body wave traveltimes and a spherically symmetric p and s wave velocity model summary the derivation of a spherically symmetric reference earth model is a subject which has recently regained interest after a decade of studies on lateral heterogeneity we now have the tools to approach the problem an iaspei working group on traveltime tables was formed in 1988 with the goal of deriving a reliable and up to date radial model of seismic velocities following the guidelines developed by that working group we have investigated the body wave traveltimes for several branches frequently reported in the isc bulletins and propose here a model for p and s wave velocities consistent with these data approximately 16000 best recorded shallow focus earthquakes are selected from 24 years of isc bulletins 1964 1987 and relocated using among others corrections for lateral heterogeneity from this dataset we derive summary traveltime curves specified for each degree of epicentral distance for p pcp p pkpab pkpbc pkikp and s to constrain velocity at the top of the outer core we include data for sks and skks sks from hales roberts 1970 1971 our discrete time distance curves are obtained by evaluating the shape of the residual density distribution independently for each 1 interval to limit contamination due to the interference of other branches precursors or late readings in the upper mantle the resulting model sp6 differs from iasp91 kennett engdahl 1991 only by slightly higher p and s velocities between 410 and 660 km in the lower mantle new velocity profiles have lower velocity gradients this is also true in comparison with prem dziewonski anderson 1981 at the top of the outer core vp is smaller than in prem and the velocity jump at the inner core boundary is reduced to the value of 0 62 km s 1 the new inner core radius is 1215 km our model predicts traveltimes that differ from the iasp91 tables considerably although generally less than 1 s except for sks and skks for which the differences are greater

radial thermo chemical structure beneath western and northern pacific from seismic waveform inversion abstract the earth s deep mantle seismic structure is dominated by two large low shear wave velocity provinces llsvps located beneath africa and the pacific while the existence of these structures has been attested by many studies and data sets their detailed nature purely thermal or thermo chemical is still a matter of debate discriminating between these hypotheses requires constraints independent from seismic velocity structure seismic shear wave attenuation measured by the quality factor q s strongly depends on temperature but not or weakly on composition it may bring key information on temperature resolving in turn the trade off between temperature and composition here we invert seismic waveform data for radial models of shear wave velocity anomalies dln v s and q s at two different locations in the pacific and from a depth of 2000 km down to the core mantle boundary cmb we show that western pacific wp models sampling the western tip of the pacific llsvp and the caroline plume cannot be explained by thermal anomalies alone but require excess in iron of 4 0 from the cmb up to 2600 km by contrast northern pacific models np if unaffected by seismic anisotropy may have a purely thermal origin based on these observations we build radial thermo chemical models at wp and np

constraints on the core mantle boundary topography from p4kp pcp differential travel times 1 p4kp pcp differential travel times are examined to infer the core mantle boundary cmb topography a total of 362 p4kp pcp times are measured with a measurement error of 0 5 s the travel times are corrected for the earth s hydrostatic ellipticity and mantle heterogeneity using a p wave tomographic model spherical harmonic expansion up to degree 4 is adopted for model parameterization the p wave velocity heterogeneity is then estimated in the lowermost 150 km of the mantle to overcome the problem of underestimation of the velocity perturbation at the base of the mantle in the global 3 d p wave mantle model subsequently the cmb topography is inferred using the residues of the above processes since the odd degree components of the cmb topography are insensitive to the p4kp pcp times only the components of degrees 2 and 4 are solved for the resultant features indicate that the maximum amplitude of the cmb topography does not exceed 2 km with an uncertainty of less than 0 5 km a numerical test confirms that the pattern of degree 4 is more reliable with less amplitude recovery the obtained degree 4 pattern shows an amplitude of less than 1 km and indicates the presence of depressions under the circum pacific the central pacific and south africa

topography at the core mantle boundary analysis of the earth s gravity field using ratio of gravity to geoid g n and cumulative degree contribution curve techniques improves resolution of the mass depth and dimensions of anomalous mass sources within the planet the source for four of the ten major geoid anomalies of the earth has large contributions from harmonic degrees 2 and 3 lie at the core mantle boundary and the greatest has a mass anomaly of 4 2 1022 grams that boundary has a density contrast 4 4 gm cm which is greater than at any other location including the contrast at the earth s surface with space thus relatively small vertical undulations of that boundary produce very large mass anomalies topographic relief of less than 3 km at the core mantle boundary occurs and accounts for the existence of the largest geoid anomalies occurring on the earth

core mantle boundary shape constraints inferred from the pressure torque acting between the core and the mantle summary in the last few years models of the flow at the top of the earth s core and of the related pressure field have been calculated from the secular variation of the geomagnetic field and core mantle topographies have been computed by seismologists a pressure torque results from the action of the pressure field on the core topography which can theoretically be computed from models of both the pressure field and the core mantle interface small scale features of the flow and of the topography are shown to be capable of contributing strongly to the pressure torque it is thus impossible to calculate the exact value of the torque from the knowledge of only the long wavelength components of the models but the interaction between the large scale components generates by itself torques two orders of magnitude larger than the torques inferred from the irregularities of the length of the day it is nevertheless possible to reconcile the topographic coupling mechanism with the length of the day observations keeping the amplitude of the core topography proposed by seismologists if an orthogonality relation between the geometry of the fluid upwellings and downwellings at the top of the core and the topography is satisfied it is shown how to compute such a topography for a given flow and close to an original topography provided by seismic tomography some consequences of the so inferred link between the fluid flow at the top of the core and the core mantle boundary topography are discussed

the westward drift of the geomagnetic field caused by length of day variation and the topography of the core mantle boundary summary we propose that the westward drift of the geomagnetic field is caused by length of day 1 o d variation through topographic coupling between the mantle and the core fluctuations of the rotation velocity of the mantle induce radial flow in the core through topographic coupling at the core mantle boundary cmb the induced flow bends the toroidal field to generate a poloidal field which appears to drift westwards an analytical model is constructed to describe this situation on the basis of a quasi geostrophic approximation we compare our model with observations of the geomagnetic field over the last several hundred years when the sectorial harmonic components of the gauss coefficients g and h are plotted on a diagram with g as the abscissa and h as the ordinate the observed trajectories follow a clockwise ellipse the clockwise motion on the g h plot signifies that the pattern of the magnetic field moves westwards this behaviour is described well by our model if 1 o d variation is assumed to consist of one fourier component we find that the period of the fourier component of 1 o d variation is about 700 yr the relationship between the ellipticity of the trajectories and the eigenfrequencies of the slow magnetohydrodynamic oscillations enables us to infer the dispersion relation to the oscillations which signifies that the strength of the toroidal field of the outer core is 50 100 gauss we derive the topography of the cmb from the directions of the axes of the ellipses the cmb topography is found to correlate well with the lower mantle structure

aspherical structure of the core mantle boundary from pkp travel times maps of aspherical structure in the vicinity of the core mantle boundary have been constructed from a tomographic analysis of p df and p ab travel times in the distance range 150 180 these maps reveal long wavelength features not previously detected by mantle tomography the peak to peak amplitude of these features is on the order of 1 s in vertical travel time too large to be explained by conventional thermal boundary layer models or dynamically supported topography on the core mantle boundary we hypothesize the existence of one or more chemical boundary layers whose large scale accumulations on the surface of the core may in some ways be analogous to continents at the earth s surface

paleoclimatic significance of late quaternary lacustrine and alluvial stratigraphy animas valley new mexico lacustrine and alluvial stratigraphic sequences in the southern animas valley of new mexico allow reconstruction of late quaternary climates four separate stands of late quaternary lake cloverdale in the southern animas valley are recorded by lacustrine shoreline deposits soils and stratigraphic evidence show that three young lake highstands occurred during the holocene and that a higher lake stand occurred 18 000 to 20 00014c yr b p fluvial systems aggraded the southern animas valley during the middle to late holocene the late quaternary stratigraphy shows that several periods during the late holocene were characterized by higher effective precipitation than at any time since the last glacial maximum

possibility of a low p wave velocity layer in the outermost core from global smks waveforms abstract a global data set consisting of 1211 smks m 2 waveforms has been analyzed to investigate the radial seismic velocity structure around the core mantle boundary cmb band passed f 0 02 0 1 hz and stacked waveforms coincide with reflectivity synthetic ones for prem very well whereas those for other global models iasp91 ak135 and sp6 yield disagreements a preliminary waveform modeling for the outermost core with prem as initial structure results in a core surface layer with a p wave velocity of 7 95 km s at the core top and thickness of 90 km based on a 3 d global s wave velocity model smks phases collected in this study mainly pierce slightly high velocity regions at the base of the mantle and the predicted differential travel time residuals for s3ks skks are expected to be negative approximately 0 5 s for the averages however positive s3ks skks residuals of which average is 0 3 s are predominant in the stacked waveforms moreover a waveform modeling for the d structure can result in a 30 km thick layer with a 10 s wave velocity reduction at the mantle bottom ulvz in which waveform fitness for the part of mainly s3ks is improved on the other hand the waveform data is not well explained by the lowermost mantle structures with thickness of several hundred kilometers these suggest that the smks data is still affected by the structure of the lowermost several tens kilometers in the mantle that is not sufficiently modeled by a global tomography although the thin low s wave velocity model is not conclusive the possibility of a low p wave velocity layer in the outermost core is remained because the waveform fitness for the part of s4ks is improved by the combination of the ulvz and a 140 km thick layer with a 0 8 p wave velocity reduction at the core top

wet conditions during the last glaciation in the chihuahuan desert alta babicora basin mexico abstract a 467 cm core b94 3 from the alta babicora basin in the chihuahuan desert northern mexico 29 n 108 w documents lake level and climate changes over the past ca 65 000 yr chronological control is provided by four accelerator mass spectrometry 14 c dates and five u series dates on diatom silica the core has been analyzed for magnetic susceptibility loss on ignition carbonate content sediment chemistry and mineralogy and pollen and diatom composition the basin was occupied by a deep freshwater lake throughout the late pleistocene which based on shoreline evidence was at least 19 m deeper than today the lake shallowed after ca 57 000 yr b p high variability typified the period between 54 600 and 38 000 yr b p probably with periodic desiccation and deflation a deep water lake was reestablished after 38 000 yr b p and persisted until ca 29 000 yr b p shallowing occurred through the last glacial maximum although the lake was still deeper than at present the pleistocene holocene transition was marked by a distinctive change in the diatom flora there is no diatom record for much of the holocene but other proxies indicate generally dry stable catchment conditions the babicora record is more akin to those from the southwest united states than it is to central america and northern south america wet conditions in the late pleistocene are attributed to winter rainfall from midlatitudes in contrast to the modern tropical summer rainfall regime

southern oscillation extremes reconstructed from tree rings of the sierra madre occidental and southern great plains abstract the el ninio southern oscillation enso is correlated with climate and tree growth over northern mexico and the southern great plains of the usa warm events favor moist cool conditions from october through march event years 0 and 1 and subsequent tree growth year 1 in the region tends to be above average the opposite climate and tree growth conditions prevail with less consistency during cold events enso sensitive tree ring chronologies from this region were selected to develop two reconstructions of the southern oscillation index soi back to 1699 for the first reconstruction a multiple regression based calibration equation between prewhitened and regionally averaged tree ring data from mexico and oklahoma and a prewhitened winter djf so index during the period 1900 71 were used to estimate the winter soi for each year from 1699 to 1971 the tree ring predictors account for 41 of the winter soi variance from 1900 to 1971 and the reconstructed so indices are significantly correl

constraints on aspherical core structure from pkikp pcp differential travel times 1 we have assembled a data set of 302 differential pkikp pcp travel times that were recorded by seismic array stations of the international monitoring system this is an order of magnitude more data than all previous pkikp pcp observations combined the differential times were measured using a beam forming and cross correlation algorithm that has a precision of 1 2 sampling intervals 0 1 s the use of differential times mitigates the effects of hypocentral errors and heterogeneities in the crust and upper mantle leaving the data sensitive to deep earth structure the pkikp pcp times are especially sensitive to variations in the thickness of the liquid outer core the data are well fit by prem with residuals having a mean of 0 29 s and a standard deviation of 0 44 s and rule out lateral variations in outer core thickness of more than 4 km most of the geographically coherent residuals can be explained by lower mantle heterogeneities and ellipticity corrections however there remain at least three regions that seem to require aspherical core structure the anomalous data are best explained by long wavelength topography on the core mantle boundary cmb with peak to peak amplitude of 3 5 km the cmb topography is most likely maintained by isostatic compensation of thickness variations in a distinct layer at the base of the mantle

contributions of la nina and el nino to middle holocene drought and late holocene moisture in the american southwest eolian landforms in estancia basin central new mexico record two episodes of extreme drought and low groundwater levels during the middle holocene 7000 5400 1 4 c yr b p followed by a rise in the water table throughthe late holocene blowouts and associated lunettes formed when water levels fell below the desiccated floor of pluvial lake estancia allowing widespread deflation elevation of the water table in the basin is regulated by a balance between evaporation from playas that occupy the blowouts and recharge of aquifers in the adjacent manzano mountains isotopic analyses of modern precipitation and groundwater reveal that recharge originates primarily as winter moisture which is amplified during el ninos and diminished during la ninas thus changes in the elevation of a reconstructed holocene water table may reflect systematic changes in el nino southern oscillation enso circulation

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

atmospheric circulation during holocene lake stands in the mojave desert evidence of regional climate change it is commonly thought that the climate conditions that supported lakes over a period of years in the mojave desert in southern california only existed before 8 000 yr bp and that the environment has been arid since1 2 here we look at a drill core in the silver lake playa at the terminus of the mojave river and find holocene lake deposits which indicate that shallow lakes existed for at least a few decades these deposits were radiocarbon dated at 3620 70 and 390 90 yr bp corresponding to the early neo glacial and the little ice age respectively3 to identify the conditions necessary to produce these holocene lake events we have examined the modern climate and hydrological patterns that produce ephemeral lakes in this usually arid watershed available data indicate that there is a link between anomalous winter atmospheric conditions over the north pacific and mojave river floods that produced ephemeral lakes in the silver lake playa and that the mojave river filters out small to medium floods and allows only the extreme floods to reach the terminal playa and leave a record of the anomalous conditions we suggest that the late holocene lakes may have resulted from persistent similar atmospheric circulation patterns and winter floods

core mantle boundary topography and its relation to the viscosity structure of the lowermost mantle abstract two large areas of anomalously low seismic velocities are visible in all tomographic models of the lowermost mantle depending on the density structure of these large low shear velocity provinces llsvps the core mantle boundary cmb will deform upwards or downwards due to isostatic and dynamic topography the latter being sensitive to the viscosity structure of the lowermost mantle heterogeneities in the viscosity structure although difficult to constrain might be especially important if the llsvps are thermochemical piles with elevated intrinsic viscosity as suggested by mineral physics based on numerical models we identify a short wavelength about 80 120 km wide up to a few km deep topographic depression that forms around the pile edges if the pile is more viscous than the surrounding mantle the depression forms when a wedge of thermal boundary layer material becomes compressed against the viscous pile and is enhanced by relative uplift of the cmb beneath the pile by plumes rising above it the depth and asymmetry of the depression constrain the magnitude of the viscosity contrast between pile and the surrounding mantle furthermore periodic plume initiation and pile collapse at the pile margin systematically modify the characteristic depression with a maximum in asymmetry and depth at the time of plume initiation core reflected waves or scattered energy may be used to detect this topographic signature of stiff thermochemical piles at the base of the mantle

global p wave tomography of earth s lowermost mantle from partition modeling 1 determining the scale length magnitude and distribution of heterogeneity in the lowermost mantle is crucial to understanding whole mantle dynamics and yet it remains a much debated and ongoing challenge in geophysics common shortcomings of current seismically derived lowermost mantle models are incomplete raypath coverage arbitrary model parameterization inaccurate uncertainty estimates and an ad hoc definition of the misfit function in the optimization framework in response we present a new approach to global tomography apart from improving the existing raypath coverage using only high quality cross correlated waveforms the problem is addressed within a bayesian framework where explicit regularization of model parameters is not required we obtain high resolution images complete with uncertainty estimates of the lowermost mantle p wave velocity structure using a hand picked data set of pkpab df pkpbc df and pcp p differential travel times most importantly our results demonstrate that the root mean square of the p wave velocity variations in the lowermost mantle is approximately 0 87 which is 3 times larger than previous global scale estimates

inference of core mantle boundary topography from isc pcp and pkp traveltimes we invert isc pcp and pkp absolute and differential traveltimes in an attempt to infer the long wavelength topography of the core mantle boundary cmb the data selection and processing methods are described and evaluated these travel time data are very noisy and the geographic distribution of the data is highly non uniform inhibiting reliable inference of cmb topography spatial averaging enhances the coherent component of the residual variance related to heterogeneity however the random component of the variance is much larger than the coherent component we show that for pcp data the coherent signal due to mantle heterogeneity overshadows that arising from the cmb and that the effects of mantle heterogeneity are mapped into our inferred cmb solutions the pcp data are not correlated across the spatial averaging bins and seem to have a strong bias due to small scale structure and or noise the non uniform geographic sampling of the data plays a role in the mapping of mantle heterogeneity onto the cmb spatial patterns of cmb models inferred from different phases do not agree amplitudes of seismically inferred cmb undulations vary greatly the sensitivity of inferred cmb models to the processing spatial averaging procedure and inversion techniques are investigated topographic amplitudes increase strongly with increasing input residual variance the power spectrum of inferred topography indicates that there are unmodelled heterogeneities that must be described with spherical harmonics of degree 6 and higher based on this work we conclude that reliable inference of long wavelength cmb topography is not likely with the current isc data set or with a spherical harmonic expansion truncated to degree and order 6

late quaternary lacustrine history and paleoclimatic significance of pluvial lake cochise southeastern arizona abstract during the latest quaternary freshwater pluvial lakes intermittently formed in the topographically closed willcox basin arizona a lacustrine sequence of six separate high stands of lake cochise is documented by stratigraphic studies 19 radiocarbon ages and supplementary evidence two stands of pluvial lake cochise older than 14 000 yr b p reached elevations above 1290 m the prominent 1274 m shoreline of lake cochise which circumscribes the basin was largely created during a high stand between 13 750 and 13 400 yr b p during the holocene water filled the willcox basin three times to an elevation slightly below the crest of the 1274 m shoreline this occurred once during the early holocene around or before 8900 yr b p and twice during the later part of the middle holocene since the middle holocene only shallow ephemeral lakes have occupied the deflated central portion of ancient lake cochise a depression known as the willcox playa the lacustrine sequence of lake cochise provides an independent evaluation of late quaternary paleoclimatic reconstructions for southern arizona and the american southwest

topographic core mantle coupling in geodynamo modeling a 3 dimensional fully nonlinear numerical geo dynamo model is used to study the core mantle coupling arising from a non hydrostatic pressure field acting on an aspherical core mantle boundary cmb based on a seismo logical model the simulation results demonstrate that the topographic coupling torque is close to being proportional to the square of the amplitude of the cmb topography we conclude that the contribution of the topographic coupling to the total torque that is responsible for the axial angular momentum transfer on decadal timescales for the decadal length of day variation is significant only if the amplitude of the cmb topography is considerably larger than 3 km inferred from seismological evidence

core mantle boundary topography from short period pcp pkp and pkkp data abstract we use a total of 839 369 pcp pkpab pkpbc pkpdf pkkpab and pkkpbc residual travel times from bull seism soc am 88 1998 722 grouped in 29 837 summary rays to constrain lateral variation in the depth to the core mantle boundary cmb we assumed a homogeneous outer core and the data were corrected for mantle structure and inner core anisotropy inversions of separate data sets yield amplitude variations of up to 5 km for pcp pkpab pkpbc and pkkp and 13 km for pkpdf this is larger than the cmb undulations inferred in geodetic studies and moreover the pcp results are not readily consistent with the inferences from pkp and pkkp although the source receiver ambiguity for the core refracted phases can explain some of it this discrepancy suggest that the travel time residuals cannot be explained by topography alone the wavespeed perturbations in the tomographic model used for the mantle corrections might be too small to fully account for the trade off between volumetric heterogeneity and cmb topography in a second experiment we therefore re applied corrections for mantle structure outside a basal 290 km thick layer and inverted all data jointly for both cmb topography and volumetric heterogeneity within this layer the resultant cmb model can explain pcp pkp and pkkp residuals and has approximately 0 2 km excess core ellipticity which is in good agreement with inferences from free core nutation observations joint inversion yields a peak to peak amplitude of cmb topography of about 3 km and the inversion yields velocity variations of 5 in the basal layer the latter suggests a strong trade off between topography and volumetric heterogeneity but uncertainty analyses suggest that the variation in core radius can be resolved the spherical averages of all inverted topographic models suggest that the data are best fit if the actual cmb radius is 1 5 km less than in the earth reference model used i e the average outer core radius would be 3478 km

whole earth tomography from delay times of p pcp and pkp phases lateral heterogeneities in the outer core or radial anisotropy in the mantle we perform tomographic inversions of compressional wave travel time measurements in order to image the laterally varying radial anisotropy of p wave velocity in the entire mantle in order to achieve an adequate sensitivity to both horizontal and vertical p velocity throughout the mantle we include in our data set travel time measurements corresponding to ray paths of very different geometries specifically we use data associated with the phases p pcp core reflected and pkpbc pkp df core refracted all extracted from the bulletins of the international seismological centre 1964 1993 pcp pkpbc and pkpdf data are also sensitive to the topography of the core mantle boundary cmb and pkp only to the velocity structure of the earth s core these circumstances require that we also allow for cmb topographic anomalies as free parameters in our inversions and investigate the possibility of the existence of lateral heterogeneities in the outer core e g ritzwoller et al 1986 stevenson 1987 wahr and de vries 1989 vasco and johnson 1998 the solution models that we propose include alternatively a laterally heterogeneous outer core a radially anisotropic mantle or both so that trade offs between those features can also be evaluated we conclude that only minor and very localized radially anisotropic anomalies are present in the middle and lower mantle the most prominent being located in the deepest 400 km under the western pacific we find a discrepancy particularly evident by a comparison between different models of the cmb topography between the travel time measurements that are sensitive to the earth s core and those that are not we show that it is not possible to explain this discrepancy only in terms of radial anisotropy of the mantle and or lateral heterogeneity of the outer core

the effects of boundary topography on convection in earth s core summary we present the first investigation that explores the effects of an isolated topographic ridge on thermal convection in a planetary core like geometry and using core like fluid properties i e using a liquid metal like low prandtl number fluid the model s mean azimuthal flow resonates with the ridge and results in the excitation of a stationary topographic rossby wave this wave generates recirculating regions that remain fixed to the mantle reference frame associated with these regions is a strong longitudinally dependent heat flow along the inner core boundary this effect may control the location of melting and solidification on the inner core boundary theoretical considerations and the results of our simulations suggest that the wavenumber of the resonant wave lr scales as ro 1 2 where ro is the rossby number this scaling indicates that small scale flow structures wavenumber in the core can be excited by a topographic feature on the core mantle boundary the effects of strong magnetic diffusion in the core must then be invoked to generate a stationary magnetic signature that is comparable to the scale of observed geomagnetic structures

p and pcp travel time tomography for the core mantle boundary pcp arrival times reported to the international seismological centre isc are inverted for vertical travel time anomaly at the core mantle boundary cmb we first invert about 2 106 isc first p arrival data for the three dimensional 3 d structure of the mantle and hypocenter locations iteratively the pcp data are corrected for the relocated hypocenters and the 3 d structure this correction has reduced the observed pcp residuals the corrected pcp data are then inverted for the vertical travel time anomalies at the cmb which are due either to velocity anomaly or topographic anomaly the spherical average of the inverted structure requires either a velocity reduction in the boundary layer just above the cmb by 3 if the layer is 20 km thick or a lowering of the cmb by 3 km the heterogeneity pattern of the cmb is drastically different from that of the overlying d layer negative vertical travel time anomalies appear in the northern high latitude region and positive anomalies appear under southeast asia and middle to south america the lateral dimensions of these anomalies are of the order of 40 the anomalies can be interpreted as either a velocity perturbation in the boundary layer 7 if the layer is 20 km thick or an undulation of the cmb 7 km various tests including the error and resolution estimations indicate that these three anomalies are the significant resolvable features of the cmb these anomalies are also corroborated by the result of the joint inversion of p and pcp for both the 3 d mantle and the 2 d cmb assuming that the layer is laterally heterogeneous in iron content and deforms at the base in isostatic equilibrium with the outer core the pcp data are also inverted for the isostatic figure of the boundary layer the velocity perturbation in this case is 5 6 if the layer is 20 km thick and the cmb undulation is only 1 2 km regardless whether the cmb heterogeneity is due to velocity and or topographic perturbations it is dominant in the zonal component of degree 2 while the degree 2 pattern in the lower mantle including the d is dominant in sectorial component

on the likelihood of post perovskite near the core mantle boundary a statistical interpretation of seismic observations abstract recent experimental studies indicate that perovskite the dominant lower mantle mineral undergoes a phase change to post perovskite at high pressures however it has been unclear whether this transition occurs within the earth s mantle due to uncertainties in both the thermochemical state of the lowermost mantle and the pressure temperature conditions of the phase boundary in this study we compare the relative fit to global seismic data of mantle models which do and do not contain post perovskite following a statistical approach our data comprise more than 10 000 pdiff and sdiff travel times global in coverage from which we extract the global distributions of dln vs and dln vp near the core mantle boundary cmb these distributions are sensitive to the underlying lateral variations in mineralogy and temperature even after seismic uncertainties are taken into account and are ideally suited for investigating the likelihood of the presence of post perovskite a post perovskite bearing cmb region provides a significantly closer fit to the seismic data than a post perovskite free cmb region on both a global and regional scale these results complement previous local seismic reflection studies which have shown a consistency between seismic observations and the physical properties of post perovskite inside the deep earth

mantle convection and core mantle boundary topography explanations and implications abstract recent seismic inferences of the topography on the core mantle boundary cmb constitute a new and potentially important constraint on the dynamics of the thermal convective circulation in the earth s mantle we employ viscous flow models for the mantle to predict the flow induced deflections of the cmb that are expected on the basis of seismic tomographic inferences of internal mantle density heterogeneity the good agreement between our theoretical predictions and the seismically inferred cmb deflections of morelli and dziewonski 1987 suggests that the latter may indeed be good approximations to the dynamically induced cmb topography the amplitude of the seismically inferred cmb topography may be construed to provide a direct constraint on the temperature derivative of seismic p wave velocity in the lower mantle and we determine a value for this parameter in the course of fitting the data that differs significantly from the value determined by laboratory measurements as appropriate for upper mantle phases the seismically observed topography on the cmb is also shown to provide evidence favouring the whole mantle convection hypothesis and in this connection we argue that deep penetration of subducted lithospheric slabs into the lower mantle is likely to be responsible for controlling many of the features of this topography

observations of pkkp precursors used to estimate small scale topography on the core mantle boundary stacks of global seismic data reveal the time and distance dependence of high frequency 1 hertz precursors to the seismic phasepkkp synthetic seismogram modeling shows that scattering from random small scale topography at the pkkpcore mantle boundary reflection point generates precursory arrivals similar to those seen in the data models with a root mean square core mantle boundary topography of 250 to 350 meters and correlation length of 7 to 10 kilometers explain the main features of the data however a systematic range dependent misfit between observed and predicted precursor power suggests that inner core scattering may contribute to the precursors

core mantle boundary topography estimated from numerical simulations of instantaneous mantle flow core mantle boundary cmb topography is estimated from numerical simulations of instantaneous mantle flow in a three dimensional spherical shell geometry density anomaly models of the earth s mantle are derived from geodynamic models constrained principally by seismic tomography results to account for the small 1 5 km cmb topographic relief inferred from recent seismological results lateral viscosity variations in the mantle compositionally dense piles in the deep mantle and a low viscosity d layer are all required for the numerical models low viscosity subducting slabs in the lower mantle serve to reduce the topographic amplitude below high velocity regions i e the circum pacific belt the existence of dense piles may cause the local topographic depressions even below low velocity regions i e the south pacific and atlantic africa regions

variability of the topographic core mantle torque calculated from core surface flow models abstract with the prospect of studying the relevance of the topographic core mantle coupling to the variations of the earth s rotation and also its applicability to constraining the core surface flow we investigate the variability of the topographic torque estimated by using core surface flow models accompanied by a uncertainty due to the non uniqueness problem in the flow inversion and b variance originating in that of geomagnetic secular variation models employed in the inversion various flow models and their variances are estimated by inverting prescribed geomagnetic models at the epoch 1980 the subsequent topographic torque is then calculated by using a core mantle boundary topography model obtained by seismic tomography the calculated axial and equatorial torques are found subject to the variability of order 1 0 19 and 1 0 20 nm respectively on which b is more effective than a the variability of the torque is attributed even to a and b of the large scale flows degrees 2 and 3 yet it still seems unlikely for the decadal polar motion with the observed amplitude to be excited exclusively by the equatorial topographic torque associated with any of reasonable core surface flow models it is also confirmed that with the topography model adopted here the axial topographic torque on a rigid annulus in the core coaxial with the earth s rotation axis associated with any of reasonable flow models is larger by two orders of magnitude than the plausible inertial torque on such cylinders this implies that any core surface flow model consistent with the topographic coupling does not exist unless the topography model is appropriately modified nevertheless the topographic coupling might provide not only a weak constraint for explaining the decadal lod variations but also the possibility to probe the core surface flow and the core dynamics

detailed investigation of a spot on the core mantle boundary using digital pcp data abstract a number of deep earthquakes from the fiji tonga region were used to study the properties of the core mantle boundary cmb the investigation was restricted to an area with a diameter of only a few hundred kilometers at the cmb the use of a cluster of neighboring events yielding redundant sampling and rather good resolution the components of the investigation are i a classical stacking procedure to search for additional reflectors near the cmb ii a deconvolution of seismograms and a stack of the inferred reflector series for the same purpose iii a matched filter method to determine pcp travel times and pcp p amplitude ratios precisely and iv a comparison with synthetic seismograms to check the consistency of derived travel time residuals and pcp amplitude variations in the presence of cmb topography while we do not see any additional reflector for epicentral distances around 35 we detect by stacking seismograms recorded at 70 an additional phase which might be caused by a reflection from the top of d we show that pcp travel time residuals must not be interpreted as being caused exclusively by cmb topography an additional investigation of synthetics provides some constraints on dominant wavelengths and corresponding amplitudes of cmb undulations for wavelengths of 50 400 km the elevation cannot exceed 2 3 km if pcp is observed in this particular area

large holocene lakes and climate change in the chihuahuan desert lake level variations preserved as beach ridges in the laguna el fresnal and laguna santa maria subbasins northern mexico record millennially spaced episodes of increased precipitation during the holocene epoch we find that the early middle and late holocene were punctuated by periods wet enough to establish large pluvial lakes in currently dry basins in the chihuahuan desert the largest dated pluvial lake covered 5650 km2 during the early holocene constructional beach ridges in these subbasins are 221 33 14c yr b p little ice age equivalent 3815 52 to 4251 59 14c yr b p early neoglacial 6110 80 to 6721 68 14c yr b p mid holocene and 8269 64 to 8456 97 14c yr b p early holocene dates that correlate with other millennially spaced wet or cold events in the northern hemisphere we attribute these wet episodes to increased precipitation cooler temperatures and reduced evaporation following southward shifts in winter storm tracks which are related to long term el nino southern oscillation variability during the holocene

variability of clay mineral composition on carolina slope nw atlantic during marine isotope stages 1 3 and its paleoceanographic significance abstract the clay mineral composition of marine isotope stages 1 3 sediments at the ocean drilling program odp site 1055 on the carolina slope consists mainly of illite kaolinite chlorite and smectite clay mineral variability is marked by a distinct increase in the relative amounts of kaolinite and smectite during the holocene and by high illite and to a lesser extent chlorite relative amounts during marine isotopic stages 2 and 3 this grouping of clay minerals as two different assemblages during different isotopic stages suggest their different source affinities the increase in the kaolinite smectite assemblage in the holocene is accompanied by a significant increase in the sedimentation rate the change in the sedimentation regime after the last glacial maximum is interpreted to be related to resuspension and advection of clays and silts by increased deep water activity over the bermuda rise followed by their transport to site 1055 on the carolina slope by shallow elements of the north atlantic deep water high amplitude variations with high illite amounts characterize marine isotope stage 3 and appear to be related to heinrich events

models of the core mantle boundary and the travel times of internally reflected core phases topographic maps of the core mantle boundary cmb obtained by inverting different subsets of international seismological centre arrival time data involving either waves refracted through the boundary pkp or waves reflected from the topside pcp do not agree we report on the implications for models of cmb structure when data from bottomside reflections notably pkkp are included in the inversion travel time residuals with respect to a reference earth model prem of all core phases are positively biased but the mean residual is different for different phases and must be accounted for prior to inversion the new data illustrate that models of cmb topography can explain only a small fraction of the data variance the new cmb model involves both topography and a laterally varying boundary layer the cmb itself is smooth compared to previous results

amplitude of the core mantle boundary topography estimated by stochastic analysis of core phases abstract the core mantle boundary cmb topography is an important parameter for constraining the models of mantle dynamics and core mantle coupling however the various large wavelength seismological models of the cmb topography which have been obtained up to now are poorly correlated moreover their maximum amplitudes vary considerably from one model to another with values ranging from 4 to 12 km these large discrepancies may be due to the difficulty to separate in the travel time anomalies the contribution of the cmb topography from that of the highly heterogeneous d region at the base of the mantle in order to better constrain the amplitude of the cmb topography we perform a stochastic analysis of the core phases which sample the cmb as transmitted waves and or as reflected waves in particular we analyse underside reflected pkkp phases which help to discriminate between cmb topography and d structure and have in addition a great sensitivity to cmb topography the other phases used are the upperside reflected waves pcp and the transmitted waves pkp the analysis is performed on the travel time residual file obtained by engdahl et al engdahl e r van der hilst r buland r p 1998 global teleseismic earthquake relocation with improved travel times and procedures for depth determination bull seismol soc am 88 722 743 after earthquake relocation and phase re identification after careful travel time data selection the stochastic analysis allows us to separate coherent signal from random signal in the data at different length scales ranging from 300 to 1500 km then the cmb topography variance is obtained at the different length scales from a joint analysis of the coherent signal in the different core phases in taking into account their different sensitivities to cmb topography the estimated cmb topography variance has a significant signal in the wavelength range that we have investigated showing that 95 of cmb topography amplitude is in the range 4 0 km for wavelengths larger than 300 km this value decreases to 1 5 km for wavelengths larger than 1200 km indicating that the long wavelength cmb topography is significantly lower than previously proposed however it has not been possible to invert the data for deriving a map of the cmb topography a checkerboard pattern analysis reveals that despite the introduction of the pkkp phase the relative contributions of d heterogenieties and cmb topography cannot be separated with the presently available data

chemical stratification at the top of earth s core constraints from observations of nutations abstract dissipation due to tidally driven flow in the earth s liquid core is detected in observations of nutations one source of dissipation is due to electromagnetic core mantle coupling but this mechanism requires a high electrical conductivity on the mantle side of the boundary and a strong radial magnetic field an alternative mechanism is viable in the presence of fluid stratification at the top of the core stratification causes the fluid close to the core mantle boundary to be trapped by the effects of topography further from the boundary the stratified fluid is swept past the mantle with the underlying tidal flow shear in the flow induces electric currents where the fluid is permeated with a radial magnetic field the resulting dissipation is only weakly dependent on the electrical conductivity of the mantle and the required strength of the radial magnetic field can be lowered for a representative calculation we explain the observed dissipation with a radial field of 0 5 mt and a mantle conductivity of 1000 s m provided the buoyancy frequency in the stratified layer is 0 09 s 1 such a strong stratification has recently been proposed on the basis of chemical interactions between the core and the mantle nutation observations support the presence of stratification to the extent that the resulting dissipation mechanism is more compatible with conventional estimates for the radial magnetic field and the mantle conductivity