chemistry of uranium thorium and radium isotopes in the ganga brahmaputra river system weathering processes and fluxes to the bay of bengal abstract the most comprehensive data set on uranium thorium and radium isotopes in the ganga brahmaputra one of the major river systems of the world is reported here the dissolved 238u concentration in these river waters ranges between 0 44 and 8 32 1 and it exhibits a positive correlation with major cations na k mg ca the 238 u cations ratio in waters is very similar to that measured in the suspended sediments indicating congruent weathering of uranium and major cations the regional variations observed in the 234 u 238 u activity ratio are consistent with the lithology of the drainage basins the lowland tributaries chambal betwa ken and son draining through the igneous and metamorphic rocks of the deccan traps and the vindhyan bundelkhand plateau have 234 u 238 u ratio in the range 1 16 to 1 84 this range is significantly higher than the near equilibrium ratio 1 05 observed in the highland rivers which drain through sedimentary terrains the dissolved 226ra concentration ranges between 0 03 and 0 22 dpm 1 the striking feature of the radium isotopes data is the distinct difference in the 228ra and 226ra abundances between the highland and lowland rivers the lowland waters are enriched in 228ra while the highland waters contain more 226ra this difference mainly results from the differences in their weathering regimes the discharge weighted mean concentration of dissolved 238u in the ganga at patna and in the brahmaputra at goalpara are 1 81 and 0 63 1 respectively the ganga brahmaputra river system constitutes the major source of dissolved uranium to the bay of bengal these rivers transport annually about 1000 tons of uranium to their estuaries about 10 of the estimated global supply of dissolved uranium to the oceans via rivers the transport of uranium by these rivers far exceeds that of the amazon although their water discharge is only about 20 of that of the amazon the high intensity of weathering of uranium in the ganga brahmapura river system can also be deduced from the 232 th 238 u and 230 th 238 u activity ratios measured in the suspended sediments 230th is enriched by about 19 in the suspended sediments relative to its parent 238u the flux of excess 230th supplied to the bay of bengal via these river sediments is 980 1012 dpm a about six times more than its in situ production from seawater in the entire bay of bengal

transport of u and th series nuclides in a baltic shield watershed and the baltic sea the transport of th ra and 210 pb from a continental source region and through an estuarine environment was investigated unlike previous studies here both short and long lived nuclide data were obtained for river inputs river water and estuarine waters the mire rich kalix river drainage basin was chosen as a typical northern shield area because this river may represent typical waters flowing into the arctic and northern seas groundwaters from bedrock and glacial tills have comparable th isotope concentrations and do not exhibit significant th isotopic shifts relative to host rocks the extensive peat deposits of the basin receive groundwater discharges and concentrate th and u but not ba and ra which cause high 230 th 232 th ratios in mire waters however mire outflows do not have a significant impact on th and ra isotopic compositions of the river overall weathering characteristics for the basin are obtained from the river data the 230 th 232 th 228 ra 226 ra and 226 ra ba river ratios are comparable to those of source rocks consistent with similar release rates of these nuclides from u th and ba bearing minerals river ratios of 230 th 238 u ar and 226 ra 238 u ar are 1 so that th and possibly ra are accumulating in the weathering regions and the weathering profile is still evolving low 228 ra 232 th ar and 226 ra 230 th ar ratios indicate that th is preferentially retained over ra river 234 th 238 u ar ratios are greater than 230 th 238 u ar ratios and suggest that in systems where river inputs are well characterized these ratios can be used to calculate th transit times through the watershed filtration data indicate that although a dominant fraction of the th is transported in the river on particles the rest is almost entirely carried by colloids the kalix river discharges into the baltic sea model calculations for the transport of th and ra isotopes in the baltic sea show that the high ratios of 234 th 238 u ar found here reflect long th residence times relative to particle scavenging of 50 d the water column 232 th budget is dominated by eolian inputs the 226 ra concentrations may be higher than those of water inflows with 60 derived from underlying sediments a greater fraction of 228 ra is derived from sediments to balance the decay of 228 ra within the water column during the 35 yr residence time of water in the baltic the baltic 228 ra 226 ra ar ratios which are relatively constant over a range of salinities are fortuitously similar to those of the river inflows

radium isotopes in the chesapeake bay 228ra and 226ra are enriched in waters of chesapeake bay through fluxes from the bottom sediments and desorption from particles entering the estuary the activity ratio of 228ra 226ra increases from 1 5 in the susquehanna river to 3 4 in waters of 15 20 salinity apparently much of the 228ra is supplied by diffusion from sediments within the bay while desorption from particles entering the bay is more important for 226ra because of the multiple inputs and long residence time of water in the bay no curvature is obvious in a plot of radium isotope concentration vs salinity

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

scavenging of thorium isotopes in the canada basin of the arctic ocean abstract investigations on the distribution of thorium isotopes in the water column have been utilized as powerful tracers to investigate the geochemical pathways of particle reactive material in the ocean we measured th 228 th 230 th 232 th and 234 th isotopes in samples collected along a horizontal profile from barrow ak to the canada basin as well as two vertical profiles in the canada basin the suspended particle concentrations were also measured to investigate the role of suspended particulate matter spm in th scavenging the spm inventory is more than an order of magnitude higher than that reported for the alpha ridge ice station indicating that the spm inventories vary over an order of magnitude in the deep arctic basins the conditional partition coefficient of th isotopes between suspended particulate matter and dissolved phase generally appears to be higher than those in non polar regions and this is attributed to more reactive suspended particulate matter the residence times of total 230 th are significantly lower than those calculated for the alpha ridge station while higher than those found in the eurasian basin and beaufort sea in the canadian basin the calculated residence time in all the deep basins of the arctic ocean varied between 13 1 and 45 1 years a reversible scavenging model applied to the distribution of th isotopes 230 th 228 th in the deep waters and 234 th 228 th in the upper 100 m indicates that the forward removal rate constant is higher than the value reported for the alpha ridge while the reverse rate constant is comparable to the alpha ridge station suggesting faster removal onto particulate matter and slower desorption of th from the particulate matter

226ra in the black sea and sea of marmara water column distributions of226ra were determined at stations in the sea of marmara and the black sea as part of the 1988 joint u s turkish black sea expedition black sea surface water226ra concentrations were a factor of three to four lower than measurements made 20 years earlier the most likely cause is increased removal of226ra and ba 35 due to increased surface biological activity a secondary effect is decreased fluvial discharge and related dimunition of inputs by desorption from fluvial suspended sediments the amount of226ra missing from the surface waters of the black sea over this period is accounted for in the high porosity surficial fluff sediment layer throughout the black sea depth profiles of226ra exhibited pronounced maxima of approximately 25 dpm 100 l at about 16 2 16 3 in the vicinity of a bacterial maximum but slightly shallower than the total dissolved mn and fe maxima 16 4 16 5 reported by lewis and landing 38 while the226ra maximum may in part be linked to the cycling of mn and fe oxyhydroxides near theo2h2s interface its distribution appears to be more plausibly explained as a result of the microbial breakdown of particulate organic matter and the subsequent release and partial dissolution of associated barite in this region a simple steady state two model has been used to obtain a semiquantitative understanding of the behavior of226ra in the black sea by incorporating reasonable estimates for the input and removal of226ra in the black sea an excellent agreement between predicted and observed 1988 226ra concentrations was achieved the model suggests that the dominant variables controlling the distribution of226ra in the black sea are riverine input and cycling with ba

on the robustness of global radially anisotropic surface wave tomography a number of recent global tomographic studies have modeled three dimensional variations in the parameters of radial anisotropy as yet there is limited agreement among such studies suggesting significant uncertainties in the models which could lead to divergent geodynamical interpretations in this study we assess the robustness of lateral variations in radial anisotropy globally in the upper mantle and in the transition zone to determine the extent to which anisotropic parameters are constrained by a data set of over 10 000 000 fundamental and higher mode surface wave dispersion measurements we carry out inversions for isotropic and radially anisotropic shear wave velocity systematically changing regularization and using three different crustal models to remove the effects of the crust on the data using crustal corrections from different crustal models has an impact on the data fit comparable or larger than that obtained by including lateral variations of radial anisotropy in the modeling moreover the use of crustal corrections from different a priori crustal models may lead to different images of radial anisotropy suggesting divergent geodynamical interpretations this work suggests that the three dimensional determination of global radial anisotropy in the earth s mantle using surface wave dispersion data is still an ongoing experiment

anisotropic shear wave velocity structure of the earth s mantle a global model 1 we combine a new large data set of surface wave phase anomalies long period waveforms and body wave travel times to construct a three dimensional model of the anisotropic shear wave velocity in the earth s mantle our modeling approach is improved and more comprehensive compared to our earlier studies and involves the development and implementation of a new spherically symmetric reference model simultaneous inversion for velocity and anisotropy as well as discontinuity topographies and implementation of nonlinear crustal corrections for waveforms a comparison of our new three dimensional model s362ani with two other models derived from comparable data sets but using different techniques reveals persistent features 1 strong 200 km thick high velocity anomalies beneath cratons likely representing the continental lithosphere underlain by weaker fast anomalies extending below 250 km which may represent continental roots 2 weak velocity heterogeneity between 250 and 400 km depths 3 fast anomalies extending horizontally up to 2000 3000 km in the mantle transition zone beneath subduction zones 4 lack of strong long wavelength heterogeneity below 650 km suggesting inhibiting character of the upper mantle lower mantle boundary and 5 slow velocity superplumes beneath the pacific and africa the shear wave radial anisotropy is strongest at 120 km depth in particular beneath the central pacific lateral anisotropic variations appreciably improve the fit to data that are predominantly sensitive to the uppermost and lowermost mantle but not to the waveforms that control the transition zone and midmantle depths tradeoffs between lateral variations in velocity and anisotropy are negligible in the uppermost mantle but noticeable at the bottom of the mantle

source and distribution of dissolved radium in the bega river estuary southeastern australia abstract measurements of the activities of the four naturally occurring radium isotopes in the surface water and porewater of an estuary have yielded information on the release of radium from sediments and on the extent of surface water porewater interaction in the estuary under low flow conditions the non conservative behaviour of dissolved radium in the estuary is almost entirely due to the flux of radium from estuarine bed sediments radium accumulates in bottom sediment porewater and is then mixed with estuarine surface water probably as a result of tidal action it is shown experimentally that the enrichment of the short lived isotopes 224ra and 223ra relative to 226ra in estuarine porewater can be explained by the repeated leaching of radium from bottom sediments by saline water and the rapid regeneration of the short lived isotope activity from their sediment bound parent nuclides the leaching of radium from bottom sediments is apparently occurring on a time scale which is long weeks months compared with the 224ra and 223ra half lives indicating that the amount of ion exchangeable radium adsorbed to the sediments is large compared with the amount dissolved in porewater by applying a simple 2 d steady state multi box model 224ra and 223ra surface water and porewater concentrations have been used to estimate the daily flux of porewater crossing the sediment water interface in the bega estuary this flux is found to be about 15 of the estuary volume

global azimuthal anisotropy in the transition zone surface wave dispersion measurements for love wave overtones carry evidence of azimuthal anisotropy in the transition zone of earth s mantle 400 to 660 kilometers deep a backus gilbert inversion of anisotropic phase velocity maps with resolution kernels mainly sensitive to the transition zone shows a robust long wavelength azimuthally anisotropic velocity structure this observation puts new constraints on the mineralogy and dynamics of the transition zone because this anisotropy may result from aligned minerals tilted laminated structures or even organized pockets of fluid inclusions

non steady state 210pb flux and the use of 228ra 226ra as a geochronometer on the amazon continental shelf the recovery of quasi cyclic 210pb profiles from the foreset region of the amazon subaqueous delta has raised questions concerning the non steady state flux of 210pb to this region of the shelf radioisotopic profiles 210pb 234th 228ra 226ra 137cs from seven kasten cores along with the analysis of grain size organic carbon and clay sized mineralogy are used to investigate the time scale and the controlling mechanisms of the 210pb cyclicity a common assumption used in 210pb geochronology is that the initial specific activity is constant with time an assumption that is invalid in this region variations of particle affinity for 210pb have been shown to affect the initial specific activity reaching the seabed in various other studies however investigations of grain size organic carbon and mineralogy have revealed no relationship with the fluctuating 210pb activity in this area a new geochronometer 228ra 226ra is used to determine sediment accumulation rates over a 30 year time scale which range from 10 to 60 cm yr in the study area spectral analysis of the 210pb cyclicity has revealed that the major component of 210pb cyclicity has a frequency of approximately 2 3 years in addition components of the 210pb cyclicity occur on annual and semi annual frequencies the controlling mechanism for the 210pb cyclicity on these time scales appears to be the seaward transport and accumulation of low activity fluid muds from the topset region

a three dimensional radially anisotropic model of shear velocity in the whole mantle summary we present a 3 d radially anisotropic s velocity model of the whole mantle saw642an obtained using a large three component surface and body waveform data set and an iterative inversion for structure and source parameters based on non linear asymptotic coupling theory nact the model is parametrized in level 4 spherical splines which have a spacing of 8 the model shows a link between mantle flow and anisotropy in a variety of depth ranges in the uppermost mantle we confirm observations of regions with vsh vsv starting at 80 km under oceanic regions and 200 km under stable continental lithosphere suggesting horizontal flow beneath the lithosphere we also observe a vsv vsh signature at 150 300 km depth beneath major ridge systems with amplitude correlated with spreading rate for fast spreading segments in the transition zone 400 700 km depth regions of subducted slab material are associated with vsv vsh while the ridge signal decreases whilethemid mantlehasloweramplitudeanisotropy vsv in the lowermost 300 km which appears to be a robust conclusion despite an error in our previous paper which has been corrected here the 3 d deviations from this signature are associated with the large scale low velocity superplumes underthecentralpacificandafrica suggestingthatvsh vsv isgeneratedinthepredominant horizontal flow of a mechanical boundary layer with a change in signature related to transition to upwelling at the superplumes

226ra and 228ra in the mixing zones of the pee dee river winyah bay yangtze river and delaware bay estuaries 226ra and 228ra have non conservative excess concentrations in the mixing zones of the pee dee river winyah bay estuary the yangtze river estuary and the delaware bay estuary laboratory experiments using pee dee river sediment indicate desorption of 226ra to increase with increasing salinities up to 20 in winyah bay desorption from river borne sediments could contribute almost all of the increases for both isotopes desorption adds only a portion of the excess 228ra measured in the yangtse river and adjacent shelf waters and delaware bay in the yangtze river the mixing zone extends over a considerable portion of the continental shelf where 228ra is added to the water column by diffusion from bottom sediments while 226ra concentrations decrease from dilution diffusion of 228ra from bottom sediments in delaware bay primarily occurs in the upper part of the bay 22 water where fine grained sediments predominate a diffusive flux for 228ra of 0 33 dpm cm 2 year was determined for delaware bay

rheology of the upper mantle a synthesis rheological properties of the upper mantle of the earth play an important role in the dynamics of the lithosphere and asthenosphere however such fundamental issues as the dominant mechanisms of flow have not been well resolved a synthesis of laboratory studies and geophysical and geological observations shows that transitions between diffusion and dislocation creep likely occur in the earth s upper mantle the hot and shallow upper mantle flows by dislocation creep whereas cold and shallow or deep upper mantle may flow by diffusion creep when the stress increases grain size is reduced and the upper mantle near the transition between these two regimes is weakened consequently deformation is localized and the upper mantle is decoupled mechanically near these depths

residence times of surface water and particle reactive 210pb and 210po in the east china and yellow seas 228ra 226ra 210pb and 210po were measured in the surface waters of the east china and yellow seas using mass balance equations for the ra isotopes we estimated the total flux of diffusion from sediments and desorption from suspended particles to be 0 1 dpm 226ra cm 2 a 1 and 1 dpm 228ra cm 2 a 1 and residence times to be 2 3 years for the waters on the east china sea shelf and 5 6 years for yellow sea waters box model calculations yielded generally congruent scavenging residence times for 210pb and 210po in the waters of 2 months on the shelf and 7 months in the kuroshio current these suggest that reactive heavy metals and pollutants discharged through rivers from the continent to the east asian continental shelf are largely deposited on the bottom sediments prior to transport to the pelagic ocean by lateral mixing

u th ra fractionation during weathering and river transport the potential of radioactive disequilibria as tracers and chronometers of weathering processes has been recognised since the 1960 s e g rosholt et al 1966 hansen and stout 1968 this interest results from the dual property of the nuclides of the u and th radioactive series 1 to be fractionated during water rock interactions and 2 to have radioactive periods of the same order of magnitude as the time constants of many weathering processes and chemical transfers to ground and river waters therefore the study of radioactive disequilibria in surface environments should help to bring information about the nature the intensity but also the time scale of the water rock interactions produced during weathering and related chemical transfers these different properties have justified many of the studies on u th series in weathering profiles and river waters rosholt 1982 scott 1982 and osmond and ivanovich 1992 gave a synthesis of the studies of u series in weathering and surface hydrology up to the 1980 s and the 1990 s respectively here we present the main directions taken in these domains over the last decade they were partly stimulated by the analytical developments made in the measurement of the medium half life nuclides of 238u series i e 234u 230th 226ra in the mid 1980 s namely the use of thermal ionisation mass spectrometry tims e g chen et al 1986 edwards et al 1987 cohen et al 1991 chabaux 1993 chabaux et al 1994 and more recently the use of mc icp ms turner et al 2001 robinson et al 2002 for u and th isotope analysis details of these developments are given in this volume goldstein and stirling 2003 compared to the radioactive counting methods previously used the new techniques permit 1 a reduction by one order of magnitude or more of the size of the sample required for

radium isotopes in the orinoco estuary and eastern caribbean sea radium isotopes provide a means of identifying the source of freshened waters in the ocean and determining the time elapsed since these waters were in the estuary we present evidence that during april waters from the amazon mixing zone pass within 50 km of the mouth of the orinoco river these amazon waters are characterized by a lower 228ra 226ra activity ratio ar than are waters from the orinoco at similar salinities during autumn the increased discharge of the orinoco displaces the freshened amazon waters seaward yet the two can be distinguished clearly within the caribbean sea waters of orinoco origin carry a characteristic radium signature including excess activities of 224ra this isotope may be used to estimate the time elapsed since the waters were removed from contact with sediments current speeds based on 224ra dating ranged from 15 to 33 cm s during april the radium isotopes also provide an assessment of sediment mixing in the estuary during low discharge april considerable mixing of older sediment by physical or biological processes or dredging maintained high activities of 228ra in the estuary and produced the highest 228ra 226ra ar s yet measured in any estuary during high discharge september a large fraction of the 228ra was derived from desorption from fresh sediment rather than mixing of older sediments activities of 224ra were high in the estuary during both high and low discharge indicating that considerable mixing of recently introduced sediment must occur during each period the high 224ra 228ra ar s measured in september imply that the same sediments were being resuspended during high discharge and that there was little exposure of older sediments which had been stored in the estuary for months to years during april 224ra and 228ra activities in the water were about equal indicating that most of the sediment being resuspended had been stored in the estuary long enough to reestablish radioactive equilibrium in the 232th decay series i e 20 years

tracing the flow rate and mixing ratio of the changjiang diluted water in the northwestern pacific marginal seas using radium isotopes we measured ra isotopes 223ra and 228ra in surface seawater of the northwestern pacific marginal seas to trace the flow rate and mixing of the changjiang diluted water cdw in the summer of 2012 based on the horizontal distribution of 223ra activities the arrival time of cdw from the river mouth to 450 km offshore northeast was estimated to be 20 35 days which is similar to that determined in previous studies moreover we successfully calculated the relative contribution of cdw at each sampling station using a salinity and 228ra diagram using this unique method we found that the relative contribution of cdw was more than 30 in most surface seawaters of the northern east china sea the yellow sea and the southern sea off korea our results suggest that cdw is of high significance in the biogeochemistry of surface seawater of these northwestern pacific marginal seas during the summer monsoon period

radium isotopes and their environmental implications in the changjiang river system abstract radium ra isotopes are widely used to trace water motion and nutrient transport in coastal oceans to enhance our understanding of ra behavior from river to ocean this study examines 226 ra and 228 ra activity concentrations in the water column suspended particulate matter and river bed surface sediments along the middle and lower changjiang river cjr basin based on two cruises in 2006 and 2008 the results show that the ra activity concentrations in the cjr basin are comparable with other rivers in the world higher ra activity concentrations and lower 228 ra 226 ra activity ratios are detected in the tributaries i e in the dongting and poyang lakes than in the mainstream the comparison of pre three gorges dam tgd data taken in 1984 and post tgd data this work illuminates the effect that a major dam has on radium transport within the river and into the estuary we estimate a riverine 226 ra flux of 1 04 0 08 tbq yr 1 extended ra datasets collected in the estuary and the east china sea show that the dissolved and sedimentary ra activity concentrations are higher in the estuary but lower in the sea whereas the particulate ra activity concentrations are larger in the river this finding indicates significant ra desorption from particulate to dissolved phase in the brackish estuary where riverborne materials are filtered and trapped overall these results help to elucidate ra transport from river to ocean and assist further application of ra isotopes in land ocean interaction studies such as submarine groundwater discharge mixing and diffusion processes and determination of water mass and or current structures in the estuarine and coastal zones

natural radionuclides in waters of the new york bight the half removal time of228th from the surface waters by settling particles tc does not change much with season except in the winter when regenerated228th as well as210pb and210po were transported back to the surface water from the bottom water and or near shore sediments the removal of228th and210pb from the surface waters of new york bight by phytoplankton zooplankton fecal pellet route is not important in the shelf but is important in the slope areas the removal of210po is almost entirely associated with the phytoplankton zooplankton fecal pellet pathway throughout the new york bight

probability of radial anisotropy in the deep mantle article i nfo it is well established that the earth s uppermost mantle is anisotropic but observations of anisotropy in the deeper mantle have been more ambiguous radial anisotropy the discrepancy between love and rayleigh waves was included in the top 220 km of prem but there is no consensus whether anisotropy is present below that depth fundamental mode surface waves for commonly used periods up to 200 s are sensitive to structure in the first few hundred kilometers and therefore do not provide information on anisotropy below higher mode surface waves however have sensitivities that extend to and below the transition zone and should thus give insight about anisotropy at greater depths but they are very difficult to measure we previously developed a new technique to measure higher mode surface wave phase velocities with consistent uncertainties these data are used here to construct probability density functions of a radially anisotropic earth model down to approximately 1500 km in the uppermost mantle we obtain a high probability of faster horizontally polarized shear wave speed likely to be related to plate motion in the asthenosphere and transition zone however we find a high probability of faster vertically polarized shear wave speed to a depth of 1500 km in the lower mantle we see no significant shear wave anisotropy this is consistent with results from laboratory measurements which show that lower mantle minerals are anisotropic but lpo is unlikely to develop in the pressure temperature conditions present in the mid mantle 2008 elsevier b v all rights reserved

non linear crustal corrections in high resolution regional waveform seismic tomography summary we compare 3 d upper mantle anisotropic structures beneath the north american continent obtained using standard and improved crustal corrections in the framework of non linear asymptotic coupling theory nact applied to long period three component fundamental and higher mode surface waveform data our improved approach to correct for crustal structure in high resolution regional waveform tomographic models goes beyond the linear perturbation approximation and is therefore more accurate in accounting for large variations in moho topography within short distances as observed for instance at ocean continent margins this improved methodology decomposes the shallow layer correction into a linear and non linear part and makes use of 1 d sensitivity kernels defined according to local tectonic structure both for the forward computation and for the computation of sensitivity kernels for inversion the comparison of the 3 d upper mantle anisotropic structures derived using the standard and improved crustal correction approaches shows that the model norm is not strongly affected however significant variations are observed in the retrieved 3 d perturbations the largest differences in the velocity models are present below 250 km depth and not in the uppermost mantle as would be expected we suggest that inaccurate crustal corrections preferentially map into the least constrained part of the model and therefore accurate corrections for shallow layer structure are essential to improve our knowledge of parts of the upper mantle where our data have the smallest sensitivity

modelling the effect of salinity on radium desorption from sediments abstract the desorption of the four naturally occurring radium isotopes 223 ra 224 ra 226 ra and 228 ra from estuarine sediments is investigated these isotopes are created within sediments by the radioactive decay of insoluble thorium parents due to competition from other ions for the occupation of adsorption sites on the sediment grains radium desorption is a function of salinity a model is developed to describe radium desorption as it is affected by salinity grain size and sediment concentration it is shown that to model the desorption of radium for a particular sediment requires the estimation of two independent parameters one of these parameters is the concentration of radium available for exchange on the sediment the other depends on major ion and radium exchange coefficients desorption experiments performed on sediments from the bed of the bega river australia are used to validate the model and to evaluate the parameters needed for model application

pressure sensitivity of olivine slip systems and seismic anisotropy of earth s upper mantle the mineral olivine dominates the composition of the earth s upper mantle and hence controls its mechanical behaviour and seismic anisotropy experiments at high temperature and moderate pressure and extensive data on naturally deformed mantle rocks have led to the conclusion that olivine at upper mantle conditions deforms essentially by dislocation creep with dominant 100 slip the resulting crystal preferred orientation has been used extensively to explain the strong seismic anisotropy observed down to 250 km depth the rapid decrease of anisotropy below this depth has been interpreted as marking the transition from dislocation to diffusion creep in the upper mantle but new high pressure experiments suggest that dislocation creep also dominates in the lower part of the upper mantle but with a different slip direction here we show that this high pressure dislocation creep produces crystal preferred orientations resulting in extremely low seismic anisotropy consistent with seismological observations below 250 km depth these results raise new questions about the mechanical state of the lower part of the upper mantle and its coupling with layers both above and below

seismic anisotropy in the mantle transition zone induced by shear deformation of wadsleyite abstract mantle flow in the earth s mantle transition zone between 410 and 660 km depth plays a key role to understand the nature of mantle convection which can be mapped by observed seismic anisotropy combined with crystallographic preferred orientations of mantle minerals although wadsleyite is the most important mineral to cause seismic anisotropy observed in the mantle transition zone there have been limited experimental data on its crystallographic preferred orientation because of experimental limitations we experimentally evaluated the preferred orientation of wadsleyite developed by shear deformation at pressure temperature conditions of the mantle transition zone 17 6 gpa and 1800 1900 k using a deformation dia apparatus the deformation experiments reveal that the 0 0 1 axis and the 0 1 0 plane of wadsleyite tend to be subparallel to the shear direction and the shear plane during deformation respectively these results demonstrate that polarization seismic anisotropy velocity contrast between horizontally polarized and vertically polarized s waves vsh vsv observed in the mantle transition zone might be attributed to the preferred orientation of wadsleyite caused by horizontal mantle flow

seismic anisotropy changes across upper mantle phase transitions the mantle transition zone is believed to play an important role in the thermochemical evolution of our planet and in its deep water cycle constraining mantle flow at these depths can help elucidate its nature and better understand mantle dynamics and the history of plate tectonics seismic anisotropy i e the directional dependence of seismic wave velocity provides us with the most direct constraints on mantle deformation its detection below 250 km depth is challenging and it is often assumed that the deep upper mantle is seismically isotropic due to a change in mantle deformation mechanism here we present a global model of azimuthal anisotropy for the top 1000 km of the mantle we used a dataset composed of fundamental and higher mode rayleigh wave phase velocity maps which provides resolution of azimuthal anisotropy to much greater depths than in previous studies our model unravels the presence of significant anisotropy in the transition zone challenging common views of mantle deformation mechanisms and reveals a striking correlation between changes in anisotropy amplitudes and in the fast direction of wave propagation where upper mantle phase transitions occur the newly found relation between anisotropy changes and phase transformations gives new insight on the nature of the mtz and suggests that the anisotropy originates from the lattice preferred orientation of anisotropic material while the interpretation of our results in terms of mantle deformation is not straightforward due to many possible scenarios possible explanations for our findings include recrystallization during phase transformations or changes in the slip system across the mtz boundaries which in turn could be explained by changes in the water content of mantle material consistent with the idea that the transition zone acts as a water filter 2013

radial anisotropy and prior petrological constraints a comparative study click here journal of geophysical research vol 115 b03303 doi 10 1029 2008jb005842 2010 for full article radial anisotropy and prior petrological constraints a comparative study c beghein 1 received 31 may 2008 revised 8 september 2009 accepted 6 october 2009 published 6 march 2010 1 radial seismic anisotropy models are traditionally obtained using empirical constraints based on laboratory experiments and petrological considerations we tested the hypothesis that such petrological constraints affect the uppermost mantle models of s wave anisotropy using a statistical approach in addition we were able to determine which model features are constrained by the data and which are dominated by the prior we focused on large scale models and found that the most likely models obtained in both cases are highly correlated this demonstrates that for the best data fitting solution the geometry of uppermost mantle radial anisotropy is not strongly affected by prior petrological constraints the amplitude of the anomalies however can change significantly the best data fitting model obtained without petrological constraints displays stronger amplitudes than the one obtained with prior this could become an issue when quantitatively interpreting seismic anisotropy models and thus emphasizes the importance of accurately accounting for parameter uncertainties and trade offs and of understanding whether the seismic data or the prior constraints the model we showed that model uncertainties are strongly affected by the prior as the relative rms uncertainties were reduced by a factor of 2 in addition we showed that while the model distributions are not necessarily gaussian a priori imposing petrological constraints can force the distributions to be narrower and more gaussian like as expected from inverse theory finally we demonstrated that the age dependence of seismic wave velocities is robust and independent of prior constraints a similar age signal exists for anisotropy but with larger uncertainties without prior constraints citation beghein c 2010 radial anisotropy and prior petrological constraints a comparative study j geophys res 115 b03303 doi 10 1029 2008jb005842 1 introduction 2 accurately modeling mantle seismic anisotropy that is the dependence of seismic wave velocity with the direction of propagation or polarization can help us under stand mantle deformation karato and toriumi 1989 kendall 2000 becker et al 2003 the coupling between lithosphere and asthenosphere silver and holt 2002 becker et al 2006b mantle composition montagner and anderson 1989 rheology becker et al 2008 and the net rotation of the lithosphere becker 2008 however despite numerous efforts to model mantle seismic anisotropy over the past 20 years uncertainties remain on its exact depth extent and lateral variations in the uppermost mantle on its presence in the transition zone and on its global nature in the d 00 layer fouch and fischer 1996 montagner and kennett 1996 ekstro m and dziewonski 1998 lay et al 1998 trampert and van heijst 2002 wookey et al 2002 gung et al 2003 panning and romanowicz 2006 earth and space sciences department ucla los angeles california usa copyright 2010 by the american geophysical union 0148 0227 10 2008jb005842 09 00 beghein and trampert 2004a 2004b beghein et al 2006 panning and romanowicz 2006 zhou et al 2006 marone et al 2007 nettles and dziewonski 2008 beghein et al 2008 3 discrepancies between models can arise for a variety of reasons to fully describe earth s elastic properties one would ideally want to determine the 21 independent ele ments of the fourth order elastic stiffness tensor at a given time and location inside earth in practice this is challeng ing because seismic data are only sensitive to subsets of those 21 elements tanimoto 1986 chen and tromp 2007 beghein et al 2008 and different types of data depend on different subsets of elastic coefficients see summary tables in chen and tromp 2007 and beghein et al 2008 in addition while some data such as shear wave splitting measurements can provide precise constraints on lateral changes in seismic anisotropy their depth resolution is very poor surface wave and free oscillation data are better suited to constrain depth changes in structure but their lateral resolution is lower than that of body waves this can yield apparent discrepancies and make model comparisons diffi cult moreover three dimensional models of seismic anisot ropy are typically obtained by data inversion which is often an ill posed and ill conditioned problem this means that b03303 1 of 17

global azimuthal seismic anisotropy and the unique plate motion deformation of australia differences in the thickness of the high velocity lid underlying continents as imaged by seismic tomography have fuelled a long debate on the origin of the roots of continents1 2 3 4 5 some of these differences may be reconciled by observations of radial anisotropy between 250 and 300 km depth with horizontally polarized shear waves travelling faster than vertically polarized ones2 this azimuthally averaged anisotropy could arise from present day deformation at the base of the plate as has been found for shallower depths beneath ocean basins6 such deformation would also produce significant azimuthal variation owing to the preferred alignment of highly anisotropic minerals7 here we report global observations of surface wave azimuthal anisotropy which indicate that only the continental portion of the australian plate displays significant azimuthal anisotropy and strong correlation with present day plate motion in the depth range 175 300 km beneath other continents azimuthal anisotropy is only weakly correlated with plate motion and its depth location is similar to that found beneath oceans we infer that the fast moving australian plate contains the only continental region with a sufficiently large deformation at its base to be transformed into azimuthal anisotropy simple shear leading to anisotropy with a plunging axis of symmetry may explain the smaller azimuthal anisotropy beneath other continents

the relationship between upper mantle anisotropic structures beneath california transpression and absolute plate motions we calculated sks splitting parameters for the california integrated seismic network in southern california we also estimated splitting in the upper 100 km using azimuthal anisotropy determined from surface waves the inferred splitting from surface waves in the mantle lithosphere is small on average 0 2 s compared with sks splitting 1 5 s and obtains a maximum value 0 5 s in the transpressive region of the big bend south of and aligned with the san andreas fault saf in contrast the sks splitting is approximately e w and is relatively uniform spatially either side of the big bend of the saf these differences suggest that most of the sks splitting is generated much deeper down to 300 400 km than previously thought probably in the asthenosphere fast directions align with absolute plate motions apm in northern and southeastern california but not in southwestern california we interpret the parallelism with apm as indicating the sks anisotropy is caused by cumulative drag of the asthenosphere by the overlying plates the discrepancy in southwestern california arises from the diffuse boundary there compared to the north where relative plate motion has concentrated near the saf system in southern california the relative motion originated offshore in the borderlands and gradually transitioned onshore to the saf system this has given rise to smaller displacement across the saf 160 180 km compared with central and northern california 400 500 km thus in southwestern california the inherited anisotropy from prior north american apm has not yet been overprinted by pacific apm

radium 228 in the south atlantic bight radium 228 in surface ocean waters is derived from sedimentary fluxes generated in estuaries salt marshes and fine grained sediments of the inner shelf thus the sup 228 ra activity of shelf waters is a measure of their interaction with these sediments mixing of the sup 228 ra signal offshore follows known circulation patterns and examination of the sup 228 ra field as determined by the sup 228 ra sup 226 ra activity ratio ar reveals details of these patterns during february sinking of chilled shelf waters carries high ar water to depths of 80 130 m just below the shelf break in april upwelling from 500 700 m depth brings low ar water to the shelf break where it replaces the high ar waters present 2 months earlier during august low ar waters at the shelf break could be traced to the surface where they separated higher ar waters both inshore and offshore of the shelf break because of such short term variability one dimensional steady state models which balance sup 228 ra decay with mixing do not provide a true measure of horizontal mixing rates fluxes necessary to support the sup 228 ra standing crop in the south atlantic bight are of the order of 2 5 x 10 sup 14 dpm yr thesemore are similar to estimates from the new york bight but 25 times lower than the apparent flux from the amazon system copyrightamerican geophysical union 1987 less

behavior of 226ra in the mississippi river mixing zone the behavior of 226ra in the mississippi river mixing zone is strongly nonconservative and includes desorption similar to that reported for the hudson pee dee and amazon rivers however dissolved and desorbed 226ra concentrations in the mississippi are 2 to 5 times greater than in the other rivers at the same salinity radium concentrations vary inversely with the water discharge rate the 226ra desorption maximum occurs at a salinity of 5 0 much lower than the 18 to 28 salinity values for the maxima of the other three rivers high concentrations of dissolved 226ra up to 82 dpm per 100 l and the low salinity values for the desorption maximum in the mississippi river result from three major factors suspended sediments include a large fraction of montmorillonite which gives the sediment a high cation exchange capacity 0 54 meq g the average suspended sediment load is large about 510 mg l and contains 1 9 dpm g desorbable 226ra the dissolved 226ra river water end member 9 6 dpm per 100 l is higher than in surface seawater the annual contribution of 226ra to the ocean from the mississippi river is 3 7 1014 dpm yr based on data from three cruises evidence of flux of 226ra from estuarine and shelf sediments is common in vertical profile sampling of the deltaic waters but is not reflected in calculations made with an apparent river water ra value extrapolated to zero salinity

strain induced seismic anisotropy of wadsleyite polycrystals and flow patterns in the mantle transition zone 1 we use forward models based on recent high pressure experimental data on mantle minerals to predict the seismic anisotropy produced by plastic strain of orthorhombic wadsleyite the dominant mineral in the upper transition zone these models predict a weak seismic anisotropy for a polycrystal of pyrolitic composition 60 wadsleyite 40 garnet at transition zone conditions 2 for p and 1 for s waves for a shear strain of 1 both p and s wave anisotropy patterns show an orthorhombic symmetry p waves propagate faster at low angle to the shear direction and slower at high angle to the shear plane s wave anisotropy is characterized by faster propagation of waves polarized at low angle to the shear direction horizontal shearing results therefore in higher velocities for horizontally propagating p waves ph and horizontally polarized s waves sh as well as in weak azimuthal variation of sv and sh velocities on the other hand vertical flow leads to higher velocities for vertically propagating p waves pv and vertically polarized s waves sv and to a weak azimuthal variation of sv velocity but to a roughly constant sh velocity analysis of global observations of seismic anisotropy in the transition zone in the light of these models supports dominant horizontal flow in the uppermost transition zone in agreement with predictions of geodynamical models that explicitly introduce phase transitions

probability density functions for radial anisotropy implications for the upper 1200 km of the mantle the presence of radial anisotropy in the upper mantle transition zone and top of the lower mantle is investigated by applying a model space search technique to rayleigh and love wave phase velocity models probability density functions are obtained independently for s wave anisotropy p wave anisotropy intermediate parameter r vp vs and density anomalies the likelihoods for p wave and s wave anisotropy beneath continents cannot be explained by a dry olivine rich upper mantle at depths larger than 220 km indeed while shear wave anisotropy tends to disappear below 220 km depth in continental areas p wave anisotropy is still present but its sign changes compared to the uppermost mantle this could be due to an increase with depth of the amount of pyroxene relative to olivine in these regions although the presence of water partial melt or a change in the deformation mechanism cannot be ruled out as yet a similar observation is made for old oceans but not for young ones where vsh s vsv appears likely down to 670 km depth and vph s vpv down to 400 km depth the change of sign in p wave anisotropy seems to be qualitatively correlated with the presence of the lehmann discontinuity generally observed beneath continents and some oceans but not beneath ridges parameter r shows a similar age related depth pattern as shear wave anisotropy in the uppermost mantle and it undergoes the same change of sign as p wave anisotropy at 220 km depth the ratio between dlnvs and dlnvp suggests that a chemical component is needed to explain the anomalies in most places at depths greater than 220 km more tests are needed to infer the robustness of the results for density but they do not affect the results for anisotropy

on the lehmann discontinuity the lehmann discontinuity 200km in the upper mantle is proposed to result from a change in the preferred orientation of olivine as a result of a change in deformation mechanism in the shallow upper mantle deformation is due to dislocation creep and results in an anisotropic structure whereas in the deeper upper mantle diffusion creep becomes dominant due to a smaller activation volume resulting in an isotropic structure for anisotropy due to dislocation glide corresponding to a horizontal flow the resultant anisotropy will yield vsh vsv and vph vpv thus for waves propagating nearly vertically e g scs waves there will be discontinuous jumps in the s and p wave velocities at this discontinuity in this model the depth of the lehmann discontinuity depends on the deformation conditions such as stress level geotherm water fugacity and grain size giving a natural explanation for its observed variation with tectonic setting

determining groundwater ra end member values for the estimation of the magnitude of submarine groundwater discharge using ra isotope tracers radium isotopes 228ra and 226ra are excellent tracers of submarine groundwater discharge sgd to estimate sgd magnitudes information on the end member values of ra concentrations in groundwater is critical however the distribution characteristics of ra in coastal aquifers are poorly understood in this study we show that ra concentrations in coastal groundwater are primarily dependent on salinity based on the data n 500 obtained from global coastal aquifers although previous end member calculations averaged all ra concentrations without considering salinity if we assume that sgd is composed mainly of seawater infiltrating the aquifer previous estimates of sgd for the atlantic ocean and the global ocean were overestimated twofold to threefold this may be similar for other applications using different ra isotopes our study highlights that the end members of ra isotopes in groundwater should be carefully considered when estimating sgd using ra isotope mass balances in the ocean

elasticity and strength of hydrous ringwoodite at high pressure oh bearing hydrous ringwoodite compressed non hydrostatically in a diamond anvil cell supports a differential stress that increases from 2 9 to 4 5 gpa over the pressure range of 6 7 13 2 gpa at room temperature this result suggests a significant water weakening effect when compared with results from similar experiments on the anhydrous counterpart kaver and duffy geophys res lett 28 2001 2691 2694 the elastic anisotropy 2c44 c11 c12 of hydrous ringwoodite is measured to be 0 87 7 throughout this pressure range similar to measured values for anhydrous ringwoodite kaver and duffy geophys res lett 28 2001 2691 2694 this lattice anisotropy cannot be explained by anelastic effects such as faulting and twinning within the structure these results suggest that hydrous minerals in the upper mantle and transition zone may have higher ductile strain rates for a fixed shear stress at high temperature resulting in stronger preferred lattice orientation this in turn may be seismically detectable which opens the possibility of using seismic anisotropy as a marker for local volatile containing areas within the upper mantle and transition zone

assessment of tomographic mantle models using spectral element seismograms summary we investigated the agreement between real seismograms and those predicted by long wavelength mantle models by looking at phase and amplitude differences we computed full synthetic seismograms using a spectral element method together with 3 d mantle models and the appropriate crustal model on top we selected differently damped mantle models to see the effect of regularization on the computed seismograms to check the phase agreement we measured time shifts between the real and synthetic surface waves and body wave phases such as p s and ss using a cross correlation technique we also compared the amplitudes of the real and synthetic seismograms to understand how well the models explain not only the phases but the whole waveforms 3 d mantle models improve the phase agreement of surface waves in particular the remaining misfit however is still so large that we cannot distinguish between different tomographic models we suggest that this is mainly due to an imperfect modelling of the crust and or source location if body waves are included which have to be addressed in future inversions amplitude mismatches are large regardless which 3 d mantle model is used we observe that 3 d scattering or focusing defocusing effects can only explain half of the surface wave amplitudes whereas body wave amplitudes are dominated by scattering effects 1 d q models particularly in the crust and upper mantle strongly affect surface wave amplitudes and have to be modelled properly

mantle anisotropy beneath northwest pacific subduction zones to assess the location strength and orientation of seismic anisotropy in the southern kuril japan and izu bonin subduction zones we analyzed shear wave splitting in local s phases and teleseismic sks phases fast directions from these phases are roughly parallel to the direction of absolute pacific plate motion wnw in izu bonin roughly parallel to the strike of the trench nne in central honshu and roughly n in the southern kuril back arc in the vicinity of sakhalin island assuming that the orientation and strength of anisotropy does not vary with depth modeling of splitting times from local s and teleseismic phases recorded at sakhalin island requires that the maximum depth of anisotropy lies between 480 km and 950 km in contrast splitting times for local s phases from events in izu bonin rule out anisotropy in the transition zone all the data are consistent with a model in which the lower transition zone 520 660 km and lower mantle are largely isotropic and in which anisotropy occurs intermittently in the upper transition zone 410 520 km possibly due to preferred orientation of spinel assuming that splitting in the upper mantle is produced by preferred orientation of olivine the observed fast directions indicate that the geometry of back arc strain varies systematically between subduction zones the relationship of fast directions and plate motions suggests that the subducting slab exerts significant control on back arc flow but that flow correlated with upper plate deformation must also exist

single crystal elasticity of β mg2sio4 to the pressure of the 410 km seismic discontinuity in the earth s mantle the complete set of elastic stiffness coefficients of a single crystal of mg2sio4 in the modified spinel wadsleyite structure are reported at eight pressures from ambient to 14 gigapascals gpa all moduli vary linearly with pressure within the resolution of the data third order birch murnaghan equation fits to aggregate elastic moduli yield values of 170 2 gpa and 4 3 0 2 for the adiabatic bulk modulus and its first pressure derivative and 115 2 gpa and 1 4 0 2 for the shear modulus and its pressure derivative these data together with previous results for mg2sio4 enable us to make the first in situ determination of the velocity increase due to the transition at the pressure 13 8 gpa of the 410 km discontinuity the room temperature velocity increase at the transition pressure is 9 8 for compressional waves and 12 4 for shear waves

on the relationship between azimuthal anisotropy from shear wave splitting and surface wave tomography 1 seismic anisotropy provides essential constraints on mantle dynamics and continental evolution one particular question concerns the depth distribution and coherence of azimuthal anisotropy which is key for understanding force transmission between the lithosphere and asthenosphere here we reevaluate the degree of coherence between the predicted shear wave splitting derived from tomographic models of azimuthal anisotropy and that from actual observations of splitting significant differences between the two types of models have been reported and such discrepancies may be due to differences in averaging properties or due to approximations used in previous comparisons we find that elaborate full waveform methods to estimate splitting from tomography yield generally similar results to the more common simplified approaches this validates previous comparisons and structural inversions however full waveform methods may be required for regional studies and they allow exploiting the back azimuthal variations in splitting that are expected for depth variable anisotropy applying our analysis to a global set ofskssplitting measurements and two recent surface wave models of upper mantle azimuthal anisotropy we show that the measures of anisotropy inferred from the two types of data are in substantial agreement provided that the splitting data is spatially averaged so as to bring it to the scale of long wavelength tomographic models and reduce spatial aliasing observed and tomography predicted delay times are significantly correlated and global angular misfits between predicted and actual splits are relatively low regional anisotropy complexity notwithstanding our findings imply that splitting and tomography yield a consistent signal that can be used for geodynamic interpretation

can the greenland climatic jumps be identified in records from ocean and land sharp jumps in climate punctuate the records from borings in the greenland ice cap during the time interval 60 000 to about 20 000 yr ago rapid fluctuations are also seen in foraminifera records for cores from the northern atlantic and in a pollen record from a core from a bog in the vosges mountains in france in this paper we present a new radiocarbon chronology for northern atlantic deep sea core v23 81 which permits comparison with the radiocarbon dated vosges mountains pollen record because of the lack of a 14c chronology for the greenland ice record and of distortions peculiar to each of the three records it is not yet possible to say whether or not the events are genetically related

mean residence time of the shelf water in the east china and the yellow seas determined by 228ra 226ra measurements increasing attention of oceanographers has recently been paied on east asian marginal seas regarding their role on the global environment yet geochemical investigations have been few to date we here report new data on the distribution of 228ra and 226ra in the surface water of the east china and the yellow seas in an effort to constrain the time necessary for the coastal and shelf waters to exchange with offshore waters such information is needed in evaluating the exchange of heat and water across the air sea interface that affects the local climate and the fate of pollutants nutrients and weathering products supplied from the continent based on the ra isotope signals we have estimated that the shelf water component contributes 20 of the tsushima current water passing through the tsushima strait and the mean residence time for the shelf water to mix with the kuroshio surface water is 2 3 years as many of materials derived from the continent such as heavy metals and the nutrients have their mean residence times less than a few months in the nearshore and shelf waters they must largely deposit on the shelf sediments prior to the transport from the shelf to the open sea by mixing

geothermal springs of the west florida continental shelf evidence for dolomitization and radionuclide enrichment abstract on the sea bed of the west florida continental shelf about 45 km ssw of ft myers florida an 85 km 2 area has been discovered in which six thermal springs discharge warm chemically altered seawater from vents and seepage zones the spring water apparently originates in the subsurface ocean around the florida platform and penetrates the highly porous strata of the platform about 500 1000 meters below sea level it percolates toward the interior of the platform and is geothermally heated to about 40 c en route then it rises along more vertical flow channels and is discharged in warm submarine springs beneath the platform several chemical processes alter the percolating seawater one process seems to be a secondary dolomitization of the limestone of the platform because in the discharging seawater magnesium is lower by 2 7 mmole kg and calcium higher by 3 6 mmole kg than in normal seawater with the same chlorinity other reactions within the sediments of the platform enrich the spring effluents 1000 fold in 226 ra 10 000 fold in 222 rn and 90 fold in 228 ra compared to the seawater surrounding the platform thus the springs may be important sources of radionuclides for the gulf of mexico the percolating seawater also loses all of its oxygen and nitrate to reduction processes loses most of its phosphate and 40 of its 238 u and roughly quadruples its silica content coastal carbonate platforms are fairly common geological features thus processes like those beneath the west florida shelf may function on a world wide basis to play an important role in the diagenesis of carbonate sediments

seismic anisotropy in the deep mantle boundary layers and the geometry of mantle convection an attempt is made to explore the geodynamical significance of seismic anisotropy in the deep mantle on the basis of mineral physics the mineral physics observations used include the effects of deformation mechanisms on lattice and shape preferred orientation the effects of pressure on elastic anisotropy and the nature of lattice preferred orientation in deep mantle minerals in dislocation creep regime many of these issues are still poorly constrained but a review of recent results shows that it is possible to interpret deep mantle seismic anisotropy in a unified fashion based on the solid state processes without invoking partial melting the key notions are i the likely regional variation in the magnitude of anisotropy as deformation mechanisms change from dislocation to diffusion creep or superplasticity associated with a change in the stress level and or grain size in the convecting mantle with a high rayleigh number and ii the change in elastic anisotropy with pressure in major mantle minerals particularly in mg fe o the results provide the following constraints on the style of mantle convection i the sh sv anisotropy in the bottom transition zone and the sv sh anisotropy in the top lower mantle can be attributed to anisotropy structures lattice preferred orientation and or laminated structures caused by the horizontal flow in this depth range suggesting the presence of a mid mantle boundary layer due to partially layered convection ii the observed no significant seismic anisotropy in the deep mantle near subduction zones implies that deformation associated with subducting slabs is due mostly to diffusion creep or superplasticity and therefore slabs are weak in the deep mantle and hence easily deformed when encountered with resistance forces and iii the sh sv anisotropy in the cold thick portions of the d layer is likely to be due to horizontally aligned shape preferred orientation in perovskite plus magnesiowustite aggregates formed by strong horizontal shear motion in the recent past

how to relate body wave and surface wave anisotropy seismic anisotropy is one of the most efficient geological and geodynamical tools for understanding the dynamics of the earth upper mantle anisotropy is evident in seismic data sets for the last 30 years primarily from surface wave dispersion curves and body wave sks data we demonstrate in this paper that surface wave and body wave derived anisotropy can be explained by the same anisotropic parameters l gc gs in the simplest case of a horizontal fast symmetry axis one application of this method is to display the sks delay time and the corresponding azimuth which can be derived from surface wave global with a lateral resolution of around 2000 km and regional lateral resolution of 350 km anisotropy tomography a global scale comparison is disappointing since there are only a few areas where both data types are correctly retrieved at the same spatial scale same lateral resolution the anisotropy is well resolved in oceanic basins from surface waves whereas most measurements of sks splitting are available below continents however there is a good agreement between surface wave and body wave anisotropy in regions where large scale processes primarily tectonic are taking place such as the western united states and in central asia in particular we show that at high frequency 1hz the observed sks delay time and azimuth depend on the order in which s waves propagate through the layers a comparison between synthetic and observed sks phases in central asia highlights the varying sensitivity of sks waves at depth we can obtain a good correlation between synthetic and observed sks by considering anisotropy only between 80 and 200 km depth in the tomographic model rather than in the entire upper mantle however it is more difficult to explain the discrepancy between synthetic and observed sks under the eastern united states where a large scale coherent sks anisotropy is present such results call for more sophisticated sks waveform modeling and for enhanced lateral resolution in anisotropic tomographic models

nonlinear crustal corrections for normal mode seismograms we present an efficient and accurate method for calculating local nonlinear crustal effects on the eigenfrequency of free oscillations of the earth we implement the method in the calculation of synthetic normal mode seismograms and demonstrate that neglecting the nonlinear crustal effects in the tomographic waveform inversion leads to velocity models that appreciably overestimate fast velocity anomalies beneath orogenic belts and cratons at 250 400 km depth online material color versions of figures

new images of the earth s upper mantle from measurements of surface wave phase velocity anomalies 1 the sensitivity of a surface wave to the lateral structure of the earth s mantle depends on the local structure and thickness of the crust particularly at short periods the strong lateral heterogeneity of the crust has a significant nonlinear effect on the surface wave sensitivity functions thus far this effect has not been quantified and tomographic studies of the upper mantle based on surface wave observations have not taken it into account on the basis of a jeffreys wentzel kramers brillouin jwkb description of surface wave propagation we find and discuss the lateral variations of the sensitivity of love and rayleigh waves to mantle seismic anomalies associated with a realistic model of the crust mooney et al 1998 we then use the resulting laterally varying sensitivity kernels to determine a new tomographic image of horizontally and vertically polarized shear velocity in the upper mantle from love and rayleigh wave phase velocity measurements periods from 35 to 300 s in terms of large scale patterns of high and low velocity our model is similar to previously published ones confirming among other features the anomalous anisotropy underlying the central pacific ekstrom and dziewonski 1998 we find however that the lateral dependence of the surface wave sensitivity kernels has a significant effect on the absolute amplitude of the anomalies which should not be neglected as we attempt to obtain higher resolution images of the upper mantle