global atmospheric teleconnections during dansgaard oeschger events during the last glacial period the north atlantic region experienced a series of dansgaard oeschger cycles in which climate abruptly alternated between warm and cold periods corresponding variations in antarctic surface temperature were out of phase with their northern hemisphere counterparts the temperature relationship between the hemispheres is commonly attributed to an interhemispheric redistribution of heat by the ocean overturning circulation changes in ocean heat transport should be accompanied by changes in atmospheric circulation to satisfy global energy budget constraints although changes in tropical atmospheric circulation linked to abrupt events in the northern hemisphere are well documented evidence for predicted changes in the southern hemisphere s atmospheric circulation during dansgaard oeschger cycles is lacking here we use a high resolution deuterium excess record from west antarctica to show that the latitude of the mean moisture source for antarctic precipitation changed in phase with abrupt shifts in northern hemisphere climate and significantly before antarctic temperature change this provides direct evidence that southern hemisphere mid latitude storm tracks shifted within decades of abrupt changes in the north atlantic in parallel with meridional migrations of the intertropical convergence zone we conclude that both oceanic and atmospheric processes operating on different timescales link the hemispheres during abrupt climate change abrupt glacial climate changes were slowly communicated between hemispheres by oceanic heat transport ice core data point to more rapid atmospheric teleconnections linking the north atlantic tropics and southern storm track

a study of mantle layering beneath the western pacific the reverberative interval of the seismogram defined as the portion following the surface wave train propagating along the minor arc and ending with the first body wave arrivals from the major arc provides an excellent window in which to observe subcritical reflections from mantle discontinuities on an sh polarized seismogram the intervals between scsn and sscsn wave groups zeroth order reverberations are approximately 15 min long and consist almost entirely of waves reflected one or more times from mantle discontinuities first and higher order reverberations we have developed signal enhancement and waveform inversion schemes to progressively extract information pertaining to mantle layering from the waveforms of zeroth and first order reverberations and have applied them to a data set of long period digital seismograms drawn from a number of tectonic regions in and around the western pacific the results place important constraints on the nature of the transition zone discontinuities measurements of the two way vertical shear wave travel time from the free surface to the 400 km and 650 km discontinuities 400 and 650 respectively yield estimates of transition zone travel time tz largely uncontaminated by outside heterogeneity variations in the path averaged estimates of tz are of the order of 5 4 s indicating significant lateral heterogeneity in the transition zone correlations of tz with 400 and lm travel time through the lower mantle are consistent with but do not require the thermal deflection of an exothermic phase transition near 400 km and an endothermic transition at 650 km with 12 km of long wavelength 500 5000 km topography on each the amount of topography estimated for the 650 km discontinuity is significantly less than predicted for a rigorously stratified mantle 100 km in a search for other mantle discontinuities we find evidence for a strong reflector possibly associated with the onset of partial melting in the upper mantle average depth of 86 km due to ambiguity inherent to the method this feature can also be interpreted as an impedance increase in the d region of the lower mantle 70 170 km above the core mantle boundary no evidence is found for a 220 km discontinuity or the d discontinuity proposed by lay and helmberger 1983 suggesting that if such features are present in the western pacific they are either small normal incidence sh reflection coefficient r 1 5 or intermittent or have lateral depth variations well in excess of 50 km

regional imprints of millennial variability during the mis 3 period around antarctica the climate of the last glacial marine isotopic stage 3 mis3 period is characterized by strong millennialscale variability with a succession of dansgaardeoeschger events first identified in greenland ice cores and associated with variations of the atlantic meridional overturning circulation amoc these abrupt events have a smooth and lagged counterpart in water stable isotopes from antarctic ice cores in this study we aim at depicting and understanding the circum antarctic expression of this millennial scale variability to illustrate the mechanisms potentially at work in the response of the southern high latitudes to an abrupt decrease of the amoc we first present results from experiments performed with the ipsl cm4 atmosphere ocean coupled model under glacial boundary conditions when the amoc is perturbed by imposing an additional freshwater flux in the north atlantic our model produces the classical bipolar seesaw mechanism generally invoked to explain the warming of the southern ocean antarctic region however this mechanism can be locally offset by faster atmospheric teleconnections originating from the tropics even though the precise location of this fast response is not coherent among different climate models our model results are confronted with a synthesis of antarctic records of ice core stable isotope and sea salt sodium including new data obtained on the taldice ice core the ipslcm4 produces a dipole like pattern around antarctica with warming in the atlantic indian sectors contrasting with an unexpected cooling in the east pacific sector the latter signal is not detected in our data synthesis both ice core data and simulations are consistent in depicting a more rapid response of the atlantic sector compared to the indian sector this feature can be explained by the gradual impact of ocean transport on which faster atmospheric teleconnections are superimposed detailed investigations of the sequence of events between different proxies are conducted in three ice cores earlier shifts in deuterium excess and significant changes in sea salt sodium fluxes in the most coastal sites taldice and edml compared to edc suggest reorganizations in local moisture sources possibly linked with sea ice cover this study demonstrates the added value of circum antarctic ice core records to characterize the patterns and mechanisms of glacial climate variability

marine isotope stage 4 in australasia a full glacial culminating 65 000 years ago global connections and implications for human dispersal patrickde deckker lee j arnold sandervan der kaars germain bayone jan berend w stuut kerstin perner raquel lopes dos santos ryu uemura martina demuro

intermittent layered convection in a model mantle with an endothermic phase change at 670 km it is not yet known whether convection in the earth s mantle occurs in cells that extend throughout the entire depth of the mantle or if the upper and lower mantle convect in independent layers both possibilities seem to be consistent with geochemical and seismologi cal observations1 4 it is generally accepted however that the seismologically observed boundary between the upper and lower mantle at 670 km depth arises from the endothermic phase change spinel perovskite mgo here we investigate the results of including this phase change in a model of mantle convection for realism and to eliminate scaling problems our calculations use commonly accepted values of geophysical parameters for a clapeyron slope y 2 106 a value close to experimental results and theoretical expectations we observe local intermittent mixing between the upper and lower mantle such behaviour may offer a way to reconcile the existence of geophysical evidence for both whole mantle and layered convection

factors influencing the capability of lava to erode its substrate application to venus there is evidence that a uniquely venusian type of channel may be at least partly erosional in origin a number of published models discuss the erosion process on various planetary bodies however these relate to lavas of specifically different compositions here a numerical model is derived to investigate the sensitivity of the erosion process to various physical parameters of the lava and substrate placing constraints on the physical characteristics required to obtain a suitably rapid erosion rate the model allows for the effects of latent heat and also considers the presence of a liquid substrate boundary layer between the lava and solid substrate the results indicate that the thermal erosion process is particularly sensitive to the melting temperature when the melting temperature is doubled from 700 to 1400 k the erosion rate decreases by approximately an order of magnitude also the erosion rate associated with a venusian initial substrate temperature is approximately 1 5 times greater than the equivalent terrestrial one mechanical erosion is considered by altering the percentage of latent heat required to melt the substrate as this percentage is varied from 0 to 100 the erosion rate decreases by about a factor of 2

long term sea surface temperature and climate change in the australian new zealand region 1 we compile and compare data for the last 150 000 years from four deep sea cores in the midlatitude zone of the southern hemisphere we recalculate sea surface temperature estimates derived from foraminifera and compare these with estimates derived from alkenones and magnesium calcium ratios in foraminiferal carbonate and with accompanying sedimentological and pollen records on a common absolute timescale using a stack of the highest resolution records we find that first order climate change occurs in concert with changes in insolation in the northern hemisphere glacier extent and inferred vegetation changes in australia and new zealand vary in tandem with sea surface temperatures signifying close links between oceanic and terrestrial temperature in the southern ocean rapid temperature change of the order of 6 c occurs within a few centuries and appears to have played an important role in midlatitude climate change sea surface temperature changes over longer periods closely match proxy temperature records from antarctic ice cores warm events correlate with antarctic events a1 a4 and appear to occur just before dansgaard oeschger events 8 12 14 and 17 in greenland

internal loading of an inhomogeneous compressible earth with phase boundaries summary we use an approach described by dehant wahr 1991 to estimate the geoid and boundary topography caused by mass loads inside the earth we show how the estimates are affected by compressibility and a radially varying density distribution and by the presence of phase boundaries with density discontinuities we compare results for earth models that have a chemical boundary at 670 km depth with those that have a phase boundary at that depth we find that the 670 km topographies predicted for models with a chemical boundary are several times larger than for models with a phase boundary depending principally on the value of the clapeyron slope in the phase case the geoid predicted in the chemical boundary case is about 30 40 per cent smaller than that predicted in the phase case the effects of compressibility and radially varying density are likely to be small we estimate the laterally varying structure inside the fluid core by computing the topography on constant potential surfaces within the core we conclude that this structure is not likely to be a complicating factor when using nutation and earth tide observations to estimate the flattening of the core mantle boundary finally we compute the inner core outer core icb topography for loading inside the mantle and for loading inside the inner core we show that in the first case the icb topography is small and dominated by long wavelengths in the second case we conclude that if the density heterogeneities inside the inner core are two orders of magnitude smaller than in the mantle the icb topography can be about 100 m peak to peak

beyond the bipolar seesaw toward a process understanding of interhemispheric coupling abstract the thermal bipolar ocean seesaw hypothesis was advanced by stocker and johnsen 2003 as the simplest possible thermodynamic model to explain the time relationship between dansgaard oeschger do and antarctic isotope maxima aim events in this review we combine palaeoclimate observations theory and general circulation model experiments to advance from the conceptual model toward a process understanding of interhemispheric coupling and the forcing of aim events we present four main results 1 changes in atlantic heat transport invoked by the thermal seesaw are partially compensated by opposing changes in heat transport by the global atmosphere and pacific ocean this compensation is an integral part of interhemispheric coupling with a major influence on the global pattern of climate anomalies 2 we support the role of a heat reservoir in interhemispheric coupling but argue that its location is the global interior ocean to the north of the antarctic circumpolar current acc not the commonly assumed southern ocean 3 energy budget analysis indicates that the process driving antarctic warming during aim events is an increase in poleward atmospheric heat and moisture transport following sea ice retreat and surface warming over the southern ocean 4 the antarctic sea ice retreat is itself driven by eddy heat fluxes across the acc amplified by sea ice albedo feedbacks the lag of antarctic warming after amoc collapse reflects the time required for heat to accumulate in the ocean interior north of the acc predominantly the upper 1500 m before it can be mixed across this dynamic barrier by eddies

recent climate and ice sheet changes in west antarctica compared with the past 2 000 years west antarctic temperature and pressure observations begin onlyin 1957 and reliable satellite observations of antarctic sea icedate to 1979 comprehensive observations of glacier dynamicsin the most rapidly changing areas were initiated in the 1990s borehole temperature data from the wais although confirmingthe recent rapid rise in temperature do not resolve decadal scalevariability in the past

evidence for a sharp 670 km discontinuity as inferred from p to s converted waves the fine structure of the upper mantle discontinuities is investigated using observations of converted waves on short period and broadband seismograms using a stacking technique to analyze the p wave coda of teleseismic records evidence is found for coherent near receiver p to s converted phases generated by the 400 and 670 km discontinuity beneath a number of the stations used in this study variations in travel time slowness and amplitude of these phases as observed among the stacks for stations of the regional seismic test network in the united states and of the network of autonomously recording seismographs in western europe are very likely the expression of upper mantle heterogeneity observations of coherent converted phases from the 400 km discontinuity are fewer in number than of phases converted at the 670 km discontinuity suggesting that the latter is more pronounced some of the seismograms especially of station rscp show extremely high amplitude p to s converted phases from the 670 km discontinuity these seismograms allow a detailed waveform comparison of the converted phase with the direct p phase and present evidence for a sharp 670 km discontinuity

a new greenland deep ice core the polar ice sheets are rich sources of information on past atmospheric conditions including paleoclimates a new deep ice core has been drilled in south greenland comparison of the oxygen isotopic profile with that from camp century and with a deep sea foraminifera record indicates that the new core reaches back to about 90 000 years before present in a continuous sequence the details in the wisconsin part of the ice core records seem to be climatically significant and the general trends reveal all of the relevant emiliani stages recorded in deep sea cores the redated camp century record suggests a dramatic termination of the eem sangamon interglacial

global mapping of topography on the 660 km discontinuity long period precursors to the ss seismic phase are used to produce global maps of topography for the discontinuity at 660 km depth in the upper mantle these maps indicate discontinuity depth variations of up to 30 km and suggest a correlation between regional depressions in the 660 km discontinuity and subduction zones a result more consistent with models in which the subducting slabs are deflected horizontally at the discontinuity than with models of unhindered slab penetration into the lower mantle

s p conversion from the transition zone beneath tonga and the nature of the 670 km discontinuity summary using more than 300 recordings of deep tonga earthquakes from the warramunga seismic array central australia we have searched for short period s p conversions from the 670 km discontinuity arriving in the p wave codas the array seismograms were stacked and plotted in sections to optimize observation of s670p yielding the following results for the northern part of tonga 17 3 18 4 s s670p phases are observed with apparent conversion depths between about 660 and 680 km for the central part 20 2 22 3 s s670p is not clearly observed for the southern part 23 1 25 7 s s670p is observed for conversion depths between about 660 and 700 km intermittent occurrence of s p conversion may result from different characteristic source radiation patterns among the three sections overall uncertainties in conversion depths resulting from unmodelled lateral velocity variations and hypocentral location errors are perhaps a few tens of kilometres the s p rays must travel either within or very close to the subducted slab because of the source receiver slab dip geometry so we conclude that large 50 km deformations of the 670 km discontinuity do not occur beneath tonga our observations appear to be inconsistent with a chemical discontinuity at 670 km but are easily explained if the 670 km discontinuity is a strongly pressure dependent and perhaps moderately endothermic phase change in both the slab and surrounding mantle

the relationship between the itcz and the southern hemispheric eddy driven jet 1 we study the effect of a thermal forcing confined to the midlatitudes of one hemisphere on the eddy driven jet in the opposite hemisphere we demonstrate the existence of an interhemispheric teleconnection whereby warming cooling the northern hemisphere causes both the intertropical convergence zone itcz and the southern hemispheric midlatitude jet to shift northward southward the interhemispheric teleconnection is effected by a change in the asymmetry of the hadley cells as the itcz shifts away from the equator the cross equatorial hadley cell intensifies fluxing more momentum toward the subtropics and sustaining a stronger subtropical jet changes in subtropical jet strength in turn alter the propagation of extratropical waves into the tropics affecting eddy momentum fluxes and the eddy driven westerlies the relevance of this mechanism is demonstrated in the context of future climate change simulations where shifts of the itcz are significantly related to shifts of the southern hemispheric eddy driven jet in austral winter the possible relevance of the proposed mechanism to paleoclimates is discussed particularly with regard to theories of ice age terminations

precise interpolar phasing of abrupt climate change during the last ice age a new ice core from west antarctica shows that during the last ice age abrupt northern hemisphere climate variations were followed two centuries later by a response in antarctica suggesting an oceanic propagation of the climate signal to the southern hemisphere high latitudes

imaging global body wave phases by stacking long period seismograms stacked record sections combining over 5 years of long period global digital seismograph network gdsn seismograms from shallow events show dozens of seismic body wave phases separate images are obtained for the vertical radial and transverse components using an automatic gain control algorithm which normalizes the amplitude of each trace prior to stacking these images provide an indication of the relative signal to noise characteristics of different parts of the gdsn data set core mantle boundary diffracted phases are particularly prominent in these stacks which show pdiff to 150 pkpdiff to 200 ppdiff to 230 sdiff to 160 and ssdiff to 220 high order s surface multiples can be seen extending through the surface wave train up to ranges of 540 two examples of possible upper mantle discontinuity reflected phases are visible one following ppp between about 95 and 125 the other following scs3 between about 210 and 240 an apparent offset between sh and sv arrivals is seen which becomes larger for the higher order s multiples sss ssss etc and appears consistent with models containing transverse isotropy in the upper mantle

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

antarctic and global climate history viewed from ice cores a growing network of ice cores reveals the past 800 000 years of antarctic climate and atmospheric composition the data show tight links among greenhouse gases aerosols and global climate on many timescales demonstrate connections between antarctica and distant locations and reveal the extraordinary differences between the composition of our present atmosphere and its natural range of variability as revealed in the ice core record further coring in extremely challenging locations is now being planned with the goal of finding older ice and resolving the mechanisms underlying the shift of glacial cycles from 40 000 year to 100 000 year cycles about a million years ago one of the great mysteries of climate science a discussion of past antarctic and global climate history as seen from antarctic ice cores with an outlook on future goals and drilling priorities

upper mantle seismic anisotropy and lithospheric decoupling the lithosphere underlying stable continental shield regions is thought to extend to a depth of 200 km compressional wave velocity models derived from a recent approximately ns refraction profile in northern australia include a pronounced high velocity zone at a depth of around 200 250 km conventional petrological interpretations in terms of mineralogical or compositional changes cannot readily explain this feature calculated velocities for the garnet pyrolite model using a derived geotherm fit the velocities above and below this high velocity zone but cannot account for the high velocity 8 6 km s 1 zone itself this feature is explained in terms of velocity anisotropy which might reflect mechanical decoupling of the continental lithosphere from the underlying mantle

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

characterization of millennial scale climate variability the oxygen isotope time series from ice cores in central greenland the greenland ice sheet project 2 gisp2 and the greenland ice core project grip and west antarctica byrd provide a basis for evaluating the behavior of the climate system on millennial time scales these time series have been invoked as evidence for mechanisms such as an interhemispheric climate seesaw or a stochastic resonance process statistical analyses are used to evaluate the extent to which these mechanisms characterize the observed time series simple models in which the antarctic record reflects the greenland record or its integral are statistically superior to a model in which the two time series are unrelated however these statistics depend primarily on the large events in the earlier parts of the record between 80 and 50 kyr bp for the shorter millennial scale dansgaard oeschger events between 50 and 20 kyr bp a first order autoregressive ar 1 stochastic climate model with a physical time scale of t 5 600 6 300 yr is a self consistent explanation for the antarctic record for greenland ar 1 with t 5 400 6 200 yr plus a simple threshold rule provides a statistically comparable characterization to stochastic resonance though it cannot account for the strong 1500 yr spectral peak the similarity of the physical time scales underlying the millennial scale variability provides sufficient explanation for the similar appearance of the greenland and antarctic records during the 50 20 kyr bp interval however it cannot be ruled out that improved cross dating for these records may strengthen the case for an interhemispheric linkage on these shorter time scales additionally the characteristic time scales for the records are significantly shorter during the most recent 10 kyr overall these results suggest that millennial scale variability is determined largely by regional processes that change significantly between glacial and interglacial climate regimes with little influence between the southern and northern hemispheres except during those largest events that involve major reorganizations of the ocean thermohaline circulation

abrupt ice age shifts in southern westerly winds and antarctic climate forced from the north the mid latitude westerly winds of the southern hemisphere play a central role in the global climate system via southern ocean upwelling carbon exchange with the deep ocean agulhas leakage transport of indian ocean waters into the atlantic and possibly antarctic ice sheet stability meridional shifts of the southern hemisphere westerly winds have been hypothesized to occur in parallel with the well documented shifts of the intertropical convergence zone in response to dansgaard oeschger do events abrupt north atlantic climate change events of the last ice age shifting moisture pathways to west antarctica are consistent with this view but may represent a pacific teleconnection pattern forced from the tropics the full response of the southern hemisphere atmospheric circulation to the do cycle and its impact on antarctic temperature remain unclear here we use five ice cores synchronized via volcanic markers to show that the antarctic temperature response to the do cycle can be understood as the superposition of two modes a spatially homogeneous oceanic bipolar seesaw mode that lags behind northern hemisphere climate by about 200 years and a spatially heterogeneous atmospheric mode that is synchronous with abrupt events in the northern hemisphere temperature anomalies of the atmospheric mode are similar to those associated with present day southern annular mode variability rather than the pacific south american pattern moreover deuterium excess records suggest a zonally coherent migration of the southern hemisphere westerly winds over all ocean basins in phase with northern hemisphere climate our work provides a simple conceptual framework for understanding circum antarctic temperature variations forced by abrupt northern hemisphere climate change we provide observational evidence of abrupt shifts in the southern hemisphere westerly winds which have previously documented ramifications for global ocean circulation and atmospheric carbon dioxide these coupled changes highlight the necessity of a global rather than a purely north atlantic perspective on the do cycle

grip deuterium excess reveals rapid and orbital scale changes in greenland moisture origin the northern hemisphere hydrological cycle is a key factor coupling ice sheets ocean circulation and polar amplification of climate change here we present a northern hemisphere deuterium excess profile covering one climatic cycle constructed with the use of 18 o and d greenland ice core project grip records past changes in greenland source and site temperatures are quantified with precipitation seasonality taken into account the imprint of obliquity is evidenced in the site to source temperature gradient at orbital scale at the millennial time scale grip source temperature changes reflect southward shifts of the geographical locations of moisture sources during cold events and these rapid shifts are associated with large scale changes in atmospheric circulation

interhemispheric anti phasing of rainfall during the last glacial period abstract we have obtained a high resolution oxygen isotopic record of cave calcite from caverna botuvera 27 13 s 49 09 w southern brazil which covers most of the last 36 thousand years ka with an average resolution of a few to several decades the chronology was determined with 46 u th ages from two stalagmites tests for equilibrium conditions show that oxygen isotopic variations are primarily caused by climate change we interpret our record in terms of meteoric precipitation changes hence the variability of south american monsoon sam intensity the oxygen isotopic profile broadly follows local insolation changes and shows clear millennial scale variations during the last glacial period with amplitudes as large as 3 but with smaller centennial scale shifts 1 during the holocene the overall record is strikingly similar to but strongly anti correlated with a number of records from the northern hemisphere we compared our record to other precisely dated contemporaneous records from hulu cave eastern china minima in 18o wet periods intense sam at our site are synchronous with maxima in 18o dry periods weak east asian monsoon eam in eastern china within precise dating errors and vice versa this anti phased precipitation relationship between two low latitude locations may be interhemispheric in extent based on comparison with records from other sites precipitation anti phasing may be related to north south shifts in the mean position of the intertropical convergence zone itcz and asymmetry in hadley circulation in two hemispheres associated not with seasonal changes as observed today but with millennial scale climate shifts the millennial scale atmospheric see saw patterns that we observe could have important controls and feedbacks on climate within hemispheres because of water vapor s greenhouse properties

can the differential sensitivity of body wave mantle wave and normal mode data resolve the trade off between transition zone structure and boundary topography abstract large scale seismic models of the three dimensional 3d variations in elastic properties will be biased by topography on mantle boundaries to the extent that volumetric and topographic structures produce similar effects in the data to date seismic inversions for global scale 3d elastic models of the mantle have largely ignored the effect that topography on the major mantle discontinuities would have on estimating these models in this paper we address three questions 1 to what extent does unmodeled structure on the 410 and 660 km boundaries bias volumetric structure in inversions based on normal mode mantle wave structure coefficients absolute s wave travel times and differential ss s travel times 2 can the differences in the sensitivity of s waves ss s phase pairs and normal mode mantle wave data be exploited to estimate transition zone volumetric models that are relatively unbiased by topographic structure 3 have current volumetric models resolved the trade off that exists between volumetric and topographic structures to address question 1 synthetic experiments were performed which show that volumetric models inferred from normal mode mantle wave data can be biased by an average value of 20 25 in r m s amplitude in the transition zone relative to current aspherical earth models depending on the model parametrization employed the average transition zone volumetric bias of models inverted from absolute travel times from both transition zone and lower mantle bottoming s rays that are imprinted with the same topographic signatures is reduced by at least by a factor of five relative to models inverted from normal mode mantle wave data alone a reduction in bias by a factor of about four is obtained using ss s phase pairs in which the ss legs bottom in both the transition zone and lower mantle the use of lower mantle bottoming s rays or ss s phase pairs with the ss legs bottoming in the lower mantle reduces the bias only by an average factor of about two to three relative to the normal mode mantle wave inversion these estimates of bias reduction can vary with the type of damping or smoothness constraints that are applied in the inversion with respect to question 2 these results suggest that in principle absolute s wave travel time data can be used to desensitize volumetric inversions to the bias caused by topography on the transition zone boundaries in practice however near source structure that contaminates s wave travel times would diminish this capability the use of ss s differential times for ss waves which bottom in the transition zone and mantle waves sensitive to transition zone structure but insensitive to the 660 km boundary should be the most effective means of overcoming the trade off to address question 3 we adopt the criterion that the combination of an unbiased volumetric model with an accurate topographic model should provide a better fit to normal mode structure coefficients than the volumetric model alone the boundary model topo660a when added to the volumetric model s12 wm13 fits the normal mode structure coefficients significantly better than the volumetric model alone however more recent models of 410 and 660 km boundary topography degrade the fit of the volumetric models s12 wm13 and sh 10c 17 to the normal mode structure coefficients suggesting that there is not yet a conclusive answer to this question

south pacific split jet itcz shifts and atmospheric north south linkages during abrupt climate changes of the last glacial period abstract a number of key paleoclimate records in the southern hemisphere midlatitudes exhibit climate changes synchronous with abrupt climate changes in the north atlantic we advance a hypothesis argued from consideration of model evidence observational climate diagnostics and atmospheric dynamics that attributes said climate changes in the southern hemisphere to a modulation in the strength of the south pacific split jet a pronounced zonally asymmetric feature of the wintertime southern hemisphere westerlies north atlantic cooling is associated with a weaker split jet characterized by weaker south pacific subtropical and subpolar jets and a strengthened midlatitude jet it leads to climate impacts over the south pacific sector that coincides with regions with observed paleoclimate changes timed to the north atlantic these circulation changes are envisioned to operate in addition to the climate impacts resulting from the oceanic bipolar seesaw a proposed global atmospheric teleconnection links north atlantic cooling to the weakening of the split jet north atlantic cooling induces a southward shift of the marine intertropical convergence zone and weakening of the asian monsoon the resulting hadley circulation change weakens the wintertime south pacific subtropical jet and which in turn leads to a weaker south pacific split jet a weaker split jet leads to a southward shift of the zero wind stress curl line implying a shift in the same sense for the south pacific subtropical front over land it leads to winter warming over new zealand winter cooling over subtropical south america drying over western patagonia and winter warming and wetting of southernmost patagonia our hypothesis also predicts reduced storminess over west antarctica similar changes but of opposite sign occur in the northern hemisphere where a stronger wintertime north pacific subtropical jet increases precipitation over the western united states

punctuated asm strengthening in late heinrich stadial from speleothem records southern china the dynamical adjustment process dap is a requisite for climate system to arrange for a new equilibrium state to evaluate the detailed structure of dap during heinrich stadial hs high resolution speleothem based asian summer monsoon asm variability was reconstructed separately covering hss 5 to 1 in these calcite 18o records a gradual increase of asm intensity is evident in each mid hs followed by a centennial scale asm stability lasting on average 590 years during late hs at this time 18o values decrease by 1 about 50 of the total hs compared with maximum values in early hs this structure of monsoonal hs in speleothem records is not reflected in temperature variations at the greenland but is well mirrored in middle to low latitudes within the northern hemisphere and even southern oceanic records it suggests that southern and low latitude climate conditions are likely important for the termination of hs thus a dap of about 400 600 years is a precondition for the asm sub system to prepare for a new equilibrium

the trade off between volumetric and topographic structure for seismic traveltimes 660 km topography and mantle structure summary synthetic body wave traveltime inversion experiments were performed to evaluate the effect of unmodelled 660km discontinuity topography on the inference of aspherical volumetric mantle structure synthetic residuals were computed for s waves refracted through a 660 km discontinuity with topography we employed a spherical harmonic parametrization for lateral variations of the topography of the 660 km discontinuity and the mantle velocity structure the radial dependence of the velocity structure in the mantle was parametrized in two ways smooth chebyshev polynomial functions and uniform shells the results of both para metrizations show that significant smearing of the input topographic signal appears in the models of volumetric mantle structure inferred from the synthetic data solving for higher order radial structure with the smooth chebyshev functions reduces the smearing but resolution is limited to the half wavelength of the radial basis functions more of the input synthetic residual variance is absorbed into the solution for volumetric structure by solving for higher order radial structure with smooth polynomials or with thinner shells directly below 660km depth however fundamental differences between the kernels for volumetric and topographic structure restrict recovery of the input signal to approximately 80 per cent this work points to the possible value of relating volumetric and topographic structure when inverting seismic observations this may be addressed by incorporating geodynamic modelling and mineral physics results into the modelling of seismic observations

trans pacific glacial response to the antarctic cold reversal in the southern mid latitudes abstract elucidating the timing and regional extent of abrupt climate events during the last glacial interglacial transition 18 11 5 ka is critical for identifying spatial patterns and mechanisms responsible for large magnitude climate events the record of climate change in the southern hemisphere during this time period however remains scarce and unevenly distributed we present new geomorphic chronological and equilibrium line altitude ela data from a climatically sensitive mountain glacier at monte san lorenzo 47 s central patagonia twenty four new cosmogenic 10be exposure ages from moraines provide a comprehensive glacial record in the mid latitudes of south america which constrain the timing spatial extent and magnitude of glacial fluctuations during the antarctic cold reversal acr 14 5 12 9 ka rio tranquilo glacier advanced and reached a maximum extent at 13 9 0 7 ka three additional inboard moraines afford statistically similar ages indicating repeated glacier expansions or marginal fluctuations over the acr our record represents the northernmost robust evidence of glacial fluctuations during the acr in southern south america documenting not only the timing of the acr maximum but also the sequence of glacier changes within this climate event based on ela reconstructions we estimate a cooling of 1 6 1 8 c at the peak of the acr the rio tranquilo record along with existing glacial reconstructions from new zealand 43 s and paleovegetation records from northwestern 41 s and central west 45 s patagonia suggest an uniform trans pacific glacier climate response to an acr trigger across the southern mid latitudes we posit that the equatorial migration of the southern westerly winds provides an adequate mechanism to propagate a common acr signal across the southern hemisphere

high resolution record of northern hemisphere climate extending into the last interglacial period high resolution record of northern hemisphere climate extending into the last interglacial period

mapping the upper mantle three dimensional modeling of earth structure by inversion of seismic waveforms a method is presented for the inversion of waveform data for the three dimensional distribution of seismic wave velocities the method is applied to data from the global digital networks international deployment of accelerometers global digital seismograph network the selected data set consists of some 2000 seismograms corresponding to 53 events and 870 paths the moment tensors of the earthquakes are determined through an iterative procedure which minimizes the corrupting influence of lateral heterogeneity a global model is constructed for shear wave velocity expanded up to degree and order 8 in spherical harmonics and described by a cubic polynomial in depth for the upper 670 km of the earth s mantle although no a priori information is incorporated the model predictions reproduce much of what is known about the dispersion of mantle waves for example high phase velocities for shields low velocities at ridges and a strong degree 2 pattern for rayleigh waves since the method makes use of complete waveforms overtone data are also included it is shown that the model is reproducible in that substantially the same model can be constructed from each half of the total data set considered independently the model shows that shields and ridges are major features in the depth interval 25 250 km the ridges of the southern pacific and the larger shields persist to 350 km but the southeast indian rise is underlain by a high velocity anomaly at this depth as is much of the mid atlantic ridge at 450 650 km the major features are a broad region of high velocities incorporating south america much of the south atlantic and parts of west africa a broad region of low velocities in the central and eastern pacific high velocities in the western pacific and a low velocity anomaly beneath the red sea and the gulf of aden in the absence of a crustal correction degrees 2 and 3 show a high positive correlation with the geoid paradoxically this is largely destroyed when the distribution in crustal thickness is taken into account spherical harmonic degrees 4 7 show a significant negative correlation

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

constraints on upper mantle discontinuities from observations of long period reflected and converted phases stacked images combining over 5 years of long period global digital seismograph network data reveal many phases associated with reflections and conversions from upper mantle discontinuities these images show travel time and amplitude relative to a reference phase which is aligned and normalized on all seismograms prior to stacking results obtained for p s ss and pp reference arrivals resolve numerous phases from discontinuities near 410 and 660 km while some of the stacks also show evidence for a weaker discontinuity near 520 km phases of particular interest include p and sh multiples resulting from topside reflections precursors to ss from underside reflections and p to sv converted phases these phases can be clearly seen both in the waveform stacks and in cross correlation analysis of individual seismograms travel times for these arrivals are converted to discontinuity depths relative to velocities in the preliminary reference earth model and empirical corrections are applied for the effects of lateral velocity variations in the upper mantle average apparent depths to the discontinuities for the different phases agree to within 3 km for the 410 km discontinuity to within 4 km for the 520 km discontinuity and to within 8 km for the 660 km discontinuity the best global averages are obtained from the ss precursor data which indicate discontinuities at 415 519 and 659 km discontinuity depths obtained from the p to sv converted phases at over 35 individual seismic stations exhibit variations of less than 20 km apparent depths to the 660 km discontinuity consistently show greater variability than depths to the 410 km discontinuity supporting recent laboratory results which indicate that the clapeyron slope for the 660 km discontinuity is significantly larger in magnitude than the slope for the 410 km discontinuity precursors to ss seen between 110 and 180 are particularly useful for mapping possible lateral variations in discontinuity depths since each arrival can be associated with a single underside reflection point apparent discontinuity depths computed from ss precursors for different tectonic regions agree to within about 5 km the ss precursors have especially good coverage near the subducting slabs in the northwest pacific analysis of apparent discontinuity depths in this area suggests the presence of a broad 1000 to 1500 km wide region near the slab in which the 660 km discontinuity is depressed by about 20 km measuring absolute amplitudes for these phases is difficult due to the large corrections required to compensate for the effects of incoherent stacking relative amplitude analysis suggests that the p and s wave impedance changes at 410 km are about 0 8 1 1 times the size of the changes at 660 km and that the contrasts at 520 km are between 0 3 and 0 6 of the changes at 410 km

topography of the 400 km discontinuity from observations of long period p400p phases summary new evidence is presented for lateral variation of the depth of upper mantle discontinuities this evidence is derived from an analysis of amplitude of p400p phases underside reflections from the 400 km discontinuity for great circle paths with bounce points in the upper mantle near the hawaii emperor hotspot chain the data are long period from wwssn sro and rstn stations the epicentral distance ranges from 95 to 115 which ensures that no other major phases arrive in the time window of interest observed p400p amplitudes are compared with synthetic amplitudes for both a laterally homogeneous model and models including 3 d topography on the discontinuities the observations permit models of topography that can be characterized by undulations of the 400 km discontinuity with wavelengths in the range 600 2000 km and peak to peak amplitude of the order of 30 40 km such long wavelength structure causes quite strong de focusing of pdp phases synthetic pdp waveforms reveal that topography not only affects its amplitude but also smears out the pdp wave causing overlap between different pdp phases this mechanism is proposed as the reason for the lack of well developed pdp phases reported in the literature the fact that the waveforms can be seriously distorted prohibits the use of pdp phases in a waveform inversion the curvature of the discontinuity can be explained as the signature of convection currents crossing the discontinuity although the data cover the hawaii hotspot no signature of a plume structure can be detected in the data probably because the resolving power of the data is too low

high resolution greenland ice core data show abrupt climate change happens in few years the last two abrupt warmings at the onset of our present warm interglacial period interrupted by the younger dryas cooling event were investigated at high temporal resolution from the north greenland ice core project ice core the deuterium excess a proxy of greenland precipitation moisture source switched mode within 1 to 3 years over these transitions and initiated a more gradual change over 50 years of the greenland air temperature as recorded by stable water isotopes the onsets of both abrupt greenland warmings were slightly preceded by decreasing greenland dust deposition reflecting the wetting of asian deserts a northern shift of the intertropical convergence zone could be the trigger of these abrupt shifts of northern hemisphere atmospheric circulation resulting in changes of 2 to 4 kelvin in greenland moisture source temperature from one year to the next

timing and structure of the younger dryas event and its underlying climate dynamics the younger dryas yd arguably the most widely studied millennial scale extreme climate event was characterized by diverse hydroclimate shifts globally and severe cooling at high northern latitudes that abruptly punctuated the warming trend from the last glacial to the present interglacial to date a precise understanding of its trigger propagation and termination remains elusive here we present speleothem oxygen isotope data that in concert with other proxy records allow us to quantify the timing of the yd onset and termination at an unprecedented subcentennial temporal precision across the north atlantic asian monsoon westerlies and south american monsoon regions our analysis suggests that the onsets of yd in the north atlantic 12 870 30 b p and the asian monsoon westerlies region are essentially synchronous within a few decades and lead the onset in antarctica implying a north to south climate signal propagation via both atmospheric decadal time scale and oceanic centennial time scale processes similar to the dansgaard oeschger events during the last glacial period in contrast the yd termination may have started first in antarctica at 11 900 b p or perhaps even earlier in the western tropical pacific followed by the north atlantic between 11 700 40 and 11 610 40 b p these observations suggest that the initial yd termination might have originated in the southern hemisphere and or the tropical pacific indicating a southern hemisphere tropics to north atlantic asian monsoon westerlies directionality of climatic recovery

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

divergent influences of the greenland and antarctica climates on the asian monsoon during a stadial to interstadial cycle abstract despite the links of asian monsoon with climates at high northern and southern latitudes it remains unclear that at which time and to what extent the asian monsoon variation is dominated by one of the two drivers throughout a greenland stadial gs to greenland interstadial gi cycle here we provide a chinese stalagmite 18o record to study their teleconnections throughout the gs 6 to gi 5 2 cycle the resemblance between the stalagmite and greenland records in timing duration and abruptness of gi 5 2 supports that the occurrence and termination of gis are paced by the northern driving force during the intervals of gi 5 2 and gs 6 however the asian monsoon fluctuated concomitantly with variation in temperature over antarctica instead of over greenland this covariation indicates dominant influences of the antarctic climate during the climatically stable intervals of stadials and interstadials this study updates our knowledge on mechanical dynamics of the asian monsoon change and global climate change throughout a gs to gi cycle

seismic wave theory in the us 1991 1994 the early 1990s saw many interesting contributions to the field of seismic wave propagation from a variety of us researchers even as many wave propagation specialists shifted their research focus away from new methods for seismogram synthesis toward data acquisition and inversion in part this shift was a natural consequence of the explosive growth in the amount and availability of high quality digital seismic data ready to be processed with interpretation tools developed in previous decades as a result observational seismology has recently made dramatic contributions toward imaging global geodynamics revealing the great variety of crustal structures and laying the foundation for a collaborative global network of broadband seismometers to monitor earthquakes and other seismic events however recent observational studies have brought to light new problems in wave propagation theory and have made some older problems more difficult to ignore

the p wave upper mantle structure beneath an active spreading centre the gulf of california summary detailed analysis of short period travel time dt d and wave form data reveals the upper mantle structure beneath an oceanic ridge to depths of 900 km more than 1400 digital seismograms from earthquakes in mexico and central america recorded at scarlet yield 1753 travel times and 58 direct measurements of dt d as well as high quality stable waveforms the 29 events combine to form a continuous record section from 9 to 40 with an average station spacing of less than 5 km first the travel times are inverted using the tau method of bessonova et al the resultant model is adjusted to agree with the experimental p values further constraints arise from the observed relative amplitudes of mantle phases which are modelled by trial and error using wkbj synthetic seismograms chapman wiggins model gca which is appropriate for western mexico north of 20 latitude is similar to existing upper mantle models for shield tectonic continental and arc trench regimes below 400 km but differs significantly in the upper 350 km gca velocities are very low in this depth range the model catches up with the others with a very large velocity gradient from 225 to 390 km this well resolved feature is consistent with the shear wave model tna for western north america of grand helmberger the abundant data from 20 to 30 control the detailed shape of the 660 km discontinuity very large velocity gradients lie both above 620 660 km and below 661 680km a 2 8 per cent velocity change

a minimum thermodynamic model for the bipolar seesaw 1 the simplest possible model is proposed to explain a large fraction of the millennial climate variability measured in the isotopic composition of antarctic ice cores the model results from the classic bipolar seesaw by coupling it to a heat reservoir in this thermal bipolar seesaw the heat reservoir convolves northern time signals with a characteristic timescale applying the model to the data of grip and byrd we demonstrate that maximum correlation can be obtained using a timescale of about 1000 1500 years higher correlations are obtained by first filtering out the long term variability which is due to astronomical and greenhouse gas forcing and not part of the thermal bipolar seesaw the model resolves the apparent confusion whether northern and southern climate records are in or out of phase synchronous or time lagged

onset of deglacial warming in west antarctica driven by local orbital forcing an annually resolved ice core record from west antarctica indicates that warming driven by local insolation resulting from sea ice decline began in that region about 2 000 years before warming in east antarctica reconciling two alternative explanations for deglacial warming in the southern hemisphere

precession modulation of the south pacific westerly wind belt over the past million years the southern westerly wind belt sww interacts with the antarctic circumpolar current and strongly impacts the southern ocean carbon budget and antarctic ice sheet dynamics across glacial interglacial cycles we investigated precipitation driven sediment input changes to the southeast pacific off the southern margin of the atacama desert over the past one million years revealing strong precession 19 23 ka cycles our simulations with 2 ocean atmosphere general circulation models suggest that observed cyclic rainfall changes are linked to meridional shifts in water vapor transport from the tropical pacific toward the southern atacama desert these changes reflect a precessional modulation of the split in the austral winter south pacific jet stream for precession maxima we infer significantly enhanced rainfall in the southern atacama desert due to a stronger south pacific split jet with enhanced subtropical subpolar jets and a weaker midlatitude jet conversely we derive dry conditions in northern chile related to reduced subtropical subpolar jets and an enhanced midlatitude jet for precession minima the presence of precessional cycles in the pacific sww and lack thereof in other basins indicate that orbital scale changes of the sww were not zonally homogeneous across the southern hemisphere in contrast to the hemispherewide shifts of the sww suggested for glacial terminations the strengthening of the jet is unique to the south pacific realm and might have affected winter controlled changes in the mixed layer depth the formation of intermediate water and the buildup of sea ice around antarctica with implications for the global overturning circulation and the oceanic storage of atmospheric co2

seismic tomography of the earth s mantle the analysis of information contained in seismograms is the most promis ing tool for retrieving the three dimensional structure of the earth s mantle and thereby improving our understanding of its dynamics the procedure through which three dimensional images of different parts of the earth s interior arc obtained by inversion of a set of observables measured on the surface of the earth is commonly termed tomography by analogy with the medical procedure with which it shares many aspects of methodology but with much less favorable experimental conditions ideally one would like to be able to model entire seismograms over a wide frequency band for a large number of uniformly distributed sources and stations in order to obtain an optimal illumination of the inhomogeneities appropriate for tomographic inversion in practice a number of difficulties and limitations arise onc is the nonuniform distribution of sources and receivers seis micity is concentrated at plate boundaries and the distribution of con tinental and island areas leaves large portions of the oceans unin strumented another difficulty is our limited ability to accurately forward model the effects of wave propagation in a realistic earth which presents departures from a spherically symmetric referencc model of both elastic anc1astic and isotropic anisotropic nature this is why until now the simplest kind of tomography travel time tomography has prevailed relying on simplifying assumptions concerning the interaction of asphericity with propagating seismic waves ray theory and the laws of geometrical optics in their

three dimensional seismic models of the earth s mantle accurate models of the distribution of elastic heterogeneity in the earth s mantle are important in many areas of geophysics the purpose of this paper is to characterize and compare quantitatively a set of recent three dimensional models of the elastic structure of the earth to assess their similarities and differences and to analyze their fit to one class of data in order to highlight fruitful directions for future research the aspherical models considered are the following m84c woodhouse and dziewonski 1984 lo2 56 dziewonski 1984 mdlsh tanimoto 1990a sh 10c 17 masters et al 1992 and s12 wm13 su et al 1994 through much of the discussion m84c and lo2 56 are combined into a single whole mantle model m84c lo2 56 the fit of each model to previously tabulated even degree normal mode structure coefficients taken from smith and masters 1989a and ritzwoller et al 1988 for multiplets along the normal mode fundamental and first second and fifth overtone branches is also presented rather than concentrating on detailed comparisons of specific features of the models analyses of these models are general and statistical in nature in particular we focus on a comparison of the amplitude and the radial and geographical distribution of heterogeneity in each model and how variations in each affect the fit to the normal mode observations in general the results of the comparisons between the models are encouraging especially with respect to the geographical distribution of heterogeneity and in the fit to the normal mode data sensitive to the upper mantle and lowermost lower mantle there remain however significant discrepancies in amplitude and in the radial distribution of heterogeneity especially near the top of the upper mantle and near the top of the lower mantle the confident use of these models to constrain compositional and dynamical information about the mantle will await the resolution of these discrepancies the factors that may be responsible for the differences in the models and or for the misfit between the observed and predicted normal mode data are divided into two types intrinsic or procedural and extrinsic or structural we discuss only three extrinsic factors at length here including errors in the reference crustal models unmodeled topography on discontinuities in the interior of the mantle and errors in the assumed relationships between shear s and compressional p heterogeneity

subducted slabs and the geoid constraints on mantle rheology and flow geoid anomalies are primarily the result of the density contrasts driving mantle convection and plate motion the total geoid anomaly resulting from a given density contrast in a convecting earth is affected by the mass anomalies associated with the flow induced deformation of the upper surface and internal compositional boundaries as well as by the density contrast itself these boundary deformations and hence the total gravity field depend on the radial distribution of effective viscosity if the internal density contrasts can be estimated as is the case for subducted slabs useful constraints can be placed on the depth and on the variation of viscosity with depth of the convecting system the degree 4 9 components of the observed long wavelength geoid are highly correlated with those predicted by a density model for seismically active subducted slabs the positive sign of the correlation requires that the effective viscosity increases with depth by a factor of 30 or more the amplitude of the correlation cannot be explained by the density contrasts associated with just the seismically active parts of subducted slabs however the amplitude can be explained if the density contrasts associated with subduction extend into the lower mantle or if old lithosphere is piled up at the base of the upper mantle beneath subduction zones to a thickness in excess of 350 km over horizontal distances of thousands of kilometers mantlewide convection in a mantle that has a viscosity increasing with depth provides a simple explanation of the long wavelength geoid anomalies over subduction zones

global glacier dynamics during 100 ka pleistocene glacial cycles ice volume during the last ten 100 ka glacial cycles was driven by solar radiation flux in the northern hemisphere early minima in solar radiation combined with critical levels of atmospheric co 2 drove initial glacier expansion glacial cycles between marine isotope stage mis 24 and mis 13 whilst at 100 ka periodicity were irregular in amplitude and the shift to the largest amplitude 100 ka glacial cycles occurred after mis 16 mountain glaciers in the mid latitudes and asia reached their maximum extents early in glacial cycles then retreated as global climate became increasingly arid in contrast larger ice masses close to maritime moisture sources continued to build up and dominated global glacial maxima reflected in marine isotope and sea level records the effect of this pattern of glaciation on the state of the global atmosphere is evident in dust records from antarctic ice cores where pronounced double peaks in dust flux occur in all of the last eight glacial cycles glacier growth is strongly modulated by variations in solar radiation especially in glacial inceptions this external control accounts for 50 60 of ice volume change through glacial cycles internal global glacier climate dynamics account for the rest of the change which is controlled by the geographic distributions of glaciers

the interaction of a subducting lithospheric slab with a chemical or phase boundary we have used a finite element model of time dependent convection to determine the conditions for penetration of the subducted plate into the lower mantle a temperature dependent and non newtonian rheology is applied to achieve platelike behavior of the upper and sinking thermal boundary layer of convection the 650 km discontinuity is taken as either a chemical or phase boundary or as a combination of both it is represented by a marker chain which effects additional buoyancy when distorted out of its equilibrium position when the compositional density contrast is greater than about 5 the descending slab is deflected sidewards at the boundary and two layer convection prevails a resulting depression of the boundary in the range of 50 200 km should be detectable with seismic methods below 5 density difference the slab plunges several hundred kilometers into the lower mantle and below 2 it will probably not stop before reaching the core mantle boundary and extensive mixing would be expected with a pure phase change a negative clapeyron slope of about 6 mpa k 60 bar k is required to establish a type of leaky double layer convection a more moderate slope can aid a small compositional density difference to prevent slab penetration into the lower mantle with the present uncertainties about the physical nature of the 650 km discontinuity a variety of convective styles appears possible on dynamical grounds

long period surface waves and mantle boundary undulations summary we address two questions with synthetic experiments 1 is there sufficient signal in long period surface wave data to provide useful constraints on topography on the internal boundaries in the upper mantle and around the transition zone 2 what is the magnitude and nature of the bias caused by undulations on internal discontinuities on inversions for volumetric structures that do not also include boundary structures the synthetic experiments rest on models of boundary topography on three upper mantle and transition zone discontinuities at 220 400 and 660 km depth the 660 km topographic model is that of shearer masters 1992 we consider the effect of two different 400 km models one is identical to shearer and master s model model a and the other is their model sign flipped model b the 220 km model is ad hoc but has been motivated by the work of revenaugh jordan 1989 1991b who argue that the 220 km discontinuity is not a global feature at all but has a continental affinity for simplicity our 220 km model is global but the jump in material properties across the discontinuity has been reduced relative prem by a factor of two and is based on a continent ocean model which is depressed up to 25 km below continents and ascends 10 km under oceans these resulting boundary models have a significant effect on long period fundamental spheroidal mode structure coefficients with a magnitude on average of 15 30per cent relative to the volumetric models m84a lo2 56 between harmonic degrees 20 l 75 inversion for volumetric structures of the structure coefficients computed with the boundary models alone devoid of volumetric structure shows that simple mantle volumetric structures can be found to fit the boundary structure coefficients nearly perfectly this demonstrates the essential trade off between boundary and volumetric mantle structures using surface wave data alone destructive interference between the effects of the 660km boundary and model b of the 400 km boundary requires a somewhat oscillatory volumetric mantle model to fit the structure coefficients but the resulting volumetric model is no more oscillatory than current models of the upper mantle both boundary models a and b produce on average a 30 per cent volumetric bias relative to m84a lo2 56 from the base of the crust to 1000 km depth the relative nature of the 400 km and 660 km undulations strongly affects the radial distribution of the volumetric bias but has less of an effect on the average size of the bias

synchronous timing of abrupt climate changes during the last glacial period abrupt climate changes during the last glacial period have been detected in a global array of palaeoclimate records but our understanding of their absolute timing and regional synchrony is incomplete our compilation of 63 published independently dated speleothem records shows that abrupt warmings in greenland were associated with synchronous climate changes across the asian monsoon south american monsoon and european mediterranean regions that occurred within decades together with the demonstration of bipolar synchrony in atmospheric response this provides independent evidence of synchronous high latitude to tropical coupling of climate changes during these abrupt warmings our results provide a globally coherent framework with which to validate model simulations of abrupt climate change and to constrain ice core chronologies

core mantle boundary topography and whole mantle convection the seismically observed topography on the core mantle boundary cmb and the velocities of the surface tectonic plates together constitute important observational constraints on viscous flow models of the earth s mantle we show here that the cmb deflections inferred by morelli and dziewonski 1987 may be explained in terms of a simple model of whole mantle convection and in the process we provide a new method for inferring the penetration depths of subducted slabs

greenland antarctic phase relations and millennial time scale climate fluctuations in the greenland ice cores abstract the greenland grip gisp2 and antarctic byrd ice cores are examined in the frequency domain with the data synchronized using common methane variations using conventional time series analysis a simple picture emerges there is low frequency periods longer than about 10 kyr coherence between the two records consistent with a simple time delay antarctica leading by 1 2 kyr two geographical data points are however inadequate to infer causality from the south to north time lag at higher frequencies in the millennial band there is no measurable average relationship between the records and they appear to represent different processes with a regional character a serious question concerns the extent to which the greenland cores reflect tracer concentration change without corresponding abrupt climate change the large abrupt shifts in ice 18 o can be rationalized as owing to wind trajectory shifts perhaps of rather modest size many different physical phenomena probably do however contribute to the record as a function of time and time scale

the deuterium excess records of epica dome c and dronning maud land ice cores east antarctica new high resolution deuterium excess d data from the two epica ice cores drilled in dronning maud land edml and dome c edc are presented here the main moisture sources for precipitation at edc and edml are located in the indian ocean and atlantic ocean respectively the more southward moisture origin for edml is reflected in a lower present day d value compared to edc the edml and edc isotopic records 18o and d show the main climate features common to the east antarctic plateau and similar millennial scale climate variability during the last glacial period however quite large 18o and d differences are observed during mis5 5 and the glacial inception with a long term behaviour a possibility for this long term difference could be related to uncertainties in past accumulation rate which are used in the glaciological models regional climate anomalies between the two sites during mis5 5 could also be consistent with the observed edml edc 18o and d gradient anomalies simulations performed with the general circulation model echam4 for different time slices provide a temporal temperature isotope slope for the edml region in fair agreement to the modern spatial slope tsite and tsource records are extracted from both ice cores using a modelling approach after corrections for past 18o seawater and elevation changes a limited impact of d on antarctic temperature reconstruction at both edml and edc has been found with a higher impact only at glacial inception the aim antarctic isotope maximum events in both ice cores are visible also after the source correction suggesting that these are real climate features of the glacial period the different shape of the aim events between edc and edml as well as some climate features in the early holocene points to a slightly different climate evolution at regional scale a comparison of our temperature reconstruction profiles with the aerosol fluxes show a strong coupling of the nssca fluxes with antarctic temperatures during glacial period and a tighter coupling of 18o and tsite with ssna flux at edml compared to edc during the glacial period and mis5 5

mapping the lower mantle determination of lateral heterogeneity in p velocity up to degree and order 6 the data from the international seismological centre bulletins for the years 1964 1979 are used to derive a three dimensional model of lateral variations of the p velocity in the lower mantle unlike previous studies in which such perturbations were represented by a blocklike parameterization we seek the solution in the form r k 0k l 0l m 0l k r kalm cos m kblm sin m plm cos some 500 000 travel time residuals for teleseismic distances from 5000 earthquakes are used in an iterative procedure to derive the coefficients a and b the maximum k and l used are 4 and 6 respectively in each iteration in addition to solving for the three dimensional structure we also relocate all the earthquakes perturb the average travel time curve and determine station corrections for over 1000 stations particular attention is given to the problem of weighting the individual observations in order to avoid as much as possible the bias due to the uneven distribution of sources and receivers the resulting model shows a high level of perturbations just below the 670 km discontinuity and just above the core mantle boundary where the maximum perturbations reach 1 1 5 of the average velocity even for this highly smoothed model at a depth of 2000 km the rms perturbations are some 3 to 4 times lower than at either of the two boundaries the model predicts well the large scale pattern of observed travel time residuals for various source regions except for the distinct effects of the subduction zones the most striking large scale three dimensional feature of the model is a ring of high velocities circumscribing the pacific basin within a depth range from 1000 km to the core mantle boundary if the proportionality between perturbations in the velocity and temperature is assumed the derived pattern of anomalies could be consistent with a two boundary layer model of convection in the lower mantle some caution however should be exercised in interpreting the results near the 670 km discontinuity where the resolving power of our data set is limited by the lack of adequate control of the upper mantle structure at the same time a significant global increase in the level of perturbations in the lowermost mantle is established beyond question the results of dziewonski et al 1977 on the correlation between the equivalent geoid and the observed geoid for the angular order numbers 2 and 3 is confirmed but there is no significant correlation for degrees from 4 to 6 these calculations are made for the rigid earth richards and hager this issue show that introduction of finite viscosity can change the sign of a geoid anomaly in addition recent studies of lateral variations in the shear velocity reveal the presence of large scale anomalies in the upper mantle thus while the inferences made here with respect to the origin of the gravest orders of the geopotential field are not conclusive they point the way in which results from seismology can be used to address some of the basic questions of geodynamics

the upper mantle degree 2 constraints and inferences from global mantle wave attenuation measurements we present the results of an analysis of global lateral variations in anelasticity of the upper mantle as measured from very long period rayleigh waves observed on the geoscope network four consecutive wave trains are used on each record to eliminate uncertainty on the amplitude at the source and to take into account in a linear manner focussing effects due to lateral variations in elastic parameters local estimates of attenuation are obtained using an inversion method based on the introduction of a correlation length as an alternative to spherical harmonics expansion comparison of phase velocity and attenuation maps obtained at various periods shows a strong correlation of high q and high velocity regions respectively low q and low velocity both for great circle and minor arc data at periods around 200 s the detailed analysis of the degree 2 pattern well resolved in the attenuation maps around this period shows that it originates in the depth range 250 500 km and indicates that of the upper mantle degree 2 pattern observed in shear velocity is likely to be of thermal origin possibly related to the topmost part of the large scale convective system corrections for dispersion due to anelastic attenuation in degree 2 lead to significant decrease of corresponding lateral variations in shear velocity and a better agreement in phase and amplitude of the upper mantle degree 2 pattern with that observed in the geoid this study shows that it is important to consider correcting presently available upper mantle tomographic models for the effects of intrinsic attenuation at long periods before performing any comparison with short period data or other geophysical parameters

lagged atmospheric circulation response in the black sea region to greenland interstadial 10 northern hemispheric high latitude climate variations during the last glacial are expected to propagate globally in a complex way investigating the evolution of these variations requires a precise synchronization of the considered environmental archives aligning the globally common production rate variations of the cosmogenic radionuclide 10be in different archives provides a tool for such synchronizations here we present a 10be record at 40 y resolution along with subdecadal proxy records from one black sea sediment core around greenland interstadial 10 gi 10 41 ka bp and the laschamp geomagnetic excursion we synchronized our 10be record to that from greenland ice cores based on its globally common production rate variations the synchronized environmental proxy records reveal a bipartite climate response in the black sea region at the onset of gi 10 first in phase with greenland warming reduced sedimentary coastal ice rafted detritus contents indicate less severe winters second and with a lag of 190 44 y an increase in the detrital k ti ratio and authigenic ca precipitation point to enhanced regional precipitation and warmer lake surface temperatures we explain the lagged climatic response by a shift in the dominant mode of atmospheric circulation likely connected with a time transgressive adjustment of the regional thermal ocean interior to interstadial conditions

links between tropical rainfall and north atlantic climate during the last glacial period the last glacial period was marked by dramatic climate fluctuations sediment records from the cariaco basin and the arabian sea suggest that cooling in the north atlantic region was tightly coupled with a southward displacement of the intertropical convergence zone and a weakening of the indian summer monsoon

reflection properties of phase transition and compositional change models of the 670 km discontinuity seismic observations of the amplitude ratio amplitude p 670p amplitude p p suggest a reflection coefficient r 7 13 for reflections from the underside of the 670 km discontinuity this can be compared with calculated reflection coefficients to constrain the nature of the mantle transition region velocity and density profiles through the transition region were calculated for olivine pyroxene garnet and olivine garnet model mineral assemblages for models with identical upper and lower mantle compositions the computed reflection coefficients from the phase transitions pertinent to the 670 km discontinuity are less than 0 5 these phase transitions produce their maximum r if they are effectively discontinuous which gives r 2 3 except for the garnet to perovskite transformation which gives r 8 in general given realistic impedance contrasts across a phase transition the change in material properties must occur over a region less than 3 km thick for r to be of the order of 5 a discontinuous change in chemical composition at 670 km depth coupled with a change in phase can also produce a reflection coefficient of the observed magnitude r 2 3 for a first order change in phase at 670 km with different iron concentrations on each side of the discontinuity and r 4 6 for cases with a discontinuous increase in silica content based on our analysis the thermal and chemical boundary layers which would occur across such a chemical discontinuity would probably have negligible effects on the reflection coefficient thus although limited by considerable uncertainties the mineralogical and seismic data suggest a two layered model of the mantle with a discontinuous change in both phase and composition at 670 km

evidence for a seismic discontinuity near 200km depth under a continental margin summary we present evidence for a seismic discontinuity near 200 km depth the lehmann discontinuity under the passive continental margin of north west australia where continental lithosphere merges into oceanic lithosphere the velocity contrast across the discontinuity is 0 2 0 3 km s and is similar to the contrast across discontinuities at similar depths in seismic models for purely continental paths to the east under central australia the discontinuity has been shown to be present under continents oceans and now at continental margins and is probably a worldwide feature

southern hemisphere westerly wind changes during the last glacial maximum paleo data synthesis changes in the strength and position of southern hemisphere westerly winds during the last glacial cycle have been invoked to explain both millennial and glacial interglacial climate fluctuations however neither paleo models nor paleodata agree on the magnitude or even the sign of the change in wind strength and latitude during the most studied glacial period the last glacial maximum lgm compared to the recent past this paper synthesizes paleo environmental data that have been used to infer changes in lgm winds data compilations are provided for changes in terrestrial moisture dust deposition sea surface temperatures and ocean fronts and ocean productivity and existing data on southern hemisphere ocean circulation changes during the lgm are summarized we find that any hypothesis of lgm wind and climate change needs to provide a plausible explanation for increased moisture on the west coast of continents cooler temperatures and higher productivity in the subantarctic zone and reductions in agulhas leakage around southern africa our comparison suggests that an overall strengthening an equatorward displacement or no change at all in winds could all be interpreted as consistent with observations if a single cause related to the southern westerlies is sought for all the evidence presented then an equatorward displacement or strengthening of the winds would be consistent with the largest proportion of the observations however other processes such as weakening or poleward shifts in winds a weakened hydrological cycle extended sea ice cover and changed buoyancy fluxes cannot be ruled out as potential explanations of observed changes in moisture surface temperature and productivity we contend that resolving the position and strength of westerly winds during the lgm remains elusive based on data reconstructions alone however we believe that these data reconstructions of environmental conditions can be used in conjunction with model simulations to identify which processes best represent westerly wind conditions during the lgm

southern hemisphere climate variability forced by northern hemisphere ice sheet topography an antarctic ice core reveals that during the last ice age the topography of northern hemisphere ice sheets shifted tropical pacific convection eastward increasing climate variability in the high southern latitudes during glacial maxima ice covered vast areas of the northern hemisphere and had a broad influence across the global climate system tyler jones and colleagues show that climate variability in the southern hemisphere was influenced by these large northern hemisphere ice sheets and was about twice as high during the last glacial maximum than in the warmer holocene that followed the ice sheets created more elevated and brighter surfaces than at present and this altered the location of tropical convection and the atmospheric pathways that link the tropics to west antarctica the results reveal the intimate climatic link between the high latitudes and the key role of the tropics as an interhemispheric mediator the presence of large northern hemisphere ice sheets and reduced greenhouse gas concentrations during the last glacial maximum fundamentally altered global ocean atmosphere climate dynamics1 model simulations and palaeoclimate records suggest that glacial boundary conditions affected the el nino southern oscillation2 3 a dominant source of short term global climate variability yet little is known about changes in short term climate variability at mid to high latitudes here we use a high resolution water isotope record from west antarctica to demonstrate that interannual to decadal climate variability at high southern latitudes was almost twice as large at the last glacial maximum as during the ensuing holocene epoch the past 11 700 years climate model simulations indicate that this increased variability reflects an increase in the teleconnection strength between the tropical pacific and west antarctica owing to a shift in the mean location of tropical convection this shift in turn can be attributed to the influence of topography and albedo of the north american ice sheets on atmospheric circulation as the planet deglaciated the largest and most abrupt decline in teleconnection strength occurred between approximately 16 000 years and 15 000 years ago followed by a slower decline into the early holocene

the spatial extent and dynamics of the antarctic cold reversal the last deglaciation was interrupted by the antarctic cold reversal proxy records and climate modelling suggest that a redistribution of oceanic and atmospheric heat caused changes in temperature and hydrology across the southern hemisphere

a mineral physics test of a pyrolite mantle an homogeneous upper mantle chemical composition defined by the pyrolite model is compatible with the observed seismic velocities such a test is now possible for depths to 670 km as a result of new mineral elasticity and phase equilibria data three specific criteria for comparing the mineralogical velocity model with that of the mantle include i the velocity jump at the 400 km discontinuity ii the velocity increase between 400 and 670 km and iii the absolute value of velocity in the 300 to 400 km region shallower than the discontinuity the first criterion is sensitive to the olivine content of the mantle and the pressure derivatives of the elastic properties of the high pressure phase the application of this criterion suggests a mantle with 40 65 olivine by volume the second criterion is sensitive to the details of the pyroxene to garnet transformation and the relative abundances of the pyroxenes and garnet the third criterion depends on details of this transformation as well as iron content and the temperature of the mantle

water isotope diffusion in the wais divide ice core during the holocene and last glacial we use a high resolution water isotope record from the west antarctic ice sheet divide ice core wdc to evaluate the effects of water isotope diffusion for the last 29 ka b p using spectral analysis of the data we determine diffusion lengths in depth and time domains the diffusion length quantifies the mean cumulative diffusive displacement of water molecules relative to their original location at time of deposition we simulate the observed signal with models and find that our understanding of processes and conditions in the ice sheet is incomplete with the effects of ice deformational thinning removed portions of the holocene record show total diffusion lengths smaller than predicted for a lower limit case of diffusion through a single ice crystal such reduced diffusion is probably due to structural features such as crusts and tortuous porosity that inhibit vapor transport in the firn in the late glacial portion of the record diffusion lengths double between 19 5 and 17 ka b p known dependencies of diffusion on climatic variables do not account for this enhancement in models and we hypothesize that it could arise from thermal gradients in the firn column impurity driven enhancement of solid ice diffusion or changes in firn grain properties that alter vapor access to open pores despite model uncertainties the wdc diffusion length chronology will be an essential input to future studies of high frequency variability in the water isotope climate record as it allows for the effects of diffusion to be removed

influence of summer sublimation on δd δ18o and δ17o in precipitation east antarctica and implications for climate reconstruction from ice cores

subtropical north atlantic temperatures 60 000 to 30 000 years ago a reconstruction of sea surface temperature based on alkenone unsaturation ratios in sediments of the bermuda rise provides a detailed record of subtropical climate from 60 000 to 30 000 years ago northern sargasso sea temperatures changed repeatedly by 2 to 5 c covarying with high latitude temperatures that were previously inferred from greenland ice cores the largest temperature increases were comparable in magnitude to the full glacial holocene warming at the site abrupt cold reversals of 3 to 5 c lasting less than 250 years occurred during the onset of two such events greenland interstadials 8 and 12 suggesting that the largest most rapid warmings were especially unstable