lithospheric and upper mantle structure of southern tibet from a seismological passive source experiment fifteen seismic stations were operated with about 20 km spacing in southern tibet across the zangbo suture the collision zone between india and asia between may and october 1994 as part of the international deep profiling of tibet and the himalaya project indepth ii for wide angle recording of the controlled source experiment and for passive earthquake recording in addition a dense deployment 4 km spacing of stations within the german depth profiling of tibet and the himalayas gedepth project also recorded a number of teleseismic earthquakes the third data source used in this study is the records of the permanent broadband station at lhasa teleseismic records have been obtained in sufficient quantity and quality to derive an image of the structure of the lithosphere and upper mantle from p to s converted phases important results are as follows the moho at 70 80 km and a second discontinuity at 50 60 km depth are observed over the entire profile south and north of the zangbo suture the data from the gedepth dense array enable the detection of inclined structures penetrating the crust at the zangbo suture a pronounced low velocity zone exists north of the zangbo suture at about 10 20 km depth the locations of the upper mantle discontinuities at 410 and 660 km depth are in agreement with the global reference model iasp91 kennett 1991 over a large region of the himalaya and southern tibet

sound velocities of majorite garnet and the composition of the mantle transition region the composition of the mantle transition region characterized by anomalous seismic wave velocity and density changes at depths of approximately 400 to 700 km has remained controversial some have proposed that the mantle transition region has an olivine rich pyrolite composition whereas others have inferred that it is characterized by pyroxene and garnet rich compositions piclogite because the sound velocities in pyrolite estimated from laboratory data are substantially higher than those seismologically observed although the velocities of the olivine polymorphs at these pressures wadsleyite and ringwoodite have been well documented those of majorite another significant high pressure phase in the mantle transition region with realistic mantle compositions have never been measured here we use combined in situ x ray and ultrasonic measurements under the pressure and temperature conditions of the mantle transition region to show that majorite in a pyrolite composition has sound velocities substantially lower than those of earlier estimates owing to strong nonlinear decreases at high temperature particularly for shear wave velocity we found that pyrolite yields seismic velocities more consistent with typical seismological models than those of piclogite in the upper to middle parts of the region except for the potentially larger velocity jumps in pyrolite relative to those observed at a depth of 410 km in contrast both of these compositions lead to significantly low shear wave velocities in the lower part of the region suggesting possible subadiabatic temperatures or the existence of a layer of harzburgite rich material supplied by the subducted slabs stagnant at these depths

moho geometry and upper mantle images of northeast india 1 images of the crust and mantle beneath northeast india obtained by 2d migration of 1000 broadband p receiver functions clearly trace a northward dipping moho from the himalayan foredeep reaching depths up to 50 km further north beneath the himalayan convergence zone also these images reveal presence of largely coherent 410 km and 660 km discontinuities that conform to the iasp91 model marginal variations in the depth of the 410 km interface are observed that appear region specific the thickness of the mantle transition zone does not deviate significantly from a global average of 250 km interestingly our results reveal consistent presence of a signal from an interface around 300 km origin of such a boundary known as x discontinuity and unrelated to the lehmann discontinuity is discussed possible presence of the x discontinuity from the indian region is reported here for the first time

seismic observations of splitting of the mid transition zone discontinuity in earth s mantle the transition zone of earth9s mantle is delineated by globally observed discontinuities in seismic properties at depths of about 410 and 660 kilometers here we investigate the detailed structure between 410 and 660 kilometers depth by making use of regional stacks of precursors to the ss phase the previously observed discontinuity at about 520 kilometers depth is confirmed in many regions but is found to be absent in others there are a number of regions in which we find two discontinuities at about 500 and 560 kilometers depth an effect which can be interpreted as a splitting of the 520 kilometer discontinuity these observations provide seismic constraints on the sharpness and observability of mineralogical phase transitions in the mantle transition zone

distinct slab interfaces imaged within the mantle transition zone oceanic lithosphere descends into earth s mantle at subduction zones and drives material exchange between earth s surface and its deep interior the subduction process creates chemical and thermal heterogeneities in the mantle with the strongest gradients located at the interfaces between subducted slabs and the surrounding mantle seismic imaging of slab interfaces is key to understanding slab compositional layering deep water cycling and melting yet the existence of slab interfaces below 200 km remains unconfirmed here we observe two sharp and slightly dipping seismic discontinuities within the mantle transition zone beneath the western pacific subduction zone that coincide spatially with the upper and lower bounds of the high velocity slab based on a multi frequency receiver function waveform modelling we found the upper discontinuity to be consistent with the mohorovicic discontinuity of the subducted oceanic lithosphere in the mantle transition zone the lower discontinuity could be caused by partial melting of sub slab asthenosphere under hydrous conditions in the seaward portion of the slab our observations show distinct slab mantle boundaries at depths between 410 and 660 km deeper than previously observed suggesting a compositionally layered slab and high water contents beneath the slab two seismic discontinuities in the mantle transition zone beneath the western pacific represent subducted slab interfaces that could be the slab moho and partially molten sub slab asthenosphere according to an analysis of seismic data

upper mantle seismic shear discontinuities of the pacific 1 shear wave impedance discontinuities are inventoried for eight paths connecting circum pacific earthquakes with seismic stations on hawaii and oahu in addition to the transition zone discontinuities we observe a consistent impedance decrease at a depth of 80 km that marks the transition from the fast seismic lid to the low velocity zone the interval over which this transition occurs is less than 30 km the requisite impedance decrease ascribed almost entirely to diminished velocities in the low velocity zone exceeds predictions for peridotite chemistries of appropriate lithospheric age in all cases the transition is better matched by lithosphere of roughly half of the true age four paths show clear evidence of the x discontinuity an impedance increase of 3 8 near 300 km depth the only viable explanations would require extensive eclogite or basalt depleted mantle in the upper mantle of the western pacific the impedance contrast of the 410 km discontinuity which depends on the modal fraction of mantle olivine does not signal the presence of either although a mixture of the two remains a possibility we find no evidence of a low velocity layer atop of the 410 km discontinuity if present it is either consistently thin 15 km highly variable in thickness topography in excess of 60 km or has little impedance contrast 2 3 with overlying mantle the apparent absence of melt atop the 410 km discontinuity an intermittent and weak 520 km discontinuity and a thinned transition zone are consistent with relatively dry conditions in the deep upper mantle of the central pacific

ps mantle transition zone imaging beneath the colorado rocky mountains evidence for an upwelling hydrous mantle abstract we analyze teleseismic p to s conversions for high resolution imaging of the mantle transition zone beneath the colorado rocky mountains using data from a dense passcal seismic broadband deployment a total of 6 021 p to s converted receiver functions are constructed using a multi channel minimum phase deconvolution method and migrated using the common converted point technique with the 3 d teleseismic p and s wave tomography models of schmandt and humphreys 2010 the image finds that the average depths of the 410 km discontinuity the 410 and 660 km discontinuity the 660 at 408 1 9 km and 649 1 6 km respectively the peak to peak topography of both discontinuities is 33 km and 27 km respectively additionally prominent negative polarity phases are imaged both above and below the 410 to quantify the mean properties of the low velocity layers about 410 km we utilize double gradient layer models parameterization to fit the mean receiver function waveform this waveform fitting is accomplished as a grid search using anelastic synthetic seismograms the best fitting model reveals that the olivine wadsleyite phase transformation width is 21 km which is significantly larger than anhydrous mineral physics prediction 4 10 km smyth and frost 2002 the findings of a wide olivine wadsleyite phase transformation and the negative polarity phases above and below the 410 suggest that the mantle at least in the 350 450 km depth range is significantly hydrated furthermore a conspicuous negative polarity phase below the 660 is imaged in high velocity region we speculate the low velocity layer is due to dehydration flux melting in an area of convective downwelling our interpretation of these results in tandem with the tomographic image of a farallon slab segment at 800 km beneath the region schmandt and humphreys 2010 is that hydrous and upwelling mantle contributes to the high standing colorado rocky mountains

seismic evidence for accumulated oceanic crust above the 660 km discontinuity beneath southern africa 1 high pressure assemblages of subducted oceanic crust are denser than the normal upper mantle but less dense than the uppermost lower mantle thus subducted oceanic crust may accumulate at the base of the upper mantle direct observational evidence for this hypothesis however remains elusive we present an analysis of a negative polarity shear wave converted from compressional wave at a seismic discontinuity near 570 600 km depth beneath southern africa the negative polarity of the converted phase indicates a 2 2 0 2 s velocity decrease with depth at the seismic discontinuity this velocity reduction is associated however with a low velocity contrast at the 660 km discontinuity the exsolution of ca perovskite in former oceanic crust at depth greater than 600 km and the associated small volume fraction of ringwoodite are plausible explanations for the apparent paradox between the negative velocity discontinuity and the low velocity contrast at the 660 km discontinuity

heterogeneous distribution of water in the mantle transition zone inferred from wavefield imaging abstract the amount of water in the earth s deep mantle is critical for the evolution of the earth mineral physics studies revealed that the mantle transition zone can store several times the volume of water in an ocean however the actual water distribution in the transition zone remains enigmatic we used the highest resolution images produced of scatterers in the north american transition zone derived from teleseismic data recorded by the earthscope transportable array we find that the transition zone is filled with previously unrecognized small scale heterogeneities that produce pervasive negative polarity signals simulations demonstrated the images can be explained by low velocity bodies shaped as distributed blobs or near vertical structures we suggest these low velocity bodies may be heterogeneously distributed water enriched subducted harzburgites produced through long term accumulation

preferential detection of the lehmann discontinuity beneath continents we perform a global survey for the presence of the lehmann discontinuity using 20 000 long period ss precursors this data set is highly sensitive to upper mantle reflectors and the coverage is more complete than in previous studies our survey indicates that the lehmann discontinuity is a local feature that is observed under continents more than twice as often as it is observed under oceans we observe significant variations in travel times and waveforms associated with this shallow mantle reflector indicating its complexity and lateral depth variations little signal is detected on the continent scale stacks of the ss precursors which provides further evidence for the intermittent and variable nature of the lehmann discontinuity

phase transformations in subducted oceanic crust and buoyancy relationships at depths of 600 800 km in the mantle abstract phase transformations in a representative oceanic crust composition morb have been studied at pressures up to 28 gpa at 1200 c and 1500 c using a multi anvil apparatus the assemblage of majorite casio 3 rich perovskite stishovite which is displayed by the morb composition at pressures above 20 gpa was found to transform further into an assemblage containing a new aluminous phase at pressures above 25 gpa this al phase has a complex composition and is also relatively sodium rich its x ray diffraction pattern is similar to that observed for mgal 2 o 4 calcium ferrite type and is tentatively indexed on the basis of an orthorhombic cell yielding a zero pressure density of 3 87 2 g cm 3 majorite garnet was found to remain as the most abundant phase at the highest pressure studied 28 gpa and does not appear likely to transform to perovskite until pressures exceed 29 30 gpa these results imply that the basaltic crust component of subducted oceanic lithosphere would be 0 1 0 2 g cm 3 less dense than surrounding mantle pyrolite between the depths of 660 800 km thereby confirming an earlier hypothesis of anderson moreover the underlying layer of harzburgite would be 0 05 g cm 3 less dense than pyrolite throughout most of this depth interval these buoyancy relationships contribute to a barrier which impedes and may sometimes prevent the entry of slabs into the lower mantle

fungal remains in pleistocene ground squirrel dung from yukon territory canada abstract fungi in dung of the arctic ground squirrel spermophilus parryii collected near dominion creek yukon territory canada have a radiocarbon age of 12 200 100 yr b p most of the fungal remains are assignable to modern taxa and most of these are either widespread saprobes or nonspecific coprophiles however specimens identified as chaetomium simile and thecaphora deformans represent fungi that may be more characteristic of rodent dung than that of other animals inviting consideration of dung fungi as a potential source of paleontological data

a ubiquitous low velocity layer at the base of the mantle transition zone global stacks of receiver functions clearly exhibit the upper mantle stratification besides the most prominent seismic discontinuities such as the moho and the 410 and 660 km discontinuities a negative discontinuity is detected at a depth of 600 km indicating a low velocity layer at the base of the mantle transition zone the slant slack technique helps to identify the primary conversions from the multiple reverberations presence of the negative 600 km discontinuity underneath both continent and ocean island stations where the crustal thickness significantly differs also precludes the possible cause of crustal reverberations we conclude that the negative 600 km discontinuity could be a global feature possibly resulted from accumulation of ancient subducted oceanic crust the x discontinuity at 300 km depth is also observed in our global stacks which can be explained by the coesite stishovite phase transformation

a review of crust and upper mantle structure beneath the indian subcontinent this study has been supported by a grant from the ministry of earth sciences moes iitkgp ckh

epibionts on shells in the sarka formation a sparsely occupied niche in the lower to middle darriwilian oretanian ordovician in the prague basin czech republic abstract epizoic invertebrates attached to shells are a rare component of the fauna of the sarka formation middle ordovician early to middle darriwilian of the prague basin barrandian area in the czech republic epibionts are known exclusively from siliceous nodules and consist of echinoderms edrioasteroids and possible crinoid holdfasts monoplacophorans and bryozoans all attached to a very small number of host taxa among them hyoliths are the strongly preferred substrate 60 of observed host shells with edrioasteroids possible crinoid holdfasts and the monoplacophoran pygmaeoconus attaching exclusively to these organisms epizoan holdfasts were also found on a unique ramose problematic fossil considered to be the first octocoral specimen from the sarka formation most of the host are interpreted have been colonized during life only a few shells were used for attachment post mortem all epibionts are attached to the outer surface of the shell with the exception of the monoplacophoran barrandicella which is attached to the internal surface of empty conulariid thecae some of the epibionts shed additional light on the mode of life of their hosts including the bivalve redonia which is shown to be semi infaunal rather than infaunal more generally epibionts in the sarka formation attest to a very low level of occupancy of this significant niche during the early stages of the great ordovician biodiversification event

dung of mammuthus in the arid southwest north america abstract the discovery of a unique organic deposit in a dry cave on the colorado plateau southern utah permits the first comparison of the physical characteristics and the diet of the dung of the extinct mammoths from the arid southwest north america with that of mammoths from siberia and northern china the only other known locations of such remains the deposit buried beneath sand and rockfall is composed primarily of mammoth dung estimated at over 300 m3 radiocarbon dates on dung boluses indicate that the mammoths frequented the cave between approximately 14 700 and 11 000 yr b p the range of ages at 2 the desiccated boluses measuring approximately 230 170 85 mm are nearly identical in size to dung from extant elephants the largest contents in the dung are stalks measuring 60 4 5 mm grasses and sedges dominated the diet although woody species were commonly eaten

seismic structure of the lithosphere beneath nw namibia impact of the tristan da cunha mantle plume northwestern namibia at the landfall of the walvis ridge was affected by the tristan da cunha mantle plume during continental rupture between africa and south america as evidenced by the presence of the etendeka continental flood basalts here we use data from a passive source seismological network to investigate the upper mantle structure and to elucidate the cretaceous mantle plume lithosphere interaction receiver functions reveal an interface associated with a negative velocity contrast within the lithosphere at an average depth of 80 km we interpret this interface as the relic of the lithosphereasthenosphere boundary lab formed during the mesozoic by interaction of the tristan da cunha plume head with the pre existing lithosphere the velocity contrast might be explained by stagnated and frozen melts beneath an intensively depleted and dehydrated peridotitic mantle the present day lab is poorly visible with converted waves indicating a gradual impedance contrast beneath much of the study area converted phases of the 410 and 660 km mantle transition zone discontinuities arrive 1 5 s earlier than in the landward plume unaffected continental interior suggesting high velocities in the upper mantle caused by a thick lithosphere this indicates that after lithospheric thinning during continental breakup the lithosphere has increased in thickness during the last 132 myr thermal cooling of the continental lithosphere alone cannot produce the lithospheric thickness required here we propose that the remnant plume material which has a higher seismic velocity than the ambient mantle due to melt depletion and dehydration significantly contributed to the thickening of the mantle lithosphere

broadband array observations of the 300 km seismic discontinuity 1 intermittent seismic discontinuities near 250 300 km depth beneath south america and the pacific basin are detected with high resolution seismic array methods that use ss and pp precursors recorded at the high lava plains seismic experiment and the earthscope transportable array the transformation of coesite to stishovite in an eclogite rich mantle composition produces a seismic discontinuity near 300 km depth lateral changes in basalt fraction of the upper mantle will thus produce an intermittent seismic discontinuity the sensitivity of the precursors to intermittent seismic structure is addressed using an axisymmetric finite difference model of wave propagation in the mantle these numerical experiments find that the precursors are sensitive to structures 500 km in lateral extent and that the observations of this discontinuity are plausibly tied to lateral variations in basaltic composition of the upper mantle related to dynamics such as plumes and subduction

ubiquitous low velocity layer atop the 410 km discontinuity in the northern rocky mountains receiver functions from three 30 station iris passcal small aperture arrays 2 15 km station spacing operated for 10 months each in the northern rocky mountains show a ubiquitous negative polarity p to s conversion just preceding the 410 km discontinuity arrival data from the three arrays were sorted into nw se and sw back azimuth quadrants and stacked to form nine quadrant stacks remarkably the negative polarity arrival npa is apparent in 8 of the 9 quadrant stacks with 7 of the 8 having well correlated waveforms each quadrant stack also contains clear p to s conversions from the 410 and 660 km discontinuities moveout analysis shows that all the major phases display the correct moveout for forward scattered p s phases the waveshapes for the seven similar npa 410 km discontinuity arrivals are modeled with a five parameter double gradient slab model that is parameterized as follows a top gradient thickness and shear velocity decrease a constant velocity layer bottom gradient thickness and shear velocity increase model misfit is assessed via a grid search over the model space using a reflectivity code to calculate synthetic seismograms model likelihood is determined by calculating 1 and 2 d marginal probability density functions pdf for the five parameters the 1 d marginals display a range of peak values although significant overlap is observed for the top gradient thickness and its associated velocity decrement from the peak value of the summary pdf we find the top velocity gradient to be sharp 6 4 km and the shear velocity decrement to be large 8 9 vs defining an effective thickness of the low velocity layer as the mean layer thickness plus half the mean gradient thicknesses the 410 low velocity layer thickness is found to be 22 km a review of changes in the physical state required to match our new 410 lvl constraints suggests that the water filter model remains an operative hypothesis to test

s to p conversions from mid mantle slow scatterers in slab regions observations of deep stagnated oceanic crust the fate of a subducted slab is a key ingredient in the context of plate tectonics yet it remains enigmatic especially in terms of its crustal component in this study our efforts are devoted to resolve slab related structures in the mid mantle below eastern indonesia the izu bonin region and the peru area by employing seismic array analysing techniques on high frequency waveform data from f net in japan and the alaska regional network and the usarray in north america a pronounced arrival after the direct p wave is observed in the recordings of four deep earthquakes depths greater than 400 km from three subduction systems including the philippines the izu bonin and the peru this later arrival displays a slightly lower slowness compared to the direct p wave and its back azimuth deviates somewhat from the great circle direction we explain it as an s to p conversion at a deep scatterer below the sources in the source region in total five scatterers are seen at depths ranging from 930 to 1500 km those scatterers appear to be characterised by an 7 km thick low velocity layer compared to the ambient mantle combined evidence from published mineral physical analysis suggests that past subducted oceanic crust possibly fragmented is most likely responsible for these thin layer compositional heterogeneities trapped in the mid mantle beneath the study regions our observations give a clue to the potential fate of subducted oceanic crust

seismic imaging of upper mantle structure with new evidence for a 520 km discontinuity stacked images of long period seismic data reveal many phases some not previously observed which are caused by reflections and conversions at discontinuities in the earth s upper mantle these images support the existence of discontinuities at 410 520 and 660km depth but no evidence is found for a significant interface at 220 km

elastic properties of hydrous ringwoodite γ phase in mg2sio4 abstract single crystal elastic properties of hydrous ringwoodite are reported for samples that were synthesized at 19 gpa and 1300 c the mg si ratio is determined to be 1 95 by epma which is slightly lower than that of ideal anhydrous ringwoodite the h 2 o content determined by sims is 2 2 wt resulting in the stoichiometry mg 1 89 si 0 97 o 4 h 0 33 the lattice parameter a 8 0786 4 a and elastic constants were determined from a single crystal with x ray diffractometry and brillouin scattering spectroscopy the crystal structure is cubic fd3m with a unit cell volume which is 0 51 larger than that of the anhydrous ringwoodite the adiabatic single crystal elastic moduli of hydrous ringwoodite in gpa are c 11 281 6 c 44 117 4 c 12 92 5 the isotropic properties of hydrous ringwoodite are k vrh 155 4 and g vrh 107 3 which are about 16 and 10 smaller than those of anhydrous ringwoodite thus hydrous ringwoodite if present drastically reduces the seismic velocity in the mantle transition zone relative to anhydrous ringwoodite in addition our results show that the anisotropy is enhanced relative to anhydrous ringwoodite using the present results we discuss the 520 km seismic discontinuity in a wet mantle transition zone

studying the 410 km and 660 km discontinuities beneath spain and morocco through detection of p to s conversions this is a contribution of the projects consolider ingenio 2010 topoiberia csd2006 00041 topomed cgl2008 03474 e bte rifsis cgl2009 09727 cgl2012 31472 and p09 rnm 5100 l bonatto benefits from a ph d jaepredoc csic grant

spores of coprophilous fungi from under the dawson tephra 25 300 14c years bp yukon territory northwestern canada based on the analysis of pollen and macrofossils zazula et al zazula g d froesne d g elias s a kuzmina s la farge c reyes a v sanborn p t schweger c e smith c a s mathewes r w 2006 vegetation buried under dawson tephra 25 300 14c yr bp and locally diverse late pleistocene paleoenvironments of goldbottom creek yukon canada palaeogeography palaeoclimatology palaeoecology 242 253 286 reported an in situ riparian grassy meadow ecosystem and regional well drained steppe tundra buried by dawson tephra ca 25 300 bp at goldbottom creek in the canadian yukon territory an additional study of fungal spores in pollen slides from that site was completed ascospores of the coprophilous sordaria type sporormiella type and podospora type appeared to be of common occurrence pointing to the former presence of herbivores the analysis of the various spore types of coprophilous fungi in pollen slides is useful and may lead to further knowledge of the role of different herbivore taxa in past ecosystems 2007 elsevier b v all rights reserved

seismically deduced thermodynamics phase diagrams for the mantle transition zone abstract seismic discontinuities at 410 and 660 km depth are usually attributed to solid phase changes within the olivine component of the mantle the clapeyron slopes 410 and 660 i e the thermal dependence of the depths of reactions have been shown experimentally to be of opposite signs yet their values are not well constrained by laboratory measurements seismological observations have not been more precise due to the difficulty to separate the competing effects of background wave velocities and of temperature on the topography of discontinuities in this study we use conversion imaging of interfaces under western us we propose a new approach to derive a seismological estimate of the clapeyron slopes with respect to 410 for the major and minor phase changes of the transition zone we obtain 660 3 mpa k 1 for 410 3 mpa k 1 we construct seismic phase diagrams of the transition zone that can be directly compared with experimental phase diagrams we also apply a z transform to better constrain the clapeyron slopes of the minor phase changes although tenuous signals in seismic phase diagrams suggest that minor phase transitions both in the olivine and the non olivine component of the mantle have visible seismic expressions they can tentatively be described as follows the 410 is overlaid at low temperature by an interface corresponding to a decrease of velocity with depth and 3 mpa k 1 the 660 widens at high temperature and is preceded at low temperature by an interface the 620 with 7 mpa k 1 a 520 is suggested with 2 3 mpa k 1 these last two interfaces correspond to velocity increases with depth at last near 590 km depth an interface may be associated with a velocity reduction showing a weak dependence on temperature 0 mpa k 1

receiver function imaging of upper mantle complexity beneath the pacific northwest united states abstract small scale topographic variations on the upper mantle seismic discontinuities provide important constraints on the thermal influences of upwellings and downwellings in geodynamically complex regions subduction of the juan de fuca plate and other tectonic processes dominating the pacific northwest united states in the cenozoic involve massive thermal flux that likely result in an upper mantle that has strong 3 d temperature variations we address the interaction of such processes in the region using receiver functions to image the upper mantle seismic discontinuities at 410 and 660 km we utilized over 15 000 high quality receiver functions gathered from 294 teleseismic earthquakes recorded at 277 regional broadband seismic stations primarily those of the earthscope usarray transportable array we find the average depths of the discontinuities to be 412 km and 658 km respectively with no obvious 520 km discontinuity detected the peak to peak range is greater on the 410 than the 660 suggesting the possibility of more significant regional dynamic processes at upper mantle depths our results are not consistent with a mantle plume below central oregon in the high lava plains region our observation of a thinner transition zone beneath the western snake river plain region however is consistent with a regional increase in mantle temperatures perhaps due to either asthenospheric flow from beneath and around the southern edge of the juan de fuca plate or to vertical flow in the form of regional mantle upwelling related to the snake river plain yellowstone hotspot track further our results are not consistent with a simple subducting juan de fuca slab morphology but rather suggest similar levels of significant complexity in slab structure found by recent regional tomographic studies we find evidence for a thickened and therefore cooler mantle transition zone beneath the wallowa idaho batholith region consistent with tomographic models which suggest broad scale increased seismic velocities to transition zone depths we speculate that this region may be the location of mantle downwelling at larger scales than previously proposed

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

stagnant slab a review a stagnant slab is a subducted slab of oceanic lithosphere subhorizontally deflected above across or below the 660 km discontinuity this phenomenon has now been widely recognized beneath subduction zones around the circum pacific and in the mediterranean collaboration of seismic and electromagnetic observations mineral physics measurements and geodynamic modeling has begun to provide a consistent picture of stagnant slab

the upper mantle transition zone beneath the ibero maghrebian region as seen by teleseismic pds phases abstract we investigate the upper mantle transition zone thickness beneath the ibero maghrebian region using teleseismic events recorded at 258 seismic stations from the iberarray of the spanish topoiberia project for this purpose the 410 km and 660 km depth discontinuities are mapped through the detection of p to s conversions in order to add consistency and robustness to the detections the final results are based on a joint analysis of receiver functions and two different cross correlation functionals discontinuity depths are determined using time corrections obtained from a 3 d velocity model this study presents the first high resolution topography maps for the 410 and 660 discontinuities which show lateral variations in the transition zone thickness beneath the study area the results are discussed to add new constraints on temperature and composition to seismic velocity anomalies observed in the transition zone beneath the controversial ibero maghrebian region we obtained a thickened transition zone beneath the alboran sea and the north balearic sea suggesting low transition zone temperatures and coinciding with the position of the high velocity anomalies in the tomographic images related to the betic alboran slab and the alpine tethys remnant slab respectively a dominantly thinned transition zone beneath the gulf of cadiz and its surroundings suggest high mantle temperatures we also found evidences of the presence of a low velocity layer atop the 410 discontinuity beneath the study area

spores of the dung fungus sporormiella increased abundance in historic sediments and before pleistocene megafaunal extinction abstract spores of the dung fungus sporormiella become abundant following the historic introduction of grazing herbivores at seven sites in the western united states during the holocene they are generally rare but at six sites sporormiella spores are abundant before the extinction of pleistocene megaherbivores ca 11 000 yr b p sporormiella spores are directly linked to extinct megaherbivores by their presence in mammoth dung preserved in bechan cave southern utah their abundance in late glacial sediments may reflect the abundance of megaherbivores during quaternary thereby indicating the age of pleistocene extinctions where other indicators are absent

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

computation of seismic profiles from mineral physics the importance of the non olivine components for explaining the 660 km depth discontinuity the recent increasing number of experimental works leads us to review the elastic properties and mineralogical transformations of mantle minerals the updated data set is used to compute seismic profiles for two petrological models along three adiabatic temperature profiles these profiles are chosen to stress out the influence of non olivine minerals on seismic parameters and to represent cold and horizontally averaged temperature profiles of the earth s mantle in a first part starting compositions of pyrolite and piclogite and a single layer convection are assumed the results clearly point out the importance of the non olivine part of the mineralogy two scenarios are found to explain the 660 km depth discontinuity

pervasive seismic low velocity zones within stagnant plates in the mantle transition zone thermal or compositional origin abstract we exploit conversions between p and s waves for large scale high resolution imaging of the mantle transition zone beneath northwest pacific and the margin of eastern asia we find pervasive reflectivity concentrated in two bands with apparent wave speed reduction of 2 to 4 about 50 km thick at the top of the transition zone and 100 km thick at the bottom this negative reflectivity associated with the scattered waves at depth is interpreted jointly with larger scale mantle tomographic images and is shown to delineate the stagnant portions of the subducted pacific plate in the transition zone with largely positive shear wave velocity contrasts the upper reflectivity zone connects to broad low velocity regions below major intra plate volcanoes whereas the lower zone coincides locally with the occurrence of deep focus earthquakes along the east asia margin similar reflectivity is found in pacific northwest of the usa we demonstrate that the thermal signature of plates alone is not sufficient to explain such features alternative explanations for these reflective zones include kinetic effects on olivine phase transitions meta stability compositional heterogeneities within and above stagnant plates complex wave propagation effects in the heterogeneous slab structure or a combination of such factors we speculate that part of the negative reflectivity is the signature of compositional heterogeneities as revealed by numerous other studies of seismic scattering throughout the mantle and that such features could be widespread across the globe

contrasting climatic histories for the snake river plain idaho resulting from multiple thermal maxima abstract ten sites near the snake river plain have consistent differences in their climatic histories sites at low elevation reflect the early holocene xerothermic of the pacific northwest whereas most climatic chronologies at high elevation indicate maximum warmth or aridity somewhat later ca 6000 yr ago this elevational contrast in climatic histories is duplicated at three sites from the central snake river plain for sites in such close proximity the different chronologies cannot be explained by changes in atmospheric circulation during the late quaternary rather the differences are best explained by the autecology of the plants involved and the changing seasonal climate the seasonal climatic sequence predicted by multiple thermal maxima explains the high and low elevation chronologies during the early holocene maximum insolation and intensified summer drought in july forced low elevation vegetation upward however moisture was not a limiting factor at high elevation where vegetation moved upward in response to increased length of growing season coincident with maximum september insolation 6000 yr ago

seismic observation of narrow plumes in the oceanic upper mantle 1 we have found seismic evidence for the existence of narrow about 200 km diameter and hot plumes in the upper mantle above the 410 km discontinuity they are located mainly under ocean islands not however under the central parts of major continents these results are obtained from comparing globally the observed travel time differences between ps conversions and ss precursors from the discontinuity at 410 km depth using published data and new observations from ocean island stations comparison of ss precursors and ps conversions however leads also to indications of significantly fewer narrow plumes in the upper mantle transition zone between 410 and 660 km depth than in the upper mantle above 410 km depth