joint inversions of seismic and geodynamic data for models of three dimensional mantle heterogeneity the seismic models of three dimensional 3 d mantle heterogeneity may be interpreted in terms of the density perturbations which drive mantle flow and thus provide important predictions of flow related observables such as the nonhydrostatic geoid the current models of global scale shear velocity heterogeneity provide reasonably good fits to the long wavelength nonhydrostatic geoid data 60 70 variance reductions but rather poor fits to the corresponding free air gravity anomalies 30 40 variance reductions this major difference is due to two factors 1 the very different amplitude spectrum of the gravity anomalies which is nearly flat for degrees l 3 the largest mismatch between the pattern of predicted and observed gravity anomalies is in the southern hemisphere this observation suggests that one reason for the poor overall match between the two fields may be that the global seismic data are not resolving sufficiently well the heterogeneity in the southern hemisphere portion of the deep mantle in contrast the gravity data provide accurate and geographically uniform constraints on the vertically integrated heterogeneity in the mantle we therefore perform a series of experiments in which we simultaneously invert a large set of seismic data which includes long period waveforms ss s and scs s differential travel times and normal mode structure coefficients and the long wavelength gravity anomaly data we thus determine whether it is possible to derive new 3 d heterogeneity models which satisfy both data sets the gravity anomaly data are interpreted in the context of spherically symmetric viscous flow models of the mantle in these inversion experiments we test several radial viscosity and ln ln profiles and thereby assess their plausibility the joint seismic geodynamic inversions reveal that it is indeed possible to greatly improve the fit to the free air gravity anomalies with variance reductions of 80 90 readily accessible while preserving the fit to the seismic data this improvement is achieved with some adjustment to the heterogeneity in the depth range 1500 2500 km where the seismic data constraints appear to be weakest the joint inversions also reveal new structures in the southern hemisphere portion of the lower mantle which apparently are not resolved by the seismic data alone

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

multiple impacts across the cretaceous tertiary boundary abstract the stratigraphy and age of altered impact glass microtektites microkrystites ejecta layers from the chicxulub crater are documented in late maastrichtian and early danian sediments in mexico guatemala belize and haiti in northeastern mexico two to four ejecta layers are present in zone cf1 which spans the last 300 ky of the maastrichtian the oldest ejecta layer is dated at 65 27 0 03 ma based on sediment accumulation rates and extrapolated magnetostratigraphy all younger ejecta layers from the maastrichtian and early danian parvularugoglobigerina eugubina zone pla l may represent repeated episodes of reworking of the oldest layer at times of sea level changes and tectonic activity the k t boundary impact event 65 0 ma is not well represented in this area due to widespread erosion an early danian pla l ir anomaly is present in five localities bochil actela coxquihui trinitaria and haiti and is tentatively identified as a third impact event at about 64 9 ma a multiimpact scenario is most consistent with the impact ejecta evidence the first impact is associated with major deccan volcanism and likely contributed to the rapid global warming of 3 4 c in intermediate waters between 65 4 and 65 2 ma decrease in primary productivity and onset of terminal decline in planktic foraminiferal populations the k t boundary impact marks a major drop in primary productivity and the extinction of all tropical and subtropical species the early danian impact may have contributed to the delayed recovery in productivity and evolutionary diversity

on crustal corrections in surface wave tomography summary mantle models from surface waves rely on good crustal corrections we investigated how far ray theoretical and finite frequency approximations can predict crustal corrections for fundamental mode surface waves using a spectral element method we calculated synthetic seismograms in transversely isotropic prem and in the 3 d crustal model crust2 0 on top of prem and measured the corresponding time shifts as a function of period we then applied phase corrections to the prem seismograms using ray theory and finite frequency theory with exact local phase velocity perturbations from crust2 0 and looked at the residual time shifts after crustal corrections residuals fall within the uncertainty of measured phase velocities for periods longer than 60 and 80 s for rayleigh and love waves respectively rayleigh and love waves are affected in a highly non linear way by the crustal type oceanic crust affects love waves stronger while rayleigh waves change most in continental crust as a consequence we find that the imperfect crustal corrections could have a large impact on our inferences of radial anisotropy if we want to map anisotropy correctly we should invert simultaneously for mantle and crust the latter can only be achieved by using perturbation theory from a good 3 d starting model or implementing full non linearity from a 1 d starting model

seismic and mineralogical structures of the lower mantle from probabilistic tomography 1 one of the most powerful approaches for understanding the 3 d thermo chemical structure of the lower mantle is to link tomographic models with mineral physics data this is not straightforward because of strong trade offs between thermal and chemical structures and their influence on seismic structures they can be reduced by mapping simultaneously perturbations of wave speeds and density anomalies and by the quantitative assessment of the accuracy and uniqueness of seismic and mineralogical data here we present new tomographic maps of low order even degree seismic structures which are an improvement on earlier models they satisfy constraints from body wave surface wave and normal mode data simultaneously thereby enhancing the spatial resolution furthermore the seismic structure at a given location is represented by a probability density function pdf which takes into account the uncertainty and non uniqueness of the solution due to modeling and data restrictions following a robust statistical procedure we fit heterogeneity of wave speeds and density from hypothetical thermo chemical models to those of our tomographic maps we thereby constrain lateral variations of temperature as well as iron silica and post perovskite concentration in terms of pdfs our work shows that large scale chemical variations are likely everywhere in the lower mantle in most of the d region post perovskite is most abundant in the circum pacific belt but near the core its lateral variation is more complex furthermore post perovskite concentration trades off with the amplitudes of temperature and silicate variations but not with their lateral distribution this might be the reason why temperature and silicate variations appear not constrained by our data in the lowermost few hundred km of the mantle

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

phase transition in mgsio 3 perovskite in the earth s lower mantle a new polymorph of mgsio3 more stable than the pbnm perovskite phase has been identified by first principles computations it has the cairo3 structure with cmcm symmetry and consists of sio3 layers intercalated by eightfoldcoordinated mg ions high temperature calculations within the quasiharmonic approximation give a volume change of f1 5 and a clapeyron slope of f7 5f0 3 mpa k at f2750 k and f125 gpa these pressure temperature p t conditions are close to those in which a phase transition in mgsio3 perovskite has been observed by in situ angle dispersive x ray diffraction measurements this transition appears to be associated with the du discontinuity d 2004 elsevier b v all rights reserved

global anisotropic phase velocity maps for higher mode love and rayleigh waves summary it is well established that the earth s uppermost mantle is anisotropic but there are no clear observations of anisotropy in the deeper parts of the mantle surface waves are well suited to observe anisotropy since they carry information about both radial and azimuthal anisotropy fundamental mode surface waves for commonly used periods up to 200 s are sensitive to structure in the first few hundred kilometres and therefore do not provide information on anisotropy below higher mode surface waves have sensitivities that extend to and beyond the transition zone and should thus give insight about azimuthal anisotropy at greater depths we have measured higher mode love and rayleigh phase velocities using a model space search approach which provides us with consistent relative uncertainties from measurement to measurement and from mode to mode from these phase velocity measurements we constructed global anisotropic phase velocity maps prior to inversion we determine the optimum relative weighting for anisotropy we present global azimuthal phase velocity maps for higher mode rayleigh waves up to the sixth higher mode and love waves up to the fifth higher mode with corresponding average model uncertainties the anisotropy we derive is robust within the uncertainties for all modes given the ray theoretical sensitivity kernels of rayleigh and love wave modes the source of anisotropy is complex but mainly located in the asthenosphere and deeper our models show a good correspondence with other studies for the fundamental mode but we have been able to achieve higher resolution

paleoatmospheric pco2 fluctuations across the cretaceous tertiary boundary recorded from paleosol carbonates in ne china abstract a dramatic change in atmospheric composition has been postulated because of global carbon cycle disruption during the cretaceous k tertiary t transition following the chicxulub impact and deccan trap eruptions here pedogenic carbonates were collected from drill core of a borehole sk 1 n straddling the late cretaceous and early paleocene strata in the songliao basin northeast china to reconstruct atmospheric co 2 concentrations using a paleosol paleobarometer our estimates for atmospheric pco 2 from paleosol carbonates range between 277 115 ppmv and 837 164 ppmv between 67 8 ma and 63 1 ma one large 66 65 5 ma and several small co 2 spikes 64 7 64 2 ma during the latest maastrichtian to earliest danian are reported here and incorporated with previously published pco 2 estimates also estimated from paleosol carbonates these co 2 spikes are attributed to one million year long emplacement of the large deccan flood basalts along with the extraterrestrial impact at the k t boundary

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

phase transitions in pyrolite and morb at lowermost mantle conditions implications for a morb rich pile above the core mantle boundary abstract subduction of mid oceanic ridge basalt morb gives rise to strong chemical heterogeneities in the earth s mantle possibly extending down to the core mantle boundary phase relations in both pyrolite and morb compositions are precisely determined at high pressures and temperatures corresponding to lowermost mantle conditions the results demonstrate that the post perovskite phase transition occurs in pyrolite between 116 and 121 gpa at 2500 k while post perovskite and sio2 phase transitions occur in morb at 4 gpa lower pressure at the same temperature theory predicts that these phase changes in pyrolite and morb cause shear wave velocity increase and decrease respectively near the northern margin of the large low shear velocity province in the lowermost mantle beneath the pacific reflections from a negative shear velocity jump near 2520 km depth are followed by reflections from a positive velocity jump 135 to 155 km deeper these negative and positive velocity changes are consistent with the expected phase transitions in a dense pile containing a mixture of morb and pyrolitic material this may be a direct demonstration of the presence of accumulations of subducted morb crust in the deep mantle

joint seismic tomography for bulk sound and shear wave speed in the earth s mantle high quality p and s travel times are now available from careful reprocessing of data reported to international agencies a restricted data set has been extracted for which comparable ray coverage is achieved for p and s and used for a joint inversion to produce a three dimensional model for shear and bulk sound velocities represented in terms of 2 2 cells and 18 layers in depth through the mantle about 106 times for each of p and s are combined to produce 312 549 summary rays for each wave type linearizing about the ak135 reference model 583 200 coupled tomographic equations are solved using an iterative partitioned scheme clear high resolution images are obtained for both bulk sound speed and shear wavespeed the bulk and shear moduli have differing sensitivity to temperature and mineral composition and so the images of the two velocity distributions help to constrain the nature of the processes which produce the variations different heterogeneity regimes can be recognised in the upper mantle the transition zone most of the lower mantle and the lowermost mantle in the upper mantle many features can be explained by thermal effects but in some orogenic zones e g western north america the opposite sense of the bulk sound and shear wave speed variation requires compositional effects or volatiles to outweigh any thermal effects in the lower mantle pronounced narrow structures which may represent remnant subduction are most marked in shear the level of large scale variations in bulk sound speed compared to shear diminishes with depth in the lower mantle reaching a minimum near 2000 km below this depth the variability of both wave speeds increases near the core mantle boundary the variations of the two wave speeds show little concordance suggesting the presence of widespread chemical heterogeneity

chicxulub impact predates k t boundary new evidence from brazos texas abstract multidisciplinary studies including stratigraphy sedimentology mineralogy and geochemistry of the new core mullinax 1 and outcrops along the brazos river and cottonmouth creek falls county texas reveal the complex history of the chicxulub impact the event deposit and the k t boundary event the k t boundary as identified by the negative 13c shift first occurrence of danian planktic foraminifera and palynomorphs occurs 80 cm above the event deposit in core mullinax 1 the underlying 80 cm interval was deposited in a shallow low oxygen environment during the latest maastrichtian as indicated by high stress microfossil assemblages small shells and burrows infilled with framboidal pyrite the underlying event deposit commonly interpreted as k t impact tsunami consists of a basal conglomerate with clasts containing chicxulub impact spherules repeated upward fining units of spherule rich sands followed by hummocky cross bedded and laminated sands which are burrowed by thalassinoides planolites and ophiomorpha and truncated by erosion this suggests a series of temporally separated storm events with re colonization of the ocean floor by invertebrates between storms rather than a series of waning tsunami generated waves the lithified clasts with impact spherules at the base of the event deposit provide strong evidence that the chicxulub impact ejecta layer predates the event deposit but was eroded and re deposited during the latest maastrichtian sea level lowstand the original chicxulub ejecta layer was discovered in a 3 cm thick yellow clay layer interbedded in undisturbed late maastrichtian clay and mudstones 40 cm below the base of the event deposit and near the base of planktic foraminiferal zone cf1 which spans the last 300 kyr of the maastrichtian the yellow clay consists of cheto smectite derived from alteration of impact glass as indicated by rare altered glass spherules with similar chemical compositions as reworked spherules from the event deposit and chicxulub impact spherules from ne mexico and haiti the brazos sections thus provide strong evidence that the chicxulub impact predates the k t boundary by about 300 kyr consistent with earlier observations in ne mexico and the chicxulub crater core yaxcopoil 1

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

temperature profile in the lowermost mantle from seismological and mineral physics joint modeling the internal structure of the core mantle boundary cmb region of the earth plays a crucial role in controlling the dynamics and evolution of our planet we have developed a comprehensive model based on the radial variations of shear velocity in the d layer the base of the lower mantle and the high p t elastic properties of major candidate minerals including the effects of post perovskite phase changes this modeling shows a temperature profile in the lowermost mantle with a cmb temperature of 3 800 200 k which suggests that lateral temperature variations of 200 300 k can explain much of the large velocity heterogeneity observed in d a single crossing phase transition model was found to be more favorable in reproducing the observed seismic wave velocity structure than a double crossing phase transition model

long period body wave traveltimes through the crust implication for crustal corrections and seismic tomography summary the traveltimes of p and sh waves through the crust depend strongly on wave period at periods longer than 10 15 s where traveltime measurements for indirect e g surface reflected and core reflected waves are typically made the traveltimes are shorter than predicted by ray theory crustal corrections often used in global mantle tomography to isolate the effects of the crust on teleseismic traveltimes may have a complex frequency dependence and influence finite frequency inversions crustal corrections for profiles of crust2 0 and the prem reference model for the mantle may be several seconds larger or smaller than ray theoretical values depending on crustal thickness crustal velocities and wave period this variability is observed in the difference times between the seismic phases ss and s and between pp and p it is therefore important to incorporate the effects of the crust on traveltimes in finite frequency tomography in order to take full advantage of the variable mantle sensitivity of body waves at different periods

three dimensional structure at the base of the mantle beneath the central pacific forward modeling of differential travel times of phases sensitive to lowermost mantle beneath the central pacific reveals lateral heterogeneity that is higher in amplitude than predicted by tomographic models a broad zone of low s velocity 4 percent with respect to standard models which may correspond to the base of a thermal plume narrows and is deflected as it extends to about 1000 kilometers above the core mantle boundary to the east of this zone a localized region of fast s velocity 5 percent suggests strong heterogeneity or anisotropy related to the presence of high pressure and temperature assemblages which may or may not involve core material its presence could also explain the observation of precursors to core reflected phases in this region

the k pg boundary at brazos river texas usa an approach by marine palynology two cores and one outcrop section from the cretaceous palaeogen k pg boundary interval at the brazos river area texas usa have been high resolution sampled and analysed fully quantitatively by marine palynology the results have been compared and integrated with data from micropalaeontology sedimentology and isotope geochemistry from the same sections within all three sections the k pg boundary defined as the base of the p0 foraminifera zone and the onset of a negative carbon isotope anomaly closely corresponds to the appearance of lowermost danian organic walled dinocysts based on various palynological proxies as well as on sedimentologic features a sequence stratigraphic subdivision of the sections is proposed from a synopsis of all three sections a long term punctuated drop of relative sea level is indicated starting at about the top of the upper cretaceous cf2 foraminifera zone and reaching a minimum within the upper p1a zone of the lower palaeogene this is followed by a prominent transgression starting within the p1a p1b boundary interval the event deposit ed a siliciclastic unit located well below the base of the p0 foraminifera zone in the present sections most probably represents a combination of both episodic relative sea level fall and lag deposit due to initial transgression it is preceded by significant fluctuations of climatic boundary conditions with cool climates correlating to heavier 13c values and increased p g ratio of dinocysts suggestive of higher marine primary productivity across the k pg boundary a prominent increase and spike of the warm temperate waters dinocysts fraction is documented represented mainly by trithyrodinium evittii which is accompanied by climatic warming and an episodic relative sea level rise thus prominent paleo environmental changes and sea level fluctuations precede and coincide with the k pg boundary proper these data are inconsistent with a single catastrophic impact as the cause for the k pg boundary event but suggest relative longer term environmental stress as finally leading to end cretaceous crisis of the biosphere between the ed and the k pg boundary proper the gradual increase to peak abundance in trilete spores demonstrates a significant time lag between these two horizons according to the distinct distribution of this peak abundance it cannot be excluded that an impact tsunami event is related to the lower portion of the ed in contrast no significant changes within most palynologic proxies are documented across the yellow clay layer below the ed documented within one of the sections and recently suggested as the original chicxulub impact ejecta horizon this questions the impact origin of this horizon or at least suggests only little consequences of this event on the ecosystem

the elastic solid solution model for minerals at high pressures and temperatures non ideality in mineral solid solutions affects their elastic and thermodynamic properties their thermobaric stability and the equilibrium phase relations in multiphase assemblages at a given composition and state of order non ideality in minerals is typically modelled via excesses in gibbs free energy which are either constant or linear with respect to pressure and temperature this approach has been extremely successful when modelling near ideal solutions however when the lattice parameters of the solution endmembers differ significantly extrapolations of thermodynamic properties to high pressures using these models may result in significant errors in this paper i investigate the effect of parameterising solution models in terms of the helmholtz free energy treating volume or lattice parameters rather than pressure as an independent variable this approach has been previously applied to models of order disorder but the implications for the thermodynamics and elasticity of solid solutions have not been fully explored solid solution models based on the helmholtz free energy are intuitive at a microscopic level as they automatically include the energetic contribution from elastic deformation of the endmember lattices a chemical contribution must also be included in such models which arises from atomic exchange within the solution derivations are provided for the thermodynamic properties of n endmember solutions examples of the use of the elastic model are presented for the alkali halides pyroxene garnet and bridgmanite solid solutions elastic theory provides insights into the microscopic origins of non ideality in a range of solutions and can make accurate predictions of excess enthalpies entropies and volumes as a function of volume and temperature in solutions where experimental data are sparse or contradictory the helmholtz free energy approach can be used to assess the magnitude of excess properties and their variation as a function of pressure and temperature the formulation is expected to be useful for geochemical and geophysical studies of the earth and other planetary bodies

reduced lattice thermal conductivity of fe bearing bridgmanite in earth s deep mantle complex seismic thermal and chemical features have been reported in earth s lowermost mantle in particular possible iron enrichments in the large low shear wave velocity provinces llsvps could influence thermal transport properties of the constituting minerals in this region altering the lower mantle dynamics and heat flux across core mantle boundary cmb thermal conductivity of bridgmanite is expected to partially control the thermal evolution and dynamics of earth s lower mantle importantly the pressure induced lattice distortion and iron spin and valence states in bridgmanite could affect its lattice thermal conductivity but these effects remain largely unknown here we precisely measured the lattice thermal conductivity of fe bearing bridgmanite to 120 gigapascals using optical pump probe spectroscopy the conductivity of fe bearing bridgmanite increases monotonically with pressure but drops significantly around 45 gigapascals due to pressure induced lattice distortion on iron sites our findings indicate that lattice thermal conductivity at lowermost mantle conditions is twice smaller than previously thought the decrease in the thermal conductivity of bridgmanite in mid lower mantle and below would promote mantle flow against a potential viscosity barrier facilitating slabs crossing over the 1000 km depth modeling of our results applied to llsvps shows that variations in iron and bridgmanite fractions induce a significant thermal conductivity decrease which would enhance internal convective flow our cmb heat flux modeling indicates that while heat flux variations are dominated by thermal effects variations in thermal conductivity also play a significant role the cmb heat flux map we obtained is substantially different from those assumed so far which may influence our understanding of the geodynamo

main deccan volcanism phase ends near the k t boundary evidence from the krishna godavari basin se india abstract recent studies indicate that the bulk 80 of the deccan trap eruptions occurred over less than 0 8 m y in magnetic polarity c29r spanning the cretaceous tertiary k t boundary determining where within this major eruptive phase the k t mass extinction occurred has remained problematic for this reason models estimating the biotic and environmental consequences have generally underestimated the rate and quantity of deccan gas emissions by orders of magnitude leading to conclusions that volcanism could not have been one of the major causes for the k t mass extinction in this study we report that the most massive deccan trap eruption occurred near the k t mass extinction these results are based on sedimentologic microfacies and biostratigraphic data of 4 9 m thick intertrappean sediments in four quarry outcrops in the rajahmundry area of the krishna godavari basin of southeastern india in this area two deccan basalt flows known as the rajahmundry traps mark the longest lava flows extending 1500 km across the indian continent and into the bay of bengal the sediments directly overlying the lower rajahmundry trap contain early danian planktic foraminiferal assemblages of zone p1a which mark the evolution in the aftermath of the k t mass extinction the upper rajahmundry trap was deposited in magnetic polarity c29n preceding full biotic recovery these results suggest that volcanism may have played critical roles in both the k t mass extinction and the delayed biotic recovery

evaluating the accretion of meteoritic debris and interplanetary dust particles in the gpc 3 sediment core using noble gas and mineralogical tracers abstract extraterrestrial et noble gases helium and neon in 35 sediment samples from central pacific core ll 44 gpc 3 demonstrate the variable flux of interplanetary dust particles idps and major meteorite impacts over the past 70 ma 21 72 ma spinel mineralogical and chemical compositions clearly distinguish major impact events from the continuous flux of idps including the well established cretaceous tertiary k t and late eocene e o impact boundaries no spinel grains with chemical or mineralogical evidence of a distinctly et origin were found in an extensive survey of background samples i e non e o or k t boundary suggesting that either the carrier grains for et noble gas occur within the fe ti oxide mineral fraction observed in this study found to include ilmenite and ulvospinel or are too small for identification by sem the presence of ilmenite and ulvospinel suggest lunar regolith is a potential source for et noble gas rich particles noble gas analysis on both the emf extractable magnetic fraction and the bulk minus emf bulk emf show that the he and ne compositions are consistent with partially degassed noble gas signatures of zero age magnetic grains z mag and stratospheric interplanetary dust particles idps conclusive evidence for a planetary ne a noble gas signature is found only in the bulk sediments at the k t boundary although all gpc 3 k t fractions bulk emf and hf digestion plot along a mixing line between planetary ne a and solar wind sw spinels from major impact boundaries k t e o exhibit dendritic texture and elevated ni consistent with previous reports in contrast to the otherwise consistent 3he signal from idps the 3he at the known impact boundaries k t and e o actually decreases these anomalously low 3he are accompanied by significantly elevated ne and significantly lower 3he 20ne solar ratios 10 lower produced by both preferentially degassing of he relative to ne at times of increased flux of larger et material degassed degassed he enriched ne profile noble gas characteristics occur in two sample intervals that do not correspond to any known impact events 47 and 71 ma explained by an influx of larger particles sem analysis of the 47 ma sample shows spinels with dendritic textures but without distinctive markers of large meteorite impacts e g elevated ni particle size increases and degassed signatures may be caused by major bolides micrometeorites comet showers or simply a flux of larger idps potentially with a different source

new evidence concerning the age and biotic effects of the chicxulub impact in ne mexico abstract in the 1990s the chicxulub impact was linked to the k t boundary by impact spherules at the base of a sandstone complex that was interpreted as an impact generated tsunami deposit since that time a preponderance of evidence has failed to support this interpretation revealing long term deposition of the sandstone complex the k t boundary above it and the primary impact spherule ejecta interbedded in late maastrichtian marls below based on evidence from mexico and texas we suggested that the chicxulub impact predates the k t boundary impact tsunami proponents have challenged this evidence largely on the basis that the stratigraphically lower spherule layer in mexico represents slumps and widespread tectonic disturbance although no such evidence has been presented the decades old controversy over the cause of the k t mass extinction will never achieve consensus but careful documentation of results that are reproducible and verifiable will uncover what really happened at the end of the crectaceous this study takes an important step in that direction by showing 1 that the stratigraphically older spherule layer from el penon ne mexico represents the primary chicxulub impact spherule ejecta in tectonically undisturbed sediments and 2 that this impact caused no species extinctions

gypsum a joint tomographic model of mantle density and seismic wave speeds 1 gypsum is a 3 d model of mantle shear wave s speeds compressional wave p speeds and density the model is developed through simultaneous inversion of seismic body wave travel times p and s and geodynamic observations while using realistic mineral physics parameters linking wave speeds and density geodynamic observations include the global free air gravity field divergence of the tectonic plates dynamic topography of the free surface and the flow induced excess ellipticity of the core mantle boundary gypsum is built with the philosophy that heterogeneity that most closely resembles thermal variations is the simplest possible solution models of the density field from earth s free oscillations have provided great insight into the density configuration of the mantle but are limited to very long wavelength solutions alternatively scaling higher resolution seismic images to obtain density anomalies generates density fields that do not satisfy geodynamic observations the current study provides a 3 d density model for the mantle that directly satisfies geodynamic and seismic observations through a joint seismic geodynamic inversion process notable density field observations include high density piles at the base of superplume structures supporting the general results of past normal mode studies however we find that these features are more localized and have lower amplitude than past studies would suggest when we consider both fast and slow seismic anomalies in gypsum we find that p and s wave speeds are strongly correlated throughout the mantle however we find a low correlation of fast s wave zones in the deep mantle 1500 km depth with the corresponding p wave anomalies suggesting a systematic divergence from simplified thermal effects in ancient subducted slab anomalies the cratonic lithosphere and d regions are shown to have strong compositional signatures however we argue that temperature variations are the primary cause of p wave speed s wave speed and density anomalies throughout most of the mantle

40k 40ar dating of the main deccan large igneous province further evidence of ktb age and short duration most mass extinctions coincide in time with outpourings of continental flood basalts cfb some 20 years ago it was shown courtillot v besse j vandamme d montigny r jaeger j j cappetta h 1986 deccan flood basalts at the cretaceous tertiary boundary earth planet sci lett 80 361 374 courtillot v feraud g maluski h vandamme d moreau m g besse j 1988 deccan flood basalts and the cretaceous tertiary boundary nature 333 843 846 duncan r a pyle d g 1988 rapid eruption of the deccan flood basalts at the cretaceous tertiary boundary nature 333 841 843 that the age of the deccan traps was close to the cretaceous tertiary kt boundary and its duration under 1 myr we have undertaken a new geochronological study using the unconventional 40k 40ar cassignol gillot technique which is particularly well suited to the potassium poor deccan lavas the mean of 4 determinations from the topmost ambenali and mahabaleshwar formations is 64 5 0 6 ma they straddle the c29r c29n reversal boundary for which they provide a new constraint the mean age of 3 determinations from the oldest jawhar formation is 64 8 0 6 ma the difference in age between top and bottom of a 3500 m composite section probably comprising 80 of the total deccan volume is statistically insignificant with the overall mean age being 64 7 0 6 ma n 7 our results are consistent with the most recent 40ar 39ar determinations knight k b renne p r halkett a white n 2003 40ar 39ar dating of the rajahmundry traps eastern india and their relationship to the deccan traps earth planet sci lett 208 85 99 knight k b renne p r baker j waight t white n 2005 reply to 40ar 39ar dating of the rajahmundry traps eastern india and their relationship to the deccan traps discussion by a k baksi earth planet sci lett 239 374 382 confirming that there should be no systematic difference between the two methods when they are used in an optimal way an earlier smaller but significant pulse of volcanism between 68 and 67 ma extending over at least 500 km in latitude in the northern part of the deccan cfb has also been identified after 2 to 3 ma of quiescence the second major phase of volcanism occurred near 65 ma expanding over most of the area covered by the first pulse and another 500 km to the south consistent with drift of india by 300 to 450 km at not vert similar 150 mm yr during the quiescence period new paleontological data from the remote rajahmundry section keller g adatte t gardin s bartolini a bajpai s humler e in prep the cretaceous tertiary boundary in deccan traps of the krishna godavari basin of southeastern india epsl to be submitted suggest that this second pulse can itself be divided into two major pulses one starting in c29r and ending at the kt boundary the second starting in the upper part of c29r and ending within c29n

normal mode and free air gravity constraints on lateral variations in velocity and density of earth s mantle with the use of a large collection of free oscillation data and additional constraints imposed by the free air gravity anomaly lateral variations in shear velocity compressional velocity and density within the mantle dynamic topography on the free surface and topography on the 660 km discontinuity and the core mantle boundary were determined the velocity models are consistent with existing models based on travel time and waveform inversions in the lowermost mantle near the core mantle boundary denser than average material is found beneath regions of upwellings centered on the pacific ocean and africa that are characterized by slow shear velocities these anomalies suggest the existence of compositional heterogeneity near the core mantle boundary

a global geoid model with imposed plate velocities and partial layering most inversions of the long wavelength geoid in conjunction with the seismic tomographic information have so far been carried out under the assumption of either purely whole mantle or perfectly layered circulation moreover modeling the lithosphere as a spherical shell with a uniform low viscosity was found to yield the best fit to the observed geoid we have tested whether a good prediction of the geoid can also be achieved by including two constraints a semipermeable behavior of the 660 km discontinuity and surface plate velocities equal to the observed ones the mass transfer between upper and lower mantle has been changed by imposing a surface density anomaly at a depth of 660 km which is proportional to the mass anomaly needed to achieve perfectly layered circulation on the top of the mantle we assume a stiff lithosphere that moves with a velocity corresponding to the observed plate motion the viscosity only varies with depth considering a simple three layer viscosity structure and changing the permeability of the 660 km interface we have obtained a satisfactory variance reduction of the geoid data 75 for degrees 2 12 the best fit to the geoid is obtained if the mass transfer across the 660 km boundary is reduced to one third in comparison with the purely whole mantle model the best fitting viscosity profile is characterized by a clearly defined asthenosphere and a viscosity increase by at least 2 orders of magnitude in the lower mantle the amplitudes of dynamic topography predicted by our model are remarkably small 100 m thus fully compatible with the observation

geographic boundary and shear wave velocity structure of the pacific anomaly near the core mantle boundary beneath western pacific we determine the geographical boundary and shear velocity structure of a very low velocity province at the base of the earth s mantle beneath western pacific we term it the pacific anomaly based on the waveform modeling and travel time analysis of scsh sh phases our seismic data are from the china national digital seismographic network the f net in japan the global seismographic network and several passcal arrays the observed scs sh differential travel time residuals allow the northwestern geographic boundary of the anomaly to be clearly defined the seismic data also suggest that the average shearvelocity reduction inside the anomaly reaches 5 in the lowermost 300km of the mantle waveform modeling of the seismic data sampling the edge of the anomaly suggests that the northwestern boundary is best characterized by a shear velocity model with a velocity jump of about 2 at about 100 145km above the core mantle boundary and a thin 30 km thick basal layer with a shear wave velocity reduction of 13 stacked seismic data sampling the middle of the anomaly however show no evidence for any internal discontinuity with a velocity decrease greater than 2 in the middle of the anomaly overall the seismic data sampling the base of the pacific anomaly can be explained by a negative shear velocity gradient from 0 to 1 top to 13 bottom in the lowermost 220 km of the mantle similar to those of a very low velocity province beneath the south atlantic ocean and the indian ocean such a strong negative shear velocity gradient can be explained by partial melting of a compositional anomaly produced early in the earth s history located within a bottom thermal boundary layer our travel time data also exhibit small scale variations inside the anomaly indicating existence of internal small scale seismic heterogeneities inside the pacific anomaly

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

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

modeling climatic effects of carbon dioxide emissions from deccan traps volcanic eruptions around the cretaceous paleogene boundary abstract warming events observed in paleotemperature proxy records within 500 kyr of the cretaceous paleogene k pg boundary have often been attributed to atmospheric co 2 increases due to deccan traps flood volcanism currently there is uncertainty in the size nature and timing of the deccan eruptions all of which lead to uncertainty in the likely climatic effects modeling the impact of deccan traps eruption on climate is complicated by the discrepancy between the lifetimes of emitted gasses and the length of the total eruptive sequence though so 2 emissions can have important climatic effects the short atmospheric lifetimes of so 2 and resulting aerosols mean these effects are unlikely to be recognized in the proxy record here we focus on the co 2 emissions and attempt to match paleotemperature proxy records with plausible emissions scenarios we also test the relevance for climate of the number length and arrangement e g increasing or decreasing size of individual eruptions as well as the total duration and size of the overall eruptive sequence we find that the number and length of individual eruptions are largely unimportant to co 2 based based climate effects but that the pattern and duration of eruption have measureable effects unsurprisingly the total emitted co 2 from the deccan traps exerts a strong control on climatic effects and better constraints on the volume of emitted gas are necessary at the high end of the uncertainty range the deccan traps eruptions are capable of generating warming events recorded in the proxy record but rates of silicate weathering above modern rates are necessary to draw down co 2 in accordance with those records

mapping the upper mantle three dimensional modeling of earth structure by inversion of seismic waveforms a method is presented for the inversion of waveform data for the three dimensional distribution of seismic wave velocities the method is applied to data from the global digital networks international deployment of accelerometers global digital seismograph network the selected data set consists of some 2000 seismograms corresponding to 53 events and 870 paths the moment tensors of the earthquakes are determined through an iterative procedure which minimizes the corrupting influence of lateral heterogeneity a global model is constructed for shear wave velocity expanded up to degree and order 8 in spherical harmonics and described by a cubic polynomial in depth for the upper 670 km of the earth s mantle although no a priori information is incorporated the model predictions reproduce much of what is known about the dispersion of mantle waves for example high phase velocities for shields low velocities at ridges and a strong degree 2 pattern for rayleigh waves since the method makes use of complete waveforms overtone data are also included it is shown that the model is reproducible in that substantially the same model can be constructed from each half of the total data set considered independently the model shows that shields and ridges are major features in the depth interval 25 250 km the ridges of the southern pacific and the larger shields persist to 350 km but the southeast indian rise is underlain by a high velocity anomaly at this depth as is much of the mid atlantic ridge at 450 650 km the major features are a broad region of high velocities incorporating south america much of the south atlantic and parts of west africa a broad region of low velocities in the central and eastern pacific high velocities in the western pacific and a low velocity anomaly beneath the red sea and the gulf of aden in the absence of a crustal correction degrees 2 and 3 show a high positive correlation with the geoid paradoxically this is largely destroyed when the distribution in crustal thickness is taken into account spherical harmonic degrees 4 7 show a significant negative correlation

very low shear wave velocity at the base of the mantle under the south pacific superswell abstract s wave velocity heterogeneity at the base of the mantle beneath the south pacific superswell has been examined using skks sks and s sdiff sks differential travel times the residuals of skks sks and s sdiff sks with respect to the preliminary reference earth model prem are compared to the residuals predicted using recently published three dimensional s wave velocity models we have obtained a three dimensional model that explains the s sdiff sks residuals well and applied this model to the correction of the skks sks residuals the corrected skks sks residuals beneath the south pacific superswell remain large and positive forward modeling reveals that these residuals can be accounted for by a 5 7 low velocity region with a lateral extent of about 500 km in the lowermost 200 km of the mantle although the magnitude of the velocity reduction and the thickness of the anomalous region trade off against one another and cannot be uniquely determined the existence of a very low s wave velocity region at the base of the mantle is a robust feature this feature lies close to the ultra low velocity zone ulvz identified by p velocity data beneath the southwestern pacific and is located just below a 2 3 low s velocity body in the lower mantle at depths of 2300 2700 km beneath the south pacific superswell

yucatan subsurface stratigraphy implications and constraints for the chicxulub impact much of the discussion about the effects of an end of cretaceous impact by a large extraterrestrial body in northwestern yucatan has been done in the context of limited and partly erroneous published data on the mesozoic stratigraphy of that area reexamination of cores and geophysical logs taken in several pemex wells has produced improved lithologic and biostratigraphic correlation of the jurassic to maastrichtian section across the northern yucatan peninsula these data suggest that major disturbance of strata by an impact would have been confined to within about 100 km of the proposed impact center near chicxulub the only unusual lithologic unit is polymict breccia which apparently was penetrated at or near the top of the cretaceous section in all the deep wells of northern yucatan this breccia in pemex wells yucatan 1 2 4 5a and 6 is composed predominantly of detrital dolomite limestone and anhydrite clasts set in dolomitized carbonate mud matrix which contains upper maastrichtian foraminifers these constituents mixed with fragments of altered glass or melt rock shocked quartz and feldspar and basement rock suggest an impact as the most likely origin for the breccia the timing of brecciation is poorly constrained by biostratigraphic data there is some evidence however that the breccia unit is overlain by about 18 m of uppermost maastrichtian marls suggesting an impact before the cretaceous tertiary boundary in addition there may have been more than one episode of breccia deposition

environmental change prior to the k t boundary inferred from temporal variation in the morphology of cheilostome bryozoans abstract fluctuations in the marine environment just prior to the k t extinction event have been inferred from several geological sections around the world most previous studies have employed isotope or trace element proxies this study uses morphological changes in erect and free living cheilostome bryozoans as a proxy to investigate environmental change through the final stages of the maastrichtian at the nye klov section in denmark the metrics used are 1 mean zooid size as a proxy for temperature 2 intracolony variation in zooid size as a proxy for degree of seasonality 3 density of defensive avicularia as a proxy for palaeoproductivity and 4 colony size and asymmetry as proxies for unfavourable environmental conditions three semi distinct phases in the benthic environment are evident the lowest 3 5 m of the roughly 4 5 m section experienced apparently normal marine conditions next low estimates of benthic seasonality and highly symmetrical and large colonies with many avicularia indicate a time of increased environmental stability subsequent to this quiescence the uppermost 20 cm of the section witnessed environmental volatility and deterioration with mean zooid sizes in all species falling dramatically suggesting a rapid warming or dysoxic event sharply increasing seasonality estimates implying unusual oceanographical states and the growth of small asymmetrical colonies with few avicularia all suggesting unfavourable conditions these data therefore indicate that strong environmental perturbations occurred just prior to the k t boundary in the danish basin such events may have contributed to biotic turnover at the k t boundary because cause and effect in macroevolution can be delayed however potential mechanisms of turnover need to be robustly tested within a detailed palaeoenvironmental framework construct from a suite of independent proxies

evolution and extinction of maastrichtian late cretaceous cephalopods from the lopez de bertodano formation seymour island antarctica one of the most expanded records to contain the final fortunes of ammonoid cephalopods is within the lopez de bertodano formation of seymour island james ross basin antarctica located at 65 south now and during the cretaceous this sequence is the highest southern latitude onshore outcrop containing the cretaceous paleogene k pg transition we present comprehensive new biostratigraphic range data for 14 ammonite and one nautiloid species based on the collection of 700 macrofossils from high resolution sampling of parallel sedimentary sections dated maastrichtian to earliest danian in age across southern seymour island we find evidence for only a single abrupt pulse of cephalopod extinction at the end of the cretaceous when the final seven ammonite species disappeared consistent with most evidence globally in the lead up to the k pg extinction in the james ross basin starting during the campanian ammonite diversity decreased overall but the number of endemic taxa belonging to the family kossmaticeratidae actually increased this pattern continued into the maastrichtian and may be facies controlled linked to changes in sea level and seawater temperature during the early maastrichtian ammonite diversity dropped significantly with only two species recorded from the basal lopez de bertodano formation on seymour island the subsequent diversification of endemic taxa and reappearance of long ranging widespread species into the basin resulted in an increase in ammonite diversity and abundance during the mid maastrichtian this was coincident with an apparent period of warming temperatures and sea level rise interpreted from palynology and sedimentology perhaps reflecting a high latitude expression of the mid maastrichtian event late maastrichtian diversity levels remained stable despite reported climatic and environmental variation ammonite diversity patterns during the maastrichtian parallel those of microfossil species such as nannofossil and planktonic foraminifera suggesting that dynamic climatic and environmental changes affected many planktonic and nektonic organisms during the latest cretaceous however we suggest that these perturbations had a minimal effect on overall diversity prior to the catastrophic extinction event at the k pg boundary

high resolution chronostratigraphy of the terrestrial cretaceous paleogene transition and recovery interval in the hell creek region montana detailed understanding of ecosystem decline and recovery attending the cretaceous paleogene boundary kpb mass extinctions is hindered by limited constraints on the pace and tempo of environmental events near the boundary to mitigate this shortcoming high resolution 40ar 39ar geochronology was performed on tephras intercalated between fossiliferous terrestrial sediments of the upper hell creek and lower fort union formations in the western williston basin of northeastern montana usa tephra samples were collected from 10 stratigraphic sections spanning an area of 5000 km2 several distinctive tephras can be correlated between sections separated spatially by as much as 60 km the tephras are thin distal deposits generally preserved only in lignite beds which are interbedded with clastic deposits yielding vertebrate faunas of lancian late maastrichtian to torrejonian early danian north american land mammal ages sanidine from 15 tephra samples was analyzed in 1649 total fusion experiments 1597 on single crystals and 12 incremental heating analyses of multigrain aliquots ages were determined for 13 distinct tephras ranging from 66 289 0 051 to 64 866 0 023 ma including only analytical uncertainties this level of precision is sufficient to resolve the ages of all of the coal beds that have served as a basis for a regional stratigraphic framework the data confirm that the hell creek fort union formational contact is diachronous and further support the age of the kpb impact layer at 66 043 0 010 ma or 0 043 ma considering systematic uncertainties application of the new results to previous magnetostratigraphic data indicates an appreciably compressed time interval between the base of chron c29r and the top of chron c28r with a maximum duration estimate of 1 421 0 066 ma most notable is the implied brevity of chron c29r with a maximum estimate of 457 54 ka and possibly as brief as 345 38 ka compared to the 710 ka estimate from the geologic time scale 2012 gts2012 further application of new results to terrestrial biostratigraphy adds higher precision to the timing and tempo of biotic change before and after the kpb our results indicate that the timing of pre kpb ecological decline is constrained to the last 200 ka of the cretaceous adding further support to the press pulse extinction hypothesis additionally the duration of the depauperate basal paleogene puercan 1 disaster fauna is confined to a 70 ka interval faunal recovery in this region indicated by the appearance of primitive members of the placental mammal radiation and the restoration of taxonomic richness and evenness occurred within 900 ka after the kpb these results show that biotic recovery after the mass extinction in the terrestrial realm was more rapid than in the marine

global mantle tomography progress status in the past 10 years abstract we review the present status of global mantle tomography and discuss two main classes of models that have been developed in the past 10 years p velocity models based on large datasets of travel times from the international seismological centre bulletins often referred to as high resolution models and s velocity models based on a combination of surface wave and hand picked body wave travel times or waveforms referred to as long wavelength models we discuss their respective strengths and weaknesses as well as progress in the resolution of other physical parameters such as anisotropy anelasticity density and bulk sound velocity using tomographic approaches we present the view that future improvements in global seismic tomography require the utilization of the rich information contained in complete broadband seismic waveforms this is presently within our reach owing to theoretical progress as well as the increase in computational power in recent years

joint seismic geodynamic and mineral physical constraints on three dimensional mantle heterogeneity implications for the relative importance of thermal versus compositional heterogeneity summary the joint interpretation of seismic and geodynamic data requires mineral physical parameters linking seismic velocity to density perturbations in the earth s mantle the most common approach is to link velocity and density through relative scaling or conversion factors r s dln dlnv s however the range of possible r s values remains large even when only considering thermal effects we directly test the validity of several proposed depthdependent conversion profiles developed from mineral physics studies for thermally varying properties of mantle materials the tests are conducted by simultaneously inverting shear wave traveltime data and a diverse suite of convection related constraints interpreted with viscous flow response functions of the mantle these geodynamic constraints are represented by surface spherical harmonic basis functions up to harmonic degree 16 and they consist of the global free air gravity field tectonic plate divergences dynamic surface topography and the excess ellipticity of the core mantle boundary cmb the tests yield an optimum 1 d thermal r s profile that is generally compatible with all considered data with the exception of the dynamic surface topography that is most sensitive to the shallow upper mantle this result is not surprising given that cratonic roots are known to be compositionally distinct from the surrounding ambient mantle moreover it is expected that the temperature dependence of the thermal r s in the upper mantle is significant due to the temperature dependence of seismic attenuation or q therefore a simple 1 d density velocity relationship is insufficient to address this problem we obtained independent estimates of the first order correction factors to the selected r s profile within the cratonic roots and in the ambient thermal upper mantle these correction factors defined as r s lnv s are highly negative within the cratons signifying considerable compositional buoyancy this result confirms that the negative buoyancy associated with the low temperatures in the cratons is significantly counteracted by the composition induced positive buoyancy resulting in near neutral buoyancy of the cratonic roots within the ambient upper mantle the correction factors are found to be positive which is consistent with the expected behaviour of the r s relationship in thermally varying upper mantle material we obtain a significantly greater reconciliation of the global gravity anomalies and dynamic surface topography when applying these laterally varying corrections compared to a simple 1 d r s relationship inversion for a fully 3 d r s relationship reveals secondary effects including additional compositional variation within the cratonic roots and the deep mantle superplume structures we estimate the relative magnitude of the thermal and compositional non thermal contributions to mantle density anomalies and conclude that thermal effects dominate shear wave and density heterogeneity throughout the non cratonic mantle we also demonstrate the potential pitfalls of scaling a purely seismically derived model to obtain density rather than performing a true joint inversion to obtain velocity and density simultaneously

closing the gap between regional and global travel time tomography recent global travel time tomography studies by zhou 1996 and van der hilst et al 1997 have been performed with cell parameterizations of the order of those frequently used in regional tomography studies i e with cell sizes of 1 2 these new global models constitute a considerable improvement over previous results that were obtained with rather coarse parameterizations 5 cells the inferred structures are however of larger scale than is usually obtained in regional models and it is not clear where and if individual cells are actually resolved this study aims at resolving lateral heterogeneity on scales as small as 0 6 in the upper mantle and 1 2 3 in the lower mantle this allows for the adequate mapping of expected small scale structures induced by for example lithosphere subduction deep mantle upwellings and mid ocean ridges there are three major contributions that allow for this advancement first we employ an irregular grid of nonoverlapping cells adapted to the heterogeneous sampling of the earth s mantle by seismic waves spakman and bijwaard 1998 second we exploit the global data set of engdahl et al 1998 which is a reprocessed version of the global data set of the international seismological centre their reprocessing included hypocenter redetermination and phase reidentification finally we combine all data used p pp and pwp phases into nearly 5 million ray bundles with a limited spatial extent such that averaging over large mantle volumes is prevented while the signal to noise ratio is improved in the approximate solution of the huge inverse problem we obtain a variance reduction of 57 1 synthetic sensitivity tests indicate horizontal resolution on the scale of the smallest cells 0 6 or 1 2 in the shallow parts of subduction zones decreasing to approximately 2 3 resolution in well sampled regions in the lower mantle vertical resolution can be worse up to several hundreds of kilometers in subduction zones with rays predominantly pointing along dip important features of the solution are as follows 100 200 km thick high velocity slabs beneath all major subduction zones sometimes flattening in the transition zone and sometimes directly penetrating into the lower mantle large high velocity anomalies in the lower mantle that have been attributed to subduction of the tethys ocean and the farallon plate and low velocity anomalies continuing across the 660 km discontinuity to hotspots at the surface under iceland east africa the canary islands yellowstone and the society islands our findings corroborate that the 660 km boundary may resist but not prevent present day large scale mass transfer from upper to lower mantle or vice versa this observation confirms the results of previous global mantle studies that employed coarser parameterizations

whole earth structure estimated from seismic arrival times the arrival times of 10 compressional and shear phases p s pp pcp ss scs pk pab pk pbc pk pdf sk sac and two sets of differential times ss s410s and ss s660s are used to infer the earth s three dimensional structure with the exception of ss sds the raw data are composed of arrival times from the international seismological centre isc catalogue the isc events have been relocated using p and s phases and the ak135 radial earth model a total of 818 655 summary arrival times constrain lateral velocity variations in the crust mantle and core we also determine relocation parameters for all events stations corrections for each phase boundary topography 410 km 660 km core mantle boundary inner core boundary and inner core anisotropy variations our model of p and s velocity variations in the crust and mantle contains the general features observed in other body wave inversions a low velocity ring around the pacific ocean basin in the depth range 35 400 km high velocity anomalies associated with subducting slabs most prominent in the depth range 660 1070 km and high velocity anomalies surrounding the pacific basin just above the core mantle boundary in the outer core there is evidence of a latitudinal variation in velocity approaching 0 6 in magnitude with lower p velocities at the poles and higher velocities near the equator this pattern is observed throughout the outer core but is most prominent in the depth range 3671 4071 km and near the inner core boundary synthetic tests indicate that outer core features are moderately resolvable and inversions of random arrival times indicates that the anomalies are a factor of 2 above the estimated parameter noise level lateral heterogeneity in the inner core trades off with anisotropy resulting in poor resolution of isotropic velocity variations anisotropy in the inner core is predominantly subparallel to the earth s axis and of the order of 3 but this value can decrease if more inner core isotropic lateral heterogeneity is allowed a comparison of structure above and below the core mantle boundary cmb indicates that velocity heterogeneity is uncorrelated across the cmb but there is a moderate correlation between cmb topograpy and outer core heterogeneity an inversion in which the pk pdf residuals were absent indicates that the latitudinal pattern of outer core velocity variations is not due to the influence of inner core velocity structure

asteroid impact vs deccan eruptions the origin of low magnetic susceptibility beds below the cretaceous paleogene boundary revisited abstract the respective roles of an asteroid impact and deccan traps eruptions in biotic changes at the cretaceous paleogene k pg boundary are still debated in many shallow marine sediments from around the world the k pg boundary is marked by a distinct clay layer that is often underlain by a several decimeter thick low susceptibility zone a previous study of the gubbio section italy lowrie et al 1990 attributed low magnetization intensity in this interval to post depositional dissolution of ferrimagnetic minerals dissolution was thought to be a consequence of downward infiltration of reducing waters that resulted from rapid accumulation of organic matter produced by mass extinctions after the k pg event we compare the magnetic properties of sediments from the gubbio section with those of the bidart section in southern france the two sections are similar in their carbonate lithology and the presence of a boundary clay and low susceptibility zone when compared to background cretaceous sediments the low susceptibility zone in both sections is marked by an absence of biogenic magnetite a decrease in total ferrimagnetic mineral content and a preferential loss of magnetite with respect to hematite features that are consistent with reductive dissolution however unlike the gubbio section where the low susceptibility zone starts immediately below the boundary clay the low susceptibility zone and the clay layer at bidart are separated by a 4 cm carbonate interval that contains abundant biogenic magnetite such separation casts doubt on a causal link between the impact and sediment bleaching more likely the low susceptibility layer marks a different environmental event that preceded the impact an episode of increased atmospheric and oceanic acidity associated with deccan traps volcanism that occurred well before the k pg impact is argued here to account for the distinct magnetic properties of the low susceptibility intervals

ridge like lower mantle structure beneath south africa recent scs s results from probing the deep structure beneath southern africa display strong delays of up to 10 s at distances beyond 90 such delays could be explained by long period tomographic models containing smooth weak features with the addition of rough strong d structure 3 9 drops in shear velocities however these structures cannot explain the sks s differentials sampling the same region to explain the sks s and scs s data sets simultaneously requires instead a large scale ridge like structure with a relatively uniform 3 reduction in shear velocity the structure is about 1000 km wide and extends at least 1200 km above d it is orientated roughly nw se and leans toward the east at latitudes from 15 to about 30 it proves difficult to explain such sharp features with thermal effects alone and thus the importance of high resolution waveform modeling to establish their existence to derive the above results we developed a special algorithm by matching simulated synthetics to observed broadband waveforms this is achieved by computing the various arrivals separately using generalized ray theory for a reference model and allowing the arrivals to shift in relative times to match the data tomographic models can then be constructed or existing tomographic models can be altered to match these data and new 2 d synthetics can be constructed as well to better fit the waveform data these updated synthetics can again be decomposed and reassembled and the process can be repeated this algorithm is applied to a combination of analog and digital data along a corridor from south america producing the high resolution 2 d model described above

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

can we resolve 3d density heterogeneity in the lower mantle we reexamine the possibility of resolving 3d density structure in the mantle by performing test inversions using a dataset of well constrained degree 2 splitting coefficients we show that degree 2 in vs and vp can be resolved and confirm that dlnvs dlnvp 2 5 at depths 2000km indicating a chemical component to large scale heterogeneity models with positively correlated with vs in the deepest mantle fit the data at least as well as those in which density and velocities are anti correlated however some rough bounds on dln dlnvs can be inferred models obtained from geodynamic considerations are acceptable in contrast to models with large negative 0 6 dln dlnvs throughout the lower mantle the most robust feature is a marked increase in dln dlnvs at the top of the lower mantle reaching a maximum around 1500 km

splitting function measurements for earth s longest period normal modes using recent large earthquakes recent megathrust earthquakes such as the 23 june 2001 peru event the sumatra events of 2004 and 2005 and the 27 february 2010 chile event have given us the opportunity to measure splitting of the longest period normal modes we use wave spectra to make robust measurements for modes 0s2 0s3 0s4 2s1 and 1s2 singlet frequencies of these modes have been measured previously using gravimeters but here we use seismic records to observe splitting functions for 0s2 and 2s1 for the first time cross coupling with nearby modes is included to account for ellipticity and rotation of the earth and results in significantly improved splitting function measurements for 0s3 0s4 and 1s2 compared with previous studies the new splitting function measurements can easily be implemented in future tomographic modelling of aspherical velocity and particularly density structure

chemically homogeneous spin transition zone in earth s lower mantle abstract the effect of spin crossover in lower mantle silicate perovskite pv and ferropericlase fp on the iron partitioning between these two phases remains uncertain mainly due to the lack of in situ experiments under the relevant pressure and temperature conditions by combining in situ synchrotron x ray absorption and diffraction measurements on synthetic ringwoodite and natural mg 0 88 fe 0 12 2 sio 4 san carlos olivine at pressures between 22 gpa and 115 gpa treated at temperatures of 1950 2300 k we found that temperature promotes iron partitioning into pv while pressure acts oppositely there is only a limited effect of spin crossover in co existing pv and fp on iron partitioning which cannot produce an appreciable chemical inhomogeneity within the earth s lower mantle spin transition zone

tomographic imaging of the lowermost mantle with differential times of refracted and diffracted core phases pkp pdiff the mapping of variations in p wave speed in the deep mantle is restricted by the uneven sampling of p waves in particular beneath the southern hemisphere to enhance data coverage we augmented the 1 6 million summary rays of p pp and pwp that we used in previous studies with differential travel times of diffracted and refracted core phases for the core refracted differential travel time residuals pkpab pkpdf and pkpab pkpbc we used 1383 cross correlated digital waveforms as well as 27 000 routinely processed bulletin data we used the waveform data to define quality criteria for the selection and reduction of the bulletin pkp data for pkpdf pdiff we only considered 543 records derived from waveform cross correlation no pcp data were used in this study we used optical ray theory to calculate the ray paths associated with the p pp pwp and pkp data which are measured at 1 hz however to account for the large fresnel zones of the low frequency 50 mhz pkpdf pdiff data we estimated the three dimensional shape of the frechet sensitivity kernels from kernels calculated by normal mode summation the use of these kernels allows us to properly distribute the sensitivity for a given seismic phase over a large mantle volume while allowing the high frequency data to constrain small scale structure the differential times are relatively insensitive to source mislocation and to structure in the shallow mantle beneath source and receivers and they have previously been interpreted exclusively in terms of lateral structure directly above the core mantle boundary cmb however images thus obtained can be contaminated by effects of small scale structure elsewhere in the mantle here we do not make a priori assumptions about the mantle source of anomalous time differentials from test inversions we conclude that both upper and lower mantle structures that are poorly resolved by p data can be mapped into the core along pkp paths but that the effect of outer core structures if any on the mantle model is small compared to the inversion of the p pp and pwp alone the inclusion of the pkpab pkpdf and pkpab pkpbc and pkpdf pdiff data improves the resolution of structure beneath 2200 km depth in particular the joint inversion puts better constraints on the long wavelength variations in the very deep mantle and yields an increase in the amplitude of velocity perturbations near the cmb that is in agreement with but still smaller than inferences from shear wave studies resolution tests indicate that in some regions the enhanced definition of structure is significant but in most regions the improvements are subtle and structure remains poorly resolved in large regions of the mantle

a strong lateral shear velocity gradient and anisotropy heterogeneity in the lowermost mantle beneath the southern pacific 1 velocity heterogeneity and seismic anisotropy in the d region beneath the southern pacific ocean is investigated using broadband shear waves from southwest pacific and south american earthquakes recorded by permanent and temporary stations in antarctica australia south america and on islands in the south pacific shear velocity perturbations vs are inferred from the differential travel times between hand picked horizontally polarized sh direct or diffracted s arrivals and vertically polarized sv sks arrivals derived patterns in vs roughly agree with global shear velocity models but reveal a stronger shear velocity lateral gradient north of approximately 52 s where vs transitions from approximately 0 5 to 1 0 relative to radially averaged reference models over less than 600 km along the base of d waveform analyses provide an even stronger constraint on the transitional region where there is a change in waveform behavior occurring over length scales less than 300 km differential travel times between skks and sks calculated by cross correlating sks with hilbert transformed skks support large vs amplitudes and help to constrain the location of the transitional region anisotropy in d ks is inferred from handpicked differential travel times between the sv and sh components of direct or diffracted s and display a slight spatial trend where the sh component precedes the sv in the north and east of the study area but arrives after sv in the center and southwest of it however there is high ks variability in the center of the study region there is no apparent correlation between vs and ks abrupt changes in the character of velocity and anisotropy in this region of d may be related to chemical heterogeneity at the boundary of the pacific superswell as well as small scale convection in the deep mantle

seismostratigraphy and thermal structure of earth s core mantle boundary region we used three dimensional inverse scattering of core reflected shear waves for large scale high resolution exploration of earth9s deep interior d and detected multiple piecewise continuous interfaces in the lowermost layer d beneath central and north america with thermodynamic properties of phase transitions in mantle silicates we interpret the images and estimate in situ temperatures a widespread wave speed increase at 150 to 300 kilometers above the coremantle boundary is consistent with a transition from perovskite to postperovskite internal d stratification may be due to multiple phase boundary crossings and a deep wave speed reduction may mark the base of a postperovskite lens about 2300 kilometers wide and 250 kilometers thick the core mantle boundary temperature is estimated at 3950 200 kelvin beneath central america a site of deep subduction the d is relatively cold t 700 100 kelvin accounting for a factor of two uncertainty in thermal conductivity core heat flux is 80 to 160 milliwatts per square meter mw m 2 into the coldest d region and 35 to 70 mw m 2 away from it combined with estimates from the central pacific this suggests a global average of 50 to 100 mw m 2 and a total heat loss of 7 5 to 15 terawatts

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

mgsio3 postperovskite at d conditions the postperovskite transition in mgsio3 at conditions similar to those expected at the d discontinuity of earth s lower mantle offers a paradigm for interpreting the properties of this region despite consistent experimental and theoretical predictions of this phase transformation the complexity of the d region raises questions about its geophysical significance here we report the thermoelastic properties of cmcm postperovskite at appropriate conditions and evidences of its presence in the lowermost mantle these are i the jumps in shear and longitudinal velocities similar to those observed in certain places of the d discontinuity and ii the anticorrelation between shear and bulk velocity anomalies as detected within the d region in addition the increase in shear modulus across the phase transition provides a possible explanation for the reported discrepancy between the calculated shear modulus of postperovskite free aggregates and the seismological counterpart in the lowermost mantle

j362d28 a new joint model of compressional and shear velocity in the earth s mantle summary the discrimination between chemical and thermal heterogeneity in the earth s mantle remains one of the most important and challenging questions to be answered by observational and theoretical geophysics to answer this question requires a thorough knowledge of the ratio between compressional and shear velocity anomalies we describe results of a joint inversion for compressional and shear velocity in the mantle using a large and diverse data set consisting of traveltimes complete waveforms and surface wave dispersion measurements a horizontal tessellation consisting of 362 spherical splines is used to parametrize the model which is approximately equivalent to a spherical harmonic of degree 18 in resolution the model contains peak variations from prem of up to 7 per cent in s velocity and 2 5 per cent in p velocity in the upper mantle these variations decrease to 1 5 and 0 6 per cent respectively at 1000 km depth and reach 2 5 and 1 0 per cent respectively in the d region the rms ratio of s to p velocity perturbations is fairly constant between 2 0 and 2 5 in the lower mantle but a local minimum in this ratio occurs at a depth of approximately 1700 km resolution tests show that the recovery of p and s velocity is not geographically uniform but also show that this amplitude ratio is well resolved between 670 and 2700 km depth we also observe a persistent negative correlation between bulk sound and shear velocity throughout the same region of the mantle in addition the model contains a minimum correlation between p and s velocity between 670 and 1100 km this feature is supported by both the favourable outcome of resolution experiments and the poor fit provided by the starting model from the inversion in which the p and s velocities are perfectly correlated to our data set of p wave traveltimes the power spectra of both p and s velocity heterogeneity are similar although we note a slightly larger dominance of degree two in the spectra for p velocity in the mid mantle where resolution is highest

evidence for strong shear velocity reductions and velocity gradients in the lower mantle beneath africa we present data which indicate that the broad low shear velocity anomaly beneath southern africa is stronger and more extensive than previously thought recordings of earthquakes in the southwestern atlantic ocean at an array of broadband seismic stations in eastern africa show anomalously large propagation time delays of the shear phases s scs and sks which vary rapidly with epicentral distance by forward modeling we estimate that the low velocity anomaly extends from the core mantle boundary about 1500 km up into the mantle and that the average shear velocity within this structure is 3 lower than in standard models such as prem strong velocity contrasts exist at its margins 2 over about 300 km these seismic characteristic are consistent with recent numerical simulations of lower mantle mega plume formation

physical properties of the yaxcopoil 1 deep drill core chicxulub impact structure mexico abstract the chicxulub structure in mexico one of the largest impact structures on earth was formed 65 ma by a hypervelocity impact that led to the large mass extinction at the k pg boundary the chicxulub impact structure is well preserved but is buried beneath a sequence of carbonate sediments and thus requires drilling to obtain subsurface information the chicxulub scientific drilling program was carried out at hacienda yaxcopoil in the framework of the international continental scientific drilling program in 2001 2002 the structure was cored from 404 m down to 1511 m through three intervals 794 m of postimpact tertiary sediments a 100 m thick impactite sequence and 616 m of preimpact cretaceous rocks thought to represent a suite of megablocks physical property investigations show that the various lithologies including the impactite units and the k pg boundary layer can be characterized by their physical properties which depend on either changes in fabric or on mineralogical variations the magnetic properties show mostly dia or paramagnetic behavior with the exception of the impactite units that indicate the presence of ferromagnetic probably hydrothermally deposited magnetite and pyrrhotite the magnetic fraction contributes mainly to enhanced magnetization in the impactite lithologies and in this way to the observed magnetic anomalies the shape and orientation of the magnetic grains are varied and reflect inhomogeneous fabric development and the influence of impact related redeposition and hydrothermal activity the chicxulub impact occurred at the time of the reverse polarity geomagnetic chron 29r and this finding is consistent with the age of the k pg boundary

constraints from seismic anisotropy on the nature of the lowermost mantle the d layer lies at the bottom of the earth s rocky mantle and separates it from the liquid metal alloy core this region extending from the core mantle boundary to a few hundred kilometres above fig 1 is geodynamically analogous to the more easily studied lithosphere at the top of the mantle the structure of d may reflect the style of lower mantle convection the nature of core mantle interaction and perhaps even the fate of subducting lithosphere1 observations of lithospheric seismic anisotropy have provided valuable insight into the nature of the upper mantle boundary layer but discussion of lower mantle seismic anisotropy has been somewhat contentious2 5 here we present evidence from seismic waves that have traversed the lowermost mantle beneath the caribbean region for a zone of seismic anisotropy below the d discontinuity which in this region lies 250 km above the core mantle boundary the anisotropy is most probably due to horizontal layering or aligned inclusions of a material with differing shear wave velocity if d is a graveyard for subducted lithosphere a plausible explanation of the anisotropy may be the contrast between cold lithospheric mantle and material that formerly constituted the oceanic crust which may have lower shear wave velocity owing to the presence of melt

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

some observations on basin evolution and dynamics sedimentary basin evolution is intimately bound in with continental lithosphere dynamics deep seismic reflection profiling has revealed images of the crust and upper mantle that suggest linkages between the shape and structure of sedimentary basins as they evolve with the rheological responses to tectonic stress of upper and lower crust and upper mantle components of the lithosphere this is well illustrated by the european geotraverse lithosphere profile from north cape norway to tunisia the alpine mountain belt has a lithospheric root as well as a crustal root by virtue of its asymmetric disposition this may be the cause of compressive and tensile forces in close association in the upper crust which have created mountain uplift and basin subsidence extension and thrusting all in juxtaposition evidence for an elastic upper layer of crust above a plastic substratum can be used as a basis for modelling basin geometry structure and stratigraphy built upon the flexural nature of the upper crust such models go a long way towards simulating the evolution of basins such as the north sea viking graben in more detail analogue structural models can reveal the geometry of basin fill in relationship to crustal movement and the relationships between structure and sedimentation patterns however sedimentary basin modelling needs to begin with the quantitative simulation of depositional environments in relation to such factors as tectonics sea level variations sediment supply and erosion in order to match sedimentary architecture only then can the range of deposits be placed in space and time within a basin environment as building blocks within a dynamically evolving structural framework to create predictive models for comparison with geological reality