nonlinear waveform and delay time analysis of triplicated core phases 1 we introduce a new method to measure differential travel times and attenuation of seismic body waves the problem is formulated as a nonlinear inverse problem which is solved by simulated annealing using this technique we have analyzed triplicated pkp waves recorded by the temporary eifel array in central europe these examples demonstrate the potential of the technique which is able to determine differential travel times and waveforms of the core phases even when they interfere on the seismograms or when additional depth phases are present the pkp differential travel times reveal the presence of large amplitude and small scale heterogeneities along the pkp ab ray paths and favor a local radial inner core model with 0 9 velocity perturbation in its top 150 km and small velocity perturbations below the quality factor in the top 300 km of this inner core region is estimated from pkp differential attenuation its preferred value is 330 with a lower bound of 75

evidence for a sharp lateral variation of velocity at the core mantle boundary from multipathed pkpab rapid changes in differential travel times between the various pkp branches have been attributed to strong lateral velocity variations at the base of the mantle differential time pkpab df residuals showing jumps of up to 2 s for paths only 100 km apart have been reported such extreme changes in travel times for signals with comparable wavelengths suggest ray bifurcation with multipaths containing slow and fast contributions to pkpab here we report on some well documented normal and multipathed pkpab phases along with rapidly varying differential time residuals observed for fiji events with paths sampling the northern edge of the mid pacific large slow structure beneath hawaii systematic analyses of these data with two dimensional models suggest the presence of a ridge shaped structure at the core mantle boundary containing abrupt reductions of p wave velocity of up to 10 this narrow ultralow velocity zone ulvz is located at the boundary between a large scale low velocity and high velocity structure corroborating recent dynamic models which propose that ulvzs preferentially occur at the boundary between hot and cold mantle domains and could be related to plume activity at hawaii

atmospheric radiocarbon calibration to 45 000 yr b p late glacial fluctuations and cosmogenic isotope production more than 250 carbon 14 accelerator mass spectrometry dates of terrestrial macrofossils from annually laminated sediments from lake suigetsu japan provide a first atmospheric calibration for almost the total range of the radiocarbon method 45 000 years before the present the results confirm the recently revised floating german pine chronology and are consistent with data from european and marine varved sediments and combined uranium thorium and carbon 14 dating of corals up to the last glacial maximum the data during the glacial show large fluctuations in the atmospheric carbon 14 content related to changes in global environment and in cosmogenic isotope production

high resolution investigation of shear wave anisotropy in d beneath the cocos plate 1 broadband shear wave signals from 16 deep south american earthquakes are used in a high resolution analysis of lowermost mantle structure beneath the cocos plate shear wave splitting of 162 scs phases is generally compatible with vertical transverse isotropy vti in the d region with little path length or spatial dependence across the 200 800 km2 area sampled assuming a 250 km thick d layer the vti has an average value of 0 63 scs s differential travel time anomalies for the same data indicate a lateral gradient in positive d transverse component velocity perturbations increasing northward s wave triplication arrivals from a shear velocity discontinuity at the top of d show splitting similar to the scs phases but with smaller magnitude vertical resolution remains limited but the data are consistent with vti material being concentrated in just the upper portion or at the top and bottom of d

observations of changing anisotropy across the southern margin of the african llsvp we present evidence for the presence of complex anisotropy in the lowermost mantle from 3 d waveform modelling of observed core diffracted shear waves that sample the southern edge of the african large low shear velocity province llsvp the anomalously strong amplitude of the sv component for the shear core diffracted phase at large distances indicates the presence of anisotropy we measure shear wave splitting parameters to determine which part of the elastic tensor is constrained by this particular data set the modelling is performed using the spectral element method the anisotropy is strong outside the llsvp weakens or rotates close to its boundary and appears to be absent inside the llsvp the presence of the llsvp margin may cause flow in the mantle to change direction the occurrence of strong anisotropy in the region of fast seismic velocities is compatible with lattice preferred orientation in post perovskite due to accommodation of flow through dislocation creep

lateglacial calendar year chronology based on annually laminated sediments from lake meerfelder maar germany abstract annually laminated sediments from lake meerfelder maar mfm in the eifel germany provide an independent calendar year timescale for the last glacial interglacial transition changes in varve thickness and varve facies reflect significant environment variations during this period the general micro facies pattern confirms that diatomaceous organic varves are favoured during warm climatic phases while cold periods generate higher allochthonous sediment fluxes an exception is the significant increase in diatom blooms at the beginning of the younger dryas six lateglacial biozones have been defined by litt and stebich quaternary international 1999 the sediment record comprises two tephra layers as chronomarkers the laacher see tephra 12 880 varve years bp is a continent wide correlation horizon while the ulmener maar tephra provides regional chronological links the beginning of the lateglacial warming is dated at 14 450 varve years vyrs bp which is regarded as the minimum age three abrupt cold spells interrupted the lateglacial interstadial at 13 800 vyrs bp oldest dryas 13 540 vyrs bp older dryas and 12 680 vyrs bp younger dryas the oldest dryas 130 years and the older dryas 190 years were rather short while the younger dryas lasted much longer 1090 years the mfm chronology was compared with ice core timescales from gisp2 and grip there is a good agreement between these timescales with the grip model age ss09 providing the best match to the mfm chronology

a 3 d spectral element and frequency wave number hybrid method for high resolution seismic array imaging we present a three dimensional 3 d hybrid method that interfaces the spectral element method sem with the frequency wave number fk technique to model the propagation of teleseismic plane waves beneath seismic arrays the accuracy of the resulting 3 d sem fk hybrid method is benchmarked against semianalytical fk solutions for 1 d models the accuracy of 2 5 d modeling based on 2 d sem fk hybrid method is also investigated through comparisons to this 3 d hybrid method synthetic examples for structural models of the alaska subduction zone and the central tibet crust show that this method is capable of accurately capturing interactions between incident plane waves and local heterogeneities this hybrid method presents an essential tool for the receiver function and scattering imaging community to verify and further improve their techniques these numerical examples also show the promising future of the 3 d sem fk hybrid method in high resolution regional seismic imaging based on waveform inversions of converted scattered waves recorded by seismic array

bio and chronostratigraphy of the lateglacial in the eifel region germany abstract in recent years enormous success has been achieved in varve counting of the eifel maar lakes but a detailed correlation with the biostratigraphy has been missing in this paper we present new palynological results of the lateglacial sequences from holzmaar lake and meerfelder maar lake based on annually laminated sediments in particular the meerfelder maar has great potential because in contrast to the holzmaar the sequence between the ulmener maar tephra 11 000 varve years bp and the laacher see tephra 12 880 varve years bp including the younger dryas is undisturbed and complete therefore we currently use the meerfelder maar chronology brauer et al 1999b as an independent varve calendar for the biostratigraphy of the lateglacial the palynological signals of both maar lakes are in good agreement and can easily be correlated with one another and with type sections type regions in northwestern germany and jutland the sequences of the eifel maar lakes have the quality of hypostratotypes with regional biozones based on an absolute time scale

using seismic waves to image earth s internal structure seismic waves generated in earth s interior provide images that help us to better understand the pattern of mantle convection that drives plate motions

evolution and environmental impacts of the eruption of laacher see volcano germany 12 900 a bp abstract laacher see volcano lsv 40 km south of bonn explosively erupted ca 6 3 km 3 of chemically strongly zoned phonolite magma probably during spring 12 900 years ago the resulting eruption column having reached at least 20 km in height the bulk of the laacher see tephra lst was deposited east of the volcano within the neuwied basin lst ashes form the most important stratigraphic marker in lateglacial deposits over much of central europe a minimum of 2 mt total s was calculated to have been released during the eruption by comparing pre and posteruptive volatile concentrations in glass inclusions and pumice matrix glasses because a separate s bearing vapor phase was probably present in the magma prior to eruption as indicated by the high melt h 2 o contents close to saturation level the actual amount of s released during the eruption could have significantly exceeded 2 mt the sulfuric acid aerosol layer resulting from the massive stratospheric s input probably resided in the stratosphere for years and most likely had a significant impact on climate and thus the environment increased precipitation in central europe and or impairment of the vegetation cover for several years is suggested by several proxies especially increased sediment supply into lakes proximal to the eruptive center major environmental impacts include an initial blast that felled trees up to 4 km away from vent fast deposition of huge tephra volumes led to the complete disruption of rhine river within the lower neuwied basin and the damming up of a lake ca 140 km 2 in areal extent sudden collapse of the temporary tephra dam caused a catastrophic flood wave downstream whose deposits are recognized as far north as bonn reworking of unconsolidated tephra deposits over several 100 km 2 was widespread and extensive generating abundant lahars and flood plain deposits a braided river system established in neuwied basin probably persisted up to several years recent reports for a second eruption of lsv are not confirmed

analysis of seismic sv waves in the core s penumbra vertically polarized s waves sv attenuate rapidly as they diffract around the earth s core whereas horizontally polarized s waves sh propagate to large distances in the core shadow zone the amplitude decay of diffracted sv signals is so strong that few studies have been made of these phases despite their acute sensitivity to velocity structure just above the core mantle boundary we analyze sv signals in the penumbra of the core s shadow finding systematic waveform complexities indicative of local stratification and apparent anisotropy at the base of the mantle with implications for the dynamic processes in this internal boundary layer

sedimentological proof and dating of the early holocene volcanic eruption of ulmener maar vulkaneifel germany a minerogeniclayer occurs in early postglacial organic sediments from five maar lakes west eifel volcanic field germany the mineralogy and stratigraphic position of this tephra suggests that it is related to the youngest german volcano ulmener maar nearby radiocarbon dating of wood from the base of the ulmener maar tephra at two locations provide ages in agreement with an accelerator mass spectrometer 14c date for the minerogenic layer from sediments of lake holzmaar situated 13 km south west of ulmener maar the mean radiocarbon age is 9 560 years bp dating by varve chronology provides an age of 10017 years vt varve time in years before 1950 or 10 895 years corrected vt based on palynology the ulmener maar tephra was deposited at the end of the preboreal high values of natural remnant magnetization intensity typical of pyroclastic material confirm that this minerogenic layer differs in composition from other clastic deposits of the sedimentary record geochemical analyses reveal increased values of total trace elements for the laacher see tephra and ulmener maar tephra an isopach map based on thickness variations of the ulmener maar tephra at five investigated maar lakes indicates that the tephra was mainly transported to the south west

geographic correlation between hot spots and deep mantle lateral shear wave velocity gradients abstract hot spot volcanism may originate from the deep mantle in regions exhibiting the earth s most pronounced lateral s wave velocity gradients these strong gradient regions display an improved geographic correlation over s wave velocities to surface hot spot locations for the lowest velocities or strongest gradients occupying 10 of the surface area of the core mantle boundary cmb hot spots are nearly twice as likely to overlie the anomalous gradients if plumes arise in an isochemical lower mantle plume initiation should occur in the hottest thus lowest velocity regions or in the regions of strongest temperature gradients however if plume initiation occurs in the lowest velocity regions of the cmb lateral deflection of plumes or plume roots are required the average lateral deflections of hot spot root locations from the vertical of the presumed current hot spot location ranges from 300 to 900 km at the cmb for the 10 30 of the cmb covered by the most anomalous low s wave velocities the deep mantle may however contain strong temperature gradients or be compositionally heterogeneous with plume initiation in regions of strong lateral s wave velocity gradients as well as low s wave velocity regions if mantle plumes arise from strong gradient regions only half of the lateral deflection from plume root to hot spot surface location is required for the 10 30 of the cmb covered by the most anomalous strong lateral s wave velocities we find that strong gradient regions typically surround the large lower velocity regions in the base of the mantle which may indicate a possible chemical in addition to thermal component to these regions

holocene climate variability in europe evidence from δ18o textural and extension rate variations in three speleothems time series o isotope profiles for three u th dated stalagmites have revealed that for much of the holocene a site on the atlantic seaboard sw ireland exhibits first order 18o trends that are almost exactly out of phase with coupled 18o curves from two southern european sites se france and nw italy in the irish stalagmite cc3 from crag cave sw ireland low 18o at 10 000 cal yr bp reflects cool conditions by the early to mid holocene 9000 6000 cal yr bp 18o had increased reflecting the onset of warmer conditions on the atlantic seaboard this shift to higher 18o was accompanied by a marked increase in the stalagmite extension rate reinforcing our interpretation that this was a period of relative warmth except for an episode of increased extension rate about 5500 yr ago 18o in the crag stalagmite exhibits a gradual decrease accompanied by declining extension rates between 7800 and 3500 cal yr bp interpreted as a cooling trend there is evidence for increases in both 18o and stalagmite extension rate in the period from 3500 cal yr bp to the present day suggesting a return to warmer conditions on the atlantic seaboard in the stalagmite from nw italy er76 grotta di ernesto trentino province the early holocene c 9200 7800 cal yr bp is characterised by high 18o probably indicative of warm and or dry conditions exceptionally low 18o from 7800 to 6900 cal yr bp at this site reflects a well defined wet phase cerin wet phase in the last three millennia this stalagmite exhibits a shift to lower 18o interpreted as some combination of cooler and or wetter conditions unlike the irish stalagmite the italian sample does not show a correlation between 18o and extension rate instead its extension rate correlates roughly with 13c presumably reflecting a climate driven vegetation change in the early holocene 18o in the french stalagmite cl26 grotte de clamouse herault province se france was low relative to its holocene average for much of the period since c 3500 cal yr bp this stalagmite exhibits higher 18o than in the early holocene suggesting warmer conditions like the irish stalagmite the french sample exhibits a well defined correlation between 18o and extension rate had drip water availability been the dominant control on 18o at this semi arid site then higher 18o would have been accompanied by lower not higher extension rates this suggests strongly that temperature rather than rainfall amount was the dominant control at this site while conclusions regarding the patterns of climate variability on a continent scale must remain tentative because of the limited number of stalagmites studied we argue that early holocene warm conditions on the atlantic seaboard irish site coincided with relatively cool conditions at the clamouse site by c 3500 yr ago the pattern appears to have been reversed

paleo environment and radiocarbon calibration as derived from lateglacial early holocene tree ring chronologies abstract we present an overview of the extended hohenheim oak chronology hoc and the dendrochronologically dated preboreal pine tree ring chronology ppc both provide an absolute annual time frame of the holocene extending into the younger dryas yd back to 11 919 bp two floating pine and larch chronologies are 14 c dated covering large parts of the lateglacial dendro ecological parameters such as ring width and stable isotope variation are used to infer past environmental conditions 14 c analyses on decadal sections provide a high precision high resolution data set for calibration of the radiocarbon time scale based on a marked change in ring width and growth pattern the yd termination is clearly identified in the german pine chronology its absolute age of 11 570 bp appears synchronous within the errors of the respective chronologies to related signals in the greenland ice cores grip gisp2 and in lacustrine varve sequences the 14 c age of the laacher see tephra lst is determined from a series of decadal tree ring samples to 11 063 12 14 c bp the calibrated range is 13 010 13 200 cal bp the climatic impact of the lst is reflected in the growth pattern of our tree ring chronologies

anisotropy in the deep earth abstract seismic anisotropy has been found in many regions of the earth s interior its presence in the earth s crust has been known since the 19th century and is due in part to the alignment of anisotropic crystals in rocks and in part to patterns in the distribution of fractures and pores in the upper mantle seismic anisotropy was discovered 50 years ago and can be attributed for the most part to the alignment of intrinsically anisotropic olivine crystals during large scale deformation associated with convection there is some indication for anisotropy in the transition zone particularly in the vicinity of subducted slabs here we focus on the deep earth the lower mantle and core where anisotropy is not yet mapped in detail nor is there consensus on its origin most of the lower mantle appears largely isotropic except in the last 200 300 km in the d region where evidence for seismic anisotropy has been accumulating since the late 1980s mostly from shear wave splitting measurements recently a picture has been emerging where strong anisotropy is associated with high shear velocities at the edges of the large low shear velocity provinces llsvps in the central pacific and under africa these observations are consistent with being due to the presence of highly anisotropic mgsio 3 post perovskite crystals aligned during the deformation of slabs impinging on the core mantle boundary and upwelling flow within the llsvps we also discuss mineral physics aspects such as ultrahigh pressure deformation experiments first principles calculations to obtain information about elastic properties and derivation of dislocation activity based on bonding characteristics polycrystal plasticity simulations can predict anisotropy but models are still highly idealized and neglect the complex microstructure of polyphase aggregates with strong and weak components a promising direction for future progress in understanding the origin of seismic anisotropy in the deep mantle and its relation to global mantle circulation is to link macroscopic information from seismology and microscopic information mineral physics through geodynamics modeling anisotropy in the inner core was proposed 30 years ago to explain faster p wave propagation along the direction of the earth s axis of rotation as well as anomalous splitting of core sensitive free oscillations there is still uncertainty about the origin of this anisotropy in particular it is difficult to explain its strength based on known elastic properties of iron as it would require almost perfect alignment of iron crystals indeed the strongly anomalous p travel times observed on paths from the south sandwich islands to alaska may or may not be due to inner core anisotropy and will need to be explained before consensus can be reached on the strength of anisotropy in the inner core and its origin

absolute dating of the late glacial period in switzerland using annually laminated sediments abstract results of a high resolution late glacial ams 14 c chronology from rotsee central switzerland have given evidence that the atmospheric radiocarbon concentration was not constant between 13 000 and 9500 yr b p this resulted in three marked phases of constant radiocarbon age at 12 700 at 10 000 and at 9500 yr b p new results of a late glacial varve chronology from soppensee central switzerland suggest that the younger two phases of constant 14 c age each had a duration of ca 400 calendar years the length of the late glacial chronozones has been calculated on the basis of these replicate varve counts and a comparison with their estimated duration in radiocarbon years shows that the estimated duration of the chronozones bolling ca 800 calendar yr vs 1000 14 c yr and younger dryas ca 900 calendar yr vs 1000 14 c yr agree with the expected time span whereas the estimated duration of the allerod chronozone ca 400 calendar yr vs 1000 14 c yr is substantially shorter than expected furthermore a tentative comparison of varve ages and 14 c ages suggests that the varve chronology is more than 1000 yr offset toward older ages from the radiocarbon chronology during the late glacial period

environmental response to climatic and volcanic events in central europe during the weichselian lateglacial this paper reviews major results of the recent german research priority program changes of the geo biosphere during the last 15 000 years a contribution to pages focussing on changes of the geo biosphere during the weichselian lateglacial different continental archives such as annually laminated lacustrine sediments floodplain sediments and speleothems were used to reconstruct environmental response to climatic changes and the laacher see eruption event at ca 12 900 cal bp special emphasis is paid to establish a reliable time control using varve counting high precision radiocarbon dating of tree ring series and ams radiocarbon dating of terrestrial plant macrofossils recovered from lacustrine sediments to correlate and synchronize large scale environmental changes and events in central europe

the limits of ray theory when measuring shear wave splitting in the lowermost mantle with scs waves observations of shear wave splitting provide unambiguous evidence of the presence of anisotropy in the earth s lowermost mantle a region known as d much recent work has attempted to use these observations to place constraints on strain above the core mantle boundary cmb as this may help map flow throughout the mantle previously this interpretation has relied on the assumption that waves can be modelled as infinite frequency rays or that the earth is radially symmetric due to computational constraints it has not been possible to test these approximations until now we use fully three dimensional generally anisotropic simulations of scs waves at the frequencies of the observations to show that ray methods are sometimes inadequate to interpret the signals seen we test simple uniform models and for a d layer as thin as 50 km significant splitting may be produced and we find that recovered fast orientations usually reflect the imposed fast orientation above the cmb ray theory in these cases provides useful results though there are occasional notable differences between forward methods isotropic models do not generate apparent splitting we also test more complex models including ones based on our current understanding of mineral plasticity and elasticity in d the results show that variations of anisotropy over even several hundred kilometres cause the ray theoretical and finite frequency calculations to differ greatly importantly models with extreme mineral alignment in d yield splitting times not dissimilar to observations t 3 s suggesting that anisotropy in the lowermost mantle is probably much stronger than previously thought potentially 10 shear wave anisotropy or more we show that if the base of the mantle is as complicated as we believe future studies of lowermost mantle anisotropy will have to incorporate finite frequency effects to fully interpret observations of shear wave splitting

broadband modeling of the 2002 denali fault earthquake on the earth simulator abstract we use a spectral element method implemented on the earth simulator in japan to simulate broadband seismic waves generated by the 3 november 2002 denali fault earthquake this mw 7 9 event is the largest strike slip earthquake in north america in almost 150 years the source model is constrained by teleseismic body waves and observed surface offsets the earthquake was initiated by a small thrust event and is well characterized by a five segment fault geometry dominated by right lateral rupture along 220 km of the denali fault we perform the three dimensional numerical simulations at unprecedented resolution and reveal significantly enhanced ground motions directivity toward the conterminous united states for both body and surface waves

deglacial changes in ocean circulation from an extended radiocarbon calibration temporal variations in the atmospheric concentration of radiocarbon sometimes result in radiocarbon based age estimates of biogenic material that do not agree with true calendar age this problem is particularly severe beyond the limit of the high resolution radiocarbon calibration based on tree ring data which stretches back only to1 2 about 11 8 kyr before present bp near the termination of the younger dryas cold period if a wide range of palaeoclimate records are to be exploited for better understanding the rates and patterns of environmental change during the last deglaciation extending the well calibrated radiocarbon timescale back further in time is crucial several studies attempting such an extension using uranium thorium dated corals3 4 5 and laminae counts in varved sediments6 7 8 9 show conflicting results here we use radiocarbon data from varved sediments in the cariaco basin in the southern caribbean sea to construct an accurate and continuous radiocarbon calibration for the period 9 to 14 5 kyr bp nearly 3 000 years beyond the tree ring based calibration a simple model compared to the calculated atmospheric radiocarbon concentration and palaeoclimate data from the same sediment core suggests that north atlantic deep water formation shut down during the younger dryas period but was gradually replaced by an alternative mode of convection possibly via the formation of north atlantic intermediate water

lowermost mantle anisotropy beneath the north pacific from differential s scs splitting summary seismic anisotropy is an important tool for studying the nature origin and dynamics of the lowermost mantle d we introduce differential s scs splitting as a tool for removing the effect of near source and near receiver anisotropy to estimate splitting accrued in the d region this is applicable to events recorded at epicentral distances between 60 and 85 near source anisotropy has often been ignored in previous studies of lowermost mantle anisotropy we apply differential s scs splitting to records from canadian national seismic network stations of western pacific earthquakes these sample the lowermost mantle beneath the north pacific the residual splitting in scs which we attribute to d shows lag times between 1 0 and 3 9 s given the near horizontal ray path of scs in d we interpret the recovered fast directions as the orientation of the fast shear wave in the plane defined by the vertical and transverse directions and observe a clearly non vti transverse isotropy with a vertical axis of symmetry style of anisotropy the largest population of results shows an approximately southeasterly dipping symmetry axis which we speculate might be explained by descending palaeoslab material being swept horizontally across the core mantle boundary towards an upwelling region beneath the central pacific non vti symmetry and the many possible contributions to d anisotropy from lower mantle minerals melt and subducted materials suggest that our understanding of the lowermost mantle could be greatly improved by trying to resolve a more general style of anisotropy

volatile composition of the phonolitic laacher see magma 12 900 yr bp implications for syn eruptive degassing of s f cl and h 2 o a volatile rich and chemically zoned phonolitic magma reservoir was tapped successively during the eruption of laacher see volcano ca 12 900 years bp and produced a tephra sequence consisting of phenocryst poor highly evolved phonolite at the base and phenocryst rich more mafic phonolite at the top the stratospheric volatile loading was estimated by comparing pre and post eruptive s f cl and h2o contents of undegassed glass inclusions and partially degassed matrix glasses glass inclusions 150 1490 ppm s and host matrix glasses 150 820 ppm s both document a strong s decrease during progressive magmatic differentiation which is interpreted to be partially caused by crystallization of hauyne the s6 stotal ratio of the pre eruptive melt increased with differentiation from 8 to 71 as indicated by s k wavelength shift measurements in glass inclusions sulfate rich highly evolved phonolitic magma was erupted during plinian and sulfide rich more mafic phonolitic magma during late phreatomagmatic phases f and cl became enriched during late stages of differentiation glass inclusions 690 4060 ppm f 1770 4400 ppm cl matrix glasses 680 3660 ppm f 2130 4330 ppm cl the most differentiated melts maximum 13 wt na2o occur only as matrix glass and are extremely f enriched 5080 8780 ppm but cl depleted 460 2820 ppm suggesting that f was retained in the melt whereas some cl was lost during pre and or syn eruptive degassing the h2o contents of glass inclusions increase irregularly with differentiation 2 5 5 7 wt matrix glasses are h2o depleted 0 2 2 8 wt compared to most glass inclusions showing that most h2o was released to the atmosphere by explosive degassing a mass balance calculation yields a syn eruptive volatile release of 1 9 tg stotal 6 6 tg cl and 403 tg h2o from the melt this is a minimum estimate since s and cl could have accumulated in a separate fluid phase as indicated by fluid inclusions in hauyne and pre eruption h2o contents close to saturation level at the likely pressure temperature conditions in the laacher see magma reservoir we estimate that at least ca 20 tg so2 were injected into the stratosphere causing a significant negative climate forcing as reflected by several paleoclimate proxies and as shown by recent modeling

late glacial climatic oscillations as recorded in swiss lake sediments regional pollen assemblage zones for the late glacial period of the swiss plateau are introduced and defined they include four major zones artemisia juniperus hippophae betula pinus paz with several subzones pollen and oxygen isotope analyses on lacustrine sediments from several lakes in the area reveal four distinct phases of climatic oscillation in the time period of 13 000 9500 yr bp the first oscillation termed the aegelsee fluctuation occurs shortly before 12 000 yr bp and varve counts suggest its duration was ca 100 yr it is characterised by a short decrease in the oxygen isotopes as well as a short increase in nap associated with a depression in birch pollen values the second oscillation which occurs in the 18o record shortly before the deposition of the laacher see tephra ca 11 000 yr bp is termed the gerzensee fluctuation it occurs during a pine dominated phase and its vegetational effects cannot be determined palynologically the most prominent regressive phase is the younger dryas biozone ca 10 700 10 000 yr bp characterised by an increase in heliophilous nap and low 18o values the younger dryas biozone can often be subdivided palynologically into two parts a first part rich in grasses and juniper and a second part with higher filipendula and birch values during the preboreal biozone another distinct oscillation is evidenced only in the oxygen isotope ratios comparison of the swiss oxygen isotope profiles with the greenland dye 3 record suggests that not only the three major shifts in the 18o curves but also the minor ones are closely comparable suggesting some common climatic control

high resolution climate signals in the bolling allerod interstadial greenland interstadial 1 as reflected in european tree ring chronologies compared to marine varves and ice core records abstract lateglacial and holocene tree ring chronologies are unique archives which provide various information on past environments on a true annual time scale changes in ring width can be related to past climate anomalies and dendrodated wood provides an ideal source for radiocarbon calibration we present a 1051 year tree ring chronology from the late glacial built from subfossil scots pines pinus sylvestris that grew in different regions of central and southern europe through a series of high precision radiocarbon measurements we obtained a floating radiocarbon chronology which allowed accurate wiggle matching to the intcal98 calibration curve the trees show a coherent pattern in ring width variations throughout central europe and extending into the mediterranean which indicates a strong external climatic factor most probably temperature during the growing season we identified major growth events which appear synchronous with events seen in isotopic and tracer signals in the greenland ice cores and with changes in the strength of upwelling in the cariaco basin

climate related variations in the composition of the late glacial and early holocene sediments of lake perespilno eastern poland abstract this paper presents an array of analyses of the laminated sediments of lake perespilno eastern poland covering the period between older dryas and preboreal using the varve chronologies pollen and 14 c data the laminated sequence of lake perespilno has been synchronised with that of lake go ciaz the length of the allerod and younger dryas periods derived from lake perespilno sediments agrees well with the previous estimates according to our data the climate in the allerod was relatively mild with some increase of precipitation and humidity after ca 13 2 kyr cal bp the beginning of the younger dryas is marked by drastic changes in temperature as well as humidity thinning of forests facilitated input of allochtonous material to the lake it seems that the groundwater level as well as lake level dropped significantly and the period of lowest water table was prolonged for ca 300 years surprisingly the abrupt warming at the beginning of the holocene is only slightly reflected in the sediments changes in sediment composition at that time seem to be controlled more by the development of surrounding vegetation than by the climate itself

varve chronology and palynology of the lateglacial in northwest germany from lacustrine sediments of hamelsee in lower saxony abstract the hamelsee is a dolina from a collapse over permian salt that occurred at the end of the pleniglacial it is more than 21 m deep and contains 18 m of lacustrine sediments the lateglacial sediments in the central part of the lake consist of 2 7 m from 15 to 17 7 m of sapropelic and silty muds which are mostly laminated and partly bioturbated minor slumps occur mainly during colder phases the subdivision of the lateglacial is based on pollen zones multidisciplinary studies of the microfacies from thin sections geochemistry pollen analysis and rotifer analysis allow environmental and palaeoclimatological interpretations to be made the biozones of the lateglacial and their boundaries are marked by rapid climatic changes though covering comparatively short periods of time they are not monotonous but characterized by minor internal climatic fluctuations the sediments of the meiendorf and oldest dryas are not laminated the allerod sediment is seasonally varved without interruption and encompasses 625 yr the eruption of the laacher see volcano 12 900 cal bp had an impact on the climate during the following 10 yr the transition from the allerod to the younger dryas took some 50 yr and it occurred 200 yr after the eruption of the laacher see volcano the abrupt change at the end of the lateglacial 11 560 cal bp occurred within less than 10 yr however many proxies reacted with a time lapse of 15 to 30 yr

correlation and synchronisation of lateglacial continental sequences in northern central europe based on annually laminated lacustrine sediments abstract the present study focusses on correlation and synchronisation of weichselian lateglacial varved lake sediments from western germany meerfelder maar eifel region northern germany hamelsee lower saxony central poland lake go ci a z and eastern poland lake perespilno by using varve chronology tephrochronology palynostratigraphy and stable isotopes comparison of the several independent time scales shows that biotic and abiotic parameters respond abruptly and quasi synchronously within the errors of the different chronologies during the younger dryas preboreal transition moreover there is a consensus about the length of the younger dryas cold stage of 1100 1150 varve years in the allerod the prominent laacher see tephra 12 880 varve years bp can be used to fix floating varve chronologies the relative duration of this biozone has been determined in meerfelder maar and hamelsee at between 625 and 670 varve years in the meerfelder maar a combination of continuous varve counting and biostratigraphy has been possible for the almost entire lateglacial the comparison between continental limnic sequences and greenland ice core records should be made on the basis of independent chronologies in both archives it is more practicable to develop regional stratotypes on the continental regions instead of simply using ice cores as stratotypes for the lateglacial for terrestrial european records in this respect annually laminated lacustrine sequences have a great potential

shear wave splitting and waveform complexity for lowermost mantle structures with low velocity lamellae and transverse isotropy 1 shear waves that traverse the lowermost mantle exhibit polarization anomalies and waveform complexities that indicate the presence of complex velocity structure above the core mantle boundary synthetic seismograms for horizontally and vertically polarized shear waves sh and sv respectively are computed using the reflectivity method for structures with low velocity sheets lamellae and for comb like models approximating long wavelength vertical transverse isotropy vti motivated by evidence for partial melt in the deep mantle lamella parameter ranges include 1 vp from 5 to 10 vs 3 vp 2 100 to 300 km thickness of vertical stacks of lamella 3 lamella spacing and thickness varying from 0 5 to 20 km and 4 lamellae concentrated near the top bottom or throughout the d region at the base of the mantle such lamellae represent in effect horizontally emplaced dikes within d excessively complex waveforms are produced when more than 20 of d volume is comprised of low velocity lamellae many lamellae models can match observed sdiff splitting 1 10 s delays of svdiff but typically underpredict scs splitting 1 4 s delays of scsv vti model parameters are selected to address d observations and include 1 0 5 to 3 anisotropy 2 discontinuous d shear velocity increases up to 3 3 d thicknesses from 100 to 300 km and 4 vti concentrated at the top bottom or throughout d vti models readily match observed splits of scs and sdiff we discuss lamellae and vti model attributes in relationship to waveform complexities splitting magnitude triplications from a high velocity d discontinuity and apparently reversed polarity svdiff onsets the possible presence of melt filled lamellae indicates that local chemical or thermal perturbations can produce regions that exceed the solidus within d such melt could occur in the bulk of d because the melt is either close to neutral buoyancy advective velocities exceed percolative velocities or both

a compositional origin to ultralow velocity zones we analyzed vertical component short period scp waveforms for 26 earthquakes occurring in the tonga fiji trench recorded at the alice springs array in central australia these waveforms show strong precursory and postcursory seismic arrivals consistent with ultralow velocity zone ulvz layering beneath the coral sea we used the viterbi sparse spike detection method to measure differential travel times and amplitudes of the postcursor arrival scsp and the precursor arrival spcp relative to scp we compare our measurements to a database of 340 000 synthetic seismograms finding that these data are best fit by a ulvz model with an s wave velocity reduction of 24 a p wave velocity reduction of 23 a thickness of 8 5 km and a density increase of 6 this 1 1 vs vp velocity decrease is commensurate with a ulvz compositional origin and is most consistent with highly iron enriched ferropericlase

the preboreal oscillation around the nordic seas terrestrial and lacustrine responses the occurrence of an early preboreal climatic cooling oscillation pbo in lacustrine and glacial records from northwest europe iceland and greenland is reviewed and documented the often subtle response of the proxy records to this oscillation in combination with its short duration make it difficult to detect owing to its chronostratigraphic position between the 10 000 9900 and 9600 9500 14 c plateaux c 11 300 11 150 calendar yr bp it is also difficult to 14 c date with precision we find that the vegetation response to the pbo varies between sites and regions in contrast to the pioneer vegetation in iceland and southern sweden the expanding birch pine forest in germany denmark was more susceptible to deteriorating growing conditions the combined lacustrine tree ring and glacial records imply that the pbo was characterised by cool and humid conditions throughout northwestern and central europe this is documented by vegetation changes decreased aquatic production increased soil erosion increased 2 h and 13 c content in tree rings readvances or stillstands of the ice sheet in norway and finland and ingression of brackish water into the baltic icelandic proxy records from lake sediments and glacial moraines imply cooler conditions than during the previous preboreal period but not as extreme as during the younger dryas greenland records suggest that the early preboreal was characterised by ice readvances as an effect of cool climate and increased precipitation in relation to the younger dryas it was not until the end of the pbo that climate was warm enough to melt the land based ice sheet this preboreal oscillation found on both sides of the nordic seas is interpreted as an effect of increased freshwater forcing on the thermohaline circulation in the nordic seas which is implied by a simultaneous and distinct rise in the atmospheric 14 c 12 c ratio a slow down of the thermohaline circulation may tempor arily have pushed the polar front further south 1997 by john wiley sons ltd preboreal oscillation thermohaline circulation lacustrine records glacial records chronology

observational evidence for diffracted sv in the shadow of the earth s core on the basis of observational evidence it has been assumed for a long time in the seismological literature that the diffracted s wave in the deep shadow of the earth s core is polarized as sh in good agreement with models that do not contain a well pronounced low velocity zone at the base of the mantle while this seems often to be the case broad band horizontal records from the geoscope network show that for some wavepaths the diffracted sv is strong enough to be clearly observed at distances between 107 and 117 moreover in some cases the diffracted sv is shifted with respect to sh by approximately a quarter period such observations should put additional constraints on the structure near the core mantle boundary among other possibilities lateral variations of the gradient at the base of the mantle allowing for regions with negative gradient and possibly azimuthal anisotropy in d should be considered in order to explain these data

three dimensional full waveform inversion of short period teleseismic wavefields based upon the sem dsm hybrid method we present a method for high resolution imaging of lithospheric structures based on full waveform inversion of teleseismic waveforms we model the propagation of seismic waves using our recently developed direct solution method spectral element method hybrid technique which allows us to simulate the propagation of short period teleseismic waves through a regional 3 d model we implement an iterative quasi newton method based upon the l bfgs algorithm where the gradient of the misfit function is computed using the adjoint state method compared to gradient or conjugate gradient methods the l bfgs algorithm has a much faster convergence rate we illustrate the potential of this method on a synthetic test case that consists of a crustal model with a crustal discontinuity at 25 km depth and a sharp moho jump this model contains short and long wavelength heterogeneities along the lateral and vertical directions the iterative inversion starts from a smooth 1 d model derived from the iasp91 reference earth model we invert both radial and vertical component waveforms starting from long period signals filtered at 10 s and gradually decreasing the cut off period down to 1 25 s this multiscale algorithm quickly converges towards a model that is very close to the true model in contrast to inversions involving short period waveforms only which always get trapped into a local minimum of the cost function

the δ18o record along the greenland ice core project deep ice core and the problem of possible eemian climatic instability over 70 000 samples from the 3029 m long greenland ice core project grip ice core drilled on the top of the greenland ice sheet summit have been analyzed for 8o a highly detailed and continuous 8o profile has thus been obtained and is discussed in terms of past temperatures in greenland we also discuss a three core stacked annual 8o profile for the past 917 years the short term 50 years variability of the annual 8o signal is found to be 1 in the holocene and estimates for the coldest parts of the last glacial are 3 or higher these data also provide insights into possible disturbances of the stratigraphic layering in the core which seems to be sound down to the onset of the eemian spectral analysis of highly detailed sequences of the profile helps determine the smoothing of the 8o signal which for the holocene ice is found to be considerably stronger than expected we suggest this is due to a process involving diffusion of water molecules along crystal boundaries in the recrystallizing ice matrix deconvolution techniques were employed for restoring with great confidence the highly attenuated annual 8o signal in the holocene we confirm earlier findings of dramatic temperature changes in greenland during the last glacial cycle abrupt and strong climatic shifts are also found within the eem sangamon interglaciation which is normally recorded as a period of warm and stable climate in lower latitudes the stratigraphic continuity of the eemian layers is consequently discussed in section 3 of this paper in terms of all pertinent data which we are not able to reconcile

seismic anisotropy in the core mantle transition zone split s waves observed at hockley texas from events in the tonga fiji region of the southwest pacific show predominantly vertically polarized shear wave sv energy arriving earlier than horizontally polarized sh energy for rays propagating horizontally through d after corrections are made for the effects of upper mantle anisotropy beneath hockley a time lag of 1 5 to 2 0 s remains for the furthest events 93 9 100 6 while the time lags of the nearer observations 90 5 92 9 nearly disappear at closer distances the s waves from these same events do not penetrate as deeply into the lower mantle and are not split these observations suggest that a patch of d beneath the central pacific is anisotropic while the mantle immediately above the patch is isotropic the thickness of the anisotropic zone appears to be of the order of 100 200 km observations of shear wave splitting have previously been made for paths that traverse d under the caribbean and under alaska sh leads sv the reverse of the hockley observations but in these areas the fact that sv leads sh in the hkt data shown here suggests a different sort of anisotropy under the central pacific from that under alaska and the caribbean the case of sh travelling faster than sv is consistent with transverse isotropy with a vertical axis of symmetry vti and does not require variations with azimuth the case of sv leading sh is consistent with transverse isotropy with a horizontal axis of symmetry hti an azimuthally anisotropic medium and with a vti medium formed by a hexagonal crystal given that mg fe sio3 perovskite appears unlikely to form anisotropic fabrics on a large scale the presence of anisotropy may point to chemical heterogeneity in the lowermost mantle possibly due to mantle core interactions

on the likelihood of post perovskite near the core mantle boundary a statistical interpretation of seismic observations abstract recent experimental studies indicate that perovskite the dominant lower mantle mineral undergoes a phase change to post perovskite at high pressures however it has been unclear whether this transition occurs within the earth s mantle due to uncertainties in both the thermochemical state of the lowermost mantle and the pressure temperature conditions of the phase boundary in this study we compare the relative fit to global seismic data of mantle models which do and do not contain post perovskite following a statistical approach our data comprise more than 10 000 pdiff and sdiff travel times global in coverage from which we extract the global distributions of dln vs and dln vp near the core mantle boundary cmb these distributions are sensitive to the underlying lateral variations in mineralogy and temperature even after seismic uncertainties are taken into account and are ideally suited for investigating the likelihood of the presence of post perovskite a post perovskite bearing cmb region provides a significantly closer fit to the seismic data than a post perovskite free cmb region on both a global and regional scale these results complement previous local seismic reflection studies which have shown a consistency between seismic observations and the physical properties of post perovskite inside the deep earth

upper and lower mantle anisotropy inferred from comprehensive sks and skks splitting measurements from india abstract in this study we investigate the upper mantle anisotropy beneath india using high quality sks and skks waveforms from 382 teleseismic earthquakes recorded at 119 broadband seismic stations in addition we present evidence for anisotropy in the d layer beneath southeast asia using sks and skks splitting discrepancies on the same seismogram during this exercise we obtain 200 new splitting measurements from 35 stations recently deployed in the indo gangetic plains igp central india and northeast india while the delay times between the fast and slow axes of anisotropy t range from 0 3 to 1 7 s the fast polarization azimuths at a majority of stations in the igp and central india coincide with the absolute plate motion of india implying shear at the base of the lithosphere as the dominant mechanism for forging anisotropy however stations in ne india reveal fast polarization azimuths mainly in the ene wsw direction suggestive of lithospheric strain induced by the ongoing indo eurasian collision our analysis for d anisotropy yielded a total of 100 sks skks pairs which can be categorized into those exhibiting i null measurements for one phase and significant splitting for the other phase ii null measurement for both the phases iii significant splitting for both the phases a pair is considered to be anomalous if the splitting difference between sks and skks is 0 5 s and the individual split time is 0 5 s using this criterion we obtain 12 measurements under category iii and 9 under category i that show a null measurement for sks and large splitting for the skks phase further we quantify the strength of the lower mantle anisotropy by correcting the skks measurement for the upper mantle anisotropy obtained by the sks phase on the same seismogram the sks delay times are found to be consistently less than skks times suggesting that the sks phases do not capture the lower mantle anisotropy in comparison to their skks counterparts seven coherent measurements thus obtained reveal measurable d anisotropy with fast polarization azimuths oriented mainly in the ene wsw direction these results suggest presence of a large region of deformation in the lowermost mantle beneath southeast asia a possible model for anisotropy in these regions could be the presence of slab material that pounded upon the core mantle boundary cmb and is experiencing large shear deformation resulting in lattice preferred orientation lpo of the lower mantle van der hilst and karason 1999 long 2009 the other possibility is the phase transformation from mgsio3 perovskite to a more stable post perovskite phase under favorable conditions which results in lpo of the lower mantle

small scale lateral shear velocity and anisotropy heterogeneity near the core mantle boundary beneath the central pacific imaged using broadband scs waves core reflected scs waves from 49 large m 5 1 deep tonga fiji events recorded in western north america are used to study a localized region of the core mantle boundary cmb under the central pacific ocean a total of 248 observations from the berkeley digital seismic network bdsn caltech united states geological survey usgs terrascope and the incorporated research institutions for seismology iris broadband arrays span epicentral distances of 73 85 scs reflection points sample a cmb patch southeast of the hawaiian islands at latitude 4 to 16 n and longitude 156 to 144 w near the proposed source location of the hawaiian plume highly variable scs travel times amplitudes waveforms and shearwave splitting indicate that the lowermost mantle in this region is heterogeneous both laterally and radially scsh sh differential travel times are on average 4 s larger than predicted by the preliminary reference earth model prem this is due to delayed scs arrivals and is largely accounted for by a model with a strong velocity decrease in d the scsh sh residuals also show a spatial trend not accounted for by a radial model indicating a lateral decrease in lower mantle shear velocity to the northeast this lateral velocity gradient appears to cause focusing of scs energy scsh sh amplitude ratios are larger than predicted by prem or by a model with a strong negative gradient with depth in d which enhances scs scs splitting corrected for lithospheric anisotropy beneath the receivers indicates spatial variations in d anisotropy with a systematic change in the orientation of the fast polarization direction from transverse to the ray path fast scsh to parallel to the ray path fast scsv along a northeast traverse these spatial trends suggest lateral gradients in the boundary layer shear flow on scale lengths of a few hundred kilometers which may be related to dynamical flow near the near the root of the large hawaiian plume

deformation crystal preferred orientations and seismic anisotropy in the earth s d layer abstract we use a forward multiscale model that couples atomistic modeling of intracrystalline plasticity mechanisms dislocation glide twinning in mgsio3 post perovskite ppv and periclase mgo at lower mantle pressures and temperatures to polycrystal plasticity simulations to predict crystal preferred orientations cpo development and seismic anisotropy in d we model the cpo evolution in aggregates of 70 ppv and 30 mgo submitted to simple shear axial shortening and along corner flow streamlines which simulate changes in flow orientation similar to those expected at the transition between a downwelling and flow parallel to the core mantle boundary cmb within d or between cmb parallel flow and upwelling at the borders of the large low shear wave velocity provinces llsvp in the lowermost mantle axial shortening results in alignment of ppv 010 axes with the shortening direction simple shear produces ppv cpo with a monoclinic symmetry that rapidly rotates towards parallelism between the dominant 100 010 slip system and the macroscopic shear these predictions differ from mgsio3 post perovskite textures formed in diamond anvil cell experiments but agree with those obtained in simple shear and compression experiments using cairo3 post perovskite development of cpo in ppv and mgo results in seismic anisotropy in d for shear parallel to the cmb at low strain the inclination of scs sdiff and skks fast polarizations and delay times vary depending on the propagation direction at moderate and high shear strains all s waves are polarized nearly horizontally downwelling flow produces sdiff scs and skks fast polarization directions and birefringence that vary gradually as a function of the back azimuth from nearly parallel to inclined by up to 70 to cmb and from null to 5 change in the flow to shear parallel to the cmb results in dispersion of the cpo weakening of the anisotropy and strong azimuthal variation of the s wave splitting up to 250 km from the corner transition from horizontal shear to upwelling also produces weakening of the cpo and complex seismic anisotropy patterns with dominantly inclined fast scs and skks polarizations over most of the upwelling path models that take into account twinning in ppv explain most observations of seismic anisotropy in d but heterogeneity of the flow at scales

the lowermost mantle beneath northern asia ii evidence for lower mantle anisotropy summary we have analysed prediffracted s waves with turning points beneath northern siberia in a study of anisotropy in the lowermost mantle deep focus earthquakes beneath the marianas izu bonin and the sea of japan recorded at stations in western europe are used a correction for upper mantle anisotropy is applied to the data comparisons of the data with synthetic data for models with and without a high velocity d layer suggest that there is a velocity discontinuity at the top of the d region and that the style of anisotropy is transversely isotropic in this region time separations between s waves on the radial and transverse component show a weak trend where the separation increases with epicentral distance a normalization of this separation with the travel distance within d 300 km thick in this region suggests that the anisotropy is uniformly distributed within this layer and has an average value of 0 5 per cent a combination of different studies which investigate the structure of the lowermost mantle beneath europe and northern siberia reveals a complicated picture tomographic models from this area and evidence of d anisotropy lower mantle scatterers reflections from a d discontinuity and ultralow velocity zones suggest two distinct regions one exhibits high velocities d anisotropy a d discontinuity and no evidence of scatterers or ultra low velocity zones these features are likely associated with the palaeosubduction of the izanagi plate well into the lowermost mantle the other region has a lower overall velocity and shows evidence of scatterers and ultra lowvelocity zones perhaps suggesting the presence of partial melt these results suggest dramatic lateral variations in the nature of the lowermost mantle beneath northern asia over a length scale of roughly 30 degrees

shdiff svdiff splitting in an isotropic earth 1 the vertically polarized svdiff seismic phase sometimes arrives later than the horizontally polarized shdiff both phases propagate in the d layer at the base of the mantle and the difference in their arrival times is usually interpreted in terms of seismic anisotropy in d using numerical simulations we demonstrate that a significant 1 s delay of svdiff relative to shdiff is present even in the iasp91 isotropic reference earth model and in a model with a high s velocity in d relative to iasp91 the delay is accompanied by strong amplitude decay of svdiff with distance this relationship resembles the effect of anelasticity in a model with a low s velocity in d the waveform of svdiff can be very different from shdiff and this difference can be mistaken for a delay of shdiff relative to svdiff in a laterally heterogeneous isotropic d svdiff can also be delayed because unlike shdiff svdiff has strong amplitude decay along fast paths its traveltime is thus dominated by slow propagation paths and tends to be positively biased for d models with an s velocity difference of 3 between two hemispheres there are regions where shdiff arrives up to 15 s earlier than the first clearly visible svdiff this is observed in a corridor several hundred kilometers wide if these effects are misinterpreted in terms of d anisotropy in actual seismic data estimates of anisotropy are biased and may contribute to the idea of seismic anisotropy being different in high and low s velocity regions of d

apparent splitting of s waves propagating through an isotropic lowermost mantle observations of shear wave anisotropy are key for understanding the mineralogical structure and flow in the mantle several researchers have reported the presence of seismic anisotropy in the lowermost 150 250 km of the mantle i e d urn x wiley jgrb media jgrb52636 jgrb52636 math 0002 layer based on differences in the arrival times of vertically sv and horizontally sh polarized shear waves by computing waveforms at a period 6 s for a wide range of 1 d and 3 d earth structures we illustrate that a time shift i e apparent splitting between sv and sh may appear in purely isotropic simulations this may be misinterpreted as shear wave anisotropy for near surface earthquakes apparent shear wave splitting can result from the interference of s with the surface reflection ss for deep earthquakes apparent splitting can be due to the s wave triplication in d urn x wiley jgrb media jgrb52636 jgrb52636 math 0003 reflections off discontinuities in the upper mantle and 3 d heterogeneity the wave effects due to anomalous isotropic structure may not be easily distinguished from purely anisotropic effects if the analysis does not involve full waveform simulations

seismic anisotropy in the lowermost mantle beneath the pacific onset time differences of up to 3 s are observed between transverse ssh and longitudinal ssv components of broadband s waves at distances of 85 to 120 for paths traversing the lowermost mantle d beneath the pacific after correction for upper mantle anisotropy ssh usually arrives earlier than ssv with the splitting increasing with distance from 100 to 120 the data yield two possible models of anisotropy 1 anisotropy may vary laterally with transverse isotropy existing in higher than average shear velocity regions beneath the northeastern pacific or 2 anisotropy may vary with depth with transverse isotropy concentrated in a thin 100 km thermal boundary layer at the base of d a few recordings at distances less than 105 show that ssv arrives earlier than ssh indicating that general anisotropy likely exists in shallower regions of d

environmental change and fluvial activity during the younger dryas in central germany abstract late pleistocene climatically induced environmental changes are reconstructed by applying a multidisciplinary approach on floodplain sediments of small to medium sized catchments in central germany radiocarbon dating pollen analysis and the presence of an accurately dated tephra layer allow the establishment of a reliable chronology the beginning of the younger dryas is marked by a change in fluvial activity that lasts for several hundred years during this period gravels and sands were deposited by a braided river system fluvial systems at this time were predominantly controlled by the climatic conditions of the surrounding uplands where the climatic deterioration led to a lowering of the forest limit and enhanced periglacial slope processes an open pine forest prevailed in the basin areas and no evidence of slope wash and solifluction was found the second part of the younger dryas is characterised by a meandering fluvial system and the deposition of overbank fines the rapid transition from younger dryas to preboreal coincides with an increase in organic deposition

the african plate a history of oceanic crust accretion and subduction since the jurassic we present a model for the jurassic to present evolution of plate boundaries and oceanic crust of the african plate based on updated interpretation of magnetic gravity and other geological and geophysical data sets location of continent ocean boundaries and age and geometry of old oceanic crust jurassic and cretaceous are updated in the light of new data and models of passive margin formation a new set of oceanic palaeo age grid models constitutes the basis for estimating the dynamics of oceanic crust through time and can be used as input for quantifying the plate boundary forces that contributed to the african plate palaeo stresses and may have influenced the evolution of intracontinental sedimentary basins as a case study we compute a simple model of palaeo stress for the late cretaceous time in order to assess how ridge push slab pull and horizontal mantle drag might have influenced the continental african plate we note that the changes in length of various plate boundaries especially trenches do not correlate well with absolute plate motion but variations in the mean oceanic crust age seem to be reflected in acceleration or deceleration of the mean absolute plate velocity

spectral element moment tensor inversions for earthquakes in southern california we have developed and implemented an automated moment tensor inversion procedure to determine source parameters for southern california earthquakes the method is based upon spectral element simulations of regional seismic wave propagation in an integrated 3d southern california velocity model sensitivity to source parameters is determined by numerically calculating the frechet derivatives required for the moment tensor inversion we minimize a waveform misfit function and allow limited time shifts between data and corresponding synthetics to accommodate additional 3d heterogeneity not included in our model the technique is applied to three recent southern california earthquakes the 9 september 2001 m l 4 2 hollywood event the 22 february 2003 m l 5 4 big bear event and the 14 december 2001 m l 4 0 diamond bar event using about half of the available three component data at periods of 6 sec and longer we obtain focal mechanisms depths and moment magnitudes that are generally in good agreement with estimates based upon traditional body wave and surface wave inversions

laacher see tephra a widespread isochronous late quaternary tephra layer in central and northern europe a late quaternary tephra layer widespread in central and northern europe resulted from explosive plinian and phreatomagmatic eruptions of the laacher see volcano 11 000 yr b p the tephra is distinguished from other late quaternary andesitic rhyolitic airfall tuff layers in northern europe and from basaltic or trachytic tuff deposits in southern europe by its phonolitic composition and abundance of sanidine plagioclase clinopyroxene amphibole and sphene the proximal tephia sequence at laacher see is divided into three main deposits the predominantly plinian deposits of lower and middle laacher see tephra llst and mlst and phreatomagmatic deposits of the upper laacher see tephra ulst the mlst member is further subdivided into beds a b and c1 c2 and c3 the chemical composition of the magma is highly differentiated phonolite in the llst to mlst b sections but mafic phonolite in the mlst c1 to ulst sections all deposits are considered to be isochronous the frequency maximum of 16 radiocarbon datings indicating an eruption about 11 000 50 yr b p distal ash was deposited in three main fans directed to the northeast lst traced up to 1 100 km distance south lst traced up to 600 km and southwest lst traced up to 100 km tephrostratigraphic correlation of the distal ash deposits is based on a the major element composition of glass shards b lithology and c heavy mineral analyses the northeastern fan consists of deposits from llst mlst b and mlst c1 eruptive phases the southern fan comprises mlst a mlst c2 and ulst deposits and the southwestern fan consists exclusively of ash from the ulst eruptive phase northeastern transport of ash during eruptive phases with high plinian eruption columns but southern and southwestern transport of ash along phases of relatively low eruption columns are interpreted in terms of prevailing southwesterly paleowinds at high altitudes tropopause level but northerly winds dominating in the lower atmosphere the laacher see eruption columns were emplaced into an atmosphere vertically zoned with respect to paleowind directions which also explains the near vent shifting of llst mlst b and mlst c1 iso pach axes from east southeast to northeast within the first 20 km of transport

evidence for normal and inhomogeneous lowermost mantle and core mantle boundary structure under the arctic and northern canada summary seismic p waves from a total of about 200 nuclear explosions in the usa the former ussr and china observed at 10 arrays and four networks in europe canada and the usa are used to analyse the structure at the base of the mantle and the core mantle boundary cmb the simple waveforms and well controlled source parameters of nuclear explosions allow one to use the events as source arrays in addition to the usual receiver array configuration a new array technique double beamforming kruger et al 1993 integrating both concepts is applied which increases the slowness resolution considerably a total of 56 source receiver combinations i e reflection points in the lower mantle or on the cmb could be analysed in five regions anomalous arrivals pdp with slowness and arrival times between those of p and pcp are observed one of these five areas svalbard region shows short period pcp p amplitude ratios which are about three times higher than those predicted by standard earth models in the severnaya zemlya region where pdp and pcp precursors were observed previously kruger et al 1993 pcp shows azimuth deviations of up to 10 for some other regions deviations of the pcp waveform from the direct p waveform are also observed these anomalous phases and the pcp waveform distortions cannot be explained with standard 1 d earth models they are probably produced by inhomogeneities in the lowermost mantle the observed variations in the waveforms are strong indications of a laterally heterogeneous structure in two depth ranges the first is the cmb and its immediate vicinity of a few tens of kilometres the second region is the depth range between about 200 and 300 km above the cmb maps of the north pole region giving the distributions of inhomogeneities in the lower mantle and on the cmb are presented these maps show evidence of strong heterogeneity of the d boundary layer and possibly also of the cmb in the same area

inferences on flow at the base of earth s mantle based on seismic anisotropy we applied global waveform tomography to model radial anisotropy in the whole mantle we found that in the last few hundred kilometers near the core mantle boundary horizontally polarized s wave velocities vsh are on average faster by 1 than vertically polarized s wave velocities vsv suggesting a large scale predominance of horizontal shear this confirms that the d region at the base of the mantle is also a mechanical boundary layer for mantle convection a notable exception to this average signature can be found at the base of the two broad low velocity regions under the pacific ocean and under africa often referred to as superplumes where the anisotropic pattern indicates the onset of vertical flow

high resolution sediment and vegetation responses to younger dryas climate change in varved lake sediments from meerfelder maar germany abstract this work focuses on a 1900 year section of varved sediments from lake meerfelder maar mfm extending from the late allerod to the preboreal varve counting provides the chronological framework and determines the length of younger dryas to 1025 1090 years a strong relation between climate change environment response and depositional processes has been found in consequence varve microfacies variations are a sensitive proxy for environment changes these are reflected for example in erosion processes within the lake s catchment minerogenic input and lake productivity diatom blooms the observed varve changes have been quantified by multiproxy analyses of physical and chemical sediment parameters with a resolution of between 8 and 40 years depending on sedimentation rate in addition high resolution palynological investigations provide the biostratigraphical subdivision based on changes in the vegetation occurring during the same time interval varve observations reveal that environment changes at the beginning and the end of the younger dryas occurred within 20 50 years furthermore sediment and vegetation changes were synchronous within the actual precision of the mfm and grip chronologies divergence of only a few decades terrestrial responses in western europe occurred quasi synchronous to temperature changes in greenland

an event stratigraphy for the last termination in the north atlantic region based on the greenland ice core record a proposal by the intimate group it is suggested that the grip greenland ice core should constitute the stratotype for the last termination based on the oxygen isotope signal in that core a new event stratigraphy spanning the time interval from ca 22 0 to 11 5 k grip yr bp ca 19 0 10 0 k 14 c yr bp is proposed for the north atlantic region this covers the period from the last glacial maximum through termination 1 of the deep ocean record to the pleistocene holocene boundary and encompasses the last glacial late glacial of the traditional northwest european stratigraphy the isotopic record for this period is divided into two stadial episodes greenland stadials 1 gs 1 and 2 gs 2 and two interstadial events greenland interstadials 1 gi 1 and 2 gi 2 in addition gi 1 and gs 2 are further subdivided into shorter episodes the event stratigraphy is equally applicable to ice core marine and terrestrial records and is considered to be a more appropriate classificatory scheme than the terrestrially based radiocarbon dated chronostratigraphy that has been used hitherto 1998 john wiley sons ltd

oxygen isotopes of lake marl at gerzensee and leysin switzerland covering the younger dryas and two minor oscillations and their correlation to the grip ice core the ratio of oxygen isotopes is a temperature proxy both in precipitation and in the calcite of lacustrine sediments the very similar oxygen isotope records from greenland ice cores and european lake sediments during the last glacial termination suggest that the drastic climatic changes occurred quasi simultaneously on an extra regional probably hemispheric scale in order to study temporal relations of the different parameters recorded in lake sediments for example biotic response times to rapid climatic changes a precise chronology is required in unlaminated lake sediments there is not yet available a method to provide a high resolution chronology especially for periods with radiocarbon plateaux alternatively an indirect time scale can be constructed by linking the lake stratigraphy with other well dated climate records new oxygen isotope records from gerzensee and leysin with an estimated sampling resolution of between 15 and 40 years match the greenlandic isotope record in many details under the assumption that the main variations in temperature and thus in oxygen isotopes occurred about simultaneously in greenland and switzerland we have assigned a time scale to the lake sediments of gerzensee and leysin by wiggle matching their stable isotope records with those of greenland ice cores which are among the best dated climatic archives we estimate a precision of 20 to 100 years during the last glacial termination

constraint on the s wave velocity at the base of the mantle some recent seismic studies have suggested the presence of a thin ultra low p wave velocity layer ulvz at the base of the mantle which is interpreted to be due to presence of partial melting partial melting would lead to a strong decrease of the s wave velocity for which there is no seismic evidence such a decrease in the s wave velocity would produce a strong precursor to sks phase from the conversion of s to p at the upper boundary of the layer we analyze records of events from the subduction zones in the south west pacific region obtained at stations in north america at the source side the converted phases propagate in the region where the ultra low p wave velocity has been found earlier our analysis demonstrates that either the s wave velocity drop in this layer is much smaller than predicted by the hypothesis of melting or the layer is so thin less than about 10 15 km that it can not be detected with our technique

synchronous radiocarbon and climate shifts during the last deglaciation radiocarbon data from the cariaco basin provide calibration of the carbon 14 time scale across the period of deglaciation 15 000 to 10 000 years ago with resolution available previously only from holocene tree rings reconstructed changes in atmospheric carbon 14 are larger than previously thought with the largest change occurring simultaneously with the sudden climatic cooling of the younger dryas event carbon 14 and published beryllium 10 data together suggest that concurrent climate and carbon 14 changes were predominantly the result of abrupt shifts in deep ocean ventilation

lowermost mantle anisotropy beneath the pacific imaging the source of the hawaiian plume we utilized recordings of seismic shear phases provided by several north american broadband seismometer arrays to provide unique constraints on shear wave anisotropy beneath the northern and central pacific ocean using a new analysis method that reduces measurement errors and enables the analysis of a larger number of available waveforms we examined relative travel times of teleseismic s and sdiff that sample a large area of lowermost mantle structure the results of this study provide evidence for small scale lateral and depth variations in shear wave anisotropy for a broad region of the lowermost mantle beneath the pacific ocean in particular we image a localized zone of anomalously strong anisotropy whose strength increases toward the top of dq beneath hawaii our results combined with a previous study of vp vsh ratios indicate that ancient subducted slab material may be responsible for observations beneath the northern pacific while lenses or layers of core mantle boundary reaction products or partial melt oriented by horizontal inflow of mantle material to the hawaiian plume source can explain observations beneath the central pacific fl 2001 elsevier science b v all rights reserved

evidence for anisotropy in the deep mantle beneath alaska we consider the possibility that the velocity structure of d is anisotropic the data we examined consist of seismograms from 9 deep japanese earthquakes recorded at wwssn receiver stations in north america the source receiver combinations span distances of 70 106 with associated s waves passing through d beneath alaska differential travel times of the s scd scs and sks phases are used to constrain the velocity structure in d shear waves refracted by d are observed beyond 72 2 and provide a sensitive measurement of the velocity structure in d beyond 93 the vertically polarized sv and horizontally polarized sh shear waves often appear distinctly split although at distances less than 89 the components are more nearly synchronous near 94 sh occurs as a double arrival sv in this range however remains a single arrival roughly synchronous with the second sh arrival we have been unable to reproduce these effects in isotropic model synthetics synthetics for transversely isotropic models have been computed that do match these waveforms the anisotropy was constrained to be only within d with a vertical symmetry axis we conclude that these observations may be explained by an anisotropic d layer the d discontinuity may be due to a transition to anisotropic mantle a few hundred kilometers above the core mantle boundary

influence of late glacial climate changes on sediment transport in the river werra thuringia germany abstract several sedimentary records of floodplain sediments in the werra valley and subcatchments have been analysed in order to investigate the influence of climate change on the sediment transport system in the sedimentary records of the subcatchments a marked change from minerogenic to organic sediments is visible about 14 6 ka b p according to palynological analyses this change occurs in a period with a remarkable rise in arboreal pollen percentages in addition a reverse change from organic to clastic sediments takes place shortly after 12 9 ka b p coincident with a decrease in arboreal pollen percentages during this period assuming that changes in vegetation cover are mainly caused by climate the variations in the sedimentary records are also attributed to these climatic oscillations in contrast to the subcatchments comparable changes in the sedimentary record are not detectable in the floodplain of the werra river this points to the conclusion that the influence of late glacial climate fluctuations on the sedimentary regime of a river with a relatively large and a more heterogeneous catchment seems less pronounced