electron spin resonance esr techniques applied to mollusc shells from south america chile argentina and implications for palaeo sea level curve esr dating of marine mollusc shells gives no evidence for a so called mid wisconsin sea level high stand in argentina and chile the idea of this high stand around 30 000 40 000 bp is still alive especially in argentina but the evidence for it is in our opinion and that of others solely based on the misinterpretation of 14c dates all esr ages from southern south america belong either to the holocene or the last interglacial or older interglacial periods apart from the regional aspect this paper deals with some methodological problems in esr dating estimates have been improved by using only mollusc shells that are still closed and in living position and by introducing a plateau screening test for the determination of the accumulated dose ad the estimate of the environmental dose has been refined by analysing the infill sediments in the shells for their u th and k contents separately from the embedding matrix special interest has been paid to the debated problem of early or continuous u uptake the u contents of recent holocene molluscs support the early uptake model a probable solution to the problem of age overestimation of holocene shells is also presented present day shells are shown to possess frequently a fully developed signal and to give ad of 2 4 in one case up to 9 gy if the holocene ad values of 14c dated shells are corrected by subtracting these recent and spurious values the resulting age estimates are more consistent with radiocarbon ages

neoproterozoic snowball earth simulations with a coupled climate ice sheet model ice sheets may have reached the equator in the late proterozoic era 600 800 myr ago according to geological and palaeomagnetic studies possibly resulting in a snowball earth but this period was a critical time in the evolution of multicellular animals posing the question of how early life survived under such environmental stress here we present computer simulations of this unusual climate stage with a coupled climate ice sheet model to simulate a snowball earth we use only a reduction in the solar constant compared to present day conditions and we keep atmospheric co2 concentrations near present levels we find rapid transitions into and out of full glaciation that are consistent with the geological evidence when we combine these results with a general circulation model some of the simulations result in an equatorial belt of open water that may have provided a refugium for multicellular animals

influence of surface topography on the critical carbon dioxide level required for the formation of a modern snowball earth abstractthe influence of continental topography on the initiation of a global glaciation i e snowball earth is studied with both a fully coupled atmosphere ocean general circulation model aogc

absence of a runaway ice albedo feedback in the neoproterozoic a runaway ice albedo feedback at low latitudes has been proposed as an essential mechanism for achieving an ice covered earth previous simulations of the neoproterozoic climate using a coupled ocean atmosphere model have failed to trigger this feedback but have also not simulated sea ice equatorward of 30 to directly test for a runaway ice albedo feedback a 1000 yr simulation of the neoproterozoic using a coupled ocean atmosphere general circulation model was completed with sea ice specified to 10 latitude in this experiment low latitude sea surface temperatures remain above freezing inhibiting sea ice advance toward the equator an ice free ocean is maintained by a low latent heat flux to the atmosphere and low ocean heat transport from low latitudes a neoproterozoic climate with low latitude sea ice is not stable once the sea ice specification was released allowing sea ice to melt the sea ice margin retreated poleward of 45 latitude these climate simulations demonstrate the resilience of earth s climate and emphasize the extreme conditions required to initiate a hard snowball earth

sedimentary challenge to snowball earth evidence from the magnetic field fossilized in sedimentary rocks suggests that more than 600 million years ago ice occupied tropical latitudes a popular explanation for these findings the snowball earth concept envisages a fully frozen earth for millions of years caused by a runaway ice albedo feedback a rapid catastrophic meltback at very high levels of atmospheric carbon dioxide is thought to have ended this extreme climatic state however sedimentary rocks deposited during these cold intervals indicate that dynamic glaciers and ice streams continued to deliver large amounts of sediment to open oceans throughout the glacial cycle the sedimentary evidence therefore indicates that despite the severity of glaciation some oceans must have remained ice free significant areas of open ocean have important implications for the survival and diversification of life and for the workings of the global carbon cycle the snowball earth concept envisages a fully frozen earth for millions of years several times during the neoproterzoic era between 1 000 and 542 million years ago however the sedimentary evidence suggests that despite the severity of glaciation some oceans must have remained ice free

global glaciation in the neoproterozoic reconciling previous modelling results 1 recent neoproterozoic climate modelling experiments using different sea ice components have yielded opposing results here we have attempted to reconcile these differences through the implementation of various representations of sea ice in the university of victoria earth system climate model in particular using a sea ice model similar to that of the fast ocean atmosphere model version 1 4 global glaciation is not achievable whereas it occurs with an energy conserving version of a more physically complete thermodynamic model we argue that using a physically incomplete representation of sea ice may be a major factor in the inability of some previous coupled models to simulate global glaciations

shear and compressional wave structure of the east pacific rise 9 10 n we use shear and compressional arrivals recorded by 45 ocean bottom seismograph receivers to model regional structure of the 9 10 n region of the east pacific rise modeling indicates that shear conversion occurs at the base of layer 2a producing ps energy that travels through oceanic crust as p and converts to s at the base of layer 2a on upgoing path and ps energy that travels as p through layer 2a and converts to s at the base of layer 2a on downgoing path arrivals the travel times of these arrivals require a shear wave velocity within layer 2a of 0 4 0 8 km s poisson s ratio of 0 46 0 49 waveform inversion was used to model the poisson s ratio structure of layers 2b and 3 for nine instruments with good shear arrivals poisson s ratio within layer 2b was highly variable with values as low as 0 24 and a mean value of 0 263 some of this variability might be due to lateral variability in layer 2a structure the mean poisson s ratio of layer 3 was 0 271 according to the cracking model of shaw 1994 the low poisson s ratios within the upper portion of layer 2b indicate that thick cracks aspect ratio 0 1 extend to depths of 1 5 1 7 km in this region two dimensional travel time modeling indicates that the southwestern portion of our study area is associated with anomalously low seismic velocities with layer 2b and layer 3 velocities reduced by up to 11 from the regional values cracking theory suggests that these low velocities could be caused by porosities of 0 3 5 5 depending on the crack aspect ratio a maximum porosity of 1 5 is predicted from our one poisson s ratio measurement in this area the anomalous velocities are located near the western discordant zone left by the 9 03 n overlapping spreading center we suggest that shearing associated with on axis rotation of the overlap basins is responsible for the low seismic velocities the pattern of anomalies indicates that faulting extends to distances of 10 15 km from the basins

waveform inversion for one dimensional near surface structure in the new madrid seismic zone waveform inversion is applied to p and s waveforms selected from microearthquakes of duration magnitude m d between 1 8 and 2 4 recorded at station parm in stahl farm missouri operated by the cooperative new madrid seismic network in the upper mississippi embayment to resolve the one dimensional 1d velocity structure of uppermost near surface soils constrained by vertical seismic profiling vsp and standard cone penetration test scpt data at nearby sites we begin by defining a gradient velocity structure in the soils a reference velocity model is constructed by grid search in which the observed p and p s wave shapes graphic 1 travel time and p p s amplitude ratio are the controlling factors in the systematic selection from among 136 000 models a waveform inverse scheme is then implemented to obtain the least spectral misfit and best waveform correlation between synthetic and observed data four simultaneous inversions of joint waveforms from p and s wave types are performed resolved four 1d final models with associated uncertainties are listed in this article the uppermost 7 m of near surface soil exhibit very low velocities through which v p and v s are in the ranges of 140 m s to 470 m s and 95 m s to 215 m s respectively this noninvasive technique demonstrates that the observed high frequency reverberations 1 17 hz of p s and p s waveforms from local microearthquakes can be utilized to provide a view of the near surface soil structure although there are trade offs between layer thickness and slowness this method explains much of the high frequency site response due to microearthquake wave propagation through the very low velocity near surface soils 1 embed inline graphic 1 gif

deepening of the axial magma chamber on the southern east pacific rise toward the garrett fracture zone a wide aperture profile along the ridge axis from 14 29 s to 13 39 s 120 km to 30 km south of the garrett fracture zone is analyzed to constrain the thickness of layer 2a and the depth to the axial magma chamber reflector five areas along the 90 km line are examined in detail with several consecutive gathers being analyzed for each area to establish the degree of consistency within each area a genetic algorithm code is used to find a best fit model from a comparison of the data and wkbj synthetic seismograms one hundred starting models are generated using a predefined set of velocity nodes with a fixed window of allowable depth variations between nodes an evolutionary process favors the better fitting models in each generation and a satisfactory misfit is usually obtained within 40 generations within individual areas the models were in good agreement with the depth of a given velocity node generally varying by not more than 20 m the depth discretization interval for the models a consistent deepening trend of the axial magma chamber amc is observed across the five areas as the garrett fracture zone is approached the depth varies from 0 99 km at area 1 which is approximately 100 km south of the garrett to 1 23 km at area 5 which is approximately 40 km south of the garrett the depth to the axial magma chamber is highly sensitive to any ship wander off axis since layer 2a thickens rapidly off axis with age for the areas examined here layer 2a is observed to be relatively constant in thickness along the axis although it is about 40 m thicker over area 5 where the axial magma chamber is deepest this variation is within the scatter of previously detailed layer 2a measurements at 13 n on the east pacific rise where an effectively constant thickness is observed this implies that layer 2a thickening is not a significant factor along this profile and that the amc deepening is real rather than apparent theoretical modeling suggests that the depth to the lid of the axial magma chamber is related to the rate of heat supply at a given location thus the gradual consistent deepening of the axial magma chamber can be taken as an indication of a slightly reduced magma supply toward the garrett fracture zone which marks a major interruption of hundreds of kilometers of continuous ridge axis the deepening may also be interpreted as a downward limb from a central injection point however there is no indication of a similar downward trend in the other southern direction furthermore there is no accompanying systematic variation in axial depth or axial volume both of which are proposed to be indicators of central injection and along axis flow

late quaternary stratigraphy glacial limits and paleoenvironments of the marresale area western yamal peninsula russia stratigraphic records from coastal cliff sections near the marresale station on the yamal peninsula russia yield new insight on ice sheet dynamics and paleoenvironments for northern eurasia field studies identify nine informal stratigraphic units from oldest to youngest the marresale formation labsuyakha sand kara diamicton varjakha peat and silt oleny sand baidarata sand betula horizon nenets peat and chum sand that show a single glaciation and a varied terrestrial environment during the late pleistocene the kara diamicton reflects regional glaciation and is associated with glaciotectonic deformation from the southwest of the underlying labsuyakha sand and marresale formation finite radiocarbon and luminescence ages of ca 35 000 to 45 000 yr from varjakha peat and silt that immediately overlies kara diamicton place the glaciation 40 000 yr ago eolian and fluvial deposition ensued with concomitant cryogenesis between ca 35 000 and 12 000 cal yr b p associated with the oleny and the baidarata sands there is no geomorphic or stratigraphic evidence of coverage or proximity of the yamal peninsula to a late weichselian ice sheet the nenets peat accumulated over the baidarata sand during much of the past 10 000 yr with local additions of the eolian chum sand starting ca 1000 yr ago a prominent betula horizon at the base of the nenets peat contains rooted birch trees ca 10 000 to 9000 cal yr old and indicates a 200 km shift northward of the treeline from the present limits corresponding to a 2 to 4 c summer warming across northern eurasia

ice age paleotopography a gravitationally self consistent theory of postglacial relative sea level change is used to infer the variation of surface ice and water cover since the last glacial maximum lgm the results show that lgm ice volume was approximately 35 percent lower than suggested by the climap reconstruction and the maximum heights of the main laurentian and fennoscandian ice complexes are inferred to have been commensurately lower with respect to sea level use of these ice age boundary conditions in atmospheric general circulation models will yield climates that differ significantly from those previously inferred on the basis of the climap data set

past climate and the role of ocean heat transport model simulations for the cretaceous a series of general circulation model experiments using global environmental and ecological simulation of interactive systems genesis were executed to evaluate the sensitivity of simulated mid cretaceous climate to small perturbations in ocean heat transport three experiments were performed 1 mixed layer ocean with no ocean heat transport zeroq 2 ocean heat transport specified as required for genesis to best match modern observations oneq and 3 doubled ocean heat transport twoq the oneq experiment represents an ocean heat transport which is actually about 15 of the values given by carissimo et al 1985 from modern observations as a sensitivity experiment these model simulations represent a doubling of the role of the ocean however relative to the observations they represent small perturbations to the total poleward heat transport in the model with the exception of the tropics no major changes in the structure of the general circulation of the atmosphere resulted from the modification of the ocean heat transport however relatively small increases in ocean heat transport resulted in a number of significant differences between simulations including tropical cooling polar warming weakened equator to pole surface temperature gradients weakened midlatitude jets decreased land sea pressure contrast and decreased midlatitude storminess the experiments indicate that changes in ocean heat transport which are well within the realm of possibility for earth history can have significant climatic impact although ocean heat transport may be a significant factor in explaining cretaceous polar warmth the changes specified in these experiments alone are not sufficient to explain the polar warmth of the cretaceous

isolation of source and receiver statics and scale factors in waveform inversion of marine refraction data in this paper i present a method that uses multiple ocean bottom instruments to separate variations in seismic amplitudes and travel times caused by near source topography and variable source amplitudes from similar variations related to the overall one dimensional seismic structure the results indicate that the standard correction for topography sediment thickness and source size can be improved with this method reducing uncertainties in the velocity solution

thermoluminescence dating of sand dunes in rajasthan india we report here a new application of thermoluminescence tl for dating sand dunes this application assumes that exposure to sunlight causes bleaching of the geological tl of a sediment to a residual value and that tl accumulation starts again when the sediment is buried under fresh deposit and is thus shielded from the sun although the tl method has been applied to loess1 2 and ocean sediments3 4 the application of this technique to sand dunes provides the first reliable dating control for dune dynamics palaeoclimatology and spread of deserts

neoproterozoic earth moon dynamics rework of the 900 ma big cottonwood canyon tidal laminae the tidal rhythmites in the proterozoic big cottonwood formation bcc of utah the neoproterozoic elatina formation of the flinders range of southern australia and the lower pennsylvanian mansfield formation of indiana indicate that the rate of retreat of the lunar orbit has been approximately constant since the late precambrian and closely matches the present epoch value determined from apollo lasers examination of a new bcc core leads to a slightly revised length of day lod and modified values for other earth moon dynamical parameters thus implying the length of day 900 million years ago reported earlier by sonett et al 1996 should be modified to 18 9 hrs

waveform inversion of time corrected refraction data summary seismic refraction data collected in land or marine surveys are often complicated by small near random time shifts of the individual seismograms relative to one another for the case of fixed receiver geometry these shifts arise primarily from basement topography often masked by sediments beneath each source although a topographic timing correction is normally applied to the seismograms the effects of poorly known topography produce residual shifts in the seismograms the application of additional time corrections statics can bias solutions computed using iterative waveform inversion because a suitable set of statics can make a poor starting velocity model appear to match the data without any correction however seismograms computed from even the best fitting laterally homogeneous model may not agree with the data to within one half period in this paper i present a formalism for iterative waveform inversion of refraction data containing residual time shifts in this method the statics are directly modelled in the synthetic seismograms and new values estimated in the inversion together with new velocity model parameters the method is similar to studies of statics in seismic reflection data but in this case values for the statics are estimated simultaneously with the velocity model with the use of suitable norms the new velocity model is required to be smooth and the statics required to be small convergence is achieved when the velocity profile s roughness the rms size of the statics and the waveform misfit are in equilibrium because the starting synthetics need not match the data to within one half period a much larger set of velocity models used for an initial guess will converge on the family of solutions than for waveform inversions in which time shifts are not used the original arbitrary starting model loses its importance after the first iteration and solutions meeting the convergence criterion appear to be independent of the particular choice of starting model numerical tests show the method to be robust an inaccurate starting model that fits the data only through a biased set of statics is accurately corrected the method is illustrated on a set of refraction data collected in the vicinity of the east pacific rise in this application two quite different starting velocity models converge to a common family of solutions

sediment deformation beneath glaciers rheology and geological consequences experiments beneath breidamerkurjokull in iceland have led to development of flow laws for the subglacial till relating strain rate to shear stress and effective pressure and assuming either bingham fluid or nonlinearly viscous fluid behavior water pressures in the till are less than ice pressures and it is suggested that this may lead to infiltration of ice into the sediment which inhibits sliding at the ice sediment interface where water pressures are equal or near to ice pressures infiltration does not occur and sliding may result a one dimensional theory of subglacial deformation is developed in which the empirical flow law is coupled with a model of subglacial hydrology and consolidation this predicts stable states in which subglacial sediment either does not deform or a dilatant deforming horizon forms with positive effective pressures at the ice bed interface or unstable states where zero or negative effective pressures are predicted time dependent analyses show that response times following perturbations of the system may be of the order of 103 years and thus that unsteady behavior may be normal on glaciers flowing over unlithified sediment beds it is suggested that the natural variability of material properties in subglacial sediment beds leads to the development of drumlins on the glacier bed it is suggested that unstable deformation at zero or negative effective stress leads to piping in subglacial sediments at the glacier terminus and the growth of sediment floored subglacial tunnels their frequency is that which is sufficient to draw down subglacial water pressures so as to prevent unstable deformation where they discharge large water volumes subglacial sediments flow laterally toward them producing tunnel valleys this sediment is then removed by water flowing along the axial tunnel tunnel valleys can be regarded as the equivalent in soft sediment areas of eskers in bedrock areas

glaciotectonised quaternary sediments at cape shpindler yugorski peninsula arctic russia implications for glacial history ice movements and kara sea ice sheet configuration the coastal cliffs of cape shpindler yugorski peninsula arctic russia occupy a key position for recording overriding ice sheets during past glaciations in the kara sea area either from the kara sea shelf or the uplands of yugorski peninsula polar urals this study on late quaternary glacial stratigraphy and glaciotectonic structures of the cape shpindler coastal cliffs records two glacier advances and two ice free periods older than the holocene during interglacial conditions a sequence of marine to fluvial sediments was deposited this was followed by a glacial event when ice moved southwards from an ice divide over novaya zemlya and overrode and disturbed the interglacial sediments after a second period of fluvial deposition under interstadial or interglacial conditions the area was again subject to glacial overriding with the ice moving northwards from an inland ice divide the age control suggests that the older glacial event could possibly belong to marine oxygen isotope stage mois 8 drenthe 300 250 ka and that the underlying interglacial sediments might be holsteinian 300 ka one implication of this is that relict glacier ice buried in sediments and incorporated into the permafrost may survive several interglacial and interstadial events the younger glacial event recognised in the cape shpindler sequence is interpreted to be of early to middle weichselian age it is suggested to correlate to a regional glaciation around 90 or 60 ka the cape shpindler record suggests more complex glacial dynamics during that glaciation than can be explained by a concentric ice sheet located in the kara sea as suggested by recent geological and model studies copyright 2003 john wiley sons ltd

precambrian climate the effects of land area and earth s rotation rate rotation rate and land area have changed significantly over the earth s history the associated climate changes are examined with a general circulation model gcm and compared with the current climate state unlike previous gcm studies clouds in the model interact with the radiation field we show that rotation rate changes can affect mean air temperatures by strongly affecting the cloud field for example an earth day length of 14 hours causes the global mean cloud fraction to decrease 20 there are also large changes in the wind field indeed at some mid latitudes the time averaged surface wind changes from westerlies to easterlies if the land masses are removed from the model and with present day rotation the global mean air temperature is 4 k higher than for the present day simulation if in addition the day length is 14 hours the global mean air temperature is 5 5 k thus the early atmosphere may not have contained the very large concentrations of co2 thought to be necessary to prevent a frozen planet higher

gcm sensitivity test using increased rotation rate reduced solar forcing and orography to examine low latitude glaciation in the neoproterozoic during the neoproterozoic period most landmasses were assembled into the super continent rodinia and parts experienced glaciation in low paleolatitudes we examine possible causes of glaciation by increasing rotation rate reducing the solar constant and carbon dioxide concentrations for an idealized super continent that is centered in the northern hemisphere tropics subtropics further we introduce a 2 km north south mountain chain in the western regions of this super continent a mixed layer ocean or prescribed time varying sea surface temperatures are used in these simulations our results show that neither an individual factor or a combination of these factors can cool temperatures enough to bring about glaciation on the tropical super continent we conclude that other factors would be needed to initiate glaciation

procedures used for luminescence dating of sediments abstract an account of the procedures for thermoluminescence tl and optically stimulated luminescence osl dating of sediments used at the nordic laboratory for luminescence dating is given the topics discussed include sampling sample preparation separation of quartz and feldspars using heavy liquids dose rate estimation and methods for estimating the equivalent dose special attention is paid to the partial bleach tl technique of mejdahl 1988 which appears to give reliable results for partially bleached sediments optically stimulated luminescence dating using stimulation by infra red ir and green light has been taken up and especially added dose methods using stimulation by ir or green light have been tried with some success some sources of error are discussed including the adverse effect of shallow traps

rupture process of the 1948 fukui earthquake m 7 1 from the joint inversion of seismic waveform and geodetic data 1 we used teleseismic body waves regional low gain strong motion seismograms and horizontal geodetic displacements from triangulation surveys to invert for the slip distribution of the 1948 fukui japan earthquake using the multiple time window method the earthquake ruptured unilaterally from a depth of 10 km to the north with very little slip 0 0 5 m evenly distributed across a fault of 24 by 18 km over a period of about 10 s at 9 s after the initiation of rupture an adjacent fault along the same strike ruptured to the south from the hypocenter to about 20 30 km toward fukui city mostly along a shallow asperity between 0 and 12 km depth the large amount of shallow slip 1 1 5 m below and within the sedimentary basin and strong rupture directivity likely contributed to the high amount of damage in fukui city we obtain a total seismic moment of 1 6 2 2 1026 dyn cm mw 6 74 6 84 which is similar to results by kanamori 1973 sagiya 1999 and kikuchi et al 1999 using portions of the same regional strong motion and geodetic data sets we needed to use very low rupture velocities 1 5 km s and a hypocenter 10 20 km north of the original location to fit the combined geodetic and seismic data sets with a single fault model we developed an alternative fault model with two segments allowing for different rupture dynamics for each segment this resulted in a slightly better fit to the data and allows for a more reasonable rupture velocity up to 2 4 km s

least squares fitting of marine seismic refraction data summary an iterative procedure is presented for fitting waveform data from a marine seismic refraction experiment the wavefunction from the explosive source is known and the crustal structure is refined using the damped least squares procedure the damping parameter serves the dual purpose of stabilizing an under constrained inversion and improving the linearity by suppressing high frequencies the synthetic seismograms and their model differentials are calculated using the wkbj seismogram method both the synthetic seismograms and the linear algebra are sufficiently straightforward that the computations can be performed on an array processor the inversion procedure is then sufficiently rapid that interactive computations are possible the technique is illustrated using the ff2 refraction data from the fanfare cruise of the scripps institution of oceanography these data had been interpreted previously by trial and error using the reflectivity method starting from two different simple models the inversion procedure obtains essentially one unique model its features are very similar to the previous model

till fabric associated with deformable beds this paper investigates the development of till fabric associated with glaciers overlying deformable beds first the till fabrics from both modern and ancient soft bed and hard bed subglacial environments are investigated and then soft bedded sites with specific strain histories are examined the results indicate that lodgement tills had a strong fabric whilst there is a wide range of much weaker fabric strengths associated with soft bed tills it is suggested from a theoretical study of soft bed till deposition that longitudinal and vertical processes in the subglacial deforming layer are important in fabric development field sites were chosen to isolate these two factors and fabrics investigated in areas of known relative shear strain and known relative thickness of the deforming layer the results indicate that 1 for a fixed deforming layer fabric strength does increase with increasing deformation except at very high strains where the development of a transverse fabric can lead to a reduction in the overall fabric strain and 2 lodgement tills and soft bed tills associated with a thin deforming layer have a high fabric strength whilst soft bed tills associated with a thick deforming layer have a low fabric strength where the deforming layer is thin clasts are oriented parallel with the glacier direction either by lodging against a hard bed or ploughing against a soft bed however where the deforming layer is thicker and shear strains are high melt out processes encourage a parallel fabric but the increased shear strain and changes in the deforming layer thickness cause the clasts to develop both a parallel and transverse fabric these processes combine to reduce the overall fabric strength till fabric studies are a further sedimentological technique to separate the depositional processes associated with a homogeneous till

fabric signature of subglacial till deformation breidamerkurjokull iceland the foreland of breidamerkurjokull iceland is the only locality where tills known to have undergone subglacial deformation are exposed till on the foreland has a two tiered structure consisting of a dilatant upper horizon c 0 5 m thick and a compact lower till these horizons correspond to the ductile deforming a horizon and the brittle ductile b horizon observed below the glacier by g s boulton and co workers the relationship between known strain history and a variety of macrofabric elements is examined for these two genetic facies of deformation till the upper horizon exhibits variable a axis fabrics and abundant evidence for clast re alignment reflecting ductile flow and rapid clast response to transient strains in contrast the lower horizon has consistently well organized a axis fabrics with a narrow range of dip values recording clast rotation into parallel with strain axes during brittle or brittle ductile shear the data indicate that till strain history imparts identifiable macrofabric signatures providing important analogues to guide the interpretation of pleistocene tills

esr dating of quaternary shells recent advances abstract this paper deals with the key problems of the esr dating of subfossil mollusc shells the estimation of the stored palaeodose of natural radiation it is shown that the use of the analytical line or peak in the integrated spectrum at g 2 0012 0 0001 with the half width 0 6 mt seems most advisable for the estimation of the relative concentration of radiation defects equivalent to the stored palaeodose in the calcareous shell skeleton optimal esr spectrometer settings have been determined results of esr dating of subfossil mollusc shells from eemian deposits of boreal transgression are discussed

the east pacific rise in cross section a seismic model in 1982 we undertook a seismic refraction experiment known as the magma expedition to examine the detailed structure of the east pacific rise near 12 50 n this segment of the rise where the full spreading rate is about 110 mm yr is near the projected trace of the o gorman fracture zone and is the site of an overlapping spreading center as well as black smoker hydrothermal activity in this paper we describe the analysis of a subset of the travel time data collected during the magma expedition namely the data from profiles which were oriented normal to the rise axis these profiles provide a data set roughly equivalent to those collected on other experiments and sample the cross sectional structure of the rise we have modeled these data using a two dimensional ray tracing program we have found that the seismic velocities in the young oceanic crust are rather high with velocity gradients of 4 0 5 5 s 1 in the uppermost crust the highest velocities at the seafloor occur beneath the rise axis itself and seem to decrease as the crust ages to 0 1 ma this decrease in velocity must result from an increase in porosity in the upper crust and may coincide with the development of abundant surface fissures as the crust spreads the decrease in velocity does not appear to penetrate deeper than about 0 5 km and may reverse itself as hydrothermal alteration fills the pores and cracks the fact that the highest velocities occur under the rise axis suggests that the hydrothermal circulation responsible for the black smokers is confined to seismically unresolved conduits a result consistent with the high temperatures and discrete nature of the vents our best model includes a magma chamber some 4 km wide and extending from the moho to within about 1 1 km of the seafloor this magma chamber is far smaller than many models for the rise axis have predicted but larger than those inferred from seismic refraction experiments at other sites on the east pacific rise these discrepancies probably arise because the magma chamber under the east pacific rise is not a steady state feature but changes with time because of hydrothermal cooling and perhaps because of an episodic supply of magma from the mantle or along the rise axis

partitioned waveform inversion and two dimensional structure under the network of autonomously recording seismographs nonlinear optimization methods are very effective in finding models that predict optimal waveform fits to observed seismograms provided the number of model parameters is small to reduce the size of the large scale two or three dimensional seismic modelling problem path integrals over seismic velocities may be defined as model parameters instead of the seismic velocities themselves this partitions the large scale optimization problem into a number of independent small scale problems and reduces the computational effort by several orders of magnitude the path integral parameters are transformed such that their correlation matrix becomes diagonal a linear inversion performs the mapping from the space of path integral parameters into the model space of seismic velocities and makes backus gilbert resolution analysis possible the method has been applied to body wave and surface wave data from the broadband portable network of autonomously recording seismographs nars in western europe using a surface wave ray approximation to generate synthetics by mode summation waveforms were fitted for frequencies as high as 50 mhz variations in s velocity as large as 6 1 have been established with a vertical resolution of 100 150 km and a horizontal resolution as small as 400 km in parts of the model although a low velocity layer is present under the southern part of the nars array such layer is very weak or absent north of the paris basin horizontal velocity gradients are well resolved down to 400 km depth

the impact of atmospheric and oceanic heat transports on the sea ice albedo instability during the neoproterozoic in order to simulate the climatic conditions of the neoproterozoic we have conducted a series of simulations with a coupled ocean atmosphere model of intermediate complexity climber 2 using a reduced solar constant of 6 and varied co2 concentrations we have also tested the impact of the breakup of the supercontinent rodinia that has been hypothesized to play an important role in the initiation of an ice covered earth our results show that for the critical values of 89 and 149 ppm of atmospheric co2 a snowball earth occurs in the supercontinent case and in the dislocated configuration respectively the study of the sensitivity of the meridional oceanic energy transport to reductions in co2 concentration and to the dislocation of the supercontinent demonstrates that dynamics ocean processes can modulate the co2 threshold value below which a snowball solution is found but cannot prevent it the collapse of the overturning cells and of the oceanic heat transport is mainly due to the reduced zonal temperature gradient once the sea ice line reaches the 30 latitudinal band but also to the freshening of the tropical ocean by sea ice melt in term of feedbacks the meridional atmospheric heat transport via the hadley circulation plays the major role all along the co2 decrease by increasing the energy brought in the front of the sea ice margin but does not appear enough efficient to prevent the onset of the sea ice albedo instability in the case of the continental configurations tested in this contribution

testing paleogeographic controls on a neoproterozoic snowball earth 1 the distribution of continents during the neoproterozoic has been hypothesized to play an important role in the initiation of an ice covered earth in this study the influence of paleogeography on the neoproterozoic climate is evaluated using a fully coupled ocean atmosphere general circulation model foam three simulations were completed with different continental distributions each simulation included a reduced solar luminosity 93 of present day and low atmospheric co2 140 ppmv model results indicate that a low latitude concentration of continents leads to lower tropical temperatures through reduced receipt of shortwave radiation and a smaller tropical greenhouse effect but does not significantly affect high latitude temperatures or sea ice extent in contrast the presence of snow covered mid and high latitude continents increases the sensible heat transport over the ocean giving rise to sea surface cooling deep water formation and an advanced sea ice margin nonetheless an ice covered earth is not simulated in these experiments

benthic foraminiferal and stable isotope evidence for a younger dryas style cold spell at the saalian eemian transition denmark abstract benthic foraminiferal and stable isotope studies of two borings from the island of anholt denmark reveal a continuous late saalian eemian sequence with an overall gradual warming interrupted by a short term temperature deterioration comparable with the younger dryas cold spell the sequence is correlated to the global deep sea stable isotope record based on environmental evidence given by foraminiferal faunas and stable isotopes the following environmental changes in the anholt cores can be attributed to the global variations 1 1 an interstadial phase the flakket interstadial which had a duration of about 1000 years is correlated to stable isotope event 6 01 2 2 the following cold spell the kattegat stadial spanning about 1000 years represents most of the interval between isotope events 6 01 and 6 0 3 3 a phase of rapid warming within about 1000 years bsotope events 6 0 and 5 53 4 4 a sea level change within the eemian between isotope events 5 53 and 5 51

approaches to the study of glacier bed deformation investigation of glacier bed conditions and subglacial processes is an obligatory requirement for the modelling and understanding of contemporary glacier behaviour past glacier behaviour glacier response to both spatial and temporal variations in bed material and glacier response to external forcing factors such as climate change the recognition of the importance of coupling between the glacier and its bed resulted in a paradigm shift in the subject and drew attention to the critical importance of subglacial deformation as opposed to simply subglacial deposition and sliding a number of methods have been adopted to measure glacier bed deformation ranging from direct observation and instrumentation the interpretation of macroscale and microscale sedimentary structures to the interpretation of glacier bedforms and glacier bed modelling this paper is concerned to describe the methods used by the two authors and co workers and attention is given to field methods the type and arrangement of sedimentary structures thin section micromorphology and microfabric and clast microfabric and glacier bedform morphology as evidence of subglacial deformation sedimentary structures macro and mesoscale are used to identify constructional deformation where deformational units are preserved one on top of another due to the formation of a thin deforming layer by low basal stresses and excavational deformation in which the deformational style is superimposed on pre existing styles due to increasing thicknesses of glacier deformation caused by increasing basal shear stresses thin section micromorphology has been used to define a range of glacial processes of which subglacial deformation is just one but has drawn attention to critical criteria such as discrete microscale shears orientated matrix kink structures and fractured grains as well as deformed intraclasts that can be used to recognise deformational processes more recently individual aggregates taking the form of rounded pebbles within till matrix have been shown to reflect rotational processes during glacial deformation and provide an additional mechanism to discrete shear within subglacial deformation and an explanation for the transport paths of subglacial materials finally measurements of sediment flow directions using clast macrofabric characteristics coupled with measurement of quantities of sediment transfered and detailed palaeoglacial properties have been used to determine the rates and scales of subglacial deformation associated with the formation of small scale glacier bedforms

the interpretation and classification of subglacially deformed materials abstract a general classification of subglacially deformed materials is proposed based on sediment properties and their relationship to styles of subglacial strain deformation till is defined as homogenized usually diamictic material formed by glacially induced shear of subsole materials three types are recognized type a formed by pervasive ductile deformation type b formed by brittle shear and comminution till produced by the reduction of void space by in situ crushing and abrasion the term glacitectonite is adopted for materials that have undergone subglacial shear but retain some of the structural characteristics of the parent material original structures may be truncated by glacitectonic fabric elements type a or distorted but not truncated type b the principles of strain and material response particle orientation mechanisms fabric development and drainage conditions are reviewed examples of deformation tills and glacitectonites from breidamerkurjokull iceland slettmarkbreen norway loch lomond scotland and east yorkshire england are used to illustrate the nature of subglacially deformed materials and their relationship to former subglacial strain and drainage conditions the geologic evidence suggests a continuum of bed strengths can be recognized in deforming substrata ranging from high strength low strain type b deformation till and comminution till to low strength high strain type a deformation till

eustatic sea level and climate changes over the last 600 ka as derived from mollusc based esr chronostratigraphy and pollen evidence in northern eurasia we reconstruct and correlate palaeoclimatic events and deposits from shelf glacial periglacial and extraglacial zones of northern eurasia over the last 600 000 years the chronostratigraphical correlation of identified palaeoenvironmental and sealevel events and corresponding horizons is based on electron spin resonance esr analysis of subfossil mollusc skeletal remains from marine freshwater and acheulian bearing cave site deposits over 230 shell samples from more than 40 sites along the continental margin of eurasian north in the black and caspian sea basins and terrestrial shells from a lower palaeolithic cavesite in the northern caucasus were dated via esr to produce a late quaternary geochronology the pleistocene composite section of the loess palaeosoil formation includes two reference sections likhvin and arapovichi from the centre of the easteuropean plain the palyno chronostratigraphic record is interpreted as the product of six warm climate high sea level events including the current interglacial and six glacial events they are presented either as complete climatic rhythms of glacial and interglacial rank or by considerable portions of climatic phytocoenotic phases constituents of the rhythm the full interglacial conditions are centred at about 580 400 310 220 and between 145 70 calendar ka a broad correspondence between long palynological sequence directly esr dated warm climate related events and other palaeoenvironmental records described in the literature has been noted for 11 upper oxygen isotope stages 11 to 1 the results obtained in this study exemplify the potential of integrated chrono climatostratigraphic sequences in linking marine and terrestrial palaeoclimate records that may eventually span the whole brunhes chron d 2002 elsevier science b v all rights reserved

neoproterozoic snowball earth dynamic sea ice over a quiescent ocean 1 low latitude sea level glacial deposits suggest the existence of snowball earth conditions in the neoproterozoic previous modeling studies have offered conflicting support for the snowball hypothesis we use a climate model of intermediate complexity including an ocean gcm and a sophisticated thermodynamic dynamic sea ice component to conduct a suite of experiments with different orbital paleogeographical configurations wind forcing and atmospheric co2 levels we show that depending on the orbital configuration and paleogeography snowball conditions prevail even with atmospheric co2 levels up to 1800 ppmv overall our modeling paradigm is consistent with the original snowball hypothesis in which an ice covered ocean surrounds a largely snow and ice free barren land with some coastal regions permitting the growth of thick glaciers

gcm response to late precambrian 590 ma ice covered continents recent coupled energy balance ice sheet modeling studies indicate that ice covered continents can be simulated for the late precambrian with 6 solar constant reduction we examine the ocean mixed layer response to such an ice sheet with the genesis 2 general circulation model and co2 levels varying from 0 5 2 5 times present the ocean ices over completely at 0 5 1 0x present levels with the final phase of sea ice growth occurring within a single model year a qualitatively different 2 5x co2 solution is close to equilibrium and yields open water between 25 n and 25 s paleolatitude the prevailing wind patterns suggest that an equatorial pacific type circulation may have developed in part of the neoproterozoic ocean

radiocarbon content of pre bomb marine mollusks and variations in the 14c reservoir age for coastal areas of the barents and kara seas russia fourteen mollusks collected alive between 1900 and 1945 from the russian barents and kara seas were analyzed by ams 14c dating to evaluate variations in the 14c marine reservoir for arctic coastal sites which is important for correcting ages in paleoenvironmental time series and advancing understanding of the exchange of carbon the 14c ages on the mollusks reveal a range of marine reservoir values r t from 159 14c yr to 764 14c yr the oldest r t values of 764 to 620 14c yr are for the bivalve portlandia arctica which often inhabit cold and low salinity waters and muddy substrates the depleted 14c content for this bivalve reflects possibly the incorporation of old carbon from freshwater inputs and or the consumption of old organic matter from the underlying sediments and pore waters other mollusks with sessile habitats and pelagic food sources gave significantly lower r t values between 159 and 344 14c yr the youngest r t values indicate enrichment in 14c and may partially reflect enhanced transfer of 14c enriched co2 from the atmosphere to the ocean surface with wind generated wave agitation this study underscores that a variety of processes can lead to variable 14c depletion and enrichment of surface waters yielding a ca 600 year age span for contemporaneous arctic mollusks there may be added uncertainty in the 14c reservoir correction for deposit feeder species such as portlandia sp and perhaps for certain benthic foraminifera e g nonion labradoricum because these taxa often incorporate old organic matter from the substrate a reservoir correction of 700 years may be more appropriate for infaunal deposit eater species particularly in glacier dominated environments mollusks and foraminifera with sessile habits and pelagic food sources should be selected preferentially for 14c dating because their shells may more closely reflect the 14c content of the global ocean mixed layer

a constant daylength during the precambrian era the semidiurnal atmospheric thermal tide would have been resonant with free oscillations of the atmosphere when the day was approximately 21 h long c 600 ma ago very large atmospheric tides would have resulted with associated surface pressure oscillations in excess of 10 mbar in the tropics near resonance the sun s gravitational torque on the atmospheric tide accelerating earth s rotation would have been comparable in magnitude to the decelerating lunar torque upon the oceanic tides the balance of the opposing torques may have long maintained a resonant approximately 21 h day perhaps for much of the precambrian because the timescale of lunar orbital evolution is not directly affected a constant daylength would result in fewer days month the hypothesis is shown not to conflict with the available stromatolitic evidence escape from the resonance could have followed a relatively abrupt global warming such as that occurring at the end of the precambrian alternatively escape may simply have followed a major increase in the rate of oceanic tidal dissipation brought about by the changing topography of the world s oceans we integrate the history of the lunar orbit with and without a sustained resonance finding that the impact of a sustained resonance on the other orbital parameters of the earth moon system would have not been large

the last glacial maximum on spitsbergen svalbard most scientists have concluded previously that the west coast of spitsbergen svalbard remained ice free during the late weichselian between 25 000 and 10 000 yr b p we conclude that the glaciation was more extensive terraces that were postulated to have been ice free are covered by a thin late weichselian till sudden drop in the marine limit and basal radiocarbon dates of raised glaciomarine sediments demonstrates that the glaciers in the main fjords isfjorden and van mijenfjorden terminated west outside of the fjord mouths basal radiocarbon dates from glaciomarine clay above till in cores from the continental shelf west of spitsbergen yielded ages of about 12 500 yr b p from which we conclude that the ice extended to the shelf edge based on the extent of amino acid diagenesis in radiocarbon dated molluscs the duration of the maximum extension of the late weichselian glaciation was short certainly less than 10 000 years during the ice free period preceding that glaciation at least back to 40 000 yr b p the glaciers on svalbard were not significantly larger than at present as shown by marine deposits close to the glacier snouts many radiocarbon dates place deglaciation of the outer coast at about 12 500 yr b p at about 10 000 yr b p the rest of the archipelago rapidly became ice free

the esr age of portlandia arctica shells from glacial deposits of central latvia an answer to a controversy on the age and genesis of their enclosing sediments abstract the occurrence of portlandia arctica shells in glacigenic sediments of central latvia had created a controversy in many publications about 1 their age ranging from the holsteinian to the late weichselian and 2 the genesis of their enclosing sediments glacial glaciomarine or marine our reinvestigation of the main object of controversy the licupe site leads to a conclusion that the sedimentary package of diamictons clays and sands containing portlandia arctica shells and marine microfossils is a large glacial raft that had been transported and deposited by the riga lobe during the weichselian the electron spin resonance esr ages on five sets of portlandia arctica shells from the licupe and daugmales tom ni sites range from 86 0 6 8 to 105 0 9 2 ka bp these esr age determinations and the cool climate indicators of the associated microflora and microfauna suggest that their source sediments probably in the gulf of riga are marine clays of early weichselian age probably correlative to the brorup interstadial in central latvia portlandia arctica shells and their enclosing clay occur resedimented or translocated in glacial deposits during weichselian glacial advances

non glacial paleoenvironments and the extent of weichselian ice sheets on severnaya zemlya russian high arctic abstract the extent of the barents kara sea ice sheet northern europe and russia during the last glacial maximum lgm in marine isotope stage mis 2 is controversial especially along the southern and northeastern russian high arctic margins we conducted a multi disciplinary study of various organic and mineral fractions obtaining chronologies with 14 c and luminescence dating methods on a 10 5 m long core from changeable lake 4 km from the vavilov ice cap on severnaya zemlya the numeric ages indicate that the last glaciation at this site occurred during or prior to mis 5d 4 early middle weichselian deglaciation was followed by a marine transgression which affected the changeable lake basin after the regression the basin dried up in late middle weichselian time ca 25 40 ka reworked marine sediments were deposited in a saline water body during the late weichselian mis 2 the basin was not affected by glaciation and lacustrine sediments were formed which reflect cold and arid climate conditions during the termination of the pleistocene and into the holocene warmer and wetter climate conditions than before led to a higher sediment input thus our chronology demonstrates that the northeastern margin of the lgm barents kara sea ice sheet did not reach the changeable lake basin this result supports a modest model of the lgm ice sheet in northern europe determined from numeric ice sheet modelling and geological investigations

arctic ocean deep sea record of northern eurasian ice sheet history the sediment composition of deep sea cores from the central arctic ocean the fram strait and the yermak plateau was analyzed for several parameters to reconstruct the history of marine paleoenvironment and terrestrial glaciation in the last 200 000 years layers with high amounts of coarse terrigenous ice rafted debris ird and often high contents of smectite were deposited during extensive glaciations in northern eurasia when ice sheets reached the northern continental margins of the barents and kara seas and discharged icebergs into the arctic ocean intercalated layers with relatively low ird and smectite contents but abundant planktic foraminifers in the coarse fraction were deposited during periods of atlantic water inflow to the arctic ocean and seasonally open waters leads in a sea ice cover with only few icebergs in the arctic ocean high ird contents in the sediments reflect the presence of ice sheets on the kara and barents seas shelves and the hinterland during the entire oxygen isotope stage 6 ca 190 130 ka in substage 5b ca 90 80 ka at the stage boundary 5 4 around 75 ka and in late stage 4 early stage 3 ca 65 50 ka these results are in excellent correlation with those from recent field work in northern scandinavia european russia siberia and on the shelves relatively low amounts of ird in central arctic ocean sediments from the late weichselian glacial maximum ca 24 18 ka correlate well with the recent reconstruction of a very limited eastern ice sheet extension during this time oxygen and carbon isotope records of planktic foraminifers from the analyzed sediment cores show a number of prominent excursions which can be interpreted as evidence for freshwater events in the arctic ocean the synchroneity of freshwater events and ird input suggests a common source strongest events were associated with deglaciations of the barents and kara seas after the ice sheets had blocked the outflow of large rivers for several millennia the outflow of freshwater from large ice dammed lakes occurred at ca 130 80 75 and 52 ka freshwater events in the central arctic ocean during the last deglaciation ca 18 ka were relatively small compared to the previous events this indicates that during most of the late weichselian glacial maximum a river outflow from northern siberia to the arctic ocean was possible atlantic water inflow to the arctic ocean and seasonally open waters in the ice leads occurred during the interglacials ofoxygen isotope stage 1 and substage 5e during several interstadials stage 3 substages 5a and 5c and to a lesser degree within stadials and glacials stages 2 4 and 6 with the exception ofthe interglacials these periods were times ofstrong ice growth on the continents as revealed by terrestrial data the coincidence suggests that open waters in the arctic ocean and the nordic seas were an important moisture source in addition to more southerly sources which fostered the growth of ice sheets on northern eurasia

lithofacies types and vertical profile models an alternative approach to the description and environmental interpretation of glacial diamict and diamictite sequences abstract increased knowledge of modern glacial depositional environments has resulted in rapidly evolving classifications of glacial tills these are based to a large degree on theoretical considerations of likely depositional processes the classifications are sophisticated and more advanced than the establishment of simple field criteria whereby individual till facies can be identified in quaternary and pre quaternary successions this situation is compounded in many quaternary terrains by the continued description of tills in terms of laboratory derived analytical data only reflecting a traditional interest in stratigraphic correlation rather than reconstruction of depositional environment detailed sedimentological logging of lithofacies is rarely undertaken there is thus considerable confusion as to what is being described or sampled when analytical data are presented for many pleistocene tills the same remarks apply to pre pleistocene tillites a lithofacies code is presented here for the rapid description and visual appraisal of field sequences or drill cores containing unconsolidated diamicts or lithified diamictites the term till is not used as it has a strict genetic definition referring to direct aggregation and deposition by glacier ice use of a four part code in conjunction with codes already published for fluvial sediments allows fundamental field properties to be depicted independent of genetic terminology and provides a firm basis for subsequent environmental interpretation and analytical work the value of this approach is illustrated by comparing a representative suite of vertical profiles of diamict assemblages deposited by modern grounded glaciers with a classic late pleistocene glacigenic sequence at scarborough bluffs ontario

the complete ordered ray expansion ii multiphase body wave tomography summary in a companion paper clarke 1993 we described a method for the calculation of body wave synthetic seismograms referred to as the complete ordered ray expansion here we present a new method for the inversion of body wave seismograms based on the core algorithm this multiphase waveform tomography allows us to incorporate many body wave arrivals from each seismogram in an inversion vastly increasing the amount of structural information that may be extracted from a set of observed traces we present an example of a synthetic inversion for a low velocity anomaly representing a mantle plume and show that our method allows us to retrieve the major features of the perturbation very successfully our computational scheme does not demand excessive storage but the amount required does grow in proportion to the number of events and stations we thus investigate the effect of partitioning the procedure by initially performing trace by trace inversions and combining the information in a final step the fact that small amplitude phases are less well constrained in the preliminary inversions leads to a somewhat degraded final image nevertheless this technique is useful in situations where a single full inversion is impractical or where data become available over an extended period of time

late quaternary stratigraphy of western yamal peninsula russia new constraints on the configuration of the eurasian ice sheet ice sheet reconstructions for the last glacial maximum in northern eurasia range from nearly complete coverage by a contiguous marine based ice sheet to large unglaciated areas stratigraphic records from yamal peninsula russia provide new insight on the eastern limit of the eurasian ice sheet in the kara sea radiocarbon and infrared stimulated luminescence ages from coastal cliff sections date the emplacement of the kara diamicton as older than ca 40 ka reflecting regional glaciation ice wedge growth peat accumulation and eolian and fluvial deposition characterized the past 40 k y and do not support coverage of yamal peninsula by an ice sheet or proximity to a glacier margin thus the late weichselian eurasian ice sheet was largely confined to barents sea and its global sea level contribution is reduced to 7 m of sea level equivalent

numerical reconstructions of the eurasian ice sheet and climate during the late weichselian abstract geological investigations undertaken through the quaternary environments of the eurasian north programme established ice sheet limits for the eurasian arctic at the last glacial maximum lgm sedimentary records of palaeo ice streams and uplift information relating to ice sheet configuration and the pattern of deglaciation ice sheet numerical modelling was used to reconstruct a history of the eurasian ice sheet compatible with these geological datasets the result was a quantitative assessment of the time dependent behaviour of the ice sheet its mass balance and climate and predictions of glaciological products including sediments icebergs and meltwater at the lgm ice cover was continuous from scandinavia to the arctic ocean margin of the barents sea to the north and the kara sea to the east in the west along the continental margin between the norwegian channel and svalbard the ice sheet was characterised by fast flowing ice streams occupying bathymetric troughs which fed large volumes of sediment to the continental margin that were deposited as a series of trough mouth fans ice streams may also have been present in bathymetric troughs to the north between svalbard and franz josef land further east however the ice sheet was thinner across the kara sea the ice thickness was predicted to be less than 300 m while on severnaya zemlya the ice cover may have been thinner at the lgm than at present it is likely that the taymyr peninsula was mainly free of ice at the lgm in the south the ice margin was located close to the shoreline of the russian mainland the climate associated with this ice sheet is maritime to the west and in stark contrast desert like in the east atmospheric general circulation modelling has revealed that such a contrast is possible under relatively warm north atlantic conditions because a circulation system develops across the kara sea isolating it from the moisture laden westerlies which are diverted to the south ice sheet decay began through enhanced iceberg calving in the deepest regions of the barents sea which caused a significant ice embayment within the bear island trough by about 12 000 years ago further iceberg calving reduced ice extent to the northern archipelagos and their surrounding shallow seas ice decay was complete by about 10 000 years ago

late proterozoic and paleozoic tides retreat of the moon and rotation of the earth the tidal rhythmites in the proterozoic big cottonwood formation utah united states the neoproterozoic elatina formation of the flinders range southern australia and the lower pennsylvanian pottsville formation alabama united states and mansfield formation indiana united states indicate that the rate of retreat of the lunar orbit is dxi dt approximately k2 sin 2delta where xi is the earth moon radius vector k2 is the tidal love number and delta is the tidal lag angle and that this rate has been approximately constant since the late precambrian when the contribution to tidal friction from the sun is taken into account these data imply that the length of the terrestrial day 900 million years ago was approximately18 hours

quantitative comparison of seismic data sets with waveform inversion testing the age dependent evolution of crustal structure studies of seismic variability play a more important role in the understanding of processes that form oceanic crust than individual velocity depth solutions found in isolation for such variability studies solutions must be contrasted with one another this is normally not straightforward owing to the lack of rigorous uncertainty estimates available for solutions here i present a method for the direct comparison of such solutions by hypothesis testing a null hypothesis is proposed that a single velocity solution is consistent with two sets of data the hypothesis is tested by seeking such a model using waveform inversion and comparing its misfit against the misfit from fully independent solutions the degree of similarity of the velocity structures can be further quantified by solving for the pair of solutions that are as similar as possible while consistent with each set of data the f test is used to evaluate the significance of each additional effective model parameter as it lends independence to the two solutions the method possesses advantages of a hypothetical monte carlo search of all models because the complete forward problem is carried out at each step yet it is tractable because the search direction is well defined the method is applied to two data sets collected near the mid atlantic ridge on 0 and 7 m y crust in order to quantify the differences between the solutions the comparison finds quantifiable differences in both layers 2 and 3 between these two regions suggesting that significant evolution of the entire crustal section has taken place in the first 7 m y

hydrography and mollusc faunas of the baltic and the white sea north sea seaway in the eemian abstract palynologically dated mollusc and cirriped faunas from a region extending from the north sea through the baltic and karelia to the arkhangelsk region show that the hydrography of the baltic was very different from the holocene for 2 2 5 ka in the early eemian a seaway existed from the barents to the north sea through karelia until it was severed at the present continental watershed to the north of lake onega after this the ladoga onega trough remained an arm of the baltic for several millennia the benthic faunas are comparable to the holocene but their boundaries were displaced much further into the baltic notable differences from the holocene are the absence or rarity of the macoma balthica biocoenosis and the presence of cold portlandia dominated biocoenoses in karelia in the belt sea and western baltic winter sea surface temperatures and salinity were higher than now by ca 6 c and 15 and the distinctly brackish top layer was missing at the same time cold bottom water 2 5 c with a tendency to anoxia characterised the karelian arm of the baltic water exchange through this area was inhibited by the constriction to the north of lake onega and the basin and threshold bathymetry water transport through the white sea baltic seaway was too sparse to play an active role in the north atlantic surface circulation or climatic change in the region the high salinity and temperature in the belt sea and western baltic persisted throughout the eemian and are explained by wider and deeper passage from the north sea to kattegat wider straits through denmark higher salinity in the north sea higher evaporation as well as more dispersed fresh water supply the advection of oceanic water into the baltic culminated in the early eemian before the carpinus zone and probably resulted in an oceanic climate in the baltic region while at the same time cold winters produced cold bottom water in the karelian arm

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

waveform inversion of seismic refraction data and applications to young pacific crust summary we present a method for constraining the velocity depth structure in the earth using seismic refraction waveform data we test the method with synthetic data from known models and apply it to a set of data collected in 1982 june from the east pacific rise at 13 n from the magma expedition in this iterative process wkbj seismograms are computed for a starting model the difference between these and the observed seismograms is used to update the model subject to physical constraints an important first step in the inverse scheme is the linearization of the wkbj seismogram equation allowing us to compute differential seismograms partial derivatives of the synthetic seismogram with respect to specific model parameters this linearization provides the means for estimating required model perturbations based on the misfit in the seismograms the choice of a suitable numerical strategy for computing an updated model is a crucial second step in formulating a working algorithm because the data contain noise synthetic seismograms can only fit the data to this noise level in this case infinitely many models fit the data to this tolerance and some of these estimates are non physical involving negative layer thicknesses a successful strategy must choose from among these possibilities a well defined physically reasonable new model in a commonly used approach to solving non linear problems the perturbation to the starting model is minimized while improving the fit to the data after several iterations the final model which possesses no special properties still tends to resemble the starting model when used with the magma data this technique essentially does not perturb the model at all a method we find much more satisfactory involves solving for the new model directly while applying physically important constraints as constraints we require the velocity gradient remain below a fixed value and penalize the roughness of the new model we thus solve for the smoothest model fitting the data to the specified misfit

global climate model high obliquity solutions to the ancient climate puzzles of the faint young sun paradox and low altitude proterozoic glaciation there is a general consensus that warm temperatures throughout precambrian were caused by very high co2 or ch4 atmospheric concentrations episodes of glaciation in the early and late proterozoic were caused by a reduction in these greenhouse gases through intensive weathering or large reduction in the methane producing bacteria population there are suggestions that these episodes were global because the continents occupied low latitudes at or near the time of glaciation here it is reported that high obliquity throughout the precambrian serves as a mechanism for producing warm temperatures throughout the archean and glaciation in the late proterozoic in this paper global climate model simulations using high obliquity values that range from 54 to 70 are undertaken these simulations use a simple 50 m slab ocean a faster rotation rate corresponding to an 18 hour day an idealized supercontinent located in the tropics which has a north south mountain chain and co2 concentrations which range from 170 to 3400 ppmv further four simulations under global ocean conditions faster rotation rate and reduced solar constant 15 and 20 using present day and high obliquity values have been undertaken the simulations show warm conditions with high obliquity but snowball earth conditions for the present day obliquity in the global ocean simulations near snowball earth conditions also occur for the tropical supercontinent simulation for an obliquity of 70 and co2 concentrations less than 1020 ppmv together these simulations show that for high obliquity the ocean remains relatively warm while land areas are cooled because of their lower heat capacity large fluxes of latent heat from the ocean balance the longwave energy lost at the top of the atmosphere thereby keeping high latitudes warm during winter seasons it is suggested from this results that lower and upper limits of 65 70 for obliquity serve as a solution for the faint young sun paradox and low latitude proterozoic finally the high obliquity climate relationship sheds some insight on the carbon isotope 13c excursions during the late proterozoic

deep drilling of glaciers in eurasian arctic as a source of paleoclimatic records abstract results of the deep drilling 80 100 m of eurasian arctic glaciers svalbard severnaya zemlya and franz josef land from the middle of the 1970s to the end of the 1990s have been discussed all spectra of the techniques used for ice core and borehole studies results of ice core dating and major paleoclimatic conclusions have been analyzed particular emphasis has been placed on the problem of accuracy and comparison of dates suggested by different authors new integrated time scale for eurasian arctic glaciers based on the ice core dating of vetreniy ice cap graham bell island and franz josef land which covers the last 700 800 years has been developed methods and results of the new paleo temperature reconstructions have been analyzed

accelerator radiocarbon dating at the molecular level molecular level 14c dating is the isolation of specific classes of molecules for their 14c dating by accelerator mass spectrometry ams complex matrices such as fossil bone are difficult to date due to their extreme chemical heterogeneity by isolating individual amino acids contaminants humates are removed and crystalline amino acids result bones with 0 1 0 2 n and collagenous compositions can be dated accurately because structural collagen is present contaminants are removable with xad resin bones with 0 1 n and non collagenous compositions yield dates hundreds to thousands of years too young because most of the preserved organic matter is exogenous accelerator 14c dates on collagenous and non collagenous bones are not comparable due to intrinsic dating inaccuracies ams 14c dating of amino acids demonstrated that 1 post 10 800 year ages for north american megafauna are due to sample contamination not holocene ages on extinct fauna 2 a clovis age 10 900 years was established for a human fossil from the anzick site montana 3 holocene ages cannot be established absolutely for many north american human fossils because the bones were non collagenous 4 accurate ages are attainable on vertebrate fossils as small as passerine birds from pacific island localities 5 well preserved bones are datable without their destruction by extracting protein with water at high temperatures and 6 stratigraphic anomalies to 45 000 years in european upper paleolithic rock shelters are recognizable by dating bone directly

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

a baseline for upper crustal velocity variations along the east pacific rise at 13 n a wide aperture profile of the east pacific rise at 13 n provides data necessary to make a high resolution seismic velocity profile of the uppermost crust along a 52 km segment of ridge crest automated and objective processing steps including p analysis and waveform inversion allow the construction of models in a consistent way so that comparisons are meaningful a continuous profile is synthesized from 70 independent one dimensional models spaced at 750 m intervals along the ridge the resulting seismic velocity structure of the top 500 m of crust is remarkable in its lack of variability the main features are a thin low velocity layer 2 a at the top with a steep gradient to layer 2b the seafloor velocity is nearly constant at 2 45 km s 3 along the entire ridge the velocity at the top of layer 2b is 5 0 km s 10 the depth to the 4 km s isovelocity contour within layer 2a is 130 20 m from 13 to 13 20 n north of which it increases to 180 m the increase in thickness is coincident with a deviation from axial linearity deval noted by both a slight change in axis depth and orientation and in geochemistry the waveform inversion providing more details plus velocity gradient information shows a layer 2a with about 80 m of constant velocity material underlain by 150 m of high velocity gradient material putting the base of layer 2a at approximately 230 m depth south of 13 20 n and about 50 m thicker north of the deval the overall lack of variability combined with other recent measurements of layer 2a thickness along and near the axis indicates that the thickness of volcanic extrusives is controlled not by levels of volcanic productivity but the dynamics of emplacement the homogeneity along axis also provides a baseline of inherent variability in crustal structure of about 10 against which other observed variations in similar regimes can be compared

the correction for the travel time effects of seafloor topography in the interpretation of marine seismic data problems associated with the application of topographic corrections to deep ocean marine seismic data are described synthetic travel time data are used to show that corrections for the delay time introduced by the water column are inferior to corrections for the true water path water path corrections place the shots at the ray entry points on the seafloor require no assumptions about the structure under investigation and maintain the correct distribution of data with range the water path correction can be most accurately applied during the stacking operation that transposes the observed data into intercept time slowness space there is then no requirement to make determinations of slowness and for experiments involving seafloor receiver and surface shots the data may be transposed to the seafloor with complete objectivity water path corrected stacks from different locations may then be compared in the sure knowledge that any observed differences are due to real structural changes and not to topographic correction errors or to invalid assumptions of lateral homogeneity

climate forcings and the initiation of low latitude ice sheets during the neoproterozoic varanger glacial interval the giss gcm was used to determine if a diverse set of climate forcings alone or in combination could have initiated the low latitude ice sheets of the varanger 600 ma glacial interval the simulations use a realistic reconstruction of the paleocontinental distribution and test the following forcings alone and in combination 6 solar luminosity decrease four atmospheric co2 scenarios 1260 315 140 and 40 ppm a 50 increase and a 50 decrease in ocean heat transports and a change in obliquity to 60 none of the forcings individually produced year round snow accumulation on low latitude continents although the solar insolation decrease and 40 ppm co2 scenarios allowed snow and ice to accumulate at high and middle latitudes combining forcings further cools the climate when solar luminosity co2 and ocean heat transports were all decreased annual mean freezing and snow accumulation extended across tropical continents no simulation would have initiated low latitude glaciation without contemporaneous glaciation at higher latitudes a finding that matches the distribution of glacial deposits but which argues against high obliquity as a cause of the varanger ice age low level clouds increased in most scenarios as did surface albedo while atmospheric water vapor amounts declined all are positive feedbacks that drive temperatures lower in the most severe scenario global snow and ice cover increased to 68 compared to 12 under modern conditions and water vapor dropped by 90 these results do not necessarily preclude a snowball earth climate scenario for the varanger glacial interval however either more severe forcings existed or radical changes occurred in the ocean atmosphere system which are unaccounted for by the gcm also as sea ice extent increased in these experiments snow accumulation began to decline because of an increasingly dry atmosphere under snowball earth conditions glaciation would be impossible since the hydrological cycle would all but cease if the atmosphere s primary moisture source were cut off

coalescent greenland and innuitian ice during the last glacial maximum revising the quaternary of the canadian high arctic abstract this paper constitutes a fundamental revision of the author s earlier research and resolves a long standing debate concerning the late quaternary history of the canadian high arctic supporting its inundation by glaciers during the last glacial maximum lgm a transect along east ellesmere island demonstrates that ice advanced northward and southward along the axis of nares strait during the late wisconsinan debouching from a central saddle in kane basin the configuration of ellesmere island and greenland ice is based on moraines meltwater channels and erratics the youngest ams 14c date obtained on shelly till suggests ice buildup after 19 ka bp whereas many other dates are younger than 30 ka bp furthermore the lowest youngest detectable amino acid ratios on shells from till postdate the robeson aminozone 70 ka bp and are separated from holocene ratios by a narrow gap wholly consistent with the occupation of the strait by late wisconsinan ice re entry of the sea throughout nares strait is shown by a series of paleogeographic maps based on geomorphic evidence and radiocarbon dates on shells associated with marine limit deglaciation at the north end of the strait occurred by 10 1 ka bp and at the south end by 9 0 ka bp nares strait may still have been blocked by ice north of kane basin at 8 ka bp however by 7 5 ka bp it provided an unobstructed seaway from the arctic ocean to baffin bay the prominent ice margins within the fiords of eastern ellesmere island previously considered to mark the last ice limit record regional stabilization of land based ice after breakup of preceding marine based margins other implications of this study are that ice buildup occurred post 30 ka bp and possibly post 19 ka bp during an interval widely assumed to be constrained by severe aridity in the high arctic and that coalescent greenland and ellesmere island ice during the lgm promoted thickening of the eastern ellesmere island ice divide causing enhanced westward flow across the island to eureka sound sizeable postglacial emergence 80 120 m on eastern ellesmere island is associated with the retreat of thick ice 1 km from the strait greater emergence in eureka sound which extends southward into the central arctic islands confirms the previously proposed axis of the innuitian ice sheet further clarification of the extent geometry and retreat of the innuitian ice sheet will provide new opportunities for glaciological and geophysical modelling and help to identify late quaternary sediment sources for the arctic ocean and baffin bay

paleomagnetic evidence for low latitude glaciation during deposition of the neoproterozoic rapitan group mackenzie mountains n w t canada the rift related rapitan group of the mackenzie mountains of northwestern canada acquired several magnetizations due to pulses of hydrothermal activity the first pulse attributed to initiation of rapitan rifting produced a widespread overprint p2 that may be reflected in the basal mount berg formation two later pulses produced overprints similar to components found in an earlier study development of iron formation and hematite pigment in the overlying sayunei formation is attributed to the second pulse represented by a paleopole n 10 sites 334 e 01 s p m 4 9 that coincides with poles of the franklin igneous events of northern canada the franklin episode suggested on geological grounds to be coeval with sayunei deposition dates the sayunei at ca 725 ma this relation implies that rifting in mackenzie mountains could be related to rifting in northern canada a third pulse reflected by a pole at 007 e 16 n n 6 sites p m 6 12 is attributed to final rifting during dep

earthquake locations and three dimensional crustal structure in the coyote lake area central california previous work on the simultaneous inversion method has been improved and extended to incorporate iterative solution for earthquake locations and laterally heterogeneous structure approximate ray tracing and parameter separation are important elements of the improved method application of the method to p wave arrival time data recorded by stations of the u s geological survey central california network yields a three dimensional model for the velocity structure of the upper crust in an area encompassing the rupture zone of the coyote lake earthquake of august 1979 very strong correlations between the velocity model and the geology and gravity and magnetic anomalies are observed improved estimates of the locations of earthquakes in the study area are also determined the relocation of explosions indicates epicentral accuracies of the order of a kilometer or better based on the revised hypocentral locations it is concluded that the san andreas fault is vertical in this area with no actual offset between the epicenters and the fault trace in contrast the calaveras has two or more active fault surfaces one nearly vertical and another dipping 75 to the northeast

middle pleistocene glaciations of the russian north abstract geological data on the pre eemian glaciations of northern russia including the latest results by the russian norwegian pechora project are synthesized in order to present evidence for comparison with other early glaciations around the arctic the bulk of evidence indicates that arctic and subarctic regions of european russia of western and central siberia during the middle pleistocene were at least 4 times covered by large ice sheets which advanced mainly from the shelf ice domes partly from fennoscandia and the putorana plateau ice accumulations in the ural mountains were insignificant and did not form any noticeable ice dispersal centres unlike the classical glaciated areas ice sheets of northern russia acted mainly on a soft perennially frozen substrate which was heavily glacitectonised the middle pleistocene ice sheets were much larger than the weichselian ones the fennoscandian ice dispersal centre was most active in northern european russia during the penultimate glaciation ois 6 when shelf centred ice domes were relatively weaker larger continental ice sheets were formed in preceding ice ages when kara sea ice dispersal centre dominated the lowland ice sheets reached their maximum extent at different stages from cromerian don glaciation in european russia to ois 8 in west siberia therefore the maximum ice limit is time transgressive in northern russia

age variations of oceanic crust poisson s ratio inversion and a porosity evolution model porosity in the oceanic crust is one of the most important factors influencing measured seismic velocities porosity is particularly important in the uppermost young crust where rapid variations in velocities with depth and crustal age are observed knowledge of the concentration and aspect ratios of inferred crack populations can be improved considerably if estimates of poisson s ratio are available from observations of compressional and shear seismic velocities p and s in this paper i present a joint seismic waveform inversion for p and s velocities are found while maximizing or minimizing poisson s ratio using a hypothesis testing mechanism i apply this method to ocean bottom hydrophone data in 140 ma atlantic crust the resulting solution corridor agrees with laboratory measurements without the low poisson s ratio anomalies at depths of 0 8 1 5 km found by spudich and orcutt 1980 and au and clowes 1984 on younger 15 ma pacific crust compiling other published p and s solutions an age dependent pattern emerges none of the solutions for crust older than 60 ma display the poisson s ratio anomaly i propose a simple crustal evolution model using thin and thick cracks to explain these observations thin cracks preferentially close at shallow depths in the crust producing the localized poisson s ratio anomaly sealing of all cracks by hydrothermal deposits as the crust ages restores the seismic velocities to consistency with laboratory measurements this model is consistent with similar models of crack populations and their evolution from shallow measurements

late weichselian ice sheets in arctic and pacific siberia abstract interpretation of space images coupled with field data on glacial geomorphology and glacial geology of northern eurasia have provided compelling evidence for a continuous late weichselian ice sheet covering the entire arctic margin of the continent in addition to the scandinavian ice sheet three ice sheets of the same order the karan east siberian and beringian are identified within the major eurasian ice sheet also there is evidence and other considerations suggesting a former ice sheet in the sea of okhotsk ice spreading centers of the ice sheets were situated on the continental shelves off the siberian coasts thus we can term the ice sheets siberian so far the conventional stratigraphic approach to the problem of siberian glaciation has proven fruitless and yielded nothing but uncertainty while geomorphological studies have discovered a wealth of glacial landforms in arctic siberia it was these landforms and their spacing that made the existence of former siberian ice sheets evident the east siberian ice sheet rested on the shelves of the eastern laptev and east siberian seas and its flow was directed radially including up the yana and indigirka rivers the beringian ice sheet was grounded on the chukchi bering and beaufort sea shelves overriding the chukchi and seward peninsulas from the north and flowing through the bering strait it was continued by a thick floating ice shelf buttressed by the submarine aleutian commander ridge the okhotsk ice sheet was grounded on the okhotsk shelf specific gemorphic signatures of marine ice sheets the glacially cut corners have been identified on all promontories jutting out into the bering and okhotsk seas the beringian and okhotsk ice sheets are consistent with the results of deep sea drilling in the adjacent north pacific during leg 145 of d v joides resolution

severnaya zemlya arctic russia a nucleation area for kara sea ice sheets during the middle to late quaternary quaternary glacial stratigraphy and relative sea level changes reveal at least four expansions of the kara sea ice sheet over the severnaya zemlya archipelago at 79 degrees n in the russian arctic as indicated from tills interbedded with marine sediments exposed in stratigraphic superposition and from raised beach sequences that occur at altitudes up to 140 m a s l chronologic control is provided by ams c 14 electron spin resonance green stimulated luminescence and aspartic acid geochronology major glaciations followed by deglaciation and marine inundation occurred during mis 10 9 mis 8 7 mis 6 5e and mis 5d 3 the mis 6 5e event associated with the high marine limit implies ice sheet thickness of 2000m only 200km from the deep arctic ocean consistent with published evidence of ice grounding at similar to 1000m water depth in the central arctic ocean till fabrics and glacial tectonics record repeated expansions of local ice caps exclusively suggesting wet based ice cap advance followed by cold based regional ice sheet expansion local ice caps over highland sites along the perimeter of the shallow kara sea including the byrranga mountains appear to have repeatedly fostered initiation of a large kara sea ice sheet with exception of the last glacial maximum mis 2 when kara sea ice did not impact severnaya zemlya and barely graced northernmost taymyr peninsula

gcm simulations of snowball earth conditions during the late proterozoic in order to simulate the snowball earth conditions that may have existed during the late proterozoic we have conducted a series of gcm simulations using a simple 50 meter slab ocean a reduced solar constant of 6 and varied co2 concentrations in this study we vary the co2 concentration from 100 to 3400 ppmv and use rotation rates corresponding to 18 and 24 hour day lengths we also examine the effects of increasing the poleward transport of heat by the oceans our results show that below a critical value of approximately 1700 ppmv of atmospheric co2 sea ice and sub freezing temperatures occur from the poles to the equator a global mean annual two meter air temperature of 221 k is found for boundary conditions of 100 ppmv atmospheric co2 6 reduction in solar forcing and a rotation rate corresponding to an 18 hour day these results confirm those of earlier studies suggesting or implying that low latitude glaciation occurred during the late proterozoic however since the ocean is the critical factor for low latitude glaciation the results should be view cautiously because of the simple slab ocean used in this study