eastern mediterranean gravity potential and the hellenic subduction process tornographic images of the mediterranean upper mantle p wave velocity structure have been used to analyse the gravity potential in the vicinity of the hellenic subduction zone the velocity anomalies are assumed to be proportional to density variations according to birch s law the effect of the topography on the geoid in the region is also calculated the results indicate that the upper mantle geoid signal probably has significant amplitudes of several metres but it correlates poorly with the observed geoid the geoid calculated from topography correlates well but has an amplitude that is too large in comparison with the observed geoid the results show that an improved understanding of the hellenic subduction zone geoid requires refinement of the moho topography so that the effect of this topography can be separated from the upper mantle signature

geochemistry of sediments of the ganges alluvial plains evidence of large scale sediment recycling the himalaya an amalgamation of different lithotectonic units brought together during and even before the himalayan orogeny has been subjected to various phases of denudation since the palaeogene sediments derived by erosion of these lithotectonic units in different proportions were delivered to the himalayan foreland the nature and proportions of sediments from various lithotectonic units delivered to the foreland varied in response to the prevailing tectonic and climatic conditions during the orogeny the geochemistry of sediments collected from floodplain river channels and reworked aeolian sediments from the delhi and moradabad areas of the ganges plain of the foreland basin indicates that they were derived from moderately weathered sedimentary sources further the results also suggest that the sediments are not subjected to any significant post depositional chemical weathering high tectonic instability in the source area and the geomorphology and cycles of aggradation and degradation in fluvial regimes in the ganges plain do not favor much chemical weathering homogenization of various lithotectonic units of the himalaya through several episodes of sedimentary reworking is shown by the rare earth elements ree geochemistry of the sediments the predominant source for the homogeneous sediments is likely the high himalayan crystalline or the tertiary sub himalayan sediments again the later was derived from high himalayan crystalline if so the ganges alluvial plain is geochemically a quaternary extension of tertiary sub himalaya the average composition of lithologies subjected to erosion in the himalaya appears to be better represented by the alluvial mud present in the plain

late pleistocene holocene eolian deposits of the southern buenos aires province argentina a preliminary model abstract the aim is to reconstruct the sedimentological history of the late pleistocene holocene eolian deposits of southern buenos aires province hence a preliminary model is proposed in order to analyze the different stages of the sedimentological cycle the deposits include sandy loess fine sand sheets and dunefields the southern buenos aires province represents the depositional area whereas the source and transport environments are located in the northern patagonian andes and northern extra andean patagonia according to the mineralogical characteristics of the material explosive volcanism is considered the predominant process of particle formation particularly as generator of pyroclastic deposits which were reworked by different erosive processes glacial fluvial eolian during the late glacial maximum and the holocene alluvial deposits which were mainly generated by fluvial erosion of volcaniclastic sedimentary units were deposited along the floodplains of the colorado and negro rivers including those distal segments located in the emerged offshore platform the fluvial regime depended upon the regime of the glaciers hence it reflected the major glacial fluctuations which took place in the andes floodplains were deflated by southwest winds transporting the material towards the southern buenos aires province sandy loess and loessial sands and to la pampa province dunefields

geochemistry of the pliocene red clay formation in the chinese loess plateau and implications for its origin source provenance and paleoclimate change abstract recent field observations and sedimentological studies suggest the eolian origin of the late tertiary red clay formation underlying the pleistocene loess soil sequence in the loess plateau thereby extending the wind blown record from 2 6 ma down to 7 0 ma in north central china to address the source provenance of the red clay major and trace element concentrations were analyzed on the samples from a north south loess transect of the loess plateau and a red clay section at jiaxian results show that there is a good agreement of both major and trace element compositions between the last glacial interglacial loess soil units and the jiaxian red clay record the ree distribution patterns of the loess soil and red clay samples are remarkably similar in shape with enriched lree and fairly flat hree profiles and clear negative eu anomaly the geochemical features of the red clay are also identical to those of upper continental crust these chemical characteristics provide further evidence for eolian sedimentation of the red clay from the relations both between na2o al2o3 and k2o al2o3 ratios and between u pb and th pb ratios the eolian loess and red clay materials must have been subject to thorough sedimentary differentiation and moderate chemical weathering in the source area the chemical similarity between soil unit s1 and the red clay implies that such an environment as occurred in the last interglacial period may have already formed in northwestern china during the late tertiary comparison of down section variations in elemental ratios with field observations of pedogenic development of the red clay suggests that cao al2o3 sr rb and sr ba are more sensitive to climate changes than magnetic susceptibility and can be used to reconstruct the climate history of the late cenozoic

role of the lithosphere in mantle convection plate geometry and kinematics generally reflect the mechanical properties of the solid lithosphere rather than those of the fluid mantle underneath and plate formation and subduction account for most of the heat transport from the earth s interior correspondingly mantle convection models must incorporate a stiff but mobile boundary layer like the lithosphere before they can reproduce the main features of mantle convection a relatively easy way to accomplish this in numerical models is to combine a temperature dependent viscosity with an imposed piecewise constant surface velocity boundary condition it is shown how surface heat flux topography gravity and geoid but not plate velocities or stresses can then be derived numerical models confirm that a lithosphere has a first order effect on the underlying flow structure for internally heated models approx imating the case of whole mantle convection the calculated surface signatures are in first order agreement with observations a level of empirical success which hitherto has not been approached by models of mantle convection companion papers exploit the observations more fully to constrain the main features of mantle convection

calcium and magnesium isotope systematics in rivers draining the himalaya tibetan plateau region lithological or fractionation control abstract in rivers draining the himalaya tibetan plateau region the 26mg 24mg ratio has a range of 2 and the 44ca 42ca ratio has a range of 0 6 the average 26 mg values of tributaries from each of the main lithotectonic units tethyan sedimentary series tss high himalayan crystalline series hhcs and lesser himalayan series lhs are within 2 standard deviation analytical uncertainty 0 14 the consistency of average riverine 26 mg values is in contrast to the main rock types limestone dolostone and silicate which range in their average 26 mg values by more than 2 tributaries draining the dolostones of the lhs differ in their 44 42 ca values compared to tributaries from the tss and hhcs the chemistry of these river waters is strongly influenced by dolostone solute mg ca close to unity and both 26 mg 1 31 and 44 42 ca 0 64 values are within analytical uncertainty of the lhs dolostone these are the most elevated 44 42 ca values in rivers and rock reported so far demonstrating that both riverine and bedrock 44 42 ca values may show greater variability than previously thought although rivers draining tss limestone have the lowest 26 mg and 44 42 ca values at 1 41 and 0 42 respectively both are offset to higher values compared to bedrock tss limestone the average 26 mg value of rivers draining mainly silicate rock of the hhcs is 1 25 lower by 0 63 than the average silicate rock these differences are consistent with a fractionation of 26 mg values during silicate weathering given that the proportion of mg exported from the himalaya as solute mg is small the difference in 26mg 24mg ratios between silicate rock and solute mg reflects the 26mg 24mg isotopic fractionation factor silicate dissolved mg between silicate and dissolved mg during incongruent silicate weathering the value of silicate dissolved mg of 0 99937 implies that in the tss solute mg is primarily derived from silicate weathering whereas the source of ca is overwhelmingly derived from carbonate weathering the average 44 42 ca value in hhcs rivers is within uncertainty of silicate rock at 0 39 the widespread hot springs of the high himalaya have an average 26 mg value of 0 46 and an average 44 42 ca value of 0 5 distinct from riverine values for 26 mg but similar to riverine 44 42 ca values although rivers draining each major rock type have 44 42 ca and 26 mg values in part inherited from bedrock there is no correlation with proxies for carbonate or silicate lithology such as na ca ratios suggesting that ca and mg are in part recycled however in spite of the vast contrast in vegetation density between the arid tibetan plateau and the tropical lesser himalaya the isotopic fractionation factor for ca and mg between solute and rocks are not systematically different suggesting that vegetation may only recycle a small amount of ca and mg in these catchments the discrepancy between solute and solid ca and mg isotope ratios in these rivers from diverse weathering environments highlight our lack of understanding concerning the origin and subsequent path of ca and mg bound as minerals in rock and released as cations in rivers the fractionation of ca and mg isotope ratios may prove useful for tracing mechanisms of chemical alteration ca isotope ratios of solute riverine ca show a greater variability than previously acknowledged the variability of ca isotope ratios in modern rivers will need to be better quantified and accounted for in future models of global ca cycling if past variations in oceanic ca isotope ratios are to be of use in constraining the past carbon cycle

boron and magnesium isotopic composition of seawater the isotopic systems of boron and magnesium are increasingly being used as proxies for a number of environmental variables and processes the isotopic composition of seawater for both systems plays a central role in these studies and is an important interlaboratory standard given the long residence times of both elements 10 7 years it is commonly assumed that seawater is isotopically homogenous for these systems yet no systematic studies currently exist here we present the b and mg isotopic composition of 26 28 seawater samples from a number of ocean basins that encompass a significant range in salinity 32 to 38 psu temperature 0 3 to 25 9c and water depth 0 to 1240 m we find no significant or systematic variation for either system in accordance with their long residence times we recommend that the mean values we report 11 b 39 61 0 04 2 s e n 28 25 mg 0 43 0 01 2 s e n 26 26 mg 0 82 0 01 2 s e n 26 be used in future studies involving mg and b isotopes copyright 2010 by the american geophysical union

geochemistry of precambrian mafic magmatic rocks of the western himalaya india petrogenetic and tectonic implications abstract precambrian sequences of the higher himalayan crystallines vaikrita group and the lesser himalaya chail jutogh and jaunsar groups in the garhwal and himachal regions of the western himalaya include abundant metamorphosed mafic lavas and dykes amphibolites a gabbroic body within the chail group has been dated at 1907 91 ma initial 87 sr 86 sr ratio of 0 7022 0 0008 by the whole rock rb sr method this age is consistent with several age data 1800 2000 ma whole rock rb sr method assigned to the granitoids and gneisses of this region these amphibolites and gabbros exhibit low ti tholeiite characteristics the lesser himalayan samples are enriched in light rare earth elements lree and large ion lithophile element lile with distinct negative nb sr p and ti anomalies conversely the vaikrita group samples are characterized by less enriched lree lile and absence of the above anomalies but have distinct positive sr anomalies the chemical characteristics of the lesser himalayan samples are remarkably similar to the basal aravalli volcanics of the nw indian shield inferred to reflect significant components from enriched mantle sources and crustal contamination samples of the vaikrita group appear to have been influenced by an asthenospheric mantle and are less contaminated as indicated by the absence of negative nb p and ti anomalies these distinct geochemical characteristics are used to demarcate the main central thrust along the vaikrita thrust the chail and jutogh groups are affiliated with the lesser himalaya and the chail group gabbro body may represent portion of one among the numerous magma chambers that fed a large precambrian magmatic province in the south of the main central thrust the similar emplacement ages 1800 2000 ma for the mafic and felsic magmatic rocks suggest that this region experienced a major episode of crustal generation and evolution in a rift environment

the deep structure of ocean ridges in a convecting mantle abstract plate tectonic models have frequently assumed that mid ocean ridges are the surface expression of the hot rising limb of a convection cell measured half spreading rates show that in any reference frame the horizontal transverse velocity of some ridges is comparable to the individual plate velocities if the above assumption is correct this observation implies that the thermal structure beneath the ridge can adjust rapidly to the changing surface configuration of the plates numerical experiments in a two dimensional convecting layer of constant viscosity fluid show that such a rapid adjustment is probably unrealistic a symmetrically spreading ridge moving horizontally is simulated by specifying a velocity boundary condition on the upper surface of the layer the other boundary conditions are uncertain so several different combinations are used in the experiments including periodic or reflecting lateral boundaries and stress free or no slip lower boundary in all the experiments the rising sheets of the convection cells are anchored firmly in the mean horizontal flow of the layer unless the mean flow coincides approximately with the ridge velocity the ridge quickly moves away from a rising sheet which is initially beneath it if the aspect ratio width depth of the convection cells is significantly greater than one this conclusion is even more evident in the earth it seems unlikely that there is a systematic deep of the order of the convecting layer thickness structure beneath the worldwide network of ocean ridges

mantle circulation models with variational data assimilation inferring past mantle flow and structure from plate motion histories and seismic tomography summary mantle convection models require an initial condition some time in the past because this initial condition is unknown for earth we cannot infer the geological evolution of mantle flow from forward mantle convection calculations even for the most recent mesozoic and cenozoic geological history of our planet here we introduce a fluid dynamic inverse problem to constrain unknown mantle flow back in time from seismic tomographic observations of the mantle and reconstructions of past plate motions using variational data assimilation we derive the generalized inverse of mantle convection and explore the initial condition problem in high resolution 3 d spherical mantle circulation models for a time period of 100 myr roughly comparable to half a mantle overturn we present a synthetic modelling experiment to demonstrate that mid cretaceous mantle structure can be inferred accurately from fluid dynamic inverse modelling assuming present day mantle structure is well known even if an initial first guess assumption about the mid cretaceous mantle involved only a simple 1 d radial temperature profile we also demonstrate that convecting present day mantle structure back in time by reversing the time stepping of the energy equation is insufficient to model the mantle structure of the past the difficulty arises because such backward convection calculations ignore thermal diffusion effects and therefore cannot account for the generation of thermal buoyancy in boundary layers as we go back in time inverse mantle convection modelling should make it possible to infer a number of flow parameters from observational constraints of the mantle

large scale mantle convection and the history of subduction a large scale pattern of mantle convention is evident in the shape of the earth s gravity field1 lateral variations in seismic velocity2 3 the distribution of hotspots4 and long wavelength bathv metric anomalies5 paradoxically these signals of mantle convection bear little resemblance to present day plate motions1 4 although they seem to be related to past subduction zone configurations6 here we quantify this relationship by correlating a time integrated history of subduction calculated in the hotspot reference frame for the past 180 myr with regions of the deep mantle inferred from other geophysical observations to be colder than average our results support suggestions6 9 that a direct relationship exists between large scale thermal heterogeneity in the lower mantle and subduction during the cenozoic and mesozoic

mineralogical variation of sr nd isotopic and elemental compositions in loess and desert sand from the central loess plateau in china as a provenance tracer of wet and dry deposition in the northwestern pacific we determined sr and nd isotopic compositions and major trace and rare earth element ree compositions of evaporite and detrital minerals in loess and desert sand from the central loess plateau clp where 80 of the loess deposits in china are found the loess exhibited a narrow range of variation in 87sr 86sr 0 715 0 001 a sequential extraction experiment on the loess however showed that water and acetic acid hoac soluble minerals mainly halite and calcite hydrochloric acid hcl soluble minerals chlorite and phosphate and hcl insoluble minerals illite kaolinite smectite vermiculite feldspar and quartz have distinct 87sr 86sr ratios approximately 0 7111 0 0004 0 7141 0 0004 and 0 7195 0 0010 respectively the water and hoac soluble ratios were regionally uniform and were close to the 87sr 86sr value of river water from the huanghe yellow river and the maximum 87sr 86sr value of spring rainwater in japan suggesting selective solution of eolian halite and calcite into the chinese river water and into the japanese rainwater the mineralogical elemental and sr nd isotopic compositions of hcl soluble and insoluble minerals in the clp loess were different from those of the northern desert sand but were indistinguishable from those of its fine grained fraction supporting the view that the loess is composed of the fine grained fraction of desert sand that was transported to the southeast in the atmosphere the 87sr 86sr end and gd yb values of hcl insoluble minerals in the clp loess were almost identical to those of airborne dust soil minerals and deep sea sediments in the northwestern pacific considered to be of eolian origin suggesting dissolution of the loess evaporite minerals during atmospheric transportation these relationships demonstrate the usefulness of sr isotopes of water and hoac soluble minerals and sr and nd isotope ratios and ree compositions of acid insoluble minerals in loess as provenance markers for wet and dry deposition respectively

aspherical earth structure from fundamental spheroidal mode data free oscillation data reveal heterogeneity in the earth s mantle whose geographical pattern is dominated by spherical harmonics of angular degree two and correlates well with the hydrostatically referenced geoid the heterogeneity can be modelled as localized in the transition zone 420 670 km depth and may be related to a large scale component of mantle convection

provenance of loess material and formation of loess deposits on the chinese loess plateau abstract results of a multiple isotopic chemical and mineralogical analysis of loess from the three northwestern inland basins the junggar basin the tarim basin and the qaidam basin and the loess plateau region of china are summarized they suggest a qualification of the conventional views that the three northwestern basins were the important source areas of the loess plateau and that the sand deserts were the primary single source of the chinese loess it is argued that the gobi stony desert in southern mongolia and the adjoining gobi and sand deserts the badain jaran desert tengger desert ulan buh desert hobq desert and mu us desert in china rather than the three inland basins are the dominant source areas of the loess plateau however although these gobi and sand deserts are regarded as the main source regions they serve as dust and silt holding areas rather than dominant producers the mountain processes including glacial grinding frost weathering salt weathering tectonic processes and some fluvial comminution in the gobi altay mts hangayn mts and the qilian mts have played an important role in producing the vast amounts of loess sized material for forming the loess plateau

palaeocontinental configurations and geoid anomalies reconstructions of past continental configurations help our understanding of the evolution of planetary surface morphology and impose valuable constraints on the models for planetary dynamics the problem is that although well established palaeogeographical maps now exist for mesozoic and later times1 2 thanks to excellent agreement between palaeomagnetic and marine magnetic deductions this is not true for earlier times mainly due to the lack of independent palaeogeophysical control over otherwise longitudinally ambiguous palaeomagnetic data the prospect of relating palaeocontinental configurations to geoid anomalies recently raised by anderson3 has thus given a fresh perspective to this problem here his continental insulation model is shown to be an inadequate explanation of the undulations of subduction compensated geoid instead the geoid highs and lows are shown to correlate well with the overall divergent and convergent plate tectonism since the middle palaeozoic and are thus ascribed respectively to the ascending and descending patterns of lower mantle convection

weathering and global denudation a negative correlation between sediment yield and weathering history as measured by the chemical alteration cia of the suspended sediment is observed for many of the world s major rivers and other regions of denudation the weathering history is a first order control on the sediment yield of such areas termed equilibrium denudation regions for other areas data scatter with either apparent increases or decreases of sediment yield for given cia values these areas are termed nonequilibrium denudation regions low sediment yields can be attributed to moderated erosion either natural or human induced and or the incorporation of unweathered glacial debris accelerated erosion resulting in high sediment yield is primarily human induced and results from cultivation and other land use each of these effects has a profound influence on global sediment discharge from the continents pre human suspended sediment discharge from the continents is estimated to be 12 6 times 10 15 g yr or about 0 6 the

effects of lithospheric rigidity on ocean floor bathymetry and heat flow the observed quasi rigid behaviour of surface plates in the course of their relative motion is a consequence of the high viscosity which obtains in the cold near surface region by assigning a particular constant velocity as the upper boundary condition in a numerical model of mantle convection we have investigated the effect of lithospheric rigidity on the variation of oceanic bathymetry and heat flow as a function of ocean floor age predicted variations of both bathymetry and heat flow at the surface of mantle wide convection cells which are partially heated from within exhibit the same qualitative behaviour as data compiled for the major ocean basins the bathymetry varies with distance x from the spreading centre initially as x sup 1 2 but subsequently flattens with respect to an x sup 1 2 reference curve whereas the heat flow closely follows an x sup 1 2 decay over most of the convection cell consequently the viability of the mechanism for sea floor flattening proposed by jarvis and peltier 1980 is increased when a constant surface velocity characteristic of rigid plates is incorporated in the model this model successfully predicts both that the bathymetry should flatten and that the heat flow should not

magnesium isotopic variations in loess origins and implications abstract loess deposits serve as important continental archives for studying quaternary climatic variations and for estimating the average chemical composition of the upper continental crust here we report high precision mg isotopic data for 19 loess samples from china argentina and europe which were previously used to estimate the composition of the upper continental crust the results show that global loess samples have heterogeneous mg isotopic compositions with 26mg per mil deviation of the 26mg 24mg ratio from the dsm3 standard values ranging from 1 64 to 0 25 and a weighted average of 0 89 which is lighter than both crust and mantle silicates mgo content and 26mg of loess positively correlate with cao al2o3 ratio suggesting a two component mixing between carbonates and secondary silicate minerals the large variation in mg isotopic composition of loess results from a combination of factors including source heterogeneity eolian sorting during transport of the loess and chemical weathering during formation of the loess deposit our results suggest that mg isotopic composition of loess has potential to be a proxy indicator to characterize the paleoclimatic change but may not represent the average mg isotopic composition of the upper continental crust due to mixing sorting of isotopically distinct components and isotope fractionation during loess deposit formation

the modern geoid and ancient plate boundaries abstract the non hydrostatic geoid is dominated by three large anomalies an area of high gravity potential in the equatorial pacific another stretching from greenland through africa to the southwest indian ocean and a semi continuous low region passing from hudson s bay through siberia to india and on to antarctica none of these three high amplitude greater than 60 m and long wavelength anomalies corresponds to present day plate boundaries however if the modern geoid is plotted over the positions of continents and plate boundaries at 125 ma b p reconstructed relative to hotspots a strong correlation emerges the modern geoidal low corresponds in position to the areas of subduction surrounding the pacific 125 ma ago the geoidal high now centered on africa is entirely contained within ancient pangaea and the equatorial pacific high overlies the location of the spreading centers preserved in the magnetic anomalies of the central pacific the most plausible cause of the large geoidal undulations is lower mantle convection only weakly coupled to plate motions the correspondence between modern geoid and ancient plate boundaries implies either that the coupling was much more intimate in the past or that there is a lag of at least 100 ma in response of the lower mantle to upper mantle conditions

resolution experiments for nw pacific subduction zone tomography we investigate the resolving power of isc neic p travel time data in tomographic inversions for the geometry of the subduction zones in the nw pacific from thermal models for the kurile japan izu bonin mariana and ryukyu slabs we generate three dimensional synthetic velocity anomalies for subducting slabs which are then projected onto a cell model for the uppermost 1400 km of the mantle using these synthetic models we compute synthetic delay times for ray paths corresponding to the source and receiver locations used for the actual data add gaussian noise invert the synthetic data and compare the resulting velocity structure to the initial synthetic models this comparison is illustrated for sections through the kuriles and the mariana arcs a variety of resolution artifacts are observed which in many cases resemble features visible in the tomographic results obtained from inverting the actual isc neic data

phanerozoic marine inundation of continents driven by dynamic topography above subducting slabs a spherical model of mantle flow constrained by the locations of trenches can be used to predict the dynamic topography of the earth s surface and hence the marine inundation of continents for past periods of high sea level the predicted geographical pattern of flooding correlates well with the geological record the high spatial correlation may result from increased plate velocities at these times leading to increased rates of subduction subsidence and inundation at convergent margins

heterogeneity and time dependence in 3d spherical mantle convection models with continental drift abstract feedback between continents and large scale mantle flow through thermal blanketing has long been surmised as a mechanism for continental drift and wilson cycles paleomagnetism provides evidence for extensive continental displacements 10 000 km on time scales of 100 200 million years comparable to an intrinsic overturn in whole mantle convection here we model continental motions in vigorous 3d spherical convection models focusing on the effects of continent size mantle heating mode and a strong increase in lower mantle viscosity continents covering 30 10 and 3 of earth s surface representative of the former supercontinent pangea present day asia and antarctica respectively are introduced into simple end member mantle convection models characterized by pure core or internal heating and uniform or layered mantle viscosity supercontinents promote temperature anomalies on the largest scales spherical harmonic degrees 1 and 2 primarily through the organization of the long wavelength convective planform inherent in models with a high viscosity lower mantle bottom heating can promote long wavelength heterogeneity by clustering plumes beneath the continent however in isoviscous models small scale structure persists away from the continent regardless of the heating mode supercontinents respond to long wavelength heterogeneity by following great circle paths with variations in velocity on time scales of 1 billion years smaller continents are unable to promote long wavelength structure and the resulting motions are governed by bursts in velocity on time scales of the order of 100 million years continental velocities are roughly a factor of 3 smaller than those in oceanic regions an observation that may help explain the observed difference in the speed of predominantly continental or oceanic plates

isotope geochemistry of quaternary deposits from the arid lands in northern china abstract quaternary deposits of various types lacustrine desert sand river loess and loess like deposits from the southwestern tarim basin north tianshan northeast china and the central loess plateau exhibit an nd 0 range from 5 to 12 8 and a 147 sm 144 nd range from 0 110 to 0 140 their nd depleted mantle model ages are between 1 50 and 1 92 ga with a mean value of 1 7 ga th sc ratios vary over a wide range from 0 1 to 2 comparison of these geochemical signatures with those of the old australian shales and modern pacific ocean dust suggests a continuous input of mantle derived material into the crust over geological time the deposits from the north tianshan show clearly different chemical and isotopic features with significantly higher sm nd high 143 nd 144 nd low 87 sr 86 sr and low la lu and th sc ratios relative to those in the deposits from the other areas this indicates a geological control on the local source which has an overall young age and an enrichment of basic components the ce 0 values of two loess samples from the central loess plateau are 1 8 and 3 9 corresponding to depleted mantle model ages of 2 2 and 3 4 ga like the rb sr system the evolution of the la ce isotopic system in the crust may be to some extent decoupled from the sm nd system due to la ce fractionation by sedimentary processes the loess deposits and the desert sand are not distinguishable in isotopic composition but show somewhat different chemical compositions we conclude that the loess can have originated from desert processes since the difference in the chemistry of the desert sand and loess may result from mineral sorting during desert processes and aeolian transportation the loess deposits on the central loess plateau chemically and isotopically resemble the desert loess deposits in the southwestern tarim basin but do not resemble those of the north tianshan accordingly the loess on the plateau may have a source region in the tarim basin but clearly not in the north tianshan

geochemistry of the surface sediments of the sulu and south china seas abstract the chemical and mineralogical composition of the surface sediments of the sulu sea a deep hydrographically restricted basin in the indonesian archipelago and the neighbouring area of the south china sea have been examined the aim of the study was to characterize the geochemistry of the sediments of the isolated oxygen deficient basin and to compare them with sediments at equivalent water depths outside the basin as a prelude to a further study of the response of the composition of the sediments to oceanographic changes including possible anoxia during the last glacial maximum the concentrations of most of the chemical constituents in the surface sediments along four transects change markedly with water depth in the sulu sea ss and the south china sea scs thus caco 3 decreases from around 60 by weight in the shallowest samples 800 1000 m to around a few percent in the deepest sample at 4203 m in the scs where the ccd lies at around 4000 m in the ss however the ccd lies much deeper so that the caco 3 contents are still approximately 40 at 4500 m depth organic c also decreases with depth in both basins from values of around 1 2 in the shallowest samples to less than 1 in the deepest samples the ratios of si mg na k co cr cu ni pb rb y zr and zn to al all decrease with increasing water depth because of the change in mineralogy and sediment texture with water depth the p al is higher in shallower water in both areas because of the higher abundance of detrital phosphorite or phosphatized limestone in these sediments the sr al ratio is also higher in shallower water reflecting the supply of aragonitic shell debris from the shallow banks and shelves around the basin the ba al ratio has a maximum at 2000 m depth in the scs due to higher primary production in overlying surface waters whereas it is uniformly low in the ss manganese is also enriched at intermediate water depths in both the ss and the scs because of the presence of oxyhydroxides and this also produces a slight enrichment in mo in the same samples because of their association in this authigenic phase the concentration of i and the i c organic ratio decreases with water depth in both areas due to the higher abundance of fresh marine organic matter at the shallow water sites which scavenges proportionally more i from sea water finally the 13 c organic values tend to decrease irregularly with increasing water depth probably reflecting a higher contribution of terrestrial organic matter in deeper water a discriminant analysis of the full data set reveals that compared with the scs samples the oxygen deficient ss sediments have higher caco 3 contents lower n and s contents and lower i c organic ba al fe al k al pb al rb al ti al v al y al and zn al ratios these contrasts are brought about by the different depths of the ccd differences in the nature of the organic matter and different types of aluminosilicate detritus in the two areas although the i c organic ratios are slightly but significantly lower in the ss which may be due to the low dissolved oxygen contents of the waters of this basin the mn and mo contents and other minor elements that respond significantly to ambient bottom water oxygen levels are not significantly different in the ss and the scs moreover the organic c contents are also indistinguishable in the two areas hence the low bottom water oxygen levels in the ss do not lead to higher organic carbon contents in the surface sediments the composition of sediments in contact with waters having even lower oxygen concentrations should be evaluated in order to conduct a more definitive test of the effect of low oxygen conditions on carbon preservation comparison of glacial interglacial sediment sections from this area must take particular care to separate the effects of source variations on those potentially caused by low oxygen levels

on some consequences and possible causes of layered mantle convection laboratory and numerical experiments are used to study convection confined to discrete layers the laboratory experiments which use the water content of a predominantly glycerine solution as a way of establishing chemical density variations show that convection will remain confined to superimposed layers when the chemical density contrast between layers is greater than the density change associated with the greatest temperature difference within the convecting system the numerical experiments show a double thermal boundary layer at the interface between layers a feature that would result in an average temperature increase of about 500 c in the mantle near 700 km depth if the lower mantle is isolated as suggested by recent geochemical models the possible causes of a layered mantle convection system include major element chemistry variations the combined effect of major element chemistry and a phase change or a phase change alone if it has a sufficiently negative clapeyron slope

regional compensation of subducted lithosphere effects on geoid gravity and topography from a preliminary model abstract it is proposed that the anomalous mass of subducted lithospheric slabs is partially compensated by two mechanisms a by stresses transmitted up the slab to the surface lithosphere causing the trench and b by stresses transmitted through the mantle wedge overlying the slab which cause a broad shallow depression results calculated from a preliminary two dimensional model show that it can give a first order account of observed topography gravity and geoid anomalies in particular the model can reconcile the large mass anomaly predicted for the slab by thermal conduction theory with the smaller than expected gravity and geoid anomalies preliminary results indicate substantial compensation by mechanism a implying rather large stresses at the top of the slab these could be important for theories of earthquake mechanisms and plate driving mechanisms roughly 50 of the possible compensation by mechanism b is indicated the resulting depression the order of 500 m deep and over 500 km wide could explain anomalously deep marginal basins and some marine transgressions of continents

can depression of the core mantle interface cause coincident magsat and geoidal lows of the central indian ocean abstract according to hide undulations at the core mantle interface distort geopotential fields that can be observed in lower order harmonics hide and malin obtained a significant correlation between long wavelength gravity and non dipole magnetic fields when the latter is displaced in longitude by 160 the central indian ocean is characterised by the largest geoidal low which correlates well with the magsat component low without any longitudinal displacement an integrated interpretation of geoidal and magnetic anomalies suggests that the source of these anomalies should be the same and may originate at the depression of the core mantle boundary the shallow curie isotherm the matching of the long wavelength of the geoidal anomaly with the vertical magsat anomaly and the identical location of the centres of the anomalies provide an excellent confirmation of hide s hypothesis but contradicts earlier geoidal low interpretations of mass deficiency in the earth s crust and mantle

summer monsoon variations over the last 1 2 ma from the weathering of loess soil sequences in china the loess soil sequence in northern china contains a near continuous record of quaternary paleoclimate magnetic susceptibility and grain size have so far been the only proxies available to address the long term changes of the east asian paleomonsoon extending back to more than one million years in this study the ratio of cbd citrate bicarbonate dithionite extractable free fe2o3 fed a measure of iron liberated by chemical weathering versus the total fe2o3 available fet was measured on samples at 10 cm intervals taken from two loess sections deposited over the last 1 2 ma variations of this index are highly consistent with other pedological indicators but in addition provide a quantitative measurement of the degree of pedogenesis in the loess plateau since chemical weathering in the region mainly depends upon summer precipitation and temperature weathering intensity primarily reflects changes in the east asian summer monsoon the new proxy has been used to document a series of summer monsoon changes of global significance which are not necessarily recorded by magnetic susceptibility

geochemical characteristics of the miocene eolian deposits in china their provenance and climate implications 1 in the loess plateau in northern china the quaternary loess soil sequences the hipparion red earth of eolian origin red clay and the miocene loess soil sequences constitute a near continuous terrestrial record of paleoclimates for the past 22 ma in this study miocene loess and paleosol samples from qinan qa i were analyzed for their major trace and rare earth element chemistry and compared with the pliopleistocene samples from xifeng with emphasis on their provenance and paleoclimatic implications the results show similar geochemical signatures for the eolian deposits of different ages and they are also comparable to the average composition of the upper continental crust these suggest that the dust materials were all derived from well mixed sedimentary protoliths which had undergone numerous upper crustal recycling processes they also support the notion of broadly similar source areas and dust transporting trajectories for different periods since the early neogene the slightly higher k2o fe2o3 and mgo concentrations and loss on ignition values and the lower na2o content in the miocene loess samples compared to their quaternary counterparts are attributable to the finer grain size of the miocene loess associated with weaker dust carrying winds in comparison with some loess in europe and america with less extensive sources eolian deposits from northern china show higher cs and lower zr and hf content this is attributable to the sorting processes from remoter sources during transportation and could be regarded as an indication of the desert origin of the loess deposits miocene paleosol samples show higher chemical index of alteration values and lower cao mgo and na2o concentrations than does the intervening loess indicating stronger weathering of the paleosols however the moderate chemical weathering of the paleosol samples indicates a constant semiarid and subhumid climatic range in northern china since the early miocene components 9791 words 8 figures 3 tables

re os isotope systematics of the taklimakan desert sands moraines and river sediments around the taklimakan desert and of tibetan soils abstract reported here are the first 187os 188os ratios and abundances of os and re for taklimakan desert sands and glacial moraines from the kunlun mountains osmium isotopic data are also reported for river sediments around the taklimakan desert river sediments from the kunlun and tianshan mountains tibetan soils and loesses from the loess plateau as well as sr and nd isotopic data for these samples the taklimakan desert sands from various regions show surprisingly homogeneous os isotopic ratios 187os 188os 1 29 0 08 and abundances os 11 ppt with some variations in re abundances re 130 to 260 ppt and 187re 188os ratios 60 to 140 the 187os 188os ratios for the taklimakan desert sands are close to the average for kunlun moraines river sediments around the taklimakan desert sands and the tibetan soils supporting the idea that the taklimakan desert sands are derived from moraines and river sediments around the desert or from tibetan soils and are homogenized by aeolian activity in the desert furthermore the os isotopic data for the sediments studied here are compared with those 187os 188os 1 04 os 32 ppt re 206 ppt 187re 188os 35 of loesses from the loess plateau reported elsewhere and it is concluded that the re os data for the loess can be used as proxy for the upper continental crust

how valid are dynamic models of subduction and convection when plate motions are prescribed a detailed comparison between fully dynamic and kinematic plate formulations has been made in models of mantle convection plate velocity is computed self consistently from fully dynamic plate models with temperature and stress dependent viscosity and preexisting mobile faults in fully dynamic models the flow is driven solely by internal buoyancy while in kinematic models the flow is driven by a combination of the prescribed surface velocity and internal buoyancy only a temperature dependent viscosity close to the effective viscosity determined from the fully dynamic models is used in the kinematic models the two types of models give very similar temperature structures and slab evolutionary histories when the effective viscosity and surface velocity are nearly identical in kinematic plate models the additional work introduced by the prescribed velocity boundary condition is apparently dissipated within the lithosphere and has little influence on the convection under the lithosphere in models with periodic lateral boundary conditions slabs sink into the lower mantle at an oblique angle and this contrasts with the vertical sinking which occurs with reflecting boundary conditions models show that we can simulate fully dynamic models with kinematic models under either periodic boundary conditions or reflecting boundary conditions

mantle convection under simulated plates effects of heating modes and ridge and trench migration and implications for the core mantle boundary bathymetry the geoid and benioff zones summary numerical convection models are presented in which plates are simulated by imposing piecewise constant horizontal velocities on the upper boundary a 4 1 box of constant viscosity fluid and two dimensional 2 d flow is assumed four heating modes are compared the four combinations of internal or bottom heating and prescribed bottom temperature or heat flux the case with internal heating and an isothermal base is relevant to lower mantle or whole mantle convection and it yields a lower thermal boundary layer which is laterally variable and can be locally reversed corresponding to heat flowing back into the core locally when scaled to the whole mantle the surface deflections and gravity and geoid perturbations calculated from the models are comparable to those observed at the earth s surface for models with migrating ridges and trenches the flow structure lags well behind the changing surface plate configurations this may help to explain the poor correlation between the main geoid features and plate boundaries trench migration substantially affects the dip of the cool descending fluid because of induced horizontal shear in the vicinity of the trench such shear is small for whole mantle convection but is large for upper mantle convection and would probably result in the tonga benioff zone dipping to the se opposite to the observed dip for the case of upper mantle convection

mg isotopic composition of carbonate insight from speleothem formation simultaneous high precision measurement of 24 mg 25 mg and 26 mg isotopic compositions were made by multiple collector inductively coupled mass spectrometry mc icp ms relative to the international standard srm980 data are presented on low mg calcite speleothems and their associated host rocks and waters from four caves one in the french alps and three in israel covering various climate conditions in addition data are presented on three dolostones and three limestones from the himalaya the overall variation is 4 13x and 2 14x in n 26 mg and n 25 mg respectively this is 35 times the uncertainty of the measurements and clearly demonstrates that the terrestrial isotopic composition of mg is not unique each speleothem shows a characteristic range of n 26 mg values that are attributed to the isotopic composition of the local water differences between the isotopic composition of mg in the water dripping from stalactites and that of the modern speleothem are interpreted as being due to mg isotopic fractionation during carbonate precipitation in the temperature range of 4 18 c the low mg calcite is enriched in light isotopes by 1 35x amu and the dependence on temperature has been found to be less than 0 02x amu c despite various geological settings the n 26 mg of the studied dolostones is 2 0 1 2x higher than the n 26 mg of the limestones all together these results suggest a strong mineralogical control and a weak temperature effect on the mg isotopic composition of carbonate a 2002 elsevier science b v all rights reserved

heavy metal distribution and environmental status of doon valley soils outer himalaya india doon valley is surrounded by two major river systems ganga and yamuna on either side with a water divide passing nearly across the centre of the valley and is sandwiched between two mountain ranges in the fragile ecological systems of the himalayan foothills in total 398 soil samples were collected from the valley in a grid pattern 1 sample per 2 km2 and investigated for their heavy metal cr cu ni pb and zn abundances that are environmentally sensitive comparison of the heavy metal abundances with the contamination threshold values ctv revealed that most of these elemental abundances in doon valley soils fall well within the range of the uncontaminated to slightly contaminated category in the case of cr and ni a sizeable number of samples exceeded the ctv 250 and 100 mg kg 1 respectively with an overall background value of 109 and 52 mg kg 1 respectively sites of high cr and ni mostly occur in the ganga catchment gc sector that includes even relatively undisturbed forestland the source of this contamination is attributed to geological factors which indicate contribution from the mafic volcanics of the lesser himalaya this is also consistent with the distribution pattern of mn and fe though their abundance levels are not alarming the background concentration of pb is low 22 mg kg 1 in doon valley soils however signs of gradual pb contamination are palpable in and around the centre of the dehra dun city and along the highways aluminium normalized heavy metal ratios were found to exhibit narrow variability in the case of cu ni and cr and had good correlation with al indicating their affinity and association with the clay minerals on the other hand pb and zn seem to be associated with non silicate sources

local background concentrations of trace elements in soils a case study in the grand duchy of luxembourg abstract background concentration values of trace elements were determined for soils in the southern part of the grand duchy of luxembourg which are developed on the bituminous shale bs the so called bifrons shale and the minette m fe oolitic sandstone substrates sampling sites with minimal anthropogenic influence were selected and soil profile samples were analysed for major and trace element contents a clear distinction is seen between the trace element content of the soils developed on the minette sandstone on the one hand and those developed on shales on the other hand for the minette soil samples most elements are present in concentrations exceeding those reported in literature for soils developed upon sandstones with for example values of up to 278 mg kg 1 as median value 123 mg kg 1 287 mg kg 1 cr median value 126 mg kg 1 and 95 mg kg 1 co median value 33 mg kg 1 in the shale soil samples concentrations are generally lower except for cu and mo these elements reach maxima of up to 66 mg kg 1 cu median value 41 mg kg 1 and 59 mg kg 1 mo median value 26 mg kg 1 the results confirm the importance of determining background concentrations locally bivariate analysis shows a linear relationship between fe content and elements such as as co and ni for the minette soils no relationship is found between trace element concentrations and clay or organic carbon oc content but this could be due to the limited variation of these factors among the studied soils a reconnaissance study with regard to the availability of the trace elements in the minette soil samples shows that these elements are quite immobile and hence of minimal threat to the ecosystem

lagging mantle convection the geoid and mantle structure abstract in numerical models of convection incorporating migration of a simulated subduction zone the main descending flow lags far behind the migrating trench and a geoid low is associated with the main descending flow this provides physical plausibility for the suggestion by chase and sprowl that present very long wavelength degree 2 4 geoid lows are associated with mesozoic trench locations and suggests further that the present long wavelength geoid deep mantle structure and hotspot distribution may be straightforward consequences of plate evolution since the upper paleozoic

geochemical differentiation of quaternary sediments from the pampean region based on soil phosphorus contents as detected in the early 20th century abstract data obtained in the early 20th century on the total phosphorus tp content in soil materials from the pampean region have been processed and plotted on maps of the region approximately 1200 subsoil data and 1400 surface horizon data obtained using the same analytical method have been processed according to differences in phosphorus content several areas have been demarcated limits and characteristics of these areas have been correlated with data from the more recent literature regarding the p distribution and with both mineralogical and sedimentological information on surface materials results suggest that a confluence of sediments from different provenances i e the andean range the pampean hills and the brazilian shield is found in the argentine pampean region

a geochemical study of loess and desert sand in northern china implications for continental crust weathering and composition abstract chemical compositional variations of desert sands and loess loess like deposits river deposits and saline lacustrine deposits collected mostly from the southern margin of the taklimakan desert china provide strong constraints on the chemical weathering processes of continental crust and also on the provenance compositions of these quaternary deposits constant but lower concentrations of al relative to the average upper continental crust ucc in these quaternary deposits could be attributed to chemical weathering of al rich minerals such as feldspar and mica and removal of clay minerals by wind and water large variations of mobile na ca mg and sr compared to relatively immobile al k and ba resulted from the chemical weathering of plagioclase and carbonate minerals salinization and carbonation of the arid lands most trace elements are not significantly fractionated their variations in absolute concentration are caused by the dilution of quartz evaporates and carbonate minerals variations in sn concentrations probably depend on relative contents of some heavy minerals high sr ba low th u k cs ratios of the saline lake deposits suggest differences in aqueous chemistry of these elements the occurrence of the m type tetrad effect a ree pattern with four convex curves of ree compositions found mainly in the river deposits is considered the result of water particle interaction during chemical weathering of crustal materials y ho ratios of these samples are significantly higher than those of chondrite and the ucc this suggests different chemical behavior of these two elements and possibly the enrichment of carbonate materials the low lree hree feature of the saline lake deposits can be interpreted as due to the different aqueous chemical behavior of the ree s however it can also be reasonably considered as the result of inheritance from their provenance since lree and lree hree variations may be related to the geological setting of the source according to some elements that are not significantly fractionated the average provenance composition of these quaternary deposits are probably characterized by enrichments of ni cr and v over fe higher la th lower la sc and th sc and lower ta w and nb w ratios compared with the ucc composition

sediment bound metallic contaminants in sydney s estuaries and adjacent offshore australia abstract almost one quarter of australia s population of 17 million and a large proportion of its industry are concentrated in the restricted area of greater metropolitan sydney heavy metal loadings of the surficial sediment which mantles the four deeply incised estuaries dissecting this region and the adjacent continental shelf are being investigated to determine potential sources and to understand the processes of dispersion and accumulation the parramatta river port jackson and georges river botany bay estuaries have been major repositories for urban industrial waste and are heavily impacted by anthropogenic metalliferous cu zn ni pb loadings the majority of estuarine areas are greatly in excess of background values the most elevated regions are the upper reaches of estuaries and bay ends generally the central sections of these estuaries are less elevated but still above background values whereas for some metals the lower reaches and mouth areas approach background values in limited parts of the inner shelf most of these metals are above background values but for the majority of the continental shelf values are close to background the strong down estuary decline in metals and lower metal levels in the marine environment is probably source related rather than due to a dilution by less enriched coarse sediment the source of offshore metal enrichment is coastal sewage outfalls and or dissolved and suspended material escaping the estuaries marginally elevated metal concentrations on the inner shelf in excess of that observed in the lower estuary are probably indicative of some sourcing from sewage outfalls in terms of public health the majority of the parramatta river port jackson estuary is in excess of guidelines and tributaries and bay ends of both estuaries are markedly in excess of these levels for all elements offshore surficial sediments are mostly within guideline levels

bending stresses in subducted lithosphere summary the flow of the mantle near subducted lithosphere applies a force dynamic load to the slab which has a normal component as well as a shear component in addition the greater density of the slab compared to the surrounding mantle results in a downward force negative buoyancy which also has a component normal to the slab because the slab is long compared to its thickness the stresses in the slab due to the normal components of the loads are accurately estimated from the bending moment we have used previous estimates of the loads to compute the bending moment in the slab as a function of distance down the slab our results indicate that the loads applied to the slab are large compared to its strength bending moments due to normal loads will not be transmitted over the length of the slab instead the slab will deflect so that the normal loads are balanced locally

high rate sea level change during the mesozoic new approaches to an old problem abstract it is generally assumed that the effects of tectonism sediment accumulation and compaction and eustasy on accommodation change cannot be untangled on a regional scale hence most reconstructions of past eustatic change have focused on global correlations here this approach is questioned in the time domain of few my and less global cycle correlation is often unreliable and the effects of isostasy on sea level show strong provincial variations in contrast on a regional scale cycle correlation is more precise and regional tectonism has predictable limitations these considerations form the base for an assessment of the poorly understood causes of high rate sea level change in the mesozoic three well documented regional semi quantitative sea level curves are chosen here for reference jurassic of britain cretaceous of russia albian of oman the absolute numbers of amplitudes are open to question but the order of magnitude rates of several meters to tens of meters per my is a robust property of all three curves even when considering large error bars variable mechanisms that cause these regressive transgressive cycles are considered but rejected with exception of the controversial issue of glacio eustasy or unknown mechanisms therefore orbital forcing of glacio eustasy is assessed freezing melting cycles of higher altitude ice shields can explain high rate sea level change with moderate amplitudes but conclusive evidence is buried beneath 3 km of east antarctic ice thus although several ambiguities of global sea level correlation are avoided by the regional approach the causes of high rate mesozoic sea level change remain poorly understood

behaviour of li isotopes during continental weathering the bidar laterite profile india weathering of large basaltic provinces is a major sink of atmospheric carbon dioxide and is also an important part of the geochemical cycle of many other elements this study aims to improve our understanding of basaltic weathering by analysis of trace elements and li isotopes in a lateritic profile developed upon the deccan traps flood basalt at its base the profile consists of unaltered basalt that has a li concentration and 7li value of 5 ppm and 2 5 respectively which is upwards progressively altered to a highly weathered laterite iron rich residuum at the surface high concentrations of fe2o3 85 wt at 11 m depth indicate the presence of a palaeowatertable the transition metals the rare earth elements rees and u are also strongly enriched in this horizon whereas fluid mobile elements such as li and rb are depleted relative to the precursor basalt and the bulk 7li value is very low 5 5 a high proportion of leachable li is found in this horizon and this is enriched in 6li relative to the bulk we suggest that leaching extracts the labile weathering products and these have high 6li as 7li has been preferentially mobilised during basalt weathering throughout the rest of the profile li concentrations and li isotope values are best explained by two component mixing between the precursor basalt and aeolian material with high li and low 7li mass balance calculations show that at least 50 of the li present in the profile could have been supplied by a mineral aerosol that has a 7li value between 0 and 1 3 the depth integrated 7li value of the profile is 0 6 which suggests that 7li has been preferentially leached from the aerosol after its deposition the source of this aeolian material is most likely to be ancient continental crust it is possibly derived from archaen proterozoic terrain to the south and southeast of the deccan and or material from central asia similar to that which forms chinese loess and which is supplied to the north pacific basin

flow in the mantle inferred from the low degree harmonics of the geopotential summary flow in the earth s mantle is inferred from continental drift its distribution may be obtained from the pattern of zones of compression and tension in the earth s crust and from the low harmonics of the geopotential the origin of the flow is held to be convection the depth to which the flow extends will be discussed the relation of the flow to the earthquake mechanism will be considered