glacier subsurface heat flux characterizations for energy balance modelling in the donjek range southwest yukon canada we apply a point scale energy balance model to a small polythermal glacier in the st elias mountains of canada in order to investigate the applicability and limitations of different treatments of the glacier surface temperature and subsurface heat flux these treatments range in complexity from a multilayer subsurface model that simulates snowpack evolution to the assumption of a constant glacier surface temperature equal to 0 c the most sophisticated model includes dry densification of the snowpack penetration of shortwave radiation into the subsurface internal melting refreezing of percolating meltwater and generation of slush layers measurements of subsurface temperature and surface lowering are used for model validation and highlight the importance of including subsurface penetration of shortwave radiation in the model using an iterative scheme to solve for the subsurface heat flux as the residual of the energy balance equation results in an overestimation of total ablation by 18 while the multilayer subsurface model underestimates ablation by 6 by comparison the 0 c surface assumption leads to an overestimation of ablation of 29 in this study where the mean annual air temperature is about 8 c

structure of a frontal cyclone a diagnostic study of a mid latitude cyclone has been carried out using routinely available numerical weather prediction model products and imagery the cyclone was intense but not exceptionally so and the results are believed to have some generality the detailed description of the cyclone structure just before it began to deepen rapidly integrates concepts from a number of researchers into a common framework a dominant feature of the cyclone close to its centre was the cloud head a region of cloud with a sharp convex outer edge which formed poleward of the main polar front cloud band the cloud head was caused by two flows that entered it from the east ascending and fanning out within it one flow the cold conveyor belt brought low wet bulb potential temperature w air back into the cloud head from low levels ahead of the warm front the other flow was due to high w air that peeled off from the base of the main warm sector airflow i e part of the warm conveyor belt and travelled in the boundary layer back towards the cyclone centre first undercutting dry air that had earlier descended from the upper troposphere called a dry intrusion and then ascending at the upper boundary of the cloud head above the cold conveyor belt the transverse circulation that gave the ascent within the cloud head also led to the cold front fracturing along its length into two separate sharp surface cold fronts with a more diffuse frontal region in between frontal fracture the two sharp surface cold fronts were associated with narrow cold frontal rainbands line convection one of the line convection segments forming the southern edge of the cloud head the overrunning of the dry intrusion in the region of the frontal fracture led to a structure known as a split front in which an upper level humidity front began to run ahead of the position of the surface cold front a feature of the cyclone at this intermediate stage in its evolution was that a large proportion of the precipitation was being generated by ascent of relatively cold air the cold conveyor belt in the cloud head as part of the thermally indirect circulation at the left exit of a developing secondary upper level jet

an improved estimate of microbially mediated carbon fluxes from the greenland ice sheet microbially mediated carbon fluxes on the surface of the greenland ice sheet gris were recently quantified by hodson and others 2010 using measurements of the surface coverage of debris cryoconite and rates of biological production associated with debris near the ice sheet margin we present updated models that do not assume the same spatial uniformity in key parameters employed by hodson and others 2010 because they make use of biomass distribution and biological production data from a 79 km transect of the gris further the models presented here also include for the first time biomass associated with both cryoconite holes and surficial algae the predicted annual carbon flux for a small 1600km2 section of ice surrounding the field transect is about four times that estimated using spatially uniform biomass and production in this area when surficial algae are included the model predicts about 11 times more carbon fixation via photosynthesis per year than the cryoconite only models we therefore suggest that supraglacial carbon fluxes from the gris have previously been underestimated by more than an order of magnitude and that the hitherto overlooked surficial algal ecosystem can be the most crucial contributor the gris is shown to be in a relatively stable state of net autotrophy according to our model and so a strong link between bare ice area and total carbon fluxes is evident the implication is a biomass feedback to surface albedo and enhanced ablation as a result climate predictions for the year 2100 show that greater carbon fixation could also result from climate warming

cryoconite hole connectivity on the wright lower glacier mcmurdo dry valleys antarctica cryoconite holes can be important sources and stores of water and nutrients on cold and polythermal glaciers and they provide a habitat for various forms of biota understanding the hydrological connectivity of cryoconite holes may be the key to understanding the transport of nutrients and biological material to the proglacial areas of such glaciers this paper aims to characterize and explain spatial variability in the connectivity of ice lidded cryoconite holes on a small piedmont glacier in the mcmurdo dry valleys through geochemical analysis of cryoconite hole waters solute concentrations in both surface and near surface ice and cryoconite holes vary greatly along the glacier centerline and all sample types displayed similar spatial patterns of variability using chloride as a tracer we estimated variations in cryoconite hole connectivity along the glacier centerline we found that a previously used mass transfer method did not provide reliable estimates of the time period for which cryoconite hole waters had been isolated from the atmosphere we attribute this to spatial variability in both the chloride content of the surface ice and surface ablation rates the approach may however be used to qualitatively characterize spatial variations in the hydrological connectivity of the cryoconite holes these results also suggest that ice lidded cryoconite holes are never truly isolated from the near surface drainage system

evolution of cryoconite holes and their contribution to meltwater runoff from glaciers in the mcmurdo dry valleys antarctica cryoconite holes are water filled holes in the surface of a glacier caused by enhanced ice melt around trapped sediment measurements on the ablation zones of four glaciers intaylorvalley antarctica show that cryoconite holes cover about 4 6 of the ice surface they typically vary in diameter from 5 to 145 cm with depths ranging from 4 to 56 cm in some cases huge holes form with 5 m depths and 30 m diameters un like cryoconite holes elsewhere these have ice lids up to 36 cm thick and melt from within each spring about one half of the holes are connected to the near surface hydrologic system and the remainder are isolated the duration of isolation estimated from the chlor ide accumulation in hole waters commonly shows ages of several years with one hole of 10 years the cryoconite holes in the mcmurdo dryvalleys create a near surface hydro logic system tens of cm below the ice surface the glacier surface itself is generally frozen and dry comparison of water levels between holes a few meters apart shows independent cycles of water storage and release most likely local freeze thaw effects control water passage and therefore temporary storage rough calculations indicate that the holes gen erate at least 13 of the observed runoff on the one glacier measured this hydrologic system represents the transition between a melting ice cover with supraglacial streams and one entirely frozen and absent of water abstract cryoconite holes are water filled holes in the surface of a glacier caused by enhanced ice melt around trapped sediment measurements on the ablation zones of four glaciers intaylorvalley antarctica show that cryoconite holes cover about 4 6 of the ice surface they typically vary in diameter from 5 to 145 cm with depths ranging from 4 to 56 cm in some cases huge holes form with 5 m depths and 30 m diameters un like cryoconite holes elsewhere these have ice lids up to 36 cm thick and melt from within each spring about one half of the holes are connected to the near surface hydrologic system and the remainder are isolated the duration of isolation estimated from the chlor ide accumulation in hole waters commonly shows ages of several years with one hole of 10 years the cryoconite holes in the mcmurdo dryvalleys create a near surface hydro logic system tens of cm below the ice surface the glacier surface itself is generally frozen and dry comparison of water levels between holes a few meters apart shows independent cycles of water storage and release most likely local freeze thaw effects control water passage and therefore temporary storage rough calculations indicate that the holes gen erate at least 13 of the observed runoff on the one glacier measured this hydrologic system represents the transition between a melting ice cover with supraglacial streams and one entirely frozen and absent of water

trapped liquid paleo porosity and formation time scale of a chromitite ortho pyroxenite cumulate section bushveld south africa to evaluate compaction and interstitial melt expulsion during cumulate formation a 20 m cumulate section including the ug2 and ug3 chromitites from a 264 m drill core through the upper critical zone of the bushveld complex south africa has been studied the cumulates in the studied section are as follows 3 m plagioclase pyroxenite to pyroxenite pegmatoid footwall pyroxenite at the lower contact to ug2 0 7 m ug2 chromitite 6 8 m pyroxenite 0 24 m ug3 chromitite 2 0 m plagioclase rich pyroxenite changing locally to norite the two 5 cm leader stringers ug3a and ug3b and 7 m total of olivine pyroxenites grading into plagioclase pyroxenites all pyroxenites are dominated by orthopyroxene opx and the cumulate sequence is topped by mottled anorthosite grading into norite stratigraphic concentrations of major and trace elements of 52 bulk rock samples were determined bulk rock mg numbers are 0 79 0 81 throughout the silicate cumulate units and 0 40 0 46 in the chromitite layers the stratigraphic distribution of six incompatible trace elements k rb ba cs zr and th has been used to determine the amount of trapped liquid ftl or paleo porosity in the cumulate rocks final porosities volume fractions based on averages from the six trace elements are 0 06 0 33 in the pyroxenites in chromitite layers trapped melt fractions of 0 12 0 36 are calculated from incompatible trace element concentrations but bulk sio2 concentrations and x ray tomography yield 0 04 0 17 higher porosities hence the bulk silicate fraction in the chromitites may not necessarily correspond to the trapped liquid fraction as poikilitic opx was crystallizing while the silicate melt still equilibrated using a previously derived experiment based model for compaction time scales gravitationally driven chemical compaction in the ug2 ug3 pyroxenite section is calculated to occur within 1 10 years this time frame corresponds to the times necessary to cool a 20 m layer by 10 50 c the temperature interval argued to encompass the liquidus and almost complete solidification compaction within a decade can in fact easily develop the paleo porosities indirectly observed today and is probably stopped by crystallization of the interstitial liquid contrary to previous assertions melt expulsion from the cumulate pile does not hinder compaction calculated permeabilities would allow for the migration of an order of magnitude higher amount of melt than has to be expelled from the 20 m pile of cumulate the pegmatoid zones in the chromitite footwalls enriched in incompatible trace elements are consistent with a collection of interstitial melts expelling from the underlying compacting pyroxenites their entrapment below the chromitite layers suggests that these act as permeability barriers this is in part due to their finer grain size compared with the pyroxenites but is mainly due to the crystallization of large poikilitic opx during compaction

a composite study of comma clouds and their association with severe weather over the great plains abstract sixty eight comma cloud systems over the great plains during two spring seasons were examined using satellite imagery and rawinsonde data composite soundings were produced for each of ten distinct parts of the comma cloud in order to describe quantitatively the atmospheric structure associated with wave cyclones that produced 585 severe weather events composite sectionals and soundings document the different kinematic and thermodynamic environment of each part of the comma cloud system relative wind isentropic analyses show air flow relative to the storm and provide additional evidence that the clear region intruding northward east of the cyclone center is an area of strong ascent of dry air with a previous history of subsidence stability computations from the mean soundings suggest that the most likely location for severe weather is near the central part of the comma tail in agreement with the tabulated severe weather reports a case study from 21 22 march 1981 is conducted to investigate

porous sidewall and sloping floor crystallization experiments using a reactive mush implications for the self channelization of residual melts in cumulates abstract tank crystallization experiments with aqueous solutions were used to explore interactions between a buoyant residual fluid and a porous cumulate at a sloping floor and a vertical wall the fluid flow path in a non reactive mush glass beads was used as a control for a vertical non reactive mush the rise of the buoyant solute depleted fluid was confined within it fluid flow paths were curved in the case of a sloping mushy floor with the upper part of the mush being principally a plume discharge zone and the lower part being a recharge zone more realistic reactive mushes were created by supercooling a nh 4 cl solution cooling against a vertical surface produced an initial mat of dendrites that were subsequently overgrown by a vertically grooved dendrite structure the grooves contained upwardly flowing residual melt while the ridges extended beyond the buoyant boundary layer into the thermally convecting solute rich part of the tank with time the grooves were covered over by a dendrite mat but remained as open channels within the mush focussing and accelerating the upflow of residual melt a buoyant boundary layer on the outside of the mush was not observed in the experiment with a sloping floor a thin mat of homogeneous mush was succeeded by a more organized deposit characterized by chimneys residual melt escaped as plumes through the chimneys chimneys formed because buoyant residual melt dissolved the mush as it rose and warmed chimney spacing increased and the number of chimneys decreased as the mush thickened dissection of the mush showed that individual chimney channels were inclined upslope so that melt residual from crystallization at the foot of the slope emerged from chimneys near the top of the slope formation of channels in cumulates would greatly increase fractionation efficiency by increasing the effective volume from which residual melts could be extracted and by increasing the rate at which residual melts could be transferred to the upper part of the chamber

structure and formation process of cryoconite granules on urumqi glacier no 1 tien shan china cryoconite granules are dark colored spherical aggregates of organic and inorganic material on glacier ice and are commonly observed on glaciers the world over the structure of cryoconite granules on urumqi glacier no 1 tien shan china was analyzed granules were distributed over the entire ice surface of the ablation area and ranged in size from 0 26 to 3 5 mm mean 1 1 mm the granule surface was densely covered with filamentous cyanobacteria microscopy of a thin section revealed various inner structures most granules had concentric layers of dense organic matter which are probably derived from annual growth of the granules by the activity of cyanobacteria the number of layers averaged 3 5 and ranged up to 7 which is likely to indicate their mean and maximum growth ages respectively some granules contained two or more subgranules showing that small granules had combined and enlarged such structures suggest that granule formation was mainly due to the activity of filamentous cyanobacteria and that the granules repeatedly grew and disintegrated over a cycle of several years on the glacier

dynamical aspects of the october storm 1987 a study of a successful fine mesh simulation a diagnostic study of a successful numerical simulation of the october storm 1987 is described this good non operational forecast was obtained using the meteorological office s fine mesh model run from a revised analysis at 00 gmt on 15 october 1987 the meteorological office s mesoscale model was also run using initial conditions at 12 gmt 15 october obtained by interpolating the above fine mesh forecast fields to examine the benefits of very high horizontal resolution 15km grid length diagnostic maps and cross sections derived are used to address two scientific issues i the role of weak moist slantwise stability or alternatively positive slantwise convective potential energy ii the dynamical significance of the striking cloud structure the cloud head seen in the satellite imagery early in the storm s life it appears that the baroclinic zone in which the storm develops is highly receptive to forcing by an upper trough and the warm boundary layer air just ahead of the main surface front is characterized by substantial slantwise convective available energy the cloud head is identified with a region of intense slantwise ascent in the warm air just above the main frontal zone its great meridional extent is associated with the upper tropospheric outflow and horizontal spreading of air which has recently undergone rapid slantwise ascent the low level temperature field exhibits throughout the development period a remarkable shape preserving thermal ridge of considerable intensity the sensitivity of the forecast to the removal of surface energy fluxes and latent heat of condensation are investigated separately it is found that on the timescale of the simulation surface energy fluxes have negligible impact on the explosive development on the other hand the contribution of latent heat release to the development appears to dominate the dry baroclinic instability process

on assessing local conditional symmetric instability from atmospheric soundings abstract the standard parcel method of assessing the susceptibility of the atmosphere to moist convection using tephigrams is extended to account for the centrifugal as well as the gravitational potential energy of the displaced air parcel this leads to a measure of the stability of the moist baroclinic atmosphere to finite slantwise reversible displacements of a two dimensional air parcel such a measure differs from previously derived measures of conditional symmetric instability which have considered only infinitesimal displacements in saturated atmospheres it is demonstrated that the combined gravitational and centrifugal potential of a two dimensional air parcel or tube can be assessed by displacing the tube slantwise along a surface of constant angular momentum and that this combined potential energy can be estimated using a single atmospheric sounding several examples of the application of this technique are presented in the context of a case study of apparent slantwise convection the result

discordant iron rich ultramafic pegmatites in the bushveld complex and their relationship to iron rich intercumulus and residual liquids the iron rich ultramafic pegmatites comprise a suite of coarse grained rocks that form discordant bodies within the layered sequence of the bushveld complex these pegmatites which are considerably more abundant than is generally realized provide evidence for the differentiation of iron rich residual melts the pegmatites are composed largely of iron rich olivine and clinopyroxene together with ti magnetite and ilmenite feldspar is characteristically absent but paradoxically the pegmatites preferentially replace anorthositic cumulates two subgroups are recognized olivine clinopyroxene pegmatite and fe ti oxide pegmatite with increased stratigraphic height the pegmatites become richer in fe ti oxides thus olivine clinopyroxene pegmatite is prevalent in the upper critical and lower main zones whereas fe ti oxide pegmatite is restricted to the upper main and upper zones zoned pegmatite with a core of fe ti oxide pegmatite is transitional between the two subgroups new whole rock and electron microprobe analyses of olivine clinopyroxene pegmatite from the upper critical and lower main zones provide convincing evidence that their composition is directly related to height cryptic compositional variations are analogous to those displayed by the layered cumulates but for a given height the pegmatites are always more evolved compositions of clinopyroxene in the pegmatites reflect a near linear relationship with height whereas cumulus pyroxenes display upward iron enrichment trends complicated by replenishment and reaction with trapped intercumulus liquid the olivine clinopyroxene pegmatite formed by magmatic replacement of earlier formed cumulates in response to infiltration of iron rich melts suitably differentiated melts comprised intercumulus and residual liquids derived from thick anorthosite layers the absence of feldspar although not fully understood is attributed to an immiscible relationship between dense iron rich melts and light silica alkali rich liquids the latter infiltrated upward to be reincorporated into the resident magma the iron rich melts however drained down into the crystallizing cumulate pile channelling along early formed fractures and joints was significant locally resulting in huge pipe like bodies of pegmatite the iron rich melts became increasingly differentiated with height partly in response to the fractional crystallization of more evolved cumulates the olivine clinopyroxene pegmatites are related to infiltration of fe ti oxide rich silicate melt whereas fe ti oxide pegmatite is ascribed to fe ti oxide liquid as originally argued by bateman 1951 the bushveld complex followed the fenner trend of almost uninterrupted iron enrichment evidence of pronounced iron enrichment is however manifested in the discordant iron rich ultramafic pegmatites several thousands of metres below the height at which iron rich cumulates are observed

cumulative deformation and original geometry of the bushveld complex abstract a structural review and re interpretation of the bushveld complex based on 254 georeferenced maps from research technical reports and geophysical surveys suggests that its current geometry differs markedly from its original geometry broadly n s flattening left lateral transpressional shearing along an interconnected fault network steepening of cumulate layering and 10 km of uplift to the north of the northern margin of the complex s main chamber were accompanied by an eastward verging escape structure burgersfort bulge and syn cooling gap areas on its western limb southward verging deformation was transmitted from the villa nora fragment via thrusting in the rooiberg area generating 50 70 km of southward displacement uplift and erosional loss of the northern limb s western portion a string of anomalous occurrences of bushveld complex units and dense areas outlined by unconstrained gravity isoshells suggest a further 170 km southward displacement through the center of the main chamber the southern limb underwent predominantly right lateral ductile offset from the western limb along the crocodile river and rustenburg faults expressed by an in line series of nnw trending magnetic highs to the south of their mapped terminations left lateral shearing and progressive splaying of a nne trending dyke swarm to the west of an inflection point provides strain markers for this offset and 35 anticlockwise rotation of the burgersfort bulge and central parts of the main chamber although recent work suggests that the complex cooled within 1 myr late intrusion syn cooling sse wards movement of its deeper ductile central parts within at least 5 10 myr after its initial intrusion the transvaalide fold and thrust event and the expulsion of fluids to form the high ti igneous suite hitis and related bodies appear to have closely post dated the complex s intrusion removal of the effects of cumulative strain produces a more equant complex shortly after its intrusion occupying an orthogonal network of normal faults and perpendicular strike slip transverse faults

assessment of conditional symmetric instability from global reanalysis data abstractslantwise convection the process by which moist symmetric instability is released has often been linked to banded clouds and precipitation especially in frontal zones within extratropica

a magmatic hydrothermal transition in arkaroola northern flinders ranges south australia from diopside titanite pegmatites to hematite quartz growth a set of palaeozoic diopside titanite veins are present in mesoproterozoic metagranites and metasediments that constitute the basement mt painter inlier of the adelaide fold belt south aus tralia these massive veins up to 1 m of pegmatitic nature contain large crystals of diopside lree y enriched titanite up to 40 cm in length and minor amounts of quartz they can be used to trace the sys tem s development from a high temperature magmatic stage through to a massive hydrothermal event the pegmatitic origin of these veins is evident from a complex fluid melt inclusion assemblage consisting of a highly saline inhomogeneous fluid and relicts of melt immiscibility of melt and heterogeneous highly saline fluids exceeding 61 eq mass nacl is preserved in primary inclusions in diopside and secondary inclusions in titanite indicating relatively shallow conditions of formation 510 20 c and 130 10 mpa graphic intergrowth of diopside and albite occurs at the contact with granitic pegmatites the system evolved into hydrothermal conditions which can be deduced from a later population of only fluid inclusions homogeneous and less saline 40 eq mass nacl trapped around 350 20 c and 80 10 mpa during quartz crystallization the conditions moved across the halite liquidus resulting in a heterogeneous mixture of brine and halite crystals which were trapped at 200 20 c and 50 10 mpa brecciation and a palaeo geother mal system overprinted the pegmatitic veins with an epithermal hematite quartz assemblage and lesser amounts of bladed calcite and fluorite in an intermit tently boiling hydrothermal system of fairly pure h2o at 100 140 c and 1 5 mpa remobilization of lrees and y from titanite and or the granitic host rock is evidenced by precipitation of apatite allanite and wakefieldite in an intermediate stage occasional incorporation of radioactive elements or minerals presumably u rich in the fluorite is responsible for radiolysis of h2 ot o h 2

the sting at the end of the tail damaging winds associated with extratropical cyclones strong surface winds often accompany the low level jets that occur along the cold fronts of extratropical cyclones but there is evidence that the strongest surface winds occur in a distinctly different part of a certain class of cyclone the most damaging extratropical cyclones go through an evolution that involves the formation of a bent back front and cloud head separated from the main polar front cloud band by a dry slot when the cyclone attains its minimum central pressure the trailing tip of the cloud head bounding the bent back front forms a hook which goes on to encircle a seclusion of warm air the most damaging winds occur near the tip of this hook the sting at the end of the tail observations of the great storm of october 1987 in south east england are re examined in some detail to study this phenomenon the cloud head is shown to have a banded structure consistent with the existence of multiple mesoscale slantwise circulations air within these circulations leaves the hooked tip of the cloud head and enters the dry slot much faster than the rate of travel of the cloud head tip implying rapid evaporation and diabatic cooling immediately upwind of the area of damaging surface winds the circumstantial evidence from the observational study leads one to hypothesize that the mesoscale circulations and the associated evaporative heat sinks may play an active role in strengthening the damaging winds regardless of how important this role may be the evolution of the cloud pattern seen in satellite imagery is a useful tool for nowcasting the occurrence and location of the worst winds copyright 2004 royal meteorological society

the mass area relationship within cryoconite holes and its implications for primary production cook j hodson a telling j anesio a irvine fynn t bellas c 2010 the mass area relationship within cryoconite holes and its implications for primary production annals of glaciology 51 56 106 110

discordant bodies from olivine bearing zones iii and iv of the stillwater complex montana evidence for postcumulus fluid migration and reaction in layered intrusions mineralogically zoned and unzoned discordant bodies composed predominately of plagioclase with up to 35 olivine occur at three different levels in olivine bearing zones iii and iv of the middle banded series of the stillwater complex the discordant bodies are elongate perpendicular to the layering of the host cumulates with slender concordant apophyses although the host olivine gabbros are foliated with tabular plagioclase the discordant bodies lack a discernible fabric and have blocky plagioclase average olivine in the host rocks is slightly more magnesian than that of the discordant bodies mg 75 8 0 7 versus mg 74 6 0 3 respectively but plagioclase compositions are indistinguishable an77 6 2 0 versus an76 6 4 3 average host and discordant bodies respectively whole rock major and trace element compositions of the discordant bodies are generally indistinguishable from cumulates with similar modal abundance however bulk compositions of anorthositic cores from the discordant bodies are enriched in k na ba sr and p we conclude that the discordant bodies formed when cooler volatile fluids or fluid rich silicate liquids moved upward and encountered a hotter undersaturated solid plus liquid assemblage continued liquid fluid fluxing increased the permeability along the flow path and focused the flow allowing the original bulk compositions to be modified and leaving plagioclase rich troctolites and anorthosites the shapes of the discordant bodies suggest that the cumulus pile had anisotropic permeability during late stage liquid fluid flow chemical and mineralogical evidence from other parts of olivine bearing zones iii and iv suggests that the processes that formed the discordant bodies may have influenced other cumulates in fact it appears that the same processes that formed the discordant bodies operated within an anorthositic layer strongly modifying the chemistry of the rock but leaving no mineralogical or textural evidence

discussion of oxygen isotope and trace element compositions of platiniferous dunite pipes of the bushveld complex south africa signals from a recycled mantle component by gunther t haase k m junge m oberthur t woelki d and krumm s

below surface ice melt on the coastal antarctic ice sheet in the jutulgryta area of dronning maud land antarctica subsurface melting of the ice sheet has been observed the melting takes place during the summer months in blue ice areas under conditions of below freezing air and surface temperatures adjacent snow covered regions having the same meteorological and climatic conditions experience little or no subsurface melting to help explain and understand the observed melt rate differences in the blue ice and snow covered areas a physically based numerical model of the coupled atmosphere radiation snow and blue ice system has been developed the model comprises a heat transfer equation which includes a spectrally dependent solar radiation source term the penetration of radiation into the snow and blue ice depends on the solar radiation spectrum the surface albedo and the snow and blue ice grain sizes and densities in addition the model uses a complete surface energy balance to define the surface boundary conditions it is run over the full annual cycle simulating temperature profiles and melting and freezing quantities throughout the summer and winter seasons the model is driven and validated using field observations collected during the norwegian antarctic research expedition nare 1996 97 the simulations suggest that the observed differences between subsurface snow and blue ice melting can be explained largely by radiative and heat transfer interactions resulting from differences in albedo grain size and density between the two mediums

hydrometeorological relationships on haig glacier alberta canada we investigate the relationships between meteorological hydrological and glaciological data collected at haig glacier alberta canada for the 2002 and 2003 ablation seasons correlation lag cross correlation and multivariate regression analyses are employed to assess the seasonal evolution of relationships between temperature temperature residuals total daily radiation albedo accumulation area ratio aar and total daily discharge qi early season melt is temperature dependent when aar remains high and the hydraulic properties of the snowpack limit both diurnal discharge variability and a rapid hydrologic response as the melt season progresses a decreasing aar and ripening of the snowpack induce a glacier wide decrease in albedo and a structured radiation discharge response is observed radiation detrended temperature values offer modest improvements over physical temperature values in multivariate regression models estimating daily discharge values using a detrended temperature indexed melt model we assess the transport efficiency of the glacial hydrologic system through a comparison of total modelled daily melt and observed discharge transport efficiency values support the notion of a purge effect during freezing events and at the end of the ablation season and suggest that it is the evolution of the supraglacial drainage system that controls diurnal discharge variability

cryoconite hole thermodynamics energy sources governing the growth of cryoconite holes have been investigated by monitoring the meteorological balance at the surface of the manatee glacier british columbia the dimensions of 125 holes and the thermal conditions within them cryoconite holes are shown to be rough indicators of the energy balance at the glacier surface both atmospheric and biological energy sources operate thereby suggesting a genetic classification of holes with solar and biological end members

geochemistry and mineralogy of the platreef and critical zone of the northern lobe of the bushveld complex south africa implications for bushveld stratigraphy and the development of pge mineralisation the northern lobe of the bushveld complex is currently a highly active area for platinum group element pge exploration this lobe hosts the platreef a 10 300 m thick package of pge rich pyroxenites and gabbros that crops out along the base of the lobe to the north of mokopane formerly potgietersrus and is amenable to large scale open pit mining along some portions of its strike an early account of the geology of the deposit was produced by percy wagner where he suggested that the platreef was an equivalent pge rich layer to the merensky reef that had already been traced throughout the eastern and western lobes of the bushveld complex wagner s opinion remains widely held and is central to current orthodoxy on the stratigraphy of the northern lobe this correlates the platreef and an associated cumulate sequence that includes a chromitite layer known as the grasvally norite pyroxenite anorthosite gnpa member directly with the sequence between the ug2 chromitite and the merensky reef as it is developed in the upper critical zone of the eastern and western bushveld implicit in this view of the magmatic stratigraphy is that similar critical zone magma was present in all three lobes prior to the development of the merensky reef and the platreef however when this assumed correlation is examined in detail it is obvious that there are significant differences in lithologies mineral textures and chemistries mg of orthopyroxene and olivine and the geochemistry of both rare earth elements ree and pge between the two sequences this suggests that the prevailing interpretation of the stratigraphy of the northern lobe is not correct the critical zone of the northern lobe cannot be correlated with the critical zone in the rest of the complex and the simplest explanation is that the gnpa platreef sequence formed from a separate magma or mixture of magmas chilled margins of the gnpa member match the estimated initial composition of tholeiitic main zone type magma rather than a critical zone magma composition where the gnpa member is developed over the ultramafic lower zone hybrid rocks preserve evidence for mixing between new tholeiitic magma and existing ultramafic liquid this style of interaction and the resulting rock sequences are unique to the northern lobe the gnpa member contains at least seven sulphide rich horizons with elevated pge concentrations some of these are hosted by pyroxenites with similar mineralogy crystallisation sequences and pd rich pge signatures to the platreef chill zones are preserved in the lowest main zone rocks above the gnpa member and the platreef and this suggests that both units were terminated by a new influx of main zone magma this opens the possibility that the platreef and gnpa member merge laterally into one another and that both formed in a series of mixing quenching events involving tholeiitic and ultramafic magmas prior to the main influx of tholeiitic magma that formed the main zone

interpretation of satellite imagery of a rapidly deepening cyclone model output and observational data are used to interpret in detail satellite images of a distinctive cloud system several hundred kilometres long which was observed before and during rapid cyclonic development that occurred over england conceptual models are forwarded which account for the structure of the major cloud areas and upper level moisture fields as observed by water vapour imagery immediately prior to and following the onset of vigorous cyclogenesis the cloud system corresponded to what r b weldon refers to as a baroclinic leaf cloud and was characterized by ana cold frontal ascent the baroclinic leaf lay within an area of enhanced ascent ahead of a major upper level trough a jet streak which originated upstream propagated rapidly around the base of the upper trough a tongue of dry air centred on the left flank of this jet streak lay immediately upstream of the leaf cloud and had a major influence on its subsequent development this intrusion of dry air was well represented by the meteorological office fine mesh model which showed it as a pronounced tongue of recently descended air with high potential vorticity which originated in the upper troposphere and lower stratosphere rapid cyclogenesis occurred as the dry intrusion overran low level air of high w within a warm conveyor belt at the southern tip of the leaf cloud interaction of the dry intrusion and baroclinic leaf cloud is shown to be particularly important in accounting for the detailed distribution of weather it also suggests a way of pinpointing the location and timing of vigorous cyclogenesis

carbon fluxes through bacterial communities on glacier surfaces there is very little information about the activity of microbial communities on the surface of glaciers though there is an increasing body of evidence to show that they strongly influence the biogeochemistry of these habitats we measured bacterial abundance and production in cryoconite holes on arctic antarctic and alpine glaciers in order to estimate the role of heterotrophic bacteria within the carbon budget of glacial ecosystems our results demonstrate an active bacterial community on the surface of glaciers with doubling times that vary from a few hours to hundreds of days depending on the glacier and position water or sediments within the cryoconite hole however bacterial production is only 2 3 of the published literature values of community respiration from similar habitats indicating that other types of microbes e g eukaryotic organisms may also play a role in the c cycle of glaciers we estimate that only up to 7 of the organic c in cryoconite sediments is utilized by the heterotrophic bacterial community annually suggesting that the surface of glaciers can accumulate organic carbon and that this c may be important for biogeochemical activity downstream to adjacent ecosystems

the merensky reef at winnaarshoek eastern bushveld complex a primary magmatic hypothesis based on a wide reef facies the merensky reef on the winnaarshoek property in the central sector of the eastern limb of the bushveld complex belongs to what may be described as a wide reef facies in which platinum group element pge mineralization is largely contained within a relatively thick succession of medium grained nonpegmatoidal feldspathic orthopyroxenite bounded top and bottom by thin chromite stringers the wide reef merensky reef facies at winnaarshoek presents the opportunity to study subtle processes which could potentially be obscured or obliterated in the thinner reef facies found in many areas of the western limb especially in places where the reef zone contains a component of pegmatoidal orthopyroxenite at winnaarshoek a pegmatoidal feldspathic orthopyroxenite is developed below the lower chromite stringer and a second pegmatoidal orthopyroxenite is intermittently developed several centimeters above the upper chromite stringer in contrast to most occurrences in the western limb however both these pegmatoidal orthopyroxenites are only sporadically mineralized if at all in addition rather than being at the base of a feldspathic orthopyroxenite unit as is the case throughout most of the western limb the merensky reef at winnaarshoek occurs toward the top of a sequence of feldspathic orthopyroxenite despite the gross relative uniformity of the merensky reef on a property wide scale at winnaarshoek we demonstrate using underground channel samples that individual intersections spaced a matter of meters apart commonly display remarkable variations in grade distribution our study is based primarily on whole rock and mineral chemical profiles from drill cores through three separate and distinctive subfacies of essentially normal i e nonpothole reef one of the drill core intersections represents typical reef in which pge mineralization forms an upper reef extending from the upper chromitite stringer approximately 1 m into the underlying feldspathic pyroxenite and a much thinner lower reef centered on the lower chromitite stringer the other two drill core intersections represent extremes in terms of the vertical distribution of the pge one having mineralization confined to the immediate vicinity of the two chromitite stringers while the other is richly mineralized throughout the reef zone peaks in pge concentrations in all three intersections are coincident with the most primitive compositions of orthopyroxene plagioclase and chromite the dominant rock forming minerals whereas the cumulate layers below major pge peaks are the most evolved as indicated by mineral compositions and elevated whole rock incompatible element concentrations furthermore the lowest grade merensky reef intersection has the most chemically evolved footwall whereas the richest intersection has a relatively primitive footwall we consider the origin of the merensky reef in the context of its position as the culmination of a sequence of several pge mineralized chromite bearing layers in the upper critical zone and suggest that the regular upward variations in the chemistry of the chromitite from one layer to the next and the overall cyclicity are indicative of complex magma replenishment and mixing processes in the chamber we apply a model of lateral streaming of magma with chromite crystallization and consequent sulfide and platinum group mineral pgm precipitation resulting from magma mixing specifically the merensky reef cannot be properly interpreted outside the context of the sequence of successive events that evolved throughout the development of the upper critical zone genetic models also need to account for lateral facies variations in the reef

convective destabilization by a tropopause fold diagnosed using potential vorticity inversion there is observational evidence to suggest that at the time tropopause folds in extra tropical cyclones are descending into the mid troposphere there is frequently a destablization of the lower troposphere to moist convection here we consider the dynamical reasons for such a linkage using so called potential vorticity pv attribution concepts in particular pv inversion is used to find the wind shear attributable to the tropopause fold itself then this shear is used to quantify the tendency to generate regions of potential instability for an observed case of a modest tropopause fold it is found that the pv anomaly contained in the tropopause fold contributed substantially to the instantaneous convective destabilization in the sense of increasing the potential instability indeed at some locations it overcame a tendency to stabilize by the atmosphere without the fold on the other hand only a fraction of the region with increased potential instability experienced sufficient large scale ascent to lead to convection and it remains to be seen whether folds in general make a significant contribution to the generation of potential instability that can actually be realized the importance of tropopause folds or indeed of tropopause depressions in general in the context of convection is that their small horizontal scale has the potential to focus and amplify any convective destabilization locally this may account for the localization of outbreaks of severe convective weather which it is important to forecast accurately currently numerical weather prediction models often do not handle these small scales very accurately

spatio temporal variability in elevation changes of two high arctic valley glaciers uncertainties in estimates of glacier and ice cap contribution to sea level rise exist in part due to poor quantification of mass balance errors particularly those resulting from extrapolation of sparse measurements centre line data are often assumed to be representative of the glacier as a whole with little attention paid to extrapolation errors or their effect on mass balance estimates here we present detailed digital elevation model dem measurements of glacier wide elevation changes over the last 40 years at two glaciers on svalbard norwegian arctic austre broggerbreen and midtre lovenbreen are shown to have lost 27 54 0 98 and 9 65 0 76 10 7 m 3 of ice respectively between 1966 and 2005 findings that we relate to trends in average summer air temperatures and winter accumulation these volume losses correspond to geodetic balances of 0 58 0 03 and 0 41 0 03 m w e a 1 respectively our analysis revealed high spatial complexity in patterns of elevation change varying between glaciers between measurement intervals and within and between elevation bins balances from extrapolated centre line geodetic data were the same within errors as those from full coverage dem differencing in the majority of comparisons yet significantly underestimated balance in three instances additionally field mass balance from centre line ablation stake data underestimated balances from full coverage geodetic measurements during three of six measurement periods these findings may support the hypothesis that field measurements under estimate svalbard glacier mass loss at least partly as a result of the failure of centre line measurements to account for glacier wide variations in ablation our results demonstrate the importance of deriving accurate interpolation functions and constraining extrapolation errors from sparse measurements

high resolution analysis of frontal fracture abstract a mesoscale array of dropwindsondes released in a small rapidly deepening frontal wave cyclone in the eastern north atlantic during the fronts 92 experiment has been assimilated into a 17 km grid mesoscale model nested within the meteorological office s operational limited area model the mesoscale model reproduced the evolving cloud pattern with cloud head and dry slot seen in satellite imagery it also revealed a well defined evolution in the three dimensional thermodynamic structure associated with the process known as frontal fracture the frontal fracture was revealed most clearly in the pattern of wet bulb potential temperature w which was distorted by an effective differential rotation the rotation increasing with height this led to backward tilted w surfaces and ana cold frontal characteristics in the cloud head to the north of the center of rotation and to forward tilted w surfaces and kata cold frontal characteristics in the dry slot to the south of the center of rota

the distribution and main geologic features of discordant bodies of iron rich ultramafic pegmatite in the bushveld complex transgressive bodies of iron rich ultramafic pegmatite composed predominantly of varying proportions of hortonolite clinopyroxene ilmenite and ti magnetite are abundantly distributed throughout the layered mafic sequence of the bushveld complex new information from field and underground studies together with interpretations of detailed magnetic surveys is presented along with an overview of the available literature some whole rock and mineralogical chemical data are also included within the bushveld complex a number of distinct but possibly genetically related groups of postcumulus ultramafic rock are recognized these include besides the main group of iron rich ultramafic pegmatite bodies of magnesian dunite and the well known platiniferous ultramafic or dunitic pipes transgressive often pipelike bodies composed almost entirely of fe ti oxides occur in the main and upper zones of the complex which although part of the iron rich ultramafic pegmatite suite are described separately composite bodies in which massive fe ti oxides occur as a core within a peripheral shell of silicate rich pegmatite are also recognized the major occurrences of iron rich ultramafic pegmatite are documented and depicted in a series of maps their distribution is subject to two major stratigraphic controls iron rich ultramafic pegmatite occurs largely above the ultramafic cumulates in the lower and lower critical zones it is abundant in the upper critical and main zones where plagioclase is a major cumulus phase but is rarely observed above the highest cumulus ti magnetite layer in the upper zone it is also evident that these rocks become increasingly iron rich toward the top of the layered sequence so that in the upper zone fe ti oxide pegmatite is the dominant variety many bodies particularly the large ones are structurally controlled and occur in disturbed areas characterized by faulting and post bushveld dikes stratigraphic magnetic anomalies in the complex may be related to anorthositic layers containing small specks of iron rich ferromagnesian silicates and fe ti oxides this iron rich material is considered to represent the accumulated and trapped late stage residual and intercumulus fluid resulting from fractional crystallization within a cyclic unit lateral migration and channeling of these pegmatitic fluids into areas of structural weakness resulted in the formation of large pipelike bodies of iron rich ultramafic pegmatite these bodies may have resulted from metasomatic replacement of preexisting cumulates or they may have crystallized directly from the pegmatitic fluids the authors believe that the pegmatitic fluids have been derived from within the crystallizing cumulate pile and it is concluded that the formation of iron rich ultramafic pegmatite is an integral feature of the layered sequence

scape charts from numerical weather prediction model fields abstract although the scape concept is strictly only meaningful for quasi two dimensional flows it can be given a precise definition for flows satisfying the conditions governing the semigeostrophic approximation for this type of balanced flow it is argued that scape corresponds to the positive area on the tephigram when the environment curve defines the thermodynamic state along a semigeostrophic vortex line a procedure for the calculation of scape as a numerical model diagnostic is described and is applied to an operational model analysis prior to the great storm which struck southern england on 16 october 1987 values of scape greater than 1000 j kg 1 are found over a large area near the embryo storm since the degree of slantwise stability or instability is widely considered to be relevant to the intensity of cyclogenesis and rainband formation scape charts may be of some utility to the bench forecaster

conditional symmetric instability in sting jet storms sting jets are transient mesoscale jets of air that descend from the tip of the cloud head towards the top of the boundary layer in severe extratropical cyclones and can lead to damaging surface wind gusts this recently identified jet is distinct from the well documented jets associated with the cold and warm conveyor belts one mechanism proposed for their development is the release of conditional symmetric instability csi here the spatial distribution and temporal evolution of several csi diagnostics in four severe storms are analysed a sting jet has been identified in three of these storms for comparison we also analysed one storm that did not have a sting jet even though it had many of the apparent features of sting jet storms the sting jet storms are distinct from the non sting jet storms by having much greater and more extensive conditional instability ci and csi csi is released by ascending air parcels in the cloud head in two of the sting jet storms and by descending air parcels in the other sting jet storm by contrast only weak ci to ascending air parcels is present at the cloud head tip in the non sting jet storm csi released by descending air parcels as diagnosed by decaying downdraught slantwise convective available potential energy dscape is collocated with the sting jets in all three sting jet storms and has a localised maximum in two of them consistent evolutions of saturated moist potential vorticity are found we conclude that csi release has a role in the generation of the sting jet that the sting jet may be driven by the release of instability to both ascending and descending parcels and that dscape could be used as a discriminating diagnostic for the sting jet based on these four case studies copyright 2011 royal meteorological society and british crown copyright the met office

solar heating rates and temperature profiles in antarctic snow and ice observations of temperature maxima at about 10 cm depth in cold antarctic snow during summer have previously been explained by proposing that solar heating is distributed with depth whereas thermal infrared cooling is localized at the surface the solid state greenhouse an increase in temperature from the surface to 10 cm depth t 4 k found by rusin 1961 on the antarctic plateau was successfully reproduced by schlatter 1972 in a combined radiative transfer and heat transfer model

near surface hydraulic conductivity of northern hemisphere glaciers the hydrology of near surface glacier ice remains a neglected aspect of glacier hydrology despite its role in modulating meltwater delivery to downstream environments to elucidate the hydrological characteristics of this near surface glacial weathering crust we describe the design and operation of a capacitance based piezometer that enables rapid economical deployment across multiple sites and provides an accurate high resolution record of near surface water level fluctuations piezometers were employed at 10 northern hemisphere glaciers and through the application of standard bail recharge techniques we derive hydraulic conductivity k values from 0 003 to 3 519 m day 1 with a mean of 0 185 0 019 m day 1 these results are comparable to those obtained in other discrete studies of glacier near surface ice and for firn and indicate that the weathering crust represents a hydrologically inefficient aquifer hydraulic conductivity correlated positively with water table height but negatively with altitude and cumulative short wave radiation since the last synoptic period of either negative air temperatures or turbulent energy flux dominance the large range of k observed suggests complex interactions between meteorological influences and differences arising from variability in ice structure and crystallography our data demonstrate a greater complexity of near surface ice hydrology than hitherto appreciated and support the notion that the weathering crust can regulate the supraglacial discharge response to melt production the conductivities reported here coupled with typical supraglacial channel spacing suggest that meltwater can be retained within the weathering crust for at least several days not only does this have implications for the accuracy of predictive meltwater run off models but we also argue for biogeochemical processes and transfers that are strongly conditioned by water residence time and the efficacy of the cascade of sediments impurities microbes and nutrients to downstream ecosystems because continued atmospheric warming will incur rising snowline elevations and glacier thinning the supraglacial hydrological system may assume greater importance in many mountainous regions and consequently detailing weathering crust hydraulics represents a research priority because the flow path it represents remains poorly constrained

recrystallization and the origin of layering in the bushveld complex abstract three rock types are distinguished in this study a the layered rocks b the reef pegmatites which are parallel to the layering and form prominent marker horizons and c the iron rich ultramafic irum pegmatites which are transgressive to the layered rocks reaction textures are observed in the layered rocks adjacent to reef pegmatites and are compared with similar reaction textures from alteration zones adjacent to and caused by the iron rich ultramafic irum pegmatites these include a clinopyroxene coronas around orthopyroxene b hornblende lamellae along 100 of orthopyroxene c orthopyroxene altered to olivine serpentine talc and or chlorite in the alteration zones adjacent to irum pegmatites the textures were generated by interaction between the pegmatite forming fluid and the wallrocks a similar origin is proposed for reaction textures in the layered rocks in the latter case they were formed by the fluid responsible for the formation of reef pegmatites in the layered rocks the reaction textures were modified to variable degrees during and after the emplacement of reef pegmatites these modifications include in leuconorites a clinopyroxene coronas overgrown by feldspar b orthopyroxene overgrown by feldspar and hornblende lamellae transforming to biotite and in olivine pyroxenites c olivine serpentine textures overgrown by a second generation of orthopyroxene the modifications suggest the subsolidus dissolution of orthopyroxene and precipitation of feldspar in leuconorites and the reverse process in pyroxenites the textures suggest considerable transfer of matter in the solid rock due to a fluid gradient orthopyroxene recrystallized in areas with a high ah2o plagioclase in regions of low ah2o this process is believed to be responsible for the stratification of the rock into pyroxene and plagioclase rich layers spherical mineral inclusions are relict textures in the three main minerals chromite orthopyroxene and plagioclase a number of similarities between the different inclusions suggest a similar origin the high content of hydrous minerals in all three suggests their formation during the emplacement of reef pegmatites these hydrous minerals were then trapped in the recrystallizing plagioclase orthopyroxene and chromite also spherical mineral inclusions witness the late crystallization of pyroxene in melanorite pyroxenite and the crystallization of plagioclase in leuconorite anorthosites after the emplacement of reef pegmatites

chemistry morphology and origin of magmatic reaction chromite stringers associated with anorthosite in the upper critical zone at winnaarshoek eastern limb of the bushveld complex

re os isotope and pd ru variations in chromitites from the critical zone bushveld complex south africa abstract rhenium osmium isotope measurements of selected chromitites from the critical zone of the bushveld igneous complex reveal that a significant component of radiogenic os was present in the magma when it formed 2 06 ga ago measured 187 os 188 os ratios increase with stratigraphic height and individual layers have similar values across the complex initial 187 os 188 os ratios for chromitites from the western bushveld decrease from 0 140 at the lg to 0 129 at the mg2 chromitite increasing to 0 153 at the ug2 chromitite the inflection at mg2 mg3 is the boundary between the lower and upper critical zone consistent with previous mineralogical and geochemical studies indicating a shift in magma composition at this point chromite separates have os values of 12 35 at 2 06 ga and are too radiogenic for a purely mantle derivation assimilation of 5 mafic granulitic lower crust in a tholeiitic parent magma can explain the radiogenic values pd ru ratios for the intergranular and chromite separate components of each layer differ by orders of magnitude but relative shifts from layer to layer are similar combined with the re os data the results suggest that pge were concentrated into chromitites by magmatic or early deuteric processes that were specific to each layer post layering upward mobilization of pge by hydrothermal processes probably did not play a significant role in pge enrichment

graphic textured inclusions in apatite evidence for pegmatitic growth in a ree enriched carbonatitic system graphic texture most common in felsic pegmatites is documented here in an unusual setting within apatite of the hoidas lake light rare earth element lree deposit an inferred distal pegmatitic hydrothermal counterpart of a carbonatite in northern saskatchewan canada the graphic texture is displayed by vermicular and cuneiform lree rich inclusions which define primary growth zones in apatite integrated electron microprobe analysis and scanning electron microscope energy dispersive x ray spectroscopy imaging document the unusual and heterogeneous compositions of the inclusions which vary between ce2o3 sio2 tho2 dominant and la2o3 nd2o3 f dominant end members here we discuss the formation of these graphic textured inclusions in the context of models suggested for the formation of graphic quartz k feldspar intergrowths in granitic pegmatites based on the similarity of these features

the impact of deformation strain on the formation of banded clouds in idealized modelling experiments experiments are performed using an idealized version of an operational forecast model to determine the impact on banded frontal clouds of the strength of deformational forcing low level baroclinicity and model representation of convection line convection is initiated along the front and slantwise bands extend from the top of the line convection elements into the cold air this banding is attributed primarily to m adjustment the cross frontal spreading of the cold pool generated by the line convection leads to further triggering of upright convection in the cold air that feeds into these slantwise bands secondary low level bands form later in the simulations these are attributed to the release of conditional symmetric instability enhanced deformation strain leads to earlier onset of convection and more coherent line convection a stronger cold pool is generated but its speed is reduced relative to that seen in experiments with weaker deformational strain because of inhibition by the strain field enhanced low level baroclinicity leads to the generation of more inertial instability by line convection for a given capping height of convection and consequently greater strength of the slantwise circulations formed by m adjustment these conclusions are based on experiments without a convective parametrization scheme experiments using the standard or a modified scheme for this model demonstrate known problems with the use of this scheme at the awkward 4 km grid length used in these simulations copyright 2008 royal meteorological society

shape and distribution analysis of merensky reef potholing northam platinum mine western bushveld complex implications for pothole formation and growth syn magmatic removal of the cumulate pile during the formation of the bushveld complex resulted in potholes erosion progressed downward in the cumulate pile resulting in a series of steep transgressive contacts between locally conformable potholed reefs in the regional pothole sub facies of the swartklip facies in the western limb of the bushveld complex the deepest of these potholes third order or fwp2 potholing occurs where the base of the merensky cyclic unit transgresses the upper pseudo reef chromitite marker horizon the base of a fwp2 pothole on northam platinum mine consists of an unconformable stringer merensky chromitite overlain by a medium grained poikilitic orthopyroxenite and underlain by either a pegmatitic harzburgite or the medium grained lower pseudo reef anorthosite detailed shape and distribution analysis of fwp2 potholes reveals underlying patterns in their shape and distribution which in turn suggest a structural control the ratio between pothole short vs long axes is 0 624 n 1 385 although the ratio increases from 0 48 to 0 61 in the long axis range 10 to 60 m then decreases from 0 61 to 0 57 from 61 to 100 m increasing again from 0 57 to 0 61 from 101 to 400 m suggesting that there is not a simple relationship between pothole shape and size shape circularity eccentricity and dendricity analysis of a subset of 638 potholes indicates that potholes with long axes 100 m have an average normalized shape that is bilobate and elongate on a 120 orientation the average aspect ratio short axis length divided by long axis length of potholes is highest for potholes with long axis lengths 100 m and lowest for potholes with long axis lengths between 35 and 60 m the most common long axis orientation for potholes with long axis lengths 100 m fractal analysis indicates that the distribution of pothole centers is controlled neither by a single nor several interacting fractal dimensions autocorrelation fry analysis of the distribution of pothole centers shows recurring pothole distribution trends at 038 070 and 110 for potholes over the full range of long axis lengths while the trends of 008 and 152 occur in potholes with long axes lengths between 60 and 100 m chi squared x2 analysis of the locations of pothole centers suggests that the distribution of small potholes is highly non uniform but becomes exponentially more uniform with increasing pothole size the model which best fits the observed shape and distribution analysis is a combination of protracted independent growth and nearest neighbor merging along specific orientations for instance the clustered distribution of original pothole centers resulted in merged potholes with long axes lengths of up to 60 m exhibiting short vs long axes ratios of 0 61 preferred orientations of 150 and alignment along 010 and 150 trends further independent growth allowed for merging of similar sized and smaller neighboring potholes generating potholes with long axes of up to 100 m in length a preferred long axis orientation of 150 and alignment along 010 040 075 and 150 subsequent preferential merging occurred along a 120 trend thereby preserving a bilobate form this implies that while pothole initiation and enlargement may be driven by a top down i e possibly thermomechanical process an underlying linear or structural catalyst control is revealed in changes in pothole shape during enlargement and furthermore in the preferred trends along which potholes merged over a considerable period possibly concomitant with adjustment of major structures in the footwall to the bushveld complex and pulses into the magma chamber

extreme hydrochemical conditions in natural microcosms entombed within antarctic ice cryoconite holes are near vertical tubes that form in the surface of glaciers when solar heated debris melts into the ice those that form in the mcmurdo dry valleys of antarctica are distinctive in that they have ice lids and are closed to the atmosphere for periods of years to decades photoautotrophs and heterotrophs grow within this closed environment perturbing the poorly buffered water chemistry yet maintaining the potential for photosynthesis microbial excretion and decomposition of organic matter produces dissolved organic carbon doc dissolved inorganic carbon ratios of 1 2 much of the dissolved nitrogen pool 80 100 exists as dissolved organic nitrogen don the don doc ratio is 1 11 mol mol typical of organic particulate material at the earth s surface the combination of photoautotrophy heterotrophy and weak chemical buffering within these microcosms promotes values of ph pco2 o2 saturation and percentage total dissolved nitrogen as don that reach 10 99 10 7 6 atm 160 and 100 respectively which are a unique combination among the surface waters on earth these ice sealed cryoconite holes could be important analogues of refugia on snowball earth and other icy planets

the formation of a clinopyroxene corona mass balance considerations abstract the formation of a cpx corona is discussed from the viewpoint of continuous reaction techniques two cases are distinguished a closed and open system the first case is based on the equation e ol w a pl u b pl v opx x c cpx y d spl y plagioclase is assumed to be a solid solution between anorthite and albite na u ca 1 w al 2 u si 2 u o 8 the other minerals are solutions of their fe and mg end members e g mg w fe 1 w 2 sio 4 for olivine spinel forms a ternary system of hercynite spinel magnetite stoichiometric coefficients are expressed in terms of the exchange parameters t u v w x y and z according to stoichiometries the net molar quantity of phaligioclase consumed equals the number of moles of clinopyroxene and spinel produced for each mole of olivine consumed there is one mole of opx produced a closed system is suggested by the ability to predict one of the mineral compositions in terms of the others using one of the six mass balances na ca mg fe al si left from the calculation of five stoichiometric coefficients this model allows the calculation of mass transfer between different shells and explains coronas from the adirondacks scandinavia and the niquel ndia layered complex brazil in the second case the system is open and is described by the equation opx x a pl u k ca 2 h si 4 f fe 2 c cpx y l na l al 3 g mg 2 the compositional variation of orthopyroxene and clinopyroxene allows the determination of c cpx fe 2 and g mg 2 which are interdependent with a known value of c and assuming a constant volume of reaction the value of a may be determined the final equation for a plagioclase composition of x an 0 75 is 1 opx 0 35 pl 0 74 ca 2 0 21 si 4 0 10 mg 2 1 cpx 0 61 al 3 0 10 fe 2 0 09 na this is the reaction responsible for the formation of cpx coronas in the bushveld complex

sensitivity of trajectory calculations to the temporal frequency of wind data abstract a mesoscale primitive equation model is used to create a 36 h simulation of the three dimensional wind field of an intense maritime extratropical cyclone the control experiment uses the simulated wind field every 15 min in a trajectory model to calculate back trajectories from various horizontal and vertical positions of interest relative to synoptic features of the storm the latter trajectories are compared to trajectories that were calculated with the simulated wind data degraded in time to 30 min 1 h 3 h 6 h and 12 h various error statistics reveal significant deterioration in trajectory accuracy between trajectories calculated with 1 and 3 h data frequencies trajectories calculated with 15 min 30 min and 1 h data frequencies yielded similar results while trajectories calculated with data time frequencies 3 h and greater yielded results with unacceptably large errors

parcel theory in three dimensions and the calculation of scape slantwise convective available potential energy scape is a measure of the degree to which the atmosphere is unstable to conditional symmetric instability csi it has until now been defined by parcel theory in which the atmosphere is assumed to be nonevolving and balanced that is two dimensional when applying this two dimensional theory to three dimensional evolving flows these assumptions can be interpreted as an implicit assumption that a timescale separation exists between a relatively rapid timescale for slantwise ascent and a slower timescale for the development of the system an approximate extension of parcel theory to three dimensions is derived and it is shown that calculations of scape based on the assumption of relatively rapid slantwise ascent can be qualitatively in error for a case study example of a developing extratropical cyclone scape calculated along trajectories determined without assuming the existence of the timescale separation show large scape values for parcels ascending from the warm sector and along the warm front these parcels ascend into the cloud head within which there is some evidence consistent with the release of csi from observational and model cross sections this region of high scape was not found for calculations along the relatively rapidly ascending trajectories determined by assuming the existence of the timescale separation

comparison of cryoconite organic matter composition from arctic and antarctic glaciers at the molecular level abstract glacier surfaces are reservoirs that contain organic and inorganic debris referred to as cryoconite solar heating of this material results in the formation of water filled depressions that are colonized by a variety of microbes and are hypothesized to play a role in carbon cycling in glacier ecosystems recent studies on cryoconite deposits have focused on their contribution to carbon fluxes to determine whether they are a net source or sink for atmospheric co 2 to better understand carbon cycling in these unique ecosystems the molecular constituents of cryoconite organic matter com require further elucidation com samples from four glaciers were analyzed by targeted extraction of plant and microbial derived biomarkers in conjunction with non targeted nmr experiments to determine the com composition and potential sources several molecular proxies were applied to assess com degradation and microbial activity using samples from greenland the canadian arctic and antarctica com from canadian john evans glacier and greenlandic leverett glacier locations was more chemically heterogeneous than that from the antarctic likely due to inputs from higher plants mosses and sphagnum as suggested by the solvent extractable alkyl lipids and sterols and the detection of lignin and sphagnum derived phenols after cupric oxide chemolysis solid state 13 c nuclear magnetic resonance nmr experiments highlighted the bulk chemical functional groups of com allowing for a general assessment of its degradation stage from the alkyl o alkyl proxy whereas solution state 1 h nmr highlighted both microbial and plant contributions to base soluble extracts from these com samples the dominance of 1 h nmr signals from microbial protein peptides in base soluble extracts of com from antarctica joyce glacier and garwood glacier phospholipid fatty acid plfa biomarker detection and the absence of plant derived biomarkers in both the solvent and cupric oxide extracts suggests that this com is dominated by microbial derived material these results indicate that com carbon composition is dependent on the local glacier environment which may have a profound impact on carbon cycling and sequestration on glacier surfaces

the relation of moist symmetric instability and upper level potential vorticity anomalies to the observed evolution of cloud heads the cloud head phenomenon has been known for some time as a precursor of rapid cyclogenesis based upon satellite imagery two stages of cloud head development are identified in this study that can be related to early and late stages of the associated cyclone weakly convex cloud heads emerge from the baroclinic zone at an early stage in cyclogenesis and strongly convex cloud heads develop in association with the rapid deepening of the cyclone trajectories are used to highlight the role of an upper level potential vorticity pv anomaly in the growth of the strongly convex cloud head and the main flow associated with the growth of the strongly convex cloud head is viewed using isentropic analysis the possible role of moist symmetric instability in the development of both weakly and strongly convex cloud heads is discussed and a new diagnostic for measuring the vertically integrated extent of realizable symmetric vrs instability based on the moist geostrophic pv is introduced royal meteorological society 2002 g j shutt s contribution is crown copyright

temporal variations of cryoconite holes and cryoconite coverage on the ablation ice surface of qaanaaq glacier in northwest greenland cryoconite holes are water filled cylindrical holes formed on ablation ice surfaces and commonly observed on glaciers worldwide temporal changes of cryoconite holes characteristically 5 cm in diameter were monitored with a time lapse interval camera over 15 d during the melting season on qaanaaq glacier in northwest greenland the holes drastically changed their dimensions and synchronously collapsed twice during the study period when the holes collapsed the coverage of cryoconite on the ice surface increased from 1 0 to 3 5 in the field of view of the camera and then decreased again to 0 4 after the holes reformed comparison with meteorological data showed that the collapses occurred in cloudy and rainy or windy weather conditions corresponding to low shortwave solar radiation 68 126 w m 2 40 55 of the incoming flux in contrast holes developed in sunny conditions correspond to high solar radiation 186 278 w m 2 63 88 results suggest that the dimensions of holes drastically changed depending on the weather conditions and that frequent cloudy warm and windy conditions would cause a decay of holes and weathering crust inducing an increase in the cryoconite coverage on the ice consequently darkening the glacier surface

self similarity in glacier surface characteristics catchment wide information on glacier snow cover depth surface albedo and surface roughness is important input data for distributed models of glacier energy balance in this study we investigate the small scale mm to 100 m spatial variability in these properties with a view to better simulating this variability in such models data were collected on midre lovenbreen a 6 km 2 valley glacier in northwest svalbard the spatial variability of all three properties was found to be self similar over the range of scales under investigation snow depth and albedo exhibit a correlation length within which measurements were spatially autocorrelated late winter and summer properties of snow depth differed with smaller depths in summer due to melt and shorter correlation lengths similar correlation lengths for snow depth and surface albedo may suggest that snow depth variation is an important control on the small scale spatial variability of glacier surface albedo for surface roughness the data highlight a possible problem in energy balance studies which use microtopographic surveys to calculate aerodynamic roughness in that the scale of the measurements made affects the calculated roughness value this suggests that further investigations of the relationships between surface form and aerodynamic roughness of glacier surfaces are needed

compositions of pegmatoids beneath the j m reef of the stillwater complex montana u s a abstract pegmatoids sporadically occur in the cumulates of the ultramafic series and norite zone i and gabbronorite zone i the lower banded series below the level of the platiniferous j m reef of the stillwater complex but are less common in norite zone ii and gabbronorite zone ii above the reef the pegmatoids are composed of the same minerals plagioclase orthopyroxene and clinopyroxene as the host cumulates and in approximately the same proportions but with grains that may be in excess of several tens of centimeters in length whole rock compositions show the pegmatoids to be modestly enriched in lithophile incompatible major and trace elements relative to the host cumulates whereas average host cumulates of norite zone i and gabbronorite zone i have a whole rock molar mg mg fe ratio of 81 1 5 and a normative an content of 85 1 5 almost all pegmatoid samples have lower values with mg mg fe ratios as low as 71 and normative an content as low as 58 these results are confirmed by electron microprobe analyses ratios of soluble insoluble lithophile trace elements e g ba zr show inconsistent trends excess sulfide in some pegmatoids strongly affects chalcophile element abundance if it is accepted that the pegmatoids define pods where late stage volatile rich fluids were locally enriched or channel ways by which fluids migrated up through the cumulate sequence then it is clear that such fluids differ from the host assemblages only in that the fluids were in equilibrium with a slightly more evolved bulk rock compositions calculated fluid compositions are relatively rich in fe and na and poor in ca and mg and are consistent with the compositional features of the pegmatoids the lack of significant difference in the ratios of soluble insoluble lithophile trace elements between the pegmatoids and the host cumulates suggests that it could be difficult to detect geochemical signatures for the former presence of igneous fluid in nonpegmatoidal cumulate rocks using these elements alone

exploration geophysics of the bushveld complex in south africa the bushveld complex consists of three roughly coeval components namely the lebowa granite suite the rashoop granophyre and the rustenburrg layered suite rls the latter being the only economically important unit the rls underlies an area of 65 000 km2 in the northeast sector of south africa and is the oldest and largest mafic layered complex in the world figure 1 it is also the largest repository of magmatic ore deposits and hosts inter alia 80 of the world s platinum group metal pgm resources and most of the chromitite resources over an east west distance of roughly 350 km the rls outcrops as three apparently discrete crescent shaped lobes the western eastern and northern limbs and lies below cover rocks along the southeastern limb the far western limb is currently not of economic significance lithologies range across mafic and ultramafic cumulates plus anorthosites pseudo stratigraphic layering exhibits dips of 8 20 toward the center of the complex the ug2 and merensky reefs constitute the major pgm bearing horizons of mining interest and fall close to the top of the 1500 m thick critical zone an assemblage of alternating leuconorite anorthosite and pyroxene rocks outcropping midway through the rls succession

numerical study of slantwise circulations in a strongly sheared prefrontal environment the simultaneous presence of convective symmetric and inertial instability in a prefrontal region of strong vertical wind shear was simulated with the non hydrostatic model moloch model diagnostics of absolute vorticity pseudo angular momentum saturated equivalent potential temperature and vorticity reveal a sequence of events that includes m adjustment followed by slanted ascent in symmetrically unstable regions becoming saturated in the later stages an idealized experiment without orography was performed to isolate the presence and role of instabilities characterizing the development the diagnosed circulation is reminiscent of a wavenumber two normal mode of dry symmetric instability while moist symmetric instability is confined to a very limited region despite the appearance of wider areas of negative moist potential vorticity the evaluation of moist thermodynamic quantities which give proper account of condensate loading is suggested as a possible resolution of this apparent inconsistency copyright 2011 royal meteorological society

seasonal changes in ground penetrating radar signature observed at a polythermal glacier bylot island canada in recent years ground penetrating radar gpr has been increasingly used for characterization of subglacial and englacial environments at polythermal glaciers the geophysical method is able to exploit the dielectric difference between water air sediment and ice allowing delineation of subsurface hydrological thermal and structural conditions more recent gpr research has endeavoured to examine temporal change in glaciers in particular the distribution of the cold ice zone at polythermal glaciers however the exact nature of temporal change that can be identified using gpr has not been fully examined this research presents the results of three gpr surveys conducted over the course of a summer ablation season at a polythermal glacier in the canadian arctic a total of approximately 30 km of gpr profiles were collected in 2002 repeatedly covering the lower 2 km of stagnation glacier bylot island 72 58 n 78 22 w comparison between profiles indicated changes in the radar signature including increased noise appearance and disappearance of englacial reflections and signal attenuation in the latter survey further an area of chaotic returns in up glacier locations which was interpreted to be a wet temperate ice zone showed marked recession over the course of the ablation season combining all the temporal changes that were detected by gpr results indicate that a polythermal glacier may exhibit strongly seasonal changes in hydrological and thermal characteristics throughout the ice body including the drainage of 17 000 m 3 of temporarily stored intra glacial meltwater it is also proposed that the liquid water content in the temperate ice zone of polythermal glaciers can be described as a fraction of a specific retention capacity copyright 2006 john wiley sons ltd

visible and near infrared reflectivity during the ablation period on peyto glacier alberta canada models for calculating glacier mass balance are sensitive to surface reflectivity variation fieldwork carried out on peyto glacier alberta canada contributes to the data set available for ice reflectivity parameterization in such models hemispherical reflectivity in the visible and near infrared parts of the solar spectrum was obtained for rock snow and three contrasting glacier surfaces to examine temporal and spatial variations glacier ice near infrared reflectivity displays only minor spatial variation 0 12 0 17 in comparison with the visible range 0 23 0 40 the latter being influenced primarily by surface impurity content surface roughness is of minor importance compared with impurities temporal variation of reflectivity was weak at all glacier ice and rock locations slight variations observed were due to changes in either solar zenith angle or cloud amount snow reflectivity displayed pronounced diurnal asymmetry and a larger response to cloud cover the minimal temporal variation in glacier ice reflectivity simplifies its parameterization this behaviour is additionally useful for satellite based measurements of the reflectivity field on larger glaciers as images obtained within a 6 h window centred on solar noon are likely to yield values which are within 2 3 of daily mean values

the 190pt 186os decay system applied to dating platinum group element mineralization of the bushveld complex south africa abstract we evaluate the 190pt 186os decay system measured by laser ablation mc icpms as a useful geochronometric tool for direct dating of platinum group minerals pgm in large mafic ultra mafic intrusions by analysing mineralised rocks from the geochronologically well constrained bushveld complex s africa mixed pgm laurite cooperite laurite platarsite sperrylite and pt fe alloy from the merensky reef yield a pt os isochron age of 1995 50 ma 2 n 48 mswd 1 16 initial 186os 188os 0 119819 0 000006 while this is 59 myr younger than the u pb zircon age for the merensky reef scoates and friedman 2008 it is consistent with recent re os ages for the platreef considering the younger age recorded by both re os and pt os systems we propose that the pt os isochron age records a late low temperature hydrothermal event affecting pgm and sulphides in the platreef and merensky reef pt rich phases may also yield single grain 190pt 186os model ages since initial 186os 188os can be well constrained a pt os model age of 2024 ma 101 ma was calculated for a merensky cooperite a pt fe alloy from the onverwacht pipe and a tweefontein hill sperrylite give pt os model ages of 2113 106 ma and 2042 102 ma respectively a pt os isochron derived from all bushveld units gives an age of 2012 47 ma 2 186os 188osi 0 119818 0 000006 with an mswd of 1 19 hence pge mineralisation in different stratigraphic horizons of the complex cannot be distinguished temporally using the pt os chronometer nonetheless the potential for the pt os system in dating events in pge mineralised systems is evident and the agreement of single grain model ages in high pt os phases with the pt os and re os isochron ages indicates their usefulness in the geochronology of these systems

polythermal glacier hydrology a review 1 the manner by which meltwater drains through a glacier is critical to ice dynamics runoff characteristics and water quality however much of the contemporary knowledge relating to glacier hydrology has been based upon and conditioned by understanding gleaned from temperate valley glaciers globally a significant proportion of glaciers and ice sheets exhibit nontemperate thermal regimes the recent growing concern over the future response of polar glaciers and ice sheets to forecasts of a warming climate and lengthening summer melt season necessitates recognition of the hydrological processes in these nontemperate ice masses it is therefore timely to present an accessible review of the scientific progress in glacial hydrology where nontemperate conditions are dominant this review provides an appraisal of the glaciological literature from nontemperate glaciers examining supraglacial englacial and subglacial environments in sequence and their role in hydrological processes within glacierized catchments in particular the variability and complexity in glacier thermal regimes are discussed illustrating how a unified model of drainage architecture is likely to remain elusive due to structural controls on the presence of water cold ice near glacier surfaces may reduce meltwater flux into the glacier interior but observations suggest that the transient thermal layer of near surface ice holds a hydrological role as a depth limited aquifer englacial flowpaths may arise from the deep incision of supraglacial streams or the propagation of hydrofractures forms which are readily able to handle varied meltwater discharge or act as locations for water storage and result in spatially discrete delivery of water to the subglacial environment the influence of such drainage routes on seasonal meltwater release is explored with reference to summer season upwellings and winter icing formation moreover clear analogies emerge between nontemperate valley glacier and ice sheet hydrology the discussion of which indicates how persistent reassessment of our conceptualization of glacier drainage systems is required there is a clear emphasis that continued integrated endeavors focused on process glaciology at nontemperate glaciers are a scientific imperative to augmenting the existing body of research centered on ice mass hydrology

frontogenesis and symmetric stability in a major new england snowstorm abstract synoptic and doppler radar data are used to study the roles of large scale frontogenetical forcing and of moist symmetric instability in the new england snowstorm of 5 6 december 1981 associated with an explosively intensifying cyclone offshore radar reflectivity patterns showed a tendency toward banded structure particularly near the leading northwestern edge of the storm only a minor portion of the snowfall however was associated with this pronounced bandedness from a set of constant pressure analyses the frontogenetical forcing was measured from the variation along the temperature gradient of the geostrophic wind component in the direction of this gradient over southeastern new england maximum forcing found near 500 mb at the outset of the storm descended to the layer between 850 and 700 mb 24 h later magnitudes were 3 7 10 10 deg m 1 s 1 observed rates of strengthening of temperature gradient were less than half this value implying relative adiabatic cooling in the rising

supraglacial weathering crust dynamics inferred from cryoconite hole hydrology water levels in cryoconite holes were monitored at high resolution over a 3 week period on austre broggerbreen svalbard these data were combined with melt and energy balance modelling providing insights into the evolution of the glacier s near surface hydrology and confirming that the hydrology of the near surface porous ice known as the weathering crust is dynamic and analogous to a shallow perched aquifer a positive correlation between radiative forcing of melt and drainage efficiency was found within the weathering crust this likely resulted from diurnal contraction and dilation of interstitial pore spaces driven by variations in radiative and turbulent fluxes in the surface energy balance occasionally causing sudden drainage events a linear decrease in water levels in cryoconite holes was also observed and attributed to cumulative increases in near surface ice porosity over the measurement period the transport of particulate matter and microbes between cryoconite holes through the porous weathering crust is shown to be dependent upon weathering crust hydraulics and particle size cryoconite holes therefore yield an indication of the hydrological dynamics of the weathering crust and provide long term storage loci for cryoconite at the glacier surface this study highlights the importance of the weathering crust as a crucial component of the hydrology ecology and biogeochemistry of the glacier ecosystem and glacierized regions and demonstrates the utility of cryoconite holes as natural piezometers on glacier surfaces

the hydrochemistry of bayelva a high arctic proglacial stream in svalbard major ion silica and ph data from a high arctic proglacial stream network are examined factor analysis shows that the major controls upon hydrochemistry in the basin include snowpack solute elution rapid alteration of minerals via surface reactions and slow incongruent silicate dissolution the importance of chemical weathering is found to increase downstream at the expense of solute acquisition from the transient snowpack data from a series of snow pits show that snowpack solute elution is characterised by the preferential elution of so42 evidence for the preferential elution of no3 is equivocal however no3 levels in proglacial runoff may be enhanced by the leaching of cryotic soils later in the ablation season chemical weathering is dominated by calcite dissolution at high elevations and a combination of calcite and dolomite dissolution in the downstream proglacial environment silicate weathering at high elevations is characterised by the enrichment of k relative to si concentrated active layer soil waters show less enrichment of k and constitute a potentially important source of dissolved silica to the river system na is also produced by silicate weathering but its mobility is believed to be largely governed by ion exchange with ca2 early in the ablation season all chemical weathering environments are characterised by high p co2 due to either respiration within the soil or at higher elevations within the catchment h provision from acid aerosol and sulphide oxidation however rates of sulphide oxidation are limited because the cold based thermal regime of the glaciers effectively prevents access of meltwaters to freshly comminuted sub glacial sulphide minerals the restricted sub glacial weathering therefore means that ice marginal and proglacial environments are the most important zones of solute acquisition by meltwaters causing significant enrichment of major ions silica and dissolved co2 within only a short distance of the contemporary ice margin

water flow through temperate glaciers understanding water movement through a glacier is fundamental to several critical issues in glaci ology including glacier dynamics glacier induced floods and the prediction of runoff from glacierized drainage basins to this end we have synthesized a conceptual model of water movement through a temper ate glacier from the surface to the outlet stream pro cesses that regulate the rate and distribution of water input at the glacier surface and that regulate water movement from the surface to the bed play important but commonly neglected roles in glacier hydrology where a glacier is covered by a layer of porous perme able firn the accumulation zone the flux of water to the glacier interior varies slowly because the firn tempo rarily stores water and thereby smooths out variations in the supply rate in the firn free ablation zone in con trast the flux of water into the glacier depends directly on the rate of surface melt or rainfall and therefore varies greatly in time water moves from the surface to the bed through an upward branching arborescent net work consisting of both steeply inclined conduits formed by the enlargement of intergranular veins and gently inclined conduits spawned by water flow along the bottoms of near surface fractures crevasses engla cial drainage conduits deliver water to the glacier bed at a limited number of points probably a long distance downglacier of where water enters the glacier englacial conduits supplied from the accumulation zone are quasi steady state features that convey the slowly varying water flux delivered via the firn their size adjusts so that they are usually full of water and flow is pressurized in contrast water flow in englacial conduits supplied from the ablation area is pressurized only near times of peak daily flow or during rainstorms flow is otherwise in an open channel configuration the subglacial drainage system typically consists of several elements that are distinct both morphologically and hydrologically an up glacier branching arborescent network of channels in cised into the basal ice conveys water rapidly much of the water flux to the bed probably enters directly into the arborescent channel network which covers only a small fraction of the glacier bed more extensive spatially is a nonarborescent network which commonly includes cav ities gaps between the glacier sole and bed channels incised into the bed and a layer of permeable sediment the nonarborescent network conveys water slowly and is usually poorly connected to the arborescent system the arborescent channel network largely collapses during winter but reforms in the spring as the first flush of meltwater to the bed destabilizes the cavities within the nonarborescent network the volume of water stored by a glacier varies diurnally and seasonally small temper ate alpine glaciers seem to attain a maximum seasonal water storage of 200 mm of water averaged over the area of the glacier bed with daily fluctuations of as much as 20 30 mm the likely storage capacity of subglacial cavities is insufficient to account for estimated stored water volumes so most water storage may actually occur englacially stored water may also be released abruptly and catastrophically in the form of outburst floods

frontogenesis in the presence of small stability to slantwise convection abstract it is often observed that despite the existence of near neutrality to slantwise convection rainbands and snowbands can persist for long periods with narrow intense updrafts producing large quantities of precipitation in many cases this is probably due to the presence of active frontogenesis which as shown here maintains such structures in order to investigate this process we use two dimensional semigeostrophic theory to solve the deformation forced frontogenesis problem for a circumstance in which the stability to moist slantwise convection is small but positive in this case numerical simulation is necessary to determine the evolution of the front several previous studies have described the role of diabatic forcing in modifying the cross frontal circulation by use of the sawyer eliassen equation in particular analytic solutions of that equation for the case of small moist symmetric stability show that a narrow updraft should occur ahead of the maximum geostrophic frontogenesis numeri

biogeochemical evolution of cryoconite holes on canada glacier taylor valley antarctica 1 the cryoconite holes of the mcmurdo dry valleys are simple closed biogeochemical systems involving water ice mineral and organic debris which serve as ecosystems for consortia of microorganisms this study is the first to document the seasonal and annual chemical evolution of solutes in cryoconite holes samples of glacier ice frozen cryoconite holes and those containing water were collected during the austral summer of 2005 2006 the isolation age was calculated from the excess cl in the holes and varied from 0 to 5 years a consistent with the last hot summer when the cryoconite holes were open to the atmosphere the holes progressively deepen with isolation age variations in dic doc k and so42 suggest that dissolution of primary minerals in the cryoconite debris cyclical precipitation and dissolution of secondary carbonates net photosynthesis over summer and net respiration during the autumnal freeze are the principal reactions which perturb the seasonal and annual solute concentrations in the holes doc is generated in the holes at a rate of 7 5 g c cm 2 a 1 and non sea salt k accumulates in frozen holes at a rate of 0 073 eq cm 2 a 1 we infer that c cycling is complex even in these otherwise simple systems

discussion of the bushveld complex south africa formation of platinum palladium chrome and vanadium rich layers via hydrodynamic sorting of a mobilized cumulate slurry in a large relatively slowly cooling subsiding magma chamber by maier et al 2013 miner deposita 48 1 56 we believe the hypothesis presented by maier et al miner deposita 48 1 56 2012 for the formation of the various ore bodies in the bushveld complex to be overly simplistic and we raise concerns that some of our work used in support of this hypothesis has been misrepresented the formation of both diverse metalliferous layers platinum group element pge reefs and ti magnetite layers and some discordant pipe ore deposits has been ascribed by maier et al to the single unifying process of hydrodynamic sorting the problem faced by authors of universal hypotheses for the bushveld complex is the sheer size and complexity of the intrusion we disagree with many aspects of the overall maier et al model and have also identified several minor errors on maps and photographs although some of these do not have a material effect on the model the nature and origin of the layering is however too complex a topic to deal with in the context of this commentary and we restrict ourselves to noting that our preferred hypothesis namely the incremental buildup of layering from numerous episodes of replenishment by different magma lineages is consistent with field relationships our hypothesis for the origin of the ultramafic hosted pge rich reefs i e lateral mixing is applicable to economically mineralized reefs mitchell and scoon econ geol 102 971 1009 2007 and poorly mineralized layers such as the pseudoreef harzburgite scoon and de klerk canad mineral 25 51 77 1987 and the chromitite layers below the ug2 scoon and teigler econ geol 89 1094 1121 1994

character of the ug2 chromitite and host rocks and petrogenesis of its pegmatoidal footwall northeastern bushveld complex the ug2 chromitite and its immediate footwall have been mapped at the centimeter scale in a characteristic section exposed in a wall in the middelpunt mine northeast bushveld complex in this region the ug2 chromitite and its coarse grained footwall rocks are underlain and overlain by plagioclase pyroxenite the ug2 chromitite itself is a massive 70 cm thick layer composed of 75 to 90 percent chromite with plagioclase as the dominant interstitial mineral immediately beneath the chromitite is the so called mixed layer a previously undescribed but distinct mappable layer 50 to 70 cm thick consisting of chromite bearing pegmatoidal pyroxenite and chromitite intimately mixed at scales of centimeters to decimeters the mixture is chaotic with no megascopic fabric and contains in bulk about 15 percent chromite the contact between the mixed layer and overlying chromitite is well defined but sinuous at scales of centimeters to decimeters based on observations of drill core and outcrop the mixed layer appears to exist throughout the central sector of the eastern bushveld and may be ubiquitous throughout the entire complex the mixed layer usually rests directly on the ug2 pegmatoid which is a well known and widespread pegmatoidal plagioclase pyroxenite the pegmatoid is much coarser grained than the underlying pyroxenite but the two lithologic units are similar in terms of mineral modes and compositions the contact between them is irregular at scales of decimeters to meters with vertical fingers of pegmatoid extending into the pyroxenite the mixed layer and ug2 pegmatoid contain rare lenses of massive anorthosite encased by continuous centimeter thick rims of chromitite a model is proposed to account for the footwall sequence of the ug2 chromitite in this model the ug2 pegmatoid formed when hydrous interstitial melt percolated up through the partially molten crystal pile accumulated beneath the preexisting compacted massive chromitite and metasomatized and partially melted the pyroxenite protolith which upon cooling recrystallized to a coarse grained rock on the basis of published experiments illustrating the effect of water on melting in mafic systems it is shown how the hydrous melt was able to penetrate the pyroxene bearing assemblages below the chromitite but unable to partially melt the chromitite itself which in consequence remained relatively impermeable the anorthosite lenses in the pegmatoid mark local areas of relatively high degrees of hydration partial melting with the anorthosite representing the crystalline residue the mixed layer formed by gradual collapse of the base of the ug2 chromitite into the less dense relatively fluid footwall the mixed layer and ug2 pegmatoid are diagnostic of the ug2 chromitite and the three lithologic units should be considered a single petrologic unit

a blue ice ecosystem on the margins of the east antarctic ice sheet freezing temperatures desiccation and high levels of solar radiation make the surface of the antarctic ice sheet one of earth s harshest habitats however our study in the vestfold hills area of east antarctica shows that favourable conditions for microbial production become established just beneath the surface of blue ice areas which collectively cover about 2 of the ice sheet periphery their translucent wind polished surface allows solar heating to create meltwater in a greenhouse type environment at depths of up to 1m melting is intensified around dark debris particles or cryoconite where we found microbiological activity to be greatest rates of photosynthesis average 2060 ngc g cryoconite 1 d 1 were adapted to low light intensities 10 of surface irradiance values and most likely dominated by cyanobacteria and chloroplastida a heterotrophic bacterial community was also found to be active within the cryoconite although average bacterial growth rates 5 7 ngc g cryoconite 1 d 1 were far lower than average community respiration 1870 ng c g cryoconite 1 d 1 the majority of the respired carbon was most likely associated with the autotrophs and several protists therefore blue ice areas constitute oases for microbial life around the periphery of earth s coldest ice sheet

delays of supraglacial runoff from differently defined microbasin areas on the peyto glacier microbasin runoff experiments were conducted on ice surfaces of the peyto glacier during the 1971 and 2008 ablation seasons when half hourly measurements of net radiation wind speed temperature vapour pressure and supraglacial runoff were obtained on warm sunny days with daily mean temperatures 6 1 10 1 c and daily total ablation amounts of 43 95 mm net radiation contributed almost 70 of the melt energy in 1971 and nearly 60 in 2008 with the remainder supplied by turbulent transfers of sensible and latent heat thus the two experiments are similar in finding that radiation dominates surface melt energy inputs to microbasin runoff outputs they differ with respect to area definitions and microbasin shapes 1 a purposely delineated rhomboidal area 30 m long 15 m wide covering 386 m2 in 1971 2 a naturally delineated elongated area approximately 600 m long 2 m wide covering an estimated 1200 m2 in 2008 three 24 hour periods beginning at 0700 h were obtained for experimental work in 1971 and 8 such periods in 2008 supraglacial runoff was modelled by passing melt energy water equivalents through a simple storage reservoir and forcing closure with measured runoff by optimising for delay time optimal times of 1 2 h were found for 1971 and 7 5 11 5 h for 2008 thus indicating a major difference between the runoff responses of the two microbasins it is possible that different basin shapes may explain the contrasting delay times perhaps because different shapes signify different flow connections to meltwater storage in the weathering crust if the elongated microbasin response is typical of the elemental hydrological response unit of the melting ice surface distributed glacier runoff models should incorporate it as part of the model structure copyright 2011 john wiley sons ltd

baroclinic instability in an environment of small stability to slantwise moist convection part i two dimensional models abstract in the semigeostrophic system the growth rate of baroclinic waves varies with the inverse square root of the potential vorticity which acts as the effective static stability recent observations in the ascent regions of middle latitude cyclones show that the effective potential vorticity for saturated air is very near zero in this paper we examine the structure and rate of growth of baroclinic cyclones when the effective potential vorticity is small for upward saturated displacements but large in regions of descent analytic solutions for two dimensional disturbances in a two layer semigeostrophic model and numerical simulations using a multilevel semigeostrophic model show that when the effective potential vorticity is small in regions of upward motion growth rates are modestly increased and the region of ascent intensifies and collapses onto a thin ascending sheet in the limit of zero moist potential vorticity the fastest growing wave has a finite growth rate which is about 2 5 times the

reply to discussion by r n scoon and a a mitchell on oxygen isotope and trace element compositions of platiniferous dunite pipes of the bushveld complex south africa signals from a recycled mantle component by t gunther k m haase m junge t oberthur d woelki s krumm