reduction in northward incursions of the south asian monsoon since 1400 ad inferred from a mt everest ice core 1 a highly resolved mt everest ice core reveals a decrease in marine and increase in continental air masses related to relatively high summer surface pressure over mongolia and reduction in northward incursions of the summer south asian monsoon since 1400 ad previously published proxy records from lower sites south of the himalayas indicate strengthening of the monsoon since this time these regional differences are consistent with a south north seesaw in convective activity in the asian monsoon region and reflect a southward shift in the mean summer position of the monsoon trough since 1400 ad the change in monsoonal circulation at 1400 ad is synchronous with a reduction in solar irradiance and the onset of the lia this demonstrates a hemispheric scale circulation reorganization at this time and the potential for future large shifts in monsoonal circulation

the deep structure of venusian plateau highlands abstract magellan gravity data confirm that several of the large tectonically deformed plateau like highlands on venus are shallowly compensated most likely by crustal thickness variations apparent depths of isostatic compensation computed in the spatial domain range from 20 to 50 km for alpha tellus ovda and thetis regiones using a two layer model for isostatic compensation alpha tellus and ovda are best represented as nearly completely compensated in crust that is regionally 20 40 km thick around these highlands with little contribution from deeper mantle sources in contrast to these three areas a stronger regional gravity high associated with thetis requires a significant upper mantle component to compensation this is evident in the spectral admittance as a pronounced deep long wavelength anomaly in the two layer isostatic model a broad deeply compensated upland underlies a shallowly compensated central block of thetis if this deep component is interpreted as a thermal anomaly the loci of maximum upwelling agree well with sites of recent extension alternatively dense mantle could be responsible for the broad gravity anomaly although a different style of dynamic coupling is called for the plateau highlands are thus physiographically and isostatically equivalent to terrestrial continents though probably not compositionally they also share the record of a long tectonic history the large regional gravity anomaly of thetis indicates that active mantle processes continue even beneath some areas tessera thought to be relics of a former geological regime the excellent agreement of modeled crustal thicknesses around alpha tellus and ovda regiones suggests that 20 40 km is a representative global value for the plains such a crust is thicker than previously estimated and about twice as thick as the expected thickness of crust produced at venusian spreading centers if crust was produced mostly at spreading centers during global resurfacing the mantle potential temperature must have been 1800 k about 100 k hotter than predicted if hotspot activity dominated lateral variations in crustal production must be smoothed out by creep of the lower crust

some effects of a dry crustal flow law on numerical simulations of coupled crustal deformation and mantle convection on venus models of coupled crustal deformation and mantle convection on venus are reevaluated in light of recent experimental evidence suggesting that the strength of dry basalt is comparable to that of olivine a previous model which assumed a relatively weak basalt flow law more appropriate for hydrated crust is compared to a similar model that assumes a strong basalt flow law more appropriate for dry crust a principal difference is the timescale for significant crustal thickening of the order of 108 years for the weak basalt model and 109 years for the strong basalt model these results can be understood in the context of theoretical scaling relations for convecting temperature dependent media which imply that in the absence of concentrated zones of near surface weakness of the type associated with plate margins on earth the lithospheric overturn time which sets the timescale required for convective thickening of crust scales dominantiy with the rayleigh number defined by the average viscosity of the lithosphere theoretical scalings as well as numerical models suggest crustal thickening timescales of the order of 108 years remain possible for high bulk mantle rayleigh numbers and effective viscosity contrasts from lithosphere to interior mantle of the order of 103 such timescales imply a relatively thin lithosphere and heat loss comparable to that of the present day earth the large values inferred for the thickness of the mechanical lithosphere on venus if correct would thus tend to favor timescales of the order of 109 years if one assumes relatively low convective mantle heat loss over an equivalent time this in turn would argue against the likelihood of highland plateaus being manifestations of crustal thickening above mantle downflows if instead one assumes that convective heat removal and implied lithospheric overturn rates were significantly higher over the last several hundred million years then highland formation due to crustal thickening cannot be ruled out based solely on the strength of dry crust

effect of state of compensation on the relaxation of crustal plateaus on venus 1 building on nunes et al 2004 which examined the end member case of viscous relaxation of fully compensated crustal plateaus on venus we now investigate viscous relaxation for the other end member case of uncompensated topography the approach consists of modeling viscous flow in a crust mantle system by means of analytic and finite element methods differential stresses produced by uncompensated plateau topography concentrate beneath the entire domain of the plateau and extend deep into the mantle loss of topography occurs via fast subsidence due to isostatic adjustment and a slower lateral flow due to spreading of the thickened crust rims arise from the isostatic adjustment and rim dimensions depend on the thickness of the competent layer controlled by the thermal state of the system rim dimensions after 1 myr are 1 km high and 300 km wide for a surface temperature of 740 k and thermal gradient of 10 k km and 600 m high and 100 km wide for hotter crustal conditions although models predict the marginal circumferential hoop folding observed at crustal plateaus they do not account for the large radial grabens also observed adding a subcrustal zone of time dependent buoyancy causes a delay in the isostatic adjustment during this period of delay viscous flow in a relatively hot crust leads to a reduction in rim amplitude once isostatic adjustment takes place only cooler conditions in this model can reproduce the wide and high rims observed at crustal plateaus on venus consequently phase transitions in the lower crust may produce sufficiently large variations in buoyancy to affect the surface topography and are more compatible with a cooler thermal state but they cannot explain the thickened crust or the interpretation of gravity data showing current isostatic support lateral variation in heat flow and thickness of the competent layer may help address the latter and needs to be explored through both lithospheric and mantle convection models

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

tropical sea surface temperatures for the past four centuries reconstructed from coral archives most annually resolved climate reconstructions of the common era are based on terrestrial data making it a challenge to independently assess how recent climate changes have affected the oceans here as part of the past global changes ocean2k project we present four regionally calibrated and validated reconstructions of sea surface temperatures in the tropics based on 57 published and publicly archived marine paleoclimate data sets derived exclusively from tropical coral archives validation exercises suggest that our reconstructions are interpretable for much of the past 400 years depending on the availability of paleoclimate data within and the reconstruction validation statistics for each target region analysis of the trends in the data suggests that the indian western pacific and western atlantic ocean regions were cooling until modern warming began around the 1830s the early 1800s were an exceptionally cool period in the indo pacific region likely due to multiple large tropical volcanic eruptions occurring in the early nineteenth century decadal scale variability is a quasi persistent feature of all basins twentieth century warming associated with greenhouse gas emissions is apparent in the indian west pacific and western atlantic oceans but we find no evidence that either natural or anthropogenic forcings have altered el nino southern oscillation related variance in tropical sea surface temperatures our marine based regional paleoclimate reconstructions serve as benchmarks against which terrestrial reconstructions as well as climate model simulations can be compared and as a basis for studying the processes by which the tropical oceans mediate climate variability and change

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

a 780 year annually resolved record of indian ocean monsoon precipitation from a speleothem from south oman 1 meteorological records of monsoon rainfall in the indian ocean are generally less than 100 years long the relative brevity of these records makes it difficult to investigate monsoon variation on decadal and centennial timescales to determine what factors influence the intensity of rainfall on these timescales or to place possible changes in the twentieth century into a broader historical context development of a geologic proxy for rainfall that records annual variation in the monsoon over much longer time periods than are covered by instrumental records would be a significant step forward we have developed an annually resolved record of monsoon rainfall variation for the past 780 years based on annual layer thickness and stable isotope analyses of a laminated stalagmite from southern arabia our results show that monsoon variation over the past century is not outside of the range of the past 800 years decreasing monsoon rainfall over the past century is related to increasing sea surface temperature in the indian ocean spectral analyses of the record are dominated by cycles that are similar to those observed in records of solar activity on centurial timescales decadal to interannual cycles in the record appear to originate in the tropical pacific ocean index terms 3344 meteorology and atmospheric dynamics paleoclimatology 3354 meteorology and atmospheric dynamics precipitation 1854 1620 global change climate dynamics 3309 1699 global change general or miscellaneous keywords monsoon paleoclimate rainfall indian ocean speleothem oxygen isotopes

sequential deformation of plains at the margins of alpha regio venus implications for tessera formation abstract the boundaries between the highly deformed tessera terrain and adjacent volcanic plains are primarily those of embayment where the tessera are stratigraphically older than the plains previous studies show that 3 of these boundaries display evidence of tectonic tilting after the emplacement of the plains one of these unusual boundaries is the western margin of alpha regio tessera a zone 100 km in width that separates the plains from the interior structures of alpha this zone is characterized by margin parallel fine scale 1 5 km fractures graben and ridges that truncate and postdate the broad scale 10 30 km ridges and troughs of the interior of alpha the western margin is embayed by several volcanic plains units that are progressively tilted and deformed by graben with closer proximity to alpha regio the earliest deformation of the plains consists of northeast trending graben 1 km in width that are similar in morphology and spacing to graben that deform intratessera plains and plains at the eastern boundary of alpha northwest trending graben then formed over an interval marked by the emplacement of two additional plains units their similarity to northwest trending structures emanating from eve corona and the lada terra rift suggests a possible genetic relationship the tilting of the plains adjacent to western alpha implies relative vertical movement of the margin either uplift of tessera or downwarping of plains subsequent to the formation and relaxation of the interior of alpha regio subsidence of plains at this locale is supported by the presence of a basin to the west of alpha surrounded by a fracture belt contiguous with western alpha thus the fractures and deformation at the western boundary of alpha may be related to the formation of a basin to the west of alpha with some influence from the northernmost extension of the lada terra rift such a basin is not present at a section along the eastern boundary of alpha regio where the origin of tilted plains remains equivocal we conclude that the deformation along the western margin of alpha regio is not directly related to the process of tessera formation but is an example of tessera modification and is consistent with the stratigraphic position of tessera as the oldest unit observed on venus

seasonal and centennial cycles of carbonate mineralisation during the past 2500 years from varved sediment in lake shira south siberia shira lake is a good representative object for detail weather climate modeling due to its local hydroclimatic information and annually laminated bottom sediments carbonate biochemical mineralization in shira lake shows not only seasonal but also centennial pulses over each 450 500 years an age depth model has been built by layer counting combined with 137cs and 14c isotopic dates thermodynamic estimation of the rock water multisystem in conformity with local conditions and source matter provides grounds for interpretation of measured geochemical parameters in sediments including environmental indicators such as temperature salinity and ph scanning x ray fluorescence for sub millimeter microstratigraphic study of varves connects geochemical oscillations of microelement content rb ti ca sr br xrd with seasonality as well as revealing centennial pulses of environmental change quantitative environmental reconstruction using multiple regression for precipitation calibrated by hydrometeorological time series year by year was carried out for the lake over the last 2500 years

evidence for decreasing south asian summer monsoon in the past 160 years from varved sediment in lake xinluhai tibetan plateau 1 we report glacial varves in the sediment of lake xinluhai tibetan plateau independent data of 137cs and 210pb indicate that these are annually deposited varves varves appear as rhythmic units of light colored silt layer capped by a dark clay layer under microscope varve thickness in lake xinluhai is sensitive to precipitation because sediment accumulation is strongly affected by monsoon rainfall in the area a general decreasing trend can be observed in the varve thickness over the past 160 years spectral analyses of the varve record are dominated by cycles which are similar to enso periodicities it implies that the decreasing trend of the south asia monsoon may be linking with enso spatially the decreasing trend can be observed across different proxy records in the south of the tibetan plateau although arguments still remain for the dynamic mechanisms and spatial rainfall difference the south asian summer monsoon could be weakened due to rising temperatures

structural analysis of a sampling of tesserae implications for venus geodynamics abstract understanding the nature and timing of tessera formation is fundamental to venus tectonic and geodynamic models tesserae are commonly considered to exhibit complex deformation histories to represent the oldest global stratigraphic unit to have formed during a global phase of tesserization and to require weak lithosphere for their formation although these characterizations of tesserae are gaining widespread mention in the literature they are essentially unsupported by an assemblage of data yet they figure prominently in the way we frame and address questions about venus tectonics thus they are quickly becoming accepted paradigms despite a lack of scientific foundation open discussion of these hypotheses or assumptions is crucial because of their implications concerning geodynamic models of the surface evolution and planetary dynamics of venus we examined tessera terrain in ishtar terra crustal plateaus and as inliers within the plains using high resolution magellan radar imagery we describe several types of tessera terrain that record a wide range of structural histories fold and s c terrains are found only in ishtar terra ribbon lava flow and basin and dome terrains reside within the interior of crustal plateaus whereas folded ribbon terrain and extended folded terrain comprise margins of crustal plateaus inliers are divisible into fracture dominated and graben dominated tesserae although some inliers host early contractional fabrics the range of deformation histories recorded by the various tessera types indicates that tesserae should not be considered a single map unit tessera deformation records local to regional surface strain patterns and reflects near surface rheology at the time of deformation progressive deformation fabrics in some tessera terrain record changes in shallow crustal rheology through time thus structural analysis of individual tessera types and tessera provinces will allow us to better understand the tectonic processes responsible for tessera formation tesserae likely formed in several tectonic environments including 1 as a result of subsurface flow in ishtar terra 2 as sequences of surface layer extension and contraction in crustal plateaus 3 as flooded crustal plateaus and 4 as densely fractured surface layers fractured as a result of corona and chasma formation

viscoelastic relaxation of topographic highs on venus to produce coronae coronae on venus are believed to result from the gravitationally driven relaxation of topography that was originally raised by mantle diapirs we examine this relaxation using a viscoelastic finite element code and show that an initially plateau shaped load will evolve to the characteristic corona topography of central raised bowl annular rim and surrounding moat stresses induced by the relaxation are consistent with the development of concentric extensional fracturing common on the outer margins of corona moats however relaxation is not expected to produce the concentric faulting often observed on the annular rim the relaxation timescale is shorter than the diapir cooling timescale so loss of thermal support controls the rate at which topography is reduced the final corona shape is supported by buoyancy and flexural stresses and will persist through geologic time development of lower flatter central bowls and narrower and more pronounced annular rims and moats enhanced by thicker crusts higher thermal gradients and crustal thinning over the diapir

interaction between the enso and the asian monsoon in a coral record of tropical climate the oxygen isotopic composition of a banded coral from the western equatorial indian ocean provides a 150 year long history of the relation between the el nino southern oscillation enso phenomenon and the asian monsoon interannual cycles in the coral time series were found to correlate with pacific coral and instrumental climate records suggesting a consistent linkage across ocean basins despite the changing frequency and amplitude of the enso however decadal variability that is characteristic of the monsoon system also dominates the coral record which implies important interactions between tropical and midlatitude climate variability one prominent manifestation of this interaction is the strong amplitude modulation of the quasi biennial cycle

mantle flow tectonics the influence of a ductile lower crust and implications for the formation of topographic uplands on venus convective flow within the interiors of terrestrial planets is a primary source of large scale topographic and tectonic features we treat the crust and mantle as a layered viscous half space and find solutions for flow driven by a buoyancy force distribution within the mantle and by relief at the surface and crust mantle boundary the crust initially responds to mantle upwelling with uplift but continued flow causes significant crustal thinning and subsidence downwelling leads to initial subsidence and subsequent crustal thickening and surface uplift changes in crustal thickness are driven by 1 vertical normal stresses due to mantle flow and 2 shear coupling of horizontal mantle flow into the crust the degree and time scale of crustal deformation are sensitive to crustal thickness and viscosity and the presence of elastic layers in the upper crust and mantle deformation can occur on time scales less than several hundred million years for reasonable values of these parameters strong or elastic upper crustal layers enhance crustal thinning or thickening while large viscosity contrasts between the lower crust and upper mantle or elastic mantle layers tend to diminish deformation for time dependent mantle flow due to a rising or sinking diapiric body crustal deformation depends upon the ratio of diapir radius to crustal thickness and the ratio of crustal to mantle viscosity a sinking diapir can lead to crustal thickening elevated topography and a specific sequence of time varying deformation while regions such as beta regio have long been suggested to result from convective upwelling in the venus mantle we suggest that a mantle downwelling origin is consistent with many of the characteristics of the compressional mountain ranges of western ishtar terra

groundwater discharge to a gobi desert lake during mid and late holocene dry periods abstract high levels of lake eastern juyanze and a wetter climate in the gobi desert nw china were recorded between 5100 and 4100 cal year bp from ostracod distribution shell geochemistry sedimentology and palynological data after about 4100 cal year bp lake levels decreased and groundwater discharge became dominant source of water during short term regressive events at about 4000 3800 and 3500 cal year bp as suggested by a significant difference in modern river water and groundwater chemistry and the geochemical signal of fossil ostracod shells the overall trend towards lower lake levels culminated in repeated episodes of desiccation of lake eastern juyanze between 3200 and 2900 cal year bp individual regressive events as well as the rapid environmental fluctuations of lake eastern juyanze at about 3000 cal year bp were likely caused by mid to late holocene temperature fluctuations recorded by the dunde ice core yao t thompson l g 1992 trends and features of climatic changes in the past 5000 years recorded by the dunde ice core annals of glaciology 16 21 24 and shifts of the southeastern asian monsoon

corona structures on venus models of origin coronae on venus are circular to elongate structures with maximum widths of 150 1000 km characterized by annuli of concentric ridges surrounding complex interiors the features have raised topography relative to the surroundings they are associated with volcanic activity and most are partially surrounded by a peripheral trough variations in morphology between individual coronae are due to differences in their stage of evolution and or differences in the relative significance of the geologic processes that occur in each stage we examine models for three processes that may be involved in corona origin and evolution 1 a hotspot or rising mantle diapir model 2 a sinking mantle diapir model and 3 gravitational relaxation of topography rising mantle diapirs are caused by heating at depth e g hotspot while sinking mantle diapirs may result from cooling or a phase change causing increased density and negative buoyancy at the base of the lithosphere the hotspot model is most consistent with the major characteristics of coronae with gravitational relaxation occurring as a modificational process the sinking mantle diapir would produce dominant central compression that has not been observed at coronae however higher resolution image and altimetry data from magellan can be used to distinguish more fully between the two models coronae in various states of formation and degradation can be identified in the venera 15 16 data suggesting that the process may be continuing today

origin of corona dominated topographic rises on venus abstract both large scale mantle upwellings comparable to terrestrial hotspots on earth and smaller scale mantle upwellings known as coronae occur on venus corona dominated rises have many of the characteristics of large scale mantle upwellings or hotspots such as broad topographic rises greater than 1000 km in diameter and large positive gravity anomalies due to the presence of clusters of three to eight coronae three large volcanic rises or hotspots on venus have been classified as corona dominated rises cdrs themis eastern eistla and central eistla regiones cdrs have been interpreted to result from the break up of a large scale plume comparison of the topographic morphology for individual coronae at themis and eastern eistla region to a model of corona evolution indicate that they are in varying stages of evolution at eastern eistla regio all the coronae have essentially the same topographic morphology consistent with a late stage of evolution and the presence of a depleted mantle layer at depth the complex deformation sequences and stratigraphic relationships both between coronae and with respect to the regional plains observed at all three rises indicate a prolonged origin this observation as well as the varying stages of evolution rule out the previously proposed interpretation of corona dominated rises as a manifestation of the break up of a large scale mantle upwelling which requires essentially simultaneous formation of the coronae instead we suggest that other large topographic rises are the manifestation of deep mantle plumes likely to originate at the core mantle boundary and that cdrs are clusters of coronae that originate at a shallower interface perhaps at an upper lower mantle boundary using top and bottom loading flexural models to fit the gravity topography admittance spectrum for each of the three cdrs yields elastic thickness estimates that are 10 15 km greater for bottom loading at longer wavelengths than top loading at shorter wavelengths estimates of elastic thickness assuming top loading are 10 12 and 22 km and 20 25 and 35 km from bottom loading for eastern eistla central eistla and themis regiones respectively we believe that the bottom loading elastic thickness estimates are more reliable because using a top loading model when both types of loading are present yields an unrealistically low elastic thickness estimate as there is no obvious source of surface loading at either themis or at eastern eistla we interpret the top loading admittance signature to be a result of delamination of the lower lithosphere depressing the surface which is consistent with the observed coronae morphologies

the implications of basalt in the formation and evolution of mountains on venus abstract the highland region of ishtar terra on venus has mountains that reach up to 11 km in height and are thought to be basaltic in composition assuming that dynamic uplift of crust to this height is unlikely we examine the topography produced by an isostatically supported thickening basaltic crust it is found that regardless of whether the crust thickens by crustal shortening or by volcanic construction the high density basalt eclogite phase transition is the limiting factor for producing significant elevation of the mountains the maximum height attained by basaltic mountains depends on the nature of the basalt eclogite phase transition without a phase transition a basaltic crust must thicken to greater than 100 km to reach heights over 10 km an instantaneous phase transition of basalt to eclogite allows a maximum topographic height of less than about 2 km however with a time lag of 100 ma owing to slow rates of solid state diffusion our calculations show that the mountains can reach elevations greater than 10 km only if they are less than 25 ma old higher temperatures within the venusian crust may decrease the extent of the stability fields of high density basalt phases and allow high topography if the thickening crust melts this can occur if the radioactive element concentrations measured on the surface of venus are uniformly distributed throughout the crust the crust thickens to greater than 65 km and the thickened crust is older than about 400 ma the conflicting results of a young age predicted for high basaltic mountains and an almost uniform surface age of 500 ma from crater populations coupled with similarities in bulk physical properties of venus and earth suggest that the basaltic surface composition found at several landing sites on the planet may not be representative of the entire crust we suggest that ishtar terra formed from the collision of continent like highly silicic cratons over a region of mantle downwelling lakshmi planum resulted from the thickening of a basaltic crust and the peripheral mountain belts formed from the collision of granitic cratons that were pulled toward a downwelling region of mantle

high resolution geochemical record for the last 1100 yr from lake toson northeastern tibetan plateau and its climatic implications abstract knowledge of the origin of past climatic variations on annual to decadal timescales is important for assessing the likelihood of the occurrence of similar variations in the future here we present a high resolution 82 m element record obtained using synchrotron radiation x ray fluorescence from the sediments of toson lake in the northeastern tibetan plateau the record which spans the last 1100 yr exhibits variability on an annual to decadal scale the results of principal component analysis of the element dataset indicates that the first three principal components reflect variations in detrital inputs via runoff and possibly eolian activity drought conditions and temperature variations respectively spectral analysis of samples scores on these components reveals strong el nino southern oscillation enso like variability at periods of 3 8 years which indicates that the regional climatic variability was effected by enso events over the last 1100 yr on the centennial time scale the element data and toc record suggest a warm and dry climate during the medieval warm period mwp and a cold and wet little ice age lia the early 20th century was a warm interval and drier than the lia but wetter than the mwp

deformation of the core mantle boundary induced by spherical shell compressible convection we derive the single mode mean field equations for spherical shell convection with variable viscosity and compressibility included core mantle boundary cmb topography produced by whole mantle convection is computed for various rheologies and different thermal boundary conditions at cmb results for long wavelength undulations show the extreme sensitivity of these signatures to rheological and equation of state parameters in order to obtain excess ellipticity of about 1 km the activation energy and volume in the lower mantle should be low and the temperatures at cmb around 4 500 k in accord with recent experimental findings

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

geological evolution of venus rises plains plumes and plateaus crustal plateaus and volcanic rises major physiographic features on venus both formed over mantle plumes crustal plateaus were produced by large degrees of plume melting beneath thin lithosphere the oldest tectonic features in crustal plateaus are ribbon like troughs indicating early uplift and tensile stretching their shallow depths suggest that surface temperature there was higher in the past widespread volcanic plains derived from the broad upwellings of internally heated mantle convection were continually erupted during the time of crustal plateau formation discrete volcanic rises younger than crustal plateaus formed over a thicker lithosphere with less plume partial melting the presence of only one transitional form indicates that the lithosphere thickened rapidly thermal and magmatic models show that the formation of these major features can be tied to an expected change in mantle convective style about 1 billion years ago

localization of gravity and topography constraints on the tectonics and mantle dynamics of venus summary we develop a method for spatio spectral localization of harmonic data on a sphere and use it to interpret recent high resolution global estimates of the gravity and topography of venus in the context of geodynamical models our approach applies equally to the simple spatial windowing of harmonic data and to variable length scale analyses which are analogous to a wavelet transform in the cartesian domain using the variable length scale approach we calculate the localized rms amplitudes of gravity and topography as well as the spectral admittance between the two fields as functions of position and wavelength the observed admittances over 10 per cent of the surface of venus highland plateaus and tessera regions are consistent with isostatic compensation of topography by variations in crustal thickness while admittances over the remaining 90 per cent of the surface rises plains and lowlands indicate that longwavelength topography is dominantly the result of vertical convective tractions at the base of the lithosphere the global average crustal thickness is less than 30 km but can reach values as large as 40 km beneath tesserae and highland plateaus we also note that an earth like radial viscosity structure cannot be rejected by the gravity and topography data and that without a mechanical model of the lithosphere admittance values cannot constrain the thickness of the thermal boundary layer of venus modelling the lithosphere as a thin elastic plate indicates that at the time of formation of relief in highland plateaus and tesserae the effective elastic plate thickness k was less than 20 km estimates of t at highland rises are consistently less than 30 km our inability to find regions with t 30 km is inconsistent with predictions made by a class of catastrophic resurfacing models

environmental history recorded over the last 70 years in biển hồ maar sediment central highlands of vietnam abstract global warming enhances atmospheric moisture loading and will likely affect the east asian monsoon system across vietnam the absence of a long written climate history from vietnam creates a reliance on geological archives of past monsoon history and regional paleoenvironmental changes to provide a framework for evaluating current climatic trends bi n h lake 14 03 n 108 00 e is a volcanic crater i e maar in vietnam s central highlands that has been accumulating sediment since the pleistocene field campaigns between 2016 and 2018 recovered an abundance of gravity and piston sediment cores extending to a depth of 15 m covering approximately the last 30 ka bp the paleoenvironmental interpretation of pleistocene and holocene sediment requires knowledge of modern lacustrine and sedimentary conditions as well as the origin and transport pathways of the catchment material here we focus on a high resolution sedimentological and geochemical reconstruction of the recent environmental history from 1950 ad to the present based on sediment cores from bi n h maar in direct comparison with local and regional weather and historic records the uppermost sedimentary record reflects a substantial anthropogenic influence such as deforestation military use crater breaching dam and sill construction and reforestation that strongly modified the maar s morphology and hydrology a strong increase in sedimentation rate during the 1960 70s in bi n h cores coincided with u s military activities and water withdrawal a breach in the maar s rim and the connection to a new external reservoir in 1983 1984 ad increased bi n h water level reforestation around most of bi n h s rim in the 1990s curbed erosion and sedimentation rate nutrient availability in bi n h declined in the past 10 years after the installation of a concrete sill between bi n h and the reservoir to limit water exchange with the more nutrient rich reservoir this paper is a calibration study on modern sediment to provide an empirical basis for the interpretation of bi n h s deeper holocene and pre holocene sedimentary record

may september precipitation in the bhutan himalaya since 1743 as reconstructed from tree ring cellulose δ18o 1 we developed a 50 year tree ring 18o chronology for each of three tree species juniperus indica larix griffithii and picea spinulosa using a total of 12 trees four trees per species from the bhutan himalaya despite originating from different species sampled at two different altitudes the 18o chronologies are highly correlated with one another r 0 76 0 89 response analyses reveal that tree ring 18o values are controlled mainly by summer precipitation irrespective of species based on these results a robust 269 year 18o chronology was established to reconstruct the amount of may september precipitation based on data from four larch trees our tree ring 18o data show significant correlations with those from other regions of the himalaya and the tibetan plateau indicating that common signals related to monsoon activity are recorded in the data however at centennial timescales our data from bhutan show normal conditions during the 20th century whereas records from sites in western nepal and the southern eastern tibetan plateau show weakening trends in monsoon intensity during the last 100 200 years the weakening trends may be the result of a reduction in the meridional sea surface temperature gradient in the indian ocean during this time at continental scales the tree ring records show that areas more from ocean basins are particularly sensitive to reduced monsoon circulation correlation analyses suggest that the el nino southern oscillation enso plays an important role in modulating summer precipitation however the teleconnected relationship disappears during the period 1951 1970 coinciding with a negative phase of the pacific decadal oscillation pdo implying interdecadal modulation of the pdo on the influence of the enso on precipitation in bhutan

venusian highlands geoid to topography ratios and their implications abstract using pioneer venus line of sight gravity data and orbit simulation procedures we have estimated apparent depths of isostatic compensation adcs for twelve venusian highland features asteria atla bell beta ovda phoebe tellus thetis and ulfrun regiones and nokomis gula and sappho montes adcs range from 50 km to 270 km half of the values are less than 100 km using these adcs we estimate geoid to topography ratios gtrs for each area to allow comparison with convection calculations and with terrestrial data for oceanic hot spots swells and plateaus the geoid is estimated in the wavenumber domain from the isostatic formula using the topography and adc for each region in the space domain the gtr is equal to the least squares slope of the linear fit of the geoid to the topography the resulting gtr range is 7 31 m km which is much higher than terrestrial oceanic values 1 to 5 m km the features fall into two distinct groups one with a gtr range of 7 13 m km and one with a range of 19 25 m km the exception is beta regio which has a gtr of 31 m km a model for thermal thinning of a 100 km thick lithosphere fits all values in the lower gtr group to within one standard deviation airy compensation could also be present but cannot fully compensate these features partial dynamic compensation of the lower gtr group in combination with lithospheric mechanisms is also possible but not required to fit the data the upper gtr range 19 25 m km can be fit with an upper mantle constant viscosity convection model the large gtr values are inconsistent with the presence of a low viscosity zone if more than one compensation mechanism is present in the regions in the higher gtr group the gtrs will be underestimated in terms of a dynamic interpretation we thus fit the convection models to the upper end of the gtr range 25 m km rayleigh numbers in the range 10 4 10 6 will produce a gtr of 25 m km when combined with conductive lid thicknesses of 85 150 km the 6 m km range in both of the gtr groups is probably due to varying degrees of crustal and thermal compensation combined with dynamic compensation in the upper gtr group the difference between terrestrial and venusian gtr ranges can be explained largely by the lack of a low viscosity zone on venus

global tectonic evolution of venus from exogenic to endogenic over time and implications for early earth processes venus provides a rich arena in which to stretch one s tectonic imagination with respect to non plate tectonic processes of heat transfer on an earth like planet venus is similar to earth in density size inferred composition and heat budget however venus lack of plate tectonics and terrestrial surficial processes results in the preservation of a unique surface geologic record of non plate tectonomagmatic processes in this paper i explore three global tectonic domains that represent changes in global conditions and tectonic regimes through time divided respectively into temporal eras impactors played a prominent role in the ancient era characterized by thin global lithosphere the artemis superstructure era highlights sublithospheric flow processes related to a uniquely large super plume the fracture zone complex era marked by broad zones of tectonomagmatic activity witnessed coupled spreading and underthrusting since arrested these three tectonic regimes provide possible analogue models for terrestrial archaean craton formation continent formation without plate tectonics and mechanisms underlying the emergence of plate tectonics a bolide impact model for craton formation addresses the apparent paradox of both undepleted mantle and growth of archaean crust and recycling of significant archaean crust to the mantle this article is part of a discussion meeting issue earth dynamics and the development of plate tectonics

paleoclimatic fingerprints of ferromanganese nodules in subtropical chinese soils identified by synchrotron radiation based microprobes abstract ferromanganese nodules are important pedogenic indicators in soil they are usually characterized by clearly expressed ring structures although these features are believed to be significant environmental fingerprints the reasons for their formation still await interpretation here we interpreted these fingerprints by the synergistic use of noninvasive synchrotron radiation based techniques and advanced analytical methods the results revealed that the soil nodules were formed due to the filling of the pore networks of the soil matrix that was mediated by both abiotic and biotic factors during alternative soil wetting drying cycles the ring structures are therefore the memories of the soil redox history the variations in elemental compositions of the ring structures indicates the soil moisture conditions at the time of formation their porosity reflects the rate of change in soil redox potential and the soil redox potential is mainly affected by precipitation

magellan observations of alpha regio implications for formation of complex ridged terrains on venus magellan images of alpha regio reveal previously undetected structures and details of the morphology of this region of complex ridged terrain we examine the complex ridged terrain of alpha regio using morphology and crosscutting relationships between structures to derive a sequence of tectonic events structures include broad 10 20 km wide linear and arcuate ridges fine scale 3 km wide ridges linear disruption zones ldzs up to several kilometers wide and numerous grabens 5 km wide and associated scarps and troughs based on their morphology we interpret the broad and fine scale ridges as compressional structures possibly folds ldzs appear to be due to small amounts of lateral shear which most commonly disrupts the older ridge fabric graben and associated structures are interpreted as extensional features they crosscut ridges and ldzs and thus appear to be the youngest structures in alpha regio this sequence of events and information on the orientation of these various structures are compared to the predictions of two models for the formation of complex ridged terrain and highlands on venus in general a hotspot model and a coldspot model the presence of compressional features along and parallel to the margins of alpha regio and the lack of any high elevation ring of extensional features are more consistent with a coldspot or roughly axisymmetric mantle downwelling mantle downwelling appears to be the most likely mode of formation of the upland of alpha regio and is likely to be important in other highland regions such as ovda and thetis regiones which are also dominated by complex ridged terrain

tessera terrain and crustal plateaus venus many workers assume that tessera terrain marked by multiple tectonic lineaments and exposed in crustal plateaus comprises a global onion skin on venus a growing body of structural mechanical magmatic gravitational topographic and geologic evidence indicates that tesserae record the local interaction of individual deep mantle plumes with an ancient globally thin venusian lithosphere resulting in local regions of thickened crust

equilibrium resurfacing of venus results from new monte carlo modeling and implications for venus surface histories abstract venus impact crater population imposes two observational constraints that must be met by possible model surface histories 1 near random spatial distribution of 975 craters and 2 few obviously modified impact craters catastrophic resurfacing obviously meets these constraints but equilibrium resurfacing histories require a balance between crater distribution and modification to be viable equilibrium resurfacing scenarios with small incremental resurfacing areas meet constraint 1 but not 2 whereas those with large incremental resurfacing areas meet constraint 2 but not 1 results of monte carlo modeling of equilibrium resurfacing strom et al 1994 is widely cited as support for catastrophic resurfacing hypotheses and as evidence against hypotheses of equilibrium resurfacing however the monte carlo models did not consider intermediate size incremental resurfacing areas nor did they consider histories in which the era of impact crater formation outlasts an era of equilibrium resurfacing we construct three suites of monte carlo experiments that examine incremental resurfacing areas not previously considered 5 1 0 7 and 0 1 and that vary the duration of resurfacing relative to impact crater formation time 1 1 suite a 5 6 suite b and 2 3 suite c we test the model results against the two impact crater constraints several experiments met both constraints the shorter the time period of equilibrium resurfacing or the longer the time of crater formation following the cessation of equilibrium resurfacing the larger the possible areas of incremental resurfacing that satisfy both constraints equilibrium resurfacing is statistically viable for suite a at 0 1 suite b at 0 1 and suite c for 1 0 7 and 0 1 areas of incremental resurfacing

800 yr long records of annual air temperature and precipitation over southern siberia inferred from teletskoye lake sediments a unique 800 yr long record of annual temperatures and precipitation over the south of western siberia has been reconstructed from the bottom sediments of teletskoye lake altai mountains using an x ray fluorescence scanner xrf providing 0 1 mm resolution timeseries of elemental composition and x ray density xrd br content appears to be broadly correlative with mean annual temperature variations because of changes in catchment vegetation productivity sr rb ratio reflects the proportion of the unweathered terrestrial fraction xrd appears to reflect water yield regime and sediment flux sedimentation is rather continuous because annual clastic supply and deposited mass are the same the artificial neural networks method was applied to convert annual sedimentary time series of xrd br content and sr rb ratio to annual records of temperature and precipitation using a transfer function comparison of these reconstructed siberian records with the annual record of air temperature for the northern hemisphere shows similar trends in climatic variability over the past 800 yr estimated harmonic oscillations of temperature and precipitation values for both historical and reconstructed periods reveal subdecadal cyclicity

a 2 300 year long annually resolved record of the south american summer monsoon from the peruvian andes decadal and centennial mean state changes in south american summer monsoon sasm precipitation during the last 2 300 years are detailed using an annually resolved authigenic calcite record of precipitation 18o from a varved lake in the central peruvian andes this unique sediment record shows that 18o peaked during the medieval climate anomaly mca from a d 900 to 1100 providing evidence that the sasm weakened considerably during this period minimum 18o values occurred during the little ice age lia between a d 1400 and 1820 reflecting a prolonged intensification of the sasm that was regionally synchronous after the lia 18o increased rapidly particularly during the current warm period cwp a d 1900 to present indicating a return to reduced sasm precipitation that was more abrupt and sustained than the onset of the mca diminished sasm precipitation during the mca and cwp tracks reconstructed northern hemisphere and north atlantic warming and a northward displacement of the intertropical convergence zone itcz over the atlantic and likely the pacific intensified sasm precipitation during the lia follows reconstructed northern hemisphere and north atlantic cooling el nino like warming in the pacific and a southward displacement of the itcz over both oceans these results suggest that sasm mean state changes are sensitive to itcz variability as mediated by western hemisphere tropical sea surface temperatures particularly in the atlantic continued northern hemisphere and north atlantic warming may therefore help perpetuate the recent reductions in sasm precipitation that characterize the last 100 years which would negatively impact andean water resources

the significance of maar volcanoes for palaeoclimatic studies in china abstract the sediments of maar lakes often provide paleoenvironmental records which are highly suitable for palaeoclimatic studies the main advantages of the records from maar lakes compared with other lacustrine records are as follows first the presence of varves annually laminated layers and tephra layers as well as other materials like terrestrial plant macrofossils charcoals organic carbon organic shells suitable for isotopic dating like ams 14c 210pb 137cs 238u 230th 40ar 39ar provides the possibility of developing several independent chronologies enabling the development of robust high resolution chronologies second maars and maar lakes can provide continuous sedimentary records spanning tens of thousands of years tianyang maar in china is hosted the longest known record spanning at least 400 kyr bp that are ideal for the investigation of climate change on different timescales including orbital millennial centennial and annual decadal third recent studies of biomarkers based on long chain alkenones lcas and glycerol dialkyl glycerol tetraethers gdgts have demonstrated the potential of maar lakes for quantitative paleotemperature reconstruction in this study we review some of the more significant achievements of palaeoclimatic research based on maar lake sediment sequences from china with the focus on chronology multiple timescales and quantitative paleoclimatic reconstruction

tree ring cellulose δ18o of fokienia hodginsii in northern laos a promising proxy to reconstruct enso 1 a tree ring cellulose 18o chronology of fokienia hodginsii from 1951 to 2002 is established by four cores from four different trees to explore its potential for reconstructing climatic variations in northern laos this chronology is the first tree ring 18o chronology from laos as well as the first for this species we compare variations of tree ring cellulose 18o and ring width between cores the results suggest that the 18o time series are more strongly correlated between different trees than ring width time series are in addition correlation analysis shows a significantly negative correlation between tree ring cellulose 18o and the may october palmer drought severity index pdsi while the tree ring width indices have a poor correlation with pdsi the spatial correlation analysis between tree ring cellulose 18o and global sea surface temperature and the time series correlation analysis between tree ring cellulose 18o and the multivariate el nino southern oscillation enso index mei during the investigated period 1951 2002 reveal that enso has an important effect on tree ring cellulose 18o in laos our results indicate that the tree ring cellulose 18o of fokienia hodginsii is a promising proxy to reconstruct pdsi in southeast asian countries such as laos and to discuss their teleconnections with global atmospheric circulations such as enso

calibration of xrf core scanners for quantitative geochemical logging of sediment cores theory and application abstract on line analysis of split sediment cores by xrf core scanners has become increasingly popular in the past decade because it allows nondestructive extraction of near continuous records of element intensities from sediment cores with a minimum of analytical effort a disadvantage of xrf core scanning relative to conventional geochemical analysis is the problematic conversion of core scanner output to element concentrations the main reason for this long standing problem is the poorly constrained measurement geometry attributable to inhomogeneity of the specimens e g variable water content and grain size distribution irregularities of the split core surface and in some setups spatial variations in thickness of an adhesive pore water film which forms directly below a protective foil covering the core surface we propose a log ratio calibration model for xrf core scanners derived from a combination of xrf spectrometry theory principles of compositional data analysis and empirical evidence the log ratio calibration model provides accurate and precise predictions of sediment composition element concentrations from xrf core scanner output with a limited number of parameters namely 2 d 1 where d equals the number of chemical elements whose concentrations are to be estimated the model can accommodate the inherent non linearity of the relation between relative intensities and concentrations which is apparent from the fact that it provides unbiased estimates an immediate corollary of our results is that log ratios of element intensities which are related to log ratios of element abundances by a simple linear transformation provide the most easily interpretable signals of relative changes in chemical composition consistent use of log ratios of element intensities or concentrations should minimise the risk of drawing erroneous conclusions from geochemical proxies the relative standard deviation precision of predicted element concentrations in core geob7920 is less than 2 stochastic simulations indicate that this level of precision can be attained with 40 randomly selected calibration specimens improved control over input errors and development of robust goodness of fit statistics allows xrf core scanning to be developed into a rigorous quantitative measurement technology the log ratio calibration equation derived in this study may be adapted to inter laboratory and inter instrument calibration as well

the evolution of impact basins viscous relaxation of topographic relief we evaluate the hypothesis that viscous relaxation has been an important process for modifying the topographic profiles of ancient large impact basins on the moon we adopt a representative topographic profile of the orientale basin the youngest large impact basin on the moon as an estimate of the initial topography of older basins of similar horizontal dimensions and we predict the topographic profiles that would result from viscous relaxation according to a number of simple analytical representations of the rheological response of the moon to surface topography at wavelengths greater than the thickness of a high viscosity lithosphere both a decrease in viscosity with depth and the partial to complete isostatic compensation of topographic relief have pronounced effects on the wavelength dependent relaxation times and must be considered in modeling viscous relaxation for features as large as impact basins the effect of a decrease in viscosity with depth is to enhance the rate of viscous relaxation at long wavelengths while the rate of relaxation decreases substantially for the fraction of long wavelength topography which is isostatically compensated as an initial condition these models are applied to two pre nectarian basins on the moon the tranquillitatis basin on the lunar nearside and the much larger and older south pole aitken basin on the farside the topographic profile of tranquillitatis after correcting for the effect of mare basalt fill is consistent with significant viscous relaxation of relief prior to the oldest episode of mare volcanism preserved as a surface unit the large topographic relief of the larger farside basin on the contrary is not consistent with significant viscous relaxation and implies a mean crustal viscosity at least a factor of 10 higher than for the central nearside over the time interval during which substantial viscous relaxation of older nearside basins such as tranquillitatis probably occurred a difference in typical crustal temperature profiles between the farside and nearside is the most likely explanation of such a viscosity difference such a nearside farside asymmetry in near surface temperature may date from the time of crust mantle differentiation and may have persisted because of the different histories of late stage impact basin formation and mare volcanism on the two hemispheres

p and pcp travel time tomography for the core mantle boundary pcp arrival times reported to the international seismological centre isc are inverted for vertical travel time anomaly at the core mantle boundary cmb we first invert about 2 106 isc first p arrival data for the three dimensional 3 d structure of the mantle and hypocenter locations iteratively the pcp data are corrected for the relocated hypocenters and the 3 d structure this correction has reduced the observed pcp residuals the corrected pcp data are then inverted for the vertical travel time anomalies at the cmb which are due either to velocity anomaly or topographic anomaly the spherical average of the inverted structure requires either a velocity reduction in the boundary layer just above the cmb by 3 if the layer is 20 km thick or a lowering of the cmb by 3 km the heterogeneity pattern of the cmb is drastically different from that of the overlying d layer negative vertical travel time anomalies appear in the northern high latitude region and positive anomalies appear under southeast asia and middle to south america the lateral dimensions of these anomalies are of the order of 40 the anomalies can be interpreted as either a velocity perturbation in the boundary layer 7 if the layer is 20 km thick or an undulation of the cmb 7 km various tests including the error and resolution estimations indicate that these three anomalies are the significant resolvable features of the cmb these anomalies are also corroborated by the result of the joint inversion of p and pcp for both the 3 d mantle and the 2 d cmb assuming that the layer is laterally heterogeneous in iron content and deforms at the base in isostatic equilibrium with the outer core the pcp data are also inverted for the isostatic figure of the boundary layer the velocity perturbation in this case is 5 6 if the layer is 20 km thick and the cmb undulation is only 1 2 km regardless whether the cmb heterogeneity is due to velocity and or topographic perturbations it is dominant in the zonal component of degree 2 while the degree 2 pattern in the lower mantle including the d is dominant in sectorial component

a 400 year record of hydroclimate variability and local enso history in northern southeast asia inferred from tree ring δ18o abstract we present here a chronology of tree ring cellulose 18 o for the period 1588 2002 based on individual measurements of seven fokienia hodginsii trees growing in northern laos response function analysis of meteorological data revealed that this oxygen isotope chronology has a significant negative correlation with monsoon season precipitation the water level of the mekong river and the palmer drought severity index pdsi our reconstructed monsoon season pdsi which accounts for 41 5 of pdsi variance showed that wetter phases occurred during the periods ad 1660 1695 and ad 1705 1790 that the main drier periods were ad 1630 1660 ad 1900 1940 and ad 1954 2002 and that there has been a trend of decreasing moisture during the monsoon season over the last 200 years a reduction in monsoon activity can also be seen in various tree ring oxygen isotope records from the himalaya tibet plateau and southeast asia rising sea surface temperatures over the tropical pacific and indian ocean could be responsible for this reduction in the asian summer monsoon by combining proxies sensitive to the el nino southern oscillation enso in northern laos and vietnam we were able to reconstruct the annual multivariate enso index mei and local enso event history and so improve our understanding of long term variations in enso and its influences on southeast asia

long term retention of impact crater topography on ganymede previous analyses into viscous relaxation of impact craters on ganymede have predicted short characteristic relaxation times of the order of 100 myr or less for larger craters in disagreement with the apparent great ages of ganymede s terrains by applying improved understanding of rheologic parameters and initial crater shapes to a viscoelastic model we calculate upper limit zero heat flow relaxation times in excess of the age of the solar system eliminating this discrepancy these very long relaxation times are not due to elastic effects but are simply due to the effective viscosity of water ice at the appropriate temperatures and stresses

seismic velocities at the core mantle boundary inferred from p waves diffracted around the core the very base of the mantle is investigated with core diffracted p wave pdiff travel times published by the international seismological centre isc for the period 1964 1987 apparent slownesses are computed for two station profiles using a difference method as the short period pdiff mostly sample a very thin layer above the core mantle boundary cmb a good approximation of the true velocity structure at the cmb can be derived from the apparent slownesses more than 27000 profiles are built and this provides an unprecedented pdiff sampling of the cmb the overall slowness distribution has an average value of 4 62 s deg which corresponds to a velocity more than 4 lower than that of most mean radial models an analysis of the residuals of absolute isc p and pdiff travel times is independently carried out and confirms this result it also shows that the degree of heterogeneities is significantly higher at the cmb than in the lower mantle a search for lateral velocity variations is then undertaken a first large scale investigation reveals the presence of coherent slowness anomalies of very large dimensions of the order of 3000 km at the cmb a tomographic inversion is then performed which confirms the existence of pronounced 8 10 lateral velocity variations and provides a reliable map of the heterogeneities in the northern hemisphere the influence of heterogeneity in the overlying mantle of noise in the data and of cmb topography is evaluated it seemingly proves minor compared with the contribution of heterogeneities at the cmb our results support the rising idea of a thin low velocity laterally varying boundary layer at the base of the d layer the two principal candidate interpretations are the occurrence of partial melting or the presence of a chemically distinct layer featuring infiltrated core material

relaxation of compensated topography and the evolution of crustal plateaus on venus 1 crustal plateaus dominant physiographic features on venus likely originate through dynamic mantle processes although a debate exists on whether they formed by mantle upwellings or downwellings regardless of the mode of formation several observations led to the hypothesis that viscous relaxation may be the driving force behind the apparent evolutionary sequence from a high standing plateau to a low standing plateau with elevated margins we apply analytic and finite element models to test this hypothesis for isostatically compensated topography as modeling of gravity data suggests that crustal plateaus are presently supported by crustal roots geotherm values 5 k km 1 combined with a surface temperature of 740 k preclude relaxation within 109 years while geotherm values 20 k km 1 can yield relaxation times of 108 years or less hence significant relaxation requires hot conditions in order to occur within the appropriate 1 gyr timescale set by crustal plateau ages we also show that a compensated plateau can either retain its shape as it relaxes or become more domical in appearance mantle temperatures 1400 k allow strong crust mantle coupling that hinders flow of crustal material from the center of the plateau and produces domical relaxed profiles higher temperatures lead to a relatively inviscid mantle and relaxation that is largely insensitive to topographic wavelength thus preserving the original topographic shape during relaxation in either case relaxation of compensated plateaus does not yield elevated rims we propose that the state of compensation must be considered variable

characteristics of sedimentary varve chronologies a review abstract we constructed a varve data base vdb to study the worldwide distribution of varved sediment records and the fidelity of their varve chronologies clusters of lakes with varved sediments exist in central north america in the canadian arctic and in northern and central europe among the 108 sites included in the vdb varved records are typically 200 500 cm long and cover a period of 1000 2000 years their varve chronologies are often based on counting of annual layers from fresh sediment surfaces and photographs or from epoxy embedded sediment blocks and thin sections the vdb indicates that chronological errors associated with counting sedimentary varves fall generally between 1 and 3 but only 57 of published records are providing quantitative error estimations for their chronologies with a careful documentation of varves and by applying radiometric dating methods to surface sediments as well as using historical events as time markers there is the potential of having centennial long varved records with a very precise age depth control however it is unrealistic to expect errors significantly below 1 from several millennia long sections we found no indication that varve chronologies would have been substantially more accurate and precise in some parts of the world than in others or statistically dependent on varve thickness or temporal extension of a varved sequence according to the vdb close to 90 of the published varve chronologies have been cross checked with some independent dating methods

varves in lake sediments a review abstract downcore counting of laminations in varved sediments offers a direct and incremental dating technique for high resolution climatic and environmental archives with at least annual and sometimes even seasonal resolution the pioneering definition of varves by de geer 1912 had been restricted to rhythmically deposited proglacial clays one century later the meaning of varve has been expanded to include all annually deposited laminae in terrestrial and marine settings under favourable basin configurations and environmental conditions limnic varves are formed due to seasonality of depositional processes from the lake s water column and or transport from the catchment area subsequent to deposition of topmost laminae the physical preservation of the accumulating varved sequence requires the sustained absence of sediment mixing for example via wave action or macrobenthic bioturbation individual sub laminae in varved lake sediments typically express contrasting colours always differ in terms of their organic chemical and or mineralogical compositions and often also differ with regard to grain size various predominating climatic and depositional conditions may result in clastic biogenic or endogenic incl evaporitic varved sediments and their mixtures to reliably establish a varve chronology the annual character of laminations needs to be determined and verified in a multidisciplinary fashion sources and influences of possible errors in varve chronologies are best determined and constrained by repeated varve counts and by including radioisotopes and correlation with historically documented events a well established varve chronology greatly enhances the scientific value of laminated limnic archives by securely anchoring the wealth of multi proxy palaeoenvironmental information in the form of time series for multidisciplinary investigations applications of varved records are discussed with special reference to advances since the 1980s these span fields like calibrating radiometric dating methods reconstructing past changes of the earth s magnetic field or detecting fluctuations in solar forcing once a varve chronology is established it can be applied to precisely date events like volcanic ash layers earthquakes or human impact as well as short and long term climate temperature precipitation wind hydroclimatic conditions or flooding and environmental changes eutrophication pollution due to their exceptional high temporal resolution and in combination with their robust and accurate internal time scale in calendar years annually laminated sediments can be regarded as one of the most precious environmental archives on the continents these records are necessary to extend temporally limited instrumental records back in time as such they have societal relevance with regard to risk assessments related to natural hazards arising from e g flooding or volcanic eruptions

observations of pkkp precursors used to estimate small scale topography on the core mantle boundary stacks of global seismic data reveal the time and distance dependence of high frequency 1 hertz precursors to the seismic phasepkkp synthetic seismogram modeling shows that scattering from random small scale topography at the pkkpcore mantle boundary reflection point generates precursory arrivals similar to those seen in the data models with a root mean square core mantle boundary topography of 250 to 350 meters and correlation length of 7 to 10 kilometers explain the main features of the data however a systematic range dependent misfit between observed and predicted precursor power suggests that inner core scattering may contribute to the precursors

tessera terrain ribbon fabrics on venus reviewed could they be dyke swarms abstract endogenic and exogenic models have been developed over the past 25 years to explain the development of extensive pervasive high aspect ratio troughs and graben like structures ribbons and short to long wavelength folds in tessera terrains commonly associated with crustal plateaus on venus many of the models are complex and appear to be both internally contradictory and inconsistent with well established geological principles they are based on what amount to assumptions regarding i the secular evolution of crustal thickness ii crustal mechanical homogeneity and isotropy iii the mechanics of ribbon formation and iv the relative timing of ribbons in the structural geological sequence of tessera terrains on the basis of both theoretical and observational considerations this contribution will suggest that coupling the formation of tessera ribbons to the emplacement of dyke swarms relatively late in the structural development of tessera terrains may provide a resolution for the apparent inconsistencies in those published models

the role of the asian winter monsoon in the rapid propagation of abrupt climate changes during the last deglaciation abstract high resolution temperature records spanning the last deglaciation from low latitudes are scarce however they are important for understanding the rapid propagation of abrupt climate events throughout the northern hemisphere and the tropics here we present a branched gdgts based temperature reconstruction from the sediments of maar lake huguangyan in tropical china the record reveals that the mean temperature during the oldest dryas was 17 8 c which was followed by a two step increase of 2 3 c to the bolling allerod a decrease to 19 8 c during the younger dryas and a rapid warming at the onset of the holocene the oldest dryas was about 2 c warmer than the younger dryas the reconstructed temperature was weighted towards the wintertime since the lake is monomictic and the mixing process in winter supplies nutrients from the lake bottom to the entire water column greatly promoting biological productivity in addition the winter biased temperature changes observed in the study are more distinctive than the summer biased temperature records from extra tropical regions of east asia this implies that the temperature decreases during abrupt climatic events were mainly a winter phenomenon within the limits of the dating uncertainties the broadly similar pattern of winter weighted temperature change observed in both tropical lake huguangyan and in greenland ice cores indicates the occurrence of tightly coupled interactions between high latitude ice sheets and land areas in the tropics we suggest that the winter monsoon especially cold surges could play an important role in the rapid transmission of the temperature signal from the arctic to the tropics

viscous relaxation of impact crater relief on venus constraints on crustal thickness and thermal gradient because of the high surface temperature on venus and the strong temperature dependence of strain rate in silicates solid state creep may be an important mechanism for the reduction of topographic relief on that planet venera 15 16 radar altimetric profiles of impact craters 30 140 km in diameter however indicate crater depths of several hundred meters suggesting that substantial relaxation has not occurred for these features on the basis of models for the viscous relaxation of impact crater topography we derive constraints on the thickness of the crust and the mean lithospheric thermal gradient beneath the craters from the mean and variance of observed crater depth as a function of crater diameter on venus and from estimates of the initial depths of fresh impact craters on venus obtained by scaling from lunar observations we formulate a statistical test for the maximum amount of viscous relaxation that is consistent with the observations at a specified confidence level we develop a general formulation for gravity driven flow in a linearly viscous fluid incorporating the densities and temperature dependent effective viscosities of distinct crust and mantle layers the statistical comparison of predicted and observed depths yields linked upper limits to crustal thickness h and thermal gradient dt dz since dt dz can be estimated from global heat loss arguments or thermal models an upper bound to h may be derived the preserved relief of the largest craters constrain h to be less than or equal to 10 20 km because the craters with measured depths appear to be representative of a larger population of impact structures in terms of regional elevation geological unit and degree of preservation of morphological detail extrapolation of this result to lowlands and rolling plains regions on a global basis is not unreasonable such an extrapolation together with isostatic considerations yields an upper limit to the crustal volume of venus of 1010 km3 this value is an order of magnitude less than the time integrated volume of crust produced on earth and implies either that the average rate of crustal generation has been much smaller on venus than on earth or that some form of crustal recycling has occurred on venus

gravitational spreading of high terrain in ishtar terra venus magellan altimetry measurements indicate that the mountain belts and plateau margins of ishtar terra have topographic slopes in the range 2 30 magellan radar images reveal that numerous sets of narrow closely spaced troughs and lineations which we interpret as graben and normal faults are located in many of these regions of high slope the orientation of many of these graben sets perpendicular to the downslope direction suggests that they formed as a result of gravitational spreading in several of the mountain belts the extensional structures are parallel to the apparent direction of shortening consistent with the interpretation that gravitational spreading occurred while mountain building was still active to explore the implications of this spreading for the tectonic evolution of ishtar terra we model the process by means of a finite element algorithm for viscoelastic deformation in a vertical section of the crust at the margin of a plateau or broad mountain range flow in the crust is assumed to be governed by a nonlinear depth dependent rheology we predict brittle failure and relaxation of topographic relief as functions of time for ranges of crustal thickness 10 30 km and thermal gradient 5 25 k km and for observed ranges in topographic elevation 1 6 km and slope 1 30 this parameter range predicts a large spread in the time scales for initial failure and significant relaxation of relief for a crustal thickness greater than 10 km a thermal gradient of 15 k km or more and average values of relief 3 km and slope 3 the topography relaxes to 25 of its original height in less than 10 m y although initial failure in the form of normal faulting on the highlands and margin and occasional shallow thrusting in the lowlands is predicted to occur much earlier values of horizontal surface strain that are likely to be observable 1 do not accumulate until significant relaxation of relief begins given the predicted rate of topographic relaxation either the crust of ishtar terra must be anomalous in some way either much stronger than predicted by standard flow laws or very thin or cold or the topographic relief and high slopes have been actively built and maintained until times significantly younger than the average crater retention age for venus of 500 ma the large apparent depth of compensation the abundance of volcanism and the uncertainty in the local age favor the view that mountain building in ishtar terra has occurred until times at least as recent as 10 ma

tessera terrain on venus a survey of the global distribution characteristics and relation to surrounding units from magellan data the tessera terrain on venus comprised of areas of high radar backscatter complex deformation patterns relative to other units and topography standing higher than surrounding plains covers 35 33 106 km2 about 8 of the surface of venus and is nonrandomly distributed being preferentially located at equatorial and higher northern latitudes with a distinct paucity below about 30 s individual tessera occurrences range in area from the lower limits of our measurements about 200 km2 up to the largest tessera ovda with an area of about 8 6 106 km2 or about 2 of the surface area of venus the size frequency distribution of tessera patches is strongly unimodal and skewed toward smaller sizes reflecting the great abundance of small tessera fragments modes of occurrence include 1 large clusters e g aphrodite terra and ishtar terra 2 arc like segments which may extend for thousands of kilometers and are either concave inward toward the major tessera cluster development or away from it 3 areas where tesserae are rare or absent which occur both as low lying plains e g guinevere planitia and as elevated regions e g atla regio tessera terrain has a bimodal elevation frequency distribution with the main peak at about 0 1 km and an additional peak at about 3 km above mean planetary radius in terms of number of occurrences however tesserae do not display a correlation with elevation at the global scale since small tessera patches commonly occupy low lying regions although tessera exhibit a range of gravity signatures many occurrences are interpreted to represent relatively shallow crustal levels of compensation tessera boundaries include type i sinuous embayed dominated by adjacent lava plains embaying tessera massifs 73 of total tesserae boundaries and type ii linear tectonic only a small percentage of the length of all boundary types show no lava embayment and could be interpreted as tectonically active for long periods subsequent to initial tessera formation occurrence of broad slopes of post tessera embayed plains away from tessera boundaries suggests that regional tilting occurred subsequent to final tessera deformation in some places several lines of evidence suggest the possibility that a widespread tessera like basement comprising at least 55 of the surface of venus is buried under a cover of lava plains a few hundred meters to as much as 2 4 km thick a wide variety of deformational structures and patterns is observed within the tessera including those representing extension compression shear and transpression in some cases the apparently complex patterns can be resolved into single event kinematic interpretations involving noncoaxial deformation e g itzpapalotl while in other cases polyphase deformation is more likely e g central ovda and thetis where relations can be determined stratigraphically earliest deformation within the tessera is primarily related to crustal shortening and compression phase i followed by pervasive extensional deformation commonly oriented normal to the strike of phase i features generally along the same principal stress direction phase ii evidence also exists for the contemporaneous formation of these distinctive deformation patterns lava plains within and adjacent to the tessera embay both of these fabrics but sometimes overlap in time with phase ii extensional deformation and with regional tilting tessera terrain as a geologic unit occupies the lowest portion of the stratigraphic column in all areas that we have observed an observation consistent with many other mapping studies we see no evidence for transitional stages between tessera and volcanic rises and or lowlands that might represent a long term sequence of upwelling or downwelling followed by crustal deformation and tessera formation no impact craters deformed by phase i deformation have yet been observed on tessera suggesting that phase i tessera deformation sufficiently intense to eradicate earlier impact craters ceased relatively abruptly somewhat before 300 500 m y ago however the starting time and thus duration of tessera formation is unknown on the basis of the very small number of on tessera craters deformed by phase ii extensional deformation this period probably did not last more than several tens of millions of years after the cessation of phase i little observable deformation of the tessera terrain appears to have taken place in the last several hundred million years during which time the vast volcanic plains were emplaced although tilting of early plains along some tessera margins is observed building on the global synthesis presented here future analyses of individual tessera occurrences will provide the detailed descriptions kinematic interpretations and strain histories necessary to assess and distinguish between the several catastrophic and uniformitarian models for tessera formation

global mapping of crustal and lithospheric thickness on venus 1 we have produced a high resolution global admittance map of venus using a spatiospectral localization technique calculating 64 800 admittance spectra evenly spaced in latitude and longitude we grouped the admittance spectra into 35 classes that we inverted for elastic crustal and lithospheric thickness using bottom loading top loading and short wavelength top loading models with long wavelength bottom loading hot spot models our best fit models identify 17 classes consistent with top loading 40 of the surface 7 with bottom loading 39 and 4 with a long wavelength bottom loading model 15 the remaining seven classes 6 of the surface generally had large amplitude top loading signatures that could not be fit with our models for the classes that were successfully modeled the estimated range of crustal 0 90 km and elastic thickness 0 100 km is larger than results for most previous studies primarily because of the incorporation of bottom loading nearly half of the planet has elastic thickness 20 km which we cannot distinguish from isostasy suggesting that these regions are tectonically inactive a key finding is that the lithospheric properties typically vary over scales of 1000 km or less except in some plains and crustal plateau regions both the scale and range of crustal and elastic thickness variations indicate that venus remains an active planet and that geologic processes may be more complex than previously recognized in many regions the correlation with surface geology is not obvious and this suggests that deformation of the lower lithosphere and crust may occur with little surface manifestation

structural and kinematic analysis of eastern ovda regio venus implications for crustal plateau formation crustal plateaus on venus constitute one of that planet s most intriguing tectonic features they host so called tessera terrain or deformed crust exhibiting two or more sets of intersecting tectonic lineaments which comprises 8 of venus surface major questions surrounding crustal plateaus and tessera terrain include 1 what is the nature of crustal plateau deformation what structures are present and what are their spatial and temporal relations 2 what is the global distribution of tessera terrain does it form a global layer under the regional plains cropping out in crustal plateaus or is it confined to crustal plateaus and 3 what are the implications of 1 and 2 for understanding the processes responsible for formation and deformation of crustal plateaus structural and kinematic analyses reveal that deformation at ovda regio venus largest crustal plateau is characterized by linear shallow steep sided troughs ribbons long arcuate long wavelength folds short mid wavelength folds and wide complex graben temporal relations between structures lead to a three phase regional strain history consisting of 1 layer normal flattening of a thin competent membrane resulting in radially oriented distributed extension ribbon formation 2 modest contraction of a thicker layer concentric folding at plateau margins interference folding in the plateau interior and 3 limited extension localized at fold crests graben formation wavelength analysis of ribbons and folds places limits on the thickness of the competent layer during each stage of deformation and reflects an early shallow brittle ductile transition bdt that deepened with time ovda s deformation topography and gravity are all spatially correlated strongly suggesting a genetic relationship between surface deformation and processes causing crustal thickening and plateau uplift thus tessera terrain at ovda regio is likely confined to the topographically high plateau and does not extend beneath the surrounding regional plains similar patterns persist at other crustal plateaus indicating that tessera terrain is in general confined to crustal plateaus and does not form a globally continuous layer ovda s structures their spatial and temporal relations the resulting strain history and the behavior of the bdt through time support a model of mantle plume upwelling in a thin lithospheric regime for crustal plateau formation and contradict previous downwelling models similarities between structural and kinematic trends at ovda regio and other crustal plateaus lend support to this idea c 1999 academic press

detailed investigation of a spot on the core mantle boundary using digital pcp data abstract a number of deep earthquakes from the fiji tonga region were used to study the properties of the core mantle boundary cmb the investigation was restricted to an area with a diameter of only a few hundred kilometers at the cmb the use of a cluster of neighboring events yielding redundant sampling and rather good resolution the components of the investigation are i a classical stacking procedure to search for additional reflectors near the cmb ii a deconvolution of seismograms and a stack of the inferred reflector series for the same purpose iii a matched filter method to determine pcp travel times and pcp p amplitude ratios precisely and iv a comparison with synthetic seismograms to check the consistency of derived travel time residuals and pcp amplitude variations in the presence of cmb topography while we do not see any additional reflector for epicentral distances around 35 we detect by stacking seismograms recorded at 70 an additional phase which might be caused by a reflection from the top of d we show that pcp travel time residuals must not be interpreted as being caused exclusively by cmb topography an additional investigation of synthetics provides some constraints on dominant wavelengths and corresponding amplitudes of cmb undulations for wavelengths of 50 400 km the elevation cannot exceed 2 3 km if pcp is observed in this particular area

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

the role of water in the deformation of dunite specimens of two natural dunites anita bay of 100 m grain size and aheirn of 900 m grain size have been deformed in experiments at temperatures between 1200 and 1400 c and constant strain rates between 10 4 and 10 6 s 1 in a gas medium deformation apparatus at 300 mpa confining pressure prior to these experiments the specimens were given a heat treatment in a controlled oxygen fugacity furnace at 1200 c for 60 hours to dehydrate the trace layer silicate minerals present and drive off adsorbed water the predried specimens are much stronger than specimens tested under wet conditions in an earlier study the predried specimens also do not show the apparent grain size dependence seen under wet conditions infrared absorption measurements on the present specimens and those of the earlier study indicate that for a given rock flow stress can be correlated with the intensity of a residual water related broad absorption the addition of water to a predried specimen results in both a mechanical reweakening and an intensifying of the broadband infrared component the weakening of the dunites under wet conditions is probably strongly influenced by the presence of small amounts of a water related species in the grain boundaries but intragranular weakening may also be contributing it is suggested that the concentration of the water related species in the grain boundaries is governed by its concentration in the small melt fraction present the amounts of water that have been found to be of importance for the strength of these dunites are of the order of 0 01 wt amounts much less than most estimates of the water content of the earth s undepleted upper mantle such water may have an important role in determining upper mantle flow behavior stress strain curves for all experiments are available with entire manuscript on microfiche order from american geophysical union 2000 florida avenue n w washington dc 20009 document b84 008 2 50 payment must accompany order

coldspots and hotspots global tectonics and mantle dynamics of venus magellan radar images and altimetry data and pioneer venus gravity data of major highlands and lowlands on venus are examined with the objective of relating these tectonic and volcanic landforms to convection within venus mantle two roughly circular lowlands atalanta and lavinia planitiae are bowl shaped depressions which contain compressional tectonic features and lack hotspot related features such as coronae and shield volcanoes they are identified as surface expressions of young mantle coldspots or regions of mantle downwelling highlands can be divided into two distinct groups volcanic rises and plateau shaped highlands volcanic rises are domical circular to elongate regions characterized by volcanic construction extensional tectonism large positive gravity and geoid anomalies centered on the highlands and large apparent depths of compensation volcanic rises include beta bell atla and western eistla regiones and are identified as surface expressions of large mantle hotspots or plumes plateau shaped highlands are steep sided elevated regions that range from circular alpha regio to elongate or irregular ovda regio tellus regio in shape their surfaces are dominated by complex ridged terrain rather than the shield volcanoes and flows that dominate volcanic rises most plateau shaped highlands contain compressional structures some of which commonly lie along their margins and have small gravity and geoid anomalies and small apparent depths of compensation compared to volcanic rises plateau shaped highlands which include those mentioned above and western ishtar terra thetis regio and phoebe regio are identified as surface expressions of mantle coldspots or regions of downwelling the highland coldspot features are elevated as a consequence of crustal thickening crustal thickening is absent or minor at lowland coldspots the importance of mantle downwelling in the tectonic deformation of venus suggests that its mantle like the mantle of earth is strongly heated from within the major venusian hotspots attest to the existence of large mantle plumes carrying heat upward from the planet s core

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

the leading mode of indian summer monsoon precipitation variability during the last millennium 1 the internally generated intraseasonal variability of the indian summer monsoon is characterized by intermittent periods of enhanced active and deficient break precipitation which produce a quasi east west precipitation dipole over the indian subcontinent here we present multicentennial length and near annually resolved reconstructions of monsoon precipitation inferred from absolute dated and instrumentally calibrated speleothem oxygen isotope records from regions central and northeast india that have diametric responses to active break monsoon circulation patterns on centennial timescales ad 1400 2008 precipitation variability from these two regions exhibit opposing behavior oscillating between periods with a persistently active dominated ad 1700 to 2007 and a break dominated ad 1400 to 1700 regime the switch between these regimes occurs abruptly within decades at a time ad 1650 1700 when a proxy record of upwelling intensity from the arabian sea suggest an abrupt increase in the monsoon winds on the basis of these observations we hypothesize that the frequency distribution of active break periods varies on centennial timescales implying a leading role of internal dynamics in governing the ism response to slowly evolving changes in the external boundary conditions

pulsating continents on venus an explanation for crustal plateaus and tessera terrains we propose that tessera terrains on venus represent continental crust that does not participate in the periodic recycling of the lithosphere through global subduction events we have studied the force balance on the boundary of a continental area that survives a global subduction event using an analytical model in the proposed model the ratio between the crustal and lithospheric mantle thicknesses controls the force balance if the crust thickness is less than 2 5 of the lithospheric mantle thickness the continental area will be compressed but if the crustal thickness is higher than 2 5 of the lithospheric mantle thickness the continental area will spread out and collapse consequently if the lithospheric mantle beneath a continental region is delaminated during a global subduction event the continent will collapse generating tessera inliers dominated by extensional tectonics but if a significant portion of lithospheric mantle remains then the continental area will be compressed generating a plateau by crustal shortening the observed plateau heights can be explained by this model a 2 km height plateau can be generated by a lithospheric mantle thickness of 40 km while a 4 km height plateau can be generated by a 90 km thick lithospheric mantle we have modelled this crustal thickening of a continental area by tectonic contraction using a thin viscous sheet approach with a newtonian viscosity for the crust the force from a hot mantle elevated during a global subduction event is enough to build up a plateau by compression in 50 ma using a viscosity for the continental crust of 1021 pa s and 200 ma for 5 1021 pa s during this compressional stage concentric fold and thrust belts are generated in the plateau continent erasing any impact craters that were present the subsequent stabilization of a new crust and lithosphere in the surrounding mantle changes the force balance allowing a moderate gravitational collapse of the plateau continent accommodated by radial grabens the pulsating continent model links for the first time the generation of crustal plateaus and the origin of the volcanic plains predicting the observed equivalent effective crater density for both terrains