stabilizing feedbacks in glacier bed erosion glaciers often erode transport and deposit sediment much more rapidly than nonglacial environments1 with implications for the evolution of glaciated mountain belts and their associated sedimentary basins but modelling such glacial processes is difficult partly because stabilizing feedbacks similar to those operating in rivers2 3 have not been identified for glacial landscapes here we combine new and existing data of glacier morphology and the processes governing glacier evolution from diverse settings to reveal such stabilizing feedbacks we find that the long profiles of beds of highly erosive glaciers tend towards steady state angles opposed to and slightly more than 50 per cent steeper than the overlying ice air surface slopes and that additional subglacial deepening must be enabled by non glacial processes climatic or glaciological perturbations of the ice air surface slope can have large transient effects on glaciofluvial sediment flux and apparent glacial erosion rate

proglacial lakes character behaviour and geological importance proglacial lakes are ubiquitous within the quaternary record and can provide exceptional breadth and depth of palaeoenvironmental information present deglaciation is increasing the number and size of proglacial lakes around the world this study provides a synthesis of knowledge on proglacial lake character and behaviour and critically evaluates the importance of proglacial lakes from a geological perspective we show how ice marginal or ice contact lakes and other distal proglacial lakes can be distinguished from each other by geomorphological sedimentological chemical and biological characteristics the key controls on proglacial lake geomorphology and sedimentology are outlined and discussed proglacial lakes can exacerbate mountain glacier and ice sheet margin ablation via mechanical and thermal stresses but very large lakes can moderate summer air temperatures and relatively retard summer ice ablation proglacial lakes interrupt meltwater flux and are very efficient sediment traps hydrological routing and consequent geomorphological activity can be radically modified by sudden drainage of proglacial lakes and resultant glacial lake outburst floods exceptionally large proglacial lake drainages affected global ocean circulation and global climate during the quaternary overall analyses of proglacial lakes can provide a valuable insight into i patterns character and behaviour of mountain glaciers ice sheets and glaciations and ii the impacts of past present and future deglaciation

the record of glacial lake champagne in kusawa lake southwestern yukon territory late pleistocene glaciers dammed glacial lake champagne in the valley of dezadeash river between a westward flowing glacier in the takhini valley and eastward flowing glaciers from the st elias complex modern kusawa lake lies in the southern extension of lake champagne geophysical and geomorphic evidence documents the deglaciation of the lake the presence of lake champagne and the postglacial sedimentary environment of the basin during the main phase of lake champagne the water level stood at 772 m in the northern part of kusawa lake and 756 m in northern dezadeash valley both probably controlled by a spillway floored at 756 m to the north into the nordenskiold river this indicates differential isostatic rebound of 0 2 m km from south to north at that time a trunk glacier occupied the southern portion of kusawa lake depositing a thick sequence of sediment in the basin a glacier in the primrose valley and the takhini trunk glacier built large deltas into lake champagne subsequently the level

sediment evacuation and glacial erosion rates at a small alpine glacier 1 glacial erosion is known to be driven by sliding which occurs in brief episodes in the melt season what is not known is how well this sliding history is reflected in the sediment output from a glacier and to what degree temporary sediment storage in the glacier smoothes the history of sediment production we document sediment evacuation rates for three seasons at bench glacier a small temperate alpine glacier in the chugach range of alaska two lines of evidence suggest that subglacial sediment storage is small strong hysteresis in the relationship between sediment and water discharge argues for sediment exhaustion from the subglacial system on seasonal and flood event timescales uplift of the glacier surface during periods of enhanced basal motion reflects intimate contact of the glacier sole with up glacier inclined stoss slopes of bedrock bumps sediment evacuation rates at bench glacier should therefore closely track glacial erosion rates over annual timescales much of the sediment emerges in a major pulse that closely follows the termination of enhanced sliding each year suggesting that the conduits and well connected cavities that allowed removal of englacial water to terminate sliding also promoted efficient transport of sediment this small glacier efficiently modifies the landscape lowering its bed by 1 2 mm yr while sliding only 1 2 m yr our sparse measurements of instantaneous bed load discharge average 41 of concurrent suspended sediment discharge assuming that half of the suspended load derives from abrasion of subglacially quarried clasts roughly two thirds of the sediment evacuated can be attributed to quarrying

the sediment delivery problem abstract the linking of on site rates of erosion and soil loss within a drainage basin to the sediment yield at the basin outlet and improved knowledge and representation of the associated processes of sediment delivery represent a major research need within the field of erosion and sedimentation and also an important scale problem in drainage basin studies this paper reviews the limitations of the sediment delivery ratio concept by considering the problems of temporal and spatial lumping and its blackbox nature some recent advances in our understanding of the sediment delivery system and its modelling are described and the lack of detailed empirical investigations is highlighted the significance of recent concern for the role of sediments in the transport of nutrients and contaminants to sediment delivery studies is introduced and the need for further work in this field is emphasised

extreme sediment delivery events recorded in the contemporary sediment record of a montane lake southern coast mountains british columbia the extreme sediment delivery regime of a montane catchment was examined through the analysis of recent lacustrine varve deposits recovered from a high density sampling program years of anomalously high sediment yield from the green lake watershed were identified over a 70 year period 1930 2000 based on whole lake varve deposit volumes intra annual sediment delivery events were categorized and described for those anomalous years using historical hydroclimatic data and the stratigraphic record observed within inflow proximal sediment cores extreme sediment delivery coincided with high discharge conditions and elevated sediment availability resulting from 1 rapid glacier recession of the early twentieth century 2 late summer and autumn rainstorm generated floods and 3 freshet floods caused by unusual snowmelt conditions the thickness and physical characteristics of varves vary among years that experienced different types of moderate and extreme sediment delivery events in relation to the defin

lake core record of grinnell glacier dynamics during the latest pleistocene deglaciation and the younger dryas glacier national park montana usa abstract few records in the alpine landscape of western north america document the geomorphic and glaciologic response to climate change during the pleistocene holocene transition while moraines can provide snapshots of glacier extent high resolution records of environmental response to the end of the last glacial maximum younger dryas cooling and subsequent warming into the stable holocene are rare we describe the transition from the late pleistocene to the holocene using a 17 000 yr sediment record from swiftcurrent lake in eastern glacier national park mt with a focus on the period from 17 to 11 ka total organic and inorganic carbon grain size and carbon nitrogen data provide evidence for glacial retreat from the late pleistocene into the holocene with the exception of a well constrained advance during the younger dryas from 12 75 to 11 5 ka increased detrital carbonate concentration in swiftcurrent lake sediment reflects enhanced glacial erosion and sediment transport likely a result of a more proximal ice terminus position and a reduction in the number of alpine lakes acting as sediment sinks in the valley

environmental reconstruction from a varve network in the southern coast mountains british columbia canada cores of annually laminated sediments varves from five lakes in the southern coast mountains of british columbia canada document clastic sediment response to climate and geomorphic change over the past 120 years interannual varve thickness correlates with annual flood magnitude interdecadal trends in varve thickness are influenced by other environmental factors such as glacier recession despite major differences in the lakes and their contributing watersheds substantial concordance is observed among the records a pronounced change in the nature of lake sedimentation accompanied by higher interannual variability occurred in 1980 the change coincides with an increase in the magnitude of autumn flooding and major re organization of the north pacific climate system these results highlight new directions for palaeoenvironmental research using varved sediment records specifically to study the magnitude and spatial extent of past hydro climatic events

century scale variability in late summer rainfall events recorded over seven centuries in subannually laminated lacustrine sediments white pass british columbia formation of annually laminated sediments in summit lake white pass british columbia is controlled by runoff generated by snowpack and glacier melt and major rainfall events the 700 yr varve record is divided into two subannual series early and late based on sedimentological criteria and sedimentary structures within each varve a comparison of recent subannual laminae with nearby meteorological records supports the interpretation they are formed by river discharge events generated by major snow and glacier melt events and large late summer rainfall events a significant correlation exists between the late subannual thickness series and the size of the largest rainfall events in late summer the long record indicates there was an abrupt increase in the thickness and frequency of major rainfall induced sedimentary events at the end of the seventeenth century in addition the frequency of laminae generated by early runoff events also increased however early subannual varve thickness component remains statistically the same as the thickness prior to the end of the seventeenth century this suggests the change in varve thickness at this time is due to increases in major late summer rainfall frequency rather than increased sediment availability caused by regional little ice age glacier advances

post little ice age record of coarse and fine clastic sedimentation in an alaskan proglacial lake many assessments of glacial sediment yield rely solely on mea surements of fine grained suspended sediment we show that sus pended sediment contributes 20 of the total clastic sediment discharge into iceberg lake an alaskan proglacial lake drainage of this lake in 1999 exposed outcrops of varved lacustrine sediment that record suspended sediment deposition since a little ice age highstand two centuries ago lake level has dropped in four abrupt spillway controlled events that are recorded stratigraphically as transient increases in the basinwide deposition of unusually fine grained sediments varve counting constrains ages of these events and hence of each abandoned shoreline and associated delta vol umes of deltaic and lake floor sediments constrain specific yields from four multidecadal intervals between a d 1825 and 1999 to tal yield ranges from 3 8 to 4 9 3 10 3 t km 22 yr 21 and most of the sediment 81 86 is sand and gravel now in the deltas a range of evidence suggests that sediment yield faithfully records up stream sediment production in this small 66 km 2 and heavily gla cierized basin implying an effective subglacial erosion rate of 1 6 mm yr these data show that estimates of total yield based upon suspended sediment alone are subject to large errors and suggest that quarrying accounts for a large fraction of the glacial erosion in this system

testing a glacial erosion rule using hang heights of hanging valleys jasper national park alberta canada 1 in most models of glacial erosion glacier sliding velocity is hypothesized to control rates of bedrock erosion if this hypothesis is correct then the elevation difference between hanging and trunk valley floors the hang height should be dictated by the relative sliding velocities of the glaciers that occupied these valleys by considering sliding velocity to be proportional to balance velocity and using mass continuity hang height is expressed in terms of glacier catchment areas slopes and widths which can be estimated for past glaciers from the morphology of glacial valleys these parameters were estimated for 46 hanging valleys and their trunk valleys in three adjacent regions of jasper national park the variability in valley morphology can account for 55 85 of the hang height variability if erosion rate scales with balance velocity raised to a power of 1 3 this correspondence is in spite of spatial variations in glaciation duration snow accumulation rates and other variables that likely affected hang heights but cannot be readily estimated and so are not included in our formulation thus it appears that balance velocity and by extension sliding velocity if the two are proportional may be a reasonable control variable for assessing erosion rate

varve and radiocarbon dating support the rapid advance of jakobshavn isbrae during the little ice age abstract large outlet glaciers draining the greenland ice sheet significantly influence overall ice sheet mass balance considerable short term years to decades retreat and fluctuations in velocity of jakobshavn isbrae western greenland illustrate the complex nature by which large outlet glaciers respond to climate change making predictions of future ice sheet change challenging to provide a longer term view centuries we investigate the geological record of jakobshavn isbrae change we use continuous sediment records from lakes that were influenced by the recent advance of jakobshavn isbrae which took place during the little ice age in particular we explore the use of annually laminated lake sediments varves to precisely constrain the advance of the ice margin as it approached its late holocene maximum extent we find that the ice margin advanced recently at least after 1650 to 1700 ad and more likely 1800 ad we suggest that during this period jakobshavn isbrae advanced at a rate that was similar to its historically documented average retreat since 1850 ad our results indicate that jakobshavn isbrae and presumably other large marine calving glaciers have the ability to advance quickly in response to climate forcing

interdecadal patterns of total sediment yield from a montane catchment southern coast mountains british columbia canada article i nfo we reconstruct sediment yield for a mountain watershed of western canada since the mid twentieth century from studies of annually laminated lake sediments delta progradation and solute transfer total yield averaged 320 40 mg km 2 a 1 and comprised 35 suspended load 50 bedload and 15 dissolved load ratios between the individual yield components varied approximately threefold at interannual timescales because of significant variability in the suspended and bedload fractions asynchronous flux in suspended and bedload fractions through time arise from differences in sediment availability and transitory sediment storage in the channel periods of elevated yield coincide with rapid glacier recession an extreme rainstorm and a landslide our results indicate that in montane environments extrapolation from even decade long monitoring programs may lead to biased projections of long term yield and delivery mode proportions if variations in sediment supply and catchment response to hydroclimatic and geomorphic controls are not considered

numerical modelling of subglacial erosion and sediment transport and its application to the north american ice sheets over the last glacial cycle abstract present day sediment distribution offers a potentially strong constraint on past ice sheet evolution glacial system models gsms however cannot address this constraint while lacking appropriate representations of subglacial sediment production and transport incorporating these elements in gsms is also required in order to quantify the impact of a changing sediment cover on glacial cycle dynamics towards these goals we present a subglacial process model hereafter referred to as the sediment model that incorporates mechanisms for sediment production entrainment transport and deposition bedrock erosion is calculated by both hallet s and boulton s abrasion laws separately and by a novel quarrying law parametrized as a function of subglacial cavity extent these process oriented erosion laws are compared against a simple empirical relationship between erosion rate and the work done by basal stress sediment entrainment is represented by philip s law for regelation intrusion and soft bed deformation is included as a subglacial sediment transport mechanism the model is driven by the data calibrated mun 3d gsm and a newly developed subglacial hydrology module the sediment model is applied to the last north american glacial cycle and predicts sediment thickness and cumulative erosion patterns results are obtained in the context of a sensitivity analysis and are compared against the present day distribution of glacigenic sediment and geological estimates of laurentide ice sheet erosion given plausible ranges for the sensitivity parameters chosen a priori based on available literature or on heuristic arguments the calculated erosion depths overlap with the geological estimates of laurentide erosion most of the runs in the sensitivity set produce unrealistically thick and continuous moraines along the eastern southern and western margins of the north american ice complex which suggests that the model overestimates sediment entrainment and thus englacial transport a realistic sediment distribution is only obtained when the entrainment rate is capped at the average basal melting rate which suggests that modelled entrainment and basal melting rates should be of the same order of magnitude

a 1500 year record of temperature and glacial response inferred from varved iceberg lake southcentral alaska we present a varve thickness chronology from glacier dammed iceberg lake in the southern alaska icefields radiogenic evidence confirms that laminations are annual and record continuous sediment deposition from a d 442 to a d 1998 varve thickness is positively correlated with northern hemisphere temperature trends and more strongly with a local 600 yr long tree ring width chronology varve thickness increases in warm summers because of higher melt runoff and sediment transport as expected but also because shrinkage of the glacier dam allows shoreline regression that concentrates sediment in the smaller lake varve thickness provides a sensitive record of relative changes in warm season temperatures relative to the entire record temperatures implied by this chronology were lowest around a d 600 warm between a d 1000 and a d 1300 cooler between a d 1500 and a d 1850 and have increased dramatically since then combined with stratigraphic evidence that contemporary jokulhlaups which began in 1999 are unprecedented since at least a d 442 this record suggests that 20th century warming is more intense and accompanied by more extensive glacier retreat than the medieval warm period or any other time in the last 1500 yr

role of transient water pressure in quarrying a subglacial experiment using acoustic emissions 1 probably the most important mechanism of glacial erosion is quarrying the growth and coalescence of cracks in subglacial bedrock and dislodgement of resultant rock fragments although evidence indicates that erosion rates depend on sliding speed rates of crack growth in bedrock may be enhanced by changing stresses on the bed caused by fluctuating basal water pressure in zones of ice bed separation to study quarrying in real time a granite step 12 cm high with a crack in its stoss surface was installed at the bed of engabreen norway acoustic emission sensors monitored crack growth events in the step as ice slid over it vertical stresses water pressure and cavity height in the lee of the step were also measured water was pumped to the lee of the step several times over 8 days pumping initially caused opening of a leeward cavity which then closed after pumping was stopped and water pressure decreased during cavity closure acoustic emissions emanating mostly from the vicinity of the base of the crack in the step increased dramatically with repeated pump tests this crack grew with time until the step s lee surface was quarried our experiments indicate that fluctuating water pressure caused stress thresholds required for crack growth to be exceeded natural basal water pressure fluctuations should also concentrate stresses on rock steps increasing rates of crack growth stress changes on the bed due to water pressure fluctuations will increase in magnitude and duration with cavity size which may help explain the effect of sliding speed on erosion rates

the new north american varve chronology a precise record of southeastern laurentide ice sheet deglaciation and climate 18 2 12 5 kyr bp and correlations with greenland ice core records new glacial varve records from long cores combined with records from key surface exposures and new radiocarbon ages have allowed the correction consolidation expansion and calibration of ernst antevs9 original new england varve chronology nevc in the connecticut valley of new england u s a the varve records have been reformulated with corrections and a new numbering system as the new north american varve chronology navc which is a continuous 5659 yr varve sequence that spans most of the last deglaciation 18 200 12 500 yr bp in the northeastern united states rates of ice recession for separate intervals terminated by abrupt glacial stillstands and readvances have been determined for western new england ice recession history is coupled to varve thickness changes that depict changes in meltwater production in the connecticut valley and show the relationship of changes in ablation rate summer climate variation to glacial readvances and periods of halted and rapid ice recession up to 300 m yr comparison of varve thickness records to greenland ice core climate records show that after 15 000 yr bp climate changes of sub century and longer scales recorded in both records appear identical and synchronous after 15 000 yr bp therefore there was a link between north atlantic climate and marginal processes of the southeastern sector of the laurentide ice sheet lis prior to 15 000 yr bp when the lis was closer to an equilibrium condition retreat rates were generally lower and changes in varve thickness and ablation were more subtle but can still be linked to ice sheet activity only weak relationships between varve thickness changes and greenland climate are evident suggesting that changes in the southeastern lis during this time may have been significantly influenced by climate patterns unique to the north american continent or ice dynamics

1857 slip on the san andreas fault southeast of cholame california sieh and jahns 1984 forecasted that the next moderate parkfield earthquake might trigger a major earthquake along a fault segment greater than 30 km long southeast of cholame their forecast assumed 1 the slip was 3 4 m in 1857 and characteristic of the segment 2 a slip rate of 3 4 cm yr and 3 full strain release in earthquakes this study represents an independent measurement of channel offsets on 1 2400 scale low sun aerial photographs and by field investigation to estimate the amount of 1857 slip although rainfall is only moderate 30 cm yr few reliable offsets of less than 20 m persist here because cattle grazing and agricultural disking of soft sediments on the steep terrain greatly aggravate erosion reconstruction of offset geometry and size depends heavily on assumptions made about the post 1857 erosion most of the apparent 3 to 4 m offsets of sieh and jahns 1984 can also be measured as 2 to 3 m larger with equal or lower uncertainty the four offsets judged as most reliable range between 5 4 and 6 7 m and the 11 offsets of medium high reliability average 5 8 0 3 m data are too sparse and ambiguous to resolve details of the 1857 slip for this segment but it is distinctly less than the 9 m of the carrizo plain and more than the 3 4 m previously estimated further trenching may refine some measurements but probability calculations for a cholame segment earthquake must allow for large observer dependent uncertainty in the 1857 slip although the probability of an m 7 cholame event seems less than that suggested by a 3 5 m characteristic earthquake model it remains among the highest in the state

unsteady late pleistocene incision of streams bounding the colorado front range from measurements of meteoric and in situ 10be 1 dating of gravel capped strath terraces in basins adjacent to western u s laramide ranges is one approach to document the history of late cenozoic fluvial exhumation we use in situ 10be measurements to date the broad surfaces adjacent to the eastern edge of the rocky mountains in colorado and compare these calculated ages with results from meteoric 10be measurements we analyze three sites near boulder colorado gunbarrel hill table mountain and pioneer that have been mapped as the oldest terrace surfaces with suggested ages ranging from 640 ka to the plio pleistocene transition our in situ 10be results reveal abandonment ages of 95 129 ka at table mountain 175 27 ka at pioneer and ages of 251 10 ka and 307 15 ka at gunbarrel hill all are far younger than previously thought inventories of meteoric 10be support this interpretation yielding ages that are comparable to table mountain and 20 lower than pioneer in situ ages we argue that lateral beveling by rivers dominated during protracted times of even moderate glacial climate and that vertical incision rates of several mm yr likely occurred during times of very low sediment supply during the few interglacials that were characterized by particularly warm climate conditions in contrast to the traditional age chronology in the area our ages suggest that the deep exhumation of the western edge the high plains occurred relatively recently and at an unsteady pace

apatite u th he signal of large magnitude accelerated glacial erosion southwest british columbia alpine glaciers are efficient agents of erosion and capable of significantly modifying topography despite recent advances in theoretical and field studies that quantify glacial erosion processes few studies have documented glacial erosion rates over long 10 6 yr or large more than tens of kilometers scales we use apatite u th he ahe and apatite fission track aft cooling ages to address the late miocene to holocene erosion history across two 60 km long transects of the heavily glaciated southern coast mountains british columbia observed ahe cooling ages from equal elevation samples range between 1 5 and 8 ma and suggest that thick alpine glaciers resulted in a 16 km shift of the highest point in the topography in the past 1 5 4 0 m y we evaluated temporal and spatial variations in erosion rates using a three dimensional thermal kinematic model that predicted ahe and aft ages at the surface for different erosion histories comparison of model predicted and observed cooling ages suggests an increase in erosion rates of as much as 300 over the past 1 5 7 m y coincident with the onset of glaciation of this range

nested temporal suspended sediment yields green lake basin british columbia canada suspended sediment yield was determined for a mountain watershed by calculating the volume of sediment stored in the watershed s outlet lake basin and monitoring sediment transport for the lake s primary inflow lake based yields for the 1931 2001 period 0 21 0 03 mg km 2 day 1 are 10 higher than estimates for the last 3000 years 0 19 0 05 mg km 2 day 1 fluvial 0 24 0 08 mg km 2 day 1 and lake based yields ca 0 21 0 07 mg km 2 day 1 are comparable over the period of overlap 1999 2001 persistence in the long term yield record is dominated by low frequency century to decadal periodicity this periodicity is shown to inflate the standard error of the sediment yield estimate for the basin elevated transport of suspended sediments following years of extreme yield is uncommon in the green lake record and indicates a sediment transport regime that is supply limited for this watershed approximately 50 years of suspended sediment monitoring would be required to reduce the uncertainty of the yield estimate to 37 of its average value a summer rainstorm in 1991 delivered more suspended sediment to the green lake basin than any other event in the last 3000 years

paleomagnetic results from cretaceous and jurassic rocks of south and southwest yunnan evidence for large clockwise rotations in the indochina and shan thai malay terranes abstract cretaceous and jurassic redbeds have been sampled from south and southwest yunnan china for paleomagnetic study the paleomagnetic directions resolved from the cretaceous rocks indicate that the xiaguan area which is located on the red river fault has not been rotated with respect to stable eurasia since at least late cretaceous times and that the jinggu mengla area 200 400 km farther south has been rotated by 46 65 clockwise the jurassic results reveal the same rotation pattern at face value the differences between the observed cretaceous paleolatitudes and those predicted from the eurasian apparent polar wander path apwp indicate a southward extrusion of the region but the amount of extrusion is not large enough to be resolved by paleomagnetic methods comparison with mesozoic paleomagnetic data from central yunnan northern thailand and northwestern borneo suggests that the red river fault is not a demarcation between unrotated and significantly rotated regions and that sundaland did not respond to the india asia collision as a single coherent unit the existing paleomagnetic data from east asia appear to indicate that the crust in central tibet and tien shan north of tarim directly in front of the indian indentor has been substantially shortened and thickened whereas the crust in the syntaxial areas has probably been largely squeezed out to further evaluate the relative importance of crustal thickening and extrusion in the tectonic evolution of the tibetan plateau paleomagnetic investigation should be intensified in the bounding zones to the indian indentor and be expanded into the cenozoic sedimentary rocks in tibet and surrounding regions

exhumation rates in the st elias mountains alaska abstract reprocessing and interpretation of seismic profiles from the gulf of alaska in the vicinity of the st elias mountains indicate that over 3000 km 3 of sediment were deposited on the ocean shelf since the end of the last glacial epoch approximately 10 000 years ago this estimate of the sediment volume does not include material moved out of the region by long shore currents transportation from the shelf to deeper waters or material trapped in fiords and basins on and near land thus an estimated sedimentation rate offshore of 7 9 mm year and the average erosion rate of 5 1 mm year from the st elias mountains derived from the estimated sedimentation rate represent minimum estimates we infer that these estimated rates are not typical of the approximately 5 m y history of mountain building in the area but rather they represent the result of efficient erosion by glaciers during this interglacial period

subglacial extensional fracture development and implications for alpine valley evolution 1 bedrock stresses induced through exhumation and tectonic processes play a key role in the topographic evolution of alpine valleys using a finite difference model combining the effects of tectonics erosion and long term bedrock strength we assess the development of near surface in situ stresses and predict bedrock behavior in response to glacial erosion in an alpine valley the matter valley southern switzerland initial stresses are derived from the regional tectonic history which is characterized by ongoing transtensional or extensional strain throughout exhumation of the brittle crust we find that bedrock stresses beneath glacial ice in an initial v shaped topography are sufficient to induce localized extensional fracturing in a zone extending laterally 600 m from the valley axis the limit of this zone is reflected in the landscape today by a valley shoulder separating linear upper mountain slopes from the deep u shaped inner valley we propose that this extensional fracture development enhanced glacial quarrying between the valley shoulder and axis and identify a positive feedback where enhanced quarrying promoted valley incision which in turn increased in situ stress concentrations near the valley floor assisting erosion and further driving rapid u shaped valley development during interglacial periods these stresses were relieved through brittle strain or topographic modification and without significant erosion to reach more highly stressed bedrock subsequent glaciation caused a reduction in differential stress and suppressed extensional fracturing a combination of stress relief during interglacial periods and increased ice accumulation rates in highly incised valleys will reduce the likelihood of repeat enhanced erosion events

reconstruction of climate and glacial history based on a comparison of varve and tree ring records from mirror lake northwest territories canada abstract this study compared the hydroclimatic signals recorded in annually laminated varved sediments and tree rings within a small study area in the selwyn mountains of the southwestern northwest territories southeastern yukon territory border region of canada the records were located immediately adjacent to each other and within 6 km of instrumental meteorological and hydrometric records which permitted a detailed analysis of climate in the area from ad 1704 to 1996 this study explored the challenges of annually resolved multi proxy hydroclimate analyses and examined how best to interpret the climate record given the differences in proxy formation and the respective signals the high frequency annual variability from both proxies showed a good match for some parts of the record e g mid 1700s and much of the 1900s but revealed different sensitivities at other times e g 1800s however both records contained a common low frequency decadal summer climate signal glacier dynamics influenced varve formation through altered sediment availability during the inferred little ice age lia maximum ice stand in the catchment from ad 1778 to 1892 and subsequent recession after this maximum as a result of reduced lia sedimentation july mean temperatures reconstructed from varve thickness during the 1800s were underestimated compared to the tree ring estimates other instances of low frequency divergence were apparent particularly during parts of the twentieth century despite the overall similarities the extended varve record also provided a glacial history for the southern selwyn mountains indicating probable glacier advances during ad 1300 1450 and ad 1600 1670 maximum ice stand during the 1800s followed by twentieth century recession

bedload to suspended load ratio and rapid bedrock incision from himalayan landslide dam lake record about 5400 cal yr bp a large landslide formed a n400 m tall dam in the upper marsyandi river central nepal the resulting lacustrine and deltaic deposits stretched n7 km upstream reaching a thickness of 120 m 14 c dating of 7 wood fragments reveals that the aggradation and subsequent incision occurred remarkably quickly 500 yr reconstructed volumes of lacustrine 0 16 km 3 and deltaic 0 09 km 3 deposits indicate a bedload to suspended load ratio of 1 2 considerably higher than the 1 10 that is commonly assumed at the downstream end of the landslide dam the river incised a new channel through 70 m of greater himalayan gneiss requiring a minimum bedrock incision rate of 13 mm yr over last 5400 yr the majority of incision presumably occurred over a fraction of this time suggesting much higher rates the high bedload ratio from such an energetic mountain river is a particularly significant addition to our knowledge of sediment flux in orogenic environments 2007 university of washington all rights reserved

sediments of ice dammed self draining ape lake british columbia the glacier damming ape lake has withdrawn from its neoglacial terminal moraines in the lake since early in this century at an average rate of 15 m a as a result the first known drainings of ape lake occurred in october 1984 and august 1986 in each event about 54 of the volume of the lake was lost through a tunnel in the damming glacier most of the remaining water was held in the deep basin of the lake behind partially breached neoglacial terminal moraines as the glaciers have withdrawn the character of the sediments has changed sediments in the deep basin of the lake are varved but the grain size especially of the summer deposits has decreased and rates of sedimentation have decreased from about 4 mm a to less than 2 mm a in shallow water deposition of varved sediments has given way to the deposition of massive sediments at rates of less than 1 mm a ice rafted debris is rare in deep water despite the presence of calving bergs during and following the first draining significant subaerial

modeling sediment transport in ice walled subglacial channels and its implications for esker formation and proglacial sediment yields sediment yields from glacierized basins are used to quantify erosion rates on seasonal to decadal timescales as well as conditions at the glacier bed and eskers hold valuable information about past subglacial hydraulic conditions in their spatial organization geometry and sedimentary structures ultimately eskers are a record of past glacio fluvial sediment transport but there is currently no physical model for this process we develop a 1 d model of morphodynamics in semicircular bedrock floored subglacial channels we adapt a sediment conservation law developed for mixed alluvial bedrock conditions to subglacial channels channel evolution is a function of melt opening by viscous heat dissipation from flowing water and creep closure of the overlying ice to which we add the closure or enlargement due to sediment deposition or removal respectively we apply the model to an idealized land terminating glacier and find that temporary sediment accumulation in the vicinity of the terminus or the formation of an incipient esker is inherent to the dynamics of the channelized water flow the alluviation of the bed combined with the pressurized channel flow produces unexpected patterns of sediment evacuation we show that the direction of hysteresis between sediment and water discharge is not necessarily linked to a supply or transport limited system as has been hypothesized for proglacial sediment yields we also find that the deposition of an incipient esker is a function of a compromise between water discharge and sediment supply but perhaps more importantly ice surface slope and the temporal pattern of water delivery to the bed

bedload component of glacially discharged sediment insights from the matanuska glacier alaska the flux of glacially derived bedload and the proportions of the suspended and bedload components carried by proglacial streams are highly debated published data indicate a large range from 30 to 75 in the bedload percentage of the total load two vents where supercooled subglacial meltwater and sediment are discharged were sam pled over the course of an entire melt season in order to quantify the flux of glacially delivered bedload at the matanuska glacier alaska the bedload component contributed by these vents for the one melt season monitored is negligible furthermore the bedload fluxes appear to be strongly supply limited as shown by the poorly correlated discharge bedload flux magnitude and grain size caliber thus in this case any attempt to employ a predictive quantitative expression for coarse sediment production based on discharge alone would be inaccurate a nonglaciated basin proximal to the matanuska glacier ter minus yielded higher bedload sediment fluxes and larger clast sizes than delivered by the two monitored vents such nonglaciated basins should not be overlooked as potentially major sources of coarse bedload that is reworked and incorporated into valley outwash usually subequal although bedload may range from 30 to 75 of the total clastic load establishing portable relationships between suspended load bedload and discharge would enhance our understanding of how and when glaciers erode for the melt season studied we show that bedload is 1 of the total clastic load and does not have any obvious correla tion to discharge at two of the main vents at the terminus of the matanuska glacier alas ka these observations coupled with high measured bedload component of adjacent non glaciated valleys inspire several working hy potheses for the observed origin of coarse bedload in the matanuska river downstream of the ice margin

late holocene glacier fluctuations in the wrangell mountains alaska four intervals of late holocene glacier advance are recognized from study of nine valley glaciers in the wrangell and westernmost st elias mountains of alaska the oldest glacial advance is recognized at the nabesna and barnard glaciers where five radiocarbon ages suggest advance as early as 2700 cal calibrated yr b p two additional radiocarbon dated advances are centered on cal yr a d 300 and the beginning of the little ice age about a d 1200 the best documented little ice age advances occurred during the mid 1600s through the 1800s and are recognized at all nine glaciers these latter advances are dated by tree rings of trees overrun by glaciers in five glacier valley by 17 radiocarbon dates and by tree ring and lichen ages from 20 moraines that were deposited during the culmination of these advances the glacial chronology is broadly similar to chronologies from adjoining alaskan mountain ranges at both coastal and interior sites for the past 3000 yr there are however differences in timing of advances during the first millennium a d the glacial history for the past 2000 yr is also consistent with temperature sensitive proxy records from interior alaska and yukon territory