basal melting of ross ice shelf from solar heat absorption in an ice front polynya ice ocean interactions at the bases of antarctic ice shelves are rarely observed yet have a profound influence on ice sheet evolution and stability ice sheet models are highly sensitive to assumed ice shelf basal melt rates however there are few direct observations of basal melting or the oceanographic processes that drive it and consequently our understanding of these interactions remains limited here we use in situ observations from the ross ice shelf to examine the oceanographic processes that drive basal ablation of the world s largest ice shelf we show that basal melt rates beneath a thin and structurally important part of the shelf are an order of magnitude higher than the shelf wide average this melting is strongly influenced by a seasonal inflow of solar heated surface water from the adjacent ross sea polynya that downwells into the ice shelf cavity nearly tripling basal melt rates during summer melting driven by this frequently overlooked process is expected to increase with predicted surface warming we infer that solar heat absorbed in ice front polynyas can make an important contribution to the present day mass balance of ice shelves and potentially impact their future stability high melt rates in a key location beneath the ross ice shelf result from a seasonal inflow of water heated in the ross sea polynya according to in situ observations

active water exchange and life near the grounding line of an antarctic outlet glacier abstract the grounding line gl of the antarctic ice sheet forms the boundary between grounded and floating ice along the coast near this line warm oceanic water contacts the ice shelf producing the ice sheet s highest basal melt rate despite the importance of this region water properties and circulations near the gl are largely unexplored because in situ observations are difficult here we present direct evidence of warm ocean water transport to the innermost part of the subshelf cavity several hundred meters seaward from the gl of langhovde glacier an outlet glacier in east antarctica our measurements come from boreholes drilled through the glacier s 400 m thick grounding zone beneath the grounding zone we find a 10 24 m deep water layer of uniform temperature and salinity 1 45 c 34 25 psu values that roughly equal those measured in the ocean in front of the glacier moreover living organisms are found in the thin subglacial water layer these findings indicate active transport of water and nutrients from the adjacent ocean meaning that the subshelf environment interacts directly and rapidly with the ocean

responses of basal melting of antarctic ice shelves to the climatic forcing of the last glacial maximum and co2 doubling abstractbasal melting of the antarctic ice shelves is an important factor in determining the stability of the antarctic ice sheet this study used the climatic outputs of an atmosphere ocean general circulation model to force a circumpolar ocean model that resolves ice shelf cavity circulation to investigate the response of antarctic ice shelf melting to different climatic conditions i e to a doubling of co2 and to the last glacial maximum conditions sensitivity experiments were also conducted to investigate the roles of both surface atmospheric change and changes of oceanic lateral boundary conditions it was found that the rate of change of basal melt due to climate warming is much greater by an order of magnitude than that due to cooling this is mainly because the intrusion of warm water onto the continental shelves linked to sea ice production and climate change is crucial in determining the basal melt rate of many ice shelves sensitivity experiments showed that changes of atmospheric heat

transport related mylonitic ductile deformation and shape change of alluvial gold southern new zealand abstract gold is a malleable metal and detrital gold particles deform via internal distortion the shapes of gold particles are commonly used to estimate transport distances from sources but the mechanisms of internal gold deformation leading to shape changes are poorly understood because of subsequent recrystallisation of the gold in situ in placer deposits which creates a rim zone around the particles with undeformed 10 m grains this paper describes samples from southern new zealand in which grain size reduction to submicrometer scale and mylonitic textures have resulted from internal ductile deformation these textures have been preserved without subsequent recrystallisation after deposition in late pleistocene holocene alluvial fan placers these mylonitic textures were imposed by transport related deformation on recrystallised rims that were derived from previous stages of fluvial transportation and deposition this latest stage of fluvial transport and deformation has produced numerous elongated gold smears that are typically 100 m long and 10 20 m wide these smears are the principal agents for transport induced changes in particle shape in the studied placers focused ion beam fib sectioning through these deformed zones combined with scanning electron microscopic sem imaging show that the interior of the gold particles has undergone grain size reduction to 500 nm and extensive folding with development of a ductile deformation fabric that resembles textures typical of mylonites in silicate rocks relict pods of the pre existing recrystallised rim zone are floating in this ductile deformation zone and these pods are irregular in shape and discontinuous in three dimensions micrometer scale biologically mediated deposition from groundwater of overgrowth gold on particle surfaces occurs at all stages of placer formation and some of this overgrowth gold has been incorporated into deformation zones these examples provide a rare view into the nature of the physical processes that accommodate gold particle shape change during sedimentary transport

on the role of coastal troughs in the circulation of warm circumpolar deep water on antarctic shelves abstractoceanic exchanges across the continental shelves of antarctica play an important role in biological systems and the mass balance of ice sheets the focus of this study is on the mechanisms responsible for the circulation of warm circumpolar deep water cdw within troughs running perpendicular to the continental shelf this is examined using process oriented numerical experiments with an eddy resolving 1 km 3d ocean model that includes a static and thermodynamically active ice shelf three mechanisms that create a significant onshore flow within the trough are identified 1 a deep onshore flow driven by the melt of the ice shelf 2 interaction between the longshore mean flow and the trough and 3 interaction between a rossby wave along the shelf break and the trough in each case the onshore flow is sufficient to maintain the warm temperatures underneath the ice shelf and basal melt rates of o 1 m yr 1 the third mechanism in particular reproduces several features revealed by moorings from m

the response of ice shelf basal melting to variations in ocean temperature abstract a three dimensional ocean general circulation model is used to study the response of idealized ice shelves to a series of ocean warming scenarios the model predicts that the total ice shelf basal melt increases quadratically as the ocean offshore of the ice front warms this occurs because the melt rate is proportional to the product of ocean flow speed and temperature in the mixed layer directly beneath the ice shelf both of which are found to increase linearly with ocean warming the behavior of this complex primitive equation model can be described surprisingly well with recourse to an idealized reduced system of equations and it is shown that this system supports a melt rate response to warming that is generally quadratic in nature this study confirms and unifies several previous examinations of the relation between melt rate and ocean temperature but disagrees with other results particularly the claim that a single melt rate sensitivity to warming is universally valid the hypothesized

antarctic thermocline dynamics along a narrow shelf with easterly winds abstractdetermining the role of southern ocean warm intermediate water for driving melting of the antarctic ice sheet is a major challenge in assessing future sea level rise analysis of 2859 ctd p

a model of ice shelf ocean interaction with application to the filcher ronne and ross ice shelves a simple analytical model has been developed to study the formation of ice shelf water isw isw is assumed to flow as a buoyancy driven layer underneath the ice shelf a relation between potential temperature and salinity in the isw layer is calculated from the mass and energy balance this temperature salinity relation is shown to depend only on the temperature and the salinity of the source water mass and to be practically independent of entrainment and melt rates the model results are obtained without making any assumptions about entrainment and melt rates the model is in good agreement with observations under the ronne ice shelf and it indicates that isw in the filchner depression is formed from western shelf water wsw with salinity higher than 34 75 practical salinity units such high salinity water is only observed in the ronne depression in the western part of the continental shelf this implies a circulation of wsw under the filchner ronne ice shelf from the ronne depression into the filchner depression similarly the model shows that the isw observed under j9 at the ross ice shelf has been formed from low salinity shelf water lssw from the eastern parts of the ross sea continental shelf lssw must therefore circulate under the eastern parts of the ross ice shelf

delayed accumulation of placers during exhumation of orogenic gold in southern new zealand abstract the giant gold placer system on the otago schist of southern new zealand was derived from mesozoic orogenic gold deposits in the underlying schist basement the core of the schist basement was exhumed in the middle cretaceous coeval with the accumulation of the oldest preserved nonmarine sedimentary rocks in the area ca 112 ma those sedimentary rocks contain quartz clasts with distinctive ductile deformation textures that were derived from structural zones in or adjacent to major orogenic gold deposits quartz textures in these structural zones are readily distinguishable from the rest of the schist belt and hence provide a fingerprint for erosion of gold the earliest sedimentary rocks on the margins of the gold bearing schist belt are immature and were derived from unoxidised outcrops in areas of high relief gold was not liberated from unoxidised basement rocks during erosion and was removed from the system without placer concentration placer concentration did not begin until about 20 million years later when oxidative alteration of gold deposits had facilitated gold grain size enhancement from micron scale primary to millimetre scale secondary subsequent erosion and recycling of gold in the early cenozoic and again in the late cenozoic caused additional concentration of gold in progressively younger deposits the klondike giant placer goldfield of canada had a similar geological history to the otago placer field and klondike placer accumulation occurred in the late cenozoic at least 70 million years after mesozoic exhumation of orogenic gold the giant placer deposit on the western slopes of the sierra nevada in california occurs in eocene and younger sedimentary rocks at least 40 million years younger than the timing of major exhumation of the source rocks circum pacific giant gold placers formed under entirely different tectonic regimes from the emplacement of their source orogenic deposits and these giant placer deposits do not form in foreland basins associated with convergent orogens formation of giant placers requires less active erosion and more subdued topography than the collisional orogenic activity that accompanied emplacement of source gold deposits in basement rocks as well as oxidative alteration of the primary deposits to liberate gold from sulfide minerals and enhance secondary gold grain size

ice ocean processes over the continental shelf of the southern weddell sea antarctica a review 1 interactions between the southern ocean and the weddell sea ice shelves are important both to the antarctic ice sheet and to the production of globally significant water masses here we review the interaction between the filchner ronne ice shelf and the shelf sea in which it floats the continental shelf processes leading to the production of weddell sea deep and bottom waters from the original off shelf source waters are discussed and a new view is offered of the initial production of high salinity shelf water data from ship based measurements at the ice front from glaciological methods and from measurements made within the sub ice shelf cavity itself are used to describe the pattern of flows beneath the ice shelf we also consider the variability observed within the cavity from tidal to interannual time scales and finish with a discussion of future research priorities in the region

instruments and methods using distributed temperature sensors to monitor an antarctic ice shelf and sub ice shelf cavity monitoring of ice shelf and sub ice shelf ocean temperatures represents an important component in understanding ice sheet stability continuous monitoring is challenging due to difficult surface access difficulties in penetrating the ice shelf and the need for long term operation of non recoverable sensors we aim to develop rapid lightweight drilling and near continuous fiber optic temperature monitoring methods to meet these challenges during november 2011 two instrumented moorings were installed within and below the mcmurdo ice shelf a sub region of the ross ice shelf antarctica at windless bight we used a combination of ice coring for the upper portion of each shelf borehole and hot point drilling for penetration into the ocean the boreholes provided temporary access to the ice shelf cavity into which distributed temperature sensing dts fiber optic cables and conventional pressure temperature transducers were installed the dts moorings provided near continuous in time and depth observations of ice and ocean temperatures to a depth of almost 800 m beneath the ice shelf surface data received document the presence of near freezing water throughout the cavity from november through january followed by an influx of warmer water reaching 150 m beneath the ice shelf base during february and march the observations demonstrate prospects for achieving much higher spatial sampling of temperature than more conventional oceanographic moorings

the structural evolution of active fault and fold systems in central otago new zealand evidence revealed by drainage patterns abstract central otago in new zealand is an area of active continental shortening in which a peneplain surface cut into schist has been deformed by folds which are developed above buried reverse faults we use the drainage patterns in this region to demonstrate various processes in fold and fault growth and interaction that would be difficult to identify by other means in particular we show 1 how simple asymmetric folds can develop into box folds 2 how apparently continuous ridges were formed by the coalescing of quite separate propagating fold and fault segments 3 evidence for the relative ages or relative uplift rates of adjacent structures and 4 evidence for the propagation direction of folds or faults as they grow the few quantitative estimates we obtain for fault propagation rates suggest an increase in length of 10 50 m per earthquake on faults about 20 km long these estimates are very uncertain but are similar in magnitude to an estimate made in nevada for a normal fault of similar size and are also similar to predicted estimates from theoretical growth models they raise the question of whether fault growth earthquake recurrence rates and climate change can interact to produce semi regular discrete features in an active landscape

recent antarctic ice mass loss from radar interferometry and regional climate modelling observed estimates of ice losses in antarctica combined with regional modelling of ice accumulation in the interior suggest that east antarctica is close to a balanced mass budget but large losses of ice occur in the narrow outlet channels of west antarctic glaciers and at the northern tip of the antarctic peninsula

detailed seismic bathymetry beneath ekstrom ice shelf antarctica implications for glacial history and ice ocean interaction the shape of ice shelf cavities are a major source of uncertainty in understanding ice ocean interactions this limits assessments of the response of the antarctic ice sheets to climate change her

the in vitro growth of gold grains gold colloids octahedral platelets and foils directly and indirectly formed from the reduction of soluble au i thiosulfate and au iii chloride complexes by iron oxidizing bacteria cyanobacteria and sulfate reducing bacteria were combined in an experimental system this system represented simplified biogeochemical conditions occurring within a fluvial environment in which placer au could occur in this study biofilm formation and physical aggregation i e sedimentation processes were critical for the accumulation of nanometer to micrometer sized au particles into grains 4 5 mm in size characterization of grain surface textures by scanning electron microscopy in association with monitoring soluble au concentrations over time suggested that dissolution and reprecipitation processes were occurring at the au grain fluid interface this laboratory model demonstrates that the biogeochemical cycling of au can contribute to the formation of anomalous enrichments such as placer au deposits

modeling ocean processes below fimbulisen antarctica 1 model simulations of circulation and melting beneath fimbulisen antarctica obtained using an isopycnic coordinate ocean model are presented model results compare well with available observations of currents and hydrography in the open ocean to the north of fimbulisen and suggest that warm deep water exists above the level of a sub ice shelf bedrock sill the principal pathway for warm waters to enter the sub ice shelf cavity the model shows a southward inflow of warm deep water over this sill and into the cavity producing a mean cavity temperature close to 1 0 c this leads to high levels of basal melting 10 m a at the grounding line of jutulstraumen and an average melting over the ice shelf base close to 1 9 m a the southward inflow is a compensating flow caused by the northward outflow of fresh cold water produced by the basal melting results on inflow and melting are difficult to validate since no in situ measurements yet exist in the cavity if such high melt rates are realistic the mass balance of fimbulisen must be significantly negative and the ice shelves along dronning maud land must contribute about 4 4 msv of melt water to the weddell sea about 15 of the total antarctic meltwater input to the southern ocean

novel monitoring of antarctic ice shelf basal melting using a fiber optic distributed temperature sensing mooring measuring basal melting of ice shelves is challenging and represents a critical component toward understanding ocean ice interactions and climate change in november 2011 moorings containing fiber optic cables for distributed temperature sensing dts were installed through the mcmurdo ice shelf antarctica 200 m and extending 600 m into the ice shelf cavity the high spatial resolution of dts allows for transient monitoring of the thermal gradient within the ice shelf the gradient near the ice ocean interface is extrapolated to the in situ freezing temperature in order to continuously track the ice ocean interface seasonal melt rates are calculated to be 1 0 mm d 1 and 8 6 mm d 1 and maximum melting corresponds to the arrival of seasonal warm surface water in the ice shelf cavity between january and april the development of continuous surface based techniques for measuring basal melting represents a significant advance in monitoring ice shelf stability and ice ocean interactions

tidal modulation of buoyant flow and basal melt beneath petermann gletscher ice shelf greenland a set of collocated in situ oceanographic and glaciological measurements from petermann gletscher ice shelf greenland provides insights into the dynamics of under ice flow driving basal melting at a site 16 km seaward of the grounding line within a longitudinal basal channel two conductivity temperature ct sensors beneath the ice base and a phase sensitive radar on the ice surface were used to monitor the coupled ice shelf ocean system a 6 month time series spanning 23 august 2015 to 12 february 2016 exhibited two distinct periods of ice ocean interactions between august and december radar derived basal melt rates featured fortnightly peaks of 15 m yr 1 which preceded the arrival of cold and fresh pulses in the ocean that had high concentrations of subglacial runoff and glacial meltwater estimated current speeds reached 0 20 0 40 m s 1 during these pulses consistent with a strengthened meltwater plume from freshwater enrichment such signals did not occur between december and february when ice ocean interactions instead varied at principal diurnal and semidiurnal tidal frequencies and lower melt rates and current speeds prevailed a combination of estimated current speeds and meltwater concentrations from the two ct sensors yields estimates of subglacial runoff and glacial meltwater volume fluxes that vary between 10 and 80 m3 s 1 during the ocean pulses area average upstream ice shelf melt rates from these fluxes are up to 170 m yr 1 revealing that these strengthened plumes had already driven their most intense melting before arriving at the study site

eddy overturning of the antarctic slope front controls glacial melting in the eastern weddell sea 1 the eastern weddell sea is characterized by narrow continental shelves and warm deep water wdw is located in close proximity to the ice shelves in this region the exchange of wdw across the antarctic slope front asf determines the rate of basal ice shelf melting here we present a unique data set consisting of 2351 vertical profiles of temperature and salinity collected by southern elephant seals mirounga leonina and a profile beneath the fimbul ice shelf fis obtained via drilling through 395 m of ice this data set reveals variations in salinity and temperature through winter and using a conceptual model of the coastal salt budget we quantify the main exchange processes our data show that modified wdw with temperatures below 1 5 c is advected onto the shelf and into the ice shelf cavities by an eddy overturning of the asf the onshore ekman flux of surface waters during summer is the main source of freshwater that leads to the formation of low salinity shelf waters in the region the modified wdw that reaches beneath the ice shelves is too cold for basal ice shelf melting to create such low salinity water a high resolution model of an idealized asf continental shelf ice shelf system supports the conclusions from the data analysis the inflow of wdw onto the continental shelf and into the ice shelf cavity occurs within a bottom boundary layer where the eddy advection in the model is particularly strong in close agreement with the observed vertical profile of temperature beneath the fis citation nost o a m biuw v tverberg c lydersen t hattermann q zhou l h smedsrud and k m kovacs 2011 eddy overturning of the antarctic slope front controls glacial melting in the eastern weddell sea j geophys res 116 c11014 doi 10 1029 2011jc006965

impact of antarctic ice shelf basal melting on sea ice and deep ocean properties 1 an approximation of antarctica s rocky and icy coastline normally forms the southern boundary in global climate models such a configuration neglects extensive ice shelf areas where ocean ice interaction initiates a net freshwater flux to the circumpolar continental shelf equal to 75 of the annual mean net precipitation in coastal seas the results of a numerical model for the southern ocean using two contrasting configurations with and without caverns beneath major antarctic ice shelves are compared they show that the freshwater flux due to deep basal melting significantly stabilizes the shelf water column in front of an ice shelf as well as downstream due to advection by the coastal current if the freshwater from the caverns is absent sea ice is thinner shelf waters are warmer and saltier and the southern ocean deep basins are flushed by denser waters

measurements beneath an antarctic ice shelf using an autonomous underwater vehicle the cavities beneath antarctic ice shelves are among the least studied regions of the world ocean yet they are sites of globally important water mass transformations here we report results from a mission beneath fimbul ice shelf of an autonomous underwater vehicle the data reveal a spatially complex oceanographic environment an ice base with widely varying roughness and a cavity periodically exposed to water with a temperature significantly above the surface freezing point the results of this the briefest of glimpses of conditions in this extraordinary environment are already reforming our view of the topographic and oceanographic conditions beneath ice shelves holding out great promises for future missions from similar platforms

on the fate of the antarctic slope front and the origin of the weddell front 1 data from the deep ocean ventilation through antarctic intermediate layers dovetail and antarctic large scale box analysis and the role of the scotia sea albatross projects are combined in the region of the antarctic peninsula and the south scotia ridge to determine the paths and transports associated with the antarctic coastal current the antarctic slope front and the weddell front the antarctic coastal current flows over the antarctic continental shelf and is not tied to a particular isobath it transports 1 sv westward to the bransfield strait its subsequent course is uncertain but we speculate that it may continue southwest along the western side of the antarctic peninsula close to the coast the antarctic slope front present almost all the way around antarctica tied to the continental slope has not previously been mapped after it reaches the tip of the antarctic peninsula we show that the frontal jet splits into two branches in the northwest powell basin where the isobaths diverge the shoreward portion of the water associated with the front lying above isobaths shallower than 1500 m is able to cross the south scotia ridge and head northwestward and retains many of the properties of the antarctic slope front it is associated with a transport of 7 sv the portion of the front constrained to lie above isobaths deeper than 1500 m becomes the weddell front this takes a path around the northern powell basin and south of the south orkney islands of the 13 sv that circulates around the southern flank of the south orkney plateau 8 sv of weddell sea deep water and warm deep water leaves the front to continue west the remaining shallow and intermediate water retroflects and returns south associated with the weddell front through the orkney passage approximately 5 7 sv is transported eastward associated with the weddell front tied closely to the southern flanks of the bruce and discovery banks above the 2000 2500 m isobaths and forming the southern boundary of the weddell scotia confluence

measurements of ice thickness and seabed topography under the fimbul ice shelf dronning maud land antarctica 1 this paper presents results from seismic measurements of the ice and water column thickness of the fimbul ice shelf in the northeastern weddell sea seismic reflection measurements were conducted at 183 stations covering most of the ice shelf seismic velocities in the ice were derived from refraction measurements at 12 stations distributed evenly across the area as well as from temperature and density data from the fimbul ice shelf velocities in the water were derived from temperature and salinity data from beneath the fimbul ice shelf ice thicknesses were found to vary between 160 m and 550 m with uncertainties up to 10 m water column thicknesses up to 900 m were found within the central ice shelf cavity and values exceed 2000 m where the ice shelf overhangs the continental slope uncertainties in water column thickness are estimated to be 60 m and are dominated by the uncertainties in the shape of the seabed ice draft and seabed elevation was derived from ice and water column thickness assuming hydrostatic pressure the resulting map of seabed elevation and water column thickness suggests that the strong westward flowing coastal current will be steered under the ice shelf and thus drive a sub ice shelf flow warm deep water does not have direct access to the ice shelf cavity while relatively cold coastal waters shallower than 500 m will interact closely with the fimbul ice shelf

intrusion of warm surface water beneath the mcmurdo ice shelf antarctica 1 a 6 month temperature record collected below mcmurdo ice shelf in 2011 2012 shows the temporal and spatial structure of the summertime warm water signal that penetrates beneath the ice shelf the strength and duration of the warm water intrusion suggest an annual melt rate at windless bight of 0 71 m yr a ross sea numerical model demonstrates a seasonal warm water pathway leading from the west side of the ross sea polynya rsp toward mcmurdo sound the warm water enters mcmurdo sound subducts beneath the ice shelf and causes accelerated summer melting temperature data were recorded using distributed temperature sensing fiber optics which gives a vertical temperature profile at a 1 m vertical resolution this study constitutes one of the first successful implementations of this technology in polar regions

topographic development of the southern alps recorded by the isotopic composition of authigenic clay minerals south island new zealand abstract the southern alps are developing as a consequence of oblique collision between the pacific and australian plates the southern alps lie on the west side of the south island of new zealand and create a massive rain shadow where greater than 12 m year of rain falls on the west coast and semiarid conditions exist to the east the rain out effect across the mountains causes precipitation west of the southern alps to have d and 18 o values averaging 30 and 5 5 whereas precipitation in the rain shadow to the east is isotopically lighter d 72 and 18 o 9 8 such large differences in the isotopic composition of precipitation would not have existed prior to the development of significant topography we have examined the topographic evolution of the southern alps using oxygen isotope analyses of authigenic kaolinites formed in the rain shadow to the east of the mountains between the cretaceous low topography and the pleistocene changes in the isotopic composition of authigenic clay minerals forming in equilibrium with meteoric water in the stratigraphic sequence record the development of southern alps topography and the resultant rain shadow our oxygen isotope analyses of authigenic kaolinites show a 5 6 decrease in the early pliocene from 18 2 in older rocks to 12 3 in younger rocks in addition smectite is abundant in all samples from the late miocene to recent but is conspicuously absent in most older rocks suggesting a change to a generally drier climate roughly coincident with the isotopic shift in kaolinites this method may be useful in unraveling timing of development of mountain belts elsewhere in the world

melting of ice shelves and the mass balance of antarctica we calculate the present ice budget for antarctica from measurements of accumulation minus iceberg calving run off and in situ melting beneath the floating ice shelves the resulting negative mass balance of 469 gt year 1 differs substantially from other recent estimates but some components are subject to high temporal variability and budget uncertainties of 20 50 annual accumulation from an earlier review is adjusted to include the antarctic peninsula for a total of 2144 gt year 1 an iceberg production rate of 2016 gt year 1 is obtained from the volume of large icebergs calculated from satellite images since 1978 and from the results of an international iceberg census project ice shelf melting of 544 gt year 1 is derived from physical and geochemical observations of meltwater outflow glaciological field studies and modeling of the sub ice ocean circulation the highest melt rates occur near ice fronts and deep within sub ice cavities run off from the ice sheet surface and from beneath the grounded ice is taken to be 53 gt year 1 less than half of the negative mass balance need come from the grounded ice to account for the unattributed 0 45 mm year 1 in the ipcc best estimate of the recent global sea level rise

high latitude oceanography using the autosub autonomous underwater vehicle a combination of measurements from autosub an autonomous underwater vehicle and ship based instruments are used to describe the oceanographic conditions beneath fimbul ice shelf antarctica the data show an intricate oceanographic regime that is suggestive of variability at seasonal or longer time scales results from a numerical model of the flow beneath the ice shelf lend support to a strong seasonal variability and data from instruments moored beneath the ice shelf show substantial temporal variations the autosub data indicate temperatures within the cavity somewhat lower than our model results would suggest thus our modeled melt rates average of 0 85 m yr21 might be overestimates although they are substantially lower than from previously published model studies 4 9 and 1 9 m yr21 the contribution of ice shelf melt in the freshwater balance of the waters upstream of the climatically important filchner ronne ice shelf needs to be reassessed

seasonal variations of the antarctic coastal ocean in the vicinity of lutzow holm bay during the period of 1990 1992 year round oceanographic observations were conducted in the vicinity of lutzow holm bay east antarctica it was found that the thickness of the winter water ww layer characterized by a cold fresh oxygen rich water exhibits its maximum in the austral fall typically 500 m and its minimum in the austral summer typically 350 400 m the associated density variation of the water column explains only about one third of the seasonal variations in sea level at the coast which suggests a large seasonal variation in barotropic coastal flow prominent freshening occurs in the ww layer during fall this appears to be caused by the accumulation of ww whose upper portion is freshened in the preceding summer these seasonal variations appear to occur every year we propose that the seasonal variation in the ww layer is mainly caused by the seasonal variation in the wind over the coastal ocean in fall the prevailing easterly wind intensifies which increases the ekman convergence of ww in the coastal ocean while in summer the opposite occurs

characteristics and origin of coarse gold in late pleistocene sediments of the cariboo placer mining district british columbia canada the cariboo placer mining district 1000 km2 sited in the interior plateau of central british columbia canada is the premier placer gold mining district of the province gold is recovered from three late pleistocene sedimentary facies postglacial fluvial gravels 25 ka this study reports the morphology size roundness sphericity of 1636 gold grains ranging in size from 0 25 to 17 mm recovered from 19 placer mines older gravels contain the smallest gold grains mean grani size 1 53 mm grains of intermediate size occur in till 2 23 mm and the coarsest gold occurs in postglacial gravels 2 34 mm with a mean of 1 93 mm for the mining district as a whole the most common grain shapes are sub rounded discoidal 14 73 of the grain population sub angular discoidal 10 88 and sub rounded sub discoidal 9 59 the most angular grains occur in postglacial gravels in situ growth of coarse angular grains is indicated by a composite grain structure consisting of aggregates of gold particles welded together by high grade ag 2 filamentous gold in situ coarsening may be reliant on organic complexing agents produced below a dense forest cover an evolutionary sequence of grain form from angular aggregates to rounded pumpkin seed grains is suggested rounded grains commonly show a crystalline structure which may result from the cold hammering of gold during transport fracturing along crystal boundaries is common gold grains may undergo cycles of coarsening rounding diagenesis and breakup in response to repeated recycling through pleistocene sedimentary environments

modeling antarctic ice shelf responses to future climate changes and impacts on the ocean 1 we investigate basal melting of all antarctic ice shelves by a circumpolar ice shelf sea ice ocean coupled model and estimate the total basal melting of 770 944 gt yr under present day climate conditions we present a comparison of the basal melting with previous observational and modeling estimates for each ice shelf heat sources for basal melting are largely different among the ice shelves sensitivities of the basal melting to surface air warming and to enhanced westerly winds over the antarctic circumpolar current are investigated from a series of numerical experiments in this model the total basal melting strongly depends on the surface air warming but is hardly affected by the change of westerly winds the magnitude of the basal melting response to the warming varies widely from one ice shelf to another the largest response is found at ice shelves in the bellingshausen sea followed by those in the eastern weddell sea and the indian sector these increases of basal melting are caused by increases of circumpolar deep water and or antarctic surface water into ice shelf cavities by contrast basal melting of ice shelves in the ross and weddell seas is insensitive to the surface air warming because even in the warming experiments there is high sea ice production at the front of the ice shelves that keeps the water temperature to the surface freezing point weakening of the thermohaline circulation driven by antarctic dense water formation under warming climate conditions is enhanced by basal melting of ice shelves

low melt rates with seasonal variability at the base of fimbul ice shelf east antarctica revealed by in situ interferometric radar measurements basal melt is a major cause of ice shelf thinning affecting the stability of the ice shelf and reducing its buttressing effect on the inland ice the fimbul ice shelf fis in dronning maud land dml east antarctica is fed by the fast flowing jutulstraumen glacier responsible for 10 of ice discharge from the dml sector of the ice sheet current estimates of the basal melt rates of the fis come from regional ocean models autosub measurements and satellite observations which vary considerably this discrepancy hampers evaluation of the stability of the jutulstraumen catchment here we present estimates of basal melt rates of the fis using ground based interferometric radar we find a low average basal melt rate on the order of 1 m yr with the highest rates located at the ice shelf front which extends beyond the continental shelf break furthermore our results provide evidence for a significant seasonal variability

measurements of ice shelf water beneath the front of the ross ice shelf using gliders measurements made by an underwater glider deployed near the ross ice shelf were used to identify the presence of ice shelf water isw which is defined as seawater with its potential temperature lower than its surface freezing point temperature properties logged by the glider included in situ temperature electrical conductivity pressure gps location at surfacings and time for most of the first 30 recorded dives of its deployment evidence suggests the glider was prevented from surfacing due to being under the ice shelf for dives under the ice shelf farthest from the ice shelf front isw layers of varying thicknesses and depth locations were observed between 2 m thick centred at 231 m depth to 93 m thick centred at 360 m for dives under the ice shelf close to the ice shelf front either no isw was observed or isw layers were centred at shallower depths 116 127 m thicker isw layers e g up to 250 m thickness centred at 421 m were observed for some glider dives in open water in front of the ross ice shelf no in situ supercooling water colder than the pressure dependent freezing point temperature was observed

wind driven spreading of fresh surface water beneath ice shelves in the eastern weddell sea solar heated fresh antarctic surface water asw is a permanent feature along the eastern weddell sea ews coast in summer down to a depth of roughly 200 m recently asw has been observed beneath the fimbul ice shelf suggesting that it might play an important role in basal melting we propose that wind driven coastal downwelling is the main mechanism that spreads asw beneath the ice shelf in this sector of antarctica we validate this hypothesis with observations scaling analyses and numerical modeling along three principle lines i data analyses of about 1500 salinity profiles collected by instrumented seals indicate that the observed freshening of the coastal water column is likely explained by the on shore ekman transport and subsequent downwelling of asw ii an analytical model of the coastal momentum balance indicates that wind driven downwelling is capable of depressing the buoyant surface water to a depth similar to the ice shelf draft and iii simulations from both idealized and regional eddy resolving numerical ice shelf ocean models support our proposition our main conclusion is that wind driven spreading of asw beneath the ice shelf occurs when downwelling exceeds the depth of the ice shelf base furthermore our study adds to the understanding of the oceanic processes at the antarctic slope front in the ews with possible implications for other sectors of antarctica

the temperature regime of fimbulisen antarctica numerical simulations of the temperature regime of the ice shelf fimbulisen antarctica are presented a vertical temperature profile s1 of fimbulisen has been measured at the extension of jutulstraumen in which the temperature decreases with depth the three dimensional steady state temperature field was computed by a finite element technique horizontal flow velocities and surface accumulation rates were derived from observations the basal melt rate distribution arose from an assumption of balance in the mass continuity equation the computed basal melt rate distribution a b indicates that the highest basal melt rates up to 15 m a 1 occur at the inflow gate of jutulstraumen and low basal melt rates 0 6 ma 1 occur in the slower moving parts where the ice shelf overhangs the continental shelf a b 1 2 m a 1 the resulting temperature field indicates that fimbulisen consists of a cold middle part built up by the extension of jutulstraumen and warmer ice masses in slow moving areas to the west and east furthermore model runs were set up in which the atmospheric temperatures increased in 1 k steps the results suggest that the warming effectively increases the temperatures throughout the ice column in the slower moving parts therefore enhancing shear at the margins of the extension of jutulstraumen

coherent eddies in the labrador sea observed from a mooring abstract during june november 1994 a mooring in the central labrador sea near the former ocean weather station bravo recorded a half dozen anomalous events that prove to be two different types of coherent eddies comparisons with simple analytical models are used to classify these events as coherent eddies on the basis of their velocity signatures the first clear examples of long lived convectively generated eddies are reported these four small radius 5 15 km eddies are exclusively anticyclonic with cold fresh middepth potential temperature and salinity s cores surrounded by azimuthal currents of 15 cm s 1 their s properties identify them unambiguously as the products of wintertime deep convection in the interior labrador sea compared with eddies in other regions these anticyclones are unusual for their strong surface expressions and composite s cores two warm cyclones are also seen these are larger radius 15 km than the anticyclones and about as energetic currents 15 cm s 1

modeling ice ocean interaction in ice shelf crevasses ocean freezing within ice shelf basal crevasses could potentially act as a stabilizing influence on ice shelves however ice ocean interaction and ocean dynamics within these crevasses are as yet poorly understood to this end an idealized 2 d model of an ice shelf basal crevasse has been developed using fluidity a finite element ocean model using an unstructured mesh a simple model of frazil ice formation and deposition has been incorporated into fluidity to better represent the freezing process model results show two different flow regimes dependent on the amount of freezing in the crevasse one driven by freezing at the top of the crevasse and the other by the ingress of meltwater from outside the crevasse in the first freezing at the top of the crevasse leads to the formation of an unstable overturning circulation due to the rejection of dense salty water in the second a buoyant layer is formed along the sides and roof of the crevasse stratifying the water column frazil ice precipitation is found to be the dominant freezing process at the top of the basal crevasse in the freeze driven case with direct freezing being dominant in the melt driven case in both cases melting occurs lower down on the walls of the crevasse due to the strong overturning circulation the freezing in ice shelf crevasses and rifts is found to be highly dependent upon ocean temperature providing a stabilizing influence on ice shelves underlain by cold waters that is not present elsewhere

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

tides of the northwestern ross sea and their impact on dense outflows of antarctic bottom water abstract moored and depth profile measurements of hydrography and currents during the 2003 2005 antarctic slope anslope program on the shelf slope region near drygalski trough sill in the northwest ross sea have revealed an energetic benthic outflow of cold dense antarctic bottom water aabw produced near the shelf break this outflow couples dense water production processes on the continental shelf to ventilation of the global deep ocean we use time series of bottom pressure and currents on the anslope moorings and a high resolution three dimensional primitive equation numerical model to explore the hypothesis that the tidal currents near the shelf break have a significant impact on outflow volume transport and hydrographic properties tidal currents exceed 1 m s 1 at spring tide over the outer shelf and upper slope tidal advection at spring tide carries water parcels 20 km across slope during a half tidal cycle sufficient to advect water from northern drygalski trough across the sill to the shelf break or similarly from the shelf break 500 m water depth down to the 1600 m isobath a tidally rectified lagrangian mean flow of order 0 05 0 1 m s 1 over the outer shelf and upper slope accompanies the spring tide thickness and water mass properties of the benthic layer over the sill and upper slope exhibit pronounced tidal cycles due to advection of hydrographic gradients and to energetic mixing that can extend more than halfway up from the seabed to the ocean surface maximum parameterized diapycnal diffusivity in the benthic layer at spring tides approaches 1 m 2 s 1 the addition of tides has a profound effect on time averaged outflows of aabw primarily through an increase in benthic layer thickness during spring tide downslope volume fluxes of aabw over the slope averaged over a tidal cycle are 2 5 times larger at spring tide than at neap indicating a need to include the effect of tides in large scale ocean models attempting to represent shelf water contributions to the global ocean circulation

variation in placer style gold morphology and gold particle behavior down gravel bed load rivers an example from the shotover arrow kawarau clutha river system otago new zealand gold eroded from quartz veins on the slopes of actively uplifting and physically weathering tributary catchments is transported to the fluvial system with little modification by mass movement mechanisms gold concentrates initially in the bed load component of spatially limited high grade primitive placers in moderate to high gradient incised juvenile valleys in which the rivers flow on or near bedrock interglacial and or uplift induced downcutting events and major floods periodically reconcentrate gold into lags on bedrock maximum gold particle size increases for the first few kilometers of primitive placers then decreases progressively downstream coarse gold 3 mm 0 5 g and gold with low flatness index 10 and is well rounded and commonly folded flattening progressively increases both the surface area to volume ratio and the entrainment potential of gold particles a reliable relationship between fluvial transport distance and flatness index maxima throughout the river system defines a critical flatness index above which gold is entrained and below which gold is not transported at given values of river gradient velocity and bed roughness both entrainment and retention of gold in primitive placer valleys are controlled by shape and mass characteristics that are largely inherited from the primary sources in transition and trunk placer valleys gold entrainment is controlled by flattening of particles to a critical flatness index state whereas gold retention is controlled by flatness index reduction to sub critical state by particle folding as well as input of relatively proximal locally derived primary or paleoplacer gold with subcritical flatness index or mass

melting and refreezing beneath roi baudouin ice shelf east antarctica inferred from radar gps and ice core data indicate that this reflector pattern is best fit by including basal melting of at least 15 cm a 1 this rate of melting is low compared with rates observed on larger ice shelves in both west and east antarctica ice cores extracted from a rift system close to the ice rise promontory show several meters of marine ice accreted beneath the shelf these observations of low rates of basal melting and limited distribution of accreted marine ice suggest that either antarctic surface water may reach the ice shelf base or that circulation beneath the shelf is likely dominated by the production of high salinity shelf water rather than the incursion of circumpolar deep water implying a weak sub shelf circulation system here many of the ice shelves located along the coast of dronning maud land are like roi baudouin ice shelf characterized by frequent ice rises and promontories therefore it is highly likely that these are also of shallow bathymetry and are subject to similarly weak side shelf basal melting and refreezing

seasonal evolution of an ice shelf influenced fast ice regime derived from an autonomous thermistor chain ice shelves strongly interact with coastal antarctic sea ice and the associated ecosystem by creating conditions favorable to the formation of a sub ice platelet layer the close investigation of this phenomenon and its seasonal evolution remains a challenge due to logistical constraints and a lack of suitable methodology in this study we characterize the seasonal cycle of antarctic fast ice adjacent to the ekstrom ice shelf in the eastern weddell sea we used a thermistor chain with the additional ability to record the temperature response induced by cyclic heating of resistors embedded in the chain vertical sea ice temperature and heating profiles obtained daily between november 2012 and february 2014 were analyzed to determine sea ice and snow evolution and to calculate the basal energy budget the residual heat flux translated into an ice volume fraction in the platelet layer of 0 18 0 09 which we reproduced by a independent model simulation and agrees with earlier results manual drillings revealed an average annual platelet layer thickness increase of at least 4 m and an annual maximum thickness of 10 m beneath second year sea ice the oceanic contribution dominated the total sea ice production during the study effectively accounting for up to 70 of second year sea ice growth in summer an oceanic heat flux of 21 w m 2 led to a partial thinning of the platelet layer our results further show that the active heating method in contrast to the acoustic sounding approach is well suited to derive the fast ice mass balance in regions influenced by ocean ice shelf interaction as it allows subdiurnal monitoring of the platelet layer thickness

a simple model of the ice shelf ocean boundary layer and current abstractocean forced basal melting has been implicated in the widespread thinning of antarctic ice shelves but an understanding of what determines melt rates is hampered by limited knowledge of the buoyancy and frictionally controlled flows along the ice shelf base that regulate heat transfer from ocean to ice in an attempt to address this deficiency a simple model of a buoyant boundary flow considering only the spatial dimension perpendicular to the boundary is presented results indicate that two possible flow regimes exist a weakly stratified geostrophic cross slope current with upslope flow within a buoyant ekman layer or a strongly stratified upslope current with a weak cross slope flow the latter regime which is analogous to the steady solution for a katabatic wind is most appropriate when the ice ocean interface is steep for the gentle slopes typical of antarctic ice shelves the buoyant ekman regime which has similarities with the case of an unstratified density current on a slope p

antarctic ice rises and rumples their properties and significance for ice sheet dynamics and evolution locally grounded features in ice shelves called ice rises and rumples play a key role buttressing discharge from the antarctic ice sheet and regulating its contribution to sea level ice rises typically rise several hundreds of meters above the surrounding ice shelf shelf flow is diverted around them on the other hand shelf ice flows across ice rumples which typically rise only a few tens of meters above the ice shelf ice rises contain rich histories of deglaciation and climate that extend back over timescales ranging from a few millennia to beyond the last glacial maximum numerical model results have shown that the buttressing effects of ice rises and rumples are significant but details of processes and how they evolve remain poorly understood fundamental information about the conditions and processes that cause transitions between floating ice shelves ice rises and ice rumples is needed in order to assess their impact on ice sheet behavior targeted high resolution observational data are needed to evaluate and improve prognostic numerical models and parameterizations of the effects of small scale pinning points on grounding zone dynamics

observations beneath pine island glacier in west antarctica and implications for its retreat thinning ice in west antarctica is currently contributing about 10 of the observed rise in global sea level observations obtained from an autonomous underwater vehicle operating beneath pine island glacier west antarctica reveal that the glacier was recently grounded on a transverse ridge in the sea floor but now warm sea water flows through the widening gap above the ridge thinning ice in west antarctica resulting from acceleration in the flow of outlet glaciers is at present contributing about 10 of the observed rise in global sea level1 pine island glacier in particular has shown nearly continuous acceleration2 3 and thinning4 5 throughout the short observational record the floating ice shelf that forms where the glacier reaches the coast has been thinning rapidly6 driven by changes in ocean heat transport beneath it as a result the line that separates grounded and floating ice has retreated inland7 these events have been postulated as the cause for the inland thinning and acceleration8 9 here we report evidence gathered by an autonomous underwater vehicle operating beneath the ice shelf that pine island glacier was recently grounded on a transverse ridge in the sea floor warm sea water now flows through a widening gap above the submarine ridge rapidly melting the thick ice of the newly formed upstream half of the ice shelf the present evolution of pine island glacier is thus part of a longer term trend that has moved the downstream limit of grounded ice inland by 30 km into water that is 300 m deeper than over the ridge crest the pace and ultimate extent of such potentially unstable retreat10 are central to the debate over the possibility of widespread ice sheet collapse triggered by climate change11 12

two years of oceanic observations below the fimbul ice shelf antarctica 1 the mechanisms by which heat is delivered to antarctic ice shelves are a major source of uncertainty when assessing the response of the antarctic ice sheet to climate change direct observations of the ice shelf ocean interaction are extremely scarce and in many regions melt rates from ice shelf ocean models are not constrained by measurements our two years of data 2010 and 2011 from three oceanic moorings below the fimbul ice shelf in the eastern weddell sea show cold cavity waters with average temperatures of less than 0 1 c above the surface freezing point this suggests low basal melt rates consistent with remote sensing based steady state mass balance estimates for this sector of the antarctic coast oceanic heat for basal melting is found to be supplied by two sources of warm water entering below the ice i eddy like bursts of modified warm deep water that access the cavity at depth for eight months of the record and ii fresh surface water that flushes parts of the ice base with temperatures above freezing during late summer and fall this interplay of processes implies that basal melting at the fimbul ice shelf cannot simply be parameterized by coastal deep ocean temperatures but instead appears directly linked to both solar forcing at the surface as well as to the dynamics of the coastal current system

stratification effects in the turbulent boundary layer beneath a melting ice shelf insights from resolved large eddy simulations abstractocean turbulence contributes to the basal melting and dissolution of ice shelves by transporting heat and salt toward the ice the meltwater causes a stable salinity stratification to form

evolution of cretaceous cenozoic quartz pebble conglomerate gold placers during basin formation and inversion southern new zealand abstract numerous auriferous fluvial quartz pebble conglomerates qpcs are present within the late cretaceous recent sedimentary sequence in southern new zealand the qpcs formed in low relief settings before during and after regional marine transgression in alluvial fan and a variety of fluvial and near shore depositional settings in particular during slow thermal subsidence associated with late cretaceous early cenozoic rifting and during the early stages of orogenic uplift following mid cenozoic marine regression qpc maturity characteristics are complex and vary with sediment transport and recycling history stratigraphic proximity to the transgressive waipounamu erosion surface and the amount of first cycle detritus incorporated during recycling for pre marine qpcs the amount of first cycle detritus varies with tectonic intensity and proximity of the depositional setting to remnant cretaceous topography for post marine qpcs it varies with tectonic intensity and proximity to late cenozoic uplift of basement ranges qpcs do not form during a single bedrock erosion sediment deposition cycle non oxidised and or oxidized groundwater alteration kaolinisation of labile minerals in immature sediment and the upper part of underlying basement and repeated sedimentary recycling are fundamental processes of qpc formation regardless of the tectonic or sedimentary settings altered immature rock disaggregates easily upon erosion and alteration clays are winnowed to leave quartz rich residues containing resistant heavy minerals such as zircon and gold detrital sulfide survives recycling if deposition and burial in saturated sediments are rapid qpcs result only if sediment recycling is not accompanied by excessive erosion of fresh basement rock uplift of many parts of the otago schist belt since late miocene has raised rocks above the water table increased erosion rates and inhibited groundwater alteration and qpc formation qpc formation is still occurring in southland where the water table is high sediments are saturated and undergoing alteration and uplift and erosion rates topography and fluvial gradients are all low the qpcs accumulate as residual gravel on the valley floors of low competence streams that are slowly incising pervasively altered dominantly late miocene pliocene immature conglomerates qpcs formation essentially represents physical and chemical lagging of precursor strata accumulation of detrital gold and other heavy minerals is an inevitable consequence and most qpcs contain some gold three types of significant gold placer have developed in the qpcs type 1 placers are essentially eluvial and or colluvial in origin and form without significant fluvial transport by residual accumulation in low competence valleys during low rate uplift fluvial incision and qpc formation type 2 placers have formed during significant fluvial transport and subsequent fluvial incision mainly in higher energy proximal and medial reaches of larger pre marine eocene and post marine fluvial systems type 3 placers formed by wave base and marine current winnowing in the shallow shelf setting during low rate regional marine transgression especially in the eocene

supergene gold transformation secondary and nano particulate gold from southern new zealand biogeochemical processes drive the transformation of gold au in surface environments in this study we assess the link between surface morphologies of au grains and supergene transformation processes with a focus on the formation of nano particulate au in temperate settings gold grains were collected from six localities across the south island of new zealand deposit styles vary from eluvial alluvial and beach placer deposits in areas of moderate to very high levels of precipitation gold grains were assessed using optical microscopy om field emission scanning electron microscopy feg sem focused ion beam scanning electron microscopy fib sem coupled with x ray dispersive analysis edxa and electron microprobe analyses epma morphologies indicative of au and ag dissolution e g grain boundary dissolution as well as abundant au neoformation and aggregation morphologies were observed on all grains the latter include a variety of secondary au morphotypes in particular nano particulate and crystalline forms as well as bacteriomorphic au sheet au and porous branched au networks pervasive dissolution features on grains from an outcropping quartz vein system as well as extensive nano particle formation on weathered quartz vein and placer grains from the west coast of new zealand s south island which is subject to very heavy orographic precipitation suggest that these climatic conditions enhance the transformation of au grains at these sites au nano particles are most abundant in the polymorphic layer i e a coating of biofilms secondary au siliceous and carbonaceous materials on the surface of transforming au grains and in soil materials associated with the grains nano particulate au is also highly abundant in carbonaceous likely exopolymeric coatings on au grains from orepuki beach suggesting that au dissolution in seawater and microbial biomineralization are important contributors to au alteration in beach placer deposits in conclusion surface morphologies of au grains from new zealand are the result of supergene transformations occurring in current environments the formation of nano particulate au which was previously thought to be evaporation driven is in these high rainfall environments likely due to other mechanisms such as biomineralization

compositional variations and morphological evolution in platinum beach placers southern new zealand morphologies of placer platinum group minerals pgm are more variable and resistant to modification during transport than placer gold grains this study documents morphological evolution of pgm placer grains during up to 120 km of transport in beach placers after river transport from inferred sources up to 200 km inland pgm morphological changes are calibrated with changes in morphology of associated placer gold most of the pgm are pt fe alloy and have been fed into the beach placer system from a large river at the western end of the beaches on the south coast of new zealand the incoming fluvial pgm suite includes os ir and ru alloys which may have been derived from distal ophiolitic sources more proximal sources have ural alaskan affinities and these contributed cooperite and braggite or sperrylite locally as well as pt fe alloy grains some pgm may have been recycled through cretaceous quaternary fluvial sediments before entering the modern placer system recycled placer pgm grains have also been derived from elevated quaternary beaches near the coastline pgm grains entering beach placers have rough surfaces with remnants of crystal faces and these evolve to smooth flakes with progressive long shore transport pgm flakes have slightly thickened rims caused by impacts by saltating sand on windy beaches and the most distal beach placers contain flakes with incipient toroidal shapes these pgm incipient toroids are poorly developed compared to accompanying well formed toroidal gold that has developed in nearly all beach placers including those on elevated quaternary beaches typical pgm and gold placer grain size decreases with increasing distance of transport from fluvial grain size of 400 1 000 to 200 microns on the most distal beaches accompanied by eastward loss of equant pgm grains and associated increase in proportion of flakes although net transport distance is 120 km in the beach placer complex frequent aeolian transport of grains from beach to dunes and subsequent recycling by storm surges substantially increased total transport distance in a dynamic windy tectonic environment

the effect of atmospheric forcing resolution on delivery of ocean heat to the antarctic floating ice shelves abstractoceanic melting at the base of the floating antarctic ice shelves is now thought to be a more significant cause of mass loss for the antarctic ice sheet than iceberg calving in this study a 10 km horizontal resolution circum antarctic ocean sea ice ice shelf model based on the regional ocean modeling system roms is used to study the delivery of ocean heat to the base of the ice shelves the atmospheric forcing comes from the era interim reanalysis 80 km resolution and from simulations using the polar optimized weather research and forecasting model 30 km resolution where the upper atmosphere was relaxed to the era interim reanalysis the modeled total basal ice shelf melt is low compared to observational estimates but increases by 14 with the higher resolution winds and just 3 with both the higher resolution winds and atmospheric surface temperatures the higher resolution winds lead to more heat being delivered to the ice shelf cavities from the adjacent ocean and an increase in the

modelling the antarctic marine cryosphere at the last glacial maximum we estimate the sea ice extent and basal melt of antarctic ice shelves at the last glacial maximum lgm using a coupled ice shelf sea ice ocean model the shape of antarctic ice shelves ocean conditions and atmospheric surface conditions at the lgm are different from those in the present day these are derived from an ice shelf ice sheet model a sea ice ocean model and a climate model for glacial simulations respectively the winter sea ice in the lgm is shown to extend up to 7 of latitude further equatorward than in the present day for the lgm summer the model shows extensive sea ice cover in the atlantic sector and little sea ice in the other sectors these modelled sea ice features are consistent with those reconstructed from sea floor sedimentary records total basal melt of antarctic ice shelves in the lgm was 2147 gt a 1 which is much larger than the present day value more warm waters originating from circumpolar deep water could be easily transported into ice shelf cavities during the lgm because the full glacial grounding line extended to shelf break regions and ice shelves overhung continental slopes this increased transport of warm water masses underneath an ice shelf and into their basal cavities led to the high basal melt of ice shelves in the lgm

gold nugget growth during tectonically induced sedimentary recycling otago new zealand abstract an alluvial fan 5 km wide and 3 5 km long has been derived over the past 65 000 years from an actively rising schist mountain range dunstan range in central otago new zealand the fan can be locally subdivided into four distinctive consecutively formed gravel sequences the earliest of which has been deformed along the range margin these gravels are pre dated by miocene mature fluvial sediments manuherikia group which have been recycled into the alluvial fan gravels during uplift all these sedimentary sequences contain gold which has been mined sporadically manuherikia gold grains up to 13 wt ag are well rounded and typically 300 m across typical grain size of gold becomes progressively coarser with each gravel sequence in the alluvial fan and nuggets up to 2 cm occur in younger gravel sequences gold grain size increase is due to addition of authigenic gold on to pre existing grains authigenic gold occurs as a cement between earlier detrital grains 1 4 wt ag as amorphous coatings and protrusions 0 1 wt ag and as overgrowths of lattice intergrowths 0 1 wt ag gold nuggets formed in the gravels have overgrown detrital minerals and authigenic clay minerals chemical accretion of nuggets may be still continuing today in parts of the alluvial fan

modelling west antarctic ice sheet growth and collapse through the past five million years changes in earth s orbit are known to influence climate shifts from cold glacials to warm interglacials how the vast west antarctic ice sheet responds to these fluctuations is uncertain but because its collapse could raise sea levels by about 5 metres of great interest naish et al have analysed the and 1b ocean sediment core extracted from beneath the ross ice shelf as part of the andrill drilling project and find evidence that the ice sheet collapsed periodically during the early pliocene 3 5 million years ago when atmospheric co2 levels were similar to or slightly higher than today s the pattern of collapse suggests an influence of approximately 40 000 year cycles in the tilt of earth s rotational axis obliquity also in this issue of nature in a numerical modelling study focused on the past 5 million years in antarctica david pollard and robert deconto combine ice sheet land supported and ice shelf water supported modelling approaches to simulate the movement of the grounding line the border between land and sea ice their results show that over the past 5 million years the west antarctic ice sheet transitioned between full intermediate and collapsed states in just a few thousand years this means that the ice sheet is likely to disintegrate if ocean temperatures in the area rise by 5 c if the west antarctic ice sheet wais melted sea levels would rise by about 5 m such changes are thought to have occurred in the past but could not be simulated by models pollard and deconto combine ice sheet with ice shelf modelling and show that over the past 5 million years the wais transitioned among full intermediate and collapsed states in only a few thousand years suggesting possible disintegration of the wais if ocean temperatures in the area rise by 5 c the west antarctic ice sheet wais with ice volume equivalent to 5 m of sea level1 has long been considered capable of past and future catastrophic collapse2 3 4 today the ice sheet is fringed by vulnerable floating ice shelves that buttress the fast flow of inland ice streams grounding lines are several hundred metres below sea level and the bed deepens upstream raising the prospect of runaway retreat3 5 projections of future wais behaviour have been hampered by limited understanding of past variations and their underlying forcing mechanisms6 7 its variation since the last glacial maximum is best known with grounding lines advancing to the continental shelf edges around 15 kyr ago before retreating to near modern locations by 3 kyr ago8 prior collapses during the warmth of the early pliocene epoch9 and some pleistocene interglacials have been suggested indirectly from records of sea level and deep sea core isotopes and by the discovery of open ocean diatoms in subglacial sediments10 until now11 however little direct evidence of such behaviour has been available here we use a combined ice sheet ice shelf model12 capable of high resolution nesting with a new treatment of grounding line dynamics and ice shelf buttressing5 to simulate antarctic ice sheet variations over the past five million years modelled wais variations range from full glacial extents with grounding lines near the continental shelf break intermediate states similar to modern and brief but dramatic retreats leaving only small isolated ice caps on west antarctic islands transitions between glacial intermediate and collapsed states are relatively rapid taking one to several thousand years our simulation is in good agreement with a new sediment record andrill and 1b recovered from the western ross sea11 indicating a long term trend from more frequently collapsed to more glaciated states dominant 40 kyr cyclicity in the pliocene and major retreats at marine isotope stage 31 1 07 myr ago and other super interglacials

supercooled southern ocean waters in cold polar waters temperatures sometimes drop below the freezing point a process referred to as supercooling however observational challenges in polar regions limit our understanding of the spatial and temporal extent of this phenomenon we here provide observational evidence that supercooled waters are much more widespread in the seasonally ice covered southern ocean than previously reported in 5 8 of all analyzed hydrographic profiles south of 55 s we find temperatures below the surface freezing point potential supercooling and half of these have temperatures below the local freezing point in situ supercooling their occurrence doubles when neglecting measurement uncertainties we attribute deep coastal ocean supercooling to melting of antarctic ice shelves and surface induced supercooling in the seasonal sea ice region to winter time sea ice formation the latter supercooling type can extend down to the permanent pycnocline due to convective sinking plumes an important mechanism for vertical tracer transport and water mass structure in the polar ocean

misuse of the term bentonite for ash beds of devonian age in the appalachian basin ash beds of devonian age in the appalachian basin have been incorrectly called bentonites the emphasis placed on the volcanic origin of the beds rather than on their mineralogical and physical characteristics apparently led to the erroneous application of the term bentonite recent study indicates that the lack of smectite in the tioga belpre and center hill bentonite beds compared with the type bentonite warrants the deletion of the term bentonite and the substitution of the term ash it is recommended that the ash beds of devonian age be referred to as the tioga ash bed the belpre ash bed and the center hill ash bed

complex network of channels beneath an antarctic ice shelf ice shelves play an important role in stabilizing the interior grounded ice of the large ice sheets the thinning of major ice shelves observed in recent years possibly in connection to warmer ocean waters coming into contact with the ice shelf base has focused attention on the ice ocean interface here we reveal a complex network of sub ice shelf channels under the fimbul ice shelf antarctica mapped using ground penetrating radar over a 100 km2 grid the channels are 300 500 m wide and 50 m high among the narrowest of any reported observing narrow channels beneath an ice shelf that is mainly surrounded by cold ocean waters with temperatures close to the surface freezing point shows that channelized basal melting is not restricted to rapidly melting ice shelves indicating that spatial melt patterns around antarctica are likely to vary on scales that are not yet incorporated in ice ocean models

stability of the west antarctic ice sheet in a warming world equivalent to 0 28 to 0 56 mm yr 1 sea level rise with the rate growing over at least the past two decades 1 4 these observations intensify concerns about the ice sheet s stability that were first raised more than four decades ago 5

observed platelet ice distributions in antarctic sea ice an index for ocean ice shelf heat flux antarctic sea ice that has been affected by supercooled ice shelf water isw has a unique crystallographic structure and is called platelet ice in this paper we synthesize platelet ice observations to construct a continent wide map of the winter presence of isw at the ocean surface the observations demonstrate that in some regions of coastal antarctica supercooled isw drives a negative oceanic heat flux of 30 wm 2 that persists for several months during winter significantly affecting sea ice thickness in other regions particularly where the thinning of ice shelves is believed to be greatest platelet ice is not observed our new data set includes the longest ice ocean record for antarctica which dates back to 1902 near the mcmurdo ice shelf these historical data indicate that over the past 100 years any change in the volume of very cold surface outflow from this ice shelf is less than the uncertainties in the measurements

a one dimensional model of ice shelf ocean interaction large scale oceanic circulation beneath antarctic ice shelves is driven by the thenmohaline differences which result from mass and energy exchange at the ice ocean interface dense saline waters are drawn underneath the ice shelves and emerge cooled and diluted as plumes of ice shelf water a simple one dimensional model of this process has been developed in which the ice shelf water plume is treated as a turbulent gravity current initiated at the inland margin by a flow of fresh meltwater emerging from beneath the grounded ice subsequent evolution of the plume as it ascends along an ice shelf base of specified geometry can be simulated the model has been applied to a flow line on ronne ice shelf antarctica to explain the observed distribution and rate of basal melting and freezing calculations indicate that the present mean melt rate of 0 6 m yr 1 would increase to 2 6 m yr 1 if the underlying water were to warm by 0 6 c this would not only lead to significant thinning of the ice shelf but could also cause a profound change in ocean circulation on the open continental shelf

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

response of southern ocean circulation to global warming may enhance basal ice shelf melting around antarctica we investigate the large scale oceanic features determining the future ice shelf ocean interaction by analyzing global warming experiments in a coarse resolution climate model with a comprehensive ocean component heat and freshwater fluxes from basal ice shelf melting ism are parameterized following beckmann and goosse ocean model 5 2 157 170 2003 melting sensitivities to the oceanic temperature outside of the ice shelf cavities are varied from linear to quadratic holland et al in j clim 21 2008 in 1 per year co2 increase experiments the total freshwater flux from ism triples to 0 09 sv in the linear case and more than quadruples to 0 15 sv in the quadratic case after 140 years at which 4 280 ppm 1 120 ppm was reached due to the long response time of subsurface temperature anomalies ism thereafter increases drastically if co2 concentrations are kept constant at 1 120 ppm varying strength of the antarctic circumpolar current acc is crucial for ism increase because southward advection of heat dominates the warming along the antarctic coast on centennial timescales the acc accelerates due to deep ocean warming north of the current caused by mixing of heat along isopycnals in the southern ocean so outcropping regions in contrast to previous studies we find an initial weakening of the acc during the first 150 years of warming this purely baroclinic effect is due to a freshening in the so which is consistent with present observations comparison with simulations with diagnosed ism but without its influence on the ocean circulation reveal a number of ism related feedbacks of which a negative ism feedback due to the ism related local oceanic cooling is the dominant one

seasonal variability of water masses and transport on the antarctic continental shelf and slope in the southeastern weddell sea 1 an array of five moorings was deployed from february 2009 to february 2010 across the antarctic shelf and slope in the southeastern weddell sea 18 w observations demonstrate the key processes responsible for variability in water masses and transport in the region rapid fluctuations in temperature and salinity throughout the year are linked with variability in wind stress over the array this causes the deepening or shoaling of the pycnocline past the depth of the moorings in the upper 500 m the seasonal cycle in salinity shows freshening in autumn with the strongest freshening at the shallowest mooring 250 m furthest on shelf the sea ice concentration over the array exceeds 90 during this period and contributes a positive salt flux into the ocean during autumn freshening begins during strong along shore easterly winds in late april 2009 this demonstrates that variations in ekman transport and wind driven mixing play a key role in determining the salinity of shelf waters around antarctica transport of the antarctic slope current also shows a seasonal cycle with a maximum during late april model simulations show the importance of along shore advection as the arrival of a fresh anomaly from upstream determines the timing of the salinity minimum at the array these processes are likely to be important for other regions around the antarctic continent

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