constraints on the correlation of p and s wave velocity heterogeneity in the mantle from p pp ppp and pkpab traveltimes we investigate the correlation of large scale p and s velocity heterogeneity in the mantle by determining how well 106 000 compressional p pp ppp and pkpab traveltimes can be explained by s wave velocity model s20rts scaled using a depth dependent factor and by a model in which the lateral p velocity variations are different we first assess the assumption that p wave traveltimes can be explained by a model in which lateral p velocity variations p are identical to s velocity variations s in model s20rts for a given depth we project s from s20rts into model s2p using a depth dependent scaling factor r defined as s r z x p we find by grid search that the highest reduction of data variance is obtained when r increases linearly from 1 25 at the surface to 3 0 at the core mantle boundary a comparison of s wave ss and p wave pp traveltimes for identical source receiver pairs also indicates that r increases with depth significantly higher variance reduction is not obtained when r is parametrized with an additional degree of freedom therefore the precise shape of r cannot be constrained by our data p and pp wave traveltime anomalies with respect to the scaled model s2p yield coherent geographic variations this indicates that there are large scale lateral p velocity variations in the mantle that are different from those in model s2p we estimate these variations by inverting p wave traveltime anomalies with respect to model s2p for a degree 12 model of p velocity heterogeneity this model p 12 s 2 p indicates where in the well sampled mantle regions we need to modify model s2p to further improve the fit to the traveltime data anomalies in p12 s 2 p exist throughout the mantle it is therefore not obvious that compositional heterogeneity is prominent in the lower 1000 km of the mantle only as suggested previously low p wave velocities in the upper mantle beneath oceans are the strongest anomalies in p12 s 2 p and explain better the delayed traveltimes of pp wave phases with oceanic surface refection points lower mantle anomalies include high and low p velocity structures beneath eastern asia and north america respectively the high p velocity anomaly in the lower mantle beneath the central pacific is consistent with the assertion made by other researchers that large scale lower mantle upwellings are not purely thermal in origin

tomography of core mantle boundary and lowermost mantle coupled by geodynamics summary we propose an innovative approach to mapping cmb topography from seismic p wave traveltime inversions instead of treating mantle velocity and cmb topography as independent parameters as has been done so far we account for their coupling by mantle flow as formulated by forte peltier this approach rests on the assumption that p data are sufficiently sensitive to thermal heterogeneity and that compositional heterogeneity albeit important in localized regions of the mantle e g within the d region is not sufficiently strong to govern the pattern of mantle wide convection and hence the cmb topography the resulting tomographic maps of cmb topography are physically sound and they resolve the known discrepancy between images obtained from classic tomography on the basis of core reflected and core refracted seismic phases since the coefficients of mantle velocity structure are the only free parameters of the inversion this joint tomography geodynamics approach reduces the number of parameters nevertheless the corresponding mantle models fit the seismic data as well as the purely seismic ones

even degree lateral variations in the earth s mantle constrained by free oscillations and the free air gravity anomaly the recent occurrence of several large earthquakes in particular the 1994 june 9 bolivia event has motivated a re examination of the earth s large scale heterogeneity from a normal mode or free oscillation perspective compared to earlier studies the number of normal mode constraints on lateral variations in the mantle has increased five fold and toroidal and cross coupled modes complement the traditional spheroidal mode data set it is demonstrated that this large collection of mode data combined with the free air gravity anomaly can reliably constrain even degree lateral variations in wave velocities as well as density we present the first whole mantle density model constrained by seismology our shear and compressional velocity models are consistent with existing models based upon traveltimes and waveforms and are reasonably well correlated throughout the mantle shear and bulk sound velocity models exhibit a gradual decrease in correlation with depth and are anti correlated near the core mantle boundary we find that lateral variations in density are poorly correlated with wave velocities and are locally anti correlated with shear velocity in the lowermost mantle the correlations between wave velocities and density suggest both a thermal and a compositional origin to lateral heterogeneity in addition to traditional maps of lateral variations in wave velocity we also present maps of lateral variations in shear and bulk moduli the inversion puts weak constraints on even degree topographic variations on the core mantle boundary the 660 km discontinuity and dynamic free surface topography finally we determine both radially and laterally varying scaling relationships including poisson s ratio

upper mantle structure below the european continent constraints from surface wave tomography and grace satellite gravity data 1 we here exploit fundamental mode rayleigh and love seismic wave information and the high resolution satellite global gravity model ggm02c to obtain a 1 1 3 d image of a upper mantle isotropic shear wave speeds b densities and c density vscoupling below the european plate 20 n 90 n 40 w 70 e the 3 d image of the density vscoupling provides unprecedented detail of information on the compositional and thermal contributions to density structures the accurate and high resolution crustal model allows us to compute a reliable residual topography to understand the dynamic implications of our models the correlation between residual topography and mantle residual gravity anomalies defines three large scale regions where upper mantle dynamics produce surface expression the east european craton the eastern side of the arabian plate and the mediterranean basin the effects of mantle convection are also clearly visible at 1 the eastern sirt embayment 2 the west african craton northern margins 3 the volcanically active region of the canarian archipelago 4 the northern edge of the central european volcanic province and 5 the northeastern part of the atlantic ocean between greenland and iceland strong connections are observed among areas of weak radial anisotropy and areas where the mantle dynamics show surface expression although both thermal and additional dependencies have been incorporated into the density model convective down welling in the mantle below the east european craton is required to explain the strong correlation between the estimated negative mantle residual anomalies and the negative residual topography

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

teleseismic p wave tomography beneath the pantanal parana and chaco parana basins se south america delimiting lithospheric blocks of the sw gondwana assemblage

insights on upper mantle melting rheology and anelastic behavior from seismic shear wave tomography in seismic tomography we map the wave speed structure inside the earth but we ultimately seek to interpret those images in terms of physical parameters this is challenging because many parameters can trade off with each other to produce a given wave speed the problem is compounded by the convention of mapping seismic structures as perturbations relative to a 1 d reference model rather than absolute wave speeds using a full waveform tomography model of europe as a case study we quantify the extent to which thermochemical and dynamic properties can be constrained using only s wave speed expressed in absolute values the wave speed distributions of this tomography model are compared with 4 million thermochemical models whose seismic properties are computed via thermodynamic modeling these models sample the full range of realistic mantle compositions including variable water and melt contents and mineral intrinsic anelasticity is taken into account intrinsic anelasticity causes waves to travel more slowly at higher temperatures leading to seismic attenuation but the sensitivity of the wave speed reduction to temperature is in turn controlled by the wave frequency global studies of surface waves indicate an anticorrelation between s wave speed and attenuation we therefore only retain thermochemical models satisfying this anticorrelation our study indicates that the frequency dependence of anelasticity depends on temperature or rheology with 0 1 being most appropriate in cold or lithospheric mantle and 0 3 in warmer regions i e the asthenosphere additionally the slowest regions require specific compositions and or a velocity weakening mechanism such as partial melting elastically accommodated grain boundary sliding or water

mantle tomography and its relation to temperature and composition we propose a new method to constrain lateral variations of temperature and composition in the lower mantle from global tomographic models of shear and compressional wave speed we assume that the mantle consists of a mixture of perovskite and magnesio wustite in a first stage we directly invert vp and vs anomalies for variations of temperature and composition using the appropriate partial derivatives or sensitivities of velocities to temperature and composition however uncertainties in the tomographic models and in the sensitivities are such that variations in composition are completely unconstrained inferring deterministic distributions of temperature and composition being currently not possible we turn to a statistical approach which allows to infer several robust features comparison between synthetic and predicted ratios of the relative shear to compressional velocity anomalies indicates that the origin of seismic anomalies cannot be purely thermal but do not constrain the amplitude of the variations of temperature and composition we show that we can estimate these variations using histograms of the relative vp and vs anomalies at a given depth we computed histograms for a large variety of cases and found that at the bottom of the mantle variations in the volumic fraction of perovskite from 14 to 10 are essential to explain seismic tomography in the mid mantle anomalies of perovskite are not required but moderate variations up to 6 can explain the observed distributions equally well these trade offs between anomalies of temperature and composition cannot be resolved by relative velocity anomalies alone an accurate determination of temperature and composition requires the knowledge of density variations as well we show that anomalies of iron can then also be resolved 2003 elsevier b v all rights reserved

constraints on lower mantle composition from molecular dynamics simulations of mgsio3 perovskite abstract we have carried out molecular dynamics simulations of mgsio 3 perovskite pv in order to determine its equation of state and elastic parameters under pressure and temperature conditions of the earth s lower mantle combining this information with density and elastic constants for mg fe o magnesiowustite mw and including the effects of iron on the density of perovskite we investigated possible compositional models for the lower mantle by calculating seismic velocities and densities as functions of depth and comparing them to the globally averaged seismological models ak135 f and prem the calculations were done along several possible geothermal gradients both adiabatic and non adiabatic and for a range of mg fe partitioning coeffcients k fe mg pv mw 0 10 0 35 our results indicate that the most probable compositional models for the lower mantle are pyrolitic with x mg 0 87 and x pv 0 54 for increasing temperatures at the top of the lower mantle the best fitting models increase slightly in both iron and silica content

seismic evidence of partial melt within a possibly ubiquitous low velocity layer at the base of the mantle three source regions show evidence of a low velocity layer that is less than 15 kilometers thick on top of the core mantle boundary and require about a 3 1 ratio of shear to compressional velocity reduction which is consistent with partial melt layer thickness is correlated with travel time residuals of the seismic phases that are most sensitive to the lowermost mantle velocity these observations suggest that the layer is thinned beneath downwellings but is present everywhere along the core mantle boundary low viscosity accompanying partial melt can localize the upwelling of warmed mantle making the low velocity layer a plausible source of mantle plumes

sensitivities of seismic velocities to temperature pressure and composition in the lower mantle we calculated temperature pressure and compositional sensitivities of seismic velocities in the lower mantle using latest mineral physics data the compositional variable refers to the volume proportion of perovskite in a simplified perovskite magnesiowustite mantle assemblage the novelty of our approach is the exploration of a reasonable range of input parameters which enter the lower mantle extrapolations this leads to realistic error bars on the sensitivities temperature variations can be inferred throughout the lower mantle within a good degree of precision contrary to the uppermost mantle modest compositional changes in the lower mantle can be detected by seismic tomography with a larger uncertainty though a likely trade off between temperature and composition will be largely determined by uncertainties in tomography itself given current sources of uncertainties on recent data anelastic contributions to the temperature sensitivities calculated using karato s approach appear less significant than previously thought recent seismological determinations of the ratio of relative s to p velocity heterogeneity can be entirely explain by thermal effects although isolated spots beneath africa and the central pacific in the lowermost mantle may ask for a compositional origin 2001 elsevier science b v all rights reserved

thermal nature of mantle upwellings below the ibero western maghreb region inferred from teleseismic tomography independent models of p wave and s wave velocity anomalies in the mantle derived from seismic tomography help to distinguish thermal signatures from those of partial melt volatiles and compositional variations here we use seismic data from sw europe and nw africa spanning the region between the pyrenees and the canaries in order to obtain a new s sks relative arrival time tomographic model of the upper mantle below iberia western morocco and the canaries similar to previous p wave tomographic results the s wave model provides evidence for 1 subvertical upper mantle low velocity structures below the canaries atlas ranges and gibraltar arc which are interpreted as mantle upwellings fed by a common lower mantle source below the canaries and 2 two low velocity anomalies below the eastern rif and betics that we interpret as the result of the interaction between quasi toroidal mantle flow induced by the gibraltar slab and the mantle upwelling behind it the analysis of teleseismic p wave and s wave arrival time residuals and the conversion of the low velocity anomalies to temperature variations suggest that the upwellings in the upper mantle below the canaries atlas ranges and gibraltar arc system may be solely thermal in nature with temperature excesses in the range 100 350 c our results also indicate that local partial melting can be present at lithospheric depths especially below the atlas ranges the locations of thermal mantle upwellings are in good agreement with those of thinned lithosphere moderate to high heat flow measurements and recent magmatic activity at the surface

bayesian inference of mantle viscosity from whole mantle density models the long wavelength geoid is sensitive to earth s mantle density structure as well as radial variations in mantle viscosity we present a suite of inversions for the radial viscosity profile using

a 3d vs model of the upper mantle beneath italy insight on the geodynamics of central mediterranean abstract we compute a high resolution vs model of the upper mantle underneath italy by inverting relative residuals of s wave arrivals from teleseisms recorded at the italian broad band national seismic network we also present a model of vp vs perturbations obtained from similarly resolved p and s wave velocity models the joint analyses of vs and vp vs models suggests that compositional variations concur with temperature variations to generate the observed velocity patterns giving deeper insight into the upper mantle structure the investigation of coherent high vs and high vp anomalies permits a comprehensive picture of the main oceanic domains involved in the puzzling subduction system of the central mediterranean slab windows and tears developed in the ionian slab are highlighted by regions of low vp low vs and high vp vs anomalies the adria continental lithosphere is characterized by high vp high vs and low vp vs anomalies that trace the sinking of eclogitized material underneath the apennines and the eastern alps chains the overall tomographic results yield the development of a new conceptual model for the alps and apennines formation in which three basic processes occurred in different time and different locations all around the alpine apennines system continental collision slab break off and delamination of the upper plate lithosphere

simultaneous inversion for 3 d variations in shear and bulk velocity in the mantle until recently most of the seismic tomographic modeling has been addressing the question of lateral heterogeneity either in p or s wave velocities the s wave velocity variations are larger and hence provide stronger signal on long period waveforms the direct p travel times being the first arrivals on the other hand are most frequently reported in the international seismological centre isc bulletins in mineral physics experiments the variation in bulk velocity is more often measured to better understand the differences between vp and vp patterns and better link the results from mineral physics to those of seismic tomography we formulate the inverse problem in terms of relative perturbations in the shear velocity vs 12 and bulk sound velocity v k 12 we use a large data set which consists of waveforms waveform derived travel times and travel times from the isc bulletins the earthquakes are relocated using corrections for lateral heterogeneity the events which cannot be reliably determined are discarded the model is defined as spherical harmonics to degree 12 horizontally and as chebyshev polynomials to order 13 radially for both shear and bulk sound velocity the inversion is performed under smoothness constraint the resolution tests and bootstrapping analysis indicate that the model is well recovered particularly at long wavelength the results indicate a much larger variability of shear than bulk sound velocity the model explains observations well the most intriguing result obtained in this study is that the variations in shear velocity and bulk sound velocity are negatively correlated in the lowermost mantle the explanation is not very clear from the mineral physics point of view it is not unlikely that this could be explained in terms of thermal variation even though we are unwilling to rule out the possibility of large wavelength compositional variations

upper mantle seismic wave velocity effects of realistic partial melt geometries we investigate seismic wave velocity reduction resulting from the presence of partial melt in the upper mantle the amount of shear and bulk modulus reduction produced by the presence of a connected network of realistically shaped and naturally organized melt inclusions is found using finite element calculations the geometries of the inclusions are taken directly from laboratory experiments of mantle melting with finite element meshes constructed to conform to these shapes the shear and bulk moduli of the composite material are found for both the unrelaxed isolated inclusions and relaxed pressure equalized inclusions cases by assigning appropriate material properties to the fluid modulus reduction from deformation simulations of a solid containing realistically shaped and ellipse shaped melt inclusions quantify the effect of melt pocket cuspateness and melt pocket organization on seismic velocity reduction the three dimensional response is estimated from two dimensional distributions of the melt phase by determining the mode ii and mode iii components of elastic modulus reduction separately and summing their effects in general cuspate and naturally organized melt inclusions cause greater velocity reduction it is shown that v and vs reduction per percent partial melt are at least 3 6 and 7 9 respectively even higher values for velocity reduction are possible above 1 melt fraction if melt exists only in tubules below 1 melt fraction the lower more conservative values of velocity reduction are 070 greater for vp and 84 greater for vs than the analytically determined values for ellipsoidal inclusions somewhat greater effects are possible if nonrandom organization of melt occurs on scales greater than our model

sedimentary carbon sulfur and iron relationships in modern and ancient diagenetic environments of the eel river basin u s a abstract depositional and diagenetic controls on the distributions of carbon sulfur and iron c s fe in modern sediments and upper pleistocene mudrocks of the eel river basin erb northern california continental margin were investigated using a combination of geochemical radioisotopic and sedimentological methods a mass balance based on down core profiles of porewater and solid phase constituents and diagenetic modeling suggests that only 12 30 of the pyrite s produced via so4 2 reduction during burial is retained in modern shelf and upper slope deposits of the erb bioturbational reoxidation of initially reduced s is inferred to be the major control on s preservation on the basis of an observed inverse relationship between pyrite s retention and biological mixing intensity estimated from profiles of excess 234th importantly these findings argue that massive depositional episodes on the shelf following floods of the eel river have a negligible long term impact on bioturbating macrofauna and the potential to affect geochemical properties of the sediments down core profiles of reactive fe3 and py fe ii for the modern deposits suggest that highly reactive fe phases are sulfidized well within 500 2000 years of burial thereby limiting later pyritization which could occur through sulfidation of less reactive phases this result explains the low 0 4 degree of pyritization dop values exhibited by both modern and ancient deposits of the erb and lends support to the notion that pyritization in aerobic continental margin sediments is largely associated with highly reactive detrital fe oxides comparable mean c s weight ratios for modern sediments 5 4 3 3 1 and mudrocks 6 9 4 5 of the erb suggest that the upper pleistocene strata reflect a geochemical environment analogous to that of the modern margin specifically the c s fe signatures shared by the modern and ancient deposits are a consequence of similar detrital fe mineralogies initial organic matter content corg 1 and composition c n 13 to 17 13corg 22 to 25o oo burial rate and importantly bioturbation intensity the findings of this study have important implications for the use of c s fe signatures as indicators of diagenetic processes in dynamic continental margin environments

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

ab initio elasticity and thermal equation of state of mgsio3 perovskite we have used high temperature ab initio molecular dynamic simulations to study the equation of state of orthorhombic mgsio3 perovskite under lower mantle pressure temperature conditions we have determined the gru neisen parameter q as a function of volume our state of the art simulations accurate to within 10 resolve the long standing controversy on thermal expansion k and gru neisen parameter of mgsio3 perovskite under ambient conditions we find the values for k and q of 1 86u10 35 k 31 and 1 51 respectively in excellent agreement with the latest experimental studies calculated elastic constants and the static equation of state at 0 k agree well with previous simulations we have found no evidence for the high temperature phase transitions of orthorhombic mgsio3 perovskite to cubic or tetragonal phases at mantle temperatures fl 2001 elsevier science b v all rights reserved

refining upon the climatic background of the early pleistocene hominid settlement in western europe barranco leon and fuente nueva 3 guadix baza basin se spain abstract the early pleistocene sites of barranco leon and fuente nueva 3 guadix baza basin se spain have yielded thousands of mode 1 or oldowan lithic artifacts both sites and one tooth in layer d formerly 5 of barranco leon today considered to be some of the earliest evidence of humans in western europe at ca 1 2 1 5 ma previous quantitative paleoclimatic reconstructions based on herpetile assemblages indicated that during the formation of these two sites the mean annual temperature and mean annual precipitation were higher than they are now in the southeastern iberian peninsula with lower continentality here we propose new climatic reconstructions where the mean monthly temperature and precipitation and the difference between the four driest months and the four rainiest months are estimated climatograms are built in order to specify the distribution and variation of temperature and precipitation during the year and the aridity indices of gaussen lautensach mayer dantin revenga and de martonne are used to characterize ombroclimatic differences according to these new climatic parameters rainfall distribution through the year shows considerably higher precipitation in every season except summer and early autumn which remain drier and thus consistent with a mediterranean climate pattern no change is observed in the duration of the aridity period which remains four months long however the value of the aridity index of de martonne is higher than 20 subhumid climate in barranco leon and fuente nueva 3 whereas today it is lower than 20 semi arid climate suggesting major changes in the ombroclimatic type these results yield a more precise scenario for the paleoclimatic conditions that prevailed during the late early pleistocene in the guadix baza basin and permit us to contrast the ages obtained from numerical dating and biochronology the very warm and humid climate reconstructed for both barranco leon and fuente nueva 3 suggests that in accordance with the numerical dating these two sites are contemporaneous with the particularly warm interglacial peaks of marine isotope stages 43 49 i e between 1 35 and 1 47 ma the similarity between reconstructed climates the high overlap between their estimated precipitation and between the difference of the driest from the rainiest season suggest that these sites may correspond to the same part of a climatic glacial interglacial cycle but because the evolutionary degree of the rodent mimomys savini shows a slightly more derived state for fuente nueva 3 than for barranco leon i e a short chronological difference between the two sites they may correspond to two consecutive warm interglacial peaks

elasticity composition and temperature of the earth s lower mantle a reappraisal the interpretation of seismological models for the earth s lower mantle in terms of chemical composition and temperature is sensitive both to the details of the methodology employed and also to uncertainties in key thermoelastic parameters especially for the dominant mg fe sio3 perovskite phase here the alternative approaches adiabatic decompression of the lower mantle for comparison at zero pressure with laboratory data and the projection of laboratory data to lower mantle p t conditions for direct comparison with seismological observations are assessed along with the equations of state on which they are based it is argued that adiabatic decompression of the lower mantle is best effected by a strategy in which consistent third order finite strain expressions are required simultaneously to fit the strain dependence of both the seismic parameter and the density this procedure accords due weight to the most robust seismological observations namely the wave speeds and reduces to an acceptable level the otherwise very strong covariance among the fitted coefficients it is demonstrated that this approach can readily be adapted to include the effects of relaxation from the hill average to the reuss lower bound of the aggregate bulk modulus which governs the radial variation of density for the projection of laboratory thermoelastic data to lower mantle p t conditions the preferred equation of state is of the mie gruneisen type involving the addition at constant volume of the pressure along a finite strain principal 300 k isotherm and the thermal pressure calculated from the debye approximation to the lattice vibrational energy a high degree of consistency is demonstrated between these alternative equations of state possible departures of the lower mantle from conditions of adiabaticity and large scale homogeneity are assessed these are negligible for the prem model bullen parameter b 0 99 0 01 but significant for ak135 b 0 94 0 02 reviving the possibility of a substantially superadiabatic temperature gradient experimentally determined thermoelastic properties of the major mg fe sio3 perovskite and mg fe o magnesiowustite phases are used to constrain the equation of state parameters employed in the analysis of seismological information although the scarcity of information concerning the pressure and temperature dependence of the elastic moduli for the perovskite phase remains a serious impediment nevertheless it is demonstrated that the combination of a pyrolite composition simplified to the three component system sio2 mgo feo with molar xpv 0 67 and xmg 0 89 and a lower mantle adiabat with a potential temperature of 1600 k consistent with the preferred geotherm for the upper mantle and transition zone is compatible with the prem seismological model within the residual uncertainties of the thermoelastic parameters for the perovskite phase in particular such consistency requires values for the perovskite phase of ks ks p t and q ln ln v t of approximately 3 8 and 2 and g t near 0 022 gpa k 1 for the lower mantle assemblage more silicic models which have sometimes been advocated can also be reconciled with the seismological data but require lower mantle temperatures which are much higher by about 700 k for pyroxene stoichiometry however the absence of seismological and rheological evidence for the pair of thermal boundary layers separating convection above and below which is implied by such models remains a formidable difficulty under these circumstances the simplest possible model that of grossly uniform chemical composition throughout the mantle is preferred

on the resolution of density anomalies in the earth s mantle using spectral fitting of normal mode data summary the resolution of the 3 d density structure in the earth s deep interior has long eluded geoscientists high quality data from digital seismic instruments emplaced this decade have renewed interest in the measurement of low frequency earth normal modes with the goal of extracting heterogeneous density structure here we perform a series of synthetic experiments aimed at investigating the resolution of lateral variations in the mantle from normal mode spectral data contamination effects between seismic velocities and density are examined in two ways 1 by using resolution matrices computed from data kernels and 2 by inverting synthetic spectra computed from realistic input earth models the first type of experiment assumes that the non linearity of the inverse problem is weak no such assumptions are necessary in the second type of tests which nevertheless concur in their results with the former these synthetic tests indicate that density structure retrieved from presently available normal mode data is not reliable contamination of seismic velocity structure into the density model space produces patterns that resemble those obtained from inversions of real data the resulting density models appear to be dependent on a combination of the starting velocity models and the model parametrization

using probabilistic seismic tomography to test mantle velocity density relationships we use a neighborhood algorithm to explore the fit to long period seismic data of a wide variety of long wavelength mantle models this approach to the global tomographic inverse problem yields probability distributions for seismic velocities density and related properties as functions of depth such distributions can be robust even when individual models are not and allow us to test several assumptions about the earth that have long been enforced a priori in inversions in particular we are able to test the paradigm ofdeep mantle heterogeneity that is dominantly thermal in origin producing velocity and density perturbations that are well correlated and have relative amplitudes given by nlnb nlnvs 6 0 5 our distributions show that such relationships are unlikely and even though the results are consistent with recent best fitting models from damped seismic inversions they demonstrate that many specific properties ofsuch models are not robust the data clearly f density perturbations that are poorly or negatively correlated with velocity heterogeneity and have amplitudes several times larger yielding nlnb nlnvs s 1 0 than damped inversions allow these characteristics are most pronounced in the upper mantle transition zone and the base ofthe lower mantle suggesting layered convection the negative density velocity correlations f at these depths imply dominantly chemical heterogeneity while the likelihood ofrelatively high amplitude density variations suggests that variable iron content is an important component ofthis heterogeneity these results which we show to be consistent with independent gravity constraints represent a profound change in the interpretation of seismic constraints in addition the distributions show that even though best fitting density models from recent inversions or our sampling are consistent with the data most specific properties of such models are not robust this implies that it is more appropriate to use seismic model distributions rather than individual models to make geodynamic and geochemical inferences

comparison of p and s station corrections and their relationship to upper mantle structure we use international seismological centre data between the years 1964 and 1994 to derive a data set of p and s station corrections by restricting the raw p and s data to those which share the same source to receiver paths we minimize any bias to the station corrections from differential p and s data distributions we find that both static and azimuth dependent terms are highly correlated in addition we perform synthetic tests to determine the sensitivity of our data to mantle structure and show that static terms are sensitive to structure at least throughout the upper mantle in determining the regression slope of the static terms we allow for regional offsets and find a best fit global slope of s to p correction a 2 85 0 19 this is lower than that obtained in previous station correction studies but is in agreement with results in the upper mantle using differential travel times and with values derived for the upper parts of the lower mantle regionalizing the data by tectonics does not give statistically distinct ratios of s to p station correction the result for shield areas however is significantly smaller than previously believed probably because of our ability to derive station corrections from similar data sets variations in the second azimuthal terms are similar in both fast direction and relative magnitude in another synthetic experiment we find that their general trends can be predicted well by isotropic models of heterogeneous velocity structure from this result and also because they do not correlate with sks splitting data we conclude that station correction second azimuthal terms should not be interpreted in terms of anisotropy

geodynamic evidence for a chemically depleted continental tectosphere the tectosphere namely the portions of earth s mantle lying below cratons has a thermochemical structure that differs from average suboceanic mantle the tectosphere is thought to be depleted in its basaltic components and to have an intrinsic buoyancy that balances the mass increase associated with its colder temperature relative to suboceanic mantle inversions of a large set of geodynamic data related to mantle convection using tomography based mantle flow models indicate that the tectosphere is chemically depleted and relatively cold to 250 kilometers depth below earth s surface the approximate equilibrium between thermal and chemical buoyancy contributes to cratonic stability over geological time

geophysical evidence for chemical variations in the australian continental mantle 1 the relative density to shear velocity scaling provides a diagnostic for the presence of compositional variations in the mantle we invert shear wave velocity from a recent 3 d model and gravity anomalies for radial profiles of of the uppermost mantle beneath australia we performed calculations for the three major tectonic provinces that constitute the continent and found significant differences between them the profile for the phanerozoic region can be explained by thermal variations alone in contrast negative values of suggest that variations in composition are important between 75 and 150 km depth in the proterozoic continental lithosphere central australia it is likely that chemical variations are also required to explain the inferences for the archean craton west australia but poor tomographic resolution precludes a definitive conclusion the scaling factors found are consistent with chemical depletion of deep precambrian lithosphere which supports a tectosphere model for the australian continental roots

anomalies of temperature and iron in the uppermost mantle inferred from gravity data and tomographic models we propose a method to interpret seismic tomography in terms of thermal and compositional anomalies in addition to the tomographic model we use gravity data which provide information on the density expressed as a relative density to shear wave velocity scaling factor ln ln vs the inferred values of are not consistent with the presence of thermal anomalies alone however simultaneous anomalies of temperature and composition explain the observations compositional anomalies can have several origins but we find the most relevant parameter to be the global volumic fraction of iron xfe fe fe mg we invert the tomographic model s16rlbm woodhouse and trampert 1995 and the density anomalies correlated to vs anomalies 0 vs v0 for anomalies of temperature t and iron fe the partial derivatives are provided by a numerical method that reconstructs density and seismic velocity for given temperatures and petrologic models vacher et al 1998 down to z 300 km depth the distribution of temperature and iron anomalies strongly depends on the surface tectonics the continental mantle below old cratons and stable platforms is colder than average and depleted in iron whereas the oceanic mantle is mostly homogeneous due to uncertainties on the reference state of the mantle error bars on t and fe reach 10 of the inverted values finally we apply these results to the stability of continental roots and test the hypothesis that the negative buoyancy induced by lower than average temperatures is balanced by the positive buoyancy induced by the depletion in iron we find that continental roots are stable only if the viscosity of the mantle is strongly temperature dependent however some uncertainties remain on the real effects and importance of rheology 2002 elsevier science b v all rights reserved

influence of thermochemical piles on topography at earth s core mantle boundary abstract numerous seismic studies reveal the presence of two large low velocity anomalies beneath africa and the central pacific efforts to characterize these anomalies have yielded a variety of interpretations over the years both isochemical and thermochemical previous interpretations have included large isochemical superplumes clusters of smaller thermal plumes and doming thermochemical superplumes a conceptual mantle model that is presently growing favor involves long lived thermochemical piles in anticipation that nutation studies will provide better topographic constraints on the core mantle boundary cmb in the future we examine the effects of thermochemical piles at this boundary in this study we perform numerical modeling of thermochemical and isochemical convection as a function of convective vigor and temperature dependent viscosity to predict the topography at earth s cmb our results show that in isochemical convection downwellings always lead to negative depressed cmb topography while upwellings cause positive elevated topography consistent with previous studies however in thermochemical convection cmb topography and its relationship to downwellings and thermochemical piles are significantly affected by temperature dependent viscosity for isoviscous or weakly temperature dependent viscosity the piles cause negative cmb topography while positive topography is often below cold downwellings however for realistic more strongly temperature dependent viscosity the most negative cmb topography is below downwellings while the topography below the piles is often slightly more positive and or flat similar to upwellings in isochemical models we also show that although thermochemical piles are intrinsically more dense the large thermal buoyancy associated with them leads to overall relative buoyancy that is on par with slabs consequently thermochemical models lead to an overall reduction in magnitude of cmb topography with respect to isochemical models

comparing p and s wave heterogeneity in the mantle from the reprocessed data set of engdahl and co workers we have carefully selected matching p and s data for tomographic imaging we assess data and model error and conclude that our s model uncertainty is twice that of the p model we account for this in our comparison of the perturbations in p and s wavespeed in accord with previous studies we find that p and s perturbations are positively correlated at all depths however in the deep mantle systematic differences occur between regions that have undergone subduction in the last 120 million years and those that have not in particular below 1500 km depth ln vs ln vp is significantly larger in mantle regions away from subduction than in mantle beneath convergent margins this inference is substantiated by wavespeed analyses with random realizations of the slab non slab distribution through much of the mantle there is no significant correlation between bulk sound and s wave perturbations but they appear to be negatively correlated between 1700 and 2100 km depth which is also where the largest differences in ln vs ln vp occur this finding supports convection models with compositional heterogeneity in the lowermost mantle

toward quantifying uncertainty in travel time tomography using the null space shuttle 1 the solution of large linear tomographic inverse problems is fundamentally non unique we suggest to explore the non uniqueness explicitly by examining the null space of the forward operator we show that with the null space shuttle it is possible to assess robustness in tomographic models and we illustrate the concept for the global p wave model mit p08 we found a broad range of acceptable solutions compatible with the travel time data the root mean square rms velocity perturbations vary from 0 2 to 0 6 in the lowermost mantle and from 0 3 to 1 3 in the upper mantle such large variations in average amplitudes prohibit meaningful inferences on temperature or chemical variations in the earth from tomographic models alone on a global scale much short wavelength structure resides in the null space of the forward operator suggesting that the data do not everywhere resolve structure on the smallest length scale 200 km allowed by the block parameterization used in mit p08 and similar models this indicates that great care should be taken when interpreting such structure as a practical measure we suggest that only those structures for which the wave speed perturbations do not change sign within the range of models permitted by the data should be considered robust with this criterion the model null space analysis shows that the high velocity anomalies in the lower mantle which are often interpreted as remnants of slabs of subducted lithosphere are required by the seismic data low velocity anomalies underneath for instance hawaii iceland and africa show varying degrees of robustness