on the in situ aqueous alteration of soils on mars abstract early 3 gy wetter climate conditions on mars have been proposed and it is thus likely that pedogenic processes have occurred there at some point in the past soil and rock chemistry of the martian landing sites were evaluated to test the hypothesis that in situ aqueous alteration and downward movement of solutes have been among the processes that have transformed these portions of the mars regolith a geochemical mass balance shows that martian soils at three landing sites have lost significant quantities of major rock forming elements and have gained elements that are likely present as soluble ions the loss of elements is interpreted to have occurred during an earlier stage s of weathering that may have been accompanied by the downward transport of weathering products and the salts are interpreted to be emplaced later in a drier mars history chemical differences exist among the sites indicating regional differences in soil composition shallow soil profile excavations at gusev crater are consistent with late stage downward migration of salts implying the presence of small amounts of liquid water even in relatively recent martian history while the mechanisms for chemical weathering and salt additions on mars remain unclear the soil chemistry appears to record a decline in leaching efficiency a deep sedimentary exposure at endurance crater contains complex depth profiles of so4 cl and br trends generally consistent with downward aqueous transport accompanied by drying while no model for the origin of martian soils can be fully constrained with the currently available data a pedogenic origin is consistent with observed martian geology and geochemistry and provides a testable hypothesis that can be evaluated with present and future data from the mars surface

the retreat of a tidewater glacier observations and model calculations on hansbreen spitsbergen based on observations and model calculations the retreat over the last two decades of hansbreen a tidewater glacier in southern spitsbergen svalbard is investigated the observations of the calving front position between 1982 and 1998 show an abrupt retreat in 1990 which is suggested to be related to a depression in the glacier bed the observed seasonal variations of the front position are mainly due to variations of the calving rate the observations of hansbreen further indicate that during periods of slow front position changes melting at the water line may play an important role in triggering the process of calving the evolution of hansbreen between 1982 and 1998 is simulated with a numerical model for the dynamics of tidewater glaciers using a flotation criterion for calving in which for each time step the part of the glacier terminus which is below a critical height above buoyancy is removed we are able to reproduce the observed rapid retreat of hansbreen through the depression in the glacier bed from the observations and model calculations we conclude that the rapid retreat is mainly an effect of basal topography in the terminus region and not a direct response to a change in mass balance

subaqueous melting of store glacier west greenland from three dimensional high resolution numerical modeling and ocean observations we present three dimensional high resolution simulations of ice melting at the calving face of store glacier a tidewater glacier in west greenland using the massachusetts institute of technology general circulation model we compare the simulated ice melt with an estimate derived from oceanographic data the simulations show turbulent upwelling and spreading of the freshwater laden plume along the ice face and the vigorous melting of ice at rates of meters per day the simulated august 2010 melt rate of 2 0 0 3 m d is within uncertainties of the melt rate of 3 0 1 0 m d calculated from oceanographic data melting is greatest at depth above the subglacial channels causing glacier undercutting melt rates increase proportionally to thermal forcing raised to the power of 1 2 1 6 and to subglacial water flux raised to the power of 0 5 0 9 therefore in a warmer climate store glacier melting by ocean may increase from both increased ocean temperature and subglacial discharge key points simulated melt agrees with estimates from a hydrographic survey of the glacier with more runoff glaciers will melt faster at constant ocean temperature 2013 american geophysical union all rights reserved

ocean forcing of the greenland ice sheet calving fronts and patterns of retreat identified by automatic satellite monitoring of eastern outlet glaciers 1 we have developed an automatic method to identify changes in the position of calving glacier margins using daily modis imagery application of the method to 32 ocean terminating glaciers in east greenland produced 26 802 margin positions for a 10 year long period 2000 2009 we report these high resolution data and show that the glaciers exhibit seasonal cycles with magnitudes of advance and retreat proportional to glacier width despite similar seasonality there is a distinct difference between the interannual trends of calving front positions north and south of 69 n all glaciers above this latitude showed very limited or no change when seasonality was excluded while glaciers south of 69 n retreated significantly between 2001 and 2005 2 3 km on average approximately 26 of the retreat of southern glaciers was regained by readvance from 2005 to 2009 to explain the latitudinal boundary of glacier dynamics we review basic climatic factors including summer and winter atmospheric forcing sea ice conditions and ocean temperature we conclude that the southern retreats were strongly influenced by warm oceanic conditions associated with increased transport of subtropical waters to the irminger sea and to fjords and coastal regions south of 69 n northern glaciers remained stable despite significant increase in runoff in this region because fjords at latitudes higher than 69 n are less exposed to subtropical waters the southern retreats illustrate sensitive behavior of calving glaciers and we hypothesize that the calving fronts retreated because they were exposed to rapid ice front melting

controls on the transport of oceanic heat to kangerdlugssuaq glacier east greenland this work was funded by nerc grant ne k014609 1 to peter nienow and andrew sole and a nerc studentship to donald slater edward hanna and david wilton acknowledge support from nerc grant ne h023402 1

transformation of an archean craton margin during proterozoic basin formation and magmatism the albany fraser orogen western australia abstract the albany fraser orogen is a well preserved example of proterozoic modification of an archean craton margin the formation of two successive basin systems accompanied by magmatism along the southern and southeastern yilgarn craton margin in western australia reflect distinct changes in tectonic regimes resulting in significant transformations of the archean craton crust provenance analysis from detrital zircons indicates that the first basin system the c 1815 1600 ma barren basin was dominantly filled with neoarchean detritus derived from the yilgarn craton and paleoproterozoic detritus derived from coeval voluminous mostly felsic magmatism the abundance of locally derived sediment deposited onto a reworked archean yilgarn craton substrate coupled with the progressively more juvenile isotopic signature of magmas indicates a largely extensional tectonic setting consistent with either a continental rift basin or a long lived backarc basin system this extensional regime led to the formation of a passive margin and oceanic basin along the craton edge the c 1600 1305 ma arid basin and formation of an ocean continent transition in the most heavily modified outboard region of the craton margin the eastern nornalup zone a change in tectonic regime is denoted by the formation of an oceanic arc the c 1410 ma loongana arc preserved in the adjoining madura province oceanic subduction led to accretion of the loongana arc onto the craton edge recorded by the presence of exotic detritus derived from this arc deposited into a foreland basin uppermost arid basin this accretion event triggered crustal thickening and the earliest phase of magmatism of stage i of the albany fraser orogeny at c 1330 ma a change to west dipping subduction beneath the accreted portion of the loongana arc placed the orogen into a backarc setting into which the remainder of the recherche supersuite mafic and felsic magmas were intruded thus the albany fraser orogen is regarded as an archean craton margin that preserves a long history of proterozoic transformation dominated by extensional processes that resulted in the formation of orogen wide basin systems accompanied by magmatism the recognition that the albany fraser orogen has always been part of the west australia craton and the tectonic regimes that have affected it provide important constraints on reconstructions of the nuna and rodinia supercontinents

linear response of east greenland s tidewater glaciers to ocean atmosphere warming predicting the retreat of tidewater outlet glaciers forms a major obstacle to forecasting the rate of mass loss from the greenland ice sheet this reflects the challenges of modeling the highly dynamic topographically complex and data poor environment of the glacier fjord systems that link the ice sheet to the ocean to avoid these difficulties we investigate the extent to which tidewater glacier retreat can be explained by simple variables air temperature meltwater runoff ocean temperature and two simple parameterizations of ocean atmosphere forcing based on the combined influence of runoff and ocean temperature over a 20 y period at 10 large tidewater outlet glaciers along the east coast of greenland we find that ocean atmosphere forcing can explain up to 76 of the variability in terminus position at individual glaciers and 54 of variation in terminus position across all 10 glaciers our findings indicate that i the retreat of east greenland s tidewater glaciers is best explained as a product of both oceanic and atmospheric warming and ii despite the complexity of tidewater glacier behavior over multiyear timescales a significant proportion of terminus position change can be explained as a simple function of this forcing these findings thus demonstrate that simple parameterizations can play an important role in predicting the response of the ice sheet to future climate warming

ice speed of a calving glacier modulated by small fluctuations in basal water pressure ice flow acceleration has played a crucial role in the rapid retreat of calving glaciers in alaska greenland and antarctica high frequency measurements of ice speed and basal water temperatures from a calving glacier in patagonia show that changes in basal water pressure by a few per cent can significantly affect ice flow speed ice flow acceleration has played a crucial role in the rapid retreat of calving glaciers in alaska1 2 greenland and antarctica3 4 glaciers that calve in water flow much faster than those that terminate on land as a result of enhanced basal ice motion where basal water pressure is high5 however a scarcity of subglacial observations in calving glaciers limits a mechanistic understanding here we present high frequency measurements of ice speed and basal water pressures from glaciar perito moreno a fast flowing calving glacier in patagonia we measured water pressure in boreholes drilled at a site where the glacier is 515 5 m thick and where more than 60 of the ice is below the level of proglacial lakes we found that the mean basal water pressure was about 95 of the pressure imposed by the weight of the overlying ice moreover changes in basal water pressure by a few per cent drove nearly 40 of the variations in ice flow speed the ice speed was strongly correlated to air temperature suggesting that glacier motion was modulated by water pressure changes as meltwater entered the system we conclude that basal water pressure in calving glaciers is important for glacier dynamics and closely connected to climate conditions

distributed subglacial discharge drives significant submarine melt at a greenland tidewater glacier this is the publisher s final pdf the article is copyrighted by american geophysical union and published by john wiley sons inc it can be found at http agupubs onlinelibrary wiley com agu journal 10 1002 28issn 291944 8007

uplift structure of the southern kapuskasing zone from 2 45 ga dike swarm displacement the ca 2 ga kapuskasing zone a 500 km long belt of uplifted crust cuts across the regional structural trend of the archean superior province in canada and disrupts the continuity of 2 45 ga matachewan dikes that form a major swarm in its vicinity as expressed by positive gravity and magnetic anomalies it appears to end about 100 km east of lake superior until now the nature of this termination has remained obscure regional variations in paleomagnetic polarity and in the intensity of feldspar clouding caused by magnetite exsolution are two physical properties of the dike swarm that depend upon postemplacement erosion level they show that the kapuskasing zone as a narrow fault bounded crustal uplift continues after sinistral offset for a further 60 km to the southwest before terminating in a major cross fault dual magnetic polarity and variable levels of feldspar clouding are features found in many other proterozoic dike swarms worldwide and therefore in principle may be used to document other examples of craton response to tectonism

mechanics of the giant radiating mackenzie dyke swarm a paleostress field modeling 1 the 1 27 ga mackenzie dyke swarm of the canadian shield is a giant radiating dyke swarm that gradually swings in orientation from n s in the focal area to nw se trends in peripheral areas in this paper we propose a new model the plug model that accounts for the paleostress contribution to the mechanism of emplacement for the mackenzie dyke swarm in the canadian shield the 1 27 ga stress field on the canadian shield calculated by the plug model explains the radiating nature of the mackenzie dyke swarm around the coppermine river lava field by local stress concentrations the parallel nature of the dyke swarm at distance more than 1000 km from the focal source can be explained by the existence of a regional tectonic stress field created by ridge push acting on the southeast margin of the canadian shield from the grenville ocean the thin elastic plate and two dimensional cross section modeling suggest that the interaction between stresses from a mantle upwelling and the grenville ocean spreading play an important role in the intrusion mechanism of the mackenzie dyke swarm the change in dyke orientation from n s trending to nw se trending is caused by coupling between resistance from the focal area plug area and a grenville ocean ridge push

geomorphologic evidence for the late pliocene onset of hyperaridity in the atacama desert the atacama desert has experienced a long and protracted period of hyperaridity that has resulted in what may be the most unusual biome on earth but the duration of this aridity is poorly constrained we reconstructed aspects of the fluvial and geochemical history of this region using integrated landscape features alluvial fans hillslope soils soil chemistry river profiles in the southern portion of the present desert topographic reconstructions of a large watershed 11 000 km 2 show deep incision and sediment removal between the late miocene and the end of the pliocene and modest to negligible incision in post pliocene times these changes in incision suggest an 50 280 reduction in river discharge which should reflect corresponding changes in precipitation changes in the nature of hillslope soils in the atacama desert indicate that in the pliocene or earlier hillslopes were mantled with silicate derived soil this mantle was stripped off and locally deposited as alluvial fans late pliocene to early pleistocene that now block or otherwise cause a rearrangement of pliocene and earlier river channels finally the hillslopes have largely accreted a soil mantle of dust and salt since the apparent late pliocene stripping suggesting a decline in annual precipitation of at least 125 mm yr 1 or more mean annual precipitation map is now 1 embedded in the long post pliocene era of salt accumulation there are a variety of features suggesting overland flow on hillslopes rills striped gravel deposits piping and water spouts and large infrequent storms that infiltrated gentle alluvial fans due to the depth of salt rich horizons despite evidence for episodes that punctuate the hyperaridity the magnitude and duration of these pluvial events have been insufficient to remove the regional accumulations of sulfate chloride and nitrate the late pliocene cessation of many fluvial features is coincident with recent research on the tropical pacific which shows that the pacific was in a permanent el nino state until ca 2 2 ma at which time sea surface temperatures offshore of south america declined greatly relative to those of the western pacific in turn setting up the present el nino southern oscillation enso climate system these observations indicate that the latest period of aridity has been prolonged and largely continuous and it appears to have occurred in step with the onset of the enso climate system beginning 2 m y ago

precambrian permafrost horizons as indicators of palaeoclimate abstract periglacial phenomena such as polygonal wedge structures and microrelief features commonly preserved in fossil permafrost horizons can provide valuable information on palaeoclimate including indications of mean annual air temperature seasonal temperature range relative precipitation and evidence for long term 104 years oscillations of climate such features are known from early and late proterozoic rocks and indicate mean annual air temperatures as low as 12 to 20 c or lower and strongly seasonal periglacial climates these palaeoclimatic data together with information on palaeolatitude and proximity to palaeo sea level may impose strong constraints on palaeoclimate models for proterozoic glaciation

the geology of the viking lander 2 site revisited abstract reevaluating the geologic history of the prior mars landing sites provides important ground truth for recent and ongoing orbital missions at the viking 2 lander vl2 site topographic measurements of relict landforms indicate that at least 100 m of sedimentary mantle material has been stripped away the observed paucity of impact craters

buoyant flexure and basal crevassing in dynamic mass loss at helheim glacier despite the role that calving plays in greenland mass loss the mechanisms of calving are poorly constrained observations of greenland s helheim glacier suggest that buoyant flexure at the glacier terminus leads to the propagation of basal crevasses and iceberg calving

equation for compaction of paleosols due to burial existing empirically derived compaction curves for marine sediments fail to estimate the compaction of paleosols flood plain sediments and peats as indicated by deformation of clastic dikes dinosaur footprints and sandstone paleochannels yet compaction estimates are vital to reconstructing paleoclimate and geochemical profiles of paleosols to modeling sediment accumulation rates of nonmarine rocks and to tectonic flexural modeling of molasse facies this paper presents a compaction equation and physically derived constants that make geologically realistic estimates of compaction of paleosols and other nonmarine sediments a protocol for the application of the equation is suggested that would allow the following equation to solve for burial compaction c as a fraction of original thickness given depth of burial d in km as c s i f e d k 1 this equation can be applied to nonmarine sedimentary rocks or paleosols given appropriate empirical data on the physical constants in the equation such as initial solidity s i initial porosity f and the corresponding curve fitting constant k data on physical constants useful for this equation are compiled here for a range of paleosol and nonmarine sediment types

the scourie dyke suite petrogenesis and geochemical nature of the proterozoic sub continental mantle the petrogenesis of bronzite picrite olivine gabbro norite and quartz tholeiite dykes which make up the 2 39 b y scourie dyke swarm cutting the archaean lewisian gneisses of n w scotland is interpreted on the basis of their major and trace element geochemistry most of the dykes bear primary amphibole and or phlogopite and with one exception are all hypersthene or quartz normative apart from one tholeiite dyke which shows relative light rare earth element depletion all the dykes show enrichment in light rare earths and large ion lithophile elements they do not however show an equivalent enrichment in other incompatible high field strength ions such as nb and ta and in this respect resemble island arc and calc alkaline basalts the different dyke types have distinctive rare earth patterns and other trace element ratios which are maintained over a range of major element compositions

convection driven melting near the grounding lines of ice shelves and tidewater glaciers subglacial meltwater draining along the bed of fast flowing marine terminating glaciers emerges at the groundingline wheretheiceeithergoesafloattoformaniceshelforterminates inacalvingface theinputof freshwater to the ocean provides a source of buoyancy and drives convective motion alongside the ice ocean interface this process is modeled using the theory of buoyant plumes that has previously been applied to the study of the larger scale circulation beneath ice shelves the plume grows through entrainment of ocean waters and the heat brought into the plume as a result drives melting at the ice ocean interface the equations are nondimensionalized by using scales appropriate for the region where the subglacial drainage rather than the subsequent addition of meltwater supplies the majority of the buoyancy forcing it is found that the melt rate within this region can be approximated reasonably well by a function that is linear in ocean temperature has a cube root dependence on the flux of subglacial meltwater and has a complex dependency on the slope of the ice ocean interface the model is used to investigate variability in melting induced by changes in both ocean temperature and subglacial discharge for a number of realistic examples of ice shelves and tidewater glaciers the results show how warming ocean waters and increasing subglacial drainage both generate increases in melting near the grounding line

partitioning effects from ocean and atmosphere on the calving stability of kangerdlugssuaq glacier east greenland we use the 7 km retreat of kangerdlugssuaq glacier kg east greenland to examine the mechanisms interactions and relative significance of atmospheric forcing and ice ocean interactions hydrographic data from 1991 1993 and 2004 show that subtropical waters are common in kangerdlugssuaq fjord kf and that surface waters were warm in 2004 relative to 1991 and 1993 the main water column was nonetheless warmest in 1991 we contend that while flow of subtropical waters into fjords provides a setting in which rapid glacier retreat can occur the triggering of retreat depends on additional environmental factors the climatic variables standing out in our study of kg and kf are air temperature and katabatic winds both had strong positive anomalies during winter 2004 05 when kg retreated we show that proglacial ice melange was absent and that fjord freeze up did not occur until 11 april 2005 due to warm and windy conditions we demonstrate that this setting is unusual and hypothesize that exposure to open water in winter months caused the retreat calculation of ice front melt rates shows that discharge of basal meltwater first from runoff and subsequently from frictional basal heating should intensify the interaction between glacier and fjord

characteristics of ocean waters reaching greenland s glaciers interaction of greenland s marine terminating glaciers with the ocean has emerged as a key term in the ice sheet mass balance and a plausible trigger for their recent acceleration our knowledge of the dynamics however is limited by scarcity of ocean measurements at the glacier ocean boundary here data collected near six marine terminating glaciers 79 north kangerdlugssuaq helheim and petermann glaciers jakobshavn isbrae and the combined sermeq kujatdleq and akangnardleq are compared to investigate the water masses and the circulation at the ice ocean boundary polar water of arctic origin and atlantic water from the subtropical north atlantic are found near all the glaciers property analysis indicates melting by atlantic water aw found at the grounding line depth near all the glaciers and the influence of subglacial discharge at depth in summer aw temperatures near the glaciers range from 4 58c in the southeast to 0 168c in northwest greenland consistent with the distance from the subtropical north atlantic and cooling across the continental shelf a review of its offshore variability suggests that aw temperature changes in the fjords will be largest in southern and smallest in northwest greenland consistent with the regional distribution of the recent glacier acceleration

seasonal to multiyear variability of glacier surface velocity terminus position and sea ice ice melange in northwest greenland glacier ice discharge which depends on ice velocity and terminus fluctuations is a primary component of greenland ice sheet mass loss some research suggests that ice melange influences terminus calving in turn affecting glacier velocity the details and broad spatiotemporal consistency of these relationships however is undetermined focusing on 16 northwestern greenland glaciers during 2009 through summer 2014 we examined seasonal surface velocity changes glacier terminus position and sea ice and ice melange conditions for a longer term analysis we also produced extended records of four glaciers from 1999 to 2014 there is a strong correspondence between seasonal near terminus sea ice melange conditions and terminus change with rigid ice melange conditions associated with advance and open water associated with retreat extended sea ice free periods and reduced rigid melange are also linked with anomalously large terminus retreat in all but one case sustained multiyear retreat of greater than 1 km during both the 15 year and 6 year records was accompanied by interannual velocity increases seasonal velocity patterns however correspond more strongly with runoff changes than terminus behavior projections of continued warming and longer sea ice free periods around greenland indicate that notable retreat over wide areas may continue this sustained retreat likely will contribute to multiyear speedup longer melt seasons and earlier breakup of melange may also alter the timing of seasonal ice flow variability

ediacaran life on land a new interpretation of fossilized soils palaeosols suggests that at least some ediacaran 625 542 million years ago organisms lived on land thus these ediacaran fossils were not animals but a fungus dominated terrestrial biota that predated vascular plants by about 100 million years ediacaran fossils 542 to 635 million years old occur worldwide in a variety of sedimentary deposits generally interpreted as shallow to deep marine origin they have been regarded as early animal ancestors of the cambrian evolutionary explosion of marine invertebrate phyla as giant marine protists and as lichenized fungi here gregory retallack raises doubts over the assumption that ediacaran organisms were marine a new interpretation of fossilized soils palaeosols from south australia suggests that at least some lived on land retallack s suggestion that some ediacaran fossils were large sessile organisms of cool dry soils is compatible with observations that ediacaran fossils are similar in appearance and preservation to lichens and other microbial colonies of biological soil crusts rather than marine animals or protists ediacaran 635 542 million years ago fossils have been regarded as early animal ancestors of the cambrian evolutionary explosion of marine invertebrate phyla1 as giant marine protists2 and as lichenized fungi3 recent documentation of palaeosols in the ediacara member of the rawnsley quartzite of south australia4 confirms past interpretations of lagoonal aeolian deposition based on synsedimentary ferruginization and loessic texture5 6 further evidence for palaeosols comes from non marine facies dilation cracks soil nodules sand crystals stable isotopic data and mass balance geochemistry4 here i show that the uppermost surfaces of the palaeosols have a variety of fossils in growth position including charniodiscus dickinsonia hallidaya parvancorina phyllozoon praecambridium rugoconites tribrachidium and old elephant skin ichnogenus rivularites7 these fossils were preserved as ferruginous impressions like plant fossils8 and biological soil crusts9 10 of phanerozoic eon sandy palaeosols sand crystals after gypsum11 and nodules of carbonate12 are shallow within the palaeosols4 even after correcting for burial compaction13 periglacial involutions and modest geochemical differentiation of the palaeosols are evidence of a dry cold temperate ediacaran palaeoclimate in south australia4 this new interpretation of some ediacaran fossils as large sessile organisms of cool dry soils is compatible with observations that ediacaran fossils were similar in appearance and preservation to lichens and other microbial colonies of biological soil crusts3 rather than marine animals1 or protists2

the tectonic relationship of two early proterozoic dyke swarms to the kapuskasing structural zone a paleomagnetic and petrographic study a tea coloured cloudiness in groundmass feldspars characterizes dykes of two early proterozoic swarms 2 45 ga matachewan 2 04 ga kapuskasing that intrude amphibolite to granulite host rocks with

ocean forcing of glacier retreat in the western antarctic peninsula in recent decades hundreds of glaciers draining the antarctic peninsula 63 to 70 s have undergone systematic and progressive change these changes are widely attributed to rapid increases in regional surface air temperature but it is now clear that this cannot be the sole driver here we identify a strong correspondence between mid depth ocean temperatures and glacier front changes along the 1000 kilometer western coastline in the south glaciers that terminate in warm circumpolar deep water have undergone considerable retreat whereas those in the far northwest which terminate in cooler waters have not furthermore a mid ocean warming since the 1990s in the south is coincident with widespread acceleration of glacier retreat we conclude that changes in ocean induced melting are the primary cause of retreat for glaciers in this region

the subglacial geology of wilkes land east antarctica wilkes land is a key region for studying the configuration of gondwana and for appreciating the role of tectonic boundary conditions on east antarctic ice sheet eais behavior despite this importance it remains one of the largest regions on earth where we lack a basic knowledge of geology new magnetic gravity and subglacial topography data allow the region s first comprehensive geological interpretation we map lithospheric domains and their bounding faults including the suture between indo antarctica and australo antarctica furthermore we image subglacial sedimentary basins including the aurora and knox subglacial basins and the previously unknown sabrina subglacial basin commonality of structure in magnetic gravity and topography data suggest that pre eais tectonic features are a primary control on subglacial topography the preservation of this relationship after glaciation suggests that these tectonic features provide topographic and basal boundary conditions that have strongly influenced the structure and evolution of the eais

the application of u pb geochronology to mafic ultramafic and alkaline rocks an evaluation of three mineral standards abstract a key to proving the accuracy of u pb age determinations by the various techniques available is routine analysis of an internal well characterized matrix matched mineral standard of known age in general there are very few well studied natural mineral standards available to the geological community in this study the nature geochemistry and u pb systematics of baddeleyite titanite and perovskite from three potential mineral standards are evaluated a compilation of concordant to near concordant analyses of a single phalaborwa baddeleyite crystal yields a precise u pb age of 2059 60 0 35 ma a large variation in uranium 51 2124 ppm thorium 0 5 73 ppm and lead 19 782 ppm concentrations and the presence of some u bearing mineral inclusions zircon monazite and baddeleyite zircon intergrowths require some caution when using aliquots of this phalaborwa baddeleyite crystal as a mineral standard the khan titanite crystal investigated here overall has a relatively uniform chemical composition e g u 584 95 ppm th 473 73 ppm pb 57 8 ppm th u 0 81 0 05 but parts of the crystal contain abundant mineral inclusions the model 1 upper intercept age of 522 2 2 2 ma for nine khan titanite analyses is considered the current best estimate for its crystallization age some variability in the u pb systematics with 207pb 206pb ages that vary between 508 and 560 ma was observed and could be related to the presence of tiny u bearing mineral inclusions such as rutile or allanite so careful selection of material from this titanite crystal is also required a single perovskite crystal from the ice river alkaline intrusion has a relatively uniform chemical composition uranium 141 25 ppm thorium 1954 270 ppm lead 41 5 ppm and th u 14 5 1 3 the u pb results for eight perovskite fractions yield a weighted average 206pb 238u age of 356 5 1 0 ma although there are always some limitations to using natural mineral crystals as u pb standards such as the presence of mineral inclusions alteration and geochemical heterogeneity the three crystals investigated here are considered suitable to be used as internal u pb standards for any of the currently used u pb dating techniques

north atlantic warming and the retreat of greenland s outlet glaciers mass loss from the greenland ice sheet quadrupled over the past two decades contributing a quarter of the observed global sea level rise increased submarine melting is thought to have triggered the retreat of greenland s outlet glaciers which is partly responsible for the ice loss however the chain of events and physical processes remain elusive recent evidence suggests that an anomalous inflow of subtropical waters driven by atmospheric changes multidecadal natural ocean variability and a long term increase in the north atlantic s upper ocean heat content since the 1950s all contributed to a warming of the subpolar north atlantic this led in conjunction with increased runoff to enhanced submarine glacier melting future climate projections raise the potential for continued increases in warming and ice mass loss with implications for sea level and climate

continental mafic dyke swarms as tectonic indicators an example from the vestfold hills east antarctica abstract the two main types of magma that were emplaced as mafic dyke swarms in the vestfold hills are high mg tholeiite hmt and fe rich tholeiite frt the former magma type is less dense than the vestfold hills felsic crust whereas the latter is more dense at deep to intermediate crustal levels therefore frt dyke emplacement requires a higher deviatoric extensional stress as is reflected in a more constant orientation palaeoproterozoic hmt frt bimodal dyke swarms were emplaced around 2 4 ga and 2240 ma the geometry and orientation of 2240 ma hmt dykes imply a low deviatoric extensional stress field inconsistent with a mantle plume origin for magmatism current petrogenetic models for hmt magmatism suggest generation at active convergent plate boundaries although this is broadly consistent with emplacement of the bimodal dyke swarms subsequent to a crustal thickening event at 2 5 ga the exact mechanisms for melting and the generation of crustal extensional stress are unknown a possible scenario involves the sinking of gravitationally unstable oceanic crust a major mesoproterozoic magmatic extensional event in the vestfold hills started with normal faulting followed by lamprophyric dyke emplacement and by two distinct episodes of frt dyke emplacement at 1380 and 1245 ma a consistent direction of the stress field during this event suggests control of a linear belt of crustal uplift by comparison with other mafic dyke swarms this is interpreted to be due to a sheet of upwelling asthenosphere elongated plume around 1 4 ga rather than a low angle detachment this upwelling led to thermal erosion of the base of the lithosphere that channelled a subsequent magma pulse at 1245 ma erosion during a long period of uplift prior to 1380 ma was followed by sedimentation during thermal subsidence as suggested by segmentation of 1245 ma dykes the lithospheric structure inherited from the mesoproterozoic magmatic extensional event was probably reactivated during two high grade tectono metamorphic events around 1 0 and 0 5 ga the vestfold hills formed part of a relatively stable craton peripheral to the mobile belt that was formed during these events

does calving matter evidence for significant submarine melt abstract during the summer in the northeast pacific ocean the alaska coastal current sweeps water with temperatures in excess of 12 c past the mouths of glacierized fjords and bays the extent to which these warm waters affect the mass balance of alaskan tidewater glaciers is uncertain here we report hydrographic measurements made within icy bay alaska and calculate rates of submarine melt at yahtse glacier a tidewater glacier terminating in icy bay we find strongly stratified water properties consistent with estuarine circulation and evidence that warm gulf of alaska water reaches the head of 40 km long icy bay largely unaltered a 10 20 m layer of cold fresh glacially modified water overlies warm saline water the saline water is observed to reach up to 10 4 c within 1 5 km of the terminus of yahtse glacier by quantifying the heat and salt deficit within the glacially modified water we place bounds on the rate of submarine melt the submarine melt rate is estimated at 9 m d 1 at least half the rate at which ice flows into the terminus region and can plausibly account for all of the submarine terminus mass loss our measurements suggest that summer and fall subaerial calving is a direct response to thermal undercutting of the terminus further demonstrating the critical role of the ocean in modulating tidewater glacier dynamics

changes in the velocity structure of the greenland ice sheet using satellite radar interferometry observations of greenland we detected widespread glacier acceleration below 66 north between 1996 and 2000 which rapidly expanded to 70 north in 2005 accelerated ice discharge in the west and particularly in the east doubled the ice sheet mass deficit in the last decade from 90 to 220 cubic kilometers per year as more glaciers accelerate farther north the contribution of greenland to sea level rise will continue to increase

rapid cooling and exhumation in the western part of the mesoproterozoic albany fraser orogen western australia abstract the albany fraser orogen of southwestern australia is an understudied orogenic belt which is interpreted to record the mesoproterozoic suturing of the yilgarn craton of western australia to the mawson craton of east antarctica during rodinia assembly previous u pb geochronology has dated peak amphibolite to granulite facies metamorphism in the orogen at ca 1180 ma here we report the first 40ar 39ar thermochronology of hornblende biotite and muscovite grains from a 360 km transect across the western albany fraser orogen and uncover a record of strikingly fast syn orogenic cooling and exhumation to the north muscovites from the northern foreland record cooling at ca 1159 ma in the central and southern domains of the orogen the biranup and nornalup zones hornblende yields ca 1169 ma cooling ages and biotite yields ca 1172 1144 ma cooling ages the new cooling ages imply that the three domains were exhumed rapidly following peak metamorphism at ca 1180 ma attained a similar structural level by ca 1159 ma and have experienced a uniform exhumation history since that time to constrain mineral closure temperatures and post peak metamorphic cooling rates we conducted a monte carlo simulation which fully propagates uncertainty and minimises error correlations modelling of cooling from hornblende to biotite closure temperatures ca 585 c and 365 c respectively in the nornalup and biranup zones yields fast cooling rates of 33 9 17 c ma and 22 5 7 c ma respectively these fast cooling rates imply rapid exhumation in an active tectonic setting undergoing peak metamorphism although the structural evolution of the albany fraser orogen remains poorly constrained the transpressional tectonic activity associated with deformation in the western part of the albany fraser orogen may have been an active driver of this fast exhumation this is distinctly different from exhumation models for granulite facies domains in other mesoproterozoic orogens which typically experience post orogenic slow 1 5 c ma cooling driven by mechanisms such as orogenic collapse and erosion we consider that the observed differences reflect the interpreted syn tectonic transpressional exhumation history of the albany fraser orogen which is an underrepresented tectonic regime in the mesoproterozoic cooling record

early to middle proterozoic dykes in the mt riiser larsen area of the napier complex east antarctica tectonic implications as deduced from geochemical studies

the vesicular layer and carbonate collars of desert soils and pavements formation age and relation to climate change abstract the vesicular fine grained a horizon av is the widespread ubiquitous surficial horizon of desert soils in diverse landforms and parent materials of varying ages now known to form mostly through accumulation of eolian dust recent studies show that dust accumulation and concomitant soil development are genetically linked to stone pavement formation changes in the magnitude of eolian activity and effective leaching related to quaternary climatic changes are also hypothesized to have influenced the evolution of the av horizon numerical modeling geochronologic and field laboratory studies elucidate the nature of pedogenic processes controlling compositional evolution of av how the changing av horizon increasingly influences soil infiltration and carbonate translocation and accumulation and the control that clasts of the evolving pavement exert on pedogenic processes results of a model that determines soil bulk chemical composition based on mixing of estimated proportions of externally derived eolian material and parent materials imply that the evolution of the soil bulk composition is strongly influenced by av horizon formation the early development of a weakly to moderately developed av horizon directly over gravelly parent material in late and middle holocene soils moderately influences soil infiltration but significant leaching of very soluble materials and some carbonate in dust are permitted in older pleistocene soils however the texturally more mature av and underlying cumulic nongravelly horizons more strongly limit the rate and depth of leaching and soil bulk composition therefore more closely approximates a simple mixture of dust and parent material other aspects of av horizon development and its relations to the pavement are evaluated through studies of pavement clasts with coatings of soil carbonate referred to as carbonate collars development of a numerical model that integrates soil hydrology a co2 production diffusion model calcite kinetics and thermodynamic considerations composition and thermal characteristics of pavement clasts and the textural and structural properties of the surface horizon provides the basis for testing a hypothesis of collar formation model results combined with results of 13c and 18o analyses of collar carbonate demonstrate how precipitation of calcite on pavement clasts and within the av is favored at a depth much shallower than that indicated by the classic carbonate depth climate relationship of jenny and leonard jenny h j leonard c d 1935 functional relationships between soil properties and rainfall soil science 38 363 381 and arkley arkley r j 1963 calculations of carbonate and water movement in soil from climatic data soil science 96 239 248 or simulated by numerical models of carbonate accumulation simultaneous development of thick carbonate collars and the av horizon requires the sustained pavement clast av horizon coupling for at least centuries to possibly millennia new thermoluminescence ages also indicate that much of the av horizon formed in the holocene and that it is certainly much younger than the older pleistocene pavements this supports the previously proposed hypothesis that increased dust flux during the pleistocene to holocene transition triggered and or greatly accelerated av horizon development an understanding of the genesis of collars provides not just an understanding of how carbonate can accumulate in surface environments but it also provides important clues into processes of pavement evolution and preservation of av horizons during long glacial periods the av horizon is not merely an insignificant surficial zone of recent dust accretion instead its development profoundly influences the genesis of desert soils and pavements

geochemical and srnd isotopic mapping of source provinces for the mawson charnockites east antarctica implications for proterozoic tectonics and gondwana reconstruction abstract large charnockite plutons 970 ma were intruded into high grade metamorphic rocks of the mawson coast area east antarctica immediately after peak granulite metamorphism in the region detailed sr nd isotopic studies suggest that charnockites in the central zone have significantly higher initial 87 sr 86 sr ratios 0 7205 0 7334 lower initial nd values 8 8 to 11 1 and older nd depleted mantle model ages tnddm 2 03 2 16 ga than those from the western and eastern zones 0 7076 0 7263 4 0 to 8 4 1 68 1 96 ga respectively the isotopic compositions of the country rocks also mimic such regional variations apart from the isotopic variations the charnockites can also be divided into two major geochemically distinctive categories i e high and low ti groups with the high ti group being characterized by higher tio2 p2o5 k2o feo na2o k2o zr nb y la ce rb ba rb sr and ga al and lower mgo cao al2o3 na2o mg cr and ni than the low ti group however the two groups show no obvious difference in terms of sr nd isotopic signatures compared with palaeozoic i type granites from the lachlan foldbelt both groups of charnockites are generally higher in tio2 p2o5 k2o zr nb y la ce and ba and lower in mgo cao sr ba and mg with the low ti charnockites being somewhat transitional between the high ti charnockites and i type granites on the spiderdiagram both charnockites and i type granites show similar degrees of negative nb anomaly implying derivation from similar nb depleted sources we consider the mawson charnockites like those in the northern prince charles mountains pcm were derived by high temperature partial melting of dry granulitic crustal sources with charnockites in the central zone being related to a significantly older source province than those in the western and eastern zones the charnockite magmatism occurred probably in response to meso neoproterozoic continental collision related to construction of the rodinia supercontinent the igneous charnockites in the mawson pcm areas and the provincial distribution of their tnddm ages and initial 87 sr 86 sr ratios are potentially important markers for precise reconstruction of the rodinia and gondwana supercontinents this study demonstrates that the isotopic systematics especially the tnddm ages in granitic rocks may provide an important means for the delineation of distinctive crustal terranes and their tectonic configuration especially in structurally complex regions

climate dependent chemical weathering of volcanic soils in iceland abstract the major element mobility within soil profiles of brown to gleyic andosols that developed under diverse climatic conditions in iceland is assessed the volcanic soils were selected from areas with good monitoring of annual temperature and precipitation and the degree of weathering and elemental behavior is compared icelandic soils are subject to high fluxes of aeolian dust and at times to tephra ejecta from volcanic eruptions the source of dust input is assessed for each profile based on comparison of the chemical signatures found in the less weathered upper horizons with those of volcanic systems supplying material to source areas results show that tio 2 al 2 o 3 and fe 2 o 3 t and mno are the least mobile species and generally found enriched within more mature horizons the mobile base cations mgo cao and na 2 o are depleted in these horizons as a result of chemical weathering during pedogenesis soils developed in colder climatic conditions with mean annual temperature mat 1 c give values for the chemical index of weathering ciw of 37 45 that reflect only weak chemical weathering soils developed in milder climates mat 2 4 c are more strongly affected by weathering ciw 50 77 the parent material has ciw values of 37 temperature is demonstrated as the dominant variable exerting control on the extent of weathering with only minor mobilization following the incipient near surface weathering stage a robust linear relationship is found between ciw and model mat mat 0 21ciw 8 93 r 2 0 81 this climofunction can deliver proxy climate estimations from volcanic soils and paleosols of basaltic origin in cool to subarctic conditions 2 to 6 c

reworking of archaean and early proterozoic components during a progressive middle proterozoic tectonothermal event in the albany mobile belt western australia abstract isotopic data derived mainly from u pb zircon ion microprobe analyses are presented for each of the three tectonic domains of the albany mobile belt western australia they show that not only the northern domain but also parts of the central domain originally crystallised about 3 100 ma ago they represent reworked archaean rocks similar to those of the adjoining yilgarn block on the basis of sm nd model ages a somewhat younger less precisely defined crustal formation event of probable early proterozoic 2100 ma age can be inferred for the southern domain and the remainder of the central domain the younger component of the albany belt is older than some previous estimates being of similar initial age to most of the early proterozoic orogenic provinces of northern and central australia interpretation of the sm nd data is not strictly based on conventional model ages for it appears that significant fractionation of the rare earth elements occurred during middle proterozoic crustal melting pegmatites and granites dated by u pb on zircon provide a temporal framework for the middle proterozoic evolution of the albany mobile belt progressive deformation incorporating foliation development up to four superimposed fold generations late tectonic granite intrusion and conjugate shear zone arrays took place under a consistent orientation of maximum compressive stress over a geologically brief time span about 1190 ma ago dextral transpression and thrusting at 1190 ma postdates granulite facies metamorphism and major deformation events in the fraser mobile belt the widespread 1190 ma rocks are of comparable age to intrusives rocks emplaced during a similar high grade metamorphic event in the bunger hills of antarctica a region that is commonly juxtaposed with the albany mobile belt in gondwana reconstructions

synchronous retreat and acceleration of southeast greenland outlet glaciers 2000 06 ice dynamics and coupling to climate a large portion of the recent increase in the rate of mass loss from the greenland ice sheet is from increased outlet glacier discharge along its southeastern margin while previous investigations of the region s two largest glaciers suggest that acceleration is a dynamic response to thinning and retreat of the calving front it is unknown whether this mechanism can explain regional acceleration and what forcing is responsible for initiating rapid thinning and retreat we examine seasonal and interannual changes in ice front position surface elevation and flow speed for 32 glaciers along the southeastern coast between 2000 and 2006 while substantial seasonality in front position and speed is apparent nearly all the observed glaciers show net retreat thinning and acceleration with speed up corresponding to retreat the ratio of retreat to the along flow stress coupling length is proportional to the relative increase in speed consistent with typical ice flow and sliding laws this affirms that speed up results from loss of resistive stress at the front during retreat which leads to along flow stress transfer large retreats were often preceded by the formation of a flat or reverse sloped surface near the front indicating that subsequent retreats were influenced by the reversed bed slope many retreats began with an increase in thinning rates near the front in the summer of 2003 a year of record high coastal air and sea surface temperatures this anomaly was driven in part by recent warming suggesting that episodes of speed up and retreat may become more common in a warmer climate

a habitable fluvio lacustrine environment at yellowknife bay gale crater mars the curiosity rover discovered fine grained sedimentary rocks which are inferred to represent an ancient lake and preserve evidence of an environment that would have been suited to support a martian biosphere founded on chemolithoautotrophy this aqueous environment was characterized by neutral ph low salinity and variable redox states of both iron and sulfur species carbon hydrogen oxygen sulfur nitrogen and phosphorus were measured directly as key biogenic elements by inference phosphorus is assumed to have been available the environment probably had a minimum duration of hundreds to tens of thousands of years these results highlight the biological viability of fluvial lacustrine environments in the post noachian history of mars

u pb zircon dating of mafic dykes and its application to the proterozoic geological history of the vestfold hills east antarctica the vestfold hills one of several archaean cratonic blocks within the east antarctic shield comprises a high grade metamorphic basement complex intruded by at least nine generations of early to middle proterozoic mafic dykes extensive u pb ion microprobe shrimp analyses of zircons derived predominantly from late stage felsic differentiates of the mafic dykes provide precise crystallisation ages for several dyke generations these new ages enable constraints to be placed on both the history of mafic magmatism in the vestfold hills and the timing of the various interspersed proterozoic deformation events in addition to demonstrating the utility of zircons derived from felsic late stage differentiates for the dating of co genetic mafic dykes this study also places doubt on previous wholerock rb sr dating of mafic dyke suites in this and other areas of east antarctica the 207pb 206pb zircon ages of 2241 4 ma and 2238 7 ma for the homogeneous and mottled norites respectively provide a younger emplacement age for associated group 2 high mg tholeiite dykes than the whole rock rb sr date 2424 72 ma originally interpreted as the age of all high mg intrusives in the vestfold hills zircon ages of 1754 16 ma and 1832 72 ma confirm the previously defined rb sr age of the group 2 fe rich tholeiites two later dyke generations the group 3 and 4 fe rich tholeiites are distinguished on the basis of field orientations and cross cutting relationships and yield zircon emplacement ages of 1380 7 ma and 1241 5 ma which also define minimum ages for two suites of lamprophyre dykes xenocrystic zircons within both felsic segregations and mafic dykes yield zircon ages of 2478 5 ma to 2740 ma indicating the presence of archaean crustal source rocks of this antiquity beneath the vestfold hills

subglacial drainage evolution modulates seasonal ice flow variability of three tidewater glaciers in southwest greenland b j d was funded in the form of a phd studentship provided by the scottish association for geosciences environment and society sages and the university of st andrews uk j m l is supported by a ukri future leaders fellowship grant no mr s017232 1 d f would like to acknowledge the support of this work through the epsrc and esrc centre for doctoral training on quantification and management of risk and uncertainty in complex systems environments grant no ep l015927 1

sensitivity of greenland ice sheet projections to model formulations physically based projections of the greenland ice sheet contribution to future sea level change are subject to uncertainties of the atmospheric and oceanic climatic forcing and to the formulations within the ice flow model itself here a higher order three dimensional thermomechanical ice flow model is used initialized to the present day geometry the forcing comes from a high resolution regional climate model and from a flowline model applied to four individual marine terminated glaciers and results are subsequently extended to the entire ice sheet the experiments span the next 200 years and consider climate scenario sres a1b the surface mass balance smb scheme is taken either from a regional climate model or from a positive degree day pdd model using temperature and precipitation anomalies from the underlying climate models our model results show that outlet glacier dynamics only account for 6 18 of the sea level contribution after 200 years confirming earlier findings that stress the dominant effect of smb changes furthermore interaction between smb and ice discharge limits the importance of outlet glacier dynamics with increasing atmospheric forcing forcing from the regional climate model produces a 14 31 higher sea level contribution compared to a pdd model run with the same parameters as for ipcc ar4

metamorphic evolution of the bunger hills east antarctica evidence for substantial post metamorphic peak compression with minimal cooling in a proterozoic orogenic event the bunger hills east antarctica experienced a low pressure granulite facies orogenic event during the proterozoic the stable coexistence of the s1 foliation parallel m1 assemblages garnet cordierite spinel ilmenite and garnet sillimanite spinel ilmenite rutile in quartz bearing pelitic gneisses is evidence for metamorphic peak pressures of around 4 kbar during m1 at temperatures of about 800 c the growth of massive reaction coronas of garnet and cordierite around hercynitic spinel and iron titanium oxides during m2 is evidence for the destabilization of the m1 assemblages during compression thermodynamic calculations on the m2 assemblages indicate formation pressures of 6 7 kbar at temperatures of about 750 c thus the gneisses from the bunger hills indicate about 2 kbar or more of compression during minimal cooling such a p t path is different from that of many other proterozoic terranes which are characterized by isobaric cooling or decompression a large charnockite body which is undeformed was intruded at 950 c towards the end of compression the low pressures during m1 can be best explained by metamorphism at mid crustal levels in thin continental crust in thin lithosphere above a thermal perturbation in the underlying asthenosphere we suggest that the compression during cooling was a result of gravitational backflow in which the action of body forces between adjacent normal thickness crust and the thin crust of the bunger hills is switched on by the thermal perturbation within such a model the timing of intrusion of the charnockite exposed in the bunger hills is consistent with its generation by partial melting during the metamorphic maximum of the lowermost crust

in situ u pb geochronology and geochemistry of a 1 13 ga mafic dyke suite at bunger hills east antarctica the end of the albany fraser orogeny abstract antarctica contains continental fragments of australian indian and african affinities and is one of the key elements in the reconstruction of nuna rodinia and gondwana the bunger hills region in east antarctica is widely interpreted as a remnant of the mesoproterozoic albany fraser orogen which formed during collision between the west australian and mawson cratons and is linked with the assembly of rodinia previous studies have suggested that several generations of mafic dyke suites are present at bunger hills but an understanding of their origin and tectonic context is limited by the lack of precise age constraints new in situ shrimp u pb zircon and baddeleyite dates of respectively 1134 9 ma and 1131 16 ma confirm an earlier rb sr whole rock age estimate of ca 1140 ma for emplacement of a major mafic dyke suite in the area existing and new geochemical data suggest that the source of the dyke involved an emorb like source reservoir that was contaminated by a lower crust like component the new age constraint indicates that the dykes post date the last known phase of plutonism at bunger hills by ca 20 million years and were emplaced at the end of stage 2 of the albany fraser orogeny in current models post orogenic uplift and progressive tectonic thinning of the lithosphere were associated with melting and reworking of lower and middle crust that produced abundant plutonic rocks at bunger hills a major episode of mafic dyke emplacement following uplift cooling and plutonic activity with increasing mantle input suggests that the dykes mark the end of a prolonged interval of thermal weakening of the lithosphere that may have been associated with continued mafic underplating during orogenic collapse if the undated olivine gabbro dykes with similar trend geochemistry and petrology at windmill islands are coeval with the ca 1134 ma dyke at bunger hills this would suggest the presence of a major dyke swarm at least 400 km in extent in such case the dykes could have been emplaced laterally from a much more distant mantle source possibly a plume and interacted with the locally heterogeneous and variably metasomatised lithosphere

precambrian tectonic history of the west australian craton and adjacent orogens this review is a synthesis of the precambrian geology of the west aus tralian craton and its fringing orogens it describes the evidence that has recently been reinvestigated and reinterpreted to reveal a long dynamic history of repeated aggregation and dispersal of crustal fragments cratons are large composite geological units of continental crust that have survived intact for significant periods of geological time they are generally tabular bodies with great areal extent millions of square kilo meters and lithospheric thickness 200 400 km orogens are narrow steep zones of tectonic and metamorphic activity associated with the deposition of sediments and the intrusion of plutonic rocks they occur within or along the margins of continents and are generally a few hundred kilometers wide thousands of kilometers long and extend to the base of the litho sphere most orogens are initially associated with compressive tectonics crustal thickening and the transient formation of mountain belts cratons and orogens may survive for very long periods of time but as part of a thin crust on a dynamic mantle they are inherently mobile and travel as rafts for great distances around the surface of the earth they may be torn apart dispersed and reassembled with other continental fragments the boundaries of these crustal fragments once established remain zones of crustal weakness that may be reactivated as intra continental mobile belts or lines along which cratons are assembled and dismembered cf hoffman 1 988

mixing relationships in the martian regolith and the composition of globally homogeneous dust abstract comparison of the chemical compositions of martian soils reveals distinct mixing trends resulting from admixture of variable amounts of sulfate chloride cement at viking landing sites and of the local andesitic rock fragments at the mars pathfinder site these trends most easily visualized in plots of oxides versus so 3 intersect approximately at a common composition thought to represent a global dust that has been homogenized by pervasive aeolian activity the source rocks that were weathered to produce the global dust are inferred to have been basalts rather than felsic rocks based on the observation that the dust lies along well established chemical weathering trends for terrestrial basalts the basaltic protolith was chemically similar e g high fe mg low al 2 o 3 to basaltic shergottite meteorites chemical changes during the weathering of martian basaltic rocks are apparently not as drastic as in terrestrial weathering perhaps because of evaporation of hydrous fluids that leave soluble components behind in the residue comparison with chemical trends for previously proposed martian soil formation mechanisms suggests that palagonitization of basalts more readily explains the dust composition than do hydrothermal alteration at higher temperatures or reactions of rocks with an acid fog produced by volcanic exhalations local or temporal processing of dust into soil involves not only cementation by salts and mixing with rock fragments but also chemical fractionations of fe 2 o 3 tio 2 presumably resulting from aeolian sorting by grain size and density if the global dust represents a broad average of the martian surficial or upper crustal composition the planet s surface geology is dominated by basaltic volcanic rocks and evaporitic salts

marine ice sheet collapse potentially under way for the thwaites glacier basin west antarctica resting atop a deep marine basin the west antarctic ice sheet has long been considered prone to instability using a numerical model we investigated the sensitivity of thwaites glacier to ocean melt and whether its unstable retreat is already under way our model reproduces observed losses when forced with ocean melt comparable to estimates simulated losses are moderate 1 mm per year of sea level rise collapse in the different simulations within the range of 200 to 900 years

geochemistry and geochronology of proterozoic tholeiite dykes of east antarctica evidence for mantle metasomatism two episodes of tholeiite dyke emplacement have been identified in archaean high grade metamorphics of the napier complex in enderby land middle proterozoic amundsen dykes are typical continental tholeiites and most of the chemical variation in individual suites can be explained in terms of different degrees of partial melting and low pressure crystal fractionation group i amundsen tholeiites were derived from a relatively homogeneous source region 1 190 200 m y ago whereas that of the group ii amundsen tholeiites was chemically and isotopically heterogeneous group ii dykes have various degrees of enrichment in incompatible elements and commonly show normalised trace element abundance patterns with negative nb anomalies these features imply variable metasomatism of the source region by a volatile rich fluid phase rather than a melt of any observed igneous composition enriched in k rb ba th and possibly la and ce early proterozoic 2 350 48 m y tholeiites were emplaced at considerable depths in the crust during the waning stages of granulite facies metamorphism and include a high mg suite of possible komatiitic affinity ranging in composition from hypersthene rich tholeiite norite to quartz rich tholeiite they tend to have higher ratios of highly to moderately incompatible elements e g k zr k ce and larger nb anomalies i e higher k nb compared with middle proterozoic tholeiites suggesting derivation from more enriched source regions isotopic data are not compatible with significant crustal contamination but constrain source metasomatism to a time immediately before emplacement metasomatism of the source region of the much younger group i tholeiites may have been contemporaneous with that of the high mg suite

regional correlation of mesoproterozoic structures and deformational events in the albany fraser orogen western australia geologic mapping and meso and microstructural analysis of a previously unstudied area in the western part of the mesoproterozoic albany fraser orogen western australia leads to a correlation of deformational events along the exposed length of the orogen the earliest deformational event d1 accompanied granulite facies metamorphism and is characterized by a locally preserved subhorizontal foliation and associated recumbent folds this event is interpreted to record northwest southeast contraction the second event d2 produced a subvertical east northeast striking foliation and variably plunging upright folds northwest vergent folds toward 333 and sparse kinematic indicators suggest southeast up tectonic transport with a dextral component of movement a third deformational event d3 is manifested by a conjugate set of mylonitic to cataclastic shear zones with a dominant dextral set oriented at 105 and a subordinate sinistral set oriented at 010 the bisector of the average orientations for the shear zones is 325 consistent with northwest southeast shortening like the d2 structures the d3 structures may represent deformation in a dextral transpressive setting d4 structures include two subvertical joint sets one oriented at 355 and the other at 105 all structures are similar in geometry and kinematics to structures near albany which have been constrained to 1190 1170 ma precambrian res 59 1992 95 as well as structures east of esperance that are associated with a major tectonothermal event at 1345 1260 ma precambrian res 102 2000 155 additional geochronological data particularly from the western albany fraser orogen are needed to resolve this apparent discrepancy there is a near one to one correlation in lithology structural style metamorphic grade kinematics and possibly timing of deformational events between the albany fraser orogen and the bunger hills of east antarctica supporting the previously suggested ties between these two areas marked dissimilarities however in model tdm ages u pb zircon dates kinematics and timing of deformation and timing of metamorphism between the albany fraser orogen and the oaxacan complex of southern mexico raise serious doubts regarding the viability of rodinia reconstructions that juxtapose those two mesoproterozoic terranes

paleomagnetism u pb geochronology and geochemistry of lac esprit and other dyke swarms james bay area quebec and implications for paleoproterozoic deformation of the superior province an extensive set of north to northwest trending diabase dykes termed the lac esprit swarm is identified in the superior province east of james bay based on geological mapping and a distinctive paleomagnetic pole 61 7 n 169 1 e dm 7 7 dp 5 5 the lac esprit swarm yields a u pb baddeleyite age of 2069 1 ma similar to that of the 2076 5 4 ma fort frances swarm of the western superior province their paleomagnetic declinations differ by 23 12 after correction to a common reference locality the difference is likely due mainly to counterclockwise rotation about a vertical axis of the fort frances area relative to the lac esprit area differential rotation of 10 20 has been proposed more locally across the kapuskasing structural zone separating the eastern and western superior province in earlier paleomagnetic studies of ca 2450 ma matachewan and 2170 ma biscotasing dyke swarms thus relative rotation may have involved the entire eastern and western superior province perhaps in response

dating sedimentary rocks using in situ u pb geochronology of syneruptive zircon in ash fall tuffs 1 mm thick absolute ages for sedimentary rocks are required to construct a temporal framework in which to decipher earth9s history the most reliable method for dating precambrian sedimentary rocks is u pb geochronology of zircon in intercalated volcanic rocks however extracting sufficient zircons involves destructive mineral separation procedures often requiring several kilograms of sample we have dated felsic tuffs 207 pb 206 pb dates between ca 2600 ma and ca 2680 ma all consistent with their stratigraphic position and available age constraints the dates represent the timing of syndepositional volcanism and provide reliable estimates for the age of deposition zircons in a 0 5 mm tuff band 15 mm above the jeerinah impact layer provide an age of 2632 7 ma which represents a close approximation for the timing of a major asteroid impact the strewn field of which is distributed across two archean cratons in situ geochronology of zircon in felsic tuffs requires only small volumes of sample and is especially useful for drill core samples where material is limited it avoids the destruction of samples associated with mineral separation retaining textural information and avoiding potential contamination this approach adds significantly to the number of sedimentary rocks that can be dated accurately

petrogenesis of plutonic rocks in a proterozoic granulite facies terrane the bunger hills east antarctica abstract several compositionally varied suites of plutonic rocks gabbro quartz monzogabbro granite quartz monzodiorite in the bunger hills were emplaced at depths of 20 km over a relatively short time interval 1170 1150 ma during the waning stages of granulite facies metamorphism the peak of which occurred 1190 ma ago alkaline syenite to granite rocks were emplaced west of the denman glacier at david island 515 ma ago most of these rocks crystallised from magmas derived by melting of heterogeneous mantle comprising at least two components nb poor long term enriched lithosphere and a nb rich oib ocean island basalt type within plate component metasomatic enrichment of the source of the plutonic rocks as well as that of 500 ma alkaline dykes may have been contemporaneous with continental crust formation in the area during the late archaean or early proterozoic much of the chemical variation of the different suites can be explained by fractional crystallisation but other processes particularly variations in the extent and or p t conditions of melting were also important crustal contamination does not appear to have been a major factor in the petrogenesis of the bunger hills magmas but cannot be entirely discounted particularly for some of the granitic rocks the more evolved david island syenitic intrusives may have been derived by melting of lower crustal rocks

the difficulties of dating mafic dykes an antarctic example archaen gneisses of the vestfold hills of east antarctica are transected by several compositionally discrete suites of tholeiitic dykes a representative of one of those suites which has been dated in the present study shows that not only rb sr whole rock isochrons but also u pb zircon techniques if not properly applied can produce erroneous crystallisation ages two zircon populations were recovered from the mafic dyke itself one of which is 2 483 9 ma in age and clearly of xenocrystic origin the other yields an age of 1 025 56 ma which is not ascribed to the magmatic crystallisation of the dyke but rather to the time that it underwent metamorphic metasomatic alteration as a response to high grade tectonism in the adjacent mobile belt it is presumed that the zircon in question formed by the breakdown of another mineral or minerals possibly magmatic baddeleyite due either to ingress of a siliceous fluid or more probably by the release of silica from the breakdown of pyroxene to amphibole a cogenetic 1 2 cm wide felsic vein of late magmatic early hydrothermal origin also contains two zircon populations again most of the grains therein which are interpreted as of xenocrystic origin grew at 2 483 9 ma however a few euhedral zircons with very high u and th contents grew at 1 248 4 ma which is taken to be the formation age of both the felsic vein and the enclosing mafic dyke

high resolution 1 km positive degree day modelling of greenland ice sheet surface mass balance 1870 2012 using reanalysis data we show results from a positive degree day pdd model of greenland ice sheet gris surface mass balance smb 1870 2012 forced with reanalysis data the model includes an improved daily temperature parameterization as compared with a previous version and is run at 1 km rather than 5 km resolution the improvements lead overall to higher smb with the same forcing data we also compare our model with results from two regional climate models rcms while there is good qualitative agreement between our pdd model and the rcms it usually results in lower precipitation and lower runoff but approximately equivalent smb mean 1979 2012 smb standard deviation in gt a 1 is 382 78 in the pdd model compared with 379 101 and 425 90 for the rcms comparison with in situ smb observations suggests that the rcms may be more accurate than pdd at local level in some areas although the latter generally compares well dividing the gris into seven drainage basins we show that smb has decreased sharply in all regions since 2000 finally we show correlation between runoff close to two calving glaciers and either calving front retreat or calving flux this being most noticeable from the mid 1990s the author s 2016

geochemical and srnd isotopic study of charnockites and related rocks in the northern prince charles mountains east antarctica implications for charnockite petrogenesis and proterozoic crustal evolution charnockite plutons were intruded into meso neoproterozoic 1000 ma high grade metamorphic zone in the northern prince charles mountains pcm east antarctica immediately after peak granulite metamorphism in the region detailed geochemical and sr nd isotopic studies were carried out on these plutons and related rocks which enables important constraints to be placed on the regional tectonic setting as well as the origin of igneous charnockites the pcm charnockites are geochemically distinctive characterised by having much higher tio2 p2o5 k2o k2ona2o zr nb y pb la ce and ba and lower mgo cao mg th u srba and rbba than for example i type granites from the lachlan foldbelt the decrease in zr nb y and ce with increasing sio2 sharply contrast with those of the i type granites isotopically the pcm charnockites are relatively uniform and evolved characterised by limited ranges of initial 87sr86sr ratios 0 7063 to 0 7100 initial nd values mainly 4 0 to 5 9 and nd depleted mantle model ages 1 60 to 1 98 ga implying derivation from pre existing crustal sources the geochemical and isotopic features are most consistent with crystal fractionation of dry hot magmas with pyroxene k feldspar plagioclase apatite zircon ilmenite and magnetite as early crystallizing phases although the involvement of a mantle derived magma via afc process cannot be ruled out we consider that the charnockitic magmas were mainly derived from pre existing subduction related crustal sources geochemically and isotopically similar to those of the i type granites the partial melting probably occurred under dry granulitic conditions at elevated temperatures 950 1050 c with orthopyroxene plagioclase and magnetite being residual phases under such conditions elements including k rb ba la ce nd zr nb and y will be strongly incompatible and partition into the melt the relatively low u and th values in the charnockites are probably due to u th depletion in the sources which may have been caused by dehydration and u th removal during amphibolite to granulite transition of the sources we consider that the pcm charnockites and related regional metamorphism resulted from meso to neoproterozoic continental collision between archaean and palaeoproterozoic cratonic blocks in east antarctica the meso neoproterozoic collision was probably a global event possibly related to construction of the rodinia supercontinent during this collisional period or earlier orogenic events in the region e g arc continent collision in the palaeo mesoproterozoic calcareous sediments formed at plate margins or back arc basins would have been tectonically transported to depth release of co2 rich fluids upon tectonic burial may have been responsible for amphibolite to granulite transition without causing dehydration melting to generate i type granites instead subsequent uplift of the dehydrated but fertile granulite crust during a period of crustal rebound may have facilitated decompression melting to produce high temperature water deficient charnockitic melts syn to post collision lithosphere delamination asthenosphere upwelling and magma underplating may have also aided in heating the lower crust above its water deficient solidus temperature

u pb id tims baddeleyite ages and paleomagnetism of 1 79 and 1 59 ga tholeiitic dyke swarms and position of the rio de la plata craton within the columbia supercontinent abstract the tandilia terrane southernmost fringe of the rio de la plata craton is an igneous and metamorphic complex produced by an accretionary orogeny 2 25 2 02 ga calc alkaline acidic dykes with e w strike and a major shear zone with similar orientation are related with the late orogeny stage as supported by field relations in a previous study the acid dykes gave 40 ar 39 ar ages of 2007 24 ma to 2020 24 ma a n and nw trending tholeiitic dyke swarm tandil swarm is also present in the tandilia terrane one sample from the nw trending subset previously gave a u pb id tims baddeleyite age of 1588 11 ma new precise u pb id tims baddeleyite dating of both n and nw trending tandil dykes yielded crystallization ages of 1589 3 ma 1588 3 ma and 1588 3 ma significantly older tholeiitic dykes known as the florida swarm occur in the northern rio de la plata craton for which a u pb id tims baddeleyite age of 1790 5 ma was previously reported consequently intermittent rifting 1 79 1 59 ga took place after tectonic stabilization of the late paleoproterozoic lithosphere proto rio de la plata craton the available geochemical data for the 1 59 ga tandil dykes define low and high tio 2 trends although only the low tio 2 subgroup is firmly dated both the tandil and florida dykes have geochemical and nd sr characteristics consistent with derivation from heterogeneous mantle sources that underwent metasomatic effects the tandil dykes may be linked with the 1 57 0 02 ga capivarita anorthosite which occurs to the east of the northern part of the craton correlatives on other crustal blocks may include those in baltica such as bimodal rock association including the breven hallefors and aland aboland diabase dykes and in the reconstructed gawler craton nw laurentia dolerites bimodal magmatism and iocg deposits contemporary within plate bimodal associations are also present in the sw amazonian craton paleomagnetic data for the 1790 ma florida dykes allows three possible reconstruction scenarios for the position of rio de la plata craton i in the southern hemisphere linked to the pampia terrane and the amazonia rio apa landmass as close as they are in present day ii in the northern hemisphere as a nearest neighbor to the reconstructed nw laurentia and gawler blocks proto australia or iii in the northern hemisphere near the boundary of amazonia proto amazonian craton and baltica positions 2 and 3 are consistent with all three criteria paleomagnetic poles and proximity to matching ca 1790 ma and 1590 ma mafic magmatism our data are consistent with the idea that the columbia supercontinent started major break up at 1 59 ga

continued evolution of jakobshavn isbrae following its rapid speedup 1 several new data sets reveal that thinning and speedup of jakobshavn isbrae continue following its recent rapid increase in speed as its floating ice tongue disintegrated the present speedup rate of 5 a 1 over much of the fast moving region appears to be a diffusive response to the initial much larger speedup near the front there is strong seasonality in speed over much of the fast flowing main trunk that shows a good inverse correlation with the seasonally varying length of a short typically 6 km floating ice tongue this modulation of speed with ice front position supports the hypothesis that the major speedup was caused by loss of the larger floating ice tongue from 1998 to 2003 analysis of image time series suggests that the transient winter ice tongue is formed when sea ice bonds glacier ice in the fjord to produce a nearly rigid mass that almost entirely suppresses calving major calving only resumes in late winter when much of this ice clears from the fjord the collapse of the ice tongue in the late 1990s followed almost immediately after a sharp decline in winter sea ice concentration in disko bay this decline may have extended the length of the calving season for several consecutive years leading to the ice tongue s collapse

elemental geochemistry of sedimentary rocks at yellowknife bay gale crater mars sedimentary rocks examined by the curiosity rover at yellowknife bay mars were derived from sources that evolved from an approximately average martian crustal composition to one influenced by alkaline basalts no evidence of chemical weathering is preserved indicating arid possibly cold paleoclimates and rapid erosion and deposition the absence of predicted geochemical variations indicates that magnetite and phyllosilicates formed by diagenesis under low temperature circumneutral ph rock dominated aqueous conditions analyses of diagenetic features including concretions raised ridges and fractures at high spatial resolution indicate that they are composed of iron and halogen rich components magnesium iron chlorine rich components and hydrated calcium sulfates respectively composition of a cross cutting dike like feature is consistent with sedimentary intrusion the geochemistry of these sedimentary rocks provides further evidence for diverse depositional and diagenetic sedimentary environments during the early history of mars

scalings for submarine melting at tidewater glaciers from buoyant plume theory abstractrapid dynamic changes at the margins of the greenland ice sheet synchronous with ocean warming have raised concern that tidewater glaciers can respond sensitively to ocean forcing understanding of the processes encompassing ocean forcing nevertheless remains embryonic the authors use buoyant plume theory to study the dynamics of proglacial discharge plumes arising from the emergence of subglacial discharge into a fjord at the grounding line of a tidewater glacier deriving scalings for the induced submarine melting focusing on the parameter space relevant for high discharge tidewater glaciers the authors suggest that in an unstratified fjord the often quoted relationship between total submarine melt volume and subglacial discharge raised to the power is appropriate regardless of plume geometry provided discharge lies below a critical value in these cases it is then possible to formulate a simple equation estimating total submarine melt volume as a function of discharge fjord temperature

in situ u pb dating of micro baddeleyite by secondary ion mass spectrometry abstract we introduce a technique for u pb dating of baddeleyite using secondary ion mass spectrometry sims in situ analysis of ng mass crystals that cannot be efficiently extracted by conventional mineral separation techniques average 207 pb 206 pb ages for precambrian baddeleyite crystals are within 206 pb 238 u ratios are corrected for instrumental fractionation calibrated through linear regression in a pb u relative sensitivity vs uo 2 u calibration plot calibration is performed on separated baddeleyite crystals 100 200 m in maximum dimension mounted in random crystallographic orientation 206 pb 238 u ages for baddeleyite from duluth gabbro fc4b and kovdor are accurate within 1 2 when averaging 15 30 individual spot analyses and relative sensitivities calibrated on phalaborwa baddeleyite the relative difference of 206 pb 238 u between large crystals and micro baddeleyite from fc4b is within 1 comparison between silicate glass and baddeleyite as well as replicate analysis of the same grains in different orientations relative to the incidence direction of the primary beam support previous evidence for bias in pb u sensitivity in baddeleyite due to variable crystal orientations we successfully utilized oxygen flooding and a uo 2 u based calibration to significantly reduce orientation dependent bias

paleosols and paleoenvironments of early mars fluviolacustrine sediments filling gale crater on mars show two levels of former exposure and weathering that provide new insights into late noachian 3 7 0 3 ga paleoenvironments of mars diagnostic features of the two successive paleosols in the sheepbed member include complex cracking patterns of surface dilation peds and cutans a clayey surface a horizon deep sand filled cracks with vertical lamination sand wedges and replacive sulfate nodules aggregated into distinct bands gypsic by horizon above bedded sandy layers sedimentary c horizon shallow gypsic horizon periglacial sand wedges and limited chemical weathering are evidence of a hyperarid frigid paleoclimate and this alternated with wetter conditions for the lacustrine parent materials in gale crater during the late noachian depletion of phos phorus vesicular structure and replacive gypsic horizons of these martian paleosols are fea tures of habitable microbial earth soils on earth and encourage further search for definitive evidence of early life on mars

geochemistry of proterozoic amphibolite dykes of commonwealth bay antarctica and possible correlations with mafic dyke swarms elsewhere in gondwanaland abstract amphibolite facies metamorphism and deformation of mafic dykes at commonwealth bay east antarctica has resulted in variable enrichment in large ion lithophile elements mainly k rb ba and u significant changes in na and sr abundances in some dykes appear to be related to alteration of plagioclase in contrast other incompatible elements ti nb p and zr were essentially immobile and together with major element compositions can be used to assess the main primary compositional features at least three distinct groups of olivine and quartz tholeiites which have systematically different abundances and ratios of relatively immobile incompatible elements were probably derived from distinct mantle source regions the chemical similarity of some of these dykes to 1600 ma gabbronorite dykes of the port lincoln area of south australia would be consistent with gondwana reconstructions based on other geological and geochronological evidence

paleomagnetism structure and longitudinal correlation of middle precambrian dykes from northwestern ontario and minnesota a primary magnetization about 2 1 ga old is well preserved in the kenora kabetogama dyke swarm in minnesota and ontario its direction the mean for 12 dykes incorporating results from 30 paleomagnetic sites is 115 9 55 6 yielding a paleomagnetic pole position of latitude 42 7 s longitude 4 2 e dm 7 3 dp 5 2 the primary nature of the magnetization can be demonstrated using geochemistry to correlate individual dykes over distances that can exceed 300 km whereas the remanence direction varies between dykes it remains constant along a dyke sufficient proof that it was formed during initial cooling of the intrusion a weak longitudinal increase in hydrous alteration mgo content and degree of magnetic overprinting is observed as the swarm is followed to the south and may reflect increasing proximity to a precambrian continental margin that underwent rifting sedimentation and deformation during the time interval 2 2 1 6 ga bp although the cause of the longitudinal changes in terms of ma

paleomagnetism and u pb geochronology of easterly trending dykes in the dharwar craton india feldspar clouding radiating dyke swarms and the position of india at 2 37 ga abstract a u pb baddeleyite age of 2367 1 ma from a diabase dyke together with previously published age data suggests that a major early proterozoic dyke swarm cuts across the structural grain of the archean dharwar craton in india paleomagnetic data suggest the swarm is at least 300 km wide and 300 km long and has a fan angle of at least 30 with convergence to the west it was originally emplaced at high latitudes and together with the widgiemooltha dykes of the yilgarn block of australia may have been a segment of a larger radiating swarm related to a long lived plume event that was active for about 50 my from 2418 to 2367 ma a regional change in the intensity of brown feldspar clouding in the dykes suggests that the dharwar craton was tilted northwards in harmony with previous observations on the structure and metamorphism of the archean rocks towards the south the brown feldspar clouding becomes more intense and locally assumes a blacker more sooty appearance the black clouding whose precise origins remain unknown is accompanied by a remagnetization and appears to coincide closely with a region of carbonatite magmatism at 800 ma and with a shear zone and change in structural trend related to pan african deformation at 550 ma paleomagnetic studies suggest that i the high coercivity part of the remanent magnetization is carried by magnetite exsolved within either brown or black clouded feldspars and ii as a more general observation diabase with brown clouded feldspar can carry a primary magnetization but not if the clouding is black

mineralogy of a mudstone at yellowknife bay gale crater mars sedimentary rocks at yellowknife bay gale crater on mars include mudstone sampled by the curiosity rover the samples john klein and cumberland contain detrital basaltic minerals calcium sulfates iron oxide or hydroxides iron sulfides amorphous material and trioctahedral smectites the john klein smectite has basal spacing of similar to 10 angstroms indicating little interlayer hydration the cumberland smectite has basal spacing at both similar to 13 2 and similar to 10 angstroms the larger spacing suggests a partially chloritized interlayer or interlayer magnesium or calcium facilitating h2o retention basaltic minerals in the mudstone are similar to those in nearby eolian deposits however the mudstone has far less fe forsterite possibly lost with formation of smectite plus magnetite late noachian early hesperian or younger age indicates that clay mineral formation on mars extended beyond noachian time

subglacial discharge driven renewal of tidewater glacier fjords the classic model of fjord renewal is complicated by tidewater glacier fjords where submarine melt and subglacial discharge provide substantial buoyancy forcing at depth here we use a suite of idealized high resolution numerical ocean simulations to investigate how fjord circulation driven by subglacial plumes tides and wind stress depends on fjord width grounding line depth and sill height we find that the depth of the grounding line compared to the sill is a primary control on plume driven renewal of basin waters in wide fjords the plume exhibits strong lateral recirculation increasing the dilution and residence time of glacially modified waters rapid drawdown of basin waters by the subglacial plume in narrow fjords allows for shelf waters to cascade deep into the basin wide fjords result in a thin boundary current of shelf waters that flow toward the terminus slightly below sill depth wind forcing amplifies the plume driven exchange flow however wind induced vertical mixing is limited to near surface waters tidal mixing over the sill increases in fjord transport of deep shelf waters and erodes basin stratification above the sill depth these results underscore the first order importances of fjord glacier geometry in controlling circulation in tidewater glacier fjords and thus ocean heat transport to the ice

changes in the dynamics of marine terminating outlet glaciers in west greenland 2000 2009 1 recent changes in the dynamics of greenland s marine terminating outlet glaciers indicate a rapid and complex response to external forcing despite observed ice front retreat and recent geophysical evidence for accelerated mass loss along greenland s northwestern margin it is unclear whether west greenland glaciers have undergone the synchronous speed up and subsequent slow down as observed in southeastern glaciers earlier in the decade to investigate changes in west greenland outlet glacier dynamics and the potential controls behind their behavior we derive time series of front position surface elevation and surface slope for 59 marine terminating outlet glaciers and surface speeds for select glaciers in west greenland from 2000 to 2009 using these data we look for relationships between retreat thinning acceleration and geometric parameters to determine the first order controls on glacier behavior our data indicate that changes in front positions and surface elevations were asynchronous on annual time scales though nearly all glaciers retreated and thinned over the decade we found no direct relationship between retreat acceleration and external forcing applicable to the entire region in regard to geometry we found that following retreat 1 glaciers with grounded termini experienced more pronounced changes in dynamics than those with floating termini and 2 thinning rates declined more quickly for glaciers with steeper slopes overall glacier geometry should influence outlet glacier dynamics via stress redistribution following perturbations at the front but our data indicate that the relative importance of geometry as a control of glacier behavior is highly variable throughout west greenland

interaction between deformation and charnockite emplacement in the bunger hills east antarctica abstract the bunger hills east antarctica have been metamorphosed at granulite facies and variably deformed by three deformation phases as well as having been intruded by a number of dyke generations and large pyroxene bearing intrusive bodies the multiple intrusion and the conspicuous pattern of the major structural features around the intrusive bodies allows a detailed relative timing history of the events to be established a layered and a massive gneiss series can be distinguished the layered gneiss series consists of metapelitic psammitic intermediate and meta basic gneisses that are interlayered on all scales the massive gneiss series is in part identical in bulk composition and has been interpreted as an intrusive equivalent to the layered series the first deformation phase d1 caused recumbent folding of both series and boudinage of an early set of mafic dykes the metamorphic temperature peak was reached during this deformation a northern charnockite body fishtail bay body was intruded during d1 and was partially deformed by it d2 occurred subsequently and is the major shortening deformation the transition from d1 to d2 is marked by intrusion of mafic dykes that are metamorphosed and folded but not boudinaged the regional orientation of the f2 axes is extremely variable over the extent of the bunger hills the f2 axes appear to wrap around the northern charnockite body this may be explained by a competency contrast between the rigid charnockite and the surrounding partially molten gneiss sequence during deformation in a dextral shear environment a large southern intrusive body lake figurnoe charnockite was intruded towards the end of this deformation phase this body is much coarser grained is undeformed and has a contact metamorphic halo around it the metamorphic pressure peak was reached at this time strong asymmetric d3 doming and late brittle fracturing occurred during subsequent cooling of the bunger hills

influence of sea ice decline atmospheric warming and glacier width on marine terminating outlet glacier behavior in northwest greenland at seasonal to interannual timescales 1 discharge from marine terminating outlet glaciers represents a key component of the greenland ice sheet mass budget and observations suggest that mass loss from northwest greenland has recently accelerated despite this the factors controlling outlet glacier dynamics within this region have been comparatively poorly studied here we use remotely sensed data to investigate the influence of atmospheric oceanic and glacier specific controls on the frontal position of alison glacier ag northwest greenland and nine surrounding outlet glaciers ag retreated by 9 7 km between 2001 and 2005 following at least 25 years of minimal change results suggest that sea ice and air temperatures influence glacier frontal position at seasonal and interannual timescales however the response of individual outlet glaciers to forcing was strongly modified by factors specific to each glacier specifically variations in fjord width and terminus type overall our results underscore the need to consider these factors in order to interpret recent rapid changes and predict the dynamic response of marine terminating outlet glaciers to atmospheric and oceanic forcing

a model for tidewater glacier undercutting by submarine melting dynamic change at the marine terminating margins of the greenland ice sheet may be initiated by the ocean particularly where subglacial runoff drives vigorous ice marginal plumes and rapid submarine melting here we model submarine melt driven undercutting of tidewater glacier termini simulating a process which is key to understanding ice ocean coupling where runoff emerges from broad subglacial channels we find that undercutting has only a weak impact on local submarine melt rate but increases total ablation by submarine melting due to the larger submerged ice surface area thus the impact of melting is determined not only by the melt rate magnitude but also by the slope of the ice ocean interface we suggest that the most severe undercutting occurs at the maximum height in the fjord reached by the plume likely promoting calving of ice above it remains unclear however whether undercutting proceeds sufficiently rapidly to influence calving at greenland s fastest flowing glaciers