a new look at the saturn system the voyager 2 images voyager 2 photography has complemented that of voyager i in revealing many additional characteristics of saturn and its satellites and rings saturn s atmosphere contains persistent oval cloud features reminiscent of features on jupiter smaller irregular features track out a pattern of zonal winds that is symmetric about saturn s equator and appears to extend to great depth winds are predominantly eastward and reach 500 meters per second at the equator titan has several haze layers with significantly varying optical properties and a northern polar collar that is dark at short wavelengths several satellites have been photographed at substantially improved resolution enceladus surface ranges from old densely cratered terrain to relatively young uncratered plains crossed by grooves and faults tethys has a crater 400 kilometers in diameter whose floor has domed to match tethys surface curvature and a deep trench that extends at least 270 around tethys circumference hyperion is cratered and irregular in shape iapetus bright trailing hemisphere includes several dark floored craters and phoebe has a very low albedo and rotates in the direction opposite to that of its orbital revolution with a period of 9 hours within saturn s rings the birth of a spoke has been observed and surprising azimuthal and time variability is found in the ringlet structure of the outer b ring these observations lead to speculations about saturn s internal structure and about the collisional and thermal history of the rings and satellites

climate instability during the last interglacial period recorded in the grip ice core isotope and chemical analyses of the grip ice core from summit central greenland reveal that climate in greenland during the last interglacial period was characterized by a series of severe cold periods which began extremely rapidly and lasted from decades to centuries as the last interglacial seems to have been slightly warmer than the present one its unstable climate raises questions about the effects of future global warming

deglacial warming of the northeastern atlantic ocean correlation with the paleoclimatic evolution of the european continent abstract isotopic micropaleontologic and pollen analyses of deep sea cores from the bay of biscay and the northeastern atlantic ocean show that the deglacial warming of this oceanic area was closely correlated with the paleoclimatic evolution of the adjacent european continent temperatures were at least as warm as those of today in the bay of biscay between 13 300 and 11 000 b p coinciding with the combined bolling allerod warm continental events a major spread of polar water occurred between 11 000 and 10 000 b p during this event which coincides with the younger dryas continental cold event marine temperatures were almost as low as those of the last glacial maximum the final deglacial warming of the norteastern atlatntic ocean occurred during the following 3000 yr

thermal hydrous and mechanical states of the mantle transition zone beneath southern africa abstract observations of p to s conversions from the seismic discontinuities near 410 and 660 km depth reveal the shoaling of the 410 km discontinuity and the deepening of the 660 km discontinuity beneath the archaean cratons in southern africa consequently the mantle transition zone is 20 km thicker than beneath post archaean regions and the global average the discontinuity structure combined with tomographically imaged seismic velocities suggests a relatively cold t 100 k and water enriched oh 20 35 saturation transition zone beneath the cratons the thicker than normal transition zone correlates with surface geology and does not follow the track of african plate motion as numerical simulations of small scale convection imply the observations suggest that cold thermal downwelling is not the primary cause of the anomalously thick transition zone beneath the cratons although it may exist in a confined area near the boundary between the kheis thrust belt and the namaqua natal belt if thermal downwelling exists its temperature anomaly at the 660 km discontinuity must be below the levels detectable by the method used in this study t 60 k the spatial correlation between surface geology high velocities beneath the cratons and the transition zone structure indicates that the cratonic keels extend to the base of the upper mantle substantially deeper than most estimates our results demonstrate the need to closely consider the effect of water content on seismic velocities beneath deep continental roots

saturn s zonal winds at cloud level we present an analysis of new cloud motion measurements in saturn s atmosphere from voyager 1 and 2 images and define a nominal zonal wind velocity vs latitude profile for the cloud top level that can be used as reference for the upcoming cassini mission improvements over previous studies include i twice the number of wind vectors used ii independent measurements of cloud tracers iii extended latitudinal coverage iv strict error control and evaluation and v a new zonal averaging methodology mean zonal wind velocities are given for the latitude range 81 to 71 planetographic in steps of 0 5 with a gap between 0 and 10 due to ring shadowing this research also includes the study of the morphology and motions of cloud systems in the equator and in the southern hemisphere which have not been described previously we give an ephemeris motion equation for several long lived features that could be observable if they survive until cassini arrives on saturn the long term behavior of the zonal wind profile is studied by comparing it with a complete set of wind velocities compiled from historical and modern ground based and hst observations significant changes are noted in the equatorial jet between this set and the nominal voyager profile we discuss in detail the possible origin for such differences

ground based observations of saturn s north polar spot and hexagon ground based observations of two conspicuous features near the north pole of saturn the polar vortex and the hexagonal wave structure were made from july 1990 to october 1991 10 years after their discovery during this period the polar spot drifted in longitude relative to system iii by 0 0353 per day on average superimposed on this mean motion the spot also underwent short term rapid excursions in longitude of up to 14 at rates of up to 1 per day the spot also exhibited irregular variations in its latitude location a combination of these data together with those obtained by voyager 1 and 2 in 1980 and 1981 shows that the spot drifted 0 0577 per day for the 11 year interval from 1980 to 1991 the large lifetime of both features indicates that they are insensitive to the strong variations in the seasonal heating of the cloud layers in the upper polar atmosphere

mantle discontinuities and temperature under the north american continental keel a ubiquitous feature of upper mantle seismic velocity models has been the presence of high velocity keels beneath stable continental interiors1 2 3 4 5 uncertainty remains however regarding the maximum depth to which continental keels extend the degree to which they have cooled the mantle that surrounds them and their role in mantle flow here we investigate thermal anomalies across the eastern margin of the north american continental keel by imaging the seismic discontinuities at depths of 410 and 660 km with compressional to shear converted waves recorded by a 1 500 km long seismometer deployment in the eastern united states the thickness of the transition zone the region nominally between depths of 410 and 660 km and the depth to the 410 km discontinuity indicate that cold keel material and sub keel downwellings must be largely confined to the upper mantle and may impinge on the transition zone only in localized regions and with thermal anomalies of less than 150 k a 20 km depression of the 660 km discontinuity to the south of the westernmost stations coincides with a region of fast velocity in the deep transition zone2 and may be associated with the remnants of the subducted farallon plate1 2 4

high temperature elasticity of iron bearing olivines the first high temperature data on the nine adiabatic elastic moduli for iron bearing olivine are reported these measurements are on two single crystal specimens of natural olivine at ambient pressure and from room temperature to a maximum of 1500 k the two specimens contain 8 and 9 modal percent fayalite which required the oxygen fugacity be controlled at high temperature to preserve their chemical stability the rectangular parallelepiped resonance apparatus was adapted to buffer the specimens from the atmosphere with a mixture of co and co2 gas a small increase 1 2 gpa in the adiabatic bulk modulus of each specimen over that of end member forsterite was found the data are high quality to extreme temperatures with good agreement found when comparing the temperature derivatives of the elastic moduli of the two specimens neither specimen exhibits measurable nonlinear temperature dependence in the computed isotropic bulk and shear moduli which is in contrast to published forsterite data the temperature derivatives of the isotropic bulk modulus ks are 1 69 1 80 10 2 gpa k 1 for the two olivine specimens and the shear modulus g derivatives are 1 38 1 36 10 2 gpa k 1 these derivatives are only slightly larger in magnitude than ks t p 1 56 10 2 and g t p 1 30 10 2 gpa k 1 found previously for iron bearing olivine over a very small temperature range there are also no significant differences between the temperature derivatives found here and the average derivatives of end member forsterite from data retrieved over a slightly larger temperature range several dimensionless parameters have been calculated from these results and are discussed in view of systematics which bear on high pressure phases in earth s transition zone one result from these systematics related to the seismic velocities in the earth and especially the shear wave velocities is that an olivine content of less than 50 is implied at the 400 km discontinuity if earth s upper mantle is isochemical furthermore the substitution of almost 10 iron for magnesium at the forsterite end of the olivine solid solution series has little effect on the dimensionless parameters or on the temperature derivatives of the elastic moduli at high temperature

δs at high pressure and dlnvs dlnvp in the lower mantle s a measure of the sensitivity of the adiabatic bulk modulus to isobaric density changes controls lateral variations of p wave velocities to estimate the effect of pressure on s we have performed lattice dynamic calculations of the cubic alkaline earth oxides we used a many body parametric hamiltonian that reproduces the available laboratory data on thermoelastic properties the calculations were carried out for p t conditions that correspond to those of the lower mantle we find that s decreases from 4 1 at standard conditions to 2 at lower mantle conditions the latter value is consistent with d l anderson s estimates based on geophysical data with s 2 and a corresponding parameter for shear modulus not decreasing with compression no partial melting is required to account for the observation that lnvs lnvp 2 in the lower mantle although more data and calculations on additional minerals are needed to draw any firm conclusions our results indicate that the relatively small lateral variations of lnvp when compared to lnvs are due entirely to compressional effects

mantle dynamics and the heat flow into the earth s continents reducedcontinental heat flow defined as the measured heat flow minus that due to radiogenic sources in the upper crust initially decreases with time after the last thermotectonic event in a region1 but then tends to stabilize at a roughly constant value which is higher than would be predicted by a conductive cooling model fig 1 here we argue that this behaviour results from the influence that continents have on mantle heat loss the finite thermal conductivity of continental crust and the fact that it is not recycled into the mantle on a large scale as is oceanic crust allows stable continental blocks to limit the local heat flux out of the mantle numerical models that allow blocks of crust to form over a mantle layer demonstrate how thermal perturbations associated with mantle convection penetrate into continents causing lateral temperature variations at their base that parallel those in the mantle just below this tends to establish a constant flux of heat from the mantle into the continents which accounts for the trend in reduced heat flow

experiments in migration processing of ss precursor data to image upper mantle discontinuity structure long period ss precursors result from underside reflections off upper mantle discontinuities by grouping and stacking global seismic data by ss bounce point location it is possible to map lateral variations in depths to the 410 and 660 km discontinuities a process analogous to common midpoint cmp stacking in reflection seismology because this method assumes horizontal reflectors energy arriving from dipping or intermittent reflectors may not be correctly imaged to address this possibility we experiment with techniques based on migration processing of shallow seismic reflection data the problem is complicated by the uneven distribution of sources and receivers for the ss precursor observations but the data are sufficiently dense beneath the northwest pacific ocean that reasonably good coverage can be obtained for this region we parameterize the model as a grid of point scatterers in latitude longitude and depth from the surface to 1000 km depth and compute travel times from each grid point to the source and receiver locations these times are used to construct a matrix equation that yields predicted ss precursor waveforms from the assumed scatterers to recover the model we experiment with both simple back projection and full inversions using a conjugate gradient method tests on noise free synthetic data generated using the same source receiver distribution as the actual data suggest that detailed resolution of discontinuity structure is possible at horizontal scales much smaller than the fresnel zone however the real data do not produce coherent results unless some degree of horizontal smoothing is imposed at least partially defeating the purpose of this approach results for the northwest pacific find structure on the 410 and 660 km discontinuities and hints of intermittent reflectors at other depths random resampling tests however suggest that most of these features are not reliably resolved with the exception of a depression on the 660 km discontinuity seen in the northwest pacific our experiments show that it is unlikely that small scale structure on the 660 km discontinuity near subducting slabs causes significant bias in maps of the large scale 660 km topography derived from long period ss precursor observations

implications of a simple mantle transition zone beneath cratonic north america many areas of old continental interiors have thick roots that extend to depths of 250 km or more in contrast to the oceans and younger continents whose lithospheric thickness is less than 100 km these cratonic roots might perturb temperatures in their surrounding mantle though the net result could be either cooling or heating both of which may lead to small scale convective flow around the root we show here using new data from a study of the seismic structure of the canadian shield that the relative positions of the 410 km and 660 km seismic discontinuities are unperturbed beneath one of the deepest and broadest cratonic roots on the earth differential arrival times and internal discontinuity structure are remarkably uniform and simple varying by little more than 0 5 s over the root s 3500 km lateral extent this implies that the root has no significant thermal effect on the underlying mantle 50 k and any small scale convection or cold mantle downwelling associated with the large free air gravity anomaly beneath the shield must be confined to the upper mantle our observations are also consistent with phase changes solely in the olivine system mg fe 2sio4 with the caveat that our method is dominantly sensitive to s wave velocity jumps alone

saturn s cloud structure and temporal evolution from ten years of hubble space telescope images 1994 2003 abstract we present a study of the vertical structure of clouds and hazes in the upper atmosphere of saturn s southern hemisphere during 1994 2003 about one third of a saturn year based on hubble space telescope images the photometrically calibrated wfpc2 images cover the spectral region between the near uv 218 255 nm and the near ir 953 1042 nm including the 890 nm methane band using a radiative transfer code we have reproduced the observed center to limb variations in absolute reflectivity at selected latitudes which allowed us to characterize the vertical structure of the entire hemisphere during this period a model atmosphere with two haze layers has been used to study the variation of hazes with latitude and to characterize their temporal changes both hazes are located above a thick cloud putatively composed of ammonia ice an upper thin haze in the stratosphere between 1 and 10 mbar is found to be persistent and formed by small particles radii 0 2 m the lower thicker haze close to the tropopause level shows a strong latitudinal dependence in its optical thickness typically 20 40 at the equator but 5 at the pole at 814 nm this tropospheric haze is blue absorbent and extends from 50 to 100 mbar to about 400 mbar both hazes show temporal variability but at different time scales first there is a tendency for the optical thickness of the stratospheric haze to increase at all latitudes as insolation increases second the tropospheric haze shows mid term changes over time scales from months to 1 2 years in its optical thickness typically by a factor of 2 such changes always occur within a rather narrow latitude band width 5 10 affecting almost all latitudes but at different times third we detected a long term 10 year decrease in the blue single scattering albedo of the tropospheric haze particles most intense in the equatorial and polar areas long term changes follow seasonal insolation variations smoothly without any apparent delay suggesting photochemical processes that affect the particles optical properties as well as their size in contrast mid term changes are sudden and show various time scales pointing to a dynamical origin

mantle discontinuity structure beneath the colorado rocky mountains and high plains analysis of mantle discontinuity structure using converted p to s pas phases beneath colorado from the program for array seismic studies of the continental lithosphere passcal rocky mountain front rmf experiment reveals significant topography at the 410 and 660 km depth discontinuities and corresponding transition zone thickness variations a stack of all radial receiver functions resolves the 410 and 660 km discontinuities at average depths of 419 and 677 km respectively imaging of lateral variations in mantle discontinuity structure is accomplished by geographically binning the pds conversion points and then stacking the receiver functions in each bin to form spatial images analogous to common depth point stacking corrections for lateral velocity heterogeneity are calculated using the local s wave tomographic model of lee and grand 1996 and a constant lnvs lnvp scaling of 1 3 this scaling value is determined from the relative scaling between teleseismic p and s wave travel time residuals measured from the rocky mountain front deployment mantle discontinuity images using 150 km square bins show 20 km of 410 km discontinuity topography 30 km of 660 km discontinuity topography and up to 40 km of transition zone thickness variation features of the discontinuity structure include a 20 km depression of the 660 km discontinuity beneath western colorado and a gradual 10 km dip of the 410 km discontinuity beneath the high plains the thickening of the transition zone beneath southwest colorado is consistent with the presence of the subducted farallon slab in this region as imaged by van der lee and nolet 1997 in general our results show that the transition zone discontinuity structure is more complex than that predicted by the simple model of olivine phase boundaries modulated by vertically coherent thermal anomalies

mid infrared mapping of jupiter s temperatures aerosol opacity and chemical distributions with irtf texes abstract global maps of jupiter s atmospheric temperatures gaseous composition and aerosol opacity are derived from a programme of 5 20 m mid infrared spectroscopic observations using the texas echelon cross echelle spectrograph texes on nasa s infrared telescope facility irtf image cubes from december 2014 in eight spectral channels with spectral resolutions of r 2000 12 000 and spatial resolutions of 2 4 latitude are inverted to generate 3d maps of tropospheric and stratospheric temperatures 2d maps of upper tropospheric aerosols phosphine and ammonia and 2d maps of stratospheric ethane and acetylene the results are compared to a re analysis of cassini composite infrared spectrometer cirs observations acquired during cassini s closest approach to jupiter in december 2000 demonstrating that this new archive of ground based mapping spectroscopy can match and surpass the quality of previous investigations and will permit future studies of jupiter s evolving atmosphere the visibility of cool zones and warm belts varies from channel to channel suggesting complex vertical variations from the radiatively controlled upper troposphere to the convective mid troposphere we identify mid infrared signatures of jupiter s 5 m hotspots via simultaneous m n and q band observations which are interpreted as temperature and ammonia variations in the northern equatorial zone and on the edge of the north equatorial belt neb equatorial plumes enriched in nh3 gas are located south east of nh3 desiccated hotspots on the edge of the neb comparison of the hotspot locations in several channels across the 5 20 m range indicate that these anomalous regions tilt westward with altitude aerosols and ph3 are both enriched at the equator but are not co located with the nh3 plumes the equatorial temperature minimum and ph3 aerosol maxima have varied in amplitude over time possibly as a result of periodic equatorial brightenings and the fresh updrafts of disequilibrium material temperate mid latitudes display a correlation between mid ir aerosol opacity and the white albedo features in visible light i e zones we find hemispheric asymmetries in the distribution of tropospheric ph3 stratospheric hydrocarbons and the 2d wind field estimated via the thermal wind equation that suggest a differing efficiency of mechanical forcing e g vertical mixing and wave propagation between the two hemispheres that we argue is driven by dynamics rather than jupiter s small seasonal cycle jupiter s stratosphere is notably warmer at northern mid latitudes than in the south in both 2000 and 2014 although the latter can be largely attributed to strong thermal wave activity near 30 n that dominates the 2014 stratospheric maps and may be responsible for elevated c2h2 in the northern hemisphere a vertically variable pattern of temperature and windshear minima and maxima associated with jupiter s quasi quadrennial oscillation qqo is observed at the equator in both datasets although the contrasts were more subdued in 2014 large scale equator to pole gradients in ethane and acetylene are superimposed on top of the mid latitude mechanically driven maxima with c2h2 decreasing from equator to pole and c2h6 showing a polar enhancement consistent with a radiatively controlled circulation from low to high latitudes cold polar vortices beyond 60 latitude can be identified in the upper tropospheric and lower stratospheric temperature maps suggesting enhanced radiative cooling from polar aerosols finally compositional mapping of the great red spot confirms the local enhancements in ph3 and aerosols the north south asymmetry in nh3 gas and the presence of a warm southern periphery that have been noted by previous authors

abrupt increase in greenland snow accumulation at the end of the younger dryas event the warming at the end of the last glaciation was characterized by a series of abrupt returns to glacial climate the best known of which is the younger dryas event1 despite much study of the causes of this event and the mechanisms by which it ended many questions remain unresolved1 oxygen isotope data from greenland ice cores2 4 suggest that the younger dryas ended abruptly over a period of about 50 years dust concentrations2 4 in these cores show an even more rapid transition 20 years this extremely short timescale places severe constraints on the mechanisms underlying the transition but dust concentrations can reflect subtle changes in atmospheric circulation which need not be associated with a large change in climate here we present results from a new greenland ice core gisp2 showing that snow accumulation doubled rapidly from the younger dryas event to the subsequent preboreal interval possibly in one to three years we also find that the accumulation rate change from the oldest dryas to the bo11ing allerod warm period was large and abrupt the extreme rapidity of these changes in a variable that directly represents regional climate implies that the events at the end of the last glaciation may have been responses to some kind of threshold or trigger in the north atlantic climate system

geoid anomalies and the structure of continental and oceanic lithospheres geoid anomalies in the wavelength range corresponding to spherical harmonies 6 to 30 are used to bring new constraints on the deep structure of the continental lithosphere on the nature of upper mantle seismic anomalies and on the thermal evolution of the oceanic lithosphere for the shallow lithospheric density anomalies considered here the geoid anomaly is shown to follow approximately the moment law derived from isostasy by a procedure of least squares inversion the geoid over old cratons is found to be equal to that over relatively young 50 myr oceanic lithosphere the oceanic geoid as a function of age follows the expected linear trend with flattening at old ages these results are independent of the degree at which the spherical harmonics are truncated if the tectosphere consisted only of a thick thermal boundary layer marked geoid lows with an amplitude larger than 20 m should be present over cratons in disagreement with the present observation the predicted geoid agrees with the observations if the negative density anomalies associated with a depleted peridotitic layer are present to the bottom of the lithosphere this analysis of the geoid confirms the petrological inferences that the deep roots observed seismically in the top upper mantle form a chemically differentiate reservoir the density of these cold roots is similar to that of mantle material elsewhere because the increase in density resulting from their lower temperature is largely balanced by the decrease in density caused by their depletion analysis in wavelength range corresponding to harmonics 6 to 30 in the oceans confirms that the geoid flattens at old ages and thus supports the view that an important amount of heat is transferred at the base of the lithosphere by small scale convection

mantle layering from scs reverberations 2 the transition zone this is the second paper in a four part sequence that examines the nature of mantle layering using the scsn phases and internal reflections observed within the reverberative interval of sh polarized seismograms mantle reflectivity profiles for 18 seismic corridors sampling portions of indonesia australia and the western and central pacific are constructed from low frequency 25 mhz scs reverberations i e scsn type phases reflected once from internal mantle discontinuities modeling the reflectivity profiles using synthetic seismograms yields precise path specific estimates of the travel times to the 410 km and 660 km discontinuities and their reflection coefficients the data are consistent with an olivine dominated model of the transition zone marked by two major phase transitions having average apparent depths of 414 2 km and 660 2 km and impedance contrasts of 0 046 0 010 and 0 072 0 010 respectively the interpath variations observed in the reverberation travel times imply that the topography on the two discontinuities is of the order of 20 km peak to peak and is negatively correlated consistent with an endothermic transition at 660 km whose clapeyron slope magnitude is comparable to that of the 410 km exothermic transition in the case of the 660 km discontinuity we have been able to supplement these long wavelength 1500 5000 km observations with an estimate of intrapath topography inferred from the frequency dependence of the 660 km reflection coefficient which gives peak to peak variations less than 40 km on scale lengths of 500 1500 km for the 660 km discontinuity the observed topography is significantly less than the dynamic topography expected for a simple compositional interface in regions of subduction however we do observed an intriguing negative correlation between the apparent depth and the reflection coefficient of the 660 km discontinuity which may involve small compositional variations plausible explanations for this correlation include heating of an initially cool chemical boundary layer gravitationally trapped above an endothermic phase transition loss of reflected energy due to local curvature or roughness of the reflector and or extreme topography occurring on a small compositional component to the discontinuity 40 of the total impedance contrast by stripping the 410 km and 660 km peaks from the composite path reflectivity profile we identify three minor reflectors at mean depths of 520 710 and 900 km the shallowest may mark the phase spinel phase transition the 710 km discontinuity can be attributed to the ilmenite gamet perovskite phase transition although changes in perovskite symmetry can potentially provide explanations for both the 710 km and 900 km features

the saturnian ribbon feature a baroclinically unstable model images of saturn from the two voyager spacecraft showed few discrete atmospheric features in the low contrast latitudinal banding over the planet one unusual feature however was a narrow dark oscillatory structure encircling the planet in the light zone at 45 to 50 north this paper presents a description of this dark structure which we call the ribbon feature a wavelength of 5710 260 km is obtained from moderate resolution images which were used to produce a 360 image of the feature higher resolution image pairs were used to examine the local circulation which shows counterclockwise rotation north of the ribbon and clockwise rotation south of it the zonal circulation measurements show that it is located in a 120 m sec 1 westerly jet but with a small 5 m sec 1 meridional velocity the infrared spectrometer results from voyager showed the greatest thermal wind sheer in this latitude region the regions adjacent to the ribbon are of similar brightness except for the uv 3400 a filter images where the northern part of this zone is lighter than the southern section from this observation we have estimated that the disturbance associated with the ribbon feature extends to at least the 200 mbar level a study of a 120 section of the ribbon feature and the associated latitudinal thermal gradient by l a sromovsky h e revercomb r j krauss and v e suomi 1983 j geophys res 88 8650 8666 led them to suggest that it was produced by some form of baroclinic instability this proposal is considered here with other alternative types of instability we conclude that the ribbon feature could arise from a baroclinic instability we then develop a numerical model of such an instability that is consistent with the observed wavelength the vertical extent of the ribbon and the local latitudinal thermal profile

410 km discontinuity sharpness and the form of the olivine α β phase diagram resolution of apparent seismic contradictions summary estimates of the thickness of the 410 km seismic discontinuity believed to be due to the olivine modified spinel transformation in olivine are as low as 4 km based on discontinuity reflectivity the seismically estimated thickness is however biased to values narrower than the true transformation interval if linear interpolation of properties is used for modelling a 5 km linear velocity gradient yields an average reflection coefficient identical to that of a 10 km transition interval based on olivine phase diagram features moreover alternative forms of the phase diagram equally consistent with experimentally determined iron magnesium partitioning can yield true transition intervals as narrow as 4 km this reconciles a discrepancy between phase equilibrium and seismic measures of discontinuity thickness in two ways 1 seismic thickness estimates are too narrow and 2 narrow transition intervals are permissible given existing phase equilibrium constraints incorporating recent results on the influence of h2o on discontinuity properties it appears that 410 km discontinuity reflectivity is much more sensitive to varying h2o concentration than to temperature suggesting that discontinuity reflectivity variations reflect changes in mantle chemistry

rheology of the upper mantle a synthesis rheological properties of the upper mantle of the earth play an important role in the dynamics of the lithosphere and asthenosphere however such fundamental issues as the dominant mechanisms of flow have not been well resolved a synthesis of laboratory studies and geophysical and geological observations shows that transitions between diffusion and dislocation creep likely occur in the earth s upper mantle the hot and shallow upper mantle flows by dislocation creep whereas cold and shallow or deep upper mantle may flow by diffusion creep when the stress increases grain size is reduced and the upper mantle near the transition between these two regimes is weakened consequently deformation is localized and the upper mantle is decoupled mechanically near these depths

ps conversions from the upper mantle transition zone beneath the canadian landmass broadband teleseismic p waveforms recorded on the canadian national seismograph network have been processed to isolate near receiver ps conversions originating from upper mantle discontinuities at approximately 410 and 660 km depth a simultaneous deconvolution procedure which removes source effects and exploits the move out characteristics of the weak ps phases is used to raise signal above the noise levels in the p coda large amplitude arrivals exhibiting times and slownesses consistent with an interpretation as ps converted phases from the 410 and 660 km discontinuities are observed at 18 and 20 of the 24 stations examined respectively the arrival times corrected for crustal heterogeneity show strong correlation with upper mantle structure above 410 km depth as determined in previous studies stations in the cordillera above low velocity mantle are characterized by residuals up to 3 3 s slow relative to the iasp91 model whereas sites overlying the high velocity root below the canadian shield are advanced by as much as 1 9 s the difference between the p660s and p410s times provides information on the structure of the transition zone beneath canadian stations the trend of the data indicates that topography on the discontinuities is at or below resolvable levels 5 km over most of the region in general accord with a previous study of the transition zone beneath continental stations but in contrast with results for the pacific basin

seismic evidence for very deep roots of continents abstract a major problem in geodynamics with seismic data is discussed how deeply do the continents penetrate into the mantle differential travel times of underside reflections from mantle discontinuities that appear as precursors to ss in large parts of the globe show a clear correlation with oceans and continents they are significantly larger beneath the asian and north american continents than underneath the neighbouring pacific from this observation we conclude that the asian and north american continents affect the mantle well below 410 km changes in the thickness of the transition zone can explain our observations which are in agreement with the hypothesis of petrological phase changes causing the 410 km and 660 km seismic discontinuities average thickness of the transition zone underneath continents is about 14 kim thicker than beneath oceans moreover our findings imply temperature variations about 100 200 k in the mantle transition zone weak reflections from 520 km depth corresponding to an impedance contrast of about 2 can be observed only in some areas of the earth while observations from other locations definitively show no signal from this depth therefore we propose that the 520 km reflector is only a regional feature

shear wave splitting continental keels and patterns of mantle flow in this study we investigated the origin of seismic anisotropy in the mantle beneath north america in particular we evaluated whether shear wave splitting patterns in eastern north america are better explained by anisotropy caused by lithospheric deformation anisotropy due to mantle flow beneath the lithosphere or a combination of both we examined new measurements of shear wave splitting from the missouri to massachusetts broadband seismometer array moma the north american mantle anisotropy and discontinuity experiment nomad as well as splitting parameters from several previous studies we developed a simple finite difference model that approximates mantle flow around a complex three dimensional continental lithospheric keel to evaluate potential anisotropy from mantle flow beneath the lithosphere in eastern north america we compared shear wave splitting observations to predicted splitting parameters calculated using this mantle flow model our results indicate that a significant portion of observed shear wave splitting in eastern north america can be explained by mantle flow around the continental keel however shear wave splitting patterns in a few regions of eastern north america indicate that a component of lithospheric anisotropy must exist particularly in regions containing the largest keel thicknesses for eastern north america as well as for splitting observations in australia europe and south america we favor a model in which anisotropy is controlled by a combination of both lithospheric deformation and subcontinental mantle flow

using moma broadband array scs s data to image smaller scale structures at the base of the mantle scs s residuals obtained at stations of the missouri to massachusetts moma temporary broadband seismic array are used to delineate variations in seismic velocity structure above the core mantle boundary cmb at scales smaller than observable with tomographic models south american earthquakes recorded at moma reveal a slow velocity anomaly that is at least as small as the limit of the resolution of scs waves about 300 km across this is modeled as being within a region of fast velocities in whole mantle models the slow scs s residuals correlate well with a peak in scs s relative amplitudes the small region of slow shear velocity at the cmb could be a pocket of lower mantle rock trapped beneath the descending farallon slab or evidence of chemical boundary layer variations

the anisotropic seismic structure of the east european platform 1 we present models for the p and s velocity structure of the upper mantle beneath the east european platform including measurements of radial anisotropy the models were obtained by full waveform inversion of 3 component broadband seismograms from strike slip earthquakes located near the edge of the platform and recorded in russia and europe we used direct and multiply reflected body wave arrivals s ss p pp combined with fundamental mode love and rayleigh waves at source receiver distances between 15 and 50 to resolve mantle structure down to the 410 km discontinuity the platform is underlain by a radially anisotropic mantle lid extending to a depth of 200 km with a largely isotropic mantle below the model has a positive velocity gradient from 41 km to 100 km depth and a relatively uniform velocity structure from 100 km to 200 km depth with high sh and ph velocities 4 77 km s 8 45 km s shear anisotropy is uniform at 5 h v from 41 to 200 km depth drops to 2 from 200 to 250 km and is isotropic below that the average shear velocity from 100 to 250 km is also uniform at 4 65 km s and the drop in anisotropy is matched by a drop in h to 4 70 km s combined with an increase in v to 4 60 km s below 250 km there is a positive velocity gradient in both p and s velocity down to 410 km p anisotropy is not well resolved but p structure mimics the sh velocity structure suggesting that p is also anisotropic within the lid

temporal behavior of cloud morphologies and motions in saturn s atmosphere saturn s atmosphere displays a variety of temporal changes in its cloud morphology at synoptic and planetary scales hemispheric long term albedo variations which are more pronounced at ultraviolet and blue wavelengths have been well established and are apparently linked to the seasonal insolation cycle short term changes of the order of months have been observed in the spectral reflectivity of the belt zone pattern mainly at mid latitudes some appear to be related to discrete spot activity but for the majority of the changes no such signs of spot activity were detected up to a resolution 2000 km in particular the ribbon structure at 45 n could have been affected by this activity and probably is a transient phenomenon the polar belts at 65 north and south and those at 75 north hexagon and south show a long term stability in their location perhaps for more than a century the most prominent belts located in the equator and the equatorial zone are persistent features of saturn s globe their main changes took place during the development of a rare planetary scale disturbance known as great white spots only five such events have been observed at repetitive regular intervals of about one saturn year since the first report in 1876 and at different latitudes from the equator to 60 n they are probably convective in origin with the seasonal heating of the upper atmosphere acting as a trigger mechanism during its life cycle of 2 3 years the disturbance shows turbulent planetary scale patterns of clouds that might be related to a wave dynamical phenomenon other distinct nonaxisymmetric cloud systems at synoptic scale are very scarce in saturn s atmosphere we observed new isolated features at several latitudes during 1990 and 1991 these spots together with those available from historical records are used as tracers for determining the latitude dependence and time variability of saturn s zonal winds the winds tend to exhibit a long term stability in their latitudinal positions and average wind speed although some significant departures from the zonal mean voyager profile have been detected during the development of the equatorial disturbance in 1990 and 1991

constraining seismic velocity and density for the mantle transition zone with reflected and transmitted waveforms we examine stacks of several seismic phases having different sensitivities to mantle transition zone structure when analyzed separately underside p and s reflections pdp and sds are suggestive of very different structures despite similar raypaths and data coverage by stacking the radial component of pdp rather than the vertical pdp we show that this difference does not result from interference from other more steeply inclined phases such as pkp and ppdpdiff in general stacks of p to s converted phases pds appear to lack evidence of a 520 km discontinuity when examined without other phases when these phases and stacked topside p reflections ppdp are analyzed jointly using a nonlinear inversion method consistent but nonunique seismological models emerge these models show that a discontinuity at 653 km depth has smaller contrasts in density and velocity than found in most previous studies a sub 660 gradient can account for the majority of this difference a 1 6 0 5 p velocity contrast and a 2 2 0 3 density contrast at 518 km depth without a s velocity contrast can explain the lack of a p520s together with robust pp520p and s520s phases for models parameterized with a finite thickness for each discontinuity the 410 km discontinuity is consistently 3 times thicker than the 660 km discontinuity

transport of late pleistocene loess particles by katabatic winds during the lowstands of the english channel studies of the submerged pleistocene conglomerate cored and dredged in the english channel show that it resulted from the cementation of stony beaches under a loess blanket although the loess cover was later washed out by younger transgressions the northern limit of the conglomerate corresponds to the original offshore extension of the loess deposits compilation of offshore and onshore altitudes of the limits of these deposits shows that loess was deposited by low level wind fields never thicker than 200 m mapping surveys show that the present limits of loess are probably close to their original boundaries the past proximity of the british ice sheet the accumulation of dusty sediments coating north facing cliffs of brittany originating in the outwash of the british irish ice sheet and in the palaeo rivers of the english channel as well as the southeastward orientation of the palaeo winds deduced from particle size analysis and heavy minerals distribution suggest that large volumes of loess were transported by strong katabatic winds blowing from the northern ice covered regions towards brittany and normandy the existence of katabatic winds is also consistent with the drift of ice rafts that transported icelandic basalts now grounded in brittany and possibly with the orientation of homo neandertalensis shelters

the great white spot and disturbances in saturn s equatorial atmosphere during 1990 a giant storm the great white spot erupted at the end of september 1990 as a localized bright cloud system close to saturn s equator its evolution produced a complex planetary disturbance which affected the whole equatorial region a month later similar spots have appeared approximately once every saturnian year about 30 earth years implying that a seasonal change in solar heating for example may be responsible for their occurrence

characterising saturn s vertical temperature structure from cassini cirs abstract thermal infrared spectra of saturn from 10 1400 cm 1 at 15 cm 1 spectral resolution and a spatial resolution of 1 2 latitude have been obtained by the cassini composite infrared spectrometer flasar f m and 44 colleagues 2004 space sci rev 115 169 297 many thousands of spectra acquired over eighteen months of observations are analysed using an optimal estimation retrieval code irwin p g j parrish p fouchet t calcutt s b taylor f w simon miller a a nixon c a 2004 icarus 172 37 49 to retrieve the temperature structure and para hydrogen distribution over saturn s northern winter and southern summer hemispheres the vertical temperature structure is analysed in detail to study seasonal asymmetries in the tropopause height 65 90 mbar the location of the radiative convective boundary 350 500 mbar and the variation with latitude of a temperature knee between 150 and 300 mbar which was first observed in inversions of voyager iris spectra hanel r and 15 colleagues 1981 science 212 192 200 hanel r conrath b flasar f m kunde v maguire w pearl j c pirraglia j samuelson r cruikshank d p gautier d gierasch p j horn l ponnamperuma c 1982 science 215 544 548 uncertainties due to both the modelling of spectral absorptions collision induced absorption coefficients tropospheric hazes helium abundance and the nature of our retrieval algorithm are quantified temperatures in the stratosphere near 1 mbar show a 25 30 k temperature difference between the north pole and south pole this asymmetry becomes less pronounced with depth as the radiative time constant for the atmospheric response increases at deeper pressure levels hemispherically symmetric small scale temperature structures associated with zonal winds are superimposed onto the temperature asymmetry for pressures greater than 100 mbar the para hydrogen fraction in the 100 400 mbar range is greater than equilibrium predictions for the southern hemisphere and parts of the northern hemisphere and less than equilibrium predictions polewards of 40 n the temperature knee between 150 300 mbar is larger in the summer hemisphere than in the winter smaller and higher at the equator deeper and larger in the equatorial belts and small at the poles solar heating on tropospheric haze is proposed as a possible mechanism for this effect the increased efficiency of ortho to para hydrogen conversion in the southern hemisphere is consistent with the presence of larger aerosols in the summer hemisphere which we demonstrate to be qualitatively consistent with previous studies of saturn s tropospheric aerosol distribution

evidence for a stagnant plume in the transition zone the structure of the mantle transition zone mtz beneath the pacific ocean is investigated by using pds waves converted from p to s from seismic discontinuities at depths d in the receiver region at stations within the south pacific superswell the pulse of p660s is anomalously broad to explain this effect we assume that the 660 km phase boundary may present a barrier for a thermochemical plume ascending from the lower mantle then the low velocity material of the plume accumulates just below this boundary and forms a large scale lens which broadens the pulse of p660s the phase boundary is uplifted by heating but the related travel time anomaly is cancelled by the increased effective width of the transition

hubble space telescope observations of the 1990 equatorial disturbance on saturn zonal winds and central meridian albedos abstract two sets of hubble space telescope hst data obtained in august 1990 and november 1990 are compared with voyager 1 and 2 data acquired in 1980 and 1981 a rare disturbance developed in the equatorial region of saturn in late september 1990 due to the large equatorial wind shear this disturbance generated inhomogeneous bright cloud structures in the equatorial region that extended to 25 north latitude in november 1990 meridional albedo scans and wind measurements from the voyager northern spring equinox and the hst northern summer observations are compared differences in average zonal winds in the images obtained with the green f547m and the methane f889n filters are interpreted as evidence of vertical shear in the zonal winds longitudinal variations in the equatorial zonal winds are interpreted as evidence of interaction between the storm nucleus generated during the disturbance and the local wind field

holocene north atlantic surface circulation and climatic variability evidence from diatom records diatom analyses have been performed on a 425cm long holocene marine sediment sequence from the north atlantic south of iceland and a 920cm sediment core spanning the last c 7500 years from skalafjord faeroe islands additional core data include stable isotopes and results from magnetic susceptibility measurements while chronostratigraphic control has been provided by ams 14c measurements the diatom records reveal distinct changes in north atlantic surface circulation and climate that have been correlated with published terrestrial and marine records initial holocene warming is dated at 9900 14c years bp and after a preboreal cold spell enhanced north atlantic current activity and warming prevailed from 9600 to 8800 14c years bp from 8800 to 8000 4c years bp the advection of warm atlantic water masses weakened presumably under atmospheric circulation conditions characterized by a dominating negative nao prior to the holocene climatic optimum 6000 5000 14c years bp distinct hydrographic gradient

upper mantle shear and compressional velocity structure of the central us craton shear wave low velocity zone and anisotropy one dimensional upper mantle velocity structure is estimated by modeling p sv and sh body waveforms sampling the central united states the resulting model features a thick mantle lid with a positive velocity gradient a significant shear wave low velocity zone 6 velocity reduction is required below the lid however the compressional velocities are not reduced the paths studied lie along the fast axes of teleseismic shear wave splitting measurements and absolute plate motion we observe splitting of upper mantle refracted shear waves with the transverse component slow relative to the radial component in order to fit the shear wave arrivals and satisfy teleseismic shear wave splitting observations we infer azimuthal anisotropy maximum 1 6 distributed throughout the mantle lid and low velocity zone with increasing anisotropy with depth

global de correlation of the topography of transition zone discontinuities analysis of long period shear wave underside reflections from upper mantle discontinuities shows the shape of the 410 km discontinuity to be neither globally nor locally correlated with that of the 660 km discontinuity the long wavelength pattern of 410 topography is dominated by degree 1 spherical harmonics and has a reasonable correlation with the seismic velocity perturbations in the mantle above it the global topography of 660 dominated by degree 2 spherical harmonics is strongly correlated with shear wave velocity variations in the transition zone the poor correlation between the observed topography of the 410 and 660 km discontinuities is inconsistent with a simple model of large scale whole mantle flow thus implying a more complex pattern which includes at least temporary accumulation of subducted material in the transition zone ringwood and irifune nature 331 1988 131 136 tackley et al nature 361 1993 699 704 the modest seismic discontinuity at 520 km is well observed when the reflections are under mid age oceans but is not observed when they are under continental shields interpretation of this observation in terms of olivine depletion in the deep continental mantle indicates the possible existence of continent ocean chemical heterogeneity at depths exceeding 500 km

earth s gravitational field seismic tomography resolves the enigma of the laurentian anomaly we demonstrate that the negative free air gravity anomaly over hudson bay is not due to glacial isostatic disequilibrium but rather to the influence of large scale mantle convection

upper mantle seismic discontinuities and the thermal structure of subduction zones the precise depths at which seismic velocities change abruptly in the upper mantle are revealed by the analysis of data from hundreds of seismometers across the western united states the boundary near 410km depth is locally elevated that near 660 km depressed the depths of these boundaries which mark phase transitions provide an in situ thermometer in subduction zones the observed temperature contrasts require at least moderate thickening of the subducting slab near 660 km depth in addition a reflector near 210 km depth may mark the bottom of the aesthenosphere

the origin and evolution of saturn s 2011 2012 stratospheric vortex the planet encircling springtime storm in saturn s troposphere december 2010 july 2011 fletcher l n et al 2011 science 332 1413 1414 sanchez lavega a et al 2011 nature 475 71 74 fischer g et al 2011 nature 475 75 77 produced dramatic perturbations to stratospheric temperatures winds and composition at mbar pressures that persisted long after the tropospheric disturbance had abated thermal infrared ir spectroscopy from the cassini composite infrared spectrometer cirs supported by ground based ir imaging from the visir instrument on the very large telescope and the mirsi instrument on nasa s irtf is used to track the evolution of a large hot stratospheric anticyclone between january 2011 and march 2012 the evolutionary sequence can be divided into three phases i the formation and intensification of two distinct warm airmasses near 0 5 mbar between 25 and 35 n b1 and b2 between january april 2011 moving westward with different zonal velocities b1 residing directly above the convective tropospheric storm head ii the merging of the warm airmasses to form the large single stratospheric beacon near 40 n b0 between april and june 2011 disassociated from the storm head and at a higher pressure 2 mbar than the original beacons a downward shift of 1 4 scale heights approximately 85 km post merger and iii the mature phase characterised by slow cooling 0 11 0 01 k day and longitudinal shrinkage of the anticyclone since july 2011 peak temperatures of 221 6 1 4 k at 2 mbar were measured on may 5th 2011 immediately after the merger some 80 k warmer than the quiescent surroundings from july 2011 to the time of writing b0 remained as a long lived stable stratospheric phenomenon at 2 mbar moving west with a near constant velocity of 2 70 0 04 deg day 24 5 0 4 m s at 40 n relative to system iii longitudes no perturbations to visible clouds and hazes were detected during this period with no direct tracers of motion in the stratosphere we use thermal windshear calculations to estimate clockwise peripheral velocities of 200 400 m s at 2 mbar around b0 the peripheral velocities of the two original airmasses were smaller 70 140 m s in august 2011 the size of the vortex as defined by the peripheral collar was 65 longitude 50 000 km in diameter and 25 latitude stratospheric acetylene c2h2 was uniformly enhanced by a factor of three within the vortex whereas ethane c2h6 remained unaffected the passage of b0 generated a new band of warm stratospheric emission at 0 5 mbar at its northern edge and there are hints of warm stratospheric structures associated with the beacons at higher altitudes p 0 1 mbar than can be reliably observed by cirs nadir spectroscopy analysis of the zonal windshear suggests that rossby wave perturbations from the convective storm could have propagated vertically into the stratosphere at this point in saturn s seasonal cycle one possible source of energy for the formation of these stratospheric anticyclones

composition of the crust in the grenville and appalachian provinces of north america inferred from v p v s ratios we use the ratios between p and s wave velocities vp vs derived from seismic refraction data to infer the composition of the crust in the grenville and the appalachian provinces of north america the crust exhibits vp vs increasing with depth from 1 64 to 1 84 there is a clear distinction between the grenville province average vp vs 1 81 and the appalachian province average vp vs 1 73 which persists at all depths the boundary between these provinces is east dipping extending for 100 km east of the champlain thrust in the appalachian province the increase in vp vs ratios with depth from 1 67 to 1 74 0 02 may reflect a normal decrease of silica content in the continental crust in the grenville province beneath the central granulite terrane an anomalous vp vs ratio of 1 82 0 02 is observed extending to a depth of 10 km this correlates with the abundance of ca plagioclase in the marcy anorthosite at greater depth 15 20 km where seismic lamination and high electrical conductivity is observed vp vs is 1 84 0 02 and correlates with the tahawus complex a layered mafic intrusion within the 25 km thick lower crust of the grenville province the vp vs is 1 84 0 02 and p velocity is 7 0 0 1 km s which are typical for plagioclase bearing rocks gabbro norite the high vp vs ratio in the grenville province has not been reported in crust of any other age since the grenville province contains 75 of the world s known anorthosites high vp vs ratio is related to high plagioclase we suggest that the composition of the grenville lower crust was significantly modified by the emplacement of the anorthosites in the mid proterozoic

temperatures winds and composition in the saturnian system stratospheric temperatures on saturn imply a strong decay of the equatorial winds with altitude if the decrease in winds reported from recent hubble space telescope images is not a temporal change then the features tracked must have been at least 130 kilometers higher than in earlier studies saturn s south polar stratosphere is warmer than predicted from simple radiative models the c h ratio on saturn is seven times solar twice jupiter s saturn s ring temperatures have radial variations down to the smallest scale resolved 100 kilometers diurnal surface temperature variations on phoebe suggest a more porous regolith than on the jovian satellites

elevation of the 410 km discontinuity beneath the central tien shan evidence for a detached lithospheric root we analyzed p sv converted phases recorded by the kyrgyzstan broadband network knet to investigate the nature of phase transitions in the upper mantle beneath the central tien shan and the kazakh shield we find that p sv phases from the 410 km discontinuity recorded by several stations located along the range front are about 2 s earlier from beneath the tien shan than they are from beneath the kazach shield because the delay times at stations located on the kazakh shield are normal and seismic velocities in the upper mantle beneath this part of the tien shan apparently are low about 4 less than the kazakh shield to the north the early arrival of the conversion from 410 km implies that this discontinuity is elevated by about 20 km because the 410 km discontinuity can be elevated by the introduction of cold material our results are consistent with a model in which the lithosphere in this area developed a root during collision that later detached and is now residing near 410 km depth

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

shear wave splitting in the mantle ps phases distribution with depth of mantle anisotropy is uncertain to better constrain this distribution we analyse records of the mantle ps phases at the grf array in germany two components of these phases sv and t are detected by stacking teleseismic records of events well distributed in azimuth and distance the detected t component signals are generated not so much by splitting of sv as by conversion from p to sh at the mantle discontinuities to reconcile these observations with previously known observations of splitting in sks at least three anisotropic layers are required the data favour a model with nne oriented fast velocity at the top of the mantle this is consistent with the results of other seismic studies beneath this layer the fast direction of anisotropy changes by around 90 our analysis suggests as well that a layer at the base of the mantle transition zone mtz is anisotropic this anisotropy might arise in the convective boundary layer between the upper and the lower mantle

imaging mantle transition zone thickness with sds ss finite frequency sensitivity kernels summary we invert differential sds ss traveltime residuals measured from stacked waveforms and finitefrequency sensitivity kernels for topography on the 410 and 660 km discontinuities this approach yields higher resolution images of transition zone thickness than previous stacking methods which simply average smooth over topographic features apparent structure measured using simple stacking is highly dependent upon the bin size of each stack by inverting for discontinuity topography with a variety of bin sizes we can more accurately calculate the true structure the inverted transition zone model is similar to simple stack models with an average thickness of 242 km but the lateral variations in thickness are larger in amplitude and smaller in scale fast seismic velocities in 3 d mantle models such as sb4l18 correlate with areas of thicker transition zone the elongated curvilinear regions of thickened transition zone that occur near subduction zones are narrow and high amplitude which suggests relatively little lateral spreading and warming of subducted lithosphere within the transition zone the anomalously thin transition zone regions are laterally narrow and not broadly continuous if these variations in transition zone thickness are interpreted as thermal in nature then this model suggests significant temperature variations on small lateral scales

receiver functions in the western united states with implications for upper mantle structure and dynamics 1 investigations into mechanisms driving surface tectonics commonly search for mantle sources but few observations constrain flow in the upper mantle and transition zone here variations in the upper mantle discontinuities at 410 km and 660 km below the western united states are revealed through mapping depths of compressional to shear wave conversions recorded by broadband seismometers the resulting image exhibits 20 and 30 km of topography on the 410 and 660 km discontinuities similar depth variations to those seen where subducting slabs of lithosphere reach the transition zone the pattern of discontinuity topography imaged here does not correlate with the surface tectonics of the rocky mountains colorado plateau or basin and range province providing no support for upward and downward flow at transition zone depths controlling surface topography and deformation in this region at least at scale lengths of a few hundred kilometers furthermore we find no clear correlation between the depths of the 410 and 660 km discontinuities undulations on the surfaces of both discontinuities appear to be spaced at distances of 800 km if the topography were due only to lateral temperature differences such differences would be comparable to those where slabs sink and might suggest separate convective flow above and below the transition zone alternatively the topography may reflect lateral variations in composition variations in the sharpness of converted phases across the region offer some support for compositional heterogeneity but the lack of a correlation between sharpness and depth casts doubt on this explanation for the variations in depth

potential vorticity of saturn s polar regions seasonality and instabilities we analyze the potential vorticity of saturn s polar regions as it is a fundamental dynamical tracer that enables us to improve our understanding of the dynamics of these regions and their seasonal variability in particular we present zonally averaged quasi geostrophic potential vorticity maps between 68 planetographic latitude and the poles at altitudes between 500 and 1 mbar for three different epochs i june 2013 early northern summer for the north polar region ii december 2008 late northern winter for both polar regions and iii october 2006 southern summer for the south computed using temperature profiles retrieved from cassini composite infrared spectrometer data and wind profiles obtained from cassini s imaging science subsystem the results show that quasi geostrophic potential vorticity maps are very similar at all the studied epochs showing positive vorticities at the north and negative at the south indicative of the dominance of the coriolis parameter 2 sin at all latitudes except near the pole the meridional gradients of the quasi geostrophic potential vorticity show that dynamical instabilities mainly due to the barotropic term could develop at the flanks of the hexagon at 78 n the jet at 73 9 s and on the equatorward flank of both polar jets there are no differences in potential vorticity gradients between the two hemispheres that could explain why a hexagon forms in the north and not in the south no seasonal variability of the potential vorticity and its meridional gradient has been found despite significant changes in the atmospheric temperatures over time

an investigation on slowness weighted ccp stacking and its application to receiver function imaging common conversion point ccp stacking of receiver functions is a widely used technique to image velocity discontinuities in the mantle the ccp imaging technique assumes that receiver functions are composed solely of p to s conversions at velocity boundaries whose depths can be mapped out through their arrival times the multiple reflections at shallow boundaries with large velocity contrasts such as the base of unconsolidated sediments and the moho can lead to artificial structures in the ccp images we develop a refined ccp stacking method that uses relative slowness as a weighting factor to suppress the multiples slowness weighted ccp stacking swccp we conduct extensive numerical tests with synthetic data to seek the best weighting scheme and to verify the robustness of the images we apply this technique to receiver function data of necessarray in china and the transportable array in western us and find most of the events in the depth range of 200 400 km shown in the regular ccp images are eliminated the swccp images on the other hand reveal a clear negative event under some parts of the two arrays indicating the presence of low velocity layer above the 410 km discontinuity which was reported by previous studies

a seasonal model of the saturnian upper troposphere comparison with voyager infrared measurements an extension of the seasonal climate model of r d cess and j caldwell 1979 icarus 38 349 357 to saturn s upper troposphere is presented the ring modulated latitudinal dependence of the insolation the ring thermal emission the oblateness of the planet the orbit eccentricity and the latitudinal variation of the internal heat flux are taken into account calculations agree closely with the temperature latitude profiles retrieved from voyager iris measurements at atmospheric levels located above the 0 2 bar pressure level they reproduce the observed large scale hemispheric asymmetry which is then shown to result from the seasonally variable insolation aerosol absorption is found to be the dominant source of atmospheric solar heating in the troposphere and the model suggests an aerosol mean unit optical depth around the 0 25 bar level in the equatorial region and around the 0 35 bar level at other latitudes the model fails to predict the retrieved temperature latitude profiles below the 0 3 bar level this discrepancy is attributed to the existence of clouds at these levels which are responsible for an additional far infrared opacity not taken into account in the temperature retrieval the cloud top altitude would be about 0 3 bar except in the 20 to 40 n region where these clouds would be confined below the 0 6 bar level the poor correlation between infrared measurements and visible images is discussed and a possible model of saturn s cloud structure is proposed

thermal structure of saturn from voyager infrared measurements implications for atmospheric dynamics abstract saturn atmospheric temperatures at the 150 mbar level retrieved from voyager iris measurements indicate the presence of small scale meridional gradients which are approximately symmetric with respect to the equator but are superposed on a large scale hemispheric thermal asymmetry under the assumption that the retrieved values at this atmospheric level represent kinetic temperatures on a constant pressure surface it is suggested that the small scale structure is produced by a meriodional circulation associated with the dissipative decay of the zonal winds with height while the hemispheric asymmetry represents a thermal response to the seasonally varying insolation the small scale gradients are correlated with zonal winds derived from voyager images at mid and high latitudes through the thermal wind relation the calculated thermal wind shears suggest a decay with height of the jet system toward a state of uniform eatward flow the existence of the approximately symmetric zonal winds and associated temperature gradients in the presence of a large scale seasonal thermal response suggests that the jet system is driven at depths substantially below the levels where seasonally modulated insolation is important p 0 5 bar

upper mantle s velocity structure of north america we have inverted fundamental and higher mode rayleigh waveforms from 685 vertical component broadband seismograms with wave paths over north america to obtain an image of the upper mantle s velocity structure down to 660 km among the well resolved features of the new model are 1 a high velocity root beneath the north american craton which extends no deeper than 250 km except near the archean core of the craton where depths of 350 km are reached 2 a weak band of low velocity along the eastern margin of the north american craton which reaches into the transition zone 3 a low velocity slab window beneath the western united states down to a depth of 300 km 4 areas of low uppermost mantle velocity beneath the cascade volcanoes the yellowstone hotspot track the colorado plateau the sierra madre occidental and the grabens bordering the jalisco block and 5 a pronounced band of high velocities in the transition zone coinciding with the expected location of the subducted trailing fragments of the farallon plate we introduce several improvements to the method of partitioned waveform inversion which was used to compute the new model rather than to correct for an estimated depth to the mohorovicic discontinuity we leave the crustal thickness as a free though yet poorly resolved parameter in the inversion we also improve the windowing and filtering operator used to select uncontaminated waveforms

phase transition and thermal equations of state of fe al bridgmanite and post perovskite implication for the chemical heterogeneity at the lowermost mantle abstract in this study we have determined the phase boundary between mg0 735fe0 21al0 07si0 965o3 bm and ppv and the thermal equations of state of both phases up to 202 gpa and 2600 k using synchrotron x ray diffraction in laser heated diamond anvil cells our experimental results have shown that the combined effect of fe and al produces a wide two phase coexistence region with a thickness of 26 gpa 410 km at 2200 k and addition of fe lowers the onset transition pressure to 98 gpa at 2000 k consistent with previous experimental results furthermore addition of fe was noted to reduce the density and bulk sound velocity v contrasts across the bm ppv phase transition which is in contrast to the effect of al using the obtained phase diagram and thermal equations of state of bm and ppv we have also examined the effect of composition variations on the and v profiles of the lowermost mantle our modeling results have shown that the pyrolitic lowermost mantle should be highly heterogeneous in composition and temperature laterally to match the observed variations in the depth and seismic signatures of the d discontinuity normal mantle in a pyrolitic composition with 10 fe and al in bm and ppv will lack clear seismic signature of the d discontinuity because the broad phase boundary could smooth the velocity contrast between bm and ppv on the other hand fe enriched regions close to the cold slabs may show a seismic signature with a change in the velocity slope of the d discontinuity consistent with recent seismic observations beneath the eastern alaska only regions depleted in fe and al near the cold slabs would show a sharp change in velocity fe in such regions could be removed to the outer core by strong core mantle interactions or partitions together with al to the high pressure phases in the subduction mid ocean ridge basalts our results thus have profound implication for the composition of the lowermost mantle

p and s wave tomography of the mantle beneath the united states mantle seismic structure beneath the united states spanning from the active western plate margin to the passive eastern margin was imaged with teleseismic p and s wave traveltime tomography including usarray data up to may 2014 to mitigate artifacts from crustal structure 5 40 s rayleigh wave phase velocities were used to create a 3 d starting model major features of the final p and s models include two distinct low velocity anomalies at depths of 60 300 km beneath the central and northern appalachians and passive margin the central appalachian low velocity anomaly coincides with eocene basaltic magmatism and the northern anomaly is located along the cretaceous track of the great meteor hot spot at depths of 300 700 km beneath the central and eastern u s large high velocity anomalies are inferred to be remnants of the farallon slab that subducted prior to 40 ma during the laramide orogeny

crustal structure across the san andreas fault southern california from teleseismic converted waves crustal structure along a 140 km long profile across the san andreas fault saf in southern california was imaged by stacking teleseismic p s converted phases recorded by a dense short period seismic array the crust mantle discontinuity moho is visible as a continuous feature at a depth around 30 km but is offset 6 to 8 km beneath the saf a small moho disruption can also be seen under the eastern california shear zone ecsz these results suggest that the saf and ecsz extend through the entire crust the moho upwarp under the san gabriel mountains indicates that the mountain ranges were lifted en masse as a result of crustal buckling under horizontal compression

the onset and growth of the 1990 equatorial disturbance on saturn abstract the development of cloud structures at low saturnian latitudes in september 1990 can be divided into three phases the onset and expansion of the initial disturbance popularly known as the great white spot the eastward and westward expansion of the bright cloud and the eventual formation of wave like structures encircling the equator the three phases of this storm are consistent with a single convective event this event is evaluated within the historical context of previous occurrences and general seasonal mechanisms are considered

saturn s zonal wind profile in 2004 2009 from cassini iss images and its long term variability abstract five years of cassini imaging science subsystem images from 2004 to 2009 are analyzed in this work to retrieve global zonal wind profiles of saturn s northern and southern hemispheres in the methane absorbing bands at 890 and 727 nm and in their respective adjacent continuum wavelengths of 939 and 752 nm a complete view of saturn s global circulation including the equator at two pressure levels in the tropopause 60 mbar to 250 mbar with the mt filters and in the upper troposphere from 350 mbar to 500 mbar with the cb filter set is presented both zonal wind profiles available at the supplementary material section show the same structure but with significant differences in the peak of the eastward jets and the equatorial region including a region of positive vertical shear symmetrically located around the equator between the 10 c

seismic shear wave reflectivity structure of the mantle beneath northeast china and the northwest pacific 1 using multiple scs reverberations we examine the mantle reflectivity structure beneath northeast china and the northwest pacific we find several upper mantle discontinuities including a melt layer with a mean thickness of 64 km atop the 410 km discontinuity present on both sides of the subducting slab near the nankai trench the transition zone contains a split 520 km discontinuity in several paths and tomographic images show stagnant slabs at this depth we believe that this may be slab related based on experimental work a negative reflector is found in one path beneath the northeast china craton at a depth of 598 km mid mantle reflectors are found in all of our paths and are present throughout a wide depth range 750 1600 km

neptune s stratospheric winds from three central flash occultations abstract we present observations of three neptune central flash events the 20 august 1985 occultation of n39 from the european southern observatory eso the 12 september 1988 occultation of n51 from pic du midi and the 8 july 1989 occultation of n55 from eso from simultaneous fits to the three central flash lightcurves we determine the shape of neptune s limb and show that winds near the 0 38 mbar level have decayed to about 0 6 0 2 times their strength at 100 mbar assuming that the latitude dependence of the stratospheric winds follows the zonal wind profile of l a sromovskyet al 1993 icarus105 110 141 from the measured decay of winds with height we find that the average vertical shear in the zonal wind between 100 and 0 38 mbar is very close to the voyager iris results for the 30 to 120 mbar region b j conrathet al 1989 science246 1454 1459 at the latitudes to which the central flashes are most sensitive we also determine the shape of the limb at the 0 7 bar level from the half light points of the atmospheric immersion and emersion light curves of five stellar occultations by neptune the winds in this pressure regime have decayed to 0 17 times their strength at the 100 mbar level and the vertical shear in the zonal wind as determined from the thermal wind equation and voyager iris measurements extends essentially unchanged all the way up to the microbar level of the stratosphere alternatively a simple oblate model fit to the planetary limb gives an oblateness of 0 0180 0 0010 and an equatorial radius ofre 25262 7 3 5 km the corresponding rotation period is 16 59 0 92 h quite similar to that of neptune s deep interior 16 11 h this near corotation suggests that there is some coupling between neptune s interior and the upper stratosphere even though the intervening atmosphere between the cloud deck and the stratosphere has strong retrograde flow

the free air gravity constraint on subcontinental mantle dynamics an outstanding geophysical issue concerns the nature and dynamical role in the mantle general circulation of the seismically fast body wave anomalies that have been tomographically imaged beneath coatinents in this paper we investigate the possibilities that these seismologically imaged roots represent either neutrally buoyant chemically distinct material or cold negatively buoyant upper mantle and transition zone downwelling flow in assessing these alternatives we first construct disaggregated models of the seismic heterogeneity in which a component associated with subcontinental fast anomalies is isolated from the global tomographic models either by employing the continent function or a new craton function we find that the use of the new craton function leads to geophysically more realistic chemical models of subcontinental heterogeneity the thermal and chemical density fields reconstructed from the disaggregated tomographic models are employed to compute the long wavelength nonhydrostatic geoid the free air gravity field and the upper mantle radial flow pattern within the framework of an anelastically compressible internal loading theory we find that the radial component of flow velocity provides useful insight into the dynamical implications of the alternative density models however since this field is not directly observable we consider the geoid and free air gravity anomaly as possible diagnostic discriminants and show that the free air gravity anomaly provides a sensitive discriminant of the gravitational differences that characterize the chemical and thermal models whereas the geoid does not by focusing on the free air gravity low over the hudson bay region of canada we are able to rule out the hypothesis that positively or neutrally buoyant subcontinental material that is chemically distinct from the surrounding mantle exists below the laurentian craton however when the fast body wave anomaly is mapped into a high density downwelling flow beneath this region we are able to fully explain the fraction of the anomalous free air gravity low which is inexplicable as a contribution associated with the existing degree of glacial isostatic disequilibrium due to the disintegration of the laurentide ice sheet this conclusion concerning the north american craton may be equally valid for other continental nuclei we explore the general tectonophysical implications of this dynamical model

the upper mantle discontinuities correlated or anticorrelated the discontinuities in the earth s upper mantle at 410km and 660km are frequently considered as phase changes which are exothermic and endothermic respectively from this follows that at laterally changing temperatures both discontinuities move in opposing vertical directions that means they are negatively correlated we attempt to test this hypothesis using travel times of seismic waves converted at both discontinuities underneath seismic stations the broadband stations used are located in areas of high e g western us and low heat flow e g central asia sclater et al 1980 we observe a clear positive correlation of the conversion times of both discontinuities which can however be explained with known lateral velocity changes above 410km from the scatter of our data we conclude that the limit of a possibly existing and negatively correlated topography of the two discontinuities is about 28km this is in contrast to recently claimed observations of about 35 km anticorrelated topography in the western pacific revenaugh jordan 1991 stammler kind 1992

saturn s cloud structure inferred from cassini iss abstract using high resolution cassini iss images with wavelengths ranging from the ultraviolet to the near infrared we have retrieved saturn s atmospheric aerosol structure and properties for a broad range of latitudes in the southern hemisphere the observations are consistent with two distinct layers of haze above a scattered cloud each layer is characterized by a vertical location an optical thickness and an effective mean particle size all of which vary with latitude the tropospheric haze is optically thickest and extends to the greatest heights 40 mbar over the equator its top surface is at significantly greater depths 150 mbar at mid latitudes the height of the haze correlates well with position of the tropopause as indicated by the temperature field beneath this haze we find a scattered denser cloud responsible for small scale contrasts at an average depth of 1 75 0 4 bar with some features as deep as 2 5 bar or greater

saturn s upper atmosphere during the voyager era reanalysis and modeling of the uvs occultations abstract the voyager 1 and 2 ultraviolet spectrometer uvs solar and stellar occultation dataset represents one of the primary pre cassini sources of information that we have on the neutral upper atmosphere of saturn despite its importance however the full set of occultations has never received a consistent nor complete analysis and the results derived from the initial analyses over thirty years ago left questions about the temperature and density profiles unanswered we have reanalyzed all six of the uvs occultations three solar and three stellar to provide an up to date pre cassini view of saturn s upper atmosphere from the voyager uvs data we have determined vertical profiles for h 2 h ch 4 c 2 h 2 c 2 h 4 and c 2 h 6 as well as temperature our analysis also provides explanations for the two different thermospheric temperatures derived in earlier analyses 400 450 k versus 800 k and for the unusual shape of the total density profile noted by hubbard et al 1997 aside from inverting the occultation data to retrieve densities and temperatures we have investigated the atmospheric structure through a series of photochemical models to infer the strength of atmospheric mixing and other physical and chemical properties of saturn s mesopause region during the voyager flybys we find that the data exhibit considerable variability in the vertical profiles for methane suggesting variations in vertical winds or the eddy diffusion coefficient as a function of latitude and or time in saturn s upper atmosphere the results of our reanalysis will provide a useful baseline for interpreting new data from cassini particularly in the context of change over the past three decades

saturn meteorology a diagnostic assessment of thin layer configurations for the zonal flow abstract voyager imaging infrared and radio observations for saturn have been recently interpreted by smith et al 1982 as an indication that the jet streams observed at the cloud tops extend to depths greater than the 104 bar level this analysis assumes a maximum latitudinal temperature contrast of a few percent a mean atmospheric rotation rate at depth given by saturn s ratio period and no variation with latitude of the bottom pressure level for the zonal flow system these assumptions are not however firmly constrained by observation the diagnostic analysis of plausible alternative configurations for saturn s atmospheric structure demonstrates that a thin weather layer system confined at mid to high latitudes to levels above 200 bar cannot be excluded by any of the available observations a quantitative estimate of the effects of moisture condensation including the differentiation of mean molecular weight suggests that these might provide the buoyancy contrasts necessary to support a thin layer flow provided that saturn s outer envelope is enriched approximately 10 times in water abundance relative to a solar composition atmosphere and strongly differentiated with latitude at the condensation level

continent ocean chemical heterogeneity in the mantle based on seismic tomography seismic models of global scale lateral heterogeneity in the mantle show systematic differences below continents and oceans that are too large to be purely thermal in origin an inversion of the geoid based on a seismic model that includes viscous flow in the mantle indicates that the differences beneath continents and oceans can be accounted for by differences in composition in the upper mantle superposed on mantle wide thermal heterogeneities the net continent ocean density differences integrated over depth are small and cause only a low flux of mass and heat across the asthenosphere and mantle transition zone

the elusive coldfinger the earth s continents can extend deep into the mantle and these continental roots are thought to be cooler than the surrounding mantle material so they could lower mantle temperature and even induce downwelling but a seismological study across eastern north america finds that there at least these features of a continental coldfinger are absent

effects of plate and slab viscosities on the geoid abstract the effects of plate rheology strong plate interiors and weak plate margins and stiff subducted lithosphere slabs on the geoid and plate motions considered jointly are examined with three dimensional spherical models of mantle flow buoyancy forces are based on the internal distribution of subducted lithosphere estimated from the last 160 ma of subduction history while the ratio of the lower mantle upper mantle viscosity has a strong effect on the long wavelength geoid as has been shown before we find that plate rheology is also significant and that its inclusion yields a better geoid model while simultaneously reproducing basic features of observed plate motion slab viscosity can strongly affect the geoid depending on whether a slab is coupled to the surface in particular deep high viscosity slabs beneath the northern pacific that are disconnected from the surface as a result of subduction history produce significant long wavelength geoid highs that differ from the observation this suggests that slabs in the lower mantle may be not as stiff as predicted from a simple thermally activated rheology if the slab model is accurate

the northern appalachian anomaly a modern asthenospheric upwelling the northern appalachian anomaly naa is an intense laterally localized 400km diameter low velocity anomaly centered in the asthenosphere beneath southern new england its maximum shear velocity contrast at 200km depth is about 10 and its compressional to shear velocity perturbation ratio is about unity values compatible with it being a modern thermal anomaly although centered close to the track of the great meteor hot spot it is not elongated parallel to it and does not crosscut the cratonic margin in contrast to previous explanations we argue that the naa s spatial association with the hot spot track is coincidental and that it is caused by small scale upwelling associated with an eddy in the asthenospheric flow field at the continental margin that the naa is just one of several low velocity features along the eastern margin of north america suggests that this process may be globally ubiquitous

general circulation and transport in saturn s upper troposphere and stratosphere abstract we present a model for the general circulation and dynamical transport in saturn s upper troposphere and stratosphere and derive the effective advective circulation and eddy transport coefficients required for use in two dimensional latitude altitude photochemistry transport models a three dimensional outer planet general circulation model opgcm is used to generate the transport data we find that the opgcm adequately captures the global scale pole to pole temperature contrast but overestimates mid and high latitude temperatures in the summer hemisphere by 5 k in addition the model reproduces the local temperature minimum seen at the equator in cassini composite infrared spectrometer cirs 0 1 mbar data but not the local maximum in 1 mbar temperatures suggesting that it is capturing the phase of saturn s semiannual oscillation associated with a temperature minimum at the equator but not the opposite phase the meridional circulation at low latitudes is found to be dominated by a seasonally reversing hadley circulation characterized by upwelling near the equator cross equatorial flow from summer to winter hemisphere and strong subsidence centered near 25 latitude in the winter hemisphere the cross equatorial flow induces an asymmetry in which the equatorial jet is found to be stronger in the winter than in the summer stratosphere the location of the subsidence near 25 n for l s 310 coincides with local maxima in acetylene diacetylene and methylacetylene mixing ratios measured by cassini cirs guerlet s fouchet t bezard b moses j i fletcher l n simon miller a a flasar f m 2010 icarus 209 682 695 this result supports the suggestion by guerlet et al 2010 that the hydrocarbon abundances are enhanced at this latitude by pronounced downward transport of hydrocarbon rich air from above the lateral eddy diffusion coefficient is found to typically be 10 5 10 6 m 2 s 1 at mid latitudes implying meridional eddy transport time scales of order 100 1000 years