ice age simulations with a calving ice sheet model abstract variations of ice sheet volume during the quaternary ice ages are simulated using a simple ice sheet model for the northern hemisphere the basic model predicts ice thickness and bedrock deformation in a north south cross section with a prescribed snow budget distribution shifted uniformly in space to represent the orbital perturbations an ice calving parameterization crudely representing proglacial lakes or marine incursions can attack the ice whenever the tip drops below sea level the model produces a large 100 000 yr response in fair agreement correlation coefficient up to 0 8 with the 18o deep sea core records to increase confidence in the results several of the more uncertain model components are extended or replaced using an alternative treatment of bedrock deformation a more realistic ice shelf model of ice calving and a generalized parameterization for such features as the north atlantic deglacial meltwater layer much the same ice age simulations and agreement with the 18o records as with the original model are still obtained the model is run with different types of forcing to identify which aspect of the orbital forcing controls the phase of the 100 000 yr cycles first the model is shown to give a 100 000 yr response to nearly any kind of higher frequency forcing although over the last 2 million yrs the model phase is mainly controlled by the precessional modulation due to eccentricity over just the last 500 000 yr the observed phase can also be simulated with eccentricity held constant a definite conclusion on the phase control of the real 100 000 yr cycles is prevented by uncertainty in the deep sea core time scales before 600 000 yr b p the model is adapted to represent west antarctica and yields unforced internal oscillations with periods of about 50 000 yr

the mutual interaction between continental scale ice sheets and atmospheric stationary waves the great continental ice sheets of the pleistocene represented significant obstacles to the northern hemisphere midlatitude westerlies they must therefore have forced large changes in the atmospheric circulation and consequently also in the patterns of accumulation and melting over the ice sheets themselves a simplified threedimensional coupled ice sheet stationary wave model is developed in order to understand the ice sheet s response to the circulation changes that it induces consistent with ice age climate simulations the ice sheet topography induces an anticyclonic circulation over the ice sheet causing a slight warming over the western slopes and a stronger cooling over the remainder the modeled feedbacks significantly affect the ice sheet configuration with the most important influences being the patterns of summer temperature and the topographically induced precipitation field the time evolution of the ice sheet is also changed by the atmospheric feedbacks and the results suggest the possibility of multiple equilibrium solutions

climatic impact of glacial cycle polar motion coupled oscillations of ice sheet mass and rotation pole position precessional motion of earth s rotation axis relative to its orbit is a well known source of long period climatic variation it is less well appreciated that growth and decay of polar ice sheets perturb the symmetry of the global mass distribution enough that the geographic location of the rotation axis will change by at least 15 km and possibly as much as 100 km during a single glacial cycle this motion of the pole will change the seasonal and latitudinal pattern of temperatures we present calculations based on a diurnal average energy balance which compare the summer and winter temperature anomalies due to a 1 decrease in obliquity with those due to a 1 motion of the rotation pole toward hudson bay both effects result in peak temperature perturbations of about 1 celsius the obliquity change primarily influences the amplitude of the seasonal cycle while the polar motion primarily changes the annual mean temperatures the polar motion induced temperature anomaly is such that it will act as a powerful negative feedback on ice sheet growth we also explore the evolution of the coupled system composed of ice sheet mass and pole position oscillatory solutions result from the conflicting constraints of rotational and thermal stability a positive mass anomaly on an otherwise featureless earth is in rotational equilibrium only at the poles or the equator the two polar equilibria are rotationally unstable and the equatorial equilibrium though rotationally stable is thermally unstable we find that with a plausible choice for the strength of coupling between the thermal and rotational systems relatively modest external forcing can produce significant response at periods of 10 4 10 6 years but it strongly attenuates polar motion at longer periods we suggest that these coupled oscillations may contribute to the observed dominance of 100 kyr glacial cycles since the mid pleistocene and will tend to stabilize geographic patterns that are suitable to glaciations

an outsider s review of the astronomical theory of the climate is the eccentricity driven insolation the main driver of the ice ages abstract although the astronomical theory of the climate also known as the milankovitch theory explains a number of key features of the paleoclimate time series especially frequency and phase distribution in terms of orbital influence on the waxing and waning of ice sheets it still faces several important unresolved challenges in this review we discuss five main challenges we believe the theory must resolve in order to survive 1 the large amplitude of the 100 ky ice age cycles 2 the switch from 41 ky to 100 ky long glaciations around 0 9 ma bp 3 the absence of significant spectral power at 413 ky over the past 1 2 ma 4 the variation in glacial cycle duration in the last 800 ky and 5 the presence of non orbital spectral peaks in the climate record but in spite of these problems we conclude that the milankovitch theory may still be a viable one at least in the original sense of hays et al science 194 1976 1121 that orbital changes somehow influenced the ice ages many investigators ourselves included appear to see the impact of orbital forcing in the data clear evidence of astronomical forcing as well as evidence which suggests that the climate system responds nonlinearly to all milankovitch orbital frequencies

laurentide ice sheet instability during the last deglaciation the factors leading to the full retreat of ice sheets during deglaciation are debated numerical modelling suggests that the laurentide ice sheet retreated only after a threshold for warming and radiative forcing was passed in the holocene

simulations of northern hemisphere ice sheet retreat sensitivity to physical mechanisms involved during the last deglaciation a 3 dimensional thermomechanical ice sheet model is used to simulate the evolution of the geometry of northern hemisphere ice sheets through the last deglaciation the ice sheet model is forced by a time evolving climatology provided by the linear interpolation through time of climate snapshots simulated by the lmd5 3 atmospheric general circulation model agcm at different periods of the last deglaciation 21 15 9 6 and 0 kyr bp the agcm is driven by insolation atmospheric co2 content ice sheet configuration and sea surface temperatures although our approach is able to produce the complete continental ice retreat our simulated deglaciation presents a phase lag with reconstructions based on observational evidences this suggests that physical mechanisms related to climate forcing and or ice sheet internal dynamics are not properly represented the influence of millennial scale forcing feedback mechanisms between ice sheet elevation and surface mass balance and parameterization of the ice flow is also tested through a set of sensitivity experiments the rapid variability has a strong impact on the evolution of the ice volume because of nonlinear effects in temperature mass balance relationships fennoscandia appears to be strongly sensitive to the small scale ice sheet instability both ice sheets are to some extent sensitive to an increased basal sliding

the role of ocean atmosphere reorganizations in glacial cycles abstract a case is made that glacial to interglacial transitions involve major reorganizations of the ocean atmosphere system such reorganizations constitute jumps between stable modes of operation which cause changes in the greenhouse gas content and albedo of the atmosphere only in this way can the rapidity of glacial terminations the hemispheric synchroneity and symmetry of mountain glaciation and the large polar air temperature and dustiness variations be accounted for if these reorganizations are driven in some fashion by orbitally induced seasonal insolation changes then the connection between insolation and climate is most likely through impacts of fresh water transport on the ocean s salinity distribution

shear wave travel time amplitude and waveform analysis for earthquakes in the kurile slab constraints on deep slab structure and mantle heterogeneity shear wave travel times amplitudes and broadband waveforms from 16 events in the kurile slab are analyzed to place further constraints on the seismic velocity heterogeneity associated with the subducted oceanic lithosphere several previous studies have proposed that the kurile slab penetrates into the lower mantle with little deformation other than a steepening of the dip angle while other studies suggest that strong slab deformations occur with perhaps negligible lower mantle penetration in an effort to detect any deep slab effects we compare anomaly patterns covering a 60 azimuthal range straddling the strike of the slab for events in three different depth intervals 500 km 350 500 km and 100 200 km s and scs pulse broadening is used as a measure of waveform complexity complexity of broadband shear waves does not vary systematically across our azimuthal range as the propagation direction changes with respect to the postulated lower mantle slab model for this region strong residual sphere travel time variations are found for shear waves from events at all depths however these variations do not display the depth dependence apparent in previous p wave analysis that has been interpreted as requiring a change in slab geometry and lower mantle penetration the same s wave residual pattern is apparent in ss depth phases and is well predicted by a recent model of shear velocity structure beneath north america a combination of deep mantle and near receiver aspherical structure can explain most of our observed shear wave travel time patterns with only a minor source depth dependent component remaining after correction individual amplitude and travel time measurements are poorly correlated however scs s amplitude ratios and scs s differential times are significantly correlated consistent with geometric focussing and defocussing effects somewhere along the raypaths the travel time behavior suggests that the responsible velocity heterogeneity is not located near the source the absence of a strong deep slab effect on the shear waves indicates that deep slab heterogeneity is less pronounced for shear waves than expected on the basis of existing p wave models derived from travel time residual sphere analysis various interpretations include estimates of vs t and compositional effects previously used to predict shear velocity structure of the deep kurile slab are in error with actual lower mantle slab anomalies being much weaker than proposed the kurile slab is so distorted in the lower mantle that no source depth dependent trends are produced for our limited focal sphere coverage or the kurile slab does not penetrate into the lower mantle

conference on the last deglaciation timing and mechanism abstract for years paleoclimatologists have held the general view that the last deglaciation began around 17 000 to 15 000 yr ago that the shape of the globally integrated deglacial curve was smoothly sigmoidal with the fastest rate of change centered around 11 000 yr ago and that the deglaciation ended around 7000 to 5000 yr ago recent studies have challenged several aspects of this consensus and have suggested that the mechanisms responsible for the deglaciation are significantly different from those previously proposed as a result an international workshop was held at airlie house in virginia during may 2 6 of 1983 to evaluate a wide range of evidence relevant to this controversy the conference results suggested that 1 the decrease in global ice volume occurred in two steps with the dating of the earlier step still in doubt but the later step occurring at about 10 000 7000 yr ago and 2 the most likely feedback mechanisms for accelerating the initial forcing by orbital variations are delayed bedrock rebound marine downdraw calving and co 2 heating

a possible station bias in travel time measurements reported to isc a station bias in the travel times of p waves may exist in the international seismological centre isc data base it appears that low gain seismographs report systematically late arrival times average station delays confirm this with low gain stations generally slower than high gain stations

processes of buildup and retreat of the greenland ice sheet sensitivity experiments were conducted to study the influence of several physical processes on the evolution of the greenland ice sheet over the last 250 000 years the experiments were carried out by means of a three dimensional thermodynamical model the mass balance is modeled by a surface energy balance model this means that variations in orbital parameters following from the astronomical milankovitch theory can be incorporated in the ablation calculations it is shown that with regard to the greenland ice sheet variations in shortwave radiation are only of minor importance for the changes in volume over time calculations of the various components of the energy balance under glacial and interglacial conditions show that the changes in absorbed shortwave radiation resulting from changes in orbital parameters are considerably smaller than the variations in the sensible heat flux caused by temperature variations thermodynamics are only of importance for the volume changes of the ice sheet if the climate is on average stable over a long period in between 110 kyr b p and 20 kyr b p the ice volume increased slowly due to the cooling of the ice which resulted in a higher effective viscosity and thus a thicker ice sheet the long response time of the thermodynamics means that the thermodynamics cannot play a crucial role during transition from a glacial to a deglacial period which occur typically within several thousands of years the mass balance height feedback has an important function in the buildup of the ice sheet after a warm period like the eemian without isostatic bedrock compensation the ablation will remain very high in some marginal parts yielding a considerably smaller ice sheet volume for the last glacial period

continuous high velocity aseismic zone beneath the izu bonin arc since 1904 eleven unusually deep earthquakes have been reported near the southern end of the izu bonin arc about 24 n 142 e these isolated events are separated from ordinary deep earthquakes in the wadati benioff w b zone of the izu bonin i b arc by a distance of about 300 km parallel to the trench axis they also are located 200 km closer to the trench than the natural extension of the shallower earthquakes above them using 3 d ray tracing a detailed analysis is performed of the difference in travel time residuals between the anomalous events and the deep events within the w b zone to the north this analysis yields higher reliability and resolution than can be obtained from ordinary tomographic imaging we find that these unusual events occurred in a high velocity zone which is connected to the northern w b zone of the i b arc the aseismic high velocity zone has a p velocity at least 3 faster than the surrounding mantle both our result and previous evidence for a horizontally lying slab west of the w b zone at 27 n e g okino et al 1989 are consistent with the existence of a prominent contortion of the subducted slab in the i b region

a time dependent ice sheet model preliminary results a numerical model of ice sheet flow is developed and preliminary results are described this model includes vertical resolution of temperature stress and strain rate which represents a considerable improvement over previous vertically averaged ice sheet models the model follows the flow of ice along a flow line within an ice sheet drainage basin longitudinal stresses and basal sliding are included basal sliding is dependent on the base shear stress and a specified distribution of basal water pressure the numerical methods used to solve the coupled set of stress and velocity equations for the static and time evolutionary cases are discussed a steady state profile simulating an ice stream is calculated for a particular set of input parameters and changes in the profile are examined for different choices of parameters preliminary studies of response behavior are completed using a simplified ice sheet geometry with a fixed terminus or grounding line the results of these studies illustrate ice sheet thinning in response to a lowered sea level or to a reduction in the extent of ice rises or pinning points within ice shelves

the ice age cycle and the deglaciations an application of nonlinear regression modelling abstract we have applied the nonlinear regression technique known as additivity and variance stabilisation avas to time series which reflect earth s climate over the last 600 ka avas estimates a smooth nonlinear transform for each variable under the assumption of an additive model the earth s orbital parameters and insolation variations have been used as regression variables analysis of the contribution of each variable shows that the deglaciations are characterised by periods of increasing obliquity and perihelion approaching the vernal equinox but not by any systematic change in eccentricity the magnitude of insolation changes also plays no role by approximating the transforms we can obtain a future prediction with a glacial maximum at 60 ka ap and a subsequent obliquity and precession forced deglaciation

the plio pleistocene 405 kyr climate cycles abstract the 405 kyr cycles in proxy records for the east asian summer monsoon and global ice volume intensified and became phase locked to eccentricity forcing at 4 5 ma we revised previous forcing mechanism hypothesis for pliocene intensification of the 405 kyr cycles in paleoclimatic records and proposed that the establishment of salinity difference between the pacific ocean and the caribbean sea at 4 5 ma triggered an ocean air positive feedback mechanism this feedback mechanism caused simultaneous climate cooling and east asian summer monsoon intensification and made the two different climate components have a clipped response to insolation forcing and thus intensified sensitivity to eccentricity variations weakening of the 405 kyr cycles in monsoon and ice volume records between 2 and 1 ma mid pleistocene transition was likely to be attributed to frequency modulation that transfer power of the 405 kyr band to the 100 kyr band which was triggered by changes in the oceanic carbon storage of the southern ocean

modeling of pleistocene european ice sheets some experiments with simple mass balance parameterizations abstract a vertically integrated ice flow model suitable for use in climate studies is formulated large continental ice sheets may be characterized by two fundamental quantities the height to width ratio and the steepness of the edge so it is natural to develop a model containing two parameters that can be chosen to give the right values of those characteristic quantities the result is a model that is close to m a w mahaffy s journal of geophysical research 81 1059 1066 1976 the model is used to study glaciation in europe dropping the level of zero mass balance creates small stable ice caps in the alps and the scandinavian mountains if the drop exceeds 600 m with respect to present day conditions the feedback between ice sheet height and mass balance becomes dominating and the fennoscandian ice sheet keeps growing it does not reach an equilibrium state within 60 000 yr an experiment simulating rapid onset of a glacial cycle shows that the growth of ice volume in europe is smaller than that in northern america j t andrews and m a w mahaffy quaternary research 6 167 183 1976 after 10 000 yr the volume of the fennoscandian ice sheet 2 10 15 m 3 is about half the volume of the laurentide ice sheet this leaves the observed sea level lowering in the period 125 000 115 000 yr b p estimates center around 50 m unexplained

the geologic record of climatic change this paper reviews the principal results from paleoclimate studies and includes background material slanted toward climate modelers the inferred temperature history of the last 4 6 billion years indicates major changes in the components of the earth s climate system a secular change in global insolation receipt is due to a 20 30 increase in solar luminosity since the formation of the earth a co2 h2o greenhouse effect may have offset the lower luminosity during early earth history inferred fluctuations of global temperature have occurred over a broad range of time scales on time scales of 106 108 years paleogeographic factors e g continental drift and sea level changes have contributed significantly to temperature changes associated with transitions between nonglacial and glacial states preliminary modeling efforts indicate that additional factors e g co2 changes in atmospheric circulation must also be considered in order to explain the origin of nonglacial climates the origin of polar ice caps may result from ocean circulation changes that were caused by plate tectonic processes fluctuations of ice volume on a time scale of 10 105 years correlate with insolation variations caused by orbital perturbations feedback interactions within the land sea air ice system e g ocean circulation changes and bedrock dynamics have been responsible for a significant modulation of the orbital signal ice ages may be due to orbitally induced temperature changes superimposed on a global cooling of terrestrial origin

integrated summer insolation forcing and 40 000 year glacial cycles the perspective from an ice sheet energy balance model although the origins of the 40 000 year glacial cycles during the early pleistocene are readily attributed to changes in earth s obliquity also having a 40 000 year period the lack of ice volume variability at precession periods 20 000 years is difficult to reconcile with most parameterizations of the insolation forcing it was recently proposed that precession s influence on glaciation is muted because variations in the intensity of summer insolation are counterbalanced by changes in the duration of the summertime but no climate model has yet been shown to generate obliquity period glacial cycles in response to the seasonal insolation forcing here we present a coupled ice sheet energy balance model that reproduces the seasonal cycle and when run over long time periods generates glacial variability in response to changes in earth s orbital configuration the model is forced by the full seasonal cycle in insolation and its response can be understood within the context of the integrated summer insolation forcing the simple fact that obliquity s period is roughly twice as long as that of precession results in a larger amplitude glacial response to obliquity however for the model to generate almost exclusively obliquity period glacial variability two other conditions must be met first the ice sheet s ablation zone must reside poleward of 60 n because insolation intensity is more sensitive to changes in earth s obliquity at high latitudes second the ablation season must be long enough for precession s opposing influences on summer and fall insolation intensity to counterbalance one another these conditions are consistent with a warm climate and a thin ice sheet where the latter is simulated as a response to subglacial sediment deformation if a colder climate is prescribed or in the absence of basal motion ice sheets tend to be larger and undergo greater precession period variability in keeping with proxy observations of late pleistocene glaciation

orbital changes and climate at the 41 000 period of orbital tilt summer insolation forces a lagged response in northern ice sheets this delayed ice signal is rapidly transferred to nearby northern oceans and landmasses by atmospheric dynamics these ice driven responses lead to late phased changes in atmospheric co2 that provide positive feedback to the ice sheets and also project late 41 k forcing across the tropics and the southern hemisphere responses in austral regions are also influenced by a fast response to summer insolation forcing at high southern latitudes at the 22 000 year precession period northern summer insolation again forces a lagged ice sheet response but with muted transfers to proximal regions and no subsequent effect on atmospheric co2 most 22 000 year greenhouse gas responses have the early phase of july insolation july forcing of monsoonal and boreal wetlands explains the early ch4 response the slightly later 22 k co2 response originates in the southern hemisphere the early 22 k ch4 and co2 responses add to insolation forcing of the ice sheets the dominant 100 000 year response of ice sheets is not externally forced nor does it result from internal resonance internal forcing appears to play at most a minor role the origin of this signal lies mainly in internal feedbacks co2 and ice albedo that drive the gradual build up of large ice sheets and then their rapid destruction ice melting during terminations is initiated by uniquely coincident forcing from insolation and greenhouse gases at the periods of tilt and precession

frequency variations of the earth s obliquity and the 100 kyr ice age cycles variations in the earth s orbital parameters modulate the seasonal distribution of solar radiation and thereby induce changes in the earth s climate1 periodicities in the geological climate record with cycles of 100 41 and 23 kyr have been linked with changes in eccentricity obliquity and precession of the equinoxes respectively2 3 but although the eccentricity does vary with a 100 kyr period the effect on the incoming solar radiation is rather weak relative to the signals from obliquity and precession variations the 100 kyr signal in the climate record should therefore be of negligible intensity yet it is observed instead to dominate the record internal nonlinear processes within the climate system have been proposed to account for this fact4 14 in contrast i show here that variations in the frequency of the obliquity cycle can give rise to strong 100 kyr forcing of climate

the origin of the 100 kiloyear ice sheet cycle in the pleistocene the response of a simple ice sheet model forced by a periodic change in the climate point exhibits a bifurcation when certain dimensionless parameters undergo realistic changes it is postulated that this bifurcation may be responsible for the initiation of large glacial cycles about 700 000 years ago following smaller higher frequency ice sheet advances and retreats before that time

the horizontally lying slab an unusual earthquake mb 6 2 occurred in the izu bonin island region in july 1982 the event occurring at a depth of 545 km was clearly separated from the main inclined seismic zone by a distance of about 200 km to diagnose the situation we have considered two possibilities 1 the event might be an unusual event in the sense that its location is unrelated to the position of the subducted slab and that there is probably no slab around it or 2 the slab at this depth might be lying horizontally on top of the 650 km discontinuity and the unusual event occurred in this horizontal part both possibilities have been modeled structurally we have computed the p wave travel times for the present event as well as for two other events located unambiguously in the inclined part of the seismic zone by employing a 3 d ray tracing program the computed travel time residuals have been compared with the observations at stations in western japan the observations are found to be consistent with the model of the horizontally lying slab our result thus suggests that the slab might be horizontal at this depth in this part of the izu bonin islands

modeling 100 000 year climate fluctuations in pre pleistocene time series a number of pre pleistocene climate records exhibit significant fluctuations at the 100 000 year 100 ky eccentricity period before the time of such fluctuations in global ice volume the origin of these fluctuations has been obscure results reported here from a modeling study suggest that such a response can occur over low latitude land areas involved in monsoon fluctuations the twice yearly passage of the sun across the equator and the seasonal timing of perihelion interact to increase both 100 ky and 400 ky power in the modeled temperature field the magnitude of the temperature response is sufficiently large to leave an imprint on the geologic record and simulated fluctuations resemble those found in records of triassic lake levels

exploring uncertainties in the relationship between temperature ice volume and sea level over the past 50 million years over the past decade efforts to estimate temperature and sea level for the past 50 ma have increased in parallel efforts to model ice sheet changes during this period have been ongoing we review published paleodata and modeling work to provide insights into how sea level responds to changing temperature through changes in ice volume and thermal expansion to date the temperature to sea level relationship has been explored for the transition from glacial to interglacial states attempts to synthesize the temperature to sea level relationship in deeper time when temperatures were significantly warmer than present have been tentative we first review the existing temperature and sea level data and model simulations with a discussion of uncertainty in each of these approaches we then synthesize the sea level and temperature data and modeling results we have reviewed to test plausible forms for the sea level versus temperature relationship on this very long timescale there are no globally representative temperature proxies and so we investigate this relationship using deep sea temperature records and surface temperature records from high and low latitudes it is difficult to distinguish between the different plausible forms of the temperature to sea level relationship given the wide errors associated with the proxy estimates we argue that for surface high latitude southern hemisphere temperature and deep sea temperature the rate of change of sea level to temperature has not remained constant i e linear over the past 50 ma although the relationship remains ambiguous for the available low latitude surface temperature data a nonlinear form between temperature and sea level is consistent with ice sheet modeling studies this relationship can be attributed to 1 the different glacial thresholds for southern hemisphere glaciation compared to northern hemisphere glaciation and 2 the ice sheet carrying capacity of the antarctic continent

consequences of pacing the pleistocene 100 kyr ice ages by nonlinear phase locking to milankovitch forcing 1 the consequences of the hypothesis that milankovitch forcing affects the phase e g termination times of the 100 kyr glacial cycles via a mechanism known as nonlinear phase locking are examined phase locking provides a mechanism by which milankovitch forcing can act as the pacemaker of the glacial cycles nonlinear phase locking can determine the timing of the major deglaciations nearly independently of the specific mechanism or model that is responsible for these cycles as long as this mechanism is suitably nonlinear a consequence of this is that the fit of a certain model output to the observed ice volume record cannot be used as an indication that the glacial mechanism in this model is necessarily correct phase locking to obliquity and possibly precession variations is distinct from mechanisms relying on a linear or nonlinear amplification of the eccentricity forcing nonlinear phase locking may determine the phase of the glacial cycles even in the presence of noise in the climate system and can be effective at setting glacial termination times even when the precession and obliquity bands account only for a small portion of the total power of an ice volume record nonlinear phase locking can also result in the observed quantization of the glacial period into multiples of the obliquity or precession periods

p and s wave travel time inversions for subducting slab under the island arcs of the northwest pacific we have observed slablike high p and s velocity anomalies around the wadati benioff zone under island arcs of the northwest pacific through travel time tomographic inversions nineteen years of international seismological centre travel time residuals for events and stations in this large region are used analyses of resolution and noise show that the images are generally well resolved the images illustrate that slab anomalies are continuous along strike in most parts of the upper mantle of the region and become contorted and generally broadened with depth near the bottom of the upper mantle fingering of the slabs including segmenting and spreading is indicated the fast anomalies associated with the japan izu bonin and mariana subduction zones tend to flatten to subhorizontal at depth while downward spreading may occur under parts of the mariana and kurile arcs the fast anomalies below 700 km are not in the shape of a single coherent sheet the principal compressional axes of focal mechanisms in the region consistently follow the downdip direction of the high velocity slab even when it bends to subhorizontal at depth the depth at which compression begins to dominate the downdip stress regime in the slab apparently depends on bending of the slab and its dip slab fingering and intense deep seismicity probably are the consequence of the slab encountering a barrier of some form around the 670 km discontinuity

a coupled marine ice stream ice shelf model theorie des variations climatiques du pleistocene a aujourd hui liees a la dynamique des couvertures marines de glace couplee aux glaces libres flottantes on neglige les effets thermodynamiques et on etudie les processus de flottaison en relation avec les rapports de densite entre la place et l eau de mer

evaluating the role of climate cooling in iceberg production and the heinrich events bond et al 1 recently presented evidence for the frequent occurrence in the pleistocene epoch of periods of massive iceberg discharge into the north atlantic ocean heinrich events each lasting a few thousand years the cause of these events is uncertain but one possibility1 is repeated advances of the laurentide ice sheet during episodes of cooler climate here i examine this idea using a model of the laurentide ice sheet driven by orbitally forced variations in insolation with and without three additional 3 000 yr long cooling episodes imposed just before the occurrence of the three most recent heinrich events hi h2 and h3 in the model the cooling event preceding hi when the climate was relatively warm and deglaciation was about to begin led to increased calving but those preceding h2 and h3 which occurred during the last ice age when the climate was very cold led to reduced iceberg calving it thus seems unlikely that the heinrich events generally reflect a direct response of the laurentide ice sheet to climate cooling

simulating the sea level imprint on marine oxygen isotope records during the middle miocene using an ice sheet climate model 1 oxygen isotopic ratios are implemented in an ice sheet climate model in order to directly compare the modeled isotopic ratio of the seawater to the high resolution isotopic records from deep sea sediment cores in the middle miocene the isotopic depletion resulting from the modeled ice sheet expansion explains the mean oxygen isotope step found in deep sea sedimentary records of 0 5 approximately 85 of the modeled increase in the isotopic composition of seawater is caused by an increase in ice volume the remainder is due to a stronger depletion in oxygen isotopes in the large ice sheet furthermore we also investigated the relation between sea level or global ice volume and the isotopic composition of seawater our experiments confirm the validity of the relation of approximately 1 enrichment per 100 m sea level lowering we further show that this relationship is r estricted by the mean ocean depth and the assumed oxygen isotopic composition of the ice sheet small deviations 10 from this general relationship occur depending on the size and the mean isotopic content of the ice sheet large continental ice sheets are more depleted in heavy oxygen isotopes and therefore reach a slightly higher ratio in contrast small ice sheets have a less depleted isotopic composition and correspondingly have a smaller effect on the isotopic composition of the ocean

travel times and station corrections for p waves at teleseismic distances approximately 3300 shallow focus earthquakes and 1000 seismic stations have been used in a study of p wave travel times and station residuals including azimuthal effects the events were selected from a catalog containing 160 000 earthquakes and those having uniform distance and azimuthal coverage were systematically relocated and used to refine p wave travel times and station corrections station corrections are provided for 994 seismic stations the station corrections involve three terms the static effect and two cosine terms with appropriate phase shifts they exhibit general consistency over broad geographic areas and where coverage is dense often show abrupt changes from one geological province to another the cos 2 terms appear to be due to upper mantle anisotropy and they correlate with the stress direction in the crust

late pliocene early pleistocene 100 ka problem 1 little is known about the 100 ka ice volume cycles immediately before 1 ma except that they are coherent with the 100 ka eccentricity cycles here we show that ice volume and ocean circulation change global carbon mass balance are phase locked and highly coupled regarding amplitude envelopes at the 100 ka band but neither is phase locked to and amplitude coupled to the 100 ka eccentricity orbital forcing signal between 3 1 ma we describe this phasing and amplitude mismatch at the 100 ka band between 3 1 ma between eccentricity forcing and paleo ice volume records as the late pliocene early pleistocene 100 ka problem a free 100 ka oscillation internal to our climate system especially those related to oscillations of atmospheric co2 concentration operating on an earth characterized by a high inertia deep thermohaline ocean that can store carbon and heat might hold the key for the late pliocene early pleistocene 100 ka problem

insolation changes caused by combination of amplitude and frequency modulation of the obliquity a forcing function is derived from the insolation signal with a bipolar pulse modulation train that is a function of earth s obliquity from this obliquity forced insolation there is a calculated variation of about 9 in the energy flux at the top of the atmosphere pulsations in the incoming solar radiation induced by the amplitude frequency coupling effect of the obliquity correlate well with continental biogenic silica data and the marine oxygen isotope record the theory lends a new perspective to the physical mechanism involving climate change studies particularly it provides a physical explanation of the 100 kyr cycle in glaciation which is distinct from the commonly considered variations in orbital eccentricity

seismic structure of the deep kurile subduction zone three dimensional modeling of p wave travel time residuals was performed to investigate the deep slab related velocity structure within the kurile subduction zone travel times of p waves from earthquakes at all depths within the kurile subduction zone to world wide standard seismograph network and canadian stations were measured instead of analyzing the travel time residuals of a single event relative travel time residuals of p waves to common stations have been analyzed and used in this study to isolate near source contributions from the total travel time residuals the effect of uncertainties in long wavelength heterogeneity far from the source in the lower mantle and the effect of uncertainties in the reference earth model have been significantly reduced by analyzing relative travel time residuals the effect of uncertainties in station corrections are eliminated by relative travel time residuals our measured p wave travel time data clearly indicate that high velocity anomalies exist in the lower mantle beneath the kurile subduction zone but they do not appear to be caused by simple slab continuation into the lower mantle the travel time modeling was accomplished using a fully three dimensional finite difference technique for wave propagation within the source region the modeling results show that high velocity anomalies extend several hundred kilometers into the lower mantle but they appear to be very complicated a large number of models consisting of simple slab continuation into the lower mantle were tested and the results show that none ofthese models was able to produce synthetics which match the observed p wave travel time data the slab related velocity anomalies in the upper mantle of the kurile subduction zone are generally consistent with the seismicity the high velocity anomalies in the lower mantle below the kurile subduction zone are broad and geometrically change from the southern to the northern kuriles below the southern kuriles the high velocity anomalies appear to be subhorizontal however beneath the central kuriles there are broad about 600 km wide high velocity anomalies on the continental side of the slab which extend several hundred kilometers into the lower mantle the high velocity anomalies found below the northern kuriles are broad and on both the continental and the ocean side of the slab and extend several hundred kilometers into the lower mantle

a coupled climate ice sheet model applied to the quaternary ice ages a coupled climate ice sheet model is used in an attempt to simulate the northern hemispheric ice sheet volume variations implied by deep sea core records over the last several 105 years the climate model consists of a global one level diffusive energy balance equation with seasonal variation and land ocean contrast using a severely truncated series of associated legendre functions the ice sheet model uses a standard vertically averaged flow law and predicts ice thickness in a north south cross section bedrock deformation under the ice is included as linear flow confined to a thin asthenospheric channel the climate model is tuned to the modern climate and tested against results from atmospheric general circulation models for 18 000 and 10 000 yr b p it is suggested that the lack of sea ice variations causes the biggest departures from the gcm results the sensitivities to the orbital perturbations of the coupled model and of the ice sheet model alone with a prescribed snow budget distribution are examined some qualitative differences are found in the latitudinal forms of these sensitivities despite these differences however the two types of models produce similar ice age simulations over the last 700 thousand years in response to the orbital perturbations in particular fairly realistic simulations with strong 100 kyr cycles similar to those obtained by adding an ice calving mechanism to the ice sheet model in pollard 1982a are again produced in the coupled model

milankovitch climatic origin of mid cretaceous black shale rhythms in central italy the earth s orbital variations reflected in pleistocene ice volume were also recorded in non glacial times carbonate production in pelagic mid cretaceous sediments quantified by calcium carbonate and optical densitometry time series reflects the orbital eccentricity and precessional cycles minimal eccentricity brought deep sea anoxia most of the sedimentary variability of this 100 myr old sequence lies within the milankovitch orbital frequency band

the impact of topographically forced stationary waves on local ice sheet climate a linear two level atmospheric model is employed to study the influence of ice sheet topography on atmospheric stationary waves in particular the stationary wave induced temperature anomaly is co

late pleistocene climatic trajectory in the phase space of global ice ocean state and co2 observations and theory we consider the evolution of three main slow response variables describing the state of the climate system during the late pleistocene global ice mass atmospheric carbon dioxide and the ocean state as measured respectively by the specmap 18o record the vostok co2 record and the north atlantic sea surface temperature record at 50 n core k708 1 which is coherent with other ocean state properties their coevolution is portrayed in the form of a trajectory in the phase space of the three variables and its projections on its three phase planes the oscillatory nature and phase lags of the variables are clearly illustrated suggesting a physical process scenario that can account for the observations the basic element of this scenario is a free near 100 kyr period oscillation driven by internal instability involving feedbacks between all three variables under the influence of earth orbital milankovitch forcing and long term tectonic co2 forcing it is shown that a phenomenological theory advanced by the authors emphasizing the role of co2 provides a credible account of the phase space trajectory

current state and future perspectives on coupled ice sheet sea level modelling the interaction between ice sheet growth and retreat and sea level change has been an established field of research for many years however recent advances in numerical modelling have shed new light on the precise interaction of marine ice sheets with the change in near field sea level and the related stability of the grounding line position studies using fully coupled ice sheet sea level models have shown that accounting for gravitationally self consistent sea level change will act to slow down the retreat and advance of marine ice sheet grounding lines moreover by simultaneously solving the sea level equation and modelling ice sheet flow coupled models provide a global field of relative sea level change that is consistent with dynamic changes in ice sheet extent in this paper we present an overview of recent advances possible caveats methodologies and challenges involved in coupled ice sheet sea level modelling we conclude by presenting a first order comparison between a suite of relative sea level data and output from a coupled ice sheet sea level model

how deep can we see the high velocity anomalies beneath the japan islands high velocity anomalies are found in the lower mantle beneath the japan islands and their vicinity through an analysis of detailed three dimensional p wave velocity structure based on a tomographic inversion of isc data beneath the sea of japan the high velocity anomalies in the lower mantle are slab like and appear to extend below the deep focus earthquakes of the high velocity pacific slab descending from the japan trench this slab like high velocity body extending into the lower mantle steepens below a depth of 500 600 km resolution analyses confirms the existence of a slab like high velocity region in the lower mantle though the depth extent is not well resolved on the other hand the pacific slab descending from the izu bonin trench seems to be fingering that is high velocity anomalies are penetrating into the lower mantle beneath latitude 26 n and not penetrating but horizontally bending in the deeper portion of the upper mantle around 29 n

travel times from earthquakes near southern kuril and their implications for the fate of subducted lithosphere abstract analysis of travel time residuals from deep and intermediate focus earthquakes along the kuril kamchatka arc has been a major impetus in advancing the hypothesis of deep slab penetration the interpretation of travel times however has been controversial the quality of arrivals reported in bulletins has also been debated using both digital and analog seismograms we precisely measured approximately 400 arrival times of teleseismic p phases for seven earthquakes most of which occurred since 1987 near southern kuril travel times from many of these events have not been analyzed by other researchers including a unique pair of shallow and deep focus earthquakes beneath the island of sakhalin in general differences between our precisely measured p arrivals and those reported by the international seismological center isc are within 0 4 s although isolated large differences do exist at common stations large differences in travel time residuals up to 1 5 s are observed between one of the deepest earthquakes in northern kuril and a deep focus event beneath the island of sakhalin located some 150 km farther west of the well defined wadati benioff zone in southern kuril significantly faster travel times up to 1 2 s or more are also observed along north northeast and south southwest azimuths for the deep focus event in northern kuril than for a shallow focus earthquake west of the sakhalin island where the effect of subducted lithosphere should be minimal in both cases the observed differential residuals between earthquakes in northern and southern portions of the arc are too large to be caused by epicenter mislocation since differential travel time residuals largely reflect differences in near source structure seismic wave speeds below depths of approximately 650 km seem to vary along this arc the magnitude of slab like anomalies in the lower mantle beneath sakhalin is probably two to three times smaller than that in northern kuril kamchatka if the pattern of travel time anomalies observed for deep focus earthquakes in northern kuril kamchatka is caused by coherent subducted slab in the lower mantle wholesale slab penetration into the lower mantle probably did not occur beneath the sakhalin there also appears to be a correlation between the state of strain within subducted lithosphere in the upper mantle and the extent of slab penetration into the lower mantle along southern kuril a net down dip extensional strain is observed within the slab at intermediate depths and persists to an unusual depth of approximately 450 km on the other hand in the northern kuril kamchatka down dip compressional strain dominates at all depths in the subducted lithosphere

the role of ocean atmosphere reorganizations in glacial cycles abstract a case is made that glacial to interglacial transitions involve major reorganizations of the ocean atmosphere system such reorganizations constitute jumps between stable modes of operation which cause changes in the greenhouse gas content and albedo of the atmosphere only in this way can the rapidity of glacial terminations the hemispheric synchroneity and symmetry of mountain glaciation and the large polar air temperature and dustiness variations be accounted for if these reorganizations are driven in some fashion by orbitally induced seasonal insolation changes then the connection between insolation and climate is most likely through impacts of fresh water transport on the ocean s salinity distribution

a paleoclimate model of northern hemisphere ice sheets abstract a model for predicting the growth and decay of ice sheets based on the astronomical theory of climate change is presented the purpose of the study in part is to isolate the role of the ice sheet physics and earth response under varying ice load by simplifying to the extreme the role of the hydrosphere atmosphere ice sheet physics and the response of the lithosphere asthenosphere under the ice load are modeled explicitly insolation anomalies taken at a fixed latitude directly force latitudinal displacement of the snow line accumulation rate a and ablation rate a evaluated at mean sea level are specificed as external constants a a decrease linearly with ice sheet elevation rough tuning of the model to the general shape of the ice volume record of the last two major glacials determines the external constants model predictions of the ages of several events in the last major glaciation compare well with the radiological ages the six glacial terminatios i vi over the last 600 000 yr are identified and the predicted ages compare reasonably well with the 18 o record for two deep sea cores a direct comparison of model power spectra of ice volume as a function of period with spectra of the 18 o record shows apparent underprediction of power near 100 000 yr when a quantitative but heuristic method for taking into account the red noise spectrum evident in the geological records is used a much more favorable comparison is possible the model prediction lends support to the hypothesis that the nonlinearity of the ice sheet physics is responsible for the 100 000 yr periodicity in the geological record of the late pleistocene

heinrich type glacial surges in a low order dynamical climate model recent studies suggest the occurrence of sporadic episodes during which the ice streams that discharge ice sheets become enormously active producing large numbers of icebergs reflected in north atlantic sea cores as heinrich events and possibly causing the partial collapse of the ice sheets to simulate the mechanism of internal thermo hydrodynamical instability implied by such behavior in the context of a more general paleoclimate dynamics model pdm we introduce a new sliding catastrophe function that can account for ice sheet surges in particular using simple scaling estimates derived from the equations of motion and thermo conductivity for ice flow we express this function in terms of the thickness density viscosity heat capacity and heat conductivity of ice analysis of the properties of this function suggests that these heinrich type instability events might be of three possible kinds the first type of event occurs in periods of glacial maximum when temperature conditions on the ice surface are extremely cold but internal friction within bottom boundary layer is also at its maximum and is strong enough to melt ice and cause its surge the second type of event may happen during an interglacial when the ice thickness is small but relatively warm climatic conditions on the upper surface of ice can be easily advected with the flow of ice to the bottom where even a small additional heating due to friction may cause melting the third and perhaps most interesting type of event is one that may occur during ice sheet growth in this period particles of ice reaching the bottom still remember the warm temperature conditions of the previous interglacial and additional heating due to increasing friction associated with the growing ice sheet may again cause melting to the extent that the upper glacier surface temperature depends on atmospheric carbon dioxide concentration this third case introduces the interesting possibility that earlier co2 concentrations may be as important for the present day climate as its current value we present results of numerical experiments demonstrating how these three kinds of instability can originate and interact with other components of the global climate system to produce variations of the heinrich event type in particular according to our model the climate system seems more vulnerable to surges during the penultimate interglacial period than in the present one which may contribute to an explanation of the recent results of the greenland ice core project

a walsh spectral comparison of oxygen δ18o and carbon isotope δ13c variations of the pleistocene bore hole eureka 67 135 from the gulf of mexico and their orbital significance abstract a new spectral analysis known as walsh spectral analysis of the oxygen 18o and carbon 13c isotopic variations during the pleistocene the past 1 7 m y reveals periodicities in milankovitch frequency bands besides three main spectral cycles eccentricity obliquity and precession the walsh spectra also reveals additional periodicity in a range of 57 000 60 000 years in case of oxygen isotopic variations only that remarkably corresponds to combined effect of obliquity and precession as suggested by berger walsh spectra of both records also exhibits significant power in a range of 29 000 34 000 years throughout the pleistocene this periodicity is also similar to the one predicted in orbital variations 29 000 31 000 years and lies in obliquity bands precessional signal during the early part of the pleistocene 0 85 1 7 m y seems to be weak however eccentricity and obliquity signals dominate throughout the pleistocene walsh spectra of both phenomena exhibits a general similarity in their spectral structures suggesting a common link in view of appearance of some unusual periodicities in walsh spectra and their striking resemblance with orbital cycles the present result further lends support to the astronomical theory of climatic change

a climate model with cryodynamics and geodynamics we present a highly simplified zero dimensional model of the climatic system the model attempts to incorporate mechanisms important on the time scale of glaciation cycles namely 104 105 years in particular the radiation balance of the ocean atmosphere the plastic flow of continental ice sheets and the viscous flow of the upper mantle are taken into account the stress is on the interaction between the cryosphere ice sheets and the asthenosphere upper mantle which had not been taken into account in previous energy balance or ice sheet models of climate weertman 1976 kallen et al 1979 the model exhibits free self sustained oscillations of an amplitude and period comparable to those found in the paleoclimatic record of glaciations viz o 10 deg k and o 104 years respectively such oscillations had already been found in the simpler model of kallen et al 1978 1979 the fact that nonlinear self sustained oscillations also obtain for the present slightly more complex model is interesting it offers mild support to the idea that unforced oscillations can actually exist in the real climatic system itself these oscillations are characterized by a quarter phase lag between ice extent and global temperature given that the climatic system is a nonlinear oscillator with a period of o 104 years it is expected that astronomical forcing at 19 000 23 000 and 41 000 years will lead to subharmonic oscillations with a period close to 100 000 years the dominant period of glaciation cycles such effects have already been noticed in the work of birchfield and weertman 1978 with an even simpler model including only glacial dynamics the careful study of the interplay between internal mechanisms and external forcing with comparable time scales represents an interesting challenge to the theory of ice ages the results of such theoretical studies can also provide guidance for observational work in particular periodicities of o 104 years in the climate record and phase lags between ice volume and temperature appear to be supported by the most recent paleogeochemical investigations ruddiman and mclntyre 1981

a simple ice sheet model yields realistic 100 kyr glacial cycles records of global ice volume for the past 700 kyr based on oxygen isotopic data from deep sea cores and reflecting mainly the changing northern hemispheric ice sheets show a dominant cycle of roughly 100 kyr period the records also show smaller amplitude oscillations with spectral peaks at roughly 40 and 20 kyr periods which are well correlated with the milankovich insolation variations due to perturbations in the earth s orbital parameters however no model has accurately simulated the 100 kyr glacial cycle recently birchfield et al 1 and oerlemans2 have obtained encouraging agreement with some features of the glacial cycle by using a simple ice sheet model with a realistic time lag in the response of the bedrock to the ice load this study extends their basic model first by including topography to represent high ground in the north improved results can then be obtained but only with unrealistic parameter values and for some aspects of the record further improvements areobtained by crudely parameterizing possible calving at the equatorward ice sheet tip during deglaciation by proglacial lakes and or marine incursions from the atlantic as emphasized by andrews3 the resulting ice volume curves agree fairly well with the observed records and their power spectra over the past 700 kyr

the mode and mechanism of the last deglaciation oceanic evidence changes in ocean temperature carbonate productivity and ice rafted detritus in the north atlantic suggest that half of the northern hemisphere ice volume at the last glacial maximum had disappeared by 13 000 yr b p despite the still extensive limits of the ice sheets this early thinning of the ice sheets occurred during a time when summer insolation values were slowly rising but when pollen evidence south of the ice margins indicates cold dry air masses we infer that this rapid early ice disintegration 16 000 13 000 yr b p was caused by oceanic mechanisms 1 rising sea level causing increased calving along ice margins 2 the chilling of the sea surface by icebergs and meltwater reducing moisture extraction by the atmosphere and transport to the ice sheets and 3 winter freezing of the low salinity meltwater layer suppressing local moisture extraction and the regional influx of moisture bearing storms from lower latitudes in winter and hence starving the ice sheets these oceanic feedback mechanisms were strongest from 16 000 to 13 000 yr b p and weaker but still active from that date until the end of deglaciation at 6000 yr b p

a new view on the driving mechanism of milankovitch glaciation cycles abstract dynamical instabilities in the glaciation and deglaciation of the ice sheets can be triggered by pulsations of the rate of change of the orbital and rotational parameters the purpose of this paper is to demonstrate that the milankovitch glaciation cycles are extremely sensitive to the rate of change of the obliquity frequency the first of our three contributions is a pedagogic principle of dynamical instability which provides a scientific basis for the driving mechanism of the glaciation cycles the second contribution is a clarification of the origin and climate significance of the obliquity related 100 kyr cycle in terms of nonlinear coupling between various orbital and rotational parameters finally the fluctuations in the size of the ice sheets are computed using the weertman birchfield watts model of ice sheets it is demonstrated that the growth and decay of the width of the ice sheets are in good agreement with the variations in the continental ice volume as seen in the late pleistocene 18o record

simulating the mid pleistocene transition through regolith removal abstract quaternary 18o ice volume proxy records show a transition from high frequency small amplitude glacial cycles to low frequency large amplitude glacial cycles this reorganization of the climate system termed the mid pleistocene transition mpt is thought to reflect a change in land ice response to orbital forcing despite no significant change in orbital cycles during this period one potential explanation for the mpt proposes that gradual erosion of high latitude northern hemisphere regolith by multiple cycles of glaciation caused a transition in ice sheet response to external forcing here we explore this regolith hypothesis using a complex earth system model we show that simulating a transition from deformable sediment to crystalline bedrock produces an evolution in ice volume response similar to proxy reconstructions of the mpt the simulated change in ice volume response is due to a combination of climate and ice flow changes with crystalline bedrock producing thicker colder ice sheets that accumulate more snowfall and have a smaller ablation zone further experiments that include transient eccentricity amplifying co2 forcing show only small differences in ice response compared to those with orbital forcing only suggesting that cycles of co2 were not the primary cause of the mpt

astronomical theory of paleoclimates and the last glacial interglacial cycle in the 19th century croll and pilgrim stressed the importance of severe winters as a cause of quaternary ice ages later on mainly during the first half of the 20th century koppen spitaler and milankovitch regarded high winter and low summer insolation as favoring glaciation after koppen and wegener related the milankovitch new radiation curve to penck and bruckner s subdivision of the quaternary there was a long lasting debate whether or not such changes in the insolation can explain the quaternary glacial interglacial cycles in the 1970s with the improvements of radiometric dating and of acquiring and interpreting the long continuous geological records with the advent of computers and with the development of more sophisticated astronomical and climate models the astronomical theory and in particular its milankovitch version were revived over the last 5 years most geological astronomical and climatological obstacles have been overcome and the causal role of the orbital variations in long term variation of climate is taken almost universally for granted models of different categories of complexity from conceptual ones to 3 d atmospheric general circulation models and 2 d time dependent models of the whole climate system have now been astronomically forced in order to test the physical reality of the astronomical theory for example experiments with both low and high resolution gcms show that the increased seasonality at 9 000 bp leads to intensified summer monsoon circulation of africa and southern asia on the other hand seasonal models for simulating the transient response of the climate system to astronomical forcing are also developed and the output of these most recent modeling efforts compares favorably with data of the past 400 000 years accordingly the model predictions for the next 100 000 years are used as a basis for forecasting how climate would evolve when forced by orbital variations in the absence of anthropogenic disturbances the long term cooling trend which began some 6 000 years ago will continue for the next 5 000 years this first temperature minimum will be followed by an amelioration around 15 000 ap after present by a cold interval centered al 23 000 ap and by a major glaciation at around 60 000 ap

simulation of the last glacial cycle by a coupled sectorially averaged climate ice sheet model 2 response to insolation and co2 variations a two dimensional climate model which links the northern hemisphere atmosphere ocean mixed layer sea ice and continents has been asynchronously coupled to a model of the three main northern ice sheets and their underlying bedrock the coupled model has been used to test the influence of several factors including snow surface albedo over the ice sheets in producing plausible ice age simulations using astronomically derived insolation and co2 data from the vostok ice core the impact of potentially important processes such as the water vapor transport clouds and deep sea circulation was not investigated in this study after several sensitivity experiments designed to identify the main mechanism governing surface temperature and ice accumulation the model is first run with ice sheet feedback by forcing it only with the astronomical insolation over the past 122 kyr large variations of ice volume are simulated between 122 and 55 kyr b p with a rapid latitudinal extension of the north american and eurasian ice sheets starting at 120 kyr b p the simulated last glacial maximum is at 19 kyr b p the model is able to simulate deglaciation as well the simulated evolution of the three northern ice sheets is generally in phase with geological reconstructions the major discrepancy between the simulation and paleoclimate reconstructions lies in the underestimation of temperature variations linked with an underestimation of the ice sheet extent and an excess in the prescribed co2 concentration sensitivity experiments show that ablation is more important to the ice sheet response than snow precipitation variations in the model a key mechanism in the deglaciation after the last glacial maximum appears to be the aging of snow which decreases its albedo the other factors which play an important role are in decreasing level of importance the ice sheet altitude insolation taiga cover and ice sheet extent a final set of experiments addresses the effects of co2 on the simulated climate of the last glacial maximum and on a new long term experiment covering the last 122 kyr this last experiment is made by forcing the model with both insolation and co2 variations this additional forcing improves the temperature and ice volume results despite the limitations inherent to the present modeling approach the sensitivity experiments performed can provide insight into the relative importance of possible mechanism responsible for the building and melting of huge ice sheets during the last glacial interglacial cycle

search for lower mantle high velocity zones beneath the deepest kuril and mariana earthquakes we perform travel time analyses for the kuril and the mariana subduction zones to investigate whether a lower mantle high velocity zone exists immediately beneath the deepest earthquakes in these regions we use p and pkikp wave arrival time data read directly and accurately from wwssn short period vertical component records and corrected for the effect of lateral heterogeneity far from the source regions using a differential residual sphere technique the corrected residuals for the deep kuril events show that a high velocity zone extends in the down dip direction of the kuril slab in the lower mantle on the other hand the residuals for the deep mariana events show no evidence for a high velocity zone in the down dip direction beneath the deepest earthquakes a reasonable interpretation of these results is that the kuril slab penetrates into the lower mantle h 670 km but that the mariana slab does not extend vertically into the lower mantle

optimal estimation of hydrographic station data and derived fields abstract optimal estimation is applied to contouring and analysis of hydrographic sections measured fields such as temperature and salinity and derived fields such as geostrophic velocity are decomposed into large scale and small scale components first a heavily sampled basin scale field is estimated and subtracted from the data a small scale field barely resolved by the hydrography is then found from the residuals the power and utility of the technique are illustrated by means of examples using a short section from the straits of florida and a transatlantic section along 24 n

vostok ice core climatic response to co2 and orbital forcing changes over the last climatic cycle vostok climate and co2 records suggest that co2 changes have had an important climatic role during the late pleistocene in amplifying the relatively weak orbital forcing the existence of the 100 kyr cycle and the synchronism between northern and southern hemisphere climates may have their origin in the large glacial interglacial co2 changes

insolation driven 100 000 year glacial cycles and hysteresis of ice sheet volume comprehensive climate and ice sheet models show that insolation and internal feedbacks between the climate the ice sheets and the lithosphere asthenosphere system explain the 100 000 year period on which the northern hemisphere ice sheets grow and shrink for the past million years earth s climate has been dominated by a 100 000 year cycle of glaciation involving the advance and retreat of the northern hemisphere ice sheets variations in insolation the solar radiation reaching earth s surface caused by constant changes in earth s orbit and orientation to the sun are closely associated with glaciations but the physical mechanisms driving the cycle have remained unclear now ayako abe ouchi and colleagues demonstrate that insolation variations interact with the size of the ice sheets and the solid earth to control the 100 000 year cycles their model shows that internal feedbacks between insolation climate and the elevation of the land beneath the ice sheet allows ice sheets to grow gradually until a maximum size is reached at that point the lowered ice sheet elevation caused by the delayed rebound of the solid earth means that a small increase in insolation can destroy the ice sheet in just a few thousand years the growth and reduction of northern hemisphere ice sheets over the past million years is dominated by an approximately 100 000 year periodicity and a sawtooth pattern1 2 gradual growth and fast termination milankovitch theory proposes that summer insolation at high northern latitudes drives the glacial cycles3 and statistical tests have demonstrated that the glacial cycles are indeed linked to eccentricity obliquity and precession cycles4 5 yet insolation alone cannot explain the strong 100 000 year cycle suggesting that internal climatic feedbacks may also be at work4 5 6 7 earlier conceptual models for example showed that glacial terminations are associated with the build up of northern hemisphere excess ice 5 8 9 10 but the physical mechanisms underpinning the 100 000 year cycle remain unclear here we show using comprehensive climate and ice sheet models that insolation and internal feedbacks between the climate the ice sheets and the lithosphere asthenosphere system explain the 100 000 year periodicity the responses of equilibrium states of ice sheets to summer insolation show hysteresis11 12 13 with the shape and position of the hysteresis loop playing a key part in determining the periodicities of glacial cycles the hysteresis loop of the north american ice sheet is such that after inception of the ice sheet its mass balance remains mostly positive through several precession cycles whose amplitudes decrease towards an eccentricity minimum the larger the ice sheet grows and extends towards lower latitudes the smaller is the insolation required to make the mass balance negative therefore once a large ice sheet is established a moderate increase in insolation is sufficient to trigger a negative mass balance leading to an almost complete retreat of the ice sheet within several thousand years this fast retreat is governed mainly by rapid ablation due to the lowered surface elevation resulting from delayed isostatic rebound14 15 16 which is the lithosphere asthenosphere response carbon dioxide is involved but is not determinative in the evolution of the 100 000 year glacial cycles

the role of ice stream dynamics in deglaciation since the mid pleistocene transition deglaciation has occurred only after ice sheets have grown large while experiencing several precession and obliquity cycles indicating that large ice sheets are more sensitive to milankovitch forcing than small ice sheets are observations and model simulations suggest that the development of ice streams in the laurentide ice sheet played an as yet unknown role in deglaciations in this study we propose a mechanism by which ice streams may enhance deglaciation and render large ice sheets more sensitive to milankovitch forcing we use an idealized configuration of the parallel ice sheet model that permits the formation of ice streams when the ice sheet is large and ice streams are sufficiently developed an upward shift in equilibrium line altitude commensurate with milankovitch forcing results in rapid deglaciation while the same shift applied to an ice sheet without fully formed ice streams results in continued ice sheet growth or slower deglaciation rapid deglaciation in ice sheets with significant streaming behavior is caused by ice stream acceleration and the attendant enhancement of calving and surface melting at low elevations ice stream acceleration is ultimately the result of steepening of the ice surface and increased driving stresses in ice stream onset zones which come about due to the dependence of surface mass balance on elevation these ice sheet simulations match the broad features of geomorphological observations and add ice stream dynamics that are missing from previous model studies of deglaciation

is the spectral signature of the 100 kyr glacial cycle consistent with a milankovitch origin spectral analysis techniques traditionally applied to proxy climate records have involved the use of specialized windowing functions because of the finite length of the record available these have the effect of artificially broadening the spectral peaks such that closely spaced peaks may not be fully resolved alternatively use of a simple boxcar window following muller and macdonald 1997b will maximize the spectral frequency resolution this is achieved at the expense of potential truncation artifacts manifested as spurious peaks in the spectra although in the spectral analyses presented here such additional peaks are not significant in addition simple methods like this are not able to place any confidence intervals on the results global ice volume proxy records obtained

pleistocene climatic variability at astronomical frequencies abstract spectral analysis of geological records show periodicities corresponding to those calculated for the eccentricity 400 and 100 ka the obliquity 41 ka and the climatic precession 23 and 19 ka it is precisely the geological observations of this bi partition of the precessional peak confirmed to be real in astronomical computations which was one of the most impressive of all tests for the milankovitch theory concerning the question of whether or not the observed cycles account for most of the climatic variability having periods in the range predicted by the astronomical theory substantial evidence from cross spectral analysis coherency analysis and modelling is provided that at least near frequencies of variation in obliquity and precession a considerable fraction of the climate variance is driven in some way by insolation changes accompanying changes in the earth s orbit the variance components centered near a 100 ka cycle dominate most ice volume records and seem in phase with the eccentricity cycle although the exceptional strength of this cycle needs a non linear amplification by the glacial ice sheets themselves and associated feedbacks as the insolation spectra change with latitudes and with the type of parameters considered the diversity of the spectra of different climatic proxy data recorded in different places of the world over different periods is used to better understand how the climate system responds to the insolation forcing this study of the physical mechanisms involved is also achieved through the analysis of the log log shape of the geological records and through the comparison in frequency domain between simulated climatic time series and proxy data the evidence both in the frequency and in the time domain that orbital influences are felt by the climate system implies that the astronomical theory may provide a clock with which to date quaternary sediments with a precision several times greater than is now possible

multiple instabilities and modes of glacial rhythmicity in the plio pleistocene a general theory of late cenozoic climatic change it has been noted that several distinct modes of glacial oscillation have existed during the past few million years ranging from low amplitude high frequency oscillations in the early pliocene through relatively high amplitude predominantly near 40 ky period oscillations in the late pliocene and early pleistocene to the major near 100 ky period oscillations of the late pleistocene in addition to other plausible mechanisms suggested previously to explain aspects of this multirhythmic phenomenon we now illustrate another possible contributor to this type of behavior based on the hypothesis that the slow response climatic system is bistable and that two kinds of internal instability may be operative along with externally imposed forcing due to earth orbital milankovitch radiation changes and slow tectonically induced changes in atmospheric carbon dioxide these two instabilities have been discussed previously one is due to positive feedback in the global carbon cycle leading to near 100 ky free oscillations of the ice sheets and the other is due to the potential for ice calving catastrophes associated with bedrock variations that can lead to oscillations of a period near 40 ky independent of obliquity forcing within the framework of a dynamical model containing the possibility for these two instabilities as well as for stable modes we show 1 how milankovitch radiative changes or stochastic forcing influencing ice sheets can induce aperiodic chaotic transitions between the possible stable and unstable modes and more significantly 2 how progressive long term tectonically induced changes in carbon dioxide acting in concert with earth orbital radiative variations in high northern hemisphere latitudes can force systematic transitions between the modes such systematic changes can result in an ice mass chronology for the past 5 my that is qualitatively similar to the observed record of global ice mass in essence we have constructed a minimum dynamical model of the late cenozoic climatic changes containing what are believed to be the main physical factors determining these changes ice mass bedrock depression atmospheric carbon dioxide concentration deep ocean thermohaline state milankovitch radiation forcing and slow tectonically induced carbon dioxide forcing this model forms the basis for a coherent theory for the complex climatic events of this long period

summer solstice solar radiation the 100 kyr ice age cycle and the next ice age modeling studies suggest that the summer solstice solar radiation is more important than the caloric half year solar radiation in producing glacial interglacial cycles because it is more representative of the energy available to melt ice during the short melt season here it is shown that the correlation between the summer solstice solar radiation and the rate of change of the oxygen isotope record is generally greater than that between the caloric half year radiation and the rate of change of the oxygen isotope record these results also suggest that the sawtoothed nature of the 100 kyr cycle may be produced by periods of relatively slow changes in ice volume punctuated by periods of rapid growth that are initiated at times of extremely low summer solstice radiation and that it is unlikely that an ice age will begin in the next 70 kyr

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

asthenospheric ice load effects in a global dynamical system model of the pleistocene climate in a previous dynamical model the late cenozoic climate variations were simulated taking into account free and forced variations of atmospheric carbon dioxide acting in concert with changes in global ice mass and the deep ocean thermal state all under the influence of the known earth orbital radiative changes this model is now extended by adding another relevant variable bedrock asthenosphere depression including its associated ice calving effects within the context of this extended model we 1 demonstrate the main results of previous bedrock ice sheet models in what we believe is the simplest possible manner 2 show how these previous models can exhibit the mid pleistocene transition with the inclusion of co2 effects 3 discuss the limitations of these previous bedrock models and 4 illustrate the possibility of removing some of these limitations and accounting for further aspects of the paleoclimate record by using the full dynamical system that includes forced and free effects of co2 as well as effects of bedrock depression and milankovitch forcing as one example of a new possibility with bedrock effects included in the full system we can obtain a solution characterized by irregularly spaced intermittent episodes in which the behavior is dominated either by near 40 kyr period oscillations or by near 100 kyr periods such as prevailed over the pleistocene

orbital signals in the diurnal cycle of radiation time of day plays a critical role in determining the dominance of the earth s orbital parameters involving eccentricity precession and obliquity extension of orbital radiation data to the diurnal cycle permits the recognition of well defined temporally separate signals corresponding to all three major orbital parameters and to combinations of these parameters past research involving computed radiation changes at the daily and seasonal scales has not shown significant eccentricity cycles results of this study show 1 at the solstices a marked change in dominance from precession for the solar elevation classes associated with early morning late afternoon to obliquity for the elevation class containing solar noon occurs in solar radiation 2 at the equinoxes cycles corresponding to those associated with eccentricity are evident in the radiation data at all individual time intervals of the day past studies have shown that the daily radiative total reflects only the influence of precession a third set of cycles at the equinoxes centered near 11 kyr corresponds to a combination of orbital signals these results imply that paleoclimatic proxy data displaying strong spectral peaks corresponding to eccentricity 100 kyr and obliquity 41 kyr such as records of global ice volume may be related to radiation changes occurring in the solar elevation class containing solar noon while paleoclimatic proxy data displaying strong spectral peaks corresponding to precession for example sea surface temperatures appear to be showing the dominance of precession on the daily total radiation

polar motion of a viscoelastic earth due to glacial cycle mass loading the growth and decay of ice sheets can change the symmetry axis of the global mass distribution and thus excite motion of the rotation axis of the earth we develop a simple normal mode expansion of the operator which converts surface load histories into polar motion histories and use it to characterize the polar motion response to arbitrary surface loading excitation in terms of gain and phase for a wide range of earth models and excitation time scales uncertainties in loading history presently limit the utility of constraints on mantle rheology which can be obtained from matching the present direction and rate of motion of the pole because of its magnitude and distance from the pole the laurentide ice sheet alone can induce motion of the rotation pole by over 100 km this will influence the pattern of incident radiation on continents and oceans and may play a significant role in limiting ice sheet growth differential motion of the mantle and core on a glacial timescale may also influence the geomagnetic field

modeling of residual spheres for subduction zone earthquakes 1 apparent slab penetration signatures in the nw pacific caused by deep diffuse mantle anomalies we have computed focal residual spheres for 145 subduction zone earthquakes along the northwest edge of the pacific using regional and global mantle velocity models from tomographic inversions the mantle models explain much of the observed residual sphere data and to a certain extent suggest the location of mantle velocity heterogeneities which are responsible for various residual sphere patterns for most deep events considered the fast slablike residual sphere anomalies are caused by diffuse heterogeneities mainly of deep lower mantle and receiver mantle origin rather than by an extension of the slab the region immediately below the deepest earthquakes depths of 650 1500 km has an effect usually smaller than or comparable to the effect of other regions of the mantle without a proper account of the teleseismic effect attributing the long wavelength anomalies of the residual sphere to near source slab effects alone or even primarily is not valid the fast bands in many observed residual spheres agree with seismicity trends once the deep mantle and receiver mantle effects are removed these may give the approximate orientation but not the depth extent of near source fast velocities for most deep earthquakes under japan the predominant fast band is subhorizontal rather than near vertical this type feature would be overlooked in conventional residual sphere studies using only steeply diving rays and cosine weighting of the data

the initiation of ice sheet growth milankovitch solar radiation variations and the 100 ky ice age cycle much work has gone into deciphering the causes of the large scale glacial interglacial variations in the climate system over the last 900 000 years while variations on the 41 thousand year ky and 23 ky time scales seem to be linearly linked to the variations in the distribution of solar radiation at the top of the atmosphere milankovitch solar radiation variations the causes of the dominant 100 ky cycle in the geologic record are still unknown one of the aspects of this cycle that is not well understood is how large scale ice sheet growth is initiated here we describe the mechanisms by which large scale ice sheet growth may have been initiated by the changes in the seasonal and latitudinal distribution of solar radiation over the past 160 ky this is done through the use of a coupled energy balance climate thermodynamic sea ice model that includes a hydrologic cycle which computes precipitation and a land surface energy balance which determines the net accumulation of snow and ice results indicate that the initiation of ice sheet growth is possible during times of extremely low summer solstice solar radiation as a result of a large decrease in ablation during the critical melt season

investigation of deep slab structure using long period s waves travel times and amplitudes of long period sh scsh ssh and sscsh phases from several deep focus earthquakes in the northwest pacific are analyzed for evidence of lithospheric slab penetration into the lower mantle inclusion of amplitude observations in the analysis provides constraints on lateral velocity gradients present in the deep slabs which are not resolvable using travel times alone travel time and amplitude residual spheres are presented for two deep focus events with good azimuthal coverage but while interesting patterns are present quantitative analysis is precluded by the lack of accurate methods for calculating synthetic long period seismograms for three dimensional slab models therefore we focus our analysis on a two dimensional approximation to the downdip geometry of the kurile and japan slabs which allows comparison of a much larger data set with accurate two dimensional synthetics as well as the use of ss and sscs travel time and amplitude patterns as empirical corrections for deep mantle structure while the limited azimuth range required for this approximation reduces the diagnostic capability of the data it also increases our confidence that the corrected data are most sensitive to the near source region the ss and sscs observations indicate that the downdip shear wave travel time pattern previously attributed to a deep slab extension is primarily caused by broad scale lower mantle heterogeneity once corrected for this structure the s wave observations do not support a simple undeformed slab steepening in dip and penetrating deep into the lower mantle beneath the kurile and japan islands as proposed by jordan 1977 and creager and jordan 1984 1986 rather the observations support shorter and or broader slab models man those previously hypothesized models which can probably be reconciled with p wave data further analysis of the complete three dimensional patterns will be required for more precise resolution of the penetration depths of these slabs if indeed any deep slab heterogeneity is actually seismically detectable

amplitude and phase of glacial cycles from a conceptual model abstract the astronomical theory of climate in which the orbital variations of the earth are taken to drive the climate changes explains many features of the paleoclimatic records nevertheless the precise link between insolation variations and climatic changes during the quaternary remains mysterious in several aspects in particular the largest sea level changes of the past million years occurred when insolation variations were minimal like during stage 11 and vice versa like during stage 7 moreover recent data from terminations ii and iii show surprising phase lead and lag between insolation and sea level variations to explain these paradoxical amplitude and phase modulations we suggest here that deglaciations started when a combination of insolation and ice volume was large enough to illustrate this new idea we present a simple conceptual model that simulates the sea level curve of the past million years with very realistic amplitude modulations and with good phase modulations

a 150 000 year climatic record from antarctic ice during much of the quaternary the earth s climate has undergone drastic changes most notably successive glacial and interglacial episodes the past 150 kyr includes such a climatic cycle the last interglacial the last glacial and the present holocene interglacial a new climatic time series for this period has been obtained using 18 o data from an antarctic ice core

a model of the antarctic ice sheet numerical modelling of ice sheets and glaciers has become a useful tool in glaciological research a model described here deals with the vertical mean ice velocity is time dependent computes bedrock adjustment and uses an empirical diagnostic relationship to derive the distribution of ice thickness in ice shelves the rate of snowfall and ice snow melt depends on the prescribed sea level temperature surface slope elevation and distance to open water the model is able to reproduce the major features of the antarctic ice sheet when it is run to a steady state for present climatic conditions the main difference with the present ice sheet is that the shallow parts of the weddell sea become covered by grounded ice

reconstruction of atmospheric co2 from ice core data and the deep sea record of ontong java plateau the milankovitch chron we provide a reconstruction of atmospheric co2 from deep sea sediments for the past 625000 years milankovitch chron our database consists of a milankovitch template of sea level variation in combination with a unique data set for the deep sea record for ontong java plateau in the western equatorial pacific we redate the vostok ice core data of barnola et al 1987 to make the reconstructions we employ multiple regression between deep sea data on one hand and ice core co2 data in antarctica on the other the patterns of correlation suggest that the main factors controlling atmospheric co2 can be described as a combination of sea level state and sea level change for best results squared values of state and change are used the square of sea level rule agrees with the concept that shelf processes are important modulators of atmospheric co2 e g budgets of shelf organic carbon and shelf carbonate nitrate reduction the square of change rule implies that on short timescales any major disturbance of the system results in a temporary rise in atmospheric co2

anomalous cyclicity in climate and stratigraphy and modeling nonlinear oscillations for geological periods lacking evidence of extensive glaciation such as the cretaceous the presence of anomalously large globally synchronous cycles in coastal onlap stratigraphy haq et al 1987 may be a new example of a nonlinear geophysical response to weak external forcing a number of analogies can be drawn between this situation and the anomalously large 100 kyr late pleistocene ice volume cycle vis a vis weak 100 kyr milankovitch insolation changes apropos of these analogies we discuss formal models for evolving free sedimentary or glacial surfaces subject to lateral mass transport and subsidence and observe that nonlinear diffusivity obtains in each case in particular the systems both involve retarded transport we then define equilibrium surfaces and show them to be closely related to either the subsidence profile or the ablation profile in the absence of mass supply feedback these equilibrium surfaces should be unconditionally stable and amplified reponses to external forcing are unlikely we then consider feedbacks on mass supply and show first how to construct a three variable glacial wedge that freely oscillates glacial models of this type suggest that a fundamental role is played in the oscillation by isostatic relaxation times this effect should be considered for sedimentary systems as well we finish with a speculation about a simple sediment supply feedback that might be associated with relative sea level changes and illustrate graphically how the multiple equilibrium points introduced by this feedback could lead to large coastal onlap shifts from relatively small perturbations to the system much more work will be needed however to determine the parameter values required to achieve such behavior and if such values are physically realistic

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