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

the development of the central european continental crust since the early archaean based on conventional and ion microprobe dating of up to 3 84 b y old detrital zircons abstract archaean components up to 3 84 b y old have been detected by ion microprobe dating of zircon from various sediments and metasediments of the european hercynides moldanubian of northeastern bavaria and the massif central the early archaean age of 3 84 b y was obtained within the euhedral core of a detrital grain from a moldanubian paragneiss the same detrital zircon grain reveals at least three metamorphic overprints at 2 59 b y b p 1 94 b y b p and 460 m y b p and at least two cycles of mechanical abrasion during surface recycling in the early proterozoic and after the pan african cycle it is thus the crystal with the most complex geological history ever detected further archaean ages for the crystallization of primary magmatic zircons probably in the course of continental crust formation in the first cycle provenances of the two studied areas were found at 3 15 2 9 2 76 and 2 65 b y b p and in a few cases between 2 6 and 2 5 b y b p early proterozoic crust forming events between about 1 75 and 2 15 b y b p play an important role especially for the sediments and metasediments of the montagne noire at the southern rim of the massif central middle and late proterozoic ages cluster around 1 0 b y b p and 600 m y b p for all analysed samples therefore the deposition of the metasedimentary precursors took place after the pan african cycle which is in line with the rb sr whole rock systematics of central european metasediments the ages of metamorphism in the different source areas could be inferred in some cases by lower intercept ages obtained on single detrital grains the following sequence coinciding with times of magmatic events has been so far established 2 59 1 94 and 1 0 b y b p and 600 m y b p thus ion probe dating is also capable of defining secondary effects in the respective provenances which are typical for orogenic cycles the ubiquitous presence of pan african detrital zircons in european metasediments is a strong indication that the detrital material was derived from gondwana however from the presence of metamorphic units including eclogitefacies rocks which probably formed at different continental margins and at various times between about 500 m y and 330 m y ago it is probable that various gondwana derived microcontinental plates contributed to the different basin fillings the gondwana derived detritus is also supported by the observation of two periods of relative quiescence or non activity around 2 3 b y ago and especially around 1 5 b y ago which is a typical feature observed in gondwana cahen et al 1984 but not in laurasia the other possible source of the detritus the lack of zircons crystallized between 3 84 and 3 15 b y ago might have different causes thus the continental crust of the european hercynides probably developed since the early archaean and grew via many crustal accretion events to its present state

review of geochemical variation in lower palaeozoic metabasites from the ne bohemian massif intracratonic rifting and plume ridge interaction

lithium behaviour and isotope fractionation during fluid rock interactions in variscan oceanic suture zones limousin ophiolite and ile de groix high pressure terrane france

laser ablation coupled with icp ms applied to u pb zircon geochronology a review of recent advances abstract improvements in the technology of laser ablation and icp ms instruments make la mc icpms a rapid precise and accurate method for u pb zircon geochronology in this review we describe the main stages of the evolution of this in situ approach from the early 1990s to the present time some key points have been progressively improved the crater size has been reduced to achieve real in situ measurements the laser wavelength has been reduced as well as the duration of each pulse in order to lower inter element fractionation the blank from the gas has to be lowered as far as possible double focusing instruments and magnetic field sectors allow flat topped peaks required for precise isotope ratio measurement to be obtained the use of a multi ion counting system significantly improves the sensitivity of the method and the static mode of integration favours the precision of measurement of the transient signal originating from a noisy laser ablated particle beam combining the use of a 213 nm uv laser and a mc icpms equipped with a multi ion counting system operating in static mode the common precisions achieved for the key ratios 207 pb 206 pb and 206 pb 238 u are better than 1 and 3 2 respectively including error propagation associated with standard normalization until now the use of a zircon standard has remained necessary to ensure the accuracy of the calculated age a strategy for common pb correction is proposed according to the age of the zircon and according to the th u ratio of the grains after recording sixteen to twenty spot analyses the precision usually achieved on the age is about 1 and even significantly better for proterozoic samples in order to show the performance achieved by modern la mc icpms geochronology we tested four zircon samples covering a wide age range from 290 to 2440 ma these new age determinations can be compared in term of precision and accuracy since they have already been dated by reference methods id tims and shrimp further developments in the technology of ion counters equipping modern mc icpms and in laser systems will certainly be applied to a large field of geochronology studies in the near future as an alternative to sims for in situ age determination

u pb zircon and monazite dating of a mafic ultramafic complex and its country rocks u pb data on zircons from the largest mafic ultramafic body 6 2 km of the french central massif sauviat sur vige yield the following age results primary magmatic crystallization of the gabbroic and peridotitic protoliths took place in the cambro ordovician 496 25 17 m y variable transformation under eclogite facies conditions was hercynian 320 29 36 m y the same age pattern derived by u pb monazite analyses was found also for the immediate country rocks i e kyanite bearing coarse grained metagranites occurring to the w and n of the sauviat massif due to the fact that there is no regional hercynian high grade metamorphism in this part of the french central massif e g duthou 1977 bernard griffiths 1975 both mafic ultramafic complex as well as immediate felsic country rocks must have been emplaced tectonically into pre hercynian acadian caledonian crustal rocks the cause for such a hercynian tectonism is thought to be due to continent continent collision of the spanish with the armorican plate

modelling the evolution of supraglacial lakes on the west greenland ice sheet margin we present a model study investigating the summer evolution of supraglacial lakes on the greenland ice margin using a one dimensional 1 d model we calculate the surface ablation for a bare ice surface and beneath supraglacial lakes for 30 days in the summers of 1999 and 2001 the surface ablation beneath the lake was enhanced by 110 in 1999 and 170 in 2001 compared with the ablation for bare ice we then use the results from the 1 d model to further model the vertical and horizontal evolution of the supraglacial lakes the results of which are compared with satellite images within the region of the ice sheet where supraglacial lakes presently occur the area covered by supraglacial lakes is found to be more or less independent of the summer melt rate but controlled by topography we therefore predict that inside this region the area covered by supraglacial lakes will remain constant even in a warmer climate however in a warmer climate surface melting will occur higher on the ice sheet where small surface slopes favour formation of large supraglacial lakes enhanced surface melting beneath such lakes is a hitherto overlooked feedback mechanism related to climate warming

early palaeozoic rift related magmatism in variscan europe fragmentation of the armorican terrane assemblage early palaeozoic bimodal rift related magmatism is widespread throughout much of the variscides of europe it is traceable from the polish sudetes to nw iberia granitic plutonism generally predates cambro ordovician bimodal magmatism in the n bohemian massif this early palaeozoic granitic plutonism was generated by partial melting of cadomian basement whereas contemporaneous alkali granites with a mantle component are typical of the nw iberian massif silurian devonian mafic magmatism in the n bohemian massif massif central and nw iberian massif is partly preserved as obducted ophiolites compositional diversity displayed by cambro ordovician mafic magmatism can be accounted for by interaction between a spreading centre and an upwelling mantle plume this indicates that combined tensional forces and mantle plume convection assisted the early palaeozoic dispersal of terranes from the n gondwana margin continued fragmentation resulted in development of an archipelago of related terranes separated by a network of seaways and formation of oceanic crust

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

the morphology of supraglacial lake ogives supraglacial lakes on grounded regions of the greenland and antarctic ice sheets sometimes produce lake ogives or banded structures that sweep downstream from the lakes using a variety of remote sensing data we demonstrate that lake ogives originate from supraglacial lakes that form each year in the same bedrock fixed location near the equilibrium line altitude as the ice flows underneath one of these lakes an image of the lake is imprinted on the ice surface both by summer season ablation and by superimposed ice lake ice formation ogives associated with a lake are sequenced in time with the downstream ogives being the oldest and with spatial separation equal to the local annual ice displacement in addition lake ogives can have decimeter to meter scale topographic relief much like wave ogives that form below icefalls on alpine glaciers our observations highlight the fact that lake ogives and other related surface features are a consequence of hydrological processes in a bedrock fixed reference frame these features should arise naturally from physically based thermodynamic models of supraglacial water transport and thus they may serve as fiducial features that help to test the performance of such models

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

transgressive wave and tide dominated barrier lagoon system and sea level rise since 8 2 ka recorded in sediments in northern bohai bay china abstract two sediment boreholes dq3 and dq4 were drilled in northern bohai bay to investigate the genetic type and sedimentary evolution process of the caofeidian barrier lagoon system and the impact of relative sea level rsl rise the core sediment was tested for grain size microfossils foraminifera and ostracods and 14c dating the holocene formation of the delta coast morphology and sedimentary types was closely associated with different phases of rsl rise fluvial facies palaeosols and limnetic facies formed successively during the latest deglaciation but did not exhibit a direct relationship with rsl rise these sediment facies were immediately followed by the littoral limnetic facies that formed during 9 5 8 2 cal ka bp when the holocene sea level reached the study area then offshore sand bars formed on the southern side and tidal sand ridges formed on the eastern and western sides forming a barrier island which suggest that the caofeidian barrier lagoon system began to form under wave and tide dominated ocean dynamics at 8 2 7 8 cal ka bp as the sea level rose at a rate of ca 11 5 mm a 1 the caofeidian barrier lagoon system reached the maximum extent and was mainly influenced by wave dominated ocean dynamics at 7 8 7 5 cal ka bp during which the sea level rose at a rate of ca 9 0 mm a 1 the caofeidian barrier lagoon system began to erode and deteriorate at 7 5 6 7 cal ka bp with a sea level rise rate of ca 2 6 mm a 1 tidal sand ridges as barrier islands formed on the shore side under the influence of tide dominated ocean dynamics rsl rise was estimated at 2 3 m over the past 6 7 ka over this period the sandy barrier islands were eroded by tides and the lagoon areas transitioned into an intertidal zone our reconstruction of rsl changes showed that the rsl rose abruptly in four stages increasing by 1 7 m 11 0 m 1 7 m and 2 3 m at 9 5 8 7 8 7 7 5 7 5 6 7 and 6 7 0 cal ka bp respectively

origin of the rheic ocean rifting along a neoproterozoic suture the rheic ocean is widely believed to have formed in the late cambrian early ordovician as a result of the drift of peri gondwanan terranes such as avalonia and carolina from the northern margin of gondwana and to have been consumed in the devonian carboniferous by continent continent collision during the formation of pangea other peri gondwanan terranes e g armorica ossa morena northwest iberia saxo thuringia moldanubia remained along the gondwanan margin at the time of rheic ocean formation differences in the neoproterozoic histories of these peri gondwanan terranes suggest the location of the rheic ocean rift may have been inherited from neoproterozoic lithospheric structures formed by the accretion and dispersal of peri gondwanan terranes along the northern gondwanan margin prior to rheic ocean opening avalonia and carolina have sm nd isotopic characteristics indicative of recycling of a juvenile ca 1 ga source and they were accreted to the northern gondwanan margin prior to voluminous late neoproterozoic arc magmatism in contrast sm nd isotopic characteristics of most other peri gondwanan terranes closely match those of eburnian basement suggesting they reflect recycling of ancient 2 ga west african crust the basements of terranes initially rifted from gondwana to form the rheic ocean were those that had previously accreted during neoproterozoic orogenesis suggesting the rift was located near the suture between the accreted terranes and cratonic northern gondwana opening of the rheic ocean coincided with the onset of subduction beneath the laurentian margin in its predecessor the iapetus ocean suggesting geodynamic linkages between the destruction of the iapetus ocean and the creation of the rheic ocean

geodynamic model for the palaeozoic crustal consolidation of western and central europe abstract the crystalline basement of western and central europe consists of a mosaic of crustal elements which were consolidated during pre grenvillian the grenvillian dalslandian morarian cadomian caledonian and hercynian orogenic cycles contemporaneous with the caledonian suturing of the precambrian laurentia greenland and fennosarmatian shields a number of gondwana derived cadomian micro cratons were accreted to the southern margin of laurasia following the late caledonian paroxysm the devonian and early carboniferous evolution of europe was dominated by continued northward subduction of the proto tethys plate at an arc trench system parallelling the southern margin of laurasia the accretion of additional gondwana derived continental fragments back arc rifting and a sinistral translation of major proportions between the laurentia greenland and the fennosarmatian sub plates the acadian and bretonian orogenies were of the pacific type the visean collision of gondwana with laurasia marked the onset of the himalayan type variscan orogeny during which collision related compressive stresses overpowered the devonian early carboniferous back arc rift systems and caused the development reactivation of a subduction zones the central armorican and saxothuringian successor basins became folded and destroyed during the latest visean whilst the variscan foredeep became scooped out in part by nappes involving the basement during the late westphalian major crustal shortening during the variscan diastrophism was accompanied by the anatectic remobilisation of subducted crustal and upper mantle material and a widespread syn and late orogenic magmatism the latest carboniferous early permian reorientation of the convergence direction between gondwana and laurasia induced the development of a complex wrench fault system transecting the variscan fold belt and an extensive post orogenic volcanism the hypotheses summarised here require confirmation by further palaeomagnetic and radiometric data and faunal analyses

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

la mc icpms and shrimp u pb dating of complex zircons from quaternary tephras from the french massif central magma residence time and geochemical implications abstract analyses of zircon grains from the queureuilh quaternary tephras pumice provide new information about their pre eruptive history u pb dating was performed in situ using two methods shrimp and la mc icpms equipped with a multi ion counting system both methods provided reliable 207pb 206pb and 206pb 238u ratios as well as u and th abundances required for u pb concordia intercept age determination after initial 230th disequilibrium correction the new la mc icpms method was validated by dating a reference zircon 61 308b and zircons from a phonolitic lava dated independently with the two techniques a time resolution of about 20 kyr for 1 ma zircon crystals was achieved for both methods the clear euhedral zircon population from queureuilh tephras is quite complex from several points of view 1 some grains are reddish or yellowish while others are colorless 2 the u and th composition changes by more than an order of magnitude and th u is generally high 1 2 3 there are three discrete ages recorded at 2 35 0 04 1 017 0 008 and 0 640 0 010 ma from the previously determined 40ar 39ar age at 0 571 0 060 ma duffell h 1999 contribution geochronologique a la stratigraphie volcanique du massif des monts dore par la methode 40ar 39ar d e a univ clermont ferrand 56 p the discontinuous zircon age populations the color of the grains and their composition we favor the following model as explanation the oldest less numerous group of reddish zircons represents xenocrystic grains resulting from assimilation of the local material during magma ascent a primitive magma chamber perhaps deep in crustal level was formed at 1 0 ma the related magma previously characterized by high th u ratio 2 2 1 1 underwent rejuvenation during ascent to a new chamber at shallow depth and or during injection of more mafic magmas during this stage at 0 64 ma the colorless zircon grains of lower th u ratio 1 3 0 5 crystallized this last stage defined the magma residence time of 70 kyr prior to eruption dated by the 40ar 39ar method however if the primitive magma is considered the magma residence time as a whole from this first stage reached 446 kyr in the light of the complex history of such magmas which commonly involves recycling of zircon grains that precipitated tens to hundreds of kyr earlier than eruptions the use of zr concentration in geochemical modeling of whole rock compositional data can be problematic

african provenance for the metasediments and metaigneous rocks of the cyclades aegean sea greece u pb geochronology on detrital and inherited zircons from metasediments as well as gneissic and metaigneous basement rocks of the cyclades aegean sea greece defines several periods of crustal growth prior to the carboniferous these ages are consistent with cycladic rocks being derived from the northern margin of gondwana there is a distinct mesoproterozoic age gap evident in zircons from the 25 samples considered this is characteristic of the west african craton and we suggest it will also prove characteristic of the aegean region this mesoproterozoic age gap may be used as a tool in terrane reconstruction to distinguish crustal material derived from different parts of gondwana

zircon u pb chronology of the nyingtri group southern lhasa terrane tibetan plateau implications for grenvillian and pan african provenance and mesozoic cenozoic metamorphism abstract the nyingtri group of the lhasa terrane in southern tibet consists dominantly of metasedimentary rocks and orthogneiss these rocks have a similar mineral paragenesis of plagioclase k feldspar biotite quartz sillimanite garnet staurolite muscovite amphibole indicating amphibolite facies metamorphic conditions inherited detrital zircons from the metasedimentary rocks show magmatic features and yield widely variable 206pb 238u ages ranging from 3300 to 50 ma the data define two prominent age populations 1200 1000 and 600 500 ma indicating that the source of the nyingtri group preserves the records of both grenville and pan african magmatic thermal events inherited magmatic zircon cores from the orthogneiss yield a crystallization age of 496 ma limiting the depositional age of the metasedimentary sequence to cambrian or older overgrowth rims on the detrital zircons from one metasedimentary rock yield a metamorphic age of 32 ma on the basis of these results together with the

the greenland ice sheet and global sea level rise the flow of several large glaciers draining the greenland ice sheet is accelerating this change combined with increased melting suggests that existing estimates of future sea level rise are too low

a two dimensional morphological model based on next generation circulation solver ii application to caofeidian bohai bay china abstract this paper is a companion of a two part papers on the development of a two dimensional morphological model curwac2d sed based on next generation circulation solver in this paper the model developed in paper i is applied to the large scale construction of near harbor industrial zone with deep water harbor by reclamation in caofeidian bohai bay which is a typical sandbar lagoon bay type tidal inlet system characteristics of tides currents waves and sediments in caofeidian are firstly analyzed with field data from comprehensive field observations carried out in 2008 these data are then used for model calibration and validation showing a good agreement between simulated results and field measurements for tides currents and suspended sediments the verified model is applied to study the effects of the construction of caofeidian embankment and the on going engineering scheme on hydrodynamics and sediment transports as compared with no constructions under three representative tide conditions and their combinations with normal and strong waves as well as on morphology evolution the simulated results indicate that the present scheme leads to morphological changes in caofeidian with acceptable rates of erosion and sedimentation however some attentions should be paid to erosion at offshore wharf in front of caofedian foreland which may lead to potentially increasing landslide risks due to steep slope and sedimentation in laogonggou creek

the late neoproterozoic magmatism in the ediacaran series of the eastern pyrenees new ages and isotope geochemistry geochronological u pb la icp ms geochemical and isotopic data from metavolcanic felsic rocks of the canigo and cap de creus massifs in the eastern pyrenees provide evidence of an ediacaran magmatic event lasting 30 ma in ne iberia these data also constrain the age of the late neoproterozoic succession in the cap de creus massif where depositional ages range from 577 to 558 ma and in the canigo massif where the data 575 568 ma represent minimum ages the geochemistry of the felsic rocks indicates that they were formed in a back arc environment and they record a fragment of a long lived subduction related magmatic arc 620 520 ma in the active northern gondwana margin the homogeneity shown by all these crustal fragments along this margin suggests that the individualization of the pyrenean basement from the iberian massif started later probably during its transition from an active to a passive margin in cambro ordovician times

surficial glaciology of jakobshavns isbrae west greenland part i surface morphology jakobshavns isbrae is a large fast moving ice stream outlet glacier in west greenland which ends at a floating calving front it drains about 6 5 of the area of the greenland ice sheet studies of its surface morphology are described in this paper the surface is relatively steep 0 01 0 03 and the thickness is large up to 2600 m along the center line clarke and echelmeyer 1989 indicating very high driving stresses 200 300 kpa the ice stream is about 6 km wide and 85 90 km long all of which is in an area of surface melting the base of the ice stream and of much of the drainage area is below sea level marginal crevasse zones have a width on the order of the width of the ice stream itself unique surficial features are ice blisters and lakes the latter have a sequence of ogive like features on their floating ice cover which can be used to determine velocity there is a pinning point near the terminus which may act as a stabilizing influence possibly playing a role in halting at least temporarily a recent retreat of the terminus ice thickness estimates at the terminus lead to a flux which is less than previously assumed by others e g bindschadler 1984 pelto and others 1989 when estimating jakobshavns isbrae s drainage basin to be nearly in balance the driving stresses on jakobshavns isbrae are an order of magnitude higher than those of the ice streams of west antarctica its crevasse patterns are much less localized its relatively unconfined terminus is more comparable to that of relatively unbuttressed ice streams such as pine island and thwaites glaciers than it is to other west antarctic ice streams which terminate in large confined ice shelves

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

morphodynamic responses to the deep water harbor development in the caofeidian sea area china s bohai bay vast bay type tidal inlets can be found along the coastal zones of china they are generally suitable for deep water channels and large harbors because of the presence of large water depth and good mooring conditions the deep channel in front of the head of caofeidian island in bohai bay china is a typical bay type tidal inlet system the tidal current a type of reverse flow makes the key contribution to maintain the deep water depth the co action of waves and tidal currents is the main dynamic force for sediment motion waves have significant influence on the sediment concentration based on the characteristics of waves tidal currents sediment and seabed evolution in caofeidian sea area a 2d mathematical model for sediment transport under influence of waves and tidal currents is developed to study the development schemes of the caofeidian harbor the model has been verified for spring and neap tides in winter as well as in summer of 2006 the calculated tidal stages flow velocities flow directions and sediment concentrations at 15 stations are in good agreement with the observations furthermore the calculated data on pattern and magnitude of sedimentation and erosion in the related area agree well with the observations this model has been used to study the effects of the reclamation scheme for caofeidian harbor on the hydrodynamic environment sediment transport and morphological changes attentions are paid to the project inducing changes of flow velocities and morphology in the deep channel at the south side of caofeidian foreland in the laolonggou channel and in various harbor basins the conclusions can provide the important foundation for the protection and use of bay type tidal inlets and the development of harbor industry

accretionary history and crustal evolution of the variscan belt in western europe abstract the variscan belt of western europe is part of a large intra paleozoic belt extending on both sides of the atlantic from the ouachitas in the us and the mauritanides in west africa to the bohemian massif in czechoslovakia and poland the belt was constructed between 500 and 250 ma as a result of convergence and collision between two main continents laurentia baltica and africa after the closure of various oceanic basins iapetus rheic and galicia massif central now recorded as discrete in part cryptic sutures which form the roots of large crystalline nappes in which are found dismembered remnants of oceanic crust and mantle the accretionary history of the variscan belt in western europe is related to the progressive closure of probably two oceanic basins the rheic in the north and another the galicia massif central in the south by respectively southward and northward intraoceanic subduction followed by obduction and intracontinental lithospheric subduction the result is a broad 700 800 km fan like divergent belt which must be considered as a classical obduction collision orogen with a long intracontinental history of about 100 150 ma its arcuate shape is probably due to the northwestward impingement of a promontory of the african continent into laurentia baltica

drainage from water filled crevasses along the margins of jakobshavn isbrae a potential catalyst for catchment expansion 1 saturated crevasses occur in local depressions within the shear margins of jakobshavn isbrae at inflections in the ice stream s flow direction spatio temporal variability of seven distinctive saturated crevasse groups was examined during the 2007 melt season the area of saturated crevasses reached its maximum extent 1 8 km2 in early july and remained largely constant until early august filling rates are correlated with regional melt production while drainage rates are highly correlated with areal extent estimates on potential drainage volume from the largest crevasse system are 9 23 10 3 km3 2 15 10 8 km3 and 4 92 10 2 km3 3 58 10 8 km3 respectively over a 16 day interval and are more than required for a distributed basal hydrologic system across this area to temporarily flood bedrock obstacles believed to control basal sliding future drainage events likely extending farther inland with warming could result in enhanced lateral mass discharge into the ice stream with implications for the dynamic evolution of the entire basin

implications of increased greenland surface melt under global warming scenarios ice sheet simulations abstract the greenland ice sheet is likely to make a faster contribution to sea level rise in a warming world than previously believed based on numerical modelling using a parameterization of recent results showing surface meltwater lubrication of ice flow zwally et al science 297 557 2002 218 documented correlation between increased ice velocity and increased surface melt as parameterized by positive degree days they argued that surface water is piped directly to the bed with little delay causing increased basal water pressures and basal sliding velocities an effect not included in recent greenland ice sheet models known to the authors using the pennsylvania state university university of chicago thermomechanical flowline model numerous simulations were conducted to test a wide range of parameter space linking surface melt with a new sliding law based on the zwally et al data under three different global warming scenarios 2 co2 4 co2 and 8 co2 comparisons to reconstructions generated with a traditional sliding parameterization illustrate an enhanced sensitivity of the ice sheet to surface warming resulting in higher ablation rates additional thinning and retreat of the margin and a reduction in ice volume leading to an increased contribution to global sea level rise

the evolution of the modern luanhe river delta north china the luanhe river delta is located at the northern coast of the bohai bay which was an enclosed sea in 1915 the luanhe river changed course to its present position and has since developed a typical fan shaped delta under fluvial wave interaction owing to the water conservancy projects constructed in the catchment the sand load that the luanhe river takes into the sea has decreased sharply since 1980 hence the evolution of the luanhe river delta can be divided into two phases the delta constructing phase between 1915 and 1979 and the para abandoned phase after 1979 in the different phases the delta characteristics are not alike the main features of the delta evolution are changes in the distribution and development of lagoons it seems that with the increase in delta construction features there is an increase in lagoon areas a decrease of lagoon areas indicates the delta dying along with further development the delta flank lagoons expand and the delta tip lagoons tend to shrink these trends are also present in the processes of the para abandoned delta phase in each different stage of development the delta has a different geometric pattern at the first stage the prolate axis of the ellipse fitted to the delta plain outline was perpendicular to the coast the delta tip had high curvature along with the delta s further development the delta tip curvature gradually decreased and the prolate axis of the ellipse turned to parallel the coast

continental crust formation seen through the sr and nd isotope systematics of s type granites in the hercynian belt of western france the isotopic compositions of sr nd and pb in leucogranites which are intercorrelated bernard griffiths et al 1985 1 may be explained by the mixing of ancient basement 1800 ma with juvenile crust late precambrian or early palaeozoic this hypothesis does not involve the existence of mid proterozoic crust as apparently indicated by thetdm model ages of the leucogranites ranging between 1600 and 1100 ma the nd isotopes reveal the crustal reworking while sr isotopes mainly record juvenile crust formation this paradox is explained by the geochemical heterogeneity of the sources involved

archean protoliths within early proterozoic granulitic crust of the west european hercynian belt possible relics of the west african craton granulites from the bay of biscay area have been studied by using u pb and sm nd methods results on four granulites suggest two major events of crustal growth and high grade metamorphism the first is archean 2 7 ga and the second is early proterozoic 1 9 ga in the hercynian belt the oldest basement remnants known so far are about 2 0 ga and are located in the northern armorican massif the 2 7 ga orthogneiss protolith age from the bay of biscay is the first direct evidence of archean crust involved in the hercynian belt major events at 2 7 1 8 and 0 6 ga in the hercynian belt suggest that european precambrian microplate s were part of the west african craton where the same sequence of events is observed

zircon u pb geochronology of ordovician magmatism in the polycyclic ruitor massif internal w alps among the middle penninic basements of the internal nw alps the ruitor massif shows the best preserved remnants of pre permian metamorphic rocks their barrovian type mineral associations are somewhat masked by the greenschist to blueschist alpine metamorphism of tertiary age four ruitor gneisses have been analysed showing geochemical characters of granitoids from orogenic zones zircon morphology also suggests magmatic protoliths and a crustal source some of the morphological zircon types suggest anatectic granites the first u pb ages on zircon for this massif have been obtained concurrently through conventional multigrain and ion microprobe dating two metavolcanic rocks at 471 5 and 468 22 ma could be slightly older than the porphyritic augen gneisses at 465 11 and 460 7 ma regional data from the other internal basement massifs suggest that the variscan event is poorly recorded except in ruitor type units ruitor and sapey gneisses belonged to the same unit nappe des pontis which was affected by a 480 450 ma event including volcanism and anatexis and ended with a late calc alkaline granite emplacement at 460 450 ma the distribution of variscan basement units roughly parallels alpine zonation

precambrian protoliths and early paleozoic magmatism in the french massif central u pb data and the north gondwana connection in the west european variscan belt abstract u pb geochronological data were collected on zircon by la mc icp ms on orthogneiss and paragneiss from the limousin area in the french massif central fmc in order to investigate the connection between the west european variscan belt and the northern margin of gondwana fifteen samples were collected in the four main tectonostratigraphic units of the fmc namely the para autochthonous unit the lower gneiss unit lgu the upper gneiss unit ugu and the thiviers payzac unit orthogneiss yield intrusion age between 521 7 and 446 6 ma considering all the results from both magmatic and metasedimentary samples two peaks at 531 and 473 ma are recognized rifting processes taking place along the north gondwana margin during the lower paleozoic went on until the end of the ordovician as suggested by the magmatic event recorded around 450 ma several maximum depositional ages were ascertained in the metasedimentary formations of the fmc as determined by the youngest detected detrital zircon crystal ranging from 604 16 ma for metasediments of the para autochthonous unit 573 12 ma for the lgu 564 9 ma for the thiviers payzac unit and 523 4 ma in the ugu minimum depositional ages are given by magmatic emplacement ages obtained in the crosscutting orthogneiss there is some evidence for a decrease of this maximum age upwards in the tectonostratigraphy detrital zircon in metasedimentary formations and inherited zircon in orthogneiss display a wide spectrum of ages with significant peaks at around 590 ma and 560 ma archean paleoproterozoic and neoproterozoic detrital zircons suggest a west african craton source of the sedimentation the large amount of neoproterozoic and lower paleozoic ages obtained in this study suggests that these periods played a significant role in the continental crustal growth history of western europe

analysis of synthetic aperture radar data collected over the southwestern greenland ice sheet analyses of the first aircraft multi frequency polarimetric synthetic aperture radar sar data acquired over the southwestern greenland ice sheet are presented data were collected on 31 august 1989 by the jet propulsion laboratory sar using the nasa dc 8 aircraft along with curvilinear patterns associated with large scale morphologic features such as crevasses lakes and streams frequency and polarization dependencies are observed in the p l and c band image products

deducing the ancestry of terranes shrimp evidence for south america derived gondwana fragments in central europe we present here an example of how the sensitive high resolution ion microprobe shrimp zircon dating method can provide a terrane specific geochronological fingerprint for a rock and thus help to reveal major tectonic boundaries within orogens this method applied to inherited zircons in a ca 580 ma metagranitoid rock from the eastern bohemian massif has provided for the first time in the central european variscan basement unequivocal evidence for mesoproterozoic and late paleoproterozoic geologic events ca 1 2 ga 1 5 ga and 1 65 1 8 ga the recognition of such zircon ages has important consequences because it implies that parts of the precambrian section of variscan central europe were originally derived from a grenvillian cratonic province as opposed to the common assumption of an african connection a comparison with previously published shrimp data suggests however that these mesoproterozoic and late paleoproterozoic zircon ages may be restricted to the moravo silesian unit in the eastern variscides whereas the saxothuringian and moldanubian zones appear to contain a typical north african i e neoproterozoic plus eburnian inherited zircon age spectrum this finding supports new tectonic concepts according to which variscan europe is composed of a number of completely unrelated terranes with extremely different paleogeographic origins the moravo silesian unit can be best interpreted as a peri gondwana terrane which was situated in the realm of the amazonian cratonic province by the late precambrian comparable to the avalonian terranes of north america and the united kingdom

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

channel response to low water levels in the pearl river delta a multi decadal analysis abstract the pearl river delta is a composite delta with one of the densest channel networks in the world to analyze the internal erosion deposition characteristics of the net channel area this paper studies the upstream evolution characteristics using digital elevation model data from local channel sections we attempt to establish the relationship between the changes in low water level and erosion rate by using the accumulated data for the main stream and hydrological stations over the past 50 years including the distributions of flow and sediment at the top of the net channel area and the erosion deposition data of a typical channel section in fact the net channel area has been in a forced erosion state artificial sand excavation for a long time forcing the flow and sediment distributions at makou and sanshui stations to adjust from 1993 to the present although the incoming sedimentation is far less than the net erosion and there are similarities and differences in the sources of the incoming flow and sediment from the main stream the internal characteristics of the upper and lower reaches of the delta have not changed substantially they have always been stable slow deposition channels the forced undercutting is concentrated within width depth ratios of approximately 50 250 which still meets the evolution characteristics of the natural channel artificial sand excavation during the 1980s tended to enhance the characteristics however the sudden increase in the undercutting rate has forced chaotic changes in the low water level since 1990 and the sediment transport rate decreased sharply the phenomena of salt tides retrogression and retrogressive erosion generally happen the water level evolution in the net channel area is chaotic and continuous undercutting is still occurring

in situ chemical dating of tectonothermal events in the french variscan belt in situ u th pb geochronology on monazite using electron probe micro analyser has been performed on migmatite in the southern french variscan massif central in order to decipher its complex history after the early visean 340 ma nappe stacking the cevennes area experienced a regional migmatization already dated 330 325 ma in northern cevennes in these rocks two monazite populations are recognized on the basis of petrology texture and geochemistry the oldest monazite generation that appears as inclusion in k feldspar is dated at 331 4 ma this age complies with that of the crustal melting experienced by the cevennes metamorphic series the youngest monazite generation is interstitial and gives an age of 320 5 ma a hydrothermal origin coeval with the peraluminous magmatism that predates the formation of the late carboniferous velay dome is proposed as a working hypothesis to account for the formation of this second monazite generation

gondwana derived microcontinents the constituents of the variscan and alpine collisional orogens abstract the european variscan and alpine mountain chains are collisional orogens and are built up of pre variscan building blocks which in most cases originated at the gondwana margin such pre variscan elements were part of a pre ordovician archipelago like continental ribbon in the former eastern prolongation of avalonia and their present day distribution resulted from juxtaposition through variscan and or alpine tectonic evolution the well known nomenclatures applied to these mountain chains are the mirror of variscan resp alpine organization it is the aim of this paper to present a terminology taking into account their pre variscan evolution at the gondwana margin they may contain relics of volcanic islands with pieces of cadomian crust relics of volcanic arc settings and accretionary wedges which were separated from gondwana by initial stages of rheic ocean opening after a short lived ordovician orogenic event and amalgamation of these elements at the gondwanan margin the still continuing gondwana directed subduction triggered the formation of ordovician al rich granitoids and the latest ordovician opening of palaeo tethys an example from the alps external massifs illustrates the gradual reworking of gondwana derived pre variscan elements during the variscan and alpine tertiary orogenic cycles

evolution of late neoproterozoic to early paleozoic tectonic elements in central and southeast european alpine mountain belts review and synthesis abstract late neoproterozoic to early paleozoic cadomian tectonic elements are widespread in the southeastern alpine mediterranean mountain belts with discontinuous exposure extending from the alps to the menderes massif in turkey the sequences include voluminous plutonic island and continental arc successions some ophiolites of variable late neoproterozoic and cambrian to early ordovician ages high to medium grade metamorphic sequences and subordinate metasediments geological and geochronological data suggest that these cadomian elements partly experienced tectonothermal activity including metamorphism and magmatism between ca 650 and 600 ma followed by further i and s type plutonism in an andean type continental margin setting ca between 570 and 520 ma subsequently a major rift zone developed which resulted in continental stretching and subsequent formation of a back arc basin which is dated as late cambrian rifting is followed by oceanic spreading which commenced ca at the cambrian ordovician boundary possibly in a back arc setting this tectonic scenario suggests that alpine mediterranean cadomian tectonic elements were accreted to gondwana during early cadomian events ca 600 650 ma subsequently they were likely part of a long lasting outer subduction zone of gondwana at the margin of a panthalassa type ocean during late neoproterozoic iii and cambrian due to back arc spreading continental pieces started to split off from gondwana ca at the cambrian ordovician boundary

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

zircon ages of the metavolcanic rocks and metagranites of the ollo de sapo domain in central spain implications for the neoproterozoic to early palaeozoic evolution of iberia dating the pre middle ordovician metavolcanic rocks and metagranites of the ollo de sapo domain has historically been difficult because of the small compositional variation the effects of the variscan orogeny and as revealed in this paper the unusually high fraction of inherited zircon components the first reliable zircon data u pb ion microprobe and pb pb stepwise evaporation indicate that the ollo de sapo volcanism spanned 495 5 ma to 483 3 ma and was followed by the intrusion of high level granites from 483 3 ma to 474 4 ma in both metavolcanic rocks and metagranites no less than 70 80 of zircon grains are either totally precambrian or contain a precambrian core overgrown by a cambro ordovician rim about 80 90 of inherited zircons are early ediacaran 602 614 ma and derived from calc alkaline intermediate to felsic igneous rocks generated at the end of the pan african arc continent collision in the villadepera region located to the west both the metagranites and metavolcanic rocks also contain meso archaean zircons 3 0 3 2 ga which ultimately originated from the west african craton in the hiendelaencina region located to the east both the metagranites and metavolcanic rocks lack meso archaean zircons but they have two different inherited zircon populations one cryogenian 650 700 ma and the other tonian 850 900 ma which suggest older than ediacaran additional island arc components the different proportion of source components and the marked variation of the 87 sr 86 sr init suggest at least tentatively that the across arc polarity of the remnants of the pan african arc of iberia trended east west with respect to the current coordinates during cambro ordovician times and that the passive margin was situated to the west

early ordovician continental break up in variscan europe ndsr isotope and trace element evidence from bimodal igneous associations of the southern massif central france abstract the high grade marvejols group and the low grade albigeois cevennes sedimentary sequence contain bimodal igneous rocks of early ordovician age which are representative of a widespread thermal event in the european variscides comparison of their nd sr isotope and trace element characteristics provides additional evidence for their origin in an ensialic extensional setting as an alternative to the back arc model proposed by previous authors we propose that these associations record a continental break up episode unrelated to contemporaneous subduction in this model the widespread early paleozoic bimodal magmatism marks the birth of a mid european oceanic arm we propose that the break up was controlled by both transtensional processes and mantle plume activity high grade associations such as the marvejols group and low grade bimodal associations such as the albigeois cevennes might be regarded as representing opposite rifted passive margins the northern marvejols type margin was involved in a late ordovician silurian subduction like process with hp ht metamorphism in contrast the southern albigeois cevennes margin remained largely unaffected possibly as part of the overriding plate of the subduction zone pseudo calc alkaline signatures unrelated to subduction processes may occur in magmatic rocks associated with continental break up episodes in this case negative nb anomalies are produced by the addition of crustal components enriched in th and lree relative to nb this inferred alternative origin of nb anomalies has important bearing on the paleogeodynamic settings based on geochemical data

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

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

transmission of basal variability to a glacier surface 1 transmission of basal variability to a glacier surface is investigated using analytical models for a linearly viscous medium the three dimensional transient response of the surface to both bedrock undulations and spatial variations in basal slipperiness for perturbations of arbitrary wavelengths is determined using perturbation methods both information transfer toward the surface and lateral transmission of horizontal stresses are strongly affected by the slip ratio that is the ratio of basal sliding to deformational velocity for any mean bedrock slope and above a minimum value of slip ratio the amplitude transfer of bedrock undulations toward the surface has a local maximum at undulation span corresponding to about 3 8 times the mean ice thickness the transmission of basal variability to a glacier surface increases quite significantly with increasing slip ratio this explains why the surfaces of fast flowing ice streams are more undulating than the slower moving bordering areas at slip ratios higher than about 100 the flow of glaciers and ice sheets becomes nonlocal in the sense that surface velocities and buildup and propagation of surface undulations cannot be calculated accurately on the basis of local thickness and slope using linearized long wave theories at these slip ratios instead of the more accurate arbitrary wavelength theory gives estimates of decay times that are an order of magnitude too small and phase velocities several times too large the problem of the propagation and decay of small amplitude surface undulations on glaciers in three dimensions is solved small amplitude surface waves on glaciers are strongly diffusive and dispersive redistribution of mass on ice sheets and glaciers is a diffusion process and it is misleading albeit not mathematically incorrect to describe the reaction of glaciers to surface perturbations in terms of a wave propagation

spatial and temporal variations in lakes on the greenland ice sheet abstract surface lakes on the greenland ice sheet provide temporary storage for meltwater that influences both the surface and basal water fluxes thus to understand the effects of variations in surface melt on ice sheet dynamics it is necessary to understand the surface hydrology we have used satellite imagery acquired at 5 day intervals to map lake initiation and cessation on two sub sections on the south west greenland ice sheet over three melt seasons 2007 2009 we observe that lake initiation is closely tied to a threshold energy input of approximately 40 18 5 positive degree days this applies to all studied melt seasons regardless of evolution and melting index anomalies lake longevity averages 24 days with little variation between different melt seasons our observed median lake area is larger than previously reported approximately 50 of all lakes have a life span of

tracing metal sources in peribatholitic hydrothermal w deposits based on the chemical composition of wolframite the example of the variscan french massif central abstract we present a complete dataset for major minor and trace elements in wolframite fe mn wo4 based on the example of several peribatholitic hydrothermal w deposits located in the variscan french massif central fmc the wolframite samples were characterized down to the micrometer scale by combining scanning electron microscopy observations and in situ chemical analysis by electron probe microanalyzer and laser ablation inductively coupled plasma mass spectrometry most samples are fe dominated wolframites and are characterized by variable fe fe mn values for the studied deposits from 0 36 to 0 86 without significant alteration features wolframites present intracrystalline variations for concentrations of several minor and trace elements principally for nb ta and sn which reflect growth zoning the minor and trace element compositions of the different wolframite samples show at first order similar enrichments in ta nb sn zn and in as well as depletions in lree li th ti zr hf pb v co and mg compared to the upper continental crust second order compositional variations are observed between the studied deposits which can be separated in four main regional geochemical groups this work highlights that the chemical composition of wolframite is controlled by both the crystallochemical parameters and the composition of the primary hydrothermal fluid the regional variations observed in the geochemical signatures of wolframite reflect local variations in the composition of the hydrothermal fluid i e variable metal sources therefore this suggests that the minor and trace element contents in wolframite likely represent direct markers of the source of the hydrothermal fluid and ultimately of the metals the comparison between the trace element compositions of wolframite with the compositions of regional granitic rocks and worldwide shales suggests that evolved peraluminous granites represent the main source of metals in peribatholitic hydrothermal w deposits from the fmc whereas metasedimentary rocks have a limited to no influence on the metal budget

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

pre variscan geological events in the austrian part of the bohemian massif deduced from u pb zircon ages in an attempt to elucidate the pre variscan evolution history of the various geological units in the austrian part of the bohemian massif we have analysed zircons from 12 rocks mainly orthogneisses by means of shrimp conventional multi grain and single grain u pb isotope dilution mass spectrometry two of the orthogneisses studied represent cadomian metagranitoids that formed at ca 610 ma spitz gneiss and ca 580 ma bittesch gneiss a metagranite from the thaya batholith also gave a cadomian zircon age 567 5 ma traces of neoproterozoic zircon growth were also identified in several other samples underlining the great importance of the cadomian orogeny for the evolution of crust in the southern bohemian massif however important magmatic events also occurred in the early palaeozoic a sample of the gfohl gneiss was recognised as a 488 6 ma old granite a tonalite gneiss from the realm of the south bohemian batholith was dated at 456 3 ma and zircon cores in a moldanubian metagranitic granulite gave similar ages of 440 450 ma this ordovician phase of magmatism in the moldanubian unit is tentatively interpreted as related to the rifting and drift of south armorica from the african gondwana margin the oldest inherited zircons in a migmatite from the south bohemian batholith yielded an age of ca 2 6 ga and many zircon cores in both moravian and moldanubian meta granitoid rocks gave ages around 2 0 ga however rocks from the moldanubian unit show a striking lack of zircon ages between 1 8 and 1 0 ga reflecting an ancestry from armorica and the north african part of gondwana respectively whereas the moravian bittesch gneiss contains many inherited zircons with mesoproterozoic and early palaeoproterozoic ages of ca 1 2 1 5 and 1 65 1 8 ga indicating a derivation from the south american part of gondwana

fracture propagation as means of rapidly transferring surface meltwater to the base of glaciers 1 propagation of water filled crevasses through glaciers is investigated based on the linear elastic fracture mechanics approach a crevasse will penetrate to the depth where the stress intensity factor at the crevasse tip equals the fracture toughness of glacier ice a crevasse subjected to inflow of water will continue to propagate downward with the propagation speed controlled primarily by the rate of water injection while the far field tensile stress and fracture toughness determine where crevasses can form once initiated the rate of water driven crevasse propagation is nearly independent of these two parameters thus rapid transfer of surface meltwater to the bed of a cold glacier requires abundant ponding at the surface to initiate and sustain full thickness fracturing before refreezing occurs

the birth of the rheic ocean early palaeozoic subsidence patterns and subsequent tectonic plate scenarios abstract new plate tectonic reconstructions of the gondwana margin suggest that the location of gondwana derived terranes should not only be guided by the models but should also consider the possible detrital input from some asian blocks hunia supposed to have been located along the cambrian gondwana margin and accreted in the silurian to the north chinese block consequently the gondwana margin has to be subdivided into a more western domain where the future avalonian blocks will be separated from gondwana by the opening rheic ocean whereas in its eastern continuation hosting the future basement areas of central europe different periods of crustal extension should be distinguished instead of applying a rather cylindrical model it is supposed that crustal extension follows a much more complex pattern where local back arcs or intra continental rifts are involved guided by the age data of magmatic rocks and the pattern of subsidence curves the following extensional events can be distinguished during the early to middle cambrian a back arc setting guided the evolution at the gondwana margin contemporaneous intra continental rift basins developed at other places related to a general post pan african extensional phase affecting africa upper cambrian formation of oceanic crust is manifested in the chamrousse area and may have lateral cryptic relics preserved in other places this is regarded as the oceanisation of some marginal basins in a context of back arc rifting these basins were closed in a mid ordovician tectonic phase related to the subduction of buoyant material mid ocean ridge since the early ordovician a new phase of extension is observed accompanied by a large scale volcanic activity erosion of the rift shoulders generated detritus armorican quartzite and the rift basins collected detrital zircons from a wide hinterland this phase heralded the opening of palaeotethys but it failed due to the silurian collision eo variscan phase of an intra oceanic arc with the gondwana margin during this time period at the eastern wing of the gondwana margin begins the drift of the future hunia microcontinents through the opening of an eastern prolongation of the already existing rheic ocean the passive margin of the remaining gondwana was composed of the galatian superterranes constituents of the future variscan basement areas remaining under the influence of crustal extension they will start their drift to laurussia since the earliest devonian during the opening of the palaeotethys ocean

rift related volcanism predating the birth of the rheic ocean ossa morena zone sw iberia abstract two very different periods of magma emplacement in the crust of the ossa morena zone early and main events in sw iberia have been previously interpreted to record a cambrian early ordovician rifting event that is thought to have culminated in the opening of the rheic ocean during the early ordovician new stratigraphic petrographic geochemical and sm nd isotope data from cambrian volcanic rocks included in six key low grade sections in both portugal and spain considerably improve our understanding of these events these data 1 confirm the existence of two rift related magmatic events in the cambrian of the ossa morena zone 2 demonstrate that the early rift related event was associated with migmatite and core complex formation in the mid upper crust and is represented by felsic peraluminous rocks the parent magmas of which were derived mainly from crustal sources and 3 show the main rift related event to be represented by a bimodal association of felsic and mafic rocks with minor amounts of intermediate rocks some of the mafic rocks show n morb affinity whereas others have oib or e morb affinities suggesting different heterogeneous mantle sources depleted and enriched asthenospheric and lithospheric plume like and non plume like the acid and intermediate rocks appear to represent hybrid mixtures of crust and mantle derived magmas this new data supports the hypothesis that the onset of rifting was associated with a process of oblique ridge trench collision we interpret the significant differences between the early and main events as reflecting the evolution from a wide rift stage with passive extension mainly accommodated by lower crust flow in a high heat flow setting to a narrow rift stage with active extension characterized by extension rates that outpaced thermal diffusion rates

inland advance of supraglacial lakes in north west greenland under recent climatic warming the inland advance of supraglacial lakes sgls towards the interior regions of the greenland ice sheet gris may have implications for the water volumes reaching the subglacial drainage system and could consequently affect long term ice sheet dynamics here we investigate changes to the areas volumes and elevation distributions of over 8000 manually delineated sgls using 44 landsat images of a 6200 km 2 sector of north west greenland over three decades 1985 2016 our results show that sgls have advanced to higher maximum 418 m and mean 299 m elevations and that there has been a near doubling of total regional sgl areas and volumes over the study period accelerating after 2000 these changes were primarily caused by an increased sgl area and volume at high 1200 m a s l elevations where sgl coverage increased by over 2750 during the study period many of the observed changes particularly the post 2000 accelerations were driven by changes to regional surface temperature anomalies this study demonstrates the past and accelerating response of the gris s hydrological system due to climatic warming indicating an urgent need to understand whether the increasingly inland sgls will be capable of hydrofracture in the future thus determining their potential implications for ice sheet dynamics

u pb geochronology and sm nd isotopic composition of proterozoic gneisses channel islands uk gneissic units of the channel islands united kingdom have traditionally been considered to be palaeoproterozoic basement to the late neoproterozoic cadomia terrane based on lithological correlations and imprecise or ambiguous isotopic data a new precise u pb date of 2061 2 ma based on analyses of single zircons for the icart granitic orthogneiss of guernsey confirms a palaeoproterozoic age of its igneous protolith the nd depleted mantle model age t dm of this gneiss is 2220 ma only slightly older than its crystallization age indicating that it represents juvenile crust other gneisses from guernsey have similar t dm ages ranging from 2210 to 2370 ma suggesting that they are also exposures of juvenile palaeoproterozoic crust a component from a penetratively deformed orthogneiss from sark previously correlated with icartian gneisses on guernsey yields a u pb zircon date of 616 4 2 ma this crystallization age demonstrates that the protolith was a cadomian intrusion and that penetrative deformation and amphibolite facies metamorphism in the northern channel islands were cadomian in age the use of gneissic features as a means of invoking the antiquity of many other undated gneissic units in the region is thus considered unreliable the presence of 2170 7 ma zircon xenocrysts within the sark orthogneiss combined with its initial e nd 615 9 5 are strong evidence that pre cadomian basement was a significant source component and that documenting the extent and age of pre cadomian basement may be possible by indirect geochemical methods

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

longitudinal variations in glacial flow theory and test using data from the byrd station strain network antarctica application de la theorie de budd 1970 des effets du lit sur les variations d ecoulement du glacier aux observations de la calotte de glace a la station byrd on utilise une rheologie visqueuse lineaire et on resout une equation biharmonique des variations des efforts et des vitesses de deformation les conditions aux limites comportent une topographie du fond avec ondulations glissements et cisaillements

electron microprobe monazite geochronology of magmatic events examples from variscan migmatites and granitoids massif central france u th pb dating of monazite with the electron probe microanalyser epma is increasingly documented as a reliable geochronological method offering high spatial resolution this method has been applied on monazite from the cevennes migmatites and granitoids from the southeast of the french massif central measurements were performed on separated grains after systematic back scattered electron bse imaging monazites from migmatites record two main ages i a protolith age of about 550 543 ma obtained on inherited cores and ii a migmatization event between 329 5 and 323 3 ma recorded by monazite rims and all other monogenetic grains monazite from the peraluminous rocles pluton yields a 318 3 ma age finally three granite dykes are dated at 333 6 318 5 and 311 5 ma the older dyke is the most deformed of them and is interpreted as linked to the migmatization event the two other dykes are geochronologically petrologically and structurally coeval with the rocles pluton the data constrain the timing of crustal melting following variscan thickening in the northern cevennes area migmatization of ordovician protoliths took place at 329 323 ma and was shortly followed by intrusion of leucogranite at 318 311 ma the study shows that epma dating of monazite can be successfully used to resolve a close succession of regional melting events

morphological responses of unsheltered channel shoal system to a major storm the combined effects of surges wind driven currents and waves abstract the storm impact on coastal environment is complex and systematic with a validated numerical model this study quantitatively investigates the effects of a historical storm process on the morphodynamic evolution of an unsheltered nanpu channel shoal system ns in the bohai sea china results show that the storm event in october 2003 induced a destructive erosion in the tidal flats during the entire storm period the channel also suffered erosion however it rapidly restored after this storm numerical estimations also confirm that wave and wind driven current dominate the erosion of tidal flat and channel respectively wind driven current can further enhance the erosion on the tidal flat by intensifying the wave current interaction while surge plays a minor role in the channel shoal evolution the morphological evolution pattern of ns varies as a function of wind direction and wind speed where three morphological evolution patterns appear as the wind speed growth in the investigated ne and nw wind directions in the study area ne strong wind usually produces flooding disasters and erosion of ns while nw strong wind can induce severe tidal flat loss and channel accretion although this study is based on a site specific model the findings will provide valuable insights into the morphological feedbacks under destructive storm and hence assist in making efficient coastal management strategies in unsheltered coasts

remote sounding of greenland supraglacial melt lakes implications for subglacial hydraulics a supraglacial lake depth retrieval function is developed based on the correspondence between moderate resolution imaging spectroradiometer modis reflectance and water depth measured during raft surveys individual lake depth area and volume statistics including short term temporal changes for greenland s southwestern ablation region were compiled for 2000 05 the maximum area of an individual lake was found to be 8 9 km 2 the maximum volume 53 0 10 6 m 3 and the maximum depth 12 2 m sampling over 0 0625 km 2 pixel areas the total lake volume reaches 1 km 3 in this region by july each year the importance of melt lake reservoirs to greenland ice sheet flow may be a feedback between abrupt lake drainage events and ice dynamics lake outburst volumes up to 31 5 10 6 m 3 d 1 are capable of providing sufficient water via moulins to hydraulically pressurize the subglacial environment since the overburden pressure at the base of a flooded moulin is greater than that provided by ice lake outburst events seem capable of exerting sufficient upward force to lift the ice sheet locally if water flow in the subglacial environment is constrained laterally considering a moulin with a 10 m 2 cross sectional area basal pressurization can be maintained over lake outburst

the role of hydrologically driven ice fracture in drainage system evolution on an arctic glacier 1 observations from ellesmere island suggest that the connection between surface and subglacial drainage on a predominantly cold glacier is made abruptly by hydrologically driven propagation of fractures from the surface to the bed where ice is 150 m thick water ponded to a depth of 6 9 m within a supraglacial stream system before establishing a permanent bed connection multiple premonitory drainage events preceded the final drainage of ponded water implying that fracturing is necessary but insufficient to establish a permanent link between surface and subglacial drainage refreezing of water that penetrates the first fractures to form may reseal the connection while flow resistance within the subglacial system may delay the onset of continuous through flow a large volume of ponded water is required to enlarge fractures sufficiently by melting to maintain continuous drainage while feedbacks between subglacial hydrology and ice dynamics may assist in maintaining the connection and initiating subglacial outflow