flow dynamics of the rutford ice stream ice drainage basin west antarctica from radar stratigraphy rutford ice stream west antarctica drains a catchment of 45 000 km 2 into the ronne ice shelf through a 26 km wide 2 4 km deep subglacial trough adjacent to the ellsworth mountains forty two per cent of its catchment boundary is common with pine island glacier where rapid change to ice dynamics is currently underway these changes may eventually affect adjacent catchments such as rutford radar sounding data were acquired over the rutford ice drainage basin that show the internal structure in particular distinctive reflector groups were identified that mark the boundaries between four different flow elements the flow margin reflector groups include curvilinear events that cross cut isochrones and are therefore likely to be post depositional these reflections may arise from crystal orientation fabrics generated by localized strain in a flow margin one of the sectors of the ice drainage basin supplies the largest share 38 of the ice volume flux through the main trunk of rutford ice stream this sector may be preferentially affected by continuing surface lowering in the pine island glacier catchment

surface velocity and mass balance of ice streams d and e west antarctica over 75 000 surface velocity measurements are extracted from sequential satellite imagery of ice streams d and e to reveal a complex pattern of flow not apparent from previous measurements horizontal and vertical strain rates calculated from surface velocity indicate that the bed experiences larger basal shear where the surface of these ice streams is rougher ten airborne radar profiles and one surface based radar profile of ice thickness make possible the calculation of mass balance for longitudinal sections of each ice stream improved data collection methods increase data density substantially reducing random errors in velocity however systematic errors continue to limit the ability of the flux differencing technique used here to resolve local variations in mass balance nevertheless significant local variations in mass balance are revealed while overall ice streams d and e are in approximate equilibrium an earlier estimate of the net mass balance for ice stream d is improved

spatial stability of ice stream d and its tributaries west antarctica revealed by radio echo sounding and interferometry it has been shown recently that ice streams are fed by fast flowing tributaries occupying well defined subglacial troughs and with shared source areas here ice penetrating radio echo sounding res data are analyzed in conjunction with ice surface velocities derived from interferometric synthetic aperture radar insar to determine the englacial properties of tributaries feeding ice stream d west antarctica all of ice stream d s tributaries are coincident with buckled internal ice sheet layers most probably deformed by the processes responsible for enhanced ice flow between the tributaries well preserved internal layers occur the data reveal that no lateral migration of the ice stream tributaries has occurred recently this is consistent with thermomechanical ice flow modelling which indicates that the flow of ice stream d is controlled by a subglacial trough and is unaffected by changes to the flow of neighbouring ice stream c

tributaries of west antarctic ice streams revealed by radarsat interferometry interferometric radarsat data are used to map ice motion in the source areas of four west antarctic ice streams the data reveal that tributaries coincident with subglacial valleys provide a spatially extensive transition between slow inland flow and rapid ice stream flow and that adjacent ice streams draw from shared source regions two tributaries flow into the stagnant ice stream c creating an extensive region that is thickening at an average rate of 0 49 meters per year this is one of the largest rates of thickening ever reported in antarctica

re os isotopic constraints on the source of platinum group minerals pgms from the vestřev pyrope rich garnet placer deposit bohemian massif abstract mineralogical studies of the heavy fraction from a holocene pyrope rich garnet placer deposit at vest ev krkonose piedmont basin bohemian massif have identified the presence of very rare grains of platinum group minerals pgm pt fe alloy grains are accompanied by os ir ru minerals native osmium iridium and ruthenium with inclusions of pt fe alloy and hongshiite ptcu this mineral assemblage is typical for several mantle settings including ophiolites the chemistry of the os ir ru minerals shows an enrichment of the pgm in ru which is typical of ophiolites the grain morphology of pgm and pyrope rich garnet mostly rounded with numerous euhedral subhedral grains does not exclude a common source in situ laser ablation mc icp ms was used to measure the re os isotopic compositions of single os rich grains which show heterogeneous subchondritic os isotopic compositions 187 os 188 os 0 12082 0 12505 0 00003 this precludes their low temperature origin and indicates derivation of platinum group elements pges essentially from mantle derived rocks without a significant contribution of crustal os the mantle model age t ma and re depletion model age t rd model ages range from 0 4 to 1 0 ga and most likely reflect a long history of melt depletion that affected the mantle sources of pgm

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

ice flow induced scattering zone within the antarctic ice sheet revealed by high frequency airborne radar to better understand how internal radar echoes depend on ice flow conditions and radar polarization we surveyed two basins in east antarctica using 179 mhz airborne radar we compared radar echoes from three ice flow conditions parallel sheet flow in the main stream of a basin convergent flow towards an ice stream and longitudinal compression by nunataks we detected a distinct zone of high radar scattering several hundred meters thick at middle depths in the latter two regions this high scattering zone was detected only when the radar polarization plane was parallel to the compression axis in ice such a high scattering zone was not found in the parallel flow region regardless of the polarization using a recently developed theory of radar scattering in ice we interpret the high scattering zone as being caused by crystal orientation fabric alternations among adjacent ice layers due to difference in horizontal strain components we argue that the spatial variation of the high scattering zone is crucial for understanding past and present flow features

switch of flow direction in an antarctic ice stream fast flowing ice streams transport ice from the interior of west antarctica to the ocean and fluctuations in their activity control the mass balance of the ice sheet the mass balance of the ross sea sector of the west antarctic ice sheet is now positive that is it is growing mainly because one of the ice streams ice stream c slowed down about 150 years ago1 here we present evidence from both surface measurements and remote sensing that demonstrates the highly dynamic nature of the ross drainage system we show that the flow in an area that once discharged into ice stream c has changed direction now draining into the whillans ice stream formerly ice stream b this switch in flow direction is a result of continuing thinning of the whillans ice stream and recent thickening of ice stream c further abrupt reorganization of the activity and configuration of the ice streams over short timescales is to be expected in the future as the surface topography of the ice sheet responds to the combined effects of internal dynamics and long term climate change we suggest that caution is needed when using observations of short term mass changes to draw conclusions about the large scale mass balance of the ice sheet

basal mechanics of ice stream b west antarctica 1 till mechanics data from laboratory geotechnical tests on till recovered from beneath ice stream b west antarctica at the upstream b camp hereinafter the upb till show that failure strength of this till is strongly dependent on effective stress but is practically independent of strain and strain rate these data support use of a coulomb plastic rheology in modeling of ice stream behavior and subglacial till deformation our testing program combined triaxial ring shear and confined uniaxial tests to investigate till strength and compressibility results show that the upb till follows closely coulomb s equation in which shear strength is a linear function of normal effective stress apparent cohesion near zero and internal friction angle equal to 24 till compressibility is best described by a logarithmic function that relates void ratio to normal effective stress in general the behavior of the upb till is consistent with other experimental evidence regarding mechanical behavior of granular materials based on our laboratory results we formulate the compressible coulomb plastic till model in which there are three interrelated primary state variables shear strength void ratio and normal effective stress this model is used in the second part of our study to simulate response of subglacial till to realistic effective stress forcings these simulations demonstrate that the model is capable of reproducing fundamental aspects of subglacial till kinematics 1 occurrence of tilt rate oscillations and negative tilt rates in tiltmeter records and 2 distributed till deformation to depths of 0 1 1 0 m beneath the ice base our laboratory and modeling results substantiate application of the compressible coulomb plastic model in simulations of the motion of ice stream b over its weak till bed

flow of ice sheets in the vicinity of subglacial peaks although changes of surface slope across a subglacial peak can be explained at least semi quantitatively on a two dimensional basis in terms of gradients of longitudinal and shear stresses along a flow line we lack appreciation of flow in the third dimension which may tend to cause lower layers to flow around rather than over a sharp peak this paper collects and surveys evidence relevant to flow in the third dimension and discusses the processes involved particularly the evidence from radio echo layering and also discusses possible reasons for the lack of radio echo layers near the base of the antarctic ice sheet

variscan vs cadomian tectonothermal activity in northwestern sectors of the tepla barrandian zone czech republic constraints from 40ar 39ar ages five muscovite concentrates from high grade pelitic metasedimentary basement rocks exposed in northwestern sectors of the tepla barrandian zone czech republic record 40ar 39ar mineral plateau ages which range between ca 376 and 362 ma hornblende concentrates from metagabbro marianske lanz complex and fine grained basement amphibolite display plateaux which define 36ar 40ar vs 39ar 40ar isotope correlation ages of ca 370 and ca 375 ma the mineral ages are interpreted to date relatively rapid cooling through appropriate argon retention temperatures following early phases of variscan early devonian regional metamorphism a slate phyllite basement sample collected within lower grade metasedimentary rocks in southeastern portions of the tepla barrandian zone is characterized by an internally discordant 40ar 39ar whole rock age spectrum which suggests partial variscan rejuvenation of intracrystalline argon systems which had cooled through appropriate argon retention temperatures following an initial regional metamorphism at or prior to ca 500 ma cadomian hornblende from undeformed diorite of the kdyn complex records a well defined 40ar 39ar age plateau which corresponds to an isotope correlation age of ca 516 ma this is interpreted to date post magmatic cooling following emplacement

geology sulfide mineralogy and petrogenesis of the angstberg ni cu pge sulfide mineralization lausitz block bohemian massif germany a potential ni cu exploration target in central europe abstract germany s largest known magmatic ni cu pge sulfide enrichment is associated with the gabbroic angstberg intrusion that is one of several small scaled ni cu sulfide bearing intrusions in the northern bohemian massif central european variscan belt the intrusions are part of a middle devonian to early carboniferous mafic ultramafic magmatic system that emplaced within cadomian granodiorites of the lausitz block the angstberg intrusion represents an exposed sub vertical dike like body 320 60 m in plan view and comprises olivine gabbronorite olivine gabbro gabbro olivine hornblende gabbro and hornblende pyroxene gabbro the hornblende bearing and hornblende free lithologies originate from at least two variably fractionated basaltic parental magma batches which differ in their ti lree hree and volatile contents fractionation probably took place in deeper seated sections of the magmatic system possibly within staging magma chamber s overall enriched incompatible trace element patterns suggest that the angstberg lithologies are linked with intra plate magmatism likely associated with a transtensional setting in the course of the variscan orogeny the previously suggested devonian age of the intrusion is confirmed by an 40ar 39ar biotite age of 373 4 0 9 ma for the olivine gabbro the magmatic sulfide mineralization is predominated by disseminated sulfides that are hosted in the olivine richest parts of the olivine gabbronorite and olivine hornblende gabbro lithologies in addition some sulfide matrix breccias up to 70 modal sulfides occur directly on the footwall contact of the intrusion to granodiorite both mineralization types are characterized by a predominance of pyrrhotite over pentlandite and chalcopyrite with minor secondary violarite and pyrite and accessory pge au and ag phases nickel rich bismuthotellurides of the melonite merenskyite solid solution series and froodite are the most common platinum group minerals disseminated sulfides are characterized by high ni moderate cu and low pt and pd tenors 4 9 wt ni 2 6 wt cu 200 800 ppb pt 200 1000 ppb pd sulfide matrix breccias feature comparable ni but lower cu pt and pd tenors 4 6 wt ni 0 3 1 wt cu 10 100 ppb pt 40 200 ppb pd fractional crystallization and wall rock assimilation of granodiorite are regarded as key factors for sulfide saturation in the magmas associated with the angstberg intrusion sulfur isotope compositions of pyrrhotite pentlandite chalcopyrite assemblages 1 4 to 0 2 vcdt propose a mantle origin of s whereas addition of crustal s cannot be completely ruled out as the s isotope values of the sulfides overlap with those of the granodiorite 7 to 4 vcdt olivine and sulfide textures suggest that disseminated sulfides likely originate from sulfide melts that were entrained in an olivine rich crystal mush during magma rise to upper levels the sulfide matrix breccias also resulted from entrained sulfide melts or from gravity driven downward percolation of sulfide melts from stratigraphically higher now eroded levels in both cases the formation of sulfide matrix breccias most likely was amplified by in situ thermomechanical erosion of the granodioritic wall rock caused by flash boiling of thermally dehydrating biotite despite spatial limitations of the mineralized zones and limited information about the architecture of the intrusions in the northern bohemian massif the angstberg intrusion as well as other neighboring magmatic sulfide occurrences may represent a new exploration target for ni and cu in central europe

nature of radio echo layering in the antarctic ice sheet detected by a two frequency experiment a two frequency radio echo sounding experiment was carried out at dome fuji the second highest dome in east antarctica and along a 1150 km long traverse line from the dome to the coast the goal was to determine the dominant causes of the radio echo internal reflections and to investigate their possible changes with depth ranges and regions from the two frequency 60 mhz and 179 mhz radio echo responses at various sites we distinguished four zones each of the zones is characterized by a dominant cause of radio echo internal reflection as follows in the pd zone changes in dielectric permittivity are mainly due to density fluctuations in the pcof zone changes in dielectric permittivity are mainly due to changes in crystal orientation fabrics and in the ca zone changes in electrical conductivity are mainly due to changes in acidity induced by past volcanic eruptions in each of these three zones the changes occur commonly along isochrones in addition a basal echo free zone the fourth zone was found to appear always below the pcof zone these four zones and their distribution suggested variations of the physical conditions within the ice sheet

re os geochemistry and geochronology of the ransko gabbro peridotite massif bohemian massif the ransko gabbro peridotite massif in eastern bohemia is a strongly differentiated intrusive complex which hosts low grade ni cu ores mainly developed close to the contact of olivine rich rocks with gabbros in troctolites and to a much lesser extent in both pyroxene and olivine gabbros and plagioclase rich peridotites gabbro troctolite peridotite and ni cu ores from the jezirka ni cu pge deposit considered to be a typical example of the liquid segregation style of mineralization were analyzed for re os concentrations and isotopic ratios seven barren and mineralized samples from the jezirka deposit yielded a re os regression of 341 5 7 9 ma mswd 69 strongly mineralized peridotite with mantle like initial 187os 188os ratio of 0 125 suggests that os as well as other pge present in the ni cu mineralization are predominantly of mantle origin on the other hand barren and low mineralized samples have radiogenic initial 187os 188os ratios of 0 14 0 16 suggesting some import of re and or radiogenic 187os most likely through contamination by continental crust during magma emplacement the re os age of the ransko massif is significantly younger than the previously suggested lower cambrian age but it is similar to and or younger than the age of metamorphism of the adjacent kutna hora crystalline complex and the moldanubian unit therefore it is likely that the emplacement of the ransko massif and its ni cu mineralization was closely connected with the late stage evolution of the kutna hora crystalline complex

airborne radar evidence for tributary flow switching in institute ice stream west antarctica implications for ice sheet configuration and dynamics despite the importance of ice streaming to the evaluation of west antarctic ice sheet wais stability we know little about mid to long term dynamic changes within the institute ice stream iis catchment here we use airborne radio echo sounding to investigate the subglacial topography internal stratigraphy and holocene flow regime of the upper iis catchment near the ellsworth mountains internal layer buckling within three discrete topographically confined tributaries through ellsworth independence and horseshoe valley troughs provides evidence for former enhanced ice sheet flow we suggest that enhanced ice flow through independence and ellsworth troughs during the mid to late holocene was the source of ice streaming over the region now occupied by the slow flowing bungenstock ice rise although buckled layers also exist within the slow flowing ice of horseshoe valley trough a thicker sequence of surface conformable layers in the upper ice column suggests slowdown more than 4000 years ago so we do not attribute enhanced flow switch off here to the late holocene ice flow reorganization intensely buckled englacial layers within horseshoe valley and independence troughs cannot be accounted for under present day flow speeds the dynamic nature of ice flow in iis and its tributaries suggests that recent ice stream switching and mass changes in the siple coast and amundsen sea sectors are not unique to these sectors and that they may have been regular during the holocene and may characterize the decline of the wais

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

geochemistry and mineralogy of platinum group elements in the ransko gabbro peridotite massif bohemian massif czech republic the ransko gabbro peridotite massif in eastern bohemia is a strongly differentiated intrusive complex of lower cambrian age the complex hosts low grade ni cu ores mainly developed close to the contact of olivine rich rocks with gabbros in troctolites and to a much lesser extent in both pyroxene and olivine gabbros and plagioclase rich peridotites the ore zone is characterized by strong serpentinization and uralitization the total ni cu locally reaches up to 4 wt anomalous concentrations of platinum group elements pge s maximum 532 ppb pd 182 ppb pt 53 ppb rh 15 ppb ru 41 ppb ir were detected in samples of cu ni and ni cu ores maximum 2 63 wt ni and 2 31 wt cu from the jezirka orebody the main ore paragenesis includes pyrrhotite pentlandite chalcopyrite cubanite pyrite magnetite mackinawite valleriite ilmenite and sphalerite during this work michenerite froodite sperrylite gold native bismuth altaite tsumoite hessite an unnamed bi ni telluride cobaltite gersdorffite and galena were newly identified the host rocks originated through partial melting of a slightly depleted mantle source with noble metals scavenged from this primitive magma prior to the development of these rocks

fast flow signature in the stagnated kamb ice stream west antarctica among the major ice streams that drain west antarctica kamb ice stream formerly called ice stream c is unique in that it stagnated 150 yr ago but its former fast flow conditions are virtually unknown here we presentsurface based radar profiles of the ice stream s undulating internal stratigraphy which records these conditions our analysis of the profiles indicates that pre stagnation flow velocities averaged over a period 740 yr exceeded 350 m yr 1 in the trunk of the ice stream this velocity constraint would be lower if the ice had been thickening higher if thinning but suggests mass loss from the ice stream catchment that is of sufficient magnitude to reverse the gain estimated for today s siple coast region analysis of other ice streams would allow comparison of velocities over millennial time scales with observations of present day velocities useful for evaluating how west antarctic ice drainage has evolved

widespread complex flow in the interior of the antarctic ice sheet it has been suggested that as much as 90 of the discharge from the antarctic ice sheet is drained through a small number of fast moving ice streams and outlet glaciers fed by relatively stable and inactive catchment areas here evidence obtained from balance velocity estimates suggests that each major drainage basin is fed by complex systems of tributaries that penetrate up to 1000 kilometers from the grounding line into the interior of the ice sheet this finding has important consequences for the modeled or estimated dynamic response time of past and present ice sheets to climate forcing

radar reflections reveal a wet bed beneath stagnant ice stream c and a frozen bed beneath ridge bc west antarctica digital airborne radar data were collected during the 1987 88 antarctic field season in nine gridded blocks covering the downstream portions of ice stream b 6 km spacing and ice stream c 11 km spacing together with a portion of ridge bc between them an automated processing procedure was used for picking onset times of the reflected radar pulses converting travel times to distances interpolating missing data converting pressure transducer readings correcting navigational drift performing crossover analysis and zeroing remanent crossover errors interpolation between flight lines was carried out using the minimum curvature method maps of ice thickness estimated accuracy 20 m and basal reflection strength estimated accuracy 1 db were produced the ice thickness map confirms the characteristics of previous reconnaissance maps and reveals no new features the reflection strength map shows pronounced contrasts between the ice streams and ridge bc and between the two ice streams themselves we interpret the reflection strengths to mean that the bed of ice stream c as well as that of ice stream b is unfrozen that the bed of ridge bc is frozen and that the boundary between the frozen bed of ridge bc and the unfrozen bed of ice stream c lies precisely below the former shear margin of the ice stream

changes in the configuration of ice stream flow from the west antarctic ice sheet surface based ice penetrating radar profiles on the northeast flank of siple dome support the hypothesis that a curvilinear scar first observed in advanced very high resolution radiometer satellite imagery represents the margin of a formerly active ice stream the scar defines the southwestern boundary of an ice stream flowing from ice stream c to ice stream d close to where it enters the ross ice shelf our studies show that the scar coincides with a trough and upward step in surface topography approximately 5 km across underlain by a zone of disturbed internal stratigraphy revealed by the radar burial depth of the disturbed zone enables us to calculate the time of shutdown as occurring prior to approximately 1 3 ka the configuration of the ice streams draining the west antarctic ice sheet into the ross ice shelf evidently changes with time and attempts to predict the evolution of the ice sheet must incorporate this observation

satellite observations of upwelling on the continental shelf south of madagascar we report on upwelling seen in satellite avhrr sea surface temperature imagery over the continental slope and shelf of southern madagascar during february and march 2000 the upwelling is concurrent with anomalously high pseudo wind stress over the region during this period however the western boundary east madagascar current which is seen over the continental slope region may contribute to the upwelling effect the upwelling covers an area of 2 longitude by 1 latitude and at its peak is about 3 5 c cooler than the local ambient sea surface temperature the paucity of in situ wind and current data in the region however prohibit a quantitative assessment of the relative forcing

a new numerical model of coupled inland ice sheet ice stream and ice shelf flow and its application to the west antarctic ice sheet we have developed a dynamic thermodynamic finite element numerical model that couples inland ice sheet ice stream and ice shelf dynamics this new model stands apart from other whole ice sheet models in its explicit treatment of ice stream flow additionally the model accounts for both horizontal and vertical advection and diffusion of temperature in the flowing ice in present day simulations of the west antarctic ice sheet wais modeled ice velocity agrees well with observed ice flow in particular the model reproduces the pattern of speed variation across ice streams although the continuous downstream speed up of ice flow cannot be reproduced without concurrent downstream variation in basal friction model thermodynamics evaluated qualitatively by model prediction of the spatial distribution of basal melting and quantitatively by comparison with ice temperature measured in boreholes at several locations are sound in particular the model reproduces the broad pattern of frozen bed inter ice stream ridges and melted bed ice streams model initialization for long time simulations is somewhat limited by computation time and by a thermodynamic feedback at sites of large viscous heating that can be a problem in heat balance only model initializations methods for averting those initialization problems are discussed the new model can accommodate a variety of boundary conditions such as various bed rheologies and is well suited to investigate the origin and evolution of wais ice streams within the context of the whole ice sheet system

chromian spinel mineralogy of the stare ransko gabbro peridotite czech republic and its implications for sulfide mineralization chromian spinels from the stare ransko gabbro peridotite czech republic display a distinct development trend from primary cr rich spinels towards ferro ferri ti enriched magnetite and magnesio alumina spinel sensu stricto spinels zoning in the spinels is thought to be the result of a combination of exsolution postdepositional diffusion and replacement processes multiphase globular silicate inclusions mainly biotite and amphibole also orthopyroxene serpentine and chlorite are present in euhedral largely homogeneous chromian spinels which occur in a matrix of poikilitic amphibole the enclosed minerals have similar compositions to the minerals in the host rock these silicate inclusions are interpreted as being due to a late magmatic introduction of a hydrous k ca rich melt phase contaminated by wall rock interaction into a semi solidified olivine pyroxenes cumulate