impact of bedrock surface topography on spatial distribution of quaternary sediments and on the flow pattern of late weichselian glaciers on the east european craton russian plain this paper presents the first high quality maps of the bedrock surface topography and the quaternary sediment distribution for the late weichselian glaciation area on the east european craton eec and the reasoning behind the estimated sediment distribution geographic information system gis based three dimensional model of the bedrock surface topography the current digital terrain model dtm and the ice flow pattern of the last glaciation provided the main source data for the analyses and interpretations in the current continental area of the western eec which was covered by the last scandinavian ice sheet sis the bedrock lies overwhelmingly 85 above the contemporary sea level with an average altitude of 54 m above sea level the topography of the bedrock surface is very well reflected in the current topography and an 80 m difference in the ice bed surface altitudes results in the topographic focusing of the ice flow and thus all late weichselian ice streams were accommodated to the depressions in the subglacial surface such depressions are characterised by a thin 10 m layer of sediment with one till bed just 50 m upslope from the bottom of the depressions the thickness of the sediment increases two to threefold and comprises five or more till layers indicating relief enhancement through subsequent glaciations the average calculated thickness of the quaternary deposits in the study area was 49 m and the total volume of sediments was 20 280 km3

interaction of mantle dregs with convection lateral heterogeneity at the core mantle boundary preliminary numerical models indicate that chemically denser material dregs at the base of the mantle would have substantial lateral variations in thickness induced by convection of the overlying mantle and might well form discontinuous aggregations below mantle upwellings a model with a density contrast of about 2 per cent and an initial uniform thickness of the denser layer of 100 km yields a discontinuous distribution with maximum thickness 230 km and bottom topography of several kilometers amplitude in reasonable accord with recent seismological observations of vertical and lateral structure heat flux out of the core is probably strongly modulated laterally by mantle convection while mantle dregs will complicate and possibly amplify this effect such modulation may be relevant to long term 10 7 10 8 year variations in the magnetic field

seismic triplication used to reveal slab subduction that had disappeared in the late mesozoic beneath the northeastern south china sea abstract we provided a new study of the seismic velocity structure of the mantle transition zone mtz beneath the northeastern south china sea using p wave triplications from two earthquakes at the central philippines recorded by the chinese digital seismic network through fitting the observed and theoretical triplications modeled by the dynamic ray tracing method for traveltimes and the reflectivity method for synthetic waveforms using grid searching method best fit velocity models based on iasp91 were obtained to constrain the p wave velocity structure of the mtz the models show that a high velocity anomaly hva resides at the bottom of mtz the hva is 215 km to 225 km thick with a p wave velocity increment of 1 0 between 450 km and 665 km or 675 km transition and increase by 2 5 3 5 at 665 km or 675 km depth the p wave velocity increment ranges from approximately 0 3 to 0 8 below the 665 km or 675 km we proposed that the hva in the mtz was caused by the broken fragments of a diving oceanic plate falling into the mtz at a high angle and or by unstable thick continental lithosphere dropping into the mtz sequentially or almost simultaneously

a new ice thickness and bed data set for the greenland ice sheet 1 measurement data reduction and errors ice thickness data collected between 1993 and 1999 using a coherent ice penetrating radar system developed at the university of kansas have been combined with data collected by the technical university of denmark in the 1970s to produce a new ice thickness grid for greenland crossover analysis was used to assess the relative accuracy of the two data sets and they were weighted accordingly and interpolated onto a regular 5 km spacing grid using a kriging interpolation procedure a high resolution land ice mask was used to help constrain the interpolation of the ice thickness data near the ice sheet margins where in the past the relative errors have been largest the ice thickness grid was combined with a new digital elevation model of the ice sheet and surrounding rock outcrops to produce a new bed elevation data set for the whole of greenland the ice thickness grid was compared with the currently available data set differences in the center of the ice sheet where the ice is thickest were of the order of a few percent near the margins however large differences of as much as a factor of 10 were found the total volume of ice contained in the ice sheet was reestimated and found to have a value of 2 93 x 10 6 km 3 the ice thickness grid was used to calculate the spatial pattern of gravitational driving stress over the ice sheet anomalous patterns of stress were found in areas that appeared to be associated with areas of rapid flow

deformation in the lowermost mantle from polycrystal plasticity to seismic anisotropy in the deep earth deformation occurs at many scales large scale convection produces subduction of slabs and upwelling of plumes in the mantle at the high temperature high pressure conditions strain is accommodated through crystal plasticity either by diffusion or the movement of dislocations slip causes crystal rotations and thus produces a characteristic pattern of crystal preferred orientation and corresponding anisotropy of physical properties at the macroscopic scale in this study we use polycrystal plasticity with experimentally derived deformation mechanisms for perovskite post perovskite and magnesiowuestite to predict texture development along streamlines in a 2d geodynamic convection model of the lowermost mantle strong preferred orientation develops during subduction and upwelling while during spreading along the core mantle boundary the orientation pattern is relatively stable from preferred crystal orientation and single crystal elastic properties bulk elastic properties can be calculated and compared with seismic observations post perovskite with predominant 001 slip and magnesiowuestite with 110 and 111 slip produce anisotropy patterns which are consistent with observed anisotropy i e fast s waves polarized parallel to the core mantle boundary and anti correlation between p and s wave anisotropies in contrast perovskite with dominant 001 slip and two post perovskite models with dominant slip on 010 and 100 produce anisotropy patterns which are inconsistent with seismic observations

balance mass flux and ice velocity across the equilibrium line in drainage systems of greenland estimates of balance mass flux and depth averaged ice velocity through the cross section aligned with the equilibrium line are produced for each of six drainage systems in greenland the estimates are based on a model equilibrium line fitted to field data and on a revised distribution of surface mass balance for the conterminous ice sheet ice drainage divides and six major drainage systems are delineated using surface topography from ers radar altimeter data ice thicknesses at the equilibrium line and throughout each drainage system are based on the latest compilation of airborne radar sounding data described elsewhere the net accumulation rate in the area bounded by the equilibrium line is 399 gt a 1 and net ablation rate in the remaining area is 231 gt a 1 excluding an east central coastal ridge reduces the net accumulation rate to 397 gt a 1 with a range from 42 to 121 gt a 1 for the individual drainage systems the mean balance mass flux and depth averaged ice velocity at the cross section aligned with the modeled equilibrium line are 0 1011 gt km 2 a 1 and 0 111 km a 1 respectively with little variation in these values from system to system in contrast the mean mass discharge per unit length along the equilibrium line ranges from one half to double the overall mean rate of 0 0468 gt km 1 a 1 the ratio of the ice mass in the area bounded by the equilibrium line to the rate of mass output implies an effective exchange time of approximately 6 ka for total mass exchange the range of exchange times from a low of 3 ka in the se drainage system to 14 ka in the ne suggests a rank as to which regions of the ice sheet may respond more rapidly to climate fluctuations

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

a comparison of synthetic seismograms for 2d structures semianalytical versus numerical teleseismic wave fields occasionally exhibit rapid changes in travel times and waveforms over distances less than several great circle degrees when observed at broadband arrays these rapid changes in wave field suggest the existence of significant structural transitions occurring over scales of several hundred kilometers or less in the mid and deep mantle although approximate analytical methods based on raytracing can be readily adapted to structures having arbitrarily small scale lengths it is important to validate their accuracy against the predictions of numerical methods here we compare synthetics from an approximate ray based method wkbj modified wkm against the pseudospectral method for a 2d model of the s velocity anomaly associated with the south african plume this model consists of a uniform 3 decrease in s velocity over a broad 10 region of the mid and deep mantle beneath south africa contiguous at its bottom with a thin 100 to 200 km thick zone of low velocity extending 30 westward toward south america along the core mantle boundary transitions between anomalous and radially symmetric structures of the test model are sharp occurring over l0 km or less sv and sh wave fields synthesized by the wkm and pseudospectral methods in this model generally agree with each other well slight mismatches in the two methods can be understood as the result of either differences in model parameterization or the effects of asymptotic approximations in the ray based wkm method

a comparison of balance velocities measured velocities and thermomechanically modelled velocities for the greenland ice sheet balance velocities for the greenland ice sheet have been calculated from a new digital elevation model accumulation rates compilation and anexisting ice thickness grid using a fully two dimensional finite difference scheme the pattern of velocities over the ice sheet is presentedand compared with velocities derived from synthetic aperture radar interferometry for part of northern greenland and a limited number ofgps data this comparison indicated that the balance velocity scheme and boundary conditions used here provide a remarkably goodrepresantation of the dynamics of the ice sheet inland from the margins it is suggested therefore that these balance velocity data couldprovide a valuable method of constraining a numerical ice sheet model the balance velocities were compared with the diagnostic velocityfield calculated from several different configurations of a numerical ice sheet model the general pattern of flow agrees well the detail however is quite different for example the large greater than 300 km ice stream in the north east is not generated by the numerical modeland much of the detailed flow pattern is completely lost due to the limited model resolution and limitations in the model physics

small post perovskite patches at the base of lower mantle primordial reservoirs insights from 2 d numerical modeling and implications for ulvzs we perform numerical experiments of thermochemical mantle convection in 2 d spherical annulus geometry to investigate the distribution of post perovskite ppv with respect to the location of primordial reservoirs of dense material in the lowermost mantle high core mantle boundary temperatures lead to strong anticorrelation between the locations of ppv and large primordial reservoirs while low values lead to a ppv layer fully covering the outer core intermediate values avoid a full ppv layer but allow ppv phase change to occur within the primordial reservoirs through interactions between cold downwellings and primordial reservoirs low viscosity weak ppv leads to the formation of long lived thin tails of primordial materials extending laterally at the edges of these reservoirs small patches of ppv also form within the primordial reservoir but are short lived if primordial reservoirs are enriched in iron these patches may provide an explanation for the ultralow velocity zones

channelized bottom melting and stability of floating ice shelves 1 the floating ice shelf in front of petermann glacier in northwest greenland experiences massive bottom melting that removes 80 of its ice before calving into the arctic ocean detailed surveys of the ice shelf reveal the presence of 1 2 km wide 200 400 m deep sub ice shelf channels aligned with the flow direction and spaced by 5 km we attribute their formation to the bottom melting of ice from warm ocean waters underneath drilling at the center of one of channel only 8 m above sea level confirms the presence of ice shelf melt water in the channel these deep incisions in ice shelf thickness imply a vulnerability to mechanical break up and climate warming of ice shelves that has not been considered previously

lateral variations in cmb heat flux and deep mantle seismic velocity caused by a thermal chemical phase boundary layer in 3d spherical convection article i nfo numerical simulations of thermo chemical multi phase compressible mantle convection in a three dimensional spherical shell are used to investigate the relationship between lateral variations in seismic shear wave velocity vs above the core mantle boundary cmb and lateral variations in heat flux across the cmb qcmb when compositional variations and the post perovskite phase transition are included for simple thermal convection the vs qcmb relationship is reasonably but not perfectly linear the post perovskite transition introduces a non linearity that amplifies fast vs anomalies in cold regions but there is still a unique mapping between vs and qcmb lateral variations in composition such as piles of dense material introduce another non linearity that affects hot upwelling regions and introduces a non uniqueness in vs qcmb if the dense material e g morb is seismically fast compared to the surrounding material in this case dense piles are ringed by sharp low vs anomalies if the cmb is covered by a global dense layer than variations in vs and qcmb are reduced but so is the mean value of qcmb in all cases the peak to peak lateral variation in qcmb is similar to or larger than twice the mean value which might create problems for generating a dynamo according to existing numerical dynamo simulations analytical scalings are developed to explain the observed trends

deeply incised submarine glacial valleys beneath the greenland ice sheet the bed topography beneath the greenland ice sheet controls the flow of ice and its discharge into the ocean a combination of sparse radar soundings of ice thickness and high resolution ice motion data suggest that many submarine ice covered valleys extend significantly deeper below sea level and farther inland than thought

implementation and initial evaluation of the glimmer community ice sheet model in the community earth system model abstractthe glimmer community ice sheet model glimmer cism has been implemented in the community earth system model cesm glimmer cism is forced by a surface mass balance smb computed in multiple elevation classes in the cesm land model and downscaled to the ice sheet grid ice sheet evolution is governed by the shallow ice approximation with thermomechanical coupling and basal sliding this paper describes and evaluates the initial model implementation for the greenland ice sheet gis the ice sheet model was spun up using the smb from a coupled cesm simulation with preindustrial forcing the model s sensitivity to three key ice sheet parameters was explored by running an ensemble of 100 gis simulations to quasi equilibrium and ranking each simulation based on multiple diagnostics with reasonable parameter choices the steady state gis geometry is broadly consistent with observations the simulated ice sheet is too thick and extensive however in some marginal regions where the smb is anomalousl

eemian interglacial reconstructed from a greenland folded ice core efforts to extract a greenland ice core with a complete record of the eemian interglacial 130 000 to 115 000 years ago have until now been unsuccessful the response of the greenland ice sheet to the warmer than present climate of the eemian has thus remained unclear here we present the new north greenland eemian ice drilling neem ice core and show only a modest ice sheet response to the strong warming in the early eemian we reconstructed the eemian record from folded ice using globally homogeneous parameters known from dated greenland and antarctic ice core records on the basis of water stable isotopes neem surface temperatures after the onset of the eemian 126 000 years ago peaked at 8 4 degrees celsius above the mean of the past millennium followed by a gradual cooling that was probably driven by the decreasing summer insolation between 128 000 and 122 000 years ago the thickness of the northwest greenland ice sheet decreased by 400 250 metres reaching surface elevations 122 000 years ago of 130 300 metres lower than the present extensive surface melt occurred at the neem site during the eemian a phenomenon witnessed when melt layers formed again at neem during the exceptional heat of july 2012 with additional warming surface melt might become more common in the future

further evidence for radial velocity anomalies in the lower mantle wright andlyons 1981 used a least squares matching technique lmm and an adaptive processing method adp to study the behaviour of slowness and azimuth measurements made on two synthetic interfering wavelets having different arrival vectors and onset times we have applied these results to the analysis of real array seismograms some of the effects generated synthetically are frequently observed on real seismograms of earthquakes recorded at yellowknife at distances close to 50 and 90 we have also processed sufficient data to illustrate how the interference phenomenon can be used to confirm the presence of radial velocity anomalies in the lower mantle numerousp arrivals from south american earthquakes at distances between 78 and 98 suggest the presence of two radial velocity anomalies at depths close to 2400 and 2730 km below the caribbean region these anomalies also appear to vary laterally

chemical analysis of ice vein microenvironments ii analysis of glacial samples from greenland and antarctica chemical constituents trapped within glacial ice provide a unique record of climate as well as repositories for biological material such as pollen grains fungal spores viruses bacteria and dissolved organic carbon past research suggests that the veins of polycrystalline ice may provide a liquid microenvironment for active microbial metabolism fueled by concentrated impurities in the veins despite these claims no direct measurements of impurity concentration in ice veins have been made using micro raman spectroscopy we show that sulfate and nitrate concentrations in the veins of glacial ice from greenland greenland ice sheet project 2 and antarctic newall glacier and a dominion range glacier core samples were 10 4 and 10 5 times greater than the concentrations measured in melted bulk core water methanesulfonate was not found in the veins consistent with its presence as particulate matter within the ice the measured vein concentration of molecular anions implies a highly acidic ph 3 vein environment with high ionic strength mm m we estimate that the vein volume provides 16 7 and 576 km 3 of habitable space within the greenland and antarctic ice sheets respectively which could support the metabolism of organisms that are capable of growing in cold high ionic strength solutions with low ph

geochemical structure of the hawaiian plume sr nd and os isotopes in the 2 8 km hsdp 2 section of mauna kea volcano 1 sr nd and os isotopic measurements were made on 110 mauna kea lava and hyaloclastite samples from the drillcore retrieved from the second phase of the hawaii scientific drilling project hsdp 2 the samples come from depths of 255 to 3098 meters below sea level span an age range from 200 to about 550 600 kyr and represent an ordered record of the lava output from mauna kea volcano as it drifted a distance of about 40 km over the magma producing region of the hawaiian hot spot the deepest oldest samples represent the time when mauna kea was closest to the center of the melting region of the hawaiian plume the sr and os isotopic ratios in hsdp 2 lavas show only subtle isotopic shifts over the 400 kyr history represented by the core neodymium isotopes nd values increase systematically with decreasing age from an average value of nearly 6 5 to an average value of 7 5 this small change corresponds to subtle shifts in 87sr 86sr and 187os 188os isotope ratios with small shifts of hf a large shift in 208pb 204pb and 208pb 207pb values and with a very large shift in he isotope ratios from r ra values of about 7 8 to values as high as 25 when mauna kea was closest to the plume core the magma source did not have primitive characteristics for nd sr pb hf and os isotopes but did have variable amounts of primitive helium the systematic shifts in nd hf pb and he isotopes are consistent with radial isotopic zoning within the melting region of the plume the melting region constitutes only the innermost highest temperature part of the thermally anomalous plume mantle the different ranges of values observed for each isotopic system and comparison of mauna kea lavas with those of mauna loa suggest that the axial region of the plume which has a radius of 20 km is a mixture of recycled subducted components and primitive lower mantle materials recently combined during the formational stages of the plume at the base of the mantle the proportions of recycled and primitive components are not constant and this requires there be longitudinal vertical heterogeneity within the core of the plume the remainder of the plume outside this plume core zone is less heterogeneous but distinct from upper mantle as represented by mid ocean ridge basalt morb the plume structure may provide a detailed view of mantle isotopic composition near the core mantle boundary

coherent radar ice thickness measurements over the greenland ice sheet we developed two 150 mhz coherent radar depth sounders for ice thickness measurements over the greenland ice sheet we developed one of these using connectorized components and the other using radio frequency integrated circuits rfics both systems are designed to use pulse compression techniques and coherent integration to obtain the high sensitivity required to measure the thickness of more than 4 km of cold ice we used these systems to collect radar data over the interior and margins of the ice sheet and several outlet glaciers we operated both radar systems on the nasa p 3b aircraft equipped with gps receivers radar data are tagged with gps derived location information and are collected in conjunction with laser altimeter measurements we have reduced all data collected since 1993 and derived ice thickness along all flight lines flown in support of program for regional climate assessment parca investigations and the north greenland ice core project radar echograms and derived ice thickness data are placed on a server at the university of kansas http tornado rsl ukans edu greenlanddata htm for easy access by the scientific community we obtained good ice thickness information with an accuracy of 10 m over 90 of the flight lines flown as a part of the parca initiative in this paper we provide a brief description of the system along with samples of data over the interior along the 2000 m contour line in the south and from a few selected outlet glaciers

inhomogeneities near the core mantle boundary inferred from short period scattered pkp waves recorded at the global digital seismograph network we analyze short period pkp precursor wave trains recorded at global digital seismograph network stations in the distance range 120 140 to infer the nature of inhomogeneities near the core mantle boundary travel times and particle motions are consistent with predictions of single scattered waves near the cmb the dominant frequencies are around 1 hz the regions best sampled are beneath indonesia north america central africa and south america based on first order scattering theory the characteristics of the wave number spectrum of the structural inhomogeneities are obtained for two hypotheses volume inhomogeneities and topographic irregularities for the range of wavelengths sampled by these data between 10 and 70 km the spectrum of inhomogeneities is best represented by a power law of index 5 3 for volume inhomogeneities and 6 8 for topographic irregularities if the scattering is due only to volumetric inhomogeneities we estimate the strength of the structural variations to be about 0 5 in p wave velocity for a 200 km thick layer if the scattering is due only to topographic irregularities the topography has an rms height of 280 m at present we can not distinguish between these two types of inhomogeneities

in situ observations of phase transition between perovskite and cairo 3 type phase in mgsio 3 and pyrolitic mantle composition abstract in situ observations of the perovskite cairo3 phase transition in mgsio3 and in pyrolitic compositions were carried out using a laser heated diamond anvil cell interfaced with a synchrotron radiation source for pure mgsio3 the phase boundary between the orthorhombic mg perovskite and cairo3 type phases in the temperature range of 1300 3100 k was determined to be p gpa 130 3 0 0070 0 0030 t 2500 k using platinum as a pressure calibrant we confirmed that the cairo3 type phase remained stable up to pressures of at least 156 gpa and temperatures of 2600 k the consistency of our results with previous theoretical calculations leads us to conclude that the 2700 km seismic discontinuity at the bottom of the lower mantle can be attributed to a phase transition to the cairo3 type phase the phase change from an orthorhombic mg perovskite to a cairo3 type bearing assemblage in a pyrolitic mantle composition was also observed at p 125 gpa which corresponds to the same mantle depth as the seismic discontinuity the phase boundary between the orthorhombic mg perovskite and cairo3 type bearing assemblage was determined to be p gpa 124 4 0 008 0 005 t 2500 k using gold as a pressure calibrant this transition boundary indicates that the temperature at a depth of 2700 km is about 2600 k and the adiabatic temperature gradient in the lower mantle is estimated to be 0 31 k km the partition coefficients and the effect of some elements on the phase equilibrium between the orthorhombic mgsio3 perovskite and cairo3 type mgsio3 were estimated from ab initio calculations our experimental and theoretical results indicate that the d layer consists of a cairo3 type bearing assemblage which is likely to have significant effect on the chemical and thermal evolution of the earth s mantle

effect of pressure and composition on lattice parameters and unit cell volume of fe mg sio3 post perovskite we obtained unit cell volumes v for fe sub 0 2 mg sub 0 8 sio sub 3 and fe sub 0 4 mg sub 0 6 sio sub 3 post perovskite ppv over a pressure p range of 80 145 gpa by in situ x ray diffraction xrd measurements in laser heated diamond anvil cells dacs our study suggests that the ppv structure is highly resistant to back transformation to pv and fe sub 0 4 mg sub 0 6 sio sub 3 ppv was preserved down to 80 gpa upon laser heating during decompression before transforming to pure perovskite pv the bulk modulus k sub 0 of ppv was determined to be 215 1 and 198 1 gpa for fe sub 0 2 mg sub 0 8 sio sub 3 and fe sub 0 4 mg sub 0 6 sio sub 3 ppv respectively with a fixed k sub 0 4 the substitution of fe for mg in ppv results in the largest expansion along the b axis among all three axes and the expansion of the b axis is nearly independent of pressure whereas the expansion along the a and c directions slightly decreases with increasing pressure combining our results with previous studies on fe sub x mg sub 1 x sio sub 3 ppv x 0 0 09 0 1 shows that the bulk modulus of ppv decreases with increasing fe content over a composition range x 0 to 0 4 varying from 231 gpa for x 0more to 198 gpa for x 0 4 with zero pressure volumes scaled to composition v sub 0 angstrom sup 3 162 2 19 5x and fixed k sub 0 4 0 the increase of fe content in ppv from x 0 to 0 4 results in a 6 3 increase in density and a 5 3 decrease in bulk sound velocity at 130 gpa less

an improved coherent radar depth sounder the university of kansas developed a coherent radar depth sounder during the 1980s this system was originally developed for glacial ice thickness measurements in the antarctic during the field tests in the antarctic and greenland we found the system performance to be less than optimum the field tests in greenland were performed in 1993 as a part of the nasa program for arctic climate assessment parca we redesigned and rebuilt this system to improve the performance the radar uses pulse compression and coherent signal processing to obtain high sensitivity and fine along track resolution it operates at a center frequency of 150 mhz with a radio frequency bandwidth of about 17 mhz which gives a range resolution of about 5 m in ice we have been operating it from a nasa p 3 aircraft for collecting ice thickness data in conjunction with laser surface elevation measurements over the greenland ice sheet during the last 4 years we have demonstrated that this radar can measure the thickness of more than 3 km of cold ice and can obtain ice thickness information over outlet glaciers and ice margins in this paper we provide a brief survey of radar sounding of glacial ice followed by a description of the system and subsystem design and performance we also show sample results from the field experiments over the greenland ice sheet and its outlet glacicrs

a shear velocity discontinuity in the lower mantle a lower mantle s wave triplication is detected using short and long period sh seismograms in the distance range 70 sup 0 to 95 sup 0 modeling of the observations with synthetic seismograms indicates that the triplication is produced by a 2 75 0 25 shear velocity increase about 280 km above the core mantle boundary the sh data from intermediate and deep focus events for three distinct source region receiver array combinations show generally consistent travel time and relative amplitude behavior of the triplication there are small systematic travel time shifts between the source regions which suggest regional variations in the depth of the discontinuity of up to 40 km

a new ice thickness and bed data set for the greenland ice sheet 2 relationship between dynamics and basal topography new data sets of bed and surface elevation have been combined with balance velocities bed topography and radar reflectivity to investigate the relationship between ice flow and bed conditions it has been found that the bed topography affects the local driving stresses and initiates the onset of streaming flow in specific areas where there are large undulations this onset region only appears significantly inland in three areas the northeast greenland ice stream kangerdlugssuaq gletscher and a region in the southwest associated with abnormally low driving stresses and an apparently unnamed but active glacier bed reflectance values below the northeast ice stream suggest that basal melting could be occurring along its length and it seems likely therefore that the enhanced flow observed inland from the onset of streaming may be due in part to basal sliding

numerical modeling of iceberg capsizing responsible for glacial earthquakes the capsizing of icebergs calved from marine terminating glaciers generate horizontal forces on the glacier front producing long period seismic signals referred to as glacial earthquakes these forces can be estimated by broadband seismic inversion but their interpretation in terms of magnitude and waveform variability is not straightforward we present a numerical model for fluid drag that can be used to study buoyancy driven iceberg capsize dynamics and the generated contact forces on a calving face using the finite element approach we investigate the sensitivity of the force to drag effects iceberg geometry calving style and initial buoyancy we show that there is no simple relationship between force amplitude and iceberg volume and similar force magnitudes can be reached for different iceberg sizes the force history and spectral content varies with the iceberg attributes the iceberg aspect ratio primarily controls the capsize dynamics the force shape and force frequency whereas the iceberg height has a stronger impact on the force magnitude iceberg hydrostatic imbalance generates contact forces with specific frequency peaks that explain the variability in glacial earthquake dominant frequency for similar icebergs top out and bottom out events have significantly different capsize dynamics leading to larger top out forces especially for thin icebergs for realistic iceberg dimensions we find contact force magnitudes that range between 5 6 1011 and 2 1014 kg m consistent with seismic observations this study provides a useful framework for interpreting glacial earthquake sources and estimating the ice mass loss from coupled analysis of seismic signals and modeling results

laterally varying reflector at the top of d beneath northern siberia summary we analyse p wave data from the french laboratoire de dctection et gcophysique l d g network of digital short period seismic stations whereas our previous paper houard nataf 1992 showed a few promising kurd events and depicted the methods we proposed to apply an extensive analysis of 32 earthquakes from the kurivkamchatka region has now been carried out by using the deconvolution technique previously described houard nataf 1992 clear and quantitative evidence of intermediate arrivals between the p and pcp waves is shown in the 75 82 distance range our approach is complementary to previous studies in the same region which used broad band data recorded at a dense narrow aperture array weber davis 1990 weber 1993 or long period data recorded at the wwssn stations gaherty lay 1992 for each station record sections of deconvolved signals from different earthquakes are built and lead us to eliminate crustal reverberations beneath l d g stations as a major explanation for these secondary arrivals a compilation of the most conclusive signals has been made it is perhaps one of the most complete pdp wave record sections since the move out with distance can be followed clearly with a good sampling over more than 5 the move out with distance of this extra pdp arrival its residual slowness value eliminates source complexities diffraction or multipathing through the kuril subduction slab as possible explanations the extra arrivals indicate the existence of a lower mantle reflector the structure of the lay discontinuity is investigated using tpdp tp residual traveltimes two different residual time versus distance tendencies are found which correlate to different geographical bounce point regions indicating the existence of lateral variations of the structure the d region is also sampled further south at distances well beyond the theoretical triplication cross over by analysing events from the honshu ryukyu islands region but only one p wave branch is observed the hodochrones and p a curves of the first p arrival are used as a complementary analysis the global prem model dziewonski anderson 1981 predicts correct p wave slownesses though its smooth lower mantle structure does not account for pdp observations this may be an indication that the lay discontinuity is a global feature however the hodochrones of the first p arrival do not allow us to test the existence of the discontinuity in the 82 85 cross over distance range

molecular characterization of dissolved organic matter associated with the greenland ice sheet abstract subsurface microbial oxidation of overridden soils and vegetation beneath glaciers and ice sheets may affect global carbon budgets on glacial interglacial timescales the likelihood and magnitude of this process depends on the chemical nature and reactivity of the subglacial organic carbon stores we examined the composition of carbon pools associated with different regions of the greenland ice sheet subglacial supraglacial proglacial in order to elucidate the type of dissolved organic matter dom present in the subglacial discharge over a melt season electrospray ionization esi fourier transform ion cyclotron resonance ft icr mass spectrometry coupled to multivariate statistics permitted unprecedented molecular level characterization of this material and revealed that carbon pools associated with discrete glacial regions are comprised of different compound classes specifically a larger proportion of protein like compounds were observed in the supraglacial samples and in the early melt season spring subglacial discharge in contrast the late melt season summer subglacial discharge contained a greater fraction of lignin like and other material presumably derived from underlying vegetation and soil these results suggest 1 that the majority of supraglacial dom originates from autochthonous microbial processes on the ice sheet surface 2 that the subglacial dom contains allochthonous carbon derived from overridden soils and vegetation as well as autochthonous carbon derived from in situ microbial metabolism and 3 that the relative contribution of allochthonous and autochthonous material in subglacial discharge varies during the melt season these conclusions are consistent with the hypothesis that given sufficient time e g overwinter storage resident subglacial microbial communities may oxidize terrestrial material beneath the greenland ice sheet

mechanisms of al3 incorporation in mgsio3 post perovskite at high pressures abstract aluminum is the fifth most abundant element in the earth s mantle yet its effect on the physical properties of the newly found mgsio3 post perovskite ppv the major mineral of the earth s d layer is not fully known in this paper large scale ab initio simulations based on density functional theory dft within the generalized gradient approximation gga have been carried out in order to investigate the substitution mechanism of al3 into ppv at high pressures we have examined three types of al substitution in ppv 6 25 mol al substitution via a charge coupled mechanism ccm 6 25 mol al substitution via oxygen vacancy mechanism ovm and an oxygen vacancy si free end member mg2al2o5 for both the ccm and ovm five models where the al atoms were put in different positions were simulated at various pressures in the range 10 150 gpa our calculations show that the most favorable mechanism is a charge coupled substitution where al3 replaces the next nearest neighbor cation pairs in the ppv structure the calculated zero pressure bulk modulus of al bearing ppv is 3 15 lower than that of the al free ppv in agreement with previous works we find that the incorporation of al2o3 slightly increases the post perovskite phase transition pressure with the al partition coefficient k 2 67 at 120 gpa and 3000 k

steady state characteristics of the greenland ice sheet under different climates the greenland ice sheet is modelled to simulate its extent and volume in warmer climates and also to find out whether the ice sheet would re form on theice free bedrock under present climatic conditions the ice sheet model is a three dimensional thermo mechanical model with a fine resolution grid thebedrock surface beneath the ice sheet was mapped using radio echo sounding measurements by the electromagnetic institute copenhagen the modelexperiments show that increased temperature will result in ice margin retreat but the ice sheet is relatively stable it takes a rather high temperature rise of atleast 6ic for the ice sheet to disappear completely which indicates that the ice sheet probably survived the last interglacial also it appears that the greenlandice sheet is not a mere relict ice mass from a previously colder climate but that the ice sheet will still re form on the bare bedrock under the present or evenslightly warmer climatic conditions

imaging the lowermost mantle d and the core mantle boundary with skks coda waves summary in our previous studies we developed a method for imaging heterogeneity at and near the core mantle boundary cmb with a generalized radon transform grt of transverse component broad band scs data and we developed a statistical model for producing images of the d discontinuity with variable confidence levels in these applications the background is smooth and perturbations are represented as contrasts here we extend the theory to allow known discontinuities such as the cmb in the background model the resulting imaging operator which is formally not a grt can be used either alone or along with scs for the imaging of lowermost mantle structure and in particular the d discontinuity with the scattered skks wavefield synthetic seismograms calculated with the wkbj method are used to test the performance of our approach as a proof of concept we transform 38 000 radial component skks waveforms into image gathers of a cmb patch beneath central america the skks image gathers and image traces are in good agreement with the image traces obtained from the grt transform of scs data

comment on reproducing earth s kernel uncertainty of the shape of the core mantle boundary from pkp and pcp travel times by p b stark and n w hengartner

a deep seismic investigation of the flemish cap margin implications for the origin of deep reflectivity and evidence for asymmetric break up between newfoundland and iberia summary seismic reflection and refraction data were acquired along the southeast margin of flemish cap at a position conjugate to drilling and geophysical surveys across the galicia bank margin the data document first order asymmetry during final break up between newfoundland and iberia an abrupt necking profile of continental crust observed off flemish cap contrasts strongly with gradual tapering on the conjugate margin there is no evidence beneath flemish cap for a final phase of continental extension that resulted in thin continental crust underlain by a strong s like reflection which indicates that this mode of extension occurred only on the galicia bank margin compelling evidence for a broad zone of exhumed mantle or for peridotite ridges is also lacking along the flemish cap margin instead anomalously thin 3 4 km thick oceanic crust is observed this crust is highly tectonized and broken up by high angle normal faulting the thin crust and rift structures that resemble the abandoned spreading centre in the labrador sea suggest that initial seafloor spreading was affected by processes observed in present day ultra slow spreading environments landwards flemish cap is underlain by a highly reflective lower crust the reflectivity most likely originates from older palaeozoic orogenic structures that are unrelated to extension and break up tectonics

radio echo sounding and ice volume estimates of western nordenskiold land glaciers svalbard as part of ongoing work to obtain a reliable estimate of the total ice volume of svalbard glaciers and their potential contribution to sea level rise we present here volume calculations with detailed error estimates for ten glaciers on western nordenskiold land central spitsbergen svalbard the volume estimates are based upon a dense net of gpr retrieved ice thickness data collected over several field campaigns spanning the period 1999 2012 the total area and volume of the ensemble are 116 06 4 53 km 2 and 10 439 0 373 km 3 respectively while the individual areas volumes and average ice thickness lie within 2 6 50 4 km 2 0 08 5 54 km 3 and 29 108 m respectively volume area scaling relationships overestimate the total volume of these glaciers by up to 35 with respect to our calculation on the basis of the pattern of scattering in the radargrams we also analyse the hydrothermal structure of these glaciers nine of the ten are polythermal while only one is entirely cold

geophysically consistent values of the perovskite to post perovskite transition clapeyron slope the double crossing hypothesis posits that post perovskite bearing rock in earth s d layer exists as a layer above the core mantle boundary bounded above and below by intersections between a curved thermal boundary layer geotherm and a relatively steep phase boundary increasing seismic evidence for the existence of pairs of discontinuities predicted to occur at the top and bottom of this layer motivates an examination of the consistency of this model with mineral physics constraints for the clapeyron slope of this phase transition using independent constraints for a lower bound on temperature in earth s deep mantle and the temperature of earth s inner core boundary we show that a post perovskite double crossing is inconsistent with plausible core temperatures for a clapeyron slope less than about 7 mpa k with the higher range of experimental values yielding better agreement with recent estimates of the melting temperature of earth s core

forward modeling of ice topography on mars to infer basal shear stress conditions 1 understanding the history of ice caps on mars could reveal important information about martian geologic and climatic history to do this an ice reconstruction model is needed that operates over complex topography and can be constrained with a limited number of free parameters in this study we developed a threshold sliding model for ice cap morphology based on the classic model of nye later incorporated into the models of reeh and colleagues we have updated the nye reeh model with a new numerical algorithm although the model was originally developed to model perfectly plastic deformation it is applicable to any ice body that deforms when a threshold basal shear stress is exceeded the model requires three inputs a digital elevation model of bed topography a mask grid that defines the position of the ice terminus and a function defining the threshold basal shear stress to test the robustness of the model the morphology of the greenland ice sheet is reconstructed using an empirical equation between threshold basal shear stress and ice surface slope the model is then used to reconstruct the morphology of ice draping impact craters on the margins of the south polar layered deposits using an inferred constant basal shear stress of 0 6 bar for the majority of the examples this inferred basal shear stress value is almost 1 3 of the average basal shear stress calculated for the greenland ice sheet what causes this lower basal shear stress value on mars is unclear but could involve the strain weakening behavior of ice

s velocities in d from diffracted sh waves at the core boundary summary the slowness of the diffracted sh wave around the core boundary is re examined the criticism that measurements from the iranian earthquake of 1968 august 31 are affected by the focal mechanism is shown to be unjustified theoretically and by comparison with analogous recordings from the imperial valley earthquake of 1979 october 15 evidence is presented that an sh ray parameter estimated from first impetus sh diffracted onsets does not represent a shear velocity averaged over a hundred kilometres depth but rather the velocity within a few tens of kilometres of the core mantle boundary in addition to a re examination of the seismograms previously used for the 1968 august 3 1 iran earthquake long period seismograms for 80 150 have been studied for two tonga earthquakes 1967 october 9 and 1977 august 11 exclusion of readings with 105 from the iranian earthquake increases the gradient from 8 68 0 13 to 9 48 0 26s deg 1 the exceptionally large ray parameter is not fully explained but a plausible hypothesis is anisotropy in shear velocity near the core mantle boundary when observations are restricted to clear arrivals at distances exceeding a few degrees from the shadow boundary reliable new estimates are obtained for the sh slowness namely 8 41 0 07s deg 1 for tonga 1967 and 8 39 0 06 for tonga 1977 the statistical uncertainty of the estimates indicates no significant average change in sh velocity at the base of the mantle along two easterly profiles 3000 km apart under the east pacific and north america for a core radius of 3485 km and a slowness of 8 4s dec 1 the average velocity near the core mantle boundary is 7 24km s 1 this value is in agreement with estimates in d by mula muller the observed attenuation of the diffracted sh ground motion amplitudes at periods of 15 20s is similar for the three profiles considered with a reduction of an order of magnitude within a distance of 25 in the shadow zone this corresponds to an equivalent decay parameter 0 10 at the core boundary

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

greenland ice sheet surface melt and its relation to daily atmospheric conditions abstractmelt area is one of the most reliably monitored variables associated with surface conditions over the full greenland ice sheet gris surface melt is also an important indicator of surface mass balance and has potential relevance to the ice sheet s global sea level contribution melt events are known to be spatially heterogeneous and have varying time scales to understand the forcing mechanisms it is necessary to examine the relation between the existing conditions and melt area on the time scales that melt is observed here we conduct a regression analysis of atmospheric reanalysis variables including sea level pressure near surface winds and components of the surface energy budget with surface melt the regression analysis finds spatial heterogeneity in the associated atmospheric circulation conditions for basins in the southern gris there is an association between melt area and high pressure located south of the denmark strait which allows for southerly flow over the western half of th

a new high resolution digital elevation model of greenland fully validated with airborne laser altimeter data a new digital elevation model of the greenland ice sheet and surrounding rock outcrops has been produced at 1 km postings from a comprehensive suite of satellite remote sensing and cartographic data sets height data over the ice sheet were mainly from ers 1 and geosat radar altimetry these data were corrected for a slope dependent bias that had been identified in a previous study the radar altimetry was supplemented with stereophotogrammetric data sets synthetic aperture radar interferometry and digitized cartographic maps over regions of bare rock and where gaps in the satellite altimeter coverage existed the data were interpolated onto a regular grid with a spacing of 1 km the accuracy of the resultant digital elevation model over the ice sheet was assessed using independent and spatially extensive measurements from an airborne laser altimeter that had an accuracy of between 10 and 12 cm in a comparison with the laser altimetry the digital elevation model was found to have a slope dependent accuracy ranging from 1 04 1 98 m to 0 06 14 33 m over the ice sheet for a slope range of 0 0 1 0 the mean accuracy over the whole ice sheet was 0 33 6 97 m over the bare rock areas the accuracy ranged from 20 to 200 m dependent on the data source available the new digital elevation model was used as an input data set for a positive degree day model of ablation the new elevation model was found to reduce ablation by only 2 compared with using an older 2 5 km resolution model which suggests that resolution induced errors in estimating ablation are no longer important

seasonal variations in greenland ice sheet motion inland extent and behaviour at higher elevations abstract we present global positioning system observations that capture the full inland extent of ice motion variations in 2009 along a transect in the west greenland ice sheet margin in situ measurements of air temperature and surface ablation and satellite monitoring of ice surface albedo and supraglacial lake drainage are used to investigate hydrological controls on ice velocity changes we find a strong positive correlation between rates of annual ablation and changes in annual ice motion along the transect with sites nearest the ice sheet margin experiencing greater annual variations in ice motion 15 18 than those above 1000 m elevation 3 8 patterns in the timing and rate of meltwater delivery to the ice bed interface provide key controls on the magnitude of hydrologically forced velocity variations at each site in the lower ablation zone the overall contribution of variations in ice motion to annual flow rates is limited by evolution in the structure of the subglacial drainage system at sites in the upper ablation zone a shorter period of summer melting and delayed establishment of a hydraulic connection between the ice sheet surface and its bed limit the timeframe for velocity variations to occur our data suggest that land terminating sections of the greenland ice sheet will experience increased dynamic mass loss in a warmer climate as the behaviour that we observe in the lower ablation zone propagates further inland findings from this study provide a conceptual framework to understand the impact of hydrologically forced velocity variations on the future mass balance of land terminating sections of the greenland ice sheet

complexity of d in the presence of slab debris and phase changes 1 a lower mantle s wave triplication with a scd branch is easily observed beneath central american and its lateral variations has been studied extensively attempts to model scd depth variations with a phase boundary based on tomography predict smooth variations e g sidorin et al 1999a recently hutko et al 2005 presents evidence of a sharper feature with a vertical step like jump of over 50 km for paths sampling the western edge of a fast lower mantle structure beneath the cocos plate we present a similar type of very anomalous scd waveforms sampling the edge further to the east we are able to model these waveforms by adopting the phase boundary model but enhancing the existing tomographic anomaly just above the velocity jump which appears to be a folded slab

localized velocity anomalies in the lower mantle summary two localized regions of velocity heterogeneity in the lower mantle with scale lengths of 1000 2000 km and 2 per cent velocity contrasts are detected and isolated through comparison of s scs p and pcp travel times and amplitudes from deep earthquakes in peru bolivia argentina and the sea of okhotsk comparison of the relative patterns of scs s differential travel times and s travel time residuals across north american wwssn and csn stations for the different source regions provides baselines for interpreting which phases have anomalous times a region of low s and p velocities is located beneath northern brazil and venezuela at depths of 1700 2700 km this region produces s wave delays of up to 4 s for signals from deep argentine events recorded at eastern north american stations the localized nature of the anomaly is indicated by the narrow bounds in azimuth 15 and take off angle 13 of the arrivals affected by it the long period s waves encountering this anomaly generally show 30 100 per cent amplitude enhancement while the short period amplitudes show no obvious effect the second anomaly is a high velocity region beneath the caribbean originally detected by jordan and lynn who used travel times from deep peruvian events the data from argentine and bolivian events presented here constrain the location of the anomaly quite well and indicate a possible short and long period s wave amplitude diminution associated with it when the travel time data are corrected for the estimated effects of these two anomalies a systematic regional variation in scs s station residuals is apparent between stations east of and west of the rocky mountains one possible explanation of this is a long wavelength lateral variation in the shear velocity structure of the lower mantle at depths greater than 2000 km beneath north america

using internal layers from the greenland ice sheet identified from radio echo sounding data with numerical models spatially extensive internal layers have been traced in airborne radio echo sounding res data collected over greenland during the late 1990s by linking internal layers within individual flight lines at crossover points it is possible to identify spatiallycontinuouslayersthatareinterpretedas isochronoussurfaces severalof thesur vey lines pass over the grip core site and this allows us to use the published gripage depth relationship to accurately date these surfaces two layers with ages of 3891 and 6956years bp have been traced over a large part of north greenland accurately dated and spatially continuous isochrones are valuable for both assimilation within and verifi cation of numerical models forexample comparison of isochronous surfaces from a nu merical simulation with those layers observed in res data can be used to inform the choice of parameters e g rheology and climate history usedto force a numerical model to demonstrate the potential of the res data two layers for north greenlandwere used to determine palaeo accumulation rates the inversion from layer depthto accumulation rate requires a three dimensional velocity field this velocity field is constructed by com biningatwo dimensionalbalance velocity fieldwithanassumedverticalstructureforthe horizontal velocity the isochronous layer derived accumulation rates were compared with the bales and others 2001 rates a larger east west gradient was found across the central ice divide for the derived accumulation rate suggesting a trend in the holocene accumulation rates for this region the layers were also compared with isochronous sur faces derived from simulations of a three dimensional thermodynamic ice sheet model using the isochronous layer derived accumulation rates to force the model improved the matchbetween modelled and observed layers

horizontal transition from fast to slow structures at the core mantle boundary south atlantic recent tomographic studies of the deep mantle have revealed large scale low velocity zones beneath africa and relatively fast velocity zones beneath south america here we conduct a concentrated core mantle boundary cmb study across this transition zone from fast to slow beneath the south atlantic deep south american earthquakes recorded in africa provide s and scs waveform data at core grazing distances of 85 105 the waveform interference pattern produced at these ranges provides an excellent sample of the cmb structure in the vicinity of the scs bounce point which can be used to map out regions of low velocity zones 10 reductions the strongest anomalies occur at the western edge roughly beneath the tristat plateau with weaker structures to the east beneath the so called great africa plume these results agree with dynamic models where strong local upwellings develop at the edges of d fast structures

the p wave velocity structure near the base of the mantle travel times from 16 events recorded at two temporary networks of short period seismometers deployed across australia have been used to study the p wave velocity structure of the mantle below depths of 1700 km the slowness distance curve derived from these times shows small deviations from the slownesses predicted from the preliminary reference earth model of dziewonski and anderson 1981 that may be significant the most important feature is a rapid decrease of more than 0 5 s degree between distances of 85 and 88 velocity filtering of seismograms from 5 nuclear explosions in kazakh recorded at these australian networks indicates the presence of two arrival branches at distances between 83 and 91 whose slownesses differ by 0 40 to 0 55 s degree and which intersect at a distance of 87 processed p wave seismograms from the yellowknife array in canada also indicate the presence of a similar phenomenon in the same distance range in two separate regions the results can be explained by a widespread rapid or sharp increase in velocity of 2 5 to 3 0 about 200 km above the core mantle boundary

traveltime sensitivity kernels for pkp phases in the mantle abstract we investigate the finite frequency effects of perturbations of compressional wave velocity on the traveltimes of pkp phases owing to their long paths in the mantle pkp phases have a large fresnel zone at the cmb and thus sample a large volume of the d layer we compute traveltime frechet derivatives of the three pkp phases in the mantle with respect to the ak135 reference model taking into account the coupling between the different branches pkp kernels do not have a simple banana doughnut shape because the different pkp branches interfere with each other and because of the presence of shadow zones and caustics in addition we compute the differential kernels ab bc bc df and ab df the geometry of pkp differential kernels in d show significant qualitative differences from those predicted by simple ray tracing this suggests that the tomographic techniques commonly used to image lower mantle structures could be improved by using 3d frechet kernels

global synthetic seismograms using a 2 d finite difference method two dimensional 2 d finite difference fd synthetics which fill the gap between fast 1 d analytic synthetics and time consuming full 3 d synthetics in our ability to model seismograms have been used in many studies we address several issues involving 2 d fd methods in generating global synthetic seismograms these include 1 interfacing point source excitation for earthquakes with 2 d fd methods 2 out of plane spreading corrections and 3 reducing the spherical earth to the flattened models the first issue is tackled using two methods a transparent source box approach and a moment tensor excitation approach where each has its own advantages moreover our source box excitation does not have the late time drift problem that occurred in previous studies the out of plane geometric spreading correction is accounted for by estimating the ray parameter and applying a post simulation filter to 2 d synthetics finally parameters of the earth flattening transformation are discussed and validated the effectiveness of this method is demonstrated by comparing our synthetics with frequency wavenumber summation normal mode and 3 d spectral element synthetics

interpreting d seismic structure using synthetic waveforms computed from dynamic models we have formulated dynamic models of processes hypothesized to give rise to observed seismological structure at the base of the mantle one dimensional seismic models are determined from thermo chemical convection calculations from which synthetic waveforms are computed of the three scenarios studied 1 a thermal slab 2 a thermal slab interacting with simple chemical layer and 3 a thermal slab interacting with a phase transition the strength of seismic phases computed from 3 best correlates with the global geography of the observed d triplication seismic structures predicted by 1 or 2 are inconsistent with the global geography and strength of the triplication our successful model motivates a new class of 1 d seismic models with two mild gradients and a small discontinuity to test this hypothesis we searched for seismic models that both contain these features and fit d seismic observations our preferred model contains a gentle positive gradient initiated 350 km above the cmb and a 1 jump in s velocity near 200 km a strong negative gradient begins about 100 km above the cmb similar to previous studies representing the lower thermal boundary layer with the reduced first order discontinuity the p wave triplication becomes too small to observe and provides a simple explanation for the observed weakness of the p in proportion to the s triplication

history of the greenland ice sheet paleoclimatic insights abstract paleoclimatic records show that the greenland ice sheet consistently has lost mass in response to warming and grown in response to cooling such changes have occurred even at times of slow or zero sea level change so changing sea level cannot have been the cause of at least some of the ice sheet changes in contrast there are no documented major ice sheet changes that occurred independent of temperature changes moreover snowfall has increased when the climate warmed but the ice sheet lost mass nonetheless increased accumulation in the ice sheet s center has not been sufficient to counteract increased melting and flow near the edges most documented forcings and ice sheet responses spanned periods of several thousand years but limited data also show rapid response to rapid forcings in particular regions near the ice margin have responded within decades however major changes of central regions of the ice sheet are thought to require centuries to millennia the paleoclimatic record does not yet strongly constrain how rapidly a major shrinkage or nearly complete loss of the ice sheet could occur the evidence suggests nearly total ice sheet loss may result from warming of more than a few degrees above mean 20th century values but this threshold is poorly defined perhaps as little as 2 c or more than 7 c paleoclimatic records are sufficiently sketchy that the ice sheet may have grown temporarily in response to warming or changes may have been induced by factors other than temperature without having been recorded

sea surface temperature control on the distribution of far traveled southern ocean ice rafted detritus during the pliocene the flux and provenance of ice rafted detritus ird deposited in the southern ocean can reveal information about the past instability of antarctica s ice sheets during different climatic conditions here we present a pliocene ird provenance record based on the 40ar 39ar ages of ice rafted hornblende grains from ocean drilling program site 1165 located near prydz bay in the indian ocean sector of the southern ocean along with the results of modeled sensitivity tests of iceberg trajectories and their spatial melting patterns under a range of sea surface temperatures ssts our provenance results reveal that ird and hence icebergs in the prydz bay area were mainly sourced from i the local prydz bay region and ii the remote wilkes land margin located at the mouth of the low lying aurora subglacial basin a series of ird pulses reaching up to 10 times background ird flux levels were previously identified at site 1165 between 3 3 and 3 0 ma our new results reveal that the average proportion of ird sourced from distal wilkes land margin doubles after 3 3 ma our iceberg trajectory melting models show that slower iceberg melting under cooling ssts over this middle pliocene interval allowed wilkes land icebergs to travel farther before melting hence declining ssts can account for a large part of the observed ird provenance record at site 1165 in early pliocene ird layers sampled at suborbital resolution around 4 6 ma we find evidence for significant increases in icebergs derived from wilkes land during very warm interglacials this is suggestive of large scale destabilization of the east antarctic ice sheet in the aurora subglacial basin as far traveled icebergs would have to overcome enhanced melting in warmer ssts our results highlight the importance of considering ssts when interpreting ird flux and provenance records in distal locations

statistical study of seismic heterogeneities at the base of the mantle from pkp differential traveltimes summary stochastic tomography can provide crucial information on the small scale structures at the base of the mantle that are still beyond the reach of deterministic imaging for this purpose we relate differential traveltime variances of core phases to the correlation function of velocity heterogeneities at the base of the mantle a global data set of pkp traveltimes is then used to invert for statistical properties of velocity perturbations in the d layer we find that the average thickness of d is 350 50 km with 1 2 0 3 per cent rms velocity perturbations the horizontal correlation length is 3 1 and the vertical correlation length 150 75 km the statistical analysis of the traveltimes of core phases reveals a much stronger energy at short wavelengths compared to global tomographic models which is in good agreement with the results of most studies of the seismic wavefield scattered at the base of the mantle based upon the analysis of pkp precursors

late cretaceous neogene trends in deep ocean temperature and continental ice volume reconciling records of benthic foraminiferal geochemistry δ18o and mg ca with sea level history 1 we reconstruct trends in ice volume and deep ocean temperature for the past 108 myr resolving variations on timescales of 2 myr and longer we use a sea level record as a proxy for ice volume a benthic foraminiferal mg cabf record as a proxy for temperature and a benthic foraminiferal 18obf record as a proxy for both this allows us to construct dual estimates of temperature and ice volume variations for the interval 10 60 ma extracting temperature from 18obf by using sea level as a proxy for ice volume to constrain the 18osw component and extracting seawater 18osw which reflects ice volume from 18obf by using mg cabf to constrain the temperature component each of these approaches requires numerous assumptions but the range of plausible solutions are concordant on timescales 2 myr and within an uncertainty of 2 c temperature and 0 4 18osw the agreement between the two approaches for the last 50 myr provides empirical justification for the use of 18obf mg cabf and sea level records as robust climate proxies our reconstructions indicate differences between deep ocean cooling and continental ice growth in the late cenozoic cooling occurred gradually in the middle late eocene and late miocene pliocene while ice growth occurred rapidly in the earliest oligocene middle miocene and plio pleistocene these differences are consistent with climate models that imply that temperatures set by the long term co2 equilibrium should change only gradually on timescales 2 myr but growth of continental ice sheets may be rapid in response to climate thresholds due to feedbacks that are not yet fully understood

high resolution ice thickness mapping in south greenland airborne radar sounding is difficult in south greenland because of the presence of englacial water which prevents the signal from reaching the bed data coverage remains suboptimal for traditional methods of ice thickness and bed mapping that rely on geostatistical techniques such as kriging because important features are missing here we apply two alternative approaches of high resolution 300 m ice thickness mapping that are based on the conservation of mass to two regions of south greenland 1 qooqqup sermia and kiattuut sermiat and 2 ikertivaq these two algorithms solve optimization problems for which the conservation of mass is either enforced as a hard constraint or as a soft constraint for the first region very few measurements are available but there is no gap in ice motion data whereas for ikertivaq more ice thickness measurements are available but there are gaps in ice motion data we show that mass conservation algorithms can be used as validation tools for radar sounding we also show that it is preferable to apply mass conservation as a hard constraint rather than a soft constraint as it better preserves elongated features such as glacial valleys and ridges

calibrating a glaciological model of the greenland ice sheet from the last glacial maximum to present day using field observations of relative sea level and ice extent we constrain a three dimensional thermomechanical model of greenland ice sheet gris evolution from the last glacial maximum lgm 21 ka bp to the present day using primarily observations of relative sea level rsl as well as field data on past ice extent our new model huy2 fits a majority of the observations and is characterised by a number of key features i the ice sheet had an excess volume relative to present of 4 1 m ice equivalent sea level at the lgm which increased to reach a maximum value of 4 6 m at 16 5 ka bp ii retreat from the continental shelf was not continuous around the entire margin as there was a younger dryas readvance in some areas the final episode of marine retreat was rapid and relatively late c 12 ka bp leaving the ice sheet land based by 10 ka bp iii in response to the holocene thermal maximum htm the ice margin retreated behind its present day position by up to 80 km in the southwest 20 km in the south and 80 km in a small area of the northeast as a result of this retreat the modelled ice sheet reaches a minimum extent between 5 and 4 ka bp which corresponds to a deficit volume relative to present of 0 17 m ice equivalent sea level our results suggest that remaining discrepancies between the model and the observations are likely associated with non greenland ice load differences between modelled and observed present day ice elevation around the margin lateral variations in earth structure and or the pattern of ice margin retreat

reconciling the ice 6g_c reconstruction of glacial chronology with ice sheet dynamics the cases of greenland and antarctica we describe a theoretical and numerical framework that has been developed to investigate the compatibility of the ice 6g c reconstruction of the glaciation histories of the greenland and antarctic ice sheets with the latest understanding of ice physics the ice 6g c reconstruction has been produced solely on the basis of the theory of the glacial isostatic adjustment gia process and it has remained an issue as to whether such reconstructions of the time dependent thickness variations of grounded continental ice sheets were compatible with physics based ice mechanical considerations our analyses focus on the evolution over the last glacial cycle of these extent ice sheet complexes and demonstrate that the gia inferred models are entirely consistent with such considerations when uncertainties in net mass balance history are taken fully into account

velocity structure of the mantle transition zone beneath the southeastern margin of the tibetan plateau abstract p wave triplications related to the 410 km discontinuity the 410 were clearly observed from the vertical component seismograms of three intermediate depth earthquakes that occurred in the indo burma subduction zone ibsz and were recorded by the chinese digital seismic network cdsn by matching the observed p wave triplications with synthetics through a grid search we obtained the best fit models for four azimuthal profiles i iv from north to south to constrain the p wave velocity structure near the 410 beneath the southeastern margin of the tibetan plateau tp a ubiquitous low velocity layer lvl resides atop the mantle transition zone mtz the lvl is 25 to 40 km thick with a p wave velocity decrement ranging from approximately 5 3 to 3 6 related to the standard earth model iasp91 an abrupt transition in the velocity decrement of the lvl was observed between profiles ii and iii we postulate that the mantle structure beneath the southeastern margin of the tp is primarily controlled by the southeastern extrusion of the tp to the north combined with the eastward subduction of the indian plate to the south but not affected by the emeishan mantle plume we attribute the lvl to the partial melting induced by water and or other volatiles released from the subducted indian plate and the stagnant pacific plate but not from the upwelling or the remnants of the emeishan mantle plume a high velocity anomaly ranging from approximately 1 0 to 1 5 was also detected at a depth of 542 to 600 km providing additional evidence for the remnants of the subducted pacific plate within the mtz

imaging of structure at and near the core mantle boundary using a generalized radon transform 2 statistical inference of singularities 1 we present the second part of our approach to high resolution imaging of deep earth s interfaces with large volumes of broadband three component seismograms we focus on the lowermost mantle also referred to as d region but the methodology can be applied more generally the first part describes the generalized radon transform grt of broadband scs data comprising main arrival precursors and coda the grt produces image gathers which represent multiple images of medium contrasts at the same image point near the base of the mantle with a method for statistical inference we use this redundancy 1 to enhance the grt images through improved recovery of weak contrasts and through suppression of spurious oscillations in the grt image gathers and 2 to provide uncertainty estimates that can be used to identify the robust features in the images using the image gathers from paper 1 as input we use mixed effects statistical modeling to produce the best estimates of reflectivity along with their uncertainty in this framework random noise in the signal is separated into white and coherent components using the geometry of the grt imaging operators and a generalized cross validation method with synthetic data we show that conventional grt images deteriorate substantially in some cases to the point at which weak reflectors can no longer be detected due to effects of uneven sampling wave phenomena that are not accounted for in the underlying single scattering approximation or errors in the assumed background wave speed model we demonstrate that even in these circumstances statistical analysis can yield adequate estimates of the true model grt imaging produces robust images of the core mantle boundary cmb beneath central america and suggests the presence of several structures in the d region in particular between 100 and 200 and between 270 and 320 km above the cmb proper most of these structures are significant at the 1 that is 68 level but at 2 95 confidence the images show at various depths above the cmb intermittent instead of laterally contiguous features

structure of the earth mantle and core the deep interior structure of the earth has been extensively analyzed using a wide variety of seismic phases and techniques during the last four years most studies have emphasized quantitative three dimensional mapping of the lateral velocity heterogeneity of the mantle and core these aspherical velocity variations are believed to be direct manifestations of thermal and compositional heterogeneity associated with convective processes a first generation of global models for the lateral velocity variations in the deep earth has been produced providing tantalizing images of large scale structures suggestive of a non steady state thermal convection system the upper 400 km of the mantle has the strongest lateral velocity variations of up to 10 for shear velocity surface wave analyses that do not require a priori regionalizations have demonstrated that there is a strong association between surface tectonic provinces and the uppermost mantle velocity variations thus the thermal and convective state of the upper 200 km of the mantle can be reliably interpreted in the context of plate tectonics the upper mantle models support the contention that continents have deep roots with differences in velocity structure from oceanic and active tectonic regions extending as deep as 400 km very long wavelength lateral velocity variations of a few percent have been detected in the transition zone at depths from 400 to 670 km as well as throughout the lower mantle these deep seated variations have little correspondence to surface tectonics and efforts to interpret their nature are just beginning the lowermost 200 km of the mantle d region has lateral velocity fluctuations comparable to those in the upper mantle and evidence has been presented for the presence of a sizable velocity discontinuity at the top of the d layer a combined thermal and compositional boundary layer roughly mirroring the lithosphere is a likely explanation for this anomalous zone the core mantle boundary appears to have significant 10 km long wavelength topography presumably sustained by dynamic stresses from deep mantle convection the inner core may have strong lateral heterogeneity or axially symmetric anisotropy suggesting a complex thermal and compositional state significant progress has been made in characterizing the frequency dependence of anelastic attenuation in the mantle in the short period body wave band models for teleseismic p wave attenuation operators have converged with t values of 0 7 1 0 s appropriate at 1 hz and t values of 0 4 0 6 s appropriate at 4 hz regional variations of attenuation are slowly being mapped out as well

further evidence for the lay discontinuity beneath northern siberia and the north atlantic from short period p waves recorded in france abstract we analyze data from the french laboratoire de detection geophysique ldg network of digital short period seismic stations the p waves from several earthquakes in the kuril okhotsk region display clear secondary arrivals in the 75 85 range which are not explained by smooth lower mantle models the move out of the arrivals across the network indicates a deep mantle origin we interpret them as waves reflected off or transmitted at the lay discontinuity about 300 km above the core mantle boundary the bounce points at this discontinuity are located beneath northern siberia slightly to the northwest of the region for which a similar structure has been reported we propose tools for investigating the lateral variations of the d structure on the scale 100 1000 km the method we propose is based on the deconvolution of the data or convolution of synthetics by a source time function derived from the data we show that the method helps identification of the waves that interact with the lay discontinuity and enables a more objective mapping of its lateral variations evidence for bumps a few kilometers high on the lateral scale of a few tens of kilometers is proposed we also probe a spot of the lower mantle beneath the north atlantic a detailed analysis of the waveforms from a chiapas earthquake suggests that the lay discontinuity is also present in that part of the world

profile of don helmberger seismological modeling is now an integral and expanding part of geophysics research and it has led helmberger to many valuable applications including modeling the characteristics of earthquakes and superplumes upwelling of the earth s mantle determining the nature of underground detonations and mapping the transition zones in the earth s mantle and core in his inaugural article published in a recent issue of pnas 1 helmberger shed light on one such transitional area in the planet an ultralow velocity zone that can be found in pockets above the core mantle boundary

flow at ice divide triple junctions 1 three dimensional full stokes modeling 1 ice domes are either axisymmetric high points along ridges or ridge triple junctions we model time dependent isothermal flow near triple junctions solving the full set of mechanical equations with a nonlinear power law rheology forcing is applied through the boundary conditions which affect flow patterns at outlets where such forcing is purely axisymmetric an axisymmetric dome is formed if a threefold symmetry in the forcing is applied the axisymmetric dome breaks up into three ridges subtending angles of 120 sets of experiments where the forcing was not exactly threefold symmetric by angle or by amplitude caused the triple junction to migrate to a new steady state here in steady state the ridges join the triple junction at nearly 120 but one ridge curves to satisfy the boundary forcing the slope pattern in the immediate dome vicinity depends only on a dimensionless parameter which is a function of the ice consistency the accumulation and the rheological power law index attempts to replicate the topography around summit greenland obtained a good fit with n 3 at a triple junction the dome is really distinct from the surrounding ridges contrary to the highest point of a single ridge divide as a consequence the raymond effect is at its strongest at the dome and weakens considerably over one ice thickness as one moves away from the flow center along the ridges leaving the dome the raymond effect is still present and decreases with the ratio of the flow across and along the ridge in the vicinity of the dome horizontal strain rates vary strongly from uniaxial to biaxial large scale effects represented in our model as fluxes at boundaries seem to be the primary controls on dome position and shape