the younger dryas in the eastern mediterranean abstract the younger dryas period is traced in pollen diagrams of the eastern mediterranean by means of radiocarbon dates the biostratigraphy in terms of pollen assemblages is compared with the absolute time scale the younger dryas as a pollen assemblage zone is at best vaguely visible in those parts that at present carry steppe or other open vegetation the part that nowadays carries forest produces pollen zones including the younger dryas which resemble those of parts of europe palynological developments in parts of the eastern mediterranean differ appreciably which implies that the vegetation developed differently also this suggests that the climatic development in the various parts of the region was not the same

buried glacier ice in southern iceland and its wider significance abstract geo electrical resistivity surveys have been carried out at recently deglaciated sites in front of three glaciers in southern iceland skei arajokull hrutarjokull and virkisjokull the results show the presence of old glacier ice beneath debris mantles of various thickness we conclude that buried glacier ice has survived for at least 50 years at virkisjokull and hrutarjokull and probably for over 200 years at skei arajokull additional data from a further site have identified a discontinuous ice core within 18th century jokulhlaup deposits photographic and lichenometric evidence show that the overlying debris has been relatively stable and hence melting of the ice at all four sites is proceeding slowly due to the heat shielding properties of the overburden the geomorphic implications are pertinent when considering the potential longevity of buried ice the possible implications for dating techniques such as lichenometry radiocarbon dating and cosmogenic surface exposure dating are also important as long term readjustments of surface forms may lead to dating inaccuracy finally it is recognised that landscape development in areas of stagnant ice topography may post date initial deglaciation by a considerable degree

glacier advances in northeastern turkey before and during the global last glacial maximum abstract our study in the basyayla valley in northeastern anatolia showed evidence of four glacier advances that built terminal and lateral moraines surface exposure dating of boulders on these moraines showed that the maximum ice extent mie was asynchronous with the global last glacial maximum lgm 22 1 4 3 thousand years ka the local mie took place at least 57 0 3 5 ka ago the extent of the basyayla glacier during this advance is not known exactly because the boulders are only preserved on a lateral moraine the next advance was prior to 41 5 2 5 ka and it descended down the valley to approximately 2320 m above sea level m a s l with a glacier length of 5 3 km during the early global lgm the basyayla glacier extended for a distance of 4 9 km down to approx 2430 m a s l the last recorded advance occurred during the global lgm this extension was 0 7 km smaller than the local mie and its terminus reached 2490 m a s l only the exposure ages of boulders in a retreat position at an altitude of approx 3045 m a s l indicate that the valley has remained ice free since the lateglacial period therefore the lateglacial extent was limited to the cirque system in the uppermost part of the catchment furthermore holocene glacier oscillations seem to be either absent or restricted to solifluction in the whole catchment and to rock glacier movements in the southern tributary of the basyayla valley system

what controls dead ice melting under different climate conditions a discussion abstract in the geological record hummocky dead ice moraines represent the final product of the melt out of dead ice processes and rates of dead ice melting in ice cored moraines and at debris covered glaciers are commonly believed to be governed by climate and debris cover properties here backwasting rates from 14 dead ice areas are assessed in relation to mean annual air temperature mean summer air temperature mean annual precipitation mean summer precipitation and annual sum of positive degree days the highest correlation was found between backwasting rate and mean annual air temperature however the correlation between melt rates and climate parameters is low stressing that processes and topography play a major role in governing the rates of backwasting the rates of backwasting from modern glacial environments should serve as input to de icing models for ancient dead ice areas in order to assess the mode and duration of deposition a challenge for future explorations of dead ice environments is to obtain long term records of field based monitoring of melt progression furthermore many modern satellite borne sensors have high potentials for recordings of multi temporal digital elevation models dems for detection and quantification of changes in dead ice environments in recent years high accuracy dems from airborne laser scanning altimetry lidar are emerging as an additional data source however time series of high resolution aerial photographs remain essential for both visual inspection and high resolution stereographic dem production

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

surface exposure ages of front range moraines that may have formed during the younger dryas 8 2 cal ka and little ice age events abstract surface exposure 10 be ages have been obtained on boulders from three post pinedale end moraine complexes in the front range colorado boulder rounding appears related to the cirque to moraine transport distance at each site with subrounded boulders being typical of the 2 km long chicago lakes glacier subangular boulders being typical of the 1 km long butler gulch glacier and angular boulders being typical of the few hundred m long isabelle glacier surface exposure ages of angular boulders from the isabelle glacier moraine which formed during the little ice age lia according to previous lichenometric dating indicate cosmogenic inheritance values ranging from 0 to 3 0 10 be ka 1 subangular boulders from the butler gulch end moraine yielded surface exposure ages ranging from 5 to 10 2 10 be ka we suggest that this moraine was deposited during the 8 2 cal ka event which has been associated with outburst floods from lake agassiz and lake ojibway and that the large age range associated with the butler gulch end moraine is caused by cosmogenic shielding of and or spalling from boulders that have ages in the younger part of the range and by cosmogenic inheritance in boulders that have ages in the older part of the range the surface exposure ages of eight of nine subrounded boulders from the chicago lakes area fall within the 13 0 11 7 10 be ka age range and appear to have been deposited during the younger dryas interval the general lack of inheritance in the eight samples probably stems from the fact that only a few thousand years intervened between the retreat of the pinedale glacier and the advance of the chicago lakes glacier in addition bedrock in the chicago lakes cirque area may have remained covered with snow and ice during that interval thus partially shielding the bedrock from cosmogenic radiation

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

correlation of late quaternary moraines impact of climate variability glacier response and chronological resolution abstract the last decade has seen a global surge in the number and resolution of well dated geomorphological reconstructions of mountain glacier fluctuations and former ice sheet extents this has been facilitated by the widespread deployment of large samples of terrestrial cosmogenic nuclide dating tcnd allowing more sophisticated probability analyses for reducing the statistical uncertainty of moraine ages and testing of correlations at local regional and hemispheric scales geochronological advances in the field laboratory and on model calibration have taken place in parallel with modelling of palaeoclimate based on glacier reconstructions and of glacier sensitivity to temperature and precipitation forcing in spite of these advances basic conceptual issues remain this review questions whether the technical leap forward in dating has encouraged us to over reach our interpretive limits particularly in terms of the concept and practice of correlation we propose that to enable reliable correlation glacier chronologies should first be examined for their climatic integrity spatial coherence and chronological robustness even with excellent dating interpretive complexity remains due to complex climate signals and glacier response characteristics and to erosion censoring of the landform record all are issues which must be addressed to avoid either spurious lumping correlation or splitting differentiation of glacier chronologies and thus misinterpretation of the climatic significance of reconstructed glacier advances we suggest that the reliability of correlation is no better than the weakest link in the chain of reasoning which connects climatic fluctuations with dated landforms and sediments improved dating means that poor chronological resolution is less likely to be the limiting factor for correlation but shifts the focus onto other less quantifiable sources of uncertainty these conceptual statements are exemplified with reference to holocene glacial chronologies of contrasting mid latitude environments in southern norway iceland and new zealand

himalayan glacial sedimentary environments a framework for reconstructing and dating the former extent of glaciers in high mountains abstract reconstructing paleoenvironmental change from glacial geologic evidence in the himalayas has been difficult because of the lack of organic material for radiocarbon dating and the problems of correctly identifying the origin of highly dissected landforms studies of the contemporary glacial depositional environments and ancient landforms and sediments in the hunza valley karakoram mountains the lahul and garhwal himalaya and the khumbu himalaya illustrate the variability in processes landforms and sediment types these studies can be used to interpret ancient landforms and sediments for paleoenvironmental reconstructions and aid in forming strategies for sampling sediments and rocks for the developing techniques of cosmogenic radionuclide crn surface exposure and optically stimulated luminescence osl dating many himalayan glaciers have thick covers of supraglacial debris derived from valley sides and such debris mantled glaciers exhibit important differences from clean glaciers both in terms of debris transport processes and the depositional landforms that they produce analysis of sediment landform associations can be used to reconstruct processes of sediment transport and deposition and the relationship between moraines and other landforms and climatic forcing cycles such analysis is of fundamental importance in guiding sampling and interpretation in crn and osl dating work

contributions and unrealized potential contributions of cosmogenic nuclide exposure dating to glacier chronology 1990 2010 abstract this paper reviews the application of cosmogenic nuclide exposure dating to glacier chronology exposure dating of glacial landforms has made an outsize impact on this field because the technique filled an obvious need that had already been recognized by glacial geologists by now hundreds of studies have used cosmogenic nuclide exposure dating to date glacial deposits and in fact it is rare to find a study of glacial geology or glacier chronology or any paleoclimate synthesis that makes use of such studies that does not involve exposure dating these developments have resulted in major contributions to glacier chronology and paleoclimate in particular i reconstructing antarctic ice sheet change ii establishing the chronology of late pleistocene and holocene glacier change in mountain regions where it was previously unknown iii establishing the broad chronological outlines of mountain glaciations prior to the last glacial maximum and iv gaining insight into subglacial erosional processes through the observation that many glaciated surfaces preserve cosmogenic nuclide inventories from long past ice free periods as well as the present one an important potential future contribution will be the application of the large data set of exposure dated glacier chronologies to better understand global and regional climate dynamics during lateglacial and holocene millennial scale climate changes however this contribution cannot be realized without significant progress in two areas i understanding and accounting for geologic processes that cause apparent exposure ages on glacial landforms to differ from the true age of the landform and ii minimizing systematic uncertainties in exposure ages that stem from cosmogenic nuclide production rate estimates and scaling schemes at present there exists an enormous data set of exposure ages on glacial deposits but these data cannot be used to their full potential in paleoclimate syntheses due to an inadequate understanding of geologic scatter and production rate uncertainties the intent of this paper is to highlight this situation and suggest some strategies for realizing this potential

timing of glaciation in the mediterranean mountains during the last cold stage recent research in the mountains of the mediterranean has attempted to establish the timing of the maximum extent of glaciers during the last cold stage several dating methods have been utilised and new dating frameworks have emerged in key areas in several places multiple dating techniques radiocarbon optically stimulated luminescence u series applied to glacial and associ ated sediments suggest that local glacier maxima preceded the global last glacial maximum lgm ca 18 14 c ka bp 21 cal ka bp by at least several thousand years studies in spain the pyrenees the maritime alps and turkey utilising cosmogenic isotope analyses 10 be and 36 cl have yielded glacial geochronologies with maxima that correlate closely with the marine isotope record of ice volume and the global lgm in some cases the use of different geochronological techniques has led to conflicting evidence for the timing of the local last glacier maxima in the same area from a palaeoclimate perspective glacier climate reconstructions for the local last glacier maxima in the mountains of central italy northern greece and western turkey indicate that climate was wet and cold in italy and greece the local glacier maxima preceded the driest part of the last cold stage as indicated by palaeoecological records by several thousand years for western turkey however recent cosmo genic exposure ages combined with glacier modelling suggest a local last glacier maximum charac terised by cold and wet conditions coinciding with the global lgm the available evidence indicates that a dichotomy is emerging not only in our understanding of the timing of glaciation across the mediterranean but also in the palaeoclimatic interpretations derived from glacial and palaeoecolo gical records copyright 2008 john wiley sons ltd

saw tooth structures and curved veins related to folds in the south central pyrenees spain two generation of folds f1 and f2 and associated structures developed in the eocene turbidites of the south central pyrenees are analyzed in this paper f1 folds are close have sub horizontal axes and southwards vergence they have an associated cleavage s1 competent layers were folded by layer parallel shortening tangential longitudinal strain some possible flexural flow and an obliquely superimposed homogeneous strain due mainly to simple shear flexural slip is also an important mechanism in the whole multilayer f2 folds are gentle and scarce they fold the s1 cleavage among the structures associated with f1 folds there are sets of veins with curved form in the competent layers the displacement of each vein gave rise usually to a step in the layer boundary so that a set of veins produces a structure that is named saw tooth structure the veins initiated as small faults that made flexural slip difficult and gave rise to a concentration of stress on the steps leading to an opening of the fractures and a propagation of them along a curved path as suggested by a simple mechanical model this propagation agrees with finite element models developed by other authors

pb o and c isotopes in silicified mooidraai dolomite transvaal supergroup south africa implications for the composition of paleoproterozoic seawater and dating the increase of oxygen in the precambrian atmosphere abstract the mooidraai dolomite formation is a unit of marine sedimentary carbonates in the upper transvaal supergroup which recorded significant changes in the composition of the earth s atmosphere and ocean previously available data had suggested that this dolomite was about 2 2 ga old and showed 13ccarb values around 0 8 pdb and hence was an exception to the positive excursion of 13ccarb values observed worldwide in marine carbonates deposited between 2 25 and 2 05 ga ago we studied the pb o c isotope systematics of drill core samples from a highly silicified and aluminosilicate free sub unit of the mooidraai dolomite formation that yields well preserved micritic dolomite grains selective leaching of the silicified dolomite revealed a significant pb isotopic contrast between the carbonate and the quartz fraction the former shows only a narrow range of 208pb 204pb 35 35 35 58 which does not correlate with 206pb 204pb indicating the absence of th derived detrital pb the carbonate bound pb defines an isochron that corresponds to an age of 2394 26 ma 2 mswd 1 8 n 22 which we interpret as the diagenetic age of the mooidraai dolomite the 13ccarb values range from 0 51 to 0 64 pdb and confirm the previous data the 18ocarb values range from 2 08 to 0 18 pdb and are amongst the heaviest reported yet from early precambrian marine sedimentary carbonates the quartz fraction is considerably lighter than the carbonate fraction and shows 18oqtz values between 9 22 and 6 73 pdb 21 40 and 23 97 smow together with evidence from fluid inclusion microthermometry suggesting that the quartz formed at minimum temperatures between 130 and 155 c this indicates that the intense silicification was post depositional comparison of 18ocarb values of silicified and non silicified mooidraai samples suggests that the post depositional silicification had sealed the system and thereby protected the primary isotopic compositions of the micritic dolomite the heavy 18ocarb values suggest that 2 4 ga ago the oxygen isotopic composition of seawater differed only little from that of today the new pb pb carbonate age for the mooidraai dolomite indicates that the upper transvaal supergroup is about 200 my older than previously thought this explains the normal carbon isotopic composition of the mooidraai dolomite and suggests that the pronounced increase of the oxygen content in the precambrian earth s atmosphere that is seen in strata underneath the mooidraai dolomite occurred before 2 4 ga ago on this background it appears unlikely that the positive excursion of the carbon isotopic ratios of 2 25 2 05 ga old marine sedimentary carbonates is related to this increase in atmospheric oxygen

timing of the last glacial maximum from observed sea level minima during the last glacial maximum ice sheets covered large areas in northern latitudes and global temperatures were significantly lower than today but few direct estimates exist of the volume of the ice sheets or the timing and rates of change during their advance and retreat here we analyze four distinct sediment facies in the shallow tectonically stable bonaparte gulf australia each of which is characteristic of a distinct range in sea level to estimate the maximum volume of land based ice during the last glaciation and the timing of the initial melting phase we use faunal assemblages and preservation status of the sediments to distinguish open marine shallow marine marginal marine and brackish conditions and estimate the timing and the mass of the ice sheets using radiocarbon dating and glacio hydroisostatic modelling our results indicate that from at least 22 000 to 19 000 calendar years before present land based ice volume was at its maximum exceeding today s grounded ice sheets by 52 5 x 10 exp 6 cu km a rapid decrease in ice volume by about 10 percent within a few hundred years terminated the last glacial maximum at 19 000 250 years

late pleistocene piedmont glaciations in the eastern mediterranean insights from cosmogenic 36cl dating of hummocky moraines in southern turkey abstract we report the presence of late pleistocene piedmont glaciers represented by the largest hummocky moraine field observed in the eastern mediterranean the piedmont glaciers originated from the geyikdag ice cap 40 km 2 situated between 2350 and 2650 m above sea level a s l central tauride mountains of turkey and deeply carved the north facing hillslopes before reaching the namaras valley 2000 2050 m a s l the hummocky moraines resulted from in situ deposition of stagnant glacier ice where debris cover was heterogeneously distributed on the glacier surface thirty four boulders from hummocky disintegration lateral and terminal moraines from the namaras valley and the tributary susam valley 2100 2200 m a s l were dated by cosmogenic 36 cl surface exposure dating the moraine ages indicate three phases of deglaciation during the late pleistocene the oldest deglaciation occurred in the namaras valley at 18 0 1 1 ka ka thousand years ago towards the end of the last glacial maximum lgm and is recorded entirely by hummocky moraines we speculate that hummocky moraine forming processes with cycles of relief inversion gave rise to boulder apparent ages up to a few thousand years younger in our study area therefore 18 0 1 1 ka should be regarded as a minimum age with a probable true age much closer to the local lgm values 20 ka as observed in the surrounding mountains paleo piedmont glaciers also deposited several lateral moraines that are 50 m higher than the hummocky moraines although the lateral moraines probably represent the build up and the hummocky moraines the final phase of the same local lgm pulse both lateral moraines started to retreat from the late glacial 14 0 2 7 ka and gradually disappeared by mid holocene 5 2 1 0 ka encompassing the younger dryas yd stadial in the susam valley the late glacial is represented by a terminal moraine 13 4 1 5 ka the glacier retreat was very fast as indicated by an almost instantaneous disappearance of 5 km long susam valley glacier represented by disintegration and hummocky moraines 14 0 1 3 ka alternatively in case the oldest boulder age is taken into account the susam valley glacier can be also attributed to the late lgm 17 ka to late glacial 14 ka transition near the exit of the susam valley a right lateral moraine was deposited 11 6 1 3 ka ago confirming the presence of the yd in geyikdag comparable glacial chronologies were obtained from other turkish and mediterranean mountains

the glacial history of the dinaric alps montenegro abstract large areas of montenegro were glaciated during the pleistocene this paper presents evidence from the massifs of central montenegro including durmitor and sinjajevina moracke planine maganik prekornica and vojnik glacial deposits have been subdivided on the basis of morphostratigraphy and soil weathering and 31 u series ages from cemented tills provided a geochronological framework the largest glaciation occurred before 350 ka when a series of conjoined ice caps over the massifs of central montenegro covered a total area of nearly 1500 km 2 these formed during mis 12 and correspond with the largest skamnellian stage glaciations in greece to the south later middle pleistocene glaciations occurred during the penultimate glacial cycle correlating with the vlasian stage in greece mis 6 when ice caps covered an area of 720 km 2 over central montenegro there is also geochronological evidence of glacial deposits dating from the interval between mis 12 and mis 6 before the interglacial complex of mis 7 this glaciation appears to have been very similar in extent to that which occurred during mis 6 the last glacial cycle in central montenegro was characterised by valley and cirque glaciers covering a total area of 49 km 2 it is very likely that glaciers have been present in the mountains of central montenegro during every glacial cycle since a small glacier still survives today the smaller glaciers of the last glacial cycle are likely to have been associated with summer temperatures that were warmer than those of earlier cold stages the striking contrast in the extent and thickness of ice cover during the cold stages of the pleistocene has an important bearing on the geomorphological and biological evolution of the balkans

surface exposure dating of late pleistocene glaciations at the dedegol mountains lake beysehir sw turkey situated on the interior side of the western central taurus mountains of sw anatolia the muslu valley in the dedegol mountains mt dipoyraz 37 37 07 n 31 19 42 e 2992 m above sea level a s l represents the lower part of a formerly glaciated valley system of 7 5 km in length surface exposure ages with cosmogenic 10be and 26al date a glacier expansion out of the muslu valley 24 3 1 8 ka ago 1 confidence interval and give evidence for pronounced glacier advances prior to the global last glacial maximum lgm 21 2 ka deglaciation commenced no later than 17 7 1 4 ka ago superimposed climatic oscillations are indicated by glacier fluctuations dated around 19 8 1 6 ka and again 13 9 2 3 ka prior to the first major amelioration of climate the late pleistocene glacier chronology from the western central taurus mountains published lake level fluctuation data of palaeolake konya and lake beysehir and nearby pollen data on the central anatolian plateau reveal a conspicuous time correlation of palaeoenvironmental change copyright 2009 john wiley sons ltd

a reevaluation of cosmogenic 36cl production rates in terrestrial rocks we have measured 36cl in a suite of 33 rock samples having well constrained exposure histories and ages the 36cl production parameters were estimated by minimizing the squared deviations between the 36cl and independent ages yielding the following production parameters spallation and muon production from ca 2940 200 atoms 36cl mole ca 1 yr 1 spallation from k 6020 400 atoms 36cl mole k 1 yr 1 and neutron production in air 586 40 fast neutrons g air 1 yr 1 the new production constants for spallation on ca and thermal neutron activation are in good agreement with previous results but that for spallation on k is about 50 larger the past decade has seen a rapid growth in the application of terrestrial cosmogenic nuclides produced by cosmic ray particles within rocks at the earth surface nuclear cross sections particularly for reactions initiated by neutrons and muons are not known with sufficient accuracy so production rates for these nuclides must be empirically calibrated using independently dated geological samples direct production estimates for 36cl were first obtained by zreda et al 1991 who measured 36c1 in mineral separates from granitic boulders on moraine crests at chiatovitch creek in the eastern white mountains of california and nevada this data set was the best available at the time but since then several factors have prompted a reevaluation of the 36cl production rates one is that zreda et al 1994 have shown that there can be significant scatter of the cosmogenic ages of individual glacial boulders toward ages younger than that of the moraine due to soil erosion a second factor is that a much wider range of samples is now available for this purpose finally recent theoretical advances in calculating production by thermal neutron absorption liu et al 1994 now permit more realistic calibration of this reaction

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