decoupling of o and pb isotope systems of uraninite in the early proterozoic conglomerates in the elliot lake district abstract both isotopic pb and o and chemical compositions were measured by two in situ techniques sims and epma for 20 m diameter areas of over forty eight individual grains of uraninite in the early proterozoic quartz pebble conglomerate uranium deposits in the elliot lake district in order to constrain the origin of uraninite and its post mineralization history together with textural observation by sem pb isotope analyses and chemical compositions of brannerite and uranothorite our data revealed a protracted uranium remobilization history for the uranium deposits in the elliot lake district all grains of uraninite examined show consistently high th contents supporting detrital origin of uraninite derived from pegmatitic rocks the unimodal distribution of th concentration of uraninite excludes the possibility that uraninite formed by multiple pathways where some grains are detrital and some are later diagenetic hydrothermal in origin the measured uraninite grains yield u pb discordia age of 1 8 ga which is much younger than the depositional age of the host huronian basin 2 45 to 2 2 ga this age closely corresponds with the age of peak metamorphism in the huronian basin suggesting that all uraninite grains completely lost its pb during this time the least texturally and chemically altered grains of uraninite yield 18 o smow values of 10 to 22 this range of 18 o value is lower than that expected for uraninite from granitic pegmatitic rocks by 10 it is concluded that the oxygen isotope ratios of uraninite is completely reset during the peak metamorphism and or by interaction with recent meteoric water the 18 o of uraninite however do not show systematic variation with th u pb u and 207 pb 206 pb ratios indicating uraninite exchanged its o isotopes without much disturbance in its chemical composition among the various geochemical signatures in uraninite high th content is the only original signature preserved since detrital deposition of uraninite both pb and o isotope systems have been disturbed in various degrees by later events the coupling decoupling of the composition pb and o isotope systematics of uraninite reflect the protracted mineralization remobilization history of the oldest uraninite

diversity in earth s early felsic crust paleoarchean peraluminous granites of the barberton greenstone belt earth9s oldest preserved granitoid crust dates back to the paleoarchean and consists predominantly of sodic tonalite trondhjemite granodiorite ttg granitoids that arose through the partial melting of hydrated metabasalts in contrast granites sensu stricto typically appear relatively late in the plutonic record of the old cratons however the existence of hadean zircons with mineral inclusion suites that are consistent with crystallization from peraluminous granitic magmas indicates that granitic rocks formed part of the earliest felsic crust although we have direct evidence this earliest felsic crust is not preserved here we present evidence of an unusual variety of markedly low cao k 2 o rich rutile bearing peraluminous granite and rhyolite that was produced concurrently with ttg magmas during three magmatic cycles in the barberton greenstone belt bgb southern africa this material is not preserved as in situ rock units but occurs as clasts within a younger conglomerate within these rocks plagioclase feldspar is a rare inclusion in zircon relative to alkali feldspar and has low anorthite contents an 2 o and sr content as well as the peraluminous character of the magmas is a consequence of phengite melting in a metagraywacke source at pressures in excess of those of plagioclase stability this process contributed to each episode of continental crustal growth through the paleoarchean to mesoarchean in the bgb despite leaving no plutonic record at the typical mid crustal level of exposure that the ttg plutons around the belt represent

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

a whiff of oxygen before the great oxidation event high resolution chemostratigraphy reveals an episode of enrichment of the redox sensitive transition metals molybdenum and rhenium in the late archean mount mcrae shale in western australia correlations with organic carbon indicate that these metals were derived from contemporaneous seawater rhenium osmium geochronology demonstrates that the enrichment is a primary sedimentary feature dating to 2501 8 million years ago ma molybdenum and rhenium were probably supplied to archean oceans by oxidative weathering of crustal sulfide minerals these findings point to the presence of small amounts of o 2 in the environment more than 50 million years before the start of the great oxidation event

chemistry of sands from the modern indus river and the archean witwatersrand basin implications for the composition of the archean atmosphere both the indus river and the witwatersrand basin contain sand with grains of detrital uraninite because this mineral is easily oxidized its presence in archean strata as a detrital particle has been used as evidence for a low oxygen atmosphere before 2 5 ga however its presence in modern sand from the indus river system has been used to argue that detrital uraninite does not provide information about the oxygen concentration of earth s early atmosphere petrographic and chemical study of sand from these two sources reveals differences that suggest the modern indus sand cannot be used as an analog for the archean witwatersrand occurrences the witwatersrand quartzites are depleted in ca mg and na indicating that the original sand from which they formed had been subjected to intense weathering the chemical index of alteration cia a commonly used indicator of degree of weathering yields an average value of about 0 80 for witwatersrand quartzites comparable to modern tropical streams such as the orinoco that drain deeply weathered terrains under tropical conditions cia 0 75 in contrast the cia for indus sand is 0 45 indicating virtually no chemical weathering the significance of archean quartz pebble conglomerates is not just that they contain unstable detrital phases like uraninitemore and pyrite but that these particles are associated with rocks whose compositions suggest intense weathering these conglomerates must have been subjected to intense weathering under tropical conditions either in their source area or at the site of deposition and the preservation of minerals like uraninite such conditions is indeed strong evidence for a low oxygen atmosphere less

assessing the ability of numerical ice sheet models to simulate grounding line migration grounding line migration is a key process affecting the stability of marine ice sheets such as the west antarctic ice sheet wais grounding line motion is often included in numerical models simulating the past and future evolution of the wais however little attention has been paid to the numerical consistency of these models the aim of this paper is to assess the ability of simple versions of existing marine ice sheet models to simulate grounding line migration in particular we investigate the response of the grounding line to external forcing and the sensitivity of the models predictions to their numerics and the mechanical coupling between ice sheet and shelf from the model comparison there is no consensus on how the grounding line should react to changes in boundary conditions a crucial finding is the strong dependency of models using a fixed grid on numerical details such as the horizontal grid size this implies that we should be very wary about grounding line predictions from such models including mechanical coupling at the grounding line does not seem to change the qualitative behavior of the models this suggests that the way the grounding line is treated in marine ice sheet models dominantly determines the grounding line dynamics we find that models that employ a moving grid to explicitly track the grounding line do not share many of the deficiencies of the fixed grid models we conclude that at present no reliable model of the grounding line is available and further model development is urgently needed

modeling the instantaneous response of glaciers after the collapse of the larsen b ice shelf following the disintegration of the larsen b ice shelf antarctic peninsula in 2002 regular surveillance of its 20 tributary glaciers has revealed a response which is varied and complex in both space and time the major outlets have accelerated and thinned smaller glaciers have shown little or no change and glaciers flowing into the remnant scar inlet ice shelf have responded with delay in this study we present the first area wide numerical analysis of glacier dynamics before and immediately after the collapse of the ice shelf combining new datasets and a state of the art numerical ice flow model we simulate the loss of buttressing at the grounding line and find a good qualitative agreement between modelled changes in glacier flow and observations through this study we seek to improve confidence in our numerical models and their ability to capture the complex mechanical coupling between floating ice shelves and grounded ice

first whiffs of atmospheric oxygen triggered onset of crustal gold cycle lack of a suitable gold source has long been regarded as a major argument against a palaeoplacer model for the exceptionally well endowed witwatersrand gold province by comparing worldwide witwatersrand type deposits occurrences ranging in age from 3 1 to 1 8 ga we propose that the primary concentration of gold in the continental crust resulted from atmospheric and biological evolution in the mesoarchaean a high au flux off the archaean land surface orders of magnitude greater than today s was a consequence of the chemistry of the mesoarchaean atmosphere and hydrosphere when early life gradually changed from anaerobic anoxygenic to oxygenic photosynthesizers at around 3 0 ga the first whiffs of free oxygen produced by photosynthesis under an overall reducing atmosphere provided the ideal trap for au dissolved in the huge reservoir in meteoric and shallow sea water oxidative precipitation of gold on the surface of the o2 producing microbes probably cyanobacteria could fix huge amounts of gold over large areas some of this microbially mediated gold is still preserved in thin kerogen layers carbon seams that typically developed on erosional unconformities scour surfaces and bedding planes in near shore environments at around 2 9 ga this gold provided the principal source for the very rich placer deposits that formed by the subsequent sedimentary reworking of the delicate microbial mats on aeolian deflation surfaces into fluvial channels and delta deposits represented by meso to neoarchaean auriferous conglomerates tectonic reworking of these first gold rich sediments by orogenic processes explains the temporal peak of orogenic type gold formation at around 2 7 to 2 4 ga with time the strongly gold enriched archaean sediments became progressively eroded covered or tectonically reworked and their role as potential source of younger placer deposits diminished this explains why witwatersrand type deposits younger than 2 4 ga are rare and far less well endowed and effectively missing in the rock record after 1 8 ga

mineral chemical study of u bearing minerals from the dominion reefs south africa the neo archean dominion reefs 3 06 ga are thin meta conglomerate layers with concentrations of u and th bearing heavy minerals higher than in the overlying witwatersrand reefs ore samples from uranium one africa s rietkuil and dominion exploration areas near klerksdorp south africa were investigated for their mineral paragenesis texture and mineral chemical composition the ore and heavy mineral assemblages consist of uraninite other uraniferous minerals fe sulphides ni co sulfarsenides garnet pyrite pyrrhotite monazite zircon chromite magnetite and minor gold sub rounded uraninite grains occur associated with the primary detrital heavy mineral paragenesis u ti u th minerals pitchblende colloform uraninite and coffinite are of secondary re mobilised origin as evidenced by crystal shape and texture most of the uranium mineralisation is represented by detrital uraninite with up to 70 2 wt uo2 and up to 9 3 wt tho2 re crystallised phases such as secondary pitchblende without th coffinite u ti and u th phases are related to hydrothermal overprint during low grade metamorphism and are of minor abundance

the early precambrian atmosphere and hydrosphere thermodynamic constraints from mineral deposits a chemical equilibrium model for the early precambrian atmosphere and hydrosphere is presented thermodynamic calculations based on observed stable minerals and mineral assemblages in banded iron formation bif pyritic uraniferous and auriferous quartz pebble conglomerates gold bearing conglomerates of the tarkwaian in ghana and red bed sediments yield the following conditions before approx 2350 ma the atmosphere was oxygen free p sub o 2 leftpointingdoubleanglequotationmark 10 super 10 bars consisted mainly of co 2 p sub co 2 2 s 10 super 7 5 bars over continents to 10 super 9 bars over oceans and hydrogen p sub h 2 10 super 9 bars over continents to 10 super 7 5 bars over oceans the high co 2 partial pressures generated acid rain and surface waters ph or 3 9 and buffered the ocean water to values around ph 6 5 to 7 in the oceans the iron minerals of the various banded iron formation facies goethite pyrite siderite in conjunction with the atmospheric co 2 partial pressure acted as a mineral buffer for p sub h 2 s and for the redox state p sub h 2 in continental weathering environments pyrite formation from ferrous iron and atmospheric h 2 s and oxidation of h 2 s to so 4 super 2 are considered the prime redox controlling mechanisms river waters which previously equilibrated with pyrite precipitated banded iron formation upon mixing with seawater mainly due to ph changes between 2350 and 2100 ma transition from pyritic to iron oxide bearing conglomerates indicates that photosynthetic oxygen production rates began to exceed oxygen consumption rates and that the redox buffers involving hydrogen sulfide and ferrous iron were overwhelmed increased oxidation potentials and increased surface water ph due to declining atmospheric co 2 partial pressures led to immobilization of iron and consequently to continental red bed formation and cessation of banded iron formation deposition

redox state of the archean atmosphere evidence from detrital heavy minerals in ca 3250 2750 ma sandstones from the pilbara craton australia the presence of detrital uraninite and pyrite in fluvial placers of the witwatersrand basin south africa has been used to infer low levels of atmospheric oxygen during the archean 2500 ma however recent studies advocate a hydrothermal origin for these minerals thereby limiting their value as constraints on the composition of the early atmosphere in contrast ca 3250 2750 ma fluvial siliciclastic sediments from the pilbara craton in australia have never undergone significant hydrothermal alteration and their heavy minerals are of unequivocal detrital origin these heavy minerals include the redox sensitive phases pyrite uraninite and gersdorffite along with more inert zircon rutile chromite and monazite locally siderite is a major constituent to 90 of the heavy mineral population with grains displaying evidence for several episodes of erosion rounding and subsequent authigenic overgrowth detrital siderite is very rare in post archean sandstones largely due to its instability in oxidizing environments however its frequent survival of prolonged transport in well mixed and therefore well aerated archean river waters that contained little organic matter strongly implies that the contemporary atmosphere was indeed much less oxidizing than at present moreover concentrations of reduced sulfur species must have been very low in surface fluids for siderite to survive repeated transportation events without pyritization

modeling the evolution of antarctic ice sheet over the last 420 000 years implications for altitude changes in the vostok region a new thermomechanical three dimensional model designed to simulate the evolution of the antarctic ice sheet over long time periods is presented this model incorporates the various types of ice flow found in antarctica relatively slow inland ice flow that is essentially due to ice deformation fast ice flow in the regions with ice streams and ice shelf flow by coupling these three types of flow it is possible to predict grounding line migration simulations covering four glacial interglacial cycles have been conducted by forcing this model with a temperature record from vostok and a sea level record from marine cores our findings indicate that grounding line migration induced by sea level changes is the primary factor governing the evolution of the antarctic ice volume on the other hand the altitude of the ice sheet surface at vostok is driven by accumulation rate variations the amplitude of the altitude change does not exceed 150 m and is very similar for all the sites located on the antarctic plateau

an integrated model for the wave induced seabed response around marine structures model verifications and applications abstract in this study an integrated model poro wssi ii for wave seabed structure interactions wssi is developed by combining i the volume averaged reynolds averaged navier stokes varans equations for wave motions in a fluid domain and the porous media flows in porous structures and ii the dynamic biot s equations for a porous elastic seabed the effects of the porous flow in seabed and marine structures and the fluid exchange at the interface between the fluid domain and solid domain on wave characteristics are considered the finite difference two step projection method and the forward time difference method are adopted to solve the varans equations the volume of fluid vof method is applied to track water free surface the finite element method and the generalized newmark method are respectively adopted for the space discretization and time discretization of the dynamic biot s equations a one way integrating method is developed to integrate the varans equations with the dynamic biot s equations several experimental data available in the literature are used to validate the integrated model an overall agreement between the numerical results and the experiment data indicates that the integrated model developed for the wssi problem is highly reliable the integrated model is then applied to investigate the dynamic response of a large scale composite breakwater on a seabed and the mechanism of wssi numerical results indicate that there are intensive fluid exchanges between the water body and the seabed and strong seepage forces in the seabed under the ocean wave loading the excessive upward seepage force leads to the liquefaction of the seabed in the region under wave trough there is a liquefaction zone in the seabed close to the bottom corner of the rubble mound which may lead to foundation instability of the composite breakwater the parametric study indicates that the wave characteristics have a significant impact on the liquefaction properties depth width and area

resonant fluidization of silty soil by water waves a new resonant stage is identified in the laboratory during the complex process of soil fluidization by water waves massive fluidization failure is consistently observed following internal resonance events inside the silt bed the weakening effect of this resonance is observed to endure long periods of consolidation a wave reloading following such a consolidation period on a preresonated soil usually results in a recurrence of a similarly massive fluidization failure analysis of pore pressure records indicates that the resonance events are localized in space and the associated wave modes are short in wavelengths it is proposed that these resonant events are a consequence of a strong channeling of the seepage flow within the silt bed

the evolving instability of the remnant larsen b ice shelf and its tributary glaciers 2015 elsevier b v following the 2002 disintegration of the northern and central parts of the larsen b ice shelf the tributary glaciers of the southern surviving part initially appeared relatively unchanged and hence assumed to be buttressed sufficiently by the remnant ice shelf here we modify this perception with observations from icebridge altimetry and insar inferred ice flow speeds our analyses show that the surfaces of leppard and flask glaciers directly upstream from their grounding lines lowered by 15 to 20 m in the period 2002 2011 the thinning appears to be dynamic as the flow of both glaciers and the remnant ice shelf accelerated in the same period flask glacier started accelerating even before the 2002 disintegration increasing its flow speed by 55 between 1997 and 2012 starbuck glacier meanwhile did not change much we hypothesize that the different evolutions of the three glaciers are related to their dissimilar bed topographies and degrees of grounding we apply numerical modeling and data assimilation that show these changes to be accompanied by a reduction in the buttressing afforded by the remnant ice shelf a weakening of the shear zones between its flow units and an increase in its fracture the fast flowing northwestern part of the remnant ice shelf exhibits increasing fragmentation while the stagnant southeastern part seems to be prone to the formation of large rifts some of which we show have delimited successive calving events a large rift only 12 km downstream from the grounding line is currently traversing the stagnant part of the ice shelf defining the likely front of the next large calving event we propose that the flow acceleration ice front retreat and enhanced fracture of the remnant larsen b ice shelf presage its approaching demise

dynamical processes involved in the retreat of marine ice sheets marine ice sheets with mechanics described by the shallow ice approximation by definition do not couple mechanically with the shelf such ice sheets are known to have neutral equilibria we consider the implications of this for their dynamics and in particular for mechanisms which promote marine ice sheet retreat the removal of ice shelf buttressing leading to enhanced flow in grounded ice is discounted as a significant influence on mechanical grounds sea level rise leading to reduced effective pressures under ice streams is shown to be a feasible mechanism for producing postglacial west antarctic ice sheet retreat but is inconsistent with borehole evidence warming thins the ice sheet by reducing the average viscosity but does not lead to grounding line retreat internal oscillations either specified or generated via a macayeal payne thermal mechanism promote migration this is a noise induced drift phenomenon stemming from the neutral equilibrium property of marine ice sheets this migration occurs at quite slow rates but these are sufficiently large to have possibly played a role in the dynamics of the west antarctic ice sheet after the glacial maximum numerical experiments suggest that it is generally true that while significant changes in thickness can be caused by spatially uniform changes spatial variability coupled with dynamical variability is needed to cause margin movement

a tutorial on the use of control methods in ice sheet modeling control methods are recommended as an efficient means to estimate undetermined physical parameters and boundary conditions and in so doing to improve the fidelity of a given ice sheet flow model to specific ice sheet velocity observations to accomplish this task the underlying dynamics of the model are inverted this permits model tuning adjustments to be represented explicitly in terms of model observation misfit

the onset of oxidative weathering traced by uranium isotopes abstract the invention of photosynthesis was a key interval in earth s history initiating major changes in the evolution of the oceans and atmosphere many studies suggest that oxygen levels were already enhanced before the paleoproterozoic great oxidation event goe 2 45 to 2 32 ga however the timing of the onset of photosynthetic oxygenation as well as spatio temporal fluctuations of atmospheric and oceanic oxygen levels are highly debated here we present a new and substantial uranium u isotope dataset comprising marine sediments e g black shales carbonates and iron rich sedimentary rocks from the hamersley basin western australia kaapvaal and zimbabwe cratons both southern africa with depositional ages ranging from 3 4 to 2 2 ga sub recent u mobilization was monitored by analyzing 234u along with 238u and detrital u contribution was monitored with al u or th u ratios samples with significant authigenic u enrichment efu 2 relative to post archean australian shale paas exhibit authigenic 238u ranging from 0 96 to 0 02 0 05 2 s d long term reproducibility which deviates markedly from typical 238u of the continental crust 0 40 to 0 20 remarkably we find increased 238u variability in the upper ghaap group 2 50 2 47 ga carbonates and black shales and the lower pretoria group 2 42 2 32 ga duitschland formation shales of the kaapvaal craton the predominantly light u isotope composition of those samples is likely explained by u isotope fractionation related to the onset of partial u mobilization during slow oxidative weathering of uraninite shortly before and during the goe the lack of low 238u values in the timeball hill formation overlying the duitschland formation indicate the onset of quantitative u mobilization during modern style oxidative weathering of uraninite and other u bearing minerals with limited u isotope fractionation these limited u isotope variations measured in the paleoproterozoic timeball hill formation further suggest that in contrast to modern oceans in the period following the goe u was largely quantitatively reduced and removed from the water column of the transvaal basin which remained mainly anoxic at depth

northern larsen ice shelf antarctica further retreat after collapse changes of larsen ice shelf antarctica and the surrounding glaciers after its collapse in 1995 were investigated using satellite radar imagery with emphasis onchanges in the glaciers which previously nourished the ice shelf north of seal nunataks the large glaciers retreated several kilometres inland of the previousgrounding line the velocity field of drygalski glacier the largest glacier in this area was mapped by means of interferograms derived from pairs of ers syntheticaperture radar images from 1995 and 1999 the main part of the glacier showed significant acceleration of flow over these 4 years with an increase of veocity upto three fold at the terminus similar accelerations were observed by means of interferometry on several other grounded glaciers suggesting that the removal ofice shelves could lead to an effect on eustatic sea level for larsen b the northernmost surviving part of larsen ice shelf the retreat of the ice front to october2000 is documented

the basal stress distribution of ice stream e antarctica inferred by control methods the irregular spatial distribution and velocity independence of basal friction derived from landsat measured surface velocity suggests that ice stream flow is not controlled by the properties of a deformable basal till alone rigid bedrock substrata may contact the base of the ice stream in small 100 km2 areas where the velocity field displays strong vorticity and where the ice stream surface appears rumpled in landsat images

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

petrography and geochronology of the pele mountain quartz pebble conglomerate uranium deposit elliot lake district canada uranium deposits older than about 2200 ma are generally hosted within quartz pebble conglomerates and are a source of economic uranium the genesis of these deposits is controversial and genetic models include hydrothermal detrital placer and modified placer petrography of the uranium mineralogy from the pele mountain quartz pebble conglomerate uranium deposit of the elliot lake district canada shows that the dominant uranium minerals are thorite th u sio4 and brannerite u ca ce ti fe 2o6 uranium lead u pb and lead lead pb pb isotopic analyses of thorite brannerite and galena were obtained using secondary ion mass spectrometry thorite has u pb age of 2489 24 in the quartzose conglomerate and 280 67 ma in the quartz arenite brannerite has a u pb age of 2403 120 ma thorite pb pb ages range from 521 19 to 248 8 ma in the arenite and from 2453 12 to 935 27 ma in the conglomerate whereas brannerite pb pb ages are between 1335 0 11 and 848 13 ma galena has a pb pb age of 2659 8 ma which likely represents the age of the source formation that produced detrital galena the rounded texture and u pb age of the conglomerate thorite corresponds to the depositional age of the host conglomerate bed of the matinenda formation suggesting a detrital origin the young u pb age of the thorite in the arenite represents the age of a resetting event the brannerite which replaces rutile has a u pb consistent with fluid events associated with the blezardian or penokean orogenic events and is likely of hydrothermal origin therefore based on the textures and variable ages of the uranium and sulfide minerals the pele mountain quartz pebble conglomerate uranium deposit is interpreted to be a modified placer type deposit

numerical modelling and data assimilation of the larsen b ice shelf antarctic peninsula in this study the flow and rheology of pre collapse larsen b ice shelf are investigated by using a combination of flow modelling and data assimilation observed shelf velocities from satellite interferometry are used to constrain an ice shelf model by using a data assimilation technique based on the control method in particular the ice rheology field and the velocities at the inland shelf boundary are simultaneously optimized to get a modelled flow and stress field that is consistent with the observed flow the application to the larsen b ice shelf shows that a strong weakening of the ice in the shear zones mostly along the margins is necessary to fit the observed shelf flow this pattern of bands with weak ice is a very robust feature of the inversion whereas the ice rheology within the main shelf body is found to be not well constrained this suggests that these weak zones play a major role in the control of the flow of the larsen b ice shelf and may be the key to understanding the observed pre collapse thinning and acceleration of larsen b regarding the sensitivity of the stress field to rheology the consistency of the model with the observed flow seems crucial for any further analysis such as the application of fracture mechanics or perturbation model experiments

metamorphism of the witwatersrand gold fields a review abstract there is widespread evidence for greenschist facies metamorphism in different rock types within all the witwatersrand gold mines the silicate mineral assemblages suggest near uniform peak metamorphic conditions equivalent to the chlorite zone in all the major witwatersrand gold fields assemblages of coexisting pyrophyllite chloritoid have been recorded in most of the gold mines and the critical assemblage of pyrophyllite chloritoid muscovite chlorite quartz tourmaline rutile pyrite is found in most gold fields kaolinite paragonite biotite pyrrhotite and arsenopyrite have more local distribution pelites shales quartzites meta conglomerates metabasalts and some dykes record these metamorphic assemblages albeit in different mineral proportions and pyrophyllite and chloritoid are recorded from many gold producing reefs andalusite kyanite garnet and or cordierite bearing assemblages indicate higher metamorphic grades in three discrete areas along the nw margin of the witwatersrand basin to the ne of the evander gold field and around the collar of the vredefort dome concentric metamorphic facies around the vredefort dome culminate in granulite facies assemblages near the centre it is important to note that there are no definitive published data constraining the temporal relationships between the granulites and amphibolites of the vredefort dome greenschist facies assemblages in the vredefort collar rocks thermal metamorphism related to intrusion of the bushveld complex and the regional greenschist facies metamorphism documented in the gold fields metamorphic conditions of 350 50 c and up to 3 kbars are inferred for the gold fields with temperatures increasing to around 700 c in granulites of the vredefort dome other geothermometric methods including illite crystallinity fluid inclusions carbon elemental ratios and vitrinite reflectance are compatible with the above estimates but are each associated with considerable temperature uncertainties fluids in equilibrium with pyrophyllite muscovite quartz pyrite paragonite are inferred during metamorphism suggesting slightly acid conditions and elevated sulphur activities regional greenschist facies metamorphism documented in the witwatersrand gold mines was accompanied by widespread deformation large fluxes of aqueous fluid and considerable element mobility the low intermediate pressure of this metamorphic terrain and the lack of large scale tectonic thickening favours underplating by mafic magmas as the likely cause of the metamorphic pattern no model for gold genesis in the witwatersrand basin is complete without accommodating post burial processes including metamorphism deformation and alteration any placer gold would be overprinted by these processes and any epigenetic gold could potentially be introduced during these periods of thermal and fluid activity it is also important to note at least two quite unusual features of the witwatersrand metamorphic assemblages the virtual absence of feldspar from the 3 km thick central rand group over 300 km and the narrow range of metamorphic grade inferred for the gold mines over this same distance

sulphur isotope evidence for an oxic archaean atmosphere the presence of mass independently fractionated sulphur isotopes mif s in many sedimentary rocks older than approximately 2 4 billion years gyr and the absence of mif s in younger rocks has been considered the best evidence for a dramatic change from an anoxic to oxic atmosphere around 2 4 gyr ago this is because the only mechanism known to produce mif s has been ultraviolet photolysis of volcanic sulphur dioxide gas in an oxygen poor atmosphere here we report the absence of mif s throughout approximately 100 m sections of 2 76 gyr old lake sediments and 2 92 gyr old marine shales in the pilbara craton western australia we propose three possible interpretations of the mif s geologic record 1 the level of atmospheric oxygen fluctuated greatly during the archaean era 2 the atmosphere has remained oxic since approximately 3 8 gyr ago and mif s in sedimentary rocks represents times and regions of violent volcanic eruptions that ejected large volumes of sulphur dioxide into the stratosphere or 3 mif s in rocks was mostly created by non photochemical reactions during sediment diagenesis and thus is not linked to atmospheric chemistry

the sequence of sediment behaviour during wave induced liquefaction this paper presents the results of an experimental investigation of the complete sequence of sediment behaviour beneath progressive waves the sediment was silty with d50 0 060 mm two kinds of measurements were carried out pore water pressure measurements across the sediment depth and water surface elevation measurements the process of liquefaction compaction was videotaped from the side simultaneously with the pressure and water surface elevation measurements the video records were then analysed to measure i the time development of the mudline ii the time development of liquefaction and compaction fronts in the sediment and iii the characteristics of the orbital motion of the liquefied sediment including the motion of the interface between the water body and the sediment the ranges of the various quantities in the tests were wave height h 9 17 cm wave period t 1 6 sec water depth 42 cm and the shields parameter 0 34 0 59 the experiments reveal that with the introduction of waves excess pore pressure builds up which is followed by liquefaction during which internal waves are experienced at the interface of the water body and the liquefied sediment the sequence of processes known from a previous investigation this sequence of processes is followed by dissipation of the accumulated excess pore pressure and compaction of the sediment which is followed by the formation of bed ripples the present results regarding the dissipation and compaction appear to be in agreement with recent centrifuge wave tank experiments as for the final stage of the sequence of processes formation of ripples the ripple steepness normalized with the angle of repose for sediment with liquefaction history is found to be the same as that in sediment with no liquefaction history

relationship between detrital zircon age spectra and the tectonic evolution of the late archaean witwatersrand basin south africa abstract shrimp u pb detrital zircon age spectra for 14 samples from the west rand and central rand groups of the witwatersrand basin indicate that felsic source rocks mostly formed at 3090 3060 and 3000 2870 ma with a smaller proportion formed prior to 3100 ma provenance evaluation indicates that source rocks have largely vanished through erosion such that greenstone terrains extending from the amalia kraaipan belt in the southwest to the murchison belt in the northeast are models of but not the actual source areas analysis of detrital zircon age modes reveals that the west rand and central rand groups record different basins the west rand group is characterised by age subpopulations with a much smaller range and less complex distribution than those in the central rand group furthermore detrital zircon age spectra for the west rand group generally decrease in complexity up section whereas they become increasingly more complex in the central rand group it is concluded that the source area for the west rand group is consistent with a passive margin setting with the decrease in complexity up section of zircon age spectra being the consequence of increasing maturity of the hinterland conversely the source area of the central rand group was far more dynamic with an increasing age range of source rocks being exposed through time pulsed tectonic rejuvenation of the source area is implied contemporaneous emplacement uplift and erosion of granitoids supports a magmatic fold thrust belt as the source and a complementary retroarc basin setting detrital xenotime grains record the ages of high u granitoids emplaced in the fold thrust belt during deposition of the central rand group and are much closer to depositional ages than are detrital zircon grains change from a passive margin to a retroarc basin across the west rand central rand boundary is emphasised by change from simple to increasingly complex detrital zircon age spectra this study establishes that detrital zircon geochronology can overcome the problem of overly quartzose compositions in archaean sandstones due to severe chemical weathering and authigenic alteration thereby enabling distinction between passive margin and retroarc basin stages through systematic changes in the age complexity of the source area

marine ice in larsen ice shelf 1 it is argued that larsen ice shelf contains marine ice formed by oceanic freezing and other mechanisms missing basal returns in airborne radar soundings and observations of a smooth and healed surface coincide downstream of regions where an ocean model predicts freezing visible imagery suggests that marine ice currently stabilizes larsen c ice shelf and implicates failure of marine flow bands in the 2002 larsen b ice shelf collapse ocean modeling indicates that any regime change towards the incursion of warmer modified weddell deep water into the larsen c cavity could curtail basal freezing and its stabilizing influence

chlorite thermometry in the witwatersrand basin constraints on the paleoproterozoic geotherm in the kaapvaal craton south africa a series of post depositional alteration events can be distinguished in the late archean witwatersrand basin they range from thrust controlled metamorphism during pre transvaal crustal thickening burial metamorphism and hydrothermal infiltration during crustal thinning and associated deposition of the 2 6 2 1 ga transvaal supergroup sediments thermal metamorphism related to the 2 06 ga bushveld igneous complex and hydrothermal alteration as a consequence of the 2 023 ga vredefort impact event the relative importance of these events varies locally with different events responsible for the maximum temperatures achieved in different parts of the basin the pre transvaal metamorphism affected the northern margin of the basin whereas the syn bushveld thermal metamorphism is particularly pronounced in the lower parts of the witwatersrand supergroup chlorite geothermometry in conjunction with fluid inclusion microthermometry and the thermodynamic modeling of equilibrium mineral assemblages was applied

the growth and concentration of uranium and titanium minerals in hydrocarbons of the carbon leader reef witwatersrand supergroup south africa abstract uranium and gold bearing pyrobitumen from the carbon leader reef in the witwatersrand basin south africa was investigated by high resolution transmission electron microscopy this study provides evidence for the in situ growth of uraninite and anatase nanocrystals in the pyrobitumen implying mobilization and concentration of uranium and titanium by formerly mobile liquid hydrocarbons individual nanocrystals of uraninite and anatase are pervasively distributed and locally isolated within the pyrobitumen matrix crystallization of uraninite and anatase led to the formation of complex nanocrystal aggregates by oriented attachment in which anatase generally provided nuclei for the growth of uraninite single nanocrystals of curite occur locally in channel ways within masses of uraninite nanocrystals consistent with later auto oxidation of uraninite and limited release of water during hydrocarbon maturation on the basis of evidence for the migration of liquid hydrocarbons in the witwatersrand basin and the presence of abundant uraninite and anatase nanoparticles in pyrobitumen a new model is proposed for the transport and concentration of uranium and titanium in the carbon leader reef according to this model liquid hydrocarbons that were circulating in the witwatersrand basin dissolved detrital u ti bearing minerals and transported the uranium and titanium until thermal degradation immobilized the hydrocarbons by solidifying them as pyrobitumen the latter process involved the release of volatiles and the destruction of bonds that may have held the uranium and titanium in solution thereby inducing the growth of individual uraninite and anatase nanocrystals and the formation of complex nanocrystal aggregates within the pyrobitumen

the oxygen geochemical cycle dynamics and stability abstract the first and possibly only major rise of atmospheric oxygen from p o 2 0 1 pal the present atmospheric level to p o 2 10 pal appears to have occurred sometime before 2 ga ago although the exact time of and the cause s for the rise have been hotly debated equally important questions on the atmospheric oxygen concern its stability especially the mechanisms regulating the atmospheric p o 2 level and the causes and magnitude of p o 2 variations since the first major rise of atmospheric oxygen previous efforts to model the p o 2 variation during the phanerozoic time have typically relied on secondary information such as the carbon and sulfur isotopic records of sedimentary rocks and on simple dynamics of the geochemical cycles of o c s and p based on box type models as a result many kinetic questions about the variation and stability of atmospheric oxygen could not have been answered here we quantitatively evaluate the dynamics and stability of atmospheric o 2 and co 2 using recent experimental data field observations and a new model for the c o coupled geochemical cycles we examine the change with time in the fluxes of various compounds o 2 co 2 phosphate organic c carbonate c c bearing reduced volcanic gases and c free reduced volcanic gases among the various reservoirs atmosphere soil surface ocean deep ocean the lower crust and mantle and upper crust under a variety of scenarios our model does not assume steady state fluxes for any of the reservoirs rather the model incorporates the kinetic experimental data on oxidation of coal a proxy for kerogen the dynamics of soil formation and erosion the kinetics of decomposition of organic matter in the oceans by aerobic and anaerobic bacteria the equilibrium ocean atmosphere carbonate model the observed relationships among the organic burial flux dissolved o 2 content of deep ocean and sedimentation rates and the three box model ocean the important parameters that strongly influence the dynamics of atmospheric o 2 are found to be a the total area of soil formation on earth b the average soil depth c the average rate of physical erosion of soils which is linked to the average rate of accumulation of clastic sediments in the oceans d the composition and flux of volcanic gas and e the level of atmospheric co 2 we develop kinetic equations linking these parameters to the production and consumption fluxes of atmospheric oxygen and also to stable p o 2 values considering the likely ranges of variations in these parameters in geologic history we suggest that the atmospheric p o 2 level is likely to have stayed within a very narrow range of 0 6 2 pal and that the entire ocean except for local euxinic basins is likely to have been basically oxygenated since the first major rise of atmospheric oxygen more than 2 ga ago

interferometric analysis of the deformation pattern of the northern larsen ice shelf antarctic peninsula compared to field measurements and numerical modeling the motion field of the northern larsen ice shelf antarctic peninsula was analyzed using radar interferometry in combination with field measurements and finite element model calculations the ice shelf between jason peninsula and seal nunataks has been in steady retreat since january 1995 model calculations suggest that the ice shelf is in a stage of irreversible retreat since the last calving events in summer 1998 99 the interferometric analysis is based on synthetic aperture radar data of the tandem mission of european remote sensing satellites ers 1and 2 in austral spring 1995 the phase contributions due to tidal motion were estimated from the vertical displacement at those parts of the grounding zone where the horizontal motion is close to zero in order to separate the vertical and horizontal motion components over the ice shelf satellite derived velocities compare well with the long term field measurements along a transverse and a longitudinal profile the real interferograms and synthetic interferograms calculated from model velocities show reasonable agreement over the main parts of the ice shelf but differ in the boundary zones where the details are not resolved by the model

the origin of the gold and uranium ores of the black reef formation transvaal supergroup south africa abstract the black reef formation is a laterally extensive sedimentary succession at the base of the transvaal supergroup the quartz pebble conglomerates host erratic concentrations of gold and uranium however in areas that are located spatially above the gold and uranium bearing reefs of the witwatersrand supergroup the concentrations can reach ore grade although the depositional environment was similar to that of the witwatersrand supergroup the processes that lead to the high gold and uranium concentrations in the black reef were quite different the black reef experienced intense post depositional hydrothermal alteration by circulating aqueous and hydrocarbon fluids oils that deposited large volumes of native gold uranium minerals and pyrobitumen the gold which is filamentous is concentrated along as ni rich surfaces of pyrite and with uraninite in pyrobitumen in both environments the gold is interpreted to be the product of chemically triggered precipitation from hydrothermal fluid s the uraninite occurs either as precipitates in pyrobitumen or is invisible in the pyrite occurring as nanoparticles in this mineral this indicates that conditions were sufficiently oxidizing for the hydrothermal transport of uranium and contrasts with the case for the witwatersrand reefs where much of the uranium is detrital luminescent radiation damage associated with vein quartz and pyrobitumen as well as the diffuse distribution pattern of uranium in the pyrobitumen may indicate that some uranium was transported by liquid hydrocarbons pyrite in the black reef sedimentary detrital and hydrothermal pyrite is highly enriched in gold uranium and other redox sensitive trace elements provenance analyses using the trace element compositions of the pyrite show that it was not the result of erosional reworking of the underlying witwatersrand reefs however the close spatial association of high gold and uranium concentrations in the black reef with ore bearing witwatersrand reefs suggests strongly that gold and uranium whether in pyrite or occurring as native gold and uraninite were hydrothermally recycled uranium may have been recycled by hydrocarbon fluids from the witwatersrand reefs

glacier acceleration and thinning after ice shelf collapse in the larsen b embayment antarctica 1 ice velocities derived from five landsat 7 images acquired between january 2000 and february 2003 show a two to six fold increase in centerline speed of four glaciers flowing into the now collapsed section of the larsen b ice shelf satellite laser altimetry from icesat indicates the surface of hektoria glacier lowered by up to 38 6 m in a six month period beginning one year after the break up in march 2002 smaller elevation losses are observed for crane and jorum glaciers over a later 5 month period two glaciers south of the collapse area flask and leppard show little change in speed or elevation seasonal variations in speed preceding the large post collapse velocity increases suggest that both summer melt percolation and changes in the stress field due to shelf removal play a major role in glacier dynamics

uranium isotope evidence for mesoarchean biological oxygen production in shallow marine and continental settings abstract oxidative weathering of continental rocks is the major source of redox sensitive elements to the ocean the timing for the initiation of oxidative weathering remains strongly debated here we report new 238 u data for the authigenic component of well preserved ca 3 07 2 90 ga marine and lacustrine shales from the mozaan and dominion groups of the kaapvaal craton south africa the 238 uauth values in marine shales 0 71 to 0 21 relative to crm 112a are lower than the bulk silicate earth bse 0 40 to 0 17 while those of lacustrine shales 0 68 to 0 05 scatter above and below the bse range in addition the 238 uauth values of marine shales deposited in shallow seawater are significantly lower than those of the shales deposited in deep seawater we propose that u vi was adsorbed onto fe mn oxides in oxic shallow water and then shuttled to the seafloor the u isotope fractionations in lacustrine shales are best explained by reduction of u vi to u iv below the sediment water interface together these observations bolster the case that a mildly oxidative weathering regime had been established at the earth s surface by 3 0 ga paving the way to the onset of aerobic metabolism in both continental and shallow marine environments

computing the volume response of the antarctic peninsula ice sheet to warming scenarios to 2200 the contribution to sea level to 2200 from the grounded mainland antarctic peninsula ice sheet apis was calculated using an ice sheet model initialized with a new technique computing ice fluxes based on observed surface velocities altimetry and surface mass balance and computing volume response using a linearized method volume change estimates of the apis resulting from surface mass balance anomalies calculated by the regional model racmo2 forced by a1b and e1 scenarios of the global models echam5 and hadcm3 predicted net negative sea level contributions between 0 5 and 12mm sea level equivalent sle by 2200 increased glacier flow due to ice thickening returned 15 of the increased accumulation to the sea by 2100 and 30 by 2200 the likely change in volume of the apis by 2200 in response to imposed 10 and 20km retreats of the grounding line at individual large outlet glaciers in palmer land southern antarctic peninsula ranged between 0 5 and 3 5mm sle per drainage basin ensemble calculations of apis volume change resulting from imposed grounding line retreat due to ice shelf break up scenarios applied to all 20 of the largest drainage basins in palmer land covering 40 of the total area of apis resulted in net sea level contributions of 7 16mm sle by 2100 and 10 25mm sle by 2200 inclusion of basins in the northern peninsula and realistic simulation of grounding line movement for ap outlet glaciers will improve future projections

a new tide model for the antarctic ice shelves and seas we describe a new tide model for the seas surrounding antarctica including the ocean cavities under the floating ice shelves the model uses data assimilation to improve its fit to available data typical peak to peak tide ranges on ice shelves are 1 2 m but can exceed 3 m for the filchner ronne and larsen ice shelves in the weddell sea spring tidal ranges are about twice these values model performance is judged relative to the 5 10 cm accuracy that is needed to fully utilize ice shelf height data that will be collected with the geoscience laser altimeter system scheduled to be launched on the ice cloud and land elevation satellite in late 2002 the model does not yet achieve this level of accuracy except very near the few high quality tidal records that have been assimilated into the model some improvement in predictive skill is expected from increased sophistication of model physics but we also require better definition of ice shelf grounding lines and more accurate water column thickness data in shelf seas and under the ice shelves long duration tide measurements bottom pressure gauge or gps in critical data sparse areas particularly near and on the filchner ronne and ross ice shelves and pine island bay are required to improve the performance of the data assimilation model

rapid collapse of northern larsen ice shelf antarctica in january 1995 4200 square kilometers of the northern larsen ice shelf antarctic peninsula broke away radar images from the ers 1 satellite complemented by field observations showed that the two northernmost sections of the ice shelf fractured and disintegrated almost completely within a few days this breakup followed a period of steady retreat that coincided with a regional trend of atmospheric warming the observations imply that after an ice shelf retreats beyond a critical limit it may collapse rapidly as a result of perturbated mass balance

zircon ion microprobe studies bearing on the age and evolution of the witwatersrand triad abstract the ventersdorp and witwatersrand supergroups and the dominion group collectively referred to as the witwatersrand triad represent some of the oldest and economically most important volcano sedimentary basins within the kaapvaal craton of southern africa precise age constraints which are vital to models aimed at understanding the formation mineralisation and evolution of these basins have been hampered by analytical and geological problems these problems have been overcome by this ion microprobe study on zircons from volcanic rocks within the three basins felsic and mafic volcanics from the ventersdorp supergroup yield ages of 2709 4 ma and 2714 8 ma respectively constraining the timing of the closure of the witwatersrand basin a maximum age for the witwatersrand basin is indicated by the age of 3074 6 ma for volcanics from the underlying dominion group thus a period of c 350 ma was available for the accumulation of the full witwatersrand basin succession unfortunately dating within the witwatersrand basin itself is not complete and future work could show that extended time breaks may exist within the basin these new data show that previous age determinations on the ventersdorp witwatersrand and dominion basins were inaccurate and that the economically important witwatersrand basin must now be recognised as archaean

the air content of larsen ice shelf 1 the air content of glacial firn determines the effect and attribution of observed changes in ice surface elevation but is currently measurable only using labor intensive ground based techniques here a novel method is presented for using radar sounding measurements to decompose the total thickness of floating ice shelves into thicknesses of solid ice and firn air or firn water the method is applied to a 1997 98 airborne survey of larsen ice shelf revealing large spatial gradients in air content that are consistent with existing measurements and local meteorology the gradients appear to be governed by meltwater induced firn densification we find sufficient air in larsen c ice shelf for increased densification to account for its previously observed surface lowering and the rate of lowering superficially agrees with published trends in melting this does not preclude a contribution to the lowering from oceanic basal melting but a modern repeat of the survey could conclusively distinguish atmosphere led from ocean led change the technique also holds promise for the calibration of firn density models derivation of ice thickness from surface elevation measurements and calculation of the sea level contribution of changes in grounded ice discharge

2001 2009 elevation and mass losses in the larsen a and b embayments antarctic peninsula we investigate the elevation and mass balance response of tributary glaciers following the loss of the larsen a and b ice shelves antarctic peninsula in 1995 and 2002 respectively our study uses modis imagery to track ice extent and aster and spot5 digital elevation models dems plus atm and icesat laser altimetry to track elevation changes spanning the period 2001 09 the measured larsen b tributary glaciers hektoria green evans punchbowl jorum and crane lost up to 160 m in elevation during 2001 06 and thinning continued into 2009 elevation changes were small for the more southerly flask and leppard glaciers which are still constrained by a larsen b ice shelf remnant in the northern embayment continued thinning of 3 m a 1 on drygalski glacier 14 years after the larsen a ice shelf disintegrated suggests that mass losses for the exposed larsen b tributaries will continue for years into the future grounded ice volume losses exceed 13 km 3 for crane glacier and 30 km 3 for the hektoria green evans glaciers the combined mean loss rate for 2001 06 is at least 11 2 gt a 1 our values differ significantly from published mass budget based estimates for these embayments but are a reasonable fraction of grace derived rates for the region 40 gt a 1

accelerated ice discharge from the antarctic peninsula following the collapse of larsen b ice shelf acceleration exceeds 27 km 3 per year and ice is thinning at rates of tens of meters per year we attribute this abrupt evolution of the glaciers to the removal of the buttressing ice shelf the magnitude of the glacier changes illustrates the importance of ice shelves on ice sheet mass balance and contribution to sea level change index terms 1827 hydrology glaciology 1863 1863 hydrology snow and ice 1827 3349 meteorology and atmospheric dynamics polar meteorology 6924 radio science interferometry 9310 information related to geographic region antarctica citation rignot e g casassa p gogineni w krabill a rivera and r thomas 2004 accelerated ice discharge from the antarctic peninsula following the collapse of larsen b ice shelf geophys res lett 31 l18401 doi 10 1029 2004gl020697

causes of pre collapse changes of the larsen b ice shelf numerical modelling and assimilation of satellite observations satellite observations revealed that beside a rapid thinning the larsen b ice shelf lbis was undergoing a significant acceleration before its collapse in 2002 this paper investigates the ice shelf acceleration between 1995 and 1999 using a combination of data assimilation and numerical modelling based on a flow model adjusted to the 1995 insar velocities perturbation experiments are performed such as ice front retreat thinning increase in tributary flow and rheological weakening furthermore an inversion for ice shelf rheology and tributary flow velocity is performed for both the 1995 and the 1999 insar velocities the perturbation experiments together with the inversion strongly suggest that the acceleration cannot solely be explained by the retreat of the ice shelf front but relies on a further significant rheological weakening of the already weak shear zones within the lbis minor tributary acceleration is found to be an effect rather than a cause of the ice shelf acceleration furthermore the observed acceleration cannot be explained by the observed recent thinning we conclude that for smaller ice shelves such as the lbis such weak shear margins play a crucial role in controlling their dynamics and are the key to understand changes in the future finally we compare the dynamic thinning likely to be associated with the observed acceleration with the observed thinning for the ice shelf as a whole this thinning accounts for 20 of the observed value which implies that factors such as enhanced basal melt were the primary cause of the observed thinning

dynamic thinning of glaciers on the southern antarctic peninsula growing evidence has demonstrated the importance of ice shelf buttressing on the inland grounded ice especially if it is resting on bedrock below sea level much of the southern antarctic peninsula satisfies this condition and also possesses a bed slope that deepens inland such ice sheet geometry is potentially unstable we use satellite altimetry and gravity observations to show that a major portion of the region has since 2009 destabilized ice mass loss of the marine terminating glaciers has rapidly accelerated from close to balance in the 2000s to a sustained rate of 56 8 gigatons per year constituting a major fraction of antarctica s contribution to rising sea level the widespread simultaneous nature of the acceleration in the absence of a persistent atmospheric forcing points to an oceanic driving mechanism

a damage mechanics assessment of the larsen b ice shelf prior to collapse toward a physically based calving law calving is a primary process of mass ablation for glaciers and ice sheets though it still eludes a general physical law here we propose a calving framework based on continuum damage mechanics coupled with the equations of viscous deformation of glacier ice we introduce a scalar damage variable that quantifies the loss of load bearing surface area due to fractures and that feeds back with ice viscosity to represent fracture induced softening the calving law is a standard failure criterion for viscous damaging materials and represents a macroscopic brittle instability quantified by a critical or threshold damage we constrain this threshold using the ice sheet system model issm by inverting for damage on the larsen b ice shelf prior to its 2002 collapse by analyzing the damage distribution in areas that subsequently calved we conclude that calving occurs after fractures have reduced the load bearing capacity of the ice by 60 10 2012 american geophysical union all rights reserved

uraniferous paleoplacers of the mesoarchean mahagiri quartzite singhbhum craton india depositional controls nature and source of 3 0 ga detrital uraninites abstract the quartz pebble conglomerate qpc hosted detrital uranium mineralization is unique in character in terms of their restricted distribution before 2 2 ga atmosphere during pre great oxidation event pre goe such qpc paleoplacer deposits over the world are good targets for moderate to high tonnage and low grade uranium deposits and more importantly for their gold content the mahagiri quartzite dated c 3 02 ga for their youngest detrital zircon population is developed unconformably over the mesoarchean singhbhum granite 3 44 ga to 3 1 ga the mahagiri quartzite includes a conglomerate pebbly sandstone dominated subaerial alluvial fan to coastal braided plain sequence in the lower parts and shallow marine mature quartz arenite in the upper parts the alluvial fan braided plain deposits in the lower parts host a number of pyritiferous and uraniferous conglomerate and pebbly sandstone beds the uraninite grains are rounded to subrounded in outline suggesting mechanical transport and detrital origin together with detrital pyrite and uraninite constitute the example of 3 0 ga paleoplacer closely comparable to the witwatersrand au u deposits epma and sem eds studies suggest that the uraninite grains are rich in th 4 wt s and ree y chemical formula calculations from epma analyses suggest uraninite grains belong to two populations with different oxidation states as revealed from y ree and cation u4 u6 apfu ratios the u contents of the detrital uraninite grains from mahagiri are significantly lower than that of the ideal stoichiometric composition of uo2 this is mainly due to higher amount of heterovalent cationic substitution by th ree y pb and ca in mahagiri qpc uraninite structures and partial alteration and metamictization of uraninites alteration due to metamictization resulted in elevated concentration of si al p and ca in more altered and metamict uraninite grains the ree pattern is typically flat with comparable lree hree concentration the high th content flat ree pattern suggests that the uraninitere presents high temperature phases 350 c and are magmatic in origin the mahagiri detrital uraninite grains suggest existence of highly felsic and k rich richer than ttg granodiorite granite monzogranite suites ggm of rocks older than 3 1 ga in the singhbhum craton

from bif to red beds sedimentology and sequence stratigraphy of the paleoproterozoic koegas subgroup south africa abstract the 2 4 ga old koegas subgroup of the transvaal supergroup contains siliciclastics and iron formations it occupies a transitional stratigraphic position between the major iron formations on the kaapvaal craton and strata with evidence for oxygenation of the environment koegas siliciclastics reflect a new phase of uplift and erosion on the craton after a period of chemical deposition paleocurrent data indicate transport to the west and northwest away from the craton however no precise source area can as yet be defined rocks were deposited in foreshore to offshore environments of a prograding delta or submarine fan system immature arkosic wackes and subarkoses occur in proximal parts and pass to terrigenous mudstones and chemical iron formations in offshore environments with low detrital input lateral facies transitions and the hierarchical stratigraphic stacking pattern of the koegas subgroup allow a sequence stratigraphic interpretation in which regressive coarse siliciclastics prograded repeatedly over transgressive units of chemical iron formations metals in the transgressive units have an intrabasinal hydrothermal source similar to underlying iron formations localized iron stained detrital units resemble red beds they reflect additional diagenetic fe remobilization from deeper water iron formations or possibly a continental fe source the second possibility would imply the presence of some free oxygen in the environment

the triggering of sub glacial lake drainage during the rapid glacier drawdown crane glacier antarctic peninsula ice surface altimetry from icesat 1 and nasa aircraft altimeter overflights spanning 2002 09 indicate that a region of lower crane glacier antarctic peninsula shows an unusual temporal pattern of elevation loss a period of very rapid drawdown 91 m a 1 between september 2004 and september 2005 bounded by periods of large but more moderate rates 23 m a 1 until september 2004 12 m a 1 after september 2005 the region of increased drawdown is 4 5 km 2 2 km based on satellite aster and spot 5 stereo image digital elevation model dem differencing spanning the event in a later differential dem the anomalous drawdown feature is not seen bathymetry in crane glacier fjord reveals a series of flat lying formerly subglacial deeps interpreted as lake sediment basins we conclude that the elevation change feature resulted from drainage of a small deep subglacial lake we infer that the drainage event was induced by hydraulic forcing of subglacial water past a downstream obstruction however only a fraction of crane glacier s increase in flow speed that occurred near the time of lake drainage derived from image feature tracking appears to be directly attributable to the event instead retreat of the ice front off a subglacial ridge 6 km downstream of the lake is likely the dominant cause of renewed fast flow and more negative mass balance in the subsequent 4 years

boundary condition of grounding lines prior to collapse larsen b ice shelf antarctica grounding zones where ice sheets transition between resting on bedrock to full floatation help regulate ice flow exposure of the sea floor by the 2002 larsen b ice shelf collapse allowed detailed morphologic mapping and sampling of the embayment sea floor marine geophysical data collected in 2006 reveal a large arcuate complex grounding zone sediment system at the front of crane fjord radiocarbon constrained chronologies from marine sediment cores indicate loss of ice contact with the bed at this site about 12 000 years ago previous studies and morphologic mapping of the fjord suggest that the crane glacier grounding zone was well within the fjord before 2002 and did not retreat further until after the ice shelf collapse this implies that the 2002 larsen b ice shelf collapse likely was a response to surface warming rather than to grounding zone instability strengthening the idea that surface processes controlled the disintegration of the larsen ice shelf

laboratory flume studies on monochromatic wave fine sandy bed interactions part 1 soil fluidization abstract tests on two fine sandy soils d50 0 134 mm and 0 092 mm under monochromatic wave actions were conducted in a wave flume of 37 m l by 1 2 m h by 1 m w to investigate characteristics of fluidized responses the pore pressure measurements demonstrate only an unfluidized response in the coarser sandy bed while in the finer one two more feature fluidized responses fluidized responses are similarly classified into resonantly and non resonantly fluidized according to foda and tzang foda m a tzang s y 1994 resonant fluidization of silty soil by water waves j geophys res 99 c10 20463 20475 at a given depth they are in principle defined by magnitude of fluidization ratio between excess pore pressure and static soil stresses and by the occurrence of a resonance event in the same test series inside the sandy bed the excess pore pressures of a fluidized response are almost initiated simultaneously their magnitudes are essentially in static balance to the integrated weight of overlaying fluidized soil layers comparisons with previously reported data from a silty bed d50 0 05 mm by foda and tzang have immediately indicated the importance of grain fraction with less fine constituents surface layers of the two sandy soils are less susceptible to fluidization resonance mechanism is evidently diminishing in a resonantly fluidized response and re fluidization becomes less potential in the subsequent tests in a resonantly fluidized response pore pressures at a given depth would start to resonantly grow from a fluidization ratio of 7 14 in a few wave cycles resonant growth subsides at a fluidization ratio of greater than 50 which value increases with depth the analyses clearly illustrate that fluidization tends to be initiated in surface layers and fast spreads into lower layers fluidization is dependent on finer constituting grains smaller shear modulus g and permeability k and thinner boundary layers in bed soils measurements of previous silt tests are analyzed to show that lower limits of wave steepness on resonantly fluidizing a soil bed increase linearly with relative water depth ranging from 0 13 to 0 23 data of present fine sand tests have preliminarily confirmed the linear trend over a fluidized sandy bed similar vivid sediment suspensions were observed during wave generations as had been reported in silt tests

pattern of retreat and disintegration of larsen b ice shelf antarctic peninsula the retreat of the larsen b ice shelf antarctic peninsula and the collapse of its northern section are analyzed using satellite images acquired between january 1995 and may 2003 over 1 week during march 2002 after a period of steady retreat since 1995 2300 km 2 of the ice shelf broke up into many small icebergs this rapid collapse occurred at the end of an exceptionally warm summer and after a multi year period of decreasing surface net mass balance ice thinning flow acceleration and widening of rifts the ice shelf area decreased from 11 512 km 2 in january 1995 to 3463 km 2 in march 2002 and 2667 km 2 in april 2003 ers synthetic aperture radar sar images were used to identify ice shelf zones with different surface morphology which generated icebergs of different sizes and shapes the pattern of retreat and break up similar to that of larsen a in 1995 suggests that fracturing enhanced by abundant surface melt played a key role in addition the recent changes of grounded and residual floating ice north of larsen b are analyzed by means of envisat advanced synthetic aperture radar asar images up to summer 2003 showing significant loss of grounded ice upstream of those ice shelf sections which disintegrated in 1995 and 2002

sinking floatation of pipelines and other objects in liquefied soil under waves abstract this paper presents the results of an experimental study where the sinking and floatation of a pipeline and other objects namely a sphere and a cube in a silt bed was investigated the bed was exposed to progressive waves two kinds of experiments were made the undisturbed flow experiments and the experiments with the structure model a pipeline a sphere and a cube in the former experiments the pore water pressure was measured across the soil depth the pore water pressure built up as the waves progressed the soil was liquefied for wave heights larger than a critical value regarding the experiments with the structure model the displacement of the structure sinking or floatation was measured simultaneously with the pore water pressure the influence of various parameters such as the initial position of the object the specific gravity the soil layer thickness and the wave height was investigated it was found that while the pipe sank in the soil to a depth of 2 3 times the pipe diameter the sphere sank to even larger depths the pipe with a relatively small specific gravity initially buried floated to the surface of the soil the drag coefficients for the objects sinking in the liquefied soil were obtained

antarctic ice sheet loss driven by basal melting of ice shelves accurate prediction of global sea level rise requires that we understand the cause of recent widespread and intensifying glacier acceleration along antarctic ice sheet coastal margins atmospheric and oceanic forcing have the potential to reduce the thickness and extent of floating ice shelves potentially limiting their ability to buttress the flow of grounded tributary glaciers indeed recent ice shelf collapse led to retreat and acceleration of several glaciers on the antarctic peninsula but the extent and magnitude of ice shelf thickness change the underlying causes of such change and its link to glacier flow rate are so poorly understood that its future impact on the ice sheets cannot yet be predicted here we use satellite laser altimetry and modelling of the surface firn layer to reveal the circum antarctic pattern of ice shelf thinning through increased basal melt we deduce that this increased melt is the primary control of antarctic ice sheet loss through a reduction in buttressing of the adjacent ice sheet leading to accelerated glacier flow the highest thinning rates occur where warm water at depth can access thick ice shelves via submarine troughs crossing the continental shelf wind forcing could explain the dominant patterns of both basal melting and the surface melting and collapse of antarctic ice shelves through ocean upwelling in the amundsen and bellingshausen seas and atmospheric warming on the antarctic peninsula this implies that climate forcing through changing winds influences antarctic ice sheet mass balance and hence global sea level on annual to decadal timescales

mass loss of larsen b tributary glaciers antarctic peninsula unabated since 2002 ice mass loss continues at a high rate among the large glacier tributaries of the larsen b ice shelf following its disintegration in 2002 we evaluate recent mass loss by mapping elevation changes between 2006 and 2010 11 using differencing of digital elevation models dems the measurement accuracy of these elevation changes is confirmed by a null test subtracting dems acquired within a few weeks the overall 2006 2010 11 mass loss rate 9 0 2 1 gt a 1 is similar to the 2001 02 2006 rate 8 8 1 6 gt a 1 derived using dem differencing and laser altimetry this unchanged overall loss masks a varying pattern of thinning and ice loss for individual glacier basins on crane glacier the thinning pulse initially greatest near the calving front is now broadening and migrating upstream the largest losses are now observed for the hektoria green glacier basin having increased by 33 since 2006 our method has enabled us to resolve large residual uncertainties in the larsen b sector and confirm its state of ongoing rapid mass loss

seasonal evolution of supraglacial lakes on a floating ice tongue petermann glacier greenland supraglacial lakes are known to trigger antarctic ice shelf instability and break up however to date no study has focused on lakes on greenland s floating termini here we apply lake boundary area and depth algorithms to landsat 8 imagery to analyse the inter and intraseasonal evolution of supraglacial lakes across petermann glacier s 81 n floating tongue from 2014 to 2016 while also comparing these lakes to those on the grounded ice lakes start to fill in june and quickly peak in total number volume and area in late june early july in response to increases in air temperatures however through july and august total lake number volume and area all decline despite sustained high temperatures these observations may be explained by the transportation of meltwater into the ocean by a river and by lake drainage events on the floating tongue further as mean lake depth remains relatively constant during this time we suggest that a large proportion of the lakes that drain do so completely likely by rapid hydrofracture the mean areas of lakes on the tongue are only 20 of those on the grounded ice and exhibit lower variability in maximum and mean depth differences likely attributable to the contrasting formation processes of lakes in each environment

larsen b ice shelf rheology preceding its disintegration inferred by a control method a new complete velocity field from satellite remote sensing is combined with numerical modeling to infer the rheology of the larsen b ice shelf before its disintegration the resulting spatial distribution of the flow parameter exhibits large variability which reflects very well observed ice shelf features this variability is explained by factors including advection of colder ice from tributary glaciers bottom melting and the presence of zones of strong shear and fracture the inferred distribution is applied to simulate numerically the flow regime ofthe ice shelf and to examine its modification by the presence of open rifts and by the retreat of ice shelf front between 1996 and 2000 results demonstrate that variable rheology is essential to understanding ice shelf evolution especially the close relationship among frontal retreat fracture ice flow acceleration and the destabilization of ice shelves copyright 2007 by the american geophysical union

hydrothermal gold mineralization in the witwatersrand basin the origin of the gold mineralization in the witwaterstrand basin of south africa the largest known gold province has been controversial for decades with arguments favouring detrital1 2 placer modified placer3 4 and hydrothermal5 6 origins here we present the results of an extensive geological study of witwatersrand rocks which show that the gold and associated uranium mineralization is hydrothermal in origin and postdates a regional high temperature alteration event alteration processes identified on a small scale can be mapped out regionally as roughly strata bound zones of acid metasomatism extending far into the basin the fluid flow responsible for this alteration was concentrated in small scale structures localized along lithological boundaries we find that the gold precipitated as a consequence of interactions of the fluid with shale derived hydrocarbons present within the basin

evolution of uranium fractionation processes through time driving the secular variation of uranium deposit types uranium deposit types have evolved considerably from the archean to the present the major global drivers were 1 change of geotectonic conditions during the late archean 2 strong increase of atmospheric oxygen from 2 4 to 2 2 ga and 3 development of land plants during the silurian other significant variations of uranium deposit types are related to unique conjunctions of conditions such as those during phosphate sedimentation in the cretaceous earth s uranium fractionation mechanisms evolved through four major periods the first from 4 55 and 3 2 ga corresponds to formation of a thin essentially mafic crust in which the most fractionated trondheimite tonalite granodiorite ttg rocks attained uranium concentrations of at most a few parts per million moreover the uranium being essentially hosted in refractory accessory minerals and free oxygen being absent no uranium deposit could be expected to have formed during this period the second period from about 3 1 to 2 2 ga is characterized by several widespread pulses of highly fractionated potassic granite strongly enriched in u th and k late in this period peraluminous granite was selectively enriched in u and to a lesser extent k these were the first granite and pegmatite magmas able to crystallize high temperature uraninite the erosion of these granitic suites liberated thorium rich uraninite which would then be concentrated in placer deposits along with pyrite and other heavy minerals e g zircon monazite fe ti oxides within huge continental basins e g witwatersrand south africa and bind river canada the lack of free oxygen at that time prevented oxidation of the uraninite which formed the oldest economic uranium deposit types on earth but only during this period the third period from 2 2 to 0 45 ga records increased oxygen to nearly the present atmospheric level tetravalent uranium from uraninite was oxidized to hexavalent uranium forming highly soluble uranyl ions in water uranium was extensively trapped in reduced epicontinental sedimentary successions along with huge quantities of organic matter and phosphates accumulated as a consequence of biological proliferation especially during the late paleoproterozoic a series of uranium deposits formed through redox processes the first of these developed at a formational redox boundary at about 2 0 ga in the oklo area of gabon all known economically significant uranium deposits related to na metasomatism are about 1 8 ga in age the high grade large tonnage unconformity related deposits also formed essentially during the late paleoproterozoic to early mesoproterozoic the last period 0 45 ga present coincided with the colonization of continents by plants the detrital accumulation of plants within continental siliciclastic strata represented intraformational reduced traps for another family of uranium deposits that developed essentially only during this period basal roll front tabular and tectonolithologic types however the increased recognition of hydrocarbon and hydrogen sulfide migration from oil or gas reservoirs during diagenesis suggests potential for sandstone hosted uranium deposits to be found within permeable sandstone older than the silurian large uranium deposits related to high level hydrothermal fluid circulation and those related to evapotranspiration calcretes are only known during this last period of time probably because of their formation in near surface environments with low preservation potential

o2 constraints from paleoproterozoic detrital pyrite and uraninite redox sensitive detrital grains such as pyrite and uraninite in sedimentary successions provide one of the most conspicuous geological clues to a different composition of the archean and early paleoproterozoic atmosphere today these minerals are rapidly chemically weathered within short transport distances prior to the rise of oxygen low o2 concentrations allowed their survival in siliciclastic deposits with grain erosion tied only to physical transport processes after the rise of oxygen redox sensitive detrital grains effectively vanish from the sedimentary record to get a better understanding of the timing of this transition we examined sandstones recorded in a scientifi c drill core from the south african 2 415 ga koegas subgroup a mixed siliciclastic and iron formation bearing unit deposited on the western deltaic margin of the kaapvaal craton in early paleoproterozoic time we observed detrital pyrite and uraninite grains throughout all investigated sandstone beds in the section indicating the rise of oxygen is younger than 2 415 ga to better understand how observations of detrital pyrite and uraninite in sedimentary rocks can quantitatively constrain earth surface redox conditions we constructed a model of grain erosion from chemical weathering and physical abrasion to place an upper limit on ancient environmental o2 concentrations even conservative model calculations for deltaic depositional systems with suffi cient transport distances approximately hundreds of kilometers show that redox sensitive detrital grains are remarkably sensitive to environmental o2 concentrations and they constrain the archean and early paleoproterozoic atmosphere to have 3 2 10 5 atm of molecular o2 these levels are lower than previously hypothesized for redox sensitive detrital grains but they are consistent with estimates made from other redox proxy data including the anomalous fractionation of sulfur isotopes the binary loss of detrital pyrite and uraninite from the sedimentary record coincident with the rise of oxygen indicates that atmospheric o2 concentrations rose substantially at this time and were never again suffi ciently low 0 01 atm to enable survival and preservation of these grains in short transport systems

redox states of archean surficial environments the importance of h2 g instead of o2 g for weathering reactions abstract redox states of the archean eon have been constrained by various lines of evidence including atmospheric photochemical and ecological models mass independent fractionations of sulfur isotopes fe depletion of paleosols and preservation of diagnostic detrital minerals although these lines of evidence present seemingly consistent upper limits on po2 g they are conceptually contradictory about the redox state of archean surficial environments atmospheric photochemical and ecological modeling studies suggest weakly reducing environments under redox states represented by moderate h2 g levels however current interpretations of fe depletion in paleosols and the preservation of detrital minerals are based on low o2 g levels at which the reducing detrital minerals are thermodynamically unstable and survive because of slow kinetics of oxidative weathering in this study we show that under the redox state indicated by the archean ph2 g range fe2 and reducing fe ii minerals are actually thermodynamically stable and have no tendency to be oxidized we emphasize that ph2 g values are orders of magnitude higher than po2 g in the archean atmosphere and that h2 g and o2 g are not in equilibrium the redox states of archean surface environments behave as though they were controlled by the more abundant h2 instead of the very low o2 as in modern anoxic basins weathering in this case should have involved non redox acidic dissolution of fe ii species or reductive reaction of fe iii species fe ii depleted paleosols and the preservation of relatively reduced detrital minerals are natural consequences of their thermodynamic stabilities in the archean eon s reducing environments rather than slow kinetics of oxidizing reactions after the appearance of oxygenic photosynthesis probably in the middle late archean locally oxygenated environments could have existed while the atmosphere as a whole remained anoxic the profile of redox states on the archean surface seems to be a reverse analogue to the modern earth although oxidizing dissolution of transition metals could happen in o2 oases quick reduction of oxyanions by abundant reductants such as aqueous fe2 fe ii minerals and h2 might have restricted riverine transport of oxyanions and potentially complicate the interpretation of signals of o2 whiffs in marine sediments

sinking of irregular shape blocks into marine seabed under wave induced liquefaction abstract the sinking of initially buried irregular blocks into the seabed under wave induced liquefaction was investigated by experimental methods pore water pressure in the soil water surface elevation time series and block displacements were measured results indicated that initiation of sinking coincides with the instant at which the accumulated pore water pressure at the bottom level of the block reaches the initial mean normal effective stress the drag forces and drag coefficients on steadily sinking irregular shaped blocks as well as spherical and cubical ones were calculated from the obtained data and compared with the available data in the literature the results show that the shape of sinking block is of minor importance as far as the kinematics and dynamics of the sinking block is concerned the conditions at which the sinking terminates are discussed in the light of experiments using the approach presented here the ultimate sinking depths are calculated for the tested cases and compared with the experimental results the calculated and measured values showed a reasonable agreement when compared finally a summary and remarks are presented to calculate the ultimate sinking depth of irregular shaped blocks for practical applications

observations of buried lake drainage on the antarctic ice sheet between 1992 and 2017 the antarctic ice sheet ais lost ice equivalent to 7 6 3 9 mm of sea level rise ais mass loss is mitigated by ice shelves that provide a buttress by regulating ice flow from tributary glaciers however ice shelf stability is threatened by meltwater ponding which may initiate or reactivate preexisting fractures currently poorly understood processes here through ground penetrating radar gpr analysis over a buried lake in the grounding zone of an east antarctic ice shelf we present the first field observations of a lake drainage event in antarctica via vertical fractures concurrent with the lake drainage event we observe a decrease in surface elevation and an increase in sentinel 1 backscatter finally we suggest that fractures that are initiated or reactivated by lake drainage events in a grounding zone will propagate with ice flow onto the ice shelf itself where they may have implications for its stability

an experimental method to verify the failure of coastal structures by wave induced liquefaction of clayey soils abstract although the dynamics involved in the liquefaction process are understood reasonably well experimental work to reproduce the sinking of structures due to liquefaction which is representative and repeatable has so far not been recorded in this work three sets of experiments were performed in an attempt to fill the gaps in the knowledge by modelling a small scale reproduction of the failure firstly an analysis of the role of the proportions of the initial fine sediment and water content is presented secondly a group of tests involving a vertical breakwater were performed and thirdly an experiment was carried out to reproduce the failure of a submerged structure on a clayey bed in the presence of waves from these experiments we were able to set thresholds for bed composition below which soil liquefaction is likely to occur it was determined that the potential to liquefy increases with the initial water content and that soils of 40 or more clay content may liquefy this methodology has proven to be repeatable allowing the reproduction of the sinking of coastal structures due to liquefaction of the underlying soil

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