influences of the depth dependence of thermal conductivity and expansivity on thermal convection with temperature dependent viscosity in this paper we carried out numerical experiments of a time dependent thermal convection in a two dimensional cartesian box of aspect ratio width height of 6 in order to study the influences on the convecting flow patterns of the spatial variations in physical properties viscosity thermal conductivity k and expansivity a series of calculations by systematically varying the magnitude of spatial variations in these properties showed that the strongly temperature dependent induces the change in flow pattern into the stagnant lid st regime regardless of the increase in k and or decrease in with depth where the convection occurs only beneath a stagnant lid of cold fluid at the top surface in particular we found that the increase in thermal conductivity k with depth significantly affects the convecting flow patterns in the presence of strong temperature dependence in compared with those with uniform k the patterns of st convection with non uniform k are characterized by i thinner top thermal boundary layers or lids and ii larger horizontal length scales of convection cells beneath the stagnant lid in addition the variation in k with depth decreases the threshold values of the temperature dependence in above which the st mode of convection takes place which may also help stabilize the convection cells of large horizontal length scales beneath stagnant lids our results may highlight the potential importance of the increase in thermal conductivity with depth or pressure in controlling the planforms of thermal convection in the mantle of terrestrial planets

the 3d numerical modeling of subduction dynamics plate stagnation and segmentation and crustal advection in the wet mantle transition zone 1 water content in the mantle transition zone mtz has been broadly debated in the earth science community as a key issue for plate dynamics in this study a systematic series of three dimensional 3d numerical simulation are performed in an attempt to verify a hypothesis regarding the behavior of subducted oceanic plate and the role of crust in the mtz under wet conditions this hypothesis is based on the seismic observations of structural variations associated with large scale flattened high velocity anomalies i e stagnant slabs in the mtz in our model weak low viscosity fault zones wfzs which presumably correspond to the fault boundaries of large subduction earthquakes are imposed on the top part of subducting plates the phase transitions of olivine to wadsleyite and ringwoodite to perovskite magnesiowustite under both dry and wet conditions are considered based on recent high pressure experiments the effect of viscosity reduction of wet garnet is also considered for oceanic crust in the mtz results show that there is a substantial difference in the behaviors of subducting plate and the trace of crustal materials between the models under dry and wet conditions under wet conditions the subducting plate tends to stagnate with a maximum lateral extent of over 1000 km the weaker and denser crustal material garnet rich is fed into the wfzs and is advected faster than the plate main body of peridotite olivine rich thus the viscosity and density contrast between the crustal materials and the peridotite permits the plate segmentation under a wet mtz condition

statistical cyclicity of the supercontinent cycle supercontinents like pangea impose a first order control on earth s evolution as they modulate global heat loss sea level climate and biodiversity in a traditional view supercontinents form and break up in a regular perhaps periodic manner in a cycle lasting several 100 myr as reflected in the assembly times of earth s major continental aggregations columbia rodinia and pangea however modern views of the supercontinent cycle propose a more irregular evolution on the basis of an improved understanding of the precambrian geologic record here we use fully dynamic spherical mantle convection models featuring plate like behavior and continental drift to investigate supercontinent formation and breakup we further dismiss the concept of regularity but suggest a statistical cyclicity in which the supercontinent cycle may have a characteristic period imposed by mantle and lithosphere properties but this is hidden in immense fluctuations between different cycles that arise from the chaotic nature of mantle flow

estimates of atmospheric processed soluble iron from observations and a global mineral aerosol model biogeochemical implications 1 desert dust deposition to the ocean may be a significant source of biogeochemically important elements specifically iron the bioavailability of iron in the oceans requires it to be in a soluble form and because atmospheric iron in desert dust is typically insoluble understanding the atmospheric processes that convert insoluble iron to more soluble forms is essential understanding these relationships is especially important in remote ocean regions where iron may be the limiting nutrient observations of soluble iron from 2001 cruise based aerosol measurements over the atlantic and pacific oceans ranged from 0 to 45 mean of 4 9 in the fine mode 2 5 m in diameter and 0 to 87 mean of 2 10 in the coarse mode we test two simple hypotheses of soluble iron enhancement in the atmosphere using a global model of mineral aerosols the first method assumes that iron solubility increases as iron is exposed to solar radiation approximating photoreduction reactions that are important pathways for enhancement of soluble iron in the presence of acidic solutions the second process imitates cloud processing of iron by increasing the amount of soluble iron when the mineral aerosol comes into contact with a cloud both methods resulted in similar average magnitudes of percent soluble iron compared to observations but did not capture specific events or have sufficient variability perhaps because the model does not include aerosol interactions between species other than mineral dust or other processes that may be important

chemical characterization of ambient aerosol collected during the southwest monsoon and intermonsoon seasons over the arabian sea anions and cations ambient aerosol samples were collected over the northern indian ocean during two 1 month long research cruises german r v meteor that took place during the intermonsoon may and sw monsoon july august of 1995 a high volume and two small volume collectors were used to collect samples which were subsequently analyzed for ferrous iron 32 elements and anions and cations the present paper focuses on the bulk aerosol material the ions while utilizing some of the trace metal data that were presented in more detail in our previous paper siefert et al 1999 data are analyzed and interpreted with the aid of principal component and multiple linear regression analyses intermonsoon samples were strongly influenced by continental material both of crustal and anthropogenic origin the crustal component 24 5 13 of the total suspended particulate mass tsp 6 0 4 4 g m 3 contained 3 2 gypsum caso 4 while more than half of the tsp 21 2 9 6 g m 3 during the sw monsoon was sea salt derived due to the strong winds prevailing during this season only 1 7 1 1 0 7 0 4 g m 3 was found to be of crustal origin sulfate so 4 2 sources were determined and quantified with linear regression analyses utilizing specific tracers for the independent variables lead pb was found to be a more reliable surrogate for anthropogenic so 4 2 compared to nitrate no 3 during the relatively polluted intermonsoon soluble calcium ca 2 served as the tracer for gypsum and methane sulfonate msa served as the tracer for biogenically derived so 4 2 during both seasons on the basis of this analysis 75 of the non sea salt sulfate nss so 4 2 0 8 0 2 g m 3 representing 2 4 of tsp was found to be of biogenic origin during the sw monsoon with the remaining 25 of anthropogenic origin during the intermonsoon nss so 4 2 accounted for 2 1 1 2 g m 3 9 2 of tsp and had a composition that was 65 anthropogenic 21 biogenic and 14 gypsum derived linear regression analyses revealed that the bio so 4 2 msa weight ratios appear to be consistent with the temperature dependence proposed by hynes et al 1986 in this case the yield of so 4 2 increased relative to msa with an increase in temperature three samples during the sw monsoon near the coast of oman showed lower temperatures due to coastal upwelling than the rest of the samples at 24 c the bio so 4 2 msa weight ratio was 6 8 0 5 the remainder of the sw monsoon samples were collected at an average temperature of 27 2 c for which the bio so 4 2 msa weight ratio was 13 5 4 4 at an average temperature of 28 9 c during the intermonsoon sampling gave a ratio of 17 7 4 8 these observations indicate a temperature dependence factor between 24 and 29 c of 2 2 i e a 2 2 increase in the ratio of bio so 4 2 msa with every degree temperature increase cl deficits determined during both seasons appear to indicate that different mechanisms may govern the observed depletion of cl in each season

transitions in thermal convection with strongly variable viscosity one of the most important material properties influencing the style of convection in the mantles of terrestrial planets is the extreme temperature dependence of viscosity three dimensional numerical convection calculations in a wide 8 8 1 cartesian box and in a spherical shell ratio of inner to outer radius of 0 55 characteristic of terrestrial planets both display two fundamental transitions as the viscosity contrast is progressively increased from unity to a factor of 105 these transitions not only mark changes in the style of deformation in the upper boundary layer from mobile lid to sluggish lid to stagnant lid but also have dramatic effects on the style planform and horizontal length scales of convection in the entire domain vertical variations of viscosity are the most important for determining the horizontal length scales of the convective patterns while lateral viscosity variations play a role in shaping the relative structures of the upwelling and downwelling flows convection in venus appears to be represented most closely by the sluggish lid regime of convection whereas the earth with plate tectonics more closely resembles the mobile lid style of convection forcing plate like characteristics onto the convective flows in the form of imposed weak zones and prescribed surface velocities results in flow patterns dominated entirely by the form used to enforce the plate like behavior and tells us little about why the mantle exhibits long wavelength heterogeneity

mantle convection with a brittle lithosphere thoughts on the global tectonic styles of the earth and venus summary plates are an integral part of the convection system in the fluid mantle but plate boundaries are the product of brittle faulting and plate motions are strongly influenced by the existence of such faults the conditions for plate tectonics are studied by considering brittle behaviour using byerlee s law to limit the maximum stress in the lithosphere in a mantle convection model with temperature dependent viscosity when the yield stress is high convection is confined below a thick stagnant lithosphere at low yield stress brittle deformation mobilizes the lithosphere which becomes a part of the overall circulation surface deformation occurs in localized regions close to upwellings and downwellings in the system at intermediate levels of the yield stress there is a cycling between these two states thick lithosphere episodically mobilizes and collapses into the interior before reforming the mobile lid regime resembles convection of a fluid with temperature dependent viscosity and the boundary layer scalings are found to be analogous this regime has a well defined nusselt number rayleigh number relationship which is in good agreement with scaling theory the surface velocity is nearly independent of the yield stress indicating that the plate motion is resisted by viscous stresses in the mantle analysis suggests that mobilization of the earth s lithosphere can occur if the friction coeycient in the lithosphere is less than 0 03 0 13 lower than laboratory values but consistent with seismic field studies on venus the friction coeycient may be high as a result of the dry conditions and brittle mobilization of the lithosphere would then be episodic and catastrophic

plume heat flow in a numerical model of mantle convection with moving plates abstract self consistent numerical models are presented for a two dimensional thermal convection with moving plates to estimate the heat flow transported by hot upwelling plumes in both internally and basally heated mantle the plume heat flow depends on the internal heating rate and convective regimes but does not exceed 30 of basal heat flow when the plates rigidly move as observed on the earth the fraction of plume heat flow to basal heat flow becomes higher when the lithosphere behaves as a stagnant lid or as a mechanically not so strong plate when estimated from the dynamic topography of plume swells above hot upwelling plumes the plume heat flow appears to be only a small fraction of the real plume heat flow as a whole the apparent plume heat flow estimated from dynamic topography remains only a few percent of the total heat flow on the surface boundary the value observed for the earth even when the basal heating rate exceeds 50 of the total heating rate such a high fraction of basal heating rate is necessary for the amplitude of topography above hot upwelling plumes to become comparable to the value observed for the earth too it is necessary to reexamine the earlier inference that the heat released from the core manifests itself as plume heat flow in the earth

geodynamically consistent inferences on the uniform sampling of earth s paleomagnetic inclinations abstract paleomagnetism is a key method to reconstruct the earth s paleogeography and thus essential for understanding tectonic evolution but it assumes that the earth s magnetic field structure has always averaged to a geocentric axial dipole gad the gad hypothesis may be tested using the observed inclination frequency distribution but only if continents sampled all of the earth s latitudes uniformly which is not known here we provide new insight into the uniform sampling problem by employing a suite of 3d spherical mantle convection models that feature the self consistent evolution of mantle convection plate tectonics and continental drift over timescales of 2 gyr or more our results suggest that continents unlikely sampled latitudes uniformly during the phanerozoic consistent with previous suggestions this finding is robust for a variety of geodynamic evolutions with different mantle and lithosphere structures at least in the absence of true polar wander for longer sampling durations uniform sampling typically becomes more feasible but may only be achieved with confidence after time scales of minimum 1 3 gyr this time scale depends on the structure of the mantle and lithosphere and may be shortest when upper mantle viscosity is small such that reduced resistive drag at the cratonic base allows for faster continental drift weak plates low plastic yield strength promote more dispersed continent configurations which tends to facilitate uniform sampling if these conditions are not met the uniform sampling time scale can easily exceed several billion years even the minimum estimate of 1 3 gyr challenges the validity of using the phanerozoic inclination frequency distribution to infer the past average magnetic field structure the approach could however still be applicable using the precambrian inclination record

plume heat flow is much lower than cmb heat flow plumes rising from the core mantle boundary cmb are often assumed to transport most or all of the heat conducted across the cmb here this assumption is explored using numerical convection models in idealized geometries that lead to a single plume under steady state or near steady state conditions plume heat transport is calculated for different internal heating rates using two methods and compared to the cmb heat flux for these conditions it is found that the heat flux transported by plumes in the upper mantle is only a fraction of the core heat flux and thus core heat flow estimates derived from observed hotspots could be multiplied by a factor of several d 2005 elsevier b v all rights reserved

generation of plate like behavior and mantle heterogeneity from a spherical viscoplastic convection model 1 how plate tectonics arises from mantle convection is a question that has only very recently become feasible to address with spherical viscoplastic computations we present mainly internally heated convection results with temperature dependent viscosity and explore parts of the rayleigh number ra yield stress y phase space as well as the effects of depth dependent y bottom heating and a low viscosity asthenosphere convective planform and toroidal poloidal velocity field ratio tpr are affected by near surface viscosity variations and tpr values are close to observed values for our most plate like models at the relatively low convective vigor that is accessible at present most models favor spherical harmonic degree one convection though models with a weaker surface viscosity form degree two patterns and reproduce tomographically observed power spectra an asthenospheric viscosity reduction improves plate like nature as expected for our incompressible computations pure bottom heating produces strong plumes that tend to destroy plates at the surface this implies that significant internal heating may be required both to reduce the role of active upwellings and to form a low viscosity zone beneath the upper boundary layer

formation of a future supercontinent through plate motion driven flow coupled with mantle downwelling flow series of high resolution numerical simulations of three dimensional mantle convection were performed to examine the interaction between the drifting continental lithospheres and the underlying mantle structure for 250 m y from the present and to predict the configuration of the future supercontinent the density anomaly of the mantle interior was determined by the seismic velocity anomaly from global seismic tomography data sets which contain well resolved subducting slabs the present day plate motion was imposed for the first stage of the simulation as a velocity boundary condition at the top surface boundary instead of a shear stress free condition the switching time from the plate motion boundary to shear stress free conditions was taken as a free parameter the results revealed that australia eurasia north america and africa will merge together in the northern hemisphere to form a new supercontinent within 250 m y from the present the continental drift was assumed to be realized by plate scale mantle flow rather than large scale upwelling plumes that is continuously moving plates at the surface for the first stage of the simulation are mechanically coupled with the subducting slabs in the mantle this enhances the underlying mantle downwelling flow as a result persistent continental drift can be reproduced for long future time periods even though top surface boundary conditions may switch in response to shear stress free conditions the configuration of the numerically reproduced future supercontinent in this study is broadly consistent with the hypothetical model of amasia as indicated by previous findings from geological correlations and a paleogeographic reconstruction

mantle convection with longest wavelength thermal heterogeneity in a 3 d spherical model degree one or two 1 the formation of longest wavelength mantle convection in the sluggish lid regime is investigated using a three dimensional spherical model the bottom rayleigh number is fixed at 107 considering temperature dependent rheology degree one dominant thermal convection occurs for both purely basal heating and mixed i e basal and internal heating modes for the purely basal heating mode degree one convection occurs when the viscosity contrast due to temperature dependent rheology is 103 104 in both boussinesq and extended boussinesq fluids however with extended boussinesq fluid degree one convection may only occur in the basal heating mode in the mixed heating mode degree one convection shifts to one with high degree modes presumably because of enhanced viscous dissipation in the highly viscous lid over up downwelling plumes the geophysically relevant degree two convection with sheet like downwellings is not observed in this study the inclusion of visco plastic rheology in the top part of the mantle breaks down degree one convection

slab pull mantle convection and pangaean assembly and dispersal two global scale mantle convection cells presently exist on earth centred on upwelling zones in the south pacific ocean and northeast africa one cell panthalassan contains only oceanic plates the other pangaean contains all the continental plates they have remained fixed relative to one another for 400 ma a transverse rheic tethyian subduction system splits the pangaean cell poloidal plate motion in the oceanic cell reflects circumferential pull of panthalassan slabs but toroidal flow in the pangaean cell reflected by vortex type motion of continents toward the altaids of central east asia throughout the phanerozoic has resulted from the competing slab pull forces of both cells the combined slab pull effects from both cells also controlled pangaean assembly and dispersal assembly occurred during palaeozoic clockwise toroidal motion in the pangaean cell when gondwana was pulled into pangaea by the ne trending rheic subduction zone forming the appalachian variscide altaid chain pangaean dispersal occurred when the rheic trench re aligned in the jurassic to form the nw trending tethyside subduction system which pulled east gondwanan fragments in the opposite direction to form the cimmerian himalayan alpine chain this re alignment also generated a new set of indian mid ocean ridge systems which dissected east gondwana and facilitated breakup 100 200 myr long phanerozoic wilson cycles reflect rifting and northerly migration of gondwanan fragments across the pangaean cell into the rheic tethyian trench pangaean dispersal was amplified by retreat of the panthalassan slab away from europe and africa which generated mantle counterflow currents capable of pulling the americas westward to create the atlantic ocean thermal blanketing beneath pangaea and related hotspot activity were part of a complex feedback mechanism that established the breakup pattern but slab retreat is considered to have been the main driving force the size and longevity of the two cells organised and maintained by long lived slab pull forces favours deep mantle convection as the dominant circulation process during the phanerozoic

vertical profiling of optical and physical particle properties over the tropical indian ocean with six wavelength lidar 2 case studies detlef muller kathleen franke frank wagner dietrich althausen albert ansmann and jost heintzenberg vertical profiling of optical and physical particle properties over the tropical indian ocena with six wavelength lidar 1 seasonal cycle journal of geophysical research atmospheres vol 106 d22 28567 28575 november 2001 doi 10 1029 2000jd900784 first published online 1 november 2001 copyright 2001 by the american geophysical union

a study of the sulfur cycle in the antarctic marine boundary layer atmospheric sampling was conducted aboard r v polar duke from march 21 to april 27 1986 in the southern drake passage and in the coastal waters west of the antarctic peninsula average atmospheric concentrations n 6 of sulfur dioxide so2 vapor aerosol methanesulfonate ch3so3 and aerosol non sea salt sulfate nss so4 were 0 36 0 22 and 0 34 nmol m 3 respectively cascade impactor data indicated that more than 85 of the masses of both ch3so3 and nss so4 were on particles of 0 25 m mean aerodynamic radius average concentrations n 8 of ch3so3 and nss so4 in precipitation were 0 09 and 0 57 m respectively box model calculations based on this limited data suggest a total sulfur removal rate from the local marine boundary layer of 0 3 7 2 mol m 2 day 1 this removal approximately balances the oceanic source of dimethylsulfide dms sulfur of 1 2 4 4 mol m 2 day 1 100 estimated by berresheim 1987 from data obtained during the same cruise the box model calculations also suggest that heterogeneous so2 oxidation may be the major pathway for dms to sulfate conversion 75 and or that other sources of sulfate may not be negligible observed mineral aerosol and 222rn concentrations were low but slightly elevated above pure remote marine background levels suggesting a weak but discernible continental character in sampled air uncertainties are large owing to the limited sampling time study area and numbers of samples obtained but overall the sulfur cycle in the coastal antarctic marine boundary layer appears not to differ remarkably from that in other remote marine regions

time dependent convection models of mantle thermal structure constrained by seismic tomography and geodynamics implications for mantle plume dynamics and cmb heat flux summary one of the outstanding problems in modern geodynamics is the development of thermal convection models that are consistent with the present day flow dynamics in the earth s mantle in accord with seismic tomographic images of 3 d earth structure and that are also capable of providing a time dependent evolution of the mantle thermal structure that is as realistic earth like as possible a successful realization of this objective would provide a realistic model of 3 d mantle convection that has optimal consistency with a wide suite of seismic geodynamic and mineral physical constraints on mantle structure and thermodynamic properties to address this challenge we have constructed a time dependent compressible convection model in 3 d spherical geometry that is consistent with tomography based instantaneous flow dynamics using an updated and revised pseudo spectral numerical method the novel feature of our numerical solutions is that the equations of conservation of mass and momentum are solved only once in terms of spectral green s functions we initially focus on the theory and numerical methods employed to solve the equation of thermal energy conservation using the green s function solutions for the equation of motion with special attention placed on the numerical accuracy and stability of the convection solutions a particular concern is the verification of the global energy balance in the dissipative compressible mantle formulation we adopt such validation is essential because we then present geodynamically constrained convection solutions over billion year timescales starting from present day seismically constrained thermal images of the mantle the use of geodynamically constrained spectral green s functions facilitates the modelling of the dynamic impact on the mantle evolution of 1 depth dependent thermal conductivity profiles 2 extreme variations of viscosity over depth and 3 different surface boundary conditions in this case mobile surface plates and a rigid surface the thermal interpretation of seismic tomography models does not provide a radial profile of the horizontally averaged temperature i e the geotherm in the mantle one important goal of this study is to obtain a steady state geotherm with boundary layers which satisfies energy balance of the system and provides the starting point for more realistic numerical simulations of the earth s evolution we obtain surface heat flux in the range of earth like values 37 tw for a rigid surface and 44 tw for a surface with tectonic plates coupled to the mantle flow also our convection simulations deliver cmb heat flux that is on the high end of previously estimated values namely 13 tw and 20 tw for rigid and plate like surface boundary conditions respectively we finally employ these two end member surface boundary conditions to explore the very long time scale evolution of convection over billion year time windows these billion year scale simulations will allow us to determine the extent to which a memory of the starting tomography based thermal structure is preserved and hence to explore the longevity of the structures in the present day mantle the two surface boundary conditions along with the geodynamically inferred radial viscosity profiles yield steady state convective flows that are dominated by long wavelengths throughout the lower mantle the rigid surface condition yields a spectrum of mantle heterogeneity dominated by spherical harmonic degree 3 and 4 and the plate like surface condition yields a pattern dominated by degree 1 our exploration of the time dependence of the spatial heterogeneity shows that for both types of surface boundary condition deep mantle hot upwellings resolved in the present day tomography model are durable and stable features these deeply rooted mantle plumes show remarkable longevity over very long geological time spans mainly owing to the geodynamically inferred high viscosity in the lower mantle

layered convection induced by phase transitions we report a systematic study on the conditions under which an endothermic phase transition can enforce layered convection two dimensional numerical calculations of convection in a domain containing a divariant phase change were performed in the framework of the extended boussinesq approximation i e considering the effects of adiabatic gradient latent heat and frictional heating in the energy equation we find that the critical value of the negative clapeyron slope which must be surpassed in order to induce layered convection decreases in magnitude with increasing rayleigh number ra in the range 104 ra 2 106 near the critical clapeyron slope vacillations between double and single layer convection or strongly leaking double layer convection are possible the breakdown into layers is influenced very little by the latent heat release but depends solely on the phase boundary deflection caused by lateral temperature differences the value of the critical clapeyron slope also seems little affected by the width of the transition zone or by its depth a possible superplastic rheology within the transition zone would tend to favor layered convection scaling the model results to the 670 km discontinuity in the earth s mantle as a possible endothermic phase boundary we estimate the critical clapeyron slope to be in the range of 4 to 8 mpa k 40 to 80 bar k the possibility that the spinel perovskite periclase transition is within this range appears to be remote but certainly cannot be neglected

effects on the long wavelength geoid anomaly of lateral viscosity variations caused by stiff subducting slabs weak plate margins and lower mantle rheology abstract instantaneous flow numerical calculations in a three dimensional spherical shell are employed to investigate the effects of lateral viscosity variations lvvs in the lithosphere and mantle on the long wavelength geoid anomaly the density anomaly model employed is a combination of seismic tomography and subducting slab models based on seismicity the global strain rate model is used to represent weak low viscosity plate margins in the lithosphere lvvs in the mantle are represented on the basis of the relation between seismic velocity and temperature i e temperature dependent rheology when highly viscous slabs in the upper mantle are considered the observed positive geoid anomaly over subduction zones can be accounted for only when the viscosity contrast between the reference upper mantle and the lower mantle is approximately 1 0 3 or lower and weak plate margins are imposed on the lithosphere lvvs in the lower mantle exert a large influence on the geoid pattern the calculated geoid anomalies over subduction zones exhibit generally positive patterns with quite high amplitudes compared with observations even when the low activation enthalpy of perovskite in the lower mantle is employed inferred weak slabs in the lower mantle may be explained in terms of recent mineral physics results highlighting the possibility of grain size reduction due to the postspinel phase transition

geodynamo reversal frequency and heterogeneous core mantle boundary heat flow abstract we analyze more than two hundred and forty magnetic polarity reversals and excursions over forty million years of time from numerical dynamos with heterogeneous boundary heat flux comparisons between a dynamo with uniform boundary heat flux dynamos with boundary heat flux patterns consisting of a single spherical harmonic and dynamos with tomographic heat flux patterns proportional to the large scale lateral variation of seismic shear wave velocity in the lower mantle reveal that boundary heat flux heterogeneity tends to increase the average frequency of polarity reversals relative to uniform boundary conditions particularly if the heterogeneity increases the average equatorial heat flux non axisymmetric spherical harmonic degree l 1 and 2 boundary heterogeneity and the seismically derived tomographic heterogeneity produce comparable reversal and excursion frequencies in our models suggesting that geomagnetic polarity reversal rates may be relatively insensitive to the non axisymmetric planform of core mantle boundary heat flux in contrast the average polarity reversal frequency in our models is quite sensitive to the total boundary heat flow and to the total heat flow at the equator with non reversing behavior at low heat flow and frequent reversals at high heat flow conditions reversal frequency also increases with the amplitude of the boundary heterogeneity although it is less sensitive to this parameter our results suggest that geomagnetic superchrons may correspond to times with reduced core mantle boundary heat flow either globally or equatorially and conversely periods with high paleomagnetic reversal frequency may correspond to times with increased heat flow at the core mantle boundary

the origin of large scale structure in mantle convection effects of plate motions and viscosity stratification convection in earth close quote s mantle is dominated by long wavelength structure as evidenced by the very open quote open quote red close quote close quote spectra of both seismic velocity heterogeneity in the deep mantle and the non hydrostatic gravity field or geoid here we show that this large scale structure may be a consequence of two factors that influence the scale of mantle convection first the existence of surface plates which tend to organize the flow second a substantial increase in lower mantle viscosity for which there is considerable independent geophysical evidence combining these two factors in 3 d spherical mantle convection models explains rather well the observed seismic spectrum of mantle heterogeneity copyright american geophysical union 1996

chemical characterization of ambient aerosol collected during the southwest monsoon and intermonsoon seasons over the arabian sea labile fe ii and other trace metals atmospheric deposition of iron fe to certain regions of the oceans is an important nutrient source of fe to the biota and the ability of the biota to uptake fe is dependent on the speciation of the fe therefore understanding the speciation of fe in the atmosphere is critical to understanding the role of fe as a nutrient source in surface ocean waters labile ferrous iron fe ii concentrations as well as total concentrations for fe and other important trace metals cations and anions were determined over the arabian sea for two nonconsecutive months during 1995 ambient aerosol samples were collected during the indian ocean intermonsoon and southwest monsoon seasons over the arabian sea sampling took place aboard the german research vessel meteor in the months of may leg m32 3 intermonsoon and july august leg m32 5 southwest monsoon both cruise tracks followed the 65th east meridian traveling for 30 days each from north to south during leg m32 3 and from south to north during leg m32 5 a high volume dichotomous virtual impactor with an aerodynamic cutoff size of 3 m was used to collect the fine and coarse aerosol fractions for metal analysis a low volume collector was used to collect aerosol samples for anion and cation analysis the analysis for labile fe ii was done immediately after sample collection to minimize any possible fe redox reactions which might occur during sample storage the analytical procedure involved filter extraction in a formate formic acid buffered solution at ph 4 2 followed by colorimetric quantification of soluble fe ii metals anions and cations were analyzed after the cruise total atmospheric aqueous labile fe ii concentrations during the intermonsoon were between 4 75 and 80 was present in the fine fraction 3 0 m during the southwest monsoon atmospheric aqueous labile fe ii concentrations were consistently below the detection limit 0 34 to 0 089 ng m 3 depending on the volume of air sampled air mass back trajectories 5 day three dimensional showed that air masses sampled during the southwest monsoon had advected over the open indian ocean while air masses sampled during the intermonsoon had advected over northeast africa the saudi arabian peninsula and southern asia these calculations were consistent with the results of the statistical analysis performed on the data set which showed that the variance due to crustal species during the intermonsoon samples was greater than the variance due to crustal species during the southwest monsoon the factor scores for the crustal components were also greater when the back trajectories had advected over the nearby continental masses principal component analysis was also performed with the intermonsoon samples where aqueous labile fe ii was above the detection limit aqueous labile fe ii did not correlate well with other species indicating possible atmospheric processing of the iron during advection

sea salt corrections and interpretation of constituent ratios in marine precipitation differentiation of sea salt and non sea salt or excess components is essential for many studies of marine aerosol and precipitation chemistry uncertainties in such calculations arise from 1 uncertainties in the composition of seawater 2 analytical uncertainties 3 the amount of dry deposited sea salt in samples 4 the validity of assuming a purely marine source for the sea salt reference species and 5 the validity of assuming no fractionation during or after production of sea salt aerosols we assessed these uncertainties and using the reduced major axis regression technique evaluated these assumptions by analyzing the composition of precipitation at amsterdam island 38 s 78 e and at bermuda 32 n 65 w precipitation on amsterdam island contained significant excess concentrations of na 2 and cl 5 relative to mg2 indirect evidence indicates that these departures from seawater ratios are probably not due to the influence of terrestrial material the excess cl was sufficient to account for approximately 20 of free acidity suggesting that scavenging of vapor phase hcl may affect the acid base chemistry of precipitation at the site volume weighted excess so42 based on na and mg2 differed by 11 a nonnegligible source of bias in estimating the wet deposition of non sea salt sulfur precipitation on bermuda contained significant concentrations of locally derived alkali and alkaline earth metals the influence of continental and non sea salt marine sources on concentrations of excess so42 was also evident these observations suggest that assumptions involved in sea salt corrections are not always satisfied and that it is therefore necessary to evaluate individual data sets using objective criteria to select the appropriate reference species

upwelling plumes superswells and true polar wander summary the geological evolution of the rotational axis of the earth is most likely controlled by internal mass redistribution within the mantle palaeomagnetic observations suggest that it is episodic in nature with periods of quasi standstill alternating with periods of faster wander here we investigate two models for the influence of mantle plumes that vary at different spatial wavelengths on the time variations of the rotational axis true polar wander tpw in the first model we represent an upwelling plume as a sphere whose radius varies as a function of the flux of material in the conduit and that traverses the mantle at the stokes velocity such a plume produces very little wander of the rotational axis we then study the effects of two superswells that mimic the ones observed with seismic tomography and conclude that a doming regime within the mantle involves significant polar wander some of the features of this tpw that are directly linked to the periodicity of doming are reminiscent of observed phases of slow and fast tpw with similar peak velocities

effects of various lithospheric yield stresses and different mantle heating modes on the breakup of the pangea supercontinent numerical simulations of three dimensional spherical mantle convection were performed to investigate the effects of various lithospheric yield stresses and two different mantle heating modes i e mixed heating from the bottom and interior and purely internal heating on the breakup of the pangea supercontinent and the subsequent continental drift for the past 200 myr results show that the continental breakup and subsequent continental drift are accomplished in mantle convection models with two different heating modes this implies that active upwelling plumes from the core mantle boundary are not necessarily required for the breakup of pangea in addition i found that the continental breakup is only realized when choosing a moderate value of the yield stress 120 mpa the bound on the yield stress derived in the present study will potentially enable the self consistent reconstruction of continental breakup and drift as well as the accompanying pattern on mantle convection since 200 ma

various influences on plumes and dynamics in time dependent compressible mantle convection in 3 d spherical shell the effects on the dynamics of compressible mantle convection from increasing rayleigh number viscosity stratification and internal heating are investigated in a time dependent 3 d spherical shell model resolution of up to spherical harmonics of degree 192 and 150 higher order finite difference grid points unevenly distributed in the radial direction have been employed surface rayleigh number varies from 5 104 to o 107 the viscosity profiles used range from constant to a depth dependent viscosity profile the adams williamson adiabatic assumption is adopted for describing the equation of state both depth dependences of thermal expansivity and thermal conductivity are included we have focused on the viscosity stratification in the lower mantle the mean temperature profiles are very sensitive to the input parameters such as the amount of internal heating and the dissipation number in the isoviscous model the number of hot plumes decreases for low rayleigh number 5 104 and increases dramatically with higher rayleigh number 106 in the model with a high viscosity zone in the lower mantle the number of plumes is small and does not change too much when the rayleigh number varies from 3 106 to 8 106 in the purely base heated viscously stratified models well organized triple junctions of the downwelling sheets are developed near the surface and interconnected cold walls of downwellings are produced around upwelling plumes the presence of internal heating increases the number of upwelling plumes and breaks up the interconnection of downwelling sheets strong viscous heating in the compressible solutions is found near the top and bottom at the higher values of rayleigh number the laterally averaged value of viscous dissipation is comparable with the chondritic strength of internal heating whether the maximum viscous heating is found in upwelling plumes or downwelling flows depends on the presence of internal heating and the mechanical boundary conditions the effects of compressibility on local structures and plume morphologies can be significant however these compressible effects are moderate on the global patterns

basaltic accumulation instability and chaotic plate motion in the earliest mantle inferred from numerical experiments 1 a series of self consistent numerical models of mantle convection with magmatism and moving plates are presented to clarify the dynamics of the strongly heated mantle of the earliest earth broad hot accumulations of subducted basaltic crusts develop on the core mantle boundary and remain there for geologic time only when the internal heating rate is below a threshold above the threshold the basaltic accumulations become unstable instead the thermal buoyancy induced by the strong internal heating repeatedly causes a massive upwelling or burst of hot materials from deep mantle the mantle burst induces vigorous magmatic activity on the surface the magmatic activity in turn damages the lithosphere and disintegrates it into smaller fragments in contrast to the well ordered plate motion observed below the threshold the lithospheric fragments chaotically move and new subduction zones sporadically develop the mantle burst and chaotic motion of lithospheric fragments effectively stir the mantle and the mantle remains chemically rather homogeneous in spite of the chemical differentiation due to the vigorous magmatism the earth s mantle convection has probably changed its regime from one dominated by frequent mantle bursts and chaotic motion of lithospheric fragments to one dominated by stable basaltic accumulations and well ordered plate motion as the internal heat source decays the suggested regime transition fits in with the earth s tectonic evolution inferred from geologic observations of the continents of various ages

low degree mantle convection with strongly temperature and depth dependent viscosity in a three dimensional spherical shell 1 a series of numerical simulations of thermal convection of boussinesq fluid with infinite prandtl number with rayleigh number 10 7 and with the strongly temperature and depth dependent viscosity in a three dimensional spherical shell is carried out to study the mantle convection of single plate terrestrial planets like venus or mars without an earth like plate tectonics the strongly temperature dependent viscosity the viscosity contrast across the shell is 10 5 makes the convection under stagnant lid short wavelength structures numerous cylindrical upwelling plumes are developed because of the secondary downwelling plumes arising from the bottom of lid this convection pattern is inconsistent with that inferred from the geodesic observation of the venus or mars additional effect of the stratified viscosity at the upper lower mantle the viscosity contrast is varied from 30 to 300 are investigated it is found that the combination of the strongly temperature and depth dependent viscosity causes long wavelength structures of convection in which the spherical harmonic degree l is dominant at 1 4 the geoid anomaly calculated by the simulated convections shows a long wavelength structure which is compared with observations the degree one l 1 convection like the martian mantle is realized in the wide range of viscosity contrast from 30 to 100 when the viscosity is continuously increased with depth at the lower mantle

magmatic sulfides and au cu ratios in porphyry deposits an experimental study of copper and gold partitioning at 850 c 100 mpa in a haplogranitic melt pyrrhotite intermediate solid solution gold metal assemblage at gas saturation abstract the behavior of copper and gold in the phase assemblage pyrrhotite po intermediate solid solution iss haplogranitic melt gold metal capsule at 850 c 100 mpa and gas saturation ah2o 0 9 has been investigated the results are presented as nernst partition coefficients the partition coefficient dcupo melt 2 6 0 3 103 1 was determined for po and melt in equilibrium with an iss of composition cu 19 1 fe 30 1 s 51 1 2 for gold the partition coefficients are dauiss melt 5 7 2 2 103 1 between iss and melt and daupo melt 140 40 1 between po and melt these imply dauiss po 42 9 1 between iss and po the oxygen fugacity for the experiments log f o 2 13 2 0 3 1 was determined by using the hydrogen sensor technique the sulfur fugacity log f s 2 1 0 0 7 1 was calculated from the composition of po in our experiments melts at iss saturation have cu fe by weight of ca 1 100 the results indicate that crystallization of iss may deplete a melt in gold and crystallization of po may deplete a melt in copper however a melt saturated with iss at a given bulk composition temperature pressure oxygen and sulfur fugacity and feo concentration will have a fixed concentration of cu preventing strong depletion of the melt in cu until iss is fractionated from the melt or changes in environmental conditions destabilize iss a thermodynamic model for the partitioning of cu and au between po and melt in terms of oxygen and sulfur fugacity indicates that dcupo melt and daupo melt may increase greatly with increasing sulfur fugacity and decreasing oxygen fugacity within the po stability field the model also suggests that destabilization of both iss and po relative to magnetite due to changes in oxygen and sulfur fugacities can result in the release of cu and au from the sulfides to the melt

degree one mantle convection dependence on internal heating and temperature dependent rheology 1 geophysical and geological observations suggest that a degree one mantle flow pattern consisting of one upwelling and one downwelling may have existed at some time in the mantles of mars the moon and perhaps even for the earth during times of supercontinent formation simple fluid experiments utilizing isoviscous rheologies predict shorter wavelength flow patterns and it is therefore important to determine fluid dynamical parameter sets which lead to larger wavelength flow patterns consistent with observations we perform a series of numerical fluid dynamics calculations in a spherical 3 d geometry in which we vary rayleigh number rheological activation parameter and the degree of internal heating in order to define which parameter choices can lead to degree one mantle convection we find that increasing the degree of internal heating increases the interior temperature of the mantle which leads to a larger viscosity contrast across the top thermal boundary layer and that degree one mantle flow occurs only in cases which exhibit internal heating utilize activation coefficients which lead to 103 or higher viscosity contrasts across the mantle and have viscosity contrasts across the top thermal boundary layer within the range of 200 3000

pāhoehoe flow cooling discharge and coverage rates from thermal image chronometry 1 theoretically and empirically derived cooling rates for active p hoehoe lava flows show that surface cooling is controlled by conductive heat loss through a crust that is thickening with the square root of time the model is based on a linear relationship that links log time with surface cooling this predictable cooling behavior can be used assess the age of recently emplaced sheet flows from their surface temperatures using a single thermal image or image mosaic this allows quantification of the variation in areal coverage rates and lava discharge rates over 48 hour periods prior to image capture for p hoehoe sheet flow at k lauea hawai i this gives coverage rates of 1 5 m2 min at discharge rates of 0 01 0 05 m3 s increasing to 40 m2 min at 0 4 0 5 m3 s our thermal chronometry approach represents a quick and easy method of tracking flow advance over a three day period using a single thermal snap shot

self consistent generation of single plume state for enceladus using non newtonian rheology the thermal dichotomy of enceladus suggests an asymmetrical structure in its global heat transfer so far most of the models proposed that obtained such a distribution have prescribed an a priori asymmetry i e some anomaly in or below the south polar ice shell we present here the first set of numerical models of convection that yield a stable single plume state for enceladus without prescribed mechanical asymmetry using the convection code stagyy in a 2 d spherical annulus geometry we show that a non newtonian ice rheology is sufficient to create a localized single hot plume surrounded by a conductive ice mantle we obtain a self sustained state in which a region of small angular extent has a sufficiently low viscosity to allow subcritical to weak convection to occur due to the stress dependent part of the rheological law we find that the single plume state is very unlikely to remain stable if the rheology is newtonian confirming what has been found by previous studies in a second set of numerical simulations we also investigate the first order effect of tidal heating on the stability of the single plume state tidal heating reinforces the stability of the single plume state if it is generated in the plume itself lastly we show that the likelihood of a stable single plume state does not depend on the thickness of the ice shell

core mantle boundary topography estimated from numerical simulations of instantaneous mantle flow core mantle boundary cmb topography is estimated from numerical simulations of instantaneous mantle flow in a three dimensional spherical shell geometry density anomaly models of the earth s mantle are derived from geodynamic models constrained principally by seismic tomography results to account for the small 1 5 km cmb topographic relief inferred from recent seismological results lateral viscosity variations in the mantle compositionally dense piles in the deep mantle and a low viscosity d layer are all required for the numerical models low viscosity subducting slabs in the lower mantle serve to reduce the topographic amplitude below high velocity regions i e the circum pacific belt the existence of dense piles may cause the local topographic depressions even below low velocity regions i e the south pacific and atlantic africa regions

chemical characterization of ambient aerosol collected during the northeast monsoon season over the arabian sea labile fe ii and other trace metals 1 ambient aerosol samples were collected over the arabian sea during the month of march of 1997 aboard the german r v sonne as part of the german joint global ocean flux study jgofs project this is the third study in a series of analogous measurements taken over the arabian sea during different seasons of the monsoon dichotomous high volume collector samples were analyzed for ferrous iron immediately after collection while trace metals anions and cations were determined upon return to the laboratory the main crustal component was geochemically well represented by the average crustal composition and amounted to 5 94 3 08 m gm 3 an additional crustal constituent of clay like character rich in water soluble ca and mg was seen in the fine fraction in air masses of arabian origin total ferrous iron concentrations varied from 3 9 to 17 2 ng m 3 and averaged 9 8 3 4 ng m 3 with 87 2 of fe ii present in the fine aerosol fraction fe ii concentrations accounted for on average 1 3 0 5 of the total fe while ferrous iron in the coarse fraction appeared to be correlated with the main crustal component the fine fe ii fraction exhibited a more complex behavior the anthropogenic contribution to the aerosol as traced by pb zn and some anions and cations was found to be considerably larger especially during the first 10 days of this cruise than in previously collected samples from the inter monsoon and southwest monsoon of 1995 index terms 0305 atmospheric composition and structure aerosols and particles 0345 4801 0365 atmospheric composition and structure troposphere composition and chemistry 4801 oceanography biological and chemical aerosols 0305 4805 oceanography biological and chemical biogeochemical cycles 1615 4875 oceanography biological and chemical trace elements keywords ferrous iron trace metals aerosol particles indian ocean citation johansen a m and m r hoffmann chemical characterization of ambient aerosol collected during the northeast monsoon season over the arabian sea labile fe ii and other trace metals j geophys res 108 d14 4408 doi 10 1029 2002jd003280 2003

on the possibility of a folded crustal layer stored in the hydrous mantle transition zone abstract previous numerical studies of mantle convection focusing on subduction dynamics have indicated that viscosity rheological heterogeneity in the subducting plates and the surrounding mantle may have a primary effect on the behavior of subducting plates meanwhile the existence of a low viscosity layer lvl beneath the mantle transition zone mtz has been suggested by several geodynamic studies we investigated the effects of a rheologically weaker crustal layer in a hydrous wet mtz with an underlying lvl on the behavior of subducting plates and determined the trace of crustal material using a numerical simulation model of subduction dynamics in three dimensional regional spherical geometry the combined effect of a rheologically weaker crustal layer and an lvl beneath the mtz produces characteristic transient behavior of the crustal layer whose trace shows a folded loop like feature in the wet mtz along with a folded subducting slab in the lvl this phenomenon is caused by mechanical interaction of the crustal layer with an upward positive bending force due to the main body of the subducting slab folding in the lvl and an enhanced downward negative buoyancy force due to an increase in density from the garnet to the post garnet phase of crustal material the transiently folded crustal layer behind the root of the stagnant slab is also observed this result is consistent with seismological evidence suggesting a piece of subducted oceanic crust in the uppermost lower mantle beneath the subducting slab in the mariana trench over half of the subducted crust is temporarily stored in the mtz however this stored crustal material begins to penetrate into the lower mantle within 10 myr a relatively short timeframe relative to the overturn time of mantle convection eventually most of the crustal material is entrained into the lower mantle but a non negligible portion remains stored in the mtz for 50 myr after the beginning of the accumulation of subducted crust in the mtz

degree 1 convection in the martian mantle and the origin of the hemispheric dichotomy 1 the surface of mars appears dramatically different between the northern and southern hemispheres any endogenic origin for this hemispheric dichotomy must involve a pattern of mantle convection that reflects the shape of the dichotomy primarily spherical harmonic degree 1 we investigated two mechanisms by which degree 1 convection may be initiated in the martian mantle 1 an endothermic phase change near the cmb and 2 viscosity layering in the mid mantle using two dimensional 2 d and 3 d spherical finite element convection models we explored the conditions under which each mechanism can produce degree 1 structures the phase transition is only effective at generating degree 1 structures when the mantle viscosity is constant or weakly temperature dependent activation energy 100 kj mol but the degree 1 pattern requires several billion years to develop increasing convective vigor in phase change models leads to reduced wavelengths for convective structures degree 1 convection can also develop in a layered viscosity mantle with temperature and depth dependent viscosity an overall sublithospheric radial viscosity variation of a factor of 100 including a factor of 8 25 jump in the midmantle can lead to formation of degree 1 structure in a timescale ranging from 100 my to several hundred my consistent with the timescale for the formation of the dichotomy neither convective vigor nor the internal heating rate greatly affects the formation of degree 1 structures we propose that degree 1 mantle convection induced by a layered viscosity structure may be responsible for the formation of the crustal dichotomy

constraints on mantle viscosity structure from continental drift histories in spherical mantle convection models abstract earth s continents drift in response to the force balance between mantle flow and plate tectonics and actively change the plate mantle coupling thus the patterns of continental drift provide relevant information on the coupled evolution of surface tectonics mantle structure and dynamics here we investigate rheological controls on such evolutions and use surface tectonic patterns to derive inferences on mantle viscosity structure on earth we employ global spherical models of mantle convection featuring self consistently generated plate tectonics which are used to compute time evolving continental configurations for different mantle and lithosphere structures our results highlight the importance of the wavelength of mantle flow for continental configuration evolution too strong short wavelength components complicate the aggregation of large continental clusters while too stable very long wavelength flow tends to enforce compact supercontinent clustering without reasonable dispersal frequencies earth like continental drift with episodic collisions and dispersals thus requires a viscosity structure that supports long wavelength flow but also allows for shorter wavelength contributions such a criterion alone is a rather permissive constraint on internal structure but it can be improved by considering continental oceanic plate speed ratios and the toroidal poloidal partitioning of plate motions the best approximation of earth s recent tectonic evolution is then achieved with an intermediate lithospheric yield stress and a viscosity structure in which oceanic plates are 103 more viscous than the characteristic upper mantle which itself is 100 200 less viscous than the lowermost mantle such a structure causes continents to move on average 2 2 1 0 slower than oceanic plates consistent with estimates from present day and from plate reconstructions this does not require a low viscosity asthenosphere globally extending below continental roots however this plate speed ratio may undergo strong fluctuations on timescales of several 100 myr that may be linked to periods of enhanced continental collisions and are not yet captured by current tectonic reconstructions

a regime diagram of mobile lid convection with plate like behavior abstract terrestrial planetary bodies that undergo solid state convection can exhibit a variety of tectonic styles from stagnant lid one plate planets to those with a mobilized lithosphere for modeled planetary convection the de facto mode of recycling of lithosphere into the planetary interior is typically achieved through 2 sided and symmetric downwelling flows however lithosphere recycling on earth occurs in a distinctly 1 sided mode known as subduction using numerical models of mantle convection in which the viscosity of planetary mantle material is strongly temperature dependent yet maintains a finite material strength as dictated by its yield stress we investigate the continuum of mobile lid convection with plate like behavior the models span a parameter space of rayleigh number and plate strength and explore convective systems with low yield stresses resulting in weak subduction hinges that bend easily and highly deformable subducted slabs three distinct modes are found to occur in convective systems with weak plates the stagnant lid mode 2 sided downwelling mode and a mode that alternates between 1 sided subduction and 2 sided downwellings we classify the style of convective downwelling for a range of models and show that mode selection strongly depends on the combination of surface mobility and strength of the downgoing plate using these measurements we have developed a regime diagram that can predict whether a particular system will be in one of those three modes

seismostratigraphy and thermal structure of earth s core mantle boundary region we used three dimensional inverse scattering of core reflected shear waves for large scale high resolution exploration of earth9s deep interior d and detected multiple piecewise continuous interfaces in the lowermost layer d beneath central and north america with thermodynamic properties of phase transitions in mantle silicates we interpret the images and estimate in situ temperatures a widespread wave speed increase at 150 to 300 kilometers above the coremantle boundary is consistent with a transition from perovskite to postperovskite internal d stratification may be due to multiple phase boundary crossings and a deep wave speed reduction may mark the base of a postperovskite lens about 2300 kilometers wide and 250 kilometers thick the core mantle boundary temperature is estimated at 3950 200 kelvin beneath central america a site of deep subduction the d is relatively cold t 700 100 kelvin accounting for a factor of two uncertainty in thermal conductivity core heat flux is 80 to 160 milliwatts per square meter mw m 2 into the coldest d region and 35 to 70 mw m 2 away from it combined with estimates from the central pacific this suggests a global average of 50 to 100 mw m 2 and a total heat loss of 7 5 to 15 terawatts

temporal evolution of the stress state in a supercontinent during mantle reorganization summary previous numerical simulation models of isoviscous mantle convection in both 2 d and 3 d geometries have revealed that the presence of a supercontinental lid produces large scale horizontal mantle flow thereby reorganizing the thermal structure of the mantle interior large scale upwelling plumes arising from the core mantle boundary cmb beneath the supercontinent are observed in some models these upwelling plumes have been hypothesized to produce tensional stresses in the supercontinent and to be responsible for the subsequent continental lifting and breakup in this study numerical simulations using a 3 d spherical shell model are conducted in order to elucidate temporal changes in stress state in an idealized supercontinent during mantle reorganization the model supercontinent is assumed to be an undeformable highly viscous supercontinental lid hvsl with respect to the mantle the mantle is heated from the base and from within the hvsl is imposed abruptly on well developed convection systems and remains fixed during the simulations i first consider the scenario in which the hvsl is imposed on isoviscous or weakly temperature dependent viscosity convection exhibiting short wavelength thermal structures the simulation results reveal that a tensional stress regime rapidly predominates throughout the hvsl within 50 100 myr in model time in response to the horizontal mantle flow beneath the hvsl the subsequent large scale plumes upwelling from the cmb also induce tensional stresses in the supercontinent but the effects of these stresses on the overall stress state appear to be secondary for most of the models investigated herein the maximum deviatoric tensional stress generated in the hvsl once convection is re established is 30 90 mpa which is probably comparable to the minimum stress magnitude required to break up the supercontinent to investigate the effects of the hvsl on other convection regimes i next consider the scenario in which an hvsl having a viscosity contrast of 10 is imposed on much longer wavelength i e degree one or degree two dominant convection systems with moderately temperaturedependent viscosity the patterns of such low degree dominant convection are affected little by the imposition of the hvsl and notable mantle reorganization does not occur on the other hand when a reasonable yielding stress is imposed at the continental margins newly formed weak upwelling plumes develop due to enhanced downwelling at the margins rather than large scale mantle reorganization however the newly upwelling plumes do not seem to produce a visible tensional stress regime in the hvsl interior and the spatial distributions of the stress regime and stress magnitude in the hvsl depend on the spatial relationship between the hvsl and the convection pattern since the series l models of the present study do not include the yield stress in the oceanic part of the lithosphere plate like behaviour and subducting plates are not actualized the results for series s models in which the oceanic lithosphere freely sinks and the convection pattern is easily organized imply that the broadly tensional regime prevailing in the supercontinent may support the penetration of mantle plumes into the supercontinent the emergence of hotspots and subsequent continental uplift and break up

plate tectonics on super earths equally or more likely than on earth abstract the discovery of extra solar super earths has prompted interest in their possible mantle dynamics and evolution and in whether their lithospheres are most likely to be undergoing active plate tectonics like on earth or be stagnant lids like on mars and venus the origin of plate tectonics is poorly understood for earth likely involving a complex interplay of rheological compositional melting and thermal effects which makes it challenging to make reliable predictions for other planets nevertheless as a starting point it is common to parameterize the complex processes involved as a simple yield stress that is either constant or has a byerlee s law dependence on pressure because the simplifying assumptions made in developing analytical scalings may not be valid over all parameter ranges numerical simulations are needed one numerical study on super earths finds that plate tectonics is less likely on a larger planet o neill and lenardic 2007 in apparent contradiction of an analytical scaling study valencia et al 2007 to try and understand this we here present new calculations of yielding induced plate tectonics as a function of planet size focusing on the idealized end members of internal heating or basal heating as well as different strength profiles and compared to analytical scalings in the present study we model super earths as simple scaled up versions of earth i e assuming constant physical properties keeping the ratio of core mantle radii constant and applying the same temperature difference between top bottom boundaries and the same internal heating rate effects that originate outside of the planet such as tidal forces meteor impacts and intense surface heating from a nearby star are not considered we find that for internally heated convection plate tectonics is equally likely for terrestrial planets of any size whereas for basally heated convection plate tectonics becomes more likely with increasing planet size this is indicated both by analytical scalings and the presented numerical results which agree with each other when scalings are adjusted to account for increasing mean density with increasing planet size plate tectonics becomes more likely with increasing planet size for all scenarios the influence of the pressure variation of viscosity thermal expansivity and conductivity may however act in the opposite sense and needs to be determined in future studies at least in the simplest case factors other than planet size such as the presence of surface water are likely most important for determining the presence or absence of plate tectonics