two dimensional modelling of subduction zone anisotropy with application to southwestern japan summary we present a series of 2 d numerical models of viscous flow in the mantle wedge induced by a subducting lithospheric plate we use a kinematically defined slab geometry approximating the subduction of the philippine sea plate beneath eurasia through finite element modelling we explore the effects of different rheological and thermal constraints e g a low viscosity region in the wedge corner power law versus newtonian rheology the inclusion of thermal buoyancy forces and a temperature dependent viscosity law on the velocity and finite strain field in the mantle wedge from the numerical flow models we construct models of anisotropy in the wedge by calculating the evolution of the finite strain ellipse and combining its geometry with appropriate elastic constants for effective transversely isotropic mantle material we then predict shear wave splitting for stations located above the model domain using expressions derived from anisotropic perturbation theory and compare the predictions to 500 previously published shear wave splitting measurements from seventeen stations of the broad band f net array located in southwestern japan although the use of different model parameters can have a substantial effect on the character of the finite strain field the effect on the average predicted splitting parameters is small however the variations with backazimuth and ray parameter of individual splitting intensity measurements at a given station for different models are often different and rigorous analysis of details in the splitting patterns allows us to discriminate among different rheological models for flow in the mantle wedge the splitting observed in southwestern japan agrees well with the predictions of trench perpendicular flow in the mantle wedge along with b type olivine fabric dominating in a region from the wedge corner to about 125 km from the trench

a kinematic model for recrystallization and texture development in olivine polycrystals the interpretation of seismic anisotropy in the mantle requires a knowledge of the relationship between the lattice preferred orientation lpo of crystals and the convective flow field in order better to understand this link we present a model for the evolution of lpo in olivine aggregates that deform by both intracrystalline slip and dynamic recrystallization dynamic recrystallization depends on the dislocation density of the grains which is a function of the applied local stress grains with a large density of dislocations lower their bulk strain energy by nucleating strain free sub grains at a rate proportional to a dimensionless nucleation parameter grains with high energy are then invaded by grains with low energy by grain boundary migration at a rate proportional to a dimensionless grain boundary mobility m the value of is constrained by observed lpo patterns in experimentally deformed olivine aggregates and m is constrained by the temporal evolution of the strength of the lpo for m 125 75 and 3 the model predictions agree well with the experimental results numerical calculations of lpo using our model are significantly faster than those based on viscoplastic self consistent or equilibrium based theories making the model especially suitable for applications for complex convective flows

seismic tomography a window into deep earth abstract the goal of this paper is to provide an overview of the current state of the art in seismic tomography and trace its origins from pioneering work in the early 1970s to its present status as the pre eminent tool for imaging the earth s interior at a variety of scales due to length limitations we cannot hope to cover every aspect of this diverse topic or include mathematical derivations of the underlying principles rather we will provide a largely descriptive coverage of the methodology that is targeted at readers not intimately familiar with the topic the relative merits of local versus global parameterization ray tracing versus wavefront tracking backprojection versus gradient based inversion and synthetic testing versus model covariance are explored a variety of key application areas are also discussed including body wave traveltime tomography surface wave tomography attenuation tomography and ambient noise tomography established and emerging trends many of which are driven by the ongoing rapid increases in available computing power will also be examined including finite frequency tomography full waveform tomography and joint tomography using multiple datasets several practical applications of seismic tomography including body wave traveltime attenuation and surface waveform are presented in order to reinforce prior discussion of theory

statistical properties of seismic anisotropy predicted by upper mantle geodynamic models 1 we study how numerically predicted seismic anisotropy in the upper mantle is affected by several common assumptions about the rheology of the convecting mantle and deformation induced lattice preferred orientations lpo of minerals we also use these global circulation and texturing models to investigate what bias may be introduced by assumptions about the symmetry of the elastic tensor for anisotropic mineral assemblages maps of elasticity tensor statistics are computed to evaluate symmetry simplifications commonly employed in seismological and geodynamic models we show that most of the anisotropy predicted by our convection lpo models is captured by estimates based on a best fitting hexagonal symmetry tensor derived from the full elastic tensors for the computed olivine enstatite lpos however the commonly employed elliptical approximation does not hold in general the orientations of the best fitting hexagonal symmetry axes are generally very close to those predicted for finite strain axes correlations between hexagonal anisotropy parameters for p and s waves show simple bilinear relationships such relationships can reduce the number of free parameters for seismic inversions if this information is included a priori the match between our model predictions and observed patterns of anisotropy supports earlier more idealized studies that assumed laboratory derived mineral physics theories and seismic measurements of anisotropy could be applied to study mantle dynamics the match is evident both in agreement between predicted lpo at selected model sites and that measured in natural samples and in the global pattern of fast seismic wave propagation directions

mantle deformation and tectonics constraints from seismic anisotropy in the western united states abstract we have examined shear wave splitting in teleseismic shear waves sks skks s from 15 stations in the western united states based on analysis of 123 records from 67 events the varied past and present tectonic styles in this region subduction transform faulting extension and stable domains are expected to cause a wide variety of anisotropic behavior and therefore make it an excellent natural laboratory fast polarization azimuths vary from e w to nearly n s and time delays t range from being barely detectable less than 0 6 s to 1 6 s most stations yielded consistent measurements independent of station event geometry the exceptions were stations situated very close to the san andreas fault which yielded well constrained but inconsistent splitting parameters these have been successfully modeled by two anisotropic layers with different horizontal symmetry axes the upper layer has a fast direction parallel to the fault and t of about 1 s the lower layer with t from 0 6 to 0 9 s is oriented e w near the san francisco bay area and ne sw in the mojave desert other measurements of e w fast are observed as far east as western nevada with large delay times of 1 3 1 5 s but disappear to the north stations in the northwestern basin and range have values of oriented at about 70 with delay times ranging from 0 7 to 1 2 s varies in other regions of the basin and range from 70 in eastern nevada to 20 in the transition zone between the colorado plateau and the basin and range with t of 1 0 s measurements are geographically coherent and in many cases changes correspond to geologic or tectonic boundaries the e w fast feature extends across several tectonic provinces but disappears near the southern boundary of the gorda plate and may therefore be related to the dynamics of plate interactions regional variations in the fast directions of anisotropy within the basin and range suggest that the present extension has not been strong enough to align the fast directions over a great depth extent fast polarization directions within the transition zone between the basin and range and colorado plateau are nearly parallel to the geologic and geophysical boundaries between the two regions and to other geophysical boundaries the size of the delay times suggests that in many of the areas studied either the mantle is almost twice as anisotropic as we estimate or the anisotropy and therefore mantle deformation extends into the sublithospheric mantle we used the two layer method to examine the effect of a postulated lower layer with fast direction parallel to the absolute motion of the north american plate and found the best estimate of contribution from a uniform layer is 0 1 s of t although a contribution of up to 0 7 s cannot be ruled out a value as low as 0 1 s suggests either that olivine is not strongly aligned in a horizontal direction in this layer or that the layer thickness is of the order of 10 km

marked decline in atmospheric carbon dioxide concentrations during the paleogene the relation between the partial pressure of atmospheric carbon dioxide pco2 and paleogene climate is poorly resolved we used stable carbon isotopic values of di unsaturated alkenones extracted from deep sea cores to reconstruct pco2 from the middle eocene to the late oligocene approximately 45 to 25 million years ago our results demonstrate that pco2 ranged between 1000 to 1500 parts per million by volume in the middle to late eocene then decreased in several steps during the oligocene and reached modern levels by the latest oligocene the fall in pco2 likely allowed for a critical expansion of ice sheets on antarctica and promoted conditions that forced the onset of terrestrial c4 photosynthesis

effect of the formation of the isthmus of panama on atlantic ocean thermohaline circulation the late cenozoic closure of the seaway between the north andsouth american continents is thought to have caused extensive changes in ocean circulation and northern hemisphere climate1 2 but the timing and consequences of the emergence of the isthmus of panama which closed the seaway remain controversial1 2 3 4 5 here we present stable isotope and carbonate sand fraction records from caribbean sediments which when compared to atlantic and pacific palaeoceanographic records indicate that the closure caused a marked reorganization of ocean circulation starting 4 6 million years ago shallowing of the seaway intensified the gulf stream and introduced warm and saline water masses to high northern latitudes these changes strengthened deep water formation in the labrador sea over the next million years as indicated by an increased deep water ventilation and carbonate preservation in the caribbean sea and favoured early pliocene warming of the northern hemisphere the evaporative cooling of surface waters during north atlantic deep water formation would have introduced moisture to the northern hemisphere although the pronounced intensification of northern hemisphere glaciation between 3 1 and 2 5 million years ago substantially lagged the full development of north atlantic deep water formation we propose that the increased atmospheric moisture content was a necessary precondition for ice sheet growth which was then triggered by the incremental changes in the earth s orbital obliquity

velocity reversals and sediment sorting in pools and riffles controlled by channel constrictions abstract keller keller e a 1971 areal sorting of bed load material the hypothesis of velocity reversal geological society of america bulletin 82 753 756 hypothesized that at high flow near bed velocities in pools exceed velocities in riffles and create pool scour pools however typically have larger cross sectional areas of flow at bankfull discharge this condition raises an inconsistency with keller s velocity reversal hypothesis and the one dimensional continuity of mass equation to address this problem a model of pool maintenance and sediment sorting is proposed that relies on constriction of flow by recirculating eddies and flow divergence over the exit slopes of pools according to the model a narrow zone of high velocity occurs in the center of pools creating scour along the downstream end of pools an uphill climb of particles up the pool exit slope promotes sediment deposition the model is tested with field and flume measurements of velocity water surface elevation and size of bed sediments in recirculating eddy influenced pools local reversals of the water surface gradient were measured in the field and a velocity reversal was created in the flume the reversals that were measured indicate higher gradients of the water surface over the upstream portions of pools and higher velocities in pools at high flow the distribution of bed sediments collected in the field also support the proposed model of pool maintenance

shear wave splitting and the pattern of mantle flow beneath eastern oregon a r t i c l e i n f o article history the tectonic and geologic setting of eastern oregon includes the volcanically active high lava plains hlp province and the accreted terrains of the blue and wallowa mountains and is bounded by the columbia river ood basalts to the north basin and range extension to the south the cascade arc to the west and stable north america to the east several models have been proposed to explain the tectonic evolution of eastern oregon and in particular the voluminous volcanic activity in the hlp but a consensus on which model fully describes the complex range of processes remains elusive measurements of the seismic anisotropy that results from active mantle ow beneath the region can provide a crucial test of such models to constrain this anisotropy here we present new sks splitting results obtained at approximately 200 broadband seismic stations in eastern oregon and the surrounding region data come from the usarray transportable array ta and two temporary experiments carried out in the hlp and in the wallowa mountains our splitting data set includes 2900 individual splitting measurements from sks phases recorded between 2006 and 2008 stations in eastern oregon exhibit signi cant shear wave splitting with average delay times at individual stations between 0 8 s and 2 7 s in the hlp nearly all observed fast directions are approximately e w while to the north in the blue and wallowa mountains there is more variability in the splitting patterns the average delay time observed at stations located in the heart of the hlp province is 2 s well above the global average of 1 s for continental regions we infer from the large split times and homogeneous fast directions that there must be signi cant active ow in a roughly e w direction in the asthenosphere beneath the hlp this inferred ow eld places a strong constraint on models that seek to explain the young tectonomagmatic activity in the region in the wallowa region the anisotropic signature is more complicated and there may be a signi cant contribution from fossil fabrics in the crust or mantle lithosphere

late pliocene greenland glaciation controlled by a decline in atmospheric co2 levels it is thought that the northern hemisphere experienced only ephemeral glaciations from the late eocene to the early pliocene epochs about 38 to 4 million years ago and that the onset of extensive glaciations did not occur until about 3 million years ago several hypotheses have been proposed to explain this increase in northern hemisphere glaciation during the late pliocene here we use a fully coupled atmosphere ocean general circulation model and an ice sheet model to assess the impact of the proposed driving mechanisms for glaciation and the influence of orbital variations on the development of the greenland ice sheet in particular we find that greenland glaciation is mainly controlled by a decrease in atmospheric carbon dioxide during the late pliocene by contrast our model results suggest that climatic shifts associated with the tectonically driven closure of the panama seaway with the termination of a permanent el nino state or with tectonic uplift are not large enough to contribute significantly to the growth of the greenland ice sheet moreover we find that none of these processes acted as a priming mechanism for glacial inception triggered by variations in the earth s orbit

constraints on subduction geodynamics from seismic anisotropy 1 much progress has been made over the past several decades in delineating the structure of subducting slabs but several key aspects of their dynamics remain poorly constrained major unsolved problems in subduction geodynamics include those related to mantle wedge viscosity and rheology slab hydration and dehydration mechanical coupling between slabs and the ambient mantle the geometry of mantle flow above and beneath slabs and the interactions between slabs and deep discontinuities such as the core mantle boundary observations of seismic anisotropy can provide relatively direct constraints on mantle dynamics because of the link between deformation and the resulting anisotropy when mantle rocks are deformed a preferred orientation of individual mineral crystals or materials such as partial melt often develops resulting in the directional dependence of seismic wave speeds measurements of seismic anisotropy thus represent a powerful tool for probing mantle dynamics in subduction systems here i review the observational constraints on seismic anisotropy in subduction zones and discuss how seismic data can place constraints on wedge slab and sub slab anisotropy i also discuss constraints from mineral physics investigations and geodynamical modeling studies and how they inform our interpretation of observations i evaluate different models in light of constraints from seismology geodynamics and mineral physics finally i discuss some of the major unsolved problems related to the dynamics of subduction systems and how ongoing and future work on the characterization and interpretation of seismic anisotropy can lead to progress particularly in frontier areas such as understanding slab dynamics in the deep mantle

alkenone and boron based pliocene pco2 records abstract the pliocene period is the most recent time when the earth was globally significantly 3 c warmer than today however the existing pco2 data for the pliocene are sparse and there is little agreement between the various techniques used to reconstruct palaeo pco2 this disagreement coupled with the general low temporal resolution of the published records does not allow a robust assessment of the role of declining pco2 in the intensification of the northern hemisphere glaciation inhg and a direct comparison to other proxy records are lacking for the first time we use a combination of foraminiferal 11b and organic biomarker alkenone derived carbon isotopes proxies to determine the concentration of atmospheric co2 over the past 5 ma both proxy records show that during the warm pliocene pco2 was between 330 and 400 ppm i e similar to today the decrease to values similar to pre industrial times 275 285 ppm occurred between 3 2 ma and 2 8 ma coincident with the inhg and affirming the link between global climate the cryosphere and pco2

small scale variations in seismic anisotropy near kimberley south africa summary we have exploited a new data set of broad band seismic waveforms recorded near the region of kimberley south africa to place new constraints on seismic anisotropy in an area of extensive mantle modification within an archaean cratonic setting the kimberley region is significant in that it represents archaean cratonic mantle that has been studied extensively via petrologic geochemical and seismic means and provides a unique opportunity to meld the results of a broad range of data sets to examine processes of crust and mantle deformation shear wave splitting observations using sks phases recorded by a dense broad band seismic array in the kimberley region exhibit consistent fast polarization directions of ne sw and splitting times that range from 0 15 s in the se to nearly 0 75 s in the nw regions of the array multichannel cross correlation of relative arrival times of teleseismic sks phases across the array reveals clear azimuthal variations and a relative arrival time range comparable to the shear wave splitting delay time range a like analysis of teleseismic pkpdf phases does not exhibit significant differences in relative arrival times across the array the combined relative arrival time and splitting analyses indicate that a significant change in the strength of anisotropic structure is required across the region our results are most consistent with a model in which variations in seismic anisotropy near the kimberley region are constrained to the lithosphere with an average anisotropic strength of 1 8 per cent extend to depths no greater than 150 km and are primarily controlled by a significant change in the strength of anisotropy within the lateral bounds of the seismic array this inferred structural boundary is near the location of the ca 2 9 gyr old colesberg magnetic lineament a prominent aeromagnetic feature interpreted as the primary suture zone joining the eastern meso archaean and western neo archaean blocks of the kaapvaal craton the proposed boundary is also located within the southern exposure of the ca 2 7 gyr old ventersdorp volcanic sedimentary sequence a key continental rifting feature in the region we hypothesize that the observed seismic anisotropy is the result of straininduced lattice preferred orientation of olivine caused by the amalgamation and deformation of the eastern and western blocks of the kaapvaal craton this collisional process mechanically weakened the region to generate the observed mantle fabric which was exploited by significant rifting events that may have subsequently enhanced the regional fabric this model is also consistent with sks splitting results for the whole of southern africa which suggest either weak anisotropic or isotropic mantle structure in the eastern kaapvaal and coherent mantle anisotropy across much of the western kaapvaal our results support the notion that seismic anisotropy beneath archaean continental regions is not created or alternatively not preserved during initial continental formation but is instead generated during subsequent significant mantle deforming events

on the structure and origin of major glaciation cycles 1 linear responses to milankovitch forcing time series of ocean properties provide a measure of global ice volume and monitor key features of the wind driven and density driven circulations over the past 400 000 years cycles with periods near 23 000 41 000 and 100 000 years dominate this climatic narrative when the narrative is examined in a geographic array of time series the phase of each climatic oscillation is seen to progress through the system in essentially the same geographic sequence in all three cycles we argue that the 23 000 and 41 000 year cycles of glaciation are continuous linear responses to orbitally driven changes in the arctic radiation budget and we use the phase progression in each climatic cycle to identify the main pathways along which the initial local responses to radiation are propagated by the atmosphere and ocean early in this progression deep waters of the southern ocean appear to act as a carbon trap to stimulate new observations and modeling efforts we offer a process model that gives a synoptic view of climate at the four end member states needed to describe the system s evolution and we propose a dynamic system model that explains the phase progression along causal pathways by specifying inertial constants in a chain of four subsystems solutions to problems involving systems of such complexity are not born full grown like athena from the head of zeus rather they evolve slowly in stages each of which requires a pause to examine data at great lengths in order to guarantee a sure footing and to properly choose the next step victor p starr

multichannel analysis of shear wave splitting a multichannel analysis is introduced to constrain seismic anisotropy from the shear wave splitting of sks and skks this technique utilizes simultaneously a set of records coming from different azimuths the splitting intensity of sks waves measured by the amplitude of the transverse component depends on the angle between the back azimuth of the earthquake and the direction of the symmetry axis and on the delay time t between the two quasi shear waves it is shown that the splitting parameters can be determined from the azimuthal dependence of the splitting intensity which is given by the first right eigenvector of the matrix containing the transverse components of all the records alternatively the splitting intensity can be measured by projecting the transverse components on the radial components derivatives experiments on synthetic seismograms demonstrate that both approaches provide robust estimates of the splitting parameters however the projection approach gives measurements that are closer to the inputs and with smaller error bars which suggests that it should be preferred when the signal to noise ratio is low the new technique is applied to analyze shear wave splitting under stations bcao bangui central african republic and bdfb brasilia brazil the analysis of 75 seismograms recorded by station bcao gives a fast polarization direction in very close agreement with the present day motion direction of the african plate comparisons with the results of previous studies obtained with other techniques and smaller data sets suggest that some of the published measurements may be strongly biased the main limitation of the multichannel analysis is a good azimuthal distribution of the seismicity maps of the azimuthal coverage were determined and it is found that the most favorable regions to study anisotropy with sks waves are a latitudinal band that goes from south america to africa the eastern part of north america and india

pleistocene deglaciation and the earth s rotation implications for mantle viscosity summary recent results from the analysis of postglacial rebound data suggest that the viscosity of the earth s mantle increases through the transition region models which fit both relative sea level and free air gravity data have viscosities which increase from a value near 1022 poise in the upper mantle beneath the lithosphere to a value of about 1023 poise in the lower mantle in this paper we analyse the effect of deglaciation upon the earth s rotation and thereby show that the observed secular trend polar wander evident in the ils ipms pole path and measurements of the non tidal acceleration of the length of day are both consistent with the viscosity profile deduced from postglacial rebound the two analyses are therefore mutually reinforcing

faults shear zones in the earth s mantle abstract geodetic data support a short term continental deformation localized in faults bounding lithospheric blocks whether major faults observed at the surface affect the lithospheric mantle and if so how strain is distributed are major issues for understanding the mechanical behavior of lithospheric plates a variety of evidence from direct observations of deformed peridotites in orogenic massifs ophiolites and mantle xenoliths to seismic reflectors and seismic anisotropy beneath major fault zones consistently supports prolongation of major faults into the lithospheric mantle this review highlights that many aspects of the lithospheric mantle deformation remain however poorly understood coupling between deformation in frictional faults in the uppermost crust and localized shearing in the ductile crust and mantle is required to explain the post seismic deformation but mantle viscosities deduced from geodetic data and extrapolated from laboratory experiments are only reconciled if temperatures in the shallow lithospheric mantle are high 800 c at the moho seismic anisotropy especially shear wave splitting provides strong evidence for coherent deformation over domains several tens of km wide in the lithospheric mantle beneath major transcurrent faults yet it cannot detect narrow strain localization zones or shallowly dipping faults seismic profiling images shallow dipping seismic reflectors in the lithospheric mantle interpreted as the continuation of normal or less frequently inverse faults in the mantle however the nature of these reflectors is poorly understood there is a large discrepancy between the scale at which deformation processes are studied a few kilometers at most due to the limited size of continuous mantle exposures at the earth s surface and the scale inferred for strain localization in the mantle from geophysical observations tens of kilometers combining data on deformation microstructures and crystal preferred orientations in naturally deformed peridotites and seismologic observations allows nevertheless discussing strain localization processes that may play a role on the development of mantle shear zones

finite frequency vectorial tomography a new method for high resolution imaging of upper mantle anisotropy summary amplitude measurements of the transverse component of sks waves the so called splitting intensity can be used to formulate a non linear inverse problem to image the 3 d variations of upper mantle anisotropy assuming transverse isotropy or hexagonal symmetry one can parametrize anisotropy by two anisotropic parameters and two angles describing the orientation of the symmetry axis these can also be written as two collinear pseudo vectors the tomographic process consists of retrieving the spatial distribution of these pseudo vectors and thus resembles surface wave vectorial tomography spatial resolution results from the sensitivity of low frequency sks waves to seismic anisotropy off the ray path the expressions for the 3 d sensitivity kernels for splitting intensity are derived including the near field contributions and validated by comparison with a full wave equation solution based upon the finite element method these sensitivity kernels are valid for any orientation of the symmetry axis and thus generalize previous results that were only valid for a horizontal symmetry axis it is shown that both lateral and vertical subwavelength variations of anisotropy can be retrieved with a dense array of broad band stations even in the case of vertically propagating sks waves

rapid early cambrian rotation of gondwana based on the history of mesozoic cenozoic plate motions as well as simple dynamical considerations a speed limit for tectonic plates has been suggested at 20 cm yr previous paleomagnetic data from the early cambrian of gondwana are confl icting but generally imply rapid motions approaching that limit herein we describe results from a continuous paleomagnetic sampling of lower to middle cambrian strata from the amadeus basin central australia we fi characteristic remanence directions that show an 60 declination shift through the section assuming a tectonically assembled gondwana supercontinent by early cambrian time this large vertical axis rotation of its australian sector corresponds to an equally large translation across paleolatitudes for its brazilian and west african sectors analysis of all high quality paleomagnetic data from gondwana both confi rms and constrains the 60 rotation to have occurred toward the end of early cambrian time at rates exceeding 16 12 8 cm yr these observations suggest that either nonuniformitarian plate tectonics or an episode of rapid true polar wander occurred during the cambrian explosion of animal life

the laurentide and innuitian ice sheets during the last glacial maximum the late wisconsinan advance of the laurentide ice sheet started from a middle wisconsinan interstadial minimum 27 30 14 c ka bp when the ice margin approximately followed the boundary of the canadian shield ice extent in the cordillera and in the high arctic at that time was probably similar to present ice advanced to its late wisconsinan stage 2 limit in the northwest south and northeast about 23 24 14 c ka bp and in the southwest and far north about 20 21 14 c ka bp in comparison to some previous reconstructions of ice extent our current reconstruction has substantially more late wisconsinan ice in the high arctic where an innuitian ice sheet is generally acknowledged to have existed in the atlantic provinces where ice is now thought to have extended to the continental shelf edge in most places and on eastern baffin island where ice probably extended to the fiord mouths rather than to the fiord heads around most of the ice margin the late wisconsinan maximum ice extent either exceeded the extent of earlier wisconsinan advances or it was similar to the early wisconsinan advance ice marginal recession prior to 14 14 ck a bp occurred mainly in deep water and along the southern terrestrial fringe however heinrich event 1 probably drew down the entire central ice surface at 14 5 14 c ka bp sufficiently to displace the labrador sector outflow centre 900 km eastward from the coast of hudson bay the onset of substantial ice marginal recession occurred about 14 14 c ka bp in the northwest southwest and south but not until about 10 11 14 c ka bp in the northeast and in the high arctic thus the period of maximum ice extent in north america generally encompasses the interval from b24 21 to 14 14 c ka bp or considerably longer than the duration of the lgm defined as occurring during a period of low global sea level as well as during a time of relative climate stability b18 14 c ka bp the interval of advance of much of the laurentide ice sheet to its maximum extent between b27 14 c ka bp and b24 14 c ka bp coincides with a suggested interval of rapid fall in global sea level to near lgmlevels r 2001 elsevier science ltd all rights reserved

closing of the central american seaway and the ice age a critical review 1 what role did the closing of the central american seaway play in enabling continental ice sheets to wax and wane over north america and fennoscandia a summary of relevant evidence presented here permits a causal relationship between them but can be interpreted to show none the common denominator of such evidence is the approximate simultaneity of that closing with global cooling and the first major ice advance at the resolution of paleoclimate however geologic evidence from central america places only weak constraints on when interchange between pacific and caribbean water ceased the strongest evidence for when north and south america became connected by a continuous land bridge the great american exchange of vertebrates assigns climate change the causal role for an arid central american climate typical of glacial times seems necessary for animals that inhabited savannas to pass through panama paleoceanographic and geochemical studies of environments and water masses in the eastern pacific western caribbean and atlantic in general call for continual change since 6 ma or earlier if evidence concurrent with global change near 3 5 3 ma seems more widespread than for other times hypothesized connections between a closed seaway and glaciation commonly call for profoundly different north atlantic ocean circulation but simulations using general circulation models provide a spectrum of differences in circulation for open and closed seaways might much not all of the evidence implicating the closing of the seaway in global climate change in fact be a consequence of that change and blind to central america s tectonic development

navigational infrastructure at the east pacific rise 9 50 n area following the 2005 2006 eruption seafloor benchmarks and near bottom multibeam surveys 1 four seafloor benchmarks were deployed with rov jason2 at frequently visited areas along the northern east pacific rise nepr ridge crest near 9 50 n within the ridge2000 epr integrated study site iss bull s eye when used in concert with established deep ocean acoustic positioning techniques these benchmarks provide navigational infrastructure to facilitate the integration of near bottom data at this site by allowing efficient and quantitative coregistration of data and observations collected on multiple dives and over multiple cruises high resolution near bottom multibeam bathymetric surveys also were conducted along and across the ridge crest to provide a morphological and geological context for the benchmark areas we describe the navigation and data processing techniques used to constrain the benchmark positions and outline operational details to effectively use benchmarks at this and other deep ocean sites where multidisciplinary time series studies are conducted the well constrained positions of the benchmarks provide a consistent geospatial framework that can be used to limit navigational uncertainties during seafloor sampling and mapping programs and enable accurate spatial coregistration and integration of observations these data are important to test a range of multidisciplinary hypotheses that seek to link geological chemical and biological processes associated with crustal accretion and energy transfer from the mantle to the hydrosphere at mid ocean ridges

resolving three dimensional anisotropic structure with shear wave splitting tomography summary shear wave splitting observations are a commonly used tool for inferring anisotropy and flow within the earth s interior here we present the development and validation of a new technique for imaging anisotropy in the upper mantle using local events shear wave splitting tomography swst the mantle is parametrized as a 3 d block model of crystallographic orientations with the elastic properties of olivine and orthopyroxene and both orthorhombic and hexagonal symmetries are tested to efficiently forward calculate splitting the christoffel equation is used to progressively split the horizontal components of a synthetic wavelet in each block of the model and predicted shear wave splitting parameters are obtained with an eigenvalue minimization technique numerically calculated partial derivatives are utilized in a linearized damped least squares inversion to solve for a best fitting model of crystallographic orientations to account for the non linear properties of shear wave splitting the inversion is applied iteratively and partial derivatives are recalculated after each iteration a starting model that incorporates information from predicted splitting parameters is found by spatially averaging fast directions and the ratio of observed to predicted splitting times models from inversions utilizing this average starting model reach lower misfit levels than do inversions with a random or uniform starting model modelling results using synthetic data from several anisotropic structures i e sharp lateral and vertical variations in anisotropy both within an idealized and a real nicaragua costa rica subduction zone illustrate the capabilities and limitations of swst with a station spacing of 25 km in an idealized subduction zone containing uniformly spaced events down to 225 km both the azimuth and dip of crystallographic axes are resolvable to a depth of 100 150 km and lateral heterogeneities in anisotropy on a scale of 50 km at arc and forearc distances from the trench are retrieved spatial resolution of anisotropy at scales of 75 km is possible further into the backarc above 150 km depth the geometry of stations and observed seismicity in the nicaragua costa rica subduction zone yields partial to good resolution at scales of 50 75 km beneath the forearc arc and limited regions of the backarc down to 100 km and resolution at coarser scales is possible in wider regions beneath the backarc given the distributions of seismic sources within many subduction zones and the advances in broad band seismic array deployments this new method offers a powerful means with which to constrain the orientation of anisotropic fabric in the upper mantle

lithospheric domains and controls on kimberlite emplacement slave province canada evidence from elastic thickness and upper mantle composition 1 we have mapped the deep structure of the slave craton by combining analysis the effective elastic thickness te with data on mantle samples from numerous kimberlites three dimensional mapping of the subcontinental lithospheric mantle sclm using mantle derived xenoliths and xenocrysts in kimberlites has shown that much of the craton is underlain by a strongly layered sclm a highly depleted upper layer low in basaltic components ca al fe is separated from a relatively fertile lower layer by a sharp boundary this boundary lies at 140 150 km depth in the lac de gras area and shallows to 100 km in the northern and southern parts of the craton weak lithosphere te 56 km in the younger eastern part of the craton is separated from the older western part by a zone of steep te gradient parallel to the major locus of kimberlite intrusion which may map the deep extension of the boundary between the two domains another strong te gradient across the kilohigok basin accompanies a marked compositional change in the upper layer of the lithospheric mantle the basin probably marks a major translithospheric fault correlations between te and mantle composition suggest that te is strongly influenced by the rheology of the upper mantle

the key role of global solid earth processes in preconditioning greenland s glaciation since the pliocene after 500 ma of absence major northern hemisphere glaciations appeared during the plio pleistocene with greenland leading other northern areas here we propose that three major solid earth processes underpinned build up of the greenland ice sheet first a mantle plume pulse responsible for the north atlantic igneous province at 60 ma regionally thinned the lithosphere younger plume pulses led to uplift which accelerated at 5 ma lifting the parts of the east greenland margin closest to iceland to elevations of more than 3 km above sea level second plate tectonic reconstruction shows a 6 northward component of greenland motion relative to the mantle since 60 ma third a concurrent northward rotation of the entire mantle and crust towards the pole dubbed true polar wander tpw contributed an additional 12 change in latitude these global geodynamic processes preconditioned greenland to sustain long term glaciation emphasizing the role of solid earth processes in driving long term global climatic transitions

rapid cenozoic glaciation of antarctica induced by declining atmospheric co2 the sudden widespread glaciation of antarctica and the associated shift towards colder temperatures at the eocene oligocene boundary 34 million years ago refs 1 4 is one of the most fundamental reorganizations of global climate known in the geologic record the glaciation of antarctica has hitherto been thought to result from the tectonic opening of southern ocean gateways which enabled the formation of the antarctic circumpolar current and the subsequent thermal isolation of the antarctic continent5 here we simulate the glacial inception and early growth of the east antarctic ice sheet using a general circulation model with coupled components for atmosphere ocean ice sheet and sediment and which incorporates palaeogeography greenhouse gas changing orbital parameters and varying ocean heat transport in our model declining cenozoic co2 first leads to the formation of small highly dynamic ice caps on high antarctic plateaux at a later time a co2 threshold is crossed initiating ice sheet height mass balance feedbacks that cause the ice caps to expand rapidly with large orbital variations eventually coalescing into a continental scale east antarctic ice sheet according to our simulation the opening of southern ocean gateways plays a secondary role in this transition relative to co2 concentration

how are vertical shear wave splitting measurements affected by variations in the orientation of azimuthal anisotropy with depth splitting measurements of teleseismic shear waves such as sks have been used to estimate the amount and direction of upper mantle anisotropy worldwide these measurements are usually made by approximating the anisotropic regions as a single homogeneous layer and searching for an apparent fast direction and an apparent splitting time t by minimizing the energy on the transverse component of the back projected seismogram in this paper we examine the validity of this assumption in particular we use synthetic seismograms to explore how a vertically varying anisotropic medium affects shear wave splitting measurements we find that weak heterogeneity causes observable effects such as frequency dependence of the apparent splitting parameters these variations can be used in principle to map out the vertical variations in anisotropy with depth through the use of frechet kernels which we derive using perturbation theory in addition we find that measurements made in typical frequency bands produce an apparent orientation direction that is consistently different from the average of the medium and weighted towards the orientation of the anisotropy in the upper portions of the model this tendency of the measurements to mimic the anisotropy at the top part of the medium may explain why shear wave splitting measurements tend to be correlated with surface geology when the heterogeneity becomes stronger multiple scattering reduces the amplitude of the tangential component seismogram and the associated splitting time so that a null result may be obtained despite the fact that the waves have travelled through a strongly anisotropic medium regardless of the amount of vertical heterogeneity we find that there is very little dependence on backazimuth for the measured fast axis direction or splitting time if the top and bottom halves of the medium average to similar fast axis directions if however the average fast axis direction in the top half of the model differs from that in the bottom half splitting time measurements will show a significant dependence on backazimuth but fast axis direction measurements will remain relatively constant

polar wandering of a dynamic earth summary the rotational behaviour of a stratified visco elastic planet submitted to changes in its inertia tensor is studied in a viscous quasi fluid approximation this approximation allows for large displacements of the earth rotation axis with respect to the entire mantle but is only valid for mass redistribution within the planet occurring on the time scale of a few million years such a motion called true polar wander tpw is detected by palaeomagneticiens assuming that the earth s magnetic field remains on average aligned with the spin axis our model shows that a downgoing cold slab induces a tpw which quickly brings this slab to the pole for a mantle of uniform viscosity the same slab is slowly moved toward the equator when a large viscosity increase with depth takes place in the mantle our model is also suitable to investigate the effects of a non steady state convection on the earth s rotation we discuss these effects using a simple mass redistribution model inspired by the pioneering paper of goldreich toomre 1969 it consists of studying the tpw induced by a random distribution of slabs sinking into the mantle for such a mass redistribution only a strongly stratified mantle can reduce the earth s pole velocity below l ma which is the upper bound value observed by palaeomagnetic investigations for the last 200 ma our model also shows that when corrected for the hydrostatic flattening the earth s polar inertia generally corresponds to the maximum inertia as it is presently observed however this may not be the case during some short time periods we also discuss the amount of excess polar flattening that can be related to tidal deceleration this frozen component is found to be negligible

shear wave splitting continental keels and patterns of mantle flow in this study we investigated the origin of seismic anisotropy in the mantle beneath north america in particular we evaluated whether shear wave splitting patterns in eastern north america are better explained by anisotropy caused by lithospheric deformation anisotropy due to mantle flow beneath the lithosphere or a combination of both we examined new measurements of shear wave splitting from the missouri to massachusetts broadband seismometer array moma the north american mantle anisotropy and discontinuity experiment nomad as well as splitting parameters from several previous studies we developed a simple finite difference model that approximates mantle flow around a complex three dimensional continental lithospheric keel to evaluate potential anisotropy from mantle flow beneath the lithosphere in eastern north america we compared shear wave splitting observations to predicted splitting parameters calculated using this mantle flow model our results indicate that a significant portion of observed shear wave splitting in eastern north america can be explained by mantle flow around the continental keel however shear wave splitting patterns in a few regions of eastern north america indicate that a component of lithospheric anisotropy must exist particularly in regions containing the largest keel thicknesses for eastern north america as well as for splitting observations in australia europe and south america we favor a model in which anisotropy is controlled by a combination of both lithospheric deformation and subcontinental mantle flow

usarray shear wave splitting shows seismic anisotropy from both lithosphere and asthenosphere north america provides an important test for assessing the coupling of large continents with heterogeneous archean to cenozoic aged lithospheric provinces to the mantle flow we use the unprecedented spatial coverage of the usarray seismic network to obtain an extensive and consistent data set of shear wave splitting intensity measurements at 1436 stations overall the measurements are consistent with simple shear deformation in the asthenosphere due to viscous coupling to the overriding lithosphere the fast directions agree with the absolute plate motion direction with a mean difference of 2 with 27 standard deviation there are however deviations from this simple pattern including a band along the rocky mountain front indicative of flow complication due to gradients in lithospheric thickness and variations in amplitude through the central united states which can be explained through varying contributions of lithospheric anisotropy thus seismic anisotropy may be sourced in both the asthenosphere and lithosphere and variations in splitting intensity are due to lithospheric anisotropy developed during deformation over long time scales

on the interpretation of sks splitting measurements in the presence of several layers of anisotropy summary concerns over the validity of expressions derived by montagner et al that link sks splitting measurements to the variation with depth of anisotropic parameters in the upper mantle have been recently expressed pointing out that the long period approximations applied by these authors may not be valid for the frequency range commonly used in sks studies and in particular that the anisotropy splitting parameters should depend on the order in which different anisotropy layers are arranged with depth we show here that indeed measurements of splitting time and fast axis direction performed at individual azimuths do depend on the order of layering however the expressions of montagner et al concern station averaged quantities that do not depend on the order of layers it is therefore correct to use these expressions in joint inversions of surface waveforms and sks station averaged splitting measurements on the other hand the depth dependent sensitivity of surface waveforms naturally provides constraints on the order of layering having clarified this confusion we extend the expressions of montagner et al to the case of a tilted axis of symmetry and non vertical incident waves and show that station averaged estimates of effective splitting parameters splitting time fast axis direction and tilt of the fast axis can be related to the integral with depth of quantities which now depend not only on the local splitting time and fast axis direction but also on the local tilt of the fast axis thus providing constraints also on the variation of the tilt with depth we show that the effective parameters used as constraints in the inversion can be obtained either from the measurement of splitting intensity or through a parameter search and cross convolution method in particular in the case when the effective tilt is significant the splitting intensity no longer presents 180 periodicity with azimuth providing a diagnostic tool for the presence of such tilts in the upper mantle thus combining body wave and surface wave observations also has the potential of constraining the variation with depth of the tilt of the fast axis of anisotropy a geodynamically important parameter

a short circuit in thermohaline circulation a cause for northern hemisphere glaciation the cause of northern hemisphere glaciation about 3 million years ago remains uncertain closing the panamanian isthmus increased thermohaline circulation and enhanced moisture supply to high latitudes but the accompanying heat would have inhibited ice growth one possible solution is that enhanced moisture transported to eurasia also enhanced freshwater delivery to the arctic via siberian rivers freshwater input to the arctic would facilitate sea ice formation increase the albedo and isolate the high heat capacity of the ocean from the atmosphere it would also act as a negative feedback on the efficiency of the conveyor belt heat pump

how robust is isotropic delay time tomography for anisotropic mantle using isotropic inversion of teleseismic p travel times computed for laterally varying anisotropic structures we investigate how anisotropy may show up in classical tomographic studies based on isotropic 3 d inversions anisotropic bodies of several tens of kilometers in the upper mantle may significantly bias the estimation of the isotropic part of a p wave velocity image the largest artifacts are introduced by anisotropic bodies with a dipping olivine a axis which could be associated with fossil subduction zones however anisotropy induced artifacts are negligible for anisotropy caused by pure shear deformation of the lithosphere due to horizontal extension or compression the negative p wave velocity anomaly arising from isotropic inversion beneath a rift zone may be enhanced if the olivine crystal s a axes are preferentially oriented parallel to the asthenospheric flow along the rift axis examination of recently published tomographic models shows that the image of the rhine graben may contain significant anisotropic artifacts

anisotropy beneath california shear wave splitting measurements using a dense broadband array we have determined the shear wave splitting parameters for a dense network of broad band stations in the western united states using high quality sks and skks waveforms with particularly high spatial resolution in the southern california region covered by the trinet seismic network the alignment of most fast polarization directions can be explained by plate tectonic extensional and compressional events we find that the overall pattern of fast directions agrees well with the pn anisotropy model by hearn that images the uppermost mantle furthermore the measured fast directions are generally orthogonal to the maximum horizontal compressive stress directions as determined from shallow crustal stress indicators world stress map this suggests that the pattern of anisotropy in the western us is predominantly uniform throughout the crust and upper mantle and that a 100 150 km thick layer as estimated from the sks delay time assuming 4 per cent anisotropy of anisotropic material has experienced coherent strain conditions and has undergone a similar deformation history a more detailed investigation reveals small scale lateral variations in anisotropy that are manifested by minor differences in splitting parameters between closely located stations as well as between sks and skks for the same station event pairs we also identify a contrast in splitting parameters between central the greater bay area and southern california in central california our measurements show evidence for variation of splitting parameters with backazimuth while in southern california the pattern of measurements can be fit adequately with a single layer anisotropy model this contrast dominates any consistent effect of the san andreas fault saf we can fit the variation of splitting parameters as a function of polarization azimuth for some stations in the vicinity of the saf better with a two layer anisotropy model than a single layer model with one thin layer having a fast direction parallel to the saf however many alternative models which could incorporate dipping axes of anisotropy lateral variation of anisotropy or a more continuous variation of fast direction with depth would be able to produce a similar fit

did glacially induced tpw end the ice age a reanalysis previous studies of earth rotation perturbations due to ice age loading have predicted a slow secular drift of the rotation axis relative to the surface geography i e true polar wander tpw of order of several degrees over the plio pleistocene it has been argued that this drift and the change in the geographic distribution of solar insolation that it implies may have been responsible for important transitions in ice age climate including the termination of ice age cycles we use a revised rotational stability theory that incorporates a more accurate treatment of the earth s background ellipticity to reconsider this issue and demonstrate that the net displacement of the pole predicted in earlier studies disappears this more muted polar motion is due to two factors first the revised theory no longer predicts the permanent shift in the rotation axis or the so called unidirectional tpw that appears in the traditional stability theory and second the increased background ellipticity incorporated in the revised predictions acts to reduce the normal mode amplitudes governing the motion of the pole we conclude that ice age induced tpw was not responsible for the termination of the ice age this does not preclude the possibility that tpw induced by mantle convective flow may have played a role in major plio pleistocene climate transitions including the onset of northern hemisphere glaciation

true to milankovitch glacial inception in the new community climate system model the equilibrium solution of a fully coupled general circulation model with present day orbital forcing is compared to the solution of the same model with the orbital forcing from 115 000 years ago the difference in snow accumulation between these two simulations has a pattern and a magnitude comparable to the ones inferred from reconstructions for the last glacial inception this is a major improvement over previous similar studies and the increased realism is attributed to the higher spatial resolution in the atmospheric model which allows for a more accurate representation of the orography of northern canada and siberia the analysis of the atmospheric heat budget reveals that as postulated by milankovitch s hypothesis the only necessary positive feedback is the snow albedo feedback which is initiated by reduced melting of snow and sea ice in the summer however this positive feedback is almost fully compensated by an increased meridionalheat transportin theatmosphereandareducedconcentrationoflow arcticclouds in contrasttosimilar previous studies the ocean heat transport remains largely unchanged this stability of the northern north atlanticcirculation is explainedby the regulatingeffect of thefreshwaterimportthroughthe naresstrait and northwest passage and the spiciness import by the north atlantic current

age spreading rates and spreading asymmetry of the world s ocean crust we present four companion digital models of the age age uncertainty spreading rates and spreading asymmetries of the world s ocean basins as geographic and mercator grids with 2 arc min resolution the grids include data from all the major ocean basins as well as detailed reconstructions of back arc basins the age spreading rate and asymmetry at each grid node are determined by linear interpolation between adjacent seafloor isochrons in the direction of spreading ages for ocean floor between the oldest identified magnetic anomalies and continental crust are interpolated by geological estimates of the ages of passive continental margin segments the age uncertainties for grid cells coinciding with marine magnetic anomaly identifications observed or rotated to their conjugate ridge flanks are based on the difference between gridded age and observed age the uncertainties are also a function of the distance of a given grid cell to the nearest age observation and the proximity to fracture zones or other age discontinuities asymmetries in crustal accretion appear to be frequently related to asthenospheric flow from mantle plumes to spreading ridges resulting in ridge jumps toward hot spots we also use the new age grid to compute global residual basement depth grids from the difference between observed oceanic basement depth and predicted depth using three alternative age depth relationships the new set of grids helps to investigate prominent negative depth anomalies which may be alternatively related to subducted slab material descending in the mantle or to asthenospheric flow a combination of our digital grids and the associated relative and absolute plate motion model with seismic tomography and mantle convection model outputs represents a valuable set of tools to investigate geodynamic problems

simple energy balance model resolving the seasons and the continents application to the astronomical theory of the ice ages a one level seasonal energy balance climate model which has explicit two dimensional land sea geography is introduced and its properties analyzed the model distinguishes land from sea surface strictly by the local thermal inertia employed the transport is governed by a smooth latitude dependent diffusion mechanism the model s seasonal cycle is solved for with and without ice feedback in each case good agreement with real data is accomplished with minimal tuning of the adjustable parameters the model sensitivity for small solar constant changes is in agreement with that of other accepted models however when the solar constant is lower a few percent the ice cap grows discontinuously from covering greenland and vicinity to a size three times larger and mostly into northern canada a similar discontinuous growth occurs when the orbital elements are changed to favor cool summers in the northern hemisphere this discontinuous sensitivity is discussed in the context of the milankovitch theory of the ice ages and the associated branch structure is shown to be analogous to the so called small ice cap instability known from simpler models although the discontinuity in sensitivity is somewhat conjectural both linear and nonlinear models have their largest sensitivity to orbital parameter changes in the northern portions of the large continents in the western hemisphere this also corresponds to the ice sheet boundary suggesting that the land sea configuration is crucial to large glacier growth irrespective of moisture budgets other experiments and implications are also discussed the remarkable paper by hays et al 1976 hereafter his presented compelling empirical evidence that continental ice sheet volume changes and sea surface temperature variations in the last 700 000 years are correlated with the changes in the earth s orbital elements theoretical arguments for such a connection date back to the last century but the most detailed and convincing arguments are due to milankovitch 1941 and the theory usually is associated with his name if the hypothesis is correct the theory and observations provide the cleanest experiment to determine the sensitivity of the earth s climate to truly external influences such a measure of climate sensitivity is of obvious importance considering the recent interest in estimating the change in climate due to other forcings such as the enhanced greenhouse effect due to increased levels of co2 in the atmosphere e g ramanathan 1981 hansen et al 1981 manabe and stouffer 1980 in effect climate models might be tested or calibrated by this natural experiment climate modeling has progressed dramatically in the last decade with the construction and testing of several general circulation models gcm s although some interesting ice age experiments have been done with these models gates 1976 manabe and hahn 1977 they still include only crude parameterizations of the slower components such as the oceans these numerical models are the ultimate tool that can be brought to bear upon the climate problem however they are almost as complicated as the climate itself hence experiments must be carefully designed and the interpretation is often difficult due to the expense of obtaining definitively long runs in parallel with gcm construction has been the development of x permanently at applied research corporation landover maryland 20785

finite strain calculations of continental deformation 1 method and general results for convergent zones a finite element method is used to calculate stresses and strain rates in a thin viscous sheet representing the continental lithosphere that is bordered in one part by an indenting boundary we calculate solutions for velocity fields crustal thickness distributions strain rates rotation time integrated vorticity and finite strain ellipsoids to show the dependence of these quantities on the controlling parameters the stress strain exponent n and the strength of the sheet relative to the gravitational forces which may be expressed as a dimensionless number the argand number ar for n 3 and ar between 1 and 10 a plateau of thickened crust is formed in front of the indenter the strain histories of individual elements within the viscous sheet may be quite complex and the finite strain at the end of deformation may bear no obvious relation to the principal stress orientations during the deformation the strain rate fields are interpreted in terms of the style of faulting that would be expected in the brittle upper crust if it were coupled to the ductile but stronger upper mantle in general the length scale of deformation increases with ar and time since the start of the collision and decreases with n for ar 3 the increase in length scale is accompanied by a change in the style of deformation from predominantly crustal thickening with thrust faulting in the brittle layer to predominantly transcurrent deformation strike slip faulting in the brittle layer

excitation of true polar wander by subduction true polar wander the global motion of the mantle relative to the earth s rotation axis is known from the analysis of palaeomagnetic data hotspot tracks and plate motions to have occurred at velocities of up to 0 5 myr 1 since the late cretaceous period1 4 we address here the longstanding question of how fast episodes of true polar wander tpw can be excited5 by analysing the impact of the distribution and activity of subduction zones on polar motion using nonlinear liouville equations which allow us to treat large excursions of the polar axis we show that unrealistically fast tpw is excited by subduction episodes unless the lower mantle has a viscosity at least 10 times that of the upper mantle this need for a viscosity increase with depth in the mantle reinforces the conclusions of previous studies on post glacial rebound and geoid anomalies theoretical creep laws and some preliminary results on tpw induced by density anomalies embedded in the mantle6 10 the lower viscosity in the upper mantle means that upper mantle density anomalies are most effective in exciting tpw changes in the pattern of subduction through time may be responsible for both episodes of fast tpw and times of quiescence in polar motion

the non commutivity of shear wave splitting operators at low frequencies and implications for anisotropy tomography summary measurements of the splitting or birefringence of seismic shear waves constitute a powerful and popular technique for characterizing azimuthal anisotropy in the upper mantle the increasing availability of data sets from dense broad band seismic arrays has driven interest in the development of techniques for the tomographic inversion of shear wave splitting data and in comparing splitting measurements with anisotropic upper mantle models obtained from other constraints such as surface wave analysis two different theoretical approaches have been developed for predicting apparent shear wave splitting parameters fast direction and delay time for models that include multiple layers of anisotropy at depth which is useful for comparing azimuthally anisotropic surface wave models with shear wave splitting measurements these approaches differ in one key aspect which is whether or not the shear wave splitting operator can be treated as commutative in this paper we investigate the theoretical source of this discrepancy and show that at frequencies relevant to most studies of upper mantle anisotropy the term that results in the non commutivity of the shear wave splitting operator in the expressions for multiple layer splitting must be retained in contrast the quantity known as the splitting intensity which is closely related to the apparent fast direction and delay time does commute at these frequencies we illustrate these inferences with forward modelling examples and discuss their implications for the tomographic inversion of shear wave splitting measurements the comparison of surface wave models with shear wave splitting observations and the joint inversion of surface wave and shear wave splitting observations for upper mantle anisotropic models

the role of co2 decline for the onset of northern hemisphere glaciation the pliocene pleistocene transition ppt from around 3 2 to 2 5 million years ago ma represented a major shift in the climate system and was characterized by a gradual cooling trend and the appearance of large continental ice sheets over northern eurasia and north america paleo evidence indicates that the ppt was accompanied and possibly caused by a decrease in atmospheric co2 but the temporal resolution of co2 reconstructions is low for this period of time and uncertainties remain large therefore instead of applying existent co2 reconstructions we solved an inverse problem by finding a schematic co2 concentration scenario that allows us to simulate the temporal evolution of key climate characteristics in agreement with paleoclimate records to this end we performed an ensemble of transient simulations with an earth system model of intermediate complexity from which we derived a best guess transient co2 scenario for the interval from 3 2 to 2 4 ma that gives the best fit between the simulated and reconstructed benthic 18o and global sea surface temperature evolution our data constrained co2 scenarios are consistent with recent co2 reconstructions and suggest a gradual co2 decline from 375 425 to 275 300 ppm between 3 2 and 2 4 ma in addition to a gradual decline the best fit to paleoclimate data requires the existence of pronounced co2 variability coherent with the 41 kyr 1 kyr 1000 years obliquity cycle in our simulations the long term co2 decline is accompanied by a relatively abrupt intensification of northern hemisphere glaciation at around 2 7 ma this is the result of a threshold behaviour of the ice sheets response to gradual co2 decrease and orbital forcing the simulated northern hemisphere ice sheets during the early pleistocene glacial cycles reach a maximum volume equivalent to a sea level drop of about 40 m both ice volume and benthic 18o are dominated by 41 kyr cyclicity our simulations suggest that before 2 7 ma greenland was ice free during summer insolation maxima and only partly ice covered during periods of minimum summer insolation a fully glaciated greenland comparable to its present day ice volume is modelled only during glacial maxima after 2 7 ma and more continuously after 2 5 ma

seismic anisotropy above and below the subducting nazca lithosphere in southern south america 1 the goal of this study is to better constrain anisotropy and mantle flow above and below the nazca slab from 28 s to 42 s through modeling of shear wave splitting in local s sks and skks sk k s phases comparisons of local s splitting times and path lengths in the slab mantle wedge and upper plate indicate that splitting times for arc and back arc stations are consistent with anisotropy in the mantle wedge but long slab paths to fore arc stations imply that slab anisotropy is also significant sk k s shear wave splitting observations and models for sub slab anisotropy show that significant anisotropy is present below the slab and that the orientation of sub slab anisotropy sometimes differs from anisotropy above the slab anisotropy both above the slab and below the slab in the south american subduction zone is consistent with mantle flow that is driven by a combination of entrainment with downgoing slab motion and flow complexity related to variations in slab shape and slab rollback

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

the initiation of northern hemisphere glaciation for well over a century scientists have speculated that variations in the atmospheric concentration of radiatively important trace gases such as carbon dioxide could control the earth s climate e g arrhenius 1 896 recent observations of significant glacial interglacial variation in atmo spheric co2 in ice cores put this idea on a firm observational as well as theoretical footing e g barnola et ai1 987 in this century as atmospheric co2 levels creep steadily upwards in response to the burning of fossil fuels and forests the specter of manmade global climate change looms before us ipcc 1990 a possibility that has heightened interest in the natural variability of climate in the past in particular the 3 c warming pre dicted for the next century has stimulated interest in past warm climates such as the middle pliocene around 3 0 million years ma ago an interval that has often been invoked as a greenhouse world budyko et al 1985 in this paper the climate transition from the warm mid pliocene around 3 2 ma to the onset of northern hemisphere ice ages around 2 4 ma is examined evidence for the initiation of significant northern hemisphere glaciation is examined as well as how this event affected climate around the globe while the cause of individual glacial interglacial oscillations is tied to milankovitch variations in the earth s orbit around the sun e g imbrie et ai 1 992 these insolation changes cannot account for the long term cooling trend which culminated in northern hemisphere glaciation in the final section of this paper mechanisms of long term climate change

numerical modelling of the upper mantle anisotropy beneath a migrating strike slip plate boundary the san andreas fault system we performed forward modelling of seismic anisotropy beneath a migrating strike slip plate boundary to 1 test if such a geodynamic context might explain teleseismic shear wave splitting data in the vicinity of the central part of the san andreas fault and 2 constrain the power of such data to unravel vertical and lateral variations in deformation patterns in the upper mantle the modelling involves five steps 1 thermomechanical modelling using a finite element code of the deformation field 2 viscoplastic self consistent modelling of the resulting olivine and pyroxene crystal preferred orientations 3 calculation of the elasticity tensors for different domains of the finite elements fes model 4 forward modelling of seismic wave propagation through the model using ray theory finite frequency theory and a full wave approach and 5 performing splitting measurements on the synthetic seismograms sks splitting data in central california are best fitted by a model with a hotter geotherm within 60 km of the plate boundary accounting for the opening of an asthenospheric window due to the northward migration of the mendocino triple junction the westward motion of the plate boundary cannot however explain the rotation of fast polarization directions east of the san andreas fault in central california comparison between modelled and measured individual shear wave splitting also implies that the homogeneity of the two layer models accounting for the observations in the vicinity of the san andreas fault indicates a sharp transition between lithospheric and asthenospheric deformations beneath this plate boundary the ability of different synthetic approaches to localize horizontally and vertically the plate boundary related deformation differs significantly splitting in ray theory synthetics closely follow variations in olivine crystallographic preferred orientations in the model in contrast splitting analysis on full wave synthetics which should be more representative of actual long period sks waves results in smooth apparent lateral variations of anisotropy the exact location and width of the plate boundary may only be retrieved by comparing fast polarization profiles obtained using a multichannel analysis on waves with different periods

evolution of cenozoic seaways in the circum antarctic region abstract a complete circum antarctic seaway did not open until both the south tasman rise cleared the oates land coast of east antarctica and drake passage opened between the southern tip of south america and the northern end of the antarctic peninsula major plate motions based on dated seafloor spreading anomalies and distinct fracture zone lineations constrain the age of the opening of a seaway between the south tasman rise and antarctica as very close to the eocene oligocene boundary with an unrestricted opening deeper than 2000 m dating from 32 ma timing of the opening of drake passage is more circumstantial because the exact motions of certain micro continental fragments are not known the motion of africa with respect to south america as well as the motion of east antarctica with respect to africa are well constrained for the cenozoic these major plate motions are used with the reasonable assumption of no cenozoic motion of the antarctic peninsula with respect to east antarctica to constrain the location of the antarctic peninsula with respect to the southern tip of south america for the critical period of late eocene to late oligocene uncertainty of motion of the south georgia and south orkney microcontinents and other possible continental fragments make an exact time for opening of drake passage difficult to ascertain even so the early oligocene position of the antarctic peninsula with respect to south america requires a through going deep water seaway to have been open at drake passage prior to 28 ma even given the unconstrained motion of various high standing crustal fragments in the scotia sea with reasonable assumptions concerning motion of the crustal fragments in the western and central scotia sea it is likely that drake passage or passage through powell basin was open to deep water circulation by 31 2 ma

global patterns of azimuthal anisotropy and deformations in the continental mantle summary we present a summary of measurements of azimuthal anisotropy in the continental mantle based on the sks technique and performed mostly with the active participation of the authors the directions of polarization of the fast quasi shear wave and the time delays between the quasi shear waves are obtained at nearly 70 locations in all continents except antarctica these data are interpreted in terms of lattice preferred orientation of olivine which is caused by deformations in the mantle the depth interval responsible for anisotropy is unknown but the data suggest that it may reach at least 300 km the fast directions in sks do not show clear correlation with the fast directions of the teleseismic p at the same seismograph stations in the regions of present day convergence the fast direction of anisotropy usually aligns with the plate boundary this correlation implies that the direction of shortening is the same in the crust and the upper mantle in the regions of rifting the inferred direction of mantle flow usually aligns with the direction of extension in the crust outside the regions of recent tectonic activity we most likely observe a combined effect of frozen anisotropy in the subcrustal lithosphere and of recently formed anisotropy in the asthenosphere on a global scale in these regions there is a positive correlation between the absolute plate velocity directions and the fast directions of anisotropy the correlation is especially strong in central and eastern parts of north america a clear absence of any evidence of large scale azimuthal anisotropy in the data of long range refraction profiling for the upper 100 km of the mantle of that region implies that the effect in sks is generated mainly at greater depths in the asthenosphere orientation of olivine at these depths reflects recent and present day flow in the mantle rather than processes of a distant geologic past

shear wave splitting as a diagnostic of variable anisotropic structure of the upper mantle beneath central fennoscandia abstract we analyze splitting of shear waves recorded during the svekalapko passive seismic experiment in south central finland to study fabrics of the mantle lithosphere of the precambrian region and thus to bring information into a debate on existence of plate tectonics or its forms in the early stage of continent formation geographical variations of the splitting parameters and their distinct dependence on direction of wave propagation through the upper mantle allow us to identify six domains of the central fennoscandian mantle lithosphere including the proterozoic archean transition and to model their fabrics by joint inversion of body wave anisotropic parameters fabrics of the archean mantle lithosphere can be approximated by a peridotite aggregate with lineation a dipping to the ne on the other hand anisotropy of the proterozoic mantle lithosphere is weaker and we model its fabric by the a c foliations dipping to the se we present a 3d self consistent anisotropic model of the proterozoic and archean upper mantle along the sw ne profile in the south central finland boundaries of inter growing wedges of the proterozoic and archean mantle lithospheres explain the longitudinal and shear wave propagation and polarization mantle xenolith ages surface wave tomography and location of the upper mantle reflectors we interpret the six anisotropic domains as fragments of mantle lithosphere retaining an old fossil olivine fabric which was created before these micro continents assembled

teleseismic shear wave splitting and lithospheric anisotropy beneath and across the altyn tagh fault sixteen 3 component 5 sec stations along a profile crossing the altyn tagh fault atf at 90 e were used to measure shear wave splitting beneath tibet s northern edge splitting delay times t of 0 3 to 1 3 sec show a marked increase in a narrow zone centered on the fault implying anisotropy in the lithospheric mantle fast polarization directions fpd swing counterclockwise from a mean azimuth of 110 e in the qaidam basin south of the fault to a mean azimuth of 60 e in the altyn mountains this swing occurs in less than 40 km both the t increase and fpd swing are consistent with ductile sinistral shear along the 40 km wide low p wave velocity anomaly 6 8 imaged to a depth of 140 km beneath the fault by teleseismic tomography this confirms that the atf guides the northeastwards extrusion of the tibet qaidam s lithosphere relative to the tarim s

pleistocene deglaciation and the earth s rotation a new analysis summary this paper is concerned with an analysis of the effect of the pleistocene glacial cycle upon the earth s rotation we demonstrate that two important geophysical observables may be explained as aspects of the rotational response to surface mass loading by ice sheets these are the astronomically observed non tidal component of the acceleration of planetary rotation and the secular drift of the rotation pole relative to the surface geography which is evident in the ils pole path the former observation is shown to provide an unambiguous constraint upon the viscosity of the planetary mantle and requires a depth dependence of this parameter which is the same as that which has previously been inferred in studies of the relative sea level variations and free air gravity anomalies associated with post glacial rebound the observed secular drift of the pole cannot provide an independent estimate of the viscosity of the mantle because it depends jointly upon mantle viscosity and lithospheric thickness with the viscosity profile fixed by the rebound data the observed speed of polar wander suggests a continental lithospheric thickness which is in excess of that appropriate for old ocean basins but nevertheless in accord with independent constraints

true polar wander during the middle paleozoic true polar wander would be recognized paleomagnetically as identical apparent polar wander paths for all surface elements the apparent polar wander paths for the late ordovician late devonian interval for laurentia baltica and gondwana have nearly identical looping shapes that can be brought into superposition the paths of these continents are well documented but even the less well known paths of south china and siberia reveal similar lengths the resulting reconstruction places the northern andean margin of south america opposite the appalachian margin of laurentia with baltica and laurentia adjoined in the fit of bullard and colleagues siberia and south china would be to the north of africa if their paleopoles are taken at face value for middle paleozoic time there is of course no information about oceanic domains butnit is interesting that all continental elements insofar as is known appear to have similar apparent polar wander tracks that can be plausibly superimposed without causing overlap in the positions of the continents albeit speculatively because of the lack of information about the oceanic elements it is suggested in this study that true polar wander may have occurred with a cumulative magnitude of about 75 during a 75 ma interval and may have been of greater magnitude than the apparent polar wander due to relative motions during the middle paleozoic this middle paleozoic rate of true polar wander appears to have been an order of magnitude greater than the average rate during the late mesozoic and tertiary

the use of direct shear waves in quantifying seismic anisotropy exploiting regional arrays abstract to overcome the potential contamination of the direct s waves by source side anisotropy in shear wave splitting analysis we describe a new approach that we call the reference station technique the technique utilizes direct shear waves recorded at a station pair and depends on maximizing the correlation between the seismic traces at reference and target stations after correcting the reference station for known receiver side anisotropy and the target stations for arbitrary splitting parameters probed via a grid search the algorithm also provides a delay time between both stations caused for example by isotropic heterogeneities synthetic tests demonstrate the stability of the estimated parameters even where variability in near surface properties thickness and velocity of sediment layer exists we applied the reference station technique to data from seismic experiments at the northern margin of tibet average splitting parameters obtained from the analysis of direct s wave results are consistent with those obtained from previous sks splitting measurements where differences exist shear wave fast polarization estimates resolved from direct s indicate a higher degree of internal consistency for closely spaced stations than those derived from sks this is probably due to the much larger number of direct s waves available for splitting measurements compared to sks for the same observational period resulting in higher quality measurements we also demonstrate the ability of the technique to provide improved splitting measurements for temporary stations by following a bootstrap approach in which only a few stations with well constrained sks splitting parameters are used as seeds to determine the splitting parameters of a large array in an iterative manner in addition the s measurements sample the anisotropic layer with different angles of incidence and back azimuths thus potentially providing additional constraints on more complicated anisotropic structures and the interstation delay times could be used for tomographic studies to reduce the bias from anisotropic structure online material multisplit software package c with instructions

mantle dynamics and seismic anisotropy abstract observations of seismic anisotropy yield some of the most direct constraints available on both past and present day deformation in the earth s mantle insight into the character of mantle flow can also be gained from the geodynamical modeling of mantle processes on both global and regional scales we highlight recent progress toward understanding mantle flow from both observations and modeling and discuss outstanding problems and avenues for progress particularly in the integration of seismological and geodynamical constraints to understand seismic anisotropy and the deformation that produces it to first order the predictions of upper mantle anisotropy made by global mantle circulation models match seismological observations well beneath the ocean basins but the fit is poorer in regions of greater tectonic complexity such as beneath continental interiors and within subduction systems in many regions of the upper mantle models of anisotropy derived from surface waves are seemingly inconsistent with shear wave splitting observations which suggests that our understanding of complex anisotropic regions remains incomplete observations of anisotropy in the d layer hold promise for improving our understanding of dynamic processes in the deep earth but much progress remains to be made in characterizing anisotropic structure and relating it to the geometry of flow geochemical heterogeneity or phase transitions major outstanding problems related to understanding mantle anisotropy remain particularly regarding the deformation and evolution of continents the nature of the asthenosphere subduction zone geodynamics and the thermo chemical state of the lowermost mantle however we expect that new seismological deployments and closer integration of observations with geodynamical models will yield rapid progress in these areas

oceanographic and climatic evolution of the miocene mediterranean deduced from nd sr c and o isotope compositions of marine fossils and sediments isotopic compositions of marine sediments and fossils have been investigated from northern basins of the mediterranean to help constrain local oceanographic and climatic changes adjacent to the uplifting alps stable c and o isotope compositions of benthic and planktonic foraminifera from the umbria marche region umc have an offset characteristic for their habitats and the changes in composition mimic global changes suggesting that the regional conditions of climate and the carbon cycle were controlled by global changes the radiogenic isotope composition of these fossil assemblages allows recognition of three distinct periods in the first period from 25 to 19 ma high nd values and low 87sr 86sr of sediments and fossils support intense tectonism and volcanism related to the opening of the western mediterranean in the second period from 19 to 13 ma the 87sr 86sr ratio of mediterranean umc deviate from the global ocean which is compatible with rapid uplift of the hinterland and intense influx of sr from mesozoic carbonates of the western apennines this local control on the seawater was driven by a humid and warm climate and indicates restricted exchange of water with the global ocean generally the nd values of the fossils are very similar to those of indian ocean water with brief periods of a decrease in the nd values coinciding with volcanic events and maybe sea level variation at 15 2 ma in the third period from 13 to 10 ma the fossils have 87sr 86sr similar to those of miocene seawater while their nd values change considerably with time this indicates fluctuating influence of the atlantic versus the paratethys and or locally evolved seawater in the mediterranean driven by global sea level changes other investigated localities near the alps and from the odp 900 site are compatible with this oceanographic interpretation however in the late early miocene enhanced local control reflecting erosion of old crustal silicate rocks near the alps results in higher 87sr 86sr

changes of the earth s rotation axis owing to advection of mantle density heterogeneities polar wander the secular motion of the earth s rotation axis relative to its surface has been studied for many years dynamical arguments1 3 show that polar wander can arise from the redistribution of mass in a plastic deformable earth the rate depending on both the rate of mass redistribution and the rate at which the earth s rotational bulge can readjust to the changing rotation axis here we use a viscosity structure obtained through geoid modelling4 a mantle flow field consistent with tomographic anomalies5 and time dependent lithospheric plate motions6 to calculate the advection of mantle density heterogeneities and corresponding changes in the degree two geoid during the cenozoic era we show that the rotation axis will follow closely any imposed changes of the axis of maximum non hydrostatic moment of inertia the resulting path of the rotation axis agrees well with palaeomagnetic results7 with the model predicting a current rate of polar motion that explains 40 of that observed geodetically8

explanations of the tertiary global cooling trend abstract a series of general circulation model experiments are performed to examine the role of paleogeography as an explanation of the tertiary global cooling trend systematic long term variations in topography and land sea distribution are potential causes of long term climate change however climate model sensitivity experiments indicate a geographically related cooling during the last 20 million years but not a long term cenozoic trend coarse resolution changes in geography throughout the tertiary may not be the explanation of the tertiary global cooling trend either a series of events some geographically related or another forcing factor most probably atmospheric co2 concentration are offered as alternative explanations of the tertiary global cooling trend

convergent cenozoic co2 history reconstructions of atmospheric carbon dioxide concentrations over the past 65 million years are heading towards consensus it is time for systematic testing of the proxies against measurements and against each other

plio pleistocene climate sensitivity evaluated using high resolution co2 records theory and climate modelling suggest that the sensitivity of earth s climate to changes in radiative forcing could depend on the background climate however palaeoclimate data have thus far been insufficient to provide a conclusive test of this prediction here we present atmospheric carbon dioxide co2 reconstructions based on multi site boron isotope records from the late pliocene epoch 3 3 to 2 3 million years ago we find that earth s climate sensitivity to co2 based radiative forcing earth system sensitivity was half as strong during the warm pliocene as during the cold late pleistocene epoch 0 8 to 0 01 million years ago we attribute this difference to the radiative impacts of continental ice volume changes the ice albedo feedback during the late pleistocene because equilibrium climate sensitivity is identical for the two intervals when we account for such impacts using sea level reconstructions we conclude that on a global scale no unexpected climate feedbacks operated during the warm pliocene and that predictions of equilibrium climate sensitivity excluding long term ice albedo feedbacks for our pliocene like future with co2 levels up to maximum pliocene levels of 450 parts per million are well described by the currently accepted range of an increase of 1 5 k to 4 5 k per doubling of co2

seismic anisotropy beneath the continents probing the depths of geology seismic anisotropy beneath continents is analyzed from shear wave splitting recorded at more than 300 continental seismic stations anisotropy is found to be a ubiquitous property that is due to mantle deformation from past and present orogenic activity the observed coherence with crustal deformation implies that the mantle plays a major if not dominant role in orogenies no evidence is found for a continental asthenospheric decoupling zone suggesting that continents are coupled to general mantle circulation

effect of plate thickness on bending radius and energy dissipation at the subduction zone hinge 1 despite the thickness of subducting oceanic plates being identified as a control on slab and hinge behavior during subduction there have been few attempts to quantify its effect with fully dynamic models this paper presents a series of dynamic laboratory experiments of progressive subduction of a narrow plate into an upper mantle reservoir in which plate thickness is systematically varied from 4 mm scaling to 20 km up to 24 mm 120 km the experiments investigate its effect on bending radius subduction kinematics and the rate of energy dissipated at the hinge as a percentage of the total potential energy released through subduction the bending radius of the hinge is found to have a strong linear relationship with plate thickness where an increase in plate thickness corresponds to an increase in bending radius subduction velocity increases with plate thickness due to the increased negative buoyancy of thicker slabs which is not compensated for by any significant increase in mantle drag on the slab this increase is almost entirely accommodated by an increase in trench retreat rate with a fivefold increase over the whole thickness range while subducting plate velocity remains relatively constant the energy dissipation ratio be bu is low across all plate thicknesses with the plates falling within a range of 4 10 during steady state subduction the models show a moderate trend toward decreasing be bu with increasing plate thickness

the influence of true polar wander on glacial inception in north america abstract the impact that long term changes in earth s rotation axis relative to the surface geography or true polar wander tpw and continental drift have had in driving cooling of high latitude north america since the eocene is explored recent reanalyses of paleomagnetic pole positions suggest a secular drift in earth s rotation axis of 8 over the last 40 myr in a direction that has brought north america to increasingly higher latitudes using modern temperature data in tandem with a simple model a reduction in the annual sum of positive degree days pdds driven by this polar and plate motion over the last 20 myr is quantified at sites in baffin island the tpw and continental drift driven decrease in insolation forcing over the last 20 myr rivals changes in insolation forcing caused by variations in earth s obliquity and precession using conservative pdd scaling factors and an annual snowfall equal to modern station observations the snowiest location in baffin island 20 myr ago had a mass balance deficit of 0 75 2 m yr 1 water equivalent thickness relative to its projected mass balance at 2 7 ma this mass balance deficit would have continued to increase as one goes back in time until 40 myr ago based on adopted paleopole locations tpw and continental drift that moved arctic north america poleward would have strongly promoted glacial inception in baffin island at 3 ma a location where the proto laurentide ice sheet is thought to have originated

on the structure and origin of major glaciation cycles 2 the 100 000 year cycle climate over the past million years has been dominated by glaciation cycles with periods near 23 000 41 000 and 100 000 years in a linear version of the milankovitch theory the two shorter cycles can be explained as responses to insolation cycles driven by precession and obliquity but the 100 000 year radiation cycle arising from eccentricity variation is much too small in amplitude and too late in phase to produce the corresponding climate cycle by direct forcing we present phase observations showing that the geographic progression of local responses over the 100 000 year cycle is similar to the progression in the other two cycles implying that a similar set of internal climatic mechanisms operates in all three but the phase sequence in the 100 000 year cycle requires a source of climatic inertia having a time constant similar to 15 000 years much larger than the other cycles similar to 5 000 years our conceptual model identifies massive northern hemisphere ice sheets as this larger inertial source when these ice sheets forced by precession and obliquity exceed a critical size they cease responding as linear milankovitch slaves and drive atmospheric and oceanic responses that mimic the externally forced responses in our model the coupled system acts as a nonlinear amplifier that is particularly sensitive to eccentricity driven modulations in the 23 000 year sea level cycle during an interval when sea level is forced upward from a major low stand by a milankovitch response acting either alone or in combination with an internally driven higher frequency process ice sheets grounded on continental shelves become unstable mass wasting accelerates and the resulting deglaciation sets the phase of one wave in the train of 100 000 year oscillations whether a glacier or ice sheet influences the climate depends very much on the scale the interesting aspect is that an effect on the local climate can still make an ice mass grow larger and larger thereby gradually increasing its radius of influence

early pleistocene glacial cycles and the integrated summer insolation forcing long term variations in northern hemisphere summer insolation are generally thought to control glaciation but the intensity of summer insolation is primarily controlled by 20 000 year cycles in the precession of the equinoxes whereas early pleistocene glacial cycles occur at 40 000 year intervals matching the period of changes in earth s obliquity the resolution of this 40 000 year problem is that glaciers are sensitive to insolation integrated over the duration of the summer the integrated summer insolation is primarily controlled by obliquity and not precession because by kepler s second law the duration of the summer is inversely proportional to earth s distance from the sun

anisotropy and p wave tomography a new approach for inverting teleseismic data from a dense array of stations summary a new formulation is given for inverting teleseismic p wave residuals in terms of both 3 d variations of seismic velocity and anisotropy of the subcrustal lithosphere two restrictive assumptions are made on the anisotropic elastic tensor to reduce the number of model parameters first the upper mantle anisotropy is modelled by a preferred orientation of olivine crystals with b and c axes randomly distributed around the a axis and second the elastic properties of polycrystalline aggregates are computed according to the voigt averaging rule the latter assumption allows us to link linearly the observed p wave residuals to the proportion of oriented crystals in a realistic polycrystalline assemblage in the inversion process the orientation of the symmetry axis is fixed either by a trial procedure or by using a priori hypotheses from independent data a linear inversion scheme is then applied to the data to retrieve the 3 d variations of two scalar parameters giving the isotropic velocity perturbation and the amount of anisotropy the inversion algorithm is tested for different source receiver configurations using several idealistic ray geometries it is concluded that the teleseismic p wave residuals could potentially carry enough information for retrieving both the 3 d isotropic velocity variations and the amount of anisotropy however from a more practical point of view by looking at the distributions of rays for two recent tomographic experiments in the southern rhine graben area and in the pyrenees it is concluded that the distributions of rays in these experiments do not fulfil the necessary requirements for retrieving reliable information about the lithospheric anisotropy additional data are thus needed to constrain the anisotropic model if the symmetry axis is set as horizontal the inversion of the p wave residual of the pyrenees leads to an east west fast p wave velocity orientation the resulting anisotropy is compatible with sks observations made in the pyrenees whatever the anisotropy is the numerical tests performed with the actual ray configurations show the robustness of the isotropic components in the inverted model a long wavelength anisotropy in the lithosphere if present should not drastically alter the isotropic 3 d velocity distributions that are obtained from classical methods of teleseismic tomography

paleolatitude of the hawaiian hot spot since 48 ma evidence for a mid cenozoic true polar stillstand followed by late cenozoic true polar wander coincident with northern hemisphere glaciation

sensitivity of mass balance of five swiss glaciers to temperature changes assessed by tuning a degree day model a degree day model is used to assess the sensitivity of the mass balance of five swiss glaciers to temperature changes the model uses temperature data extrapolated from nearby climate stations and is tuned by varying precipitation to make the model fit the observed distribution of mass balance with altitude once the model is tuned the effect of temperature change is simulated by recalculating the mass balance with the same parameters as before but with a temperature increase of 1 c throughout the year the largest mass balance changes involving increased ablation of 1 m w e a 1 c 1 occur at the snout with a progressively smaller increase with altitude the area averaged sensitivities for the five glaciers are 0 7 to 0 9 m w e a 1 c 1 if annual precipitation also increased by 20 it would partly offset the effect of the 1 c higher temperatures but could not compensate for it