nb in hawaiian magmas constraints on source composition and evolution abstract published th and nb data for basalts from 11 volcanoes from the hawaiian islands and emperor seamount chain yield a nearly uniform concentration ratio of nb th 15 2 1 7 the basalts include tholeiites with nb concentrations ranging down to 4 ppm and alkali basalts with a maximum nb concentration of 127 ppm however nephelinites and melilitites from the honolulu volcanic series have much lower nb th ratios and are excluded from this analysis using the average th u ratio of 3 2 for the five volcanoes of the island of hawaii this corresponds to a nb u 48 6 5 3 which is indistinguishable from the mean value of a worldwide sample of oib and morb nb u 47 10 given by direct nb and u analyses in contrast the earth s primordial mantle had a nb th ratio of 8 6 and a nb u ratio of 30 these data demonstrate that none of the hawaiian volcanoes not even the isotopically most nearly primitive koolau series from oahu has refractory elements derived from an undifferentiated or primordial portion of the mantle an even less likely source is recycled continental crust which has a mean nb u ratio of 10 a consideration of the chromatographic theory of metasomatism and comparison with metasomatic rocks observed in the continental crust show that large scale metasomatism in the mantle which would be needed to account for the very large total volume of hawaiian emperor magmas is expected to produce a strongly zoned mantle yielding chemically highly heterogeneous magmas unless the metasomatic column has reached a steady state the expected heterogeneity is difficult to reconcile with observed chemical homogeneity of the main stage tholeiitic magmas the late stage alkali basalts and the nephelinites must be excluded from this consideration because their isotopic composition shows that they are derived from the depleted lithosphere or asthenosphere the consequence of a steady state metasomatic column would be that the tholeiitic melt itself represents the metasomatic fluid the model of recycled oceanic crust and lithosphere is consistent with the chemical and isotopic data the mantle that originally produced this oceanic crust was already depleted and rehomogenized as a result of prior separation of continental crust uniform source ratios of nb u and ba rb were established during this rehomogenization process the general enrichment of the hawaiian source and the chemical and isotopic heterogeneities found in the present day lavas were produced during the differentiation into oceanic crust and lithosphere secondary pb 1bpb model ages indicate that this crust was probably produced 1 ga ago

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

alternatives to core mantle boundary topography abstract nuclear explosions at the french test sites in the south pacific have been recorded at the regional seismic networks in the western united states a total of approximately 600 good quality short period seismograms were selected from an initial set of 900 the dense receiver spacing particularly in california allows the division of the networks into numerous subarrays and the data from each subarray can be analysed using conventional array techniques such as beam forming and frequency wave number analysis the seismic rays provide dense sampling of a small area of the core mantle boundary cmb beneath the eastern pacific from 5 to 13 n and 133 to 126 w pcp has a shallow incidence angle on the cmb at an epicentral distance of 65 and d is sampled over a large swathe from 2 s to 24 n and from 134 to 123 w beam traces have been formed for the p and pcp phases for each of the subarrays using slowness and azimuth values derived from array methods good quality beams have been selected for further analysis by applying a matched filter to the beamed traces an accurate set of pcp p differential travel times has been determined and compared with iasp91 the residuals show a smooth trend over the area sampled for the given epicentral distances p and pcp have similar paths through the crust and upper mantle so we make the assumption that near source and near receiver structures have negligible effect on the differential times we also assume that the midmantle is relatively homogenous many previous studies have interpreted pcp p residuals in terms of cmb topography by mapping the residuals directly to topography a range in elevation of 40 km over a lateral extent of about 700 km results alternatively we attempted to interpret the residuals in terms of velocity heterogeneity in d a range of d models have been tested including lateral velocity variation in a d layer with constant thickness the pcp p residuals may be explained by allowing the p wave velocity to vary laterally by a factor of 2 over the region sampled with most of the region being marginally fast this suggests that d heterogeneity has large spectral power at relatively short wavelengths in this scenario previous tomographic inversions for cmb topography will have large errors since in these inversions corrections for d structure are based on low order spherical harmonic models

morphodynamics of barchan and transverse dunes using a cellular automaton model 1 we analyze the morphodynamics of bedforms produced under unidirectional flow conditions by a lattice gas cellular automaton designed to study sediment transport in this model the same instability is responsible for the formation of dunes on flat sand beds and the initiation of superimposed bedforms on dune slopes in transverse dune fields secondary bedforms increase crestline sinuosity and number of defects end of crestlines on the other hand avalanches and lateral grain motions tend to eliminate these defects to produce more regular crestlines lateral fluxes of sediment are also essential for stabilizing the shape of isolated barchan dunes we measure the propagation speed of superimposed bedforms on steady state barchan dunes and show how they contribute to the formation and detachment of smaller barchans along horns the model predicts that barchan dunes are not scale invariant and that their shape varies with respect to the strength of the flow in addition we show that the increase in bed shear stress between the ground and the crest is proportional to the dune aspect ratio finally we present a general methodology for estimating the sediment flux over the brink from dune aspect ratio and flow velocity using these fluxes we rescale the propagation speed of different generations of bedforms to verify that dunes and secondary bedforms are dynamically identical despite more fluctuations than in the case of isolated barchan dunes all these geometric and dynamical relationships hold for a population of dunes with complex dune dune interactions

formation and evolution of a population of strike slip faults in a multiscale cellular automaton model summary this paper describes a new model of rupture designed to reproduce structural patterns observed in the formation and evolution of a population of strike slip faults this model is a multiscale cellular automaton with two states a stable state is associated with an intact zone in which the fracturing process is confined to a smaller length scale an active state is associated with an actively slipping fault at the smallest length scale of a fault segment transition rates from one state to another are determined with respect to the magnitude of the local strain rate and a time dependent stochastic process at increasingly larger length scales healing and faulting are described according to geometric rules of fault interaction based on fracture mechanics a redistribution of the strain rates in the neighbourhood of active faults at all length scales ensures that long range interactions and non linear feedback processes are incorporated in the fault growth mechanism typical patterns of development of a population of faults are presented and show nucleation growth branching interaction and coalescence the geometries of the fault populations spontaneously converge to a configuration in which strain is concentrated on a dominant fault in these numerical simulations the material properties are uniform so the entire process of fault development arises spontaneously from the pattern of interactions between the elements of the system over time furthermore homogenization of the strain rate along faults and structural regularization of the fault trace can be quantified by analysis of the output patterns the time dependent stochastic process allows relocation of faults by branching from bends and irregularities of the fault traces this relocation mechanism involves partitioning of the strain and competition between faults possible relationships between the seismic regime and the geometry of the fault population suggest that the fault system may be attracted by a critical point towards this critical point the correlation length i e the length of the largest fault increases the proportion of large event is higher and larger faults are able to slip at lower stresses

systematics of the equatorward diffuse auroral boundary dmsp f2 and dmsp f4 precipitating electron data are used to determine statistically the systematic variations of the equatorward diffuse auroral boundary with kp as a function of local time this work extends a previous study of dawn and dusk boundaries to the noon and midnight regions the boundaries are well ordered by kp in the night sector but show increasingly greater scatter from dawn to postnoon in the noon sector it is often the case that no diffuse aurora is discernible within detector sensitivity the equatorward diffuse auroral boundary is well fit by a circle at each activity level the center of the circle is offset from the geomagnetic pole and the radius of the circle increases with increasing magnetic activity the circular fits are projected to the ecliptic plane by using the mead fairfield magnetic field model where they are identified with the inner edge of the plasma sheet in order to make comparison with volland stern type convection electric field predictions previously found variations of the electric field with kp are confirmed including a rotation of the axis of symmetry away from the dawn dusk meridian all equatorward auroral boundaries from dmsp for 1978 were compiled using the equations for boundary variations with kp each evening sector boundary was projected to a midnight boundary the projected midnight boundary serves as an index of auroral activity and an indicator of the strength of the large scale magnetospheric convection electric field the auroral boundary index is given in monthly plots for 1978

geochemistry and models of mantle circulation geochemical data help to constrain the sizes of identifiable reservoirs within the framework of models of layered or whole mantle circulation and they identify the sources of the circulating heterogeneities as mainly crustal and or lithospheric but they do not decisively distinguish between different types of circulation the mass balance between crust depleted mantle and undepleted mantle based on 143 nd 144 nd nb u and ce pb and the concentrations of very highly incompatible elements ba rb th u and k shows that ca 25 70 by mass of depleted mantle balances the trace element and isotopic abundances of the continental crust this mass balance reflects the actual proportions of mantle reservoirs only if there are no additional unidentified reservoirs evidence on the nature and ages of different source reservoirs comes from the geochemical fingerprints of basalts extruded at mid ocean ridges and oceanic islands consideration of nd and he isotopes alone indicates that ocean island basalts oibs may be derived from a relatively undepleted portion of the mantle this has in the past provided a geochemical rationale for a two layer model consisting of an upper depleted and a lower undepleted primitive mantle layer however pb isotopic ratios and nb u and ce pb concentration ratios demonstrate that most or all oib source reservoirs are definitely not primitive models consistent with this evidence postulate recycling of oceanic crust and lithosphere or subcontinental lithosphere recycling is a natural consequence of mantle convection this cannot be said for some other models such as those requiring large scale vertical metasomatism beneath oib source regions unlike other trace elements nb ta and pb discriminate sharply between continental and oceanic crust forming processes because of this the primitive mantle value of nb u 30 ce pb 9 has been fractionated into a continental crustal nb u 12 ce pb 4 and a residual mantle morb mid ocean ridge basalt plus oib source nb u 47 ce pb 25 these residual mantle values are uniform within about 20 and are not fractionated during formation of oceanic crust by using these concentrations ratios as tracers it can be shown that the possible contribution of recycled continental crust to oib sources is limited to a few percent therefore recycling must be dominated by oceanic crust and lithosphere or by subcontinental lithosphere oceanic crust normally bears a thin layer of pelagic sediment at the time of subduction and this is consistent with oib sources that are dominated by subducted oceanic crust with variable but always small additions of continental material primordial 3 he 36 ar and excess 129 xe in oceanic basalts demonstrate that the mantle has been neither completely outgassed nor homogenized but they do not constrain the degree of mixing or the size of reservoirs also helium does not correlate well with other isotopic data and may have migrated into the basalt source from other regions the high 3 he 4 he ratios found in some oibs suggest that even though the basalts are not derived from primordial mantle their sources may be located close to a reservoir rich in primordial gases this leads to models in which the oib sources are in a boundary layer within the mantle the primordial helium migrates into this layer from below the interpretation of the rare gas data is still quite controversial it is often argued that the upper mantle is a well homogenized reservoir but the data indicate heterogeneities on scales ranging from 10 to 10 6 m the 206 pb 204 pb ratios in the oceanic m antle range from 17 to 21 which is similar to the range in most continental rocks the degree of mixing cannot be directly inferred from these data unless the size and composition of the heterogeneities and the time of their introduction into the system are known the relative uniformity of nb u and ce pb ratios in the otherwise heterogeneous morb and oib sources indicates that this reservoir was indeed homogenized after the separation of the continental crust and that the observed isotopic and chem ical heterogeneities were introduced subsequently overall the results are consistent with but do not prove a layered mantle where the upper layer contains both morb and oib sources and the lower primitive mantle is not sampled by present day volcanism alternative models such as those involving a chemically graded mantle have not been sufficiently explored

on a small scale roughness of the core mantle boundary abstract the roughness of the core mantle boundary on the scale of centimeters to tens of meters is modelled using a cellular automata method square cells of the size of grains of the mantle material on a 2 d grid can be in one of three states corresponding to mantle silicate or oxide core fluid saturated in light element and unsaturated core fluid the dynamical process of evolution is defined as a stationary stochastic process without memory transitions of doublets of cells from one state to another are governed by parameters representing the rates of physical processes dissolution and crystallization at the cmb and diffusion of the light element in the core fluid with reasonable values of the parameters the boundary roughens on the scale of grains and a boundary layer of saturated fluid of a few tens of centimeters thick soon appears at the interface an undulation with dominant wavelength of the order of a few tens of meters eventually appears an interaction of the roughness of the cmb with the fluid flow in the core is considered as possible

geochemistry of kauai volcanics and a mixing model for the origin of hawaiian alkali basalts a comprehensive model is developed to explain the major trace element and strontium and neodymium isotopic characteristics of alkali basalts from hawaii the model is similar to that of chen and frey 1983 in that it requires mixing of a small melt fraction of morb source material with another component to generate the alkalic suite of a particular hawaiian volcano it differs from the chen and frey model in that the other end member must be different from primitive mantle if it is to be consistent with both trace element and isotopic data alkali basalts and tholeiites from kauai analyzed in this study show a nearly complete transition in sr and nd isotopes there is a relatively well constrained array on a nd sr isotope correlation plot that can be explained by two component mixing of kauai tholeiite magma and a small amount of melt of east pacific rise source rock after corrections are made for fractional crystallization involving primarily clinopyroxene and olivine the sr and ba concentrations of kauai lavas plot along mixing curves defined by the above sources providing positive tests of the mixing hypothesis implications of this model are 1 the main source of hawaiian shield building tholeiites is a mixture of subducted crust primitive mantle and depleted asthenosphere that has been homogenized prior to melting 2 early alkalic volcanism as at loihi seamount will be characterized by greater isotopic heterogeneity than will late stage alkali basalt production and 3 there are two fundamentally distinct types of alkalic lavas erupted towards the end of magmatism at a given hawaiian volcano one represents smaller degrees of melting of the same source that generated shield building tholeiites kohala type the other derives from the mixed source discussed in this paper haleakala kauai type

gravimetric and magnetic anomalies produced by dissolution crystallization at the core mantle boundary using the unique datasets provided by satellite observations correlated temporal variations in gravity and magnetic fields over a large area extending from the atlantic to the indian ocean have been recently reported mandea et al 2012 on a time scale of few years to a decade both field variations may be linked to changes at the top of the core here we propose that in addition to the topography generated by the convection in the mantle the core mantle boundary cmb may be in a dynamic equilibrium state mainly controlled by a dissolution crystallization process of the mantle silicate rocks in the liquid alloy of the core due to the resulting continuous changes in cmb topography anomalies of hundreds of ngals and tens of ntyr 2 might be produced by the corresponding mass redistribution and the secondary flow generated by the associated pressure field numerical modeling and both gravimetric and magnetic anomaly magnitudes suggest a rate of centimeters per year and a large range of length scales for the changes in the topography at the cmb

chemical aspects of the formation of the core the interpretation of data on lead isotopes in mantle material indicates that some of the lead has been pumped into the core during geological time such a continuous lead loss yields an increase of the u pb ratio in the mantle quantitative modelling of this effect permits an evaluation of the core s growth curve in the preferred model 85 of the core would have been formed in the early period of the earth s history 50 200 ma after 4 55 ga ago and 15 later on and its accretion continues some of the consequences of such a process are reviewed

small scale changes of core mantle boundary reflectivity studied using core reflected pcp abstract short period pcp phases from earthquakes in the western aleutians recorded at the canadian yellowknife array yka sample the core mantle boundary cmb beneath the alaskan kenai peninsula and the cook inlet the pcp to p amplitude ratios for these events show large variability with some amplitude ratios more than an order of magnitude larger than predicted by radial earth models such as iasp91 or prem the amplitude ratios vary laterally with pcp core mantle boundary reflection points showing a 1 region with exceptionally large amplitude ratios it is likely that the variability in relative pcp amplitudes is due in large part to cmb topography other mechanisms such as radiation pattern mantle attenuation focusing and defocusing and core mantle boundary velocity and density perturbations could cause variations on scale lengths of less than a degree as found in this study but are unable to produce variations as large as those we observe

travel times and station corrections for p waves at teleseismic distances approximately 3300 shallow focus earthquakes and 1000 seismic stations have been used in a study of p wave travel times and station residuals including azimuthal effects the events were selected from a catalog containing 160 000 earthquakes and those having uniform distance and azimuthal coverage were systematically relocated and used to refine p wave travel times and station corrections station corrections are provided for 994 seismic stations the station corrections involve three terms the static effect and two cosine terms with appropriate phase shifts they exhibit general consistency over broad geographic areas and where coverage is dense often show abrupt changes from one geological province to another the cos 2 terms appear to be due to upper mantle anisotropy and they correlate with the stress direction in the crust

viscoelastic mantle density heterogeneity and core mantle topography summary starting with the model of mantle density heterogeneity derived by ricard et al 1993 which uses plate motion reconstructions under the assumption that subducted slabs sink vertically into the mantle we model the temporal evolution of these internal loads we investigate the viscoelastic deformation of the earth due to this time dependent excitation source using a love number formalism for a linear viscoelastic mantle with a maxwell model of rheology we are especially interested in the deep earth viscoelastic deformation such as the radial displacement that appears at the core mantle boundary and we compare the calculated topography with that proposed by seismologists

setting the length and time scales of a cellular automaton dune model from the analysis of superimposed bed forms 1 we present a new 3 d cellular automaton model for bed form dynamics in which individual physical processes such as erosion deposition and transport are implemented by nearest neighbor interactions and a time dependent stochastic process simultaneously a lattice gas cellular automaton model is used to compute the flow and quantify the bed shear stress on the topography local erosion rates are assumed to be proportional to the shear stress in such a way that there is a complete feedback mechanism between flow and bed form dynamics in the numerical simulations of dune fields we observe the formation and the evolution of superimposed bed forms on barchan and transverse dunes using the same model under different initial conditions we perform the linear stability analysis of a flat sand bed disturbed by a small sinusoidal perturbation comparing the most unstable wavelength in the model with the characteristic size of secondary bed forms in nature we determine the length and time scales of our cellular automaton model thus we establish a link between discrete and continuous approaches and open new perspectives for modeling and quantification of complex patterns in dune fields

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

global 30 240 kev proton precipitation in the 17 18 april 2002 geomagnetic storms 1 patterns 1 global 30 240 kev proton precipitation patterns during the 17 18 april 2002 magnetic storm events were generated using newly developed 3 hour data products of the medium energy proton and electron detector meped on board the noaa 15 and 16 satellites the observational data set was sorted in 1 degree corrected magnetic latitude cml and 8 min magnetic local time mlt bins to achieve global coverage the missing mlt data were filled by interpolation techniques in each 1 degree cml interval the resulting global 30 240 kev proton precipitation maps available on a 3 hour cadence provide new information on the development and variability in the structure of incident high energy protons on a timescale commensurate with the growth and decay of the plasma sheet and ring current the change of energetic proton precipitation patterns in the april 2002 storms is quantified in terms of three aspects hemispheric integrated total particle energy input midnight proton oval equatorward boundary and position of proton precipitation peak in a general sense as magnetospheric activity intensified the midnight equatorward boundary tended to move to the lower latitude while the precipitation peak moved equatorward and westward away from midnight in agreement with ring current motion it is well illustrated that energetic proton precipitation patterns can serve as a valuable diagnostic tool for investigating the inner magnetospheric activity

earth degassing mantle metasomatism and isotopic evolution of the mantle excess 3 he in mantle derived rocks relative to atmospheric abundance shows that degassing of earth is a continuing process the importance of chemical changes induced by fluids from the deep mantle on the oceanic and continental lithosphere has been recognized increasingly in recent years i propose that the parent daughter element fractionation caused by such fluids dominates the sr nd and pb isotope evolution of the mantle in the second half of earth9s history a simple two layered mantle model based on such a premise seems to be consistent with the present knowledge of the sr nd pb and he isotopic characteristics of mantle derived rocks the model predicts two populations of intraplate oceanic island volcanism primary fluid induced volcanism and secondary volcanism caused by remelting of ancient metasomatized domains the model implies a new interpretation of the nd sr isotopic covariations in oceanic basalts

plume formation in the d double prime layer and the roughness of the core mantle boundary numerical simulations of the formation of thermal plumes in the d layer at the base of the earth s mantle show that plumes are initiated by coalescence of small scale convective instabilities within the low viscosity region immediately above the core mantle boundary cmb these instabilities support topographic roughness on the cmb having horizontal scales of 20 50 km and provide a source for scattered p waves seen as precursors to the phases pkikp and pkkp the calculated structure of fully developed plumes emerging from d consists of 5 50 cm yr 1 flow confined to 50 100 km thick vertical conduits with strongly temperature dependent viscosity plumes exhibit time dependent behaviour including upward propagating solitary conduit waves which may contribute to episodicity in hotspot volcanism

modeling the shape and evolution of normal fault facets facets formed along the footwalls of active normal fault blocks display a variety of longitudinal profile forms with variations in gradient shape degree of soil cover and presence or absence of a slope break at the fault trace we show that a two dimensional process oriented cellular automaton model of facet profile evolution can account for the observed morphologic diversity the model uses two dimensionless parameters to represent fault slip progressive rock weathering and downslope colluvial soil transport driven by gravity and stochastic disturbance events the parameters represent rock weathering and soil disturbance rates respectively scaled by fault slip rate both can be derived from field estimated rate coefficients in the model s transport limited regime slope gradient depends on the ratio of disturbance to slip rate with a maximum that represents the angle of repose for colluvium in this regime facet evolution is consistent with nonlinear diffusion models of soil mantled hillslope evolution under the weathering limited regime bedrock becomes partly exposed but microtopography helps trap some colluvium even when facet gradient exceeds the threshold angle whereas the model predicts a continuous gradient from footwall to colluvial wedge under transport limited behavior fully weathering limited facets tend to develop a slope break between footwall and basal colluvium as a result of reduced transport efficiency on the rocky footwall slope to the extent that the model provides a reasonable analogy for natural facets its behavior suggests that facet profile morphology can provide useful constraints on relative potential rates of rock weathering soil disturbance and fault slip

p and s wave reflections from anomalies in the lowermost mantle summary seismic p and s waves recorded at the grf array in germany are used to study the inhomogeneous structure of the boundary layer d at the base of the mantle the use of the seismic array allows the detection of small scale anomalies in the lowermost mantle the lateral resolution attainable is about 100 to 400 km i e more than 10 times better than with tomographic methods the analysis of 13 years of grf broad band array data yields 255 events with high signal to noise ratio which are used to map the lowermost mantle 74 of these events show anomalous p waves pdp which arrive 3 6 s after the direct p wave and have a slowness 0 7 0 8 s smaller than the slowness of direct p other events close to the events with pdp do not have such an anomalous phase using slowness traveltime amplitude and waveform information it is demonstrated that pdp is caused by an anomalous lower mantle velocity structure below the turning point of the p wave if the pp phase i e a p wave first reflected at the free surface near the source is used together with the p phase distinct and well sepaarted p velocity anomalies can be determined under the nansen basin the kara sea and northern siberia the areas of the bounce points of pdp in the lower mantle have a lateral extension of about 100 by 200 km but this is not the size of the anomaly since the resolution of p waves at 1 hz in this depth is 130 km by 260 km fresnel zone the accuracy to which the depth of the reflector 2612 km under the nansen basin and 2605 km under northern siberia respectively can be determined for 1 d models is 10 to 20 km the velocity contrast at the lower mantle discontinuity is about 3 per cent 1 to 1 5 per cent areas which have velocity fluctuations smaller than about 1 per cent can not be detected as anomalous areas 2 d models of the anomalies reveal the range of adequate models and possible trade offs if the lateral extension of the anomaly is about 7 the reflector has to be 40 km deeper than in the 1 d model if the dip of the reflector in the lowermost mantle is only about 1 5 it is difficult to resolve if the reflector is tilted towards the source or towards the receiver for the anomaly under the nansen basin deviations from the great circle path are observed for pdp indicating 3 d effects below the three anomalies the core mantle boundary cmb can be located by pcp with a depth in agreement with standard earth models under northern siberia has a region where p and s velocity anomalies coincide but also a region where only an s velocity anomaly but no p velocity anomaly is observed this results in changes of the poisson ratio from 5 9 per cent to 4 8 per cent 1 5 per cent across the discontinuity in the lowermost mantle for the regions studied the analysis of the s waves s sds and scs reveals two s velocity anomalies in the lowermost mantle under northern siberia the first anomaly coincides with the p velocity anomaly under northern siberia and can be explained by a 1 d model with a reflector depth of 2610 km 15 km and a velocity contrast of 2 3 per cent the second s velocity anomaly is in an area where no p velocity anomaly can be detected the corresponding 1 d s velocity model has a reflector depth of 2575 km 15 km and an s velocity contrast of 2 6 per cent the smallest structures that can be resolved with the s waves in this depth are about twice as large as for the p waves i e 230 km by 460 km fresnel zone the joint analysis of p and s waves therefore shows a region with a p velocity anomaly together with weak indications for an s velocity anomaly nansen basin and a second region with a p but no s anomaly kara sea the lowermost mantle

core mantle interactions abstract recent studies of the magnetic field at the core mantle boundary have revealed fixed sites of either static magnetic features or persistent secular variation this suggests part of the magnetic field behaviour is controlled by the mantle the most plausible mechanism for core mantle interaction is thermal coupling although topography may also be significant the magnetic sites coincide with anomalies in lower mantle seismic velocity as determined from tomography and density as determined by flow models of mantle convection constrained by tomography and the geoid some magnetic features coincide with subduction zones particularly those near indonesia they may be caused by bumps on the core mantle boundary beneath trenches paleomagnetic pole positions suggest the magnetic behaviour has persisted for at least 5 myr as would be expected if it were controlled from the mantle

thorium uranium ratio of the earth through the study of komatiites and ophiolites remnants of the old mantle we review the evolution of the lead system in the depleted mantle through time using the 230th232th vs 208pb 206pb diagram 208pb 206pb radiogenic lead ratio we constrain the th u ratio of the bulk silicate earth to 4 2 this value is also in full agreement with ratios inferred from the study of the various types of meteorites using this value in the 208pb 206pb 143nd144nd diagram bulk earth plots on the oceanic basalt correlation line as previously guessed a comparison with sr and nd data shows that the th u fractionation correlates with that of rb sr and nd sm whereas the u pb fractionation is independent of other ratios

mapping the lower mantle determination of lateral heterogeneity in p velocity up to degree and order 6 the data from the international seismological centre bulletins for the years 1964 1979 are used to derive a three dimensional model of lateral variations of the p velocity in the lower mantle unlike previous studies in which such perturbations were represented by a blocklike parameterization we seek the solution in the form r k 0k l 0l m 0l k r kalm cos m kblm sin m plm cos some 500 000 travel time residuals for teleseismic distances from 5000 earthquakes are used in an iterative procedure to derive the coefficients a and b the maximum k and l used are 4 and 6 respectively in each iteration in addition to solving for the three dimensional structure we also relocate all the earthquakes perturb the average travel time curve and determine station corrections for over 1000 stations particular attention is given to the problem of weighting the individual observations in order to avoid as much as possible the bias due to the uneven distribution of sources and receivers the resulting model shows a high level of perturbations just below the 670 km discontinuity and just above the core mantle boundary where the maximum perturbations reach 1 1 5 of the average velocity even for this highly smoothed model at a depth of 2000 km the rms perturbations are some 3 to 4 times lower than at either of the two boundaries the model predicts well the large scale pattern of observed travel time residuals for various source regions except for the distinct effects of the subduction zones the most striking large scale three dimensional feature of the model is a ring of high velocities circumscribing the pacific basin within a depth range from 1000 km to the core mantle boundary if the proportionality between perturbations in the velocity and temperature is assumed the derived pattern of anomalies could be consistent with a two boundary layer model of convection in the lower mantle some caution however should be exercised in interpreting the results near the 670 km discontinuity where the resolving power of our data set is limited by the lack of adequate control of the upper mantle structure at the same time a significant global increase in the level of perturbations in the lowermost mantle is established beyond question the results of dziewonski et al 1977 on the correlation between the equivalent geoid and the observed geoid for the angular order numbers 2 and 3 is confirmed but there is no significant correlation for degrees from 4 to 6 these calculations are made for the rigid earth richards and hager this issue show that introduction of finite viscosity can change the sign of a geoid anomaly in addition recent studies of lateral variations in the shear velocity reveal the presence of large scale anomalies in the upper mantle thus while the inferences made here with respect to the origin of the gravest orders of the geopotential field are not conclusive they point the way in which results from seismology can be used to address some of the basic questions of geodynamics

layered mantle convection a model for geoid and topography the long wavelength geoid and topography are dynamic effects of a convecting mantle the long wavelength geoid of the earth is controlled by density variations in the mantle and has been explained by circulation models involving whole mantle flow however the relationship of long wavelength topography to mantle circulation has been a puzzling problem in geodynamics we show that the dynamic topography is mainly due to density variations in the upper mantle even after the effects of lithospheric cooling and crustal thickness variation are taken into account layered mantle convection with a shallow origin for surface dynamic topography is consistent with the spectrum small amplitude and pattern of the topography layered mantle convection with a barrier about 250 km deeper than the 670 km phase boundary provides a self consistent geodynamic model for the amplitude and pattern of both the long wavelength geoid and surface topography

p velocity structure of the core mantle boundary region inferred from pkp ab pkp bc differential travel times pkp ab pkp bc differential travel time residuals from the bulletins of the international seismological centre are inverted to retrieve the p velocity anomalies in the lowermost 300 km of the mantle we show that a realistic core mantle boundary cmb topography may have only a marginal contribution to the residuals the block parametrization used allows us to discard poorly sampled areas prior to a least squares inversion the main features are negative anomalies beneath eurasia and north america and a positive anomaly beneath the southwest pacific the inferred map is not consistent with other maps resulting from previous p and s velocity studies which are correlated at long wavelengths with each other the preferred explanation for this challenging inconsistency is the presence of a thin heterogeneous layer just above the cmb

separating and quantifying ionospheric responses to proton and electron precipitation over svalbard on 16 december 2001 a variable and structured aurora associated with a period of high solar wind velocity and low solar wind density was recorded through optical radar and particle measurements

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

partial melting in a thermo chemical boundary layer at the base of the mantle abstract seismological detections of complex structures in the lowermost mantle boundary layer the d region motivate a conceptual model of a compositionally stratified thermo chemical boundary layer tcbl within which lateral temperature variations sustained by large scale mid mantle flow cause variations of partial melt fraction partial melt fractions of from 0 to 30 in the tcbl occur due to the eutectic of the boundary layer material lying below the high temperature at the core mantle boundary cmb coupled with the presence of steep thermal gradients across the boundary layer regions of the tcbl with the highest temperatures have extensive partial melting producing lateral chemical variations and strong effects on seismic velocities and boundary layer dynamics this tcbl concept provides a relatively simple framework that can plausibly account for diverse seismological observations such as predominance of large scale volumetric elastic wave velocity heterogeneity in the boundary layer laterally extensive but intermittent abrupt shear and compressional velocity increases and decreases at the top of the d region small scale topography of d velocity discontinuities thin ultra low velocity zones at the cmb widespread shear wave anisotropy in d bulk sound velocity anomalies detected in low shear velocity regions of d and large scale upwellings from the most extensively melted regions of the boundary layer neutral or negative buoyancy of the partial melt in the tcbl is required along with an increase in bulk modulus and some density increase of the boundary layer material relative to the overlying mantle more complex models in which the partially melted chemical boundary layer is laterally displaced by mid mantle downwellings are also viable but these appear to require additional special circumstances such as a phase change efficient segregation of slab crustal material or abrupt onset of anisotropy in order to account for rapid velocity increases at the top of d the precise nature of the compositional anomaly or anomalies in d and the eutectic composition of this zone are yet to be determined but it appears likely that partial melting within the lowermost mantle plays a paramount role in the observed seismic velocity heterogeneity increased resolution geodynamic calculations of dense boundary layer structures with partial melting and attendant viscosity reductions and chemical heterogeneity are needed to quantify this tcbl concept

characterizing the 18 april 2002 storm time sawtooth events using ground magnetic data 1 beginning during the last half of the main phase and extending for over 12 hours the 18 april 2002 magnetic storm is dominated by sawtooth oscillations we examine the ground magnetic signatures of these events at both high latitudes and low latitudes the first immediate observation is that the magnetic signatures from one sawtooth to another can show considerable variation thus the current systems associated with these events show considerable variation in location pattern and strength from one oscillation to the next the characteristics of the magnetic disturbance for each sawtooth has little to distinguish it from the typical substorm expansion magnetic disturbance field except that some of the sawteeth disturbance profiles are broader in local time than observed during typical substorms the injection region determined from the energetic particle flux measurements at geostationary orbit is generally broad extending across the entire tail and past the terminators into the dayside the injection region is usually wider than the magnetic disturbance profile at the peak of the dipolarization there appears to be no relationship between the magnetic disturbance profile and the location and strength of the injection boundary finally sawtooth events appear to be driven by a steady solar wind driver and to develop during a period of already enhanced ring current

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

mean sediment residence time in barchan dunes when a barchan dune migrates the sediment trapped on its lee side is later mobilized when exposed on the stoss side then sand grains may undergo many dune turnover cycles before their ejection along the horns but the amount of time a sand grain contributes to the dune morphodynamics remains unknown to estimate such a residence time we analyze sediment particle motions in steady state barchans by tracking individual cells of a 3 d cellular automaton dune model the overall sediment flux may be decomposed into advective and dispersive fluxes to estimate the relative contribution of the underlying physical processes to the barchan shape the net lateral sediment transport from the center to the horns indicates that dispersion on the stoss slope is more efficient than the convergent sediment fluxes associated with avalanches on the lee slope the combined effect of these two antagonistic dispersive processes restricts the lateral mixing of sediment particles in the central region of barchans then for different flow strengths and dune sizes we find that the mean residence time of sediment particles in barchans is equal to the surface of the central longitudinal dune slices divided by the input sand flux we infer that this central slice contains most of the relevant information about barchan morphodynamics finally we initiate a discussion about sediment transport and memory in the presence of bed forms using the advantages of the particle tracking technique

development and steady states of transverse dunes a numerical analysis of dune pattern coarsening and giant dunes we investigate the development and steady states of transverse dunes for ranges of flow depths and velocities using a cellular automaton dune model subsequent to the initial bed instability dune pattern coarsening is driven by bed form interactions collisions lead to two types of coalescence associated with upstream or downstream dominant dunes in addition a single collision ejection mechanism enhances the exchange of mass between two adjacent bed forms throughpassing dunes the power law increases in wavelength and amplitude exhibit the same exponents which are independent of flow properties contrary to the wavelength dune height is limited not only by flow depth but also by the strength of the flow superimposed bed forms may propagate and continuously destabilize the largest dunes we identify three classes of steady state transverse dune fields according to the periodicity in crest to crest spacing and the mechanism of size limitation in all cases the steady state is reached and maintained through the dynamic equilibrium between flow strength and dune aspect ratio in the limit of low flow strength where it becomes the primary factor of size limitation the bed shear stress in the dune trough regions is close to its critical value for motion inception comparisons with natural dune fields suggest that many of them may have reached a steady state finally we infer that the sedimentary patterns in the model may be used to bring new constraints on the development of modern and ancient dune fields