accelerator mass spectrometer ages for the younger dryas event in atlantic canada abstract in atlantic canada bulk sediment dates for the onset of a late glacial cooling widely regarded as the younger dryas event are highly variable ranging from 14 300 270 to 10 800 100 yr b p we present the first accelerator mass spectrometer ams 14c dates from atlantic canada at or close to the boundaries of this event from six sites in new brunswick and nova scotia the mean of five dates places the onset of this cooling at ca 10 770 yr b p the inclusion of a sixth perhaps anomalously old date changes the mean to 10 880 yr b p the termination averages three dates ca 10 000 yr b p these dates place the timing of the younger dryas event in atlantic canada closer in line with the traditional chronozone boundaries of 11 000 and 10 000 yr b p in northwest europe

buckling instabilities of subducted lithosphere beneath the transition zone a sheet of viscous fluid poured onto a surface buckles periodically to generate a pile of regular folds recent tomographic images beneath subduction zones together with quantitative fluid mechanical scaling laws suggest that a similar instability can occur when slabs of subducted oceanic lithosphere encounter enhanced resistance to penetration at 660 km depth beneath subduction zones such as central america and java the fast anomalies in elastic wavespeed are too broad 400 km to be explained by simple compressional thickening of subducted lithosphere that is initially 50 100 km thick by contrast a periodic buckling mechanism can account for both the characteristic wedge shapes of the anomalies and for their widths which agree closely with those predicted by the scaling laws in addition to providing a plausible physical interpretation of tomographic observations beneath some subduction zones buckling instabilities have important consequences for other aspects of global geodynamics such as tectonic plate reconstructions interpretation of geoid anomalies and modeling of secular changes in earth s moment of inertia

investigating the linear and nonlinear stationary wave response to anomalous north american snow cover abstractcontinental scale snow cover represents a broad thermal forcing on monthly to intraseasonal time scales with the potential to modify local and remote atmospheric circulation a previous gcm study reported robust transient eddy responses to prescribed anomalous north american na snow cover the same set of experiments also indicated a robust upper level stationary wave response during spring but the nature of this response was not investigated until now here the authors diagnose a deep snow induced tropospheric cooling over na and hypothesize that this may represent a pathway linking snow to the stationary wave response a nonlinear stationary wave model is shown to reproduce the gcm stationary wave response to snow more accurately than a linear model and results confirm that diabatic cooling is the primary driver of the stationary wave response in particular the total nonlinear effects due to cooling and its interactions with transient eddies and orography are shown to be essential fo

a numerical sensitivity study of the influence of siberian snow on the northern annular mode abstractthe climate version of the general circulation model action de recherche petite echelle grande echelle arpege climat is used to explore the relationship between the autumn siberian snow and the subsequent winter northern annular mode by imposing snow anomalies over siberia as the model presents some biases in the representation of the polar vortex a nudging methodology is used to obtain a more realistic but still interactive extratropical stratosphere in the model free and nudged sensitivity experiments are compared to discuss the dependence of the results on the northern stratosphere climatology for each experiment a positive snow mass anomaly imposed from october to march over siberia leads to significant impacts on the winter atmospheric circulation in the extratropics in line with previous studies the model response resembles the negative phase of the arctic oscillation the well documented stratospheric pathway between snow and the arctic oscillation operates in the nudged experiment

response of interior north america to abrupt climate oscillations in the north atlantic region during the last deglaciation several broad scale climatic oscillations during the last deglaciation are well documented in regions around the north atlantic ocean this paper reviews empirical evidence for these deglacial climatic oscillations from non coastal north america and discusses implications for testing climatic simulations and for understanding the cause and transmittal mechanisms paleoclimatic interpretation of oxygen isotope records from several small sites in the eastern great lakes region indicates a classic deglacial climatic sequence that is comparable with records from europe and greenland the climatic events as interpreted from crawford lake oxygen isotopes include the bolling allerod boa warming at 12 700 14c bp a warm boa at 12 500 10 920 14c bp an intra allerod cold period shortly before 11 000 14c bp a cold younger dryas yd climate reversal at 10 920 10 000 14c bp the holocene warming at 10 000 14c bp a brief preboreal oscillation pb at 9650 14c bp and an early holocene cold event at 7500 14c bp 8200 cal bp some of these events were also evident from changes in upland and aquatic vegetation and sediment lithology the pronounced yd climatic reversal has been documented from pollen records along the ecotones at this time and from glacier readvances in the great lakes region along the rocky mountains from alberta to colorado the yd event is indicated by alpine glacier advance and or shift in timberline vegetation in minnesota and upper new york early holocene climatic instability is also suggested by oxygen isotope records and or varve thickness the regional variations in evidence for the yd and other events in north america suggest that climatic oscillations may have different expressions in paleo records depending on geographic location and characteristics of a particular site the extent and magnitude of these climatic oscillations across north america suggest that these oscillations are an expression of climatic change that was probably widespread rather than locally induced by a nearby glacier the location of these sites implies that climatic signals were likely carried over the northern hemisphere through the atmosphere as indicated by general circulation models we hypothesize that the lack of evidence for a cold yd in interior north america west of the great lakes region and east of the rocky mountains was caused by the trapping of cold arctic air mass north of the laurentide ice sheet and by uninterrupted northward expansion of warm caribbean air this strongly contrasted climate also produced non analogous biological assemblages

vegetation et fluctuations climatiques postglaciaires sur la cote septentrionale gaspesienne quebec the postglacial ca 13 000 bp vegetational and climatic history is reconstructed from the pollen analytical study of the sediments of two lakes located in coastal northern gaspe peninsula j arrive lake altitude 56 m was submerged by the goldthwait sea until about 11 400 bp euloge lake 83 m in altitude and less than 1 km from the shore escaped the marine invasion before 10 400 bp the vegetation very scarce at first evolved to a herb tundra that lasted for over a millenium a cold climatic oscillation correlative with the younger dryas is identified at both sites between 10 700 and 10 400 bp during this herb tundra phase after 10 400 bp dwarf birch and willows colonized the sites leading to a shrub tundra the afforestation of the area began only around 9300 bp with the late arrival of spruce and aspen this phase was interrupted at 8650 bp by the upsurge of green alder in response to another cold oscillation since 7250 bp the present forested landscape finally developed in three steps characterized by i a high diversity of forest species until 2500 bp ii a loss of diversity until 1000 bp that favoured spruce and balsam fir and iii an increase of heliophilous trees shrubs and herbs these reconstructions contribute to the late glacial palaeogeography of eastern north america through the unravelling of conditions close to the retreating ice

dispersion of p waves in subducted lithosphere evidence for an eclogite layer cold subducted lithosphere has relatively fast seismic velocity which leads to early arrivals for some event station paths the effect is very large for events in the tonga kermadec deep seismic zone recorded at certain new zealand stations these particular arrivals are very high frequency 3 hz or greater and sometimes resemble two distinct phases the later arrival appearing at about the time predicted by jeffreys bullen tables data from the digital station snzo in wellington confirm the travel time results of the analog stations and furthermore show frequencies above 5 hz much higher than can be seen on analog records and up to 4 dispersion in the range 1 8 hz energy in the second phase which is often absent at snzo is mainly 1 2 hz the digital data support the idea proposed earlier that the effect is caused by propagation through a thin slab which passes only short wavelength waves the essential features of the wave propagation are modeled by acoustic waves in a one dimensional high velocity slab the waveforms produced by the model are discussed in terms of the leaky modes of the system and calculated by a reflectivity method a very thin 15 km uniform slab provides the required dispersion but the waves are heavily attenuated and would never be observed at teleseismic distances a thicker slab allows the energy through but does not give enough dispersion altering the variation of velocity across the slab provides the required dispersion if a thick high velocity layer with wave speed increasing gradually with height is overlain by a thin lid of even higher velocity for the models considered the lid thickness must lie in the range 6 15 km and be continuous from a depth of about 50 km to the bottom of the earthquake zone the thick layer could arise from the thermal anomaly in the subducted lithosphere the thin lid may be the gabbroic part of the subducted crust that has transformed to eclogite

observations of body wave multipathing from broadband seismograms evidence for lower mantle slab penetration beneath the sea of okhotsk we present observations of anomalously broad and complex sh waveforms from two deep focus earthquakes in the southern kurils to digital world wide standard seismograph network and regional seismic test network stations in north america by making use of broadband three component seismograms and relying on a variety of phases one can limit the class of possible explanations for these phases in particular it is shown that the anomalous arrivals are not due to source complexity near receiver structure a triplication from a lower mantle velocity gradient a low q zone in the mid lower mantle or shear wave splitting we show that these arrivals correspond to relatively fast paths and have enhanced amplitudes indicating that they are most probably caused by multipathing due to interaction with a high velocity lower mantle feature below the source region we examine the hypothesis that this feature may be a lower mantle extension of the kuril slab ray tracing experiments show that multipathing is indeed expected along the azimuths corresponding to the north american stations used in this study for a realistic slab model observations of reversed polarity arrivals are consistent with such an explanation where it appears that two rays comprising the arrival emerge from different points on the focal sphere one test of this explanation is that slab induced multipathing should not be seen along back of slab oceanic azimuths examinations of seven events for which the stations are in this azimuth range do not exhibit the same broadening and complexity of the sh waveforms identification of slab induced multipathing associated with deep focus events has two important consequences first it appears to require slab penetration beyond the 650 km discontinuity at least 300 400 km into the lower mantle under the sea of okhotsk second these phases may act as a contaminant when attempting to study other seismic phenomena such as discontinuities low q zones near receiver structure or source complexity

seismic velocity structure of the subducting pacific plate in the izu bonin region although numerous studies have been made on the contrast between subducting slabs and surrounding mantle little is known about the internal structure of the oceanic lithosphere after it descends into the mantle at subduction zones the velocity structure within the slab can be expected to be heterogeneous based on thermal and petrological considerations in this study observed travel time residual data from a spatially dense seismic network above deep earthquakes in the izu bonin region are compared with three dimensional ray tracing calculations the data are inconsistent with a homogeneous slab model and consistent with a heterogeneous slab model with regional velocity variations the residual data can be explained by a model that has a velocity gradient within the slab in the subducting pacific plate the velocity near the center of the slab is faster than that near the upper boundary and gradually decreases toward the bottom of the plate a model with a velocity decrease of 3 as predicted by a thermal profile explains the observed data

fine scale structure of the mid mantle characterised by global stacks of pp precursors abstract subduction zones are likely a major source of compositional heterogeneities in the mantle which may preserve a record of the subduction history and mantle convection processes the fine scale structure associated with mantle heterogeneities can be studied using the scattered seismic wavefield that arrives as coda to or as energy preceding many body wave arrivals in this study we analyse precursors to pp by creating stacks recorded at globally distributed stations we create stacks aligned on the pp arrival in 5 distance bins with range 70 120 from 600 earthquakes recorded at 193 stations stacking a total of 7320 seismic records as the energy trailing the direct p arrival the p coda interferes with the pp precursors we suppress the p coda by subtracting a best fitting exponential curve to this energy the resultant stacks show that pp precursors related to scattering from heterogeneities in the mantle are present for all distances lateral variations are explored by producing two regional stacks across the atlantic and pacific hemispheres but we find only negligible differences in the precursory signature between these two regions the similarity of these two regions suggests that well mixed subducted material can survive at upper and mid mantle depth to describe the scattered wavefield in the mantle we compare the global stacks to synthetic seismograms generated using a monte carlo phonon scattering technique we propose a best fitting layered heterogeneity model brt2017 characterised by a three layer mantle with a background heterogeneity strength 0 8 and a depth interval of increased heterogeneity strength 1 between 1000 km and 1800 km the scalelength of heterogeneity is found to be 8 km throughout the mantle since mantle heterogeneity of 8 km scale may be linked to subducted oceanic crust the detection of increased heterogeneity at mid mantle depths could be associated with stalled slabs due to increases in viscosity supporting recent observations of mantle viscosity increases due to the iron spin transition at depths of 1000 km

orographic constraints on a modeled siberian snow tropospheric stratospheric teleconnection pathway previous modeling studies have identified a teleconnection pathway linking observation based early season siberian snow perturbations to a modulation of the winter arctic oscillation ao mode in this study the key role of orography in producing this modeled teleconnection is explicitly investigated using numerical experiments analogous to the previous studies the climatic response to the same snow perturbation is investigated under modified orographic barriers in southern and eastern siberia reducing these barriers results in a weakening of the prevailing orographically forced region of stationary wave activity centered over siberia as well as the snow forced upward wave flux anomaly that initiates the teleconnection this diminished anomaly propagates upward but does not extend into the stratosphere to weaken the polar vortex consequently poleward refraction of upper tropospheric waves and downward propagation of coupled wave mean flow anomalies which ultimately produce the negative winter ao response fail to develop thus the mountains represent an orographic constraint on the snow ao teleconnection pathway by reducing the orographic barrier the snow forced influx of wave energy remains in the troposphere and instead produces a hemispheric scale equatorward wave refraction

test of the lower mantle slab penetration hypothesis using broadband s waves bioadband s and scs displacement pulses from deep focus earthquakes in the sea of okhotsk recorded at analog and digital stations in north america and europe show com plexities produced by velocity heterogeneity in the lower mantle the slab beneath the sea of okhotsk strikes n40oe while the stations used in this study range from n40ow to n70oe allowing a test of the hypothesis that multipathing in a lower mantle slab extension gives rise to the waveform complexities comparisons of the s and scs displacement pulses at a given station indicate that in many cases the direct s pulses have a broader more complex waveform than the scs pulses we parameterized the waveform complexity by the energy tempo al centroid difference cs csc s between the s and scs pulses the largest cs csc s anomalies are observed along the strike of the slab there is also a correlation between the cs csc s values and scs s travel time residuals for stations along the strike ofthe slab and no correlation for off strike azimuths however cs csc s anomalies are ob served over the entire azimuth range including azimuths to the southwestern u s for which the raypaths should quickly exit any deep slab structure these data suggest that lateral velocity heterogeneities both near the source possibly due to slab pene tration and deep in the mantle near the s wave turning points contribute to the s and scs complexity

the caribbean anomaly short wavelength lateral heterogeneity in the lower mantle data from the archean proterozoic transect experiment 1989 apt89 are used to constrain smaller scale velocity heterogeneity in the lower mantle beneath the caribbean we observe large variations in travel time residuals for paths from south america to portable broadband and short period stations distributed along a 1500km long transect in central north america after accounting for near source and near reciever contributions we find variations as large as 3s for p and 8s for s for rays that bottom around 1200km depth this rapid spatial variation in travel times suggests that we are seeing the western edge of the caribbean anomaly a feature previously observed by several investigators the gradient appears to be about 300km wide assuming a path length of 1500km through the anomaly suggested from previous studies we find that p p 2 1 and s s 3 6 which for s waves is about three times the estimate from long period body waves the relative velocity ratio for this anomaly is ln s ln p 1 7 a value slightly larger than typical laboratory values but lower than previous recent seismological studies of lower mantle heterogeneity with increasing turning depth the observed anomaly appears to decrease in size although it continues to be visible for shear waves with bottoming depths as great as 2200km the presence of multiple direct s phases for several records of one of the events studied suggests multipathing induced by the large observed lateral velocity gradients required by the travel time data

study on impact of the south asian summer monsoon on the down valley wind on the northern slope of mt everest 1 based on the observational data from the hest2006 campaign in the rongbuk valley on the northern slope of mt everest in june 2006 the variation of surface wind in the rongbuk valley and possible impact of the south asian summer monsoon sasm on the surface wind are analyzed during the observation campaign sasm experienced a break stage and an active stage in middle and late june 2006 respectively the surface wind variation measured in the rongbuk valley is closely related to the sasm evolution with a strong down valley wind coinciding with the break stage of sasm and a weak down valley wind with the active stage it is suggested that the impact of sasm on the surface wind on the northern slope of mt everest can be attributed to change of radiation conditions

seismic observation of a high velocity slab 1200 1600 km in depth a slablike high velocity region 1200 1600 km beneath the east coast of north america apparently has a half width of 450 km and a lateral extent of 3000 km and a depth extent of more than 800 km this slab lies beneath the boundary where the farallon plate had been subducting for up to 120 ma the high velocity anomaly has been inferred previously from travel time studies this study uses body waveforms to verify and refine its structure the anomalous region has a peak shear wave velocity 1 5 greater than the surrounding region and compressional velocity 0 6 greater the width and smoothness of this anomaly is constrained by the complexity observed in sh waveforms from deep earthquakes beneath south america observed at gdsn stations in north america this slab is about 5 times broader and 5 times weaker in its velocity anomaly than most slabs observed in the upper mantle this observation implies that if this slab has passed through the 650 km discontinuity which is the most likely explanation for the observation it has been spread by more than simply thermal conductivity the possibility that there is two layer convection with thermal coupling is not ruled out however as thermal coupling could lead to a similar pattern

late quaternary paleohydrology of the gulf of st lawrence quebec canada based on diatom analysis abstract postglacial sediments from three cores raised from the gulf of st lawrence along the path of the laurentian channel were analyzed for diatoms a sequence of four distinct zones was defined from oldest to youngest 1 characterized by low diatom concentrations this zone is mainly represented by heavily silicified species delphineis surirella stephanopyxis turris coscinodiscus radiatus and paralia ornata that were probably deposited by glacial drift in a glaciomarine environment from about 18 000 to 14 000 years bp 2 this postglacial sedimentation zone deposited around 13 000 years bp is marked by high diatom concentrations with cold water marine species thalassiosira antarctica resting spores and porosira glacialis the upper part of the zone suggests a gradual warming with the appearance of temperate water species bacterosira bathyomphala and chaetoceros spp both resting spores and vegetative cells 3 an important cooling of surface temperatures is noticed by the resurgence of cold species porosira glacialis thalassiosira antarctica resting spores and thalassiosira hyalina this zone is associated with the younger dryas chronozone dated from 10 800 to 10 300 years bp this interval is also characterized by two marked freshwater pulses coming from the outflow of laurentide ice sheet meltwater on the surface layer of the goldthwait sea although well defined in the northwestern gulf area core 90 031 015 the influence of this meltwater decreases considerably or even disappears in cabot strait core 89 007 111 and offshore core 90 031 044 evidence of reduced postglacial glacial runoff on surface water supports the conclusion that it could not have generated the younger dryas cooling episode 4 the diatom assemblages from 10 000 years bp through recent correspond to the progressive establishment of modern like surface water conditions

a look at the date of snowmelt and correlations with the arctic oscillation spring snow cover across arctic lands has on average retreated 5 days earlier since the late 1980s compared to the previous 20 years however it appears that since about the late 1980s the date the snowline first retreats north during the spring has changed only slightly in the last 20 years snow has not been disappearing significantly earlier snowmelt changes observed since the late 1980s have been step like unlike the more continuous downward trend seen in arctic sea ice extent at 70 n several longitudinal segments of 10 show significant negative trends while only two longitudinal segments at 60 n show significant trends one positive and one negative these variations appear to be related to variations in the arctic oscillation ao when the springtime ao is strongly positive snow melts earlier when it is strongly negative snow disappears later in the spring the winter ao is less straightforward at higher latitudes 70 n a positive ao during the winter months is correlated with later snowmelt but at lower latitudes 50 n and 60 n a positive wintertime ao is correlated with earlier snowmelt if the ao during the winter months is negative the reverse is true similar stepwise changes since the late 1980s have been noted in sea surface temperatures and in phytoplankton abundance as well as in snow cover

extratropical influences on the inter annual variability of south asian monsoon the effects of extratropical dynamics on the interannual variations in south asian monsoon sam are examined based on ncep ncar reanalysis and cru precipitation data a conditional maximum covariance analysis is performed on sea level pressure 200 hpa geopotential heights and the sam rainfall by removing the linear effects of el nino southern oscillation from the fields it is found that two modes provide a strong connection between the upper level circulation in the atlantic european region and sam rainfall the circumglobal teleconnection cgt and the summer north atlantic oscillation snao the structures in the 200 hpa heights of both modes in the atlantic region are similar in the atlantic region and their southeastward extension to south asia sa also corresponds to upper level ridges in their positive phases in slightly different positions nevertheless the influence of both modes on sam rainfall is distinct whereas a positive cgt is related to a widespread increase of rainfall in sam a positive snao is related to a precipitation dipole with its positive phase over pakistan and the negative phase over northern india the physical mechanisms for the influence of cgt and snao on sam are related to the upper level geopotential anomaly which affects the amplitude and position of the low level convergence the small displacements of the centers of these responses and the low level cold advection from the north east of sa in case of snao explain the differences in the corresponding sam rainfall distributions these findings are confirmed with the relatively high resolution coupled climate model ec earth which gives confidence in the physical basis and robustness of these extratropical variability modes and their influence on the south asian monsoon rainfall

controls of the structure of subducted slabs numerical simulations of subducting slabs are formulated in which the shape and dip of the slab are determined by the dynamics of the flow rather than imposed a priori the dip of slabs is a function of the time since the initiation of subduction slabs fold develop a kink in dip and thicken on entry into a high viscosity lower mantle comparison of the simulations with seismic observations suggest that the lower mantle is at least 10 30 times more viscous than the upper mantle

the climate of north america and adjacent ocean waters ca 6 ka the climate of north america and the adjacent ocean at 6000 bp was estimated using five independent approaches using pollen data the terrestrial climate was estimated by the movement of ecozone boundaries and by the method of modern analogues both analyses indicate warmer temperatures in the western great lakes area and the northern great plains a model of sphagnum dominated peatland initiation when forced by canadian climate model 6 ka output projected a cooler and or wetter climate for continental western north america contrary to this a reconstruction of the distribution of sphagnum dominated peatlands in western canada indicates that they were located north of their modern distribution suggesting warmer and or drier conditions at 6000 bp this interpretation is strengthened by observations of lower lake levels at 6000 bp in western canada this drier climate may have been associated with warmer conditions as indicated by the quantitative climate reconstructions in general eastern north a

the relative influence of laurentide and local ice sheets during the last glacial maximum in the eastern chic chocs range northern gaspe peninsula quebec geochemical signatures erratic dispersal and striae indicate glacial flows in the north central gaspe peninsula exclusively from appalachian sources except for the coastal fringe during the last glacial maximum lgm ice from the monts mcgerrigle flowed northward and northwestward over the summits of the eastern chic chocs range and the coastal plateau to the gulf of st lawrence lesser flows also occurred to the north from mont albert and the western chic chocs range where intersecting striae were noted the older flows are associated with a locally developed ice cap the rare occurrences of striae and erratics and morphometric and lithological characteristics of summit diamictons imply limited basal erosion by both local and regional ice caps this is postulated to result from early protection of rock surfaces by the initial buildup of thin frozen based ice in the eastern chic chocs range a compact till mantle in the cols and on the coastal plateau and striae parallel to col directions indica

on isc pp and pp data and their use in delay time tomography of the caribbean region summary a major problem in p delay time tomography is the inhomogeneous sampling of mantle structure by the p wave ray paths resulting in low resolution in images of large regions of the upper mantle incorporation of pp and pp phases can improve the quality and reliability of tomographic images because they 1 sample earth structure not ordinarily sampled by direct p phases 2 add rays that are oblique to rays of direct phases which is especially important where the latter sample mantle structure in selected directions and 3 pp data better constrain the earthquake focal depths pp traveltimes have often been used in combination with p data in differential traveltime studies we show that the assumptions and approximations necessary for this approach are problematic and that they can be avoided when the p pp and p data are used in tomographic inversion we investigated the applicability of pp and pp delay times to the tomographic study of the aspherical mantle structure below the caribbean region the success of the application of data of the later arriving reflected waves depends critically on the quality of these data we examined possible sources of error in the isc pp and pp data and assessed the contribution to the delay times used in this study for the caribbean region analyses of the isc pp and pp delay times do not reveal biases due to effects of pp waveform distortions the asymmetry of the reflections or due to misidentifications of phases that reflect at a surface other than that assumed the noise level of pp and pp data is high with respect to data of the direct p wave this is accommodated by weighting with the inverse of the variance of the data of each of the three phases the independent information that is revealed from the pp and pp data results in modifications of tomographic images based solely on p data these modifications are important if the tomographic images are being used to understand the geodynamical history of convergent margins in the caribbean region we investigated the effect of adding data of later arriving phases to the isc p data with sensitivity tests we inverted synthetic delay times to which we added gaussian noise with a standard error typical for the data of the three seismic phases these tests demonstrate that the image resolution of shallow mantle structure is enhanced significantly by the incorporation of later arriving phases due to the absence of seismicity below 200 km the resolution improved less at deeper levels below the caribbean region in some poorly constrained parts of the solution the test results even indicate an apparent decrease of resolution this is explained by changes in the rate of convergence of the inversion algorithm in mantle regions where the effective sampling of structure improved by the addition of pp and pp wave ray paths the convergence was speeded up at the expense of the convergence rate in regions where fewer or no pp or pp wave ray paths were added a shortcoming of the resolution tests used in our study is that some specific problems of reported delay times are not reflected in synthetic data we observe that isc delay times of later arriving phases are not necessarily consistent with the reported hypocentral parameters as most isc reported earthquake locations are computed from direct p wave data for the pp data the inconsistency with event location not only results in a decrease of focal depths during relocation but in a bias of the imaged velocity perturbations as well this property of reported data is not modelled in resolution tests

waveform modeling of the slab beneath japan 1 the tomographic p wave model for the japan subduction zone derived by zhao et al 1994 has two very striking features a slab about 90 km thick with p wave velocities 3 6 higher than the surrounding mantle and a mantle wedge with 6 low velocity anomalies we study three component seismograms from more than 600 hi net stations produced by two earthquakes which occurred in the downgoing pacific plate at depths greater than 400 km we simulate body wave propagation in the three dimensional 3 d p wave model using 2 d finite difference fdm and 3 d spectral element sem methods as measured by cross correlation between synthetics and data the p wave model typically explains about half of the traveltime anomaly and some of the waveform complexity but fails to predict the extended sh wave train in this study we take advantage of the densely distributed hi net stations and use 2 d fdm modeling to simulate the p sv and sh waveforms our 2 d model suggests that a thin elongated low velocity zone exists atop the slab extending down to a depth of 300 km with an s wave velocity reduction of 14 if a thickness of 20 km is assumed further 3 d sem simulations confirm that this model explains a strong secondary arrival which cannot easily be imaged with standard tomographic techniques the low velocity layer could explain the relatively weak coupling associated with most subduction zones at shallow depths 50 km generally involving abundant volcanic activity and silent earthquakes and it may also help to further our understanding of the water related phase transition of ultramafic rocks and the nature of seismicity at intermediate depths 70 300 km

comparison of marine and terrestrial holocene climatic reconstructions from northeastern north america quantitative climate reconstructions based on marine dinocysts and terrestrial pollen sequences are consistent through the holocene in northeastern north america principal components analysis pca indicates a large scale climate signal in the dinocysts and pollen the combined and separate analyses of marine and nearby terrestrial pollen sequences from hudson bay labrador and the st lawrence estuary differentiate tun dra boreal forest and deciduous forest assemblages in time and space these analyses indicate that the marine pollen record reflects vegetation changes of the regional terrestrial environment and allows direct correlations between the marine and terrestrial stratigraphies sea surface temperatures estimated from dinocysts and terres trial air temperatures from pollen using the method of modern analogs show that the three regions had differing climate histories associated with their location with respect to deglaciation and air mass boundaries high frequency climatic changes reconstructed

relation between eurasian snow cover snow depth and the indian summer monsoon an observational study abstract satellite derived snow cover data for 22 yr and snow depth data for 9 yr over eurasia have been analyzed to reexamine the possible relation of snow with the indian summer monsoon in contrast to the previous studies that use snow cover averaged over all of eurasia as a single number the frequency of occurrence of snow at each grid point over eurasia is correlated with the indian summer monsoon rainfall thus specific geographical regions over eurasia that are responsible for the well known inverse relationship between eurasian snow cover and indian monsoon rainfall are delineated it is found somewhat surprisingly that western eurasia is the only geographical region for which a significant inverse correlation exists between winter snow cover and subsequent summer monsoon rainfall however composites for high and low snow cover over eurasia show spatially homogeneous large scale patterns of snow cover and surface temperature anomalies winters of high and low snow cover for eurasia are found t

comparative radiocarbon dating of terrestrial plant macrofossils and aquatic moss from the ice free corridor of western canada in order to assess the reliability of aquatic moss for radiocarbon dating sup 14 c analyses were performed on a stratigraphic series of terrestrial plant macrofossils and samples of drepanocladus crassicostatus from a small hard water lake ph 8 2 in the ice free corridor of alberta all sup 14 c dating was done by using accelerator mass spectrometry mazama ash provided an independent chronological control the aquatic bryophyte samples consistently produced sup 14 c ages significantly older than the terrestrial macrofossils the relation between the radiocarbon dates from the macrofossils and the moss was not linear and age differences ranged from approximately 1400 to 6400 yr the sup 14 c content of d crassicostatus growing in the lake at present was less than 85 modern despite the apparent inability to take up sup 14 c deficient carbon by the direct incorporation of bicarbonate the bryophytes clearly do not provide reliable material sup 14 c dating the sup 14 c deficiency of aquatic mosses may be explained by the generation of sup 14 c deficient co sub 2 through isotopic exchange the formation of co sub 2 from bicarbonate by chemical processes and metabolic co sub 2 production these results demonstrate the potential unreliability of sup 14 c dates from aquatic mosses and raise serious concernsmore about the deglaciation dates from the ice free corridor that were obtained from aquatic drepanocladus less

regional change in snow water equivalent surface air temperature relationship over eurasia during boreal spring present study investigates local relationship between surface air temperature and snow water equivalent swe change over mid and high latitudes of eurasia during boreal spring positive correlation is generally observed around the periphery of snow covered region indicative of an effect of snow on surface temperature change in contrast negative correlation is usually found over large snow amount area implying a response of snow change to wind induced surface temperature anomalies with the seasonal retreat of snow covered region region of positive correlation between swe and surface air temperature shifts northeastward from march to may a diagnosis of surface heat flux anomalies in april suggests that the snow impact on surface air temperature is dominant in east europe and west siberia through modulating surface shortwave radiation in contrast atmospheric effect on swe is important in siberia and russia far east through wind induced surface sensible heat flux change further analysis reveals that atmospheric circulation anomalies in association with snowmelt over east siberia may be partly attributed to sea surface temperature anomalies in the north atlantic and the atmospheric circulation anomaly pattern associated with snowmelt over russia far east has a close association with the arctic oscillation

wisconsinan late glacial environmental change in nova scotia a regional synthesis a synthesis of the interpreted vegetational and climatic history of nova scotia during the wisconsinan late glacial period 14 9 ka bp is presented and described in 500 year intervals the concomitant regional deglaciation history of the region is also described a synthesis chart is provided which includes a qualitative curve of temperature reconstructions for the late glacial period

seismic strain rate and deep slab deformation in tonga seismic strain rates estimated from volume averaged moment tensor data are combined with the strike dip and extent of the subducting lithosphere obtained from residual sphere analysis and seismicity to map deformation in the deep tonga slab tonga strain rate orientation tensors constructed from the harvard centroid moment tensor catalog are consistent with slab deformation models in which the lithosphere encounters resistance to penetration at the 650 km discontinuity those which sample lithospheric volumes from 18 s to 25 s at 500 700 km depth indicate that the axis of maximum shortening lies within 15 of the slab s downdip azimuth and plunge the discrepancy between shortening and slab dip may be larger in northernmost tonga 17 s 18 s where slab morphology is less well constrained but overall the slab is compressing roughly parallel to its trajectory tonga strain rate orientation tensors also indicate horizontal strain both parallel and perpendicular to the strike of the slab the proportion of along arc strain rate decreases from northern to southern tonga the scalar rate of seismic strain peaks at certain latitudes but its values averaged over northern and central tonga and southern tonga are roughly equal strain rate data may be related nonuniquely to constraints on slab structure and finite strain inferred from seismicity and travel time data assuming that aseismic and seismic flow have the same geometry and that the total rate of strain is constant along the arc the degree of cross strike advective thickening increases to the south while along arc strain decreases if along arc strain rate exclusively produces southward shear of the deeper lithosphere and the amount of shear is estimated from the offset during subduction of the northern edge of the slab and the louisville ridge thickening of at least 50 is required in central tonga this lower bound improves the resolution of minimum slab width provided by travel time observations if 50 thickening occurs an upper bound on the estimated rate of seismic moment release is large enough to account for scalar strain rate in the seismogenic core of the slab larger thickening factors or flow in the outer regions of the slab require aseismic deformation assuming that the observed travel time residuals are the result of a single coherent slab a synthesis of seismicity strain rate and travel time data indicate a slab structure in northern tonga which curves sharply to the west at the base of the upper mantle but which strikes roughly to the north at shallower depths and in the lower mantle such a contortion in the deep lithosphere is consistent with the subduction zone configuration prior to 8 m y and the tectonic history of the region

waveform complexity in teleseismic broadband sh displacements slab diffractions or deep mantle reflections transverse component bodywave ground displacements from deep focus earthquakes exhibit complexities which are not accounted for by standard radially symmetric earth models this anomalous behavior is examined using 123 broadband teleseismic sh displacement recordings reconstituted from short and long period or broadband seismograms for 11 events in the kurile subduction zone the greatest complexity involving an extra arrival between the s and scs phases at epicentral distances greater than 72 has a strong distance dependence and little azimuthal variation with respect to the source region favoring an interpretation as a deep mantle triplication rather than a near source slab diffraction

seismic modelling of subduction zones with inhomogeneity and anisotropy i teleseismic p wavefront tracking summary traveltimes and amplitudes for p waves from a 500 km deep source in the kuril subduction zone have been synthesized by ray tracing in smooth 3 d models that allow general anisotropy and inhomogeneity the aim is to compare the effects of proposed anisotropy in or near slabs with those of lateral heterogeneity alone to concentrate on these effects the source position slab thickness 90 km dip 63 and velocity anomaly 5 per cent are held constant results are presented for isotropic models with slab penetration to 670km and 1000 km anisotropic models with 670 km deep slabs have anisotropy within the slab anderson 1987 and in a 10 wedge above the slab ribe 1989a mckenzie 1979 the resulting wavefront topology is never as simple as that in a laterally homogeneous reference earth and there is strong model dependence of shadow zones caustics and areas of multipathing rays are traced through slab models defined by 3 d cubic spline interpolation of up to 21 elastic constants outside the slab region 1 d ray tracing through prem and spherical trigonometry are used to complete the ray path the results illustrate the importance of using both traveltime and amplitude information when interpreting slab structure from teleseismic data some anisotropic slab models have been found which produce large 2 s traveltime residuals that are similar in many parts of the world to those for the deep isotropic model but the amplitude patterns are substantially different the model with a deep isotropic slab produces a narrow band of large traveltime residuals 3 s and high amplitudes in a region across northern canada this feature is due to the focusing of rays that have travelled down the high velocity core of the deep slab regions where ray theory fails i e caustics are obvious through multipathing and amplitude singularities hilbert transforms and airy type decay caustics should be observed in many places if the models presented are good representations of the earth multipathing in along strike regions is a pervasive feature of the models considered and the degree of such multipathing is highly dependent on the nature of the slab boundary velocity gradients the model with anisotropy above the slab produces multipathing traveltime triplication in the down dip region i e a narrow region through europe identifying such non linear or catastrophic features in teleseismic data is potentially more diagnostic than linearized interpretations automated inversion overall the results show that a range of conservative models representing a range of structural theories can encompass a wide range of wavefront consequences drs m weber and v cerven are thanked for helpful reviews of the manuscript advice and reprints from dr k fujita are appreciated and dr d l anderson is thanked for suggesting the anisotropic slab model with no isotropic velocity anomaly the authors acknowledge the support of nserc canada through operating grant number a1465 j m k is supported by an amoco postgraduate scholarship and an ontario graduate scholarship

importance of the reference model in linearized tomography and images of subduction below the caribbean plate in linearized delay time tomography the choice of a one dimensional reference model is very important for correct mapping of seismic anomalies we demonstrate that the use of an inadequate background model results in artifacts in tomographic images which may give rise to erroneous interpretations particularly the absence or presence of upper mantle discontinuities in the reference model influences the mapping of structures e g subducting slabs at the transition between upper and lower mantle new tomographic images of the mantle below the caribbean region are presented these show the extent of the lesser antilles subduction zone down to 600 km which is well below the seismic zone a high velocity anomaly is imaged from the mid american trench to lower mantle depths 1400 km

effects of multiple phase transitions in a three dimensional spherical model of convection in earth s mantle numerical models of mantle convection that incorporate the major mantle phase changes of the transition zone reveal an inherently three dimensional flow pattern with cylindrical features and linear features that behave differently in their ability to penetrate the 670 km discontinuity the dynamics are dominated by accumulation of cold linear downwellings into rounded pools above the endothermic phase change at 670 km depth resulting in frequent avalanches of upper mantle material into the lower mantle the effect of the exothermic phase transition at 400 km depth is to reduce the overall degree of layering by pushing material through the 670 km phase change resulting in smaller and more frequent avalanches and a wider range of morphologies large quantities of avalanched cold material accumulate above the coremantle boundary cmb resulting in a region of strongly depressed mean temperature at the base of the mantle the 670 km phase change has a strong effect on the temperature field with three distinct regions being visible 1 the upper mantle containing linear downwellings and pools of cold material in the transition zone and characterized by a high amplitude long wavelength spectrum 2 the midmantle containing quasi cylindrical avalanche conduits and characterized by a low amplitude broad spectrum and 3 the deep mantle containing large pools of cold avalanched material and characterized by a high amplitude ultra red i e long wavelength spectrum the effect on the velocity field is very different flow penetration across the 670 km phase change is strongly wavelength dependent with easy penetration at long wavelengths but strong inhibition at short wavelengths thus when comparing numerical models with long wavelength seismic tomography diagnostics based on the density field such as the radial correlation function are much more sensitive to the effects of phase transitions than those based on the velocity field the amplitude of the geoid is not significantly affected by the partial layering because the contribution from the strong heterogeneity in the transition zone is almost perfectly balanced by the contribution from deflection of the 670 km discontinuity avalanches are associated with geoid lows however a more complex viscosity structure is required to correctly match the sign of the geoid over slabs in earth

simultaneous travel time inversion for earthquake location and subduction zone structure in the central aleutian islands a combined location and velocity inversion technique is applied to travel time data from 151 well recorded central aleutian earthquakes the data include p and s wave arrivals at stations of a local network and p pp and sp wave arrivals at teleseismic stations after correction for crustal structure at the reflection points the depth phases pp and sp provide important constraints on subduction zone structure not ordinarily resolved by other types of data the structure is assumed to vary only across the arc and in depth it is parameterized with cubic splines by specifying the velocity at each point of a gridded arc cross section to avoid ray tracing it is assumed that the velocity part of the problem is linear preliminary results show that deep earthquakes apparently occur within a narrow downdip zone near the upper surface of the descending slab in contrast with previous studies which have located them within the presumably stronger and colder inner core of the slab a slab thickness of 80 100 km and a downdip length of about 400 km well below the deepest seismic activity are indicated the slab is characterized by seismic velocities as much as 11 higher than the surrounding mantle in its upper portions and 4 6 higher at depth a sharp velocity gradient and lower velocities occur directly beneath the volcanic arc near the top of the slab the slab anomaly appears to spread out and fall off very slowly with depth these are probably not real effects but consequences of the omission of three dimensional ray tracing

on the three dimensional propagation of stationary waves abstract a locally applicable nonzonally averaged conservation relation is derived for quasi geostrophic stationary waves on a zonal flow a generalization of the eliassen palm relation the flux which appears in this relation constitutes it is argued a useful diagnostic of the three dimensional propagation of stationary wave activity this is illustrated by application to a simple theoretical model of a forced rossby wave train and to a northern hemisphere winter climatology results of the latter procedure suggest that the major forcing of the stationary wave field derives from the orographic effects of the tibetan plateau and from nonorographic effects diabatic heating and or interaction with transient eddies in the western north atlantic and north pacific oceans and siberia no evidence is found in the data for wave trains of tropical origin forcing by the orographic effects of the rocky mountains seems to be of secondary importance

mapped plant macrofossil and pollen records of late quaternary vegetation change in eastern north america abstract macrofossil presence absence maps and isopoll maps in 3000 year intervals show how ranges and abundance maxima for 12 eastern north american tree and shrub taxa picea p glauca p mariana larix laricina abies balsamea tsuga canadensis pinus strobus p resinosa p banksiana betula papyrifera b alleghaniensis b series humiles have changed from the last glacial maximum to the present the macrofossil maps corroborate patterns shown by the isopoll maps and provide spatial detail and taxonomic resolution the macrofossils confirm the inference from pollen data that unglaciated southeastern north america was cooler during the last glacial maximum 18 and 15 ka than simulated by the cohmap experiments with the ncar ccm0 general circulation model the geographic distribution of macrofossil occurrences during the late glacial 12 and 9 ka indicate that migration lag of boreal species did not occur at regional to subcontinental scales and that pollen assemblages lacking modern analogs resulted from climate gradients lacking modern analogs early holocene 9 and 6 ka macrofossil maps show rapid northward expansion of tree species ranges into deglaciated regions the data also show modest contractions of northern range limits of temperate species and expansions of southern range limits of boreal species in response to cooling trends during the late holocene 3 and 0 ka comparison of modern macrofossil maps of nine of these taxa with corresponding range maps confirm that the macrofossils record the geographic ranges accurately comparison of the modern macrofossil maps with maps of tree growing stock volume shows that for some taxa abies tsuga macrofossil occurrences were most frequent in regions of maximum tree abundance comparison of modern isopoll maps with the modern range and growing stock volume maps indicate that in contrast to the macrofossil data the pollen data provide poorer resolution of range limits for most taxa but better indications of abundance maxima and minima within the ranges

cooling in the gulf of st lawrence and estuary region at 9 7 to 7 2 14c ka 11 2 8 0 cal ka palynological response to the pbo and 8 2 cal ka cold events laurentide ice sheet air mass circulation and enhanced freshwater runoff abstract an examination of post younger dryas yd pollen stratigraphies in the gulf of st lawrence and st lawrence estuary region reveals features in the pollen records that represent breaks in the normal vegetation succession widespread vegetation suppression and a delay in migration of plant taxa between 9 7 and 7 2 14 c ka 11 2 and 8 0 cal ka the domination of alnus crispa at sites bordering the st lawrence estuary gulf region in gaspesie and northern new brunswick within this timeframe represents a diversion from the typical vegetation progression from picea and or populus or picea betula to pinus and or betula and signifies a shift to a cooler drier climate coinciding with the a crispa expansion and domination in that region was the contraction of picea populations in other areas in southwestern new brunswick and eastern and southeastern nova scotia picea was replaced by the first appearance of tree birch b papyrifera whereas in western and southwestern newfoundland picea gave way to a resurgence of shrub birch betula glandulosa the picea contraction and immediate resurgence of betula represents cooling and is reliably dated at 9720 110 14 c bp 10 800 11 240 cal bp in southwest newfoundland this first post yd episode of widespread cooling is correlated with the north atlantic preboreal oscillation pbo centered around 9650 14 c bp 10 900 11 180 cal bp in the adjacent great lakes region sites exposed to winds from the gulf of st lawrence in eastern new brunswick prince edward island and northern nova scotia show a lingering persistence of picea and delay in arrival of pinus to 8 0 and 7 7 14 c ka 9 0 and 8 4 cal ka yet pinus was dominant as early as 9 4 14 c ka 10 6 cal ka in southwestern new brunswick at the same time tundra vegetation persisted at high elevations in western and southwestern newfoundland only to be replaced by upslope migration of shrub birch heath by 8 14 c ka prolonged broad scale cooling to 8 14 c ka and to as late as 7 7 14 c ka extended up to 200 km inland in areas exposed to the st lawrence estuary and gulf region and was in response to strong cold dry anticyclonic winds coming off the retreating laurentide ice sheet in combination with enhanced freshwater runoff through the gulf of st lawrence the end of the period of prolonged cooling and onset of regional warming coincided with the diversion of western canada runoff and agassiz ojibway drainage to hudson bay and reduced effect or final break up of the laurentide ice sheet several sites document a subsequent cold shift that interrupted regional warming at 7650 to 7200 14 c bp 8400 to 8000 cal bp and which is variously represented by the suppression of pinus and resurgence of picea sometimes with a crispa quebec maritime region or by an abrupt decrease of picea and resurgence of betula western newfoundland this second post yd cool interval is equated with the 8200 cal bp cold event registered in the greenland ice isotopic record

modeling teleseismic p wave propagation in the upper mantle using a parabolic approximation teleseismic waves propagating in the upper mantle are subject to considerable distortion due to the effects of laterally heterogeneous structure the magnitude and scale of velocity contrasts representative of features such as subducted slabs may be such that wave diffraction becomes an important process in this case forward modeling methods based on high frequency asymptotic approximations to the wave equation will not accurately describe the wavefield a method is introduced to model the propagation of teleseismic p waves in a laterally heterogeneous upper mantle that accounts for distortion of the initial portion of the wavefield including the effects of multipathing and frequency dependent diffraction the method is based on a parabolic approximation to the wave equation that is solved in the time domain on a finite difference grid which mimics the expected pattern of energy flow in a reference velocity field numerical examples for a simple two dimensional subducting slab model demonstrate the application of the method and illustrate the effects of multipathing and diffraction which dominate waveform distortion at high and low frequencies respectively

imaging subducted slab structure beneath the sea of okhotsk with teleseismic waveforms the structure of subducted slabs is not well imaged in most global tomographic models with anomalies typically less than 1 synthetic waveforms for such models are not noticeably different from their 1d reference models in contrast we find that observed teleseismic waveforms sampling slabs in the down dip direction display multipathing features indicative of substantial structure such waveform complexity patterns appear distinctly different for various source locations with outer rise events generally displaying relatively simple waveforms here we model these waveform patterns for a portion of the kuril subduction zone the best fitting slab models have relatively fast cores 5 with smooth edges compatible with thermal modeling in shape complete seismograms for these 2d models indicate that multibounce p and s waves are slab sensitive and can be used in refining models

slab diffraction of s waves seismograms are synthesized by superposition of gaussian beams for teleseismic s waves radiated by deep focus earthquakes located within the zone of high seismic velocities that define a descending lithospheric slab the s wave synthetics demonstrate a phenomenon of slab diffraction that acts to lengthen the apparent pulse width of the s wave low frequency energy leaks out of the slab and travels to the receivers along paths in the lower velocity region surrounding the slab seismograms synthesized in fully three dimensional models of slab structure show that slab diffraction strongly depends on the azimuth of the receiver with respect to strike of the slab slab diffraction lengthens the pulse width of s waves in the entire azimuthal range on the dipping side of the slab as the azimuth approaches the strike of the slab the diffracted phase may evolve into a higher frequency impulsive phase which arrives after the minimum time arrival some slab models predict the slab phase to become a superposition of multipathed ray theoretical arrivals as the azimuth approaches the strike of the slab as the azimuth varies through the strike of the slab toward the side opposite the direction of dip the slab phase is rapidly extinguished rapid variations in the complexity of s waveforms recorded along azimuths parallel to the strike of the slab are consistent with the predictions of synthetic seismograms evidence of slab diffraction in the s waveforms of the deepest focus earthquakes supports the hypothesis of slab penetration beyond 650 km depth broadband s waveforms from deep focus earthquakes in the kuril kamchatka slab suggest that this slab penetrates below 650 km and that it may suffer some distortion in shape and advective thickening below this depth

amplitude distribution of seismic waves for laterally heterogeneous structures including a subducting slab summary we demonstrate the effects that the subducting plate 3 d velocity and q structures of the upper mantle beneath japan can have on the amplitudes of seismic waves observed above the inhomogeneous structure and we explore the possibility of finding the fine velocity and q structures by means of the amplitude distribution the methods used for the computation of the seismograms are the ray tracing and also gaussian beams which give more accurate amplitudes at caustics and shadow zones first we construct a slab velocity model by calculating the temperature distribution within and around the subducting slab and investigate the effects of relative source location within the slab possible detachment of shallow and deep parts of the slab and elevation and depression of the 400 km discontinuity within the slab the possibility of detecting the location of the leading edge of the subducted slab is also examined by calculating the amplitude distribution of incident plane waves the distribution of the ray amplitudes is also calculated for a velocity model including the pacific plate subducting beneath japan we finally adopt a 3 d velocity and q structure model including the subducting pacific plate and philippine sea plate in the uppermost mantle beneath japan obtained from previous studies the calculations for incident plane waves reveal that there may be large variations of the amplitudes on the surface it is found that the maximum amplitude distribution of waveforms calculated by the gaussian beam method for most of the above cases gives a smoothed pattern of the corresponding ray amplitudes and that the amplitudes for low frequency waves are smoother than those of high frequencies from all these calculations we find that the amplitude distributions are very sensitive to the velocity heterogeneities suggesting that the amplitude distribution may be used to obtain the source location within the slab and fine velocity and q structures of the upper mantle near the subduction zone

the fate of slabs inferred from seismic tomography and 130 million years of subduction the volume and location of subducted plate since 130 ma are reconstructed from magnetic anomalies and updated finite rotation parameters describing relative motions in the ocean basins and between plates and hotspots the area of subducted plate is calculated from the relative motions between overriding and overridden plates along the length of convergence in the hotspot reference frame in 5 ma intervals ridge locations are reconstructed by rotating magnetic anomalies from their present to their former positions the distances between trenches and ridges at a certain point of time in the spreading direction are thus estimated and are used to determine the ages of trenches according to the spreading history recorded in present day oceans the thickness of the oceanic plate is obtained from the age based on the half space cooling model about 3 45 10 10 km 3 of oceanic plate has been subducted during the past 130 ma with maximum accumulation beneath southeast eurasia at spherical harmonic degree l 2 where subduction flux peaks excellent correlation is found between four seismic tomographic models at the top of the lower mantle about 800 1100 km deep and predicted slab locations for some seismic models excellent correlation is also found at other degrees in that depth range some models also have good correlations with predicted slab locations in the deep mantle at l 2 direct comparison between subduction and seismic tomographic patterns at 800 1100 km shows that subduction history correlates with tomography very well in terms of location and amplitude of anomalies cold downwellings which may be related to slabs appear to be trapped in the mesosphere or middle mantle correlations between seismic tomography and subduction over different time periods support this conclusion there are also some correlations in the upper mantle and in the deep lower mantle although they are generally not as significant as those in the 800 1100 km depth range there may be a significant boundary in the mantle near the 800 1100 km depth from a geodynamic and chemical point of view the lower mantle may start at a depth closer to 1000 km than to 670 km

rupture processes of large deep focus earthquakes from inversion of moment rate functions we determine the source time histories of five recent 1994 1996 large deep focus earthquakes using a method that inverts for fault slip from far field moment rate functions the moment rate functions are obtained through the deconvolution of multiple body wave phases using broadband records from global seismic network gsn and geoscope stations tests of this method on synthetic data indicate that it is successful in determining the low frequency rupture characteristics of deep earthquakes under a variety of complicating conditions we find that source parameters such as average rupture velocity and stress drop are highly variable among the events studied and that some unusual characteristics exhibited by the june 9 1994 great bolivian earthquake are also found for other events comparison of the slip distributions with background seismicity and aftershock locations indicates that most of the moment release for large deep focus earthquakes is probably occurring within the active slab interior this provides further evidence that temperature controlled mechanisms such as transformational faulting play a large role in deep earthquake faulting most of the events studied also show a tendency for horizontal rupture propagation suggesting that isobaric processes may be an important factor in controlling progression of the rupture large gaps in the slip distributions point to possible occurrence on multiple fault planes

wisconsinan late glacial environmental change in newfoundland a regional review the distribution of 75 lake and bog sites from which pollen stratigraphical evidence relevant to the late quaternary of newfoundland has been obtained is presented the detailed records of lithology pollen and diatoms supplemented by geochemical data are discussed for three of the sites southwest brook lake woody hill brook pond and leading tickles and interpretations made are based on vegetational history relative climatic changes and extent of ice cover radiocarbon data from 14 sites are summarised a summary chart of late glacial environmental changes including a qualitative palaeotemperature curve is provided

relative impacts of siberian and north american snow anomalies on the winter arctic oscillation 1 numerical model mean climatic response to realistic land surface snow forcings is evaluated for two different forcing regions siberia and north america the atmospheric teleconnection pathway and negative winter ao mode response produced by the siberia forcing described by the authors in previous studies is not produced by the comparable extent north america forcing it is shown that the combination of a large snow forced local diabatic heating anomaly over a region of substantial stationary wave activity is required to produce strong upward wave activity flux anomalies which initiate the teleconnection pathway these features are unique to siberia making it a critical region for reproducing the snow winter ao statistical relationship evident in the observational record

effects of environmental change on scree slope development throughout the postglacial period in the chic choc mountains in the northern gaspe peninsula quebec abstract the combined study of present day processes and stratigraphic data has permitted the reconstruction of the dynamics of scree slopes of the northern gaspe peninsula throughout the postglacial period in this region liberated progressively from beneath an ice cover between 13 500 and 10 000 years bp the scree slopes have to be seen as an integral part of a regional geosystem evolution of these slopes has been rapid influenced by local paraglacial conditions glacio isostatic rebound glacio eustatic fluctuations and re equilibration of glacially over steepened rockwalls which operated against a backdrop of late glacial and holocene climatic fluctuations during the younger dryas and part of the early holocene period as the foot slopes emerged from beneath the goldthwait sea the basal part of several scree slopes advanced onto marine terraces as lobate rock glaciers under the influence of a periglacial climate characterised by permafrost many scree slopes continued to transfer debris downslope after regional establishment of a closed forest cover at ca 7250 years bp forest colonisation in the early pre emergent phase of the postglacial period was retarded due to constant replenishment of the debris removed from the foot slopes by marine processes in the later post emergent phase development of a complete forest cover has only been possible on slopes where the summit rockwall segment has been completely eliminated a condition not yet fulfilled for many geomorphologically active scree slopes of the region in fact both of these paraglacial influences have been diachronous on a regional scale advance upslope of the forest front on the scree slopes appears to have been slow difficult and subject to periodic regressions of possible climatic origin as indicated by numerous buried soils in colluvial stratigraphic sequences and for the past 150 years by dendro ecological studies stratigraphic exposures along with direct observation of slope events have revealed the operation of a large variety of debris transfer processes including niveo aeolian sedimentation and frost coated clast flows the latter representing an important process first recognised on the scree slopes of gaspesie

postglacial palaeophytogeography of the eastern st lawrence river watershed and the climatic signal of the pollen record abstract pollen analytical studies of the sediments of over 150 lakes and bogs in southern quebec have led to the indentification of a limited number of sequential types or temporal suites of the various pollen assemblages within each of three main stages of the postglacial development of vegetation the non arboreal stage the afforestation stage and the forest stage these sequential types within any of the three stages show distinct geographical and chronological distributions in southern quebec during the last 12 000 yr when compared in a broader geographic context they demonstrate a strong differentiation of the pollen record within and around the great lakes st lawrence river watershed during the last 15 000 yr b p amongst the most striking gradients identified stand the late glacial perichamplainian gradients ca 11 500 10 000 yr b p and the early holocene gradients the former involve seaward and northeastward clines from closed coniferous forests to tundras and the latter northeastward changes in the nature and duration of the afforestation stage the modern pattern of even the broader vegetational gradients in southern quebec was not attained before 3000 yr b p palynological reconstruction of the various regional paleoclimates highlights the interaction of local regional and global climatic forcings through time mesoscale climate modelling and palaeoclimatology should find boundary conditions in those regional reconstructions

effects of continental scale snow albedo anomalies on the wintertime arctic oscillation the ncar cam3 gcm with prescribed satellite based snow albedo sa is used to investigate the remote effects of snow cover on northern hemisphere nh winter climate a pair of 100 ensemble member experiments are integrated through the autumn winter season with prescribed high and low sa over eurasia ea similar to other non cam gcm studies using prescribed snow mass anomalous ea snow albedo produces a wave activity pulse that propagates into the stratosphere culminating in a negative phase arctic oscillation ao like surface response this occurs for idealized but representative sa anomalies as well as for more realistic sa anomalies similar experiments over north america na and the entire nh are also performed unlike prior studies anomalous na snow yields a significant ao signal here the local na surface cooling elicits a transient eddy response which propagates downstream to eurasia resulting in significant but short lived cooling and upward propagating wave activity over siberia a negative ao like response develops primarily confined to the stratosphere upper troposphere which eventually gives way to a tropospheric ao like response of the opposite phase due to equatorward wave refraction and wave divergence reanalysis data support this na snow positive ao response snow forcing experiments for the whole nh however yield a weakened ao signal this is due to a muted wavenumber 1 and 2 response caused by destructive interference between the background stationary wave and the corresponding rossby wave response which results in negligible wave activity reaching the stratosphere

contrasting eurasian spring and summer climate anomalies associated with western and eastern eurasian spring snow cover changes spring snow cover variations over eurasia consist of a same sign component and a west east dipole component the same sign component is dominant during late 1970s through 1980s and the two components are equally important during 1990s and 2000s present study contrasted eurasian spring and summer climate anomalies associated with western and eastern eurasian spring snow cover variations and between 1975 1989 and 1993 2007 and found notable differences and obvious interdecadal changes in eurasian climate anomalies after 1990 spring surface temperature anomalies display an obvious west east contrast pattern consistent with a prominent atmospheric wave pattern over eurasia the temperature anomalies are larger corresponding to western than eastern eurasian snow cover change snow albedo effect plays a main role in temperature change during 1975 1989 whereas atmospheric processes also contribute largely to temperature change during 1993 2007 large soil moisture anomalies occur corresponding to western eurasian snow cover and during 1975 1989 the snow hydrological effect on summer precipitation is mainly detected over the west siberian plain where soil moisture anomalies persist over 3 months over most of other eurasian regions atmospheric circulation pattern plays a dominant role in summer surface temperature and precipitation variations the relationship of indian summer rainfall to western eurasian spring snow cover was weakened since the mid 1990s and the correlation of northeast china summer rainfall with local spring snow cover changed from positive to negative around 1990

holocene vegetation dynamics of anticosti island quebec and consequences of remoteness on ecological succession abstract the postglacial vegetation history of anticosti island in the gulf of st lawrence quebec was reconstructed from pollen analysis of three sites providing the first paleoecological data for the island the earliest plant communities 8550 7770 14c yr b p 9500 8500 cal yr b p were characterized by a high representation of green alder alnus crispa indicating an open environment with sparse spruce picea between 7770 and 3700 14c yr b p 8500 4000 cal yr b p the island was colonized by mixed deciduous and coniferous forests dominated by paper birch betula papyrifera and spruce a major increase in the abundance of balsam fir abies balsamea occurred ca 3700 14c yr b p 4000 cal yr b p indicating development of the closed fir spruce forests as they exist today the island s remoteness from continental seed plant sources influenced plant colonization afforestation and ecological succession the gulf of st lawrence slowed plant migration and tree species had to spread from the gaspe peninsula repeated drops in fir pollen percentages during the last 4 000 years may reflect the activity of defoliating insects

seismic constraints on the morphology of deep slabs residual sphere images from deep earthquakes not only detect the presence of slab associated velocity anomalies but also lend insight into the flow and deformation of lithosphere subducted into the lower mantle we have compared travel times from deep events in the kuril and mariana arcs with the seismic velocity anomalies implied by kinematical models that thicken the slab perpendicular to its plane by reducing the vertical velocity of the flow with depth we assume that the details of the deformation whether the slab buckles imbricates fragments etc are averaged out along the ray paths and hence our models constrain the scale not the mode of slab thickening the deep event travel times are best fit by undeformed models but the ability of the residual sphere method to resolve slab thickness is limited by ray bending effects although the mariana times are consistent with advective thickening factors of 5 or more factors larger than 3 are ruled out by the kuril data for all models examined the data require that slab material extends to depths of 900 1000 km global tomographic models and regional studies which delineate high velocity anomalies in the lower mantle beneath zones of cenozoic subduction are consistent with our results as is recent work on pulse distortion by slab gradients comparison of observed and predicted rates of seismic moment release suggests that if substantial advective thickening does occur it is largely aseismic

thermodynamically consistent decompression implications for lower mantle composition estimates of the composition of the lower mantle are commonly obtained by a process of adiabatic decompression in which zero pressure mantle parameters are determined with a least squares fit of an equation of state to mantle densities we show that this procedure may lead to biased estimates of the zero pressure density and compressibility of the mantle the bias can be eliminated or minimized by subjecting the equation of state fit to a thermodynamic constraint at zero pressure such inversions provide equations of state that are compatible with the lower mantle as well as the thermal properties of selected mineral models we used this method to evaluate assemblages containing mg fe sio3 perovskite and mg fe o with fe fe mg ratios as high as 0 2 thermodynamic systematics were used to place bounds on the anderson gruneisen parameter s for perovskite we find a fairly large range of acceptable models for the composition of the lower mantle they range from uppermantle like compositions to assemblages that are enriched in both si and fe relative to the upper mantle however a pure perovskite lower mantle whether enriched in fe or not seems improbable small fe enrichments of the order of 1 given density contrasts across the 670 km discontinuity that are probably insufficient to prevent convective mixing of the upper and lower mantles we therefore suggest that unless the mantle is homogeneous in its major oxide components the lower mantle is likely to be enriched in fe by at least several percent

eurasian snow cover variability and northern hemisphere climate predictability we present observational evidence demonstrating dynamic interactions and feedbacks between multi seasonal snow cover and winter time circulation anomalies over mid high latitudes the cooling effect of snow cover is associated with a strengthened and more expansive siberian high with more frequent topographically constrained intrusions west and north early season snow cover variability leads to altered general circulation patterns consistent with the dominant mode of winter variability observed in the northern hemisphere troposphere the implications of the surface atmosphere coupling for seasonal to interannual predictability are also discussed

modeled climate state and dynamic responses to anomalous north american snow cover abstract the radiative and thermal properties of widespread snow cover anomalies have the potential to modulate local and remote climate over monthly to seasonal time scales in this study physical and dynamical links between anomalous north american snow conditions and northern hemisphere climate are examined a pair of 40 member ensemble agcm experiments is run with prescribed high and low snow forcings over north america during the course of an entire year ey the difference between the two ensemble averages reflects the climatic response to sustained ey snow forcing local surface responses over the snow forcing occur in all seasons and a significant remote surface temperature response occurs over eurasia during spring a hemispheric scale transient eddy response to ey forcing also occurs which propagates downstream from the forcing region to eurasia and then reaches a maximum in extent and amplitude in spring the evolution of this transient eddy response is indicative of considerable downstr

sudden changes in north atlantic circulation during the last deglaciation sudden changes in the flow of warm atlantic surface waters into the norwegian sea occurred frequently during the last deglaciation typically involving shifts in sea surface temperature of 5 c in fewer than 40 years these led to equally large and rapid changes in atmospheric temperatures and to shifts in atlantic deep thermo haline circulation and ice sheet melting rates

waveform effects of a high velocity subducted slab creager and jordan 1986 propose that penetration of subducting slabs under the kurile islands and other subduction zones to depths of at least 1000 km is necessary to explain travel time anomalies of deep earthquakes such penetration would also be expected to affect the amplitudes and waveforms of the body waves from earthquakes synthetic seismograms appropriate for a record section in a plane perpendicular to the strike of the slab are presented using a coupled finite difference and kirchhoff method our shear wave version of the compressional wave velocity structure of creager and jordan 1986 produces an amplitude decrease up to a factor of four and waveform broadening up to 20 seconds for sh arrivals with a take off angle pointing straight down the slab slabs that extend only 300 km below the earthquake but are half as thick and twice as anomalously fast as creager and jordan s 1986 velocity model will roughly preserve the travel time variation pattern and show less waveform broadening but produce first arrivals that are emergent slabs that become thicker with depth show less waveform broadening reconciliation of the amplitude waveform distortion and timing of body waves from deep events is necessary to understand the geometry of slabs in the upper mantle and near and below the 650 km discontinuity

wisconsinan late glacial environmental chage in new brunswick a regional synthesis a synthesis is presented of the history of deglaciation vegetation cover and climate in new brunswick canada during that part of the wisconsinan late glacial period 12 9 ka bp for which detailed records exist curves of inferred temperature variations in the region during this period are presented based upon qualitative interpretations of plant records and quantitative estimates based upon fossil chironomid assemblages

localized velocity anomalies in the lower mantle summary two localized regions of velocity heterogeneity in the lower mantle with scale lengths of 1000 2000 km and 2 per cent velocity contrasts are detected and isolated through comparison of s scs p and pcp travel times and amplitudes from deep earthquakes in peru bolivia argentina and the sea of okhotsk comparison of the relative patterns of scs s differential travel times and s travel time residuals across north american wwssn and csn stations for the different source regions provides baselines for interpreting which phases have anomalous times a region of low s and p velocities is located beneath northern brazil and venezuela at depths of 1700 2700 km this region produces s wave delays of up to 4 s for signals from deep argentine events recorded at eastern north american stations the localized nature of the anomaly is indicated by the narrow bounds in azimuth 15 and take off angle 13 of the arrivals affected by it the long period s waves encountering this anomaly generally show 30 100 per cent amplitude enhancement while the short period amplitudes show no obvious effect the second anomaly is a high velocity region beneath the caribbean originally detected by jordan and lynn who used travel times from deep peruvian events the data from argentine and bolivian events presented here constrain the location of the anomaly quite well and indicate a possible short and long period s wave amplitude diminution associated with it when the travel time data are corrected for the estimated effects of these two anomalies a systematic regional variation in scs s station residuals is apparent between stations east of and west of the rocky mountains one possible explanation of this is a long wavelength lateral variation in the shear velocity structure of the lower mantle at depths greater than 2000 km beneath north america

modelling d structure beneath central america with broadband seismic data deep south american events recorded at distances between 70 and 96 are used to constrain the lower mantle structure beneath central america employing data from the terrascope bdsn iris and cnsn canadian national seismic network broadband arrays we obtain a 1 d shear model with a 200 km thick d layer with a 3 velocity jump at the top and a negative gradient within the layer from broadband waveform modelling of sh polarized s scs and scd the shear structure above d is similar to the preliminary reference earth model prem dziewonski and anderson 1981 preliminary reference earth model physics of earth planetary interiors 25 297 356 in terms of dt d and s sks differential times sv synthetics for this model assuming a prem core produce a satisfactory fit to the data except at the larger ranges to the north where the waveforms display rapid variation this feature is indicative of lateral variation of the type suggested by grand 1994 where the discontinuity becomes less well defined synthetics computed for a simplified 2 d cross section through this structure produce the scd phase and simple s at larger distances while scd is easily observed in the shear wave data there is little evidence of its counterpart in the p data by testing several models with different p velocity jumps at the same depth as the d jump in shear structure we conclude that the velocity jump must be less than 1 in fact prem provides an excellent fit both in timing and waveform shapes and therefore we conclude that lower mantle structure beneath central america shows clear evidence for a discontinuity in s velocity but not in p

mapping the tonga slab we applied residual sphere analysis to 25 intermediate and deep focus earthquakes in the tonga subduction zone and mapped variations in slab strike and dip depth extent and deep lateral advection at all latitudes along the arc as in the northwest pacific travel time data from intermediate and deep focus events are well modeled by simple thermal slab structures and a thermal coefficient of velocity p t of 0 5 0 2 m s k i e peak velocity perturbations of about 3 7 in all regions where the seismicity extends to depths of 600 km or more the shortest thermal slab models that cannot be rejected at the 90 confidence level reach depths of 750 km northern tonga to 850 km southern tonga these models however require a value of p t 1 5 m s k 3 times the value suggested by laboratory studies if the magnitude of p t is constrained to be less than 1 0 m s k the minimum penetration depth varies from 800 km to 900 km the travel time data are equally consistent with slab structures that deform at the base of the upper mantle thickening by a factor of 3 and models that contain no advective thickening inferred slab structure varies considerably throughout the subduction zone and the nearly ubiquitous match between the observed seismicity and detailed undulations in slab strike and dip is evidence that residual spheres do reflect near source heterogeneity travel times in southern tonga 21 5 s 27 s are consistent with and in some cases require slab models that bend to a steeper dip at depth this phenomenon is also observed in parts of the northwest pacific but is not required in central 19 s 21 5 s and northern 13 s 19 s tonga the residual spheres of both intermediate and deep focus events suggest a sharp bend in slab strike at 25 s this feature coincides with a kink in strike observed at all depths in the seismicity and occurs at the intersection of the trench and the louisville ridge northern tonga is the locus of considerable complexity in deep slab structure seismicity strain rate and travel time data are consistent with although they do not uniquely require a slab structure in northern tonga which curves sharply to the west at the base of the upper mantle but which strikes to the north at shallower depths and in the lower mantle the contorted segment would have begun subducting roughly 10 m y ago during a major plate reorganization in northernmost tonga a decrease in the maximum depth of seismicity and inferred slab structure provides evidence for southward shear of deep material relative to the shallow slab compared to data from the northwest pacific where travel time observations also suggest slab penetration below the deepest earthquakes tonga seismicity and residual spheres show more complexity at depth and may indicate more deformation internal to the slab these observations argue against convective models which are rigorously stratified at 650 km depth but they are consistent with a discontinuity in mantle properties such as an increase in viscosity or a small percent variation in chemistry if it merely resists and deforms the descending slabs without acting as a complete barrier to flow

ecosystem response to lateglacial and early holocene climate oscillations in the great lakes region of north america abstract fossil pollen plant macrofossils gastropods and elemental and stable isotope geochemistry in a sediment core from twiss marl pond southern ontario canada were used to document climate oscillations during the last glacial interglacial transition 13 000 8500 14 c bp and understand their ecological effects chronology was provided by ams 14 c dating and regional pollen correlation oxygen isotope 18 o results from mollusc shells chara encrustations and bulk carbonates show a classic climate sequence of a warm bolling allerod boa at 12 500 10 920 14 c bp a cold younger dryas yd at 10 920 10 000 14 c bp the holocene warming at 10 000 14 c bp a brief preboreal oscillation pb at 9650 14 c bp and a possible gerzensee killarney g k cooling shortly before 11 000 14 c bp clay sediments at the base of the core contain high herb and shrub pollen and abundant arctic alpine plant macrofossils indicating a treeless tundra with severe soil erosion in watershed during the boa warm period authigenic marl began to be deposited and picea woodland became established the establishment of picea woodland after peaks of 18 o and of carbonate accumulation suggests a lagged response of upland vegetation to boa warming in contrast the occurrence of warmth loving aquatics najas flexilis and typha latifolia at that time indicates sensitive responses of aquatic plants the yd cooling is indicated by a 1 5 negative excursion in 18 o an increase in minerogenic matter and higher concentrations of erosion derived elements al na k ti and v pollen data show no forest transformation in response to yd cooling which is attributed to the insensitive nonecotonal vegetation at that time however more openings in the forests and increased erosion in the watershed are indicated by a slight increase of herb pollen high concentrations of erosion elements and a pediastrum peak the onset of the holocene was marked by an abrupt increase of 2 in 18 o and the replacement of picea woodland by pinus dominated forest the picea recurrence at 9650 14 c bp demonstrates sensitive response of ecotonal vegetation to the pb climate oscillation which is also indicated by 0 4 negative excursion of 18 o these new results suggest the importance of multiproxy records for reliable paleoclimate reconstruction reevaluation and revised chronologies of previously published sites gage street and nichols brook in the eastern great lakes region show their major 18 o shifts correlative to the yd and pb oscillations as documented from twiss marl pond and nearby crawford lake the sequence and magnitude of climatic oscillations from these sites match in detail with records from the atlantic seaboard suggesting that these oscillations are an expression of broad scale probably global climate change rather than local meltwater induced climate cooling