late pliocene and quaternary glaciations of new zealand abstract evidence of late pliocene glaciations is fragmentary the ross glaciation in northwest south island is correlated with the first marine faunal evidence of marked cooling in the wanganui basin in southern north island it is inferred to be in the 2 6 2 4 ma age range the porika glaciation in a separate area of northwest south island is younger judged by palynology and is correlated with a cooling 2 2 2 1 ma bp inferred from stratigraphic evidence of lowered sea level in the wanganui basin a gap of 1 5 ma in the record of glaciations is largely filled by wanganui basin marine sediments although an unconformity cuts out the record between ca 1 45 and 1 0 ma bp since then the many sedimentary cycles caused by sea level changes are presumed to be glacio eustatic in origin uplifted interglacial marine terraces back to marine oxygen isotope stage 17 and river terraces formed by cold climate aggradation are linked into the cyclic sequence through loess and tephra in their cover beds east of the south island cores from the dsdp site 594 in the southwest pacific record the earliest glaciation and higher in the core nine periods of south island glaciation in the last 0 7 ma onshore because of uplift and erosion only the last four glaciations assigned to oxygen isotope stages 10 8 6 and 4 2 are identified being named nemona waimaunga waimea and otira glaciations they are best known in north westland northwest south island where the relations of glacial outwash and interglacial shoreline deposits are displayed the late otira glaciation stage 2 is radiocarbon dated as beginning somewhat before 22 3 ka bp culminating ca 18 ka bp and ending at ca 14 ka bp when rapid deglaciation began a relatively small advance at ca 12 ka bp may have been followed by little change until after 8 6 ka bp recession was then followed by a period of advance from ca 5 ka bp until recent centuries and a subsequent recession that still continues

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

zircon 4he 3he thermochronometry abstract multiple thermochronometric methods are often required to constrain time continuous rock exhumation for studying tectonic processes or development of km scale topography at earth s surface here we explore 4he 3he thermochronometry of zircon as a method for constraining continuous time temperature t t paths of individual samples through a temperature range that is complementary to methods such as 40ar 39ar thermochronometry of k feldspar and 4he 3he thermochronometry of apatite for different cooling rates and diffusion domain size the temperature sensitivity of zircon 4he 3he thermochronometry ranges from slightly less than 100 c to slightly greater than 250 c a typical sample provides continuous thermal constraints over 100 c within that range outside these temperatures 4he in zircon will either be quantitatively retained or completely lost by volume diffusion as proof of concept we present stepwise release 4he 3he spectra and associated u and th concentration maps measured by laser ablation icp ms analysis of individual crystal aliquots of fish canyon tuff fct zircon and of a more complex setting in the sierra nevada batholith that experienced reheating from a proximal basaltic intrusion the little devil s postpile ldp the fct zircon 4he 3he release spectra are consistent with a 4he spatial distribution dominated by alpha ejection from crystal surfaces the spatial distributions of u and th measured in the same crystals do not substantially influence 4he 3he release spectra that are predicted for the known thermal history even when incorporating spatially variable diffusivity due to accumulation of radiation damage conversely the ldp 4he 3he release spectra are strongly influenced by the observed parent nuclide zonation a three dimensional 3d numerical model of 4he production and diffusion which incorporates crystal geometry u and th zonation and spatially variable he diffusion kinetics substantially improves the fit between measured and modeled 4he 3he release spectra for the independently known thermal history of the sample we conclude that zircon 4he 3he thermochronometry provides accurate thermal constraints through geologic time but certain applications of this technique may require 3d numerical modeling approaches

late cenozoic relief evolution under evolving climate a review abstract the present review paper is an attempt to summarize quantitative evidence of late cenozoic changes in topographic relief different meanings of the word relief as it is commonly used and detail the metrics used to quantify it we then specify methodological tools used to quantify relief change primarily low temperature thermochronometry and terrestrial cosmogenic nuclides and analyze published evidence for different regions our review first shows that relief changes and rates of changes are more important at mid than high or low latitudes and appear to be insensitive to mean precipitation rates it also show that relief change is positive relief increases in most of the reported cases 80 we subsequently define two functional relationships between relief and erosion depending on the chosen definition of relief and propose a conceptual model of landscape memory we conclude following others that erosion rates depend non linearly on relief evolution itself being a function of the spatial distribution and rates of erosion the relief increases documented in this review may be to erosion rate increases during the same timescales lastly we discuss the importance of glacial and periglacial processes on late cenozoic relief and erosion rate changes and stress the importance of frost shattering and glacial erosion at mid and high latitudes

episodic reductions in bottom water currents since the last ice age although north atlantic deep water formation was greatly reduced during the last glacial maximum bottom water currents were as vigorous as at present however they were weakened during periods of north atlantic surface freshening a strong correlation can be seen between bottom water current strength and greenland air temperature records thus confirming a close connection between ocean circulation and abrupt climate change past changes in the freshwater balance of the surface north atlantic ocean are thought to have influenced the rate of deep water formation and consequently climate1 2 although water mass proxies are generally consistent with an impact of freshwater input on meridional overturning circulation3 there has been little dynamic evidence to support this linkage here we present a 25 000 year record of variations in sediment grain size from south of iceland which indicates vigorous bottom water currents during both the last glacial maximum and the holocene period together with reconstructions of north atlantic water mass distribution vigorous bottom currents suggest a shorter residence time of northern source waters during the last glacial maximum relative to the holocene period the most significant reductions in flow strength occur during periods that have been associated with freshening of the surface north atlantic the short term deglacial oscillations in bottom current strength are closely coupled to changes in greenland air temperature with a minimum during the younger dryas cold reversal and a maximum at the time of rapid warming at the onset of the holocene our results support a strong connection between ocean circulation and rapid climate change

rock slope failure in the british mountains abstract in this first full review of extant quaternary rock slope failure rsf in the british mountains we provide a near complete inventory of 1082 sites 40 being rock slope deformations 40 arrested rockslides and 20 rock avalanches current rsf activity is negligible and this relict population is predominantly paraglacial with a parafluvial minority its spatial distribution is perplexing with rsf density varying greatly both regionally and locally in the scottish highlands eight main clusters account for 76 of rsf area in 15 of the montane area local concentrations occur in all the british ranges across high and low relief in core and peripheral locations and on varied geological and glaciological domains as conversely do extensive areas of sparsity even in similar lithologies generic interpretations are thus precluded geology is only a secondary control an association with concentrated erosion of bedrock ceb is proposed as a driver of intensified slope stresses ceb is most evident at those glacial breaches of main divides where the most vigorous recent incision is inferred and also in some trough heads a clear association between rsfs and these late developing breaches is demonstrated in the highlands in 42 localities with sparsity away from them it is also seen in seven lake district localities glaciological models identify ice sheet volatility capable of driving breach ramification high magnitude paleoseismic events are generally unlikely to have provoked rsf clusters a few candidates are considered rsf has been underrated as an agent of mountain landscape evolution in britain its spatio temporal incidence may assist in calibrating regional ice sheet models and in assessing climate change impacts we argue that the ceb rsf association has global relevance in identifying primary drivers of mass movement in bedrock

rate of fluvial incision in the central alps constrained through joint inversion of detrital 10be and thermochronometric data abstract we integrate constraints from published detrital thermochronometry and detrital 10 be concentrations in a single catchment to infer how erosion is shaping alpine topography we apply this analysis to the codera watershed in the bergell intrusion central european alps where rivers have incised into a glacial valley because thermochronometric ages of bedrock samples increase with elevation a distribution of detrital ages in river sediment provides a tracer for the source elevations of sediment within the catchment while detrital 10 be concentrations provide constraints on rates of erosion we find that modern erosion rates within the fluvial portion of the landscape are too low to permit the inferred 500 m of incision during the most recent interglacial based on the spatial pattern of modern erosion rates we predict that if the incised fluvial valley was formed during interglacial periods only it has formed over the last approximately 400 000 years

cirques have growth spurts during deglacial and interglacial periods evidence from 10be and 26al nuclide inventories in the central and eastern pyrenees cirques are emblematic landforms of alpine landscapes the statistical distribution of cirque floor elevations is used to infer glacial equilibrium line altitude and the age of their frontal moraines for reconstructing glacial chronologies very few studies however have sought to measure cirque floor and supraglacial ridgetop bedrock downwearing rates in order to confront these denudation estimates with theoretical models of quaternary mountain landscape evolution here we use 10 be nuclide samples n 36 from moraines bedrock steps and supraglacial ridgetops among a population of cirques in the east central pyrenees in order to quantify denudation in the landscape and detect whether the mountain topography bears any relevance to the glacial buzzsaw hypothesis minimum exposure ages meas obtained for a succession of moraines spanning the oldest dryas to the holocene produced a deglaciation chronology for three different pyrenean ranges maladeta bassies and carlit based on a series of corrections calibrations and chronostratigraphic tuning procedures meas on ice polished bedrock exposures were further used to model denudation depths at nested timescales during the wurm the younger dryas and the holocene results show that subglacial cirque floor denudation was lower during glacial periods wurm 10 mm ka than during deglacial and interglacial periods tens to hundreds of mm ka the relative inefficiency of glacial denudation in the cirque zone during the wurm would have resulted from i cold based and or ii low gradient glaciers situated in the upper reaches of the icefield and or from iii glacier load starvation because of arrested clast supply from supraglacial rockslopes situated in the permafrost zone denudation peaked during the younger dryas and holocene glacial advances a time when cirque glaciers became steeper warmer based and when frost cracking weakened supraglacial ridgetops thus enhancing sub glacial erosion by providing debris to the sliding glacier base cirques therefore grow faster during more temperate periods of cirque glaciation than under full glacial conditions another key finding was the very low rates of ridgetop lowering averaged over the wurm and holocene 10 25 mm ka a comparison of the denudation rates obtained from the cirque zone with regional estimates of crustal uplift indicates that the alpine topography is not in a steady state the low intensity of glacial denudation failed to bring the topography to a buzzsaw equilibrium state

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

freezing of ridges and water networks preserves the gamburtsev subglacial mountains for millions of years once an ice sheet grows beyond a critical thickness the basal thermal regime favors melting and development of subglacial water networks subglacial water is necessary for bedrock erosion but the exact mechanisms that lead to preservation of subglacial topography are unclear here we resolve the freezing mechanisms that lead to long term high altitude preservation across the gamburtsev subglacial mountains in east antarctica analyses of a comprehensive geophysical data set reveal a large scale water network along valley floors the ice sheet often drives subglacial water up steep topography where it freezes along high ridges beneath thinner ice statistical tests of hypsometry show the gamburtsevs resemble younger midlatitude mountains indicating exceptional preservation we conclude that the gamburtsevs have been shielded from erosion since the latest eocene 34 ma these freezing mechanisms likely account for the spatial and temporal patterns of erosion and preservation seen in other glaciated mountain ranges

deciphering the driving forces of erosion rates on millennial to million year timescales in glacially impacted landscapes an example from the western alps in many regions tectonic uplift is the main driver of erosion over million year myr timescales but climate changes can markedly affect the link between tectonics and erosion causing transient variations in erosion rates here we study the driving forces of millennial to myr scale erosion rates in the french western alps as estimated from in situ produced cosmogenic 10be and a newly developed approach integrating detrital and bedrock apatite fission track thermochronology millennial erosion rates from 10be analyses vary between 0 27 and 1 33 m kyr similar to rates measured in adjacent areas of the alps significant positive correlations of millennial erosion rates with geomorphic measures in particular with the lgm ice thickness reveal a strong transient morphological and erosional perturbation caused by repeated quaternary glaciations the perturbation appears independent of myr scale uplift and erosion gradients with the effect that millennial erosion rates exceed myr scale erosion rates only in the internal alps where the latter are low 0 4 km myr these areas moreover exhibit channels that clearly plot above a general linear positive relation between myr scale erosion rates and normalized steepness index glacial erosion acts irrespective of rock uplift and thus not only leads to an overall increase in erosion rates but also regulates landscape morphology and erosion rates in regions with considerable spatial gradients in myr scale tectonic uplift our study demonstrates that climate change e g through occurrence of major glaciations can markedly perturb landscape morphology and related millennial erosion rate patterns even in regions where myr scale erosion rates are dominantly controlled by tectonics

incorporating 3 d parent nuclide zonation for apatite 4he 3he thermochronometry an example from the appalachian mountains the ability to constrain km scale exhumation with apatite 4he 3he thermochronometry is well established and the technique has been applied to a range of tectonic and geomorphic problems however multiple sources of uncertainty in specific crystal characteristics limit the applicability of the method especially when geologic problems require identifying small perturbations in a cooling path here we present new 4he 3he thermochronometric data from the appalachian mountains which indicate significant parent nuclide zonation in an apatite crystal using la icpms measurements of u and th in the same crystal we design a 3 d model of the crystal to explore the effects of intra crystal variability in radiation damage accumulation we describe a numerical approach to solve the 3 d production diffusion equation using our numerical model and a previously determined time temperature path for this part of the appalachians we find excellent agreement between predicted and observed 4he 3he spectra our results confirm this time temperature path and highlight that for complex u and th zonation patterns 3 d numerical models are required to infer an accurate time temperature history in addition our results provide independent and novel evidence for a radiation damage control on diffusivity the ability to exploit intra crystal differences in 4he diffusivity i e temperature sensitivity greatly increases the potential to infer complex thermal histories

the influence of burial heating on the u th he system in apatite grand canyon case study article history thermochronological data can constrain the cooling paths of rocks exhumed through the uppermost 1 2 km of earth s crust and have thus been pivotal in illuminating topographic development over timescales 0 1 ma however in some cases different methods have led to conflicting conclusions about timing of valley scale exhumation here we investigate the case of western grand canyon usa where different thermochronological datasets have been interpreted to record very different timings of canyon incision 70 ma versus 5 ma we present a method to assess key assumptions in these constraints and demonstrate that burial heating conditions of basement rocks in the mesozoic can result in incomplete annealing of radiation damage in apatite in turn this has a dramatic effect on the temperature sensitivity of the apatite u th he system and its ability to record post burial exhumation the possibility of incomplete annealing resolves the apparent conflict in time temperature paths inferred over the last 70 ma although it requires temperatures during burial that are lower than predicted by apatite fission track data a refinement of parameters that prescribe the kinetics of damage annealing and related control on 4 he diffusivity in apatite would account for this discrepancy specifically if alpha recoil damage anneals at a lower rate than fission tracks at a given temperature these effects will be important for any application of the apatite u th he system in geologic settings that experienced prolonged residence 10 ma between 50 150 c the approaches developed here provide means to assess these effects

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

quantifying the diffusion kinetics and spatial distributions of radiogenic 4he in minerals containing proton induced 3he apatite titanite and olivine samples were bombarded with a 150 mev proton beam to produce 10 8 atoms mg of spallation 3he high precision stepped heating experiments were then performed in which the artificial 3he and for apatite and titanite the natural radiogenic 4he were measured to characterize the diffusive behavior of each isotope helium 3 diffusion coefficients are in excellent agreement with concurrently and or previously determined he diffusion coefficients for each mineral our results indicate that proton induced 3he is uniformly distributed and that radiation damage associated with a proton fluence of 5 x 10 14 protons cm 2 does not cause noticeable changes in 4he diffusion behavior in at least apatite and titanite proton induced 3he can therefore be used to establish he diffusion coefficients in minerals with insufficient natural helium for analysis or those in which the natural 4he distribution is inhomogeneous in addition step heating 4he 3he analysis of a mineral with a uniform synthetic 3he concentration provides a means by which a natural 4he distribution can be determined

glacial steady state topography controlled by the coupled influence of tectonics and climate glaciers and rivers are the main agents of mountain erosion while in the fluvial realm empirical relationships and their mathematical description such as the stream power law improved the understanding of fundamental controls on landscape evolution simple constraints on glacial topography and governing scaling relations are widely lacking we present a steady state solution for longitudinal profiles along eroding glaciers in a coupled system that includes tectonics and climate we combined the shallow ice approximation and a glacial erosion rule to calculate ice surface and bed topography from prescribed glacier mass balance gradient and rock uplift rate our approach is inspired by the classic application of the stream power law for describing a fluvial steady state but with the striking difference that in the glacial realm glacier mass balance is added as an altitude dependent variable from our analyses we find that ice surface slope and glacial relief scale with uplift rate with scaling exponents indicating that glacial relief is less sensitive to uplift rate than relief in most fluvial landscapes basic scaling relations controlled by either basal sliding or internal deformation follow a power law with the exponent depending on the exponents for the glacial erosion rule and glen s flow law in a mixed scenario of sliding and deformation complicated scaling relations with variable exponents emerge furthermore a cutoff in glacier mass balance or cold ice in high elevations can lead to substantially larger scaling exponents which may provide an explanation for high relief in high latitudes

structure and kinematics of oblique continental convergence in northern fiordland new zealand abstract southeast of the alpine fault in fiordland new zealand late tertiary faults envelope exposures of early cretaceous high p 12 16 kbar granulite facies orthogneisses these exposures allowed us to examine how pre cenozoic structures influenced variations in the degree and style of kinematic partitioning within a zone of oblique continental convergence fault slip data and kinematic modelling show that oblique slip and reverse displacements preferentially were partitioned east of the alpine fault onto curved ne striking surfaces and moderately dipping 30 52 n and nw striking surfaces respectively approximately 3 0 5 5 km of exhumation and 6 5 7 0 km of oblique dextral displacement occurred on faults that display palm tree style geometries and subhorizontal detachments in profile this heterogeneous style contrasts with the near horizontal slip on subvertical surfaces that define the alpine fault north of milford sound the principal axes of instantaneous strain we determined using faults are compatible with principal stress axes derived from earthquakes our data indicate that mechanical anisotropies created by inherited structures controlled the locations and highly variable geometries of faulting since 7 9 ma the effects of these pre existing structures and a focusing of contractional deformation along the northeast margin of fiordland resulted in an unusually high degree of strike slip partitioning across the southernmost onshore segment of the alpine fault

low temperature thermochronology and thermokinematic modeling of deformation exhumation and development of topography in the central southern alps new zealand 1 apatite and zircon u th he and fission track ages were obtained from ridge transects across the central southern alps new zealand interpretation of local profiles is difficult because relationships between ages and topography or local faults are complex and the data contain large uncertainties with poor reproducibility between sample duplicates data do form regional patterns however consistent with theoretical systematics and corroborating previous observations young neogene ages occur immediately southeast of the alpine fault the main plate boundary structure on which rocks are exhumed partially reset ages occur in the central southern alps and older mesozoic ages occur further toward the southeast zircon apparent ages are older than apatite apparent ages for the equivalent method three dimensional thermokinematic modeling of plate convergence incorporates advection of the upper pacific plate along a low angle detachment then up an alpine fault ramp adopting a generally accepted tectonic scenario for the southern alps the modeling incorporates heat flow evolving topography and the detailed kinetics of different thermochronometric systems and explains both complex local variations and regional patterns inclusion of the effects of radiation damage on he diffusion in detrital apatite is shown to have dramatic effects on results geometric and velocity parameters are tuned to fit model ages to observed data best fit is achieved at 9 mm a 1 plate convergence with pacific plate delamination on a gentle 10 se dipping detachment and more rapid uplift on a 45 60 dipping alpine fault ramp from 15 km depth thermokinematic modeling suggests dip slip motion on reverse faults within the southern alps should be highest 22 km from the alpine fault and much lower toward the southeast

similar rates of modern and last glacial ocean thermohaline circulation inferred from radiochemical data today the ocean thermohaline circulation transports half of the 231pa produced by radioactive decay in the atlantic ocean water column to the southern ocean this export respectively imparts low and high 231pa 230th ratios to the surface sediments of these oceans ocean sediments from the last glacial maximum bear a similar isotopic fingerprint implying that advection of north atlantic deep intermediate water into the circumpolar deep water of the southern ocean occurred at a similar or slightly higher rate during the last glacial period

positive feedbacks associated with erosion of glacial cirques and overdeepenings the principal points of water input to a glacier are the bergschrund in cirques and crevasse fields lower on the glacier crevasse fields commonly occur over convexities at the heads of overdeepenings in glacier beds the amplitude of subglacial water pressure fluctuations is large just down glacier from these points of water input erosion by quarrying is likely in such areas erosion is thus inferred to be localized on the headwalls of cirques and overdeepenings in the case of overdeepenings this leads to a positive feedback process in which a perturbation in the bed causes crevassing at the surface resulting in erosional forces that accentuate the perturbation when subglacial water flows up an adverse bed slope leading out of a cirque or overdeepening much of the viscous energy dissipated is used to warm the water to keep it at the pressure melting temperature as the ice thins and the pressure decreases in such situations subglacial conduits are maintained by high water pressures rather than by melting of conduit walls in the limit water pressures apparently become so high that water is forced out along the ice bed interface and the conduits collapse the products of erosion are then no longer flushed out and a protective till layer accumulates by limiting erosion on such adverse bed slopes this till layer controls the geometry of these overdeepened basins

a poisoned chalice deep sea sediments often carry information about past temperatures in the upper ocean directly above their location but as mccave cautions in this perspective interpretation of the sediments can be complicated by deep sea processes for example material transported laterally in the deep ocean may distort the upper ocean signal in the sediments a report by ohkouchi illustrates such processes at bermuda rise the study yields a detailed understanding of deep sea sediment transport processes aiding the interpretation of sediment records at this site and sounding a warning for others

controls of initial topography on temporal and spatial patterns of glacial erosion abstract here we investigate the influence of initial pre glacial topography on spatial and temporal patterns of glacial erosion using numerical surface process modelling including a higher order ice sheet model first we consider glacier dynamics when simulating glaciation in two real landscapes representing plateau type topography southeast australia and characteristic steady state fluvial topography southern taiwan we find that the different initial landscape configurations result in distinctly different ice configurations and patterns of basal sliding the sliding patterns are controlled by ice configuration and the resulting basal shear stresses and by the thermal properties at the base of the ice we then investigate how these characteristic patterns of basal sliding control glacial erosion and long term landscape evolution using synthetic representations of the two landscapes the two landscape configurations result in markedly different spatial and temporal patterns of glacial erosion however the resulting landscapes may have similar morphology irrespective of initial landscapes and glacial erosion patterns being significantly different the numerical experiments also suggest that in addition to basal temperature basal shear stress is important in restricting long term glacial erosion which is relevant for the preservation of landforms during glaciations specifically pre glacial landforms may be eroded although they are initially protected by cold based ice when the ice configuration promotes significant basal shear stress glacial erosion at the edge of a plateau like landscape in contrast pre glacial landforms may be preserved irrespective of the ice being warm based when low gradients in the ice surface act to limit basal shear stress

230th normalization an essential tool for interpreting sedimentary fluxes during the late quaternary there is increasing evidence indicating that syndepositional redistribution of sediment on the seafloor by bottom currents is common and can significantly affect sediment mass accumulation rates notwithstanding its common incidence this process generally referred to as sediment focusing is often difficult to recognize if redistribution is near synchronous to deposition the stratigraphy of the sediment is not disturbed and sediment focusing can easily be overlooked ignoring it however can lead to serious misinterpretations of sedimentary fluxes particularly when past changes in export flux from the overlying water are inferred in many instances this problem can be resolved at least for sediments deposited during the late quaternary by normalizing to the flux of 230th scavenged from seawater which is nearly constant and equivalent to the known rate of production of 230th from the decay of dissolved 234u we review the principle advantages and limitations of this method notwithstanding its limitations it is clear that 230th normalization does provide a means of achieving more accurate interpretations of sedimentary fluxes and eliminates the risk of serious misinterpretations of sediment mass accumulation rates

evolution of lumpy glacial landscapes at 10 100 m scales glaciated landscapes are lumpy sporting rocky bedforms with abraded upvalley faces and sharp quarried downvalley edges using models that operate at two time scales i address glacial landform development at these scales in order to explore the dependence of bed topography and erosion rate on glacier and rock properties sliding at the bed a prerequisite for both abrasion and quarrying is strongly modulated by water pressures short time scale models designed to capture the transient hydro sliding system beneath a temperate glacier quantify the temporal and spatial pattern of effective stress on the glacier bed the bed is subjected to repetitive stressing focused on the edges of bed steps the frequency and magnitude of stress events increasing with ice thickness the numerical model of bed evolution at longer time scales incorporates both abrasion and quarrying of fracture bound blocks the probability of quarrying of a block by a particular stress event is taken to depend inversely upon both block size and the depth of the niche in which it sits upglacier migrating rock bedforms with 10 100 m wavelengths inevitably emerge reflecting efficient quarrying of blocks from downvalley facing steps in the bed the relative importance of abrasion and quarrying and bed roughness are controlled by fracture spacing and major steps in the valley floor are attributable to transitions in fracture spacing

evidence for a warm last glacial maximum in the nordic seas or an example of shortcomings in uk 37 and uk 37 to estimate low sea surface temperature the occurrence of ice free conditions in the nordic seas during the last glacial maximum has been demonstrated using microfossil and biomarker approaches an accurate picture of the prevailing oceanographic conditions may be more difficult to ascertain thus uncertainties for most proxies increase at the cold extreme of temperature calibrations and glacial sediments contain small amounts of microfossils and biomarkers here we discuss sea surface temperature estimates derived from alkenone indices for the last glacial maximum in the nordic seas that provide a surprising scenario where surface temperatures were warmer than at present we conclude that the uk37 and uk37 estimates may not be interpreted in terms of sea surface temperature as several factors may have conspired to bias the molecular signal however using an alternative index based on the abundance of the c37 4 alkenone we estimate a maximum temperature of 6 c

high magnitude and rapid incision from river capture rhine river switzerland 1 landscape evolution is controlled by the development and organization of drainage basins as a landscape evolves drainage reorganization events can occur via river capture or piracy whereby one river basin grows at the expense of another the river downstream of a capture location will generate a transient topographic response as the added water discharge increases sediment transport and erosion efficiency this erosional response will propagate upstream through both the captured and original river basins here we focus on quantifying the impact of drainage reorganization along the rhine aare river system 45 000 km2 during the late pliocene early pleistocene where gravel remnants indicate total incision of 650 m during the last 4 2 myr in the region of the recent aare rhine confluence we develop a numerical model of drainage capture to quantify the range of possible magnitudes of erosion and the transient river response resulting from the reorganization of the rhine river the model accounts for both fluvial incision and sediment transport our model estimates 400 800 m of river elevation change lowering profiles during the last 4 myr due to river capture events providing an important component to the recent exhumation budget of the swiss alpine foreland the model indicates a rapid response to capture events re equilibration timescale of 1 myr the predicted incision magnitudes are consistent with incision measured from the elevation of pliocene and early pleistocene river gravels suggesting that across northern switzerland a significant amount of incision can be explained by drainage reorganization

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

hydrography and sedimentation under the deep western boundary current on bjorn and gardar drifts iceland basin abstract the interaction of northern component waters overflowing the iceland scotland ridge with sediments further south in the iceland basin has been responsible for highly variable sedimentation patterns since at least the late miocene 5 million years ago and has formed both bjorn and gardar drifts presently this process is maintained by a flux of approximately 3 sv measured along the south icelandic slope of iceland scotland overflow water isow with a mean regional flow direction to the sw which affects sediment transport on bjorn drift below 1350 m depth and over most of the gardar drift coriolis deflection of isow flow causes it to move as a deep western boundary current dwbc in the iceland basin which redistributes sediments under the influence of seabed topography here hydrographic and sedimentological data are used to illustrate the effects and extent of the vigorous dwbc activity the study presents a baseline against which inferred changes in the past can be assessed north of 58 n bottom nepheloid layers with heavy suspended sediment loads are often encountered particularly under the denser and faster flowing core of isow on the eastern slope of gardar drift in this area the presence of migrating regular sedimentary waves is seen on 3 5 khz profiles this is consistent with both the near bottom flow velocity probably in excess of 10 cm s 1 estimated by the geostrophic method and the presence of well sorted foraminiferal sand on the seabed south of 58 n dilute bottom nepheloid layers associated with more tranquil conditions near the seabed inferred from photographic and sedimentological evidence show that currents on gardar drift are weak although most of the south iceland basin is mantled by both regular and irregular non migratory mudwaves the absence of migrating structures with the exception of current intensification associated with discrete basement highs indicates that more sluggish currents characterise this region throughout the region highly variable mean holocene sediment accumulation rates are inferred 2 40 cm ka 1 as a result of the dwbc interacting with the topography

u th ne and multidomain u th he systematics of a hydrothermal hematite from eastern grand canyon a hydrothermal hematite from the redwall limestone in eastern grand canyon arizona usa was analyzed for u th he and u th ne systematics the dense polycrystalline aggregate has u and th concentrations of 16 ppm each neon produced by particle capture on 18 o is abundant in the sample it constitutes 90 of total 21 ne and indicates a u th ne age of 217 5 ma this value provides a lower bound for the formation age of the sample but detailed interpretation is not possible given the current absence of neon diffusion data for hematite the 130 2 ma u th he age is much younger than the ne age and therefore must be a cooling age highly reproducible step heating he release experiments reveal the existence of a spectrum of diffusion domain sizes spanning about 3 orders of magnitude in radius corresponding to he closure temperatures ranging from 0 to 150 c these domains may correspond to crystallites in the specimen which are observed to range from smaller than a few nm to a few m 4he 3he age spectra acquired from the same experiments define a distinctive sigmoidal shape with step ages increasing monotonically from 0 to 210 ma this pattern clearly records the presence of the multiple domains as the hematite acquired its radiogenic helium coupling of the 4he 3he spectra and the diffusion data allow modeling of permissible time temperature paths experienced by the hematite these paths can be compared with independent thermal history reconstructions for eastern grand canyon characterized by 170 m yr of sedimentary burial followed by regional unroofing and canyon incision beginning at 80 ma the unroofing portion of the path inferred from the hematite is in good agreement with apatite u th he and fission track data the results permit rapid cooling in late cretaceous early tertiary time suggest a subsequent period of slow cooling through temperatures of 50 70 c with accelerated cooling ensuing after 20 ma in contrast the hematite data demand temperatures in some part s of the mesozoic that are at least 100 c hotter than expected based on burial depth estimates this may indicate that the sample experienced a reheating event associated with the passage of fluids similar to those observed at nearby mineralized breccia pipes overall these data provide motivation for further work to understand he and ne behavior in hematite and their potential use in thermochronometry

grain size effects on th 230 xs inventories in opal rich and carbonate rich marine sediments abstract excess thorium 230 230thxs as a constant flux tracer is an essential tool for paleoceanographic studies but its limitations for flux normalization are still a matter of debate in regions of rapid sediment accumulation it has been an open question if 230thxs normalized fluxes are biased by particle sorting effects during sediment redistribution in order to study the sorting effect of sediment transport on 230thxs we analyzed the specific activity of 230thxs in different particle size classes of carbonate rich sediments from the south east atlantic and of opal rich sediments from the atlantic sector of the southern ocean at both sites we compare the 230thxs distribution in neighboring high vs low accumulation settings two grain size fractionation methods are explored we find that the 230thxs distribution is strongly grain size dependent and 50 90 of the total 230thxs inventory is concentrated in fine material smaller than 10 m which is preferentially deposited at the high accumulation sites this leads to an overestimation of the focusing factor and consequently to an underestimation of the vertical flux rate at such sites the distribution of authigenic uranium indicates that fine organic rich material has also been re deposited from lateral sources if the particle sorting effect is considered in the flux calculations it reduces the estimated extent of sediment focusing in order to assess the maximum effect of particle sorting on we present an extreme scenario in which we assume a lateral sediment supply of only fine material comparing 230thxs normalized mass fluxes biased by sorting effects with uncorrected mass fluxes we suggest that 230thxs normalization is still a valid tool to correct for lateral sediment redistribution however differences in focusing factors between core locations have to be evaluated carefully taking the grain size distributions into consideration

focused pliocene quaternary exhumation of the eastern pamir domes western china abstract the kongur shan and muztagata domes are the most prominent topographic features in the eastern pamir indenter corner in spite of several previous bedrock thermochronology studies the extent processes and mechanisms of dome exhumation remain unclear we present 1158 detrital zircon fission track dzft ages from modern glacier and river sediments along the domes to better constrain the spatial distribution of exhumation our thermochronologic dataset from the chakragil massif shows a sharp increase in the proportion of dzft ages 10 ma from basins that drain the peak region to those further to the north which suggests that the kalagile fault forms the northern boundary of the rapidly exhuming dome structure a cluster of middle miocene dzft ages in these samples documents pre doming cooling possibly associated with motion on the main pamir thrust and or oytag fault large fractions of dzft grain ages

the available enthalpy cycle ii applications to idealized baroclinic waves the local available enthalpy cycle proposed in part i of this paper is applied to document energetics of three numerical simulations representing life cycles of idealized baroclinic waves an improved temporal numerical scheme defined in part i is used in this study together with the arpege ifs model using a t42 triangular truncation a 45 n and 200 hpa dry unstable jet is constructed with the most unstable mode at zonal wave number 8 energetic impacts of both horizontal and vertical diffusion schemes are determined separately the role of ageostrophic winds within the ekman layer is investigated leading to an explanation for large observed values for the dissipation terms and to a new formulation of the potential energy conversions the magnitudes of these new conversion terms are compared with those of the usual barotropic and baroclinic conversions a new version for the available enthalpy cycle is proposed it is suitable for open systems and it includes explicitly the potential energy component as a transitional reservoir finally some results from intensive observing period 15 of the fronts and atlantic storm track experiment are compared with those from the idealized diabatic experiment copyright 2003 royal meteorological society

past extent of sea ice in the northern north atlantic inferred from foraminiferal paleotemperature estimates 1 using 150 core top samples we developed a conservative but fairly robust new measure to reconstruct past changes in maximum seasonal sea ice distribution in the northern north atlantic hitherto a major unknown the proxy is based on similarity maximum modern analog technique simmax estimates of threshold temperatures near the sea surface sst today almost 100 of all sites with sst 2 5 c during summer 0 4 for winters 1978 1987 and 0 75 c for little ice age winters lie seaward of the sea ice margin when applied to 60 sediment records of the last glacial maximum this proxy shows that peak glacial sea ice was far more restricted than in the classic climap project members 1981 reconstruction during glacial summer sea ice only covered the arctic ocean and western fram strait the northern north atlantic and nordic seas were largely ice free and thus formed a high latitude moisture source for the continued buildup of continental ice sheets in contrast sea ice spread far south across the iceland faeroe ridge during glacial winter with an inferred patch of sea ice also in the central east atlantic near the center of the azores high a broad ice free channel extended from 50 to 60 n forming an ideal site for large scale convection of glacial upper north atlantic deep water the extreme seasonality in glacial sea ice formation and melt in the nordic seas implies a major consumption of the regional energy income

glacial erosion promotes high mountains on thin crust abstract every mountain range on earth is unique in its tectonic evolution but quaternary climate cooling resulted in alpine landscapes with globally similar landforms such as horn shaped peaks cirques and overdeepened valleys on a global scale the impact of glacial and periglacial processes on the geometry of mountain ranges and the relative efficiency of glacial and fluvial erosion are still discussed to address these issues we analyse a novel worldwide dataset of more than 16 000 mountain peaks we relate peak height and steepness to mean elevation and crustal thickness we show that the geometry of mountain ranges systematically varies with latitude and thus with climate for peaks of similar height the steepness increases towards high latitudes while mean elevation and thickness of the supporting crust decrease modelling flexural isostasy based on the topographic load of mountain ranges demonstrates that the crustal thickness required to support peaks of similar height dramatically declines with intensity of glacial erosion in concert with latitudinal trends in peak steepness mean elevation and crustal thickness this shows that quaternary climate cooling reduces the topographic load by transforming the geometry of mountain landscapes from fluvial to glacial conditions we conclude that the prevalence of glacial erosion in cold climate reduces the topographic load of mountain ranges and enables the growth of high mountain peaks even on moderately over thickened crust

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

effective closure temperature in leaky and or saturating thermochronometers abstract the classical equation of closure temperature t c in thermochronometry dodson 1973 assumed i no storage limitation for the accumulating radiogenic product ii a negligible product concentration at the initial temperature of cooling t 0 and iii a negligible product loss at the final present day temperature t p a subsequent extension ganguly and tirone 1999 provided a simple correction for systems cooling from an arbitrary t 0 at which presence of an initial concentration profile may affect final concentrations here we use a combination of analytical and numerical solutions to derive a general expression for the effective closure temperature in i systems which cool between arbitrary initial and final temperatures potentially still suffering from thermal product loss at t p termed leaky and ii systems which may contain a physical limit on the maximum amount of product that can be stored termed saturating while all conservative results can be easily reproduced an extended use of our formulation provides meaningful effective closure temperatures even when the standard calculation schemes fail for a first order loss radiometric system governed by k t s exp e r t where e j mol 1 and s s 1 are the arrhenius parameters and r is the gas constant we find that the effective closure temperature t c t 0 t p is given by t c t 0 t p 1 t p r e k p ln 1 k p k p e k p k p k 0 1 where k 0 and k p s 1 are shorthand for k t 0 and k t p respectively s 1 the production rate s a time constant and a z the upper incomplete gamma function under conventional conditions our solution reduces to dodson s formula although the solution strictly applies only to systems where 1 t increases linearly with time it is nevertheless a useful approximation for a broad range of cooling functions in systems where closure occurs close to the system s initial final thermal boundary conditions we clarify the use and the meaning of t c t 0 t p by drawing a comparison between i a hypothetical application of apatite u pb dating t c 450 c on venus mean surface temperature of 450 c leaky behaviour and ii the recently introduced thermochronometric application of optically stimulated luminescence osl dating on earth both leaky and saturating behaviour

late cenozoic exhumation model of new zealand impacts from tectonics and climate abstract late cenozoic convergence between the australian and pacific plates has led to extensive crustal deformation and exhumation in new zealand over the past five decades the convergence associated exhumation history of the palaeozoic mesozoic new zealand basement has been studied using various thermochronometric techniques here we compile 992 published ages and invert them to estimate the temporal and spatial variation in exhumation rates since 25 ma which we then interpret with a plate tectonic reconstruction model the results reveal widespread increases in the exhumation rates at 6 4 ma a time correlated with accelerations in the relative plate motion the absolute increases in exhumation rates are however not uniformly distributed over new zealand since the plio pleistocene the inferred models suggest variable response of exhumation to global cooling in the non glaciated eastern north island the hikurangi subduction margin the million year average exhumation rates have remained relatively low 5 km myr have been observed in the collision zone of the central southern alps of south island where both tectonic deformation and precipitation rates are highest in fiordland the puysegur subduction margin despite the weak crustal deformation the exhumation rates have been high 0 5 3 km myr which we suggest is due to intensified glacial erosion our findings highlight the determining role of tectonics in setting up the regional exhumation pattern but both strong tectonic deformation and efficient erosion are required to maintain rapid exhumation rates the influence of climate on mountain erosion can only be distinguished in regions under slow to mediate exhumation

the sediment budget of an alpine cirque cirques form and evolve as glaciers attack the bed and subaerial processes dismantle the surrounding walls collectively these processes which can make a cirque longer or deeper or both profoundly influence near divide regions of glaciated mountains and yet are rarely studied in a systematic way toward this end we developed a theoretical framework for the sediment budget of a cirque that includes sediment sources transport pathways and storage elements we quantified each component of the sediment budget using field measurements and remote sensing data of a glaciated alpine cirque in british columbia canada the cirque has a plan view area of 1 64 km 2 and relief of 780 m our budget values which correspond to a period of substantial glacier retreat are based on measurements reflecting time intervals ranging from 1 yr to 80 yr we report errors as a range enclosed in parentheses analogous to 95 confidence bounds on average 1640 250 7950 metric tons of rock are released by the headwall each year nearly 90 of this debris leaves the wall as small rockfalls or in snow avalanches our field observations indicated that snow avalanches originating as cornice failures are currently the most important transport process on the headwall we estimated the mass of debris transported annually by the glacier to the foreland using 1 the volume and age of the foreland ground moraine and 2 the product of rock mass per unit volume of ice and glacier velocity over the past several decades the glacier delivered 6440 1180 14930 tons yr to the foreland via forward ice motion and margin retreat mostly in subglacial till or sediment rich basal layers less than 3 of the glacierborne sediment flux traveled as supraglacial debris 170 50 320 tons yr at present sediment evacuation from the cirque occurs in a single meltwater stream we monitored water discharge and suspended sediment concentration in this stream between 29 june and 28 august 2007 by season s end 650 80 1860 tons of sediment had passed our gauging station equivalent to an erosion rate of 0 2 0 03 0 70 mm yr when averaged over the glacier bed approximately one third of the total annual streamborne sediment transport occurred over a 2 d period during the first major melt event of the year using our budget relations and flux magnitudes we estimate the glacier is removing between 1240 and 2470 tons of rock from its bed per year a rate equivalent to 0 5 0 9 mm yr of erosion glacierwide the headwall by comparison is being worn away horizontally at 1 2 0 2 5 9 mm yr thus our results suggest that the headwall is retreating at rates roughly equivalent to vertical incision by the glacier our sediment budget results demonstrate that the wide variety of sediment sources and transport processes active in cirques necessitates a holistic view of cirque formation one that most morphometric range scale and glacial erosion analyses ignore

plio pleistocene increase of erosion rates in mountain belts in response to climate change here we review an ensemble of observations that point towards a global increase of erosion rates in regions of elevated mountain belts or otherwise high relief since the onset of northern hemisphere glaciation about 2 3 ma during that period of earth s history atmospheric co2 concentrations may have dropped and global climate cooled and evolved towards high amplitude oscillating conditions that are associated with the waxing and waning of continental ice sheets in the northern hemisphere we argue for a correlation between climate change and increased erosion rates and relief production which we attribute to some combination of the observed cooling onset of glaciation and climatic oscillation at orbital timescales in our view glacial erosion played a major role and is driven by the global cooling furthermore analyses of the sedimentary fluxes of many mountain belts show peaks of erosion during the transitions between glacial and inter glacial periods suggesting that the variable climatic conditions have also played a role

glacial overdeepenings in the swiss alps and foreland spatial distribution and morphometrics abstract overdeepenings ods are erosional features that have been excavated below the regional sea fluvial base level to produce closed topographic basins accessing bedrock topography and od volume is often challenging hence despite constituting major landscape features and being widespread in paleo glaciated regions ods have been overlooked and the subglacial processes involved in their evolution have remained debated in the swiss alpine foreland and valleys ods are commonly found filled with water or large volumes of sediment using a gis matlab approach based on topographic datasets and bedrock contour curves we mapped the spatial distribution of ods in switzerland and adjacent areas in the ice free alpine areas the majority of the mapped ods occurs in very low bedrock erosional resistance where ods are larger wider and shallower than in medium to high bedrock erosional resistance domains evidencing a strong lithological control on od geometry longitudinal asymmetry and hypsometric integral suggest a dominance of quarrying during od evolution and for selected glacial catchments headward erosion propagation or high sediment evacuation efficiency od surface data surface and length can be tentatively used for extracting od subsurface metrics depths nested valleys and first order volume estimates our data seem to indicate that ods may initiate as multiple small nested valleys and progress to a single and connected depression transversal cross sections also suggest a negative feedback between the erosion potential for deep carving and the presence of low resistance bedrock where subglacial meltwater infiltration could have played a key role in od evolution although insightful relationships have been evidenced for ods in the swiss alps and foreland we have also observed a high spatial variability in key od metrics such as surface area and depth this results in general first order interpretations at regional scale but currently prevent to quantitatively constrain physical subglacial processes at their origin comparisons with existing od datasets under present day ice greenland antarctica and modern swiss glaciers place our results in a broader context and allow a step forward in our understanding of the complex patterns and mechanisms of sub glacial erosion and resulting landforms

the relationships between tectonics climate and exhumation in the central andes 18 36 s evidence from low temperature thermochronology abstract the central andes between 18 s and 36 s latitude strike north south for 2000 km along the chilean subduction margin cross several climate zones from hyperarid to humid and exhibit mean elevations in excess of 4000 m a s l here we investigate the relationships between tectonics climate and exhumation by inverting low temperature thermochronological data compiled from the literature 824 ages from 549 samples and new data 238 ages from 146 samples to quantify the exhumation rate history of the central andes since 80 ma our inferred exhumation rates west of the drainage divide and between 18 and 32 s did not exceed 0 25 km ma such low exhumation rates are consistent with low shortening rates and arid conditions in this region local pulses of exhumation occurred only during the eocene as response to active deformation and during the miocene probably as response to uplift of the western andean slope east of the drainage divide between 18 and 28 s the observed exhumation pattern reflects the onset and eastward propagation of deformation here exhumation occurred locally since the middle to late eocene in the eastern cordillera and the altiplano puna and subsequently affected larger parts of these regions and the north western sierra pampeanas during the oligocene in the early miocene 20 ma the interandean zone started exhuming and at 12 10 ma exhumation propagated into the subandean zone enhanced shortening rates and intensified precipitation along the eastern deformation front associated with the onset of the south american monsoon led to increased exhumation rates in the eastern interandean and the subandean zones in the plio pleistocene 0 6 km ma higher pleistocene exhumation rates are also observed in the northern sierra pampeanas 1 5 km ma that can be related to rock uplift along steep reverse faults coupled with high precipitation south of 32 s on the western side exhumation rates in the principal cordillera increased from ca 0 25 km ma in the miocene to rates locally exceeding 2 km ma in the pleistocene whereas the tectonic regime in the southern principal cordillera remained unchanged since the late miocene these higher rates are likely associated with enhanced erosion resulting from intensified pleistocene precipitation and glacial growth in this region reinforced by isostatic rock uplift and active tectonics our study shows that the onset of exhumation correlates mainly with the initiation of horizontal shortening and crustal thickening whereas the magnitude of exhumation is largely set by the amount of precipitation and glacial erosion and by the style of deformation which is controlled by inherited structures and the amount of sediments in the foreland

apatite u th he thermochronometry using a radiation damage accumulation and annealing model helium diffusion from apatite is a sensitive function of the volume fraction of radiation damage to the crystal a quantity that varies over the lifetime of the apatite using recently published laboratory data we develop and investigate a new kinetic model the radiation damage accumulation and annealing model rdaam that adopts the effective fission track density as a proxy for accumulated radiation damage this proxy incorporates creation of crystal damage proportional to production from u and th decay and the elimination of that damage governed by the kinetics of fission track annealing the rdaam is a version of the helium trapping model hetm shuster d l flowers r m and farley k a 2006 the influence of natural radiation damage on helium diffusion kinetics in apatite earth planet sci lett 249 148 161 calibrated by helium diffusion data in natural and partially annealed apatites the chief limitation of the hetm now addressed by rdaam is its use of he concentration as the radiation damage proxy for circumstances in which radiation damage and he are not accumulated and lost proportionately from the crystal by incorporating the rdaam into the hefty computer program we explore its implications for apatite u th he thermochronometry we show how u th he dates predicted from the model are sensitive to both effective u concentration eu and details of the temperature history the rdaam predicts an effective he closure temperature of 62 c for a 28 ppm eu apatite of 60 m radius that experienced a 10 c ma monotonic cooling rate this is 8 c lower than the 70 c effective closure temperature predicted using commonly assumed durango diffusion kinetics use of the rdaam is most important for accurate interpretation of u th he data for apatite suites that experienced moderate to slow monotonic cooling 1 0 1 c ma prolonged residence in the helium partial retention zone or a duration at temperatures appropriate for radiation damage accumulation followed by reheating and partial helium loss under common circumstances the rdaam predicts u th he dates that are older sometimes much older than corresponding fission track dates nonlinear positive correlations between apatite u th he date and eu in apatites subjected to the same temperature history are a diagnostic signature of the rdaam for many but not all thermal histories observed date eu correlations in four different localities can be explained with the rdaam using geologically reasonable thermal histories consistent with independent fission track datasets the existence of date eu correlations not only supports a radiation damage based kinetic model but can significantly limit the range of acceptable time temperature paths that account for the data in contrast these datasets are inexplicable using the durango diffusion model the rdaam helps reconcile enigmatic data in which apatite u th he dates are older than expected using the durango model when compared with thermal histories based on apatite fission track data or other geological constraints it also has the potential to explain at least some cases in which u th he dates are actually older than the corresponding fission track dates

pliocene erosional pulse and glacier landscape feedbacks in the western alaska range abstract pliocene pleistocene glaciation modified the topography and erosion of most middle and high latitude mountain belts because the evolution of catchment topography controls long term glacier mass balance and erosion hence characterizing how erosion rates change during repeated glaciations can help test hypothesized glacier erosion landscape feedbacks across a range of settings to better understand how glaciations and landscapes coevolve on geologic timescales i quantify erosion rates in the glaciated western alaska range with low temperature thermochronometric data and modeling zircon u th he and apatite fission track data suggest mountain building was underway by the early miocene in contrast lower temperature apatite u th he age elevation and grain age kinetic data indicate that erosion accelerated coincident with regional pliocene glaciation ca 4 ma furthermore erosion rates calculated within an eroding half space indicate slow erosion at a rate 0 3 km m y before 4 2 ma an initial pulse of rapid erosion at a rate of 1 0 1 6 km m y during 4 2 2 9 ma and more moderate erosion at a rate of 0 4 0 7 km m y since 2 9 ma the initial erosion pulse suggests a significant transient landscape adjustment to the introduction of efficient glacial erosion the subsequent decrease in pleistocene erosion rates is consistent with a negative feedback between continuing glaciation and glacier size erosivity if glacial erosion outpaces rock uplift glacier erosion decreases over time as topography mass balance valley gradients and ice flux are reduced these findings imply that in areas of moderate rock uplift rates the onset of local plio pleistocene glaciation may have been punctuated by an initial pulse of rapid landscape change after which change became more gradual

last glacial maximum climate in new zealand inferred from a modelled southern alps icefield abstract we present a simulation of the new zealand southern alps icefield at the last glacial maximum lgm c 30 000 20 000 calendar years ago ka in an attempt to constrain the climate of that period we use a 500 m resolution ice sheet model parameterised using empirical glaciological climatological and geological data specific to the model domain to simulate the entire southern alps icefield we find that an lgm cooling of at least 6 6 5 c is necessary to bring about valley glaciers that extend beyond the mountains however climate topography thresholds related to the elevation and hypsometry of individual catchments control the gradient of the rate of glacier expansion in the domain and in order to remain within geologically reconstructed lgm limits we find that the lgm cooling was most likely associated with a precipitation regime up to 25 drier than today wetter than present scenarios give rise to equilibrium line depressions and ice extents that are incompatible with empirical evidence these results perhaps indicate that either the westerly air masses affecting new zealand during the lgm were drier than today or that they were weaker or zonally displaced with respect to present

accelerated subglacial erosion in response to stick slip motion subglacial stick slip motion speeds erosion by hydrofracturing and in other ways as determined from analysis of the growing body of field data microearthquake monitoring commonly detects subglacial earthquakes likely mostly from stick slip motion of debris laden ice over bedrock source parameters show that many quakes cause enough motion to greatly lower water pressure in cavities on the lee sides of bedrock steps we calculate that the resulting expansion of higher pressure water in nearby cracks promotes hydrofracturing with even relatively small cracks growing unstably under thick glaciers and all cracks growing faster than for aseismic behavior this mechanism also helps generate the step like topography favoring block plucking this stick slip glacier erosion hypothesis suggests that the erosion rate will increase with ice thickness as well as basal shear stress ice flow velocity and water supply

knickpoint evolution on the yarlung river evidence for late cenozoic uplift of the southeastern tibetan plateau margin a salient geomorphic feature of the yarlung river is its abundance of large knickpoints which in many cases coincide with north south trending rifts across one of these rifts near the town of jiacha the yarlung falls nearly 500 m from an elevation of 3500 m over 80 river kilometers making this the second largest knickpoint on the river we propose that the jiacha knickpoint represents a wave of incision migrating upstream through the drainage network in response to a downstream base level fall not a disturbance in the channel to due rift tectonics longitudinal profile slope area and chi analysis of yarlung river tributaries and those of several major rivers in southeastern tibet indicate several knickpoints are present at 3500 m elevation all resulting from a single regional scale base level fall retreat rates calculated from celerity modeling indicate that the jiacha knickpoint was located at the upstream edge of the namche barwa massif at 10 ma a history consistent with apatite 4he 3he thermochronometry data and thermokinematic modeling from that region these data suggest the yarlung river has flowed in its present course through this area since at least 10 ma and imply that at least 500 m of incision occurred within this canyon over this time period the spatial scale of these observations suggests that these knickpoints resulted from surface uplift of southeastern tibet of 500 to 2500 m just prior to 10 ma additionally our mapped knickpoint locations indicate that reorganization of the drainage network just east of the namche barwa massif occurred prior to this time

the glacially overdeepened trough of the salzach valley austria bedrock geometry and sedimentary fill of a major alpine subglacial basin abstract overdeepened valleys are unambiguous features of glacially sculpted landscapes they result from erosion at the bed of the glacier and their size and shape is determined by ice dynamics and the characteristics of the underlying bedrock major overdeepened valleys representing vertical bedrock erosion of several hundreds of meters are characteristic features of many trunk valleys in the formerly glaciated parts of the alpine mountain belt the thick sedimentary fill usually hinders attempts to unravel bedrock geometry formation history and fill characteristics based on reflection seismic data and core logs from multiple deep drillings we construct a detailed bedrock model of the lower salzach valley trough one of the largest overdeepened valleys in the european alps the analysed overdeepened structure characterized by a strongly undulating topography two reaches of enhanced erosion can be identified and are suggested to be related to variations in bedrock erodibility and a triple glacier confluence the sedimentary fill shows clear characteristics of rapid infilling and subaqueous fan delta deposits indicate a strong influence of tributary streams associated surface lowering of the valley floor had a major impact on tributary stream incision but also on the available ice accumulation area at subsequent glaciations the extent to which fills of earlier glaciations have been preserved from erosion during the last glacial maximum remains ambiguous and demands further exploration to our knowledge the presented bedrock model is one of the best defined of any major overdeepened trunk valley

numerically simulating alpine landscapes the geomorphologic consequences of incorporating glacial erosion in surface process models abstract a numerical model that simulates the evolution of glaciated mountain landscapes is presented by employing a popular sliding based glacial erosion model many common glacial landforms are created the numerical model builds on earlier work as it is fully two dimensional and employs the first order forcing on mountain evolution these forcings include tectonic uplift isostasy hillslope processes fluvial processes and glacial processes a climate dependent model of ice dynamics is employed to determine ice coverage and ice flux two simulations are presented one with generic model parameters and a second with parameters that correspond to conditions in the southern alps of new zealand landforms that are produced by the model include climate climate dependent elevation lowering similar to what might be expected by a glacial buzz saw valley overdeepening terminal moraines and valley retreat the model also predicts that current rates of sedimentation are higher than the long term average and that several tens of thousands of years are required for the landscape to adjust to a change in the dominant erosional forcing therefore glaciated orogens are unlikely to achieve topographic steady state over milankovitch timescales

intermittent glacial sliding velocities explain variations in long timescale denudation abstract quantifying controls on glacial erosion over geologic timescales is central to understanding the role of cenozoic climate change on the development of modern mountain belts yet the mechanisms that produce the distinct relief and topography visible in glaciated regions remain poorly constrained we test the hypothesis that commonly assumed glacial sliding parameterizations control denudation rates over geologic timescales we do this by modeling glacier dynamics over a glacial interglacial cycle and compare with a dense dataset of u th he thermochronometer derived denudation rates from the southern coast mountains bc results indicate zones of rapid quaternary erosion correspond to locations where the model predicts the highest averaged sliding velocities the results are consistent with the hypothesis that sliding influences the rate of glacial erosion regression between sliding predicted by the model and erosion rates shows a statistically significant correlation r 2 0 6 the coefficient of the regression 10 5 is smaller than previous estimates based on data from much shorter timescales the model results also reveal that for a specific location active subglacial sliding and hence erosion occurs for only 10 20 of a glacial interglacial cycle suggesting high temporal variations in erosion rates this intermittency of erosion requires instantaneous erosion rates to be greater than long term averages explaining how timescale averaging can impact estimates of glacial erosion rates

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

signatures of late pleistocene fluvial incision in an alpine landscape uncertainty regarding the relative efficacy of fluvial and glacial erosion has hindered attempts to quantitatively analyse the pleistocene evolution of alpine landscapes here we show that the morphology of major tributaries of the rhone river switzerland is consistent with that predicted for a landscape shaped primarily by multiple phases of fluvial incision following a period of intense glacial erosion after the mid pleistocene transition 0 7 ma this is despite major ice sheets reoccupying the region during cold intervals since the mid pleistocene we use high resolution lidar data to identify a series of convex reaches within the long profiles of 18 tributary channels we propose these reaches represent knickpoints which developed as regional uplift raised tributary bedrock channels above the local fluvial baselevel during glacial intervals and migrated upstream as the fluvial system was re established during interglacial periods using a combination of integral long profile analysis and stream power modelling we find that the locations of 80 of knickpoints in our study region are consistent with that predicted for a fluvial origin while the mean residual error over 100 km of modelled channels is just 26 3 m breaks in cross valley profiles project toward the elevation of former end of interglacial channel elevations supporting our model results calculated long term uplift rates are within 15 of present day measurements while modelled rates of bedrock incision range from 1 mm yr for low gradient reaches between knickpoints to 6 10 mm yr close to retreating knickpoints typical of observed rates in alpine settings together our results reveal approximately 800 m of regional uplift river incision and hillslope erosion in the lower half of each tributary catchment since 0 7 ma

rock uplift and erosion rate history of the bergell intrusion from the inversion of low temperature thermochronometric data the bergell intrusion european alps was one of the first locations where thermochronometry was used to resolve changes in erosion rate yet relating these changes to variations in climate or in local tectonics remains challenging one approach that enables changes in erosion rate to be related to changes in climate or rock uplift rate is to utilize landscape evolution models as topographic response to these forcing parameters is unique furthermore low temperature thermochronometric systems have the potential to resolve topography through time and thus topographic response we present new u th he data for samples collected across 2 km of relief from the bergell the ages range from 2 to 16 ma and define an age elevation with an apparent exhumation rate of 0 1 km myr in order to infer erosion rates we use a thermokinematic model to solve the heat equation in the crust track material points through time and predict thermochronometric data paleo topography and erosion rate are parameterized using the stream power model we find that rock uplift rates were 0 4 km myr from 25 to 20 ma and subsequently decreased to 0 05 km myr this results in a gradual decrease in erosion rate from rates of 0 4 to 0 1 km myr a recent increase in rock uplift rate at 4 ma to 0 6 km myr is required to explain the youngest ages and high topographic relief

thorium 230 normalized particle flux and sediment focusing in the panama basin region during the last 30 000 years 1 application of the 230 th normalization method to estimate sediment burial fluxes in six cores from the eastern equatorial pacific eep reveals that bulk sediment and organic carbon fluxes display a coherent regional pattern during the holocene that is consistent with modern oceanographic conditions in contrast with estimates of bulk mass accumulation rates mars derived from core chronologies two nearby sites less than 10 km apart which have different mars show nearly identical 230 th normalized bulk fluxes focusing factors derived from the 230 th data at the foot of the carnegie ridge in the panama basin are 2 in the holocene implying that lateral sediment addition is significant in this part of the basin new geochemical data and existing literature provide evidence for a hydrothermal source of sediment in the southern part of the panama basin and for downslope transport from the top of the carnegie ridge the compilation of core records suggests that sediment focusing is spatially and temporally variable in the eep during oxygen isotope stage 2 ois 2 from 13 27 ka bp focusing appears even higher compared to the holocene at most sites similar to earlier findings in the eastern and central equatorial pacific the magnitude of the glacial increase in focusing factors however is strongly dependent on the accuracy of age models we offer two possible explanations for the increase in glacial focusing compared to the holocene the first one is that the apparent increase in lateral sediment redistribution is partly or even largely an artifact of insufficient age control in the eep while the second explanation which assumes that the observed increase is real involves enhanced deep sea tidal current flow during periods of low sea level stand

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

canyon incision and knickpoint propagation recorded by apatite 4he 3he thermochronometry abstract in many regions of the world deeply incised canyons demonstrate the net effects of physical processes active at earth s surface in response to surface uplift low temperature thermochronometry can constrain the timing and rates of bedrock incision which is necessary for relating canyon incision to surface uplift over geological timescales we analyzed four samples from the cotahuasi ocona canyon system in southwest peru using 4 he 3 he thermochronometry and we present a new inversion model to identify continuous low temperature cooling histories that are consistent with the observed data derived cooling histories limit the onset of fluvial incision to 13 to 8 ma this is in agreement with previously reported interpretations based on a three dimensional thermal model interpolation to a much larger set of conventional apatite u th he ages however because 4 he 3 he thermochronometry constrains an independent cooling history for each sample the results also permit testing of landscape evolution models with greater spatial variability in exhumation compared to those models that can be tested by a geographically scattered set of conventional u th he ages the different cooling histories of the four samples require asynchronous incision along at least part of the canyon system that is best explained by headward propagation of fluvial incision by knickpoint migration

determination of sediment provenance at drift sites using hydrogen isotopes and unsaturation ratios in alkenones the large 20 hydrogen isotopic gradient in surface waters of the northwest atlantic ocean is exploited to track changes in the source of alkenones to the bermuda rise sediment drift cultures of the predominant alkenone producing coccolithophorid e huxleyi were grown in deuterium enriched seawater and shown to possess alkenones with a d h ratio that closely tracked the water d h ratio r 2 0 999 n 5 isotopic enrichments with a fractionation factor between 0 732 and 0 775 a hydrogen isotopic depletion of 193 3 n 9 was measured in alkenones from suspended particles relative to seawater in the subpolar and subtropical northwest atlantic ocean this value was used to calculate the water d values in which alkenones from bermuda rise sediment were synthesized and by extension the water mass in which they were produced applying this technique we find that 60 to 100 of the alkenones in late holocene bermuda rise sediment were produced in deuterium depleted subpolar water to the northwest of the drift to reconcile values of the alkenone unsaturation ratio u k 37 a widely used proxy for sea surface temperature with the d values of alkenones in late holocene sediments from the bermuda rise at least three sources of sediment must be invoked a cold very isotopically depleted source almost certain to be the scotian margin a warm moderately isotopically depleted source likely to be the northwestern edge of the subtropical gyre and a cold isotopically enriched source which we hypothesize to be the subpolar waters overlying the main branch of north atlantic deep water flowing southwest from the nordic seas copyright 2005 elsevier ltd paleoclimate records with sufficient length and temporal resolution to study the occurrence and causal mechanisms of abrupt climate change are exceedingly rare rapidly deposited ocean sediments provide the best archive for studying these events through geologic time but such sites in the open ocean are limited to sediment drift deposits such as the bermuda rise in the northwest atlantic using multiple climate proxies in a single core is becoming more common in high resolution pa leoclimate investigations but a major potential concern for this approach arises from the possibility that the fine fraction of sediment e g 63 m and the climate proxies within it may represent conditions far from the deposition site here we propose and test the hypothesis that hydrogen isotope ratios of alkenones a class of lipids from phytoplankton may provide insight into the source of fine grained sediment because of their restricted sources broad geographic distri bution and excellent preservation properties alkenones are of particular interest in the emerging field of compound specific hydrogen isotopic analysis the sedimentary abundances ex tents of unsaturations and isotopic compositions of alkenones provide quantitative and near continuous records we isolated alkenones from cultured unicellular algae haptophyte emil iania huxleyi surface ocean particulate material and open ocean sediments to determine the extent and variability of hydrogen isotopic fractionation in the di tri and tetraunsat urated c37 compounds we then compared the d values of the alkenones in surface sediments between the bermuda rise and the scotian margin above which a large 20 d gradient exists we determined the fractionation between alkenones from suspended particulate samples and the water in which the phytoplanton lived and examined the variability of alkenone d values during the medieval warm period little ice age climate transition cf keigwin 1996 at the bermuda rise

glacial abrasion and sliding their dependence on the debris concentration in basal ice a model of bedrock abrasion by a temperate glacier is developed that takes into account the significant impeding effect of scattered rock fragments at the ice rock interface on glacier sliding an increase in the debris concentration above a relatively low value can reduce the abrasion rate by decreasing both the flux of rock fragments and the effective forces which press fragments against the bed abrasion of a simple wavy bed is shown to be most rapid for a relatively low debris concentration above which the abrasion varies inversely with the debris content a similar concentration dependence of abrasion is expected for beds of arbitrary geometry but low roughness sliding glaciers will tend to abrade until the basal ice is nearly stagnant at which point rocks of all sizes start to lodge against the bed

glacial and periglacial buzzsaws fitting mechanisms to metaphors the buzzsaw hypothesis refers to the potential for glacial and periglacial processes to rapidly denude mountains at and above glacier equilibrium line altitudes elas irrespective of uplift rates rock type or pre existing topography here the appropriateness of the buzzsaw metaphor is examined alongside questions of the links between glacial erosion and elas and whether the glacial system can produce low relief surfaces or limit summit heights plateau fragments in mountains on both active orogens and passive margins that have been cited as products of glacial and periglacial buzzsaw erosion instead generally represent dissected remnants of largely inherited pre glacial relief summit heights may correlate with elas but no causal link need be implied as summit erosion rates are low cirque headwalls may not directly abut summits and on passive margins cirques are cut into pre existing mountain topography any simple links between elas and glacial erosion break down on passive margins due to topographic forcing of ice sheet growth and to the km scale vertical swaths through which elas have shifted through the quaternary glaciers destroy rather than create low relief rock surfaces through the innate tendency for ice flow to be faster thicker and warmer along valleys the glacial buzzsaw cuts down

inversion of thermochronological age elevation profiles to extract independent estimates of denudation and relief history i theory and conceptual model abstract we determine to what extent low temperature thermochronology data in particular from age elevation profiles provide independent and quantitative estimates on denudation rates and relief development thermochronological age elevation profiles have been widely used to infer exhumation histories however their interpretation has remained inherently one dimensional neglecting potential effects of lateral offsets between samples furthermore the potential effects of transient topography on crustal isotherms and consequently on thermochronological data have not yet been addressed in detail we investigate this problem with the aim of deriving independent estimates of both denudation rates and relief history from low temperature thermochronometers measuring the relative uncertainties on these parameters and finally constraining the timing of potential variations in denudation rate and or relief development we adopt a non linear inversion method combining the three dimensional thermal kinematic model pecube which predicts thermal histories and thermochronological ages from an input denudation and relief history with an inversion scheme based on the neighbourhood algorithm we use synthetic data predicted from imposed denudation and relief histories and quantitatively assess the resolution of thermochronological data collected along an age elevation profile our results show that apatite fission track aft ages alone do not provide sufficient quantitative information to independently constrain denudation and relief histories multiple thermochronometers apatite u th he ahe ages and or track length measurements combined with aft ages are generally successful in constraining denudation rates and timing of rate changes the optimum combination of thermochronometers varying with the input scenario relief change or varying denudation rates however relief changes can only be quantified and precisely constrained from thermochronological age elevation profiles if the rate of relief growth is at least 2 3 times higher than the background denudation rate this limited resolution is due to the depth of the closure isotherm between 70 and 110 c for the aft and ahe systems which only partly record topographic change new thermochronometers such as 4 he 3 he or osl that are sensitive to lower temperatures may be the key for resolving this issue

alkenone temperature anomalies in the brazil malvinas confluence area caused by lateral advection of suspended particulate material alkenone temperatures derived from suspended particulate organic material which was collected in austral summer 2001 from surface waters 5 m south of the brazil malvinas confluence deviate from measured temperatures by 4 to 7 c when uk 37 ratios are converted to temperature using the muller et al 1998 calibration and up to 3 c when using the calibration of conte et al 2006 in contrast alkenone temperatures determined from particulate material sampled north of the confluence reveal close correspondence to in situ temperatures or show slightly higher values we suggest that the southern samples are biased by suspended organic detritus originating from the cold subpolar waters of the northward flowing malvinas current whereas the northern samples carry an uk 37 signal of tropical subtropical origin transported southward with the brazil current on the basis of surface ocean transport pathways and velocities simulated with the large scale geostrophic lsg ocean general circulation model we identify areas of the world ocean where alkenone temperatures are potentially biased to higher or lower values due to long particle residence times and lateral advection by surface currents

miocene development of alpine glacial relief in the patagonian andes as revealed by low temperature thermochronometry abstract apatite thermochronometry and synthetic maps of ages and rates for thermochronometric data are used to estimate the timing of incision of valley relief in the andes central patagonia offers a unique location to study the feedbacks between long term climate topography and erosion due to the high relief and well resolved mid latitude glacial history new apatite u th he ages from two vertical transects and two 4he 3he release spectra in the fjord network around 47 s reveal fast cooling 15 30 c ma from 10 to 5 ma samples currently at the surface cooled below 35 c by 5 ma indicating slow cooling and little erosion in those regions since 5 ma we show that these very low temperature thermochronometric data are useful indicators of changes in topography and insensitive to deep thermal processes such as migration of the chile triple junction map based predictions of the thermochronometric signatures of disparate topographic scenarios show the distribution of sample data necessary to resolve the timing of relief change comparisons to predicted cooling ages and rates indicate that our new apatite he data are most consistent with a pulse of early glacial incision with much of the observed valley relief in patagonia carved between 10 and 5 ma early onset of glaciation in patagonia is supported by glacial till with bracketing ages of 7 4 and 5 ma we therefore conclude that the observed thermochronometric signal of fast cooling from 10 to 5 ma is likely due to an increase in valley relief coinciding with these early glaciations in the andes in other glaciated areas at lower latitudes studies have found a dramatic increase in valley relief at 1 ma this timing has generated the idea that incision of glacial valleys may be related to the mid pleistocene transition when the global glacial cycle changed from 40 to 100 ka periods our results from a higher latitude indicate an alternative that glacial valleys incised rapidly after the onset of alpine glaciation

scaling the teflon peaks rock type and the generation of extreme relief in the glaciated western alaska range 1 parts of the alaska range alaska usa stand in prominent exception to the glacial buzzsaw hypothesis which postulates that terrain raised above the ela is rapidly denuded by glaciers in this paper we discuss the role of a strong contrast in rock type in the development of this exceptional terrain much of the range is developed on pervasively fractured flysch with local relief of 1000 1500 m and mean summit elevations that are similar to modern snow line elevations in contrast cretaceous and tertiary plutons of relatively intact granite support the range s tallest mountains including mt mckinley or denali at 6194 m with 2500 5000 m of local relief the high granitic peaks protrude well above modern snow lines and support many large glaciers we focus on the plutons of the denali massif and the kichatna mountains to the west we use field observations satellite photos and digital elevation data to demonstrate how exhumation of these plutons affects glacier longitudinal profiles the glacial drainage network and the effectiveness of periglacial processes in strong granite steep smooth valley walls are maintained by detachment of rock slabs along sheeting joints these steep walls act as low friction surfaces teflon efficiently shedding snow simple scaling calculations show that this avalanching may greatly enhance the health of the modern glaciers we conclude that in places such as denali unusual combinations of rapid tectonic uplift and great rock strength have created the highest relief in north america by enhancing glacial erosion in the valleys while preserving the peaks

glacial erosion status and outlook glacier erosion rates range across orders of magnitude and much of this variation cannot be attributed to basal sliding rates subglacial till acts as lubricating fault gouge or sawdust and must be removed for rapid subglacial bedrock erosion such erosion occurs especially where and when moulin fed streams access the bed and are unconstrained by supercooling or other processes streams also may directly erode bedrock likely with strong time evolution erosion is primarily by quarrying aided by strong fluctuations in the water system driven by variable surface melt and by subglacial earthquakes debris bed friction significantly affects abrasion quarrying and general glacier flow frost heave drives cirque headwall erosion as winter cold air enters bergschrunds creating temperature gradients to drive water flow along premelted films to growing ice lenses that fracture rock and the glacier removes the resulting blocks recent subglacial bedrock erosion and sediment flux are in many cases much higher than long term averages over glacial cycles evolution of glacial valley form feeds back strongly on erosion and deposition most of this is poorly quantified with parts open to argument glacial erosion and interactions are important to tectonic and volcanic processes as well as climate and biogeochemical fluxes motivating vigorous research

spatial and temporal variations of glacial erosion in the rhone valley swiss alps insights from numerical modeling the present day topography of the european alps shows evidence of intense glacial reshaping however significant questions regarding alpine landscape evolution during glaciations still persist in this study we focus on the rhone valley swiss alps and use a numerical model to estimate patterns and magnitudes of glacial erosion comparing modeling results on a reconstructed pre glacial topography and the present day landforms we find that the landscape response to glaciation is more complex than a simple buzzsaw mechanism by which glacial erosion sets the height of mountain ranges or increase of relief due to localized valley incision instead glacial erosion propagates headward as the landforms evolve from a fluvial to a glacial state leading to an initial increase of local relief followed by subsequent erosion at high elevations it has also been proposed that the mid pleistocene climatic transition of glacial interglacial oscillations from periods of 40 kyr with symmetric shapes to periods of 100 kyr with asymmetric shapes promoted glacial erosion within the alps although this change of climate periodicity may have contributed to enhance glacial erosion our results suggest that other factors such as an increase in rock uplift and or progressive climate cooling are required to explain enhanced glacial carving at view the mathml source 1ma

identification of erosional mechanisms during past glaciations based on a bedrock surface model of the central european alps the bedrock topography beneath the quaternary cover provides an important archive for the identification of erosional processes during past glaciations here we combined stratigraphic investigations of more than 40 000 boreholes with published data to generate a bedrock topography model for the entire plateau north of the swiss alps including the valleys within the mountain belt we compared the bedrock map with data about the pattern of the erosional resistance of alpine rocks to identify the controls of the lithologic architecture on the location of overdeepenings we additionally used the bedrock topography map as a basis to calculate the erosional potential of the alpine glaciers which was related to the thickness of the lgm ice we used these calculations to interpret how glaciers with support by subglacial meltwater under pressure might have shaped the bedrock topography of the alps we found that the erosional resistance of the bedrock lithology mainly explains where overdeepenings in the alpine valleys and the plateau occur in particular in the alpine valleys the locations of overdeepenings largely overlap with areas where the underlying bedrock has a low erosional resistance or where it was shattered by faults we also found that the assignment of two end member scenarios of erosion related to glacial abrasion plucking in the alpine valleys and dissection by subglacial meltwater in the plateau may be adequate to explain the pattern of overdeepenings in the alpine realm this most likely points to the topographic controls on glacial scouring in the alps the flow of lgm and previous glaciers were constrained by valley flanks while ice flow was mostly divergent on the plateau where valley borders are absent we suggest that these differences in landscape conditioning might have contributed to the contrasts in the formation of overdeepenings in the alpine valleys and the plateau

radiocarbon and 230th data reveal rapid redistribution and temporal changes in sediment focussing at a north atlantic drift abstract in locations of rapid sediment accumulation receiving substantial amounts of laterally transported material the timescales of transport and accurate quantification of the transported material are at the focus of intense research here we present radiocarbon data obtained on co occurring planktic foraminifera marine haptophyte biomarkers alkenones and total organic carbon toc coupled with excess thorium 230 230thxs measurements on four sediment cores retrieved in 1649 2879 m water depth from two such high accumulation drift deposits in the northeast atlantic bjorn and gardar drifts while 230thxs inventories imply strong sediment focussing no age offsets are observed between planktic foraminifera and alkenones suggesting that redistribution of sediments is rapid and occurs soon after formation of marine organic matter or that transported material contains negligible amounts of alkenones an isotopic mass balance calculation based on radiocarbon concentrations of co occurring sediment components leads us to estimate that transported sediment components contain up to 12 of fossil organic matter that is free of or very poor in alkenones but nevertheless appears to consist of a mixture of fresh and eroded fossil material considering all available constraints to characterize transported material our results show that although focussing factors calculated from bulk sediment 230thxs inventories may allow useful approximations of bulk redeposition they do not provide a unique estimate of the amount of each laterally transported sediment component furthermore our findings provide evidence that the occurrence of lateral sediment redistribution alone does not always hinder the use of multiple proxies but that individual sediment fractions are affected to variable extents by sediment focussing

an evaluation of 14c age relationships between co occurring foraminifera alkenones and total organic carbon in continental margin sediments 1 radiocarbon age relationships between co occurring planktic foraminifera alkenones and total organic carbon in sediments from the continental margins of southern chile northwest africa and the south china sea were compared with published results from the namibian margin age relationships between the sediment components are site specific and relatively constant over time similar to the namibian slope where alkenones have been reported to be 1000 4500 years older than co occurring foraminifera alkenones were significantly similar to 1000 years older than co occurring foraminifera in the chilean margin sediments in contrast alkenones and foraminifera were of similar age within 2 sigma error or better in the nw african and south china sea sediments total organic matter and alkenone ages were similar off namibia age difference toc alkenones 200 700 years chile 100 450 years and nw africa 360 770 years suggesting minor contributions of preaged terrigenous material in the south china sea total organic carbon is significantly 2000 3000 years older owing to greater inputs of preaged terrigenous material age offsets between alkenones and planktic foraminifera are attributed to lateral advection of organic matter physical characteristics of the depositional setting such as seafloor morphology shelf width and sediment composition may control the age of co occurring sediment components in particular offsets between alkenones and foraminifera appear to be greatest in deposition centers in morphologic depressions aging of organic matter is promoted by transport age offsets are correlated with organic richness suggesting that formation of organic aggregates is a key process