ice flow sensitivity to boundary processes a coupled model study in the vostok subglacial lake area antarctica several hundred subglacial lakes have been identified beneath antarctica so far their interaction with the overlying ice sheet and their influence on ice dynamics are still subjects of investigation while it is known that lakes reduce the ice sheet friction towards a free slip basal boundary condition little is known about how basal melting and freezing at the lake ice interface modifies the ice dynamics thermal regime and ice rheology in this diagnostic study we simulate the vostok subglacial lake area with a coupled full stokes 3 d ice flow model and a 3 d lake circulation model the exchange of energy heat and mass at the lake ice interface increases decreases the temperature in the ice column above the lake by up to 10 in freezing melting areas resulting in a significant modification of the highly nonlinear ice viscosity we show that basal lubrication at the bottom of the ice sheet has a significant impact not only on the ice flow above the lake itself but also on the vicinity and far field while the ice flow crosses vostok subglacial lake flow divergence is observed and modelled the heterogeneous basal mass balance pattern at the lake ice interface intensifies this divergence instead of interactive coupling between the ice flow model and the lake flow model only a single iteration is required for a realistic representation of the ice water interaction in addition our study indicates that simplified parameterizations of the surface temperature boundary condition might lead to a velocity error of 20 for the area of investigation

assessing the subglacial lake coverage of antarctica lakes beneath the antarctic ice sheet are known to decrease traction at the ice base and therefore can have a great impact on ice dynamics however the total extent of antarctic subglacial lakes is still unknown we address this issue by combining modeling and remote sensing strategies to predict potential lake locations using the general hydraulic potential equation we are able to reproduce the majority of known lakes as well as predict the existence of many new and so far undetected potential lakes to validate our predictions we analyzed ice penetrating radar profiles from radio echo sounding flights acquired over 1994 2013 in dronning maud land east antarctica and this led to the identification of 31 new subglacial lakes based on these findings we estimate the total number of antarctic subglacial lakes to be 1300 a factor of three higher than the total number of lakes discovered to date we estimate that only 30 of all antarctic subglacial lakes and 65 of the total estimated lake covered area have been discovered and that lakes account for 0 6 of the antarctic ice bed interface

systematics of ni and co in olivine from planetary melt systems lunar mare basalts the systematics of co and ni in olivine from six apollo 12 olivine basalts were studied by sims techniques and correlated with major and minor element data obtained with the electron microprobe our results together with previous studies demonstrate that one of these basalts 12009 was extruded as a liquid onto the lunar surface and was parental to five other cumulates 12075 12020 12018 12040 and 12035 the concentrations of ni in zoned olivine phenocrysts behave as expected for a highly compatible element with initial estimated d ni 9 9 on the other hand co concentrations in olivine vary hardly at all and show flat patterns across crystals that retain normal zoning trends for mg fe mn and ni the explanation for this behavior is not that d co nearly equal 1 our estimated d co 4 or that co zoning has been erased by rapid diffusion of co compared to the other elements e g mg and fe that still show normal zoning the explanation for the behavior as originally suggested by kohn et al 1989 is that the increase in d co with crystallization exactly balances the depletion of co in the melt this decoupling of the behavior of ni and co in olivine results in significant increases in co ni with crystallization

estimating the depth and shape of subglacial lake vostok s water cavity from aerogravity data 1 we use aerogravity data to estimate the water depth of subglacial lake vostok in east antarctica the inversion produces the first bathymetry map covering the entire lake lake vostok consists of two sub basins separated by a ridge with very shallow water depths the deeper southern sub basin is approximately double the spatial area of the smaller northern sub basin the close correlation between the pattern of basal melting and freezing and the bathymetric structure has important ramifications for the water circulation and the sediment deposition we estimate the lake volume to be 5400 1600 km3

erosion by tectonic carving in the concordia subglacial fault zone east antarctica abstract in this work we present the analysis of the footwall morphology of the concordia subglacial extensional fault in the east antarctic craton the concordia fault is a regional fault zone that extends for almost 200 km the displacement up to 1800 m and the listric geometry were recognized by numerical modeling of the resulting hangingwall bedrock morphology and is responsible for the marked asymmetry that characterizes the corresponding scarp in the concordia subglacial trench the portion of the footwall in the proximity of the master fault exhibits an excavated morphology about 500 m deep and up to 5 km wide showing strong correlation with the master fault displacement we excluded a predominant glacial and fluvial origin of this morphology considering i the sharp topography of the concordia fault suggesting that the fault activity started after the onset of the ice sheet ii the ice sheet bedrock contact is characterized by a general negligible erosion deposition rates still allowing clast removal iii the lack of significant deposits in the concordia trench we hence explored the possibility that this morphology may result from the combined action of fault induced fracturing and passive clast removal and scattering by flow and plastic deformation within the ice sheet we introduced the term tectonic carving for this process our modeling shows that tectonic carving relates to the relative fracture intensity in the concordia fracture zone that corresponds to the envelope of master and secondary fault damage zones fracture intensity depends on the frequency and the displacement of secondary faulting and can be approximated by a normal distribution using a monte carlo modeling approach we selected the set of parameters that best fits the data set with the carving theoretical curve the final results of the monte carlo analysis show a root mean square of about 50 meters comparable with the data resolution this analysis demonstrates a method to unravel the presence of fracture zones in similar weak erosional environments

highly siderophile element depletion in the moon abstract coupled 187 os 188 os and highly siderophile element hse os ir ru pt pd re abundance data are reported for apollo 12 12005 12009 12019 12022 12038 12039 12040 apollo 15 15555 and apollo 17 70135 mare basalts along with mare basalt meteorites la paz icefield lap 04841 and miller range mil 05035 these mare basalts have consistently low hse abundances at 2 10 5 to 2 10 7 the chondritic abundance the most magnesian samples have broadly chondrite relative hse abundances and chondritic measured and calculated initial 187 os 188 os the lower abundances and fractionated hse compositions of more evolved mare basalts can be reproduced by modeling crystal liquid fractionation using rock melt bulk partition coefficients of 2 for os ir ru pt and pd and 1 5 for re lunar mare basalt bulk partition coefficients are probably higher than for terrestrial melts as a result of more reducing conditions leading to increased hse compatibility the chondritic relative abundances and chondritic 187 os 188 os of the most primitive high mgo mare basalts cannot readily be explained through regolith contamination during emplacement at the lunar surface mare basalt compositions are best modeled as representing 5 11 partial melting of metal free sources with low os ir ru pd 0 1 ng g 1 pt 0 2 ng g 1 re 0 01 ng g 1 and s 75 g g 1 with sulphide melt partitioning between 1000 and 10 000 apollo 12 olivine pigeonite and ilmenite normative mare basalts define an imprecise 187 re 187 os age of 3 0 0 9 ga with an initial 187 os 188 os of 0 107 0 010 this age is within uncertainty of 147 sm 143 nd ages for the samples the initial os isotopic composition of apollo 12 samples indicates that the source of these rocks evolved with re os within 10 of chondrite meteorites from the time that the mantle source became a system closed to siderophile additions to the time that the basalts erupted similarity in absolute hse abundances between mare basalts from the apollo 12 15 and 17 sites and from unknown regions of the moon la paz mare basalts mil 05035 indicates relatively homogeneous and low hse abundances within the lunar interior low absolute hse abundances and chondritic re os of mare basalts are consistent with a late accretion addition of 0 02 wt of the moon s mass to the mantle prior to the formation of the lunar crust late accretion must also have occurred significantly prior to cessation of lunar mantle differentiation 4 4 ga to enable efficient mixing and homogenization within the mantle low lunar hse abundances are consistent with proportionally 40 times more late accretion to earth than the moon disproportional late accretion to the two bodies is consistent with the small 182 w excess 21 28 ppm measured in lunar rocks compared to the silicate earth

a large deep freshwater lake beneath the ice of central east antarctica in 1974 75 an airborne radio echo survey of ice depths over central east antarctica led to the discovery of a sub ice lake of unknown depth and composition with an area of about 10 000km2 and lying beneath 4km of ice1 in 1993 altimetric data from satellite measurements2 provided independent evidence of the lake s areal extent thus confirming it to be the largest known sub ice lake by an order of magnitude here we analyse new altimetric and radio echo data along with existing seismic data3 to show that the lake is deep mean depth of 125 m or more and fresh and that it has an area that exceeds previous estimates by about 50 dimensions comparable with those of lake ontario we estimate that the residence time of the water in the lake is of the order of tens of thousands of years and that the mean age of water in the lake since deposition as surface ice is about one million years regional ice dynamics can be explained in terms of steady state ice flow along and over the lake

comment on the comment by m j siegert on a numerical model for an alternative origin of lake vostok and its exobiological implications for mars by n s duxbury et al 1 there is some debate on the origin of subglacial lake vostok and whether the water within the present subglacial lake system contains biota that survived the buildup of the antarctic ice sheet one theory suggests that lake vostok existed as a preglacial lake before glaciation of the continent at around 15 ma ago survived the subsequent period of ice sheet growth and remained stable beneath the thick ice cover to the present day duxbury et al 2001 another hypothesis challenges this view by stating that the early phase of build up would have resulted in ice grounding throughout the trough which the lake now occupies siegert 2004 2 according to duxbury et al 2001 the lake could have survived the growth of the ice sheet as long as the preglacial lake was more than 53 m deep in their model subglacial lake vostok is considered as an approximate closed system however if lake vostok existed as a preglacial lake the closure of the lake vostok system must have happened between the initiation of the ice sheet and the establishment of a stable ice cover in east antarctica siegert 2004 therefore the dynamic evolution of the ice sheet and its interaction with the preglacial lake should be accounted for in determining whether subglacial lake vostok is a direct remnant of a preglacial surficial lake 3 siegert 2004 demonstrates that the preglacial lake could not have survived the buildup of the antarctic ice sheet water flow beneath an ice mass is controlled by the hydraulic potential gradient pg rigas rw ri gab where ri and rw are the density of ice and water and as and ab are the surface and basal slope respectively shreve 1972 if the magnitude of surface slope is larger than one tenth the basal slope water can be driven out of a topographic depression and flow uphill two icesheet model studies huybrechts 1993 deconto and pollard 2003 show that during the inception and growth of the antarctic ice sheet the ice sheet margin was situated close to lake vostok the steep margin of the ice sheet across lake vostok implies high subglacial hydraulic potential gradients which led siegert 2004 to conclude that basal water is evacuated as the steep ice sheet margin progresses over the lake however none of the above models take into account the interaction of the ice sheet with the underlying lake whether it be a preglacial or a subglacial lake and only consider an ice sheet more or less frozen to the underlying bedrock 4 below i present a physical mechanism that allows for subglacial lake vostok to survive the buildup of the ice sheet by taking into account the interaction of the ice sheet with the preglacial subglacial lake this hypothesis is supported by the fact that the surface of a preglacial lake as well as the interface between a subglacial lake and the overriding ice sheet can be regarded as a slippery spot

new 3d bathymetry and sediment distribution in lake vostok implication for pre glacial origin and numerical modeling of the internal processes within the lake abstract a new distribution of water and unconsolidated sediments in subglacial lake vostok east antarctica was developed via inversion of airborne gravity data constrained by 60 seismic soundings a model was developed for host rock with a density of 2550 kg m 3 that was inferred from prior 2d modeling our 3d bathymetry model of lake vostok corresponds better with seismic data rms of 125 m than two previous models based on the same gravity dataset the good match in both water and sediment thicknesses between the gravity model and seismic measurements confirms two major facts about lake vostok 1 the lake is hosted by sedimentary rocks and 2 the bottom of the lake is covered with a layer of unconsolidated sediments that does not exceed 300 m in the southern basin and thickens almost to 400 m in the northern basin our new bathymetry model suggests much shallower water thicknesses up to twice the previous estimates in the middle and northern parts of the lake while the water layer is thicker in the southern basin numerical modeling of the internal processes in the lake reveals the relevance of our new bathymetry model to the basal mass balance a significant decrease in transport is observed in the shallower northern basin as well as a decrease of 33 in the turbulent kinetic energy however only minor differences were observed in the distribution of the calculated freezing and melting zones compared to previous models estimates for the sedimentation rates for six possible mechanisms were made possible sedimentation mechanisms are 1 fluvial and periglacial i e those that are active prior to the establishment of a large subglacial lake 2 deposition due to overlying ice sheet including melting out of the ice as well as bulldozering by the overriding ice and 3 suspended sediments from subglacial water flow including those deposited by periodical subglacial outbursts the estimates for these mechanisms show that unconsolidated sediments of the observed thickness are most consistent with a lake that existed before glaciation

ice cover landscape setting and geological framework of lake vostok east antarctica abstract lake vostok located beneath more than 4 km of ice in the middle of east antarctica is a unique subglacial habitat and may contain microorganisms with distinct adaptations to such an extreme environment melting and freezing at the base of the ice sheet which slowly flows across the lake controls the flux of water biota and sediment particles through the lake the influx of thermal energy however is limited to contributions from below thus the geological origin of lake vostok is a critical boundary condition for the subglacial ecosystem we present the first comprehensive maps of ice surface ice thickness and subglacial topography around lake vostok the ice flow across the lake and the landscape setting are closely linked to the geological origin of lake vostok our data show that lake vostok is located along a major geological boundary magnetic and gravity data are distinct east and west of the lake as is the roughness of the subglacial topography the physiographic setting of the lake has important consequences for the ice flow and thus the melting and freezing pattern and the lake s circulation lake vostok is a tectonically controlled subglacial lake the tectonic processes provided the space for a unique habitat and recent minor tectonic activity could have the potential to introduce small but significant amounts of thermal energy into the lake

ice flow field over lake vostok east antarctica inferred by structure tracking abstract here we present a flow field for the east antarctic ice sheet over lake vostok based on tracking structures across the lake inherited from the western upstream shoreline the structures distinctive peaks and troughs identified in the ice sheet internal layers are imaged in ice penetrating radar data collected in a systematic grid over the lake these structures are resolved in three distinct internal layers at depths between 900 and 3750 m ridges in the internal layers are associated with prominent topographic highs along the western shoreline and troughs are correlated with shoreline topographic lows lake vostok is bisected by a topographic ice divide south of this topographic divide the ice flow is dominantly nnw to sse and north of the divide the ice flow is w to e the flow over the lake is also deflected by the small 0 02 southward lake surface slope accretion ice lake water frozen to the bottom of the ice sheet is preferentially imaged along flowlines emanating from topographic ridges the coincidence of the accretion ice reflector with our flowlines both provides independent support for our new flow field and suggests focused accretion along the western shoreline global positioning system gps data provide additional independent support for our new flow field in the southern lake the new flow field will be crucial for subsequent analyses of mass exchange between the ice sheet and the lake identifying areas of melting above the lake and estimating residence times

bathymetry and geological structures beneath the ross ice shelf at the mouth of whillans ice stream west antarctica modeled from ground based gravity measurements 1 new gravity data reveal important geologic controls on the location and behavior of the grounding zone of whillans ice stream wis west antarctica grounding zones of ice sheets and contiguous ice shelves are important for understanding ice sheet dynamics as key processes that influence the grounded ice and its discharge into the ocean occur in these regions here we model the bathymetry and shallow geological structures beneath the ross ice shelf in an embayment of the wis grounding zone using gravity data collected on the ground in conjunction with seismic and radar data we find that the region of shallow ocean water 50 m is extensive oceanographic models suggest that grounding zones exhibiting thin ocean cavities with gently sloping ice ocean interfaces are likely to be tidally well mixed leading to slower basal melting than would occur in a thicker stratified water column beneath the ocean water column we model a fault and a sedimentary basin in a half graben filled with two layers of sediments the total thickness of the sediment layer is 900 to 1250 m in the half graben and 600 to 800 m on the upthrown block and the basement depth is no more than 2000 m we observe that the upper softer sediment is thinnest near the modern grounding line and may possibly pinch out near our grid and that the modeled fault is roughly parallel to part the grounding we therefore hypothesize that the wis grounding line stabilized in its current location in part due to the subglacial geology

physical processes in subglacial lake whillans west antarctica inferences from sediment cores abstract the hydrologic system beneath the antarctic ice sheet is thought to influence both the dynamics and distribution of fast flowing ice streams which discharge most of the ice lost by the ice sheet despite considerable interest in understanding this subglacial network and its affect on ice flow in situ observations from the ice sheet bed are exceedingly rare here we describe the first sediment cores recovered from an active subglacial lake the lake known as subglacial lake whillans is part of a broader dynamic hydrologic network beneath the whillans ice stream in west antarctica even though floods pass through the lake the lake floor shows no evidence of erosion or deposition by flowing water by inference these floods must have insufficient energy to erode or transport significant volumes of sediment coarser than silt consequently water flow beneath the region is probably incapable of incising continuous channels into the bed and instead follows preexisting subglacial topography and surface slope sediment on the lake floor consists of till deposited during intermittent grounding of the ice stream following flood events the fabrics within the till are weaker than those thought to develop in thick deforming beds suggesting subglacial sediment fluxes across the ice plain are currently low and unlikely to have a large stabilizing effect on the ice stream s grounding zone

theoretical framework and diagnostic criteria for the identification of palaeo subglacial lakes abstract the antarctic ice sheet is underlain by numerous subglacial lakes which comprise a significant and active component of its hydrological network these lakes are widespread and occur at a range of scales under a variety of conditions at present much glaciological research is concerned with the role of modern subglacial lake systems in antarctica another approach to the exploration of subglacial lakes involves identification of the geological record of subglacial lakes that once existed beneath former ice sheets this is challenging both conceptually in identifying whether and where subglacial lakes may have formed and also distinguishing the signature of former subglacial lakes in the geological record in this work we provide a synthesis of subglacial lake types that have been identified or may theoretically exist beneath contemporary or palaeo ice sheets this includes a discussion of the formative mechanisms that could trigger onset of or drain subglacial lakes these concepts provide a framework for discussing the probability that subglacial lakes exist ed beneath other palaeo ice sheets indeed we conclude that the former mid latitude ice sheets are likely to have hosted subglacial lakes although the spatial distribution frequency and type of lakes may have differed from today s ice sheets and between palaeo ice sheets given this possibility we propose diagnostic criteria for identifying palaeo subglacial lakes in the geological record these criteria are derived from contemporary observations hydrological theory and process analogues and provide an observational template for detailed field investigations

rapid discharge connects antarctic subglacial lakes a hidden subglacial world consisting of more than 140 lakes lies beneath the antarctic ice sheet it is widely assumed that these lakes are a static ecosystem cut off from each other and the rest of the world far from it according to a 16 month study of surface elevation changes in the region changes indicating a rapid discharge of water from one lake to another were detected and it is possible that the entire subglacial drainage basin is flushed through periodically by massive water transfers observation of surface elevation changes over 16 months in an area of the east antarctic ice sheet overlying subglacial lakes suggests a rapid discharge of one lake to at least two other lakes involving a water volume of 1 8 km3 in conflict with previous expectations that subglacial lakes have long residence times and slow circulations the existence of many subglacial lakes1 provides clear evidence for the widespread presence of water beneath the east antarctic ice sheet but the hydrology beneath this ice mass is poorly understood2 such knowledge is critical to understanding ice flow basal water transfer to the ice margin glacial landform development and subglacial lake habitats here we present ice sheet surface elevation changes in central east antarctica that we interpret to represent rapid discharge from a subglacial lake our observations indicate that during a period of 16 months 1 8 km3 of water was transferred over 290 km to at least two other subglacial lakes while viscous deformation of the ice roof above may moderate discharge the intrinsic instability of such a system3 suggests that discharge events are a common mode of basal drainage4 if large lakes such as lake vostok or lake concordia1 are pressurizing it is possible that substantial discharges could reach the coast5 6 our observations conflict with expectations that subglacial lakes have long residence times and slow circulations2 7 8 and we suggest that entire subglacial drainage basins may be flushed periodically the rapid transfer of water between lakes would result in large scale solute and microbe relocation and drainage system contamination from in situ exploration is therefore a distinct risk

ice flow velocities over vostok subglacial lake east antarctica determined by 10 years of gnss observations repeated global navigation satellite systems gnss observations were carried out at 50 surface markers in the vostok subglacial lake east antarctica region between 2001 and 2011 the horizontal ice flow velocity vectors were derived with accuracies of 1 cma and 0 58 representing the first reliable information on ice flow kinematics in the northern part of the lake within the lake area ice flow velocities do not exceed 2ma the ice flow azimuth is southeast in the southern part of the lake and turns gradually to east northeast in the northern part in the northern part as the ice flow enters the lake at the western shore the velocity decreases towards the central lake axis then increases slightly past the central axis in the southern part a continued acceleration is observed from the central lake axis across the downstream grounding line based on the observed flow velocity vectors and ice thickness data mean surface accumulation rates are inferred for four surface segments between ridge b and vostok subglacial lake and show a steady increase towards the north

salinity impact on water flow and lake ice in lake vostok antarctica 1 lake vostok isolated from direct exchange with the atmosphere for millions of years provides a unique so far inaccessible habitat by using a numerical model and recent geometry information the lake circulation was investigated for different salinites for freshwater thermally driven circulation occurs resulting from pressure dependent melting point differences at the inclined ice ceiling north to south ice pumping provides a steady supply of glacial water the weak circulation is driven by very small density contrasts but requires no unusual geothermal input for low salinity conditions however circulation intensifies occupying the entire lake the maximum amplitudes of melting freezing increase by about 50 and melting extends further south for both conditions approximately 200 m of refrozen ice accumulates beneath vostok station the lake habitat will be affected clearly by salinity it is essential to establish the specific chemistry for comprehending this unique environment and planning in situ experiments

origin and fate of lake vostok water frozen to the base of the east antarctic ice sheet the subglacial lake vostok may be a unique reservoir of genetic material and it may contain organisms with distinct adaptations but it has yet to be explored directly the lake and the overlying ice sheet are closely linked as the ice sheet thickness drives the lake circulation while melting and freezing at the ice sheet base will control the flux of water biota and sediment through the lake here we present a reconstruction of the ice flow trajectories for the vostok core site using ice penetrating radar data and global positioning system gps measurements of surface ice velocity we find that the ice sheet has a significant along lake flow component persistent since the last glacial maximum the rates at which ice is frozen accreted to the base of the ice sheet are greatest at the shorelines and the accreted ice layer is subsequently transported out of the lake using these new flow field and velocity measurements we estimate the time for ice to traverse lake vostok to be 16 000 20 000 years we infer that most vostok ice analysed to date was accreted to the ice sheet close to the western shoreline and is therefore not representative of open lake conditions from the amount of accreted lake water we estimate to be exported along the southern shoreline the lake water residence time is about 13 300 years

comment on a numerical model for an alternative origin of lake vostok and its exobiological implications for mars by n s duxbury i a zotikov k h nealson v e romanovsky and f d carsey

subglacial bathymetry and sediment layer distribution beneath the pine island glacier ice shelf west antarctica modeled using aerogravity and autonomous underwater vehicle data

abundance of highly siderophile elements in lunar basalts controlled by iron sulfide melt the moon accreted meteoritic material towards the end of solar system formation quantification of this late accretion requires an estimation of the abundance of highly siderophile or iron loving elements in the lunar mantle as lunar mantle samples are not available estimates are derived from lunar basalt compositions but the melting phase relations needed to derive the mantle composition are poorly constrained here we present sulfur solubility measurements from laboratory experiments combined with thermodynamic calculations which show that the lunar basalt source is likely to be saturated in a sulfur poor iron rich sulfide melt that concentrates some highly siderophile elements more than others we found that the observed range in the ratios of highly siderophile elements in primitive lunar basalts is much smaller than expected from residual sulfide control alone instead the elemental ratios are consistent with mixing between primary sulfide saturated melts and minute 1 amounts of lunar regolith that contain impact debris although the composition of some samples suggests a highly depleted lunar mantle the exact level of depletion is unclear because mixing trajectories overlap at the inferred level of regolith contamination we conclude that the composition of the lunar mantle is veiled by regolith contamination of the lunar basalts if so highly siderophile element abundances in lunar mantle derived materials cannot be used to determine the mass of material accreted late onto the moon the abundance of iron loving elements in moon rocks cannot indicate the amount of late accretion onto the moon according to experiments and thermodynamic calculations constraining the behaviour of these elements under lunar mantle conditions

technologies for retrieving sediment cores in antarctic subglacial settings accumulations of sediment beneath the antarctic ice sheet contain a range of physical and chemical proxies with the potential to document changes in ice sheet history and to identify and characterize life in subglacial settings retrieving subglacial sediments and sediment cores presents several unique challenges to existing technologies this paper briefly reviews the history of sediment sampling in subglacial environments it then outlines some of the technological challenges and constraints in developing the corers being used in sub ice shelf settings e g george vi ice shelf and larsen ice shelf under ice streams e g rutford ice stream at or close to the grounding line e g whillans ice stream and in subglacial lakes deep under the ice sheet e g lake ellsworth the key features of the corers designed to operate in each of these subglacial settings are described and illustrated together with comments on their deployment procedures

modelling flow and accreted ice in subglacial lake concordia antarctica more than 150 subglacial lakes have been discovered in antarctica so far due to obvious challenges with exploration numerical modelling remains one of the major tools to acquire information about those hard to access objects until now only the huge lake vostok has been investigated in detail this paper focuses on lake concordia the second largest subglacial lake in antarctica over which substantial geophysical data has been collected this lake is covered by about 4000 m ice and is located near dome c in order to apply numerical models to the hard to access antarctic subglacial lakes decent geometries and boundary conditions are required in this study we present the results of airborne gravity inversion suggesting that this lake has an area of 617 km2 a volume of 31 km3 and a maximum water column thickness of 126 m this bathymetry is used as geometry input for an established 3d numerical lake flow model to simulate the circulation and basal mass balance compared to our model studies of subglacial lake vostok we obtain a general circulation pattern that is significantly weaker due to the smaller size of the lake and of reversed orientation due to the reversed ice surface tilt the modelled mean horizontal and vertical velocities are in the order of 0 2 mm s and 0 5 m s respectively the larger molecular convective velocity estimations 1 35 0 13 mm s and 0 81 0 08 mm s are similar to lake vostok s the modelled average melting and freezing rates are 4 3 1 1 mm a and 1 1 0 3 mm a respectively and the corresponding fresh water gain is 58 27 dm3 s integration of the modelled freezing and melting along prescribed ice flow lines allows us to calculate the distribution and thickness of accreted ice at the ice sheet bottom we estimate a volume of 2 6 2 0 km3 8 3 8 2 of the total lake volume occupying the north eastern corner of the lake covering an area of 159 48 km2 26 9 of the total lake area with about 16 800 7600 yr the residence time of the lake s water is significantly shorter than lake vostok s

sensitivity of subglacial lake vostok s flow regime on environmental parameters abstract subglacial lakes provide unique habitats but the exact nature of physical and geochemical conditions are still a matter of debate and await direct sampling of water due to its isolation from external atmospheric forcing other environmental parameters influence the flow characteristics within the lake in this study we use an improved treatment of the physical processes at the ice water boundary interface to identify and quantify the impact of 1 the geothermal heat flux 2 the heat flux from the lake into the ice 3 the influence of the salinity of the lake water and 4 the ice thickness on the size of the freezing area and the freeze melt rates we show that the modelled basal mass imbalance that is the produced melt water minus the re frozen water depends on the geothermal heating as well as the heat flux into the ice the circulation and the temperature distribution within subglacial lake vostok are rather stable against variations of geothermal heat flux heat flux into the ice sheet salinity of the lake and small changes of the ice thickness above the lake however the flow regime for any subglacial lake with less than 2000 m ice thickness above will be substantially different from those that experience higher pressures this is because the buoyancy temperature relationship reverses at this depth

recovery lakes east antarctica radar assessment of sub glacial water extent 1 a fast flowing tributary of recovery ice stream penetrates more than 500 km into the interior of east antarctica recent satellite based studies found surface features in the onset area of this tributary that indicate a significant subglacial hydraulic system including four large smooth basins the typical surface expression of large subglacial lakes as well as eleven smaller areas over which ice sheet surface elevations rapidly changed by discharge filling of subglacial water here we present the first ice penetrating radar evidence of subglacial conditions in this area we identified a distinct ice water interface only over a limited area within the boundaries of the investigated large smooth basins previously hypothesized to be water filled lakes the radar characteristics in some areas are similar to those of a drained lake indicating that parts of the bed are wet but not a typical lake we also find evidence for discrete water bodies outside of the lake boundaries the lines of evidence indicate that the northern most two recovery lakes have recently drained

magmatic processes that produced lunar fire fountains reanalysis of the apollo 15 a b and c green glass beads from slide 15426 72 led to the discovery of patchy highly vesicular glass rims adhering to beads these rims are high in s and ni and low in mgo but otherwise compositionally similar to the green glasses we find that these rims represent a unique melt composition that places constraints on lunar magmatic processes combining the compositional data for the vesicular glass rims with new minor element data for the green glass beads leads us to hypothesize that the vesicular rim fluid originated near the green glass source at about 2 2 gpa these findings support the theory of a heterogeneous lunar mantle and suggest that sulfur did not drive the eruption from depth but that degassing volatiles into the vacuum did drive the final fire fountain eruption

airborne gravity over lake vostok and adjacent highlands of east antarctica lake vostok and a 1200 km transect were the targets of aerogeophysical surveys in east antarctica during the austral summer of 2000 2001 the measurement of gravity anomalies for geologic studies was the primary goal a total of 24 459 line km of data were acquired favorable weather aircraft navigation and instrument performance contributed to excellent data quality multiple carrier phase gps solutions to determine aircraft induced accelerations were available for each flight raw gravity and gps position solutions were initially filtered to compensate for hardware filtering within the gravity meter filtering of remaining high frequency noise was accomplished with a spatial moving average smoother due to upward continuation effects imposed by the ice cover the theoretically estimated minimum resolvable gravity feature size for the lake vostok survey is 8 km consistent with an analysis of power spectra comparing the gravity signal to noise calculated from geographically repeated lines comparison of gravity results with subice topography indicates that the gravity data are sensitive to real features including the existence of major crustal structures repeated lines and crossovers were analyzed to estimate uncertainties for the lake vostok data set with both of these repeatability measures indicating relative accuracy in the 2 mgal range for the unleveled data and 1 mgal after leveling

modelling accreted ice in subglacial lake vostok antarctica 1 the distribution and thickness of accreted ice at the ice lake interface of subglacial lake vostok east antarctica is calculated conflating various sources 1 the modelled basal mass balance at the ice lake interface based on two different bathymetry models 2 different ice flow trajectories obtained from satellite interferometry and ice penetrating radar measurements and 3 reasonable ice flow velocity our results show that the accreted ice distribution is highly sensitive to the ice draft and to the used flow line directions the volume and thickness of the accreted ice depends significantly on the ice flow velocity according to our modelling we estimate the accreted ice area volume and mean thickness to be 10 800 500 km2 980 200 km3 and 90 45 m respectively for an ice flow velocity of 3 6 m a only about 36 2 of lake vostok s surface is in contact with meteoric ice melted by about 2 65 0 10 cm a this has impacts on the sedimentation rate and the supply of nutrients oxygen and or other components only present in meteoric ice but not in accreted lake ice we estimate the residence time of the lake water at about 32 000 4000 years

application of a three dimensional numerical model to lake vostok an antarctic subglacial lake the circulation in two water column thickness scenarios for lake vostok a subglacial lake in central east antarctica is investigated using a three dimensional numerical model in a scenario with a constant water column thickness the distance between the sloping ice sheet which caps the lake and the lake bed the circulation is barotropic but with a more realistic water column thickness the flow is baroclinic the different circulations result from the geothermal heat flux warming shallow regions of the realistic lake more efficiently thus forming a different horizontal density structure the differences and uncertainties between the two scenarios highlight the importance that the largely unknown water column thickness and geothermal heat flux have in determining the circulation in the lake

natural occurrence of reidite in the xiuyan crater of china the high pressure minerals of reidite and coesite have been identified in the moderately shock metamorphosed gneiss shock stage ii 3545gpa and the strongly shock metamorphosed gneiss shock stage iii 4555gpa respectively from the polymict breccias of the xiuyan crater a simple impact structure 1 8km in diameter in china reidite in the shock stage ii gneiss displays lamellar textures developed in parental grains of zircon the phase transformation of zircon to reidite likely corresponds to a martensitic mechanism no coesite is found in the reidite bearing gneiss the shock stage iii gneiss contains abundant coesite but no reidite is identified in the rock coesite occurs as acicular dendritic and spherulitic crystals characteristic of crystallization from shock produced silica melt zircon in the rock is mostly recrystallized the postshock temperature in the shock stage iii gneiss is too high for the preservation of reidite whereas reidite survives in the shock stage ii gneiss because of relatively low postshock temperature reidite does not occur together with coesite because of difference in shock induced temperature between the shock stage ii gneiss and the shock stage iii gneiss

geodetic observations of ice flow velocities over the southern part of subglacial lake vostok antarctica and their glaciological implications summary in the austral summer seasons 2001 02 and 2002 03 global positioning system gps data were collected in the vicinity of vostok station to determine ice flow velocities over lake vostok ten gps sites are located within a radius of 30 km around vostok station on floating ice as well as on grounded ice to the east and to the west of the lake additionally a local deformation network around the ice core drilling site 5g 1 was installed the derived ice flow velocity for vostok station is 2 00 m a 1 0 01 m a 1 along the flowline of vostok station an extension rate of about 10 5 a 1 equivalent to 1 cm km 1 a 1 was determined this significant velocity gradient results in a new estimate of 28 700 years for the transit time of an ice particle along the vostok flowline from the bedrock ridge in the southwest of the lake to the eastern shoreline with these lower velocities compared to earlier studies and hence larger transit times the basal accretion rate is estimated to be 4 mm a 1 along a portion of the vostok flowline an assessment of the local accretion rate at vostok station using the observed geodetic quantities yields an accretion rate in the same order of magnitude furthermore the comparison of our geodetic observations with results inferred from ice penetrating radar data indicates that the ice flow may not have changed significantly for several thousand years

a numerical model for an alternative origin of lake vostok and its exobiological implications for mars in connection with recent galileo images of the jovian satellite europa there has been a significantly increased interest in the subglacial lake vostok in central east antarctica since the theoretical prediction by zotikov 1961 of the existence of lakes one of which later was named vostok under 4 km of ice in antarctica and its confirmation by radar measurements from aircraft in 1974 robin et al 1977 and from the ers 1 satellite in 1993 it was tacitly assumed that the lake originated from the basal melting of ice e g zotikov 1961 1977 1986 kapitsa et al 1996 recently zotikov and duxbury 2000 proposed an alternative hypothesis that if lake vostok had existed on the surface before the antarctic perennial glaciation began 5 30 myr ago then it was buried under the thermally protecting ice and never froze all the way to the bottom in this study we support our hypothesis by comprehensive numerical modeling and evaluate its interesting implications for mars and exobiology our numerical experiments on the sensitivity of the freezing depth to different upper boundary temperatures and different internal heat flows and the recent mars global surveyor mars orbiter laser altimeter mola data support our model of the origin of possible subpolar water on mars versus the old basal melting hypothesis we also propose here experimental tests for our hypothesis as a result of our computations we conclude that lake vostok has survived the antarctic glaciation without freezing completely if it were initially an open lake with a surprisingly small water depth greater than 53 m the current water depth at a single point is as much as 500 m verkulich et al 1996 this maximum freezing depth of 53 m was reached in only 3300 years when the total upper ice cover was only 600 m the computed critical water depth is applicable to other antarctic lakes under a thick ice cover for the martian internal heat flow we have computed the critical water depth to be 63 m the existence of the lake before the accumulation of ice in antarctica implies that possible microorganisms and their remnants in the lake can be older than the antarctic ice sheet i e older than 5 30 myr if future observations find water under the martian polar caps this subpolar water according to our model has originated from lakes that were on the martian surface before the formation of mars polar caps and have not completely frozen such environments would be ideal candidates for extant or extinct life on the red planet this argues for a drilling mission at the martian polar caps

advances in modelling subglacial lakes and their interaction with the antarctic ice sheet subglacial lakes have long been considered hydraulically isolated water bodies underneath ice sheets this view changed radically with the advent of repeat pass satellite altimetry and the discover

penetration of antarctic subglacial lakes by vhf electromagnetic pulses information on the depth and electrical conductivity of basal water bodies owing to the high level of absorption of very high frequency radio waves in water previous investigators of airborne radio echo sounding res data from antarctica have assumed that the depth of subglacial lakes cannot be measured directly by this method however we have identified a number of res returns from beneath the ice water interface at the surface of eight subglacial lakes that we have interpreted as being reflected from the lake floor the returns allow us to measure the depth of subglacial lakes since the velocity of radio waves in water 33 4 m s 1 is relatively unaffected by electrical conductivity attenuation of radio waves within water is controlled largely by its electrical conductivity consequently by examining the decay of the radio wave amplitude with depth we can gain information about the conductivity of subglacial water bodies our results indicate that the minimum water depths of eight subglacial lakes vary between 8 and 21 m the lakes from which our depth measurements were taken are distributed widely around the ice sheet thus it may be concluded for the first time that antarctic subglacial water bodies are generally at least several meters in depth by examining the attenuation of radio waves through subglacial water the electrical conductivity of the water is estimated to be extremely low i e fresh pure water

a comment on the equation of state and the freezing point equation with respect to subglacial lake modelling the empirical equation of state eos allows the calculation of the density of water in dependence of salinity temperature and pressure water density is an important quantity to determine the internal structure and flow regime of ocean and lakes hence its exact representation in numerical models is of utmost importance for the specific simulation results the three parameters namely salinity temperature and pressure have a complex interdependency on the eos whether warmer water parcels sink or rise therefore depends on the surrounding salinity and pressure the empirical equation of freezing point eofp allows to calculate the pressure and salinity dependent freezing point of water both equations are necessary to model the basal mass balance below antarctic ice shelves or at the ice water interface of subglacial lakes this article aims three tasks first we comment on the most common formulations of the eos and the eofp applied in numerical ocean and lake models during the past decades then we describe the impact of the recent and selfconsistent gibbs thermodynamic potential formulation of the eos and the eofp on subglacial lake modelling finally we show that the circulation regime of subglacial lakes covered by at least 3000 m of ice in principle is independent of the particular formulation in contrast to lakes covered by a shallower ice sheet like e g subglacial lake ellsworth however as modelled values like the freezing and melting patterns or the distribution of accreted ice at the ice lake interface are sensitive to different eos and eofp we present updated values for subglacial lake vostok and subglacial lake concordia c 2010 elsevier b v all rights reserved

location for direct access to subglacial lake ellsworth an assessment of geophysical data and modeling ellsworth is 14 7 km 3 1 km with an area of 28 9 km 2 lake depth increases downlake from 52 m to 156 m with a water body volume of 1 37 km 3 the ice thickness suggests an unusual thermodynamic characteristic with the critical pressure boundary intersecting the lake numerical modeling of water circulation has allowed accretion of basal ice to be estimated we collate this physiographic and modeling information to confirm that lake ellsworth is ideal for direct access and propose an optimal drill site the likelihood of dissolved gas exchange between the lake and the borehole is also assessed citation woodward j a m smith n ross m thoma h f j corr e c king m a king k grosfeld m tranter and m j siegert 2010 location for direct access to subglacial lake ellsworth an assessment of geophysical data and modeling geophys res lett 37 l11501 doi 10 1029 2010gl042884

water exchange between the subglacial lake vostok and the overlying ice sheet it has now been known for several years that a 200 km long lake called lake vostok lies beneath the ice sheet on which sits vostok station in antarctica the conditions at the base of the ice sheet above this subglacial lake can provide information about the environment within the lake including the likelihood that it supports life here we present an analysis of the ice sheet structure from airborne 60 mhz radar studies which indicates that distinct zones of basal ice loss and accretion occur at the ice water interface subglacial melting and net ice loss occur in the north of the lake and across its 200 km long western margin whereas about 150 m of ice is gained by subglacial freezing in the south this indicates that significant quantities of water are exchanged between the base of the ice sheet and the lake waters which will enrich the lake with gas hydrates cause sediment deposition and encourage circulation of the lake water