ice age simulations with a calving ice sheet model abstract variations of ice sheet volume during the quaternary ice ages are simulated using a simple ice sheet model for the northern hemisphere the basic model predicts ice thickness and bedrock deformation in a north south cross section with a prescribed snow budget distribution shifted uniformly in space to represent the orbital perturbations an ice calving parameterization crudely representing proglacial lakes or marine incursions can attack the ice whenever the tip drops below sea level the model produces a large 100 000 yr response in fair agreement correlation coefficient up to 0 8 with the 18o deep sea core records to increase confidence in the results several of the more uncertain model components are extended or replaced using an alternative treatment of bedrock deformation a more realistic ice shelf model of ice calving and a generalized parameterization for such features as the north atlantic deglacial meltwater layer much the same ice age simulations and agreement with the 18o records as with the original model are still obtained the model is run with different types of forcing to identify which aspect of the orbital forcing controls the phase of the 100 000 yr cycles first the model is shown to give a 100 000 yr response to nearly any kind of higher frequency forcing although over the last 2 million yrs the model phase is mainly controlled by the precessional modulation due to eccentricity over just the last 500 000 yr the observed phase can also be simulated with eccentricity held constant a definite conclusion on the phase control of the real 100 000 yr cycles is prevented by uncertainty in the deep sea core time scales before 600 000 yr b p the model is adapted to represent west antarctica and yields unforced internal oscillations with periods of about 50 000 yr

erosion of the laurentide region of north america by glacial and glaciofluvial processes collection of seismic reflection data from continental margins and ocean basins surrounding north america makes it possible to estimate the amount of material eroded from the area formerly covered by laurentide ice sheets since major glaciation began in north america a minimum estimate is made of 1 62 106 km3 or an average 120 m of rock physically eroded from the laurentide region this figure is an order of magnitude higher than earlier estimates based on the volume of glacial drift cenozoic marine sediments and modern sediment loads of rivers most of the sediment produced during laurentide glaciation has already been transported to the oceans the importance of continental glaciation as a geomorphic agency in north america may have to be reevaluated evidence from sedimentation rates in ocean basins surrounding greenland and antarctica suggests that sediment production sediment transport and possibly denudation by permanent ice caps may be substantially lower than by periodic ice caps such as the laurentide low rates of sediment survival from the time of the permo carboniferous and precambrian glaciations suggest that predominance of marine deposition during some glacial epochs results in shorter lived sediment because of preferential tectonism and cycling of oceanic crust versus continental crust

global ice sheet system interlocked by sea level abstract denton and hughes 1983 quaternary research 20 125 144 postulated that sea level linked a global ice sheet system with both terrestrial and grounded marine components during late quaternary ice ages summer temperature changes near northern hemisphere melting margins initiated sea level fluctuations that controlled marine components in both polar hemispheres it was further proposed that variations of this ice sheet system amplified and transmitted milankovitch summer half year insolation changes between 45 and 75 n into global climatic changes new tests of this hypothesis implicate sea level as a major control of the areal extent of grounded portions of the antarctic ice sheet thus fitting the concept of a globally interlocked ice sheet system but recent atmospheric modeling results manabe and broccoli 1985 journal of geophysical research 90 2167 2190 suggest that factors other than areal changes of the grounded antarctic ice sheet strongly influenced southern hemisphere climate and terminated the last ice age simultaneously in both polar hemispheres atmospheric carbon dioxide linked to high latitude oceans is the most likely candidate shackleton and pisias 1985 atmospheric carbon dioxide orbital forcing and climate in the carbon cycle and atmospheric co2 natural variations archean to present e t sundquest and w s broecker eds pp 303 318 geophysical monograph 32 american geophysical union washington d c but another potential influence was high frequency climatic oscillations 2500 yr it is postulated that variations in atmospheric carbon dioxide acted through an antarctic ice shelf linked to the grounded ice sheet to produce and terminate southern hemisphere ice age climate it is further postulated that milankovitch summer insolation combined with a warm high frequency oscillation caused marked recession of northern hemisphere ice sheet melting margins and the north atlantic polar front about 14 000 14c yr b p this permitted renewed formation of north atlantic deep water which could well have controlled atmospheric carbon dioxide w s broecker d m peteet and d rind 1985 nature london 315 21 26 combined melting and consequent sea level rise from the three warming factors initiated irreversible collapse of the interlocked global ice sheet system which was at its largest but most vulnerable configuration

the cryptotephra record of the marine isotope stage 12 to 10 interval 460 335 ka at tenaghi philippon greece exploring chronological markers for the middle pleistocene of the mediterranean region abstract precise chronologies that allow direct correlation of paleoclimate archives are a prerequisite for deciphering the spatiotemporal characteristics of short term climate variability such chronologies can be established through the analysis of tephra layers that are preserved in the respective sedimentary archives here we explore the yet untapped tephrochronological potential of the eastern mediterranean region for the middle pleistocene specifically for the interval spanning marine isotope stages mis 12 10 460 335 ka high resolution cryptotephra analysis was carried out on peat cores spanning the mis 12 10 interval that have been recovered from the iconic climate archive of tenaghi philippon ne greece eighteen primary cryptotephras were identified and major and trace element analyses of single glass shards from all cryptotephras were performed in order to correlate them with their eruptive sources the results suggest origins from both italian and aegean arc volcanoes specifically one cryptotephra layer could be firmly correlated with the cape therma 1 eruption from santorini which makes it the first distal tephra finding for this eruptive event while eight further cryptotephras could be tentatively correlated with their volcanic or even eruptive sources the provenance of another nine cryptotephras as yet remains unknown the relatively large number of cryptotephras that could not be assigned to specific volcanic sources and eruptive events reflects the still considerable knowledge gap regarding the geochronology and geochemistry of proximal tephra deposits from the middle pleistocene of the central and eastern mediterranean regions due to the lack of well dated middle pleistocene eruptions we provide age estimates for all cryptotephra layers identified in the mis 12 10 interval at tenaghi philippon based on high resolution pollen data from the same cores while eight of the identified cryptotephras were deposited within mis 12 438 427 ka one cryptotephra was deposited at the onset of mis 11 419 ka five cryptotephras during the younger part of mis 11 391 367 ka and four cryptotephras during mis 10 359 336 ka the high number of cryptotephras from multiple sources as recorded in the mis 12 10 interval at tenaghi philippon highlights the key role of this archive for linking tephrostratigraphic lattices for the middle pleistocene of the central and eastern mediterranean regions

a first order global model of late cenozoic climatic change ii further analysis based on a simplification of co2 dynamics a model developed recently for the long term variations of global ice mass carbon dioxide and mean ocean temperature through the late cenozoic is simplified by hypothesizing a new equation for the co2 variations containing one less adjustable parameter but retaining the essential physical content of the previous equation including nonlinearity and the potential for instability by assuming plausible time constants for the glacial ice mass and global mean ocean temperature and setting the values of six adjustable parameters rate constants a solution for the last 5 my is obtained displaying many of the features observed over this period including the transition to the near 100 ky major ice age oscillations of the late pleistocene in obtaining this solution it is also assumed that variations in tectonic forcing lead to a reduction of the equilibrium co2 concentration perhaps due to increased weathering of rapidly uplifted mountain ranges over this period as a consequence of this co2 reduction the model dynamical system can bifurcate to a free oscillatory ice age regime that is under the pacemaker influence of earthorbital milankovitch forcing expanded discussions are given of the surface temperature variations accompanying the evolution of ice co2 and ocean temperature and of the bifurcation properties of the model from both mathematical and physical viewpoints

insolation values for the climate of the last 10 million years new values for the astronomical parameters of the earth s orbit and rotation eccentricity obliquity and precession are proposed for paleoclimatic research related to the late miocene the pliocene and the quaternary they have been obtained from a numerical solution of the lagrangian system of the planetary point masses and from an analytical solution of the poisson equations of the earth moon system the analytical expansion allows the direct determination of the main frequencies with their phase and amplitude numerical and analytical comparisons with the former astronomical solution ber78 are performed so that the accuracy and the interval of time over which the new solution is valid can be estimated the corresponding insolation values have also been computed this analysis leads to the conclusion that the new values are expected to be reliable over the last 5 ma in the time domain and at least over the last 10 ma in the frequency domain

bifurcation structure and noise assisted transitions in the pleistocene glacial cycles 1 the glacial cycles are attributed to the climatic response of the orbital changes in the irradiance to the earth these changes in the forcing are too small to explain the observed climate variations as simple linear responses nonlinear amplifications of the orbital forcing are necessary to account for the glacial cycles here an empirical model of the nonlinear response is presented from the model it is possible to assess the role of stochastic noise in comparison to the deterministic orbital forcing of the ice ages the model is based on the bifurcation structure derived from the climate history it indicates the dynamical origin of the mid pleistocene transition from the 41 ka world to the 100 ka world the dominant forcing in the latter is still the 41 ka obliquity cycle but the bifurcation structure of the climate system is changed the model suggests that transitions between glacial and interglacial climate are assisted by internal stochastic noise in the period prior to the last five glacial cycles while the last five cycles are deterministic responses to the orbital forcing

an electron microprobe study of the u th pb systematics of metamorphosed monazite the role of pb diffusion versus overgrowth and recrystallization abstract an electron microprobe emp was used to obtain u th pb chemical ages from monazite that underwent an amphibolite facies metamorphic overprint in the monashee complex southern canadian cordillera u pb isotope dilution dating of single grains yielded discordant ages that plot in linear arrays on a concordia diagram upper intercepts of the arrays indicate primary growth in the proterozoic and cretaceous and lower intercepts are coeval with a tertiary metamorphic event the microprobe s high spatial resolution 2 m and ability to quickly differentiate between domains with greatly contrasting u th and pb concentrations made it the ideal tool for investigating the cause of the u pb isotopic discordance the microprobe dating shows that proterozoic monazite lies directly adjacent to generally small

binge purge oscillations of the laurentide ice sheet as a cause of the north atlantic s heinrich events ice rafted debris in sediment cores from the north atlantic suggests that the laurentide ice sheet lis periodically disgorged icebergs in brief but violent episodes which occurred approximately every 7 000 years here i propose that heinrich events i e what these episodes are called were caused by free oscillations in the flow of the laurentide ice sheet which arose because the floor of hudson bay and hudson strait is covered with soft unconsolidated sediment that forms a slippery lubricant when thawed the proposed heinrich event cycle has two phases the growth phase occurs when the sediment is frozen and the lis is stranded immobile on a rigid bed the volume of the lis slowly grows during this phase at a rate dictated by snow accumulation the purge phase occurs when the basal sediment thaws and a basally lubricated discharge pathway i e an ice stream such as those which occur in west antarctica today developes through hudson strait the volume of the lis rapidly equilibrates to the reduced basal friction during this phase by dumping icebergs into the labrador sea the periodicity t k sl2g 2 7000 years of the proposed heinrich event cycle is a function of the thermal conductivity and diffusivity of ice k and respectively the atmospheric sea level temperature sl in degrees celsius and the excess geothermal heat flux defined by g g k where is the atmospheric lapse rate and g is the geothermal heat flux agreement between the predicted t and the apparent periodicity implied by the marine record is the main virtue of the free oscillation mechanism i propose an alternative mechanism in which heinrich events are forced by periodic variations in external climate is implausible because periodic atmospheric temperature perturbations are strongly attenuated with depth in an ice sheet

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

numerical modeling of advective transport of saturated deforming sediment beneath the lake michigan lobe laurentide ice sheet abstract we investigated the dynamics of advective transport of saturated fine grained deforming sediment beneath the late wisconsinan lake michigan lobe lml of the laurentide ice sheet using a numerical flowline model sediment rheology is modeled using a general constitutive law for mass movement of fine grained material geotechnical tests on selected tills deposited by the lml in northern illinois suggest an effective viscosity of about 5 109 pa s for fully saturated sediment and mild nonlinearity in the stress strain relation numerical calculations of sediment flux based on conservative parameters and assumptions yield estimates on the order of 100 m3 y meter width of the lobe suggesting that advective transport could account directly for at least 25 of the total sediment flux estimated on the basis of field evidence less restrictive parameters and assumptions would increase sediment flux attributable to both advective transport as well as other ice velocity dependent sediment transport processes modeling results also suggest that maximum advective transport rates might be associated with viscosities up to an order of magnitude higher than the viscosity exhibited by fully saturated sediment these results thus suggest that subglacial sediment deformation could account for a substantial portion of total sediment flux in areas with fine grained glacial till and that transport rates are strongly sensitive to the subglacial hydrologic processes that control basal water pressure and sediment viscosity

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

geologic and topographic controls on fast flow in the laurentide and cordilleran ice sheets ice streams are fast flowing arteries which play a vital role in the dynamics and mass balance of present day ice sheets although not fully understood fast flow dynamics are intimately coupled with geologic topographic thermal and hydrologic conditions of the underlying bed these are difficult observables beneath contemporary ice sheets hindering elucidation of the processes which govern ice streaxn behavior for past ice sheets the problem is antithetic geologic evidence of former ice streams exists but spatim and temporal histories are uncertain however detailed knowledge of bed geology and topography is available in many places we take advantage of this information to compile terrain characteristics relevant to fast flow dynamics in the laurentide and cordilleran ice sheets using seed points where fast flowing wisconsinan ice has been geologically inferred discriminant analysis of a suite of north american geologic and topographic properties yields a concise measure of ice bed coupling strength our analysis suggests that the interior plains and continental shelf regions of north america have low basal coupling relative to areas of variable relief or exposed bedrock in the cordillera and on the canadian shield we conclude that the interior plains and continental shelves are both topographically and geologically predisposed to large scale basal flows i e ice streaxns or surge lobes this result holds independent of whether the mechanism of fast flow is sediment deformation or decoupled sliding over the bed

changes in the silt and clay size mineralogy of sediments at ocean drilling program site 645b baffin bay site 645b ocean drilling program odp leg 105 was cored in 2010 m water depth off the eastern margin of baffin island a 279 m core was dated as extending back to ca 2 ma samples of sediment were analyzed for their silt and clay size mineralogy using x ray diffraction techniques constrained cluster analysis on these data defined three units which indicated that significant changes in mineralogies occurred at approximately 75 m below sea floor bsf ca 0 5 ma and 155 m bsf ca 1 15 ma these boundaries coincide closely with units ia ib and ib ii as designated in the odp shipboard reports and which were based on detrital carbonate and sand contents unit ii 1 2 2 ma is characterized by zero clay size calcite and high amounts of clay size smectite and silt size mica unit ib 0 5 1 15 ma is noteworthy for the first presence of clay size calcite high amounts of detrital dolomite and a peak in clay size kaolinite the uppermost unit ia 0 0 5 ma is characterized by consistently high value

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

the evolution of pleistocene climate a time series approach abstract in the later part of the pleistocene variations in global ice volume have been dominated by an approximate 100 000 year cycle analysis of 2 myr long oxygen isotope record from an equatorial pacific core indicates that this is true only for the last 900 000 years prior to this time the amplitude of the 100 000 year cycle is much reduced as is the variance of all oscillations with periods greater than 60 000 years based on results of time series analysis of this 2 myr long record the pleistocene glacial cycles can be divided into three sections 1 the late pleistocene 0 900 kyr b p where the variations in the isotope record are dominated by the 100 000 year cycle 2 the middle pleistocene 900 1450 kyr b p in which low frequency components are not as important as in the later period of the pleistocene and 3 the early pleistocene late pliocene 1450 2000 kyr b p where general reductions of importance at all frequencies is seen as compared to the later intervals recent modeling efforts which describe variations in global ice volume show that the dominant low frequency component observed in the late pleistocene can result from different time constants for the rate of glacial growth and decay in response to variations in the earth s orbital parameters it is hypothesized that during the early pleistocene the rate of growth and decay of glaciers were more similar and that continental erosion by successive glacial advances lowered the land surface in areas of ice cap formation to below sea level when the ice caps became marine based more rapid decay of the ice became possible

effects of chemical weathering and sorting on the petrogenesis of siliciclastic sediments with implications for provenance studies the bulk chemical compositions mineralogy and mineral proportions of sands and muds of the mallacoota basin in southeastern australia reflect the composition of weathering profiles mantling source rocks rather than bedrock muds contain abundant clay minerals that are virtually absent from the source rocks but abundant in the weathering profiles sands are strongly enriched in quartz relative to source rocks even in the headwaters of the fluvial system demonstrating that feldspar destruction occurs by in situ chemical weathering within profiles and before detritus enters the fluvial system k feldspar is proportionally more abundant in fluvial sands than in source rocks because plagioclase is more rapidly destroyed than either quartz or k feldspar in weathering profiles subsequent erosion and sorting produce sands enriched in quartz with high k feldspar plagioclase ratios relative to source rocks the composition of plagioclase incorporated into the fluvial sands is also controlled by chemical weat

a one dimensional model of continental ice volume fluctuations through the pleistocene implications for the origin of the mid pleistocene climate transition abstract a new one dimensional zonally averaged high resolution spherical model of the accumulation and flow of continental ice sheets in response to variations in insolation forcing has been constructed the model includes a detailed representation of the influence of the isostatic adjustment of the earth under the weight of the ice at a rate determined by the viscosity of the planetary mantle with this model we address the issue as to whether the combination of physical processes that it includes is sufficient to allow us to reconcile the variations of continental ice volume that are inferred to have dominated the pleistocene epoch on the basis of the analysis of 18o data from deep sea sedimentary cores we use high resolution data from the newly analyzed odp site 677 and from dsdp site 607 as principal observational constraints these datasets reveal a marked transition in climate to have occurred at mid pleistocene time about 900 000 years bp in order to understand this transition we find that we

plio pleistocene time evolution of the 100 kyr cycle in marine paleoclimate records to constrain theories for the dynamic evolution of global ice mass through the late neogene it is important to determine whether major changes in the record were gradual or rapid of particular interest is the evolution of the near 100 kyr ice age cycle in the middle pleistocene we describe and apply a new technique based on multiple taper spectral analysis which allows us to model the time evolution of quasi periodic signals this technique uses both phase and amplitude information and enables us to address the question of abrupt versus gradual onset of the 100 kyr periodicity in the middle pleistocene we analyzed three long 2 6 m y time series from deep sea drilling project dsdp site 607 midlatitude atlantic and ocean drilling program odp site 677 equatorial pacific the published time scales for these sites differ significantly influencing the interpretation of the mid pleistocene climate transition when the data series are expressed on the same time scale there is evidence for a coherent 18o signal at both sites in the eccentricity and obliquity frequency bands consistent with variations in global ice volume as the causative factor if the shaddeton et al 1990 time scale for odp 677 is accepted the amplitude match between the 18o obliquity cycle and the 65 n insolation derived from the recent astronomical solution of laskar 1988 1990 is excellent for times t 2 3 ma if the ruddiman raymo time scale for dsdp 607 is accepted the 41 kyr 18o cycle has enhanced amplitude between 1 0 and 1 5 ma relative to the late pleistocene t 1 0 ma implying a nonlinear earth system response to obliquity insolation cycles we do not find compelling evidence for an abrupt change in the 100 kyr 18o signal although we cannot rule it out at this time if the shackleton et al 1990 time scale for odp 677 is accepted our three 18o records are consistent with a low amplitude 100 kyr cycle between 1 2 and 2 6 ma whose local period of oscillation alternates between the 95 and 124 kyr eccentricity periods and a gradual increase in the 100 kyr 18o signal between 0 5 amd 1 0 ma the dsdp 607 time scale is more favorable to an abrupt jump in amplitude for the 95 kyr 18o envelope but not in the 124 kyr envelope rather long period 18o fluctuations appear phase locked with the 124 kyr eccentricity cycle some 300 400 kyr prior to its growth in amplitude and phase lock with the 95 kyr eccentricity cycle in the late pleistocene

north american glacial meltwater history for the past 2 3 m y oxygen isotope evidence from the gulf of mexico a high resolution 18 o record from the northeastern gulf of mexico ocean drilling program leg 100 hole 625b provides a history of surface waters for the past 5 35 m y negative 18 o anomalies between 0 3 and 3 0 in planktic foraminifera beginning at 2 30 ma can only have resulted from the discharge of isotopically light glacial meltwater from the mississippi river meltwater anomalies implicate either the periodic presence of north american mid latitude glacial ice since the late pliocene or a drainage system very different from that of the present

rheological nonlinearity and flow instability in the deforming bed mechanism of ice stream motion contrary to what has recently been assumed in modeling the proposed deforming bed mechanism for the rapid motion of antarctic ice streams the rheology of water saturated till is probably highly nonlinear according to information from soil mechanics and preliminary experiments on till from the base of ice stream b the equivalent flow law exponent n is probably as high as 100 and the nonlinearities of the shear stress and effective pressure dependences are closely linked the high nonlinearity has important consequences for the deforming bed mechanism a flow system operating by this mechanism can be unstable as a result of feedback from the generation of basal water by shear heating of basal till the short term feedback effect is analyzed for a perturbation in a model ice stream in which the basal meltwater is transported through a distributed system of narrow gap conduits at the ice till interface although the analysis is approximate and some of the system parameters are poorly known the results suggest that the deforming bed mechanism is unstable for n 20 the apparent lack of such an instability in the currently active ice streams implies that their motion is controlled not by the deforming bed mechanism but by some other as yet unidentified mechanism

coupled energy balance ice sheet model simulations of the glacial cycle a possible connection between terminations and terrigenous dust we apply a coupled energy balance ice sheet climate model in an investigation of northern hemisphere ice sheet advance and retreat over the last glacial cycle when driven only by orbital insolation variations the model predicts ice sheet advances over the continents of north america and eurasia that are in good agreement with geological reconstructions in terms of the timescale of advance and the spatial positioning of the main ice masses the orbital forcing alone however is unable to induce the observed rapid ice sheet retreat and we conclude that additional climatic feedbacks not explicitly included in the basic model must be acting in the analyses presented here we have parameterized a number of potentially important effects in order to test their relative influence on the process of glacial termination these include marine instability thermohaline circulation effects carbon dioxide variations and snow albedo changes caused by dust loading during periods of high atmospheric aerosol concentration for the purpose of these analyses the temporal changes in the latter two variables were inferred from ice core records of these various influences our analyses suggest that the albedo variations in the ice sheet ablation zone caused by dust loading may represent an extremely important ablation mechanism using our parameterization of dirty snow in the ablation zone we find glacial retreat to be strongly accelerated such that complete collapse of the otherwise stable laurentide ice sheet ensues the last glacial maximum configurations of the laurentide and fennoscandian complexes are also brought into much closer accord with the ice 3g reconstruction of tushingham and peltier 1991 1992 and the ice 4g reconstruction of peltier 1994 when this effect is reasonably introduced

properties and genesis of a soil and the underlying gibbsite bearing saprolite cape breton island canada brownish yellow saprolite derived from granite gneiss is exposed beneath 1 2 m of reddish brown till along a roadcut in the cape breton highlands the strongly acid soil is similar to other podzolic soils of the region except for the presence of gibbsite in the silt and clay fractions in the saprolite feldspar and biotite have weathered to poorly crystalline kaolinite gibbsite and a minor component of iron oxide the poorly crystalline kaolinite does not occur in the overlying till the evidence presented is consistent with the view that the area was covered either by cold based or by stagnant warm based glaciers that eroded the pre quaternary surface only slightly and deposited a thin discontinuous veneer of till derived mainly from local bedrock and saprolite weathering products from the saprolite are probably incorporated commonly in tills of the area during the formation of soils in these tills the poorly crystalline kaolinite and most of the gibbsite are either transformed to other minerals

sensitivity of a greenland ice sheet model to ice flow and ablation parameters consequences for the evolution through the last climatic cycle sensitivity experiments are conducted to test the influence of poorly known model parameters on the simulation of the greenland ice sheet by means of a three dimensional numerical model including the mechanical and thermal processes within the ice two types of experiments are performed steady state climatic conditions and simulations over the last climatic cycle with a climatic forcing derived from the grip record the experiments show that the maximum altitude of the ice sheet depends on the ice flow parameters deformation and sliding law coefficients geothermal flux and that it is low when the ice flow is fast on the other hand the maximum altitude is not sensitive to the ablation strength and consequently during the climatic cycle it is driven by changes in accumulation rate the ice sheet extension shows the opposite sensitivity it is barely affected by ice flow velocity and the ice covered area is smaller for large ablation coefficients for colder climates when there is no ablation the ice sheet extension depends on the sea level an interesting result is that the variations with time of the altitude at the ice divide summit do not depend on the parameters we tested the present modelled ice sheets resulting from the climatic cycle experiments are compared with the present measured ice sheet in order to find the set of parameters that gives the best fit between modelled and measured geometry it seems that compared to the parameter set most commonly used higher ablation rate coefficents must be used

climate evolution in the pliocene and pleistocene from marine sediment records and simulations internal variability versus orbital forcing three deep sea 18o records are compared in the temporal and spectral domain with each other a simple ice sheet and bedrock climate model simulation and insolation anomalies at 65 n the model is one dimensional it represents the accumulation and ablation processes of ice its viscoplastic flow and the isostatic adjustment of bedrock under it being forced solely by orbital insolation anomalies the objective of the comparison is to determine the importance of internal variability arising from feedback between the ice sheets and bedrock through the late pliocene and pleistocene in the simulations and in the climate record it is found that a considerable part of the spectral variability at suborbital frequencies i e below the precessional and obliquity frequencies as seen in the sediment cores is simulated by the model simulated variability near 100 kyr has its period determined by the difference between two precessional frequencies the model s internal variability mechanism has a period near 30 50 kyr but is highly damped still it enhances suborbital variability near 100 kyr and 300 kyr and appears to induce nonstationarity of the orbital and suborbital peaks on even longer time scales since the insolation forcing exhibits no noticeable peaks at suborbital or lower frequencies it appears that the nonstationarity in the model simulations as well as the suborbital peaks themselves are due to coupling between model nonlinearities and the orbital forcing since the variability at 100 kyr in the deep sea records is correlated with the simulated variability such a control by the orbital forcing may be also operating in the climate system the absence from the simulation of a trend toward larger ice sheets and colder climate seen throughout the pleistocene in the sediment record is possibly due to slow tectonics and co2 forcing changes not included in the model

water pressure coupling of sliding and bed deformation ii velocity depth profilesr b alley 130 139water pressure coupling of sliding and bed deformation iii application to ice stream b antarctica

slow physics of large continental ice sheets and underlying bedrock and its relation to the pleistocene ice ages very simple two component models of the earth s cryosphere lithosphere system are analyzed the models consist of a continental ice sheet coupled to a responding earth model because of relatively long response times of the bedrock at least over shorter wavelengths long period of the order of 50 000 years internal or free oscillations in the system occur over a wide range of parameters and parameterizations the oscillatory mechanism operates primarily in the ablation zone and in particular at the firn line these free oscillations could play a dynamic role in the response of the climate system to eccentricity forcing a simple analysis presented strongly suggests that for the simplest rheological models of the earth response to pleistocence ice sheets elastic response is confined to a layer considerably less than the thermal thickness of the lithosphere furthermore because of the striking lack of geological evidence for large depressions of bedrock at the leading edge of growing continental ice sheets predicted by viscoelastic models of the earth more sophisticated rheological models of the earth s uppermost layer appear necessary

origin of the middle pleistocene transition by ice sheet erosion of regolith the transition in the middle pleistocene 0 9 ma seen in 18o deep sea core records from relatively low amplitude high frequency 41 kyr to high amplitude low frequency 100 kyr ice volume variations under essentially the same orbital forcing can be attributed to a change from an all soft bedded to a mixed hard soft bedded laurentide ice sheet through glacial erosion of a thick regolith and resulting exposure of unweathered crystalline bedrock a one dimensional ice sheet and bedrock model which includes transport of sediment and ice by subglacial sediment deformation demonstrates that a widespread deforming sediment layer maintains thin ice sheets before the transition which respond linearly to the dominant 23 and 41 kyr orbital forcing progressive removal of the sediment layer eventually causes a transition to thicker ice sheets whose dominant timescale of change 100 kyr reflects nonlinear deglaciation processes in model simulations over the last 3 ma initialized with no ice and a uniform 50 m sediment layer the time series of ice volume and extent agree in several important aspects with the observed records

numerical reconstruction of a soft bedded laurentide ice sheet during the last glacial maximum we used a numerical ice sheet model to reconstruct the north american laurentide ice sheet during the last glacial maximum our model simulates ice sheet conditions that can be specified experimentally as either a rigid substrate hard bed or a wet deformable till soft bed basal sliding is excluded we use geologic records of former basal ice sheet processes to prescribe the distribution of hard and soft beds our reconstruction of the laurentide ice sheet is significantly lower in ice surface height and contains less ice volume than the climap maximum reconstruction in contrast our reconstruction agrees well with the ice 4g reconstruction both in height and volume because the ice 4g reconstruction is based on the inversion of relative sea level data whereas our reconstruction is based on glacial geology and ice mechanics this agreement suggests that soft beds provide a glaciological mechanism to explain the shape and volume of the laurentide ice sheet that is most consistent with observations of relative sea level change and other geodynamic considerations

numerical modeling of subglacial sediment deformation implications for the behavior of the lake michigan lobe laurentide ice sheet we apply a numerical model of the late wisconsin circa 20 000 years b p lake michigan lobe lml laurentide ice sheet to investigate how fine grained subglacial sediment might influence lobe behavior particularly rapid millennial scale marginal oscillations observed in the geologic record over the canadian shield we assume a rigid bed hard bedded basal boundary condition in areas overlain by fine grained sediment soft bedded the base of the ice is coupled to a deformable sediment layer using a rate dependent stress strain law geotechnical tests of clay rich till deposited by the lml provide control for sediment rheologic parameters simulated cross sectional profiles are consistent with reconstructions from geologic evidence time dependent simulations suggest that a soft bedded lobe could have reached steady state in about 20 000 years or less in contrast to 50 000 to 60 000 years for an otherwise identical hard bedded lobe a soft bedded lobe with sediment viscosity at the experimentally determined value is about twice as responsive to millennial scale shifts in accumulation or ablation as a nonsliding hard bedded lobe but in both cases the response is slower than that indicated by the geologic record results suggest that while strong millennial scale changes in accumulation and ablation can produce responses in hard bedded or soft bedded ice that are consistent with the geologic record changes in subglacial sediment viscosity even relatively modest changes whether independent or in conjunction with climate change might more readily account for millennial and submillennial scale fluctuations of the lobe margin these observations do not exclude a role for sliding but they do provide some perspective from which to evaluate relative contributions of the various processes that influence lobe behavior

nonstationary phase of the plio pleistocene asian monsoon paleoclimate records indicate that the strength of the asian summer monsoon is sensitive to orbital forcing at the obliquity and precession periods 41 000 and 23 000 years respectively and the extent of northern hemisphere glaciation over the past 2 6 million years the timing phase of strong monsoons has changed by 83 degrees in the precession and 124 degrees in the obliquity bands relative to the phase of maximum global ice volume inferred from the marine oxygen isotope record these results suggest that one or both of these systems is nonstationary relative to orbital forcing

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

simulation of the last glacial cycle by a coupled sectorially averaged climate ice sheet model 2 response to insolation and co2 variations a two dimensional climate model which links the northern hemisphere atmosphere ocean mixed layer sea ice and continents has been asynchronously coupled to a model of the three main northern ice sheets and their underlying bedrock the coupled model has been used to test the influence of several factors including snow surface albedo over the ice sheets in producing plausible ice age simulations using astronomically derived insolation and co2 data from the vostok ice core the impact of potentially important processes such as the water vapor transport clouds and deep sea circulation was not investigated in this study after several sensitivity experiments designed to identify the main mechanism governing surface temperature and ice accumulation the model is first run with ice sheet feedback by forcing it only with the astronomical insolation over the past 122 kyr large variations of ice volume are simulated between 122 and 55 kyr b p with a rapid latitudinal extension of the north american and eurasian ice sheets starting at 120 kyr b p the simulated last glacial maximum is at 19 kyr b p the model is able to simulate deglaciation as well the simulated evolution of the three northern ice sheets is generally in phase with geological reconstructions the major discrepancy between the simulation and paleoclimate reconstructions lies in the underestimation of temperature variations linked with an underestimation of the ice sheet extent and an excess in the prescribed co2 concentration sensitivity experiments show that ablation is more important to the ice sheet response than snow precipitation variations in the model a key mechanism in the deglaciation after the last glacial maximum appears to be the aging of snow which decreases its albedo the other factors which play an important role are in decreasing level of importance the ice sheet altitude insolation taiga cover and ice sheet extent a final set of experiments addresses the effects of co2 on the simulated climate of the last glacial maximum and on a new long term experiment covering the last 122 kyr this last experiment is made by forcing the model with both insolation and co2 variations this additional forcing improves the temperature and ice volume results despite the limitations inherent to the present modeling approach the sensitivity experiments performed can provide insight into the relative importance of possible mechanism responsible for the building and melting of huge ice sheets during the last glacial interglacial cycle

the mid pleistocene climate transition a deep sea carbon isotopic perspective five 13c records from the deep ocean extending back to 1 3 ma were examined in order to constrain changes in mean ocean carbon isotope composition and thermohaline circulation over the 41 to 100 ka climate transition these data show that significant perturbations in mean ocean carbon chemistry were associated with the mid pleistocene climate transition notable features of the last 1 3 myr are 1 a pronounced 0 3 decrease in mean ocean 13c between 0 9 and 1 0 myr followed by a return to pre 1 0 ma values by 400 ka b p which we propose was due to the onetime addition of isotopically depleted terrestrial carbon to the ocean possibly associated with an increase in global aridity and decrease in the size of the biosphere across the 41 to 100 ka transition 2 no change in the atlantic pacific a p 13c gradient over the last 1 3 myr suggesting no change in mean ocean nutrient content accompanied the addition of light carbon and 3 stronger vertical nutrient fractionation in the north atlantic in the middle pleistocene between sites 607 and 552 suggesting weaker north atlantic deep water formation at this time relative to the early and late pleistocene we also find evidence for a more pronounced deep recirculation gyre in the western north atlantic basin in the early brunhes as evidenced by aging of deep northern basin water site 607 relative to deep water in the equatorial atlantic site 664

hydraulic run away a mechanism for thermally regulated surges of ice sheets by using a simple parameterized model of thermomechanically coupled flow in cold ice sheets together with a physically based sliding law which includes a description of basal drainage we show that relationships between ice flux and ice thickness can realistically be multi valued and hence that hydraulically induced surges can occur we term this mechanism hydraulic run away as it relies on the positive feed back between sliding velocity and basal melt production for this feed back to operate it is essential that water pressure increases with water storage this is consistent with various recent ideas concerning drainage under ice sheets be it through a system of canals a distributed film or a subglacial aquifer for confined flows such as valley glaciers e g trapridge glacier or topographically constrained ice streams e g hudson strait in the laurentide ice sheet which are underlain by sufficiently deformable sediment we can expect thermally regulated surges to occur while in a laterally unconfined drainage basin such as that which flows into the ross ice shelf we might expect ice streams to develop

characteristics of residual and colluvial soils developed on granite and of the associated pre wisconsin landforms in north central new brunswick an area in north central new brunswick of approximately 20 km2 where tors are common was found to be undisrupted by glaciation the soils in this area are developed on grus and appear to be of pr

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

irregular oscillations of the west antarctic ice sheet model simulations of the west antarctic ice sheet suggest that sporadic perhaps chaotic collapse complete mobilization of the ice sheet occurred throughout the past one million years the irregular behaviour is due to the slow equilibration time of the distribution of basal till which lubricates ice sheet motion this nonlinear response means that predictions of future collapse of the ice sheet in response to global warming must take into account its past history and in particular whether the present basal till distribution predisposes the ice sheet towards rapid change

deformation till in the great lakes region implications for rapid flow along the south central margin of the laurentide ice sheet structural and lithologic data indicate that while deposited under actively moving ice considerable portions of three muddy calcareous subglacial tills in the great lakes region probably experienced some component of ductile deformation viscous till flow and ductile shear are invoked to explain a combination of features such as recumbent isoclinal folds unlithified sediment clasts mixed ostracode shells reversed stone lee ends girdle and transverse fabrics irregular stone pavements fine striae following stone curves and inconsistent stone striae and a axes deforming fine textured subglacial till is considered as a subhorizontal shear zone rheologically layered with associated structures in descending order ductile e g isoclinal folds brittle ductile e g fissility and brittle e g till wedges rheology would be controlled mainly by till pore water content matrix texture and stone content spatial and temporal superposition of rheologies and subglacial processes probably occur

the mid pleistocene climate transition onset of 100 ka cycle lags ice volume build up by 280 ka the mid pleistocene climate transition mpt is the complex climatic change which brought the late pleistocene ice ages we explore the mpt in the time and frequency domains by new methods of time series analysis high resolution oxygen isotope records reveal that the ice volume related increase in 18o mean amplitude 0 29 0 05 1 transition midpoint 922 12 ka duration 40 9 ka significantly preceded the abrupt increase in the amplitude of the 100 ka cycle at 641 9 ka this finding can be quantitatively simulated using a simple ice bedrock model in which due to the additional ice the calving threshold is exceeded the simulated calving events prior to 650 ka are separated by 77 ka whereas after 650 ka they occur pseudo periodically with a mean period of nearly 100 ka the cause for the delay of the 100 ka calving cycle was not a slow bedrock relaxation rather the coincidental combination of insolation existing ice mass and bedrock depression