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

toward a boot strap hypothesis of plate tectonics feedbacks between plates the asthenosphere and the wavelength of mantle convection abstract the solid earth system is characterized by plate tectonics a low viscosity zone beneath plates the asthenosphere and long wavelength flow in the convecting mantle we use suites of numerical experiments to show 1 how long wavelength flow and the operation of plate tectonics can generate and maintain an asthenosphere and 2 how an asthenosphere can maintain long wavelength flow and plate tectonics plate subduction generates a sub adiabatic temperature gradient in the mantle which together with temperature dependent viscosity leads to a viscosity increase from the upper to the lower mantle this allows mantle flow to channelize in a low viscosity region beneath plates an asthenosphere forms dynamically flow channelization in turn stabilizes long wavelength convection the degree of dynamic viscosity variations from the upper to the lower mantle increases with the wavelength of convection and drops toward zero if the system transitions from plate tectonics to a single plate planet the plate margin strength needed to initiate that transition increases for long wavelength cells long wavelength flow allows plate tectonics to exist over a wider range of plate margin strength the coupled feedbacks allow for a linked causality between plates the asthenosphere and the wavelength of mantle flow with none being more fundamental than the others and the existence of each depending on the others under this hypothesis the asthenosphere is defined by an active process plate tectonics which maintains it and is maintained by it and plate tectonics is part of an emergent self sustaining flow system that bootstraps itself into existence

observed parameters for turbidity current flow in channels reserve fan lake superior abstract fine grained tailings discharged from a taconite ore processing operation near the shore of lake superior produced turbidity currents that transported the sediment from a small delta into deep water at silver bay minnesota deposition over nearly 20 years produced a sublacustrine fan with two prominent channels during 1972 and 1973 a current meter anchored 5 m above the lake floor adjacent to one of the channels recorded episodic turbidity current flow events lasting as long as two weeks to understand flow parameters for turbidity currents better a short term experiment within a channel on reserve fan in 1975 measured those variables not previously directly observed for channelized turbidity currents flow thickness flow density and concurrent velocity the observed flow thickn ss approximately 16 m is nearly four times the channel depth calculations using the average flow speeds 8 to 12 cm sec and the dilute concentration of the flow as measured during the experiment yield a value for the drag coefficient that is in remarkable agreement with estimated values commonly used for deriving speeds of turbidity currents using dimensions of submarine channels and properties of the sediments

is a pyrolitic adiabatic mantle compatible with seismic data in this paper the simplest average physical model of a mantle convecting as a whole i e following an adiabatic temperature gradient with a single composition pyrolite with phase transitions is tested directly against global seismic data instead of against spherically symmetric seismic models constraints from seismic data on average velocities and lower mantle velocity gradients are hard to reconcile with an adiabatic pyrolitic mantle given the current state of knowledge of elastic and anelastic mineral parameters at high pressure and temperature this physical model generally gives a a stronger baseline offset between upper and lower mantle average travel time residuals than allowed by the data and b an insufficient decrease in velocity with depth in the lower mantle above 2500 km we tested 105 upper and 105 lower mantle models that were selected randomly within the mineral parameter uncertainties only 2 lower mantle models and 24 upper mantle models yield whole mantle seismic structures that are compatible with global isc p and s travel times and central frequencies of toroidal and spheroidal fundamental modes with angular order higher than 18 to improve the fit to the seismic data the physical model would require a a lower velocity transition zone composition than dry pyrolite at least around continents and subduction zones as well as b a gradual change in physical state of the lower mantle that decreases the velocity depth gradient e g a superadiabatic temperature gradient

the missing silica sink revisiting the marine sedimentary si cycle using cosmogenic 32si burial of biogenic silica bsitotal in high sedimentation rate continental margins remains highly uncertain cosmogenic 32si t1 2 140 years can be used to trace the fates of bsitotal postdeposition including as opal bsiopal and diagenetically altered opal bsialtered the latter dominantly authigenic clay bsiclay to determine the magnitude and form of bsitotal storage in coastal sediments conventional operational leaches targeting bsiopal and bsialtered including bsiclay were modified for large scale samples necessary for measurement of 32si the 32si activity was used to estimate total biogenic silica burial bsitotal bsiopal bsialtered in several depositional settings gulf of papua gulf of mexico long island sound and in the previously studied amazon guianas deltaic system in subtropical and temperate regions 32si was detected in both traditional biogenic silica leaches bsiopal and residual authigenic clays traditional bsiopal and modified operational leaches designed to target the most reactive authigenic silicates bsialtered consistently underestimate authigenic clay formation bsiclay and thus the magnitude of bsitotal burial in temperate coastal zones and subtropical deltas by 2 4 fold in tropical deltas 32si activities in the residual fraction after removal of bsiopal demonstrate rapid and almost complete alteration of initial bsiopal to new forms most likely bsiclay globally 4 5 4 9 tmol yr si may be trapped in marine nearshore deposits as rapidly formed clay bsiclay 100 of the missing silica sink in the marine silica budget

the combined effects of continents and the 660 km depth endothermic phase boundary on the thermal regime in the mantle abstract we explore a wide range of parameter space to investigate the dynamical interaction between effects due to surface boundary conditions representing continental and oceanic lithosphere and the endothermic phase boundary at 660 km depth in simple two dimensional convection calculations phase boundary induced mantle layering is strongly affected by the wavelength of convective flows and mixed surface boundary conditions strongly increase the horizontal wavelength of convection our study shows that for mixed cases the effects of the surface boundary conditions dominate the effects of the phase boundary we show that the calculations with complete continental coverage have the most significantly decoupled lower and upper mantle flows and substantial thermal and mechanical layering unlike the free slip case where the surface heat flow decreases substantially with increasing magnitude of the clapeyron slope surface heat flow is shown to be almost independent of the clapeyron slope for mixed boundary condition cases although very different when not layered models with free and mixed surfaces have very similar planforms with very large aspect ratio flows when run with large magnitudes of the clapeyron slope we also calculate the critical boundary layer rayleigh number as a measure of the thermal resistance of the surface boundary layer our results show that the thermal resistance in the oceanic and the continental regions of the mixed cases are similar to fully free and no slip cases respectively we find that even for purely basally heated models the mantle becomes significantly subadiabatic in the presence of partial continental coverage this is due to the significant horizontal advection of heat that occurs with very large aspect ratio convection cells

true polar wander in mantle convection models with multiple mobile continents a b s t r a c t a r t i c l e i n f o the geologic record supports numerous instances during which continents apparently moved at speeds signi cantly faster than any of today s tectonic plates while the time dependence of convective driving forces likely explains some such observations rapid motions of large continents in particular are often attributed to true polar wander tpw in order to gauge the potential for connections between continents mantle temperature anomalies and polar motion we present the rst calculations of tpw derived from models that couple mantle convection with multiple mobile continents we nd that the aggregation and dispersal of supercontinents can lead to two types of tpw driven either by a well developed hot upwelling axis that creates a stable maximum moment of inertia or by the homogenization of mantle thermal structure following continent dispersal that leads to destabilization of the principal axis and possible large magnitude polar wander these supercontinent modulated thermal heterogeneities drive model tpw events as large as 90 at rates of up to 2 5 ma 1 such magnitudes and speeds are greater than those attained in similar models lacking continents but comparable to those for episodes inferred from paleomagnetic data for some large continents in the past published by elsevier b v on behalf of international association for gondwana research

mapping spherical seismic into physical structure biases from 3 d phase transition and thermal boundary layer heterogeneity summary earth s mantle is to a very good first approximation spherically symmetric with lateral deviations in seismic velocities and density of only a few per cent this observation has led to the common assumption that average radial seismic models reflect the mantle s average physical structure we test this assumption by using a set of dynamically generated mantle structures and comparing seismic velocities for the average radial physical state with laterally averaged seismic velocities the thermal and thermochemical dynamic circulation models are earth like in terms of convective vigour thermal structure and geographical pattern of heterogeneity we find that in general averaged seismic structure is not distinguishable from the seismic structure of the physical average within the uncertainty bounds of seismic reference models an exception is near phase boundaries where phase boundary topography broadens the averaged seismic jump relative to the discontinuity at physical reference conditions in an inversion for 1 d seismic structure where narrow discontinuities are imposed these biases may map into a lower jump and substantially stronger velocity gradients above and below the interface than are actually present other small biases in averaged structure occur in thermal boundary layers including those that form above chemical piles these biases are caused by large lateral variations in temperature not compositional variability

constraints on lower mantle composition from molecular dynamics simulations of mgsio3 perovskite abstract we have carried out molecular dynamics simulations of mgsio 3 perovskite pv in order to determine its equation of state and elastic parameters under pressure and temperature conditions of the earth s lower mantle combining this information with density and elastic constants for mg fe o magnesiowustite mw and including the effects of iron on the density of perovskite we investigated possible compositional models for the lower mantle by calculating seismic velocities and densities as functions of depth and comparing them to the globally averaged seismological models ak135 f and prem the calculations were done along several possible geothermal gradients both adiabatic and non adiabatic and for a range of mg fe partitioning coeffcients k fe mg pv mw 0 10 0 35 our results indicate that the most probable compositional models for the lower mantle are pyrolitic with x mg 0 87 and x pv 0 54 for increasing temperatures at the top of the lower mantle the best fitting models increase slightly in both iron and silica content

non adiabaticity in mantle convection seismic observations indicate that earth s lower mantle is homogeneous as revealed by smooth depth variations of the bulk sound velocity and bullen s inhomogeneity parameter being close to one here we show with 3d spherical convection simulations that it should also be non adiabatic because a significant proportion of mantle heat sources is internal the computer simulations predict non adiabaticity of 100 300 degrees for the lower mantle geotherm with 1 01 implying that the temperature drop across the core mantle boundary exceeds 1000 degrees

solidus and liquidus profiles of chondritic mantle implication for melting of the earth across its history abstract we investigated the melting properties of a synthetic chondritic primitive mantle up to core mantle boundary cmb pressures using laser heated diamond anvil cell melting criteria are essentially based on the use of x rays provided by synchrotron radiation we report a solidus melting curve lower than previously determined using optical methods the liquidus curve is found between 300 and 600 k higher than the solidus over the entire lower mantle at cmb pressures 135 gpa the chondritic mantle solidus and liquidus reach 4150 150 k and 4725 150 k respectively we discuss that the lower mantle is unlikely to melt in the d layer except if the highest estimate of the temperature profile at the base of the mantle which is associated with a very hot core is confirmed therefore recent suggestions of partial melting in the lowermost mantle based on seismic observations of ultra low velocity zones indicate either 1 a outer core exceeding 4150 k at the cmb or 2 the presence of chemical heterogeneities with high concentration of fusible elements our observations of a high liquidus temperature as well as a large gap between solidus and liquidus temperatures have important implications for the properties of the magma ocean during accretion not only complete melting of the lower mantle would require excessively high temperatures but also below liquidus temperatures partial melting should take place over a much larger depth interval than previously thought in addition magma adiabats suggest very high surface temperatures in case of a magma ocean that would extend to more than 40 gpa as suggested by siderophile metal silicate partitioning data such high surface temperature regime where thermal blanketing is inefficient points out to a transient character of the magma ocean with a very fast cooling rate

provenance stratigraphic architecture and hydrogeologic influence of turbidites on the mid ocean ridge flank of northwestern cascadia basin pacific ocean abstract the northwestern edge of cascadia basin north pacific ocean is unusual because late pliocene to holocene turbidites lap onto juvenile oceanic crust of the juan de fuca ridge subsidence of the ridge flank combines with irregular westward progradation of the turbidite facies to create a stratigraphic section that coarsens and thickens upward the sand provenance is mixed individual turbidity currents have funneled into the area through several shelf slope and abyssal floor conduits including vancouver valley juan de fuca channel barkley canyon and nitinat canyon local bathymetric blockage deflection and reflection of flow paths by basement relief remobilization by intrabasinal submarine slides and debris flows episodic channel switching and sporadic overbank flooding have combined to produce erratic recurrence intervals for the turbidity currents only the tallest basement highs have remained isolated from turbidite deposition during the last 500 000 years spatial and temporal differences in sediment accumulation are important because they modulate the three dimensional responses of compaction and consolidation those changes in physical properties govern where and when hydraulic communication with the underlying basement shuts down the basal hemipelagic layer of cascadia basin transforms to an effective hydrologic seal seepage rates 1 mm yr once the sediment basalt interface is buried by 100 150 m of strata rapid accumulation of turbidites therefore accelerates the hydrogeologic conversion of igneous basement from open to sealed

mantle circulation models with sequential data assimilation inferring present day mantle structure from plate motion histories data assimilation is an approach to studying geodynamic models consistent simultaneously with observables and the governing equations of mantle flow such an approach is essential in mantle circulation models where we seek to constrain an unknown initial condition some time in the past and thus cannot hope to use first principles convection calculations to infer the flow history of the mantle one of the most important observables for mantle flow history comes from models of mesozoic and cenozoic plate motion that provide constraints not only on the surface velocity of the mantle but also on the evolution of internal mantle buoyancy forces due to subducted oceanic slabs here we present five mantle circulation models with an assimilated plate motion history spanning the past 120 myr a time period for which reliable plate motion reconstructions are available all models agree well with upper and mid mantle heterogeneity imaged by seismic tomography a simple standard model of whole mantle convection including a factor 40 viscosity increase from the upper to the lower mantle and predominantly internal heat generation reveals downwellings related to farallon and tethys subduction adding 35 bottom heating from the core has the predictable effect of producing prominent high temperature anomalies and a strong thermal boundary layer at the base of the mantle significantly delaying mantle flow through the transition zone either by modelling the dynamic effects of an endothermic phase reaction or by including a steep factor 100 viscosity rise from the upper to the lower mantle results in substantial transition zone heterogeneity enhanced by the effects of trench migration implicit in the assimilated plate motion history an expected result is the failure to account for heterogeneity structure in the deepest mantle below 1500 km which is influenced by jurassic plate motions and thus cannot be modelled from sequential assimilation of plate motion histories limited in age to the cretaceous this result implies that sequential assimilation of past plate motion models is ineffective in studying the temporal evolution of core mantle boundary heterogeneity and that a method for extrapolating present day information backwards in time is required for short time periods of the order of perhaps a few tens of myr such a method exists in the form of crude backward convection calculations for longer time periods of the order of a mantle overturn a rigorous approach to extrapolating information back in time exists in the form of iterative nonlinear optimization methods that carry assimilated information into the past through the use of an adjoint mantle convection model

is the mantle geotherm subadiabatic internal heating and secular cooling are expected to decrease the geothermal gradient away from an adiabat a scaling analysis that is quantified on the basis of published numerical models indicates that the interior geotherm should be nearly isothermal through the lower mantle if all heating is from within for the more realistic condition that the mantle is partly heated from within and partly from below we still expect the average thermal gradient to be significantly subadiabatic through much of the interior of the mantle values of the seismologically observable inhomogeneity parameter 1 04 to 1 07 should characterize the subadiabatic regions of the lower mantle

a mineralogical model for density and elasticity of the earth s mantle 1 we present a thermodynamic model of high pressure mineralogy that allows the evaluation of phase stability and physical properties for the earth s mantle the thermodynamic model is built from previous assessments and experiments in the five component cfmas system cao feo mgo al2o3 sio2 including mineral phases that occur close to typical chemical models of the mantle and reasonable mantle temperatures in this system we have performed a system gibbs free energy minimization including pure end member phases and a nonideal formulation for solid solutions solid solutions were subdivided into discrete pseudocompounds and treated as stoichiometric phases during computation of chemical equilibrium by the simplex method we have complemented the thermodynamic model with a model of shear wave properties to obtain a full description of aggregate elastic properties density bulk and shear moduli that provide a useful basis for the consideration of seismic and geodynamic models of the earth s mantle the thermodynamic model described here is made available for research and training purposes through a web interface http www earthmodel org we examine its validity in light of experiments from mineral physics and briefly discuss inferences for mantle structure

case for periodic colossal jokulhlaups from pleistocene glacial lake missoula two classes of field evidence firmly establish that late wisconsin glacial lake missoula drained periodically as scores of colossal jokulhlaups glacier outburst floods 1 more than 40 successive flood laid sand to silt graded rhythmites accumulated in back flooded valleys in southern washington hiatuses are indicated between flood laid rhythmites by loess and volcanic ash beds disconformities and nonflood sediment between rhythmites are generally scant because precipitation was modest slopes gentle and time between floods short 2 in several newly analyzed deposits of pleistocene glacial lakes in northern idaho and washington lake beds comprising 20 to 55 varves average 30 40 overlie each successive bed of missoula flood sediment these and many other lines of evidence are hostile to the notion that any two successive major rhythmites were deposited by one flood they dispel the notion that the prodigious floods numbered only a few the only outlet of the 2 500 km 3 glacial lake missoula was through its great ice dam and so the dam became incipiently buoyant before the lake could rise enough to spill over or around it like grimsvotn iceland lake missoula remained sealed as long as any segment of the glacial dam remained grounded when the lake rose to a critical level 600 m in depth the glacier bed at the seal became buoyant initiating underflow from the lake subglacial tunnels then grew exponentially leading to catastrophic discharge calculations of the water budget for the lake basin including input from the cordilleran ice sheet suggest that the lakes filled every three to seven decades the hydrostatic prerequisites for a jokulhlaup were thus re established scores of times during the 2 000 to 2 500 yr episode of last glacial damming j harlen bretz9s spokane flood outraged geologists six decades ago partly because it seemed to flaunt catastrophism the concept that lake missoula discharged regularly as jokulhlaups now accords bretz9s catastrophe with uniformitarian principles

profiles of the bullen parameter from mantle convection modelling the state of adiabaticity in the mantle can be assessed by using the bullen parameter profile derived from seismology we have employed the extended boussinesq convection models in a cartesian 2 d domain to extract the bullen parameter as a function of depth in both base heated and internally heated configurations and for both constant and variable thermal conductivity we also studied models with depth dependent thermal expansivity our results show that the values of the bullen parameter for a rayleigh number of 10 6 lie close to one with sometimes excursions out to 0 9 and 1 1 they are sensitive to the mode of heating and the depth dependences of the thermal expansivity and changes of thermal conductivity we obtained a subadiabatic geotherm above the lower boundary layer in all the models with constant thermal expansivity but a decrease of thermal expansivity with depth can result in a subadiabatic geotherm below the upper boundary layer thus the profiles of the bullen parameter have a definite potential of being useful in constraining the physical parameters and flow structures associated with mantle convection fl 2000 elsevier science b v all rights reserved

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

extensive deposits on the pacific plate from late pleistocene north american glacial lake outbursts abstract one of the major unresolved issues of the late pleistocene catastrophic flood events in the northwestern united states e g from glacial lake missoula has been what happened when the flood discharge reached the ocean this study compiles available 3 5 khz high resolution and airgun seismic reflection data long range sidescan sonar images and sediment core data to define the distribution of flood sediment in deepwater areas of the pacific ocean upon reaching the ocean at the mouth of the columbia river near the present day upper continental slope sediment from the catastrophic floods continued flowing downslope as hyperpycnally generated turbidity currents the turbidity currents resulting from the lake missoula and other latest pleistocene floods followed the cascadia channel into and through the blanco fracture zone and then flowed west to the tufts abyssal plain a small part of the flood sediment which was stripped off the main flow at a bend in the cascadia channel at its exit point f

estimates of heat flow in the deep mantle based on the power requirements for the geodynamo 1 regeneration of the magnetic field by convection in the core places demands on heat flow into the base of the mantle if the heat flow is too low thermal convection is shut off and the rate of generation of compositional buoyancy by the solidification of the core becomes too low to sustain the geodynamo conversely a large heat flow causes rapid growth of the inner core so that convection prior to the appearance of the inner core must be sustained by thermal buoyancy alone the attendant requirements on primordial heat become more severe as the age of the inner core decreases we show that estimates of the present day heat flow satisfy the power requirements for the geodynamo however the resulting thermal history is incompatible with estimates of mantle temperatures prior to 3 ga this discrepancy can be resolved by accumulating radioactive isotopes in d or adding heat sources to the core

deep sea sedimentary record of the late wisconsin cataclysmic floods from the columbia river new results from ocean drilling program site 1037 and u s geological survey high resolution seismic reflection profiles confirm the great thickness fast deposition rate distant source and convolute path of turbidites that fill the escanaba trough the rift valley of the southernmost segment of the gorda ridge accelerator mass spectrometry 14 c measurements provide the first direct dating of the escanaba trough turbidites demonstrating an average deposition rate faster than 10 m k y between 32 and 11 ka and as fast as 15 m k y during the oxygen isotope stage 2 lowstand in the upper 60 m of sediment the petrology of turbidite sand beds which are as much as 12 m thick show that the dominant source for the turbidites is from the columbia river which is more than 800 km to the north rather than from the much closer rivers of northern california new high resolution seismic reflection profiles show that except for areas of very recent volcanism the entire escanaba trough below 3200 m water depth is floored by the turbidite sequence that was cored in the upper 60 m at site 1037b the ages of the upper 120 m of turbidites correspond with the ages of channeled scabland deposits associated with latest quaternary jokulhlaups from glacial lake missoula the age and source characteristics suggest that these megaturbidite beds in escanaba trough are most likely deposits formed by hyperpycnally generated turbidity currents as the largest of the lake missoula floods entered the sea

the deep earth may not be cooling down abstract the earth is a thermal engine generating the fundamental processes of geomagnetic field plate tectonics and volcanism large amounts of heat are permanently lost at the surface yielding the classic view of the deep earth continuously cooling down contrary to this conventional depiction we propose that the temperature profile in the deep earth has remained almost constant for the last 4 3 billion years the core mantle boundary cmb has reached a temperature of 4400 k in probably less than 1 million years after the moon forming impact regardless the initial core temperature this temperature corresponds to an abrupt increase in mantle viscosity atop the cmb when 60 of partial crystallization was achieved accompanied with a major decrease in heat flow at the cmb then the deep earth underwent a very slow cooling until it reached 4100 k today this temperature at or just below the mantle solidus is suggested by seismological evidence of ultra low velocity zones in the d layer such a steady thermal state of the cmb temperature excludes thermal buoyancy from being the predominant mechanism to power the geodynamo over geological time an alternative mechanism to sustain the geodynamo is mechanical forcing by tidal distortion and planetary precession motions in the outer core are generated by the conversion of gravitational and rotational energies of the earth moon sun system mechanical forcing remains efficient to drive the geodynamo even for a sub adiabatic temperature gradient in the outer core our thermal model of the deep earth is compatible with an average cmb heat flow of 3 0 to 4 7 tw furthermore the regime of core instabilities and or secular changes in the astronomical forces could have supplied the lowermost mantle with a heat source of variable intensity through geological time episodic release of large amounts of heat could have remelted the lowermost mantle thereby inducing the dramatic volcanic events that occurred during the earth s history in this scenario because the moon is a necessary ingredient to sustain the magnetic field the habitability on earth appears to require the existence of a large satellite

turbulent 3 d thermal convection in an infinite prandtl number volumetrically heated fluid implications for mantle dynamics summary the structure and time dependence of 3 d thermal convection in a volumetrically heated infinite prandtl number fluid is examined for high values of the rayleigh number the methods employed allow the numerical experiments to proceed for long enough times to derive good estimates of mean and fluctuating parts of the structure an iterative multirigid method to solve for the buoyant incompressible viscous flow at each time step of the energy equation is a novel aspect of the methodology a simple explicit time step of the energy equation is utilized that vectorizes well on serial computers and which is ideally suited to massively parallel computers numerical experiments were carried out for rayleigh numbers from 3 106 to 3 107 in a cartesian region with a prescribed temperature at the top boundary and an adiabatic bottom boundary over this complete range of rayleigh number the flow structure consists dominantly of cold nearly axisymmetric plumes that migrate horizontally sweeping off the cold thermal boundary layer that forms at the top of the convecting fluid plumes disappear by coalescing with other plumes new plumes are created by thermal boundary layer instability sheet plumes form only occasionally and do not penetrate to significant depths in the fluid plumes have sizes comparable to the thickness of the thermal boundary layer and an average spacing comparable to the fluid depth no persistent large scale motion in the fluid can be identified its absence may reflect the large subadiabatic stratification that develops beneath the thermal boundary layer as cold plumes penetrate to the bottom boundary without thermally equilibrating with surrounding fluid we consider the possible implications for convection in planetary mantles and for the existence of plate tectonics

effects of multiple phase transitions in a three dimensional spherical model of convection in earth s mantle numerical models of mantle convection that incorporate the major mantle phase changes of the transition zone reveal an inherently three dimensional flow pattern with cylindrical features and linear features that behave differently in their ability to penetrate the 670 km discontinuity the dynamics are dominated by accumulation of cold linear downwellings into rounded pools above the endothermic phase change at 670 km depth resulting in frequent avalanches of upper mantle material into the lower mantle the effect of the exothermic phase transition at 400 km depth is to reduce the overall degree of layering by pushing material through the 670 km phase change resulting in smaller and more frequent avalanches and a wider range of morphologies large quantities of avalanched cold material accumulate above the coremantle boundary cmb resulting in a region of strongly depressed mean temperature at the base of the mantle the 670 km phase change has a strong effect on the temperature field with three distinct regions being visible 1 the upper mantle containing linear downwellings and pools of cold material in the transition zone and characterized by a high amplitude long wavelength spectrum 2 the midmantle containing quasi cylindrical avalanche conduits and characterized by a low amplitude broad spectrum and 3 the deep mantle containing large pools of cold avalanched material and characterized by a high amplitude ultra red i e long wavelength spectrum the effect on the velocity field is very different flow penetration across the 670 km phase change is strongly wavelength dependent with easy penetration at long wavelengths but strong inhibition at short wavelengths thus when comparing numerical models with long wavelength seismic tomography diagnostics based on the density field such as the radial correlation function are much more sensitive to the effects of phase transitions than those based on the velocity field the amplitude of the geoid is not significantly affected by the partial layering because the contribution from the strong heterogeneity in the transition zone is almost perfectly balanced by the contribution from deflection of the 670 km discontinuity avalanches are associated with geoid lows however a more complex viscosity structure is required to correctly match the sign of the geoid over slabs in earth

turbidite megabeds in an oceanic rift valley recording jokulhlaups of late pleistocene glacial lakes of the western united states abstract escanaba trough is the southernmost segment of the gorda ridge and is filled by sandy turbidites locally exceeding 500 m in thickness new results from ocean drilling program odp sites 1037 and 1038 that include accelerator mass spectrometry ams 14c dates and revised petrographic evaluation of the sediment provenance combined with high resolution seismic reflection profiles provide a lithostratigraphic framework for the turbidite deposits three fining upward units of sandy turbidites from the upper 365 m at odp site 1037 can be correlated with sediment recovered at odp site 1038 and deep sea drilling program dsdp site 35 six ams 14c ages in the upper 317 m of the sequence at site 1037 indicate that average deposition rates exceeded 10 m k yr between 32 and 11 ka with nearly instantaneous deposition of one 60 m interval of sand petrography of the sand beds is consistent with a columbia river source for the entire sedimentary sequence in escanaba trough high resolution acoustic strat

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

status of glacial lake columbia during the last floods from glacial lake missoula abstract the last floods from glacial lake missoula montana probably ran into glacial lake columbia in northeastern washington in or near lake columbia s sanpoil arm lake missoula floods dating from late in the fraser glaciation produced normally graded silt beds that become thinner upsection and which alternate with intervals of progressively fewer varves the highest three interflood intervals each contain only one or two varves and about 200 400 successive varves conformably overlie the highest flood bed this sequence suggests that jokulhlaup frequency progressively increased until lake missoula ended and that lake columbia outlasted lake missoula the upper grand coulee lake columbia s late fraser age outlet contains a section of 13 graded beds most of them sandy and separated by varves that may correlate with the highest missoula flood beds of the sanpoil river valley the upper grand coulee also contains probable correlatives of many of the approximately 200 400 succeeding varves as do nearby parts of the columbia river valley this collective evidence casts doubt on a prevailing hypothesis according to which one or more late fraser age floods from lake missoula descended the columbia river valley with little or no interference from lake columbia s okanogan lobe dam

melting temperature of the earth s mantle and core earth s thermal structure abstract although the temperature at the top of the lower mantle is well constrained by phase equilibrium data for the transformation of transition zone minerals to the denser perovskite polymorphs the temperature distribution in the lower mantle is poorly known models depend strongly on the assumptions of the amount of internal heating and the viscosity profile new melting data on iron to pressures of the outer core 2 mbar and the observed strong decrease of eutectic melting depression in the fe feo fes system with increasing pressure however tightly constrain the temperature at the inner core boundary to slightly less than 5000 k this estimate can be reconciled with all recent static melting measurements on iron and lays within the uncertainty of shock temperature measurements the resulting temperature at the core mantle boundary of about 4000 k then requires a large temperature gradient at the bottom of the lower mantle of about 1500 k recent findings of the very high melting temperatures o

rapid south atlantic spreading changes and coeval vertical motion in surrounding continents evidence for temporal changes of pressure driven upper mantle flow the south atlantic region displays 1 a topographic gradient across the basin with africa elevated relative to south america 2 a bimodal spreading history with fast spreading rates in late cretaceous and eo oligocene and 3 episodic regional uplift events in the adjacent continents concentrated in late cretaceous and oligocene here we show that these observations can be linked by dynamic processes within earth s mantle through temporal changes in asthenosphere flow beneath the region the topographic gradient implies westward pressure driven mantle flow beneath the basin while the rapid spreading rate changes on order 10 million years require significant decoupling of regional plate motion from the large scale mantle buoyancy distribution through a mechanically weak asthenosphere andean topographic growth in late miocene can explain the most recent south atlantic spreading velocity reduction arising from increased plate boundary forcing associated with the newly elevated topography but this mechanism is unlikely to explain the late cretaceous tertiary spreading variations as changes in andean paleoelevation at the time are small we propose an unsteady pressure driven flow component in the asthenosphere beneath the south atlantic region to explain the late cretaceous tertiary spreading rate variations temporal changes in mantle flow due to temporal changes in regional mantle pressure gradients imply a correlation of horizontal and vertical motions we find that this prediction from our models agrees with geologic and geophysical observations of the south atlantic region including episodes of passive margin uplift regional basin reactivation and magmatic activity

joint seismic geodynamic and mineral physical constraints on three dimensional mantle heterogeneity implications for the relative importance of thermal versus compositional heterogeneity summary the joint interpretation of seismic and geodynamic data requires mineral physical parameters linking seismic velocity to density perturbations in the earth s mantle the most common approach is to link velocity and density through relative scaling or conversion factors r s dln dlnv s however the range of possible r s values remains large even when only considering thermal effects we directly test the validity of several proposed depthdependent conversion profiles developed from mineral physics studies for thermally varying properties of mantle materials the tests are conducted by simultaneously inverting shear wave traveltime data and a diverse suite of convection related constraints interpreted with viscous flow response functions of the mantle these geodynamic constraints are represented by surface spherical harmonic basis functions up to harmonic degree 16 and they consist of the global free air gravity field tectonic plate divergences dynamic surface topography and the excess ellipticity of the core mantle boundary cmb the tests yield an optimum 1 d thermal r s profile that is generally compatible with all considered data with the exception of the dynamic surface topography that is most sensitive to the shallow upper mantle this result is not surprising given that cratonic roots are known to be compositionally distinct from the surrounding ambient mantle moreover it is expected that the temperature dependence of the thermal r s in the upper mantle is significant due to the temperature dependence of seismic attenuation or q therefore a simple 1 d density velocity relationship is insufficient to address this problem we obtained independent estimates of the first order correction factors to the selected r s profile within the cratonic roots and in the ambient thermal upper mantle these correction factors defined as r s lnv s are highly negative within the cratons signifying considerable compositional buoyancy this result confirms that the negative buoyancy associated with the low temperatures in the cratons is significantly counteracted by the composition induced positive buoyancy resulting in near neutral buoyancy of the cratonic roots within the ambient upper mantle the correction factors are found to be positive which is consistent with the expected behaviour of the r s relationship in thermally varying upper mantle material we obtain a significantly greater reconciliation of the global gravity anomalies and dynamic surface topography when applying these laterally varying corrections compared to a simple 1 d r s relationship inversion for a fully 3 d r s relationship reveals secondary effects including additional compositional variation within the cratonic roots and the deep mantle superplume structures we estimate the relative magnitude of the thermal and compositional non thermal contributions to mantle density anomalies and conclude that thermal effects dominate shear wave and density heterogeneity throughout the non cratonic mantle we also demonstrate the potential pitfalls of scaling a purely seismically derived model to obtain density rather than performing a true joint inversion to obtain velocity and density simultaneously

the efficiency of plate tectonics and nonequilibrium dynamical evolution of planetary mantles consideration of the structure of dynamical equilibria in terrestrial planets using simplified descriptions of the relevant heat transport processes rigid lid convection plate tectonics and heat pipe volcanism reveals that if the efficiency of plate tectonic heat transport decreases at higher mantle temperature then it cannot govern quasi equilibrium dynamical evolution and the system is always evolving away from the plate tectonic regime a planet on which plate tectonics is less efficient at higher temperature stays in heat pipe mode longer spends less time undergoing plate tectonics and has a low and ever decreasing urey number during this phase these conclusions are based solely on the structure of the equilibria in a system with less efficient plate tectonics in the past and are independent of the mechanisms leading to this behavior commonly used quasi equilibrium approaches to planetary thermal evolution are likely not valid for planets in which heat transport becomes less efficient at higher temperature

bullen s parameter η a link between seismology and geodynamical modelling valuable information concerning the style of mantle density stratification can be gleaned from the spatial distribution of bullen s parameter by means of numerical modelling of 2 d cartesian convection and by monitoring this with the 2 d field of the local bullen s parameter values which were obtained by post processing the convection results we show that the local adiabaticity is seriously influenced by the presence of an endothermic phase transition at 670 km depth in this situation the upper mantle suffers much more from non adiabatic effects than the lower mantle we have also employed the 3 d distributions of density and seismic velocities from the model of ishii and tromp based on free oscillation observations and constructed 3 d local bullen s parameter for a spherical earth in the lower mantle we find that there is a striking similarity in the range of magnitudes of the local bullen s parameter calculated from convection and those inferred from the splitting of seismic free oscillations the morphologies of subadiabatic regions in the deep mantle under africa and the central pacific would suggest a thermal chemical nature of the superplumes their vertical extent is limited to around 400 km above the core mantle boundary cmb underneath the icelandic area in the north atlantic such an subadiabatic region in the deep mantle above the cmb cannot be discerned we conclude that the mantle is not as adiabatic as has commonly been held fl 2002 elsevier science b v all rights reserved

are mantle plumes adiabatic abstract the issue concerning the state of adiabaticity of mantle plumes has been examined in a cartesian two dimensional box with an aspect ratio of six we have investigated in the quasi steady state regime high rayleigh number convection with both depth dependent viscosity and thermal expansivity for both the boussinesq and the extended boussinesq approximations we have also assessed the influence of various forms of thermal conductivity and internal heating we have generalized the classical bullen s parameter equation from one dimension to multidimensions for assessing the local state of adiabaticity inside plumes and in their surroundings we have extracted from the local geotherms and the local thermodynamic properties the corresponding bullen s parameter profiles and the two dimensional maps portraying the state of adiabaticity in the mantle histograms characterizing the frequency of adiabaticity are also employed for quantification purposes in general superadiabatic thermal gradients are found inside the thick plume limbs and sometimes along the central part of the plume the centers of plume heads are subadiabatic or nearly adiabatic but the edges of the plume heads are strongly subadiabatic alternating strips of subadiabaticity and adiabaticity are found in the downwellings the ambient mantle outside the plumes is generally adiabatic and is sometimes perforated with islands of marked deviations from adiabaticity

widespread uplift and trapdoor faulting on galapagos volcanoes observed with radar interferometry volcanic uplift caused by the accumulation of magma in subsurface reservoirs is a common precursor to eruptions1 2 but for some volcanoes uplift of metres or more has not yet led to an eruption3 here we present displacement maps of volcanoes in the galapagos islands constructed using satellite radar interferometry that might help explain this dichotomy we show that all but one of the seven volcanoes on the islands of isabela and fernandina deformed during 1992 99 cerro azul and fernandina erupted4 5 6 during the observation period and show evidence of inflation co eruptive deflation and shallow dyke intrusion in contrast the largest volcano sierra negra has not erupted yet exhibits spatially and temporally variable deformation with a maximum uplift of 2 7 m between 1992 and 1999 which can be modelled by a shallow inflating sill inflation during 1997 98 however was accompanied by trapdoor faulting on a steeply dipping fracture system within the caldera repeated trapdoor faulting over geological time has formed an arcuate intra caldera ridge within sierra negra and may have acted to relax stresses above the magma chamber inhibiting summit eruptions similar processes may help explain large uplift unaccompanied by eruptive activity at other volcanoes

vertical tectonics in northern escanaba trough as recorded by thick late quaternary turbidites escanaba trough the southernmost segment of the gorda ridge is filled by as much as 500 m of late quaternary turbidite and hemipelagic sediment coring at deep sea drilling project site 35 and ocean drilling program odp sites 1037 and 1038 together with 4 5 khz deep tow and 3 5 khz surface ship seismic reflection profiles enable a distinct pattern of reflections to be mapped throughout escanaba trough in the upper part of this sediment fill the uppermost 80 m of turbidite sediment which includes at least 11 turbidity current events were deposited in 3200 m the turbidity currents were trapped upon entering escanaba trough resulting in all of the sediment in suspension in the flows being deposited the thickness of the turbidite layers reflects both the flow thickness and the vertical grain concentration within the flow that deposited the layer variations in the turbidite thickness with respect to water depth can be used to estimate the degree of relative vertical movement within the floor of escanaba trough in the area of hydrothermal activity near odp site 1038 uplift of as much as 140 m has occurred over the past 8 kyr

eruptive and tectonic history of the endeavour segment juan de fuca ridge based on auv mapping data and lava flow ages high resolution bathymetric surveys from autonomous underwater vehicles abe and d allan b were merged to create a coregistered map of 71 7 km2 of the endeavour segment of the juan de fuca ridge radiocarbon dating of foraminifera in cores from three dives of remotely operated vehicle doc ricketts provide minimum eruption ages for 40 lava flows that are combined with the bathymetric data to outline the eruptive and tectonic history the ages range from modern to 10 700 marine calibrated years before present yr bp during a robust magmatic phase from 10 700 yr bp to 4300 yr bp flows erupted from an axial high and many flowed 5 km down the flanks some partly buried adjacent valleys axial magma chambers amcs may have been wider than today to supply dike intrusions over a 2 km wide axial zone summit seamount formed by 4770 yr bp and was subsequently dismembered during a period of extension with little volcanism starting 4300 yr bp this tectonic phase with only rare volcanic eruptions lasted until 2300 yr bp and may have resulted in near solidification of the amcs the axial graben formed by crustal extension during this period of low magmatic activity infrequent eruptions occurred on the flanks between 2620 1760 yr bp and within the axial graben since 1750 yr bp this most recent phase of limited volcanic and intense hydrothermal activity that began 2300 yr bp defines a hydrothermal phase of ridge development that coincides with the present day 1 km wide amcs and overlying hydrothermal vent fields

the scotia sea gateway no outlet for pacific mantle abstract the scotia sea in the south atlantic holds a prominent position in geodynamics because it has been proposed as a potential outlet of asthenosphere from under the shrinking pacific into the mantle beneath the opening atlantic shear wave splitting and geochemical studies have previously tested this hypothesis here we take a different approach by calculating present day dynamic topography of the region in search for a systematic trend in dynamic topography decreasing from west to east in response to a flow related pressure gradient in the sublithospheric mantle to this end we reconstruct the kinematic history of the scotia sea which is characterized by complex back arc spreading processes active on a range of time scales our plate reconstructions allow us to derive an oceanic age grid and to calculate the associated residual dynamically maintained topography of the scotia sea by comparing present day isostatically corrected topography with that predicted from our reconstruction the results provide no indication for a systematic trend in dynamic topography and we conclude that the material needed to supply the growing subatlantic mantle must be derived from elsewhere

clay mineralogy of surface sediments as a tool for deciphering river contributions to the cariaco basin venezuela 1 the mineralogical composition of 95 surface sediment samples from the cariaco basin continental shelf and orinoco delta was investigated in order to constrain the clay mineral main provenance and distribution within the cariaco basin the spatial variability of the data set was studied using a geo statistical approach that allows drawing representative clay mineral distribution maps these maps are used to identify present day dominant sources for each clay mineral species in agreement with the geological characteristics of the main river watersheds emptying into the basin this approach allows 1 identifying the most distinctive clay mineral species ratios that determine particle provenance 2 evaluating the respective contribution of local rivers and 3 confirming the minimal present day influence of the orinoco plume on the cariaco basin sedimentation the tuy unare and neveri rivers are the main sources of clay particles to the cariaco basin sedimentation at present the tuy river is the main contributor of illite to the western part of the southern cariaco basin continental shelf the unare river plume carrying smectite and kaolinite has a wide westward propagation whereas the neveri river contribution is less extended providing kaolinite and illite toward the eastern cariaco basin the manzanares araya tortuga and margarita areas are secondary sources of local influence these insights shed light on the origin of present day terrigenous sediments of the cariaco basin and help to propose alternative explanations for the temporal variability of clay mineralogy observed in previously published studies