hydrological characteristics of the drainage system beneath a surging glacier a rare combination of natural circumstances permits assessment of current theories on water flow beneath glaciers outburst floods from the subglacial lake grimsvotn in iceland took place before during and after surging of skei ararjokull the glacier beneath which the outburst floods drain the observable drainage patterns associated with these floods show the different nature of the basal water conduit system of the glacier during surge and non surge phases during surge conditions basal water is dispersed slowly across the bed in a distributed drainage system but when the glacier is not surging water is transported rapidly through a system of tunnels

a coupled sheet conduit mechanism for jokulhlaup propagation 1 the largest glacier outburst flood jokulhlaup ever recorded in iceland occurred in 1996 and came from subglacial lake grimsvotn in vatnajokull ice cap among other noteworthy features this flood was characterized by an unprecedentedly high lake level prior to flood initiation extremely rapid linear rise in lake discharge delay between the onset of lake drainage and floodwater arrival at the glacier terminus formation of short lived supraglacial fountains and initially unchannelized outbursts of floodwater at the terminus observations suggest that the 1996 flood propagation mechanism was fundamentally different than that of previously observed floods from grimsvotn we advance a new model whereby floodwater initially propagates in a turbulent subglacial sheet which feeds a nascent system of conduits this model is able to explain key observations made of the 1996 jokulhlaup and may shed light on other outburst floods that do not conform to the standard model

seasonal acceleration of inland ice via longitudinal coupling to marginal ice we use an ice flow model to demonstrate how flow variations initiated in the marginal zone of an ice sheet affect flow farther inland through longitudinal along flow coupling our findings allow for an alternate interpretation of seasonal accelerations observed near the equilibrium line of the greenland ice sheet zwally and others 2002 we demonstrate that these observations can be explained by accelerations initiated up to 12 km closer to the margin where the ice is 40 thinner is heavily crevassed experiences a seasonal doubling of velocity and where the ablation rate surface meltwater flux and ice temperature are likely higher our modeling and observations suggest that conditions and processes normally found near ice sheet margins are adequate for explaining the observations of zwally and others 2002 this and considerations of the likely subglacial hydrology in the marginal zone lead us to suggest that seasonal accelerations may have limited impact on ice sheet mass balance even in the face of climate warming

the role of subglacial water in ice sheet mass balance subglacial water can significantly affect the velocity of ice streams and outlet glaciers of ice sheets depending on the geometry and capacity of the subglacial hydrologic system increased surface melting in greenland over the coming decades may influence the ice sheet s mass balance furthermore subglacial lakes in antarctica can modulate ice velocities and act as nucleation points for new fast flowing ice streams in the coming decades significant changes in the polar regions will increase the contribution of ice sheets to global sea level rise under the ice streams and outlet glaciers that deliver ice to the oceans water and deformable wet sediments lubricate the base facilitating fast ice flow the influence of subglacial water on fast ice flow depends on the geometry and capacity of the subglacial hydrologic system water moving rapidly through a well connected system of conduits or channels will have little impact on ice sheet velocities but water injected into a spatially dispersed subglacial system may reduce the effective pressure at the base of the ice sheet and thereby trigger increased ice sheet velocities in greenland the form of the subglacial hydrologic system encountered by increasing surface melt water will determine the influence of changing atmospheric conditions on ice sheet mass balance in antarctica subglacial lakes have the capacity to both modulate velocities in ice streams and outlet glaciers and provide nucleation points for new fast ice flow tributaries climate models of ice sheet responses to global change remain incomplete without a parameterization of subglacial hydrodynamics and ice dynamics

stability of an upland gravel bed stream swinhope burn northern england some upland gravel bed streams often undergo frequent channel change in response to floods and changes in sediment supply however in others where local conditions permit long term channel planform stability can be observed this paper uses evidence from swinhope burn in upper weardale northern england to demonstrate relative channel stability over a 180 year period channel planform change is identified using seven historical maps and air photographs dating from 1815 to 1991 the 1 4 km study reach has retained a stable meandering pattern over a period of 180 years with a temporary but dramatic change to a straight low sinuosity partly divided channel at some point between 1815 and 1856 identifiable on the 1844 tithe map channel planform stability observed over the historic period is related in part to the low channel gradient upstream of a cross valley moraine situated at the lower end of the study reach this moraine has led to partial closure of the valley system and the development of a small upstream floodplain or sedimentation zone average width of 150 m the resulting low channel gradient 0 012 inhibits coarse bedload transport and encourages overbank sedimentation cohesive banks promote lateral channel stability and the wide floodplain reduces potential coupling between the channel and valley side slope sediment sources the probable cause of the observed channel planform change is the combination of a succession of four major floods in the river wear catchment during the 1820s and episodic inputs of sediment generated by upstream metal mining between 1823 and 1846 large floods in the past 40 years have produced very little evidence of lasting channel change however locally a channel avulsion has been documented following a flood in february 1997 this represents a useful modern analogue for previous mechanisms of channel change

a multicomponent coupled model of glacier hydrology 1 theory and synthetic examples 1 basal hydrology is acknowledged as a fundamental control on glacier dynamics especially in cases where surface meltwater reaches the bed for many glaciers at midlatitudes basal drainage is influenced by subaerial englacial and subsurface water flow one of the major shortcomings of existing basal hydrology models is the treatment of the glacier bed as an isolated system we present theoretical and computational models that couple glacier surface runoff englacial water storage and transport subglacial drainage and subsurface groundwater flow each of the four model components is represented as a two dimensional vertically integrated layer that communicates with its neighbors through water exchange governing equations are derived from the law of mass conservation and are expressed as a balance between the internal distribution of water and external sources the numerical exposition of this theory is a time dependent finite difference model that can be used to simulate glacier drainage in this paper we outline the theory and conduct simple tests using an idealized glacier geometry in the companion paper the model is tailored to trapridge glacier yukon territory canada where results are compared with measurements of subglacial water pressure

pleistocene collapse of the west antarctic ice sheet some glacial sediment samples recovered from beneath the west antarctic ice sheet at ice stream b contain quaternary diatoms and up to 108 atoms of beryllium 10 per gram other samples contain no quaternary diatoms and only background levels of beryllium 10 less than 10 6 atoms per gram the occurrence of young diatoms and high concentrations of beryllium 10 beneath grounded ice indicates that the ross embayment was an open marine environment after a late pleistocene collapse of the marine ice sheet

progressive increase in ice loss from greenland 1 laser altimeter measurements over greenland show increasing thickening rates above 2000 m reflecting increasing snowfall in a warming climate but near coastal thinning rates have increased substantially since the mid 1990s and net mass loss more than doubled from an average of 4 50 gt yr 1 between 1993 4 and 1998 9 to 57 105 gt yr 1 between 1998 9 and 2004 this increasing trend is very similar to findings from independent mass budget studies but differs widely from ers radar altimeter results this may result from limitations associated with the large ers footprint over sloping and undulating surfaces that typify fast narrow glaciers where thinning is most pronounced

an ice shelf model test based on the ross ice shelf a standard numerical experiment featuring the ross ice shelf antarctica is presented as a test package for the development and intercomparison of ice shelfmodels the emphasis of this package is solution of stress equilibrium equations for an ice shelf velocity consistent with present observations as ademonstration we compare five independently developed ice shelf models based on finite difference and finite element methods our results suggest thatthere is little difference between finite element and finite difference methods in capturing the basic large scale flow features of the ice shelf we additionallyshow that the fit between model and observed velocity depends strongly on the ice shelf temperature field for which there is presently little observationalcontrol the main differences between model results are due to the equations being solved the boundary conditions at the ice front and the discretisationmethod finite element vs finite difference

an active subglacial water system in west antarctica mapped from space satellite laser altimeter elevation profiles from 2003 to 2006 collected over the lower parts of whillans and mercer ice streams west antarctica reveal 14 regions of temporally varying elevation which we interpret as the surface expression of subglacial water movement vertical motion and spatial extent of two of the largest regions are confirmed by satellite image differencing a major previously unknown subglacial lake near the grounding line of whillans ice stream is observed to drain 2 0 cubic kilometers of water into the ocean over 3 years while elsewhere a similar volume of water is being stored subglacially these observations reveal a wide spread dynamic subglacial water system that may exert an important control on ice flow and mass balance

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

recent ice sheet growth in the interior of greenland a continuous data set of greenland ice sheet altimeter height from european remote sensing satellites ers 1 and ers 2 1992 to 2003 has been analyzed an increase of 6 4 0 2 centimeters per year cm year is found in the vast interior areas above 1500 meters in contrast to previous reports of high elevation balance below 1500 meters the elevation change rate is 2 0 0 9 cm year in qualitative agreement with reported thinning in the ice sheet margins averaged over the study area the increase is 5 4 0 2 cm year or 60 cm over 11 years or 54 cm when corrected for isostatic uplift winter elevation changes are shown to be linked to the north atlantic oscillation

recent dramatic thinning of largest west antarctic ice stream triggered by oceans a growing body of observational data suggests that pine island glacier pig is changing on decadal or shorter timescales these changes may have far reaching consequences for the future of the west antarctic ice sheet wais and global sea levels because of pig s role as the ice sheet s primary drainage portal we test the hypothesis that these changes are triggered by the adjoining ocean specifically we employ an advanced numerical ice flow model to simulate the effects of perturbations at the grounding line on pig s dynamics the speed at which these changes are propagated upstream implies a tight coupling between ice sheet interior and surrounding ocean

bedmap a new ice thickness and subglacial topographic model of antarctica measurements of ice thickness on the antarctic ice sheet collected during surveys undertaken over the past 50 years have been brought together into a single database from these data a seamless suite of digital topographic models have been compiled for antarctica and its surrounding ocean this includes grids of ice sheet thickness over the grounded ice sheet and ice shelves water column thickness beneath the floating ice shelves bed elevation beneath the grounded ice sheet and bathymetry to 60 s including the sub ice shelf cavities these grids are consistent with a recent high resolution surface elevation model of antarctica while the digital models have a nominal spatial resolution of 5 km such high resolution is justified by the original data density only over a few parts of the ice sheet the suite does however provide an unparalleled vision of the geosphere beneath the ice sheet and a more reliable basis for ice sheet modeling than earlier maps the total volume of the antarctic ice sheet calculated from the bedmap grid is 25 4 million km3 and the total sea level equivalent derived from the amount of ice contained within the grounded ice sheet is 57 m comprising 52 m from the east antarctic ice sheet and 5 m from the west antarctic ice sheet slightly less than earlier estimates the gridded data sets can be obtained from the authors

regional scale fluid flow and element mobility in barrow s metamorphic zones stonehaven scotland the geochemistry of metamorphic quartz vein formation in barrow s index mineral zones north of stonehaven scotland was investigated in order to assess regional fluid flow and mass transfer metamorphic grade in the dalradian metasediments increases to the north northwest away from the highland boundary fault hbf and associated ophiolitic rocks of the highland border complex hbc passing through the chlorite chl biotite bt garnet grt chloritoid cld and staurolite st zones syn metamorphic fluid infiltration at 462 8 8 ma breeding et al in am mineral 89 1067 2004 produced considerable quartz veining vein abundance varies from about 5 to 15 volume percent of the outcrops veins tend to be more abundant in metapelitic layers than in metapsammitic ones metamorphic veins are surrounded by centimeter to decimeter wide zones of chemical and mineralogical alteration selvages porphyroblasts particularly bt grt cld and st are typically larger in selvages than in wallrocks distal to veins the altered selvages underwent fluid driven addition of na ca and sr and loss of k rb and ba alteration is most intense within 750 m of the hbf but is still very significant at the northern end of the field area some 2 km away mg fet fet total iron was either unchanged or increased due to alteration silica was added at some chl and bt zone localities near the hbf pb mass transfer was variable although pb was added at a number of locations rare earth elements ree were generally immobile but light ree and possibly heavy ree were lost at one field site the gain of na and ca and loss of k promoted the growth of plagioclase at the expense of micas particularly muscovite in selvages and wallrock inclusions throughout the field area and probably some calcite and or dolomite growth directly adjacent to the hbf the ca gains were also critical for epidote production in the bt zone gains of ca and increases in mg fet helped to stabilize grt at the expense of cld and st in some selvages hornblende and cummingtonite were discovered in strongly altered metapelitic rocks at one cld zone locality the metasomatism puts important constraints on the processes of mass transfer and suggests two models for regional fluid flow in the first model fluid flow in a direction of increasing temperature downward along the hbf added na and ca and removed k from the dalradian in the second model fluid flow upward from the hbc transported na and ca into the overlying dalradian and at the same time stripped out k the latter model is favored because it can most readily account for silica addition near the hbf but neither model can be ruled out at present in either case the veins represent fractures that transmitted very large time integrated fluid fluxes of at least 104 m3 fluid m2 rock consequently the veins were conduits for regional fluid flow that caused considerable open system chemical and mineralogical alteration during metamorphism

fast flow signature in the stagnated kamb ice stream west antarctica among the major ice streams that drain west antarctica kamb ice stream formerly called ice stream c is unique in that it stagnated 150 yr ago but its former fast flow conditions are virtually unknown here we presentsurface based radar profiles of the ice stream s undulating internal stratigraphy which records these conditions our analysis of the profiles indicates that pre stagnation flow velocities averaged over a period 740 yr exceeded 350 m yr 1 in the trunk of the ice stream this velocity constraint would be lower if the ice had been thickening higher if thinning but suggests mass loss from the ice stream catchment that is of sufficient magnitude to reverse the gain estimated for today s siple coast region analysis of other ice streams would allow comparison of velocities over millennial time scales with observations of present day velocities useful for evaluating how west antarctic ice drainage has evolved

glacier surge mechanism 1982 1983 surge of variegated glacier alaska the hundredfold speedup in glacier motion in a surge of the kind the kind that took place in variegated glacier in 1982 1983 is caused by the buildup of high water pressure in the basal passageway system which is made possible by a fundamental and pervasive change in the geometry and water transport characteristics of this system the behavior of the glacier in surge has many remarkable features which can provide clues to a detailed theory of the surging process the surge mechanism is akin to a proposed mechanism of overthrust faulting

melting and freezing beneath the ross ice streams antarctica we have estimated temperature gradients and melt rates at the bottom of the ice streams in west antarctica measured velocities were used to include the effects of horizontal advection and strain heating in the temperature model and to determine shear heating at the bed our modeled temperatures agree well with measured temperatures from boreholes in regions of steady flow we find that ice stream tributaries and the inland ice account for about 87 of the total melt generated beneath the ross ice streams and their catchments our estimates indicate that the ice plains of whillans ice stream and ice stream c even when active have large areas subject to basal freezing confirming earlier estimates that import of water from upstream is necessary to sustain motion the relatively low melt rates on whillans ice stream are consistent with observations of deceleration over the last few decades and suggest a shutdown may take place in the future possibly within this century while there are pockets of basal freezing beneath ice streams d and e there are larger areas of basal melt that produce enough melt to more than offset the freezing which is consistent with inferences of relatively steady flow for these ice streams over the last millennium

substantial contribution to sea level rise during the last interglacial from the greenland ice sheet during the last interglacial period the eemian global sea level was at least three metres and probably more than five metres higher than at present1 2 complete melting of either the west antarctic ice sheet or the greenland ice sheet would today raise sea levels by 6 7 metres but the high sea levels during the last interglacial period have been proposed to result mainly from disintegration of the west antarctic ice sheet3 with model studies attributing only 1 2 m of sea level rise to meltwater from greenland4 5 this result was considered consistent with ice core evidence4 although earlier work had suggested a much reduced greenland ice sheet during the last interglacial period6 here we reconsider the eemian evolution of the greenland ice sheet by combining numerical modelling with insights obtained from recent central greenland ice core analyses our results suggest that the greenland ice sheet was considerably smaller and steeper during the eemian and plausibly contributed 4 5 5 m to the sea level highstand during that period we conclude that the high sea level during the last interglacial period most probably included a large contribution from greenland meltwater and therefore should not be interpreted as evidence for a significant reduction of the west antarctic ice sheet

continental ice in greenland during the eocene and oligocene the eocene and oligocene epochs approximately 55 to 23 million years ago comprise a critical phase in earth history an array of geological records supported by climate modelling indicates a profound shift in global climate during this interval from a state that was largely free of polar ice caps to one in which ice sheets on antarctica approached their modern size however the early glaciation history of the northern hemisphere is a subject of controversy here we report stratigraphically extensive ice rafted debris including macroscopic dropstones in late eocene to early oligocene sediments from the norwegian greenland sea that were deposited between about 38 and 30 million years ago our data indicate sediment rafting by glacial ice rather than sea ice and point to east greenland as the likely source records of this type from one site alone cannot be used to determine the extent of ice involved however our data suggest the existence of at least isolated glaciers on greenland about 20 million years earlier than previously documented at a time when temperatures and atmospheric carbon dioxide concentrations were substantially higher