the occurrence of hydromagnesite in bottom sediments from lake siling central tibet implications for the correlation among δ18o δ13c and particle density abstract a core 305 cm long taken from lake siling in central tibet is subdivided on the basis of carbonate composition into a dolomite zone 275 255 cm depth a dolomite aragonite zone 255 245 cm depth an aragonite zone 245 147 5 cm depth and a hydromagnesite aragonite zone 147 5 30 cm depth most core samples contain clay minerals quartz and low mg calcite six samples on terraces near the lakeshore consist of hydromagnesite with heavy o and c stable isotopic ratios and low density the hydromagnesite aragonite zone is characterized by a strong correlation among 18o 4 9 2 3 13c 5 5 8 0 and particle density 2 40 2 68 g cm3 this occurs when carbonates from the underlying aragonite zone characterized by lighter o and c isotopic ratios and high density are plotted against terrace samples of hydromagnesite the correlation in the hydromagnesite aragonite zone is attributed to the mixing of hydromagnesite with the aragonite zone sediments in varying proportions when compared with available 14c tams ages it appears that hydromagnesite was added to bottom sediments at about 5000 years bp at that time the lake began to shrink and formed lacustrine terraces because evaporation dominated over precipitation around the lake the depositional rate of the hydromagnesite aragonite zone 0 6 mm year was about six times faster than that of the aragonite zone 0 1 mm year due to formation of hydromagnesite within an arid climate the plausible sources of four carbonate minerals are as follows 1 aragonite is an inorganic carbonate from the lake water 2 hydromagnesite is an evaporitic carbonate on terraces 3 dolomite is a diagenetic carbonate and 4 low mg calcite is a detrital carbonate

stable isotope and radiocarbon balances of two tibetan lakes sumxi co longmu co from 13 000 bp hydrologic and climatic changes in western tibet since 13 000 bp are inferred from mineralogical and isotopic analyses of lacustrine sediments from the sumxi longmu co basin the interpretation is based on isotope data from the modern system carbon isotope chemistry is complex because of the presence of old limestones on the watershed sediments contain detrital and authigenic calcites the proportions of which are quantitatively estimated by x ray diffractometry stable isotope contents of the authigenic calcite are reconstructed from mass balance equations the high 13c contents up to 6 are attributed to an enhancement of calcite precipitation from water surface during freezing periods the 18o content is related to residence time and evaporation the validity of 14c dates obtained from different types of material is discussed our record in good agreement with paleobiological data from the same material shows the sudden establishment of wet warm conditions at 10 000 bp which culminated at 7500 6000 bp after a brief reversal event at 7900 bp maximum aridity for the whole holocene is recorded at 4300 bp

ice dams outburst floods and glacial incision at the western margin of the tibetan plateau a 100 k y chronology from the shyok valley karakoram some of the largest and most erosive floods on earth result from the failure of glacial dams while potentially cataclysmic ice dams are recognized to have repeatedly formed along ice sheet margins much less is known about the frequency and longevity of ice dams caused by mountain glaciers and their impact on landscape evolution here we present field observations and results from cosmogenic nuclide dating that allow reconstructing a 100 k y long history of glacial damming in the shyok valley eastern karakoram south asia our field observations provide evidence that asia s second longest glacier the siachen once extended for over 180 km and blocked the shyok river during the penultimate glacial period leading to upstream deposition of a more than 400 m thick fluvio lacustrine valley fill 10 be depth profile modeling indicates that glacial damming ended with the onset of the eemian interglacial and that the shyok river subsequently incised the valley fill at an average rate of 4 7 m k y 1 comparison with contemporary ice dammed lakes in the karakoram and elsewhere suggests recurring outburst floods during the aggradation period while over 25 cycles of fining upward lake deposits within the valley fill indicate impounding of floods from farther upstream despite prolonged damming the net effect of this and probably earlier damming episodes by the siachen glacier is dominated by glacial erosion in excess of fluvial incision as evidenced by a pronounced overdeepening that follows the glaciated valley reach strikingly similar overdeepened valleys at all major confluences of the shyok and indus rivers with karakoram tributaries indicate that glacial dams and subsequent outburst floods have been widespread and frequent in this region during the quaternary our study suggests that the interaction of karakoram glaciers with the shyok and indus rivers promoted valley incision and headward erosion into the western margin of the tibetan plateau

anisotropic behavior of grip ices and flow in central greenland abstract mechanical tests have been performed on strongly textured ice samples coming from a wide range of depths from 1328 down to 2868 m of the greenland ice core project grip for different sample orientations with respect to the prescribed stress in this way two directional viscosities corresponding to the easy glide and to the hard glide orientations were determined along the core the viscoplastic anisotropy gradually increases down to a depth of 2600 m and slightly decreases below revealing a clear correlation between rheology and texture the experimental mechanical response compares well with that predicted by the viscoplastic self consistent vpsc model the vpsc model is also applied to ice samples that exhibit an axisymmetric texture to show in more detail the sensitivity of the rheology to specific texture parameters this leads to a number of recommendations for future mechanical tests on anisotropic samples a large scale ice flow model is finally used to estimate the influence of ice anisotropy on the flow along the grip gisp2 flow line the particular mechanical behavior of deep grip ices in the stress regime corresponding to an ice divide leads to deformation rates that are highly sensitive to bedrock topography and texture pattern this feature is likely to favour the formation of stratigraphic disturbances in deep ice layers as observed in the last 300 m of both grip and gisp2 cores

glacio meteorological and isotopic studies along the egig line central greenland the geographical distribution of firn temperature annual accumulation rate as well as deuterium and oxygen 18 content in the firn were determined along an east west transect through central greenland this study is based on isotopic and chemical analyses of shallow firn cores at 18 sites along the egig line and high precision firn temperature measurements in 17 steam drilled boreholes along the eastern part of the transect the firn temperatures at 15 m depth range from 31 6 c at dome grip 3230 m a s l to 11 4 c at caecilia nunatak eastern ice margin at 1600 m a s l and 18 c at t05 near the western ice margin at 1900 ma s l the temperature altitude gradient changes from 0 7 c 100 m 1 in the ice divide region to 1 1 c 100 m 1 in the eastern part of the dry snow zone the temperature latitude gradient in the central part of the egig line is 0 7 c lat 1 the average annual accumulation decreases significantly from the west 47 cm a 1 water equivalent at t05 towards the ice divide 20 25 cm a 1 water equivalent from t99 to t43 accumulation rates are constantly low east of the ice divide 23 17 cm a 1 water equivalent thus dividing central greenland into two climatologically different regions the average 18 o and d values along the whole egig line reflect the well known temperature dependence for greenland very well e g 18 o t m 0 69 c 1 different regression lines for the western and eastern part however should be applied unlike the mean annual temperature the isotopic minimum along the egig line lies east of the ice divide this geographical distribution supports the choice of different water vapour trajectories in central greenland for the west and for the east significant parts of the water precipitated over the western slope are attributed to cyclonic systems entering greenland from the west the deuterium excess shows no significant geographical trend but a uniform seasonal variation at all sites along the egig line suggesting equal contributions from vapour source areas of the water precipitated over central greenland

pollen and diatom inferred climatic and hydrological changes in sumxi co basin western tibet since 13 000 yr b p although the tibetan plateau greatly influences the atmospheric circulation of the nortbern hemisphere few continuous paleoclimatic records are available from the plateau a 13 000 yr pollen and diatom record from the sumxi longmu co basin in western tibet gives information on major changes both in regional vegetation and in local hydrology after the basin first filled ca 13 000 yr b p a dry spell occurred about 10 500 yr b p within the interval spanned by the european younger dryas chronozone a major environmental change occurred suddenly at 10 000 yr b p with the establishment of wet conditions and was followed by a long term trend toward maximum aridity which lasted approximately 6000 yr short term oscillations are superimposed on this general climatic change with a major reversal event about 8000 yr b p and a second wet pulse leading to a maximum lake volume ca 7500 6000 yr b p maximum aridity occurred 4300 yr b p the major environmental fluctuations recorded at sumxi longmu co appear in phase with climatic changes recognized in north tropical africa suggesting that the 8000 to 7000 yr b p event was caused by an abrupt disequilibrium in the climatic system as was the younger dryas and possibly the 4300 yr b p event

holocene lake stages and thermokarst dynamics in a discontinuous permafrost affected region north eastern tibetan plateau abstract sediments of a thermokarst system on the north eastern tibetan plateau were studied to infer changes in the lacustrine depositional environment related to climatic changes since the early holocene the thermokarst pond with a length of 360 m is situated in a 14 5 6 km tectonically unaffected intermontane basin which is underlain by discontinuous permafrost a lake sediment core and bankside lacustrine onshore deposits were analysed additionally fossil lake sediments were investigated which document a former lake level high stand the sediments are mainly composed of marls with variable amounts of silt carbonate micrite and organic matter on the basis of sedimentological grain size data geochemical xrf mineralogical xrd and micropaleontological data ostracods and chironomide assemblages a reconstruction of a paleolake environment was achieved lacustrine sediments with endogenic carbonate precipitation suggest a lacustrine environment since at least 19 0 cal ka bp however because of relocation and reworking processes in the lake the sediments did not provide distinct information about the ultimate formation of the lake the high amount of endogenic carbonate suggests prolonged still water conditions at about 9 3 cal ka bp ostracod shells and chironomid head capsules in fossil lake sediments indicate at least one former lake level high stand which were developed between the early and middle holocene from the late holocene the area was possibly characterized by a lake level decline documented by a hiatus between lacustrine sediments and a reworked loess or loess like horizon after the lake level decline and the following warming period the area was affected by thermally induced subsidence and a re flooding of the basin because of thawing permafrost

analysis of scarp profiles evaluation of errors in morphologic dating morphologic analysis of scarp degradation can be used quantitatively to determine relative ages of different scarps formed in cohesionless materials under the same climatic conditions scarps of tectonic origin as well as wavecut or rivercut terraces can be treated as topographic impulses that are attenuated by surface erosional processes this morphological evolution can be modelled as the convolution of the initial shape with erosion or degradation function whose width increases with time such modeling applies well to scarps less than 10m high formed in unconsolidated fanglomerates to a good approximation the degradation function is gaussian with a variance measuring the degree of rounding of the initial shape this geometric parameter can be called the degradation coefficient a synthetic experiment shows that the degradation coefficient can be obtained by least squares fitting of profiles levelled perpendicular to the scarp gravitational collapse of the free face is accounted for by assuming initial scarp slopes at the angle of repose of the cohesionless materials 30 35 uncertainties in the measured profiles result in an uncertainty in degradation coefficient that can be evaluated graphically because the degradation coefficient is sensitive to the regional slope and to three dimensional processes gullying loess accumulation stream incision etc a reliable and accurate determination of degradation coefficient requires several long profiles across the same scarp the linear diffusion model of scarp degradation is a gaussian model in which the degradation coefficient is proportional to numerical age in that case absolute dating requires only determination of the propotionality constant called the mass diffusivity constant for holocene scarps a few meters high in loose alluvium under arid climatic conditions mass diffusivity constants generally range between 1 and 6 m 2 kyr morphologic analysis is a reliable method to compare ages of different scarps in a given area and it can provide approximate absolute ages of holocene scarplike landforms

a 13 000 year climate record from western tibet although the tibetan plateau is important in influencing the atmospheric circulation of the northern hemisphere1 3 there are only a few continuous palaeoclimate records available and these are limited to the plateau s northeastern margin4 6 here we present a 13 000 yr record from sumxi co western tibet constructed from both lake core and shoreline studies which shows that conditions in the early middle holocene were warmer and wetter than at present these results confirm model predictions of an intensified monsoon over the region at 9 000 yr bp owing to an orbitally induced increase in summer insolation7 8 we also find evidence for warm humid pulses at 12 500 and 10 000 yr bp in phase with the steps of the last deglaciation and for a return to cold dry conditions at 11 10 000 yr bp none of which can be explained by orbital variations the existence of the cold episode confirms that the cooling associated with the younger dry as event occurred in continental china6 9 and provides further evidence of the global nature of this event10

environmental conditions in the donggi cona lake catchment ne tibetan plateau based on factor analysis of geochemical data abstract we present a methodological approach for the characterization of spatial variations of environmental and transport conditions based on geochemical data we analyzed the geochemical characteristics of terrestrial sediments on an alluvial fan and adjacent areas in the eastern part of the donggi cona catchment the geochemical characteristics of the sediment samples were measured using x ray fluorescence spectroscopy a factor model based on nine elements sr rb zn fe mn v ti ca and k gave the best results using factor analysis our results show that factor analysis of geochemical data can explain spatial variations in the catchment even within a relatively small area with more or less constant climatological conditions the four most important factors of our model explain 68 4 of the total variance within the dataset the four factors represent a carbonate weathering and two redox condition signals respectively the spatial distribution of factor loadings points to regions with specific environmental conditions these regions show a certain carbonate input or production factor 1 specific weathering rates factor 2 or specific redox conditions factors 3 and 4 the factor loadings of the first and second factor are used to reclassify the data into four groups being dune loess lake and fluvial gravel sediments this reclassification confirms or improves the prior field classification in 70 of all cases for 23 of the samples the model gave a different interpretation the factor loadings in these cases represent the origin of the material rather than its present sediment type thereby giving valuable information about sediment provenance since the third and fourth factor seem more dependent on location they do not differentiate between different sediment types multivariate statistics of the geochemistry of terrestrial sediments i allows the subdivision of samples into different sediment types ii indicates dominant regions for weathering carbonate production and manganese washout and iii gives suggestions about the provenance of the sediments this is the first detailed study of geochemical parameters on the tibetan plateau not focusing on lake cores resulting in a spatial characterization of the local geochemistry this gives insight in the sediment transport connections between the catchment and the lake illustrating the value of terrestrial sediments as additional indicator of environmental variations

pulsed strain release on the altyn tagh fault northwest china abstract earthquake recurrence models assume that major surface rupturing earthquakes are followed by periods of reduced rupture probability as stress rebuilds although purely periodic time or slip predictable rupture models are known to be oversimplifications a paucity of long records of fault slip clouds understanding of fault behavior and earthquake recurrence over multiple ruptures here we report a 16 kyr history of fault slip including a pulse of accelerated slip from 6 4 to 6 0 ka determined using a monte carlo analysis of well dated offset landforms along the central altyn tagh strike slip fault atf in northwest china this pulse punctuates a median rate of 8 1 1 2 0 9 mm a and likely resulted from either a flurry of temporally clustered mw 7 5 ground rupturing earthquakes or a single large mw 8 2 earthquake the clustered earthquake scenario implies rapid re rupture of a fault reach 195 km long and indicates decoupled rates of elastic strain energy accumulation versus dissipation conceptualized as a crustal stress battery if the pulse reflects a single event slip magnitude scaling implies that it ruptured much of the atf with slip similar to or exceeding the largest documented historical ruptures both scenarios indicate fault rupture behavior that deviates from classic time or slip predictable models

recumbent folding of divide arches in response to unsteady ice divide migration arches in stratigraphic layers directly under a flow divide raymond bumps are predicted by models of steady ice sheet flow and have been observed in several ice domes here we model the evolution of these layers when a formerly stationary divide migrates rapidly to a new position then again becomes stationary leaving the arched layers in a flank position as they are then carried downstream with the flow these abandoned arches can develop into recumbent folds these folds can occur over a wide range of divide migration speeds the shearing flow that produces these recumbent folds also distributes the folded layers over a wide distance downstream from the original divide location if the divide offset is abrupt pre cores or material lines comprising core relative isochrones can be used to quickly identify portions of an abandoned raymond bump that would be overturned at any future ice core site downstream if as appears to be the case in greenland the divide is never stable long enough to produce a mature arch folds of this type would not occur the most likely place to find such folds might be the flank of an ice ridge bounded by unsteady ice streams

fluvial terrace riser degradation and determination of slip rates on strike slip faults an example from the kunlun fault china 1 the use of displaced fluvial terrace risers to determine slip rates along strike slip faults depends on knowing when risers become passive markers typically chronologic constraints from terrace deposits only place bounds on this age consequent slip rates may be highly uncertain along the kunlun fault in northeastern tibet we demonstrate that consideration of riser degradation can augment terrace chronology and improve the precision of slip rate estimates multiple offset risers and 14c age determinations from terrace treads yield a narrow range of slip rates of 4 5 0 5 m kyr at one site but permit a wide range of allowable rates 3 10 m kyr in an adjacent drainage using a locally calibrated sediment transport rate of 3 2 5 4 m2 ka we show that riser morphology at this latter site is consistent with degradation since abandonment of the upper terrace tread our analysis indicates that slip rates at this site are 5 2 m kyr

u series dating of the travertine depositing near the rongma hot springs in northern tibet china and its paleoclimatic implication abstract high resolution age data obtained by travertine dating is necessary for the reconstruction of paleoclimate and palaeohydrology near rongma hot springs in northern tibet u series dating of a travertine cone depositing in the study area was used the results show that the u contents of the travertine samples range from 0 298 to 1 363 ppm and the 234 u 238 u is in the rage of 1 475 1 700 indicating that the deposition of travertine near the rongma hot springs is stable and continuous the age of the travertine samples ranges from 11 500 to 4600 a corresponding to the first stage of mis 1 combination of the first stage of mis with the effect of monsoon climate of the indian ocean in the same period implies a warm and humid climate during the deposition of the travertines in the study area the u series age range in the rongma hot springs area is coincident with that of the travertine in southern tibet the age data detected from travertine cone help in understanding the evolution of hydrogeology of the hot spring area high resolution age data must be acquired before travertine can be confidently used as a paleoclimatic archive

electrical conductivity measurements from the gisp2 and grip greenland ice cores the direct current electrical conductivity of glacial ice depends on its acidity1 3 and can also indicate changes in climate as ice formed in cold dusty periods has a high concentration of alkaline dust1 4 5 which significantly reduces the conductivity6 7 compared to warmer less dusty periods here we present electrical conductivity records for the greenland ice sheet project 2 gisp2 and greenland ice core project grip ice cores drilled 28 km apart to enable direct comparison of the results the upper parts of both records are consistent with previous evidence from other greenland cores4 8 12 for a stable greenland climate during the holocene and a series of warm events punctuating the last glacial period however there is a significant discrepancy between the two records in the bottom 10 of the cores calling into question recent reports of climate variability in the last interglacial4 8 and the penultimate glaciation8 at this stage it is too early to say what exactly is causing the discrepancy although ice flow may have introduced some discontinuities into the records further work will be necessary to establish how much climatic information it will eventually be possible to extract from the lower parts of the two cores

the buildup of the ice sheet in central greenland a study of the isotopic and gas composition of the basal silty ice recovered by the greenland ice core project grip core indicates that local ice formed in the absence of the greenland ice sheet is still preserved at summit such ice developed most probably within a peat deposit in a permafrost environment this local ice was subsequently intimately mixed with glacier ice from an advancing ice sheet progressing on the site this is in agreement with the highland origin and windward growth hypothesis for ice sheet development not for an in situ or regional growth from snowbanks the basal ice from the grip core possibly dates back to the original buildup of the greenland ice sheet 2 4 million years ago

ice fabric evolution process understood from anisotropic distribution of a axis orientation on the grip greenland ice core to better understand how ice fabric evolves in polar ice sheets we use x ray diffraction to measure ice crystal orientations x ray measurement equipment which can measure the orientation of the c axis and a axis of each crystal in a thin section with high measurement accuracy was developed in this study we present a axes orientation distribution of the deep part of the grip greenland summit ice core at some depths we find an anisotropic distribution of a axes orientation long term uniaxial compression tests are also carried out on the grip ice core to investigate the ice fabric evolution process the c axis orientation distribution develops into a stronger single maximum as the strain increases up to about 20 strain we find that the a axes of each grain also tend to cluster close to nearly a mutual direction we discuss the development process of ice fabrics taking into consideration the distribution of the a axis orientations it is suggested that these fabrics may be attributed to a local simple shear deformation in the deep part of an ice sheet

a bound on the viscosity of the tibetan crust from the horizontality of palaeolake shorelines abstract palaeoshorelines around four large lakes in central tibet record a latest pleistocene to holocene high stand during which the lakes were filled 150 200 m more deeply than they are at present gps measurements of shoreline elevations around zhari namtso show that they are horizontal to within 2 m at the 2 level measurements of height made by combining shuttle radar topographic mission elevations with google earth imagery of shorelines around zhari namtso tangra yumtso taro tso and ngangla ringtso show that all the palaeoshorelines are horizontal within measurement uncertainty support of the lake loads by elastic stresses can explain the horizontality of the shorelines only if the equivalent elastic thickness of the crust exceeds 15 25 km the observations are more plausibly explained by support of the lake loads through viscous stresses in the middle to lower crust this support requires that the viscosity of the middle to lower crust is at least 10 19 10 20 pa s these values are consistent with estimates from studies of post seismic relaxation after large earthquakes of the region and are higher by two orders of magnitude than would permit significant lateral flux of material through a channel in the middle to lower crust

evaluating the size and extent of paleolakes in central tibet during the late pleistocene subhorizontal lake shorelines allow a geodynamic test of the size and extent of a hypothesized paleo lake in central tibet the east qiangtang lake eql during the last interglacial period marine isotope stage mis 5e reconstructions based on relict lake deposits suggest that the eql would have been 400 m deep and over 66 000 km2 models of flexural rebound driven by lake recession predict that shorelines near the eql center at the present day location of siling co would have rebounded 60 90 m above their initial elevation new 36cl chronology of the highest relict shorelines around siling co indicates that they reflect lake levels between 110 190 ka these shorelines however are presently 300 m below their predicted elevations implying a substantially smaller water load our results reveal that the expansion of tibetan lakes during mis 5e was relatively limited instead individual lakes were supplied by river networks much as they are today

deformation in the vicinity of ice divides calculs numeriques aux elements finis de la variation de la vitesse horizontale avec la profondeur au voisinage d une difference de la glace symetrique isotherme et ou glissement sur un lit plat

sensitivity of the ice divide position in greenland to climate change model calculations of depth age relations for deep ice cores in central greenland are sensitive to stability of the ice divide position in addition the folding of layers observed in the deep ice could be instigated by divide migration changing the velocity and particle paths of ice flow we use simple steady state calculations to show that lateral divide migration of between 10 km and 50 km and elevation change of approximately 100 m is likely on glacial interglacial time scales enough to affect model dating the ice divide location appears to be most sensitive to the position of the ice sheet margins by contrast the ice divide elevation is most sensitive to the accumulation rate the temperature profile and the ice stiffness profile

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

the flickering switch of late pleistocene climate change polar ice contains a unique record of past climate variations previous greenland ice cores have documented relatively warm interstadial periods during the last glaciation and short centuryscale returns to colder conditions during the glacial to interglacial warming see for example ref 1 these climate features have also been observed to varying degrees in ocean sediment cores2 4 and terrestrial pollen and insect records5 7 here we report electrical conductivity measurements from a new greenland ice core which confirm these previous observations and also reveal a hitherto unrecognized mode of rapid climate variation fluctuations in ice conductivity on the scales of 5 20 years reflect rapid oscillations in the dust content of the atmosphere this flickering between two preferred states would seem to require extremely rapid reorganizations in atmospheric circulation

crustal rheology of southern tibet constrained from lake induced viscoelastic deformation we probe the rheology of the tibetan lithosphere using the rebound that accompanied climate driven lake level variations at the modern decadal time scale we used deformation around siling tso measured from insar at the millennial time scale we use holocenepaleoshorelines around siling tso and zhari nam tso we use chronological constraints from the literature and digital elevation models to constrain their ages and geometry we observe a small post highstand subsidence of the area near the center of mass of the paleolake load and a low amplitude short wavelength outer bulge in the context of a model consisting of an elastic lid over a viscous channel with a rigid base these observations preclude the existence of a thick low viscosity channel and require a thin elastic lid based on monte carlo inversion we constrain the range of possible equivalent elastic thickness of the lid 20 km and a 20 km thick channel with lower crustal viscosity 10 20 pa s in both cases the interferences in space of the deformation signals induced by the lakes geometry and in time through the viscoelastic response to the lake level variations results in limited distortion of the paleo shorelines while the elastic lid in the upper crust needs in any case to be thin 20 km inferred in some previous studies of holocene paleoshorelines in the longer term the effective elastic thickness of the lithosphere must drop asymptotically to the value of the elastic lid in the upper crust 10 km this explains the low effective elastic thickness derived from gravity studies

abrupt increase in greenland snow accumulation at the end of the younger dryas event the warming at the end of the last glaciation was characterized by a series of abrupt returns to glacial climate the best known of which is the younger dryas event1 despite much study of the causes of this event and the mechanisms by which it ended many questions remain unresolved1 oxygen isotope data from greenland ice cores2 4 suggest that the younger dryas ended abruptly over a period of about 50 years dust concentrations2 4 in these cores show an even more rapid transition 20 years this extremely short timescale places severe constraints on the mechanisms underlying the transition but dust concentrations can reflect subtle changes in atmospheric circulation which need not be associated with a large change in climate here we present results from a new greenland ice core gisp2 showing that snow accumulation doubled rapidly from the younger dryas event to the subsequent preboreal interval possibly in one to three years we also find that the accumulation rate change from the oldest dryas to the bo11ing allerod warm period was large and abrupt the extreme rapidity of these changes in a variable that directly represents regional climate implies that the events at the end of the last glaciation may have been responses to some kind of threshold or trigger in the north atlantic climate system

potential for stratigraphic folding near ice sheet centers lack of agreement between the deep portions of the greenland icecore project grip and greenland ice sheet project ii gisp2 ice cores from central greenland suggests that folds may disrupt annual layering even near ice divides we use a simple kinematic flow model to delineate regions where slope disturbances wrinkles introduced into the layering could overturn into recumbent folds and where they would flatten leaving the stratigraphic record intact wrinkles are likely to originate from flow disturbances caused internally by inhomogeneities and anisotropy in the ice rheological properties rather than from residual surface structures sastrugi or from open folds associated with transient flow over bed topography if wrinkles are preferentially created in anisotropic ice under divides where the resolved shear stress in the easy glide direction can be weak and variable then the deep intact climate record at dye 3 may result from its greater distance from the divide alternatively the larger simple shear at dye 3 may rapidly overturn wrinkles so that they are not recognizable as folds the ice core record from siple dome may be intact over a greater fraction of its depth compared to the central greenland records if its flat bedrock precludes fluctuations in the stress orientation near the divide

climate variability on the south eastern tibetan plateau since the lateglacial based on a multiproxy approach from lake naleng comparing pollen and non pollen signals a multi proxy lateglacial environmental record is described from lake naleng 31 10 degrees n 99 75 degrees e 4200 m above sea level situated on south eastern tibetan plateau to gain deeper insights into the hydrological and palaeoclimate development since 17 7 cal ka bp palynological reconstructions of variations in mean annual precipitation map and temperature mat sedimentological data and sediment chemistry including weathering indicators provide a multi faceted picture of local and regional environmental changes since the lateglacial principal component analyses of all parameters provide information on interrelationships between each parameters which help to evaluate their traceability to temperature and precipitation and to estimate their usability as proxy indicators for local and or regional variations during the lateglacial from 17 7 to 14 0 cal ka bp lake naleng experienced cold and dry climate conditions with low biological productivity and supply of unaltered fine grained material due to the high supply of glacier milk during the second half of the lateglacial climate conditions changed abruptly increases in mat from 4 to 2 2 degrees c and map from 500 mm to 820 mm between 14 0 and 13 0 cal ka bp indicate a climate amelioration this time interval can be correlated to the bolling allerod b a warming period in the north atlantic region and is followed by the younger dryas cold reversal indicated by abrupt decreases of mat from 2 2 to 5 degrees c and map from 820 to 650 mm the onset of the holocene at about 11 5 cal ka bp is indicated by rises in reconstructed mat from 5 to about 03 degrees c and map from 600 mm to 950 mm which led to an increased supply of weathered material and higher biological productivity between 5 0 and 3 0 cal ka bp mat increases to about 0 2 degrees c and map rises to maximum values of about 1000 mm followed by slightly decreasing mat and map between 3 0 and 0 cal ka bp the biogeochemical parameters total organic carbon toc c n delta c 13 org and weathering indicators e g the chemical index of alteration cia and sr ba are directly erosion of soils or indirectly changing provenance related to moisture availability on the south eastern tp and shows matching regional climate oscillations since the lateglacial in comparison to other lateglacial records from the tp map reconstructions from lake naleng indicate wetter climate conditions in the south eastern part of the tp and dryer conditions farther away from moisture sources c 2015 elsevier ltd all rights reserved

comparison of oxygen isotope records from the gisp2 and grip greenland ice cores recent results1 2 from the greenland ice core project grip summit ice core suggest that the climate in greenland has been remarkably stable during the holocene but was extremely unstable for the time period represented by the rest of the core spanning the last two glaciations and the intervening eemian inter glacial here we present the complete oxygen isotope record for the greenland ice sheet project 2 gisp2 core drilled 28 km west of the grip core we observe large rapid climate fluctuations throughout the last glacial period which closely match those reported for the grip core however in the bottom 10 of the cores spanning the eemian interglacial and the previous glacia tion there are significant differences between the two records it is possible that ice flow may have altered the chronological sequences of the stratigraphy for the bottom part of one or both of the cores considerable further work will be necessary to evaluate the likelihood of this and the extent to which it will still be possible to extract meaningful climate information from the lowest sections of the cores

cosmogenic 10be inferred lake level changes in sumxi co basin western tibet abstract most tibetan lakes are surrounded by conspicuous regressive shorelines attesting to high water levels in the past concentrations of the in situ produced cosmogenic radionuclide 10 be in bedrock from the three highest terraces surrounding sumxi co situated in western kunlun indicate that the highest lake level appeared before 10 000 11 500 yr and most likely between 11 000 and 12 800 yr younger ages for the two lower terraces imply regression of sumxi co during the early mid holocene the concurrency of the highest lake level with orbitally induced maximum northern hemisphere summer insolation suggests that the increase in water supply to sumxi co was most likely associated with increased recharge from melting glaciers we conclude that the enhanced indian monsoon during the early holocene did not penetrate the tibetan plateau and affect the northwestern part of tibet significantly

response of the east antarctica ice sheet to sea level rise a nonsteady ice flow model that explicitly includes longitudinal stresses is used to study the effects of marginal changes on the interior portions of an ice sheet sea level rise is found to cause a wave of thinning to propagate upglacier and this wave is slowed diffused and damped at the ice divide at dome c in central east antarctica post wisconsinan sea level rise is modeled as having caused 110 m thinning with response now 70 complete if accumulation rates increased by 10 at the same time then calculated net thinning has been only 75 m ice sheet response to changes in sea level and accumulation rate is relatively rapid response to temperature changes is an order of magnitude slower response times are shorter if the marginal position of an ice sheet is controlled

hydrological and geomorphological basin and catchment characteristics of lake nam co south central tibet abstract for lake nam co south central tibetan plateau geomorphological features and hydrological characteristics have been compiled to provide an overview of the landscape character information is based on primary data collected during field campaigns in the summers of 2005 and 2006 and secondary data as available from satellite images nam co is located at the intersection of the westerlies the winter monsoon the south west asian monsoon and the east asian monsoon it is situated in a graben structure landforms of the drainage basin are highly influenced by tectonics superimposed by fluvial and periglacial processes and locally by glacial and aeolian processes its drainage basin is endorheic thus lake level is primarily controlled by the balance between input through precipitation or inflow streams and evaporation as the output the significance of lake water evaporation is evidenced by the salinity and measurable increase of 2h and 18o of the lake water when compared to the inflow streams and precipitation maximum conductivities total 431 s mm 1 for the inflowing river waters and 1920 s mm 1 for the lake water overall the solute contents are low and characterise nam co as a pristine environment dominant ions are calcium and bicarbonate in river waters sulphate contents are relatively high in nam co and the lake is also characterised by high boron values a significant decrease in lake water salinity during the past two decades is likely related to increased freshwater input

greenland ice core records and rapid climate change long ice cores from greenland yield records of annually resolved climate change for the past ten to twenty thousand years and decadal resolution for one hundred thousand years or more these cores are ideally suited to determine the rapidity with which major climate changes occur the termination of the younger dryas which marks the end of the last glacial period appears to have occurred in less than a human lifetime in terms of oxygen isotopic evidence a proxy for temperature in less than a generation 20 years for dust content and deuterium excess proxies for winds and sea surface conditions and in only a few years for the accumulation rate of snow similarly rapid changes have been observed for stadial interstadial climate shifts dansgaard oeschger cycles which punctuate the climate of the last glacial period these changes appear to be too rapid to be attributed to external orbital forcings and may result from internal instabilities in the earth s atmosphere ocean system or periodic massive iceberg discharges associated with ice sheet instability heinrich events in contrast the holocene climate of the arctic appears to have been relatively stable however the potential for unstable interglacials with very rapid shortlived climatic deteriorations has been raised by results from the lower part of the grip ice core these results have not been confirmed by other ice cores notably the nearby gisp2 core evidence from other records of climate during the eemian interglacial have yielded mixed results and the potential for rapid climate change during interglacial periods remains one of the most intriguing gaps in our understanding of the nature of major quaternary climate change

sensitivity of a greenland ice sheet model to ice flow and ablation parameters consequences for the evolution through the last climatic cycle sensitivity experiments are conducted to test the influence of poorly known model parameters on the simulation of the greenland ice sheet by means of a three dimensional numerical model including the mechanical and thermal processes within the ice two types of experiments are performed steady state climatic conditions and simulations over the last climatic cycle with a climatic forcing derived from the grip record the experiments show that the maximum altitude of the ice sheet depends on the ice flow parameters deformation and sliding law coefficients geothermal flux and that it is low when the ice flow is fast on the other hand the maximum altitude is not sensitive to the ablation strength and consequently during the climatic cycle it is driven by changes in accumulation rate the ice sheet extension shows the opposite sensitivity it is barely affected by ice flow velocity and the ice covered area is smaller for large ablation coefficients for colder climates when there is no ablation the ice sheet extension depends on the sea level an interesting result is that the variations with time of the altitude at the ice divide summit do not depend on the parameters we tested the present modelled ice sheets resulting from the climatic cycle experiments are compared with the present measured ice sheet in order to find the set of parameters that gives the best fit between modelled and measured geometry it seems that compared to the parameter set most commonly used higher ablation rate coefficents must be used

steady state characteristics of the greenland ice sheet under different climates the greenland ice sheet is modelled to simulate its extent and volume in warmer climates and also to find out whether the ice sheet would re form on theice free bedrock under present climatic conditions the ice sheet model is a three dimensional thermo mechanical model with a fine resolution grid thebedrock surface beneath the ice sheet was mapped using radio echo sounding measurements by the electromagnetic institute copenhagen the modelexperiments show that increased temperature will result in ice margin retreat but the ice sheet is relatively stable it takes a rather high temperature rise of atleast 6ic for the ice sheet to disappear completely which indicates that the ice sheet probably survived the last interglacial also it appears that the greenlandice sheet is not a mere relict ice mass from a previously colder climate but that the ice sheet will still re form on the bare bedrock under the present or evenslightly warmer climatic conditions

holocene lake level variations on the qinghai tibetan plateau paleoshorelines indicative of multiple high lake stages can be found around many lakes on the qinghai tibetan plateau qtp conspicuous paleoshorelines associated with the most recent highstands can be readily observed on satellite images and during field investigations however earlier paleoshorelines have been identified at only a few sites around these lakes due to a lack of clear shoreline features and limited spatial extension of those that can be identified we investigated past lake highstands using published chronologies aster dem google earth and arcgis 9 2 software these data suggest that 1 paleoshorelines of the most recent lake highstand were formed during the holocene 2 during this highstand lake levels reached more than 90 60 90 and 30 60 m higher than present in the central gangdise and western qiangtang areas while in the eastern qiangtang holxil yushu and northeastern qtp areas highstands did not exceed 30 m above modern 3 during the early holocene some lakes on the southwestern qtp combined to form large lakes while lakes in the inland areas of the qtp were only a little larger during the holocene high lake level stages

dating quaternary fault scarps formed in alluvium using morphologic parameters abstract ages of fault scarps as well as those other types of transport limited slopes can be estimated by comparing their morphology with the morphology of scarps of known age age estimates are derived by fitting the scarp profiles to synthetic profiles generated using a diffusion equation or alternatively by classification using a linear discriminant function the usefulness of morphology derived age estimates depends on the relative importance of non age related morphologic variation data from more than 200 scarp profiles demonstrate that morphologic variation not related to scarp age can introduce significant uncertainties into morphology derived age estimates

evolution of permafrost on the qinghai xizang tibet plateau since the end of the late pleistocene 1 the present distribution of permafrost on the qinghai xizang tibet plateau qtp is largely a relict of the permafrost formed during the late pleistocene it has been degrading and shrinking in areal extent under the fluctuating climates with a general trend of warming during the holocene the major criteria for the occurrence of relict permafrost include the remnants of ancient buried permafrost relict permafrost tables thawed sandwiches taliks thick layered ground ice and periglacial phenomena such as pingo scars cryoturbations primary sand and clayey silt wedges ice wedge casts aeolian sand dunes and loesses thick layers of peat and humic soils on the basis of 14c dating of soils comprehensive analyses and comparisons of the spatiotemporal distribution of relict and modern permafrost and periglacial phenomena the evolution of permafrost and periglacial environments since the late pleistocene was divided into seven stages 1 the cold period at the end of the late pleistocene 35 000 to 10 800 years b p 2 the period of significant climatic change during the early holocene 10 800 to 8500 7000 years b p 3 the megathermal period in the middle holocene 8500 7000 to 4000 3000 years b p 4 the cold period in the late holocene 4000 3000 to 1000 years b p 5 the warm period in the later holocene 1000 to 500 years b p 6 the little ice age 500 to 100 years b p and 7 the recent warming period 100 years b p to present the conditions for permafrost development distribution and the paleoclimates and paleoenvironments are discussed for each stage

stochastic perturbation of divide position perturbation of divide position is considered by a linearization about the vialov nye solution and also about related solutions with 0 1 relief relaxation times of one sixteenth the fundamental thickness accumulation rate time scale are found for the vialov nye configuration while substantial basal topography can halve the rate of relaxation steady divide position is most sensitive to anti symmetric accumulation rate distributions near the divide but transient divide motion is most strongly excited by anti symmetric accumulation rate variations halfway between the margin and the divide relaxation times for the antarctic peninsula divide position are estimated to be around 200 years while the larger greenland ice sheet has a divide position relaxation time of around 600 years modelling accumulation rate as a white noise process permits analysis of divide perturbation as a stochastic ornstein uhlenbeck process where the standard deviation of the response is proportional to the standard deviation of the forcing if observed accumulation rate variability in the antarctic peninsula were antisymmetric about the divide it would be sufficient to force the divide position to fluctuate with standard deviation 10 20 times the depth of the ice sheet there appears to be sufficient noise to cause raymond bumps to be spread significantly more data on the statistical variation of accumulation with position are needed random forcing will increase the complexity of any fold structures created in the divide region and in particular the number of such structures intersecting any borehole

oxygen and hydrogen isotopic ratios of water in precipitation beyond paleothermometry paleoclimatologists face a dilemma no sedimentary proxy is a pure recorder of quantitative climate information yet climate modelers and policy makers increasingly seek quantitative comparisons between instrumentally documented possibly anthropogenic climate changes and those produced naturally in the past in the second edition of his influential book bradley 1999 p 6 discussed the calibration of proxy records to learn past climate changes calibration involves using modern climatic records and proxy materials to understand how and to what extent proxy materials are climate dependent it is assumed that the modern relationships observed have operated unchanged throughout the period of interest the principle of uniformitarianism in other words one relates the characteristics of sediment to climate at different places for one time or at a place for short times and then uses that relation plus characteristics of older sediments to estimate the climatic conditions that produced those sedimentary characteristics bradley 1999 then extensively discussed the difficulties in applying this methodology in a complex world with imperfect recorders nonetheless the goal of using calibrated proxies for quantitative as well as qualitative paleoclimatic reconstruction is clear a prominent recent use of calibrated paleoclimatic data is the assessment of whether the probably anthropogenic warming of the latter 20th century moved beyond the band of natural variability of the prevailing climate bradley 2000 combined recent instrumental records with several longer proxy based reconstructions of surface temperature including that of mann et al 1999 based on this composite data set bradley 2000 argued that temperatures in the late 20th century were unique in the context of the entire millennium the proxy records were primarily based on tree ring data but included isotopic and major element geochemistry of corals and occurrence of melt layers and isotopic ratios of water in ice cores however broecker 2001 questioned the basis for the reconstruction of

towards a gis assessment of numerical ice sheet model performance using geomorphological data a major difficulty in assimilating geomorphological information with ice sheet models is the lack of a consistent methodology to systematically compare model output and field data as an initial step in establishing a quantitative comparison methodology automated proximity and conformity analysis apca and automated flow direction analysis afda have been developed to assess the level of correspondence between modelled ice extent and ice marginal features such as end moraines as well as between modelled basal flow directions and palaeo flow direction indicators such as glacial lineations to illustrate the potential of such an approach an ensemble suite of 40 numerical simulations of the fennoscandian ice sheet were compared to end moraines of the last glacial maximum and the younger dryas and to glacial lineations in northern sweden using apca and afda model experiments evaluated in this manner were ranked according to level of correspondence such an approach holds considerable promise for optimizing the parameter space and coherence of ice flow models by automated quantitative assessment of multiple ensemble experiments against a database of geological or glaciological evidence

holocene climate development on the central tibetan plateau a sedimentary record from cuoe lake abstract we present a climate record for central tibet spanning the time between 10500 and 1600 cal years bp from a 5 3 m long sediment core from cuoe lake the analyses show that high sr concentration high toc content high c n ratio and enriched 13c of lake sediments corresponded to stronger chemical weathering high productivity high lake level and warmer wetter climate and vice versa as revealed by the variations of these proxies the climatic evolution during this period can be divided into five stages and several substages these are a cold dry stage before 10140 cal years bp a transition stage from 10140 to 8560 cal years bp an optimum stage warm wet from 8560 to 5750 cal years bp a cool dry stage from 5750 to 4000 cal years bp a transition stage from 4000 to 3000 cal years bp and a drying up stage after 3000 cal years bp these changes are presumably reflections of variations in indian monsoon intensity which according to that interpretation should have been enhanced from 8560 cal years bp slightly weakened after 5750 cal years bp and gradually diminished since 3000 cal years bp in this region

palaeoclimatic interpretation of a topographic profile across middle holocene regressive shorelines of longmu co western tibet most lakes and closed basins in tibet are surrounded by abandoned shorelines attesting for high water levels in the past two such lakes longmu co and sumxi co western tibet were the focus of detailed palaeoclimatic work during the 1989 sino french expedition based on the study of lake sediments and analysis of a piston core here it is shown that additional information may be deduced from the abandoned shorelines around longmu co a 1800 m long topographic profile was levelled from 75 m up to 230 m above the present lake elevation 5008 m on which the shorelines form a flight of horizontal terraces separated by more or less degraded scarps the elevations of the terraces were spaced regularly about 1 35 m except for 3 5 m high lacunae reccurring every 15 16 m the potential annual evapo transpiration was computed based on meteorological data collected at tien shui hai from 13 08 89 to 11 09 90 using a modified penman formula a value of 1 60 0 3 m yr was obtained under close system conditions no outlet and a synthetic curve of lake regression was derived which appears to be consistent with the measured shoreline heights it is concluded that the 1 35 m terraces represent annual regression of the lake level by contrast the 3 5 m high lacunae might reflect the periodic return about every 9 12 yr of particular climatic conditions responsible for a perturbation of the annual process of shoreline formation all the shorelines of this profile have formed probably within one century during a continuous and abrupt regression of the lake such evolution has also been identified in the sediments of a piston core collected at sumxi co and dated at about 6 5 5 kyr b p the lake level must have been at its highest stand shortly before that major regression the mechanism responsible for this middle holocene high stand and for the major sudden drop of the lake level remains uncertain

potential forcing mechanisms of holocene lake level changes at nam co tibetan plateau inferred from the stable isotopic composition of shells of the gastropod radix the timing of lake level fluctuations on the tibetan plateau and their relationship with climatic changes is still under debate and the main reason for this is the lack of suitable archives for reconstructing the paleohydrology and paleoclimatology of the lakes here we present the results of analyses of the shell geochemistry of radix sp from an exposed terrace of nam co lake on the south central tibetan plateau optically stimulated luminescence osl dating reveals that deep water lacustrine sediments formed between ca 4 4 and 2 2 ka suggesting a high and stable lake level significantly above the present the results of sr ca 13c and 18o analyses of the fossil shells of radix sp indicate that during the mid to late holocene lake level variations in nam co were mainly controlled by variations in the indian summer monsoon a trend of decreasing evaporation also played an important role comparison with other results suggests a consistent pattern of mid to late holocene lake level changes acro

geometry kinematics and landscape characteristics of an active transtension zone karakoram fault system southwest tibet abstract we investigate the style of active transtensional deformation and characteristics of the landscape along the southern segment of the karakoram fault system through field mapping structural analysis and examination of digital topography srtm aster multi spectral aster landsat 7 and panchromatic corona photography imagery our data suggests that wrench dominated transtension is occurring within a 90 km wide zone simple shear within the zone is accommodated by right lateral r and p shears as well as left lateral r shears vertical shortening is accommodated by north striking extensional shear zones and horizontal shortening is accommodated by east west trending transtension related corrugations drainage divides strike at high angles and subparallel to the transtension zone those that strike at high angles to the zone can be explained by movement along north striking extensional fault systems while the geometry of drainage divides subparallel to the zone are consistent with uplift of corrugations as well as rapid uplift of mountain ranges by normal right slip faulting

late quaternary hydrological changes inferred from lake level fluctuations of nam co tibetan plateau china abstract this paper discusses the range of lake level changes using hydro acoustic techniques and sediment core data for lake nam co in central tibet china this second largest tibetan lake is situated at an elevation of 4718 m a s l lake level changes with different high stands documented by beach remnants are widespread on the tibetan plateau and also in lake nam co they indicate significant changes in the hydrological system triggered by the monsoonal circulation using hydro acoustic techniques submerged beach remains at approximately 10 15 m 30 m 45 m 70 m and 95 m below present day lake level were detected indicating a more negative water balance than today the 10 15 m lake level is dated to an age equal to the little ice age all deeper lake levels are assumed to have been formed prior to 10000 cal bp folded older sediments detected in the hydro acoustic profiles indicate a possible larger extent of the glaciers during or prior to the last glacial maximum lgm this is followed by a low stand or even by possible desiccation of the entire lake basin during or after the lgm

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

late cenozoic right lateral strike slip faulting in southern tibet field observations collected during the sino french himalaya tibet cooperation project 1980 1982 show that right lateral faulting between southern and northern tibet roughly along the chord joining the eastern and western syntaxes of the himalayan arc is a major component of the active tectonics north of the himalayas just north of the well developed rifts of southern tibet we studied three right lateral 100 200 km long strike slip faults forming an en echelon array along that chord evidence for earthquake surface breaks was found on each of these faults for the m 8 beng co or damxung strike slip earthquake of november 18 1951 rupture parameters could be constrained to be 8 2 m and l 90 km offset glacial and postglacial morphological markers 105 years and 104 years b p imply displacement rates of the order of 1 2 cm yr on the three faults the en echelon fault zone appears to extend westward 2000 km across the plateau to the karakorum fault and is thus referred to as the karakorum jiali fault zone kjfz fast rates of slip on it and on faults along the northern rim of tibet appear to contrast with the slower rates of deformations in the interior of the plateau our estimates of slip rates on the strike slip and normal faults of southeastern tibet suggest that eastward extrusion of north tibet between the right lateral kjfz and the left lateral altyn tagh and kun lun faults absorbs at least 30 of the present convergence between india and asia we propose an instantaneous velocity model of the active tectonics of asia compatible with such faulting rates and geometries evidence for tertiary right lateral strike slip movements in southern tibet and around the eastern syntaxis of the himalayas is also found in the structural record offsets of various large scale geological features imply that these movements have been large

lake evolution during the past 30 000 years in china and its implications for environmental change abstract taking china as a whole lake expansion occurred 500 b c 0 a d 650 950 a d and 1250 1650 a d over the past 3000 yr the expansions were mainly due to climatic fluctuations and correspond to frequent flood reports three regional characteristics of lake development have been recognized 1 fluctuations of near coastal lakes and those in other regions as well were out of phase during 500 b c 500 a d which possibly resulted from out of phase fluctuations of climate and sea level on 10 to 100 yr time scale 2 as the lower yellow river channel migrated southward the lakes in the south of the north china plain formed and reached their largest sizes much later than those in the north 3 for the lakes in the middle and lower yangtze river valley the higher a lake basin lies above a neighboring river and the further the lake is from the river s mouth in general the younger the lake is through land reclamation the ancient chinese have changed most of the lakes greatly however human activities were also constrained in certain ways by the natural changes of the lakes reclamation usually was banned and farmland was abandoned to repair reservoirs while lakes were expanding

sensitivity of the divide position at siple dome west antarctica to boundary forcing a linearized perturbation about a two dimensional vialov nye icesheet profile is used to investigate the sensitivity of the divide position at siple dome west antarctica to small changes in the accumulation pattern and in the elevation of its lateral boundaries at the margins of ice streams c and d relaxation time scales for the icesheet surface and divide position are derived from the perturbation theory for siple dome these time scales are short 450 800 years for surface adjustment and 200 350 years for divide position adjustment these short time scales indicate that siple dome responds quickly to forcing at its boundaries therefore the recent migration of the siple dome divide determined from previous work is probably a response to an ongoing sustained forcing rather than a response to a long past climate event such as the transition from the last glacial maximum to the holocene based on our analysis the inferred rate of migration of the siple dome divide could be attained be

modelling hillslope evolution linear and nonlinear transport relations many recent models of landscape evolution have used a diffusion relation to simulate hillslope transport in this study a linear diffusion equation for slow quasi continuous mass movement e g creep which is based on a large data compilation is adopted in the hillslope model transport relations for rapid episodic mass movements are based on an extensive data set covering a 40 yr period from the queen charlotte islands british columbia a hyperbolic tangent relation in which transport increases nonlinearly with gradient above some threshold gradient provided the best fit to the data model runs were undertaken for typical hillslope profiles found in small drainage basins in the queen charlotte islands results based on linear diffusivity values defined in the present study are compared to results based on diffusivities used in earlier studies linear diffusivities adopted in several earlier studies generally did not provide adequate approximations of hillslope evolution the nonlinear transport relation was tested and found to provide acceptable simulations of hillslope evolution weathering is introduced into the final set of model runs the incorporation of weathering into the model decreases the rate of hillslope change when theoretical rates of sediment transport exceed sediment supply the incorporation of weathering into the model is essential to ensuring that transport rates at high gradients obtained in the model reasonably replicate conditions observed in real landscapes an outline of landscape progression is proposed based on model results hillslope change initially occurs at a rapid rate following events that result in oversteepened gradients e g tectonic forcing glaciation fluvial undercutting steep gradients are eventually eliminated and hillslope transport is reduced significantly

rifting processes and ice flow modulation observed on mertz glacier east antarctica we investigated the evolution of two major rifts cutting across mertz glacier tongue east antarctica using a combination of satellite images and 60 day sets of gps data from two stations deployed either side of the western rift in 2007 the eastern rift began to open in the early 1990s and the western rift initiated in 2002 in conjunction with the collision of a large iceberg with the tongue velocity time series derived from the 2007 gps data exhibited strong variations at tidal periods modulated by sea surface height and sea surface slope and reproduced here with a conceptually simple model we found that opening of the western rift in 2002 leads to a dramatic change in behavior of the tongue as the large range in velocity 700 2400 m a 1 observed in 2000 was largely reduced 1075 1225 m a 1 in 2007 opening of the western rift decoupled the glacier from the transverse loading on the tongue driven by east west tidal circulation this loading previously induced time varying lateral drag which caused the large velocity range our results suggest changes in the mechanical behavior of an ice tongue impact the dynamics of the outlet glacier system and should be considered in longer term mass balance evaluations

first tectonic geomorphology study along the longmu gozha co fault system western tibet abstract the longmu gozha co left lateral strike slip fault system lgcf is located in remote western tibet forming a triple junction with both the altyn tagh fault atf and the karakorum fault kf the two major strike slip faults in the region the ashikule gozha co and longmu co faults are clear and distinct left stepping en echelon faults together forming the lgcf system although poorly documented quantifying its activity remains a key problem to understand the kinematics and the tectonic history of the westernmost tibetan plateau indeed the karakax fault nw segment of the atf lgcf and kf together control the tectonics of western tibet which itself controls the extrusion of tibet towards the east with the lgcf acting as a natural boundary for eastward motion of the tibetan plateau due to india s northward impingement the lgcf system shows clear and impressive morphological indications of left lateral active shear that we quantify using field measurements terrestrial lidar along with 10 be surface exposure dating our data suggest a slip rate 9 and 8 mm yr respectively yielding a few mm yr of extension accommodated most likely in the ashikule graben and surroundings located between the atf and karakax faults numerous evidences of recent tectonic related events are present in the vicinity such as the 1951 volcanic eruption as well as the 2008 and 2014 ms 7 3 yutian earthquakes attesting of its high activity in addition the lgcf s en echelon geometry and identical direction with the atf as well as smaller geological offsets and lower slip rate compared to those on the surrounding faults suggest that this segment of the atf may be the most recent

intra seasonal hydrological processes on the western tibetan plateau monsoonal and convective rainfall events at 7 5 ka abstract the tibetan plateau and the himalayas influence monsoonal circulation patterns and represent the water towers of humanity modern coupling of atmospheric circulation and the hydrological cycle over and on the plateau can be observed and delineated and the factors that control lake levels can be identified recent monitoring of lakes has shown that many have grown for decades due to higher meltwater inflow or stronger rainfall from different sources depending on the location the long term dynamics of lakes however can be best described by using high resolution climate archives we focus here on the often controversial discussion of holocene lake development and use the bangong co drainage basin on the western tibetan plateau as a case study we aim to identify the factors that influence lake level such as monsoonal or convective precipitation and meltwater to accomplish this shells of the aquatic gastropod genus radix were collected from an early middle holocene sediment sequence in the nama chu subcatchment of the eastern bangong co and sclerochronlogical isotope patterns for five shells were obtained at a weekly to submonthly resolution our data suggest that at ca 7 5 ka monsoonal rainfall was higher than it is today however summer precipitation was not continuous but occurred as extended moisture pulses this result implies that the northern boundary of the sw asian monsoon was similar to that today we identify convective rainfall events significantly stronger than today and relate these events to higher soil moisture and larger lake surface areas under higher insolation conditions the regional amount of meltwater corresponds with the westerly derived winter snowfall exceptionally heavy 13c values archived in the shells were likely at least partly triggered by biogenic methane production we suggest that our approach is suitable for studying other lake systems on the tibetan plateau from which fossil radix shells can be obtained

tracer transport in the greenland ice sheet constraints on ice cores and glacial history the climate history and dynamics of the greenland ice sheet are studied using a coupled model of the depositional provenance and transport of glacier ice allowing simultaneous prediction of the detailed isotopic stratigraphy of ice cores at all the major greenland sites adopting a novel method for reconstructing the age depth relationship we greatly improve the accuracy of semi lagrangian tracer tracking schemes and can readily incorporate an age dependent ice rheology the larger aim of our study is to impose new constraints on the glacial history of the greenland ice sheet leading sources of uncertainty in the climate and dynamic history are encapsulated in a small number of parameters the temperature and elevation isotopic sensitivities the glacial interglacial precipitation contrast and the effective viscosity of ice in the flow law comparing predicted and observed ice layering at ice core sites we establish plausible ranges for the key model parameters identify climate and dynamic histories that are mutually consistent and recover the past depositional elevation of ice cores to ease interpretation of their climatic records with the coupled three dimensional model of ice dynamics and provenance transport we propose a method to place all the ice core records on a common time scale and use discrepancies to adjust the reconstructed climate history analysis of simulated grip ice layering and borehole temperature profiles confirms that the grip record is sensitive to the dynamic as well as to the climatic history but not enough to strongly limit speculation on the state of the greenland ice sheet during the eemian in contrast our study indicates that the dye 3 and camp century ice cores are extremely sensitive to ice dynamics and greatly constrain eemian ice sheet reconstructions we suggest that the maximum eemian sea level contribution of the ice sheet was in the range of 3 5 4 5 m

conjugate strike slip faulting along the bangong nujiang suture zone accommodates coeval east west extension and north south shortening in the interior of the tibetan plateau this is the published version copyright 2002 american geophysical union all rights reserved

isotopic tracing of the dissolved u fluxes of himalayan rivers implications for present and past u budgets of the ganges brahmaputra system u activity ratios have been measured in the dissolved loads of selected rivers from the himalayan range in central nepal and from the bangladesh as well as in some rain waters a few european and asian rivers have also been analyzed for their u activity ratios the data confirm the negligible effect of rainwater on the budget of dissolved u in river waters the results also indicate that rivers on each himalayan structural unit have homogeneous and specific u isotope compositions i 234u 238u activity ratios slightly lower than unity in the dissolved load of the streams draining the tethyan sedimentary series tss ii values slightly higher than unity for waters from the high himalaya crystalline hhc and the lesser himalaya lh iii systematically higher 234u 238u activity ratios for waters from the siwaliks thus u activity ratios in association with sr isotopic ratios can be used to trace the sources of dissolved fluxes carried by these rivers coupling of u with sr isotope data shows 1 that the u carried by the dissolved load of the himalayan rivers mainly originates from u rich lithologies of the tss in the northern formations of the tibetan plateau and 2 that the elemental u and sr fluxes carried by the himalayan rivers at the outflow of the highlands are fairly homogeneous at the scale of the himalayan chain rivers flowing on the indian plain define a different trend from that of the himalayan rivers in the u sr isotopic diagram indicating the contribution of a specific floodplain component to the u and sr budgets of the ganges and the brahmaputra the influence of this component remains limited to 10 to 15 percent for the u flux but can contribute 35 to 55 of the sr flux the variations of the sr and u fluxes of the ganges brahmaputra river system in response to climatic variations have been estimated by assuming a temporary cut off of the chemical fluxes from high altitude terrains during glacial episodes this scenario would significantly decrease the dissolved u flux of the ganges brahmaputra river system and increase its u activity ratio such a climatic dependence of the himalayan u flux could induce a periodic variation of the mean u activity ratio of the world rivers on glacial interglacial timescales

fault slip rates and initiation age based on diffusion equation modeling wasatch fault zone and eastern great basin models of the evolution of fault scarp morphology provide time elapsed since slip initiated on a faulted surface and may therefore provide more accurate estimates of slip rate than the rate calculated by dividing scarp offset by the age of the ruptured surface to accomplish this task linear and nonlinear models of sediment transport are calibrated from the morphology of lake bonneville shoreline scarps and fault scarps formed by multiple surface rupturing earthquakes along the wasatch fault zone wfz profile modeling of scarps formed by several events distributed through time is done using a constant slip rate csr solution and yields a value of a 1 2 slip rate diffusivity time elapsed since slip initiated on a fault is determined by establishing a value for and measuring total scarp offset csr nonlinear modeling 0 2 8 1 1 m2 kyr wfz of faults along the west slope of the oquirrh mountains indicates a slip rate of 0 1 mm yr since 50 to 65 ka which is corroborated by cosmogenic dating 10be 26al age 75 ka the slip rate along the west flank of the stansbury mountains varies from 0 04 to 0 2 mm yr for time frames of 10 to 100 ka with the most recent rupture on the northern portion of the fault zone 10 ka scarp analysis of the southern end of the nephi segment wfz suggests either temporal clustering or variable slip rate as indicated by differences in the short term 1 3 mm yr for 4 3 ka versus long term 0 4 mm yr for 70 ka slip rates

evidence for general instability of past climate from a 250 kyr ice core record recent results1 2 from two ice cores drilled in central greenland have revealed large abrupt climate changes of at least regional extent during the late stages of the last glaciation suggesting that climate in the north atlantic region is able to reorganize itself rapidly perhaps even within a few decades here we present a detailed stable isotope record for the full length of the greenland ice core project summit ice core extending over the past 250 kyr according to a calculated timescale we find that climate instability was not confined to the last glaciation but appears also to have been marked during the last interglacial as explored more fully in a companion paper3 and during the previous saale holstein glacial cycle this is in contrast with the extreme stability of the holocene suggesting that recent climate stability may be the exception rather than the rule the last interglacial seems to have lasted longer than is implied by the deep sea specmap record4 in agreement with other land based observations5 6 we suggest that climate instability in the early part of the last interglacial may have delayed the melting of the saalean ice sheets in america and eurasia perhaps accounting for this discrepancy

a holocene climatic record from arid northwestern china abstract the history of climatic changes during the interval 8500 3000 cal yr b p has been reconstructed from stratigraphical and chronological studies and the results of total organic carbon toc total inorganic carbon tic element composition pollen and stable isotope analyses of a section along the hongshui river in the southern tengger desert nw china the record suggests that from 8450 bottom of the studied section to 7500 yr b p the climate was characterized by instability from 7500 to 5070 yr b p the climatic conditions improved and can be divided into two parts a warm humid spell between 7290 and 6380 yr b p during which the average temperature was 3 4 c higher than that of today and a warm dry spell lasting from 5950 to 5720 yr b p the climate deteriorated between 6380 and 5950 yr b p from 5720 to 5070 yr b p the temperature decreased but humidity increased an abrupt temperature drop occurred between 5340 and 5290 yr b p that indicated the decline of the warmer and humid mid holocene climate from 5070 yr b p onward the climate oscillated significantly and there were three large temperature decreases coinciding with high mountain glacier advances between 5070 4670 yr b p 4300 3740 yr b p and 3410 3230 yr b p top of the section respectively the climatic fluctuations recorded in the southern tengger desert appear in phase with climatic changes recognized in the tibetan plateau suggesting that the period between 7290 and 6380 yr b p was the most warm humid spell one extremely dry event occurred at ca 3000 yr b p and subsequently the fluvial lacustrine depositional process terminated and wind action prevailed in the area both of these features can be attributed to the rapid strengthening and weakening of the summer monsoon circulation which are closely connected with global changes

peregrinations of the greenland ice sheet divide in the last glacial cycle implications for central greenland ice cores abstract the superb quality of the climate chronology archived in the summit greenland ice cores grip gisp2 testifies that the greenland ice sheet divide has been generally stable through the last glacial cycle the ice sheet has experienced a broad range of paleoclimate conditions ice sheet margin configurations and internal dynamical adjustments in glacial interglacial transitions however it is unlikely that the summit region escaped shifts in ice divide position geometry elevation and flow characteristics details of this dynamical history are important to several aspects of ice core studies the magnitudes of pure and simple shearing reconstruction of vertical ice velocity the explicit location of the ice divide and the divide residence time at different locations are all of interest in interpretation of climatic variables and physical properties of ice in the ice cores we apply a three dimensional thermomechanical ice sheet model to examine the evolution of these dynamical variables over the last 160 kyr in central greenland while a high elevation ice dome is present in the summit region throughout the simulation ice divide migrations of up to 150 km are predicted all points in the vicinity of the summit ice cores including the modern divide have been subject to flowline shifts and variable non zero shear deformation during the adjustment from glacial to holocene conditions from ca 10 ka to the present modelled divide peregrinations and strain rate history are consistent with the observed disturbance of deep ice in the grip and gisp2 ice cores which has muddled paleoclimate reconstructions for the last interglacial eemian period in greenland dynamical excursions are also evident north of the modern summit where the ngrip ice core is currently being drilled dahl jensen et al j glaciol 43 1997 300 306 however the prevailing flow direction and deformation regime at the ngrip site are much more stable than those at grip and gisp2 in the simulations combined with the greater depth of ice at this site this lends cautious optimism to the hope that eemian ice at ngrip may contain an intact record of eemian climate

diagenesis of analcime bearing reservoir sandstones the upper permian pingdiquan formation junggar basin northwest china abstract the junggar basin is one of the largest and most important oil producing basins in china in which upper permian lacustrine oil shales are among the thickest and richest source rocks in the world the upper permian pingdiquan formation was deposited predominantly in fan delta sequences within a lacustrine setting in the northeastern junggar basin the pingdiquan formation sandstones volcanic and feldspathic litharenites constitute the principal oil reservoirs whereas the interbedded black shales are the predominant oil source rocks the early diagenetic minerals in the sandstones include siderite pyrite analcime albite calcite and trace amounts of halite late diagenetic minerals include k feldspar ankerite and minor mixed layer clay minerals a similar diagenetic sequence as recognized in the intercalated mudrocks which contain abundant analcime albite and microcrystalline dolomite early authigenic mineral formation e g calcite versus analcime albite was controlled by alternating periods of lower and higher salinity in the lacustrine environment cementation by siderite analcime calcite and albite substantially occluded sandstone porosity early in diagenesis however extensive dissolution of analcime cement and detrital feldspars during burial produced significant secondary porosity in the sandstones improving their reservoir potential organic acids generated during oil shale maturation may be responsible for secondary porosity in the interbedded sandstones

holocene lake level variations of longmu co western qinghai tibetan plateau longmu co is surrounded by conspicuous regressive shorelines attesting to high water levels in the past in this study we dated paleoshoreline sediments around the lake using optically stimulated luminescence osl infrared stimulated luminescence irsl and radiocarbon dating then explored the forces that drove holocene lake level variations we found that 1 osl irsl ages are consistent with geomorphic sequences and some extant cosmogenic and u th shoreline ages however inorganic evaporite crust 14c ages are much older than corresponding osl irsl ages due to varying lake reservoir effects and should be used with caution 2 the highest lake levels of longmu co 160 145 m above present lasted from 12 8 to 11 3 ka two periods of abrupt lake level declines occurred at 11 3 and 7 4 ka lake level dropped 44 and 45 m respectively with a gradual regression of 18 m between these two intervals the lake expanded moderately from 7 to 5 5 ka then declined once more after 5 5 ka with this regression accelerating during the last 1 5 ka 3 the deglacial lake level transgression was likely due to glacier melt waters the indian summer monsoon reached the longmu co watershed and brought precipitation during the early holocene middle to late holocene lake regression was caused by the retreat of the monsoon and consequent environmental deterioration

temperature accumulation and ice sheet elevation in central greenland through the last deglacial transition we present a combined heat and ice flow model constrained by measurements of temperature in the greenland ice sheet project 2 gisp2 borehole and by the gisp2 18o record and depth age scale which determines a history of temperature accumulation rate and ice sheet elevation for the past 50 000 years in central greenland important results are that the temperature increase from average glacial to holocene conditions was large approximately 15 c with a 20 c warming from late glacial to holocene that the average accumulation rate during the last glacial maximum between 15 and 30 kyr b p was 5 5 to 7 cm yr 1 approximately 25 of the modern accumulation rate that long term 500 1000 years averaged accumulation rate and temperature have been inversely correlated during the most recent 7 millennia of the holocene and that the greenland ice sheet probably thickened during the deglacial transition the inverse correlation of accumulation rate and temperature in the mid and late holocene suggests that the greenland ice sheet is more prone to volume reduction in a warmed climate than previously thought and demonstrates that accumulation rate is not a reliable proxy for temperature the elevation history of the ice sheet is poorly constrained by the model and independent evidence is needed we also present a simple estimate of the response time for thinning of the interior region of an ice sheet due to retreat of its margins this was approximately 1900 years for central greenland during deglaciation

flow at ice divide triple junctions 1 three dimensional full stokes modeling 1 ice domes are either axisymmetric high points along ridges or ridge triple junctions we model time dependent isothermal flow near triple junctions solving the full set of mechanical equations with a nonlinear power law rheology forcing is applied through the boundary conditions which affect flow patterns at outlets where such forcing is purely axisymmetric an axisymmetric dome is formed if a threefold symmetry in the forcing is applied the axisymmetric dome breaks up into three ridges subtending angles of 120 sets of experiments where the forcing was not exactly threefold symmetric by angle or by amplitude caused the triple junction to migrate to a new steady state here in steady state the ridges join the triple junction at nearly 120 but one ridge curves to satisfy the boundary forcing the slope pattern in the immediate dome vicinity depends only on a dimensionless parameter which is a function of the ice consistency the accumulation and the rheological power law index attempts to replicate the topography around summit greenland obtained a good fit with n 3 at a triple junction the dome is really distinct from the surrounding ridges contrary to the highest point of a single ridge divide as a consequence the raymond effect is at its strongest at the dome and weakens considerably over one ice thickness as one moves away from the flow center along the ridges leaving the dome the raymond effect is still present and decreases with the ratio of the flow across and along the ridge in the vicinity of the dome horizontal strain rates vary strongly from uniaxial to biaxial large scale effects represented in our model as fluxes at boundaries seem to be the primary controls on dome position and shape

holocene climatic change and the nomadic anthropocene in eastern tibet palynological and geomorphological results from the nianbaoyeze mountains abstract our study provides detailed information on the lateglacial landscape and vegetation development of tibet based on a suite of geomorphological and palynological proxy data from the nianbaoyeze shan on the eastern margin of the tibetan plateau 33 n 101 e 3300 4500 m asl we reconstruct the current state as a function of climate history and the longevity of human influence study results constrain several major phases of aeolian sedimentation between 50 15 ka and various glacier advances during the late pleistocene the holocene and the little ice age increased aeolian deposition was primarily associated with periods of more extensive glacial ice extent fluvial and alluvial sediment pulses document an increase of erosion starting at 3926 79 cal yr b p coinciding with cooling neoglacial and a growing anthropo zoogenic influence evidence for periglacial mass movements indicate that the late holocene cooling started at around 2000 cal yr b p demonstrating increased surface activity under the combined effects of human influence and climate deterioration the onset of peat growth generally depended on local conditions that include relief meso climate and in more recent times also on soil compaction due to animal trampling we distinguish three initiation periods of peat growth 12 700 10 400 cal yr b p for flat basins inside last glacial terminal moraines 7000 5000 cal yr b p for the main valley floors and 3000 1000 cal yr b p for the higher terrace surfaces the holocene vegetation history started with an open landscape dominated by pioneer shrubs along braided rivers picea juniperus abies and betula trees accompanied by a successive closing of the vegetation cover by poaceae cyperaceae and herbs 9800 8300 cal yr b p first signs of nomadic presence appear as early as 7200 cal yr b p when temperatures were up to 2 c warmer than today forest remained very patchy with strong local contrasts during the following cooling phase 5900 2750 cal yr b p the natural vegetation was transformed by nomadic grazing to bistorta rich kobresia pygmaea pastures modern nomadic migration routes were established at least 2200 years ago overgrazing and trampling led to the shrinking of bistorta and the spreading of annual weeds short lived cold events 8000 6200 3500 cal yr b p impacted on the vegetation only temporarily as the transformation of the natural poaceae rich vegetation into kobresia pastures modified the influence of the tibetan plateau hot plate on the monsoon system our data even point to an early start of a nomadic anthropocene nearly 6000 years ago against the background of a very long grazing history modern tibet must be seen as a cultural landscape

reaction of the accumulation zone portions of glaciers to climatic change the response of the accumulation regions of glaciers to changes in accumulation rate and in surface temperature is calculated by considering perturbations to the ice flow the analysis is limited to glaciers in which flow is dominated by internal shear and not by bottom sliding and for which flowlines are geographically parallel in general glaciers begin to alter thickness immediately after a change in accumulation rate but the effect of a change in surface temperature is delayed by the time for this temperature change to penetrate to depth in the glacier a warming leads to glacial thinning the amount and timing of the response is very different for different glaciers characteristic times are on the order of tens of years for mountain glaciers and tens of thousands of years for the east antarctic ice sheet as an example the theory is applied to the ice sheet near byrd station antarctica for hypothetical changes in surface accumulation rate and temperature rates and amounts of thickness change are calculated and it is found that the measured thinning is probably too fast to have been caused by climatic variation alone the effects of hypothetical climatic variations on the depth age and depth temperature relationships through the ice sheet are also calculated it is hoped that the theory will prove valuable in the interpretation of future drill hole results

relative dating of quaternary moraines rongbuk valley mount everest tibet implications for an ice sheet on the tibetan plateau abstract relative dating studies applied to high altitude moraines 5000 5500 m in the rongbuk valley on the northern flank of mt everest reveal strong contrasts in the weathering characteristics of the boulders exposed along moraine crests these differences serve to define three intervals of major pleistocene glaciation that on the basis of the degree of weathering are interpreted to extend back to at least the penultimate glaciation and probably encompass at least one still older glaciation either interpretation indicates that some of these moraines are considerably older than their previously assigned ages the magnitude of equilibrium line lowering during neoglacial and late pleistocene times is calculated to be ca 50 100 and 350 450 m respectively the data described here are incompatible with the recently proposed model kuhle 1987 for large scale ice sheet development on the tibetan plateau the reconstructed equilibrium line lowering in the everest region is only 30 of that cited in the ice sheet model moreover the flow patterns and geometry of the former rongbuk glaciers are in opposition to those proposed by the model based on the data from the everest region it appears that valley glaciation rather than ice sheet growth characterized the southern margin of the tibetan plateau during the middle and late pleistocene glaciations