melting curve of mg fe sio3 perovskite to 96 gpa evidence for a structural transition in lower mantle melts the melting temperature of mg fe sio3 perovskite the dominant mineral phase of the earth s lower mantle has been measured to a pressure of 96 10 gpa below 60 gpa we find good agreement with the previous measurement of heinz and jeanloz 1987a and above 60 gpa the melting curve of silicate perovskite has a small positive slope of 19 5 5 5 k gpa the melting point of mg0 9fe0 1 sio3 perovskite is 3800 300 k at 96 gpa by extrapolation to 136 gpa it is 4500 500 k at the core mantle boundary these values provide an upper limit to the geotherm through the solid mantle and they are compatible with recent estimates of the temperature in the core being high 4500 k at the core mantle boundary our melting curve implies that the volume change on melting increases at 60 gpa from vm 0 0 0 2 cm mole to vm 0 16 0 03 cm mole we derive an equation of state for the melt from our data and find that a highly coordinated structure is required to explain the molar volume of liquid mg fe sio3 at pressures above 20 gpa

decorrelation in interferometric radar echoes a radar interferometric technique for topographic mapping of surfaces implemented utilizing a single synthetic aperture radar sar system in a nearly repeating orbit is discussed the authors characterize the various sources contributing to the echo correlation statistics and isolate the term which most closely describes surficial change they then examine the application of this approach to topographic mapping of vegetated surfaces which may be expected to possess varying backscatter over time it is found that there is decorrelation increasing with time but that digital terrain model generation remains feasible the authors present such a map of a forested area in oregon which also includes some nearly unvegetated lava flows such a technique could provide a global digital terrain map

radarfacies and sedimentological analysis study of sedimentary substrate from an archaeological site shell mound southern brazil integrated results of gpr ground penetrating radar and sedimentological analysis are presented for the jabuticabeira ii archaeological site shell mound santa catarina brazil by means of rada

internal heating and thermal constraints on the mantle we have carried out numerical simulations to study the average temperature and thermal structure of an internally heated mantle it is found that in an incompressible mantle the hottest parts are localized in regions of slow flow in the middle and upper mantle whereas in a compressible mantle they occur over laterally extensive regions near the core mantle boundary cmb for chondritic concentrations of heat producing elements hpe s the average thermal conductivity in the mantle must be high to avoid heating the core and the temperature of the cmb tcmb is low to avoid large scale melting the mantle may have been extensively molten in the archaean

the influences of lower mantle viscosity stratification on 3d spherical shell mantle convection we have studied the dynamical effects of a stratified viscosity profile on time dependent mantle circulations by using a 3d boussinesq spherical shell convection model this particular viscosity profile taken from the inversion of geoid data has the distinct feature of having a large viscosity maximum in the middle lower mantle surface rayleigh numbers ranging from 5 104 to 8 106 have been considered there are several distinct differences in both the morphologies of the plumes and the sinking currents from constant viscosity models in a viscously stratified lower mantle plumes are formed invariably only after the cold downwelling has reached the bottom the mean mantle temperature is reduced and this allows for the sinking of very cold material into the deep mantle the number of upwelling plumes is reduced and the overall circulation is stabilized with well organized triple junctions of cold sheets developed the presence of internal heating increases the number of upwelling plumes and breaks up the network of downwelling sheets these plumes become larger in size and have a hotter interior because of the nearly stationary character

axisymmetric spherical shell models of mantle convection with variable properties and free and rigid lids axisymmetric spherical shell numerical simulations of mantle convection were carried out to investigate the influence of two end member surface stress conditions stress free and rigid these correspond approximately to a subducting or a rigid lithosphere and can be seen as end member models of the surface of venus our model assumed an effective rayleigh number of 3 106 similar to that for earth and included uniform internal heating and depth dependent thermal expansivity and thermal conductivity k the simulations utilized a newtonian viscosity which was constant or varied with depth and or temperature we show how the temperature speed and vorticity fields change qualitatively and quantitatively with surface temperature surface stress condition internal heating and viscosity distribution among the significant results we find that a rigid lid and viscosity which increases with depth both promote steady large scale circulation with smaller scale circulation in the upper mantle

long wavelength s wave velocity structure throughout the mantle summary long wavelength s wave velocity anomalies are obtained for the entire mantle by inverting 6ooo long period 40 100s sh body waves and lo00 long period 100 500 s love waves in the shallow mantle the patterns of fast and slow velocity anomalies correlate well with age and tectonics in the depth range of 400 1000 km we find two antipodal fast velocity anomalies one in the western pacific and the other in south america they form a fairly strong i 2 anomaly moreover the 1 2 anomaly is the largest spherical harmonic component throughout the entire mantle except for in the layer of 1000 1300 km depth a fast velocity anomaly near the caribbean previously reported by several other investigators is found between the depths of 1000 and 1300km with diminishing amplitudes it can be traced to canada and the aleutians and may be related to old subducted slabs in the same depth interval similar size anomalies can be found in the middle of the pacific and in the indian ocean these could be checked by more detailed studies to judge the reliability of this study heterogeneity power has a peak at the surface and a secondary peak at the bottom of the mantle d consistent with the existence of chemical and or thermal boundary layers at the base of the mantle we also find a fast velocity anomaly which surrounds the pacific ocean this feature has been found by other researchers previously but its interpretation is difficult due to the trade off between velocity anomaly and core mantle boundary deformation

geoid and topography of earth like planets a comparison between compressible and incompressible models for different rheologies abstract a systematic study of 2d axisymmetric spherical shell models of compressible and incompressible mantle convection with constant and variable viscosity and constant and depth dependent thermodynamic properties is presented to account for compressibility effects we employ the anelastic liquid approximation in the case of variable viscosity an arrhenius law with strongly temperature and pressure dependent viscosity is considered we show that assuming compressible convection with depth dependent thermodynamic properties strongly influence the geoid undulations using compressible convection with constant thermodynamic properties is physically inconsistent and may lead to spurious results for the geoid and convection pattern in addition we examine the impact of compressibility as well as different rheologies on the power law relation that connects the nusselt number to the rayleigh number we discover that the power law index of the nu ra relationship is controlled by the rheology independent of which approximation is used instead the bound of this relation is controlled by a combination of different approximation and rheology

the interaction of plumes with the transition zone under continents and oceans abstract the dynamics of mantle flow with phase transitions can be influenced by the presence of a thick continental lithosphere especially for the 400 km phase transition we have studied the effects of the continental lithosphere on the mantle convection with the two major phase transitions at 400 km and 660 km a 2 d model with an aspect ratio 8 box was used in which the surface was divided into two domains oceanic and continental the continental lithosphere is modeled by a highly viscous 200 km thick lid which is 100 times stiffer than the oceanic part we also consider the consequences of plate reconfiguration by switching instantaneously the underlying rheological structure of the oceanic and continental domains a depth dependent viscosity with an additional jump at 660 km depth was employed other depth dependent properties employed are the thermal expansivity and the thermal diffusivity both internal and basal heating configurations were studied our results show that there are distinct differences in the mode of interaction of plumes with the transition zone under continents and oceans much larger stationary plumes are developed in the lower mantle under continents and erupt from time to time into the upper mantle creating a very hot continental upper mantle plumes under oceans are weaker but produce fast horizontal jets when the plumes impinge upon the surface very large plumes are formed in the internally heated cases because of the presence of depth dependent properties in the lower mantle creating a thick bottom boundary layer this thick thermal boundary layer is swept upward under the continents by a large scale flow in the lower mantle these results can be important for the interpretation of seismic tomography under africa and the central pacific

mantle downwelling and crustal convergence a model for ishtar terra venus the ishtar terra region contains the highest topography known on venus over 10 km above mean planetary radius as well as abundant tectonic features many of apparently compressional origin these characteristics suggest that ishtar is a crustal convergence zone overlying downwelling mantle in order to explore quantitatively the implications of this hypothesis for ishtar s origin we present models of viscous crustal flow driven by gradients in topography assuming a free slip surface boundary condition we find that if the crustal convergence hypothesis is correct then the crustal thickness in the plains surrounding ishtar can be no more than about 25 km thick this upper bound assumes a cold 10 k km 1 geotherm and the stiffest available diabase flow law if the geothermal gradient is larger or the rheology is weaker the crust must be even thinner for net crustal convergence to be possible this upper bound is in good agreement with several independent estimates of crustal thickness of 15 30 km in the plains of venus based on modeling of the spacing of tectonic features and of impact crater relaxation if the surface layer of venus provides a no slip boundary then our models allow the crustal thickness in the plains to be up to 50 km but the likely existence of faults that cut through the crust makes a no slip surface layer unlikely our upper bound on crustal thickness is much less than that derived from an airy isostasy model of ishtar s gravity anomaly much of the observed gravity anomaly must be due to density anomalies in the mantle beneath ishtar although we treat ishtar as a crustal convergence zone our crustal flow model shows that under some circumstances near surface material may actually flow away from ishtar providing a possible explanation for grabenlike structures in fortuna tessera

thermal expansion measurements at very high pressure systematics and a case for a chemically homogeneous mantle new data on thermal expansivity of iron to over 400 kbar in laser and electrically heated diamond anvil cells and on mgo and forsterite from spectroscopic data to over 200 kbar and previously measured adiabats boehler 1982 imply that at high pressure may be described by a constant value of 1n 1nv t 5 5 0 5 this behavior is supported by previous measurements at lower pressures on a large variety of materials densities of magnesio wustite and silicate perovskite in a pyrolitic ratio calculated at lower mantle conditions using a finite strain equation and at high pressures using the above systematics match those in prem through the entire lower mantle making a strong case for a uniform mantle

holocene interplay between a dune field and coastal lakes in the quiaios tocha region central littoral portugal coastal dune fields and dune body associated lakes were studied through sedimentological malacological and palynological analyses lake deposits and palaeosoil horizons gave new information about phases of aeolian activity and dune building coastline and sea level vegetation history and cultural impact after a period of intense aeolian activity late pleistocene that limited soil forming processes a wet aeolian field recorded by podzolized sands and interdune wetland deposits was present in the region around 5000 to 4000 yr bp sea level reached the current level and the coastline was situated c 1 km further inland compared with the present situation subsequently lagoon barrier island systems developed c 4000 yr bp vegetation in the region during this time was a pine oak forest between 4000 and 1600 yr bp the forest was replaced by a semi natural heathland later the heathland was degraded allowing dune migration and finally formation of a desert like landscape this process was probably a c

the impact of variability in the rheological activation parameters on lower mantle viscosity stratification and its dynamics the recently discovered spin state crossover in iron in major mantle minerals at high pressures should exert dramatic influences on transport properties such as the activation creep parameters in the deep mantle wentzcovitch et al 2009 have computed the elastic parameter of ferropericlase that is affected by this crossover the bulk modulus at high realistic conditions using first principles density functional theory plus hubbard u as a consequence of the spin crossover there is a significant softening of the bulk modulus and an attendant reduction of the activation energy in the creep law using a parameterized model capturing the basic physics we have studied the dynamical consequences within the framework of a 2 d cartesian convection model our models reveal a series of asthenospheres and are characterized first by a low viscosity channel below the 670 km boundary second asthenosphere caused by an increase of temperature and decrease in grain size reduction superplascity with the post spinel transition which results in partially layered convection further the variability of the rheological parameters leads to the prevalent formation of viscosity minimum at a depth of about 1600 km third asthenosphere caused by spin crossovers followed by a viscosity hill in the bottom half of the lower mantle the latter due to the increase of the activation energy in some low spin irons at the bottom of the mantle our numerical simulations reveal a tendency to the formation of small scale convection below the 670 km boundary and bigger stabilized plumes and or plume clustering in the deep lower mantle other material properties at the bottom of the mantle decrease of thermal expansivity radiative thermal conductivity also exert significant influence on the multi scale lower mantle plume dynamics

effects of temperature and mantle dynamics on estimates of the thermal conductivity in the deep mantle abstract estimates are made of the thermal conductivity ki of the deep mantle as a function of the temperature jump t across a thermal boundary layer at the core mantle boundary cmb three types of boundary layer corresponding to differing physical processes at the cmb are investigated and the effect of temperature dependent thermal conductivity is calculated for each the consequences of lateral heterogeneities are considered results indicate that if the temperature jump t seen by most of the mantle at the cmb is large 1350 k then ki cannot be more than 6 w m 1k 1 if the conductivity is constant or 8 w m 1k 1 if it is temperature dependent if t is small 350 k then ki may be 16 w m 1k 1 or higher the dynamics of the boundary layer is important results suggest that hot spot plumes are not the only mechanism for removing heat from the core

evidence from sites in western ireland of late holocene changes in coastal environments the presence of sand layers interleaved with biogenic material in coastal sedimentary sequences from back barrier areas has often been used as evidence of storm surge or tsunami action during the holocene this paper discusses such stratigraphies from two adjacent sites in western ireland in order to assess the validity of this inference detailed examination of the sand layers indicates that while some of the sand beds at one of the sites were deposited by overwash other processes including aeolian deposition and fluvial reworking of aeolian sediments were also important in sand redistribution in the back barrier areas in these cases local controls including site drainage characteristics barrier morphology and available sediment supply appear to have been important in site evolution a regional signal of storm frequency was not detected

lagoa da apulia a residual lagoon from the late holocene nw coastal zone of portugal the lagoa da apulia is a unique feature in the nw coastal zone of portugal a remaining form from a lagoon complex system that during the late holocene was dominant in the region this system was mainly neotectonically controlled occupying a depressed area bounded by faults on a palaeozoic rocky lower platform today observable on beaches at low tide with the intention of knowing the main architecture of the palaeo lagoon geophysical prospecting with gpr and resistivity was carried out accordingly six cores were taken and the sedimentary and mineralogical facies and diatom and foraminifer contents were analysed and five rich organic layers were dated by radiocarbon analysis with the data an evolutionary environment reconstruction model was created for this palaeo lagoon and the main structural features of the neighbouring area

tectonic controls on the stratigraphic development of the rifted taipei basin a late quaternary marine influenced inland half graben abstract we establish the three dimensional stratigraphic architecture of the taipei basin and its spatiotemporal palaeoenvironmental development during the past 50 kyr by analysing 36 borehole cores and 177 age dates we calculate the rates of basin subsidence from the borehole data at depths where radiocarbon age dates are available our results indicate that during the eustatic sea level falling period 35 20 ka low rates of sediment supply and or rapid basin subsidence controlled sedimentation leading to a change in the depositional environment from gravelly braided rivers through sandy braided rivers to meandering rivers with falling eustatic sea level during the early stage of eustatic sea level rise 20 10 2 ka balanced rates of sediment supply eustasy and basin subsidence maintained the meandering river environment rapid sea level rise led to the initial appearance of estuarine facies at 10 2 ka and widened the distribution of the estuarine environment after 8 5 ka however the coeval phases of rapid basin subsidence 10 6 10 2 ka and 9 8 5 ka promoted the first appearance and widening of the estuary respectively after 7 ka when the eustatic sea level and rate of basin subsidence remained relatively stable sufficient sediment supply gradually infilled the estuary the established stratigraphy improves our understanding of the factors controlling stratigraphic development in marine influenced inland half grabens specifically in the taipei basin with high rates of sediment supply accommodation space in the taipei basin was governed by eustatic fluctuations with modulations by basin subsidence and sediment supply since the last glacial period

effect of the mid mantle viscosity and phase transition structure on 3d mantle convection abstract recent geophysical evidence shows some sort of layering in the lower part of the mantle transition zone down to a depth of 1000 km seismic observations have revealed a sharp reflector surface at around 900 or 1000 km depth for which a possible explanation can be given in terms of a new phase transition of the lower mantle constituent minerals furthermore new results from the inversion of the oceanic geoid show the existence of a second low viscosity zone lvz somewhere between 660 and 1000 km depth the existence of the second lvz may be linked to the mid mantle phase transitions the phase and viscosity stratification of the transition zone have been included in a series of 3d convection simulations in a 4 4 1 rectangular box with a surface rayleigh number of 2 10 7 beneath the well known 400 and 660 km phase changes we assumed a hypothetical weak endothermic transition at 1000 km in some of our models the principal controlling factor of the style of mantle convection is still the 660 km endothermic transition which sets up a partial or full barrier to flow causing stratified circulation we used various viscosity profiles with emphasis on the model containing the second lvz the main consequence of this zone is to enhance flow layering many plumes can emanate from the transition zone and small scale instabilities develop in the second lvz when the 1000 km endothermic phase transition is included these instabilities can grow only at a few places but then they form strong downwellings two distinct types of penetrative deep downwellings can be present at the same time one which crosses the whole transition zone and another one which crosses only the 660 km discontinuity and stops at 1000 km at least temporarily this can explain seismological observations which suggest that subducted slabs can be retarded not only by the 660 km boundary but also by some deeper obstacle near 1000 km depth

intense local toroidal motion generated by variable viscosity compressible convection in 3 d spherical shell strong dynamical effects arising from lateral viscosity variations are found near the heads of fast upwelling plumes approaching the top boundary for unsteady 3 d compressible convection in a spherical geometry at an average rayleigh number of o 106 the temperatures at the center of fast upwelling plume are very hot due to the positive feedback interaction between viscous dissipation and temperature dependence of the viscosity although the laterally averaged ratio of toroidal to poloidal energy varies only around 10 the local toroidal velocities can reach about 40 of the poloidal velocities the strongest toroidal motions always take place right above or close to the fast upwelling plume heads and behave as vortical flows with time this finding may explain the origins of microplate rotation in regions with hotspot activity e g easter island which shows no direct relation to the transform faulting system near the mid ocean ridge

the timescales of plume generation caused by continental aggregation abstract to understand the thermal evolution of the mantle following the aggregation of non subductable thick continental lithosphere we study a numerical model in which a supercontinent simulated by high viscosity raft hvr covers a part of the top surface of a convection layer we model infinite prandtl number convection either in a three dimensional 3d spherical shell 3d rectangular box aspect ratios 8 and 4 or two dimensional 2d rectangular box aspect ratio 8 and except for the hvr we specify a constant viscosity the hvr which has a viscosity higher than that of its surrounding is instantaneously placed on the top surface of a well developed convection layer and its position is fixed our results from 3d spherical shell cases with and without phase transitions show the emergence of a large plume characterized by a long wavelength thermal anomaly a degree one pattern for a pangea like geometry we analyze the volume averaged temperature under the hvr t c the remaining oceanic area t o and total area t m to determine the timescale of plume generation the difference between t c and t o t co and t m show the existence of two characteristic timescales t co exhibits an initial rapid increase and may become constant or continue to gradually increase meanwhile t m shows a similar behavior but with a longer timescale we find that these timescales associated with the increase of t co and t m can be attributed to the formation of large scale flow i e plume and response of the whole system to the emplacement of the hvr respectively for 3d spherical cases we find that the timescale of plume generation is 1 2 gyr if the rayleigh number is 10 6 to determine the effects of the viscosity of the hvr 2d versus 3d modeling and the effects of the internal heating we have also studied 2d and 3d rectangular box cases a factor of about two variation exists in the timescale of plume generation it appears that the timescale becomes greater for a smaller amount of internal heating this may be attributed to the time dependent flow caused by the internal heating for 2d cases we find that the timescale of the high rayleigh number 10 7 case is shortened by a factor of three to five when compared to the ra 10 6 case which is consistent with the simple boundary layer theory this may imply that well developed plumes may arise with the timescale of 0 2 to 0 4 gyr for ra 10 7

dynamical consequences on fast subducting slabs from a self regulating mechanism due to viscous heating in variable viscosity convection the authors have studied 2 d time dependent convection for a rheology which is both non newtonian and temperature dependent strong effects associated with viscous heating are found in the downwelling sheets which are heated on both sides with an intensity around o 10 sup 2 times the chondritic value the magnitude of viscous heating increases strongly with the subduction speed the slab interior is weakened by viscous heating and slab breakoff then takes place this process provides a self regulating mechanism for governing the speed of intact slabs able to reach the deep mantle timescales associated with viscous heating are quite short a few million years internal heating by radioactivity decreases the amount of shear heating 13 refs 5 figs

mantle convection with internal heating and pressure dependent thermal expansivity recent laboratory work 1 suggests that the thermal expansivity of the mantle decreases strongly with pressure determines the buoyancy of plumes and the rate at which plumes lose their thermal signature as they rise and so may be expected to have a strong influence on the temperature and velocity structure of the mantle numerical simulations were conducted on convection in an internally heated compressible mantle including constant and pressure dependent and thermal conductivityk they show that a pressure dependent allows the existence of thermal plumes rising from the core mantle boundary cmb however the plumes are weak and dissipate higher in the mantle

characterization and dating of coastal deposits of nw portugal minho neiva area a record of climate eustasy and crustal uplift during the quaternary abstract this study presents the characterization and numerical dating of quaternary coastal deposits of nw portugal located between the mouths of the minho and neiva rivers they record continental small alluvial fans and streams and transitional aeolian dunes interdune ponds estuary sandy and gravelly beaches paleoenvironments quartz and k feldspar optically stimulated luminescence osl dating is employed as well as ams 14 c dating a staircase of coastal terraces abrasion shore platforms was identified altimetry a s l and ascribed to the following probable marine isotope stages mis t1 20 18 m mis11 t2 ca 13 m mis9 t3 9 3 7 3 m mis7 t4 5 5 4 5 m mis5 t5 3 5 2 0 m mis5 the terraces have some preserved sedimentary facies that includes coeval beach sediments on the lowest four a late pleistocene to holocene sedimentary cover comprises four sub units a the lower sub unit corresponding to ferruginous stream deposits and aeolian dunes dated ca 67 61 ka mis4 probably related with sub humid to arid mid cold conditions b on the slopes the lower sub unit is overlapped by sandy silty colluvium and sandy alluvial deposits dated ca 56 28 ka mis3 and probably reflecting cold mid cold and wet dry climate conditions c this sub unit is topped by soliflucted lobes and sandy silty silty deposits recording cold and dry climate dated 20 13 ka mis2 and d a top sub unit dated to 16 18th century recording little ice age events consisting of fluvial sediments coeval with temperate climate evolving to aeolian dunes from the maunder minimum cold windy dry conditions

a mantle plume model for the equatorial highlands of venus the equatorial highlands of venus consist of four main structures atla beta ovda and thetis regiones each has a circular to oval shaped planform and rises 4 5 km above the mean planetary radius these highlands are associated with long wavelength geoid highs with amplitudes ranging from 35 m at ovda to 120 m at atla they also contain topographic valleys interpreted as extensional rift zones and beta is known to contain shield volcanoes these characteristics are all consistent with the equatorial highlands being formed by mantle plumes an alternative model in which ovda and thetis are interpreted as spreading centers analogous to terrestrial mid ocean ridges fails to explain most of the observed geoid anomalies and topography in these regions some smaller highlands such as bell regio eistla regio and the hathor innini ushas region may also be plume related but most coronae are unlikely to be the direct result of plume activity we have modeled plumes using a cylindrical axisymmetric finite element code and a depth dependent newtonian rheology we compare our model results with profiles of geoid and topography across atla beta ovda and thetis our best model fits are for beta and atla assuming whole mantle convection and that earth and venus have similar mantle heat flows venus must lack an earth like low viscosity zone in its upper mantle in order satisfy the observed geoid and topography for these features this conclusion is consistent with the long wavelength admittance spectrum of venus and with the observed differences in the slopes of the geoid spectra for the two planets one explanation for the different viscosity structures of the two planets could be that the mantle of venus is drier than earth s mantle

mantle dynamics in mars and venus influence of an immobile lithosphere on three dimensional mantle convection numerical calculations of fully three dimensional convection in constant viscosity compressible spherical shells are interpreted in terms of possible convective motions in the mantles of venus and mars the shells are heated both internally and from below to account for radiogenic heating secular cooling and heat flow from the core the lower boundary of each of the shells is isothermal and shear stress free as appropriate to the interface between a mantle and a liquid outer core the upper boundary of each of the shells is rigid and isothermal as appropriate to the base of a thick immobile lithosphere calculations with shear stress free upper boundaries are also carried out to assess the role of the rigid surface condition the ratio of the inner radius of each shell to its outer radius is in accordance with possible core sizes in both venus and mars a calculation is also carried out for a mars model with a small core to simulate mantle convection during early core formation different relative proportions of internal and bottom heating are investigated ranging from nearly complete heating from within to almost all heating from below the rayleigh numbers of all the cases are approximately 100 times the critical rayleigh numbers for the onset of convection cylindrical plumes are the prominent form of upwelling in the models independent of the surface boundary condition so long as sufficient heat derives from the core thus major volcanic centers on mars such as tharsis and elysium and the coronae and some equatorial highlands on venus may be the surface expressions of cylindrical mantle plumes the form of the downwelling sheets is significantly affected by the rigid boundary in that the sheets are more irregular in their horizontal structure than when the top boundary is shear stress free in the mainly heated from within models the downwelling sheets are also shorter and less temporally durable when the top boundary is rigid than when it is stress free thus the free motion of plates on the earth facilitates extensive durable convective currents that drive the plates while the stiffening of the lithospheres on mars and venus promotes a style of convection that is not particularly effective in breaking the lithosphere into plates in the rigid top cases the upper boundary layer surrounding the plumes appears to be interspersed with downwelling currents emanating radially from the plumes axes these currents may establish a stress field at the base of hotspot swells that could lead to radial fractures such as those on tharsis models with rigid upper boundaries have higher interior temperatures than do similar models with shear stress free upper boundaries on this basis venus not only has a higher surface temperature than earth but it would have a hotter mantle as well upwelling plumes are more numerous when the outer boundary is rigid as compared with shear stress free flows dominated by a few stong plumes occur when the proportion of basal heating is large thus if the martian crustal dichotomy were caused by a convective system dominated by spherical harmonic degree l 1 then the convection may have been driven strongly from below by a heating pulse accompanying core formation or from the flow of heat from an early hot core if a convective mechanism is responsible for the crustal dichotomy then the dichotomy is likely a very ancient feature however the small core models we consider consistently produce a convective pattern with a dominant l 2 signature which does not correlate with the martian crustal dichotomy a yet smaller core may be necessary to yield the l 1 pattern the small core convective pattern does correlate with the geoid and topography signatures of tharsis which have strong l 2 components and the model produces dynamic uplift comparable to the total topography of the tharsis rise models with larger cores i e with the probable inner to outer radius ratio of mars present mantle generate 4 km of uncompensated topography similar to estimates of the uncompensated tharsis topography thus the tharsis rise could have achieved its full height early in the evolution of mars by mantle plume dynamic uplift at present the uncompensated portion of the tharsis rise topography can be accounted for by dynamic uplift obviating the need for elastic support the present compensated portion of the tharsis topography could be attributed to volcanic or magmatic crustal thickening or depletion of the underlying mantle large core models appropriate to the present mars produce too many plumes to account for just two major volcanic centers i e tharsis and elysium mantle plume activity could be focussed beneath regions like tharsis if fracturing or thinning of the lithosphere in these regions has facilitated magma and heat transport across the lithosphere a similar mechanism could be responsible for clustering of coronae on venus there are no deep seated active linear upwellings in the venus models that could be associated with linear spreading centers in aphrodite if linear spreading is actually occurring in aphrodite the phenomenon is probably a shallow passive one similar to mid ocean ridges on earth

rapid least squares inversion of apparent resistivity pseudosections by a quasi newton method1 a fast inversion technique for the interpretation of data from resistivity tomography surveys has been developed for operation on a microcomputer this technique is based on the smoothness constrained least squares method and it produces a two dimensional subsurface model from the apparent resistivity pseudosection in the first iteration a homogeneous earth model is used as the starting model for which the apparent resistivity partial derivative values can be calculated analytically for subsequent iterations a quasi newton method is used to estimate the partial derivatives which reduces the computer time and memory space required by about eight and twelve times respectively compared to the conventional least squares method tests with a variety of computer models and data from field surveys show that this technique is insensitive to random noise and converges rapidly this technique takes about one minute to invert a single data set on an 80486dx microcomputer

effects of depth dependent properties on the thermal anomalies produced in flush instabilities from phase transitions abstract the effects of depth dependent thermal expansivity viscosity and thermal conductivity on mantle convection with phase transitions have been examined with two dimensional finite element simulations in an aspect ratio four the model includes a core mantle thermal coupling boundary condition which takes into play the secular cooling of the core by the overlying mantle flow initial surface rayleigh numbers between 2 10 7 and 10 8 have been considered with time the surface rayleigh number decreases to a value upon which a transition takes place from layered to single cell convection this tumultuous period is marked by large scale coherent breakthrough of cold material trapped in the transition zone all the way to the base of the mantle and a violent reaction of hot plume in the upper mantle both hot and cold anomalies have large magnitudes cold anomalies with temperatures exceeding 1000 k are found at the base of the mantle the magnitude of the cold anomalies is largest with all three depth dependent properties the timescales of this catastrophic event are between 20 and 50 myr with the longest being produced by the model with depth dependent expansivity and viscosity results from these simple two dimensional cartesian models represent a lower bound to the larger cold thermal anomalies potentially capable of being generated in spherical shell convection models

late pleistocene and holocene environmental changes in the coastal zone of northwestern portugal this paper examines the quaternary changes in the coastal zone of northwestern portugal through the relationship between sediments and landforms by interpolating the available data from geomorphology lithostratigraphy sedimentology and geochronology it was possible to reconstruct existing palaeoenvironments the oldest of which date back to the last interglacial the main landforms are a high and a low platform bounded by scarps river sands and silty sediments formed in a lacustrine environment antas formation mis 5 are associated with the high platform two deposits are associated with the low platform the cepaes formation mis 3 consisting of a lower set of river sand beds and an upper gravel beach bed and the agucadoura formation mis 1 consisting of holocene lagoonal beds copyright 2006 john wiley sons ltd

measurement of the melting curve of mg0 9fe0 1sio3 at lower mantle conditions and its geophysical implications an experimental determination of the melting of mg0 9fe0 1sio3 under lower mantle conditions is presented samples were compressed and laser heated nd yag in a mao bell type diamond cell the temperature fields generated in the sample are measured using a simple tomographic technique and spectroradiometer mg0 9fe0 1sio3 in the perovskite structure melts at 3000 300 k independently of pressure over the range of 22 65 gpa the melting curve of mg0 9fe0 1sio3 represents an experimental upper bound upon the geotherm from 700 to 1500 km this melting curve corresponds to a homologous temperature of 0 70 or 0 85 for one or two layer mantle convection respectively this is consistent with either one or two layer mantle convection and a lower mantle of constant viscosity based upon homologous temperature arguments

post glacial relaxation of a viscously stratified compressible mantle summary we investigate the post glacial relaxation phenomenon of a viscously compressible non self gravitating spherical shell model we have developed analytical solutions to an exponentially depth dependent viscosity with various types of compressible density models such as those with an exponential dependence of the radius and an algebraic root dependence of the radius the solutions of a viscously compressible multilayered model with constant thermodynamic properties and viscosity are developed by the propagator matrix method our results show that inferences of deep mantle viscosity from post glacial rebound would be hampered by mantle compressibility for long wavelength harmonics because of the smaller excitation of compressible eigenfunctions in the lower mantle the relaxation times and velocity fields are more sensitive to higher viscosity contrast for the exponentially varying viscosity than for models with discrete jumps in the viscosity structure stress fields associated with deglaciation are extremely sensitive to the underlying mantle viscosity structure large stresses o 102 bar can still remain at the deglaciated region if the lower mantle viscosity is o 1023p the presence of a hard garnet layer in the transition zone will induce a smaller viscosity in the lower mantle in the viscosity trade off with the standard two layer model

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

phase transitions in the martian mantle implications for the planet s volcanic history the influences of phase transitions on martian mantle convection and melting in the martian mantle have been studied with an axisymmetric spherical shell model an extended boussinesq model in which viscous and adiabatic heatings are included has been used there are depth dependences in the thermal expansivity and gravity which taken together decrease by a factor of between 2 and 3 across the martian mantle the two destabilizing exothermic phase transitions olivine to spinel and and spinel transitions above the martian core mantle boundary cmb accelerate the mantle flow and result in an amplification and superheating of plumes in mars the additional consideration of the endothermic phase transition spinel to perovskite transition which was only likely present in the early evolution of the planet has little influence on martian mantle convection in this model strong localized viscous heating is generated near the cmb and underneath the lithosphere because here the flow bends over sharply a possible volcanic evolution of mars can be derived from a comparison of the temperature fields with the solidus of anhydrous peridotite if the cooling of the martian mantle is taken into account in the early evolution the mantle was molten to a high degree along the plume axis which possibly resulted in a strong differentiation of the mantle as the planet cooled down the region of melt generation receded to where the maximum of viscous heating occurred at the cmb and immediately underneath the lithosphere upon further cooling the deep mantle melt source region became subsolidus the most recent volcanism on mars was most likely generated at a shallow depth below the lithosphere

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

phase transitions in the martian mantle implications for partially layered convection abstract numerical simulations of mantle convection in mars using an axisymmetric spherical shell model show partial layering caused by the two exothermic olivine spinel phase transitions an extended boussinesq approximation has been used in which viscous dissipation adiabatic heating and cooling and latent heat are included the rayleigh number ra has been varied between 5 105 and 108 the partial layering with the vertical velocity at the exothermic phase transitions varying strongly in space and time is the result of two opposing effects the enhanced buoyancy of the phase boundaries by thermal anomalies and the impeding influences from the latent heat release or consumption the effect of the latent heat is stronger in mars than the earth because of the comparatively low pressure gradient in the martian mantle and the smaller excess temperature of upwellings and downwellings the time series of the mean vertical mass transport across the phase transitions show oscillations between blocking and acceleration of the flow the amplitude and the oscillations in the time series increase with increasingra because of the partial layering the planet will cool more slowly and less uniformly than suggested by thermal evolution models with parameterized convection in addition the number of strong mantle plumes is reduced to only a few upwellings such a pattern is suggested for mars by the existence of two pronounced volcanic centers tharsis and elysium this could also cause a strong time dependence in the martian volcanic activity the latent heat release causes the mantle temperature to increase across each transition by about 50 k and produces a hot lower mantle and a liquid core we have tested the case of a 85 350 km thick perovskite layer at the core mantle boundary a layer thicker than about 300 km would convect separately and induce leaking to the mantle above at a significantly smaller rate compared to the layers induced by the olivine spinel phase boundaries for a perovskite layer smaller than about 300 km the convective vigor near the core mantle boundary decreases with the layer thickness

c band repeat pass interferometric sar observations of the forest properties of ers 1 c band repeat pass interferometric sar information for a forested area are studied the intensity information is rather limited but including coherence and effective interferometric sar insar height more information about the forest parameters can be obtained via satellite such information is also important for correction of insar derived topographic maps coherence properties have been used to identify forested nonforested areas and the interferometric effective height of the forest determined by comparison to a dem of the area the authors have developed a model to relate basic forest properties to insar observations these show that the coherence and interferometric effective height of a forested area change between image pairs the model demonstrates how these properties are related to the temporal decorrelation and the scattering from the vegetation canopy and the ground surface gaps in the vegetation are found to be important in the characterization of boreal forests

viscous heating a potential mechanism for the formation of the ultralow velocity zone abstract the detection of several ultralow velocity zones ulvz on the core mantle boundary cmb together with the recent estimates of the lower mantle solidus would suggest that some sort of melting may occur inside the upwellings in the deep mantle we have studied the possibilities of viscous heating as a mechanism for producing favorable conditions leading to melting inside deep mantle upwellings a two dimensional model in an aspect ratio 3 box with temperature and pressure dependent rheology and two major phase transitions has been employed both viscous and adiabatic heating have been included and various amounts of internal heating have been applied viscous heating can heat up the interior of the deep mantle plumes to around 100 degrees above the cmb temperature this amount of temperature rise is sufficient to cause melting inside upwellings in the deep lower mantle we propose here that viscous heating plays an important role in producing conditions leading to the formation of the ulvz massive melting within rising plumes might have been more prevalent in the past

a temperature profile of the mantle transition zone the temperature distribution across the transition zone has been determined on the high pressure phase equilibria in the system mg2sio4 fe2sio4 recently determined by katsura and ito 1988 and ito and takahashi 1988 the temperature at the base of the transition zone 655 km depth is estimated to be about 1600 c assuming that the spinel dissociation is responsible for the sharp seismic discontinuity implied by study of reflected waves this estimation would be little modified for the chemically stratified mantle models since the lower mantle is characterized by the stability of perovskite the temperatures in the region from 350 km to 655 km depth are calculated for a peridotitic mantle model assuming an adiabatic temperature gradient the and transformations in mg fe 2sio4 make the temperature profile steep in the depth regions of 390 415 km and 500 530 km respectively the temperature would increase from about 1400 c to 1600 c over the depths interval 350 between 655 km

delayed cooling of the earth s mantle due to variable thermal conductivity and the formation of a low conductivity zone abstract the thermal conductivity of mantle materials decreases with higher temperatures and increases with greater pressure due to phonon mechanism but increases with temperature for radiative transfer this trade off attribute allows for the formation of a low conductivity zone lcz within the top thermal boundary layer in mantle convection we have studied with a two dimensional 2 d cartesian model the potential of this low conductivity zone in retarding secular cooling of the mantle for a longer period of time than models with constant thermal conductivity using a recently proposed model for thermal conductivity and an adiabatic boundary condition for the bottom of the mantle we have carried out a set of numerical experiments within the framework of the extended boussinesq approximation for constant viscosity depth dependent thermal expansivity and variable thermal conductivity with surface rayleigh numbers between around 106 and around 107 we have employed internal heating between two and four times the chondritic level and half life time values of 2 5 ga and 5 ga the cooling rate of the mantle can be decreased by the feedback interaction between mantle conductivity and internal heating which gives rise to slower sinking cold currents the retardation time increases with the strength of radiogenic heating and can be as long as a couple of billion years for high initial heating rates four times the chondritic value however increasing the radiative contribution of the conductivity speeds up the cooling process the less the radiative contribution is the more mantle cooling would be retarded in the course of adiabatic cooling we find the paradoxical situation in which the effective rayleigh number of the mantle can actually increase with time we suggest then that because of the possibilities for the formation of low conductivity zones adjacent to the thermal boundary layers which bring forth unexpected consequences mantle thermal conductivity should be considered as a fundamentally important factor in constraining the thermal evolution of both the core and mantle

change detection on alaska s north slope using repeat pass ers 1 sar images measurements of the c band wavelength 5 cm radar cross section of an area in the brooks range foothills on the north slope of alaska using images from the ers 1 satellite that show significant temporal changes occur these changes are strongly correlated with elevation and hill slope orientation and are greatest on some of the elevated areas and weaker in river drainages by constructing difference images using various image pairs and by analyzing climatological and hydrological data from the site it is concluded that the radar backscatter changes are largely due to changes in soil and vegetation liquid water content induced by freeze thaw events the correlation with topography in the difference images arises from the dependence of vegetation organic layer thickness and volumetric water content on hill slope position and orientation these results demonstrate the viability of radar backscatter intensity comparisons using repeat pass images as a means of change detection

morpho chronology of coastal dunes in medoc a new interpretation of holocene dunes in southwestern france abstract previous work on the coastal dunes in the medoc region was carried out by the brgm marionnaud j m 1972 carte geologique de la france 1 50000 feuille st vivien de medoc soulac sur mer 729 730 orleans brgm notice explicative par j dubreuilh j m marionnaud 1973 45 pp dubreuilh j marionnaud j m 1973 carte geologique de la france 1 50000 feuille lesparre medoc le junca 753 754 orleans brgm notice explicative par j dubreuilh j m marionnaud p andreieff 1973 47 pp they divided the dune systems into four chronological generations based on their morphology and comprising i isolated barchans 5100 yrs bp ii parabolic dunes 5100 3000 yrs bp iii localized areas of barchan dunes 2300 200 yrs bp and iv present day littoral dunes this paper describes the dune morphology and presents a revised chronology for the medoc dune systems based on historical records and a number of dated palaeosols this paper confirms the existence of two generations buffault p 1942 histoire des dunes maritimes de la gascogne imprimerie delmas bordeaux 446 pp which are constituted by six different dune forms the first of these generations is termed primary and consists of coalescent compound climbing parabolic dunes naturally fixed by vegetation possibly mobile around 5000 3500 b p or 3000 2300 bp the primary dunes were invaded by a second system named modern which consists of simple barchans isolated compound barchanoid ridges a field of coalescent compound barchanoid ridges simple parabolic dunes and compound digitate parabolic dunes all of which are believed to have been active between 1500 and 200 bp the highest degree of dune development occurs in the south of the area where the maximum expanse of sand deposits occurs on the continental shelf in this region the sequence of dunes inland from the coast consists of the littoral foredune compound digitate parabolic dunes simple barchans or isolated compound barchanoid ridges and coalescent compound barchanoid ridges of increasing height and linearity further landwards the barchanoid forms are interpreted as the amalgamation of landward migrating parabolic forms which arose as blowouts in the littoral foredunes the large heights of the barchans can be explained by favourable conditions of sand accumulation due to the presence of the slacks locally called ledes the hourtin lake and most importantly the vegetated primary dune system which curtailed landward sand transport past this point to the south of the study area inland of the coalescent compound barchanoid ridges belonging to the modern dune system is a field of older coalescent compound climbing parabolic dunes of lower height belonging to the primary system

tomography the geoid and plate motions the dynamics of the earth s mantle and its relationship to mantle structure as revealed by seismic tomography the geoid and plate motions are discussed in a critical review of u s research from the period 1987 1990 sections are devoted to plates and mantle convection seismic anomalies flow and the geoid subducted slabs and the 670 km discontinuity the physical properties of the mantle plumes and computational and experimental modeling efforts also included is a comprehensive bibliography for the period 248 refs

the tectonic evolution of western ishtar terra venus the origin and evolution of ishtar terra is modeled using numerical simulations of crust mantle interaction on venus based on our modeling we favor a two phase evolutionary sequence for ishtar the first phase is driven by bulk mantle flow due to thermal convection and results in the formation of a thickened crustal block over a mantle downwelling the second phase a result of the thermal perturbation exerted on the mantle by the region of thickened crust is driven by the sinking of a lithospheric slab this slab driven tectonic pulse leads to low angle subduction and orogenesis at the peripheries of the crustal block throughout its evolution ishtar remains coupled to flow within the deep mantle due to the lack of an asthenosphere and ultimately bulk mantle flow comes to dominate as the effects of the slab driven pulse wane

a dynamic model of venus s gravity field unlike earth long wavelength gravity anomalies and topography correlate well on venus venus s admittance curve from spherical harmonic degree 2 to 18 is inconsistent with either airy or pratt isostasy but is consistent with dynamic support from mantle convection a model using whole mantle flow and a high viscosity near surface layer overlying a constant viscosity mantle reproduces this admittance curve on earth the effective viscosity deduced from geoid modeling increases by a factor of 300 from the asthenosphere to the lower mantle these viscosity estimates may be biased by the neglect of lateral variations in mantle viscosity associated with hot plumes and cold subducted slabs the different effective viscosity profiles for earth and venus may reflect their convective styles with tectonism and mantle heat transport dominated by hot plumes on venus and by subducted slabs on earth convection at degree 2 appears much stronger on earth than on venus a degree 2 convective structure may be unstable on venus but may have been stabilized on earth by the insulating effects of the pangean supercontinental assemblage

postglacial sealevels on a spherical self gravitating viscoelastic earth effects of lateral viscosity variations in the upper mantle on the inference of viscosity contrasts in the lower mantle abstract previous studies of lateral viscosity variations on postglacial rebound often neglect the spherical shape of the earth and the self gravity of the solid earth and oceans in this paper the consistent sealevel equation is solved by the coupled laplace finite element method for a spherical self gravitating incompressible maxwell earth it confirms the importance of self gravity in the oceans on sealevel computation near the ice margin where the data are most sensitive to the existence of lateral variations in lithospheric thickness and asthenospheric viscosity the effects of lateral variations in lower mantle viscosity asthenospheric viscosity and lithospheric thickness are investigated this paper confirms the finding of earlier investigations the combined effect of lateral viscosity variations in the lower mantle with reverse viscosity contrasts in the upper mantle is also investigated for sealevel data near the center of rebound the effect of lateral viscosity contrast in the lower mantle can be masked by the existence of a reverse lateral viscosity contrasts in the upper mantle however this is not the case for sealevel data just outside the ice margin

change detection for thematic mapping by means of airborne multitemporal polarimetric sar imagery the paper addresses the detection of changes in multitemporal polarimetric radar images focusing on small objects and narrow linear features the images were acquired at c and l band by the airborne emisar system it is found that the radar intensities are better suited for change detection than the correlation coefficient and the phase difference between the co polarized channels in the case of linear features there is no obvious difference between the c and l bands and slight variations of the flight tracks are acceptable at look angles larger than 35 degrees theoretical detection thresholds are evaluated from the statistical distribution of the intensity ratio due to speckle for the linear features and for urban environments the observed thresholds are larger than the theoretical predictions this is interpreted as an effect of radar intensity variations on length scales smaller than the spatial image resolution the signature of urban areas is very sensitive to deviations between the flight tracks and the sensitivity is larger at c band than at l band on the other hand the intensity contrast between buildings and the urban background is smaller at l band and larger at c band for change detection thresholds may have to be chosen separately for each object class because the intensity ratios of different object classes vary differently as a function of time

compressible convection in a viscous venusian mantle finite element simulations of axisymmetric spherical shell compressible convection were carried out to investigate the effect of various surface boundary conditions in a venusian mantle we employed a thermal expansivity which decreased with depth a uniform viscosity an order of magnitude greater than the earth s and zero and chondritic quantities of internal heating as long as hot plumes from the core mantle boundary were strong the convection pattern was typical of that for variable flow that is it was characterized by steady upflowing regions unsteady collections of downflowtng plumes and large aspect ratio cells increases in the internal heating or the temperature t0 at the lop of the convecting layer weakened the hot plumes and therefore decreased the width of the cells a rigid surface increased the internal temperature and also decreased the widih of convection cells extensive regions of subadiabaticity were found in the mantle we compare our results with those for fully three dimensional convection under similar conditions schubert et al 1990

holocene evolution of relative sea level and local mean high water spring tides in belgium a first assessment abstract holocene age depth data from the belgian coastal plain basal peat data particularly are assessed in terms of local water and tide levels basal peat formation was generally induced by sea level rise effects of local seepage were limited and significant river gradient effects were absent no relation was found between the timing of basal peat inception and substrate permeability in general edaphic moisture conditions were unfavourable for organic accumulation prior to sea level related rise of the groundwater table error envelopes for the minimum level of local mean high water spring tides and for upper and extreme lower relative mean sea level limits during the last 9400 solar years 8400 yrs b p are presented these indicate that the rate of relative sea level rise showed marked retardations at ca 7500 7000 yrs cal b p 6650 6100 yrs b p and ca 5500 5000 yrs cal b p 4400 yrs b p a eustatic explanation appears to be plausible particularly for the former prior to ca 4000 yrs cal b p ca 3600 yrs b p the relative sea level rise in belgium differs noticeably from that established for the western netherlands with both diverging progressively back in time this implies that considerable differential crustal movement occurred between the two areas in the early holocene

clay minerals from the sedimentary cover from the northwest iberian shelf abstract the northern iberian margin is a typical example of a continental margin subjected to seasonal highly energetic regime waves and tides and receiving inputs of continental sediments via riverine discharges the principal goal of this study has been to use clay minerals as indicators of sedimentary dynamics in the open shelf system the distributions of clay mineral in the top layer of the sedimentary cover are shown to be related to their continental sources but also reflect the influences of winter storms and longshore currents in determining the pathways of sediment transport the mineralogical composition of the material issuing from the rivers is very similar to the general mineralogical composition of the fine fractions of the seabed sediments those deposits that are directly influenced by riverine discharges have higher contents of kaolinite 20 whereas those that are not have higher contents of illite 80 the available data indicate no significant quantities of terrigenous particles are being discharged from the spanish rias therefore we conclude that physical processes are controlling the clay mineral distributions and that despite contributions from the minho river the main source of fine detrital particles to the shelf region is the douro river discharge these particles settle on the middle shelf below the 60 m isobath during storm events these particles are re suspended and advected northwards to the galician shelf or into deeper domains thus the distributions of the clays indicate there is a net transport of fine sediments both northwards and off shelf

convection driven tectonics on venus an analysis is presented for convective stress coupling to an elastic lithosphere as it applies to tectonic deformation on venus theoretical solutions are introduced for the response of a mathematically thick elastic plate overlying a newtonian viscous medium with an exponential depth dependence of viscosity a green s function solution is obtained for the viscous flow driven by a harmonic density distribution at a specified depth it is shown that during the transient phase of the solution the field quantities stress velocity displacement in both media depend upon the rheology of both media whereas in steady state the solutions become uncoupled due to this uncoupling stresses obtained from a purely viscous solution can be applied as boundary conditions at the base of an isolated elastic plate i show for such a plate that 1 shear stress imparted at the base of the lower boundary can induce large in plane stretching mode forces and 2 these forces can be estimated directly from free air gravity anomalies without detailed knowledge of the density distribution in the underlying flow free air gravity anomalies for venus imply in plane forces of about 2 3 1012 n m in plane forces of this magnitude are capable of extensive disruption of the venusian lithosphere to demonstrate this an elastic plastic analysis is carried out for the deformation of a model venus lithosphere the calculation is based on a lithospheric strength envelope and uses gravity and topography as a guide to the magnitude of stresses imparted by convective flow the results predict that dynamic uplift of venusian topography must be accompanied by extensive brittle failure and viscous flow in the lithosphere geoid anomalies and geoid to topography ratios for venusian topographic features are large compared to values for thermal swells in earth s ocean basins this suggests that dynamic support of topography must be an important process on venus further a low viscosity zone is probably not present beneath the venusian lithosphere a comparative analysis suggests that tectonic deformation of the lithosphere associated with dynamically supported topography on earth is unimportant

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

radiocarbon calibration curve spanning 0 to 50 000 years bp based on paired 230 th 234 u 238 u and 14 c dates on pristine corals abstract radiocarbon dating is the most widely used dating technique in the world recent advances in accelerator mass spectrometry ams and sample preparation techniques have reduced the sample size requirements by a factor of 1000 and decreased the measurement time from weeks to minutes today it is estimated that more than 90 percent of all measurements made on accelerator mass spectrometers are for radiocarbon age dates the production of 14 c in the atmosphere varies through time due to changes in the earth s geomagnetic field intensity and in its concentration which is regulated by the carbon cycle as a result of these two variables a radiocarbon age is not equivalent to a calendar age four decades of joint research by the dendrochronology and radiocarbon communities have produced a radiocarbon calibration data set of remarkable precision and accuracy extending from the present to approximately 12 000 calendar years before present this paper presents high precision paired 230 th 234 u 238 u and 14 c age determinations on pristine coral samples that enable us to extend the radiocarbon calibration curve from 12 000 to 50 000 years before present we developed a statistical model to properly estimate sample age conversion from radiocarbon years to calendar years taking full account of combined errors in input ages and calibration uncertainties our radiocarbon calibration program is publicly accessible at http www radiocarbon ldeo columbia edu along with full documentation of the samples data and our statistical calibration model

relative stability on a tectonically active coast paleoenvironment during the last 7000 years at lake kohangapiripiri wellington new zealand abstract lake kohangapiripiri is a small coastal lake in fitzroy bay wellington new zealand it is situated in a tectonically active area that has well documented records of co seismic uplift the lake is separated from the sea by a gravel barrier and is partially surrounded by wave cut cliffs indicating that it was once an inlet lake sediments were analysed to determine the age of the marine to freshwater transition believed to have taken place as a result of recent uplift diatoms particle size organic carbon and radiocarbon data indicate changes in barrier height water depth and marginal wetland occurrence the data also provide evidence that the lake has been freshwater for 7000 years we propose that a large barrier has been in place between the lake and sea at least since the culmination of the post glacial marine transgression ca 7000 years ago comparison of the lake record with holocene earthquake records from surrounding locations shows that the sedimentary environment of the lake has been relatively insensitive to sudden changes in base level and disturbance of the catchment previous studies have identified earthquake related features in lake sediments however this study provides an example of a lacustrine environment subjected to large co seismic uplift events that does not record conclusive evidence of the events

extended boussinesq thermal chemical convection with moving heat sources and variable viscosity we have studied with an aspect ratio four box the thermal chemical convective evolution with strongly temperature and depth dependent viscosity and moving heat sources within the extended boussinesq framework in which both adiabatic and viscous heating are included and a depth dependent thermal expansivity is assumed in the equation of state our focus is to show how this type of mantle evolution with an averaged ra of 0 106 may develop with a linear chemical stratification and a uniformly hot mantle as an initial condition the effects of extended boussinesq and depth dependent thermal expansivity are to prevent the effective destruction of the chemical heterogeneities our results show that this initial condition would after the age of the earth lead to a lava lamp mode consisting of a thick chemically stratified and intensely internally heated layer with a thickness of around a quarter of the whole mantle thickness however in this isolated internally convecting layer exceedingly high temperatures greater than 4500 k would be reached in the deep mantle plumes can be launched from the top of this thick denser layer this lava lamp stage would give way to the formation of denser hill like structures at the core mantle boundary then upwellings with deep lower mantle origins can be induced by the interaction of the downwellings with the d layer our simulations show the possibility for some long range mass transfer interaction between these widely separated chemical hills promoted by the fast horizontal flow induced by the sinking currents along the low viscosity zone due to temperature dependent rheology at the core mantle boundary

the effects of multiple phase transitions on venusian mantle convection the effects of multiple phase transitions on the dynamics of the venusian mantle have been investigated with a two dimensional finite element method a depth dependent thermal expansivity has been employed in a purely basal heating configuration with an aspect ratio of four the addition of the olivine spinel transition promotes layered convection more so than models with a single spinel perovskite phase change the shifts in the phase transition depths and the presence of a conductive lid increase the tendency of the venusian mantle circulation to flow through the transition zone the potentially lower rayleigh number in venus from lack of volatiles will also enhance whole mantle convection the style of convection in venus may have changed from layered convection in the past to whole mantle convection today this transition might have been responsible for the major resurfacing event on venus

mantle convection and crustal evolution on venus venus is probably similar to the earth in that recycling of basaltic crust has been induced by the development of cratons combinations of siliceous crust with high mg fe mantle venus has less remaining energy sources at depth but still enough to to support the great plateaus and to deliver appreciable heat close to the surface by convection the main differences of venus from the earth arise from its lack of water rather than higher temperatures lack of water plus lower stress levels due to lesser energy makes the upper mantle more viscous and hence venus tectonics more driven by bulk mantle rather than boundary layer characteristics making tectonics difficult to infer from magellan imagery will be shallower and more widespread layers of weakness in crustal rocks obscuring the mantle driven patterns

hotspots basalts and the evolution of the mantle the trace element concentration patterns of continental and ocean island basalts and of mid ocean ridge basalts are complementary the relative sizes of the source regions for these fundamentally different basalt types can be estimated from the trace element enrichment depletion patterns their combined volume occupies most of the mantle above the 670 kilometer discontinuity the source regions separated as a result of early mantle differentiation and crystal fractionation from the resulting melt the mid ocean ridge basalts source evolved from an eclogite cumulate that lost its late stage enriched fluids at various times to the shallower mantle and continental crust the mid ocean ridge basalts source is rich in garnet and clinopyroxene whereas the continental and ocean island basalt source is a garnet peridotite that has experienced secondary enrichment these relationships are consistent with the evolution of a terrestrial magma ocean

the influences of surface temperature on upwellings in planetary convection with phase transitions abstract the importance of surface temperature for mantle convection appears with the presence of adiabatic heating and cooling and the release and consumption of latent heat in the presence of phase transitions for some planetary bodies these effects cannot be neglected the dimensionless surface temperature t 0 which is the ratio between the temperature at the top of the convective region and the temperature drop across the mantle is close to one for mars and venus for the earth t 0 lies between 0 2 and 0 5 the dynamical influence of t 0 is especially poignant for internally heated convection with temperature dependent viscosity there is a tight coupling between the magnitude of the temperature field and the viscosity itself we have studied temperature dependent viscosity convection for both low t 0 0 2 and high t 0 1 2 situations and with internal heating in mantle convection with two upper mantle phase transitions our results show that within this range of t 0 there exist two regimes for the evolution of upwellings in the mantle in transient situations plume plume collisions lead to the formation of megaplumes for high t 0 regimes but are less likely to do so for low t 0 in the long term regime plumes with low t 0 are prone to develop from the transition zone with a supply of hot material coming from the shallow lower mantle in systems with high t 0 however long lived plumes tend to have deeper mantle origins in quasi layered situations high t 0 may act as a positive feed back mechanism in inducing powerful hot upwellings into the upper mantle