changes in the salinity and porosity of sea ice samples during shipping and storage a theoreti cal examination of salinity and porosity changes introduced in sea ice samples by brine expulsion and gas entrapment caused by thermal cycling during shipping and storage shows that in extreme cases such effects can be significant resulting in 15 reductions in porosity 1 more representative scenarios give porosity changes of less than 2 which assuming that ice property variations scale with n 1 2 result in property variations of less than 1

a two phase approach to the simulation of the combined isotope salinity signal of marine ice a modified boundary layer box diffusion model is used to simulate the production of the combined oxygen isotope salinity signal of a body of marine ice the final signal is considered to be the result of fractionation and segregation processes occurring first upon the formation of frazil crystals in suspension and second during their accumulation and consolidation due to the advancement of a freezing front the model results are compared to oxygen isotope and salinity values in a 45 m marine ice core from the nansen ice shelf antarctica parameters more suitable to the modeling of marine ice as opposed to sea ice are introduced and conclusions are drawn on the origin of the impurity characteristics of marine ice including the still unresolved question of its anomalously low salinity

frazil ice formation in an ice shelf water plume 1 we present a model for the growth of frazil ice crystals and their accumulation as marine ice at the base of antarctic ice shelves the model describes the flow of buoyant water upward along the ice shelf base and includes the differential growth of a range of crystal sizes frazil ice formation starts when the rising plume becomes supercooled initially the majority of crystals have a radius of similar to0 3 mm and concentrations are below 0 1 g l depending on the ice shelf slope which controls the plume speed frazil crystals increase in size and number typically crystals up to 1 0 mm in radius are kept in suspension and concentrations reach a maximum of 0 4 g l the frazil ice in suspension decreases the plume density and thus increases the plume speed larger crystals precipitate upward onto the ice shelf base first with smaller crystals following as the plume slows down in this way marine ice is formed at rates of up to 4 m yr in some places consistent with areas of observed basal accumulation on filchner ronne ice shelf the plume continues below the ice shelf as long as it is buoyant if the plume reaches the ice front its rapid rise produces high supercooling and the ice crystals attain a radius of several millimeters before reaching the surface similar ice crystals have been trawled at depth north of antarctic ice shelves but otherwise no observations exist to verify these first predictions of ice crystal sizes and volumes

borehole imagery of meteoric and marine ice layers in the amery ice shelf east antarctica a real time video camera probe was deployed in a hot water drilled borehole through the amery ice shelf east antarctica where a total ice thickness of 480 m included at least 200 m of basal marine ice down looking and side looking digital video footage showed a striking transition from white bubbly meteoric ice above to dark marine ice below but the transition was neither microscopically sharp nor flat indicating the uneven nature at centimetre scale of the ice shelf base upstream where the marine ice first started to accrete marine ice features were imaged including platelet structures cell inclusions entrained particles and the interface with sea water at the base the cells are assumed to be entrained sea water and were present throughout the lower 100 150m of the marine ice column becoming larger and more prevalent as the lower surface was approached until near the base they became channels large enough that the camera field of view could not contain them platelets in the marine ice at depth appeared to be as large as 1 2 cm in diameter particles were visible in the borehole meltwater probably marine and mineral particles liberated by the drill but their distribution varied with depth

gully annealing by aeolian sediment field and remote sensing investigation of aeolian hillslope fluvial interactions colorado river corridor arizona usa abstract processes contributing to development of ephemeral gully channels are of great importance to landscapes worldwide and particularly in dryland regions where soil loss and land degradation from gully erosion pose long term land management problems whereas gully formation has been relatively well studied much less is known of the processes that anneal gullies and impede their growth this study of gully annealing by aeolian sediment spanning 95 km along the colorado river corridor in glen marble and grand canyon arizona usa employed field and remote sensing observations including digital topographic modelling results indicate that aeolian sediment activity can be locally effective at counteracting gully erosion gullies are less prevalent in areas where surficial sediment undergoes active aeolian transport and have a greater tendency to terminate in active aeolian sand although not common examples exist in the record of historical imagery of gullies that underwent infilling by aeolian sediment in past decades and evidently were effectively annealed we thus provide new evidence for a potentially important interaction of aeolian hillslope fluvial processes which could affect dryland regions substantially in ways not widely recognized moreover because the biologic soil crust plays an important role in determining aeolian sand activity and so in turn the extent of gully development this study highlights a critical role of geomorphic ecologic interactions in determining arid landscape evolution

relations between rainfall runoff induced erosion and aeolian deposition at archaeological sites in a semi arid dam controlled river corridor process dynamics in fluvial based dryland environments are highly complex with fluvial aeolian and alluvial processes all contributing to landscape change when anthropogenic activities such as dam building affect fluvial processes the complexity in local response can be further increased by flood and sediment limiting flows understanding these complexities is key to predicting landscape behavior in drylands and has important scientific and management implications including for studies related to paleoclimatology landscape ecology evolution and archaeological site context and preservation here we use multi temporal lidar surveys local weather data and geomorphological observations to identify trends in site change throughout the 446 km long semi arid colorado river corridor in grand canyon arizona usa where archaeological site degradation related to the effects of upstream dam operation is a concern using several site case studies we show the range of landscape responses that might be expected from concomitant occurrence of dam controlled fluvial sand bar deposition aeolian sand transport and rainfall induced erosion empirical rainfall erosion threshold analyses coupled with a numerical rainfall runoff soil erosion model indicate that infiltration excess overland flow and gullying govern large scale centimeter to decimeter scale landscape changes but that aeolian deposition can in some cases mitigate gully erosion whereas threshold analyses identify the normalized rainfall intensity defined as the ratio of rainfall intensity to hydraulic conductivity as the primary factor governing hydrologic driven erosion assessment of false positives and false negatives in the dataset highlight topographic slope as the next most important parameter governing site response analysis of 4 years of high resolution four minute weather data and 75 years of low resolution daily climate records indicates that dryland erosion is dependent on short term storm driven rainfall intensity rather than cumulative rainfall and that erosion can occur outside of wet seasons and even wet years these results can apply to other similar semi arid landscapes where process complexity may not be fully understood copyright 2015 john wiley sons ltd

geomorphic threshold conditions for ephemeral gully incision abstract ephemeral gully erosion in cultivated land is an important source of sediment that is frequently being overlooked and not accounted for in soil erosion studies furthermore little is known about the factors controlling ephemeral gully erosion in this paper the available information on the initiation and location of ephemeral gullies is summarised focusing on the relationship between the upslope drainage area a and the critical slope gradient s cr for ephemeral gully initiation by plotting the non linear relationship between critical slope gradient measured immediately upstream of the incision head and drainage area at the incision head for gullied sites it was possible to draw a straight line on log log paper through the lower most points for each of the available datasets representing a critical slope area relationship for incision consequently below this line no incision occurs this line or critical relation can also be written as a power function between critical slope and area although many factors vary between the different datasets the exponent of the drainage area relationship 0 4 showed very little variation the observed critical slope area relationship can be related to a simple model of channel initiation by overland flow furthermore this relationship can be used to identify potentially unstable sites where control measures should be undertaken

catchment scale analysis of the effect of topography tillage direction and unpaved roads on ephemeral gully incision high resolution airphotographs and digital elevation models dems were used to study areas prone to gully incision in a small agricultural catchment in northern israel data are analyzed to understand better the integrated effect of environmental and human factors on gully incision the effect of flow accumulation slope unpaved roads density and tillage direction on gully width and length is found discontinuous and topography in the form of the topographic threshold was found to be the most dominant factor on gully incision in the catchment scale the topographic threshold coefficients observed in yehezkel catchment were lower than those found in previous studies due to the effect of large rainfall events during the early winter season when the soil in fields is still unprotected by crops in addition a survey of previous studies shows that in cultivated areas there is relationship between the topographic threshold coefficients a multiplier and b exponent the influence of unpaved roads and tillage direction on gully incision was also found significant and consistent during the two seasons observed while the non parametric test showed that it added to the prediction in 15 of the gullies incision points density plot show that of the two parameters the unpaved roads were found to have greater influence on gully head location than tillage direction studying the phenomenon on the catchment scale allows better understanding of the effect of spatial variation on gully head location copyright 2009 john wiley sons ltd

properties of a marine ice layer under the amery ice shelf east antarctica the amery ice shelf east antarctica undergoes high basal melt rates near the southern limit of its grounding line where 80 of the ice melts within 240 km of becoming afloat a considerable portion of this later refreezes downstream as marine ice this produces a marine ice layer up to 200 m thick in the northwest sector of the ice shelf concentrated in a pair of longitudinal bands that extend some 200 km all the way to the calving front we drilled through the eastern marine ice band at two locations 70 km apart on the same flowline we determine an average accretion rate of marine ice of 1 1 0 2 m a 1 at a reference density of 920 kg m 3 between borehole sites and infer a similar average rate of 1 3 0 2 m a 1 upstream the deeper marine ice was permeable enough that a hydraulic connection was made whilst the drill was still 70 100 m above the ice shelf base below this marine close off depth borehole video imagery showed permeable ice with water filled cavities and individual ice platelets fused together while the upper marine ice was impermeable with small brine cell inclusions we infer that the uppermost portion of the permeable ice becomes impermeable with the passage of time and as more marine ice is accreted on the base of the shelf we estimate an average closure rate of 0 3 m a 1 between the borehole sites upstream the average closure rate is faster at 0 9 m a 1 we estimate an average porosity of the total marine ice layer of 14 20 such that the deeper ice must have even higher values high permeability implies that sea water can move relatively freely through the material and we propose that where such marine ice exists this renders deep parts of the ice shelf particularly vulnerable to changes in ocean properties

equations for determining the gas and brine volumes in sea ice samples on a etabli des equations utilisables pour determiner le taux de gaz inclus dans la glace de mer a partir de mesures de densite de salinite et de temperature dans des gammes de temperatures entre 2 c et 30 c inversement ces relations peuvent etre utilisees pour calculer la densite de la glace de mer en fonction de la temperature et de la salinite en prenant en compte a la fois la presence de gaz et de cristaux de sel dans la glace on donne aussi des equations qui permettent le calcul du volume de gaz et d eau salee dans la glace a des temperatures autres que celle pour laquelle la densite a ete determinee

assessing grain size correspondence between flow and deposits of controlled floods in the colorado river u s a abstract flood deposited sediment has been used to decipher environmental parameters such as variability in watershed sediment supply paleoflood hydrology and channel morphology it is not well known however how accurately the deposits reflect sedimentary processes within the flow and hence what sampling intensity is needed to decipher records of recent or long past conditions we examine these problems using deposits from dam regulated floods in the colorado river corridor through marble canyon grand canyon arizona u s a in which steady peaked floods represent a simple end member case for these simple floods most deposits show inverse grading that reflects coarsening suspended sediment a result of fine sediment supply limitation but there is enough eddy scale variability that some profiles show normal grading that did not reflect grain size evolution in the flow as a whole to infer systemwide grain size evolution in modern or ancient depositional systems requires sampling enough deposit profiles that the standard error of the mean of grain size change measurements becomes small relative to the magnitude of observed changes for simple steady peaked floods 5 10 profiles or fewer may suffice to characterize grain size trends robustly but many more samples may be needed from deposits with greater variability in their grain size evolution

an ice shelf mechanism for heinrich layer production the effect of an ice shelf in the labrador sea on ice rafted sediment delivery to the glacial north atlantic is investigated using a finite element numerical model of ice shelf flow discharge into the shelf from hudson strait creates a thick central core extending downstream into the shelf that is flanked by relatively thin ice melting at the base of the deep keel would produce cool fresh water which would rise to refreeze along the keel s flanks debris deposited by melting deep central ice could create the heinrich layers observed in the labrador sea while the debris rich ice protected by basal freezing on the flanks of the thick ice plume could transport sediment over large distances heinrich layers would be produced solely by external climate forcing without changes in ice sheet flow the mechanism is plausible if the lifetime of the ice shelf is about 1000 years

effects of river regulation on aeolian landscapes colorado river southwestern usa 1 connectivity between fluvial and aeolian sedimentary systems plays an important role in the physical and biological environment of dryland regions this study examines the coupling between fluvial sand deposits and aeolian dune fields in bedrock canyons of the arid to semiarid colorado river corridor southwestern usa by quantifying significant differences between aeolian landscapes with and without modern fluvial sediment sources this work demonstrates for the first time that the flow and sediment limiting effects of dam operations affect sedimentary processes and ecosystems in aeolian landscapes above the fluvial high water line dune fields decoupled from fluvial sand supply have more ground cover biologic crust and vegetation and less aeolian sand transport than do dune fields that remain coupled to modern fluvial sand supply the proportion of active aeolian sand area also is substantially lower in a heavily regulated river reach marble grand canyon arizona than in a much less regulated reach with otherwise similar environmental conditions cataract canyon utah the interconnections shown here among river flow and sediment aeolian sand transport and biologic communities in aeolian dunes demonstrate a newly recognized means by which anthropogenic influence alters dryland environments because fluvial aeolian coupling is common globally it is likely that similar sediment transport connectivity and interaction with upland ecosystems are important in other dryland regions to a greater degree than has been recognized previously

simulation of subice shelf melt rates in a general circulation model velocity dependent transfer and the role of friction two parameterizations of turbulent boundary layer processes at the interface between an ice shelf and the ocean beneath are investigated in terms of their impact on simulated melt rates and feedbacks the parameterizations differ in the transfer coefficients for heat and freshwater fluxes in their simplest form they are assumed constant and hence are independent of the velocity of ocean currents at the ice shelf base an augmented melt rate parameterization accounts for frictional turbulence via transfer coefficients that do depend on boundary layer current velocities via a drag law in simulations with both parameterizations for idealized as well as realistic cavity geometries under pine island ice shelf west antarctica significant differences in melt rate patterns between the velocity independent and velocity dependent formulations are found while patterns are strongly correlated to those of thermal forcing for velocity independent transfer coefficients melting in the case of velocity dependent coefficients is collocated with regions of high boundary layer currents in particular where rapid plume outflow occurs both positive and negative feedbacks between melt rates boundary layer temperature velocities and buoyancy fluxes are identified melt rates are found to increase with increasing drag coefficient inline image in agreement with plume model simulations but optimal values of cd inferred from plume models are not easily transferable uncertainties therefore remain both regarding simulated melt rate spatial distributions and magnitudes

ocean circulation beneath the ronne ice shelf the intimate thermal contact between the base of antarctic ice shelves and the underlying ocean enables changes in climate to have a rapid impact on the outflow of ice from the interior of antarctica1 2 furthermore water modified by passage under ice shelves particularly in the weddell sea is believed to be an important constituent of antarctic bottom water3 a water mass that can be observed as far north as 50 n in the deep oceans4 antarctic bottom water is both cold and oxygen rich and plays an important part in the cooling and ventilation of the world s oceans because of the difficulty in gaining access the oceano graphic regime beneath ice shelves is very poorly sampled5 by successfully drilling through the ice however we were able to obtain oceanographic data from beneath the largest antarctic ice shelf the ronne filchner ice shelf in the southern weddell sea we find that our data agree well with the predictions of a relatively simple oceanographic plume model of sub ice shelf circulation6 this model can therefore be used with some confidence to investigate the links between climate changes ice shelf melting and bottom water production

marine ice formation in a suture zone on the larsen c ice shelf and its influence on ice shelf dynamics 1 antarctic ice shelves are fed primarily by the glaciers flowing into them downstream of promontories separating these glaciers supercooled water can rise and freeze into suture zones leading to the accretion of marine ice marine ice bodies have been found in several antarctic ice shelves but little is known about their detailed geometry rate of accretion or influence on ice dynamics in this study we investigate marine ice in a suture zone downstream of the joerg peninsula in the southern part of the larsen c ice shelf antarctic peninsula we present ground penetrating radar data from which we infer the base of the meteoric ice and in combination with gps data and assuming hydrostatic equilibrium estimate marine ice thickness within a suture zone we show that the joerg peninsula suture zone contains marine ice layer which is increasing in thickness along flow from 140 m to 180 m over 20 km implying an average basal accretion rate of 0 5 m a 1 in our study area we examined the impact of this inferred marine ice on ice shelf dynamics by modeling the suture zone within an ice flow model the results which replicate observed surface velocities and strain rates show that the warmer and thus softer ice of the suture zone serves to channel shear deformation this enables decoupling of neighboring flow units with different flow velocities while maintaining the structural integrity of the ice shelf

gullying and erosion control at archaeological sites in grand canyon arizona gully erosion of cultural sites in grand canyon national park is an urgent management problem that has intensified in recent decades potentially related to the effects of glen canyon dam we studied 25 gullies at nine sites in grand canyon over the 2002 monsoon erosion season to better understand the geomorphology of the gully erosion and the effectiveness of erosion control structures ecs installed by the park under the direction of the zuni conservation program field results indicate that hortonian overland flow leads to concentrated flow in gullies and erosion focused at knickpoints along channels as well as at gully heads though groundcover type soil shear strength and permeability vary systemat ically across catchments gradient and to a lesser degree contributing drainage area seem to be the first order controls on gully extent location of new knickpoints and ecs damage the installed ecs do reduce erosion relative to reaches without them and initial data suggest woody checkdams are preferable to rock linings but maintenance is essential because damaged structures can exacerbate erosion topographic data from intensive field surveys and detailed photogrammetry provide slope contributing area data for gully heads that have a trend consistent with previous empirical and theoretical formulations from a variety of landscapes the same scaling holds below gully heads for knickpoint and ecs topographic data with threshold coefficients the lowest for gully heads slightly higher for knickpoints and notably higher for damaged ecs these topographic thresholds were used with 10 cm digital elevation models to create simple predictive models for gully extent and structure damage the model predictions accounted for the observed gullies but there are also many false positives purely topographical models are probably inadequate at this scale and application but models that also parameterize the variable soil properties across sites would be useful for predicting erosion problems and ecs failure copyright 2006 john wiley sons ltd

a process based model for channel degradation application to ephemeral gully erosion abstract ephemeral gullies are important features of soil erosion yielding large amounts of sediment and dissecting the landscape in spite of their agricultural and environmental importance gully erosion is not usually considered in routine schemes for predicting soil loss an event oriented process based model for stream degradation has been adapted for the description of ephemeral gully erosion the initial channel is considered as prismatic formed in a uniform soil profile defined by the watershed swale and receives upstream and lateral runoff the resulting gradually varied flow is computed by the standard step method whereas the erosion assumed transport limited is attributed first to bed erosion and the remainder is dedicated to the bank erosion at the end of each time step channel shape is redefined accordingly with the computed erosion in each reach a sensitivity analysis revealed that particle density particle size and roughness coefficient are the key parameters a calibration of the model simulating the incision of an ephemeral gully in a small watershed with highly erodible soil allowed a proper estimation of soil loss and the gully cross section shapes along the channel with realistic values of the calibrated parameters

discontinuous ephemeral streams abstract many ephemeral streams in western north america flowed over smooth valley floors before transformation from shallow discontinuous channels into deep arroyos these inherently unstable streams of semiarid regions are sensitive to short term climatic changes and to human impacts because hillslopes supply abundant sediment to infrequent large streamflow events discontinuous ephemeral streams appear to be constantly changing as they alternate between two primary modes of operation either aggradation or degradation may become dominant attainment of equilibrium conditions is brief disequilibrium is promoted by channel entrenchment that causes the fall of local base level and by deposition of channel fans that causes the rise of local base level these opposing base level processes in adjacent reaches are maintained by self enhancing feedback mechanisms the threshold between erosion and deposition is crossed when aggradational or degradational reaches shift upstream or downstream extension of entrenched reaches into channel fans tends to create continuous arroyos upvalley migration of fan apexes tends to create depositional valley floors with few stream channels less than 100 years is required for arroyo cutting but more than 500 years is required for complete aggradation of entrenched stream channels and valley floors discontinuous ephemeral streams have a repetitive sequence of streamflow characteristics that is as distinctive as sequences of meander bends or braided gravel bars in perennial rivers the sequence changes from degradation to aggradation headcuts concentrate sheetflow a single trunk channel conveys flow to the apex of a channel fan braided distributary channels end in an area of diverging sheetflow and converging sheetflow drains to headcuts the sequence is repeated at intervals ranging from 15 m for small streams to more than 10 km for large streams lithologic controls on the response of discontinuous ephemeral streams include 1 amount and size of sediment yielded from hillslopes 2 infiltration capacity of valley floor alluvium that influences both the unit stream power available for upstream headcut migration and the attenuation of flashy streamflow events by riparian vegetation in sheetflow reaches and 3 cohesiveness of alluvium which affects headcut and streambank morphology and rates of arroyo extension downcutting and widening initiation of arroyo cutting may be too complex to be attributed to a single cause such as change in mean annual precipitation or grazing by livestock but is most likely associated with a decrease in density of protective plant cover on hillsides and along valley floors relatively larger unit stream power makes downstream reaches more susceptible to initial entrenchment during floods than headwaters reaches and favors persistent arroyos entrenchment continues until an equilibrium longitudinal profile is briefly attained then channel widening occurs streambanks are undercut and aggradation begins

a sediment transport model for incision of gullies on steep topography 1 we have conducted surveys of gullies that developed in a small steep watershed in the idaho batholith after a severe wildfire followed by intense precipitation we measured gully length and cross sections to estimate the volumes of sediment loss due to gully formation these volume estimates are assumed to provide an estimate of sediment transport capacity at each survey cross section from the single gully forming thunderstorm sediment transport models commonly relate transport capacity to overland flow shear stress which is related to runoff rate slope and drainage area we have estimated the runoff rate and duration associated with the gully forming event and used the sediment volume measurements to calibrate a general physically based sediment transport equation in this steep high shear stress environment we find that a shear stress exponent of 3 corresponding to drainage area and slope exponents of m 2 1 and n 2 25 match our data this shear stress exponent of 3 is approximately 2 times higher than those for bed load transport in alluvial rivers but is in the range of shear stress exponents derived from flume experiments on steep slopes and with total load equations the concavity index of the gully profiles obtained theoretically from the area and slope exponents of the sediment transport equation c m 1 n agrees well with the observed profile concavity of the gullies our results although preliminary because of the uncertainty associated with the sediment volume estimates suggest that for steep hillslopes such as those in our study area a greater nonlinearity in the sediment transport function exists than that assumed in some existing hillslope erosion models which calculate sediment transport capacity using the bed load equations developed for rivers

crystal size variations in eemian age ice from the grip ice core central greenland continuous measurements of ice crystal size have been carried out on an 80 m sequence between 2790 and 2870 m depth in the grip ice core from central greenland the ice in this interval is at present considered to originate from the eemian interglacial period the record reveals that the crystal size in ice older than 100 000 yr is highly dependent on climatic conditions at the time of snowfall this dependence shows up as a strong correlation between delta 18 o values and crystal size throughout the eemian as well as a negative correlation between crystal size and several soluble and insoluble impurities although high resolution impurity records are available from selected parts of the eemian ice the study is not conclusive on which impurities are most effective in slowing grain growth it is shown that the normal grain growth process commonly observed in the upper few hundred metres of polar ice sheets does not yield grain sizes compatible with observed ones at this depth in the ice sheet even in those parts of the eemian ice where impurity drag effects are not present polygonization of crystals within the ice sheet and the nucleation and rapid growth of new grains at relatively high temperatures in the lowest part probably play an important role in producing the observed grain size variations the relevance of possible flow disturbances of the grip eemian climatic record for the results presented is discussed briefly

importance of slope gradient and contributing area for optimal prediction of the initiation and trajectory of ephemeral gullies abstract ephemeral gully erosion rates are controlled by various factors out of which topography plays an important role this study investigates the possibility to predict the location of ephemeral gullies using two topographic attributes only i e local slope gradient s and upslope contributing area per unit length of contour a s an inverse relationship between s and a s in which the relative importance of the area exponent was varied was evaluated as to its performance in predicting the location of ephemeral gullies for three intensively cultivated catchments in the belgian loess belt this model is easily applicable as it only uses information on topography which can be derived from a digital elevation model and land use predicted locations of ephemeral gullies were confronted with the locations recorded over 5 years of field observations for all catchments a high relative area exponent relative to the slope exponent was required to yield good results the optimal relative area exponent ranged from 0 7 to 1 5 however the quality of the prediction did not vary significantly when changing the exponent within this range an average relative area exponent of ca 1 is appropriate for all three study areas and may therefore be proposed for similar areas under similar conditions a striking discrepancy was found between the high relative area exponent required to predict optimally the trajectory of the gullies and the low relative area exponent 0 2 required to identify the spots in the landscape where ephemeral gullies begin this indicates that spots in the landscape where gullies start are more controlled by slope gradient while the presence of concavities control the trajectory of the gullies until the slope gradient is too low and sediment deposition starts no relationship was found between the frequency of the occurrence of ephemeral gullies and the percentage of predicted gully pixels for each frequency class this indicates that frequently occurring gullies at the same location are not easier to predict than accidental gullies

ice composition evidence of marine ice transfer along the bottom of a small antarctic ice shelf the existence of marine ice transfer along the underside of the hell s gate ice shelf victoria land is indicated by an isotopic and chemical study of ice cores because of top surface ablation the marine ice formed at the ice shelf ocean interface ultimately appears at shelf surface a succession of congelation platelet and frazil ice is shown to occur the combined study of stable isotope composition and of the sodium content of these different ice types proves to be a valuable tool for specifying the ice shelf ocean interactions in this area two different freezing zones separated by a melting rone exist the parent water for the frazil ice is meltwater from congelation ice which appears in the upstream zone

grain growth in polar ice i theory many observations regarding grain growth in ice sheets are glaciologically interesting but imperfectly understood here we develop the theory of grain growth in ice that is not deforming rapidly and in the succeeding paper we use this theory to explain observations from glacial ice in the absence of significant strain energy the driving force for grain growth arises from grain boundary curvature grain growth is slowed by the interaction of grain boundaries with extrinsic materials microparticles bubbles and dissolved impurities if the driving force for growth is not large enough to cause boundaries to separate from an extrinsic material then the grain boundary velocity is determined by the velocity characteristic of the extrinsic material low velocity regime if the driving force is large enough to cause separation then boundaries migrate more rapidly than the extrinsic material high velocity regime but the net driving force is reduced through transient pinning by the extrinsic material polar ice is typically in the low velocity regime relative to dissolved impurities and the high velocity regime relative to microparticles and bubbles cross sectional area of grains is predicted to increase linearly with time under most but not all circumstances

a probabilistic approach for channel initiation 1 the channel head represents an important transition point from hillslope to fluvial processes there is a nonlinear threshold transition across the channel head with sediment transport much larger in channels than on hillslopes deterministic specific catchment area a thresholds for channel initiation sometimes dependent on slope s in the form of as c have been suggested in this paper the channel initiation problem is viewed probabilistically with a spatially variable probability of channel initiation that depends on slope specific catchment area and the probability distributions of median grain size surface roughness and excess rainfall rate the channel initiation threshold c is cast as a random variable to characterize the variability of as at channel heads using field measurements from the idaho batholith we show that median grain size measurements at each channel head explain a significant part of the observed variability of as we then characterize the variability of model inputs median grain size roughness and excess rainfall using probability distributions and show that the probability distribution of area slope threshold derived from these inputs matches the probability distribution of area slope thresholds measured at field channel head locations a gamma probability distribution provides a reasonable match to the distributions of area slope threshold measured and modeled at channel heads in our study area and in other published channel head data

numerical simulations of the profile properties of undeformed first year sea ice during the growth season a simulation scheme is developed that estimates salinity profiles for first year sea ice during the growth season as a function of the growth history of the ice the model considers the dependence of the initial ice salinity on ice growth velocity and sea water salinity and also the subsequent drainage of brine from the ice the equation for ice growth assumes a linear temperature profile within the ice and is driven by surface heat balance equations that are based on smoothed climatic data for the central arctic basin the estimated salinity profiles are in good agreement with natural profiles although temperature and salinity profiles depend upon the time of the year when ice growth is initiated the brine volume profiles which they specify are essentially a unique function of ice thickness a conclusion that holds even when the insulative effects of snow are considered the temperature and brine volume profiles are then utilized to calculate the ice strength and elastic modulus profiles which in turn specify the composite mechanical properties of the ice sheets through the application of a theory developed by assur 1967 significant differences which are largest for thin ice sheets are observed between ice sheet properties as calculated using composite plate theory and properties calculated from uniform plate theory and average ice properties these results provide a justification for the practice common within the ice modeling community of parameterizing the mechanical behavior of pack ice on the basis of the ice thickness distribution

predicting the topographic limits to a gully network using a digital terrain model and process thresholds a digital terrain model is used with process thresholds to predict the extent of a stable gully network in a 5 km2 catchment of the southeastern highlands of australia the model developed by dietnch et al 1992 1993 predicts the topographic controls on channel networks and interprets these in terms of a critical shear stress for channel incision c applied by saturation overland flow we adapt the model slightly to compare the shear stress applied by hortonian overland flow to that applied by saturation overland flow the limits to gully erosion in the catchment are controlled strongly by a topographic threshold that has an inverse relationship between upslope catchment area and local gradient the topographic threshold for channel incision is reproduced using a simple model of hortonian overland flow and a c appropriate for incision into a degraded grass surface c 245 dyn cm2 this is consistent with historical evidence for the timing of gully erosion the study confirms a strong topographic control on the extent of the channel network in a catchment significantly different from the western north america catchments where the topographic threshold was first demonstrated despite its simplicity the model for incision by overland flow appears capable of distinguishing the hydrological processes responsible for channel incision when these are reflected in the relationship between channel network and landscape morphology the model requires relatively simple inputs suggesting it may be useful for mapping gully erosion hazard in actively eroding catchments

fabrics in polar ice sheets development and prediction fabrics in polar ice sheets provide a record of deformational history and control the viscosity of ice during further deformation they affect geophysical sensing of ice sheets and provide an accessible analogue to fabric development during deformation of other geological and engineering materials a new synthesis of experimental and theoretical results shows that c axis fabrics are quantitatively related to cumulative strain and stress state in ice sheets for the full range of likely flow patterns basal shear divergent flow and parallel flow cause c axes to rotate toward the vertical axis whereas convergent flow causes c axes to rotate toward a vertical plane transverse to flow

experimental fracture and mechanical properties of antarctic ice preliminary results an experimental study of the fracture mechanics and rheology of ice from the ronne ice shelf antarctica is currently being undertaken the apparent critical stress intensity factor or apparent fracture toughness k q for crack propagation has been measured using a three point bend method for inducing crack growth perpendicular to the axis of cylindrical ice core specimens tensile crack nucleation under applied uniaxial compressive stress has also been evaluated both methods have allowed a profile of ice elastic and fracture properties with depth through the ice shelf to be constructed preliminary results indicate that the measured elastic modulus increases with depth through the firn and upper meteoric ice before reaching a constant value in the deeper dense meteoric and basal marine ice the resistance to fracture as measured by changes in apparent fracture toughness and crack nucleation stress increases with depth right through the firn and meteoric ice layers a simple fracture mechanics model applied to the ronne ice shelf indicates that crevasses form from small surface cracks less than 40 cm deep which quickly grow to depths of 40 60m and then remain stable

first access to the ocean beneath ekstromisen antarctica by means of hot water drilling a hot water drill developed during the past 2 years at alfred wegener institute was used to penetrate the ekstrom ice shelf several times near the georg von neumayer station the drilling operation was successful and the initial large diameter 35 cm allowed easy access to the ocean one hole was used to install an ultrasonic echo sounder which recorded the ablation at the ice shelf bottom continuously another hole was used for emplacement of a thermistor string throughout the 237 m thick ice shelf for ice temperature measurements several ctd profiles in the 175 m deepwater column and the analysis of water samples provided valuable data for the understanding of ice shelf ocean interactions

observed and modelled distributions of channel and gully heads with examples from se spain and belgium abstract in soil erosion by overland flow sediment discharge is driven by discharge and gradient against the resistance of the soil and vegetation the form of the relationship which is taken as a sum of power law terms may then be inferred from several data sources 1 ephemeral gully head location 2 permanent channel head location 3 slope profile form 4 consistency with evolutionary models and 5 erosion plot data the paper focuses on 1 to 4 in order to understand how far these data constrain the form of the dependence on discharge and gradient field data from mapping a set of ephemeral gully heads after storms show a weak inverse relationship between channel head gradient and catchment area which flattens towards an upper threshold gradient of about 25 these relationships are compared with those theoretically obtained for a range of sediment transport laws which have been previously used and which are analysed to forecast both overall slope profile form and the location of stream heads the theory predicts that the location of ephemeral gully heads produced in a single storm will differ from the long term average stream head location there is a fairly good relationship between theoretical and observed locations associated with the form of the sediment transport law for a single event the location depends on storm magnitude which cannot be well defined from the data sets creating a spread of data with an envelope curve defined for the largest events the empirically acceptable sediment transport equations have also been used to analyse families of slope profiles in a small area and used within a landscape evolution model to generate landscapes which are compatible with those observed in the field and provide confirmation of the long term viability of the expressions obtained to forecast the direction style and rates of landscape evolution in response to land use or climate drivers

further evidence of changes in the hydrological regime of the river paraguay part of a wider phenomenon of climate change abstract analysis of flow measured at 20 sites rainfall measured at 36 sites and a 95 year record of water level at one site in the basin of the river paraguay area 1095 10 3 km 2 showed that the flow regime during the approximate period 1960 1970 differed substantially from the flow regime both before and after the long record of ladario water levels suggested that the changes between one period and the next were considerably abrupt and that the periods differed not only in terms of mean water levels but also in terms of the year to year correlational structure within the record despite the fragmentary nature of rainfall records from 36 sites an explanation for the increased flows since 1970 was found in the increases of rainfall as assessed in terms of the frequency of annual rainfalls more than the long term mean rainfall there was some degree of consistency in the change of rainfall pattern across the paraguay basin as a whole a detailed examination of daily rainfall characteristics at two gauges where records were fairly complete showed that during the 1960 1970 period when river flows were low dry spells were more persistent and on days when rain did fall the amounts of rain were generally smaller the results obtained were compared with results obtained by other researchers using flow records from the rivers paraguay parana negro and uruguay in the la plata basin and rainfall records from other parts of south america there is now strong evidence of changes in the runoff regime of the la plata basin during the last 40 years not all of which can be attributed to land use change as there is complementary evidence of change in rainfall regime the results were also compared with findings from the congo basin which appears to exhibit changes in flow regime that are a mirror image of those found for the paraguay at ladario

tributary debris fans and the late holocene alluvial chronology of the colorado river eastern grand canyon arizona bouldery debris fans and sandy alluvial terraces of the colorado river developed contemporaneously during the late holocene at the mouths of nine major tributaries in eastern grand canyon the age of the debris fans and alluvial terraces contributes to understanding river hydraulics and to the history of human activity along the river which has been concentrated on these surfaces for at least two to three millennia poorly sorted coarse grained debris flow deposits of several ages are interbedded with overlie or are overlapped by three terrace forming alluviums the alluvial deposits are of three age groups the striped alluvium deposited from before 770 b c to about a d 300 the alluvium of pueblo ii age deposited from about a d 700 to december 1900 and the alluvium of the upper mesquite terrace deposited from about a d 1400 to 1880 two elements define the geomorphology of a typical debris fan the large inactive surface of the fan and a smaller entrenched active debris flow channel and fan that is about one sixth the area of the inactive fan the inactive fan is segmented into at least three surfaces with distinctive weathering characteristics these surfaces are conformable with underlying debris flow deposits that date from before 770 b c to around a d 660 a d 660 to before a d 1200 and from a d 1200 to slightly before 1890 respectively based on late 19th century photographs radiocarbon and archaeologic dating of the three stratigraphically related alluviums and radiocarbon dating of fine grained debris flow deposits these debris flows aggraded the fans in at least three stages beginning about 2 8 ka if not earlier in the late holocene several main stem floods eroded the margin of the segmented fans reducing fan symmetry the entrenched active debris flow channels contain deposits 100 yr old which form debris fans at the mouth of the channel adjacent to the river early and middle holocene debris flow and alluvial deposits have not been recognized as they were evidently not preserved adjacent to the river or are buried by younger deposits

the conservation of the shahr e zohak archaeological site central afghanistan geomorphological processes and ecosystem based mitigation abstract the archaeological remains of shahr e zohak are part of the bamiyan valley which has been recognized by unesco as world heritage and is famous for hosting the main heritage of the buddhist culture in afghanistan the site comprises the remains of the zohak fortress which is placed on a steep hill at the confluence of the bamiyan and kalu rivers the fortress is protected by ramparts built along the steep cliffs bounding the site which are equipped with several watchtowers the citadel is protected by three more orders of walls and is located on the topmost part of the hill all the structures are made of mudbricks placed on top of stony foundations due to the prolonged exposure to weathering the lack of conservation measures and the misuse during war periods many constructions collapsed or are prone to collapse a new topography 1 m contour lines of the site was produced using drone derived 3d photogrammetry combined with gps data then a detailed geomorphological survey of the whole site was carried out in order to identify the main geomorphic processes acting on the land surface and structures gis analysis allowed defining the internal drainage system of the studied area the site is affected by incised erosional phenomena on the eastern side while the hilltop is mainly hit by diffuse erosion and soil mobilization during snowmelt monument deterioration is coupled with the lack of an adequate drainage system to collect runoff ramparts located on the steep hillslopes are severely affected by gully erosion and siphoning which cause depressions infilled by eroded and weathered building material the access path is locally eroded or buried by debris cones the western margin of the plateau has been rapidly retreating due to collapses while the citadel is in danger due to diffuse or gully erosional processes developed on all its sides a mitigation strategy with low environmental impact ecosystem based approach is proposed in order to adopt sustainable systemic and cost effective tools for soil conservation in order to improve the environmental resilience of the site

ice ocean interaction on ronne ice shelf antarctica detailed glaciological studies have been completed at 28 sites lying on an approximate flow line extending 760 km across ronne ice shelf parameters measured at each location include ice velocity thickness principal strain rates surface elevation temperature and accumulation rate the data have been used in a steady state model to derive the basal mass flux and the temperature profile with depth at each site from the principles of mass and energy conservation these calculations indicate basal melting in excess of 1 m yr 1 over the first 100 km of the flow line downstream of the grounding line where the ice shelf is between 1200 m and 1600 m thick the maximum melt rates in this region occur near the inland margin and exceed 4 m yr 1 melting continues at a lesser rate over the next 200 km before freezing commences freezing then dominates up to the final 100 km before the ice front resulting in the accumulation of a layer of basal sea ice up to 50 m thick this is rapidly removed as melt rates increase to over 6 m yr 1 at the ice front the cumulative effect of this pattern of basal accumulation and ablation is the wastage by melting of 85 of the mass discharged across the grounding line before it reaches the ice front supporting evidence for a layer of saline ice underlying the ice shelf is provided by the strength of basal radar reflections observed during radio echo sounding of ice thickness in the region where mass balance calculations suggest an accretion of basal sea ice reflection coefficients are consistently low ranging from 6 db to 38 db most of these weak reflections are believed to originate from the true base of the ice shelf the additional energy loss being the result of increased attenuation of the radar signal within the saline layer the derived pattern of basal melting and freezing is consistent with a simple model of sub ice shelf oceanic circulation involving a deep thermohaline convection cell dense saline water which is formed over the continental shelf during winter when the sea surface freezes drains into the deepest parts of the sub ice shelf cavity at the inland margin where this water mass comes into contact with the ice shelf its temperature is 1 c above the local pressure freezing point melting of ice results producing a buoyant outflow of cold relatively fresh water along the ice shelf base basal freezing occurs towards the ice front where the ascending water becomes supercooled this circulation has important implications for the production of antarctic bottom water and for the response of the ice shelf to driving stresses through the temperature dependent viscosity of ice

experiments on headcut growth and migration in concentrated flows typical of upland areas experiments were conducted to examine soil erosion by headcut development and migration in concentrated flows typical of upland areas in a laboratory channel packed sandy loam to sandy clay loam soil beds with preformed headcuts were subjected to simulated rain followed by overland flow the rainfall produced a well developed surface seal that minimized surface soil detachment during overland flow soil erosion occurred exclusively at the headcut and after a short period of time a steady state condition was reached where the headcut migrated at a constant rate the scour hole morphology remained unchanged and sediment yield remained constant a fourfold increase in flow discharge resulted in larger scour holes yet aspect ratio was conserved a sediment bed was deposited downstream of the migrating headcut and its slope depended weakly on flow discharge

large woody debris physical process and riparian forest development in montane river networks of the pacific northwest abstract we present a conceptual biogeomorphic model of riparian forest development in montane river networks the role of physical process in driving the structure composition and spatial distribution of riparian forests is examined we classify the drainage network into disturbance process based segments including 1 debris flow and avalanche channels 2 fluvial and debris flow channels and 3 fluvial channels riparian forests are shown to be significant in the development of channel morphology through the stabilization of active floodplains and as sources of large woody debris olwd lwd is operationally defined as wood 0 1 m diameter and 1 m length lwd plays a key role in the development of montane riparian forests lwd deposited in the active channel and floodplain provides sites for vegetation colonization forest island growth and coalescence and forest floodplain development riparian forest patterns parallel the distribution of hillslope and fluvial processes through the network riparian forest structure composition and spatial distribution through the network are driven by the major disturbance processes including 1 avalanches 2 debris flows and 3 flooding riparian forest patterns also reflect the action of lwd in the organization and development of forested floodplains in gravel bedded montane river networks the focus of our examples are montane river networks of the pacific northwest usa

predicting the susceptibility to gully initiation in data poor regions permanent gullies are common features in many landscapes and quite often they represent the dominant soil erosion process once a gully has initiated field evidence shows that gully channel formation and headcut migration rapidly occur in order to prevent the undesired effects of gullying there is a need to predict the places where new gullies might initiate from detailed field measurements studies have demonstrated strong inverse relationships between slope gradient of the soil surface s and drainage area a at the point of channel initiation across catchments in different climatic and morphological environments such slope area thresholds s a can be used to predict locations in the landscape where gullies might initiate however acquiring s a requires detailed field investigations and accurate high resolution digital elevation data which are usually difficult to acquire to circumvent this issue we propose a two step method that uses published s a thresholds and a logistic regression analysis lr s a thresholds from the literature are used as proxies of field measurement the method is calibrated and validated on a watershed close to the town of algiers northern algeria where gully erosion affects most of the slopes the gullies extend up to several kilometres in length and cover 16 of the study area first we reconstruct the initiation areas of the existing gullies by applying s a thresholds for similar environments then using the initiation area map as the dependent variable with combinations of topographic and lithological predictor variables we calibrate several lr models it provides relevant results in terms of statistical reliability prediction performance and geomorphological significance this method using s a thresholds with data driven assessment methods like lr proves to be efficient when applied to common spatial data and establishes a methodology that will allow similar studies to be undertaken elsewhere

meteoric and marine ice crystal orientation fabrics from the amery ice shelf east antarctica the northwestern sector of the amery ice shelf east antarctica has a layered structure due to the presence of both meteoric ice and a marine ice layer resulting from sub shelf freezing processes crystal orientation fabric and grain size data are presented for ice cores obtained from two boreholes 70 km apart on approximately the same flowline multiple maxima crystal orientation fabrics and large mean grain sizes in the meteoric ice are indicative of stress relaxation and subsequent grain growth in ice that has flowed into the amery ice shelf strongly anisotropic single maximum crystal orientation fabrics and rectangular textures near the base of the 200 m thick marine ice layer suggest accretion occurs by the accumulation of frazil ice platelets crystal orientation fabrics in older marine ice exhibit vertical large circle girdle patterns influenced by the complex stress configurations that exist towards the margins of the ice shelf post accumulation grain growth and fabric development in the marine ice layer are restricted by a high concentration of brine and insoluble particulate inclusions differences in the meteoric and marine ice crystallography are indicative of the contrasting rheological properties of these layers which must be considered in relation to large scale ice shelf dynamics

textural characteristics and impurity content of meteoric and marine ice in the ronne ice shelf antarctica the texture and physical properties of an ice core recovered to 215 m depth from the ronne ice shelf antarctica have been studied with regard to formation and transformation of the ice at a depth of 152 8 m a sharp discontinuity marks the transition between meteoric ice accumulated from above and marine ice accreted from below as testified by electrolytical conductivity and stable isotope measurements as well as geophysical field surveys automated image analysis of thin sections indicates that the decrease in grain boundary density and the increase in grain cross sectional area with depth is commensurate with though not necessarily caused bythermodynamically driven grain growth down to 120m depth corresponding to a vertical strain of roughly 65 as computed with a simple temperature history particle path model the observed increase of grain boundary density i e a decrease of grain size with age in the marine ice is in part explained by the thermal history of this layer sediment inclusions at the top of the marine ice layer affect the observed grain boundary density profile by inhibiting grain growth and dynamic recrystallization this may allow some conclusions on the role of temperature particulate inclusions stress and strain rate in controlling the grain size evolution of deforming ice supplementing earlier laboratory experiments conducted at much shorter time scales salinities 0 026 parts per thousand brine volumes 0 09 0 2 parts per thousand and solid salt concentrations have been computed from electrolytical conductivity measurements mean of 51 0 x 10 6 s cm 1 for the marine ice an assessment of salt incorporation and desalination rates shows that these low salinities can at present only be explained by a unique densification mechanism of under water ice crystals at the base of the ice shelf

evidence for basal marine ice in the filchner ronne ice shelf the filchner ronne ice shelf which drains most of the marine based portions of the west antarctic ice sheet is the largest ice shelf on earth by volume the origin and properties of the ice that constitutes this shelf are poorly understood because a strong reflecting interface within the ice and the diffuse nature of the ice ocean interface make seismic and radio echo sounding data difficult to interpret1 2 ice in the upper part of the shelf is of meteoric origin but it has been proposed2 5 that a basal layer of saline ice accumulates from below here we present the results of an analysis of the physical and chemical characteristics of an ice core drilled almost to the bottom of the ronne ice shelf we observe a change in ice properties at about 150 m depth which we ascribe to a change from meteoric ice to basal marine ice the basal ice is very different from sea ice formed at the ocean surface and we propose a formation mechanism in which ice platelets in the water column accrete to the bottom of the ice shelf

massive ancient sea ice strata and preserved physical structural characteristics in the ward hunt ice shelf two sea ice layers one measured as 9 m thick the other at least 12 m thick and estimated to be 24 5 m thick have been located by ice core drilling in the west ward hunt ice shelf to examine the preservation of physical structu ral characteristics over long time intervals the crystal structure and brine volumes in the sea ice which possibly dates back to about 3000 bp have been studied the structural characteristics are immediately recognizable as those of undeformed congelation sea ice accreted by stefan growth brine volumes in the ancient sea ice are higher than those in modern multi year ice at the same temperature the preservation of brine over a time span of hundreds to thousands of years is attributed to an absence of surface meltwater to effect brine flushing and the very slow even negligible action of gravity drainage brine pocket migration and brine expulsion the congelation structures indicate that sea ice can grow by the stefan accretion mechanism to thicknesses exceeding the equilibrium thickness 2 5 5 m of most undeformed multi year ice the observed physical structu ral characteristics of the ward hunt sea ice strongly suggest that many of the properties attained by sea ice are permanent and not affected by slow acting physical processes

flow law hypotheses for ice sheet modeling ice flow modeling requires a flow law relating strain rates to stresses in situ but a flow law cannot be measured directly in ice sheets microscopic processes such as dislocation glide and boundary diffusion control both the flow law for ice and the development of physical properties such as grain size and c axis fabric these microscopic processes can be inferred from observations of the physical properties and the flow law can then be estimated from the microscopic processes a review of available literature shows that this approach can be imperfectly successful interior regions of large ice sheets probably have depth varying flow law constants with the stress exponent n for power law creep less than 3 in upper regions and equal to 3 only in deep ice n probably equals 3 through most of the thickness of ice shelves and ice streams

large flux of iron from the amery ice shelf marine ice to prydz bay east antarctica the antarctic continental shelf supports a high level of marine primary productivity and is a globally important carbon dioxide co2 sink through the photosynthetic fixation of co2 via the biological pump sustaining such high productivity requires a large supply of the essential micronutrient iron fe however the pathways for fe delivery to these zones vary spatially and temporally our study is the first to report a previously unquantified source of concentrated bioavailable fe to antarctic surface waters we hypothesize that fe derived from subglacial processes is delivered to euphotic waters through the accretion fe storage and subsequent melting fe release of a marine accreted layer of ice at the base of the amery ice shelf ais using satellite derived chlorophyll a data we show that the soluble fe supplied by the melting of the marine ice layer is an order of magnitude larger than the required fe necessary to sustain the large annual phytoplankton bloom in prydz bay our finding of high concentrations of fe in ais marine ice and recent data on increasing rates of ice shelf basal melt in many of antarctica s ice shelves should encourage further research into glacial and marine sediment transport beneath ice shelves and their sensitivity to current changes in basal melt currently the distribution volume and fe concentration of antarctic marine ice is poorly constrained this uncertainty combined with variable forecasts of increased rates of ice shelf basal melt limits our ability to predict future fe supply to antarctic coastal waters

microstructure and electrical properties of marine ice and its relationship to meteoric ice and sea ice a 215 m ice core has penetrated the central part of the ronne ice shelf 76 59 s 52 16 w the core consisted of meteoric ice above 152 8 m depth below this the ice was bubble free and of seawater origin the salinity of the layer below 152 8 m depth is less than 0 05 ppt very much lower than typical sea ice the ice represents an unusual source of ice intermediate in salinity and some physical properties between meteoric ice and sea ice the structure of the ice from four different depths where salinities vary by a factor of 4 has been investigated using a scanning electron microscope sem the fabric and grain structures are unusual and seem to depend on the impurity concentration in the ice chemical impurity localization has also been investigated the dielectric properties of the ice show that the ice has a transitional behavior from the linear dependence of conductivity on chloride concentration found in meteoric ice to the less predictable sea ice behavior the behavior may be interpreted in terms of the structure of the ice the higher salinity samples show convoluted grain boundaries small grain size and brine inclusions there are indications that the brine inclusions were liquid at temperatures between 10 c and 30 c by contrast the lower salinity samples possess larger grain sizes and show no evidence of brine inclusions all samples show brine concentrated at triple grain junctions and also along two grain boundaries in higher salinity samples the dielectric properties of the lower salinity samples are well described by a jaccard mechanism with l defects created in proportion to salinity that also describes the behavior of meteoric ice higher salinity samples exhibit lower conductivity than would be seen if l defects continued to be created in proportion to salinity the dielectric and structural data are consistent with a solubility limit of about 300 m for cl in the ice lattice

ice shelf ocean interactions under the amery ice shelf seasonal variability and its effect on marine ice formation 1 marine ice is an important factor in ice shelf stability an extensive marine ice layer is present under the amery ice shelf ais east antarctica this paper documents observations on the seasonal variability of the ais ocean interaction beneath the marine ice layer we focus on data collected during 2002 through a borehole at am01 100 km from the ice shelf calving front and use additional data from two other boreholes to complement the study at am01 the top 20 m of the water column is super cooled almost year round protecting the marine ice layer and promoting frazil ice formation the mixed layer thickness varies from 50 m in february to at least 160 m by june as the water column cools and freshens high salinity shelf water hssw abruptly arrives at am01 in june august as an eddy like flow we suggest that the flow characteristics are a result of baroclinic instabilities in addition the inflow of hssw results in a steepening of the isopycnals that enhances the upwelling of ice shelf water this study documents for the first time a seasonal signal in the formation of marine ice under the ais our results highlight the vulnerability of the marine ice layer to ocean variability with potential consequences for the overall ice shelf mass balance

borehole evidence for a thick layer of basal ice in the central ronne ice shelf the ronne ice shelf floating between berkner island and the antarctic peninsula is the biggest ice shelf in antarctica it drains an estimated 1 2 106km2 of the antarctic ice sheet much of it resting on bedrock below sea level consequently the balance and dynamics of this ice shelf is of importance to antarctic glaciology especially with regard to the integration of this part of the cryo sphere into the global processes that control the climate of the earth extensive radio echo sounding res by robin and others revealed reflections in the central part of the ronne ice shelf at the relatively shallow depth of 100 200 m below surface1 the interpretation of these echoes which varied in strength was ambiguous and the possibility of internal reflecting horizons was thoroughly discussed but after surface elevation measurements by radar altimeter from drifting balloons appeared to fit the presence of thin ice it was decided to base a thickness map of the ronne ice shelf on these res echoes1 4 we now present direct observational evidence from boreholes that the total ice thickness is much greater than mapped and that the shallow res reflections therefore do come from internal horizons

platelet ice the southern ocean s hidden ice a review basal melt of ice shelves is not only an important part of antarctica s ice sheet mass budget but it is also the origin of one of the most peculiar types of sea ice found in the polar oceans platelet ice in many regions around coastal antarctica tiny ice crystals form and grow in supercooled plumes of ice shelf water releasing heat into the surrounding ocean they usually rise towards the surface eventually becoming trapped under an ice shelf as marine ice frequently masses of those crystals are advected out of the ice shelf cavity and accumulate below a solid sea ice cover to form a semiconsolidated layer when the overlying sea ice grows into this so called sub ice platelet layer the loose crystals are consolidated adding additional thickness to the sea ice these phenomena are generally referred to as platelet ice although confusion about the terminology is widespread in the literature the presence of platelet ice has a profound impact on sea ice properties and processes in several regions of antarctica with numerous implications for the local polar marine biosphere most notably sub ice platelet layers provide a stable sheltered nutrient and food rich habitat which usually results in a highly productive and uniquely adapted ecosystem it has also been hypothesised that platelet ice may be an indicator of the state of an ice shelf although comprehensive time series are limited to the ross sea this paper clears up the terminology by providing exact definitions of the relevant terms we review platelet ice formation observational methods as well as geographical and seasonal occurrence the physical properties and ecological implications are merged in a way understandable for physicists and biologists alike to lay the foundation for the interdisciplinary research that is necessary to tackle the current knowledge gaps

crystalline texture of the 2083 m ice core at vostok station antarctica crystalline texture and c axis orien tation of the 2083 m ice core at vostok station covering more than 150 kyear reveal the existence of strong anisotropies changes in crystal size with depth are compatible with the growth of grains driven by the free energy of grain boundaries a smaller growth rate appears to be associated with cold periods a gradual increase in the horizontal elongation of grains was observed between 350 and 680 m but the mean value of the coefficient of the linear dimensional orientation of grains does not cha nge below 700 m the e axis orientation of ice grains tends to orientate perpendicular to the direction of the elongation of grains forming a vertical girdle patt ern this charac teristic fabric has been interpreted as resulting from the gradual rotation of grains by basal glide under uniaxial longit udi nal tension the rotation of grains was calcu lated with respect to the total strain simulating the form ation of the girdle fabric pattern the fabric enhance ment factor was calculated at various depths it appears that vostok ice hardens gradua lly