a new look at the saturn system the voyager 2 images voyager 2 photography has complemented that of voyager i in revealing many additional characteristics of saturn and its satellites and rings saturn s atmosphere contains persistent oval cloud features reminiscent of features on jupiter smaller irregular features track out a pattern of zonal winds that is symmetric about saturn s equator and appears to extend to great depth winds are predominantly eastward and reach 500 meters per second at the equator titan has several haze layers with significantly varying optical properties and a northern polar collar that is dark at short wavelengths several satellites have been photographed at substantially improved resolution enceladus surface ranges from old densely cratered terrain to relatively young uncratered plains crossed by grooves and faults tethys has a crater 400 kilometers in diameter whose floor has domed to match tethys surface curvature and a deep trench that extends at least 270 around tethys circumference hyperion is cratered and irregular in shape iapetus bright trailing hemisphere includes several dark floored craters and phoebe has a very low albedo and rotates in the direction opposite to that of its orbital revolution with a period of 9 hours within saturn s rings the birth of a spoke has been observed and surprising azimuthal and time variability is found in the ringlet structure of the outer b ring these observations lead to speculations about saturn s internal structure and about the collisional and thermal history of the rings and satellites

impact of bedrock surface topography on spatial distribution of quaternary sediments and on the flow pattern of late weichselian glaciers on the east european craton russian plain this paper presents the first high quality maps of the bedrock surface topography and the quaternary sediment distribution for the late weichselian glaciation area on the east european craton eec and the reasoning behind the estimated sediment distribution geographic information system gis based three dimensional model of the bedrock surface topography the current digital terrain model dtm and the ice flow pattern of the last glaciation provided the main source data for the analyses and interpretations in the current continental area of the western eec which was covered by the last scandinavian ice sheet sis the bedrock lies overwhelmingly 85 above the contemporary sea level with an average altitude of 54 m above sea level the topography of the bedrock surface is very well reflected in the current topography and an 80 m difference in the ice bed surface altitudes results in the topographic focusing of the ice flow and thus all late weichselian ice streams were accommodated to the depressions in the subglacial surface such depressions are characterised by a thin 10 m layer of sediment with one till bed just 50 m upslope from the bottom of the depressions the thickness of the sediment increases two to threefold and comprises five or more till layers indicating relief enhancement through subsequent glaciations the average calculated thickness of the quaternary deposits in the study area was 49 m and the total volume of sediments was 20 280 km3

self organization in the thermomechanical flow of ice sheets this paper uses a three dimensional thermomechanical ice sheet model to investigate the initiation and evolution of ice streams within ice sheets ice streams are distinguished within general ice sheet flow by their relatively fast of the order of kilometers per year velocity and are thought to be crucial in determining the response of ice sheets to climatic change we show that streaming can arise solely as a consequence of internal feedback between ice sheet flow and temperature inhomogeneities in topography and or geology are not therefore necessary conditions for ice stream development this spatial patterning replaces the large scale surging reported from previous modeling work in one horizontal dimension we suggest that surging is restricted to situations where ice flow is channelized e g valley glaciers and that it is replaced by streaming where horizontal ice flow is unconstrained ice sheets the ice streams in some of our experiments show switching behavior reminiscent of that known from the siple coast ice streams west antarctica the cause appears to be the competition for ice discharge and drainage basin area between adjacent ice streams this may lead to the gradual reduction of a stream s discharge below the threshold necessary to generate enough frictional heat to maintain warm viscous ice at the ice stream bed in this case the stream will stagnate abruptly which then prompts localized ice thickening and the initiation of a new stream in a different location

a shallow convective model for jupiter s alternating wind bands 1 the dynamics of a thin atmospheric layer 1 340 bar below the cloud tops of jupiter is simulated with a three dimensional nonlinear transformed spectral model the gas is compressible and stratified commensurate with the observed emitted radiation from jupiter s atmosphere the transport of the planet s internal energy in the lower region is taken to be convective but solar heating is not accounted for in qualitative agreement with observations the model produces alternating wind bands and a dominant prograde equatorial jet the zonal wind speed at the equator is within a factor of 2 of the observed values

the link between climate warming and break up of ice shelves in the antarctic peninsula a review of in situ and remote sensing data covering the ice shelves of the antarctic peninsula provides a series of characteristics closely associated with rapid shelf retreat deeply embayed ice fronts calving of myriad small elongate bergs in punctuated events increasing flow speed and the presence of melt ponds on the ice shelf surface in the vicinity of the break ups as climate has warmed in the antarctic peninsula region melt season duration and the extent of ponding have increased most break up events have occurred during longer melt seasons suggesting that meltwater itself not just warming is responsible regions that show melting without pond formation are relatively unchanged melt ponds thus appear to be a robust harbinger of ice shelf retreat we use these observations to guide a model of ice shelf flow and the effects of meltwater crevasses present in a region of surface ponding will likely fill to the brim with water we hypothesize building on weertman 1973 hughes 1983 and van der veen 1998 that crevasse propagation by meltwater is the main mechanism by which ice shelves weaken and retreat a thermodynamic finite element model is used to evaluate ice flow and the strain field and simple extensions of this model are used to investigate crack propagation by meltwater the model results support the hypothesis

influence of glacier hydrology on the dynamics of a large quaternary ice sheet the influence of glacier hydrology on the time dependent morphology and flow behaviour of the late weichselian scandinavian ice sheet is explored using a simple one dimensional ice sheet model the model is driven by orbitally induced radiation variations ice albedo feedback and eustatic sea level change the influence of hydrology is most marked during deglaciation and on the southern side of the ice sheet where a marginal zone of rapid sliding thin ice and low surface slopes develops such a zone is absent when hydrology is omitted from the model and its formation results in earlier and more rapid deglaciation than occurs in the no hydrology model the final advance to the glacial maximum position results from an increase in the rate of basal sliding as climate warms after 23000 yr bp channelised subglacial drainage develops only episodically and is associated with relatively low meltwater discharges and high hydraulic gradients the predominance of iceberg calving as an ablation mechanism on the northern side of the ice sheet restricts the occurrence of surface melting lack of meltwater penetration to the glacier bed in this area means that ice flow is predominantly by internal deformation and the ice sheet adopts a classical parabolic surface profile

thermal structure of jupiter s atmosphere near the edge of a 5 μm hot spot in the north equatorial belt thermal structure of the atmosphere of jupiter was measured from 1029 km above to 133 km below the 1 bar level during entry and descent of the galileo probe the data confirm the hot exosphere observed by voyager 900 k at 1 nanobar the deep atmosphere which reached 429 k at 22 bars was close to dry adiabatic from 6 to 16 bars within an uncertainty 0 1 k km the upper atmosphere was dominated by gravity waves from the tropopause to the exosphere shorter waves were fully absorbed below 300 km while longer wave amplitudes first grew then were damped at the higher altitudes a remarkably deep isothermal layer was found in the stratosphere from 90 to 290 km with t 160 k just above the tropopause at 260 mbar there was a second isothermal region 25 km deep with t 112 k between 10 and 1000 mbar the data substantially agree with voyager radio occultations the voyager 1 equatorial occultation was similar in detail to the present sounding through the tropopause region the voyager iris average thermal structure in the north equatorial belt neb approximates a smoothed fit to the present data between 0 03 and 400 mbar differences are partly a result of large differences in vertical resolution but may also reflect differences between a hot spot and the average neb at 15 4 bars probe descent velocities derived from the data are consistently unsteady suggesting the presence of large scale turbulence or gravity waves however there was no evidence of turbulent temperature fluctuations 0 12 k a conspicuous pause in the rate of decrease of descent velocity between 1 1 and 1 35 bars where a disturbance was also detected by the two radio doppler experiments implies strong vertical flow in the cloud seen by the probe nephelometer at p 0 6 bar measured temperatures were 3 k warmer than the dry adiabat possible evidence of radiative warming this could be associated with a tenuous cloud detected by the probe nephelometer above the 0 51 bar level for an ammonia cloud to form at this level the required abundance is 0 20 solar

geometry bed topography and drainage system structure of the haut glacier d arolla switzerland as part of an integrated study of the hydrology meltwater quality and dynamics of the haut glacier d arolla switzerland the glacier s drainage network structure was determined from patterns of dye recovery in 342 injection experiments conducted from 47 moulins distributed widely across the glacier this structure was compared with theoretical predictions based upon reconstructed patterns of water flow governed by a the subglacial hydraulic potential surface and b the subglacial bedrock surface these reconstructions were based on measurements of ice surface and bedrock topography obtained by a combination of ground survey and radio echo sounding techniques the two reconstructions simulate the drainage system structures expected for a closed channels in which water is pressurized by the overlying ice and b gravity driven open channel flow the closed channel model provides the best fit to the observed structure even though theoretical calculations suggest that under summer discharge conditions open channel flow may be widespread beneath the glacier possible reasons for this apparent discrepancy are discussed

groundwater flow beneath ice sheets part ii its impact on glacier tectonic structures and moraine formation abstract meltwater flowing as groundwater from beneath the margin of an ice sheet determines the distribution of sub surface heads and effective pressures a subglacial groundwater flow model is used together with an ice sheet loading model to compute the magnitudes and directions of the principal effective stresses in the subsurface from which the distribution of different types of sub surface failure in the subglacial and proglacial zones are deduced zones of hydrofracturing shear fracture and pervasive shear failure are distinguished beneath the ice sheet divide area intact rocks of high tensile strength may fail hydrofracturing and liquefaction are two coupled processes which lead to the formation of upward filled and downward filled sediment dykes and till wedges quicksand conditions are developed where strong vertical seepage pressures occur producing sediment diapirism it is suggested that subglacial permeability magnitude may be the product of a self organising process certain types of moraine extrusion moraines are suggested to be a consequence of upward movement and surface extrusion of sediment driven by rising groundwater it is suggested that groundwater over pressure associated with narrow proglacial permafrost plates are conductive to the formation of large push moraines and that many large ancient and modern examples are produced in this setting

the effect of the subglacial water pressure on the sliding velocity of a glacier in an idealized numerical model in order to interpret observed short term variations of the sliding velocity of a glacier the effect of a variable subglacial water pressure on the sliding velocity has been studied using an idealized numerical model in particular the transient stages of growing or shrinking water filled cavities at the ice bedrock interface were analysed it was found that the sliding velocity was larger when cavities were growing than when they had reached the steady state size for a given water pressure the smallest sliding velocities occurred while cavities were shrinking when cavitation is substantial a small drop of water pressure below the steady state value e g by 0 5 bar can temporarily cause backward sliding a limiting water pressure at which sliding becomes unstable is derived the consequences of more realistic assumptions than those of the model are discussed

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

the variability of holocene climate change evidence from varved lake sediments varved sediments from a lake near the present forest prairie border in northwestern minnesota provide an annual record of climate change for the last 10 400 years climate sensitive mineral chemical and biological components show that the mid holocene dry interval between 8500 and 4000 years ago is asymmetrical and actually consists of two distinct drier pulses separated by a moister interval that lasted about 600 years cyclic fluctuations with periods of several hundred years were abrupt and persistent throughout the holocene and are most clearly recorded within the two drier pulses

southwest scandinavia 40 15 kyr bp palaeogeography and environmental change twelve palaeogeographical reconstructions illustrate environmental changes at the southwest rim of the scandinavian ice sheet 40 15 kyr bp synchronised land sea and glacier configurations are based on the lithostratigraphy of tills and intertill sediments dating is provided by optically stimulated luminescence and calibrated accelerator mass spectrometry radiocarbon an interstadial sequence ca 40 30 kyr bp with boreo arctic proglacial fjords and subarctic flora and occasional glaciation in the baltic was succeeded by a last glacial maximum sequence ca 3020 kyr bp with the closure of fjords and subsequent ice streams in glacial lake basins in kattegat and the baltic steadily flowing ice from sweden bordered the norwegian channel ice stream a deglaciation sequence ca 20 15 kyr bp indicates the transgression of arctic waters retreat of the swedish ice and advance of baltic ice streams succeeded by a return to interstadial conditions when ameliorated ice free conditions prevailed in maritime regions glaciers advanced through the baltic and when interstadial regimes dominated the baltic glaciers expanded off the norwegian coast the largest glacier extent was reached in the north sea around 29 kyr bp about 22 kyr bp in denmark and ca 18 kyr bp in the baltic our model provides new data for future numerical and qualitative landform based models copyright c 2003 john wiley sons ltd less

climate driven hydrologic transients in lake sediment records multiproxy record of mid holocene drought abstract oxygen and carbon isotopes trace element concentrations and ostracode paleoecology are used with ground water flow modeling to assess the magnitude of climate driven hydrologic changes during mid holocene time for elk lake grant county minnesota during mid holocene time two extended droughts were recorded in the sediment record from elk lake at 5385 and 5221 calendar yr bp a drop in lake level to 14 9 m below the 1994 level and 10 0 m below the maximum 1930s decline at about 5385 calendar yr bp is interpreted for elk lake and its water table based on heterocypris fretensis occurrence and higher 18 o 13 c and mg ca values numerical modeling suggests this low water event could have resulted from conditions drier than those of the 1930s drought sustained over a time frame of approximately a century the later drought event at approximately 5221 calendar yr bp is marked by a shift in the dominant source of water to elk lake from surface run off to ground water from the till aquifer the shift in source water is identified by lower 18 o values and the presence of limnocythere staplini estimates of climatic sensitivity for holocene lakes should not be directly based on modern conditions but must take into account the changing shape of the lake basin through time and its effect on the paleohydrology

results from the eismint model intercomparison the effects of thermomechanical coupling this paper discusses results from the second phase of the european ice sheet modelling initiative eismint it reports the intercompartison of ten operational ice sheet models and uses a series of experiments to examine the implications of thermomechanical coupling for model behaviour a schematic circular ice sheet is used in the work which investigates both steady states and the response to stepped changes in climate the major finding is that radial symmetry implied in the experimental design can under certain circumstances break down with the formation of distinct regularly spaced spokes of cold ice which extended from the interior of the ice sheet outward to the surrounding zone of basal melt these features also manifest themselves in the thickness and velocity distributions predicted by the models they appear to be a common feature to all of the models which took part in the intercomparison and may stem from interactions between ice temperature flow and surface form the exact nature of these features varies between models and their existence appears to be controlled by the overall thermal regimne of the ice sheet a second result is that there is considerable agreement between the models in their predictions of global scale response to imposed climate change

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

groundwater supported evapotranspiration within glaciated watersheds under conditions of climate change this paper analyzes the effects of geology and geomorphology on surface water groundwater interactions evapotranspiration and recharge under conditions of long term climatic change our analysis uses hydrologic data from the glaciated crow wing watershed in central minnesota usa combined with a hydrologic model of transient coupled unsaturated saturated flow hydrat2d analysis of historical water table 1970 1993 and lake level 1924 2002 records indicates that larger amplitude and longer period fluctuations occur within the upland portions of watersheds due to the response of the aquifer system to relatively short term climatic fluctuations under drought conditions lake and water table levels fell by as much as 2 4 m in the uplands but by 1 m in the lowlands the same pattern can be seen on millennial time scales analysis of holocene lake core records indicates that moody lake located near the outlet of the crow wing watershed fell by as much as 4 m between about 4400 and 7000 yr bp during the same time water levels in lake mina located near the upland watershed divide fell by about 15 m reconstructed holocene climate as represented by hydrat2d gives somewhat larger drops 6 and 24 m for moody lake and lake mina respectively the discrepancy is probably due to the effect of three dimensional flow a sensitivity analysis was also carried out to study how aquifer hydraulic conductivity and land surface topography can influence water table fluctuations wetlands formation and evapotranspiration the models were run by recycling a wet year 1985 87 cm annual precipitation over a 10 year period followed by 20 years of drier and warmer climate 1976 38 cm precipitation model results indicated that groundwater supported evapotranspiration accounted for as much as 12 10 cm of evapotranspiration the aquifers of highest hydraulic conductivity had the least amount of groundwater supported evapotranspiration owing to a deep water table recharge was even more sensitive to aquifer hydraulic conductivity especially in the lowland regions these findings have important implications for paleoclimatic studies because the hydrologic response of a surface water body will vary across the watershed to a given climate signal

holocene climatic change in the northern midwest pollen derived estimates mapping of holocene pollen data in the midwestern united states has revealed several broadscale vegetational changes that can be interpreted in climatic terms these changes include 1 the early holocene northward movement of the spruce dominated forest and its later southward movement after 3000 yr b p and 2 the eastward movement of the prairie forest border into southwestern wisconsin by 8000 yr b p and its subsequent westward retreat after 6000 yr b p when certain basic assumptions are met multiple regression models can be derived from modern pollen and climate data and used to transform the pollen record of these vegetational changes into quantitative estimates of temperature or precipitation to maximize the reliability of the regression equations we followed a sequence of procedures that minimize violations of the assumptions that underlie regression analysis reconstructions of precipitation during the holocene indicated that from 9000 to 6000 yr b p precipitation decreased by 10 to 25 over much of the midwest while mean july temperature increased by 0 5 to 2 0 c at 6000 yr b p precipitation was less than 80 of its modern values over parts of wisconsin and minnesota after 6000 yr b p precipitation generally increased while mean july temperature decreased in the north and increased in the south the time of the maximum temperature varies within the midwest and is earlier in the north and later in the south

geometry and flow conditions of subglacial water at south cascade glacier washington state u s a an analysis of tracer injections tracers were injected into south cascade glacier to determine the flow condition and geometry of the subglacial water system results indicate that two distincts drainage basins on the glacier feed the two main streams flowing from the glacier in the largest basin two parallel drainage networks exist one englacial and the other subglacial

a thermomechanical model of ice flow in west antarctica this study uses a three dimensional thermo mechanical model to investigate the internal flow dynamics of the west antarctic ice sheet wais the model allows ice thickness flow and temperature to interact freely its domain is prescribed as that of the present day grounded wais realistic present day climatic and topographical boundary conditions are employed the analysis of a series of experiments pays particular attention to the location and dynamics of concentrations of ice flow ice streams underlying topographic troughs are crucial in determining the strength and location of these concentrations of flow the flow pattern generated by subglacial troughs is made more distinct by the inclusion of ice flow temperature coupling the inclusion of sliding leads to the generation of limit cycles in the ice flow they are concentrated around the present day ice streams b and c of the siple coast and have a period of 5 to 10 ky there appears to be competition between several preferred ice flow pathways in this area the two end members of the flow regime are a strong ice stream c with a weakened ice stream a b complex and strong ice streams a and b with a dormant ice stream c ice streams appear to require ice discharges above a certain threshold in order to maintain frictional heat generation and fast flow individual ice streams can therefore interact through changes in catchment area size a reduction in catchment area reduces the volume of ice entering a stream and can cause stagnation as the amount of frictional heating falls

sliding with cavity formation we present a model for the determination of a sliding law in the presence of sub glacial cavitation this law determines the basal stress at a clean ice bedrock interf ace in terms of the velocity and effective pressure the method is based on an exact solut ion of the nye kamb linearly viscous sliding problem with cavities and uses ideas of lliboutry 979 to construct via re norma lization methods an approximate law for gene ral bedrock form we show that for a bedrock whose spectrum has a power law behaviour one obtains a sliding law which gives the basal shear stress proportional to a power of the velocity and to a power of the effective pressure the effect of subgl acial cavitation on the drainage system is exam ined using recent ideas of kamb for sufficiently high velocities drainage through a r6thlisberger tunnel system is unstable and drai nage takes place through the linked system of cavities this leads to a reduction of the effective pressure and by taking account of this one can rewrite the sliding law in terms of st ress and velocity only this sliding law can be multi v alued and it is suggested that this unde rlies the dynamic phenomenon of surges

the eurasian ice sheet formation and collapse resulting from natural atmospheric co2 concentration variations finite element model simulations of ice sheet dynamics are conducted to investigate the role of natural atmospheric co2 variations obtained from ice core bubbles in creating climatic conditions necessary to glaciate the eurasian arctic effects of co2 variations are sufficient to support extensive marine ice sheet and ice shelf development at the last glacial maximum and to deglaciate the region during the holocene from approximately 14 ka to 9 ka the ice sheet coverage simulated for the last glacial maximum agrees with evidence for extensive ice sheet grounding over the shallow continental shelves of the barents kara laptev and east siberian seas an ice shelf also forms over the arctic ocean and greenland and norwegian seas

sea level change glacial rebound and mantle viscosity fornorthern europe northwestern europe remains a key region for testing models of glacial isostasy because of the good geological record of crustal response to the glacial unloading since the time of the last glacial maximum models for this rebound and associated sea level change require a detailed knowledge of the ice sheet geometry including the ice thickness through time existing ice sheet reconstructions are strongly model dependent and inversions of sea level data for the mantle response may be a function of the model assumptions thus inverse solutions for the sea level data are sought that include both ice and earth model parameters as unknowns sea level data from fennoscandia the north sea the british isles and the atlantic and english channel coasts have been evaluated and incorporated into the solutions the starting ice sheet for fennoscandia is based on a reconstruction of a model by denton hughes 1981 that is characterized by quasi parabolic cross sections and symmetry about the load centre both global northwestern europe as a whole and regional subsets of the data solutions have been made for earth model parameters and ice height scaling parameters the key results are as follows 1 the response of the upper mantle to the changing ice and water loads is spatially relatively homogenous across scandinavia the north sea and the british isles 2 this response can be adequately modelled by an effective elastic lithosphere of thickness 65 85 km and by an effective upper mantle viscosity from the base of the lithosphere to the 670 km depth seismic discontinuity of about 3 4 1020 pa s the effective lower mantle viscosity is at least an order of magnitude greater 3 the ice thickness over scandinavia at the time of maximum glaciation was only about 2000 m much less than the 3400 m assumed in the denton hughes model 4 the ice profiles are asymmetric about the centre of the ice sheet with those over the western part being consistent with quasi parabolic functions whereas the ice heights over the eastern and southern regions increase much more slowly with distance inwards from the ice margin

equilibrium analysis of groundwater vadose zone interactions and the resulting spatial distribution of hydrologic fluxes across a canadian prairie the spatial distribution and magnitude of time averaged surface runoff evapotranspiration net recharge and groundwater divergence are constrained by mutual dependence on the shape and position of the water table the water table position impacts the partitioning of rainfall by bounding the moisture profile at depth and creating a potential source of capillary rise to the root zone by coupling a water table dependent vadose zone model that time averages over event scale surface fluxes to a regional groundwater model modflow 96 we are able 1 to estimate the unique shape and position of the water table for which net recharge percolation additions or capillary rise losses is balanced by the divergence of the underlying groundwater flow field 2 to estimate the distribution of recharge and discharge to and from an aquifer and 3 to estimate the spatial distribution of the long term mean partitioning of rainfall into evapotranspiration runoff and infiltration parameters of the coupled model include soil physical properties permeability porosity tension saturated matric potential and brooks and corey pore size distribution index basin topography aquifer hydraulic conductivity and statistical moments of the probability distributions of meteorological variables storm intensity storm duration time between storms and potential evaporation the recharge and discharge areas predicted by the equilibrium model without specifying a priori the water table shape are in good agreement with field estimates by toth 1966 for a canadian prairie indicating the importance of accounting for groundwater vadose zone interactions and lateral groundwater redistribution in watershed modeling

on the role of mechanical energy in maintaining subglacial water conduits at atmospheric pressure recent theo retical studies of glacier hydrology have assumed that subglacial conduits are completely filled with water under steady state conditions this however is not necessarily the case where discharges are larger than a few tens of liters per second and the down glacier slope of the bed is more than a few degrees the potential energy released by water descending this slope may be capable of melting the walls of a subglacial conduit many times faster than the conduit can close by plastic flow of the ice as a result the pressure in such tunnels may normally be atmospheric or possibly even at the triple point pressure if there is no open connection to the glacier surface simple calcu lations suggest that such pressures in subglacial conduits may be more common than herclofore anticipated the positions of such open conduits may be unstable to small per turbations in discharge or ice veloci ty this is because the mechanical energy available in excess of that needed to balance closure can instead offset the gene ral flow of the ice cond ui ts can thus trend diago nally across the direction of ice aow if an increase in the angle which such a conduit makes with the ice flow direction also results in an increase in slope of he conduit more mechanical energy will become available resulting in a positive feedback process subglacial channels at atmospheric pressure may inauence the origin and morphology of certain glacial landforms such as eskers and plastically molded features resume sur le role de l inergie newtonnienne dam le maintien du riseau aquiflrl sous glaciaire cl la pression almosph irique de recentes etudes theoriques d hydrologie glaciaire ont suppose que le reseau sous glaci aire clait corn pletement rempli d eau en condition d cquil ibre stable ceci neanmoins n est pas necess airement le cas lorsque les debits sonl superieurs a quelques dizaines de litres par seconde et que la pente du lit du glacier est superieure a quelques degres i energie potentielle mise en oeuvre par i eau descendant celle pente peut ire capable de fondre les parois d un chenal sous glaciaire beaucoup plus vite que le chenal ne peut se refermer par le auage plastique de la glace il en resuhe que la pression dans de tels chenaux peut nor rnalement etre atrnospherique

on the thermal regime of an arctic valley glacier a study of white glacier axel heiberg island n w t canada from 1974 to 1981 a total of 32 bore holes was drilled on white glacier and vertical ice temperature profiles measured the data obtained allowed the construction of three longitudinal and four transverse profiles of the two dimensional temperature distributions thus an extensive layer of temperate or near temperate ice was discovered close to the bedrock in the lowest part of the glacier tongue it was also found that the temperature distribution cannot be in a steady state since there is a temperature minimum 100 150 m below the glacier surface in the accumulation area a simple numerical model calculation shows that this minimum can be mostly explained by the general climatic warming since 1880 the 10 m temperatures show diffuse relations to climatic conditions and balance zones a simple scheme for extrapolating surface temperatures is discussed

unsaturated zone wedge beneath a large natural lake lake belle taine 480 ha in north central minnesota receives on average 76 000 m3 d 1 of water from little sand creek but has no outlet water seeps out of the lake flows through glacial outwash and discharges into three nearby lakes with stages 13 14 m lower than belle taine seepage meter data indicate water is seeping out of belle taine at velocities up to 263 cm d 1 hydraulic head measurements made in the lake bed indicate the sediments are unsaturated beneath a portion of the lake edge and a wedge of unsaturated sediments extends beneath the lake bed as much as 20 m from the shoreline at the shoreline the water table is as much as 6 7 m below the lake surface modeling results of a similar hypothetical setting indicate that the horizontal extent of an unsaturated zone beneath a lake depends on 1 the permeability contrast between the outwash and the lake bed 2 anisotropy 3 lake bed slope and 4 thickness of the lower permeability lake bed sediments although rarely documented unsaturated sediments beneath a lake may not be extremely uncommon similar much smaller unsaturated zone areas also have been observed beneath two other lakes in minnesota

hydrologic response of the crow wing watershed minnesota to mid holocene climate change in this study we have integrated a suite of holocene paleoclimatic proxies with mathematical modeling in an attempt to obtain a comprehensive picture of how watersheds respond to past climate change a three dimensional surface water groundwater model was developed to assess the effects of mid holocene climate change on water resources within the crow wing watershed upper mississippi basin in north central minnesota the model was first calibrated to a 50 yr historical record of average annual surface water discharge monthly ground water levels and lake level fluctuations the model was able to reproduce reasonably well long term historical records 1949 1999 of water table and lake level fluctuations across the watershed as well as stream discharge near the watershed outlet the calibrated model was then used to reproduce paleo groundwater and lake levels using climate reconstructions based on pollen transfer functions from williams lake just outside the watershed computed declines in mid holocene lake levels for two lakes at opposite ends of the watershed were between 6 and 18 m simulated streamflow near the outlet of the watershed decreased to 70 of modern average annual discharge after 200 yr the area covered by wetlands for the entire watershed was reduced by 16 the mid holocene hydrologic changes indicated by these model results and corroborated by several lake core records across the crow wing watershed may serve as a useful proxy of the hydrologic response to future warm dry climatic forecasts ca 2050 made by some atmospheric general circulation models for the glaciated midwestern united states

mid holocene hydrologic model of the shingobee watershed minnesota abstract a hydrologic model of the shingobee watershed in north central minnesota was developed to reconstruct mid holocene paleo lake levels for williams lake a surface water body located in the southern portion of the watershed hydrologic parameters for the model were first estimated in a calibration exercise using a 9 yr historical record 1990 1998 of climatic and hydrologic stresses the model reproduced observed temporal and spatial trends in surface groundwater levels across the watershed mid holocene aquifer and lake levels were then reconstructed using two paleoclimatic data sets ccm1 atmospheric general circulation model output and pollen transfer functions using sediment core data from williams lake calculated paleo lake levels based on pollen derived paleoclimatic reconstructions indicated a 3 5 m drop in simulated lake levels and were in good agreement with the position of mid holocene beach sands observed in a williams lake sediment core transect however calculated paleolake levels based on ccm1 climate forcing produced only a 0 05 m drop in lake levels we found that decreases in winter precipitation rather than temperature increases had the largest effect on simulated mid holocene lake levels the study illustrates how watershed models can be used to critically evaluate paleoclimatic reconstructions by integrating geologic climatic limnologic and hydrogeologic data sets

possible solar forcing of century scale drought frequency in the northern great plains a 2100 yr decadal resolution salinity and aridity proxy record of lacustrine ostracode shell mg ca ratios from a closed basin lake in the northern great plains shows statistically significant periodicities of 400 200 130 and 100 yr these periodicities are similar to the three principal solar oscillation periods 420 218 143 yr as inferred from the atmospheric radiocarbon record suggesting strong solar forcing of century scale drought frequency with a fundamental 400 yr period and its 2nd 3rd and 4th harmonics our proxy record correlates visually and statistically cross spectral analysis with the atmospheric 14 c record solar proxy and the gisp2 greenland ice sheet project two 18 o record climate proxy showing that solar minima are in phase with drought periods in the northern great plains and cold periods in greenland this spectral similarity together with phase correlation indicates a possible teleconnection of century scale global climate fluctuations through common solar forcing

a theory of glacier surges we propose a model of glacier flow that is capable of explaining temperate glacier surges the laws of conservation of mass and momentum are supplemented by the prescription of a sliding law that gives the basal shear stress as a function of the basal velocity u and the effective pressure n the effective drainage pressure n is determined by a simple study of the subglacial hydraulic system following rothlisberger we determine n nr for the case of drainage through a single subglacial tunnel alternatively following kamb we find that the corresponding theory for a linked cavity drainage system yields n nk nr furthermore the stability of each drainage system depends on the velocity u such that for large enough u there is a transition from tunnel to cavity drainage consequently one can write n n u we then find that the sliding law u is multivalued and hence so also is the flux depth relation q q h an analysis of the resulting system of equations is sketched for large enough accumulation rates a glacier will undergo regular relaxation oscillations resembling a surge the surge is triggered at the point of maximum stress from this point two hydraulic transition fronts travel up and down glacier to calculable boundary points the speed of propagation is the order of 50 metres an hour at these fronts the tunnel drainage system collapses and a high water pressure cavity drainage system is installed this activated zone has high velocities and quickly relaxes surges to a quasi equilibrium state this relaxation is much like opening a sluice gate in that a large wave front propagates forward behind this wave front the velocity can decay oscillatorily and thus the flow can be compressive we conclude with some discussion of the effects of seasonal variation and of prospects for the current theory s applicability to soft bedded glaciers

reconstruction of past lake levels and their relation to groundwater hydrology in the parkers prairie sandplain west central minnesota abstract the parkers prairie sandplain lies along the prairie forest border in west central minnesota an area with ample palynological evidence of significant environmental change during the late pleistocene and holocene past lake levels provide independent evidence of environmental change over the same temporal and spatial scales as pollen analysis we used the method established by digerfeldt 1986 1988 to determine the lowest past levels of three closed basin lakes in the sandplain the method uses a combination of evidence from macrofossils coarse minerogenic matter and organic matter from a transect of cores taken in the near shore region to reconstruct past lake levels lower than present carbonate content magnetic susceptibility and pollen content in the cores provided supplementary evidence and aided cross correlation among the cores each of the three lakes had its lowest level between about 7200 and 6700 yr b p lake levels ranged from 2 8 m to 6 2 m lower than modern levels the greatest lake level lowering occurred for the lake farthest from the river that drains the sandplain the least lake level lowering occurred for the lakes close to the river this spatial pattern of lake level lowering implies that a lowering of the regional water table caused by a reduction in groundwater recharge was the ultimate cause of the lower lake levels the results demonstrate that surface water hydrology is not the sole determinant of lake level and that there are possible dangers in paleoclimatic interpretations of past lake levels based on the assumption of hydrologic isolation of basins

flow variability in the scandinavian ice sheet modelling the coupling between ice sheet flow and hydrology abstract there is increasing geologic evidence for periodic flow variability within large ice sheets manifested as spatially and temporally variable areas of fast ice flow and resulting in the very complex patterns of lineations observed in formerly glaciated areas however many ice sheet models do not replicate this behaviour a possible reason for this is that such models do not include a detailed treatment of basal hydrology changes in the character of sub glacial drainage systems are believed to cause surges in valley glaciers recent ice sheet models which have included basal hydrology or at least a link between basal velocity and the presence of water at the bed often show flow variability however these models have typically assumed a deformable bed or have made no assumptions about the nature of the bed whilst these assumptions seem applicable to areas close to the former margins of quaternary ice sheets they are less applicable to interior areas these areas typically show thin or scanty till cover over eroded bedrock and the presence of eskers which are indicative of drainage in sub glacial tunnels we have developed a two dimensional time dependent ice sheet model that includes hard bed basal hydrology this allows calculation of sub glacial water pressures and the use of a water pressure dependent sliding law to calculate ice sheet velocities when used to simulate the weichselian scandinavian ice sheet with late quaternary climate and sea level as forcing functions this model develops localised areas of fast flowing ice which vary in extent and in distance of penetration into the interior of the ice sheet both spatially and temporally the behaviour of these lobes depends crucially on the influence of the evolving ice sheet topography on the routing of subglacial water flow due to the resulting variations in the subglacial hydraulic potential which drive the water flow bedrock topography also has some influence but fast flow areas are not confined to obvious topographic troughs a relatively thin ice sheet with low surface slopes is produced in areas experiencing fast ice flow generally two to four separate areas of fast flow can be recognised and these are similar in size and shape to the lobes identified in some geologically based reconstructions of the scandinavian ice sheet within the fast flowing areas sub glacial drainage is typically in a cavity based system however tunnel based drainage is predicted to have extended up to 150 km from the ice sheet margin particularly during deglaciation because of the changes in ice sheet topography associated with fast flow and the resulting changes in the pattern of sub glacial water flow the model predicts that these fast flowing lobes interacted in complex ways and exhibited quasi periodic switching between fast and slow flow

basal hydraulic system of a west antarctic ice stream constraints from borehole observations pressure and tracer measurements in boreholes drilled to the bottom of ice stream b west antarctica are used to obtain information about the basal water conduit system in which high water pressures are developed these high pressures presumably make possible the rapid movement of the ice stream pressure in the system is indicated by the borehole water level once connection to the conduit system is made on initial connection here also called breakthrough to the basal water system the water level drops in a few minutes to an initial depth in the range 96 117 m below the surface these water levels are near but mostly somewhat deeper than the flotation level of about 100m depth water level at which basal water pressure and ice overburden pressure are equal which is calculated from depth density profiles and is measured in one borehole the conduit system can be modelled as a continuous or somewhat discontinuous gap between ice and bed the thickness of the gap has to be about 2 mm to account for the water level drop on breakthrough and about 4 mm to fit the results of a salt tracer experiment indicating downstream transport at a speed of 7 5 mm s 1 the above gap conduit model is however ruled out by the way a pressure pulse injected into the basal water system at breakthrough propagates outward from the injection hole and also by the large hole to hole variation in measured basal pressure which if present in a gap conduit system with 2 or 4 mm would result in unacceptably large local water fluxes an alternative model that avoids the e objections called the gap opening model involves opening a gap as injection proceeds starting with a thin film the injection of water under pressure lifts the ice mass a round the borehole creating a gap 3 or 4 mm wide at the ice bed interface evaluated quantitatively the gap opening model accounts for the volume of water that the basal watery system accepts on breakthrough which obviates the gap conduit model in order to transport basal meltwater from upstream it is then necessary for the complete hydraulic model to contain also a network of relatively large conduits of which the most promising type is the canal conduit proposed theoretically by walder and fowler 1994 flat low conduits incised into the till 0 1 m deep and perhaps 1 m wide with a flat ice roof the basal water pressure data suggest that the canals are spaced 50 300 m apart much closer than r tunnel would be the deepest observed water level 117 m i the most likely to reflect the actual water pressure in the canals corresponding to a basal effective pressure of 1 6 bar in this interpretation the shallower water levels are affected by loss of hydraulic head in the narrow passageway s that connect along the bed from borehole to canal s once a borehole has frozen up and any passageways connecting with canals have become closed a pressure sensor in contact with the unfrozen till that underlies the ice will measure the pore pressure in the till given enough time for pressure equilibration this pressure varies considerably with time over the equivalent water level range from 100 to 113 m basal pressure sensors 500 m apart report uncorrelated variations whereas sensors in boreholes 25 m apart report mostly but not entirely well correlated variations of unknown origin in part of the record remarkable anticorrelated variations are interspersed with positively correlated one and there are rare abrupt excursions to extreme water levels as deep as 125m and as shallow as 74 m a diurnal pressure fluctuation intermittently observed may possibly be caused by the ocean tide in the ross sea the lack of any observed variation in ice stream motion when large percentagewise variations in basal effective pressure were occurring according to our data suggests that the observed pressure variations are sufficiently local and so randomly variable from place to place that they are averaged out in the process by which the basal motion of the ice stream is determined by an integration over a large area of the bed

basal sliding of ice stream b west antarctica a tethered stake apparatus is used to measure basal sliding in a borehole on ice stream b west antarctica about 300 km upstream east from its grounding line near the head of the ross ice shelf a metal stake emplaced at the top of a layer of unfrozen till underlying the ice is connected by a tether line to a metering unit that measures the tether line as it is pulled out from the borehole by the stake as a result of basal sliding the measured sliding motion includes any actual slip across the ice till interface and may include in addition a possible contribution from shear deformation of till within about 3 cm of the interface this 3 cm figure follows from a qualitative model of the movements of the stake in the course of the experiment based on features of the record of apparent sliding alternative but less likely models would increase the figure from 3 cm to 10 cm or 25 cm in any case it is small compared to the seismically inferred till thickness of 9 m measured apparent sliding averages 69 of the total motion of 1 2 m d 1 over 26 days of observation if a 3 5 day period of slow apparent sliding 8 of the total motion is included in the average the occurrence of the slow period raises the possibility that the sliding motion switches back and forth between c 80 and c 8 of the total motion on a time scale of a few days however it is likely that the period of slow apparent sliding represents instead a period when the stake got caught on the ice sole if the slow period is therefore omitted the indicated average basal sliding rate is 83 of the total motion in either case basal sliding predominates as the cause of the rapid ice stream motion in the last 2 days of observation the average apparent sliding rate reached 1 17 m d 1 essentially 100 of the motion of the ice stream if till deformation contributes significantly to the ice stream motion the contribution is concentrated in a shear zone 3 cm to possibly 25 cm thick at the top of the 9 m thick till layer these observations if applicable to the west antarctic ice sheet in general pose complications in modeling the rapid ice streaming motion

late wisconsin ice surface profile calculated from esker paths and types katahdin esker system maine abstract values of the gradient of the former ice surface can be inferred at points along a flow line from deviations of esker paths or transitions in esker type and numerically integrated to give the profile a profile calculated in this way shows that during formation of the katahdin esker system about 12 700 yr ago the ice thickness at distances of 10 20 50 100 and 140 km from the terminus which is about two thirds of the distance to the ice divide was approximately 200 300 600 750 and 900 m the terminal reach was computed by assuming an unknown uniform basal drag and matching the profile to its known elevation at the terminus and known gradient 10 km upglacier correction for isostatic rebound based on the elevations of contemporaneous deltas and of the marine limit proved unnecessary because the tilt due to the difference in uplift at the two ends of the profile is only 0 1 m km 1 with other plausible assumptions as to sea levels in the past elevations of the marine limit or exact location of the terminus the profile could be as much as roughly 100 m higher it hits mount katahdin about 500 m below its summit which is at 1600 m in agreement with the geological evidence farther west the steepening of the upper part of the profile suggests that the mountain dammed and diverted the ice basal drag computed from the profile varies from about 20 kpa 0 2 bar near the terminus to 30 kpa 0 3 bar at 100 km to 70 kpa 0 7 bar at 140 km the relatively low values away from the influence of mount katahdin agree with independent evidence from deep sea cores of substantial late wisconsin ice sheet thinning without comparable areal reduction the method has potential for application over wide areas that were occupied by the laurentide and scandinavian ice sheets

irregular glacial interstadials recorded in a new greenland ice core the greenland ice sheet offers the most favourable conditions in the northern hemisphere for obtaining high resolution continuous time series of climate related parameters profiles of 18o 16o ratio along three previous deep greenland ice cores1 3 seemed to reveal irregular but well defined episodes of relatively mild climate conditions interstadials during the mid and late parts of the last glaciation but there has been some doubt as to whether the shifts in oxygen isotope ratio were genuine representations of changes in climate rather than artefacts due to disturbed stratification here we present results from a new deep ice core drilled at the summit of the greenland ice sheet where the depositional environ ment and the flow pattern of the ice are close to ideal for core recovery and analysis the results reproduce the previous findings to such a degree that the existence of the interstadial episodes can no longer be in doubt according to a preliminary timescale based on stratigraphic studies the interstadials lasted from 500 to 2 000 years and their irregular occurrence suggests complexity in the behaviour of the north atlantic ocean circulation

flow pattern evolution of the last british ice sheet we present a 10 stage reconstruction of the evolution in ice flow patterns of the last british ice sheet from build up to demise derived from geomorphological evidence 100 flowsets identified in the subglacial bedform record drumlins mega scale glacial lineations and ribbed moraine are combined with ancillary evidence erratic transport paths absolute dates and a semi independently reconstructed retreat pattern to define flow patterns ice divides and ice sheet margins during build up maximum glaciation and retreat overprinting and cross cutting of landform assemblages are used to define the relative chronology of flow patterns and a tentative absolute chronology is presented based on a collation of available dates for ice advance and retreat the ice flow configuration of the last british ice sheet was not static some ice divides were remarkably stable persisting through multiple stages of the ice sheet evolution whereas others were transient features existing for a short time and or shifting in position 10s km the 10 reconstructed stages of ice sheet geometry capture two main modes of operation first as an integrated ice sheet with a broadly n s orientated ice divide and second as a multi domed ice sheet orientated parallel with the shelf edge a thick integrated ice sheet developed as ice expanded out of source areas in scotland to envelop southerly ice caps in northern england and wales and connect with the irish ice sheet to the west and the scandinavian ice sheet across the north sea following break up of ice over the north sea ice streaming probably drove mass loss and ice sheet thinning to create a more complex divide structure where ice flow patterns were largely controlled by the form of the underlying topography ice surface lowering occurred before separation of and retreat to multiple ice centres centred over high ground we consider this 10 stage reconstruction of the evolution in ice sheet configuration to be the simplest palaeo glaciological interpretation of the flowsets identified from the geomorphological record and their relative timing this empirically based reconstruction of flow pattern geometry provides a framework for more detailed local and regional studies and numerical modelling to provide robust explanations of the observed changes in ice sheet structure in terms of climate and glacial dynamics as a minimum numerical model outputs should be able to reproduce the identified flowset patterns in space and satisfy their chronological order

hydrogeologic controls on the groundwater interactions with an acidic lake in karst terrain lake barco florida transient groundwater interactions and lake stage were simulated for lake barco an acidic seepage lake in the mantled karst of north central florida karst subsidence features affected groundwater flow patterns in the basin and groundwater fluxes to and from the lake subsidence features peripheral to the lake intercepted potential groundwater inflow and increased leakage from the shallow perimeter of the lake bed simulated groundwater fluxes were checked against net groundwater flow derived from a detailed lake hydrologic budget with short term lake evaporation computed by the energy budget method discrepancies between modeled and budget derived net groundwater flows indicated that the model underestimated groundwater inflow possibly contributed to by transient water table mounding near the lake recharge from rainfall reduced lake leakage by 10 to 15 times more than it increased groundwater inflow as a result of the karst setting the contributing groundwater basin to the lake was 2 4 ha for simulated average rainfall conditions compared to the topographically derived drainage basin area of 81 ha short groundwater inflow path lines and rapid travel times limit the contribution of acid neutralizing solutes from the basin making lake barco susceptible to increased acidification by acid rain

patterns of hydroclimatic change in the rocky mountains and surrounding regions since the last glacial maximum abstract the paleohydrologic record of western north america since the last glacial maximum reveals a wide range of hydroclimatic variability in time and space to improve the understanding of abrupt hydroclimatic shifts and millennial scale hydrologic changes in the central rocky mountains we reconstruct the lake level histories of two small lakes in the beartooth and bighorn mountains in northern wyoming over the past 17 ka to do so we use ground penetrating radar gpr and sediment cores to track the elevations of shoreline sediments within the lakes through time we compare the stratigraphies with those from four other lakes in wyoming and colorado and find widespread evidence for a terminal pleistocene drought from 15 to 11 ka an early holocene humid period from 11 to 8 ka and mid holocene aridity from 8 to 5 5 ka the northern wyoming lakes also provide evidence of high levels in the pleistocene possibly before ca 15 ka and rapid hydroclimatic changes that may have correlated with heinrich event 1 ca 16 8 ka we place the changes in a broad context by summarizing and mapping water level changes from 107 additional previously studied lakes important patterns include 1 extensive drying across the western u s after 15 ka 2 sub regional differences during the pleistocene holocene transition 3 a north south contrast from 9 to 6 ka consistent with a northward shift in storm tracks as the influence of the laurentide ice sheet diminished and 4 rapid increases in effective moisture across much of western north america from 6 to 4 ka

palaeoglaciology of an ice sheet through a glacial cycle the european ice sheet through the weichselian abstract satellite images provide unique means of identifying large scale flow generated lineations produced by former ice sheets they can be interpreted to reconstruct the major elements which make up the integrated large scale structure of ice sheets ice divides ice streams interstream ridges ice shelves calving bays the evolving palaeoglaciological structure of the european ice sheet during its decay from the last glacial maximum lgm is reconstructed by reference to these components and in the context of a new map showing isochrons of retreat during the retreat phase in particular the time dependent dynamic evolution of the ice sheet and the pattern of ice stream development are reconstructed crossing lineations are widespread the older ones are suggested to have formed during molten bed phases of ice sheet growth and preserved by frozen bed conditions during the glacial maximum particularly in areas which lay during deglaciation beneath ice divides and inter ice stream ridges both areas of slow flow and possibly frozen bed conditions four phases of growth a1 to a4 and five phases of decay r1 to r5 are used to describe the major climatically and dynamically determined stages in the evolution of the ice sheet through the last glacial cycle the growth and decay patterns are quite different and associated with major shifts in the ice divide reflecting growth from the fennoscandian mountains and decay away from marine influenced margins these patterns were determined by the locations of nucleation areas spatial patterns of climate and calving at marine margins the prevalence of streaming within the retreating ice sheet suggests that the mean elevation of the ice sheet was lower than predicted from glaciological models which do not include streaming and that this might reconcile glaciological models and earth rheology models which infer paleao ice sheet thickness by inverting sea level data

a theory of ice sheet surges a simplified model of a two dimensional ice sheet is described it includes basal ice sliding dependent on the basal water pressure which itself is described by a simple theory of basal drainage we show that this simple but sophisticated model predicts surges of the ice mass in realistic circumstances and we describe these surges by solving the problem numerically we also are able to describe some parts of the surge analytically the numerical solution of the model is a delicate matter and highlights pitfalls to be avoided if more complicated models are to be solved successfully

dynamics of the siple coast ice streams west antarctica results from a thermomechanical ice sheet model most of the ice leaving the antarctic ice sheet is discharged to the sea by ice streams an understanding of the dynamics of these ice streams is therefore crucial if we are to predict the future evolution of the ice sheet and its effect on global sea level the behavior of ice streams a to e of the siple coast west antarctica has attracted much attention with evidence of recent stagnation and numerous relict ice flow features here i present results from a three dimensional thermodynamic ice sheet model of the west antarctic ice sheet wais the model generates variability in the flow of the siple coast ice streams reminiscent of that mentioned above this is caused by competition between several preferred ice flow pathways in the area individual ice streams interact through changes in catchment area size a reduction in catchment area reduces the flow of ice entering a stream and can cause stagnation through the associated reduction in deformational heating

a highly mobile laurentide ice sheet revealed by satellite images of glacial lineations satellite images of the north american continent reveal previously unrecognized patterns of crossing drift lineations reflecting unexpected shifts of centres of mass of the laurentide ice sheet throughout the last glacial cycle such shifts must must have been associated with major global atmospheric circulation changes

banded surface flow maintained by convection in a model of the rapidly rotating giant planets in three dimensional numerical simulations of a rapidly rotating boussinesq fluid shell thermally driven convection in the form of columns parallel to the rotation axis generates an alternately directed mean zonal flow with a cylindrical structure the mean structure at the outer spherical surface consists of a broad eastward flow at the equator and alternating bands of westward and eastward flows at higher latitudes in both hemispheres the banded structure persists even though the underlying convective motions are time dependent these results although still far from the actual motions seen on jupiter and saturn provide support for theoretical suggestions that thermal convection can account for the remarkable banded flow structures on these planets

fennoscandian palaeoglaciology reconstructed using a glacial geological inversion model the evolution of ice sheet configuration and flow pattern in fennoscandia through the last glacial cycle was reconstructed using a glacial geological inversion model i e a theoretical model that formalises the procedure of using the landform record to reconstruct ice sheets the model uses mapped flow traces and deglacial melt water landforms as well as relative chronologies derived from cross cutting striae and till lineations as input data flow trace systems were classified into four types i time transgressive wet bed deglacial fans ii time transgressive frozen bed deglacial fans iii surge fans and iv synchronous non deglacial event fans using relative chronologies and aggregation of fans into glaciologically plausible patterns a series of ice sheet configurations at different time slices was erected a chronology was constructed through correlation with dated stratigraphical records and proxy data reflecting global ice volume geological evidence exists for several discrete ice sheet configurations centred over the scandinavian mountain range during the early weichselian the build up of the main weichselian fennoscandian ice sheet started at approximately 70 ka and our results indicate that it was characterised by an ice sheet with a centre of mass located over southern norway this configuration had a flow pattern which is poorly reproduced by current numerical models of the fennoscandian ice sheet at the last glacial maximum the main ice divide was located overthe gulf of bothnia a major bend in the ice divide was caused by outflow of ice to the northwest over the lowest part of the scandinavian mountain chain widespread areas of preserved pre late weichselian landscapes indicate that the ice sheet had a frozen bed core area which was only partly diminished in size by inward transgressive wet bed zones during the decay phase

a groundwater model to explain past lake levels at parkers prairie minnesota usa the response of a lake level to a shift in climate depends on characteristics of surficial and groundwater hydrology that are unique to that lake determination of the past levels of several lakes rather than just one can help provide a more precise determination of past hydrology and climate for a given region the levels of several closed basin lakes in the parkers prairie sandplain in west central minnesota were lowest about 7 ka a steady state analytic element groundwater model was used to quantify the effect of changing regional groundwater recharge n on the altitude of the water table surrounding these lakes because overland flow on the sandplain is limited the surficial hydrology of the lakes can be simplified to the net quantity lake evaporation minus precipitation e p both n and e p were manipulated to force the model water table to coincide with the palaeolake levels model results indicate that lake levels at 7 ka can be explained primarily by reducing n to about 50 60 mm yr 1 40 5

the interaction of lakes with variably saturated porous media numerical simulation of variably saturated porous media indicates that groundwater recharge is variable in time and space depending on the thickness of the unsaturated zone through which infiltrating water must move the resulting complex transient groundwater flow systems have significant impact on contiguous surface water in very permeable media small local closed groundwater flow systems can develop and dissipate within a few weeks to several months after major recharge these have a direct effect on contiguous surface water by alternately causing seepage to and seepage from the surface water the transient nature of these flow systems indicates that reversals of the direction of groundwater flow may be common in less permeable media the same complex flow systems may occur but the time for development and dissipation is much greater for example it is conceivable that small local flow systems may exist for many months or years as a result of major recharge therefore directions of flow in such systems are more stable and the effect on contiguous surface water also is more stable the findings of this study indicate that wells and groundwater quality sampling sites need to be carefully located to define accurately water table configuration groundwater recharge direction of seepage through the beds of surface water bodies and complex geochemical processes related to changing directions of groundwater flow

holocene environmental and climatic change in the northern great plains as recorded in the geochemistry of sediments in pickerel lake south dakota the sediments in pickerel lake northeastern south dakota provide a continuous record of climatic and environmental change for the last 12000 yr sediments deposited between 12 and 6 ka radiocarbon show extreme variations in composition oxygen and carbon isotopic composition of bulk carbonate carbon isotopic composition of organic matter and magnetic susceptibility these variations re ect changes in sources of moisture regional vegetation types precipitation evaporation balance ground and surface water in ux water residence time erosion lake productivity water level and water temperature the total carbonate content of late pleistocene sediments steadily increased from 20 at the base of the core to as much as 80 in sediments deposited between 11 and 9 ka by about 8 ka the total carbonate content of the sediments had declined to about 40 where it remained with little variation for the past 8 kyr suggesting relatively stable conditions there are marked increases in values of d13c and d18 oi n bulk carbonate and d13c of organic matter in sediments deposited between 10 and 6 ka as evaporation increased and the vegetation in the watershed changed from forest to prairie this shift toward more 18o enriched carbonate may also re ect a change in source or seasonality of precipitation during this early holocene interval the organic carbon oc content of the sediments remained relatively low 2 3 but then increased rapidly to 4 5 between 7 and 6 ka re ecting the rapid transition to a prairie lake the oc content uctuates slightly between 4 and 6 in sediments deposited over the past 6 kyr like oc and total carbonate most variables measured show little variation in the 13 m of sediment deposited over the past 6 kyr particularly when compared with early holocene variations although the magnetic susceptibility of this upper 13 m of sediment is generally low 10 si units the upper six meters of the section is marked by striking 1 m cycles ca 400 500 yr periodicity in susceptibility these cycles are interpreted as being due to variations in the in ux of eolian detrital clastic material century scale cyclic variations in di erent proxy variables for aridity and eolian activity from sediments deposited over the past 2000 yr in other lakes in the northern great plains as well as in sand dune activity suggest that aridity cycles were the dominant feature of late holocene climate of the northern great plains 2000 elsevier

stable isotopic composition of precipitation in the semi arid north central portion of the us great plains abstract where data are available hydrologic studies may use stable oxygen and hydrogen isotopes to investigate groundwater surface water interaction groundwater recharge and advective diffusive transport to estimate groundwater ages or to unravel paleohydrology such studies require that the isotopic composition of precipitation be known as precipitation is a major input to groundwater and surface water systems oxygen 18 18 o and deuterium 2 h data for precipitation are lacking for the semi arid portion of the north central great plains of the us and thus there is need to establish additional meteoric water lines as isotope input functions across the region as well as to develop better understanding of the isotopic climate linkages that control oxygen and hydrogen isotope ratios in precipitation this study examines the 18 o and 2 h composition of precipitation for a representative site near north platte nebraska in the semi arid north central great plains during the years 1989 through 1994 oxygen 18 values range from 30 5 to 1 7 deuterium values range from 228 to 11 yearly arithmetic mean values for the north platte station are 9 8 and 71 respectively weighted yearly means for 18 o and 2 h over the 6 year period were 9 6 and 69 respectively north platte values show a strong isotopic enrichment between winter and summer precipitation and a strong 18 o t correlation r 2 0 845 for mean monthly values of 0 54 per degree celsius the local meteoric water line for the site is 2 h 7 66 18 o 4 96

zonal flow formation in the earth s core as zonal flows are common in turbulent convection regimes such as in the atmospheres of giant planets and the earth s oceans they might also be present in the earth s outer core which is thought to be in a turbulent state takehiro miyagoshi and colleagues have investigated numerical simulations of the earth s geodynamo with lower viscosities than most previous simulations have been able to use and have identified a new convection regime of core flow that exhibits a dual structure inner sheet like radial plumes surrounded by westward cylindrical zonal flow thus convection in the earth s outer core may therefore resemble that seen in planetary atmospheres and oceans more closely than was expected zonal jets are common in nature and are spontaneously generated in turbulent systems because the earth s outer core is believed to be in a turbulent state it is possible that there is zonal flow in the liquid iron of the outer core by investigating numerical simulations of the geodynamo with lower viscosities than most previous simulations have been able to use a convection regime of the outer core is now found that has a dual structure comprising inner sheet like radial plumes and an outer westward cylindrical zonal flow zonal jets are very common in nature well known examples are those in the atmospheres of giant planets and the alternating jet streams found in the earth s world ocean1 zonal flow formation in nuclear fusion devices is also well studied2 a common feature of these zonal flows is that they are spontaneously generated in turbulent systems because the earth s outer core is believed to be in a turbulent state it is possible that there is zonal flow in the liquid iron of the outer core here we report an investigation at the current low viscosity limit of numerical simulations of the geodynamo we find a previously unknown convection regime of the outer core that has a dual structure comprising inner sheet like radial plumes and an outer westward cylindrical zonal flow we numerically confirm that the dual convection structure with such a zonal flow is stable under a strong self generated dipole magnetic field

intercomparison of simulated global vegetation distributions in response to 6 kyr bp orbital forcing abstract the response of ten atmospheric general circulation models to orbital forcing at 6 kyr bp has been investigated using the biome model which predicts equilibrium vegetation distribution as a diagnostic several common features emerge a reduced tropical rain forest as a consequence of increased aridity in the equatorial zone b expansion of moisture demanding vegetation in the old world subtropics as a consequence of the expansion of the afro asian monsoon c an increase in warm grass shrub in the northern hemisphere continental interiors in response to warming and enhanced aridity and d a northward shift in the tundra forest boundary in response to a warmer growing season at high northern latitudes these broadscale features are consistent from model to model but there are differences in their expression at a regional scale vegetation changes associated with monsoon enhancement and high latitude summer warming are consistent with palaeoenvironmental observations but the simulated sh

climate driven hydrologic transients in lake sediment records calibration of groundwater conditions using 20th century drought abstract the effect of 20th century drought on groundwater and lakes in a climatically sensitive area of the northern great plains grant county western minnesota was investigated by analysis of lake sediments and historical air photos in conjunction with groundwater flow simulations drought caused an observed 4 0 to 5 1 m lake head decline between 1923 and 1938 all but three either dried completely or declined to 210 pb dated sediment core was analyzed for ostracode species distribution and geochemistry of c rawsoni shells 18 o 13 c mg ca sr ca mg ca and 13 c increased during drought at the same time that salinity tolerant ostracodes thrived suggesting increasing salinity however 18 o decreased during drought anti correlative with mg ca a numerical model for transient response of groundwater flow to drought modeled after that of 1923 38 was constrained by drought end recessional strand lines observed on air photos of one lake with strand line elevations inferred from bathymetry simulated lake water level declines in the first 15 drought years were consistent with strand line elevations the rate of decline was exponential at drought onset slowing as water in lakes and wetlands receded to below land surface and lake evaporation declined a near steady state was attained between 40 and 50 simulation years after onset of drought with the water table from 1 5 m below the level of dry lakebeds and only the two deepest lakes still holding water they were however greatly constricted in area and depth simulated evaporation fluxes were reduced by over 50 within 15 years despite increased et rates due to lake area constriction and water table declines both relate to land surface morphology beneath and around lakes model results suggest that lake bed elevations exert control on the rate and ultimate level to which groundwater is depressed by drought the deepest lakes in any region will tend to become the focus of groundwater flow during sustained drought once lakes around it dry out the unexpected finding of decreasing shell 18 o in response to drought is interpreted as some combination of source and or evaporation reduction effects such as seasonality of recharge precipitation or reduced et flux from wetlands that seep back into lakes

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

lakes groundwater and palaeohydrology in the sahel of ne nigeria evidence from hydrogeochemistry the geochemistry of natural waters from rain soil the unsaturated zone shallow aquifers lakes and the stratified aquifer system of the chad basin sediments has been investigated to interpret modern hydrological processes and to reconstruct the palaeohydrology of ne nigeria a type region of the southern sahel recharge to the confined middle and lower aquifers recorded in ne nigeria occurred between 24 and 18 6 ka bp prior to the last glacial maximum the mean annual temperature at this time derived from dissolved noble gas ratios was at least 6 c cooler than at the present day this groundwater is not coupled to the active modern recharge cycle and was not reactivated during the holocene wet phases as elsewhere in the saharan region a reflection of changing lake levels and or of changing climatic regimes the absence of groundwater recharge at the time of the last glacial maximum supports other evidence for aridity at this time present day direct recharge rates in the manga grasslands are high mean 44 mm a 1 however regional recharge in ne nigeria at the present day is even higher 60 mm a 1 emphasizing the importance of infiltration from surface runoff as input to groundwater the present study confirms that the shallow aquifer in the region contains significant renewable groundwater resources however the confined aquifer of the chad basin clearly contains palaeowater and this declining artesian basin will need careful conservation prior to a return to traditional water use methods via improved management of the renewable waters in the shallow phreatic aquifers

evidence for massive discharges of icebergs into the north atlantic ocean during the last glacial period sediments in the north atlantic ocean contain a series of layers that are rich in ice rafted debris and unusually poor in foraminifera1 here we present evidence that the most recent six of these heinrich layers deposited between 14 000 and 70 000 years ago record marked decreases in sea surface temperature and salinity decreases in the flux of planktonic foraminifera to the sediments and short lived massive discharges of icebergs originating in eastern canada the path of the icebergs clearly marked by the presence of ice rafted detrital carbonate can be traced for more than 3 000 km a remarkable distance attesting to extreme cooling of surface waters and enormous amounts of drifting ice the cause of these extreme events is puzzling they may reflect repeated rapid advances of the laurentide ice sheet perhaps associated with reductions in air temperatures yet temperature records from greenland ice cores appear to exhibit only a weak corresponding signal moreover the 5 10 000 yr intervals between the events are inconsistent with milankovitch orbital periodicities raising the question of what the ultimate cause of the postulated cooling may have been

evidence for general instability of past climate from a 250 kyr ice core record recent results1 2 from two ice cores drilled in central greenland have revealed large abrupt climate changes of at least regional extent during the late stages of the last glaciation suggesting that climate in the north atlantic region is able to reorganize itself rapidly perhaps even within a few decades here we present a detailed stable isotope record for the full length of the greenland ice core project summit ice core extending over the past 250 kyr according to a calculated timescale we find that climate instability was not confined to the last glaciation but appears also to have been marked during the last interglacial as explored more fully in a companion paper3 and during the previous saale holstein glacial cycle this is in contrast with the extreme stability of the holocene suggesting that recent climate stability may be the exception rather than the rule the last interglacial seems to have lasted longer than is implied by the deep sea specmap record4 in agreement with other land based observations5 6 we suggest that climate instability in the early part of the last interglacial may have delayed the melting of the saalean ice sheets in america and eurasia perhaps accounting for this discrepancy

a comparison of different ways of dealing with isostasy examples from modelling the antarctic ice sheet during the last glacial cycle the bedrock isostatic response exerts a strong control on ice sheet dynamics and is therefore always taken into account in ice sheet models this paperreviews the various methods normally used in the ice sheet modeling community to deal with the bedrock response and compares these with a moresophisticated full earth model each of these bedrock treatments five in total are coupled with a three dimensional thermomechanical ice sheet model underthe same forcing conditions to simulate the antarctic ice sheet during the last glacial cycle the outputs of the simulations are compared on the basis of thetime dependent behavior for the total ice volume and the mean bedrock elevation during the cycle and of the present rate of uplift over antarctica thiscomparison confirms the necessity of accounting for the elastic bending of the lithosphere in order to yield realistic bedrock patterns it furthermoredemonstrates the deficiencies inherent to the diffusion equation in modeling the complex deformation within the mantle nevertheless when characteristicparameters are varied within their range of uncertainty differences within one single method are often of the same order as those between the variousmethods this overview finally tries to point out the main advantages and drawbacks of each of these methods and to determine which one is mostappropriate depending on the specific modeling requirements

vertical wind shear on jupiter from cassini images 1 multifilter images of jupiter acquired by the cassini imaging science subsystem iss are used to derive zonal winds at altitudes above and below the visible cloud deck small features unique to the ultraviolet images of iss are tracked to get the systematic high altitude zonal winds comparison between the zonal winds from ultraviolet images and the vertical profile of zonal winds from the cassini composite infrared spectrometer cirs shows that the zonal winds from the ultraviolet images are from a pressure level that is 0 2 scale heights higher than the pressure level of the zonal winds from continuum band images deeper zonal winds at different latitudes of the equatorial region are measured by tracking cloud features observed within hot spots on continuum band images the deeper zonal winds in this study extend the measurement of the galileo probe to different latitudes of the equatorial region comparison between the galileo probe and this study suggests that these fast moving clouds within hot spots are deeper than 3 bars and are therefore probably water clouds

mega scale glacial lineations and cross cutting ice flow landforms landsat images reveal a previously unsuspected large scale pattern of streamlining within drift that is assumed to reflect former phases of ice flow such a glacial grain can be regarded as a landform assemblage comprised of a number of components drumlins and megaflutes form part of the pattern but in addition there are two previously undocumented ice moulded landform elements streamlined lineations of much greater proportions referred to as mega scale glacial lineations and a distinctive cross cutting topology within the grain the ice moulded landform assemblage is described and illustrated with reference to examples from canada possible modes of genesis of such landforms are discussed and their glaciological implications outlined the discovery of this pattern indicates the pervasive nature of subglacial deformation of sediment and demands a radical re interpretation of ice sheet dynamics

flow mechanism of glaciers on soft beds subhourly measurements of bed deformation bed shear strength subglacial water pressure and surface speed at storglaciaren a glacier in northern sweden showed that the shear strain rates of the bed decrease during periods of high water pressure and surface speed high water pressures appear to be accompanied by a reduction in the coupling of ice with the bed that is sufficient to reduce or eliminate shearing the instability of large ice masses may result from similar decoupling rather than from pervasive bed deformation as has been commonly thought

antarctic ice streams a review an ice stream is a part of an inland ice sheet that flows rapidly through the surrounding ice the distinction between an ice sheet and an outlet glacier is clear in principle but muddy in practice many antarctic glaciers are intermediate in character the ross ice streams which flow through the west antarctic inland ice into the ross ice shelf are distinct in character differing even from other ice streams in the marine ice sheet of west antarctica their surface elevation profiles are low their bed slopes are low and smooth and their driving stresses diminish monotonically downglacier in transverse profile they are broader in relation to ice thickness and exhibit shallower subglacial troughs than other ice streams many models for the fast sliding of glaciers have been applied to the ross ice streams most have included in some form a reduction in basal drag resulting from a lesser effective than glaciostatic pressure at the bed most models are only semi physical at best so need to be tuned by fitting to assumed parameters at some place or places in the real ice sheet the recent discovery of a very small effective pressure beneath one ice stream consequently has led to some gross errors in the velocities predicted by the models the difficulty may be resolved if it is true as recent experiments suggest that ice stream b and by extrapolation other ross ice streams as well slide on a deforming bed that absorbs most or all of the differential motion between the ice and the bedrock