elasticity and lattice dynamics of enstatite at high pressure the behavior of synthetic powdered 57 fe enriched enstatite mg 0 980 fe 0 020 5 mg 0 760 fe 0 240 si 2o 6 has been explored by x ray diffraction xrd and nuclear resonant inelastic x ray scattering nrixs the pbca structured enstatite sample was compressed in fine pressure increments for our independent xrd measurements one structural transition between 10 1 and 12 2 gpa has been identified from the xrd data the xrd reflections observed for the high pressure phase are best matched with space group p2 1 c we combine density functional theory with mossbauer spectroscopy and nrixs to understand the local site symmetry of the fe atoms in our sample a third order birch murnaghan bm3 equation of state fitting gives k t0 103 5 gpa and k t0 13 2 for the pbca phase at 12 gpa a bm3 fitting gives k t12 220 10 gpa with k t12 8 4 for the p2 1 c phase nrixs measurements were performed with in situ xrd up to 17 gpa the partial phonon density of states dos was derived from the raw nrixs data and from the low energy region of the dos the debye sound velocity was determined we use the equation of state determined from xrd and debye sound velocity to compute the isotropic compressional v p and shear v s wave velocities of enstatite at different pressures our results help constrain the high pressure properties of pbca structured enstatite in the earth s upper mantle we find that candidate upper mantle phase assemblages containing pbca structured enstatite are associated with shear velocity gradients that are higher than the average earth model prem but lower than regional studies down to about 250 km depth

iron partitioning in a pyrolite mantle and the nature of the 410 km seismic discontinuity pyrolite1 is a hypothetical mixture of distinct minerals which iswidely believed to represent the composition of the earth s mantle the main pressure induced phase transformations of the olivine component of pyrolite occur at about 13 5 gpa to and 24 gpa to mgsio3 rich perovskite magnesiowustite 2 3 which are thought to be responsible for the seismic discontinuities at 410 and 660 km depths in the mantle recent seismological studies however have demonstrated that the 410 km seismic discontinuity is sharper in some areas than that expected from the to transformation in mantle olivine with a fixed composition4 5 6 7 moreover some mineral physics studies suggest that the seismic velocity jump at the 410 km discontinuity is inconsistent with that associated with the to transformation in olivine8 9 here we present a phase equilibria study of a material having pyrolite composition at pressures of 6 16 gpa we found that the iron content in olivine changes significantly with increasing pressure as a result of the formation of a relatively iron rich majorite phase at these pressures this variation in iron content can overcome or at least reduce both of the above difficulties encountered with the pyrolite model of mantle composition by showing that the component mineral systems cannot be treated as separate

the orthorhombic to high p monoclinic phase transition in mg fe pyroxenes can it produce a seismic discontinuity the orthorhombic to high p monoclinic phase transition in mg fe sio 3 pyroxene with a mantle relevant composition x fs 0 1 is expected to occur at 300 km depth woodland and angel 1997 however the divariant nature of the phase transition in the mg fe system leaves the question open as to whether this transition occurs over a narrow enough pressure interval to cause a seismic discontinuity new experimental results with binary mg fe pyroxenes constrain the divariant loop to be 0 2 gpa wide at the composition of x fs 0 4 and on the order of 0 15 gpa for a mantle relevant composition this implies that the phase transition will be complete over a depth interval of about 5 6 km in the mantle and it is concluded that the divariant loop of the orthorhombic to high p monoclinic phase transition in mg fe sio 3 pyroxene is indeed narrow enough to produce a jump in seismic velocities the experimentally observed metastable behavior of orthopyroxene could further reduce the effective depth interval of this phase transition the expected location of this phase transition coincides with a small magnitude seismic discontinuity the x discontinuity occasionally observed in seismic profiles at 300 km depth and thus provides a viable petrologic explanation for the origin of this discontinuity if it truly exists

little ice age cold interval in west antarctica evidence from borehole temperature at the west antarctic ice sheet wais divide 1 the largest climate anomaly of the last 1000 years in the northern hemisphere was the little ice age lia from 1400 1850 c e but little is known about the signature of this event in the southern hemisphere especially in antarctica we present temperature data from a 300 m borehole at the west antarctic ice sheet wais divide results show that wais divide was colder than the last 1000 year average from 1300 to 1800 c e the temperature in the time period 1400 1800 c e was on average 0 52 0 28 c colder than the last 100 year average this amplitude is about half of that seen at greenland summit grip this result is consistent with the idea that the lia was a global event probably caused by a change in solar and volcanic forcing and was not simply a seesaw type redistribution of heat between the hemispheres as would be predicted by some ocean circulation hypotheses the difference in the magnitude of the lia between greenland and west antarctica suggests that the feedbacks amplifying the radiative forcing may not operate in the same way in both regions

molecular dynamics simulations of mgsio3 enstatite and seismic discontinuity in the upper mantle 1 the seismic velocities for three kinds of enstatite low clinoenstatite l cen orthoenstatite oen and high pressure clinoenstatite hp cen which is a major component in the upper mantle were estimated using molecular dynamics simulations based on the interatomic potential model the calculated seismic velocities vp and vs for oen are slower than those of l oen up to 6 gpa and 1000 k the velocities in hp cen are faster than those in oen at the transition pressure between oen and hp cen furthermore anisotropy of the seismic velocity in particular vp parallel to 100 001 010 is observed we conclude that the transitions in enstatites are related to the seismic discontinuity observed at depths less than 410 km and that enstatite is as an important a mineral as olivine for the mineralogical model and for the anisotropic propagation of seismic waves in the earth s upper mantle

metastable high pressure transformations of orthoferrosilite fs82 high pressure single crystal x ray diffraction experiments with natural ferrosilite fs82 fe

in situ elasticity measurement for the unquenchable high pressure clinopyroxene phase implication for the upper mantle ks t 0 017 1 gpa k 1 and g t 0 015 1 gpa k 1 our results show that the elastic velocities of hp cen are not only higher than those of oen but also higher than olivine at upper mantle conditions according to these new data the elastic velocities for the depleted mantle composition with 20 vol of hp cen would result in 0 8 and 1 6 in difference for p and s waves respectively when oen data were used in previous calculations newly measured results also suggest that it is possible to distinguish undepleted and depleted composition models in the upper mantle citation kung j b li t uchida and y wang 2005 in situ elasticity measurement for the unquenchable high pressure clinopyroxene phase implication for the upper mantle geophys res lett 32 l01307 doi 10 1029 2004gl021661

insignificant change in antarctic snowmelt volume since 1979 1 surface snowmelt is widespread in coastal antarctica satellite based microwave sensors have been observing melt area and duration for over three decades however these observations do not reveal the total volume of meltwater produced on the ice sheet here we present an antarctic melt volume climatology for the period 1979 2010 obtained using a regional climate model equipped with realistic snow physics we find that mean continent wide meltwater volume 1979 2010 amounts to 89 gt y 1 with large interannual variability 41 gt y 1 of this amount 57 gt y 1 64 is produced on the floating ice shelves extending from the grounded ice sheet and 71 gt y 1in west antarctica including the antarctic peninsula we find no statistically significant trend in either continent wide or regional meltwater volume for the 31 year period 1979 2010

the nature of the lehmann discontinuity from its seismological clapeyron slopes abstract while the existence of the lehmann discontinuity at 220 km in earth s mantle has been known for over 40 years from seismology it is still debated what causes this discontinuity here we report measurements of seismological clapeyron slopes for the lehmann discontinuity which are key to explaining the seismic discontinuity as either a manifestation of phase transitions or of other mineral physical processes the clapeyron slopes are measured by correlating discontinuity depths with local velocity perturbations from a tomographic model assuming that the velocity perturbations are solely due to temperature variations we find that in most regions the lehmann discontinuity is characterised by a regionally varying negative seismological clapeyron slope reflections from greater depths 250 350 km which we associate with the x discontinuity appear in clusters without a clear clapeyron slope in seeking mineralogical explanations of the lehmann and x discontinuities we can reject hypotheses in disagreement with these observations known phase transitions in the upper mantle above 400 km depth all have positive clapeyron slopes in the case of the lehmann discontinuity the only remaining hypothesis for a negative clapeyron slope is that it represents the transition in deformation mechanism from dislocation to diffusion creep the only exception is the middle east region where a positive claperyon slope is observed which could be explained by a phase transition from coesite to stishovite in the case of the x discontinuity we are not aware of mineral physical mechanisms possessing the required behaviour

phase relations of peridotites under h2o saturated conditions and ability of subducting plates for transportation of h2o phase relations of peridotites under h2o saturated conditions up to 28 gpa and 2600 c have been clarified based on the high pressure experimental results in the mgo feo sio2 h2o mgo sio2 al2o3 h2o cao mgo al2o3 sio2 h2o and natural systems based on the phase assemblages deduced and the chemical compositions of the phases the maximum h2o contents for two bulk rock compositions lherzolite and harzburgite have been calculated using mass balance then the potential ability of subducting plates for transportation of h2o is discussed by simple models combining the phase relations and the thermal structures of subduction zones as a function of the age of subducting plate a 15 130 ma the subduction velocity u0 2 25 18 cm year and angle 30 and 60 and the mantle potential temperature tp 1300 and 1350 c the results confirm the importance of choke point a cusp around 6 2 gpa and 550 c at which the stability field of major hydrous phases is minimized in terms of temperature although the choke point is exceeded by the mg sursassite bearing assemblage that has the maximum h2o content of 0 4 0 7 wt around 5 7 gpa and 550 700 c if the geotherm of the coldest part across the subducting plate passes below the choke point maximum 4 6 wt h2o can subduct whereas h2o less than 0 4 0 7 wt can subduct above the choke point the critical conditions are clarified as follows if 30 and tp 1300 c a 15 ma with u0 9 cm year or a 30 ma with u0 2 25 cm year is the critical condition if 60 and tp 1300 c or 30 and tp 1350 c the critical conditions slightly shift to an older age or a greater velocity e g a 30 ma with u0 3 5 cm year the slab thermal parameter au0sin is useful to predict roughly the critical condition however within the younger plate the spatial extent of a cold region with major hydrous phases is thinner hence less h2o is subducted even with the same slab thermal parameter this indicates that the age of the plate and the subduction velocity act differently on the h2o transportation ability of subducting plates

lower satellite gravimetry estimates of antarctic sea level contribution a new model of glacial isostatic adjustment used in conjunction with grace satellite data suggests that ice loss from antarctica is contributing 0 19 millimetres per year to global mean sea level which is substantially less than previous grace based estimates estimates for recent mass loss from antarctica vary widely making it difficult to calculate any subsequent contributions to sea level rise one prominent approach uses the grace gravity recovery and climate experiment satellites to detect changes in gravity but the final estimates are dependent on the modelled response of the continent to changes in ice loading over time and to changes in lithospheric elastic rebound matt king and colleagues apply a new model of this glacial isostatic adjustment to the grace data and estimate antarctic mass loss between 2002 and 2010 to be about 69 gigatonnes per year much lower than previous estimates and equivalent to an annual sea level rise of about 0 2 millimetres recent estimates of antarctica s present day rate of ice mass contribution to changes in sea level range from 31 gigatonnes a year gt yr 1 ref 1 to 246 gt yr 1 ref 2 a range that cannot be reconciled within formal errors3 time varying rates of mass loss2 4 5 6 contribute to this but substantial technique specific systematic errors also exist3 in particular estimates of secular ice mass change derived from gravity recovery and climate experiment grace satellite data are dominated by significant uncertainty in the accuracy of models of mass change due to glacial isostatic adjustment7 8 gia here we adopt a new model of gia developed from geological constraints which produces gia rates systematically lower than those of previous models and an improved fit to independent uplift data9 after applying the model to 99 months from august 2002 to december 2010 of grace data we estimate a continent wide ice mass change of 69 18 gt yr 1 0 19 0 05 mm yr 1 sea level equivalent this is about a third to a half of the most recently published grace estimates2 5 which cover a similar time period but are based on older gia models plausible gia model uncertainties and errors relating to removing longitudinal grace artefacts destriping confine our estimate to the range 126 gt yr 1 to 29 gt yr 1 0 08 0 35 mm yr 1 sea level equivalent we resolve 26 independent drainage basins and find that antarctic mass loss and its acceleration is concentrated in basins along the amundsen sea coast outside this region we find that west antarctica is nearly in balance and that east antarctica is gaining substantial mass

improved methods for pca based reconstructions case study using the steig et al 2009 antarctic temperature reconstruction abstracta detailed analysis is presented of a recently published antarctic temperature reconstruction that combines satellite and ground information using a regularized expectation maximization algorithm though the general reconstruction concept has merit it is susceptible to spurious results for both temperature trends and patterns the deficiencies include the following i improper calibration of satellite data ii improper determination of spatial structure during infilling and iii suboptimal determination of regularization parameters particularly with respect to satellite principal component retention this study proposes two methods to resolve these issues one utilizes temporal relationships between the satellite and ground data the other combines ground data with only the spatial component of the satellite data both improved methods yield similar results that disagree with the previous method in several aspects rather than finding warming concentrated in west antarctica the authors fi

density patterns of the upper mantle under asia and the arctic comparison of thermodynamic modelling and geophysical data based on the modified thermodynamic model we calculate the density of olivine pyroxene mixtures at different proportions of molecular masses in minerals at temperatures of 1000 1700 k and pressures up to 17 gpa the calculated density of the olivine pyroxene mixtures is compared with the prem model and the average density patterns of the asian and arctic regions estimated from the data on the rayleigh wave group velocity dispersion in upper mantle conditions taking into account the characteristic temperature profiles of different tectonic units in asia tibetan plateau khangay dome baikal rift and southern siberian platform and the arctic chukotka fold belt barents sea northern greenland and eurasia basin we discuss density patterns and features of the mantle composition underneath structures and to depths of 200 km where the most significant variations in their density patterns are observed the results obtained suggest some insights into the geodynamic evolution of the lithosphere in the study area the calculated density characteristics of the components in the upper mantle and the modelling of their composition are compared with average density patterns under the tectonic units in order to show the possibilities of previously developed equations of state it has been shown that the composition 0 9 mg0 9fe0 1 2sio4 0 1mgsio3 describes the prem model well in addition to the average velocity patterns of the study regions variations in mineralogical compositions are revealed between stable structures and the areas with high levels of tectonic and seismic activity the present paper could be of interest to geophysicists and mineralogists as it researches overlapping topics for both sciences indicating the relationship between seismicity and features of the deep structure of the upper mantle

changes in antarctic temperature wind and precipitation in response to the antarctic oscillation output of a 14 year integration with a high resolution 55 km 55 km regional atmospheric climate model is used to study the response of antarctic near surface climate to the antarctic oscillation aao the periodical strengthening and weakening of the circumpolar vortex in the southern hemisphere in spite of the relatively short record wind temperature and precipitation show widespread and significant aao related signals when the vortex is strong high aao index northwesterly flow anomalies cause warming over the antarctic peninsula ap and adjacent regions in west antarctica and the weddell sea in contrast cooling occurs in east antarctica the eastern ross ice shelf and parts of marie byrd land most of the annual temperature signal stems from the months march august the spatial distribution of the precipitation response to changes in the aao does not mirror temperature changes but is in first order determined by the direction of flow anomalies with respect to the antarctic topography when the vortex is strong high aao index the western ap becomes wetter while the ross ice shelf parts of west antarctica and the lambert glacier basin east antarctica become drier

electrical conductivity of enstatite as a function of water content implications for the electrical structure in the upper mantle abstract the electrical conductivity of ca free aluminous enstatite with various water contents has been determined at a pressure of 3 gpa in a kawai type multi anvil apparatus impedance spectroscopy was performed for both hydrogen doped and undoped samples in a frequency range from 0 1 hz to 1 mhz to examine the effect of water on conductivity two conduction mechanisms were identified for hydrogen undoped samples at temperature of 1000 1723 k and for hydrogen doped samples at relatively lower temperature range of 500 900 k to minimize dehydration of samples for the hydrogen undoped samples the activation enthalpy is around 1 9 ev at the higher temperatures range 1300 k suggesting that the dominant charge transfer mechanism is fe 2 fe 3 hopping small polaron conduction for the hydrogen doped samples measured below 900 k the activation enthalpy decreases from 1 11 to 0 70 ev and the conductivity values systematically increase with increasing water content suggesting that proton conduction is the dominant conduction mechanism taking hopping conduction and water content dependence of activation enthalpy for proton conduction into account all electrical conductivity data were fitted to the formula 0 h exp h h kt 0 p c w exp h p 0 c w 1 3 kt where 0 is pre exponential factor c w is the water content in weight percent h is the activation enthalpy h p 0 is the activation enthalpy for proton conduction at very low water concentration is the geometrical factor k is the boltzmann constant t is absolution temperature and subscripts h and p represent hopping and proton conductions respectively using the present results a laboratory based conductivity depth profile in the earth s upper mantle has been constructed as a function of water content comparison of our model with the currently available geophysical observations beneath the eastern pacific rise indicates that hydrous aluminous enstatite cannot account for the high conductivity anomaly at the top of the asthenosphere as well as hydrous olivine

phase transformations in a harzburgite composition to 26 gpa implications for dynamical behaviour of the subducting slab abstract the mineralogy adopted by a depleted harzburgite composition has been studied over the pressure interval 5 26 gpa at temperatures of 1300 1400 c the pyroxene garnet component of the harzburgite composition harzburgite minus 82 wt olivine transforms to majorite garnet by 18 19 gpa and further disproportionates to the assemblage of garnet stishovite mg 2 sio 4 spinel above 20 gpa at still higher pressures first ilmenite 22 24 gpa and then perovskite mgsio 3 24 26 gpa are found to coexist with garnet garnet disappears at 26 gpa and almost complete transition to perovskite is achieved at this pressure the mineral proportions and density profiles in the subducting oceanic lithosphere modelled by a combination of 80 harzburgite 20 primitive morb compositions are calculated as a function of depth under conditions isothermal with surrounding pyrolite mantle and also for a temperature distribution in which the slab is substantially cooler than surrounding mantle to below 700 km under isothermal conditions the slab has a density similar to surrounding mantle to a depth of 600 km however between 600 and 700 km the slab is up to 0 08 g cm 3 denser than surrounding mantle this is caused primarily by the higher alumina content in pyrolite as compared to harzburgite which causes the transition to perovskite in pyrolite to occur at substantially higher pressures than in harzburgite the presence of alumina also smears out the garnet perovskite transition in pyrolite over a depth interval of 50 km whereas this transformation is much sharper in the harzburgite composition calculations based on the observed phase equilibria also show that a subducted cool slab remains much denser by 0 1 0 3 g cm 3 than surrounding mantle to a depth of 700 km but possesses a density similar to surrounding mantle below this depth these results have important implications for the dynamical behaviour of slabs possessing different thermal regimes when they encounter the 670 km discontinuity and also for the nature of this discontinuity

central west antarctica among the most rapidly warming regions on earth the west antarctic ice sheet is contributing to sea level rise but temperature trends in the region have remained uncertain a complete temperature record for byrd station in central west antarctica spanning from 1958 to 2010 establishes west antarctica as one of the fastest warming regions globally

experimental evidence for the survival of augite to transition zone depths and implications for subduction zone dynamics ca mg rich clinopyroxenes are abundant in earth s upper mantle and subduction zones experimental studies on the thermoelastic properties of these minerals at simultaneous high pressure and high temperature are important for constraining of the composition and structure of the earth here we present a synchrotron based single crystal x ray diffraction study of natural diopside dominated augite ca 0 89 na 0 05 mg 0 06 mg 0 74 fe 0 11 al 0 14 ti 0 01 si 1 88 al 0 12 o 6 00 at p and t to 27 gpa and 700 k the experiment simulates conditions in cold subducting slabs and results indicate that augite is stable over this pressure and temperature range a third order high temperature birch murnaghan equation was fit with the pressure volume temperature data yielding the following thermoelastic parameters k t0 111 1 gpa k t0 4 1 1 k 0 t p 0 008 5 gpa k and t 4 1 10 5 k 1 2 3 10 8 k 2 t a strain analysis shows that the compression along the three principal stress directions is highly anisotropic with 1 2 3 1 98 2 43 1 00 additionally high pressure structural refinements of room temperature polyhedral geometry bond lengths and o3 o3 o3 angle were investigated to 27 gpa at ambient temperature pressure dependences of polyhedral volumes and distortion indicate that the substitution of al 3 for si 4 significantly changes the compressional behavior of the t o 4 tetrahedron in augite density calculations of this augite along a subducting slab geotherm suggest that augite as well as other common clinopyroxenes would promote slab stagnations at transition zone depths if they are metastably preserved in significant quantities

discovery of clinoenstatite in the luobusa ophiolitic mantle peridotite recovered from a drill hole tibet abstract clinoenstatite is an uncommon mineral and has been documented in sulu ultrahigh pressure uhp garnet pyroxenite however clinoenstatite clen was rarely reported in the previous studies of ophiolitic mantle peridotites the studied ophiolitic peridotites are recovered from a drilling hole of the luobusa ophiolitic massif tibet that is unique in occurrences of super reducing uhp minerals in both chromitite and peridotite our petrographic and tem studies reveal abundant clen as lamellae associated with diopside exsolution in orthoenstatite oren from the luobusa mantle peridotite in addition termination of clen lamellae and guinier preston zones nanometer scale precipitates with compositions between enstatite and diopside are also observed in the oren both clen lamellae and host oren have nearly identical mg0 9fe0 1 2si2o6 compositions the clen lamellae in host oren from two core samples have even numbers of 9 a 1 0 0 fringes without twins and are oriented parallel to 1 0 0 of the host oren the crystallographic data of the clen are d100 9 06 a r1 d020 4 34 a r2 d120 4 03 a r3 r1 r2 90 r1 r3 64 and r2 r3 26 whereas the host oren have d100 18 1 a r1 d020 4 48 a r2 d120 4 34 a r3 r1 r2 90 r1 r3 77 and r2 r3 13 no antiphase domain boundaries are observed in tem images it probably indicates that the clinoenstatite is the low p polymorph of clinoenstatite lclen with p21 c symmetry the exsolution of diopside and the presence of guinier preston zones are attributed by cooling of primary high t ca bearing very minor oren the transformation of lclen from oren may have occurred mainly by cooling along the p t boundaries between the oren and lclen stability fields shear stress only enhances the transition the occurrence of clen lamellae in oren and the existence of super reducing uhp minerals in peridotite provide an additional constraint for a very deep mantle origin of the luobusa peridotite

topography of the transition zone seismic discontinuities phase transformations in mantle mineralogies probably cause the transition zone seismic discontinuities nominally at 410 520 and 660 km depth thermodynamic principles govern phase transformations making them sensitive to changes in the mantle s ambient conditions through the thermodynamic response changes in temperature or composition shift the transformation to a different pressure creating topography on a level discontinuity with this use as an exploratory tool for the mantle in mind we review the basic seismological and phase equilibrium concepts underlying the detection and interpretation of the seismic wave arrivals associated with the transition zone discontinuities reviewing the evidence for and against the phase transition model we conclude that it is viable and describe how discontinuities have been and can be used to probe the physical and chemical state of the transition zone

mineralogy of the martian interior up to core mantle boundary pressures in order to determine the mineralogy of the martian interior along a high temperature areotherm multianvil experiments have been performed with a model martian mantle composition up to 23 5 gpa the dreibus and wanke 19851 martian mantle composition yields an upper mantle that consists of olivine clinopyroxene orthopyroxene garnet at pressures up to 9 gpa above 9 gpa orthopyroxene is no longer present the transition zone is marked by the appearance of spinel at 13 5 gpa up to 15 gpa clinopyroxene and majorite coexists with phase and or spinel by 17 gpa clinopyroxene is entirely replaced by majorite and the modal abundance of spinel increases at the expense of phase the dominant assemblage throughout most of the transition zone is spinel majorite two experiments completed in the perovskite stability field indicate that the lower mantle consists of mg fe silicate perovskite magnesiowustite and majorite casio 3 perovskite is not present in these experiments both the presence of a martian lower mantle i e an mg fe silicate perovskite bearing zone and the phase assemblage stable in the martian lower mantle are very sensitive to the temperature profile of the interior a low temperature profile may stabilize stishovite in the lower mantle or it may lead to the absence of the lower mantle because of the higher transition pressure required for forming perovskite at lower temperatures regardless of the temperature profile assumed the martian upper mantle and transition zone will account for a larger proportion of the planet s interior than is the case for the earth s interior because of the smaller size of mars

orthoenstatite clinoenstatite phase transformation in mgsio3 at high pressure and high temperature determined by in situ x ray diffraction implications for nature of the x discontinuity 1 in the upper mantle the x discontinuity at the depths of 240 to 340 km is observed in many localities but its origin is under debate the phase transformation of mg fe sio3 from orthopyroxene to high pressure clinopyroxene has been proposed to cause this discontinuity in this study in situ x ray diffraction experiments were performed and the orthoenstatite high pressure clinoenstatite c2 c phase boundary in the mgsio3 system was determined all the experiments were carried out using a kawai type multianvil apparatus installed at bl04b1 in spring 8 hyogo japan two types of experiments were carried out 1 phase observation experiments at varying pressures and temperatures and 2 quench experiments at fixed pressure and temperature the phase boundary determined precisely by integrating the results of both sets of experiments was found to be p gpa 0 0035t c 3 7 using a recent mgo pressure scale the orthoenstatite high pressure clinoenstatite phase boundary at 1400 c determined here is 0 7 1 0 gpa lower than previous reports based on quench experiments the phase transformation of orthopyroxene is expected to occur at 260 km depth along a typical mantle geotherm and the shallower part of the observed x discontinuity can be explained by this phase transformation

effect of h2o on upper mantle phase transitions in mgsio3 is the depth of the seismic x discontinuity an indicator of mantle water content the mantle x discontinuity usually assigned to positive seismic velocity reflectors in the 260 330 km depth range has proved difficult to explain in terms of a single mineralogical phase transformation in part because of its depth variability the coesite to stishovite transition of sio sub 2 matches deeper x discontinuity depths but requires 5 10 free silica in the mantle to match observed impedance contrast the orthoenstatite oen to high pressure clinoenstatite hpcen transformation of mgsio sub 3 also broadly coincides with depths of the x but requires chemically depleted and orthoenstatite rich lithology at 300 km depth in order to match observed seismic impedance contrast on the basis of high pressure infrared spectroscopy x ray diffraction and raman spectroscopy we show that 1300 ppm variation of h sub 2 o content in mgsio sub 3 can displace the transition of low pressure clinoenstatite lpcen to hpcen by up to 2 gpa similar to previous quench experiments on the oen to hpcen phase transition where about 30 45 km 1 0 1 5 gpa of deflection could occur per 0 1 wt h sub 2 o if the mantle x discontinuity results from pyroxene transitions in a depleted harzburgite layer because of the strong influence of minor amounts of water on the transformation boundary the depth of the mantle x discontinuity couldmore serve as a potentially sensitive indicator of water content in the upper mantle less

seasonal climate information preserved in west antarctic ice core water isotopes relationships to temperature large scale circulation and sea ice as part of the united states contribution to the international trans antarctic scientific expedition itase a network of precisely dated and highly resolved ice cores was retrieved from west antarctica the itase dataset provides a unique record of spatial and temporal variations of stable water isotopes d 18 o and dd across west antarctica we demonstrate that after accounting for water vapor diffusion seasonal information can be suc cessfully extracted from the itase cores we use meteo rological reanalysis weather station and sea ice data to assess the role of temperature sea ice and the state of the large scale atmospheric circulation in controlling seasonal average water isotope variations in west antarctica the strongest relationships for all variables are found in the cores on and west of the west antarctic ice sheet divide and during austral fall during this season positive isotope anomalies in the westernmost itase cores are strongly related to a positive pressure anomaly over west antarctica low sea ice concentrations in the ross and amundsen seas and above normal temperatures analyses suggest that this seasonally distinct climate signal is due to the pronounced meridional oriented circulation and its linkage to enhanced sea ice variations in the adjacent southern ocean during fall both of which also influence local to regional temperatures

climate variability in the amundsen and bellingshausen seas satellite data reveal a 20 decline in sea ice extent in the amundsen and bellingshausen seas in the two decades following 1973 this change is negatively correlated with surface air temperatures on the west side of the antarctic peninsula which have increased 0 58c decade21 since the mid 1940s the recession was strongest during summer when monthly average minima in 1991 92 removed much of the incipient multiyear ice over the continental shelf this would have lowered the regional mean ice thickness impacting snow ice formation brine production and vertical heat flux the northern ice edge contracted by 18 of latitude in all seasons from 1973 79 to 1987 93 returning toward mean conditions in 1993 95 the decline included multiyear cycles of several years in length superimposed on high interannual variability a review of atmospheric forcing shows winds consistent with mean and extreme ice extents and suggests links to larger scale circulation changes in the south pacific historical ocean measurements are sparse in this sector but mixed layer depths and upper pycnoclines beneath the sea ice resemble those in the weddell sea weaker surface currents or changes in the upwelling of circumpolar deep water on the continental shelf could have contributed to the anomaly persistence

understanding the sam influence on the south pacific enso teleconnection the relationship between the el nino southern oscillation enso and the southern hemisphere annular mode sam is examined with the goal of understanding how various strong sam events modulate the enso teleconnection to the south pacific 45 70 s 150 70 w the focus is on multi month multi event variations during the last 50 years a significant p 0 10 relationship is observed most marked during the austral summer and in the 1970s and 1990s in most cases the significant relationship is brought about by la nina el nino events occurring with positive negative phases of the sam more often than expected by chance the south pacific teleconnection magnitude is found to be strongly dependent on the sam phase only when enso events occur with a weak sam or when a la nina el nino occurs with a positive negative sam phase are significant south pacific teleconnections found this modulation in the south pacific enso teleconnection is directly tied to the interaction of the anomalous enso and sam transient eddy momentum fluxes during la nina sam and el nino sam combinations the anomalous transient momentum fluxes in the pacific act to reinforce the circulation anomalies in the midlatitudes altering the circulation in such a way to maintain the enso teleconnections in la nina sam and el nino sam cases the anomalous transient eddies oppose each other in the midlatitudes overall acting to reduce the magnitude of the high latitude enso teleconnection

rapid recent warming on rutford ice stream west antarctica from borehole thermometry 1 the antarctic peninsula has warmed faster than the global average rate of warming during the last century due to limited availability of long term meteorological records the geographical extent of this rapid warming is poorly defined we collected borehole temperature measurements in the upper 300 m of rutford ice stream west antarctica and employed an inverse modeling scheme with a heat diffusion advection equation to determine the recent surface temperature history of the borehole position our results reveal recent warming of 0 17 0 07 c decade 1 since 1930 this result suggests that at least in an attenuated form the rapid warming observed over the antarctic peninsula extends as far south as rutford ice stream this result agrees with other recent results that show a warming trend across much of the west antarctic ice sheet

full resolution 3d gpr imaging noninvasive 3d ground penetrating radar gpr imaging with submeter resolution in all directions delineates the internal architecture and processes of the shallow subsurface full resolution imaging requires unaliased recording of reflections and diffractions coupled with 3d migration processing the gpr practitioner can easily determine necessary acquisition trace spacing on a frequency wavenumber f k plot of a representative 2d gpr test profile quarter wavelength spatial sampling is a minimum requirement for full resolution gpr recording an intensely fractured limestone quarry serves as a test site for a 100 mhz 3d gpr survey with 0 1 m 0 2 m trace spacing this example clearly defines the geometry of fractures in four different orientations including vertical dips to a depth of 20 m decimation to commonly used half wavelength spatial sampling or only 2d migration processing makes most fractures invisible the extra data acquisition effort results in image volumes with submeter resolution both