comment on barrell et al reconciling the onset of deglaciation in the upper rangitata valley southern alps new zealand quaternary science reviews 203 2019 141 150

comment on slip rate measurements on the karakorum fault may imply secular variations in fault motion chevalier et al 1 1 presented cosmic ray exposure dates for glacial deposits offset by movement along the karakorum fault they inferred a late quaternary slip rate on this fault 10 7 0 7 mm year that is higher than rates reported in recent field studies 2 2 the reported rate

dating plio pleistocene glacial sediments using the cosmic ray produced radionuclides 10be and 26al we use the cosmic ray produced radionuclides 26al and 10be to date plio pleistocene glacial sediment sequences these two nuclides are produced in quartz at a fixed ratio but have different decay constants if a sample is exposed at the surface for a time and then buried by overburden and thus removed from the cosmic ray flux the 26al 10be ratio is related to the duration of burial we first attempted to date pre wisconsinan tills by measuring 26al and 10be in fluvial sediments beneath them and applying the method of burial dating which previous authors have used to date river sediment carried into caves this method however requires simplifying assumptions about the 26al and 10be concentrations in the sediment at the time of burial we show that these assumptions are not valid for river sediment in glaciated regions 26al and 10be analyses of such sediment do not provide accurate ages for these tills although they do yield limiting ages in some cases we overcome this difficulty by instead measuring 26al and 10be in quartz from paleosols that are buried by tills we use a more general mathematical approach to determine the initial nuclide concentrations in the paleosol at the time it was buried as well as the duration of burial this technique provides a widely applicable improvement on other means of dating plio pleistocene terrestrial glacial sediments as well as a framework for applying cosmogenic nuclide dating techniques in complicated stratigraphic settings we apply it to pre wisconsinan glacial sediment sequences in southwest minnesota and eastern south dakota pre wisconsinan tills underlying the minnesota river valley were deposited 0 5 to 1 5 ma and tills beneath the prairie coteau in eastern south dakota and adjacent minnesota were deposited 1 to 2 ma

fitting degradation of shoreline scarps by a nonlinear diffusion model the diffusion model of degradation of topographic features is a promising means by which vertical offsets on holocene faults might be dated in order to calibrate the method we have examined present day profiles of wave cut shoreline scarps of late pleistocene lakes bonneville and lahontan it may be assumed that these scarps were initially at least as steep as the angle of repose offsets range from 1 to 12 m and present slope angles range from 9 to 29 a parameter called apparent diffusion age defined as half the mean square horizontal extent of the slope function of each profile is plotted as a function of scarp offset the points show a clear trend of apparent age increasing nearly linearly with offset if linear diffusion held and scarps were initially vertical apparent diffusion age would be the same for all the shoreline profiles the increasing trend can only partly be explained by nonvertical initial scarp slope and therefore the rate of transport of material downslope must increase significantly faster than a linear law in the range of slopes spanned by the data the transport law must become linear at small slope to reduce scatter between profiles with varying ambient slopes the transport law adopted for the purpose of dating is k0 1 5s2 where s is local slope the transport coefficient k0 is correlated inversely with fan slope suggesting that there is a dependence on the particle size distribution a table is included that allows easy application of the model to scarps with simple initial shape

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

insights into alpine moraine development from cosmogenic 36cl buildup dating abstract we have used cosmogenic 36cl surface exposure dating to determine apparent construction ages of late pleistocene moraines in the sierra nevada the white mountains and the wind river range all in the western united states the inferred glacial chronologies from the various localities all exhibit certain characteristics 1 local records are fragmentary and deposits of some glacial advances are always missing no location has deposits of all glaciations and no glacial advance is recorded at all locations this is due either to unfavorable conditions for glacier development at some times or to obliteration of earlier deposits by later more extensive glaciers 2 most moraines have young apparent exposure ages with only a few older than the last glacial cycle this may be due to young true ages of these deposits erosion of moraine surfaces or obliterative overlap and covering of older deposits by younger ones 3 many deposits that were previously correlated e g based on their stratigraphic positions are not correlative they may represent different stades and sometimes even different glaciations similarly some previously uncorrelated deposits have the same exposure ages and may be correlative 4 clusters of several distinct moraines of nearly the same age are present at most locations these clusters suggest that alpine moraines are formed during short deposition episodes that last between several hundred and several thousand years

cosmogenic radionuclide dating of glacial landforms in the lahul himalaya northern india defining the timing of late quaternary glaciation the timing of glaciation in the lahul himalaya of northern india was ascertained using the concentrations of cosmogenic 10be and 26al from boulders on moraines and drumlins and from glacially polished bedrock surfaces five glacial stages were identified sonapani i and ii kulti batal and chandra of these cosmogenic exposure ages were obtained on samples representative of the batal and kulti glacial cycles stratigraphical relationships indicate that the sonapani i and ii are younger no age was obtained for the chandra glacial advance batal glacial stage deposits are found throughout the valley indicating the presence of an extensive valley glacial system during the kulti stage glaciers advanced ca 10 km beyond their current positions moraines produced during the batal stage ca 12 15 5 ka are coeval with the northern hemisphere late glacial interstadial bolling allerod deglaciation of the batal glacial stage was completed by ca 12 ka and was followed by the kulti glacial stage during the early holocene at ca 10 11 4 ka on millennial time scales glacier oscillations in the lahul himalaya apparently reflect periods of positive mass balance coincident with times of increased insolation during these periods the south asian summer monsoon strengthened and or extended its influence further north and west thereby enhancing high altitude summer snowfall copyright 2001 john wiley sons ltd

cosmogenic nuclide exposure ages and glacial history of late quaternary ross sea drift in mcmurdo sound antarctica 3he 10be and26al surface exposure ages were determined for rock samples from late quaternary drift sheets deposited on the coast of mcmurdo sound antarctica by the ross sea ice sheet the youngest drift previously believed to be late wisconsin in age has exposure ages of 8 106 kyr 19 samples these results suggest that the apparent late wisconsin ice margin may represent a number of ice sheet advances during the last glacial period 3he exposure ages from analogous stratigraphically older drift range from 104 to 567 kyr seven samples and clearly distinguish the youngest drift from older deposits comparison of10be and3he concentrations in ross sea drift quartz samples indicates significant3he loss in only one sample suggesting that cosmogenic3he is generally retained in young antarctic quartz samples exposure age 100 kyr of appropriate grain size in two of the five quartz samples analyzed significant excesses of26al relative to10be preclude meaningful26al exposure age calculations these excesses may be due to nucleogenic26al produced as a byproduct of uranium and thorium decay

evidence for restricted ice extent during the last glacial maximum in the koryak mountains of chukotka far eastern russia field evidence in the koryak mountains lake mainitz region of far eastern russia supports three pleistocene glacial advances the early wisconsinan and pre wisconsinan glaciations are represented by broad lobate moraines extending as much as 30 km north of the koryak mountains field evidence demonstrates that the terminal lateral and medial moraines as well as meltwater channels dead ice topography kettles and outwash plains mark the extent of ice during the last glacial maximum lgm during which glaciers reached no more than 20 km beyond their present limits those emanating from the southern koryak mountains may have reached the bering sea numerical and relative dating techniques support these results and test the theoretical models of the lgm in western beringia erratics on moraines and glaciofluvial terraces common to all valleys at 13 13 8 m above river level yield 36 cl exposure ages ranging from 10 08 to 25 78 ka the koryak mountains lake mainitz record of glaciation is spatially and temporally consistent with the glaciation pattern across central beringia found in other terrestrial and marine records glacier growth in the koryak mountains was sustained by possible increased summer sea surface temperatures and precipitation in the northwest pacific region evidence from the northern koryak mountains lat 64 n long 177 e indicates that the extent of ice in western beringia was limited to mountain and valley glaciers during the lgm this fieldbased research contradicts m g grosswald s theoretical beringian ice sheet hypothesis

degradation of unconsolidated quaternary landforms in the western north america abstract one of the major goals of geomorphology is to understand the rate of landscape evolution and the constraints that erosion sets on the longevity of land surfaces the latter has also turned out to be vital in modern applications of cosmogenic exposure dating and interpretation of lichenometric data from unconsolidated landforms because the effects of landform degradation have not been well documented disagreements exist among researchers regarding the importance of degradation processes in the dating techniques applied to exposures here we show that all existing qualitative data and quantitative markers of landform degradation collectively suggest considerable lowering of the surface of unconsolidated landforms over the typical life span of quaternary moraines or fault scarps degradation is ubiquitous and considerable even on short time scales of hundreds of years on steeply sloping landforms these conservative analyses are based entirely on field observations of decreasing slope angles of landforms over the typical range of ages in western north america and widely accepted modeling of landscape degradation we found that the maximum depth of erosion on fault scarps and moraines is on average 34 of the initial height of the scarp and 25 of the final height of the moraine although our observations are limited to fault scarps and moraines the results apply to any sloping unconsolidated landform in the western north america these results invalidate the prevailing assumption of no or little surface lowering on sloping unconsolidated landforms over the quaternary period and affirm that accurate interpretations of lichen ages and cosmogenically dated boulder ages require keen understanding of the ever present erosion in our view the most important results are twofold 1 to show with a large data set that degradation affects universally all sloping unconsolidated landforms and 2 to unambiguously show that even conservative estimates of the total lowering of the surface operate at time and depth scales that significantly interfere with cosmogenic exposure and lichen dating

chronology for fluctuations in late pleistocene sierra nevada glaciers and lakes mountain glaciers because of their small size are usually close to equilibrium with the local climate and thus should provide a test of whether temperature oscillations in greenland late in the last glacial period are part of global scale climate variability or are restricted to the north atlantic region correlation of cosmogenic chlorine 36 dates on sierra nevada moraines with a continuous radiocarbon dated sediment record from nearby owens lake shows that sierra nevada glacial advances were associated with heinrich events 5 3 2 and 1

the timing of the last glacial maximum in australia abstract late pleistocene glaciation of australia was restricted to the snowy mountains and the tasmanian highlands glaciers were most extensive in tasmania where ice caps formed on the central plateau and west coast ranges and systems of valley and cirque glaciers formed on surrounding mountains to investigate the timing of maximum glacier advance during the last glacial maximum lgm we dated boulders on 18 moraines in 8 glaciated areas using cosmogenic 36cl and 10be we sampled moraines deposited by cirque and valley glaciers in a range of climate types and lithologies over a 7 latitudinal and a 1430 m altitudinal transect exposure ages for lgm moraine boulders group tightly in the range 17 20 ka boulders from the hamilton moraine often treated as the lgm type moraine range in age from 190 350 ka indicating that it was deposited well before the last glaciation collectively the exposure ages show that southeastern australia was glaciated briefly during and after the lgm and deglaciated well before the holocene ice retreated from the terminal moraines after the period of coldest sea surface temperatures in the southwest pacific ocean no evidence was found for readvance of glaciers during the antarctic cold reversal or the younger dryas period

modification of wave cut and faulting controlled landforms from a casual observation that the form of degraded fault scarps resembles the error function this investigation proceeds through an elementary diffusion equation representation of landform evolution to the application of the resulting equations to the modern topography of scarplike landforms the morphologic observations can be analyzed either in the form of one or more cross strike elevation profiles or in the form of the slope offset plot a point plot of maximum scarp slope versus scarp offset working with either or both of these data representations for nine geologic structures which range in age from 3 to 400 ka b p and in offset from 1 to 50 m we apply analytical solutions for the vertical initial value scarp the vertical continuous offset scarp and the finite slope initial value scarp the model calculations are intrinsically ambiguous yielding as the final answer only the product t in the case of the initial value problem or the product a 1 in the case of the repeated faulting problem here t is the age of a single scarp forming event 2a is the vertical slip rate and is the mass diffusivity a single profile across three sea cliffs along the santa cruz california coast is analyzed as three separate initial value problems a reasonably constrained age for the sea cliff standing above the highway 1 platform returns 11 gkg 1 gkg 1 m2 ka with this we can date the two older sea cliffs in fact we do the converse age estimates for these two older sea cliffs based on a uniform rate of uplift both yield the same as for the lower sea cliff we treat a single profile of the raymond fault in pasadena san marino in terms of the repeated faulting problem for it the uplift rate of r crook and others yields 16 gkg the very substantial preexisting offset across the raymond fault must have been buried leveled some 230 ka b p when the modern topography began to form our analysis of the lake bonneville shoreline scarps reveals a dependence of t on 2a suggestive of nonlinear modification processes this appearance is treated with the finite slope initial value scarp model to determine 1 1 gkg for the lake bonneville shoreline scarps the suggestion of m n machette that approximately 100 000 year old meter high scarps are unobservable in weakly consolidated alluvial terranes of the basin and range and rio grande rift valley provinces can be formulated as 1 gkg the coincidence between this inequality and the lake bonneville shoreline is striking and it suggests that the value of 1 gkg may be generally applicable as a good first approximation to the modification of alluvial terranes within the semiarid regions of the western united states the lake bonneville shoreline is the basis for dating four sets of fault scarps in west central utah the drum mountains fault scarps can be modeled in several different circumstances but the most likely interpretation is that these fault scarps formed as the result of a single episode of normal faulting 3 6 to 5 7 ka b p the younger age is associated with quite low initial slope angles 25 the other three sets of fault scarps show no evidence for finite initial value slopes fault scarps along the eastern base of the fish springs range are very young 3 ka b p we estimate the age of fault scarps along the western flank of the oquirrh mountains to be 32 ka b p which meets the weak geologic constraint that they be older than the lake bonneville shoreline fault scarps along the northeastern margin of the sheeprock mountains are even older 53 ka b p an intriguing consequence of our single event analysis of these scarps is that an 11 5 m offset occurred in a single earthquake

uncertainties in slip rate estimates for the mission creek strand of the southern san andreas fault at biskra palms oasis southern california this study focuses on uncertainties in estimates of the geologic slip rate along the mission creek strand of the southern san andreas fault where it offsets an alluvial fan t2 at biskra palms oasis in southern california we provide new estimates of the amount of fault offset of the t2 fan based on trench excavations and new cosmogenic 10be age determinations from the tops of 12 boulders on the fan surface we present three alternative fan offset models a minimum a maximum and a preferred offset of 660 m 980 m and 770 m respectively we assign an age of between 45 and 54 ka to the t2 fan from the 10be data which is significantly older than previously reported but is consistent with both the degree of soil development associated with this surface and with ages from u series geochronology on pedogenic carbonate from t2 described in a companion paper by fletcher et al this volume these new constraints suggest a range of slip rates between 12 and 22 mm yr with a preferred estimate of 14 17 mm yr for the mission creek strand of the southern san andreas fault previous studies suggested that the geologic and geodetic slip rate estimates at biskra palms differed we find however that considerable uncertainty affects both the geologic and geodetic slip rate estimates such that if a real discrepancy between these rates exists for the southern san andreas fault at biskra palms it cannot be demonstrated with available data

lateglacial and early holocene glacier stages new dating evidence from the meiental in central switzerland abstract in order to improve our understanding of the glacier development in central switzerland after the last glacial maximum lgm we have investigated moraine sequences of the meiental meien valley after detailed field mapping 34 rock samples from different moraines were dated with cosmogenic 10be the results indicate that the investigated moraines were deposited mainly in multi phased advances during the younger dryas and the early holocene this is in contrast to previous studies suggesting that these moraines represent equivalents of the older egesen and daun stadials our research shows that factors like the moraine stratigraphy geomorphologic characteristics and equilibrium line altitudes elas can be highly variable at the local scale which can make a stadial correlation of moraines between different valleys or regions within the alps without age constraints ambiguous and unreliable against this background the interregional use of elas ela depressions for the parallelization of paleo glacier extents throughout the alps according to traditional concepts of the lateglacial deglaciation seems to produce erroneous results geomorphologic observations and relative age relationships in moraine successions based on stratigraphic principles remain the basis for this kind of studies but as we show here it is necessary to base local to regional comparisons of moraine sequences additionally on exposure ages to gain a more reliable understanding of the alpine glacier development after the lgm

evaluating the timing of former glacier expansions in the tian shan a key step towards robust spatial correlations the timing of past glaciation across the tian shan provides a proxy for past climate change in this critical area correlating glacial stages across the region is difficult but cosmogenic exposure

accuracy of cosmogenic ages for moraines analyses of all published cosmogenic exposure ages for moraine boulders show an average age range of 38 between the oldest and youngest boulders from each moraine this range conflicts with the common assumption that ages of surface boulders are the same as the age of the landform the wide spread in boulder ages is caused by erosion of the moraine surface and consequent exhumation of fresh boulders a diffusion model of surface degradation explains the age range and shows that a randomly sampled small set of boulders n 3 7 will always yield a lower age limit for the moraine the model indicates that for identical dating accuracy six to seven boulders are needed from old and tall moraines 40 000 100 000 yr 50 100 m initial height but only one to four boulders from small moraines 20 000 100 000 yr 10 20 m by following these guidelines the oldest obtained boulder age will be 90 of the moraine age 95 probability this result is only weakly sensitive to a broad range of soil erosion rates our analysis of published boulder ages indicates that 3 of all moraine boulders have prior exposure and 85 of these boulders predate the dated moraine

precision of terrestrial exposure ages and erosion rates estimated from analysis of cosmogenic isotopes produced in situ the concentration of cosmogenic isotopes produced within mineral grains varies with both the exposure age and erosion rate of the rock surface in principle therefore exposure age and erosion rate may be determined by analyzing two cosmogenic isotopes from the same sample provided the erosion rate is constant it is also possible to find either age or erosion rate from one isotope if the other parameter can be determined independently simple mathematical models predict the precision and accuracy of the exposure ages and erosion rates the results provide insight into the use of cosmogenic isotopes and a framework to optimize dating experiments and predict tractable geologic questions the precision and accuracy of the exposure ages and erosion rates depend on the precision of the measured isotope concentrations the half lives of the isotopes and the age and erosion rate of the sampled rock surface they also vary with the analytic strategy exposure age estimates from some isotope pairs approach measurement precision for late pleistocene surfaces eroding at 1 cm kyr 1 uncertainties in erosion rate for the same surfaces may be better than 0 2 cm kyr 1 for older surfaces the upper limits for both age and erosion rate estimates become infinite pairing of isotopes with different half lives may give misleading results unless the erosion rate is constant because they record different erosion histories analyzing a single isotope removes this source of error but is appropriate only if either the age or erosion rate can be well determined independently if the erosion rate is poorly constrained ages for some realistic situations may be inaccurate by 50 or more even though their precision may approach the measurement precision although it is important to understand the limitations of cosmogenic dating it is equally important not to lose sight of the potential of this powerful tool for quantitative geomorphologic studies

devising quality assurance procedures for assessment of legacy geochronological data relating to deglaciation of the last british irish ice sheet this contribution documents the process of assessing the quality of data within a compilation of legacy geochronological data relating to the last british irish ice sheet a task undertaken as part of a larger community based project britice chrono that aims to improve understanding of the ice sheet s deglacial evolution as accurate reconstructions depend on the quality of the available data some form of assessment is needed of the reliability and suitability of each given age s in our dataset we outline the background considerations that informed the quality assurance procedures devised given our specific research question we describe criteria that have been used to make an objective assessment of the likelihood that an age is influenced by the technique specific sources of geological uncertainty when these criteria were applied to an existing database of all geochronological data relating to the last british irish ice sheet they resulted in a significant reduction in data considered suitable for synthesis the assessed data set was used to test a bayesian approach to age modelling ice stream retreat and we outline our procedure that allows us to minimise the influence of potentially erroneous data and maximise the accuracy of the resultant age models

cosmogenic 36cl and 10be ages of quaternary glacial and fluvial deposits of the wind river range wyoming we measured cosmogenic 36 cl in 56 samples from boulders on moraines and fluvial terraces in the vicinity of the wind river range wyoming we also measured 10 be in 10 of the same samples most of the 10 be ages were in good agreement with the 36 cl ages indicating that rock surface erosion rates were very low the oldest moraine investigated the type sacagewea ridge site yielded only a limiting minimum age of 232 ka the oldest moraines in the type bull lake complex also could be constrained only to 130 ka the main sequence of type bull lake moraines yielded age distributions indicating deposition within the intervals 130 to 100 ka and 120 to 100 ka the best estimates are closer to the upper limits of these ranges and associated uncertainties are in the range of 10 to 15 these uncertainties could permit deposition in either marine isotope stage 6 or stage 5d we found no evidence of glacial deposits dating to marine isotope stage 4 both bull lake age moraines from fremont lake on the opposite side of the wind river range and boulders on a fluvial terrace above the wind river gave age distributions very similar to that of the second oldest bull lake advance ca 130 to 100 ka the distribution of boulder ages for pinedale moraines at bull lake indicated deposition between 23 and 16 ka nearly identical to the distribution of 10 be ages previously reported for the type pinedale moraines at fremont lake

cosmogenic 10be and 26al ages for the last glacial maximum eastern baffin island arctic canada a chronology for the last glacial maximum based on cosmogenic exposure dating in the pangnirtung fjord area eastern baffin island arctic canada is at odds with many decades of land based glacial stratigraphic research most previous chronologies focused primarily on relative weathering criteria and suggested that ice extent was restricted during the late wisconsinan ca 24 8 ka in contrast by directly dating glacial features we conclude that late wisconsinan ice was far more extensive than previously believed there are 36 gneissic boulders and 8 samples of ice molded gneissic bedrock that yield late wisconsinan 10 be and 26 al exposure ages for the last glaciation of pangnirtung fjord the prominent duval moraines which were previously interpreted to represent a significant early wisconsinan 100 60 ka ice advance on southern cumberland peninsula were actually deposited between 24 and 9 ka two boulders from a raised glaciomarine delta stratigraphically related to the duval moraines date to about 10 ka two recessional moraines and striated bedrock along pangnirtung fjord as well as erratics on the floor of the kolik river valley a tributary to pangnirtung fjord indicate that deglaciation began between 12 and 9 ka in situ produced 10 be and 26 al abundances indicate that ice filled pangnirtung fjord for about 15 k y either continuously or intermittently prior to 10 ka which is compatible with 14 c chronologies for adjacent cumberland sound thus our data support other recent studies that suggest the northern and southern margins of the laurentide ice sheet were generally in phase during the latest wisconsinan contrary to earlier interpretations

timing and extent of late quaternary glaciation in the western himalaya constrained by 10be moraine dating in garhwal india glacial chronologies from the himalayan region indicate various degrees of asynchronous glacial behavior part of this has been related to different sensitivities of glaciers situated in contrasting climatic compartments of the orogen but so far field data in support for this hypothesis is lacking here we present a new 10 be derived glacial chronology for the upper tons valley in western garhwal india and initial results for the pin and thangi valleys in eastern himachal pradesh these areas cover a steep gradient in orographic precipitation and allow testing for different climatic sensitivities our data provide a record of five glacial episodes at w16 ka w11 12 ka w8 9 ka w5 ka and 1 ka in the thangi valley our results indicate a glacial episode at w19 ka but no data are available for younger glacial deposits in this valley at their largest mapped extent w16 ka the two main glaciers in the upper tons valley joined and descended down to w2500 m asl which represents a drop of w1400 m compared to the present day glacial extent during the holocene the two largest glaciers produced distinct glacial landforms that allowed us to reconstruct changes in the equilibrium line altitude ela over w20 km north south distance that is presently associated with a steep gradient in rainfall we observe that ela changes have been consistently w2 times higher for the glacier located in a presently wetter climate pointing at different climate sensitivities related to the amount of precipitation that they receive at regional scale our data is in reasonable agreement with other published glacial chronologies from the western himalaya and suggest that glacial advances during the holocene have been largely synchronous in this region comparison of glacial chronologies from the western himalaya with other palaeoclimatic proxy data suggests that long term changes in glacial extents are controlled by glacial interglacial temperature oscillations related to the waxing and waning of the large northern hemisphere ice sheets while the timing of millennial scale advance and retreat cycles are more directly related to monsoon strength

late pleistocene cosmogenic 36cl glacial chronology of the southwestern ahklun mountains alaska thirty two cosmogenic 36cl surface exposure ages constrain the timing of two late pleistocene glacial advances in the western ahklun mountains southwestern alaska boulders were sampled from one early wisconsin sensu lato and six late wisconsin moraines deposited by ice cap outlet glaciers and local alpine glaciers four moraine boulders deposited during an extensive early wisconsin ice cap outlet glacier advance have a mean surface exposure age of 60 300 3200 yr a moraine deposited by an ice cap outlet glacier during the restricted late wisconsin advance has a mean surface exposure age of 19 600 1400 yr five moraines deposited by late wisconsin alpine glaciers have mean ages that range between 30 000 and 17 000 yr the 36cl ages are consistent with limiting 14c and thermoluminescence ages from related deposits and indicate that ahklun mountains glaciers reached their most extensive position of the last glaciation early during the late pleistocene in contrast to the deep sea isotopic record of global ice volume 1

10be exposure age chronology of the last glaciation in the krkonose mountains central europe abstract a new chronology of the last glaciation is established for the krkonose giant mountains central europe based on in situ produced 10be in moraine boulders exposure ages and schmidt hammer rebound values obtained for terminal moraines on the northern and southern flank of the mountains suggest that the oldest preserved moraines represent early phases of the last glacial maximum lgm large moraines at the outlet of the snowy cirques niezne kotly and in the middle part of the upa ob i d l trough were deposited around 21 ka while a series of smaller moraines above the lgm deposits represent readvances that occurred no later than 18 1 0 6 ka 15 7 0 5 ka 13 5 0 5 ka and 12 9 0 7 ka an exposure age of 13 8 0 4 ka obtained for protalus ramparts at the foot of the upska jama cirque headwall indicates that glaciers advanced only in north to east facing cirques during the lateglacial the last glacier fluctuation was synchronous with the younger dryas cold event the timing of local glacier advances during the last glacial episode correlates with the late weichselian glacier phases in the alps and in the bavarian bohemian forest

wisconsinan refugia and the glacial history of eastern baffin island arctic canada coupled evidence from cosmogenic isotopes and lake sediments new data constrain the timing and character of glaciation of eastern baffin island arctic canada lake sediments radiocarbon dated to between 14 and 52 ka coupled to 10 be and 26 al exposure ages 35 ka from adjacent moraines and bedrock demonstrate that some coastal uplands remained above the limits of continental glaciation throughout oxygen isotope stage 2 at lower elevations exposure ages from a flight of distinct lateral moraines range from 10 to 35 ka and imply that low gradient outlet glaciers reached the outer coast during the late wisconsinan collectively these results reconcile terrestrial evidence for the presence of ice free refugia through the late wisconsinan with marine evidence for extensive outlet glaciers in the major fiords and bays of eastern baffin island

cosmogenic 3he and 10be chronologies of the late pinedale northern yellowstone ice cap montana usa cosmogenic 3 he and 10 be ages measured on surface boulders from the moraine sequence deposited by the northern outlet glacier of the yellowstone ice cap indicate that the outlet glacier reached its terminal position at 16 5 6 0 4 3 he ka and 16 2 6 0 3 10 be ka respectively concordance of these ages supports the scaled production rates used for 3 he 118 6 6 6 6 atoms g 21 y r 21 and 10 be 5 1 6 0 3 atoms g 21 y r 21 62s at high latitudes at sea level two recessional moraines upvalley from the terminal moraine have mean ages of 15 7 6 0 5 10 be ka and 14 0 6 0 4 10 be ka respectively and a late glacial flood bar was deposited at 13 7 6 0 5 10 be ka these cosmogenic chronologies identify a late pinedale glacial maximum in northern yellowstone that is significantly younger than previously thought and they suggest deglaciation of the yellowstone plateau by 14 10 be ka

cosmogenic nuclide chronology of pre last glacial maximum moraines at lago buenos aires 46 s argentina at lago buenos aires argentina 10 be 26 al and 40 ar 39 ar ages range from 190 000 to 109 000 yr for two moraines deposited prior to the last glaciation 23 000 16 000 yr ago two approaches maximum boulder ages assuming no erosion and the average age of all boulders and an erosion rate of 1 4 mm 10 3 yr both yield a common estimate age of 150 000 140 000 yr for the two moraines the erosion rate estimate derives from 10 be and 26 al concentrations in old erratics deposited on moraines that are n760 000 yr old on the basis of interbedded 40 ar 39 ar dated lavas the new cosmogenic ages indicate that a major glaciation during marine oxygen isotope stage 6 occurred in the mid latitude andes the next five youngest moraines correspond to stage 2 there is no preserved record of a glacial advance during stage 4 the distribution of dated boulders and their ages suggest that at least one major glaciation occurred between 760 000 and n200 000 yr ago the mid latitude patagonian glacial record which is well preserved because of low erosion rates indicates that during the last two glacial cycles major glaciations in the southern andes have been in phase with growth and decay of northern hemisphere ice sheets especially at the 100 000 yr periodicity thus glacial maxima are global in nature and are ultimately paced by small changes in northern hemisphere insolation d 2005 university of washington all rights reserved

cosmogenic 36cl accumulation in unstable landforms 2 simulations and measurements on eroding moraines cosmogenic 36cl surface exposure ages obtained for multiple boulders from single landforms are usually characterized by a variance larger than that of the analytical methods employed this excessive boulder to boulder variability progressively more profound with increasing age of landforms is due to removal of soil and gradual exposure of boulders at the surface in our gradual exposure model boulders are initially buried in moraine matrix with time erosion lowers the moraine surface and the boulders are gradually exposed to cosmic rays because the cosmic ray intensity changes with depth the boulders are subjected to variable production rates of the cosmogenic 36cl initial depth of boulders and their chemical composition are variable which results in different amounts of the accumulated cosmogenic 36cl and thus different apparent ages of boulders the shape of the resulting distribution of the apparent ages and the coefficient of variation depend on the erosion depth while the first moment is a function of the true surface age and the erosion depth these properties of the apparent age distributions permit calculation of the surface age the erosion depth and also the average erosion rate we tested the model calculations using 26 boulders from a late pleistocene moraine at bishop creek sierra nevada california the set exhibited a bimodal distribution of the 36cl surface exposure ages we interpreted the older mode as the result of gradual exposure and the younger one as the result of surficial processes other than soil removal the 10 samples that constitute the older mode produced a distribution which closely matches the modeled distribution calculated using an age of 85 kyr and erosion depth of 570 g cm 2 this age is the same as an independent estimate obtained from cation ratio studies and the calculated erosion depth is very close to the erosion depth of 600 g cm 2 based on a simple analytical model of soil erosion these results indicate that our statistical model adequately describes effects of soil erosion on accumulation of cosmogenic 36cl the approach can be used to simultaneously obtain the true landform age and the erosion rate from apparent 36cl ages and thereifore may help in evaluation of surface exposure ages of eroding landforms

geomorphological evidence and 10be exposure ages for the last glacial maximum and deglaciation of the velka and mala studena dolina valleys in the high tatra mountains central europe abstract 10be exposure age chronology of the last glaciation was established in a key area at the southern slopes of the high tatra mts western carpathians in situ produced 10be in moraine boulders glacially transformed bedrock surfaces and rockfall accumulations constrains the timing of the last glacial maximum lgm glacier expansion and provides chronological evidence for the post lgm decay of one of the largest paleoglaciers in the range the uncertainty weighted mean age of 22 0 0 8 ka obtained for the terminal moraine in the forefield of the velka studena dolina valley indicates that the oldest moraine was deposited close to the global lgm this finding confirms that well preserved moraines in the range were formed during the last glacial cycle and that glaciation on the southern flank of the range was more extensive than earlier in the last glacial cycle the maximum glacier extent correlates with late wurmian weichselian glacier phases in the alps the bavarian bohemian forest and the krkonose mts but probably postdates the period of maximum glaciation in the southern carpathians re advance moraines at the mouth of the velka studena dolina valley and in the middle part of the mala studena dolina valley were deposited no later than around 20 5 ka and 15 5 ka respectively the timing of these advances is broadly synchronous within the high tatra mts as well as with glacier advances in the alps bavarian bohemian forest and krkonose mts 10be exposure ages obtained from glacially transformed bedrock surfaces range between 20 5 1 7 ka and 10 7 0 3 ka constraining the onset and the final phase of the deglaciation surface exposure dating of four rockfall accumulations produced uncertainty weighted mean ages of 20 2 1 2 ka 17 0 0 7 ka 16 5 0 4 ka and 15 6 0 7 ka these ages indicate that activation of rock slope failures occurred under paraglacial conditions within a few centuries up to 1400 years after the formation of re advance moraines

nature and timing of quaternary glaciation in the himalayan tibetan orogen abstract much effort has been made in recent years to define the timing and extent of quaternary glaciation throughout the himalayan tibetan orogen these studies are challenging because of the logistical and political inaccessibility of the region and the inherent problems associated with the application of numerical dating techniques nevertheless the studies are providing abundant evidence for significant glacial advances throughout the last several glacial cycles and are beginning to accurately define the extent and timing of glaciation in selected regions studies are showing that himalayan tibetan glaciers in arid regions during the last glacial cycle reached their maximum extent early in the cycle and that global last glacial maximum glacier advances were significantly less extensive however along the more monsoonal influenced greater himalaya there is increasing evidence to suggest that glaciation was more extensive later in the last glacial cycle but this has yet to be fully assessed in addition the new studies are showing that throughout most himalayan tibetan regions significant glacier advances occurred during the lateglacial and early holocene with minor advances in some regions during the mid holocene the still relatively poor chronological control in the himalayan tibetan orogen however makes it difficult to construct correlations across the region and with regions elsewhere in the world this in turn makes it hard to assess the relative importance of the different climatic mechanisms that force glaciation across the himalayan tibetan orogen and to quantify paleoclimate change in this high altitude subtropical region the lateglacial and holocene glacial records however are particularly well preserved in several himalayan tibetan regions glacial successions such as these have the greatest potential to be examined in detail using newly developing numerical dating and geomorphic and sedimentologic methods to derive high resolution terrestrial records of glaciation that will help in paleoclimatic reconstruction for high altitude subtropical regions

revised deglaciation history of the pietrele stânisoara glacial complex retezat mts southern carpathians romania abstract although geomorphological evidences of quaternary glaciations of the southern carpathians were extensively studied and discussed the limited number of chronological studies resulted in poor and controversial knowledge on the age of glaciations and deglaciation of the area we use new and recalculated in situ produced 10be surface exposure ages of glacial landforms to shed light on the age of the maximum glacial extent and the glacier oscillations during the last deglaciation process on the northern side of the retezat mountains according to our data the maximum ice extent documented by preserved moraines occurred around 21 0 1 5 0 8 ka coincident with the global last glacial maximum lgm the deglaciation process during the lateglacial was characterized by two glacial advances at 18 6 0 8 0 9 and 16 3 0 6 0 6 ka inferred stabilization date of the penultimate glacial stage at 15 2 0 8 0 7 ka was closely followed by the abrupt warming at the onset of the bolling allerod documented by a local chironomid based temperature reconstruction the last small glacier advance was dated to 13 5 0 4 0 5 ka these recessional readvance phases agree with other european glacial chronologies

timing and nature of alluvial fan and strath terrace formation in the eastern precordillera of argentina abstract sixty eight 10 be terrestrial cosmogenic nuclide tcn surface exposure ages are presented to define the timing of alluvial fan and strath terrace formation in the hyper arid san juan region of the argentine precordillera this region is tectonically active and numerous fault scarps traverse quaternary landforms the three study sites marquesado strath complex loma negra alluvial fan and carpinteria strath complex reveal a history of alluvial fan and strath terrace development over the past 225 ka the marquesado complex q3 m surface dates to 17 3 ka whereas the loma negra q1 ln q2 ln q3 ln q4 ln and q5 ln surfaces date to 24 3 ka 48 2 ka 65 13 ka 105 21 ka and 181 29 ka respectively the carpinteria complex comprises eight surfaces that have been dated and include the q1 c 23 3 ka q2 c 5 5 ka q3a c 25 12 ka q3b c 29 15 ka q4 c 61 12 ka q5 c 98 18 ka q6 c 93 18 ka and q7 c 212 37 ka 10 be tcn depth profile data for the loma negra alluvial fan complex and carpinteria strath terrace complex as well as osl ages on some carpinteria deposits aid in refining surface ages for comparison with local and global climate proxies and additionally offer insights into inheritance and erosion rate values for tcns 10 10 4 10 be atoms g of sio 2 and 5 m ma 1 respectively comparison with other alluvial fan studies in the region show that less dynamic and older preserved surfaces occur in the carpinteria and loma negra areas with only younger alluvial fan surfaces preserved both to the north and south these data in combination with that of other studies illustrate broad regional agreement between alluvial fan and strath terrace ages which suggests that climate is the dominant forcing agent in the timing of terrace formation in this region

long term shoreline retreat rates on whidbey island washington usa abstract long term retreat rates of puget sound s unconsolidated sediment shorelines have been difficult to quantify and little systematic research has been completed to constrain retreat in this area we put forward a new application of cosmogenic 10be exposure dating to assess long term shoreline retreat on whidbey island wa by dating lag boulders exposed on the shore platform as the shoreline erodes production of 10be in shoreline boulders is modulated by both tidal submergence and topographic shielding from the retreating bluff by modeling the combined effect of these variables on 10be production the timing of exposure can be determined and used to calculate long term 103 104 yr bluff retreat rates in rare cases retreat rates are underestimated due to inherited 10be within the study area average retreat rates ranged between 0 and 8 cm yr 1 our results demonstrate the utility of cosmogenic nuclides for determining long term shoreline retreat rates in areas with thick sediment cover where large numbers of samples can be collected and where the pre depositional history of the boulders is uncomplicated

spatio temporal evaluation of the groundwater quality in kafr al zayat district egypt eighteen groundwater well sites located in kafr al zayat egypt were sampled monthly from january 2009 to november 2011 for microbial content mn 2 fe 2 total dissolved solids tds total hardness no3 and turbidity the data were analyzed combining the integrated use of factor and cluster analyses as well as the geostatistical semi variogram modeling the prime objectives were to assess the groundwater suitability for drinking to document the factors governing the spatio tempral variability and to recognize distinctive groundwater quality patterns to help enable effective sustainability and proactive management of the limited resource the groundwater microbial mn 2 fe 2 tds and total hardness contents violated the drinking water local standards while the turbidity and the nitrate content complied with them factor analysis indicated that the microbial content is the most influential factor raising the variability potential followed in decreasing order by mn2 fe2 tds no3 turbidity and finally the total hardness turbidity resulting from urban and agricultural runoff was strongly associated with most of the quality parameters quality parameters fluctuate sporadically without concrete pattern in space and time while their variability scores peak in november every year three spatially distinctive quality patterns were recognized that were consistent with and affected by the cumulative effects of the local topography depth to water table thickness of the silty clay cap layer surface water and groundwater flow direction and hence the recharge from contaminated surface canals and agricultural drains copyright 2013 john wiley sons ltd

precise cosmogenic 10be measurements in western north america support for a global younger dryas cooling event the inner titcomb lakes moraine in the wind river mountains wyoming has been dated with a precision of 5 using accelerator mass spectrometric measurements of cosmogenic 10be in surface samples from nine boulders resting on its crest the age of this moraine is 11 4 0 5 to 13 8 0 6 ka the range in age is due to the uncertainty in the 10be production rate and uncertainties in rock erosion rate and shielding of cosmic rays by snow additional exposure ages on bedrock and boulder surfaces that are stratigraphically younger and older than the inner titcomb lakes moraine conform with this age the exposure ages of the inner titcomb lakes moraine overlap the north atlantic younger dryas event 12 8 0 2 to 11 5 0 3 ka

asynchronous glacier dynamics during the antarctic cold reversal in central patagonia abstract we present 14 new 10be cosmogenic nuclide exposure ages quantifying asynchronous readvances during the antarctic cold reversal from glaciers in the baker valley region of central patagonia we constrain glacier and ice dammed palaeolake dynamics using a landsystems approach concentrating on outlet glaciers from the eastern northern patagonian icefield npi and monte san lorenzo msl soler glacier npi produced lateral moraines above lago bertrand from 15 1 0 7 to 14 0 0 6 ka when it dammed the drainage of lago general carrera buenos aires through rio baker at a bedrock pinning point at this time soler glacier terminated into the 400 m deseado level of the ice dammed palaeolake later calluqueo glacier msl deposited subaerial and subaqueous moraines in the salto valley near cochrane at 13 0 0 6 ka these moraines were deposited in an ice dammed palaeolake unified through the baker valley lago chalenko 350 m asl the salto valley glaciolacustrine landsystem includes subaqueous morainal banks ice scoured bedrock glacial diamicton plastered onto valley sides perched delta terraces kame terraces ice contact fans palaeoshorelines and subaerial push and lateral moraines boulders from the subaqueous salto moraine became exposed at 12 1 0 6 years indicating palaeolake drainage these data show an asynchronous advance of outlet glaciers from the northern patagonian icefield and monte san lorenzo during the antarctic cold reversal these advances occurred during a period of regional climatic cooling but differential moraine extent and timing of advance was controlled by topography and calving processes

beryllium 10 dating of the duration and retreat of the last pinedale glacial sequence accurate terrestrial glacial chronologies are needed for comparison with the marine record to establish the dynamics of global climate change during transitions from glacial to interglacial regimes cosmogenic beryllium 10 measurements in the wind river range indicate that the last glacial maximum marine oxygen isotope stage 2 was achieved there by 21 700 700 beryllium 10 years and lasted 5900 years ages of a sequence of recessional moraines and striated bedrock surfaces show that the initial deglaciation was rapid and that the entire glacial system retreated 33 kilometers to the cirque basin by 12 100 500 beryllium 10 years

middle to upper pleistocene morphostructural evolution of the nw coast of sicily thermoluminescence dating and palaeontological stratigraphical evaluations of littoral deposits abstract infralittoral and terrigenous middle to upper pleistocene deposits outcropping at the nw coast of sicily gulf of castellammare and peninsula of capo san vito were investigated in order to recognise glacio eustatically caused sea level highstands and the interference between tectonics eustasy and local conditions of the coast the results originating from a stratigraphically important area were obtained by palaeontological stratigraphical analyses physical age determination with the thermoluminescence method and by detailed morphostructural studies the marine deposits situated at 3 40 m a s l mainly provide an oligotypic molluscan fauna referable to modern sgcf sfhn and sfbc biocoenoses of the infralittoral zone which live on various substrates in water of moderate to strong energy the senegalese molluscan forms providing the palaeocommunity of tyrrhenian age oxygen isotope substage 5e is comparable with the modern ap biocoenose which live on hard substrates in water of moderate energy the age determinations and estimates of the terraces indicate at most four glacio eustatic sea level highstands one perhaps two highstands between 800 ka and 250 ka isotope stages 24 8 a further event between 250 ka and 190 ka stage 7 and the youngest highstand event between 140 ka and 75 ka stage 5 the last glaciation stage 4 2 is represented by a terrigenous sequence which suggests rapid alternation of morphodynamically stable and unstable intervals as well as changes in moisture and seasonality of climate the terraces are affected by ne sw orientated faults causing brittle deformation and uplifting in rates of approximately 0 14 0 25 m ka with increasing intensity towards the east the oxygen isotope stratigraphy indicates 7 warming events during the past 800 ka the morphostructural pattern of the nw coast of sicily gives evidence for the sensible interference between local coastal conditions like sediment drift sea water currents and sediment supply on the one hand the absolute rise of eustatic sea level and the absolute uplift rate on the other hand it is obvious that eustatic oscillations deduced from deep sea sediment values does not automatically coincide with those recorded in the inner shelf area

timing of glacial retreat in the wicklow mountains ireland conditioned by glacier size and topography this paper presents 170 schmidt hammer exposure ages from moraine boulders and glacially sculpted bedrock to reveal the post last glacial maximum lgm history of the wicklow mountains ireland these data suggest that large ice masses survived for 4 7 ka after retreat of the irish sea ice stream and were sustained by summit ice fields until 16 6 ka post lgm retreat was dynamic with re advance moraines deposited in response to heinrich stadial 1 however these events reflect short term ice front oscillations 1 ka during the long term retreat phase retreat from re advance positions was synchronous across the range and paced by climate with time progressive deglaciation from low to high elevation in contrast marked asynchroneity in the timing of younger dryas deglaciation is closely linked to snow redistribution and indicates that for small cirque glaciers 1 km2 topography can exert the primary control on glacier survival this result has important implications for palaeoclimate econstructions as cirque glacier dynamics may be unrelated to climate this is further complicated by post depositional processes which can result in moraine ages e g 10be which post date retreat future palaeoclimate studies should prioritise cirque glaciers where snow contributing areas are small and where postdepositional disturbance is limited matrix poor boulder rich moraines

reconstruction of equilibrium line altitudes for tropical and sub tropical glaciers past fluctuations of tropical and sub tropical glaciers provide important palaeoclimate proxies for regions where other forms of evidence are rare however published equilibrium line altitude ela estimates for tropical and sub tropical glaciers at the lgm vary widely reflecting the diversity of methods and approaches employed by different research groups this complicates regional and global comparisons of ela estimates and emphasises the need for standardised methods the distinctive character of tropical and sub tropical glaciers however means that standard methods for reconstructing former glacier limits elas and palaeoclimate need to be adapted for local conditions many methods of ela reconstruction explicitly or implicitly make assumptions about glacier mass balance gradients and care needs to be taken that the choice of accumulation area ratios aars balance ratios brs and terminus to head ratios thars is appropriate as such indices are influenced by climatic regime debris cover and other factors ela reconstructions should employ multiple methods and should be cross checked and fully reported to allow assessment of the accuracy of ela estimates reliable glacier chronologies are equally important dating should be based on multiple radiometric techniques wherever possible and method of dating the type of material dated and the context of the date must all be reported

relative dating of moraines based on landform degradation lee vining canyon california abstract cross sectional topographic profiles of lateral moraines formed during different glaciations were measured at lee vining canyon california to determine whether the degree of moraine degradation could be quantified and used for relative dating logarithmic plots of the value of a slope parameter against cross sectional area show distinct linear trends for moraines of different relative ages the slopes of these trends suggest that the degradation processes are not among those that can be modeled with the linear diffusion equation

deglaciation and landscape history around annapurna nepal based on 10be surface exposure dating abstract the high himalaya is a key area for tectonic geomorphological and climate studies because of its extreme relief and location at the transition zone between areas with abundant monsoonal precipitation and the arid semiarid tibetan plateau we present 10be surface exposure ages on 22 boulders from the annapurna area in nepal the ages improve understanding of the late quaternary landscape history and the geomorphological processes operating in this part of the himalaya although our study is reconnaissance in nature it highlights the importance of catastrophic events such as landslides and debris flows for denudation of high mountains holocene exposure ages for the dhampu chooya landslide 4 1 0 6 ka and for 600 m of alluviation in kali gandaki valley 2 1 0 6 ka for example indicate the frequent occurrence and extent of catastrophic events and their implications for natural hazards we also offer an explanation for the differences in late quaternary glacial chronologies at closely spaced study sites in the nepal himalaya topographically controlled and spatially variable precipitation in the himalaya determines the sensitivity of glaciers to changes in temperature and precipitation accordingly some glaciers advanced in phase with northern hemisphere ice sheets whereas others reached their maximum extent at times of increased monsoonal precipitation during marine isotope stage 3 and the early holocene

effect of height and orientation microclimate on geomorphic degradation rates and processes late glacial terrace scarps in central idaho terrace scarps can serve as a nearly ideal natural laboratory for the study of the evolution of slopes this paper examines the effects of scarp size height and orientation microclimate by keeping constant variables such as age lithology and regional climate if a scarp degrades as a closed system and downslope movement is directly proportional to surface gradient the evolution of the scarp is modeled by the diffusion equation for a group of scarps of same age and known starting angle the diffusion equation model predicts the relation between maximum scarp angle and scarp height h late pleistocene terrace scarps now as steep as 33 25 as well as measured angles of repose for sand and gravel require a starting angle as steep as 33 5 for latest pleistocene idaho and utah scarps as h increases is gentler more degraded than modeled by the diffusion equation with a constant rate coefficient the degradation rate coefficient c increases tenfold with scarp height it should not change with scarp height if downslope movement is solely determined by surface gradient to the first power soil wash appears to be responsible for this departure from the diffusion equation model for transport rate by soil wash is a function of scarp size height south facing scarps are less vegetated and more degraded than north facing scarps for scarps 2 m high the degradation rate c on s facing scarps is 2 times that on n facing scarps for 10 m scarps it is 5 times the observed dependence of the rate coefficient c on scarp height can be removed by normalizing c to values for west facing scarps of the same height the residual c values calculated by this method correlate well with differences in incident solar radiation resulting from the different scarp orientations and maximum gradients this correlation demonstrates the importance of orientation on slope processes and their rates through the differences in freeze thaw cycles soil moisture and vegetative cover scarp morphology may be used to estimate age if one accounts for the effects of climate and for scarp height orientation and lithology for example using the dated bonneville shoreline scarps for calibration and comparing only scarps of equal height we estimate the drum mountains fault scarps to be 9 000 yr old this age is about twice that produced by previous diffusion equation calculations that have not accounted for the height as we have here but it is the same as independent geologic estimates of their age

glacial sequence near mccall idaho weathering rinds soil development morphology and other relative age criteria abstract the sequence of glacial deposits near mccall idaho previously assigned to the pinedale and bull lake glaciations contains deposits of four different ages these ages are defined by multiple relative age criteria including weathering rinds soil development surface rock weathering morainal morphology and loess stratigraphy the thickness of weathering rinds on basaltic clasts is statistically representative and reproducible and can be used to estimate numerical ages following in order of decreasing relation to age are soil development surface rock weathering and moraine morphology the glacial deposits near mccall appear to correspond to times of high worldwide ice volume indicated by the marine oxygen isotope record pilgrim cove and mccall deposits both assigned to the pinedale glaciation are late wisconsin in age perhaps 14 000 and 20 000 years respectively they represent a rare case in which deposits of pinedale age can be separated by relative age data timber ridge deposits assigned to the bull lake glaciation have subdued but well preserved morainal morphology relative age data indicate that they are pre wisconsin in age probably about 140 000 150 000 years old although we cannot exclude an older age williams creek deposits are clearly distinct from and intermediate in age between mccall and timber ridge deposits weathering rinds and the inferred ages of the other deposits suggest an early wisconsin age for williams creek deposits

trough geometry was a greater influence than climate ocean forcing in regulating retreat of the marine based irish sea ice stream marine terminating ice streams are a major component of contemporary ice sheets and are likely to have a fundamental influence on their future evolution and concomitant contribution to sea level rise to accurately predict this evolution requires that modern day observations can be placed into a longer term context and that numerical ice sheet models used for making predictions are validated against known evolution of former ice masses new geochronological data document a stepped retreat of the paleo irish sea ice stream from its last glacial maximum limits constraining changes in the time averaged retreat rates between well defined ice marginal positions the timing and pace of this retreat is compatible with the sediment landform record and suggests that ice marginal retreat was primarily conditioned by trough geometry and that its pacing was independent of ocean climate forcing we present and integrate new luminescence and cosmogenic exposure ages in a spatial bayesian sequence model for a north south 173km transect of the largest marine terminating ice stream draining the last british irish ice sheet from the south and east coasts of ireland initial rates of ice margin retreat were as high as 300 600 m a 1 but retreat slowed to 26 m a 1 as the ice stream became topographically constricted within st george s channel a sea channel between ireland to the west and great britain to the east and then stabilized retreating at only 3 m a 1 at the narrowest point of the trough during the climatic warming of greenland interstadial 2 gi 2 23 3 22 9 ka later retreat across a normal bed slope during the cooler conditions of greenland stadial 2 was unexpectedly rapid 152 m a 1 we demonstrate that trough geometry had a profound influence on ice margin retreat and suggest that the final rapid retreat was conditioned by ice sheet drawdown dynamic thinning during stabilization at the trough constriction which was exacerbated by increased calving due to warmer ocean waters during gi 2

chronology of taylor glacier advances in arena valley antarctica using in situ cosmogenic 3he and 10be abstract in situ produced cosmogenic nuclides provide a new technique for constraining exposure ages of glacial deposits in situ3he and 10be in quartz sandstone boulders from arena valley southern victoria land antarctica provide chronological constraints for a sequence of moraines taylor ii ivb moraines related to expansions of taylor glacier and the east antarctic ice sheet mean 3he ages are 113 000 45 000 yr 208 000 67 000 yr 335 000 187 000 yr and 1 2 0 2 myr for taylor ii iii iva and ivb moraines respectively mean 1 corresponding mean 10be ages for taylor ii and ivb moraines are 117 000 51 000 yr and 2 1 0 1 myr for the older deposits the 3he ages are probably lower limits due to diffusive loss although the exposure ages appear consistent with the few previous age estimates particularly with an isotope stage 5 age for taylor ii each moraine exhibits a broad age distribution the distribution probably results from a variety of factors which may include prior exposure to cosmic rays 3he loss erosion postdepositional boulder movement and radiogenic production of 3he nonetheless the exposure ages provide direct chronological constraints for the moraine sequence and suggest a maximum thickening of taylor glacier relative to the present ice surface of 500 m since the late pliocene early pleistocene

scarp degraded by linear diffusion inverse solution for age under the assumption that landforms unaffected by drainage channels are degraded according to the linear diffusion equation a procedure is developed to invert a scarp profile to find its diffusion age diffusion age having dimension length 2 is the product of diffusivity times chronological age the second moment of scarp slope grows linearly with age this fact together with an assumption about initial scarp shape allows the inverse determination of diffusion age age found assuming a vertical initial scarp is termed apparent age any nonvertical initial scarp profile has a nonzero initial apparent age true diffusion age differs from apparent age by a fraction of scarp offset squared the inverse procedure applied to synthetic data yields the following rules of thumb evidence of initial scarp shape has been lost when apparent age reaches twice its initial value if a scarp is formed by two events the inversion gives their offset weighted mean age with an error that is a fraction of offset squared a scarp that appears to have been formed by one event may have been formed by two with an interval between them as large as apparent age after scarps of two fault traces have diffused to appear as one the error in inferred age may be as large as half the apparent age variation of apparent age along strike would indicate multiple fault traces the simplicity of scarp profile measurement and this inversion makes profile analysis attractive if linearity of the flow law time for a free face to be reduced to the angle of repose and variation of diffusivity with climate and material could be established profile analysis would become a reliable dating technique

lateglacial and holocene cosmogenic surface exposure age glacial chronology and geomorphological evidence for the presence of cold based glaciers at nevado sajama bolivia comparisons of palaeo equilibrium line altitudes between the western and eastern cordilleras in the central andes are commonly based on the assumption that the tall outermost moraines visible in remotely sensed images of the western cordillera date to the last glacial maximum lgm however field investigation and geomorphic mapping at nevado sajama bolivia indicates the tall moraines are relic features with shorter moraines overlying and in some cases extending beyond them 36cl exposure ages from the shorter moraines suggest that they date to lateglacial times ca 16 9 10 2 ka although lateglacial deposits have been found throughout the central andes the extent of these deposits relative to lgm deposits varies both between the western and eastern cordilleras and north to south along the western cordillera in the western cordillera in the zone of easterly winds the lateglacial appears to be the most extensive glacial advance of the last glacial cycle geomorphic evidence also suggests that some lateglacial moraines were deposited by cold based ice a previously unreported finding in the tropical andes retreat from other glacial features occurred at about 7 0 4 4 ka and 4 7 3 3 ka these are the first directly dated holocene glacial deposits in the western cordillera of bolivia and their presence suggests that the mid holocene may not have been as warm and dry as previously thought copyright 2008 john wiley sons ltd

chronology of quaternary glaciations in east africa abstract a new glacial chronology for equatorial east africa is developed using in situ cosmogenic 36 cl measured in 122 boulders from moraines on mount kenya and kilimanjaro the oldest deposits sampled on kilimanjaro yield a limiting 36 cl age of 360 calendar kyr all 36 cl ages are in calendar years cal kyr or cal yr on mount kenya the oldest moraines give ages of 355 420 kyr liki i and 255 285 kyr teleki given the uncertainty in our 36 cl ages the liki i moraine may correspond to either marine isotope stage 10 or 12 whereas the teleki moraine correlates with stage 8 there is no evidence for stage 6 on either mountain the liki ii moraines on mt kenya and moraines of the fourth glaciation on kilimanjaro give ages of 28 3 kyr and 20 1 kyr respectively they represent the last glacial maximum lgm and correlate with stage 2 of the marine isotope record a series of smaller moraines above the lgm deposits record several readvances that occurred during the late glacial on mt kenya these deposits date to 14 6 1 2 kyr liki iia 10 2 0 5 kyr liki iii 8 6 0 2 kyr liki iiia and 200 yr lewis the corresponding deposits on kilimanjaro have mean ages of 17 3 2 9 kyr fourth glaciation saddle 15 8 2 5 kyr little glaciation saddle and 13 8 2 3 kyr fourth glaciation kibo these data indicate that the climate of the tropics was extremely variable at the end of the last glacial cycle

effect of far field slope on morphologic dating of scarplike landforms the principal finding of this paper is that the far field slope has a first order effect on model age determinations of scarplike landforms in weakly consolidated terrains observationally this can be demonstrated in two ways using the lake bonneville and lahontan shoreline scarps as separate and combined data sets use of the reduced scarp slope tan theta sub s minus b where theta sub s is the maximum scarp angle and b is the far field or fan slope instead of tan theta sub s alone as the measure of scarp slope measurably reduces separation between the two data sets induced by different average fan slopes for the two data sets and significantly reduces scatter in the slope offset plot for both the separate and combined data sets theoretically the argument can be put even more strongly at least within the range of linear and nonlinear diffusion models that the authors consider here together with a mathematical transformation of the empirical approach of r c bucknam and r e anderson when one correctly takes into account the far field slope one will basically get the same age determination no matter which of these models one uses conversely without accounting properly for the effect of far field slope one is virtuallymore guaranteed to get an erroneous age determination no matter which model is used less

glacial chronology and palaeoclimate in the bystra catchment western tatra mountains poland during the late pleistocene abstract deglaciation chronology of the bystra catchment western tatra mountains has been reconstructed based on 10be exposure age dating fourteen rock samples were collected from boulders located on three moraines that limit the horizontal extent of the lgm maximum advance and the lateglacial recessional stage the oldest preserved maximum moraine was dated at 15 5 0 8 ka an age that could be explained more likely by post depositional erosion of the moraine such scenario is supported by geomorphologic and palaeoclimatological evidence the younger cold stage is represented by well preserved termino lateral moraine systems in the kondratowa and sucha kasprowa valleys the distribution of the moraine ridges in both valleys suggest a complex history of deglaciation of the area the first late glacial re advance lg1 was followed by a cold oscillation lg2 that occurred at around 14 0 0 7 13 7 1 2 ka glaciers during both stages had nearly the same horizontal extent however their thickness and geometry changed significantly mainly due to local climatic conditions triggered by topography controlling the exposition to solar radiation the lg1 stage occurred probably during the pre bolling cold stage greenland stadial 2 1a however the lg2 stage can be correlated with the cooling at around 14 ka during the greenland interstadial 1 gi 1d older dryas this is the first chronological evidence of the older dryas in the tatra mountains the ela of the maximum bystra glacier was located at 1480 m a s l in accordance with the ela in the high tatra mountains during the lgm during the lg1 and lg2 stages the ela in the catchment rose up to 1520 1530 m a s l and was located approximately 100 150 m lower than in the eastern part of the massif climate modelling results show that the bystra glacier maximum advance could have advanced in the catchment when mean annual temperature was lower than today by 11 12 c and precipitation was reduced by 40 60 this is in accordance with lgm conditions previously reported for the high tatras during the lg1 and lg2 stages the temperature decrease in the study area reached 10 c and precipitation was lower by 30 compare to modern conditions this resulted in slightly higher accumulation 20 30 in the western tatra mountains compare to the high tatra mountains

the timing and cause of glacial activity during the last glacial in central tibet based on 10be surface exposure dating east of mount jaggang the xainza range abstract mountain glaciers are sensitive to climate change and can provide valuable information for inferring former climates on the tibetan plateau tp the increasing glacial chronologies indicate that the timing of the local last glacial maximum lgm recorded across the tp is asynchronous implying different local influences of the mid latitude westerlies and asian summer monsoon in triggering glacier advances however the well dated sites are still too few especially in the transition zone between regions controlled by the two climate systems here we present detailed last glacial chronologies for the mount jaggang area in the xainza range central tibet with forty three apparent 10be exposure ages ranging from 12 4 0 8 ka to 61 9 3 8 ka these exposure ages indicate that at least seven glacial episodes occurred during the last glacial cycle east of mount jaggang these include a local lgm that occurred at 61 9 3 8 ka possibly corresponding to marine isotope stage 4 mis 4 subsequent glacial advances at 43 2 2 6 ka and 35 1 2 1 ka during mis 3 one glacial re advance standstill at mis3 2 transition 29 8 1 8 ka and three glacial re advances standstills that occurred following mis 3 at 27 9 1 7 ka 21 8 1 3 ka and 15 1 0 9 ka the timing of these glacial activities is roughly in agreement with north atlantic millennial scale climate oscillations heinrich events suggesting the potential correlations between these abrupt climate changes and glacial fluctuations in the mount jaggang area the successively reduced glacial extent might have resulted from an overall decrease in asian summer monsoon intensity over this timeframe

quaternary glaciation of gurla mandhata naimon anyi abstract the quaternary glaciation of gurla mandhata naimona nyi an impressive isolated dome shaped massif situated in a remote region of southern tibet was examined using glacial geomorphic methods and dated using 10 be terrestrial cosmogenic nuclides the oldest moraines representing an expanded ice cap that stretched 5 km into the foreland of the gurla mandhata massif likely formed during marine isotope stage mis 10 or an earlier glacial cycle another glacial expansion of coalescing piedmont type occurred during the early part of the last glacial cycle or the penultimate glacial cycle after which glaciers became restricted to entrenched valley types impressive latero frontal moraines at the mouths of most valleys date to the early part of the last glacial cycle and mis 3 a succession of six sets of moraines within the valleys and up to the contemporary glaciers show that glaciers advanced during the lateglacial early holocene neoglacial and possibly little ice age the change of style of glaciation throughout the latter part of the quaternary from expanded ice caps to deeply entrenched valley glaciers might reflect 1 climatic controls with reduced moisture supply to the region over time and or 2 tectonic controls reflecting increase basin subsidence due to detachment faulting and enhanced valley incision

cosmic ray production rates of 10be and 26al in quartz from glacially polished rocks we have studied the concentrations of 10be and 26al in quartz crystals extracted from glacially polished granitic surfaces from the sierra nevada range these surfaces were identified with the glacial advance during the tioga period 11 000 years ago our measurements yield the most accurate estimates to date for the absolute production rates of these nuclides in sio2 due to cosmic ray nucleons and muons for geomagnetic latitudes 43 8 44 6 n and altitudes 2 1 3 6 km the estimates are relatively free from uncertainties in snow cover since we studied a suite of rock surfaces inclined 0 75 with respect to the horizontal the principal uncertainty arises due to the lack of a precise date for the glacial retreat event about 10 the 26al 10be ratio at production 6 0 is determined more accurately since the exposure age of the samples is 2 orders of magnitude smaller than the mean lives of the two nuclides production rates of 10be and 26al in quartz at other latitudes and altitudes in the troposphere can be determined from the present measurements by scaling using the known altitude and latitude dependence of cosmic ray fluxes of nucleons and negative muons knowledge of the production rates of these nuclides is a prerequisite for their application in erosion and geomorphological studies

late tertiary to late quaternary record in the mackenzie mountains northwest territories canada stratigraphy paleosols paleomagnetism and chlorine 36 a stratigraphic sequence of unconsolidated sediments ranging in age from late pliocene to late pleistocene is recorded in the canyon ranges of the mackenzie mountains three of the sections katherine creek little bear river and inlin brook expose bedrock and tertiary gravel overlain by colluvium and a multiple till sequence of montane origin separated by paleosols and capped by a till of laurentide origin the sections are correlated on the basis of lithology paleosol development paleomagnetism and chlorine dating of surface boulder erratics a formal stratigraphic nomenclature is proposed for the deposits of this region the sequence of glacial tills separated by paleosols reflects a long record of glacial interglacial cycles soil properties from the oldest paleosol to modern soil show a general decrease in the degree of soil development suggesting a progressive deterioration of interglacial climatic conditions a normal reverse normal sequence of remanent magnetization was determined within th

regional mid pleistocene glaciation in central patagonia abstract southern south america contains a glacial geomorphological record that spans the past million years and has the potential to provide palaeoclimate information for several glacial periods in earth s history in central patagonia two major outlet glaciers of the former patagonian ice sheet carved deep basins 50 km wide and extending over 100 km into the andean plain east of the mountain front a succession of nested glacial moraines offers the possibility of determining when the ice lobes advanced and whether such advances occurred synchronously the existing chronology which was obtained using different methods in each valley indicates the penultimate moraines differ in age by a full glacial cycle here we test this hypothesis further using a uniform methodology that combines cosmogenic nuclide ages from moraine boulders moraine cobbles and outwash cobbles 10 be concentrations in eighteen outwash cobbles from the moreno outwash terrace in the lago buenos aires valley yield surface exposure ages of 169 269 ka we find 10 be inheritance is low and therefore use the oldest surface cobbles to date the deposit at 260 270 ka which is indistinguishable from the age obtained in the neighbouring lago pueyrredon valley this suggests a regionally significant glaciation during marine isotope stage 8 and broad interhemispheric synchrony of glacial maxima during the mid to late pleistocene finally we find the dated outwash terrace is 70 100 ka older than the associated moraines on the basis of geomorphological observations we suggest this difference can be explained by exhumation of moraine boulders

quaternary history and landscape evolution of a high altitude intermountain basin at the western end of the himalayan tibetan orogen waqia valley chinese pamir abstract quaternary valley fills and landforms in the waqia valley of the chinese pamir were examined using geomorphic mapping geomorphic and sedimentological analysis of landforms and sediments and cosmogenic 10be surface exposure dating six sets of moraines m 1 to m 6 are identified and date to the penultimate or an older glacial cycle m 1 penultimate glacial cycle m 2 early last glacial and probably marine oxygen isotope stage mis 4 m 3 mis 2 m 4 late glacial m 5 and early holocene m 6 younger moraines are also present but these were not examined in detail alluvial fans and outwash river terraces are present throughout the valley four surfaces on alluvial fans and river terraces were dated to 580 ka t 1 120 ka t 2 100 ka t 3 and 70 ka t 4 large landslide and lacustrine deposits are common within the valley fills and represent periods of major slope instability and possible damming of drainages glacial processes dominate the western side of the valley landsliding is the leading process on its eastern side defining the nature and timing of landform formation in this valley highlights the punctuated nature of landform evolution in the himalaya and tibet and provides a framework for considering the abundant valley fill successions elsewhere in the orogen

timing of multiple late quaternary glaciations in the hunza valley karakoram mountains northern pakistan defined by cosmogenic radionuclide dating of moraines moraines and associated landforms in the upper hunza valley karakoram mountains northern pakistan provide an excellent record of multiple glaciations during the late quaternary glaciers advanced at least eight times by using 10 be and 26 al surface exposure dating on moraine boulders and scoured bedrock we determined the timing of glaciation for four of these glacial advances ca 54 7 43 2 ka borit jheel glacial stage ca 25 7 21 8 ka ghulkin i glacial stage ca 18 4 15 3 ka ghulkin ii glacial stage and ca 10 8 9 0 ka batura glacial stage for two of the older advances the yunz and shanoz glacial stages our data set a limit of 60 ka although at present the uncertainties in dating make it impossible to describe unequivocally the climate processes controlling glaciations in the hunza valley the results suggest that precipitation changes related to oscillations in the southwest asian monsoonal system combine with cooling that is broadly associated with heinrich events to cause glacial advances in this region

surface dating of dynamic landforms young boulders on aging moraines the dating of landforms is crucial to understanding the evolution history and stability of landscapes cosmogenic isotope analysis has recently been used to determine quantitative exposure ages for previously undatable landform surfaces a pioneering application of this technique to date moraines illustrated its considerable potential but suggested a chronology partially inconsistent with existing geological data consideration of the dynamic nature of landforms and of the ever present processes of erosion deposition and weathering leads to a resolution of this inconsistency and more generally offers guidance for realistic interpretation of exposure ages

middle pleistocene glaciation in patagonia dated by cosmogenic nuclide measurements on outwash gravels the well preserved glacial record in argentine patagonia offers a 1 ma archive of terrestrial climate extremes in southern south america these glacial deposits remain largely undated beyond the range of radiocarbon dating at ca 40 ka dating old glacial deposits several 105 a by cosmogenic surface exposure methods is problematic because of the uncertainty in moraine degradation and boulder erosion rates here we show that cobbles on outwash terraces can reliably date old glacial deposits in the lago pueyrredon valley 47 5 s argentina favorable environmental conditions e g aridity and strong winds have enabled continuous surface exposure of cobbles and preservation of outwash terraces the data demonstrate that nuclide inheritance is negligible and we therefore use the oldest surface cobbles to date the deposit 10be concentrations in outwash cobbles reveal a major glacial advance at ca 260 ka concurrent with marine isotope stage 8 mis 8 and dust peaks in antarctic ice cores a 10be concentration depth profile in the outwash terrace supports the age and suggests a low terrace erosion rate of ca 0 5 mm ka 1 we compare these data to exposure ages obtained from associated moraines and find that surface boulders underestimate the age of the glaciation by 100 ka thus the oldest boulders in this area do not date closely moraine deposition the 10be concentration in moraine cobbles help to constrain moraine degradation rates these data together with constraints from measured 26al 10be ratios suggest that all moraine boulders were likely exhumed after original deposition we determine the local last glacial maximum lgm occurred at 27 25 ka consistent with the maximum lgm in other parts of patagonia

application of a combination of dating techniques to reconstruct the lateglacial and early holocene landscape history of the albula region eastern switzerland landforms in val mulix and the albula region in eastern switzerland offer a detailed insight into the period between the oldest dryas until the early holocene to better understand lateglacial and holocene climate change in the central alps glacial moraines polished bedrock and periglacial rock glacier landforms were dated using a combined approach of numerical cosmogenic 10be and relative schmidt hammer weathering rind thickness dating techniques at high elevation sites near the last glacial maximum lgm trimline 10be exposure ages of glacially modified bedrock are between 11 2 ka and 13 5 ka this suggests the persistence of long lasting small local ice caps after the breakdown of the lgm ice domes or alternatively a reformation of ice perhaps during the younger dryas in val mulix we obtained one of the first ages for the daun stadial 14 7 ka moraines 14 9 1 8 ka supporting a pre bolling chronological position the age is in excellent agreement with the age of a boulder from an egesen i moraine located up valley which we postulate may be a daun moraine that was re occupied during the egesen stadial a boulder from an egesen ii moraine gave an age of 10 7 ka which is similar to ages of egesen ii moraines at other sites in the alps 10be ages from boulders found on a relict rock glacier in val mulix indicate that the main active phase lasted from the lateglacial until the early holocene the derived mean annual flow rate is of the order of decimetres which is in accordance with values stated in the literature based on measuring active rock glaciers in the alps exposure ages from a glacially polished rock barrier showed that this area was ice free at the end of the younger dryas 9 0 0 7 ka and 11 9 0 9 ka the polished bedrocks are located a few hundred meters down valley from the little ice age lia moraines this gives direct evidence of a fast ice retreat towards the end of the younger dryas with glacier length variations that did not exceed the 1850 ad extension little ice age maximum surface exposure dating is however limited by several methodological constraints the choice of suitable snow depths plays a crucial role in the calculation of the 10be ages shielding of surfaces from cosmic rays by snow can significantly influence the exposure age and variations in the estimated annual snowfall in the albula region since the lgm is therefore a potential source of considerable uncertainty in our measurements while the measurement of weathering rind thicknesses turned out to be an appropriate tool to support the reconstruction of lateglacial landscape evolution schmidt hammer r values were less helpful the r values enabled a temporal distinction of landforms within the holocene lia moraine active rock glaciers but not within the lateglacial from a methodological point of view the different dating methods enabled a cross checking an extended interpretation of the data and a more accurate estimate of the possible sources of error

moraine preservation and boulder erosion in the tropical andes interpreting old surface exposure ages in glaciated valleys cosmogenic dating provides a long awaited means of directly dating glacial deposits that pre date the last glacial cycle although the potential benefits of longer chronologies are obvious the greater uncertainty associated with older cosmogenic ages may be less readily apparent we illus trate the challenges of developing and interpreting a long chronology using our data from the peruvian andes we used surface exposure dating with cosmogenic radionuclides crns 10 be and 26 al to date 140 boulders on moraines in valleys bordering the junin plain 11 s 76 w in central peru our chronology spans multiple glacial cycles and includes exposure ages greater than 1 million years which indicate that long term rates of boulder erosion have been very low interpreting the chronology of moraines for glaciations that predate the last glacial cycle is complicated by the need to consider boulder erosion and exhumation surface uplift and inheritance of crns from pre vious exposure intervals as an example we recalculate exposure ages using our boulder erosion rates 0 3 0 5 metres per million years and estimated surface uplift rates to emphasise both the chal lenges involved in interpreting old surface exposure ages and the value of chronological data even with large uncertainties when reconstructing the palaeoclimate of a region copyright 2005 john wiley sons ltd

cosmogenic chlorine 36 chronology for glacial deposits at bloody canyon eastern sierra nevada deposits from mountain glaciers provide an important record of quaternary climatic fluctuations but have proved difficult to date directly a chronology has been obtained for glacial deposits at bloody canyon california by measurement ofthe accumulation of chlorine 36 produced by cosmic rays in boulders exposed on moraine crests the accumulation ofchlorine 36 indicates that episodes of glaciation occurred at about 21 24 65 115 145 and 200 ka thousand years ago although the timing of the glaciations correlates well with peaks of global ice volume inferred from the marine oxygen isotope record the relative magnitudes differ markedly the lengths of the moraines dating from 115 ka and 65 ka show that the early glacial episodes were more extensive than those during the later wisconsin and indicate that the transition from interglacial to full glacial conditions was rapid

glacial lake evolution and atlantic pacific drainage reversals during deglaciation of the patagonian ice sheet abstract modelling experiments of drainage events from proglacial lakes of the rio baker catchment central patagonia 46 48 s indicate that atlantic pacific drainage reversals may have caused freshwater forcing of regional climate however much of the region remains unmapped in detail and available geochronological data is equivocal leading to multiple published palaeolake evolution models we evaluate these models through new geomorphological mapping from the baker valley cosmogenic dating of moraine boulders that demonstrates an antarctic cold reversal ice readvance that blocked drainage through the rio baker an altitudinal based review of published geochronology and regional analysis of shoreline glacio isostasy and palaeolake levels we use these datasets to present a new regional palaeolake evolution model underpinned by bayesian age modelling we demonstrate that 103 km3 of freshwater was released to the pacific over at least 6 drainage events from before 15 3 15 0 cal yr bp to the early holocene the final stages of lake drainage involved catastrophic flooding along the baker valley evidenced by high magnitude flood landforms such as boulder bars likely caused by failure of large valley floor moraine dams we place these drainage events in the context of late quaternary meltwater pathways associated with advance retreat of the patagonian ice sheet and early human occupation across the region although broad patterns of ice retreat and lake formation may be similar across patagonia driven by southern hemisphere palaeoclimate regional topographic settings likely resulted in spatial and temporal heterogeneity of atlantic pacific drainage reorganisation across southernmost south america

a high elevation holocene pollen record from iztaccihuatl volcano central mexico pollen data from 3860 m in the agua el marrano valley on the northwest side of iztaccihuatl volcano southeastern part of the basin of mexico 198n combined with data on the glacial chronology and tephrochronology document holocene climatic and vegetation changes near the present timberline glacial advances occurred between 12 000 and 10 500 cal yr bp milpulco 1 and between 8300 and 7300 cal yr bp milpulco 2 a 450 cm core was drilled in sediments accumulated in a milpulco 1 moraine depression the sequence covers the last 11 000 cal yr bp according to three 14 c dates and the presence of a distinctive pumice deposit dated elsewhere at 5700 cal yr bp the pollen record of agua el marrano shows barren conditions after glacier retreat followed by a gradual colonization by elements of the alpine grasslands around 10 900 cal yr bp this plant community remained until c 7200 cal yr bp between c 7200 cal yr bp and 6500 cal yr bp coniferous forest grew close to or at the site but alpine grasslands were re established between 6500 cal yr bp and c 5000 cal yr bp apparently in connection with mid holocene dry conditions recorded in central mexico the upper timberline attained its modern altitude 4000 m in the last 3000 years during this period no significant changes are observed in the pollen spectra comparison with other palaeoecological records from central mexico suggests that the timberline fluctuated in response to both temperature and precipitation changes during the holocene