chronology of late wisconsinan glaciation in middle north america abstract we propose a chronology of late wisconsinan glacial fluctuations in middle north america from alberta to wisconsin based on radiocarbon dates derived solely from wood previous chronologies of the southwestern margin of the north american continental ice sheet have depended to a considerable degree on radiocarbon dates from fine grained organic sediment this material is commonly contaminated with older carbon resulting in chronologic confusion by using only dates from wood much of the confusion disappears however because of the scarcity of wood dates only four of the sixteen identified fluctuations are accurately dated an advance into iowa about 14 000 to 13 500 bp an advance into south dakota and iowa about 12 300 bp an advance into the lake michigan basin about 11 700 bp and an advance into the lake superior basin about 9900 bp in addition the beginning of late wisconsinan glaciation before 20 000 bp is fairly well documented none of the fluctuations in the western part of the region are accurately dated

the selection of sites for paleovegetational studies the judicious selection of sites for paleovegetational and paleoclimatic studies permits paleoecologists to answer specific research questions that go beyond primary descriptions of past vegetation we present a model that describes the relationship between basin size and pollen source area and predicts the proportions of local extralocal and regional pollen sampled by lake basins of different size the distinctive sampling properties of lakes peats and small hollows can be exploited to provide details of pattern in paleovegetation so long as attention is given to the limitations and problems of these types of sites combinations of site types in a single study most fully exploit the information contained in sediments

a reconstruction of moorhead and emerson phase environments along the eastern margin of glacial lake agassiz rainy river basin northwestern ontario mapping of quaternary geology in the rainy river lowland northwestern ontario resulted in discovery of several fossil bearing localities organic remains are associated with both the moorhead and

hybrid modeling of p sv seismic motion at inhomogeneous viscoelastic topographic structures a new hybrid two step method for computation of p sv seismic motion at inhomogeneous viscoelastic topographic structure is presented the method is based on a combination of the discrete wavenumber dw finite difference fd and finite element fe methods in the first step the dw method is used to calculate the source radiation and wave propagation in the background 1d medium in the second step the fd fe algorithm is used to compute the wave propagation along the topographic structure the accuracy of the method has been separately tested for inclusion of the attenuation and for inclusion of the free surface topography through numerical comparisons with analytical and independent numerical methods the method is a generalization of the hybrid dw fd method of zahradnik and moczo 1996 for localized structures with a flat free surface numerical computations for a ridge sediment valley and the ridge neighboring the sediment valley show that a ridge can considerably influence the response of the neighboring sediment valley this means that the neighboring topographic feature should be taken into account even when we are only interested in the valley response

2d finite difference viscoelastic wave modelling including surface topography we have pursued two dimensional 2d finite difference fd modelling of seismic scattering from free surface topography exact free surface boundary conditions for the particle velocities have been derived for arbitrary 2d topographies the boundary conditions are combined with a velocity stress formulation of the full viscoelastic wave equations a curved grid represents the physical medium and its upper boundary represents the free surface topography the wave equations are numerically discretized by an eighth order fd method on a staggered grid in space and a leap frog technique and the crank nicholson method in time in order to demonstrate the capabilities of the surface topography modelling technique we simulate incident point sources with a sinusoidal topography in seismic media of increasing complexities we present results using parameters typical of exploration surveys with topography and heterogeneous media topography on homogeneous media is shown to generate significant scattering we show additional effects of layering in the medium with and without randomization using a von karman realization of apparent anisotropy synthetic snapshots and seismograms indicate that prominent surface topography can cause back scattering wave conversions and complex wave patterns which are usually discussed in terms of inter crust heterogeneities

a 3d discrete numerical elastic lattice method for seismic wave propagation in heterogeneous media with topography 1 a three dimensional elastic lattice method for the simulation of seismic waves is presented the model consists of particles arranged on a cubic lattice which interact through a central force term and a bond bending force particle disturbances are followed through space by numerically solving their equations of motion a vacuum free surface boundary condition is implicit in the method we demonstrate that a numerical implementation of the method is capable of modelling seismic wave propagation with complex topography this is achieved by comparing the scheme against a finite difference solution to the elastodynamic wave equation the results indicate that the scheme offers an alternative 3d method for modelling wave propagation in the presence of strong topography and subsurface heterogeneity we apply the method to seismic wave propagation on mount etna to illustrate its applicability in modelling a physical system

recognition of a cretaceous outlier in northwestern ontario borehole f 88 33 located near rainy river ontario intersected cretaceous nonmarine clastic sediments this is the first documented occurrence in ontario of cretaceous sediments associated with the western interior lithologie and heavy mineral analyses were used to differentiate this unit from the overlying quaternary sediments seventy five species of fossil angiosperm pollen gymnosperm pollen spores megaspores and algal cysts were recovered from borehole f 88 33 and used to date the pre quaternary sediments as late albian to early cenomanian the occurrence of these nonmarine sediments in northwestern ontario helps to better define the limits of cretaceous sedimentation in the western interior

2d surface topography boundary conditions in seismic wave modelling new formulations of boundary conditions at an arbitrary two dimensional 2d free surface topography are derived the top of a curved grid represents the free surface topography while the grid s interior represents the physical medium the velocity stress version of the viscoelastic wave equations is assumed to be valid in this grid however the rectangular grid version attained by grid transformation is used to model wave propagation in this work in order to achieve the numerical discretization we show the detailed solution of the particle velocities at the free surface resulting from discretizing the boundary conditions by second order finite differences fds the resulting system of equations is spatially unconditionally stable the fd order is gradually increased with depth up to eighth order inside the medium staggered grids are used in both space and time and the second order leap frog and crank nicholson methods are used for time stepping we simulate point sources at the surface of a homogeneous medium with a plane free surface containing a hill and a trench applying parameters representing exploration surveys we present examples with a randomly realized surface topography generated by a 1d von karman function of order 1 viscoelastic simulations are presented using this surface with a homogeneous medium and with a layered randomized medium realization all generating significant scattering

new curvilinear scheme for elastic wave propagation in presence of curved topography we study a new curvilinear scheme for wave propagation modelling in presence of topography the discrete scheme takes advantage of recent developments in areoacoustics our new scheme relies on the conventional grid coupled with optimized filters to remove numerical noise in case of strong material heterogeneity we used non centred stencils for free surface implementation and optimized the explicit runge kutta scheme for the time differencing we performed a complete theoretical stability and dispersion analysis of the discrete scheme finally we illustrate the numerical accuracy of the new scheme by intensive experiments

warm core ring dynamics derived from satellite imagery a reinterpretation of the life history of warm core ring 82 b is constructed from drifter trajectories hydrographie profiles and satellite derived sea surface temperature fields an analysis of the observations strongly suggests this ring is fundamentally a dipole structure for much of its existence the dipole has unequal strength vortices with the cyclone being the weaker component quantification of the dipole indicates the anticyclone is oval shaped and rotates at approximately 7 7 per day while the cyclone revolves around the anticyclone at about 14 5 per day yielding an orbit period of approximately 25 days for the cyclone the elusive part of the dipole explanation is in interpreting its surface signature as a function of time as the cyclone appears to disappear when in proximity to the steep continental slope vortex modeling suggests that when the weaker cyclonic eddy is placed close to a boundary the cyclone is drawn into a filament as it is advected through the gap between the anticyclone and the boundary this means the cyclone is either in a filamentation state or undergoing the straining associated with filamentation approximately three fourths of the time which is in complete agreement with the satellite imagery in addition the 82 b dipole configuration is sufficiently robust to survive continuous interaction with the continental slope and the onset of a gulf stream interaction although only warm core ring 82 b is analyzed in detail and found to be a vortex pair dipole configurations are found in the warm core ring 81 f time series as well

glacial lake agassiz its northwest maximum extent and outlet in saskatchewan emerson phase abstract six different lines of evidence support the hypothesis that glacial lake agassiz expanded an additional 70 000 km2 over that previously mapped in northwestern saskatchewan and that the lake discharged out the northwestern clearwater outlet then through glacial lake mcconnell and mackenzie river to the arctic ocean elevations of formerly unmapped 1 strandlines and 2 glaciolacustrine sediments between the previously mapped northwest limit of lake agassiz and the clearwater lower athabasca spillway indicate that water extended 170 km farther northwest recently mapped strandlines at elevations up to 60 m above the previously mapped extent of lake agassiz can be traced along 3 isobases to the mouth of the spillway based upon 4 six radiocarbon dates recovered from spillway flood deposits in the athabasca river valley and its late pleistocene delta the 5 clearwater spillway was cut at 9 9 ka bp this date is synchronous with the initiation of the emerson phase 9 9 ka bp in the southern lake agassiz basin and 6 coincides with the position of the laurentide ice sheet at the cree lake moraine 10 ka bp along the northern margin of the lake following closure of the eastern outlets at the onset of the emerson phase lake agassiz transgressed toward the northwest into the deglaciated and isostatically depressed glacial foreland in the churchill river valley to an elevation of 490 m the pre flood elevation of the churchill mackenzie drainage divide at the head of the clearwater lower athabasca spillway the beaver river moraine an earthen drainage divide was breached resulting in lowering lake agassiz 52 m to a stable elevation at 438 m the lake discharged 21 000 km3 of water into the arctic ocean that raised global sea level by 6 cm

gulf stream warm core ring collection an introduction the collection of papers that follows evolved out of presentations given at a warm core rings workshop at the alton jones conference center university of rhode island in august 1984 at that time it became clear that a number of investigators in the warm core rings program had reached a stage where they were completing the analysis of their individual data sets and models some of these studies were interrelated and others were not our aim in collecting these investigations is not to present an integrated synthesis of gulf stream warm core rings but rather to provide a forum where some of our initial results can be focused

elastic wave propagation simulation in the presence of surface topography summary we present a 2 d numerical modelling algorithm based on a pseudo spectral method which accounts for surface topography the modelling scheme uses fourier derivative operators for spatial differencing in the horizontal direction whereas a chebyshev operator is used for vertical derivatives the incorporation of surface topography is achieved by mapping a rectangular grid onto a curved grid modelling of surface topography is important to study near surface effects of wave propagation in field seismic situations since diffraction and scattering at rough surfaces are non ray effects and can only be understood as wave phenomena static corrections cannot always account for these effects and some care has to be taken in their interpretation the presented modelling algorithm can serve as a powerful tool for the study of wave propagation phenomena in the vicinity of non planar surfaces or interfaces

3d free boundary conditions for coordinate transform finite difference seismic modelling new alternative formulations of exact boundary conditions for arbitrary three dimensional 3d free surface topographies on seismic media have been derived they are shown to be equivalent to previously published formulations thereby verifying the validity of each set of formulations the top of a curved grid represents the free surface topography while the interior of the grid represents the physical medium we assume the velocity stress version of the viscoelastic wave equations to be valid in this grid before transforming the equations to a rectangular grid in order to perform the numerical discretization we apply the latter version of the equations for seismic wave propagation simulation in the medium the numerical discretization of the free surface topography boundary conditions by second order finite differences fds is shown as well as the spatially unconditional stability of the resulting system of equations the fd order is increased by two for each point away from the free surface up to eight which is the order used in the interior we use staggered grids in both space and time and the second order leap frog and crank nicholson methods for wavefield time propagation an application using parameters typical of teleseismic earthquakes and explosions is presented using a 200 x 100 km 2 area of real topography from southwestern norway over a homogeneous medium a dipping plane wave simulates a teleseismic p wave incident on the surface topography results show conversion from p to rg short period fundamental mode rayleigh waves in the steepest and or roughest topography as well as attenuated waves in valleys and fjords the codes are parallelized for simulation on fast supercomputers and pc clusters to model high frequencies and or large areas

the age of the hartman moraine and the campbell beach of lake agassiz in northwestern ontario fluctuations in the level of lake agassiz are dated at two sites in northwestern ontario a radiocarbon date on a modern shell sample indicates dates on freshwater molluscs from the area are about 440 100 years gsc 3281 too old due to the hard water effect an adjustment of 400 years to two fossil freshwater mollusc dates of 11 400 410 gsc 3114 and 10 400 100 years bp gsc 2968 makes them compatible with radiocarbon dates on wood from deposits in other parts of the lake agassiz basin the two new dates indicate the beginning of the low water moorhead phase of lake agassiz started about 11 000 years bp the high water emerson phase started when the water level rose to form the upper campbell beach approximately 10 000 years bp the red clay widely distributed throughout northwestern ontario was deposited during the emerson phase when the ice margin lay along the hartman dog lake and marks moraines

detecting dipole ring separatrices with zebra palettes two new analysis methods applicable to remote sensing of the sea surface are described the first is a zebra false color palette that combines a slowly varying rainbow with a rapid gray scale sinusoid and the other is a routine that tracks the thermal separatrix or dividing line between two of more circulation regimes used together a significant increase in data retention is achieved compared to traditional algorithms based on frontal digitizations these two methods are applied to historical infrared data of gulf stream rings from the western north atlantic the ring data indicate anticyclonic rings usually have at least one cyclonic companion and are more appropriately modeled as dipoles several examples of cyclonic rings in the sargasso sea are seen with anticyclonic companions and are also believed to be dipoles the separatrix method is shown to provide reliable rotation rates for these vortex systems

3 d finite difference elastic wave modeling including surface topography three dimensional finite difference fd modeling of seismic scattering from free surface topography has been pursued we have developed exact 3 d free surface topography boundary conditions for the particle velocities a velocity stress formulation of the full elastic wave equations together with the boundary conditions has been numerically modeled by an eighth order fd method on a staggered grid we give a numerical stability criterion for combining the boundary conditions with curved grid wave equations where a curved grid represents the physical medium with topography implementation of this stability criterion stops instabilities from arising in areas of steep and rough topographies we have simulated scattering from teleseismic p waves using a plane vertically incident wavefront and real topography from a 40 40 km area centered at the noress array of seismic receiver stations in southeastern norway synthetic snapshots and seismograms of the wavefield show clear conversion from p waves to rg sh

rings in the ocean certain robust long lived eddy features in t he world s ocean have come to be k nown as rings the nomenclature stems from fuglister 1 1972 who c oined t he name based on the c ircle or ring of gulf stream waters t hat surrounded a class of eddies formed from t he g ulf stream through the pinch off of meanders here it s uffices to define rings as intense eddies or vortices that represent a wrapped up piece of a major ocean c urrent they t ranslate over space scales of hundreds to thousands of k ilometers and exist for periods lasting from months to years rings move t hrough the ocean carrying anomalies s imilar to the contrasts in physical biological and chemical properties observed across the major ocean c urrents while the term ring is t ied to fuglister t he existence ofthese features was evident at least as early as t he 1930s when iselin 2 1936 1940 encountered strong cold anomalies in the region south of the gulf stream that we would identify as c old corc cyclonic rings today iselin fully rec ognized these features as part of the highly time dependent eddy motion in the ocean later iselin fuglister 1948 were the first to survey what is now k nown as a warm core anticyc lonic ring there are tw o types of rings cold core rings are made up of w aters from the poleward s ide of c urrents like the gulf stream that are introduced to

small scale cyclones on the periphery of a gulf stream warm core ring small scale cyclones found around gulf stream warm core ring 82b are investigated by using infrared satellite images and current information obtained with an acoustic doppler velocimeter currents in these cyclones reveal speeds ranging from 20 to 80 cm s one small cyclone or ringlet found in june 1982 was studied extensively by removing the basic rotational velocities of 82b the azimuthal velocity field for this ringlet was used with the gradient current equation to calculate the absolute dynamic topography at 100 dbar it was found that the ringlet was 13 dyn cm lower than its surroundings in addition neglect of the centrifugal term would have changed the dynamic topography of the ringlet by 30 from a comparison with ctd data the absolute reference level was determined and a vertical profile of horizontal currents was calculated for the ringlet other cyclones were found throughout the slope water region around warm core ring 82b with observable lifetimes of 1 to 2 weeks the northeast quadrant of 82b was a favored generation site for ringlets two cyclones were observed to form in this region and were advected anticyclonically around 82b typically at any one time six cyclones with diameters of approximately 40 to 50 km can be detected north of the gulf stream by using satellite images

age and paleoecological significance of an early postglacial fossil assemblage near marathon ontario canada organic materials recovered from deltaic deposits confined to the black river valley near marathon ontario provide information on lake level history and local ecology for the region radiocarbon samples provide the first age determinations ca 8200 bp for the post minong iii iv lake phases along the northeast corner of the lake superior basin a minimum of 49 coleoptera taxa and 22 vascular plant taxa indicate coniferous forest and riparian communities overbank deposition along rivers flowing from the ice margin provided temporary ponds colonized by sedges and other aquatic plants and animals many records for both animals and plants illustrate occurrences of western and arctic disjunct species in early holocene time some western coleoptera might still be present as disjuncts in the marathon region

late albian kiowa skull creek marine transgression lower dakota formation eastern margin of western interior seaway u s a abstract an integrated geochemical sedimentological project is studying the paleoclimatic and paleogeographic characteristics of the mid cretaceous greenhouse world of western north america a critical part of this project required to establish a temporal framework is a stratigraphic study of depositional relationships between the albian cenomanian dakota and the upper albian kiowa formations of the eastern margin of the western interior seaway wis palynostratigraphic and sedimentologic analyses provide criteria for the dakota formation to be divided into three sedimentary sequences bounded by unconformities d0 d1 and d2 that are recognized from western iowa to westernmost kansas the lowest of these sequences defined by unconformities d0 and d1 is entirely upper albian and includes the largely nonmarine basal dakota lower part of the nishnabotna member strata in western iowa and eastern nebraska and the marine kiowa formation to the southwest in kansas the gravel rich fluvial deposits of the basal part of the nishnabotna member of the dakota formation correlate with transgressive marine shales of the kiowa formation this is a critical relationship to establish because of the need to correlate between marine and nonmarine strata that contain both geochronologic and paleoclimatic proxy data the basal gravel facies up to 40 m thick in western iowa aggraded in incised valleys during the late albian kiowa skull creek marine transgression in southeastern nebraska basal gravels intertongue with carbonaceous mudrocks that contain diverse assemblages of late albian palynomorphs including marine dinoflagellates and acritarchs this palynomorph assemblage is characterized by occurrences of palynomorph taxa not known to range above the albian kiowa skull creek depositional cycle elsewhere in the western interior and correlates to the lowest of four generalized palynostratographic units that are comparable to other palynological sequences elsewhere in north america tidal rhythmites in mudrocks at the ash grove cement quarry in louisville cass county nebraska record well developed diurnal and semimonthly tidal cycles and moderately well developed semiannual cycles these tidal rhythmites are interpreted to have accumulated during rising sea level at the head of a paleoestuary that experienced at least occasional mesotidal conditions this scenario places the gravel bearing lower part of the nishnabotna member of the dakota formation in the mouth of an incised valley of an upper albian transgressive systems tract deposited along a tidally influenced coast furthermore it provides a depositional setting consistent with the biostratigraphic correlation of the lower part of the nishnabotna member of the dakota formation to the marine kiowa formation of kansas

2d finite difference elastic wave modelling including surface topography1 a 2d numerical finite difference algorithm accounting for surface topography is presented higher order dispersion bounded cost optimized finite difference operators are used in the interior of the numerical grid while non reflecting absorbing boundary conditions are used along the edges transformation from a curved to a rectangular grid achieves the modelling of the surface topography we use free surface boundary conditions along the surface in order to obtain complete modelling of the effects of wave propagation it is important to account for the surface topography otherwise near surface effects such as scattering are not modelled adequately even if other properties of the medium for instance randomization can improve numerical simulations inclusion of the surface topography makes them more realistic

elastic modelling on a grid with vertically varying spacing1 we present a discrete modelling scheme which solves the elastic wave equation on a grid with vertically varying grid spacings spatial derivatives are computed by finite difference operators on a staggered grid the time integration is performed by the rapid expansion method the use of variable grid spacings adds flexibility and improves the efficiency since different spatial sampling intervals can be used in regions with different material properties in the case of large velocity contrasts the use of a non uniform grid avoids spatial oversampling in regions with high velocities the modelling scheme allows accurate modelling up to a spatial sampling rate of approximately 2 5 gridpoints per shortest wavelength however due to the staggering of the material parameters a smoothing of the material parameters has to be applied at internal interfaces aligned with the numerical grid to avoid amplitude errors and timing inaccuracies the best results are obtained by smoothing based on slowness averaging to reduce errors in the implementation of the free surface boundary condition introduced by the staggering of the stress components we reduce the grid spacing in the vertical direction in the vicinity of the free surface to approximately 10 gridpoints per shortest wavelength using this technique we obtain accurate results for surface waves in transversely isotropic media

3 d elastic modeling with surface topography by a chebychev spectral method the 3 d numerical chebychev modeling scheme accounts for surface topography the method is based on spectral derivative operators spatial differencing in horizontal directions is performed by the fourier method whereas vertical derivatives are carried out by a chebychev method that allows for the incorporation of boundary conditions into the numerical scheme the method is based on the velocity stress formulation the implementation of surface topography is done by mapping a rectangular grid onto a curved grid boundary conditions are applied by means of characteristic variables the study of surface effects of seismic wave propagation in the presence of surface topography is important since nonray effects such as diffractions and scattering at rough surfaces must be considered several examples show this the 3 d modeling alogrithm can serve as a tool for understanding these phenomena since it computes the full wavefield

plant and insect fossils from nipissing sediments along the goulais river southeastern lake superior fossiliferous sediments have long been known to occur along the banks of the goulais river radiocarbon dating has established their time of deposition as late in the nipissing transgression sedimentary environments and fluvial conditions in the ancestral river channel shaped fossil assemblages pollen reflects regional upland forests of pinus and picea with other trees and shrubs such as quercus fraxinus acer ulmus and alnus forested peatlands fens and emergent and submerged aquatic communities were common in lowland valleys and along the floodplain of the river these reconstructions are supported by aquatic insect fossils and hygrophilous taxa such as micropeplus sculptus and elaphrus olivaceus and also are confirmed by plant macrofossils such as myrica gale triadenum virginicum dulichium arundinaceum typha brasenia schreberi potamogeton spirillus and possibly zizania palustris there is no evidence for regional climatic change during this portion of holocene time

a comparison of a hydrodynamic lens model to observations of a warm core ring analysis of advanced very high resolution radiometer satellite images has shown long period fluctuations in ring eccentricity for 82 b concurrent shipboard measurements as well as satellite data also suggest that this anticyclonic ring was often paired with one or more cyclonic eddies furthermore satellite image analysis and recent numerical simulations suggest strong deformation in the fluid between the anticyclone and the cyclone the purpose here is to investigate a causal relation between the time dependent environmental deformation and the eccentricity fluctuations to this end we attempt to parameterize the effects of the cyclone as environmental forcing on the anticyclone using a nonlinear non quasi geostrophic hydrodynamic lens model our results suggest that the strong deformation in the environmental flow could be responsible for long period fluctuations in eccentricity other model ring features such as spin vorticity 2 and rotation rate of 82 b are discussed and compared to observations another significant feature of the model results is the presence of superinertial frequencies in the solution fields since current mesoscale observation techniques are unable to resolve such high frequencies it is unknown whether or not this result is realistic

evidence for an albian hudson arm connection between the cretaceous western interior seaway of north america and the labrador sea numerous researchers have alluded to the existence of a cretaceous hudson arm connection between the labrador sea and the western interior seaway of north america however the evidence for this marine connection has been circumstantial in this paper we present sedimentary geochemical data that indicate a marine influence in the albian mattagami formation of the moose river basin james bay lowlands ontario the facies associations between dinoflagellate bearing laminated mudstones fluvial sandstones and early pyrite mineralization are interpreted to indicate deposition in the central basin of an estuary we use the facies association between the estuarine fill and coeval kaolinitic paleosols in the moose river basin and in similar deposits in quebec and labrador to reconstruct a southern shoreline of the albian hudson arm to the cretaceous western interior seaway we suggest that development of the hudson arm connection between the labrador sea and the cretaceous western interior seaway may be related to a regional extensional regime associated with rifting between labrador and greenland and the passage of eastern north america over cretaceous hotspots

petrology of pliocene mississippi river alluvium provenance implications abstractpliocene mississippi river terrace gravels the upland complex crop out east and west of the present mississippi river in the northern mississippi embayment as the only sedimentary unit in the northern mississippi embayment deposited between the end of the eocene and the onset of glaciation its origin provides ground truth about conditions that existed in the heartland of north america within this 30 m yr interval recent studies concluded that the pliocene mississippi river originated in what is now southern canada and that the upland complex is the remnant of a much larger deposit that once extended from canada to the gulf of mexico is there some evidence in the petrology of the upland complex that can confirm this conclusion to determine the provenance of the upland complex we sampled 18 exposures analyzed 50 thin sections and obtained source terrane age data from its zircons upland complex gravel came from proximal early paleozoic carbonate rocks containing bedded and nodular chert