at what depth is the eemian layer expected to be found at neem no continuous record from greenland of the eemian interglacial period 130 115 ka bp currently exists however a new ice core drill site has been suggested at 77 449 n 51 056 w in north west greenland north eemian or neem radio echo sounding images and flow model investigations indicate that an undisturbed eemian record may be obtained at neem in this work a two dimensional ice flow model with time dependent accumulation rate and ice thickness is used to estimate the location of the eemian layer at the new drill site the model is used to simulate the ice flow along the ice ridge leading to the drill site unknown flow parameters are found through a monte carlo analysis of the flow model constrained by observed isochrones in the ice the results indicate that the eemian layer is approximately 60m thick and that its base is located approximately 100m above bedrock

southeast greenland high accumulation rates derived from firn cores and ground penetrating radar despite containing only 14 of the greenland ice sheet by area the southeastern sector has the highest accumulation rates and hence receives 30 of the total snow accumulation we present accumulation rates obtained during our 2010 arctic circle traverse derived from three 50 m firn cores dated using geochemical analysis we tracked continuous internal reflection horizons between the firn cores using a 400 mhz ground penetrating radar gpr gpr data combined with depth age scales from the firn cores provide accumulation rates along a 70 km transect we followed an elevation gradient from 2350 to 1830m to understand how progressive surface melt may affect the ability to chemically date the firn cores and trace the internal layers with gpr from the firn cores we find a 52 0 43 m w e a 1 increase in average snow accumulation and greater interannual variability at the lower site than the upper site the gpr profiling reveals that accumulation rates are influenced by topographic undulations on the surface with up to 23 variability over 7 km these measurements confirm the presence of high accumulation rates in the southeast as predicted by the calibrated regional climate model polar mm5

a full coverage high resolution topographic model of greenland computed from a variety of digital elevation data in this paper a new high resolution elevation model of greenland including the ice sheet as well as the ice free regions is presented it is the first published full coverage model computed with an average resolution of 2 km and providing an unprecedented degree of detail the topography is modeled from a wide selection of data sources including satellite radar altimetry from geosat and ers 1 airborne radar altimetry and airborne laser altimetry over the ice sheet and photogrammetric and manual map scannings in the ice free region the ice sheet model accuracy is evaluated by omitting airborne laser data from the analysis and treating them as ground truth observations the mean accuracy of the ice sheet elevations is estimated to be 12 13 m and it is found that on surfaces of a slope between 0 2 and 0 8 corresponding to approximately 50 of the ice sheet the model presents a 40 improvement over models based on satellite altimetry alone on coastal bedrock the model is compared with stereo triangulated reference points and it is found that the model accuracy is of the order of 25 35 m in areas covered by stereo photogrammetry scannings and between 200 and 250 m elsewhere

time dependent models of single and double layer mantle convection one outstanding problem of great geophysical importance is the vertical extent of convection in the earth s mantle1 steady state models of convection in the mantle have yet to produce a description of the mantle which is consistent with geochemical and geophysical observations here we present time dependent numerical models of two dimensional convection which simulate mantle convection starting from a fluid initially at rest with a purely conductive temperature profile we now find that for a rayleigh number ra of 107 the model experiences a transient period of double layer convection which lasts on the order of hundreds of millions of years when scaled to the earth s mantle it is suggested that transient periods of double layer convection in the earth s mantle may have existed in the past whether or not such a period exists today

the morphodynamic modelling of tidal sand waves on the shoreface an artificial sand wave on the dutch shoreface of the north sea has been studied in conditions with relatively strong tidal currents in the range of 0 5 to 1 m s and sediments in the medium sand size range of 0 2 to 0 5 mm the sand wave is perpendicular to the tidal current and has a maximum height and length of the order of 5 m and 1 km respectively the sand wave is dynamically active and shows migration rates of the order of a few metres per year a numerical morphodynamic model delft3d model has been used to simulate the morphological behaviour of the sand wave in the north sea this model approach is based on the numerical solution of the three dimensional shallow water equations in combination with a surface wave propagation model wind waves and the advection diffusion equation for the sediment particles with online bed updating after each time step the model results show that the sand wave grows in the case of dominant bed load transport weak tidal currents relatively coarse sediment small roughness height low waves and that the sand wave decays in the case of dominant suspended transport strong currents relatively fine sediment large roughness height storm waves

significant effect of sediment cohesion on delta morphology delta morphology is thought to be controlled by factors such as river discharge tides and waves numerical modelling shows that sediment cohesion also strongly influences the development of a delta s characteristics the morphologies of the world s deltas are thought to be determined by river discharge tidal range and wave action 1 more recently sea level rise 2 3 and human engineering4 have been shown to shape delta evolution the effects of factors such as sediment type and the overall amount of sediment carried by rivers are considered secondary4 5 6 in particular the role of sediment cohesion which is controlled by sediment size and type of vegetation is unclear here we use a numerical flow and transport model7 8 9 10 to show that sediment cohesiveness also strongly influences the morphology of deltas we find that holding all other factors constant highly cohesive sediments form bird s foot deltas with rugose shorelines and highly complex floodplains whereas less cohesive sediments result in fan like deltas with smooth shorelines and flat floodplains in our simulations sediment cohesiveness also controls the number of channels that form within the deltas and the average angle of bifurcation of those channels as vegetation generally acts as a cohesive agent we suggest that deltas that formed before the expansion of land plants in the devonian period should show fan like characteristics a finding consistent with the limited data from the sedimentological record11

whole mantle convection and the thermal evolution of the earth abstract three recent results in the fields of geophysics and geochemistry have combined to rejuvenate the study of the earth s thermal history the first and perhaps most important of these is the new understanding of the planetary accretion process which has been realized as a consequence of work by v s safronov and others this work shows that the terrestrial planets accrete on a time scale of 10 8 y by the accumulation of planetesimal sized objects and that they probably form sufficiently hot to induce immediate differentiation of iron from silicates the second important recent contribution has been from geochemistry and concerns the net complement of radioactivity of the earth as a whole although the net complement of radioactive elements has usually been inferred on the basis of the assumption of equilibrium with the surface heat flow a priori analysis based upon elemental condensation temperatures in the primitive solar nebula yields a model in which the net radioactivity is at least a factor of two less than would be required to explain the mean surface heat flow there are good reasons to believe that this model is valid for the earth the third result has been the successful development of a technique for incorporating the influence of convective heat transfer in a thermal evolution model which does not require explicit solution of the dynamical equations this paper begins with an analysis of the mantle convective circulation which is required to understand plate tectonics and continental drift an analysis which presents the arguments for and against the crucial notion that the main circulation has a depth scale equal to the mantle thickness using the idea of parameterized convection the radial heat transfer by the mantle convective circulation is included in a thermal history model of the earth this model turns out to contain within it an interesting explanation of the fact that no crustal rocks with ages in excess of 3 8 ga have ever been found an observation that has heretofore proven enigmatic

surface mass balance of the greenland ice sheet from climate analysis data and accumulation runoff models we used surface climate fields from high resolution 0 56 0 56 european centre for medium range weather forecasts ecmwf operational analyses 1992 98 together with meteorological and glaciological models of snow accumulation and surface meltwater runoff retention to produce novel maps of greenland ice sheet gis net accumulation net runoff and surface mass balance smb we compared our runoff maps with similar scaled runoff melt minus refreezing maps based on passive microwave satellite data our gross spatial temporal patterns of runoff compared well with those from the satellite data although amounts of modelled runoff are likely too low mean accumulation was 0 287 0 307 m a 1 and mean runoff was 0 128 0 151 m a 1 averaged across the w abdalati t l mote gis mask corresponding mean smb was 0 159 0 156 m a 1 with considerable interannual variability standard deviation 0 11 m a 1 primarily due to variations in runoff considering best estimates of current iceberg calving overall the gis is probably currently losing mass our study shows great promise for meaningfully modelling smb based on forthcoming second generation ecmwf re analysis era 40 data and comparing the results with ongoing laser radar measurements of surface elevation this should help elucidate to what extent surface elevation changes are caused by short term smb variations or other factors e g ice dynamics

the impact of wave induced longshore transport on a delta shoreface system abstract wave induced longshore sediment transport is a major basinal process that reworks fluvial sediment supply along a delta shoreface depositional system however the effects of the variable counterbalance between supply and wave reworking have not been sufficiently acknowledged in understanding paleoshoreline evolution and stratigraphic concepts such as sequence stratigraphy and autostratigraphy with a numerical shoreline model we quantify the relation between wave induced longshore sediment transport and shoreline orientation under conditions of steady sea level a case study of the po delta shoreface system reveals that a decrease in delta progradation rate can in part be considered as an autogenic response to steady wave conditions offshore long term wave induced transport can be quantified by the volume of sediment redistribution as such we propose a revised definition of accommodation space by integrating the effect of wave induced longshore transport wave induced longshore sediment transport has a significant impact on progradation of deltaic and adjacent shoreface shorelines and needs to be considered while interpreting shallow marine stratigraphy

recent accumulation variability in northwest greenland from ground penetrating radar and shallow cores along the greenland inland traverse accumulation is a key parameter governing the mass balance of the greenland ice sheet several studies have documented the spatial variability of accumulation over wide spatial scales primarily using point data remote sensing or modeling direct measurements of spatially extensive detailed profiles of accumulation in greenland however are rare we used 400mhz ground penetrating radar along the 1009 km route of the greenland inland traverse from thule to summit during april and may of 2011 to image continuous internal reflecting horizons we dated these horizons using ice core chemistry at each end of the traverse using density profiles measured along the traverse we determined the depth to the horizons and the corresponding water equivalent accumulation rates the measured accumulation rates vary from 0 1mw e a in the interior to 0 7mw e a near the coast and correspond broadly with existing published model results though there are some excursions comparison of our recent accumulation rates with those collected along a similar route in the 1950s shows a 10 increase in accumulation rates over the past 52 years along most of the traverse route this implies that the increased water vapor capacity of warmer air is increasing accumulation in the interior of greenland

finite amplitude convection through a phase boundary summary a finite amplitude numerical model of convective motions through a phase boundary is studied the phase change is assumed to occur over a finite vertical transition zone in the fluid and thus a continuously stratified model is considered the motions are driven by horizontal temperature gradients on the boundaries or super critical vertical stratification in the case of rayleigh instability the numerical results reproduce the neutral curve previously obtained in the analytic solution of the linearized problem when the parameters of the problem are chosen to simulate the olivine spinel transition in the upper mantle the following results are obtained 1 the vertical scale of motion is the entire depth of the fluid 2 the horizontal scale is not significantly changed from the case of a single phase fluid 3 the amplitude of the motion is not significantly changed due to buoyancy changes at the phase boundary being offset by corresponding sources and sinks of latent heat and 4 the phase boundary varies in depth by as much as 30 km when the vertical velocities are of the order of 10 1 cm yr 1

on bar growth and decay during interannual net offshore migration abstract multiple bar systems often show a cyclic net offshore directed migration with return periods on the order of years generally a bar is generated near the shoreline grows in height and width while migrating offshore before finally decaying at the seaward limit of the surf zone based on a three year hindcast of a bar cycle at noordwijk netherlands and on additional synthetic runs using a wave averaged cross shore process model this study identifies the dominant mechanisms that govern the bar amplitude growth and decay during net interannual offshore migration the bar amplitude response is particularly sensitive to the water depth above the bar crest h xb and the angle of wave incidence these variables largely control the amount of waves breaking on the bar and the strength and cross shore distribution of the associated longshore current the longshore current has its maximum landward of the bar crest inducing additional stirring of sediment on the landward bar slope and trough the enhanced sediment concentration in the trough region shifts the cross shore transports peak landward of the bar crest forcing bar amplitude growth during offshore migration for increased h xb values wave breaking becomes less frequent consequently reducing the influence of the longshore current on sediment stirring therefore the resulting dominance of the cross shore current results in a sediment transport peak at the bar crest causing bar amplitude decay all four types of bar response viz all combinations of onshore offshore migration and bar amplitude growth decay can occur for a single wave height and wave period combination depending on h xb and additional hindcast runs in which the wave direction was assumed time invariant confirmed that h xb and largely control the transient bar amplitude response

on the upscaling of process based models in deltaic applications abstract process based numerical models are increasingly used to study the evolution of marine and terrestrial depositional environments whilst a detailed description of small scale processes provides an accurate representation of reality application on geological timescales is restrained by the associated increase in computational time in order to reduce the computational time a number of acceleration methods are combined and evaluated for a schematic supply driven delta static base level and an accommodation driven delta variable base level the performance of the combined acceleration methods is evaluated by comparing the morphological indicators such as distributary channel networking and delta volumes derived from the model predictions for various levels of acceleration the results of the accelerated models are compared to the outcomes from a series of simulations to capture autogenic variability autogenic variability is quantified by re running identical models on an initial bathymetry with 1 cm added noise the overall results show that the variability of the accelerated models fall within the autogenic variability range suggesting that the application of acceleration methods does not significantly affect the simulated delta evolution the time scale compression method the acceleration method introduced in this paper results in an increased computational efficiency of 75 without adversely affecting the simulated delta evolution compared to a base case the combination of the time scale compression method with the existing acceleration methods has the potential to extend the application range of process based models towards geologic timescales

a numerical model of coupled subcontinental and oceanic convection abstract at least two different types of convection should exist in the earth s mantle a large scale flow which is revealed by the creation and subduction of oceanic crust and a circulation of unknown scale under the continents which does not approach the surface the effects of these coexisting and interacting modes on mantle dynamics and thermal regime are investigated by means of a finite element model system of two adjacent cells which are coupled thermally and mechanically at their interface an a priori viscosity distribution is applied in such a way that the surface layer of the longer cell can move with almost uniform velocity while it is immobile on the shorter cell the results suggest that the pattern of sub oceanic convection is stabilized to some extent by the frame of the highly viscous but moving lithosphere whereas convection under continents is more complex and less stable most of the sub continental heat is transferred into the oceanic cell this causes an imbalance of surface heat flow which is well in accordance with recent determinations of global heat loss distribution furthermore the model reflects at least semiquantitatively the continent ocean differences in lithospheric thickness mean topographic level and upper mantle temperatures while the average geotherms below ocean and continent diverge by up to 400 k at moderate depth they nearly coincide below 250 km depth thus giving no support for deep reaching continent ocean differences these results refer to convection penetrating the lower mantle if the circulation is confined to moderate depth the continental upper mantle is too hot compared with the oceanic however this may be due to the strict separation of the flow on either side the model results stimulate some speculations on the possible form of whole mantle convection

stress and flow beneath island arcs summary the flow pattern stress distribution topography and gravity anomalies were computed from numerical models having density and viscosity distributions resemblant to the aleutian arc the results were compatible with the hypothesis that the excess density of the slab drives its descent and that hydrodynamic forces are responsible for topographic and gravity highs over the outer rise seaward of the trench and the frontal arc and lows over the trench in models with simple distributions of rheological parameters the force from the slab was transmitted directly upward producing a negative gravity anomaly over the arc material with low resistance to flow was needed along the fault plane above the slab or within the crust of the frontal arc and within the wedge of asthenosphere above the slab to reduce that force and to allow the horizontal lithosphere to move with the slab models with the resistance to flow thus reduced had outer rises deep trenches horizontal tension seaward of the trench horizontal compression under the trench and downdip tension in the slab free air gravity anomalies which are the sum of between deflections of the free surface due to hydrodynamic forces and direct attractions from the masses driving the flow were not fit excellently by any of the models in part because the coarse grid used precluded accurate representation of the fault zone above the slab and the frontal arc an alternate to the hypothesis that about 5 kb of stress on the fault plane is needed to produce an outer rise is offered by these models shear stress between the slab and the island arc was always below 700 bars in the more successful models if the density distribution was scaled to match the topography of the trench this is much less than the 2000 bars stresses needed if frictional heating causes island arc volcanism

principles of regression and transgression the nature of the interplay between accommodation and sediment supply the rate of change of accommodation at the shoreline a and the rate of sediment supply s are the primary factors controlling regression and transgression in the geological record or retreat and advance of a coastal depositional system the generally accepted notion here referred to as the a s ratio concept stating that the migration of a shoreline is controlled principally by the magnitude of the ratio of a to s is insufficient and somewhat misleading partly owing to the problem of dimensional confusion the interplay of a and s inevitably results in autoretreat of the shoreline whereby the seaward advance of any shoreline is halted and is subsequently turned to landward retreat provided there is a continuous rise of relative sea level how effective the autoretreat process is depends for a given period of relative sea level rising upon the length of potential time period for the seaward advance of shoreline and is proportional to a 2 s and a super 1 5 s super 0 5 in two and three dimensional sediment dispersal modes respectively autoretreat becomes more effective as a increases and or s decreases a functions more critically to the effectiveness than s does the a s ratio concept is only approximately applicable when autoretreat is less effective i e with very low a and or very high s in this sense the autoretreat theory and the a s ratio concept are complementary to each other the autoretreat mechanism is expected to be substantially useful not only in fluvial deltas but also in many other depositional systems e g barrier islands whose evolution has been considered in terms of the a s ratio concept

can bed load transport drive varying depositional behaviour in river delta environments abstract understanding the processes and conditions at the time of deposition is key to the development of robust geological models which adequately approximate the heterogeneous delta morphology and stratigraphy they represent we show how the mechanism of sediment transport the proportion of the sediment supply transported as bed load vs suspended load impacts channel kinematics delta morphology and stratigraphy to at least the same extent as the proportion of cohesive sediment supply this finding is derived from 15 synthetic delta analogues generated by processes based simulations in delft3d the model parameter space varies sediment transport mechanism against proportions of cohesive sediment whilst keeping the total sediment mass input constant proximal morphology and kinematics previously associated with sediment cohesivity are also produced by decreasing the proportion of bed load sediment transport however distal depositional patterns are different for changes in sediment transport and sediment load cohesivity changes in sediment transport mechanisms are also shown to impact clinoform geometry as well as the spatiotemporal scale of autogenic reorganisation through channel avulsions we conclude that improving insight into the ratio of bed load to suspended load is crucial to predicting the geometric evolution of a delta

role of morphological variability in the evolution of nearshore sandbars abstract computations using a depth averaged morphological process based model of a double nearshore bar system have been used to test the hypothesis that bathymetries with small variability adapt more easily to new hydrodynamic conditions than bathymetries with distinctly imprinted crescentic patterns the computations are used to investigate the assumption that nearshore bathymetries tend to evolve toward a rip channelled pattern matching concurrent constant hydrodynamic forcing if these conditions prevail for an extended period of time in each computation an initially alongshore uniform double barred bathymetry seeded with a small random bed level perturbation was forced by two sequential constant hydrodynamic conditions for each set of conditions four different computations show the effect of a later transition moment and thus more distinctly evolved patterns on the level of adaptation to the second condition after the transition to the second condition different hydrodynamic circulations occur due to differences in the bathymetry at the transition moments depending on how pronounced the existing features were at the moment of transition these circulations either reinforce the existing bathymetric pattern or allow the bathymetry to evolve to a new rip chanelled pattern with a spacing similar to the one that occurs if the second condition had been applied from the start as hydrodynamic conditions generally change more rapidly than the adaptation time which is at least in the order of days it is highly unlikely that observed rip channel distances match length scales expected for concurrent hydrodynamic conditions consistent with field observations e g holman et al 2006 it is therefore concluded that nearshore patterns are formed by a combination of both the antecedent morphology and thus antecedent hydrodynamics and the current local hydrodynamic conditions next to factors like sediment characteristics

contribution to the glaciology of northern greenland from satellite radar interferometry interferometric synthetic aperture radar insar data from the ers 1 and ers 2 satellites are used to measure the surface velocity topography and grounding line position of the major outlet glaciers in the northern sector of the greenland ice sheet the mass output of the glaciers at and above the grounding line is determined and compared with the mass input we find that the grounding line output is approximately in balance with the input except for the three largest glaciers for which the mass loss is 4 3 km3 ice year 1 or 11 8 of the mass input along the coast we detect a systematic retreat of the grounding lines between 1992 and 1996 with insar which implies that the outlet glaciers are thinning the inferred coastal thinning is too large to be explained by a few warm summers glacier thinning must be of dynamic origin that is caused by spatial and temporal changes in ice velocity iceberg production from the glaciers is uncharacteristically low it accounts for only 8 of the ice discharge to the ocean about 55 of the ice is lost through basal melting 5 8 m ice year 1 on average from the underside of the floating glacier tongues that are in contact with warm ocean waters mass losses are highest in the first 10 km of floating ice where ice reaches the greatest depths and basal melting is 3 times larger than on average only a small increase in basal melting would suffice to disintegrate the floating glacier tongues copyright 2001 by the american geophysical union

the salt dome problem a multilayered approach the dynamical evolution of buoyancy driven geological systems is mainly influenced by two physical parameters density and equivalent viscosity salt structures are an appropriate geological representation of these systems important variations of these parameters are directly related to changes in mass and thus the internal structures themselves and their changes in time become significant parameters in order to decipher these structures in nature by means of experiments it is therefore necessary to extend model calculations on intrusion processes to multilayer systems a two dimensional finite element program has been developed to simulate the rise of individual diapirs through multi layered overburdens exhibiting newtonian viscous rheologies this program includes the following features arbitrarily shaped interfaces between different fluid boundaries layers of finite thickness and subsidence of the model within the model frame as well as time and depth dependent material properties this paper is concerned with single diapirs intruding upbuilt multilayered overburdens the effect of a subsiding salt base due to pure downbuilding and differential loading of gravitationally unstable distributions of mass variable density and viscosity of the overburden with depth control the shape and ascent rates of diapiric bodies the subsidence history of salt structures dominates the development of maximum horizontal pressure gradients with time and thus the ascend histories of diapirs

quantitative models of sedimentary basin filling summary quantitative modelling of the filling of sedimentary basins was begun in earnest in the 1960s dozens of themes and variations have been proposed since then and have yielded an abundance of idealized stratigraphic patterns as functions of both imposed changes and basin properties post plate tectonic modelling began with rigid lid models which show the stratigraphic signature of subsidence variation this work introduced the connection between stratigraphy and the rheology of the lithosphere rigid lid models are the simplest type of geometric model in which the sediment surface is assigned prescribed geometries usually corresponding to different depositional environments these can reproduce many aspects of overall stratal geometry but are formally restricted to relatively long timescales for which quasi steady surface topography can be assumed so called dynamic models attempt to represent the morphodynamics of the sediment surface by abstracting and averaging short term transport processes most of the dynamic models proposed to date can be seen as special cases of a single general morpho dynamic equation the most important result of the first wave of quantitative basin filling models is that even relatively simple models can produce reasonable stratal patterns we now have a wide array of tools for exploring scenarios searching for general behaviours and effects and making initial quantitative predictions we have also learned that basin response to external forcing as recorded in stratigraphy can be as sensitive to the characteristics of the basin as to the forcing the main brake on the development of basin modelling is not computing power but lack of methods and data for testing the models we have already developed physical experiments which are only just beginning are one means of doing this experimental stratigraphy is a bridge to quantitative field tests which will require collaboration among academic researchers from a wide range of areas and between academia and industry on projects of greater scale and degree of integration than we have seen so far the advancement of quantitative sedimentary geology will also require significant changes in the way the subject is taught at all levels

phase boundaries in finite amplitude mantle convection summary phase boundaries are included in dynamical finite element models of mantle convection they are represented by point chains which act as additional sources of buoyancy forces when distorted and as additional source or sink of heat the influence of the exothermic olivine spinel transition is studied in both shallow and deep convection models the flow is only slightly enhanced by the transition the increase of temperature due to latent heat release is step like in the deep model in the case of shallow convection it is more diffuse other quantities like ocean floor topography gravity anomalies and stress distribution are no more than moderately affected in a further investigation the effect of spinel post spinel transition whether endothermic or exothermic on deep convection is examined the effect on the flow is negligibly small in both cases

numerical approach to problems of gravitational instability of geostructures with advected material boundaries summary we present a numerical approach for solving 2 d mantle flow problems where the chemical composition changes abruptly across intermediate boundaries the method combines a galerkin spline technique with a method of integration over regions bounded by advected interfaces to represent discontinuous variations of material parameters it allows direct approximation of a natural free surface position instead of a posteriori calculation of topography from the normal stress at the upper free slip boundary we formulate a model where a viscous incompressible fluid filling a square box is divided into layers not necessarily horizontal by advected boundaries across which the density and viscosity change discontinuously no slip or free slip conditions are assumed at the model sides the suggested approach being eulerian avoids the difficulties due to material discontinuities at intermediate boundaries like the moho or the earth s surface and is also free from the deficiencies of the lagrangian approach always resulting in mesh distortion we present two geophysical cases analysed by this technique the first case concerns the formation of sedimentary basins under the effects of heavy bodies sinking in the asthenosphere and of load due to sedimentary infills the second case demonstrates the evolution of salt diapirs and shows how their growth is affected by a laterally inhomogeneous sedimentary layer this numerical approach is well suited for problems of gravitational instability with discontinuities of density and viscosity across advected boundaries

the impacts of mantle phase transitions and the iron spin crossover in ferropericlase on convective mixing is the evidence for compositional convection definitive new results from a yin yang overset grid based control volume model high resolution seismic tomographic images from several subduction zones provide evidence for the inhibition of the downwelling of subducting slabs at the level of the 660 km depth seismic discontinuity furthermore the inference of old 140 myr sinking slabs below fossil subduction zones in the lower mantle has yet to be explained we employ a control volume methodology to develop a new anelastically compressible model of three dimensional thermal convection in the mantle of a terrestrial planet that fully incorporates the influence of large variations in material properties the model also incorporates the influence of 1 multiple solid solid pressure induced phase transitions 2 transformational superplasticity at 660 km depth and 3 the high spin low spin iron spin transition in ferropericlase at midmantle pressures the message passing interface parallelized code is successfully tested against previously published benchmark results the high resolution control volume models exhibit the same degree of radial layering as previously shown to be characteristic of otherwise identical 2 d axisymmetric spherical models the layering is enhanced by the presence of moderate transformational superplasticity and in the presence of the spin crossover in ferropericlase stagnation of cold downwellings occurs in the range of spin crossover depths 1700 km although this electronic spin transition has been suggested to be invisible seismically recent high pressure ab initio calculations suggest it to have a clear signature in body wave velocities which could provide an isochemical explanation of a seismological signature involving the onset of decorrelation between vp and vs that has come to be interpreted as requiring compositional layering

glaciological investigations in the crete area central greenland a search for a new deep drilling site the results of the 1984 85 post gisp campaigns in central greenland are presented eight ice cores were obtained some spanning up to 360 years we present i geographical positions and elevations at the drill sites

mantle phase transitions and layered chaotic convection we report preliminary results from a sequence of very high resolution simulations of the mantle convection process based upon an axisymmetric anelastically compressible spherical model with both olivine spinel and spinel post spinel phase transitions our analyses strongly suggest that at earth like rayleigh numbers near 107 the circulation may be quite strongly layered although a very long timescale transient exists that is characterized by brief excursions into the whole mantle state we suggest the use of a new diagnostic tool that may be employed to determine whether or not the layered state is characteristic of the present day earth

characterization of river delta shorelines 1 coastline shape is among the basic geometric properties of the earth s surface yet defining and quantifying coastline shape precisely remains challenging here we propose new methods based on the dilation and erosion operators of mathematical morphology mm for the characterization of delta shorelines a class of open coasts we demonstrate the suitability of mm based methods for noise removal and shoreline identification from digital images of river deltas of varying geometric complexity we also show that a global a regional and a local shoreline parameter provide a first step in distinguishing among river wave and tidal dominated delta shapes

the fate of descending slabs oceanic lithosphere the stiff plate formed by the cooling of chemically diffcrentiated material that rises and mclts under mid ocean ridges and spreads laterally in the process called sea floor spreading has an ephemeral residence at the surface of the earth typically within 100 200 ma densi fication of the old oceanic lithosphere leads to a dynamic instability causing it to sink into the interior descending as a layered slab with highly anom alous thermal and chemical characteristics relative to the surrounding ambient mantle the slab heats as it sinks but the warming is sluggish slow to overcome the effect of more than 100 ma of cooling at the surface and the negative buoyancy of the slab continues to drive its descent increasing pressure and temperature with depth drive various chemical reactions in the slab including expulsion of water and other volatiles accumulated at the surface and destabilization of associated hydrous phases suites of mineralogical phase transformations occur within the slab s crustal and depleted mantle components because the slab has been processed through the melting zone at the ridge partially hydrated and cooled significantly the phase equilibria within the slab differ from those of the surrounding mantle near 400 km depth an exothermic phase transformation occurs with mg fe 2si04 olivine in the slab converting to the high pressure modified j spinel structure the spinel transition helps to drive the slab onward although a central core of metastable olivine may persist to greater depths with eventual transformational faulting

mechanics of river mouth bar formation implications for the morphodynamics of delta distributary networks 1 in this paper we use observational data and numerical modeling to present a new explanation for the formation of river dominated delta networks observational data from deltas throughout the world show that distributary channel widths depths and lengths decrease nonlinearly with successive bifurcations trends in width and depth are an outcome of hydraulic geometry scaling the trend in channel length is a consequence of delta growth analyses of serial maps show that the positions of bifurcations are the fossilized locations of river mouth bars also called middle ground and distributary mouth bar in front of old delta channel mouths therefore the trend in channel length can be explained through the mechanics of river mouth bar formation and evolution we use delft3d a coupled hydrodynamic and morphodynamic model to simulate the process of river mouth bar formation within an expanding turbulent jet in front of distributary channel mouths our results describe in detail the formation and evolution of a river mouth bar system and demonstrate that the distance to the river mouth bar is proportional to jet momentum flux and inversely proportional to grain size therefore channel length decreases down delta because with each successive bifurcation the jet momentum flux decreases these results can be used to predict the future evolution of river dominated deltas and can be used to aid in hydrocarbon exploration of these depositional environments

bifurcation modelling in a meandering gravel sand bed river the rhine bifurcation at pannerden forms the major distribution point for water supply in the netherlands distributing not only water and sediment but also flooding risks and navigability its morphological stability has been a concern for centuries we present experiences from more than two decades of numerical morphological modelling of this bifurcation with a gravel sand bed and a meandering planform successive computations have shown the importance of upstream approach conditions the necessity to include physical mechanisms for grain sorting and alluvial roughness and the need to assume a thicker active layer of the river bed than is suggested by laboratory flume experiments using a constant discharge the active layer must be thicker in the model to account for river bed variations due to higher frequency discharge variations that are filtered out in morphological modelling we discuss limitations in calibration and verification but argue that notwithstanding these limitations 2d and 3d morphological models are valuable tools not only for pragmatic applications to engineering problems but also for revealing the limitations of established knowledge and understanding of the relevant physical processes the application of numerical models to the pannerden bifurcation appeared to reveal shortcomings in established model formulations that do not pose particular problems in other cases this application is therefore particularly useful for setting the agenda for further research copyright 2012 john wiley sons ltd

interferometric estimation of three dimensional ice flow using ascending and descending passes satellite radar interferometry sri provides an important new tool for determining ice flow velocity interferometric measurements made from a single track direction are sensitive only to a single component of the three component velocity vector observations from along three different track directions would allow the full velocity vector to be determined a north south looking synthetic aperture radar sar could provide these observations over large portions of the globe but not over large areas of the polar ice sheets the authors develop and demonstrate a technique that allows the three component velocity vector to be estimated from data acquired along two track directions ascending and descending under a surface parallel flow assumption this technique requires that there are accurate estimates of the surface slope which are also determined interferometrically to demonstrate the technique the authors estimate the three component velocity field for the ryder glacier greenland their results are promising although they do not have yet ground truth data with which to determine the accuracy of their estimates

influence of sea ice treatment in a regional climate model on boundary layer values in the fram strait region abstract the influence of two simple descriptions for the sea ice distribution on boundary layer values is investigated by comparing model results from the regional climate model remo with measured data in the fram strait in april 1999 one method for determining the sea ice distribution in remo is to diagnose the sea ice cover from the prescribed surface temperature and allow each grid cell to be either completely free of ice or completely covered by ice remo original the other one is to employ a partial sea ice concentration in each remo grid cell with the input data derived from satellite data remo partial surface fluxes are average values of the ice and water partial fluxes there is a clearly better agreement between measured and simulated surface and boundary layer temperatures and humidities when using remo partial compared to remo original the closed ice cover in remo original leads to downward sensible heat fluxes over ice whereas the ice cover with leads and polynyas in remo partial lead

controls on river delta formation insights from numerical modelling abstract primary hydraulic and sedimentary controls on river deltas were previously postulated in classification schemes in the field of sedimentary geology based on many field observations however detailed mechanistic models were restricted to the river dominated delta class analogous to the concept of morphological conditioning here we show that antecedent stratigraphy controls morphometrics of prograding river dominated delta distributary networks under steady sea level further we use coupled hydrodynamic morphodynamic stratigraphic detailed numerical modelling to assess the influence of wind generated waves and tides on clastic river delta formation our synthetic simulations show that deltas forming under mere riverine forcing prograde via sequences of mouth bar induced flow bifurcation and upstream channel shifting windwave action suppresses sequestration of fine sediments on the developing delta plain entailing relatively smooth shorelines perturbed by a limited number of distributary channels in contrast tide influenced river deltas are found to prograde mainly via lengthening of initially formed relatively stable distributaries as well as being characterized by cyclicity in deposits interbedding of sands and silts these results provide a framework for physics based river delta modelling under various environmental conditions particularly our findings suggest that relatively low energetic basin conditions can already significantly impact morphological and stratal patterns of prograding river deltas

the interaction of a subducting lithospheric slab with a chemical or phase boundary we have used a finite element model of time dependent convection to determine the conditions for penetration of the subducted plate into the lower mantle a temperature dependent and non newtonian rheology is applied to achieve platelike behavior of the upper and sinking thermal boundary layer of convection the 650 km discontinuity is taken as either a chemical or phase boundary or as a combination of both it is represented by a marker chain which effects additional buoyancy when distorted out of its equilibrium position when the compositional density contrast is greater than about 5 the descending slab is deflected sidewards at the boundary and two layer convection prevails a resulting depression of the boundary in the range of 50 200 km should be detectable with seismic methods below 5 density difference the slab plunges several hundred kilometers into the lower mantle and below 2 it will probably not stop before reaching the core mantle boundary and extensive mixing would be expected with a pure phase change a negative clapeyron slope of about 6 mpa k 60 bar k is required to establish a type of leaky double layer convection a more moderate slope can aid a small compositional density difference to prevent slab penetration into the lower mantle with the present uncertainties about the physical nature of the 650 km discontinuity a variety of convective styles appears possible on dynamical grounds

an experimental study of subduction and slab migration results of fluid dynamical experiments on the behavior of subducted slabs are presented that address two important characteristics of subduction slab penetration through the transition zone and horizontal slab migration cold negatively buoyant molded slabs of concentrated sucrose solution with viscosities of 3 5 106 p were introduced into a more dilute two layered sucrose solution representing the upper and lower mantle the transition zone was modeled by a step increase in both density and viscosity ratios of slab thickness to upper layer depth the relative viscosities in each layer and the effective rayleigh number were close to mantle values the initial configuration consisted of two plates separated by a trench gap with one plate attached to a dipping slab simulating a developing subduction zone with an overriding plate two different boundary conditions were used at the trailing ridge end of the subducting plate 1 fixed end corresponding to zero ridge motion and 2 free end corresponding to zero ridge resistance based on results from 15 experiments with various combinations of upper layer lower layer and slab densities u l s and viscosities we find that slab penetration depth is sensitive to both the normalized slab density anomaly r s l s u and the dip angle three penetration regimes are recognized r 0 5 in which the slab sinks into the lower layer without distortion 0 2 r 0 5 which permits limited penetration the amount dependent on dip angle in this regime the slab develops a thick root just beneath the transition zone and r 0 2 in which the slab is deflected by the transition and little or no penetration occurs retrograde slab motion and trench migration occurred in nearly every case with the overriding plate moving parallel to but faster than the trench causing the gap between subducting and overriding plates to close with time prograde trench motion was never observed and the trench remained stationary only in the extreme case free ridge condition with equal upper and lower layer densities these results suggest that the transient extension found in many back arc basins may be a direct consequence of the tendency for retrograde subduction and the amount of extension may be governed in part by the degree of slab penetration through the transition zone

evaluation of recent precipitation studies for greenland ice sheet the retrieval of an accurate spatial and temporal record of contemporary greenland precipitation is a uniquely challenging task because of the extreme variability in both atmospheric processes and the resulting precipitation distribution over relatively small spatial scales a comparison of precipitation data sets composed of monthly mean values from recent studies shows a convergence on the general features of the long term spatial patterns but substantial disagreement on the temporal variability both regionally and for all of greenland there is general agreement on a long term greenland average of about 35 cm yr 1 and on long term values for regional scales although differences for outlying data sets exceed 50 of the observed glaciological estimate for particular regions a fundamental problem is the inadequate topographic representation of greenland in the numerical analyses nearly all of the data sets are overly dry for high elevation areas as seen from comparisons with glaciological observations from summit the east central region of greenland is found to be particularly susceptible to the temporal discontinuities in data sets which employ operational analyses in contrast there is strong agreement among all methods on the temporal variability for the west central region over a 15 year period from the comparison it is concluded that none of the data sets is able to capture all of the regional scale features in general however the deficiencies of each data set are readily identifiable from comparison and evaluation in the context of circulation features agreement among the methods on particular regions and timescales gives increased confidence in drawing conclusions related to aspects of greenland s precipitation climatology in particular an enhanced precipitation retrieval method is found to be less susceptible to data artifacts than other methods using operational analyses in the north anomalously high precipitation is associated with cyclonic development near the fram strait for west central greenland the close agreement among methods is related to the dominant contribution of the mean circulation

a mini surge on the ryder glacier greenland observed by satellite radar interferometry satellite radar interferometry reveals that the speed of the ryder glacier increased roughly threefold and then returned to normal 100 to 500 meters year over a 7 week period near the end of the 1995 melt season the accelerated flow represents a substantial though short lived change in ice discharge during the period of rapid motion meltwater filled supraglacial lakes may have drained which could have increased basal water pressure and caused the mini surge there are too few velocity measurements on other large outlet glaciers to determine whether this type of event is a widespread phenomenon in greenland but because most other outlet glaciers are at lower latitudes they should experience more extensive melting making them more susceptible to meltwater induced surges

recent greenland accumulation estimated from regional climate model simulations and ice core analysis the accumulation defined as precipitation minus evaporation over greenland has been simulated with the high resolution limited area regional climate model hirham4 applied over an arctic integration domain this simulation is compared with a revised estimate of annual accumulation rate distribution over greenland taking into account information from a new set of ice core analyses based on surface sample collections from the north greenland traverse the region with accumulation rates below 150 mm yr21 in central northwest greenland is much larger than previously assumed and extends about 500 km farther to the south it is demonstrated that good agreement between modeled and observed regional precipitation and accumulation patterns exists particularly concerning the location and the values of very low accumulation in the middle of greenland the accumulation rates in the northern part of greenland are reduced in comparison to previous estimates these minima are connected with a prevailing blocking high over the greenland ice sheet and katabatic wind systems preventing humidity transports to central greenland the model reasonably represents the synoptic situations that lead to precipitation maxima of precipitation and accumulation occur at the southwestern and southeastern coasts of greenland and are connected with cyclonic activity and the main storm tracks around greenland the central region of the greenland ice sheet acts as a blocking barrier on moving weather systems and prohibits cyclones moving from west to east across this region and thus prevents moisture transports

modelled channel patterns in a schematized tidal inlet abstract tidal inlets in the dutch wadden sea show typical morphological features i e westward oriented main inlet channel and ebb tidal delta the objective of this study is to find the governing physical processes of these morphological features the study uses a 2dh process based morphodynamic model delft3d on a schematized model domain with dimensions similar to the ameland inlet in the dutch wadden sea starting from a flat bed the models are forced by tides only short term simulations are made to explore the hydrodynamic characteristics and initial sedimentation and erosion patterns long term morphodynamic simulations are employed to investigate the governing parameters of the main inlet channel and the ebb tidal delta evolution sensitivity of the evolution is described in terms of initial inlet width 1 0 km and 3 5 km direction and asymmetry of tidal forcing m2 m4 transport formulations van rijn 1993 engelund and hansen 1967 and relative position of the tidal basin with respect to the inlet east existing middle and west the results tend to produce morphological features typical to the ameland inlet the direction of tidal forcing is the main governing parameter to the present orientation of the main inlet channel and the ebb tidal delta the model results generally prove the conceptual hypotheses that describe the orientation of the main inlet channel and the ebb tidal delta

precipitation over greenland and its relation to the north atlantic oscillation the equation method based on an equivalent isobaric geopotential height in coordinates has been used to retrieve the precipitation over greenland this approach is designed to accurately represent the topographic effects of the greenland ice sheet on atmospheric motion and precipitation the 11 year mean precipitation from 1985 to 1996 over all of greenland is 376 mm yr 1 which is close to the long term mean precipitation of 346 mm yr 1 estimated from glaciological data the precipitation over all of greenland shows that the largest value in 1986 is 472 mm yr 1 and the smallest value in 1995 is 309 mm yr 1 the major interannual variability of the atmospheric circulation in the north atlantic can be represented by the variation of the north atlantic oscillation nao index which is most pronounced during winter it is found that if the nao index increases the total precipitation over greenland decreases and vice versa the correlation coefficient between these two series for 1985 1995 is 0 75 the mean precipitation over southern greenland where the majority of precipitation falls is more closely related to the nao index in winter and their correlation coefficient is 0 80 this relationship can be understood from the composite maps of sea level pressure and greenland precipitation for the high and low index months during months of high nao index values the icelandic low is strong during months of low nao index values the monthly mean low is located to the southwest of greenland over the labrador sea precipitation amounts over the southeast coast of greenland are about 100 mm larger during the low nao index months than the high nao index months precipitation over all of greenland during the low nao index months is higher there are significant downward trends in annual precipitation from 1985 1995 for all of greenland and its southern and central west coastal regions amounting to about 3 per year

new precipitation and accumulation maps for greenland annual total precipitation and the annual accumulation on the greenland ice sheet are evaluated and presented in two maps the maps are based on accumulation measurements of 25 1 pits and cores obtained from the upper accumulation zone and precipitation measurements made at 35 meteorological stations in the coastal region to construct the accumulation map the annual precipitation was split into solid and liquid precipitation components annual total precipitation exceeding 2500 mm w e occurs on the southeastern tip of greenland while the minimum precipitation is estimated to occur on the northeastern slope of the ice sheet the mean annual precipitation for all of greenland is 340 mm w e the largest annual accumulation of about 1500 mm w e is found on the glaciers in the southeastern corner of greenland while the smallest accumulation is found on the northeastern slope of the ice sheet west of danmarkshavn the mean accumulation on the greenland ice sheet is estimated at 310 mm w e the regional difference in accumulation is examined with respect to the 850 hpa m bar level circulation the present surface topography is found to play an important role in determining regional accumulation on the ice sheet

cross sectional stability of tidal inlets a comparison between numerical and empirical approaches article i nfo a process based morphodynamic model is used to simulate the evolution of tidal inlets towards equilibrium when subjected to a range of tide and wave conditions specific attention is given to the evolution of the cross sectional area of the inlet channel equilibrium values of cross sectional areas are found by approaching the equilibrium from two initial conditions i e by starting an experiment with an initial value of the cross sectional area that is smaller than the estimated equilibrium value and an initial value that is larger than the estimated equilibrium value similar to nature a good correlation between cross sectional area and tidal prism is found this suggests that the physical mathematical formulations in the model used to describe the hydrodynamic and sedimentary process are a good representation of the morphodynamics in the real world and that the conceptual idea of escoffier on the cross sectional stability of inlets is valid

an experimental study of the effects of base level change on fluvial coastal plain and shelf systems abstract several of the concepts of sequence stratigraphic models were tested by experiments in a large flume using different gradients for successive runs a physical model of a fluvial drainage basin coastal plain continental shelf and continental slope was constructed in all experiments initial gradients of the coastal plain and shelf were identical systematic raising and lowering of the water surface simulated the changes in relative sea level i e base level that are similar to most sequence stratigraphic models base level changes significantly affected the shelf area but they had little effect on the fluvial drainage basin during base level falls fluvio deltaic progradation occurred fluvial aggradation occurred only during periods of base level stillstand or rise because of the slow rate of head ward erosion of incised valleys a significant time gap existed from the time base level first fell below the shelfbreak and the development of a cross shelf bypass valley during this lag time deposition occurred on the exposed shelf to form a fluvial braid plain concurrently channels developed at the shelfbreak and grew by headward erosion at various rates continuous incision and widening of these channels formed valleys these valleys were not linked directly to the main drainage basin unt l ultimately one captured the flow from that drainage at that point other valleys were abandoned and ceased to grow during the subsequent base level rise the single large cross shelf valley filled with coarser sediments while the other valleys partially filled with finer suspended load sediments the main cross shelf valley also supplied coarser sediments to the slope