive and persist for several hours on the basis of these observations we suggest that calving events at jakobshavn isbrae can stimulate seiches or basin eigenmodes in both ilulissat icefjord and disko bay our observations furthermore indicate that coastal land based seismometers located near calving termini e g as part of the new greenland ice sheet monitoring network glisn can aid investigations into the largely unexplored oceanographic consequences of iceberg calving

step wise changes in glacier flow speed coincide with calving and glacial earthquakes at helheim glacier greenland this is the publisher s version also available electronically from http onlinelibrary wiley com

large scale changes in greenland outlet glacier dynamics triggered at the terminus the recent dramatic retreat of many outlet glaciers of the greenland ice sheet has raised concerns over greenland s contribution to future sea level rise simulations with a numerical ice flow model indicate that the recent rates of mass loss in greenland s outlet glaciers are transient and should not be extrapolated into the future

variation of thermal conductivity and heat flux at the earth s core mantle boundary the two convective systems that dominate earth s internal dynamics meet at the boundary between the rocky mantle and metallic liquid core energy transfer between processes driving plate tectonics and the geodynamo is controlled by thermal conduction in the lowermost mantle d we use atomic scale simulations to determine the thermal conductivity of mgsio3 perovskite and post perovskite under d conditions and probe how these two convective systems interact we show that the thermal conductivity of post perovskite 12 w mk is 50 larger than that of perovskite under the same conditions 8 5 w mk and is anisotropic with conductivity along the a axis being 40 higher than conductivity along the c axis this enhances the high heat flux into cold regions of d where post perovskite is stable strengthening the feedback between convection in the core and mantle reminiscent of the situation in the lithosphere there is potential for deformation induced texturing associated with mantle convection to modify how the mantle is heated from below we test this by coupling our atomic scale results to models of texture in d and suggest that anisotropic thermal conductivity may help to stabilise the roots of mantle plumes over their protracted lifetime

thermal diffusivity of olivine under upper mantle conditions summary thermal diffusivity of olivine with mg xsfeo i 2si04 composition is measured at pressures up to 9 gpa and temperatures up to 1700 k using the angstrom method in a cylindrical geometry the experimental results demonstrate that lattice thermal diffusivity increases with increasing pressure at a small rate 46 per cent increase over 10 gpa the experimental data also suggest that radiative thermal diffusivity may decrease with increasing pressure thermal diffusivity in the upper mantle has almost constant values of 7 8 x

synchronous retreat and acceleration of southeast greenland outlet glaciers 2000 06 ice dynamics and coupling to climate a large portion of the recent increase in the rate of mass loss from the greenland ice sheet is from increased outlet glacier discharge along its southeastern margin while previous investigations of the region s two largest glaciers suggest that acceleration is a dynamic response to thinning and retreat of the calving front it is unknown whether this mechanism can explain regional acceleration and what forcing is responsible for initiating rapid thinning and retreat we examine seasonal and interannual changes in ice front position surface elevation and flow speed for 32 glaciers along the southeastern coast between 2000 and 2006 while substantial seasonality in front position and speed is apparent nearly all the observed glaciers show net retreat thinning and acceleration with speed up corresponding to retreat the ratio of retreat to the along flow stress coupling length is proportional to the relative increase in speed consistent with typical ice flow and sliding laws this affirms that speed up results from loss of resistive stress at the front during retreat which leads to along flow stress transfer large retreats were often preceded by the formation of a flat or reverse sloped surface near the front indicating that subsequent retreats were influenced by the reversed bed slope many retreats began with an increase in thinning rates near the front in the summer of 2003 a year of record high coastal air and sea surface temperatures this anomaly was driven in part by recent warming suggesting that episodes of speed up and retreat may become more common in a warmer climate

tides and the flow of rutford ice stream west antarctica 1 surface speeds of rutford ice stream west antarctica are known to vary by around 10 20 depending on location with a fortnightly periodicity corresponding to a springneap tidal cycle the reasons for these periodic variations in flow are unclear here the possible role of tidal stress transmission upstream of the grounding line in affecting rates of basal motion is investigated it is found that nonlinear rheological effects within the till when coupled with transmission of tidal stresses within the ice that are linearly related to tidal amplitude can give rise to the type of periodic oscillations in flow observed this nonlinear interaction between tidal forcing and till deformation increases the mean ice flux across the grounding line by a few percent above what might be expected in the absence of tidal forcing periodic velocity fluctuations of this type have not been observed on other ice streams however modeling suggests that this may be due to lack of data and that such flow variations are likely to be common features of active ice streams draining into the ronne ice shelf as well as of other ice streams subjected to similar tidal forcing

subduction flux modulates the geomagnetic polarity reversal rate abstract ascertaining the cause of variations in the frequency of geomagnetic polarity reversals through the phanerozoic has remained a primary research question straddling paleomagnetism and geodynamics for decades numerical models suggest the primary control on geomagnetic reversal rates on 10 to 100 ma timescales is the changing heat flux across the core mantle boundary and that this is itself expected to be strongly influenced by variations in the flux of lithosphere subducted into the mantle a positive relationship between the time dependent global subduction flux and magnetic reversal rate is expected with a time delay to transmit the thermal imprint into the lowermost mantle we perform the first test of this hypothesis using subduction flux estimates and geomagnetic reversal rate data back to the early paleozoic subduction area flux estimates are derived from global full plate tectonic models and are evaluated against independent subduction flux proxies based on the global age distribution of detrital zircons and strontium isotopes a continuous phanerozoic reversal rate model is built from pre existing compilations back to 320 ma plus a new reversal rate model in the data sparse mid to early paleozoic cross correlation of the time dependent subduction flux and geomagnetic reversal rate series reveals a significant correlation with a time delay of 120 ma with reversals trailing the subduction flux this time delay represents a value intermediate between the seismologically constrained time expected for a subducted slab to transit from the surface to the core mantle boundary 150 300 ma and the much shorter lag time predicted by some numerical models of mantle flow 30 60 ma while the reason for this large discrepancy remains unclear it is encouraging that our novel estimate of lag time represents a compromise between them although important uncertainties in our proposed relationship remain these results cast new light on the dynamic connections between the surface and deep earth and will help to constrain new models linking mantle convection the thermal evolution of the lowermost mantle and the geodynamo

tide modulated ice motion and seismicity of a floating glacier tongue in east antarctica we recorded the ice motion and icequakes on the floating part of langhovde glacier in east antarctica to better understand the dynamic behavior of ice shelves and floating tongues diurnal and semi diurnal variations in ice motion and seismicity were simultaneously observed at all four global navigation satellite system and three seismic stations over 2 weeks the short term along flow ice motion is explained by the elastic response of the glacier to ocean tide induced hydrostatic stress variations which decayed at a rate of 0 8 km 1 toward the grounding line we observed a large number of icequakes during mid rising and high tides that covered a broad frequency range and formed two major groups of events centered at 10 and 120 hz respectively the hourly occurrence rates were 500 events h 1 with the observed seismicity consistent with fracture due to floating tongue bending we also observed minor secondary peaks at high ice speeds which could reflect surface cracking due to stretching or basal friction our observation demonstrates that tidal modulation was the main factor to fracture the floating tongue of langhovde glacier

an integrated structure from motion and time lapse technique for quantifying ice margin dynamics fine resolution topographic data derived from methods such as structure from motion sfm and multi view stereo mvs have the potential to provide detailed observations of geomorphological change but have thus far been limited by the logistical constraints of conducting repeat surveys in the field here we present the results from an automated time lapse camera array deployed around an ice marginal lake on the western margin of the greenland ice sheet fifteen cameras acquired imagery three times per day over a 426 day period yielding a dataset of 19 000 images from these data we derived 18 point clouds of the ice margin across a range of seasons and successfully identified calving events ranging from 234 to 1475 m2 in area and 815 8725 m3 in volume induced by ice cliff undercutting at the waterline and the collapse of spalling flakes low ambient light levels locally reflective surfaces and the large survey range hindered analysis of smaller scale ice margin dynamics nevertheless this study demonstrates that an integrated sfm mvs and time lapse approach can be employed to generate long term 3 d topographic datasets and thus quantify ice margin dynamics at a fine spatio temporal scale this approach provides a template for future studies of geomorphological change

volume change of jakobshavn isbrae west greenland 1985 1997 2007 following three decades of relative stability jakobshavn isbrae west greenland under went dramatic thinning retreat and speed up starting in 1998 to assess the amount of ice loss we analyzed 1985 aerial photos and derived a 40 m grid digital elevation model dem we also obtained a 2007 40 m grid spot dem covering the same region comparison of the two dems over an area of 4000 km 2 revealed a total ice loss of 160 4k m 3 with 107 0 2 km 3 in grounded regions 0 27 mm eustatic sea level rise and 53 4k m 3 from the disintegration of the floating tongue comparison of the dems with 1997 nasa airborne topographic mapper data indicates that this ice loss essentially occurred after 1997 with 0 7 5 6 km 3 between 1985 and 1997 and 160 7k m 3 between 1997 and 2007 the latter is equivalent to an average specific mass balance of 3 7 0 2 m a 1 over the study area previously reported thickening of the main glacier during the early 1990s was accompanied by similar magnitude thinning outside the areas of fast flow indicating that the land based ice continued reacting to longer term climate forcing

the age of the inner core abstract the energy conservation law when applied to the earth s core and integrated between the onset of the crystallization of the inner core and the present time gives an equation for the age of the inner core in this equation all the terms can be expressed theoretically and given values and uncertainties of all relevant physical parameters the age of the inner core can be obtained as a function of the heat flux at the core mantle boundary and the concentrations in radioactive elements it is found that in absence of radioactive elements in the core the age of the inner core cannot exceed 2 5 ga and is most likely around 1 ga in addition to have an inner core as old as the earth concentrations in radioactive elements needed in the core are too high to be acceptable on geochemical grounds

acceleration of jakobshavn isbrae triggered by warm subsurface ocean waters observations over past decades show a sudden switch of jakobshavn isbrae a large outlet glacier feeding a deep ocean fjord on greenland s west coast from slow thickening to rapid thinning in 1997 this switch is associated with a doubling in glacier velocity hydrographic data show a concurrent sudden increase in subsurface ocean temperatures along the entire west coast of greenland suggesting that the changes in jakobshavn isbrae were triggered by the arrival of relatively warm water originating from the irminger sea observations over the past decades show a rapid acceleration of several outlet glaciers in greenland and antarctica1 one of the largest changes is a sudden switch of jakobshavn isbrae a large outlet glacier feeding a deep ocean fjord on greenland s west coast from slow thickening to rapid thinning2 in 1997 associated with a doubling in glacier velocity3 suggested explanations for the speed up of jakobshavn isbrae include increased lubrication of the ice bedrock interface as more meltwater has drained to the glacier bed during recent warmer summers4 and weakening and break up of the floating ice tongue that buttressed the glacier5 here we present hydrographic data that show a sudden increase in subsurface ocean temperature in 1997 along the entire west coast of greenland suggesting that the changes in jakobshavn isbrae were instead triggered by the arrival of relatively warm water originating from the irminger sea near iceland we trace these oceanic changes back to changes in the atmospheric circulation in the north atlantic region we conclude that the prediction of future rapid dynamic responses of other outlet glaciers to climate change will require an improved understanding of the effect of changes in regional ocean and atmosphere circulation on the delivery of warm subsurface waters to the periphery of the ice sheets

distinct seismic anisotropy at the centre of the earth abstract although arrival times of pkp df or pkikp phases from epicentral distances between 120 and 173 are consistent with an anisotropy model obtained from a joint inversion of normal mode and body wave data pkp df measurements from nearly antipodal distances between 173 and 180 exhibit deviations from this model we demonstrate that this behaviour is coherent and that the characteristics associated with these data are still those of transverse isotropy we also perform a systematic search for the location of the symmetry axis results suggest that the axis may be significantly tilted away from the geographic pole however higher quality data are required to draw a firm conclusion inversion of the data with the symmetry axis at the geographic pole yields a model of anisotropy that is considerably different from that of the inner core between 300 and 1220 km radius these observations imply that there is a seismically distinct region at the centre of the earth with radius of about 300 km

spatial and temporal melt variability at helheim glacier east greenland and its effect on ice dynamics this is the publisher s version also available electronically from http onlinelibrary wiley com

dynamical consequences in the lower mantle with the post perovskite phase change and strongly depth dependent thermodynamic and transport properties article i nfo we have carried out numerical simulations of large aspect ratio 2 d mantle convection with the deep phase change from perovskite pv to post perovskite ppv using the extended boussinesq approximation for a fluid with temperature and pressure dependent viscosity we have investigated the effects of various ppv phase parameters on the convective planform heat transport and mean temperature and viscosity profiles since ppv is expected to have a relatively weak rheology with respect to pv and a large thermal conductivity we have assumed that the transition from pv to ppv is accompanied by both a reduction in viscosity by 1 to 2 orders of magnitude and by an increase in thermal conductivity by a factor of 2 furthermore we have analyzed the combined effects of a strongly decreasing thermal expansivity in pv and steeply increasing thermal conductivity according to recent evidence from high pressure experiments and first principle calculations as long as the thermal expansivity and conductivity are constant ppv exerts a small but noticeable effect on mantle convection it destabilizes the d layer causes focusing of the heat flux peaks and an increase of the average mantle temperature and of the temporal and spatial frequency of upwellings when the latest depth dependent thermal expansivity and conductivity models are introduced the effects of ppv are dramatic on the one hand without ppv we obtain a very sluggish convective regime characterized by a relatively cool mantle dominated by large downwellings that tend to stagnate beneath the transition zone with ppv on the other hand we observe an extremely significant increase of the average mantle temperature due to the formation of large sized and vigorous upwellings that in some cases tend to cluster thus forming superplumes if a very large thermal conductivity at the core mantle boundary is assumed k 20 wk 1 m 1 we obtain a quasi steady state regime characterized by large and stable plumes with long lifetimes the combination of strongly depth dependent expansivity and conductivity is a viable mechanism for the formation of long wavelength long lived thermal anomalies in the deep mantle even if a low viscosity ppv atop the core mantle boundary is included

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

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

analysis of low frequency seismic signals generated during a multiple iceberg calving event at jakobshavn isbrae greenland 1 we investigated seismic signals generated during a large scale multiple iceberg calving event that occurred at jakobshavn isbrae greenland on 21 august 2009 the event was recorded by a high rate time lapse camera and five broadband seismic stations located within a few hundred kilometers of the terminus during the event two full glacier thickness icebergs calved from the grounded or nearly grounded terminus and immediately capsized the second iceberg to calve was two to three times smaller than the first the individual calving and capsize events were well correlated with the radiation of low frequency seismic signals 0 1 hz dominated by love and rayleigh waves in agreement with regional records from previously published glacial earthquakes these low frequency seismic signals had maximum power and or signal to noise ratios in the 0 05 0 1 hz band similarly full waveform inversions indicate that these signals were also generated by horizontal single forces acting at the glacier terminus the signals therefore appear to be local manifestations of glacial earthquakes although the magnitudes of the signals twice time integrated force histories were considerably smaller than previously reported glacial earthquakes we thus speculate that such earthquakes may be a common if not pervasive feature of all full glacier thickness calving events from grounded termini finally a key result from our study is that waveform inversions performed on low frequency calving generated seismic signals may have only limited ability to quantitatively estimate mass losses from calving in particular the choice of source time function has little impact on the inversion but dramatically changes the earthquake magnitude accordingly in our analysis it is unclear whether the smaller or larger of the two calving icebergs generated a larger seismic signal

on the increase in thermal diffusivity caused by the perovskite to post perovskite phase transition and its implications for mantle dynamics the thermal diffusivity of perovskite and post perovskite cairo3 has been measured at elevated pressure and temperatures up to 600 c using the x radiographic angstrom method at high temperatures we find that the thermal diffusivity of post perovskite is slightly below twice that of isochemical perovskite over the temperature range investigated assuming a similar effect occurs in mgsio3 post perovskite the effect of the contrasting thermal transport properties between perovskite and post perovskite on mantle dynamics has been investigated using simple two dimensional convection models these show a reduction in extent and increase in depth of post perovskite lenses as well as increased core mantle boundary heat flux broader upwellings and more vigorous downwellings when compared to the reference constant a case

changes in ice front position on greenland s outlet glaciers from 1992 to 2007 1 previous studies in greenland show that retreat of tidewater glaciers may be linked to recent increases in ice loss raising greenland s contribution to sea level rise we examined ice front changes of 203 tidewater glaciers land terminating glaciers and glaciers terminating with ice shelves to understand greenland glacier behavior over three periods 1992 2000 2000 2006 and 2006 2007 we observed synchronous ice sheet wide increases in tidewater retreat during 2000 2006 relative to 1992 2000 coinciding with a 1 1 c increase in mean summer temperature at coastal weather stations rates of retreat for the southeast and east slowed during 2006 2007 when temperatures were slightly cooler than the 2000 2006 average our work suggests that regional greenland tidewater retreat responds strongly to climate change with higher temperatures corresponding to increasing retreat and helps confirm a link between ice thickness velocity and ice front position

inferences on flow at the base of earth s mantle based on seismic anisotropy we applied global waveform tomography to model radial anisotropy in the whole mantle we found that in the last few hundred kilometers near the core mantle boundary horizontally polarized s wave velocities vsh are on average faster by 1 than vertically polarized s wave velocities vsv suggesting a large scale predominance of horizontal shear this confirms that the d region at the base of the mantle is also a mechanical boundary layer for mantle convection a notable exception to this average signature can be found at the base of the two broad low velocity regions under the pacific ocean and under africa often referred to as superplumes where the anisotropic pattern indicates the onset of vertical flow

ice flow modulated by tides at up to annual periods at rutford ice stream west antarctica 1 for over 2 years we have collected gps data 40 km upstream of the grounding line on rutford ice stream west antarctica here we examine deviations from mean downstream flow although the record is incomplete during winter there is clear modulation of flow at semi diurnal diurnal two weekly semi annual and annual ocean tidal frequencies this is the first observation of ice stream flow variations over such a long time period and such a wide range of frequencies the ice stream flows fastest at equinoxes when there are two semi diurnal tides of equal magnitude and slowest at solstices when one of the semi diurnal tides has lower amplitude the sensitivity of the downstream flow is greatest to the long period forcing which suggests that ice stream velocity may be affected by future changes in sea level if so this effect would provide a feedback whereby rising sea levels could increase ice stream velocity and hence discharge

continued evolution of jakobshavn isbrae following its rapid speedup 1 several new data sets reveal that thinning and speedup of jakobshavn isbrae continue following its recent rapid increase in speed as its floating ice tongue disintegrated the present speedup rate of 5 a 1 over much of the fast moving region appears to be a diffusive response to the initial much larger speedup near the front there is strong seasonality in speed over much of the fast flowing main trunk that shows a good inverse correlation with the seasonally varying length of a short typically 6 km floating ice tongue this modulation of speed with ice front position supports the hypothesis that the major speedup was caused by loss of the larger floating ice tongue from 1998 to 2003 analysis of image time series suggests that the transient winter ice tongue is formed when sea ice bonds glacier ice in the fjord to produce a nearly rigid mass that almost entirely suppresses calving major calving only resumes in late winter when much of this ice clears from the fjord the collapse of the ice tongue in the late 1990s followed almost immediately after a sharp decline in winter sea ice concentration in disko bay this decline may have extended the length of the calving season for several consecutive years leading to the ice tongue s collapse

weakening of calcium iridate during its transformation from perovskite to post perovskite magnesium silicate perovskite the dominant mineral in the lower mantle is thought to transform into a post perovskite phase in the mantle s lowermost region laboratory experiments suggest substantial weakening could occur during the transformation from perovskite to post perovskite which could explain the anomalous physical properties of the lowermost mantle

seasonal variation in velocity before retreat of jakobshavn isbrae greenland 1 using repeat pass satellite feature tracking the dynamic behaviour of jakobshavn isbrae which drains around 7 of the greenland ice sheet is investigated in improved detail through its recent period of thinning acceleration and retreat the departure in 1995 from the normal seasonal invariance in velocity near the grounding line suggests a possible change in dynamics well before the subsequent thinning phase the initiation of the acceleration and retreat is pinpointed to spring 1998 and may have been prompted by the same mechanism that gave rise to the spring 1995 flow increase

seasonal to decadal scale variations in the surface velocity of jakobshavn isbrae greenland observation and model based analysis 1 using new data we build upon the nearly two decade long record of observations from jakobshavn isbrae to investigate the processes driving its dynamic evolution while winter flow speed has not increased substantially over the last three winters there remains a strong seasonal variation in flow speed that coincides with a cycle of summer thinning and winter thickening we relate changes in glacier speed to geometry through variations in basal traction and horizontal stresses using ice flow models constrained by satellite and airborne observations these results suggest that the bed provides little flow resistance along the main trough within about 20 km of the terminus while the loss of buttressing from the retreat of grounded and floating ice likely contributed to the initial speedup other processes are of comparable significance at seasonal to decadal time scales from analysis of the models we hypothesize that thinning induced change in basal effective pressure is the dominant process influencing near terminus behavior while diffusive processes drive the upstream response the apparent need for the terminus to thin to near flotation before it can calve may limit the rate at which retreat occurs our analysis of the processes controlling the speed suggests little potential for further large acceleration thinning and elevated speeds may continue at rates similar to present however putting the glacier on course to retreat to the head of its deep trough in about a century at which point it likely would stabilize with a thinner terminus

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

fluidity a fully unstructured anisotropic adaptive mesh computational modeling framework for geodynamics we present a new computational modeling framework fluidity for application to a range of two and three dimensional geodynamic problems with the focus here on mantle convection the approach centers upon a finite element discretization on unstructured simplex meshes which represent complex geometries in a straightforward manner throughout a simulation the mesh is dynamically adapted to optimize the representation of evolving solution structures the adaptive algorithm makes use of anisotropic measures of solution complexity to vary resolution and allow long thin elements to align with features such as boundary layers the modeling framework presented differs from the majority of current mantle convection codes which are typically based upon fixed structured grids this necessitates a thorough and detailed validation which is a focus of this paper benchmark comparisons are undertaken with a range of two and three dimensional isoviscous and variable viscosity cases in addition model predictions are compared to experimental results such comparisons highlight not only the robustness and accuracy of fluidity but also the advantages of anisotropic adaptive unstructured meshes significantly reducing computational requirements when compared to a fixed mesh simulation

palaeomagnetic field intensity variations suggest mesoproterozoic inner core nucleation the earth s inner core grows by the freezing of liquid iron at its surface the point in history at which this process initiated marks a step change in the thermal evolution of the planet recent computational and experimental studies have presented radically differing estimates of the thermal conductivity of the earth s core resulting in estimates of the timing of inner core nucleation ranging from less than half a billion to nearly two billion years ago recent inner core nucleation high thermal conductivity requires high outer core temperatures in the early earth that complicate models of thermal evolution the nucleation of the core leads to a different convective regime and potentially different magnetic field structures that produce an observable signal in the palaeomagnetic record and allow the date of inner core nucleation to be estimated directly previous studies searching for this signature have been hampered by the paucity of palaeomagnetic intensity measurements by the lack of an effective means of assessing their reliability and by shorter timescale geomagnetic variations here we examine results from an expanded precambrian database of palaeomagnetic intensity measurements selected using a new set of reliability criteria our analysis provides intensity based support for the dominant dipolarity of the time averaged precambrian field a crucial requirement for palaeomagnetic reconstructions of continents we also present firm evidence for the existence of very long term variations in geomagnetic strength the most prominent and robust transition in the record is an increase in both average field strength and variability that is observed to occur between a billion and 1 5 billion years ago this observation is most readily explained by the nucleation of the inner core occurring during this interval the timing would tend to favour a modest value of core thermal conductivity and supports a simple thermal evolution model for the earth