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

ice front variations and speed changes of calving glaciers in the southern patagonia icefield from 1984 to 2011 patagonian icefields are losing volume and their loss is due partly to rapid changes in their outlet glaciers that terminate in lakes or the ocean despite this key influence from outlet glaciers relatively few of these calving glaciers have had high frequency measurements on their frontal variations and ice speed changes we describe here recent frontal variations and ice speed changes of all 28 major calving glaciers in the southern patagonia icefield spi including ice speed maps covering approximately half of the entire icefield the analysis is based on satellite data from 1984 to 2011 over this period only the two termini of glaciar pio xi advanced of the remaining glacial fronts 12 changed less than 0 5 km but 17 retreated at least 0 5 km in the latter group three glacial fronts glaciar jorge montt hps12 and upsala retreated over 6 km averaged over all 31 glacial fronts of the calving glaciers the front positions retreated 1 56 km median is 0 71 km along the flowline within 20 km of the front the ice speeds up to 5900 200 m a 1 except for regions showing large acceleration or deceleration the mean speed over the measured area decreased by 30 m a 1 from 1984 to 2011 the three most rapidly retreating glaciers showed much larger acceleration near the calving front suggesting that ice dynamics drive their rapid retreat thus we see retreat as a long term trend for the calving glaciers in the spi with behavior that implies a dynamically controlled rapid recession that may explain the recently reported volume change of the spi

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

bathymetric control of tidewater glacier mass loss in northwest greenland abstract it has been suggested that fjord geometry could be an important contributor to the observed mass loss variability in greenland by modulating the flow of warm water to marine terminating glaciers new gravity derived bathymetry of greenlandic fjords confirms the link between the grounding line depth and rates of glacier mass loss a relationship previously predicted only in ice models we focus on two neighboring glaciers to minimize differences in external forcing and therefore isolate the role of the fjord bathymetry tracy glacier has a deeper grounding line and has been retreating since 1892 with a contemporary mass budget of 1 63 gt a 1 heilprin glacier has a shallower grounding line depth a stable ice terminus and a mass budget of only 0 53 gt a 1 because of its deeper grounding line tracy has more ice in contact with warm subsurface water leaving it more vulnerable to changes in ocean forcing and therefore mass loss

force perturbation analysis of recent thinning and acceleration of jakobshavn isbrae greenland observations between 1997 and 2001 of a 30 velocity increase and up to 60 m thinning of downstream parts of jakobshavn isbrae greenland immediately following calving of about 4 km of its 15 km floating ice tongue suggest that acceleration may have been initiated by the calving assuming that the force perturbation associated with such weakening is swiftly transmitted far up glacier i develop equations to estimate the perturbation initially the observed changes are consistent with the comparatively small perturbation associated with the calving thereafter it was probably sustained by thinning of the remaining ice tongue at rates of about 80 m a 1 otherwise the force perturbation would soon have been balanced by reduction in the hydrostatic driving force for longitudinal creep as the glacier thinned with velocities dropping to their former values the calculated force perturbation increases to a maximum about 10 km inland of the grounding line consistent with decreasing weight forces as the glacier thins over bedrock that slopes uphill seawards further inland it progressively decreases probably because marginal drag increased as the glacier accelerated both here and on the floating tongue marginal ice appears to have been softened by the influence of locally intense shear on ice temperature and or fabric more recent observations show continued acceleration and thinning and most of the remaining ice tongue calved away in april 2003 so thinning is likely to continue

short term flow dynamics of a retreating tidewater glacier leconte glacier alaska u s a an analysis of motion in the terminus region of leconte glacier alaska u s a delineates mechanisms that are important to tidewater glacier stability this glacier is undergoing rapid retreat since 1995 it has retreated 2 km and thinned 125 m at the present location of the terminus ice velocities in this region are quite high at the terminus they exceed 27 m d 1 our analysis reveals that fluctuations in speed and surface elevation are forced by ocean tides surface melt and precipitation the near terminus ice speed is 180 out of phase with the tide such that high tide corresponds to low speed smaller melt forced diurnal variations in speed are present throughout the lowermost 7 km speed ups in excess of 10 of the mean often occur after rainstorms as a result of increased basal water pressure and storage but the relation is not simple the velocity does not vary over the spring and summer seasons

recurring dynamically induced thinning during 1985 to 2010 on upernavik isstrom west greenland this is the publisher s version also available electronically from http onlinelibrary wiley com

tidewater glaciers frontal flow acceleration and basal sliding a numerical glacier flow model finite element method is used to suggest the processes that control the flow behind the calving front of a tidewater glacier the model is developed for grounded calving glaciers and includes an effective pressure dependent sliding law the sliding law is implemented by adding a soft basal layer with a variable viscosity the model is applied on hansbreen a tidewater calving glacier in svalbard comparison between modeled surface velocities and observed velocity data shows good agreement we conclude that the flow of a grounded calving glacier can be modeled with an effective pressure dependent sliding law

spread of ice mass loss into northwest greenland observed by grace and gps 1 greenland s main outlet glaciers have more than doubled their contribution to global sea level rise over the last decade recent work has shown that greenland s mass loss is still increasing here we show that the ice loss which has been well documented over southern portions of greenland is now spreading up along the northwest coast with this acceleration likely starting in late 2005 we support this with two lines of evidence one is based on measurements fromthegravityrecoveryandclimateexperiment grace satellite gravity mission launched in march 2002 the other comes from continuous global positioning system gps measurements from three long term sites on bedrock adjacent to the ice sheet the grace results provide a direct measure of mass loss averaged over scales of a few hundred km the gps data are used to monitor crustal uplift caused by ice mass loss close to the sites the grace results can be used to predict crustal uplift which can be compared with the gps data in addition to showing that the northwest ice sheet margin is now losing mass the uplift results from both the gps measurements and the grace predictions show rapid acceleration in southeast greenland in late 2003 followed by a moderate deceleration in 2006 because that latter deceleration is weak southeast greenland still appears to be losing ice mass at a much higher rate than it was prior to fall 2003 in a more general sense the analysis described here demonstrates that gps uplift measurements can be used in combination with grace mass estimates to provide a better understanding of ongoing greenland mass loss an analysis approach that will become increasingly useful as long time spans of data accumulate from the 51 permanent gps stations recently deployed around the edge of the ice sheet as part of the greenland gps network gnet citation khan s a j wahr m bevis i velicogna and e kendrick 2010 spread of ice mass loss into northwest greenland observed by grace and gps geophys res lett 37 l06501 doi 10 1029 2010gl042460

present status of the geochronology of the precambrian rocks of rajasthan abstract age determinations mostly by rb sr analyses of the precambrian rocks of rajasthan by us and by others are summarized and discussed broad periods of acid magmatism at 1 3000 2900 m y 2 2600 2500 m y 3 2000 1900 m y 4 1700 1500 m y and 5 850 750 m y were identified the oldest rocks in the area are the yet undated banded gneisses bgc east of udaipur intruded by the untala granite dated at 2950 m y and hence of mid archean age the basal status hitherto attributed to the berach granite dated at 2600 m y is no longer tenable the radiometric control on the beginning and duration of the overlying aravalli supergroup is not yet satisfactory though a lower limit at 2000 m y is indicated heron s original delhi rocks have recorded two magmatic events widely separate in space and time while the earliest granitic activity at 1600 m y is recorded only in the alwar basin in the northeast the younger activity between 850 750 m y is widespread as shown by the nearly concordant ages of erinpura type granites along the aravalli mountain range and the malani rhyolites in the western plains of the aravalli range

greenland ice sheet mass balance distribution of increased mass loss with climate warming 2003 07 versus 1992 2002 we derive mass changes of the greenland ice sheet gis for 2003 07 from icesat laser altimetry and compare them with results for 1992 2002 from ers radar and airborne laser altimetry the gis continued to grow inland and thin at the margins during 2003 07 but surface melting and accelerated flow significantly increased the marginal thinning compared with the 1990s the net balance changed from a small loss of 7 3g t a 1 in the 1990s to 171 4g t a 1 for 2003 07 contributing 0 5 mm a 1 to recent global sea level rise we divide the derived mass changes into two components 1 from changes in melting and ice dynamics and 2 from changes in precipitation and accumulation rate we use our firn compaction model to calculate the elevation changes driven by changes in both temperature and accumulation rate and to calculate the appropriate density to convert the accumulation driven changes to mass changes increased losses from melting and ice dynamics 17 206 gt a 1 are over seven times larger than increased gains from precipitation 10 35 gt a 1 during a warming period of 2 k 10 a 1 over the gis above 2000 m elevation the rate of gain decreased from 44 to 28 gt a 1 while below 2000 m the rate of loss increased from 51 to 198 gt a 1 enhanced thinning below the equilibrium line on outlet glaciers indicates that increased melting has a significant impact on outlet glaciers as well as accelerating ice flow increased thinning at higher elevations appears to be induced by dynamic coupling to thinning at the margins on decadal timescales

hydrogeochemical zonation and its implication for arsenic mobilization in deep groundwaters near alluvial fans in the hetao basin inner mongolia summary high as groundwater has been found in shallow aquifers of the flat plain of the hetao basin but little is known about as concentration in deep groundwaters around piedmont areas which are the major drinking water resources one hundred and three groundwater samples from wells with depths 50 m and seven samples from one multi level monitoring well 89 m in depth were analyzed for chemical compositions and 18o and d isotopes to examine the geochemical processes controlling as mobilization according to hydrogeological setting chemical and isotopic characteristics of deep groundwater three distinguished hydrogeochemical zones are delineated including recharge oxic zone zone i groundwater flow moderate reducing zone zone ii and groundwater flow reducing zone zone iii zone i is located in proximal fans in the recharge area with oxic conditions where low as groundwater generally occurs in zone ii located in the intermediate between the fans and the flat plain with fe reduction predominated groundwater as is moderate zone iii lies in the flat plain with the occurrence of so 4 2 reduction where high as groundwater is mostly found this indicates that release of as to groundwater is primarily determined by reduction sequences arsenic is immobilized in o 2 no 3 reduction stage in zone i and released in fe reducing conditions of zone ii and displays a significant elevated concentration in so4 reducing stage in zone iii dissolution of carbonate minerals occurs in zone i while ca2 and mg2 are expected to precipitate in zone ii and zone iii in the multi level monitoring well both chemical and isotopic compositions are dependent of sampling depths with the similar trend to the hydrogeochemical zonation along the flow path the apparent increases in d and 18o values in zone iii reveal the possibility of high as shallow groundwater recharge to deep groundwater it indicates that deep groundwaters in proximal fans have low as concentrations and are considered as safe drinking water resources in the hetao basin however high as concentration is frequently observed in deep groundwater in the flat plain which should be routinely monitored in order to avoid chronic as poisoning

aerial photographs reveal late 20th century dynamic ice loss in northwestern greenland global warming is predicted to have a profound impact on the greenland ice sheet and its contribution to global sea level rise recent mass loss in the northwest of greenland has been substantial using aerial photographs we produced digital elevation models and extended the time record of recent observed marginal dynamic thinning back to the mid 1980s we reveal two independent dynamic ice loss events on the northwestern greenland ice sheet margin from 1985 to 1993 and 2005 to 2010 which were separated by limited mass changes our results suggest that the ice mass changes in this sector were primarily caused by short lived dynamic ice loss events rather than changes in the surface mass balance this finding challenges predictions about the future response of the greenland ice sheet to increasing global temperatures

high resolution bed topography mapping of russell glacier greenland inferred from operation icebridge data detailed maps of bed elevation and ice thickness are essential for understanding and projecting the evolution of the ice sheets such maps are traditionally obtained using airborne radarsounding profiler data interpolated onto regular grids using geostatistical tools such as kriging here we compare three mapping techniques applied to a dense radar survey of russell glacier west greenland by nasa operation icebridge 1 radar tomography rt processing of the radar data to map the bed elevation 2 interpolation of radar derived thickness by ordinary kriging kr and 3 reconstruction of ice thickness based on the principles of mass conservation mc combining radar sounding profiler and ice motion data rt eliminates ambiguities caused by off nadir reflections but is spatially limited kr yields a standard error in bed elevation of 35 m but large errors 300m a 1 in flux divergence when combined with ice motion data mc yields a comparable performance in bed elevation mapping and errors smaller than 1m a 1 in flux divergence when the number of radar sounding tracks is reduced the performance of kr decreases more rapidly than for mc our study site shows that mc is capable of maintaining precision levels of 60m at 400m posting with flight tracks separated by 5 km 2014 publishing technology

flow dynamics of tidewater glaciers a numerical modelling approach the dynamics of grounded tidewater glaciers is investigated with a time dependent numerical flow model which solves the full equations for the stress and velocity fields and includes a water pressure dependent sliding law the calving criterion implemented in the model shifts the calving front at each time step to the position where the frontal ice thickness exceeds flotation height by a prescribed value with this model the linear relation between calving rate and water depth proposed on empirical grounds is qualitatively reproduced for the situation of a slowly retreating or advancing terminus but not for situations of rapid changes length changes of tidewater glaciers i e especially rapid changes are dominantly controlled by the bed topography and are to a minor degree a direct reaction to a mass balance change thus accurate information on the near terminus bed topography is required for reliable prediction of the terminus changes due to climate changes the results also confirm the suggested cycles of slow advance and rapid retreat through a basal depression rapid changes in terminus positions preferably occur in places where the bed slopes upwards in the ice flow direction

rapid thinning of lake calving yakutat glacier and the collapse of the yakutat icefield southeast alaska usa both lake calving yakutat glacier 337 km 2 alaska usa and its parent icefield 810 km 2 are experiencing strong thinning and under current climate conditions will eventually disappear comparison of digital elevation models shows that yakutat glacier thinned at area averaged rates of 4 76 0 06 m w e a 1 2000 07 and 3 66 0 03 m w e a 1 2007 10 simultaneously adjacent yakutat icefield land terminating glaciers thinned at lower but still substantial rates 3 79 and 2 94 m w e a 1 respectively for the same time periods indicating lake calving dynamics helps drive increased mass loss yakutat glacier terminates into harlequin lake and for over a decade sustained a 3 km long floating tongue which started to disintegrate into large tabular icebergs in 2010 such floating tongues are rarely seen on temperate tidewater glaciers we hypothesize that this difference is likely due to the lack of submarine melting in the case of lake calving glaciers floating tongue ice losses were evaluated in terms of overall mass balance and contribution to sea level rise the post little ice age collapse of yakutat icefield was driven in part by tidewater calving retreats of adjacent glaciers the lake calving retreat of yakutat glacier a warming climate and by the positive feedback mechanisms through surface lowering

improved ice loss estimate of the northwestern greenland ice sheet 1 we estimate ice volume change rates in the northwest greenland drainage basin during 2003 2009 using ice cloud and land elevation satellite icesat laser altimeter data elevation changes are often reported to be largest near the frontal portion of outlet glaciers to improve the volume change estimate we supplement the icesat data with altimeter surveys from nasa s airborne topographic mapper from 2002 to 2010 and nasa s land vegetation and ice sensor from 2010 the airborne data are mainly concentrated along the ice margin and thus have a significant impact on the estimate of the volume change our results show that adding airborne topographic mapper and land vegetation and ice sensor data to the icesat data increases the catchment wide estimate of ice volume loss by 11 mainly due to an improved volume loss estimate along the ice sheet margin furthermore our results show a significant acceleration in mass loss at elevations above 1200m both the improved mass loss estimate along the ice sheet margin and the acceleration at higher elevations have implications for predictions of the elastic adjustment of the lithosphere caused by present day ice mass changes our study shows that the use of icesat data alone to predict elastic uplift rates biases the predicted rates by several millimeters per year at gps locations along the northwestern coast

rapid retreat and acceleration of helheim glacier east greenland 1 a significant amount of the measured coastal thinning of the greenland ice sheet may be due to recent acceleration of outlet glaciers using remote sensing we measured two major periods of speedup on helheim glacier between 2000 and 2005 that increased peak speeds from approximately 8 to 11 km yr these speedups coincided with rapid retreats of the calving front totaling over 7 5 km the glacier also thinned by over 40 m from 2001 to 2003 retreat of the ice front appears to decrease resistance to flow and concentrates the gravitational driving force over a smaller area farther up glacier acceleration may be a delayed response to surface draw down and steepening of the glacier s main trunk if the 2005 speedup also produces strong thinning then much of the glacier s main trunk may un ground leading to further retreat citation howat i m i joughin s tulaczyk and s gogineni 2005 rapid retreat and acceleration of helheim glacier east greenland geophys res lett 32 l22502 doi 10 1029 2005gl024737

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

glacier dynamics near the calving front of bowdoin glacier northwestern greenland to better understand recent rapid recession of marine terminating glaciers in greenland we performed satellite and field observations near the calving front of bowdoin glacier a 3 km wide outlet glacier in northwestern greenland satellite data revealed a clear transition to a rapidly retreating phase in 2008 from a relatively stable glacier condition that lasted for 20 years ice radar measurements showed that the glacier front is grounded but very close to the floating condition these results in combination with the results of ocean depth soundings suggest bed geometry in front of the glacier is the primary control on the rate and pattern of recent rapid retreat presumably glacier thinning due to atmospheric and or ocean warming triggered the initial retreat in situ measurements showed complex short term ice speed variations which were correlated with air temperature precipitation and ocean tides ice speed quickly responded to temperature rise and a heavy rain event indicating rapid drainage of surface water to the bed semi diurnal speed peaks coincided with low tides suggesting the major role of the hydrostatic pressure acting on the calving face in the force balance these observations demonstrate that the dynamics of bowdoin glacier are sensitive to small perturbations occurring near the calving front