numerical reconstructions of lgm climate across the eurasian arctic abstract although limits of the eurasian ice sheet at the last glacial maximum lgm are known relatively well from geological evidence the palaeoclimate coexistent with this ice sheet has yet to be identified confidently a simple ice sheet model has been used to determine lgm climate conditions by finding the ice sheet mass balance configuration that permits model results to match with geological data relating to the extent of lgm ice cover these conditions are compared with atmospheric general circulation model agcm calculations of the climate at the lgm which reveal the meteorological conditions associated with the ice sheet mass balance the western margin of scandinavia is characterised by a maritime climate influenced by the nearby warm seasonally sea ice free ocean conditions however further east the warming effect of the north atlantic diminishes and an extreme polar desert climate dominates across the kara sea similar to east antarctica today the lgm climate that we establish is in contrast to many previous reconstructions involving snow laden warm westerly winds originating from the north atlantic affecting the entire eurasian arctic severely cold conditions across the eurasian arctic are compatible with a stable circulation cell over the kara sea that isolates the region climatically and or cold dry westerly winds from the fennoscandian ice sheet that flow across the rest of the eurasian arctic

the morphology structural evolution and significance of push moraines abstract push moraines glaciotectonic ice marginal moraines have a restricted distribution at modern glacier margins and consequently are of potential value in reconstructing pleistocene ice sheets providing data both on former glaciodynamics and on the palaeoenvironment of the glacial foreland to the wider earth science community push moraines are of interest as analogues for thin skin tectonics within orogenic belts this paper reviews the morphology structural geology formation and significance of push moraines the morphological and structural characteristics of small seasonal push moraines through to large multi crested examples produced by sustained glacier advances are reported before the primary controls on push moraine formation are examined these controls include the nature of the applied glacial stress field the presence and properties of decollement horizons and the shear strength and rheology of the glacial foreland a conceptual model of push moraine formation is introduced in which the range of observed morphological and structural forms are viewed within a matrix defined by the main variables which control their formation the absence of consensus over which of these variables is of greatest importance currently limits the significance of push moraines in palaeoglaciological research as a consequence this review emphasises the need for future research in order to realise the true potential of push moraine in the reconstruction of pleistocene environments

late glacial events in the central russian arctic abstract investigation of glacigenic diamictons and landforms in the central russian arctic has established that continental glaciation was widespread during late glacial time diamictons in west central siberia containing marine foraminifera and mollusca include glacitectonic deformation features and are associated with glacial landforms these sediments are interpreted as basal tills formed by continental glaciation which expanded from the exposed kara sea shelf southwards along the western flank of the ural mountains piedmont glaciers advancing westward from the urals were obstructed by the southward flowing ice and were areally restricted simultaneously piedmont glaciers advancing eastward from the crests of the urals were able to expand without restriction alpine glaciation within the urals was areally restricted and culminated simultaneously with the maximum extent of the southward flowing shelf based ice in west siberia rapid disintegration of an early weichselian ice sheet by marine calving was followed by slow stagnation of the dissected mass throughout the remainder of the weichselian in contrast glaciation in the pechora basin reached its maximum during the middle late weichselian at least 30 000 years after the glacial maximum in western siberia the transition to holocene climatic conditions was influenced by the presence of remanent decaying glacial masses

subglacial drainage eskers and deforming beds beneath the laurentide and eurasian ice sheets glaciological theory predicts that the subglacial drainage network at the base of gently sloping ice sheets resting on deforming sediment should consist of many wide shallow probably braided canals distributed along the ice sediment interface rather than an arborescent network of relatively few large tunnels as would develop over a rigid substrate a corollary prediction examined here is that eskers which form in large subglacial tunnels should be rare where subglacial bed deformation occurred but they may be relatively common where the bed was rigid bed deformation would be most likely where subglacial till was relatively continuous fine grained and of low permeability that is in regions where till is derived primarily from underlying sedimentary bedrock but unlikely where discontinuous coarse grained high permeability till was derived from underlying crystalline bedrock the observed distribution of eskers in areas covered by the laurentide and eurasian british scandinavian and barents sea ice sheets during the last glaciation shows that most eskers occur over crystalline bedrock overlain by discontinuous high permeability till but are rare or absent over sedimentary bedrock overlain by fine grained low permeability till thus matching reasonably well our prediction glaciological theory and geologic evidence indicate that esker systems on a subcontinental scale are time transgressive sedimentological evidence for a canal drainage system appears to be present in fine grained tills where eskers are largely absent

true amplitude seismic migration a comparison of three approaches knowledge of elastic parameter compressional and shear velocities and density contrasts within the earth can yield knowledge of lithology changes elastic parameter contrasts manifest themselves on seismic records as angle dependent reflectivity interpretation of angle dependent reflectivity or amplitude variation with offset avo on unmigrated records is often hindered by the effects of common depth point smear incorrectly specified geometrical spreading loss source receiver directivity as well as other factors it is possible to correct some of these problems by analyzing common reflection point gathers after prestack migration provided that the migration is capable of undoing all the amplitude distortions of wave propagation between the sources and the receivers a migration method capable of undoing such distortions and thus producing angle dependent reflection coefficients at analysis points in a lossless isotropic elastic earth is called a true amplitude migration the principles of tr

late glacial landforms of wollaston peninsula victoria island northwest territories product of ice marginal retreat surge and mass stagnation an analysis of glacial landforms on a regional scale leads to an interpretation of the dynamics of late wisconsinan glaciation on wollaston peninsula victoria island northwest territories the glacial record is dominated by four adjacent belts of landforms i ground moraine till plains and ice marginal drainage features ii hummocky moraine iii lateral and shear moraine and iv streamlined landforms the landform belts are considered as representing four distinct glacial ice conditions or regimes 1 ice margin retreat during extending flow of thin active ice 2 marginal ice stagnation following compressional flow 3 a surging ice margin producing massive shear moraines and 4 large scale flooding and mass ice stagnation following a surge these landform belts were arranged in zones by topographically controlled glacial dynamics the latter two defining a former ice stream glaciological inferences can be extended by examining the sediments and processes that produced each landform set

the russian component of an arctic ice sheet during the last glacial maximum abstract the extent of late weichselian glaciation in arctic russia remains a problem our model of 1995 1998 suggests a continuous and long lived ice sheet centered on the kara sea other reconstructions suggest a smaller and diachronous glaciation the vast majority of dates support the model of restricted glaciation we show that these dates are inconsistent with the geologic record of ice flow across the kara barents divide with second order glacial geology of kola peninsula and with evidence for glacial surges they are also inconsistent with late weichselian climate of the arctic with eurasian continental paleohydrology including meltwater drainage systems cataclysmic megafloods major transgressions of the caspian sea and with the entire paleogeographic context of northern eurasia we believe that due to impeded ventilation of the pleistocene arctic ocean these dates are often too old and that recycling and contamination of the samples would also contribute to the dating errors destructive impacts of late glacial ice sheet surges and megafloods upon the arctic glacial sequences would also aggravate the situation by ignoring the dates until these problems are addressed we find that the field evidence supports continuous glaciation of arctic russia at the lgm and that the resulting ice sheet was part of an arctic ice sheet that included an ice shelf in the arctic ocean and ice sheets in greenland and north america we present 3 d reconstructions of the arctic ice sheet and with an enlarged antarctic ice sheet we show that the ice volume is equivalent to an lgm sea level that was 130 135 m lower than at present the reconstructions depict the arctic ice sheet before and after massive thawing of the bed that allows partial gravitational collapse of the overlying ice with a minimal change in ice volume

a simple exact method of 3 d migration theory recently a heuristic and empirical analysis of a method of three dimensional 3 d migration has been presented by gibson larner and levin it consists of sets of two dimensional 2 d migrations in orthogonal directions in this paper we examine this two pass method using scalar wave theory and show its action to be identical to that of so called full 3 d migration for constant velocity media the demonstration of equivalence is carried out in both the frequency space and frequency wavenumber domains the latter providing a particularly transparent view of the mechanism of the two pass approach we then extend our analysis to consider how various approximate algorithms used in practical implementations affect the validity of our conclusion

major end moraines of younger dryas age on wollaston peninsula victoria island canadian arctic implications for paleoclimate and for formation of hummocky moraine some of the most extensive and massive end moraines of younger dryas age 11 10 14c ka bp yet recognized in north america occur on wollaston peninsula of victoria island on the western part of the peninsula numerous closely spaced end moraines formed in the interval starting 11 100 100 radiocarbon years ago and ending about 10 500 10 200 years ago net recession was generally slow throughout and was punctuated by moraine building and at least two readvances recession is mapped with a resolution that is approximately decadal the moraines form an orderly nested succession and are consistently associated with westward shedding of meltwater which formed a sequence of marine limit deltas we lack firm independent proxy climate evidence needed to assess whether these moraines formed because of cold younger dryas climate rather than because of controls such as topographic setting and water depth but climatic control seems probable the moraines evidently retain glacier ice cores as do most similarly

implementation of elastic reverse time migration using wavefield separation in the frequency domain summary considerable effort has been devoted to the migration of multicomponent data in elastic media with wavefield separation techniques being the most successful most of this work has been carried out in the time domain in this paper we formulate a multicomponent migration technique in the frequency domain reverse time migration can be viewed as the zero lag cross correlation between virtual source and back propagated wavefields cross correlating the helmholtz decomposed wavefields rather than directly correlating the vector displacement fields results in sharper more interpretable images contaminated by fewer crosstalk artefacts the end products are separate p and s wave and if desired ps and sp migration images we test our migration algorithm on synthetic seismic data generated using the seg eage salt dome overthrust and marmousi 2 models we correctly image the location and shape of the target zone for oil exploration using these data sets furthermore we demonstrate that our new migration technique provides good images even when the initial velocity model is only approximate

a numerical investigation of ice lobe permafrost interaction around the southern laurentide ice sheet permafrost existed around and under marginal parts of the southern laurentide ice sheet during the last glacial maximum the presence of permafrost was important in determining the extent form and dynamics of ice lobes and the landforms they produced because of influences on resistance to basal motion and subglacial hydrology we develop a two dimensional time dependent model of permafrost and glacier ice dynamics along a flowline to examine i the extent to which permafrost survives under an advancing ice lobe and how it influences landform development and hydrology and ii the influence of permafrost on ice motion and surface profile the model is applied to the green bay lobe which terminated near madison wisconsin during the last glacial maximum simulations of ice advance over permafrost indicate that the bed upstream of the ice sheet margin was frozen for 60 200 km at the glacial maximum permafrost remained for centuries to a few thousand years under advancing ice and penetrated sufficiently deep tens of meters into the underlying aquifer that drainage of basal meltwater became inefficient likely resulting in water storage beneath the glacier our results highlight the influence of permafrost on subglacial conditions even though uncertainties in boundary conditions such as climate exist

refined modeling and14c plateau tuning reveal consistent patterns of glacial and deglacial14c reservoir ages of surface waters in low latitude atlantic atlantic surface water reservoir ages

from the hagedoorn imaging technique to kirchhoff migration and inversion the seminal 1954 paper by j g hagedoorn introduced a heuristic for seismic reflector imaging that heuristic was a construction technique a string construction or ruler and compass method for finding reflectors as an envelope of equal traveltime curves defined by events on a seismic trace later kirchhoff migration was developed this method is based on an integral representation of the solution of the wave equation for decades kirchhoff migration has been one of the most popular methods for imaging seismic data parallel with the development of kirchhoff wave equation migration has been that of kirchhoff inversion which has as its objectives both structural imaging and the recovery of angle dependent reflection coefficients the relationship between kirchhoff migration inversion and hagedoorn s constructive technique has only recently been explored this paper addresses this relationship presenting the mathematical structure that the kirchhoff approach adds to hagedoorn s constructive method and showing the relationship between the two

fresnel volume migration of single component seismic data standard implementations of kirchhoff prestack depth migration psdm distribute the recorded wavefield along two way traveltime isochrons and an image is generated by constructive interference of these isochrons along the actual reflector elements beside the recent developments of wave equation based approaches kirchhoff psdm is still considered widely as a state of the art technique in obtaining high quality images of the subsurface particularly for highly irregular survey layouts and target oriented imaging tasks however for sparse sampling or limited aperture the resulting image is affected by significant migration noise as a result of limited constructive interference of the back propagated wavefield some modifications have been proposed to reduce these artifacts these modifications include constructing a specular path of wave propagation derived from estimates of the emergent angle of coherent phases in the seismogram section and the mainly heuristic restriction of the imaging operator to the neighborhood of that wavepath our approach uses fresnel volumes to restrict the migration operator in a physically frequency dependent way using the emergent angle at the receiver determined by a local slowness analysis a ray is propagated into the subsurface the back propagation of the wavefield is restricted to the vicinity of this ray according to its approximated fresnel volume this so called fresnel volume migration approach enhances image quality significantly compared with standard kirchhoff psdm because of the inherent focusing and the restriction of the back propagation to the region around the actual reflection point

an integrated approach to the study of glaciolacustrine landforms and sediments a case study from hagavatn iceland abstract this paper describes the landform assemblage associated with the neoglacial fluctuation of hagafellsjokull eystri into proglacial hagavatn iceland this landform assemblage is dominated by three components firstly a series of moraines formed both by thrusting within the glacier margin and by proglacial subglacial deformation the internal sedimentary architecture of these moraines ranges from stacked sediment slabs separated by listric thrusts to complexly folded and faulted sequences all in glaciolacustrine sediments the second component is a morainal bank and associated ice contact delta the morainal bank consists of a series of low amplitude ridges composed of laminated silts and clays which have been deformed proglacially during the neoglacial ice advance 1890 this morainal bank helped stabilise the 1890 ice limit within the proglacial lake and was the focus for subaqueous outwash first in the form of a subaqueous out wash fan and subsequently as an ice contact delta the third component is a series of lake bed kame terraces formed by rapid sedimentation in a canal like lake formed as the ice retreated away from the push moraines a range of glaciolacustrine facies is present within the lake bed kame terraces and the sedimentary architecture records a complex pattern of ice marginal oscillations and changes in lake level this detailed case study provides an important analogue for the interpretation of both glaciolacustrine and glaciomarine landform sediment assemblages of pleistocene age and illustrates a range of styles of glaciolacustrine sedimentation and glaciotectonic deformation

pleistocene glacitectonic landforms and sediments around central magellan strait southernmost chile evidence for fast outlet glaciers with cold based margins abstract proglacial and subglacial glacitectonic sediments and landforms around the margins of the strait of magellan southernmost chile record the advance and retreat of outlet glaciers of the patagonian ice cap during the last glacial cycle the spatial arrangement of glacitectonic landforms in the area is inferred to have resulted from the advance of ice lobes with cold based margins and wet based interiors as the ice advanced subglacial basins were excavated beneath the glacier margins and the eroded material was pushed up into thrust moraines probably because frozen bed conditions formed a thermal dam against the free drainage of subglacial meltwater these ice marginal glacitectonic landforms were then overridden and streamlined into drumlins and flutes as thicker wet based ice passed over the area evidence for permafrost near sea level in patagonia during the last glaciation suggests that mean annual temperatures were several degrees lower than indicated by recent modelling studies the results indicate that future modelling experiments should incorporate more realistic basal boundary conditions particularly the presence of a weak deforming layer at the glacier bed to improve climatic reconstructions of southern south america

theory of glacial erosion transport and deposition as a consequence of subglacial sediment deformation a theory of erosion transport and deposition of unlithified sediments by glaciers is presented it predicts the large scale areal distribution of zones and rates of erosion and deposition in time and space through a complete glacial cycle together with the resultant intensity of large scale lincations drumlins which will be incised in the landscape the theory also predicts the dispersal patterns of subglacial lithologies together with the form of dispersal trains derived from distinctive sources and the vertical and horizontal distribution of lithologies within a till it predicts major erosional discontinuities within tills and the formation of boulder pavements it suggests that the dominant proportion of the lowland tills produced by pleistocene mid latitude ice sheets was generated by subglacial deformation and explains why they are predominantly fine grained the theory is based on an analysis of glacier dynamic processes and therefore can be used to infer the dynamic behaviour of former ice sheets from the distribution of tills and their lithologic composition

depositional history of the north taymyr ice marginal zone siberia a landsystem approach the sediment landform associations of the northern taymyr peninsula in arctic siberia tell a tale of ice sheets advancing from the kara sea shelf and inundating the peninsula probably three times during the weichselian in each case the ice sheet had a margin frozen to its bed and an interior moving over a deforming bed the north taymyr ice marginal zone ntz comprises ice marginal and supraglacial landsystems dominated by thrust block moraines 2 3 km wide and large scale deformation of sediments and ice large areas are still underlain by remnant glacier ice and a supraglacial landscape with numerous ice walled lakes and kames is forming even today the proglacial landsystem is characterised by subaqueous e g deltas or terrestrial e g sandar environments depending on location altitude and time of formation dating results osl c 14 indicate that the ntz was initiated ca 80 kyr bp during the retreat of the early weichselian ice sheet and that it records the maximum limit of a middle weichselian glaciation ca 65 kyr bp during both these events proglacial lakes were dammed by the ice sheets part of the ntz was occupied by a thin late weichselian ice sheet 20 12 kyr bp resulting in subaerial proglacial drainage copyright c 2002 john wiley sons ltd

pleistocene periglacial eolian deposits in southwestern alaska sedimentary facies and depositional processes abstract pleistocene periglacial eolian sediments are widespread in the nushagak holitna and upper kuskokwim lowlands of southwestern alaska these sediments comprise mainly sand sheet deposits and sand loess intergrades with subordinate sand dune deposits and loess sand sheet deposits range from 1 sharply defined parallel low angle laminae formed by migrating wind ripples to 2 irregular subhorizontal strata which reflect migration of poorly segregated wind ripples accumulation on a sparsely vegetated surface adhesion on a quasi planar bed and or niveo eolian deposition sand loess intergrades represent alternating bed load and suspension deposition related mainly to the stochastic and seasonal variability of former wind systems the dominance of sand sheet deposits over deposits of well formed dunes in the pleistocene eolian record of southwestern alaska reflects the limited availability of loose dry sand in this former periglacial environment immobilization of sand after deposition as low relief sheets resulted from a seasonally variable combination of ice cementation sparse vegetation high water tables and snow cover

hagedoorn told us how to do kirchhoff migration and inversion migration and inversion as i expound on it is often lost on my audience because of my tendency to couch my explanations in mathematical terms well i really do come at these problems from the mathematical rather than the geo physical point of view and it is kind of hard for us leopards to change our spots on the other hand i really would like to expand my audience of kirchhoff inversion aficionados so i am always looking for alternative ways to explain our mathematical esoterica chalk this article up as an attempt to do just that

a platform for kirchhoff data mapping in scalar models of data acquisition kirchhoff data mapping kdm is a procedure for transforming data from a given input source receiver configuration and background earth model to data corresponding to a different output source receiver configuration and background model the generalization of nmo dmo datuming and offset continuation are three examples of kdm applications this paper describes a platform for kdm for scalar wavefields the word platform indicates that no calculations are carried out in this paper that would adapt the derived formula to any one of a list of kdms that are presented in the text platform formulae are presented in 3d and in 2 5d for the latter the validity of the platform equation is verified within the constraints of high frequency asymptotics by applying it to a kirchhoff approximate representation of the upward scattered data from a single reflector and for an arbitrary source receiver configuration the kdm formalism is shown to map this kirchhoff model data in the input source receiver configuration to kirchhoff data in the output source receiver configuration with one exception the method does not map the reflection coefficient thus we verify that asymptotically the ray theoretical geometrical spreading effects due to propagation and reflection including reflector curvature are mapped by this formalism consistent with the input and output modelling parameters while the input reflection coefficient is preserved in this sense this is a true amplitude formalism as with earlier kirchhoff inversion a slight modification of the kernel of kdm provides alternative integral operators for estimating the specular reflection angle both in the input configuration and in the output configuration thereby providing a basis for amplitude versus angle analysis of the data

frequency selective design of the kirchhoff migration operator1 integral migration techniques perform a sum over an aperture of input traces to obtain output at a single point the length of the aperture is limited by a spatial nyquist criterion which typically prohibits imaging very steep dips at very high frequencies without generating severe migration artifacts migration operator aliasing for time domain kirchhoff migration this can be a fatal shortcoming the standard way to address this problem is to interpolate traces spatially before migration this reduces the trace spacing thereby increasing the frequency content which can be migrated without aliasing at steep dips an alternative remedy to the operator aliasing problem is to modify the phase response of the kirchhoff migration operator this operator is frequency selective across the migration aperture it passes all temporal frequencies of the input traces in the innermost portion of the aperture referring to the shallow dips and gradually cuts out the higher frequencies as it approaches the outer portion of the aperture thus while all frequencies of the input data contribute to the shallow dip portion of the migrated image only the permissible low frequencies of the input data contribute to imaging the steepest dips using a simple realization of a frequency selective kirchhoff migration operator this technique is illustrated on a synthetic data set involving greater than vertical dips

the last ice sheet of the kara sea terrestrial constraints on its age abstract the ice dispersal features on the siberian mainland unanimously indicate that the last ice domes were positioned on the kara sea shelf and adjacent coastal plains the age of the last glacial maximum evokes much controversy the problem may be solved by sorting out the available radiocarbon dates both statistically and using geological common sense over 200 finite radiocarbon dates have been analysed 77 of the entire population represent sediments overlying the topmost till including dating the most reliable organics such as mammoth remains the rest of the dates 23 have been obtained from the underlying sediments mostly on materials readily prone to redeposition and contamination such as driftwood and shells the last culmination of the shelf ice sheet must have occurred beyond the range of the conventional radiocarbon method and after the last warm water sea transgression correlative to the eemian the early weichselian age for the last kara sea ice sheet is also supported by the lack of proglacial lacustrine sediments with finite radiocarbon dates in periglacial west siberia