marine ice sheet stability we examine the stability of two dimensional marine ice sheets in steady state the dynamics of marine ice sheets is described by a viscous thin film model with two stefan type boundary conditions at the moving boundary or grounding line that marks the transition from grounded to floating ice one of these boundary conditions constrains ice thickness to be at a local critical value for flotation which depends on depth to bedrock at the grounding line the other condition sets ice flux as a function of ice thickness at the grounding line depending on the shape of the bedrock multiple equilibria may be possible using a linear stability analysis we confirm a long standing heuristic argument that asserts that the stability of these equilibria is determined by a simple mass balance consideration if an advance in the grounding line away from its steady state position leads to a net mass gain the steady state is unstable and stable otherwise this also confirms that grounding lines can only be stable in positions where bedrock slopes downwards sufficiently steeply

grounding line movement and ice shelf buttressing in marine ice sheets 1 understanding the dynamics of marine ice sheets is integral to studying the evolution of the antarctic ice sheet in both the short and long terms an important component of the dynamics grounding line migration has proved difficult to represent in numerical models and most successful attempts have made use of techniques that are only readily applicable to flow line models however to capture the stress transmission involved in another important component the buttressing of a marine ice sheet by its ice shelf the transverse direction must also be resolved we introduce a model that solves the time dependent shelfy stream equations and makes use of mesh adaption techniques to overcome the difficulties typically associated with the numerics of grounding line migration we compare the model output with a recent benchmark for flow line models and show that our model yields an accurate solution while using far less resources than would be required without mesh adaption we also show that the mesh adapting techniques extend to two horizontal dimensions experiments are carried out to determine how both ice shelf buttressing and ice rises affect the marine instability predicted for an ice sheet on a foredeepened bed we find that buttressing is not always sufficient to stabilize such a sheet but collapse of the grounded portion is still greatly delayed we also find that the effect of an ice rise is similar to that of narrowing the ice shelf

direct measurements of floodwater infiltration into shallow alluvial aquifers the relationship between floodwater infiltration and groundwater recharge of a shallow alluvial aquifer in a hyper arid desert was studied in the arava valley israel the study implemented an experimental system that simultaneously monitors all three hydrological domains controlling the recharge process 1 floodwater stages on land surface 2 water content variations in the deep vadose zone and 3 groundwater response to the infiltration this experimental system makes use of a new technique for installing flexible time domain reflectometry probes for continuous monitoring of the vadose zone moisture profiles water infiltration was monitored in controlled infiltration experiments and during a natural flood the monitoring setup allowed real time tracking of the wetting front propagation velocities and assessment of water fluxes through the vadose zone the results revealed a complex infiltration process that includes matrix and preferential flow together with lateral flow

influence of late quaternary climatic changes on geomorphic and pedogenic processes on a desert piedmont eastern mojave desert california radiocarbon dating of late quaternary deposits and shorelines of lake mojave and cation ratio numerical age dating of stone pavements dorn 1984 on the adjacent soda mountains piedmont provide age constraints for alluvial and eolian deposits these deposits are associated with climatically controlled stands of lake mojave during the past 15 000 yr six alluvial fan units and three eolian stratigraphic units were assigned ages based on field relations with dated shorelines and piedmont surfaces as well as on soil geomorphic data all but one of these stratigraphic units were deposited in response to time transgressive climatic changes beginning approximately 10 000 yr ago increased eolian flux rates occurred in response to the lowering of lake mojave and a consequent increase in fine sediment availability increased rates of deposition of eolian fines and associated salts influenced pedogenesis stone pavement development and runoff infiltration relations by 1 enhancing mechanical weathering of fan surfaces and hillslopes and 2 forming clay and silt rich surface horizons which decrease infiltration changes in alluvial fan source areas from hillslopes to piedmonts during the holocene reflect runoff reduction on hillslopes caused by colluvial mantle development and runoff enhancement on piedmonts caused by the development of less permeable soils inferred increased in early to middle holocene monsoonal activity resulted in high magnitude paleo sheetflood events on older fan pavements this runoff triggered piedmont dissection which in turn caused increased sediment availability along channel walls thus runoff infiltration changes during the late quaternary have occurred in response to eolian deposition of fines pedogenesis increased sheetflood activity in the holocene and vegetational changes which are related to many complicated linkages among climatic change lake fluctuations and eolian hillslope and alluvial fan processes

glacial flow of floating marine ice in snowball earth 1 simulations of frigid neoproterozoic climates have not considered the tendency of thick layers of floating marine ice to deform and spread laterally we have constructed a simple model of the production and flow of marine ice on a planetary scale and determined ice thickness and flow in two situations when the ocean is globally ice covered hard snowball and when the tropical waters remain open soft snowball in both cases ice flow strongly affects the distribution of marine ice flowing ice probably carries enough latent heat and freshwater to significantly affect the transition into a snowball earth climate we speculate that flowing marine ice rather than continental ice sheets may be the erosive agent that created some neoproterozoic glacial deposits

marine ice sheet dynamics part 2 a stokes flow contact problem we develop an asymptotic theory for marine ice sheets from a first principles stokes flow contact problem in which different boundary conditions apply to areas where ice is in contact with bedrock and inviscid sea water along with suitable inequalities on normal stress and boundary location constraining contact and non contact zones under suitable assumptions about basal slip in the contact areas the boundary layer structure for this problem replicates the boundary layers previously identified for marine ice sheets from depth integrated models and confirms the results of these previous models the interior of the grounded ice sheet can be modelled as a standard free surface lubrication flow while coupling with the membrane like floating ice shelf leads to two boundary conditions on this lubrication flow model at the contact line these boundary conditions determine ice thickness and ice flux at the contact line and allow the lubrication flow model with a contact line to be solved as a moving boundary problem in addition we find that the continuous transition of vertical velocity from grounded to floating ice requires the presence of two previously unidentified boundary layers one of these takes the form of a viscous beam in which a wave like surface feature leads to a continuous transition in surface slope from grounded to floating ice while the other provides boundary conditions on this viscous beam at the contact line

chronology of tectonothermal activity in the western avalon zone of the newfoundland appalachians the ackley granite was intruded into variably deformed and metamorphosed rocks within the gander and avalon zones of the newfoundland appalachians biotite and hornblende from the pluton record internally concordant 40ar 39ar age spectra that define total gas dates of 352 356 ma these are similar to a previously reported 355 5 ma rb sr whole rock isochron age and indicate relatively rapid post magmatic cooling a feature consistent with the post kinematic character of the pluton in the western avalon zone recrystallized sericitic metavolcanic rocks from the late precambrian love cove and marys town groups display variably discordant whole rock 40ar 39ar age spectra however the samples yield well defined plateau ages of 353 400 ma 11 samples which are interpreted to closely date the regional metamorphism and associated deformation in the western avalon zone this chronology is similar to that of the easternmost gander zone indicating that deformation and metamorphism in the western avalon zone

west antarctic ice streams draining into the ross ice shelf configuration and mass balance extensive airborne radar soundings closely tied to accurately located ground stations made over a portion of the west antarctic inland ice sheet and the ross ice shelf have enabled us to map the boundaries of the ice streams and their flow bands on the ross ice shelf the surfaces of the active ice streams a and b are heavily crevassed but there are no visible crevasses on ice stream c however the existence of numerous crevasses at a depth of about 35 m implies that ice stream c ceased to be active about 250 years ago there is a complex zone at the head of ice stream b that suggests that ice stream b is currently widening and advancing toward the interior of the ice sheet the grounding lines between the inland ice and the ice shelf were mapped from the surface elevations using the requirement of hydrostatic balance and the character of the radar echoes the inland ice is more extensive than previously mapped and includes a large weakly grounded area downstream of ice streams a and b where the surface elevations are barely above hydrostatic equilibrium crary ice rise extends 70 km upstream of the previously mapped position to relate surface elevation data from satellite observations to sea level three geoidal models were tested we conclude that the gem 10c model is the best of the three for this section of antarctica a new map of surface elevation shows deep marginal troughs and longitudinal ridges along ice streams a and b but not on ice stream c whose surface contours show signs of stagnation ice stream c shows a strongly positive and ice stream b a significantly negative net balance rate ice streams a and d also show a significantly negative net flux whereas ice streams e and f do not the overall net balance of this part of the west antarctic inland ice is suggestively negative 23 15 km3 yr 1 from the measured flux into the ross ice shelf and previous measurements we calculate an average basal melt rate from beneath the ice shelf of 0 12 0 03 m yr 1

revealing sediment sources mixing and transport during erosional crater evolution in the hyperarid negev desert israel abstract to better understand the sedimentary history of the erosional crater of makhtesh hazera in the hyper arid negev desert of southern israel we have measured concentrations of in situ 10 be in alluvial sediments from the active drainage system and from abandoned alluvial terraces and dated them using optically stimulated luminescence osl these sedimentary sequences suggest changes in the drainage system behavior over time and show a periodic pattern in which periods of sediment aggradation alternate with periods of degradation through incision and erosion in alluvial terrace sediments 10 be concentrations were combined with osl ages to gain insight of the temporal framework of sediment deposition and shed light on the process of sediment storage osl and simple 10 be exposure ages of terrace sediments give deposition ages between 340 ka to 50 ka at which time the youngest alluvial terrace was incised deposition was interrupted twice at 300 ka and 160 ka when periods of rapid incision caused the abandonment of the active fluvial surface and the stabilization of a younger and lower fluvial surface in the active drainage system 10 be concentrations suggest several possible quartz sources of which the lower cretaceous sandstone bedrock exposed at the base of the cliff enclosing the makhtesh is the most dominant the results suggest that sediment eroding form these cliffs are conveyed through the active alluvial channels without significant sediment contribution from alluvial terraces or from eroded bedrock exposed within the makhtesh in contrast to our measurements in the active drainage system 10 be concentrations in alluvial terrace sediments record significant storage within the makhtesh we suggest that the geometry of the makhtesh mainly the disproportion between the size of the makhtesh and its narrow outlet leads to significant accumulation of sediment within the makhtesh episodic breaching of the barrier is followed by rapid and short lived incision into the stored sediment the long residence time of sediment within the makhtesh results in high measured 10 be concentrations at the end of each of these short incision episodes when channels are already carved into the alluvial sediment and terraces are formed the channels only convey sediment directly eroded from the bounding cliff of the makhtesh a situation presently observed

runoff generation rill erosion and time scales for hyper arid abandoned alluvial surfaces the negev desert israel abstract sequences of abandoned alluvial surfaces are ubiquitous desert landscapes considered as responding to quaternary timescale climatic fluctuations and tectonic activity the post depositional evolution surficial properties of these alluvial surfaces change with time a surface clast breakdown and development of desert pavement b dust and salts accumulation c reg gravelly desert soil evolution in the present study we focus on the hydrological response and the resulting erosional processes to these changes in surface properties over a sequence of late pleistocene holocene abandoned alluvial surfaces in nahal evrona catchment at the hyperarid southern arava valley north of the towns of eilat israel and aqaba jordan the methods used in the study are a simulated rainfall experiments to determine hydrological characteristics b terrestrial laser scanning tls to determine surface roughness c osl dating for the chronology d surface and soil descriptions and measurements and grain size analyses to determine texture of the uppermost av horizon of the reg soil e gis analysis of the new drainage network developing over the older abandoned alluvial surfaces the results show that with time roughness decreases infiltration decreases to about 8 9 relative to present rates point runoff generation increases 5 fold and as a result rill and gully erosion initiated finally leading to the destruction of the alluvial surface under the current hyper arid climate in the southern negev desert between 30 and 50 ka of surface weathering and soil development are required before significant runoff is generated and 100 ka until effective runoff initiates rill and gully erosion over these abandoned alluvial surfaces this local source runoff and gully erosion essentially destroys the alluvial surfaces in the southern arava valley within a period of 400 500 ka roughness extracted from spaceborne lidar scanning which is inversely related to surface age directly reduces friction and resistance to runoff can provide an estimate for the hydrological characteristics of the surfaces this can support applications such as rainfall runoff models risk assessment etc

large scale ice flow over a viscous basal sediment theory and application to ice stream b antarctica recent seismic studies of ice stream b antarctica and field analysis of mid latitude glacial deposits suggest that deformable basal sediments e g water saturated till are important in determining ice sheet flow if the ratio of till viscosity to effective ice viscosity is small vertical shear associated with horizontal flow is confined to the deforming bed alone ice flow over a deformable bed is thus akin to that of floating ice shelves because ice shelves flow over inviscid seawater for some antarctic ice streams and possibly for portions of the late wisconsin ice sheets in north america and eurasia basal drag associated with deforming basal sediment does not induce significant vertical gradients of horizontal velocity instead basal drag affects the flow as if it were a horizontal body force balanced by longitudinal and transverse deviatoric stress gradients comparison of the observed flow of ice stream b to finite element simulations incorporating a viscous basal till suggest that a simple till theology is sufficient to explain the current velocity profile these simulations also highlight the importance of horizontal deviatoric stress in regions where driving stress and basal stress do not locally balance while bed deformation is critical to ice stream existence sensitivity tests suggest that ice shelf back pressure is still a crucial control affecting ice stream response to atmospheric and oceanic climate

evidence of past fluctuations on stancomb wills ice tongue antarctica preserved by relict flow stripes this paper presents a newly derived velocity field of the brunt ice shelf and stancomb wills ice tongue in east antarctica velocity is derived using automated feature tracking techniques on radarsat 1 imagery from the 1997 antarctic mapping mission 1 amm 1 and 2000 modified antarctic mapping mission mamm we measure velocities up to 1350 m a 1 at the ice front closer investigation of the velocity field and radar images reveals an obvious divergence of contemporary flowlines from relict flow stripes along a portion of stancomb wills ice tongue analysis of relict flow stripes can reveal glacial events in the past assuming relatively steady flow since the event occurred we utilize the current velocity field to reconstruct former stripe configuration we believe the flowline deviation points to a change in dynamics that occurred approximately 140 years ago perhaps due to thinning and an associated shift in grounding line position this seems to be supported by historic observations and bottom topography data

relict sheetflood bed forms on late quaternary alluvial fan surfaces in the southwestern united states late pleistocene and early holocene alluvial fan surfaces in southeastern california are locally mantled by narrow widely spaced transverse to slope bands of fine gravel and coarse sand these features are interpreted to be relict bed forms of high magnitude sheetfloods that inundated abandoned alluvial fan surfaces the majority of these sheetflood events apparently occurred in the latest pleistocene to middle holocene and produced meso bed forms with wavelengths of 2 6 m these flood events mobilized sediment with mean grain sizes of 2 8 mm on inactive fan surfaces under calculated velocities of 30 60 cm s and may be responsible for increased dissection of upper piedmonts and widespread alluvial fan deposition in the lower piedmont regions an older sheetflood event produced macro bed forms with wavelengths of 20 80 m on a pedogenic caco 3 cemented piedmont in southwestern arizona both scales of bed forms display characteristics of confined flow fluvial bed forms which form under diverse flow regimes that cause either megaripples or transverse ribs

the effect of bottom boundary conditions in the ice sheet to ice shelf transition zone problem uniqueness of the transition zone solution is discussed it is argued that when it is assumed that the stresses are continuous at the grounding line the ice shelf solution at the grounding line should possess a zero slope in order to avoid issues caused by the mathematical singularity of the solution the same technique as that used to study the transition zone is applied to a similar problem of lifting an elastic sheet from a rigid substrate which allows a better physical understanding this exemplifies the effect of the bottom boundary condition of no vertical motion imposed up to a fixed grounding line position while the forcing parameters such as the flow rate vary its effect is to produce multiple solutions due to suppression of the separation mechanism ice sheet motion is determined by the transition zone dynamics that couple the grounded part of an ice sheet to its floating part the ice shelf as the shallow ice approxima tion makes equations governing the grounding part of the ice sheet simpler it is convenient to decouple the grounded ice sheet problem from the ice shelf problem this leads to the necessity of imposing boundary conditions at the grounding line although it has been argued that due to its low stress the presence of the ice shelf does not constrain the ice sheet dynamics in any way apart from the condition of hydrostatic equilibrium at the grounding line hindmarsh 1993 here we discuss the other possibility namely that the ice sheet motion is uniquely determined by its external and initial conditions the latter implies two boundary conditions at the grounding line that together with the condition of zero mass flux at the ice divide close the isothermal ice sheet problem described by a non linear parabolic second order differential equation solved between the ice divide and the unknown grounding line position to date no work has been able to show uniqueness of the transition zone solution that apart from the hydrostatic equilibrium condition would allow us to impose an additional unique boundary condition at the grounding line to relate the ice thickness to the mass flow rate the progress in doing so has been hindered by the complexity of the problem especially due to a singularity of the solution at the grounding line caused by a jump in the bottom boundary

the effect of buttressing on grounding line dynamics determining the position and stability of the grounding line of a marine ice sheet is a major challenge for ice sheet models here we investigate the role of lateral shear and ice shelf buttressing in grounding line dynamics by extending an existing boundary layer theory to laterally confined marine ice sheets we derive an analytic expression for the ice flux at the grounding line of confined marine ice sheets that depends on both local bed properties and non local ice shelf properties application of these results to a laterally confined version of the mismip 1a experiment shows that the boundary condition at the ice shelf front i e the calving law is a major control on the location and stability of the grounding line in the presence of buttressing allowing for both stable and unstable grounding line positions on downwards sloping beds these results corroborate the findings of existing numerical studies that the stability of confined marine ice sheets is influenced by ice shelf properties in contrast to unconfined configurations where grounding line stability is solely determined by the local slope of the bed consequently the marine ice sheet instability hypothesis may not apply to buttressed marine ice sheets

a coupled marine ice stream ice shelf model theorie des variations climatiques du pleistocene a aujourd hui liees a la dynamique des couvertures marines de glace couplee aux glaces libres flottantes on neglige les effets thermodynamiques et on etudie les processus de flottaison en relation avec les rapports de densite entre la place et l eau de mer

stages and rate of the gravel shattering process by salts in desert reg soils abstract gravel shattering by salts is a common process in gravelly young to mature desert reg soils the shattered gravel is found on the surface and in the b and c horizons down to a maximum depth of 50 cm field studies on the process of salt weathering in deserts are scarce therefore the objectives of this study are to present the stages of development of the gravel shattering process by salts in desert reg soils and to discuss factors influencing the rate of development under natural conditions prevailing in the negev desert the studied reg soils were developed on two alluvial fan chronosequences in the negev desert the fan of nahal ze elim and the fan of nahal shehoret the development of the shattering process can be divided into five stages a e based on chemical physical and micromorphological features of the reg soils stages a d very young yo young reg soils represent the early holocene to the present some 14 000 years salinity values are between 0 27 16 mmho cm and the percentage of the shattered gravel is between 20 and 70 the shattered gravel however still maintains its original shape stage e mature reg soil represents the middle pleistocene to early holocene or 500 000 years salinity values at this stage reach 62 mmho cm although part of the gravel does not keep its original shape and starts to disperse in the profile the percentage of shattered gravel is only 75 80 it was found that the process of salt shattering in reg soils does not depend on concentrations or mineralogy of the salts but on the micro environmental conditions of the soil profile the process requires extreme and rapid variations of temperatures and moisture conditions such as found close to the soil surface to cause salt crystallization in a confined system with time the sealing of the soil surface through the development of desert pavement and a gravel free b horizon results in a change in the soil micro environment this change impedes the process irrespective of the amounts concentrations and types of salt minerals which exist in the profile the fact that in extreme arid regions the gravel shattering process develops primarily close to the reg soil surface suggests that it can be used to identify buried reg soils and to reconstruct climatic and micro environmental conditions of paleosols

ice shelf response to ice stream discharge fluctuations i unconfined ice tongues abstract unsteady thickness and flow observed on the ross ice shelf are thought to result primarily from fluctuations of ice streams and outlet glaciers that feed the ice shelf at its inland boundaries ice stream discharge fluctuations constitute an independent means of forcing unsteady ice shelf behavior and their effect must be distinguished from those of oceanic and atmospheric climate to understand ice shelf change ice stream generated fluctuations of a one dimensional ice shelf are found to propagate through the ice shelf environment along the two characteristics of the hyperbolic governing equations one characteristic permits instantaneous transmission of grounding line velocity changes to all points downstream the other characteristic represents slow transmission of grounding line thickness changes along lagrangian particle paths

the effect of a new drag law parameterization on ice shelf water plume dynamics abstract a drag law accounting for ekman rotation adjacent to a flat horizontal boundary is proposed for use in a plume model that is written in terms of the depth mean velocity the drag law contains a variable turning angle between the mean velocity and the drag imposed by the turbulent boundary layer the effect of the variable turning angle in the drag law is studied for a plume of ice shelf water isw ascending and turning beneath an antarctic ice shelf with draft decreasing away from the grounding line as the isw plume ascends the sloping ice shelf ocean boundary it can melt the ice shelf which alters the buoyancy forcing driving the plume motion under these conditions the typical turning angle is of order 10 over most of the plume area for a range of drag coefficients the minus sign arises for the southern hemisphere the rotation of the drag with respect to the mean velocity is found to be significant if the drag coefficient exceeds 0 003 in this case the plume body propagates farther

ice sheet grounding line dynamics steady states stability and hysteresis 1 the ice sheet ice shelf transition zone plays an important role in controlling marine ice sheet dynamics as it determines the rate at which ice flows out of the grounded part of the ice sheet together with accumulation this outflow is the main control on the mass balance of the grounded sheet in this paper we verify the results of a boundary layer theory for ice flux in the transition zone against numerical solutions that are able to resolve the transition zone very close agreement is obtained and grid refinement in the transition zone is identified as a critical component in obtaining reliable numerical results the boundary layer theory confirms that ice flux through the grounding line in a two dimensional sheet shelf system increases sharply with ice thickness at the grounding line this result is then applied to the large scale dynamics of a marine ice sheet our principal results are that 1 marine ice sheets do not exhibit neutral equilibrium but have well defined discrete equilibrium profiles 2 steady grounding lines cannot be stable on reverse bed slopes and 3 marine ice sheets with overdeepened beds can undergo hysteresis under variations in sea level accumulation rate basal slipperiness and ice viscosity this hysteretic behavior can in principle explain the retreat of the west antarctic ice sheet following the last glacial maximum and may play a role in the dynamics of heinrich events

10be exposure ages of ancient desert pavements reveal quaternary evolution of the dead sea drainage basin and rift margin tilting abstract early to late pleistocene 10 be exposure ages of abandoned surfaces in the negev desert reveal the regional drainage evolution history and its relationship with the subsidence of the western margin of the dead sea rift the dated desert paved surfaces have developed over originally westward flowing rivers which were abandoned by early pleistocene and whose relicts are now progressively tilted towards the rift axis the slow and non destructive subsidence coupled with extreme hyperaridity enabled the preservation of these ancient surfaces along some of the main water divides in the negev nearly irrespective of their distance from the rift axis constraints on the tilting history are obtained from analyzing the spatial pattern of the exposure ages suggesting subsidence rates as low as 120 300 m ma 1 in the southern arava valley since the late pliocene it is shown that the transition from the pliocene to current drainage pattern occurred over a short period during the early pleistocene and that the governing fluvial response that followed the delineation of current basins is represented by a continuous spectrum of ages of inset terraces

aggradation incision transition in arid environments at the end of the pleistocene an example from the negev highlands southern israel abstract one of the most significant environmental processes that occurred at the transition from the last glacial phase into the present inter glacial phase in arid regions was the shift from aggradation to incision in the drainage systems this is evident by the sharp transition from a fluvial regime depositing fine grained sediment within the wadis to intensive incision which formed gullies and narrow channels that dissected the late pleistocene sediments in order to investigate this transition we studied three small scale basins in the arid region of the negev highlands southern israel although the selected basins drain toward different base levels their geomorphological parameters particle size distribution of alluvial units and their osl ages are similar sediments from the penultimate glacial cycle are found in patches in the bigger catchments fluvial loess was widely deposited since at least 67 ka until after 28 ka covering valleys and slopes between 28 and 24 ka loess was washed from the slopes into the channels exposing the underlying colluvium at 24 ka erosion began with the transport of slope colluvium as gravels into the valleys that eroded the underlying loess sediments incision became dominant at 12 ka and is still ongoing and intensifying dust and reworked loess continued to be deposited during the main incision stages it is proposed that the transition from aggradation to incision was controlled by rates of loess supply and removal until 24 ka dust choked the drainage system and only after reduction in dust supply was erosion and incision possible it began first on the slopes and then in the channels our results show that an increase in precipitation is not a prerequisite for initiation of incision as is often assumed similar processes are described in other arid zones around the world

on the coupled response to ice shelf basal melting ice shelf basal melting is tightly coupled to ice shelf morphology ice shelves in turn are coupled to grounded ice via their influence on compressive stress at the grounding line ice shelf buttressing here we examine this interaction using a local parameterization that relates the basal melt rate to the ice shelf thickness gradient this formulation permits a closed form solution for a steady state ice tongue time dependent numerical simulations reveal the spatial and temporal evolution of ice shelf ice stream systems in response to changes in ocean temperature and the influence of morphology dependent melting on grounding line retreat we find that a rapid 1 year re equilibration in upstream regions of ice shelves establishes a spatial pattern of basal melt rates relative to the grounding line that persists over centuries coupling melting to ice shelf shape generally but not always increases grounding line retreat rates relative to a uniform distribution with the same area average melt rate because upstream ice shelf thickness gradients and retreat rates increase nonlinearly with thermal forcing morphology dependent melting is more important to the response of weakly buttressed strongly forced ice streams grounded on beds that slope upwards towards the ocean e g those in the amundsen sea

terminating the 100 kyr ice age cycle we report a simulation of the most recent 100 000 year glaciationdeglaciation cycle of the late pleistocene ice age a simulation that delivers an ice sheet chronology that is in close accord with that inferred from the geological record our analyses are performed with a reduced model of the climate system that incorporates significant improvements to the representation of both climate forcing and mass balance response in a previously described theory based upon a coupled one level energy balance model ebm and vertically integrated ice sheet model ism the theory fully incorporates the influences of orbital insolation forcing glacial isostatic adjustment and variations in the atmospheric concentrations of greenhouse gases it correctly predicts the main geographical regions of the northern hemisphere that were glaciated at last glacial maximum 21 000 years ago as well as the abrupt termination of the glacial epoch that occurred subsequently the latter feature of the ice age cycle is obtained without the need to incorporate unconstrained and therefore controversial physical processes into the model a limitation of all previous attempts to understand this global scale climate cycle our analyses suggest that the radiative impact on surface glaciation due to the changing atmospheric concentration of co2 is critical to the ability of the model to deliver a synthetic history of glaciation and deglaciation that is in accord with inferences based upon surface geological and geomorphological evidence with the incorporation of this influence model predicted ice thickness distributions at last glacial maximum lgm are very similar to those of the recently described ice 4g reconstruction that was based upon the inversion of postglacial relative sea level histories

large floods during late oxygen isotope stage 3 southern negev desert israel abstract slackwater deposits were found in a cave in the nahal netafim catchment 35 km 2 near the head of the gulf of aqaba in the southern negev israel the sedimentological record includes 27 large paleofloods dated by infrared stimulated luminescence to 33 000 29 000 years ago the scatter of the ages and their large uncertainties prevented an assessment of the exact duration of the record and the specific timing of each flood bayesian analysis was used to adjust the dense dating results so that the time interval between the first and last flood deposit preserved in the cave could be estimated minimum peak discharges were reconstructed based on the estimated elevation of the late pleistocene channel bed as indicated by fluvio pedogenic layers found near the cave the average frequency of these large floods 200 600 m 3 s 1 for the period between 33 000 and 29 000 years ago is about 1 flood per 150 years while for the mid late holocene it is only 1 large flood per 1000 years eight floods out of the 27 recorded deviate from the envelope curves of mid late holocene paleofloods and measured floods in the hyperarid negev desert indicating a different hydroloclimatological regime the anomalous large floods are hypothesized to have resulted from an increase in regional rainfall intensity and or duration attributed to increased frequency of the red sea trough low pressure system that affects the region available records indicate that the northern negev and areas farther north in israel controlled by mediterranean pressure systems were wetter 40 20 ka bp at the same time the southern negev probably in response to the red sea trough system also experienced short episodes of more and or larger rainstorms the timing of these episodes in both the northern and the southern negev towards the last glacial maximum points to a potential synchronous strengthening of both the mediterranean and red sea systems currently acting at different seasons these episodes of increased storminess in the area were brief and were not able to alter the general hyperarid conditions in this area

ice dynamics at the mouth of ice stream b antarctica field data collected at the mouth of ice stream b show that the flow dynamics of this region are distinctly different than either the major portion of the ice stream upstream or the ice shelf downstream surface slopes in this region are as low as ice shelf surface slopes yet with the exception of patches of ice which may be floating the ice is grounded basal shear stress is negligible the resistance to flow being partitioned between shear at the sides and longitudinal gradients of longitudinal and transverse stress the surface is generally crevasse free features similar to ice rises are observed upstream of the grounding line their origin is uncertain but they move at velocities comparable to the surrounding ice the flow is laterally extensive and longitudinally compressive but there are large local variations of the strain rate from the regional trends the boundary between the two major tributaries to ice stream b followed with the radar is characterized by a band of strain rates much smaller than average detailed measurements at the downstream b network highlight this local variability of strain rates but confirm that there is a strong correlation between surface topography and strain rates the strain rates indicate that the undulating topography is locally generated the lower elevation ice is thicker and moves faster a velocity profile across the crevassed northern margin shows that the decrease of velocity toward the edge is nearly linear a calculation of ice stream discharge at this location agrees closely with two rather rough estimates of balance flux and is considerably larger than a third estimate the discharge of ice stream b does not appear to be significantly out of balance with published estimates of total ice accumulation within the present catchment basin

influences of eolian and pedogenic processes on the origin and evolution of desert pavements well developed desert pavements are present above eolian deposits that mantle flows of the cima volcanic field located in the mojave desert california soil stratigraphic data and geochemical data demonstrate that eolian and pedogenic processes play major roles in the evolution of these pavements eolian dust 1 accelerates mechanical fragmentation of flow rock providing the source material for pavements and 2 accumulates slowly below basaltic colluvium in flow depressions eventually promoting development of cumulate soils below the evolving stone pavement an increase in dust flux during the holocene has raised ancient pleistocene pavements as much as 20 cm above the former land surface the results of our studies demonstrate for the first time that most desert pavements do not form by deflation by overland flow or by upward migration of stones through a slowly formed clayey argillic horizon desert pavements are born and maintained at the surface

permanent quaternary hyperaridity in the negev israel resulting from regional tectonics blocking mediterranean frontal systems although various episodes of wet quaternary climates have been suggested in studies of the negev desert here we demonstrate that reg soils developed on flat alluvial surfaces and sensitive to minor changes in precipitation indicate that the southern negev has been permanently hyperarid at least since the middle pleistocene the wetter episodes were restricted to the northern negev currently mildly arid gypsic saline reg soils that de veloped on stable surfaces during 200 k y in the southern negev are cumulative and polygenetic but none exhibits even the weakest development of calcic horizons under current climate conditions in the negev calcic soils are widespread in areas with rainfall 80 mm yr 21 the fact that the hyperarid 50 mm yr 21 southern negev lacks any calcic horizons indicates that it never experienced an average rainfall of 80 mm yr 21 during the middle late pleistocene whereas the northern negev was wetter this negev dichotomy is explained by the main physical features controlling rainfall over the negev i e the southward decreasing depth of the atmospheric boundary layer with distance from the mediterranean and the altitude of the central negev highland 1000 m the interaction between these two features often prevents the passage of rain clouds into the southern negev the holocene and pleistocene gypsic salic soil distribution across the negev closely matches the current circumstances we suggest that hyperaridity has prevailed over the southern negev since the last stages of the uplift of the central negev highland in the late pliocene early pleistocene

force mass and energy budgets of the crary ice rise complex antarctica the stress mass and energy dissipation budgets of crary ice rise are analyzed using field data collected during the 1983 1985 austral summers and in previous field programs in addition the net back pressure and ice discharge rate along the grounding lines of ice streams are calculated to assess the effect of the ice rise on the surrounding flow comparison of the ice rise budgets with the analysis of grounding line data confirms the influence of the ice rise on ice sheet stability and suggests that crary ice rise may have formed recently in response to an acceleration of one of the ice streams it is concluded that feedback between ice stream acceleration and ice rise formation may control the future evolution of the above ice stream and promote long term grounding line stability in the face of strong natural fluctuations

marine ice sheet dynamics the impacts of ice shelf buttressing marine ice sheets are continent scale glacial masses that lie partially submerged in the ocean as applies to significant regions of antarctica and greenland such ice sheets have the potential to destabilise under a buoyancy driven instability mechanism with considerable implications for future sea level this paper and its companion present a theoretical analysis of marine ice sheet dynamics under the effect of a potentially dominant control of the buttressing force generated by lateral stresses on the downstream floating component of the ice sheet the ice shelf the analysis reveals critical conditions under which ice shelf buttressing suppresses the buoyancy driven collapse of an ice sheet and elucidates the implications of lateral stresses on grounding line control and overall ice sheet structure integrations of a suitably simplified quasi two dimensional model are conducted yielding analytical results that provide a quick assessment of steady state balances for a given ice sheet configuration an analytical balance equation describing the spectrum of marine ice sheet flow regimes spanning zero to strong ice shelf buttressing is developed it is determined that the dynamics across this spectrum exhibits markedly different flow regimes and structural characteristics for sufficient buttressing the grounding line occurs near to where a lateral drag controlled section of the ice shelf meets the bedrock implying an independent control of the grounding line by the ice shelf the role of basal stresses is relegated to controlling only the thickness of the ice sheet upstream of the grounding line with no significant control of the grounding line itself it is further demonstrated that lateral stresses are responsible for inducing additional secondary contacts between the ice shelf and the bedrock downstream of the grounding line resulting in a rich variety of additional steady states these inducements generate a further stabilising mechanism that can fully suppress grounding line retreat and eliminate otherwise irreparable hysteresis effects the results provide a conceptual framework for numerical and observational interpretation of marine ice sheet dynamics and clarifies the manner in which ice shelves can control grounding line positions independently it is thus indicated that a full resolution of the fine details of the flow of ice shelves and the processes controlling their erosion and disintegration is necessary for the confident forecasting of possible ice sheet collapse over the course of the next few centuries

influence of relative surface age on hydraulic properties and infiltration on soils associated with desert pavements abstract av horizons found in desert pavement environments are known to evolve pedogenically over geologic time this study was conducted to determine whether increased pedogenic development of the av vesicular horizon over relative time impacts the hydraulic properties of individual soil peds and the mechanism of infiltration as inferred by dye patterns we examined peds from the av horizons associated with desert pavements that mantled three different alluvial deposits with different relative surface ages qf5 10 ka qf3 50 100 ka and qf2 10 50 ka and included an additional surface qf6 4 ka for the dye studies we hypothesized that increases in the development of the av over time would lead to a more structured soil surface with greater potential flow between soil peds and lower hydraulic conductivity of the soil peds themselves results showed that average k s and of the qf5 peds were significantly greater than estimated for the qf2 and qf3 peds although k s was greater for the qf5 peds the steady state infiltration rate was equal for the qf3 qf2 and qf5 surfaces perhaps indicating a reduction in matrix flow through soil peds and an increase in interped flow between soil peds studies were also conducted using dyed water and a tension disc infiltrometer set to saturation following the tests the soil was excavated in 2 cm increments and photographed the dye patterns for the qf6 indicated that the water moved rapidly through the soil matrix into deeper soil and without any preferential flow around peds the qf5 exhibited more dye stained soil at depth than seen in the qf2 and qf3 perhaps due to higher ped conductivity qualitative observations on the qf2 and qf3 suggest that water flowed primarily along the ped faces and then toward the ped interiors the results suggest that infiltration through av horizons evolves from a matrix dominated process on the younger soils to a preferential flow dominated process on older surfaces

role of small ice shelves in sea level rise 1 recent observations show marked acceleration and thinning of tributary ice streams following shrinkage or loss of a few of the many small ice shelves that ring antarctica and greenland our model experiments which adopt a simplified geometry show that calving from or partial disintegration of a small ice shelf has more effect on tributary ice streams than does loss of a berg of equal length from a larger ice shelf our results which are consistent with the recent observations indicate that even small ice shelves can provide important restraint on tributary flow from ice sheets implying that characterization and modeling of the full range of ice shelves will be required for accurate projection of ice sheet changes and their influence on sea level

evolution of reg soils in southern israel and sinai abstract reg soils mostly camborthids and gypsiorthids cover some 15 of the negev and sinai deserts analytical data of seven profiles on depositional surfaces of increasing age in the negev show clear relationships between ages of surfaces and soil profile features exemplified in the development of the cambic salic and gypsic horizons the youngest soil a coarse desert alluvium soil typic torriorthent of dry wadi beds up to a few thousand years old has little profile differentiation and is generally non saline or slightly saline on the higher terraces connected with the lisan formation about 70 000 12 000 years b p clear profile differentiation and the beginning of development of cambic horizons can be discerned evident in their colour textural p differences the soils are already saline and somewhat gypsiferous both soils are not yet defined as regs because of their negligible or restricted profile differentiation reg soils on the older and higher geomorphic surfaces in the arava rift valley several hundred thousand years old have well differentiated profiles with cambic salic and gypsic horizons below their stony desert pavements typical reg soils have light coloured vesicular horizons over mellow very pale brown loam frequently with laminar structure almost completely stone free layers the reddish brown loamy to clay loam b horizons are very saline and show intensive salt weathering of gravels gypsic and petrogypsic horizons occur at depth such reg soils represent the stable surfaces and soils of deserts and were developed over a long period of desert weathering

use of multiparameter relative age methods for age estimation and correlation of alluvial fan surfaces on a desert piedmont eastern mojave desert california numerical and calibrated age determinations of the late quaternary alluvial fan deposits of the soda mountains piedmont in the mojave desert provide an opportunity to study the utility of the multiparameter relative age ra method for distinguishing and mapping geomorphic surfaces on a desert piedmont most ra parameters could not discriminate between deposits of holocene age although pavements have formed over locally significant parts of surfaces as young as middle holocene several parameters including lithologic composition particle size soil development and varnish cover permit distinguishing between holocene surfaces and late pleistocene surfaces statistically significant differences in initial particle size and lithology of the deposits inferred to be the result of complex interaction among hillslope alluvial fan and eolian processes and climatic change create conditions unfavorable for use of most ra techniques in contrast soil profile development and varnish cover data are successful in discrimination among deposits of holocene and pleistocene age this is attributed to the development of pedogenic features and varnish that are strongly dependent on dust influx and to the relatively minor dependence of these features on differences in the depositional character of the fan

alternative ice shelf equilibria determined by ocean environment 1 dynamic and thermodynamic regimes of ice shelves experiencing weak 1 m year 1 to strong 10 m year 1 basal melting in cold bottom temperature close to the in situ freezing point and warm oceans bottom temperature more than half of a degree warmer than the in situ freezing point are investigated using a 1 d coupled ice ocean model complemented with a newly derived analytic expression for the steady state temperature distribution in ice shelves this expression suggests the existence of a basal thermal boundary layer with thickness inversely proportional to the basal melt rate model simulations show that ice shelves afloat in warm ocean waters have significantly colder internal ice temperatures than those that float in cold waters our results indicate that in steady states the mass balance of ice shelves experiencing strong and weak melting is controlled by different processes in ice shelves with strong melting it is a balance between ice advection and basal melting and in ice shelves with weak melting it is a balance between ice advection and deformation sensitivity simulations show that ice shelves in cold and warm oceans respond differently to increase of the ocean heat content ice shelves in cold waters are more sensitive to warming of the ocean bottom waters while ice shelves in warm waters are more sensitive to shallowing of the depth of the thermocline

late quaternary weathering erosion and deposition in nahal yael israel an impact of climatic change on an arid watershed in their seminal paper in 1979 bull and schick proposed a conceptual model for the geomorphic response to pleistocene to holocene climate change based on the hyperarid nahal yael watershed in the southern negev desert in this model the change from semiarid late pleistocene to hyperarid early holocene climates reduced vegetation cover increased the yield of sediment from slopes and accelerated aggradation of terraces and alluvial fans the model is now over 30 yr old and during this time chronologic paleoenvironmental and hydrogeomorphic research has advanced here we reevaluate the model using data acquired in nahal yael over the 30yr since the original model was proposed recent studies indicate that the late pleistocene climate was hyperarid and a transition from semiarid to hyperarid climates did not occur the revised chronology reveals a major 35 20 ka episode of accelerated late pleistocene sediment production on slopes with lower rates probably already at ca 50ka due to increased frequency of wetting drying cycles caused by frequent extreme storms and floods between 35 and 27 ka without lag time these sediments were transported and aggraded in depositional landscape components fluvial terraces and alluvial fans this intensified sediment production and delivery phase is unrelated to the pleistocene holocene transition the depositional landforms were rapidly incised between 20 and 18 ka since and or soon after this last glacial maximum lgm incision most material leaving the basin originated from sediments stored in depositional landforms and was not produced from bedrock using these new data we propose a revision to the bull and schick model in this hyperarid environment our revision suggests that the model should include the frequent storms and floods responsible for a late pleistocene pulse of intense weathering due to numerous cycles of wetting and drying on slopes and coeval sediment transport to fluvial terraces and alluvial fans we also discuss the common use and pitfalls of using the bull and schick conceptual model to explain observations in diverse arid environments usually without sufficient data on basin specific stratigraphic chronologic paleoenvironmental and paleoclimatic information

marine ice sheet dynamics part 1 the case of rapid sliding marine ice sheets are continental ice masses resting on bedrock below sea level their dynamics are similar to those of land based ice sheets except that they must couple with the surrounding floating ice shelves at the grounding line where the ice reaches a critical flotation thickness in order to predict the evolution of the grounding line as a free boundary two boundary conditions are required for the diffusion equation describing the evolution of the grounded ice thickness by analogy with stefan problems one of these conditions imposes a prescribed ice thickness at the grounding line and arises from the fact that the ice becomes afloat the other condition must be determined by coupling the ice sheet to the surrounding ice shelves here we employ matched asymptotic expansions to study the transition from ice sheet to ice shelf flow for the case of rapidly sliding ice sheets our principal results are that the ice flux at the grounding line in a two dimensional ice sheet is an increasing function of the depth of the sea floor there and that ice thicknesses at the grounding line must be small compared with ice thicknesses inland these results indicate that marine ice sheets have a discrete set of steady surface profiles if they have any at all and that the stability of these steady profiles depends on the slope of the sea floor at the grounding line

concentrated flow erosion rates as affected by rock fragment cover and initial soil moisture content abstract concentrated flow experiments using a small hydraulic flume and a constant flow discharge and bed slope have been conducted in order to investigate the effects of rock fragment cover rc on sediment yield for an initially wet and an initially air dry loamy topsoil the experimental results indicate that rc reduces concentrated flow erosion rates e in an exponential way i e e e brc which is similar to previously reported relations for other water erosion processes such as interrill erosion and sheet rill erosion measured on runoff plots the decay rate b of this exponential relationship increased throughout the experiments because of scour hole development and bed armouring the concentrated flow erosion rates and b values also depend on the initial moisture content of the topsoil depending on rc mean concentrated flow erosion rates were 20 to 65 less on initially wet compared to initially air dry topsoils the mean value for b was 0 032 for the initially wet but only 0 017 for the initially air dry topsoil indicating that a rock fragment cover is less efficient in reducing concentrated flow erosion rates when the topsoil is initially air dry than when it is initially wet the results help explain the data scatter in reported relationships between rc and interrill rill erosion rates they also indicate that a given surface rock fragment cover will offer more protection to wet topsoils than to dry topsoils which are very common in mediterranean environments event based water erosion models should incorporate effects of antecedent soil moisture content as well as those of rc on concentrated flow erosion rates