frechet kernels for imaging regional earth structure based on three dimensional reference models high resolution images of three dimensional 3d seismicstructuresare not only of scientific interest but also of practical importance in predicting strong ground motion after large earthquakes given the source and station distributions resolutions in current regional seismic tomography studies have been limited by two types of simplifying practices the adoption of high frequency approximations such as the ray theory and the use of one dimensional 1d reference starting models we have developed a new approach to compute accurate finite frequency 3d frechet sensitivity kernels of observed travel time and amplitude anomalies relative to 3d reference models in our approach we use a fourth order staggered grid finite difference method to model the seismic wave propagation in 3d media and the reciprocity property of the green s tensor to reduce the number of numerical simu lations this approach accounts for the perturbations in compressional and shear wave speeds in the same way leading to a capability of inverting for the shear wave speed directly from seismic data the algorithm is readily parallelized to allow for realistic regional high resolution 3d tomography inversions we have implemented the algorithm for the southern california earthquake center scec community velocity model scec cvm 3 0 a complex 3d model for southern california in cluding a number of sedimentary basins by enabling the inversion of 3d structural perturbations to 3d reference models our approach provides a practical means of iteratively solving the nonlinear regional tomography problems

climate records covering the last deglaciation the oxygen 18 oxygen 16 ratio of molecular oxygen trapped in ice cores provides a time stratigraphic marker for transferring the absolute chronology for the greenland ice sheet project gisp ii ice core to the vostok and byrd ice cores in antarctica comparison of the climate records from these cores suggests that near the beginning of the last deglaciation warming in antarctica began approximately 3000 years before the onset of the warm bolling period in greenland atmospheric carbon dioxide and methane concentrations began to rise 2000 to 3000 years before the warming began in greenland and must have contributed to deglaciation and warming of temperate and boreal regions in the northern hemisphere

isotope calibrated greenland temperature record over marine isotope stage 3 and its relation to ch4 large temperature variations on millennial time scales in greenland characterised the last ice age abrupt warmings known as dansgaard oeschger do events can be traced in the 18oice record of greenland ice cores however it has been shown that 18oice is not a direct temperature proxy measurements of the isotopic composition of gases trapped in the ice can be used to calibrate the paleothermometer here we present a continuous temperature record based on high resolution 15n measurements and firn model studies it covers a sequence of 9 do events 9 17 during the time period from 38 to 64 kyr bp for which temperature changes of 8 to 15 c were estimated the difference between the modern and the glacial 18oice t relationship can be explained by a combination of source temperature changes and changes in the annual distribution of precipitation a detailed comparison of the temperature evolution with reconstructions of the atmospheric methane ch4 concentration shows that ch4 rises lag temperature increases at the onset of do events by 25 to 70 yr within data resolution the strong correlation between greenland temperature and ch4 on millennial and submillennial time scales suggests that variations on these time scales were probably of hemispheric extent

δ15n of n2 in air trapped in polar ice a tracer of gas transport in the firn and a possible constraint on ice age gas age differences with respect to gas transport the fim near the surface of an ice sheet can be divided into three zones the uppermost is a convective zone located just below the surface of the ice sheet in which the air is rapidly flushed by convective exchange with the overlying atmosphere below the convective zone there is a diffusive air column in which diffusion is rapid but there is no convection between the bottom of the diffusive air column and the bubble close off region there may be a nondiffusive zone in which diffusion is so slow that negligible gas transport occurs the diffusive air column is characterized by progressive enrichment with depth of 15n in n2 and heavy isotopes of gases in general as predicted using the barometric equation in this paper we present data on the 15n of n2 in recently trapped air samples from 12 ice cores along with numerous downcore samples from byrd vostok and dome c bubble close off depths for these cores calculated from a densification model ranged from 51 to 114 meters below the surface mbs we used these data and the barometric equation to calculate the thickness of the diffusive air column and found that it comprised 46 to 93 of the total firn thickness at our study sites paleo close off depths calculated from the densification model for glacial sections of byrd vostok and dome c are 15 25 m deeper than close off depths today diffusive column heights calculated from 15n varied in a more complex manner the diffusive column height at byrd appears to have decreased from 74 m during the last glacial period to 50 m during the holocene at vostok and dome c the diffusive column height calculated from 15n increase from about 65 m during the last glacial period to about 80 m in the holocene we use records of surface temperature and co2 at byrd and vostok along with their respective chronologies to constrain the ice age gas age difference age throughout the section of the vostok ice core corresponding to the last glacial termination in principle age values calculated from these data can be used to discriminate whether gas in the fim mixes to the bubble close off depth or to a depth equivalent to the diffusive column height in practice however uncertainties in the chronology of byrd and vostok are too great to allow us to distinguish between these two possibilities one can only say that at the time of the last termination age for vostok was between 3 and 10 5 kyr previous estimates fall within this range

research note insights into the data dependency on anisotropy an inversion prospective while velocity contrasts are responsible for most of the events recorded in our data the long wavelength behavior of the velocity model is responsible for the geometrical shape of these events for isotropic acoustic materials the wave dependency on the long wave propagation and short scattering wavelength velocity components is stationary with the propagation angle on the other hand in representing a transversely isotropic with a vertical symmetry axis medium with the normal moveout velocity the anellepticity parameter the vertical scaling parameter and the sensitivity of waves vary with the polar angle for both the long and short wavelength features of the anisotropic dimensionless medium parameters and for horizontal reflectors at reasonable depths the long wavelength features of the model is reasonably constrained by the long offsets whereas the short wavelength features produce very week reflections at even reasonable offsets thus for surface acquired seismic data we could mainly invert for smooth responsible for the geometrical shape of reflections on the other hand while the long wavelength components mildly affects the recorded data its short wavelength variations can produce reflections at even zero offset with a behavior pattern synonymous to density the lack of the long wavelength information will mildly effect focusing but will cause misplacement of events in depth with low enough frequencies very low we may be able to recover the long wavelength using full waveform inversion however unlike velocity the frequencies needed for that should be ultra low to produce long wavelength scattering based model information as perturbations do not exert scattering at large offsets for a combination given by the horizontal velocity and the diving wave influence of is absorbed by the horizontal velocity severely limiting the influence on the data and full waveform inversion as a result with a good smooth estimation for example from tomography we can focus the full waveform inversion to invert for only the horizontal velocity and maybe as a parameter to fit the amplitude this is possibly the most practical parametrization for inversion of surface seismic data in transversely isotropic with vertical symmetry axis media

climate correlations between greenland and antarctica during the past 100 000 years the ice cores recovered from central greenland by the grip1 2 and gisp23 projects record 22 interstadial warm events during the part of the last glaciation spanning 20 105 kyr before present the ice core from vostok east antarctica records nine interstadials during this period4 5 here we explore links between greenland and antarctic climate during the last glaciation using a high resolution chronology derived by correlating oxygen isotope data for trapped o2 in the gisp2 and vostok cores we find that interstadials occurred in east antarctica whenever those in greenland lasted longer than 2 000 years our results suggest that partial deglaciation and changes in ocean circulation are partly responsible for the climate teleconnection between greenland and antarctica ice older than 115 kyr in the gisp2 core shows rapid variations in the 18o of o2 that have no counterpart in the vostok record the age depth relationship and thus the climate record in this part of the gisp2 core appears to be significantly disturbed

eight glacial cycles from an antarctic ice core the antarctic vostok ice core provided compelling evidence of the nature of climate and of climate feedbacks over the past 420 000 years marine records suggest that the amplitude of climate variability was smaller before that time but such records are often poorly resolved moreover it is not possible to infer the abundance of greenhouse gases in the atmosphere from marine records here we report the recovery of a deep ice core from dome c antarctica that provides a climate record for the past 740 000 years for the four most recent glacial cycles the data agree well with the record from vostok the earlier period between 740 000 and 430 000 years ago was characterized by less pronounced warmth in interglacial periods in antarctica but a higher proportion of each cycle was spent in the warm mode the transition from glacial to interglacial conditions about 430 000 years ago termination v resembles the transition into the present interglacial period in terms of the magnitude of change in temperatures and greenhouse gases but there are significant differences in the patterns of change the interglacial stage following termination v was exceptionally long 28 000 years compared to for example the 12 000 years recorded so far in the present interglacial period given the similarities between this earlier warm period and today our results may imply that without human intervention a climate similar to the present one would extend well into the future

wave equation migration velocity analysis i theory we present a migration velocity analysis mva method based on wavefield extrapolation similarly to conventional mva our method aims at iteratively improving the quality of the migrated image as measured by the flatness of angle domain common image gathers adcigs over the aperture angle axis however instead of inverting the depth errors measured in adcigs using ray based tomography we invert image perturbations using a linearized wave equation operator this operator relates perturbations of the migrated image to perturbations of the migration velocity we use prestack stolt residual migration to define the image perturbations that maximize the focusing and flatness of adcigs our linearized operator relates slowness perturbations to image perturbations based on a truncation of the born scattering series to the first order term to avoid divergence of the inversion procedure when the velocity perturbations are too large for born linearization of the wave equation we do not invert directly the image perturbations obtained by residual migration but a linearized version of the image perturbations the linearized image perturbations are computed by a linearized prestack residual migration operator applied to the background image we use numerical examples to illustrate how the backprojection of the linearized image perturbations i e the gradient of our objective function is well behaved even in cases when backprojection of the original image perturbations would mislead the inversion and take it in the wrong direction we demonstrate with simple synthetic examples that our method converges even when the initial velocity model is far from correct in a companion paper we illustrate the full potential of our method for estimating velocity anomalies under complex salt bodies

mantle circulation models with variational data assimilation inferring past mantle flow and structure from plate motion histories and seismic tomography summary mantle convection models require an initial condition some time in the past because this initial condition is unknown for earth we cannot infer the geological evolution of mantle flow from forward mantle convection calculations even for the most recent mesozoic and cenozoic geological history of our planet here we introduce a fluid dynamic inverse problem to constrain unknown mantle flow back in time from seismic tomographic observations of the mantle and reconstructions of past plate motions using variational data assimilation we derive the generalized inverse of mantle convection and explore the initial condition problem in high resolution 3 d spherical mantle circulation models for a time period of 100 myr roughly comparable to half a mantle overturn we present a synthetic modelling experiment to demonstrate that mid cretaceous mantle structure can be inferred accurately from fluid dynamic inverse modelling assuming present day mantle structure is well known even if an initial first guess assumption about the mid cretaceous mantle involved only a simple 1 d radial temperature profile we also demonstrate that convecting present day mantle structure back in time by reversing the time stepping of the energy equation is insufficient to model the mantle structure of the past the difficulty arises because such backward convection calculations ignore thermal diffusion effects and therefore cannot account for the generation of thermal buoyancy in boundary layers as we go back in time inverse mantle convection modelling should make it possible to infer a number of flow parameters from observational constraints of the mantle

measurements of isotope and elemental ratios of air from polar ice with a new on line extraction method 1 a recently developed continuous on line extraction and analyzing technique for air trapped in ice cores has been improved and tested the technique allows fast high resolution measurements of the isotopic and elemental ratios of the main air components nitrogen oxygen and argon continuous measurements of up to 1 m long ice sections can be performed with a resolution of 3 cm running average for nitrogen and oxygen isotope ratios the accuracy and reproducibility are comparable to conventional melt extraction techniques it depends on the desired resolution and the composition of the ice for either pure bubble or complete bubble free ice the reproducibility and accuracy for a resolution of 3 cm are 0 02 for 15n 0 05 for 18o and 0 06 for 17o furthermore ar o2 1 o2 n2 ar n2 both 5 8 and 36ar 0 2 0 3 can be determined as well the precision of elemental ratios is good enough to resolve variations that are detected in ice at depths where air bubbles form clathrate hydrates surprisingly at these depths the elemental ratios show roughly annual variations probably due to strong fractionations between air in clathrate hydrates and air in bubbles on the basis of our measuring precision we cannot exclude similar effects for isotope ratios

experimental determination of the diffusion rate of deuterated water vapor in ice and application to the stable isotopes smoothing of ice cores abstract the stable isotope records of d and 18o in ice cores show that the isotopic gradients are smoothed out with time by diffusion transport of hydrogen and oxygen atoms through solid ice is slow whereas water vapor diffusion through the interconnected porosity is much faster smoothing occurs preferentially in the upper layers of firn where the density is lowest and is responsible for the gradual alteration of the isotopic stratigraphy results for the diffusion of water vapor in ice are presented they were obtained from laboratory experiments with diffusion couples prepared using artificial snow with different d h values the samples were allowed to diffuse for about one year at a controlled temperature before being cut into thin sections and analyzed with a mass spectrometer the effects of both temperature and density were investigated the measured diffusion coefficients are fully consistent with water vapor diffusion through the ice porosity the computed values are less than the diffusion coefficient of hdo in free air and imply a tortuosity factor in the range 3 2 6 5 depending on the sample density the influence of the grain sizes on the timescale of the isotopic homogenization between the vapor phase and the ice matrix was studied we show that for grain sizes up to 1 mm in diameter solid diffusion within the grains is not a limiting factor and therefore the isotopic equilibrium between the vapor and the solid phases can be considered as immediate the diffusion model developed to compute the diffusion coefficients was further applied to investigate the real case of isotope smoothing in ice cores the smoothing rate is highly dependent on the wavelength of the isotopic signal linked to the accumulation rate and on temperature finally the model was applied to tritium deposition in antarctica from a continuous record of artificial fallout at the south pole 1954 1978 the result shows that in spite of large gradients the initial distribution underwent only limited smoothing that did not exceed 10

abrupt increase in greenland snow accumulation at the end of the younger dryas event the warming at the end of the last glaciation was characterized by a series of abrupt returns to glacial climate the best known of which is the younger dryas event1 despite much study of the causes of this event and the mechanisms by which it ended many questions remain unresolved1 oxygen isotope data from greenland ice cores2 4 suggest that the younger dryas ended abruptly over a period of about 50 years dust concentrations2 4 in these cores show an even more rapid transition 20 years this extremely short timescale places severe constraints on the mechanisms underlying the transition but dust concentrations can reflect subtle changes in atmospheric circulation which need not be associated with a large change in climate here we present results from a new greenland ice core gisp2 showing that snow accumulation doubled rapidly from the younger dryas event to the subsequent preboreal interval possibly in one to three years we also find that the accumulation rate change from the oldest dryas to the bo11ing allerod warm period was large and abrupt the extreme rapidity of these changes in a variable that directly represents regional climate implies that the events at the end of the last glaciation may have been responses to some kind of threshold or trigger in the north atlantic climate system

orbital tuning chronology for the vostok climate record supported by trapped gas composition abstract we present data on the o2 n2 ratios of trapped gas samples for the entire length of the vostok climate record as in other cores o2 n2 ratios in these samples are less than the atmospheric ratio by a small and variable amount because o2 is selectively excluded during the gas trapping process and because o2 is also preferentially lost in poorly preserved core samples samples younger than 150 ka have large and variable o2 depletions samples older than 200 ka have o2 n2 ratios that replicate well and vary smoothly with depth we plot o2 n2 ratios of well replicated samples older than 160 ka using a chronology derived by matching the 18o of paleoatmospheric o2 18oatm to northern hemisphere june insolation on this timescale o2 n2 varies coherently with local 78 s summertime insolation based on time series analysis of the o2 n2 record and the dynamics of snow metamorphism at the surface we conclude that summertime insolation influences physical properties of ice grains that control the degree of o2 exclusion during bubble closeoff o2 n2 in vostok is thus arguably a property that records local summertime insolation and can be used to test independent chronologies for the core we show that the 18oatm chronology supported by the coincidence of o2 n2 ratios with insolation is also compatible with recent radiometric dating of corals from high sea stands we further successfully test the 18oatm tuning chronology by showing that it predicts a chronology for the gisp2 core which is essentially indistinguishable from the standard gisp2 chronology and therefore in excellent agreement with the radiometric chronology of hulu cave china an accurate chronology for the vostok ice core is now in place

3d coupled acoustic elastic migration with topography and bathymetry based on spectral element and adjoint methods abstractin the context of the adjoint method we considered 3d coupled acoustic elastic migration in the presence of surface topography and or bathymetry isotropic elastic imaging involves three primary kernels related to mass density and shear and bulk moduli and various secondary kernels for example related to p wave impedance and compressional and shear wave speeds similar to reverse time migration these kernels reflect the constructive interference between a forward wavefield generated by active sources and an adjoint wavefield triggered by simultaneously back propagating recorded reflections from all receivers forward and adjoint wavefields were simulated using a spectral element method which due to its weak nature captures free surface topography in land surveys and bathymetry in marine acquisition to avoid storing the entire 3d forward wavefield required for calculating its interaction with the adjoint wavefield we only saved information on domain boundaries and reconstructed the forw

elastic full waveform inversion application using multicomponent measurements of seismic data collection abstractrecent computational improvements allowed us to simulate elastic wavefields in a 3d manner and undertake the challenge of executing elastic full waveform inversion efwi the 3d seg eage overthrust synthetic data were used to run the initial tests which included using all three components for the simulation the inversion targeted two regions the channel system and the overthrusted zone which proved the effectiveness of efwi to delineate geology in terms of vp and vs velocity fields for the field data experiment to demonstrate the technologies we elected to use a gulf of mexico ocean bottom cable data set which allowed us to take advantage of relatively large offsets along with the 4c acquisition the input data were minimally processed mostly through noise removal and the initial model was a gaussian smoothed version of grid tomography output which is done by a prestack migrated gather flattening process during ewfi a multiscale approach was followed to ensure convergence and the earl

oxygen isotope and palaeotemperature records from six greenland ice core stations camp century dye 3 grip gisp2 renland and northgrip oxygen isotope variations spanning the last glacial cycle and the holocene derived from ice core records for six sites in greenland camp century dye 3 grip gisp2 renland and northgrip show strong similarities this suggests that the dominant influence on oxygen isotope variations reflected in the ice sheet records was regional climatic change differences in detail between the records probably reflect the effects of basal deformation in the ice as well as geographical gradients in atmospheric isotope ratios palaeotemperature estimates have been obtained from the records using three approaches i inferences based on the measured relationship between mean annual 18o of snow and of mean annual surface temperature over greenland ii modelled inversion of the borehole temperature profile constrained either by the dated isotopic profile or iii by using monte carlo simulation techniques the third of these approaches was adopted to reconstruct holocene temperature variations for the dye 3 and grip temperature profiles which yields remarkably compatible results a new record of holocene isotope variations obtained from the northgrip ice core matches the grip short term isotope record and also shows similar long term trends to the dye 3 and grip inverted temperature data the northgrip isotope record reflects i a generally stronger isotopic signal than is found in the grip record ii several short lived temperature fluctuations during the first 1500 yr of the holocene iii a marked cold event at ca 8 2 ka the 8 2 ka event iv optimum temperatures for the holocene between ca 8 6 and 4 3 ka a signal that is 0 6 stronger than for the grip profile v a clear signal for the little ice age and vi a clear signal of climate warming during the last century these data suggest that the northgrip stable isotope record responded in a sensitive manner to temperature fluctuations during the holocene copyright 2001 john wiley sons ltd

a 135 000 year vostok specmap common temporal framework the object of the present study is to introduce a means of comparing the vostok and marine chronologies our strategy has been to use the 18o of atmospheric o2 denoted 18oatm from the vostok ice core as a proxy for the 18o of seawater denoted 18osw our underlying premise in using 18oatm as a proxy for 18osw is that past variations in 18osw an indicator of continental ice volume have been transmitted to the atmospheric o2 reservoir by photosynthesizing organisms in the surface waters of the world s oceans we compare our record of 18oatm to the 18osw record which has been developed from studies of the isotopic composition of biogenic calcite 18oforam in deep sea cores we have tied our 18oatm record from vostok to the specmap timescale throughout the last 135 kyr by correlating 18oatm with a 18osw record from v19 30 results of the correlation indicate that 77 of the variance is shared between these two records we observed differences between the 18oatm and the 18osw records during the coldest periods which indicate that there have been subtle changes in the factors which regulate 18oatm other than 18osw our use of 18oatm as a proxy for 18osw must therefore be considered tentative especially during these periods by correlating 18oatm with 18osw we provide a common temporal framework for comparing phase relationships between atmospheric records from ice cores and oceanographic records constructed from deep sea cores our correlated age depth relation for the vostok core should not be considered an absolute vostok timescale we consider it to be the preferred timescale for comparing vostok climate records with marine climate records which have been placed on the specmap timescale we have examined the fidelity of this common temporal framework by comparing sea surface temperature sst records from sediment cores with an antarctic temperature record from the vostok ice core we have demonstrated that when the southern ocean sst and antarctic temperature records are compared on this common temporal framework they show a high degree of similarity we interpret this result as supporting our use of the common temporal framework for comparing other climate records from the vostok ice core with any climate record that has been correlated into the specmap chronology

enclosure of air during metamorphosis of dry firn to ice if cold firn has reached a density of about 0 55 mg m 3 further densificaron occurs by a sintering process which increases the contact surface between the firn grains the pore volume is decreasing continuously but the firn remains permeable to air up to a density of 0 82 mg m 3 at about this density the remaining air in the pore volume is closed off in isolated bubbles we are interested in the age and the age distribution of the air enclosed in bubbles relative to the age of the surrounding ice and are investigating the development of the pore volume in firn a newly constructed measuring device allows the field measurement of the amount of air which is already enclosed in bubbles of firn samples measurements have been made during summer 1983 in greenland and during winter 1983 84 at the south pole the results are discussed and compared with results obtained with a simplified statistical sintering model using some results of percolation theory

visual stratigraphic dating of the gisp2 ice core basis reproducibility and application annual layers are visible in the greenland ice sheet project 2 ice core from central greenland allowing rapid dating of the core changes in bubble and grain structure caused by near surface primarily summertime formation of hoar complexes provide the main visible annual marker in the holocene and changes in cloudiness of the ice correlated with dustiness mark wisconsinan annual cycles both markers are evident and have been intercalibrated in early holocene ice layer counts are reproducible between different workers and for one worker at different times with 1 error over century length times in the holocene reproducibility is typically 5 in wisconsinan ice age ice and decreases with increasing age and depth cumulative ages from visible stratigraphy are not significantly different from independent ages of prominent events for ice older than the historical record and younger than approximately 50 000 years visible observations are not greatly degraded by brittle ice or many other core quality problems allowing construction of long consistently sampled time series high accuracy requires careful study of the core by dedicated observers

variation in n2 o2 ratio of occluded air in dome fuji antarctic ice ancient atmospheric gases are trapped in polar ice sheets the gas molecules are stored in air bubbles at shallow depth and are incorporated into clathrate hydrates below a depth at which the hydrostatic pressure becomes greater than the formation pressure of the air clathrate hydrate significant gas fractionation has been found from measurements of the depth profile of the n2 o2 composition ratios in clathrate hydrates and air bubbles of vostok antarctic ice to investigate the effect of the ice condition on the fractionation process we measured the n2 o2 ratios in clathrate hydrates and air bubbles from dome fuji antarctic ice using raman spectroscopy the results showed that the n2 o2 ratios in the clathrate hydrates of the dome fuji ice are slightly lower than those of the vostok ice although the tendency of the variation of the n2 o2 ratio with depth is similar the difference in the n2 o2 ratio between the dome fuji ice and the vostok ice for the transition zone is attributed to the difference of the ice temperature and the snow accumulation rate on the other hand it is concluded that the difference in the bubble free ice zone was caused by gas loss from the ice core after coring the n2 o2 ratio of clathrate hydrate increases after coring because of higher diffusion rate and lower dissociation pressure of o2 than of n2 our data suggest that the effect of gas loss in the dome fuji ice is relatively small and so the gas composition in the dome fuji ice can be a precise paleoenvironmental indicator

delta15n measurements as a calibration tool for the paleothermometer and gas ice age differences a case study for the 8200 b p event on grip ice isotopic composition measurement on atmospheric nitrogen 15n trapped in ice cores is established as a gravitational enrichment indicator recently it was discovered that thermal diffusion has a significant influence on this parameter during periods of large temperature fluctuations severinghaus et al 1998 here we present results for the 8200 b p event the largest temperature excursion during the holocene which underpin this view and extend it to medium range temperature changes the thermal diffusion component is about one third of the gravitational enrichment for this event the shortness of this cooling event does not significantly affect the gravitational enrichment as documented by rather constant close off depths therefore the 15n residuals can be directly used to estimate temperature changes hence calibrating the paleothermometer since our 15n record can resolve temperature variations as short as a few decades it complements the calibration technique of borehole temperature measurements for which the resolution decreases rapidly with increasing age we obtained a relation 18oice temperature of 0 30 0 11 k which agrees nicely with the temporal dependence for cold periods about 0 30 k and is significantly lower than today s greenland spatial relation 0 67 k this corresponds to a range of the surface temperature shift for the 8200 b p event in central greenland between 5 4 and 11 7 k with a best estimate of 7 4 k by matching the 15n variations with 18oice variations we estimate the mean gas ice age difference to be 209 15 years which is in good agreement with model estimates

multiparameter full waveform inversion of multicomponent ocean bottom cable data from the valhall field part 2 imaging compressive wave and shear wave velocities

the dole effect and its variations during the last 130 000 years as measured in the vostok ice core we review the current understanding of the dole effect the observed difference between the delta sup 18 o of atmospheric o sub 2 and that of seawater and its causes extended the record of variations in the dole effect back to 130 kyr before present using data on the delta sup 18 o of o sub 2 obtained from studying the vostok ice core and discuss the significance of temporal variations the dole effect reflects oxygen isotope fractionation during photosynthesis respiration and hydrologic processes evaporation precipitation and evapotranspiration our best prediction of the present day dole effect 20 8 per thousand is considerably lower than the observed value 23 5 per thousand and we discuss possible causes of this discrepancy during the past 130 kyr the dole effect has been 0 05 per thousand lower than the present value on average the standard deviation of the dole effect from the mean has been only per thousand and the dole effect is nearly unchanged between glacial maxima and interglacial periods the small variability in the dole effect suggests that relative rates of primary production in the land and marine realms have been relatively constant most periodic variability in the dole effect is in the precession band suggesting that changes in this global biogeochemical termmore reflects variations in low latitude land hydrology and productivity or possibly variability in low latitude oceanic productivity 65 refs 4 figs 3 tabs less

reverse time migration migration of stacked or zero offset sections is based on deriving the wave amplitude in space from wave field observations at the surface conventionally this calculation has been carried out through a depth extrapolation we examine the alternative of carrying out the migration through a reverse time extrapolation this approach may offer improvements over existing migration methods especially in cases of steeply dipping structures with strong velocity contrasts this migration method is tested using appropriate synthetic data sets

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

anisotropic full waveform inversion with tilt angle recovery abstractby allowing spatial variations in the direction of the anisotropic fast axis tilted transverse isotropy tti helps to image complex or steeply dipping structures without a priori geologic constraints however recovery of all the anisotropic parameters can be nontrivial and nonunique we adopt two methods for tti inversion with tilt angle recovery one based on the familiar voigt parameters and another based on the so called chen and tromp parameters known from regional and global seismology these parameterizations arise naturally in seismic wave propagation and facilitate straightforward recovery of the tilt angle and anisotropic strength in numerical experiments with vertical transversely isotropic starting models and tti target models we find that the voigt as well as the chen and tromp parameters allow quick and robust recovery of steeply dipping anticlinal structures

p and s wave receiver function imaging of subduction with scattering kernels a method for scattered wave imaging in 3d with both teleseismic p and s wave receiver function data is introduced the approach relies on body wave scattering kernels that are derived from the adjoint data sensitivity kernels which are typically used for full waveform tomography the forward problem is approximated using ray theory yielding a computationally efficient imaging algorithm that can resolve dipping and discontinuous interfaces using both p and s wave receiver functions travel time fields for the incident teleseismic arrivals and the receiver point sources are obtained by solving the eikonal equation using a fast marching code which can handle a 3d reference velocity model an energy stable finite difference method is used to simulate elastic wave propagation in a 2d hypothetical subduction zone model the resulting synthetic p and s wave receiver function datasets are used to validate the imaging method the kernel images are compared with those generated by the generalized radon transform and common conversion point stacking methods these results demonstrate the potential of the kernel imaging approach for constraining lithospheric structure in complex geologic environments with sufficiently dense recordings of teleseismic data potential imaging targets include short wavelength compositional variations in the mantle wedge and the slab s lithosphere asthenosphere boundary

age difference between polar ice and the air trapped in its bubbles air entrapped in bubbles formed in cold ice has essentially the same composition as that of the atmosphere at the time of bubble formation the analysis of dated ice samples therefore enables the history of atmospheric composition to be investigated 1 3 the age of the entrapped air is however not the same as that of the surrounding ice because air bubbles only become isolated from the atmosphere during the transition from firn to ice typically the age of the ice at this transition is between 100 and 3 000 yr depending mainly on firn temperature and snow accumulation rate the mean age difference between ice and enclosed air as well as the age distribution width for a given sample are especially important for the investigation of the anthropogenic increase of co2 and trace gases in the atmosphere over the last centuries and for the comparison of climatic parameters recorded in the ice with parameters recorded in the bubbles for siple station antarctica this age difference and age distribution width were deduced from the bubble volume measured as a function of depth the values are 95 yr and 22 yr respectively

finite frequency kernels for wave propagation in porous media based upon adjoint methods s u m m a r y we determine finite frequency kernels for wave propagation in porous media based upon adjoint methods these sensitivity kernels may be obtained based upon two numerical simulations for each source one calculation for the current model and a second adjoint calculation that uses time reversed signals at the receivers as simultaneous fictitious sources the adjoint equations for a porous medium are identical to the usual biot equations with the exception of the adjoint source term which involves time reversed measurements of the differences between simulations and data isotropic poroelastic wave propagation is governed by eight primary model parameters which appear in the original biot equations these parameters include four moduli the shear modulus of the frame plus three bulk moduli we consider two alternative parameterizations one involving density normalized moduli corresponding to squared wave speeds and a second involving the poroelastic shear and compressional wave speeds the alternative parameterizations lead to density sensitivity kernels that are small reflecting the fact that the traveltime of a poroelastic wave is governed by wave speed not density we systematically investigate and illustrate the sensitivity of the fast and slow compressional waves and the shear wave to the poroelastic model parameters using a 2 d spectral element method the poroelastic sensitivity kernels presented herein form the basis of tomographic imaging and inversion in porous media

the 4d seismic signature of oil water contact movement due to natural production in a stacked turbidite reservoir combined time lapse reservoir simulation and seismic modelling has been performed on both 1d and 3d models of a channelized turbidite reservoir the models have been built using core log laboratory and seismic data from the nelson field central north sea as a template oil and water movement in the main channels channel margins and interchannel regions is investigated with a particular focus being the effect of poor net to gross the analysis confirms that saturation effects dominate the response whilst stress sensitivity effects play a minor role the trough peak signature in the seismic difference volumes formed by the sweep of the water can be continued and mapped slightly further than the channel margins this characteristic 4d signature remains roughly intact despite the complicated depositional architecture and accurately delineates the area of moved fluid but it needs additional calibration to combat the detrimental influence of the low net to gross signal strength is largely dependent on reservoir quality but is also compromised by the net to gross fluid distribution and more critically by the exact timing of the seismic survey for example a region of bypassed oil zone remains undetected as it forms early during the production this work demonstrates that to understand fully the 4d signature at a quantitative level requires adequate knowledge of the fluid properties but also more critically the geology

the δ18o of atmospheric o2 from air inclusions in the vostok ice core timing of co2 and ice volume changes during the penultimate deglaciation one important question about the co2 climate connection is how increasing atmospheric pco2 levels affect climate and thereby the mass balance of continental ice sheets a record of atmospheric co2 variations over the last 160 000 years has recently been constructed by analyzing the trapped gas in the vostok ice core barnola et al 1987 the relationship between changes in atmospheric co2 and the size of the continental ice sheets has been difficult to ascertain because the co2 record is obtained from ice cores while the ice volume record has been constructed from the stable isotopic composition of biogenic caco3 in deep sea sediment cores in order to compare these two records in a more precise manner we present a record of the isotopic composition 18o of atmospheric o2 trapped in the vostok ice core and propose that it may be considered a proxy for the 18o of seawater and hence ice volume having a record of atmospheric co2 along with a continental ice volume proxy in trapped air in the same ice core allows us to compare the timing of changes in these two parameters with little uncertainty in the relative ages of important events our results suggest that during the penultimate glacial termination atmospheric co2 began to increase at least 3 kyr before the initial introduction of meltwater to the oceans possible errors in the relative age depth curve of the vostok ice core and uncertainties in the influence of biological and hydrologic processes on the 18o of atmospheric o2 introduce some uncertainty into our conclusions however our results are in general agreement with the observed phase relationship between atmospheric co2 and ice volume imbrie et al 1984 imbrie et al 1989 inferred from changes in the 13c difference between contemporaneous planktonic and benthic forams deposited in deep sea cores and the specmap ice volume record

age scale of the air in the summit ice implication for glacial interglacial temperature change the air occluded in ice sheets and glaciers has in general a younger age defined as the time after its isolation from the atmosphere than the surrounding ice matrix because snow is first transformed into open porous firn in which the air can exchange with the atmosphere only at a certain depth firn ice transition the pores are pinched off and the air is definitely isolated from the atmosphere the firn ice transition depth is at around 70 m under present climatic conditions at summit central greenland the air at this depth is roughly 10 years old due to diffusive mixing whereas the ice is about 220 years old this results in an age difference between the air and the ice of 210 years this difference depends on temperature and accumulation rate and did thus not remain constant during the past we used a dynamic firn densification model to calculate the firn ice transition depth and the age of the ice at this depth and an air diffusion model to determine the age of the air at the transition past temperatures and accumulation rates have been deduced from the 18o record using time independent functions we present the results of model calculations of two paleotemperature scenarios yielding a record of the age difference between the air and the ice for the greenland ice core project grip and the greenland ice sheet project two gisp2 ice cores for the last 100 000 years during the holocene the age difference stayed rather stable around 200 years while it reached values up to 1400 years during the last glaciation for the colder scenario the model results are compared with age differences obtained independently by matching corresponding climate events in the methane and 18o records assuming a very small phase lag between variations in the greenland surface temperature and the atmospheric methane the past firn ice transition depths are compared with diffusive column heights obtained from 15n of n2 measurements the results of this study corroborate the large temperature change of 20 to 25 k from the coldest glacial to holocene climate found by evaluating borehole temperature profiles

the greenland ice sheet project 2 depth age scale methods and results the greenland ice sheet project 2 gisp2 depth age scale is presented based on a multiparameter continuous count approach to a depth of 2800 m using a systematic combination of parameters that have never been used to this extent before the ice at 2800 m is dated at 110 000 years b p with an estimated error ranging from 1 to 10 in the top 2500 m of the core and averaging 20 between 2500 and 2800 m parameters used to date the core include visual stratigraphy oxygen isotopic ratios of the ice electrical conductivity measurements laser light scattering from dust volcanic signals and major ion chemistry gisp2 ages for major climatic events agree with independent ages based on varve chronologies calibrated radiocarbon dates and other techniques within the combined uncertainties good agreement also is obtained with greenland ice core project ice core dates and with the specmap marine timescale after correlation through the 18 o of o 2 although the core is deformed below 2800 m and the continuity of the record is unclear we attempted to date this section of the core on the basis of the laser light scattering of dust in the ice

wave equation traveltime inversion this paper presents a new traveltime inversion method based on the wave equation in this new method designated as wave equation traveltime inversion wt seismograms are computed by any full wave forward modeling method we use a finite difference method the velocity model is perturbed until the traveltimes from the synthetic seismograms are best fitted to the observed traveltimes in a least squares sense a gradient optimization method is used and the formula for the frechet derivative perturbation of traveltimes with respect to velocity is derived directly from the wave equation no traveltime picking or ray tracing is necessary and there are no high frequency assumptions about the data body wave diffraction reflection and head wave traveltimes can be incorporated into the inversion in the high frequency limit wt inversion reduces to ray based traveltime tomography it can also be shown that wt inversion is approximately equivalent to full wave inversion when the starting velocity model is

a method for precise measurement of argon 40 36 and krypton argon ratios in trapped air in polar ice with applications to past firn thickness and abrupt climate change in greenland and at siple dome antarctica we describe a method for measuring the 40ar 36ar ratio and the 84kr 36ar ratio in air from bubbles trapped in ice cores these ratios can provide constraints on the past thickness of the firn layer at the ice core site and on the magnitude of past rapid temperature variations when combined with measured 15n 14n both variables contribute to paleoclimatic studies and ultimately to the understanding of the controls on earth s climate the overall precision of the 40ar 36ar method 1 standard error of the mean is 0 012 for a sample analyzed in duplicate corresponding to 0 6 m in reconstructed firn thickness we use conventional dynamic isotope ratio mass spectrometry with minor modifications and special gas handling techniques designed to avoid fractionation about 100 g of ice is used for a duplicate pair of analyses an example of the technique applied to the gisp2 ice core yields an estimate of 11 3k of abrupt warming at the end of the last glacial period 15 000 years ago the krypton argon ratio can provide a diagnostic of argon leakage out of the bubbles which may happen naturally during bubble close off or artifactually if samples are warmed near the freezing point during core retrieval or storage argon leakage may fractionate the remaining 40ar 36ar ratio by 0 007 per change in 84kr 36ar introducing a possible bias in reconstructed firn thickness of about 2 m if thermal diffusion is not accounted for or 6 m if thermal diffusion effects are quantified with measured 15n 14n reproducibility of 84kr 36ar measured in air is about 0 2 1 standard error of the mean but is about 1 for ice core samples ice core samples are systematically enriched in 84kr 36ar relative to atmosphere by 5 probably reflecting preferential size dependent exclusion of the smaller argon atom during bubble entrapment recent results from the siple dome ice core reveal two climate events during the last deglaciation including an 18 m reduction in firn thickness associated with an abrupt warming at sometime between 18 and 22 kyr bp and a partial or total removal of the firn during an ablation event at 15 3 kyr bp

increasing illumination and sensitivity of reverse time migration with internal multiples reverse time migration is a two way time domain finite frequency technique that accurately handles the propagation of complex scattered waves and produces a bandlimited representation of the subsurface structure that is conventionally assumed to be linear in the contrasts in model parameters because of this underlying linear singlescattering assumption most implementations of this method do not satisfy the energy conservation principle and do not optimally use illumination and model sensitivity of multiply scattered waves migrating multiply scattered waves requires preserving the non linear relation between the image and perturbation of model parameters i modify the extrapolation of source and receiver wavefields to more accurately handle multiply scattered waves i extend the concept of the imaging condition in order to map into the subsurface structurally coherent seismic events that correspond to the interaction of both singly and multiply scattered waves this results in an imaging process referred to here as non linear reverse time migration it includes a strategy that analyses separated contributions of singly and multiply scattered waves to a final non linear image the goal is to provide a tool suitable for seismic interpretation and potentially migration velocity analysis that benefits from increased illumination and sensitivity from multiply scattered seismic waves it is noteworthy that this method can migrate internal multiples a clear advantage for imaging challenging complex subsurface features e g in salt and basalt environments the results of synthetic seismic imaging experiments including a subsalt imaging example illustrate the technique

greenland temperature response to climate forcing during the last deglaciation greenland ice core water isotopic composition 18 o provides detailed evidence for abrupt climate changes but is by itself insufficient for quantitative reconstruction of past temperatures and their spatial patterns we investigate greenland temperature evolution during the last deglaciation using independent reconstructions from three ice cores and simulations with a coupled ocean atmosphere climate model contrary to the traditional 18 o interpretation the younger dryas period was 4 5 2 c warmer than the oldest dryas due to increased carbon dioxide forcing and summer insolation the magnitude of abrupt temperature changes is larger in central greenland 9 to 14 c than in the northwest 5 to 9 c fingerprinting a north atlantic origin simulated changes in temperature seasonality closely track changes in the atlantic overturning strength and support the hypothesis that abrupt climate change is mostly a winter phenomenon

vertical seismic profile migration using the kirchhoff integral wave equation migration can form an accurate image of the subsurface from suitable vsp data the image s extent and resolution are determined by the receiver array dimensions and the source location s experiments with synthetic and real data show that the region of reliable image extent is defined by the specular zone of illumination migration is achieved through wave field extrapolation subject to an imaging procedure wave field extrapolation is based upon the scalar wave equation and for vsp data is conveniently handled by the kirchhoff integral the migration of vsp data calls for imaging very close to the borehole as well as imaging in the far field this dual requirement is met by retaining the near field term of the integral the complete integral solution is readily controlled by various weighting devices and processing strategies whose worth is demonstrated on real and synthetic data

16 c rapid temperature variation in central greenland 70 000 years ago variations in the29n2 28n2 ratio of air bubbles trapped in polar ice cores and their relation to variations of the 18o 16o of the ice allow past surface temperature variations and ice age gas age differences to be determined high resolution measurements of29n2 28n2 in dansgaard oeschger event 19 around 70 000 years before the present in ice from central greenland show that at the beginning of the event the ice age gas age difference was 1090 100 years with the use of a combined firn densification temperature and gas diffusion model the 18oice temperature coefficient was determined to be 0 42 0 05 per mil per kelvin this coefficient implies a mean surface temperature change of 16 0 kelvin between 14 3 and 18 1 kelvin which differs substantially from values derived from borehole temperatures and modern spatial 18oice surface temperature correlations

co2 and o2 n2 variations in and just below the bubble clathrate transformation zone of antarctic ice cores co2 measurements on the epica european project for ice coring in antarctica dml ice core in depth levels just below the bubble ice clathrate ice transformation zone 1230 2240 m depth were performed in the youngest part 1230 1600 m they reveal variations of up to 25 ppmv around the mean atmospheric concentration within centimetres corresponding to a snow deposition interval of a few years similar results are found at corresponding depth regions of the dome c and the talos dome ice cores since we can exclude all hitherto known processes altering the concentration of co2 in ice cores we present a hypothesis about spatial fractionation of air components related to episodically increasing clathrate formation followed by diffusion processes from bubbles to clathrates this hypothesis is supported by optical line scan observations and by o2 n2 measurements at the same depth where strong co2 variations are detected below the clathrate formation zone this small scale fractionation process is slowly smoothed out most likely by diffusion regaining the initial mean atmospheric concentration although this process compromises the representativeness of a single co2 measurement on small ice samples in the clathrate formation zone of an ice core it does not affect the mean atmospheric co2 concentration if co2 values are averaged over a sufficiently long depth scale 10 cm in case of the epica dml ice core

full 3d tomography for the crustal structure of the los angeles region we apply full 3d tomography f3dt to a regional dataset in the los angeles area in f3dt the starting model as well as the model perturbation is three dimensional and the sensitivity frechet kernels are computed using numerical schemes that account for the full physics of 3d wave propagation f3dt can account for the nonlinearity of structural inverse problem through iteration thus providing the most efficient means for assimilating seismic observations into dynamic ground motion models we have successfully applied a scattering integral si formulation of f3dt to improve a 3d elastic structure model the southern california earthquake center scec community velocity model version 3 0 cvm3 0 with lowest shear velocity clamped at 1 km sec in the los angeles region our data are time and frequency localized measurements of phase delay anomalies relative to synthetics computed from the 3d elastic starting model the frechet kernels for our measurements were computed by convolving the earthquake wave fields generated by point earthquake sources with the receiver green tensors rgts which are the spatial temporal fields produced by three orthogonal unit impulsive point forces acting at the receiver locations we inverted 7364 phase delay measurements of p and s body waves by using the lsqr method the revised 3d model laf3d provides substantially better fit to the observed waveform data than the 3d starting model to our knowledge this study is the first successful application of f3dt using real data in structural seismology

elastic imaging and time lapse migration based on adjoint methods we have drawn connections between imaging in exploration seismology adjoint methods and emerging finite frequency tomography all of these techniques rely on spatial and temporal constructive interference between observed and simulated waveforms to map locations of structural anomalies modern numerical methods and computers have facilitated the accurate and efficient simulation of 3d acoustic an elastic and poroelastic wave propagation using a 2d cross section of the seg eage salt model we have determined how such waveform simulations might be harnessed to improve onshore and offshore seismic imaging strategies and capabilities we have found that the density sensitivity kernel in adjoint tomography is related closely to the imaging principle in exploration seismology and that in elastic modeling the impedance kernel actually is a better diagnostic tool for reflector identification the shear and compressional wave speed sensitivity kernels in adjoint tomography are related closely to finite frequency banana doughnut kernels and these kernels are well suited for mapping larger scale structure i e for transmission tomography these ideas have been substantiated by addressing problems in subsalt time lapse migration

migration by extrapolation of time dependent boundary values migration of an observed zero offset wavefield can be performed as the solution of a boundary value problem in which the data are extrapolated backward in time this concept is implemented through a finite difference solution of the two dimensional acoustic wave equation all depths are imaged simultaneously at time 0 the imaging condition and all dips right up to vertical are correctly migrated numerical examples illustrate this technique in both constant and variable velocity media

quantification of rapid temperature change during do event 12 and phasing with methane inferred from air isotopic measurements the description of rapid climatic changes during the last glacial period at high northern latitudes has been largely documented through greenland ice cores that are unique climatic and environmental records however greenland ice isotopic records are biased temperature proxies and it is still a matter of debate whether changes in the high latitudes lead or lag rapid changes elsewhere we focus here on the study of the mid glacial dansgaard oeschger event 12 45 ky bp associated to a large d 18 oice change in the grip greenland ice core project ice core we use combined measurements of ch4 d 15 n and d 40 ar in entrapped air associated with a recently developed firn densification and heat diffusion model to infer i the phasing between methane and temperature increases and ii the amplitude of the temperature change our method enables us to overcome the difficulty linked with rapid accumulation change in quantifying the temperature change we obtain a 12f2 5 8c temperature increase at the beginning of do event 12 thus confirming that the conventional use of water isotopes in the greenland ice cores largely underestimates the actual amplitude of rapid temperature change in central greenland in agreement with previous studies methane and temperature increase are in phase at the sampling resolution of that part of our profile 90 years d 2004 elsevier b v all rights reserved

multiparameter full waveform inversion of multicomponent ocean bottom cable data from the valhall field part 1 imaging compressional wave speed density and attenuation multiparameter full waveform inversion fwi is a challenging quantitative seismic imaging method for lithological characterization and reservoir monitoring the difficulties in multiparameter fwi arise from the variable influence of the different parameter classes on the phase and amplitude of the data and the trade off between these in this framework choosing a suitable parametrization of the subsurface and designing the suitable fwi workflow are two key methodological issues in non linear waveform inversion we assess frequency domain visco acoustic fwi to reconstruct the compressive velocity vp the density or the impedance ip and the quality factor qp from the hydrophone component using a synthetic data set that is representative of the valhall oil field in the north sea we first assess which of the vp and vp ip parametrizations provides the most reliable fwi results when dealing with wide aperture data contrary to widely accepted ideas we show that the vp parametrization allows a better reconstruction of both the vp and ip parameters first because it favours the broad band reconstruction of the dominant vp parameter and secondly because the trade off effects between velocity and density at short to intermediate scattering angles can be removed by multiplication to build an impedance model this allows for the matching of the reflection amplitudes while the broad band velocity model accurately describes the kinematic attributes of both the diving waves and reflections then we assess different inversion strategies to recover the quality factor qp in addition to parameters vp and a difficulty related to attenuation estimation arises because on the one hand the values of qp are on average one order of magnitude smaller than those of vp and and on the other hands model perturbations relative to the starting models can be much higher for qp than for vp and during fwi in this framework we show that an empirical tuning of the fwi regularization which is adapted to each parameter class is a key issue to correctly account for the attenuation in the inversion we promote a hierarchical approach where the dominant parameter vp is reconstructed first from the full data set i e without any data preconditioning to build a velocity model as kinematically accurate as possible before performing the joint update of the three parameter classes during a second step this hierarchical imaging of compressive wave speed density and attenuation is applied to a real wide aperture ocean bottom cable data set from the valhall oil field several geological features such as accumulation of gas below barriers of claystone and soft quaternary sediment are interpreted in the fwi models of density and attenuation the models of vp and qp that have been developed by visco acoustic fwi of the hydrophone data can be used as initial models to perform visco elastic fwi of the geophone data for the joint update of the compressive and shear wave speeds

fractionation of gases in polar ice during bubble close off new constraints from firn air ne kr and xe observations gas ratios in air withdrawn from polar firn snowpack show systematic enrichments of ne n2 o 2 n2 and ar n2 in the firn ice transition region where bubbles are closing off air from the bubbles in polar ice is correspondingly depleted in these ratios after accounting for gravitational effects gas in the bubbles becomes fractionated during the process of bubble close off and fractionation may continue as ice cores are stored prior to analysis we present results from firn air studies at south pole and siple dome antarctica which add ne kr and xe measurements to the suite of observations ne o2 and ar appear to be preferentially excluded from the shrinking and occluding bubbles and these gases therefore accumulate in the residual firn air creating a progressive enrichment with time and depth in firn air early sealing of gases by thin horizontal impermeable layers into a nondiffusive zone or lock in zone greatly enhances this enrichment a simple model of the bubble close off fractionation and lock in zone enrichment fits the data adequately the model presumes that fractionation is caused by selective permeation of gas through the ice lattice from slightly overpressured bubbles the effect appears to be size dependent because ne o2 and ar have smaller effective molecular diameters than n2 and fractionation increases strongly with decreasing size ne is fractionated 34 2 times more than o2 in south pole firn air and reaches an enrichment of 90 in the deepest sample the large atoms kr and xe do not appear to be fractionated by this process despite the large size difference between the two gases suggesting a threshold atomic diameter of 3 6a above which the probability becomes very small that the gas will escape from the bubble these findings have implications for ice core and firn air studies that use gas ratios to infer paleotemperature chronology and past atmospheric composition

evidence for substantial accumulation rate variability in antarctica during the holocene through synchronization of co 2 in the taylor dome dome c and dml ice cores high resolution records of atmospheric co2 concentration during the holocene are obtained from the dome concordia and dronning maud land antarctica ice cores these records confirm that the co2 concentration varied between 260 and 280 ppmv in the holocene as measured in the taylor dome ice core however there are differences in the co2 records most likely caused by mismatches in timescales matching the taylor dome timescale to the dome c timescale by synchronization of co2 indicates that the accumulation rate at taylor dome increased through the holocene by a factor two and bears little resemblance to the stable isotope record used as a proxy for temperature this result shows that different locations experienced substantially different accumulation changes and casts doubt on the often used assumption that accumulation rate scales with the saturation vapor pressure as a function of temperature at least for coastal locations d 2004 elsevier b v all rights reserved

finite frequency effects in global surface wave tomography summary we compare traditional ray theoretical surface wave tomography with finite frequency tomography using 3 d born sensitivity kernels for long period fundamental mode dispersion measurements the 3 d kernels preserve sidelobes beyond the first fresnel zone and fully account for the directional dependence of surface wave scattering and the effects of timedomain tapering and seismic source radiation tomographic inversions of love and rayleigh phase delay measurements and synthetic checkerboard tests show that 1 small scale s wave velocity anomalies are better resolved using finite frequency sensitivity kernels especially in the lowermost upper mantle 2 the resulting upper mantle heterogeneities are generally stronger in amplitude than those recovered using ray theory and 3 finite frequency tomographic models fit long period dispersion data better than ray theoretical models of comparable roughness we also examine the reliability of 2 d phase velocity sensitivity kernels in global surface wave tomography and show that phase velocity kernels based upon a forwardscattering approximation or previously adopted geometrical simplifications do not reliably account for finite frequency wave propagation effects 3 d sensitivity kernels with full consideration of directional dependent seismic scattering are the preferred method of inverting long period dispersion data finally we derive 2 d boundary sensitivity kernels for lateral variations in crustal thickness and show that finite frequency crustal effects are not negligible in long period surface wave dispersion studies especially for paths along continent ocean boundaries unfortunately we also show that in global studies linear perturbation theory is not sufficiently accurate to make reliable crustal corrections due to the large difference in thickness between oceanic and continental crust

formation of air clathrate hydrates in polar ice sheets heterogeneous nucleation induced by micro inclusions to investigate factors influencing nucleation of air clathrate hydrates in polar ice sheets we have performed high resolution mapping of the distributions of soluble impurities air bubbles and air hydrate crystals versus depth in the dome fuji antarctic ice significant correlation observed between the concentrations of air inclusions and impurities in ice along with frequent occurrence of impurities inside hydrate crystals suggest that micro inclusions promote hydrate nucleation in the ice matrix our observations also show that the diffusive macroscopic scale redistribution of air constituents in ice in the bubble hydrate transition zone is controlled by the original sedimentary layering of soluble impurities acting as nucleation helpers the results of this study are important for the correct interpretation of high resolution gas analyses of ice cores and for better understanding the global bubble to hydrate transformation process in polar ice sheets

wave equation migration velocity analysis ii subsalt imaging examples subsalt imaging is strongly dependent on the quality of the velocity model however rugose salt bodies complicate wavefield propagation and lead to subsalt multipathing illumination gaps and shadow zones which cannot be handled correctly by conventional traveltime based migration velocity analysis mva we overcome these limitations by the wave equation mva technique introduced in a companion paper and demonstrate the methodology on a realistic synthetic data set simulating a salt dome environment and a gulf of mexico data set we model subsalt propagation using wave paths created by one way wavefield extrapolation those wave paths are much more accurate and robust than broadband rays since they inherit the frequency dependence and multipathing of the underlying wavefield we formulate an objective function for optimization in the image space by relating an image perturbation to a perturbation of the velocity model the image perturbations are defined using linearized prestack residual migration thus ensuring stability relative to the first order born approximation assumptions synthetic and real data examples demonstrate that wave equation mva is an effective tool for subsalt velocity analysis even when shadows and illumination gaps are present

improved amplitude preservation for prestack depth migration by inverse scattering theory a prestack reverse time migration image is not properly scaled with increasing depth the main reason for the image being unscaled is the geometric spreading of the wavefield arising during the back propagation of the measured data and the generation of the forward modelled wavefields this unscaled image can be enhanced by multiplying the inverse of the approximate hessian appearing in the gauss newton optimization technique however since the approximate hessian is usually too expensive to compute for the general geological model it can be used only for the simple background velocity model we show that the pseudo hessian matrix can be used as a substitute for the approximate hessian to enhance the faint images appearing at a later time in the 2d prestack reverse time migration sections we can construct the pseudo hessian matrix using the forward modelled wavefields which are used as virtual sources in the reverse time migration by exploiting the uncorrelated structure of the forward modelled wavefields and the impulse response function for the estimated diagonal of the approximate hessian although it is also impossible to calculate directly the inverse of the pseudo hessian when using the reciprocal of the pseudo hessian we can easily obtain the inverse of the pseudo hessian as examples supporting our assertion we present the results obtained by applying our method to 2d synthetic and real data collected on the korean continental shelf

implications of ice core smoothing for inferring co2 flux variability 1 ice core records are commonly used to infer information about past variability of co2 fluxes because of processes involved in enclosing this air in ice ice core records are a smoothed representation of the actual past atmospheric variations as such there is a limit to how much information ice core measurements can contain about flux variability on short timescales with a numerical model of the firn processes we quantify this smoothing and describe how it can be reproduced with pulse response functions we generate and make available pulse response functions for co2 at the de08 site on law dome antarctica we discuss implications of the smoothing for inferring co2 flux variability from the law dome ice core record in particular we look at results from an intercomparison of terrestrial biosphere models over the twentieth century and show how much of the co2 variability would be reflected in the law dome ice core record we also smooth atmospheric 13co2 from a study that compared fixed and varying isotopic discrimination we find that the impact of changing discrimination shown previously to be large on interannual timescales is small on the decadal scales accessible from ice core records

a revised 1000 year atmospheric δ13c co2 record from law dome and south pole antarctica 1 we present new measurements of 13c of co2 extracted from a high resolution ice core from law dome east antarctica together with firn measurements performed at law dome and south pole covering the last 150 years our analysis is motivated by the need to better understand the role and feedback of the carbon c cycle in climate change by advances in measurement methods and by apparent anomalies when comparing ice core and firn air 13c records from law dome and south pole we demonstrate improved consistency between law dome ice south pole firn and the cape grim tasmania atmospheric 13c data providing evidence that our new record reliably extends direct atmospheric measurements back in time we also show a revised version of early 13c measurements covering the last 1000 years with a mean preindustrial level of 6 50 finally we use a kalman filter double deconvolution to infer net natural co2 fluxes between atmosphere ocean and land which cause small 13c deviations from the predominant anthropogenically induced 13c decrease the main features found from the previous 13c record are confirmed including the ocean as the dominant cause for the 1940 a d co2 leveling our new record provides a solid basis for future investigation of the causes of decadal to centennial variations of the preindustrial atmospheric co2 concentration those causes are of potential significance for predicting future co2 levels and when attempting atmospheric verification of recent and future global carbon emission mitigation measures through coupled climate carbon cycle models