c band radar polarimetry useful for detection of icebergs in sea ice this paper is focused on investigations of polarimetric c band radar signatures of icebergs in sea ice covered ocean regions the main objective is to assess the potential improvement of iceberg detection when using radar polarimetry the dominant backscattering mechanisms of icebergs are deduced by evaluating different polarimetric parameters magnitudes of the cross polarization ratios the correlation coefficients between hh and vv polarized signals and the entropy alpha parameters indicate a strong contribution of volume scattering in many cases over most icebergs the phase differences between hh and vv polarization are larger than zero spatial patterns of the polarimetric parameters differ from iceberg to iceberg and between different parameters on some bergs they only exhibit slight variations whereas on others they show noiselike textures but also more systematic changes are observed occasionally radar intensities of icebergs are of similar magnitude as those of sea ice only for a number of these cases the combined use of the investigated polarimetric parameters together with intensity improves the discrimination performance between icebergs and sea ice

ice pumps and their rates an ice pump is a heat engine driven by the change of freezing point with pressure which will melt ice at depth in the ocean and deposit it at a shallower location it is self starting calculations of the maximum magnitude of this effect are made which show good agreement with field data available for sea and lake ice the discussion is applied to the general case of a moving pack ice sheet with a well mixed surface layer and to floating ice shelves the rate of melt from an 11 m deep pressure ridge keel due to ice pumping is estimated as 26 cm year and that from the front of the ross ice shelf at mcmurdo sound antarctica is estimated as 5 m year for the level of water movement noted in the authors field observations far from the ice front pumping between shelf areas of different thickness will still occur with tidal motion providing the necessary water exchange but its magnitude is now limited by the ability to remove the potentially stable layer of melt water out of the system it is important to realize that the pumping does not depend on the availability of sensible heat in the water column and its effects are additional to any melting caused by the advection of warmer water to the ice water interface

study of the impact of ice formation in leads upon the sea ice pack mass balance using a new frazil and grease ice parameterization abstractleads are cracks in sea ice that often form because of deformation during winter months leads expose the ocean to the cold atmosphere resulting in supercooling and the formation of frazil ice crystals within the mixed layer here the authors investigate the role of frazil ice formation in leads on the mass balance of the sea ice pack through the incorporation of a new module into the los alamos sea ice model cice the frazil ice module considers an initial cooling of leads followed by a steady state formation of uniformly distributed single size frazil ice crystals that precipitate to the ocean surface as grease ice the grease ice is pushed against one of the lead edges by wind and water drag that the authors represent through a variable collection thickness for new sea ice simulations of the sea ice cover in the arctic and antarctic are performed and compared to a model that treats leads the same as the open ocean the processes of ice formation in the new module slow down the refreezing o

early plate tectonics versus single plate tectonics on mars evidence from magnetic field history and crust evolution 1 the consequences of an early epoch of plate tectonics on mars followed by single plate tectonics with stagnant lid mantle convection on both crust production and magnetic field generation have been studied with parameterized mantle convection models thermal history models with parameterized mantle convection not being dynamo models can provide necessary but not sufficient conditions for dynamo action it is difficult to find early plate tectonics models that can reasonably explain crust formation as is required by geological and geophysical observations and allow an early magnetic field that is widely accepted as the cause for the observed magnetic anomalies dating of crust provinces and topography and gravity data suggest a crust production rate monotonically declining through the noachian and hesperian and a present day crust thickness of more than 50 km plate tectonics cools the mantle and core efficiently and the core may easily generate an early magnetic field given a sufficiently weak mantle rheology plate tectonics can explain a field even if the core is not initially superheated with respect to the mantle because the crust production rate is proportional to temperature however an early efficient cooling will frustrate later crust production and therefore cannot explain for example the absence of prominent magnetic anomalies in the northern crustal province and the northern volcanic plains in the early hesperian voluminous crust formation following plate tectonics is possible if plate tectonics heat transfer is inefficient but then the crust growth rate has a late peak about 2 ga b p which is not observed these models also require a substantial initial superheating of the core to allow a dynamo if one accepts the initial superheating then as we will show a simple thermal evolution model with monotonic cooling of the planet due to stagnant lid mantle convection underneath a single plate throughout the evolution can better reconcile early crust formation and magnetic field generation

physical structural and isotopic characteristics and growth processes of fast sea ice in lutzow holm bay antarctica a sea ice ocean study was conducted off queen maud land and enderby land antarctica from 1990 to 1991 by the japanese antarctic research expedition observations of multiyear land fast sea ice were made in lutzow holm bay over a period of 2 years to determine the snow and ice characteristics and ice growth processes the snow depth in the bay reached large values of 1 0 to 1 5 m during the winter season at offshore locations from the analysis of ice thickness measurements it is confirmed that the fast ice with deep snow cover grew little in winter but substantially thickened during the summer months on the basis of ice core structure salinity and stable isotopic composition we conclude that the summer growth was caused by upward growth at the top of the ice to which snow ice and superimposed ice formation contribute these processes were the primary contributors to sea ice growth and characteristics only where the snow accumulation was large in areas of low snow accumulation there was no surface growth superimposed ice formation on sea ice in antarctica has not been reported previously evidence for snow cover melting which is a prerequisite for superimposed ice formation was also found

a model for planetesimal meltdown by 26al and its implications for meteorite parent bodies the melting of planetesimals heated by 26 al has been modelled using a new finite difference method that incorporates convection as an example we consider a planetesimal with a radius of 64 km which accretes instantaneously at t 0 75 myr after the formation of calcium aluminum rich inclusions from cold 250 k nebular dust with 50 porosity at t 0 9 myr t 700 k the planetesimal shrinks to a radius of 50 km due to sintering at t 1 2 myr t 1425 k the fully insulated interior deeper than a few kilometers starts to melt and at t 1 5 myr t 1725 k with 50 melting convection starts by t 2 myr the planetesimal is a globe of molten convecting slurry inside a thin residual crust from about t 2 5 myr the crust thickens rapidly as the power of 26 al fades planetesimals probably melt in this manner when they accrete before t 1 3 myr and are large enough to insulate themselves r 20 km for accretion at t 0 rising to 80 km at t 1 3 myr melting behavior will also be affected by the level of 60 fe in nebular dust by the extent of devolatilization reactions and basalt segregation during heating and by gradual accretion the model suggests that a the parent bodies of differentiated meteorites had accreted before about t 1 5 to 2 myr and before most chondritic parent bodies had formed and b that molten planetesimals may be a source for chondrule melt droplets

partitioning and budget of li be and b in high pressure metamorphic rocks abstract partitioning and budget of li be and b in high pressure metamorphic rocks from the island of syros greece were studied using secondary ion mass spectrometry inductively coupled plasma optical emission spectrometry and prompt gamma neutron activation analysis partitioning between coexisting mineral phases was found to be rather constant and independent of element concentrations for several mineral pairs apparent partition coefficients vary in a narrow range while concentrations vary by more than an order of magnitude hence it was possible to establish sets of inter mineral partition coefficients for li be and b among 15 different high pressure minerals this data set provides important information on the behaviour of the light elements in different lithologies within subducting slabs from the onset of metamorphism to the eclogite stage it is essential for modelling trace element and isotope fractionation during subduction and dehydration of oceanic crust

inner core anisotropy and heterogeneity level an analysis of core phase travel times is performed in order to estimate the relative contributions of heterogeneities and anisotropy and their distribution inside the inner core a stochastic analysis performed on the equatorial paths which are poorly sensitive to anisotropy indicates that the inner core exhibits a heterogeneity level smaller than 0 3 at all depths on scale lengths larger than 200 km this result extended to all path orientations suggests a predominant effect of the anisotropy on travel times anomalies a simultaneous inversion of absolute and differential core phase travel times for any path orientation favours a simple inner core model in which a large homogeneous structure with 3 uniform anisotropy is asymmetrically surrounded by a homogeneous isotropic layer with a thickness of 100 200 km on the western hemisphere increasing to 400 km over a large part of the eastern hemisphere

polynya dynamics a review of observations and modeling 1 polynyas are nonlinear shaped openings within the ice cover ranging in size from 10 to 105 km2 polynyas play an important climatic role first winter polynyas tend to warm the atmosphere thus affecting atmospheric mesoscale motions second ocean surface cooling and brine rejection during sea ice growth in polynyas lead to vertical mixing and convection contributing to the transformation of intermediate and deep waters in the global ocean and the maintenance of the oceanic overturning circulation since 1990 there has been an upsurge in polynya observations and theoretical models for polynya formation and their impact on the biogeochemistry of the polar seas this article reviews polynya research carried out in the last 2 decades focusing on presenting a state of the art picture of the physical interactions between polynyas and the atmosphere sea ice ocean system observational and modeling studies the surface heat budget and water mass transformation within these features are addressed

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

equatorial anisotropy in the inner part of earth s inner core from autocorrelation of earthquake coda the speed of seismic waves passing through the earth s inner core varies with direction analysis of earthquake seismic data suggests that this directional dependence differs between innermost and outer inner core

test of the innermost inner core models using broadband pkikp travel time residuals 1 two different innermost inner core imic models have recently been proposed each with different radii and different orientation and strength of anisotropy in order to test these imic models we have systematically assembled a high quality dataset of broadband pkikp data in the epicentral distance range of 150 to 180 our new dataset consists of 1 100 individually measured absolute pkikp arrival times also pkikp bottoming points and ray paths in the inner core are well distributed in the east west and north south hemispheres we invert this dataset for two layer models of anisotropy in the inner core and compare the fits obtained with those predicted from the existing imic models our results show that if there is an imic its radius is most likely around 500 km rather than the 300 km originally proposed and is in better agreement with the layering in the inner core based on pkikp waveform modeling

coupled core mantle thermal evolution of early mars 1 several arguments point out that at the end of planetary accretion the core of mars was likely to be much hotter than its mantle resulting in the formation of a completely or partially molten thermal boundary layer at the base of the mantle here we address the following questions how did the superheated core cool and what role did it play in the early mantle dynamics of mars we divide the coupled core mantle evolution of early mars into two stages during the first stage vigorous convection within the molten boundary layer removes the heat from the core so that the boundary layer expands up as the boundary layer gets thicker the temperature of the layer decreases eventually the temperature of the molten boundary layer drops down to the temperature for the rheological transition melt fraction 40 within 100 years this stage is described by a parameterized convection approach the second stage is modeled in spherical shell geometry using the fully three dimensional finite element code citcoms a single plume superplume forms by the instability of the thermal boundary layer the superplume stage lasts much longer on the scale of millions to hundreds of millions of years depending on the mantle viscosity during both stages of evolution the heat flux can easily satisfy the requirements for the dynamo

lawsonite composition and zoning as tracers of subduction processes a global review abstract lawsonite is an abundant hydrous mineral in oceanic crust sediments and metasomatic rocks at depths of 45 to 300 km in most subduction zones but it is rarely preserved in the geologic record because it commonly transforms to epidote and other minerals during prograde or retrograde metamorphism owing to the significance of lawsonite for water and element cycling in subduction zones occurrences of fresh lawsonite in blueschist and more rarely eclogite provide important opportunities to determine lawsonite composition zoning and inclusion suites and to use this information to reconstruct reaction history during subduction and exhumation in this review we use new and published data to document lawsonite composition in eight of the nine known lawsonite eclogite localities in which fresh lawsonite coexists with garnet omphacite in the rock matrix as well as the composition of lawsonite inclusions in six of seven known sites in which lawsonite occurs only as inclusions in garnet in eclogite facies rocks that lack matrix lawsonite as lawsonite blueschist is much more common than lawsonite eclogite we survey the composition of lawsonite in representative localities of blueschist including blueschist associated with eclogite lawsonite bearing epidote bearing and blueschist not associated with eclogite at current exposure levels included in this review are metabasaltic rocks silica and carbonate rich metasedimentary rocks metasomatic rocks and lawsonite rich veins this dataset demonstrates that lawsonite composition is a sensitive indicator of reaction history during subduction and exhumation and specifically of fluid rock interaction with implications for element cycling in subduction zones furthermore most exhumed lawsonite eclogite records slab surface conditions that correspond to the location where the slab mantle interface transitions from decoupled to coupled and therefore provides key insights into the thermal history and dynamics of subduction zones

seasonal variations in the properties and structural composition of sea ice and snow cover in the bellingshausen and amundsen seas antarctica sixty three ice cores were collected in the bellingshausen and amundsen seas in august and september 1993 during a cruise of the r v nathaniel b palmer the structure and stable isotopic composition 18 o 16 o of the cores were investigated in order to understand the growth conditions and to identify the key growth processes particularly the contribution of snow to sea ice formation the structure and isotopic composition of a set of 12 cores that was collected for the same purpose in the bellingshausen sea in march 1992 are reassessed frazil ice and congelation ice contribute 44 and 26 respectively to the composition of both the winter and summer ice core sets evidence that the relatively calm conditions that favour congelation ice formation are neither as common nor as prolonged as the more turbulent conditions that favour frazil ice growth and pancake ice formation both frazil and congelation ice layers have an av erage thickness of 0 12 m in winter evidence that congelation ice and pancake ice thicken primarily by dynamic processes the thermodynamic development of the ice cover relies heavily on the formation of snow ice at the surface of floes after sea water has flooded the snow cover snow ice layers have a mean thickness of 0 20 and 0 28 m in the winter and summer cores respectively and the contribution of snow ice to the winter 24 and summer 16 core sets exceeds most quantities that have been reported previously in other antarctic pack ice zones the thickness and quantity of snow ice may be due to a combination of high snow accumulation rates and snow loads environmental conditions that favour a warm ice cover in which brine convection between the bottom and top of the ice introduces sea water to the snow ice interface and bottom melting losses being compensated by snow ice formation layers of superimposed ice at the top of each of the summer cores make up 4 6 of the ice that was examined and they increase by a factor of 3 the quantity of snow entrained in the ice the accumulation of superimposed ice is evidence that melting in the snow cover on antarctic sea ice floes ran reach an advanced stage and contribute a significant amount of snow to the total ice mass

geodynamic estimates of the viscosity of the earth s inner core recent seismological studies1 2 have suggested that the inner core is rotating relative to the bulk of the earth a situation which according to numerical simulations3 may be sustained by convective flow in the liquid outer core on the other hand large gravitational forces due to the heterogeneous distribution of mass in the earth s mantle should be sufficient to keep the inner core aligned with the mantle4 here i show that the differential rotation of the inner core can be reconciled with these strong gravitational forces by allowing the shape of the inner core to adjust as it rotates so permitting an estimate of the effective viscosity of this innermost region of the earth the inferred rotation rate constrains the viscosity of the inner core to be less than 1016 pa s or greater than 1020 pa s as two different dynamical regimes are possible the viscosity estimates for these two regimes have very different implications for the origin of seismic anisotropy in the inner core

seasonality of southern ocean sea ice the sea ice cover of the southern ocean undergoes a rapid decrease from mid november to mid january the atmosphere to ocean heat flux is insufficient to account for this melting even in the presence of the high percentage of open water characteristic of the southern ocean sea ice it is estimated that sea air heat exchange in the 60 to 70 s zone can account for roughly 50 of the required spring heating the remainder must be supplied by the relatively warm deep water residing below the southern ocean pycnocline deep to surface water heat flux is accomplished by upwelling of the pycnocline due to the regional ekman divergence of the surface layer and by cross pycnocline mixing it is estimated that the required vertical mixing coefficient for the pycnocline is 1 5 cm2 s this value is considered as realistic in view of the relatively weak pycnocline which is a consequence of low levels of fresh water input to the surface layer it is suggested that the magnitude of fresh water input is a major factor in determining the degree of seasonality of southern ocean sea ice

waveform search for the innermost inner core abstract waveforms of the pkikp seismic phase are inconsistent with the effects of a sharp boundary at 300 km radius of an inner most inner core seismograms synthesized for the distance range 150 to 180 in models having a discontinuity at 300 km radius in the inner core exhibit focused diffractions around the innermost sphere at antipodal range that are inconsistent with observed pkikp waveforms a few examples however can be found of pkikp complexity near 165 consistent with a triplication created by a 475 km radius discontinuity a rapid change in the magnitude and variance of seismic attenuation of pkikp whose rays turn between 400 and 600 km radius is also observed consistent with an innermost sphere of low regionally uniform seismic attenuation models predicting waveforms in agreement with observed waveforms have either a transition in elastic and anelastic properties spread over a depth interval greater than 100 km or an innermost sphere of differing elastic properties that exceeds 450 km radius the weak but suggestive evidence for an innermost inner core may simply represent a transition in fabric spread over 100 km or more centered near 500 km radius this change may signify the end stage of solidification flow and re crystallization resulting in the highest ordering and largest grain sizes of intrinsically anisotropic crystals

fluid dynamics of local martian magma oceans the evolution of a melt region produced by a large impact during mars formation is addressed while some impact induced melt is redistributed during crater excavation sufficiently large impacts much larger than basin forming impacts generate an intact melt region which is retained beneath the excavation zone i e a local magma ocean local magma ocean evolution depends on the effective rheology controlling large scale deformation of the solid part of the planet mechanism of crystallization and melt region size within the uncertainties of various parameters two scenarios are possible for sufficiently weak rheology or large melt region size evolution is characterized by rapid extrusion and formation of a global magma ocean for sufficiently strong rheology or small melt region size in situ crystallization to a partially molten solid state occurs prior to isostatic adjustment subsequent to in situ crystallization local magma ocean evolution depends on melt region size and efficiency of lateral redistribution compared to bulk conductive cooling for large melt regions lateral spreading occurs via plastic deformation and results in an asymmetric global partial melt layer for small melt region size viscous spreading viscous can result in bulk cooling below the solidus prior to formation of a global layer a hypothesis for the origin of the hemispherical crustal dichotomy and tharsis rise is suggested the dichotomy is associated with a global partial melt layer produced by evolution of a large local magma ocean after dichotomy formation evolution of a second smaller local magma ocean is related to tharsis development

turbulent metal silicate mixing fragmentation and equilibration in magma oceans abstract much of the earth was built by high energy impacts of planetesimals and embryos many of these impactors already differentiated with metallic cores of their own geochemical data provide critical information on the timing of accretion and the prevailing physical conditions but their interpretation depends critically on the degree of metal silicate chemical equilibration during core mantle differentiation which is poorly constrained efficient equilibration requires that the large volumes of iron derived from impactor cores mix with molten silicates down to scales small enough to allow fast metal silicate mass transfer here we use fluid dynamics experiments to show that large metal blobs falling in a magma ocean mix with the molten silicate through turbulent entrainment with fragmentation into droplets eventually resulting from the entrainment process in our experiments fragmentation of the dense fluid occurs after falling a distance equal to 3 4 times its initial diameter at which point a sizable volume of ambient fluid has already been entrained and mixed with the dense falling fluid contrary to previous assumptions we demonstrate that fragmentation of the metallic phase into droplets may not be required for efficient equilibration turbulent mixing by drastically increasing the metal silicate interfacial area may result in fast equilibration even before fragmentation efficient re equilibration is predicted for impactors of size small compared to the magma ocean depth in contrast much less re equilibration is predicted for large impacts in situations where the impactor core diameter approaches the magma ocean thickness

petrology of hp metamorphic veins in coesite bearing eclogite from western tianshan china fluid processes and elemental mobility during exhumation in a cold subduction zone abstract a petrological study was carried out for high pressure hp veins which cut through the host coesite bearing eclogites at habutengsu kebuerte in western tianshan china the results place constraints on the origin and property of metamorphic fluids during subduction zone metamorphism omphacite clinozoisite and quartz dominated veins occur on centimeter to meter scales within lens shaped and layered eclogites or cutting into the country rocks of garnet phengite schists coesite bearing eclogites mainly consist of fibrous fine grained omphacite and porphyroblastic garnet with minor amounts of amphibole mainly barroisite clinozoisite white mica mainly paragonite and rutile the veins are pronouncedly coarse grained compared to the host eclogites and commonly consist of quartz clinozosite rutile white mica phengite and paragonite and garnet with or without omphacite titanite apatite carbonate mainly dolomite and glaucophane fluid inclusions are abundant in vein omphacite titanite and apatite but are rare in the equivalent minerals of host eclogites rounded vein garnets usually occur close to the sharp interface of vein and host eclogite omphacite in the veins is characterized by its euhedral form surrounded by quartz or coarse bladed aggregates in contrast to the fibrous or patchy one suggesting dynamic recrystallization in the host rocks omphacite in both veins and host eclogites has similar jadeite contents jd 40 50 indicating formation at eclogite facies metamorphic conditions vein phengite uniformly contains certain amounts of ba with maximum bao content of 3 16 4 25 wt suggesting that ba was mobilized during the exhumation of uhp rocks specific textures of vein minerals such as the enclosure of magnesite or calcite in dolomite rutile in titanite and the occurrence of zoned ba rich phengite indicate the chemical variability of channelized fluids over time based on zr content in rutile and the presence of paragonite omphacite phengite glaucophane as well as the little deformed textures of hp veins it is estimated that the vein forming fluids would flow at about 1 3 2 1 gpa and 540 580 c corresponding to the stage of retrograde eclogite facies recrystallization during exhumation of the uhp eclogites that formed at peak p t conditions of 2 7 gpa and 460 520 c the hp veins occur as a consequence of a regional tectonothermal event triggering breakdown of lawsonite within the uhp eclogites based on the petrology of vein minerals it is inferred that the hp fluids were enriched in si ca na al and ba this suggests that these elements could be mobilized during the retrograde metamorphism of uhp eclogites in a cold subduction zone coeval pervasive flow of hp metamorphic fluids through the uhp eclogites at this stage may be an important process to eliminate most mineralogical evidence of the uhp metamorphism

development and properties of sea ice in the coastal regime of the southeastern weddell sea from october to december 1986 a program consisting of sea ice core analysis in combination with sea ice observations was carried out from the icebreaker r v polarstern as part of the winter weddell sea project the ship operated in the central and southeastern weddell sea with interests focusing on the ice shelf front between 70 s and 77 s where a system of coastal polynyas extended for several tens to hundreds of kilometers opened and maintained despite temperatures well below freezing by offshore wind forcing three wind controlled ice formation regimes may be distinguished 1 for wind speeds 10 m s the ice cover is heavily ridged and rafted 50 and formation of granular ice predominates of the total length of cores sampled in the coastal regime 32 consisted of columnar and 35 of granular ice in addition we observed two unique ice classes along the coast platelet ice formed through consolidation of underwater ice platelets underneath the solid ice cover contributed 11 to our core samples another 22 consisted of intermediate columnar granular ice which originates under the dynamic hydrographic conditions of the near ice shelf region

a multi phase model of runaway core mantle segregation in planetary embryos abstract a classic scenario of core formation suggests that growing proto planets are heated by the impacts of accreting planetesimals at their surface until their shallow layers reach the melting temperature of their metallic components or even of the silicates in this partially molten shell metal and silicates differentiate and the metallic phase ponds on top of the still undifferentiated inner planet later a gravitational instability brings dense metallic diapirs to the center of the planet we test this multi phase scenario by using a formalism that self consistently accounts for the presence of solid silicates solid and liquid iron at each point of the mixture an average velocity and a separation velocity of the solid and liquid phases are defined the energy balance accounts from the changes in potential energy associated with the segregation we show that core formation starts before a significant melting of the silicates as soon as impact heating is large enough to reach the melting temperature of the metallic component segregation proceeds in a few thousand years by a runaway process due to the conversion of gravitational energy into heat that occurs necessarily in all undifferentiated embryos of moon to mars sizes the first metallic diapirs leave behind them a trailing conduit along which most of the further melting occurs the cores of large planets do not form at the end of accretion but must result from the merging of the already differentiated hot cores of embryos

earth and mars early thermal profiles abstract the extent of formation heating for the earth and mars has been evaluated assuming that the terrestrial planets accumulated from planetesimals the main result is that even if a long accumulation time is assumed 100 ma it is possible to obtain a planetary structure with a large melted shell taking into account the role played by massive projectiles which upon reaching depths of several kilometres are able to deposit heat significantly below the planetary surface internal temperatures sufficient for the downward migration of the liquid iron alloy have been obtained

depth dependence of anisotropy of earth s inner core both body wave pkp travel times creager 1992 song and helmberger 1993a mcsweeney and creager 1993 shearer 1994 and fits to the splitting of core modes tromp 1993 show general agreement that the top 300 km of inner core is very anisotropic the anisotropy displays axial symmetry around the earth s spin axis with the polar direction 3 faster than the equatorial direction one key problem now is the depth dependence of the inner core anisotropy here we extend our polar path studies to include both long period and short period modeling for the pkp phases at ranges 120 to 173 arrivals from the top of the inner core pkikp and reflections from the inner core boundary pkikp can be observed distinctly in short period records at ranges 130 to 140 and as waveform distortions in the long period records at ranges 130 to 146 these waveforms provide a new set of data for examining the topmost 150 km of the inner core which is not well sampled by the previous body wave travel times record sections of waves traversing the inner core nearly parallel to the earth s spin axis polar paths from three events two beneath the south sandwich islands and one along the macquarie ridge recorded at world wide standardized seismograph network canadian network and long range seismic measurements stations are analyzed our results suggest that the top 150 km of the inner core is only weakly anisotropic less than 1 with strong evidence indicating that the top 60 km is not anisotropic at all

consequences of giant impacts in early mars core merging and martian dynamo evolution a giant impact is an increasingly popular explanation for the formation of the northern lowland on mars it is plausible that at the impact time both mars and the impactor were differentiated with solid silicate mantles and liquid iron cores such a large impact likely resulted in merging of the cores of both bodies a process which will have implications on the thermal state of the planet we model the evolution of the martian mantle following a giant impact and characterize the thermochemical consequences of the sinking of an impactor s core as a single diapir the impact heating and the viscous heating induced during the core merging may affect the early thermal state of mars during several tens of million years our results show that large viscosity contrasts between the impactor s core and the surrounding mantle silicates can reduce the duration of the merging down to 1 kyr but do not modify the merging temperature when the viscosity contrast between the diapir and the surrounding silicates is larger than a factor of 1000 the descent of the diapir can lead to some entrainment of the relatively shallow silicates to deepest regions close to the core mantle boundary finally the direct impact heating of martian core leads to thermal stratification of the core and kills the core dynamo it takes on the order of 150 200 myr to reinitiate a strong dynamo anew the merging of the impactor s core with the martian core only delays the reinitiation of the dynamo for a very short time

cs rb ba systematics in phengite and amphibole an assessment of fluid mobility at 2 0 gpa in eclogites from trescolmen central alps eclogites from trescolmen that contain abundant hydrous minerals phengite amphibole paragonite zoisite talc apatite show petrographic evidence for fluid infiltration under conditions of 2 0 to 1 8 gpa 650 c large ion lithophile elements lile e g cs rb ba and sr were analysed by in situ techniques in all eclogite mineral phases in order to characterize the behaviour of fluid mobile elements at high pressure in situ analysis of carefully chosen metamorphic assemblages circumvents the problem of partial late stage alteration which can severely influence the calculated element budgets of whole rock samples phengite is the dominant host for cs rb and ba in both eclogite and adjacent garnet mica schist samples and incorporates 90 of the budgets of these elements in whole rocks lile contents of phengites in phengite rich rocks are likely to record the cs rb and ba rb ratios of their host rock protoliths the lile patterns of eclogite are consistent with protoliths derived from basalt that underwent seafloor alteration whereas those of mica schist are almost identical to average upper continental crust in contrast lile patterns of eclogite samples that lack phengite but do contain amphibole are unlike any plausible protolith but are identical to those of amphibole in phengite bearing samples this observation points to homogenization of the lile in different lithologies which we correlate with petrographic evidence for fluid infiltration because phengite in garnet mica schist has a strong capacity to buffer the fluid with respect to cs rb and ba homogenization of amphiboles is best explained by fluid infiltration from the surrounding metapelites into eclogite bodies implying at least metre scale fluid mobility the amphibole homogenization can be most easily modelled by a pervasive open system fluid flux through the eclogites possibly facilitated by ductile deformation during the early stages of uplift simple calculations give minimum fluid rock ratios of 0 001 to 0 004 demonstration of the mobility of very small volumes of fluid through eclogite is an important prerequisite of many subduction zone models that try to explain across arc variations in trace element geochemistry the low fluid rock ratios from this study are not in contrast with oxygen isotope heterogeneities reported from other eclogite localities fluid mobile elements such as cs rb and ba are more sensitive indicators of small volume fluid rock interaction and are therefore potentially valuable for understanding fluid infiltration processes in systems where oxygen isotope shifts are not large enough to be detectable

forming a moon with an earth like composition via a giant impact in the giant impact theory the moon formed from debris ejected into an earth orbiting disk by the collision of a large planet with the early earth prior impact simulations predict that much of the disk material originates from the colliding planet however earth and the moon have essentially identical oxygen isotope compositions this has been a challenge for the impact theory because the impactor s composition would have likely differed from that of earth we simulated impacts involving larger impactors than previously considered we show that these can produce a disk with the same composition as the planet s mantle consistent with earth moon compositional similarities such impacts require subsequent removal of angular momentum from the earth moon system through a resonance with the sun as recently proposed

lower mantle dynamics with the post perovskite phase change radiative thermal conductivity temperature and depth dependent viscosity the new post perovskite phase near the core mantle boundary has important ramifications on lower mantle dynamics we have investigated the dynamical impact arising from the interaction of temperature and depth dependent viscosity with radiative thermal conductivity up to a lateral viscosity contrast of 10 4 on both the ascending and descending flows in the presence of both the endothermic phase change at 670 km depth and an exothermic post perovskite transition at 2650 km depth the phase boundaries are approximated as localized zones we have employed a two dimensional cartesian model using a box with an aspect ratio of 10 within the framework of the extended boussinesq approximation our results for temperature and depth dependent viscosity corroborate the previous results for depth dependent viscosity in that a sufficiently strong radiative thermal conductivity plays an important role for sustaining superplumes in the lower mantle once the post perovskite phase change is brought into play this aspect is especially emphasized when the radiative thermal conductivity is restricted only to the post perovskite phase these results revealed a greater degree of asymmetry is produced in the vertical flow structures of the mantle by the phase transitions mass and heat transfer between the upper and lower mantle will deviate substantially from the traditional whole mantle convection model streamlines revealed that an overall complete communication between the top and lower mantle is difficult to be achieved 2005 elsevier b v all rights reserved

towards an estimation of sub sea ice platelet layer volume with multi frequency electromagnetic induction sounding ice platelet clusters modify the heat and mass balance of sea ice near antarctic ice shelves and provide a unique habitat for ice associated organisms the amount and distribution of these ice crystals below the solid sea ice provide insight into melt rates and circulation regimes in the ice shelf cavities which are difficult to observe directly however little is known about the circum antarctic volume of the sub sea ice platelet layer because observations have mostly been limited to point measurements in this study we present a new application of multi frequency electromagnetic em induction sounding to quantify platelet layer properties combining in situ data with the theoretical response yields a bulk platelet layer conductivity of 1154 271msm 1 and ice volume fractions of 0 29 0 43 calibration routines and uncertainties are discussed in detail to facilitate future studies our results suggest that multi frequency em induction sounding is a promising method to efficiently map platelet layer volume on a larger scale than has previously been feasible

differentiated planetesimal impacts into a terrestrial magma ocean fate of the iron core abstract the abundance of moderately siderophile elements iron loving e g co ni in the earth s mantle is 10 to 100 times larger than predicted by chemical equilibrium between silicate melt and iron at low pressure but it does match expectation for equilibrium at high pressure and temperature recent studies of differentiated planetesimal impacts assume that planetesimal cores survive the impact intact as concentrated masses that passively settle from a zero initial velocity and undergo turbulent entrainment in a global magma ocean under these conditions cores greater than 10 km in diameter do not fully mix without a sufficiently deep magma ocean we have performed hydrocode simulations that revise this assumption and yield a clearer picture of the impact process for differentiated planetesimals possessing iron cores with radius 100 km that impact into magma oceans the impact process strips away the silicate mantle of the planetesimal and then stretches the iron core dispersing the liquid iron into a much larger volume of the underlying liquid silicate mantle lagrangian tracer particles track the initially intact iron core as the impact stretches and disperses the core the final displacement distance of initially closest tracer pairs gives a metric of core stretching the statistics of stretching imply mixing that separates the iron core into sheets ligaments and smaller fragments on a scale of 10 km or less the impact dispersed core fragments undergo further mixing through turbulent entrainment as the molten iron fragments rain through the magma ocean and settle deeper into the planet our results thus support the idea that iron in the cores of even large differentiated planetesimals can chemically equilibrate deep in a terrestrial magma ocean

the innermost inner core of the earth evidence for a change in anisotropic behavior at the radius of about 300 km since the discovery of the inner core in 1936 no additional spherical subshell of the earth has been observed based on an extensive seismic data set we propose the existence of an innermost inner core with a radius of 300 km that exhibits a distinct transverse isotropy relative to the bulk inner core specifically within the innermost inner core the slowest direction of wave propagation is 45 from the east west direction in contrast the direction of the slowest wave propagation in the overlying inner core is east west the distinct anisotropy at the center of the earth may represent fossil evidence of a unique early history of inner core evolution

investigations on physical and textural properties of arctic first year sea ice in the amundsen gulf canada november 2007 june 2008 ipy cfl system study we report sea ice temperature and bulk salinity measurements as well as textural analysis from 33 first year drift and fast ice stations sampled between november 2007 and june 2008 in the southern beaufort sea amundsen gulf canadian arctic during the international polar year circumpolar flaw lead ipy cfl system study we use this significant dataset to investigate the halo thermodynamic evolution of sea ice from growth to melt a strong desalination phase is observed over a small time window in the spring using calculated proxies of sea ice permeability brine volume fraction and of the intensity of brine convection rayleigh number we demonstrate that this phase corresponds to full depth gravity drainage initiated by a restored connectivity of the brine network with warming in the spring most stations had a textural sequence typical of arctic first year ice with granular ice overlying columnar ice unusual textural features were observed sporadically sandwiched granular ice platelet ice and draped platelet ice we suggest that turbulence in leads and double diffusion in strong brine plumes following the refreeze of cracks are plausible mechanisms for the formation of these textures

earth s core formation aided by flow channelling instabilities induced by iron diapirs abstract the core formation mechanism remains poorly known an unstable gravitational configuration of a dense molten metallic layer overlying a cold chondritic protocore is predicted by most studies which leads to the formation of a rayleigh taylor rt instability recent results dahl t w 2005 turbulent mixing during planet accretion and core formation interpretation of the hf w chronometer and implications for the age of the moon m sc thesis university of copenhagen indicate that additionally iron cores of predifferentiated planetesimals are also able to plunge mostly intact into the cold protocore and create large iron diapirs for both scenarios we propose the application of the stress induced melt channelling mechanism stevenson d j 1989 spontaneous small scale melt segregation in partial melts undergoing deformation geophys res lett 16 1 067 1 070 in the region surrounding an incipient iron diapir we therefore perform numerical experiments solving the two phase two composition flow equations within a 2d rectangular box with symmetrical boundary conditions we apply the compaction boussinesq approximation cba and include a depth dependent gravity for simplicity we use a constant viscosity for the solid phase and a melt fraction dependent rheology for the partially molten region around the diapir we investigate the physical conditions under which the melt channels can form and whether they are applicable to the early earth as a result for sufficiently small melt retention numbers iron rich melt channels develop within a region of approximately twice the diapir s size this could lead to effective draining of the surrounding region and might initiate cascading daughter diapirs the region of the protocore drained by this cascading mechanism is expected to significantly increase with depth and thus indicates an effective mechanism to also extract iron melt from deeper parts of the initially chondritic protocore this mechanism could effectively accelerate the process of core formation

u th pa ra study of the kamchatka arc new constraints on the genesis of arc lavas abstract the 238u 230th 226ra and 235u 231pa disequilibria have been measured by mass spectrometry in historic lavas from the kamchatka arc the samples come from three closely located volcanoes in the central kamchatka depression ckd the most active region of subducted related volcanism in the world the large excesses of 226ra over 230th found in the ckd lavas are believed to be linked to slab dehydration moreover the samples show the uncommon feature of 230th 238u activity ratios both lower and higher than 1 the u series disequilibria are characterized by binary trends between activity ratios with 231pa 235u ratios all 1 it is shown that these correlations cannot be explained by a simple process involving a combination of slab dehydration and melting we suggest that they are more likely to reflect mixing between two end members a high magnesia basalt hmb end member with a clear slab fluid signature and a high alumina andesite haa end member reflecting the contribution of a slab derived melt the u th ra characteristics of the hmb end member can be explained either by a two step fluid addition with a time lag of 150 ka between each event or by continuous dehydration the inferred composition for the dehydrating slab is a phengite bearing eclogite equilibrium transport or dynamic melting can both account for 231pa excess over 235u in hmb end member nevertheless dynamic melting is preferred as equilibrium transport melting requires unrealistically high upwelling velocities to preserve fluid derived 226ra 230th a continuous flux melting model is also tested in this model 231pa 235u is quickly dominated by fluid addition and for realistic extents of melting this process cannot account for 231pa 235u ratios as high as 1 6 as observed in the hmb end member the involvement of a melt derived from the subducted oceanic crust is more likely for explaining the haa end member compositions than crustal assimilation melting of the oceanic crust is believed to occur in presence of residual phengite and rutile resulting in no 226ra 230th disequilibrium and low 231pa excess over 235u in the high alumina andesites consequently it appears that high alumina andesites and high magnesia basalts have distinct origins the former being derived from melting of the subducted oceanic crust and the latter from hydrated mantle it seems that there is no genetic link between these two magma types in contrast with what was previously believed

the ice thickness distribution across the atlantic sector of the antarctic ocean in midwinter during the first leg of the winter weddell sea project antarktis v 2 cruise of f s polarstern the entire width of the antarctic sea ice zone was traversed in the vicinity of 0 longitude in the period july 18 to september 10 1986 ice thicknesses were measured by direct drilling and by helicopter profiling using an exstar 100 mhz impulse radar system in addition aerial photography of the ice cover was done from 100 to 2000 m altitude using a 70 mm aerial camera mounted in the helicopter the results of the point measurements drilling are reported in this paper together with an indication of how the radar and photography data will be used to extend them so as to yield area averaged ice thickness distributions it was found that the main ice type across the entire width of the ice cover was consolidated pancake ice occurring in vast floes this formed out of a 250 km wide band at the advancing ice edge which comprised a concentrated field of individual pancakes in a matrix of frazil ice preferred thicknesses of undeformed floes were 40 60 cm of ice covered with 5 15 cm of snow the individual pancakes attained almost all of this thickness before consolidation subsequent congelation growth was slow estimated at 0 4 cm d 1 the floes contained much small scale roughness on the upper and lower surfaces due to rafting of pancakes at the time of consolidation but pressure ridging was modest except in the far south a few very thick 8 11 m multiyear floes were observed embedded in the pack at latitudes beyond 66 s

crystal structure stable isotopes δ18o and development of sea ice in the ross amundsen and bellingshausen seas antarctica the crystal structure and oxygen isotopic composition of ice cores obtained from floes at the end of summer in the eastern ross sea the amundsen sea and the western bellingshausen sea were investigated to determine the ice growth processes and conditions that contribute to sea ice development in the eastern pacific sector of the southern ocean the isotope data indicate that a moderate amount of snow contributes to the development of the sea ice however even the combined use of isotopes and crystal structure analysis does not unambiguously explain the means by which all of the snow is entrained in the ice nevertheless it seems clear that much of the snow is contained in granular snow ice that results from seawater flooding of floes and the base of the snow cover the snow cover in the ross amundsen region was as much as 2 m deep and supported by 7 to 8 m thick floes primarily composed of frazil ice in the bellingshausen region the snow cover and the floes were thinner than in the ross amundsen region the bellingshausen cores were composed primarily of multiple layers of frazil and congelation ice in addition in both regions there were numerous tipped or inclined blocks of congelation ice and layers of rafted nilas in the cores the data indicate that the sea ice develops by multiple mechanisms in a turbulent environment

development of sea ice in the weddell sea we report on the development and physical properties of sea ice in the central and eastern weddell sea the investigations were part of the winter weddell sea project 1986 which extended over the months of july through december major elements of the glaciological part of this study included continuous shipborne observations of sea ice conditions and occasional helicopter reconnaissance flights extensive measurements of snow and ice thicknesses at daily ice stations and detailed analyses of sampled ice cores from each ice station textural investigations of the sampled ice revealed the dominance of frazil ice in the central weddell sea and the occurrence of an additional ice class called platelet ice together with the commonly known frazil and congelation ice in the coastal region of the eastern weddell sea these results in combination with the visual ice observations reveal two major mechanisms for sea ice generation in the antarctic which were not sufficiently well accounted for in previous investigations in the central weddell sea a cycle of pancake ice formation and its growth into consolidated floes seems to be the dominant process of the advancing sea ice edge in the coastal waters the growing sea ice cover consists to a considerable degree of ice platelets which are formed in the underlying water column in front of the ice shelf edges thus congelation ice growth which is mainly controlled by atmospheric thermodynamic forcing seems to be of less importance in the central and south eastern weddell sea than for example in the arctic basin

physical and biological controls on dms p dynamics in ice shelf influenced fast ice during a winter spring and a spring summer transitions we report the seasonal and vertical variations of dimethylsulfide dms and its precursor dimethylsulfoniopropionate dmsp in fast ice at cape evans mcmurdo sound antarctica during the spring summer transition in 2011 and winter spring transition in 2012 we compare the variations of dms p observed to the seasonal evolution of the ice algal biomass and of the physical properties of the ice cover with emphasis on the ice texture and brine dynamics isolated dms and dmsp maxima were found during both seasonal episodes in interior ice and corresponded to the occurrence of platelet crystals in the ice texture we show that platelet crystals formation corresponded in time and depth to the incorporation of dinoflagellates strong dmsp producers in the ice cover we also show that platelet crystals could modify the environmental stresses on algal cells and perturb the vertical redistribution of dms p concentrations we show that during the winter spring transition in 2012 the dms p profiles were strongly influenced by the development and decline of a diatom dominated bloom in the bottom ice with dmsp variations remarkably following chl a variations during the spring summer transition in 2011 the increase in brine volume fraction influencing ice permeability on warming was shown to trigger 1 an important release of dms to the under ice water through brine convection and 2 a vertical redistribution of dmsp across the ice

thermodynamics of the system fe feo mgo at high pressure and temperature and a model for formation of the earth s core summary recent shock wave data of jeanloz ahrens have established the presence of a phase transformation in fe0 94 o occurring at approximately 70 gpa which has profound implications for the phase behaviour of the geophysically important system feo mgo at high pressure we calculate the phase diagram based on the available shock wave data for feo and mgo and suggest that an increase of pressure in the system feo mgo will result in a gradual exsolution of an almost pure feo high pressure phase hpp leaving an iron depleted fe mg o rocksalt bl phase using current geophysical data exsolution of feo hpp from the rocksalt phase in the present day lower mantle is calculated to occur only in the region immediately overlying the core before core segregation took place however the increased iron content of the rocksalt phase could place the exsolution region at lesser depths perhaps 2000 km extrapolation of the solubility data of oxygen in liquid iron indicates that the liquid miscibility gap in the system fe feo at atmospheric pressure should close at approximately 4250k we construct schematic phase diagrams for the system fe feo at high pressure by considering the effect of pressure upon end member melting points upon the extent of liquid immiscibility and upon eutectic temperatures and compositions we suggest that at high pressure p 90 gpa the phase diagram of fe feo becomes qualitatively similar to the system fe fes displaying complete liquid miscibility and a eutectic temperature well below the melting point of pure iron on the basis of our inferences regarding behaviour in the system fe feo mgo at high pressure we have constructed a model for core segregation assuming the earth to have accreted from the primordial solar nebula as a relatively homogeneous mixture of metallic iron and silicate phases core segregation involving oxygen as the principal light alloying element in the core would commence at a depth where pressure is sufficiently high to cause exsolution of feo hpp from the rocksalt phase and temperature is sufficiently high to allow formation of an fe feo hpp melt without extensively melting the silicate phases a gravitational instability arises leading to vertical differentiation of the earth as molten blobs of the metal phase sink downwards to form the core and the residual depleted silicate material coalesces to form large bodies which rise diapirically upwards to form the mantle

understanding oblique impacts from experiments observations and modeling natural impacts in which the projectile strikes the target vertically are virtually nonexistent nevertheless our inherent drive to simplify nature often causes us to suppose most impacts are nearly vertical recent theoretical observational and experimental work is improving this situation but even with the current wealth of studies on impact cratering the effect of impact angle on the final crater is not well understood although craters rims may appear circular down to low impact angles the distribution of ejecta around the crater is more sensitive to the angle of impact and currently serves as the best guide to obliquity of impacts experimental studies established that crater dimensions depend only on the vertical component of the impact velocity the shock wave generated by the impact weakens with decreasing impact angle as a result melting and vaporization depend on impact angle however these processes do not seem to depend on the vertical component of the velocity alone finally obliquity influences the fate of the projectile in particular the amount and velocity of ricochet are a strong function of impact angle

the age of the inner core abstract the energy conservation law when applied to the earth s core and integrated between the onset of the crystallization of the inner core and the present time gives an equation for the age of the inner core in this equation all the terms can be expressed theoretically and given values and uncertainties of all relevant physical parameters the age of the inner core can be obtained as a function of the heat flux at the core mantle boundary and the concentrations in radioactive elements it is found that in absence of radioactive elements in the core the age of the inner core cannot exceed 2 5 ga and is most likely around 1 ga in addition to have an inner core as old as the earth concentrations in radioactive elements needed in the core are too high to be acceptable on geochemical grounds

sea ice structure and biological activity in the antarctic marginal ice zone ice cores obtained during october november 1981 from weddell sea pack ice were analyzed for physical chemical and biological parameters frazil ice which is associated with dynamic turbulent conditions in the water column predominated 70 both floe thickness and salinity indicate ice which is less than 1 year old chemical analyses particularly with regard to the nutrients revealed a complex picture phosphate values are scattered relative to the dilution curve nitrate and silicate values are lower than expected from simple scaling with salinity and suggest diatom growth within the ice nitrite values are higher in the ice than in adjacent waters frazil ice formation which probably concentrates algal cells from the water column into ice floes results in higher initial chlorophyll a concentrations in the ice than in adjacent waters this mechanical concentration is further enhanced by subsequent reproduction within the ice ice core chlorophyll ranged from 0 09 to 3 8 mg m3 comparable to values previously reported for this area but significantly lower than values for antarctic coastal fast ice the dominance of frazil ice in the weddell is one of the major differences between this area and others consequently we believe that ice structural conditions significantly influence the biological communities in the ice

microfabric analysis by manual and automated stereological procedures a methodological approach to antarctic tillite fabric analysis is commonly used to infer former movement directions of diamictons however analysis techniques are typically time consuming and partly subjective stereological analysis provides an alternative for objective determination of the preferred orientation of particles the ability of manual and automated stereological procedures to determine the preferred orientation of particles 120 4000 m in tillite is assessed using core samples from mount feather mcmurdo dry valleys antarctica orthogonal sets of vertical thin sections were subjected to directed secant analysis involving the determination of the number of intersections between particle outlines and a rotating series of parallel lines the manual analysis reveals a weakly developed three dimensional tilt angle plunge of 68 this reflects the depth averaged subglacial deformational structure of the deposit because the preferred orientation signal is weak in the automated set up this signal is obscured by the effect of digitization hampering the reliable assessment of the orientation direction the study shows microfabric variability within the studied cored section that is interpreted as a shear fabric reliable macrostructural information that might constrain ice flow directions requires a larger number of randomly or systematically drawn core samples from which averaged microstructural fabrics can be derived

observations of platelet ice growth and oceanographic conditions during the winter of 2003 in mcmurdo sound antarctica 1 platelet ice is the name given to ice crystals that nucleate in the ocean and grow either at depth or loosely attached to the ice water interface related to the proximity of ice shelves it is known to form in supercooled seawater this study details the conditions for its growth during the austral winter of 2003 in mcmurdo sound antarctica a key finding is that the presence of platelet ice in the sea ice cover is conclusively linked to the time history of the appearance of ice crystals in the water column monitored using the strength of the backscattered signal from an acoustic doppler current profiler as a proxy generally these crystals appeared from mid may as water near the interface became supercooled this near surface supercooling appears to be related to a simultaneous abrupt change in the structure of the upper 250 m of the water column from one that behaved dynamically and contained both the warmest and coolest measured temperatures to one in which the water was essentially isothermal near its surface freezing temperature near surface ocean temperature was also affected by tidal mixing and by an increase in the thickness and density over the course of winter of the surface mixed layer created by salt rejection during ice growth these processes allowed cold water trapped by buoyancy in a band at the base of the mixed layer in early winter to gain access to the ice water interface by midwinter this band of cold water probably had its origin beneath the mcmurdo ice shelf

equilibrium and disequilibrium trace element partitioning in hydrous eclogites trescolmen central alps elements pb th and u whereas phengite hosts ba and is along despite the widespread presence of hydrous phases in subductionwith clinopyroxene and paragonite an important phase for b related systems experimental d trace element values for many however because of their low modal abundance in eclogite facies hydrous phases are lacking to fill this gap we present a set of rocks phengite and paragonite do not control the b whole rock budget d values where min indicates amphibole zoisite we infer that estimated d values from equilibrium phengite paragonite or apatite derived from equilibrium parageneses assemblages can be used as a good approximation for partition of eclogites from trescolmen central alps switzerland these coefficients under the given pt conditions 650 c and 2 0 data can be combined with experimental data for d gpa deformation induced dynamic recrystallization seems to be to estimate d values for the hydrous phases thus circumventing the driving process necessary for a close approach towards trace experimental problems with the direct determination of such values element equilibrium this process should work efficiently during we analysed li be b sr y zr nb ba ce nd sm pb th subduction close to the slab mantle interface affecting sediments and u in coexisting phases by laser ablation microprobe inductively and low t altered mid ocean ridge basalt but is of less importance coupled plasma mass spectrometry many of the values are extremely inside the slab knowledge of d values from equilibrium assemblages low for example nb ba ce th and u are in the lower ppb determined in this study is useful not only for large scale subductionrange in clinopyroxene attainment of equilibrium was evaluated zone modelling but also as an aid in the selection of equilibrium by textural and major and trace element characteristics nonsubassemblages in imperfectly equilibrated samples so that a equilibrated assemblages are common in most eclogite localities crystallization sequence can be derived including trescolmen and using such samples would lead to the derivation of erroneous values for equilibrium partitioning however four of the 10 studied eclogites from trescolmen having homogeneous

formation of a solid inner core during the accretion of earth the formation of an inner core during the accretion of earth is investigated using self gravitating and compressible earth models formed by accreting a total of 25 or 50 moon to mars size planetary embryos the impact of an embryo heats the proto earth s interior differentially more below the impact site than elsewhere the rotating core dynamically overturns and stratifies shortly after each impact creating a spherically symmetric and radially increasing temperature distribution relative to an adiabatic profile merging of an embryo to the proto earth increases the lithostatic pressure that results in compressional temperature increase while further enhances the melting temperature of the core causing solidification a total of 36 thermal evolution models of the growing proto earth s core are calculated to investigate effects of major physical parameters no solidification is considered in the first 21 models where modified two body escape velocities are used as the impact velocities of the embryos at the end of accretion temperatures in the upper part of the core are significantly different among these models whereas temperatures in the deeper parts are similar the core solidification considered in the remaining 15 models where impact velocities higher than the modified two body escape velocities are adopted drastically changes the temperature distribution in the deeper parts of the core all of the models produce partially solidified stiff inner cores 1000 2100 km in radius at the end of accretion where the solid fraction is larger than 50 the innermost of the stiff inner cores are completely solidified to radii 250 1500 km

thermal evolution of a growing mars 1 we developed a new numerical model which can simulate the thermal history and metal silicate separation of a growing mars in this model the thermal disturbance caused by planetesimal impacts is calculated for each impact event by taking into account the effects of shock heating crater excavation and isostatic rebound a metallic blob is assumed to form at the base of a magma pond if an impact site is heated above the melting temperature sinking of the metal blobs is traced assuming stokes velocity their coalescence during sinking is treated by a monte carlo approach a series of simulations is carried out assuming that mars is formed by the runaway growth from a swarm of planetesimals as was suggested by recent numerical simulations of the planetary accretion process our numerical results show that 1 no global magma ocean is formed during accretion 2 metal silicate separation takes place without global scale melting and 3 instead of a metallic core a metal rich layer is formed at the late stage of accretion

seasonal development of the properties and composition of landfast sea ice in the gulf of finland the baltic sea 1 the seasonal development of the structure salinity and stable oxygen isotopic composition 18o of landfast sea ice was studied during the winter seasons 1999 2001 in the gulf of finland in the baltic sea the main focuses were on the seasonal and the interannual variability in ice properties and composition and on the contribution of meteoric ice to sea ice mass balance results provide a first statistical description of the seasonal evolution of sea ice in mild ice climate conditions the ice has a characteristic structure with an upper granular ice layer composed almost exclusively of superimposed ice and snow ice averaging at 20 35 of the total ice thickness the remaining is composed of columnar or intermediate granular columnar ice depending on growth conditions while salinity shows a uniform profile through the ice 18o shows lower values in the surface because of meteoric ice formation the thin ice cover is susceptible to changes in atmospheric conditions and rapid changes in ice salinity are connected to changes in the ice thermal regime and flooding the contribution of meteoric ice varied from 0 to 35 by mass depending on season and year superimposed ice formation is a recurring process and significantly contributed to ice growth up to 20 by mass especially late in the season during snowmelt freeze cycles liquid precipitation also cause formation of intermittent superimposed ice layers at these latitudes the contribution of meteoric ice to sea ice mass balance is largely dependent on the amount and timing of snow accumulation and timing of snowmelt freeze processes which all showed large year to year variation the conditions presented here may start to occur at higher latitudes if global warming continues

the high conductivity of iron and thermal evolution of the earth s core abstract we measured the electrical resistivity of iron and iron silicon alloy to 100 gpa the resistivity of iron was also calculated to core pressures combined with the first geophysical model accounting for saturation resistivity of core metal the present results show that the thermal conductivity of the outermost core is greater than 90 w m k these values are significantly higher than conventional estimates implying rapid secular core cooling an inner core younger than 1 ga and ubiquitous melting of the lowermost mantle during the early earth an enhanced conductivity with depth suppresses convection in the deep core such that its center may have been stably stratified prior to the onset of inner core crystallization a present heat flow in excess of 10 tw is likely required to explain the observed dynamo characteristics

thickness structure and properties of level summer multiyear ice in the eurasian sector of the arctic ocean in august and september 1991 thickness structure and properties of level multiyear ice were studied at 66 locations in the eurasian sector of the arctic ocean the mean ice thickness was 2 86 m with 0 31 m of freeboard including 0 08 m deteriorated ice of mean density 370 kg m 3 on the basis of the study of ice cores 61 of the ice cover consisted of undeformed columnar ice the remaining 39 consisted of a mixture of ice types including frazil 18 and deformed ice 9 through microstructural studies six main classes of pores could be identified the mean density of the ice cover increased from 720 kg m 3 at the top to 880 kg m 3 below 0 4 m depth sea ice salinities mean value 2 1 correlate with ice thickness on average salinity profiles exhibit a linear increase from values close to 0 at the top to 2 at 1 m depth with less steep salinity gradients below sampling from different depths within the ice cover indicates that the brine in summer sea ice is strongly stratified the influence of meltwater percolation is evident with salinities around 5 and ph values 15 and 8 at greater depths brine volumes ranging from 80 to 150 are controlled by the thermodynamic equilibrium between the solid and liquid phases gas volumes decrease from 200 at the top to 50 below 1 m depth pore microstructure is highly variable even on small scales salinity and other ice properties do not vary to a large degree between different regions the evolution of level multiyear sea ice is discussed with particular reference to hidden occurrence of deformed ice and the importance of ablation processes

mars paleomagnetic field as the result of a single hemisphere dynamo mars crustal magnetic field was most likely generated by dynamo action in the planet s early history unexplained characteristics of the field include its strength concentration in the southern hemisphere and lack of correlation with any surface features except for the hemispheric crustal dichotomy we used numerical dynamo modeling to demonstrate that the mechanisms proposed to explain crustal dichotomy formation can result in a single hemisphere dynamo this dynamo produces strong magnetic fields in only the southern hemisphere this magnetic field morphology can explain why mars crustal magnetic field intensities are substantially stronger in the southern hemisphere without relying on any postdynamo mechanisms

a low variance mineral assemblage with talc and phengite in an eclogite from the saxonian erzgebirge central europe and its p t evolution a light coloured fine grained eclogite sample from near the village of hammerunterwiesenthal in the erzgebirge nw bohemian massif preserves the low variance mineral assemblage of garnet omphacite phengite talc amphibole clinozoisite quartz rutile and accessory phases porphyroblasts of amphibole clinozoisite and phengite formed during a late stage iii of metamorphism paragonite joined the assemblage late in this stage iiib the chemical zonation of the minerals was carefully studied various geothermobarometric methods were applied especially involving phengite and talc the constrained p t path for the eclogite starts at about 480 c and 25 kbar stage ib followed by a significant temperature rise stage ii at slightly increasing pressure at the peak p t conditions of 720 c and 27 kbar blastesis of amphibole clinozoisite and phengite was caused by infiltrating hydrous fluids the resulting density reduction may have allowed buoyant uplift of the eclogite subsequently significant cooling occurred at high pressures stage iiib is characterized by p t conditions around 520 c and 18 kbar at reduced water activities this unusual late p t evolution might explain the freshness of the eclogite including the preservation of chemical zonation on the micrometre scale

a model of metal silicate separation on growing planets abstract the thermal evolution of planets during their accretionary growth is strongly influenced by impact heating the temperature increase following a collision takes place mostly below the impact location in a volume a few times larger than that of the impactor impact heating depends essentially on the radius of the impacted planet when this radius exceeds 1000 km the metal phase melts and forms a shallow and dense pool that penetrates the deep mantle as a diapir to study the evolution of a metal diapir we propose a model of thermo chemical readjustment that we compare to numerical simulations in axisymmetric spherical geometry and with variable viscosity we show that the metallic phase sinks with a velocity of order of a stokes velocity the thermal energy released by the segregation of metal is smaller but comparable to the thermal energy buried during the impact however as the latter is distributed in a large undifferentiated volume and the former potentially liberated into a much smaller volume the diapir and its close surroundings a significant heating of the metal can occur raising its temperature excess by at most a factor of 2 or 3 when the viscosity of the hot differentiated material decreases the proportion of thermal energy transferred to the undifferentiated material increases and a protocore is formed at a temperature close to that of the impact zone

heat partitioning in terrestrial planets during core formation by negative diapirism article i nfo we investigate the gravitational heat partitioning between silicate and metallic phases during core formation by negative diapirism in terrestrial planets we model the dynamic evolution of an iron diapir sinking through a solid silicate proto mantle our calculations include viscous heating and strong temperature and composition dependence of viscosity we show that the diapir mean temperature evolves towards an asymptotic value td which depends on the efficiency of viscous heating and heat transfer between the diapir and its surroundings and on the viscous rheology we derived a simple analytic model that captures the thermal evolution observed in our numerical experiments this model can be applied to determine the heat distribution within terrestrial planets with a large number of diapirs during their growth and early differentiation we show that the partitioning of core formation heating during negative diapirism depends strongly on the size distribution of the iron diapirs a large number of small sinking iron diapirs favors metal silicate heat exchange and leads to a relatively hot core which allows a sustainable dynamo as on earth a small number of large diapirs have the opposite effect and could correspond to a mars like planet with an early dynamo that is no longer active in all cases negative diapirism tends to leave the lowermost mantle significantly hotter than its upper part which could trigger large amounts of melting in the early deep mantle

thermal perturbations caused by large impacts and consequences for mantle convection united states national aeronautics and space administration planetary geology and geophysics program

platelet ice the southern ocean s hidden ice a review basal melt of ice shelves is not only an important part of antarctica s ice sheet mass budget but it is also the origin of one of the most peculiar types of sea ice found in the polar oceans platelet ice in many regions around coastal antarctica tiny ice crystals form and grow in supercooled plumes of ice shelf water releasing heat into the surrounding ocean they usually rise towards the surface eventually becoming trapped under an ice shelf as marine ice frequently masses of those crystals are advected out of the ice shelf cavity and accumulate below a solid sea ice cover to form a semiconsolidated layer when the overlying sea ice grows into this so called sub ice platelet layer the loose crystals are consolidated adding additional thickness to the sea ice these phenomena are generally referred to as platelet ice although confusion about the terminology is widespread in the literature the presence of platelet ice has a profound impact on sea ice properties and processes in several regions of antarctica with numerous implications for the local polar marine biosphere most notably sub ice platelet layers provide a stable sheltered nutrient and food rich habitat which usually results in a highly productive and uniquely adapted ecosystem it has also been hypothesised that platelet ice may be an indicator of the state of an ice shelf although comprehensive time series are limited to the ross sea this paper clears up the terminology by providing exact definitions of the relevant terms we review platelet ice formation observational methods as well as geographical and seasonal occurrence the physical properties and ecological implications are merged in a way understandable for physicists and biologists alike to lay the foundation for the interdisciplinary research that is necessary to tackle the current knowledge gaps

the inner inner core of the earth texturing of iron crystals from three dimensional seismic anisotropy abstract we present a three dimensional model of the seismic anisotropy and texturing of iron crystals in the inner core the form of the anisotropy changes suddenly at slightly less than half of the inner core radius the outer part is composed of iron crystals of a single phase with different degrees of preferred alignment along the spin axis of the earth the inner part may be composed of a different phase of crystalline iron or have a different pattern of alignment