toward a boot strap hypothesis of plate tectonics feedbacks between plates the asthenosphere and the wavelength of mantle convection abstract the solid earth system is characterized by plate tectonics a low viscosity zone beneath plates the asthenosphere and long wavelength flow in the convecting mantle we use suites of numerical experiments to show 1 how long wavelength flow and the operation of plate tectonics can generate and maintain an asthenosphere and 2 how an asthenosphere can maintain long wavelength flow and plate tectonics plate subduction generates a sub adiabatic temperature gradient in the mantle which together with temperature dependent viscosity leads to a viscosity increase from the upper to the lower mantle this allows mantle flow to channelize in a low viscosity region beneath plates an asthenosphere forms dynamically flow channelization in turn stabilizes long wavelength convection the degree of dynamic viscosity variations from the upper to the lower mantle increases with the wavelength of convection and drops toward zero if the system transitions from plate tectonics to a single plate planet the plate margin strength needed to initiate that transition increases for long wavelength cells long wavelength flow allows plate tectonics to exist over a wider range of plate margin strength the coupled feedbacks allow for a linked causality between plates the asthenosphere and the wavelength of mantle flow with none being more fundamental than the others and the existence of each depending on the others under this hypothesis the asthenosphere is defined by an active process plate tectonics which maintains it and is maintained by it and plate tectonics is part of an emergent self sustaining flow system that bootstraps itself into existence

isothermal isobaric mineral equilibria in braunite rhodonite johannsenite calcite bearing assemblages from northern apennine metacherts italy abstract johannsenite coexisting with rhodonite mol casio 3 up to 24 25 and calcite mol mnco 3 up to 4 occurs in veins and fracture fillings in braunite quartz hematite deposits within metacherts at gambatesa mine val graveglia italy primary sedimentary mn oxide and hydroxide ores re equilibrated to braunite quartz layers and developed mn silicate and carbonate assemblages during folding and ca co 2 metasomatism under prehnite pumpellyite facies metamorphic conditions the observed braunite rhodonite johannsenite and calcite bearing assemblages equilibrated at constant p and t as a result of progressive infiltration of ca rich co 2 bearing relatively reducing hydrous fluids the observed paragenetic equilibria are discussed in a o 2 co 2 isothermal isobaric diagram

greenland flow variability from ice sheet wide velocity mapping using radarsat synthetic aperture radar data we have mapped the flow velocity over much of the greenland ice sheet for the winters of 2000 01 and 2005 06 these maps provide a detailed view of the ice sheet flow including that of the hundreds of glaciers draining the interior the focused patterns of flow at the coast suggest a strong influence of bedrock topography differences between our two maps confirm numerous early observations of accelerated outlet glacier flow as well as revealing previously unrecognized changes the overall pattern is one of speed up accompanied by terminus retreat but there are also several instances of surge behavior and a few cases of glacier slowdown comprehensive mappings such as these at regular intervals provide an important new observational capability for understanding ice sheet variability

fracture propagation to the base of the greenland ice sheet during supraglacial lake drainage surface meltwater that reaches the base of an ice sheet creates a mechanism for the rapid response of ice flow to climate change the process whereby such a pathway is created through thick cold ice has not however been previously observed we describe the rapid 2 hours drainage of a large supraglacial lake down 980 meters through to the bed of the greenland ice sheet initiated by water driven fracture propagation evolving into moulin flow drainage coincided with increased seismicity transient acceleration ice sheet uplift and horizontal displacement subsidence and deceleration occurred over the subsequent 24 hours the short lived dynamic response suggests that an efficient drainage system dispersed the meltwater subglacially the integrated effect of multiple lake drainages could explain the observed net regional summer ice speedup

persistent flow acceleration within the interior of the greenland ice sheet we present surface velocity measurements from a high elevation site located 140 km from the western margin of the greenland ice sheet and 50 km into its accumulation area annual velocity increased each year from 51 78 0 01 m yr 1 in 2009 to 52 92 0 01 m yr 1 in 2012 a net increase of 2 2 these data also reveal a strong seasonal velocity cycle of up to 8 1 above the winter mean driven by seasonal melt and supraglacial lake drainage sole et al 2013 recently argued that ice motion in the ablation area is mediated by reduced winter flow following the development of efficient subglacial drainage during warmer faster summers our data extend this analysis and reveal a year on year increase in annual velocity above the equilibrium line altitude where despite surface melt increasing it is still sufficiently low to hinder the development of efficient drainage under thick ice

geological consequences of super sized earths 1 the discovery of terrestrial scale extrasolar planets and their calculated abundance in the galaxy has prompted speculation on their surface conditions and thermal structure both are dependent on the tectonic regime of a planet which is itself a function of the balance between driving forces and the resistive strength of the lithosphere here we use mantle convection simulations to show that simply increasing planetary radius acts to decrease the ratio of driving to resisting stresses and thus super sized earths are likely to be in an episodic or stagnant lid regime this effect is robust when associated increases in gravity are included as the more dominant effect is increased fault strength rather than greater buoyancy forces the thermo tectonic evolution of large terrestrial planets is more complex than often assumed and this has implications for the surface and conditions habitability of such worlds

a preliminary investigation of the influence of basal and surface topography on supraglacial lake distribution near jakobshavn isbrae western greenland the spatial distribution of supraglacial lakes has been hypothesized to be dominantly controlled by the component of surface roughness influenced by basal topography basal topography and surface roughness profiles within the jakobshavn isbrae drainage basin in western greenland acquired from an ice penetrating radar echo sounder were analysed through fourier and wavelet decompositions spectral analyses of basal to surface transfer under a range of ice thickness were compared with spatial distribution of lakes mapped from high resolution landsat imagery fourier analysis identifies dominant signals in both the basal and surface profiles to be ranging between wavelengths of 1 25 12 5 km the strongest peaks of transfer of basal signals to the surface were identified at wavelengths 11 5 km wavelet analysis identifies these peaks with thicker 1200 1400 m and thinner 500 700 m ice respectively and also identifies surface frequencies not present in the basal signal indicating some influence from other factors spatial autocorrelation analysis of supraglacial lake distribution indentifies high correlations at 1 9 5 6 11 24 and 30 km lags lags at 5 6 and 11 km correspond to dominant frequencies present in the basal and surface profiles over thinner and thicker ice respectively these 5 and 11 km frequency components present in the basal topography surface roughness and lake distribution are within the theoretical transfer window of basal to surface transfer as a function of ice thickness the transect analysed in this study does not contain wavelengths in the surface structure that are less than the ice thickness even though there are relatively higher frequencies present in the surface profile between 0 and 15 km that have less power in the basal profile copyright 2011 john wiley sons ltd

characterizing englacial drainage in the ablation zone of the greenland ice sheet rapid local drainage of surface meltwater to the base of the greenland ice sheet is thought to result in surface velocity variations as far inland as the equilibrium zone zwally and others 2002 ice penetrating radar surveys throughout this region allow us to characterize englacial drainage features that appear as vertically stacked diffraction hyperbolae in common offset profiles these data are used with a radar simulation model which allows for variations in geometry penetration depth and infill material to understand the characteristics of these hyperbolae and the likelihood that they are produced by moulins we find only a moderate correlation between the locations of these possible moulins and supraglacial lakes indicating that many lakes drain over the surface of the ice sheet or do not contain sufficient water to reach the bed through moulin formation we find a strong correlation between moulin location in the ablation region and elevated along flow tension due to flow over rough bedrock which generates surface crevassing and provides an entry point for meltwater although theory suggests that moulins may form anywhere on the ice sheet given sufficient meltwater input our data suggest that they are far more common in the ablation zone than near or inland from the equilibrium line

greenland supraglacial lake drainages triggered by hydrologically induced basal slip a dense network of gps observations shows that rapid lake drainage events on the western greenland ice sheet are preceded by period of ice sheet uplift and or enhanced basal slip surface lakes are common during the warm season on the greenland ice sheet especially in middle to lower elevations these lakes can drain rapidly often preceded by a hydro fracturing event the mechanism behind the hydro fracturing and thus the connection between the lakes and the broader greenland hydrological system has been unclear now laura stevens and colleagues use a dense network of gps observations to show that rapid lake drainage events at their study site in western greenland were preceded by uplift events likely from water entering the basal system from neighboring moulins vertical conduits connecting the surface and bedrock the uplift created sufficient stress in the ice sheet to open the hydro fractures thus allowing the rapid lake drainage although speculative the authors suggest that the comparative lack of crevassing in upper elevations might limit the geographical spread of the uplift fracturing mechanism water driven fracture propagation beneath supraglacial lakes rapidly transports large volumes of surface meltwater to the base of the greenland ice sheet1 these drainage events drive transient ice sheet acceleration1 2 3 and establish conduits for additional surface to bed meltwater transport for the remainder of the melt season1 4 5 6 although it is well established that cracks must remain water filled to propagate to the bed7 8 9 the precise mechanisms that initiate hydro fracture events beneath lakes are unknown here we show that for a lake on the western greenland ice sheet drainage events are preceded by a 6 12 hour period of ice sheet uplift and or enhanced basal slip our observations from a dense global positioning system gps network allow us to determine the distribution of meltwater at the ice sheet bed before during and after three rapid drainages in 2011 2013 each of which generates tensile stresses that promote hydro fracture beneath the lake we hypothesize that these precursors are associated with the introduction of meltwater to the bed through neighbouring moulin systems vertical conduits connecting the surface and base of the ice sheet our results imply that as lakes form in less crevassed interior regions of the ice sheet10 11 12 13 14 where water at the bed is currently less pervasive5 14 15 16 the creation of new surface to bed conduits caused by lake draining hydro fractures may be limited

modeling supraglacial water routing and lake filling on the greenland ice sheet 1 during more intense melt years supraglacial lakes on the greenland ice sheet are found to fill more rapidly and to drain earlier in the melt season they also develop at higher latitudes and elevations since the rapid drainage of lakes has been shown to enhance basal sliding and since the lake volume can play an important role in determining if and when it drains understanding and modeling the processes which control lake filling rates forms a key development in successfully modeling the possible impacts of lake drainage events on ice motion we have developed a surface water routing and lake filling model and applied it to a 100 km2area of the paakitsoq region west greenland the model takes the time series of calculated runoff melt minus refreezing over the area and calculates flow paths and water velocities over the snow ice covered surface routing the water into topographic depressions which can fill to form lakes runoff is calculated from a distributed surface energy balance model coupled to a subsurface model which calculates changes in temperature density and water content in the snow firn and upper ice layers and hence refreezing and therefore net runoff the model is calibrated against field measurements of a filling lake in our study area during june 2011 and can be used to calculate the filling rate of the instrumented lake with a high degree of accuracy the filling rate of the instrumented modeled lake depends on melt and routing from the immediate lake catchment and from overflowing lakes in upstream catchments

theoretical contemporary observational and palaeo perspectives on ice sheet hydrology processes and products publisher elsevier articletitle theoretical contemporary observational and palaeo perspectives on ice sheet hydrology processes and products journaltitle earth science reviews articlelink http dx doi org 10 1016 j earscirev 2016 01 010 content type article copyright copyright 2016 elsevier b v all rights reserved

plate tectonics on super earths equally or more likely than on earth abstract the discovery of extra solar super earths has prompted interest in their possible mantle dynamics and evolution and in whether their lithospheres are most likely to be undergoing active plate tectonics like on earth or be stagnant lids like on mars and venus the origin of plate tectonics is poorly understood for earth likely involving a complex interplay of rheological compositional melting and thermal effects which makes it challenging to make reliable predictions for other planets nevertheless as a starting point it is common to parameterize the complex processes involved as a simple yield stress that is either constant or has a byerlee s law dependence on pressure because the simplifying assumptions made in developing analytical scalings may not be valid over all parameter ranges numerical simulations are needed one numerical study on super earths finds that plate tectonics is less likely on a larger planet o neill and lenardic 2007 in apparent contradiction of an analytical scaling study valencia et al 2007 to try and understand this we here present new calculations of yielding induced plate tectonics as a function of planet size focusing on the idealized end members of internal heating or basal heating as well as different strength profiles and compared to analytical scalings in the present study we model super earths as simple scaled up versions of earth i e assuming constant physical properties keeping the ratio of core mantle radii constant and applying the same temperature difference between top bottom boundaries and the same internal heating rate effects that originate outside of the planet such as tidal forces meteor impacts and intense surface heating from a nearby star are not considered we find that for internally heated convection plate tectonics is equally likely for terrestrial planets of any size whereas for basally heated convection plate tectonics becomes more likely with increasing planet size this is indicated both by analytical scalings and the presented numerical results which agree with each other when scalings are adjusted to account for increasing mean density with increasing planet size plate tectonics becomes more likely with increasing planet size for all scenarios the influence of the pressure variation of viscosity thermal expansivity and conductivity may however act in the opposite sense and needs to be determined in future studies at least in the simplest case factors other than planet size such as the presence of surface water are likely most important for determining the presence or absence of plate tectonics

seasonal speedup of the greenland ice sheet linked to routing of surface water we use interferometric synthetic aperture radar observations recorded in a land terminating sector of western greenland to characterise the ice sheet surface hydrology and to quantify spatial variations in the seasonality of ice sheet flow our data reveal a non uniform pattern of late summer ice speedup that in places extends over 100 km inland we show that the degree of late summer speedup is positively correlated with modelled runoff within the 10 glacier catchments of our survey and that the pattern of late summer speedup follows that of water routed at the ice sheet surface in late summer ice within the largest catchment flows on average 48 faster than during winter whereas changes in smaller catchments are less pronounced our observations show that the routing of seasonal runoff at the ice sheet surface plays an important role in shaping the magnitude and extent of seasonal ice sheet speedup

modeling subglacial water routing at paakitsoq w greenland 1 the functioning of the greenland ice sheet s subglacial drainage system and its effect on ice dynamics have been inferred largely from hypothetical hydrology dynamics models and from analysis of satellite data and in situ gps measurements despite this there is still uncertainty about how the surface hydrology interacts with the subglacial drainage system and affects basal water pressures and ice flow especially over annual time scales to address this we developed a high spatial 100 m and temporal 1 h resolution distributed physically based subglacial hydrological model and applied it to the paakitsoq region western greenland the model is driven with moulin input hydrographs calculated by a surface routing and lake filling draining model forced ultimately with distributed hourly runoff calculated by a surface mass balance model key outputs from the model are spatially and temporally varying subglacial water pressures and proglacial stream hydrographs early in the melt season short spikes in water pressure lasting less than a day occur as a result of lake drainage events during midsummer there are sustained periods of high water pressure lasting days to weeks even at times when the subglacial system is inferred to be predominantly efficient later in the summer large diurnal fluctuations in water pressure occur with peaks regularly exceeding ice overburden pressure superimposed on a gradually declining trend these phenomena support the results of previous hypothetical modeling efforts and inferences drawn from gps measurements

granite and gabbrodiorite associated skarn deposits of nw iran abstract field and laboratory studies show that there are two types of skarn deposits in nw iran granite associated type i and gabbrodiorite associated type ii granite associated deposits are accompanied by cu and fe mineralisation whereas mn and fe are the main ore metals in gabbrodiorite associated skarn deposits there are some differences in the mineralogy of these skarn deposits bixbyite piemontite and cr bearing garnet are found only in gabbrodiorite associated skarns whereas idocrase occurs only in granite associated deposits type ii skarns show exoskarn features whereas some type i skarns have developed endoskarn as well evidence of boiling of hydrothermal fluid can be seen in both types and seems to be a common mechanism of mineral deposition gabbrodiorite associated skarns show higher f o 2 than granite associated deposits based on mineralogical and textural evidence mineralisation in both groups has started from about 550 c early formed anhydrous minerals have begun to be replaced by hydrous minerals from about 400 c it seems that due to low fluid content in the gabbrodioritic magma heated meteoritic water in the surrounding volcanoclastic and tuffaceous rocks was the main source of hydrothermal solution in the gabbrodiorite associated skarn system

envisat derived elevation changes of the greenland ice sheet and a comparison with icesat results in the accumulation area abstract we show for the first time over the greenland ice sheet that an along track method for deriving rates of elevation change can successfully be applied to envisat radar altimetry data 2002 2010 the results provide improved resolution and coverage compared to previous results obtained from cross over methods also we find that temporal changes in the elevation change rate can be derived from envisat data and show clear examples of this by generating five year running means for selected areas of the greenland ice sheet for a period between 2003 and 2009 the elevation of the ice sheets was measured by both the laser altimeter on board icesat and the radar altimeter on board envisat we compare rates of elevation change derived from icesat and envisat for this time span in which both sensors were operating we focus on the area above the equilibrium line altitude in order to specifically derive information on snow parameters a comparison of the elevation changes observed by the two sensors shows a complex pattern which can be explained regionally by model output describing the changes in both firn air content and accumulation rates

convection scaling and subduction on earth and super earths super earths are the smallest class of discovered extra solar planets owing to their relatively small mass some might resemble earth and perhaps be habitable because of the connection to habitability through thermal evolution we investigate the tectonic regime of massive terrestrial planets two independent studies o neill c lenardic a oct 2007 geological consequences of super sized earths grl 34 l19204 valencia d o connell r j sasselov d d nov 2007a inevitability of plate tectonics on super earths apjl 670 l45 l48 have reached opposing conclusions about the likelihood of plate tectonics here we offer possible reasons for the discordant findings and address three key aspects of sustaining plate tectonics deformation on faults negative buoyancy and energy dissipation during subduction we show that in general the ratio of driving force to plate resistance increases with planetary mass this is a consequence of increasing convective stresses thinning plates and similar plate structure we conclude that even though the strength of dry and wet of faults increases for massive terrestrial planets the convective stress increases even more allowing deformation to take place also despite shorter timescales for plate cooling rocky super earths achieve negative buoyancy at subduction zones finally by investigating the effects of energy dissipation during subduction we find that massive terrestrial planets dissipate less energy during subduction and hence provide a positive feedback to sustain active lid tectonics in conclusion rocky super earths have more favorable conditions than earth for the subduction of plates and hence for sustaining plate tectonics

seasonal evolution of supra glacial lakes on the greenland ice sheet abstract a survey of supra glacial lakes on the western margin of the greenland ice sheet reveals a seasonally driven hydrological system culminating in widespread lake drainage in late summer we used satellite imagery to study the evolution of 292 lakes across two sites totalling 22 000 km 2 in area during 2001 the lakes combined area increased to 75 5 km 2 by the beginning of july over the following 25 days an area totalling 36 3 5 km 2 drained from 216 lakes at one study site we used meteorological data and a positive degree day model to calculate the volume of water generated by melting in the lake catchments based on this estimate the mean depth of filling lakes surveyed rose from 1 5 0 7 m on 7th july to 3 9 1 1 m on 1st august in agreement with a value for one lake of 4 4 0 9 m we have derived from airborne altimetry during this 25 day period we estimate that 38 18 10 7 m 3 of water drained from the surface at this site and that there was an average water flux of 1 3 0 3 m 3 s 1 passing through each lake that drained completely

subglacial water drainage storage and piracy beneath the greenland ice sheet meltwater drainage across the surface of the greenland ice sheet gris is well constrained by measurements and modeling yet despite its critical role knowledge of its transit through the subglac

seasonal speedup along the western flank of the greenland ice sheet it has been widely hypothesized that a warmer climate in greenland would increase the volume of lubricating surface meltwater reaching the ice bedrock interface accelerating ice flow and increasing mass loss we have assembled a data set that provides a synoptic scale view spanning ice sheet to outlet glacier flow with which to evaluate this hypothesis on the ice sheet these data reveal summer speedups 50 to 100 consistent with but somewhat larger than earlier observations the relative speedup of outlet glaciers however is far smaller 15 furthermore the dominant seasonal influence on jakobshavn isbrae s flow is the calving front s annual advance and retreat with other effects producing outlet glacier speedups an order of magnitude larger seasonal melt s influence on ice flow is likely confined to those regions dominated by ice sheet flow

an improved mass budget for the greenland ice sheet extensive ice thickness surveys by nasa s operation icebridge enable over a decade of ice discharge measurements at high precision for the majority of greenland s marine terminating outlet glaciers prompting a reassessment of the temporal and spatial distribution of glacier change annualmeasurements for 178 outlet glaciers reveal that despite widespread acceleration only 15 glaciers accounted for 77 of the 739 29 gt of ice lost due to acceleration since 2000 and four accounted for 50 among the top sources of loss are several glaciers that have received little scientific attention the relative contribution of ice discharge to total loss decreased from 58 before 2005 to 32 between 2009 and 2012 as such 84 of the increase in mass loss after 2009 was due to increased surface runoff these observations support recentmodel projections that surface mass balance rather than ice dynamics will dominate the ice sheet s contribution to 21st century sea level rise

evolution of supra glacial lakes across the greenland ice sheet abstract we used 268 cloud free moderate resolution imaging spectroradiometer modis images from 2003 and 2005 2007 to study the seasonal evolution of supra glacial lakes in three different regions of the greenland ice sheet lake area estimates were obtained by developing an automated classification method for their identification based on 250 m resolution modis surface reflectance observations widespread supra glacial lake formation and drainage is observed across the ice sheet with a 2 3 week delay in the evolution of total supra glacial lake area in the northern areas compared to the south west the onset of lake growth varies by up to one month inter annually and lakes form and drain at progressively higher altitudes during the melt season a positive correlation was found between the annual peak in total lake area and modelled annual runoff high runoff and lake extent years are generally characterised by low accumulation and high melt season temperatures and vice versa our results indicate that in a future warmer climate meehl g a stocker t f collins w d friedlingstein p gaye a t gregory j m kitoh a knutti r murphy j m noda a raper s c b watterson i g weaver a j zhao z c 2007 global climate projections in climate change 2007 the physical science basis contribution of working group i to the fourth assessment report of the intergovernmental panel on climate change solomon s d qin m manning z chen m marquis k b averyt m tignor h l miller eds cambridge university press cambridge united kingdom and new york ny usa greenland supra glacial lakes can be expected to form at higher altitudes and over a longer time period than is presently the case expanding the area and time period over which connections between the ice sheet surface and base may be established das s joughin m behn m howat i king m lizarralde d bhatia m 2008 fracture propagation to the base of the greenland ice sheet during supra glacial lake drainage science 5877 778 781 with potential consequences for ice sheet discharge zwally h j abdalati w herring t larson k saba j steffen k 2002 surface melt induced acceleration of greenland ice sheet flow science 297 218 221

metallogenic model of the trepca pb zn ag skarn deposit kosovo evidence from fluid inclusions rare earth elements and stable isotope data the trepca pb zn ag skarn deposit 29 mt of ore at 3 45 pb 2 30 zn and 80 g t ag is located in the kopaonik block of the western vardar zone kosovo the mineralization hosted by recrystallized limestone of upper triassic age was structurally and lithologically controlled ore deposition is spatially and temporally related with the postcollisional magmatism of oligocene age 23 26 ma the deposit was formed during two distinct mineralization stages an early prograde closed system and a later retrograde open system stage the prograde mineralization consisting mainly of pyroxenes hd54 100jo0 45di0 45 resulted from the interac tion of magmatic fluids associated with oligocene 23 26 ma postcollisional magmatism whereas there is no direct contact between magmatic rocks and the mineralization the deposit is classified as a distal pb zn ag skarn abundant pyroxene reflects low oxygen fugacity 10 31 bar and anhydrous environment fluid inclu sion data and mineral assemblage limit the prograde stage within a temperature range between 390 and 475 c formation pressure is estimated below 900 bars isotopic composition of aqueous fluid inclusions hosted by hedenbergite d 108 to 130 18o 7 5 8 0 mn enriched mineralogy and high ree content of the host carbonates at the contact with the skarn mineralization suggest that a magmatic fluid was modified during its infiltration through the country rocks the retrograde mineral assemblage comprises ilvaite magnetite arsenopyrite pyrrhotite marcasite pyrite quartz and various carbonates increases in oxygen and sulfur fugacities as well as a hydrous character of min eralization require an open system model the opening of the system is related to phreatomagmatic explosion and formation of the breccia arsenopyrite geothermometer limits the retrograde stage within the temperature range between 350 and 380 c and sulfur fugacity between 10 8 8 and 10 7 2 bars the principal ore minerals galena sphalerite pyrite and minor chalcopyrite were deposited from a mod erately saline ca na chloride fluid at around 350 c according to the isotopic composition of fluid inclusions hosted by sphalerite d 55 to 74 18o 9 6 to 13 6 the fluid responsible for ore deposition was dominantly meteoric in origin the 34s values of the sulfides spanning between 5 5 and 10 point to a magmatic origin of sulfur ore deposition appears to have been largely contemporaneous with the retrograde stage of the skarn development postore stage accompanied the precipitation of significant amount of carbonates including the travertine deposits at the deposit surface mineralogical composition of travertine varies from calcite to siderite and all carbonates contain significant amounts of mn decreased formation temperature and depletion in the ree content point to an influence of ph neutralized cold ground water and dying magmatic system

manganiferous pyroxenes and pyroxenoids from three pb zn cu skarn deposits samples from the pb zn cu skarns of m ci villina italy valle del temperino italy and empire mine new mexico usa have been analysed for their pyroxenes and pyroxenoids the samples were collected immediately adjacent to the marble skarn replacement front all contain manganiferous pyroxenoids and manganeserich ca pyroxenes the pyroxenes from each deposit form distinct groups of compositions within the diopside hedenbergite johannsenite triangle with no apparent miscibility gap diopside contents usually are below 15 mole percent fibrous bustamite occurs as monomineralic zones in the empire and in the temperino deposit although rhodonite may be a primary phase in some samples from the empire mine it is commonly of secondary origin in the empire mine and in the civillina deposit its formation from manganiferous clinopyroxenes is either due to increasing mn activity in the hydrothermal skarn solution or to higher x co2 in the vapour phase when rhodonite is formed within clinopyroxenes as submicroscopic lamellae that eventually replace the whole host crystal resulting compositions lie in the miscibility gap between rhodonite and bustamite textural relations indicate the replacement reaction johannsenite co2 rhodonite calcite quartz equilibrium temperatures for this reaction have been calculated by using estimated thermochemical data for johannsenite giving a t eq 385 c for x co2 0 1 at p tot 1 kbar taking into consideration the reduced activity of mn in rhodonite and of ca in calcite both buffered by the johannsenite the temperature is increased for about 15 c at x co2 0 01 at lower temperatures where johannsenite is stable the x co2 is confined to values below 0 01 despite the mineralogical similarities of the three deposits differences in the development of the manganiferous skarns can be depicted

the energetics and convective vigor of mixed mode heating velocity scalings and implications for the tectonics of exoplanets the discovery of large terrestrial 1 earth mass me to 10 me extrasolar planets has prompted a debate as to the likelihood of plate tectonics on these planets canonical models assume classic basal heating scaling relationships remain valid for mixed heating systems with an appropriate internal temperature shift those scalings predict a rapid increase of convective velocities vrms with increasing rayleigh numbers ra and non dimensional heating rates q to test this we conduct a sweep of 3 d numerical parameter space for mixed heating convection in isoviscous spherical shells our results show that while vrms increases with increasing thermalrait does so at a slower rate than predicted by bottom heated scaling relationships further the vrms decreases asymptotically with increasing q these results show that independent of specific rheologic assumptions e g viscosity formulations water effects lithosphere yielding the differing energetics of mixed and basally heated systems can explain the discrepancy between different modeling groups high temperature or young planets with a large contribution from internal heating will operate in different scaling regimes compared to cooler temperature or older planets that may have a larger relative contribution from basal heating thus differences in predictions as to the likelihood of plate tectonics on exoplanets may well result from different models being more appropriate to different times in the thermal evolution of a terrestrial planet as opposed to different rheologic assumptions as has often been assumed

the pressure weakening effect in super earths consequences of a decrease in lower mantle viscosity on surface dynamics 1 the quest to identify habitable planets has raised interest in the surface dynamics of terrestrial bodies in this context super earths a new class of exoplanets have become of special interest in the past decade scalings to super earth sizes when compared to the earth suggest changes to convective stresses and mantle temperatures which can cause either an increase in surface mobility or in plate resistance mantle viscosity which depends on temperature stress and pressure plays a critical role in both cases new mineralogical assumptions suggest that the viscosity in super earths acts differently than in the earth and what had been assumed for super earths in planets larger than the earth pressure will become so high that pressure weakening and a decrease of viscosity in the lowermost mantle results we present a numerical convection study featuring this viscosity decrease and find that this leads to a reduction in surface mobility

hydrologic drainage of the greenland ice sheet a simple hydrologic drainage network for the greenland ice sheet is modelled from available digital elevation models dems of bedrock and surface topography and assumptions of hydrostatic water pressure uniform hydraulic conductivity and no conduit flow within the ice sheet as such it is a first order model best suited for broad scale hydrological assessment results identify 293 distinct hydrologic basins 185 117 000 km2 together with their realized wet and unrealized dry drainage patterns intersection with 1991 2000 polar mm5 pmm5 mesoscale climate model hindcasts of meltwater runoff suggest that these basins route varying amounts of water to the ice edge ranging from 0 to 16 km3 annually and totalling 242 km3 year for the entire ice sheet regionally speaking average annual volumetric meltwater production km3 year is highest in southwest and lowest in northeast greenland with greater hydrologic activity in western regions than in eastern regions for a given latitude the extent to which meltwater truly reaches the ice margin as modelled is difficult to test however the simulated flow outlet locations show qualitative agreement with the locations of 460 observed meltwater outlets proglacial lakes streams and rivers and sediment plumes into fjords mapped continuously along the ice sheet perimeter on average about 36 of the modelled drainage network was activated i e received water over the 1991 2000 study period remaining areas barring dynamic changes to ice surface topography would presumably activate if surface melt penetrates deeper into the ice sheet interior both new datasets are freely available for scientific use at the national snow and ice data center ftp sidads colorado edu pub datasets parca nsidc 0372 hydrologic outlets ftp sidads colorado edu pub datasets parca nsidc 0371 hydrologic sub basins copyright 2009 john wiley sons ltd

constraints on the lake volume required for hydro fracture through ice sheets 1 water filled cracks are an effective mechanism to drive hydro fractures through thick ice sheets crack geometry is therefore critical in assessing whether a supraglacial lake contains a sufficient volume of water to keep a crack water filled until it reaches the bed in this study we investigate fracture propagation using a linear elastic fracture mechanics model to calculate the dimensions of water filled cracks beneath supraglacial lakes we find that the cross sectional area of water filled cracks increases non linearly with ice sheet thickness using these results we place volumetric constraints on the amount of water necessary to drive cracks through 1 km of sub freezing ice for ice sheet regions under little tension lakes larger than 0 25 0 80 km in diameter contain sufficient water to rapidly drive hydro fractures through 1 1 5 km of subfreezing ice this represents 98 of the meltwater volume held in supraglacial lakes in the central western margin of the greenland ice sheet

surface melt induced acceleration of greenland ice sheet flow ice flow at a location in the equilibrium zone of the west central greenland ice sheet accelerates above the midwinter average rate during periods of summer melting the near coincidence of the ice acceleration with the duration of surface melting followed by deceleration after the melting ceases indicates that glacial sliding is enhanced by rapid migration of surface meltwater to the ice bedrock interface interannual variations in the ice acceleration are correlated with variations in the intensity of the surface melting with larger increases accompanying higher amounts of summer melting the indicated coupling between surface melting and ice sheet flow provides a mechanism for rapid large scale dynamic responses of ice sheets to climate warming

geoframework coupling multiple models of mantle convection within a computational framework solver coupling can extend the capability of existing modeling software and provide a new venue to address previously intractable problems a software package has been developed to couple geophysical solvers demonstrating a method to accurately and efficiently solve multiscale geophysical problems with reengineered software using a computational framework pyre pyre is a modeling framework capable of handling all aspects of the specification and launching of numerical investigations we restructured and ported citcoms a finite element code for mantle convection into the pyre framework two citcoms solvers are coupled to investigate the interaction of a plume at high resolution with global mantle flow at low resolution a comparison of the coupled models with parameterized models demonstrates the accuracy and efficiency of the coupled models and illustrates the limitations and utility of parameterized models

flow variability in the scandinavian ice sheet modelling the coupling between ice sheet flow and hydrology abstract there is increasing geologic evidence for periodic flow variability within large ice sheets manifested as spatially and temporally variable areas of fast ice flow and resulting in the very complex patterns of lineations observed in formerly glaciated areas however many ice sheet models do not replicate this behaviour a possible reason for this is that such models do not include a detailed treatment of basal hydrology changes in the character of sub glacial drainage systems are believed to cause surges in valley glaciers recent ice sheet models which have included basal hydrology or at least a link between basal velocity and the presence of water at the bed often show flow variability however these models have typically assumed a deformable bed or have made no assumptions about the nature of the bed whilst these assumptions seem applicable to areas close to the former margins of quaternary ice sheets they are less applicable to interior areas these areas typically show thin or scanty till cover over eroded bedrock and the presence of eskers which are indicative of drainage in sub glacial tunnels we have developed a two dimensional time dependent ice sheet model that includes hard bed basal hydrology this allows calculation of sub glacial water pressures and the use of a water pressure dependent sliding law to calculate ice sheet velocities when used to simulate the weichselian scandinavian ice sheet with late quaternary climate and sea level as forcing functions this model develops localised areas of fast flowing ice which vary in extent and in distance of penetration into the interior of the ice sheet both spatially and temporally the behaviour of these lobes depends crucially on the influence of the evolving ice sheet topography on the routing of subglacial water flow due to the resulting variations in the subglacial hydraulic potential which drive the water flow bedrock topography also has some influence but fast flow areas are not confined to obvious topographic troughs a relatively thin ice sheet with low surface slopes is produced in areas experiencing fast ice flow generally two to four separate areas of fast flow can be recognised and these are similar in size and shape to the lobes identified in some geologically based reconstructions of the scandinavian ice sheet within the fast flowing areas sub glacial drainage is typically in a cavity based system however tunnel based drainage is predicted to have extended up to 150 km from the ice sheet margin particularly during deglaciation because of the changes in ice sheet topography associated with fast flow and the resulting changes in the pattern of sub glacial water flow the model predicts that these fast flowing lobes interacted in complex ways and exhibited quasi periodic switching between fast and slow flow

high resolution mapping of supra glacial drainage pathways reveals link between micro channel drainage density surface roughness and surface reflectance this paper reports on the use of a small unmanned aerial vehicle suav carrying a standard compact camera to construct a high resolution orthomosaic om and digital elevation model dem over the lower reaches of the glacier midtre lovenbreen svalbard structure from motion sfm techniques were used to build the om and dem and together these reveal insights into the nature of supra glacial drainage major meandering supra glacial drainage pathways show clear dynamism via meander cutoffs and abandoned channels in addition the imagery reveals a very extensive network of smaller channels that may well carry substantial amounts of water this network of channels is in part controlled by the structure of the glacier but in turn these channels have a significant impact on the ice surface roughness of the ice surface is higher where channels are most extensive in addition we find a relationship between channel density and surface reflectance such that greater channel density is associated with lower reflectance values given the role of surface reflectance and roughness in the energy balance of glaciers it is therefore apparent that extensive networks of small supra glacial channels across such glaciers have the potential to have an important impact on energy exchanges between the atmosphere and the ice surface copyright 2015 john wiley sons ltd

limits to future expansion of surface melt enhanced ice flow into the interior of western greenland moulins are important conduits for surface meltwater to reach the bed of the greenland ice sheet it has been proposed that in a warming climate newly formed moulins associated with the inland migration of supraglacial lakes could introduce surface melt to new regions of the bed introducing or enhancing sliding there by examining surface strain rates we found that the upper limit to where crevasses and therefore moulins are likely to form is 1600m this is also roughly the elevation above which lakes do not drain completely thus meltwater above this elevation will largely flow tens of kilometers through surface streams into existing moulins downstream furthermore results from a thermal ice sheet model indicate that the 1600m crevassing limit is well below the wet frozen basal transition 2000m together these data sets suggest that new supraglacial lakes will have a limited effect on the inland expansion of melt induced seasonal acceleration key points greenland ice sheet meltwater volumes are increasing notably at high elevations low strain rates limit the likelihood of moulin formation at high elevations high elevation meltwater will reach an already wet bed at lower elevations

seasonal variations in greenland ice sheet motion inland extent and behaviour at higher elevations abstract we present global positioning system observations that capture the full inland extent of ice motion variations in 2009 along a transect in the west greenland ice sheet margin in situ measurements of air temperature and surface ablation and satellite monitoring of ice surface albedo and supraglacial lake drainage are used to investigate hydrological controls on ice velocity changes we find a strong positive correlation between rates of annual ablation and changes in annual ice motion along the transect with sites nearest the ice sheet margin experiencing greater annual variations in ice motion 15 18 than those above 1000 m elevation 3 8 patterns in the timing and rate of meltwater delivery to the ice bed interface provide key controls on the magnitude of hydrologically forced velocity variations at each site in the lower ablation zone the overall contribution of variations in ice motion to annual flow rates is limited by evolution in the structure of the subglacial drainage system at sites in the upper ablation zone a shorter period of summer melting and delayed establishment of a hydraulic connection between the ice sheet surface and its bed limit the timeframe for velocity variations to occur our data suggest that land terminating sections of the greenland ice sheet will experience increased dynamic mass loss in a warmer climate as the behaviour that we observe in the lower ablation zone propagates further inland findings from this study provide a conceptual framework to understand the impact of hydrologically forced velocity variations on the future mass balance of land terminating sections of the greenland ice sheet

evolution of melt pond volume on the surface of the greenland ice sheet 1 the presence of surface meltwater on ice caps and ice sheets is an important glaciological and climatological characteristic we describe an algorithm for estimating the depth and hence volume of surface melt ponds using multispectral aster satellite imagery the method relies on reasonable assumptions about the albedo of the bottom surface of the ponds and the optical attenuation characteristics of the ponded meltwater we apply the technique to sequences of satellite imagery acquired over the western margin of the greenland ice sheet to derive changes in melt pond extent and volume during the period 2001 2004 results show large intra and interannual changes in ponded water volumes and large volumes of liquid water stored in extensive slush zones

a decadal investigation of supraglacial lakes in west greenland using a fully automatic detection and tracking algorithm abstract the sudden drainage of supraglacial lakes has been previously observed to initiate surface to bed hydrologic connections which are capable of enhancing basal sliding in regions of the greenland ice sheet where ice thickness approaches 1 km in this study we develop a robust algorithm which automatically detects and tracks individual supraglacial lakes using visible satellite imagery to document the evolution of a population of west greenland supraglacial lakes over ten consecutive melt seasons validation tests indicate that the algorithm is highly accurate 99 0 of supraglacial lakes can be detected and tracked and 96 3 of reported lakes are true supraglacial lakes with accurate lake properties such as lake area and timing of formation and drainage investigation of the interannual evolution of supraglacial lakes in the context of annual melt intensity reveals that during more intense melt years supraglacial lakes drain more frequently and earlier in the melt season additionally the lake population extends to higher elevations during more intense melt years exposing an increased inland area of the ice sheet to sudden lake drainage events these observations suggest that increased surface meltwater production due to climate change will enhance the spatial extent and temporal frequency of lake drainage events it is unclear whether this will ultimately increase or decrease the basal sliding sensitivity of interior regions of the greenland ice sheet

melt induced speed up of greenland ice sheet offset by efficient subglacial drainage fluctuations in surface melting are known to affect the speed of glaciers and ice sheets but their impact on the greenland ice sheet in a warming climate remains uncertain although some studies suggest that greater melting produces greater ice sheet acceleration others have identified a long term decrease in greenland s flow despite increased melting here we use satellite observations of ice motion recorded in a land terminating sector of southwest greenland to investigate the manner in which ice flow develops during years of markedly different melting although peak rates of ice speed up are positively correlated with the degree of melting mean summer flow rates are not because glacier slowdown occurs on average when a critical run off threshold of about 1 4 centimetres a day is exceeded in contrast to the first half of summer when flow is similar in all years speed up during the latter half is 62 16 per cent less in warmer years consequently in warmer years the period of fast ice flow is three times shorter and overall summer ice flow is slower this behaviour is at odds with that expected from basal lubrication alone instead it mirrors that of mountain glaciers where melt induced acceleration of flow ceases during years of high melting once subglacial drainage becomes efficient a model of ice sheet flow that captures switching between cavity and channel drainage modes is consistent with the run off threshold fast flow periods and later summer speeds we have observed simulations of the greenland ice sheet flow under climate warming scenarios should account for the dynamic evolution of subglacial drainage a simple model of basal lubrication alone misses key aspects of the ice sheet s response to climate warming

supraglacial melt channel networks in the jakobshavn isbrae region during the 2007 melt season supraglacial channels are an important mechanism for surface water transport over the ablation zone of western greenland the first assessment of the spatio temporal distribution of surface melt channels and their relationship to supraglacial lakes over the jakobshavn isbrae region of western greenland was analysed using landsat enhanced thematic mapper plus panchromatic images during the 2007 melt season a total of 1188 melt channels were delineated and show an increase in the number of melt channels throughout the season reaching a peak on 9 august water filled melt channels advanced to a maximum elevation of 1647 m on 9 august and attained a minimum average slope of 0 009 on 8 july the ablation zone demonstrates two hydrologic modes where crevasse and moulin terminating channels dominate at elevations 800 m development of higher order networks is interrupted by flow divergence due to partitioning of melt water into vertical infiltration through moulins and crevasse fields prevalent at lower elevations tributary and connector networks between 800 and 1200 m in elevation are correlated with fewer lake occurrences lower surface velocities 50 m a 1 and ice flow dominated by internal deformation over basal sliding high order channels are associated with lake basins that exceed melt water storage capacity evolution of channel networks is coupled to changes in melt water production runoff and ice dynamics with implication for the englacial and subglacial environments 2013 the authors hydrological processes published by john wiley sons ltd

modelling channelized surface drainage of supraglacial lakes supraglacial lakes can drain to the bed of ice sheets affecting ice dynamics or over their surface relocating surface water focusing on surface drainage we first discuss observations of lake drainage in particular for the first time lakes are observed to drain 70 km across the nivlisen ice shelf east antarctica inspired by these observations we develop a model of lake drainage through a channel that incises into an ice sheet surface by frictional heat dissipated in the flow modelled lake drainage can be stable or unstable during stable drainage the rate of lake level drawdown exceeds the rate of channel incision so discharge from the lake decreases with time this can prevent the lake from emptying completely during unstable drainage discharge grows unstably with time and always empties the lake model lakes are more prone to drain unstably when the initial lake area the lake input and the channel slope are larger these parameters will vary during atmospheric warming induced ablation area expansion hence the mechanisms revealed by our analysis can influence the dynamic response of ice sheets to warming through their impact on surface water routing and storage

an increase in crevasse extent west greenland hydrologic implications 1 we compare high resolution 1985 and 2009 imagery to assess changes in crevasse extent in the sermeq avannarleq ablation zone west greenland the area occupied by crevasses 2 m wide significantly increased 13 4 over the 24 year period this increase consists of an expansion of existing crevasse fields and is accompanied by widespread changes in crevasse orientation up to 45 we suggest that a combination of ice sheet thinning and steepening are responsible for the increase in crevasse extent we examine the potential impact of this change on the hydrology of the ice sheet we provide a first order demonstration that moulin type drainage is more efficient in transferring meltwater fluctuations to the subglacial system than crevasse type drainage as enhanced basal sliding is associated with meltwater pulses an increase in crevasse extent can therefore be expected to result in a net decrease in basal sliding sensitivity an increase in crevasse extent may also accelerate cryo hydrologic warming and enhance surface ablation

scaling laws for convection with temperature dependent viscosity and grain damage numerical experiments of convection with grain damage are used to develop scaling laws for convective heat flow mantle velocity and plate velocity across the stagnant lid and plate tectonic regimes three main cases are presented in order of increasing complexity a simple case wherein viscosity is only dependent on grainsize a case where viscosity depends on temperature and grainsize and finally a case where viscosity is temperature and grainsize sensitive and the grain growth or healing is also temperature sensitive in all cases convection with grain damage scales differently than newtonian convection due to the effects of grain damage for the fully realistic case numerical results show stagnant lid convection fully mobilized convection that resembles the temperature independent viscosity case and partially mobile or transitional convection depending on damage to healing ratio rayleigh number and the activation energies for viscosity and healing applying our scaling laws for the fully realistic case to earth and venus we demonstrate that increasing surface temperature dramatically decreases plate speed and heat flow essentially shutting down plate tectonics due to increased healing in lithospheric shear zones as proposed previously contrary to many previous studies the transitional regime between the stagnant lid and fully mobilized regimes is large and the transition from stagnant lid to mobile convection is gradual and continuous thus planets could exhibit a full range of surface mobility as opposed to the bimodal distribution of fully mobile lid planets and stagnant lid planets that is typically assumed

internal geometry and evolution of moulins storglaciaren sweden depart ment of physical geog raphy university of stoc kholm s 106 91 stockholm sweden abstract the initial conditions needed for formation of moulins are crevasses and a supply of melt water water pouring into a crevasse may fill it until it ove rflows at the lowest point which is normally near the margin however as the crev asse deepens it intersects englacial chann els through which the water can drain these chann els may be finger tip tributaries in a dendritic system such as that described by shreve 1972 and observ ed by raymond and harrison 1975 when the cre vasse closes heat in the melt water keeps the conne ction open and a moulin is formed the englacial channel enlarges rapidly by melt ing utilizing mechanical energy released by the descending water descents into moulins and mapping of stru ctures exposed at the surface after many years of melting demonstrate that the drainage channels leading down from the bottoms of the moulins have incli nations of 0 450 from the vertical these channels trend in the dire ction of the original crevasse but appear to be deeper than the expected depth of the crevasse they have not

direct observations of evolving subglacial drainage beneath the greenland ice sheet simultaneous observations of moulins and boreholes in western greenland show that water delivery to the base of the ice sheet via moulins affects short term ice velocity fluctuations but not late season ice velocity decelerations increased meltwater delivery to the base of the greenland ice sheet will increase the ice sheet velocity accelerating its inevitable rush towards the ocean and subsequent sea level rise or will it debate on this topic is at the forefront of cryospheric research but has been hampered by the lack of simultaneous observations of hydraulic head in moulins vertical shafts that deliver water to the base of the ice sheet and boreholes which monitor basal water pressure lauren andrews and colleagues now provide these observations from a small region in western greenland and reveal that water delivery by moulins into channelized basal flow is indeed linked to short term fluctuations in ice velocity late season decelerations in ice velocity however seem to be controlled by changes in unchannelized flow rather than any shifts in the moulin system seasonal acceleration of the greenland ice sheet is influenced by the dynamic response of the subglacial hydrologic system to variability in meltwater delivery to the bed1 2 via crevasses and moulins vertical conduits connecting supraglacial water to the bed of the ice sheet as the melt season progresses the subglacial hydrologic system drains supraglacial meltwater more efficiently1 2 3 4 decreasing basal water pressure4 and moderating the ice velocity response to surface melting1 2 however limited direct observations of subglacial water pressure4 5 6 7 mean that the spatiotemporal evolution of the subglacial hydrologic system remains poorly understood here we show that ice velocity is well correlated with moulin hydraulic head but is out of phase with that of nearby 0 3 2 kilometres away boreholes indicating that moulins connect to an efficient channelized component of the subglacial hydrologic system which exerts the primary control on diurnal and multi day changes in ice velocity our simultaneous measurements of moulin and borehole hydraulic head and ice velocity in the paakitsoq region of western greenland show that decreasing trends in ice velocity during the latter part of the melt season cannot be explained by changes in the ability of moulin connected channels to convey supraglacial melt instead these observations suggest that decreasing late season ice velocity may be caused by changes in connectivity in unchannelized regions of the subglacial hydrologic system understanding this spatiotemporal variability in subglacial pressures is increasingly important because melt season dynamics affect ice velocity beyond the conclusion of the melt season8 9 10

large and rapid melt induced velocity changes in the ablation zone of the greenland ice sheet continuous global positioning system observations reveal rapid and large ice velocity fluctuations in the western ablation zone of the greenland ice sheet within days ice velocity reacts to increased meltwater production and increases by a factor of 4 such a response is much stronger and much faster than previously reported over a longer period of 17 years annual ice velocities have decreased slightly which suggests that the englacial hydraulic system adjusts constantly to the variable meltwater input which results in a more or less constant ice flux over the years the positive feedback mechanism between melt rate and ice velocity appears to be a seasonal process that may have only a limited effect on the response of the ice sheet to climate warming over the next decades

persistent englacial drainage features in the greenland ice sheet 1 surface melting on the greenland ice sheet is common up to 1400 m elevation and in extreme melt years even higher water produced on the ice sheet surface collects in lakes and drains over the ice sheet surface via supraglacial streams and through the ice sheet via moulins water delivered to the base of the ice sheet can cause uplift and enhanced sliding locally here we use ice penetrating radar data to observe the effects of significant basal melting coincident with moulins and calculate how much basal melt occurred we find that more melting has occurred than can be explained by the release of potential energy from the drainage of surface meltwater during one melt season suggesting that these moulins are persistent for multiple years we find only a few persistent moulins in our study area that drain the equivalent of multiple lakes per year and likely remain active over several years our observations indicate that once established these persistent moulins might be capable of establishing well connected meltwater drainage pathways

supraglacial lake spatial structure in western greenland during the 2007 ablation season 1 increases in surface melt rates along the greenland ice sheet margin have accelerated meltwater production resulting in supraglacial lakes that can drain rapidly and enhance basal sliding in this study supraglacial lakes in western greenland are mapped from multitemporal landsat panchromatic imagery during the 2007 melt season the analysis models lake spatial structure and establishes sequencing of drainage from 6 june through 9 august 2007 results indicate high spatial clustering at lower elevations between 100 and 700 m with minimum lake to lake distances of 100 500 m and lake sizes 0 5 km2 at higher elevations 700 1200 m clustering decreases with minimum distances on the order of 900 1400 m and lake sizes between 0 5 and 1 km2 lakes at elevations above 1200 m exhibit less clustering and smaller areas clustering was spatially heterogeneous within elevation bands with high clustering between ice streams and fewer lakes along shear margins spatial patterns of lake drainage throughout the 2007 melt season exhibit strong spatial coherency the magnitude of clustering reveals patterns caused by surface melt production and water routing through surface streams as well as in situ drainage cluster patterns are influenced by proximity to crevasses fields shear margins as well as topographic structure

seasonal evolution of subglacial drainage and acceleration in a greenland outlet glacier an acceleration of ice mass loss has been observed near the margin of the greenland ice sheet partly as a result of faster ice motion observations by gps receivers reveal high seasonal variability in ice motion with summer motion up to 220 higher than winter background levels the greenland ice sheet contains enough water to raise sea levels by 7 m however its present mass balance and future contribution to sea level rise is poorly understood1 accelerated mass loss has been observed near the ice sheet margin partly as a result of faster ice motion2 3 4 surface melt waters can reach the base of the ice sheet and enhance basal ice motion5 6 however the response of ice motion to seasonal variations in meltwater supply is poorly constrained both in space and time here we present ice motion data obtained with global positioning system receivers located along a 35 km transect at the western margin of the greenland ice sheet throughout a summer melt season our measurements reveal substantial increases in ice velocity during summer up to 220 above winter background values these speed up events migrate up the glacier over the course of the summer the relationship between melt and ice motion varies both at each site throughout the melt season and between sites we suggest that these patterns can be explained by the seasonal evolution of the subglacial drainage system similar to hydraulic forcing mechanisms for ice dynamics that have been observed at smaller glaciers

seismicity on the western greenland ice sheet surface fracture in the vicinity of active moulins we analyzed geophone and gps measurements collected within the ablation zone of the western greenland ice sheet during a 35 day period of the 2011 melt season to study changes in ice deformation before during and after a supraglacial lake drainage event during rapid lake drainage ice flow speeds increased to 400 of winter values and icequake activity peaked at times 7 days after drainage this seismicity developed variability over both diurnal and longer periods 10 days while coincident ice speeds fell to 150 of winter values and showed nightly peaks in spatial variability approximately 95 of all detected seismicity in the lake basin and its immediate vicinity was triggered by fracture propagation within near surface ice 330m deep that generated rayleigh waves icequakes occurring before and during drainage frequently were collocated with the down flow west end of the primary hydrofracture through which the lake drained but shifted farther west and outside the lake basin after the drainage we interpret these results to reveal vertical hydrofracture opening and local uplift during the drainage followed by enhanced seismicity and ice flow on the downstream side of the lake basin this region collocates with interferometric synthetic aperture radar measured speedup in previous years and could reflect the migration path of themeltwater supplied to the bed by the lake the diurnal seismic signal can be associated with nightly reductions in surface melt input that increase effective basal pressure and traction thereby promoting elevated strain in the surficial ice

mineralization of luziyuan pb zn skarn deposit baoshan yunnan province sw china evidence from petrography fluid inclusions and stable isotopesy l yang and othersmineralization process of luziyuan pb zn skarn deposit the luziyuan pb zn skarn deposit located in the baoshan narong dongzhi block metallogenic belt in sw china is hosted by marble and slate in the upper cambrian shahechang formation three skarn zones have been identified from the surface 1495 m above sea level asl to a depth of 1220 m asl zone 1 consists of chlorite actinolite calcite quartz zone 2 of rhodonite actinolite fluorite quartz calcite and zone 3 contains garnet rhodonite actinolite fluorite quartz calcite the deposit formed in four distinct mineralization stages an early anhydrous skarn garnet rhodonite and bustamite stage stage 1 a hydrous skarn actinolite and chlorite stage stage 2 an early quartz coarse barren quartz veins stage stage 3 and a late sulphide forming fine sulphide bearing quartz veins stage stage 4 the stage 1 skarn forming fluid temperature was at least 500 c according to the geothermometer with rhodonite bustamite trace elements measured by laser ablation inductively coupled plasma mass spectrometry la icp ms a decrease in ore fluid temperatures with time is consistent with the decreases in the 18 o fluid and d fluid values from stage 3 to 4 this trend suggests that the ore fluid was mainly derived from magmatic water and mixed with large amounts of meteoric water during mineralization the 34 s values of stage 4 chalcopyrite sphalerite and galena are similar to those of an ordovician gypsum layer and together with the high salinity fluids in stage 4 indicate the dissolution of evaporites in the luziyuan region overall the results of this study suggest that the luziyuan deposit is a distal pb zn skarn deposit that formed in response to multi stage alteration associated with a combination of magmatic water and meteoric water

the influence of mantle internal heating on lithospheric mobility implications for super earths abstract super earths a recently discovered class of exoplanets have been inferred to be of a similar rock and metal composition to the earth as a result the possibility that they are characterised by the presence of plate tectonics has been widely debated however as the super earths have higher masses than earth it is assumed that they will also have higher rayleigh numbers and non dimensional heating rates accordingly we conduct a systematic 2d study to investigate the influence of these parameters on the surface behaviour of mantle convection the main focus of our work considers the response of surface motion to the mantle s internal heating however we also include an analysis of other parameters scaling with planet mass such as viscosity in agreement with the findings of valencia and o connell 2009 and van heck and tackley 2011 we find plate like surface mobilisation for increased rayleigh numbers but increasing the internal heating leads to the formation of a strong stagnant lid because the mantle heating effects thermally activated viscosity additionally viscosity is affected by the increased pressures and temperatures of super earths in total our findings indicate that surface mobility will likely be reduced on super sized earths our numerical models show that the interior temperature of the convecting system is of vital importance in planets with a hotter interior plate tectonics is less likely

fast draining lakes on the greenland ice sheet 1 the rapid drainage of supraglacial lakes around the ablation zone of the greenland ice sheet forms an important link between water at the surface and the ice sheet base allowing surface meltwater to reach the bed and hence increase glacial velocity the conduits formed by lake drainages may remain open during the remainder of the melt season providing a pathway for further meltwater to reach the base we investigated the drainage behavior of lakes from all regions of the greenland ice sheet for the period 2005 2009 we mapped the evolution of 2600 lakes from 3704 modis images detecting a mean of 263 drainage events per year of which 61 occurred in the south west region only 1 of lake drainages occurred in the rapidly thinning south east region our results show marked differences between the hydrology of the different regions of the ice sheet with few lake drainages occurring in the regions where the highest dynamic mass loss is occurring in the south west and north east lake drainages are common and could impact glacier dynamics in the south east they are rare and are thus unlikely to do so