eastern mediterranean surface water nd during eemian sapropel s5 monitoring northerly mid latitude versus southerly sub tropical freshwater contributions abstract a well developed sapropel s5 was deposited in the eastern mediterranean during the last interglacial eemian 124 119 ka freshwater contributions to the basin at this time can be traced using the isotopic composition of nd in planktonic foraminifera this enables differentiation between radiogenic sources to the south under the influence of the african monsoon and unradiogenic sources to the north relating to the mid latitude westerlies here we compare new nd data from a core in the southeast aegean sea with published data from the ionian and levantine seas shifts towards more radiogenic nd in the lower and middle parts of sapropel s5 are most pronounced in the ionian sea record with nd and 18og ruber co varying more closely here than in the levantine and aegean seas this is consistent with a freshwater source proximal to the ionian sea site likely indicating a substantial reactivation of rivers flowing northward from the central saharan watershed the lack during s5 deposition of a noticeable shift towards more unradiogenic nd in the aegean record would exclude a large influx of water from the northern borders of the eastern mediterranean during sapropel deposition these findings support a scenario whereby the last interglacial eastern mediterranean was influenced strongly by the remote effects of an intensified african monsoon with more local precipitation in the northern borders contributing relatively little to the sea surface composition

a geodynamic model of mantle density heterogeneity using cenozoic and mesozoic plate motion reconstructions we derive a model of present day mantle density heterogeneity under the assumption that subducted slabs sink vertically into the mantle the thermal buoyancy of these slabs is estimated from the observed thermal subsidence cooling of oceanic lithosphere slab velocities in the upper mantle are computed from the local convergence rate we assume that slabs cross the upper lower mantle interface and continue sinking into the lower mantle with a reduced velocity for a velocity reduction factor between 2 and 5 our slab heterogeneity model is as correlated with current tomographic models as these models are correlated with each other we have also computed a synthetic geoid from our density model for a viscosity increase of about a factor of 40 from the upper to lower mantle our model predicts the first 8 spherical harmonic degrees of the geoid with statistical confidence larger than 95 and explains 84 of the observed geoid assuming that the model c21 and s21 terms are absent due to a long relaxation time for earth s rotational bulge otherwise 73 of the geoid variance is explained the viscosity increase is consistent with our velocity reduction factor for slabs entering the lower mantle since downwelling velocities are expected to scale roughly as the logarithm of viscosity loge 40 3 7 these results show that the history of plate tectonics can explain the main features of the present day structure of the mantle the dynamic topography induced by this heterogeneity structure consists mainly of about 1 km amplitude lows concentrated along the active continental margins of the pacific basin our model can also be used to predict the time variation of mantle heterogeneity and the gravity field we find that the age of the geoid defined as the time in the past before which the geoid becomes uncorrelated with the present geoid is about 50 m y our model for the history of the degree 2 geoid which is equivalent to the history of the inertia tensor should give us a tool to study the variations in earth s rotation pole indicated in paleomagiietic studies

buckling instabilities of subducted lithosphere beneath the transition zone a sheet of viscous fluid poured onto a surface buckles periodically to generate a pile of regular folds recent tomographic images beneath subduction zones together with quantitative fluid mechanical scaling laws suggest that a similar instability can occur when slabs of subducted oceanic lithosphere encounter enhanced resistance to penetration at 660 km depth beneath subduction zones such as central america and java the fast anomalies in elastic wavespeed are too broad 400 km to be explained by simple compressional thickening of subducted lithosphere that is initially 50 100 km thick by contrast a periodic buckling mechanism can account for both the characteristic wedge shapes of the anomalies and for their widths which agree closely with those predicted by the scaling laws in addition to providing a plausible physical interpretation of tomographic observations beneath some subduction zones buckling instabilities have important consequences for other aspects of global geodynamics such as tectonic plate reconstructions interpretation of geoid anomalies and modeling of secular changes in earth s moment of inertia

polar wandering and paleomagnetism an important hypothesis of paleomagnetism is that apparent polar wander the motion of the paleomagnetic pole relative to a continent or plate is caused entirely by plate motion relative to the mesosphere which is the relatively stronger and slowly deforming mantle beneath the astheno sphere recent research has challenged this hypothesis and suggests that polar wandering i e true polar wandering affects apparent polar wander apw paths the debate about polar wandering has been revived by studies comparing paleomagnetic poles to plate motion relative to hotspots morgan 1 98 1 what makes the new work interesting is that the under lying hotspot model makes many predictions that are testable by available and obtainable data one prediction that the hawaiian hotspot has shifted southward over cenozoic time has been severely tested and confirmed kana et al 1 978 kana 1 980 gordon cape 1 98 1 gordon 1 982 recent tests have mainly but not entirely taken the form of a reexamination of existing data in the light of the new model future tests can and should be made however by obtaining high quality data from paleomagnetic study of critical time intervals already identified by analysis of existing data this paper discusses what polar wandering is and reviews how the available paleomagnetic data and plate reconstructions have been used to investigate past polar wandering implications of work to date possible causes of polar wandering and promising directions of future research are also discussed

haskell 1935 revisited the haskell 1935 value of 1021 pa s for mantle viscosity is a classic and enduring constraint on the rheology of the earth s interior we revisit this inference using spherically symmetric self gravitating maxwell viscoelastic earth models our inference is based on both forward and inverse analyses of decay times associated with uplift at two sites considered by haskell angerman river sweden and oslo norway rather than the raw relative sea level rsl data at these sites we demonstrate that predictions of the decay time associated with the angerman river data are insensitive to variations in both the late pleistocene ice load history and the lithospheric thickness of the earth model the predictions at oslo are sensitive to both these inputs and hence decay times at this site provide a remarkably robust constraint on mantle viscosity we derive a constraint on the average viscosity of the mantle of 0 65 1 10 1021 pa s where the average resolved by the data encompasses a region which extends from the base of the lithosphere to a depth of near 1400 km this indicates that many previous analyses which have invoked the haskell value of 1021 pa s as a constraint on the average upper mantle i e above 670 km depth viscosity alone have misinterpreted the resolving power of the inference furthermore our analysis indicates that a number of apparently contradictory inferences of viscosity based on fennoscandian data satisfy the new rigorous interpretation of the haskell constraint finally we demonstrate how the ambiguity in the upper mantle lower mantle viscosity contrast associated with the haskell average may be reduced by invoking decay time constraints estimated from rsl curves in hudson bay a preliminary inversion of decay times at angerman river and richmond gulf in hudson bay suggests a contrast of approximately an order of magnitude between the average viscosities of the upper mantle and the top 1000 km of the lower mantle however a conclusive analysis in this regard must await the determination of consistent decay time estimates for the hudson bay region

polar wander and the cambrian joseph l kirschvink et al 1 1 propose that true polar wander tpw occurred during the cambrian 520 to 535 ma this conclusion is based on i apparently high drift rates from gondwana laurentia ii an anomalous pole reading from siberia and iii reinterpretation of polarity from a

limited true polar wander as evidence that earth s nonhydrostatic shape is persistently triaxial earth s spin axis follows the maximum moment of inertia axis of mantle convection with some delay due to adjustment of the rotational bulge here we compute this axis for geodynamic models based on subduction history assuming constant slab sinking speed with another contribution due to thermochemical piles for a wide range of parameters a large shift of 90 degrees is predicted around 80 90 ma it can be largely attributed to a change in circum pacific subduction from predominantly in the north and south towards east and west actual amounts of true polar wander are much smaller pointing towards additional inertia tensor contributions possibly due to slabs in the lowermost mantle below both polar regions these slabs would have been subducted before 150 ma when plate motions in the panthalassa basin are largely unknown matching predicted and observed true polar wander can serve at constraining such plate motions

polar wandering in mantle convection models we calculate polar motion in models of 3 d spherical mantle convection at rayleigh numbers up to 108 which include internal heating radial viscosity variations and an endothermic phase change isoviscous models yield rapid polar motion of order 3 myr but a factor of 30 increase in viscosity with depth reduces the rate of polar motion to about 0 5 myr due to stabilization of the large scale pattern of convection avalanching due to an endothermic phase change causes pulsating inertial interchange polar excursions of order 80 110 and of duration 20 70 myr a layered viscosity model with an endothermic phase change yields only one inertial interchange event in 600 million years these models show that the slow observed rate of post paleozoic true polar wander is not incompatible with higher rates inferred for earlier times

dynamic mantle density heterogeneities and global geodetic observables summary we investigate the influence of mantle dynamics on low degree deformations of the earth at geological timescale we first compute surface deformations and discuss the analytical form of the tangential surface displacement induced by internal loads in a reference frame related to the centre of mass of the planet we use the theoretical love numbers formalism since the earth has a viscoelastic behaviour at geological timescale then we quantify degree one and degree two deformations induced by upwelling domes and subducted plates sinking into the mantle we use a simple model in which the slabs are modelled as blobs diving vertically through the mantle and in which the domes are assumed to be stable over the last 120 ma their location is modelled from seismic tomography within the lower mantle the temporal evolutions of the j2 gravitational potential coefficient and of the geocentre motion are plotted since 120 ma we find that 1 the mantle density heterogeneities within the mantle can explain the present day non hydrostatic flattening of the earth however they vary at a too slow timescale to significantly perturb the coefficient 2 although there is a significant discrepancy of about a few hundred metres between the centre of figure and the centre of mass of the earth the secular variation of the geocentre motion is one order of magnitude smaller than the one induced by surface loads

scaling rates of true polar wander in convecting planets and moons abstract mass redistribution in the convecting mantle of a planet causes perturbations in its moment of inertia tensor conservation of angular momentum dictates that these perturbations change the direction of the rotation vector of the planet a process known as true polar wander tpw although the existence of tpw on earth is firmly established its rate and magnitude over geologic time scales remain controversial here we present scaling analyses and numerical simulations of tpw due to mantle convection over a range of parameter space relevant to planetary interiors for simple rotating convection we identify a set of dimensionless parameters that fully characterize true polar wander we use these parameters to define timescales for the growth of moment of inertia perturbations due to convection and for their relaxation due to true polar wander these timescales as well as the relative sizes of convective anomalies control the rate and magnitude of tpw this analysis also clarifies the nature of so called inertial interchange tpw events and relates them to a broader class of events that enable large and often rapid tpw we expect these events to have been more frequent in earth s past

evidence for a 20 tilting of the earth s rotation axis 110 million years ago true polar wander tpw the shift of the earth s rotation axis with respect to the entire globe is most probably due to mass redistribution in the earth s mantle as a result of convection using a new rigorously selected palaeomagnetic database gathering only directions obtained from magmatic rocks we find that tpw has been clearly intermittent over the last 200 ma with two long periods of strict standstill from the present to 80 ma and from approximately 150 to 200 ma a single period of shifting is observed between 80 and about 150 ma ago this period culminates around 110 ma ago in an 20 abrupt tilting during which an angular speed exceeding 5 ma 0 5m yr may have been reached assuming that the time averaged geomagnetic field is axial our results indicate that the changes in the position of the rotation axis and therefore in the inertia tensor of the earth are intermittent we suggest that a major reorganization of the mass distribution in the earth s mantle occurred in the lower cretaceous this event concomitant with plume hyperactivity at the earth s surface and probable drastic changes at the core mantle boundary attested by the inhibition of geomagnetic reversals suggests unmixing of upper and lower mantle by avalanching of upper mantle material down to the core mantle boundary the astonishingly strict stability of the time averaged position of the rotation axis before and after this episode of shifting implies the existence of some steady convection which does not modify the large scale distribution of mass within the mantle given the intermittence of mantle avalanching we suggest that these long periods of stability correspond to the temporary reestablishment of a basically two layered convection system within the mantle

the geological significance of the geoid data are being used to study terrestrial phenomena at scales from tens of thousands of kilometers down to hundreds or even tens of kilometers this paper focuses on the interpretation of global gravity in the form of the geoid and concentrates on what has already been learned from it about the structure of the earth at long to intermed iate wavelengths the longest wavelengths of the gravity field are dominated by density distributions that must be supported by very large scale convective processes i attempt here to document the claim that these density anomalies are both deep and old at slightly shorter length scales the geoidal anomalies give very useful constraints on possible subduction processes and lead to the conclusion that density contrasts in the subducting slabs must be regionally partially compensated and supported from below at yet shorter length scales satellite altimetry results give insight into the thermal and mechanical structure of the oceanic litho sphere seeming to favor plate over thermal boundary layer models of cooling at the shortest length scales the location and mode of compen sation of individual seamounts can be studied

pleistocene deglaciation and the earth s rotation implications for mantle viscosity summary recent results from the analysis of postglacial rebound data suggest that the viscosity of the earth s mantle increases through the transition region models which fit both relative sea level and free air gravity data have viscosities which increase from a value near 1022 poise in the upper mantle beneath the lithosphere to a value of about 1023 poise in the lower mantle in this paper we analyse the effect of deglaciation upon the earth s rotation and thereby show that the observed secular trend polar wander evident in the ils ipms pole path and measurements of the non tidal acceleration of the length of day are both consistent with the viscosity profile deduced from postglacial rebound the two analyses are therefore mutually reinforcing

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

reassessment of isotaches compression concept and isotaches consolidation models abstracta substantial body of laboratory and field observations on compression and consolidation of soft clay and silt deposits as well as organic soils suggest a soil behavior according to hypoth

mediterranean climate and oceanography and the periodic development of anoxic events sapropels mediterranean sapropels are layers with elevated organic carbon concentrations that contrast with surrounding sediments which are organic poor sapropels occur quasi periodically in sedimentary sequences of the last 13 5 million years and exist both in the eastern and western mediterranean sub basins they have been the subject of extensive study based on records from both short conventional and long ocean drilling program sediment cores and from a wide variety of uplifted marine sediment sequences on the basin margins and islands previous syntheses in the 1990s and 2000s have discussed how the formation of sapropels is commonly ascribed to deep sea anoxia enhanced export productivity or a combination of these effects however a wealth of new evidence and insights has emerged during the past 1 2 decades based on traditional and novel proxy data as well as modelling which has revealed intriguing new aspects and nuances to the reconstructed conditions hence it is timely to present a new synthesis of current understanding of the processes behind the formation of sapropels which have over the past decade also become a matter of commercial interest in sub salt hydrocarbon exploration in this review we present a context of modern mediterranean climate and oceanography followed by an integrated assessment of the growing understanding of climatological and ocean circulation changes that were associated with sapropel deposition we find that sapropels predominantly formed during astronomically timed episodes when climatic and oceanographic conditions and ecological responses broadly preconditioned the basin for sapropel deposition there is strong correspondence with times of monsoon intensification fuelling runoff from north africa into the mediterranean sea while preconditioning due to sea level rise and regional precipitation and runoff may have contributed as well within these broad episodes of surface buoyancy gain and resultant decline in deep water ventilation specific deposition under dysoxic anoxic or even euxinic conditions occurred within a clearly dynamic system that was characterised by complex spatial and depth dependent patterns gradients with distinct temporal variability on at least decadal to centennial millennial timescales in the final section we evaluate the implications of different modes of deep water removal from silled basins to investigate why sapropels are more frequently and often more intensely developed in the eastern mediterranean than in the western mediterranean

erratum to holocene climate optimum and last glacial maximum in the mediterranean the marine oxygen isotope record marine geology 153 1999 57 75

polar wandering of a dynamic earth summary the rotational behaviour of a stratified visco elastic planet submitted to changes in its inertia tensor is studied in a viscous quasi fluid approximation this approximation allows for large displacements of the earth rotation axis with respect to the entire mantle but is only valid for mass redistribution within the planet occurring on the time scale of a few million years such a motion called true polar wander tpw is detected by palaeomagneticiens assuming that the earth s magnetic field remains on average aligned with the spin axis our model shows that a downgoing cold slab induces a tpw which quickly brings this slab to the pole for a mantle of uniform viscosity the same slab is slowly moved toward the equator when a large viscosity increase with depth takes place in the mantle our model is also suitable to investigate the effects of a non steady state convection on the earth s rotation we discuss these effects using a simple mass redistribution model inspired by the pioneering paper of goldreich toomre 1969 it consists of studying the tpw induced by a random distribution of slabs sinking into the mantle for such a mass redistribution only a strongly stratified mantle can reduce the earth s pole velocity below l ma which is the upper bound value observed by palaeomagnetic investigations for the last 200 ma our model also shows that when corrected for the hydrostatic flattening the earth s polar inertia generally corresponds to the maximum inertia as it is presently observed however this may not be the case during some short time periods we also discuss the amount of excess polar flattening that can be related to tidal deceleration this frozen component is found to be negligible

stability of the rotation axis in high resolution mantle circulation models weak polar wander despite strong core heating 1 growing evidence points to a substantial heat flow across the core mantle boundary cmb but the rotational stability of strongly bottom heated mantle flow with prominent upwelling plumes is poorly known here we calculate polar motion for the past 100 myr induced in a new class of isochemical high resolution mantle circulation models mcms with earth like convective vigor and up to 12 tw core heat flux our mcms include internal heating and a simple three layer viscosity profile associated with the lithosphere 1023 pa s and the upper 1021 pa s and the lower mantle 1023 pa s separated at 100 and 650 km depth respectively a published mantle mineralogy model in the pyrolite composition consistent with our assumption of whole mantle flow allows us to relate thermal to density variations in a thermodynamically self consistent way all models yield modest polar motion on the order of 0 5 myr 1 or less in accordance with paleomagnetic data and agreeing with a number of studies that demonstrate the stabilizing effect of the rotational bulge although a substantially reduced lower mantle viscosity would increase this rate the good agreement between mcm and seismic mantle heterogeneity lends independent support for our viscosity profile as otherwise slabs in the mcm would rapidly sink to depth levels where they are tomographically not observed in general there is good agreement between the long wavelength geoids predicted from our mcms and recent satellite derived models of earth s geoid correlation coefficient of around 0 4 but noticeable differences at intermediate wavelengths for example in the western pacific and in africa suggest the use of gravity data to distinguish between competing plate reconstruction models

phanerozoic hot spot traces and paleogeographic reconstructions of the siberian continent based on interaction with the african large low shear velocity province abstract we review intraplate magmatism in siberia and its folded surroundings from 480 ma to the present we describe several large igneous provinces lips and the intervals in which they were continuously formed within the limits of the siberian continent the altay sayan early paleozoic magmatic area 598 446 ma the altay sayan lip 408 393 ma the viluy lip 380 350 ma the barguzin vitim lip 310 275 ma the late paleozoic rift system of central asia 318 242 ma the siberian traps and west siberian rift system 250 249 ma the east mongolian and west trans baikalian lip 228 195 ma and a number of various aged late mesozoic and cenozoic rift zones and magmatic areas from 160 ma to the present day following lawver and muller 1994 kharin 2000 lawver et al 2002 and chernysheva et al 2005 we accept the position of the icelandic hot spot under the siberian trap area at the permo triassic boundary that enables us to estimate the geographic coordinates of the siberian trap location at 250 ma they are the same as iceland today presently the icelandic hot spot is situated above the african large low shear velocity province llsvp that indicates a hot mantle plume we suggest a set of paleogeographic reconstructions of the siberian continent for which we evaluate the paleolatitude based on the paleomagnetic data and estimate the paleolongitude position by placing siberia above the african llsvp furthermore we estimate the geographic coordinates for other ancient hot spots in the framework of the african llsvp that we consider to be responsible for the intraplate magmatism during different time periods in the phanerozoic eon available rare element and isotopic characteristics of the intraplate magmatic rocks of siberia enable us to determine three primary sources moderately depleted mantle prema enriched mantle em i and em ii of the mantle origin magma we propose that the model explains the interaction of the hot mantle plume including hot spots plume tails with the siberian intraplate magmatism areas throughout the phanerozoic eon

an inversion for radial viscosity structure using seismic tomography using a uniform velocity density scaling we have inverted the seismically inferred 3 dimensional structure of the whole manfie for the radial viscosity structure which best fits the geoid we are able to obtain 60 72 variance reductions for three different s wave tomographic models the resulting viscosity structures are remarkably similar showing a high viscosity lid a low viscosity zone in the transition region and a high viscosity lower manfie a resolution analysis indcates that the viscosity structure in the upper manfie is well resolved by the data however the resolution in the lower manfie is poorer our models axe in general agreement with previous studies except that our inversions prefer a low viscosity layer at 400 670 km as opposed to 100 400 kin

long term true polar wander of the earth including the effects of convective processes in the mantle and continental drift summary long term true polar wander tpw of the earth was examined by taking into account the effects of simplified convective processes in the earth s mantle and continental drift the tpw for a given viscoelastic earth model is wholly determined by both the magnitude of non forcing elements of moment of inertia i 11 i 22 and i 33 and i 12 element of product of inertia and the rates for forcing elements di 13 dt and di 23 dt the forcing rates are largely related to time dependent convective processes in the mantle and also continental drift in this study i examined the tpw on a convecting mantle with oscillating moments of inertia inferred from a convective process of alternating degree one and degree two structure changes of mantle convection by zhongetal 2007 in the phase for a relatively hydrostatic earth corresponding to largely degree one planform the predicted tpw is sensitive to the viscosity structure of the mantle particularly to the lithospheric viscosity structure and its magnitude may be larger than 30 even for the forcing rates with 10 30 kg m 2 myr 1 related to continental drift dickman 1979 in the phase for a non hydrostatic earth characterized by largely degree two planform such as for the present day however the tpw is less sensitive to the viscosity structure and its magnitude may be 10 at most these results may provide quantitative constraints on examining relationship between rheological structure and convective processes in the mantle continental drift and tpw

the rotational stability of a convecting earth assessing inferences of rapid tpw in the late cretaceous summary we outline a linearized rotational stability theory for predicting the time dependence of true polar wander tpw on a maxwell viscoelastic body in response to mantle convective loading the new theory is based on recent advances in ice age rotation theory a comparison between predictions based on the new theory and analytic expressions for equilibrium infinite time tpw on planetary models with elastic lithospheres demonstrates that the linearized theory can in the case of loading at mid latitudes predict tpw of over 20 to better than 5 per cent accuracy we present predictions of tpw for loading with periodic and net ramp up time histories moreover we compare the time dependence of tpw under assumptions consistent with the canonical equilibrium stability theory adopted in most previous analyses of convection induced tpw and a stability theory that includes two effects that have not been considered in previous geophysical analyses 1 the so called remnant rotational bulge associated with the imperfect reorientation of the rotational bulge due to the presence of an elastic lithosphere and 2 a stable over the timescale of the forcing excess ellipticity as a first application of the new theory we consider recent inferences of rapid order 1 myr tpw motion of amplitude 10 20 during the late cretaceous we conclude that excursions of this amplitude and timescale are physically implausible

theoretical constraints on true polar wander 1 for the present geologic epoch true polar wander tpw is relatively small but simple theoretical considerations suggest that it could have been larger in other epochs in this work we use scaling arguments to assess the qualitative behavior of tpw and a simple maxwell model to analytically describe how changes in mass anomalies translate into tpw unlike previous work we derive simple analytical estimates of tpw based on the characteristic amplitudes and timescales for changes in the moment of inertia we find estimates for both the amplitude and speed of tpw as a function of earth properties the following four main factors influence how large the maximum tpw can be the geological timescale over which the desired tpw occurs tpw the viscosity structure of the mantle which yields a weighted average viscosity the characteristic amplitude of the nonhydrostatic changes in the moment of inertia c and the added moment of inertia due to the equatorial bulge c a for the nominal values 3 1022 pa s and c c a 0 003 the maximum tpw is 61 over 100 myr and 8 over 10 myr the maximum tpw speed is only sensitive to c and c a and is 2 4 myr 1 for the nominal values tpw is shown to act as a low pass filter rapid changes in moment of inertia produce smaller and delayed tpw a consequence of this is that inertial interchange tpw does not have a different character than tpw tpw can have an important contribution to plate motions over relatively long timescales but not over shorter timescales our simple approach allows us to assess whether multiple tpw events are possible but the major uncertainty continues to be the mantle viscosity structure

true polar wander abstract a re evaluation of the existence of true polar wander tpw since the late cretaceous and a comparison among the various approaches are made using updated paleomagnetic hotspot and relative motion datasets previous attempts to determine the existence of tpw had resulted in different conclusions comparison of hotspot locations and paleomagnetic poles required significant pole motion although lithospheric plate displacement analysis yielded insignificant motion however these earlier determinations cannot be directly compared to find the reason for the discrepancies because each used different datasets for this study the different approaches are applied to a single updated model with three alternative relative motions of east and west antarctica although the results are model dependent in general there was not significant motion of the pole relative to the lithosphere 1 5 since the early tertiary but a large motion 10 12 relative to the hotspot framework it is unlikely that errors in the determinations could account for this disagreement the a95 of the plate reconstruction is about 3 the uncertainty in antarctica motion is estimated to no larger than 3 and cumulative errors in the relative plate motions may also amount to 3 only if all these errors are present in the maximum estimated amount and in the same direction could they account for the 10 12 gap between the two approaches this conclusion of pole motion relative to the hotspots but not the lithosphere may indicate an independent shift of the mesosphere relative to the lithosphere or mantle roll of hargraves and duncan

a resonance in the earth s obliquity and precession overthe past 20 myr drivenbymantle convection the motion of the solar system is chaotic to the extent that the precise positions of the planets are predictable for a period of only about 20 myr ref 1 the earth s precession obliquity and insolation parameters over this time period1 2 3 4 5 6 can be influenced by secular variations in the dynamic ellipticity of the planet which are driven by long term geophysical processes such as post glacial rebound5 7 8 9 10 here we investigate the influence of mantle convection on these parameters we use viscous flow theory to compute time series of the earth s dynamic ellipticity for the past 20 myr and then apply these perturbations to the nominal many body orbital solution of laskar et al 5 we find that the convection induced change in the earth s flattening perturbs the main frequency of the earth s precession into the resonance associated with a secular term in the orbits of jupiter and saturn5 and thus significantly influences the earth s obliquity we also conclude that updated time series of high latitude summer solar insolation diverge from the nominal solution for periods greater than the past 5 myr our results have implications both for obtaining precise solutions for precession and obliquity and for procedures that adopt astronomical calibrations to date sedimentary cycles and climatic proxy records

early tertiary subduction zones and hot spots hot spots and subduction zones may control the position of the earth s rotation axis paleomagnetic reconstructions in the hot spot framework require a relative motion of about 10 between the hot spot reference frame and the earth s spin axis since the early tertiary if the viscosity of the mantle permits a changing distribution of mass anomalies can control the position of the earth s rotation axis as it tracks the maximum principal axis of the perturbations the present plate geometry should give the best indication of what features are associated with the controlling density anomalies for the present geoid anomalies indicate that subducting slabs are major mass contributions removing these effects a simpler residual geoid remains with highs strongly correlated with hot spot locations although neither the hot spots alone nor subducting slabs alone have maximum principal axes near the present pole adding the contributions of the two gives a combined axis within a degree of the pole suggesting that these two effects control the location of the spin axis to establish whether changes in plate geometry could account for the observed polar motion the locations of subduction zones are reconstructed in the hot spot framework for the early tertiary a time of significantly different plate motions for this reconstruction the combined maximum principal axis of the subductionhot spot systems is not at the present pole but about 10 away in a location similar to the pole position required by paleomagnetic studies this suggests that changes in subduction geometry may cause a shift in the position of the rotation pole and furthermore that inertia tensor effects may be the link between plate velocities and the spin axis

true polar wander an analysis of cenozoic and mesozoic paleomagnetic poles a true polar wander path is presented for the past 180 ma tracing the relative motion between the hotspot framework and the geomagnetic poles deduced from worldwide paleomagnetic data a relative displacement of 22 10 is observed the motion between the two reference frames has not been smooth episodes of rapid motion on the order of 80 100 mm yr include the present 65 50 ma 115 85 ma and 180 160 ma periods of slow or little polar motion less than 20 mm yr are observed at 50 5 ma and 160 115 ma the effect of plate motions is removed using morgan s plate motion model a19 this model is tied to a hotspot reference frame assumed to be fixed in the mantle the rotated paleopoles are grouped by ages and then averaged they are found to cluster about points distinct from the north pole as defined by the hotspots these points trace out a path of motion between the rotation axis and the hotspots or mantle this is interpreted as a shifting of the entire earth in response to a change in the principal axes of the moment of inertia of the mantle paleomagnetic poles of the past 7 m y are examined independently this eliminates a dependence on the fixed hotspot hypothesis since plate motions are essentially negligible for this time span there is a deviation of 5 2 between the present geographic north pole and the average of the paleopoles at 5 ma this is too great to attribute to plate motion interpreted as true polar wander the rate of this polar motion is about 1 m y astronomical measurements for the past 80 years have recorded actual motion of the earth s rotational axis up to 1 m y in the direction of eastern canada this agrees in rate with paleomagnetic polar motion of the past 7 m y and in both rate and direction for the past 2 m y

excitation of true polar wander by subduction true polar wander the global motion of the mantle relative to the earth s rotation axis is known from the analysis of palaeomagnetic data hotspot tracks and plate motions to have occurred at velocities of up to 0 5 myr 1 since the late cretaceous period1 4 we address here the longstanding question of how fast episodes of true polar wander tpw can be excited5 by analysing the impact of the distribution and activity of subduction zones on polar motion using nonlinear liouville equations which allow us to treat large excursions of the polar axis we show that unrealistically fast tpw is excited by subduction episodes unless the lower mantle has a viscosity at least 10 times that of the upper mantle this need for a viscosity increase with depth in the mantle reinforces the conclusions of previous studies on post glacial rebound and geoid anomalies theoretical creep laws and some preliminary results on tpw induced by density anomalies embedded in the mantle6 10 the lower viscosity in the upper mantle means that upper mantle density anomalies are most effective in exciting tpw changes in the pattern of subduction through time may be responsible for both episodes of fast tpw and times of quiescence in polar motion

new insights into mantle convection true polar wander and rotational bulge readjustment the earth s rotation axis is constantly tracking the main inertia axis of the planet that evolves due to internal and surface mass rearrangements this motion called true polar wander tpw is due to mantle convection on the million years time scale most studies assumed that on this long time scale the planet readjusts without delay and that the earth s rotation axis and the maximum inertia direction of mantle convection mid mc coincide we herein overcome this approximation that leads to inaccurate tpw predictions and we provide a new treatment of earth s rotation discussing both analytical and numerical solutions we obtain an average tpw rate in the range 0 5 1 5 myr 1 and a sizeable offset of several degrees between the rotation axis and the mid mc this is in distinct contrast with the general belief that these two axes should coincide or that the delay of the readjustment of the rotational bulge can be neglected in tpw studies we thus clarify a fundamental issue related to mantle mass heterogeneities and to tpw dynamics

reconstructing the cenozoic evolution of the mantle implications for mantle plume dynamics under the pacific and indian plates article history the lack of knowledge of the initial thermal state of the mantle in the geological past is an outstanding problem in mantle convection the resolution of this problem also requires the modelling of 3 d mantle evolution that yields maximum consistency with a wide suite of geophysical constraints quantifying the robustness of the reconstructed thermal evolution is another major concern to solve and estimate the robustness of the time reversed inverse problem of mantle convection we analyse two different numerical techniques the quasi reversible qrv and the backward advection bad methods our investigation extends over the 65 myr interval encompassing the cenozoic era using a pseudo spectral solution for compressible flow thermal convection in 3 d spherical geometry we find that the two dominant issues for solving the inverse problem of mantle convection are the choice of horizontally averaged temperature i e geotherm and mechanical surface boundary conditions we find in particular that the inclusion of thermal boundary layers that yield earth like heat flux at the top and bottom of the mantle has a critical impact on the reconstruction of mantle evolution we have developed a new regularisation scheme for the qrv method using a time dependent regularisation function this revised implementation of the qrv method delivers time dependent reconstructions of mantle heterogeneity that reveal 1 the stability of pacific and african large low shear velocity provinces llsvp over the last 65 myr 2 strong upward deflections of the cmb topography at 65 ma beneath the north atlantic the south central pacific the east pacific rise epr and the eastern antarctica 3 an anchored deep mantle plume ascending directly under the epr easter and pitcairn hotspots throughout the cenozoic era and 4 the appearance of the transient reunion plume head beneath the western edge of the deccan plateau at 65 ma our reconstructions of cenozoic mantle evolution thus suggest that mantle plumes play a key role in driving surface tectonic processes and large scale volcanism 2014 elsevier b v all rights reserved

mantle dynamics geoid inertia and tpw since 120 myr abstract we investigate the effect of internal masses redistributions on the position of the earth rotational pole for the last 120 myr we use a geodynamic model based on plate reconstructions that estimates the location and rate of subducted slabs under the assumption that they sink vertically into the mantle ricard et al 1993b our model also takes into account the effect of large scale upwellings domes derived from an analysis of seismic tomography their location is assumed to remain stable with time we then compute the geoid associated with the time dependent mantle density heterogeneities in order to reconcile the computed and observed geoids we investigate the influence of the depth down to which the subducted pacific plates beneath the americas present a significant density contrast with respect to the surrounding mantle on the present day geoid and propose a plate model in which we obtain a variance reduction greater than 0 9 for the degree 2 we show that the vertical oscillation of domes within mantle only modulates the amplitude of the associated geoid the temporal variation of the mantle density heterogeneities is consequently essentially due to changes in the subduction history the temporal evolution of the principal inertia axis pia of the earth derived from our model the rotational axis is aligned to the maximum pia is then investigated and finally compared to estimations of tpw both the maximum and intermediate pias have moved in a plane perpendicular to africa along a circle corresponding to the low of geoid induced by subduction around the pacific the minimum pia seems to be relatively stable since 120 myr and close to the maximum degree 2 geoid high under africa this can account for observed tpw or african apw in the last 200 myr

an explanation for earth s long term rotational stability paleomagnetic data show less than approximately 1000 kilometers of motion between the paleomagnetic and hotspot reference frames that is true polar wander during the past 100 million years which implies that earth s rotation axis has been very stable this long term rotational stability can be explained by the slow rate of change in the large scale pattern of plate tectonic motions during cenozoic and late mesozoic time provided that subducted lithosphere is a major component of the mantle density heterogeneity generated by convection therefore it is unnecessary to invoke other mechanisms such as sluggish readjustment of the rotational bulge to explain the observed low rate of true polar wander

the cause for the north south orientation of the crustal dichotomy and the equatorial location of tharsis on mars the crustal dichotomy and the tharsis rise are the most prominent topographic features on mars the dichotomy is largely an expression of different crustal thicknesses in the northern and southern hemispheres while tharsis is centered near the equator at the dichotomy boundary however the cause for the orientation of the dichotomy and the equatorial location of tharsis remains poorly understood here we show that the crustal thickness variations associated with the dichotomy may have driven true polar wander establishing the north south orientation of the dichotomy very early in martian history such a reorientation that placed the dichotomy boundary near the equator would also have constrained the tharsis region on the dichotomy boundary to have originated near the equator we present a scenario for the early generation and subsequent reorientation of the hemispheric dichotomy although the reorientation is independent of the formation mechanism our results also have implications for the sharply different remanent magnetizations between the two hemispheres

mantle stratification and long term polar wander there is accumulating evidence1 2 that the average location of the earth s magnetic pole has moved relative to the deep mantle over the past 200 myr this phenomenon of true polar wander may be caused by the advance and retreat of ice sheets3 or by mass redistribution in the earth s interior due to changes in the pattern of mantle convection4 5 new analyses1 2 6 of polar wander data show a significant shift of the pole in the late cretaceous but this period is thought to have been too warm for glaciation to have occurred thus most of the true polar wander must be due to mass movements in the mantle here we show by analysing the appropriate equations for polar wander that both viscosity and chemical stratification in the mantle are important in determining the rate of polar wander the dynamical effects of chemical stratification and high viscosity 1023 poise in the lower mantle are to decrease polar wander speed to a level consistent with the averaged velocities inferred from palaeomagnetic data whole mantle convection models without non adiabatic density jumps at 670 km depth and with viscosities less than 1023 p would produce wandering rates in excess of 5 per myr

cenozoic plate driving forces past studies of plate driving forces have concluded that the forces due to subducted slabs in the upper mantle and those due to the thickening of the oceanic lithosphere are the principal driving forces we reexamine the balance of driving forces for the present day and extend our analysis through the cenozoic using an analytical torque balance method which accounts for interactions between plates via viscous coupling to the induced mantle flow we use an evolving mantle density heterogeneity field based on the last 200 myr of subduction to drive plate motions an approach which has proven successful in predicting the present day mantle heterogeneity field we find that for plausible upper mantle viscosities the forces due to subducted slabs in the cenozoic and mesozoic account for in excess of 90 of plate driving forces and those due to lithospheric thickening for less than 10

plate motions and the geomagnetic field ii jurassic to tertiary summary the analysis of the time averaged geomagnetic field is extended back to 200ma palaeomagnetic poles from the major plates have been carefully selected from recent compilations of reliable results for each region these were returned with their corresponding sampling sites to their locations at the estimated dates of magnetization in a fixed hotspots framework the corrected results were then grouped into 20ma windows at intervals of 10ma representing the past 100ma and 40ma windows at 30ma intervals for the more sparse 100 200ma data global means and fisher statistics were calculated for each window having included the axial quadrupole in the calculation the value of this coefficient which gave the maximum value for the fisher precision parameter tightest grouping of poles was taken as representative of each interval the results indicate that a small axial quadrupole of the same sign as the axial dipole may have persisted throughout the cenozoic this is equivalent to a northward offset axial dipole field far sided effect during the late cretaceous this component appears to have changed sign with respect to the dipole negative values seem to have obtained throughout the cretaceous long normal polarity interval corresponding to a southward offset dipole near sided effect the data distribution is inadequate for the resolution of the quadrupole at earlier times and zero values cannot be discounted little relative motion is implied between the hotspots and the geomagnetic axis for the past 90 ma the global mean polar path curving around the predicted fixed hotspots pole at a distance of typically 5 latitude with little sign of rapid tertiary polar wander as implied by studies of pacific data alone between 100 and 200 ma however there is a clear difference between the two reference frames amounting to 17 19 in the jurassic this may reflect motion of the mantle relative to the geomagnetic axis but may also include errors due to inaccurate determination of hotspot tracks and inter hotspot motion

modeling cenozoic sedimentation in the central equatorial pacific and implications for true polar wander high productivity along the central pacific equator has left thick deposits of calcareous and siliceous ooze west of 102 w these have deposited on the pacific plate which has been drifting northwards and therefore the thickest deposits are displaced north of the equator this study attempts to simulate the sediment distribution using information from deep sea drilling project dsdp cores and models for pacific plate motion the dsdp core data are used to account for accumulation rate variations with time paleolatitude and paleodepth accounting for variable carbonate dissolution the model predicts the distribution on 35 50 ma seafloor relatively well and demonstrates that the sedimentary bulge is distorted by variable dissolution over a regional bathymetry gradient due to the northern edge of the south pacific superswell a westerly deepening of the carbonate compensation depth and sedimentary lysocline evolution are proposed which improve the model for seafloor 30 ma and older the degree of match between model and data provides a test of the amount of northward drift of the pacific plate and assuming a model for pacific hotspots motion of true polar wander tpw motion of the hotspot frame relative to the earth s spin axis assuming the duncan and clague 1985 model for pacific hotspots motion the sedimentary modeling favors tpw models which have little or no effect on the central pacific paleoequator over the past 30 ma we also explore implications of interhotspot motion and the suggestion that the bend in the hawaiian volcanic chain represents an abrupt change in pacific hotspot motion within the last 5 ma

pattern of lobate scarps on mercury s surface reproduced by a model of mantle convection numerical simulations suggest that lobate scarps on the surface of mercury could have resulted from compressive stresses induced by convection within the planet s mantle in addition to those generated by global contraction mercury is the smallest and least tectonically active of the terrestrial planets1 2 although mercury s ancient cratered surface resembles the moon it has the largest ratio of metallic core to silicate mantle among the terrestrial planets3 as well as an internal magnetic field4 images from the mariner 10 spacecraft reveal lobate scarps so called because of their curved or scalloped edges which have been interpreted to be high angle thrust faults5 6 7 8 resulting from a period of global contraction a range of mechanisms has been invoked to explain the stresses leading to global contraction including cooling and core formation5 9 tidal effects due to gravitational interactions with the sun10 mantle convection11 and the impact that formed the caloris basin12 here i present numerical simulations of the three dimensional nature of convection within mercury s silicate mantle the model yields a regularly spaced pattern of convection in which upwelling regions of the mantle assume linear sheet like shapes at low latitudes and a nearly hexagonal pattern near the poles the distribution of resultant surface stresses is consistent with the observed pattern of lobate scarps suggesting that the compressive features record an ancient pattern of mantle convection11 in addition to global contraction the gravity and topographic data returned from the messenger 11 mission13 will help test this hypothesis

divergent mediterranean seawater circulation during holocene sapropel formation reconstructed using nd isotopes in fish debris and foraminifera abstract the recurrent deposition of organic rich sapropel layers in the eastern mediterranean sea ems has been attributed to deep water stagnation and enhanced biological production however the underlying climatic interactions paleoceanographic processes and associated ventilation dynamics are still debated here we present a basin wide reconstruction of circulation systematics during sapropel s1 formation 10 8 6 1 kyr bp using the nd isotope composition e nd as paleo seawater tracer our e nd data from fish debris and foraminifera tests are remarkably radiogenic compared to today and spatially and temporally constant these results predominantly reflect enhanced nile versus atlantic contributions and indicate that ems deep water stagnation prevailed below 800 m water depth during s1 formation additional e nd records obtained from bulk sediment leachates show that such stagnation was preconditioned thousand s of years prior to s1 initiation by intensified nile flooding whereas it terminated with a basin wide deep water renewal in addition decoupling of deep waters between the ems and western mediterranean sea wms is evident for the s1 period using a box model for nd in the ems the observed e nd distribution can be most adequately explained by a 2 fold increase in nile discharge and a 50 decrease in the ems wms exchange that was mostly limited to the surface waters alone the corresponding circulation during s1 was more sluggish and shallow for the ems but largely unaffected for the wms this implies that deep water stagnation is a prerequisite for sapropel formation

geoid anomalies in a dynamic earth in order to obtain a dynamically consistent relationship between the geoid and the earth s response to internal buoyancy forces we have calculated potential and surface deformation love numbers for internal loading these quantities depend on the depth and harmonic degree of loading they can be integrated as green functions to obtain the dynamic response due to an arbitrary distribution of internal density contrasts spherically symmetric self gravitating flow models are constructed for a variety of radial newtonian viscosity variations and flow configurations including both whole mantle and layered convection we demonstrate that boundary deformation due to internal loading reaches its equilibrium value on the same time scale as postglacial rebound much less than the time scale for significant change in the convective flow pattern by calculating relaxation times for a series of spherically symmetric viscous earth models for uniform mantle viscosity the geoid signature due to boundary deformations is larger than that due to internal loads resulting in net negative geoid anomalies for positive density contrasts geoid anomalies from intermediate wavelength density contrasts are amplified by up to an order of magnitude geoid anomalies are primarily the result of density contrasts in the interior of convecting layers density contrasts near layer boundaries are almost completely compensated layered mantle convection results in smaller geoid anomalies than mantle wide flow for a given density contrast viscosity stratification leads to more complicated spectral signatures because of the sensitivity of the dynamic response functions to model parameters forward models for the geoid can be used to combine several sources of geophysical data e g subducted slab locations seismic velocity anomalies surface topography to constrain better the structure and viscosity of the mantle