quaternary tectonic response to intensified glacial erosion in an orogenic wedge active orogens are thought to behave as internally deforming critical taper wedges that are in rough long term equilibrium with tectonic influx and erosional outflux spatial and temporal variations in climate are therefore hypothesized to have a significant influence on denudation topography and deformation of orogens thereby affecting wedge taper however the impact of the most severe transition in northern hemisphere climate during the cenozoic era the onset of glaciation has hitherto not been empirically documented here we analyse the spatial patterns of denudation and deformation and their temporal variations in the heavily glaciated st elias orogen in southern alaska low temperature thermochronometry thermokinematic modelling and offshore seismic reflection and borehole data suggest that the global scale intensification of glaciation in the middle pleistocene epoch enhanced glacier growth and caused ice streams to advance to the edge of the continental shelf this led to focused denudation across the subaerial reaches of the orogen and burial of the actively deforming wedge toe by the eroded sediment we propose that this climatically driven mass redistribution forced a structural reorganization of the orogen to maintain critical taper our empirical results thus support decades of numerical model predictions of orogenesis and provide compelling field evidence for the significant impact of climate change on tectonics intense glaciation during the middle pleistocene epoch led to focused denudation and mass redistribution within the st elias orogen in southern alaska and resulted in structural reorganization of the orogen the tectonic response of this orogen to climate change is consistent with predictions of numerical models

thermochronometry reveals headward propagation of erosion in an alpine landscape glacial erosion of mountain ranges produces spectacular alpine landscapes and by linking climate with tectonics influences a broad array of geophysical phenomena although the resultant landforms are easily identified the timing and spatial pattern of topographic adjustment to pleistocene glaciations remain poorly known we investigated topographic evolution in the archetypal glacial landscape of fiordland new zealand using u th he thermochronometry we find that erosion during the past 2 million years removed the entire pre pleistocene landscape and fundamentally reshaped the topography erosion focused on steep valley segments and propagated from trunk valleys toward the heads of drainage basins a behavior expected if subglacial erosion rate depends on ice sliding velocity the fiordland landscape illustrates complex effects of climate on earth s surface morphology

modeling the tor dotted crests bedrock edges and parabolic profiles of high alpine surfaces of the wind river range wyoming abstract broad low curvature surfaces common to many alpine landscapes and particularly well expressed in the laramide ranges of the western usa appear to be roughly in steady state with respect to the current surface processes uniform thin order 1 m regolith covers the crystalline bedrock with the exception of occasional tors at ridgecrests the surfaces are commonly edged by bedrock lips at the tops of cliffs that bound the intervening glacial troughs regolith production is low order 10 m year on bare bedrock and reaches a maximum under tens of centimeters of regolith all of these characteristics may be accounted for with a numerical model that is faithful to the periglacial surface processes presently operating on these surfaces hillslope transport is modeled assuming the dominant process is frost creep transport efficiency depends linearly upon local slope and appropriately vanishes in the absence of regolith frost frequency scales the rate of motion while the ratio of frost penetration depth to regolith thickness dictates the degree to which the process is curtailed by available regolith initial conditions include initial mean slope and bedrock roughness and a uniform regolith cover boundary conditions are imposed by dictating that the edges of the calculation space be lowered at a rate appropriate for glacial incision broad convex surfaces emerge over time scales of order 1 million years all surfaces rapidly decouple from the bounding glacial troughs as the high negative curvature at the edges disallows accumulation of regolith these bedrock edges lower at a few microns per year while the troughs lower at many times this rate as the bedrock edges therefore serve as the effective boundary condition for the remaining surface the surfaces operate independently of both lowering and widening of the glacial troughs for a reasonable set of transport and weathering rates the final steady state landscape is sensitive to the initial condition in particular the steeper the initial landscape the more likely tors are to develop as the high curvature disallows accumulation of regolith these tors in turn limit the lowering rate of the crest and the landscape retains significant local relief low relief initial conditions more likely lead to parabolic surfaces that lack tors that tors are more likely on the crests reflects the high initial curvature at the crest tors on the side slopes are eliminated the present high surfaces in the laramide ranges of the western usa imply operation of periglacial surface processes over millions of years the occasional tors imply a steeper rather than flatter initial landscape the strong decoupling of surfaces from glacial troughs justifies their use as local geomorphic markers that allow calculation of relief production since glacial incision was initiated on the other hand their present low curvature does not require a landscape history in which these surfaces were once joined as an initially widespread erosion surface

the role of ocean atmosphere reorganizations in glacial cycles abstract a case is made that glacial to interglacial transitions involve major reorganizations of the ocean atmosphere system such reorganizations constitute jumps between stable modes of operation which cause changes in the greenhouse gas content and albedo of the atmosphere only in this way can the rapidity of glacial terminations the hemispheric synchroneity and symmetry of mountain glaciation and the large polar air temperature and dustiness variations be accounted for if these reorganizations are driven in some fashion by orbitally induced seasonal insolation changes then the connection between insolation and climate is most likely through impacts of fresh water transport on the ocean s salinity distribution

some geological implications of average quaternary glacial conditions analyses of quaternary landscape evolution tend to focus on events associated with culminations of glacial and interglacial ages but for most of quaternary time environments were intermediate in character average quaternary glacial conditions based on assessment of the marine isotope record approximated those near the stage 2 stage 1 transition and during substages 5b and 5d isotope stages 3 and 4 and substages 5a d lie within one standard deviation of the mean value under average glacial conditions ice sheets lying over northern north america and europe were much more contracted than their full glacial counterparts and the distribution of mountain glaciers reflected a snowline depression of some 500 m geomorphic processes operating under average quaternary conditions contributed importantly to landscape evolution examples of landscapes that may represent such average conditions include cirques and fluvial deposits of the pacific northwest the fjords and strandflat of western norway and atolls of the tropical oceans by examining the geologic record from the perspective of average conditions rather than those of climatic extremes added insight can be gained regarding the evolution of quaternary landscapes

numerical simulations of glacial valley longitudinal profile evolution glaciers shape alpine landscapes they broaden valley bottoms enhance local valley relief generate multiple steps overdeepen valley floors and cause tributary valleys to hang these distinctive glacial signatures result from 10 4 10 5 yr of erosion during which swings in climate drive advances and retreats of alpine glaciers we use a numerical model of glacial erosion to explore the development of the longitudinal profiles of glaciated valleys the model is driven by the past 400 k y of variable climate because both sliding speed which dictates abrasion rate and water pressure fluctuations which strongly modulate quarrying rate should peak at the equilibrium line altitude ela we expect the locus of most rapid erosion to follow the transient ela simulations of a single glacial valley show rapid flattening of the longitudinal profile inclusion of a tributary glacier creates a step immediately downvalley of the tributary junction that persists over multiple glaciations and commonly leaves the tributary valley hanging steps and overdeepenings result from an increase in ice discharge immediately below the tributary junction which is accommodated primarily by increased ice thickness and hence sliding rate the size of the step increases with the ratio of tributary to trunk ice discharge while the height of a hanging valley reflects the difference in the time integrated ice discharge in tributary and trunk valleys and therefore increases as the discharge ratio decreases

positive degree day factors for ablation on the greenland ice sheet studied by energy balance modelling ice ablation is related to air temperature by the positive degree day factor variations of the positive degree day factor in west greenland are studied using an energy balance model to simulate ablation under different conditions degree day factors for simulated and measured ice ablation at nordbogletscher and qaman rssup sermia agree well with values around 8 mm d 1 c 1 degree day factors for snow are less than half those for ice energy balance modelling shows that degree day factors vary with summer mean temperature surface albedo and turbulence but there is only evidence of large positive degree day factors at lower temperatures and with low albedo 0 3 the greatest effect of albedo variations 0 3 0 7 is at lower temperatures while variations in turbulence have greater effect at higher temperatures current models may underestimate runoff from the greenland ice sheet by several tenths because they use a degree day factor for melting ice that is too small for the colder parts of the ice sheet i e the upper ablation area and the northerly margin

the relative efficacy of fluvial and glacial erosion over modern to orogenic timescales it has been debated whether rivers or glaciers are more effective agents of erosion a global compilation of erosion rates reveals that both are capable of generating rates of erosion that match or exceed the highest rates of rock uplift since the late nineteenth century it has been debated whether rivers or glaciers are more effective agents of erosion1 the dramatic landscapes associated with glaciated terrain have often led to the argument that glaciers are more erosive than rivers and recent studies have documented the topographic signature of an ice controlled limit of mountain height known as the glacial buzz saw 2 3 here we present a new global compilation of erosion rates which questions the conventional view of glaciers and erosion in regions of rapid tectonic uplift erosion rates from rivers and glaciers both range from 1 to over 10 mm yr 1 indicating that both are capable of generating erosion rates matching or exceeding the highest rates of rock uplift moreover a comparison of erosion rates over timescales ranging from 101 to 107 years indicates that glacial erosion tends to decrease by one to two orders of magnitude over glacial cycles whereas fluvial erosion rates show no apparent dependence on time we conclude that tectonics controls rates of both fluvial and glacial erosion over millennial and longer timescales and that the highest rates of erosion 10 mm yr 1 generally result from a transient response to disturbance by volcanic eruptions climate change and modern agriculture

global climate and tectonic controls on the denudation of glaciated mountains abstract significant uncertainty exists concerning the efficiency of alpine glacial erosion relative to fluvial and hillslope processes latitudinal variations in temperature are important for determining the extent of glaciers as are the rates of tectonic uplift that influence the elevation and hence temperatures that glaciers can form the acute sensitivity of glacial erosion to temperature has complicated previous interpretations because temperatures must be cool enough to maintain ice yet warm enough to allow glacial sliding here we quantify the influence of climate and tectonics on glacial landscape evolution with a coupled glacial fluvial and hillslope landscape evolution model that systematically explores variations in rock uplift rate and periodic variations in climate i e glacial interglacial periods over million year time scales emphasis is placed on understanding when a particular climate is either more e g buzzsaw conditions or less erosive than its preglacial landscape results indicate that the erosional efficiency of glaciers varies as a function of latitudinal controlled temperature and rock uplift rate an order of magnitude increase in erosion rates occurs in some scenarios for both localized valley bottom erosion and short term one glacial period durations of glaciation however when averaged over the entire landscape for 2 ma increases in glacial erosion are typically less than double that of the preglacial landscape in some scenarios average glacial erosion rates are less than preglacial rates due to either small inefficient glaciers or extensive cold based glaciation model predictions are compared with a compilation of long term denudation rates from glaciated mountain ranges and indicate models perform well at explaining patterns of glacial erosion efficiency the findings presented here have clear implications for the impact of glaciations on the evolution of landscapes including 1 the climatic window in which glaciers are more erosive compared to pre glacial rates 2 spatial and temporal variations in denudation that can lead to pulses of erosion and 3 predictions of glacial erosional efficiency at different latitudes we conclude that latitudinal and elevation dependent variations in temperature control the efficiency of glacial denudation and explain discrepancies between previous studies

rapid glacial erosion at 1 8 ma revealed by 4he 3he thermochronometry alpine glaciation and river incision control the topography of mountain ranges but their relative contributions have been debated for years apatite 4 he 3 he thermochronometry tightly constrains the timing and rate of glacial erosion within one of the largest valleys in the southern coast mountains of british columbia canada five proximate samples require accelerated denudation of the klinaklini valley initiating 1 8 0 2 million years ago ma at least 2 kilometers of overlying rock were removed from the valley at 5 millimeters per year indicating that glacial valley deepening proceeded 6 times as fast as erosion rates before 1 8 ma this intense erosion may be related to a global transition to enhanced climate instability 1 9 ma

the influence of alpine glaciation on the relief of tectonically active mountain belts a surface processes model incorporating large scale fluvial processes local hill slope processes and glacial erosion is used to investigate the effect of glaciation on the relief of a fast growing narrow orogen the results of several simulations show that glaciers concentrate erosion near peaks fluvial erosion is reduced downstream of glaciers glaciers with frozen bases can lead to an increase in relief and the net affect of glaciation in the absence of widespread basal freezing is to reduce relief

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

a new surface processes model combining glacial and fluvial erosion we have developed a new surface processes model incorporating large scale fluvial processes local hill slope processes and glacial erosion ice thickness and velocity are calculated under a shallow ice approximation simulation experiments in fast growing orogens comparing the efficiencies of fluvial and glacial erosion where the two are operating simultaneously over several glacial cycles show that glacial landscapes can support greater ice masses than fluvial landscapes glacial valley and lake shapes create a disequilibrium between landform and land forming process that leads to pulses of high erosion at the end of glacial periods glacial erosion rates can reach a constant value in a uniformly growing orogen and glacial erosion is capable of eroding drainage divides when the ice is moderately thick

cenozoic landscape evolution of the convoy range to mackay glacier area transantarctic mountains onshore to offshore synthesis on the basis of geomorphic mapping we reconstruct the landscape evolution of the convoy range to mackay glacier area of the rifted margin of the transantarctic mountains and compare the land record with that obtained from offshore marine sedimentary rocks in the ross sea three landform assemblages reflect 1 fluvial planation and dissection 2 local glaciation under temperate conditions and 3 overriding by the east antarctic ice sheet overall landscape evolution is typical of a passive continental margin contrasts in morphology between the convoy range and the adjacent dry valleys and royal society mountains reflect the varying location of the initial drainage divide in relationship to the rifted coast in a wider synthesis we draw the following conclusions for the 260 km long mcmurdo sector of the transantarctic mountains denudation since rifting at ca 55 ma has removed a wedge 4 7 km thick at the coast declining inland to 1 km most denudation occurred in the eocene from planation and incision of river valleys near the coast a subsequent pulse of denudation most rapid at 34 31 ma and declining until ca 17 ma coincided with further crustal extension and a change from cool temperate to polar climate during the same interval there was a progressive decrease in landscape modification by warm based glaciers and or rivers between 14 8 and 13 6 ma the maximum overriding antarctic ice sheet flowed northeastward across the mountains leaving meltwater features crossing high altitude saddles and areally scoured bedrock near the coast since 13 6 ma the landscape has seen little change under a hyperarid polar climate either by local glaciers or subaerially fragile middle miocene surficial deposits still survive there has been tectonic stability since the middle miocene

orographic precipitation and the relief of mountain ranges 1 the influence of climate on the relief of mountain ranges has long been supposed but has received little quantitative investigation the simplified system of a longitudinal river profile in a bedrock channel coupled to a physically based model of orographic precipitation is used to explore how orographically influenced patterns of precipitation affect the relief of fluvially sculpted mountain ranges it is shown that the impact on the orography depends on the nature of the precipitation regime significant changes in relief also occur for imposed temperature changes of realistic magnitude although the ability of the relief to respond to any given change depends sensitively on its response time and the timescale of the forcing

glaciation as a destructive and constructive control on mountain building glacial erosion is thought to have an important role in controlling mountain height a common perception is that glaciers are powerful erosive agents inhibiting the ability of active mountains to rise significantly above the elevation of permanent glaciation through a process known as the glacial buzzsaw but now a study of erosion patterns and climate data from the late cenozoic in the glaciated patagonian andes suggests that glaciation can also have the reverse effect in effect making mountains higher at these polar latitudes glaciation acts as a protective layer preserving the uplifting mountains from erosion and allowing them to reach heights well above those predicted had a glacial buzzsaw been active it has been suggested that glacial erosion has an important role in controlling mountain height here spatial and temporal patterns of erosion in the glaciated patagonian andes have been assessed with the help of a low temperature thermochronologic data set the results show that between 38 s and 49 s accelerated erosion at the onset of widespread glaciation limits mountain height but at higher latitudes glaciation protects the landscape from erosion and leads to growth in mountain height and width theoretical analysis predicts that enhanced erosion related to late cenozoic global cooling can act as a first order influence on the internal dynamics of mountain building leading to a reduction in orogen width and height1 2 3 the strongest response is predicted in orogens dominated by highly efficient alpine glacial erosion producing a characteristic pattern of enhanced erosion on the windward flank of the orogen and maximum elevation controlled by glacier equilibrium line altitude3 4 where long term glacier mass gain equals mass loss however acquiring definitive field evidence of an active tectonic response to global climate cooling has been elusive5 here we present an extensive new low temperature thermochronologic data set from the patagonian andes a high latitude active orogen with a well documented late cenozoic tectonic climatic and glacial history data from 38 s to 49 s record a marked acceleration in erosion 7 to 5 myr ago coeval with the onset of major patagonian glaciation6 and retreat of deformation from the easternmost thrust front7 the highest rates and magnitudes of erosion are restricted to the glacial equilibrium line altitude on the windward western flank of the orogen as predicted in models of glaciated critical taper orogens where erosion rate is a function of ice sliding velocity3 8 in contrast towards higher latitudes 49 s to 56 s a transition to older bedrock cooling ages signifies much reduced late cenozoic erosion despite dominantly glacial conditions here since the latest miocene6 the increased height of the orogenic divide at these latitudes well above the equilibrium line altitude leads us to conclude that the southernmost patagonian andes represent the first recognized example of regional glacial protection of an active orogen from erosion leading to constructive growth in orogen height and width

controls of glacial valley spacing on earth and mars abstract low order alpine glacial valleys on earth commonly have a characteristic spacing of 1 3 km here we develop analytic and numerical solutions of a coupled numerical model for alpine glacial flow and subglacial bedrock erosion to quantitatively determine the controls on glacial valley spacing assuming an initially undissected landscape an initially fluvially dissected landscape and an initially cratered landscape the characteristic spacing of glacial valleys produced by the model is controlled by a competition between the thickening of ice in incipient glacial valleys which acts to enhance flow and valley deepening and viscous sidewall drag which acts to limit flow and deepening the glacial valley spacing that represents the best compromise between these two competing effects is found to be a function of valley slope the threshold basal shear stress for ice motion the effective ice viscosity a bed friction parameter and gravity this model framework provides the preliminary basis for understanding the relative spacing of glacial valleys on earth and mars on mars montane glacial valleys have widths and spacings that are approximately 10 20 times larger than those on earth model results suggest that this difference is predominantly a consequence of lower bed slopes and larger temperature controlled ice viscosities of glacial ice on mars compared to earth

glacial effects limiting mountain height the height of mountain ranges reflects the balance between tectonic rock uplift crustal strength and surface denudation tectonic deformation and surface denudation are interdependent however and feedback mechanisms in particular the potential link to climate are subjects of intense debate spatial variations in fluvial denudation rate caused by precipitation gradients are known to provide first order controls on mountain range width crustal deformation rates and rock uplift moreover limits to crustal strength are thought to constrain the maximum elevation of large continental plateaus such as those in tibet and the central andes there are indications that the general height of mountain ranges is also directly influenced by the extent of glaciation through an efficient denudation mechanism known as the glacial buzzsaw here we use a global analysis of topography and show that variations in maximum mountain height correlate closely with climate controlled gradients in snowline altitude for many high mountain ranges across orogenic ages and tectonic styles with the aid of a numerical model we further demonstrate how a combination of erosional destruction of topography above the snowline by glacier sliding and commensurate isostatic landscape uplift caused by erosional unloading can explain observations of maximum mountain height by driving elevations towards an altitude window just below the snowline the model thereby self consistently produces the hypsometric signature of the glacial buzzsaw and suggests that differences in the height of mountain ranges mainly reflect variations in local climate rather than tectonic forces

significant increase in relief of the european alps during mid pleistocene glaciations the effect of expanded glaciation on the relief of mountain ranges is debated chronometric and model data from the european alps suggest that relief in the rhone valley was enhanced as a result of the increased incision of the valley following the mid pleistocene climate transition

modeling topographic and climatic control of east west asymmetry in sierra nevada glacier length during the last glacial maximum 1 glaciers draining westward from the sierra nevada divide california during the last glacial maximum lgm were 7 times longer than east draining glaciers we address the degree to which this difference may be attributed to the topographic asymmetry of the west tilted sierran block and the climate asymmetry resulting from orographic modification of pacific ocean storms we simulate kilometer scale glaciers within the 50 50 km kings canyon region of the southern sierra by employing a two dimensional numerical model that is driven by simple spatially variable climates and treats ice transport by deformation sliding and avalanching in numerical experiments we match simulated termini to lgm moraine positions to constrain the parameters of different climate scenarios the 38 km long lgm glacier in kings canyon was reproduced by a climate specified by an equilibrium line altitude ela of 3170 m a mass balance gradient of 0 01 m yr m and a maximum positive balance of 2 m yr this climate generates much shorter average 6 km long east draining glaciers that however overshoot the lgm moraines by 1 km roughly 97 of the e w difference in glacier lengths can therefore be attributed to topographic asymmetry alone a second experiment suggesting a 120 m higher ela of 3290 m east of the divide can explain the shorter east draining glaciers an experiment in which orographic precipitation is explicitly simulated and melt is prescribed using a positive degree day algorithm matches both kings canyon and the average east draining glacier length with an lgm climate that was 5 6 c cooler and 2 times wetter than the modern sierra nevada

geomorphic limits to climate induced increases in topographic relief recognition of the potential for strong dynamic coupling between atmospheric and tectonic processes has sparked intense cross disciplinary investigation and debate on the question of whether tectonics have driven long term climate change or vice versa it has been proposed that climate change might have driven the uplift of mountain summits through an isostatic response to valley incision because isostasy acts to compensate mean elevations the debate hinges on the question of whether climate change can significantly increase topographic relief or more precisely increase the volume of missing mass between summits and ridges here we show that in tectonically active mountain ranges geomorphic constraints allow only a relatively small increase in topographic relief in response to climate change thus although climate change may cause significant increases in denudation rates potentially establishing an important feedback between surficial and crustal processes neither fluvial nor glacial erosion is likely to induce significant isostatic peak uplift

cenozoic range front faulting and development of the transantarctic mountains near cape surprise antarctica thermochronologic and geomorphologic constraints 1 the transantarctic mountains tam define the western flank of the west antarctic rift system cape surprise near the shackleton glacier in the central tam is the only location along the range s 3500 km length where upper paleozoic beacon supergroup strata are down faulted to near sea level previous studies have inferred a range front master normal fault accommodates extension and rock uplift across the tam front in this region the history of rock uplift is debated suggested as early as mesozoic typically cenozoic or even pliocene or younger structural observations apatite fission track aft thermochronology and geomorphologic mapping undertaken within the tam front east of the shackleton glacier indicate extension faulting and denudation was mostly late eocene late oligocene and likely into the early miocene an exhumed aft partial annealing zone is found at the coast and traced 50 km inland the base of that exhumed partial annealing zone indicates denudation accelerated at 40 ma near the coast and at 30 35 ma on the inland side of the tam front vertically offset aft isochrones across the tam front reveal a step faulted architecture rather than a single master fault the cumulative vertical offset of the 55 ma isochrone is 2 3 2 7 km compared to 2 4 2 6 km offset of beacon strata indicating that all significant normal faulting is cenozoic and not related to mesozoic extension within the west antarctic rift system denudation from 26 ma to 14 ma produced a locally preserved erosion surface within the tam front erosion surface remnants indicate the tam front has undergone minimal internal deformation and only 290 790 m of surface uplift along offshore faults since the middle miocene

bedrock incision rock uplift and threshold hillslopes in the northwestern himalayas the topography of tectonically active mountain ranges reflects a poorly understood competition between bedrock uplift and erosion dating of abandoned river cut surfaces in the northwestern himalayas reveals that the indus river incises through the bedrock at extremely high rates 2 12 mm yr 1 in the surrounding mountains the average angles of hillslopes are steep and essentially independent of erosion rate suggesting control by a common threshold process in this rapidly deforming region an equilibrium is maintained between bedrock uplift and river incision with landsliding allowing hillslopes to adjust efficiently to rapid river down cutting

a comparison of decadal climate oscillations in the north atlantic detected in observations and a coupled gcm abstract decadal climate variations in the atlantic ocean found in observational fields and a coupled general circulation model cgcm are investigated in particular physical processes responsible for the phase reversal are compared observed and modeled decadal variations have dominant periodicities around 12 3 and 9 9 yr respectively both variations show similar spatial features sea surface temperature sst anomalies in the western subtropical atlantic sandwiched by those with opposite sign to the north and south and a dipole of sea level pressure anomalies which resemble the north atlantic oscillation their temporal evolutions are however different from each other suggestive of different dynamics of the oscillation in the observations sst and surface layer 0 100 m temperature anomalies move eastward from the subtropical western atlantic to the european coast along the gulf stream northward propagation of sst anomalies are also found along the western boundaries including the gulf of me

effect of alpine glaciation on thermochronometer age elevation profiles 1 low temperature thermochronometers are widely used to quantify exhumation histories typically by interpreting sample cooling age elevation relationships however the effects of specific geomorphic processes on age elevation profiles are seldom considered we integrate apatite u th he ahe apatite fission track and zircon u th he thermochronometry with numerical modeling to determine the effect of glacial erosion on an age elevation profile from the heavily glaciated southern coast mountains british columbia ahe data show a distinct break in slope in ageelevation between 1900 2100 m we interpret this break in slope as an acceleration of erosion associated with the onset ofalpineglaciation weusea3 dthermo kinematic modelto constrain pre and synglacial erosion rates results indicate a preglacial erosion rate of 0 4 mm yr that accelerated by a factor of 2 since 6 ma we propose that glacial valley widening and deepening are responsible for the observed nonlinear ahe age elevation profile citation densmore m s t a ehlers and g j woodsworth 2007 effect of alpine glaciation on thermochronometer age elevation profiles geophys res lett 34 l02502 doi 10 1029 2006gl028371

late cenozoic uplift of denali and its relation to relative plate motion and fault morphology apatite fission track analysis of samples that cover a 4 kilometer vertical section from the western flank of denali mount mckinley north america9s highest mountain suggests that the mountain massif was formed by rapid uplift 1 kilometer per million years beginning 6 million years ago ma uplift was a result of the morphology of the denali fault and a change in motion of the pacific plate with respect to north america at 5 ma which created opposing tangential vectors of relative movement along the fault and forced the intervening crustal blocks upward

controls on the patterns of topography and erosion rate in a critical orogen 1 we solve for the steady state patterns of erosion rate and topography in a critical wedge to understand the mutual adjustment of tectonics erosion and topography in the absence and presence of spatial variations in precipitation rate we consider steady state systems in which tectonics favors a critical topographic form assumed to be the mean elevation across the range and in which surface erosion by rivers and hillslopes operates on a two dimensional landscape we find that 1 a nonuniform critical topographic form implies a nonuniform pattern of ridge valley relief and hence a nonuniform pattern of erosion rate and 2 when the system is forced by local variations in precipitation rate maintenance of the critical topographic form requires a local response of rock uplift that greatly dampens changes in topography we apply these concepts to the western side of the olympic mountains of washington state where mean elevation ridge valley relief and precipitation rates increase from the coast to the topographic crest of the range we find that the main control on the erosion rate pattern is the pattern in mean elevation and the amount of precipitation in contrast the pattern of precipitation is only a minor control as a whole our work demonstrates an approach for developing the theoretical context that is necessary for interpreting spatial associations between patterns in topography precipitation and erosion in natural orogens

evolution and paleohydrology of glacial lakes barlow and ojibway abstract the evolution of lakes barlow and ojibway two large ice contact glacial lakes that covered parts of northern ontario and western quebec from about 10 1 8 0 ka bp during the retreat of the laurentide ice sheet was reconstructed using deglaciation landforms ice flow indicators inscribed on bedrock 14 c dates of early postglacial vegetation accumulated in lake basins the distribution of mysis relicta a biological indicator of lacustrine submergence varve chronology and a detailed photogrammetric survey of the lake limit although the rate of ice retreat varied greatly throughout the basin the average rate in lake barlow ojibway was 450 m year rates of ice retreat deduced from varve chronology were assessed using predictable relationships between the lake limit the shoreline gradient and the rate of relative uplift over a 590 km long profile the gradient on the maximum elevation of the lake defines a parabola that is attributed to restrained rebound although in part it could be due to the position of the profile with respect to the overall uplift pattern generated by the ice sheet the life span of the lake determined from varve chronology 2110 years shows excellent agreement with the age difference obtained between that of marine shells overlying lake ojibway sediments at its northern end in hudson bay and the oldest radiocarbon ages on basal organics in lakes in the southern part of the barlow ojibway basin both lakes drained eastward through the ottawa river between 10 1 and 8 0 ka bp and the average yearly discharge to the north atlantic ocean during the last 1000 1500 years is estimated at about 946 km 3 at break up of the ice sheet in southern hudson bay at about 8 0 ka bp lake ojibway drained abruptly releasing an estimated 114 396 km 3 of freshwater into the tyrrell sea and raising sea level by about 30 cm

influence of a glacial buzzsaw on the height and morphology of the cascade range in central washington state usa abstract analysis of climatic and topographic evidence from the cascade range of washington state indicates that glacial erosion limits the height and controls the morphology of this range glacial erosion linked to long term spatial gradients in the ela created a tilted planar zone of 373 cirques across the central part of the range peaks and ridges now rise 600 m above this zone hypsometric analysis of the region shows that the proportion of land area above the cirques drops sharply and mean slopes 30 indicate that the areas above the cirques may be at or near threshold steepness the mean plus 1 relief of individual cirque basins 570 m corresponds to the 600 m envelope above which peaks rarely rise the summit altitudes are set by a combination of higher rates of glacial and paraglacial erosion above the ela and enhanced hillslope processes due to the creation of steep topography on the high precipitation western flank of the cascades the dominance of glacial and hillslope erosion at altitudes at and above the ela may explain the lack of a correspondence between stream power erosion models and measured exhumation rates from apatite u th he thermochronometry

erosion rates during rapid deglaciation in icy bay alaska 1 contemporary glacial erosion rates based on sediment yields in southeast alaska merit considerable attention because they are unsurpassed worldwide and they significantly exceed long term exhumation rates in the region two issues are likely to contribute to these high rates contemporary sediment yields in fjords 1 have generally been overestimated by failing to account for the considerable input of subaerially derived material and 2 are exceptionally high because tidewater glaciers in southeast alaska have been anomalously dynamic and erosive during the past century of rapid retreat to investigate these influences and to quantify the rate at which tyndall glacier erodes its basin we present seismic data defining the volume of sediments in taan fjord icy bay we subtract the contribution of subaerially derived sediments from the fjord sediment package to determine the sediment yield directly from tyndall glacier during the most recent period of retreat 1962 1999 using a numerical model of proglacial glacimarine sedimentation we then calculate the annual sediment yield from and the corresponding erosion rate of tyndall glacier during this period which averages 28 5 mma 1 a strong correlation emerges between glacial retreat rates and glacial sediment yields implying that most contemporary sediment yield data from retreating tidewater glaciers may correspond to contemporary erosion rates that are a factor of 3 5 1 5 higher than in the long term hence we estimate the long term erosion rate for tyndall glacier to be 9 2 mma 1

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

glacial hydrology and erosion patterns a mechanism for carving glacial valleys article i nfo article history most glacial erosion models assume that erosion rates are proportional to ice sliding velocity while recent studies have shown that water plays a major role in modulating sliding velocities the impact it might have on erosion rates is still unclear here we incorporate subglacial hydrology into a glacial erosion model that is based on a sliding rule our results explicitly highlight that adding subglacial hydrology has profound impacts on the temporal and spatial patterns of glacial erosion first it suggests that erosion might mainly occur during melting seasons when subglacial water pressure is large and effective pressure is low i e before the channels fully develop and reduce the water pressure at the ice bedrock interface which promotes sliding and erosion second the distribution of erosion is bimodal with a peak at the equilibrium line altitude and as we demonstrate enhanced erosion at much lower altitudes within the ablation area where water due to melting abounds this has important implications because it explains why glacial erosion can simultaneously

response of glacier basal motion to transient water storage the speed of a glacier is affected most by sudden jumps in the water supply to the glacier but it goes back to previous levels if high water inputs are sustained because the glacier s plumbing system adjusts

pleistocene relief production in laramide mountain ranges western united states gently sloped summits and ridges collectively referred to as summit flats are abundant in many laramide ranges in the western united states the erosion rate of summit flats is 10 m m y on the basis of the concentrations of cosmogenic radionuclides because erosion rates in valleys between summit flats are an order of magnitude faster relief within these ranges is currently increasing by about 100 m m y if summit flat erosion is slower than rock uplift driven by the isostatic response to valley erosion then this relief production could result in increased summit elevations the mean depth of material eroded from a smooth surface fit to existing summit flats varies from 280 to 340 m in four laramide ranges based on geographic information system gis analyses of digital elevation models this erosion would result in a maximum of 250 300 m of rock uplift assuming airy isostasy however because the laramide ranges examined here are narrow relative to the flexural wavelength of the lithosphere erosionally driven rock uplift is limited to 50 100 m over the past several million years summit erosion would approximately offset this rock uplift therefore we conclude that summit elevations have remained essentially constant even though several hundred meters of relief has been produced on the basis of valley and summit erosion rates and the average depth of erosion we estimate that relief production in laramide ranges began at ca 3 ma we hypothesize that this relief production was climatically driven and was associated with the onset or enhancement of alpine glaciation in these ranges

geologic constraints on bedrock river incision using the stream power law denudation rate in unextended terranes is limited by the rate of bedrock channel incision often modeled as work rate on the channel bed by water and sediment or stream power the latter can be generalized as ka m s n where k represents the channel bed s resistance to lowenng whose variation with lithology is unknown a is drainage area a surrogate for discharge s is local slope and m and n are exponents whose values are debated we address these uncertainties by simulating the lowering of ancient river profiles using the finite difference method we vary m n and k to match the evolved profile as closely as possible to the corresponding modern river profile over a time period constrained by the age of the mapped paleoprofiles we find at least two end member incision laws ka 0 3 0 5 s for australian rivers with stable base levels and k f a 0 1 0 2 s n for rivers in kauai subject to abrupt base level change the long term lowering rate on the latter expression is a function of the frequency and magnitude of knickpoint erosion characterized by k f incision patterns from japan and california could follow either expression if they follow the first expression with m 0 4 k varies from 10 7 10 6 m 0 2 yr for granite and metamorphic rocks to 10 5 10 4 m 0 2 yr for volcaniclastic rocks and 10 4 10 2 m 0 2 yr for mudstones this potentially large variation in k with lithology could drive strong variability in the rate of long term landscape change including denudation rate and sediment yield

increase in late neogene denudation of the european alps confirmed by analysis of a fission track thermochronology database abstract a sharp increase in deposited sediment volume since pliocene times has been observed worldwide and in particular around the european alps this phenomenon has been linked to a rise in denudation rates controlled by an increase of either climatic or tectonic forcing observation of in situ cooling histories for orogens is critical to assess the reality of the inferred increase in denudation rates and to determine whether this phenomenon is widespread or localized at active tectonic structures we exploit the unique density of fission track ages in the western european alps to reconstruct cooling isoage surfaces and to estimate exhumation rates on the orogen scale between 13 5 and 2 5 ma our novel technique is based on the association of isoage contours with age elevation relationships it uses map view interpolation enabling a spatio temporal analysis of exhumation rates over the entire western alps the resulting exhumation histories reconstructed for eight areas of the western alps display strong similarities in timing and rates with orogen wide average denudation rates inferred from sediment volumes this consistency validates the use of both techniques for the study of an orogen characterized by strong relief and high recent exhumation rates we conclude that exhumation rates in the western alps have increased more than twofold since late miocene times this increase may have been locally modulated by the distinct response of different tectonic units

the thermochronological record of tectonic and surface process interaction at the yakutat north american collision zone in southeast alaska we investigate the material fluxes in space and time as a result of exhumation and erosion processes at the ongoing yakutat north american collision in southeast alaska many thermochronologic studies using a variety of sampling strategies are challenged by the widespread ice cover that limit field observations and accessibility this paper reviews new and published low temperature thermochronological data from southeast alaska to give a comprehensive interpretation of the exhumation patterns through time and how they are influenced by surface processes and climate change we find that the southeastern margin of alaska was exhumed and eroded long before the late miocene pliocene yakutat collision but since the beginning of the subduction of the yakutat lithosphere in the oligocene early miocene today there is a distinct pattern of exhumation in southeast alaska with a localized very rapid and deep seated exhumation at the yakutat plate corner st elias syntaxis where strike slip motion changes to convergence exhumation is also rapid but less deep along the dextral fairweather fault and in the evolving fold and thrust belt we present a re interpretation of the exhumation pattern in the fold and thrust belt and suggest that mass transport by exhumation is parallel to the observed active thrust faults and oblique to the suture zone and orogenic strike the locus of most rapid exhumation migrated from northwest to southeast with recent exhumation occurring near the st elias syntaxis exhumation of the chugach terrane rocks is still active however to a lesser degree than on the south side of the orogen where precipitation rates are much higher the wrangellia terrane to the north has experienced little exhumation and has essentially formed the backstop for terrane accretion in southeast alaska since the early cretaceous apatite u th he ages give the first evidence that rocks of the wrangell range have only been recently uplifted and eroded as a consequence of the continuing yakutat collision in general the thermochronology in southeast alaska reveals that climate variations across the region as well as changes through time have a limited influence on the pattern of erosion and that the location of deep exhumation is primarily influenced by tectonic processes